Excited State Intramolecular Proton Transfer (ESIPT)
z '., to Aromatic Carbon
Matthew Joseph Lukeman
B.Sc. (Honours), St. Francis Xavier University, 1999
A Dissertation Submitted in Partial Fulfilment of the Requirements for the Degree of
Doctor of Philosophy in the Department of Chemistry
0 Matthew Joseph Lukeman, 2003
University of Victoria
All rights reserved. This dissertation may be reproduced in whole or in part, by photocopying or by other means, without the permission of the author.
Supervisor: Dr. Peter C. Wan
Abstract
Excited state intramolecular proton transfer (ESIPT) reactions occur when the acidity and basicity of groups on the same molecule are sufficiently enhanced on
excitation to permit a proton transfer between these sites. Most studies have been carried out for proton transfers between heteroatoms, though ESIPTs to carbon atoms (alkenyl or alkynyl) have been reported, and these give rise to new and predictable products. This Thesis outlines the discovery of a new class of ESIPT reactions in which aromatic carbon atoms act as the proton accepting group. The reaction either leads to deuterium exchange of the starting material, or gives new products not readily accessible by thermal means.
The photochemical deuterium incorporation at the 2' and 4'-positions of 2- phenylphenol(18) and equivalent positions of related compounds has been studied in D20-CH,CN solutions with varying D,O content. Predominant exchange was observed at the 2'-position with an efficiency that is independent of D 2 0 content. The data are
consistent with an exchange mechanism that involves ESIPT from the phenol to the 2'- carbon position of the benzene ring not containing the phenol, to generate the
corresponding keto tautomer (an o-quinone methide (QM)). This is the first explicit
example of a new class of ESIPT in which an acidic phenolic proton is transferred to an sp2- hybridized carbon of an aromatic ring. Examination of related derivatives led to postulation of a mechanism of ESIPT that requires an initial hydrogen bonding
interaction between the phenol proton and the benzene n-system. '
. . * 111
(59 and 60) was studied. Irradiation of these derivatives in 1 :9 H,O-CH,CN gave rise to dihydrobenzoxanthene products. Deuterium exchange was observed in both these products and recovered starting material when D,O was the co-solvent. The reactivity is
rationalized by a mechanism involving solvent-mediated ESIPT processes to the 2'- and
7'-positions of the naphthyl ring for both 59 and 60. Evidence for the intermediacy of QM intermediates was obtained by laser flash photolysis (LFP) experiments. These reactions represent the first examples of net product formation (other than simple deuterium exchange) resulting from ESIPT to an aromatic carbon atom.
The photochemistry of naphthols 10, 62, and 63 has been studied in aqueous solution with the primary aim of exploring the viability of such compounds for
naphthoquinone methide (NQM) generation. Our results show these derivatives give rise to three types of NQMs (61,88, and 64) on irradiation. Photolysis of the parent 1 -
naphthol (10) in neutral aqueous solution gave 1,5-NQM 61 as well as the non-KekulC 1,s-NQM 88, both via the process of water-mediated ESIPT, based on observation of deuterium exchange at the 5- and 8-positions, respectively, on photolysis in D,O-CH,CN.
A transient assignable to the 1,5-NQM 61 was observed by LFP experiments. Evidence
is presented that suggests a related ESIPT reaction operates in 1 -pyreno1 (89). The more
conjugated I ,5-NQM 64 was formed efficiently via photodehydroxylation of 62; isomeric
63 was unreactive. The efficient photosolvolytic reaction observed for 62 opens the way to design related naphthol systems for application as photoreleasable protecting groups by virtue of the long-wavelength absorption of the naphthalene chromophore.
Table of Contents
. .
Abstract. . .
11 Tableofcontents. . .
v ListofTables. . .
ix ListofFigures. . .
x . . Listofschemes. . .
xvll ... List of Important Abbreviations . . . xvlli List of Structures. . .
xixAcknowledgements
. . .
xxi...
Dedication. . .
xxll 1 Chapter 1-
Introduction 1 .I Basic Photophysical Processes. . .
11.2 Changes in Nuclear Configuration
. . .
. 21.2.1General
. . .
2. . .
1.2.2 Singlet States of Rigid Aromatic Hydrocarbons . 4. . .
1.2.3 Singlet Excited States of Biaryl Hydrocarbons 5 1.2.3.1 Fluorescence of Biaryls with Low Steric Repulsion. . .
6. . .
1.2.3.2 Fluorescence of Biaryls with Medium Steric Repulsion 7 . . . 1.2.3.3 Fluorescence of Biaryls with High Steric Repulsion 9 1.3 Effects of Substitution of Aromatic Hydrocarbons. . .
1 1 1.3.1General. . .
11vi
1.3.2 Reactions Arising from Enhanced Electron Donation in S,
. . .
121.2.3.1 Photosolvolysis of Benzylic Compounds
. . .
121.2.3.2 Photoprotonation of Aromatic Rings
. . .
131.3.3 Reactions Arising from Enhanced Electron Withdrawal in S,
. . .
191.3.3.1 Photogeneration of Nitrobenzyl Carbanions
. . .
191.3.3.2 Photoredox Behaviour of Hydroxyrnethylanthraquinone
. . .
201.4 Enhanced Acidity of Hydroxyarenes in S,
. . .
201.5 Excited State Intramolecular Proton Transfer (ESIPT) . . . 27
1.5.1General
. . .
271.5.2 Intrinsic ESIPT
. . .
281.5.3 Water-Mediated ESIPT
. . .
301.5.4 ESIPT to Carbon Atoms
. . .
321.5.5 ESIPT to Aromatic Carbon . . . 38
1.6 Proposed Research
. . .
39Chapter 2 . Excited State Intramolecular Proton Transfer to Aromatic Carbon in Simple o-Hydroxybiaryls 2.1 Introduction
. . .
46. . .
2.2 Materials 47 2.3 Product Studies. . .
482.4 Fluorescence Measurements
. . .
62vii
2.6 Mechanism of Deuterium Incorporation
. . .
68Chapter 3
.
Photocyclization of o-Naphthyl Hydroxyarenes3.1 Introduction . . . 74
. . .
3.2 Synthesis and Materials - 7 5
. . .
3.3 Ground State Geometries of Substrates 76
. . .
3.4 UV-vis Studies 80 . . . 3.5 Product Studies 83. . .
3.6 Fluorescence Spectroscopy 92. . .
3.7 Laser Flash Photolysis 101
. . .
3.8 Mechanism of Reaction 107
Chapter 4 . Excited State Intramolecular Proton Transfer in 1-Naphthol and 1-Pyrenol
. . . 4.1 Introduction 116
. . .
4.2 Materials 119. . .
4.3 Product Studies 119. . .
4.3.1 Deuterium Exchange Studies of 10 and 9 119
. . .
4.3.2 Photodehydration of 62 and 63 124
. . .
4.3.3 Deuterium Incorporation Studies of 89 126
. . .
4.4 Fluorescence Studies 129
. . .
... V l l l
. . .
4.5 Mechanism of Reaction 142. . .
4.5.1 ESIPT in 10 142. . .
4.5.2 Photodehydroxylation of 62 and ESIPT of 63 144
. . . 4.5.3 ESIPT in 89 147 4.6 Summary
. . .
151 Chapter 5-
Experimental 5.1 General. . .
153 . . . 5.2 Materials 154 . . .5.2.1 Common Laboratory Reagents 154
. . .
5.2.2 Synthesis 154. . .
5.3 Product Studies 161. . .
5.3.1General 161. . .
5.3.2 Results of Product Studies 162
. . .
5.3.3 UV-vis Studies 166
. . .
5.3.4 Reaction Quantum Yields 166
. . .
5.4 Steady State Fluorescence and Lifetime Measurements 168
. . .
5.5 Laser Flash Photolysis (LFP) 170
. . .
5.6 X-Ray Crystallography 172
. . .
Table 1.1 Table 1.2 Table 2.1 Table 2.2 Table 3.1 Table 3.2 Table 3.3 Table 4.1 Table 4.2 Table 5.1 Table 5.2 Table 5.3 Table 5.4 1X List of Tables
Excited State pKa Values for 2-Naphthols . . . 25
Acidity Data for Phenylphenols
. . .
26Deuterium Exchange Data for 18
. . .
- 5 0 Photophysical and Photochemical Parameters for Hydroxybiphenyls . . . 6 1 Calculated Ground State Structural Parameters for Hydroxybiaryls . . . . 77Product Quantum Yields for 58 . 60
. . .
91Relative Eneries of Chromophores in Hydroxybiaryls . . . 114
Photochemical and Photophysical Parameters for Naphthols . . . 125
Summary of LFP Data for 1-Naphthols . . . 141
Reaction Quantum Yield of Reactive Compounds
. . .
168Fluorescence Quantum Yields . . . 170
Fluorescence Lifetimes . . . 171
Fig. 1.1 Fig. 1.2 Fig. 1.3 Fig. 1.4 Fig. 1.5 Fig. 1.6 Fig. 2.1 Fig. 2.2 Fig. 2.3 Fig. 2.4 X List of Figures
Absorption and fluorescence spectra of anthracene in cyclohexane.
. . . .
. 4 n-Bond orders and relevant valence structures for biphenyl in So(left) and S, (right).
. . .
. 5 Absorption and fluorescence emission spectra of biphenyl. . . 7 Absorption and fluorescence emission spectra of 1,l I-binaphthyl incyclohexane
. . .
8 Absorption and fluorescence emission spectra of 9-phenylanthracene in cyclohexane. . .
10Orbital diagrams of the HOMO (top) and LUMO (bottom) of
Phenol (left) and 10 (right) as predicted by HMO theory. Lobe
diameters are proportional to the orbital coefficient at that atom,
and the shading represents the phase . . . . 2 2 Expanded aromatic region of the 'H NMR (360 MHz) of 18 before
(bottom) and after (top) 1 h irradiation in 1 : 1 D,O-CH,CN.
. . .
. 4 9Plot showing the dependence of deuterium exchange at the 2'-position (. , solid line) and the 4'-position ( 0 , dashed line)
of 18 on irradiation time in 1 : 1 D20-CH,CN.
. . .
. 5 1 Plot showing the dependence of photochemical deuteriumexchange at the 2-position
(m,
solid line) and 4'-position (*, dashedline) for 18 on concentration of D 2 0 (in CHJN). . . . . 5 4 Plot showing the efficiency of exchange at the 2'-position of 18 (A, solid
Fig. 2.5 Fig. 2.6 Fig. 2.7 Fig. 2.8 Fig. 2.9 Fig. 3.1 Fig. 3.2
line) after photolysis in solutions containing varying amounts of CH,OD (in CH,CN), as measured by 'H NMR, Data for exchange of the same position of 18-OD in neat cyclohexane is
also shown (m). Dashed line indicates the expected exchange
efficiencies at low CH,OD concentrations if H,O contamination couldbeeliminated
. . .
Plot showing the efficiency of exchange at the 6-position of 55
after irradiation in solutions containing varying D 2 0 concentrations (in CH3CN), as determined by 'H NMR (360 MHz).
. . .
AM1 geometry optimized structures of 18 (top) and 52 (bottom).
. . .
Fluorescence emission spectra of 18 in CH3CN containing different amounts of water (amounts given in legend). Emission band at 420 nrn at high water concentrations is assigned to phenolate 18-. . . .
Fluorescence emission spectra of 52 in CH3CN containing different
amounts of water (amounts given in legend). Emission at 390 nm
. . .
at high water content is assigned to phenolate 52-.
Triplet-Triplet absorption spectra of 18 (*, dashed line), 19
(0, solid line), and 2 (x, dotted line) detected by LFP in CH,CN
(3,
purged). Data points were taken immediately after the laser pulse. . 67 AM 1 geometry optimized structures of 58 (top), 59 (middle),. . .
and60(bottom) 78
Fig. 3.3 Fig. 3.4 Fig. 3.5 Fig. 3.6 Fig. 3.7 Fig. 3.8 Fig. 3.9 xii
of 58 ( 0 , dashed line), 59 (0, dotted line), and 60 (A, solid line),
as predicted by AM1 semi-empirical calculations (Hyperchem 7). . . 79
. . .
X-ray crystal structure of 58.
UV-vis traces showing the phototransformation of 59 with on
photolysis in 1 :9 H20-CH,CN. Traces were recorded after 0,
0.25, 0.5, 1 , 2 , 3 , 5, and 7 min of irradiation time. . .
UV-vis spectra showing the phototransformation of 60 with
photolysis time. Traces recorded after 0,0.33, 1, 1.7,2.3, 3,
3.7, and 8.3 min of photolysis (A 300nrn, 1:9 H,O-CH,CN,
argonpurged).
. . .
82500 MHz 'H NMR spectrum of 79 (in CDCl,), formed on
irradiation of 59 in 1 :9 H20-CH,CN (argon purged). Assignments
were made with the aid of the COSY spectrum. . . - 8 4
360 MHz 'H NMR spectra of 59 (in acetone-d,) before (bottom)
and after (top) I h irradiation in 1 : 1 D20-CH,CN (argon purged), showing the decrease in the integrated area of the peak at 7.90 ppm
assignedtoH,.
. . .
86500 MHz 'H NMR spectrum of 81, formed on irradiation of 60
in 1 :9 H,O-CH,CN. Partial assignment made with aid of COSY
spectrum.
. . .
87500 MHz 'H NMR spectra of 60 after 10 (bottom) and 30 (top)
Fig. 3.10 Fig. 3.1 l Fig. 3.12 Fig. 3.13 Fig. 3.14 Fig. 3.15 Fig. 3.16 Fig. 3.17 ... X l l l
integrated area of the peak at 8.02 assigned to H,.
. . .
89Plot showing the dependence of the yield of W 2 ' D ( 0 ,
thin line) and 79-7'0 (m, thick line) (formed on irradiation
of 59) on the concentration of D 2 0 present in solution. . . 90 Excitation and emission spectra of 58 in neat CH3CN (thick
lines) and 58- in H,O pH 12 (thin lines).
. . .
93Fluorescence excitation and emission spectra of 59 in neat CH3CN.
. .
. 9 4 Fluorescence quenching traces of 59 by added water (inCH3CN). Traces were recorded in 0 , 4 , 6 , 8, 10, 12, 16,
and 40% water (vlv in CH3CN).
. . .
. 9 5Fluorescence quenching traces of 59 by added water (in CH,CN). Traces were recorded in 30,40,60, 70, 80, 90,
and 100% water (v/v in CH3CN). . . . 9 5 Plot showing the quenching effects of water content on
the fluorescence emission intensity from 59 at 357 nm
(+, solid line) and 59- (A, dashed line). Intensities for 59-
were increased 1 0-fold for clarity. . . 96
Fluorescence excitation and emission spectra for 60 in neat
CH3CN (thick lines) and for 60- in 1 : 1 H20-CH,CN, pH 12. . . . 9 8
Fluorescence quenching traces of 60 with added water
(in CH3CN). Traces shown are taken in solutions with water
Fig. 3.18 Fig. 3.19 Fig. 3.20 Fig. 3.21 Fig. 3.22 Fig. 3.23 Fig. 4.1
Stem-Volmer plot for quenching of the fluorescence of 60
with added water (in CH3CN). Line drawn is fit to the
. . .
equation 411 = 1
+
K,,[H,0]'.7.Transient absorption spectra observed on LFP of 59 in 1 :9 CH3CN (+) and in neat CH3CN ( 0 ) (0, purged,
A,,
308 nm).Absorption data was taken immediately after the laser pulse. . .
Decay trace for the absorption at 530 nm observed on LFP of 59 showing the short lived species at early times and a second, longer lived speces at longer times.
. . .
Transient absorption spectrum observed on LFP of 60 in 1 :9
H20-CH3CN (A,, 308 nrn, 0, purged). Data points were taken
immediately after the laser flash. . .
Biphasic decay of the absorption at 550 nm observed on LFP of 60 in 1 :9 H,O-CH3CN (A,, 308 nm, 0, purged). Line drawn is least squares fit to the equation AOD = A
+
B(e-k't)+
C(e'k2t), and givesl;, = 580 ps and T,= 120 ps.. . .
Plot showing the variation of ESIPT quantum yields with the
difference in singlet energies of the two aryl rings.
. . .
500 MHz 'H NMR spectra of 10 before (bottom) and after (top)
20 min irradiation at 300 nm in I: l D,O-CH,CN, showing decreases
in the integrated areas of the peaks at 7.80 and 8.22 ppm assigned
xiv 100 103 104 105 lo6 114
xv
Fig. 4.2 Plot of % deuterium exchange at the 5- (A, solid line) and 8- (m,
dashed line) positions of 10 with varying solution pD
(1:1D20-CH,CN)
. . .
122Fig. 4.3 Plot of % deuterium exchange at the 5- (A, solid line) and 8- (m,
dashed line) positions of 10 with varying [D20] in CH,CN. . . . 123
Fig. 4.4 500 MHz 'H NMR spectra of 89 before (bottom) and after (after)
irradiation at 350 nrn in 1 : 1 D,O-CH,CN (2 h), showing decreases in the area of the peaks at 8.14, 8.12, and 7.99 ppm assigned to ring
protons at the 8-, 6-, and 3-positions, respectively.
. . .
127Fig. 4.5 Plot of the sum of the % deuterium exchange at the 3-, 6-, and
8-positions of 89 as a function of [D,O], as determined by MS.
. . .
128Fig. 4.6 Representative fluorescence quenching traces of 63 by added
water in CH,CN (A,, 300 nm). Water concentrations are given
in the legend. Naphtholate (633 emission grows in at 465 nm with
increasing water content. . . 130
Fig. 4.7 Representative fluorescence quenching traces of 62 with added
water in CH,CN (A,, 300 nrn). Water concentrations are given
in the legend. Naphtholate (623 emission grows in at 460 nrn
withaddedwater
. . .
131Fig. 4.8 Modified Stern-Vomer plots for steady-state fluorescence
quenching of 62 (A) and 63 ( 0 ) by added water (in CH,CN).
xvi
Fig. 4.9 Transient absorption observed on LFP of 10 in 1 : 1 H20-CH3CN
(a) and neat CH3CN ( 0 ) (A,, 308,0, purged for both). All
absorption values were taken immediately after the laser pulse. . . 135
Fig. 4.10 Transient absorption observed on LFP of 62 in 1 : 1 H20-CH3CN
(A) and in neat CH3CN (+) (A,, 308 nm, 0, purged).
. . .
137Fig. 4.1 1 Transient absorption observed on LFP of 63 in 1 : 1 H20-CH,CN
(m, solid line) and neat CH3CN (0, dashed line) (A,, 308,0, purged). . 138
Fig. 4.12 Quenching plot showing the increase in decay rate constant
of 64 with added ethanolamine. Line is the least squares fit,
and gives k, = 1.0 f 0.1 x lo6 M%-I.. . . 139
Fig. 4.13 Dynamic quenching plot of 61 (formed on LFP of 10) with
added ethanolamine. Line drawn is the least squares fit to the data and gives k, = 9.0 f 0.1 x 10' M-Is-'. Outline point was
xvii List of Schemes
Scheme 1.1 Jablonski Diagram
. . .
1. . .
Scheme 1.2 Thermal and Photochemical Deuterium Exchange of 2 16 Scheme 1.3 Resonance Structures for Substituted Biphenyls. . .
17. . .
Scheme 2.1 Synthesis of 52 47. . .
Scheme 2.2 Summary of Photochemistry of 18 69 Scheme 3.1 Scheme 3.2 Scheme 3.3 Scheme 3.4 Scheme 3.5 Scheme 3.6 Scheme 4.1 Scheme 4.2 Scheme 4.3 . . . Synthesis of Phenylboronic Acid 75. . .
Synthesisof76and58 75 Synthesis of 77 and 59. . .
76 Synthesisof78and60. . .
76 Summary of Photochemistry of 59. . .
108 Summary of Photochemistry of 60. . .
110 Summary of Photochemistry of 10. . .
143 Summary of Photochemistry of 62. . .
144 Summary of Photochemistry of 63. . .
146xviii List of Important Abbreviations
COSY EDG EWG ESPT ESIPT HFIP HOMO HRMS LFP LUMO MO MS NOESY YAG Correlated Spectroscopy Electron Donating Group Electron Withdrawing Group Excited State Proton Transfer
Excited State Intramolecular Proton Transfer
1,1,1,3,3,3-Hexafluoro-2-propanol
Highest Occupied Molecular Orbital High Resolution Mass Spectrum Laser Flash Photolysis
Lowest Unoccupied Molecular Orbital Molecular Orbital
Mass Spectrum
Nuclear Overhauser Effect Spectroscopy Yttrium Aluminum Garnet
xix
xxi Acknowledgements
First and foremost I wish to acknowledge Dr. Peter Wan for his guidance during
my Ph.D. studies. His wisdom and continuous optimism helped me maintain my focus and enthusiasm, and I sincerely thank him for the many opportunities that he afforded me over the past four years.
I wish to acknowledge my peers, past and present, in the Wan group for their friendship and insight: Dr. Darryl Brousmiche, Musheng Xu, James Morrison, John Cole, Sierra Rayne, Kaya Forest, Ryan Sasaki, Devin Mitchell, Lawrence Huck, and Mitch Flegel.
I wish to thank the members of my supervisory committee, for their helpful discussions: Dr. Tom Fyles, Dr. Robin Hicks, Dr. Juan Ausio, and Dr. James Pincock.
I wish to acknowledge Dr. Cornelia Bohne and Dr. Luis Netter for help and
training with the LFP system. Thanks also to Dr. Becky Chak for crystal structure
determination, Dr. David MacGillivray for MS analysis, and Chris Greenwood for her many helpful discussions and for carrying out numerous specialized NMR experiments. I wish to acknowledge the rest of the faculty, support staff, and students in the Chemistry Department for their support during my time here.
I wish to thank all of my friends who have made my time here enjoyable on a social level as much as on an academic level, especially: Craig, Mike, Gwen, Tamara, Steve, Joe and Kelly, Jessica and Jaida, Paul, Jackie, and Emily. Also thanks go to my family for their constant support, and for countless entertaining discussions, emails, and 'the pipe': Mom, Dad, Greg, Jen, Jack, Mark, and Ryan.
xxii
Most importantly I wish to acknowledge my wife Nicole for supporting me in my
decision to study in Victoria, and for her unwavering love, patience and encouragement throughout the past ten years.
Lastly, I wish to acknowledge the University of Victoria and NSERC for
xxiii Dedication
For my friends and family, Nicole, and Chimpy.
Chapter 1 Introduction
1.1 Basic Photophysical Processes
The absorption of light to promote a ground state molecule to its singlet-excited state (S,) is often manifested by profound changes in the electronic and nuclear
configuration of the molecule, which can open up new reactive pathways not available to the ground state species. In order to be able to predict the fates of excited states, it is important to understand the basic processes that can occur during the lifetime of the excited state. The most important of such processes are summarized in the Jablonski diagram presented in Scheme 1.1. These pathways include intersystem crossing (ISC, a change in multiplicity), internal conversion (IC, vibrational relaxation), and fluorescence (emission of a photon). These processes are often very fast, and as a result, few excited singlet state species have lifetimes longer than 10 nanoseconds. Triplet excited states
tend to be longer lived, many having lifetimes of several microseconds or longer. Of particular interest to organic chemists are chemical reactions that can occur from excited states (singlet or triplet) to give new products. Reactions from excited states must be very fast in order to compete with the other deactivational pathways, especially for singlet excited states. There are several well studied reaction types that are known to proceed from excited states, including isomerizations, electron and proton transfers, pericyclic reactions, fragmentations, hydrogen abstractions, dimerizations, and others.
A number of experimental methods exist that can offer insight into the fate of the excited states produced. Standard characterization methods (NMR, MS, IR, UV-vis, etc.) can be carried out on the products of photochemical reactions. Steady-state fluorescence spectroscopy gives information on the proportion of excited states that deactivate through emission of a photon, and provides information regarding the changes in nuclear
configuration of the molecule on excitation. Time-resolved fluorescence offers information regarding the rate of fluorescence (kJ and the lifetime of the excited state.
Nanosecond laser flash photolysis (LFP) allows direct detection of short-lived
intermediates (-20 ns I
z
I 1 s), including triplet excited states, allowing measurement ofboth their UV-vis spectra and their rates of decay. The role of triplet excited states can be further assessed by triplet quenching and sensitization techniques.
1.2 Changes in Nuclear Configuration on Excitation 1.2.1 General
3
Excitation of a molecule to S, leads to the rapid promotion of an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). According to the Franck-Condon (FC) principle, the motion of electrons is fast (k -1 015 s-') relative to the motion of nuclei (k -1 O I 3 - 10' s-I). Therefore, immediately
following excitation, the molecule has the same nuclear configuration and solvent cage as it did prior to excitation. A molecule in this state, termed the FC excited state, rapidly undergoes thermal relaxation to its lowest vibrational level (k
-
1013 - 1014 s-I). This is followed by a reorganization of its nuclear geometry and solvent cage (k-
10" - 10'' s-I) to accommodate the electronic perturbation. Once this reorganization is complete, the molecule is said to be in its equilibrium excited state, and it is this state which we refer to when we say singlet excited state or S,. It is from the equilibrium excited state that the relevant photophysical and photochemical processes take place.Fluorescence spectroscopy provides information about the geometric changes that have taken place on excitation to S,. For example, some compounds show broad and unstructured absorption, but give highly structured fluorescence emission. It can be inferred from this information that the equilibrium excited state possesses a more rigid geometry than the ground state. Similarly, most compounds show fluorescence emission with their 0-0 band at energies that are at longer wavelengths (lower energies) than the 0- 0 band of the corresponding absorption spectrum. The magnitude of the shift in the 0-0 band energy, termed the Stokes shift, is indicative of the extent to which the excited state geometry has changed from the ground state geometry. Inspection of the absorption and
fluorescence spectra for a given molecule remains the most reliable source for information on its excited state geometry.
1.2.2 Singlet Excited States of Rigid Aromatic Hydrocarbons
For unsubstituted rigid aromatic molecules (e.g. benzene, naphthalene,
anthracene, etc.), the changes in nuclear geometry and solvation on excitation are slight. As a result, these compounds usually show structured emission with very small Stokes shifts. Because the ground and excited state geometries are so similar in these respects,
250 300 350 400 450 500 550
Wavelength
(nm)
Figure 1.1 - Absorption and fluorescence spectra of anthracene in cyclohexane, data taken from ref. 1.
5
the absorption and emission spectra usually show a 'mirror image' relationship whereby the vibrational fine structure that is visible in the absorption spectrum is reproduced in the fluorescence emission spectrum. The absorption and emission spectra of anthracene in
cyclohexane are reproduced in Figure 1.1. The extremely small Stokes shift and the
structured absorption and emission are clearly visible.
1.2.3 Singlet Excited States of Biaryl Hydrocarbons
The ground state geometries of simple biaryl hydrocarbons are dictated by the subtle interplay of two opposing factors. Steric repulsion between groups ortho to the biaryl bond promote a twisting of the biaryl bond away from planarity to larger dihedral angles, while electronic considerations favour a more planar conformation in order to enhance conjugation between the two rings. In biphenyl, for example, these factors balance out to give an equilibrium geometry in which the dihedral angle between the
Ground State (So) Excited State (Sf) Figure 1.2 - n-Bond orders and relevant valence structures for biphenyl in So (left) and S, (right).
6
rings is -30" in the ground state2. On excitation, however, there is a tendency towards planarity. Imamura and Hoffman2, using SCF-CI calculations on the n-electrons of biphenyl, have calculated the approximate n-bond orders in both the ground and excited states, and have determined that the n-bond order increases from 0.26 in the ground state to 0.48 in the excited state (Figure 1.2). They rationalized the move towards planarity on excitation by the increased double bond character of the biaryl bond in S,. The authors propose a quinoid valence structure for the singlet excited state of biphenyl, as shown in Figure 1.2.
1.2.3.1 Fluorescence of Biaryl Hydrocarbons with Low Steric Repulsion
Biaryls with only hydrogen atoms at the positions ortho to the biaryl bond, such as
biphenyl or 2-phenylnaphthalene, are considered to have relatively low steric repulsion with respect to rotation about the biaryl bond. In the case of biphenyl, while the equilibrium twist angle is known2 to be -30•‹, the energy well corresponding to the twisting motion is shallow. At room temperature, molecules with a large variety of twist angles are present in significant amounts. A result of this conformational flexibility is that the absorption spectrum is broad and structureless. This is apparent in the absorption
spectrum of biphenyl that presented in Figure 1.3. As previously mentioned, on
excitation to the singlet excited state, biphenyl undergoes a twist to planarity due to the
increased double bond character in S,. The large change in nuclear configuration
Absorption Fluorescence
225 250 275 300 325 350 375 400
Wavelength
(nm)
Figure 1.3
-
Absorption and fluorescence emission spectra of biphenyl in cyclohexane, from ref. 1.fluorescence emission. The increased double bond character of the biaryl bond imparts a more rigid geometry to the S, state. As a result, the fluorescence emission is more structured than the absorption spectrum. The large Stokes shift and enhanced vibrational fine structure is visible for biphenyl in Figure 1.3.
1.2.3.2 Fluorescence of Biaryl Hydrocarbons with Medium Steric Repulsion Compounds that contain few and small functional groups substituted at the positions ortho to the biaryl bond have a higher degree of steric repulsion associated with
ground state equilibrium twist angle closer to 90" and a steeper potential well. The absorption spectrum usually has a narrower bandwidth as a result of the steeper potential well, and while the absorption is still typically unstructured, some vibrational fine structure may be visible for some substrates.
Although the ground states of molecules in this class of biaryls are more rigid than for those in the previous class, the excited state geometries tend to be looser. While a planar (or more planar) geometry for these molecules is still the most stable conformation in the excited state, the increased steric repulsion causes the potential energy well around this equilibrium excited state geometry to be much broader. As a result, the geometry of
300 350 400 450
Wavelength (nm)
Figure 1.4
-
Absorption and fluorescence emission spectra of 1 ,l '-binaphthyl in cyclohexane, from ref. 1.9
the excited state is looser and more prone to twisting. These effects are reflected in the fluorescence emission spectrum: there is generally still a sizeable Stokes shift, as
significant twisting still occurs in the excited state, although the emission is usually broad and unstructured.
Typical examples of this class are 1 -phenylnaphthalene and 1,l '-bisnaphthene. The absorption and fluorescence spectra for the latter are presented in Figure 1.4. The rigidity of the ground state of 1,l'-bisnaphthalene is manifested in the vibrational fine structure that is visible in the absorption spectrum. On excitation, the rings twist to a planar (or more planar) state that is structurally loose. The geometry change is evidenced in the large Stokes shift, and the lack of rigidity of the excited state can be inferred from the broad and structureless emission.
1.2.3.3 Fluorescence of Biaryls with High Steric Repulsion
When one aryl ring of a biaryl hydrocarbon is substituted at both positions ortho
to the biaryl bond, the barrier to rotation to planarity becomes very high. In such cases, the strong steric hindrance dominates the energetics of the aryl-aryl twisting potential energy well for both the ground and excited states. The ground state typically has a very rigid geometry with the two aryl rings perpendicular to each other. While the electronic drive to move to a planar orientation is much higher in the excited state, the high steric repulsion associated with this process usually forbids any significant twisting. Thus, S,
and S, often have similar geometries, and small Stokes shifts are often encountered. Excitation of molecules in this class leads to a widening of the potential energy well
10
around their equilibrium twist geometry. As a result, the vibrational fine structure in the fluorescence emission spectrum is broadened somewhat relative to the absorption
spectrum. The extent of this broadening decreases as the steric bulk at the ortho positions increases.
The absorption and fluorescence emission spectra of 9-phenylanthracene, a typical
example of a molecule in this class, are shown in Figure 1.5. As both positions on the
phenyl ring ortho to the biaryl bond are unsubstituted, this is a less severe example of this class of steric repulsion. Indeed, laser-induced fluorescence spectroscop~ and ab initio calculations4 both indicate that the equilbrium geometry of the ground state of 9-
290 350 410 470 530
Wavelength (nm)
Figure 1.5
-
Absorption and fluorescence emission spectra of 9-phenylanthracene in cyclohexane, from ref. 1.1 1
phenylanthracene is
90•‹,
as expected, but in the excited state, the equilibrium geometry possesses a dihedral twist angle of 60". The fluorescence emission spectrum shows a small Stokes shift, resulting primarily from the 30" twist. While the emission exhibits vibrational fine structure, it is broadened relative to the absorption, suggesting that the geometry of the excited state is somewhat looser than that of the ground state.1.3 Effects of Substitution of Aromatic Hydrocarbons 1.3.1 General
The addition of functional groups to aromatic hydrocarbons can have varying effects on their photophysics and photochemistry. Addition of simple alkyl groups often has little effect on the chromophore, except in cases where the ground state or excited state geometries are altered because of the additional steric congestion. Other functional groups can drastically change the rates of the relevant photophysical and photochemical processes. Addition of heavy atoms (ex. Br, C1, etc.), nitro, carbonyl, and other
functional groups enhances the rates of intersystem crossing. Substituents containing heteroatoms can introduce the possibility that n-n* excited states will play a role in the photochemistry and photophysics. Many functional groups enhance the oscillator strength of optical transitions of aromatic hydrocarbons, leading to higher molar absorptivities and higher fluorescence rate constants.
Of particular interest to many organic chemists are changes in the relative electron donating and accepting properties of attached functional groups on excitation, for it is changes in these properties that are often responsible for excited state reactivity. In
12 general, the electron donating properties of electron donating groups (EDGs) are greatly enhanced on excitation, leading to strong charge transfer from the EDG into the aromatic n-system. Electron withdrawing groups (EWGs) generally become much better EWGs in the excited state, causing strong charge transfer from the aromatic n-system to the
functional group.
1.3.2 Reactions Arising from Enhanced Electron Donation in S,
The relative electron donating abilities of EDGs in the ground state are quantified in terms of their Hammett parameters. A number of different Harnmett parameters are available for a given substituent, depending on whether it is substituted meta (om) o r p a r a (0,) to the reaction center, and whether the pertinent reaction proceeds via a cationic (0') or anionic (o-) intermediate. Such quantitative parameters are not available for
substituents in the excited state.
1.3.2.1 Photosolvolysis of Benzylic Compounds
Zimmerman and Sandel noticed interesting substitution effects on the photochemical fates of benzyl acetates5. The ordinary mode of reaction for these compounds is homolytic cleavage of the benzylic C - 0 bond on irradiation to yield a radical pair. The authors noticed that when EDGs were substituted at thepara and meta positions, photosolvolysis products were obtained that were interpreted as arising from heterolytic cleavage of the C - 0 bond to yield the acetate anion and a benzyl cation. The authors rationalized this behaviour as being due to enhanced electron donation of the
EDG in the excited state which promotes the heterolytic cleavage by stabilizing the resultant cationic intermediate. The authors examined a number of differently substituted derivatives and found a peculiar result: the substituent effect shows behaviour that is the reverse of typical ground state behaviour. They found that derivatives with methoxy
groups at the para position primarily gave photoproducts resulting from homolytic
cleavage, while those substituted at the meta position(s) gave proportionately higher
amounts of the photosolvolysis products. For the derivative in which both meta positions
are substituted with methoxy groups, only the photosolvolysis product was observed. The authors termed the reverse substituent effect the "meta effect", and rationalized the effect
initially with simple Hiickel molecular orbital (HMO) calculations, and decades later with more sophisticated ab initio calculations6.
While some of the conclusions of Zimmerman and Sandel have received criticism, primarily from the Pincock g r o ~ p ~ - ~ , it remains a stark experimental fact that many
excited state reactions of substituted benzene compounds involving cationic intermediates
are selectively enhanced by EDGs at the meta and ortho positions, but not those at the
para positions. These reactions include, but are not limited to, the photoprotonation of
alkenes and alkynes, photoprotonation of aromatic rings, and photodehydroxylation of arylmethyl alcohols. These reaction types will be reviewed in sections to follow.
1.3.2.2 Photoprotonation of Aromatic Rings
In addition to promoting reactions of substituents on aromatic rings, EDGs can
14
been donated into the aromatic n-system causes the ring positions to be more reactive towards electrophiles. The simplest electrophile is of course the proton (or hydronium ion in aqueous solution), and the simplest electrophilic aromatic substitution reaction is the protonation of an aromatic ring, followed by deprotonation of the same site. This reaction is very difficult to achieve in the ground state. The basicity of aromatic carbon atoms is very low in the ground state. Even for the highly activated 1,3,5-
trimethoxybenzene, the pK, is e~timated'~," to be approximately -6, a value inaccessible
by aqueous acids. Several reports'2-20 have appeared which demonstrate that many
aromatic compounds undergo protonation much more readily when electronically excited, although much of this early work has lacked quantitative or mechanistic detail.
A detailed mechanistic study of the photoprotonation of several 1,3-dialkoxy- substituted benzenes in dilute aqueous H2S04 (or D,SO,) was undertaken by Wan and co- w o r k e r ~ . ~ ' It was found that protonation proceeded from S, primarily to the 2-position (and to a much smaller extent the 5-position) of the benzene ring to give the
corresponding cyclohexadienyl cation, as shown for 1 in Eq. [l .I]. Deprotonation of this
OEt OEt
.
. . OEtI
1
1-20
intermediate ultimately resulted in hydrogenldeuterium exchange of the 2-position to form 1-20. Thermal deuterium exchange required much higher acid concentrations, and showed a different regioselectivity, with exchange occurring exclusively at the 4-position.
The regiochemistry of both the ground and excited state reactivities were correctly predicted with simple HMO calculations. Involvement of the singlet excited state as being the reactive state was inferred from the observation of fluorescence quenching by acid, and the fluorescence titration curves gave p k * values of
-
0.1-
0.5, depending onthe substrate. The quantum yields for deuterium exchange of the derivatives at pH = pK,
ranged from 0.10 to 0.25.
McClelland and coworker^^^.^^ studied several compounds with alkoxy groups in
the 1- and 3-positions and found that many of these same derivatives underwent similar hydrogeddeuterium exchange in HFIP with the same regiochemistry reported by Wan and coworkers2'. LFP of each compound in HFlP allowed observation of a single intermediate with
A,,,
= 395 - 460 nm (depending on the nature of the substituents).This transient was assigned to the corresponding cyclohexadienyl cation resulting from photoprotonation at the 2-position, providing direct evidence for the proposed reaction pathway. Subsequent work by McClelland and L e d 4 showed that this reaction can be exploited for desilylation. Photolysis of 1,3-dimethoxybenzenes substituted at the 2- position with alkyl and aryl-substituted silyl groups gave 1,3-dimethoxybenzene and the corresponding silyl alcohol or ether, depending on the solvent used. The reaction is thought to proceed via initial protonation of the 2-position on excitation by the acidic solvent to yield the P-silyl-substituted cyclohexadienyl cation, followed by selective cleavage of the C-Si bond over the C-H bond.
Shi and Wan2' studied substituted biphenyls for the possibility of photochemical protonation of the aromatic ring in the excited state. They chose 4-phenylphenol (2) and
16 4-methoxybiphenyl(3) to study since it has been previously established that alkoxy (and hydroxy) substituents activate aromatic rings towards photoprotonation. Thermal
exchange studies of 2 in 30% D2S0,-CH,C02H solution showed that deuterium exchange
was only observed at the positions ortho to the activating substituent to give 2 - 2 0
(Scheme 1.2). Irradiation of 2 in more weakly acidic solutions led to exclusive exchange
on the unsubstituted ring (90% at the 2'-position to give 2-2 'D and 10% at the 4-position to give 2-4 'D) (Scheme 1.2). Similar results were observed for thermal and
Scheme
1.2
photochemical exchange of 3. These results suggest an excited state where there is strong
charge transfer from the OH or OCH, substituent to the unsubstituted ring. This state can
17
This example clearly demonstrates how the electronic distribution can be very different for the ground and excited states of a given molecule.
Scheme 1.3
The prototropic behaviour of excited methoxynaphthalenes was first studied by
Shizuka and T ~ b i t a ~ ~ p * ~ . They observed that the fluorescence emission of 1
-
methoxynaphthalene (4) was efficiently quenched by the addition of acid, whereas the
quenching of the fluorescence of 2-methoxynaphthalene (5) was slight. While
S
fluorescence quenching by acid is suggestive of the photoprotonation of the aromatic ring, the authors sought further evidence through deuterium exchange studies similar to those performed for the methoxy-substituted benzenes and biphenyls. The results of the thermal (ground state) exchange studies were as expected: strongly acidic refluxing solutions were required to effect exchange, and for 4, exchange was exclusively at the 2- position, while for 5, exchange was exclusively at the 1 -position. The authors found that
milder. The regiochemistry of the exchange was also different; exchange occurred primarily at the 5-position to form 4-50 (Eq. [1.2]), and in smaller amounts at the 8-
position (approximately a 10: 1 ratio). The quantum yield for the reaction (in 4: 1 D20-
CH,CN, [D,O'] = 0.1 M) was measured to be 0.24, whereas the quantum yield dropped
by a factor of ten under triplet sensitization. This implied that the singlet excited state is primarily responsible for the reaction, although a minor contribution from the triplet state
cannot be excluded on this basis. The authors propose that the reaction proceeds through
the intermediate carbocation 6 - 5 0 although they were without direct evidence for its
presence. Contrasting with this reactivity, 5 did not undergo measurable exchange when
photolyzed under the same conditions as for 4. The fluorescence quenching of 5 by acid
for this derivative was also much less than that observed for 4. This difference has been attributed to the fact that the electronic distribution in 5 is more diffuse, whereas the charge transfer for 4 is more localized (see Section 1.4). Evidence for the intermediacy of
carbocation 6 was later provided by McClelland and coworkers23 who were able to detect
a transient on LFP of 4 in HFIP with two A,,, at 360 and 550 nrn that they assigned to the corresponding cyclohexadienyl cation obtained by photoprotonation at the 5-position. No transient was observed for 5 under the same conditions, mirroring the results of Shizuka and T ~ b i t a . ~ ~ , ~ ~
1.3.3 Reactions Arising from Enhanced Electron Withdrawal in S, 1.3.3.1 Photogeneration of Nitrobenzyl Carbanions
An excellent testament to the enhanced electron withdrawing power of the nitro group in the excited state is provided by the work of Wan and M ~ r a l i d h a r a n . ~ ~ They
prepared a number of meta and para-substituted nitroaromatics that were designed to
yield carbanionic intermediates on photolysis. Substrates substituted at the benzylic carbon with alcohol, ketal, or acetate groups all underwent C-C bond heterolysis via
similar mechanisms to give the desired intermediate. For example, m- andp-
nitrophenethyl alcohols underwent a base-catalyzed photo-retro-aldol reaction when irradiated to give the aldehyde and carbanion products (Eq. [1.3]), and were completely
inert in the ground state. The driving force behind the formation of the carbanions was interpreted as being a result of the strong electron withdrawing nature of the nitro group in the excited state, strongly stabilizing the developing negative charge at the benzylic site of the transition state through resonance. The authors found the meta-substituted
derivatives to be much more active towards carbanion formation that the corresponding para-substituted derivatives, demonstrating that the "meta effect" proposed by
1.3.3.2 Photoredox Behaviour of Hydroxymethylanthraquinone
When the two carbonyl groups of anthraquinone become excited, they become stronger electron withdrawing groups, causing the molecule to become very electron poor. A recent p ~ b l i c a t i o n ~ ~ demonstrates the chemical consequences of this enhanced electron withdrawing ability on excitation. Irradiation of hydroxymethyl substituted
anthraquinone 7 in the presence of water leads to formation of the redox product 8 (Eq.
[1.4]) with very high efficiency (@ w 0.8). A mechanistic proposal was presented in
which the primary photochemical step is deprotonation of the benzylic C-H bond by water. Formation of the anion in this way was rationalized based on the strong electron withdrawing ability of the anthraquinone in the excited state.
1.4 Enhanced Acidity of Hydroxyarenes in S,
Hydroxyarenes experience a rather dramatic increase in their acidity when they are excited to their first excited singlet states. For example, 2-naphthol (9) experiences an
enhancement in acidity of around 7 orders of magnitude on ex~itation,'~ going from
having a ground state pK, of 9.45 to an excited state p ~ , * of 2.8. The enhancement is
even more dramatic for 1-naphthol which goes from pK,(S,) = 9.2 to pK,(S,) =
solutions, the protonated form is favoured in the ground state, but on excitation, deprotonation to water often occurs readily, via a process termed excited state proton transfer (ESPT). The origin of the enhanced acidity of hydroxyarenes in S, is perhaps best explained through a simple HMO approach. Simple hydroxyarenes possess a HOMO with a nonbonding lobe on the oxygen atom with a large coefficient, whereas the corresponding LUMO is localized not on the oxygen, but in the ring system. On
photoexcitation, one electron transfers from the HOMO to the LUMO, causing a decrease in electron density at the oxygen atom, and therefore a decrease in the basicity of this
oxygen atom. The orbital coefficients for the HOMO and LUMO of phenol and 10 are
presented in Figure 1.6. In this figure, the diameter of the orbital is proportional to the orbital coefficient for a given atom, and the shading of the lobe represents its phase. It is clear in both cases that the orbital coefficient at the oxygen atom is large for the HOMO, whereas in the case of the LUMO, the electron density is more localized in the ring. Carbon atoms having high LUMO coefficients (e.g. the 5- and 8-carbons of 1 -naphthol) will also become more basic in S,, and is responsible for such chemistry as described in Section 1.3.2.2.
The differing excited state acidities of isomeric hydroxyarenes is often justified by the invocation of free electron model developed in large part by Platt.31 According to this
Phenol
a
LUMO0-3
HOMOFigure 1.6 - Orbital diagrams of the LUMO (top) and HOMO (bottom) of phenol (left) and 10 (right), as predicted by HMO theory. Lobe diameters are proportional to the orbital coefficient and shading represents the phase.
model, the lowest singlet excited states of many aromatic compounds have two possible states, labeled 'La and 'L,. The relative energies of the two states for a given compound are a function of the substituents present, and the symmetry of the system. If there is an appreciable difference between the energies of the two states, the lower energy state will dominate the photochemistry and photophysics, and if the states are of similar energy, state mixing is observed and both states contribute.
Naphthalene possesses 'La and 'L, states that are polarized along the short and long axes, respectively, and these states are nearly degenerate, with the 'L, band having a slightly lower energy. By introducing a hydroxyl group to the 1-position, this substituent
23 is aligned with the short axis of naphthalene, and the energy of the 'La state is lowered relative to the 'L,. Similarly, a hydroxyl group at the 2-position lowers the energy of the 'L, band relative to that of the 'La state. The 'La state of the naphthols is much more polarized than in the 'L, state, owing to enhanced charge transfer away from the oxygen for the former, whereas the electron density in the latter is more diffuse. Thus, because the S, state of 10 is 'La and that of 9 is 'L,, the excited state of the former is much more polarized, with more charge transfer away from the oxygen. This is manifested in the
lower pKa* of 10 relative to 9 (pKa* = 0.4 and 2.8, respectively).30 The charge density in
the 'La state is localized primarily on the carbon atoms at the 5- and 8-positions. This effect is also manifested by the quenching of the excited states of 1030,32 and 4 (see Section 1.3.2.2) by aqueous acid via protonation of the ring carbon atoms at the 5- and 8-
positions.
Tolbert and H a u b r i ~ h ~ ~ , ~ ~ have taken advantage of the fact that the 'La states of the naphthols are strongly polarized with electron donation directed primarily towards the 5- and 8-positions in their quest for compounds with very high excited state acidities, or "super photoacids". They synthesized a number of naphthols substituted at various ring
positions with cyano groups. Initial studies focused on 5-cyano-1 -naphthol (11) and 5,8-
excited state activity relative to 10, including ESPT to non-aqueous solvents such as
DMSO. These derivatives had undesirable fluorescence emission characteristics in protic
solvents, complicating their study. It was presumed that this was due to a solvent- mediated excited state intramolecular proton transfer reaction (ESIPT) (see Section
1 S . 3 ) . These complications were avoided in the cyano-substituted 2-naphthol
derivatives, which had better fluorescence proper tie^'^. The authors examined 5-, 6-, 7-,
and 8-cyano-2-naphthols (13,14,15,16) and 5,s-dicyano-2-naphthol(17), all of which
showed enhanced acidity in the excited state. The excited-state acidities of all of these compounds were estimated by Forster analysis, fluorescence titration, and analysis of the dynamics of proton transfer, with data presented in Table 1.1. Ail derivatives studied were stronger acids in S, than the parent 9, with those possessing substituents at the 5- and 8-positions giving the lowest p&* values. The most acidic derivative was 17 for which the pK,* is estimated to be as low as -4.5. Indeed, this derivative showed efficient ESPT to solvent methanol. Derivatives 13 and 16 also showed ESPT to methanol,
Table 1.1 - Excited State pKa Values @Ka*) for 2-Naphthols. Compound 9 13 14 15 16 17 Average pKal
Data determined by the aDebye-Smouluchowski equation (DSE), "Forster analysis, and by "fluorescence titration. All data taken from ref. 35.
although with a diminished efficiency, and the other derivatives showed no signs of deprotonation in this solvent. The stronger electron withdrawing effects of substituents at the 5- and 8-positions suggests that the excited states of the cyanonaphthols have
considerable 'La character. Gas-phase emission spectroscopy of 13 by Knochenmuss and c o - ~ o r k e r s ~ ~ showed evidence of vibronic coupling that may be responsible for the inversion of the 'L, and 'La states.
Townsend and S ~ h u l m a n ~ ~ also invoke Platt's free electron model to justify the excited state acidities of the phenylphenol series. The authors observed that both 2- phenylphenol(18) and 3-phenylphenol(19) efficiently undergo prototropic dissociation from S, in aqueous solution, whereas the extent of deprotonation of 4-phenylphenol(2) in S, is much lower than that of the other isomers, and is completely quenched by the
addition of even small amounts of an organic co-solvent (e.g. 5% (vlv) ethanol). This is despite the fact that 2 is the strongest acid of the series in the ground state. The relevant photophysical information for the three isomers is presented in Table 1.2, from which it is evident that of the three isomers, 2 shows the highest p&* of the series, and the lowest deprotonation rate constant. The authors rationalize the differences between the compounds by applying the free electron model. The S, state of these compounds is classified as a 'L, state that is polarized along the short symmetry axis of the biphenyl backbone. For 18 and 19, the hydroxyl substituents are reasonably well aligned (If: 30") with the polarization axis of the 'L, state, and as a result are themselves
Table 1.2 - Acidity Data for Phenylphenols.
'Ground and excited state acidity constants, taken from ref. 37
,Excited state deprotonation rate constants, from ref. 37. Compound 18 19 2 p' a (SOT 10.1 9.6 9.5 PK, (SJa 1.15 1.36 1.83 kDb ( X lo9 S-') 2 1 8.5 2.2
2 7
more strongly polarized in the excited state, giving rise to their higher acidity. For 2, the hydroxyl substituent lies perpendicular to the 'L, polarization axis, precluding strong coupling of the hydroxyl donating ability with the inherent polarization of the excited biphenyl chromophore. The differing photoprototropic properties of the phenylphenol series, while adequately predicted by the free electron model, are also completely consistent with ordering that would be expected according to the "meta effect" proposed
by Zimmerman (Section 1.3.2.1) which relies on a more traditional HMO rationalization. According to the "meta effect", the electron withdrawing properties of the phenyl ring
will have the most effect on substituents located at the ortho and meta positions, with a
much lower effect on those at the para position. While the charge transfer nature has
been demonstrated for the excited states of 2 and its methyl ether analogue 3 (Section
1.3.2.2), no similar study to date has been performed for the ortho and meta isomers.
1.5 Excited State Intramolecular Proton Transfer (ESIPT) 1.5.1 General
While charge transfer away from the oxygen atom is ultimately responsible for the enhanced acidity of phenols, by corollary, the atom(s) to which this charge is transferred should become more basic. Indeed, there are many examples of systems where, on excitation, a position becomes sufficiently basic to permit a proton transfer between the phenol and the basic site through a process termed excited state intramolecular proton transfer (ESIPT). In nearly all examples of this process, the acidic functionality is a phenol or aromatic amine, and the basic position is a heteroatom, such as a carbonyl
oxygen or a heterocyclic nitrogen atom. In such cases, the proton transfer is between heteroatoms, and gives rise to the excited state tautomer. For the purposes of this discussion, the ESIPT reactions discussed are mechanistically classified into two types, intrinsic and solvent-mediated, though Kasha3* has proposed a classification system in which the latter type is further subdivided into three types. Several reviews offer comprehensive coverage on ESIPT between h e t e r ~ a t o m s . ~ ~ - ~ ~
1.5.2 Intrinsic ESIPT
Many examples of ESIPT occur between atoms that are spatially very close together, often such that there exists a hydrogen bond between the acidic hydrogen and the basic electron pair. ESIPTs in such systems occur directly between the proton donor and the proton acceptor, aided by the hydrogen bond present in the ground state, and usually require very little movement of nuclei. Solvent is not required in these cases to mediate the proton transfer. ESIPTs of this type are termed intrinsic and give the fastest rates (usually subpicosecond) and show quantum yields for ESIPT near unity.
The first reported ESIPT reaction is an example of an intrinsic type and is operative in methyl salicylate (20).45 The author noticed an unusually large Stokes shift
in the fluorescence emission spectrum of 20 while the methyl ether derivative 21 showed
2 9
the expected mirror-image relationship between the absorption and fluorescence spectra. He proposed that on excitation, an intrinsic ESIPT reaction occurred between the oxygen atoms according to Eq. [1.5] to give 22*, and that the fluorescence emission that he
observed was from this tautomeric form. Observation of fluorescence from the tautomer can only arise if the ESIPT reaction is adiabatic, which is often the case in ESIPT reactions.
Because of the high rates and high efficiencies often encountered, systems involving intrinsic ESIPTs have found widespread commercial use, with many other
potential applications being developed. Tinuvin P (23) undergoes very efficient ESIPT to
give the corresponding excited state photo-tautomer, which on relaxation very rapidly returns to starting material via a reverse proton transfer. The energy of the absorbed photon is quickly dissipated as heat, leaving the system chemically unchanged. These photoprototropic properties have lead to the widespread use of this compound as a
30 photostabilizer in plastics to lessen the effects of UV irradiation on the material.46 Kasha et al. examined 3-hydroxyflavone (24) for possible use as a laser dye. The excited
phototautomer generated by ESIPT of 24 shows strong fluorescence (<D, = 0.36 in
methylcyclohexane) and is generated from 24* with a time constant of 8 p ~ . ~ ' The authors discovered that 24 was a very effective laser dye, giving peak laser powers comparable to the best of the widely used coumarin based dyes and rhodamine-6G, and gave emission
that was tunable between 518-545 nm (he, = 337.1 nm).
1.5.3 Water-mediated ESIPT
A second type of ESIPT between heteroatoms occurs when the acidic site is distal to the basic site, prohibiting the ground state hydrogen bond which is prerequisite to the intrinsic ESIPT. In such cases, molecules of water or some other hydroxylic solvent are required to mediate the proton transfer. Well studied examples of systems that exhibit this type of ESIPT are those of 7-hydroxyflavone (25) and 7-hydroxyisoflavone (26).
The possibility that an ESIPT process might be operative for 25 was first proposed
by Schipfer et aL4' after they observed long-wavelength fluorescence emission (h,,, 540 nm) when 25 was photoexcited in acidic aqueous or methanolic media. After ruling out
the possibility that the fluorescence was due to the corresponding anion, the authors proposed that it was instead from either an exciplex or the excited phototautomer 27* that
would arise from an ESIPT reaction (Eq. [1.6]).
Additional insight was provided by Itoh et al. who used two-step laser excitation (TSLE) fluorescence studies to examine the system.49 They showed that the long- wavelength emission band that was observed for this system in methanol shows two exponential decays (T = <0.2 and 0.7 ns). The authors ascribe the bimodal fluorescence decay as resulting from the formation of two different phototautomers (27 and 28), both resulting from solvent mediated ESIPT processes. The fluorescence emission from both
3 2
tautomers showed a second order dependence on the concentration of methanol present in solution, suggesting that two molecules of methanol are required to mediate the proton transfer to generate the tautomers. LFP allowed observation of both suspected
phototautomers, which showed absorption maxima at 380 and 420 nm with lifetimes of -400 ns (for 28) and 60 ps (for 27). Using the same experimental methodology, the authors later showed that two distinct solvent mediated ESIPT processes occur for the isomeric 7-hydroxyisoflavone (26) as well.50
Nearly a decade later, the authors publish a somewhat revised mechanistic picture for the ESIPT process of 25 and 26 in methan01.~' After comparison of the absorption and fluorescence spectra of the anion of 25 (25'), formed by dissolving 25 in basic methanol, the authors conclude that the short lived contributors to the long-wavelength fluorescence emission band are not 28 or 29, but are actually the anions that result from ESPT to solvent (25- and 26-). Armed with picosecond time-resolved fluorescence date, the authors observe separate rise times for the growth of the anion and tautomer, ruling out a step-wise mechanism for the formation of the phototautomer in which the anion is an intermediate. Instead, the ESIPT process is thought to proceed in a concerted fashion.
1.5.4 ESIPT to Carbon Atoms
While examples of ESIPT between heteroatoms abound, relatively few examples have been reported to carbon atoms. This is probably because of the lack of a strong hydrogen bond interaction between OH or NH acids and carbon atoms, and the typically low basicity of carbon-based functional groups. Examples of ESIPT to carbon atoms are
of particular interest because the reverse proton transfer from the tautomer (quinone methide (QM) when ESIPT is to carbon) to regenerate starting material appears to be much slower than for the heteroatomic case. As a result, the QM thus formed is often sufficiently long lived for other reaction pathways to compete with the reverse proton transfer. In many cases, the ESIPT process is irreversible, and the QMs are completely trapped. Their long lifetime often also permits their detection by LFP techniques, providing the experimentalist with a valuable tool for mechanistic exploration.
The first reaction reported in which an ESIPT to carbon was the primary
photochemical step was the photohydration of o-hydroxyphenylacetylene (30) to give o-
hydroxyacetophenone (31), presumably via en01 32 (Eq. [1.7]). While initially reported
by Ferris and Antonu~ci:~ the authors did not recognize the reaction as an ESIPT. A
more detailed mechanistic investigation was later carried out by Yates and coworkers on this reaction, and the corresponding photohydration of o-hydroxystyrene (33) (Eq.
33
35
36
[ l .8]).53 They found that the efficiency of the photohydration of30 was independent of the solution pH in the 0 to 7 range, whereas for derivatives in which the hydroxyl group
was replaced with a methoxy substituent, efficient photohydration required strongly acidic solutions. Because the pK,* for 30 is expected to lie within this range, the authors
proposed that the photohydration for this derivative was not a result of protonation of the
acetylene by aqueous acid, but instead resulted from ESIPT from the phenol OH to the
P-
carbon of the acetylene (or vinyl substituent for 33) to give QM intermediate 34 (or 35
from 33, Eq. [I .8]). Nucleophilic attack by water in each case leads to the observed product (31 or 36).
In a subsequent publication~4 the authors examined more closely the geometric
requirements of the ESIPT reaction for 33. When the a - H atom is replaced with a methyl group (to make 37), the quantum yield of photohydration rises markedly ( 0 = 0.19 for 33,
0 = 0.41 for 37). The authors suggested that the primary reason for the increased
reaction efficiency is because the steric bulk imparted by the methyl group forces a twisted orientation of the alkene group and the aromatic ring, which allows better overlap between the acidic proton and the alkene .n-electrons. This enhancement of the ground state hydrogen bond between the proton donor and proton acceptor is usually a
3 5 requirement for intrinsic proton transfers. Experimental evidence for the existence of this hydrogen bonding interaction was provided by the authors who observed two separate OH bands in the infrared spectrum of 33. One band was assigned to the 'free' hydroxyl group and the second assigned to hydroxyl groups that are engaged in hydrogen bonding with the styryl moiety.
Direct evidence for the intermediacy of QMs in the photohydration reactions was
provided by Foster et al. using LFP?' LFP of 38 in neat or aqueous CH3CN gave rise to a
transient with
A,,,
= 305 and 400 nm. The transient was formed during the time of thelaser flash (1 0 ns) and no appreciable decay was observed during the time limits of the instrumentation used (100 p ) . The authors assigned the transient to that of 39, resulting from ESIPT in 38 (Eq. [I .9]). This QM has similar structural characteristics to those
proposed by Yates et al.53,54 Moreover, the observation of this transient in neat
acetonitrile confirms that the ESIPT reaction is of the intrinsic type, since this solvent is not known to be capable of mediating proton transfers.
Fisher and Wan extended the ESIPT reactions of hydroxyaryl alkenes to rneta and
para isomer^.'^.'^ They reported efficient photohydration of meta derivatives 40 and 41
(@ = 0.22 and 0.24, respectively) while thepara derivative 42 reacted
