Exploratory studies of photocyclization and photosolvolysis of biaryl methanols

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EX PLO RA TO RY STU DIES O F PH O T O C Y C LIZA T IO N AND PH O TO SO LV O LY SIS OF BIARYL M ETH A N O LS

by Yijian Shi

B Sc., South China University o f Technology, 1983 M.Sc., South China University o f Technology, 1988 A Dissertation Submitted in Partial Fulfillment o f the

Requirements for the Degree o f DOCTOR OF PHILOSOPHY in the Department o f Chemistry We accept this dissertation as conforming

to the required standard

_____________________________________

Dr. P. C. Wan, Supervisor (Department o f Chemistry)

Dr. A. D. Kirk, Departmental Member (Department o f Chemistry)

s, Dqpartmen

Dr. T. M. Fyles, Departmental Member (Department o f Chemistry)

Dr. R. Keeler, Oi^side Member (Department o f Physics and Astronomy)

W ^ eigh,/Çxtemal Examiner (McMaster University) © Yijian Shi, 1997 University o f Victoria

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ABSTRACT

The photocyclization and photosolvolysis of a series o f hydroxy-substituted biaryl methanols (HOAr-Ar'CHiOH) have been studied. The proposed mechanism involves deprotonation of the HOAr moiety and heterolytic cleavage of the C -0 bond o f the hydroxymethyl group (CH2OH) in the first excited singlet state (Si), to give biaryl quinone

methide intermediates, which subsequently cyclize to the corresponding chromene product and/or react with solvent to give the solvolysis product. The formation of these quinone methide intermediates is facilitated by the excited state planarization and the subsequent charge polarization (negative charge transferred firom the HOAr ring into the Ar'CH^OH ring) of the biaryl. Although both of these processes are influenced by steric and electronic factors, the latter turns out to have a more significant effect on reaction efficiency.

When the geometry of the molecule goes fi’om a twisted conformation to a more planar form, the molecule becomes more conjugated and thus gains delocalization energy. This energy is generally larger for the planarization in the S i state than in the ground state. When it is large enough to overcome the steric repulsion for twisting, biaryls can planarize efficiently in the excited state, which is true for most biphenyl systems. However, it is shown that the deprotonated forms ( OAr-Ar'CHzOH) of these biaryls have an even larger driving force for Si planarization than the neutral forms (HOAr-Ar’CH^OH). Thus, biaryls with naphthalene ring(s) joined at the 1-position which do not planarize efficiently

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U1

in the neutral form can still reach a more planar geometry after adiabatic deprotonation of the phenolic hydroxy group in St. That is, the photocyclization o f these molecules proceeds via initial adiabatic deprotonation from the twisted S| state, followed by twisting (to the planar form) and subsequent charge polarization which expels the hydroxy group at the benzylic position (Ar'CHzOH), to give the required quinone methide intermediate.

The o,o'-biaryl quinone methides derived from the o,o'-substituted biaryl methanols are very short-lived due to rapid intramolecular ring closure and are therefore not detectable by nanosecond laser flash photolysis. The o,p’- and p.p'-biphenyl quinone methides, however, do not cyclize and as expected, are readily observable by nanosecond laser flash photolysis. When the benzylic hydroxy group is replaced by other leaving groups, the reaction can be used, in principle, as a photodeprotecting reaction and also to photogenerate acid.

These and other results of the Thesis have uncovered many interesting mechanistic details of this new class of reaction and hence have increased our general knowledge of the excited state behavior of aromatic molecules.

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Examiners:

Dr. P. C. Wan, Supervisor (Department of Chemistry)

Dr. A /D . Kirk, Departmental Member (Department of Chemistry)

Dr. T. M. Fyles, Departmental Member (Department of Chemistry)

Dr. R. Keeler, Outside Member (Department of Physics and Astronomy)

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1.6 Pr o p o se d St u d ie s 5 2 CH A PTER TW O SYNTHESIS O F SUBSTRATES 57 2.1 Bia r y l Me th a n o ls WITH Na ph t h a l e n e RiNG(s) 57 2.1.1 Synthesis of 2-[2’-(HydroxymethyI)-r-Naphthyl]phenol (53) 57 2.1.2 Synthesis of 1-[2’-(Hydroxymetliyl)phenyI]-2-naphthoI (55) 60 2.1.3 Synthesis of 2-Hydroxy-2’-Hydroxym ethyl-l,r-Binaphthyl (54) 62 2.1.4 Synthesis of 2’-Hydroxy-2-Bromomethyl-l,r-Binaphthyl (61) 6 6

2 .2 Sy n t h e siso f 2 - (2 ’-Hy d r o x y p h e n y l)b e n z y l Al c o h o l (4 4 ) a n d 2 - (2 ’-Hy d r o x y-

ph e n y l) -a,a-Diph e n y lben zy la lc o h o l(56) 6 8

2.3 Ad d it io n a l Biph en y l Meth a n o ls 73

2 .4 Mo d e l Co m p o u n d s 77

2.4.1 Synthesis of 2-(2'-MethyIphenyl)phenol (57) and 2-(2’-Methyl-l ’-naphthyI)-phenol (58) via Photochemical Reduction of 44 and 53 with NaBH, 77

2.4.2 Synthesis of 60, 62, and 63 78

2.4.3 Synthesis of 2-Hydroxy-2’-M ethyI-l,r-Binaphthyl (59) 79

CH A PTER TH REE

PH O TO CY CLIZA TIO N OF BIARYL M ETH A N O LS 82

3.1 Pr o d u c t St u d ie s 82

3.1.1 Photolysis of 2-[2’-(Hydroxymethyl)-r-naphthyl]phenol (53) 82 3.1.2 Photolysis of l-[2 ’-(Hydroxymethyl)phenyl]-2-Naphthol (55) 89 3.1.3 Photolysis of 2-Hydroxy-2’-Hydroxym ethyl-l,r-Binaphthyl (54) 92 3.1.4 Photolysis of 2-(2’-Hydroxyphenyl)-cx,a-diphenylbenzyl Alcohol (56) 97 3.1.5 Photolysis of 2-Bromomethyl-2’-Hydroxy-1,1’-Binaphthyl (61) 100

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vu

3.1.6 Photocyclization in the Crystalline State 102

3 .2 Q u a n t u m Yields 106

3.3 St e a d y St a t e Fl u o r e sc e n c e 110

3.3.1 Biphenyl Systems 112

3.3.2 Biaryls with Naphthalene Ring(s) 115

3 .4 Fl u o r e sc e n c e Qu a n t u m Yields 122

3.4.1 <D/in 100% CHjCN 123

3.4.2 H2O Effect on Cy of Model Compounds 126

3 .5 Fl u o r e sc e n c e Lifetim es 128 3.5.1 In 100% CHjCN 128 3.5.2 In Aqueous Solution 130 3.6 Iso t o pe Effect 132 3 .7 Su m m a r y AND Dis c u s s io n 136 C H A PT E R FO U R

PH O T O PR O T O N A TIO N O F BIPHENYLS AND

PH O TO SO LV O LY SIS O F BIPH EN Y L M ETHANOLS 144 4.1 Ch a r g e Dist r ib u t io no f Ph e n y l ph e n o l sa n d De r iv a t iv e sin Soa n d Si 144

4.1.1 4-Phenylphenol (3) and Derivatives 144

4.1.1.1 Ground State D-H Exchange 144

4.1.1.2 Excited State D-H Exchange 147

4.1.2 2-PhenyIphenol (8 ) 152

4.1.3 Fluorescence Quenching Study on 4-Methoxybiphenyl (4) 155

4.1.4 Laser Flash Photolysis Study 158

4.1.4.1 4-Methoxybiphenyl (4) 158

4.1.4.2 4-PhenyIphenol (3) 160

4 .2 Ph o t o so l v o l y sis OF Biph e n y l Me th a n o l s 162

4.2.1 Product Studies 162

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4.2.1.2 Photolysis of 5 0 in Aqueous Methanol 167 4.2.1.3 Photolysis of 4 9 and 52 in Aqueous Solution 169

4.2.2 Steady State Fluorescence Quantum Yields 173

4.3 La s e rf l a s h Ph o t o l y s i s- Q M Tr a n sie n t Spec tr a 176 4.3.1 4-[2'-(hydroxymethyl)phenyl]phenol (48) 176 4.3.2 4-[4*-(Hydroxymethyl)phenyl]phenol (50) 182 4.3.3 3-[2’-(Hydroxymethyl)phenyl]phenol (49) 185 4.4 Su m m a r y 185 CHAPTER FIVE EXPERIMENTAL 189 5.1 In str u m en ta t io n 189 5.2 Co m m o n La b o r a t o r y Re a g e n t s 190 5.3 Ma t e r ia l s 190 5.3.1 l-Bromo-2-Methylnaphthalene (65) 191 5.3.2 l-Bromo-2-(BromomethyI)nz^hthalene (6 6) 191 5.3.3 l-Bromo-2-(HydroxymethyOnaphthalene (67) 192 5.3.4 1 -Bromo-2-Naphthoic Acid (6 8) 192 5.3.5 Esters 70,74, and 77 193 5.3.6 Qiromenones 7 1 ,7 5 ,7 6 , and 78 194 5.3.7 6H-Benzo[c]chromenone (81) 196

5.3.8 Reduction of Qiromenones by LiAlHi 197

5.3.9 Methylphenyl Boronic Acid 8 6 199

5.3.10 Synthesis of 90 and 91 via Suzuki Coupling 199 5.3.11 Synthesis of 92 and 93 via NBS Bromination 200

5.3.12 Biphenyl Methanols 48 - 52 201

5.3.13 (X,a-Diphenyl-2-(2'-Hydroxyphenyl)benzyl alcohol (56) 204

5.3.14 Model C onfounds 57-60 and 62-63 205

5.3.15 2-Bromometh>i -2 ’-Hydroxy - l , l ’-Binaphthyl (61) 207

5.4 Pr o d u c t St u d ie s 208

5.4.1 Photolysis of 2 -[2 ’-(Hydroxymethyl)phenyl]phenol (44) 208 5.4.2 Photolysis of 2-[2’-(Hydroxymeth^-r-Naphthj^phenol (53) 209 5.4.3 Photolysis of 6H-Naphtho[2,l-c]chromene (96) 210 5.4.4 Photolysis of l-[2 ’-(HydroxymethyOphenyl]-2-Naphthol (55) 212

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IX

5.4.5 Photolysis of 5H-Dibenzo[c4]chromene (99) 213

5.4.6 Photolysis of 2-Hydroxy-2’-Hydroxyinethyi-1,1’-Binaphthyi (54) 213 5.4.7 Photolysis of 4H-Benzo[f]naphtho[2,l-c]chromene (80) 215 5.4.8 Photolysis of 2-Bromomethyl-2’-Hydroxy-l,r-Binaphthyl (61) 216 5.4.9 Photolysis of a,a-Dçhenyl-2-(2'-HydroxyphenyOben2yl alcohol (56) 216 5.4.10 Photolysis of 6,6-Dçhenyl-6 H-Benzo[c]chromene (104) 218

5.4.11 Photolysis of 4-[2’-(Hydroxymethyl)phenyl]phenol (48) 218 5.4.12 Photolysis of 3-[2’-(Hydroxymethyl)phenyl]phenol (49) 219 5.4.13 Photolysis of 4-[4’-(Hydroxymethyl)phenyI]phenol (50) 221 5.4.14 Photolysis of 4-[2’-(Hydroxyinethjd)phenyl]anisole (51) 221 5.4.15 Photolysis of 3-[2’-(Hydroxymethjd)phenyl]anisole (52) 222 5 .5 Is o t o p e Effect 222 5 .6 De u t e r iu m-Hy d r o g e n Ex c h a n g e Ex pe r im e n t s 223

5.6.1 Ground State Protonation 223

5.6.2 Excited State Protonation 224

5 .7 La s e r FLa sh Photolysis 226

5 .8 Fl u o r e s c e n œ Lifetime Me a s u r e m e n t s 227

5 .9 St e a d y Statef l u o r e sc e n c ea n d Fl u o r e sc e n c e Qu a n t u m Yield

Me a s u r e m e n t s 2 29

5 .1 0 Qu a n t u m Yield Me a s u r e m e n t s 229

5.10.1 Quantum Yields for Photocyclization in 1:1 CH3CN-H2O. 229

5.10.2 Quantum Yields for Photosolvolysis of Biphenyl Systems. 230

5 .1 1 X -Ra y Cr y sta llo g r a ph y 231

REFER EN C ES 233

LIST OF TABLES

T ab le 1.1 Acid-base Property of Phenylphenols in S, 13 T able 1.2 UV Absorption Spectral Properties of the A Band vs. Twist

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Table 3.1 Solvent Dependence on Photocyclization Quantum Yields 107 T able 3.2 Fluorescence Data of Phenylphenols and Derivatives 113 T able 3.3 Fluorescence Data of Biaryls with Naphthalene Ring(s) 117 T able 3.4 Fluorescence Quantum Yields (O/) in 100% CH3CN 124

Table 3.5 Fluorescence Lifetime and Estimated Excited State Intramolecular

Proton Transfer Rate (Lh) for Some Biaryls in 100% CH3CN 130

Table 4.1 Fluorescence quantum yields for Some Biphenyls 175 Table 5.1 Additional ^ D a ta for Some Biaryls 226 Table 5.2 Extinction Coefficients (e) of Chromene Products 229

Table 5.3 X-Ray Crystallography Data 231

LIST OF FIGURES

Figure 1.1 UV Absorption Spectra of Benzene, Toluene and Phenol in CH3CN 20 Figure 1.2 Potential Energy (£,) vs. Twist Angle (0) for Some Biaryls 28 Figure 1.3 Four-State Diagram Illustrating the Frank-Condon Principle 30 Figure 1.4 Potential Energy (£,) vs. Twist Angle (0) for 2-Phenylnaphthalene (20)

in So and Si 37

Figure 1.5 Fluorescence Spectrum of 2-Phenylnaphthalene (20) 38 Figure 1.6 Fluorescence Spectrum of 9-Phenylanthracene 39 Figure 1.7 Si Potential Energy vs. Twist Angle for Biaryls with Moderate Ej 41 F igure 1.8 Fluorescence Spectrum of 1-Phenylnaphthalene (19) 41 Figure 2.1 Crystal Structure of 2-(2'-Hydroxymethyl-1 '-naphthyl)phenol (53) 59

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XI

Figure 2.2 Crystal Structure of 2-Hydroxy-2’-HydroxymethyI-1,1 ’-Binaphthyl (54) 65 Figure 2 3 NMR of 2-Hydroxy-2’-HydroxyniethyI-1,1 ’-Binaphthyl (54) and

2-Hydroxy-2’-B rom om ethyl-l,l’-Binaphthyl (61) 67 Figure 2.4 NMR of 2-(2’-HydroxyphenyI)-a,a-DiphenylbenzyI Alcohol (56) 70 Figure 2.5a Crystal Structure of 2-(2’-HydroxyphenyI)-a,a-DiphenylbenzyI

Alcohol (56). 71

Figure 2 3 b Crystal Structure of 2-(2’-Hydroxyphenyl)-a,a-Diphenylbenzyl

Alcohol (56). (Dimer) 72

Figure 2.6 Crystal Structure of 4-[2’-(Hydroxymethyl)phenyl]phenol (49) 75 Figure 2.7 Crystal Structure of 3-[2’-(Hydroxymethyl)phenyl]phenol (48) 76 Figure 2.8 Crystal Structure of 4H-Benzo[f]naphtho[2,l-c]chromene (80) 81 Figure 3.1 Conversion vs. Time for Photolysis of 2-(2Hydroxymethyl-l

’-Naphthyl)phenol (53) in CH3CN and 1:1 CH3CN-H2O 84 Figure 3.2 UV Traces Observed on Photolysis of 2-(2Hydroxymethyl-1

’-Naphthyl)phenol (53) in CH3CN 8 8

Figure 3.3 Product Yield vs. Time for Photolysis of 1 -[2 ’

-(Hydroxymethyl)-phenyl]-2-Naphthol (55) in C H 3Œ and 1:1 CH3CN-H2O 91 Figure 3.4 Product Yield vs. Time for Photolysis of

2-Hydroxy-2’-Hydroxymethyl-1,1 ’-Binaphthyl (54) in Different Solvents 94 Figure 3.5 UV Traces Observed on Photolysis of

2-Hydroxy-2’-Hydroxymethyl-1.1 ’-Binaphthyl (54) in 1:1 CH3CN-H2O 95

Figure 3.6 Conversion vs. Time for Photolysis o f

2-Hydroxy-2’-Hydroxymethyl-1.1 ’-Binaphthyl (54) in CD3CN 96

Figure 3.7 UV Traces Observed on Photolysis of 2-(2’ -Hydroxypheny

1)-a,a-Diphenylbenzyl Alcohol (56) 98

Figure 3.8 UV Traces Observed on Photolysis of

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F igure 3.9 F igure 3.10 F igure 3.11 F igure 3.12 F igure 3.13 F igure 3.14 F igure 3.15 F igure 4.1 F igure 4.2 F igure 4.3 F igure 4.4 F igure 4.5 F igure 4.6 F igure 4.7 F igure 4.8

UV Traces of the Solid State Photocyclization of 2-(2’-

Hydroxyphenyl)-<x,a-Diphenylbenzyl Alcohol (56) 106 Fluorescence Spectrum of 2-(2’-Methyl-1 ’-Naphthyl)phenol (58) 118 Fluorescence Spectrum of 2-Hydroxymethyl Naphthalene (109) 118 Fluorescence Spectrum of l-(2 ’-MethyIphenyl)-2-Naphthol (60) 119 Fluorescence Spectrum of 2-Hydroxy-2’-Methyl-1,1’-Binaphthyl (59) 121 H2O Effect on <!>/ of 5 8 ,5 9 , and 60 127

Isotope Effect on Photocyclization of

2-Hydroxy-2’-Hydroxymethyl-1,1 ’-Binaphthyl (54) 136

‘H NMR of 4-(2’-Methylphenyl)phenol (63) Before and After

Ground State Deuteration 146

‘H NMR of 4-Phenylphenol (3) Before and After Excited State

Deuteration 149

‘H NMR of 2-Phenylphenol (8) Before and After Excited State

Deuteration 154

Fluorescence Quenching of 4-Methoxybiphenyl (4) by FT in H2O 156

Stem-Volmer Plot for Fluorescence Quenching of 4-Methoxybiphenyl (4) by H* in H2O

Transient Spectrum on Photolysis of 4-Phenylphenol (3) in 1:1 CH3CN-5% H2SO4

F igure 4.9 Transient Spectrum of QM 129

F igure 4.10 Solvent Dependence on the Decay of QM 129

F igure 4.11 Stem-Volmer Plot for Quenching of QM 136 by H2NCH2CH2OH

157 Transient Spectrum on Photolysis of 4-Methoxybiphenyl (4) in HFP 159 Transient Spectrum on Photolysis of 4-Phenylphenol (3) in HFP 161

162 177 178 179

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x m

F igure 4.12 pH Dependence on the Yield of QM 129 181

Figure 4.13 Power Dependence on the Yield of QM 129 at pH 12 181 Figure 4.14 Transient Spectrum o f QM 136 in H2O 184

Figure 4.15 Stem-Volmer Plot for Quenching of QM 136 by H2NCH2CH2OH 184

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Acknowledgm ents

I would like to thank my supervisor Dr. Peter Wan for his guidance and helpful advice during my studies. I would also like to thank my former and present colleagues. Dr. Deepak Shukla, Dr. Dave Budac, Cheng Yang, Bing Guan, Guangzhong Zhang (Geoff), Angela Bianco, Beverly Barker, Li Diao, Maike Fisher, Darryl Brousmiche, Kai Zhang, and Sarah Baker, who helped make research in the group enjoyable. Special thanks to my wife, Zihui (Christy), for her help and understanding.

Finally, I am indebted to the University of Victoria for giving me the opportunity to study here and for financial support

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XV

Dedication

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INTRODUCTION

1.1 Prologue

Chemical reactions can be accomplished either thermally or photochemically. Thermal reactions are initiated by absorption of heat which increases the translational, rotational, and vibrational energies of the reactant. Collisions at certain orientations between molecules with thermal energy higher than the necessary activation energy are able to distort the electronic structure of the molecules and lead to electronic reorganization, namely chemical reaction, to produce the product. Absorption of a photon by a molecule, however, promotes an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), which subsequently initiates chemical reaction by direct disturbance of the electronic structure of the molecule. As a result, the thermal vs. the photochemical reactivities for the same molecule are generally different. For example, irradiation of 1,3-butadiene in cyclohexane with 254 nm ultraviolet (UV) light gives the intramolecular ring-closure product efficiently (Eq. 1.1), while heating the molecule to 500 - 600 K gives predominately the Diels-Alder cycloaddition product (Eq. 1.2). Therefore, in terms of reactivity, a molecule in the excited state behaves as a different species. The fact that many confounds react differently in the excited state can be attributed to the different electronic structure and the extraordinary high energy of the electronically excited state, which generally cannot be

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O (1.1)

( 1.2)

achieved thermally. For exait^Ie, the Si state of 1,3-butadiene has an energy corresponding to 113 kcal mol'*. To achieve the same energy thermally, the molecules need to be heated to a tenrçerature of 10^ ~ 10“* K which is usually impossible in practice without destroying the conçound. Another prominent advantage o f photochemical reaction is that ±ermodynamically inçossible reactions (reactions with AG > 0) can sometimes be accomplished photochemically.

Generally, the electronic excitation of a molecule can affect the chemistry in the following ways:

R edistribute E lectron Density The change in charge distribution of an aromatic molecule in the excited state can be on a first approximation understood using simple Hiickel Molecular Orbital (HMO) theory. According to this theory, the tr-electron density Qr on the r"' carbon is given by

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coefiBcient o f the /“* orbital at the carbon atom. When an electron is promoted from the HOMO into the LUMO, the net charge change on the r'* carbon is given by

A<7, = {

Z

n

,

c

-

}

^

d

"

)

As Chomoj^ is normally different from Cujmoj^ , the excited state charge distribution is normally different from the ground state. It was noticed by Kuz’min et aL* a long time ago that the basicity of aromatic hydrocarbons in the singlet and triplet excited states is higher by many orders of magnitude than in the ground state (So). Spillane^ and Lodder et aL^ showed by deuterium-hydrogen exchange experiments that the basicity o f the meta position o f mono-substituted benzenes bearing electron donating groups (e.g. methyl, hydroxy, and methoxy) increases dramatically upon electronic excitation. The para- position of these molecules, however, becomes much less basic. Bie and Havinga"* calculated the charge distribution and the localization energy for electrophüic attack of a proton to phenol in the first excited singlet state (Si). According to their calculation, the meta and ortho-positions of the molecule have higher electron density and lower localization energy than the para-position. These results predict that the meta and ortho positions should be more reactive than the para-position towards excited state protonation, which is consistent with the experimental observations. Poly-substituted benzenes have also been shown to be photochemically protonated at position(s) different from that expected under thermal conditions.^'^'^ Wan and coworkers* found that simple

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HMO calculation can also be used to rationalize the regioselectivity for photoprotonation of these molecules. Generally, it is believed that these reactions occur in Si. In some cases, however, both triplet and singlet states are involved. When the aromatic compound possesses a high intersystem crossing yield, the reaction proceeds mainly via the triplet state.’

In a neutral molecule, when some positions become electron rich in Si, other positions become electron deficient (more electrophüic). This enhanced electrophilicity in the Si state can sometimes change the reactivity of the aromatic rings dramaticaUy. For example, many aromatic molecules such as naphthalene (1), biphenyl (2), and many of

their derivatives which are “inert” to nucleophilic attack in the ground state, can undergo nucleophilic substitutions with normal nucleophUes in Sj

Another result o f this charge redistribution is that the polarity of the molecule in the excited state is also different fix)m that of the ground state. For example, the a position of naphthalene (I) is more electron rich in S„ whüe the P position is more basic in S i.‘® This indicates that naphthalene (I) is polarized along the long axis in Si.*’ Similarly, biphenyl (2) has also been shown to polarize longitudinally in Si.** More recently, Shi and Wan*’ showed that biphenyls 3 and 4 are also highly polarized in Si with most o f the negative charge residing in the benzene ring not bearing the substituent

3: R=H 4: R=CH,

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charge redistribution is the change o f charge density on the substituents. It is well known that electronic excitation makes the ArOH protons more acidic and the conjugated (to a aromatic ring) carbonyl group more basic. For example, the p^T, of phenol is 10 in So and 4 in Si, while the p ^ , of benzoic acid is - 4.2 in So and 6 - 10 in Si.^° The stronger acidity

observed for phenols and naphthols in S i is due to the enhanced electron-donating effect o f the oxygen atom making the proton more acidic. Carbonyl groups, however, become better electron-withdrawing upon excitation. As a result, carbonyl groups are generally more basic in Si.^‘ Similar acid-base property change is also observed for other non oxygen acids and bases such as the sulfur and nitrogen substituted analogs. Wan and cow orkers^ reported that the acidity o f some carbon acids increased by up to 32 pATa units upon excitation, making the benzylic C-H protons acidic enough to be deprotonated by H2O. Yates and coworkers^ found that the rate of protonation of some functional groups

can be increased by 11 - 14 orders o f magnitude in Si. When irradiated in a proper solvent, these molecules can undergo excited state proton transfer (ESPT) (yide infra).

Alter M olecular G eom etry A family of molecules that undergo significant geometry change in the excited state is the biphenyls. It is well known that these molecules are twisted in the ground state but planar (or more planar) in the excited state (Eq. 1.5). Generally, data related to the ground state molecular conformation can be obtained fi’om x-ray analysis, UV-Vis spectrophotometry, NMR spectroscopy, and molecular mechanics calculations. However, methods to determine the geometry of Si are fairly limited due to the difficulty of probing very short-lived species. Stokes shift data provide a qualitative

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6

measurement of the molecular geometry change between So and Si . It is a simple but powerful tool to probe for geometry changes upon electronic excitation {vide infra).

twisted 2

ht) ♦>

planar 2

(1.5)

1.2 Proton T ransfer in the Excited State

The first example of ESPT was discovered by W eber^ in 1931. He observed that both the absorption and fluorescence p e c tra of l-naphthylamine-4-suIphonate were pH dependent, viz., the spectra shift considerably at certain pHs. It was not until twenty years later that Fôrster^ first realized that this phenomenon is the result of an excited state mfermolecular proton transfer (ESIerPT). Several years later, Weller^^ reported the first example o f excited state m/ramolecular proton transfer (ESIraPT). He found that the fluorescence emission of methyl salicylate (5a) showed a large Stokes shift. When the acidic phenolic proton was substituted by a methyl group, this unusually large Stokes shift disappeared and the fluorescence emission showed the expected mirror image relationship with the absorption spectrum. He suggested that the fluorescence emission observed for methyl salicylate (5a) is due to am excited state isomer 5b formed via ESIraPT (Eq. 1.6). Thereafter, extensive studies have been carried out in the field o f excited state proton transfer, especially the intramolecular version, due to important application available for this class of photoreaction.

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5a hu 1 ESIraPT

r ^ T

?

0 _ 5b (1.6)

1^.1 Acid-Base P ro p e rty o f th e Excited State

The acid-base property o f a molecule can be measured by its dissociation constant {Ka). Such constants for the ground state can be easily determined by a variety of available techniques. In the excited state, however, the traditional methods used for ground state measurement are not valid anymore. The dfficulties in measuring excited state dissociation constants come from the fact that the concentration of the excited state is much lower than the ground state and the lifetime of the excited state is usually very short. Therefore, direct determination o f the acid-base property o f Si challenges both ultra-fast experimental technique as well as highly sensitive detection technology. Due to these technical problems, the excited state dissociation constants ( Ka) reported in the early days were all determined via indirect methods (vide infra). Even nowadays when ultra-fast laser systems are available for direct determination of the excited state proton transfer rates, these traditional methods are still very useful.

One o f the most widely known methods is the Fôrster cycle developed by Forster^’ in the early 5 0 ’s. He related the acid-base dissociation of the ground state to that of the excited state for a general acid-base pair by a four-state cycle:

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AH *

I'

A H

A H * A H .

A'*

+

H*

A- +

H*

Schem e 1.1 F our-S tate F o rste r Cycle

In condensed phase, the volume change is negligible. Therefore, Equation 1.7 can be obtained from Scheme 1.1.

Ea + A H j * = Eb + A H j (1.7)

By assuming that the entropy change for the dissociation reaction in Si is the same as that in So, Equation 1.7 can be rewritten as

AG - AG* = AHd - AHd* = Ea - Eb (1.8)

Theoretically, Ea and Eb can be determined from the absorption or fluorescence emission spectra of ± e corresponding species by N Hc v a and N Hc v b, respectively. For very dilute solution, since AG = AG^ and AG* ~ AG°*, it immediately follows that

pKa - pKa = NhcivK - Ub) / (2.303/?T) (1.9)

where, N is the Avogadro's number, h is the Planck's constant, c is the velocity o f light, R is the universal gas constant, and 7 is the absolute tenperature. Ka is usually available, therefore K / can be calculated using Equation 1.9. In practice Va and Vb are estimated from absorption and/or emission spectra. The accuracy of this equation depends on how

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entropy change in both the excited and ground states could be invalid and the pKa* determined by this method would be larger than the real value.

Another alternative method based on steady state fluorescence intensity was developed by Weller.^ In this method, it is assumed that an acid-base equilibrium in the excited state is established:

A ‘* 4-

H*

k / ‘+k'.

AH *

h v kf+k^

A H

'

Scheme 1.2 E xcited S tate Acid-base E quilibrium

Based on the steady-states approximation. Equations 1.10 and 1.11 can be obtained:

( 1. 10)

k-HT^o ( 1. 11)

where (p and q> ' are the fluorescence quantum yields o f the undissociated {AH) and dissociated (A") forms (Scheme 1.2) in certain pH, respectively; <po is the quantum yield of the undissociated form in absence o f the deprotonation reaction; q>o ' is the quantum yield

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1 0

o f the dissociated form in alkaline solution; Lh and ku are the rate constants of

deprotonation and protonation in the excited state, respectively; A/ and kd are the rate constants for fluorescence emission and non-radiative deactivation, respectively, for the undissociated form, and kf ' and kd ' for the dissociated form; Xo=l / ( ^ +1 /) and To ' = 1 I {k f '+ kd '). If ç<Po '/<p '(Po is plotted against [H*], it is possible to extract k ^ , kn , and the dissociation constant Ka (= k ^ Iku) from Equation 1.11. This method requires the establishment of an acid-base equilibrium in Si. For systems that do not meet this condition, a modified equation has been derived.^ ‘

The Forster cycle is a thermodynamic determination o f pAT,* and thus provides no direct information about the kinetics of ES FT. However, when in conjunction with certain theoretical models o f proton transfer (PT), such as the Marcus t h e o r y , t h e intersecting modeL^^ and the Eigen m odel,^ the thermodynamic property AG° does show some correlations with the rate o f proton transfer. Earlier studies in this area made extensive use of Forster cycle and fluorescence titration methods.

Recent instrumental advances in time-resolved spectroscopy have made direct kinetic measurements of ESPT possible. By using pico- and femto-second laser systems, some ultra-fast ESPT^^ rates can been measured directly and thus the dynamic determination o f pAT,* becomes possible. With deprotonation (k^O and protonation {k») rates, dissociation constant in the excited state can be calculated by pAT,* = - Igik-v/ka). Usually, the pAT,* obtained by dynamical analysis, Forster cycle, and fluorescence titration experiments have different values. Such discrepancies can sometimes approach several pAT, units.

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1 ^ .2 Interm olecular Proton T ra n sfe r

ESIerPT usually involves transferring a proton from the substrate to a proton acceptor, generally the solvent molecule, in the excited state. Therefore the rate o f proton transfer depends on both the acidity o f the substrate and the bacisity o f the acceptor. In general, the more basic the acceptor and the more acidic the proton o f the substrate are, the faster is the observed proton transfer rate. ESIerPT in polar solvents proceeds via charge transfer type of transition state with formation of solvated ions as the products. Thus rearrangement of the solvent molecules in the solvent shell may also affect the ESIerPT rate. The slower reactions can be as slow as lO’ s ' in which the rate is limited by the reorientation of the polar solvent molecules. Faster reactions, however, can have rate constant as large as 1 0" s ' which is limited by diffusion. It is generally observed that most

ESIerPT proceeds adiabadcally in the relaxed excited singlet state.

Naphthols and D erivatives 1-Naphthol (6) and 2-naph±ol (7) and their

derivatives are among the molecules which have been most thoroughly studied.^ In the ground state, the two molecules have similar pAT.'s. In the excited state, however, it is found that I-naphthol (6) has lower pAT.' (~ 0.5 for 6 and 2.78 for 7) and much faster

ESIerPT rate (2.5 x I0 '° s ' for 6 and I.I x 10® s ' for 7 in H^O) than 2-naphthol ( 7 ) . ^

These differences have been attributed to the different nature of the S, state o f the two molecules. The 2-naphthol (7), similar to naphthalene (1), has a 'U, type o f Si which is polarized along the longitudinal axis of the naphthalene ring, whereas the transversely

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1 2

’L. = S,

(Arrows denote the transition moment axis)

polarized ‘L, state is believed to have more charge transfer character and higher energy than the ‘U state. For 1-naphthoI (6) and its conjugated base (6a), however, the C-O bond is perfectly aligned with the transition moment axis of the ‘L, state and thus the energy of the ‘L, is lowered more significantly than that o f the 'l* state. It has been shown that the Si state of the neutral molecule 6 has both ‘Lb and ‘L, character and the St o f I- naphtholate ion (6a) is predominated by ‘L, in polar media.” Webb et aL^* suggested that there is an inversion of the Si state (‘L, has lower energy than the ‘Lb state) for 6 and 6a. Tsutsumi and Shizuka^’ showed that the Si state o f 1-methoxynaphthalene also has similar features. Since the Si transition moment of 6 (‘LO is perfectly aligned with the C-O bond while that of 7 (Si = ‘Lb) is aligned at 30® with the C-O bond, the electron density on the oxygen atom o f 6 decreases more than that of 7 upon electronic excitation. Therefore, the ArOH proton o f 6 is more acidic and the rate o f deprotonation is faster. Huppert et aL“° studied 2-naphthol (7) and the sulfonated derivatives and found that a plot o f the logarithm of deprotonation rate constant against pAT,* is linear.

Phenylphenols It was reported by Bridges et aL^‘ more than thirty years ago that 2-phenylphenol (8) and 3-phenylphenol (9) showed efiScient ESIerPT upon electronic excitation to Si, while the para-isomer, 4-phenylphenol (3), did not. Townsend and

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Schulman^^ have recently reinvestigated the acid-base property o f these nx)lecules in more detail They found that 4-phenylphenol (3) does proceed via proto tropic dissociation in Si but to a much lesser extent (larger pAT." and smaller k^d than its ortho and meta-isomers 8 and 9 (Table 1.1). Similarly, p-cyanophenol is also found to have larger pAT»* and smaller

Table 1.1 Acid-base Properties o f Phenylphenols in Si

(Compound k~a (10* s ') (10‘° M ' s ') pAT. pA:.* _y (ns) 8 21±2 3.Q±0.1 10.09±0.05 1.15±0.04 0.5 ±0.1 9 8.5±0.1 1.9±0.1 9.62+0.01 1.36±0.02 1.5 ±0.1 3 2.2+0.2 1.5±0.1 9.56+0.03 1.83±0.04 1.2 ±0.1

is the deprotonation rate of the phenol molecule in Si: & is the rate of protonation of the phenolate ion in Si; AT. is the ground state dissociation equilibrium constant and superscript denotes the Si state; X/ is the fluorescence lifetime. Data in this table is obtained from reference 42.

k-H than the ortho and meta-cyanophenols.^^ These authors rationalized this phenomenon by treating these molecules as disubstituted benzenes using Platt’s free-clectron model** According to this m o d el the lowest excited singlet state o f disubstituted benzenes with an electron withdrawing and an electron donating group is the *Lb state which is known to polarize transversely. Since the acidity o f the phenolic proton depends on the extent of charge transfer (from the oxygen atom to the benzene ring), and this is dependent on the alignment o f the C-O bond with the transition axis of the Si (*Lb) state, the ArOH acidity depends on the angle between the C-O bond and the transition moment axis. The C-O

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14

bonds of 8 and 9 lie at 30" with respect to the ‘Lb<—‘A transition moment axis while this bond is perpendicular to the transition moment o f St in the case of 4-phenylphenol (3). Consequently, the OH group of 4-phenylphenol (3) is less polarized than that of 8 and 9 in S i , which therefore explained why 3 has smaller Lh and greater pKa.

CL^<r- ’A) = S,

< ►

transition moment axis 3: para

8: ortho 9: me ta

1.2.3 Intram olecular Proton T ra n sfe r

The term ESIraPT applies when both the proton donor and proton acceptor reside at the same molecule. Reactions o f this type usually involve transferring an acidic proton from an oxygen atom to a more basic oxygen or nitrogen acceptor, to give a tautomer o f the substrate. From this point o f view, ESIraPT is a tautomerization reaction in the Si state. Kasha**^ has classified ESIraPT reactions into four classes according to the mechanism of reaction:

(i) The intrinsic in tram olecular transfer is an ultra-rapid process where the proton is transferred across an internal H-bond between the H atom being transferred and

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capable of exhibiting intrinsic intramolecular transfer are usually those in which a pseudo five- or six-member ring can be formed via an internal H-bond between the donor and the acceptor. However, it should be noted that such molecules do not always exhibit intrinsic proton transfer because interaction with H-bonding solvents may break the internal H- bond thus eliminating this proton transfer mechanism.

(ii) T h e concerted b ip ro to n ic transfer involves a cooperative double proton transfer within a cyclic complex or dimer. In this class, the proton donor and the proton

acceptor are so far apart that it is inpossible to form an internal H-bond. Therefore a mediator is required to transport the proton. This mediator can be either the substrate itself (via a dimer) or other component (containing an O H group) in the solution. The first example discovered to exhibit this transfer mechanism is 7-azaindole (10).“^ This mechanism is shown in Equation 1.12.

H 10a

ESIraPT • H

O —H ( 1. 12)

N— H lOb

(ill) T h e static a n d dynam ic catalysis of proton transfer applies when the transporter formed a complex with the substrate via H-bonds. As in the case o f class (ii), forming cyclic complexes can be considered as an example of static catalysis. When the transporter (the catalyst) is only singly H-bonded to the substrate, a dynamic catalysis may

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1 6

occur. The ESIraPT observed for hunichrome (11) was found to proceed via a dynamic catalysis mechanism catalyzed by pyridine (Eq. 1.13).47

11a H NH P T

Q

H NH 11b (1.13)

(iv) T he proton-relay tautom erization is applied when the donor and the acceptor of the molecule in class (ii) and (iii) are separated further apart so that the proton transfer involves a multi-proton-bridged solvate. The ESIraPT o f 7-Hydroxyquinolone^ (12a) is an example of this type (Eq. 1.14).

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(1.14)

Strictly speaking, only the intrinsic ESIraPT is the real mrramolecular proton transfer and ail the rest are some variations of i/j/ermolecular proton transfer reactions. The rate constants for these reactions are usually very large, in the range o f 10‘° - 10*^ s'*, which are generally faster than those o f simple ESIerPT. As a result, rapid ESIraPT sometimes occurs in the non-relaxed excited state.^’ Occasionally, ESPT is accompanied by non-radiative deactivation o f the excited molecule.^* Direct dynamic measurements of these processes rely on picosecond and femtosecond time-resolved spectroscopic techniques. With recent advances in ultra-fast laser technologies, signUBcant progress has been made in understanding these reactions.^*

1 3 Electronic Absorption Spectra and Ground State Molecular Geometry

In biaryl systems, the energies of the k orbitals is affected by the extent of the conjugation of the two aromatic rings and thus the twist angle. Therefore, the electronic absorption spectra of these systems depend on the dihedral angle of the molecule. When the two rings are coplanar, conjugation is maximal and the molecule has lower HOMO- LUMO gap (minimum excitation energy). Any deviation from the planar geometry wül

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1 8

thus increase the HOMO-LUMO gap and blue-shifts the absorption spectrum to a corresponding extent The perpendicular geometry minimizes the conjugation and thus is expected to have the highest excitation energy. Steric repulsion between the substituents at positions ortho to the C-C bond joining the two aromatic rings is generally the main reason for such deviations.

Generally, the twist angle can affect the shape of the absorption spectrum and the intensity and position of the conjugation band {vide infra). It is generally observed that smaller twist angle normally gives a more red-shifted and more intense conjugation band and vice versa. When the twist angle is big enough (close to perpendicular) to eliminate any conjugation, the spectrum appears as the simple overlapping of the spectra o f the individual chromophores comprising the molecule. Therefore by conçaring the spectrum o f a biaryl molecule to those o f the corresponding simple yom atic molecules, information related to the ground state molecular conformation of the biaryl can be inferred. Suzuki^”’^* successfully rationalized these observed experimental effects using LCAO molecular orbital method. The twist angle 0 for biphenyl (2) estimated by this method agrees very well with that inferred from other experimental data.^'

1.3.1 Spectral C haracteristics of Simple A rom atic Molecules

Benzene an d M onosubstituted Benzenes The UV absorption spectrum of benzene consists of three bands. Klevens and Platt‘S have denoted these bands as the *Lb«-‘A (256 nm), ^L.4-'A (203 nm), and ‘B«—‘A (180 nm) according to the free-electron

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modeL According to this theory, the *B<—‘A is symmetry-allowed and thus has the highest intensity, while the other two transitions are symmetry-forbidden and therefore the intensities are much weaker. Experimentally, the ‘L. and 'L<, bands are weaker than the band by ca. one and three orders o f magnitude, respectively. Introducing a substituent on the benzene ring can reduce the symmetry o f the molecule and thus intensify the forbidden bands in a corresponding ex ten t It is generally observed that monosubstitution red-shifts all bands to similar extents and intensifies the symmetry forbidden 'l* band. Such effects can be further intensified when the substituent contains lone pairs, double or triple bonds, or phenyl groups which can conjugate with the benzene moiety. On the other hand, complex substituents can considerably increase the possibility of conversion of the electronic energy to the vibrational energy o f the substituents, and this increases the internal conversion rate and decreases the lifetime of Si. Since the width o f an energy state is inversely proportional to the lifetime of the state, the result o f substitution with complex groups is a broadening o f the absorption band and an obscuring of the structure of the spectrum.

This substitution effect is perfectly demonstrated by the absorption spectra of benzene, toluene, and phenol (Fig. 1.1). It can be seen from Figure 1.1 that the hydroxy group of phenol red shifts and intensifies the 'L* band much more than the methyl group does. The spectra of toluene and phenol are also less structured than that of benzene as expected.

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2 0 S a

I

<

0.7 - -Phenol - 'Toluene - Benzene 0.6 0.5 0.4 0.3 0.2 0.1 0 210 230 250 270 290 Wavelength (nm)

Figure 1.1 A bsorption Spectra of S.SxlO'^M Benzene, T oluene, an d Phenol in 100% CH3CN

D isubstituted Benzenes When benzene is disubstituted, the spectral shift depends on both the type of the substituents (electron donating or withdrawing) and their relative positions. It is well-known that the para-disubstitution pattern o f opposite types of substituents tends to enhance the effect of the individual substituent. The observed spectral shift is greater than the sum of those caused by the individual substituents in the corresponding monosubstituted benzenes. In contrast, when two substituents of the same type are opposed, these effects seem to cancel out to some ex ten t.S im ilarly , ortho and meta-disubstituted benzenes with same type of substituents have less effect than that with opposite type o f substituents.^^ Interestingly, complementary disubstitution (with opposite

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type o f substituents) at ortho-, meta-, and para-position have similar effects on the and ‘B bands, but the ‘L. band is affected most remarkably by the para-substitution. Both the spectral shift and e of the ‘L. band are much greater for the para-substitution than the

ortho- and meta-substitution. This is believed to be due to the charge transfer character of the ‘L, excited state. For example, it is shown that the transition moment of ‘L«<—‘A of 4- hydroxybenzoic acid (13) is collinear with the longitudinal symmetry axis of the molecule while the transition is perpendicular to it. Consequently, the ‘L, state can be most stabilized by the para-substitution o f the OH and carboxylic group and hence is most red- shifted and intensified. With stronger electron donating and/or withdrawing groups, this charge transfer character is further enhanced and therefore the 'L . band is more red-shifted and intensified. For example, the NH2/NO2 pair shifts the *L,<—*A band (-170 nm) much

more than the OH/CHO pair does (80 nm ).^

CT 13 O" 0 = OH (%<— ’A)

Naphthalene an d D erivatives The UV absorption spectrum of naphthalene (I) also consists of three major bands in the accessible ultraviolet, the ‘Bb band (221 nm), the ‘L, band (286 nm), and the ‘Lb band (312 nm). Similar to benzene, the *Lb<-‘A transition has the longest wavelength and is polarized along the long axis of the molecule. The

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2 2

*Bb<—*A transition is also longitudinally polarized, but the *L,<—‘A band is polarized transversely along the short axis o f the molecule. Thus one will expect that substitution at 1-, 4-, 5-, and 8-positions will primarily affect the transversely polarized ‘L, band, while the longitudinally polarized 'Lt band should be red-shifted and intensified by substitution at 2-, 3-, 6-, or 7-position. Many substituted naphthalenes have been studied and found to agree with this prediction very welL^^ When compared to benzene, the energy difference between the ‘L, and ^Lb states in naphthalene is much smaller.*® As a result, state inversion occurs more readily upon proper substitution (at the a position).

1.3.2 Geometry of Biphenyl and Derivatives

According to X-ray crystal analysis, the biphenyl molecule is planar or nearly planar in the crystalline state. In solution and in the gas phase, however, the molecule is twisted, with a dihedral angle o f ca. 25° and 45°, respectively. This molecule has a very intense and structureless conjugation band (also referred as the A band) with Xmax = 253 nm in crystalline state, 247 nm in solution, and ca. 238 nm in the gas state.*' This band is so strong (e^ at X^x is ca. 100 fold greater than that o f benzene and toluene) and broad

(200 nm - 300 nm) that any weaker band in this region is hidden.*’ Berlman'® inferred that this band was composed of three bands similar to those observed in benzene, a weak transition similar to the *Lb<—'A and two stronger transitions similar to the 'L,<—'A and 'B<—'a . The broad and structureless features of this band suggest that the conformation of

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the molecule is by no means rigid, and the dihedral angle value only reflects the most probable equilibrium conformation.

Introduction o f substituents into the 2-, 2’-, 6-, and 6’-positions is expected to increase the steric interference between the two benzene rings and thus increases the twist angle. Concurrently, the Xmax of the A band is blue shifted and its intensity decreases. For example, the A band o f 2-methyIbiphenyl is markedly blue shifted and Emaz decreases by about 40% if compared to biphenjd. When an additional methyl group is introduced, in the case of 2,2 ' -dimethylbiphenyl, the A band is further blue shifted so that it exhibits no distinct maximum o f the A band. These spectral properties of the A band and estimated dihedral angles (0) for these molecules are summarized in Table 1.2. In the case of bimesityl (14), the UV spectrum in solution doesn’t show the A band at all In contrast, the weak *Lb<—‘a band which is hidden in biphenyl is now recovered conçletely and the spectrum is very similar to that of mesitylene (15), except that is approximately twice as big as that of 15.^’ This observation clearly shows that the 7t electron interaction between the two benzene rings is almost completely eliminated in 14.

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T able 1.2 T h e UV A bsorption S pectral Properties o f the A band Biphenyl Solvent Xrn«(nm) enaut(M’^cm‘‘) 8

Unsubstituted Ethanol 247.7 16600 25”

2-Methyl- Ethanol 235 10500 59”

2,2’ Dimethyl- Ethanol 227 6800 72”

Note: Data shown in this table are from reference 51.

24

If one can force the two benzene rings to a planar conformation, the molecule will have maximal conjugation. In fact, this is observed experimentally in o,o’-bridged biphenyls. Jones^® con çared the UV spectra of 9,10-dihydrophenanthrene (16), and 4,5- methylene-9,10-dihydrophenanthiene (17) to biphenyl (2). He found that the spectra of these compounds have similar intensity as biphenyl but are red-shifted by about 20 nm due to a more planar geometry and better conjugation- Although it is well known that introduction of methyl groups into benzene will also lead to a spectral red shift, it is obvious that the methylene groups in these molecules alone cannot account for such a big shift. Therefore, geometry changes should be involved. Suzuki** compared the spectra of a series of o ,o ’-bridged biphenyls. It was found that the spectra o f 9H-fluorene (18) and 9,10-dihydrophenanthrene (16) are in fact very similar, except that the former is more structured indicating a more rigid ground state. As the number o f the bridging carbons increases, the geometry o f the biphenyl moiety is gradually forced away from the planar geometry, which results in a progressive blue spectral shift and diminishing of the A band.

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When the number o f bridging carbons equals to five, the A band is almost entirely diminished.*

18

1.3.3 G eom etry o f Phenyinaphthalene and Derivatives

Phenylnaphthalenes The UV spectra of 1-phenyinaphthalene (19) and 2- phenylnaphthalene (20) are found to be very differenL The phenyl group at the 1-position intensifies and red-shifts the transversely polarized *L, band and diminishes all vibrational structures. This alteration of the *L, band is sufficient to obscure and submerge the *Lb band. However, the effect is not as great as one would have expected for a phenyl group. It has been pointed out by Jones^’ that the spectrum of 1-phenyinaphthalene (19) is somewhat similar to a spectrum calculated by addition o f the separate spectra of benzene and naphthalene. This is obviously due to the steric interference between the H atom at the 2-position of the phenyl group and the H at the 8-position of the naphthalene moiety that seriously hinders the planarization of the molecule. As a result, the conjugation in this system is very limited. In contrast, 2-phenylnaphthalene (20) has almost no steric hindrance, therefore the phenyl group and the naphthalene ring are expected to gain much

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2 6

better conjugation. Consistent with this prediction is the large spectral red-shifts o f the longitudinal polarized ^Bb ( > 20 nm !) and ‘L«, bands observed for this molecule.^

B inaphthyls The steric interference in 1-phenyinaphthalene (19) is further enhanced in l . l ’-binaphthyl (21). The UV absorption spectrum o f 21 closely resembles that of naphthalene which suggests that there is little conjugation between the two naphthalene rings. In the system with less steric hindrance, the l,2 ’-binaphthyl (22), all bands are more difihise due to the overlap o f differently shifted bands from the 1- substitution and 2-substitution. For the system o f minimal steric hindrance, the 2 ,2 ’- binaphthyl (23), all bands are significantly red-shifted and intensified, indicating a strong conjugation effect. Detail discussion on the spectral characteristics of these polycyclic hydrocarbons can be found in references 54 and 61. All these facts suggest ± a t 1- phenylnaphthalene (19), l , l ’-binaphthyl (21) are highly twisted, whereas the 2- phenylnaphthalene (20) and 2,2 ’-binaphthyl (23) are much more planar in the ground state.

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2 1

1.3.4 Theoretical Calculation o f Tw ist Angle 6

From the above discussion, it can be seen that there are two type o f energies, the repulsion energy £ , due to intramolecular steric interference and the it-electron delocalization energy £* due to conjugation, which can affect the equilibrium geometry of a biaryl molecule. The former (E J is positive and has a maximum in the planar geometry (9 = 0°) while the latter is negative and reaches a minimum at 0 = 0°. Generally, it is assumed that the total potential energy Ei is the sum of these two contributions. That is,

£ ,= £ . + £ , (1.15)

The equilibrium twist angle will be that corresponding to the minimum o f £*. Therefore, the equilibrium geometry of a biaryl molecule will thus depends on the relative magnitude of £* and £ , . When £* » - £ * , £ , « £ , , the molecule is expected to be highly twisted (0 = 90°). In contrast, if £ * < - £ %, the molecule is expected to have a more planar geometry (0° < 0 < 90°).

Gamba et aL“ calculated the total potential energy curve (£ J vs. the twist angle for some phenyl and naphthyl substituted naphthalenes (Fig. 1.2). Their results suggest that the equilibrium twisted angles o f 20 and 23 are much smaller (35° ~ 40°) than those of

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2 8

compound 19, 21, and 22 (~ 90“)- Qualitatively, these results agree very well with those obtained from the UV spectra of these molecules.

o a o b • c x d >. <a c LU □ e CO c o Q. 0 30 60 90 120 150 180 Twist Angle (0)

Figure 1.2 Calculated Potential Energy (EJ vs. Twist Angle for Some Biaryls. Energy curves produced based on data from reference 62.

Curve-a: 20; curve-b: 23; curve-c: 22; curve d: 19; curve-e: 21.

1.4 Fluorescence Emission Spectrum and Si Geometry

It has been seen how the UV absorption spectrum of a biaryl molecule relates to the molecular geometry o f the ground state. All the above rationalizations are based on the Franck-Condon principle (vide in fr a )^ which states that a molecule preserves its nuclear conformation of the initial state during any electronic transition. That is, the initial state obtained immediately after excitation of a molecule has the ground state geometry. In

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contrast to absorption, fluorescence emission concerns a transition from S, to So. According to the Franck-Condon principle, the emitting species should also conserve its excited-state nuclear conformation immediately after emitting a photon. Therefore, the emission spectrum should also refkct some conformational information regarding the Si state. A comparison o f the spectral characteristics o f the absorption and fluorescence emission spectra can thus provide qualitative information related to the conformational difference between Si and So . For biaryls, the transition energies of absorption and emission depend critically on the twist angle o f So and Si, respectively. Therefore, the energy difference between the absorption and fluorescence emission spectra of a molecule can to some extent reflect this conformational deviation between So and S i . This energy difference is generally defined as the Stokes shift which will be discussed in more detail in the following section.

1.4.1 The Franck-Condon Principle and Stokes Shift

Based on the fact that nuclear motion of a molecule is much slower (lO " to 10‘^ s ') than electronic motion (~10'^ s '), Franck and Condon^ suggested that a molecule immediately after excitation conserves its ground-state geometry. This state is known as the Franck-Condon (FC) excited state. Since the rate for fluorescence emission (10* to 10^ s ') is normally much slower than internal conversion (10‘^ to 10'^ s ') and nuclear motion, molecules in this state rapidly lose excess vibrational energy by thermal relaxation and return to the lowest vibrational level, namely the FC Si state. After subsequent nuclear

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30

relaxation, solvent cage relaxation, the nmlecule reaches a more stable conformation and solvent cage before emitting any fluorescence. This is known as the equilibrium (EQ) Si state (generally known as Si). Emitting a photon from the EQ Si (again, much faster than any vibrational and nuclear relaxation), the molecule ends up at the FC S® state which has the same geometry and solvent cage as the EQ S i . Finally the molecule in this unstable FC So undergoes thermal relaxation and returns to EQ S® (usually known as S®). These statements, known as the Franck-Condon principle, are illustrated by a four-state diagram shown in Figure 1.3 . S.(FQ S,(EQ) ’/solvatioQ relaxalioa geooieuy/solvatiof^ leUxatioo S,(EQ) S.(FC)

Fig. 1.3 Four-State Diagram DIustrating the Franck-Condon Principle

It is obvious from this diagram that the 0-0 band o f absorption is always at shorter wavelength (higher energy) than the 0-0 band o f fluorescence emission. Therefore the emission spectrum always has lower energy than the absorption spectrum. This energy

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difference is the sum o f the energy loss between FC Si and EQ S | and that between FC So and EQ So , which will depend on both the molecular conformation and solvation o f the four states. For a non-polar molecule in non-polar solvent, the contribution from the polarity change is generally small, so that the contribution from the geometry change dominates. Therefore, this energy loss reflects primarily the molecular conformational change. For a system in which both effects are significant, the conformational contribution can still be qualitatively inferred by comparing to a rigid reference molecule (without geometry change) which experiences similar polarity change when electronically excited. Generally, if a molecule experiences large conformational and/or polarity changes (thus marked change in solvation) upon electronic excitation, a large energy loss will be observed, and vice versa. Stokes^ was the first person who noticed this phenomenon. He defined this energy loss (riff) by the difference between Vo (the wave number o f the line o f symmetry between the absorption and the fluorescence emission curve) and v eg (the wave number of the center o f gravity o f the fluorescence emission curve) (Eq. 1.16), which is generally known as Stokes loss or Stokes shift

= (1.16)

where (ffcg ) is given by _ j v f i v ) d v

j n v ) dv

where f ( v ) represents the photon flux per unit wave number increment o f the fluorescence curve at a particular wave number ff. In practice, ffo is usually arbitrarily

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32

taken as the intersection between the absorption and fluorescence emission spectra recorded-*®

Another inportant spectral property related to the Si state is the spectral width (w) of the fluorescence emission spectrum, which is defined as

w = ( ü ® - (1. 18)

where i7® is the second moment defined by

Usually, the magnitude of w reflects the rigidity of the Si state. A smaller value of w usually indicates a rigid Si and vice versa.

1.4.2 Molecular Geometry and Spectroscopic Behavior

Similar to the way that twist angle of ground state influences the absorption spectrum, the fluorescence emission spectrum is also affected by the S i twist angle 0 in the same manner. That is, smaller Si twist angle corresponds to more red-shifted emission spectrum, and vice versa. Moreover, the spectral width and the structure o f the spectrum are also affected- Generally, if Si and So have different conformations, either the absorption or the fluorescence spectrum or both will be broadened. On the other hand, a transition originated from a more planar geometry results in a narrower and more structured spectrum than that originated from a less planar state. Berhnan^ discussed and summarized the correlation between the molecular planarity and the spectroscopic

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properties for some aromatic compounds. He classified these molecules into five different classes;

1) P lan ar in both Si an d So (no geom etry change) Molecules o f this type have rigid and planar geometry. Single aromatic conçounds and 0,0 ’-bridged biphenyl systems

such as 9H-fiuorene (18) are examples o f this class. The spectral characteristics for this type o f molecules are small Stokes shift, narrow (small w) and highly structured absorption and emission spectra.

2) N onplanar in both Si an d So (no geom etry change) The energy barrier for twisting (to the planar form) o f these molecules is so high that the Si planarization is seriously inhibited. Exançles can be found in multiple ortho-substituted biphenyls such as mesitylene (14) and 9- and/or 10-phenyl substituted anthracenes. The spectrum o f the former resembles that of toluene while the latter resembles that of anthracene very much. Stokes shift is usually small for these molecules. More examples can be found in the fluorescence spectra o f bianthranyl, 9,10-dinaphthyl anthracene, and 9,10-diphenyl anthracene which are all similar to anthracene. Since both So and Si are rigid, the absorption and emission spectra are slightly structured and w is usually small.

3) N onplanar in So, p la n ar in Si Biphenyl and most mcta, para-substituted and some ortho-substituted biphenyls belong to this class. These molecules have large geometry change upon electronic excitation and have a relatively rigid S i . The absorption spectrum is usually diffuse while the emission spectrum is structured and narrow. The Stokes shift is usually large.

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Table 1.3 Spectral Classifications of Aromatic Molecules^

Class 1 Class 2 Class 3 Class 4 Class 5

planar both states nonplanar both states nonplanar ground state, planar excited state nonplanar ground state, more nonplanar excited state

planar ground state, nonplanar excited state Absorption spectrum Well structured

Red shifted Narrow Slightly structured Blue shifted Narrow Diffuse Blue shifted Broad Slightly structured Blue shifted Broad Structured Red shifted Narrow Fluorescence spectrum Well structured

Red shifts Narrow Slightly structured Blue shifted Narrow Structured Red shifted Narrow Diffuse Red shifted Broad Diffuse Red shifted Broad

Mirror similarity Yes Sometimes No No No

Stokes loss Small Small Large Large Large

Concentration

Sensitive Yes No No No No

Large Small Small Small Large

Large Small Large Small Small

Example Anthracene 3,4-Benzophenanthrene Biphenyl 1 , 1-Diphenylcthylene Fluoranthene? t_{Taken from ref. 65}

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4) N o n p lan ar in So , m ore n o n p lan ar in Si 1,1-Diphenylethylene belongs to this class. The twist angle of the ground state is estimated to be c.a. 44°.“ It becomes more twisted when it is excited to S i .

5) P la n a r in S«, no n p lan ar in Si Some ring-chain systems such as trans-stilbene belong to this group. It is believed that trans-stilbene is planar in the ground state^ but twisted in Si.“

The expected spectral characteristics of these molecules are summarized in Table 1.3.

1.4.3 Fluorescence Spectra for Biaryl Molecules

The excited state planarization of biphenyl can be rationalized from simple HMO calculation which shows that the LUMO o f the molecule over the C-C bond joining the two benzene rings has bonding character (coefiBcients of both carbon atoms have same signs) whereas the HOMO is anti-bonding (coefiBcients have opposite signs). Therefore, promotion o f an electron from the HOMO to the LUMO orbital will increase the bond order (from 0.337 in So to 0.595 in SO of the bond joining the benzene rings. From the

Ground State First Excited State

Exploratory studies of photocyclization and photosolvolysis of biaryl methanols

INFORMATION TO USERS

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TABLE OF CONTENTS

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LIST OF FIGURES

Dedication

INTRODUCTION

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