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ADVANCES IN PHOTOCHEMISTRY

Volume 26

Editors

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

GUNTHER VON BUNAU Physikalische Chemie, Universitat Siegen, Germany

WILLIAM S. JENKS Department of Chemistry, Iowa State University, Ames, Iowa

A Wiley Interscience Publication

JOHN WILEY & SONS, INC.

New York Chichester Weinheim Brisbane Singapore Toronto


Volume 26


Volume 26

Editors

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

GUNTHER VON BUNAU Physikalische Chemie, Universitat Siegen, Germany

WILLIAM S. JENKS Department of Chemistry, Iowa State University, Ames, Iowa

A Wiley Interscience Publication

JOHN WILEY & SONS, INC.

New York Chichester Weinheim Brisbane Singapore Toronto

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10 9 8 7 6 5 4 3 2 1

CONTRIBUTORS

Viktor Jarikov Eastman Kodak CO. 1999 Lake Avenue Rochester, NY 14650-2 1 10

Horst Kisch Institut fur Anorganishe Chemie I1

der Universitat Erlangen-Numberg Lehrstuhl fur Allgemeine und Anorganische Chemie

D-91058 Erlangen, 02.03.00 Egerlandstr 1 Denmark

Catherine J. Murphy Dept. of Chemistry and Biochemistry University of South Carolina 730 Main Street Columbia, SC 29208

Douglas C. Neckers Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403

John P. Toscano Dept. of Chemistry Johns Hopkins University 3400 N. Charles Street Baltimore, MD 21218

V

PREFACE

Volume 1 of Advances in Photochemistry appeared in 1963. The stated purpose of the series was to explore the frontiers of photochemistry through the medium of chapters written by pioneers who are experts. As editors we have solicited articles from scientists who have strong personal points of view, while encouraging critical discussions and evaluations of existing data. In no sense have the articles been simply literature surveys, although in some cases they may have also ful- filled that purpose.

In the introduction to Volume 1 of this series, the editors noted developments in a brief span of prior years that were important for progress in photochemistry: flash photolysis, nuclear magnetic resonance, and electron spin resonance. A quarter of a century later, in Volume 14 (1998), the editors noted that since then two developments had been of prime significance: the emergence of the laser from an esoteric possibility to an important light source, and the evolution of computers to microcomputers in common laboratory use of data acquisition. These developments strongly influenced research on the dynamic behavior of excited state and other transients. We can look forward to significant developments to be included by the end of another quarter century.

With the increased sophistication in experiment and interpretation, photoche- mists have made substantial progress in achieving the fundamental objective of photochemistry: elucidation of the detailed history of a molecule that absorbs radiation. The scope of this objective is so broad and the systems to be studied are so many that there is little danger of exhusting the subject. We hope that this series will reflect the frontiers of photochemistry as they develop in the future.

DOUGLAS C. NECKERS DAVID H. VOLMAN GUNTHER VON BUNAU

Bowling Green, Ohio Davis, California Siegen, Germany

vii

CONTENTS

Photochemistry of Triarylmethane Dye Leuconitriles VIKTOR v. JARIKOV AND DOUGLAS c. NECKERS

1

Structure and Reactivity of Organic Intermediates as Revealed by Time-Resolved Infrared Spectroscopy 41

JOHN P. TOSCANO

Semiconductor Photocatalysis for Organic Synthesis HORST f i S C H

93

Photophysical Probes of DNA Sequence-Directed Structure and Dynamics 145

CATHERINE J. MURPHY

Index 219

Cumulative Index, Volumes 1-26 227

ix

PHOTOCHEMISTRY OF TRIARYLMETHANE DYE

LEUCONITRILES*

Viktor V. Jarikov and Douglas C. Neckers Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

CONTENTS'

I. Formation of color A. Introduction B. Oxidation of leuco dyes C. D. Miscellaneous E. Thermal bleaching reaction F. Applications

A. Homolysis versus heterolysis B. Fatigue of triarylmethane dyes C. Di-n-methane rearrangement D. Triplet state reaction of a TAM dye leuconitrile

Ionization in the first singlet excited state

11. Other reactions

*Contribution #421 from the Center for Photochemical Sciences 'Note that the glossary of terms and abbreviations and structures of the representative compounds is included at the end of this chapter.

Advances in Phozochemiszry, Volume 26. Edited by Douglas C. Neckers, Gunther von Bunau, and William S. Jenks ISBN 0-47 1 -39467-X 8 2001 John Wiley & Sons, Inc.

1

2 PHOTOCHEMISTRY OF TRIARYLMETHANE DYE LEUCONITRILES

111. Conclusions References Glossary

I. FORMATION OF COLOR

A. Introduction

The photosensitivity of Crystal Violet leucocyanide resulting in formation of the dye was first noticed but was not recognized as such by Hantzsch and Osswald [ 11 in 1900, when they were studying the reaction between dyes and anions to produce colorless species. The ionic dissociation of a triarylmethane dye leuconitrile (TAM-CN) induced by ultraviolet radiation to form the corresponding dye cation (TAMf) is a photochemical analog of the cation-forming step in an SNl reaction and has been known since 1919, when it was discovered and immediately recognized by Lifschitz and Joffe [2-41. In numerous studies it has been confirmed that the absorption spectra of TAM carbocations produced photochemically from leuconitriles exactly match those of the corresponding ground-state cations, which are usually formed from carbinol bases in the presence of acid [5-71. The quantum yield of dye formation from Crystal Violet leuconitrile (CVCN) and Mala- chite Green leuconitrile (MGCN) in ethanol was reported in 1931 [8] to be unity and since then it has been confirmed to vary from 0.9 to 1 .O in numerous publica- tions (Scheme 1.1 ; see Glossary for structures) [9-131.

Quantum yields of 0.93 [ 141 and 0.96 [ 131 for ionization of MGCN in acetonitrile were reported as well. It has been shown that minimizing the water content of the solvent is important in obtaining reliable quantum yields [15]. Also, when

hv - 1

Scheme 1.1. Photoionization of Crystal Violet leuconitrile.

FORMATION OF COLOR 3

determining the quantum yields of dye formation, one should take into account the ease of adsorption of TAM dyes to glass surfaces [ 161, the photosensitivity of the produced dye cations, and deviations from Beer's law [ 171, which CV cation exhibits at concentrations greater than 5 x 10-6M. Fischer et al. [12] discussed how easily the apparent extinction coefficient of TAM dyes, such as MG, could vary. Note that the cyanide anion is also formed upon heterolytic dissociation, and it has been reported that the quantum yields of its formation were 1.0 for ethanolic solutions of CVCN and MGCN [18]. According to Bertelson [19], the photoionization reaction is normally independent of the excitation wave- length, light intensity, and temperature.

After the light is removed from solutions of leuconitriles, the color slowly fades in the dark, giving rise to the colorless products: dye leuconitrile, dye carbinol base [20] where water is present, and dye ether base [21] in alcohols [22]. The carbinol and ether bases, which are called Zeucohydroxide and leucoether, respectively, undergo efficient photoionization as well [ 11, 23-27].

The dye-forming photoreaction for a series of TAM dye leuconitriles has been shown by Holmes [ 11,22,28] and others [5,27] to depend on the polarity of the solvent. Holmes reported that no dye formation occurs when dielectric constant ( E ) is less than 4.7. However, at this E the formation of a TAM dye begins sud- denly, and the efficiency of formation increases rapidly as E becomes larger. This was assessed quantitatively and confirmed later by Spears et al. [ 131, who showed that the quantum yield for ionization of MGCN increased from 0.15 to 0.91 due to faster formation and slower recombination of the ion pair with a change in E

from 6 to 18 for a mixture of ethyl acetate and acetonitrile (see later). Sporer [4] studied the photochemistry of a series of TAM dye leuconitriles in

polar and nonpolar solvents [29]. He showed that hexahydroxyethylated Basic Fuchsin leuconitrile photoionizes with a quantum efficiency of 0.95 in ethanol, but only 0.25 in the more polar water. This unexpected observation was confirmed by others [ 151 but still requires an explanation. Other water-soluble dye leucocyanides should be examined to confirm the generality of this phenomenon. Sporer also contrasted the fluorescent and photochemical properties of the naphthyldiarylmethane dye leucocyanide, Victoria Blue B leucocyanide (VBCN), and N-phenyl- 1 -naphthylamine and suggested that ionization does not occur from either the excited singlet or triplet state. Sporer assumed a TAM+ should be formed adiabatically, that is, 'VBCN*+'VB+*, and he dis- counted a singlet state reaction because no fluorescence of the dye was detected. On the other hand, he discarded the triplet state reaction because the photocolora- tion was dependent on E of the solvent and independent of the presence of oxygen, both of which are uncharacteristic of a triplet state reaction. Sporer proposed that photoionization follows internal conversion of S I to a vibrationally excited ground state because VBCN and other naphthyldiarylmethane dye derivatives photoionize at higher temperatures only.


B. Oxidation of Leuco Dyes

Numerous studies showed the possibility of dye cation formation from a radical cation of dye leuco derivatives Dye-X’, which was produced in several ways (A=acceptor; the last step may be assisted by other species, solvent, or light):

Photoinduced electron transfer: Dye-X + hv + Dye-X* + A + Dye-X+ + A -

Photooxidation:

Photoejection of an electron: Pulse radiolysis: In all cases:

A + hv + A* + Dye-X + Dye-X+ + A - SzOi- + hv + 2SOi- + Dye-X +

Dye-X+ + SO:- Dye-X + nhv + Dye-X+ + esolv, n = 1,2. . . Solvent” + Dye-X -+ Dye-Xf Dye-X + +Dye+ + X

For over 50 years it has been commonly observed in halogenated hydrocarbon solvents that the photoionization of TAM dye leuco derivatives proceeds more effectively than expected on the basis of the polarity of the solvent, and the thermal bleaching of TAM’ is extremely inefficient [ 161. In the course of our study, we found the same effect for photoionization of CVCN and MGCN in dichloromethane, chloroform, and carbon tetrachloride [30]. The formation of dye cation in these systems may occur via two pathways, depending on which component absorbs the light. One pathway is by oxidation of the excited leuco dye by the halocarbon solvent. The other is by reaction of free radicals generated from organic polyhalogenated compounds (or from silver halide [3 11) by ultraviolet or visible light with arylamine-type TAM leuco dyes. The latter method was parti- cularly studied for preparation of polyarylmethane dyes and used for a negative print-out process [ 16,321. The generality of the concept was demonstrated with a wide array of dye leucobases and aromatic amines in the presence of various halogenated compounds, such as carbon tetrachloride and tetrabromide, iodo- form, polybrominated and polychlorinated ethanes, butanes, and arenes. Irrever- sible dye formation related to such free radical photographic processes has been observed for mixtures of TAM dye leuconitriles and polyhalogenated materials. They have been used for “fixing” the color in photochromic systems by many others [16, 331.

Halocarbon-sensitized oxidation of amines, in both the ground and the excited state, is a well-known and very general reaction [34]. Even moderately halogenated solvents like dichloromethane are known to quench the singlet excited states of substituted benzenes [35]. For example, based on the observation of the reaction intermediates, MacLachlan [34] presented a mechanism for halocarbon-sensitized oxidation of the singlet and triplet excited states of Ethyl Crystal Violet leucobase (ECVH), tris( p-N,N-diethylaminopheny1)methane. He


Or 3DH* + CC14 -+ DH" + CCl,* -+ D+ + HC1+ CClz + C1-

DH + CC1,'('CC13 + C1-) -+ DHf' + -CCl3 + C1-

DH+* + D' .+ D+ +DH

DH" + D' + H+

Scheme 1.2. Mechanism for halocarbon-sensitized oxidation of the singlet and triplet excited states of leuco Ethyl Crystal Violet presented by MacLachlan; Df is a TAM dye and DH is a leucobase.

suggested that the ECVH radical cation undergoes either hydrogen atom transfer or electron transfer with the solvent radical anion and forms the dye cation (Scheme 1.2). The formation of HCl and the high efficiency of coloration are supported by observations of others that the quantum yield of dye formation from CV leucohydroxide (CVOH) when irradiated in carbon tetrachloride varies from 2.0 to 3.0 [36]. MacLachlan's mechanism is a photoalternative to one- step hydride transfer suggested by Abeles et al. [37] for the ground state oxidation of dye leucobases by various oxidants. Although hydride transfer has been believed for a long time [38], significant evidence exists now that the net hydride-transfer reaction for the oxidation of TAM dye leucobases by various organic oxidants in the ground state proceeds via several intermediates. The initial charge-transfer complex relaxes into a leucobase radical cation and oxi- dant radical anion, which upon proton transfer is transformed into a radical pair and subsequently into an ion pair [39, 401.

However, another way exists by which an excited state of TAM dye leuco derivative may generate a radical cation, viz. by photoejection of an electron. If a solvated electron (esolv) forms, halogenated organic solvents would efficiently scavenge it with rate constants on the order of 108-1010 M-' s-', subsequently producing solvent radicals and anions. In accord, several workers proposed that the photoionization of TAM leuconitriles in polar solvents originates from the triplet state by electron loss from the amine, forming a solvated electron and a TAM radical cation, which then proceeds to dye cation [41, 421.

The color-forming reactions of MGCN were studied by ps and ns pulse radiolysis in various solvents by Bobrowski and Grodkowski and their co-workers [43-451. In the absence of oxygen, the three species observed were assigned to MGCN* (360nm), M G (400nm), and MG+ (620 nm). However, in chlorocar-

bon solvents where the only solute-solvent cation reaction occurs, M G was replaced with MGCN'+ (480 nm), which was the single product of the reaction of parental solvent cationic species with MGCN [45]. The assignment is consistent with the findings of other workers who assigned 470-500 nm absorption to the DMA+ and analogous radical cations [34, 41, 46, 471. In the presence of oxygen and polar solvents, MG+ was formed in two steps with the slow step being an oxidation of M G by RO; , hydrox yalkylperoxyl radicals derived from

3


alcohols. The slow step is complete in tens of ps and has a rate constant of about lo9 M-I s - ' [44]. The fast formation of MG+ (ROH' +MGCN+MGCN'+ + MG') is complete in tens of ns, and it is speculated that the excess of vibrational energy in MGCN+ causes this fast dissociation. Overall, radiolytic ionization of MGCN was demonstrated to be at least an order of magnitude less efficient than photoionization. The same conclusion was reached for the radiolytic heterolysis of BFCN [48]. Many other examples were presented by Dorfman [49] who generated radical cations of arylmethyl halides using pulse radiolysis.

In a recent paper some assignments made by Bobrowski and Grodkowski were questioned by Bhasikuttan et a1 [50]. These workers studied the reduction of a series of CV- and MG-type dyes by diphenylketyl radical and hydrated electrons in pulse radiolysis experiments. They showed that both M G and MGH' have two absorption bands at 340 and 400 nm. Also, it was observed that the dye radicals exhibit second-order decay without bleach recovery. This suggested that the radicals probably dimerize rather than disproportionate.

Other examples where the dye cation was produced from the radical cation of a leuco dye include photooxidation of TAM dye leucolactones by iodonium salts that produce colored cations with high efficiency (Scheme 1.3) [51, 521. Color formation was observed from the irradiation of TAM leucolactones alone in acetonitrile. It was suggested that the formation of S1(7cx*) was followed by the homolytic cleavage of the lactone P-bond and by subsequent electron transfer [53]. Because the quantum yield for the color formation was low ( N

lop4), it was suggested that krecomb >> bt (Scheme 1.4).

C. Ionization in the First Singlet Excited State

In contrast to the proposal that a TAM dye cation is formed from the radical cation of the dye leuco derivative, Herz [54] presented evidence that the first

hv kq[ Phzl+X-] L = 'L*(nn*) - L"+ Ph2I'+ X-

1 / I k ~ . \ 3L* LH' + R' Ph' + Phl

hv

32 ' LH' + R' Ph' + Phl

Scheme 1.3. Photooxidation of the TAM dye leucolactones by iodonium salt; L is a TAM leucolactone and RH is the solvent.

hv p-scission e-T H-donor CVL 4- l(nn*) - biradical - ion Pair CVLH'

TS 1 k c o h CVL

3cvL*

Scheme 1.4. Color formation in irradiation of TAM dye leucolactones in acetonitrile: CVL is crystal violet leucolactone.


excited singlet state is the precursor of ionization as a result of his mechanistic studies of structure-reactivity relationships. The concept of S being a precursor to heterolysis is supported by many studies of photodissociation reactions in aryl- CH2-X compounds [55 ] . Herz utilized fluorescence yields and ionization yields determined for a series of MG-type dye leuconitriles with different substituents on the phenyl ring to demonstrate the point. He showed that efficiency of photoionization is reduced when the aryl chromophore with the lowest excited state energy efficiently fluoresces (e.g., dialkylaminonaphthyl group) or undergoes an enhanced nonradiative transition (e.g., halophenyl, acetophenonyl, and nitro- phenyl moieties). Herz also showed that ultrafast intramolecular singlet-singlet energy transfer between the three insulated chromophores of a TAM-CN system places the excitation energy on the chromophore with the lowest S1 energy before any other processes occur. Herz considered that the lowest excited singlet state of the N,N-dimethylanilino component of MGCN was nn*. In contrast, other workers concluded that the S1 of N,N-dimethylaniline (DMA) is of nn* nature, which is perturbed by the lone pair on nitrogen (perturbation -20%) [56]. However, one has to note that because the unshared electrons on nitrogen by conjugation become incorporated in the aromatic n electron system, an excitation that shifts electronic charge from nitrogen into the aromatic system is really just another nn* transition. It is quite different from the process in ketones in which nonbond- ing electrons are promoted to orbitals that are orthogonal to those from which the transition originates (G. S. Hammond, personal communication, 1999).

With the introduction of more sophisticated spectroscopic and photochemical techniques, especially measurements on the nanosecond time scale, new informa- tion was obtained. Brown and Cosa [57], who compared the fluorescence lifetimes and quantum yields for MGCN and DMA in cyclohexane, ethanol, and acetonitrile, confirmed Herz's conclusions. These workers showed that the compounds behaved similarly in cyclohexane and concluded that intersystem crossing and fluorescence are characteristics of the photophysical behavior of both DMA and MGCN in solvents of low polarity. In solvents of medium and high polarity the photophysics of 'MGCN* changes to include ionic photodissociation. They reported fluorescence lifetimes of <2OOps for MGCN in ethanol and acetonitrile. Brown and Cosa concluded that MGCN photoionizes from its first excited singlet state, with the electronic excitation energy being transferred from the chromophore to the vibrational modes of the central carbon atom-nitrile bond, which undergoes heterolysis.

A comparative spectroscopic and photochemical study of Brilliant Green leuconitrile (BGCN) and N,N-diethyl-p-toluidine (DET) at room and low temperature and in polar and nonpolar solvents was conducted by Geiger et al. [15]. At 77 K the spectroscopic and photochemical properties of BGCN were solvent independent. However, at room temperature the quantum yield of ionization was 0.8-0.9 in ethanol and 25% lower in both acetonitrile and 4% watedethanol.


It was noted that in cyclohexane BGCN disappears with 4 =0.02. Because DET displayed similar behavior in cyclohexane, it was postulated that the last photoreaction is the known cleavage of the nitrogen-alkyl bond [4]. A theoretical model for BGCN ionization was proposed to describe the lack of dye formation in nonpolar solvents. According to this model, at some point along the (TO* C- CN potential energy surface, a crossing or an avoided crossing occurs with the S,(nn*) potential energy surface [58], which results in an activation energy barrier on the S, surface along the C-CN bond stretching coordinate. The energy initially localized in S1 must be shuttled to the GO* state across the barrier to produce dissociation. In polar solvents, the barrier is reduced if the polar (TO* state increases its dipole moment before dissociation [59], and crossing (or avoided crossing) is either solvent assisted (early transition state) or solvent induced (polar solvent is required to create the surface crossing or avoided surface crossing). In nonpolar solvents the barrier is high because the increase of the dipole is inhibited, and ionization does not occur. Also, it was concluded that BGCN could not be sensitized by aromatic and nonaromatic ketones because this compound is an efficient triplet quencher via a mechanism other than energy transfer, most possibly via a charge-transfer interaction or electron transfer of the aniline moiety to the triplet sensitizer.

The model described by Geiger et al. is consistent with conclusions of Larson et al. [60], who, on the basis of molecular orbitals (MO) calculations, tried to explain why benzyl acetates and halides undergo heterolysis from TI and benzyl- ammonium structures from S 1. They found that immediately before dissociation, as the C-X bond elongates, the 3nn* state rapidly approaches the 3n0* state in the case of acetates and halides and 300* in the case of ammonium salt because there is no lone pair of electrons in the latter structure. It was suggested that an efficient spin inversion process leads to ion pair formation from the 3n0* state, and no such channel for the 300* state exists.

When picosecond flash photolysis became available, several reports appeared on the mechanism of photoionization of TAM dye leuco compounds. On the basis of the ultrafast absorption changes at 625nm, Cremers and Cremers [61] suggested that MGCN and BGCN undergo rapid cleavage in ethanol and glycerol to produce the corresponding dye cations initially in a pyramidal conformation (t < 35 ps). The latter relaxes via solvent-restricted motion into the more stable propeller-shaped planar conformation in 120 ps in ethanol and in 2-5 ns in glycerol. The subsequent nanosecond intensity changes were attributed to conforma- tional changes in the dye in the ground state. These workers also measured the absorbance spectrum over the 550-680 nm range at delays of 0.25 and 5.7 ns to identify the MG+ spectrum.

These conclusions were challenged by Manring and Peters [23], who studied the photochemistry of trityl-X compounds and Malachite Green leuco derivatives in polar and nonpolar solvents. They proposed that the initially formed absor-


bance at 625nm, observed by Cremers and Cremers, was due to S1 of MGX (where X is OH, OMe, or CN), because the SI of MGX, MG leucobase (MGH), and N,N-dimethyl-p-toluidine (DMT), a reasonable model, exhibits a wide 550- 700nm absorption band centered at 610nm. The initial absorption of S1 at 6 10 nm was formed within 30-50 ps after the flash. They proposed that the subsequent increase in absorption at 610 and 625 nm in polar solvents occurred due to the ionization of S1 to form MG+ and X - . This secondary rise was concomi- tant with spectrum sharpening as it was taking on the normal appearance of the MG+ spectrum centered at 610nm as well. The rate of S1 decay matched the rate of MG + formation. For example in the case of MGOMe, S I decayed and MG+ appeared with the lifetime of 2 ns in 90% t-butanol/lO% methanol and 400 ps in methanol. In nonpolar solvents, S1 of MGX decayed with a lifetime of 2-6ns, and no color was formed. Also, these workers concluded that the dominant factor in the rates of heterolytic cleavage in polar solvents is the electron affinity of the leaving group, that is, stability of X -, rather than the bond dissociation energy. This conclusion is consistent with the excited state potential energy surface dia- gram suggested by Geiger et al. I151 because the activation energy barrier to cleavage directly reflects the stability of the products. This is also in line with the solvent effects predicted by Ottolenghi [62], who assumed that the extra energy of 70-105 kcal/mol gained by solvation of the ion pair by the polar solvents is sufficient to cause the ionization, which is absent in nonpolar solvents.

In an extensive study of MGCN photoionization and the solvation dynamics of MG'CN- by Spears et al., the radiative lifetime, ion pair formation lifetime, and the quantum yields of fluorescence, intersystem crossing, and ion pair formation (@fl, +T, and +ip, respectively) were quantitatively shown to depend on the solvent dielectric constant [ 131. The ratio of radiative and nonradiative rates of SI relaxation was assumed essentially solvent independent as it is for DMA. The rise of the transient absorbance at 600 nm was found to be biexponential. The long component of absorbance rise time ranged from 1860 ps in ethyl acetate to 95 ps in methanol. The remarkable agreement was found for the kinetics of the long component and fluorescence decay, which decisively established that MG' was cre- ated directly from the S I of MGCN. This conclusion agrees with the important commentary by Michl[63] on the qualitative association of the o-+ o* excitation energy with the heterolytic bond strength for the singlet excited state and with the homolytic bond strength for the triplet excited state. That the slow component of the ionization rate greatly accelerated with increasing dielectric constant in aprotic solvents was rationalized as a rate-limiting behavior controlled by solvation of the ionic transition state. The ionization rate was modeled utilizing classic solvation energies of a forming dipole in a dielectric environment. The modeling pro- vided the transition-state dipole moment of 0.54-0.7. The short component of the absorption rise at 600 nm ranged from 6 ps in ethyl acetate to 1 1-1 8 ps in polar solvents and was not seen in the fluorescence decay kinetics. In contrast


to Manring and Peters [23], Spears and co-workers [13] interpreted the dual rise at 600nm as originating from MGf and not from S1 because the fast component was in large yield and would have existed in the fluorescence decay data. There- fore, the fast component was assigned in agreement with Cremers and Cremers [61] and others [ 151 to the initial contact ion pair where MGf is in partially con- jugated pyramidal geometry, which then undergoes geometric and charge reorganization to generate planar MG+ . The fast component displayed rates that decreased with increasing dielectric constant, and this was rationalized as due to a classic solvent reorganization barrier. The solvent must rearrange from an orientation stabilizing localized C 'CN- tetrahedral dipole to an orientation favoring a planar delocalized MG +CN- ion pair. The solvent reorganization barrier is smaller in lower dielectric constant solvents than in higher dielectric constant solvents when the delocalized ion pair is favored by a larger amount of energy over the localized pair in the lower polarity solvent than it is in the higher polarity solvent.

Spears et al. [ 131 suggested that the fast process originated from the S2 and had a much lower activation energy barrier for ionization. They speculated that S2 undergoes rapid internal conversion to high vibrational levels of S,, which then suffers both vibrational relaxation and fast ionization to form MG+CN- contact ion pair in tetrahedral geometry (Scheme 1.5). The latter is formed in < 1 ps and then either relaxes into the planar MG+ with delocalized charge or recombines, reducing the yield of ionization. To explain the solvent specificity of the ionization quantum yield, these workers suggested that the fast process has substantial recombination in solvents of low and medium polarity, which is reduced to a noncompetitive process in polar solvents. In other words, recombination yields in the fast process depend on the dielectric constant, vibrational relaxation, and longitudinal dielectric relaxation of the solvent. The slow process

I -

Scheme 1.5. The mechanism for photoionization of MGCN proposed by Spears et al.


has significant recombination only in solvents of low dielectric constant that is enhanced by the poor ionic stabilization of the developing ion pair.

In a later paper, Miller, Spears and co-workers [64] reported a comprehensive study of the rates for photoionization of MGCN measured as a function of solvent and temperature. The photoionization rates were used as a unique and sensitive probe for characterization of the intrinsic properties of the various solvents and their mixtures. The rate constants for the ionization (or dissociative intramolecular electron transfer), bt, were calculated according to the following equation from the fluorescence kinetic traces, which were monoexponential:

where kf is the experimental fluorescence rate constant, kfl is the radiative component (3.6 x lo7 s-I), and kT is the intersystem crossing rate (2.9 x 10' s-I; nonradiative decay to So, if any, is included here). The values of bt varied from 10" to lo7 s-' over the range of solvents studied. The activation energy for electron transfer displayed an intrinsic nature and was constant, averaging - 1 kcal/mol for the mixtures of aprotic solvents. In alcohols the activation energy ranged from 2 to 4 kcal/mol and consisted of a dominant viscosity-related barrier, which depends on alcohol monomer orientation times, and the intrinsic barrier, -0.5 kcal/mol. This means in the case of protic solvents that specific solvent motion controls the crossing of transition-state barrier. Because the intrinsic barrier was uniformly small, it was proposed that it is not solvent specific over the wide range of polarity ( E > 6), but becomes large in nonpolar solvents because no electron transfer occurs in toluene and cyclohexane. This conclusion supports the early observations of Holmes and the theoretical model of Geiger et al.

Spears and co-workers concluded that ionization proceeds via intramolecular electron transfer, which is nonadiabatic with only a barrier from solvent viscosity. They suggested that favorable changes in solvent configurational entropy (solvent gating) are necessary to achieve molecular distortion, which is brought about by the large amplitude motion that changes the bond angle at the central carbon atom. The distortion gives rise to a partial ionic charge development followed by electron transfer and ionic dissociation.

Similar partial ionic charge development or intramolecular partial charge transfer among the aryl groups leads to a different kind of dissociation that has been proposed by Okamoto et al. [65] to occur in many TAM structures, includ- ing TAM-CNs. This work is discussed later in the section dealing with other reactions of TAM derivatives.

We have recently shown that upon 300-nm irradiation, CVCN gives rise to the CV+CN- ion pair in solvents with E 2 -5 [30, 661. Because the fluorescence quantum yield decreases with increasing solvent polarity while the efficiency of coloration increases, we concluded that S I undergoes ionic dissociation. The


rate constants and quantum yields of ionization agree well with those reported for MGCN and other similar leuconitriles.

D. Miscellaneous

Several studies have appeared on the direct heterolysis of a C-C bond covalently linking a stable carbocation and stable carbanion. It was shown by picosecond laser flash photolysis that S of [ 3 4 1,2,3-triphenylcyclopropenyl)](4’-cyanophe- ny1)malononitrile (i) (Scheme 1.6) undergoes charge transfer, giving rise to a dipolar state, which then either relaxes into TI of triphenylcyclopropenyl group or undergoes heterolysis to produce an ion pair [67]. This example may represent fertile ground for examining the solvent and substituent effects on the partition- ing between forming a biradical (TI) or an ion pair. In another study it was suggested that nitrobenzyl carbanion and phenylacetal carbocation are produced via heterolysis of the triplet excited states of nitrobenzyl acetals (ii) (Scheme 1.6) [68]. Both ions were observed in a nanosecond flash photolysis experiment [69]; therefore uncertainty still exists over the events at shorter times ( < 10 ns).

The ionization of TAM dye leuco derivatives has also been studied in a micellar environment. For example, the enhancing participation of the carboxylate group in the ionization kinetics of Rhodamine B leuconitrile was studied in ionic micellar solutions of ammonium chloride surfactants [70]. Several binding mod- els were generated, which included ionic association of the ionized form of the dye carboxy-leuconitrile and surfactants. Also, the properties of MG leucobase substituted with long hydrocarbon chain and an electron transfer reaction with Fe3 + were investigated [7 11. Critical micelle concentrations of sulfate and ammonium surfactants in methanol solutions were estimated using the MG leucobase/Fe3 + as a probe.

We conclude this section with the idea that novel TAM+ structures have been synthesized, for example, 2,6,10-tris(dialkylamino)trioxatriangulenium dye (see Glossary) [72], a hybrid between CVand trioxatriangulenium cation, which is an exceptionally stable carbenium ion. This carbocation is about 10 orders of magnitude more thermodynamically stable than CV + on the scale of reactivity of the cation toward hydroxide ions. Other examples of the extraordinary stable carbo-

i ii

Scheme 1.6. Typical examples of compounds containing covalently bound stable carbocation and carbanion for the C-C bond heterolysis studies.


cations include the azulene analog of TAM cation [73], TAM dye ethynylogous [74], highly crowded @is( 1- and 2-naphthy1)methyl cations [38], highly stable and crowded ( 1 -pyrenyl)diphenylmethyl cation [75], a-(thio)carbamoyl-substituted aryl cations [76], N-alkylarylnitrilium cations [77], and novel derivatives of the Victoria Blue series of dyes containing the 3,6-bis(dimethylamino)fluore-9-y1 moiety [78], which affords extended conjugation. On the other hand, structures that contain more than one TAM unit recently have been reported to exhibit intri- guing chemistry. For example, a novel tricolor electrochromic system based on the reversible C-C bond cleavage in hexaarylethanes (two different TAM units) has been reported to exhibit hysteretic color changes [79]. Other examples are polymeric TAM dyes made from modified polystyrene [80] and polymeric CV+ [81] where CV moiety is pendent from a polyester backbone. The photochemical behavior of the leuconitriles of these novel structures may provide interesting aspects to the study and significance of the photoionization reaction.

E. Thermal Bleaching Reaction

Thermal bleaching, sometimes called the dark reaction or fading, is an intimately related subject of photoionization. Bleaching occurs when the photogenerated dyes react reversibly, or less often irreversibly, with solvent or with nucleophiles such as azide ion, water, and primary amines. The dyes may turn colorless, or fade, while reacting in the ground state or in the excited state, in the latter case upon absorption of visible light. Ground state reactions were studied by Ritchie [82] and co-workers with mixing and stopped-flow techniques, Dorfman [49] by pulse radiolysis, McClelland et al. with nanosecond laser flash photolysis [24,83, 841, and by others [82, 85-87]. McClelland and co-workers [88, 891 compre- hensively examined the subject and developed several structure-reactivity and reactivity-selectivity correlations. They also studied steric effects in detail. The photogeneration of carbocations and the study of their subsequent thermal reactions have been reviewed [16, 26, 55, 901.

Ritchie [82] studied the reactions of relatively stable cations with heteroatom nucleophiles mostly in water and derived a simple relationship [91] of the rate constant for the combination of a cation with a given nucleophile (excluding amines [84]), k, and a reference nucleophile. b:

The parameter N + reflects the nature of a nucleophile and the reaction condi- tions, and Eq. 2 has been shown valid for stable cations. However, the parameter N, does not depend on the nature of the electrophile, thus disconnecting the selectivity and reactivity of it. Therefore, N + overestimates the selectivity for


reactive cations and Eq. 2 does not hold for cations that react with solvent with rate constants > 1 x lo5 s-l [26,90]. A plot of log of rate constants for reaction of trityl cation with different nucleophiles versus N + gives a slope of 0.33 as opposed to 1 .O for the more stable methoxy- and dimethylamino-substituted ana- logs. A smaller slope reflects a lower selectivity in the reactivity of trityl cation [881.

When several nucleophilic species bleach a relatively less stable cation, the reactivity-selectivity model fits competition kinetics data. This study afforded the use of the azide and thiolate anion reaction (diffusion controlled k-5 x lo9 M-’ s-’) as a “clock” for obtaining absolute rate constants for less stable cations or, on the other hand, for more stable cations in the presence of more reactive solvents or nucleophiles [83, 921. Using flash photolysis, McClelland et al. [83,93] concluded that the “azide clock” method produces excellent estimates of rate constants for various nucleophiles. However, several studies revealed the retarding effect of the addition of water on the reaction, which involves hydration of such nucleophiles as amines, halides, and acetates and makes them less reactive than expected on the basis of their nucleophilicity [90]. The rate constants for reaction of dye cations with solvents and nucleophiles were shown to be consistent with the theory of diffusional encounter followed by the formation of ion pair: For more stable cations the ion pair-forming step was rate determining, and for the less stable the diffusion step was rate limiting. Naturally, for less stable cations the rate constants become independent of substituent. For more stable cations, the influence of a R donor substituent was divided into resonance and polar or inductive contributions, and each was treated separately because o +

measures alone underestimate the resonance interaction in the fully formed cations [88]. As the reaction proceeds from cation to transition state the resonance interaction of substituent with the positive charge decreases at a much greater rate than does the polar interaction. A rate-equilibrium plot of log k versus pKR based on combined data for ui- and diarylmethyl cations was shown to be linear for the families of different substituted cations, and the slope of 0.6 was found to be, remarkably, unchanged in the range of 23 PKR units [7].

The thermal bleaching reaction was recently shown, through linear free enthalpy relationships, to be useful in designing organic reactions of the electrophile-nucleophile type and predicting the rates of these reactions [94]. Study of the reactions of di- and triarylmethyl cations with various nucleophiles afforded a scale of nucleophilicity in weakly polar non-nucleophilic solvents [94] and pro- vided support for a relationship with the Ritchie [82] N, scale of nucleophilicity [87] in water.

Note that bleaching can also occur via a nucleophilic attack by solvent or by a nucleophile like N3- and CN- on the excited state of a cation. For example, for the excited states of 9-phenylxanthyl cation and related cationic structures, such reactions are shown to have rate constants several orders of magnitude higher

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