Article pubs.acs.org/JPCA

Kinetics of C−C and C−H Bond Cleavage in Phenyl Alkane Radical Cations Generated by Photoinduced Electron Transfer Douglas Cyr† and Paritosh Das* Physical Sciences Department, Cameron University, Lawton, Oklahoma 73505, United States S Supporting Information *

ABSTRACT: Employing nanosecond laser flash photolysis, we determined the relative importance of two fragmentation modes, namely, C−C bond cleavage and deprotonation, for the radical cation of 1,1,2,2-tetraphenylethane photogenerated by electron transfer to cyanoaromatic singlet excited states in acetonitrile at room temperature. Analysis of the kinetic data for this phenyl alkane suggests that the C−C bond cleavage dominates over the deprotonation by a ratio of about 2:1. In addition, the deprotonation kinetics of diphenylmethane, 1,1-diphenylethane, triphenylmethane, and several phenyl-substituted alcohols have been investigated. To aid identification and characterization, experiments based on two laser pulses in tandem (308 and 337.1 nm) were performed to probe fluorescence and photochemistry of the transient radicals formed as products of radical ion fragmentation. The first-order rate constants for growth of transient absorptions due to fragmentation-derived radicals were measured to be ≥1 × 106 s−1. Activation parameters, with activation enthalpies in the range 10−18 kJ/mol and activation entropies between −60 and −91 J/(mol.K), are also reported for fragmentation kinetics of radical cations of several systems under study.

1. INTRODUCTION Chemistry via photoinduced electron transfer (PET) can be very rich owing to the involvement of a variety of transient intermediates, namely, singlet and triplet excited states, radical ion pairs, exciplexes, solvated radical ions, and radicals.1−18 In addition, secondary transient products can be formed and implicated as a result of bimolecular interaction (quenching) of these intermediates with various species, including oxygen. In the recent past, considerable interest has been shown6,15−29 in using time-resolved techniques such as laser flash photolysis and pulse radiolysis to elucidate the mechanisms of chemical transformations arising from photoassisted electron transfer. Much of the focus6,11,15−18 in recent fast-kinetic studies of photoinitiated electron transfer has been on the dynamics of contact and solvent-separated radical ion pairs in picosecond and shorter time domains. However, to obtain a full picture of the chemistry arising from PET, there is still a need for studying processes involving solvated radical ions in the nanosecond and longer time domains. The work presented in this report was performed in a continuation of our interest in time-resolved kinetic investigations of photochemical reactions initiated by electron transfer (ET).19−29 In our work as well as that by several other groups, cyanoaromatic systems, such as 1,4-dicyanonaphthalene (DCN), 1,2,4,5-tetracyanobenzene (TCB), and 9,10dicyanoanthracene, have been widely used as excited-state electron acceptors.15−29,34−37 In the work reported herein, using nanosecond laser flash photolysis, we studied the fragmentation behavior of radical cations produced as a result of electron transfer from several phenyl-substituted alkanes and © 2014 American Chemical Society

alcohols to the lowest singlet excited state of two cyanoaromatics, namely, DCN and TCB. In particular, this undertaking was desirable to resolve an issue regarding the fate of the radical cation of 1,1,2,2,-tetraphenylethane (TPE). In ref 24, the radical cation of TPE was claimed to undergo C−C bond cleavage to diphenylmethyl carbocation (Ph2CH+) and diphenylmethyl radical (Ph2CH•), with the latter contributing to the growth of an intense transient absorption observed at 320−335 nm. Reference 34 contains a comment by its authors that the observation of the growth of the short-wavelength transient absorption could possibly be interpreted in terms of deprotonation of TPE radical cation to diphenyl(diphenylmethyl)methyl radical (Ph2C•−CHPh2), which would be spectrally indistinguishable from Ph2CH•. Apparently, this comment followed from the fact that the C−C bond cleavage of TPE was not observed34 in the course of its 1,4dicyanobenzene-photosensitized steady-state photolysis in acetonitrile−methanol mixture at ambient temperatures. The donor substrates under study are as follows: (1) 1,1,2,2tetraphenylethane (TPE), Ph2CH−CHPh2; (2) diphenylmethane (DPME), Ph2CH2; (3) 1,1-diphenylethane (1,1DPE), Ph2CH−CH3; (4) triphenylmethane (TPME), Ph3CH; (5) benzyl alcohol (BZA), PhCH2OH; (6) diphenylmethanol (DPML), Ph 2 CHOH; (7) triphenylmethanol (TPML), Ph3COH. Received: August 24, 2014 Revised: October 27, 2014 Published: October 27, 2014 11155

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2. EXPERIMENTAL SECTION 2.1. Materials. 1,4-Dicyanonaphthalene (DCN) was prepared by heating 1,4-dibromonaphthalene (Eastman) with cuprous cyanide and pyridine according to a modification of the procedure37,38 described for conversion of 1-bromonaphthalene to 1-cyanonaphthalene. The crude product was purified by column chromatography on neutral alumina followed by recrystallization from toluene. 1,2,4,5-Tetracyanobenzene (TCB) (Aldrich) was recrystallized twice from ethanol. 1,1,2,2-Tetraphenylethane (TPE), purchased from Columbia Organic Chemicals, was recrystallized from methanol. Diphenylmethane (DPME), 1,1-diphenylethane (1,1-DPE), and benzyl alcohol (BZA), all from Aldrich, were used as received. Diphenylmethanol (DPML), from Aldrich, was sublimed twice before use. Triphenylmethanol (TPML), also from Aldrich, was recrystallized from ethanol. Di-tert-butyl peroxide (DTBP) from MC&B was purified by passing through an alumina column. Acetonitrile (Aldrich, Gold Label) was distilled over P2O5 under an atmosphere of nitrogen. Benzene and methanol were of spectral grade. Pyridine (Aldrich) was distilled under reduced pressure. Lithium perchlorate, tetra-n-butyl ammonium perchlorate, and tetraethylammonium perchlorate, all from Aldrich, were used as received. 2.2. Nanosecond Laser Flash Photolysis. For nanosecond laser flash photolysis experiments (performed at the Radiation Laboratory, University of Notre Dame), appropriately attenuated outputs from the following two laser systems were used: UV-400 Molectron nitrogen (337.1 nm, 2−3 mJ, ∼8 ns) and Lambda-Physik EMG 101 excimer (Xe−Cl, 308 nm, ≤20 mJ, ∼20 ns) . A description of the kinetic spectrophotometer and the data collection system is available in previous publications from the Radiation Laboratory.39−41 The time domains for observation of transient species were typically in the range from 100 ns to 100 μs. Usually a front-face configuration was employed at an angle of ∼20° between the directions of laser pulses and the monitoring light. To minimize interference from fluorescence, in some experiments use was made of 1 cm × 1 or 1 cm × 0.3 cm quartz photolysis cells in which the laser excitation pulses and the monitoring light met at a right angle. Time-resolved fluorescence or transient absorption spectra were measured wavelength by wavelength using a flow system in which a rectangular quartz photolysis cell (path length ≈ 3 mm) was continuously fed with the solution under experiment from a reservoir. Kinetics measurements at specific monitoring wavelengths made use of static cells (quartz, path length ≈ 2 or 10 mm) containing 2−4 mL of the solutions that were shaken between laser shots. Except when the effect of oxygen was meant to be studied, the solutions were deoxygenated by bubbling high-purity argon or nitrogen. Unless otherwise noted, all laser flash photolysis experiments were performed in fluid solutions at room temperature (24 °C). The ground-state absorbances of the solutions at laser wavelengths were typically 0.1−0.3. The ground-state absorption spectra were recorded in a Cary 219 spectrophotometer with a 1 nm band pass. The steady-state fluorescence quenching experiments were carried out in a PerkinElmer LS5 spectrofluorimeter with a perpendicular configuration for excitation and emission. For temperature-dependence studies, the photolysis cell was surrounded by a Dewar-type quartz jacket fitted with flat windows. Preheated or precooled nitrogen gas was allowed to flow into the jacket around the cell. By regulating the flow of

the nitrogen gas, the temperature, monitored with a thermocouple probe, could be controlled within ±1 °C.

3. RESULTS 3.1. Transient Absorption Spectra and Kinetics. In acetonitrile, the lowest energy band systems of the ground-state absorption spectra of the two cyanoaromatics we studied, namely, 1,4-dicyanonaphthalene (DCN) and 1,2,4,5-tetracyanobenzene (TCB), occupy the spectral region 265−350 and 275−325 nm, respectively. Thus, 308 nm laser pulses were suitable for photoexcitation of these two cyanoaromatics at less than millimolar concentrations in the presence of the donor substrates under study at 10−60 mM without interference from direct excitation of the latter. In addition, we performed several dual-laser-pulse experiments with the purpose of identifying the radical cation-derived transient products (absorbing strongly at 320−340 nm) based on their fluorescence and photochemical behavior. In these experiments, in tandem with the primary 308 nm “synthesis” laser pulses, we used 337.1 nm “photolysis” laser pulses delayed from the former by 5−10 μs. In a previous study,24 it has been shown that fluorescence of DCN in acetonitrile is quenched by TPE and related aryl alkyl substrates (e.g., aryl pinacols and pinacol ethers) with bimolecular rate constants (ksq) in the limit of diffusion control. Specifically, the Stern−Volmer quenching constant (KsSV) is 94 M−1 for TPE; this gives42 a value of 9.9 × 109 M−1 s−1 for ksq. On the basis of these data, TPE at concentrations of 20−30 mM would quench the DCN singlet (1DCN*) to the extent of 65−74%. The extents of quenching of 1DCN* by other donors at the concentrations used in this work are estimated to be similar (i.e., above 50%). As for TCB, its lowest singlet excited state reduction potential is more favorable for electron transfer43 than that of 1DCN*. In support of this, we measured the Stern−Volmer quenching constant (KsSV) to be 262 M−1 for TPME as a quencher for steady-state TCB fluorescence in acetonitrile (compared to 44 M−1 with 1DCN*).48 In addition, with TCB, formation of ground-state electron donor−acceptor (EDA) complexes with the donor substrates under study is expected to be facile. Photoexcitation of such complexes would also lead to radical ions concomitantly with bimolecular quenching of the uncomplexed excited state by the donors. In the following subsections, we present results on transient absorption phenomena on the nanosecond to microsecond time scales following 308 nm laser pulse excitation of DCN and TCB in the presence of 10−50 mM concentrations of various phenyl-substituted alkanes and alcohols in acetonitrile at 24 °C. On the basis of evidence from supporting experiments, we also offer plausible interpretations of these phenomena in relation to steps 1−7 (below) that show the various relevant processes in the nanosecond to microsecond time domain following photoexcitation of the cyanoaromatics by nanosecond laser pulses. In eqs 1−7, TPE and DCN have been used as representative donor and excited-state acceptor, respectively. DCN* + TPE → 1(DCN−•/TPE+•)

1

(Formation of radical ion pair)

(1)

(DCN−•/TPE+•) → DCN + TPE

1

(Back electron transfer) 11156

(2)

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(DCN−•/TPE+•) → 3DCN* + TPE

1

(ET‐assisted intersystem crossing)

(3)

(DCN−•/TPE+•) → 2 DCN−• + 2 TPE+•

1

(Ion separation) 2

(4)

(CC bond cleavage)

TPE+• → 2 Ph 2CH• + Ph 2CH+

(5) 2

+•

TPE

→ Ph 2C CHPh 2 + H 2

•

+

(Deprotonation) (6)

2

TPE+• + 2 DCN−• → DCN + TPE (Diffusional back electron transfer)

(7)

3.1.1. 1,1,2,2-Tetraphenylethane. The transient absorption spectra observed at 0.25 and 1.3 μs following 308 nm laser pulse excitation of DCN in the presence of 20−30 mM TPE in deoxygenated acetonitrile are shown in Figure 1. These spectra

Figure 2. Representative kinetic traces of transient absorption at (A) 325, (B) 385, and (C) 440 nm following 308 nm laser pulse excitation of DCN in the presence of 20 mM TPE in acetonitrile.

from the emission could be substantially reduced to reveal the fast, in-pulse formation of the 385 nm species. The transient species observed with a sharp peak at 385−390 nm (Figure 1) is assigned to the solvated radical anion of DCN formed as a result of the fast dissociation of the ion pair, DCN−•/TPE+• in the subnanosecond time domain (step 4). This assignment is based on the similarity of the absorption spectral features, decay kinetic behavior, and oxygen sensitivity with those reported in the literature23,24 for the radical anion. The growth of the intense transient absorption with a maximum at 325−330 nm (Figure 2A), similar to that observed in an earlier study,24 is attributable, at least in part, to the diphenylmethyl radical (Ph2CH•) produced as a result of C−C bond cleavage in the TPE radical cation (step 5). The observed transient absorption spectrum, decay kinetics on a longer time scale, and oxygen sensitivity of the decay agree well with such behaviors well known for this extensively studied radical.51−56 The ratio of the maximum absorbances at 330 and at 390 nm is found to be 2.1 ± 0.2, which agrees very well with the ratio of molar absorption coefficients of Ph2CH• and DCN−• (namely, 4.2 × 104 and 2.2 × 104 M−1 cm−1, respectively).23,51,52 Additional evidence in support of the occurrence of step 5 is presented later. As pointed out by Okamoto and Arnold,34 the radical formed through deprotonation of TPE radical cation (i.e., step 6) should be spectrally similar to Ph2CH•. The hydrocarbon radical cations (CH+•) are recognized57−63 to be superacids with pKa values in the range from 0 to −30. Specifically, the pKa’s of the phenyl-substituted methanes, Ph2CH2 and PhCH3, have been estimated57 to be −25 and −20, respectively (in dimethyl sulfoxide). The pKa of toluene radical cation in acetonitrile has also been reported to be very

Figure 1. Transient absorption spectra observed at 0.25 and 1.3 μs following 308 nm laser pulse excitation of DCN in the presence of 20 mM TPE in acetonitrile. (Inset) Kinetic trace of decay of fluorescence resulting from subsequent 337.1 nm laser pulse excitation of the transient absorbance produced by 308 nm laser pulse excitation of DCN in the presence of 25 mM TPE in acetonitrile.

have the following principal features: (a) a sharp, intense band system with absorption maximum at 325−330 nm characterized by a first-order growth that is completed over ∼3 μs, (b) a sharp but relatively less intense peak at 385−390 nm due to a transient species that is formed within the laser pulse and decays slowly on a >microsecond time scale, (c) rapid growth followed by decay over ∼2 μs of a transient species with absorption maximum at 440 nm, and (d) minor, slowly decaying transient absorptions with maxima at ∼460 nm formed within the laser pulse. Three kinetic traces showing the decay/formation characteristics of the 325, 385, and 440 nm species are presented in Figure 2. It should be noted that the apparent initial growth observed for the 385 nm species (Figure 2B) is due to a contribution from the tail of the unquenched portion of DCN fluorescence; this was verified by laser flash experiments with the same solutions in the absence of the monitoring light (that is, with the monitoring light blocked from entering into the photolysis cell.) Also, experiments in square cells (1 cm × 1 cm) in a right-angle geometry between laser pulses and monitoring light showed that the interference 11157

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maximum (2.2 × 10 4 and 4.4 × 10 4 M −1 cm − 1 , respectively)23,51,52 and ΔAmax,DCN−•. and ΔAmax,Ph2CH+ are the corresponding maximum absorbances. Using the absorbance data from our experiments with DCN and TPE, we estimated a value of 0.7 ± 0.1 for ϕC−C. Thus, the deprotonation (i.e., step 6) accounts for about 30% of the decay mode of TPE radical cation. Interestingly, we obtained a similar value for ϕC−C on the basis of a comparison of absorbances at 325−330 and 440 nm if the molar absorption coefficient of the radical, Ph2C•− CHPh2, from deprotonation (step 6) was assumed to be the same as that of Ph2CH• at 325−330 nm (namely, 4.4 × 104 M−1 cm−1).51,52 The minor absorption band system observed at ∼460 nm (Figure 1) is due to DCN triplet produced in part from direct intersystem crossing and in part through electron-transferinduced intersystem crossing (step 3). Its spectral location, decay kinetics, and oxygen sensitivity match similar properties noted for the DCN triplet in earlier studies.23,24 The transient phenomena observed with TCB as the excitedstate electron acceptor and TPE as the ground-state donor are essentially similar to those described above for DCN. TCB is known71 to form ground-state electron-donor−acceptor (EDA) complexes with aryl alkanes. Thus, as noted earlier, the radical cation of TPE in this case is probably produced in part from the excitation of the EDA complex at the laser excitation wavelength (308 nm) used. Presented in Figure S1 (see Supporting Information), the transient absorption spectra consist of an intense absorption band system at 325−330 nm due to radicals derived from TPE radical cation and a minor band system around 440 nm due to diphenylmethyl carbocation. The TCB radical anion exhibits its major absorption maximum at 460 nm. Additionally, minor bands are indicated for TCB−• at shorter wavelengths, namely, 440, 360, and 345 nm. These spectral features agree with those reported in the literature15,50,72,73 for TCB−•. It should be noted that the ratio (3:1) of the observed maximum absorbances at 330 and 460 nm agrees well with that of the molar absorption coefficients of Ph2CH+ and TCB−• at their wavelength maxima (4.4 × 104 M−1 cm−1 at 330 nm51,52 and 1.35 × 104 M−1 cm−1 at 460 nm,72 respectively). A small but nonnegligible difference was noted between the transient absorption phenomena from TCB and DCN in the presence of some of the donors studied. A minor component of the transient absorbance growing over a long microsecond time scale was observed with the former but not with the latter. Though small (∼10% of the total absorbance), this slow component was particularly conspicuous at the maximum (i.e., 460 nm) corresponding to TCB−•. We chose not to investigate it in any detail.74 However, because of the complication arising from it in experiments with TCB, the kinetic data associated donor radical cations were obtained from experiments with DCN only. We sought additional evidence in support of the occurrence of C−C bond cleavage in TPE radical cation leading to diphenylmethyl radical by carrying out experiments in which the transient species with λmax at ∼330 nm was subjected to photoexcitation with a second 337.1 nm laser pulse that was delayed by a few microseconds with respect to the initial 308 nm laser pulse. Two representative traces of the relatively longlived fluorescence decay observed in experiments with DCN and TCB as excited-state acceptors are shown in the insets of Figures 1 and S1, Supporting Information. The doublet− doublet fluorescence of Ph2CH•, with lifetimes in the range of

negative, namely, −12 by Green et al.61 and −13 by Nicholas and Arnold.63 On the basis of a similarly negative pKa of TPE radical cation, one would expect it to undergo facile deprotonation to the radical, Ph2C•−CHPh2, the transient spectrum of which would overlap strongly with that of Ph2CH•. Thus, using transient absorption spectra alone, one cannot decide if the observed band system at 325−330 nm is due to Ph2CH• entirely, partially, or not at all. Convincing evidence in support of C−C bond cleavage in TPE radical cation comes from the observation of the transient absorption with a maximum at ∼440 nm (Figure 1), characterized by its initial growth followed by decay (Figure 2C). This transient absorption is assignable to the diphenylmethyl carbocation, Ph2CH+, formed from step 5. In acetonitrile at room temperature, this carbocation is known51,64−68 to exhibit an absorption maximum at 435 nm and decay by first-order kinetics with a rate constant of 3.2 × 106 s−1. The first-order decay of diphenylmethyl cations in acetonitrile has been attributed to formation of a nitrilium ion68 (eq 8) and shown to be sensitive to the presence of nucleophiles such as water. The reported51,52,64−68 absorption maxima of Ph2 CH + in other solvents, namely, water, concentrated sulfuric acid, nitromethane, and 1,2-dichloroethane, are in the same spectral region (440−453 nm). Ph 2CH+ + CH3CN → CH3CN+CHPh 2

(8)

An analysis of the formation/decay kinetics of the ∼440 nm species observed in our experiments (Figure 2C) was performed in terms of two consecutive first-order processes using a formation rate constant of (3.7 ± 0.6) × 106 s−1 (i.e., same as that of formation of the 330 nm species, i.e., radical(s) produced in parallel from TPE radical ion fragmentation) and a decay rate constant of 3.2 × 106 s−1. The computed maximum of the 440 nm transient absorption was found to be at 0.29 ± 0.05 μs following the laser pulse; this agreed reasonably well with the location of the maximum, 0.23 ± 0.05 μs, observed in several of our experiments. The best fit of the kinetic trace in Figure 2C into consecutive decay/formation kinetics led to an estimate of (4.1 ± 0.5) × 106 and (3.6 ± 0.4) × 106 s−1 for the formation and decay rate constant, respectively.69 The computed formation rate constant agrees, within experimental errors, with that measured for formation of the radical(s) at 330 nm. Thus, both absorption spectral and kinetic behavior of the observed 440 nm species strongly supports its assignment as the carbocation, Ph2CH+. An estimate of the extent to which the C−C bond cleavage mode (step 5) contributes to the fragmentation of the TPE radical cation could be made on the basis of a calculation of the maximum plateau absorbance that the diphenylmethyl cation (Ph2CH+) would have if it did not decay following its formation. On the basis of the rate constants of 4.1 × 106 and 3.5 × 106 s−1 for formation and decay, respectively, this extrapolated plateau absorbance was estimated to be 2.6 times larger than that at the observed maximum at 440 nm.69,70 This factor was then used in eq 9 (below) to calculate the fraction (ϕc−c) of the decay of TPE radical cation precursor that occurred via C−C bond cleavage. ϕC − C =

ΔA max,Ph 2CH+ εmax,DCN−• k5 = 2.6 ΔA max,DCN−• εmax,Ph 2CH+ k5 + k 6

(9)

In eq 9, εmax,DCN−• and εmax,Ph2CH+ are molar absorption coefficients of DCN−• and Ph2CH+ at their absorption 11158

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100−260 ns in fluid solutions at room temperature, has been well characterized in the literature.53−55,75,76 The fluorescence spectrum of the 330 nm transient that we recorded in a pointby-point manner under excitation by 337.1 nm laser pulses agreed in both location (500−600 nm) and structural features with that published by Bromberg et al.54 This combined with the observed relatively long fluorescence lifetimes of 210−250 ns77 provided strong evidence in support of a substantial contribution of Ph2CH• to the transient absorption at 325−330 nm. Unlike the remarkably long-lived doublet excited state of Ph2CH•, its substituted analogs, e.g., Ph2C•R* (R = Ph, Me, or cyclo-Pr), have been shown to possess short lifetimes at room temperature in fluid solutions.53−55,76 The short lifetimes are attributed to a large contribution of intramolecular photochemistry, namely, electrocyclic ring closure, facilitated by an increased twist angle of the phenyl rings owing to substituents bulkier than H.53−55,76 Accordingly, the radical (Ph2C•− CHPh2) derived from deprotonation of TPE radical cation could not be responsible for our observed long-lived fluorescence. We present later the results of our two-laser experiments in the absorption mode showing significant depletion of the absorbance of the two radicals, Ph2C•R* (R = Me and Ph), produced via deprotonation of 1,1-DPE and TPME radical cations, respectively, as well as concomitant increase in absorbance at 450−500 nm due to photoproduct formation in both cases. To examine the extent to which Ph2CH• is spectrally distinct from Ph2C•−CHPh2, we performed experiments in which the radicals in question were generated via hydrogen abstraction by tert-butoxy radical and acetone triplet. As in a previous study,78 the tert-butoxy radical was conveniently produced by laser flash photolysis of a mixture of benzene and di-tert-butyl peroxide (DTBP) (4:1, v/v). Similarly, the 308 nm laser flash photolysis of a 1:9 mixture (v/v) of acetone and acetonitrile produced acetone triplet that abstracted hydrogen from TPE. t ‐BuOOBu‐t + hv → 2t ‐BuO• •

t ‐BuO → CH3COCH3 +

CH3•

(first order)

Figure 3. Transient absorption spectra observed upon 308 nm laser pulse excitation of (A) 10 mM DPME solution in benzene:DTBP mixture (4:1, v/v), (B) 10 mM TPE solution in benzene:DTBP mixture (4:1, v/v), and (C) 10 mM TPE in acetone:acetonitrile mixture (1:9, v/v). Times at which the spectra were recorded were 1, 1.2, and 1.2 μs, respectively, following the laser pulse. (Insets) (a) Kinetic trace of fluorescence at 560 nm resulting from 337.1 nm laser pulse excitation of the transient absorbance produced by 308 nm laser pulse excitation 10 mM DPME solution in benzene:DTBP mixture; (b) kinetic trace of emission at 560 nm resulting from 337.1 nm laser pulse excitation of the transient absorbance produced by 308 nm laser pulse excitation 10 mM TPE solution in benzene:DTBP mixture.

two radicals was not sufficiently pronounced to be definitive in distinguishing between them. Notably, however, the transient spectra observed upon laser flash photolysis of DCN or TCB in the presence of TPE in acetonitrile displayed the shortwavelength maximum at a wavelength closer to the λmax of Ph2CH•, indicating qualitatively that C−C bond cleavage in TPE radical cation was more important than its deprotonation. Employing two-laser excitations (308 and 337.1 nm), we probed the emission characteristics of the transient radicals produced from DPME and TPE by H abstraction by tert-butoxy radical. As expected for Ph2CH• from DPME, a relatively longlived, strong fluorescence with a lifetime of ∼300 ns (see inset a in Figure 3) was observed at 500−600 nm. In contrast, in the case of the radical from TPE, practically no emission was detected (except for a very short-lived, weak emission of spurious origin, see inset b in Figure 3). The lack of fluorescence in the case of the radical, Ph2C•−CHPh2, from TPE is in agreement with similar behaviors of doublet excited states of substituted radicals, Ph2C•R (R = Ph, Me, or cyclo-Pr), discussed earlier in this subsection. In light of our conclusion that the deprotonation to the substituted radical Ph2C•−CHPh2 constitutes about 30% of TPE radical ion decay in acetonitrile, it was desirable to conduct a few two-laser experiments (308 and 337.1 nm) in the absorption mode on the radical products from TPE radical cation. The purpose was to show the occurrence of photobleaching expected for Ph2C•−CHPh2. A few experiments performed with 1TCB* as the acceptor and TPE as the donor in acetonitrile showed a small amount of 337.1 nm laserinduced depletion of transient absorbance at 325 nm and a concomitant increase of transient absorbance at 490 nm. At the former wavelength, partial recovery of the transient absorbance was noticed on a nanosecond time scale that matched kinetically with the fluorescence decay of Ph2CH•*. Similarly, at 490 nm, a portion of the increased absorbance from 337.1

(10) (11)

t ‐BuO• + Ph 2CHCHPh 2 → t ‐BuOH + Ph 2C•CHPh 2 (CH3)2 CO + hv → 1(CH3)2 CO* → 3(CH3)2 CO*

(12) (13)

(CH3)2 CO* + Ph 2CHCHPh 2

3

→ (CH3)2 C•OH + Ph 2C•CHPh 2

(14)

The transient absorption spectra observed at ∼1 μs following laser flash photolysis (308 nm) of benzene/DTBP mixture and acetone in the presence of TPE and of benzene−DTBP mixture in the presence of diphenylmethane (DPME) are presented in Figure 3. The radicals produced as a result of H abstraction from both TPE and DPME are spectrally similar. These spectra exhibit sharp, intense absorption maxima in the spectral region 310−335 nm. Close inspection, however, reveals a small difference. While the radical from DPME (i.e., Ph2CH•) shows its absorption maximum at 330 nm in benzene−DTBP mixture, λmax of the radical from TPE (i.e., Ph2C•−CHPh2) is slightly blue shifted (i.e., at 315 nm in benzene−DTBP mixture and 325 nm in acetonitrile). Admittedly, in the light of the experimental error (±5 nm) in our transient absorption spectral measurements, the difference in the spectra of the 11159

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at 450−500 nm (Figure 4). These were assigned to the DCN triplet formed through direct as well as electron-transferassisted intersystem crossing (step 3). 3.1.3. 1,1-Diphenylethane. We undertook the study of 1,1diphenylethane (1,1-DPE) since its radical cation could presumably undergo both deprotonation and C−C bond cleavage. The transient absorption spectra observed upon 308 nm laser flash photolysis of DCN in the presence of 27 mM 1,1-DPE in acetonitrile are presented in Figure 5. The inset of

nm photoexcitation underwent decay on the time scale of that of Ph2CH• emission. These nanosecond absorption phenomena, attributable to Ph2CH•*, were followed by small, longer lived components of depletion and of increased absorbance at the two wavelengths, respectively. Apparently, these longer lived absorbance changes were due, in a large part, to the photochemistry of Ph2C•−CHPh2. The magnitudes of these changes, however, were small, understandably because Ph2C•− CHPh2 was a minor component of the mixture of transients that were photoexcited at 337.1 nm. In addition, Ph2C•− CHPh2 with its absorption maximum blue shifted relative to that of Ph2CH• probably absorbed less light at the 337.1 nm laser excitation wavelength. 3.1.2. Diphenylmethane. The 308 nm laser flash photolysis of DCN or TCB in the presence of 25−30 mM diphenylmethane (DPME) in acetonitrile results in formation of transient species that are assignable to cyanoaromatic radical anions and diphenylmethyl radical, Ph2CH•. The latter is formed as a result of deprotonation of DPME radical cation (eq 15) on a microsecond time scale. Ph 2CH 2+• → Ph 2CH• + H+

(15)

Figure 4 shows the transient spectra observed at 0.5−3 μs following laser pulse excitation of DCN. The radical, Ph2CH•, Figure 5. Transient absorption spectra observed at 0.5 and 1.5 μs following 308 nm laser pulse excitation of DCN in the presence of 27 mM 1,1-DPE in acetonitrile. (Inset) Kinetic trace for formation of transient absorption at 325 nm following 308 nm laser pulse excitation of DCN in the presence of 27 mM 1,1-DPE in acetonitrile.

this figure shows a kinetic trace of the first-order growth of the transient absorption at 325 nm. The sharp, intense absorption maximum (325 nm) in the short wavelength region is assignable to either or both of the radicals that could be formed from the radical cation of 1,1-DPE (eqs 16 and 17). Ph 2CHCH3•+ → Ph 2C•CH3 + H+

(16)

Ph 2CHCH3•+ → Ph 2CH• + CH3+

(17)

However, two-laser experiments (based on 308 and 337.1 nm in tandem) showed that the observed radical product from 1,1DPE radical cation was nonemitting. Thus, it could not possibly be the diphenylmethyl radical, ruling out C−C bond cleavage (eq 17) as a fragmentation mode. In addition, as detailed in Figure 6 with 1TCB* as acceptor, the transient absorption spectra recorded at ∼0.2 μs before and at ∼0.2 μs after the 337.1 nm “photolysis” laser pulse showed considerable depletion of absorbance at 315−340 nm, accompanied by enhancement of absorbance at 450−500 nm. Thus, 337.1 nm photoexcitation of the 325 nm transient species resulted in formation a photoproduct with an absorption band system at 430−500 nm. This is distinctly indicated by the difference spectrum C in Figure 6. In the insets of Figure 6 are presented kinetic traces that also clearly show the depletion of absorbance at 330 nm and its enhancement at 460 nm. These results point to the pronounced photochemistry of the radical product formed from deprotonation (step 16) and support its assignment as Ph2C•−CH3 (in line with the discussion presented earlier for substituted diphenylmethyl radicals). 3.1.4. Triphenylmethane. As expected, the 308 nm laser flash photolysis of DCN or TCB in the presence of triphenylmethane (TPME) in acetonitrile resulted in facile formation of the radical, Ph3C•, via fast deprotonation of

Figure 4. Transient absorption spectra observed at 0.5 and 1.5 μs following 308 nm laser pulse excitation of DCN in the presence of 30 mM DPME in acetonitrile. (Inset) Kinetic trace of growth of transient absorption at 325 nm following 308 nm laser pulse excitation of DCN in the presence of 30 mM DPME in acetonitrile.

is characterized by its sharp, intense absorption maximum at 330 nm (Figure 4). The transient spectra observed with 1TCB* as acceptor are presented in Figure S2, Supporting Information. The inset in Figure 4 shows its first-order growth over ∼3 μs. The radical anions of DCN and TCB, formed within the laser pulse, display their major maxima at 390 and 460 nm, respectively (Figures 4 and S2, Supporting Information). Identification of these transients was confirmed on the basis of spectral and kinetic similarity with those reported in the literature.15,22,23 Two-laser experiments based on 308 and 337.1 nm laser pulses in tandem led to the observation of the relatively long-lived fluorescence (500−600 nm, τF ≈ 300 ns) characteristic of the diphenylmethyl radical. A representative kinetic trace of the emission is presented in the inset of Figure S2, Supporting Information. As in the case of TPE (discussed earlier), with 1DCN* as the acceptor, additional minor transient absorptions were observed 11160

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Figure 7. Transient absorption spectra observed at 1.2 and 1.75 μs following 308 nm laser pulse excitation of TCB in the presence of 25 mM TPME in acetonitrile with a 337.1 nm laser pulse photoexciting the transients at 1.4 μs after the 308 nm pulse. (C) Difference of transient absorptions after and before 337.1 nm photolysis. (Insets) Representative kinetic traces showing (a) bleaching at 320 nm and (b) photoproduct formation at 490 nm as a result of 337.1 nm laser photolysis of transient(s).

Figure 6. Transient absorption spectra observed at (A) 3.5 and (B) 4.0 μs following 308 nm laser pulse excitation of TCB in the presence of 25 mM 1,1-DPE in acetonitrile with a 337.1 nm laser pulse photoexciting the transient at 3.7 μs after the 308 nm pulse. (C) Difference of transient absorptions after and before 337.1 nm photolysis. (Insets) Representative kinetic traces showing (a) bleaching at 330 nm and (b) photoproduct formation at 460 nm as a result of 337.1 nm laser photolysis of transient(s).

we performed laser flash photolysis experiments with three phenyl-substituted alcohols as electron donors for 1TCB* and 1 DCN* in acetonitrile. With benzyl alcohol (BZA), the electron transfer to both of the two singlet excited-state cyanoaromatic acceptors resulted in formation of phenyl hydroxymethyl radical, PhC•HOH. This occurred within the laser pulse, suggesting that deprotonation of BZA radical cation to phenyl hydroxymethyl radical was too fast to be observed over nanoseconds. The transient absorption spectra are shown in Figure S4 (Supporting Information) with 1TCB* as acceptor. The phenyl hydroxymethyl radical is characterized by a strong absorption band system at 260−300 nm with a sharp maximum at 275 nm (Figure S4, Supporting Information). This spectrum agrees with the published transient spectra of the ketyl radical, PhC•(CH3)OH, produced as a result of H abstraction by the acetophenone triplet.82,83 The absorption band systems at longer wavelengths in Figure S4, Supporting Information, are primarily due to TCB−• and in a small part due to PhC•HOH. The results of our experiments with diphenylmethanol (DPML) as a donor for 1TCB* and 1DCN* were similar to those observed with benzyl alcohol. The transient absorption spectra from 308 nm laser flash photolysis of TCB in the presence of 50 mM DPML in acetonitrile are presented in Figure S5 (Supporting Information). The observed transient absorption spectra are composed of those of diphenyl hydroxymethyl radical (Ph2C•OH) with band systems at 320−340 and 500−580 nm and TCB radical anion with band systems at 440−470 and 340−390 nm. For both, the observed spectra are in agreement with those reported in the literature.15,72,84−86 At room temperature, deprotonation of DPML radical cation occurred within nanoseconds after the laser pulse. At subambient temperatures (i.e., at and below −15 °C), however, we were able to observe the fast growth of the absorbance of the radical with lifetimes ranging from 100 to 250 ns as the temperature was lowered from −15 to −45 °C. In the case of triphenylmethanol (TPML), electron transfer to 1TCB* or 1DCN* occurred, as evidenced by formation the

TPME radical cation. As shown in Figure S3, Supporting Information, the radical is characterized by a sharp absorption maximum at 335 nm. The spectral and kinetic characteristics observed by us agreed with those reported in the literature for this well-known radical.52,79−81 In our flash photolysis experiments, the radical was formed within the laser pulse, suggesting that the deprotonation took place on a time scale shorter than a nanosecond. Experiments at subambient temperatures (e.g., −50 °C) did not reveal any growth of the transient absorption at ≥50 ns time domains. We examined the photochemistry of Ph3C• by performing two-laser experiments (308 and 337.1 nm in tandem) with 1 TCB* as the electron acceptor from TPME. The results are presented in Figure 7. The transient absorption spectra at ∼0.2 μs before the 337.1 nm laser pulse and at ∼0.2 μs after it display, respectively, a pronounced decrease of absorbance at 320−350 nm and a substantial, concomitant increase in absorbance at 400−550 nm. The difference spectrum (C in Figure 7) clearly reveals formation of a photoproduct with an absorption spectrum at 400−550 nm. The kinetic traces in the insets of Figure 7 demonstrate the loss and enhancement of absorbance in the two spectral regions as a result of the 337.1 nm photolysis of Ph3C•. Dual-laser experiments on Ph3C• in aqueous solution by Faria and Steenken81 showed that as a result of photolysis by 248 or 308 nm laser pulses the radical underwent extensive photoionization to Ph3C+ with an efficiency of 75−85%. This work also revealed a small extent of photochemical transformation, i.e., formation of the cyclization product, 4a,4b-dihydro-9-phenylfluorenyl radical, in agreement with the work of Bromberg et al.53,54 The features of the difference spectrum (Figure 7C) observed in our experiment in acetonitrile demonstrate that photocyclization is favored in this solvent. 3.1.5. Benzyl Alcohol (BZA), Diphenylmethanol (DPML), and Triphenylmethanol (TPML). For the sake of comparison, 11161

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cyanoaromatic radical anions. The transient spectra with 1 TCB* as acceptor are presented in Figure 8. In addition to

Figure 9. Eyring plots for the temperature dependence of C−C bond cleavage and/or deprotonation kinetics of the radical cations of (I) TPE, (II) DPME, (III) 1,1-DPE, and (IV) DPML.

different. A close look at the plots corresponding to TPE (I in both Figures 9 and S6, Supporting Information) reveals a slight curvature. However, our attempt to fit the data to an appropriate double-exponential equation did not yield any meaningful results. 3.3. Quenching Studies. A limited number of experiments were carried out to study the effects of several reagents, namely, perchlorate salts, methanol and pyridine, which could potentially act as quenchers for some of the steps involved in radical cation formation and fragmentation processes. The objective of this work was to corroborate the assignments of the observed transient species while keeping in mind that the effects of the quenchers might originate in multifarious manners, e.g., interactions with photoexcited states of cyanoaromatics, radical cations and their ion-pair precursors, and products of radical ion fragmentation. 3.3.1. Perchlorate Salts. We studied the effects of three perchlorate salts, namely, lithium perchlorate, triethylammonium perchlorate (TEAP), and tri-n-butyl ammonium perchlorate (TBAP), at 0.10 M on DCN-photosensitized formation and fragmentation of radical cations of TPE, 1,1DPME, and DPM in acetonitrile. The effects of the three salts, presumably arising from the ion pairing of the radical cation with perchlorate anion, were nearly identical. Under 308 nm laser pulse excitation of DCN + 20 mM TPE in acetonitrile, the presence of lithium perchlorate, TEAP, and TBAP, each at 0.10 M in the solution, caused a slight enhancement (4−9%) in the transient absorption due to radical products at 325 nm. The first-order rate constants for growth of the same transient absorption also showed a small increase, namely, by 18−30%. In the presence of the salts, the growth−decay phenomena of transient absorbance due to the diphenylmethyl carbocation (Ph2CH+) at 440 nm disappeared completely, suggesting that the carbocation either was not formed in the presence of the salts or was kinetically quenched by them. The carbocation is known to be quenched by various inorganic anions (e.g., halides and nitrate) with diffusion-controlled rate constants in

Figure 8. Transient absorption spectra observed at 0.5 and 2.0 μs following 308 nm laser pulse excitation of TCB in the presence of 25 mM TPML in acetonitrile.

the absorbances at 400−480 nm due to TCB radical cation, prominent transient absorptions are evident at 330−450 nm. The latter are best assigned to the radical cation of TPML formed within the laser pulse. 3.2. Temperature Dependence of Radical Cation Fragmentation Kinetics. The first-order rate constants for growth of radicals as a result of the fragmentation of the radical cations from TPE, DPME, and 1,1-DPE at room temperature (24 °C) are given in Table 1. As our kinetic data suggest, fragmentation of TPE radical cation comprises C−C bond cleavage and deprotonation in parallel. For DPME and 1,1-DPE radical cations, deprotonation is the sole mode of fragmentation. Also included in Table 1 is the rate constant for deprotonation of the radical cation of DPML at the highest temperature (−17 °C) at which we could measure the rate reliably. It should be noted that the rate constant data in Table 1 were obtained from experiments that were carried out in optically dilute solutions at relatively low 308 nm laser pulse intensities for excitation. In experiments in which high concentrations of the transient radicals were produced, the rate constants obtained from best fits of the growth profiles to first-order kinetics were found to be 5−30% higher. We studied the temperature dependence of the fragmentation kinetics by measuring the rate constants at various temperatures in the range from −45 to 75 °C. The Eyring and Arrhenius plots are presented in Figures 9 and S6 (Supporting Information), respectively. The activation parameters obtained from these plots are compiled in Table 1. In light of the two parallel modes contributing to fragmentation of the TPE radical cation, one would expect a nonlinear Arrhenius (or Eyring) plot if the activation parameters for the two modes are substantially

Table 1. First-Order Rate Constants (at 24 °C) and Activation Parameters for C−C Bond Fragmentation of Radical Cations in Acetonitrile

a

radical cation substrate

fragmentation mode

k at 24 °C, 106 s−1 a

1,1,2,2,-tetraphenylethane diphenylmethane 1,1-diphenylethane diphenylmethanol

C−C cleavage and deprotonation deprotonation deprotonation deprotonation

3.7 1.5 1.4 7.8b

Ea, kJ/mol 19.4 18.1 13.2 12.0

± ± ± ±

0.7 0.6 0.4 1.8

ΔH‡, kJ/mol 17.7 15.8 10.9 10.1

± ± ± ±

0.7 0.6 0.4 1.6

ΔS‡, J/mol-K −60 −73 −91 −72

± ± ± ±

2 2 1 7

With ±15%. bAt −17 °C. 11162

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the vicinity of 2 × 1010 M−1 s−1 in acetonitrile.87 In comparison to TPE, in the presence of the salts, the enhancement of the yields of radicals in terms of plateau absorbance at 325 nm and of their first-order formation kinetics was spectacular in the case DPME and 1,1-DPE. For 1,1-DPE, the increase in the rate constant for growth of Ph2C•−CH3 was about 3-fold (i.e., from 1.1 × 106 to 3.2 × 106 s−1) and that in the plateau absorbance (at 325 nm) was by ∼52%. In the case of DPME, the rate constant for growth of Ph2CH• absorption also increased dramatically from 2.0 × 106 s−1 to ∼8 × 106 s−1 and the corresponding plateau absorbance increased by ∼46%. In the absence of detailed data, it was premature to attempt to interpret the effects of ion pairing. Nevertheless, the differences between the salt effects on TPE radical cation and those on its DPME and 1,1-DPE analogs were worth noting. 3.3.2. Methanol. A brief study of the effects of methanol (MeOH) was of interest in light of the fact that it has been used as a nucleophile to trap Ph2C+ in electron-transfer photochemical studies.34,35 The 308 nm laser flash photolysis of DCN in the presence of 20 mM TPE and 0.082 M MeOH showed that the first-order rate constant for growth of 325 nm absorbance increased slightly from 3.3 × 106 (without MeOH) to 3.7 × 106 s−1 (with MeOH). This increase in rate, though small, suggested modest reactivity of TPE radical cation with the alcohol. The rate increase was accompanied by a decrease in plateau absorbance to the extent of 18%. Pertinent to our assignment of the 440 nm transient as diphenylmethyl carbocation, the growth−decay profile absorbance at 440 nm attributable to this species was totally absent in the experiment with MeOH. Methanol is a moderately strong quencher of the carbocation with a reported87 rate constant of 1.2 × 109 M−1 s−1 in acetonitrile. With [MeOH] = 0.082 M, the lifetime of the carbocation would be shorter than 10 ns, rendering it undetectable in our experiment. With DPME and 1,1-DPE radical cations, the increase in the growth rate constant for 325 nm absorbance in the presence of 0.080 M MeOH was more pronounced (i.e., by a factor of 2) than in the case of TPE. The corresponding plateau absorbances in the presence of MeOH were higher than those in its absence by 10−20%. The behavior of TPE radical cation in the presence of MeOH was again noticeably different from that of DPME and 1,1-DPE radical cations. 3.3.3. Pyridine. Like methanol, pyridine (Pyr) is also expected to act as a nucleophile interacting with diphenylmethyl cation. A few 308 nm laser flash photolysis experiments with DCN and TPE in the presence of 3.1 mM pyridine showed the disappearance of the growth/decay phenomena of transient absorbance at 440 nm (due to Ph2CH+) in the presence of pyridine. Pyridine at 3.1 mM also caused a noticeable decrease (by 25%) in the absorbance due to DCN radical anion (monitored at 385 nm). This decrease could be explained in part in terms of the quenching of the 1DCN* by pyridine.88 Note that the laser flash photolysis (308 nm) of DCN in the presence of pyridine alone (i.e., with no TPE) did not show any significant absorbance due to DCN radical anion at 385−390 nm.89 Interestingly, however, at the millimolar concentration of pyridine used, the yield of the transient radical products at 325 nm was almost completely suppressed (i.e., by ∼90%) and the usual fast growth of the transient absorption at this wavelength was replaced by a considerably slower growth of much weaker absorption with a lifetime of ∼4 μs. Similar effects of pyridine at millimolar concentrations were also noted with DPME and 1,1-DPE as the donor substrate for 1DCN*.

For DPME radical cation, in particular, measurements of the growth kinetics of Ph2CH• at varying submillimolar pyridine concentrations (0−0.31 mM) led to an estimate of the rate constant at 9.0 × 109 M−1 s−1 for the bimolecular quenching of the radical cation by pyridine. In these measurements, a progressive decrease in the absorbance due to Ph2CH• was also noticed with increasing [Pyr].

4. DISCUSSION One of the objectives of this work was to establish or rule out C−C bond cleavage as a significant mode of fragmentation of TPE radical cation produced under photoinitiated electrontransfer sensitization by cyanoaromatics. Our detailed transient spectral and kinetic data generated for this system combined with the findings from two-laser experiments provided both qualitative and quantitative evidence in support of the occurrence of extensive C−C bond cleavage. At the same time, the data also suggest that this fragmentation mode alone, leading to Ph2CH• and Ph2CH+, cannot account for all of the observed transient absorbance observed at 325−330 nm. This absorbance is assignable to either or both of the spectrally similar radicals, Ph2CH• and Ph2C•−CHPh2, the latter being the product of deprotonation. Our detailed kinetic analysis of the absorbance data at 325−330 nm relative to that at the absorption maximum of diphenylmethyl carbocation (440 nm) led to the conclusion that the deprotonation mode constitutes a minor component (30%) of the fragmentation. These results regarding TPE agree with those reported in ref 35 on DCNsensitized photolysis (steady state) of TPE in acetonitrile− methanol (3:1). However, in light of our finding that C−C cleavage occurs even at subambient temperatures well below 10 °C, it is not clear why the product from C−C cleavage (i.e., methyl diphenylmethyl ether) was not observed34 in the course of 1,4-cyanobenzene-senstized photolysis in acetonitrile− methanol at 10 °C. It is possible that in the presence of the very high concentration of methanol (6.2 M) used in the work of Okamoto et al.34,35 deprotonation competed more favorably with the cleavage mode. The significant enhancement of decay kinetics of DPME and 1,1-DPE radical cations observed by us at much lower [MeOH], i.e., 0.08 M, lends support to this explanation. As discussed in the Results, the radical cations of hydrocarbons (CH+•) are recognized to be very strong acids with negative pKa values (from 0 to −30) in solutions of dimethyl sulfoxide and acetonitrile.57−63 In particular, the radical cations of phenyl-substituted alkanes such as toluene and diphenylmethane have highly negative pKa values in the range from −12 to −25. In light of this property,90 it is not surprising that we observed very fast deprotonation kinetics of radical cations of phenyl-substituted alkanes and alcohols with associated firstorder rate constants higher than 1 × 106 s−1 (at room temperature). For TPE radical cation also, with one-third of the fragmentation apportioned to deprotonation, the rate constant associated with it is ∼1 × 106 s−1. The reported proton loss rate constants for radical cations of methyl-substituted benzenes are in the range from 104 to 107 s−1 in water at room temperature.61,91 In particular, the rate constant for deprotonation of toluene radical cation in aqueous solution has been measured to be 1.0 × 107 s−1 at 298 K.91 Our measured values of room-temperature deprotonation rate constants, ∼106 s−1, for phenyl alkane radical cations in the less nucleophilic (basic) solvent acetonitrile92 are very reasonable. 11163

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The steady-state photolysis work34,35 on 1,4-dicyanobenzenephotosensitized C−C bond-cleavage reaction of TPE in acetonitrile:methanol mixture (3:1) gave an estimate of the activation energy of the rate-determining step at 7.2 kcal/mol (i.e., 30.1 kJ/mol). The activation energy measured with DCN as photosensitizer was slightly lower (28.0 kJ/mol).35 It was suggested35 that these energies represented a minimum value, close to the dissociation energy of the central C−C bond in TPE radical cation. The Arrhenius activation energy (19.4 kJ/ mol) that we measured for fragmentation of the TPE radical cation in acetonitrile is considerably lower. Obviously, a significant difference in the relative contributions of the two modes of fragmentation in the different solvents used in the two studies could be responsible for the observed lack of agreement on activation energy.93 Notably, the activation energies measured for deprotonation of the radical cations of DPME, 1,1-DPE, and DPML are smaller than the activation energy of fragmentation of the TPE radical cation (Table 1). This suggests that the extent of increase in the rate of C−C bond cleavage in the TPE radical cation due to an increase in temperature would be more pronounced than that in its deprotonation rate. The C−C bond cleavage and deprotonation (i.e., C−H cleavage) in the radical cations studied in this work result from intramolecular electron transfer from the σ orbitals associated with these bonds to the highest singly occupied molecular orbital (SOMO) of the phenyl groups in the radical cations. For the most efficient overlap between these orbitals that would facilitate the electron transfer, the σ bond should be in a perpendicular orientation with respect to the plane of the phenyl rings.4,94 The σ-bond cleavage kinetics of a radical cation would be relatively slow if its most stable conformation does not correspond to the favorable conformation for the σbond/π-system overlap.4,94 In order to shed light on these structural aspects, we have undertaken95 a computational study of the structures and energetics of the phenyl alkane radical cations. Our preliminary results from DFT/UB3LYP/6-31g-full optimization calculations (solvent: acetonitrile) using Gaussian09 show that in the most stable conformation of the TPE radical cation the C−C bond in the β position with respect to a phenyl moiety is nearly perpendicular to the plane of the phenyl ring, while in the most favorable case, the β-C−H bond is off from such orientation by nearly 30°. This can explain, at least partially, why C−C bond cleavage is favored over deprotonation in the case of TPE radical cation. DFT calculations on other phenyl alkane systems and using more extensive basis sets are currently in progress. While our data from the quenching studies are not comprehensive enough to explain most of the observations we made, some discussion is in order for the results we obtained on pyridine quenching. As a proton acceptor, pyridine is expected to contribute to the radical cation deprotonation step (eq 18). Ph 2CH 2+• + C5H5N → Ph 2CH• + C5H5NH+

scale to produce an equilibrated mixture of the complex, Ph2CH• and C5H5NH+ (eq 19). Ph 2CH 2+• + C5H5N → Complex ⇌ Ph 2CH• + C5H5NH+

(19)

The intermediacy of a complex formed between pyridine and radical cations of phenylthioacetic acids has, in fact, been implicated4 in the mechanism of electron-transfer-induced decarboxylation of these thioacids. Obviously, the pyridine quenching effects on phenyl alkane radical cation fragmentation behaviors deserve further investigation.

5. CONCLUSIONS Using nanosecond laser flash photolysis, a kinetic study was performed on the fragmentation behaviors of radical cations of several phenyl-substituted alkanes and alcohols. Radical cations were generated in acetonitrile via electron transfer to singlet excited states of 1,4-dicyanonaphthalene (DCN) and 1,2,4,5tetracyanobenzene (TCB). The major findings from this study are as follows. (1) Fragmentation of the radical cation of 1,1,2,2-tetraphenylethane (TPE) in acetonitrile at 24 °C occurs with a first-order rate constant of ∼4 × 106 s−1 via both carbon−carbon bond cleavage and deprotonation. The former mode dominates over the latter by a factor of 2:1 approximately. (2) The radical cations of diphenylmethane (DPME) and 1,1-diphenylethane (1,1-DPE) decay solely by deprotonation with first-order rate constants in the vicinity of 1 × 106 s−1 (at room temperature). (3) The deprotonation kinetics of triphenylmethane (TPME), diphenylmethanol (DPML), and benzyl alcohol (BZA) at room temperature were too fast to be resolved in our study (with rate constants > 1 × 107 s−1). (4) The activation energies (Arrhenius) and activation enthalpies (Eyring) of fragmentation kinetics are in the ranges 12−19 and 10−18 kJ/mol, respectively. The activation entropies (Eyring) range from −60 to −91 J/ (mol·K) (5) The quenching effects of pyridine on the decay behaviors of radical cations of TPE, DPME, and 1,1-DPE suggest the involvement of a radical cation/pyridine complex in the quenching mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

■

(18)

In agreement, our experiments with the DCN/DPME system showed an enhancement in growth kinetics of the radical in the presence of submillimolar pyridine concentrations. However, the involvement of eq 18 should not result in a decrease in the plateau absorbance due to the product radical, Ph2CH•. At the low concentrations used, the quenching effect of pyridine on 1 DCN* was negligible. It is possible that pyridine initially forms a complex with the radical cation which decays on a longer time

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Chevron Products Company, Richmond, California 94801, United States. Notes

The authors declare no competing financial interest. 11164

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(18) Wang, Y.; Haze, O.; Dinnocenzo, J. P.; Farid, S.; Farid, R. S.; Gould, I. R. Bonded Exciplexes. A New Concept in Photochemical Reactions. J. Org. Chem. 2007, 72, 6970−6981. (19) Davis, H. F.; Chattopadhyay, S. K.; Das, P. K. Photophysical Behavior of Exciplexes of 1,4-Dicyanonaphthalene with Methyl- and Methoxy-Substituted Benzenes. J. Phys. Chem. 1984, 88, 2798−2603. (20) Kumar, C. V.; Chattopadhyay, S. K.; Das, P. K. Geminate Reverse Electron Transfer in Photogenerated Ion-Pair. Mechanism of 1,4-Dicyanonaphthalene Sensitized Ylide Formation from Stilbene Oxides. J. Chem. Soc., Chem. Commun. 1984, 1107−1109. (21) Davis, H. F.; Lohray, B. B.; Gopidas, K. R.; Kumar, C. V.; Das, P. K.; George, M. V. Photoinduced Electron Transfer Reactions of 2(3H)-Furanones and Bis(benzofuranones). Spectral and Kinetic Behavior of Radicals and Radical Cations. J. Org. Chem. 1985, 50, 3685−3692. (22) Reichel, L. W.; Griffin, G. W.; Muller, A. J.; Das, P. K.; Ege, S. N. Photoinduced Electron Transfer C-C Bond Cleavage Reactions. Oxidations and Isomerizations. Can. J. Chem. 1984, 62, 424−436. (23) Das, P. K.; Muller, A. J.; Griffin, G. W. Photoinduced Electron Transfer Processes Involving Substituted Stilbene Oxides. J. Org. Chem. 1984, 49, 1977−1985. (24) Davis, H. F.; Das, P. K.; Reichel, L. W.; Griffin, G. W. Electron Transfer Sensitized C-C Bond Cleavage. Facile Homolytic Fission via Back Electron Transfer in Photogenerated Ion-Pairs. J. Am. Chem. Soc. 1984, 106, 6968−6973. (25) Das, P. K.; Muller, A. J.; Griffin, G. W.; Gould, I. R.; Tung, C. H.; Turro, N. J. Photosensitization via Charge Transfer or Reversible Electron Transfer. Oxirane Isomerization and Sulfur Dioxide Extrusion. Photochem. Photobiol. 1984, 39, 281−285. (26) Davis, H. F.; Das, P. K.; Griffin, G. W.; Timpa, J. D. Mechanistic Aspects of 1,4-Dicyanonaphthalene Singlet Sensitized Phototransformation of Aryl Glycopyranosides. J. Org. Chem. 1983, 48, 5256−5259. (27) Timpa, J. D.; Legendre, M. G.; Griffin, G. W.; Das, P. K. Photoinduced Electron Transfer Reactions of Aryl Glycosides. Carbohydr. Res. 1983, 117, 69−80. (28) Bhattacharyya, K.; Das, P. K. Photosensitized Ring Opening in Phenyl Oxiranes. Res. Chem. Intermed. 1999, 25, 645−665. (29) Cyr, D. R.; Shrestha, S.; Das, P. K. Photochemical Ring Opening in 2,3-Diphenyl Aziridines. Transient-Spectral and Kinetic Behavior of Azomethine Ylides and Related Photointermediates. J. Phys. Chem. A 2013, 117, 12332−12349. (30) Baciocchi, E.; Bietti, M.; Manduchi, L.; Steenken, S. Oxygen versus Carbon Acidity in the Side-Chain Fragmentation 2-, 3-, and 4Arylalkanol Radical Cations in Aqueous Solution: The Influence of the Distance between the OH Group and the Aromatic Ring. J. Am. Chem. Soc. 1999, 121, 6624−6629. (31) Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Rates of C-S Bond Cleavage in tert-Alkyl Phenyl Sulfide Radical Cations. Org. Lett. 2006, 8, 641−644. (32) Baciocchi, E.; Del Giacco, T.; Giombolini, P.; Lanzalunga, O. Aryl Sulfoxide Radical Cations. Generation, Spectral Properties and Theoretical Calculations. Tetrahedron 2006, 81, 6566−6573. (33) Baciocchi, E.; Del Giacco, T.; Elisei, F.; Gerini, M. F.; Lapi, A.; Liberali, P.; Uzzoli, B. Steady-State and Laser Flash Photolysis Study of the Carbon-Carbon Bond Fragmentation Reactions of 2-Aryl Sulfanyl Alcohol Radical Cations. J. Org. Chem. 2004, 69, 8323−8330. (34) Okamoto, A.; Arnold, D. R. Radical Ions in Photochemistry. 16. The Photosensitized (Electron Transfer) C-C Bond Cleavage Reaction of Radical Cations. Can. J. Chem. 1985, 63, 2340−2342. (35) Okamoto, A.; Snow, M. S.; Arnold, D. R. Photosensitized (Electron Transfer) Carbon-Carbon Bond Cleavage of Radical Cations. The Diphenylmethyl System. Tetrahedron 1986, 42, 6175− 6187. (36) Albini, A.; Arnold, D. R. Radical Ions in Photochemistry. 6. The Photosensitized (Electron Transfer) Ring Opening of Aryloxiranes. Can. J. Chem. 1978, 56, 2985−2993. (37) Arnold, D. R.; Maroulis, A. Radical Ions in Photochemistry. 3. The Photosensitized (Electron Transfer) Cleavage of β-Phenethyl Ethers. J. Am. Chem. Soc. 1976, 98, 5931−5937.

ACKNOWLEDGMENTS Generous financial support for this work was obtained from the Harvard L. and Judith D. Tomlinson of Duncan Endowed Lectureship in Physical Sciences and the Frontiers in Chemistry Endowed Lectureship in Chemistry at Cameron University. The experimental work presented in this report utilized laser flash photolysis equipment and other instrumental facilities at the Notre Dame Radiation Laboratory (NDRL). P.D. is thankful to Drs. Gordon Hug, Ian Carmichael (Director, NDRL), and G. N. R. Tripathi for help and hospitality during part of the work.

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(90) The very negative pKa values in solution also render the kinetics of the deprotonation step essentially irreversible. (91) Sehested, K.; Holeman, J. Reactions of the Radical Cations of Methylated Benzene Derivatives in Aqueous Solution. J. Phys. Chem. 1978, 82, 651−653. (92) Minegishi, S.; Kobayashi, S.; Mayr, H. Solvent Nucleophilicity. J. Am. Chem. Soc. 2004, 126, 5174−5181. (93) It is possible that the mechanism of radical ion fragmentation is different in methanol-rich solvents and may involve the intermediacy of an adduct of the radical cation with the alcohol. (94) Baciocchi, E. Side-Chain Reactivity of Aromatic Radical Cations. Acta Chem. Scand. 1990, 44, 645−652. (95) The authors are thankful to the referee for suggesting the computational work. (96) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision C.01; Gaussian, Inc.: Wallingford, CT, 2010.

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