J . Phys. Chem. 1984,88, 3516-3520
3516
we have examined the aromatic character in the excited states of benzene and its derivatives using the bond order approach proposed by Jug40 which is applicable to a wide variety of molecules. Aromaticity indices are the bond orders of the weakest ring bonds, marked with an asterisk in the figures. There is a marked decrease of aromaticity in excited states of benzenoid systems. Furthermore, the presence of a substituent reduces the aromatic character in all the electronic states, though the influence of substituents is small in the ground state and IBzustate. The excited states of lBZuorigin are seen to be moderately aromatic with the indices ranging from 1.52 to 1.58. The lBlu state in benzene is moderatley aromatic as that of the 'Bzu state while the triplet state (3Bl,) is weakly aromatic, the index being 1.40. However, in phenol, aniline, and toluene, the excited states of both 'Blu and 3Bluorigin have indices in the range 1.27-1.34, which indicates that these states are essentially nonaromatic. The indices of nitrobenzene are close to those of benzene. The notable differences that occur in the aromatic properties of the various electronic excited states may be explained from the structures of the excited states. The excitation to the states of 'BZuorigin results in weakening of the ring bonds as manifested by the elongation of the bonds in the entire ring, thus making the state less aromatic than the ground state, i.e. moderately aromatic. The situation is similar in the 'Blu state of benzene, but in the 3Blustate, benzene undergoes both bond stretching and contraction to a larger extent and consequently the 3Blustate is only weakly aromatic. However, in the states of lBlu and 3Bluparentage of the derivatives, the ring current formation is practically interrupted owing to the fact that the ring bonds around the ipso carbon attain essentially u character on excitation. The influence on aromaticity by other structural factors such as angular and out-of-planar deformations seems to be negligible as indicated by the negligible differences in the numerical values of the aromaticity indices in the 'Bzu state of benzene and its derivatives. The nonaromatic nature is slightly more pronounced in the triplet than in the singlet B,, state for all the molecules, as expected from the magnitudes of the ipsmrtho bond lengths. Clearly, the triplet states are more reactive with respect to addition reaction, since stability arising (39) J. Aihara, J . Am. Chem. Soc., 98, 6840 (1976). (40) K. Jug, J . Org. Chem., 48, 1344 (1983).
from the aromaticity concept is virtually absent. The pronounced "diradicaloid" structure of the triplet state is also indicative of its reactive n a t ~ r e . ~The ' switch over from a weakly aromatic triplet in benzene to the nonaromatic triplet in the derivatives may very well explain the rather sharp drop in the lifetime of triplet ~ calculations states in the vapor phase on s ~ b s t i t u t i o n . ~Our predict such a trend in the lifetime of the 'A, state also. Conclusion. The SINDO1 method reproduces the different kinds of experimental data on the excited states of benzene and its derivatives successfully. For the IBz state, it predicts quinoidal character in phenol, aniline, and toluene and antiquinoidal character in fluoro- and nitrobenzene, in agreement with experimental result^.^-^ Also, the out-of-plahe deformations of the amino hydrogens in ground and excited IBz states agree well with experimental s t ~ d i e s . ~The , ~ ,A, ~ ~singlet and triplet states have pronounced quinoidal character. A t the site of substitution the ring is fairly nonplanar in the singlet, less so in the triplet state. Except in the toluene triplet, the substituent atoms suffer substantial out-of-plane deformation. The role of intramolecular charge transfer in the studied excited a-T* states appears negligible, whereas experimental dataz1 seemed to support such a charge transfer in the 'Al state of nitrobenzene. The general pattern of a-T mixing and substituent participation is such that nitrobenzene should be no exception. Aromaticity is only moderate or lost in the excited states, particularly upon substitution. Another interesting feature is, the repulsive character of the T, state of nitrobenzene. It is caused by an excitation to an orbital which is antibonding for the C N bond. It explains naturally the lack of phosphorescence' in this molecule.
Acknowledgment. The compptations were performed with the CYBER76 at Universitat Hannover. Partial support of this work by Deutsche Forschungsgemeinschaft is gratefully acknowledged. Registry No. Benzene, 7 1-43-2; fluorobenzene, 462-06-6; phenol, 108-95-2; benzenamine, 62-53-3; methylbenzene, 108-88-3;nitrobenzene, 98-95-3. (41) J. Michl in 'Semiempirical Methods of Electronic Structure Calculation", Part B, G. A. Segal, Ed., Plenum Press, New York, 1977. (42) R. Bonneau, M. E. Sime, and D. Phillips, J . Photochem., 8, 239 (1978).
Picosecond Dynamics of the Photodissociation of Triarylmethanes Lewis E. Manring and Kevin S. Peters* Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 (Received: January 19, 1984)
The photochemistry of triarylmethanes (TAM) are studied by picosecond absorption spectroscopy. Two series of TAMS are investigated, (Ia-f) triphenylmethane (Ph3CX, X = H, OH, OCH,, CI, Br, and SCH3 for Ia-f, respectively) and (IIa-c) malachite green ((Me2NPh)2PhCX,X = H, OH, and OCH3 for IIa-c, respectively). The TAMS are studied in polar and nonpolar solvents. The amount of heterolytic(TAM'X-) and homolytic (TAM-X.) cleavage after 266-nm excitation is determined for the various conditions studied. As expected, the amount of heterolytic bond cleavage increases in polar solvents. Furthermore, the amount of cleavage is more dependent on the electron affinity (EA(X.)) than on the bond dissociation energy (BDE(TAM-X)) of the leaving group. The importance of EA(X-) is noted even in nonpolar solvents where only homolytic cleavage is observed, suggesting the homolytic cleavage occurs via initial TAM-X bond heterolysis to give (TAM'X-) with subsequent electron transfer to yield (TAM-X.).
The photodissociation of triarylmethanes (TAM) has been extensively studied.'qz It is known that, depending on conditions, excited-state triarylmethanes can undergo either homolytic cleavage to give radicals',2 (eq 1) or heterolytic cleavage to give
a triarylcation and the corresponding anion (eq 2)., Absorption of light by the triarylmethanes may be viewed as being localized on a single aromatic ring. Therefore, the absorption spectrum of triphenylmethane (Ia) is almost identical with that
(1) Lewis, G . N.; Lipkin, D.; Nagel, T. T. J . Am. Chem. Soc. 1944, 66, 1579. (2) Porter, G.; Strachan, E. Trans. Faraday SOC.1958, 54, 1595.
1151.
0022-3654/84/2088-35 16$01.50/0
(3) Harris, L.; Kaminsky, J.; Simard, R. G . J . Am. Chem. SOC.1935,57,
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3517
Photodissociation of Triarylmethanes
b .CI
cu-
I .
500
,
I
I
600
700
'
Figure 1. (a, -)
Spectrum generated by 266-nm excitation of Ia in CH3CN. (b, ---) Spectrum generated by 266-nm excitation of Ib or IC in CH3CN.
I,
x
kcal/mol more energy available for bond cleavage; furthermore, direct formation of the singlet ground-state products (TAM+, X-) is spin forbidden from 3TAM*. For triphenylmethyl compounds, it is not known whether cleavage (homolytic or heterolytic) occurs from the singlet or triplet excited states. We report here a picosecond laser spectroscopic study of two series of TAMS (I and 11), in both polar and nonpolar solvents. By observation of the absorption spectra of products formed within 50 ps after laser excitation the relative importance of the various pathways of Scheme I are deduced.
=
a,
H
b,
OH
11,
x
=
a,
H
c, OCH3
b,
OH
d,
C1
c , OCH3
e,
Br
f , SCH3
-
of toluene: and the lowest excited singlet (SI*) energies of both are 104 kcal/mol. The triplet (TI*) energies of Ia and toluene are also the same, -81 kcal/m01.~ Similarly, the lowest excited state of leucomalachite green (IIa) has the energy localized on one of the dimethylaniline rings5 and has the same singlet and triplet energies as N,N-dimethyl-p-toluidine, 87 and 74 kcal/mol, respectively? The excited singlet triarylmethane ('TAM*) formed by photolysis can produce radicals or ions by a number of possible paths as depicted in Scheme I. In Scheme I, kHoM and kHET are the Scheme I 'TAM*
k m
+ XTAM+ + X-
TAM-
k m
(3) (4)
kiu
'TAM*
k'kiOM
(5)
+
'TAM* TAM- X* (6) rates of homolytic and heterolytic cleavage, respectively, of the C-X bond from 'TAM*, kiscis the rate of intersystem crossing of 'TAM* to triplet TAM (3TAM*), and KrHOMis the rate of homolytic cleaage of the C-X bond from 3TAM*. For malachite green leuocyanide (11, X = CN), it is believed that heterolytic cleavage occurs predominantly from the 'TAM* rather than 3TAM*.3-7*8 This is to be expected since the singlet state has 13
-
(4) Watson, F. H., Jr.; El-Bayoumi, M.A. J. Chem. Phys. 1971,555464.
( 5 ) Geiger, M.W.; Turro, N. J.; Waddel, W. H. Photochem. Photobiol. 1977, 25, 15. (6) Based on absorption and emission spectra from ref 6. (7) Hen, M. L. J . Am. Chem. SOC.1975, 97, 6777. (8) Brown, R. G.; Cosa, J. Chem. Phys. Lett. 1977, 45, 429.
Experimental Section Chemicals. Acetonitrile (Mallinckrodt) was distilled from P205. Cyclohexane (Kodak ACS, Spectro) was used as received. Methanol (Mallinckrodt) and tert-butyl alcohol (Aldrich) were distilled from K2C03. Triphenylmethane (Ia), triphenylcarbinol (Ib), triphenylmethyl bromide (Ie), triphenylmethyl mercaptan (1 f), malachite green leucohydride (IIa), malachite green hydroxide (IIb), and malachite green chloride (IIc) (Aldrich) were either recrystallized (ligroin Ie, f, IIb; 95% ethanol IA), sublimed (Ia), or wed as received (Ib, IIc). Triphenylmethyl chloride (Id) (Fisher) was recrystallized from ligroin. Triphenylmethyl methyl ether (IC) was prepared by adding 2.5 g of K2C03to 10 g of triphenylmethyl chloride in 50 mL of dry methanol. The resulting IC was recrystallized from methanol (mp 82-82.5 OC, lite9mp 82.6-82.9 "C). Solutions of malachite green methyl ether (IIc) in C H 3 0 H , 90% C H 3 C N / 1 0 % CH'OH, and 90% (CH3)3COH/10% C H 3 0 H were prepared by adding a stock solution of 5.0 X lo-' M malachite green chloride with 0.05 M NaOMe to 9 parts CH'OH, CH'CN, or (CH3)3COH. The malachite green chloride reacts with methoxide to form the colorless methyl ether. It was shown that the excess methoxide had no effect on the kinetics of either malachite green cation (11+) appearance or SI* disappearance (vide infra) by monitoring solutions at varying methoxide concentrations. The concentration of triarylmethane in all solutions was 5 X 10-4-1 X M. The laser apparatus has been described in detail elsewhere.1° All solutions were degassed by passing N, through them (2 min) prior to laser photolysis and were continuously stirred during photolysis. Results (C6H5)3CXRe~ult~ in CH3CN. Laser excitation (266 nm, 0.1 mJ) of triphenylmethane(Ia) in CH3CN produced a species which (9) Norris, J. F.; Young, R. C . J . Am. Chem. SOC.1930, 52, 753. (10) Simon, J. D.; Peters, K. S. J. Am. Chem. SOC.1983, 1058 4875.
3518
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
Manring and Peters
0.404
I 450
,
I 550
I
I
650
Figure 2. Spectrum generated by 266-nm excitation of Id in CH3CN. 500
660
760
Figure 4. Spectrum generated by 266-nm excitation of 20% toluene in C6H12.
500
600
700
A"m
Figure 3. Spectrum generated by 266-nm excitation of Id in C6HI2.
shows a broad absorbance from 550 to -700 nm with a A, at -640 nm (Figure la). The transient formed within the 25-ps laser pulse and decayed with a lifetime of 1.3 f 0.3 ns. Similar laser excitation of triphenylcarbinol (Ib) and triphenylmethyl methyl ether (IC) also gave species with broad absorbances from 500 to 650 nm with=,.A,, of 610-620 nm (Figure lb) both of which decayed with lifetimes of 10 f 2 ns. Laser excitation of triphenylmethyl chloride (Id) in CH3CN produced a species within the laser pulse with an absorbance A,, of 420-435 nm (Figure 2). The species thus formed showed no decay in the first 50 ns. We attribute the observed spectrum to triphenylmethylcarbocation, (C6H5)3C+,on the basis of the similarity between our observed spectra and known spectra for (C6H5)$+ with peaks a t 415 and 419 nm." Similar laser excitation of triphenylmethyl bromide (Ie) in CH3CN gave (C6H5)3C+within the laser flash which also showed no decay in the first 50 ns after formation. Upon laser excitation of triphenylmethyl mercaptan (If) in CH3CN, no absorbances appeared between 410 and 700 nm up to 50 ns after the laser flash. (C6H5)3CXResults in C6HIZ.Laser excitation of Ia in cyclohexane (C6Hl2)produced a transient with a broad absorbance from 550 to 700 nm (A, 640 nm) and a lifetime of 1.8 f 0.2 ns. Both I b and IC also gave the same transient in C6H12 as in CH3CN (broad absorbance from 500 to 650 nm), both with lifetimes of 10 f 2 ns. Similar photolysis of Id gave a species with a A,, at 510 nm (Figure 3) formed within the laser pulse which did not exhibit any sign of decaying even 50 ns after laser excitation. We attribute this spectrum to triphenylmethyl radical (C6HJ3C., on the basis of its similarity to the known (C6H5)3C-spectrum.l2 In C6Hl2, photolysis of neither Ie nor If produced an absorbance from 410 to 700 nm.
-
(11) Jones, R. L.; Dorfman, L. H. J . Am. Chem. SOC.1974, 96, 5715. (12) Ting, L. C.; Weissman, S. Ii. J . Chem. Phys. 1954, 22, 21.
I
L A
i
I
500
600
700
A"rn
Figure 5. (a) Spectrum generated by 266-nm excitation of IIa in CH,CN. (b) Spectrum generated by 266-nm excitation of DMT in CH,CN.
Photolysis of toluene at concentrations similar to those studied with Ia-f did not produce an absorbance between 410 and 700 nm in either CH3CN or C6H12(although all of the laser light was absorbed by the solution). However, 266-nm excitation of a 20% toluene solution in C6H12 produced a species within 50 ps, with a broad absorbance centered at 550 nm (Figure 4). This spectrum is very similar to that seen upon two-photon excitation of neat toluene (Arnm = 560 nm) and attributed to a toluene e ~ c i r n e r . ' ~ J ~ (MezNPh)2PhCXResults in CH3CN. Laser excitation of malachite green leucohydride (IIa) in CH3CN produced a species of 610 nm within 50 ps after laser exwith an absorption A, citation (Figure 5a) which decayed with a lifetime of 2.1 f 0.4 ns. This spectrum is assigned to a S,* S1* transition on the basis of its similarity to the species observed after photolysis of N,N-dimethyl-p-toluidine (DMT) (Figure 5b). Compounds similar to DMT such as N,N-dimethylaniline are known to have singlet lifetimes of 2.4-3.8 ns8,15depending on solvent. Furthermore, DMT should not form excimers within 50 ps at the concentrations used M) to obtain the spectrum in Figure 5b. Photolysis of malachite green hydroxide (IIb) in CH3CN showed a spectrum with an absorption A, of 610 nm, 30 ps after the laser flash (Figure 6a). The somewhat broad spectrum
-
(13) Hamanoue, K.; Hidaka, T.; Nakayama, T.; Teranishi, H. Chem. Phys. Lett. 1981, 82, 55. (14) Masuhara, H.; Miyasaka, H.; Ikeda, N.; Mataga, N. Chem. Phys. Lett. 1981, 82, 59. (15) Berlrnan, I. B. "Handbook of Fluorescence Spectra of Aromatic Molecules", 2nd ed.; Academic Press: New York, 1971.
. . Photodissociation of l'riarylmethanes
v 500
I
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3519
I.
I
700
600
500
?m
I
I 700
I
600
Anm
Figure 6. Spectrum generated by 266-nm excitation of Ilb: (a, -) 50 ps after laser pulse; (b, --) 2 ns after laser pulse.
Figure 8. Spectrum generated by 266-nm excitation of IIC in 90% tert-butyl alcohol/lO% CH30H at (a) 50 ps and (b) 9 ns. TABLE I: Bond Dissociation Energies for PhCHIX and Electron Affinities for X BDE(PhCH2-X),' kcal/mol EA(X.)," eV X H 85 0.80
OH CH,O
c1 Br
CH,S
78 68-70' 69 55 54
1.83'
1.57' 3.62 3.36 1.88C
aValues taken from ref 18 unless otherwise noted. *Estimated from observed similarities in bond energies for C-OCH3 and C-C1 bonds. CReference19.
?um
Figure 7. Spectrum generated by 266-nm excitation of IIa in C6HI2.
initially observed sharpens at later times and by 2 ns (Figure 6b) takes on the charactistics shape of the malachite green cation (11'). (Me2NPh)2PhCXResults in C6Hl,. Excitation of IIa in C6H12 gives a transient which shows a broad absorption from 500 to 700 nm with a A, of 610 nm (Figure 7). The absorption at 610 nm decays with a lifetime of 6.3 f 1.O ns. Excitation of IIb gives is red shifted to 633 nm and a transient similar to IIa; the A, the lifetime is 1.6 X 0.2 ns. (Me2NPh)2PhCXResults in Protic and Mixed Protic-Aprotic Solvent Systems. Photolysis of IIa in 90% CH3CN/10% C H 3 0 H (by volume) produced a transient with A, at 600 nm which decayed with a lifetime of 2.1 f 0.3 ns. This spectrum is slightly blue shifted relative to that obtained in pure CH3CN but otherwise has a very similar shape. Excitation of IIb in 90% CH3CN/10% CH30H gave results similar to those of IIb in CH3CN, except the broader spectrum initially observed sharpened to the 11' spectrum faster, within 300 ps. Excitation of malachite green methyl ether (IIc) in 90% CH3CN/10% C H 3 0 H also showed an initial broad absorption at 50 ps (Amx = 610 nm, A = 0.03) which within 2 ns grows into the distinct II+ spectrum (A, = 610 nm, A = 0.25). Assuming the absorbance due to 11' at 610 nm is much greater than that due to the S,* S1* transition, the lifetime for II+ appearance is 1 f 0.1 ns. The methyl ether (IIc) was also studied in 90% (CH3)3COH/10% C H 3 0 H and in pure CH30H. Excitation of IIc in 90% (CH3)3COH/10% C H 3 0 H also gave an initial broad absorbance at 50 ps (A, = 610 nm, A = 0.03), which by 9 ns takes on the appearance of the 11' spectrum (A, = 610 nm, A = 0.22) (Figure 8). Again, assuming the absorbance due to 11' at 610 nm is much greater than that due to the S,* S1* transition, the lifetime for 11' appearance is 2.1 f 0.3 ns. Since the broad absorption due to the S,* SI*transition extends to 680 nm, where 11+absorbs very little, the rate of S1* disappearance a t 680 nm can be determined. The lifetime for SI*decay was thus determined to be 2.2 f 0.3 ns, similar to the lifetime for 11' appearance, 2.1 f 0.3 ns. The agreement between these values indicates that, as expected, SI*is a precursor to 11'. Excitation
-
-
-
of IIc in C H 3 0 H gave a much faster lifetime for II+ appearance. Even at 50 ps (A, = 610 nm, A = 0.09) the spectrum has considerable 11' character, which by 1 ns is due to only 11' (A, = 610 nm, A = 0.560). The lifetime of appearance of 11' from IIc in C H 3 0 H is 0.4 f 0.05 ns. Discussion The results with the malachite green type compounds, IIa-c, are most consistent with initial formation of the first excited singlet at 610 state, Sl*(II), which has a visible absorbance with A,, nm (Figures 5a, 6a, and 7) based on the similarity between this S,* SI* transition for IIa and DMT (Figure 5, a and b). Subsequent to SI*formation, the excited species can either decay back to ground-state I1 (kd) or heterolytically cleave (kHET,Scheme I) to yield 11' and X-. The results indicate that kHETis dependent on both solvent and leaving group. In cyclohexane, only SI* is observed and no 11' is formed from 1Ia-c. This result is consistent with the anticipated instability of the ion pair 11'- X- in the nonpolar environment. The importance of the stability of the resulting ions is further demonstrated by the fact that Sl*(IIc) ionizes with kHET 2.5 X 10' SS1 - kd in CH30H but Slows to 1.o X 10' s-l - kd in 90% CH3CN/10%CH30H (where kd is the sum, all processes other than kHET which lead to SI* decay). The CH30- formed is stabilized more by hydrogen bonding in C H 3 0 H than in 90% CH3CN/ 10% CH30H. The effect of the leaving group X is best discussed in relation to the data in Table I. Table I lists the bond energies for C6H5CH,-X bondsL6and the electron affinities of X.. Cleavage of &*(Ha) to 11' and H- was not observed in any of the solvents studied, We interpret this result as being a manifestation of the strong C-H bond (BDE(PhCH,-H) = 85 kcal/mol; the strongest PhCH2-X bond in the compounds studied), and the low electron affinity of the hydrogen atom (EA(CH.))
-
--
-
(16) The bond energies for I-X and 11-X are not known; however, it is assumed that the C,H5CH2-X bond energies will accurately parallel to I-X and 11-X bond energies. (17) Cremers, D. A,; Cremers, T. L. Chern. Phys. Lett. 1983, 94, 102. (18) "Handbook of Chemistry and Physics"; CRC Press: Cleveland, OH, 1914. (19) Janousek, B. K.; Brauman, J. I. "Gas Phase Ion Chemistry"; Bowers, M. T., Ed.; Academic Press: New York, 1979.
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
3520
T2
X-
X-
V
SI
Figure 9. Energy surface diagram for heterolysis of TAM from SI*.E,' = small barrier for loss of good leaving groups in polar solvents. Ea2= large barrier for cleavage in nonpolar solvents or with poor leaving
groups. = 0.80 eV). The results also show that IIb cleaves much faster (reaction complete in -300 ps) than IIc (reaction 90% complete in -2 ns) in 90% CH3CN/10% C H 3 0 H . If it is assumed that HO- and CH30- are similarly solvated and that the PhCH,OH and PhCH2-OCH3bond strengths are similar, the increased rate of S,*(IIb) cleavage relative to SI*(IIc) can be attributed to the higher electron affinity of HO. (1.83 eV) as compared to CH30. (1.57 eV). The conclusion that the electron affinitity of the leaving group is a dominant factor in the rates of cleavage of IIb,c is strengthed by the fact that the PhCH2-OCH3 bond (68-70 kcal/mol) is actually weaker than the PhCH2-OH (78 kcal/mol); however, since the relative solvation energies of HO- and Ch30in 90% CH3CN/10% C H 3 0 H are not known, it is possible that ion solvation is a dominate effect in this case as it is in cyclohexane (see above). The observed dependence of cleavage on product stability is consistent with the excited-state potential energy surface diagram proposed by Geiger et al.,5 shown in Figure 9. The crossing of the higher lying surface with the a-a* surface is dependent on the stability of the products 11' and X-. The activation barrier to cleavage, E,, directly reflects the stability of the products. Recently, it has been reported that malachite green leucocyanide (IICN) undergoes very fast cleavage in ethanol or glycerol to give II+ initially in a pyramidal conformation which then via a solvent restricted motion forms the more stable propeller c~nformation.'~ The proposed intermediacy of a pyramidal cation was based on the observation of a fast rise in absorbance at 610 nm ( t l p < 50 ps) followed by a slower rise (120 ps to complete formation of 11') after 266-nm excitation of IICN in ethanol. This result contrasted with that in glycerol where the very fast appearance at 610 nm (tl,2 < 50 ps) is followed by a much slower rise (2 ns to complete formation of II+). It was suggested that the immediately observed absorbance at 610 nm is the result of some overlap between the aryl group a-electron systems immediately after photoionization with II+ still in the pyramidal configuration
Manring and Peters characteristic of IICN. The increased viscous drag of the glycerol (relative to ethanol) was suggested to cause slower attainment of the propeller conformation; hence, the slower rise in the 610-nm absorption. On the basis of our results, it seems likely that the initially formed absorbance at 610 nm is due to S1*(IICN) and the subsequent rise in absorption is due to the cleavage of S,*(IICN) to 11' and CN-. The effect of viscosity on II+ appearance could just as easily be attributed to a hydrodynamic effect on the crossing of the barrier associated with cleavage. We attribute the broad absorbance between 550 and 700 nm from 266-nm exciation of Ia, Ib, or IC in CH3CN and C6H12 to the singlet state of the triarylmethanes, Sl*(I). This assignment is based on the immediate appearance of the transient after excitation coupled with the reported singlet state lifetime for la of 21.7 nse4 Excitation of toluene does not give a similar transient. = 550) at high The observation of a broad absorbance (A, concentrations of toluene (due to exciplex formation)14 suggests that the broad absorption from Ia-c is due to intramolecular overlap between the phenyl rings. The reason for the discrepancy between the literature value of 21.7 ns and our value of -2 ns for the singlet lifetime of Ia is not certain. When Id or Ie is photolyzed in CH,CN, I+ appears within the 25-ps laser pulse, and S1* is not observed. It is apparent from the data in Table I that this result is due primarily to the large electron affinities of C1. and Br.which stabilize the ion pair I'X-. The increased stability of I+X- correspondingly decreases E, (Figure 9) and results in ionization, within the time resolution of our apparatus, 25 ps. In contrast, If, which should have the lowest bond energy of the compounds studied (la-f), does not form I+. The lack of I+ from If might be anticipated on the basis of the low electron affinity of CH3S- (EA(CH3S.) = 1.88 eV). However, an increase in kist (Scheme I) could also account for the absence of I+. It is expected that sulfur will cause a heavy atom promoted increase in kkc,and we do not observe any &*(If) from excitation of If. When Id is photolyzed in C6H12,I. appears within the 25-ps laser pulse and SI*(Id) is not observed. The data in Table I indicate that IC, Id have similar bond energies, and yet only Id gives homolytic cleavage in C6H12. If we estimate that kHoMfor Id is 2 4 X 1Olo s-l (no S1*(Id) is observed within the 25-ps resolution of our apparatus) and that kHoMfor IC is 5 1 X lo7 s-l (based on the observed lifetime of S1*(Ic) being 10 ns and the assumption that we would have seen 10% conversion of IC to I-), then we can estimate that AE, for homolysis of Id relative to IC is 1 4 . 5 kcal/mol. Unfortunately, bond dissociation energies are not known for IC and Id; however, they are usually closer than 4.5 kcal/mol for C-OCH, vs. C-C1 bonds. It is possible initial cleavage of Id in C6H12 is heterolytic with very fast back electron transfer to give the observed radicals (eq 7). Subsequent to [Ph3C-C1]*
k m
C6Hi2
[Ph3C+Cl-]
-
Ph3CCl.
(7)
heterolytic cleavage, relaxation to the homolytic surface ensues, a process that formally corresponds to an electron transfer in the ion pair to form a radical pair. Neither Ie nor If shows any &*(I) or I. when photolyzed in C6H12. This again is attributed to heavy atom promotion of k,,,. We anticipate that the heavy atom effect on Ie will be similar in CH3CN and C6H12, and yet I+ is observed in CH3CN and I. is and ) k ~ s c> not observed in C6H12, Le., kist < ~ H E T ( C H ~ C N k ~ o ~ ( C 6 H 1 2This ) . result is not inconsistent with eq 7 since we would anticipate that kHET(C6H12) < kHET(CH3CN). It is also apparent that eq 6 of Scheme I (homolysis of T,*(I)) does not occur with Ie or If since no I. is seen after photolysis.
Acknowledgment. This work is supported by a grant from the National Science Foundation, CHE-8 1175 19. K.S.P. acknowledges support from the Henry and Camille Dreyfus Foundatior, for a teacher-scholar grant and the Alfred P. Sloan Foundation. Registry No. Ia, 519-73-3; Ib, 76-84-6; IC, 596-31-6; Id, 76-83-5; Ie, 596-43-0; If, 62575-83-1; IIa, 129-73-7; IIb, 510-13-4; Ilc, 32315-05-2; CH30,2143-68-2.
