J. Phys. Chem. 1984, 88, 3508-3516
3508
A/B ratio upon water adsorption at -196 O C , followed by heating to room temperature. The overall raction scheme is as follows: on Pt
e c
0 L
C02----CO+O
G .-
on titania
2.0
0
LL L
c02
a
+ 2e-
+
02-
-
02-
(2)
co32-
(3)
2Ti3+ + 2Ti4+
I
f
1.0 u0 .-+
e
m \ U I
O1
10
I
I
I
I
40 50 Concentration of Ti+3 (%)
20
30
Figure 6. A relation between the concentration of Ti3+and the A/B intensity ratio of the Ti LMM Auger fine structure.
On Ar-ion-bombarded titania alone, where the intensity ratio was 1.75, no conversion of adsorbed C02into CO3-was observed and no A/B intensity ratio change was obtained. This result indicates that transformation of COz to CO?- species occurs only in the presence of Pt. In addition, no change was observed in the
(1)
+ 2e-
(4) The adsorbed carbon dioxide molecule dissociates on Pt to form a CO molecule and an oxygen atom. The oxygen species pick up electron supplied by the oxidation reaction of Ti3+to Ti4+. The 02-species reacts with adsorbed COz to deposit on the titania support as a C032- species. It is worth mentioning that the dissociation of C 0 2can be detected only in the presence of the titania support since this reaction has never been detected in unsupported Pt metal at low temperatures. CO formation by the water-gas shift reaction between CO, and adsorbed hydrogen does occur but not below room temperature. It is very interesting that COzdissociates and changes into CO and oxygen on TiOz-supported Pt catalysts; however, this reaction is unfortunately not catalytic, because the C032-species are strongly held on the support and can be removed by evacuation only at temperatures as high as 500 O C . If the oxygen species produced by C 0 2 dissociation could be consumed and/or removed from the surface faster than the subsequent reaction with adsorbed COz to make up C032-species, the reaction could then take place catalytically. Registry No. CO2, 124-38-9;Pt, 7440-06-4; TiO,, 13463-67-7.
Structures and Properties of Excited States of Benzene and Some Monosubstituted Benzenes E. J. Padma Malar and Karl Jug* Theoretische Chemie, Universitdt Hannover, 3000 Hannover 1, Federal Republic of Germany (Received: January 18, 1984)
Configuration interaction (CI) calculations using the semiempirical SINDO1 approach were carried out for the structural optimization of low-lying singlet and triplet excited states in benzene, fluorobenzene,phenol, aniline, toluene, and nitrobenzene, The equilibrium geometries, adiabatic excitation energies, charges, dipole moments, and degree of aromaticity are discussed for the various excited states. Planarity vs. nonplanarity of the ring and the substituents are investigated in connection with delocalized (ring) or localized (substituent) excitation. The calculated results compare well with the available experimental data.
Introduction Although considerable attention has been focused on the spectral analyses of benzene and a large number of its derivatives, the structures and the properties of these benzenoid systems in their excited states are not clearly understood.' Structural rearrangement accompanying electronic excitation in benzene was first detected by de Groot and van der Waals in 1963 when they noticed from the magnetic resonance spectrum (EPR) that the lowest triplet state of benzene is distorted from the regular hexagonal structure.2 Since then a few experimental studies3" on the (1) C. J. Seliskar, 0.S. Khalil, and S. P. McGlynn in "Excited States", Vol. 1, E. C. Lim, Ed., Academic Press, New York, 1974. (2) M. S.de Groot and J. H. van der Waals, Mol. Phys., 6, 545 (1 963). (3) H. D. Bist, J. C. D. Brand, and D. R. Williams, J . Mol. Spectrosc.,
24, 413 (1967).
(4) J. Christoffersen,J. M. Hollas, and G. H. Kirby, Mol. Phys., 16, 441
(1969). (5) K.-T. Huang and J. R. Lombardi, J . Chem. Phys., 52, 5613 (1970).
monosubstituted benzenes have revealed that the first excited singlet state (lB2) has appreciable quinoidal character in phenol and aniline. However, in fluorobenzene it is inferred from an examination of dipole moments that the 'B, state has less quinoidal character than in the ground state.5 Furthermore, experimental evidence also indicates that the first excited singlet states in some of the benzene derivatives are n ~ n p l a n a r . ~ @Although ~ these experimental results help in understanding the structures of excited states in benzenoid systems, they are at present not capable of providing complete structural details. Experimental techniques used are subjected to limitations like low resolution of rotational (6) E. D. Lipp and C. J. Seliskar, J . Mol. Spectrosc., 87, 242 (1981). (7) R. A. Coveleskie and C. S. Parmenter, J. Mol. Sperrrosr., 86, 86 (1981) --, \ - -
(8) J. C. D. Brand, D. R. Williams, and T.J. Cook, J . Mol. Spectrosc., 20, 359 (1966). (9) J . Christoffersen, J. M. Hollas, and G. H. Kirby, Mol. Phys., 18, 451 (1970).
0022-365418412088-3508$01.50/0 0 1984 American Chemical Society
Excited States of Benzene and Its Derivatives band structure. Also, one frequently has to invoke model assumptions about the molecules like an unchanged ring framework and the rigid-rotor approximation.l0>” Hence, it is mandatory to resort to theoretical studies to get a better understanding of the excited states of these aromatic systems. In this work we have primarily been concerned with the optimization of the structures of the two low-lying excited singlet states and the lowest triplet state in benzene and its monosubstituted derivatives: fluorobenzene, phenol, aniline, toluene and nitrobenzene. Of the different substituents, F, OH, and N H z possess lone-pair electrons, but F is known to act as a weak electron acceptor while the remaining two are considered to be electron donors; C H 3 on one hand typifies a substituent lacking lone-pair electrons and on the other hand acts as electron donor by hyperconjugative effect, i.e. overlap of c bonds with the T system; the NO2 group is well-known for its remarkably strong electron-accepting nature. We feel that these molecules comprise an appropriate set for examining the excited states of benzene and its monosubstituted derivatives in general. We wish to investigate the change of geometry upon excitation and substitution. Besides the determination of the structures and transition energies, we have also examined the other facets such as degree of aromaticity and dipole moments in the excited states. Rigorous ab initio calculations would be quite expensive for structural optimization of the excited states of benzenoid systems. Nakajima et a1.I2 have examined the excited-state geometries of conjugated hydrocarbons on the basis of a variable bond length S C F formalism on the Pariser-Parr-Pople a-electron level. Their approach permits only bond length distortion and is hence inadequate for predicting deviations from planarity, if any, in the excited states. Moreover, it is not suitable to study the substituted benzenes where deviations in the bond angles are known to occur in the ground and excited state^."-'^,'^ An alternate approach which has already proved successful in studying excited states of small molecules15and large m o l e c ~ l e s ’ having ~ ~ ’ ~ aromatic as well as nonaromatic rings is based on the semiempirical SINDOl l 7 method. This method is applied in the present work to study the excited states of the benzenoid systems.
Computational Method and Results We use the configuration interaction (CI) version of the SINDO1 method” to examine the excited states of benzene and its monosubstituted derivatives. The procedure for optimizing the structure of the excited states has been described in detail prev i o ~ s l y .The ~ ~search ~ ~ ~ is performed with a Newton-Raphson procedure where the first and second derivatives of the energy with respect to the internal coordinates are determined by a difference method. The tolerances for convergence are 0.001 A for bond lengths, 0 . 2 O for bond angles, and 0 . 5 O for dihedral angles. In practice, the CI calculations on the excited states are preceded by an initial optimization of the closed-shell S C F ground state using the SINDOl approximation. We have started the calculations on the excited states with a vertical excitation involving a 17X 17 CI which includes all singly excited configurations arising from the two highest occupied orbitals to the four lowest virtual orbitals. Besides the ground state there are eight excited singlets and eight triplets generated. The configurations having mixing coefficients larger than 0.05 are selected for the various low-energy excited states. In the subsequent steps we have also checked excitations from the next two occupied orbitals and included those additional configurations which give a significant contribution to (10) K. Keith Innes in “Excited States”, Vol. 2, 1, E. C. Lim, Ed., Academic Press, New York, 1975. (11) T. Cvitas, J. M. Hollas, and G.H. Kirby, Mol. Phys., 19, 305 (1970). (12) T. Nakajima, A. Toyoa, and M. Kataoka, J . Am. Chem. SOC.,104, 5610 (1982). (13) A. Domenicano and P. Murray-Rust, Tetrahedron Lett., 24, 2283 (1979). (14) E. von Nagy-Felsobuki, R. D. Topsom, S. Pollack, and R. W. Taft, J . Mol. Struct., 88, 255 (1982). (15) P. C. Mishra and K. Jug, Theor. Chim. Acta, 61, 559 (1982). (16) K. Jug and G.Hahn, J . Comput. Chem., 4, 410 (1983). (17) D. N. Nanda and K. Jug, Theor. Chim. Acta, 57, 95 (1980).
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3509 the considered excited state. Orbital inversions that may occur in the course of optimization and configurational functions that become important in distorted geometries are appropriately taken into account. We denote the atoms in the phenyl frame by the numbering shown below: x12
I
The following geometrical constraints are imposed in the process of energy minimization: (i) The carbon atoms 1, 2, 4, and 5 lie in a plane which is taken as the reference plane. (ii) The phenyl frame in the derivatives other than phenol maintains a mirror plane which is perpendicular to the reference plane. With the abovementioned geometrical restrictions, we have optimized two lowlying excited singlet states and the lowest triplet state. In the case of nitrobenzene our calculations show that the low-lying excited singlet and triplet states originate from the substituent fragment of the molecule. Hence, in nitrobenzene we have calculated higher excited states which involve ring a-electron excitation in both singlet and triplet manifolds. We denote by SI,S, ... the optimized excited singlet states in the order of increasing energy. The triplet states are denoted in the same manner as T,, Tz, etc.
Results and Discussion Transition Energies. Adiabatic excitation energies, obtained as differences between the total energies of the independently optimized ground and excited states, are presented in Table I for benzene and its derivatives. For substituted benzenes we use the ring symmetry to label the states. This fragment can be best described by Cz, in planar arrangements and C, in nonplanar arrangements. The symmetry assignments of the considered states in DZh,C,,, and C, are related in the following way: As seen from Table I, the calculated adiabatic excitation energies for the ~ a triplet * states, the second singlet state of benzene ]Blur and the analogous ‘Al states in the derivatives agree fairly well with the experimental values. Our transition energies for the singlet states of lBlu origin are better than the C N D O results which are parametrized to reproduce the first excited singlets. However, in the first excited singlet state IBz, of benzene and the corresponding ‘Bz states in the derivatives, the calculated adiabatic energies are higher than the experimental values by about 1 eV. Our calculations on the lB2 state show that the stabilization resulting from the relaxation process is very small in the derivatives of benzene although there is appreciable reorganization in the ring structure. Probably we get poor correlation in the lBZ state, reflected by the rather large deviation from the experimental transition energy. Experimental s t ~ d i e s ~have , ~ ~shown ~ ~ -that, ~ ~ in the F, OH, NH2, and CH, derivatives, substitution does not alter the order of the energy levels of S1 and S., That is, ‘B2 is the first excited singlet and ‘A, is the second excited singlet state similar to the situation in benzene. In the case of fluorobenzene our calculations show as expected that the ‘Bz state is lower in energy than the ‘A, state. But the energy difference between them is very small, probably because the stabilization energy resulting from energy (18) K. Kimura and S. Nagakura, Mol. Phys., 9, 117 (1965). (19) J. Philis, A. Bolovinos, G. Antritsapoulous, E. Pantos, and P. Tsekeris, J . Phys. E , 14, 3621 (1981). (20) A. Bolovinos, J. Philis, E. Pantos, P. Tsekeris, and G. Antritsopoulos, J . Mol. Spectrosc., 94, 55 (1982).
3510 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
Malar and Jug
TABLE I: Excited-State Energies (eV) of Substituted Benzenes
excitn energy vertical
Td3BlU)
ring sYm D2h
SI(%”) S2(’BIU)
D6h D6h
sub-
stituent H
state
main excitn dbzg)-n*(bd n(elg)-n*(ezu) n(elg)-n*(e2u)
cs
CS CS
n(a’)-n* (a’) 5+1/a2)-.*(a2/bJ n(a’)-n*(a’) n(a’)-n* (a’) n(a’)-a* (a’) a(a’/ a”)-n* (a”/ a’) *(a’)-r*(a’) r(a’)-n* (a’) n(a’/a’’)-“r*(a’’/a’)
c 2 0
n(bi)-n*(bi)
c 2 0
cs CS CS
c, c,
CS CS
n(a’)-n* (a’) n(a’/a’’)--?T*(a’’/a’)
c 2 0
4 a I )-a* (a I ) a(a’)-r* (a’) d a I )-n*(b2) n-cr*(al) ?r(a’)-a*(a’) 4bl/a2)-.*(az/bl)
cs
C2” c 2 0
CS
C2”
ab initio 3.83’ 5.00“ 7.64”
this
adiabatic,
semiemp
work
3.76 4.7b 5.26 3.0*
4.53 6.46 6.99 3.72 5.81 6.55
this work 3.56 5.82 6.39
exptl 3.89c 4.89c 6.18‘
2.99 5.78 5.85
4.69‘ 6.21‘
4.68
3.75 5.62 5.71
2.91 5.17 5.55
4.51h
4.78 4.49
3.79 5.55 5.77
3.10 4.86 5.64
3.08’ 5.41 4.22k
5.18 4.68
3.88 6.32 5.81
3.36 5.52 5.73
3.60’ 5.83m 4.65m
4.48 4.27 5.42 5.70
1.06 3.45 3.87 4.95 5.70 6.10
4.38“ 3.65”
3.221
5.11“
P. J. Hay and I. Shavitt, J . Chem. Phys., 60, 2865 (1 974). J. Del Bene and H. H. Jaffe, J . Chem. Phys., 48, 1807 (1 968). J. P. Doering, J. Chem. Phys., 67,4065 (1977). dR. Bonneau, M. E. Sime, and D. Phillips, J . Photochem., 8, 239 (1978). CReference19. fC. A. Parker and C. G. Hatchard, Analyst (London),87, 664 (1962). 8Reference 35. hReference3. ‘E. C. Lim and S. K. Chakrabarti, J . Chem. Phys., 47, 4726 (1967). /Reference 18. kReference4. ‘G. N. Lewis and M. Kasha, J. Am. Chem. SOC.,66, 2100 (1944). mReference20. “Reference 21.
minimization is only 0.03 eV in the IB, state while it is 0.70 eV in the ‘Al state. In aniline and toluene we find the order of adiabatic energy reversed between lB2 and lA1; Le., ‘A, becomes SI and lB2 becomes S2. As pointed out later in this section, considerable mixing from some a-u* excitations is found to bring down the energy of the ‘A, state. According to our calculation, SI of phenol is predicted to arise from a A-u* transition and is a ]BI state. This state is followed by the ‘B2 state. Existence of a low-lying T U * state is detected recently in polyfl~orobenzenes.~~ In the case of penta- and hexafluoro derivatives, it is seen that the A-u* state lies below the ‘A, state.19 Our calculations on phenol clearly indicate the probable existence of a low-lying A-u* transition in phenol. Experimental assignment of the many lowlying states in nitrobenzene is ambiguousaZ1 The low-energy singlet and triplet states in nitrobenzene which originate essentially from the NOz fragment are discussed later. In benzene the lowest triplet state T1 arises from the a-a* excitations b2, b3, and bl, a, in DZhsymmetry. The major LUMO contribution to this state comes from the H O M O excitation bz, b,, and is basically a 3Blustate. It may be pointed out that in DShsymmetry TI has equal contribution from both excitations (elg e2,). Structural optimization leading to the DZhsymmetry results in well-separated occupied orbitals bl, and bzr and unoccupied orbitals b,, and a,. It is noticed that the contribution of the excitation bl, a, is small with a CI coefficient of 0.12. Ti in the isoelectronic series with substituents F, OH, NH2, and CH, and T2 in nitrobenzene are analogous to the TI state in benzene. They are predominantly PA* in origin. The dominant excitation involved is from HOMO LUMO which, bl type. The contribution from in Cz, approximation, is of b, the excitation a2 a2 is much smaller than the corresponding a, in TI of benzene. It is found that the a-a* excitation bl, triplet of the F, OH, and N H z derivatives are mixed with a small contribution from a a-u* excitation of bl a l origin. In toluene and nitrobenzene this excitation is absent, but in nitrobenzene T2 is mixed to some extent by a ~ - a * excitation arising from a A orbital localized largely in the nitro group.
-
-
-
-+
+
--
-
-
+
(21) S.Nagakura, M. Kojima, and Y. Maruyama, J . Mol. Spectrosc., 13, 174 (1964).
Calculations show that in toluene the interaction of the methyl group with the frontier orbitals is practically negligible and T, remains virtually as a pure A-a* state. However, the substituents F, OH, NH2, and NO2 interact strongly with the T1 state as evidenced by significant ?r as well as u contribution of the substituent group in the dominant excitation bl b,. The interaction of the substituents with the ring is found to cause significant structural distortions in the excited states, in particular, around the substituted carbon atom. The a-n* ‘B2 state in substituted benzenes has two main excitations, bl a2 and a2 b,. Appreciable involvement of T U * excitation is noticed in the lB2 states of phenol and aniline. It is seen that the interaction of the substituent group with the ‘B, state is much less marked than in the TI state. In fluorobenzene and in nitrobenzene the ‘B2 states do not have any u-A mixing because planarity is preserved in fluorobenzene and the nitro group becomes orthogonal. In the latter case there is no contribution from the NO2 group in the excited state as expected from the structure. In the ‘Al states of the substituted benzenes corresponding to the ‘Blustate in benzene, the origin of the excitations involved is the same as that in the TI(,Al) states. A noteworthy difference in the singlet state is that the interaction of the substituent and hence the u-a mixing in the ‘A, state is more pronounced than that in the ,A1 state. Even in toluene, significant mixing of u electrons occurs centered around the ipso carbon. Because of the strong substituent interaction in the ‘Al state, it attains pronounced TU* character, which is eventually responsible for the considerable stabilization of this state in comparison with the ‘B, state. S1 in phenol arises from v u * excitation involving the HOMO and a low-energy u* level. The energy of this state is lower than that of the a-n* singlet states of phenol because the u* level lis lower in energy than the a* orbital a2 and is very close to the LUMO. The first triplet state T, of nitrobenzene is found to be a repulsive state originating from a o-u* transition localized mainly on the substituent fragment. The u* orbital is antibonding with respect to the C N bond. de Mayo22has observed that GLC-
-
-
-
(22) P. de Mayo, unpublished result (quoted in ref 1).
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3511
Excited States of Benzene and Its Derivatives
STRUCTURE OF THE I
119.8
“7 1.429
a = 180.0 6 = 180.0
180.0 = 180.0
a =
B
7
6
a = 180.0
= 176.0
B = 180.0
Figure 1. Bond lengths (A), bond angles, and dihedral angles CY = c&!&Ic6and j3 = c2clc6xl2 (deg) in the ground states of fluorobenzene, phenol, aniline, toluene, and nitrobenzene. The weakest ring bonds are marked with an asterisk.
purified samples of nitrobenzene do not phosphoresce. A weak phosphorescence reported a t -2.6 eV was therefore considered spurious.’ Our study attributes the nonemissive behavior of the triplet states of nitrobenzene to the presence of the dissociative TI state. The emission from higher triplet states ends up in T1 which will result in simultaneous dissociation. The following explanation is given for the unduly small dissociation energy in the TI state: Our calculations and experimentsz3reveal that in nitrobenzene the C-N bond is weaker than a C-N single bond. Excitation to TI weakens further the C N bond but strengthens the N-0 bonds. This is seen from the changes in the magnitudes of appropriate bond orders, which in turn show inverse relation with bond lengths.24 Since bond length is related to bond energy, it is possible to estimate the bond energy directly from bond order by using standard values. Such an estimation of bond energy for the C-N bond in the optimized structure of TI of nitrobenzene shows that the value is very close to 1.06 eV. This is a convincing evidence for the low dissociation energy in the T1state of nitrobenzene. Our calculations show that the lowest singlet state S1(lBZ)in b2 localized nitrobenzene originates from UT* excitation a, largely on the nitro group. The adiabatic excitation energy of this state is predicted to occur at 3.87 eV. The second singlet state S2(:A1),having an excitation energy of 4.95 eV, arises from the excitation of n electrons of oxygens to a ring u* orbital of a, symmetry. It is seen that the nitro group has substantial contribution to the u* orbital. Assignment of the observed transitions in nitrobenzene is not conclusive.21 Three transitions measured at 3.65, 4.38, and 5.1 1 eV are attributed to arise from n-*, FR*, and F R * transitions, respectively.21 Polarization measurementszs show that the bands at 3.65 and 5.11 eV are polarized parallel to the principal molecular axis and the 4.38-eV band is polarized perpendicular to the principal axis. This indicates that the band at 4.38 eV may be identified as the transition S1(’B2)of our calculation. The transition at 3.65 eV may be identified as the n-a* transition, i.e. S2of our calculation. It is very likely that the transition at 5.1 1 eV arises from the R-R* transition in which the R oribtals
-
(23) J. Trotter, Acta Crystallogr., 12, 884 (1959); Tetrahedron, 8, 13 (1960). (24) K.Jug, Theor. Chim. Acta, 51, 331 (1979). (25) H. Labhart, Tetrahedron Suppl., 2, 19, 223 (1963).
of the ring are involved in the excitation. Our calculations locate this transition, ‘Al(r-a*), at 5.70 eV. Structure of the Excited States. 1. Triplet State. The present study shows that the lowest triplet state of benzene T, has a planar quinoidal structure of DZhsymmetry. The calculations indicate that in the triplet state there occurs a rearrangement in the R electrons of the ring resulting in the formation of partial single and double bonds. The four bonds “forming the shoulders of the ring” are elongated each by 0.07 A, and the two bonds “forming its sides” are shortened each by 0.05 A with respect to the ground state. The electronic structure that emerges is of the 1,4-diradical type having a quinoidal form:
H
1.368
1.0721
H de Groot and van der Waals2 have concluded from electron magnetic resonance spectra that benzene in its lowest triplet state possesses lower symmetry than trigonal symmetry. From a qualitative analysis of the phosphorescence spectrum of benzene on the basis of the Franck-Condon principle,26Redlich and Holt arrived at a quinoidal structure for the T1 state of benzene. Our result is consistent with these experimental observations. Calculation of Nakajima et al.lz based on bond length distortion also leads to the same conclusion though the quinoidal character is somewhat smaller than that obtained in the present study. In the lowest R-T* triplet of the benzene derivatives, the nature of the substituent seems to be uncritical in determining the bond length distortions of the phenyl frame as demonstrated in Figure 2. In general, we observe that the deviations from the structure of the benzene triplet are largely centered around the substituted carbon atom. Irrespective of the nature of the substituent, there appears a rather large extension of the bond between the ipso and (26) 0. Redlich and E. K. Holt, J . Am. Chem. SOC.,67, 1228 (1945).
3512 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
;"
STRUCTURE OF THE
1.349
1.074
Malar and Jug
STATE
r d TRIPLET
110.8 b.363
3A1
118.7 122,6 1. 7
1.479 117.8 a = 182.0 E = 131.0
a = 181.8
5 ; 134.3
1.399
ti a = 180.0 B = 180.0
a = 173.6
6 =-148.6
i 122.2
1.523
%
,,*
n
Figure 2. Same as Figure 1 for the r x * 3AIstates.
the ortho positions. The distance between these carbon atoms ranges from 1.517 A in nitrobenzene to 1.543 8, in phenol. In fact, this bond approaches that of the Q bond. The bond lengths ortho-meta and meta-para are in the order of 1.36 and 1.475 A, respectively, and they are not much different from the values of the benzene triplet. The deviations in the internal ring bond angles are in the pattern noted below for all molecules considered: The ring angles are reduced at the ipso and meta positions while they are increased at the ortho and para positions as compared to the respective bond angles in the ground state (Figure 1). The bond angle distortions are minimum in the toluene triplet. Large angular distortions appear usually at the ipso and para positions of the ring. The changes are in the order of -4.5O at the ipso carbon of phenol and fluorobenzene and -3.2 and -2O respectively for nitrobenzene and aniline. The differences in the ring bond angle at the para carbon amount to about +3O in aniline and phenol and +2O in fluorobenzene and nitrobenzene. The deviations are generally smaller a t the ortho and meta positions. Dihedral angles (Y and /3 defined in the figures provide a measure of the out-of-plane distortions. It is seen that the phenyl frame in the TI state is planar in toluene. The phenyl ring is nearly planar in the triplet state of fluorobenzene and phenol, having a distortion of about 2O. In aniline and nitrobenzene the substituted carbon is out of the reference plane by 6.4 and 1 7 O , respectively. The i so carbons in these molecules are displaced by 0.09 and 0.23 , respectively, from the reference plane. Since the valence numbers2' for the ipso carbon are 3.27 for C&, 3.26 for C6HSCH,, 3.20 for C6H5MH2,3.07 for C6H50H, , diradical character is much and 3.05 for C6HSNo2and C ~ H S Fthe less in benzene and toluene than in the other compounds, which may explain their nonplanarity. The geometry of the methyl group in the F T * triplet state of toluene does not change notably compared to the case for the ground state. However, in the F, OH, NHz, and NOz derivatives the heteroatom attached to the ring undergoes a drastic out-ofplane displacement resulting in significant deviation of the positions of the substituent atoms. The changes in the dihedral angle of the heteroatom attached to the ring in aniline, fluorobenzene, phenol, and nitrobenzene are respectively 31.4,45,49, and 60'.
8:
(27) M. S. Gopinathan and K. Jug, Theor. Chirn. Acta, 63, 497, 5 1 1 (1983); K. Jug, submitted for publication in J . Am. Chem. SOC.
In aniline and nitrobenzene, the distortion at the ipso carbon is also having a considerable impact in dislocating the heteroatom attached to the ring. The displacements of the heteroatoms from the reference plane are in the order
NO2 (1.45) > _OH (0.93) > E(0.91) > FJH2 (0.72) The numerical values are in angstroms (A) and correspond to the out-of-plane displacements of the heteroatoms underlined. Obviously, in these cases the out-of-plane displacements of the remaining substituent atoms (except the F derivative) are much more pronounced. The values in angstroms are as shown: NQ2 (1.90) OB (1.16) N u 2 (1.32) I t must be emphasized that the positions of the oxygen atoms of nitrobenzene are subjected to radical reorientation in the *-A* triplet state. The resultant orientations of oxygen atoms around the nitrogen atom are very similar to those of the amino hydrogens. That is, both the oxygen atoms are twisted upward from the reference plane. In the aniline triplet, it is clear that the nitrogen atom tends to be pyramidal in nature as in the ground state. In the F, OH, NH2, and CH3 derivatives, the phenylsubstituent bond is contracted as compared with the ground-state value in the order
F (0.015) < OH
-
CH3 (0.025) < NH2 (0.035)
The numbers inside parentheses are the reduction in Ph-X bond lengths in angstroms. The value is small in fluorobenzene, but in the other molecules it is substantial. The shortening of this bond in conjunction with the changes in ring bond lengths suggests that there may exist a noticeable contribution from structure B
X
A
0
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3513
Excited States of Benzene and Its Derivatives
STRUCTURE OF THE
II- SINGLET
STATE lBp I
1.430 a = 180.0 5 = 180.0
B
t-
tP c(
=
= 183.5
184.0
8 = 173.3
P
' '
1.248
Fw Figure 3. Same as Figure 1 for the
T-T*
u
-
IO".
B = 180.
'B2states.
having an extended conjugation besides the diradicaloid structure
A in phenol, aniline, and toluene. In nitrobenzene, structure B is ruled out since the Ph-X bond length is not showing any shrinkage in the triplet. 2. Singlet States. Our calculations predict that in the two low-lying excited singlet states SI(lB2,,)and S2('B1,) of benzene the D6hsymmetry is retained, but the carbon-arbon bond lengths are increased. The values are 1.442 and 1.438 A respectively for the lBzuand IBlu states. The carbon-hydrogen bond lengths are respectively 1.078 and 1.079 A. Experimental studies on the lB2, state based on both the vibrational and rotational band contour a n a l y s e ~conclude ~ ~ , ~ ~ that this state has an expanded regular hexagonal structure. Our structure for the lBzu state is in accordance with these findings, and the C-C and C-H bond lengths compare favorably with the experimental values 1.435 and 1.070 A, Nakajima et a1.I2have also arrived at the same conclusion for the lBZustate with a slightly lower value of 1.428 A for the ring bond length. The structure of the S2 state is not conclusive from experimental studies. Liehr30argues that the close proximity of the l B l u and lElu states in benzene causes a noteworthy destabilization of the hexagonal structure in the lBlu state by the Jahn-Teller effect and predicts that the structure is antiquinoidal in nature. On the other hand, a quinoidal structure is proposed for the S2state to account for the probable involvement of this state in the formation of Dewar benzene from benzene by phot~lysis.~'Our study shows that the energy of the lBlu state is increased by quinoidal, antiquinoidal, and out-of-plane distortions. Consequently, S2 prefers the D6h symmetry. The geometries of the states of lBlu origin in the benzene derivatives are depicted in Figure 3. Although it is clear that, in the various derivatives except nitrobenzene, the ring bond lengths increase like those in the parent molecule, the following important features are manifested: (i) In the OH, NH2, and CH3 derivatives the ring bond lengths are in the order ipso-ortho > meta-para > ortho-meta. Such a trend in the ring bond lengths giving rise to quinoidal character is apparent also in the ground states of all the derivatives we have (28) D.P. Craig, J . Chem. Soc., 2146 (1950). (29) J. H. Callomon, T. M. Dunn, and I. M. Mills, Philos. Trans. R. SOC. London, Ser. A , 259, 499 (1966). (30) A. D. Liehr, Z . Naturforsch., A , 16, 641 (1961). (31) D. Bryce-Smith,A. Gilbert, and D. A. Robinson, Angew. Chem., Inf. Ed. Engl., 10, 745 (1971).
considered as revealed by Figure 1. But the quinoidal character in the ground state is less prominent. (ii) In fluorobenzene the ring bond lengths are nearly equal in the 'B2 state. In fact, the ortho-meta distance becomes slightly larger than the remaining two. Thus, there is a definite reversal of the trend noticed in case (i). (iii) The situation in the 'B2 state of nitrobenzene is rather unique in the sense that all the ring bond lengths are decreased by excitation and the ring attains a pronounced antiquinoidal character. In the series with substituents F, OH, NH2, and CH, the phenyl-substituent bond length is reduced in the excited state of lBzu origin as compared to the case for the ground state. The shrinkage in the phenyl-substituent distance (in angstroms) is in the order
> NH2 (0.021) > CH3 (0.013) > F (0.008) This distance is slightly higher (by 0.003 A) in the 'B2 state of OH (0.031)
nitrobenzene than the corresponding ground-state value. Cvitas et al." have estimated the contraction in the phenyl-substituent bond length for some monosubstituted benzenes in the 'B2 state by assuming that the geometry of the phenyl frame is unchanged from that of the benzene lBzu state except for the internal ring angle a t the ipso carbon. Their results for phenol, aniline, and fluorobenzene are respectively 0.044, 0.080, and 0.009 A. The surprisingly good agreement between the theoretical and experimental values in fluorobenzene may be attributed to nearly equal ring bond lengths which justify the assumption made by Cvitas et al." In the other examples the calculations show that the ring bond lengths are no longer equal; hence, the results obtained by Cvitas et al." which force the structural changes to concentrate only on the two parameters mentioned above may not be realistic. Nevertheless, their result and the calculated values of the present study are in accord with a higher rotational barrier in the 'B, state in phenol which is estimated to be about 3.5 times greater than in the ground state.3 Obviously, these results are indicative of formation of partial double-bond character between the ring and the substituent in the IB2 state in phenol and presumably in aniline. The reduction in the phenyl-substituent bond length is comparatively small in toluene, which is in agreement with a very low barrier obtained in recent rotational ana lyse^.^^,^^ (32) M. A. Leugers and C. J. Seliskar,J. Mol. Specfrosc.,91, 150 (1982).
3514 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 TABLE II: Dipole Moments (D) in Substituted Benzenes' sub-
stituent So S(IB2) s(%) F 1.48 (1.66)b 0.94 (1.96)b 4.24 OH 1.42 (1.45)b 1.81 (1.65)b NH2 1.85 (1.53)b 1.95 (2.38)b 3.95 CH3
NO1
0.36 3.81 (4.00)'
0.48 3.25
1.75 5.17 (8.18)d
S('B1) 3.87
V3~d 1.89 1.74 2.71 0.36 5.07
"Experimental values in parentheses. W. Liptay in "Excited States", Vol. 1, E. C. Lim, Ed., Academic Press, New York, 1974. 'M. B. Ledger and P. Suppan, Spectrochim. Acta, Part A , 23A, 641 (1971). d T . Abe, Bull, Chem. SOC.Jpn., 38, 1914 (1965). From the analysis of our data, it is clear that the quinoidal character is increased in the 'B, states of phenol, aniline, and toluene compared to the case for the respective ground states. In fluorobenzene, the 'B2 state has less quinoidal character than in the ground state, and it tends to be antiquinoidal. In nitrobenzene, it is obvious that the 'B2 state possesses antiquinoidal geometry. Our conclusions regarding the bond length distortions in the phenyl ring of the substituted benzenes in the 'B2 state are in complete agreement with the existing experimental result^.^-^ Ring bond length expansion in the 'B2 state is demonstrated in fluorobenzene from the reduction of the vibrational-mode wavenumber in the a l and b2 modes.6 Analyses of the changes in rotational constants and dipole moments accompanying the excitation to the 'B2 state clearly emphasize that in phenol and aniline there is an increase in quinoidal c h a r a ~ t e r while ~ - ~ in fluorobenzene there is a decrease in quinoidal c h a r a ~ t e r .Lombardi ~ et aLShave proposed that the 'B2 state has a dipolar structure having the positive pole at the substituent and the negative pole at the para carbon. From a comparison of our results with the experimental observations, it may be possible to generalize that the electron-donating substituents will increase the quinoidal character in the 'B, state of substituted benzenes. However, the trend is reversed with electron-accepting substituents; Le., the molecule will have less quinoidal character on excitation to the 'B2 state. Powerful electron acceptors such as the nitro group may influence the ring to have antiquinoidal structure in the 'B2 state. It is seen that the angular deformations in the phenyl frame and the substituent portion are normally much less than 1' in the 'B2 state. Exceptions are noticed at the para position of phenol and para and meta positions of nitrobenzene. In phenol the internal ring angle at the para position and the external angle C2C3H9are deformed by +2.2 and -2', respectively. The corresponding changes at the para position of nitrobenzene are respectively +3.7 and -3.8'. In addition to these, the internal angles at the meta positions of the nitrobenzene are narrowed by 1.9'. The structural parameters of the substituent groups in the 'B2 state closely resemble the ground-state values. A shift in the atomic positions of the entire group occurs particularly for nonplanar situations, but the relative positions are intact for practical purposes: see following paragraphs. The resemblance seems natural since the 'B2 state originates from the ~ - a transition * pertaining to the ring and is supported by the recent rotationtorsion band contour analysis of Leugers et al.32which reveals that the changes in methyl group structure are small in toluene on excitation to the 'B2 state. Table I1 shows that fluorobenzene is planar in the 'B, state, which is in agreement with the recent conclusion of Lipp et aL6 deduced from extended vibrational term level assignment. For phenol, vibrational analysis3 suggests that a nonplanar configuration is improbable for the 'B, state. Our results show that the ring is near planar with the substituted carbon lying below the reference plane by 0.02 A (with out-of-plane distortion 1.5'). Both atoms in the OH group are displaced above the reference plane by 0.05 A. The out-of-plane deformation from the reference plane amounts to 2', a deviation from planarity too small to be detected by experiment. (33) M. A. Leugers and C. J. Seliskar, J. Mol. Spectrosc., 91, 209 (1982).
Malar and Jug A few spectroscopic a n a l y ~ e son ~ ~aniline * ~ ~ ~are centered around the configuration of amino hydrogens relative to the rest of the molecule. Brand et ale8noticed that the energy levels of the inversion vibration were highly anharmonic in the ground state and slightly anharmonic in the excited state 'B2. The out-of-plane deformation of amino hydrogens was estimated to be 46' in the ground state, but that in the excited state could not be determined and was referred as "quasi-planar".* A smaller deviation from planarity is suggested in the excited state in view of the smaller degree of anharmonicity of the inversion vibration. Rotational band contour analyses have provided the values 37 and 30' respectively for the nonplanarity of amino hydrogens in the ground state34and the 'B, state: on the basis of the model assumption that the phenyl frame maintains the regular hexagonal structure of the corresponding states in benzene. According to the SINDO1 calculations, in the ground state of aniline the nitrogen is out of the ring plane by 4' and the H N H angle is 104.9'. In the 'B2 state the ring is nonplanar by 4.0 and 4.5' respectively at the ipso and para carbons, but the H N H angle is 105.5' similar to the ground-state angle. Our numerical values of 34.0 and 27.2O as a measure of nonplanarity of the amino group from the reference plane in the ground state and in the excited state are in fair agreement with those from the rotational ana lyse^,^^^^ Le., 37 and 30°, respectively. Since the calculated structures show that the structure of the NH, group is not affected much by excitation, it is clear that the pyramidal character of the nitrogen preferred in the ground state of aniline is preserved in the excited state. Thus, the only probable explanation for the observed reduction in the degree of nonplanarity* centers at the deviation of the ipso carbon in the opposite direction as found to be the case according to our calculations. From the above arguments it seems very likely that our structures depict closely the realistic situation. In the 'B2 state of nitrobenzene, the nitro group is orthogonal as in the ground state. The energy difference between the planar and orthogonal arrangements of the NOz group is about 15 kcal/mol, which indicates that the orthogonal structure of the NO2 group is the preferred structure for the 'B2 state. In the case of the ground state the rotated structure is favored more than the planar geometry by about 2.5 kcal/mol according to our calculation, and it contradicts with the crystallographic analysis of nitrobenzene showing that it is planar.23 Figure 4 shows that the structural distortions in the 'A, state in the substituted benzenes are essentially uniform. The ring attains pronounced quinoidal character which is found to be between those of the IB2 and 3A1states. The change in the phenyl-substituent bond length is more marked than in the other excited states. Thus, the shrinkages in fluorobenzene, toluene, and aniline are respectively 0.016, 0.030, and 0.055 A whereas in nitrobenzene there is a sharp increase in the bond length by 0.022 A. Variations in the internal ring angles show the following pattern: The ring angle at the para carbon is increased by about 4'. The internal angles at the meta positions are decreased by about 2O. At the ortho positions the changes in the internal angle are small in the F and NH2 derivatives while a decrease of 1 and 2' respectively are met with in toluene and nitrobenzene. In these two molecules, changes are small at the ipso carbon, but a reduction of about 2' is noticed in fluorobenzene and aniline. The 'A, state of all these molecules is nonplanar. The dominant contribution of u electrons at the ipso carbon in the predominant bl excitation may be responsible for the rather significant bl out-of-plane distortion in the ipso carbon. The out-of-plane distortion, in angstroms, of the ipso carbon is found to be in the order NH, (0.17) C F (0.22) CH3 (0.22) < NO2 (0.26)
-
N
The deviation from planarity in the substituent atoms in F, "2, and NO, is similar to those of the corresponding 3Al states, although the nonplanarity of the nitro group is somewhat less than in the triplet state. In the 'A, state of toluene, the methyl group (34) D. G. Lister, J. K. Tyler, J. H. Hog, and N. W. Larsen, J . Mol. (1974).
Struct., 23,253
Excited States of Benzene and Its Derivatives
The Journal ofphysical Chemistry, Vol. 88, No. 16, 1984 3515
/1
a
382
121.8
a = 167.8 6 =-146.4
a = 163.0
6
=
230.0
/I
,543
/1.513 120.9
115.4 1*500
123.4
077 117.3 a = 196.2 6 = 135.0
1 I
a = 159.1 E = 230.5
rI
Figure 4. Same as Figure 1 for the T-T* 'Al states except for phenol.
is distorted significantly, resulting in the location of the methyl carbon atom 1.17 A above the phenyl plane. The relative positions of the methyl hydrogen atoms are also altered as reflected by changes in the dihedral angles, which show a change of about 10' from the ground-state values. Considerable structural deformations in the methyl group are also indicated by the large changes in the bond angles C6ClZHl3and C6ClZHl5.In aniline the angle C6NlZHl3is increased by about 10' and the dihedral angles of the amino hydrogens are decreased by about 24' to 10.2'. These results reflect that there is a decrease in the pyramidal character in the aniline 'A, state. Experimental data on elucidation of structures in the 'A, state are rather sparse. The absence or weakness of the 0-0 transition and the shape of the FranckCondon envelope of the 'Al band in tolueneZoare attributed to originate from a considerable distortion in the structure of the 'A, state of toluene. This observation is consistent with our predictions. The a-u* state 'B, in phenol is predicted to be planar. In contrast to the structures of the 'Al and 'BZ states, in this state the ring structure does not possess any C,( I)symmetry, Nevertheless, a well-defined quinoidal pattern in the ring structure is apparent. The phenyl-0 bond length is reduced by 0.062 A, which is greater than that in the other states we have considered so far. The internal angle at the para position is widened by 5.7' while the ring angles at the meta carbons are narrowed by about 3'. The changes at the ortho positions are respectively 1 and 2'. The ring angle at the ipso carbon is narrowed by about 3'. The COH angle is 116.1' in the 'B1 state while in the ground state it is 110.4'. The C C H angles are also altered up to about 4'. Major changes in the structures of SI and Szin nitrobenzene are manifested in the substituent fragment since these states originate primarily from excitations within the substituent. The impact of the substituent in the S1 state causes a slight increase in the quinoidal character of the ring accompanied by a small reduction in the average ring bond length while in the S2 state the ring attains a small antiquinoidal behavior together with a reduction in average bond length. In the S1state the bond lengths between hydrogen and ortho as well as meta carbon atoms are reduced noticeably. The C N bond length is considerably increased in the SI and S2 states respectively by 0.107 and 0.231 A. The NO bond is elongated in the S1 state by 0.014 A whereas it is shortened in the S2 state by 0.018 A. The angle subtended by the nitrogen atom with the two oxygen atoms is widened by 28' in SI while it is narrowed by 6.4' in Sz. The NOz group is
orthogonal in both these states, and the dihedral angles remain the same as the ground-state value. Charges and Dipole Moments. An examination of charges obtained in our calculations shows that in the ground state the substituent group has a net negative charge in phenol, aniline, fluorobenzene, and nitrobenzene. In toluene, the methyl group has a small positive charge. In the 'BZ state there is a small transfer of negative charge from the substituent to the ring. This behavior is observed by experimental studies on phenol, aniline, and flu~robenzene.~ In the 'A, state we find that the substituent withdraws electrons from the ring by a small extent. In the triplet state electron transfer from the substituent to the ring is noticed in the F, OH, and NH2 derivatives. The trend is reversed in toluene and nitrobenzene. Our study shows that the magnitude of charge transfer is very small in all the a-a* states considered. This confirms that the participation of the charge-transfer component in the a - ~ *excited states in substituted benzenes is negligible. Del Bene and Jaffe35have arrived at the same conclusion for the excited singlet states in phenol, aniline, and toluene on the basis of CNDO calculations. However, our conclusion that the a-a* excited states in nitrobenzene possess negligible charge-transfer character questions the validity of the results of Nagakura et a1.:' who have assigned the a-a*('Al) band at 5.1 1 eV as an intramolecular charge-transfer band. Dipole moments collected in Table 11 reflect that the values are increased on excitation except in the 'BZstates of fluorobenzene and nitrobenzene. Experimental study5 indicates an increase in the dipole moment in the 'Bz state of fluorobenzene. In the other cases, our values agree satisfactorily with the experimental values that are available. Aromaticity of the Excited States. A characteristic of the benzenoid systems is their exceptional stability in the ground state which is attributed to arise from the so-called aromaticity concept. The determination of the degree of aromaticity in the excited states of these systems is of considerable interest from both experimental and theoretical viewpoints. In the literature different kinds of indices are used as a measure of a r ~ m a t i c i t y . ~In ~ -this ~ ~ work (35) J. Del Bene and H. H. Jaffe, J . Chem. Phys., 49, 1221 (1968). (36) 0.E.Polansky and G. Derflinger, Int. J . Quantum Chem., 1, 379 (1967). (37) E. D. Bergmann and I. Agranat in "The Jerusalem Symposia on Quantum Chemistry and Biochemistry", Vol. 111, E. D. Bergmann and B. Pullman, Eds., Israel Academy of Sciences and Humanities, Jerusalem, 1971. (38) I. Gutman, M. Milun, andN. Trinajstic, MATCH, No. 1, 171 (1975).
3516
J . Phys. Chem. 1984,88, 3516-3520
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 states in the vapor phase on s ~ b s t i t u t i o n . ~Our ~ calculations 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 021 38 (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
