5235
J . Phys. Chem. 1985, 89, 5235-5239
the geometry of the quinone anion is distorted. The excited state of ZnPPc involved in electron transfer is probably a singlet. The decay kinetics of the light-induced EPR signals of ZnPPc depends on the site of attachment of the picryl moiety to the porphyrin. The symmetry matching of the donor and acceptor orbitals is favored for electron transfer and this happens in all the 0-,m-, and p-substituted picrylporphyrins though the distances vary from 6 to 11 A. More experiments are planned to elucidate the role of conformers in governing the rates of forward and backward electron transfers in these donor-acceptor systems.
and quinone systems.45 The reduction mode of nitroaromatic compounds proceeds through a single step in contrast to quinones which exhibit semiquinone intermediates depending on the pH of the medium.46 In the light-induced radical pair of ZnPPc, the picryl anion radical displays EPR features suggesting the out-of-plane disposition of the nitro groups with reference to the picryl plane, in contrast to the free T N B anion radical indicating steric constraints of the covalently attached picryl group. The change in geometry of the acceptor anion is considered as a disadvantage since both forward and recombination rates are influenced by these factors.47 The important difference between the nitroaromatic and quinone acceptor is that the change in geometry of the former arises from the orientational features of the covalent linkage while in the quinones the intrinsic nature of
Acknowledgment. This work has been supported by the Department of Science and Technology, Government of India, New Delhi, and G.B.M. thanks the Council of Scientific and Industrial Research, New Delhi, for a fellowship. Registry No. pPPc, 98541-07-2; mPPc, 98541-08-3; oPPc, 9854109-4; ZnTPP, 14074-80-7; ZnpPPc, 98541-1 1-8; ZnmPPc, 98541-12-9; ZnoPPc, 98541-1 1-8; CuTPP, 141 72-9 1-9; CupPPc, 98541 13-0; CumPPc, 98541-14-1; CuoPPc, 98541-15-2; PCIs, 10026-13-8; picric acid, 88-89-1; o-hydroxybenzaldehyde, 90-02-8; m-hydroxybenzalde, 100-83-4; p-hydroxybenzalde, 123-08-0; picryl chloride, 88-88-0; benzaldehyde, 100-52-7; pyrrole, 287-97-8; (0-hydroxypheny1)triphenylporphyrin, 98541- 10-7; (m-hydroxyphenyl)triphenylporphyrin,873775 1-3; @-hydroxyphenyl)triphenylprophyrin, 87345-22-0.
(45) The values of ground-state association constants for zinc(I1) mesoarylporphyrin derivatives with 1,4-benzoquinone (Yamada, S.;Sata, T.; Kano, K.; Ogawa, T. Photochem. Photobiol. 1983,37,257-262) and TNB (ref 1 1 of this article) in acetone are found to be 0.7 and 10 M-I, respectively. (46) (a) Evans, D. H. In 'Encyclopedia of Electrochemistry of Elements"; Bard, A. J., Lund, A., Eds.; Marcel Dekker: New York, 1973; Vol. XII, Chapter 1, (b) Kemula, W.; Krygowski, T. M. Ibid. Vol. XIII, Chapter 2. (47) (a) Holten, D.; Windsor, M. W.; Parson, W. W.; Gouterman, M. Phorochem. Phorobiol. 1978, 28,951-961. (b) Reference 36 of this article.
-
Structures and Properties of Excited States of Some Para-Disubstituted Benzenes E. J. Padma Malart and Karl Jug* Theoretische Chemie, Universitat Hannover, 3000 Hannover 1 , Federal Republic of Germany (Received: March 19, 1985)
Configuration interaction calculations with the semiempirical MO method SINDO1 were carried out to optimize the structures of low-lying singlet and triplet states of the following para-disubstituted benzenes: p-difluorobenzene,p-quinol, p-fluoroaniline, and p-nitroaniline. The effect of para substitution on the quinoidal character of the excited states is investigated. Adiabatic excitation energies, equilibrium geometries, dipole moments, and aromaticity are presented and discussed.
Introduction Electronic excitation is accompanied by redistribution of electrons. Frequently, this is manifested by dramatic changes in structure and other properties such as charge densities, dipole moments etc.' In cyclic *-electron systems the degree of aromaticity is also altered by electronic e x c i t a t i ~ n . ~The ~ ~ nodal pattern of the virtual MO which the electron occupies upon excitation is different from that of the occupied MO from which the electron is excited. Our recent studies reflect that changes in bond length and aromatic character can easily be understood by an examination of the nodal patterns of the M O s involved in the e ~ c i t a t i o n . ~The nodal characteristics are incorporated in the bond order values which in turn influence the variation of free valence. Recently, we have calculated the structures and properties of excited states of monosubstituted benzenes3at the SINDOl l e ~ e l . ~ . ~ It is found that the calculations reproduce satisfactorily the experimental data that are available. It was predicted that in the IB2 state electron donating substituents cause an increase in the quinoidal character of the phenyl frame while electron acceptors lead to antiquinoidal character. This is in complete agreement with experimental finding^.^-^ The out-of-plane deformations of the amino hydrogens of aniline in the ground and 'B2 states agree well with experimental studies.10," The A, singlet and triplet states have pronounced quinoidal character. Major Present address: Department of Inorganic and Physical Chemistry, Quantum Chemistry Laboratory, Indian Institute of Science, Bangalore 560012, India.
0022-3654/85/2089-5235$01.50/0
structural deformations, both planar and nonplanar, are found to be centered around the ipso carbon. For the distortion of the triplet state of benzene a model of vibronic coupling was already suggested.I2 ESR experiments13 on the aniline triplet found quinoidal character and a loss of the in-plane axis of symmetry. In this work, we examine the effects of para disubstitution on the structures and properties of the excited states of benzene. We study the extent of both in-plane and out-of-plane distortions exerted by the para substituent on the low-lying triplet and singlet states. We are particularly interested in understanding whether or not the antiquinoidal character introduced in the 'B, state by electron acceptors F and NOz is retained when the ring takes up a second substituent at the para position. We have chosen the (1) Keith Innes, K. In "Excited States"; Lim, E. C., Ed.; Academic Press: New York, 1975; Vol. 2. (2) Jug, K.; Hahn, G.J . Comput. Chem. 1983, 4, 410. (3) Malar, E. J. P.; Jug, K, J . Phys. Chem. 1984, 88, 3508. (4) Malar, E. J. P.; Jug, K. Tetrahedron, in press. (5) Nanda, D. N.; Jug, K. Theor. Chim. Acta 1980, 57, 95. (6) Jug, K.; Nanda, D. N . Theor. Chim. Acta 1980, 57, 107, 131. (7) Bist, H. D.; Brand, J. C. D.; Williams, D. R. J . Mol. Spectros. 1967, 24, 413. (8) Christoffersen, J.; Hollas, J. M.; Kirby, G. H. Mol. Phys. 1969, 16, 441. (9) Huang, K.-T.; Lombardi, J. R. J . Chem. Phys. 1970, 52, 5613. (10) Brand, J. C. D.; Williams, D. R.; Cook, T. J. J . Mol. Spectrosc. 1966, 20, 359. (1 1) Lister, D. G.;Tyler, J. K.; Hog, J. H.; Larsen, N. W. J . Mol. Srruct. 1970, 23, 253. (12) Vergracht, P. J.; van der Waals, J. H. Mol. Phys. 1977, 33, 1507. ( 1 3 ) van Noort, H. M.; Vergracht, P. J.; Herbich. J.; van der Waals, J. H.Chem. Phys. Lert. 1980, 71, 5.
0 1985 American Chemical Society
5236 The Journal of Physical Chemistry, Vol. 89, No. 24, 1985
derivatives p-difluorobenzene (p-DFB), p-quinol, p-fluoroaniline @-FA), and p-nitroaniline (p-NA). p-DFB and p-quinol are examples of symmetrically disubstituted benzenes: the substituents are electron acceptors in the former and electron donors in the latter. In p-FA and p-NA one substituent (NH,) is an electron donor while the other one is an electron acceptor. The numbering system used in this study is as shown: Xa
Xb
\
/ x7
X
F (2)OH (1)
(3)NH;! (4) NH2
Y F
(p-DFB) OH (p-quinol) F (P-FA) NO2 (p-NA)
We assume in our calculations that the atoms C,, C3, Cs,and C6 lie in a plane which is taken as the reference plane. Another important objective of this work is to explore the nature of the lowest triplet state of p-NA. The nature of the triplet state of nitrobenzene is discussed in earlier work^.^.'^ It is known that p-NA phosphorescesi5J6while nitrobenzene and other derivatives of nitrobenzene with weakly polar substituents do not phosphoresce.l5 In our earlier study3 we have found that the lowest triplet state TI of nitrobenzene is a u-u* state localized on the NO2 group contrary to what is commonly believed as a n-a* state." The u* orbital is antibonding with respect to the C-N bond. Structural optimization reveals that Ti(u-u*) of nitrobenzene dissociates into phenyl and NO,. This shows that the population of the Ti state of nitrobenzene results in simultaneous dissociation. This may explain the lack of phosphorescence in nitrobenzene. The situation in the derivatives of nitrobenzene is expected to be similar to that in nitrobenzene because it is known from earlier studies that the other common substituents do not introduce any drastic change in the parent m o l e c ~ l e . ~In , ' ~this context, we are interested to study the low-lying triplets of p - V A since it is reported that this system phosphoresces.1s~16From earlier experimental work it was concluded that the isoelectronic system p-nitrophenol does not phosphoresce16though the electronic nature of the ipso substituent OH is very similar to that of the NH, group. However, this can now be attributed to an inattention to excitation wavelengths. According to most recent experimental studies'* pnitrophenol both fluoresces and phosphoresces (in both polar and nonpolar solvents), but the intensity and the structure of these luminescences are critically dependent on excitation wavelength. For most compounds studied previo~sly'~ dual luminescence is now manifested. Since excitation wavelengths were not given in the earlier study,15 emission data are suspect.
Computation Procedure We use the configuration interaction (CI) version of the SINDO1 method5 to examine the excited states of disubstituted benzenes. The procedure for optimizing the structure of excited states has been described in detail previo~sly.'~The search is performed with a Newton-Raphson procedure where the first and second derivatives of the energy with respect to all internal coordinates are determined by a difference method. The tolerances for convergence are 0.001 A for bond lengths, 0.2' for bond angles, and 0.5' for dihedral angles. The CI calculations of the excited (14) Seliskar, C. J.; Khalil, 0.S.; McGlynn, S. P. In "Excited States"; Lim, E. C., Ed.; Academic Press: New York, 1974; Vol. 1. (15) Carsey, T. P.; Findley, G. L.; McGlynn, S . P. J . Am. Chem. SOC. 1979,101, 4502 (16) Khalil, 0. S., unpublished work, quoted in ref 14. (17) Bigelow, R. W.; Freund, H. J.; Dick, B. Theor. Chim.Acra 1983, 63, 177
(18) Carswell, L., Findley, G. L., to be published. (19) Mishra, P. C.; Jug, K . Theor. Chim. Acra 1982, 61, 559.
Malar and Jug states are preceded by an initial optimization of the closed shell S C F ground state. The calculations on excited states are started with a vertical excitation involving 17 configurations including the ground state and all singly excited configurations arising from excitations of electrons from the two highest occupied orbitals to the four lowest vacant orbitals. Besides the ground states eight singlets and eight triplets are generated. Usually the low-lying delocalized A-s* and a-u* singlet and triplet states are incorporated in such a study. We have optimized the lowest triplet state Tl and the two s-s* singlets which are analogous to the 'Bzu and 'Blu states of benzene. When NO2 is present as a substituent, the ordering of the MO levels is altered significantly as compared to that in the parent molecule benzene; particularly the few highest occupied MO's are intermingled with the NO2 group orbitals. It is seen that in p-nitroaniline the localized nitro group orbitals lie in between the delocalized ring s-MO's a, and bl. Hence it is natural that the low-lying states of p-NA are dominated by the nitro group orb i t a l ~ . ~ ~We , ~ 'have examined these states, in addition to the delocalized ring FA* states. Although the ring symmetry is varied in the excited states of benzene derivatives, we use in the discussions the state representations in C,,point group. The symmetry assignments of the considered states in D2h,C,,,and C, are related as Bl,-Al-Ar, B2u-B2-Aff, B,,-BI-A', A,-A,-A".
Results and Discussion Excitation Energies. Since the purpose of this study was the comparison of the lowest singlet and triplet states resulting from a-a* excitation, we have optimized these states together with the ground states. Vertical and adiabatic excitation energies are listed in Table I. Adiabatic excitation energies are obtained as differences between the total energies of the independently optimized ground and excited states. Experimental and other calculated values refer to vertical excitations. In all four molecules the lowest triplet was found to be of 3A, symmetry resulting from bi-bl excitation with a,-a2 admixture. There is good agreement with the available experimental and theoretical data as far as the assignment and absolute value is concerned. There are two a-a* excitations giving rise to the singlets 'B2 and 'Al. We find the latter sequence in all vertical excitations, but reversal of the sequence in p-quinol, p-FA, and p-NA in adiabatic excitations. The absolute values of the excitation energies seem uniformly too high for the 'Bz state, but agreement should be better for the 'Al state as can be seen from previous work on monosubstituted benzenes. Here it counts that ground-state and 'Al excited-state geometry differ appreciably whereas 'B2 is still similar to the ground state. The experimental value for p-NA was obtained on a solution of p-NA in methylcyclohexane at room temperature. There are frequently PCT* excitations between the low-lying ?M* excitations, but their geometries were not optimized. They do not seem to play an important role in the excitation process, because of low oscillator strength. Also the lack of substantial deformation of the benzene ring is accompanied by little u-a mixing as a consequence. Unlike nitrobenzene3 a vertical u-u* excitation in p N A is 0.6 eV higher than the s-a* excitation of TI. So a repulsive state resulting from such an excitation has lesser influence on the phosphorescence behavior, because the crossing can take place only at large C N distances. Since the lowest-lying triplet is the 3Al state resulting from a a-a* excitation with an excitation energy in the expected range, phosphorescence from this state is allowed and observed. Structures in the Ground State. In Table I1 we have presented the optimized structures of the ground states. It is seen that the ring bonds adjacent to the substituted carbon atoms are lengthened by about 0.02-0.04 A as compared to the values in benzene where the CC bond lengths are calculated as 1.419 A and the C H bond lengths as 1.080 A. There is a slight shortening of the central bonds Cz-C3 and Cs-C6. This indicates that there exists some amount of quinoidal character in the ground states of para-disubstituted benzenes. Such a trend is noticed in p-methylphenol
The Journal of Physical Chemistry, Vol. 89, No. 24, 1985 5231
Para-Disubstituted Benzenes TABLE I: Excited-State Enereies (eW of Disubstituted Benzenes
TABLE 11: Bond Lengths (A), Bond Angles, and Dihedral Angles (deg) of Ground States of Para-Disubstituted Benzenes
internal coordinate
p-DFB
molecule p-quinol p-FA
p-NA
TABLE III: Bond Lengths (A), Bond Angles, and Dihedral Angles (deg) of the *--A* Triplet States )A,
internal coordinate
p-DFA
molecule pquinol p-FA
p-NA
1.435 1.417 1.435 1.080 1.080 1.365 1.365
1.446 1.416 1.446 1.080 1.080 1.390 1.390 0.978 0.978
1.453 1.416 1.435 1.079 1.079 1.439 1.366 1.018
1.450 1.418 1.433 1.080 1.080 1.437 1.522 1.018 1.248
1.519 1.346 1.519 1.076 1.076 1.349 1.349
1.527 1.347 1.527 1.076 1.076 1.360 1.360 0.975 0.975
1.558 1.350 1.476 1.077 1.077 1.389 1.351 1.016
1.508 1.360 1.513 1.077 1.077 1.410 1.525 1.014 1.246
121.4 119.3 119.3 121.4 119.3
118.9 120.5 120.5 118.9 115.5 110.4
117.4 121.2 121.2 117.4 113.2 110.5
125.6 110.4
117.7 121.1 120.0 120.0 121.0 109.5 104.9 120.0 118.0 124.0
119.7 120.1 120.1 119.7 116.0
119.3
118.2 120.9 119.7 120.6 120.6 109.2 104.4 119.7
116.0
122.0 110.5
115.5 120.8 120.9 121.1 117.8 113.6 105.4 119.4
117.6 121.1 119.7 117.0 120.8 109.3 105.7 116.8 118.6 122.7
180.0 180.0
180.0 180.0 0.0 0.0
182.5 185.5 -37.5
183.2 182.8 -36.1 90.0
176.3 151.5
177.4 151.0 21.5 177.4 151.0 21.5
182.4 143.8 -12.7 178.5 177.5
177.1 193.3 -37.5 161.5 236.0 -18.5
by M I N D 0 / 3 calculationsZo and in p-nitroaniline by a b initio calculations.21 Our calculations show that both p-FA and p N A are nonplanar. The nonplanarity of the amino hydrogens is nearly the same as that in aniline.3 Although experimental studies show that the amino group is nonplanar in P - F A , ~the ~ ab initio studies indicate that it is planar in P-NA.~O According to the present calculations at the S I N D O l level it is found that the presence of the NOz group a t the para position does not bring about planarity in the amino group. The present calculations also predict that the nitro group is perpendicular to the ring plane. This is probably a shortcoming of the ZDO approximation. The energy (20) Eckert-MaksiE, M. 2.Nafurforsch., A 1982, 3 7 4 688. (21) Politzer, P.; Abrahmsen, L.; Sjoberg, P. J. Am. Chem. SOC.1984,106, 855.
(22) Christoffersen, J.; Hollas, J. M.; Kirby, G. H.Mol. Phys. 1970, 18, 451. (23) Vergracht, P. J.; Kooter, J. A.; van der Waals, J. H. Mol. Phys. 1977, 33, 1523.
176.3 151.5
difference between the planar and perpendicular forms is within 2 kcal/mol. Structures in Excited States. Little experimental information is available on excited-state geometries of para-disubstituted benzenes. ESR data on the triplet of p-xyleneZ2 suggested a distortion of DZhsymmetry due to the crystal field. No decision could be made on either quinoidal or antiquinoidal shape of the free molecule. In Table I11 we have collected the structural parameters for the T-T* triplet state 3A,. Our study reveals that major structural reorganization takes place in this state. The central ring bonds (ortho-meta) are shortened to 1.346-1.360 A with bond orders about 1.95. Thus, these central bonds are close to double bonds. The ipso-ortho and meta-para bonds approach single bond lengths. Considerable structural changes are noticed in thk substituent fragment also. The dihedral angles given in Table I11 reflect that the substituent atoms undergo significant
5238
The Journal of Physical Chemistry, Vol. 89, No. 24, 1985
out-of-plane distortions. The displacements of the substituent atoms from the reference plane are in the order F(p-FA) 0.03 < NH,@-NA) 0.32 C OH 0.52 < F@-DFB) 0.53 < NH,@-FA) 0.76 < NO2 1.40; NH&-NA) -0.12 < OH0.61 < NH,@-FA) 1.37 < NO2 1.83. The minus sign indicates a displacement in the opposite direction. The numerical values are in A and correspond to the out-of-plane distortion of the italicized atom. It may be remarked that in p-FA and p-NA the substituents which cause major distortions are this amino and the nitro groups, respectively. Thus it is seen that in the triplet state of p-FA, the fluorine atom causes essentially no influence in the ring structure and the para-meta bond lengths are nearly identical with those in the triplet of the monosubstituted benzene^.^ In the a-r* triplet of p-NA the carbon atom adjacent to the nitro group is nonplanar by 0.25 8,and both oxygen atoms of the nitro group are oriented above the reference plane. To complement the study atomic valence numbers along the lines suggested by Gopinathan and Jugz4 and extended to CIZ5 were calculated. These allow to establish the extent of diradical character of a molecular state.26 Whereas the valence numbers of normal carbon atoms are close to 4, diradicals show a substantial reduction of valence numbers of two or more atoms. If the effect is localized, two atoms of the diradical would have valence numbers close to 3. The valence numbers for the substituted carbons in p-DFB and p-quinol are 3.14 and 3.22, respectively. In p F A and p-NA the valence numbers are 3.16 and 3.49, respectively, for the ipso carbons and 3.40 and 3.12, respectively, for the para carbons. These numerical values clearly reflect the dominating influence of the amino group in p F A and the nitro group in p N A . The pattern of the changes in ring bond lengths and the valence numbers indicate that the a-a* triplet state can be considered as a 1,4 diradical with quinoidal geometry. X
Malar and Jug TABLE I V Bond Lengths (A), Bond Angles, and Dihedral Angles (deg) of the T-T* Sinelet States 'B,
internal coordinate
To support this claim further we have also calculated the spin densities for the carbon atoms in para position at the substituent sites. The values are 0.61 in p D F B , 0.55 in pquinol, 0.47 (ipso) and 0.58 in p-FA, and 0.62 (ipso) and 0.45 in p-NA. The rest of the spin density is mainly on the other ring atoms. The ringsubstituent bonds undergo a reduction in bond lengths in the range 0.015-0.050 A. In p-NA, the C-NO, bond is lengthened to a small extent. The structures of the 'B2 states are presented in Table IV. It is seen that all the ring bonds are lengthened by about 0.01-0.02 A in p-DFB, p-quinol, and p-FA. In the case of p-NA, the four bonds adjacent to the two substituted carbon atoms are shortened as compared to those in the ground state. The central ring bonds in p-NA are elongated by 0.01 A. As far as the quinoidal pattern is concerned, it is found that the 'B2 states of all the derivatives possess quinoidal character. In p-quinol, the quinoidal character is increased in the 'B, state as compared to that in the ground state. There is not much change in the quinoidal character of the 'B, states of p D F B and p F A relative to the case in the ground states. However, the presence of the NO2 substituent in p-NA causes a considerable reduction of the quinoidal character in the IB, state. It is clear from this study that the antiquinoidal character introduced in the *B2states of fluorobenzene and nitrobenzene3-' disappears when a second substituent is present in the para position. The ring-substituent bond is reduced by about 0.01-0.02 A in the 'B, states. Bond angle distortions in the IB2 states are usually of the order of l o . It is found that p-quinol retains planarity in the 'B2 state. In p-FA and p-NA the dihedral angles show that the deviations (24) Gopinathan, M. S.; Jug, K. Theor. Chim. Acta 1983, 63, 497, 5 1 1 (25) Jug, K. J . Compur. Chem. 1984.5, 5 5 5 . ( 2 6 ) Jug, K. Tetrahedron t e r r . 1985, 26, 1437.
p-guinol
122.5 118.7 118.7 122.5 118.6
120.4 119.8 119.8 120.4 115.3 110.7
118.6
124.3 110.7
176.5 176.0
180.0 180.0 0.0 180.0 180.0 0.0
176.5 176.0
1.461 1.427 1.461 1.073 1.073 1.367 1.367 0.975 0.975
p-FA 1.462 1.432 1.445 1.418 1.353 1.015
p-NA 1.444 1.429 1.431 1.074 1.074 1.422 1.513 1.012 1.248
119.4 120.2 119.1 121.8 120.0 110.3 104.7 119.1
119.2 120.3 119.5 121.0 120.2 109.8 105.9 119.5 117.7 124.6
183.0 184.2 -36.3 181.2 181.5
184.5 182.4 -35.8 181.0
178.0 90.0
TABLE V Bond Lengths (A), Bond Angles, and Dihedral Angles (deg) of the K--A* Singlet States 'Al
molecule
internal
coordinate
Y
molecule p-DFB 1.446 1.430 1.446 1.074 1.074 1.355 1.355
p-DFA 1.477 1.376 1.477 1.077 1.077 1.348 1.348
p-quinol 1.481 1.372 1.48 1 1.075 1.075 1.345 1.345 0.976 0.976
p-FA 1.511 1.392 1.434 1.08 1 1.077 1.380 1.359 1.010
p-NA 1.463 1.400 1.467 1.080 1.082 1.405 1.605 1.013 1.240
123.2 118.4 118.4 123.2 118.3
119.7 120.2 120.2 119.7 115.3 1 14.0
116.5 120.4 119.5 123.1 120.7
117.2
124.2 114.0
118.3 120.8 119.5 117.8 120.8 111.6 107.4 117.9 118.0 123.5
180.0 193.0
180.0 169.3 5.0 180.0 169.3 5.0
180.0 193.0
109.1 118.5
188.5 153.5 -1 1.0 179.0 180.7
178.0 181.5 -29.5 162.4 229.8 -18.7
from the ground-state values are within 2'. But we find that nonplanarity is more marked in the 'B, state of p-DFB. In this case the ring is nonplanar by 3S0. The substituted carbon atoms are distorted by 0.04 A above the reference plane and the fluorine atoms are shifted below the reference plane by 0.04 A. The nonplanarity predicted the 'B2 state of p-difluorobenzene is in agreement with the inference made from vibrational analysis.22 Table V presents the structures of the A,(?nr*) singlet states. The distortions in the singlet state are quite similar to the corresponding triplet 3Al. The quinoidal pattern in the ring is less pronounced in the singlet than in the triplet. The valence numbers are nearly the same in both states. In the unsymmetrically disubstituted derivatives p-FA and p-NA, the substituents controlling
The Journal of Physical Chemistry, Vol. 89, No. 24, 1985 5239
Para-Disubstituted Benzenes TABLE VI: Dipole Moments (Debye) with Experimental and Other Calculated Values in Parentheses
states p-DFB p-quinol
p-FA p-NA
0 0 2.42 4.41 (6.31')
0.34 0.80 2.40 7.02 (9.91b)
0.30 0 3.15 5.54
0.78 0.68 2.77 12.06 (16.3')
Reference 13. * Reference 15. Reference 12. the distortions are N H 2 and NO2, respectively. The acceptor end in p-FA remains unaffected as compared to the ground state. It is seen that in the 'A, state of p-NA the amino hydrogens are displaced below the reference plane and the nitro group above the reference plane. The nonplanar distortions of the substituent fragments are less marked in the 'Al state than in the case of 3AI. The displacements from the reference plane in A for the atoms italicized are F@-FA) -0.03 M-12@-NA) 0.06 < OH 0.21 < F@-DFB) 0.27 < NH,@-FA) 0.65 < NO2 1.33 OH 0.28 < NH2@-NA) -0.40 < NH2(p-FA) 1.12 < NO2 1.69. There is considerable shrinkage of the ringsubstituent bonds in the 'A, a-a* state, except for the C-NO2 bond in p-NA which is lengthened by 0.078 A. The changes in the ring-substituent bonds in the 'A, states are more pronounced than the case in the triplet states 3A1. A comparison of the structures of the a-a* excited states of the substituted benzenes reflects the following interesting feature. In general, para disubstitution of the ring brings down the distortion a t the substituent end as compared to the situation in monosubstituted benzenes. Dipole Moments and Charges. The dipole moments for the four optimized states are listed in Table VI. There is more similarity in the dipole moments of the ground state with those of 'B2 than with 'Al or 3Al. Particularly striking is the large dipole moment of 12 D for 'A, of p-NA. A breakdown of the dipole moment in hybrid moments of atom polarization and chargetransfer component yields 90% of the latter. There is a substantial shift of charge from the site of the N H 2 group and adjacent carbon to the N O 2 group. In the ground states the electronegativity of the elements dominates the charge distribution. F, 0, and N atoms attract electrons from the adjacent atoms leaving the ring carbon at the substituent positions all positive with net charges of 0.27-0.1 7 except for the carbon adjacent to NO2 which is slightly negative (-0.01) because of the inductive effect of the oxygens. The next neighbor ring carbons, i.e., those between the substituent sites, are slightly negative with net charges -0.02 to -0.05 except the carbon in meta position to the NO2 group which carries a positive net charge of 0.05. There is little modification in this pattern in the 3A, state of p-FB, p-quinol, and p-FA except that the polarization in the ring is increased. The same holds for the singlet states of these three compounds. In p-NA the charge is shifted from the carbon at the NO2 group to the meta carbon for 3A, and 'Bz, leaving the former atom positive by 0.10 and 0.05 and the latter atom positive by 0.01 and negative by -0.03. In the
'A, state this site is not appreciably affected compared to the ground state. However, in this state there is a much more significant shift from the N H 2 site to the NO, giving rise to the increase dipole moment discussed above. Aromaticity. We had previously found2s3a reduction of the degree of aromaticity2' in the excited states of aromatic systems. This reduction is expected because the change in occupation from occupied to virtual a orbitals causes an increase in antibonding between adjacent carbon atoms.4 This trend is also observed in the compounds studied here, but the magnitude of the reduction is dependent on state and substituent. Whereas the aromaticity index in the ground states is between 1.70 and 1.72, close benzene's 1.75, we find the 'B, states moderately aromatic with indices between 1.54 and 1.56. In 'Al the aromaticity is further reduced with indices between 1.3 1 for p-FA and between 1.43 and 1.45 for the rest. It is only natural that also the triplets 3A1show little aromaticity with indices ranging from 1.29 for p-FA to 1.36-1.38 for the rest. In consequence there is an increased tendency for addition reactions in the sequence of the three excited states for all four molecules. Conclusion
We have investigated the lowest triplet and the two lowest singlets of the four para-disubstituted benzenes p-DFB, p-quinol, p-FA, a n d p N A for T-T* excitations. The sequence of excitations agrees with experimental and other theoretical work and the excitation energies seem satisfactory. For the triplet manifold in nitrobenzene we found a dissociative u-u* excitation close to the lowest-lying a-a* excitation, which might explain the phosphorescence behavior via internal conversion. The vertical repulsive u-u* excitation in p-NA is distinctively higher in energy and cannot couple efficiently with the a-a* excitation. So the phosphorescence of p-NA and the lack of phosphorescence of nitrobenzene have a natural explanation. Geometry optimization of the excited states reveals that the 3Al state is most affected leading to a quinoidal form of the ring and to out-of-plane distortions of the substituent atoms. This situation is similar to the one in the monosubstituted benzenes studied previ~usly.~ From the valence numbers and the spin densities of the carbon atoms in para position at the substituent sites these states can be best described as 1,4 diradicals. The quinoidal character of the ring is less pronounced in the singlet states. In general, para disubstitution reduces distortion at the substituent end as compared to the situation in monosubstituted benzenes. From the dipole moments the large moment of 12 D of the 'Al state of p-NA should be mentioned. It is due to charge transfer from the N H 2 to the NO2 site. Aromaticity is decreased in all excited states, mostly in the triplet. Acknowledgment. We thank Deutsche Forschungsgemeinschaft for partial support of this work. The calculations were performed with the CYBER 76 at Universitat Hannover. Registry No. p-DFB, 540-36-3; p-FA, 37 1-40-4; p-NA, 100-01-6;
p-quinol, 123-31-9. (27) Jug, K. J . Org. Chem. 1983, 48, 1344.
