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Electronic Spectra and Molecular Structure of Biphenyl and Para-Substituted Biphenyls in a Supersonic Jet Yuichiro Takei, Tetsuya Yamaguchi, Yoshihiro Osamura, Kiyokazu Fuke, and Koji Kaya* Department of Chemistry, Faculty of Science and Technology, Keio University, 3- 14- 1 Hiyoshi, Kohokuku, Yokohama 223, Japan (Received: March 27, 1987; In Final Form: July 21, 1987)
The electronic excitation spectra have been measured for jet-cooled biphenyl and para-substituted biphenyls in order to explore the electronic structures and their molecular conformations. The difference of the molecular structures between the ground and excited states is analyzed from the low-frequency progression which is assigned to the torsional vibration. The result shows that the perturbation of the substituents at the para position does not affect the structures of biphenyl derivatives in both ground and lowest excited singlet states. The ab initio MO calculations also support this fact and confirm that the molecular conformation is determined by subtle balance of the interactions of two phenyl rings. The equilibrium torsional angle of biphenyls is estimated to be about 40° in the ground state and nearly Oo in the lowest excited state. It is found that the character of the lowest excited singlet state of biphenyl is described by the linear combination of the lBlu state of benzene.
Introduction The molecular structure of biphenyl has been the subject of numerous investigations on the torsional angle between two phenyl rings, as a basic problem of structural organic and physical chemistry.' While the molecular structure of biphenyl in the crystalline state at ordinary temperature is known to be planar,2 the crystal undergoes phase transition to have a nonplanar structure in the low-temperature conditiona3+ The several optical studies in solution suggest that biphenyl has ca. 15-30' twisted structure from the planar geometry.'*15 The electron diffraction studies on the gaseous biphenyl conclude that the torsional angle is about 450.16J7 The twisted structure of biphenyl has been explained by the balance between the hydrogen-hydrogen repulsion in the ortho position of two phenyl rings and the electron delocalization effect. The former destabilization brings two aromatic rings perpendicular, while the latter stabilization tends to keep the .n-electrons in the same plane. These two opposing effects keep balance with each other so subtly that the dihedral angle might exhibit substantial variation with the change of the environment (gas, liquid, or solid phase). The ab initio calculation predicts relatively small barrier height of torsional motion, ca. 2-2.5 kcal/mol.I8 Biphenyl in various phases shows structureless electronic spectrum because of vibrational and rotational congestion, except for the low-temperature solid, which gives no precise information
on the structure of the molecules. Murakami, Ito, and Kayalg reported the electronic spectra of biphenyl and biphenyl-dlo in a supersonic jet which exhibit well-resolved vibrational structure. By the analysis of the long progression of the torsional vibration, it was suggested that the two phenyl rings in biphenyl may have a coplanar conformation in the excited electronic state in contrast to the twisted structure in the ground state. The electronic spectra of 9-phenylanthracenezoand bianthryl*I in supersonic jets also exhibit progressions of a torsional vibration and effective torsional potentials in So and S, have been so determined as to simulate the observed spectra. The present work aims to make the structural problem of biphenyl more quantitative by the analysis of the electronic spectra of para-substituted biphenyls from experiment and by the theoretical point of view. The substitution of a functional group to the para position of biphenyl does not affect the moment of inertia about the principal axis. Therefore, changes in the frequency and the progression pattern of torsional vibration in the electronic spectra of parasubstituted biphenyl may inform us of the direct information on the structural change in the ground and excited state. We have studied the electronic spectra of jet-cooled four-substituted biphenyls by use of multiphoton ionization (MPI) and laser-induced fluorescence (LIF) methods. They are, 4,4'-dichlorobiphenyl, 4,4'-difluorobiphenyl, 4-hydroxybiphenyl, and 4-biphenylcarbonitrile in addition to biphenyl and biphenyl-d,,. The experimental results were analyzed on the basis of ab initio calculation for the ground state of biphenyl and its derivatives.
(1) Newman, M. S. Steric Effects in Organic Chemistry; Wiley: New York, 1956; p 484. (2) Hochstrasser, R. M.; Sung, H. N. J . Chem. Phys. 1977, 66, 3265. (3) Cailleau, H.; Baudour, J. L.; Zeyen, C. M. E. Acta Crystallogr., Sect.
Experimental Section The experimental setup was the same as described in the previous paper.22 The gas mixture of heated sample (typically 340 K) and 3 atm of He was expanded into the expansion chamber through a 400-pm orifice at a repetition rate of 7 Hz and crossed by an output beam of the second harmonic of a N2 laser excited dye laser. The ions of biphenyl derivatives generated after the 1-photon resonant 2-photon ionization process were mass selected by a T O F mass spectrometer and the ion current from the channeltron multiplier was fed into a preamplifier and was averaged by a boxcar integrator. Dispersed fluorescence spectrum of biphenyl-d,, was measured by the use of a Spex 0.75-m monochromator using the second harmonic of a YAG laser excited
8 1979,835,426. (4) Cailleau, H.; Girard, A.; Moussa, F.; Zeyen, C. M. E. Solid State Commun. 1979, 29, 259. ( 5 ) Cailleau, H.; Moussa, F.; Mons, J. Solid State Commun. 1979,31,521.
( 6 ) Cailleau, H.; Moussa, F.; Zeyen, C. M. E.; Bouillot, J. Solid State Commun. 1980, 33,407. (7) Atake, T.; Chihara, H. Solid State Commun. 1980, 35, 13 1. (8) Atake, T.; Saito, K.; Chihara, H. Chem. Lett. 1983, 493. (9) Saito, K.; Atake, T.; Chihara, H. Chem. Lett. 1984, 531. (10) Suzuki, H. Bull. Chem. SOC.Jpn. 1959, 32, 1340. (11) Kurland, R. J.; Wise, W. B. J. Am. Chem. SOC.1964, 86, 1877. (12) Eaton, V. J.; Steele, D. J . Chem. Soc., Faraday Trans. 2 1973, 69, 1601. (13) Lim, E. C.; Li, Y. H. J . Chem. Phys. 1970, 52, 6416. (14) Uchimura, H.; Tajiri, A,; Hatano, M. Chem. Phys. Lett. 1975, 34, (15) Uchimura, H.; Tajiri, A,; Hatano, M. Bull. Chem. SOC.Jpn. 1981,
(19) Murakami, J.; Ito, M.; Kaya, K. J . Chem. Phys. 1981, 74, 6505. (20) Werst, D. W.; Gentry, W. R.; Barbara, P. F. J . Phys. Chem. 1985,
54, 3219. (16) Bastiansen, 0.;Samdal, S. J . Mol. Struct. 1985, 128, 59, 95, 115. (17) Bastiansen, 0. Acta Chem. Scand. 1949, 3, 408. (18) Hifelinger, G.; Regelmann, C. J. Comput. Chem. 1985,6, 368; 1987, 8 , 1057.
89, 729. Werst, D. W.; Londo, W. F.; Smith, J. L.; Barbara, P. F. Chem. Phys. Lett. 1985, 118, 367. (21) Yamasaki, K.; Arita, K.; Kazimoto, 0.;Hara, K. Chem. Phys. Lett. 1986, 123, 277. (22) Fuke, K.; Kaya, K. Chem. Phys. Lett., 1983, 94, 97.
34.
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The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
Takei et al. 58 cm-1 n
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a1
60cm-‘
WAVELENGTH inm)
35800
36000
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WAVENUMBER icrr-’ I
Figure 1. (a) 1-Photon resonant 2-photon ionization spectrum of bi-
phenyl-dlo.(b) Calculated Franck-Condon Factor corresponding to the torsional progression of biphenyl-d,o.
Figure 2. Dispersed fluorescence spectrum of biphenyl-d,,, excited at the
36 316-cm-’ band.
c;
dye laser (Quanta-Ray DCR-2). 4,4’-Difluorobiphenyl was synthesized from h y d r o q ~ i n o n eand ~ ~ was purified by sublimations. All the other samples were obtained commercially and were purified by repeated sublimations.
m
r---
Theoretical Calculation Ab initio S C F MO calculation was carried out with STO-3G minimal basis set for biphenyl and its para-substituted derivatives. The geometrical conformations and their energies of biphenyl, 4,4’-dichlorobiphenyl, 4,4’-difluorobiphenyl, and 4,4‘-biphenylcarbonitrile in the ground state were calculated by using the analytical gradient optimization technique implemented in GAMFSS program system.24 The electronic structure in the excited states was estimated with singly excited configuration interaction (SECI) method.
Electronic Spectra Murakami, Ito, and Kaya19 obtained the electronic spectra of biphenyl and biphenyl-dlo using the MPI method. The observed long progression of the torsional vibration in the spectra was explained in terms of the geometrical change of biphenyl along the dihedral angle of two phenyl rings between the ground and excited electronic states. By introducing a functional group at the para position of biphenyl, one might expect the perturbation effect on the structure which is reflected in the electronic spectra of biphenyl derivatives. In the electronic spectra of biphenyl, there exist two or three progression series of torsional vibration. The assignments of these progressions have been attempted in the previous paper using one- and two-photon absorption spectra.I9 Although these progression series also appear in the spectra of biphenyl derivatives, i&is difficult to assign these progressions using one-photon absorption spectra only. Thus we discuss only the main progression in the present paper. Table I summarizes the torsional frequencies of substituted biphenyls which will be discussed below. Biphenyl-d,,. Figure l a shows the 1-photon resonant 2-photon ionization spectrum of biphenyl-d,,. As seen in Figure l a , the progression starts at 36 075 cm-I. The long progression of the 60-cm-I vibration seen in Figure l a has been already assigned to the torsional vibrati0n.l’ The dispersed fluorescence spectrum of biphenyl-d,, excited at 36316 cm-l band is shown in Figure 2. The spectrum exhibits only one prominent band at 58 cm-I shifted from the band at the excitation wavelength and it can be
( 2 3 ) Schiemann, G.; Winkelmuller, W. Org. Synrh. 1943, Collect. Vol. 11, 188. (24) Dupuis, M.; Spangler, D.;Wendoloski, J. J. NRCC Software Catalog Vol. 1, program No. QGOl ‘GAMESS”, 1980.
IAVELLN5TH tnmj
Figure 3. 1 -Photon resonant 2-photon ionization spectrum of 4,4‘-di-
chlorobiphenyl. assigned to 1-0 transition of the torsional vibration in the ground state. In order to interpret the observied spectral pattern, the Franck-Condon factor (FCF) was calculated for the transitions from the v” = 0 of the ground state to individual vibronic levels of the excited state. By comparison of the calculated result with the observed spectral intensity distribution, the best fit of the calculated FCF shown in Figure l b was obtained by assuming that the dihedral angles of So and SI are 38’ and 0’ and that the 36 316-cm-’ band corresponds to the u” = 0 to u ’ = 17 transition. FCF calculation for various dihedral angles gives appreciable deviation from the observed spectral pattern when the angle exceeds 5’ from the planarity in the excited state. The calculated FCF for the transitions from u’ = 17 in the excited state to vibrational levels in the ground state also supports that the observed prominent band of 58 cm-’ in the fluorescence spectrum corresponds to the one quantum of torsional vibration in the ground state. According to the calculation, one also expects the appreciable bands at v‘ = 3, 6, and 9 in addition to the strongest v” = 1 band. However, poor S/N ratio of the spectrum in the figure does not allow one to confirm the calculation. Consequently, biphenyl would have a coplanar structure (dihedral angle = 0 f 5 ’ ) in the excited state in contrast to the twisted structure in the ground state, and the observed MPI spectrum of biphenyl-d,, is explained by the progression of the torsional vibration in which v‘extends from 13 to 27. 4,4’-Dichlorobiphenyl. As seen in Figure 3, 1-photon resonant 2-photon ionization spectrum of 4,4’-dichlorobiphenyl exhibits the long progression of 62 cm-’ which also can be assigned to the torsional vibration in the excited states. Apart from the band positions (the band starts at 35 945 cm-I), the spectral feature and the torsional frequency are very similar to biphenyl. It is, therefore, concluded that this molecule has more or less the same geometry as biphenyl in both the ground and excited states. 4,4 ‘-Difluorobiphenyl. Figure 4 displays 1-photon resonant 2-photon ionization spectrum of 4,4’-difluorobiphenyl. As seen in the figure, the spectrum starts at around 34 525 cm-’ and a
Spectra of Biphenyl
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 519
70 cm-'
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290
289
201 286 285 284 WAVELENGTH lnml
288
283
282
281
Figure 4. 1-Photon resonant 2-photon ionization spectrum of 4,4'-difluorobiphenyl.
296
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294
293
292
291
290
289
280
287
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285
-
WAVELENGTH I n m I
56 rm-'
torsional angle and the stabilization energy do not change when the para substituents are introduced in biphenyl. The fact that the dihedral angle is not affected by any substituent suggests that the interaction between two phenyl rings through C-C bond is small. It is noticeable that the bond lenght (1.5 1 A) of bridging C-C bond is almost that of the single bond and that the distance between ortho hydrogens in planar conformation is relatively large (1.96 A). These results clearly show that the twisted structure of biphenyl is determined by the subtle balance of the phenylphenyl interactions. The energy of the ortho hydrogen repulsion is competitive to the T-T interaction which would be the polarization effect rather than delocalization one. In order to obtain the vibrational frequencies of biphenyl, we have performed the vibrational analysis by using analytical gradient method. Table I11 shows all frequencies of biphenyl calculated with the STO-3G basis set. The torsional frequency in the ground state of biphenyl and biphenyl-dlowas obtained to be 70 and 63 cm-I, respectively. The observed frequency of 58 cm-I of biphenyl-dloin its dispersed fluorescence is in good agreement with the theoretical calculation. We have confirmed that the potential curvature of the torsional mode does not change much for all para-substituted biphenyls. Moreover, the substitution at para position of biphenyl does not affect the bond distance of C1-Cl, (1.505-1.508 A) as is shown in Table 11. 4,4'-Difluorobiphenyl has the shortest C1-Cl, bond length (1.505 A) among the molecules we have studied. The observed frequency 70 cm-l in the excited state of difluorobiphenyl is 10 cm-l larger than other biphenyls. This fact may suggest the shorter CI-ClJ bond length in the excited state as well as in the ground state of difluorobiphenyl.
Character of the Lowest Excited Singlet State of Biphenyl
LENGTH l n m l
Figure 5. Fluorescence excitation spectra of (a) 4-hydroxybiphenyl and (b) 4-biphenylcarbonitrile.
long progression of the torsional vibration with the spacing of 70 cm-I is observed. The progression pattern is similar to other biphenyl derivatives. The spectrum also contains another progression series of torsional mode starting at 252 cm-' above the first series. In addition to these progressions, a rather intense band appears at 827 cm-I above the 34 525-cm-I band. Above this band, the spectral feature begins to be very congested. As judged from the torsional progression, the dihedral angles in both the ground and excited states of difluorobiphenyl are more or less the same as those of other biphenyls. 4-Hydroxybiphenyl and 4-Biphenylcarbonitrile. Parts a and b of Figure 5 are the LIF spectra of jet-cooled 4-hydroxybiphenyl and 4-biphenylcarbonitrile, respectively. The spectra consist of 58-cm-' (band starts at 33 850 cm-I) and 56-cm-I (band starts at 35 025 cm-') vibrational progressions for hydroxybiphenyl and biphenylcarbonitrile, respectively. Again, these molecules are suggested to have dihedral angles similar to biphenyl in both ground and excited states.
Molecular Structure and Vibrational Frequencies for Ground-State Biphenyls In order to understand not only the above spectral data but also the nature of phenyl-phenyl conjugation, we have carried out the ab initio calculation using STO-3G basis set on biphenyl derivatives. Table I1 summarizes the molecular structures of biphenyl and para-substituted biphenyl and their energies in the ground state. As seen in the table, the most stable conformation of biphenyl has the dihedral angle of 38' and this equilibrium structure is stabilized by 2.1 and 2.4 kcal/mol from the coplanar and perpendicular conformations, respectively. Note that the
There have been several precise theoretical studies on the excited electronic states of biphenyl, where the low-lying two excited states are assigned to 'Bjg and IB2,,in DZh~ y m m e t r y . ~One ~ - ~of~ the important problems to be solved is that these lowest states can be correlated with either the lBzuor 'Blu states of benzene with D6h symmetry. In the case of the B1, (S2)state of benzene as the origin, one can expect the change of electronic character due to the substituent because the intramolecular charge-transfer (CT) character is increased by the introduction of substituents like C1, F, and so on. In the case of B2,, character (SI state) of benzene as the origin of the low-lying state of biphenyl, C T character can be neglected by the s u b s t i t ~ t i o n . ~In~ the latter case, one may expect no appreciable substitution effect upon the electronic spectra. Since no distinct change in the feature of the electronic spectra is observed by para substitution to biphenyl as is described above, it seems to be reasonable to identify the observed excited state of biphenyl to be originated from the lBzustate of benzene. The character of the excited state of biphenyl discussed above is also confirmed by analyzing the wave function calculated with the SECI method. The wave function of the lowest excited singlet (IB3 in D2 symmetry) state is described as a linear combination of the lBzustate of benzene, and this state becomes degenerate at the 90" twisted conformation. Although this state is the lowest among the excited singlet states at the twisted geometries, the SECI calculation gives the result that the lBlu state is the lowest at the planar (DZh)conformation because of large mixing of the T molecular orbitals between two phenyl rings. The IB1, state involves the excitation from HOMO to LUMO at DZhgeometry and the wave function of this state consists of the linear combination of the IB1, state of benzene. Since the SECI method offers only qualitative feature, further rigorous calculation would be necessary in order to obtain the reliable excitation energies and potential energy surfaces. (25) (26) (27) (28) (29) (30)
Imamura, A,; Hoffmann, R. J . A m . Chem. SOC.1968, 90, 5379. Ducker, R. P.; McClain, W. M. J . Chem. Phys. 1974, 61, 2609. Sagiv, J.; Yogev, A.; Mazur, Y . J . A m . Chem. SOC.1977.99, 6861. McLaughlin, T. G.; Clark, L. B. Chem. Phys. 1978, 31, 11. Dick, B.; Hohlneicher, G. Chem. Phys. 1985, 94, 131. Kimura, K.; Nagakura, S. Mol. Phys. 1965, 9, 117.
580 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
Takei et al.
J . Phys. Chem. 1988, 92, 581-586 TABLE 111: Fundamental Frequencies of Biphenyl-b Obtained by ab Initio Calculation A , cm-I B,, cm-’ B1,cm-’ E?, cm-’ 70 348 484 857 1035 1160 1192 1217 1385 1508 1814 1948 3711 3734 3747
477 702 1032 1I 5 3 1180 1190 1234 1377 1777 1939 3710 3732 3746
109 416 566 707 839 885 1108 1164 1206 1278 1374 1564 1743 1921 3724 3738
143 310 642 721 837 939 1126 1160 1206 1273 1374 1543 1705 1903 3725 3740
Noting that the biphenyls have similar low vibrational frequencies about the torsional mode in both the ground state and excited state, we may conclude that both electronic states have similar potential curvature about the rotation of phenyl rings, and both states would have similar molecular structures except the twisting angle. In order to induce the appreciable change in the electronic spectra of biphenyl derivatives, the introduction of two different types of substituent such as NH2 and NO2in different phenyl rings might be necessary.
58 1
Concluding Remarks Both the observation of electronic spectra and the ab initio calculation of biphenyls show that the substitution at para position does not affect the electronic character and molecular structure in both the ground and lowest excited states. The C1-CI, bond connecting two phenyl rings is almost single bond, and the molecular conformation may be determined by the subtle balance between ortho-hydrogen repulsion and 7-7 interaction. While the twisting angle of biphenyls studied here is ca. 40° in the ground state, the potential minimum would be located at nearly planar conformation in the excited state. We can conclude that the lowest excited singlet state of biphenyls has a character of S1 (IB2,,) state of benzene from the analysis of the wave function. Acknowledgment. The ab initio calculation was carried out at the computer center of the Institute for Molecular Science using HITAC M-680H. We thank Mr. N. Sato for the computation of Franck-Condon factor and Mr. M. Aoyagi for the preliminary M O calculation. This work is partly supported by the Grantin-Aid for Scientific Research from Ministry of Education, Science and Culture. Registry No. 4,4’-Dichlorobiphenyl, 2050-68-2; 4,4’-difluorobiphenyl, 398-23-2; 4-hydroxybiphenyl, 92-69-3; 4-biphenylcarbonitrile, 2920-38-9; biphenyl, 92-52-4; biphenyl-d,,,, 1486-01-7. Supplementary Material Available: Table lisiting position of individual vibronic bands in the spectra of biphenyl-dlo,4,4’-difluorobiphenyl, 4,4’-dichlorobiphenyl, 4-hydroxybiphenyl, and 4-biphenylcarbonitrile ( 5 pages). Ordering information is given on any current masthead page.
Resonance Raman Studies of Monomerlc and Oxygen-Bridged Dimeric Iron Octaethylchlorin Nancy J. Boldt and David F. Bocian* Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 1521 3 (Received: May 22, 1987: In Final Form: July 14, 1987)
Resonance Raman (RR) spectra are reported for OECFeCl and (OECFe),O (OEC = trans-octaethylchlorin) with excitation in the B, Q,, and Qy absorption regions. All of the observed in-plane chlorin skeletal modes are assigned by analogy to those of NiOEC. Both the symmetric and antisymmetric Fe-0-Fe stretching modes are observed in the RR spectra of the iron chlorin dimer. The frequencies which are observed for these two modes yield F A stretching and stretchstretch interaction constants which are, in general, slightly larger than those which are observed for (QEPFe)20 (OEP = octaethylporphyrin). The larger force constants for the chlorin relative to the porphyrin dimer are attributed to increased 7 bonding in the bridging ligand of the former complex. The RR intensity enhancement patterns for OECFeCl and (OECFe)20are nearly identical upon excitation in the Qy region and also upon excitation in the Q, region. In contrast, the intensity patterns for the monomer and dimer are distinctly different upon Soret excitation. These trends are different from those observed for OEPFeCl and (OEPFe),O in which the RR intensity enhancement patterns are the same upon B-state excitation as well as upon Q-state excitation. It is suggested that the differences which are observed in the B-state RR intensity enhancement patterns of the iron chlorin monomer and dimer could reflect the presence of excitonic interactions between the €3 excited states of the low-symmetry dimer.
Introduction A number of biological functions are mediated by proteins and enzymes which contain iron dihydroporphyrins (chlorins) rather than iron porphyrins. For example, green heme proteins,’12 m y e l o p e r ~ x i d a s e s, ~ ~l f~m~y~o g l o b i n ,s~~~l ~ f h*e~m o g l o b i n ,and ~~*~ (1) Babcock, G. T.; Ingle, R. T.; Oertling, W. A,; Davis, J. C.; Averill, B. A.; Hulse, C. L.; Stufkens, D. J.; Bolscher, B. G. J. M.; Wever, R. Biochim. Biophys. Acra 1985, 828, 58. (2) Andersson, L. A.; Loehr, T. M.; Lim, A. R.; Mauk, A. G. J . Biol. Chem. 1984, 259, 15340. (3) Sibbet, S.S.;Hurst, J. K. Biochemistry 1984, 23, 3007. (4) Stump, R. F.; Deanin, G. G.; Oliver, J. M.; Shelnutt, J. A. Biophys. J . 1987, 51, 605. (5) Morell, D. B.; Chang, Y.; Clezy, P. S. Biochim. Biophys. Acta 1967, 136, 121.
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microbial hemes d and dI39*-l1all contain reduced pyrrole prosthetic groups. Modification of the basic tetrapyrrole structure results in significant alteration of the photophysical, redox, and (6) (a) Peisach, J.; Blumberg, W. E.; Adler, A. Ann. N.Y Acad. Sci. 1973, 206, 310. (b) Berzofsky, J. A.; Peisach, J.; Blumberg, W. E. J . Biol. Chem. 1971, 246, 3367. (7) Brittain, T.; Greenwood, C.; Barber, D. Biochim. Biophys. Acta 1982, 705, 26. (8) Lemberg, R.; Barrett, J. In Cytochromes; Academic: London, 1973; pp 233-245. (9) Timkovich, R.; Cork, M. S.;Gennis, R. B.; Johnson, P. Y . J. A m . Chem. SOC.1985, 107, 6060. (10) (a) Chang, C. K. J . Biol. Chem. 1985, 260,9520. (b) Chang, C. K.; Wu, W. J . Biol. Chem. 1986, 261, 8593. (1 1) Chang, C. K.; Barkigia, K. M.; Hanson, L. K.; Fajer, J. J. Am. Chem. SOC.1986, 108, 1352.
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