J . Phys. Chem. 1988, 92, 5120-5122
5120
rotation, and internal motion of the olefin should be approximately and 60%,20respectively. These values do 5-10%,14 13-1 not sum to 100%because different molecular systems are involved in the various experiments. The trajectory data given in Tables 11, 111, and VI1 are in reasonable accord with these results. Although there are a number of points of agreement between experiment and theory, the comparisons indicate that several features of the empirical potential surface are in error. The agreement between the calculated onset for the addition reaction with ethylene and that measured by Grover et d6for benzene 5%,z1923
suggests that the potential barrier for the addition reaction (R3) is too large. Comparison with the activation energy reported by Kapralova et aL3 also indicates this to be the case. The fact that the energy partitioning into H F vibrational modes is less than that obtained in the chemiluminescence studiesz1provides evidence that the transition state predicted by the empirical potential for four-center H F elimination is too productlike. Registry No. CH2FCH2F, 624-72-6; CH,=CHF, 7664-39-3.
75-02-5; H F ,
Fluorescence from the l1BuState of Diphenylhexatriene: Inversion of the IIB, and 2lA, Levels in CS, Bryan E. Kohler* and Takao Itoh Department of Chemistry, University of California. Riverside, California 92521 (Received: January 14, 1988)
Fluorescence and fluorescence excitation spectra of diphenylhexatrienehave been measured at 77 K and at rmm temperature in different solvents. As the polarizability of the solvent increases, the energy of the llB, state decreases much more rapidly than does the energy of the 2lA, state. At an effective polarizability (n2 - l)/(n2 + 2) 0.45 the llB, state crosses below the 2'A, state. Thus, the majority of the fluorescence can originate from either the 2'A, state or the llB, state depending on the polarizability of the matrix. In CSzat 77 K diphenylhexatriene fluoresces entirely from the llB, state. Franck-Condon analysis of the 77 K fluorescence shows that displacements of the C-C and C=C stretching modes are significantly smaller in the l'B, state than they are in the 2lA, state.
Introduction The ordering of excited singlet states in diphenylpolyenes has been the focus of a number of experimental studies since Hudson and Kohler reported that the lowest energy excited singlet state in diphenyloctatetraene was the 2'A, state and not the l'B, state as had been previously believed.l~zThis state ordering, 2IA, below 1 B,, obtains for diphenylbutadiene,j" diphenylhe~atriene,~.' diphenyloctatetraene,6 and, presumably, longer diphenylpolyenes in the gas phase. In the condensed phase in solvents with low polarizability (n-alkanes and hydrocarbon glasses) the 2'A, state clearly lies lower than the 1IB, state in diphenylhexatriene,s-ll diphenyloctatetraene,l,*diphenyldecapentaene, and diphenyldodecahexaene.lz In diphenylbutadiene these states are nearly isoenergetic, so that the apparent order can differ in different experiment^.'^-^^ In hydrocarbon solution the difference between diphenylhexatriene 2lA, and 1'B, excitation energies is sufficiently (1) Hudson, B. S.; Kohler, B. E. Chem. Phys. Lett. 1972, 14, 299. (2) Hudson, B. S.; Kohler, B. E. J . Chem. Phys. 1973, 59, 4984. (3) Heimbrook, L. A,; Kohler, B. E.; Spiglanin. T. A. Proc. Narl. Acad. Sci. U.S.A. 1983, 80, 4580. (4) Shepanski, J. F.; Keelan, B. W.; Zewail, A. H. Chem. Phys. Lett. 1983, 103, 9. (5) Horwitz, J. S.; Kohler, B. E.; Spiglanin, T. A. J . Chem. Phys. 1985, 83, 2186. (6) Itoh, T.; Kohler, B. E. J . Phys. Chem. 1988, 92, 1807. (7) Kohler, B. E.; Spiglanin, T. A. J . Chem. Phys. 1984, 80, 5465. (8) Hudson, B. S.; Kohler, B. E.; Schulten, K. Excited States 1982, 6, 1. (9) Birks, J. B.;Tripathi, G. N. R.; Lumb, M. D. Chem. Phys. Lett. 1978, 33, 185. (10) Alford, P. C.; Palmer, T. F. Chem. Phys. Lett. 1982, 86, 248. ( 1 1 ) Felder, T. C.; Choi, K.-J.; Topp, M. R. J . Chem. Phys. 1982, 64, 175. (12) Horwitz, J. S.; Itoh, T.; Kohler, B. E.; Spangler, C. W. J. Chem. Phys. 1987, 87, 2433. (13) Bennett, J. A,; Birge, R. R . J . Chem. Phys. 1980, 73, 4234. (14) Velsko, S. P.; Fleming, G. R. Chem. Phys. Lett. 1982, 76, 3553. (15) Goldbeck, R. A.; Twarowski, A. J.; Russell, E. L.; Rice, J. K.; Birge, R R.; Switkes, E.; Kliger. D. S. J . Chem. Phys. 1982, 7 7 , 3319.
0022-3654/88/2092-5 120$01.50/0
TABLE I: Solvent Dependence of Origin Band Positions for Room-Temperature Solutions of Diphenylhexatriene a = l'B, abs IIB, em (n2 - I ) / origin," origin! solvent n (n2 + 2) cm-1 cm-1 vacuum 1.oooo 0 29 160 per fluorohexdne 28050 1.2515 0.1588 pentane 1.3575 0.2193 27 170 26380 2:l pentane:benzene 1.4122 0.2489 26880 26120 0.2618 26770 26010 1: 1 pentane:benzene 1.4367 1:2 pentane:benzene 1.4596 0.2737 26530 25920 26390 25820 1.5011 0.2947 benzene 25580 1.6319 0.3567 cs2
2lA, abs
origin,c cm-1 25 740 25 150 24930 24850 24820 24785 24735
'For l'B, absorption, i~= (29800 - 11 700a) cm-I. *for I1B, emission, (29090 - 11 700a) cm". 'For 2IA, emission, 5 = (25710 - 3420a) cm-'. I=
small ( N 1000 cm-]) that dual fluorescence can be observed.10J618 Even though room-temperature hydrocarbon solutions of diphenylhexatriene and diphenyloctatetraene fluoresce from both the 2'Ag and l'B, states, the l'B, fluorescence is almost completely obscured by the much stronger 2lA, fluorescence, and little beyond the approximate shape of the origin band can be extracted. It is well established that the llB, excitation energy decreases significantly with increasing solvent polarizability (approximately lo4 cm-' per unit change in (nz - l)/(n2 2)), while the 2IA, excitation energy is more nearly c o n ~ t a n t . ~ Thus, ~ * ~ in ~ ~a ~ ~ ~ sufficiently polarizable solvent it should be possible to invert the 2'Ag and l'B, levels of diphenylhexatriene and observe the full
+
(16) Alford, P. C.; Palmer, T. F. J . Chem. SOC.,Faraday Trans. 2 1983, 79, 433.
(17) Itoh, T.; Kohler, B. E. J . Phys. Chem. 1987, 91, 1760. (18) Allen, M. T.; Miola, L.; Whitten, D. G. J . Phys. Chem. 1987, 91, 6099. (19) Jones, G . R.; Cundall, R. B. Chem. Phys. Lett. 1987, 126, 129. (20) D'Amico, K. L.; Manos, C.; Christensen, R. L. J . Am. Chem. SOC. 1980, 102, 1777.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5121
1 IB, State of Diphenylhexatriene
YI C
al C
L)
u
0
2 ; >
25000
m
I 24000 I8000
20080
22000
24000
26000
Wavenumbers 23000 0.00
0.10
8.20
0.30
0.40
0.50
Polarizability
Figure 1. Origin band centers as a function of solvent polarizability (n2 + 2). Band centers for fluorescence from the 2'A, state (*), fluorescence from the llB, state (+), and excitation to the l'B, state (I) were determined as described in ref 17. Reading from left to right, the points are for the vapor, perfluorohexane, pentane, 2:l by volume pentane and benzene, 1:l by volume pentane and benzene, 1:2 by volume pentane and benzene, benzene, and CS2 solution. - l)/(n2
I'B, fluorescence spectrum. This has been accomplished at room temperature by applying high pressure to hexane and toluene solutions of diphenylhe~atriene.'~ The realization of this experimental goal for 77 K solutions and the Franck-Condon analysis of the resulting spectrum are reported in this paper.
Experimental Section Diphenylhexatriene obtained from Aldrich Chemical Co. was purified by repeated recrystallizations. Spectroscopic grade solvents were obtained from Mallinckrodt Inc. or Fluka Chemical Corp. Fluorescence and fluorescence excitation spectra were measured with a SPEX D M l B spectrofluorimeter. Samples were contained in l-cm path length quartz cuvettes for the room-temperature experiments and in a cell made from 8-mm quartz tubing that was immersed in liquid N, for the 77 K experiments. Emission spectra were corrected for the spectral sensitivity of the monochromator and photomultiplier, and excitation spectra were corrected for the variation of the intensity of the exciting light with wavelength. Digital data were transferred to a Hewlett Packard 98 16 microcomputer for analysis. Results The dependence of 2'A, and 1'B, excitation energies on solvent polarizability for solutions of diphenylhexatriene is easily measured. Reasonably accurate estimates of fluorescence and fluorescence excitation origin band centers can be derived by the Gaussian fitting procedure described previo~sly.'~Solvent polarizabilities are approximated by (n2- l)/(n2 + 2), where n is the refractive index tabulated in the CRC Handbook. Figure 1 shows the origin band centers determined by fitting Gaussians to the room-temperature spectrum for diphenylhexatriene 2'A, and 1'B, fluorescence and l'B, fluorescence excitation as a function of solvent polarizability. The data shown in Figure 1 are summarized in Table I. As expected, the l'B, fluorescence and fluorescence excitation origins show significant and similar shifts to lower energy with increasing solvent polarizability, while the 2lA, fluorescence origin shifts only slightly. Extrapolation of these data predicts that the order of the 2lA, and l'B, levels will be inverted in a solvent of polarizability (n2- l)/(n2 2) E 0.45. This can be achieved in CS2 at low temperatures, taking advantage of the high intrinsic polarizability and the fact that polarizability scales with density, which increases with decreasing temperature. For CS, the density of the crystal is 1.554 g cm-3 (ref 21), which would
+
(21) Donnay, J. D. H.; Donnay, G.; Cos, E. G.; Kennard, 0.;King, M. V. Crysral Dura, Determinatiue Tables; Monograph No. 5 ; American Crystallographic Association: New York, 1963; p 539.
in
28000
30000
l/cm
Figure 2. Fluorescence and fluorescence excitation spectra for hexane at 77 K (upper curves) and CS2 at 77 K (lower curves) solutions of diphenylhexatriene. increase the polarizability to at least -0.44. Fluorescence and fluorescence excitation spectra of diphenylhexatriene in hexane and in CS2 at 77 K are shown in Figure 2. As the host polarizability increases, the 1'B, excitation origins shift to lower energy. Although the position of the fluorescence origin in pentane, hexane, or benzene is almost the same, there is a significant shift to lower energy in CS2. In CS2 there is good overlap between fluorescence and fluorescence excitation origins, and the spectra show a reasonable mirror image relationship. The Franck-Condon envelope of the fluorescence seen in CS2is qualitatively different from that of the fluorescence seen with pentane, hexane, or benzene hosts, resembling more closely the Franck-Condon envelope of the fluorescence excitation spectrum. All of this is consistent with the idea that the diphenylhexatriene fluorescence seen in CS2 at 77 K comes from the llB, state, which has crossed below the 2'A, state to become the lowest excited singlet state, SI.
Discussion Fluorescence from diphenylhexatriene in CS2 at 77 K is assigned as llBu(Sl) llAg(So)because of the following: (1) There are mirror image symmetry and good overlap between the fluorescence and the llB, excitation spectrum. (2) The fluorescence origin at 23 500 cm-' is reasonably close to the energy extrapolated from the dependence of llB, energy on solvent polarizability (29090 - (1 1 700 X 0.44) = 23 900 cm-I). (3) The Franck-Condon envelope of the fluorescence spectrum in CS2 is significantly narrower than that seen when pentane, hexane, or benzene is the solvent. This narrower envelope closely resembles the mirror image of the envelope for l'A, to l'B, absorption. This assignment is confirmed by the following analysis. Assuming a linear relationship between excitation energy and solvent polarizability, the energy of the 1 IB, excitation origin E( 1 lB,) and the energy of the 2'A, fluorescence origin E(2'A.J will be given by
-
E(llB,) = Eo(llB,) - K2a
(1)
E(21Ag) = E'(2'A.J
(2)
- Kla
where E O ( l'B,) and E0(21A,) are, respectively, the energies of the 1'B, excitation origin and 2'Ag fluorescence origin in the vapor phase, Kl and K, are constants, and a is the solvent polarizability. Rearranging, we have E(l'B,) = Eo(l'B,)
- K2(AEo - AE)/(K2
- Kl)
(3)
E(2'Ag) = E0(21A,) - Kl(AEo - AE)/(K, - K1)
(4)
where AEo = E0(l1B,) -E0(2'Ag) and AE = E(llB,) -E(2'Ag). The energies of the fluorescence excitation and fluorescence origins are plotted against A E in Figure 3. The data are well fit by relations 3 and 4 except for the values measured in CS2. The large deviation for the points measured in CS2 is due to the fact that the emitting state has changed from 2IAg to l'B,.
5122
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988
Kohler and Itoh
x
-
e
M
C
al
+ c
H
ld
I 24888
o
ma
iaaa
15aa maa ?sa8 Stokes S h i f t
Wavenumbers 3899
3599
Figure 3. l'B, excitation energy (origin band centers from 77 K fluorescence excitation spectra plotted as I) and 2'A, excitation energy (origin band centers from 77 K fluorescence spectra plotted as *) versus the apparent Stokes shift or difference between these numbers. From left to right the solvents are CS2, benzene, hexane, pentane, perfluorohexane, and the vapor phase.
TABLE II: Mode Displacements in Units of (amu)'/* A from Franck-Condon Fits' to the 77 K Spectra solvent C-C C=C C-C C=C
mode fluorescence fluorescence excitation excitation
hexane 0.200 f 0.002 0.213 f 0.001 0.170 f 0.005 0.192 i 0.003
benzene 0.209 f 0.001 0.207 f 0.001 0.168 f 0.004 0.194 f 0.002
0.168 0.172 0.163 0.177
CS2 f 0.001 f 0.001 f 0.003b f O.OOlb
a Three independent harmonic modes assumed to have ground-state frequencies of 525, 1154, and 1601 cm-I and excited-state frequencies of 540, 1265, and 1585 cm-I, respectively. Low-frequency mode not included in the fit.
The Franck-Condon envelope of the fluorescence spectrum in CS2 is markedly different from that seen in benzene or hexane. To treat this difference more quantitatively, we have fit the spectra measured at 77 K by spectra calculated for three separable harmonic modes by varying the mode displacements and the width of the Gaussian line-shape function. The mode frequencies were fixed at 525, 1154, and 1601 cm-' in the ground state and at 540, 1265, and 1585 cm-I in the excited state. Since the computed Franck-Condon factors are much less sensitive to frequency changes than to displacements, the exact choice of frequencies is somewhat ambiguous. The frequencies used are consistent with frequencies extracted from high-resolution studies. The observed spectra are well fit as is shown in Figure 4. The results of this analysis are summarized in Table 11. The displacements of the
in
l/cm
Figure 4. Calculated (dashed line) and measured (solid line) fluorescence spectra for diphenylhexatriene in benzene a t 77 K. The sticks represent the spectrum before convolution with the Gaussian line-shape function. The calculated spectrum was determined by adjusting the electronic energy, the Gaussian width, the C-C and C=C displacements, and an overall scale factor. The inclusion of a third mode (-530 cm-') improved the calculated line shape but had no effect on the best-fit C-C and C=C displacements.
C-C and C=C modes in CS2 as obtained from the fluorescence spectrum differ significantly from those obtained when hexane or benzene was the solvent but correspond quite closely to the displacements obtained by fitting the excitation spectra. This further supports the assignment of SI in CS2as llB, since this is exactly what is expected if in going from hydrocarbon solvent to CS, the emitting state changes from 2'A, to llBu.
Conclusion Although in the vapor phase or in a hydrocarbon matrix at 77 K the 2 ' 4 state of diphenylhexatriene is the lowest energy excited singlet state, in a CS2 matrix the lowest excited state is llBu. That is, when the host polarizability is sufficiently high, the order of the 2'A and l'B, levels reverses so that the emission originates from l8BU in a highly polarizable host (a 2 0.45). The Franck-Condon analysis of the excitation and emission spectra at 77 K shows that displacement of the C-C and C=C stretching modes of polyene chain is smaller in the l'B, state than in the 2'AB state. All the available information on band positions and spectral shapes as a function of host properties is consistently accounted for by assuming that the 2IA, and llB, levels invert in CS2. Acknowledgment. This work was supported in part by grants from the National Science Foundation (CHE 8514873) and the National Institutes of Health (EY 06466). Registry No. CS2,75-1 5-0; diphenylhexatriene, 1720-32-7; benzene, 7 1-43-2; hexane, 110-54-3; pentane, 109-66-0; perfluorohexane, 355-42-0.
