J . Phys. Chem. 1987, 91, 1040-1047
1040
Raman sDectra and the G C analyses are in agreement with the photochemical reactions expected in the presence of a triplet sensitizer and support the idea that triplet species are intermediates in the sensitized photoisomerizations. The time-resolved resonance Raman spectra of the lowest triplet states produced from the E and Z isomers of 2,Sdimethyl1,3,5-hexatriene are similar but show clear differences. On this basis it is concluded that these triplet states must have different geometries or that different mixtures of triplet species are produced from the two isomers. The simplest explanation to this fact is that the triplet species formed from both isomers are twisted around the central carbon-carbon bond and that the differences in conformation around C-C single bonds in the ground state are preserved upon excitation to the lowest triplet state. The twisted geometry corresponds to a diallylic structure and is in agreement with theoretical predictions. Moreover, the preservation of ground-state conformations around C< single bonds in the excited state indicates that the NEER principle applies to the lowest triplet states of E-DMH and Z-DMH. In addition, through a comparison between hexatriene, heptatriene, octatriene, and 2J-dimethylhexatriene a partial assignment is suggested for the resonance
Raman spectra of the lowest triplet states of thesecompounds.
Note Added in Proof. Recent investigation^^^ show that cZt rather than cZc is the predominant conformation of (Z)-2,5-dimethyl- 1,3,5-hexatriene in the ground state. Although this is different from our assumption in the present paper, our qualitative arguments and conclusions are unaltered by this change. Acknowledgment. We thank G. Olsen for help with the excimer laser and H. Egsgaard for help with the G C analyses. We thank Dr. 0. F. Nielsen, Chemical Laboratory V, University of Copenhagen, for recording of ground-state Raman spectra and for fruitful discussions, and Drs. K. B. Hansen and A. H. Sillesen for a continual improvement of the instrumentation. We are grateful to Drs. R. McDiarmid and M. H. Baron for sending us preprints prior to publication. The work was partially supported by the Danish Natural Science Research Council. Registry No. (E)-2,5-Dimethyl-l,3,S-hexatriene, 41233-74-3; (Z)2,s-dimethyl-1,3,5-hexatriene,49839-76-1. (74) Brouwer, A. M.; Cornelisse, J.; Jacob, H. J. C. Tetrahedron, submitted for publication.
Time-Resolved Resonance Raman Spectroscopy of the Lowest Excited Triplet States of the E and Z Isomers of 1,3,5-Hexatriene Frans W. Langkilde, Niels-Henrik Jensen, and Robert Wilbrandt* Chemistry Department, R i m National Laboratory, DK-4000 Roskilde, Denmark (Received: June 24, 1986)
The lowest excited triplet states of the E and Z isomers of 1,3,5-hexatriene were produced by laser flash photolysis using acetone as sensitizer. Acetone was excited at a wavelength of 308 nm and the time-resolved resonance Raman spectra of the lowest excited triplet states of the triene isomers were obtained by subsequent probing in resonance with the T, T1 absorption at 315 nm. The resonance Raman spectra of the triplet states of the two isomers were identical within limits of error. Five transient Raman bands were observed from both isomers. Bands were detected for the E isomer at 1570, 1270, 1234, 1200, and 1106 cm-'and for the Z isomer at 1569, 1270, 1236, 1199, and 1107 cm-'. The apparent first-order decay rate constants of the triplet states were 1.09 X lo's-' for (E)-hexatriene and 1.04X lo7s-' for (a-hexatriene. Ground-state Raman spectra and GC analysis of samples before and after the sensitized photolysis indicated a higher efficiency of Z E than of E Z isomerization from the triplet states. From these overall results a common relaxed geometry of the lowest triplet states of the E and Z isomers of 1,3,5-hexatrienewith a twisted central bond seems the most likely structure in solution. Isomerization efficiencies suggest that the structure of the twisted triplet species is closer to that of the E than to that of the Z isomer.
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1. Introduction Short polyenes have attracted renewed interest in the past decade.] The state ordering of the singlet manifold has been the main issue in these studies. Another fundamental question, albeit less extensively studied, is that of the properties of the potential energy surfaces in ground and excited states. 1,3,5-Hexatriene is the simplest polyene with a central double bond and therefore of particular fundamental interest. Of the two isomers, (E)-1,3,5-hexatriene (E-HT) is known to have a predominantly planar tEt conformation with respect to double and single bonds in the ground state, whereas (Z)-1,3,5-hexatriene (Z-HT) has been reported to have a tZt conformation being twisted by 10' around the central double bond.2 Additional less stable s-cis conformers have been found in low-temperature matrix isolation s t u d i e ~ . ~ . ~ ( I ) Hudson, B. S.; Kohler, B. E.; Schulten, K. In Excited States: Lim, E. C., Ed.; Academic: New York, 1982; Vol. 6, pp 1-95. (2) Traetteberg, M. Acta Chem. Scand. 1968, 22, 628, 2294. ( 3 ) Furukawa, Y . ;Takeuchi, H.; Harada, I.; Tasumi, M. J. Mol. Struct. 1983, 100, 341.
(4) Tasumi, M.; Nakata, M. J. Mol. Struct. 1985, 126, 1 1 I.
0022-3654/87/2091-l040$01 S O / O
Of the excited singlet states of hexatriene the l1BUtstate is the one best characterized by optical ab~orption,~-~ electron loss,8and preresonance Raman9 spectroscopy, whereas attempts to locate the 2lA,- state have been unsuccessful10 partly because of the complete absence of fluorescence from hexatriene.6 The question of state ordering is still a matter of discussion, and the potential energy surfaces in both the 1]B,+ and the 2]A; states are largely unknown from an experimental point of view. The triplet states of hexatriene have been studied by direct singlet-triplet absorptionsJ' and electron loss spectroscopy.8 From the former the vertical triplet energy of the lowest triplet state ( 5 ) Minnaard, N. G.; Havinga, E. Red. Trau. Chim. 1973, 92, 1179. (6) Gavin Jr., R. M.; Risemberg, S.;Rice, S. A. J . Chem. Phys. 1973, 58, 3160. (7) Leopold, D. G.; Pendley, R. D.; Roebber, J. L.; Hemley, R. J.; Vaida, V. J . Chem. Phys. 1984,81,4218. (8) McDiarmid, R.; SabljiE, A,; Doering, J. P. J . Am. Chem. SOC.1985, 107, 826. (9) Myers, A. B.; Mathies, R. A,; Tannor, D. J.; Heller, E. J. J. Chem. Phys. 1982, 77, 3857. (10) Fujii, T.; Kamata, A,; Shimizu, M.; Adachi, Y . ;Maeda, S. Chem. Phys. Lett. 1985, 115, 369. (11) Evans, D.F. J . Chem. Soc., 1960, 1735.
0 1987 American Chemical Society
Spectra of 1,3,5-Hexatriene
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
1041
of E-HT was determined as 196.1 kJ/mol and that of Z-HT as 199.4 kJ/mol in methylene iodide. N o detailed data on triplettriplet absorption spectra or the triplet lifetime of hexatriene are given in the literature. However, the lowest triplet state has been reported to show a T, TI absorption maximum around 3 15 nm and a lifetime around 300 ns for neoalloocimeneI2 and (E,E)1,3,5-heptatriene.13 A number of theoretical calculations on excited states of hexatriene have appeared in the literature. These include both ab initio and semiempirical calculation^.^"^^ Of particular interest in the context of the present work are results on triplet states. The energies of the two lowest triplet states of E-HT are 261.5 and 416.8 kJ/mol from ab initio work.I4 The shape of the potential energy surface of the lowest triplet state along selected internal coordinates has been investigated theoretically by a number of 1000 700 iew im author^.^^.'^^'^ The two most recent calculations agree in the WAVENUMBER/cm-l prediction of a twisting around the central carbon-carbon bond Figure 1. Raman spectra of Ar-saturated solutions of (A-G) 5.2 X lo-’ which results in a diallylic structure and stabilizes the lowest triplet M E - H T in acetonitrile containing 0.57 M acetone (solution I), and state by 2416 or 7 kJ/mol’* compared to the planar biradical (A’-G’) 1.0 X tO-* M 2 - H T in acetonitrile containing 0.58 M acetone structure. In an earlier c a l c ~ l a t i o nthe ~ ~planar biradical, a di(solution 11). A and A’, spectra of fresh solutions obtained with probe allylic, and a methylenepentadienyl structure were found to be pulse only. 9-F and 9°F’: at delays of B and B’, 60 ns; C and C’, 100 isoenergetic. ns; D and D’, 200 ns; E and E’, 300 ns; F and F’, 1.06 gs between pump Vibrational spectroscopy has been widely used to characterize and probe pulses. G and G’, probe-only spectra obtained after the repolyenes in ground as well as excited states. The Raman and IR cording of spectra A-F and A‘-F‘. Pump wavelength 308 nm, probe wavelength 315 nm. S, solvent; GE, ground state of E-HT; GZ, ground spectra of E-HT and Z-HT in their ground states have been state of 2 - H T . Each spectrum obtained by averaging 50 scans from reported and analyzed by a number of In recent individual pulses. years it has become possible as well to obtain vibrational spectra from excited triplet states25of molecules in solution by using the excitation has been rationalized taking three main categories of technique of time-resolved resonance Raman spectroscopy, initially effects into account: (i) a-electronic interactions, (ii) nonbonded developed in our laboratory.26 The resonance Raman spectra interactions (steric hindrance), and (iii) influence of the of a number of polyenes such as carotenoid^,*^*^^ r e t i n a l ~ , ~ ~ . ~ ~ ground-state conformational equilibrium. Considerations involving d i p h e n y l b ~ t a d i e n e ,and ~ ~ recently (E,E)-I,3,5-heptatriene,13 the last effect have led to the conclusion of nonequilibration of (E,E,E)-2,4,6-0ctatriene,~~ (E)-2,5-dimethylhexatriene, and excited rotamers (NEER) in excited singlet states. The main (2)-2,5-dimethylhe~atriene~~ in excited triplet states have been photochemical reaction from triplet states of hexatriene has been reported. reported to be E Z or Z E isomerization leading to a The photochemistry of hexatrienes has been reviewed in detail photostationary-state distribution containing -25% Z-HT when p r e v i o ~ s l y .The ~ ~ photochemical product distribution upon direct biacetyl is used as ~ e n s i t i z e r .Other ~ ~ products possibly include 1,3-~yclohexadieneand its dimer^.^^,^^ In the present paper the triplet states of E-HT and Z-HT are produced by triplet energy transfer from acetone in a laser flash (12) Gorman, A. A,; Hamblett, I. Chem. Phys. Lett. 1983, 97, 422. (13) Langkilde, F. W.; Wilbrandt, R.; Jensen, N.-H. Chem. Phys. Lett. photolysis experiment. The resonance Raman spectra of E-HT 1984, 111, 372. and Z-HT in their triplet states are recorded and the photoisom(14) Nascimento, M. A. C.; Goddard 111, W. A. Chem. Phys. 1979, 36, erization from one isomer to the other with the triplet state as 147; Chem. Phys. Lett. 1979, 60, 197. intermediate is observed by Raman spectroscopy and gas chro(15) BonaEiE-Koutecky, V.; Ishimaru, S. J . Am. Chem. SOC.1977, 99, matography. A preliminary report on part of this work has been 8134. published previo~sly.~’ (16) Ohmine, I.; Morokuma, K. J . Chem. Phys. 1980, 73, 1907. +-
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(17) Lasaga, A. C.; Aerni, R. J.; Karplus, M. J . Chem. Phys. 1980, 73, 5230. (18) Said, M.; Maynau, D.; Malrieu, J.-P. J . Am. Chem. SOC.1984,106, 580. (19) Tai, J. C.; Allinger, L. J . Am. Chem. SOC.1976, 98, 7928. (20) Lippincott, E. R.; White, C. E.; Sibilia, J. P. J . Am. Chem. SOC.1958, 80, 2926. (21) Lippincott, E. R.; Kenney, T. E. J . Am. Chem. SOC.1962,84,3641. (22) RakoviE, D.; Stepanyan, S. A.; Gribov, L. A.; Panchenko, Yu.N. J. Mol. Struct. 1982, 90, 363. (23) Curry, B. U. Ph.D. Thesis, University of California, Berkeley, CA, 1983. (24) McDiarmid, R.; Sabljic, A. J . Phys. Chem., submitted for publication. (25) Wilbrandt, R.; Jensen, N.-H.; Pagsberg, P.; Sillesen, A. H.; Hansen, K. B. Nature 1978, 276, 167. (26) Wilbrandt, R.; Pagsberg, P.; Hansen, K. B.; Weisberg, C. V. Chem. Phys. Lett. 1975, 36, 76. (27) Jensen, N.-H.; Wilbrandt, R.; Pagsberg, P. B.; Sillesen, A. H.; Hansen, K. B. J . Am. Chem. SOC.1980, 102, 7441. (28) Wilbrandt, R.; Jensen, N.-H. In Time-resolved Vibrational Spectroscopy; Atkinson, G. H., Ed.; Academic: New York, 1983; pp 273-285. (29) Wilbrandt, R.; Jensen, N.-H. J . Am. Chem. SOC.1981, 103, 1036. (30) Wilbrandt, R.; Jensen, N.-H.; HouEe-Levin, C. Photochem. Photobioi. 1985, 41, 175. (31) Wilbrandt, R.; Grossman, W. E. L.; Killough, P. M.; Bennett, J. E.; Hester, R. E. J . Phys. Chem. 1984, 88, 5964. (32) Langkilde, F. W.; Jensen, N.-H.; Wilbrandt, R. Chem. Phys. Lett. 1985, 118, 486. (33) Langkilde, F. W.; Jensen, N.-H.; Wilbrandt, R.; Brouwer, A. M.; Jacobs, H. J. C. J . Phys. Chem., preceding paper in this issue.
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2. Materials and Methods Acetonitrile (Merck, LiChrosolv), cyclopentane (Fluka, UV spectroscopy grade), and acetone (Ferak, p.a.) were used as received. Solvents or mixtures of solvents with acetone were purged with Ar for 35 min before use. 1,3,5-Hexatriene was obtained from Aldrich as a mixture of the isomers. The two isomers were separated by using preparative gas chromatography as described in ref 38 with a few modifications. A stainless steel tube 5 m long with 6 mm i.d. was packed with 10% P,P-dipropylpropanenitrile on 60/80 mesh Chromosorb W,AW (No. 7800 B, Mikrolaboratoriet) and installed in a Perkin-Elmer F21 preparative gas chromatograph. Temperatures used were injection 75 OC, column 50 OC, manifold oven (split) 100 OC, and connection tube from oven to manifold 90 OC. Carrier gas was N2. The separated isomers were collected under N2 in cold traps in a dry ice/2propanol bath and retrieved from the traps by washing these with small Ar-saturated batches of acetonitrile or cyclopentane. The (34) Jacobs, H. J. C.; Havinga, E. Adv. Photochem. 1979,11, 305-373. (35) Minnaard, N. G.; Havinga, E. Recl. Trau. Chim. 1973, 92, 1315. (36) Chuang, M.; Zare, R. N . J . Chem. Phys. 1985, 82, 4791. (37) Langkilde, F. W.; Jensen, N.-H.; Wilbrandt, R. In Time-resolved Vibrational Spectroscopy; Laubereau, A,, Stockburger, M., Eds.; Springer Proc. Phys. Springer-Verlag: West Berlin, 1985; Vol. 4, p 175. (38) Hwa, J. C. H.; de Benneville, P. L.; Sims, H. J. J . Am. Chem. SOC. 1960, 82, 2537.
1042 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Langkilde et al.
concentrations of E-HT and 2 - H T were determined by UV absorption spectroscopy (see section 3) and proper amounts of acetone and acetonitrile/cyclopentane added to the solutions. All manipulations, including transfer to sample cells, were performed in a glovebox under Ar. The purity of the resulting materials was analyzed by analytical GC, using the chromatographic system described above or in some cases an analytical gas chromatograph equipped with a BPO/OV-l phase capillary column as described p r e v i o ~ s l y .The ~ ~ solutions of E-HT contained less than 1% of the 2 isomer. The solutions of 2-HT were 98% in the 2 isomer, 2% in the E isomer. The isomeric composition was measured again after the samples had been exposed to laser pulses during the time-resolved Raman measurements. However, as no internal standard was used, G C allowed only a determination of the concentrations of E-HT and 2-HT relative to each other but not of the absolute concentrations of the isomers. Time-resolved Raman spectra were recorded in the same way as described in detail in the preceding Le., by using a pump wavelength of 308 nm and a probe wavelength of 315 nm. Individual Raman spectra were obtained by automatic averaging over 50 single-pulse spectra using a rotating cell. Further details of the procedures used in constructing the spectra are given in section 3.
3. Results Pulsed Raman spectra were recorded of (I) 5.2 X 10-3 M (E)-hexatriene in an Ar-saturated solution of 0.57 M acetone in M (2)-hexatriene in an Ar-satacetonitrile and (11) 1.0 X urated solution of 0.58 M acetone in acetonitrile. Corresponding experiments were carried out using cyclopentane as solvent. Hexatriene concentrations were obtained from the optical absorption spectra mentioned in section 2 of solutions without acetone by using the maximum extinction coefficients in n-heptane from the 1iteratu1-e.~As the procedures in the treatment of spectra were modified somewhat compared to the previous paper due to qualitative differences in the appearance of the spectra, they shall be described in detail. Recorded Raman spectra are shown in Figure 1. Spectra obtained from (Qhexatriene, Le., solution I, with different time delays between pump and probe laser pulses are shown on the left-hand side. Each spectrum was obtained by computer averaging of 50 single-pulse spectra. All seven spectra were recorded using the same sample, Le., without changing solution between experiments. The sequence of experiments was as follows: (1) recording of a spectrum (A) by using the probe laser only; (2) recording of a total of six spectra by using both pump and probe lasers at various time delays between the laser pulses; of these the five spectra at 60 (B), 100 (C), 200 (D), and 300 (E) ns and 1.06 p s (F) delay are shown in Figure 1; (3) again recording a spectrum (G) by using the probe laser only. Corresponding spectra were obtained from 2-HT Le., solution 11, and are shown in Figure 1 on the right-hand side (labeled A’ to G’). All spectra in Figure 1 are scaled in the same way, i.e., relative intensities can be compared directly (apart from small variations in probe laser intensity). However, the differences in the concentration of E-HT and Z-HT should be noted. As an overall result the transient spectra B to F indicate the appearance of a number of new Raman bands and of a transient absorbance compared to the probe-only spectrum of Figure 1A. Both the new bands and the absorbance decay within a few hundred nanoseconds. Qualitatively similar results are seen from Figures 1B’ to 1F’. In order to investigate whether products different from the starting material had built up during the two series of experiments shown in Figure 1, the first spectrum (A) of Figure 1 is subtracted from the last one (G). The result is shown in Figure 2A. Before the subtraction procedure spectrum 1A was multiplied by 1.03, this value being different from 1 due to variations in probe laser power. It was chosen because it gives the best cancellation of solvent bands in the resulting difference spectrum (Figure 2A). The corresponding difference spectrum, obtained by substraction of spectrum 1A’ from 1G’ is shown in Figure 2A’. In Figure 2
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Figure 2. Difference (A and A’) and ground-state (B and B’) Raman spectra from E-HT (solution I, spectra A and B) and Z-HT (solution 11, spectra A‘and B’), A: 1G - 1.03 X 1A. A‘: 1G’- 0.98 X 1A’. A and A’ scaled up by a factor -6.5 compared to the lower spectra B and B’. B and B’: after subtraction of bands from acetonitrile and acetone. S, solvent. Probe wavelength 315 nm.
the difference spectra are scaled up relative to spectra A and A‘ of Figure 1. Finally, for comparison, the ground-state Raman spectra of E-HT (Figure 2B) and 2-HT (Figure 2B’), obtained with the probe laser alone, from fresh solutions I and I1 are shown as well. In these latter two spectra, bands from the solvent and sensitizer have been subtracted leading to small artifacts at the positions (1372 and 917 cm-I) of the strongest solvent bands. Gas chromatographic analysis provided an independent measure of the relative concentrations of E-HT and 2 - H T before and after the series of experiments of Figure 1. For both isomers this analysis showed a conversion of the starting isomer to the other, the ratio of E-HT:Z-HT after the experiments of Figure 1A-G being 0.92:0.08 and after those of Figure lA’-G’ being 0.09:0.91. No signs of species other than the hexatriene isomers could be identified in the chromatograms, although results from Raman measurements (see below) clearly showed a substantial disappearance of hexatriene during photolysis. In Figure 3 transient Raman bands are inspected more closely with respect to positions and intensities. For this purpose spectra with improved signal to noise ratio were obtained and two different subtraction procedures were used and evaluated in the construction of transient difference spectra. These details shall be described now. For the Raman spectra shown in Figure 3 a sequence of four individual spectra, each obtained with 50 laser pulses, was recorded for each of four samples of fresh solution contained in the rotating cell. Such a sequence consisted of the following: (1) a spectrum recorded with the probe laser only (as in spectra 1A and IA’); (2) a spectrum recorded with both pump and probe lasers with a time delay of 60 ns between the two laser pulses (as spectra 1B and 1B’); (3) a spectrum recorded with both pump and probe lasers, however, with a time delay of 1.06 ps between the two laser pulses (as spectra 1F and 1F’); and finally (4) a remeasurement of the first spectrum with the probe laser alone (as spectra 1G and 1G’). Each of the four kinds of spectra was averaged over the four sequences of experiments on fresh solutions by using the procedures described previously33and the resulting spectra are shown in Figure 3. Figure 3A shows the first probe-only spectrum from E-HT (solution I), 3B the pump-and-probe spectrum with 60 ns delay, and 3C the one with 1.06 ps delay. Figure 3D shows a constructed
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1043
Spectra of 1,3,5-Hexatriene
TABLE I: Vibrational Wavenumbers (cm-I) of Observed Preresonance-Enhanced Raman Bands Assigned to Modes of ap and a1 Symmetry in the Ground States and of Observed Transient Bands in the Triplet States of E-HT and 2-HTin the Spectral Region 800-1700 cm-I ground state triplet state E-HT in CH,CN Z-HT in CH$N E-HT in CH$N 2-HT in CH$N E-HT in C5Hlo Z-HT in C5HI0 1627 vs 1625 vs 1570 vs 1569 vs 1569 vs 1569 vs 1576 w 1572 w 1270 s 1270 s 1267 s 1267 s 1398 w 1398 w 1234 w 1236 w 1233 w 1232 w 1288 m 1319 m 1200 m 1199 m 1200 m 1197 m 1191 s
1248 s 1082 w
1139 vw’ 1106 s 600 vw‘
1106 s
1107 s 600 vwa
1106 s
‘Doubtful. I
1
WAVENUMBERIcm-’
Figure 3. Raman spectra of (A-E): E-HT (solution I), (A’-E’): 2-HT (solution 11). A-C and’A‘-C‘: each spectrum obtained from 200 scans in total from four independent series of experiments on fresh solutions (see text); A and A‘, with the probe pulse only; B and B’, at a delay of 60 ns between pump and probe pulses; C and C’, at a delay of 1.06 ps betweem pump and probe pulses. D, E, D’, and E’, time-resolved difference spectra. D, B - 0.48 X A; E, B - 0.48 X C; D’, B’ - 0.41 X A‘; E’, B’ - 0.44 X C’. Spectra B and B’ are scaled up by a factor 2 and spectra D, E, D’, and E’ by a factor 20 compared to spectra A, C, A‘, and C’. S , solvent.
difference spectrum obtained by multiplying spectrum 3A by 0.48 and subtracting this rescaled spectrum from 3B; Figure 3E shows a constructed difference spectrum obtained by multiplying spectrum 3C by 0.48 and subtracting this rescaled spectrum from 3B. Spectra 3A’ to 3E’ are obtained from Z-HT (solution 11) in the same manner. G C analysis was performed for Figure 3 as described for Figure 1. The ratio of E-HT:Z-HT was greater than 0.99:O.Ol before, and was 0.98:0.02 after the experiments of Figure 3A-C, and 0.02:0.98 before, 0.05:0.95after, the experiments of Figure 3A%’. In Figure 4 the two types of difference spectra were averaged for each isomer. Figure 4A shows the average of 3D and 3E and spectrum 4A’ the corresponding average of 3D’ and 3E’. The reasons for using this procedure shall be discussed in section 4. The positions of Raman bands of ground-state E-HT and Z-HT determined from the spectra of Figure 2B,B’ and the transient bands determined from Figure 4A,A’ are listed in Table I. Apart from the spectral region shown in Figures 1-4 spectra were recorded down to 300 cm-I. No transient Raman bands which can be quoted with certainty were seen from either E-HT or Z-HT in this region. However, a very weak feature may be present around 600 cm-’ for both isomers. To investigate the time behavior of the transient spectra the intensity of the strongest transient Raman band at 1570 cm-I from E-HT relative to the strong solvent band at 1372 cm-’ is determined by integration of the respective Raman bands in the spectra B to E of Figure 1, and the logarithm of the ratio is plotted as a function of time in Figure 5 along with the corresponding result for Z-HT. Time-resolved Raman spectra were also recorded by using cyclopentane as solvent. The transient absorbance, as seen from the Raman spectra, was lower by a factor of 1.6 than in aceto-
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Figure 4. Final difference spectra assigned to the triplet states of (A): E-HT (solution I) and (A’): Z-HT (solution 11). A, average of spectra 3D and 3E; A‘, average of spectra 3D’and 3E’.
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Figure 5. Logarithmic plot of the normalized ratio of the integrated intensity of the strongest transient Raman band at 1570 cm-’ to that of the solvent band at 1372 cm-I as a function of time delay between pump and probe pulses for E-HT (obtained from spectra 1B-E) and Z-HT (from spectra 1B’-E’). Solid line, E-HT; dashed line, 2-HT.
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nitrile, and the intensity of the transient spectra was somewhat lower. This can probably be explained by a T, TI absorption spectrum shifted to the UV compared with that in acetonitrile. A consequence of this shift is less absorbance at 3 15 nm and lower resonance enhancement of the transient Raman spectra. Apart
1044 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
from these differences the spectra were very similar to those obtained in acetonitrile. The positions of the bands observed in cyclopentane are tabulated in Table I as well. They are subject to a somewhat larger uncertainty than those in acetonitrile. 4. Discussion
The general mechanisms leading to the production of the triplet states of trienes have been discussed in detail in the preceding paper. As the absence of methylation of hexatriene at the 2- and 5-positions is not expected to change any of the arguments given there, the reader is referred to that discussion. Hence we assume in the following that the excitation of acetone by the pump pulse leads to the production of the triplet states of E-HT and Z-HT. Direct excitation of hexatriene can be neglected under the present conditions: the absorbance of acetone over 1 cm path length at the pump laser wavelength of 308 nm was -2 at the concentrations used, while that of hexatriene (measured independently) was less than 0.02. Hence more than 99% of the pump energy is expected to be absorbed by the sensitizer. The Raman spectra at different time delays between pump and probe pulses shown in Figure 1 for both E-HT and Z-HT and the probe-only spectra before and after the series of experiments allow some qualitative conclusions to be drawn. (i) At 60 ns delay between pump and probe pulses (Figure 1 B,B’) the intensity of the spectrum is attenuated by a factor of -2 compared to the probe-only case (Figure 1A,A’). This attenuation disappears at longer time delays. We attribute this effect to the production and TI subsequent decay of triene triplets causing a transient T, absorbance at the probe wavelength of 315 nm and possibly at higher wavelengths where the Raman bands are detected, in correspondence with the experiments on other trienes reported earlier. The qualitative appearance of this transient absorbance is similar for E-HT and 2-HT. This shall be discussed in more detail below. (ii) A number of new Raman bands not seen in the probe-only spectra (Figure 1A,A’) appear in the pump-and-probe spectra (Figure 1B-F and 1B’-F’), They are most intense at the shortest time delays (Figure 1B,B’) and decrease in intensity at longer delays. These are also attributed to the triplet states of the respective isomers of hexatriene. Details regarding their positions and relative intensities shall be discussed below. (iii) M In spite of the low concentrations of trienes used (5.2 X for E-HT and 1 .O X M for Z-HT) Raman bands from the ground states of both isomers are seen with considerable intensities. This is due to preresonance enhancement as the wavelength of excitation (315 nm) is relatively close to the first strong dipoleallowed electronic transition (the absorption maxima of E - H T and 2 - H T in acetonitrile are both found at 256 nm). Taking the different concentrations of E-HT and Z-HT into account it is seen that the peak intensity of the strongest Raman band around 1626 cm-l is weaker by a factor of 1.4 for Z-HT compared to E-HT and the band from Z-HT at 1248 cm-’ is weaker by a factor of 1.9 than the band at 1191 cm-I from E-HT. These differences are considerably smaller than in the case of (E)- and (2)-2,5dimethylhe~atriene,~~ a fact which is probably explained by the higher degree of planarity and the different conformation around C-C single bonds of Z - H T compared to (2)-2,5-dimethylhexatriene. (iv) In the context of the present work the fact that the ground-state Raman spectra of both E - H T and Z-HT are clearly seen, together with the presence of characteristic bands for each isomer at positions where no bands from the other isomer are seen, is very useful. This allows us to analyze the isomeric purity of the solutions by inspection of the ground-state Raman bands of the two hexatriene isomers. As an example the probe-only spectra for 2-HT before (Figure 1A’) and after (Figure IC’) the series of transient experiments can be compared. Even without using subtraction procedures it is clearly seen that the band at 1248 cm-l characteristic for Z-HT has decreased in intensity, while a new band around 1191 cm-I, characteristic of E-HT has built up after the experiments, indicating a photoisomerization of Z-HT to E-HT during the recording of the transient spectra. The quantitative details of this isomerization shall now be discussed together with results obtained by G C analysis of some of the
-
-
-
Langkilde et al. samples before and after the time-resolved Raman experiments. Photochemistry. In order to obtain quantitative results regarding the efficiency of photoisomerization of the two isomers of hexatriene into each other and into other possible photoproducts the differences between Raman spectra obtained before and after transient experiments were investigated more closely by constructing the difference spectra shown in Figure 2. In spectra 2A and 2A‘ both negative and positive bands are seen. As the subtraction procedure to obtain these spectra is carried out such that solvent bands cancel out as exactly as possible, the appearance of negative bands in this type of spectra indicates the disappearance of solute molecules and thus conversion into other species. In Figure 2A,A’ negative bands are seen at positions characteristic for E-HT and Z-HT, respectively. E-HT shows a characteristic strong Raman band at 1191 cm-I and Z-HT one at 1248 cm-I, none of which overlap with any band of considerable intensity of the other isomer. Consequently the intensity of these negative bands relative to that of the corresponding positive bands in Figure 1A,A’ or 2B,B’ can be used to determine the amount of the starting isomer that has disappeared during the sequence of transient experiments consisting of 300 pump-and-probe pulses. On the other hand the positive bands at the same positions in Figure 2A,A’ allow us to determine the amount of the isomer different from the starting one that is produced during the same sequence of experiments. The following results are obtained from an analysis of the Raman bands at 119 1 and 1248 cm-’ in Figure 2A,A’ by using the factor 1.9 mentioned above. (i) Experiments starting M) with E-HT: -20% of the starting concentration (5.2 X M has disappeared, while 0.7 X of E-HT, Le., 1.04 X M 2-HT has been formed. (ii) Experiments starting with Z-HT: -21% of the starting concentration (1.0 X lo-* M) of 2-HT, i.e., M Z-HT has disappeared, while 1.3 X low3M E-HT 2.1 X has been formed. These results are subject to some error in the determination of relative band intensities and it should be noted that the values for the isomers formed as products are subject to a somewhat larger uncertainty than those for the isomers that disappeared, as the former involves use of the above-mentioned intensity factor of 1.9 for the bands at 1191 and 1248 cm-I. The experimental data for the disappearance of material show that the amount of hexatriene that disappeared is twice as high in experiments with Z-HT as in those with E-HT. In spite of the higher starting concentration of Z-HT relative to E - H T approximately the same number of triplet hexatriene molecules is expected to be produced in the two series of experiments as the triplet energy transfer from acetone to both E-HT and 2 - H T is expected to occur with an efficiency close to unity. This is supported by the similar transient absorbance observed from E-HT and Z-HT as estimated from the similar factors used in the subtraction procedures of Figure 3D, E and D’, E’. Further assuming that E-HT and Z-HT form a common relaxed triplet state (see below), the differences in product formation lead us to conclude that the observed consumption of hexatriene does not occur via the relaxed triplet state. This conclusion is consistent with the results found in the sensitized photoisomerization of retinals39and &carotenem obtained with various sensitizers other than acetone, where no consumption of the polyene took place. With these considerations in mind the data can be used to estimate isomerization quantum yields from the lowest excited triplet state, using the equation
-
N =P
+ 4ETT+ 4zTT
with
4ET
+ 4ZT = 1
where N is the number of acetone triplet molecules produced in the absence of hexatriene, P is the number of product molecules (39) Jensen, N.-H.; Wilbrandt, R.; HoueC-Levin, C.; Bensasson, R. V. In Primary Processes in Biologv and Medicine; Bensasson, R. V., Jori, G., Land. E. J., Truscott, T. G., Eds.; Plenum: New York, 1985; p 105. (40) Jensen, N.-H.;Nielsen, A. B.; Wilbrandt, R. J . Am. Chem. SOC.1982, 104, 6117.
Spectra of 1,3,5-Hexatriene produced different from hexatriene, and Tis the number of relaxed triplet-state molecules of hexatriene which partition into the ground states of the E and Z isomers with quantum efficiencies +ET and C$zT, respectively. These equations are valid under the following conditions. (1) The number of photons absorbed by the sensitizer is identical for experiments with E-HT and 2 - H T . (2) The ground-state absorbance of E-HT and Z-HT,at the pump wavelength of 308 nm is negligible relative to that of the sensitizer. This has been discussed above. (3) The quantum yield of triplet energy transfer from acetone to either E-HT or Z-HT is close to unity. This has been discussed in detail in the previous paper.33 (4) E-HT and Z-HT form a common relaxed triplet state (see below), which upon decay partitions into the respective ground states, giving rise to the observed conversion between the isomers. ( 5 ) Products accounting for the observed consumption of hexatriene are entirely formed by processes competing in some way with the formation of the relaxed triplet state of hexatriene. As possible processes one could think of the quenching of excited singlet acetone molecules by reactions with ground-state hexatriene, the formation of charge-transfer exciplexes between acetone and hexatriene, the formation of free radicals4’ from acetone reacting with ground-state hexatriene, or finally reactions of hexatriene via nonrelaxed triplet states. (6) The ratio of the photostationary concentrations of E-HT and Z-HT is independent of this product formation. This holds only if product formation occurs at a much slower rate than isomerization. (7) No mechanisms other than that via the relaxed triplet state are active in inducing isomerization from one isomer to the other. Under these assumptions an analysis of the obtained data shows that of the number of photons absorbed by acetone in the experiments with E-HT, 26% lead to photoisomerization, 13% to products other than hexatriene, and the remaining 61% to the return from the triplet to the ground state of E-HT. In the experiments with Z-HT 49% lead to photoisomerization, 30% to product formation, and 21% to the return to ground-state Z-HT. Hence, the ratio of partitioning of the relaxed triplet state into ground-state E-HT and Z-HT C$ET/C$zT is found to be 2.3. This yields 30% Z-HT and 70% E-HT in the photostationary state in reasonable qualitative agreement with the values of 25% and 75% reported in the l i t e r a t ~ r e .In ~ ~view of the assumptions made it is obvious that the resulting ratio for and @should be regarded as tentative. In particular assumption 6 is certainly not rigorously correct and assumption 7 is doubtful in view of the unknown pathways of product formation, which possibly may lead to photoisomerization as well. However, as the aim of the present work is mainly a spectroscopic investigation of the lowest triplet state of hexatriene, a more thorough discussion of the triplet state photochemistry must await more extensive experimental results. Such work is in progress. If the results from G C analysis of the solutions used to obtain Figure 1 are compared with those from the Raman spectra of Figure 2A,A’, it is seen that a somewhat higher isomerization efficiency is derived from the Raman data than from the G C analysis for both isomers of HT. This may be qualitatively explained by an incomplete mixing of the solution under the Raman experiments with the rotating cell, leading to higher local product concentrations at the top of the cell, where the pump laser beam enters and where the Raman measurements are made, than at the bottom. In spite of the considerable amount of products other than hexatriene formed, no such products were detected in either Raman spectra (Figure 2A,A‘) or G C analysis. The absence of any detectable Raman bands from products points to species with either lower exctinction coefficients and/or absorption spectra shifted to the UV compared to hexatriene, leading to a lower Raman intensity. Evaluation of Transient Raman Spectra. The conversion of as much as 21% of the starting isomer of hexatriene to the other isomer and to other species after 300 pump-and-probe pulses indicated that less pulses should be used to obtain reliable transient (41) Leuschner, R.; Fischer, H . Chew. Phys. Lett. 1985, 121, 554
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1045 trjplet Raman spectra of the pure isomers. Hence the series of experiments described to obtain Figure 3 was performed. Both Raman and GC analyses performed in the same way as described for Figure 2 showed that not more than 5% of the starting isomer had disappeared after any of the sequences of experiments underlying Figure 3. This means that the crucial transient spectra (Figure 3B,B’) were obtained from samples with an isomeric purity in the bulk solution better than -95%. Although, as pointed out previ~usly,~~ the local concentration of photoproducts in the volume irradiated by the laser beam is probably higher than in the bulk solution, we feel confident that the exchange of the irradiated volume of solution from pulse to pulse by the rotation of the cell is sufficient to ascertain reliable spectra from each of the separate isomers under the present conditions. To determine exact positions and intensities in transient spectra, two types of difference spectra were constructed; these are shown in Figure 3, D and E, for E-HT and in Figure 3, D’ and E’, for Z-HT. Although these difference spectra are subject to some errors to be discussed below, this subtraction method has proven most useful to extract information on weak transient Raman bands from the spectra obtained. In both types of difference spectra it is seen that using the scaling factors indicated in the figure caption the solvent band at 1372 cm-’ is undersubtracted and that at 917 cm-’ oversubtracted suggesting that the subtraction is approximately correct in the region in between where all of the weak transient Raman bands occur. This type of error in subtraction is due to the wavelength dependence of the transient T, T, absorbance of hexatriene as discussed previously for the other trienes.33 The wavelengths corresponding to these two solvent bands are 329 and 324 nm and we thus conclude that the T, T, absorption spectra of both E-HT and Z-HT extend to a wavelength of at least 324 nm. Additional points should be noted for each of the two kinds of difference spectra 3D and 3E (and 3D’ and 3E’): When constructing spectra 3D and 3D’, in the spectrum to be subtracted (3A and 3A’) bands from the starting isomer are slightly overrepresented and bands from the product isomer after isomerization are underrepresented, as spectra 3A and 3A’ are recorded before 3B and 3B‘. This situation is reversed in the construction of the difference spectra 3E and 3E’. Hence both types of spectra contain errors, but by adding them together, these errors are cancelled out to a large extent. These errors are seen in spectra 3D and 3E’ as a slight oversubtraction at 1191 cm-’, Le., the position of a band of E-HT, and as a slight undersubtraction at 1248 cm-’, the position of a band of Z-HT. Vice versa, in spectra 3E and 3D’ the band at 1191 cm-’ is slightly undersubtracted and that at 1248 cm-I slightly oversubtracted. The final spectra 4A and 4A‘ are used to determine wavenumbers and relative intensities of the transient triplet Raman bands of E-HT and Z-HT. These are tabulated in Table I. An important question in the light of the previous paper is whether the triplet Raman spectra from E-HT and Z-HT are identical with or different from each other. The spectra shown in Figure 4, and the wavenumbers and intensities of Raman bands given in Table I, indicate that within limits of error, the triplet resonance Raman spectra from E-HT and Z-HT are identical. W e note that there seems to be a very weak band in spectrum 4A from E-HT at 1 139 cm-I which is not seen in spectrum 4A’ from Z-HT, but considering its low intensity, this cannot be taken as evidence against identical triplet spectra of E-HT and Z-HT. This conclusion of identical spectra is in contrast to the results for (E)- and (Z)-2,5dimethylhexatriene discussed in the previous paper. The simplest interpretation of this result is that either E-HT or Z-HT or both, having tEt and tZt conformations in their ground states, respectively, undergo torsion around the central double bond upon excitation to the lowest triplet state and thereby either find a common minimum along this torsional coordinate or populate different minima which are in equilibrium with each other. This interpretation is supported by the identical rates of triplet decay derived from the Raman spectra of the two isomers (see below). Alternatively, various minima, separated by energy barriers sufficiently high to prevent equilibration within the triplet lifetime at room temperature, may
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1046 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Langkilde et al.
be populated on the complex triplet potential energy surface. As the identity of the triplet Raman spectra, however, would suggest that identical distributions are obtained starting from either E-HT or Z-HT, this possibility seems less likely. In contrast to the case of 2,5-dimethyl- 1,3,5-hexatriene the Z and Z E isomerization are clearly seen efficiencies of E to be different for 1,3,5-hexatriene both from the Raman spectra and from the analysis by GC. Assuming a common relaxed triplet state for E-HT and Z-HT on the basis of the transient Raman spectra reported here, the different isomerization efficiencies seem to indicate a minimum of the triplet energy surface which is shifted relative to the maximum of the ground-state energy surface toward the tEt-conformation, Le., to smaller torsional angles of the central carbon-carbon bond. If several minima in equilibrium with each other are populated these could include one with essentially tEt conformation and one with a twisted central carbon-arbon bond. Tentative Spectral Assignment. The Raman spectra of (E)and (Z)-hexatriene in the ground state have been reported prev i o ~ s l and y ~ ~have ~ ~recently been reassigned on the basis of normal coordinate a n a l y ~ i and s ~ ~improved ~ ~ ~ spectra.24 Furthermore the preresonance Raman spectrum of E-HT has been discussed in detail by Myers et aL9 Our present work shows the preresonance Raman spectra of E-HT and Z-HT (Figure 2B,B'), and in additional experiments not shown here, the normal Raman spectra of the two isomers were remeasured by using an excitation wavelength of 514.5 nm. The preresonance Raman spectrum of Z-HT has not been reported before. The strongest preresonance enhancement in this spectrum is seen for the bands at 1625 and 1248 cm-'. Somewhat less enhanced is the band at 1082 and even less the bands at 1319 and 1398 cm-l. The band at 1082 cm-' has been previously assignedz3 to the symmetric C-C stretching mode in Z-HT, while the band at 1248 cm-' has been assigned to a CCH bending mode. The stronger enhancement of the latter compared to the former appears surprising as it contrasts with the enhancement pattern for longer polyenes such p-~arotene,~' where both these modes are resonance enhanced, however, with the symmetric C-C stretching mode usually the more intense of the two. Although an assignment of the transient spectra at the present stage has some character of speculation, a few points should be made. The general pattern of band positions and intensities of the transient spectra is completely different from the ground-state spectra of both E-HT and Z-HT. In the transient spectra 4A and 4A' the appearance of the strongest band at 1570 cm-I, the second strong band at 1270 cm-I, and finally the strong band at 1106 cm-* are notable. The strongest band at 1570 cm-' can be assigned to the C=C stretching mode being shifted by 57 and 56 cm-' from the corresponding bands of the ground states of E-HT and Z-HT, respectively. This large shfit is about the same as seen in heptatriene and and o ~ t a t r i e n but e ~ ~smaller than in 2,5-dimeth~lhexatriene~~ indicates a considerable weakening of the C==C bonds in the triplet state compared with the ground state. The shift is about twice as large in hexatriene as in &carotene.*' The position of the strong band at 1270 cm-' is in a region where normally C C H in-plane bending modes occur. Hence an assignment of this band to 6 C C H seems possible. An alternative possibility, however, could be an assignment as a C-C stretching mode of the formal C-C single bond of the ground state being strengthened in the triplet state. A bond with a bond order between a formal single and double bond is suggested by the calculations of Ohmine et a1.16 It should be noted that the position of this band is weakly dependent on methyl substitution: One terminal methyl group as in heptatriene shifts this band 2 cm-' downward, and two methyl groups as in octatriene shift it by 6 cm-'. The position of the transient band at 1106 cm-' is in a region characteristic for a C-C single bond stretching mode. Compared to the Y C-C modes of E-HT and Z-HT in their ground states, 1106 cm-l is lower by 85 cm-' than that of E-HT and higher by
25 cm-I than that of Z-HT, provided the previously given assignment~*~ are accepted. This band seems shifted to 1125 cm-] in heptatriene, to 1136 cm-' in octatriene, and to around 1 150 cm-' in 2,5-dimethylhexatriene. From the theoretical calculations of Ohmine et an assignment to the central C-C stretching mode (the formal double bond in the ground state) seems the most reasonable one. If this tentative assignment is accepted, it should be noted that methylation seems to increase the wavenumber of the C-C single bond stretching mode. The fact that this band increases in wavenumber upon increase of chain length might then be interpreted by a larger r-electron delocalization leading to larger single-bond orders. Of the two remaining bands around 1200 and 1234 cm-' the position of the first one is influenced only little by the presence of terminal methyl groups, which suggests an assignment as a CCH bending mode. On the other hand, the wavenumber of 1200 cm-' seems rather low for a 6 C C H mode. The weakest band at 1234 cm-l is slightly dependent on terminal methylation being shifted upward by 1 I cm-' with one methyl and by 18 cm-I with two methyl groups. Finally it should be noted that no hydrogen out-of-plane vibrations in the region around 900-1 000 cm-l and no bands at lower wavenumbers (e.g., torsional or in-plane CCC bending modes) were detected with certainty in the transient spectra. Concluding this discussion it should be stressed that the assignments given are tentative indeed and subject to future verification or modification. However, a general picture of the studied trienes evolves; it shows very large changes in the resonance Raman spectra of the lowest triplet states relative to the ground states. This of course is an indication that these molecules are free to undergo large geometrical changes upon excitation. Considerably weaker Raman spectra and smaller changes are, for example, expected for corresponding cyclic trienes. Although the triplet spectra in principle contain information not only of the lowest TI but also of the higher T, state involved in the electronic transition, this information cannot as yet be extracted because of the lack of knowledge of the vibrational modes in the TI state at present. Kinetics. A discussion on the extraction of kinetic data from transient Raman experiments has been given in the previous paper.33 Figure 5 shows the kinetic plot for the strongest transient Raman band in the spectra of E-HT and Z-HT of Figure 1. A least-squares fit to determine the slopes of straight lines through the experimental points yields first-order rate constants for the decay of triplet states of k = 1.09 X lo7 s-' for E-HT and k = 1.04 X 1O7 s-' for Z-HT. As our data from previous measurements of decay rate constants on other triplet species in general indicate first-order decay rate constants from Raman measurements which are somewhat higher than values derived from triplet-triplet optical absorption spectroscopy, the present values should be regarded as upper limits rather than exact first-order rate constants. Noteworthy, however, is the apparent agreement in the decay rate constants for E-HT and Z-HT which further supports the evidence for identical relaxed triplet states of the two isomers. Detailed conclusions on kinetics, however, must await measurements by time-resolved optical absorption spectroscopy of the T, T I absorption spectrum of hexatriene.
-
-+
(42) Saito, S.; Tasumi, M. J . Raman Specfrosc. 1983, 14, 310
-
5. Conclusions The time-resolved resonance Raman spectra of the lowest excited triplet states of the two isomers of 1,3,5-hexatriene were recorded. A strong transient absorbance was seen in the Raman spectra extending at least over the region 315-324 nm and attributed to the triplet-triplet absorption of hexatriene. The resonance Raman spectra of the two isomers in their triplet states were identical within limits of error. First-order rate constants for the decay of triplet hexatriene as determined from the transient Raman spectra were k = 1.09 X lo7 s-l for E-HT and 1.04 X lo7 s-I for Z-HT and thus identical with each other within limits of error. These findings together strongly suggest that E-HT and Z-HT form a common vibrationally and conformationally relaxed triplet state or that various minima on the triplet potential energy surface in equilibrium with each other are populated. This result
J . Phys. Chem. 1987,91, 1047-1054 is discussed in the context of that of the previous paper on ( E ) and (Z)-2,5-dimethylhexatriene,where the corresponding spectra were shown to be different from each other. The present results support the previously discussed validity of the principle of nonequilibration of excited rotamers (NEER) for the triplet states of these molecules in solution. Relative E Z and Z E isomerization efficiencies were obtained from ground-state Raman spectra and GC analysis. These indicated a considerably higher Z E than E Z efficiency of isomerization from the triplet state, a result suggesting that the minimum of the triplet potential energy surface along the torsional coordinate of the central C - C double bond is shifted toward the planar tEt conformation relative to the maximum of the corresponding ground-state potential energy surface. In the case of a population of various minima in equilibrium on the triplet potential energy surface, the different Z and Z E isomerization suggest at least two rates of E minima, one with an essentially planar tEt conformation and the other with a twisted central carbon-carbon bond. Furthermore
-
-
-
-
-+
-
1047
the efficiency of conversion of hexatriene to other photoproducts was seen to be larger for 2-HT than for E-HT. The mechanisms of this formation of photoproducts are not presently known; however, mechanisms not involving the relaxed triplet state of hexatriene must be active. A rather tentative assignment of the resonance Raman spectra of the triplet states of E-HTand Z-HT, based partly on available calculations in the literature, was attempted.
Acknowledgment. We thank AIS Ferrosan for a generous loan of the preparative gas chromatograph, Helge Egsgaard and Ole Jargensen for invaluable help with the G C apparatus and the purification of samples, and Gert Olsen for help with the excimer laser. W e also thank K. B. Hansen for continual support with the development of the experimental facility. This work was partly supported by the Danish National Science Research Council. Registry No. (Z)-1,3,5-Hexatriene,2612-46-6; (E)-1,3,5-hexatriene, 821-07-8.
Electrochemistry and Spectroscopy of Ortho-Metalated Complexes of I r ( III)and Rh( I II) Y. Ohsawa,? S. Sprouse,*K. A. King,*M. K. DeArmond,? K. W. Hanck: and R. J. Watts** Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, and Department of Chemistry, University of California, Santa Barbara, California 93106 (Received: July 10, 1986; In Final Form: October 8, 1986)
The electrochemical and UV-visible spectroscopic properties of Rh(II1) and Ir(II1) complexes of the ortho-metalating (NC) ligands, 2-phenylpyridine (ppy) and benzo[h]quinone (bzq), have been studied. Cyclic voltammetric studies of several of the dimeric species, [M(NC),CI],, indicate metal-centered oxidation occurs at moderate potentials. Cationic monomers of the type M(NC),(NN)+ where (NN) = 2,2'-bipyridine or 1,lO-phenanthroline have been prepared by reaction of the chelating ligands with the parent dimers. Cyclic voltammetric studies of these monomers indicate that several reversible ligand-centeredreductions are generally observed and that the chelating ligand is more easily reduced than is the ortho-metalating ligand. Spectroscopic studies of the mixed ligand monomers indicate that dual emissions from MLCT states associated with the ortho-metalating and chelating ligands occur in the Ir(II1) complexes whereas a single emission from a ligand-localized excited state is observed in the Rh(II1) complexes. These results are discussed in terms of electronic and nuclear coupling factors analogous to those encountered in descriptions of bimolecular energy and electron-transfer processes.
Introduction The photophysics, photochemistry, and electrochemistry of several d6 diimine complexes of Ru(I1) can be rationalized if the optical and redox orbitals are considered to be localized (spatially isolated upon a single propellar blade of the tris and bis chelate systems. This model facilitates semiquantitative correlation of optical and electrochemical data.3 For example, cyclic voltammetric reduction wave^^-^ can be readily associated with a specific ligand in a mixed-ligand complex, [RuL2L'I2+. Further, the ESR method:,' previously used to assess the dynamics of electron hopping in the reduced [Ru(L-)L2]+ species, can in conjunction with electronic spectroscopys and resonance Raman methodsg produce details of the hopping barrier asymmetry for the mixed ligand complexes. The isoelectronic d6 Ir(II1) and Rh(II1) complexes have been less thoroughly probed than the Ru(I1) analogues, no doubt in part due to the more difficult chemistry, but also because of the success of the Ru(I1) complexes as photocatalysts. Nevertheless, the mixed-ligand [Rh(bpy),(phen)] 3+ and [Rh(phen),(bpy)] 3+ species (bpy = 2,2'-bipyridine, phen = 1,lo-phenanthroline) were earlier identified'O as producing a dual M* emission of the spatial isolated orbital type, one involving the phen system and the other 'North Carolina State Universitv. *University of California.
the bpy system. Emissions of mixed-ligand analogues derived from the parent I r ( b ~ y ) , ~and + I r ( ~ h e n ) ~c o~m + p l e x e ~ ~ might ~ - ' ~ also be expected to show dual emissions, but these would be difficult to prepare and have not yet been reported. Although the photoselection m e t h ~ d ' ~used J ~ to identify the spatially isolated (1) DeArmond, M. K.; Carlin, C. M. Coord. Chem. Reu. 1981, 36, 325. (2) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. Reu., in press. (3) Ohsawa, Y.; Hanck, K. W.; DeArmond, M. K. J . Electroanal. Chem. 1984, 175, 229. (4) Rillema, D. P.; Allen, G.; Meyer, T. J., Conrad, D. Inorg. Chem. 1983, 22, 1617. (5) Morris, D. E.; Ohsawa, Y.; Segers, D. P.; DeArmond, M. K.; Hanck, K. W. Inorg. Chem. 1984, 23, 3010. ( 6 ) Motten, A. G.; Hanck, K. W.; DeArmond, M . K. Chem. Phys. Lerf. 1981. 79. 541. (7) Morris, D. E.; Hanck, K. W.; DeArmond, M. K. J . Am. Chem. SOC. ~
~~
1983, 105, 3032. (8) Heath, G. A.; Yellowlees, L. J.; Braterman, P. S. Chem. Phys. Lerf. 1982, 92, 646. (9) Angel, S. M.; DeArmond, M. K.; Donohoe, R. J.; Hanck, K. W.; Wertz, D. W. J . Am. Chem. SOC.1984, 106, 3688. (10) Halper, W.; DeArmond, M. K. J . Lumin. 1972, 5, 225. (11) Flynn, C.; Demas, J. J . Am. Chem. SOC.1974, 96, 1959. (12) Flynn, C.; Demas, J. J . Am. Chem. SOC.1975, 97, 1988. (13) Watts, R. J.; Harrington, J.; Van Houten, J. J . Am. Chem. SOC.1977,
99, 2179. (14) Fujita, I.; Kobayashi, H. Inorg. Chem. 1973, 12, 2758.
0022-3654/87/2091-1047$01.50/00 1987 American Chemical Society
