J. Phys. Chem. 1993,97, 1515-1520
1515
The Lowest Excited Singlet State of 1,4-Diphenyl-1,3-cyclopentadiene Xiaopei Ci, Bryan E. Kohler,’ and Thomas A. Shaler Department of Chemistry, University of California, Riverside, California 92521-0403
Soren Moller Department of Chemistry, University of California, Santa Cruz, California 95064
W.Atom Yee Department of Chemistry, Santa Clara University, Santa Clara, California 95053 Received: September 18, 1992; In Final Form: October 26, 1992
We report a high-resolution excitation spectrum for 1,4-diphenyl- 1,3-~yclopentadienecooled in a supersonic expansion. Thespectrum, which was obtained by the useof two-color resonance-enhanced multiphoton ionization techniques, has an origin at 27 008 cm-I. The dependence of absorption and fluorescence spectra on solvent and the vapor phase absorption spectrum shows that the lowest energy excited singlet state in the isolated molecule is the 2A state. The order of excited singlet states reverses in the condensed phase: the 1B state is found to be SIin solvents with refractive indices ranging from 1.25 to 1.56. The lifetime of the 2A state in the isolated molecule could not be accurately determined, though an upper bound of 10 ns could be placed on it. These results give insight into the effect that an s-cis conformation in a polyene has on its electronic structure.
1. Introduction The thesis that conjugated linear polyenes constitute one of the most important groups of organic compounds is supported by the fact that many biochemical processes depend on their special photochemical and thermochemical properties. Linear polyenes (the retinals) are the chromophores for such important lightdriven processes as visual transduction14 and energy production in Halobacterium halobium and linear polyenes (the polyene steroid hormone vitamin D) mediate photochemical transformations in the ~ k i n . 5 - ~The special thermal chemistry of linear polyenes is also of biochemical importance as, for example, in the case of the leukotrienes which are intracellular metabolites produced from phospholipid precursors as part of the inflammatory response.8 Increasing attention is being paid to the polyenes for their potential material applications as conducting polymers and as nonlinear optical media.9-’3 To understand the thermochemical, photochemical,nonlinear optical, and transport properties of linearly conjugated molecular structures, we must develop a detailed picture of linear polyene electronic structure and the response of this electronic structure to physical and chemical perturbation. As a result of high-resolution spectroscopic studies, a great deal is known about the electronic structure of simple linear polyenes. Of specialimportance was the discovery that in polyene hydrocarbons there is an excited singlet state, the 2IA, state, that is lower in energy than the 1IB, state that is predicted by single determinant HartrebFock theory to be the lowest energy excited singlet state.I4 If the polyene has Czhsymmetry so that states can in fact be properly labeled as A, and B,, then dipole transitions between the ground state and the lowest energy excited state are forbidden. For those polyenes that fluoresce, this selection rule manifests itself as an energy gap between the origins of strong absorption to the SzlIB, state and emission, which occurs from the lower lying 2IA, state.l”17 Because relaxation to the lowest energy singlet state is usually faster than photochemistry, the 2IA, state is expected to determine the photochemical consequences of electronic excitation. The link between the properties of this state and the photochemistry of polyenes is still being developed.
When strict centrosymmetry is removed, either by the addition of a nonconjugated substituent group or the introduction of a double bond cis linkage, changes in the excitation energies of transitions from SOto SIand S2 states are relatively minor. For example, when trans,trans-octatetraeneis compared to a,wdimethyloctatetraene or to cis,trans-octatetraene, the change in the excitation energies is less than 300 cm-1.18-20 Similarly, insignificant changes are found in the excitation energies from hexatriene to heptatriene2’ and from tetradecaheptaene to he~adecaheptaene.]~ Thus, we refer to the SO,SI,and SZstates in asymmetric polyenes as 1A, 2A, and 1B, respectively,to indicate the close relationship of these states to the llA,, 2’A,, and llB, states of the unsubstituted all-trans precursor. Despite the importance of s-cis conformations in the photochemical and material properties22 of polyenes, little is known about the effect of s-cis conformations on the 2A state. This reflects the fact that there are numerous difficulties that must be overcome to study s-cis conformational isomers. The barrier interconverting s-cis and s-trans isomers is very low, approximately 3 kcal/mol for simpleunsubstituted polyene^.^^*^^ The freeenergy difference between simple unsubstituted s-trans and s-cis isomers is 2-3 kcal/m01,~3~Z~ with s-trans being the more stable. Therefore, at room temperature the equilibrium distribution of conformers contains at most 2% of the less stable s-cis form. Because of the low barrier it is not possible to chromatographically isolate these isomers as it is for double-bond isomers. Further, the low barrier for s-cis to s-trans interconversion makes it impossible to obtain a low temperature sample for high-resolution spectroscopy that contains detectable amounts of s-cis conformers since almost any cooling process occurs too slowly to prevent equilibration of the mixture. To overcome these difficultiesand obtain high resolutionspectra for s-cis conformers of polyenes, some ingenious methods have been devised. One method is to photochemically generate the s-cis conformer in a low temperature matrix as a photoproduct from the corresponding s-trans molecule. This has been done for 1,3,5,7-octatetraene,where it was found that the excitation energy of the 2A state of the s-cis conformer was about 1300cm-I lower than that of the s-trans precursor.Zs Another method for obtaining a nonequilibrium mixture of conformersis by seeding the molecule
0022-3654/93/2097-1515$04.00/00 1993 American Chemical Society
Ci et al.
1516 The Journal of Physical Chemistry, Vol. 97,No. 8, 1993
in a supersonic expansion. This method together with the technique of mass-selective detection of multiphoton ionization has been used to obtain the high resolution 1A 2A excitation spectrum of an s-cis conformer of 2,4,6-heptatriene.21 In this case the red shift of the 1A 2A 0-0 transition for the s-cis conformer was only 50 cm-I. Another method for generating s-cis polyene conformations is to synthesize the molecule such that part of the polyene chain is locked in an s-cis configuration. A good way to do this is to connect parts of the polyene chain with a chain of saturated carbonsto make a carbocyclicringthat has limited conformational mobility. The absorption spectra of several steroids containing short polyene segments embedded in the ring system have been recorded, and the effect of an s-cis linkage on the energy of the 1B state can be predicted.26 Less is known about the relative state ordering of the 2A and 1B states in such systems. The effect of s-cisconformationson the transition energy to the excited states in vitamin D has been reported recently.' This paper presents the results of high-resolution studies of 1,4-diphenyl-1,3-cyclopentadienc (DPCP). The technique by which we have measured these spectra is resonance-enhanced multiphoton ionization with mass-selective detection. A distinct advantage of multiphoton ionization over fluorescencetechniques is the ability to detect ions mass-selectively. This ability is especially important for detecting weakly absorbing states such as the 2A states of polyenes and has governed our selection of techniques here. Our results, together with previous highresolution studies of DPB,2'32* substantially increase our understanding of how conformation affects linear polyene electronic structure and provide the first example of the determination of the electronic state ordering in an isolated polyene fixed in an s-cis conformation.
-
-
u
DPCP
2. Experimental Section
Our apparatus for obtaining excitation spectra of jet-cooled moleculesby resonanceenhanced two-photon ionization has been described in detail.29 As in our previous experiments we have employed 2-3 atm of helium as the carrier gas for the expansion. These experiments required a slight modification of the pulsed valve for the introduction of diphenylcyclopentadiene into the supersonic expansion. The fmal retaining ring of the 0.5-mm nozzle was replaced by a small stainless steel block that had been drilled along all three face normals. A 0.5 mm diameter hole went through the block along the face normal that is parallel to the valve axis to become the new nozzle through which the carrier gas expanded. A 0.5 mm diameter hole along a second face normal went approximately */3 through the block, intersecting the nozzle hole and a 10 mm diameter hole through the block along the third face normal that passed just under the nozzle hole. Sample was placed into the 10 mm diameter hole which was then plugged at both ends withGC injection septa. Although at a given temperature the equilibrium partial pressure of sample is highest when the sample is on the high pressure side of the valve, under our operating conditions this arrangement gave us higher sample densities than those obtained with the sample in a reservoir on the high pressure side of the valve. For these experiments the valve, including this extension, was heated to 70 OC. The excitation energy at maximum absorption for the 1A 1B transition is significantly lower for diphenylcyclopentadiene vapor than for diphenylbutadiene vapor (30 960 cm-1 versus 32 500 cm-l).30 Although the ionization potential of diphenylcyclopentadiene is not known, it is almost certainly larger than
-
2 times 30 960 cm-I, so it is unlikely that a one-color [l + 11 experiment is possible with this molecule. In fact no resonance enhancement could be detected when using only one color. Therefore, it was necessary to do the experiment with two colors. As in the past,29one color was produced by a Spectra Physics PDL-2 dye laser pumped by a Spectra Physics pulsed Nd:YAG laser (upgraded from DCR-3 unstable resonator to GCR-3 filled in beam). The laser dyes used in this study were LDS 698, Rhodamine 610, Rhodamine 590, and Fluorescein 548, all of which were obtained from Exciton. Wavelengths in the range of 3400-3530 A were generated by frequency doubling the dye laser output in a 58O KD*P crystal. The longer wavelengths were obtained by nonlinear mixing of the Nd:YAG 1064-nm fundamental with the dye laser output in a 58" KD*P crystal. The dye laser output provided the source of tunable light to excite the molecules, while a pulse of 193-nm photons from a Lambda Physik Model LPX l05i ArF excimer laser provided the boost up to the ionization contin~um.~'During an experiment the excimer laser was run at energies of 15 mJ per 10-ns pulse (beam area ca. 1 cm2at the molecular beam), while the tunable dye laser was run near full power (ca.0.5 mJ per 8 4 s pulse with a beam whose confocal diameter at the sample was ca. 50 Mm). Since one of the lasers could be delayed with respect to the other, a convenient method for determining excited-state lifetimes was available. Lifetimes were measured as described p r e v i o ~ s l y . ~ ~ J ~ As usual, scans taken with different dyes were linked by linearly scaling one scan to obtain a least-squares fit to the other in the region where they overlap. Spectra were obtained by scanning the dye laser while monitoring the yield of molecular ions ( m / z = 218). Data were taken in steps of 0.05 A in the range 37203500 A and in 0.2-A steps from 3500 to 3400 A, with the signal being averaged for 8 s at each point. For low-resolutionspectroscopy,standard techniques wereused to measure absorption spectra with a Varian DMS-100 spectrophotometer and to measure fluorescence excitation and emission spectra with a Spex Fluorolog spectrofluorometer. The sample for measuring the low-resolution absorption spectrum of diphenylcyclopentadiene vapor was an 8 cm path length quartz cell which, after introducing the ca. 50 mg of solid sample, was evacuated, backfilled with 1 atm of argon, wrapped with heating tape, and heated to about 70 "C. Preparation of DPCP is described elsewhere.33 For condensed phase experiments n-hexane and methanol solvents were Optima Grade from Fisher Scientific. Decahydronaphthalene, 3-methylpentane, and 1,Zethanedithiol were reagent grade materials obtained from Aldrich, reagent grade toluene was from Fisher, and perfluorohexane was obtained from Alfa. All of these solvents were used without further purification after we verified that they were free of absorbing or fluorescent impurities that could contribute to the measured spectra. 3. Results and Discussion 3.1. Low-Resolution Spectroscopy. A comparison of fluorescence excitation and emission spectra of 1,4-diphenyl-1,3cyclopentadiene (DPCP) in n-hexane at room temperature and at 77 K is shown in Figure 1. At room temperature the absorption spectrum of DPCP shows no resolved vibrational fine structure. The absorption maximum occurs at 356 nm, and there is a barely perceptible shoulder at about 385 nm. Compared to the room temperature absorption spectrum of all-trans- 1,rl-diphenyl-1,3butadiene (DPB),j4 the absorption maximum is red-shifted by about 1200 cm-l as might be expected by analogy with other polyenes where absorption spectra are known for both the s-trans and s-cis conformations.35 As is evident in Figure 1, cooling increases resolution;at 77 K the fluorescence excitation spectrum shows distinct vibrational structure. At 77 K the strongly allowed fluorescence excitation spectrum overlaps the fluorescence spectrum. This is in striking contrast
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1517
Singlet State of 1,CDiphenyl-1,3-cyclopentadiene
31@0--
t
\
c x
-
\
YI
c u c
.L c (
e
.I
.3
.2
(nm-1 )/(n*nt2) Figure 2. Crosses show the energy at maximum absorption (cm-I) versus (n2 - l)/(n2 + 2) for DPCP in the five different solvents of Table I. The slope of the best fit line is -12 300 cm-1, and they intercept is 31 050 cm-I. The energy at the 1B origin versus (n2 - l)/(n2 2) is plotted as #. (With the higher resolution that obtains at 77 K, we established that the difference between the origin and the absorption maximum was ca. 1800 cm-I. The points at n = 0 and n = 1.3749 are just the excitation energy for maximumabsorption minus 1800cm-I.) The IAMexcitation energy at n = 1.0 is plotted as *: the line from this point has a slope of -1200 cm-I per unit (n2 - l)/(n2 2).
+
3E00
4000
Havelength in
5000
A
Figure 1. Fluorescence excitation and emission spectra of DPCP in n-hexane at room temperature (top) and at 77 K (bottom). Excitation spectra are on the left and emission are on the right. Excitation spcctra were recorded while detecting the emission at 460 nm. Emission spectra were recorded while exciting at 350 nm.
to the gap between strongly allowed fluorescence excitation and fluorescence that is observed for other polyenes and raises the possibility that SImay be the 1B rather than the 2A state. A second possibility is that the 2A state is SI,but it is only slightly lower in energy than the 1B state. In this case the 1A to 2A transition would be greatly enhanced by intensity borrowing from the nearly degenerate 1B state, and the intense peaks on the red edge of the excitation spectrum would, in fact, belong to the 2A state, from which the emission would originate. Which of these two possibilities is correct may be decided by examining the solvent shift behavior of excitation and emission. The effect of molecular environment on the low-lying excited singlet states of polyenes is well-known.14 Since the 1A 1B transition has a large transition dipole moment, it shifts by a rather large amount as the polarizability of the solvent is changed. A plot of the transition energy to the 1B state versus the quantity (n2- l)/(n2+ 2), wherenis theindexofrefraction for thesolvent, typically has a slope -10 000cm-I per unit change in (n2- l)/(n2 2). Thus, in n-hexane the 1B excitation energy is shifted about 2500 cm-I to lower energy from the gas phase value. On the other hand, the solvent-induced shifts of the excitation energy of the covalent 2A state are relatively small, typically in the range of 1WlOOO cm-l per unit change in (n2 - l)/(n2 + 2). The observation of different solvent shift behavior for excitation and emissionspectraclearly signals that excitation and emissionshould be attributed to different excited states, at least in some of the solvents. If the 2A excitation energy in the isolated molecule is only slightly below the 1B excitation energy, it is possible that the ordering of the 1B and 2A states could reverse on going from the gas phase to a polarizablesolvent. These considerationswere the motivation for a study of the solvent shift behavior of DPCP fluorescence and fluorescence excitation spectra. Excitation energies for DPCP absorption and fluorescence excitation band maxima obtained in five different solvents whose refractive indices measured at the sodium D line ranged from 1.25 to 1.56 are presented in Table I and plotted against (n2 l)/(nZ+ 2)in Figure2. Fitting theabsorptiondata bytherelation Y K(n2 - l)/(nZ + 2) + YO predicts gas phase absorption maximum to be at 31 050 cm-l (322 nm). This value is in good
-
+
-
+
TABLE I: Absorption Maximcr of DPCP in Room Temperature Solutions with Various Solvents solvent perfluorohexane met h a noI hexane MDT* 1,2-ethanedithiol
n2OD
1.2515 1.3290 1.3749 1.475 1.5580
A,,
nm 340.7 352.0 355.6 358.8 368.9
MDT = 1:l:l by volume mixture of 3-methylpentane, decalin, and toluene. Kohler, B. E.; Spangler, C. W.; Westerfield, C. J . Chem. Phys. 1991, 94, 908.
agreement with the experimentally measured gas phase absorption maximum of 322.9 nm. The slope K is -12 300 cm-I per unit change in (n2- l)/(n* 2) indicating that the 1B state in DPCP is slightly more sensitive to solvent environment than is usual for linear polyenes (k = -10 OOO C ~ - I ) , ~ To* get an estimate of the position of the origin of the 1B state in the gas phase, the energy differencebetween the excitation energy for maximum absorption and the red shoulder in the hexane solution spectra, 1800 cm-I, was subtracted from the gas phase maximum. The resulting estimate of 29 200 cm-I (342 nm) for 1B excitation origin in the gas phase corresponds nicely to the shoulder seen on the red edge of the gas phase absorption spectrum. Fluorescence excitation and emission spectra were measured for 77 K solutions of DPCP in each of the solvents listed in Table I. In each case there was overlap between the origins of the excitation and emission spectra. The solvent shift behavior of all of the bands in the excitation and emission spectra were the same, indicating that in these solvents the 1B states is SI.For the excitation energy of the 2A state we obtain only a lower bound of about 27 OOO cm-1. Although it is believed that the 2A state is SIfor condensed phase solutionsof DPB, the energy gap between the 1B and 2A states was found to be only 130cm-1.36 Given this, the observation that the introduction of an s-cis linkage causes a reversal in the ordering of excited singlet states is not especially surprising. Does the 1B state persist as SIin the gas phase as well? This question is answered in the negative by the high resolution studies described in the next section. 3.2. MPI Spectroscopy of DPCP in a Free Jet Expaadon. The resonance enhancement profile for the production of
+
Ci et al.
1518 The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 Wavelength i n nm 365
-t-t
368
t++
t
I
t
1
355
I
I
I
I
j
358
I
I
I
I
t
IT
,
TABLE II: Peak Pasitions Relative to 0-0 Bind at 27 00 cm-I, Intensities (Peak Heights),md Assignmeats of Bands Seen in Resonrace-Eab.aad Two-Color Two-Photon Ionization Spectrum of Jet-Cooled 1,4-Diphenyl-l,lcyclopeotadiene peak position (cm-')
"tdLd-
L-
8
1
1
sea
1
L
- + , , , . A
ism
1888
Wavenumbers i n l k m
Figure 3. Resonanceenhanced two-color two-photon ionization spectrum of jet-cooled DPCP. The origin is at 27 008 cm-I.
I 278
378
328
Wavelength i n nm
Figure 4. Vapor phase absorption spectrum of DPCP at 70 "C plotted over the measured photoionization spectrum.
molecular ions of DPCP by two-color laser irradiation of a seeded supersonic helium expansion is given in Figure 3. By varying the intensities and timing of the two lasers, we established that photoionization followed from the absorption of one photon from the tunable dye laser with subsequent ionization coming from absorption of one 193-nm photon from the ArF excimer laser. As long as the cross section for transitions from excited state levels to the continuum is independent of energy (a very good approximation for the limited energy range explored here), the spectrum in Figure 3 is equivalent to a high-resolution absorption spectrum. A comparison of the high-resolution spectrum with that of the low-resolution, high-temperature vapor-phase absorption spectrum is shown in Figure 4. The lowest energy band seen in the resonance-enhanced twocolor (1 + 1) photoionization spectrum of DPCP is at 370.26 nm (27 008 cm-I). No additional features could be found in careful scans that extended 500cm-l to theredof this band which, together with fact that the peak at 370.26 nm is the most intense one in the spectrum, signals that it is the origin band. Because absorptions from the ground state to both the 2A and 1B states are symmetry allowed, there is no precedent for supposing that the spectrum should be built on a false origin, so we assign it as the 0-0 band of the SO SI transition. Two aspects of the spectra in Figures 3 and 4 immediately catch the eye. One is the presenceof relatively intense long progressions in a low frequency mode (242.9 cm-I). The second is the onset of a very high density of lines with an associated underlying broad feature beginning around 1100 cm-I above the origin. Table I1 summarizes the wavelengths for the maximum of each band together with our assignment. The most strongly coupled fundamental is 242.9 cm-l for which a progression out to the sixth overtone shows only slight deviations from harmonic
-
0 33.6 53.4 75.4 116.5 134.2 152.6 199.2 242.9 262.2 27 1.9 287.6 298.7 318.1 345.0 358.5 38 1.8 444.2 454.0 474.3 484.2 494.8 5 19.0 545.5 560.7 600.3 620.9 640.8 687.5 693.6 728.2 756.7 809.2 841.7 883.7 957.8 970.3 1007.9 1081.9 1126.1 1215.0 1328.6 1457.7 1514.5 1661.7 1683.1
relative intensity
assignment
1.o
0 4
0.06 0.06 0.1 1 0.25 0.07 0.06
VI
0.10 0.75 0.08 0.09 0.07 0.06 0.1 1 0.11 0.25 0.17 0.20 0.07 0.16 0.45 0.07 0.10 0.06 0.1 1 0.11 0.06 0.12 0.10 0.07 0.38 0.06 0.08 0.25 0.12 0.08 0.10 0.07 0.13 0.11 0.15 0.13 0.13 0.14 0.20 0.18
deviation from harmonic approx.
Y2
VI v4 Y3 VI
v3
+ Y4 + Y4
+2.5 +7.3
+ v6
-4.6
+ Y6
-0.2
Y6
0 VI Y8 Y9 Y3
4 0 v4
+ v6
-0.9
VI I
VI2 VI3 VI4
2Y6 Y4
+ YII
+ 2Y6 + VI4 Y3 + 2Y6
-1.6 -3.5
VI
-0.4
VI
-4.2
Y4
+ 2Y6
-0.5 -2.0
VI3 VI6 Y11
VI8
3Yh
2Vl I 3Y6 3V6
+ Y3 + v4
2v12 2Y14 4Y6
4V6 4V6
+ +
VI Y4
-0.5 -6.9 +5.1 -3.5 -4.7
+9.2 -1.3 +2.7 -6.2
VI9 5V6
-2.0
6v6 6&
+0.3 +3.7
5V6 + Y4
+
Y2
-2.4
Y20 Y2 I
behavior. Modes corresponding to the C-C single bond stretch and C = C double bond stretch, which are intense vibronic transitions in the spectra of other polyenes, are not so prominent in the spectrum of DPCP. Attempts were made to measure the lifetime of the molecule in different vibronic levels of SI.These were not successful since we found that the decay time of the signal, even at the origin, was shorter than the 8-11s pulse width of the excitation laser. A consequenceof the relatively short lifetime is that the intensities reported for the MPI spectrum have a greater unccrtainty than they would were the lifetime longer. This is because small fluctuations in the timing of the two laser pulses on the nanosecond time scalecould produce significantintensitychanges. In addition fluctuations in the power of the excimer laser have an effect on the kinetics of the ionization step and can result in uncertainty in the measured intensities. In practice the uncertainty was gauged by repeating scans of the same regions. The intensities reported in the spectrum are estimated to be uncertain by about f25%. 3.2.1. State Ordering. Our determination of which excited singlet state, 2A or lB, is responsible for the isolated molecule spectrum shown in Figure 3 is based on the solvent shift behavior
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1519
Singlet State of 1,4-Diphenyl-1,3-~yclopentadiene and the absorption spectrum of DPCP vapor discussed in section 3.1. The vapor phase absorption spectrum has its maximum intensity at 323 nm and there is a broad shoulder on the red edge of the absorption band at 340 nm. The assumption that this red edge shoulder is the 1A 1B 0-0 band is supported by the solvent shift study which, as described in section 3.1., gave 342 nm as the estimate for the wavelength of the DPCP 1A 1B 0-0 band in the gas phase. Since this is more than 2200 cm-I higher in energy than the SO SIorigin in Figure 3, the excited state responsible for the MPI spectrum must be the 2A state. Thus, we have a very interesting case where, in the gas phase, the lowest energy excited singlet state is the 2A state but, because of the greater stabilization of the 1B state, the state ordering reverses in the condensed phase. If we make the reasonable assumption that the excitation energy of the 2A state does not significantly change with changing solvent environment, then from the solvent shift studies we estimate that the 2A and 1B states are degenerate in a solvent whose refractive index is about 1.25. Although this means that the two states should be approximately degenerate in perfluorohexane solution, it should not be possible to see the weakly absorbing 2A state until it is significantly lower in energy than the 1B state. We now address the question of the relative absorption intensities that might be expected for the 1A 1B and 1A 2A transitions. Using simple Hiickel molecular orbital theory, we are able to make a rough prediction of the relative intensities of the 1A 1B and 1A 2A transitions in DPCP. The descriptionof the electronic states of linear polyenesusing Hiickel MO's with empirically determined mixing of two configurations has been shown to yield accurate predictions of the 1A 2A and 1A 1B transition energies.37 The calculation of the transition dipole moment matrix from the calculated MO coefficients was made by assigning C-C single bond lengths of 1.45 A, C = C double bond lengths of 1.34 A, and C-C bond lengths in the phenyl rings of 1.397 A. All bond angles were assumed to be 120O. The calculated magnitudes of (1AIpIlB) and (lAlr(2A) for DPCP are 8.061 and 1.394 D, respectively, where the 1A 1B transition is taken to be the HOMO LUMO transition and the 1A 2A transition is described by two doubly excited configurations as described previ0usly.3~The intensity ratio of the transitions is therefore predicted to be (8.061/1.394)2 = 30: 1. If we perform the same calculation on trans-DPB, we find that the intensity of the 1A 1B transition compared to that ofDPCPis (11.772/8.061)2 = 2.13,whichisinreasonablygood agreement with the observed ratio. A rough integration of the MPI spectrum of DPCP reveals that the ratio of the area of the broad feature to the sum of the areas of the sharp peaks near the origin is of the same order of magnitude as that predicted by the theory for the transitions to the 1B and 2A states respectively, thus lending further support to our assignment. 3.2.2. Comparison of DPCP with DPB. State Ordering. The orderingoftheexcitedstatesofisolatedDPCP(a weaklyabsorbing 2A state lower in energy than the strongly allowed 1B state) is the same as it is for isolated DPB and other polyenes. The energy gap between the 2A and 1B states in isolated DPCP is estimated to be about 1800 cm-I, somewhat larger than the 1200-cm-' gap observed for gas phase DPB.27J8J0 Since we have established that the 1B state is lower than the 2A state for DPCP in the condensed phase, either the excitation energy of the 1B state in DPCP is more sensitive to solvent than is the excitation energy of the 1B state of DPB or the excitation energy of the 2A state in DPCP is less sensitive to solvent than is the excitation energy of the 2A state in DPB or both. The solvent shift studies suggest that the primary cause is that the 1B excitation energy is more sensitive to solvent environment in DPCP (the naive idea that because DPB is in a more extended conformation and should therefore have greater electron redistribution in the transition to the 1B state would have suggested the opposite).
-
-
-
-
-
-
-
-
-
-
-
-
-.
-
Transition Energies. The origin of the 1A 2A transition in isolated DPB, as determined by two-photon fluorescenceexcitation of DPB seeded into a supersonic He expansion,is located at 29 6 14 cm-I. The origin for the 1A 2A transition in isolated DPCP is at 27 008 cm-I, about 1400 cm-I lower in energy. As can be seen by comparing the measured 2A excitation energy of hexatriene to that of heptatriene (methylhexatriene), adding alkyl groups to the polyene chain should result in only a small red shift (ca. 150 cm-l in this case).21 Thus, it is reasonable to assume that most of this red shift is a result of the polyene portion of the molecule being in the s-cis conformation. The 1400-cm-1 red shift of the 2A excitation energy on going from DPB to DPCP is very close to the 1300-cm-I red shift of the 2A excitation energy of octatetraene in the condensed phase that results from the introduction of a s-cis linkage. Ground-State Structures. We have calculated the groundstate structures of DPCP and s-cis DPB by molecular mechanics (PC-MODEL 4.0, Serena Software, MMX force field with *-calculation). Due to the rigidity of the cyclopentadiene ring, DPCP is calculated to have a rigorously planar carbon skeleton: no stable minima could be found for structures that have the phenyl rings twisted about the C-C single bond that attaches them to the ring or that have a nonplanar five-membered ring. This is not the case for s-cis DPB where two stable minima are predicted to exist. One of the minima has a nonplanar s-cis diene portion (37O CEC-C=C dihedral angle) with no twist of the phenyl groups, that is the phenyl ring and the double bond to which it is attached are coplanar. The other minimum has a twist about both the diene C-C single bond (15 O C = C - C = C dihedral angle) and the phenyl rings are slightly twisted (14O dihedral angle between the plane of the phenyl ring and the Cphenyl-C=Cplane). These calculations support our belief that the MPI spectrum obtained for DPCP is associated with a single conformer. Since little is known about how the 2A and 1B excitation energies change as polyenes are made nonplanar, what the state ordering will be for s-cis DPB remains an open question. Since we might expect that the s-cisconformation of octatetraene is also nonplanar, and since the excitation energy of the 2A state was red shifted by about the same amount as e red-shift on going from DPB to DPCP, it appears that neit er the relative state ordering nor the state excitation energy is greatly affected by small deviations of the carbon skeleton from planarity. Certainly a more detailed investigation of the effect of single bond twisting on the relative excited singlet state energies is in order and is an area that we are actively pursuing. Lifetimes. Isolated DPB has a fluorescencedecay time of 100 ns at the two-photon origin. We have found that for DPCP the excited-state lifetime as measured by multiphoton ionization is at least an order of magnitude shorter. This dramatic decrease in lifetime may have its origins in photochemistry on the 2A state potential surface. In the condensed phase the introduction of a s-cis linkage into trans,rrans-octatetraenedecreases the lifetime from 226 to 29 ns. Part of this decrease is due to the fact the transition is no longer symmetry forbidden but, since the lifetime of cis,trans-octatetraene is still 70 ns,3*there must be other effects. The shorter lifetime of the s-cis conformation excited state may be the result of a Woodward-Hoffman-allowed 4r-electron disrotatory electrocyclization according to eq 1. By symmetry,
-
r
the ground state of the ring-closure product correlates with the 2A state of DPCP. The fact that such a photochemical reaction can occur has been shown by Van-Tamelen and co-workers upon UV irradiation of c~clopentadiene.3~ We hypothesize that this ring closure may be occurring on the 2A excited state surface to give a highly vibrationally excited diphenylbicyclopenteneground
1520 The Journal of Physical Chemistry, Vol. 97, No. 8, 1993
state, which is incapable of being ionized. In order for this ring closing reaction to be feasible, there should be a significant displacement along a vibrationalcoordinateinthe 2Astaterelative to the ground state that involves out-of-planemotion of the fivemembered ring. The most vibronically active mode observed in the spectrum has a frequency of 242.9 cm-I, which might be tentatively assigned as the vibrationalmode along the ring closing reaction coordinate. This is corroborated by QCFF/PI CI theoretical calculations that have been reported by Zgierski and Zcrbetto for 1,3-cyclopentadiene, where the same mode was calculated to have a frequency of 228 cm-I.@ 3.2.3. Comparison 4th Other s-Cis Polyenes. The results for DPCP in terms of the energy change of the excited singlet states relative to thecorresponding s-trans compound (DPB) agree nicely with what has been reported previously for octatetraene in a low temperature hydrocarbon matrix. That is, the energies of both the 1B and 2A states shift to the red by about 1300 cm-I in the s-cis compounds. The results however seem to be at variance with the difference in 2A state excitation energies previously reported for heptatriene, where a 50-cm-1 blue shift of the s-cis conformer relative to the s-trans conformer was suggested. Also of interest is a comparisonwithcyclopentadiene. Although the ordering of the excited singlet states for this molecule has not been unambiguously established, a resonance Raman study by Shang and Hudson has shown that resonances for both the 1B and 2A states occur in the same energy region, suggesting a near degeneracy of these two states in the isolated molecule.41
4. Summary and Conclusions High resolution resonance-enhanced multiphoton ionization spectra of jet-cooled DPCP together with solvent shift studies and static vapor absorption have firmly established that the lowest energy excited singlet state in the isolated molecule is the 2A state. The 0-0 band of the 1A 2A transition occurs at 27 008 cm-I. Forty-six lines in the spectrum have been assigned to 21 separate fundamentalsand 24 combinations/overtones. The most active vibronic mode in the 2A state is a low frequency mode at 242.9 cm-I, which has a relatively long progression out to sixovertones. The onset of a very broad feature ca. 1200cm-’ above the 0-0 might correspond to the 1B state. The lifetime of the molecule in the 2A state is shorter than we are able to measure accurately with our 10-ns laser pulses, which might be due to photochemical ring closing in the 2A state. With respect to the effect of an s-cis conformer on the excited singlet state ordering and the S I S 2energy gap, our results are in agreement with previous results on s-cis-octatetraene in condensed phase, where both the 1B and the 2A states are redshifted by ca. 1300 cm-’ relative to the all-trans conformer. However the change in the energy of the 2A state in s-cis- versus s-trans-heptatriene, only 50 cm-1, is significantly different. Further experiments are underway in order to uncover any predictable trends in s-cis polyene excitation energies.
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Acknonledgme~~t. This work was supported by theNSF (Grant No. CHE-8803916), the NIH (Grant No. EY-06466), and the PRF (Grant No. ACS-PRF 22777-AC6,7).
Ci et al.
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