Electronic spectroscopy of jet-cooled phenylbutadienyltrimethylcyclohexene

30 September 1994

CHEMICAL PHYSICS

LETTERS

ELSEVIER

Chemical Physics Letters 228 ( 1994) 9-l 4

Electronic spectroscopy of jet-cooled phenylbutadienyltrimethylcyclohexene J.P. Finley, J.R. Cable Department of Chemistry and Centerfor Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, USA Received 20 June 1994

Abstract The resonanceenhanced ionization spectrum of a trimethylcyclohexene compound in which the cyclohexene ring is formally conjugated to a phenylbutadienyl system has been recorded in a supersonic jet expansion. Two strong origin transitions, associated with different ground-state conformers, were identified, indicating that the electronic transition occurs with little change in torsional geometry. The frequencies at which these origin transitions occur indicate that the torsional angle between the cyclohexene double bond and the phenylbutadienyl system is 15 a from perpendicular. Similar electronic and steric forces are expected to govern the ground-state torsional geometry of the p-ionylidene ring in retinal.

1. Introduction The conformation of the P_ionylidene ring in the retinal chromophore has been widely investigated, particularly with regard to its effect on the location of the maximum in the electronic absorption spectrum. While the protonated Schiff base of retinal absorbs around 440 nm in a methanol solution, the same chromophore when incorporated into the bacteriorhodopsin protein exhibits a red-shift in its absorption spectrum of 5 100 cm-’ [ 11. Roughly one-fourth of this shift has been attributed to a change in the torsional conformation of the terminal conjugated cyclohexene ring with respect to the remainder of the conjugated polyene chromophore [ 2 1. Evidence for the change in conformation comes primarily from solution and solid-state NMR [ 3-5 1. In a previous study we investigated the torsional conformation of 2- (trans-@tyryl)- 1,3,3-trimethylcyclohexene (STMC, see Fig. I), a compound having a trimethylcyclohexene ring in conjugation with a styryl group [ 61. Vibronically resolved electronic

spectroscopy was used to characterize the torsional potential in both the ground and lowest-lying excited singlet electronic states of the molecule in the isolated and low temperature conditions of a supersonic jet expansion. In STMC the equilibrium value of the torsional angle, r, between the cyclohexene ring and the styryl system is determined by the same type of interplay between electronic and steric forces which govern the conformation of the l3-ionylidene ring in retinal. Under isolated conditions, the torsional angle was found to be 17” from perpendicular in the ground electronic state and essentially unchanged in the excited electronic state. This ground state value is considerably closer to the perpendicular limit than the 40”-60” from planarity generally inferred from

0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved XSDIOOO9-2614(94)00915-5

PBTMC

STMC

Fig. 1. Structures of PBTMC and STMC.

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J.P. Finley, J.R. Cable /Chemical PhysicsLetters228 (1994) 9-14

condensed phase studies of various retinals, including X-ray crystallography [ 7-9 1. The small change in torsional conformation that occurs following electronic excitation is also at odds with previous studies which assign much of the broadening in the electronic spectrum of retinal to unresolved FranckCondon activity in a low-frequency torsional mode

[lOIIn the present study, the investigation is extended to 2-(all-trans-8-phenylbutadienyl)-l,3,3-trimethylcyclohexene (PBTMC, see Fig. 1) which contains an additional double bond in the polyene portion of the molecule. The practical advantages of working with these phenyl derivatized compounds instead of the unsubstituted polyenes include the ease with which they can be synthesized and their chemical stability. The motivation for extending the studies to PBTMC is that as the length of the polyene chain is increased, the electronic structure should more closely reflect that of a polyene. Previous analysis of the vibronic structure in the spectrum of STMC, concluded that the electronic excitation to the lowest lying excited singlet state was localized primarily on the phenyl ring. Recent work by Buma et al. [ 111 on trans-lphenyl-1,3-butadiene (phenylbutadiene) and trans, trans- 1-phenyl- 1,3,5-hexatriene (phenylhexatriene) bears directly on this hypothesis. Here it was found that the SitSo transition in phenylbutadiene was primarily localized on the phenyl ring but that in phenylhexatriene the excitation was considerably delocalized into the polyene chain. Thus the nature of the S, e Se transition in PBTMC might be expected to be quite different than in STMC, since an S, state with considerable delocalization of the excitation into the polyene should be accessible in a molecular conformation where the cyclohexene double bond is in conjugation with the phenylbutadienyl system.

2. Experimental Mass-resolved resonant two-photon ionization spectroscopy was employed to measure the Sits, excitation spectrum of PBTMC. The supersonic molecular beam apparatus and time-of-flight mass spectrometer have been described previously [ 12 1. The sample of PBTMC was vaporized into the He carrier gas in a reservoir heated to 130 oC. The gas was then

expanded from a pulsed nozzle, also heated to 13O”C, having a 1 mm diameter orifice. Excitation and ionization were accomplished using a single color beam generated by frequency doubling the output of a tunable dye laser pumped with the 532 nm second harmonic of a pulsed Nd : YAG laser. Before crossing the molecular beam, the fluence of the unfocused laser beam was reduced to 0.4 mJ/cm2 with a combination of optical filters. PBTMC was synthesized in a four step procedure starting from p-ionone. First, the methyl ketone was oxidized to a carboxylic acid in a haloform reaction using NaOCl following the procedure of van den Tempel and Huisman [ 13 1. Subsequent reduction of the acid to an alcohol with LiAlH4 followed by oxidation with Mn02 yields the aldehyde analog of pionone. Two different types of Wittig reactions were then employed in an attempt to produce all-trans PBTMC. In the first attempt, benzyltriphenylphosphonium chloride was deprotonated in THF using butyllithium and then the aldehyde in THF was added to the reaction mixture. This procedure was found to yield a 60: 40 mixture of trans and cis isomers. The second attempt used the Horner-Emmons moditication of the Wittig procedure and involved refluxing a mixture of diethylbenzylphosphonate, NaNH,, and the aldehyde in THF for 3 h. Distillation of the crude product at 90” C and 0.15 Torr gave a brown solid that was identified by GC-MS and ‘H NMR as being an approximately 13 : 1 mixture of two isomers. ‘H NMR coupling constants confirmed a trans regiochemistry for the dominant isomer.

3. Results The upper panel of Fig. 2 displays the mass-resolved resonance-enhanced ionization spectrum of PBTMC from 32796 to 33146 cm-’ when seeded into an expansion of 4 atm He. The frequency axis gives the relative displacement from the strongest line in the spectrum at 32846 cm-‘. Rhodamine 640 was used as the laser dye to cover this wavelength range and the fluence of the unfocused excitation beam was maintained near 0.4 mJ/cm2. The ionization signal plotted in Fig. 2 has been occurred for variation in the laser power by dividing the measured ion signal by the simultaneously recorded relative laser power.

J.P. Finley, J.R. Cable /Chemical Physics Letters 228 (I 994) 9-14

-50

0

50

100

FREQUENCY

150

200

250

300

(CM-‘)

Fig. 2. Origin regions of the resonant two-photon ionization spectra of PBTMC, upper panel, and STMC, lower panel, seeded in a supersonic jet expansion. Frequencies are relative to the strongest line in each spectrum which for PBTMC occurs at 32846 cm-’ and for STMC occurs at 34449 cm-‘.

For comparison with our earlier work, the lower panel in Fig. 2 displays the spectrum of STMC between 34399 and 34749 cm-’ recorded under similar conditions [ 6 1. Again the frequency axis corresponds to the displacement from the strongest line in the spectrum, here at 34449 cm-‘. Although incorporation of an additional double bond shifts the electronic transition by 1603 cm-‘, the two spectra display remarkably similar vibronic structure. Both have a single line preceding the strongest transition by a small frequency interval and then a series of weaker low-frequency peaks which follow. In PBTMC this first peak is located 15 cm-’ to the red of the strongest line while in STMC the two lines are separated by only 11 cm-‘. In our previous investigation of STMC , the possibility that the lowest frequency peak at - 11 cm-’ in the resonance-enhanced ionization spectrum was a hot band was tested and ruled out when no change in the spectrum was observed on switching the carrier gas from He to Ar or on changing the stagnation pressures with either gas. Instead, optical power saturation studies revealed that the two lowest frequency lines in the spectrum arose from different ground state conformers having different populations. The structural similarity between PBTMC and STMC clearly suggests that this might also be the explanation for the two similar peaks in the longer derivative. To test this, the laser fluence was decreased from 0.4 to 0.2 mJ/cm* by placing a microscope slide in the optical

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path of the laser beam. Changes in the relative intensities of several peaks were observed, most noticeably in the peak at 37 cm-’ and the doublet centered at 95 cm-‘. The intensity of the peak at 37 cm-‘, measured relative to that of the strongest transition, dropped by 25Ofoat the lower power and the two members of the 95 cm-’ doublet displayed comparable intensities. However, the relative intensities of the strongest peak and the one 11 cm-’ to the red remained constant. These observations are all consistent with the population of two ground state conformers in the supersonic expansion having origin transitions separated by 11 cm- ‘. Even at the low powers used to record the spectrum in Fig. 2, the stronger vibronic transitions, such as the origins and the 37 cm-’ peak, are saturated, having intensities that reflect primarily their ground state population. As the power is lowered, Franck-Condon factors also influence the observed intensity. Thus the 37 cm-’ peak appears to be a vibronic transition associated with the stronger origin transition and the two members of the 95 cm-’ doublet appear to originate from different ground state conformers. Attempts to distinguish the origins of some of the weaker vibronic features by saturating at higher laser power proved difficult because of an increasing broad background signal that made measurement of the relative peak intensities difficult.

4. Discussion The location of the origin transition in PBTMC gives considerable insight into its torsional geometry. The absence of any long vibrational/torsional progressions indicates that there is little change in geometry upon excitation of PBTMC to its lowest lying excited singlet state and thus the origin transition corresponds to the vertical transition. Since the energy of the vertical transition will depend on the degree to which the cyclohexene double bond is conjugated to the rest of the R electron system, with a suitable model the torsional geometry can be determined from the measured origin transition energy. In the perpendicular limit, r=90°, the vertical transition energy of PBTMC would be expected to be similar to the origin transition energy of phenylbutadiene or phenylpentadiene. Phenylpentadiene

J. P. Finley, J. R. Cable / ChemicalPhysicsLetters228 (I 994) 9-l 4

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provides a better comparison because the inductive shift of an alkyl group is also taken into account. For this reason, the resonance enhanced two-photon ionization spectrum of phenylpentadiene was recorded and the origin region of S, t S,,transition is displayed in Fig. 3. The origin transition is assigned at 33144 cm-‘. Buma et al. [ 111 have reported the origin transition energy of phenylbutadiene at 33315 cm-’ and so the inductive effect of a methyl substitution results in a red-shift of the origin by 17 1 cm-‘. In the limit of a planar conformation, the vertical transition energy of PBTMC would be expected to lie near the origin transition energy of phenylhexatriene, 29097 cm-’ [ 111, although small perturbations from the inductive effects of an alkyl substitution would again be expected. From the measured origin frequency of 32846 cm-’ in PBTMC it is clear that the torsional conformation of this compound is much closer to the perpendicular than the planar limit. A summary of these vertical transition energies

33100

33200

FREQUENCY

33300

33400

(CM-‘)

Fig. 3. Origin region of the resonant two-photon ionization spectrum of phenylpentadiene.

as well as those in compounds related to STMC is provided in Table 1. As was shown in the earlier work on STMC, the energy for the vertical transition to the lowest singlet excited state is expected to vary with the torsional angle, r, through a 1 -cos( 22) dependence [ 61. This expectation originates from the observation that the semiempirical calculations of the ground state potential energy for single bond torsion in an unhindered phenyl polyene display this dependence and that if similar behavior is found in the excited state, the energy difference will also vary as 1- cos (22). Taking the experimental origin transition energy of phenylpentadiene as the perpendicular, unconjugated limit and the experimental origin transition energy of phenylhexatriene with a - 17 1 cm- ’ correction for an alkyl inductive effect, the vertical Sic So transition energy in PBTMC would be expected to display a dependence on r as given below: E(r)=2108cm-‘[1-cos(2r)]+28926cm-*.

(1)

Since PBTMC appears to lie much closer to the perpendicular limit, the exact location of the planar limit will have little effect on the predicted value of the vertical transition energy. The vertical transition energy as given by Eq. ( 1) is plotted in Fig. 4 as a function of the torsional angle. As a check on this assumed functional form, the excitation energies of the lowest lying singlet excited state in phenylheptatriene constrained to have its terminal double bond at torsional angles of 90”) 100 ‘, 110” and 120” were calculated semiempirically using the MOPAC program with the AM1 Hamil-

Table 1 S,+S,, vertical transition frequencies for some phenylpolyenes Phenylpolyene

Transition frequency (cm-‘)

styrene tram-&methylstyrene STMC transl-phenyl-1,3-butadiene all-tram- 1-phenyl- I ,3pentadiene PBTMC all-tram-1-phenyl-1,3,5-hexatriene

34760 b 34585 c 34449 d 33315 = 33144 f 32846 ’ 29097 =

Shift from methyl substitution (cm-‘) ’ -175

-171

a Shift in origin frequency on addition of a terminal methyl group to the phenylpolyene of the same length. ‘Ref. [14]. cRef. [15]. dRef. [6]. ‘Ref. [II]. ‘Thiswork.

J.P. Finley, J.R. Cable /Chemical PhysicsLetters228 (1994) 9-14

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from a perpendicular configuration, four possible conformations of PBTMC are possible, two distorted s-cis and two distorted s-trans. With guidance from the semiempirical results, the two s-cis conformations at r = +_7 5 o are taken to be the more likely conformations of PBTMC which give rise to the two origin transitions observed in the electronic spectrum of the supersonically cooled molecule.

5. Conclusion 90

105

120 Torsional

135

150

165

180

Angle (degrees)

Fig. 4. Variation of the S,+So vertical transition energy in phenylheptatriene as a function of the torsional angle T.The solid line plots the function E( T) given in Eq. ( 1). Solid circles correspond to the experimentally observed origin transition in all-transphenylpentadiene (r= 90” ) and the origin transition in all-transphenylhexatriene (r= 180” ) minus 171 cm-’ to account for methyl substitution. Open circles represent the calculated vertical transition energies corrected to give the proper value at the perpendicular limit (7=90” ).

tonian ‘. The results of these calculations, corrected to have the experimental value of phenylpentadiene, 33 144 cm-‘, at r= 90” are shown as open circles on the plot. Fairly good agreement with the 1 - cos (2r) model is seen. According to Eq. ( 1 ), the measured vertical transition energy of 32846 cm-’ in PBTMC corresponds to an equilibrium torsional angle 15’ from the perpendicular limit. Using a similar analysis, the torsional angle in STMC was determined to be 17 ’ from a perpendicular conformation [ 6 1. A direct estimation of the equilibrium torsional angle in PBTMC was also made by identifying the minimum energy structure in a semiempirical AM1 calculation. These calculations find two minimum energy structures having torsional angles of 50” and -47”, considerably smaller than the value estimated from the vertical transition energy. The two structures indicated by the semiempirical calculations correspond to torsion away from the planar conformation in opposite directions, with both structures considered to have distorted s-cis conformations. From the experimental determination that T is 15o ’ MOPAC version 5.00, Quantum Chemistry Program Exchange No. 455, Department of Chemistry, Indiana University, Bloomington, IN, USA.

The S1+--Sophotoionization spectrum of PBTMC reveals that the torsional angle between the cyclohexene double bond and the phenylbutadienyl system is 15” from perpendicular in the ground electronic state and that two distinguishable conformers are present. The relatively uncomplicated photoionization spectrum in Fig. 1 also reveals that the torsional geometry of the lowest singlet excited state is relatively unchanged from that of the ground electronic state. Thus the very large steric forces arising from the methyl groups on the cyclohexene ring dictate the structure in both of these electronic states. This occurs in S, despite the suggestion that an electronic state having considerable delocalization of the excitation into the polyene should be accessible at a planar conjugated conformation [ 111, a possibility not available to the shorter compound, STMC, previously investigated.

Acknowledgement Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research.

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