Geometric effects in the excited states of conjugated trienes

Volume 60, number 3

GEOMETBIC

CHEMICAL

PHYSICS

1.5January 1979

LIxrERs

EFFECkS IN THE EXCZTED STATES OF CONJUGATED

TBTENES *

fohn R ANDREWS + and Bruce S. HUDSON ff mpartment of Chemikay~ Stanford UniversitY, Sta?lfo_~ ~ifornh H305* USA Received1 August 1978 Revisedmauuscriptreceived2 October i978

The fluorescence spectraand intrinsicIif’etimesof two constrainedtienes [cholesta4,6,8(14)-tsiene and choIesta-5,7, 9(11)-triene_3fllj indicatethat the lowest energysinglet-singlettransitionof the s_rrrmscis,s-~ 4,6,8(14) trieneis a weak transitionto a ‘A&ike excited state while the s-cis,tmns,s-cis5,7,9(11) trienehas a lcwest transitionwhichis strongIy aIlowedto a ‘B&ke~excited state.

a, hIrocb.lction Alkyl substituted Linear conjugated polyenes with six [l], five [l-3] and four [4] double bonds have a 1% state [s-lo] as their lowest excited singlet state. In these species the transition from the ground state to the 1% excited state is slightly Iower in energy than the strongly allowed transition to the l S, excited state. However, as the polyene chain Iengtb decreases, the gap between these two excited states decreases 143. The relative energy of these excited states in trienes has not been determined_ This excited state or&r is of interest because of its relevance to the photochemistry of trienes and because its determination provides a test of appro-ximate molecular orbital l &eories [8-lL]_ Linear polyenes have intrinsic fluorescence lifetimes which are much longer than the value obtained by integrating their strong absorption spectra according to the formula of Strickler and Berg [3-7,12211, This is due to tie fact that the fluorescence ori* This work was supportedby ffie NationalInstitutesof Health (GrantFXO1518)* Presentaddress:Departmat of Chemistry,Un.iver&tyof Penn!zylvalSL Pmadelpa Peml!W1vania 19104,USA *NIB Re~~ch CareerDev*lopmeutAward- (GM 00284). Prfzsent addtess: Dfqiutment of Chemistry,Universityof Oregon, Eugene,Ore+pn97403, USA.

380

ginates in the low energy 1% excited state. The intrinsic lifetime is therefore a measure of the lAg’ to IA, ground state oscillator strength rather than the 1K to 1% ground state transition strength. For example, the intrinsic lifetime for the dimethyl tetraene, decatetraene, is 500 11scompared to the value of 1.8 ns obtained from its integrated absorption [4] _ There are now numerous cases where the existence of the low lying 1q excited state has been demonstrated by high resolution [3-7,221 or two-photon spectroscopy [23,24] and these same polyenes have long intrinsic fluorescence lifetimes. There are no exceptions to the correlation between the presence of the 1% state as the lowest excited sin$et and an intrinsic fluorescence LIfetime which is signifkantly longer than the integrated absorption value. Therefore, the presence or absence of the 1% state as the lowest energy excited singlet state can be determined from the measurement of a compounds intrinsic fluorescence lifetime and its absorption spectrum_ Another important aspect of polyene spectroscopy is that there is a separation between the absorption and emission qectra. This is particularly striking in those cases where these spectra have resolved vibrational structure. Mull&en [25] noted that this separation was inconsistent with the assignment of the lowest energy transition as the lA8 to IF& transition unless the excited state was SevereIy distorted. This sepa-

Volume6O.number3

CEEMICAL.

PHYSICSLETTERS

ration is, in fact, due to the low intensity of the low lying 1% to 1% transition in comparison to the lAa to 1% trausitioa It is effectively absent from the low resolution absorption spectrum. If a polyene has a lowest energy IB, excited state there should be overlap between its absorption and emission spectra. There has been interest in the hypothesis that polyenes are severely distorted ia their excited state. The very long intrinsic fluorescence lifetime of linear polyenes has been ascribed to this hypothetical distortion by severalauthors [13,1$17,18,21]. Fluorescence lifetime studies have been limited to open chain linear polyenes which may have considerable flexibility. There have been no reports of the fluorescence properties of polyenes which are constrained to a particular geometry by a sigma bond system. The relative order of the two low lying excited states of linear polyenes may depend on the polyene chain stereochemistry. This is particularly likely for short polyenes because of the small energy difference between the excited states_Both the double bond (cis,trans) geometry and the single bond (s&,s-truns) geometry are likely to be important [l&26,2?]. In order to have a polyene with a completely specified geometry it must be constrained by a sigma system. In this paper we report the fluorescence and absorption properties of two conjugated trienes which are constrained to a single conformation by a cholestane ring system. The molecules, cholesta4,6,8(14)triene (I) and cholesta-5,7Q(l I)-triene-3@Ol(II), have nearly planar chromophores with n-system geometries thataresjrans,cis,~ans ands-cis,c&,s2nzns,respectively.

15

January1979

2. Rxperhnental

Cholesta4,6,8(14)-triene was purchased from Steraloids. Purification was accomplished by TLC on 10% silvernitrate-silica gel. The identity of the material was confhmed by its W absorption spectrum 1281. Cholesta-5,7,9(1l)triene-3&l)l benzoate was prepared by the method of Ruyle et al. [29] _Hydrolysis of the benzoate ester with alcoholic KOH afforded the product which was identified by its IJV and IR spectra [30]. Absorption spectra were recorded on a Car-y14 recording spectrophotometer, CD spectra were recorded on a Jasco J-4 spectrophotometer, and fluorescence spectra were recorded on a Spex fluorometer. Quantum yields were measured relative to PPO in cyclohexane (@ = 1-O) [3 1] . The lifetime measurements were performed on an Ortec single photoncounting instrument. The data were corrected for scattered light and deconvoluted for the lamp pulse. 3. Results The room temperature absorption spectra of I (in isopentane) and II (in cyclohexane) are shown in fig. 2. The absorption of the 4,6,8-triene(l) is stronger

381

Volume 60, number3

CHEMICAL PiiYsrcs LETTERS

EXCITATION

Fii3_Ihe

excitationandemissionspectmofchoksta5,7,9(1l)_~en~3~1CLI)inisopentaneat77 K.

Fig_4_Theexcitation

andemissionspectraof

cholesta-

4,6,8(14)-triene(I)in isopentaaeat 77 K. cf= 0.73) and higher in energy than the 5,7,9-triene (II) (f= O-26) by 4400 cm-l_ The spectrum of I with a peak near 280 run is quite similar to the spectra of open chain trienes; the spectrum of iI is unusuaily red-shifted. The fluorescence and excitation spectra of the 5,7,9-hiem in isopentane at 77 K are shown in fig. 3. The common origin of absorption and emission is at 28600 cm-l _ The fluorescence quantum yield under these conditions is OS to 1.0. The measured fluorescence lifetime is 2.6 + 02 as_ This gives a radiative iifetime of 2.4 to 5.6 11swhich is within experimental error the same as the Wetime predicted from the integrated absorption spectrum which is 5 2 1 11s.The fluorescence and excitation spectra of the 4,6,8txiene(I) (fig. 4) are not vibronically resolved. There is a separation of 3900 cm-1 between the absorption and emission maxima, The fluorescence quantum yield in isopentane at 77 K is 0.13 i 0.05 and the measured fluorescence Xfetime is l-7 k 03 11sunder the same conditions. This yields a measured radiative lifetime of 7.8 to 25 11s.The lifetime expected from integration of the strong absorption band is 1.7 ns. The CD spectrum of the 4,6,&tiene(I) has a single maximum in the 210 to 340 nm range corresponding closely to the absorption spectrum_ The CD spectrum of the 5,7$tricne@) also has a positive maximum corresponding to the absorption band and a negative band in the 280 to 240 mn region with a minirnm at 245 nm_

382

4. Dkarssion Cholesta-5,79(1 I)triene(II) shows overlap of the strong origins of absorption and fluorescence, a rough mirror image symmetry between absorption and fluorescence and a lifetime obtained from the integrated strong absorption which agrees within experimental error with the measured radiative lifetime. This data is consistent only with assignment of the lowest excited singlet state as the analog of the 1BU state of symmetric linear polyenes. Cholesta4,6,8(14)-triene(I) shows a gap between its absoIptionand emissionspectraincompaSon to the case for cholesta-5,7,9(1 I)-triene(II)_ This separation between absorption and emission is evidence for the presence of a low energy forbidden transition in the 4,6,8-triene. However, the Jack of vibrational resolution of the spectra of fig. 4 makes this conclusion less convincing than it is for many other polyenes [l71 where the zero-zero lines are resolved. An ahernative interpretation is that there is a weak origin line at about 3 15 nm for a single electronic transition. However, this interpretation is inconsistent with the fact that the abso_@ion transition is indisputedly strongly allowed, that other polyenes (e.g., hexatriene and the 5,7,9-triene) have strong origin lines and that this polyene has restricted conformational freedom. The fluorescence emission lifetime is 4 to 14 times longer than the value obtained from integration of its

Volume 60, number 3

15 January1979

CJSEMICAL PHYSICS LETTERS

strong absorption spectrum. This observation demonstrates that the lowest energy excited singlet state of this conjugated triene is the forbidden rransition correlating with the 14 state of symmetric linear plyenes. This observation also demonstrates that the long intrinsic fluorescence lifetime of pqlyenes is not related to their conformational flexibility since this polyene is essentially rigid. These two compounds have the same number of alkyl substituents and a similar substitution pattemThe contrasting results obtained with cholesta-5,7,9(1 l)triene and cholesta4,6,8(14)-triene demomtrate that the state order of conjugated trienes depends on the geometry of the z-electron chain. In the s-fffzns,ck,strrrnstriene(I) the origin of the weak, l&-like state is located at about 3.9 eV (320 nm) and ‘&e origin of the strong, l E&-l&e state in solution at room temperature is at about 4.3 eV (290 run). For the s-+5s,czksnans triene(II) the B, origin is at about 3.65 eV (340 nm) at room temperature (3.55 eV at 77 K)_ The position of the 1%~like state in (II) is not known but may correspond to the negative CD peak whose origin appears to be at about 4.5 eV (375 run). Introduction of an s-cis bond therefore lowers the ~BJike state relative to the ground state by 0.65 eV and may raise the l&J excitation energy. These results are in qualitative agreement with the recent calculations of Alliger and Tai [ 111 so far as the reversal of the excited state order is concerned. A similar effect has been noted in calculations on retinal 1261. A series of PPP double CI calculations were performed for three idealized hexatriene geometries (table l)_ These calculations do not include the erffects of alkyl substitution, solvent shift or nonnearest neighbor interactions. The vertical allowed transition for Z~Q?ZS and cis hex&Gene in the gas phase is at 5.1 to 5.2 eV in agreement with the calculated values. The observed vertical excitation energy for the s-tmns,cik,s&ms steroid triene(I) is 4.4-4.5 eV_ The difference between these two experimental values (ca. 0.7 &V) is attriiuted to alkyl substituent and solvent effects. The PPP doubleCl calculationspredict that the allowed transition at 5.15 eV (f = 0.99) for the s~,c&,s~ans hexatriene will move to 4.87 eV (f= 038) upon introduction of an s&is bond. This decreased intensiQ and red-shift are observed experimentally wlth the steroid trienes but the observed redshift is twice as large as the calculated value. This

Table 1 1,3,5hexafsieneexcitationenergiesfrom PPP double CI calculationsa) otrmrs, cis,

s-tram 4.73 CO) 5.18 (1.07)

4.69 (0) 5.15 (0.99)

4.68 (0) 4.87 (0.38)

a) Excitationenergiesin eV. Oscillatorstrengthsare gives in parentheses.
means that either the alkyl substituent effects cannot be considered constant for these two steroid trienes or the PPP calculation is missing an important contribution to the scis red-shift. This extra red-shift has a magnitude of approximately 0.3 eV. The observed gap between the forbidden and allowed transitions in the s*ans,cis,s-trans steroid(I) is about 0.4 eV in agreement with the PPP calculation for the corresponding hexatriene geometry. The gap calculated for the strans,cis,s-& hexatriene geometry is only 0.2 eV. The extra red-shift, whatever its or&k, is therefore sufficient to cause the state order to switch for this compound even if the forbidden transition remains at the same energy. En conclusion, the data presented above demonstrates (1) that the long intri_nsiclifetime of polyenes is not related to their confonnational flexibility, (2) at least one polyene has “normal” behavior with respect to its intrinsic lifetime and low resolution spectral overlap, (3) the state order in trienes depends on the chromophore geometry, mediated by alkyl substituent effects, and (4) at least one azkyl substituted triene has a forbidden transition as its lowest energy singlet excitation.

References [ 1 ] RL. Christensen2nd

B-E. Kohler, J. Phys. Chem. 80

(1976) 2197.

[2] R.L. Christensen and B.E. Kohler, J. Chem. Phys. 63 (1975) 1837.

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Volume 60, number 3 131J. Andrews,

Thesis, Stanford University,

CHEMICAL PHYSICS LETTERS Stanford,

CA

(1978).

[43 J. Andre-and B. Hudson, Chem. Phys. Letters 57 (1978) 600. C51B. Hudson ard B.E. KohIer, Ann Rev. Phys Chem. 25 (1974) 437. C61B. Hudson and B_E Rohier, J_Chem, Phys_59 (1973) 4984. 171 B. Hudson and B.E. KohIer, Chem. Phys L.etters 14 (1972) 299_ and M_ Karplus, Chem. Phys Letters 14 181 K. Schulten (1972) 305, iSI K_ SchuIten,L Ohmine and M. KarpIus,J. Chea Phys 64 (1976) 4422 iN L Ohmine, M_ KarpIusand K_ SchuIten, J. Cnem. Phys. 68 (1978) 2298_ ill1 N-L Allinger and J-C_TG J. Am_ Chem. Sot 99 (1977) 4526. 11% SJ. Strickk and EA Berg, J. Chem. Phys. 37 (1962) 814. 1131J-B. Birks and D_J_Dyson, Proc Roy_ Sot A275 (1963) 135. 1141J_P.Dalle and B. Rosx~berg,Photochem-PhotobioL12 (1970) 151_ t151 J_B_Birks and D_J_S_Birch, Chem_Phyr Letters 31 (1975) 608. Cl61 J_B_Birks,Z_ Physik_Chem_ lOl(l976) 91_ RB_ Cuna J-R Lockwood and T-F_ 1171ED_ C&em PaImer,J. whys Che-rn_ 79 (1975) 1369.

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[18] B_S_Neporent, Zb_ Fiz. Khim- 37 (1963) 236. [19] TM. Garb, C Weismzn,JX, McVey arid SA Rice, J_ Chem. Phys. 68 (1978) 522. 1201LA.. SkIar,B.S. Hudson. k-LPetersenand J. Diamond, Biochemistry16 (1977) 813. [21] A_J_Thompsos, J. Chem, Phys_Sl(l969) 4106. [22] W_M_Hetheriugtoon IJI, Thesis,Stanford University, Stanford, CA (1977). [23] H.LB. Fang, R.J. Thrash and G.E. Leroi, J. Ch_en~. Phys. 67 (1977) 3389. [24] RR B&e, J.k Bennett,B&L Pierceand T_M Thomas, I. Am_ C&m_ Sot 100 (1978) 1533. [25] R_S_M&l&en, J_Chem, i?hys_7 (1939) 364. [26] RR_ Birge, K. Schultenand M. Karphs, Chea Phys, Letters 31(1975) 451. 1271k Warshel and hf. Ka@us, J_Am_ Chem. Sot. 96 (19?4) 5677. [28] F_ von Hunziker,FX MiilIer,ICG. RenteIer2nd & Schaiteger, HeEv.Chim. Acta 38 (1955) 1316. [29] W.V. Ruyle, T.A_ Jacob, J&L Chemerda,E.M. Chamber- D-W_ Rosenberg,GE Sita, R-L- Erickson, L&f. Aliminosaand hf. Tishler,J_Am_ Chem, Sot. 76 (1953) 2601. [30] A_ W&us and 0. Linsert, JustusLiebigs AM. Chem. 465 (1928) 148. 1311L Bzrhnan, Handbook of fluorescencespectraof arematicmolecules(Academic Press,New York, 1971) p_ 291.