Fluorescence excitation spectra of the S, states of isolated trienes

The study of the low lying states of linear polyenes is vitally important for the understanding of photobiological processes, including energy transfer in photosynthesis, cistram photoisomerization in visual systems and photosynthetic bacteria, and the photochemistry of vitamin D.le5 The complexity of the naturally occurring polyenes inhibits both computational and experimental investigations of these molecules. The research emphasis thus has moved towards model systems with similar but simpler chromophores and analogous photochemical behaviors.‘,’ The relative simplicity and availability of dienes, trienes, and tetraenes has meant that they have received much attention from experimentalists and theoreticians. Tetraenes have yielded a considerable amount of experimental data on the structure and dynamics of linear polyenes in the Sr and S, states in solution and under isolated conditions.1V6-9 However, theoretical calculations on tetraenes and longer polyenes are limited by the unusually large amount of electron correlation required to describe accurately the S, states.“* Higher level calculations have been applied to dienes and trienes,*‘1°-i5 but theoretical predictions of S, state structure and dynamics have been difficult to verify. The weak, symmetry forbidden S, -Se (2 ‘A,+ 1 ‘A,) transition is not easily detected in absorption, and, in contrast to tetraenes and longer polyenes, the apparent lack of emission in dienes and trienes prevents the study of their S, states by more sensitive fluorescence techniques. l6 The fast nonradiative decay in dienes and trienes has been attributed to enhancement of coupling between the Sr and Sa states by distortions of the Sr states from their planar ground state geometries.2’10’1*~13 Current experimental and theoretical evidence for fast nonradiative decay in trienes has led to the consensus that triene Sr states do not fluoresce.1V2,17 Encouraged by the recent observation of fluorescence from the Sr states of tetraenes and pentaenes,6.9 and the measurement of the resonance enhanced multiphoton ionization (REMPI) S, -So spectra of several trienes by Buma et aZ.,18*19 we have attempted to observe fluorescence from several trienes under isolated conditions. We report here the successful observation of the fluorescence excitation (FE) spectra of the Si state of hexatriene and ocatriene. The FE spectra, emission lifetimes, and relative emission quantum yields for trienes and tetraenes allow some preliminary conjectures on the dynamics of the S, states in short, isolated polyenes. The experimental setup used to measure fluorescence excitation spectra under collision free conditions has been described previously.6 The apparatus consists of (i) a vacuum chamber equipped with a pulsed valve, for production of the molecular beam, (ii) an excimer-pumped dye laser for excitation of the molecule under study, and (iii) emission collection optics and detection electronics. Hexatriene (Aldrich) contained a mixture of cisand trans-isomers and was used as received. The octatriene sample was prepared by the Wittig reaction between hexadienal (Aldrich) and ethyltriphenylphosphonium bromide (Fluka) using a method similar to that described previously for the synthesis of alZ-trans-decatetraene.6 Analysis of the sample by GC/MS and HPLC showed it to contain an isomeric mixture which was 90% all-trans-octatriene together with at least two cis-isomers. The trienes were coexpanded with He carrier gas through the 500 ,um orifice of the pulsed valve into the vacuum chamber. The sample reservoir was maintained at room temperature, which gave sufficient vapor pressures of both trienes to observe FE spectra in a molecular beam. The excitation source was an excimer-pumped dye laser system. The frequency-doubled output of a rhodamine 6G dye was used to excite the Sr states of the trienes. The frequency and time resolution was limited by the -0.3 cm-’ band width and 15 ns pulse width of the laser. The data were acquired for 10 laser shots for each frequency interval of -0.3 cm ‘. The dye laser wavelength was calibrated against the optogalvanic spectrum of neon. The laser and molecular beams crossed 15 mm from the nozzle orifice. The laser beam was gently focused so as to maximize the signal without saturating the transitions. The fluorescence was collected by an f/l quartz lens and detected by a PMT tube. The signal was gated and integrated with a boxcar and then passed to a microcomputer along with a monitor of the laser intensity for normalization and signal averaging. Fluorescence decays traces were recorded and averaged 200 times using a digital oscilloscope and transferred to a computer for analysis. The FE spectrum of isolated hexatriene is shown in Fig. 1. The positions and relative intensities of the peaks are given in Table I. Based on the assignment of the REMPI spectrum, the FE spectrum is due to the &-isomer.” The origin region consists of two major peaks separated by -5.7 cm‘. This doublet shows further structure due to partially resolved rotational features. Changing the expansion conditions affects the rotational structure, however, the relative integrated intensities of the two bands are not affected. The rotational line shapes of the origin bands are consistent with a predominantly parallel transition of cis-hexatriene. The ‘A, + ‘A, transition is allowed for a molecule with a C2, symmetry, however, a perpendicular band is expected. This spectrum appears to be deriving strength by intensity borrowing from the S,+-So transition as is the case for all-trans polyenes for