Fluorescence excitation spectra of the S, states of isolated trienes

photoisomerization in visual systems and photosyn- thetic 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 chro- mophores 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 How- ever, theoretical calculations on tetraenes and longer poly- enes 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 struc- ture 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 tet- raenes 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 nonradi- ative 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 ion- ization (REMPI) S, -So spectra of several trienes by Buma