THE ELECTRONIC SPECROSCOPY OF LINEAAR POLYENES
Loading...
Date
1973
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
A recent study of the highly resolved absorption and fluoresence spectra of dephenyloctatetraene in mixed crystal environments has revealed a weak, low-energy singlet-singlet transition below the well known strongly allowed $\pi\pi^{*}$ $transition.^{1}$ The $\pi$ states of linear polyenes have $ B_{u}$ and $ A_{g}$ symmetry in the $ C_{2h}$ point group. The lowest energy excited singlet state of diphenyloctatetraene has $ A_{g}$ symmetry and the transition to the ground state is therefore formally forbidden. The strongly allowed transition involving a $ B_{u}\Pi^{*}$ state is at slightly higher energy. The experimental work will be briefly reviewed. In previous work it was assumed that the lowest singlet-singlet transition of polyenes such as diphenyloctatetraen is the strongly allowed one. This assignment results in three difficulties involving polyene fluorescence, namely, (1) there is a separation between the vibrationless origins of absorption and fluorescence, (2) the absorption and fluorescence spectra shift differently when the solvent is changed and (3) the measured intrinsic fluorescence lifetime is much longer than that calculated from the observed absorption spectrum. These three phenonmena are consistent with the new state order since the fluorescence is due to the forbidden $ A_{g}$ to $ A_{g}$ transition while the main absorption is due to the allowed $ A_{g}$ to $ B_{u}$ transition. Conversely, an examination of the fluorescence behavior of a variety of linear polyenes has shown that the state order established for diphenylocatatraene is common to all linear polyenes. This conclusion is in agreement with recent theoretical work. The separation between the polyene excited states increases sharply as the chain length increases. At the same time, the oscillator strength for the $ A_{g}$ to $ A_{g}$ transition decreases sharply so that the excited state lifetime increases. A simple model relating these two trends will be presented. Long polyenes such as lycopene and $\beta$-carotene are expected to have a very low energy, very low energy, very weak singlet-singlet transition. Mixed crystal results on thses molecules will be presented. $^{1}$ B. Hudson and B. Kohler, hem phys. Lett. 14 -, 299 (1972)
Description
B. Hudson and B. Kohler, hem phys. Lett. 14 -, 299 (1972)
Author Institution: Department of Chemistry, Stanford University; Department of Chemistry, Harvard University
Author Institution: Department of Chemistry, Stanford University; Department of Chemistry, Harvard University