Franck–Condon analysis of the 1 1A g→1 1B u absorption in linear polyenes with two through six double bonds

Abstract

The vibronic intensity distribution at room temperature and 77 °K of the 1 1Ag→1 1Bu transition for the series of linear polyenes with two through six conjugated double bonds has been measured and analyzed. These inhomogeneously broadened solution spectra can be described reasonably using only two harmonic normal modes of vibration: single and double bond stretches at ∼1200 and ∼1600 cm−1, respectively. Excited state normal coordinate displacements were determined by a least-squares fitting procedure. For both the double and single bond modes the estimated change in bond length upon excitation decreases as the length of the polyene chain increases. The bandwidth of these spectra decreases with increasing polyene length. This is well described by assuming that in solution a range of conformations differing by very small torsions about the terminal single bonds has been frozen in. The Franck–Condon analysis indicates that under these conditions all of the polyenes are approximately planar in the 1Bu state, ruling out the argument that the lack of fluorescence in butadiene and hexatriene is due to the polyene framework being significantly twisted in the lowest energy 1Bu state.