ELECTRONIC STATES OF LINEAR POLYENES - HIGH-RESOLUTION SPECTROSCOPY OF CIS-1,3,5,7,9-DECAPENTAENE AND TRANS-1,3,5,7,9-DECAPENTAENE

High resolution fluorescence, one-photon excitation and two-photon exc itation spectra of cis- and trans-1,3,5,7,9,-decapentaene have been me asured in n-alkane matrices at 4.2 K and vibronic progressions have be en analyzed. Trans-decapentaene in decane occupies a centrosymmetric s ite and one-photon transition occurs via mixing with a low frequency p romoting vibrational mode. Cis-decapentaene can occupy two different s ites in the crystal lattice of heptane with comparable probabilities. Some vibrations associated with the cis-double bond are observed in th e emission spectra of cis-decapentaene. The totally symmetric double b ond stretch vibration has a higher frequency in the excited 2 (1)A(g) State (approximate to 1790 cm(-1)) than in the ground 1 (1)A(g) state (approximate to 1600 cm(-1)). The appearance of another double bond vi brational band around 1530 cm(-1) in the excitation spectra of trans-d ecapentaene may be evidence of substantial mixing of normal vibrationa l modes in the first excited 2 (1)A(g) electronic state. The two-photo n excitation spectra of the 1 (1)A(g) -->2 (1)A(g) electronic transiti on of trans-decapentaene in n-decane and n-heptane exhibit vibronic pr ogressions based on overtones of the totally symmetric double bond str etch vibrational mode. Five overtones of this mode are observed in the region 24 700-33 000 cm(-1) and the vibronic progression appears to b e nearly harmonic. No other A, electronic states with an appreciable t wo-photon cross section have been observed in the region below 40000cm (-1). (C) 1998 American Institute of Physics.