Photoconduction Activation Energies in cis-trans Isomers of β-Carotene

Abstract

The activation energies for photoconduction have been measured in all trans and 15–15′ cis β‐carotene powders and in β‐carotene glass consisting of a mixture of isomers. The average values of a number of measurements of each are all trans, 0.37 ev; 15–15′ cis, 0.20 ev; isomerized glass, 0.19 ev. The values predicted by the triplet state theory of photoconduction are 0.35 (or 0.53), 0.18, and 0.18 ev, respectively. The larger activation energy (two or three vibrational quanta) of the all trans is attributed to the thermal energy necessary to allow an intersystem crossing from the first singlet excited state (about 2.6 ev) to the triplet state (about 3.0 ev). The activation energy of the cis‐trans forms of the molecule (1 vibrational quanta) correspond to intersystem crossing from the ``cis peak'' state (about 3.5 ev) to the triplet state. The excitation spectrum of photoconduction in 15–15′ cis β‐carotene and isomerized glass, peaks in the ``cis peak'' region, in agreement with this assignment. These results, that intersystem crossing occurs more readily from the cis peak state than the first singlet excited state, provide an explanation for the shape of the photocurrent excitation spectra in β‐carotene and for the lack of phosphorescence in all trans lycopene as reported by Lewis and Kasha.