Fluorescence spectra second singlet excited state

Excited States. The first absorption region in the ultraviolet spectrum of cycloheptatriene extends from 3300 to 2250 A. The absorption is intense and the transition is presumably a ir — ir excitation from the ground singlet to an upper singlet state. The radiative lifetime has been calculated to be about 10 sec. No fluorescence has actually been observed. There are no data available on the triplet states, but by comparison with the singlet — triplet absorption spectrum of 1,3,5-hexa-triene, which has been recorded by Evans, it is reasonable to assume that l,3,o-cycloheptatriene has its lowest triplet state at 47 kcal./mole or less. A second triplet state has also been reported for 1,3,5-hexatrienc by Evans. ... 

This description of the relative spectral linewidths of the lowest excited toi states applies to the whole family of aromatic hydrocarbons. It also applies to the manifold of triplet jui states. In the case of benzene, Burland, Castro and Robinson 24> and Burland and Castro 25> have used phosphorescence and delayed fluorescence excitation techniques, respectively, to measure the absorption spectrum of the lowest triplet state, 3Biu of ultrapure crystals at 4 K. The origin is located at 29647 cm-1. Unlike all the earlier studies on the lowest singlet triplet absorption spectrum, this was not an 02 perturbation experiment. Here widths of less than 3 cm-1 were obtained. This result should be compared with the much broader bands 150-1 observed for the suspected second triplet ZE i in 5 cm crystals of highly purified benzene 26>. The two triplet states are separated by 7300 cm"1. 

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