Kinetics of the Excited Singlet and Triplet States of Vapor-Phase Phenanthrene

Abstract

The lifetime, τ=1/k, of the first excited singlet state of vapor-phase phenathrene has been measured as a function of pressure and excitation wavelength. Excitation in the first singlet absorption band (3400–3100 Å) resulted in τ=50±2 nsec and excitation in the second singlet band resulted in τ=45±2 nsec. The singlet-to-triplet intersystem-crossing efficiency, ΦST=kST/k, was measured and found to be ΦST=0.88±0.05. From the measured absorption and emission spectra, the fluorescence efficiency, ΦF=kF/k, was calculated to be ΦF=0.07. Internal conversion from the lowest excited singlet state to the ground state was shown to be very inefficient (ΦIC<0.05). All parameters were found to be independent of phenanthrene pressure. It is also shown that triplet—triplet annihilation to give delayed fluorescence is at least 20 times more efficient than any other second-order triplet process. The first-order triplet decay constant β was found to be β=3.3×102 sec−1.