THE S(0)-]S(3) TRANSITION IN 1,2-BENZANTHRACENE - EXPERIMENTAL-EVIDENCE FOR THE STATISTICAL LIMIT OF VIBRONIC COUPLING OF S(3) WITH S(2)

The spectroscopic properties of an excited singlet electronic state S( n) (n greater-than-or-equal-to 1) of an aromatic compound depend on th e type of vibronic coupling of S(n) with S(n-1). The main spectroscopi c criterion for the statistical limit of vibronic coupling is a defini te 0,0 transition S0.0-->S(n,0) whose homogeneous lineshape is Lorentz ian. This criterion is satisfied by the S0-->S3 absorption band of 1,2 -benzanthracene. In the excitation spectrum of the S1-->S0 fluorescenc e from isolated 1,2-benzanthracene molecules in a supersonic jet, the origin of the S0-->S3 absorption band consists of a single Lorentzian line with a width (fwhm) of 39 cm-1. Practically the same Lorentzian w idth is obtained with 1,2-benzanthracene in polymethylmethacrylate by transient spectral hole-burning. The corresponding lifetime of S3,0 is tau3,0=136 fs. The same order of magnitude Of tau3,0 is obtained from the quantum yield of the S3-->S0 fluorescence and the calculated radi ative lifetime Of S3.