The S₀→S₃ transition in 1, 2-benzanthracene. Experimental evidence for the statistical limit of vibronic coupling of S₃ with S₂

The spectroscopic properties of an excited singlet electronic state S_n (n≥1) of an aromatic compound depend on the type of vibronic coupling of S_n with S_n-1. The main spectroscopic criterion for the statistical limit of vibronic coupling is a definite 0,0 transition S_0,0→S_n0 whose homogeneous lineshape is Lorentzian. This criterion is satisfied by the S₀→S₃ absorption band of 1,2-benzanthracene. In the excitation spectrum of the S₁→S₀ fluorescence 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 width is obtained with 1,2-benzanthracene in polymethylmethacrylate by transient spectral hole-burning. The corresponding lifetime of S_3,0 is τ_3,0=136 fs. The same order of magnitude of τ_3,0 is obtained from the quantum yield of the S₃→S_O fluorescence and the calculated radiative lifetime of S₃.