Absorption and fluorescence excitation spectra of 9-(N-carbazolyl)-anthracene:: Effects of intramolecular vibrational redistribution and diabatic transitions involving electron transfer

The absorption and fluorescence excitation spectra of 9-(N-carbazolyl)-anthracene (C9A) in vibronically excited S-1 states are measured and calculated by means of a simple model. Accordingly, C9A is excited from torsional states \0(j)] of the electronic ground-state So to diabatic torsional states \1l] of the bright electronically excited state S-1, which are coupled to states \2l] of the dark electronically excited-state S-2. In addition, all torsional states are coupled to the other vibrations of C9A. The model parameters are adapted from bur previous papers yielding good agreement of the experimental and theoretical fluorescence emission spectrum and fluorescence lifetimes of C9A. The present additional agreement for the experimental and theoretical absorption and fluorescence excitation spectra confirms the simple model, which implies rather weak couplings of the torsional bright state S-1 but strong coupling of the dark state S-2 to the other vibrations of C9A, respectively. This points to different electronic structures of these excited states. This conjecture is confirmed by quantum chemical calculations based on density functional theory (DFT) that reveal the covalent structure of S-1, in contrast with the TICT (twisted intramolecular charge transfer) behavior of S-2.