Fluorescence lifetimes of 9-(N-carbazolyl)-anthracene: Effects of intramolecular vibrational redistribution and electronic transitions in coupled bright and dark states

The fluorescence lifetimes of 9-(N-carbazolyl)-anthracene (C9A) in selected vibronic states \(1) over tilde l] are calculated by means of a simple model. Specifically, the states \(1) over tilde l] are excited to the bright electronic state S-1 which is coupled to a dark state X, as well as to the l-th vibrational excitation of the torsional mode of C9A. The model takes into account the torsional moments of inertia of C9A, the empirical diabatic torsional potentials, the potential couplings, the dipole couplings, and the vibrational couplings between the torsion and the other vibrations of C9A. The corresponding model parameters are fit to the experimental spectra and fluorescence lifetimes of Monte [J. Chem. Phys. 98, 2580 (1993)]. Three competing processes are described by the model, i.e.: (i) fluorescence from doublets of states \(1) over tilde l, +] and \(1) over tilde l, -] with + and - parity; (ii) intramolecular vibrational redistribution IVR from the torsion to the other vibrational modes; and (iii) the coupling between states S-1 and X. The resulting fluorescence lifetimes decrease systematically from ca. 20 ns for \(1) over tilde 0] to ca. 6 ns for \(1) over tilde 50], with an exceptional decrease to ca. 7 ns for intermediate states such as \(1) over tilde 24], with energies close to the crossings of the diabatic potentials of the bright and dark states S-1 and X. These systematic and exceptional trends agree well with the experimental results of Monte , and they are explained and interpreted as consequences of the three processes (i)-(iii) within our model. Accordingly, the fluorescence lifetimes contain some information about the intramolecular dynamics, e.g., IVR from the torsion to the other vibrations should be faster in the dark state X than in the bright state S-1, where it proceeds on the extraordinarily long time scale of several ns. (C) 2000 American Institute of Physics. [S0021-9606(00)00820-5].