Looking for the species responsible of the anomalous fluorescence band in ortho-, meta- and para-(di-tert-butylamino)benzonitrile

In spite of the amount of effort devoted to the study of the photophysics of intramolecular charge transfer systems, there are still controversial points of debate. One of them is the geometrical and electronic structure of the species responsible of the anomalous fluorescence band shown by some compounds. DTABN is a derivative of the prototype system, ABN, that can shed some light in this controversy due to the geometrical constrains that its bulky substituents must impose. We have studied here the ortho-, meta- and para- isomers of DTABN using the strategy of combining QM/MM (CASSCF/UFF) and CASPT2 methods. Our results show that the steric repulsions of the bulky amino substituents impose a non-planar geometry even in the ground state. This constrain favors the twisted structure of the charge transfer excited state relative to the locally excited state in all three isomers, predicting anomalous fluorescence for all of them. Our results are in good agreement with the experimental data available for the para- and meta-isomers.