Absence of Dual Fluorescence in Cyano Isomers and Di-Cyano Derivatives Related to N,N-Dimethyl-4-Aminobenzonitril: A DFT/MRCI Study

A combination of density functional theory and multi-reference configuration interaction method (DFT/MRCI) is used to investigate the excited state intramolecular charge transfer (ICT) reaction for the ortho- and meta-cyano-isomers of {N,N}-dimethyl-4-aminobenzonitril (DMABN), 2-CDMA and 3-CDMA, and two di-cyano derivatives of DMABN, 3,4-DCDMA and 3,5-DCDMA. Experimentally, a red-shifted emission was only observed for DMABN and attributed to an ICT state. The other systems emit fluorescence solely from a locally excited state. The results of our computations show that the emission properties of all systems are best explained by the TICT (twisted intramolecular charge transfer) model: The first excited state of the investigated systems is of ICT character and its energy increases with increasing the twist angle about the donor-acceptor single bond. This finding is contrary to the stabilization in energy observed for DMABN. Also in contrast to DMABN, the dipole moment of both the first and second excited states increases for increasingly decoupled geometries, resulting in two highly polar twisted states. Consequently, only the TICT state of DMABN becomes the global minimum on the first excited state energy surface resulting in an ICT emission. The PICT model (planar ICT) is also discussed for explanation of the emission properties of this class of molecules. This model requires a small energy gap between the first and second excited state and the formation of a planar ICT state as the energy minimum of the first excited state. A significantly larger energy gap ΔE(S₂-S₁) between the first and second excited state found in the calculations for the investigated derivatives in respect to DMABN is in favor of the PICT hypothesis. However, the planarization energies are comparable for all systems including DMABN and this rules out the PICT model as a general explanation for the experimentally observed emission properties.