Density Functional Study of Excited Charge Transfer State Formation in 4-(N,N-Dimethylamino)benzonitrile

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Abstract

The excited singlet states of 4-(N,N-dimethylamino)benzonitrile (DMABN), 4-N,N-dimethylaminobenzaldehyd (DMABA), and methyl 4-N,N-dimethylaminobenzoate (DMABME) are studied by a combination of density functional theory and configuration interaction approaches (DFT/SCI). DMABN is investigated in more detail as the best known model system showing dual fluorescence in polar solvents. Because the origin of the second red-shifted fluorescence is still not settled definitely, we consider three commonly discussed geometric relaxation pathways which generate low lying intramolecular charge transfer states (TICT, WICT, and RICT). In general, the results of the DFT/SCI calculations for excitation energies, oscillator strengths, and dipole moments compare favorably with either experimental results or data from very elaborate theoretical CASPT2 calculations. For DMABN we find a global minimum structure in the first excited state with a twisted (60°) but not pyramidalized dimethylamino group. The barrier for TICT state formation from the lowest 1B2 state is computed to be 2.3 kcal/mol. The calculated vertical fluorescence band energy of 3.4 eV is in good agreement with experimental data (3.2 eV). The rehybridization of the cyano group according to the RICT model leads also to an excited-state minimum with strong charge-transfer character. However, this RICT state lies higher in energy than the TICT state and is furthermore associated with a large barrier (17.6 kcal/mol). The results of the DFT/SCI calculations for the keto derivatives DMABA and DMABME reveal, in agreement with experiment, quite a small splitting between the lowest Lb and La states (≈0.15 eV).

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