Comparative Theoretical Study on Charge-Transfer Fluorescence Probes:  6-Propanoyl-2-(N,N-dimethylamino)naphthalene and Derivatives

The efficient fluorescence probe 6-propanoyl-2-(N,N-dimethylamino)naphthalene (PRODAN) shows a significant Stokes shift in polar solvents. Neither experimental nor theoretical studies performed up to now yield a definitive description of the character of the emissive state. Results of the recently developed DFT/SCI method are compared to semiempirical calculations with the spectroscopic ZINDO/S parametrization and the AM1/CISD Hamiltonian which was not parametrized for computation of spectroscopic properties. The most reliable results are obtained with the DFT/SCI and AM1/CISD approaches. The absorption spectrum of PRODAN is excellently reproduced by the DFT/SCI calculations, and the AM1/CISD results are superior to ZINDO/S. These results allow the assignment of the red-shifted fluorescence band to both emission of a highly polar dimethylamino twisted intramolecular charge transfer state (N-TICT) and a geometrically unchanged planar intramolecular charge transfer (PICT) state. A comparison with 2-(N,N-dimethylamino)naphthalene (DAN) and 2-propanoylnaphthalene (PRON) shows that each of the donor or acceptor substituents alone in combination with rotation about the naphthalene-substituent single bond is sufficient to generate a low lying charge-transfer state with a large dipole moment.