THEORETICAL DESCRIPTION OF SOLVENT EFFECTS ON FLUORESCENCE-SPECTRA OFBULKY CHARGE-TRANSFER COMPOUND DMA-DMPP

Several theoretical models are compared to reproduce the spectroscopic fluorescence shift of 4-(4'-N, l-1,7-diphenyl-bis-pyrazolo-[3,4-b;4'3 'e]-pyridine (DMA-DMPP) in different solvents. DMA-DMPP is used as a m odel compound because it shows a large shift in emission energy for so lvents of various polarities and dual fluorescence in polar protic sol vents. Although the simple Onsager model is not able to reproduce the experimental results, the self-consistent reaction field (SCRF) model with extension to excited states based on the AM1 Hamiltonian yields e xcellent agreement. According to the latter model, the red-shifted emi ssion band can be related to a highly polar charge transfer state with out geometrical rearrangements, whereas the normal (short wavelength) emission is attributed to emission from an excited state with increase d conjugation in a flattened geometry. A supramolecular approach with six molecules of water surrounding the solute can explain satisfactori ly the two distinct fluorescence bands. In protic solvents, the emitti ng CT state shows additional stabilization of the locally excited stat e with a planar conformation. (C) 1998 John Wiley & Sons, Inc.