CONSIDERATIONS ON THE DIPOLE-MOMENT OF MOLECULES FORMING THE TWISTED INTRAMOLECULAR CHARGE-TRANSFER STATE

Several derivatives of dual-fluorescing dimethylaminobenzonitrile (DMABN) are compared regarding their ground and excited-state dipole moments extracted from electro-optical emission and solvatochromic measurements. The comparison includes bridged model compounds as well as compounds with different degrees of steric hindrance to planarity, as well as ester and carbazolyl derivatives. The results are discussed on the basis of the twisted intramolecular charge transfer (TICT) model, the most widely accepted model. They show that (i) the dipole moment of the A (TICT) state (long-wavelength band) shows only little solvent sensitivity (polarizability), (ii) this observed small dependence of the dipole moment on solvent polarity can be considered as the result of overlap of two fluorescence bands or of solvent-induced changes in the mean deviation from the ideal TICT geometry ("atomic polarizability"), (iii) the dipole moment of the B state (short-wavelength band) is considerably smaller than that of the A band, (iv) the order of the dipole moment values obtained from solvatochromy is the same as from theoretical calculations: mu(gp) < mu(ep)(1L(b)) < mu(ep)(1L(a)) < mu(eT), (v) ester derivatives show a reduced dipole moment with respect to the corresponding nitriles and (vi) sterically crowded compounds seem to possess a reduced dipole moment. The possible reasons are discussed. The agreement and the differences between the two methods are discussed and compared with results from dielectric loss measurements and from quantum-chemical calculations.