CRYSTALLOGRAPHY AND AM1 AND INDO/S CALCULATIONS ON ELECTRONIC GROUND AND EXCITED SINGLET-STATES OF 2-[PARA-(DIMETHYLAMINO)PHENYL]-3,3-DIMETHYL-3H-INDOLE - SOLVENT EFFECTS ON THE ABSORPTION AND FLUORESCENCE-SPECTRA

Absorption and fluorescence spectra, fluorescence quantum yields (phi-F), and lifetimes (tau-F) were obtained for 2-[p-dimethylamino)phenyl]-3,3-dimethyl-3H-indole (1) in a series of n-alkanes and n-alcohols, along with the absorption spectrum in the vapor phase. AMPAC and INDO/S calculations as well as crystallographic measurements were also performed on this molecule, using AMPAC for the geometry optimization and INDO/S for electronic transitions calculations and conformational analysis. The structure of 1 was established by single-crystal structure analysis. The refinement converged to a final R value of 0.052 for 1144 observed reflections. The unit cell has dimensions a = 7.978 (4), b = 5.980 (2), c = 16.461 (9) angstrom and beta = 103.09 (4)-degrees, and the space group is P2(1). It is shown that AMPAC predicts a final geometry similar to that obtained from the crystallographic investigation, while INDO/S calculates transition energies corresponding to those observed in the absorption spectrum of 1 in the vapor phase. It is also shown that INDO/S calculates ground- and excited-state dipole moments similar to those obtained from spectral shift data. The results also indicate that the dimethylanilino ring (Ph(c)) has a certain freedom of rotation in the molecule at room temperature, this motion representing an important deactivation path of the S1 excited state. As the viscosity increases in the n-alkane series, the rotation amplitude of the Ph(c) moiety is reduced, resulting in a decrease of k(nr) and consequently an increase of phi-F. The k(nr) dependence on a fractional power of the viscosity has shown a friction-limited rate for the rotamers inside the energy barrier at room temperature. On the other hand, phi-F remains constant as the n-alcohol viscosity varies because of a more hindered rotation of the Ph(c) ring in protic solvents. It is finally shown that the fluorescence spectra are always larger than the absorption spectra in n-alkanes whereas their widths are almost the same in n-alcohols. The "bandwidth effect" observed on the spectra in n-alkanes is interpreted in terms of a vibronic interaction taking place between the S1 and S2 close-lying excited states. On the opposite, such an effect is not observed for 1 in n-alcohols because of the greater stabilization of the more polar S1 excited state. This solvent effect increases the S1-S2 energy gap and should therefore weaken the vibronic coupling between them.