Electronic and molecular structure of 2-phenyl-3,3-dimethyl-3H-indole molecular fluorescence probe and the molecular structure of its dimer

Abstract

Abstract Crystallographic measurements as well as AMPAC and INDO/S calculations were performed on 2-phenyl-3,3-dimethyl-3 H -indole ( 1 ). This fluorescent molecular probe was shown to dimerize in the solid and the X-ray structure shows that the dimer consists of two monomers linked by covalent bonds between the nitrogen atom (N1) of one molecule to the carbon atom C2 of the other molecule, and vice versa, thus forming a four-membered ring. It was observed that the dimer of 1 is labile in the presence of traces of acid and reverts back to the monomer. It is shown that AMPAC, while predicting good bond distances and angles, failed to predict the correct dihedral angle between the Ph c ring and the indolic moiety. The semiempirical INDO/S method coupled to absorption and fluorescence spectral data in more than 18 non-polar, polar aprotic and/or protic solvents showed that the Ph c ring should librate within a range of about 20° around the equilibrium dihedral angle of approximately 45° at room temperature in the ground electronic state of the molecule. This libration is responsible for the fact that the fluorescence quantum yields (φ F ) and lifetimes (τ F ) of 1 vary from 2.4 × 10 −4 to 0.12 and from 1 to 520 ps, respectively, in the various solvents investigated as a result of viscosity dependent fast internal conversion. Rates of internal conversion are shown to be dependent upon the bulk viscosity of the solvent as opposed to the microviscosity (free volume effect of the solvent), so that the Forster—Hoffmann mechanism (φ F = Cϵ 2 3 ) with an identical value of C applies in all solvents. This molecule may then be regarded as an excellent viscosity probe for heterogeneous systems and polymers in a very large range of viscosities.