CONFORMATIONAL-ANALYSIS AND ELECTRONIC SPECTROSCOPY OF DONOR-ACCEPTOR 3H-INDOLE DERIVATIVES IN NONAQUEOUS MEDIA - COMBINED EXPERIMENTAL AND THEORETICAL APPROACH

A joint-experimental and theoretical approach was used to study the molecular structure through spectroscopic and photophysical properties of four substituted derivatives of the molecular probe 2-phenyl-3,3-dimethyl-3H-indole in a series of nonpolar, polar aprotic, and polar protic solvents. The intramolecular charge delocalization (ICT) states and electron transfer (TICT) states are shown to play different roles depending on the solvation medium. Substitution of the indole ring at position 5 by a cyano group is found to increase the planarity of the ground-state molecule by increasing drastically the charge delocalization throughout the molecular frame including the phenyl ring. INDO/S molecular orbital calculations showed that the TICT (twisted intramolecular charge-transfer) states in these molecules are not the lowest excited singlet state in the gas phase but that a crossing of the energy surfaces should occur in polar media. Comparison between the absorption and fluorescence spectra in various polar and nonpolar media revealed conformational changes from a twisted form in the ground state to a more planar geometry in the first relaxed excited state in polar media only. The TICT states have not been observed experimentally. Reasons for this are discussed.