CONFORMATIONAL-ANALYSIS AND ELECTRONIC SPECTROSCOPY OF DONOR-ACCEPTOR3H-INDOLE DERIVATIVES IN NONAQUEOUS MEDIA - COMBINED EXPERIMENTAL ANDTHEORETICAL APPROACH

A joint-experimental and theoretical approach was used to study the mo lecular structure through spectroscopic and photophysical properties o f four substituted derivatives of the molecular probe 2-phenyl-3,3-dim ethyl-3H-indole in a series of nonpolar, polar aprotic, and polar prot ic solvents. The intramolecular charge delocalization (ICT) states and electron transfer (TICT) states are shown to play different roles dep ending on the solvation medium. Substitution of the indole ring at pos ition 5 by a cyano group is found to increase the planarity of the gro und-state molecule by increasing drastically the charge delocalization throughout the molecular frame including the phenyl ring. INDO/S mole cular orbital calculations showed that the TICT (twisted intramolecula r charge-transfer) states in these molecules are not the lowest excite d singlet state in the gas phase but that a crossing of the energy sur faces should occur in polar media. Comparison between the absorption a nd fluorescence spectra in various polar and nonpolar media revealed c onformational changes from a twisted form in the ground state to a mor e planar geometry in the first relaxed excited state in polar media on ly. The TICT states have not been observed experimentally. Reasons for this are discussed.