Electronic transition moment directions in indoloindoles: the use of orientation amplifiers

Indolo[3,2-b]indole and its four derivatives are studied by linear dichroism in stretched polymeric sheets, fluorescence anisotropy in glassy solutions, and by quantum chemical calculations. Poor alignment of the parent molecule in the stretched polymers does nor allow the determination of the transition moment directions by standard procedures. However, this is made possible if long inert substituents are introduced into the molecule. Their role is to increase the alignment without modifying the electronic structure of the chromophore. For a well-oriented chromophore, principal orientation factors and polarizations of transitions may be obtained. The results show that for the four lowest observed excited singlet states of indolo[3,2-b]indole, all the pi pi* character, substitution by carboethoxy or alkyl groups does not alter the directions of the transition moments. (C) 1999 Elsevier Science B.V. All rights reserved.