HETEROCYCLES AS DONOR AND ACCEPTOR UNITS IN PUSH-PULL CONJUGATED MOLECULES .1.

The synthesis and spectroscopic Investigation of a number of push-pull ethenes in which the donor moiety is represented by a pi-excessive fi ve membered heterocycle (pyrrole, indole and thiophene) and the accept or group is a pi-deficient heterocyclic azine ring (pyridine, pyrazine , pyrimidine, pyridazine) are described. The Intramolecular charge tra nsfer in both the neutral compounds and the corresponding N-alkylpyrid inium triflates is discussed and confirmed on the basis of three diffe rent descriptors, Delta lambda(Pb)(Het), Delta lambda(n)(+), and Delta lambda(solv2)(solv1), that take into account the substitution of a ph enyl with a heterocyclic donor ring, charge effects and solvatochromis m, respectively. According to the Delta lambda(Pb)(Ilet) descriptor, t he intramolecular charge transfer in the described diheteroarylethenes increases upon increasing the electron-withdrawing capacity of the ac ceptor, sustained by the presence of either more than one nitrogen ato m or the positive charge in the heterocyclic azine. The described pyri dinium derivatives belong to the rarely investigated class of dimethin e cyanine dyes. The response of the C-13 and N-15 NMR chemical shift d ata appears to be less clear because of the low sensitivity of the NMR probes to remote substitution. (C) 1997 by John Wiley & Sons, Ltd.