Heterocycles as donor and acceptor units in push-pull conjugated molecules. Part 1

The synthesis and spectroscopic investigation of a number of push-pull ethenes in which the donor moiety is represented by a π-excessive five-membered heterocycle (pyrrole, indole and thiophene) and the acceptor group is a π-deficient heterocyclic azine ring (pyridine, pyrazine, pyrimidine, pyridazine) are described. The intramolecular charge transfer in both the neutral compounds and the corresponding N-alkylpyridinium triflates is discussed and confirmed on the basis of three different descriptors, Δλ^Het_Ph, Δλ⁺_n, and Δλ^solv1_solv2, that take into account the substitution of a phenyl with a heterocyclic donor ring, charge effects and solvatochromism, respectively. According to the Δλ^Het_Ph descriptor, the intramolecular charge transfer in the described diheteroarylethenes increases upon increasing the electron-withdrawing capacity of the acceptor, sustained by the presence of either more than one nitrogen atom or the positive charge in the heterocyclic azine. The described pyridinium derivatives belong to the rarely investigated class of dimethine cyanine dyes. The response of the ¹³C and ¹⁵N NMR chemical shift data appears to be less clear because of the low sensitivity of the NMR probes to remote substitution.