Mutual interference between intramolecular proton transfer sites through the adjoining π-conjugated system in Schiff bases of double-headed, fused salicylaldehydes.

We synthesized two constitutionally isomeric bis(iminomethyl)-2,6-dihydroxynaphthalenes, namely, α,α-diimines 1 and β,β-diimines 2, which can be formally represented as fused salicylaldimines with resonance-assisted hydrogen-bonding sites. Spectroscopic data show that the OH/OH, NH/OH, and NH/NH forms of 1 were in equilibrium in solution and that the proportion of the NH-bearing tautomers increased as the solvent polarity increased. The UV spectra of thin solid films of 1 with various types of hydrogen-bonding networks differed from one another, and the spectral profiles were markedly temperature dependent, whereas the spectra of 1 in the molten state showed quite similar profiles. In contrast, 2 existed predominantly as the OH/OH form irrespective of the solvent polarity or crystal packing. Quantum chemical calculations suggest that the difference between the probabilities of intramolecular proton transfer in 1 and 2 can be explained in terms of the interplay between the resonance-assisted hydrogen-bonding sites and the adjoining π-conjugated system.