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.