Fluorescence quenching by pyridine and derivatives induced by intermolecular hydrogen bonding to pyrrole-containing heteroaromatics

Photophysical studies of a series of over 20 compounds based on pyrrole, indole, and carbazole chromophores were carried out in protic, nonpolar, and polar solvents. Absorption spectra revealed the formation of groundstate hydrogen bonding with protic solvent partners. The equilibrium constants were determined by spectrophotometric titration. Strong fluorescence quenching was observed when azaaromatic proton acceptors, pyridine and quinoline, were used as protic solvents. No quenching occurs for nonaromatic protic partners such as dimethyl sulfoxide, morpholine, and piperidine, even though the ground-state equilibrium constants are not smaller. The rates of quenching in pyridine solutions at 293 K span a range from 1.2 x 10(9) to 5.9 x 10(10) s(-1) and are sensitive to minor structural variations. The mechanism of quenching involves an electron transfer from a photoexcited chromophore to a hydrogen-bonded partner, followed by a rapid internal conversion (back electron transfer) to the ground state. The quenching rates are larger for systems with stronger hydrogen bonds. This model was confirmed by theoretical time-dependent DFT and semiempirical studies, in which the pattern of excited states of an isolated chromophore was compared with that of a hydrogen-bonded pyridine complex. For the latter, low-lying charge transfer (CT) states were predicted, with an electron transferred to pyridine.