Excited state intramolecular proton transfer of 2-phenyl,3-hydroxybenzo[g]quinolones in solution and in G4 supramolecular hydrogels

Abstract

The 2-Phenyl,3-Hydroxybenzo[g]quinolone (2P3HBQ) fluorophore and its derivative, 2-(4′-hydroxyphenyl)-3-hydroxybenzo[g]quinolone (2HP3HBQ),exhibit well-separated dual emission peaks attributable to Excited State Intramolecular Proton Transfer (ESIPT), thus offering promise as useful visible-absorbing ratiometric fluorescence probes. Time-resolved emission measurements in polar and H-bonding solvents reveal very long time-constants extending up to ∼2 ns, ostensibly associated with the ESIPT process, although the latter has traditionally been known to be ultra-fast for ordinary ESIPT molecules. These unusually long time-constants suggest that ESIPT of 2P3HBQ and 2HP3HBQ involves a large activation barrier, resulting from intermolecular H-bonding with the solvent. This is supported by temperature-dependent fluorescence studies, which show that the ESIPT rates in a polar solvent differ significantly over a range of temperatures, implying that solvent‧‧‧fluorophore intermolecular H-bonding is prevalent at lower temperatures but get disrupted at higher temperatures. Our studies were extended up to 2P3HBQ molecules incorporated inside G4 Supramolecular hydrogels via host–guest type non-covalent binding. ESIPT observed in the gels differ from that in solution state on several counts, reflecting the effect of confinement on the photophysics of the gel-bound fluorophores.