Spectral features and excited-state transformations of hydroxy derivatives of 4'N,N-dimethylaminoflavone in PVA films and on plasmonic platforms

Compounds which undergo excited-state intramolecular proton-transfer (ESIPT) have been widely applied in biophysical investigations as fluorescence sensors enabling several analytical signals. Low fluorescence quantum yields in protic media seem to be, however, their main drawback. In this study, we investigated spectral features of 4'-N,N-dimethylaminoflavone derivatives containing hydroxyl groups at positions 3 and/or 7 in protic solid media-poly(vinyl alcohol) (PVA) films at various concentrations and under conditions of fluorescence intensity enhancement by plasmonic resonance. Our experimental findings indicate that, due to ESIPT and susceptibility of the investigated compounds to the polarity and hydrogen-bonding ability of the medium, various tautomeric species localized in different domains of PVA can be distinguished in the ground and electronically excited states, which determines multiple spectral parameters of such materials. According to the evaluated rate constants, relaxation and aggregation processes as well as proton-transfer transformations occurring in the excited state in PVA films are relatively slow and proceed in the nanosecond time domain. Under the conditions of plasmonic resonance, the multiband fluorescence intensity of compounds increases up to 7 times, whereas rates of excited-state processes including ESIPT increase up to 15 times. Due to acceleration of proton transfer, plasmon resonance can be successfully applied for enhancement of dual fluorescence of ESIPT sensors.