Achieving solvent specific charge transfer damped excited state proton transfer by controlling chain length and charge donor torsion: Implications in nanomolar Fe3+sensing

The interplay of excited state photophysical phenomena like excited state charge transfer (ESICT) and excited state proton transfer (ESIPT) in tailor made molecular systems has been a fascinating field of research. In the current work, a detailed photophysical dissection of a chalcone (E)-1-(2-hydroxyphenyl)-3-(4-(piperidin-1-yl) phenyl)prop-2-en-1-one (HPPP) is reported. HPPP could probably show both or either of ESIPT and ESICT due to the presence of a piperidinyl ring as a charge donor and six-membered carbonyl-phenolic -OH Hydrogen bonded system (platform for ESIPT), the latter evidenced from the crystal structure. In order to disentangle the photophysical events, detailed spectrodynamic experiments were performed for control compound (E)-1-(2-methoxyphenyl)-3-(4-(piperidin-1-yl)phenyl)prop-2-en-1-one (MePPP), which bears only charge transfer moiety. Careful comparison led to the conclusion that ESICT is fully suppressed and ESIPT is favored for HPPP in organic solvents except DMF and DMSO, where ESICT is dominant over ESIPT. The practical utility of HPPP was explored by fluorimetric nanomolar sensing (limit of detection: 30 nM) of aqueous Fe3+ ion by the mode of emission quenching in DMF medium. Practical utility of HPPP was demonstrated by detection of Fe3+ on paper strips. The current work serves the dual purpose of boosting the understanding of emissive dyes showing coupled photophysical phenomena and construction of efficient fluorogenic sensors for biorelevant ions like Fe3+.