Solvent-dependent photoacidity state of pyranine monitored by transient mid-infrared spectroscopy

Abstract

We investigate with femtosecond mid-infrared spectroscopy the vibrational-mode characteristics of the electronic states involved in the excited-state dynamics of pyranine (HPTS) that ultimately lead to efficient proton (deuteron) transfer in H2O (D2O). We also study the methoxy derivative of pyranine (MPTS), which is similar in electronic structure but does not have the photoacidity property. We compare the observed vibrational band patterns of MPTS and HPTS after electronic excitation in the solvents: deuterted dimethylsulfoxide, deuterated methanol and H 2O/D2O, from which we conclude that for MPTS and HPTS photoacids the first excited singlet state appears to have charge-transfer (CT) properties in water within our time resolution (150 fs), whereas in aprotic dimethylsulfoxide the photoacid appears to be in a non-polar electonic excited state, and in methanol (less polar and less acidic than water) the behaviour is intermediate between these two extremes. For the fingerprint vibrations we do not observe dynamics on a time scale of a few picoseconds, and with our results obtained on the O-H stretching vibration we argue that the dynamic behaviour oberved in previous UV/Vis pump-probe studies is likely to be related to solvation dynamics. © 2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.