Concurrent ultrafast twisting and proton transfer photoreactions in new pyrano[2,3-c]pyrazole derivatives

Pyrano[2,3-c]pyrazole derivatives are a class of compounds exhibiting dual solvent-dependent fluorescence. This interesting and potentially useful optical property is attributed to the excited state intramolecular proton transfer (ESIPT). We have investigated excited state dynamics of these molecules in detail using femtosecond time-resolved fluorescence and transient absorption spectroscopy. We found that when the compounds containing methoxy groups in a phenyl ring are dissolved in a polar protic solvent (methanol), they undergo excited state twisting that competes with the ESIPT reaction. Additionally, the dumping of the tautomer stimulated emission allowed us to populate a short-lived ground-state tautomer and track a ground-state proton transfer (GSIPT) back reaction. We found that the GSIPT decays on the sub-picosecond to picosecond time scale, and a fast process is more pronounced in less polar solvents.