Intramolecular proton transfer in 4-methyl-2,6-diformyl phenol and its derivative studied by femtosecond transient absorption spectroscopy

Dynamics of excited state intramolecular proton transfer (ESIPT) and solution phase fluorescence properties of 4-methyl-2,6-diformyl phenol (MFOH) and its derivative, 4-methyl-2,6-diacetyl phenol (MAOH), have been monitored by nanosecond time correlated single photon counting and femtosecond transient absorption techniques. Time evolution of the transient absorption data reveals that both the compounds undergo an ultrafast proton transfer, yet with a significantly slower rate constant of about 4–5 × 1012 s−1 compared to the other intramolecularly hydrogen bonded systems reported recently. This is explained on the basis of large skeletal rearrangement necessary for the proton transfer to occur. The ultrafast ESIPT process is followed by rapid intramolecular vibrational relaxation (IVR) with a time constant of few picoseconds before fluorescence emission occurs from the proton transferred keto structure in the excited state potential energy surface.