Photophysical properties of substituted intramolecularly hydrogen bonded compounds: A combined experimental and theoretical study

Photophysical properties of prototype excited state intramolecular proton transfer (ESIPT) system 4-methyl-2,6-diformyl phenol (MFOH) and its derivatives were studied by steady state and time-resolved fluorescence spectroscopy as well as by ab-initio quantum chemical calculation. It has been found that nonradiative decay process is the most important deactivation channel in all the cases and the hydrogen bonded enol conformer is stable in the ground state whereas, the proton transferred keto form is energetically favoured in the S₁(ππ^*) state. However, the net gain in stabilization in the process of ESIPT is almost unaffected by the substitution. The reversal of stability in the excited state was explained on the basis of the nature of frontier molecular orbital in all the cases. Intrinsic reaction coordinate analysis showed that drastic change in nonbonded interoxygen distance R(O-O) in the proton transfer pathway causes the switch over from the enol to keto configuration. A close comparison of several properties like molecular geometry, hydrogen bond strength and atomic charge in different derivatives of MFOH were found to be consistent and in good agreement with the experimental results obtained from time-resolved fluorescence experiments.