Experimental and computational study on photophysical properties of substituted o-hydroxy acetophenone derivatives: Intramolecular proton transfer and solvent effect

The intramolecular proton transfer reaction, relative stability of different conformers arising from the internal torsional motion and effect of solvent on the photophysical properties in the ground (S0) as well as first excited singlet (S1) state is studied for two prototype intramolecularly hydrogen bonded o-hydroxy acetophenone analogues, namely 2-acetyl-4-methyl-6-nitrophenol (I) and 2-acetyl-4-chloro-6-nitrophenol (II) by steady state and time-resolved fluorescence spectroscopy and density functional theory. In addition to the enol (E) structure suitable for excited state intramolecular proton transfer (pro-ESIPT), two non-ESIPT enol conformers (E1 and E2) are also predicted in the ground state. Exothermic ESIPT from E results largely Stokes shifted tautomeric emission for both the compounds. On the other hand, the excited state photophysics of E1 or E2 is largely controlled by the nature of the solvent. TDDFT calculation with B3LYP/6-31++G(d,p) method reproduces the experimental results in these systems much better than hybrid functionals like BHandHLYP, MPWPW91 and MPWB1K.