Photoinduced intramolecular charge transfer precedence over Enol-Keto isomerization in a molecular system: Spectroscopic exploration

In this article we report excited state properties of Methyl-4-(diphenylamino)-2-hydroxybenzoate (DPHB), a newly-designed synthetic molecule having both the intramolecular charge transfer (ICT) and excited state intramolecular proton transfer (ESIPT) sites. Competitive photoinduced processes in this molecular system have been extensively studied in different solvents with varying polarity, hydrogen bonding capability using steady state absorption and emission and time-resolved emission spectroscopy. Spectral comparison of DPHB with two other more or less similar synthetic compounds 4-(diphenylamino)-2-methoxybenzoate (DPMB) and methyl 4(diphenylamino)benzoate (DPB), both with only charge transfer channel and with Methyl salicylate (MS) having only proton transfer channel supports preferential charge transfer over proton transfer (PT) reaction in DPHB. The molecule DPHB shows high energy emission from the locally excited state (similar to 400 nm) and solvent dependent Stokes shifted charge transfer emission (>450 nm) in polar aprotic solvents where ICT emission is slower (>6ns) than the emission from the locally excited state (<2ns). The dynamics of the excited state is comparatively faster in polar protic solvents due to solute-solvent H-bonding interaction. Structural calculation and theoretical PECs both along the donor twist coordinate and PT coordinate at Density Functional Theory (DFT) level also supports favourable charge transfer with low excited state energy barrier.