Intramolecular charge transfer in coumarin based donor-acceptor systems: Formation of a new product through planar intermediate

• Photophysical behavior and ICT of anil derivatives of coumarinyl amine were studied.
• Multi-parametric regression analysis shows the importance hydrogen bonding (HB).
• HB formation with solvent coupled with ICT leads to the formation of a new isomer.
• This process goes through a planar intermediate and instantaneous in S1 state.
• However, in the ground state, it involves a barrier height ∼50 kJ mol−1.

Fluorescence behavior and intramolecular charge transfer (ICT) property of N,N′-dimethylamino substituted anil derivative of coumarinyl amine was studied by steady state and time-resolved fluorescence spectroscopy in combination with density functional theory calculation. Quantitative analysis of the effect of solvent hydrogen bonding on several spectral parameters was done using multiple regression analysis based on Kamlet–Taft model. The present system shows an additional long wavelength fluorescence band almost instantaneously in presence of methanol in sharp contrast to the model unsubstituted analogue. This new fluorescence band is believed to be due to the formation of a new isomer (cis-type) through a planar intermediate. While there is a substantial barrier (∼50 kJ mol−1) in the ground state, the isomerization process in the excited state in polar-protic environment proceeds very fast and believed to be catalyzed by hydrogen bond formation at the imine nitrogen atom in combination with stabilization of the resulting species through ICT.

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