Intramolecular charge transfer-induced solvatochromism and large Stokes shifts of furocoumarins

Abstract

A series of furocoumarins (4a-h) was synthesized via a multi-component reaction and characterized by FTIR, 1H NMR, 13C NMR and ESI-MS. Compound 4g was further characterized with single-crystal X-ray diffraction for the molecular packing and interaction analysis. The photophysical and photochemical properties of the series have been experimentally and theoretically investigated, focusing on the absorption and fluorescence properties in solvents of varying polarity. Geometry optimization, HOMO-LUMO energy gap, NBO analyses and potential nonlinear optical (NLO) responses for all furocoumarins (4a-h) have been computed using DFT/TD-DFT method. The experimental results reveal that absorption and fluorescence of furocoumarins are highly sensitive to the solvent polarity and inter or intramolecular interactions. Furocoumarin exhibits strong solvatochromism through the intramolecular charge transfer (ICT) process, large Stokes shift up to 142 nm in polar solvents, long emission wavelength up to 520 nm in polar protic solvent through large polarization and relative high quantum efficiencies up to 94% in DMSO. Compound 4h exhibits Stokes shift value of 197 nm in toluene. The charge transfer character was correlated with the Lippert-Mataga and McRae plots while the effect of substituents on the fluorescence quantum yield was described by the Hammett plot. Based on the theoretical computations, furocoumarins with strong electron donating substituents (4b and 4d) possess high frequency-dependent first order hyperpolarizabilities, β in the range of 21.32–27.34 × 10−30 esu, suggesting their possible use in the NLO application. Fluorescence quenching in 4c, 4g and 4h were caused by site-specific hydrogen bonding strengthening and molecular packing that led to the formation of excimer species. This work will be valuable for the exploitation of ICT and solvatochromic effects and future molecular engineering of organic materials for practical utilities endowing low cost, facile and greener approach.