Investigation of first hyper-polarisability molecular switches between enol-keto equilibrium of phenyl benzodifurantrione: a DFT-based computational study.

The contrast of the second-order nonlinear optical response between the enol (anti and syn) and keto tautomeric equilibrium of 7-Phenyl-7H-benzo[1,2-b;4,5-b']difuran-2,3,6-trione (PBDT) with donor/acceptor substitution at the para position of phenyl group have been computed involving electron correlation, frequency dispersion, and solvent polarity effects. The Hyper Rayleigh Scattering (HRS) first-hyperpolarisability contrast between the enol and keto forms for PBDT and Nitro-substituted PBDT is 3.5 and 2.8, respectively. However, in NMe2-substituted PBDT, a large contrast (~12 times) of HRS first hyper-polarisability between enol (anti and syn) and keto form is observed. Present study reveals that electron donor group (-NMe2) in PBDT plays a pivotal role for obtaining very high contrast in the first hyper-polarisability (ßHRS and ßvec) values between the enol and keto forms compare to the electron acceptor (-NO2) group substitution at the same position. The solvent effect also plays a significant role in further improving the first hyper-polarisability value and its switching behaviour between the enol and keto forms of NMe2-PBDT. NLO switching contrast between the enol and keto forms of donor/acceptor substituted PBDT is also explained using the electronic features including UV-Vis absorption spectra, HOMO-LUMOelectron densities, optical energy gaps, charge transfer contribution etc. [ABSTRACT FROM AUTHOR]