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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]

Potential energy (PE) curves along the proton transfer co-ordinate of ground and excited states of 9-Hydroxy-phenalen-1-one (9HPO) and 6-Hydroxy-benzo[de]anthracen-7-one (6HBAO) have been studied using DFT-CAM-B3LYP/6-311G(d,p) and TD-DFT-CAM-B3LYP/6-31G(d) level of theory, respectively. The choice of these two molecules is based on their symmetric and asymmetric nature for their intramolecular H-bonded six-membered ring systems. Ground and excited states of PE calculations show that due to lower potential energy barrier, 9-Hydroxy-phenalen-1-one (9HPO) and 6-Hydroxy-benzo[de]anthracen-7-one (6HBAO) can be in equilibrium with their enol and keto tautomeric forms. Computed values of global reactivity descriptors, that is, average polarisability (αav) and chemical hardness (η) along the proton transfer co-ordinate of both the molecules are very small although their variations conform with their respective optimum principles. Inspired by the significant variation of the first hyperpolarisabilty along the proton transfer co-ordinate of both the compounds, it is concluded that the first hyperpolarisability can serve as a better parameter to identify enol or keto tautomeric forms in enol keto equilibrium. The variation of average first hyper polarisability (βav) along the proton transfer co-ordinate follows the optical gap rule in both the molecules. [ABSTRACT FROM AUTHOR]