Effect of Hydroxyl Group on Photo-Physical Properties and Dipole Moments of Fluorescent Dyes: An Experimental and Computational Approach.

In this work, structurally similar, (E)-N'-(2-hydroxybenzylidene)-3,5-di-tert-butyl-2-hydroxybenzohydrazide (A) and (E)-N'-(2-4-dihydroxybenzylidene)-3,5-di-tert-butyl-2-hydroxybenzohydrazide (A-OH) dyes dissolved in general solvents have been studied to explore photo-physical properties, employing solvatochromic shift method, thereby determining their dipole moments in the ground (μ) and excited (μ) states. The molecule A shows a bathochromic shift of fluorescence emission maxima in aprotic solvents whereas a hypsochromic shift in protic solvents. Interestingly, A-OH follows a hypsochromic shift in both protic and aprotic solvents with increasing solvent polarity. The effect of hydroxyl substituent on UV-Visible absorption, fluorescence emission, and dipole moment of the titled organic molecules was explained. Theoretical methods such as Bilot-Kawski method for determination of μ and μ and Bakshiev, Kawski-Chamma-Viallet, Lippert-Mataga equations for μ, and Reichardt method for the difference between μ and μ were employed. It is observed that μ is higher than that of μ for both the molecules, and interestingly, upon substituting an additional hydroxyl group the value of μ has increased while μ is decreased. The DFT calculations have been performed to support experimental results by employing DFT/B3LYP/6-311G + (d) and TD-DFT/B3LYP/6-311G + (d) method using Gaussian09 software. The electrophilic and nucleophilic sites on the molecules were studied with the help of MEP. The NBO analysis results show that the interaction N24 (σ) → C22-O23 (π*) is found to be stronger in both the molecules with energy 68.90 kJ/mol and the effect of hydroxyl group is also discussed on the basis of HOMO and LUMO.