Experimental and theoretical electronic absorption spectra of 2,4-diphenyl-1,5-benzothiazepine and its derivatives: Solvatochromic effect and time dependent density functional theory approach

Electronic spectra of 2,4-diphenyl-1,5-benzothiazepine and some of its derivatives in 1,2-dichloromethane and ethanol are investigated experimentally and theoretically using the time dependent density functional theory (TD-DFT) method at the B3LYP/6-311G** level of the theory. The origin of the spectrum of the parent compound is found to be an additive one. The observed ultra violet (UV) spectra in both solvents show two bands S1 in the range between 312-334 nm and S2 in the range between 248-272 nm. The solvent effect is investigated experimentally and theoretically and a blue shift is observed, which is explained in terms of a hydrogen bond model between the solvent and the most negative site of the solute (N atom). This theoretical model is robust in reproducing the experimental blue shift and calculating the hydrogen bond energy and hydrogen bond length. The extent of delocalization and charge transfer processes of the studied compounds is estimated and discussed in terms of natural bond orbital (NBO) analysis and second order perturbation interactions (E-2) between donors and acceptors. The effect of substituents of the studied compounds in both solvents shows a noticeable red shift attributed to hyperconjugation effects of the pi electron systems of the different moieties.