Quantum chemical insights into electronic, spectroscopic, and NLO properties of 7-chloro-benzothiophene-2-carboxylic acid

This study presents DFT/TD-DFT investigations of 7-chloro-1-benzothiophene-2-carboxylic acid (7-Cl-BTCA), focusing on its electronic, spectroscopic, and NLO properties. The system demonstrates well-organized donor-acceptor sites that facilitate charge transfer from the heterocyclic network to the COOH group. The band gap (Egap) of the compound is found to be 4.26 eV. The NLO analysis reveals a first hyperpolarizability of 1.63×10−29 esu, much larger (44 times) than that of the reference compound. Furthermore, Cl substitution at C4 and C5, as well as an increase in the number of substituents, produces modest changes in the nonlinear optical response of the compound. In particular, substitution at C5 makes the compound slightly more NLO-active. The compound exhibits a λmax at 205 nm corresponding to the S0→S12 transition, whose details are further analyzed through hole-electron distributions following excitation. Overall, these comprehensive investigations provide theoretical insights into the potential of 7-Cl-BTCA as an organic scaffold for NLO materials and other optical applications.