LAUSR

Abstract Thieno[2,3-b]thiophene incorporating enaminone (4) was synthesized and characterized by various spectroscopic tools such as 1H NMR, 13C NMR, UV–vis spectroscopy, infrared (IR) and X-ray single crystal crystallography. Density Function Theory (DFT) optimization of molecular structure of 4 was obtained using B3LYP/6-31G(d,p) basis set and the geometrical parameters showed good agreement with the X-ray experimental data. Also, natural bond orbital (NBO) calculations were used to calculate the intramolecular charge transfer (ICT) interactions among the most significant natural orbitals as well as the natural atomic charges. The high LP(N) → BD*(2)C-C/BD*(2)C-N ICT interaction energies indicated strong electron delocalization from the lone pair (LP) of the N-atoms to π*-NBOs of the neighboring C C and C N bonds. In addition, Time Domain-Density Function Theory (TD-DFT) calculations of the absorption spectrum of 4 predicted that the lowest energy transition is mainly due to HOMO → LUMO excitation (85%), having a wavelength at maximum absorption of 350.5 nm (f = 0.4072) with about 10% relative accuracy compared to the experimental value (391 nm). Furthermore, the most reactive electrophilic and nucleophilic sites in the compound were shown by the molecular electrostatic potential map. The results show strong positive electrostatic potential regions related to the electropositive bivalent sulfur atoms.