Linear and Nonlinear Optical Responses of Nitrobenzofurazan-Sulfide Derivatives: DFT-QTAIM Investigation on Twisted Intramolecular Charge Transfer

In this study, we report on the green fluorescence exhibited by nitrobenzofurazan-sulfide derivatives (NBD-S_i, i = 1–4). The optical responses of these studied compounds in a polar methanol solvent were simulated by the use of time-dependent density functional theory (TD-DFT) employing the Becke-3-Parameter-Lee–Yang–Parr (B3LYP) functional along with the 6-31G(d,p) basis set. The computed energy and oscillator strength (f) results complement the experimental results. The band gap was calculated as the difference between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). Additionally, the density of states (DOS) was computed, providing a comprehensive understanding of the fundamental properties of these materials and further corroborating the experimental data. When the experimental data derived from ultraviolet/visible (UV/visible) and fluorescence spectroscopic techniques and those from simulated spectra are analyzed, the extracted values match up adequately. In addition, the NBD-sulfide compounds exhibit a large Stokes shift up to 85 nm in a polar methanol solvent. They are hypothesized to represent a novel paradigm of excited-state intramolecular charge transfer (ICT). To understand the intrinsic optical properties of NBD-S_i materials, an ICT was identified, and its direction within the molecule was evaluated using the ratio of β^vect and β_total, values extracted from the computed nonlinear optical (NLO) properties. Moreover, the reduced density gradient (RDG)-based noncovalent interactions (NCIs) were employed to characterize the strength and type of NBD-Si interactions. Furthermore, noncovalent interactions were identified and categorized using the Quantum Theory of Atoms in Molecules (QTAIM) analysis. Ultimately, the combination of Hirshfeld surface analysis and DFT calculations was utilized to enhance the characterization and rationalization of these NCIs.