Molecular engineering of D-π-A-type structures based on nitrobenzofurazan (NBD) derivatives for both organic solar cells and nonlinear optical response

The electronic, photo-physical, and nonlinear optical (NLO) properties of small molecules from nitrobenzofurazan (NBD) derivatives have been fine-tuned through the introduction of various electron-donating groups (OH, OCH3, and N(CH3)2). These designed small molecules follow a donor-linker-acceptor (D-π-A) motif, featuring p-phenylene as the electron donor, thiophene as the π-conjugated bridge, and NBD as the electron acceptor. To achieve this, we conducted density functional theory (DFT) and its time-dependent counterpart, TD-DFT calculations. Our exploration of the structure–property relations among the molecular parameters revealed that the introduction of donor groups has a notable influence on lowering the highest occupied molecular orbital (HOMO) energy levels. Analysis of the absorption–emission spectra showed a prominent π → π* band followed by a weaker n → π* band. Remarkably, three compounds substituted with electron-donor group's exhibit anti-Stokes fluorescence (ASF) emission. Moreover, the relatively weak emission band associated with the charge transfer n → π* transition of the nitro-functionalized molecules is quenched, similar to the π → π* electronic transition. Consequently, these compounds display a lower bandgap energy, resulting in a broad optical absorption profile with emissions spanning from green to yellow. Notably, when tuning the NLO properties, push-type groups exhibit demonstrate exceptionally high hyperpolarizabilities. Furthermore, our investigation unveiled excellent photovoltaic properties, with these compounds achieving an impressive power conversion efficiency (PCE) of approximately 7 %. Additionally, we conducted complementary topological analyses, including Molecular Electrostatic Potential (MEP), Quantum Theory of Atoms in Molecules (QTAIM), Reduced Density Gradient (RDG), Electron Localization Function (ELF), and the Localized Orbital Locator (LOL), to gain insights into both intra- and intermolecular interactions. These findings strongly underscore the effectiveness of modifying the electron-donating group capability in D-π-A conjugated systems as a strategy for enhancing the optoelectronic properties of organic photovoltaic (PV) devices, as well as their NLO responses.