Tuning the BOAPY derivatives for enhancing the NLO properties using flanking groups – A DFT & TD-DFT study

Abstract

The quest for organic molecules with tunable optoelectronic properties for specific purposes is still continuing and computational designing strategies gain momentum in recent years. In this context, BOAPY core moiety attracts our attention due to its excellent photophysical properties and structural flexibility. Therefore, here we made an attempt to further enhance the photophysical properties of BOAPY molecule by introducing electron donating tuner group such as –OCH₃, –NH₂, –NH(CH₃) and –N(CH₃)₂ as well as electron withdrawing flanking groups such as –F, –CF₃ and –CN, one on each side of the tuner group. Totally, we have designed 25 molecules and analyzed their optical and Non-Linear Optical (NLO) properties using Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations. These calculations are done at M06-2X/6-31+G(d,p) level of theory. The wavelength dependent NLO properties of the designed molecules are also determined. Our studies reveal that the HOMO-LUMO energy gap (ΔE) decreases with an increase in the electron donating nature of the tuner group. The electron withdrawing flanking groups such as –CF₃ and –CN are acting well in reducing the ΔE value. The simulated absorption spectra of the designed molecules show that the λ_max increases with the electron donating power of the tuner group. Electron Localization Function (ELF) and Localized Orbital Locator (LOL) topological analyses are done to ascertain the presence of delocalization in the designed molecules. Transition Density Matrix (TDM) Analysis is also done to quantify the intramolecular charge transfer. The NLO calculations show that all the designed molecules show superior NLO characteristics, with enhanced <α>, β_total and <γ> values, surpassing urea, which is a standard NLO material. The wavelength dependent NLO calculations of a selected molecule reveal that the dynamic NLO descriptors fluctuate with wavelength and they are reaching their highest values at 450 nm. In summary, this study laid the foundation for the designing principles to tune the BOAPY derivatives to get enhanced optoelectronic and NLO properties.