Theoretical Insights into the Electronic Structure and NBO Features Driving Nonlinear Optical Enhancement in the Donor–π–acceptor System of OPPh3py

Abstract Organic materials capable of intramolecular charge transfer (ICT) are of considerable interest for photonic, biophysics and medicine applications because of their strong nonlinear optical (NLO) responses and favourable electro-optic properties. In this work, diphenyl-[2-(2-pyridylaminomethyl)phenyl]phosphine oxide (OPPh3py) was investigated as a potential NLO material using experimental characterizaion and density functional theory (DFT) calculations. Structural and electronic properties were examined through molecular electrostatic potential (MEP) analysis, frontier molecular orbitals (FMOs), natural bond orbital (NBO) calculations and intramolecular hydrogen-bonding interactions. Static and frequency-dependent NLO properties were evaluated at λ=∞, 1906, 1341, 1064, 625 and 532 nm. The molecule exhibits pronounced HOMO–LUMO separation, strong donor–acceptor interactions and a stabilizing N38–H39⋯O2 intramolecular hydrogen bond that promotes efficient charge transfer. NBO analysis further confirmed extensive charge delocalization throughout the molecular framework. As a result, OPPh3pydisplays a large dipole moment, high polarizability and enhanced first- and second-order hyperpolarizabilities compared with the benchmark p-nitroaniline. These results demonstrate that OPPh3py combines electronic stability with strong NLO activity, highlighting its potential for future advanced optical and electronic applications.