Spectroscopic studies and Non-Linear optical response through C/N replacement and modulation of electron Donor/Acceptor Units on naphthyridine derivatives

This study explores the nonlinear optical (NLO) and photophysical properties of newly designed naphthyridine derivatives by density functional theory (DFT). The first hyperpolarizability (β_tot), a key indicator of NLO activity, varies significantly depending on the substituent groups. N-substituted compounds (IUB-N series) generally show lower β_tot values, while compounds with electron donor/acceptor groups (IUB-P series) demonstrate a broader range, with IUB-A-02 achieving the highest β_tot value of 16,362 a.u. due to the presence of two -NH₂ groups. TD-DFT analysis confirms key electronic transitions, mostly from HOMO to LUMO, with absorption wavelengths (λmax) ranging from 349.596 to 440.692 nm for the IUB-P series. The introduction of electron-donor groups considerably boosts absorption, particularly in IUB-P-06, with highest λ_max and oscillator strength (f_o) signifying excellent light absorption capabilities. The calculated light harvesting efficiency (LHE) correlates strongly with f_o values, IUB-N-01 to IUB-N-05 exhibiting higher LHE than the unsubstituted IUB. Additionally, lower radiative lifetimes (τ) for the modified compounds indicate faster decay, useful for applications in photodynamic therapy and fluorescence imaging. Lower transition energy (ΔE) and higher f_o values contributed to greater first hyperpolarizability (β_o). IUB-P-06, with two -NH₂ donor groups, shows the lowest ΔE (2.81 eV) and a correspondingly high β_o (60218.89 a.u.). Whereas IUB-A-02 exhibits the highest β_o (68907.84 a.u.) due to its large dipole moment change (Δμ = -6.37 D). Among N-substituted compounds, IUB-N-01 exhibits the highest charge density. IUB-P-06 has the highest charge density and electron-hole separation due to electron donor/acceptor groups, indicating a higher degree of internal atomic localization. This enhanced charge separation further confirms the superior performance of these compounds in NLO applications. In conclusion, this comprehensive analysis spanning ESP, TD-DFT, TLM, LHE, and TDM demonstrates that the studied naphthyridine derivatives possess promising NLO properties and exhibit strong potential for use in optoelectronics, photovoltaics, photodynamic therapy, and other advanced optical technologies.