Enhancing Near-Infrared Two-Photon Absorption of Aza-Boron-Dipyrromethene Compounds Through Intramolecular Charge Transfer Via Electron Donating Substitution

The nonlinear optical properties of aza-borondipyrromethene (aza-BODIPY) derivatives modified with 4-methoxyphenyl, 2,4-dimethoxyphenyl, and 4-N,N-diphenylaminophenyl groups at the 1 and 7 positions of the core structure are evaluated. The effects of substituents and solvent polarity (tetrahydrofuran (THF) and chloroform (CHCl₃₎) on the linear absorption and fluorescence properties are systematically investigated. The target aza-BODIPYs exhibit higher absorption in CHCl₃ compared to THF. While solvent polarity exerted a minimal effect on the absorption and fluorescence intensity, the fluorescence in BOD2 and BOD3 is markedly quenched as a result of the electron-donating methoxy groups and the pronounced intramolecular charge transfer (ICT) characteristics of the diphenylamine unit. Moreover, all compounds shows strong near-infrared two-photon absorption (TPA) behavior. The TPA cross-sections of BOD1 and BOD3 are measured as 61 and 269 GM at 1000 nm, respectively, and BOD3 showed the highest value due to the enhanced ICT efficiency. Density functional theory calculations are also performed to study the thermodynamic and photophysical properties in different media. The findings indicated an enhancement in molecular stability and a reduction in HOMO–LUMO gaps, particularly in THF. Among the compounds, BOD3 stood out with its low energy gap and high polarizability, and its theoretical NLO parameters are in strong agreement with the experimental TPA findings.