DFT investigation of functional group effects on the structure of tetraphenyl porphyrin for enhanced nonlinear optical properties

This study investigates four donor-pi-acceptor (D-pi-A) tetraphenyl porphyrin derivatives (UAF1-UAF4) with systematically modified functional groups, revealing how structural changes govern optoelectronic and nonlinear optical (NLO) properties. Geometric harmonic oscillator model of aromaticity (HOMA) and electronic quantum theory of atom in molecules (QTAIM) aromaticity analyses show UAF1-UAF2 maintain strong aromaticity (HOMA = 0.940-0.941), while intermediate value of UAF4 (0.935) balances conjugation and polarizability. NBO analysis identifies critical intramolecular charge transfer pathways, with UAF4 exhibiting optimized donor-acceptor interactions (E(2) = 5.54 kJ/mol). Frontier orbital analysis (FMO) reveals narrow HOMO-LUMO gap (4.01 eV) for UAF4, supported by TD-DFT absorption spectra, showing redshifted transitions at 375 nm. The most striking finding is the exceptional nonlinear optical response of UAF4, exhibiting a static first hyperpolarizability (beta tot = 3.37 x 104 a.u.) that surpasses UAF1 by 455-fold. Dynamic hyperpolarizability calculations at telecom-relevant wavelengths (1460/1907 nm, confirm practical utility, with UAF4 maintaining high beta HRS values (8348/6401 a.u.). This performance stems from its balanced electronic structure, evidenced by electrostatic potential (ESP) maps showing optimal charge separation and unit sphere representations demonstrating anisotropic NLO activity. These results establish UAF4 as a prime candidate for optical switching and frequency conversion devices, while providing definitive design principles for porphyrin-based NLO materials: (1) moderate aromaticity preserves conjugation while enabling polarization, (2) asymmetric push-pull groups enhance charge transfer, and (3) delocalization must be tuned to match target wavelengths. The combined computational approach demonstrates how molecular engineering can optimize materials for advanced photonic applications.