Second harmonic generation (SHG) plays a crucial role in modern nonlinear optics, with applications spanning nonlinear photonics and material science. Organic molecular aggregates, owing to their distinct morphology, are promising candidates for nonlinear optical applications. One such aggregate is formed by zwitterionic meso-tetra(4-sulfonatophenyl) porphyrin (TPPS4), which self-assembles into chiral nanotubes under acidic conditions (pH ≤ 4). In this study, we model the nonlinear optical properties of TPPS4 nanotubes using quantum chemistry methods.
We construct two chiral nanotube models — one with clockwise and the other with counterclockwise chirality — and compute their SHG responses using our in-house quantum chemistry package, QCFP. Density functional theory (DFT) calculations, performed using CAM-B3LYP 6-311G(d,p) in water (PCM), reveal significant differences in dipole moments between the Z1 and Z2 TPPS4 monomers in both ground and excited states. These dipole moments are then used to evaluate the SHG response of each nanotube. Calculated SHG response spectra indicate that intensity varies with the polarization and propagation direction of the incident wave. Our findings also suggest that chirality plays a role in the strength of the SHG response. These findings enhance our understanding of the nonlinear optical behavior of TPPS4 aggregates and highlight their potential for applications in tunable photonic devices.