Computational investigation on non-linear optical properties of hexaphyrin and core modified hexaphyrins

Abstract: Expanded porphyrin-based (Hexaphyrins) sensitizers are promising due to their excellent light harvesting feature in dye-sensitized solar cell (DSSC). We calculated the low-lying excitations of expanded porphyrins (EPs) as hexaphyrin and core modified hexaphyrin structures using Time-Dependent Density Functional Theory. Our calculation showed the EPs (both hexaphyrin and core modified hexaphyrin) have broad range of absorption band suitable for harvesting the visible and near infrared region of solar spectrum. All EPs studied here satisfy the energy condition of singlet fission (SF). SF is the process in which the theoretical limit of Shockley-Quiesser (SQ) (33\%) can be overcome in single junction solar cell. The non-linear optical properties like first hyper polarizability $\beta$ and second order hyper polarizability $\gamma$ were calculated using coupled perturbed Hartree-Fock approach. From the second order NLO properties we carried out degenerate four wave mixing (DFWM) component ($\gamma^{(2)}(-\omega;\omega,\omega,-\omega$)) and finally quadratic non linear refractive indices of these EPs are calculated. Calculation showed EPs are promising as organic dye for the opto-electronic applications and useful for high efficiency DSSC and also useful for potential NLO materials as their hyper polarizabilities showed higher order non linearities.