Role of Dipolar Interactions in Fine-Tuning the Linear and Nonlinear Optical Responses in Porphyrins

The variation in the ground-state dipole moment, polarizability and the 1 st hyperpolarizability for non-chelated porphyrins are studied with increase in the inter-dipolar angles between the pyrrole rings. The different dipolar orientations are realized through twisting of one of the ring along the porphyrin plane. Computations performed on both conformational and configurational isomers of porphyins lead to a variety of structures. For the conformational distortions, the dipole-moment (μ), the polarizability (α) and the 1 st hyperpolarizabilities (β) increase with increase in the distortions. This is understood on the basis of a simple excitonic picture wherein the splitting due to dipolar interactions reduces with increase in the distortions which effectively reduces the optical gap for the system. The computations reveal that dihedral twists within the conventional porphyrin structure provide strategies to design molecules with enhanced linear and nonlinear response functions.