Electron–Hole Recombination Is Suppressed by Breaking the Ring Planarity in Porphyrin Nanorings: Density Functional Atomistic Simulation

Abstract

Porphyrin nanorings get enormous attention as potential photovoltaic materials due to their unique and tunable optoelectronic properties. Distribution of charge in porphyrin nanorings can alter the photovoltaic performance. We investigate the photodynamics of two porphyrin nanorings, i.e., fused and meso nanorings, to observe the role of charge delocalization on carrier relaxation dynamics. Employing nonadiabatic molecular dynamics within the framework of the density functional tight binding theory, we demonstrate that the meso nanoring exhibits six times longer exciton lifetime compared to the fused nanoring. Charges are more localized at the band edge states of the meso nanoring compared to fused nanoring and reduce the orbital overlap between electron and hole wave functions. As a result, localization of the charge weakens the nonadiabatic coupling, resulting in delayed electron–hole (e−h) recombination. Participation of low-frequency electron-vibrational modes and rapid decoherence at the energy gap further extends the exciton lifetime. Additional β conjunctions between the porphyrin dimer in the fused nanoring facilitate the charge delocalization throughout the nanoring because fusions between the porphyrin dimer hold the circular planarity of the nanoring. Quick charge delocalization creates a strong orbital overlap between the ground and the excited states, resulting in quick charge recombination. Further, the simulated exciton binding energy and charge transition rate support our results. We demonstrate that e−h recombination is dependent on the planarity of the porphyrin nanoring. Our simulations give light on the effect of charge localization on the carrier relaxation dynamics by tuning the geometry of the porphyrin nanoring and provide valuable guidance to design high-performance porphyrin and organic conjugated system-based optoelectronic appliances.