Quantum Dynamics of Charge Carriers in Fullerenes Encapsulated by Covalent Organic Polyhedra: Choice of Fullerene Matters
Shrabanti Mondal, Uttam Kumar Chowdhury, Md Habib, Shriya Gumber, Ranjan Das, Thomas Frauenheim, Ritabrata Sarkar, Oleg V. Prezhdo, Sougata Pal
Abstract
High Resolution Image Download MS PowerPoint Slide Charge separation is at the heart of solar energy applications, and efficient materials require fast photoinduced electron transfer (ET) and slow charge recombination (CR). Using time-dependent self-consistent charge density functional tight-binding theory combined with nonadiabatic (NA) molecular dynamics, we report a detailed analysis of ET and CR in hybrids composed of photoactive covalent organic polyhedra (COP) and encapsulated fullerenes. The ET occurs on a subpicosecond time scale and accelerates with increasing fullerene diameter, C 60 to C 70 to C 84 . As the fullerene size increases, the π-electron system available for interaction with the COP grows, the fullerene-COP separation decreases, and the number of fullerene states available to accept the photoexcited electron increases, accelerating the ET. In comparison, the CR occurs on a nanosecond time scale and correlates with the length of the fullerene shortest axis because the relevant fullerene state is polarized in that direction. The largest and least symmetrical C 84 exhibits the fastest ET and the slowest CR, making COP@C 84 the most promising hybrid. Both high-frequency bond stretching and bending vibrations and low-frequency breathing modes are involved in the ET and CR processes, with more modes present in the C 84 system due its lower symmetry. The 10–20 fs vibrationally induced coherence loss in the electronic subsystem contributes to long lifetimes of the charge-separated states. The comprehensive investigation of the structure–property relationship of the charge carrier dynamics in the COP@fullerene hybrids provides a detailed atomistic understanding of interfacial ET processes and generates guidelines for rational design of high-performance materials for solar energy and related applications.