Quantum simulation of carrier dynamics in nanographene-fused carbaporphyrin tweezers@C60 heterojunctions: role of dihedral-angle engineering

The molecular structures and electronic properties of semiconductors are two fundamental concepts in photovoltaic research, as they directly control charge-carrier behaviour. We used nonadiabatic (NA) quantum simulation to demonstrate geometry-dependent excited-state dynamics in nanographene-fused carbaporphyrin tweezers (NCT) and encapsulated C60 by engineering the dihedral angle between the nanographene planes. Herein, we investigate the photoinduced ultrafast electron transfer (ET) occurring on the sub-picosecond timescale in dipyrromethene (D)-NCT@C60, in good agreement with experiment. The simulated average dihedral angle between nanographene planes is 56.73° in D-NCT@C60. We observed that the dihedral-angle configurations of the tweezers in aza-dipyrromethene (AD)-NCT@C60 (49.84°) and 2,2-bipyrrole (BP)-NCT@C60 (64.96°) play a subtle role on the ET. The identical ET pattern aligns with the comparable energy offset and NA coupling values between donor and acceptor states. In contrast, the dihedral angle variation greatly influenced the electron-hole recombination rate at the band edges. In BP-NCT@C60, electrostatic interaction between linker and C60 confines the C60 to the cavity of the tweezers, while nanographene-C60 interaction increases the linker-C60 separation in AD-NCT@C60. Also, a lower dihedral angle limits energy level reduction of C60 and hinders the conjugation of participating orbitals, widening the band gap. The larger band gap, longer spatial charge separation and weak NA coupling at the band edges suppress the recombination rate in AD-NCT@C60. In this theoretical study, we highlight the impact of dihedral angle on the carrier dynamics of NCT@C60 heterojunctions to realize an efficient photovoltaic device.