The possibility and rate of charge separation (CS) in donor−bridge−acceptor molecules mainly depend on two factors: electronic coupling and solvent effects. The question of how CS occurred in two identical chromophores is fundamental, as it is particularly interesting for potential molecular electronics applications and the photosynthetic reaction centers (RCs). Conjugated bridge definitely plays a crucial role in electronic coupling. To determine the bridgemediated charge separation dynamics between the two identical chromophores, the isomeric N-annulated perylene diimide dimers (para-BDNP and meta-BDNP) with different conjugated bridge structures have been comparatively investigated in different solvents using femtosecond transient absorption spectra (fs-TA). It is found that the charge separation is disfavored in weak polar solvent, whereas direct spectroscopic signatures of radicals are observed in polar solvents, and the rate of charge separation increases as the solvent polarity increasing. To our surprise, the rate of charge separation in m-BDNP is more than an order of magnitude slower than that in p-BDNP, although there is a larger negative ΔG CS in m-BDNP. The slow CS rate that occurred in m-BDNP mainly results from the intrinsic destructive interference of the wave function through the meta-substituted bridge. The roles of solvent effects in free energy and electronic coupling for charge separation are further identified with quantum calculations.

As one of the most promising nonfullerene acceptors for organic photovoltaics, perylene diimide (PDI)-based multibranched molecules with twisted or three-dimensional (3D) geometric structures have been developed, which effectively increase the power conversion efficiency (PCE) of organic solar cells. Understanding the structure–property relationships in multichromophoric molecular architectures at molecular and ultrafast time levels is a crucial step in establishing new design principles in organic electronic materials. For this, photodriven excited-state symmetry-breaking charge separation (SB-CS) of PDI-based multichromophoric acceptors has been proposed to improve the PCE by reducing the self-aggregation of the planar PDI monomer. Herein, we investigated the intramolecular excited-state SB-CS and charge recombination (CR) dynamics of two symmetric phenyl-methane-based PDI derivatives, a twist dimer PM-PDI2 (phenyl-methane-based PDI dimer) and a 3D configuration tetramer PM-PDI4 (phenyl-methane-based PDI tetramer), in different solvents using ultrafast femtosecond transient absorption (fs-TA) spectroscopy and quantum chemical calculations. The quantum chemical calculations and steady-state spectra show that the two PDI derivatives undergo conformational changes upon excitation, leading to their emission states that have the characteristics of partial charge-transfer (CT) exciton in all solvents. Based on the evolution of the fs-TA data, it is observed that the evolution from the CT state to SB-CS state is disfavored in a weak polar solvent, whereas clear SB-CS spectroscopic signatures of cationic and anionic PDI are observed in polar solvents. Faster CS and slower CR processes of PM-PDI4 are observed in comparison to those of PM-PDI2. The crowded space in the 3D structure shortens the distance between the branches, leading to a stronger electronic coupling at the lowest excited state and a larger negative Gibbs free energy change of PM-PDI4 relative to that of PM-PDI2, which benefits the charge separation among PDI units in PM-PDI4. Besides, the 3D structure of PM-PDI4 also restricts rotation to a surface crossing region between the excited state and ground state, thus inhibiting nonradiative CR process and increasing the CS state lifetime. Our results suggest that the kinetics of CS and CR processes are strongly related to the molecular geometric structure, and the excited-state symmetry breaking in the 3D structure acceptor has superior photogenerated charge and photovoltaic properties from the perspective of ultrafast dynamics.

Recently, an organic synthetic strategy based on hybridized local and charge transfer (HLCT) character has been attracting much attention because of its potential for designing high-efficiency organic light-emitting diode materials. In this work, two novel molecules, N,N-diphenyl-4-phenol-(1-phenyl-1H-phenanthro­[9,10-d]­imidazol-2-yl)­biphenyl-4-amine (TPA-PPI-OH) and N,N-diphenyl-4′-(1-phenyl-1H-phenanthro­[9,10-d]­imidazol-2-yl)-[1,1′-biphenyl]-4-amine (TPA-PPI), were investigated by quantum chemical calculations, steady-state spectroscopy, and femtosecond transient absorption spectroscopy (fs-TA) to explore the nature of HLCT. Computational results and steady-state spectra suggest that the lowest excited state is dominated by local excitation (LE) character in low-polar toluene (TOL), whereas the charge transfer (CT) character plays the main role in high-polar acetonitrile (ACN) for both TPA-PPI-OH and TPA-PPI. Relative to TPA-PPI, TPA-PPI-OH shows less sensitivity to solvent polarity with higher quantum yields because of the more planar geometric structure, fabricated by inserting an additional intramolecular hydrogen bond (H-bond) to enhance the inflexibility of the molecule. Ultrafast fs-TA clearly shows the conversion of excited states from LE to CT with the increase of solvent polarity. The stimulated emission is mainly from the LE-dominated lowest excited state in low-polar TOL, whereas CT dominates the final relaxation process in high-polar ACN because of strong solvation. Furthermore, the excited states being dominated by LE and CT simultaneously in medium-polar tetrahydrofuran is observed, while the quick equilibrium LE ↔ CT is established just after a femtosecond pulse excitation, indicating the typical HLCT character. The excited state deactivation process of TPA-PPI-OH is faster than that of TPA-PPI, which is attributed to the higher proportion of the LE component and the additional vibrational decay paths induced by the H-bond in TPA-PPI-OH. The results herein offer a guidance to understand the solvent-modulated excited state deactivation mechanism of HLCT molecules.