Excited-State Symmetry-Breaking Charge Separation Dynamics in Multibranched Perylene Diimide Molecules

Abstract

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.

Excited State Charge Separation in an Azobenzene‐Bridged Perylenediimide Dimer – Effect of Photochemical Trans‐Cis Isomerization

“…These results tracked those reported in the literature for PDI spectroelectrochemistry. [ 1k , 3a ] The small red‐shift observed in the case of PDI .+ compared to that reported for pristine PDI .+ could be attributed to the presence of azobenzene. Using spectroelectrochemical data, differential spectrum of the charge separation product, PDI .+ –PDI .− was constructed as shown in Figure S17c (spectrum of the cation plus anion minus neutral compound).…”

Linker dependent symmetry breaking charge separation in 9,10-bis(phenylethynyl)anthracene dimers

“…S2, ESI†), which is similar to the observation in perylene diimide oligomers in which the short-lifetime component was ascribed to that from the relaxed singlet state (S 1 ) to the SB-CS state. 12…”

“…Slow recombination of the CS state could provide sufficient time for charge separation at the donor/acceptor interface. 12 Weakly polar surroundings are closer to the real situation in the solid layer of photoelectric devices. However, to date, for most SB-CS materials, SB-CS only could proceed in polar solvents, and the recombination rate of the CS state is also fast.…”

“…However, to date, for most SB-CS materials, SB-CS only could proceed in polar solvents, and the recombination rate of the CS state is also fast. 11–18 This is disadvantageous for applying SB-CS materials in organic solar cells for improving the photoelectric conversion efficiency. Thus, developing new SB-CS materials with faster rates and slow recombination in nonpolar or weakly polar solvents is urgent for its practical application in organic solar cells.…”

Diastereoselective formation of homochiral flexible perylene bisimide cyclophanes and their hybrids with fullerenes

“…SBCS is unfavorable in non-polar toluene due to the positive free energy of charge separation. 55,56 Finally, it is with a time delay of 4.1 ns that the SBCS state populates the PBI triplet excited state as the fourth and nal species. 118 ms is the lifetime, by which the triplet excited state decays back to the ground state.…”