Symmetry-Breaking Charge Separation in Molecular Constructs for Efficient Light Energy Conversion

Abstract

The generation of electron–hole radical pair at the active layer of organic photovoltaics through symmetry-breaking charge separation (SB-CS) has a crucial role in enhancing open-circuit voltage (V oc) and thereby increasing power conversion efficiency. Since the SB-CS materials achieve intramolecular charge separation with a negligible energetic driving force and decelerated charge recombination (CR) rate, SB-CS has been subjected to extensive experimental and theoretical studies. This Focus Review assesses the fundamentals of photosynthetic reaction centers, especially the “special pair”, and discusses how covalent control over the geometric arrangement, surrounding dielectric medium, and substitutions on multichromophoric perylenediimide architecture affects the energy landscape of SB-CS and CR. We systematically summarize the kinetically favored undesirable radiative and non-radiative deactivation channels of SB-CS and CR processes on diverse chromophoric arrangements. Here, we suggest new rational design principles to fine-tune the electron-transfer dynamics at the molecular level to improve the performance of light–energy conversion devices.

Excimer evolution hampers symmetry-broken charge-separated states

“…[8][9][10][11][12][13] Accomplishing SB-CS in a multichromophoric system is analogous to radical pair formation in silicon semiconductors, where exciton binding energy between hole and electron is overcome by thermal energy in silicon semiconductors. 3 The fundamental understanding of structureproperty correlation and related excited-state dynamics among the molecular architectures has enabled researchers to rationalize the design of organic photovoltaics (OPVs). [14][15][16][17][18] The conventional donor-acceptor based OPVs generally exhibit low open-circuit voltages (𝑽 𝑶𝑪 ) due to the significant energy difference between the optical band gap and charge-separated state at the interface.…”

“…3,[25][26][27][28][29][30] Among the various weakly coupled chromophoric assemblies investigated for SB-CS, multichromophoric perylenediimide (PDI) architectures are widely explored due to their exciting optoelectronic properties. 3,24,27,31 Out of the several interesting PDI molecular constructs reported to date, the spiro-conjugated orthogonal arranged PDI dimer (Sp-PDI2) reported by our group exhibited a prolonged SB-CS state (𝑘 𝑆𝐵-𝐶𝑆 /𝑘 𝐶𝑅 =2647 in acetonitrile). 32 Ultrafast SB-CS in PDI dimers was first experimentally demonstrated by Wasielewski and coworkers in cofacial and head-to-tail stacked PDI dimers.…”

“…Symmetry-breaking charge separation (SB-CS) is a photoexcitedstate process by which a pair of identical chromophores forms a charge-separated state with the electron and hole localized on different chromophoric units. [1][2][3][4][5][6][7] Since discovering their existence in photosynthetic reaction centers, SB-CS processes have received tremendous attention. [8][9][10][11][12][13] Accomplishing SB-CS in a multichromophoric system is analogous to radical pair formation in silicon semiconductors, where exciton binding energy between a hole and an electron is overcome by thermal energy in silicon semiconductors.…”

“…16,19,20 However, the energy loss during SB-CS is low compared to that during charge separation (CS) in donor-acceptor systems. 3,21 Moreover, SB-CS processes in solid-states, transition metal complexes, and polymers are a progressing topic of interest and have been widely explored in weakly coupled multichromophoric systems. [2][3][4][22][23][24] Developing robust organic materials exhibiting optimized SB-CS and recombination dynamics is hence pivotal for the advancement of organic photovoltaics.…”

Solvent dielectric delimited nitro–nitrito photorearrangement in a perylenediimide derivative

“…22 It is rationalized, that the photodissociation step is probably characterized by a nitritoaromatic intermediate (-ONO), whose experimental characterization is still in debate and therefore not observed hitherto. 11 Our persistent efforts in understanding the novel non-covalent interactions in unique chromophoric architectures 23,24 and deciphering the diverse photoexcited state processes in multichromophoric assemblies, [25][26][27][28] motivated us to dwell into the realm of linkage isomers, thereby unravelling the photoexcited mechanism responsible for the isomerization. This report highlights the first unequivocal evidence of linkage isomerism in a nitroperylenediimide chromophore.…”