Aggregation-Induced Radical

Since the discovery of conductive polymers,organic semiconductors have developed rapidly in the past 40 years.Among them,low bandgap semiconductors have the characteristics of absorption up to the infrared region,high mobility and low exciton binding energy,which shows great application prospects in photoelectric devices such as organic solar cells and organic field-effect transistor.Low band gap quinoidal polycyclic aromatic hydrocarbons(PAHs)and donor-acceptor(D-A)type semiconductors are two typical low band gap semiconductor materials.With the bandgap decreasing,the quinoidal PAHs have open shell single / triplet electronic structure.The driving force for the formation of radicals in these molecules is derived from the restoration of molecular aromaticity.However,D-A type low bandgap semiconductors have been recognized as closed-shell electronic structures.Meanwhile,the radicals in it have been recognized from impurities,defects,polarons,oxygen-doped states,etc.In 2017,our group(J.Phys.Chem.C 2017,121,8579-8588)discovered that all organic low bandgap semiconductor materials have quinoidal-diradical(Q-D)structure.Based on the previous work of our group,in order to further study the source of free radicals in D-A low bandgap small molecules,we further explored the essential structure of D-A low bandgap small molecules by studying the electronic ground state and radical formation mechanism of D-A low bandgap semiconductor materials.First,series of D-A type low bandgap semiconductor molecules based on pyrrolopyrrole diketone(DPP)units were synthesized.By studying the relationship between the electron paramagnetic resonance(ESR)signal intensity and the molecular structure and molecular normal temperature nuclear magnetic signal,we proposed the radicals in the D-A type low bandgap molecules based on DPP unit are derived from the molecular essential structure rather than impurities or defects.With the help of X-ray single crystal diffraction and theoretical calculations,we found that the D-A type low bandgap semiconductors based on DPP units have an open-shell quinoidal-diradical(Q-D)structure.Variable temperature nuclear magnetic,variable temperature electron spin resonance and superconducting quantum interference showed that the D-A type low bandgap semiconductors based on DPP unit have open-shell singlet electronic ground state.In order to understand the mechanism of the formation of its QD structure,we studied the molecular aggregation state,and we found that the molecular aggregation state can induce D-A type low bandgap molecules to form Q-D structure,so we proposed the mechanism of aggregation-induced radical to explain the formation of its Q-D structure.This provides a new perspective to understand the essential structure of D-A type low band gap semiconductors.Secondly,we studied the radicals of D-A type photovoltaic donor materials and nonfullerene acceptors.We found that the donor materials have strong ESR signals,while the acceptors have very weak ESR signals.Theoretical calculations indicated that the rings in the donor molecular skeleton don't have strong aromaticity,while the core of the non-fullerene acceptor has strong aromaticity.As for the donors,the rings are easy to lose part of the aromaticity and exhibit quinoidal characteristic,therefore,donors can form a Q-D structure.In addition,the aggregation state of molecule can promote the further stability of its Q-D structure.As for the non-fullerene acceptors,the fused ring aromatic core converted into a quinoidal structure will lose lots of the aromaticity,which means non-fullerene acceptors are difficult to form a Q-D structure.Finally,we studied the relationship between the radical signal strength of the acceptor and the performance of the photovoltaic device based on the acceptor.We found that the performance of the organic solar cells(OSCs)based on the acceptor with strong radical signal is poor,while the OSCs based on the acceptor with weak radical signal is good.