Transient Magneto-optical Spectroscopic Study Of The Singlet Fission Process

In some organic molecules,the energy of singlet excited state is about twice of that of triplet excited state.One photo-excited singlet state may be converted into a pair of triplet population,known as singlet fission.Singlet fission doubles the quantum efficiency for photon-to-electron,holding the potential to boost the efficiency of solar energy conversion.The theoretical limit of the power conversion efficiency for a single-junction solar cell may increases from 33%to 45%when singlet fission can be ideally implanted.In order to search singlet fission sensitizers with spectral coverage suitable for practical device applications,it is essential to establish a comprehensive picture of the mechanism of singlet fission.In available literatures,a multi-exciton state of correlated triplet pair has been accepted as the intermediate connecting the photo-excited singlet and two triplet states.In this work,we focus on understanding the generation and dissociation dynamics of the intermediate state for the intermolecular and intramolecular process of singlet fission using magneto-optical spectroscopy.By distinguishing the triplet pair state with weak and strong exchange interaction using the magnetic field effect,the critical role of spatially-separated triplet pair state has been identified in the tetracene crystal and oligomers.Moreover,the highly efficient triplet pair generation has been successfully applied for singlet oxygen generation.The major findings are summarized below:1?By using magneto-quantum beating spectroscopy,a direct channel of singlet fission from singlet state to the weakly-coupled triplet pair state has been established.In theory,the projection of triplet pair state on the singlet has been systematically studied using Merrifield model.The dependences of quantum beating spectra on the strength and alignment of the magnetic field suggest that the quantum beat spectrum probes the fine levels of weakly-coupled triplet pair state,i.e.,the spatially-separated state.The experimental results suggest the existence of a direct intermolecular singlet fission channel from the singlet to the spatially-separated triplet pair state,which explains the previous debates on the temperature dependence of the fission rate for singlet fission in tetracene films.The results suggest the spatially-separated triplet pair state is a key intermediate for the direct channel of the intermolecular singlet fission mechanism.2?The generation and dissociation dynamics of the intermediate state of triplet pairs in tetracene oligomers have been investigated using transient magneto-optical spectroscopy.We have shown that together with the adjacent triplet pair state where two triplet excitons reside on adjacent tetracene units,the spatially separated ~1(T…T)state acts as intermediate states for efficient intramolecular singlet fission processes in covalently linked tetracene trimer or larger arrays.Different from the adjacent triplet pair state,the generation dynamics of the weakly exchange coupled state is strongly dependent on the magnetic field.Benefiting from the weak inter-triplet interaction,the triplet pair dissociation for the spatially-separated triplet pair state is much more efficient than that for the adjacent triplet pair state,which promotes the efficiency of free triplet generation from 21%for the dimer to 85%(124%)for the trimer(tetramer),respectively.Our results provide an alternative route for highly efficient triplet pair dissociation,representing an important step towards practical applications of intramolecular singlet fission materials since the free triplets can be harvested much more efficiently than the triplet pair states through the interfacial charge transfer.Moreover,the rate of the singlet fission from the singlet to the adjacent triplet pair increases with the polarity of the solvent,which suggests the charge-transfer state mediates the singlet fission process.This finding provides more insights into the intramolecular singlet fission mechanism.3?Highly efficient singlet oxygen generation has been demonstrated using the singlet fission process in the intramolecular singlet fission molecules.Singlet oxygen is the key agent for the photodynamic therapy.The conventional photo-sensitizer is based on the intersystem crossing,which limits the singlet oxygen yield to 100%per photon absorbed.Here we show this limit can be potentially overcome by employing the multiplication effect of triplet state using singlet fission.We synthesize a series of oligomers of thieno[3,2-b]thiophene bridged perylene and demonstrate the intramolecular singlet fission process in these oligomers.Quantitative experiments further suggested a high quantum efficiency(>140%)of singlet oxygen generation per absorbed photon is enabled by a combination of?179%yield of the triplet pair generation,?80%of the triplet pair dissociation,and near unit triplet energy transfer from free triplet excitons to oxygen.The finding in this work represents a new scheme for singlet oxygen generation that can surpass the efficiency limit in the conventional intersystem-crossing-based approach.These merits are favorable for optimizing the photodynamic therapy agents with reduced dose of drugs.