Exploring The Charge Transfer Mediated Singlet Fission

Singlet fission(SF)is a spin-allowed multiexciton generation processs,by which a photoexcited singlet exciton is splitted into two triplet excitons.It has been proposed that SF can break the theoretical limit of power conversion efficiency for single junction solar cells.Exploring SF based solar cells has been constrained by a lack of materials capable of efficient SF.Exploring new material design principle is highly desirable for the development of novel SF-capable materials with strong light absorption and high triplet yields.To this end,isoindigo and bay--annulated indigo based derivatives were used to study the charge transfer mediated SF,because the indigo unit has a strong absorption coefficient and is easy-to-functionalize.The main contents of this thesis are detailed as follow:Firstable,an isoindigo based conjugated polymer(IIDDT-Me),which shows a donor-acceptor type of structure,was selected as an archetype compound to provide further proof that the charge-transfer state plays a vital role in mediating the SF process.The calculation results showed that the quinoid resonance structure of IIDDT-Me can effectively lower the triplet energy level,thus the energy requirement(E(S₁)?2E(T₁))for SF is well satisfied.Transient absorption studies revealed that dynamic equilibrium of the singlet state(S₁),the coexistence state of the singlet and triplet pair(S₁+TT),and the triplet exciton pair(TT)is formed during SF,and the corresponding lifetimes are?7.8 ps,?58 ps,and?190 ps.In addition,transient fluorescence studys showed that triplet pair(TT)is emissive,and the emission wavelength red-shifted over time.Highly efficient intramolecular SF with a triplet yield up to 160%-200%was confirmed in the dilute solution of IIDDT-Me.Further studies of the exciton dynamics in the aggregated state suggested that intramolecular SF was completely quenched,which was attributed to the decreased singlet energy due to aggregation of polymer chains.Secondly,since isoindigo has been proved as a promising motif for constructing novel SF materials,we further investigate the potentional application of bay-annulated indigo derivatives mediated by charge transfer.Compounds 2T-BAI-2TC12,BAI-1Tx-1Ty and TPA-3D-BAI,whose synthesis have been reported in the literature,were used for this study.Their exciton dynamics were systematically studied by using ultrafast spectroscopy technology,and BAI was used as a control compound.Transient absorption studies revealed that compound TPA-3D-BAI is capable of efficient intramolecular SF with a triplet yield of 143%.Transient fluorescence spectra show that triplet pair state generated by SF in TPA-3D-BAI can also undergo radiative decay.The efficient SF in TPA-3D-BAI can be rationalized with the strong intramolecular charge transfer interaction,as well as the reduced triplet energy level due to the increased diradical character.Furthermore,according to the Baird 4n electron rule,the4n electronic structure of the five member ring within BAI structure can stabilize the triplets generated by SF.In summary,our studies further proved that the intramolecular charge transfer state plays an important role in mediating singlet fission.In addition,isoindigo and bay-annulated indigo are very promising building blocks for SF,and their corresponding derivatives,IIDDT-Me and TPA-3D-BAI,are capable of highly efficient intramolecular SF.Our result can help understand the underlying mechanism of SF,and provide new insight into exploring the structure-function relationship in singlet fission.