Design And Investigation Of Small Molecules For Singlet Fission Sensitizers

Singlet fission(SF)is a spin-allowed photophysical process in which a high-energy singlet exciton splits into two low-energy triplet excitons.Although it was observed fifty years ago,it has been renewed interest of researchers until 2006.It is expected that the ulitimation of this process can overcome the Shockley-Quessier limit of single-junction solar cells and improve the energy coversion efficiency from 33%to 46%.Therefore,it has been the research focus in the field of photo-to-electric conversion.However,only a limited number of SF chromophores with efficient triplet yields have been found,including tetracene,pentacene and their derivatives.Michl et al.proposed that diradicaloids are also promising SF chromorphores besides polyacenes.Recently,efficient SF has been found in D-A polymers and small molecules with D-A-D electronic structures.The resonance structure between aromatic form and quinoidal form in these systems makes them to be diradicaloids.The common building blocks of these organic polymers include diketopyrrolopyrrole(DPP),isoindigo(IID),bithiophene(BT)and so on.Therefore,seeking new SF chrmophores based on these building blocks is necessary.In this work,structure modifications of these building blocks have been made to get a series of compounds based on DPP,IID,BT and benzodipyrrolidone(BDP).The correlations between molecular structures,diradical character and SF relevant excited state energies were explored,to get suitable SF chromophores and feasible modification strategies,and suitable SF sensitizers further.The main contents of this thesis include the following points:1.Singlet fission sensitizers based on quinoidal bithiopheneEfficient SF has been found in quinoidal bithiophene end-capped with dicyanomethylene groups(QBT).However,it has rather low E(T₁),which hinders its application as SF sensitizer.Modifications to the molecular structure of QBT were performed to optimize the SF relevant excited-state energy levels and its diradical character.It includes chalcogen replacement,fusing of the heterocyclic ring between the two thiophene rings,and introduction of side-substituents.Detailed analysis focused on the correlations between the molecular structures of the QBT derivatives and their diradical character y₀,bond length alternation(BLA),the frontier molecular orbital energy levels,and SF relevant excited-state energy levels.The results show that typical electron-donating substituents,particularly groups introduced at the inner?-positions of the thiophene ring,can increase E(T₁)and reduce the energy loss of SF significantly under the premise of exothermic SF.2.Singlet fission sensitizers based on diketopyrrolopyrroleAryl-substituted DPPs can undergo SF efficiently after photoexcitation.In order to find suitable DPP-based SF sensitizers for solar cells,we designed a series of DPP derivatives by varying aromatic substituents,including changing the conjugation and constitution of aromatic substituents,as well as introducing side-substituents on the aromatic units.Detailed analysis focused on the molecular structures,the frontier molecular orbitals,multiple diradical characters and SF relevant excited-state energy levels.The results indicate that aromatic substituents containing no more than two aromatic rings and modification of the aromatic rings with side-substituents are both practical ways to get suitable SF sensitizers for solar cells.3.Singlet fission sensitizers based on benzodipyrrolidoneBDP has been widely used as electron-acceptor for organic polymers.Here we desiged a series of BDP and its analogue BDF derivatives to investigate their SF properties.It includes that attaching aromatic groups with different electron-donating abilities,and further modifying the central benzene ring of BDP core with inserting heteroatoms or connecting side-substituents.Detailed analysis focused on molecular structures,BLA,diradical character and SF relevant excited-state energie levels.The results indicate that BDP and BDF core with stronger electron-accepting ability and/or aromatic groups having weaker electron-donating abilities could increase E(T₁).Thiophene and thiozale attached derivatives show relatively appropriate E(T₁)for SF sensitizers.4.Singlet fission sensitizers based on isoindigo and indigoEfficient SF has been found in isoindigo-based polymers and indigo fulfill the energetic requirement of SF as proposed by Michl et al..Here a series of isoindigo and indigo derivatives has been designed to investigate SF properties of isolated isoindigo and indigo derivatives.It includes that modifying the five-member rings and the benzene rings,respectively.Detailed analysis focused on molecular structures,the frontier molecular orbitals energy levels and SF relevant excited-state energy levels.The results indicate that the replacement of benzene rings with thiophene or thiazole rings gets appropriate E(T₁)for SF sensitizers.