Non-orthogonal Configuration Interaction Study on the Effect of Thermal Distortions on the Singlet Fission Process in Photoexcited Pure and B

Identifier:  imarina:9327605

Handle: https://hdl.handle.net/20.500.11797/imarina9327605

Authors:  López X; Straatsma TP; Sánchez-Mansilla A; de Graaf C

Abstract:
The present computational work analyzes singlet fission(SF) asa pathway for multiplication of photogenerated excitons in crystallinepolyacenes. Our study explores the well-known crystalline pentacene(C22H14) and the prospective and potentiallyinteresting doped B,N-pentacene (BC20NH14).At the molecular level, the singlet fission process involves a pairof neighboring molecules and is based on the coupling between an excitedsinglet state (S1S0) and two singlet-coupledtriplets ((T1T1)-T-1), which, typically,is influenced by an intermolecular charge transfer state. Taking datafrom periodic density functional theory and ab initio wave functioncalculations, we applied the non-orthogonal configuration interactionmethod to determine electronic coupling parameters. The comparisonof the results for both equilibrium structures reveal smaller SF couplingfor pentacene than for B,N-pentacene by a factor of & SIM;5. Introductionof the dynamic behavior to the crystals (vibrations, thermal motion)provides a more realistic picture of the effect of the disorder atthe molecular level on the SF efficiency. The coupling values associatedto out-of-equilibrium structures show that most of the S1S0/(T1T1)-T-1 couplings remainvirtually constant or slightly increase for pentacene when moleculardisorder is introduced. Homologous calculations on B,N-pentacene showa generalized decrease in the coupling values, notably if large phonondisplacements are considered. A few of the structures analyzed featuremuch larger SF coupling if some distortion results in (nearly) degeneratecharge transfer and excited singlet and triplet states. For theseparticular situations, an acceleration of the SF process could occuralthough in competition with electron-hole separation as analternative pathway.