Molecular Design And Theoretical Study Of The Character For Singlet Fission

Singlet fission (SF) is a sort of multi-exciton generation process, which is a photoexcited singlet exciton splits to two triplet excitons in an organic chromophore material. Theoretically, the doubling effect of triplet excitons with a long lifetime produced by the split of singlet will increase the efficiency of the theoretical photoelectric conversion to 47.7%, which break through the traditional Shockley-Queisser record 33.7%. As a result, the study of singlet fission has attracted wide attention in the scientific and engineering fields. In this paper, a series of novel aromatic pyrene ring derivatives were designed, and the relationship between the structure of chemical modification and the properties of singlet splitting was by the SF energy matching condition and the characteristics of multiple diradical.(1) The singlet fission molecules with multiple diradical character were designed by introducing bisfuran ring and cyano into pyrene aromatic rings. The multiple diradical character (yi) was calculated by using the approximately spin-projected spin-unrestricted Hartree-Fock method. The time dependent density functional theory (TDDFT) with Tamm-Dancoff approximation, thecomplete active space self-consistent field (CASSCF) method and the N-electron valence state perturbation theory (NEVPT2) method were used to investigate the relationship between the chemical structure and the energy level of SF. The calculation results show that PDF-5 is nearly satisfied with the SF energy condition with diradical character. In cyano substituted derivatives, PDFC-4 and PDFC-5 are satisfied with the energy level matching conditions and show obviously diradical character (y0> 0.46) and unapparently tetraradical character by odd electronic density calculations. best candidate of the singlet fission. Calculations suggest that PDFC-4 and PDFC-5 be good candidates of the singlet fission.(2) We design theoretically small molecule candidates for singlet fission (SF) molecules with multiple diradical character by replacing pairs of adjacent carbons with BN isoelectronic pairs in pyrene. The TDDFT method is used to calculate the energy levels of SF on the basis of the multiple diradical character (yi). The CASSCF method and the NEVPT2 method were performed to confirm the energy results. The multiple diradical character calculations show that the derivatives have no diradical character with a pair of BN substituent and don't meet the conditions of SF. While double BN substituted derivatives with two head-to-tail (N?B) BN fragments form two captodatively stabilized radical centers, nba-2 and nba-3, and the two separated BN unit also produce a total captodatively stabilized radical centers. The excitation energy calculation results suggest that the energy level matching conditions are satisfied in nba-2 and nba-3, in which the two separated BN unit produce a total captodatively stabilized radical centers.