Dark 2A_g State as Pathway for Intramolecular Singlet Fission: a DMRG Study
【摘要】：The fact that the double-excitation(dark 2Ag state) lies below single excitation(1Bu) discovered experimentally by Kohler  had been a long-standing puzzle in quantum chemistry solved by Karplus et al  by considering electron correlation at the configuration interaction level. Nevertheless, targetting such 2Ag state for large molecule becomes intractable because of the multireference difficulty for the dark state. Density matrix renormalization group theory has been shown to be nearly exact for conjugated linear chain, even for the low-lying excited states  which was the very first application of DMRG in quantum chemistry. Very recently, intramolecular singlet fission(i SF) phenomena has been discovered by Busby et al. for a donor-acceptor copolymer . Regarding the role of the dark 2Ag state, two conflicting mechanisms have been proposed:(i) pros mechanism: the dark state 2Ag symmetry broken which led to direct photoabsorption followed by splitting into two triplet states , nanmely 2Ag1Bu is essential;(ii) cons mechanism: then the dark 2Ag state serves as a deactivation pathway, detrimental for iSF , namely, 2Ag1Bu is required for iSF. We here propose another mechanism. Through DMRG computation and spin correlation function analysis, we conclude that(i) 2Ag is of intrinsically TT character and(ii) it serves as an active pathway for iSF instead of deactivation;(iii) and according to this model, the much lower i SF quantum yield for oligonmer is due to the larger non-adiabatic coupling between 2Ag and 1Ag than in polymer.