Exact wave packet dynamics of singlet fission in unsubstituted and substituted polyene chains within long-range interacting models

Abstract

Singlet fission (SF) is a potential pathway for significant enhancement of efficiency in organic solar cells (OSC). In this paper, we study singlet fission in a pair of polyene molecules in two different stacking arrangements employing exact many-body wave packet dynamics. In the noninteracting model, the SF yield is absent. The individual molecules are treated within Hubbard and Pariser-Parr-Pople (PPP) models and the interaction between them involves transfer terms, intersite electron repulsions and site-charge-bond-charge repulsion terms. Initial wave packet is constructed from excited singlet state of one molecule and ground state of the other. Time development of this wave packet under the influence of intermolecular interactions is followed within the Schrödinger picture by an efficient predictor-corrector scheme. In unsubstituted Hubbard and PPP chains, 2 1 A excited singlet state leads to significant SF yield while the 1 1 B state gives negligible fission yield. On substitution by donor-acceptor groups of moderate strength, the lowest excited state will have sufficient 2 1 A character and hence results in significant SF yield. Because of rapid internal conversion, the nature of the lowest excited singlet will determine the SF contribution to OSC efficiency. Furthermore, we find the fission yield depends considerably on the stacking arrangement of the polyene molecules.

Optical Projection and Spatial Separation of Spin-Entangled Triplet Pairs from the S1 (21 Ag–) State of Pi-Conjugated Systems

“…Various strategies have been suggested for biasing the fate of S 1 (2 1 A g -) toward triplet production over internal conversion, such as engineering an enhanced ''triplet-pair'' weight into the S 1 (2 1 A g -) wavefunction with donor-acceptor copolymers, 26 and exploiting polymer-polymer interactions to extract long-lasting triplet states. 46,47 Interestingly, when SF does occur in systems expected to have relevant S 1 (2 1 A g -) states, the optical signatures of this intervening singlet are not observed, 18,[48][49][50][51][52] and its proposed role in fission remains controversial. 26,35,53,54 Although, several groups have shown that some polymers, all the way back to polyacetylene 35,39 have triplet-pair-like states similar to 2 1 A g -, 48,55 the general design strategy has been to avoid ''dark'' 2 1 A g states for efficient fission.…”

Theoretical Modeling of Singlet Fission

“…On the other hand, molecular torsion of photoexcited polyenes has been suggested as a potential mechanism able to lower the rather strong triplet–triplet binding energy . Despite recent advances in the development of theoretical models, , much work is necessary in order to achieve a deep understanding of the intricacies and specificities of singlet fission in polyenes and carotenoids. In particular, there is still some debate on the nature of low-lying dark electronic states in carotenoids, which does not allow one to establish a well-defined singlet fission mechanism for these systems.…”

Theoretical Studies of Singlet Fission: Searching for Materials and Exploring Mechanisms

“…Prodhan and Ramasesha conducted a quantum dynamics study on SF in polyenes . The polyenes include 1,3‐butadiene, 1,3,5‐hexatriene, and 1,3,5,7‐octatetraene.…”