On couplings and excimers: lessons from studies of singlet fission in covalently linked tetracene dimers

Abstract

Electronic factors controlling singlet fission (SF) rates are investigated in covalently linked dimers of tetracene. Using covalent linkers, relative orientation of the individual chromophores can be controlled, maximizing the rates of SF. Structures with coplanar and staggered arrangements of tetracene moieties are considered. The electronic structure calculations and three-state kinetic model for SF rates provide explanations for experimentally observed low SF yields in coplanar dimers and efficient SF in staggered dimers. The calculations illuminate the role of the excimer formation in SF process. The structural relaxation in the S1 state leads to the increased rate of the multi-exciton (ME) state formation, but impedes the second step, separation of the ME state into independent triplets. The slower second step reduces SF yield by allowing other processes, such as radiationless relaxation, to compete with triplet generation. The calculations of electronic couplings also suggest an increased rate of radiationless relaxation at the excimer geometries. Thus, the excimer serves as a trap of the ME state. The effect of covalent linkers on the electronic factors and SF rates is investigated. In all considered structures, the presence of the linker leads to larger couplings, however, the effect on the overall rate is less straightforward, since the linkers generally result in less favorable energetics. This complex behavior once again illustrates the importance of integrative approaches that evaluate the overall rate, rather than focusing on specific electronic factors such as energies or couplings.

Exciton localization in excited-state dynamics of a tetracene trimer: a surface hopping LC-TDDFTB study

“…[46][47][48][49] As a drawback, analogous to linear response TDDFT, the LC-TDDFTB method does not allow for description of doubly excited states (see, e.g., refs. [50][51][52], which are important for processes such as singlet fission, and which can be calculated with, e.g., spin-flip configuration interaction approaches [53][54][55] or multireference perturbation theory, 52,56 which generally are much more time-demanding. Thus, by using LC-TDDFTB, we constrain the manifold of excited states to the singly excited singlet states, and focus on the dynamics within this subset.…”

Multireference exciplex binding energies: Basis set convergence and error

“…Singlet fission pathways, in which two electron‐hole pairs are obtained from a singlet excitation, often involve exciplex formation. Optimizing singlet fission in organic materials has the potential to increase photovoltaic efficiency …”

“…Optimizing singlet fission in organic materials has the potential to increase photovoltaic efficiency. [7][8][9][10] Characterizing aromatic exciplexes involves a number of challenges for electronic structure methods. Several methods fail to give the correct ordering for the valence π !…”

Mechanism of Spin-Exchange Internal Conversion: Practical Proxies for Diabatic and Nonadiabatic Couplings

“…We note here that Feng and Krylov also tested the effect of orbital overlap on the coupling strength, and found only a weak correlation between them. 46 However, they considered the NAC strength, while our derivation in this sections is for the diabatic coupling. Moreover, with our derivation, we found a direct relation between the diabatic coupling an the product of orbital overlaps (eq (8)), while in their work they took the sum of the orbital overlaps instead.…”