Symmetry-Directed Control of Electronic Coupling for Singlet Fission in Covalent Bis–Acene Dimers

Abstract

While singlet fission (SF) has developed in recent years within material settings, much less is known about its control in covalent dimers. Such platforms are of fundamental importance and may also find practical use in next-generation dye-sensitized solar cell applications or for seeding SF at interfaces following exciton transport. Here, facile theoretical tools based on Boys localization methods are used to predict diabatic coupling for SF via determination of one-electron orbital coupling matrix elements. The results expose important design rules that are rooted in point group symmetry. For Cs-symmetric dimers, pathways for SF that are mediated by virtual charge transfer excited states destructively interfere with negative impact on the magnitude of diabatic coupling for SF. When dimers have C2 symmetry, constructive interference is enabled for certain readily achievable interchromophore orientations. Three sets of dimers exploiting these ideas are explored: a bis-tetracene pair and two sets of aza-substituted tetracene dimers. Remarkable control is shown. In one aza-substituted set, symmetry has no impact on SF reaction thermodynamics but leads to a 16-fold manipulation in SF diabatic coupling. This translates to a difference of nearly 300 in kSF with the faster of the two dimers (C2) being predicted to undergo the process on a nearly ultrafast 1.5 ps time scale.

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

“…BT1‐ trans and BT1‐ cis are two such structures. In the third paper of this series, Damrauer and Snyder evaluated the effective eg ( ge )– tt couplings to be 1.1 and 2.0 meV for BT1‐ trans and ‐ cis , respectively. Despite the small magnitudes of all transfer integrals in BT‐1‐ trans , the constructive interference between the two CT branches gives an effective coupling comparable to that of BT1‐ cis .…”

Design of singlet fission chromophores with cyclic (alkyl)(amino) carbene building blocks

“…One obstacle for the application of the SF phenomenon in photovoltaics is that only a small number of molecules meet the two criteria: e.g., tetracene, 7 pentacene, 8-10 some of their derivatives, [11][12][13][14][15][16][17][18][19][20][21] perylenediimide, [22][23][24] and some polyenes. [25][26][27] Especially, research of SF is "overwhelmingly limited" to tetracene and pentacene.…”

The hierarchical and perturbative forms of stochastic Schrödinger equations and their applications to carrier dynamics in organic materials

“…It is noted that through artificial design strategies such as changing the symmetry of covalent dimer, breaking local crystal structures, controlling side chains or elements, one can effectively tune the couplings and . The consequence of J‐ and H‐aggregates in aggregate spectra can be further borrowed to guide the tuning of and .…”

“…Much attention has been given to the SF process recently for the potential of overcoming the Shockley–Queisser limit in single‐junction photovoltaic devices . Several works have found that the quantum interference between different pathways of the SF process, such as the direct and indirect CT‐mediated pathways, can have strong impact on SF rates . Interestingly, there is an analogous process in aggregate spectra, where a simple dimer model are denoted as H (J)‐aggregates when the direction of transition dipole moments of monomer are perpendicular (parallel) to the line connecting the the centers of monomer, with the higher (lower) excitonic states optically bright as revealed in the molecular spectra.…”