Search for a Small Chromophore with Efficient Singlet Fission: Biradicaloid Heterocycles

Abstract

Of the five small biradicaloid heterocycles whose S(1), S(2), T(1), and T(2) adiabatic excitation energies were examined by the CASPT2/ANO-L-VTZP method, two have been found to meet the state energy criterion for efficient singlet fission and are recommended to the attention of synthetic chemists and photophysicists.

Guidance for Mutual Disposition of Chromophores for Singlet Fission

“…Indeed, all efficient SF sensitizers known up to 2010 were alternant hydrocarbons or their simple derivatives,4 and the first chromophore designed specifically for SF was a biradicaloid 14–17. Several others have since been calculated to meet the conditions (1) and (2),18–20 for another one, the fulfillment of condition (1) was not only calculated but also confirmed experimentally,21 and a more detailed theoretical analysis has been published 22,23…”

Tuning of the stability and energy levels of singlet exciton fission relevant excited states of pentacenes by site-specific substitution

“…The E SF calculated from E(S 1 ) − 2E(T 1 ) were used to predicate the driving force for SF, which is also the heat released during SF [4].…”

“…However, a close to zero value of E SF is desired for reducing the energy loss of SF in the photon to electron transfer process in solar cells [4]. On the other hand, the energy of T 1 (E(T 1 )) should smaller than the energy gap between 3 O 2 and 1 O 2 (0.98 eV) to avoid the generation of reactive singlet oxygen and undergoing cycloaddition subsequently [39,40].…”

“…In order to obtain an efficient SF, this process must be fast enough to compete effectively with other radiative and nonradiative deactivation pathways, therefore, an exoergic value for E SF (E(S 1 ) − 2E(T 1 )) is desirable. However, excessive exoergicity for SF would waste much energy and then loss of solar cell efficiency [4], so isoergicity is more favorable.…”

Potential Use of Squarates and Croconates as Singlet Fission Sensitizers

“…In the quest for new SF materials, molecules with an intermediate diradical character have been proposed as one of the main classes of promising SF sensitizers. This strategy was proposed by Michl and has been successfully used afterwards . The viability of this approach, that is, the design of SF sensitizers by means of their diradicaloid character, has been also demonstrated experimentally ,,,.…”