Exploiting Excited-State Aromaticity To Design Highly Stable Singlet Fission Materials

Abstract

Singlet fission, the process of forming two triplet excitons from one singlet exciton, is a characteristic reserved for only a handful of organic molecules due to the atypical energetic requirement for low energy excited triplet states. The predominant strategy for achieving such a trait is by increasing ground state diradical character; however, this greatly reduces ambient stability. Herein, we exploit Baird's rule of excited state aromaticity to manipulate the singlet−triplet energy gap and create novel singlet fission candidates. We achieve this through the inclusion of a [4n] 5-membered heterocycle, whose electronic resonance promotes aromaticity in the triplet state, stabilizing its energy relative to the singlet excited state. Using this theory, we design a family of derivatives of indolonaphthyridine thiophene (INDT) with highly tunable excited state energies. Not only do we access novel singlet fission materials, they also exhibit excellent ambient stability, imparted due to the delocalized nature of the triplet excited state. Spin-coated films retained up to 85% activity after several weeks of exposure to oxygen and light, while analogous films of TIPS-pentacene showed full degradation after 4 days, showcasing the excellent stability of this class of singlet fission scaffold. Extension of our theoretical analysis to almost ten thousand candidates reveals an unprecedented degree of tunability and several thousand potential fission-capable candidates, while clearly demonstrating the relationship between triplet aromaticity and singlet−triplet energy gap, confirming this novel strategy for manipulating the exchange energy in organic materials.

Toward an understanding of electronic excitation energies beyond the molecular orbital picture

“…In the diagrams, we see that the lines pertaining to the excitation hole (red) interact with the operator, and algebraically this is represented by the term ∑ a C ia C ja , which we identify with the hole density matrix γ h according to Eq. (27). Furthermore, we find that the second row in Fig.…”

“…[23][24][25] Some rules exist, e.g. minimising the spatial overlap between the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) for delayed fluorescence 1, 22 and to exploit diradical character in the ground state 26 or aromaticity in the triplet state 27 for singlet fission but a more comprehensive theoretical framework is not available and unwanted side-effects are hard to exclude. 28 More specific cases of the breakdown of the MO picture are concerned with exciton correlation in conjugated polymers.…”

Structure and photophysics of indigoids for singlet fission: Cibalackrot

“…A different conclusion has been reached in very recent study by Fallon et al 62 of a group of solids whose molecular structures are nearly identical with that of 2 (they carry alkylthienyl instead of phenyl substituents), and this might be viewed as a discrepancy. Tacitly assuming that thin films of these materials are isotropic or that the observed singlet-singlet and triplet-triplet transition moments are parallel, the authors used transient absorption measurements to find a triplet yield of ~60% at 1 ns after initial excitation (in the absence of a kinetic analysis, the initial yield remains unknown).…”

“…In support of their claim of SF in their variously substituted thienyl analogs of 2, the authors quote results of DFT calculations of the relative size of ÄE(T 1 ) and ÄE(S 1 ), acknowledging that they will be modified in the solid. 62 The computational results are systematically biased in favor of SF by the authors' choice to use vertical excitation energies for ÄE(S 1 ) and adiabatic excitation energies for ÄE(T 1 ). For illustration, consider the results for isolated 2 at the B3LYP/6-311G level: for ÄE(S 1 ), the adiabatic value is 2.31 and the vertical, 2.42 eV, and for ÄE(T 1 ), the values are 1.20 and 1.23 eV.…”

Singlet fission in a hexacene dimer: energetics dictate dynamics

“…[10][11][12][13][14][15][16][17][18] Dimers serve as model systems to study singlet ssion. They represent the fundamental smallest number of chromophores required for SF and varying the connectivity between the chromophores can lead to insightful structure-property relationships of the constrained excitons, from the generation, [19][20][21][22] separation, [23][24][25][26] and recombination [27][28][29][30] of triplet states, to the elucidation of the bound triplet pair state. [31][32][33][34] It has been established that molecular vibrations play a key role in mediating singlet ssion in both oligoacene molecular crystals (intermolecular singlet ssion, xSF) and in acene intramolecular singlet ssion (iSF) compounds.…”