Theoretical Molecular Design of Phenanthrenes for Singlet Fission by Diazadibora-Substitution

Captodative Effect Facilitates the Excitation in Diboron Molecule (CAAC)2 B2 (SH)2

Given the extraordinary versatility in chemical reactions and applications, boron compounds have gained increasing attentions in the past two decades. One of the remarkable advances is the unprecedented preparation of unsaturated boron species. Notably, Braunschweig et al. found that the cyclic (alkyl)(amino) carbenes (CAACs) stabilized diboron molecules (CAAC)₂ B₂ (SR)₂ host unpaired electrons and exist in the 90°-twisted diradical form, while other analogues, such as N-heterocyclic carbenes (NHCs), stabilized diboron molecules prefer a conventional B=B double bond. Since previous studies recognized the differences in the steric effect between CAAC and NHC carbenes, here we focused on the role of thiol substituents in (CAAC)₂ B₂ (SR)₂ by gradually localizing involved electrons. The co-planarity of the thiol groups and the consequent captodative effect were found to be the culprit for the 90°-twisted diradical form of (CAAC)₂ B₂ (SR)₂ . Computational analyses identified two forces contributing to the π electron movements. One is the "push" effect of lone pairs on the sulfur atoms which boosts the π electron delocalization between the BB center and CAACs. The other is the π electron delocalization within each (CAAC)B(SR) fragment where the pull effect originates from the π electron withdrawal by CAACs. There are two such independent and orthogonal push-pull channels which function mainly in individual (CAAC)B(SR) fragments. This enhanced π push-pull effect in the triplet state facilitates the electronic excitation in (CAAC)₂ B₂ (SR)₂ by reducing the singlet-triplet gap.

Dicarbonyl anthracenes and phenanthrenes as singlet fission chromophores

Singlet fission is a highly desired process in photovoltaic devices as it can significantly enhance photoelectric conversion efficiency. Exploitation of this process in photovoltaics is hindered by the lack of appropriate chromophores. We used mixed-reference spin-flipping time-dependent density functional theory to investigate five di-carbonyl anthracenes and phenanthrenes, with the purpose to design singlet fission chromophores. Two molecules were found to be promising candidates. For all the dicarbonyl molecules, the oxygen lone pair orbitals were found to be involved in the excited states that are relevant to singlet fission.

Computational predictions of adaptive aromaticity for the design of singlet fission materials

The concept of adaptive aromaticity has been demonstrated as an alternative strategy for the design of singlet fission materials.

Theoretical Study on Singlet Fission in Aromatic Diaza s-Indacene Dimers

We theoretically show that diaza (N₂)-substitution to s-indacene with 4n π-electrons, by which the number of π-electrons in N₂-s-indacene amounts to 4n+2, is a new strategy to design efficient singlet fission (SF) molecules. By N₂-substitution, the diradical character and the exchange integral are found to be tuned moderately, leading to satisfying the excitation energy level matching condition for SF with a large triplet excitation energy. On the basis of the effective electronic coupling related to the SF rate, we explore the optimal slip-stack dimer packings for fast SF. Their underlying mechanisms are well understood from the odd-electron density, resonance structure, and frontier orbital distribution, as the functions of the N₂-substituted positions. Furthermore, aromaticities of N₂-s-indacenes are evaluated explicitly on the basis of the magnetically induced current. Although N₂-s-indacenes display strengths of aromaticities similar to that of anthracene, a local decrease in aromaticity is found to correlate to the spatial feature of diradical character, i.e., odd-electron density. The present findings not only newly propose N₂-s-indacenes as feasible SF molecules but also contribute to comprehending the interplay between aromaticity and diradical electronic structures contributing to SF.

Diboron- and Diaza-Doped Anthracenes and Phenanthrenes: Their Electronic Structures for Being Singlet Fission Chromophores

We used quantum chemistry methods at the levels of mixed-reference spin-flip time-dependent density functional theory and multireference perturbation theory to study diboron- and diaza-doped anthracenes and phenanthrenes. This class of structures recently surged as potential singlet fission chromophores. We studied electronic structures of their excited states and clarified the reasons why they satisfy or fail to satisfy the energy criteria for singlet fission chromophores. Many studied structures have their S₁ states not dominated by HOMO → LUMO excitation, so they cannot be described using the conventional two site model. This is attributed to frontier orbital energy shifts induced by the doping and different charge-transfer energies in different one-electron singlet excitations or, in other words, different polarizations of hole and/or particle orbitals in their S₁ and T₁ states. There is a mirror relation between the orbital energy shifts induced by diboron- and diaza-dopings, which together with alternant hydrocarbon pairings of occupied and unoccupied orbitals, leads to more mirror relations between the excited states of the two types of doped structures.