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

Abstract

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.

Modulating singlet fission through interchromophoric rotation

“…17 This theory was previously applied by Nakano and co-workers to study SF efficiency in a wide range of coupled chromophoric pairs. 17,34,[36][37][38]…”

Is azulene's local aromaticity and relative stability driven by the Glidewell–Lloyd rule?

“…8 This rule establishes that the population of p-electrons in conjugated polycyclic systems tends to form the smallest 4n+2 groups and to avoid the formation of the smallest 4n groups and has been successfully used to indicate the influence of local aromaticity on the stability of indole and its most known isomers. 9 Additionally, by applying this rule, the aromaticity patterns of more complex systems such as heteroatomic s-indacene and fuzed derivatives, [10][11][12] among others, 13,14 can be studied, allowing, as mentioned, the design of compounds with photovoltaic properties and even biological activity. 13,15 One of the best-known compounds with photovoltaic properties is azulene.…”

Optimizing through-space interaction for singlet fission by using macrocyclic structures

“…3–9 To break the 45% power conversion efficiency limit of conventional photovoltaic devices, 1 scientists have devoted great efforts to unravelling the SF mechanisms 10–17 and discovering more effective SF materials. 18–26…”

“…[3][4][5][6][7][8][9] To break the 45% power conversion efficiency limit of conventional photovoltaic devices, 1 scientists have devoted great efforts to unravelling the SF mechanisms [10][11][12][13][14][15][16][17] and discovering more effective SF materials. [18][19][20][21][22][23][24][25][26] An efficient SF process requires sufficiently large electronic coupling between two chromophores in addition to satisfying the energy level matching conditions (E(S 1 ) Z 2E(T 1 ) and E(T 2 ) Z 2E(T 1 )). 18 Usually, electronic couplings originate from close packing between adjacent chromophores in intermolecular singlet fission (xSF, see in Fig.…”