Theoretical design of conjugated diradicaloids as singlet fission sensitizers: quinones and methylene derivatives

Abstract

The electronic structures of 206 carbonyls and methylene derivatives based on conjugated cyclic hydrocarbons are computationally studied in this work using theoretical methods of quantum chemistry. The singlet open-shell nature of the ground state and its influence on the low-lying excited states is analyzed for 90 carbonyl (quinone, Q), 90 methylene (quinone dimethide, QDM) and 26 carbonyl-methylene (quinone methide, QM) mixed derivatives in the pursuit of new promising candidates for singlet fission sensitizers. Non-negligible diradical character is observed for most of the studied molecules, which is mainly determined by the nature and the relative position of the substituting groups in the bare rings. In general, the methylene group enhances to a greater extent the diradical character and the following trend is observed: y(QDM) > y(QM) > y(Q). This feature leads to a decrease in the energy of the S → S and, especially, the S → T transitions, facilitating the accomplishment of the singlet fission energetic conditions: 2T-S ≤ 0 (C1) and 2T-T < 0 (C2). For all the methylene derivatives, these transitions have π → π* character, while some carbonyl-containing molecules, in particular those with low diradical character, show transitions with n → π* character, due to the presence of the lone pairs of the oxygen atom. From the total set of 206 molecules analyzed, 10 molecules with intermediate diradical character may be considered as potential candidates to undergo singlet fission efficiently.

Elucidating the Relationship between Multiradical Character and Predicted Singlet Fission Activity

“…[24]). Our results support the current biradicaloid‐focused direction regarding SF molecule design . Those points close to E(S 1 ) – 2⋅E(T 1 )≥0 should be the best candidates for SF and do appear to have a weak diradical character.…”

“…Our results support the current biradicaloid-focused direction regarding SF molecule design. [9,19,[54][55][56] Those points close to E (S 1 ) -2•E(T 1 ) � 0 should be the best candidates for SF and do appear to have a weak diradical character. Those which generously satisfy the criterion (E(S 1 ) -2•E(T 1 ) > 0.5), become more diradical, showing that SF is still possible for stronger biradicaloids but will, in general, be less efficient as alternative relaxation channels may become more prevalent.…”

Singlet Fission in Carbene‐Derived Diradicaloids

“…Until today, most of the SF materials are, however, built around the acene platform, that is, ubiquitous tetracene and pentacene as well as their derivatives . In stark contrast, investigations on diradicaloids remain hitherto largely restrained to computational studies . Notable exceptions are 1,3‐diphenylisobenzofurans and zethrenes, which both were also spectroscopically characterized …”

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 ,,,.…”

“…This strategy was proposed by Michl [57] and has been successfully used afterwards. [66][67][68][69][70][71][72][73][74] The viability of this approach, that is, the design of SF sensitizers by means of their diradicaloid character, has been also demonstrated experimentally. [75,76,77,78] In this sense, squarates and croconates appear as potential candidates to undergo SF efficiently.…”