Evolution of the electronic structure in open-shell donor-acceptor organic semiconductors

Abstract

Most organic semiconductors have closed-shell electronic structures, however, studies have revealed open-shell character emanating from design paradigms such as narrowing the bandgap and controlling the quinoidal-aromatic resonance of the π-system. A fundamental challenge is understanding and identifying the molecular and electronic basis for the transition from a closed- to open-shell electronic structure and connecting the physicochemical properties with (opto)electronic functionality. Here, we report donor-acceptor organic semiconductors comprised of diketopyrrolopyrrole and naphthobisthiadiazole acceptors and various electron-rich donors commonly utilized in constructing high-performance organic semiconductors. Nuclear magnetic resonance, electron spin resonance, magnetic susceptibility measurements, single-crystal X-ray studies, and computational investigations connect the bandgap, π-extension, structural, and electronic features with the emergence of various degrees of diradical character. This work systematically demonstrates the widespread diradical character in the classical donor-acceptor organic semiconductors and provides distinctive insights into their ground state structure-property relationship.

Air Stable Chalcogen-Doped Rubicenes with Diradical Character

“…The existence of a weak long tail is in accordance with the previous diradical species. 5,[27][28][29][30][31] In addition, the absorption peaks of O-rubicene are slightly bathochromically shifted to those of S-rubicene, while blue shifted to those of Se-rubicene, which agree well with the B3LYP/6-31G(d,p)-calculated electronic transitions for O-, S-, and Se-rubicenes (Fig. S10).…”

Stable dinitrile end-capped closed-shell non-quinodimethane as a donor, an acceptor and an additive for organic solar cells

“…It is noteworthy that the radical anion will more readily recover into its aromatic neutral form compared to p-quinodimethane in Scheme 1b, due to the well-known aromatic stabilization energy effect detected in previous Chichibabin type diradicaloids. 56 Interestingly, we detected that the donors show an open-shell structure, and the efficient NFAs including ITIC and Y6-series have closed-shell structures as these NFAs exhibit no or extremely weak ESR signal, while the poor acceptors including CN-TDPP and others have open-shell structure as they exhibit a strong ESR signal and bipolar transport properties. 55–58…”

“…56 Interestingly, we detected that the donors show an open-shell structure, and the efficient NFAs including ITIC and Y6-series have closed-shell structures as these NFAs exhibit no or extremely weak ESR signal, while the poor acceptors including CN-TDPP and others have open-shell structure as they exhibit a strong ESR signal and bipolar transport properties. 55–58…”

“…In our previous work, we have disclosed the open-shell radical character of donor–acceptor (D–A) type organic semiconductors. 55–58 As presented in Scheme 1, the donors of OSCs including PCE10 and small molecules show a quinoidal radical resonance structure which will produce a radical cation and recover the aromatic neutral form during the operation of OSCs (Scheme 1a). 59–62 In previous studies, p -quinodimethane CN-TDPP was reported to be a typical diradicaloid with an open-shell singlet ground state and showed poor performance in OSCs as the radical anion will be stabilized while it formed the aromatic resonance structure (Scheme 1b).…”

“…59–62 In previous studies, p -quinodimethane CN-TDPP was reported to be a typical diradicaloid with an open-shell singlet ground state and showed poor performance in OSCs as the radical anion will be stabilized while it formed the aromatic resonance structure (Scheme 1b). 56,57 Based on the above considerations, we proposed a new strategy to construct a closed-shell aromatic (non-quinodimethane) backbone with dinitrile end-capping groups for the design of NFAs in OSCs (Scheme 1c), and this will provide the expected high stability than IC-based NFAs. It is noteworthy that the radical anion will more readily recover into its aromatic neutral form compared to p-quinodimethane in Scheme 1b, due to the well-known aromatic stabilization energy effect detected in previous Chichibabin type diradicaloids.…”