Singlet Fission in a para-Azaquinodimethane-Based Quinoidal Conjugated Polymer

Abstract

The exploitation of singlet fission (SF) in photovoltaic devices is restricted by the limited number of SF materials available and the conflicting requirement of intermolecular interactions to satisfy both efficient SF and subsequent triplet extraction. Intramolecular SF (iSF) represents an emerging alternative, and may prove simpler to implement in devices. On account of the excellent chemical structure tunability and solution processability, conjugated polymers have emerged as promising candidates for iSF materials despite being largely underexplored. It remains a significant challenge to develop SF-capable conjugated polymers and achieve efficient dissociation of the formed triplet pairs simultaneously. In this contribution, we present a new iSF material in a para-azaquinodimethane-based quinoidal conjugated polymer. Using transient optical techniques, we show that an ultrafast iSF process dominates the deactivation of excited state in such polymer, featuring ultrafast population (< 1 ps) and stepwise dissociation of triplet pairs. Notably, these multiexciton states could further diffuse apart to produce long-lived free triplets (tens of μs) in strongly coupled aggregates in solid thin film. Such findings not only introduce a new iSF-active conjugated polymer to the rare SF material family, but also shed unique insight into interchain interaction-promoted triplet pair dissociation in aggregates of conjugated polymers, thus open new avenues for developing next-generation SF-based photovoltaic materials.

Molecular Design Strategy for Practical Singlet Fission Materials: The Charm of Donor/Acceptor Decorated Quinoidal Structure

“…[42][43][47][48] The SF behavior of AQs has recently been disclosed in a quinoidal conjugated polymer system featuring an ultrafast and efficient intramolecular SF process. 28 These outcomes inspire us to take the monomeric AQ skeletons as examples to explore the proposed quinoidal structure design strategy for practical SF material in Scheme 1. Here we use thiophene as the varied peripheral donor units to decorate and stabilize the AQ quinoidal core, and screen the target molecule with the suitable energy levels and the high oscillator strength based on preliminary TDDFT calculations (for details about molecular design, see section 4 of SI).…”

“…[5][6][7][8] Unfortunately, the scope of SF-active chromophores is still limited with the dominant species being vulnerable acenes and their derivatives, which becomes a major roadblock for the much-needed progress in the field. [1][2][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Moreover, strong absorption with absorption coefficient above 10 5 M −1 cm −1 is needed in the practical SF materials to reduce device thickness and mitigate problems with triplet exciton diffusion. 1-2, 5, 8-9 Only several chromophores previously reported meet this criterion, such as Perylenediimide, 20,29 Terrylenediimide, 16 and some Zethrene diradicaloids.…”

“…Femtosecond transient absorption (fs-TA) spectroscopy and nanosecond laser flash photolysis (ns-TA) measurements were all performed using previously described instruments and experimental conditions. 25,28,[45][46] The details for kinetic analysis and global fitting are provided in SI.…”

Quinoidal conjugated polymers with open-shell character

“…77 Interestingly, the quinoidal-aromatic copolymer P10 was reported recently as a good singlet fission candidate. 95 The fulfillment of energetic requirements for singlet fission is primarily ascribed to the distinctive biradical character of the heterocyclic quinoidal core. 96,97 P10 was found to produce long-lived triplet pairs efficiently in strongly coupled polymer thin films, showing the promising potential of quinoidal conjugated polymers in developing next-generation singlet-fission-based organic solar cells.…”

An unprecedented quinoid–donor–acceptor strategy to boost the carrier mobilities of semiconducting polymers for organic field-effect transistors

“…39,[50][51][52][53] Recently, para-azaquinodimethane (p-AQM), emerged as the smallest stable quinoidal building block that is free of annulated aromatic units or electron-withdrawing carbonyl groups. [54][55][56][57] In addition, p-AQM adopts a true quinoidal structure in the ground state, which is distinct from pro-quinoidal units, e.g., thieno [3,4-b]thiophene (TbT). 58 Markedly different from other ground-state quinoidal building blocks (Figure 1a), 46,53,59,60 p-AQM is neither strongly electron-rich nor electron-deficient, acting as a unique class of electroactive building blocks aside from the traditional electron donors and acceptors for tuning frontier orbital energies of conjugated polymers.…”