Flashing light at the “dark” triplet states in organic semiconductors

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The triplet (high-spin) excitons play key role in photophysical processes occurring in the organic materials. It stems from the uniquely long lifetime of the triplets dictated by the weak spin-orbit coupling together with spin-forbidden transition to the ground state. Their management plays a key role not only in the industry to achieve high external quantum yield of electroluminescence in current commercial organic lightemitting devices, but also at much larger scale in the nature – e.g. for photo-protection in plants. In the present work we focus on the phenomenon of singlet fission (SF) – generation of a pair of triplet excitons (2T1) out of one photogenerated singlet exciton (S1) in organic semiconductors: 2S0 + hν → S0+S1 → 1(TT) → 2T1 It was envisaged as a way to increase the theoretical limit of energy conversion in solar photovoltaic cells, since both triplets can possibly undergo a charge separation and thus contribute the photocurrent with two charge carriers instead of only one. Using time-resolved transient absorption and fluorescence spectroscopies to probe the photophysical processes, we identified fast singlet fission in a metallosupramolecular polymer formed by the coordination of Zn2+ ions to bis(terpyridine-4ʹ-yl)terthiophene ligand (Fig. 1). In this case, since S1 is energetically lower than the coupled triplet-pair state, the system requires photoexcitation to the next optical state (Sn) in order to be able to access the intermediate 1(TT) state and fission into two triplets (2T1). The zinc-supramolecular polymer exhibits relatively short lifetimes for the fission triplets due to triplet−triplet recombina������on, as is commonly observed in polymers. Our effort to expand the basis of SF materials continues with synthesis of two derivatives of 3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TDPP), inspired by the recently reported derivative of diketopyrrolopyrrole showing triplet yield in excess of 100% [5]. By varying the thin film preparation method, we influenced the resulting rate constant for triplet formation (Fig. 2). We observed behavior that is consistent with the SF phenomenon, i.e. decreasing rate with increasing energy difference |E(S1)– 2E(T1)| [3]. However, triplet yield quantification resulted in large uncertainties. Therefore, the production mechanism of the triplet excitons remains equivocal.

intersystem crossing, singlet fission, triplet exciton, transient absorption

RAIS, D.; PFLEGER, J.; MENŠÍK, M.; PANTHI, Y.; VALA, M.; KRAJČOVIČ, J.; STŘÍTESKÝ, S.; WEITER, M.

12. 9. 2018

Vysoké učení technické v Brně, Fakulta chemická

Purkyňova 464/118, 612 00 Brno

978-80-214-5488-0

7th Meeting on Chemistry and Life 2018. Book of abstracts

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