Functionalized diketopyrrolopyrrole compounds for NIR-to-visible photon upconversion

Abstract

Triplet–triplet annihilation (TTA) mediated NIR-to-vis photon upconversion (UC) operating under incoherent and low power excitation is increasingly attractive for various applications including photovoltaics. Unfortunately, efficient TTA emitters/annihilators for this spectral range are still scarce as the current ones (e.g. rubrene) typically suffer from strong singlet fission and poor stability. In this work, a series of alternative annihilators based on differently functionalized diketopyrrolopyrrole (DPP) derivatives are thoroughly assessed by systematically elucidating their key UC properties such as UC quantum yield and threshold, triplet lifetime, triplet energy transfer (TET) efficiency and statistical probability (f) of a singlet generation from two triplets via TTA. All the studied DPP compounds demonstrated the feasibility for TTA with probability factor f (4.4–15.6%) close to that of rubrene and moderate UC yield (up to 3.8% out of 50%). The factor f was found to nicely correlate with the energy gap T₂–2T₁ suggesting the energy losses to the T₂ state to be mainly responsible for the diverse f of DPPs. The alkyl-strapped derivatives outperformed the non-strapped ones in terms of UC efficiency and threshold as a result of enhanced probability factor f, TET efficiency and triplet annihilation radius. Although shorter triplet lifetimes of DPP derivatives compared to rubrene implies enlarged UC threshold, DPP derivatives can be considered as one of the promising stable alternatives to a rubrene annihilator for NIR-to-vis TTA-UC.