Multinuclearity Effect of Yb(III) Complexes as Sensitizers for Near-Infrared-to-Visible Triplet–Triplet Annihilation Upconversion

NIR-to-visible upconversion (UC) has garnered substantial attention owing to its promising applications in biomedicine and solar energy conversion. Although organo-Yb(III) complexes as photosensitizers can overcome the threshold of large anti-Stokes shifts in NIR-to-visible UC based on triplet–triplet annihilation (TTA), improving the quantum yields remains a formidable challenge. Herein, focusing on the Yb3+-complex/Rubrene coupling that emits yellow light (λem = 560 nm) under NIR excitation (980 nm), we systematically investigate the multinuclearity effect induced by three Yb(III) complex sensitizers, namely, binuclear [Yb2(Salen)2(H2Salen)Cl2(MeOH)2] (Yb-1), trinuclear [Yb3(Salen)3(HSalen)(OH)2] (Yb-2), and tetranuclear [Yb4(Salen)4(NO3)2(CO3)(DMF)2] (Yb-3). The progression from binuclear to tetranuclear architecture systematically enhances 980 nm absorbance, suppresses nonradiative loss, and improves energy transfer efficiency, culminating in a maximal upconversion intensity for tetranuclear Yb-3 (absorbance = 28.6 M–1cm–1, UC energy transfer efficiency = 4.0%, relative intensity 1.67 times larger than Yb-1 and 1.25 times large than Yb-2). This work provides key insights for the rational design of high-performance Yb3+-complex/Rubrene TTA-UC systems.