Nanoengineering Triplet–Triplet Annihilation Upconversion: From Materials to Real-World Applications

Abstract

Using light to control matter has captured the imagination of scientists for generations, as there is an abundance of photons at our disposal. Yet delivering photons beyond the surface to many photoresponsive systems has proven challenging, particularly at scale, due to light attenuation via absorption and scattering losses. Triplet−triplet annihilation upconversion (TTA-UC), a process which allows for low energy photons to be converted to high energy photons, is poised to overcome these challenges by allowing for precise spatial generation of high energy photons due to its nonlinear nature. With a wide range of sensitizer and annihilator motifs available for TTA-UC, many researchers seek to integrate these materials in solution or solid-state applications. In this Review, we discuss nanoengineering deployment strategies and highlight their uses in recent state-of-the-art examples of TTA-UC integrated in both solution and solid-state applications. Considering both implementation tactics and application-specific requirements, we identify critical needs to push TTA-UC-based applications from an academic curiosity to a scalable technology.

A simple yet stable molybdenum(0) carbonyl complex for upconver-sion and photoredox catalysis

“…The 1 MLCT absorption bands of 1 and 2 show strong solvatochromic shifts that can be linearly correlated with the empirical solvent polarity parameter ET( 30) 69 (see inset in Figure 2, Figure S39). In fact, the solvent sensitiviy of the lowest energy 1 MLCT absorption in 1 and 2 is almost identical (Figure S39). 71 Negative solvatochromism indicates a higher polarity of the ground state (GS) compared to the 1 MLCT excited state.…”

“…The 3 MC state of 2 (2.20 eV) features two bent Mo-CO bonds (166°). The energies of high-spin 5 MC states were calculated at 4.07 and 4.14 eV for 1 and 2, respectively, which makes them irrelevant for the photophysics following low-energy 1 MLCT excitation (Table S10). In solution, 1 showed a high phosphorescence quantum yield  with 1.2 % being the highest value obtained in deaerated toluene (Table 3).…”

“…The development of photoactive metal complexes with earthabundant metals holds great promise to enable sustainable photocatalysis, (solar) energy conversion and light emitting diodes. [1][2][3][4] Hence, considerable efforts have been undertaken to explore this compound class in the recent years 5,6 with creative designs utilizing excited states of different characters like metal-to-ligand (MLCT), 7 ligand-to-metal, ligand-to-ligand charge transfer, and metal-centered (MC) states, 8,9 covering zirconium, [10][11][12][13][14] vanadium, [15][16][17][18] chromium, [19][20][21][22][23][24][25][26] iron, [27][28][29] manganese, [30][31][32] cobalt, 33,34 copper [35][36][37] , molybdenum, 38 -42 and zinc as metal centers 43,44 amongst others. However, elaborate ligand scaffolds are often required to unlock the desired long-lived excited states for photoactivity with earth-abundant metals.…”

Controlling the durability and optical properties of triplet–triplet annihilation upconversion nanocapsules

“…1,16 On the other hand, using a nonlinear process can allow for precise high energy photon generation at depth. 17–21…”

“…1,16 On the other hand, using a nonlinear process can allow for precise high energy photon generation at depth. [17][18][19][20][21] One such nonlinear process, triplet-triplet annihilation upconversion (TTA-UC), exploits excitonic states in organic semiconductors to convert two low energy photons into one high energy photon (anti-Stokes shift). 13,20 Briefly, a sensitizer (typically a metalated porphyrin 13,17 ) absorbs low energy photons, whose energy is transferred to annihilator molecules (often acenes 13,17 ) via triplet energy transfer.…”