Multiple Resonance Thermally Activated Delayed Fluorescence Sensitizers Enable Green-to-Ultraviolet Photon Upconversion: Application in Photochemical Transformations

Abstract

Efficient visible-to-ultraviolet (UV) triplet-triplet annihilation upconversion (TTA-UC) with large anti-Stokes shift is highly promising for solar-powered and indoor applications. Nonetheless, the excitation wavelength is confined to the blue region (< 450 nm) mainly due to large energy loss during triplet sensitization, resulting in reduced photon utilization efficiency in practical scenarios. Herein, a series of multiple resonance thermal activated delayed fluorescence (MR-TADF) compounds are developed as purely organic sensitizers for the purpose of energy-loss reduction, which also feature intense absorbance at visible region, high intersystem crossing efficiencies, and long triplet lifetimes. By pairing the MR-TADF sensitizers with appropriate acceptors, green-to-UV TTA-UC systems were realized with anti-Stokes shift up to 1.05 eV, upconversion quantum yield up to 8.6% and threshold excitation intensity as low as 9.2 mW cm -2 in solution. The TTA-UC pairs were applied as internal or external source of UV photons to trigger energy-demanding photopolymerization and photoligation reactions even under excitation of low-power-density green LED light, revealing the broad utility

Manipulating Exciton Dynamics toward Simultaneous High-Efficiency Narrowband Electroluminescence and Photon Upconversion by a Selenium-Incorporated Multiresonance Delayed Fluorescence Emitter

“…The relationship of the T 1 energy level between photosensitizers and acceptors is summarized in Figure A. Albeit the triplet energy transfer process in the BN–Se /1-TAN system was thermodynamically unfavorable, a small energy barrier (Δ E TTET = −0.01 eV) could still permit efficient thermal activation of TTET and open up the possibility for larger anti-Stokes shifts with 1-TNA as an acceptor …”

“…In brief, the satisfactory I th herein was a joint result of intense molar absorptivity, high Φ ISC , and excellent Φ TTET . The simultaneous enhancement of Φ UC′ and reduction of I th clearly highlighted the advantage of a Se-incorporated MR-TADF photosensitizer …”

“…The concept of multiresonance TADF (MR-TADF) proposed by Hatakeyama and co-workers in 2016 has effectively addressed the aforementioned shortcomings . Molecules exhibiting MR-TADF are typically constructed with boron/nitrogen (B/N)-based polycyclic frameworks, which utilize the opposite resonance effects of B and N atoms to realize the atomic separation of the FMOs, leading to small Stokes shift and extremely sharp emission. ,− In addition, the short-range charge transfer (SR-CT) character endows these compounds with intense oscillator strength and consequentially high absorptivity and luminescence efficiency. , Owing to these properties, MR-TADF emitters covering the entire color gamut have been developed and applied in high-performance narrowband OLEDs. − Meanwhile, our group recently demonstrated that the MR-TADF compounds could also function as a new category of pure organic photosensitizers in TTA-UC to actualize green-to-UV upconversion with large anti-Stokes shift values, thanks to their minimized energy loss for triplet sensitization and large molar extinction coefficients (ε > 10 4 M –1 cm –1 ) . Despite these advances, the performance of current MR-TADF materials is still hampered by insufficient ISC and RISC because of small S 1 –T 1 spin–orbital coupling (SOC) as well as non-negligible Δ E ST values (∼0.1 eV).…”

Spatially Controlled UV Light Generation at Depth Using Upconversion Micelles

“…Annihilators for TTA-UC are usually small organic molecules like acenes, whereas sensitizers be a range of materials such as metallic complexes, thermally activated delayed fluorescence (TADF) molecules, and inorganic nanoparticles. 20,27,28 For UC emission in the UV, a range of sensitizer and annihilator pairs for both blue-to-UV UC [33][34][35][36][37][38][39][40][41][42][43] and green-to-UV UC 44 demonstrate the flexibility of materials options, excitation wavelengths, and emission ranges available to accommodate distinct requirements for different light-driven applications. Yet despite this apparent versatility, sensitizers and annihilators used for UV light generation via TTA-UC are usually incompatible with aqueous environments.…”

Versatile, stable, and air-tolerant triplet–triplet annihilation upconversion block copolymer micelles

“…[14][15][16] Compared with the rare earth-based UC process, TTA-UC usually requires low excitation light energy and shows high upconversion quantum efficiency. [17][18][19] Moreover, the excitation wavelength and emission wavelength can be adjusted independently by selecting different combi-nations of energy donors and acceptors, allowing for the conversion of light with different wavelengths into high-energy light. 20 However, the wide applications of TTA-UC systems in the solution state have not been realized, and are mainly restricted by three factors: (1) the distinct solubilities of chromophore pairs in solvents, significantly impeding their wider applications; (2) limited stability of the chromophore pairs, especially under dilution, impairing the short-distance energy transfer between the donor and the acceptor; and (3) low tolerance toward air, since the triplet state is readily quenched by dissolved oxygen.…”