Triplet-triplet-annihilation (TTA) upconversion is an anti-Stokesprocess which is usually implemented by using a sensitizer and an acceptor. The sensitizer could absorb low-energy photons and transfer its exciting energy to the acceptor. Then the two acceptors with triplet excited state (T1) would be annihilated to produce one singlet excited state (S1), which would consequently relax to ground state (S0) and emit a high-energy photon. Compared with two-photon absorption and lanthanide step-wise absorption of multiple photons, many TTA systems could emit upconversion luminescence with a high quantum yield (>10%) at a low excitation powerdensity (<100 mW cm-2), which could be realized under the incoherent light or even sunlight. Therefore, TTA upconversion has attracted wide attention and exhibits potential prospects in solar energy utilization, photocatalysis, biological imaging, and phototherapy. Among numerous types of sensitizers, the thermally activated delayed fluorescence (TADF) molecules exhibit a small energy gap between S1and T1. This would theoretically reduce the energy loss during the ISC and increase the anti-stocks shift. So far, however, the maximumquantum yield of TTA upconversion sensitized by a TADF molecule is only 11.2%. Therefore, it is of great value and desire to develop novel TADF sensitizers or other new methods for metal-free TTA upconversion.
Optical microcavities combining small size with high Q-factor are very attractive for a wide range of applications in low-threshold lasers, optical waveguides, and high-performance filters.These mirrorless structures of whispering gallery mode(WGM) microcavities display strong optical confnement due to total internal reflection at the cavity periphery. Their radial symmetry allows light to recirculate numerous times, enabling enhanced light-matter interactions.
Recently, Professor Xuewen Shu's group at Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology in collaboration with Dr. Zhu Zece of Wuhan Textile University and Associate Professor Liu Yuan of Guangdong University of Technology synthesized anew type of TADF sensitizer molecule, BTZ-DMAC-4Br, andcreativelypioneered a design scheme based on WGM optical microcavity to effectively improve the TTA up-conversion efficiency. As shown in Fig. 1, the mixture of BTZ-DMAC-R (1 mM) and DPA (5 mM) in toluene filled in a quartz capillary with 0.2-mm inner diameter under the excitation of a 520 nm laser. The ends of this capillary were sealed with a little UV curing adhesive to prevent the inner solution from air. Reasonably adjusting the incident angle of the excitation light beam, itcould be incident on the inner walltangentially. The refractive index of toluene is 1.49 at 520 nm, which is slightly higher than that of quartz (1.46). So it is possible to form WGM modes in the inner wall of cylindrical quartz capillary, and the incident light propagates along the inner wall in the form of an energy loop, which can increase the interaction between the light and the medium in the microcavity. An upconversion quantum yield of 24.6% is observed in this work, which is a dozen times higher than that measured in conventional cuvettes. To the best of knowledge, this is the highest upconversion effciency achieved in TADF sensitized TTA systems.
Figure 1. a) Schematic of the light transmission in a WGM microcavity filled with the toluene solution of BTZ-DMAC-4Br/BTZ-DMAC and DPA. b) Main construction of the optical system for measuring TTA upconversion in microcavities.
Figure 2.TTA upconversion quantum yield versus the excitation intensity in different inner diameter capillaries at different excitation intensities for the sensitizer molecules of a) BTZ-DMAC-4Br and b) BTZ-DAMC, respectively.
The results of the TTA upconversion characteristics of different sensitizers in capillaries with different sizes, materials, and structures in this work show that the WGM optical microcavity can not only effectively improve the TTA upconversion efficiency, but also greatly reduce the TTA upconversion excitation threshold. Its function is not dependent on the sensitizers and has universal applicability for any sensitizers. The proposed scheme provides a new way for further research on the combination of optical microcavity and material chemistry. Besides the WGM microcavities, many other types of optical cavities may also be used to construct highly effcient and low-threshold photonic devices for the application of TTA upconversion.
This work was published on Advanced Functional Materials and was supported by the National Key R&D Program of China (2018YFE0117400), National Natural Science Foundation of China (NSFC) (52073109, 61775074), etc. The first authors are Dr. Mengmeng Han and Prof. Yuan Liu, and the corresponding authors are Prof. Xuewen Shu and Dr. Zece Zhu.
Full paper link:https://doi.org/10.1002/adfm.202104044
