Triplet-sensitized photon upconversion in deep eutectic solvents

Abstract

Photon upconversion (UC) is a technology that can increase solar utilization efficiencies in broad photoenergy conversion systems by converting lower-energy photons into usable higher-energy photons. Recently, UC using triplet-triplet annihilation (TTA) of organic molecules has drawn attention because it is presently the only method applicable to weak and noncoherent light. To date, many attempts have been made to realize this UC technology in forms suitable for applications, but they typically suffer from either high cost or insufficient stability and/or safety of materials. Recently, a new class of liquid called deep eutectic solvents (DESs) has emerged as low-cost green fluids that possess low toxicity and vapor pressure, biodegradability, and high thermal stability. DESs have been proposed as an alternative to ionic liquids. This article develops triplet-sensitized UC samples using DESs that are found to be suitable solvents for this purpose, attaining a new materials platform for UC with the aforementioned advantages. The high thermal stability of the samples is qualitatively confirmed and their UC quantum yields are determined to be 0.11-0.21 (based on the definition that the maximum quantum yield is 0.5) depending on the DES composition. The triplet lifetime of the emitter 9,10-diphenylanthracene increases with DES viscosity, resulting in unique kinetics. Analysis of photophysical experimental results allows the relevant physics governing the performance of this sample system to be determined and discussed. Overall, a novel UC platform that simultaneously achieves high thermal stability, low cost, and environmental friendliness is developed using DESs as the solvent.

Annihilation Versus Excimer Formation by the Triplet Pair in Triplet–Triplet Annihilation Photon Upconversion

“…Comparing to previous upconversion work with the annihilator 9,10-bis(phenylethynyl)anthracene (BPEA) we note that in low-viscosity solvents the highest upconversion yield is less than 2%, whereas in high-viscosity solvents (poly(ethylene glycol) (PEG)) the yield is 15%. 55 − 57 In this study, it was argued that the extended lifetime of the triplet pair intermediate allowed for a higher probability of obtaining a favorable geometry. 56 …”

Visible-to-ultraviolet (<340 nm) photon upconversion by triplet–triplet annihilation in solvents

“…We further assume that the second-order rate constant between E* molecules for the TTA process (k 2 ) is close to k diff (i.e., k 2 E k diff ), which was found to be a quantitatively good approximation. 36 Although the degradation phenomenon considered here is transient, the timescales of the above-described kinetics are much shorter than those of the change of the UC emission intensity. Therefore, at each instantaneous moment during continuous photoirradiation, the quasi-steady-state approximation is considered to hold well for E*,…”

van der Waals solid solution crystals for highly efficient in-air photon upconversion under subsolar irradiance

“…[1][2][3] However, liquid samples have risks of freezing, evaporation, and ignition; although some samples have high stability even against a flame. 14,15 Furthermore, the large gas diffusivity in liquids requires suppression of damaging singlet oxygen. 12,13,[16][17][18] To be free from these issues, a solid UC system is being quested [19][20][21][22] where triplet energy transfer (TET) from the sensitizer to the annihilator and triplet exciton diffusion should be efficiently designed (Fig.…”