Molecularly Dispersed Donors in Acceptor Molecular Crystals for Photon Upconversion under Low Excitation Intensity

Abstract

For real-world applications of photon upconversion based on the triplet-triplet annihilation (TTA-UC), it is imperative to develop solid-state TTA-UC systems that work effectively under low excitation power comparable to solar irradiance. As an approach in this direction, aromatic crystals showing high triplet diffusivity are expected to serve as a useful platform. However, donor molecules inevitably tend to segregate from the host acceptor crystals, and this inhomogeneity results in the disappointing performance of crystalline state TTA-UC. In this work, a series of cast-film-forming acceptors was developed, which provide both regular acceptor alignment and soft domains of alkyl chains that accommodate donor molecules without segregation. A typical triplet sensitizer, Pt octaethylporphyrin (PtOEP), was dispersed in these acceptor crystals without aggregation. As a result, efficient triplet energy transfer from the donor to the acceptor and diffusion of triplet excitons among regularly aligned anthracene chromophores occurred. It resulted in TTA-UC emission at low excitation intensities, comparable to solar irradiance.

Kinetically controlled crystal growth approach to enhance triplet energy migration-based photon upconversion

“…[12][13][14][15]47,48 For further improvement of the current method, we consider that the key is to suppress the formation of defect sites that deactivate the triplet excitons. 28 The development of approaches to circumvent this issue is under way in our laboratory.…”

“…In this perspective, it is natural to develop triplet energy migration-based photon upconversion (TEM-UC), 10 in which triplet excitons effectively diffuse in densely organized molecular assemblies without molecular diffusion. [21][22][23][24][25][26][27][28][29][30][31][32] Among various assembly systems, molecular crystals with ordered chromophore arrangements should be promising for achieving fast TEM. However, the crystalline systems have suffered from the aggregation of donor molecules and their segregation in acceptor crystals, which caused poor TET efficiency.…”

“…However, the crystalline systems have suffered from the aggregation of donor molecules and their segregation in acceptor crystals, which caused poor TET efficiency. 4 Despite efforts to solve this problem, 28,29,[32][33][34] the development of a simple and rational strategy to molecularly accommodate donor molecules in acceptor crystals is still anticipated.…”

Triplet–Triplet Annihilation Upconversion in a MOF with Acceptor‐Filled Channels

“…The intersection gives a threshold value of I thresh =329 mW cm −2 (Figure b). This I thresh is larger than that of the best systems reported ( I thresh few mW cm −2 ), but much smaller than that of typical two‐component porphyrin‐DPA systems ( I thresh several kW cm −2 ) …”

“…Rapid drying of these materials yields a favorable distribution of the sensitizer in the annihilator matrix and lowers I thresh to 6.8 Wcm −2 . Optimization of the annihilator further reduces phase segregation and leads to an even lower I thresh of 1.7 mW cm −2 . However, the interface between the chromophores and the relative orientation of the chromophores is difficult to control in less ordered systems.…”