Taku Ogawa

Highly Efficient Photon Upconversion in Self-Assembled Light-Harvesting Molecular Systems

To meet the world’s demands on the development of sunlight-powered renewable energy production, triplet–triplet annihilation-based photon upconversion (TTA–UC) has raised great expectations. However, an ideal highly efficient, low-power, and in-air TTA–UC has not been achieved. Here, we report a novel self-assembly approach to achieve this, which enabled highly efficient TTA–UC even in the presence of oxygen. A newly developed lipophilic 9,10-diphenylanthracene-based emitter molecule functionalized with multiple hydrogen-bonding moieties spontaneously coassembled with a triplet sensitizer in organic media, showing efficient triplet sensitization and subsequent triplet energy migration among the preorganized chromophores. This supramolecular light-harvesting system shows a high UC quantum yield of 30% optimized at low excitation power in deaerated conditions. Significantly, the UC emission largely remains even in an air-saturated solution, and this approach is facilely applicable to organogel and solid-film systems.

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

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, PtII 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

Abstract. Improving efficiency of triplet–triplet annihilation-based photon upconversion (TTA-UC) in crystalline media is challenging because it usually suffers from the severe aggregation of the donor (sensitizer) molecules in acceptor (emitter) crystals. We show a kinetically controlled crystal growth approach to improve donor dispersibility in acceptor crystals. As the donor–acceptor combination, a benchmark pair of platinum(II) octaethylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA) is employed. A surfactant-assisted reprecipitation technique is employed, where the concentration of the injected PtOEP–DPA solution holds the key to control dispersibility; at a higher PtOEP–DPA concentration, a rapid crystal growth results in better dispersibility of PtOEP molecules in DPA crystals. The improvement of donor dispersibility significantly enhances the TTA-UC quantum yield. Thus, the inherent function of donor-doped acceptor crystals can be maximized by controlling the crystallization kinetics.

Donor-Acceptor-Collector Ternary Crystalline Films for Efficient Solid-State Photon Upconversion

It is pivotal to achieve efficient triplet-triplet annihilation based photon upconversion (TTA-UC) in the solid-state for enhancing potentials of renewable energy production devices. However, the UC efficiency of solid materials is largely limited by low fluorescence quantum yields that originate from the aggregation of TTA-UC chromophores and also by severe back energy transfer from the acceptor singlet state to the singlet state of the triplet donor in the condensed state. In this work, to overcome these issues, we introduce a highly fluorescent singlet energy collector as the third component of donor-doped acceptor crystalline films, in which dual energy migration, i.e., triplet energy migration for TTA-UC and succeeding singlet energy migration for transferring energy to a collector, takes place. To demonstrate this scheme, a highly fluorescent singlet energy collector was added as the third component of donor-doped acceptor crystalline films. An anthracene-based acceptor containing alkyl chains and a carboxylic moiety is mixed with the triplet donor Pt(II) octaethylporphyrin (PtOEP) and the energy collector 2,5,8,11-tetra- tert-butylperylene (TTBP) in solution, and simple spin-coating of the mixed solution gives acceptor films of nanofibrous crystals homogeneously doped with PtOEP and TTBP. Interestingly, delocalized singlet excitons in acceptor crystals are found to diffuse effectively over the distance of ∼37 nm. Thanks to this high diffusivity, only 0.5 mol % of doped TTBP can harvest most of the singlet excitons, which successfully doubles the solid-state fluorescent quantum yield of acceptor/TTBP blend films to 76%. Furthermore, since the donor PtOEP and the collector TTBP are separately isolated in the nanofibrous acceptor crystals, the singlet back energy transfer from the collector to the donor is effectively avoided. Such efficient singlet energy collection and inhibited back energy transfer processes result in a large increase of UC efficiency up to 9.0%, offering rational design principles toward ultimately efficient solid-state upconverters.

Simple and Versatile Platform for Air-Tolerant Photon Upconverting Hydrogels by Biopolymer–Surfactant–Chromophore Co-assembly

Exploration of triplet-triplet annihilation based photon upconversion (TTA-UC) in aqueous environments faces difficulty such as chromophores insolubility and deactivation of excited triplets by dissolved oxygen molecules. We propose a new strategy of biopolymer-surfactant-chromophore coassembly to overcome these issues. Air-stable TTA-UC with a high upconversion efficiency of 13.5% was achieved in hydrogel coassembled from gelatin, Triton X-100 and upconverting chromophores (triplet sensitizer and emitter). This is comparable to the highest UC efficiency observed to date for air-saturated aqueous UC systems. Moreover, this is the first example of air-stable TTA-UC in the form of hydrogels, widening the applicability of TTA-UC in biological applications. The keys are two-fold. First, gelatin and the surfactant self-assemble in water to give a developed hierarchical structure with hydrophobic domains which accommodate chromophores up to high concentrations. Second, thick hydrogen-bonding networks of gelatin backbone prevent O2 inflow to the hydrophobic interior, as evidenced by long acceptor triplet lifetime of 4.9 ms. Air-stable TTA-UC was also achieved for gelatin with other nonionic surfactants (Tween 80 and Pluronic f127) and Triton X-100 with other gelling biopolymers (sodium alginate and agarose), demonstrating the versatility of current strategy.