Triplet–triplet annihilation photon upconversion (TTA-UC) is a process in which triplet excitons combine to form emissive singlets and holds great promise in biological applications and for improving the spectral match in solar energy conversion. While high TTA-UC quantum yields have been reported for, for example, red-to-green TTA-UC systems, there are only a few examples of visible-to-ultraviolet (UV) transformations in which the quantum yield reaches 10%. In this study, we investigate the performance of six annihilators when paired with the sensitizer 2,3,5,6-tetra(9H-carbazol-9-yl)benzonitrile (4CzBN), a purely organic compound that exhibits thermally activated delayed fluorescence. We report a record-setting internal TTA-UC quantum yield (Φ_UC,g) of 16.8% (out of a 50% maximum) for 1,4-bis((triisopropylsilyl)ethynyl)naphthalene, demonstrating the first example of a visible-to-UV TTA-UC system approaching the classical spin-statistical limit of 20%. Three other annihilators, of which 2,5-diphenylfuran has never been used for TTA-UC previously, also showed impressive performances with Φ_UC,g above 12%. In addition, a new method to determine the rate constant of TTA is proposed, in which only time-resolved emission measurements are needed, circumventing the need for more challenging transient absorption measurements. The results reported herein represent an important step toward highly efficient visible-to-UV TTA-UC systems that hold great potential for driving high-energy photochemical reactions.

Exciton coupling between the transition dipole moments of ordered dyes in supramolecular assemblies, so-called J/H-aggregates, leads to shifted electronic transitions. This can lower the excited state energy, allowing for emission well into the near-infrared regime. However, as we show here, it is not only the excited state energy modifications that J-aggregates can provide. A bay-alkylated quaterrylene was synthesized, which was found to form J-aggregates in 1,1,2,2-tetrachloroethane. A combination of superradiance and a decreased nonradiative relaxation rate made the J-aggregate four times more emissive than the monomeric counterpart. A reduced nonradiative relaxation rate is a nonintuitive consequence following the 180 nm (3300 cm^–1) red-shift of the J-aggregate in comparison to the monomeric absorption. However, the energy gap law, which is commonly invoked to rationalize increased nonradiative relaxation rates with increasing emission wavelength, also contains a reorganization energy term. The reorganization energy is highly suppressed in J-aggregates due to exciton delocalization, and the framework of the energy gap law could therefore reproduce our experimental observations. J-Aggregates can thus circumvent the common belief that lowering the excited state energies results in large nonradiative relaxation rates and are thus a pathway toward highly emissive organic dyes in the NIR regime.

Triplet–triplet annihilation photon upconversion (TTA-UC) is a process in which low-energy light is transformed into light of higher energy. During the last two decades, it has gained increasing attention due to its potential in, e.g., biological applications and solar energy conversion. The highest efficiencies for TTA-UC systems have been achieved in liquid solution, owing to that several of the intermediate steps require close contact between the interacting species, something that is more easily achieved in diffusion-controlled environments. There is a good understanding of the kinetics dictating the performance in liquid TTA-UC systems, but so far, the community lacks cohesiveness in terms of how several important parameters are best determined experimentally. In this perspective, we discuss and present a “best practice” for the determination of several critical parameters in TTA-UC, namely triplet excited state energies, rate constants for triplet–triplet annihilation ( $$k_{{{\text{TTA}}}}$$k TTA ), triplet excited-state lifetimes ( $$\tau_{{\text{T}}}$$τ T ), and excitation threshold intensity ( $$I_{{{\text{th}}}}$$I th ). Finally, we introduce a newly developed method by which $$k_{{{\text{TTA}}}}$$k TTA , $$\tau_{{\text{T}}}$$τ T , and $$I_{{{\text{th}}}}$$I th may be determined simultaneously using the same set of time-resolved emission measurements. The experiment can be performed with a simple experimental setup, be ran under mild excitation conditions, and entirely circumvents the need for more challenging nanosecond transient absorption measurements, a technique that previously has been required to extract $$k_{{{\text{TTA}}}}$$k TTA . Our hope is that the discussions and methodologies presented herein will aid the photon upconversion community in performing more efficient and manageable experiments while maintaining—and sometimes increasing—the accuracy and validity of the extracted parameters.

In the strive to develop triplet–triplet annihilation photon upconversion (TTA-UC) to become applicable in a viable technology, there is a need to develop upconversion systems that can function well in solid states. One method to achieve efficient solid-state TTA-UC systems is to replace the intermolecular energy-transfer steps with the corresponding intramolecular transfers, thereby minimizing loss channels involved in chromophore diffusion. Herein, we present a study of photon upconversion by TTA internally within a polymeric annihilator network (iTTA). By the design of the annihilator polymer and the choice of experiment conditions, we isolate upconversion emission governed by iTTA within the annihilator particles and eliminate possible external TTA between separate annihilator particles (xTTA). This approach leads to mechanistic insights into the process of iTTA and makes it possible to explore the upconversion kinetics and performance of a polymeric annihilator. In comparison to a monomeric upconversion system that only functions using xTTA, we show that upconversion in a polymeric annihilator is efficient also at extremely low annihilator concentrations and that the overall kinetics is significantly faster. The presented results show that intramolecular photon upconversion is a versatile concept for the development of highly efficient solid-state photon upconversion materials.

We report the first example of direct far-red endothermic triplet sensitizedZ-to-Ephotoswitching of azobenzene derivative (Azo1) in a condensed phase of a liquid Azo1 co-assembled within a liquid surfactant-protein bioplastic film in air.

Single-molecule fluorescence emission of certain positive photochromic systems such as diarylethenes have been exploited for biological imaging and optical memory storage applications. However, there is a lack of understanding if negative photochromic systems can be used for such type of applications. Hence, to explore the potential of negative photochromic molecules for possible optical memory storage applications, we have here synthesized and studied a series of four norbornadiene–quadricyclane (NBD–QC) photoswitching molecules. These molecules feature either linearly conjugated or cross-conjugated pi-electron systems. Upon photoisomerization, the UV-vis absorption spectra of the molecules revealed a strong blue shift in the QC-form, with a photoisomerization quantum yield close to 80% for the cross-conjugated systems. In contrast, a strong intrinsic emission (up to Φ_f = 49%) for the linearly conjugated compounds in the NBD form was observed. Upon light-induced isomerization, the emission was completely turned off in the QC-form in all the compounds studied. Further, the robustness of the system was evaluated by performing several switching cycles. Under nitrogen, the emission can be turned off and recovered with almost no loss of emission. We also show that the QC-form can be photochemically triggered to convert back to the NBD-form using a low energy UV light (340 nm), allowing an all optical conversion to both species. The demonstrated properties can make the NBD–QC system attractive for potential applications such as optical memory storage devices.