Kinetics of photon upconversion by triplet-triplet annihilation: a comprehensive tutorial

A Nonlinear Photochromic Reaction Based on Sensitizer-Free Triplet-Triplet Annihilation in a Perylene-Substituted Rhodamine Spirolactam

Nonlinear photochromic reactions that work with weak incoherent light are important for molecular operations with high spatial resolution and multiple photofunctions based on single molecules. However, nonlinear photochromic compounds generally require complex molecular design, restricting accessibility in various fields. Herein, we report nonlinear photochromic properties in a perylene-substituted rhodamine spirolactam derivative (Rh-Pe), which is synthesized from rhodamine B in facile procedures. Direct excitation of Rh-Pe produces the triplet excited state via the charge-transfer (CT) state. The triplet excited state causes triplet-triplet annihilation to bring the generation of the intensely colored ring-open form with nonlinear behavior. Furthermore, green- and red-light-induced photochromism was achieved in Rh-Pe using triplet sensitizers, although Rh-Pe can be directly excited only by ultraviolet and blue light. Our findings are expected to contribute to the development of photofunctional materials showing nonlinear behavior and low-energy light responsivity.

Triplet properties and intersystem crossing mechanism of PtAg28 nanocluster sensitizers achieving low threshold and efficient photon upconversion

Ligand-protected metal nanoclusters have emerged as a promising platform for providing sensitizers for triplet-triplet annihilation upconversion (TTA-UC). Herein, we report [PtAg28(BDT)12]4- (PtAg28; BDT = 1,3-benzenedithiolate) as a sensitizer enabling TTA-UC at low excitation intensities. PtAg28 exhibits a long-lived triplet state (approximately 7 μs) generated with a 100% intersystem crossing (ISC) quantum yield. The mechanism driving this efficient ISC was unveiled with the aid of theoretical calculations. Specifically, the S1-T1 ISC reveals a small spin-orbit coupling (SOC) matrix element, attributed to their similar electron configuration. In contrast, the T2 state, which is energetically close to S1, features a hole distribution derived from the Py superatomic orbital of the icosahedral Pt@Ag12 core. This distribution enables direct SOC based on the orbital angular momentum change from the S1 state with a Pz-derived hole distribution. Consequently, the efficient ISC was rationalized by the S1 → T2 → T1 pathway. The T1 state possesses a metal core-to-surface metal charge transfer character, facilitating triplet energy transfer and conferring superior sensitization ability. Leveraging these characteristics, the combination of PtAg28 sensitizer with a 9,10-diphenylanthracene annihilator/emitter attained an extremely low UC threshold of 0.81 mW cm-2 at 532 nm excitation, along with efficient green-to-blue TTA-UC with an internal quantum yield (ΦUCg) of 12.2% (50% maximum). This results in a pseudo-first-order TTA process with strong UC emission under 1-sun conditions.

Enhancing NIR-to-visible upconversion in a rigidly coupled tetracene dimer: approaching statistical limits for triplet-triplet annihilation using intramolecular multiexciton states

Important applications of photon upconversion through triplet-triplet annihilation require conversion of near-IR photons to visible light. Generally, however, efficiencies in this spectral region lag behind bluer analogues. Herein we consider potential benefits from a conformationally well-defined covalent dimer annihilator TIPS-BTX in studies that systematically compare function to a related monomer model TIPS-tetracene (TIPS-Tc). TIPS-BTX exhibits weak electronic coupling between chromophores juxtaposed about a polycyclic bridge. We report an upconversion yield ϕUC for TIPS-BTX that is more than 20× larger than TIPS-Tc under comparable conditions (0.16%). While the dimer ϕUC is low compared to bluer champion systems, this yield is amongst the largest so-far reported for a tetracenic dimer system and is achieved under unoptimized conditions suggesting a significantly higher ceiling. Further investigation shows the ϕUC enhancement for the dimer is due exclusively to the TTA process with an effective yield more that 30× larger for TIPS-BTX compared to TIPS-Tc. The ϕTTA enhancement for TIPS-BTX relative to TIPS-Tc is indicative of participation by intramolecular multiexciton states with evidence presented in spin statistical arguments that the 5TT is involved in productive channels. For TIPS-BTX we report a spin-statistical factor f = 0.42 that matches or exceeds values found in champion annihilator systems such as DPA. At the same time, the poor relative efficiency of TIPS-Tc suggests involvement of non-productive bimolecular channels and excimeric states are suspected. Broadly these studies indicate that funneling of photogenerated electronic states into productive pathways, and avoiding parasitic ones, remains central to the development of champion upconversion systems.

The statistical probability factor in triplet mediated photon upconversion: a case study with perylene

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process where two low-energy photons are converted into one higher-energy photon. A crucial component for an efficient upconversion process is the statistical probability factor (f), defined as the probability of the formation of a high-energy singlet state upon coupling of two low-energy triplet states. Theoretically, f depends on the energy level distribution, molecular orientation, inter-triplet exchange coupling of triplet dyads, and spin-mixing of resulting spin states (singlet, triplet, and quintet). However, experimental values of f for acene-based annihilators have been subject to large variations due to many factors that have resulted in the reporting of different f values for the same molecule. In this work, we discuss these factors by studying perylene as a case study annihilator, for which by far the largest variation in f = 16 to 100% has been reported. We systematically investigated the TTA-UC of PdTPBP:perylene, as a sensitizer-annihilator pair and obtained the experimental f = 17.9 ± 2.1% for perylene in THF solution. This limits the maximum TTA-UC quantum yield to 9.0% (out of 50%) for this annihilator. We found that such a low f value for perylene is largely governed by the energy-gap law where higher non-radiative losses due to the small energy gap between 2 × T1 and T2 affect the probability of singlet formation. Interestingly, we found this observation true for other acene-based annihilators whose emission ranges from the UV to the yellow region, thus providing a blueprint for future design of efficient TTA-UC systems.

Tetracene Dimers: A Platform for Intramolecular Down- and Up-conversion

Photon energy conversion can be accomplished in many different ways, including the two opposing manners, down-conversion (i.e., singlet fission, SF) and up-conversion (i.e., triplet-triplet annihilation up-conversion, TTA-UC). Both processes have the potential to help overcome the detailed balance limit of single-junction solar cells. Tetracene, in which the energies of the lowest singlet excited state and twice the triplet excited state are comparable, exhibits both down- and up-conversion. Here, we have designed meta-diethynylphenylene- and 1,3-diethynyladamantyl-linked tetracene dimers, which feature different electronic coupling, to characterize the interplay between intramolecular SF (intra-SF) and intramolecular TTA-UC (intra-TTA-UC) via steady-state and time-resolved absorption and fluorescence spectroscopy. Furthermore, we have used Pd-phthalocyanine as a sensitizer to enable intra-TTA-UC in the two dimers via indirect photoexcitation in the near-infrared part of the solar spectrum. The work is rounded off by temperature-dependent measurements, which outline key aspects of how thermal effects impact intra-SF and intra-TTA-UC in different dimers.

Triplet quenching pathway control with molecular dyads enables the identification of a highly oxidizing annihilator class

Metal complex - arene dyads typically act as more potent triplet energy donors compared to their parent metal complexes, which is frequently exploited for increasing the efficiencies of energy transfer applications. Using unexplored dicationic phosphonium-bridged ladder stilbenes (P-X2+) as quenchers, we exclusively observed photoinduced electron transfer photochemistry with commercial organic photosensitizers and photoactive metal complexes. In contrast, the corresponding pyrene dyads of the tested ruthenium complexes with the very same metal complex units efficiently sensitize the P-X2+ triplets. The long-lived and comparatively redox-inert pyrene donor triplet in the dyads thus provides an efficient access to acceptor triplet states that are otherwise very tricky to obtain. This dyad-enabled control over the quenching pathway allowed us to explore the P-X2+ photochemistry in detail using laser flash photolysis. The P-X2+ triplet undergoes annihilation producing the corresponding excited singlet, which is an extremely strong oxidant (+2.3 V vs. NHE) as demonstrated by halide quenching experiments. This behavior was observed for three P2+ derivatives allowing us to add a novel basic structure to the very limited number of annihilators for sensitized triplet-triplet annihilation in neat water.

Stable and low-threshold photon upconversion in nondegassed water by organic crystals

Photon upconversion (UC) is a technology that converts lower-energy photons (longer wavelength light) into higher-energy photons (shorter wavelength light), the opposite of fluorescence. Thus, UC is expected to open a vast domain of photonic applications that are not otherwise possible. Recently, UC by triplet-triplet annihilation (TTA) between organic molecules has been studied because of its applicability to low-intensity light, although the majority of such studies have focused on liquid samples in the form of organic solvent solutions. To broaden the range of applications, solid-state UC materials have been an active area of research. We recently developed air-stable, high-performance molecular UC crystals that utilize a stable solid-solution phase of bicomponent organic crystals. This article begins with a brief overview of previous challenges in developing and improving solid-state TTA-UC materials. Then, we briefly review and explain the concept as well as advantages of our molecular solid-solution UC crystals. We applied these organic crystals for the first time to a water environment. We observed blue UC emission upon photoexcitation at 542 nm (green-yellow light) and then measured the excitation intensity dependence as well as the temporal stability of the UC emission in air-saturated water. In nondegassed water, these organic crystals were stable, functioned with a low excitation threshold intensity of a few milliwatts per square centimeter, and exhibited high photo-irradiation durability at least over 40 h; indicating that the developed organic crystals are also viable for aqueous conditions. Therefore, the organic crystals presented in this report are expected to extend the domain of UC-based photonic applications in practical water systems including in vivo diagnostic, clinical, and therapeutic applications.

Triplet-triplet annihilation mediated photon upconversion solar energy systems

Solar energy harvesting is among the best solutions for a global transition toward carbon-neutral energy technologies. The existing solar energy harvesting technologies like photovoltaics (PV) and emerging molecular concepts such as solar fuels and molecular solar thermal energy storage (MOST) are rapidly developing. However, to realize their full potential, fundamental solar energy loss channels like photon transmission, recombination, and thermalization need to be addressed. Triplet-triplet annihilation mediated photon upconversion (TTA-UC) is emerging as a way to overcome losses due to the transmission of photons below the PV/chromophore band gap. However, there are several challenges related to the integration of efficient solid-state TTA-UC systems into efficient devices such as: wide band absorption, materials sustainability, and device architecture. In this article, we review existing work, identify and discuss challenges as well as present our perspective toward possible future directions.

High-Efficiency Near-Infrared-to-Visible Photon Upconversion in Poly(vinyl alcohol) Porous Film

Triplet-triplet annihilation photon upconversion (TTA-UC) has received significant attention in energy harvesting applications such as solar cells. The realization of high upconversion (UC) performance in the form of films is a crucial factor for the incorporation of this technology into large-area devices. Herein, we propose a porous UC film prepared by an emulsification method with a poly(vinyl alcohol) (PVA) aqueous solution and a toluene solution of chromophores (rubrene/Pd-tetraphenyltetraanthraporphyrin pair) that achieved considerable UC performance in the near-infrared (NIR) (810 nm) to visible (560 nm) range with a maximum quantum yield of 3.7% (out of 50%). Notably, the films displayed a UC emission when using an NIR light-emitting diode as a low-power-density noncoherent light source, which was confirmed by the naked eye. Excitation-power-dependent time-resolved photoluminescence measurements showed almost identical triplet lifetimes of emitter species for the films and toluene solutions; however, lower threshold intensities (I_th = 1-2 W/cm²) were observed for the films than those of the solutions (I_th = ∼10 W/cm²). An evaluation of the remaining toluene in the film and UC emission behavior in liquid nitrogen suggested that the chromophores exist as an amorphous solid in the pores, thus enabling hybrid triplet energy transfer (chromophore mobility based and exciton migration) in this unique film. The presented methodology can be generalized to other wavelengths and can enable diverse applications of the TTA-UC technology.

Water-Stable Upconverting Coordination Polymer Nanoparticles for Transparent Films and Anticounterfeiting Patterns with Air-Stable Upconversion

Photon upconversion (UC) based on triplet-triplet annihilation is a very promising phenomenon with potential application in several areas, though, due to the intrinsic mechanism, the achievement of diffusion-limited solid materials with air-stable UC is still a challenge. Herein, we report UC coordination polymer nanoparticles (CPNs) combining sensitizer and emitter molecules especially designed with alkyl spacers that promote the amorphous character. Beyond the characteristic constraints of crystalline MOFs, amorphous CPNs facilitate high dye density and flexible ratio tunability. To show the universality of the approach, two types of UC-CPNs are reported, exhibiting highly photostable UC in two different visible spectral regions. Given their nanoscale, narrow size distribution, and good chemical/colloidal stability in water, the CPNs were also successfully printed as anticounterfeiting patterns and used to make highly transparent and photostable films for luminescent solar concentrators, both showing air-stable UC.

Blue-to-UVB Upconversion, Solvent Sensitization and Challenging Bond Activation Enabled by a Benzene-Based Annihilator

Several energy-demanding photoreactions require harsh UV light from inefficient light sources. The conversion of low-energy visible light to high-energy singlet states via triplet-triplet annihilation upconversion (TTA-UC) could offer a solution for driving such reactions under mild conditions. We present the first annihilator with an emission maximum in the UVB region that, combined with an organic sensitizer, is suitable for blue-to-UVB upconversion. The annihilator singlet was successfully employed as an energy donor in subsequent FRET activations of aliphatic carbonyls. This hitherto unreported UC-FRET reaction sequence was directly monitored using laser spectroscopy and applied to mechanistic irradiation experiments demonstrating the feasibility of Norrish chemistry. Our results provide clear evidence for a novel blue light-driven substrate or solvent activation strategy, which is important in the context of developing more sustainable light-to-chemical energy conversion systems.

Triplet-Triplet Annihilation via the Triplet Channel in Crystalline 9,10-Diphenylanthracene

Delayed fluorescence resulting from triplet-triplet annihilation in crystalline 9,10-diphenylanthracene was observed by means of steady-state fluorescence measurements under magnetic fields of ≤10 T. At five specific magnetic fields, four peaks and one dip in the magnetic field dependence of fluorescence intensity were observed, proving that exchange-coupled triplet pairs were generated in the course of triplet-triplet annihilation. The dip was in the opposite direction predicted for singlet channel triplet-triplet annihilation. Further analysis using the stochastic Liouville equation confirmed that the closest exchange-coupled triplet pair in crystalline 9,10-diphenylanthracene is quenched via both triplet channel and singlet channel triplet-triplet annihilation.

Triplet-triplet annihilation photon upconversion from diphenylhexatriene and ring-substituted derivatives in solution

We report that diphenylhexatriene (DPH) and its ring-substituted derivatives act as emitter molecules in triplet-triplet annihilation photon upconversion (TTA-UC). A palladium porphyrin derivative, meso-tetraphenyl-tetrabenzoporphine palladium complex (PdTPBP), which acts as a sensitiser in TTA-UC, and DPH derivatives were dissolved in tetrahydrofuran (THF). The solution showed blue-green to green UC emission under photoexcitation at 640 nm in a nitrogen atmosphere. The UC quantum efficiency (η_UC) values of the DPHs were estimated, with (E,E,E)-1,6-bis[4-(di-2-picolylamino)phenyl]hexa-1,3,5-triene (pico DPH) showing the highest. In addition, the quantum yields of triplet energy transfer (TET) and triplet-triplet annihilation (TTA), which are elementary processes in TTA-UC, were estimated, as well as the triplet lifetimes of each DPH derivative. The results indicate that the TTA process governs the value of η_UC.

Best practice in determining key photophysical parameters in triplet-triplet annihilation photon upconversion

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 ([Formula: see text]), triplet excited-state lifetimes ([Formula: see text]), and excitation threshold intensity ([Formula: see text]). Finally, we introduce a newly developed method by which [Formula: see text], [Formula: see text], and [Formula: see text] 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 [Formula: see text]. 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.

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

Triplet-sensitized photon upconversion (UC) has been proposed for broad applications. However, the quest for superior solid materials has been challenged by the poor exciton transport often caused by low crystallinity, a small crystal domain, and aggregation of triplet sensitizers. Here, we demonstrate substantial advantages of the van der Waals solid solution concept to yield molecular crystals with extraordinary performance. A 0.001%-order porphyrin sensitizer is dissolved during recrystallization into the molecular crystals of a blue-fluorescent hydrocarbon annihilator, 9-(2-naphthyl)-10-[4-(1-naphthyl)phenyl]anthracene (ANNP), which contains bulky side groups. This attempt yields millimeter-sized, uniformly colored, transparent solid solution crystals, which resolves the long-standing problem of sensitizer aggregation. After annealing, the crystals exhibit unprecedented UC performance (UC quantum yield reaching 16% out of a maximum of 50% by definition; excitation intensity threshold of 0.175 sun; and high photostability of over 150 000 s) in air, which proves that this concept is highly effective in the quest for superior UC solid materials.

CdSe nanocrystal sensitized photon upconverting film

Here, films using CdSe nanocrystal (NC) triplet photosensitizers in conjunction with diphenylanthracene (DPA) emitters were assembled to address several challenges to practical applications for solution-based photon upconversion. By using poly(9-vinylcarbazole) as a phosphorescent host in this film, volatile organic solvents are eliminated, the spontaneous crystallization of the emitter is significantly retarded, and ∼1.5% photon upconversion quantum yield (out of a maximum of 50%) is obtained. Transient absorption spectroscopy on nanosecond-to-microsecond time scales reveals this efficiency is enabled by an exceptionally long triplet lifetime of 3.4 ± 0.3 ms. Ultimately, we find the upconversion efficiency is limited by incomplete triplet–triplet annihilation, which occurs with a rate 3–4 orders of magnitude slower than in solution-phase upconversion systems.

Here, films using CdSe nanocrystal (NC) triplet photosensitizers in conjunction with diphenylanthracene (DPA) emitters doe for the conversion of green to blue light.