Dane R. McCamey

The Hückel and Mandado rules explain the emergence of triplet–triplet annihilation in a functionalized amide-containing chromophore.

Singlet fission (SF), which has applications in areas ranging from solar energy to quantum information, relies critically on transitions within a multi-spin manifold. These transitions are driven by fluctuations in the spin–spin exchange interaction, which have been linked to changes in nuclear geometry or exciton migration. While simple calculations have supported this mechanism, to date, limited efforts have been made to model realistic fluctuations, which are informed by the actual structure and properties of physical materials. In this paper, we develop a modular computational pipeline for calculating SF spin dynamics by way of electronic structural calculations, molecular dynamics, and numerical models of spin dynamics. The outputs of this pipeline aid in the interpretation of measured spin dynamics and allow us to place constraints on geometric fluctuations, which are consistent with these observations.

Singlet fission (SF) and triplet-triplet annihilation upconversion (TTA-UC) are photophysical processes which, respectively, result in the division of one high energy singlet state into two lower-energy triplet states, and vice versa. These processes are components of strategies to overcome the efficiency limits of photovoltaic devices, by funneling more energy into silicon solar cells. Molecules featuring a fused π -conjugated ring system interrupted by amide groups have recently attracted interest for singlet fission owing to their greater stability than acene-based chromophores. While aromaticity has been regularly used to design chromophores for both SF and TTA-UC and rationalize results, debate continues about the relevance and application of the concept. Here, we describe the spectroscopic properties of novel derivatives of a 5-6-5 fused-ring chromophore with a central 8 π 1,4-diacyl-1,4-dihydropyrazine ring. Density functional theory calculations suggest that the parent chromophore does not meet the key thermodynamic criterion for SF, i.e. E (S ₁ ) ≥ 2 E (T ₁ ), but that modifying the carbonyl groups via microwave-assisted thionation and subsequent silver(I)-promoted condensation will substantially modulate the E (T ₁ )/ E (S ₁ ) ratio. Through photophysical measurements including time-resolved absorption and magneto-photoluminescence, we demonstrate that intersystem crossing followed by triplet-triplet annihilation occurs upon excitation of a dicyanomethylene-substituted derivative. We propose an explanation for these observations in terms of Mandado’s (2 n + 1) aromaticity rule, which provides guidance for future designs of multiexcitonic chromophore systems based on the cyclic amide motif.

Photochemical upconversion has several potential applications in optoelectronics. However, there is yet to be demonstrated a rational approach to high efficiencies in nanoscale solid-state devices. Here we demonstrate that the liquid triplet fusion medium, 9,10-bis( n -octyl-diisopropylsilylethynyl)anthracene (NODIPS-An), which behaves as a solid on excitonic timescales, can achieve high-efficiency upconversion on the nanoscale. Owing to its amorphous structure, there are highly-coupled sites that trap upconverted states and prevent the back transfer that plagues nanoscale upconversion systems. With NODIPS-An filling the pores of a sensitized alumina nano-scaffold, we achieve an absolute photon upconversion efficiency of 8.2(4)%. The majority of efficiency losses are attributed to fluorescence and triplet energy transfer yields, and thus the self-trapping of excitons within the triplet fusion medium is demonstrated to effectively prevent the back transfer of excitons to the sensitizer. Strategies are proposed to improve the fluorescence yield and triplet energy transfer to pursue higher efficiencies in nanoscale solid-state photochemical upconversion devices.

Photon upconversion has potential applications in light–emitting diodes, photocatalysis, bio‐imaging, microscopy, 3D printing, and photovoltaics. Bulk lead‐halide perovskites have emerged as promising sensitisers for solid‐state photon upconversion via triplet–triplet annihilation due to their excellent optoelectronic properties. In this system, a perovskite‐sensitiser absorbs photons and subsequently generates triplet excitons in an adjacent emitter material, where triplet–triplet annihilation can occur, allowing for the emission of higher energy photons. However, a major loss pathway in perovskite‐sensitised upconversion is the back‐transfer of singlet excitons from the emitter to the sensitiser via Förster Resonance Energy Transfer. In this investigation, a 2D perovskite spacer layer is introduced between the bulk perovskite‐sensitiser and rubrene emitter to mitigate back‐transfer of singlet excitons. This modification reveals the inherent balance between efficient triplet exciton transfer across the interface with a potential barrier vs the mitigation of near‐field back‐transfer by increasing the distance between the sensitiser and singlet excitons in the emitter. Notably, the introduction of this spacer layer enhances the relative upconversion efficiency at lower excitation power densities while also sustaining performance over extended timescales. This work represents significant progress toward the practical applications of perovskite‐sensitised photon upconversion.

Magnetic field effects play a key role in the characterisation of organic electronic materials. In particular, the microscopic processes underlying singlet fission and triplet-triplet annihilation have been clarified in recent years, through the observation and subsequent theoretical explanation of magnetic field effects in these processes. Here, we study magnetic field effects on the luminescence of 5,12-Bis((triisopropylsilyl)ethynyl) tetracene: a quintessential material in which both singlet fission and triplet-triplet annihilation occur. By performing highly sensitive modulated-field magneto-photoluminescence experiments, we find magnetic field effect lineshapes of unprecedented detail, allowing a precise characterisation of triplet exciton dynamics. By additionally filtering these magnetic field effects by luminescence wavelength, we can identify the historically elusive excimer state by its unique magnetic field signatures. Finally, we show that a theoretical explanation of these field effects requires a model that explicitly accounts for quantum coherences, which highlights the importance of detailed spin dynamics modelling in understanding magnetic field effects, even at room temperature.

Singlet fission photovoltaics enable the extraction of two electron-hole pairs from each higher-energy photon while leveraging a mature technology such as crystalline silicon (c-Si) as the underlying cell. Significant steps have been made in the development of singlet fission silicon photovoltaics, but all examples reported to date use tetracene as the singlet fission material. Tetracene is photochemically unstable and therefore unsuitable for commercial applications. Here we demonstrate singlet fission-derived triplet exciton transfer to c-Si from photochemically stable dipyrrolonaphthyridinedione (DPND) derivatives, with a thin layer of tin oxide as the passivation layer. An overlayer of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was found to improve interface passivation further. These observations demonstrate that singlet fission photovoltaic devices can be made using stable and commercially viable materials.

The successful development of optoelectronic devices is contingent on a detailed understanding of interactions between light and excited energy states in photoactive materials. In 2D perovskites, excitons are the dominant photogenerated species and their energetic structure plays a pivotal role, governing photon absorption and emission processes. In these materials, dark exciton states can undergo photoluminescence due to relaxation of selection rules and this process can be modulated by an external magnetic field, enabling unambiguous identification of the exciton fine structure. Previous reports of magneto‐optical spectroscopy on 2D perovskites are restricted to the macroscopic response, where key information is lost regarding the microscopic heterogeneity of the photoluminescence. Here, magneto‐optical microscopy is used for the first time on perovskite materials to elucidate the spatial variation of exciton emission processes. In 2D perovskite thin films, regions of localized bright and dark exciton populations are distinguished, correlated to the film morphology. In single crystals, dark excitons become localised at the edges, where excitons can be trapped in two distinct types of sub‐gap states. This work represents significant progress in understanding the properties of exciton emission in 2D perovskites, which is crucial for the development and optimization of optoelectronic technology.

Photochemical upconversion has several potential applications in optoelectronics. However, there is yet to be demonstrated a rational approach to high efficiencies in nanoscale solid state devices. Here we demonstrate that the liquid triplet fusion medium, 9,10-bis(n-octyl-diisopropylsilylethynyl)anthracene (NODIPS-An), which behaves as a solid on excitonic time-scales, can achieve high efficiency upconversion on the nanoscale. Due to its amorphous structure, there are highly-coupled sites that trap upconverted states and prevent the back-transfer that plagues nanoscale upconversion systems. With NODIPS-An filling the pores of a sensitized alumina nano-scaffold, we achieve an absolute photon upconversion efficiency of 7.0%. The efficiency losses are entirely attributed to fluorescence yield and triplet energy transfer, and thus the self-trapping of excitons within the triplet fusion medium is demonstrated to effectively prevent the back transfer of excitons to the sensitizer. Strategies are proposed to improve the fluorescence yield and triplet energy transfer to pursue higher efficiencies in nanoscale solid-state photochemical upconversion devices.

Photon upconversion has potential applications in light-emitting diodes, photocatalysis, bio-imaging, microscopy, 3D printing, and photovoltaics. Bulk lead-halide perovskite films have emerged as promising sensitisers for solid-state photon upconversion via triplet-triplet annihilation due to their excellent optoelectronic properties. In this system, a perovskite sensitiser absorbs photons and subsequently generates triplet excitons in an adjacent emitter material, where triplet-triplet annihilation can occur allowing for the emission of higher energy photons. However, a major loss pathway in perovskite-sensitised upconversion is the back-transfer of singlet excitons from the emitter to the sensitiser via Förster Resonance Energy Transfer. In this investigation we introduce a 2D perovskite spacer layer between the bulk perovskite sensitiser and a rubrene emitter to mitigate back-transfer of singlet excitons from rubrene to the bulk perovskite sensitiser. This modification reveals the inherent balance between efficient triplet exciton transfer across the interface with a potential barrier versus the mitigation of near-field back-transfer by increasing the distance between the sensitiser and singlet excitons in the emitter. Notably, the introduction of this spacer layer enhances the relative upconversion efficiency at lower excitation power densities while also sustaining performance over extended timescales. This work represents significant progress toward the practical applications of perovskite-sensitised photon upconversion.