Efficient Visible to Near-UV Photochemical Upconversion Sensitized by a Long Lifetime Cu(I) MLCT Complex

Luminescent Fe(III) Complex Sensitizes Aerobic Photon Upconversion and Initiates Photocatalytic Radical Polymerization

Light energy conversion often relies on photosensitizers with long-lived excited states, which are mostly made of precious metals such as ruthenium or iridium. Photoactive complexes based on highly abundant iron seem attractive for sustainable energy conversion, but this remains very challenging due to the short excited state lifetimes of the current iron complexes. This study shows that a luminescent Fe(III) complex sensitizes triplet-triplet annihilation upconversion with anthracene derivatives via underexplored doublet-triplet energy transfer, which is assisted by preassociation between the photosensitizer and the annihilator. In the presence of an organic mediator, the green-to-blue upconversion efficiency ΦUC with 9,10-diphenylanthracene (DPA) as the annihilator achieves a 6-fold enhancement to ∼0.2% in aerated solution at room temperature. The singlet excited state of DPA, accessed via photon upconversion in the Fe(III)/DPA pair, allows efficient photoredox catalytic radical polymerization of acrylate monomers in a spatially controlled manner, whereas this process is kinetically hindered with the prompt DPA. Our study provides a new strategy of using low-cost iron and low-energy visible light for efficient polymer synthesis, which is a significant step for both fundamental research and future applications.

Steric and Electronic Influence of Excited-State Decay in Cu(I) MLCT Chromophores

ConspectusFor the past 11 years, a dedicated effort in our research group focused on fundamentally advancing the photophysical properties of cuprous bis-phenanthroline-based metal-to-ligand charge transfer (MLCT) excited states. We rationalized that, by gaining control over the numerous factors limiting the more widespread use of CuI MLCT photosensitizers, they would be readily adopted in numerous light-activated applications given the earth-abundance of copper and the extensive library of 1,10-phenanthrolines developed over the last century. Significant progress has been achieved by recognizing valuable structure-property concepts developed by other researchers in tandem with detailed ultrafast and conventional time-scale investigations, in-silico-inspired molecular designs to predict spectroscopic properties, and applying novel synthetic methodologies. Ultimately, we achieved a plateau in exerting cooperative steric influence to control CuI MLCT excited state decay. This led to combining sterics with π-conjugation and/or inductive electronic effects to further exert control over molecular photophysical properties. The lessons gleaned from our studies of homoleptic complexes were recently extended to heteroleptic bis(phenanthrolines) featuring enhanced visible light absorption properties and long-lived room-temperature photoluminescence. This Account navigates the reader through our intellectual journey of decision-making, molecular and experimental design, and data interpretation in parallel with appropriate background information related to the quantitative characterization of molecular photophysics using CuI MLCT chromophores as prototypical examples.Initially, CuI MLCT excited states, their energetics, and relevant structural conformation changes implicated in their photophysical decay processes are described. This is followed by a discussion of the literature that motivated our research in this area. This led to our first molecular design in 2013, achieving a 7-fold increase in excited state lifetime relative to the current state-of-the-art. The lifetime and photophysical property enhancement resulted from using 2,9-branched alkyl groups in conjunction with flanking 3,8-methyl substituents, a strategy we adapted from the McMillin group, which was initially described in the late 1990s. Applications of this newly conceived chromophore are presented in solar hydrogen-producing photocatalysis, photochemical upconversion, and photosensitization of [4 + 4] anthracene dimerization of potential interest in thermal storage of solar energy in metastable intermediates. Ultrafast transient absorption and fluorescence upconversion spectroscopic characterization of this and related CuI molecules inform the resultant photophysical properties and vice versa, so the most comprehensive structure-property understanding becomes realized when these experimental tools are collectively utilized to investigate the same series of molecules. Computationally guided structural designs generated newly conceived molecules featuring visible light-harvesting and 2,9-cycloalkane substituted complexes. The latter eventually produced record-setting excited state lifetimes in molecules leveraging both cooperative steric influence and electronic inductive effects. Using photoluminescence data from structurally homologous CuI MLCT excited states collected over 44 years, an energy gap correlation successfully modeled the data spanning a 0.3 eV emission energy range. Finally, a new research direction is revealed detailing structure-photophysical property relationships in heteroleptic CuI phenanthroline chromophores that are photoluminescent at room temperature.

Photonic crystal-assisted sub-bandgap photocatalysis via triplet-triplet annihilation upconversion for the degradation of environmental organic pollutants

This study explores novel approaches to enhance photocatalysis efficiency by introducing a photonic crystal (PC)-enhanced, multi-layered sub-bandgap photocatalytic reactor. The design aims to effectively utilize sub-bandgap photons that might otherwise go unused. The device consists of three types of layers: (1) two polymeric triplet-triplet annihilation upconversion (TTA-UC) layers converting low-energy green photons (λEx = 532 nm, 2.33 eV) to high-energy blue photons (λEm = 425 nm, 2.92 eV), (2) a platinum-decorated WO3 layer (Eg = 2.8 eV) serving as a visible-light photocatalyst, and (3) a PC layer optimizing both TTA-UC and photocatalysis. The integration of the PC layer resulted in a 1.9-fold increase in UC emission and a 7.9-fold enhancement in hydroxyl radical (•OH) generation, achieved under low-intensity sub-bandgap irradiation (17.6 mW cm-2). Consequently, the combined layered structure of TTA/Pt-WO3/TTA/PC achieved a remarkable 38.8-fold improvement in •OH production, leading to outstanding degradation capability for various organic pollutants (e.g., 4-chlorophenol, bisphenol A, and methylene blue). This multi-layered sub-bandgap photocatalytic structure, which uniquely combines TTA-UC and PC layers, offers valuable insights into designing efficient photocatalytic systems for future solar-driven environmental remediation.

Axial Coordinated Manganese(III) Porphyrin/Tetraazaporphyrin - 4-(10-phenylanthracen-9-yl)Pyridine Dyads: Self-Assembly, Structure and Spectral Properties in Ground and Excited States

Self-assembly of new donor-acceptor systems based on (5,10,15,20-tetraphenylporphinato)manganese(III)/(5,10,15,20-tetra-4-tert-butylphenylporphinato)manganese(III)/(octakis(4-tert-butylphenyl)tetraazaporphinato)manganese(III) acetate ((AcO)MnTPP/(AcO)MnTBPP/(AcO)MnTAP) and 4-(10-phenylanthracen-9-yl)pyridine (PyAn) was studied using fluorescence spectroscopy and mass spectrometry. It was found that the coordination complexes of 1 : 1 composition (dyads) are formed in toluene. The spectral properties, the chemical structures and redox behavior of the dyads were described using 1H NMR, IR, ESR spectroscopy and cyclic voltammetry, respectively. The dynamic processes and the characteristics in the excited state of the dyads were obtained using the femtosecond transient absorption spectroscopy method. Density functional theory (DFT), time-dependent DFT methods were used to elucidate the dyad electronic structures and to establish the differences in their frontier molecular orbitals. The analysis of the lambda parameter and the distance of hole-pair interaction was indicated more favorable charge transfer between the macrocycle and the axial PyAn fragment in (AcO)(PyAn)MnTAP. The calculated values of the zero-field splitting parameters D and E/D, together with the g tensors of the lowest spin-orbit state for (AcO)MnTPP and (AcO)(PyAn)MnTPP were obtained using the combination of DFT and Multireference Perturbation Theory (CASSCF/NEVPT2) simulations. The data obtained develop the fundamental basis in the field of photovoltaics and show the prospects for the study of molecular systems of this class.

Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes

Utilization of renewable energies for catalytically generating value-added chemicals is highly desirable in this era of rising energy demands and climate change impacts. Artificial photosynthetic systems or photocatalysts utilize light to convert abundant CO2, H2O, and O2 to fuels, such as carbohydrates and hydrogen, thus converting light energy to storable chemical resources. The emergence of intense X-ray pulses from synchrotrons, ultrafast X-ray pulses from X-ray free electron lasers, and table-top laser-driven sources over the past decades opens new frontiers in deciphering photoinduced catalytic reaction mechanisms on the multiple temporal and spatial scales. Operando X-ray spectroscopic methods offer a new set of electronic transitions in probing the oxidation states, coordinating geometry, and spin states of the metal catalytic center and photosensitizers with unprecedented energy and time resolution. Operando X-ray scattering methods enable previously elusive reaction steps to be characterized on different length scales and time scales. The methodological progress and their application examples collected in this review will offer a glimpse into the accomplishments and current state in deciphering reaction mechanisms for both natural and synthetic systems. Looking forward, there are still many challenges and opportunities at the frontier of catalytic research that will require further advancement of the characterization techniques.

Triplet-triplet annihilation photon upconversion-mediated photochemical reactions

Photon upconversion is a method for harnessing high-energy excited states from low-energy photons. Such photons, particularly in the red and near-infrared wavelength ranges, can penetrate tissue deeply and undergo less competitive absorption in coloured reaction media, enhancing the efficiency of large-scale reactions and in vivo phototherapy. Among various upconversion methodologies, the organic-based triplet-triplet annihilation upconversion (TTA-UC) stands out - demonstrating high upconversion efficiencies, requiring low excitation power densities and featuring tunable absorption and emission wavelengths. These factors contribute to improved photochemical reactions for fields such as photoredox catalysis, photoactivation, 3D printing and immunotherapy. In this Review, we explore concepts and design principles of organic TTA-UC-mediated photochemical reactions, highlighting notable advancements in the field, as well as identify challenges and propose potential solutions. This Review sheds light on the potential of organic TTA-UC to advance beyond the traditional photochemical reactions and paves the way for research in various fields and clinical applications.

Luminescent and Photoredox-Active Molybdenum(0) Complexes Competitive with Isoelectronic Ruthenium(II) Polypyridines

Ruthenium(II) complexes with chelating polypyridine ligands are among the most frequently investigated compounds in photophysics and photochemistry, owing to their favorable luminescence and photoredox properties. Equally good photoluminescence performance and attractive photocatalytic behavior is now achievable with isoelectronic molybdenum(0) complexes. The zero-valent oxidation state of molybdenum is stabilized by carbonyl or isocyanide ligands, and metal-to-ligand charge transfer (MLCT) excited states analogous to those in ruthenium(II) complexes can be established. Microsecond MLCT excited-state lifetimes and photoluminescence quantum yields up to 0.2 have been achieved in solution at room temperature, and the emission wavelength has become tunable over a large range. The molybdenum(0) complexes are stronger photoreductants than ruthenium(II) polypyridines and can therefore perform more challenging chemical reductions. The triplet nature of their luminescent MLCT states allows sensitization of photon upconversion via triplet-triplet annihilation, to convert low-energy input radiation into higher-energy output fluorescence. This review summarizes the current state of the art concerning luminescent molybdenum(0) complexes and highlights their application potential. Molybdenum is roughly 140 times more abundant and far cheaper than ruthenium, hence this research is relevant in the greater context of finding more sustainable alternatives to using precious and rare transition metals in photophysics and photochemistry.

First-Row d6 Metal Complex Enables Photon Upconversion and Initiates Blue Light-Dependent Polymerization with Red Light

Photosensitizers for sensitized triplet-triplet annihilation upconversion (sTTA-UC) often rely on precious heavy metals, whereas coordination complexes based on abundant first-row transition metals are less common. This is mainly because long-lived triplet excited states are more difficult to obtain for 3d metals, particularly when the d-subshell is only partially filled. Here, we report the first example of sTTA-UC based on a 3d6 metal photosensitizer yielding an upconversion performance competitive with precious metal-based analogues. Using a newly developed Cr0 photosensitizer featuring equally good photophysical properties as an OsII benchmark complex in combination with an acetylene-decorated anthracene annihilator, red-to-blue upconversion is achievable. The upconversion efficiency under optimized conditions is 1.8 %, and the excitation power density threshold to reach the strong annihilation limit is 5.9 W/cm2 . These performance factors, along with high photostability, permit the initiation of acrylamide polymerization by red light, based on radiative energy transfer between delayed annihilator fluorescence and a blue light absorbing photo-initiator. Our study provides the proof-of-concept for photon upconversion with elusive first-row analogues of widely employed precious d6 metal photosensitizers, and for their application in photochemical reactions triggered by excitation wavelengths close to near-infrared.

Efficient Triplet-Triplet Annihilation Upconversion Sensitized by a Chromium(III) Complex via an Underexplored Energy Transfer Mechanism

Sensitized triplet-triplet annihilation upconversion (sTTA-UC) mainly relies on precious metal complexes thanks to their high intersystem crossing (ISC) efficiencies, excited state energies, and lifetimes, while complexes of abundant first-row transition metals are only rarely utilized and with often moderate UC quantum yields. [Cr(bpmp)2 ]3+ (bpmp=2,6-bis(2-pyridylmethyl)pyridine) containing earth-abundant chromium possesses an absorption band suitable for green light excitation, a doublet excited state energy matching the triplet energy of 9,10-diphenyl anthracene (DPA), a close to millisecond excited state lifetime, and high photostability. Combined ISC and doublet-triplet energy transfer from excited [Cr(bpmp)2 ]3+ to DPA gives 3 DPA with close-to-unity quantum yield. TTA of 3 DPA furnishes green-to-blue UC with a quantum yield of 12.0 % (close to the theoretical maximum). Sterically less-hindered anthracenes undergo a [4+4] cycloaddition with [Cr(bpmp)2 ]3+ and green light.

Low power threshold photochemical upconversion using a zirconium(iv) LMCT photosensitizer

The current investigation demonstrates highly efficient photochemical upconversion (UC) where a long-lived Zr(iv) ligand-to-metal charge transfer (LMCT) complex serves as a triplet photosensitizer in concert with well-established 9,10-diphenylanthracene (DPA) along with newly conceived DPA-carbazole based acceptors/annihilators in THF solutions. The initial dynamic triplet-triplet energy transfer (TTET) processes (ΔG ∼ -0.19 eV) featured very large Stern-Volmer quenching constants (K SV) approaching or achieving 105 M-1 with bimolecular rate constants between 2 and 3 × 108 M-1 s-1 as ascertained using static and transient spectroscopic techniques. Both the TTET and subsequent triplet-triplet annihilation (TTA) processes were verified and throughly investigated using transient absorption spectroscopy. The Stern-Volmer metrics support 95% quenching of the Zr(iv) photosensitizer using modest concentrations (0.25 mM) of the various acceptor/annihilators, where no aggregation took place between any of the chromophores in THF. Each of the upconverting formulations operated with continuous-wave linear incident power dependence (λ ex = 514.5 nm) down to ultralow excitation power densities under optimized experimental conditions. Impressive record-setting η UC values ranging from 31.7% to 42.7% were achieved under excitation conditions (13 mW cm-2) below that of solar flux integrated across the Zr(iv) photosensitizer's absorption band (26.7 mW cm-2). This study illustrates the importance of supporting the continued development and discovery of molecular-based triplet photosensitizers based on earth-abundant metals.

Iridium(III) Sensitisers and Energy Upconversion: The Influence of Ligand Structure upon TTA-UC Performance

Six substituted ligands based upon 2-(naphthalen-1-yl)quinoline-4-carboxylate and 2-(naphthalen-2-yl)quinoline-4-carboxylate have been synthesised in two steps from a range of commercially available isatin derivatives. These species are shown to be effective cyclometallating ligands for Ir^III , yielding complexes of the form [Ir(C^N)₂ (bipy)]PF₆ (where C^N=cyclometallating ligand; bipy=2,2'-bipyridine). X-ray crystallographic studies on three examples demonstrate that the complexes adopt a distorted octahedral geometry wherein a cis-C,C and trans-N,N coordination mode is observed. Intraligand torsional distortions are evident in all cases. The Ir^III complexes display photoluminescence in the red part of the visible region (668-693 nm), which is modestly tuneable through the ligand structure. The triplet lifetimes of the complexes are clearly influenced by the precise structure of the ligand in each case. Supporting computational (DFT) studies suggest that the differences in observed triplet lifetime are likely due to differing admixtures of ligand-centred versus MLCT character instilled by the facets of the ligand structure. Triplet-triplet annihilation upconversion (TTA-UC) measurements demonstrate that the complexes based upon the 1-naphthyl derived ligands are viable photosensitisers with upconversion quantum efficiencies of 1.6-6.7 %.

A Photorobust Mo(0) Complex Mimicking [Os(2,2'-bipyridine)3]2+ and Its Application in Red-to-Blue Upconversion

Osmium(II) polypyridines are a well-known class of complexes with luminescent metal-to-ligand charge-transfer (MLCT) excited states that are currently experiencing a revival due to their application potential in organic photoredox catalysis, triplet-triplet annihilation upconversion, and phototherapy. At the same time, there is increased interest in the development of photoactive complexes made from Earth-abundant rather than precious metals. Against this background, we present a homoleptic Mo(0) complex with a new diisocyanide ligand exhibiting different bite angles and a greater extent of π-conjugation than previously reported related chelates. This new design leads to deep red emission, which is unprecedented for homoleptic arylisocyanide complexes of group 6 metals. With a ³MLCT lifetime of 56 ns, an emission band maximum at 720 nm, and a photoluminescence quantum yield of 1.5% in deaerated toluene at room temperature, the photophysical properties are reminiscent of the prototypical [Os(2,2'-bipyridine)₃]²⁺ complex. Under 635 nm irradiation with a cw-laser, the new Mo(0) complex sensitizes triplet-triplet annihilation upconversion of 9,10-diphenylanthracene (DPA), resulting in delayed blue fluorescence with an anti-Stokes shift of 0.93 eV. The photorobustness of the Mo(0) complex and the upconversion quantum yield are high enough to generate a flux of upconverted light that can serve as a sufficiently potent irradiation source for a blue-light-driven photoisomerization reaction. These findings are relevant in the greater contexts of designing new luminophores and photosensitizers for use in red-light-driven photocatalysis, photochemical upconversion, light-harvesting, and phototherapy.

Annihilator dimers enhance triplet fusion upconversion

Optical upconversion is a net process by which two low energy photons are converted into one higher energy photon. There is vast potential to exploit upconversion in applications ranging from solar energy and biological imaging to data storage and photocatalysis. Here, we link two upconverting chromophores together to synthesize a series of novel tetracene dimers for use as annihilators. When compared with the monomer annihilator, TIPS-tetracene, the dimers yield a strong enhancement in the triplet fusion process, also known as triplet-triplet annihilation, as demonstrated via a large increase in upconversion efficiency and an order of magnitude reduction of the threshold power for maximum yield. Along with the ongoing rapid improvements to sensitizer materials, the dimerization improvements demonstrated here open the way to a wide variety of emerging upconversion applications.

Applications and Prospects for Triplet-Triplet Annihilation Photon Upconversion

Triplet-triplet annihilation photon upconversion (TTA-UC) is a photophysical process in which the energy of two photons are combined into a single photon of higher energy. While this strategy has been recognized in applications ranging from bioimaging to solar energy conversion, its uses in synthetic organic chemistry have not been extensively developed. Here, we present a short tutorial on the theoretical underpinnings and the design principles of TTA-UC systems. Selected applications are then discussed to highlight key features of the TTA mechanism, along with a prospective discussion of the potential of these mechanisms to enable innovation in photocatalysis, methods development, and synthetic organic chemistry.

Enhancing the Visible-Light Absorption and Excited-State Properties of Cu(I) MLCT Excited States

A computationally inspired Cu(I) metal-to-ligand charge transfer (MLCT) chromophore, [Cu(sbmpep)₂]⁺ (sbmpep = 2,9-di(sec-butyl)-3,8-dimethyl-4,7-di(phenylethynyl)-1,10-phenanthroline), was synthesized in seven total steps, prepared from either dichloro- or dibromophenanthroline precursors. Complete synthesis, structural characterization, and electrochemistry, in addition to static and dynamic photophysical properties of [Cu(sbmpep)₂]⁺, are reported on all relevant time scales. UV-Vis absorption spectroscopy revealed significant increases in oscillator strength along with a concomitant bathochromic shift in the MLCT absorption bands with respect to structurally related model complexes (ε = 16 500 M^-1 cm^-1 at 491 nm). Strong red photoluminescence (Φ = 2.7%, λ_max = 687 nm) was observed from [Cu(sbmpep)₂]⁺, which featured an average excited-state lifetime of 1.4 μs in deaerated dichloromethane. Cyclic and differential pulse voltammetry revealed ∼300 mV positive shifts in the measured one-electron reversible reduction and oxidation waves in relation to a Cu(I) model complex possessing identical structural elements without the π-conjugated 4,7-substituents. The excited-state redox potential of [Cu(sbmpep)₂]⁺ was estimated to be -1.36 V, a notably powerful reductant for driving photoredox chemistry. The combination of conventional and ultrafast transient  absorption and luminescence spectroscopy successfully map the excited-state dynamics of [Cu(sbmpep)₂]⁺ from initial photoexcitation to the formation of the lowest-energy MLCT excited state and ultimately its relaxation to the ground state. This newly conceived molecule appears poised for photosensitization reactions involving energy and electron-transfer processes relevant to photochemical upconversion, photoredox catalysis, and solar fuels photochemistry.

Luminescent Dinuclear Copper(I) Complexes as Potential Thermally Activated Delayed Fluorescence (TADF) Emitters: A Theoretical Study

The excited state properties of a series of binuclear NHetPHOS-Cu(I) complexes (NHetPHOS) have been investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). It is shown that experimental trends observed in powder, generally explored via S₁ and T₁ excited state energetics and S₁ ⇔ T₁ intersystem crossing (ISC) efficiency, are hardly analyzed on the basis of excited state properties calculated in solution. Indeed, several local minima corresponding to various structural deformations are evident on the lowest excited state potential energy surfaces (PES) when solvent correction is applied, leading to a four-state thermally activated delayed fluorescence (TADF) mechanism. In contrast, preliminary simulations performed in the solid point to the reduction of nuclear flexibility and consequently to a rather simple two-state model.

Multimolecular assemblies on high surface area metal oxides and their role in interfacial energy and electron transfer

High surface area metal oxides offer a unique substrate for the assembly of multiple molecular components at an interface.

Artificial photosynthesis by light absorption, charge separation, and multielectron catalysis

We highlight recent novel approaches in the field of artificial photosynthesis. We emphasize the potential of a highly modular plug-and-play concept that we hope will persuade the community to explore a more inclusive variety of multielectron redox catalysis to complement the proton reduction and water oxidation half-reactions in traditional solar water splitting systems.

A long-lived cuprous bis-phenanthroline complex for the photodynamic therapy of cancer

An earth-abundant cuprous bis-phenanthroline photosensitizer showed potential use in the photodynamic therapy of cancer.

Photoinduced structural distortions and singlet–triplet intersystem crossing in Cu(i) MLCT excited states monitored by optically gated fluorescence spectroscopy

Comprehensive analysis of the photo-induced structural distortions and singlet–triplet intersystem crossing dynamics of a series of Cu(i) phenanthroline chromophores.

Photochemical upconversion in water

The current investigation achieves photochemical upconversion in pure water using combinations of water soluble Ru(ii) metal-to-ligand charge transfer (MLCT) sensitizers in concert with 9-anthracenecarboxylate (AnCO₂⁻) and 1-pyrenecarboxylate (PyCO₂⁻).

Synergistically enhanced upconversion luminescence in Li+-doped core–shell-structured ultrasmall nanoprobes for dual-mode deep tissue fluorescence/CT imaging

The development of upconversion luminescence that allows for multimodal imaging in terms of resolution and penetration depth using a single system is attracting increasing interest for use in clinical molecular imaging and diagnostics.

Liquid PEG Polymers Containing Antioxidants: A Versatile Platform for Studying Oxygen-Sensitive Photochemical Processes

This article proposes the exploitation of widely available, inexpensive, innocuous "green" liquid polyethylene glycol (PEG) polymers containing the oxygen scavenger oleic acid (OA) as promising media for studying oxygen-sensitive photochemical processes. Here we report the successful application of this media to detailed investigations of triplet-sensitized photochemical upconversion, previously established as being readily poisoned by dissolved oxygen. Three different PEG materials were investigated with increasing molecular weight from 200 to 600 g/mol, coded as PEG-200, PEG-400, and PEG-600. These fluidic polymers facilitate an oxygen-depleted environment in comparison to commonly employed organic solvents while providing high solubility and diffusion for the dissolved chromophores. Moreover, the low oxygen permeation afforded by these PEG solvents allows them to remain deoxygenated in open containers under ambient conditions for extended time periods. OA, 9,10-dimethylanthracene (DMA), and 2,5-dimethylfuran (DMF) are shown to efficiently and quantitatively consume dissolved oxygen in the PEG environment in the presence of the photoactivated triplet sensitizer platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP). Oxygen consumption was directly correlated with systematically increasing sensitizer excited-state lifetimes that eventually reach the same plateau as achieved through extensive N2 sparging. Diffusion-controlled bimolecular triplet-triplet energy transfer quenching between PtTPBP and the acceptor/annihilator 9,10-bisphenylethynylanthracene (BPEA) was observed in all three PEG formulations investigated. Subsequent triplet-triplet annihilation, between triplet excited BPEA acceptors, achieves bright and stable upconverted singlet fluorescence from BPEA with no decrease in intensity over 20 h under ambient conditions. In the champion composition (PEG 200), the upconversion quantum efficiency reached 31% under conditions where triplet-triplet annihilation was maximized. This is in stark contrast for the same upconverting pair measured in toluene under ambient conditions, which rapidly decomposes upon exposure to visible light. To illustrate that these PEG compositions could be translated into a suitable solid-state format, these viscous solutions were embedded in a transparent polyurethane polymer shell yielding a flexible and long-term stable upconverting cell that can be manipulated for possible real-world applications. Although the current investigation focused on photochemical upconversion, the oxygen-depleted environments developed here can be utilized to study a plethora of oxygen-intolerant photochemical reactions.

Photon upconversion with directed emission

Photon upconversion has the potential to increase the efficiency of single bandgap solar cells beyond the Shockley Queisser limit. Efficient light management is an important point in this context. Here we demonstrate that the direction of upconverted emission can be controlled in a reversible way, by embedding anthracene derivatives together with palladium porphyrin in a liquid crystalline matrix. The system is employed in a triplet-triplet annihilation photon upconversion scheme demonstrating controlled switching of directional anti Stokes emission. Using this approach an emission ratio of 0.37 between the axial and longitudinal emission directions and a directivity of 1.52 is achieved, reasonably close to the theoretical maximal value of 2 obtained from a perfectly oriented sample. The system can be switched for multiple cycles without any visible degradation and the speed of switching is only limited by the intrinsic rate of alignment of the liquid crystalline matrix.

Neighbour-sensitized near-infrared emission of new Nd(iii) and Er(iii) complexes with 1-(anthracene-2-yl)-4,4,4-trifluoro-1,3-butanedione

Solvatochromic effects on the luminescence of the anthracene pendant and the sensitized NIR luminescence of Nd(iii) and Er(iii).

Ultrafast structural flattening motion in photoinduced excited state dynamics of a bis(diimine) copper(i) complex

The intriguing ultrafast photoinduced structural change dynamics of a prototypical Cu(i) complex, namely, [Cu(dmp)₂]⁺ (dmp = 2,9-dimethyl-1,10-phenanthroline), is investigated based on the theoretical analysis of static and dynamical calculations at the all-atomic level.

MLCT sensitizers in photochemical upconversion: past, present, and potential future directions

This frontier presentation highlights the historical development of MLCT sensitizers in photochemical upconversion while indentifying current state-of-the-art and exciting opportunities in this arena moving towards the future.