Photochemical Upconversion Research Articles

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%, λmax = 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.

Photochemical upconversion is suppressed by high concentrations of molecular sensitizers.

Photochemical upconversion uses sensitized triplet-triplet annihilation in bimolecular compositions to convert lower energy photons to higher energy photons. For high efficiency under low illumination, usually a high sensitizer concentration is desirable. However, here we show that the upconversion sensitizer can reduce the emitter triplet lifetime by dynamic quenching, with rate constants on the order 106 M-1 s-1, leading to diminishing returns beyond a certain concentration. These results serve as a warning to designers of photochemical upconvertors that higher concentrations of sensitizers are not necessarily beneficial to upconversion performance.

Photochemical upconversion in water.

A major challenge remaining in photochemical upconversion lies in identifying appropriate chromophore combinations that function in pure water in the absence of hydrophobic hosts or surfactant additives. The current investigation achieves this goal using combinations of water soluble Ru(ii) metal-to-ligand charge transfer (MLCT) sensitizers in concert with 9-anthracenecarboxylate (AnCO2-) and 1-pyrenecarboxylate (PyCO2-) in neat H2O.

Enhanced Triplet–Triplet Annihilation Upconversion in Dual-Sensitizer Systems: Translating Broadband Light Absorption to Practical Solid-State Materials

Photochemical upconversion (UC) of low-energy photons that would otherwise be wasted could drastically improve the efficiency of solar technologies by allowing them to harness a greater fraction of the solar spectrum. Although UC through the triplet–triplet annihilation (TTA) mechanism operates efficiently under low-power irradiation such as sunlight, its ability to improve solar device efficiencies is limited by the narrow light absorption bands of its sensitizer chromophores. This bottleneck on UC performance can be overcome by employing multiple sensitizers in tandem, but such an approach has thus far been studied exclusively in solution-based TTA-UC systems requiring intensive deoxygenation and sealing procedures. This study presents the first dual-sensitizer TTA-UC system in a solid-state host suitable for practical applications. We fabricate thin polyurethane films containing two benchmark TTA-UC sensitizers in a range of different concentrations and characterize their red-to-blue and green-to-blue ...

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.

G-Quadruplex Supramolecular Assemblies in Photochemical Upconversion.

Parallel, tetramolecular G-quadruplex (G4) DNA possessing TINA monomer, (R)-1-O-[4-(1-pyrenylethynyl)phenylmethyl]glycerol, were synthesised and evaluated in complexes with tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)3 ](2+) , and the Zn(2+) derivative of 5,10,15,20-tetrakis-(1-methyl-4-pyridyl)-21 H,23H-porphine, ZnTMpyP4. UV/Vis, fluorescence, and circular dichroism (CD) spectroscopy showed that the use of G4-DNA as a template resulted in the effective communication between the ligands and the TINA molecule that was covalently attached to the 5'-end and between T and dG at the 5'-end of the dTG4 T sequence. Only one G4-DNA possessing the TINA molecule at the 5'-end of the dTG4 T sequence was able to yield a green-to-blue photochemical upconversion (PUC, λem =420 nm) in the presence of [Ru(bpy)3 ](2+) upon excitation at 500 nm. Different DNA secondary structures can thus be used in DNA-based assemblies for PUC and the way of attachment of chromophores to DNA plays a pivotal role for the creation of a photosynthetic centre.

Materials Integrating Photochemical Upconversion.

This review features recent experimental work focused on the preparation and characterization of materials that integrate photochemical upconversion derived from sensitized triplet-triplet annihilation, resulting in the conversion of low energy photons to higher energy light, thereby enabling numerous wavelength-shifting applications. Recent topical developments in upconversion include encapsulating or rigidifying fluid solutions to give them mechanical strength, adapting inert host materials to enable upconversion, and using photoactive materials that incorporate the sensitizer and/or the acceptor. The driving force behind translating photochemical upconversion from solution into hard and soft materials is the incorporation of upconversion into devices and other applications. At present, some of the most promising applications of upconversion materials include imaging and fluorescence microscopy, photoelectrochemical devices, water disinfection, and solar cell enhancement.

DNA-Based Assemblies for Photochemical Upconversion.

In the present study DNA was used as a scaffold for the supramolecular assembly of organic chromophores for photochemical upconversion (PUC). Initially, a green-to-blue PUC was observed using free chromophores in solution: tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)3](2+), which acts as a long-wavelength absorber (λex = 500 nm), and an in situ energy donor to an acceptor (R)-1-O-[4-(1-pyrenylethynyl)phenylmethyl]glycerol (PEPy or TINA monomer), which acts as an annihilator and short-wavelength photoemitter (λem = 420 nm). Then, DNA duplexes possessing TINA monomers were synthesized, and complexes with [Ru(bpy)3](2+) were investigated. In contrast to the dynamic interactions of [Ru(bpy)3](2+) with TINA monomer free in solution, ground-state complex formation was the predominant mechanism of interaction between [Ru(bpy)3](2+) and DNA duplexes bearing two TINA monomers at the 5' ends as shown by fluorescence, circular dichroism (CD), and UV-vis spectroscopy studies. The use of TINA-modified DNAs led to PUC occurring at concentrations significantly lower than that for free chromophores in solution: 2.5 μM [Ru(bpy)3](2+) and 5.0 μM TINA-modified duplex in the DNA-based systems in aqueous buffer versus 46 μM [Ru(bpy)3](2+) and 4.6 mM TINA monomer for the free donor and acceptor in DCM, respectively. Providing vast capabilities of DNA in the development of novel photonic systems as a result of the controllable organization of various chromophores, this study opens a new perspective for the development of DNA-based light-harvesting systems using PUC.

Deuteration of Perylene Enhances Photochemical Upconversion Efficiency

Photochemical upconversion via triplet-triplet annihilation is a promising technology for improving the efficiency of photovoltaic devices. Previous studies have shown that the efficiency of upconversion depends largely on two rate constants intrinsic to the emitting species. Here, we report that one of these rate constants can be altered by deuteration, leading to enhanced upconversion efficiency. For perylene, deuteration decreases the first order decay rate constant by 16 ± 9% at 298 K, which increases the linear upconversion response by 45 ± 21% in the low excitation regime.

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

The current investigation compares the photochemical upconversion sensitization properties of two long lifetime Cu(I) metal-to-ligand charge transfer (MLCT) chromophores to 3 distinct anthryl-based triplet acceptors. The sensitizers [Cu(dsbtmp)2](PF6) (1, dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) and [Cu(dsbp)2](PF6) (2, dsbp = 2,9-di(sec-butyl-1,10-phenanthroline) were selectively excited in the presence of anthracene, 9,10-diphenylanthracene (DPA), and 9,10-dimethylanthracene (DMA) in degassed dichloromethane solutions. In all instances, triplet energy transfer was observed from selective excitation of the Cu(I) MLCT chromophore to each respective anthryl species. The bimolecular triplet-triplet energy transfer quenching rate constants were extracted from dynamic Stern-Volmer analyses in each case, yielding values below the diffusion limit in dichloromethane. However, the Stern-Volmer quenching constants (KSV's) were sizable enough (up to ∼2300 M(-1) with 1 as a sensitizer) to support efficient photochemical upconversion. As such, visible to near-UV photochemical upconversion was observed in every instance, along with the anticipated quadratic-to-linear incident light power dependence when pumping at 488 nm. The latter verified that it is indeed sensitized triplet-triplet annihilation responsible for the generation of the anthryl-based singlet fluorescence. Photochemical upconversion quantum efficiencies were evaluated using a relative actinometric method as both a function of incident light power density as well as anthryl acceptor/annihilator concentration. When 1 was used as the sensitizer, upconversion quantum yields as large as 9.2% and 17.8% were observed for DMA and DPA, respectively. Finally, the combination of 1 with DMA was shown to be quite robust, showing no obvious signs of decomposition during 12 h of continuous 488 nm photolysis.

Altering molecular photophysics by merging organic and inorganic chromophores.

Photofunctional molecules and assemblies lie at the heart of many important fundamental processes in nature, and researchers have generated multitudes of artificial chromophores intended to mimic these naturally occurring systems. As dynamic spectroscopic techniques are becoming more widely available, ultrafast techniques in particular, substantial insight continues to be gleaned from the initial photon stimulation event through internal conversion, structural rearrangements, intersystem crossing, energy migration, electron transfer events, and ultimately regeneration of the ground state chromophores in both naturally occurring and inspired chromophores. This Account details research endeavors motivated by the concept that merging organic and inorganic chromophores can lead to new molecules exhibiting novel excited state properties. Moreover, these excited state properties can be fundamentally understood using combinations of static and dynamic spectroscopic tools, yielding systematic improvements to molecules poised for application in diverse research areas including light-harvesting, lifetime engineering, photocatalysis, and photochemical upconversion. Initial explorations focused on utilizing Förster energy transfer processes in Ru(II)-based metal-organic chromophores for solar light-harvesting while maintaining long excited state lifetimes. This eventually led to molecules exhibiting triplet-triplet energy transfer between energetically proximate triplet states featuring thermally activated photoluminescence from the upper charge transfer excited state with markedly extended lifetimes. Interest in systematically producing long-lived excited states with concomitant large Stokes shifts inspired the development of numerous Pt(II) bipyridyl and terpyridyl acetylide charge transfer chromophores featuring ultrafast intramolecular energy migration, high quantum yield ligand-localized phosphorescence at room temperature, and synthetically tunable excited state absorption properties. This structural motif also made it possible to access the triplet excited states of perylenediimide chromophores, permitting quantitative examination of internal conversion and intersystem crossing processes in these complex molecules. The generation of new metal-organic structures featuring unique photophysics appears limitless and simply requires the continued ingenuity of researchers.

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

This treatment 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.

Highly efficient photochemical upconversion in a quasi-solid organogel

Photochemical upconversion in a quasi-solid gel proceeds with an identical efficiency to an otherwise identical liquid composition.

Photochemical upconversion: present status and prospects for its application to solar energy conversion

We review the application of sensitized triplet–triplet annihilation upconversion to broaden the harvested spectrum of solar energy conversion devices.

Photochemical upconversion in metal-based octaethyl porphyrin–diphenylanthracene systems

This paper studies photochemical upconversion in solutions of octaethyl porphyrin (OEP) and diphenyl anthracene (DPA).

Photochemical Upconversion: The Primacy of Kinetics.

Incoherent photochemical upconversion is a process by which low-energy light can be converted into a higher-energy form with promising applications in solar energy conversion and storage, photocatalysis, biological imaging, and photochemical drug activation. Despite intensive research in recent years, there remains an underappreciation of the chemical kinetics that controls the efficiency of the upconversion process. Here, we provide a brief overview of research into photochemical upconversion and provide a tutorial to guide the design of efficient upconversion compositions. We further provide our perspective on where this area of research is heading and how very efficient systems will be developed.

Enhancing solar cells with photochemical upconversion

Photovoltaics have the potential to become a major contributor to future sustainable energy generation. To this end, device efficiency needs to be brought beyond present physical limits. Solar cells cannot absorb light with less energy than the bandgap (absorption threshold of the photovoltaic absorber material), and consequently this energy source is usually uncaptured. Upconversion (UC) devices harvest those unused subthreshold photons behind the solar cell, create one higher-energy photon out of (at least) two transmitted photons, and radiate upconverted light back toward the solar cell, thus expanding the usable solar spectrum (see Figure 1). Key requirements for UC units are broad absorption and high UC quantum yield under even the low-intensity incoherent illumination that is available to solar energy conversion devices. Upconversion was proposed as a means to increase solar cell efficiency about a decade ago,1 and research has since then been primarily focused on UC processes occurring in lanthanide-doped glasses or nanoparticles.2 Although transitions between the rare earth 4f orbitals lie in an energy range promising for combination with common crystalline silicon solar cells, UC systems based on erbium (Er) or ytterbium (Yb) suffer from weak absorption by Laporte-forbidden transitions, a narrow absorption bandwidth, and the resulting low efficiency. Recently, triplet-triplet annihilation (TTA) in organic molecules has been demonstrated to be an alternative UC mechanism highly promising for solar energy application.3 It proceeds efficiently even for low photon flux and displays highly tunable absorption characteristics. The key to high quantum yield from TTA-UC lies in the combination of two different classes of organic molecules dissolved together in organic solvents or in a polymeric host matrix.4 A socalled sensitizer species, which in many cases is a metalated porphyrin, absorbs a low-energy photon (process 1 in Figure 2) and undergoes fast intersystem crossing (ISC, process 2), whereby Figure 1. Principle of a solar cell assisted by upconversion (UC). The creation of usable electron-hole pairs in the photovoltaic absorber material (left) is augmented by light, usually transmitted unused, which is converted to higher photon energies in the UC unit (right).

Photochemical Upconversion: A Physical or Inorganic Chemistry Experiment for Undergraduates Using a Conventional Fluorimeter

Photochemical upconversion is a regenerative process that transforms lower-energy photons into higher-energy light through two sequential bimolecular reactions, triplet sensitization of an appropriate acceptor followed by singlet fluorescence producing triplet–triplet annihilation derived from two energized acceptors. This laboratory directly investigates this phenomenon using the facile photo-induced triplet–triplet energy-transfer reaction between the benchmark inorganic chromophore tris(2,2′-bipyridyl)ruthenium(II), [Ru(bpy)3]2+, and 9,10-diphenylanthracene (DPA). Selective green excitation of [Ru(bpy)3]2+ in the presence of DPA results in two observations, the (dynamic) quenching of the [Ru(bpy)3]2+ photoluminescence in the red and the production of upconverted DPA singlet fluorescence in the blue. The quadratic nature of the incident light power dependence for the latter process is easily realized by using the percent transmission characteristics of neutral density filters placed in the excitation beam prior to sample illumination. In a single laboratory period, students are able to directly visualize the upconverted fluorescence generated and gain experience in various aspects of fluorescence spectroscopy, triplet energy-transfer processes, quenching, and reaction molecularity.

Orange-to-blue and red-to-green photon upconversion with a broadband absorbing copper(i) MLCT sensitizer

A synthetically facile and earth abundant Cu(I) metal-to-ligand charge transfer sensitizer was successfully incorporated into two distinct photochemical upconversion schemes, affording red-to-green and orange-to-blue wavelength conversions.

Near-IR phosphorescent metalloporphyrin as a photochemical upconversion sensitizer

The phosphorescent metalloporphyrin sensitizer PtTPTNP (TPTNP = tetraphenyltetranaphtho[2,3]porphyrin) has been successfully coupled with perylenediimide (PDI) or rubrene utilized as triplet acceptors/annihilators to upconvert 690 nm incident photons into yellow fluorescence through sensitized triplet-triplet annihilation at overall efficiencies in the 6-7% range while exhibiting exceptional photostability.