Oxidative two-state photoreactivity of a manganese(IV) complex using near-infrared light.

Highly reducing or oxidizing photocatalysts are a fundamental challenge in photochemistry. Only a few transition metal complexes with Earth-abundant metal ions have so far advanced to excited state oxidants. All these photocatalysts require high-energy light for excitation, and their oxidizing power has not been fully exploited due to energy dissipation before reaching the photoactive state. Here we demonstrate that the complex [Mn(dgpy)2]4+, based on Earth-abundant manganese and the tridentate 2,6-diguanidylpyridine ligand (dgpy), evolves to a luminescent doublet ligand-to-metal charge transfer (2LMCT) excited state (1,435 nm, 0.86 eV) with a lifetime of 1.6 ns after excitation with low-energy near-infrared light. This 2LMCT state oxidizes naphthalene to its radical cation. Substrates with extremely high oxidation potentials up to 2.4 V enable the [Mn(dgpy)2]4+ photoreduction via a high-energy quartet 4LMCT excited state with a lifetime of 0.78 ps, proceeding via static quenching by the solvent. This process minimizes free energy losses and harnesses the full photooxidizing power, and thus allows oxidation of nitriles and benzene using Earth-abundant elements and low-energy light.

Competitive Charge Separation Pathways in a Flexible Molecular Folda-Dimer.

We report the photophysical properties of a molecular folda-dimer system PDI-AnEt2-PDI, where the electron-donating N,N-diethylaniline (AnEt2) moiety bridges two electron-accepting perylene diimide (PDI) chromophores. The conformationally flexible PDI-AnEt2-PDI adopts either an open (two PDIs far apart) or folded (two PDIs within π-stacking distance) conformation, depending on the solvent environment. We characterized the photoinduced charge separation dynamics of both open and folded forms in solvents of varying polarity. The open form undergoes charge separation to give PDI•--AnEt2•+-PDI (Bridge electron transfer) independent of solvent polarity. The folded form exhibits two charge separation photoproducts, yielding both PDI•--AnEt2•+-PDI and PDI•--AnEt2-PDI•+, the latter of which is formed via symmetry-breaking charge separation (SBCS) between the two π-stacked PDI chromophores. Our results further indicate that the conformational flexibility of the folda-dimer leads to unexpected excimer formation in some open form conditions. In contrast, no excimer formation is observed in the folded form, indicating that this geometry preferentially yields the SBCS instead. Our results provide insight into how conformationally flexible folda-dimer systems can be designed and built to tune competitive photophysical pathways.

Ultrafast Surface-Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water-Splitting Photocatalyst.

A critical step in photocatalytic water dissociation is the hole-mediated oxidation reaction. Molecular-level insights into the mechanism of this complex reaction under realistic conditions with high temporal resolution are highly desirable. Here, we use femtosecond time-resolved, surface-specific vibrational sum frequency generation spectroscopy to study the photo-induced reaction directly at the interface of the photocatalyst TiO2 in contact with liquid water at room temperature. Thanks to the inherent surface specificity of the spectroscopic method, we can follow the reaction of solely the interfacial water molecules directly at the interface at timescales on which the reaction takes place. Following the generation of holes at the surface immediately after photoexcitation of the catalyst with UV light, water dissociation occurs on a sub-20 ps timescale. The reaction mechanism is similar at pH 3 and 11. In both cases, we observe the conversion of H2 O into Ti-OH groups and the deprotonation of pre-existing Ti-OH groups. This study provides unique experimental insights into the early steps of the photo-induced dissociation processes at the photocatalyst-water interface, relevant to the design of improved photocatalysts.

Nanoscale Thermometry of Plasmonic Structures via Raman Shifts in Copper Phthalocyanine.

Temperature measurements at the nanoscale are vital for the application of plasmonic structures in medical photothermal therapy and materials science but very challenging to realize in practice. In this work, we exploit a combination of surface-enhanced Raman spectroscopy together with the characteristic temperature dependence of the Raman peak maxima observed in ß-phase copper phthalocyanine (ß-CuPc) to measure the surface temperature of plasmonic gold nanoparticles under laser irradiation. We begin by measuring the temperature-dependent Raman shifts of the three most prominent modes of ß-CuPc films coated on an array of Au nanodisks over a temperature range of 100-500 K. We then use these calibration curves to determine the temperature of an array of Au nanodisks irradiated with varying laser powers. The extracted temperatures agree quantitatively with the ones obtained via numerical modeling of electromagnetic and thermodynamic properties of the irradiated array. Thin films of ß-CuPc display low extinction coefficients in the blue-green region of the visible spectrum as well as exceptional thermal stability, allowing a wide temperature range of operation of our Raman thermometer, with minimal optical distortion of the underlying structures. Thanks to the strong thermal response of the Raman shifts in ß-CuPc, our work opens the opportunity to investigate photothermal effects at the nanoscale in real time.

Symmetry-breaking charge transfer and intersystem crossing in copper phthalocyanine thin films.

Intermolecular interactions in π-stacked chromophores strongly influence their photophysical properties, and thereby also their function in photonic applications. Mixed electronic and vibrational coupling interactions lead to complex potential energy landscapes with competitive photophysical pathways. Here, we characterize the photoexcited dynamics of the small molecule semiconductor copper pthalocyanine (CuPc) in solution and in thin film, the latter comprising two different π-stacked architectures, α-CuPc and ß-CuPc. In solution, CuPc undergoes ultrafast intersytem crossing (ISC) to the triplet excited state. In the solid state, both α-CuPc and ß-CuPc morphologies exhibit a mixing between Frenkel and charge-transfer excitons (Frenkel-CT mixing). We find that this mixing influences the photophysical properties differently, based on morphology. In addition to ISC, α-CuPc demonstrates symmetry-breaking charge transfer, which furthermore depends on excitation wavelength. This mechanism is not observed in ß-CuPc. These results elucidate how molecular organization mediates the balance of competitive photexcited decay mechanisms in organic semiconductors.

Grain engineering for improved charge carrier transport in two-dimensional lead-free perovskite field-effect transistors.

Controlling crystal growth and reducing the number of grain boundaries are crucial to maximize the charge carrier transport in organic-inorganic perovskite field-effect transistors (FETs). Herein, the crystallization and growth kinetics of a Sn(II)-based 2D perovskite, using 2-thiopheneethylammonium (TEA) as the organic cation spacer, were effectively regulated by the hot-casting method. With increasing crystalline grain size, the local charge carrier mobility is found to increase moderately from 13 cm2 V-1 s-1 to 16 cm2 V-1 s-1, as inferred from terahertz (THz) spectroscopy. In contrast, the FET operation parameters, including mobility, threshold voltage, hysteresis, and subthreshold swing, improve substantially with larger grain size. The optimized 2D (TEA)2SnI4 transistor exhibits hole mobility of up to 0.34 cm2 V-1 s-1 at 295 K and a higher value of 1.8 cm2 V-1 s-1 at 100 K. Our work provides an important insight into the grain engineering of 2D perovskites for high-performance FETs.

Pseudo-Octahedral Iron(II) Complexes with Near-Degenerate Charge Transfer and Ligand Field States at the Franck-Condon Geometry.

Increasing the metal-to-ligand charge transfer (MLCT) excited state lifetime of polypyridine iron(II) complexes can be achieved by lowering the ligand's π* orbital energy and by increasing the ligand field splitting. In the homo- and heteroleptic complexes [Fe(cpmp)2 ]2+ (12+ ) and [Fe(cpmp)(ddpd)]2+ (22+ ) with the tridentate ligands 6,2''-carboxypyridyl-2,2'-methylamine-pyridyl-pyridine (cpmp) and N,N'-dimethyl-N,N'-di-pyridin-2-ylpyridine-2,6-diamine (ddpd) two or one dipyridyl ketone moieties provide low energy π* acceptor orbitals. A good metal-ligand orbital overlap to increase the ligand field splitting is achieved by optimizing the octahedricity through CO and NMe units between the coordinating pyridines which enable the formation of six-membered chelate rings. The push-pull ligand cpmp provides intra-ligand and ligand-to-ligand charge transfer (ILCT, LL'CT) excited states in addition to MLCT excited states. Ground and excited state properties of 12+ and 22+ were accessed by X-ray diffraction analyses, resonance Raman spectroscopy, (spectro)electrochemistry, EPR spectroscopy, X-ray emission spectroscopy, static and time-resolved IR and UV/Vis/NIR absorption spectroscopy as well as quantum chemical calculations.

Molecular ruby: exploring the excited state landscape.

The discovery of the highly NIR-luminescent molecular ruby [Cr(ddpd)2]3+ (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine) has been a milestone in the development of earth-abundant luminophors and has led to important new impulses in the field of spin-flip emitters. Its favourable optical properties such as a high photoluminescence quantum yield and long excited state lifetime are traced back to a remarkable excited state landscape which has been investigated in great detail. This article summarises the results of these studies with the aim to create a coherent picture of the excited state ordering and the ultrafast as well as long-timescale dynamics. Additional experimental data is provided to fill in gaps left by previous reports.

Role of Linker Functionality in Polymers Exhibiting Main-Chain Thermally Activated Delayed Fluorescence.

Excellent performance has been reported for organic light-emitting diodes (OLEDs) based on small molecule emitters that exhibit thermally activated delayed fluorescence. However, the necessary vacuum processing makes the fabrication of large-area devices based on these emitters cumbersome and expensive. Here, the authors present high performance OLEDs, based on novel, TADF polymers that can be readily processed from a solution. These polymers are based on the acridine-benzophenone donor-acceptor motif as main-chain TADF chromophores, linked by various conjugated and non-conjugated spacer moieties. The authors' extensive spectroscopic and electronic analysis shows that in particular in case of alkyl spacers, the properties and performance of the monomeric TADF chromophores are virtually left unaffected by the polymerization. They present efficient solution-processed OLEDs based on these TADF polymers, diluted in oligostyrene as a host. The devices based on the alkyl spacer-based TADF polymers exhibit external quantum efficiencies (EQEs) ≈12%, without any outcoupling-enhancing measures. What's more, the EQE of these devices does not drop substantially upon diluting the polymer down to only ten weight percent of active material. In contrast, the EQE of devices based on the monomeric chromophore show significant losses upon dilution due to loss of charge percolation.

Reduced Intrinsic Non-Radiative Losses Allow Room-Temperature Triplet Emission from Purely Organic Emitters.

Persistent luminescence from triplet excitons in organic molecules is rare, as fast non-radiative deactivation typically dominates over radiative transitions. This work demonstrates that the substitution of a hydrogen atom in a derivative of phenanthroimidazole with an N-phenyl ring can substantially stabilize the excited state. This stabilization converts an organic material without phosphorescence emission into a molecular system exhibiting efficient and ultralong afterglow phosphorescence at room temperature. Results from systematic photophysical investigations, kinetic modeling, excited-state dynamic modeling, and single-crystal structure analysis identify that the long-lived triplets originate from a reduction of intrinsic non-radiative molecular relaxations. Further modification of the N-phenyl ring with halogen atoms affects the afterglow lifetime and quantum yield. As a proof-of-concept, an anticounterfeiting device is demonstrated with a time-dependent Morse code feature for data encryption based on these emitters. A fundamental design principle is outlined to achieve long-lived and emissive triplet states by suppressing intrinsic non-radiative relaxations in the form of molecular vibrations or rotations.