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.

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.

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.

Strongly Red-Emissive Molecular Ruby [Cr(bpmp)2]3+ Surpasses [Ru(bpy)3]2.

Gaining chemical control over the thermodynamics and kinetics of photoexcited states is paramount to an efficient and sustainable utilization of photoactive transition metal complexes in a plethora of technologies. In contrast to energies of charge transfer states described by spatially separated orbitals, the energies of spin-flip states cannot straightforwardly be predicted as Pauli repulsion and the nephelauxetic effect play key roles. Guided by multireference quantum chemical calculations, we report a novel highly luminescent spin-flip emitter with a quantum chemically predicted blue-shifted luminescence. The spin-flip emission band of the chromium complex [Cr(bpmp)2]3+ (bpmp = 2,6-bis(2-pyridylmethyl)pyridine) shifted to higher energy from ca. 780 nm observed for known highly emissive chromium(III) complexes to 709 nm. The photoluminescence quantum yields climb to 20%, and very long excited state lifetimes in the millisecond range are achieved at room temperature in acidic D2O solution. Partial ligand deuteration increases the quantum yield to 25%. The high excited state energy of [Cr(bpmp)2]3+ and its facile reduction to [Cr(bpmp)2]2+ result in a high excited state redox potential. The ligand's methylene bridge acts as a Brønsted acid quenching the luminescence at high pH. Combined with a pH-insensitive chromium(III) emitter, ratiometric optical pH sensing is achieved with single wavelength excitation. The photophysical and ground state properties (quantum yield, lifetime, redox potential, and acid/base) of this spin-flip complex incorporating an earth-abundant metal surpass those of the classical precious metal [Ru(α-diimine)3]2+ charge transfer complexes, which are commonly employed in optical sensing and photo(redox) catalysis, underlining the bright future of these molecular ruby analogues.

Role of pH in the synthesis and growth of gold nanoparticles using L-asparagine: a combined experimental and simulation study.

The use of biomolecules as capping and reducing agents in the synthesis of metallic nanoparticles constitutes a promising framework to achieve desired functional properties with minimal toxicity. The system's complexity and the large number of variables involved represent a challenge for theoretical and experimental investigations aiming at devising precise synthesis protocols. In this work, we use L-asparagine (Asn), an amino acid building block of large biomolecular systems, to synthesise gold nanoparticles (AuNPs) in aqueous solution at controlled pH. The use of Asn offers a primary system that allows us to understand the role of biomolecules in synthesising metallic nanoparticles. Our results indicate that AuNPs synthesised in acidic (pH 6) and basic (pH 9) environments exhibit somewhat different morphologies. We investigate these AuNPs via Raman scattering experiments and classical molecular dynamics simulations of zwitterionic and anionic Asn states adsorbing on (111)-, (100)-, (110)-, and (311)-oriented gold surfaces. A combined analysis suggests that the underlying mechanism controlling AuNPs geometry correlates with amine's preferential adsorption over ammonium groups, enhanced upon increasing pH. Our simulations reveal that Asn (both zwitterionic and anionic) adsorption on gold (111) is essentially different from adsorption on more open surfaces. Water molecules strongly interact with the gold face-centred-cubic lattice and create traps, on the more open surfaces, that prevent the Asn from diffusing. These results indicate that pH is a relevant parameter in green-synthesis protocols with the capability to control the nanoparticle's geometry, and pave the way to computational studies exploring the effect of water monolayers on the adsorption of small molecules on wet gold surfaces.

Insight into Electron Transfer from a Redox Polymer to a Photoactive Protein.

Biohybrid photoelectrochemical systems in photovoltaic or biosensor applications have gained considerable attention in recent years. While the photoactive proteins engaged in such systems usually maintain an internal charge separation quantum yield of nearly 100%, the subsequent steps of electron and hole transfer beyond the protein often limit the overall system efficiency and their kinetics remain largely uncharacterized. To reveal the dynamics of one of such charge-transfer reactions, we report on the reduction of Rhodobacter sphaeroides reaction centers (RCs) by Os-complex-modified redox polymers (P-Os) characterized using transient absorption spectroscopy. RCs and P-Os were mixed in buffered solution in different molar ratios in the presence of a water-soluble quinone as an electron acceptor. Electron transfer from P-Os to the photoexcited RCs could be described by a three-exponential function, the fastest lifetime of which was on the order of a few microseconds, which is a few orders of magnitude faster than the internal charge recombination of RCs with fully separated charge. This was similar to the lifetime for the reduction of RCs by their natural electron donor, cytochrome c2. The rate of electron donation increased with increasing ratio of polymer to protein concentrations. It is proposed that P-Os and RCs engage in electrostatic interactions to form complexes, the sizes of which depend on the polymer-to-protein ratio. Our findings throw light on the processes within hydrogel-based biophotovoltaic devices and will inform the future design of materials optimally suited for this application.

Experimental Observation of Strong Exciton Effects in Graphene Nanoribbons.

Graphene nanoribbons (GNRs) with atomically precise width and edge structures are a promising class of nanomaterials for optoelectronics, thanks to their semiconducting nature and high mobility of charge carriers. Understanding the fundamental static optical properties and ultrafast dynamics of charge carrier generation in GNRs is essential for optoelectronic applications. Combining THz spectroscopy and theoretical calculations, we report a strong exciton effect with binding energy up to ∼700 meV in liquid-phase-dispersed GNRs with a width of 1.7 nm and an optical band gap of ∼1.6 eV, illustrating the intrinsically strong Coulomb interactions between photogenerated electrons and holes. By tracking the exciton dynamics, we reveal an ultrafast formation of excitons in GNRs with a long lifetime over 100 ps. Our results not only reveal fundamental aspects of excitons in GNRs (strong binding energy and ultrafast exciton formation etc.) but also highlight promising properties of GNRs for optoelectronic devices.

Energy dissipation mechanisms in the FCPb light-harvesting complex of the diatom Cyclotella meneghiniana.

Transient absorption spectroscopy has been applied to investigate the energy dissipation mechanisms in the nonameric fucoxanthin-chlorophyll-a,c-binding protein FCPb of the centric diatom Cyclotella meneghiniana. FCPb complexes in their unquenched state were compared with those in two types of quenching environments, namely aggregation-induced quenching by detergent removal, and clustering via incorporation into liposomes. Applying global and target analysis, in combination with a fluorescence lifetime study and annihilation calculations, we were able to resolve two quenching channels in FCPb that involve chlorophyll-a pigments for FCPb exposed to both quenching environments. The fast quenching channel operates on a timescale of tens of picoseconds and exhibits similar spectral signatures as the unquenched state. The slower quenching channel operates on a timescale of tens to hundreds of picoseconds, depending on the degree of quenching, and is characterized by enhanced population of low-energy states between 680 and 710 nm. The results indicate that FCPb is, in principle, able to function as a dissipater of excess energy and can do this in vitro even more efficiently than the homologous FCPa complex, the sole complex involved in fast photoprotection in these organisms. This indicates that when a complex displays photoprotection-related spectral signatures in vitro it does not imply that the complex participates in photoprotection in vivo. We suggest that FCPa is favored over FCPb as the sole energy-regulating complex in diatoms because its composition can more easily establish the balance between light-harvesting and quenching required for efficient photoprotection.

Asymmetric Covalent Triazine Framework for Enhanced Visible-Light Photoredox Catalysis via Energy Transfer Cascade.

Complex multiple-component semiconductor photocatalysts can be constructed that display enhanced catalytic efficiency via multiple charge and energy transfer, mimicking photosystems in nature. In contrast, the efficiency of single-component semiconductor photocatalysts is usually limited due to the fast recombination of the photogenerated excitons. Here, we report the design of an asymmetric covalent triazine framework as an efficient organic single-component semiconductor photocatalyst. Four different molecular donor-acceptor domains are obtained within the network, leading to enhanced photogenerated charge separation via an intramolecular energy transfer cascade. The photocatalytic efficiency of the asymmetric covalent triazine framework is superior to that of its symmetric counterparts; this was demonstrated by the visible-light-driven formation of benzophosphole oxides from diphenylphosphine oxide and diphenylacetylene.

Enhanced photoluminescence properties of a carbon dot system through surface interaction with polymeric nanoparticles.

HYPOTHESIS: Carbon dot systems are highly surface sensitive fluorescent nanomaterials. In the presence of specific molecules or ions, the fluorescence properties can be strongly influenced. Often their fluorescent properties are activated or strongly enhanced through passivation agents such as polymer coatings. While several passivating polymers have been directly attached to the carbon dot systems, the interaction of carbon dot systems with the polymer surface of colloids has not been investigated as a way to activate or enhance the photoluminescent properties. Here, we show for the first time that the interaction of carbon dot systems with polymer colloids can strongly enhance the fluorescent properties of the carbon dot systems. EXPERIMENTS: To introduce carbon dot - polymer nanoparticle interactions, carbon dots are either generated directly in a microwave assisted synthesis in the presence of negatively charged polystyrene nanoparticles (in situ) or synthesized in the microwave separately and mixed afterwards with polymer nanoparticles (mixing). For the carbon dot system synthesis, chitosan, 1,2-ethylenediamine, and acetic acid are used as precursors. The produced carbon dot - polymer nanoparticle system are characterized by scanning electron microscopy, transmission electron microscopy, and flow cytometry measurements, and their interaction is assessed by fluorescence spectroscopy and fluorescence lifetime measurements. FINDINGS: We show that depending on the synthesis route (in situ or mixing), the carbon dot systems are either covalently attached (in situ) or electrostatically bound (mixing) to the surface of the nanoparticles. Regardless of the preparation methods of the investigated carbon dot - polymer nanoparticle system and the interaction (chemical or physical) with the surface, the fluorescence intensity is strongly enhanced and the fluorescence lifetime prolonged. These findings indicate a stabilization of the radiative trap states of carbon dot systems through interaction with the surface of the particles.

Evidence for coherent mixing of excited and charge-transfer states in the major plant light-harvesting antenna, LHCII.

LHCII, the major light harvesting antenna from plants, plays a dual role in photosynthesis. In low light it is a light-harvester, while in high light it is a quencher that protects the organism from photodamage. The switching mechanism between these two orthogonal conditions is mediated by protein dynamic disorder and photoprotective energy dissipation. The latter in particular is thought to occur in part via spectroscopically 'dark' states. We searched for such states in LHCII trimers from spinach, at both room temperature and at 77 K. Using 2D electronic spectroscopy, we explored coherent interactions between chlorophylls absorbing on the low-energy side of LHCII, which is the region that is responsible for both light-harvesting and photoprotection. 2D beating frequency maps allow us to identify four frequencies with strong excitonic character. In particular, our results show the presence of a low-lying state that is coupled to a low-energy excitonic state. We assign this to a mixed excitonic-charge transfer state involving the state with charge separation within the Chl a603-b609 heterodimer, borrowing some dipole strength from the Chl a602-a603 excited states. Such a state may play a role in photoprotection, in conjunction with specific and environmentally controlled realizations of protein dynamic disorder. Our identification and assignment of the coherences observed in the 2D frequency maps suggests that the structure of exciton states as well as a mixing of the excited and charge-transfer states is affected by coupling of these states to resonant vibrations in LHCII.

Interplay between Long-Range Crystal Order and Short-Range Molecular Interactions Tunes Carrier Mobility in Liquid Crystal Dyes.

We investigated the influence of molecular packing on the optical and electrical properties of the liquid crystalline dye 4,7-bis[5-(2-fluoro-4-pentyl-phenyl)-2-thienyl]-2,1,3-benzothiadiazole (FPPTB). FPPTB is crystalline at room temperature, exhibits a nematic phase at temperatures above 149 °C and is in an isotropic melt at temperatures above 230 °C. Solution processed FPPTB films were subject to thermal annealing through these phase transition temperatures and characterized with X-ray diffraction and polarized optical microscopy. Cooling FPPTB films from the nematic and isotropic phases increased crystal domain size, but also induced local structural variations in the molecular packing of crystalline FPPTB. The decrease in long-range order was correlated with an increase in short-range π-π interactions, leading to changes in molecular aggregation which persisted even when the FPPTB films were cooled to room temperature. Annealing-induced changes in molecular aggregation were confirmed with optical spectroscopy. The carrier mobility in FPPTB films increased over 2 orders of magnitude from (2.2 ± 0.4) × 10-5 cm2 V-1 s-1 in as-spun films to µ = (5.0 ± 0.8) × 10-3 cm2 V-1 s-1 in films cooled from the isotropic melt. We discuss the relationship between thermal stability and high carrier mobility values in terms of the interplay between long-range molecular order and increased π-π interactions between molecular pairs in the FPPTB film.

Controlling Charge Carrier Overlap in Type-II ZnSe/ZnS/CdS Core-Barrier-Shell Quantum Dots.

We describe the synthesis and spectroscopic characterization of colloidal ZnSe/ZnS/CdS nanocrystals, which exhibit a type-II electronic structure and wave function overlap that is strongly dependent on the thickness of the ZnS barrier. Barrier thickness is controlled by both the amount of deposited material and the reaction and annealing temperature of CdS shell growth. The results show that a single monolayer of ZnS mitigates the overlap significantly, while four and more monolayers effectively suppress band edge absorption and emission. Transient absorption spectra reveal a broad distribution of excitons with mixed S and P symmetry, which become allowed due to alloy formation and contribute to charge carrier relaxation across the barrier. We present a model of the core/shell interface based on cation diffusion, which allows one to estimate the extent of the diffusion layer from optical spectra.