Lattice Symmetry-Guided Charge Transport in 2D Supramolecular Polymers Promotes Triplet Formation.

Singlet-to-triplet intersystem crossing (ISC) in organic molecules is intimately connected with their geometries: by modifying the molecular shape, symmetry selection rules pertaining to spin-orbit coupling can be partially relieved, leading to extra matrix elements for increased ISC. As an analog to this molecular design concept, the study finds that the lattice symmetry of supramolecular polymers also defines their triplet formation efficiencies. A supramolecular polymer self-assembled from weakly interacting molecules is considered. Its 2D oblique unit cell effectively renders it as a coplanar array of 1D molecular columns weakly bound to each other. Using momentum-resolved photoluminescence imaging in combination with Monte Carlo simulations, the study found that photogenerated charge carriers in the supramolecular polymer predominantly recombine as spin-uncorrelated carrier pairs through inter-column charge transfer states. This lattice-defined recombination pathway leads to a substantial triplet formation efficiency (≈60%) in the supramolecular polymer. These findings suggest that lattice symmetry of micro-/macroscopic structures relying on intermolecular interactions can be strategized for controlled triplet formation.

Nonadiabatic Charge Transfer within Photoexcited Nickel Porphyrins.

Metalloporphyrins with open d-shell ions can drive biochemical energy cycles. However, their utilization in photoconversion is hampered by rapid deactivation. Mapping the relaxation pathways is essential for elaborating strategies that can favorably alter the charge dynamics through chemical design and photoexcitation conditions. Here, we combine transient optical absorption spectroscopy and transient X-ray emission spectroscopy with femtosecond resolution to probe directly the coupled electronic and spin dynamics within a photoexcited nickel porphyrin in solution. Measurements and calculations reveal that a state with charge-transfer character mediates the formation of the thermalized excited state, thereby advancing the description of the photocycle for this important representative molecule. More generally, establishing that intramolecular charge-transfer steps play a role in the photoinduced dynamics of metalloporphyrins with open d-shell sets a conceptual ground for their development as building blocks capable of boosting nonadiabatic photoconversion in functional architectures through "hot" charge transfer down to the attosecond time scale.

Symmetry-Breaking Charge Transfer in Metal-Organic Frameworks.

High quantum-yield charge carrier generation from the initially prepared excitons defines a key step in the light-harvesting and conversion scheme. Photoinduced charge transfer in molecular electron donor-acceptor assemblies is driven by a sizable ΔG0, which compromises the potential of the generated carriers. Reminiscent of the special pair at the reaction center of the natural light-harvesting complex, symmetry-breaking charge transfer (SBCT) within a pair of identical struts of metal-organic framework (MOF) will facilitate the efficient generation of long-lived charge carriers with maximized potentials without incorporating any foreign redox species. We report SBCT in pyrene-based zirconium metal-organic framework (MOF) NU-1000 that leads to efficient generation of radical ions in a polar solvent and bound CT states in a low-polar solvent. The probe unveils the role of the low-lying non-Franck-Condon excitonic states as intermediates in the formation of the SBCT state from the initially prepared Franck-Condon S1 states. Ultrafast and transient spectroscopy─probed over 200 fs-30 µs time scale─evinces a kSBCT = (110 ps)-1 in polar media (εs = 37.5) forming solvated radical ions with recombination rate kCR = (∼45 ns)-1. A slower rate with kSBCT = (203 ps)-1 was recorded in low-polar (εs = 7.0) solvent manifesting a bound [TBAPy•+ TBAPy•-] state with kCR ≈ (17 µs)-1. This discovery, along with other unique photophysical features relevant to light harvesting, should define a MOF-based platform for developing heterogeneous artificial photon energy conversion systems.

Ultrafast Jahn-Teller Photoswitching in Cobalt Single-Ion Magnets.

Single-ion magnets (SIMs) constitute the ultimate size limit in the quest for miniaturizing magnetic materials. Several bottlenecks currently hindering breakthroughs in quantum information and communication technologies could be alleviated by new generations of SIMs displaying multifunctionality. Here, ultrafast optical absorption spectroscopy and X-ray emission spectroscopy are employed to track the photoinduced spin-state switching of the prototypical complex [Co(terpy)2 ]2+ (terpy = 2,2':6',2″-terpyridine) in solution phase. The combined measurements and their analysis supported by density functional theory (DFT), time-dependent-DFT (TD-DFT) and multireference quantum chemistry calculations reveal that the complex undergoes a spin-state transition from a tetragonally elongated doublet state to a tetragonally compressed quartet state on the femtosecond timescale, i.e., it sustains ultrafast Jahn-Teller (JT) photoswitching between two different spin multiplicities. Adding new Co-based complexes as possible contenders in the search for JT photoswitching SIMs will greatly widen the possibilities for implementing magnetic multifunctionality and eventually controlling ultrafast magnetization with optical photons.

Axial vs equatorial: Capturing the intramolecular charge transfer state geometry in conformational polymorphic crystals of a donor-bridge-acceptor dyad in nanosecond-time-scale.

Two conformational polymorphs of a donor-bridge-acceptor (D-B-A) dyad, p-(CH3)2N-C6H4-(CH2)2-(1-pyrenyl)/PyCHDMA, were studied, where the electron donor (D) moiety p-(CH3)2N-C6H4/DMA is connected through a bridging group (B), -CH2-CH2-, to the electron acceptor (A) moiety pyrene. Though molecular dyads like PyCHDMA have the potential to change solar energy into electrical current through the process of photoinduced intramolecular charge transfer (ICT), the major challenge is the real-time investigation of the photoinduced ICT process in crystals, necessary to design solid-state optoelectronic materials. The time-correlated single photon counting (TCSPC) measurements with the single crystals showed that the ICT state lifetime of the thermodynamic form, PyCHDMA1 (pyrene and DMA: axial), is ∼3 ns, whereas, for the kinetic form, PyCHDMA20 (pyrene and DMA: equatorial), it is ∼7 ns, while photoexcited with 375 nm radiation. The polymorphic crystals were photo-excited and subsequently probed with a pink Laue x-ray beam in time-resolved x-ray diffraction (TRXRD) measurements. The TRXRD results suggest that in the ICT state, due to electron transfer from the tertiary N-atom in DMA moiety to the bridging group and pyrene moiety, a decreased repulsion between the lone-pair and the bond-pair at N-atom induces planarity in the C-N-(CH3)2 moiety, in both polymorphs. The Natural Bond Orbital calculations and partial atomic charge analysis by Hirshfeld partitioning also corroborated the same. Although the interfragment charge transfer (IFCT) analysis using the TDDFT results showed that for the charge transfer excitation in both conformers, the electrons were transferred from the DMA moiety to mostly the pyrene moiety, the bridging group has little role to play in that.

Toward Improved Charge Separation through Conformational Control in Copper Coordination Complexes.

The continued development of solar energy as a renewable resource necessitates new approaches to sustaining photodriven charge separation (CS). We present a bioinspired approach in which photoinduced conformational rearrangements at a ligand are translated into changes in coordination geometry and environment about a bound metal ion. Taking advantage of the differential coordination properties of CuI and CuII, these dynamics aim to facilitate intramolecular electron transfer (ET) from CuI to the ligand to create a CS state. The synthesis and photophysical characterization of CuCl(dpaaR) (dpaa = dipicolylaminoacetophenone, with R = H and OMe) are presented. These ligands incorporate a fluorophore that gives rise to a twisted intramolecular charge transfer (TICT) excited state. Excited-state ligand twisting provides a tetragonal coordination geometry capable of capturing CuII when an internal ortho-OMe binding site is present. NMR, IR, electron paramagnetic resonance (EPR), and optical spectroscopies, X-ray diffraction, and electrochemical methods establish the ground-state properties of these CuI and CuII complexes. The photophysical dynamics of the CuI complexes are explored by time-resolved photoluminescence and optical transient absorption spectroscopies. Relative to control complexes lacking a TICT-active ligand, the lifetimes of CS states are enhanced ∼1000-fold. Further, the presence of the ortho-OMe substituent greatly enhances the lifetime of the TICT* state and biases the coordination environment toward CuII. The presence of CuI decreases photoinduced degradation from 14 to <2% but does not result in significant quenching via ET. Factors affecting CS in these systems are discussed, laying the groundwork for our strategy toward solar energy conversion.

Structural Capture of η1-OSO to η2-(OS)O Coordination Isomerism in a New Ruthenium-Based SO2-Linkage Photoisomer That Exhibits Single-Crystal Optical Actuation.

Recent discoveries of a range of single-crystal optical actuators are feeding a new form of materials chemistry, given their broad range of potential applications, from light-induced molecular motors to light sensors and optical-memory media. A series of ruthenium-based coordination complexes that exhibit sulfur dioxide linkage photoisomerization is of particular interest because they exhibit single-crystal optical actuation via either optical switching or nano-optomechanical transduction processes. We report the discovery of a new complex in this series of chemicals, [Ru(SO2)(NH3)4(3-fluoropyridine)]tosylate2 (1), which forms an η1-OSO photoisomer with 70% photoconversion upon the application of 505 nm light. The uncoordinated oxygen atom in this η1-OSO photoisomer impinges on one of the arene rings in a neighboring tosylate counter ion of 1 just enough that incipient nano-optomechanical transduction is observed. The structure and optical properties of this actuator are characterized via in situ light-induced single-crystal X-ray diffraction (photocrystallography), single-crystal optical absorption spectroscopy and microscopy, as well as single-crystal Raman spectroscopy. These materials-characterization methods were also used to track thermally induced reverse isomerization processes in 1. One of these processes involves an η1-OSO to η2-(OS)O transition, which was found to proceed sufficiently slowly at 110 K that its structural mechanism could be determined via a time sequence of photocrystallography experiments. The resulting data allowed us to structurally capture the transition, which was shown to occur via a form of coordination isomerism. Our newfound knowledge about this structural mechanism will aid the molecular design of new [RuSO2] complexes with functional applications.

Triple Emission of 5'-(para-R-Phenylene)vinylene-2-(2'-hydroxyphenyl)benzoxazole (PVHBO). Part I: Dual Emission from the Neutral Species.

The effects of 5'-(para-R-phenylene)vinylene (PV) substituents on the emission properties of 2-(2'-hydroxyphenyl)benzoxazole (HBO) are analyzed using steady-state and time-resolved absorption and emission spectroscopies in addition to quantum chemical calculations. All members in the series of PVHBOs are capable of excited-state intramolecular proton transfer (ESIPT) with a solvent sensitivity that is typical of a HBO derivative to produce a normal (aka enol) emission and an excited-state tautomer (aka keto) emission. These two emission bands of the neutral dyes are discussed in the current paper. The intermolecular proton transfer, i.e., the deprotonation, of a PVHBO results in the third band of the triple emission, which is described in the succeeding paper. The placement of an electron-withdrawing substituent R on the PVHBO scaffold increases the intensity of the keto emission relative to the enol emission in hydrogen-bonding solvents. The R substituents do not significantly alter the wavelengths of the enol and keto emission bands, which are located in the blue and green regions, respectively, of the visible spectrum. The ultrafast time-resolved spectroscopies and quantum chemical calculations offer explanations on how the R group and the solvent affect the enol and keto emission properties (i.e., wavelength, lifetime, fluorescence quantum yield, and relative ratio of their emissions). The key findings include the following: (1) the emission energies of both enol and keto forms are not sensitively dependent on the R substituent and (2) the solvent-engaged enol excited state is quenched more efficiently as the R substituent becomes more electron-withdrawing. A PVHBO acts as a fusion of HBO and stilbenoid that intersect at the hydroxyphenyl moiety. Depending on the solvent and other environmental conditions, PVHBOs may exhibit the ESIPT property of HBO or the substituent-dependent emission of stilbenoid. This paper and the succeeding article provide a photophysical model of PVHBOs to explain the wavelengths and relative abundances of the three emission bands (enol, keto, and anion) that these compounds are able to produce. Judicial selection of the environmental factors may drive the emission of a PVHBO into the spectral regions of blue, green, and, in a couple of cases, orange or red.

Triple Emission of 5'-(para-R-Phenylene)vinylene-2-(2'-hydroxyphenyl)benzoxazole (PVHBO). Part II: Emission from Anions.

This paper is the second part of a study on the effects of a substituted 5'-phenylenevinylene (PV) functionality on the emission properties of 2-(2'-hydroxyphenyl)benzoxazole (HBO)─a dye that is known for excited-state intramolecular proton transfer. The topical compounds are referred to as PVHBOs, each of which is a structural fusion of HBO and a 4-hydroxy-4'-R-stilbene fluorophore that occurs at the hydroxyphenyl moiety. Therefore, the resulting fusion fluorophore manifests the properties of one component or the other, as governed by its interactions with the environment. In part I (the preceding paper), PVHBOs are divided into two groups depending on whether the R substituent is electron-donating/neutral (group I) or electron-withdrawing (group II). The difference in absorption and emission properties between groups I and II is explained based on observations from spectroscopic experiments (both steady-state and time-resolved) and quantum chemical calculations. In the current paper, the same set of tools is applied to characterize the photophysical properties of the conjugate bases─that is, the anions─of PVHBOs. The emission energy of the anion of any group I compound, where the R substituent is either electron-donating or neutral, is situated between those of the neutral enol and keto forms. The emission of the anion of any given group II compound, on the other hand, has a lower energy than both the enol and keto emissions. The frontier molecular orbitals (i.e., HOMO, LUMO, and LUMO + 1) of a PVHBO localized on either HBO or stilbenoid are impacted by the substituent R and the solvent/additive differently, which leads to the differences in the optical properties of group I and II PVHBOs in both neutral and anion forms.

Ultrafast sorting: Excimeric π-π stacking distinguishes pyrene-N-methylacetamide isomers on the ultrafast time scale.

Pyrene based molecules are inclined to form excimers through self-association upon photoexcitation. In this work, the pyrene core is functionalized with the N-methylacetamide group at the position 1 or 2 to develop pyren-1-methylacetamide (PyMA1) and pyren-2-methylacetamide (PyMA2), respectively. Upon photoexcitation with 345 nm, a portion of molecules in PyMA1 and PyMA2 solutions at ≥1.0 mM have formed static excimers. The steady state spectroscopic measurements suggest that, whether it is the dimerization of molecules in the ground state (GS) or in excimer formation, characteristic signs are more pronounced in PyMA1 than its isomeric counterpart, PyMA2. The shift of the excimer band in their respective emission spectra suggests that the extent of overlap in π-π stacking is greater for PyMA1 than for PyMA2 in the excited state. The optimized geometry of dimers in toluene shows that the overlapping area between the pyrene moieties in π-π stacking between the dimers is greater for PyMA1 than for PyMA2 in GS. The natural bond orbital analysis with the optimized GS geometries shows that the stabilization/interaction energy between the dimers in π-π stacking is higher in PyMA1 compared to PyMA2 in toluene. The transient absorption (TA) measurements in toluene over the fs-ps regime (fs-TA) showed that the formation of static excimers with pre-associated dimers in PyMA1 happens in ∼700 fs whereas the excimers for the pre-associated dimers in PyMA2 have formed in slightly slower time scale (∼1.95 ps). Contrary to what was observed in solution, the extent of overlap in π-π stacking is lower for PyMA1 dimers (∼17%) than for PyMA2 dimers (∼37%) in single crystals.

Synthetic Ligand Selection Affects Stoichiometry, Carrier Dynamics, and Trapping in CuInSe2 Nanocrystals.

CuInSe2 nanocrystals exhibit tunable near-infrared bandgaps that bolster utility in photovoltaic applications as well as offer potential as substitutes for more toxic Cd- and Pb-based semiconductor compositions. However, they can present a variety of defect states and unusual photophysics. Here, we examine the effects of ligand composition (oleylamine, diphenylphosphine, and tributylphosphine) on carrier dynamics in these materials. Via spectroscopic measurements such as photoluminescence and transient absorption, we find that ligands present during the synthesis of CuInSe2 nanocrystals impart nonradiative electronic states which compete with radiative recombination and give rise to low photoluminescence quantum yields. We characterize the nature of these defect states (hole vs electron traps) and investigate whether they exist at the surface or interior of the nanocrystals. Carrier lifetimes are highly dependent on ligand identity where oleylamine-capped nanocrystals exhibit rapid trapping (<20 ps) followed by diphenylphosphine (<500 ps) and finally tributylphosphine (>2 ns). A majority of carrier population localizes at indium copper antisites (electrons), copper vacancies (holes), or surface traps (electrons and/or holes), all of which are nonemissive.

Photoinduced Charge Transfer with a Small Driving Force Facilitated by Exciplex-like Complex Formation in Metal-Organic Frameworks.

Photoinduced charge transfer (PCT) is a key step in the light-harvesting (LH) process producing the redox equivalents for energy conversion. However, like traditional macromolecular donor-acceptor assemblies, most MOF-derived LH systems are designed with a large ΔG0 to drive PCT. To emulate the functionality of the reaction center of the natural LH complex that drives PCT within a pair of identical chromophores producing charge carriers with maximum potentials, we prepared two electronically diverse carboxy-terminated zinc porphyrins, BFBP(Zn)-COOH and TFP(Zn)-COOH, and installed them into the hexagonal pores of NU-1000 via solvent-assisted ligand incorporation (SALI), resulting in BFBP(Zn)@NU-1000 and TFP(Zn)@NU-1000 compositions. Varying the number of trifluoromethyl groups at the porphyrin core, we tuned the ground-state redox potentials of the porphyrins within ca. 0.1 V relative to that of NU-1000, defining a small ΔG0 for PCT. For BFBP(Zn)@NU-1000, the relative ground- and excited-state redox potentials of the components facilitate an energy transfer (EnT) from NU-1000* to BFBP(Zn), forming BFBP(Zn)S1* which entails a long-lived charge-separated complex formed through an exciplex-like [BFBP(Zn)S1*-TBAPy] intermediate. Various time-resolved spectroscopic data suggest that EnT from NU-1000* may not involve a fast Förster-like resonance energy transfer (FRET) but rather through a slow [NU-1000*-BFBP(Zn)] intermediate formation. In contrast, TFP(Zn)@NU-1000 displays an efficient EnT from NU-1000* to [TFP(Zn)-TBAPy], a complex that formed at the ground state through electronic interaction, and thereon showed the excited-state feature of [TFP(Zn)-TBAPy]*. The results will help to develop synthetic LHC systems that can produce long-lived photogenerated charge carriers with high potentials, i.e., high open-circuit voltage in photoelectrochemical setups.

Controlled Fabrication of Quality ZnO NWs/CNTs and ZnO NWs/Gr Heterostructures via Direct Two-Step CVD Method.

A novel and advanced approach of growing zinc oxide nanowires (ZnO NWs) directly on single-walled carbon nanotubes (SWCNTs) and graphene (Gr) surfaces has been demonstrated through the successful formation of 1D-1D and 1D-2D heterostructure interfaces. The direct two-step chemical vapor deposition (CVD) method was utilized to ensure high-quality materials' synthesis and scalable production of different architectures. Iron-based universal compound molecular ink was used as a catalyst in both processes (a) to form a monolayer of horizontally defined networks of SWCNTs interfaced with vertically oriented ZnO NWs and (b) to grow densely packed ZnO NWs directly on a graphene surface. We show here that our universal compound molecular ink is efficient and selective in the direct synthesis of ZnO NWs/CNTs and ZnO NWs/Gr heterostructures. Heterostructures were also selectively patterned through different fabrication techniques and grown in predefined locations, demonstrating an ability to control materials' placement and morphology. Several characterization tools were employed to interrogate the prepared heterostructures. ZnO NWs were shown to grow uniformly over the network of SWCNTs, and much denser packed vertically oriented ZnO NWs were produced on graphene thin films. Such heterostructures can be used widely in many potential applications, such as photocatalysts, supercapacitors, solar cells, piezoelectric or thermal actuators, as well as chemical or biological sensors.

Nanooptomechanical Transduction in a Single Crystal with 100% Photoconversion.

Materials that exhibit nanooptomechanical transduction in their single-crystal form have prospective use in light-driven molecular machinery, nanotechnology, and quantum computing. Linkage photoisomerization is typically the source of such transduction in coordination complexes, although the isomers tend to undergo only partial photoconversion. We present a nanooptomechanical transducer, trans-[Ru(SO2)(NH3)4(3-bromopyridine)]tosylate2, whose S-bound η1-SO2 isomer fully converts into an O-bound η1-OSO photoisomer that is metastable while kept at 100 K. Its 100% photoconversion is confirmed structurally via photocrystallography, while single-crystal optical absorption and Raman spectroscopies reveal its metal-to-ligand charge-transfer and temperature-dependent characteristics. This perfect optical switching affords the material good prospects for nanooptomechanical transduction with single-photon control.

Rapid build up of nanooptomechanical transduction in single crystals of a ruthenium-based SO2 linkage photoisomer.

Single-crystal nanooptomechanical transduction occurs in [Ru(SO2)(NH3)4(H2O)]chlorobenzenesulfonate2, reaching maximal levels within 40 s at 100 K when photostimulated by 505 nm light. Its in situ light-induced crystal structure reveals the molecular origins of this optical actuation: 26.0(3)% of the η1-SO2 ligand photoconverts into an η1-OSO photoisomer which, in turn, induces a 49.6(9)° arene ring rotation in its neighbouring counter ion.

Low-energy optical switching of SO2 linkage isomerisation in single crystals of a ruthenium-based coordination complex.

Single crystals that behave as optical switches are desirable for a wide range of applications, from optical sensors to read-write memory media. A series of ruthenium-based complexes that exhibit optical switching in their single-crystal form via SO2 linkage photoisomerisation are of prospective interest for these technologies. This study explores the optical switching behaviour in one such complex, trans-[Ru(SO2)(NH3)4(H2O)]tosylate2 (1), in terms of its dark and photoinduced crystal structure, as well as its light and thermal decay characteristics, which are deduced by photocrystallography, single-crystal optical absorption spectroscopy and microscopy. Photocrystallography results reveal that a photoisomerisation level of 21.5(5)% is achievable in 1. Biphasic photochromic crystals of 1 were generated by applying green and then red light to switch on and off the η2-(OS)O photoisomer in different regions of a crystal. Heat is a known alternative to its thermal decay, whereby a method is demonstrated that employs optical absorption spectra to determine its activation energy of 30 kJ mol-1. This low-energy barrier to optical switching agrees well with computational studies on 1, as well as being comparable to activation energies in ruthenium-based nitrosyl linkage photoisomers that also display solid-state optical switching.

Photophysical implications of ring fusion, linker length, and twisting angle in a series of perylenediimide-thienoacene dimers.

Perylenediimide (PDI) derivatives have been widely studied as electron acceptor alternatives to fullerenes in organic photovoltaics (OPVs) because of their tunable absorption in the visible range, inexpensive synthesis, and photochemical stability. A common motif for improving device efficiency involves joining multiple PDIs together through electron-rich linkers to form a twisted acceptor-donor-acceptor molecule. Molecular features such as ring fusion are further employed to modify the structure locally and in films. These synthetic efforts have greatly enhanced OPV device efficiencies, however it remains unclear how the increasingly elaborate structural modifications affect the photophysical processes integral to efficient photon-to-charge conversion. Here we carry out a systematic study of a series of PDI dimers with thienoacene linkers in which the twist angle, linker length, and degree of ring fusion are varied to investigate the effects of these structural features on the molecular excited states and exciton recombination dynamics. Spectroscopic characterization of the dimers suggest that ring fusion causes greater coupling between the donor and acceptor components and greatly enhances the lifetime of a thienoacene to PDI charge transfer state. The lifetime of this CT state also correlates well with the linker-PDI dihedral angle, with smaller dihedral angle resulting in longer lifetime. DFT and two-photon absorption TDDFT calculations were developed in-house to model the ground state and excited transitions, providing theoretical insight into the reasons for the observed photophysical properties and identifying the charge transfer state in the excited state absorption spectra. These results highlight how the longevity of the excited state species, important for the efficient conversion of excitons to free carriers in OPV devices, can be chemically tuned by controlling ring fusion and by using steric effects to control the relative orientations of the molecular fragments. The results provide a successful rationalization of the behavior of solar cells involving these acceptor molecules.

Directly measuring the structural transition pathways of strain-engineered VO2 thin films.

Epitaxial films of vanadium dioxide (VO2) on rutile TiO2 substrates provide a means of strain-engineering the transition pathways and stabilizing of the intermediate phases between monoclinic (insulating) M1 and rutile (metal) R end phases. In this work, we investigate structural behavior of epitaxial VO2 thin films deposited on isostructural MgF2 (001) and (110) substrates via temperature-dependent Raman microscopy analysis. The choice of MgF2 substrate clearly reveals how elongation of V-V dimers accompanied by the shortening of V-O bonds triggers the intermediate M2 phase in the temperature range between 70-80 °C upon the heating-cooling cycles. Consistent with earlier claims of strain-induced electron correlation enhancement destabilizing the M2 phase our temperature-dependent Raman study supports a small temperature window for this phase. The similarity of the hysteretic behavior of structural and electronic transitions suggests that the structural transitions play key roles in the switching properties of epitaxial VO2 thin films.

Surface immobilized copper(I) diimine photosensitizers as molecular probes for elucidating the effects of confinement at interfaces for solar energy conversion.

Heteroleptic copper(i) bis(phenanthroline) complexes with surface anchoring carboxylate groups have been synthesized and immobilized on nanoporous metal oxide substrates. The species investigated are responsive to the external environment and this work provides a new strategy to control charge transfer processes for efficient solar energy conversion.

The Effect of Periodic Spatial Perturbations on the Emission Rates of Quantum Dots near Graphene Platforms.

The quenching of fluorescence (FL) at the vicinity of conductive surfaces and, in particular, near a 2-D graphene layer has become an important biochemical sensing tool. The quenching is attributed to fast non-radiative energy transfer between a chromophore (here, a Quantum Dot, QD) and the lossy graphene layer. Increased emission rate is also observed when the QD is coupled to a resonator. Here, we combine the two effects in order to control the emission lifetime of the QD. In our case, the resonator was defined by an array of nano-holes in the oxide substrate underneath a graphene surface guide. At resonance, the surface mode of the emitted radiation is concentrated at the nano-holes. Thus, the radiation of QD at or near the holes is spatially correlated through the hole-array's symmetry. We demonstrated an emission rate change by more than 50% as the sample was azimuthally rotated with respect to the polarization of the excitation laser. In addition to an electrical control, such control over the emission lifetime could be used to control Resonance Energy Transfer (RET) between two chromophores.