Nonradiative Relaxation Rates Research Articles

Twisted Structure Induced Solid-State Fluorescence and Room-Temperature Phosphorescence from Furan-Based Carbon Dots.

Boron doping can effectively induce solid-state fluorescence (SSF) in carbon dots (CDs); however, research on the intrinsic mechanism underlying this phenomenon is lacking. Herein, a design strategy for boron-doped furan-based CDs is proposed, CDs with aggregation-induced emission (AIE) properties are synthesized, and the mechanism by which boron atom dopants induces SSF and room-temperature phosphorescence (RTP) is elucidated. The morphology and structural characterization of the CDs indicate that boron doping leads to structural twisting of the CDs. The AIE phenomenon of CDs arises from the inhibition of the twisted structure motions and a reduction in the nonradiative relaxation rate during the aggregation process. In addition, CDs with twisted structures exhibit a smaller overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), effectively reducing the singlet-triplet splitting energy (ΔEST). CDs embedded in microcrystalline cellulose (MCC) exhibit green RTP because the nonradiative transitions are suppressed, and the excited triplet species remain stable. For the first time, this study reveals the structure-activity relationship between the twisted structure and optical properties of CDs, providing a new approach for the preparation of solid-state light-emitting CDs.

Combustion synthesis and photoluminescence of Ca2SrWO6:Dy3+ double perovskite: An approach for white light emission

Luminescent Ca2SrWO6:Dy3+ double perovskites are synthesized via the combustion route of material synthesis, wherein urea is taken as fuel. The as-prepared phosphor series comprehensively characterized for their structural and photoluminescence (PL) properties. The Rietveld refinement of XRD data revealed monoclinic crystallized microcrystals with mixed particles of the size of less than 1 µm, studied via SEM. Ca2SrWO6:Dy3+ displayed standard Dy3+ transitions 4F9/2–6H15/2 (blue emission) and 4F9/2–6H13/2 (yellow emission) collectively resulted in white light emission, confirmed via CIE study. Under 278 and 388 excitations, 2 mol% doping of Dy3+ found to be optimum concentration, beyond which concentration quenching is observed due to dipole–dipole interaction. The Y/B ratio ∼1 shows potential to produce white light and make phosphor suitable for WLED applications. The Ca2SrWO6:Dy3+ phosphor displayed notably high thermal stability with PL intensity persisted 77.6 % at LED burning temperature (150 °C) when compared to PL intensity attained at room-temperature, which is basically due to the rigid perovskite structure. In addition, the PL decay lifetime of 4F9/2 state was calculated by utilizing decay curves. Subsequently, the PL quantum efficiency and non-radiative relaxation rate are determined. The theoretical model of Auzel’s fit function is used to calculate to quantum efficiency and was found to be 93.27 % for Ca2SrWO6:2 % Dy3+ phosphor. Moreover, the photometric results reveals that the CIE points are lies in white region of CIE diagram with comparable CCT. The collective results indicated the utility of the phosphor for potential WLED applications.

Tuned quantum cutting efficiency by Li+/Na+ molar content in LixNa1-xGd(MoO4)2:Er3+/Yb3+ phosphors

Quantum cutting materials have garnered significant interest due to their potential applications in enhancing photoelectric conversion efficiency in devices such as solar cells. The host matrices that accommodate rare earth ions play a pivotal role in determining the optical transition properties, which are crucial for quantum cutting. In this study, both Er3+ single-doped and Er3+/Yb3+ co-doped LixNa1-xGd(MoO4)2 (x = 0, 0.01, 0.1, 0.3, 0.5, 0.7, 0.9, 0.99 and 1.0) were synthesized via a high-temperature solid-state reaction to tune the optical transitions properties and enhance quantum cutting efficiency. Optical transition intensity parameters (Judd-Ofelt parameters) of Er3+ were calculated and shown to vary significantly with the Li+/Na+ molar content, demonstrating that these optical properties can be effectively tuned by adjusting the sample composition. The reliability of these calculations was confirmed by comparing the radiative transition rate ratios between two green emissions of Er3 obtained through both theoretical and experimental methods. A two-step energy transfer mechanism was identified as the basis for quantum cutting process, and the relevant energy transfer and nonradiative relaxation rates were determined. Notably, the quantum cutting efficiency reached a peak value of 155.65 % in Li0.5Na0.5Gd(MoO4)2 sample, indicating that successful tuning of the sample's optical transition properties effectively enhanced its quantum cutting efficiency. These findings underscore the viability of tuning the optical properties of rare earth-doped materials to design highly efficient quantum cutting materials.

Reductive Dynamic and Static Excited State Quenching of a Homoleptic Ruthenium Complex Bearing Aldehyde Groups.

A new homoleptic Ru polypyridyl complex bearing two aldehyde groups on each bipyridine ligand, [Ru(dab)3](PF6)2, where dab is 4,4'-dicarbaldehyde-2,2'-bipyridine, was synthesized, characterized, and utilized for iodide photo-oxidation studies. In acetonitrile (CH3CN) solution, the complex displayed an intense metal-to-ligand charge transfer (MLCT) absorbance maximum at 475 nm (ε = 22,000 M-1 cm-1) and an infrared (IR) band at 1712 cm-1 assigned to the pendent aldehyde groups. Visible light excitation in air-saturated solution resulted in room temperature photoluminescence (PL) with a maximum at 675 nm, a quantum yield, ϕPL = 0.048, and an excited state lifetime, το = 440 ns, from which radiative and nonradiative relaxation rate constants were extracted, kr = 9.1 × 104 s-1 and knr = 1.8 × 106 s-1. Pulsed visible light excitation yielded transient UV-vis and IR absorption spectra consistent with an MLCT excited state; relaxation occurred with the maintenance of two isosbestic points in the visible region, and a lifetime that agreed with that measured by time-resolved PL. Cyclic voltammetry studies in a CH3CN solution with 0.1 M TBAPF6 electrolyte revealed a quasi-reversible oxidation, E°(RuIII/II) = +1.25 V vs. Fc+/0, and three sequential one-electron reductions at -1.10, -1.25, and -1.54 V vs. Fc+/0. An excited state reduction potential of E°(Ru*2+/+) = +0.89 V vs. Fc+/0 was estimated with the Rehm-Weller expression. Titration of tetrabutylammonium iodide, TBAI, into a CD3CN solution of [Ru(dab)3](PF6)2 resulted in significant shifts in the aldehyde H atom and 3,3'-biypridyl resonances that were analyzed with a 1:1 equilibrium model, from which Keq = 460 M-1 was extracted, increasing to 5800 M-1 when the solvent was changed to acetone-d6. Iodide titrations resulted in a significant quenching of the [Ru(dab)3]*2+ lifetime and quantum yield in both CH3CN and acetone solvents. In CH3CN, the quenching was mainly dynamic and well described by the Stern-Volmer model, from which a quenching rate constant, kq, of 4.5 × 1010 M-1 s-1 and an equilibrium constant, Keq, of 8.3 × 103 M-1 were obtained. In acetone, the static quenching pathway by iodide was greatly enhanced, with a Keq of 1.2 × 104 M-1 and a higher kq of 9.2 × 1010 M-1 s-1.

Near infrared emissions from both high efficient quantum cutting (173%) and nearly-pure-color upconversion in NaY(WO4)2:Er3+/Yb3+ with thermal management capability for silicon-based solar cells

Raising photoelectric conversion efficiency and enhancing heat management are two critical concerns for silicon-based solar cells. In this work, efficient Yb3+ infrared emissions from both quantum cutting and upconversion were demonstrated by adjusting Er3+ and Yb3+ concentrations, and thermo-manage-applicable temperature sensing based on the luminescence intensity ratio of two super-low thermal quenching levels was discovered in an Er3+/Yb3+ co-doped tungstate system. The quantum cutting mechanism was clearly decrypted as a two-step energy transfer process from Er3+ to Yb3+. The two-step energy transfer efficiencies, the radiative and nonradiative transition rates of all interested 4 f levels of Er3+ in NaY(WO4)2 were confirmed in the framework of Föster-Dexter theory, Judd-Ofelt theory, and energy gap law, and based on these obtained efficiencies and rates the quantum cutting efficiency was furthermore determined to be as high as 173% in NaY(WO4)2: 5 mol% Er3+/50 mol% Yb3+ sample. Strong and nearly pure infrared upconversion emission of Yb3+ under 1550 nm excitation was achieved in Er3+/Yb3+ co-doped NaY(WO4)2 by adjusting Yb3+ doping concentrations. The Yb3+ induced infrared upconversion emission enhancement was attributed to the efficient energy transfer 4I11/2 (Er3+) + 2F7/2 (Yb3+) → 4I15/2 (Er3+) + 2F5/2 (Yb3+) and large nonradiative relaxation rate of 4I9/2. Analysis on the temperature sensing indicated that the NaY(WO4)2:Er3+/Yb3+ serves well the solar cells as thermos-managing material. Moreover, it was confirmed that the fluorescence thermal quenching of 2H11/2/4S3/2 was caused by the nonradiative relaxation of 4S3/2. All the obtained results suggest that NaY(WO4)2:Er3+/Yb3+ is an excellent material for silicon-based solar cells to improve photoelectric conversion efficiency and thermal management.

Photo-physical characteristics of color N3-center in diamond studied via UV femtosecond-laser pumped luminescence.

Micro-joule UV-range (350-415 nm) femtosecond-laser pulses generated via frequency-doubled parametric conversion of 525-nm 150-fs pulses of Yb-glass laser were used for "hot" photoluminescence excitation in a diamond plate enriched by blue-emitting N3-centers (zero-phonon line, ZPL, at 415 nm). Photoluminescence spectra acquired in the range of 400-500 nm exhibited wavelength-independent well-resolved ZPL and phonon progression bands, where the involved phonons possessed the only energies of 0.09 eV (LA-phonons) and 0.15 eV (softened LO/TO-phonons), potentially, as a result of a Clemens decay mechanism. Photoluminescence yield in the ZPL and other phonon bands exhibited the power slope of 1.8 at lower energies and ≈1 at higher energies. The transition zone at fluence ∼1014-15 photons/cm2 was related to the saturation of the pumped resonance transition and the slower non-radiative vibrational relaxation to the ZPL-related excited electronic state and the nanosecond spontaneous photoluminescence transition to the ground state. As a result, the absorption cross section σ(370-390 nm) ≈1·10-15 cm2 and concentration [N3] ≈6·1014 cm-3 were determined along with the ZPL absorption cross section σ(415 nm) ≈2.5·10-15 cm2, and the non-radiative vibrational relaxation rate was estimated, providing altogether the crucial information on lasing possibilities in N3-doped diamonds.

When do tripdoublet states fluoresce? A theoretical study of copper(II) porphyrin.

Open-shell molecules rarely fluoresce, due to their typically faster non-radiative relaxation rates compared to closed-shell ones. Even rarer is the fluorescence from states that have two more unpaired electrons than the open-shell ground state, since they involve excitations from closed-shell orbitals to vacant-shell orbitals, which are typically higher in energy compared to excitations from or out of open-shell orbitals. States that are dominated by the former type of excitations are known as tripdoublet states when they can be described as a triplet excitation antiferromagnetically coupled to a doublet state, and their description by unrestricted single-reference methods (e.g., U-TDDFT) is notoriously inaccurate due to large spin contamination. In this work, we applied our spin-adapted TDDFT method, X-TDDFT, and the efficient and accurate static-dynamic-static second order perturbation theory (SDSPT2), to the study of the excited states as well as their relaxation pathways of copper(II) porphyrin; previous experimental works suggested that the photoluminescence of some substituted copper(II) porphyrins originate from a tripdoublet state, formed by a triplet ligand π → π* excitation antiferromagnetically coupled with the unpaired d electron. Our results demonstrated favorable agreement between the X-TDDFT, SDSPT2 and experimental excitation energies, and revealed noticeable improvements of X-TDDFT compared to U-TDDFT, not only for vertical excitation energies but also for adiabatic energy differences. These suggest that X-TDDFT is a reliable tool for the study of tripdoublet state fluorescence. Intriguingly, we showed that the aforementioned tripdoublet state is only slightly above the lowest doublet excited state and lies only slightly higher than the lowest quartet state, which suggests that the tripdoublet of copper(II) porphyrin is long-lived enough to fluoresce due to a lack of efficient non-radiative relaxation pathways; an explanation for this unusual state ordering is given. Indeed, thermal vibration correlation function (TVCF)-based calculations of internal conversion, intersystem crossing, and radiative transition rates confirm that copper(II) porphyrin emits thermally activated delayed fluorescence (TADF) and a small amount of phosphorescence at low temperature (83K), in accordance with experiment. The present contribution is concluded by a few possible approaches of designing new molecules that fluoresce from tripdoublet states.

Revisiting the luminescence properties of Er3+, Tm3+ and Ho3+ doped Bi4Ge3O12 rod-type crystal fibers for the laser application

BGO single crystals doped with Er3+, Tm3+ and Ho3+ rare-earth ions were prepared in the form of rod-type crystal fibers via the micro-pulling down technique to perform an extensive investigation of their visible, near- and mid-infrared luminescence properties and to really decide on their interest as solid-state laser media in the various spectral domains. Revisiting and completing the absorption spectra already available in the literature led to the derivation of more reliable “effective” radiative lifetimes and branching ratios. Registration of the fluorescence decays of the most important emitting levels versus dopant concentrations led for the first time to an unambiguous determination of “intrinsic” emission lifetimes – corresponding to negligible contributions from inter-ionic energy-transfers - and non-radiative multiphonon relaxation rates. Such data also allowed for the determination of the dominant phonon energy which comes into play in the electron-phonon coupling responsible for these multiphonon relaxation processes and for its lattice vibration origin, i.e. a high phonon energy of the order of 820 cm−1 corresponding to the coupling between bending vibrations of (GeO4)4- tetrahedra and vibrations corresponding to motions of these (GeO4)4- tetrahedra against the Bi3+ ions for which the rare-earth dopant ions substitute. Using the above data and different methods for the calibration of the registered emission spectra in cross section unit and for the derivation of gain cross section curves finally show that mid-infrared emissions of Er:BGO, Tm:BGO and Ho:BGO around 1.55 μm, 1.9 μm and 2 μm, respectively, occur with comparable gain cross sections as in the most important reference mid-infrared laser materials, although over slightly shifted thus complementary spectral domains. It is shown that potentialities also exist with Tm:BGO in the blue, deep-red and near-infrared spectral domains around 470 nm, 800 nm and 1.45 μm, by pumping the crystals with conventional pump sources and using some suitable co-dopants to avoid detrimental self-terminating processes.

Chemical applications of hybridized light-matter states (a review)

Interactions between light and matter are a fundamental part of chemical sciences responsible for basic photophysical processes such as phosphorescence and fluorescence. However, these photophysical phenomena occur in the "weak" limit of interaction between light and matter in which the photon and molecule interact with each other without the former fundamentally changing the physical properties of the latter. By constructing a Fabry-Perot cavity, which traps light of a certain frequency, then placing a molecule in a cavity that undergoes a molecular electron transition at the frequency of the trapped light, scientists can force strong light-matter interaction. This interaction occurs if the exchange between the light of the cavity mode and the molecule's excited state is faster than the decay rate of either state, forming a hybrid light-matter state known as a polariton. The photophysical properties of these polariton states have been of interest to scientists due to the possibility that they can allow for the modification of the reactivity of molecules without the addition of functional groups or modification of the surrounding environment. Of particular interest is the ability of polaritons to influence the potential energy surface of molecules, with polaritons showing the ability to both, suppress the photochemical reaction in molecules such as spiropyran and stilbene, while also enhancing the nonradiative relaxation rate of porphyrins. Due to their photonic nature, polaritons have also shown the ability to facilitate long range energy transfer processes in organic dye molecules. This review focuses on discussing these recent advances in a chemistry context as well as the optical design of cavities required to sustain polaritons.

A quantitative model of multi-scale single quantum dot blinking

We present a fundamentally new model of colloidal semiconductor quantum dot blinking. The blinking is caused by fluctuations of the non-radiative exciton relaxation rate, induced by variations of the electron–phonon coupling value.

Fermi energy dependence of ultrafast photoluminescence from graphene

The application of graphene in new light-emitting devices has been extensively studied since the demonstration of the ultrafast luminescence from single-layer graphene. The control of luminescence using doping techniques is crucial for these applications. In particular, for the application of graphene in flexible and wearable devices, electrochemical doping is a promising approach, and its influence on luminescence properties of the resulting material needs to be examined. In this study, we demonstrate the effect of the electrochemical doping of graphene using an ion gel on the photoluminescence (PL) of graphene at the emission energy ℏω of 0.9 eV. The Fermi energy EF of graphene was controlled from +40 to −560meV, and femtosecond PL was observed. The PL intensity was maximum when EF was −440meV (|EF|≈ℏω/2). This trend of the PL intensity is due to (i) an increase in the PL emission rate owing to the doping-induced empty states in the valence band acting as the final states of the radiative relaxation of hot electrons and (ii) an increase in the non-radiative relaxation rate owing to the acceleration of carrier–carrier scattering by the doping-induced increase in the density of states around the EF.

Rare earth elements as a source of impurities in doped chalcogenide glasses

Elemental and heterophase impurities in some rare-earth elements (Ce, Pr, Nd, Tb, and Dy) and features of their influence on optical and emission properties of doped chalcogenide glasses are determined. The main impurity elements in REEs are: W, Mo, Fe, Mg, Ti and some other metals (up to 8400 ppmw), fluorine and oxygen in the form of oxofluorides and oxides (up to 680 ppmw), and hydrogen in dissolved form (up to 68 ppmw). In doped Ge20Sb10Ga5Se65 glasses, the REEs are the main source of impurities of d-transition metals, non-metals (Si, F), gas-forming impurities (H, O), and inclusions. Heterogeneous inclusions, the source of which is REEs, make the greatest contribution to optical losses in doped chalcogenide glasses. Impurities of hydrogen in the form of SeH groups, oxygen in the form of dissolved oxides, d-transition metal ions, and REE analogues affect the rate of non-radiative relaxation of the excited state of REE.

Rationalizing the structural changes and spectra of manganese and their temperature dependence in a series of garnets with first-principles calculations

Detailed first-principles calculations have been carried out to study the stabilization, excitation and luminescence mechanisms of the ion ${\mathrm{Mn}}^{3+}$ in a series of ${A}_{3}{B}_{2}{B}_{3}^{\ensuremath{'}}{\mathrm{O}}_{12}$ garnet hosts. The formation energy shows that ${\mathrm{Mn}}^{3+}$ is dominant and is situated at the octahedral $B$ site. The excited states, excitation, and emission energies of ${\mathrm{Mn}}^{3+}$ have then been calculated. The calculated energy levels of ${\mathrm{Mn}}^{3+}$ confirm that the red emission is due to the ${}^{5}{\mathrm{T}}_{2}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}{\phantom{\rule{4pt}{0ex}}}^{5}{\mathrm{E}}^{\ensuremath{'}}$ transition and the near-infrared (NIR) emission arises from the ${}^{1}{\mathrm{T}}_{2}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}{\phantom{\rule{4pt}{0ex}}}^{3}{\mathrm{T}}_{1}$ transition. The populations of the ${}^{5}{\mathrm{T}}_{2}$ and ${}^{1}{\mathrm{T}}_{2}$ excited states and the corresponding radiative rates lead to the temperature dependence of the red to NIR emission. Furthermore, the adiabatic potential energy surfaces along the ${A}_{1g}$ and ${E}_{g}$ moeity modes of [${\mathrm{MnO}}_{6}$] have been calculated and fitted well in the harmonic approximation. The high activation energy for ${\mathrm{Mn}}^{3+}$ indicates a low nonradiative multiphonon relaxation rate of ${}^{5}{\mathrm{T}}_{2}$ to ${}^{3}{\mathrm{T}}_{1}$. Hence, the ionization process was considered, and we show that it is responsible for the luminescence quenching of ${\mathrm{Mn}}^{3+}$, so that the luminescence has rarely been reported experimentally. This work illustrates a well-designed approach based on the density-functional theory framework to predict the optical transition properties of the transition metal ion ${\mathrm{Mn}}^{3+}$ by calculating the structural distortions due to the Jahn-Teller effect, the optical transitions, quenching processes and the influence of pressure.

An atomically precise silver nanocluster for artificial light-harvesting system through supramolecular functionalization.

Designing an artificial light-harvesting system (LHS) with high energy transfer efficiency has been a challenging task. Herein, we report an atom-precise silver nanocluster (Ag NC) as a unique platform to fabricate the artificial LHS. A facile one-pot synthesis of [Cl@Ag16S(S-Adm)8(CF3COO)5(DMF)3(H2O)2]·DMF (Ag16) NC by using a bulky adamantanethiolate ligand is portrayed here which, in turn, alleviates the issues related to the smaller NC core designed from a highly steric environment. The surface molecular motion of this NC extends the non-radiative relaxation rate which is strategically restricted by a recognition site-specific supramolecular adduct with β-cyclodextrin (β-CD) that results in the generation of a blue emission. This emission property is further controlled by the number of attached β-CD which eventually imposes more rigidity. The higher emission quantum yield and the larger emission lifetime relative to the lesser numbered β-CD conjugation signify Ag16 ∩ β-CD2 as a good LHS donor component. In the presence of an organic dye (β-carotene) as an energy acceptor, an LHS is fabricated here via the Förster resonance energy transfer pathway. The opposite charges on the surfaces and the matched electronic energy distribution result in a 93% energy transfer efficiency with a great antenna effect from the UV-to-visible region. Finally, the harvested energy is utilized successfully for efficient photocurrent generation with much-enhanced yields compared to the individual components. This fundamental investigation into highly-efficient energy transfer through atom-precise NC-based systems will inspire additional opportunities for designing new LHSs in the near future.

Determination of radiative and multiphonon non-radiative relaxation rates of upconversion materials.

The radiative and multiphonon non-radiative relaxation rates of lanthanide ions are intrinsic parameters to characterize the optical properties, which are the basic data for the theoretical model and numerical simulation of lanthanide upconversion systems. However, there are complex energy transfer processes, such as energy migration, energy transfer upconversion, and cross-relaxation in the lanthanide-doped upconversion materials, so it is difficult to accurately measure the intrinsic radiative and multiphonon relaxation rates. Therefore, a method to determine the relaxation rates of multi-level upconversion systems is proposed based on multi-wavelength excitation and level-by-level parameter calculations in this paper. For a dilute doped multi-level luminescence system excited at low powers, a model based on the measurements of steady-state emission spectra and luminescence decay curves is established through the macroscopic rate equations at multi-wavelength excitation, which can be used for the level-by-level calculation of the multi-level radiative and multiphonon relaxation rates. With the dilute doped β-NaYF4:Er3+ six-level luminescence system as an example, the measurement method and the model are introduced in detail. Under the experimental conditions of neglecting the energy transfer effect between ions, the materials are excited by five lasers with central wavelengths of 1523 nm, 980 nm, 808 nm, 660 nm, and 520 nm to form five subsystems. The steady-state emission spectra and luminescence decay curves of the luminescence system excited by each wavelength were recorded. The intrinsic relaxation rates including 11 radiative relaxation rates and 4 multiphonon relaxation rates in the β-NaYF4:Er3+ six-level system were determined based on the established model and method, which experimentally verified the applicability of the method proposed in this paper. This work will provide basic data for the analysis and regulation of the luminescence properties of lanthanide upconversion systems.

Exciton Delocalization Counteracts the Energy Gap: A New Pathway toward NIR-Emissive Dyes.

Exciton coupling between the transition dipole moments of ordered dyes in supramolecular assemblies, so-called J/H-aggregates, leads to shifted electronic transitions. This can lower the excited state energy, allowing for emission well into the near-infrared regime. However, as we show here, it is not only the excited state energy modifications that J-aggregates can provide. A bay-alkylated quaterrylene was synthesized, which was found to form J-aggregates in 1,1,2,2-tetrachloroethane. A combination of superradiance and a decreased nonradiative relaxation rate made the J-aggregate four times more emissive than the monomeric counterpart. A reduced nonradiative relaxation rate is a nonintuitive consequence following the 180 nm (3300 cm–1) red-shift of the J-aggregate in comparison to the monomeric absorption. However, the energy gap law, which is commonly invoked to rationalize increased nonradiative relaxation rates with increasing emission wavelength, also contains a reorganization energy term. The reorganization energy is highly suppressed in J-aggregates due to exciton delocalization, and the framework of the energy gap law could therefore reproduce our experimental observations. J-Aggregates can thus circumvent the common belief that lowering the excited state energies results in large nonradiative relaxation rates and are thus a pathway toward highly emissive organic dyes in the NIR regime.

Emission kinetics of HITC laser dye on top of arrays of Ag nanowires

Abstract We have grown arrays of silver nanowires in pores of anodic alumina membranes (metamaterials with hyperbolic dispersion at λ ≥ 615 nm), spin coated them with the dye-doped polymer (HITC:PMMA), and studied the rates of radiative and nonradiative relaxation as well as the concentration quenching (Förster energy transfer to acceptors). The results were compared to those obtained on top of planar Ag films and glass (control samples). The strong spatial inhomogeneity of emission kinetics recorded in different spots across the sample and strong inhibition of the concentration quenching in arrays of Ag nanowires are among the most significant findings of this study.

Two-Photon Excited Fluorescence Dynamics in Enzyme-Bound NADH: the Heterogeneity of Fluorescence Decay Times and Anisotropic Relaxation.

The dynamics of polarized fluorescence in NADH in alcohol dehydrogenase (ADH) in buffer solution has been studied using the TCSPC spectroscopy. A global fit procedure was used for determination of the fluorescence parameters from experiment. The interpretation of the results obtained was supported by ab initio calculations of the NADH structure. A theoretical model was developed describing the polarized fluorescence decay in ADH-NADH complexes that considered several interaction scenarios. A comparative analysis of the polarization-insensitive fluorescence decay using multiexponential fitting models has been carried out. As shown, the origin of a significant enhancement of the decay time in the ADH-NADH complex can be attributed to the decrease of nonradiative relaxation rates in the nicotinamide ring in the conditions of the apolar binding site environment. The existence of a single decay time in the ADH-NADH complex in comparison with two decay times observed in free NADH was attributed to a single NADH unfolded conformation in the ADH binding site. Comparison of the experimental data with the theoretical model suggested the existence of an anisotropic relaxation time of about 1 ns that is related with the rotation of fluorescence transition dipole moment due to the rearrangement of the excited state NADH nuclear configuration.

Hot Carrier Dynamics at Ligated Silicon(111) Surfaces: A Computational Study.

We provide a case-study for thermal grafting of benzenediazonium bromide onto a hydrogenated Si(111) surface using ab initio molecular dynamics (AIMD) calculations. A sequence of reaction steps is identified in the AIMD trajectory, including the loss of N2 from the diazonium salt, proton transfer from the surface to the bromide ion that eliminates HBr, and deposition of the phenyl group onto the surface. We next assess the influence of the phenyl groups on photophysics of hydrogen-terminated Si(111) slabs. The nonadiabatic couplings necessary for a description of the excited-state dynamics are calculated by combining ab initio electronic structures and reduced density matrix formalism with Redfield theory. The phenyl-terminated slab shows reduced nonradiative relaxation and recombination rates of hot charge carriers in comparison with the hydrogen-terminated slab. Altogether, our results provide atomistic insights revealing that (i) the diazonium salt thermally decomposes at the surface allowing the formation of covalently bonded phenyl group, and (ii) the coverage of phenyl groups on the surface slows down charge carrier cooling driven by electron-phonon interactions, which increases photoluminescence efficiency at the near-infrared spectral region.

Evaluating the anharmonicity contributions to the molecular excited state internal conversion rates with finite temperature TD-DMRG.

In this work, we propose a new method to calculate molecular nonradiative electronic relaxation rates based on the numerically exact time-dependent density matrix renormalization group theory. This method could go beyond the existing frameworks under the harmonic approximation (HA) of the potential energy surface (PES) so that the anharmonic effect could be considered, which is of vital importance when the electronic energy gap is much larger than the vibrational frequency. We calculate the internal conversion (IC) rates in a two-mode model with Morse potential to investigate the validity of HA. We find that HA is unsatisfactory unless only the lowest several vibrational states of the lower electronic state are involved in the transition process when the adiabatic excitation energy is relatively low. As the excitation energy increases, HA first underestimates and then overestimates the IC rates when the excited state PES shifts toward the dissociative side of the ground state PES. On the contrary, HA slightly overestimates the IC rates when the excited state PES shifts toward the repulsive side. In both cases, a higher temperature enlarges the error of HA. As a real example to demonstrate the effectiveness and scalability of the method, we calculate the IC rates of azulene from S1 to S0 on the ab initio anharmonic PES approximated by the one-mode representation. The calculated IC rates of azulene under HA are consistent with the analytically exact results. The rates on the anharmonic PES are 30%-40% higher than the rates under HA.