Nonradiative Deexcitation Research Articles

Enhancing Photothermal Energy Transduction through Inter- and Intramolecular Interactions of Multiple Two-Photon Dyes Appended onto Calix[4]arene.

Organic dyes-based photothermal agents (OPTAs) have received increasing attention as alternative to inorganic materials due to their higher biocompatibility and extensive diversification. Maximizing nonradiative deexcitation channels is crucial to improve the photothermal conversion efficiency (PCE) of OPTAs. This is typically achieved through individual molecular design or collective enhancement using supramolecular strategies. Furthermore, photothermal therapy (PTT) generally relies on linear one-photon absorption of the light source by the OPTA, with less consideration given to nonlinear two-photon absorption (2PA) strategies, despite their potential benefits. Here, a synergistic strategy, which combines intramolecular and intermolecular quenching, is employed to maximize the photothermal efficiency of diphenylamino-substituted distyryl dicyanobenzene (DSB), an outstanding two-photon-absorbing chromophore. One to three DSB units have been introduced on the conic p-tert-butyl-calix[4]arene (CX), serving as a preorganizing platform to allow aggregate formation and promote intramolecular quenching within the multichromophoric systems. Importantly, the multichromophoric molecules had very high two-photon absorption capabilities with cross sections (δ2PA) reaching maximal values of 3290 GM at 810 nm. Experimental data accompanied by large-scale molecular dynamics simulations and time-dependent density functional theory calculations shed light onto the interaction mechanism in those multiple DSB-appended CX compounds to rationalize their optical properties. Then, the formulation with Pluronic F127 amphiphile yields water-dispersible nanoprecipitates (Nps), in which the PCE is further maximized and the photobleaching is reduced due to the combination of intra- and intermolecular quenching. The high two-photon absorption in the near-infrared (NIR) window associated with the high PCE of these nanosized OPTAs could serve as a basis to future in vivo 2P-PTT applications.

Multi-anionic polymer templated aggregation induced emission of berberine and its application for protamine sensing

In recent years exploration of aggregation induced emission from the naturally occurring luminogens has garnered significant attention across the globe due to their widespread applications in diverse fields of modern-day research. Herein, we delineate multi-anionic polymer, polystyrene sulfonate (PSS) templated aggregation induced emission (AIE) of a medically important and naturally origin alkaloid, berberine (BBR) and its application towards the quantification of an important bio-analyte, protamine (Pr). Cationic BBR at the vicinity of the polyanionic PSS, partially loses its positive charge which causes the reduction in the electrostatic repulsion between the adjacent dye molecules, promoting their aggregation via π-π staking over the polymer surface. This substantially changes the microenvironment of the dye surrounding, leading to the lowering in its structural swiftness and tendency of intramolecular charge transfer (ICT) state formation. Thus, the dye exhibits amplified emission in the aggregated state. Increase of average lifetime and sluggish nature of anisotropy decay of BBR in the presence of PSS support structural confinement and weakening of non-radiative deexcitation pathways for the dye through the formation of BBR-PSS supramolecular assembly. The formed BBR-PSS assembly displays high sensitivity towards external parameters like ionic strength, temperature but unresponsive to a wide range of pH. Importantly, BBR-PSS has been found to be highly sensitive and selective towards an important polypeptide, Pr. Owing to its high positive charge density, Pr disintegrates the BBR-PSS complex, producing fluorescence turn-off response for the BBR-PSS complex which enables us to quantify Pr in aqueous solution. Notably, this AIE based complex is quite efficient to detect protamine even in 1% human serum matrix as low as ∼ 89 nM in spite of presence of various other interfering bioanalytes.

Reflecting the Quality Degradation of Engine Oil by the Thermal Diffusivity: Radiative and Nonradiative Analyses.

Ageing of engine oil is an important issue determining the engine life and performance. The present work attempts to delineate the ageing-induced changes in engine oil through the mode-mismatched dual-beam thermal lens (MMDBTL) technique and other conventional spectroscopic techniques. For the analyses, engine oil samples were collected after every 200 km of runtime. As the thermal diffusivity is related to the nonradiative deexcitation upon optical absorption, comprehensive radiative and nonradiative analyses were carried out. The Ultraviolet-Visible, Fourier transform infrared, and Nuclear magnetic resonance spectroscopic analyses point to the structural modification as a result of the breaking of the long-chain hydrocarbons into ketones, aldehydes, esters, and other compounds. This modifies the absorption pattern, which can also be understood from the nonlinear refractive index study using the Z-scan technique. The compositional variations associated with the degradation upon ageing, the length of the hydrocarbon chain, and the formation of newer molecules account for the enhancement of the thermal diffusivity revealed through the MMBDTL techniques. The complementary nature of the radiative and nonradiative emission is understood from the fluorescence study. Thus, the study reveals the possibility of thermal diffusivity measurement as an effective tool for the quality monitoring of engine oil.

Pressure influence on excitonic luminescence of CsPbBr3 perovskite.

This study investigates the effect of hydrostatic pressure on the luminescence properties of CsPbBr3 single crystals at 12 K. The luminescence at the edge of the band gap reveals a structure attributed to free excitons, phonon replica of the free excitons, and Rashba excitons. Changes in the relative intensity of the free and Rashba excitons were observed with increasing pressure, caused by changes in the probability of nonradiative deexcitation. At pressures around 3 GPa, luminescence completely fades away. The red shift of the energy position of the maximum luminescence of free and Rashba excitons in pressure ranges of 0-1.3 GPa is attributed to the length reduction of Pb-Br bonds in [PbBr6]4- octahedra, while the high-energy shift of the Rashba excitons at pressures above 1.3 GPa is due to [PbBr6]4- octahedra rotation and changes in the Pb-Br-Pb angle.

PH assisted modulation in the binding affinity for BODIPY-benzimidazole conjugate with anionic cyclodextrin

Modulation in the properties of chromophoric guest dyes involving non-covalent supramolecular interactions of various macrocyclic hosts are the intense topics of current research. In this study we report interaction of a sulfobutylether-β-cyclodextrin (SBEβCD) host in modulating the photophysical as well as prototropic properties of an inhouse synthesized BODIPY-benzimidazole conjugate dye, abbreviated as BDZ. The SBEβCD host displays substantially different binding strengths for the protonated and unprotonated forms of the prototropic BDZ dye, inducing a large upward pKa shift for the dye. Furthermore, structural confinement of the dye upon binding with the host leads to substantial enhancement in the fluorescence intensity and lifetime for both the prototropic forms of the dye. This can be attributted to the reduction in the non-radiative deexcitation process for the bound dye. Fluorescence enhancements are also accompanied with large increase in fluorescence anisotropy decay times, substantiating dye-host binding. Since unprotonated BDZ in free state is extremely weak in emission due to its intramolecular charge transfer (ICT) character, its fluorescence enhancement on binding to SBEβCD is found to be more pronounced than protonated BDZ. Importantly, emission of BDZ–SBEβCD system appears in the green region of visible spectrum, implying the immense prospect of the system for fluorescence-based sensor and imaging applications in different biological studies.

Supramolecular modulation in photophysical features of berberine and its application towards ATP sensing

In this study we report supramolecular interaction of the cationic dye berberine (BBR) with the highly substituted polyanionic cyclodextrin derivative, sulfobutylether beta cyclodextrin (SBE10βCD). Though interaction of BBR with parent β-cyclodextrin (βCD) host is very weak, its interaction with SBE10βCD host is significantly strong with an association constant Ka as large as ∼2.67 × 105 M−1. Strong binding of BBR with SBE10βCD is governed by cooperative effect of electrostatic attraction and enhanced hydrophobic interaction caused by extended SBE10βCD cavity for bound BBR dye. In the BBR-SBE10βCD complex, lower micropolarity and rigid microenvironment of host cavity cause a large decrease both in structural agility and intramolecular charge transfer (ICT) state formation of bound BBR. Accordingly, the non-radiative de-excitation of excited BBR in the dye-host complex is reduced significantly, causing its emission intensity to increase quite largely. The BBR-SBE10βCD complex is found to be highly sensitive towards ionic strength of the medium. This property is further explored to form a ternary complex, BBR-SBE10βCD-Cd2+, with largely reduced fluorescence intensity. Interestingly, this ternary complex exhibits selective fluorescence turn-on in response to an important bio-analyte, adenosine triphosphate (ATP). Present study, thus, demonstrates a very strong supramolecular complex, BBR-SBE10βCD, which is found to be very useful in the ATP detection in the presence of Cd2+, involving the ATP responsive fluorescence.

Photoluminescence of the Eu3+-Activated YxLu1−xNbO4 (x = 0, 0.25, 0.5, 0.75, 1) Solid-Solution Phosphors

Eu3+-doped YxLu1−xNbO4 (x = 0, 0.25, 0.5, 0.75, 1) were prepared by the solid-state reaction method. YNbO4:Eu3+ and LuNbO4:Eu3+ crystallize as beta-Fergusonite (SG no. 15) in 1–10 μm diameter particles. Photoluminescence emission spectra show a slight linear variation of emission energies and intensities with the solid-solution composition in terms of Y/Lu content. The energy difference between Stark sublevels of 5D0→7F1 emission increases, while the asymmetry ratio decreases with the composition. From the dispersion relations of pure YNbO4 and LuNbO4, the refractive index values for each concentration and emission wavelength are estimated. The Ω2 Judd–Ofelt parameter shows a linear increase from 6.75 to 7.48 × 10−20 cm2 from x = 0 to 1, respectively, and Ω4 from 2.69 to 2.95 × 10−20 cm2. The lowest non-radiative deexcitation rate was observed with x = 1, and thus LuNbO4:Eu3+ is more efficient phosphor than YNbO4:Eu3+.

Lanthanide doped nanoheaters with reliable and absolute temperature feedback

The development of selective and controlled photo-thermal therapies requires luminescent nanoparticles capable of simultaneous heating and contactless thermal sensing. Until now, thermal therapies have suffered from a lack of control over the absolute temperature of the treated tissue because the nanothermometers used for thermal feedback, based on a spectral analysis of emitted radiation, were affected by the inhomogeneous extinction of the tissues. This work shows how this deficiency can be overcome by using core-shell-shell nanostructures doped with lanthanide ions (Nd3+ and Yb3+). Thermal reading was achieved from the analysis of the Yb3+ luminescence lifetime whereas simultaneous heating was achieved thanks to the non-radiative deexcitations of Nd3+ ions. Simple proof-of-concept experiments show the great potential of these lanthanide-doped nanostructures for the development of in vivo photo-thermal treatments with absolute and reliable thermal feedback.

Molecular collisions or resonance energy transfer in lipid vesicles? A methodology to tackle this question

In this work, molecular interactions in a lipid membrane are discussed through fluorescence spectroscopy data, both experimentally and theoretically. In particular, the fluorescence quenching mechanisms between the fluorescent probe 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan) and the potential drug 2-nitrobenzaldehyde-thiosemicarbazone (2-TSC) were studied, both inserted in a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DMPC) model membrane. The fluorescence intensity and the lifetime of Laurdan decrease dramatically in the presence of 2-TSC, in both gel and fluid phases of the DMPC bilayer. It is shown here how to identify the correct quenching mechanism, by conducting a careful analysis of the fluorescence data. The analysis of the bimolecular constant values obtained through the Stern-Volmer equation, considering the collisional mechanism, made clear the incompatibility of the obtained values with estimated diffusion coefficients for Laurdan and 2-TSC inserted into lipid bilayers. On the other hand, using the Förster’s theory of resonance energy transfer (FRET) we obtained results in good agreement with the already known dynamic characteristics of a DMPC bilayer, at its both gel and fluid phases. Through spectroscopy data and computational calculation, Förster distance, energy transfer efficiency and distance distribution were obtained for the donor/acceptor pair Laurdan/2-TSC, at both gel and fluid phases of the bilayer. The distance distribution reflects the occurrence of FRET involving donor/acceptor pairs in the same leaflet of the lipid bilayer and pairs in opposite leaflet, and these results are in good agreement with our previous proposal about the lateral organization and position of Laurdan and 2-TSC molecules in a DMPC bilayer. All these results lead us to conclude that FRET between the donor Laurdan and the acceptor 2-TSC is the mechanism responsible for non-radiative deexcitation of Laurdan. The methodology used here could be extended to other pairs of donor/acceptor molecules, to contribute to the knowledge about their localizations in lipid membranes.

Stimuli Responsive Confinement of a Molecular Rotor Based BODIPY Dye inside a Cucurbit[7]uril Nanocavity.

Present study reports the interaction of a molecular rotor based BODIPY dye, 8-anilino-BODIPY (ABP), with a versatile macrocyclic molecule, cucurbit[7]uril (CB7), investigated through various techniques such as ground-state absorption, steady-state fluorescence, time-resolve emission, proton NMR, and quantum chemical studies. Although BODIPY dyes have widespread applications due to their intriguing photochemical properties, studies on their noncovalent interactions with different macrocyclic hosts, especially regarding their supramolecularly induced modulations in photophysical properties are very limited. The investigated BODIPY dye, especially its protonated ABPH+ form (pH ∼ 1), shows a large fluorescence enhancement on its interaction with the CB7 host, due to large reduction in the structural flexibility for the bound dye, causing a suppression in its nonradiative de-excitation process in the excited state. Unlike ABPH+, the neutral ABP form (pH ∼ 7) shows considerably weaker interaction with CB7. For ABPH+-CB7 system, observed photophysical results indicate formation of both 1:1 and 1:2 dye-to-host complexes. Plausible geometries of these complexes are obtained from quantum chemical studies which are substantiated nicely from 1H NMR results. The response of the ABPH+-CB7 system toward changing temperature of the solution have also been investigated elaborately to understand the potential of the system in different stimuli-responsive sensor applications.

Controlling Photosynthetic Light Harvesting at the Single Protein Level

The light-harvesting complexes of oxygenic photosynthetic organisms have to balance two vital functions: efficient transport of photoexcitations to the photochemical reaction center under low levels of solar radiation and efficient photoprotection under intense solar radiation and other stress conditions, during which photoexcitations are rapidly dissipated as heat. Most light-harvesting proteins bind a dense arrangement of chromophores, ensuring that subtle protein conformational changes are often sufficient to switch between the complexes’ light-harvesting and photoprotective functions. Single-molecule spectroscopy allows one to follow these switches in real time. I will show how this technique has led to the discovery of new light-harvesting and photoprotective states in various photosynthetic light-harvesting complexes and how the population equilibrium between these states can be finely tuned at the single protein level by the incident photon flux, the pH, the chromophore arrangement, and by controlling the interaction with small, accessory proteins. I will also demonstrate how the light-harvesting efficiency of these complexes can be actively controlled through interactions with localized surface plasmons from metallic nanostructures to dramatically alter the radiative and nonradiative deexcitation rates.

Warm white broadband emission and tunable long lifetimes in Yb3+ doped Gd2O3 nanoparticles

Upconversion luminescence materials have stimulated growing attention in photovoltaic cell owning to the ability of converting a few low-energy photons into a single high-energy photon. However, there still remains a daunting challenge to achieve upconversion emission covering the entire visible spectrum in a single material. Here, an incandescent broadband white upconversion emission covering the entire visible region is realized in a single Gd2O3:Yb3+ nanoparticles by utilization of adverse multiphonon relaxation. Importantly, tunable long fluorescence lifetime can be synchronously implemented in the single material, where the lifetime range spans one order of magnitude. Furthermore, benefiting from the broadband emission, the luminescence intensity achieves 482-fold improvement as the power increase from 0.53 W to 1.51 W. These broadband emissions are enabled by engineering the pathways of nonradiative de-excitation via multiphonon relaxation. Based on the kinetic analysis and mechanism research, the photon avalanche is demonstrated to play an important role for broadband emission. The warm white emission together with its tunable long lifetime provides a new avenue for developing single emitter-based white-light-emission materials for next-generation display and lighting technologies.

Photo-to-thermal conversion: effective utilization of futile solid-state carbon quantum dots (CQDs) for energy harvesting applications

Dominant non-radiative electronic de-excitation in fluorescence quenched CQDs in the solid state results in excellent photo-to-thermal transduction in the system.

Effect of doping mechanism on photogenerated carriers behavior in Cu-doped ZnSe/ZnS/L-Cys core-shell quantum dots

Cu-doped ZnSe/ZnS/L-Cys core–shell QDs are prepared by both nucleation doping and growth doping in an aqueous synthesis method. Transport of photogenerated free charge carriers (FCCs) in these Cu-doped QDs is probed via a combination of surface photovoltaic (SPV), photoacoustic (PA), and electric-field-induced SPV techniques, supplemented by the UV–VIS absorption spectrum and Raman spectrum. The results confirm that the two doping mechanisms result in different doping locations and microelectronic structures of the Cu-doped QDs. The distinctive microelectronic structure of the QDs prepared by nucleation doping, as compared with those prepared by growth doping, results in a number of favorable SPV characteristics. For example, the QDs prepared by nucleation doping exhibit a higher SPV response intensity at 600 nm because of a higher concentration of photogenerated FCCs. The ratio of the strongest SPV response and the strongest PA signal of the QDs prepared by nucleation doping is up to 2.41 times greater than those of the QDs prepared by growth doping. This is because the greater numbers of photogenerated FCCs in the QDs prepared by nucleation doping generate the PV effect rather than the PA effect that is caused by a nonradiative de-excitation process. The position of the shoulder peak of the SPV response at a long wavelength of the QDs prepared by nucleation doping is significantly red-shifted compared with that of the QDs prepared by growth doping, leading to a broader SPV response range in the visible region. The QDs prepared by nucleation doping have a more obvious donor feature than those prepared by growth doping.

Nonradiative Excited-State Decay via Conical Intersection in Graphene Nanostructures.

Chemical groups are known to tune the luminescent efficiencies of graphene-related nanomaterials, but some species, including the epoxide group (-COC-), are suspected to act as emission-quenching sites. Herein, by performing nonadiabatic excited-state dynamics simulations, we reveal a fast (within 300 fs) nonradiative excited-state decay of a graphene epoxide nanostructure from the lowest excited singlet (S1 ) state to the ground (S0 ) state via a conical intersection (CI), at which the energy difference between the S1 and S0 states is approximately zero. This CI is induced after breaking one C-O bond at the -COC- moiety during excited-state structural relaxation. This study ascertains the role of epoxide groups in inducing the nonradiative recombination of the excited electron-hole, providing important insights into the CI-promoted nonradiative de-excitations and the luminescence tuning of relevant materials. In addition, it shows the feasibility of utilizing nonadiabatic excited-state dynamics simulations to investigate the photophysical processes of the excited states of graphene nanomaterials.

Effect of Al co-doping on the luminescence properties of Nd3+-doped TiO2 thin films

Co-doping Aluminium (Al) with neodymium (Nd) is shown to have a sensitizing effect on the rare earth luminescence in thin film titanium dioxide (TiO2) host materials. Whilst the Al co-doping has the effect of quenching the direct (resonantly) excited, steady state photoluminescence from Nd3+ emitting centers, for indirect (non-resonant) excitation, the emission is enhanced with increasing Al. Time-resolved PL measurements reveal a decrease in both the excitation/de-excitation rates (increases in lifetime) with increasing Al co-doping, confirming the indirect nature of excitation and implying a suppression of non-radiative de-excitation of the Nd3+ centers in TiO2. Suppression of non-radiative processes by Al is considered to be the result of reduced rare earth clustering, which is known to promote de-excitation via energy migration and thus has important implications for improved efficiency infra-red LED materials.

Pharmacological application of the thermal lens technique: a thermal diffusivity study

The photothermal phenomenon has emerged as a potential tool for the nondestructive evaluation of thermal and optical properties of materials. Thermal analysis of drugs is an unavoidable part of preformulation study, as temperature variations can induce structural changes of the constituents of drugs. Techniques based on photothermal phenomena are highly sensitive, as only the absorbed radiation contributes to the signal. Periodic illumination and subsequent nonradiative de-excitation generate thermal lens signals of various types within and around the sample. Variation of thermal diffusivity with a concentration of the commonly used drug terbutaline is studied through the single-beam thermal lens technique. The ultraviolet–visible spectrum of the drug shows strong absorption around 500 nm, which suggests the possible wavelengths that can be used for the study. It is found that concentration of the drug in liquid form decides its thermal stability, as its thermal diffusivity varies with concentration. The study gives information about the optimum value for the concentration of the drug noted above for which the chance of thermal stability is high.

Three- and Two-Photon NIR-to-Vis (Yb,Er) Upconversion from ALD/MLD-Fabricated Molecular Hybrid Thin Films.

We report blue, green, and red upconversion emissions with strongly angular-dependent intensities for a new type of hybrid (Y,Yb,Er)–pyrazine thin films realized using the atomic/molecular layer deposition thin-film fabrication technology. The luminescence emissions in our amorphous (Y,Yb,Er)–pyrazine thin films of a controllable nanothickness originate from three- and two-photon NIR-to-vis excitation processes. In addition to shielding the lanthanide ions from nonradiative de-excitation, the network of interconnected organic molecules serves as an excellent matrix for the Yb3+-to-Er3+ excitation energy transfer. This suggests a new approach to achieve efficient upconverting molecular materials with the potential to be used for next-generation medical diagnostics, waveguides, and surface-sensitive detectors.

Temperature quenching in LAB based liquid scintillator

The effect of temperature changes on the light output of LAB based liquid scintillator is investigated in a range from -5 to 30,^{circ }C with alpha -particles and electrons in a small scale setup. Two PMTs observe the scintillator liquid inside a cylindrically shaped aluminum cuvette that is heated or cooled and the temperature dependent PMT sensitivity is monitored and corrected. The alpha -emitting isotopes in dissolved radon gas and in natural Samarium (bound to a LAB solution) excite the liquid scintillator mixtures and changes in light output with temperature variation are observed by fitting light output spectra. Furthermore, also changes in light output by compton electrons, which are generated from external calibration gamma -ray sources, is analysed with varying temperature. Assuming a linear behaviour, a combined negative temperature coefficient of {(-0.29 pm 0.01)}{,%/^{circ }}hbox {C} is found. Considering hints for a particle type dependency, electrons show {(-0.17 pm 0.02)}{,%/^{circ }}hbox {C}, whereas the temperature dependency seems stronger for alpha -particles, with {(-0.35 pm 0.03)}{,%/^{circ }}hbox {C}. Due to a high sampling rate, a pulse shape analysis can be performed and shows an enhanced slow decay component at lower temperatures, pointing to reduced non-radiative triplet state de-excitations.

Bright Photon Upconversion on Composite Organic Lanthanide Molecules through Localized Thermal Radiation.

Converting low-energy photons via thermal radiation can be a potential approach for utilizing infrared (IR) photons to improve photovoltaic efficiency. Lanthanide-containing materials have achieved great progress in IR-to-visible photon upconversion (UC). Herein, we first report bright photon, tunable wavelength UC through localized thermal radiation at the molecular scale with low excitation power density (<10 W/cm2) realized on lanthanide complexes of perfluorinated organic ligands. This is enabled by engineering the pathways of nonradiative de-excitation and energy transfer in a composite of ytterbium and terbium perfluoroimidodiphosphinates. The IR-excited thermal UC and wavelength control is realized through the terbium activators sensitized by the ytterbium sensitizers having high luminescence efficiency. The metallic molecular composite thus can be a potential energy material in the use of the IR solar spectrum for thermal photovoltaic applications.