Nonradiative Decay Rate Research Articles

Influence of modifier oxides on spectroscopic features of Nd2O3 doped PbO-Ro2O3–WO3–B2O3 glasses (with Ro2O3 = Sb2O3, Al2O3, and Bi2O3)

The influence of modifier oxides (Ro2O3=Sb2O3, Al2O3, and Bi2O3) on the spectral analysis of PbO-Ro2O3–WO3–B2O3 glasses doped with Nd3+ ions is investigated in this work. Samples were prepared by the melt quench method, and their optical, luminescence, and physical properties were examined. From the measured properties, it is observed that the density and refractive index of the studied glasses vary non-linearly with the concentration of Nd2O3, leading to the formation of cross-linked networks under the polaraisability of the oxide glasses. The optical band gap (E0) and Urbach energy (ΔE) was determined by using UV–visible spectra; it also varies non-linearly based on the modifier oxides and the formation of non-bridging oxygen ions. The oscillator strengths of the absorption bands were determined by the Judd-Ofelt model, and the three intensity parameters Ω2, Ω4, and Ω6 were calculated for each glass. From these parameters, the transition probability (Aτ), radiative life time (), branching ratio (βr), quantum efficiency (η), and non-radiative decay rate (WNR) were calculated. From these results, it can be concluded that the studied glasses can be used for laser and optical applications.

A study on the modification of azole rings to regulate the transition dipole moment, MLCT and T1 structural distortion of 2-pyridyl-azole copper (I) complexes for high phosphorescence performance

Heterocyclic ligands based on 2-pyridyl-azole, have become some of the ideal ligands for metal complexes due to their flexible structure and wide wavelength tunability. In this paper, a series of Cu(N∧N) (P∧P) complexes are studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The results indicate that the combination of incorporation of N atoms and substitution of electron donor-acceptor groups can effectively control the type and the proportion of the lowest triplet states (T1) phosphorescent transition. A higher matching degree between the effective spin-orbit coupling (SOC) matrix elements and the transition dipole moments (The Sn state has both effective SOC and large transition dipole moments of Sn→S0 transition) brings about a larger transition dipole moment MTa of T1a→S0 transition, and further results in the fastest radiative decay rate (kr) for complex 2, although the main transition of which is internal-ligand charge transfer (ILCT). In addition, the non-radiative decay rate (knr) of complex 2 is lower than other complexes of series 3 (the reorganization energy produced by in-plane bending vibration of C–H bond in six-membered ring is larger than complex 2), so this complex has the highest quantum yield. Overall, the above analyses illustrate that despite the participation of Cu(I) in transition as an indispensable factor for the radiative transition, the higher component ratio does not necessarily lead to a faster kr. In addition, it is also essential to have a well-matched large transition dipole moments of the singlet states Sn→S0 transition involved in the effective SOC. Consequently, in the development of the 2-pyridyl-azole Cu(I) luminescent materials, the high quantum yield materials can be designed by adjusting the substituent at C3 position and the strength of electron donor-acceptor groups at C5 position to balance the charge transfer transition modes.

Interplay of Radiative and Non Radiative Rate Constants in the Photophysics of Fluorescent Proteins

Genetically encodable fluorophores like Fluorescent Proteins (FPs) have been engineered to provide better contrast, localization and specificity. Red-FPs (RFPs) are known to have an extended chromophore through an acylimine moiety with respect to their blue shifted and generally brighter counterparts. The molecular brightness is based on the product of the extinction coefficient (EC) and fluorescence quantum yield (QY). Development of brighter FPs has focused on increasing the QY, which is directly proportional to fluorescence lifetime (FL), a time constant corresponding to the inverse sum of the ensemble radiative and non-radiative decay rate constants. We have performed directed evolution of RFPs through random and site-directed mutagenesis on a multi-parametric microfluidic sorting system capable of FL and brightness selections. We also applied site directed mutagenesis on RFPs based on structural analysis to investigate the structural effects on FL and QY. The selected mutants derived from mCherry and FusionRed show that the improvement in FL brings about an expected linear increase in QY (∼3.2 fold for mCherry and ∼2.3 fold for FusionRed). This can be attributed to significant decrease in the non-radiative rate constant, and a modest improvement in radiative rate constant that is supported by hypsochromic shifts and changes in absorption profiles. Both approaches reveal that the improvement in molecular brightness with increased FL is mainly governed by a decrease in the non-radiative rate constant in conjunction with improved absorption (narrower absorption spectrum and higher ECmax). This strategy forms the basis of the best photophysical route to engineer brighter RFPs.

Insights on aggregation induced room temperature phosphorescence properties: A QM/MM study

Organic room temperature phosphorescence (RTP) molecules have attracted significant attention recently due to their promising application in security, bioimaging, sensing and organic light emitting diodes (OLEDs). Nevertheless, limited RTP molecules were reported until now and most of RTP phenomena were observed in crystal. Moreover, the underlying mechanisms of RTP still remain ambiguous. In this paper, excited-state properties of CZs-CN with RTP features in three crystals are theoretically studied using the combined quantum mechanics and molecular mechanics (QM/MM) method. Moreover, the photophysical properties of CZs-CN in solvent are also investigated by polarizable continuum model (PCM). The mechanism of aggregation induced RTP is revealed which is mainly contributed by the enhanced phosphorescence rates in aggregation, rather than the traditional restricted intramolecular motion mechanisms which usually bring decreased non-radiative decay rates. In addition, different crystal structures could induce different phosphorescence properties, and the emission spectra of the molecule in three crystals and THF are explained reasonable. Furthermore, dimers are confirmed to be involved in the emission when the intermolecular interaction is strong enough in aggregation. Our theoretical results provide inner perspective for aggregation induced RTP mechanism and could help one to better understand the different light-emitting properties in aggregation and solvent.

Design of Schottky barrier for plasmon-induced hot-electron passivation of defect environments of semiconductor quantum dots

Metal oxide plasmonic metafilms consisting of a Au/Si Schottky barrier in close vicinity of a Si/Al oxide charge barrier can suppress defect-induced non-radiative decay rates of semiconductor quantum dots, enhancing their emission efficiency beyond what the near field enhancement of metallic nanostructures (Purcell effect) can offer (Sadeghi et al 2017 Nanotechnology 29 015402). In this paper we study the impact of the efficiency of the hot electron transfer across the Schottky barrier on such plasmon-induced suppression of the impact of the defect environments. For this the emission intensity and dynamics of quantum dots on such metafilms are studied as the structural features of the Schottky barrier are controlled. We consider the Si layer that separates the Schottky barrier from the charge barrier is either intrinsically undoped, p-doped, or n-doped. Our results show the metafilms with n-type Si can elongate the emission lifetime of the quantum dots the most, suggesting a superior quarantine of excitons against the defect environments. This highlights the fact that n-type Schottky barriers can more efficiently capture the hot electrons generated via non-radiative decay of plasmons. This allows an improved plasmon-induced screening of the excitons against the defect environment.

Morphology and doping concentration effect on the luminescence properties of SnO2:Eu3+ nanoparticles

Morphology and Eu3+ doping effect on structural and photoluminescence properties of tin dioxide nanoparticles obtained by co-precipitation and hydrothermal methods are reported and analyzed for the first time. The samples were characterized by means of transmission electron microscopy (TEM), powder X-ray diffraction (XRD), specific surface area (SSA) estimation. TEM, XRD and SSA analyses showed that in the case of co-precipitation method the nanoparticles were spherical. Hydrothermal treatment leads to formation of cubic nanoparticles. An average particle size increased from 3 to 5 nm and from 6 to 11 nm along with increase of Eu3+ concentration for spherical and cubic nanoparticles, respectively. Steady-state and kinetics photoluminescence properties of nanophophors with different morphology were studied and compared. Radiative and nonradiative decay rates and Judd-Ofelt parameters were calculated using the model of 4f–4f intensity theory. It was shown, that preferred positions of substitution in SnO2 host differ depending on Eu3+ doping concentration and particle morphology.

A highly emissive AIE-active luminophore exhibiting deep-red to near-infrared piezochromism and high-quality lasing.

Further development of high-efficiency and low-cost organic fluorescent materials is intrinsically hampered by the energy gap law and spin statistics, especially in the near-infrared (NIR) region. Here we design a novel building block with aggregation-induced emission (AIE) activity for realizing highly efficient luminophores covering the deep-red and NIR region, which originates from an increase in the orbital overlap and electron-withdrawing ability. An organic donor–acceptor molecule (BPMT) with the building block is prepared and can readily form J-type molecular columns with multiple C–H⋯N/O interactions. Notably, such synthesized materials can emit fluorescence centered at 701 nm with extremely high photoluminescence quantum yields (PLQYs) of 48.7%. Experimental and theoretical investigations reveal that the formation of the hybridized local and charge-transfer (HLCT) state and substantial C–H⋯N/O interactions contribute to a fast radiative decay rate and a slow nonradiative decay rate, respectively, resulting in high PLQYs in the solid state covering the NIR range. Remarkably, such BPMT crystals, as a first example, reveal strong-penetrability piezochromism along with a distinct PL change from the deep-red (λmax = 704 nm) to NIR (λmax = 821 nm) region. Moreover, such typical AIE-active luminophores are demonstrated to be a good candidate as a lasing medium. Together with epoxy resin by a self-assembly method, a microlaser is successfully illustrated with a lasing wavelength of 735.2 nm at a threshold of 22.3 kW cm−2. These results provide a promising approach to extend the contents of deep-red/NIR luminophores and open a new avenue to enable applications ranging from chemical sensing to lasing.

Solid-State Fluorescence Enhancement of Bromine-Substituted Trans-Enaminone Derivatives

Halogen bonding, as a kind of intermolecular interaction, has rarely been used to tune solid-state emission properties of luminescent materials, especially fluorescent materials. Herein, three trans-enaminone (TE) derivatives (nonbrominated TE, monobrominated BrTE, and tribrominated Br3TE) with aggregation-induced emission property have been designed and synthesized. Two types of BrTE crystals (BrTE-B and BrTE-G) with different fluorescence properties were obtained. It was observed that their solid-state fluorescence has been enhanced by the formation of halogen bonding. In particular, the crystal BrTE-G containing Br…π interactions exhibits a fluorescence quantum yield (9.6%) nearly sevenfold higher than BrTE-B, the crystal without halogen bonding (1.4%), and fivefold higher than the nonbrominated TE derivative (2.1%). By careful inspection of the single-crystal data and theoretical calculations, the high fluorescence quantum yield of BrTE-G appears to be due to halogen-bonding interactions as well as multiple stronger intermolecular interactions which may restrain molecular motions, leading to the reduced nonradiative decay rate and the enhanced radiative decay rate. Additionally, increasing the number of bromine substituents may further promote the radiative decay rate, explaining therefore the higher fluorescence quantum yield (12.5%) of Br3TE.

Aggregation of Metal-Nanoparticle-Induced Fluorescence Enhancement and Its Application in Sensing.

Fluorescence-based detection methods have been widely utilized in various applications. Materials that display aggregation-induced emission (AIE) are excellent fluorescence probes to offer high contrast ratio. Chromophore-conjugated plasmonic metal nanoparticles (NPs) have been recently found to display significantly enhanced fluorescence emission upon the formation of aggregates. This new type of AIE enhancement has a totally different working mechanism. It is based on aggregation-induced plasmon coupling of metal NPs to enhance the fluorescence intensity of chromophores by increasing both the excitation efficiency and radiative decay rates, instead of reducing nonradiative decay rates as in typical AIE. AIE enhancement of chromophore-conjugated metal NPs results in a dramatic change in fluorescence intensity from severely quenched fluorescence to significantly enhanced fluorescence upon aggregate formation. It offers excellent contrast ratio and is attractive for developing platforms for highly sensitive sensing and imaging applications with reduced background. This mini-review summarizes the basic working principle and recent progress in fluorescence enhancement by coupled metal NPs on the single-molecule level, aggregation-induced plasmon coupling enhanced fluorescence of chromophore-conjugated metal NPs, and their applications in sensing. Perspectives on further utilization of this interesting phenomenon for various biomedical applications have also been discussed.

Aggregation-induced fluorescence of the luminol-terbium(III) complex in polymer nanoparticles for sensitive determination of thrombin.

A fluorometric method is described for the determination of thrombin. Polymer nanoparticles containing the luminol-terbium(III) complex (luminol-Tb) were prepared where luminol acts as the bridging ligand, and Tb(III) acts as the central metal ion. Thrombin possesses a large number of electrons donating groups that coordinate with luminol-Tb. Following coordination, the rigidity of the linker is increased, and this decreases the non-radiative decay rate and induces an increase in fluorescence intensity at 430nm. Hence, thrombin can be fluorometrically determined. The detection limit of thrombin is as low as3.5 pM (at an SNR of 3). This is about 10 times lower than assays using an aptamer. The method was applied in the determination of thrombin in human serum via thestandard addition method and gave satisfying results. Graphical abstractSchematic representation of the preparation of the luminol-Tb(III) complex in a nanoparticle host by the self-assembly of luminol and Tb(III) ions. Thrombin readily coordinates with the luminol-Tb(III) system, and this results in particle aggregation. The blue fluorescence of luminol increases strongly, and this effect provides the basis for fluorometric determination of thrombin.

Primary amines enhance triplet energy transfer from both the band edge and trap state from CdSe nanocrystals.

In this study, the role that primary amines play during triplet energy transfer from photoexcited CdSe nanocrystals (NCs) was examined. Colloidally synthesized CdSe NCs were placed in varying concentrations of 1-propyl- or 1-octylamine, with and without 2-anthracenecarboxylic acid transmitter ligands attached. This primary amine increases upconversion quantum yield approximately 5-fold. Further addition of amine does not improve photon upconversion, as CdSe NC photoluminescence (PL) increases at the expense of triplet energy transfer. Transient absorption measurements show that the amine plays three key roles. Primary amines enhance NC PL by decreasing the nonradiative decay rate, increase the rate of triplet energy transfer, and enable the broad trap state in these CdSe NCs to participate in triplet photosensitization.

Hydrogen Bonding-Induced Morphology Dependence of Long-Lived Organic Room-Temperature Phosphorescence: A Computational Study.

Organic room-temperature phosphorescence (RTP) is generally only exhibited in aggregate with strong dependence on morphology, which is highly sensitive to the intermolecular hydrogen bonding interaction. Here, 4,4'-bis(9H-carbazol-9-yl)methanone (Cz2BP), emitting RTP in a cocrystal consisting of chloroform but not in the amorphous nor in the crystal phase, was investigated to disclose the morphology dependence through molecular dynamics simulations and first-principles calculations. We find that the strong intermolecular C═O···H-C hydrogen bonds between Cz2BP and chloroform in cocrystals decrease the nonradiative decay rate of T1 → S0 by 3-6 orders of magnitude due to the vibronic decoupling effect on the C═O stretching motion and the increase of (π,π*) composition in the T1 state. The former is responsible for high efficiency and the latter for long-lived RTP with a calculated lifetime of 208 ms (exp. 353 ms). Nevertheless, the weak hydrogen bonds cannot cause any appreciable RTP in amorphous and crystal phases. This novel understanding opens a way to design organic RTP materials.

Effects of β-bromine substitution and core protonation on photosensitizing properties of porphyrins: Long wavelength photosensitizers

In this study, a series of β-brominated meso-tetraphenylporphyrins, H2TPPBrx (x = 2, 4, 6 and 8) with remarkably red shifted absorption bands towards the longer wavelengths of the visible light known as therapeutic window and their diprotonated analogues were used as photosensitizers in the aerobic oxidation of cyclooctene to investigate the influence of degree of β bromination on the photocatalytic activity and oxidative stability of H2TPP. Furthermore, the β brominated porphyrins showed significantly decreased fluorescence quantum yields (φF) and radiative decay rate constants but increased non-radiative decay rate constants. The highest catalytic activity was observed in the case of the partially brominated porphyrins, H2TPPBr2 and H2TPPBr4. While, H2TPPBr2 and H2TPPBr4 were as stable as H2TPP, the oxidative degradation of H2TPPBr6 and H2TPPBr8 was remarkably higher than that of the non-brominated porphyrin. A good correlation was observed between the photocatalytic activity of H2TPPBrx and their diprotonated species and the singlet oxygen quantum yields (ϕΔ = 0.02–0.21 and 0.12 to 0.59, respectively), determined chemically through the reaction of singlet oxygen with 1,3-diphenylisobenzofuran (DPBF). In order to examine the effects of steric hindrance on the photocatalytic properties of the brominated porphyrins, β tetra-brominated derivative of meso-tetra(2-methylphenyl)porphyrin, H2T(2-Me)PPBr4 with bulky methyl substituents at the ortho positions was prepared and compared with H2TPPBr4. The steric hindrance caused by the presence of four methyl groups could improve the oxidative stability of the photosensitizer by 30 to 50% at the cost of significant loss of photocatalytic activity. On the other hand, diprotonation of H2TPPBrx with weak and strong acids (CH2ClCOOH and CF3COOH) was found to be an efficient approach to overcome the extensive oxidative degradation of these photosensitizers, with concomitant red shift of the absorption bands to 451–490 and 674–743 nm, in the Soret and Q band regions, respectively.

Blue Phosphorescent trans-N-Heterocyclic Carbene Platinum Acetylides: Dependence on Energy Gap and Conformation.

A series of 11 complexes of the type trans-(NHC)2Pt(CC-Ar)2 (where NHC = N-heterocyclic carbene) have been synthesized and their photophysics characterized. The complexes display moderately efficient deep blue to green phosphorescence from a triplet excited state that is localized mainly in the aryl acetylide ligand (CC-Ar). The emission energy varies with the substituent on CC-Ar, with the highest energy emission for Ar = 4-pyridyl. The emission quantum efficiency and lifetime for the series decreases with increasing emission energy (Eem), and the effect is identified as arising from an increase in the nonradiative decay rate (knr) with Eem. Temperature-dependent emission lifetime studies for three complexes give activation energies for the nonradiative decay process ∼1000 cm-1, and the thermally activated decay process is attributed to crossing to a nonemissive metal-centered (d-d) excited state. At a low temperature, two different emission progressions are observed. Density functional theory calculations suggest that the triplet energy varies with the torsion of the aryl acetylide rings relative to the plane defined by the PtC4 unit (where C = the carbon atoms bonded to Pt). The multiple emission is ascribed to emission from complexes differing with respect to the aryl acetylide ring torsion. Ultrafast transient absorption spectroscopy reveals a fast relaxation (∼5 ps) that may also be due to aryl acetylide ring torsional relaxation in the triplet excited state.

Modifying the Nonradiative Decay Dynamics through Conical Intersections via Collective Coupling to a Cavity Mode.

The coupling of a molecular ensemble to the confined electromagnetic modes of a microcavity can strongly modify the photophysics and photochemistry of the molecules upon photoexcitation. We investigate here how collective coupling effects lead to modifications of the mechanisms and rates of photochemical processes, in particular, photodissociation and nonradiative decay in NaI and pyrazine, respectively. We show that, after direct excitation into the lower polaritonic states, the lower-energy light-matter hybrid states, the dynamics of the molecular ensemble coupled to light is very similar to the dynamics of the corresponding isolated molecules. Conversely, excitation into the upper polaritonic states results in more complex dynamics that involve as a first step the population transfer toward the manifold of intermediate dark states. These dynamics differ substantially from those of the isolated molecules and may result in measurable time delays for nonradiative decay or excited-state reaction mechanisms. Similarly, we describe how addition of a buffer of nonreactive molecules coupled to the cavity mode can be used to delay the onset of the photochemical processes of the reactive part of the ensemble, where the buffer medium is more effective in inhibiting the reactive process than only reactive molecules in the cavity.

Some recent advances in ab initio calculations of nonradiative decay rates of point defects in semiconductors

In this short review, we discuss a few recent advances in calculating the nonradiative decay rates for point defects in semiconductors. We briefly review the debates and connections of using different formalisms to calculate the multi-phonon processes. We connect Dr. Huang’s formula with Marcus theory formula in the high temperature limit, and point out that Huang’s formula provide an analytical expression for the phonon induced electron coupling constant in the Marcus theory formula. We also discussed the validity of 1D formula in dealing with the electron transition processes, and practical ways to correct the anharmonic effects.

A theoretical study on spectroscopic properties and quantum yields of chiral-at-metal cyclometalated Pt(II) complexes

A new kind of chiral-at-metal Pt(II) complexes have been explored with theoretical methods. These chiral complexes are synthesized with achiral chelate ligands and the platinum(II) ions serve as the stereo center. To explore the intersystem crossing (ISC) and radiative decay process, we investigated the absorption and phosphorescence property, spin-orbit coupling (SOC) matrix element, major intersystem crossing channel and phosphorescent transition rate. For the temperature-dependent nonradiative decay process, the metal-centered excited state (3MC), the transition state (TS), and the minimum energy crossing point (MECP) were investigated with theoretical calculation. The way of temperature-independent nonradiative decay which has weak influence on nonradiative decay rate was also compared through the calculation of reorganization energy (λ) between the ground state (S0) and the lowest triplet excited state(T1).

Enhancement of sub-bandgap light absorption in perovskite semiconductor films via critical coupling.

Light absorption in semiconductors is a fundamental problem that has broad impact on a wide range of fields. However, it is intrinsically limited by the bandgap energy of the semiconductor. Herein, we study the enhancement of sub-bandgap light absorption in inorganic-organic hybrid perovskite semiconductor films via critical coupling. This is achieved at large incidence angles by balancing radiative and nonradiative decay rates in a planar multilayer structure. We found that a very small loss in the semiconductor layer can result in substantial light absorption. This simple but general method can be used to enhance the optical and optoelectronic responses of semiconductors below the bandgap energy.

Molecular aggregation stimulated tunable emission behaviour of functionalized 1,8-naphthalimides

The integrated influence of linkers and solvent polarity on tuning the emission of 1,8-naphthalimide (NI) aggregates is investigated. Naphthalimides are systematically functionalized with vinyl, ethynyl and direct link connecting conjugated aromatic units via Pd-catalyzed cross coupling reactions. The molecules self-assembled as nano-aggregates in binary solvent system and exhibited aggregation induced enhanced emission. Field emission scanning electron microscopy and dynamic light scattering analysis proved the well-defined nanostructured morphology of the NI aggregates. Fluorescence life-time studies demonstrated the effect of aggregation on their radiative and non-radiative decay rates. Among the compounds synthesized, ethynyl linked NI exhibited significantly enhanced emission due to the limited intermolecular motion. A binary solvent system comprising of tetrahydrofuran/H2O facilitated enhanced emission and higher quantum yield of the NI in their aggregated phase. Detailed photophysical and computational studies of the compounds indicated that structural variation and solvent polarity together have a significant impact on tuning the emission behaviour of NIs.

Photophysical Behavior and Fluorescence Quenching of l-Tryptophan in Choline Chloride-Based Deep Eutectic Solvents.

Intrinsic fluorescence from l-tryptophan (l-Trp) is routinely used to obtain insight into the structural features and dynamics of proteins and enzymes. In contrast to aqueous enzymology, different parameters that control and influence the behavior of proteins and enzymes in nonaqueous media depend heavily on the solvent. Detailed analysis of the intrinsic fluorescence from l-Trp dissolved in two deep eutectic solvents (DESs), reline and glyceline, prepared by mixing salt choline chloride with H-bond donors urea and glycerol, respectively, in a 1:2 molar ratio within 298.15-358.15 K temperature range, is presented. Fluorescence emission maxima of l-Trp dissolved in DESs show bathochromic shift with increasing temperature. In comparison to water and several organic solvents, the fluorescence quantum yields of l-Trp in both DESs are significantly higher. While the rates of nonradiative decay in the two DESs are comparable and increase with increasing temperature, radiative decay rates are independent of temperature and are higher in glyceline than in reline, resulting in a higher fluorescence quantum yield of l-Trp in glyceline. Excited-state emission intensity decays of l-Trp fit best to a double exponential model irrespective of the identity of the DES and temperature. Average lifetime decreases with increasing temperature due to increased thermal deactivation; however, this decrease is much slower in DESs as compared to that in water. Both steady-state fluorescence anisotropy and rotational reorientation times for l-Trp are governed by the inherent complexity of the DESs as solubilizing milieu resulting in noncompliance to simple hydrodynamic treatment. Fluorescence quenching of l-Trp by acrylamide in reline is purely dynamic in nature. This is in contrast to the aqueous media where the decrease in fluorescence is a combined result of both dynamic and static quenching. The quenching within reline is fairly efficient considering the high viscosity of the medium. Significantly lower activation energy of the bimolecular quenching process as compared to the activation energy of the viscous flow indicates facilitation of the electron/charge transfer quenching of l-Trp by acrylamide within the ionic environment offered by reline. The effect of high viscosity is partly overcome by the strongly ionic environment of reline during the electron/charge transfer between l-Trp and acrylamide. The results highlight the structural complexity of these DESs especially within the cybotactic region of the probe, which is absent in common molecular solvents of similar high viscosity.