Self-quenching Process Research Articles

Dual-mode luminescence and anti-counterfeiting application of YVO4: Eu3+@SiO2@CDs nanocomposites

With the combination of carbon quantum dots (CDs) and rare earth-doped fluorescent materials, multi-mode luminescence can be achieved in fluorescent ink, thereby enhancing its anti-counterfeiting security coefficient. However, the self-quenching process in CDs currently hinders their application in the field of anti-counterfeiting. In this study, we employed simple chemical method to prepare the SiO2-encapsulated YVO4:Eu3+/CDs nanocomposites, which greatly improved the stability of the composite and achieved dual-mode solid-state luminescence. Quick response code anti-counterfeiting patterns were fabricated using screen printing technology, and dual-mode luminescence performance with red and blue emission was achieved under excitation at 254 nm and 365 nm respectively. The YVO4:Eu3+@SiO2@CDs nanocomposites showed stable dual-mode luminescent performance in both solid-state and solvent environment, demonstrating great potential in anticounterfeiting and information protection application.

An efficient peroxymonosulfate activator derived from metal-organic frameworks: The application of Cu, Mg co-doping strategy

The application of metal-organic frameworks (MOFs)-based catalysts in Fenton-like reactions attracts increasing attention, however, improving the efficiency of these catalysts remains a challenge. Herein, a highly efficient peroxymonosulfate (PMS) activator (CuMg@Fe-MOFs-500) derived from MOFs was successfully synthesized. The results indicated that the Cu and Mg co-doping strategy employed in the fabrication of CuMg@Fe-MOFs-500 significantly enhanced the PMS activation performance, achieving a tetracycline (TC) degradation efficiency of 96.8 % within 60 min. The incorporation of the doped Mg and Cu significantly increased the contents of the oxygen vacancy (OV) and low-valent copper (Cu0 and Cu+), respectively. The interaction between the OV and Cu0/Cu+ became a key factor during the PMS decomposition process. Furthermore, for the first time, the OV content was observed to be positively and negatively linearly correlated with the reaction rate constant (k) and the charge transfer resistance (Rct) of the prepared PMS activators, respectively. Quenching experiments and electron paramagnetic resonance (EPR) tests demonstrated that the predominant reactive species generated during the PMS activation process were SO4•-, •OH, O2•-, and 1O2. It was also shown that the 1O2 primarily originated from O2•- disproportionation, and a self-quenching process between SO4•- and •OH was also observed. The intermediates produced during pollutant degradation induced by PMS-based Fenton-Like reaction exhibited reduced toxicity, as demonstrated by ecotoxicity analysis. This study presented a novel approach for the fabrication of catalyst derived from bimetallic doped MOFs, characterized by a high OV content, for the application in PMS-based Fenton-like reactions.

Spectroscopic and Thermographic Qualities of Praseodymium-Doped Oxyfluorotellurite Glasses.

The thermal stability of oxyfluorotellurite glass systems, (65-x)TeO2-20ZnF2-12PbO-3Nb2O5-xPr2O3, doped with praseodymium was examined. The different concentrations of praseodymium oxide (x = 0.5 and 2 mol%) were applied to verify the thermal, optical and luminescence properties of the materials under study. The relatively high values of the Dietzel (ΔT) and Saad-Poulain (S or H') thermal stability factors determined using a differential thermal analysis (DTA) indicate the good thermal stability of the glass matrix, which gradually improves with the content of the active dopant. The temperature dependence of optical spectra in the temperature range 300-675 K for the VIS-NIR region was investigated. The involved Pr3+ optical transition intensities and relaxation dynamic of the praseodymium luminescent level were determined. The ultrashort femtosecond pulses were utilized to examine a dynamic relaxation of the praseodymium luminescent levels. Although the measured emission of the Pr3+ active ions in the studied glass encompasses the quite broad spectral region, the observed luminescence may only be attributed to 3PJ excited states. As a result, the observed decrease in the experimental lifetime for the 3P0 level along with the increasing activator content was identified as an intensification of the Pr-Pr interplay and the associated self-quenching process. The maximum relative sensitivities (Sr) estimated over a relatively wide temperature range are ~0.46% K-1 (at 300 K) for FIR (I530/I497) and 0.20% K-1 (at 600 K) for FIR (I630/I497), which seems to confirm the possibility of using investigated glasses in optical temperature sensors.

Excited state properties of 5-fluoro-4-thiouridine derivative†.

The excited state properties of thionated 5-fluorouridine (2',3',5'-tri-O-acetyl-5-fluoro-4-thiouridine; ta5F4TUrd), synthesized with Lawesson's reagent, have been intensively investigated with nanosecond transient absorption spectroscopy, time-resolved thermal lensing, near-infrared emission, and quantum chemical calculation. The intrinsic triplet lifetime of ta5F4TUrd was determined to be μs in acetonitrile, and the formation quantum yield of the excited triplet state was as large as . The quenching rate constants of the triplet ta5F4TUrd by the dissolved oxygen molecule and by the self-quenching process were found to be nearly equal to the diffusion-controlled rate of acetonitrile. The quantum yield of the singlet molecular oxygen produced through energy transfer between the triplet ta5F4TUrd and the dissolved oxygen, , was successfully determined to be under the oxygen-saturated condition. From the oxygen concentration dependence of the value, the fraction of triplet ta5F4TUrd quenched by dissolved oxygen which gives rise to the 1 O2 * formation, , was successfully obtained to be , which was the largest among the thionucleobases and the thionucleosides reported so far. This could be due to the lower energy and/or the ππ* character of the triplet state.

Kinetics of Photoreactive 4,4'-Dimethylbenzophenone Nanocrystals: Relative Contributions to Triplet Decay from Intermolecular H-Atom Transfer and Reductive Charge Transfer Quenching.

Crystals of 4,4'-dimethylbenzophenone (DMBP) are known to react by intermolecular H atom transfer, followed by radical pair recombination. To determine the contribution of the H atom transfer reaction to the deactivation of the triplet ketone, transient absorption spectra and kinetics were obtained using aqueous nanocrystalline suspensions. Single-exponential lifetimes of ca. 1185 ns with no deuterium isotope effect and inefficient product formation suggest that the reaction does not contribute significantly to the kinetics of triplet decay. By contrast, the observed lifetime is consistent with previous observations with p,p'-disubstituted benzophenones that undergo an efficient self-quenching process by a reductive charge transfer mechanism.

Mechanism toward Turn‐on of Polysaccharide‐Porphyrin Complexes for Fluorescence Probes and Photosensitizers in Photodynamic Therapy in Living Cells

β-(1,3-1,6)-D-Glucan, λ-carrageenan, tamarind gum, and pullulan can dissolve various porphyrin derivatives via the formation of complexes in water using a high-speed vibration milling method. The aqueous solutions of the resulting complexes exhibit long-term stability. Despite the adverse effects of the self-quenching process, notable fluorescence and improved photodynamic activity of the polysaccharide-complexed porphyrin derivatives were observed in the presence of liposomes, micelles, cyclodextrins, and HeLa cells. It was noted that the type of porphyrins was more important than the type of polysaccharides present in the complex. Porphyrin self-aggregates were monodispersed in the lipid membranes of the liposomes and lysosomes. The polysaccharide-complexed porphyrin derivatives showed increased photodynamic activity toward HeLa cells under photoirradiation between 610 and 740 nm.

Combating Concentration Quenching in Upconversion Nanoparticles.

Lanthanide-doped upconversion nanoparticles (UCNPs) are a special class of luminescent nanomaterials that convert multiwavelength near-infrared (NIR) excitation into tunable emissions spanning the deep ultraviolet (UV) to NIR regions. In addition to large anti-Stokes shift, UCNPs also feature a sharp emission bandwidth, long excited-state lifetime, as well as high resistance to optical blinking and photobleaching. Therefore, UCNPs have been identified as promising candidates to solve many challenging problems in fields ranging from biological imaging and therapeutics to photovoltaics and photonics. Nevertheless, the progress of utilizing an upconversion process is being hindered by the limited emission intensity, principally due to low oscillator strength in these nanoparticles. UCNPs essentially resemble the optical characteristics of their bulk counterparts, which take advantage of electronic transition within the 4f configuration of the lanthanide dopants to realize photon energy conversions. In general, a high dopant concentration promotes upconversion luminescence by providing a high density of optical centers to collect and to sustain the energy of the excitation light. However, an increase in dopant concentration induces self-quenching processes that offset the emission gain and may eventually result in attenuation of the overall emission intensity. This phenomenon known as concentration quenching represents a major obstacle to constructing bright UCNPs. In recent years, advances in nanoparticle research have led to the emergence of several strategies for mitigating energy loss at elevated dopant concentrations. In consequence, doping high levels of lanthanide ions in UCNPs has become a viable solution to boosting the emission intensity of photon upconversion. On account of extensive energy exchange interaction in heavily doped UCNPs, the spectrum tunability of photon upconversion is also greatly enhanced. These advances have largely expanded the scope of upconversion research. To provide guidelines for enhancing upconversion through heavy doping, we attempt to review recent advances in the understanding and control of concentration quenching in UCNPs. With significant advancements made in the chemical synthesis, we are now able to exquisitely control the doping of lanthanide ions in various nanoparticles of well-defined size, morphology, and core-shell structure. We show that, by confining energy transfer in nanostructured host materials in conjunction with innovative excitation schemes, concentration quenching of upconversion luminescence is largely alleviated. As a result, unusually high dopant concentrations can be used to construct UCNPs displaying high brightness and large anti-Stokes shift. We demonstrate that the development of heavily doped UCNPs enables advanced bioimaging and photonic applications that can hardly be fulfilled by conventional UCNPs comprising low concentrations of lanthanide dopants.

Uniform, Scalable, High-Temperature Microwave Shock for Nanoparticle Synthesis through Defect Engineering

Summary Here we demonstrate a thermal shock synthesis method triggered by microwave irradiation for the rapid synthesis of nanoparticles on reduced graphene oxide (RGO) substrate. With properly controlled reduction, RGO has high electrical conductivity while maintaining functional groups, leading to an extremely efficient microwave absorption of ∼70%. The high utilization of microwaves results in the ability to raise the temperature to 1,600 K in just 100 ms, which is followed by rapid quenching to room temperature. The defects on the RGO are crucial for achieving this record-high microwave-induced temperature as these defects play a fundamental role in absorbing the radiation as well as the self-quenching mechanism. By loading precursors onto RGO, we can utilize rapid temperature change to synthesize nanoparticles. The nanoparticles are ∼10 nm with uniform distribution. This facile, rapid, and universal synthesis technique has the potential to be employed in large-scale production of nanomaterials and suggests a new direction for nanosynthesis.

Comparative study of optical properties of Ho3+ -doped RE2O3-Na2O-ZnO-TeO2 glasses

Tellurite glasses of the composition xHo2O3-(7-x)RE2O3−3Na2O-25ZnO-65TeO2 were prepared by the melt quenching-technique. Absorption (300 K) and fluorescence (300 K) spectra as well as the lifetimes from excited states are presented and discussed in details. The analysis of the absorption spectrum at 300 K has been carried out for determination of phenomenological Ωλ (λ = 2, 4, 6) Judd-Ofelt parameters. Based on the Judd-Ofelt theory, the radiative transition probabilities (AT), branching ratios (βR) and radiative lifetimes (τR) of the emitting levels of Ho3+ ion have been determined. The experimental lifetime of the 5I7 level has been significantly reduced in comparison to the radiative lifetime. It was found that the quenching of emission from the 5I7 level of the Ho3+ ion is due to self-quenching process. The stimulated and effective emission cross-sections for the 5I7→5I8 transition have been calculated. The fluorescence from the 5S2 level is strongly dependent on the activator concentration. The obtained results show that among three types of glasses, Ho3+-doped Y2O3-Na2O-ZnO-TeO2 glasses are the most promising candidates for a 2 µm laser host as well as an efficient phosphor with green luminescence.

"Rigid" Luminescent Soft Materials: Europium-Containing Lyotropic Liquid Crystals Based on Polyoxyethylene Phytosterols and Ionic Liquids.

Soft materials of europium β-diketonate complexes constructed in lyotropic liquid crystals (LLCs) mediated by ionic liquids (ILs) are impressive for their excellent luminescence performance and stability. For the aim to further improve their mechanical processability and luminescent tunablility, the polyoxyethylene phytosterols (BPS-n) were introduced here as structure directing agents to prepare relatively "rigid" lamellar luminescent LLCs in 1-butyl-3-methyl-imidazolium hexafluorophosphate by doping europium β-diketonate complexes with different imidazolium counterions. As a result of the solvophobic sterol ring structure of BPS-n, the more effective isolation and confinement effects of europium complexes could be achieved. The longest fluorescence lifetime and the highest quantum efficiency reported so far for europium containing lyotropic organized soft materials were thus obtained. Changing the molecular structures of BPS-n with different oxyethylene chains or doped complexes with imidazolium counterions of different alkyl chain lengths, the spacings of lamellar LLC matrixes and position of dispersed complexes became tunable. The measured luminescent and rheological properties for such composite LLCs showed a dependence on the rigidity and isolation capability afforded by sterol molecules. It was also found that the increase of counterion alkyl chain length would weaken the LLC matrix's confinement and isolation effects and therefore exhibit the deteriorated luminescence performance. The enhanced luminescence efficiency and stability of doped BPS-n LLCs reflected the excellent segregation of europium complexes from each other and therefore the reduced self-quenching process. The obtained results here present the designability of LLC matrixes and their great potential to promote achieving the luminescence tunability of soft materials.

Concentration Quenching Behavior of Thermally Activated Delayed Fluorescence in a Solid Film

Concentration-dependent photophysical properties were investigated using an approach for more rigorous interpretation through well-defined concentration quenching equations derived from an optical exciton model. These calculations verified that the solid-state self-quenching process of the 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzne (4CzIPN) emitter in a host matrix is controlled by the Dexter energy transfer model. By extension, we found that the reduction in the radiative singlet decay rate (krS) can be ascribed to the stabilization of excited states. This process may increase the efficiency of intersystem crossing as the doping concentration of 4CzIPN increases in the host material in a solid matrix. Furthermore, the nonradiative triplet decay process (knrT) could be an important consideration for estimating the exact concentration quenching rate constant value and the efficiency of reverse intersystem crossing corresponding to experimental photoluminescence behaviors with the various doping conc...

An Improved Convergent Model for Single-Photon Avalanche Diodes

An improved convergent model for single-photon avalanche diodes (SPADs) is presented, where the qualitative property of SPADs operating in different reverse-biased voltages is represented by an equivalent circuit, and the accurate current–voltage ( $I$ – $V$ ) quantitative characteristic of SPADs is described by Verilog-A language. For the detector operated in different modes, a piecewise fitting method is employed for a higher fit coherence. A pseudo-max/min function method is adopted to solve the convergence problem in the piecewise curve fitting. The proposed model can simulate the ignition of the detector due to photon absorption, and the self-sustaining and self-quenching processes. The fitting accuracy of the SPAD $I$ – $V$ measurement data reaches 99%. Furthermore, the proposed model can be feasibly implemented into the electronics design automation tools, such as Spectre in Cadence, to support the integration simulation between circuits and SPAD detectors.

Photosensitized H2 Production Using a Zinc Porphyrin-Substituted Protein, Platinum Nanoparticles, and Ascorbate with No Electron Relay: Participation of Good's Buffers.

Development of efficient light-driven splitting of water, 2H2O → 2H2 + O2, often attempts to optimize photosensitization of the reductive and oxidative half-reactions individually. Numerous homogeneous and heterogeneous systems have been developed for photochemical stimulation of the reductive half reaction, 2H+ + 2e- → H2. These systems generally consist of various combinations of a H+ reduction catalyst, a photosensitizer (PS), and a "sacrificial" electron donor. Zinc(II)-porphyrins (ZnPs) have frequently been used as PSs for H2 generation, but they are subject to various self-quenching processes in aqueous solutions. Colloidal platinum in nanoparticle form (Pt NP) is a classical H+ reduction catalyst using ZnP photosensitizers, but efficient photosensitized H2 generation requires an electron relay molecule between ZnP and Pt NP. The present report describes an aqueous system for visible (white) light-sensitized generation of H2 using a protein-embedded Zn(II)-protoporphyrin IX as PS and Pt NP as H+ reduction catalyst without an added electron relay. This system operated efficiently in piperazino- and morpholino-alkylsulfonic acid (Good's buffers), which served as sacrificial electron donors. The system also required ascorbate at relatively modest concentrations, which stabilized the Zn(II)-protoporphyrin IX against photodegradation. In the absence of an electron relay molecule, the photosensitized H2 generation must involve formation of at least a transient complex between a protein-embedded Zn(II)-protoporphyrin IX species and Pt NP.

Origin of efficiency roll-off for FIrpic based blue organic light-emitting diodes and implications on phosphorescent molecule design

This work reports our effort on understanding the efficiency roll-off in blue phosphorescent organic light-emitting diodes (OLEDs), based on bis[3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)]-iridium (FIrpic) doped in 4,4-N,N-dicarbazole-biphenyl (CBP) host. The performance of a set of blue phosphorescent OLEDs with different FIrpic dopant concentrations was analyzed. Unusually, the device having a 30-nm-thick pure FIrpic emissive layer with a high luminous efficiency of 7.76 cd/A at 100 mA/cm2 was obtained. Theoretical calculation, based on the density functional theory, reveals that the exciton self-quenching process is suppressed due to the strong Coulomb repulsion between FIrpic molecules. It is found that the efficiency roll-off in FIrpic-based OLEDs is closely related to the exciton quenching induced by the CBP+ radical cations that are present in the CBP host.

Surface microrelief and hardness of laser hardened and ultrasonically peened AISI D2 tool steel

This paper is focused on experimental analysis of the effects of laser surface hardening (LSH) combined with subsequent ultrasonic impact treatment (UIT) on the surface microrelief, hardness and microstructure in near-surface layers of AISI D2 high-chromium, cold worked tool steel. The LSH provides fast heating of the near-surface layers to the temperature above that of the phase transformation and temperature then rapidly cools by a self-quenching process. The formed heat affected zone is hardened thanks to the rapid heating/cooling process affecting microstructure, phase composition and carbide formation. Conversely, the UIT induces multiple impact loads providing severe plastic deformation of near-surface layers, and the hardening occurs by a dislocation mediated process. The optimal parameters of each process were determined to obtain maximum hardness and regular surface microrelief. Further, complete analysis of the effect of combined treatment (LSH+UIT) on the surface hardness, microhardness depth profile and surface microrelief was performed. Results show that the combined LSH+UIT process provides almost triple, double and a 10% increase in hardness in comparison with those of the initial, UIT-processed and LSH-treated states, respectively. The surface microrelief, waviness and roughness parameters were respectively diminished after LSH+UIT by approx. 50, 65, and 90%. XRD analysis was carried out after LSH and LSH+UIT processes, which showed essential α-Fe peak broadening due to the formation of microstrains (0.27% and 0.47%, respectively) and reduction in crystallite size (84nm after LSH+UIT). Favorable compressive residual stresses (−205MPa and −409MPa, respectively) were also observed in the near-surface layers of ~350 and 80μm thick, respectively. The obtained results demonstrate that the combined LSH+UIT process is a feasible surface treatment for the quality improvement of the tool steel components including both the surface microrelief and hardness characteristics.

Solar cells sensitized with near-infrared absorbing dye: Problems with sunlight conversion efficiency revealed in ultrafast laser spectroscopy studies

Complete solar cells sensitized with cyanine dye MK-245, having near-infrared absorption band optimized for tandem devices, are prepared, characterized and studied by time-resolved laser spectroscopy techniques. The photocurrent of the cell correlates with the amount of residual transient absorption signal, pointing to the poor electron injection as the main factor limiting the sunlight conversion efficiency. The measurements show that the lifetime of the excited state is drastically reduced from about 300 ps in solution to single picoseconds when the dyes are attached to titania nanoparticles. It is due to the self-quenching process that effectively competes with electron injection, even under the presence of co-adsorbent. In addition, high concentration of lithium cations in the electrolyte is necessary to improve electron injection quantum yield. The obtained results should be valid for a large class of similar compounds and reveal the main problems in obtaining sufficient performance of near-infrared absorbing dyes.

Nd:YAG at its 50th anniversary: Still to learn

The paper presents a critical review of the actual knowledge on Nd:YAG and brings new data on the spectroscopic properties and energy transfer self-quenching processes and on their implication in laser emission.

Optical processes in (Y,Bi)VO4 doped with Eu3+ or Pr3+

Zircon and fergusonite-type vanadates either undoped or doped with Eu3+ or Pr3+ are synthesized in the system (Y,Bi)2O3-V2O5 by solid state and coprecipitation procedures. Their optical properties are investigated at 300 and 77 K and the luminescence mechanisms are discussed on the basis of energy level schemes that combine the host and the dopant states. Fergusonite BiVO4 is shown to glow in the deep red region at 77 K upon excitation at 450 nm and shorter wavelengths. Host sensitization is demonstrated in Eu3+-doped fergusonite BiVO4 and zircon BiVO4 at 77 K, but lost as temperature is raised to 300 K. The origin of this effect is addressed by considering the nature of the host-band edge states and self-quenching processes. The near-UV excited luminescence in the system (Y, Bi)VO4:Pr3+ (zircon) consists of the yellow bandlike emission of the zircon host and of the characteristic red 1D2 → 3H4 emission lines of Pr3+ in vanadates. The relative contribution of these features can be fine-tuned at room temperature by adjusting the composition of the materials or the excitation wavelength.

Study of the effect of aliphatic and π-conjugated systems on the photophysical properties of polypyridinic Ruthenium II complexes as potential semiconductor materials for iTMC type LEC

The photophysical properties of polypyridinic Ruthenium complexes, as potential semiconductor materials for iTMC (ionic transition metal complex) type LECs have been studied. Substituted 2,2′-bipyridine ligands were used to study the effects of conjugated and aliphatic chains on the properties of the complexes, especially on the photophysical properties. Specifically, the N^N type ligands 4,4′-bis[2-hydroxy-2-(phenyl)ethyl]-2,2′-bipyridine (1), 4,4′-bis(α-styrene)-2,2′-bipyridine (2) and 4,4′-diphenylethyl-2,2′-bipyridine (3) were synthesized, and used to prepare the corresponding [RuII(bpy)2(N^N)](PF6)2 complexes. All three ligands contain a phenyl group as substituent for bpy, but with different residues as bridges between both: ligands 1 and 3 have free rotating connecting groups, while 2 is more rigid due to the styryl double bond. From the achieved results it was observed that, as expected, a conjugation on the ligand produces complexes with bands shifting toward lower energy regions, due to the electronic communication between the phenyl and bipyridine groups. On the contrary, in the absence of this conjugation, as is the case of the complexes with ligands 1 and 3, absorptions and emissions bands are very similar to the corresponding complex with unsubstituted bpy. Therefore, complexes with ligands 1 and 3 seem to be promising for LEC devices, due to the free rotations of the connecting aliphatic chain. This should preserve the properties as emission color, and advantages of Ru(bpy)32+ photoluminescence, but increasing the efficiency when used in a device, avoiding crystallization and diminishing self-quenching processes.

Relaxation of the T1 excited state of 2-thiothymine, its riboside and deoxyriboside-enhanced nonradiative decay rate induced by sugar substituent

Abstract UV absorption, circular dichroism and emission spectroscopy as well as nanosecond and femtosecond transient absorption measurements were used to characterize the excited states of 2-thiothymine (2TT), its riboside (2TTR) and deoxyriboside (2TTD) in acetonitrile (ACN) solution. The lowest triplet state (T1) could be observed exclusively in the experiments. Upon excitation to higher singlet states (λexc = 266 nm), the T1 (ππ*) states of the investigated compounds were formed on an ultrafast time scale (kT ≥ 3 × 1012 s−1) with an efficiency approaching unity (ϕT = 0.9 ± 0.1). These T1 states were characterized by their intrinsic lifetimes ( τ T 0 ) and rate constants of self-quenching (kSQ), phosphorescence (kP), and nonradiative processes (∑kNR). In the series of compounds studied only 2TT exhibited a weak room-temperature phosphorescence, and the spectrum of this 2TT emission was recorded for the first time. In the absence of self quenching, nonradiative processes (NR) are the dominant (ϕNR ∼ 0.9) channel of the decay of 2TT and its nucleosides in their T1 states. Despite the chromophore being the same in all of the compounds studied, the decay dynamics of the nucleosides’ T1 states differed considerably from that of the T1 state of 2TT. The ∑kNR values determined for the derivatives containing a ribosyl (in 2TTR) or a deoxyribosyl (in 2TTD) substituent in the position α to the thiocarbonyl group were an order of magnitude larger, and as a consequence, the lifetimes ( τ T 0 ) shorter (by factor of 14 and 22 for 2TTD and 2TTR, respectively) as compared to 2TT. The reason for the enhanced rate of nonradiative decay in the nucleosides is discussed based on the results obtained from additional experiments including the determination of the T1 lifetime for the deuterated derivative of 2TTR and the intermolecular quenching of 2TT by ribose molecules. The factors which might be responsible for this substituent effect (on the nonradiative decay) such as an intramolecular or an intermolecular H bond formation involving sugar OH groups as well as a reversible H abstraction from the sugar α-substituent in the nucleosides appear to be not important.