Nonradiative Decay Rate Research Articles

Europium Complexes: Luminescence Boost by a Single Efficient Antenna Ligand.

We advance the concept that a single efficient antenna ligand substituted in or added to an otherwise weakly luminescent europium complex is enough to significantly boost its luminescence. Our results, on the basis of photophysical measurements on 5 novel europium complexes and 15 known ones, point in the direction that ligand dissimilarity and ligand diversity are all concepts that clearly play a fundamental role in the luminescence of europium complexes. We show that it is important that a symmetry breaker ligand exists in the complex to enhance ligand dissimilarity and ligand diversity, all mainly affecting the nonradiative decay rate by reducing it. Because the presence of at least one antenna ligand is also obviously necessary, the optimal and the most cost-effective situation can be achieved by adding a single coordination symmetry breaker that is also an efficient antenna, such as 1-(2-thenoyl)-3,3,3-trifluoroacetone or 4,4,4-trifluoro-1-phenyl-1,3-butanedione. In such cases the quantum efficiency, η, is decidedly boosted, as can be verified by going from complex [EuCl2(TPPO)4]Cl·3H2O with η = 0% to the novel complex [EuCl2(BTFA)(TPPO)3], where TPPO stands for triphenylphosphine oxide, with η = 62%.

Energy Gap Law for Exciton Dynamics in Gold Cluster Molecules.

The energy gap law relates the nonradiative decay rate to the energy gap separating the ground and excited states. Here we report that the energy gap law can be applied to exciton dynamics in gold cluster molecules. Size-dependent electrochemical and optical properties were investigated for a series of n-hexanethiolate-protected gold clusters ranging from Au25 to Au333. Voltammetric studies reveal that the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of these clusters decrease with increasing cluster size. Combined femtosecond and nanosecond time-resolved transient absorption measurements show that the exciton lifetimes decrease with increasing cluster size. Comparison of the size-dependent exciton lifetimes with the HOMO-LUMO gaps shows that they are linearly correlated, demonstrating the energy gap law for excitons in these gold cluster molecules.

Fluorescence and absorption characteristics of p-xylene: applicability for temperature measurements

This paper explores the characteristics of absorption and fluorescence spectra of p-xylene within the temperature range that is frequently encountered during the mixture formation in internal combustion engines. At 266 nm wavelength, the p-xylene absorption cross section shows a mean value of (3.4 ± 0.2) × 10−19 cm2/molecule within the temperature range from 423 to 623 K in N2. As expected, fluorescence peak intensity decreases by a factor of 3 when the temperature increases by 100 K, due to a increasing non-radiative decay rate of excited state at increasing temperatures. In addition, the suitability of p-xylene for temperature measurements in the gas phase via the single-wavelength excitation (at 266 nm) two-color detection laser-induced fluorescence imaging is explored. Combinations of spectral detection bands were compared and the combination of 320/289 nm provides the best temperature performance with a relative error of 2.6% within the investigated temperature range. It is also shown that the temperature field measurement has not been strongly affected by the laser attenuation.

Probing the Influence of Disorder on Lanthanide Luminescence Using Eu-Doped LaPO4 Nanoparticles.

Lanthanide-doped nanocrystals (NCs) differ from their bulk counterparts due to their large surface to volume ratio. It is generally assumed that the optical properties are not affected by size effects as electronic transitions occur within the well-shielded 4f shell of the lanthanide dopant ions. However, defects and disorder in the surface layer can affect the luminescence properties. Trivalent europium is a suitable ion to investigate the subtle influence of the surface, because of its characteristic luminescence and high sensitivity to the local environment. Here, we investigate the influence of disorder in NCs on the optical properties of lanthanide dopants by studying the inhomogeneous linewidth, emission intensity ratios, and luminescence decay curves for LaPO4:Eu3+ samples of different sizes (4 nm to bulk) and core–shell configurations (core, core–isocrystalline shell, and core–silica shell). We show that the emission linewidths increase strongly for NCs. The ratio of the intensities of the forced electric dipole (ED) and magnetic dipole (MD) transitions, a measure for the local symmetry distortion around Eu3+ ions, is higher for samples with a large fraction of Eu3+ ions close to the surface. Finally, we present luminescence decay curves revealing an increased nonradiative decay rate for Eu3+ in NCs. The effects are strongest in core and core–silica shell NCs and can be reduced by growth of an isocrystalline LaPO4 shell. The present systematic study provides quantitative insight into the role of surface disorder on the optical properties of lanthanide-doped NCs. These insights are important in emerging applications of lanthanide-doped nanocrystals.

Fluorescence Behavior of Schiff Base-N, N'-bis(salicylidene) Trans 1, 2-Diaminocyclohexane in Proteinous and Micellar Environments.

Fluorescence properties of N, N'-bis(salicylidene) trans 1, 2-diaminocyclohexane (H 2 L) is used to probe the anionic (SDS), cationic (CTAB) and nonionic (TX-100) micelles as well as in serum albumins (BSA and HSA) and chicken egg white lysozyme (LYZ) by steady state and picosecond time-resolved fluorescence spectroscopy. The fluorescence band intensity was found to increase with concomitant blue-shift with gradual addition of different surfactants. All the experimental results suggest that the probe molecule resides in the micelle-water interface rather than going into the micellar core. However, the penetration is more towards the micellar hydrocarbon core in nonionic surfactant (TX-100) while comparing with ionic surfactants (SDS and CTAB). Several mean microscopic properties such as critical micelle concentration, polarity parameters and binding constant were calculated in presence of different surfactants. The decrease in nonradiative decay rate constants in micellar environments indicates restricted motion of the probe inside the micellar nanocages with increasing fluorescence emission intensity and quantum yields. Further in this work, we also investigated the interaction behavior of the probe with different proteins at low concentrations under physiological conditions (pH = 7.4). Stern-Volmer analysis of the tryptophan (Trp) fluorescence quenching data in presence of probe reveals Stern-Volmer constant (Ksv) as well as bimolecular quenching rate constant (Kq). The binding constant as well as the number of binding sites of the probe with proteins were also monitored and found to be 1:1 stoichiometry ratio.

Efficient Inorganic Perovskite Light-Emitting Diodes with Polyethylene Glycol Passivated Ultrathin CsPbBr3 Films.

Efficient inorganic perovskite light-emitting diodes (PeLEDs) with an ultrathin perovskite emission layer (∼30 nm) were realized by doping Lewis base polyethylene glycol (PEG) into CsPbBr3 films. PEG in the perovskite films not only physically fills the crystal boundaries but also interacts with the perovskite crystals to passivate the crystal grains, reduce nonradiative recombination, and ensure efficient luminance and high efficiency. As a result, promoted brightness, current efficiency (CE), and external quantum efficiency (EQE) were achieved. The nonradiative decay rate of the PEG:CsPbBr3 composite film is 1 order of magnitude less than that of the neat CsPbBr3 film. After further optimization of the molar ratio between CsBr and PbBr2, a peak CE of 19 cd/A, a maximum EQE of 5.34%, and a maximum brightness of 36600 cd/m2 were achieved, demonstrating the interaction between PEG and the precursors. The results are expected to offer some helpful implications in optimizing the polymer-assisted PeLEDs with ultrathin emission layers, which might have potential application in see-through displays.

Highly absorbing multispectral near-infrared polymer nanoparticles from one conjugated backbone for photoacoustic imaging and photothermal therapy.

Semiconducting polymers with specific absorption are useful in various applications, including organic optoelectronics, optical imaging, and nanomedicine. However, the optical absorption of a semiconducting polymer with a determined structure is hardly tunable when compared with that of inorganic semiconductors. In this work, we show that the optical absorption of polymer nanoparticles from one conjugated backbone can be effectively tuned through judicious design of the particle morphology and the persistence length of polymers. Highly absorbing near-infrared (NIR) polymers based on diketopyrrolopyrrole-dithiophene (DPP-DT) are synthesized to have different molecular weights (MWs). The DPP-DT polymer with a large molecular weight and high persistence length exhibited remarkably high optical absorption with a peak mass extinction coefficient of 81.7Lg-1cm-1, which is one of the highest value among various photothermal agents reported to date. Particularly, the polymer nanoparticles with different sizes exhibit broadly tunable NIR absorption peaks from 630 to 811nm. The PEGylated small polymer dots (Pdots) show good NIR light-harvesting efficiency and high non-radiative decay rates, resulting in a relatively high photothermal conversion efficiency in excess of 50%. Thus, this Pdot-based platform can serve as promising photothermal agents and photoacoustic probes for cancer theranostics.

“Oxygen quenching” in Eu-based thermographic phosphors: Mechanism and potential application in oxygen/pressure sensing

The effect of ambient oxygen concentration on thermographic phosphor thermometry was investigated to uncover the “oxygen quenching” mechanism of Eu-based phosphors using a lifetime-based measurement system. The phosphors being studied were the Eu doped yttria stabilized zirconias (YSZs) with Y3+ concentrations of 0, 8, 12 and 57mol%, having monoclinic, tetragonal, cubic and δ crystalline lattice, respectively. To vary the ambient oxygen concentration, three different gas phases (air, oxygen and nitrogen with methane) were used. It was found that the phosphorescent lifetimes and intensities of all four phosphors were sensitive to the ambient oxygen concentration, while the reference phosphor, Y2SiO5:Eu, was not influenced. The “oxygen quenching” phenomenon observed in Eu-doped YSZs was attributed to the oxygen vacancies in the phosphors. In addition, the four YSZ:Eu phosphors exhibited different sensitivity to ambient oxygen concentration. The oxygen sensitivity was found to be closely related to the site symmetry of Eu3+, which was previously determined by the phosphorescent spectra. The site symmetry of Eu3+ strongly affected the charge-transfer states and therefore determined the non-radiative decay rate. The current findings provided guidance for the phosphor selection and sensor design for oxygen/pressure sensing in high-temperature environment.

Photophysics of a coumarin based Schiff base in solvents of varying polarities

The present work reports detailed photophysics of a coumarin based Schiff base, namely, (E)-7-(((8-hydroxyquinolin-2-yl)methylene)amino)-4-methyl-2H-chromen-2-one (HMC) in different solvents of varying polarity exploiting steady state absorption, fluorescence and time resolved fluorescence spectroscopy. The dominant photophysical features of HMC are discussed in terms of emission from an intramolecular charge transfer (ICT) excited state. Molecular orbital (MO) diagrams as obtained from DFT based computational analysis confirms the occurrence of charge transfer from 8′-hydroxy quinoline moiety of the molecule to the coumarin part. The notable difference in the photophysical response of HMC from its analogous coumarin (C480) lies in a lower magnitude of fluorescence quantum yield of the former, particularly in the solvents of low polarity, which is rationalized by considering the higher rate of non-radiative decay of HMC in apolar solvents. Phosphorescence emission as well as phosphorescence lifetime of HMC has also been reported in 77K frozen matrix.

Solvent effects on the photophysical properties of distyrylnaphthalene-based conjugated oligoelectrolytes

In this study we have characterized electronic structure and photophysical properties of the new-synthetized distyrylnaphthalene DSNN derivatives in solvents of different polarity and viscosity with the use of both experimental (absorption and fluorescence spectroscopy) and theoretical (TD DFT(B3LYP)/PCM) methods. The obtained results were interpreted in terms of the reasonable scenario for the change in both geometrical conformation and charge distribution upon photoexcitation. Direct excitation of DSNN conformers with non-planar geometries concerning the conjugated backbone leads to the locally excited Franck-Condon excited states LE FC, from which the ICT state is formed. Most probably, the formation of this ICT state involves planarization and quinoidization of the conjugated core. The emissive non-relaxed LE and relaxed ICT states differ in geometry, charge distribution, polarity as well as in radiative and nonradiative decay rates. Environmental effects have a profound effect on the rate of ICT formation and also on the relative stability of these states. Thus, controlling the environment can be a very effective way of tuning the linear and nonlinear responses of the compounds studied.

Effect of 2,4,6-tri(2-pyridyl)-1,3,5-triazine on visible and NIR luminescence of lanthanide tris(trifluoroacetylacetonates)

We here report the first example of a praseodymium complex of the type [Pr(tfaa)3(tptz)] (tfaa = trifluoroacetylacetone and tptz = 2,4,6-tri(2-pyridyl)-1,3,5-triazine), the photophysical properties of the praseodymium complex (visible and NIR region), as well as of the analogous neodymium (NIR region), europium and terbium complexes (visible region), investigated in the solid state. We showed that the ancillary tptz ligand has detrimental effect on the photophysical properties of the terbium complex, showing a 2.5-fold shorter observed lifetime as compared to the hydrated binary complex [Tb(tfaa)3H2O] (τobs = 220 μs). However, it has a positive effect on the NIR emission of the neodymium complex (τobs = 10.33 μs and ϕLn = 3.2%) and visible emission of the europium complex (τobs = 678.54 μs and ϕLn = 89.45%). The Judd-Ofelt parameters (Ω2 and Ω4), the radiative (Arad) and non-radiative (Anrad) decay rates and the energy transfer (WET) and back energy transfer (WBT) rates were calculated and discussed in detail for the europium complex. The high value of Ω2 (30.88 × 10−20 cm2) indicates a greater degree of covalency in the europium – ligand bonds, on the other hand, the higher value of Ω4 (22.36 × 10−20 cm2) could be due to π–π interactions between the non-coordinating arms of the tptz co-ligand. The impact of each ligand (anionic tfaa and neutral tptz) on the photophysical properties in the case of the europium complex was ascertained theoretically by the chemical partition of the radiative decay rate (Arad) and it confirms that the neutral ancillary tptz ligand is solely contributing approximately one third of the Arad (∼32%) with a value of 402.78 s−1 while the three anionic tfaa ligands are contributing ∼68% of the Arad with a value of 855.13 s−1.

A theoretical study of the low-lying excited states and the photophysics of dimethoxy curcumin in cyclohexane and acetonitrile

Two important aspects of the photophysics of dimethoxy curcumin in solvents are theoretically analysed. First, the large Stokes shift in cyclohexane and in acetonitrile is calculated using time-dependent density functional theory with large basis set. Second, a possible explanation is given for the considerable decrease of the nonradiative decay rate in changing from the nonpolar cyclohexane to polar acetonitrile. The solvent is treated using the polarizable continuum model. We analyse different isomers and obtain the absorption transition energies. Similarly, after obtaining the relaxed geometry of the excited state, the emission transition is obtained. In both cases, a red shift of the singlet transition is obtained with increasing solvent polarity. In contrast, the low-lying triplet states are essentially solvent independent. The calculated solvent shift in the emission is in very good agreement with experiment. The Stokes shifts are also in fair agreement with experiment when long-range or dispersion-corrected functionals are used. There are three calculated triplet states lying below the S 1 state. As they are π → π* states, they cannot participate in the intersystem crossing by the El-Sayed rule. However, a fourth triplet state of n → π* nature is found only slightly above the Franck–Condon position of the S 1 state, and the S 1 → T 4 separation increases with the solvent polarity, this being a possible explanation for the decrease of the nonradiative transition.

Acetone photophysics at 282 nm excitation at elevated pressure and temperature. II: Fluorescence modeling

This is the second in a series of two papers that presents an updated fluorescence model and compares with the new experimental data reported in the first paper, as well as the available literature data, to extend the range of acetone photophysics to elevated pressure and temperature conditions. This work elucidates the complete acetone photophysical model in terms of each and every competing radiative and non-radiative rate. The acetone fluorescence model is then thoroughly examined and optimized based on disparity with recently conducted elevated pressure and temperature photophysical calibration experiments. The current work offers insight into the competition between non-radiative and vibrational energy decay rates at elevated temperature and pressure and proposes a global optimization of model parameters from the photophysical model developed by Thurber (Acetone Laser-Induced Fluorescence for Temperature and Multiparameter Imaging in Gaseous Flows. PhD thesis, Stanford University Mechanical Engineering Department, 1999). The collisional constants of proportionality, which govern vibrational relaxation, are shown to be temperature dependent at elevated pressures. A new oxygen quenching rate is proposed which takes into account collisions with oxygen as well as the oxygen-assisted intersystem crossing component. Additionally, global trends in ketone photophysics are presented and discussed.

Theoretical Study of Conversion and Decay Processes of Excited Triplet and Singlet States in a Thermally Activated Delayed Fluorescence Molecule

Quantitative understanding of the photophysical processes is essential for developing novel thermally activated delayed fluorescence (TADF) materials. Taking as an example 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene, a typical TADF-active molecule, we calculated the interconversion and decay rates of the lowest excited singlet and triplet states at different temperatures as well as the prompt and delayed fluorescence efficiencies at 300 K at the first-principles level. Our results can reproduce well the experimentally available data. It is found that the reverse intersystem crossing rate (kRISC) is sharply increased by 3 orders of magnitude, while the other rates increase slightly or remain unchanged when the temperature rises from 77 to 300 K. Importantly, kRISC reaches up to 1.23 × 106 s–1 and can compete with the radiative and nonradiative decay rates of S1 (1.11 × 107 and 2.37 × 105 s–1) at 300 K, leading to an occurrence of delayed fluorescence. In addition, our calculations indicate that it i...

Excited state intersystem crossing and the relaxation dynamics of phosphorescent Ir(III) complexes bearing bipyridine-based C^N ligand

Abstract The photo-deactivation mechanism of heteroleptic Ir(III)(C^N)2 (LX) complexes [(dfpypy)2Ir(pic)](1) and [(dfppy)2Ir(pic)](2) [where dfpypy = 2′,6′-difluoro-2,3′-bipyridine, dfppy = 2-(2,4-difluorophenyl)pyridine, pic = 2-picolinate] were elucidated by density functional theory/time-dependent density functional theory (DFT/TD-DFT) to rationalize the nature of the significant differences in luminescence efficiency between 1 and 2. Specifically, the radiative deactivation was formulated through the calculation of zero-field splitting (ZFS) and the radiative decay rate constant (kr) based on spin-orbit coupling (SOC). Meanwhile, the non-radiative deactivation was estimated by considering the structural distortion, the SOC between the emitting state and the ground state, as well as the thermal deactivation process via metal-centered excited 3MC state. Based on the results, 1 has much smaller SOC matrix elements between T1 and S0 than 2, which determines its small non-radiative decay rate constant, thus one may understand why 1 has higher phosphorescence quantum efficiency than 2. To further explore the structure-property relationship of Ir(III) complexes, four other new complexes 3–6 were designed by incorporating trimethylphenyl(R1), phenyl(R2), ter-butyl (R3), diphenylamine(R4) to the pyridine rings of dfpypy ligand of 1, respectively. Through analyzing, complex 4 with larger radiative decay rate and smaller non-radiative decay rate may be considered as a potential candidate as robust blue-emitting material applied in organic light emitting diodes (OLEDs).

Excited-State Electron and Energy Transfer Dynamics between 2D MoS2 and GO/RGO for Turn ON BSA/HSA Sensing

We report excited-state electron and energy transfer processes from two-dimensional MoS2 to graphene oxide (GO) and reduced graphene oxide (RGO) and also extend its application toward turn ON sensing of bovine serum albumin (BSA) and human serum albumin (HSA). Steady-state fluorescence results reveal the quenching of MoS2 QDs by GO and RGO. The fluorescence quenching efficiency is found to be more in the presence of GO than with RGO. The obtained Stern–Volmer plot suggests the occurrence of both static and dynamic quenching processes. The electron transfers from photoexcited MoS2 QDs to GO and RGO cause reduction and electron storage reactions, respectively. Time-resolved fluorescence results show significant reduction in the lifetime of a fast decay component of MoS2 upon addition of GO and RGO, which provides evidence for the excited-state energy transfer interaction in addition to electron transfer between MoS2 and GO/RGO. The calculated nonradiative decay rate indicates that the energy transfer proces...

Reduction of non-radiative decay rates in boro-tellurite glass via silver nanoparticles assisted surface plasmon impingement: Judd Ofelt analysis

We report an enhancement in the absorption and emission cross-section of erbium ions (Er3+) inside zinc-boro-tellurite glass due to the impingement of silver (Ag) nanoparticles (NPs) into the host matrix. The impacts of varying Ag NPs concentration on the spectroscopic, structural, and thermal properties of such glass samples are evaluated. Synthesized glass samples are characterized using imaging and spectroscopic tools. XRD spectra confirmed the amorphous nature of glass samples and EDX spectra detected appropriate elements present in the glass including Ag. TEM micrographs revealed the distribution of Ag NPs with average size ~8.4nm. FTIR spectra exhibited the fundamental stretching vibrations modes of glass network. Two surface plasmon resonance (SPR) peaks of Ag are divulged at 550 and 580nm. Bonding parameters displayed the ionic nature of the Er−O metal-ligand linkages. Judd-Ofelt analysis is performed to calculate the radiative transition probability, stimulated emission cross-section, radiative lifetime and branching ratio for the excited levels of Er3+ ions in the prepared glass system. Intensity parameters and quality factors are calculated. Silver NPs impingement is found to augment the luminescence intensity by a factor as much as 3.32 times than the glass without NPs. The decay curves for the 4S3/2 level of Er3+ ion are recorded under 476nm excitations, where the emission at 550nm is registered. The lifetime is elongated from 6.36μs to 9.07μs with increasing silver NPs concentration from 0.0 to 0.9mol%. Present glass compositions are established to be promising for the development of photonic devices.

Synthesis, characterization and photo-physical properties of Eu(III) complexes and its luminescent thin films

Luminescent chiral europium(III) complexes EuLRCl3 (1) and EuLSCl3 (2) were synthesised [where LR and LS are schiff-base ligands of 2,9-dialdehyde 1,10-phenanthroline and (R) and (S) isomer of 2-amino-3-phenylpropanol]. These europium complexes depicting intense “red luminescence” upon incorporating them in polyvinyl alcohol (PVA) matrices a series of transparent, flexible thin films were fabricated. These complexes 1, 2 and the films 1a-1c, 2a-2c were investigated for their photophysical properties such as radiative (ARAD) and non-radiative (ANR) decay rates, lifetimes (τobs), radiative lifetime (τRAD), intrinsic quantum yield (ΦLn), energy transfer efficiencies (Φsen) and total quantum yields (Φoverall). This PVA material at higher concentration demonstrates an interesting LRET (Luminescence Resonance Energy Transfer) behaviour. Based on the detailed investigation of the photo-luminescent properties, a plausible mechanism was proposed for energy transfer process.

Effect of n-propyl substituents on the emission properties of blue phosphorescent iridium(iii) complexes.

Ligand substitution is often used for tuning the emission color of phosphorescent iridium(iii) complexes that are used in organic light-emitting diodes. However, in addition to tuning the emission color, the substituents can also affect the radiative and non-radiative decay rates of the excited state and hence the photoluminescence quantum yield. Understanding the substituent effect is therefore important for the design of new iridium(iii) complexes with specific emission properties. Using (time dependent) density functional methods, we investigate the substituent effect of n-propyl groups on the structure, emission color, and emission efficiency of fac-tris(1-methyl-5-phenyl-[1,2,4]triazolyl)iridium(iii) based phosphorescent complexes by comparing the calculated results for structural models with and without the n-propyl substituents. We find that attachment of the n-propyl groups increases the length of three Ir-N bonds, and although the emission color does not change significantly, the radiative and non-radiative rates do, leading to a prediction of enhanced blue phosphorescence emission efficiency. Furthermore, the calculations show that the attachment of the n-propyl groups leads to a larger activation energy to degradation and the formation of dark states.

TDDFT study on recognition mechanism for the oxygen sensing of the cyclometalated platinum (II) complex

The influence of oxygen molecule on the luminescent properties of a cyclometalated Pt(II) complex Lxp1, was investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Analysis of frontier molecular orbitals and electronic configuration indicated that the highest-occupied molecular orbital of the Lxp1 has a significant mixture of metal Pt (d) as well as 2-phenylpyridine and acetyl acetone(π). The lowest-unoccupied orbital of the Lxp1 primarily locates on π* of 2-phenylpyridineligands. The emission mechanism of the cyclometalated Pt(II) complex Lxp1 is assigned to the mixing of ligand-to-metal charge transfer and ligand-to-ligand charge transfer. The emission mechanism of the Lxp1–O2 complex can be attributed to the charge transfer from the oxygen molecule to the luminescent material Lxp1. Our study showed that intermolecular hydrogen bonding between the Lxp1 and oxygen molecule was strengthened by the calculation of electronic excitation, leading to a luminescence-decreasing phenomenon. The calculation of the radiative and non-radiative decay rate constants of the Lxp1 and the Lxp1–O2 complex demonstrates that the phosphorescence from T1-S0 of the Lxp1 would alter to the internal conversion from T1-T0 of the Lxp1–O2 complex. This alteration further explains the luminescence quenching phenomenon of the cyclometalated Pt(II) complex Lxp1 after interacting with oxygen molecule.