Triplet State Quantum Yields Research Articles

Twisted Benzothieno-Fused BOPHY Dyes as Triplet Photosensitizers with Long-Lived Triplet Excited States.

The synthesis of a series of photostable [b]-benzothieno-fused BOPHY derivatives is reported via one-pot condensation of formylated isoindoles or formylindoles with hydrazine and subsequent boron complexation. These dyes show strong absorption in the deep red region and acceptable fluorescence quantum yields (∼30%). The two fused benzothiophene moieties are slightly deviated from the BOPHY core (with dihedral angles of 6.1 and 10.2°). This slightly twisted conformation brings an enhanced spin-orbit coupling and a reduced energy band gap between singlet and triplet states. The enhanced intersystem cross process endows these series of dyes with a good singlet oxygen quantum yield (up to 63%), a high triplet-state quantum yield (up to 78%), and a long lifetime value (up to 127 μs). Density functional theory calculations indicate that the transition from S1 to T2 states is crucial for triplet-state formation, highlighting their high efficiency in intersystem crossing. The calculated triplet electron spin surface reveals a widespread distribution of triplet states across the conjugated molecular structure, which enhances the Dexter mechanism for triplet energy transfer in these BOPHY photosensitizers. These findings are helpful for thorough understanding of the fundamental ISC process and developing triplet photosensitizers.

Origin of Intersystem Crossing in Red-Light Absorbing Bodipy Derivatives: Time-Resolved Transient Optical and Electron Paramagnetic Resonance Spectral Studies with Twisted and Planar Compounds.

We studied the intersystem crossing (ISC) property of red-light absorbing heavy atom-free dihydronaphtho[b]-fused Bodipy derivatives (with phenyl group attached at the lower rim via ethylene bridge, taking constrained geometry, i.e., BDP-1 and the half-oxidized product BDP-2) and dispiroflourene[b]-fused Bodipy (BDP-3) that have a twisted π-conjugated framework. BDP-1 and BDP-3 show strong and sharp absorption bands (i.e., ε = 2.0 × 105 M-1 cm-1 at 639 nm, fwhm ∼491 cm-1 for BDP-3). BDP-1 is significantly twisted (φ = 21.6°), while upon mono-oxidation, BDP-2 becomes nearly planar on the oxidized side (φ = 3.5°). Interestingly, BDP-2 showed efficient ISC (triplet state quantum yield, ΦT = 40%) due to S1/T2 state energy matching. Long-lived triplet excited state was observed (τT = 212 μs in solution and 2.4 ms in polymer matrix), and ISC takes 4.0 ns. Differently, twisted BDP-1 gives weak ISC only 5%, ISC takes 7.7 ns, and the triplet state is populated only with addition of ethyl iodide. Time-resolved electron paramagnetic resonance spectra of BDP-1 revealed the coexistence of two triplet states, with drastically different zero-field splitting D parameters of -2047 MHz and -1370 MHz, respectively, along with varying sublevel population ratios. We demonstrate that the ISC is not necessarily enhanced by torsion of the π-conjugation framework; instead, S1/Tn state energy matching is more efficient to induce ISC even in compounds that have planar molecular structures.

Multi‐Resonance Thermally Activated Delayed Fluorescence Molecules for Triplet‐Triplet Annihilation Upconversion

AbstractTriplet‐triplet annihilation upconversion (TTA‐UC) has made significant progress in recent years in several key applications, including solar energy harvesting, photocatalysis, stereoscopic 3D printing, and disease therapeutics. In TTA‐UC research, photosensitizers serve the vital function of harvesting low‐energy photons. The photophysical characteristics of photosensitizers, including absorbance, triplet state quantum yield, triplet state energy level, triplet state lifetime, etc., determine the performance of TTA‐UC. Thus, the study of photosensitizers has been a key aspect of TTA‐UC. In recent years, multi‐resonance thermally activated delayed fluorescence (MR‐TADF) molecules have received extensive attention due to their excellent photophysical properties and electroluminescent device performance. MR‐TADF molecules not only present a narrow energy gap between the singlet and triplet excited states, but also have stronger absorption and better wavelength regulation than conventional TADF molecules. Nowadays, the preliminary attempts in TTA‐UC using MR‐TADF molecules as photosensitizers have resulted in the development of green to ultraviolet, blue to ultraviolet, and even near‐infrared to blue emission. This concept will summarize the research progress of MR‐TADF molecules as photosensitizers in TTA‐UC, analyzing the challenges and giving possible solutions. Finally, we prospect the future development of MR‐TADF molecules as photosensitizers, including the molecular design as well as the possible application areas.

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.

Radical enhanced intersystem crossing mechanism, electron spin dynamics of high spin states and their applications in the design of heavy atom-free triplet photosensitizers.

Heavy atom-free triplet photosensitizers (PSs) can overcome the high cost and biological toxicity of traditional molecular systems containing heavy atoms (such as Pt(II), Ir(III), Ru(II), Pd(II), Lu(III), I, or Br atoms) and, therefore, are developing rapidly. Connecting a stable free radical to the chromophore can promote the intersystem crossing (ISC) process through electron spin exchange interaction to produce the triplet state of the chromophore or the doublet (D) and quartet (Q) states when taking the whole spin system into account. These molecular systems based on the radical enhanced ISC (REISC) mechanism are important in the field of heavy atom-free triplet PSs. The REISC system has a simple molecular structure and good biocompatibility, and it is especially helpful for building high-spin quantum states (D and Q states) that have the potential to be developed as qubits in quantum information science. This review introduces the molecular structure design for the purpose of high-spin states. Time-resolved electron paramagnetic resonance (TREPR) is the most important characterization method to reveal the properties of these molecular systems, generation mechanism and electron spin polarization (ESP) of the high spin states. The spin polarization manipulation of high spin states and potential application in the field of quantum information engineering are also summarized. Moreover, molecular design principles of the REISC system to obtain long absorption wavelength, high triplet state quantum yield and long triplet state lifetime are introduced, as well as applications of the compounds in triplet-triplet annihilation upconversion, photodynamic therapy and bioimaging. This review is useful for the design of heavy atom-free triplet PSs based on the radical-chromophore molecular structure motif and the study of the photophysics of the compounds, as well as the electron spin dynamics of the multi electron system upon photoexcitation.

Wavelength dependent excited state dynamics observed in canonical pyrimidine nucleosides

Epidemiological evidence indicates that damage to DNA/RNA initialized by ultraviolet (UV) radiation is associated with skin cancer. Wavelength dependence of DNA photodamage was proposed as early as 1990s and demonstrated later on. Unraveling the photo-activated dynamics involved in related reactions is essential. However, studies aimed at uncovering the wavelength dependent excited state dynamics in canonical pyrimidine nucleosides have not received enough attention. In this work, excitation wavelength dependent excited state dynamics of 2′-deoxy-thymidine (dThd) and oxy-uridine (Urd) are investigated in acetonitrile solutions by femtosecond broadband transient absorption spectroscopy. Varying the excitation wavelength leads to a significant difference in the branching of the excited state population at the Franck-Condon (FC) region, resulting higher fluorescence quantum yield with 285 nm pump but higher triplet state quantum yield under 267 nm excitation. Based on our results, a vibronic coupling regulated excited state relaxation mechanism is proposed. This mechanism information is important for understanding the formation of harmful photoproducts for DNA/RNA with different wavelength UV excitations.

Photobleaching Kinetics and Effect of Solvent in the Photophysical Properties of Indocyanine Green for Photodynamic Therapy.

Indocyanine green is an attractive molecule for photodynamic therapy due to its near infrared absorption, resulting in a higher tissue penetration. However, its quantum yields of the triplet and singlet state have been reported to be low and then, reactive oxygen species are unlikely to be formed. Aiming to understand the ICG role in photodynamic response, its photobleaching behavior in solution has been studied under distinct conditions of CW laser irradiation at 780 and 808 nm, oxygen saturations and solvents. Sensitizer bleaching and photoproduct formation were measured by absorption spectroscopy and analyzed using the PDT bleaching macroscopic model to extract physical parameters. ICG photobleaching occurs even at lower oxygen concentrations, indicating that the molecule presents more than one way of degradation. Photoproducts were produced even in solution of less than 4 % oxygen saturation for both solvents and excitation wavelengths. Also, the amplitude of absorption related to J-dimers was increased during irradiation, but only in 50 % PBS solution. The formation of photoproducts was enhanced in the presence of J-type dimers under low oxygen concentration, and the quantum yields of triplet and singlet states were one order of magnitude and two times higher, respectively, when compared to ICG in distilled H2 O.

Determination of excited triplet states of dissolved organic matter using chemical probes: A comparative and mechanistic study

Dissolved organic matter (DOM) plays an important role in the biogeochemical cycle in natural waters. The determination and characterization of the excited triplet state of DOM (3DOM*) have attracted much attention recently. However, the underlying differences of determined 3DOM* through different pathways are not yet fully understood. In this study, the differences and underlying mechanisms of the determined 3DOM* using 2,4-hexadien-1-ol (HDO) through an energy transfer pathway and 2,4,6-trimethylphenol (TMP) through an electron transfer pathway, were investigated. The results showed that the determined quantum yields of 3DOM* (Φ3DOM*) for four commercial and four isolated local DOMs are different using HDO ((0.04 ± 0.00) × 10−2 to (2.9 ± 0.17) × 10−2)) and TMP ((0.08 ± 0.01) × 10−2 to (1.2 ± 0.17) × 10−2), respectively. For 17 DOM-analogs, significant differences were also observed with the quantum yields of their 3DOM* determined using HDO (ΦHDO) and the triplet-state quantum yield coefficients determined using TMP (fTMP). It indicates the different reactivity of TMP and HDO with the excited triplet of the chromophores with different structures within the isolated DOM. Based on the experimental and predicted values of fTMP and ΦHDO for different DOM-analogs, the impact of substituents on differences in 3DOM* values were further revealed. These results demonstrated that the levels of 3DOM* depended on the chemical functionalities present in the DOM-analogs.

Biocompatible Aza-BODIPY-Biotin Conjugates for Photodynamic Therapy of Cancer.

With an objective to develop efficient photosensitizers to cancerous tissues, we synthesized two novel biocompatible sensitizers based on aza-BODIPYs incorporated with heavy atoms and biotin moieties. The bioconjugates DPR2a and DPR2b exhibited a favorable absorption range (600-750 nm) with excellent triplet-state quantum yields (up to 79%) and singlet oxygen generation yields (up to 75%). In vitro photobiological investigations employing MDA-MB-231 breast cancer cell lines exhibited rapid cellular uptake, negligible dark toxicity, and high photocytotoxicity. The mechanism of cell death of these systems was predominantly due to the mitochondrial damage, leading to apoptosis mediated via the generation of singlet oxygen-triggered reactive oxygen species. The in vivo studies with the representative conjugate DPR2a employing female NOD/SCID mice models showed inhibition in tumor growth and significantly decreased tumor volume post photodynamic therapy (PDT) treatment. Our results validate that both DPR2a and DPR2b with iodine incorporation exhibit favorable and superior photophysical and photobiological aspects and demonstrate thereby their potential applications in imaging and PDT of cancer.

Photochemical Processes of Superbase Generation in Xanthone Carboxylic Salts

AbstractPhotobase generators are species that allow the photocatalysis of various reactions, such as thiol‐Michael, thiol‐isocyanate, and ring‐opening polymerization reactions. However, existing compounds have complex syntheses and low quantum yields. To overcome these problems, photobase generators based on the photodecarboxylation reaction were developed. We synthesized and studied the photochemistry and photophysics of two xanthone photobase, their carboxylic acid precursors, and their photoproducts to understand the photobase generation mechanism. We determined accurate quantum yields of triplet states and photobase generation. The effect of the intermediate radical preceding the base release was demonstrated. We characterized the photophysics of the photobase by femtosecond spectroscopy and showed that the photodecarboxylation process occurred from the second excited triplet state with a rate constant of 2.2×109 s−1.

Photochemical Processes of Superbase Generation in Xanthone Carboxylic Salts.

Photobase generators are species that allow the photocatalysis of various reactions, such as thiol-Michael, thiol-isocyanate, and ring-opening polymerization reactions. However, existing compounds have complex syntheses and low quantum yields. To overcome these problems, photobase generators based on the photodecarboxylation reaction were developed. We synthesized and studied the photochemistry and photophysics of two xanthone photobase, their carboxylic acid precursors, and their photoproducts to understand the photobase generation mechanism. We determined accurate quantum yields of triplet states and photobase generation. The effect of the intermediate radical preceding the base release was demonstrated. We characterized the photophysics of the photobase by femtosecond spectroscopy and showed that the photodecarboxylation process occurred from the second excited triplet state with a rate constant of 2.2×109 s-1 .

Spiro rhodamine-coumarin compact electron donor-acceptor dyads: synthesis and spin-orbit charge transfer intersystem crossing.

We prepared spiro rhodamine (RB)-coumarin (Cou) compact electron donor-acceptor dyads (RB-Cou-CF3 and RB-Cou-CN), to study the charge transfer (CT) and spin-orbit CT intersystem crossing (SOCT-ISC). The π-conjugation planes of the rhodamine and coumarin units in both dyads are in nearly orthogonal geometry (dihedral angle: 86.3°). CT state emission was observed for RB-Cou-CF3 (at 550nm) and RB-Cou-CN (at 595nm). Although the fluorescence of the pristine coumarin units (fluorescence quantum yields ΦF = 59%) was quenched in the dyads (ΦF = 0.5 ~ 1.1% in n-hexane), the triplet state quantum yields of the dyads are also low (singlet oxygen quantum yield, ΦΔ = 2.3-7.5% in n-hexane). Nanosecond transient absorption spectra show that the 3Cou* state was formed, which shows a triplet state lifetime of 11-15.6μs. The proposed photophysical path for the dyads is as follows: RB-1Cou* → RB+•-Cou-• → RB-3Cou*. The low SOCT-ISC yield is attributed to the slightly lower charge-transfer state energy (1.94eV in toluene) as compared to the 3Cou* state energy (2.23eV) and the shallow potential energy curve (PEC) at energy minima of the dyads. This work indicates that orthogonal conformation of donor-acceptor units is inadequate for achieving efficient SOCT-ISC. These results are useful for studying charge separation and intersystem crossing of electron donor/acceptor dyads.

Novel selenium-containing photosensitizers for near-infrared fluorescence imaging-guided photodynamic therapy.

Benzopyran nitrile dyes cannot be used as qualified photosensitizers due to the low quantum yield of triplet state. The benzopyran derivatives containing selenium instead of oxygen atom based on the heavy atom effect are expected to become potential agents for photodynamic therapy. In this paper, a series of selenium-containing photosensitizers (PSX) were prepared according to this strategy. PSX can effectively produce both singlet oxygen and superoxide anions upon laser irradiation. PSX exhibited the emission wavelength at 500-800nm and near-infrared (NIR) fluorescence imaging in HeLa cells. Excellent biocompatibility and phototoxicity further indicated that PSX could be used as efficient photosensitizers for NIR fluorescence imaging and photodynamic therapy.

Wavelength trends of photoproduction of reactive transient species by chromophoric dissolved organic matter (CDOM), under steady-state polychromatic irradiation

The formation quantum yields of photochemically produced reactive intermediates (PPRIs) by irradiated CDOM (in this study, Suwannee River Natural Organic Matter and Upper Mississippi River Natural Organic Matter) decrease with increasing irradiation wavelength. In particular, the formation quantum yields of the excited triplet states of CDOM (3CDOM*) and of singlet oxygen (1O2) have an exponentially decreasing trend with wavelength. The •OH wavelength trend is different, because more effective •OH production occurs under UVB irradiation than foreseen by a purely exponential function. We show that the parameter-adjustable Weibull function (which adapts to both exponential and some non-exponential trends) is suitable to fit the mentioned quantum yield data, and it is very useful when CDOM irradiation is carried out under polychromatic lamps as done here. Model calculations suggest that, thanks to the ability of CDOM to also absorb visible radiation, and despite its decreasing quantum yield of •OH generation with increasing wavelength, CDOM would be able to trigger •OH photogeneration in deep waters, to a higher extent than UVB-absorbing nitrate or UVB + UVA-absorbing nitrite.

Comprehensive Investigation of the Photophysical Properties of Alkynylcoumarin Gold(I) Complexes.

Six gold(I) complexes (R3P-Au-Coum) containing three different alkynylcoumarin chromophores (Coum) with different electron-donating and electron-withdrawing characteristics and two different water-soluble phosphanes (PR3 = PTA (a) and DAPTA (b)) have been synthesized (1a,b, unsubstituted coumarin; 2a,b, 4-methyl substituted coumarin; 3a,b, 3-chloro and 4-methyl substituted coumarin). A comprehensive study of the photophysical properties of the R3P-Au-Coum, together with their propynyloxycoumarin precursors 1-3, was performed in solution at room and low temperatures. Spectral and photophysical characteristics of the R3P-Au-Coum essentially depend on the electronic characteristics of the propynyloxycoumarin ligand. The presence of the Au(I) atom was found to be responsible for an increase of the intersystem crossing, with triplet state quantum yield values, ϕT, ranging from ∼0.05 to 0.35 and high coumarin phosphorescence quantum yield values for derivatives 1 and 2; fluorescence dominates the deactivation in derivatives 3. Efficient singlet oxygen photosensitization was observed for the new compounds 3a,b. From TDDFT calculations, the relevant HOMO and LUMO of the compounds, i.e., those involved in the transitions, are dominated by the frontier orbitals associated with the coumarin core. The Au(I)-phosphane structure introduces a new transition assigned to an intraligand transition involving the phosphane ligand, and π(C≡C) system, to the p orbitals of phosphorus and gold atoms.

Twisted BODIPY derivative: intersystem crossing, electron spin polarization and application as a novel photodynamic therapy reagent

The photophysical properties of a heavy atom-free BODIPY derivative with a twisted π-conjugated framework were studied. Efficient intersystem crossing (ISC quantum yield: 56%) and an exceptionally long-lived triplet state were observed (4.5 ms in solid polymer film matrix and 197.5 μs in solution). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy and DFT computations confirmed the delocalization of the triplet state on the whole twisted π-conjugated framework and the zero-field-splitting (ZFS) D parameter of D = -69.5 mT, which is smaller than that of 2,6-diiodoBODIPY (D = -104.6 mT). The electron spin polarization (ESP) phase pattern of the triplet state TREPR spectrum of the twisted BODIPY is (a, a, e, a, e, e), which is different from that of 2,6-diiodo BODIPY (e, e, e, a, a, a), indicating that the electron spin selectivity of the ISC of the twisted structure is different from that of the spin orbital coupling effect. According to the computed spin-orbit coupling matrix elements (0.154-1.964 cm-1), together with the matched energy of the S1/Tn states, ISC was proposed to occur via S1→T2/T3. The computational results were consistent with TREPR results on the electron spin selectivity (the overpopulation of the TY sublevel of the T1 state). The advantage of the long-lived triplet state of the twisted BODIPY was demonstrated by its efficient singlet oxygen (1O2) photosensitizing (ΦΔ = 50.0%) even under a severe hypoxia atmosphere (pO2 = 0.2%, v/v). A high light toxicity (EC50 = 1.0 μM) and low dark toxicity (EC50 = 78.5 μM) were observed for the twisted BODIPY, and thus the cellular studies demonstrate its potential as a novel potent heavy atom-free photodynamic therapy (PDT) agent.

A photosensitizer with conformational restriction for enhanced photodynamic therapy.

A rigid hemicyanine CSZ-J and a flexible molecule ESZ-J were synthesized. In particular, the conformationally restrained CSZ-J had higher fluorescence quantum yields, longer fluorescence lifetimes and higher triplet state quantum yields. CSZ-J could generate highly cytotoxic ROS simultaneously via type I and type II processes. This will contribute to the design and development of new photosensitizers in the future.

3,5-Anthryl-Bodipy dyad/triad: Preparation, effect of F-B-F induced conformation restriction on the photophysical properties, and application in triplet-triplet-annihilation upconversion.

We prepared a series of compact Bodipy-anthryl electron donor/acceptor triads and dyads by attaching anthryl moieties at the 3-,5-positions of the Bodipy core, with a novel conformation restriction approach, to study the spin-orbit charge transfer intersystem crossing (SOCT-ISC). The conformation restrictions are imposed by the BF2 unit of Bodipy without invoking the previously reported method with 1,7-dimethyl or 1,3-dimethyl groups. Our new approach shows a few advantages, including the stronger electron accepting ability of the methyl-free Bodipy core (reduction potential anodically shifted by +0.3 V vs the methylated Bodipy), red-shifted absorption (by 21 nm), and longer triplet state lifetime (372 µs vs 126 µs). The effects of the different mutual orientations of the electron donor and acceptor on ultraviolet-visible absorption, fluorescence, triplet state quantum yields, and lifetimes were studied. Triads with orthogonal geometries show higher singlet oxygen quantum yields (ΦΔ = 37%) than those with more coplanar geometries. Since the non-radiative decay for the S1 state is significant in the parent Bodipy chromophore (ΦF = 6.0%), we propose that in dyads/triads, the charge separation and recombination-induced ISC outcompete the non-radiative decay to the ground state, which is new in the study of SOCT-ISC. Density functional theory computation indicated a shallow torsion potential energy curve as compared to the meso-anthryl-Bodipy dyad analog, which may contribute a low triplet state quantum yield of the new dyads/triads. Triplet-triplet annihilation upconversion was performed with the electron donor/acceptor dyads as the triplet photosensitizer, with an upconversion quantum yield of 12.3%.

In vitro antiproliferative effect of four ball-type phthalocyanines linked by t-butylcatechol and high singlet oxygen production

Four ball-type phthalocyanines (H2Pc, MgPc, CoPc, and ZnPc) containing 4-tert-butylcatechol substituents at alpha-positions were synthesized from 3,3′-(4-tert-butyl-1,2-phenylene)bis(oxy)diphthalonitrile. The structures were elucidated by elemental analysis and ultraviolet-visible, Fourier-transform infrared, proton nuclear magnetic resonance, and Maldi-Tof mass spectroscopies. The photophysicochemical properties of H2Pc, MgPc, and ZnPc were investigated in dimethylsulphoxide. The fluorescence quantum yields (ΦF), triplet state quantum yields (ΦT), and singlet oxygen quantum yields (ΦΔ) with respect to H2Pc, MgPc and ZnPc were experimentally obtained. Furthermore, the degradation quantum yield (Φd) was evaluated for all complexes. Finally, the magnetic circular dichroism spectra of the four Pcs were obtained. Among these ball-type phthalocyanines, MgPc had the highest triplet lifetime and ZnPc had the highest singlet oxygen quantum yield. Moreover, the in vitro antiproliferative effect of ball-type phthalocyanines (H2Pc, MgPc, CoPc, and ZnPc) was evaluated on MCF-7 cell line originated from luminal A breast cancer and MDA-MB-231 cell line originated from triple negative breast cancer. For this purpose, cell viability MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was used. For all compounds, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, 25.6, 51.2, 102.4 µM concentrations were applied to both cell lines. There was a significant decrease in cell viability for MCF-7 and MDA-MB-231 cells (p < 0.05).

Method for monitoring singlet oxygen quantum yield in real time by time resolved spectroscopy measurement.

The singlet oxygen quantum yield (ΦΔ) was monitored in real time through time resolved spectroscopy measurement, using gadolinium labeled hematoporphyrin monomethyl ether (Gd-HMME) as photosensitizer. According to the kinetics equations of singlet oxygen generation and reaction, ΦΔ was related to phosphorescence lifetime (τp). Through measuring τp of Gd-HMME in different oxygen conditions, the radiation transition property of first exited triplet state (T1) was monitored; combined with the triplet state quantum yield (ΦT) determined by linear fitting the ΦΔ, which was measured in different oxygen content using a relative measurement, ΦΔ can be determined in real time. The identification of anoxia during the treatment of photodynamic therapy (PDT) by this method is also presented.