Twenty-fold Enhancement of Gadolinium-Porphyrin Phosphorescence

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Twenty-fold Enhancement of Gadolinium-Porphyrin Phosphorescence at Room Temperature by Free Gadolinium Ion in Liquid Phase Lixin Zang, Huimin Zhao, Yangdong Zheng, Feng Qin, Jianting Yao, Ye Tian, Zhiguo Zhang, and Wenwu Cao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08783 • Publication Date (Web): 21 Nov 2015 Downloaded from http://pubs.acs.org on November 30, 2015

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The Journal of Physical Chemistry

Twenty-fold Enhancement of Gadolinium-Porphyrin Phosphorescence at Room Temperature by Free Gadolinium Ion in Liquid Phase ◇

Lixin Zang,† Huimin Zhao,† Yangdong Zheng, Feng Qin, † Jianting Yao,‡ Ye Tian,‡ ,* Zhiguo Zhang,†,* and Wenwu Cao†,§,* †

Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin 150080, China

◇

‡

Department of Physics, Harbin Institute of Technology, Harbin 150001, China

Division of Cardiology, the First Affiliated Hospital, Harbin Medical University, Harbin 150001, China

§

Department of Mathematics and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA

*Corresponding Authors: Ye Tian*

Email: *[email protected].

Zhiguo Zhang*


Wenwu Cao*


ACS Paragon Plus Environment

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ABSTRACT: The influence of free gadolinium ion (Gd3+) on room temperature phosphorescence (RTP) of gadolinium labeled hematoporphyrin monomethyl ether (Gd-HMME) was studied. Twenty-fold enhancement of Gd-HMME RTP by the titration of free Gd3+ was achieved. We found that the absorption from the ground (S0) to singlet excited states of GdHMME did not change with the addition of Gd3+, which means that the phosphorescence quantum yield has been tuned from 1.4% to 28%. According to the excitation spectra, the transition possibility from S0 to the lowest triplet excited state (T1) is increased by 1.2-fold because of the heavy atom effect of free Gd3+. The phosphorescence lifetime of Gd-HMME with free Gd3+ is 7.0-fold greater than that of Gd-HMME itself, which demonstrates that the nonradiative processes of Gd-HMME is decreased by Gd3+ with the rate constant of non-radiative processes decreasing from 2.2(2)×105 s-1 to 2.8(2)×104 s-1. This sharp decrease is mainly responsible for the huge enhancement of Gd-HMME RTP. The reason for the decrease of nonradiative processes is due to the formation of a rigid microenvironment, which protects GdHMME from being quenched.

KEYWORDS: heavy atom effect, states mixing, phosphorescence enhancement, excitation spectrum, lifetime.


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1. INTRODUCTION Room temperature phosphorescence (RTP) has been developed for applications in many fields, including electroluminescence,1,2 solar cells,3-7 photocatalysis,8-11 luminescent molecular probes,12,13 triplet-triplet annihilation based upconversions14-22 and optical sensing23 with several important advantages23, such as larger Stokes’ shift, longer lifetime24,25, better selectivity23, etc. However, the practical use of RTP in liquid phase was limited due to its relatively low intensity because phosphorescence is a spin-forbidden process26,27. In contrast to fluorescence, phosphorescence transition in porphyrin itself does not belong to an allowed electric dipole transition. This means that the phosphorescence transition rate is very small causing the weak intensity.28-30 It is very hard to detect phosphorescence from free-base porphyrins although phosphorescence of free-base porphyrin was once observed by Tsvirko et al31. Deoxygenation methodology, rigid microenvironmental systems32,33, and heavy atom effect (HAE)34-43 are the main method to enhance phosphorescence emission in fluid solution. Minaev et al has theoretically demonstrated that the existence of a central metal ion can induce RTP emission from porphyrins.41 The rapid development of RTP in liquid phase is benefited from the exploitation of metalloporphyrins. 44-47 Most studies about metalloporphyrins phosphorescence have focused on Pt(II)- and Pd(II)-porphyrins45,46, there are very few reports on RTP of gadolinium labeled porphyrins48. Gadolinium labeled porphyrins are potential multifunctional agents, i.e., as Magnetic Resonance Imaging (MRI) contrast agents, oxygen sensors49-51 and photosensitizers used in photodynamic therapy. Gadolinium porphyrins have relatively high quantum yield of triplet states because of HAE43 and special energy levels of Gd3+ ([Xe]4f7), i.e., the lowest excited energy level is above the first excited singlet and triplet states of porphyrin. Phosphorescence of


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Gd(III) Tetraphenylporphyrin (Gd-TPP) has been studied recently52 by Kopylova et al. Density functional theory calculations of few porphyrin lanthanide complexes phosphorescence lifetime were carried out by Minaev's group41 and 3.6 microseconds had been obtained for Eu-complex. Besides, the phosphorescence of Gd-TPP in ethanol solutions and in thin films has been reported for optical oxygen sensing52. The phosphorescence quantum yield of Gd-TPP is only 10-3 at room temperature.52 In our previous studies, gadolinium labeled hematoporphyrin monomethyl ether (Gd-HMME) was demonstrated to display relative strong RTP in an air-saturated solution53. States mixing between the ground state and triplet excited state of Gd-HMME was produced by the coordination of Gd3+.54 However, the phosphorescence quantum yield of Gd-HMME (1.4% in air-saturated solution49) is inferior compared with widely used Pt(II)- and Pd(II)-porphyrins, the enhancement of Gd-porphyrins RTP in solutions is desirable for many practical applications. Herein, we report a significant enhancement of Gd-HMME RTP by adding free Gd3+ ions. The systematic experimental investigations of free Gd3+ ions’ effect on the phosphorescence enhancement and the lifetimes of the first triplet state (T1) are presented. Substantial enhancement of Gd-HMME RTP by free Gd3+ was found by phosphorescent spectroscopic analysis. To investigate the enhancement mechanism, the effect of free Gd3+ on states mixing of Gd-HMME was studied by excitation spectral analysis. 2. EXPERIMENTAL SECTION 2.1 Materials. Metal salts, including anhydrous GdCl3 and GdCl3·6H2O, were purchased from Aladdin Industrial Co. and were used without further purification. Hematoporphyrin monomethyl ether (HMME) was obtained from Shanghai Xianhui Pharmacuetical Co., Ltd., and methanol was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. as the solvent. High purity nitrogen was from Harbin Liming Co., Ltd.


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2.2 Preparation of samples. Gd-HMME was synthesized by a method described by Srivastava.55 The mixture of 6 g imidazole, 9.6 mg HMME and excess anhydrous GdCl3 (25 mg) was added into a 250 ml three-necked bottle with argon flow protection for 30 min before synthesis. Subsequently, the mixture was heated and kept at 200 ℃ and stirred magnetically for 2 hours protected with argon flow. Then the mixture was dissolved with methanol to get 10 ml 1.2 mg/ml (1.5 mM) Gd-HMME methanol solution after cooling down to room temperature. GdHMME with different concentrations of Gd3+ (GdCl3·6H2O) in methanol solutions was added into a silica cuvette to be measured. The solvation was performed at room temperature under 1 atm. 2.3 Measurements. A diode laser centered at 405 nm was used as the excitation light. Luminescence spectra of Gd-HMME with different concentrations of Gd3+ were recorded by a miniature fiber optic spectrometer (Ocean Optics USB2000). Each spectrum was obtained from the average of five independent measurements. All luminescence spectral measurements were performed at room temperature, presenting the same geometry for recording. UV-visible absorption spectra were recorded using a miniature fiber optic spectrometer (Ocean Optics QE65000) equipped with a deuterium lamp based on Beer-Lambert law. Excitation spectra of Gd-HMME with free Gd3+ were measured by monitoring the emission at the wavelength of 720 nm. A xenon lamp was used as the broadband light source. The grating with a 0.5 nm spectral resolution was used to produce monochromatic light from 670 nm to 694 nm. The sample to be measured in a quartz curette was put in a collimating light path. The emission signals were recorded via a photomultiplier tube (PMTH-S1-R212) placed in the perpendicular direction attached to an interference filter centered at 720 nm.


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To determine the lifetime of phosphorescence, the decay profile was measured. A square wave (10 kHz) was given to a diode laser controller (Thorlabs ITC510) to control a diode laser centered at 405 nm (Thorlabs TCLDM9). Phosphorescence signals were recorded by a grating spectrometer (Zolix Omni-λ300) and amplified by a photomultiplier tube (Zolix PMTH-S1R212) with a high voltage power supply (Zolix HVC1800). The time-resolved signal was averaged with a digital phosphor oscilloscope (Tektronix DPO5054) and the decay curve was sent to a personal computer for lifetime calculations by fitting the decay curve to an exponential function using adjustable parameters. 3. THEORETICAL BASIS Due to the coordination of gadolinium ion (Gd3+), Gd-HMME displays phosphorescence emission centered at 712 nm and 790 nm with the quantum yield of 1.4%.49 Compared with HMME, the spin-forbidden transition rule from T1 to S0 was broken for Gd-HMME. Fig. 1 presents the possible energy transfer process and the chemical structure of Gd-HMME. When a Gd-HMME molecule is excited by a 405 nm laser (hν), it absorbs photons and transfers from the ground state S0 to an excited singlet state (S1, S2…), then rapidly decays to the bottom of S1. The molecule either transfers from S1 to S0 with fluorescent emission (F) or to T1 by intersystem crossing (ISC, 80%54). Due to the coordination of Gd3+, there are states mixing between the singlet and triplet states of HMME, which is demonstrated by the direct absorption (AD) from S0 to T1.50 Gd-HMME in T1 returns to S0 with the phosphorescence emission (P, 1.4%), nonphosphorescent transition and energy transfer to surrounding quenchers (Q). The quantum yield of phosphorescence emission (ΦP) can be described as.

ΦP = ΦT

kP kP + knP + ∑ kq,m [Q]m

(1)


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where ΦT is the first triplet state formation quantum yield of Gd-HMME, kp, knp are rate constants of phosphorescence transition, non-phosphorescent transition, and Σkq,m[Q]m is the sum of all effective mono- and bi-molecular quenching rate constants of phosphorescence. [Q]m represents the concentration of each quencher. The sum of knP and Σkq,m[Q]m is the total rate constant of non-radiative processes of phosphorescence. Here the reciprocal of the sum (kp+knP+Σkq,m[Q]m) is identical to the observed phosphorescence lifetime (τP). From Eq. (1), it can be seen that ΦP can be improved by means of two ways. One is to increase

ΦT and kP by HAE, which mixes pure singlet and triplet states to produce states with a mixed character in spin multiplicity. Another is to reduce non-radiative rate constants (knP, Σkq,m[Q]m). 4. RESULTS AND DISCUSSION 4.1 Influence of coordination Gd3+ on the energy levels of HMME. The observation of phosphorescence emission from Gd-HMME indicates that the effect of central metal gadolinium ion on HMME is significant. To further investigate the influence of central Gd3+ on the energy levels of HMME, UV-visible absorption spectra of Gd3+, HMME and Gd-HMME at the same concentrations in methanol were measured as shown in Fig. S1 in the Supporting Information. There is a new absorption peak at around 325 nm from Gd-HMME. This indicates that GdHMME belongs to irregular metalloporphyrin56, similar to Pt(II)- and Pd(II)-porphyrins. That is, the central Gd3+ produces relative strong impact on the energy level structure of HMME. This strong impact of the central Gd3+ indicates that free Gd3+ may also produce influence on the energy transfer process of Gd-HMME. 4.2 Enhancement of Gd-HMME RTP by free Gd3+. To investigate the influences of free Gd3+ on the energy levels of Gd-HMME, luminescence spectra of Gd-HMME with different


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concentrations of Gd3+ were measured. Fig. 2(a) shows the luminescence spectra of Gd-HMME (48 µM) with various concentrations of Gd3+. It can be seen that the phosphorescence intensity at 712 nm increases with the concentration of Gd3+ while the fluorescence emission remains unchanged. Fig. 2(b) shows the calibration of the phosphorescence intensity versus the concentration of Gd3+. A linear relationship of the phosphorescence intensity versus the concentration of Gd3+ was obtained over the range from 0 to 19.2 mM. We can see that the phosphorescence intensity of Gd-HMME with 19.2 mM Gd3+ was approximately twenty-fold greater than that of Gd-HMME itself. In addition, when the concentration of Gd3+ became more than 80 mM, the RTP intensity went into a plateau and the original transparent solution looked cloudy after precipitation. Similar phenomenon was also observed in other published works27, 32. UV-visible absorption spectra of Gd-HMME with and without free Gd3+ were also measured as shown in Fig. 3. It is noted that the existence of Gd3+ did not change the UV-visible absorption spectrum. The Qx band in Gd-HMME has a vibronic origin and by this reason the absence of any shift for it is quite natural57. There is also no change in the Soret band upon adding Gd3+. This indicates that addition of Gd3+ has no influence on the electronic structure of Gd-HMME. It is worthwhile to point out that the phosphorescence quantum yield is tuned from 1.4% to 28% according to the 20-fold greater phosphorescence intensity and unchanged absorption of the excitation light. This high quantum yield of phosphorescence is comparable to that of metalloporphyrins with bright phosphorescence emission, such as Pt(II)- and Pd(II)coproporphyrin (about 10% and 20%).35 4.3 Mechanism of the phosphorescence enhancement. To understand the mechanism of the enhancement, the effect of free Gd3+ on the transition process of Gd-HMME was investigated. States mixing between singlet and triplet states of Gd-HMME was found in our previous work


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according to the direct absorption from the ground (S0) to triplet excited states (T1) centered at 686 nm and 711 nm.54 The influence of the free Gd3+ on the absorption from S0 to T1 was investigated to study the effect of free Gd3+ on states mixing. Excitation spectra for Gd-HMME (48 µM) with different concentrations of Gd3+ were recorded as shown in Fig. 4. The excitation spectra were recorded from 678 nm to 694 nm by monitoring at the wavelength of 720 nm. The excitation band centered at 686 nm was also found in the excitation spectra of Gd-HMME with different concentrations of Gd3+. Thus, the energy levels of T1 of Gd-HMME were not changed by the free Gd3+. On the other hand, the intensity of the excitation band increases monotonously with the concentration of Gd3+. That is, the free Gd3+ enhanced the absorption from S0 to T1 of Gd-HMME. The enhancement of the absorption from S0 to T1 is a direct manifestation of the classical external heavy atom effect. Therefore, the states mixing between singlet and triplet states of Gd-HMME was intensified because of the HAE of free Gd3+. ΦT and the rate constant of the phosphorescence emission kp are dependent of the degree of states mixing because the increase of the states mixing is accompanied by the increase of ΦT and kp simultaneously. Hence, we may conclude that the increase of states mixing induced by the HAE of free Gd3+ is one of the reasons for the increase of RTP emission. On the other hand, it can be seen that the increase of the direct absorption from S0 to T1 is about 1.2-fold, which is not enough to induce the 20-fold phosphorescence intensity enhancement. Besides, ΦT of Gd-HMME has reached 80%. For this high quantum yield of T1, there can be only minor increase (less than 1.25-fold) with the increase of states mixing. Therefore, the increase of states mixing is not the only reason for the greatly enhanced RTP emission. To further understand the mechanism of the enhancement, lifetimes of the phosphorescence from Gd-HMME with various concentrations of Gd3+ were measured. Fig. 5 shows the


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phosphorescence decay curves of Gd-HMME (48 µM) and Gd-HMME (48 µM) with 19.2 mM Gd3+. The inset of Fig. 5 shows the calibration of fitted lifetimes versus the concentration of Gd3+. It can be seen that lifetimes of Gd-HMME phosphorescence increase monotonously with the concentration of Gd3+. The lifetime of Gd-HMME with 19.2 mM Gd3+ (30.6 µs) is 7.0-fold greater than that of Gd-HMME itself (4.4 µs). This means that non-radiative process from T1 and the energy transfer from T1 to quenchers decreases upon the titration of Gd3+, that is, the total non-radiative rate constant (knP+Σkq,m[Q]m) decreases with the concentration of free Gd3+. The decrease of the non-radiative processes also enhanced Gd-HMME RTP emission. We believe that the decrease of the non-radiative processes is caused by the formation of a rigid environment upon titration of Gd3+. The formation of such rigid microenvironment decreases the nonradiative process from T1 and protects Gd-HMME from being quenched in solutions. We believe that the titration of Gd3+ affects the aggregation form5 of imidazole to enhance Gd-HMME RTP, which will be further studied later. Finally, the effects of free Gd3+ on the energy transfer processes of Gd-HMME were analyzed using the obtained results above. Based on the values of ΦP (1.4%) and ΦT (80%) of Gd-HMME 54

, the ratio of the term knP+Σkq,m[Q]m and kp is 56.1. Upon the titration of 19.2 mM Gd3+, kp

increases to 1.2kp and the term kp+knP+Σkq,m[Q]m decreases from 0.23(2) µs-1 to 0.033(2) µs-1. From simple calculations, the term knP+Σkq,m[Q]m decreases from 2.2(2)×105 s-1 to 2.8(2)×104 s-1. Therefore, the increase of kp and the decrease of the term knP+Σkq[Q] are both responsible for the enhancement of Gd-HMME RTP. The increases of kp is caused by the enhanced states mixing (HAE of Gd3+) while the decrease of the term knP+Σkq[Q] is due to the formation of rigid microenvironment. We found that the latter is the main cause for the RTP enhancement. The


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change of the energy transfer process of Gd-HMME upon the addition of Gd3+ is summarized in Figure 6. In Figure 6, the “*” represents the physical parameters after the addition of Gd3+. 5. SUMMARY AND CONCLUSIONS In summary, twenty fold enhancement of RTP from Gd-HMME at 712 nm by the titration of free Gd3+ was found and the enhancement mechanism was analyzed. The phosphorescence quantum yield was tuned from 1.4% to 28%. Spectral analysis indicates that the transition possibility from S0 to the lowest triplet excited state (T1) is increased by 1.2-fold, induced by the heavy atom effect of free Gd3+. The phosphorescence lifetime of Gd-HMME with Gd3+ is 7.0-fold greater than that of Gd-HMME itself, which means that the total non-radiative processes of Gd-HMME were decreased by Gd3+. From calculations, the sum of rate constants of non-radiative and quenching processes of Gd-HMME decreases from 2.2(2)×105 s-1 to 2.8(2)×104 s-1. This is a result of the formation of a rigid microenvironment upon titration of free Gd3+. The rigid microenvironment protects Gd-HMME from being quenched by oxygen and decreases the nonradiative process of Gd-HMME. Therefore, the increase of states mixing and the decrease of non-radiative processes are both responsible for the enhancement of Gd-HMME RTP, while latter is the main cause. AUTHOR INFORMATION Corresponding Author *Email: *[email protected] *[email protected] *[email protected] ACKNOWLEDGMENT


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This research was supported in part by National Key Basic Research Program of China (973 Program, Grant No. 2013CB632900) and the National Natural Science Foundation of China (Grant No. 61308065). ASSOCIATED CONTENT Supporting information UV-visible absorption spectra of Gd3+, HMME and Gd-HMME at the same concentrations; The measurement of phosphorescence quantum yield of Gd-HMME at 712 nm. This Supporting information is available free of charge via the Internet at http://pubs.acs.org REFERENCES 1. Yeh, T. S.; Chub, C. S.; Lob, Y. L.; Highly Sensitive Optical Fiber Oxygen Sensor Using Pt(II) Complex Embedded in Sol-Gel Matrices. Sens. Actuators B 2006, 19, 701–707. 2. Wang, X.; Meier, R. J.; Link, M.; Wolfbeis, O. S. Photographing Oxygen Distribution, Angew. Chem. Int. Ed. 2010, 49, 4907–4909. 3. Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M. Enhance the Optical Absorptivity of Nanocrystalline TiO: Film with High Molar Extinction Coefficient Ruthenium Sensitizers for High Performance Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 10720–10728. 4. Borgstrom, M.; Shaikh, N.; Johansson, O.; Anderlund, M.; Styringet, S,; Akermark, B.; Magnuson, A.; Hammarstrom, L. Light Induced Manganese Oxidation and Long—Lived Charge Separation in a Mn2II,II-RuII(bpy)3-Acceptor Triad. J. Am. Chem. Soc. 2005, 127, 17504–17515.


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Figure 1 by Zang et al.


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Figure Captions: Figure 1. The energy level schematic diagram and the chemical structure of Gd-HMME. Figure 2. (a). Luminescence spectra of Gd-HMME (48 µM) with various concentrations of Gd3+ and that of GdHMME itself. (b). Calibration of phosphorescence intensity at 712 nm versus the concentration of Gd3+. Figure 3. UV-visible absorption spectra of 48 µM Gd-HMME (black) and 48 µM Gd-HMME with 19.2 mM Gd3+ (red). Figure 4. Excitation spectra of Gd-HMME with different concentrations of Gd3+ detected at the wavelength 720 nm. Figure 5. Decay curves of Gd-HMME phosphorescence with and without Gd3+. Inset: the calibration of fitted lifetimes versus the concentration of Gd3+. Figure 6. The comparison of possible energy transfer processes of Gd-HMME (left) and Gd-HMME with Gd3+ (right).


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Table of Contents Image

BRIEFS Twenty-fold enhancement of Gd-HMME RTP was achieved by the titration of free Gd3+. The phosphorescence quantum yield has been tuned from 1.4% to 28%. According to the excitation spectra, the transition possibility from S0 to the lowest triplet excited state (T1) is increased 1.2fold because of the heavy atom effect of free Gd3+. The phosphorescence lifetime of Gd-HMME with free Gd3+ is 7.0-fold greater than that Gd-HMME itself, which demonstrates that the nonradiative processe of Gd-HMME is decreased by Gd3+ with the rate constant decreasing from 2.2×105 s-1 to 2.8×104 s-1. This sharp decrease is mainly responsible for the enhancement of GdHMME RTP.


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Figure 1. Energy level schematic diagram and the chemical structure of Gd-HMME. 101x112mm (300 x 300 DPI)


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Figure 2. (a). Luminescence spectra of Gd-HMME (48 µM) with various concentrations of Gd3+ as well as that of Gd-HMME itself. 558x431mm (150 x 150 DPI)



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Figure 2. (b). Calibration of phosphorescence intensity at 712 nm versus the concentration of Gd3+. 594x419mm (150 x 150 DPI)


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Figure 4. Excitation spectra of Gd-HMME with different concentrations of Gd3+ detected at the wavelength 720 nm. 594x419mm (150 x 150 DPI)


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Figure 5. Decay curves of Gd-HMME phosphorescence with and without Gd3+. Inset: the calibration of fitted lifetimes versus concentration of Gd3+. 594x419mm (150 x 150 DPI)



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Figure 6. The comparison of possible energy transfer process between Gd-HMME (left) and Gd-HMME with Gd3+ (right). 217x94mm (300 x 300 DPI)


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Table of Contents Image 162x117mm (300 x 300 DPI)


Twenty-fold Enhancement of Gadolinium-Porphyrin Phosphorescence

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