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Nov 21, 2015 - Twenty-fold enhance- ment of Gd-HMME RTP by the titration of free Gd3+ was achieved. We ..... *E-mail: [email protected]. *E-mail: ...
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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*,†,∥ †

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, Pennsylvania 16802, United States ‡

S Supporting Information *

ABSTRACT: The influence of free gadolinium ion (Gd3+) on roomtemperature 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 Gd-HMME 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 nonradiative processes decreasing from 2.2(2) × 105 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 Gd-HMME from being quenched. 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)porphyrins,45,46 and there are very few reports on RTP of gadolinium labeled porphyrins.48 Gadolinium labeled porphyrins are potential multifunctional agents, that is, as magnetic resonance imaging (MRI) contrast agents, oxygen sensors,49−51 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); that is, the lowest excited energy level is above the first excited singlet and triplet states of porphyrin. Phosphorescence of 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

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 upconversions,14−22 and optical sensing23 with several important advantages,23 such as larger Stokes’ shift, longer lifetime,24,25 better selectivity,23 etc. However, the practical use of RTP in liquid phase was limited due to its relatively low intensity because phosphorescence is a spin-forbidden process.26,27 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 al.31 Deoxygenation methodology, rigid microenvironmental systems,32,33 and heavy atom effect (HAE)34−43 are the main methods to enhance phosphorescence emission in fluid solution. Minaev et al. have theoretically demonstrated that the existence of a central metal ion can induce RTP © 2015 American Chemical Society

Received: September 8, 2015 Revised: November 20, 2015 Published: November 21, 2015 28111

DOI: 10.1021/acs.jpcc.5b08783 J. Phys. Chem. C 2015, 119, 28111−28116

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The Journal of Physical Chemistry C out by Minaev’s group,41 and 3.6 μs was obtained for Eucomplex. Besides, the phosphorescence of Gd-TPP in ethanol solutions and in thin films has been reported for optical oxygen sensing.52 The phosphorescence quantum yield of Gd-TPP is only 10−3 at room temperature.52 In our previous studies, gadolinium labeled hematoporphyrin monomethyl ether (GdHMME) was demonstrated to display relative strong RTP in an air-saturated solution.53 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 as compared to widely used Pt(II)- and Pd(II)porphyrins, and 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.

tube (PMTH-S1-R212) placed in the perpendicular direction attached to an interference filter centered at 720 nm. 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-S1-R212) 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 Because of the coordination of gadolinium ion (Gd3+), GdHMME displays phosphorescence emission centered at 712 and 790 nm with the quantum yield of 1.4%.49 As compared to HMME, the spin-forbidden transition rule from T1 to S0 was broken for Gd-HMME. Figure 1 presents the possible energy

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. 2.2. Preparation of Samples. Gd-HMME was synthesized by a method described by Srivastava.55 The mixture of 6 g of imidazole, 9.6 mg of 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 °C and stirred magnetically for 2 h protected with argon flow. The mixture then was dissolved with methanol to get 10 mL of 1.2 mg/mL (1.5 mM) Gd-HMME methanol solution after being cooled to room temperature. Gd-HMME 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 GdHMME 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 the 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 to 694 nm. The sample to be measured in a quartz curet was put in a collimating light path. The emission signals were recorded via a photomultiplier

Figure 1. Energy level schematic diagram and the chemical structure of Gd-HMME.

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). Because of 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 + k nP + ∑ kq,m[Q]m

(1)

where ΦT is the first triplet state formation quantum yield of Gd-HMME, kp, knp are rate constants of phosphorescence transition, nonphosphorescent transition, and ∑kq,m[Q]m is the sum of all effective mono- and bimolecular 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 nonradiative processes of phosphor28112

DOI: 10.1021/acs.jpcc.5b08783 J. Phys. Chem. C 2015, 119, 28111−28116

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The Journal of Physical Chemistry C escence. Here, the reciprocal of the sum (kp + knP + ∑kq,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; and another is to reduce nonradiative 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 Figure S1. There is a new absorption peak at around 325 nm from Gd-HMME. This indicates that Gd-HMME belongs to irregular metalloporphyrin,56 similar to Pt(II)- and Pd(II)-porphyrins. That is, the central Gd3+ produces a 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 concentrations of Gd3+ were measured. Figure 2a 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. Figure 2b 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 20-fold greater than that of GdHMME 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. A similar phenomenon was also observed in other published works.27,32 UV−visible absorption spectra of Gd-HMME with and without free Gd3+ were also measured as shown in Figure 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 natural.57 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

Figure 2. (a) Luminescence spectra of Gd-HMME (48 μM) with various concentrations of Gd3+ and that of Gd-HMME 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).

Gd-HMME was found in our previous work according to the direct absorption from the ground (S0) to triplet excited states (T1) centered at 686 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 Figure 4. The excitation spectra were recorded from 678 to 694 nm by monitoring at the 28113

DOI: 10.1021/acs.jpcc.5b08783 J. Phys. Chem. C 2015, 119, 28111−28116

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Figure 5. Decay curves of Gd-HMME phosphorescence with and without Gd3+. Inset: Calibration of fitted lifetimes versus the concentration of Gd3+.

Figure 4. Excitation spectra of Gd-HMME with different concentrations of Gd3+ detected at the wavelength 720 nm.

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 on 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 a 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. Figure 5 shows the phosphorescence decay curves of Gd-HMME (48 μM) and Gd-HMME (48 μM) with 19.2 mM Gd3+. The inset of Figure 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 nonradiative process from T1 and the energy transfer from T1 to quenchers decreases upon the titration of Gd3+; that is, the total nonradiative rate constant (knP + ∑kq,m[Q]m) decreases with the concentration of free Gd3+. The decrease of the nonradiative processes also enhanced Gd-HMME RTP emission. We believe that the decrease of the nonradiative processes is caused by the formation of a rigid environment upon titration of Gd3+. The formation of such a 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. On the basis of the values of ΦP (1.4%) and ΦT (80%) of Gd-HMME,54 the ratio of the term knP + ∑kq,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 + ∑kq,m[Q]m decreases from 0.23(2) to 0.033(2) μs−1. From simple calculations, the term knP + ∑kq,m[Q]m decreases from 2.2(2) × 105 to 2.8(2) × 104 s−1. Therefore, the increase of kp and the decrease of the term knP + ∑kq[Q] are both responsible for the enhancement of GdHMME RTP. The increases of kp is caused by the enhanced states mixing (HAE of Gd3+), while the decrease of the term knP + ∑kq[Q] is due to the formation of a rigid microenvironment. We found that the latter is the main cause for the RTP enhancement. The change of the energy transfer process of GdHMME 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 A 20-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 GdHMME with Gd3+ is 7.0-fold greater than that of Gd-HMME itself, which means that the total nonradiative processes of GdHMME were decreased by Gd3+. From calculations, the sum of rate constants of nonradiative and quenching processes of GdHMME decreases from 2.2(2) × 105 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 GdHMME from being quenched by oxygen and decreases the nonradiative process of Gd-HMME. Therefore, the increase of states mixing and the decrease of nonradiative processes are both responsible for the enhancement of Gd-HMME RTP, while the latter is the main cause. 28114

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Figure 6. Comparison of possible energy transfer processes of Gd-HMME (left) and Gd-HMME with Gd3+ (right).



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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08783. UV−visible absorption spectra of Gd3+, HMME, and GdHMME at the same concentrations; measurement of phosphorescence quantum yield of Gd-HMME at 712 nm (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This research was supported in part by the National Key Basic Research Program of China (973 Program, Grant no. 2013CB632900) and the National Natural Science Foundation of China (Grant no. 61308065).



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DOI: 10.1021/acs.jpcc.5b08783 J. Phys. Chem. C 2015, 119, 28111−28116

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DOI: 10.1021/acs.jpcc.5b08783 J. Phys. Chem. C 2015, 119, 28111−28116