Mechanism of Gadolinium Doping Induced Room-Temperature

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On the Mechanism of Gadolinium Doping Induced Room Temperature Phosphorescence from Porphyrin Huimin Zhao, Lixin Zang, Hua Zhao, Feng Qin, Zhongwei Li, Zhiguo Zhang, and Wenwu Cao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00328 • Publication Date (Web): 28 Apr 2015 Downloaded from http://pubs.acs.org on May 3, 2015

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The Journal of Physical Chemistry

On the Mechanism of Gadolinium Doping Induced Room Temperature Phosphorescence from Porphyrin ◇

Huimin Zhao,† Lixin Zang,† Hua Zhao, Feng Qin,† Zhongwei Li,‡ Zhiguo Zhang,†,* and Wenwu Cao†,§,* †

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

◇

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

‡

School of Electrical Engineering and Automation, Harbin Institute of Technology, Harbin 150001, China

§

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

*Corresponding Authors: Zhiguo Zhang*

Email: *[email protected]; Telephone number: 0451-86402639

Wenwu Cao*

Email: *[email protected]; Telephone number: 0451-86402639

ACS Paragon Plus Environment

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ABSTRACT: The spin-orbit coupling mechanism was generally used to explain heavy atom effect induced room temperature phosphorescence (RTP). Here, we demonstrate that the mechanism of RTP induced by Gd3+ from hematoporphyrin monomethyl ether (HMME) is due to the mixing of singlet (S) and triplet (T) states. The spin-forbidden transition between S and T states was partly allowed due to the states mixing, as indicted by the direct absorption corresponding to transition from S0 to T1, which was observed for the first time from RTP. The quantum yield of T1 was determined to be 0.80 and the percent of each energy transfer process was determined. While there is no non-radiative relaxation from HMME to Gd3+ because of the large energy gap between the excited and ground states of Gd3+. The special energy level of Gd3+ as well as the states mixing between S and T states produced the strong phosphorescence emission. The population and deactivation of triplet states in heavy atom induced RTP can be used to analyze the mechanism of phosphorescent emission.

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


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1. INTRODUCTION In the past few decades, room temperature phosphorescence (RTP) in the liquid state has evolved into a sensitive and versatile tool in analytical chemistry especially after the development of heavy atom-induced RTP (HAI-RTP) methodology.1-3 The mechanism of heavy atom effect (HAE) may be explained by the spin-orbit coupling. Singlet and triplet states of molecules can be perturbed by the nearby nuclear magnetic field of heavy atoms, which mixes pure singlet and triplet states to produce states with a mixed character in spin multiplicity.4 HAE makes the intersystem crossing (ISC) from the lowest excited singlet state (S1) to the lowest triplet state (T1) to occur and the forbidden singlet-triplet transition possible.4 RTP from metalloporphyrins, such as Pt(II)-5,6 and Pd(II)-porphyrins7,8, in solutions is induced by the HAE of metal ions. Applications of molecular triplet states include imaging of oxygen9,10, photodynamic therapy11,12 (PDT), etc., which could benefit from imposing heavy atoms to increase the population of triplet states13,14. Molecular triplet states were also reported to be controlled by weak magnetic fields.15,16 For low temperature phosphorescence, induced S1 to T1 intersystem crossing and S0 to T1 absorption have been observed in HAI-RTP of alkanones and azoalkanes based on zeolites.17 The ISC from S1 to T1 and the absorption from S0 to T1 were favored at 77 K by Tl+ on solid phase.17 However, the influence of HAE on the absorption related to triplet states for room temperature phosphorescence in solutions is rarely reported. Recently, we found that gadolinium ion (Gd3+) labeled hematoporphyrin monomethyl ether (Gd-HMME) displays relatively strong RTP in methanol without any protective media.18 The phosphorescence of Gd-HMME (712, 790 nm) has been used for oxygen sensing in our earlier work.19,20 The phosphorescence of Gd-HMME based on filter paper can be quenched effectively by surrounding molecular oxygen with good response. The phosphorescence intensity decreased


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monotonously with oxygen concentration. The production of RTP resulted from the strong paramagnetism, HAE and special energy levels of Gd3+. The lowest excited energy level of Gd3+ is above both S1 and T1 of HMME, which avoids the non-radiative relaxation from HMME to Gd3+.18 However, the mechanism and physical picture of the HAE on generating the room temperature phosphorescence of HMME has not been discussed. In this work, the mechanism of RTP induced by Gd3+ from HMME was investigated. The quantum yield of T1 was measured and the percent of each energy transfer process was determined. The direct absorption from the singlet ground state (S0) to T1 was measured by excitation spectrum. The change of the properties of singlet and triplet sates induced by Gd3+ was studied. The mechanism of the generation of RTP from HMME by Gd3+ doping was discussed. 2. EXPERIMENTAL SECTION 2.1 Sample preparation. Gd-HMME was synthesized by a method published by T.S. Srivastava.21 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. Then, the mixture was heated and kept at 200 oC and stirred magnetically for 2 hours protected with argon flow. The mixture was dissolved with methanol to get 10 ml 1.2 mg/ml Gd-HMME methanol solution after cooling down to room temperature. 1.2 mg/ml (1.5 mM) Gd-HMME methanol solution was added into the quartz curette for measuring. 2.2 Measurements. UV-visible absorption spectra were measured with a miniature fiber optic spectrometer (Ocean Optics QE65000) equipped with a deuterium lamp. Absorption spectra were calculated from the Beer-Lambert law. A diode laser centered at 405 nm was used as light source. Luminescence spectra were recorded by another fiber optic spectrometer (Ocean Optics


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USB2000). To determine the lifetime of fluorescence, a pulse laser centered at 392 nm with 1 MHz was used as light source. Fluorescence signals were recorded by a spectro-fluorometer (HORIBA, Fluoromax-4). The fluorescence lifetime evaluation was performed by fitting the decay curve to an exponential function using adjustable parameters. To determine the lifetime of phosphorescence, 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 Omniλ300) 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 determination. The lifetime evaluation was performed by fitting the decay curve to an exponential function using adjustable parameters. The calculation of quantum yield of the first triplet state (Φt) at room temperature was performed from low-temperature measurements according to a published work22. Gd-HMME was placed into a heating stage. Liquid nitrogen was used to cool the heating stage by a liquid nitrogen pump (LINKAM, LNP94/2). The temperature of the heating stage was monitored from 298 K to 153 K using LINKAM TMS94 equipment with error of ±0.1 K. Excitation spectrum of Gd-HMME monitored at 790 nm was measured. 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 500 to 720 nm. 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 with an attached interference filter centered at 800 nm. 3. RESULTS AND DISCUSSION


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Figure 1 shows the typical normalized absorption (red) and photoluminescence (blue) spectra of HMME (solid) and Gd-HMME (dot) in methanol solution. It can be seen that the spectrum of HMME comprises a Soret band (between 380 and 420 nm) and four Q bands (499, 530, 568 and 614 nm). Meanwhile, the measured Soret band peak of Gd-HMME is at 406 nm and two Q band peaks are at 538 nm and 571 nm. The molar absorption coefficients of Gd-HMME were determined in the Supporting Information (SI). The absorbance of 406 nm was 11 times larger than that of 571 nm19 as shown in Fig. 1. HMME has two emission peaks at 623 nm and 687 nm, which are well known fluorescence. Meanwhile, Gd-HMME has four photoluminescence peaks. Two peaks at 585 nm and 626 nm are weak fluorescence18 with the lifetime of 10 ns as shown in Fig. 2(a). Their intensities decreased significantly compared with that of HMME in luminescent spectra.19 Strong red shift photoluminescence peaks in the very near-infrared region (712, 790 nm) were observed at room temperature, which was revealed to be phosphorescence19 with a lifetime of 3 µs (Gd-HMME, 50 µM) as shown in Fig. 2(b). The lifetime of phosphorescence is 290 times longer than that of fluorescence analyzed from Fig. 2. Quantum yield of the first triplet state T1 ( Φt ) at room temperature was obtained. According to an earlier work22, Φt can be expressed as

Φt =

Φpτ p′ Φp′τ p

where Φp

Φt′ ~

Φpτ p′ Φp′τ p

and τ p

(1-Φf ′ ),

are

(1)

phosphorescence

quantum

yield

and

lifetime

at

room

temperature; Φp′ , τ p′ , Φf ′ and Φt′ are phosphorescence quantum yield, lifetime, fluorescence quantum yield and quantum yield of T1 at a low temperature (153 K) , respectively. As the non-


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radiative relaxation from S1 to S0 can be ignored at the low temperature, the substitution of

1-Φf ′ for Φt′ is reasonable. To obtain the quantum yield of T1, the fluorescence quantum yield of Gd-HMME at low

temperature ( Φf ′ ) was measured. Φf ′ was determined to be 10-4, which can be neglected. Φt′ was almost unity at low temperature. Phosphorescence quantum yield ( Φp , Φp′ ) and lifetime values ( τ p , τ p′ ) at both room temperature and low temperature were determined. Φp for Gd-HMME at 712 nm was determined to be 0.014 (Supporting Information). Cooling from room temperature to the low temperature (153 K), the phosphorescence intensity and lifetime increased 3.53 and 2.81 fold for Gd-HMME (1.5 mM) as shown in Fig. 3. From Eq.1, Φt was 0.80 for Gd-HMME. From these data, we can conclude that 0.01% of the absorbed photons were converted to fluorescence, approximately 20% involved in non-radiative transitions from S1 to S0, and 80% transferred from S1 to T1. Figure 4 shows the chemical structures and energy-level diagrams of HMME and Gd-HMME as well as energy-level diagram of Gd3+. As indicated in the left part of Fig. 4, when a HMME molecule is excited by a 405 nm laser, it transfers from S0 to an excited singlet state and rapidly decays to the bottom of first excited singlet state S1 by a relaxation process. There are two deactivation pathways: (1) the molecule may transfer from S1 to its S0 with fluorescent emission; (2) it may undergo intersystem crossing (ISC) to produce a molecule in the first excited triplet state T1. The fluorescence quantum yield of HMME at room temperature was determined to be 3.2% (Supporting Information). It is well known that the quantum yield of triplet state for HMME is relatively high because there is an overlap between S1 and T1. Thus, the ISC process is dominated by phonon-assisted non-radiation relaxation. The transition from T1 to S0 is spin-


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forbidden. Usually, the molecule in T1 is deactivated by non-radiative relaxation or the interactions with surrounding molecules such as oxygen. In the interaction, the oxygen molecule may be excited from the triplet ground to a singlet state. Singlet oxygen species are highly toxic to cells, which is the basic principle of PDT. Different from HMME, under the excitation of the 405 nm laser, Gd-HMME has strong phosphorescence (phos, quantum yield 1.4%) emission and weak fluorescence emission (quantum yield 0.01%) as shown in the right part of Fig. 4. The phosphorescence emission was identical to the transition from T1 to S0, which is not found in HMME. It is hard to know the percent of each energy transfer process for HMME. The phosphorescence emission provides the possibility to study the energy transfer process quantitatively. The quantum yield of the first excited triplet state ( Φt ) was measured to be 0.80 based on the phosphorescence emission. Although the spin-forbidden transition was partially allowed, there is no obvious increase of Φt in Gd-HMME compared with that of HMME. We believe that the reason is due to the negligible transition from S1 to T1, which is proportional to ν 3 ( ν , the frequency of the transition). Therefore, the formation of T1 is still dominated by the phonon-assisted non-radiative relaxation. The phosphorescence emission indicates the change of the deactivation of T1 in Gd-HMME resulted from the HAE of Gd3+. To analyze the HAE of Gd3+, the character of Gd3+ was discussed. Generally, the RussellSaunders coupling scheme is used in rare earth ions. Under the Russell-Saunders coupling scheme, the states of rare earth ions can be described by spectral term 2S+1LJ, where S is the total spin quantum number, L depends on the orbital quantum number (L), J is the total angular momentum quantum number. 4f7 configuration of trivalent gadolinium ion (Gd3+) is the configuration with the lowest energy. The energy states of 4f7 configuration includes the ground


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state 8S7/2, excited state 6PJ (J=3/2, 5/2, 7/2) and 6IJ (J=7/2, 9/2, 11/2, 13/2, 15/2, 17/2), etc. The energy of the first excited state 6P7/2 is about 32000 cm−1. Therefore, Gd3+ is usually used to study triplet states of some macrocyclic organics23. The energy of S1 (16000 cm−1) and T1 (14000 cm−1) for HMME are much lower than that of 6P7/2 for Gd3+. Thus, there is almost no energy transfer from HMME to Gd3+. On the other hand, the spin of seven uncoupled electrons for Gd3+ in the ground state is in the same direction. Consequently, the magnetic moment of Gd3+ is relatively large. The nuclear magnetic field increases the spin-orbit coupling in Gd-HMME. As a result, the triplet state of Gd-HMME acquires some singlet character as a result of mixing with the singlet state and vice versa. This is explained as states mixing.24 Therefore, the wave function of the lowest triplet state T1 can be rewritten as Ψ ′(T1 ) = Ψ (T1 ) + Σ an Ψ ( S n ). n

(2)

Where Ψ′(T1 ) is the wave function of T1 mixed with characters of singlet states after the doping of Gd3+; Ψ (T1 ) is the wave function of pure T1; Ψ ( Sn ) is the wave function of different singlet states; an is the mixing coefficient between T1 and different singlet states. The phosphorescence emission happens in Gd-HMME because of the HAE and special energy levels of Gd3+. From another point of view, the properties of Gd-HMME energy levels are mainly dominated by that of HMME after the combination of Gd3+ and HMME. The two main reasons are as follows: First, as a rare earth element, the energy levels of Gd3+ belong to the 4f shell. It is well known that the intra-4f electronic transitions of rare-earth ions are parity forbidden. Second, the energy transfer between Gd3+ and HMME is almost zero. Consequently, the effect of Gd3+ on the properties of Gd-HMME energy levels is relatively weak. The extent of states mixing enabled by HAE of Gd3+ is weak, mainly reflected in the following two aspects: (1) although there are


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transitions between triplet and singlet states, T1 still has a lifetime as long as microseconds; (2) if the effect of Gd3+ is strong enough, fluorescence emission from S1 should be extinguished via the intersystem crossing from S1 to T1.The fact that fluorescence emission from S1 is decreased but still exists reflects the relatively weak effect of Gd3+. To further certify this proposed physical picture, the transition from S0 to T1 was investigated. The transition from S0 to T1 is hard to be observed directly by recording the absorption spectrum because of the relatively small extent of states mixing. Generally, rate constant of a transition is inversely proportional to the corresponding natural radiation lifetime of the excited state. Because the phosphorescence lifetime of T1 is 290 times longer than that of fluorescence, the phosphorescence rate constant of the transition from T1 to S0 should be 290 times smaller than fluorescence transition rate constant. Therefore, the absorbance of direct absorption from S0 to T1 should be two or three orders of magnitude weaker than that at 406 nm. For such a weak absorption, excitation spectrum was used instead of absorption spectrum due to the high detecting sensitivity for luminescence17. The measurement of excitation spectrum of Gd-HMME monitored at 790 nm emission in the range of 500-720 nm was performed as shown in Figure 5. According to Fig. 5, there are four bands centered at 538, 571, 686 and 711 nm. The observed bands centered at 538 and 571 nm in the excitation spectrum correspond well to the absorption bands with the previous reported UVvisible absorption spectrum of Gd-HMME17, which reflects the transition from S0 to S1. The new-found bands centered at 686 nm and 711 nm reflect the transition from S0 to T1. To make sure that these two new-found bands are indeed due to the absorption from S0 to T1, the emission spectrum (blue) and excitation spectrum (red) of Gd-HMME were presented in the inset figure of Fig. 4. Generally, the excitation spectrum which is identical to absorption spectrum reflects the


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structures of excited states, while the luminescence spectrum reflects the structure of the ground state. As illustrated in the inset, these two spectra have a mirror-image relationship. This agrees well with the proposal that there can be direct transition from S0 to T1. In addition, the absorbance from S0 to T1 was about 28 times smaller than the absorbance at 571 nm according to calculations based on Fig. 4. This means that the absorbance from S0 to T1 is about two orders of magnitude weaker than the absorbance at 406 nm. This also verified that the absorption bands in the excitation spectrum shown in the inset figure of Fig. 5 were derived from the direct absorption from S0 to T1. These results demonstrate that Gd-HMME molecules could absorb photons and transfer from S0 to T1 directly. Therefore, the energy-level diagram of Gd-HMME as well as the transition for fluorescence and phosphorescence in Fig. 4 was further described as Fig. 6. The schematic line describing the direct absorption process from S0 to T1 was added to reflect the states mixing. Figure 6 also shows the percent of each energy transfer process. The quantum yield of singlet oxygen for Gd-HMME is 0.40 reported in our recently published work18. Based on the measured values of Φt (0.80) and Φp (0.014), the percent of the non-radiative transition from T1 to S0 is calculated to be 38.6%. To understand this physical picture visually, the generation of the phosphorescence from Gd-HMME can be described by a pail-water-hole fable shown in Table of Contents Image. Because of the states mixing as well as the special energy levels of Gd3+, strong phosphorescence emission from T1 to S0 was produced from HMME after coordination of Gd3+. 4. CONCLUSIONS The doping of Gd3+ resulted in the direct S0 to T1 absorption from HMME. Singlet and triplet sates were mixed because of the doping of Gd3+. The quantum yield of T1 was obtained to be 0.80 and the percent of each energy transfer process was determined. The special energy level of


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Gd3+ did not produce non-radiative relaxation from HMME to Gd3+. Therefore, the special energy level of Gd3+ as well as the states mixing between triplet and singlet states produced the strong phosphorescence emission from T1 to S0. The physical picture of production and deactivation of triplet states in HAI-RTP can be used to analyze the detailed process of phosphorescent emission. ASSOCIATED CONTENT

Supporting information The measurement of absorption coefficients for Gd-HMME in methanol; the measurement of phosphorescence quantum yield of Gd-HMME at 712 nm; the measurement of fluorescence quantum yield of HMME at 626 nm. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *Email: *[email protected] *[email protected] ACKNOWLEDGMENT This research was supported in part by National Key Basic Research Program (973 Program, Grant No. 2013CB632900) and the National Natural Science Foundation of China (Grant No. 61308065). REFERENCES


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Interactionwith Axial Ligands, Bioinorg. Chem. 1978, 8, 61-76. 22. Ramasamy, S. M.; Hurtubise, R. J. Room-Temperature Luminescence Properties of pAminobenzoic Acid Adsorbed on Sodium Acetate-Sodium Chloride Mixtures, Anal. Chem.

1987, 59, 2144-2148. 23. Hurtubise, R. J.; Thompson, A. L.; Hubbard, S. E. Solid-Phase Room-Temperature Phosphorescence, Anal. Lett. 2005, 38, 1823-1845. 24. Sato, S.; Wada, M. Relations between Intramolecular Energy Transfer Efficiencies and Triplet State Energies in Rare Earth β-diketone Chelates, B. Chem. Soc. Jpn. 1970, 43, 19551962.


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Table of Contents Image

BRIEFS The generation of RTP from Gd-HMME was described as a pail-water-hole fable shown in the left. Excited states are just like pails. Water could be pumped from S0 into S1. Water in S1 could drain out to S0 and T1 via holes on the pail. Upon the doping of Gd3+, a hole was bored on the bottom of T1. Water can flow from T1 to S0. The direct absorption from S0 to T1 in Gd-HMME was observed by excitation spectrum shown in the lower right.


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Figure 1 by Zhao et al.


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Figure Captions: Figure 1. Typical normalized absorption (red) and photoluminescence (blue) spectra of HMME (solid) and GdHMME (dot) in methanol solution. Figure 2. The decay curves of Gd-HMME (50 µM) fluorescence at 626 nm (a) and phosphorescence at 712 nm (b) in air-saturated methanol solution. Figure 3. The emission spectra (a) and decay curves (b) of Gd-HMME (1.5 mM) phosphorescence at room temperature and low temperature (153 K). Figure 4. The chemical structures and energy-level diagrams of HMME and Gd-HMME as well as energy-level diagram of Gd3+. Figure 5. Excitation spectra of Gd-HMME monitored at 790 nm. Inset figure shows the mirror-image relationship between excitation (red) and phosphorescence (blue) spectra of Gd-HMME. For the emission spectrum excitation was kept at 405 nm and for the excitation spectrum the emission was monitored at 790 nm. Figure 6. Energy-level diagrams of Gd-HMME and Gd3+ as well as the percent of each possible energy transfer process.


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Left: The pail-water-hole fable describing the generation of RTP from Gd-HMME; Upper right: The chemical structure of Gd-HMME; Lower right: The direct absorption from S0 to T1 in Gd-HMME. 178x144mm (150 x 150 DPI)


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Typical normalized absorption (red) and photoluminescence (blue) spectra of HMME (solid) and Gd-HMME (dot) in methanol solution. 594x419mm (150 x 150 DPI)



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The decay curves of Gd-HMME (50 µM) fluorescence at 626 nm in air-saturated methanol solution. 594x419mm (150 x 150 DPI)


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The decay curves of Gd-HMME (50 µM)phosphorescence at 712 nm in air-saturated methanol solution. 594x419mm (150 x 150 DPI)



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The emission spectra of Gd-HMME (1.5 mM) phosphorescence at room temperature and low temperature (153 K). 142x100mm (150 x 150 DPI)


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The decay curves of Gd-HMME (1.5 mM) phosphorescence at room temperature and low temperature (153 K). 142x100mm (150 x 150 DPI)



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The chemical structures and energy-level diagrams of HMME and Gd-HMME as well as energy-level diagram of Gd3+. 181x116mm (300 x 300 DPI)


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Excitation spectra of Gd-HMME monitored at 790 nm. Inset figure shows the mirror-image relationship between excitation (red) and phosphorescence (blue) spectra of Gd-HMME. For the emission spectrum excitation was kept at 405 nm and for the excitation spectrum the emission was monitored at 790 nm. 594x419mm (150 x 150 DPI)



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Energy-level diagrams of Gd-HMME and Gd3+ as well as the percent of each possible energy transfer process. 144x139mm (300 x 300 DPI)


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Mechanism of Gadolinium Doping Induced Room-Temperature

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