Near-Infrared Photoluminescence Properties of Endohedral Mono

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Near-Infrared Photoluminescence Properties of Endohedral Mono- and Di-Thulium Metallofullerenes Zhiyong Wang, Noriko Izumi, Yusuke Nakanishi, Takeshi Koyama, Toshiki Sugai, Masayoshi Tange, Toshiya Okazaki, and Hisanori Shinohara ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07780 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016

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Near-Infrared Photoluminescence Properties of Endohedral Monoand Di-Thulium Metallofullerenes Zhiyong Wang,† Noriko Izumi,† Yusuke Nakanishi,† Takeshi Koyama,‡ Toshiki Sugai,†,# Masayoshi Tange,§ Toshiya Okazaki,§ and Hisanori Shinohara*,†

†

Department of Chemistry and Institute for Advanced Research, Nagoya University, Nagoya

464-8602, Japan. ‡

Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan.

§

Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial

Science and Technology (AIST), Tsukuba 305-8565, Japan.

ABSTRACT The optical properties of endohedral metallofullerene molecules can be tuned by changing the fullerene size as well as the number of metal atoms inside the fullerene cages. In this work we have synthesized and isolated a series of mono- and di-thulium metallofullerenes, including Tm@C82 (isomers I, II, III, IV), Tm@C88 (I-IV), Tm2@C82 (I-III) and (Tm2C2)@C82 (I-III). Near-infrared photoluminescence is observed from the thulium metallofullerenes. By changing the number of Tm ion in the fullerene cage, we have found that one can vary and tune the photoluminescence from 1,200 to 1,300-2,000 nm observed for Tm2+ (4f13) in Tm@C88 and Tm3+ (4f12)

in (Tm2C2)@C82, respectively. The

photoluminescence intensity depends sensitively on the fullerene cages. (Tm2C2)@C82 (III) exhibits the highest photoluminescence intensity among the three structural isomers because of its large HOMO-LUMO energy gap.

Keywords: Metallofullerene, Thulium, Photoluminescence, Lanthanide, Fullerene

Light-emitting lanthanide endohedral metallofullerenes (EMFs) are one of the most 1

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interesting optical applications of fullerene materials since, in fullerene cages, the valence of the encapsulated lanthanide ions oftentimes varies depending on the number of metal species entrapped as well as on the size of the fullerene cage.1 Such an unique feature of lanthanide EMFs provides salient contrast to the existing chelate-type lanthanide photoluminescent materials. Generally, most of the lanthanide trivalent ions, Ln3+, are luminescent, either phosphorescent (Sm, Eu, Gd, Tb, Dy, Tm) or fluorescent (Pr, Nd, Ho, Er, Yb) or both.2 During the past decade, growing interests for the near-infrared (NIR) emission of the lanthanide ions have stemmed from a broad range of applications such as optical fibers, lasers and amplifiers for telecommunication, and biological imaging agents.2-4 Lanthanide EMFs can emit NIR photoluminescence (PL) through the f-f transition of metal ions inside fullerene cages. However, the PL would be significantly quenched if the fullerene cages have strong absorption bands in the NIR region. This precludes efficient PL to be observed as an entire EMF molecule even though the entrapped lanthanide ions are photoluminescent. Up to date, only erbium (Er) EMFs have been reported to exhibit NIR PL.5-15 The observed PL at 1.5 µm from Er2@C826, 7 and (Er3N)@C809 corresponds to the 4I13/2 → 4I15/2 transition of trivalent Er3+ ion. In our previous study, we reported the synthesis and purification of all the major isomers (I, II, III) of (Er2C2)@C82 and [email protected] One of the most important observations is that a di-erbium-carbide EMF, (Er2C2)@C82 (III), exhibits an intense PL from the encapsulated Er3+ at 1,520 nm compared to other di-erbium EMFs Er2@C82 (I, II, III). The results have been interpreted in terms of widening of the HOMO-LUMO energy gap of the C82 cage when C2 is inserted between the two Er3+, which results in the reduction of absorbance at ca. 1,500 nm of the C82 cage. The PL intensity of (E2C2)@C82 (III) is thus enhanced significantly compared to other Er-EMFs. Here, we report the observation of NIR photoluminescence from thulium (Tm) EMFs. We have synthesized and isolated three isomers of di-Tm-carbide EMFs (Tm2C2)@C82 (I-III) together with the known Tm2@C82 (I-III). Moreover, four isomers of Tm@C88 (I-IV) have been isolated. The NIR PL at 1,300-2,000 nm from Tm3+ (3H4 → 3F4 and 3F4 → 3H6 transitions) and at 1,200 nm from Tm2+ (2F5/2 → 2F7/2 transition) are observed from these 2


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metallofullerenes. These PL from Tm3+ and Tm2+ can be applied to future light-emitting devices.

RESULTS AND DISCUSSION Separation and Isolation of Thulium Metallofullerenes The mono- and di-thulium EMFs were separated and isolated by the multistage high performance liquid chromatography (HPLC) method. The overall HPLC separation scheme is described in Supporting Information (Figure S1). In the first-stage HPLC separation by using a Buckyprep column, four isomers of Tm@C82 elute together with C82 and C84, and the fractions after C86 provide Tm2@C82 (I-III), (Tm2C2)@C82 (I-III) and Tm@C88 (I-IV). Details of further purification are described in Supporting Information (Figure S2-4). The typical purities of the current thulium EMFs are more than 99.9% except for Tm@C88 (III, IV) whose retention times are accidentally overlapped with that of empty C92. Previous studies reported the presence of three isomers of Tm@C82, and these isomers have been assigned as I, II, III based on the corresponding HPLC (increasing) retention times.16-18 However, in the present study, we have found a new isomer of Tm@C82 that actually has turned out to be the ‘correct’ isomer II of Tm@C82 in reference to the UV-vis-NIR absorption spectra of the reported four isomers of Ca@C82 (I-IV).19

Absorption and Emission Properties of Mono-Thulium Metallofullerenes UV-vis-NIR absorption spectra of four isomers of Tm@C82 in CS2 are shown in Figure 1. The absorption spectra of each isomer of Tm@C82 (I-IV) are different from each other. The spectrum of the newly found isomer of Tm@C82 (II) in the present study shows pronounced peaks at 510, 660, and 1,000 nm, whose onset of the spectrum is found at about 1,710 nm. Generally the UV-vis-NIR absorption spectrum of a EMF is similar to that of another EMF when the fullerene cage and the valence state of the entrapped metal atoms are the same.1 For example, metallofullerenes M@C2v(9)-C82 with different trivalent rare earth metals show very similar absorption bands with each other.20 On the basis of this famous empirical rule, the molecular symmetries of Tm@C82 (I-IV) can be inferred from the known Ca@C82 (I-IV) 3


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isomers whose symmetries have already been determined as Cs, C2, C3v, C2v, respectively, by 13

C NMR measurements.21 In fact, all of four absorption spectra of Tm@C82 are very similar

to those of the four isomers of Ca@C82, indicating the validity of the above correspondence. The molecular symmetries of four isomers of Tm@C82 (I-IV) are thus assigned to be Cs, C2, C3v, C2v, respectively. The molecular symmetry of Tm@C82 (I) is consistent with that determined by our single-crystal X-ray diffraction study.22

Figure 1. (a-d) UV-vis-NIR absorption spectra of Tm@C82 (I, II, III, IV) in CS2. The dotted lines are vertical expansions of the spectra.

UV-vis-NIR absorption spectra of four isomers of Tm@C88 in CS2 are shown in Figures 2a-d. Importantly, we have observed sharp and strong PL from Tm@C88 (III, IV) at 1,200 nm together with small additional peaks at 1,150 nm and 1,250 nm, which are shown in Figure 2e-h. The light emission can be identified as the 2F5/2 → 2F7/2 transition of Tm2+ ion.23 A similar PL feature has been reported in the [SrB4O7:Tm2+] compound.24 Interestingly, isomers I and II of Tm@C88 and four isomers of Tm@C82 do not emit PL, which suggests that Tm2+ PL depends sensitively on these fullerene cages.

4


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Figure 2. (a-d) UV-vis-NIR absorption spectra of Tm@C88 (I, II, III, IV) in CS2. The dotted lines are vertical expansions of the spectra. (e-h) PL spectra of Tm@C88 (I, II, III, IV) excited at 420 nm in CS2.

One of the most important findings for the present absorption/emission spectra is the direct excitation of the encapsulated Tm2+ ion in C88 fullerene cages. Figure 3a shows the absorption and excitation spectra of Tm@C88 (III). The absorption intensity of Tm@C88 (III) gradually decreases from visible to NIR region, which is a characteristic feature for absorption of fullerene cages. There is a similar trend for the excitation spectrum except that a strong peak at 825 nm is observed. This observation suggests that an indirect and direct excitation mechanism may coexist for causes of the photoluminescence of Tm@C88 (III). The similarity in absorption and excitation spectra in visible region implies that the energy transfer from C88 fullerene cage to Tm2+ ion is responsible for the photoluminescence, whereas the presence of the strong peak at 825 nm suggests that Tm2+ ion in this C88 isomer can directly absorb excitation photons (the direct excitation). The direct excitation mechanism is confirmed by the fact that the observed emission and absorption of Tm@C88 (III) are in an exact mirror image with each other as shown in Figure 3b. Figures 3c and d show the emission/excitation 2-dementional contour plots of Tm@C88 (III) and Tm@C88 (IV), respectively. The plots clearly illustrate the presence of strong PL emissions from Tm2+ ions in these fullerene isomers. An excitation peak at 825 nm is observed for both Tm@C88 (III) and Tm@C88 (IV). 5


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Figure 3. (a) UV-vis-NIR absorption and excitation spectra of Tm@C88 (III) in CS2. (b) Absorption and PL spectra in the range of 1000~1300 nm for Tm@C88 (III). (c, d) Emission/excitation 2-dementional contour plots of Tm@C88 (III) and Tm@C88 (IV) in CS2.

A direct excitation of an encapsulated Er3+ in Er3N@C80 has been reported by Briggs and co-workers.10 They observed a weak series of emission of Er3+ from 1,530 to 1,570 nm resulting from 4I13/2 → 4I15/2 transition at a low temperature of 5 K. Most of the previous observations

of

photoluminescence

from

metallofullerenes

were

carried

out

by

photoexcitation of the fullerene cage states followed by relaxation to the ionic states of the encapsulated metal atom(s) (indirect excitation mechanism). The present observation of the strong photoluminescence via the direct excitation/emission of the encapsulated Tm2+ ions in C88 cages is an important and unique example in metallofullerene photophysics.

Absorption and Emission Properties of Di-Thulium Metallofullerenes UV-vis-NIR absorption spectra of isomers of Tm2@C82 (I-III) and (Tm2C2)@C82 (I-III) in CS2 solution are shown in Figures 4a-c and 4d-f, respectively. Similar absorption spectra of C82-based di-metal EMFs have been reported for erbium (Er) and yttrium (Y).11,25,26 Interestingly, all of the absorption spectra of pure di-metal EMFs, M2@C82 (I-III) (M = Tm, Er, Y), are almost exactly the same with each other. Furthermore, all of the absorption spectra 6


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of metal-carbide EMFs, (M2C2)@C82 (I-III) (M = Tm, Er, Y), are also almost exactly the same with each other. According to the empirical rule previously described existing between absorption spectra and molecular structures for EMFs, the cage symmetry among Tm, Er and Y EMFs can be considered as the same for each corresponding isomers of M2@C82 (I-III) and (M2C2)@C82 (I-III). 13

C NMR measurements have been reported for Y2@C82 (III) and (Y2C2)@C82 (I-III) by

Inoue et al.,25, 26 which shows these isomers have C3v(8), Cs(6), C2v(9) and C3v(8) fullerene cages, respectively, suggesting that Tm2@C82 (III) and (Tm2C2)@C82 (I-III) also have C3v(8), Cs(6), C2v(9) and C3v(8) fullerene cages, respectively. The molecular symmetry of Tm2@C82 (I) cage has been regarded as Cs(6) based on our recent X-ray diffraction study.27 Moreover, Tm2@C82 (II) may have C2v(9) cage inferred from a similarity of the corresponding absorption spectra between Tm2@C82 (II) and (Tm2C2)@C82 (II). The schematic molecular structures of Tm2@C82 (I-III) and (Tm2C2)@C82 (I-III) are shown in Figure 4g.

Figure 4. (a-f) UV-vis-NIR absorption spectra of Tm2@C82 (I-III) and (Tm2C2)@C82 (I-III) in CS2. The dotted lines are vertical expansions of the spectra. (g) Schematic molecular structures of Tm2@C82 (I-III) and (Tm2C2)@C82 (I-III). Red and blue spheres are thulium and carbon atoms inside fullerene cages, respectively. 7


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In contrast to the sharp PL of mono-Tm EMFs such as Tm@C88 (III, IV) already described, di-Tm EMFs exhibit broad and (mostly) weak PL in the range of 1,500~2,000 nm for (Tm2C2)@C82 (I, III), and the other isomers of Tm2@C82 and (Tm2C2)@C82 do not show any PL (excited at 800 nm, Figure 5). The observed PL intensity of (Tm2C2)@C82 (III) is more than an order of magnitude stronger than that of (Tm2C2)@C82 (I). The PL from (Tm2C2)@C82 (I, III) corresponds to 3F4 → 3H6 transition of Tm3+ ion. When (Tm2C2)@C82 (I, III) metallofullerenes are excited at 405 nm, a series of additional PL peaks in the range of 1,300~1,600 nm are observed (Figure 6). These peaks correspond to 3

H4 → 3F4 transition of Tm3+ ion. Similar PL features have been reported in the [Tm3+:CaF2]

compound.28 In this compound, the photoluminescence between 1,300 and 1,600 nm have only been observed in diluted low concentrations of Tm3+ in CaF2 matrices due to an effective intersystem crossing from one Tm3+ to another. Because of the absence of this energy transfer in Tm-EMFs to any matrices as such, we have found that (Tm2C2)@C82 (I, III) can exhibit PL irrespective of the concentration.

Figure 5. (a-f) PL spectra of Tm2@C82 (I-III) and (Tm2C2)@C82 (I-III) excited at 800 nm in CS2. The upper lines are vertical expansions of the spectra. The peaks denoted with asterisks are from quartz cell. 8


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Figure 6. (a-b) PL spectra of (Tm2C2)@C82 (I, III) excited at 405 nm in CS2. The dotted lines are vertical expansions of the spectra. The peaks denoted with asterisks (1,620 and 2,030 nm) are from the excitation source.

Photoluminescence Mechanisms of Thulium Metallofullerenes On the basis of the preceding discussion, the PL mechanisms are proposed as follows. For the di-thulium metallofullerene (Tm2C2)@C82, the absorption is primarily achieved by the C82 fullerene cage and an efficient energy transfer occurs from excited states of the C82 cage to the encapsulated Tm3+ (the indirect excitation, Figure 7a). According to the experimental results presented in Figures 5 and 6, the PL spectra are dependent on the excitation energy. When (Tm2C2)@C82 is excited at 800 nm, energy transfer occurs from the excited states of the C82 cage to the 3F4 state of the Tm3+ ion. The 3F4 state would then decay to the 3H6 state by emitting a 1,800 nm photon. When (Tm2C2)@C82 is excited at 405 nm, energy transfer from the C82 cage to the 3H4 state of the Tm3+ ion should be occurring. In this situation, light emissions from 3H4 → 3F4 (1,460 nm) and 3F4 → 3H6 (1,800 nm) transitions are observed. The corresponding schematic diagram is presented in Figure 7a. On the other hand, for the mono-thulium metallofullerene Tm@C88, both direct and indirect excitations are responsible for the photoluminescence. The direct excitation of encapsulated Tm2+ ion is followed by an internal conversion via 4f125d1 → 2F5/2, which 9


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finally provides 2F5/2 → 2F7/2 PL at 1,200 nm (Figure 7b).

Figure 7. Proposed PL mechanism for (a) di-thulium EMF (Tm2C2)@C82 and (b) mono-thulium EMF Tm@C88.

The PL properties of di-thulium metallofullerenes correlate strongly with their HOMO-LUMO energy gaps. As can be seen in the UV-vis-NIR absorption spectra in Figure 4, the HOMO-LUMO energy gaps (inferred from the absorption onsets in Figure 4) of Tm2@C82 (I-III) and (Tm2C2)@C82 (I-III) are 0.68, 0.50, 0.89, 0.83, 0.53 and 0.99 eV, respectively. The PL would be quenched significantly by the fullerene cages with small HOMO-LUMO gaps. Thus no PL was observed for Tm2@C82 (II) and (Tm2C2)@C82 (II). The PL intensity of (Tm2C2)@C82 (III) is the strongest among the di-thulium metallofullerenes because of its large HOMO-LUMO energy gap. Tm2@C82 (III) has a relatively large HOMO-LUMO gap, but we did not observe photoluminescence from Tm2@C82 (III). The situation in Tm EMFs is different from that in Er EMFs. For Er EMFs, both Er2@C82 (III) and (Er2C2)@C82 (III) emit lights at 1.5 µm. Further studies are needed to gain a better understanding on the nonluminescent nature of Tm2@C82 (III).

CONCLUSIONS Mono-

and

di-thulium

metallofullerenes

Tm@C82

(I-IV),

Tm2@C82

(I-III),

(Tm2C2)@C82 (I-III) and Tm@C88 (I-IV) have been synthesized and purified. The molecular symmetries of Tm@C82 (I-IV), Tm2@C82 (I-III) and (Tm2C2)@C82 (I-III) have been assigned by comparison with the UV-vis-NIR absorption spectra of other metallofullerenes reported with

known

structures. NIR

photoluminescence

is observed

from

the

thulium

metallofullerenes. The thulium atoms are in divalent and trivalent states in mono- and 10


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di-thulium metallofullerenes, respectively. Thus (Tm2C2)@C82 and Tm@C88 exhibit photoluminescence at 1,300-2,000 (3H4 → 3F4 and 3F4 → 3H6 transitions of Tm3+) and 1,200 nm (2F5/2 → 2F7/2 transition of Tm2+), respectively.

EXPERIMENTAL SECTION Synthesis and Purification Tm-EMFs were prepared by using the direct current arc-discharge method.1,29,30 A Tm/graphite composite rod (0.8 mol% of Tm) and a pure graphite rod were used as the anode and the cathode, respectively. Tm-EMFs and empty fullerenes were extracted from the arc-discharge produced soot by successive sonication in o-xylene and CS2. Then Tm@C82 (I-IV), Tm2@C82 (I-III), (Tm2C2)@C82 (I-III) and Tm@C88 (I-IV) were separated and purified by using the multistage HPLC method. Five different types of HPLC columns were used for the separation and purification, including 5PYE (20 mm diameter × 250 mm, Nacalai Tesque), Buckyprep (20 mm diameter × 250 mm, Nacalai Tesque), Buckyclutcher-I (21.1 mm diameter × 500 mm, Regis Chemical), Buckyprep-M (20 mm diameter × 250 mm, Nacalai Tesqu) and 5PBB (20 mm diameter × 250 mm, Nacalai Tesque). Characterization LD-TOF mass spectrometry and HPLC analyses were used to examine the purity of the EMFs. The mass spectra were obtained on a Shimadzu AXIMA CFR mass spectrometer. UV-vis-NIR absorption spectra of Tm2@C82 (I-III), (Tm2C2)@C82 (I-III), Tm@C82 (I-IV) and Tm@C88 (I-IV) were measured in CS2 solution using a JASCO V-570 spectrophotometer. PL measurements for Tm@C88 (I-IV) in CS2 solution were performed on a Shimadzu NIR-PL system (CNT-RF) equipped with a liquid nitrogen cooled InGaAs detector array. The EMFs Tm@C88 (I-IV) were excited by a Xe lamp at 420 nm. Two lasers at 405 and 800 nm were used to excite the di-thulium EMFs Tm2@C82 (I-III) and (Tm2C2)@C82 (I-III). The slit widths used were 20 nm for both excitation and emission. The typical scan step was 1 nm. Excitation spectrum for Tm@C88 (III) in CS2 solution was obtained by using a Shimadzu NIR-PL system (CNT-RF). The slit width and scan steps were 20 nm and 10 nm, respectively, for both excitation and emission. All the absorption and photoluminescence spectra were measured at 11


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room temperature.

Supporting Information. HPLC separation details and LD-TOF mass spectra of thulium EMFs. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *E-mail: [email protected]. Present Addresses #

Present address, Department of Chemistry, Toho University.

Acknowledgment This work has been supported by the Grant-in-Aid for Scientific Research S (22225001) of MEXT, Japan. HS thanks Hiroaki Iijima (Nagoya University) for experimental help.

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11. Ito, Y.; Okazaki, T.; Okubo, S.; Akachi, M.; Ohno, Y.; Mizutani, T.; Nakamura, T.; Kitaura, R.; Sugai, T.; Shinohara, H. Enhanced 1520 nm Photoluminescence from Er3+ Ions in Di-Erbium-Carbide Metallofullerenes (Er2C2)@C82 (Isomers I, II, and III). ACS nano 2007, 1, 456-462. 12. Tiwari, A.; Dantelle, G.; Porfyrakis, K.; Ardavan, A.; Briggs, G. A. D. Temperature-Dependent Photoluminescence Study of ErSc2N@C80 and Er2ScN@C80 Fullerenes. Physica Status Solidi (B) 2008, 245, 1998-2001. 13. Dantelle, G.; Tiwari, A.; Rahman, R.; Plant, S. R.; Porfyrakis, K.; Mortier, M.; Taylor, R. A.; Briggs, G. A. D. Optical Properties of Er3+ in Fullerenes and in β-PbF2 Single-Crystals. Opt. Mater. 2009, 32, 251-256. 14. Plant, S. R.; Dantelle, G.; Ito, Y.; Ng, T. C.; Ardavan, A.; Shinohara, H.; Taylor, R. A.; Briggs, G. A. D.; Porfyrakis, K. Acuminated Fluorescence of Centres in Endohedral Fullerenes through the Incarceration of a Carbide Cluster. Chem. Phys. Lett. 2009, 476, 41-45. 15. Popov, A. A.; Yang, S. F.; Dunsch, L. Endohedral Fullerenes. Chem. Rev. 2013, 113, 5989-6113. 16. Kirbach, U.; Dunsch, L. The Existence of Stable Tm@C82 Isomers. Angew. Chem., Int. Ed. 1996, 11, 2380-2383. 17. Kodama, T.; Ozawa, N.; Miyake, Y.; Sakaguchi, K.; Nishikawa, H.; Ikemoto, I.; Kikuchi, K.; Achiba, Y. Structural Study of Three Isomers of Tm@C82 by

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Endohedral Metallofullerenes. Chem. Mater. 2000, 12, 3222-3226.

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Near-Infrared Photoluminescence Properties of Endohedral Mono

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