The effect of fullerenes C₆₀ and C₇₀ on the photo- and triboluminescence of terbium sulphate crystallohydrate in the solid phase

Abstract

It was found that the addition of C₆₀ and C₇₀ fullerenes to Tb₂(SO₄)₃·8H₂O crystals led to a decrease in the photo- and triboluminescence intensities of the Tb³⁺ ion, without affecting the positions of its luminescence maxima. An increase in the intensity of the emission band ascribed to fullerene at 630–850 nm, relative to that without terbium sulphate, was also observed in the photoluminescence spectra. The phosphor luminescence was quenched in a mechanical mixture with the quenchers and luminescence sensitization of the quenchers occurred, indicating that quenching took place in the solid phase by a mechanism similar to that observed in solution; this is due to radiationless energy transfer from the excited Tb³⁺ ion to the C₆₀ and C₇₀ molecules. The excitation energy is probably transferred through the contacting crystal surfaces of the phosphor and quencher.

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Introduction

Currently, fullerene materials are attracting great attention because they can be used in diverse optically active systems such as solar energy converters, laser shutters, and photocells.^1–7 Studying the effects of luminescence activation and quenching in fullerene compositions has become important. Fullerenes are good quenchers of electron-excited states of various organic compounds in solution.^8–11 The quenching of luminescence of trivalent lanthanide ions with fullerenes has also been observed in solution.^12,13 The authors observed highly efficient quenching reactions with fullerenes in solution that have rate constants close to, or even exceeding, that determined by the diffusion limit of 10¹⁰ L mol⁻¹ s⁻¹.¹⁴ Considering this fact, we assumed that quenching of various phosphors with fullerenes could also occur in solid-phase compositions; we observed earlier the quenching of photoluminescence (PL) and triboluminescence (TL, the luminescence of solids during their destruction)¹⁵ in mixtures of terbium sulphate and sodium nitrite crystals,¹⁶ and the latter is an effective luminescence quencher of lanthanide ions.¹⁴ The aim of this work is to verify this assumption, i.e. to study how crystalline C₆₀ and C₇₀ fullerene additives affect the spectral characteristics and luminescence intensity of excited Tb³⁺ ions for the photo- and triboluminescence of crystalline terbium sulphate. The effect of fullerenes on the luminescence of dinitrogen molecules in the air surrounding the crystals (nitrogen or gas component in TL spectrum)^17,18 will also be examined jointly with the effect of fullerenes on the luminescence of the terbium sulphate crystals (solid-state component in TL spectrum).^15,17–19

Experimental section

Reagent-grade polycrystalline C₆₀ (99.5% Sigma-Aldrich), C₇₀ (98% Sigma-Aldrich), Tb₂(SO₄)₃·8H₂O (99.99% Lanhit) and NaNO₃ were used. Samples of the Tb₂(SO₄)₃·8H₂O crystals (200 mg) were placed in a steel cylindrical cell (25 mm in diameter) with a quartz window at the bottom. For TL excitation, a 4-blade PTFE rod rotating at 1000 rpm was used (the TL setup has been previously described).^16,17,20 The measurement of luminescence was performed while grinding the terbium sulphate crystals with a mixer at the bottom of the steel cylindrical cell at room temperature (295 K) in an air atmosphere, and adding C₆₀, C₇₀, and NaNO₃ crystals. The total intensities of the gas and solid-state spectral components of the TL were estimated by using different light filters in a special setup equipped with an “FEU-39” light detector. In the case of photoluminescence, the crystals and their mixtures (after thorough mixing) were placed in a standard 1 × 1 cm quartz cell and the radiation was detected in the reflection mode. Photoluminescence spectra were recorded with a Fluorolog-3 (Horiba Jobin Yvon) spectrofluorometer (model FL-3-22) equipped with double-grating monochromators, dual lamp housing with a 450 W xenon lamp and a photomultiplier tube detector (Hamamatsu R928P). The PL and excitation spectra were corrected in all cases for source intensity (lamp and grating) and emission spectral response (detector and grating) with the standard instrument correction provided in the instrument software. The UV-visible absorption spectra were recorded on a Perkin Elmer Lambda 750 spectrophotometer using 1 cm quartz cells.

Results and discussion

Fig. 1 shows the dependence of the PL intensity of the terbium sulphate crystals on the amount of C₆₀ and C₇₀ fullerenes. As can be seen, the addition of fullerenes to the terbium sulphate crystals leads to a strong quenching of the excited Tb³⁺ ion. Thus, the PL intensity of terbium in the presence of fullerene at a 10 : 1 ratio is about 0.5% of the original intensity (Fig. 1), while the positions of the Tb³⁺ luminescence maxima corresponding to the electronic transitions ⁵D₄ →⁷F_j (j = 0…6) are not shifted (Fig. 2).

Fig. 1 The dependence of the PL intensity of Tb₂(SO₄)₃·8H₂O (200 mg) on the amount of C₆₀ and C₇₀ added, λ_exc = 488 nm and λ_lum = 543 nm.

Fig. 2 Spectra illustrating the luminescence quenching of Tb₂(SO₄)₃·8H₂O with C₆₀. Fluorolog-3, λ_exc = 370 nm and Δλ = 0.5 nm.

Such effective quenching of the PL of terbium sulphate crystals in a mechanical mixture with fullerenes is probably due to the quenching mechanism in the solid phase being similar to that observed in solution,¹³i.e. it occurs due to nonradiative energy transfer from the excited Tb³⁺ ion to the fullerene molecule. The increase in the luminescence intensities of the C₆₀ and C₇₀ fullerenes (Fig. 3), observed in the crystal mixture upon PL excitation in the resonance band of Tb³⁺ at 488 nm and measured in both absorption and emission spectra, also confirms this fact. In a mixture of crystals, the energy is transferred through the contact surfaces of the phosphor and quencher.

Fig. 3 PL spectra for the emission bands of fullerenes C₆₀ (a) and C₇₀ (b): (1) pure fullerene without additives and (2) solid-phase mixture of fullerene with Tb₂(SO₄)₃·8H₂O at a 1 : 10 ratio; (3) excitation spectra of pure fullerene without additives. For PL spectra, λ_exc = 488 nm; for excitation spectra, λ_lum = 750 nm (C₆₀) and 810 nm (C₇₀). Fluorolog-3, Δλ = 2 nm. The insets show the absorption spectra of fullerenes C₆₀ (1 × 10⁻⁴ M) and C₇₀ (1 × 10⁻⁵ M) in toluene at room temperature (295 K).

Thus, the PL intensity of fullerenes mixed with terbium sulphate is approximately twice as high as the PL intensity of pure C₆₀ and C₇₀ fullerenes. In addition, a hypsochromic shift of the luminescence maximum is observed in the PL spectrum of C₆₀ in the presence of Tb₂(SO₄)₃·8H₂O crystals (Fig. 3a). We may assume that the interaction between phosphor and quencher molecules takes place in a similar fashion to the complexation process between them that leads to the redistribution of electron density. As a result, the HOMO–LUMO energy gap of the fullerenes increases, as this gap is responsible for the electronic excitation.^21,22 To ascertain the detailed mechanism of the effect of terbium sulphate on the PL of C₆₀, further studies are required.

The quenching of excited Tb³⁺ ions is also observed in the TL of a mixture of Tb₂(SO₄)₃·8H₂O crystals with fullerenes. However, as compared with PL, the decrease in the luminescence intensity for TL is not so strong. Fig. 4 shows a plot of the intensity of the gas-phase (N₂ is an emitter)^15,17,23,24 and solid-state (Tb³⁺ is an emitter) components of the TL spectrum of Tb₂(SO₄)₃·8H₂O against the quantity of C₆₀ and C₇₀ crystals added.

Fig. 4 The dependence of the intensities of the nitrogen (*N₂) and solid-state (*Tb³⁺) components of the TL spectrum of Tb₂(SO₄)₃·8H₂O (200 mg) on the amount of C₆₀ and C₇₀ added. Optical (260–400 nm) and interference (λ = 546 nm) filters were used to extract N₂ and Tb³⁺ luminescence, respectively.

The TL intensity sharply decreases by half initially, but further addition of fullerene leads to a slight decrease in luminescence intensity. For example, when 20 mg of fullerene is added to 200 mg of terbium sulphate, the TL intensity is five times lower than the initial value, whereas the intensity becomes 350 times lower for PL (Fig. 1).

Considering that electronic phenomena underlie the luminescence caused by the destruction of inorganic lanthanide salts,^15,17 we may assume that in the case of adding fullerenes, their effect is most likely due to the change in the electrophysical properties of the crystal mixture. The small size and high degree of surface curvature of the fullerene molecules, as well as the electrical properties of their crystals (fullerenes are semiconductors with a bandgap of about 1.5–2 eV),^25–27 contribute to the formation of strong local electric fields, resulting in stable electron emission.^28,29

The strong electric fields arising during the mechanical destruction of crystals on the interface between the terbium sulphate and fullerene, as well as high mobility and charge carrier concentrations, apparently prevent the effective deactivation of the electronically excited states of the Tb³⁺ ion involved in PL.

To confirm the hypothesis that the electrophysical properties of the quencher crystals influence the efficiency of the quenching of TL, we additionally observed that the addition of dielectric sodium nitrate crystals to terbium sulphate in the same ratio (1 : 10) leads to almost complete quenching of TL (Fig. 5), although for PL, quenching of the excited Tb³⁺ ions with sodium nitrate is not observed (Fig. 5, inset). This corresponds to the fact that in aqueous solution, there is no true (except for that caused by the absorption of exciting radiation) quenching of luminescence of the terbium aqua-ion by NO₃⁻ anions.³⁰

Fig. 5 The dependence of the intensities of the nitrogen (*N₂) and solid-state (*Tb³⁺) components of the TL spectrum of Tb₂(SO₄)₃·8H₂O (200 mg) on the amount of NaNO₃ added. Optical (260–400 nm) and interference (λ = 546 nm) filters were used for the separation of N₂ and Tb³⁺, respectively. The inset shows the PL spectra of Tb₂(SO₄)₃·8H₂O without (solid line) and with NaNO₃ as an additive (short dotted line). Fluorolog-3, λ_exc = 370 nm, Δλ = 0.5 nm.

In this case, upon TL, adding sodium nitrate, in contrast to fullerenes, leads to a decrease in the conductivity and magnitude of the effective charge accumulated on the terbium sulphate crystal surface.

Conclusions

Thus, addition of C₆₀ and C₇₀ fullerene crystals to crystalline terbium sulphate leads to strong quenching of the PL of the Tb³⁺ ion, which takes place via nonradiative energy transfer to the fullerene molecule. Sensitization of the fullerene luminescence by terbium also confirms this fact. This mechanism of quenching of terbium luminescence is clearly seen for TL. Furthermore, considering that fullerenes quench nitrogen molecules in addition to the luminescence of terbium ions, there is another process of quenching occurring on the crystal surface:

[C₆₀/C₇₀]_solid + [N₂*]_gas → [C₆₀/C₇₀]_solid + [N₂]_gas + Δ,

where Δ is the thermal energy transferred to the crystal.

In the TL process, the quenching effect of fullerenes on the luminescence of terbium ions is weaker than in the PL process, and it is probably due to the electrical nature of the luminescence upon mechanical destruction of the crystals.

Acknowledgements

We are grateful to Ph. D. D. I. Galimov for his help with the experiments. This work was financially supported by the Russian Foundation for Basic Research (project numbers 14-02-31019 mol_a and 14-02-97015).

Notes and references

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