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Directional Energy Migration in Nanoparticles of Crystalline Metal Complexes Elia Previtera, Antoine Tissot,* Robert W. Johns, and Andreas Hauser* Inorganic and coordination-based materials show promise for solar-energy harvesting and conversion when synthesized at the nanoscale.[1,2] Since excitation energy transfer plays an important role for such applications, understanding how these processes are affected by crystallite size is crucial to design novel materials.[3] Typically, materials showing long-range efficient directional energy migration are targeted.[4] In this Communication, we describe the preparation of both nano- and microscale crystallites of the 3D oxalate network [Ru(bpy)3][NaCr(ox)3], ox = C2O42−, bpy = 2,2′-bipyridine,[5] and discuss the influence of crystal size, so far unknown, on energy migration within the 2 E state of the [Cr(ox)3]3− chromophores.[6] The nanocrystals were prepared via the reverse micelle technique using sodium dioctyl sulfosuccinate (NaAOT)[7] as surfactant. They were analyzed for size distribution with transmission electron microscopy (TEM) and for crystallinity with powder X-ray diffraction (XRD) using synchrotron radiation (for details see the Experimental Section and Supporting Information). Figure 1a shows TEM images of a series of crystal sizes achieved by varying the water-to-surfactant ratio W0. For all preparations, the crystals are tetrahedral and the corresponding powder X-ray diffraction patterns (Figure 1b) reveal all samples to be crystalline and can be indexed to the cubic chiral space group P213 with a = 15.510 Å (for a graphic representation of the crystal structure see Figure S1 in the Supporting Information).[5] For comparison, a scanning electron microscopy (SEM) image and the corresponding powder diffraction pattern of a polycrystalline sample with crystallites of around 4 µm in size obtained by precipitation from water are included in Figure 1. In accordance with the Scherrer equation,[8] the diffraction peaks broaden as size decreases from a full width at half maximum (FWHM) of 0.05° for the 4 µm crystallites to 0.11° for the smallest 140 nm particles at 2θ = 5.8°. Figure 1c shows how the average crystals size, expressed by the edge length
E. Previtera, Dr. A. Tissot,[+] Prof. A. Hauser Département de Chimie Physique Université de Genève 30 quai Ernest Ansermet, 1211, Genève 4, Switzerland E-mail: [email protected]; [email protected] R. W. Johns Department of Chemistry University of California Berkeley, CA 94720, USA [+] Present address: Institut Lavoisier de Versailles, UMR 8180, Université de Versailles, Saint-Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France.
DOI: 10.1002/adma.201405179
Adv. Mater. 2015, DOI: 10.1002/adma.201405179
of the tetrahedra, depends on the water-to-surfactant ratio W0 (for details see Figure S1–S3 in the Supporting Information), thus allowing for size-controlled synthesis of nanocrystals from 140 to 1000 nm via W0. Finally, we note that for the smallest nanocrystals, that is, for an edge length of 140 nm, the shortest distance from the center to one of the faces is less than 30 nm. [Ru(bpy)3][NaCr(ox)3] is dark red due to the strongly absorbing spin-allowed metal–ligand charge transfer (1MLCT) transition of the [Ru(bpy)3]2+ chromophore centered at 468 nm (21 400 cm−1).[9] The characteristic luminescence from the corresponding 3MLCT state of [Ru(bpy)3]2+ is completely quenched within less than 1 µs by excitation energy transfer to the [Cr(ox)3]3− chromophores.[10] In fact, the weak and broad spinallowed ligand-field transition of the [Cr(ox)3]3− chromophore, the 4A2 → 4T2 transition centered at 543 nm (18 400 cm−1)[6,11] serves as acceptor level for this energy transfer, which, following intersystem crossing, results in strong luminescence from the spin-flip 4A2 → 2E transition of [Cr(ox)3]3− at 694.5 nm (14 400 cm−1). Figure 2a shows the excitation spectra for this luminescence at 1.4 K of crystallites of different sizes in the region of the transition itself. For the largest crystallites, that is, those with 4 µm edge length, two sharp lines corresponding to the zero-field split components of the electronic origins of the 2E state and generally referred to as the R-lines[12] with a FWHM of 4 cm−1 are clearly resolved with a splitting of 13.6 cm−1. For the 2.5 µm crystallites, the FWHM increases to 8 cm−1 and a broad tail begins to appear on the low-energy side. For the still smaller crystallites of edge lengths 670 and 140 nm, the FWHM increases still further and the peaks become asymmetric with increasing tails toward low energies. The inhomogeneous line width thus increases asymmetrically toward lower energy with decreasing size, suggesting that the 4A2−2E energy difference of the [Cr(ox)3]3− chromophores closer to surface is slightly lower than the one of the complexes in the bulk. Figure 2b shows the luminescence spectra at 4.2 K of the same four samples upon irradiation at 532 nm, that is, into the 4A → 4T transition of [Cr(ox) ]3− as well as into the low-energy 2 2 3 tail of the 1MLCT and the 3MLCT transitions of [Ru(bpy)3]2+. For the 4 µm crystallites, the spectrum consists essentially of the electronic origin at 694.6 nm (14 396 cm−1) of the lower energy component, the R1 line, of the 2E → 4A2 emission. The small splitting of 1.3 cm−1 barely resolved in the spectrum is due to the zero-field splitting of the 4A2 ground state. In the emission spectrum of the 2.5 µm crystallites, the intensity of the sharp R1 line decreases and a broad new band appears at lower energy, which does not have a counterpart in the absorption spectrum. The relative intensity of the sharp band with respect to the broad low-energy band decreases still further with decreasing crystal size. This is a first indication that energy is
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Figure 1. a) TEM and SEM images of the nano- and microcrystals of [Ru(bpy)3][NaCr(ox)3]. b) Powder X-ray diffraction patterns of the nano- and microcrystals at 10 K. c) Average edge-length of the nano- and microcrystals as function of the water-to-surfactant ratio W0.
migrating from high-energy sites to low-energy sites within the inhomogeneous distribution causing energy to be immediately funneled to the low-energy surface sites serving as final energy acceptors. Figure 2c shows the quasisteady-state fluorescence line narrowing (FLN) spectra at 1.4 K obtained upon site-selective irradiation into the center of the R1 line at 14 396.0 cm−1 by tuning a narrow-band Ti–sapphire laser to that energy and
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using an alternating chopper in order to prevent excitation light from entering into the monochromator.[13,14] Near the excitation energy, the spectra consist of a series of sharp lines spaced by the zero-field splitting of the 4A2 ground state of 1.3 cm−1. For isolated chromium(III) complexes, one would expect only three sharp lines in the FLN spectrum[14,15] and not four or five as observed in the present case (see inset of Figure 2c). The additional lines are due to efficient resonant energy migration
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Adv. Mater. 2015, DOI: 10.1002/adma.201405179
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within the inhomogeneously broadened R1 line implying different components of the R1 transition.[6,11,14,16] For the 4 µm sample, this series of sharp lines is the only signal observed. Similar to the non-selectively excited luminescence, for smaller crystallites, the intensity in the region of the maximum of the R1 absorption is much weaker and instead the unstructured broad low-energy band appears. The observation of the resonant energy transfer at the center of the R1 absorption for all samples indicates that the inhomogeneous broadening in the core only slightly increases when the particle size decreases. The lowenergy tail in the absorption spectra of the smaller crystallites therefore indeed corresponds to chromophores progressively nearer to the surface. Since these low-energy sites constitute final energy acceptors, the energy thus migrates preferentially from the initially excited core chromophores to the surface. In order to substantiate the above conclusion, Figure 3 shows the luminescence decay curves for pulsed irradiation into the maximum of the R2 line at 14 409 cm−1 with a 1 µs pulse at a temperature of 1.4 K. Excitation into the upper component of the 2E state is followed by a relaxation to the lower component, that is,
Figure 2. a) Excitation spectra of [Ru(bpy)3][NaCr(ox)3] particles of different sizes at 1.4 K (see below for experimental details). b) Luminescence spectra of particles of different sizes at 4.2 K after excitation at 532 nm. c) Steady-state FLN spectra at an excitation energy of 14 396.0 cm−1 at 1.4 K, inset: enlarged presentation of the sharp line structure around the laser wavelength.
Adv. Mater. 2015, DOI: 10.1002/adma.201405179
Figure 3. a) Comparison between time-resolved luminescence kinetics of 4 µm MPs, 2.5 µm MPs, 670 nm NPs, and 140 nm NPs; luminescence at 14 394 ± 2 cm−1; laser excitation at 14 409.1 cm−1; 1 µs irradiation at 1.4 K. b) Comparison between time-resolved luminescence kinetics of 2.5 µm MPs, 670 nm NPs, and 140 nm NPs; luminescence at 14 371 ± 2 cm−1; laser excitation at 14 409.1 cm−1; 1 µs irradiation at 1.4 K.
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the R1 line within
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Directional Energy Migration in Nanoparticles of Crystalline Metal Complexes Elia Previtera, Antoine Tissot,* Robert W. Johns, and Andreas Hauser* Inorganic and coordination-based materials show promise for solar-energy harvesting and conversion when synthesized at the nanoscale.[1,2] Since excitation energy transfer plays an important role for such applications, understanding how these processes are affected by crystallite size is crucial to design novel materials.[3] Typically, materials showing long-range efficient directional energy migration are targeted.[4] In this Communication, we describe the preparation of both nano- and microscale crystallites of the 3D oxalate network [Ru(bpy)3][NaCr(ox)3], ox = C2O42−, bpy = 2,2′-bipyridine,[5] and discuss the influence of crystal size, so far unknown, on energy migration within the 2 E state of the [Cr(ox)3]3− chromophores.[6] The nanocrystals were prepared via the reverse micelle technique using sodium dioctyl sulfosuccinate (NaAOT)[7] as surfactant. They were analyzed for size distribution with transmission electron microscopy (TEM) and for crystallinity with powder X-ray diffraction (XRD) using synchrotron radiation (for details see the Experimental Section and Supporting Information). Figure 1a shows TEM images of a series of crystal sizes achieved by varying the water-to-surfactant ratio W0. For all preparations, the crystals are tetrahedral and the corresponding powder X-ray diffraction patterns (Figure 1b) reveal all samples to be crystalline and can be indexed to the cubic chiral space group P213 with a = 15.510 Å (for a graphic representation of the crystal structure see Figure S1 in the Supporting Information).[5] For comparison, a scanning electron microscopy (SEM) image and the corresponding powder diffraction pattern of a polycrystalline sample with crystallites of around 4 µm in size obtained by precipitation from water are included in Figure 1. In accordance with the Scherrer equation,[8] the diffraction peaks broaden as size decreases from a full width at half maximum (FWHM) of 0.05° for the 4 µm crystallites to 0.11° for the smallest 140 nm particles at 2θ = 5.8°. Figure 1c shows how the average crystals size, expressed by the edge length
E. Previtera, Dr. A. Tissot,[+] Prof. A. Hauser Département de Chimie Physique Université de Genève 30 quai Ernest Ansermet, 1211, Genève 4, Switzerland E-mail: [email protected]; [email protected] R. W. Johns Department of Chemistry University of California Berkeley, CA 94720, USA [+] Present address: Institut Lavoisier de Versailles, UMR 8180, Université de Versailles, Saint-Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France.
DOI: 10.1002/adma.201405179
Adv. Mater. 2015, DOI: 10.1002/adma.201405179
of the tetrahedra, depends on the water-to-surfactant ratio W0 (for details see Figure S1–S3 in the Supporting Information), thus allowing for size-controlled synthesis of nanocrystals from 140 to 1000 nm via W0. Finally, we note that for the smallest nanocrystals, that is, for an edge length of 140 nm, the shortest distance from the center to one of the faces is less than 30 nm. [Ru(bpy)3][NaCr(ox)3] is dark red due to the strongly absorbing spin-allowed metal–ligand charge transfer (1MLCT) transition of the [Ru(bpy)3]2+ chromophore centered at 468 nm (21 400 cm−1).[9] The characteristic luminescence from the corresponding 3MLCT state of [Ru(bpy)3]2+ is completely quenched within less than 1 µs by excitation energy transfer to the [Cr(ox)3]3− chromophores.[10] In fact, the weak and broad spinallowed ligand-field transition of the [Cr(ox)3]3− chromophore, the 4A2 → 4T2 transition centered at 543 nm (18 400 cm−1)[6,11] serves as acceptor level for this energy transfer, which, following intersystem crossing, results in strong luminescence from the spin-flip 4A2 → 2E transition of [Cr(ox)3]3− at 694.5 nm (14 400 cm−1). Figure 2a shows the excitation spectra for this luminescence at 1.4 K of crystallites of different sizes in the region of the transition itself. For the largest crystallites, that is, those with 4 µm edge length, two sharp lines corresponding to the zero-field split components of the electronic origins of the 2E state and generally referred to as the R-lines[12] with a FWHM of 4 cm−1 are clearly resolved with a splitting of 13.6 cm−1. For the 2.5 µm crystallites, the FWHM increases to 8 cm−1 and a broad tail begins to appear on the low-energy side. For the still smaller crystallites of edge lengths 670 and 140 nm, the FWHM increases still further and the peaks become asymmetric with increasing tails toward low energies. The inhomogeneous line width thus increases asymmetrically toward lower energy with decreasing size, suggesting that the 4A2−2E energy difference of the [Cr(ox)3]3− chromophores closer to surface is slightly lower than the one of the complexes in the bulk. Figure 2b shows the luminescence spectra at 4.2 K of the same four samples upon irradiation at 532 nm, that is, into the 4A → 4T transition of [Cr(ox) ]3− as well as into the low-energy 2 2 3 tail of the 1MLCT and the 3MLCT transitions of [Ru(bpy)3]2+. For the 4 µm crystallites, the spectrum consists essentially of the electronic origin at 694.6 nm (14 396 cm−1) of the lower energy component, the R1 line, of the 2E → 4A2 emission. The small splitting of 1.3 cm−1 barely resolved in the spectrum is due to the zero-field splitting of the 4A2 ground state. In the emission spectrum of the 2.5 µm crystallites, the intensity of the sharp R1 line decreases and a broad new band appears at lower energy, which does not have a counterpart in the absorption spectrum. The relative intensity of the sharp band with respect to the broad low-energy band decreases still further with decreasing crystal size. This is a first indication that energy is
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Figure 1. a) TEM and SEM images of the nano- and microcrystals of [Ru(bpy)3][NaCr(ox)3]. b) Powder X-ray diffraction patterns of the nano- and microcrystals at 10 K. c) Average edge-length of the nano- and microcrystals as function of the water-to-surfactant ratio W0.
migrating from high-energy sites to low-energy sites within the inhomogeneous distribution causing energy to be immediately funneled to the low-energy surface sites serving as final energy acceptors. Figure 2c shows the quasisteady-state fluorescence line narrowing (FLN) spectra at 1.4 K obtained upon site-selective irradiation into the center of the R1 line at 14 396.0 cm−1 by tuning a narrow-band Ti–sapphire laser to that energy and
2
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using an alternating chopper in order to prevent excitation light from entering into the monochromator.[13,14] Near the excitation energy, the spectra consist of a series of sharp lines spaced by the zero-field splitting of the 4A2 ground state of 1.3 cm−1. For isolated chromium(III) complexes, one would expect only three sharp lines in the FLN spectrum[14,15] and not four or five as observed in the present case (see inset of Figure 2c). The additional lines are due to efficient resonant energy migration
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Adv. Mater. 2015, DOI: 10.1002/adma.201405179
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within the inhomogeneously broadened R1 line implying different components of the R1 transition.[6,11,14,16] For the 4 µm sample, this series of sharp lines is the only signal observed. Similar to the non-selectively excited luminescence, for smaller crystallites, the intensity in the region of the maximum of the R1 absorption is much weaker and instead the unstructured broad low-energy band appears. The observation of the resonant energy transfer at the center of the R1 absorption for all samples indicates that the inhomogeneous broadening in the core only slightly increases when the particle size decreases. The lowenergy tail in the absorption spectra of the smaller crystallites therefore indeed corresponds to chromophores progressively nearer to the surface. Since these low-energy sites constitute final energy acceptors, the energy thus migrates preferentially from the initially excited core chromophores to the surface. In order to substantiate the above conclusion, Figure 3 shows the luminescence decay curves for pulsed irradiation into the maximum of the R2 line at 14 409 cm−1 with a 1 µs pulse at a temperature of 1.4 K. Excitation into the upper component of the 2E state is followed by a relaxation to the lower component, that is,
Figure 2. a) Excitation spectra of [Ru(bpy)3][NaCr(ox)3] particles of different sizes at 1.4 K (see below for experimental details). b) Luminescence spectra of particles of different sizes at 4.2 K after excitation at 532 nm. c) Steady-state FLN spectra at an excitation energy of 14 396.0 cm−1 at 1.4 K, inset: enlarged presentation of the sharp line structure around the laser wavelength.
Adv. Mater. 2015, DOI: 10.1002/adma.201405179
Figure 3. a) Comparison between time-resolved luminescence kinetics of 4 µm MPs, 2.5 µm MPs, 670 nm NPs, and 140 nm NPs; luminescence at 14 394 ± 2 cm−1; laser excitation at 14 409.1 cm−1; 1 µs irradiation at 1.4 K. b) Comparison between time-resolved luminescence kinetics of 2.5 µm MPs, 670 nm NPs, and 140 nm NPs; luminescence at 14 371 ± 2 cm−1; laser excitation at 14 409.1 cm−1; 1 µs irradiation at 1.4 K.
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