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Multi-Step Host−Guest Energy Transfer Between Inorganic Chalcogenide-Based Semiconductor Zeolite Material and Organic Dye Molecules Dan-Dan Hu,† Jian Lin,† Qian Zhang,† Jia-Nan Lu,† Xiao-Yan Wang,† Yan-Wei Wang,† Fei Bu,‡ Li-Feng Ding,§ Le Wang,*,† and Tao Wu*,† †

The Key Lab of Health Chemistry and Molecular Diagnosis of Suzhou, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China ‡ Department of Chemistry, University of California, Riverside, California 92521, United States § Department of Chemistry, Xi’an Jiaotong-Liverpool University, Suzhou Dushu Lake Higher Education Town, Jiangsu 215123, China S Supporting Information *

ABSTRACT: We herein present the first case of energy transfer process in an inorganic chalcogenide-based semiconductor zeolite material (coded as RWY) serving as UV−vis light-harvesting host. A multistep vectorial energy transfer assay was fabricated by encapsulating acridine orange (AO) molecules into the RWY porous framework and further covering the formed capsules with rhodamine B (RhB) molecules. The UV high-energy excitations absorbed by RWY host were channeled to AO molecules and then onto RhB molecules to give rise to visible-light emission. The steady-state fluorescence and confocal microscope as well as fluorescent dynamics of emission reveal successfully the process of multistep vectorial energy transfer. This inorganic-host-involved energy transfer process has never been observed in an insulating oxide-based zeolite host system. Therefore, chalcogenide-based semiconductor zeolites could be a class of promising host materials to be further explored in the field of energy transfer and electron transfer between inorganic host and organic guest.

1. INTRODUCTION The design and fabrication of high-efficiency light harvesting antennae are of the very essence for artificial solar energy utilization.1−3 To construct the systems with ideal energy efficiency necessitates two criteria: efficient light harvesting and the excited energy transduction with low decay rates. The first criterion is usually achieved by an aggregation of closely positioned light-absorption units in which the increased absorption cross-section greatly enhances the photon capture. Therefore, the exited energy transduction within such massive chromophores aggregate turns to be the major concern. To achieve the efficient vectorial energy migration, such ensemble of chromophores needs to function in a cooperative fashion hence not only to be delicately matched in energy levels, but also structurally highly ordered as well. In the past decade, various intelligently designed artificial systems were synthesized.4−9 Among them, host−guest system is a facile approach to achieve the ordered assembly for the chromophores. Induced by host−guest interactions and space confinement effects, the predesigned alignments of light-absorbing units could be readily accomplished. Zeolites, typically referred as oxide-based zeolites and regarded as a class of porous crystalline materials that possess highly ordered channels, great structural diversity, and good thermal and hydrothermal stabilities have been employed as the matrix material in most of these studies.10−18 It should be noted that the continuous processes on energy absorption, © XXXX American Chemical Society

migration, and transfer in these oxide-based zeolite matrices mainly occur in the guest chromophores without inorganic host involved (Scheme 1a). This is because insulating silicate and phosphate zeolites intrinsically lack either the UV- or visiblelight absorption or fluorescent property to be involved in a cooperative absorption−transduction energy cascade. However, it is generally accepted that host-involved energy transfer process can facilitate the enhancement of energy transfer efficiency due to much close contact between host and guest, which has been substantially exemplified in the organic host material system19−23 and never observed in pure inorganic zeolite hosts. Actually, the highly structural and thermal stability of rigid inorganic hosts, compared to flexible organic hosts, makes them good host candidates. Therefore, exploration of process on excitation energy harvesting and transportation with novel pure inorganic host material involved is of great importance and yet still remains a great challenge over the years. Chalcogenide-based zeolites are a unique family of materials constructed by ME4 tetrahedral units (M stands for main-group metal ions such as Ga, In, Ge, or Sn; E for chalcogen atoms such as S, Se, or Te), where all the chalcogen atoms are Received: March 29, 2015 Revised: May 9, 2015

A

DOI: 10.1021/acs.chemmater.5b01158 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

vessel and heated to 190 °C and kept for 6 days. When the autoclave was cooled to room temperature, pale yellow crystals were obtained together with amorphous powder. The mixture products were then ultrasonicated and washed with ethanol (95%) to float away small particles of amorphous powder; dodecahedral crystals were recovered with a yield of ∼67% according to gallium amount. 2.3. Removal of Protonated TAEA Template Molecules for Pore Opening. Ion exchange with 10 mL of CsCl (aq) (1 mol/L) was repeated on the same batch of RWY-TAEA crystals two times under 85 °C to completely remove H+TAEA cations in the pores of RWY. After the ion exchange, crystals of RWY-Cs were collected by filtration and vacuumdried under room temperature. 2.4. Preparation of Acridine Orange Included Framework RWY⊃AO. The encapsulation of AO molecules into RWY framework was achieved via a second-time ion exchange. RWY-Cs crystals (40 mg) were emerged into 5 mL of AO solution with different concentrations (10−5, 10−6, and 10−7 mol/L, respectively) for 2 days under room temperature. As the impregnation time prolonged, the color of RWY crystals turned darker from pale yellow to bright orange, and the orange solution slowly faded away, which indicates the inclusion of AO into the RWY frameworks. The actual amounts of AO encased into RWY framework were 5× 10−5, 5× 10−6, and 5 × 10−7 mmol, respectively, which were measured by UV−vis absorption spectroscopy. The sample after ion exchange was washed with ethanol and water several times to remove the redundant surface-adsorbed AO molecules. The sample was then vacuum filtrated and dried under room temperature. 2.5. Crystal Surface Modification with Rhodamine B. The AO encapsulated crystals RWY⊃AO were further emerged in RhB solution with different concentrations (10−5, 10−6, and 10−7 mol/L, respectively) for 3 days under room temperature. There was no obvious change observed for the color of solution phase upon the long-time soaking. The actual amounts of RhB adsorbed onto the surface of RWY were 1.15 × 10−5, 5× 10−6, and 5 × 10−7 mmol, respectively, which were measured by UV−vis absorption spectroscopy. The obtained surface modified crystals [RhB-(RWY⊃AO)] were vacuum filtrated and dried at room temperature. 2.6. Material Characterizations. The as-synthesized and dye-adsorbed RWY crystalline samples were characterized by power X-ray diffraction that was collected on a desktop diffractometer (D2 PHASER, Bruker, Germany) using Cu−Kα radiation operated at 30 kV and 10 mA. Variable temperature powder X-ray diffraction (PXRD) data were collected on a diffractometer (D8, ADVANCE, Bruker, Germany) equipped with high-temperature in situ vacuum reaction attachment at a heating rate of 5 °C/min. Elemental analysis (C, H, and N) was performed by VARIDEL III Elemental analyzer. Nitrogen adsorption−desorption isotherms were measured on Micromeretics ASAP 2020 HD88. The distribution of different chromophors was characterized by a confocal laser scanning microscope (CLSM) with a 405 nm excitation laser source. Absorption spectra of dye and dye-loaded RWY were recorded with SHIMADZU UV-3600 UV−vis spectrograph. Fluorescence spectra of the host−guest antenna system were measured at 25 °C using a Fluorolog-3 spectrofluorometers with a 450 W Xe lamps as light source. Time-resolved fluorescence behaviors were measured via time-correlated single-photon counting (TCSPC) technique.

Scheme 1. Energy Transfer Processes (a) without and (b) with Host Matrix Involved

biconnected just as oxygen bridges in traditional oxide-based zeolites.24−28 Interestingly, the long M−E bond length comparing to that of Si/Al−O bond results in the formation of supertetrahedral clusters M4E10, which are exact fragments of cubic ZnS-type lattice. As such, these chalcogenide-based zeolites are capable of integrating uniform porosity with various physical properties such as semiconductivity, photoluminescence, photocatalytic activity, and electrochemical behavior.29−34 Grafted with the functionalities that could hardly be achieved in traditional oxide-based zeolites, these semiconducting chalcogenide-based zeolites may provide a whole new opportunity for framework-involved processes such as energy transfer between inorganic host and organic guest (Scheme 1b). In this study, we presented the first case study of fluorescent chalcogenide-based zeolite involved energy transduction, where the fluorescent zeolite host (coded as RWY, originally named as UCR-20-GaGeS with RWY framework type code)25 acts as the UV-light absorber and passes the exited energy down to visiblelight absorbing chromophors. Interestingly, a vectorial specific second-order energy transfer is achieved by encapsulating the first-order acceptor (blue range absorbing dye molecules acridine orange, abbreviated as AO) inside the framework but screening out the second-order acceptor (longer wavelengths absorbing molecule rhodamine B, abbreviated as RhB) via poresize regulations.

2. EXPERIMENTAL SECTION 2.1. Materials. Sulfur (S, >99%, powder), germanium oxide (GeO2, HP, powder), gallium metal (Ga, AP, bulk), cesium chloride (CsCl, 99%, solid), tris(2-aminoethyl)amine (TAEA, 96%, liquid), AO (55−60%, power), and RhB (AR, powder) were all used as received without any further purification. 2.2. Synthesis of Chalcogenide-Based Zeolite RWYTAEA. Fluorescent chalcogenide-based zeolite material RWY was synthesized according to literature method with minor modifications.2 By mixing small pieces of gallium metal (82.7 mg), germanium oxide (109.1 mg), sulfur (222.1 mg), and 2.1719 g of TAEA in a Teflon-lined autoclave under room temperature with vigorous stir for about 30 min, an opaque suspension was formed. The suspension was then sealed in the B

DOI: 10.1021/acs.chemmater.5b01158 Chem. Mater. XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION 3.1. Fabrication and Characterizations of Antennae System. RWY host, dye entrapped RWY⊃AO, and RhB(RWY⊃AO) were synthesized as described above. The PXRD pattern of prepared materials displayed sharp peaks, indicating the crystallinity of the host frameworks were well preserved during the post-treatment of ion exchange (Figure S1, Supporting Information). However, at higher temperature, the structure of RWY or RhB-RWY was destructed at 573 K; RWY⊃AO was destructed at 623 K. RWY⊃AO composites showed preferable thermostability compared to original RWY or RhB-RWY (Figure 1), which can be explained by dyes inside

Figure 2. (a) The inner pore surface of RWY framework; (b) a sodalite (SOD) cage in RWY; (c) the window aperture size of 12membered ring and 8-membered ring in SOD cage; (d) the size of RhB and AO molecules.

AO molecule (approximately 5.1 Å). Thus, the AO molecules could penetrate into the inner voids of the host. In fact, the AO inclusion was readily achieved under room temperature with a very fast inclusion rate. An intense orange color of 5 mL 10−5 M AO solution gradually faded away within hours upon exposure to RWY adsorber. However, on the contrary, bulkier luminescent molecule RhB could not pass through RWY windows because none of its cross-section could properly be fitted into the RWY channels. The control experiment of RhB inclusion was performed, and indeed, there was neither obvious color change observed for the dye solution nor any detectable nitrogen content in elemental analysis measurement (Table S1, Supporting Information). As such, a multistep energy cascade system could be constructed by encapsulating AO in the UV absorbing RWY host; however, RhB only covers the exterior of each capsule particle. The controlled chromophore distribution was further verified by the confocal laser scanning fluorescent images of the modified RWY crystals. As presented in Figure 3, in a crosscut crystal, the green section representing AO molecules is evenly distributed throughout the RWY host, which proves the AO molecules dominantly distributed in the pore space. However, RhB molecules are only adsorbed on the crystal surface since the red section contributed from RhB only luminates on the surface of the solid. 3.2. Fluorescent Characteristics of Antennae System. RWY-Cs shows a broad absorption band at UV region. Upon excitation at 370 nm, RWY-Cs emitted blue luminescence centered at 420 nm (Figure 4a, black curve). The excitation spectrum monitored at 420 nm displayed a peak at 370 nm (Figure S4, Supporting Information). On the other hand, diluted AO aqueous solution absorbed at around 490 nm (Figure 4a, red curve), and the emission spectrum showed a peak at 525 nm (Figure 4b, pink curve), which is close to 550

Figure 1. PXRD patterns of dye-loaded RWY at different temperatures: (a) RhB-RWY; (b) RWY⊃AO.

pores limiting the liberty vibration of framework at higher temperature. Gas sorption experiments with N2 at 77 K displayed a typical type-I sorption isotherm, which reveals the typical microporous nature of RWY framework (Figures S2 and S3, Supporting Information). The Brunauer−Emmett−Teller (BET) surface area was evaluated to be 555.3186, 530.2990, and 541.6651 m2 g−1 for RWY-Cs, RWY⊃AO, and RhB-RWY, respectively. The decrease in N2 uptake amount and specific surface area are reasonable since the void space of RWY host was occupied or blocked as a result of inclusion of dye molecules. The larger window of microporous RWY host was determined to be 5.9 Å (excluding the van der Waals radii of the pore wall) from the single-crystal diffraction data as shown in Figure 2. It is slightly wider than the shorter dimension of C

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between each donor−acceptor pair of RWY-AO and AO−RhB are shown as green and orange shaded areas, respectively. In relation to this energy matching as observed, a control experiment showed that neither AO nor RhB is emissive upon excitation at 370 nm, which makes the selective excitation of the RWY-Cs framework feasible. Also, the spectral overlaps suggest such an RhB-(RWY⊃AO) system could serve as a multistep energy transfer cascade since the overlaps of absorption band with the emission band would in principle favor the fluorescence resonance energy transfer (FRET) caused by the electronic transition dipole moment matching. In a typical cascade system, 5 × 10−5/5 × 10−6/5 × 10−7 mmol AO chromophore was loaded to 40 mg of RWY host crystals. These crystals were then exposed to 5 mL of RhB aqueous solution (10−5 M, 10−6 M, and 10−7 M respectively) for surface modification. RWY⊃AO emitted bright fluorescence at 525 nm (bright green representing) upon excitation of the RWY framework material at 405 nm (Figure 5a1). As expected, only dim fluorescence (red representing, Figure 5a2) at 605 nm from the RhB was observed in RhB-RWY due to the low spectral overlapping. These observations proved that the excitation energy was more efficiently transferred from the RWY framework to AO than to RhB. Also, as shown in Figure 5, panel a (3−5), as the AO ratio decreased in RhB-(RWY⊃AO) system, the observed fluorescence turned weaker together with the loss of green light fraction. More detailed quantitative fluorescent spectra were taken on a series of samples containing different AO and RhB ratios (Figure 5b,c; Figure S5, Supporting Information). As shown in Figure 5, panel b, an obvious strong emission peak around 550 nm was observed in AO-loaded RWY system, which was absent from the original RWY host. Upon keeping the AO amount constant and increasing RhB ratio, the 550 nm peak was drastically weakened with the appearance of a new emission peak at 620 nm. As well, upon decreasing AO inclusion amount but maintaining RhB amount as indicated in Figure 5, panel c, the AO emission peak around 550 nm gradually disappeared. Together with the decrease of AO emission, the energy available to be transferred to RhB decreased as well, which hence resulted in a weaker, longer wavelength emission at around 620 nm. Such AOdependent RhB emission confirms the AO chromophor is the very necessary energy transfer pathway from RWY host to the final acceptor RhB. The microporous RWY framework in RhB(RWY⊃AO) serves as an energy funnel that harvests highenergy excitations from the UV region and channels it to AO and then onto RhB. 3.3. Energy Transfer Efficiency within RhB-(RWY⊃AO). The luminescence of RhB-(RWY⊃AO) is highly dependent on its content, which could be fine tuned from bright green to dark orange. Quantitative measurements using fluorescence spectroscopy showed a dependent growth in the peak intensity at 525 nm as the AO content increased while maintaining RhB content. The efficiency of energy transfer is of great concern in such antenna system. Thus, to evaluate the energy transfer efficiency of RhB-(RWY⊃AO) system, fluorescence decay profiles were investigated (Figure 6). Upon nano-light-emitting diode (LED) pulse exposure, the major fluorescence peaks were monitored in different samples of RWY host, RWY⊃AO, and RhB-(RWY⊃AO). All the monitored peaks exhibited a three-component decay profile as shown in Table S2 of the Supporting Information. Upon inclusion of AO, the weighted lifetime of RWY fluorescence decreased from 3.50 to 3.08 ns.

Figure 3. Confocal laser scanning graphs of dye-loaded RWY in a crosscut single crystal. (a): AO (green) mainly existing in the pores of RWY; (b): RhB (red) mainly existing on the surface of RWY and slightly stopper at the pore entrances; (c): overlap of panels a and b.

Figure 4. (a) The overlap of AO absorption spectrum and RWY emission spectrum (green). (b) The overlap of RhB absorption spectrum and AO emission spectrum (orange).

nm where RhB chromophore showed its absorption maximum (Figure 4b, blue curve). The absorption−emission overlap D

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Figure 6. Fluorescence dynamics of emission at different emission center: (a) RWY and RWY⊃AO (λem = 420 nm); (b) RWY⊃AO and RhB-(RWY⊃AO) (λem = 532 nm), (c) RhB-(RWY⊃AO) at different temperature; (d) temperature-dependent energy transfer efficiency between RWY and AO for RhB-(RWY⊃AO) host−guest system (λem = 420 nm).

dipole moment matching and resonance transfer.35 These results present an unprecedented speedy energy transduction for fluorescent zeolite and are rare for inorganic host evolved cooperative light-harvesting.

4. CONCLUSIONS In summary, we here presented the first showcase of lightharvesting inorganic semiconductor zeolite by designing energy-donating porous framework as UV-antenna that encapsulated energy-accepting AO molecules. A multistep vectorial energy transfer assay could be fabricated by further covering RWY⊃AO capsules with RhB. The chalcogenidebased zeolite RWY is capable of excitation energy transduction over the crystalline semiconducting framework and allows for rapid carrier transportation with very high mobility. Moreover, RWY zeolite screens off UV light to protect the visible-region absorbing organic chromophores from photobleach in a lowenergy-loss fashion by channeling the excited energy to final acceptors. Because of such unique characteristics of rapid triggered and high-efficiency vectorial energy migration and transfer, we believe chalcogenide-based zeolites are a class of promising host materials to be further explored in lightharvesting and even be extended to the field of electron transfer between host and guest.

Figure 5. (a) Visual demonstration of energy transfer: CLSM image of dye-loaded RWY with 405 nm laser as the excitation source (1) RWY⊃AO, (2) RhB-RWY, (3) RhB(10−5)-[RWY⊃AO(10−5)], (4) RhB(10−5)-[RWY⊃AO(10−6)], (5) RhB(10−5)-[RWY⊃AO(10−7)]; (b) the steady-state emission spectra of RhB(10−5)-RWY with different AO concentrations under excitation wavelength of 370 nm; (c) the steady-state emission spectra of RWY⊃AO(10−5) with different RhB concentrations under excitation wavelength of 370 nm. The values in bracket represent dye concentration (in mol/L) of solution to be used for ion exchange.

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The average lifetime of AO emission was shortened with 0.43 ns from 5.04 ns once the RWY⊃AO capsules were surfacemodified by RhB. The acceleration on decay rates clearly indicates the fluorescent energy transfer process, and the time scale required for the energy transfer is on the nanosecond level. According to the following equation, ηET = 1 − τDA/τD,5 the energy transfer efficiency on each transfer step of RWY to AO and AO to RhB is calculated to be 12% and 9%, respectively. Interestingly, the transfer efficiency of RWY to AO step is highly temperature dependent and showed a dramatic decrease upon cooling (Figure 6c; Table S3, Supporting Information). This phenomenon could be explained that the more fixed AO molecules under lower temperature lack the opportunity to freely rotate to a special orientation to provide

ASSOCIATED CONTENT

S Supporting Information *

Additional PXRD, nitrogen adsorption and desorption isotherms, steady-state excitation and emission spectra, elemental analysis, and fluorescent dynamic data analysis. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01158.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21271135), a start-up fund (Q410900712) from Soochow University, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Young Thousand Talented Program.

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DOI: 10.1021/acs.chemmater.5b01158 Chem. Mater. XXXX, XXX, XXX−XXX