Orderly Assembled with Layered Double Hydroxide Nanosheets

Article pubs.acs.org/JPCC

Phosphorescent Sensor Based on Iridium Complex/ Poly(vinylcarbazole) Orderly Assembled with Layered Double Hydroxide Nanosheets: Two-Dimensional Föster Resonance Energy Transfer and Reversible Luminescence Response for VOCs Yumei Qin, Jun Lu,* Shuangde Li, Zhen Li, and Shufang Zheng State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P. Box 98, 100029, Beijing, P. R. China S Supporting Information *

ABSTRACT: The application study of phosphorescence complexes is important for further investigation and exploration of novel optofunctional materials. In this work, neutral poly(vinylcarbazole) (PVK) and tris[2-(4,6difluorophenyl)pyridinato-C 2,N]iridium(III) (Ir(F2ppy) 3) were assembled with LDH nanosheets to form ordered ultrathin films (UTFs). These inorganic/organic composite UTFs exhibited cyan luminescence from Ir(F2ppy)3, peaking at 471 and 491 nm, due to triplet metal-to-ligand charge transfer and ligand-centered states, respectively. Under PVK maximal excitation at 294 nm, the photoluminescence spectra of the UTFs showed emission from Ir(F2ppy)3 rather than from PVK, demonstrating PVK transfer of resonance energy to Ir(F2ppy)3. Temporal luminescence spectroscopy revealed that the phosphorescence lifetime of Ir(F2ppy)3 molecules increased to 885 ns and fluorescence lifetime of PVK fell to 1.39 ns in the UTFs, which was typical character of the FRET process. This FRET process occurred within the interlayers of LDH nanosheets and can be described as a two-dimensional (2D) process with high efficiency (0.892). Moreover, it was found that the presence of volatile organic compound (VOC) vapors can interrupt this 2D process because of the unique hydrophobic character of the organic interlayers within the UTFs. This function was utilized for a phosphorescent sensor that enables reversible two-state photoemission switching (ON for cyan light of Ir(F2ppy)3 vs OFF for blue light of PVK). This phosphorescence sensor demonstrated a fast, highly sensitive, and reversible response toward common VOCs, and this 2D FRET with energy transfer involving the D(singlet) → A(triplet) process was more efficient compared with that of singlet−singlet processes. That is, to get the same luminescence intensity, these UTFs can be realized by less frequent excitation of PVK or lower concentrations of Ir(F2ppy)3 compared with intrinsic excitation of Ir(F2ppy)3.

1. INTRODUCTION Design and preparation of chemical sensors have become one of the most intense topics in academic research due to their versatile performance and valuable applications.1,2 Fabricating novel and efficient sensors for harmful chemical organic solvents is very essential not only for academic interest but also for the safety of the environment. Unfortunately, some chemical sensors nowadays seem to be complicated and irreversible and severely limit their potential application. Thus, developing low-cost and reliable sensors for detecting volatile organic compounds (VOCs) is an attractive research objective and plays a crucial role in industrial, agricultural, and medical fields. Föster resonance energy transfer (FRET) is the transfer of excited-state energy from a donor (D) to a proximal acceptor (A), which results from the long-range dipole−dipole D/A interactions.3,4 It depends on the molar ratio of the acceptor to the donor and the extent of the emission spectral overlap of the © 2014 American Chemical Society

donor with the absorption spectrum of the acceptor as well as on the distance between them. The FRET technology has been utilized particularly in biological and optoelectronic applications such as fluorescent probe and organic light-emitting diodes (OLEDs).5 The newly emerging two-dimensional FRET nanomaterials have drawn considerable attention because of high FRET rate and unique optical, electronic, and mechanical properties.6 Phosphorescent sensors based on the FRET procedure are rarely reported for sensing VOCs. Luminescent transition-metal complexes are appealing for their utility in varied fields as a result of their adjustable physicochemical properties.7−11 In contrast to traditional fluorescent materials, which are singlet state emitters, d6, d8, and d10 heavy-metal phosphorescent Received: June 3, 2014 Revised: August 13, 2014 Published: August 13, 2014 20538

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suspension. The quartz glass substrates were cleaned in a mixed solution of concentrated H2SO4/H2O2 (v:v = 7:3) and anhydrous alchohol for 30 min each and then washed by deionized water thoroughly. The quartz substrate was immersed in the LDH colloidal suspension (1 g·L−1) for 10 min and washing several times with deionized water, and then the quartz substrate was immersed in PVK (1 g·L−1) or Ir(F2ppy)3@PVK (Ir(F2ppy)3 0.01−0.1 g·L−1) toluene solution for 10 min and washed several times with deionized water. Multilayer thin films of (PVK/LDH)n and (Ir(F2ppy)3@PVK/ LDH)n UTFs were fabricated by alternately depositing LDH nanosheets and PVK or Ir(F2ppy)3@PVK for n cycles. The fabricated UTFs were dried under a nitrogen gas flow for 5 min at room tempreture. The Ir(F2ppy)3@PVK drop-casted films for comparative research were prepared by solvent evaporation. 2.3. Sample Characterization. UV−vis absorption spectra were obtained on a Shimadzu U-3600 spectrophotometer. Fluorescence excitation and emission spectra were recorded on a RF-5301PC fluorospectrophotometer. X-ray diffraction patterns (XRD) of the UTFs in the dry air and in the toluene vapor were obtained with a Rigaku 2500VB2+ PC diffractometer using Cu Kα radiation (λ = 1.541844 Å, 2θ = 0.5−8°), at 40 kV, 50 mA. A scanning electron microscope (SEM Zeiss Supra 55) was used to investigate the morphology of UTFs. Atomic force microscope (AFM) software (Digital Instruments, Version 6.12) was used to obtain the surface roughness data. The decay curves were obtained on an Edinburgh Instruments FLS980 fluorimeter, and the weighted average fluorescence lifetime was calculated by a multiple-exponential fitting with Edinburgh F980 instrument software.

complexes are triplet state emitters and widely utilized in photocatalysis, chemosensors, and biological imaging. Phosphorescent cyclometalated iridium(III) complexes (d6) are characterized by their metal-to-ligand-change-transfer (MLCT) transition leading to phosphorescent emission and have attracted increasing attention in applications such as luminescent sensors for oxygen,12 heavy metal ions,13 anions,14 and amino acids.15 The strong spin−orbit coupling of the Ir(III) metal ion leads to efficient intersystem crossing between the singlet and triplet excited states, thus endowing iridium(III) complexes with excellent characteristics, such as large Stokes shifts, sensitive luminescence properties in response to external environment changes, tunable luminescence color across the visible spectrum (from blue to red), long excited-state lifetimes (typically on the order of microseconds), and high luminescent quantum yield. The relatively long phosphorescent lifetimes, compared to those of purely organic luminophores (nanoseconds), endow Ir(III) complexes with the ability of good time resolution, and their phosphorescent signal can be recognized from fluorescent backgrounds easily and fast.16−19 During the past decade, nanomaterials with microscopic scales have attracted tremendous attention because of their unique and appealing optical and electrical properties. Materials with a two-dimensional (2D) layered structure, such as layered double hydroxides (LDHs), one significant type of functional inorganic layered material, have shown to be a successful host material for preparing a large variety of inorganic/organic hybrids. It is generally expressed by the formula [M2+1−xM3+x(OH)2](An−)x/n·mH2O, where M2+ and M3+ are metal cations and An− is a charge-compensating anion.20 LDHs can be widely used as an inorganic matrix because of its high stability and versatility in chemical composition. Previous studies have illustrated that blue fluorescent poly(N-vinylcarbazole) (PVK) and the phosphor cyclometalated iridium(III) are suitable FRET D/A pairs.21−26 In this study, Mg−Al LDH exfoliated nanosheets were chosen as the inorganic layer, and the iridium(III) complex (Ir(F2ppy)3) intertwined with PVK as the intact organic functional layer; all of them were alternately assembled to form ultrafine thin films (UTFs) by the LbL assembly method based on hydrogen bonding. Within the organic layer of the UTFs, the twodimensional (2D) FRET process between PVK (D) and Ir(F2ppy)3 phosphorescent dyes (A) was realized and its character was investigated. Moreover, this 2D FRET process can be interrupted when common organic vapors are present, a function that can be applied to sensing VOCs. Therefore, the Ir(F2ppy)3@PVK/LDH hybrid is a promising phosphorescent sensor candidate having an integrated 2D FRET process.

3. RESULTS AND DISCUSSION 3.1. Assembly Characterization of (Ir(F2ppy)3 @PVK/ LDH)n Thin Films. The fabrication of the Ir(F2ppy)3@PVK/ LDH UTF was based on the hydrogen bonding interaction between PVK@Ir(F2ppy)3 and LDH nanosheets. Figure 1

Figure 1. Assembly of the (Ir(F2ppy)3@PVK/LDH)n UTFs.

2. EXPERIMENTAL SECTION 2.1. Materials. Tris[2-(4,6-difluorophenyl)pyridinatoC2,N]iridium(III) (Ir(F2ppy)3, C33H18F6IrN3, MW = 762.72, 96%) and poly(N-vinylcarbazole) (PVK, (C14H11N)n, MW = 90 000, 99%) were purchased from Aldrich. Reagents including Mg(NO3)2·6H2O, Al(NO3)3·9H2O, urea, tetrahydrofuran (THF), formamide, acetone, chloroform, toluene, H2SO4, and H2O2 were purchased from Beijing Chemical Co. Ltd. Decarbonated and deionized water was used in the experiment. 2.2. Preparation of (Ir(F2ppy)3@PVK/LDH)n Thin Films. The method of synthesis and exfoliation of Mg2Al−LDHs were similar to the paper previously reported.20 A 0.1 g amount of Mg2Al−LDH was shaken in 100 mL of formamide solution for 24 h to produce an exfoliated Mg2Al−LDH nanosheet colloidal

shows the molecular structure of PVK and Ir(F2ppy)3 and the assembly process of UTFs. It can be reasonably supposed that the carbazole N atom of PVK can form a hydrogen bond with the hydroxyl group of LDH nanosheets or H2O molecules in the interlayers and that this hydrogen bonding interaction can bind the luminescent organic layer and LDH inorganic layer together to form an orderly hybrid film. The assembly process of the multilayer thin films fabricated by alternate deposition of PVK@Ir(F2ppy)3 and LDHs on quartz substrate was monitored by the UV−vis absorption spectra. Figure 2A shows UV−vis absorption spectra of UTF-n: the absorption bands at ca. 233, 297 nm were identified as 20539

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Figure 3. Small-angle XRD pattern of UTF-50 recorded in the atmosphere (black) and in toluene vapor (red).

pounds,29 the 4 nm spacing between LDH layers indicated that the PVK was arranged in a multilayer way. The top-view SEM images show that the UTFs are continuous and the side-view SEM images show that the UTFs’ thickness with different bilayer numbers is approximately 66 nm (n = 15) and 130 nm (n = 30), respectively (Figure 4,

Figure 2. (A) UV−vis spectra and (B) fluorescence spectra of UTF-n (n = 3, 6, 9, ..., 27); (inset) absorbance or emission intensity versus the number of bilayers, n.

π−π* and n−π* optical transitions in the carbazole group of PVK,27 and 345 nm is the characteristic absorption of singlet metal-to-ligand charge transfer (1MLCT) transition of Ir(F2ppy)3.28 The inset in Figure 2 shows the absorbance and the Ir(F2ppy)3 cyan photoluminescence of UTFs increased proportionally with the number of bilayers (n), which demonstrated that a regular and stepwise film growth was achieved with the hydrogen bonding method, although it is a fairly weak intermolecular interaction in contrast to the electrostatic interactions widely applied for LDH-based UTF LbL assembly. The S/N ratio in the absorption spectra (Figure 2A) was poorer because the thickness of the as-prepared ultrathin film was not greater than 200 nm, for which the optical absorption was weak. 3.2. Morphology and Structure of (Ir(F2ppy)3 @PVK/ LDH)n Thin Films. The alternate assembly of Ir(F2ppy)3@ PVK and LDH nanosheets would result in a periodic orderly UTF along its normal direction. The small-angle X-ray diffraction (XRD) pattern (Figure 3a) revealed a Bragg peak at 2θ = 1.97°, confirming that the UTF has an ordered periodic layered structure vertical to the quartz substrate, and the basal spacing was about 4.46 nm. The S/N ratio in the XRD pattern (Figure 3) was poorer because, although the assembly cycles were up to 50, the ordering of the ultrathin film in the normal direction was not very good. It is known that the exfoliated LDH nanosheet has a thickness of 0.48 nm, which implies that the thickness of the organic monolayer was about 4 nm throughout the 50 bilayers. Through calculation (by Gaussian software), Ir complex molecular length was about 0.5 nm, PVK molecular width was about 0.8 nm, and because the polymer tends to assume a planar conformation in laminar com-

Figure 4. Top- and side-view SEM images (top) and tapping-mode AFM images (bottom) of UTFs for n = 15, 30.

top). Thus, the UTF thickness of one bilayer can be calculated to be 4.35 nm, which is comparable to the value of 4.46 nm XRD observation. Consequently, the spacing between the two assembled functional organic molecules should be controlled relatively accurately in the range of several nanometers, which is suitable for the FRET process. The typical top-view AFM images (5 μm × 5 μm) demonstrate that the as-prepared UTF morphology surface is microscopically continuous and uniform (Figure 4, bottom), with the root-mean-square roughness of 5.441 nm (n = 15), 10.475 nm (n = 30). In this work, many neutral polymers (e.g., PMMA) were tried to assemble with LDH nanosheets; however, only PVK, which is also a carrier of the Ir complex, can be assembled with LDH nanosheets to form ultrathin films with good homogeneity and continuity. Therefore, this interaction of PVK with the host layers can be attributed to hydrogen bonding between the N atoms of PVK and the H atoms of interlayer H2O or hydroxides. The above results proved that the UTFs assembled by hydrogen-bonding interaction are integral and homogeneous and are comparable with those films assembled by electrostatic interaction. 20540

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3.3. The 2D FRET Process in (Ir(F2ppy)3@PVK/LDH)n Thin Films. PVK and Ir(F2ppy)3 molecules are well studied FRET D/A pairs due to the overlapping of the PVK emission and Ir(F2ppy)3 excitation spectra (Figure S1 in Supporting Information, SI). To optimize the luminescence of the UTFs, the Ir(F2ppy)3 weight concentration dependence was studied in the range from 1 to 10 wt % (based on PVK 1g·L−1). Figure 5

in the FRET distance. According to our work, in the 2D interlayers the distance between PVK and Ir complex was suitable for the FRET process, and the homogeneity dispersion of Ir complex into PVK avoided the fluorescence quenching, which was not possible for the PVK/Ir complex solution. Above all, the solid-state form of Ir(F2ppy)3@PVK complex (UTFs or drop-casted films) could attain FRET rate higher than that in solution (Figure S3 in SI), and the UTFs have more merits, compared with drop-casted films, such as uniformity, tuning thickness, and response to VOCs. It has been reported that the acceptor coupled with the donor via FRET displayed a noticeable enhancement in emission intensity in contrast to that in the absence of donor.3 As shown in Figure 7, it can be speculated that the

Figure 5. Fluorescence spectra of the (PVK@Ir(F2ppy)3/LDH)10 film with 294 nm PVK intrinsic excitation. (Ir(F2ppy)3 doping concentration in PVK: 1−10 wt %).

shows the PL spectra of (Ir(F2ppy)3@PVK/LDH)n UTFs with different Ir(F2ppy)3 doping concentrations. The emission spectra of the film were almost identical to those of Ir(F2ppy)3 in toluene (Figure S1 in SI), demonstrating that the optical properties of the Ir complex were maintained in the UTF. With exclusive PVK excitation of 294 nm, the emission peak at 375 nm of PVK was almost quenched when the Ir(F2ppy)3 doping concentration was greater than 2 wt %, illustrating that the FRET process had efficiently taken place above 2 wt % doping concentration. Increasing the doping concentrations also increased the brightness of the films, and optimal brightness was acquired at 8 wt % doping concentration. Beyond 8 wt % the brightness fell, which suggested that tripet−triplet annihilation30 and concentration quenching dominated thereinto.31 This consequence can be visualized in the photograph taken under a UV lamp (Figure 6).

Figure 7. Fluorescence spectra of (PVK/LDH)25 film excited at 294 nm, and (Ir(F2ppy)3@PVK/LDH)25 film excited at 294 and 434 nm.

excitation energy of PVK is obviously transferred to Ir complex when using the PVK intrinsic excitation of 294 nm, leading to the predominant Ir(F2ppy)3 cyan phosphorescence, with simultaneous PVK 375 nm fluorescence quenching. However, the emission intensity upon excitation at the intrinsic excitation 434 nm of Ir(F2ppy)3 had lower efficiency as compared to that in UTF-25. The phosphorescence intensity increased by over 10 times when excited at PVK excitation, which confirmed the intermolecular singlet to triplet (S−T) 2D energy transfer occurring effectively within the interlayers of LDHs. The FRET process in UTF-25 was further explored by its luminescence decay dynamics. The FRET process in a D/A system can be confirmed from steady-state luminescence quenching, the faster luminescence decay of the donor, and slower decay for the acceptor.32 The weighted-average lifetime was obtained by multiple-exponential fitting of the luminescence decay curves (Table S1, Figures S4 and S5 in SI). From the luminscence lifetime by exponential fitting of the decay profile of PVK, it can be found that the 12.85 ns at 407 nm of (PVK/LDH)25 UTF fell to 1.39 ns at 375 nm of (Ir(F2ppy)3@ PVK/LDH)25 (Figure 8A). The 471 nm phosphorescence lifetime of Ir(F2ppy)3 in UTF-25 increases to 885 ns as shown in Figure 8B, far more than that of its toluene solution (29.28 ns) and even more than that of the Ir(F2ppy)3@PVK (8 wt %) drop-casted film (529 ns). The considerably reduced luminescence lifetime of PVK and the enhanced one of Ir(F2ppy)3 was another dynamic manifestation of the FRET process in the confined 2D environment (Figure 8A). According to the fomula,4 the FRET efficiency of UTF-25

Figure 6. Photographs of the (Ir(F2ppy)3@PVK/LDH)10 films (doping concentration: 1−10 wt %), taken under UV irradiation at 364 nm (top) and at 254 nm (bottom), respectively.

In contrast, it was found that the Ir(F2ppy)3/PVK organic solution with the same doping concentration range showed lower luminescent intensity (Figure S2 in SI). The energy transfer process in this inorganic/organic assembly layered material was proved to be more efficient than that in organic solution, because of the immobilization effect of the LDH host layers that restrict the spacing between the donor and acceptor 20541

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As shown in Figure 9A, the emission spectrum of UTF-25 in the atmosphere (black) and in toluene vapor (red) depicts that

Figure 8. (A) The luminescence decay curves of PVK (at 407 nm) of (PVK/LDH)25 (a); PVK (at 375 nm) (b) and Ir(F2ppy)3 (at 471 nm) of UTF-25 (c) excitated at 294 nm; inset is the magnified decay curve in the range of 0 to 50 ns. (B) The luminescence decay curves of Ir(F2ppy)3: in toluene solution (at 473 nm) excitated at 434 nm (a); in Ir(F2ppy)3@PVK (8 wt %) drop-casted films (at 471 nm) (b); in UTF-25 (at 471 nm), both excitated at 294 nm (c).

Figure 9. (A) Fluorescence spectra of UTF-25 in the atmosphere (black) or in toluene vapor (red) excited at 294 nm. (B) Five reversible fluorescence response cycles.

was calculated to be 0.892 in the atmosphere by using the weighted-average lifetime of PVK with or without doping of Ir(F2ppy)3. 3.4. Reversible Luminescence Response of (Ir(F2ppy)3@PVK/LDH)n Thin Films for VOCs. The above disscussion clarifies that the 2D FRET between PVK and Ir(F2ppy)3 was realized within the interlayer of UTFs, and the spacing between them was less than 10 nm according to Figure 3. In addition, it was reported that the 2D FRET process of the UTFs could exhibit an adjustable OFF/ON fluorescence signal when contacting volatile organic compound (VOC) vapors.33 In other words, when putting the UTFs into a sealed cuvette full of VOC vapor, the VOCs will infiltrate into the hydrophobic interlayer to enlarge the interlayer spacing and also the space between PVK and Ir(F2PPy)3 and then interrupt the FRET process; when the UTF was back into the dry air, the FRET behavior would reversibly recover. On the basis of these phenomena, these UTFs can be utilized as luminescence senors for common VOCs. Specifically, the reversible fluorescence response cycle measurements of the UTFs were carried out in a sealed quartz cuvette by exposure to four different VOCs (toluene, tetrahydrofuran (THF), chloroform, acetone) for 2 min or back to the atmosphere, whereafter checked by fluorescence spectroscopy, respectively. This cycle can be repeated several times. The cyan phosphorescence emission of Ir(F2ppy)3 could be quenched (OFF state) or recovered (ON state) when the UTFs contacted or avoided the VOCs, namely absorption or desorption of VOCs from the interlayers of LDHs.

the Ir(F2ppy)3 cyan phosphorescence with 294 nm excitation due to the FRET process was almost quenched (OFF) and the PVK fluorescence (375 nm) recovered immediately when the films were exposed to toluene vapor. If returned into the atmosphere, the toluene molecules desorb and the Ir(F2ppy)3 cyan phosphorescence will recover again (ON). Furthermore, this adjustable OFF/ON fluorescence signal exhibited good reversibility and repeatability, as displayed in Figure 9B. It was found that the response of the UTF to other organic solvents such as chloroform, acetone, and THF was very similar to that of toluene (Figures S6−8 in SI). This universality and reversible response toward VOCs indicates that UTFs can be applied to VOC detection. To investigate the principle of response to VOCs, small-angle XRD was used. The XRD pattern (in Figure 3) of the UTF in toluene vapor (red) indicated that the basal spacing was expanded to 10.14 nm. That is to say, the toluene molecules diffuse into the interlayer, swell the basal spacing of UTF, and finally isolate Ir(F2ppy)3 from PVK molecules.34,35 Thus, the spacing between PVK and Ir(F2ppy)3 augment beyond the typical FRET distance (less than 10 nm), suppress the 2D FRET process, and finally largely restore the PVK luminescence. In contrast, the drop-casted Ir(F2ppy)3@PVK films exhibited irreversible and dull response to VOCs (Figure S9 in SI). Schematic illustration of the reversible fluorescence signal to VOC vapor in the UTFs is presented in Figure 10. Therefore, the 2D FRET process in the UTFs could be interrupted or rebuilt by the VOCs’ adsorption or desorption 20542

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Figure 10. Schematic illustration of the reversible fluorescence response to VOC vapor in the (Ir(F2ppy)3@PVK/LDH)n UTFs.

anticipated that further investigation of how optical functional UTFs with this type of 2D FRET process interact with VOC molecules will provide novel ideas for the design and preparation of LDH-based sensors that can be used in industrial, agricultural, and biological fields.

because of the reversible and lossless physical process in the LDHs. The PVK and Ir(F2ppy)3 molecules scattered uniformly within the interlayer of the rigid LDH matrix and their FRET in the 2D confined environment endowed the UTF with the ability for luminescence response to VOCs. If the FRET process between PVK and Ir(F2ppy)3 of the UTF can be regulated by external stimuli, i.e., VOC vapors, and exhibit a two-state fluorescence signal (ON or OFF), then such a luminescent UTF can be used to detect VOCs. Furthermore, this 2D FRET process involving the fluorescent donor and phosphorescent acceptor have some merits, one of which is the good excitation efficiency due to the distinct lifetime difference between PVK and Ir(F2ppy)3 (1.39 ns vs 885 ns). This means that the long phosphorescence lifetime for Ir(F 2 ppy) 3 can be antijamming to other luminescence and requires less frequent excitation for PVK to obtain a certain luminescence intensity for determination compared with that of fluorescent donor/fluorescent acceptor FRET systems; thus, VOCs can be detected more easily. The longer phosphorescence lifetime is more suitable for the instantaneous detection of VOCs in the temporal range of 300 ns with a UV nanosecond scale pulse, which is consistent with environmental protection principles, such as energy saving, minimizing UV light irradiation, and extending the working life of UTF sensors by minimizing the photoaging.

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ASSOCIATED CONTENT

S Supporting Information *

The PL excitation and emission spectra of PVK and Ir(F2ppy)3 toluene solution; fluorescence spectra of the Ir(F2ppy)3@PVK toluene solution of different concentrations; table of fluorescence lifetimes of PVK and Ir(F2ppy)3 in films and in toluene solution; fluorescence decay curves of (PVK/LDH)25 and (Ir(F2ppy)3@PVK/LDH)25 UTFs; fluorescence spectra of (Ir(F2ppy)3@PVK/LDH)25 in acetone, THF, or chloroform vapor or the atmosphere; fluorescence spectra of Ir(F2ppy)3@ PVK drop-casted film in the atmosphere or VOC vapor. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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4. CONCLUSION In summary, Ir(F2ppy)3@PVK/LDH thin films have been fabricated by LbL deposition based on hydrogen-bonding interaction, and the application of the films as phosphorescence sensors for VOCs was explored. The UV−vis absorption, fluorescence spectra, and small-angle XRD demonstrated that the thin films were uniform and well-organized. The rigid LDH nanosheets isolated Ir(F2ppy)3 molecules from each other and thus improved the luminous intensity by suppressing the molecular interaction. In addition, these UTFs displayed a twostate reversible fluorescence signal (cyan vs blue emission) by means of controlling the 2D energy transfer process by exposing the films to VOCs. The FRET route from PVK to Ir(F2ppy)3 in the confined 2D layers can be effectively hindered by common VOCs because of their selective penetration into the organic interlayers of the LDH nanosheets. The reversible and rapid luminescence response of the UTF toward VOC vapor means that UTFs could be applied as phosphorescent sensors. Moreover, these UTF sensors with long phosphorescent lifetimes have more merits, including minimizing UV excitation, antijamming other luminescence, and instantaneous detection, compared with those of fluorescence sensors. Therefore, this work shows a successful example for the fabrication of a phosphorescent sensor based on LDHs. It is

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China, the 973 Program (grant no.: 2014CB932101), 111 Project (grant no.: B07004), Program for Changjiang Scholars and Innovative Research Team in University (IRT1205), and New Century Excellent Talents in University (NCET-11-0560).

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REFERENCES

(1) Zampolli, S.; Elmi, I.; Ahmed, F.; Passini, M.; Cardinali, G. C.; Nicoletti, S.; Dori, L. Selectivity Enhancement of Metal Oxide Gas Sensors Using a Micromachined Gas Chromatographic Column. Sens. Actuators, B 2005, 105, 400−406. (2) Tomchenko, A. A.; Harmer, G. P.; Marquis, B. T. Detection of Chemical Warfare Agents Using Nanostructured Metal Oxide Sensors. Sens. Actuators, B 2005, 108, 41−45. (3) Förster, Th. Intermolecular Energy Transference and Fluorescence. Ann. Phys. (Leipzig, Ger.) 1948, 55−75. (4) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer/ Plenum: New York, 1999; pp 443−452. The FRET efficiency η was measured by quenching of donor fluorescence using η = 1− (Fij/Fi) = 1 − ⟨τij⟩/⟨τi⟩, where Fij and Fi are intensities and ⟨τij⟩ and ⟨τi⟩ are the average lifetimes of donor i in the presence and absence of acceptor j. (5) Chen, C. H. Evolution of Red Organic Light-Emitting Diodes: Materials and Devices. Chem. Mater. 2004, 16, 4389−4400. 20543

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(6) Christina, L. T.; Makarov, D. E. Two-Dimensional Fluorescence Resonance Energy Transfer as a Probe for Protein Folding: A Theoretical Study. J. Chem. Phys. 2008, 128, 115102−115114. (7) Cook, M. J.; Lewis, A. P.; McAuliffe, G. S. G.; Skarda, V.; Thomson, A. J.; Glasper, J. L.; Robbins, D. J. Luminescent Metal Complexes. Part 1. Tris-chelates of Substituted 2,2′-Bipyridyls with Ruthenium(II) as Dyes for Luminescent Solar Collectors. J. Chem. Soc., Perkin Trans. 2 1984, 1293−1301. (8) Higgins, B.; DeGraff, B. A.; Demas, J. N. Luminescent Transition Metal Complexes as Sensors: Structural Effects on pH Response. Inorg. Chem. 2005, 44, 6662−6669. (9) Lo, K. K. W.; Hui, W. K.; Chung, C. K.; Tsang, K. H. K.; Ng, D. C. M.; Zhu, N.; Cheung, K. K. Biological Labelling Reagents and Probes Derived from Luminescent Transition Metal Polypyridine Complexes. Coord. Chem. Rev. 2005, 249, 1434−1450. (10) Slinker, J.; Bernards, D.; Houston, P. L.; AbruÇ a, H. D.; Bernhard, S.; Malliaras, G. G. Solid-State Electroluminescent Devices Based on Transition Metal Complexes. Chem. Commun. 2003, 2392− 2399. (11) Kirch, M.; Lehn, J. M.; Sauvage, J. P. Hydrogen Generation by Visible Light Irradiation of Aqueous Solutions of Metal Complexes. An Approach to the Photochemical Conversion and Storage of Solar Energy. Helv. Chim. Acta 1979, 62, 1345−1384. (12) DeRosa, M. C.; Hodgson, D. J.; Enright, G. D.; Dawson, B.; Evans, C. E. B.; Crutchley, R. J. Iridium Luminophore Complexes for Unimolecular Oxygen Sensors. J. Am. Chem. Soc. 2004, 126, 7619− 7626. (13) Brandel, J.; Sairenji, M.; Ichikawa, K.; Nabeshima, T. Remarkable Mg2+-Selective Emission of an Azacrown Receptor Based on Ir(III) Complex. Chem. Commun. 2010, 46, 3958−3960. (14) Zhao, Q.; Li, F. Y.; Liu, S. J.; Yu, M. X.; Liu, Z. Q.; Yi, T.; Huang, C. H. Highly Selective Phosphorescent Chemosensor for Fluoride Based on an Iridium(III) Complex Containing Arylborane Units. Inorg. Chem. 2008, 47, 9256−9264. (15) Chen, H. L.; Zhao, Q.; Wu, Y. B.; Li, F. Y.; Yang, H.; Yi, T.; Huang, C. H. Selective Phosphorescence Chemosensor for Homocysteine Based on an Iridium(III) Complex. Inorg. Chem. 2007, 46, 11075−11081. (16) Thomas, K. R. J.; Velusamy, M.; Lin, J. T.; Chien, C. H.; Tao, Y. T.; Wen, Y. S.; Hu, Y. H.; Chou, P. T. Efficient Red-Emitting Cyclometalated Iridium(III) Complexes Containing Lepidine-Based Ligands. Inorg. Chem. 2005, 44, 5677−5685. (17) Tan, W. J.; Zhang, Q.; Zhang, J. J.; Tian, H. Near-Infrared Photochromic Diarylethene Iridium(III) Complex. Org. Lett. 2009, 11, 161−164. (18) Lu, W.; Mi, B. X.; Chan, M. C. W.; Hui, Z.; Che, C. M.; Zhu, N.; Lee, S. T. Light-Emitting Tridentate Cyclometalated Platinum(II) Complexes Containing σ-Alkynyl Auxiliaries: Tuning of Photo- and Electrophosphorescence. J. Am. Chem. Soc. 2004, 126, 4958−4971. (19) Lowry, M. S.; Bernhard, S. Synthetically Tailored Excited States: Phosphorescent, Cyclometalated Iridium(III) Complexes and Their Aapplications. Chem.Eur. J. 2006, 12, 7970−7977. (20) Yan, D.; Lu, J.; Ma, J.; Qin, S.; Wei, M.; Evans, D. G.; Duan, X. Layered Host-Guest Materials with Reversible Piezochromic Luminescence. Angew. Chem., Int. Ed. 2011, 50, 7037−7040. (21) Lee, C. L.; Lee, K. B.; Kim, J. J. Polymer Phosphorescent LightEmitting Devices Doped with Tris(2-phenylpyridine) Iridium as a Triplet Emitter. Appl. Phys. Lett. 2000, 77, 2280−2282. (22) Lee, C. L.; Das, R. R.; Kim, J. J. Polymer-Based Blue Electrophosphorescent Light-Emitting Diodes Using a Bisorthometalated Ir(III). Chem. Mater. 2004, 16, 4642−4646. (23) Lamansky, S.; Djurovich, P. I.; Abdel Razzaq, F.; Garon, S.; Murphy, D. L.; Thompson, M. E. Cyclomtalated Ir Complexes in Polymer Organic Light-Emitting Diodes. J. Appl. Phys. 2002, 92, 1570−1575. (24) Kwon, T. H.; Kim, M. K.; Kwon, J.; Shin, D. Y.; Park, S. J.; Lee, C. L.; Kim, J. J.; Hong, J. I. Highly Efficient Light-Harvesting System Based on a Phosphorescent Acceptor Coupled with Dendrimer

Donors via Singlet−Singlet and Triplet−Triplet Energy Transfer. Chem. Mater. 2007, 19, 3673−3680. (25) Kwon, T. H.; Kim, H. J.; Hong, J. I. Phosphorescent Thymidine Triphosphate Sensor Based on a Donor-Acceptor Ensemble System Using Intermolecular Energy Transfer. Chem.Eur. J. 2008, 14, 9613−9619. (26) Kwon, T. H.; Kwon, J.; Hong, J. I. Signal Amplification via Intramolecular Energy Transfer Using Tripodal Neutral Iridium(III) Complexes upon Binding to Avidin. J. Am. Chem. Soc. 2008, 130, 3726−3727. (27) Cui, K. M.; Tria, M. C.; Pernites, R.; Binag, C. A.; Advincula, R. C. PVK/MWNT Electrodeposited Conjugated Polymer Network Nanocomposite Films. ACS Appl. Mater. Interfaces 2011, 3, 2300− 2308. (28) Zhao, Q.; Cao, T.; Li, F.; Li, X.; Jing, H.; Yi, T.; Huang, C. A Highly Selective and Multisignaling Optical-Electrochemical Sensor for Hg2+ Based on a Phosphorescent Iridium(III) Complex. Organometallics 2007, 26, 2077−2081. (29) Aharon, E.; Kalina, M.; Frey, G. L. Inhibition of Energy Transfer between Conjugated Polymer Chains in Host/Guest Nanocomposites Generates White Photo- and Electroluminescence. J. Am. Chem. Soc. 2006, 128, 15968−15969. (30) Baldo, M. A.; Adachi, C.; Forrest, S. R. Transient Analysis of Organic Electrophosphorescence. II. Transient Analysis of TripletTriplet Annihilation. Phys. Rev. B 2000, 62, 10967−10977. (31) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151−154. (32) Achermann, M.; Jeong, S.; Balet, L.; Montano, G. A.; Hollingsworth, J. A. Efficient Quantum Dot−Quantum Dot and Quantum Dot−Dye Energy Transfer in Biotemplated Assemblies. ACS Nano 2011, 5, 1761−1768. (33) Li, Z.; Lu, J.; Li, S. D.; Qin, Y. M. Orderly Ultrathin Films Based on Perylene/poly(N-vinyl carbazole) Assembled with Layered Double Hydroxide Nanosheets: 2D Fluorescence Resonance Energy Transfer and Reversible Fluorescence Response for Volatile Organic Compounds. Adv. Mater. 2012, 24, 6053−6057. (34) Parkinson, P.; Aharon, E.; Chang, M. H.; Dosche, C.; Frey, G. L.; Köhler, A.; Herz, L. M. Dimensionality-Dependent Energy Transfer in Polymer-Intercalated SnS2 Nanocomposites. Phys. Rev. B 2007, 75, 165206−165211. (35) Aharon, E.; Breuer, S.; Jaiser, F.; Köhler, A.; Frey, G. L. Effect of the Solvent on the Conformation of Isolated MEH-PPV Chains Intercalated into SnS2. ChemPhysChem 2008, 9, 1430−1436.

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dx.doi.org/10.1021/jp505448d | J. Phys. Chem. C 2014, 118, 20538−20544