Solid-State Phosphorescence-to-Fluorescence Switching in a

Letter pubs.acs.org/Langmuir

Solid-State Phosphorescence-to-Fluorescence Switching in a Cyclometalated Ir(III) Complex Containing an Acid-Labile Chromophoric Ancillary Ligand: Implication for Multimodal Security Printing Dong Ryeol Whang,† Youngmin You,‡ Weon-Sik Chae,§ Jeongyun Heo,‡ Sehoon Kim,‡ and Soo Young Park*,† Downloaded via UNIV OF SUSSEX on August 12, 2018 at 12:27:22 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Center for Supramolecular Optoelectronic Materials, WCU Hybrid Materials Program and Department of Materials Science and Engineering, Seoul National University, Seoul, Korea ‡ Center for Theragnosis, Korea Institute of Science and Technology, Seoul, Korea § Korea Basic Science Institute, Gangneung, Korea S Supporting Information *

ABSTRACT: In this study, we have demonstrated the reconstruction of encrypted information by employing photoluminescence spectra and lifetimes of a phosphorescent Ir(III) complex (IrHBT). IrHBT was constructed on the basis of a heteroleptic structure comprising a fluorescent N∧O ancillary ligand. From the viewpoint of information security, the transformation of the Ir(III) complex between phosphorescent and fluorescent states can be encoded with chemical/ photoirradiation methods. Thin polymer films (poly(methylmethacrylate), PMMA) doped with IrHBT display long-lived emission typical of phosphorescence (λmax = 586 nm, τobs = 2.90 μs). Meanwhile, exposure to HCl vapor switches the emission to fluorescence (λmax = 514 nm, τobs = 1.53 ns) with drastic changes in both the photoluminescence color and lifetime. Security printing on paper impregnated with IrHBT or on a PMMA film containing IrHBT and photoacid generator (triphenylsulfonium triflate) enables the bimodal readout of photoluminescence color and lifetime.

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INTRODUCTION Security printing is a method to prevent unauthorized copying or fraud.1 Multiple encryptions are highly preferred in order to prohibit duplication of the original data. Information readout by photoluminescence offers a promising approach to multiple encryption because its dual modality comprising spectra and lifetimes is an intrinsic property and is difficult to reproduce without correct information about the original chromophores. In this context, luminescent patterning with different spin multiplicity will be the most effective because of the drastic difference in phosphorescence and fluorescence lifetimes. Tailoring and manipulating the luminescence properties of organometallic complexes by control of coordination between a ligand and a metal center have been extensively investigated so far.1a,2 However, chemical transformation in the solid state, which is demanded for security printing application, is rather challenging because it requires high activation energy for the diffusion of reagents and generally proceeds only under high temperature and pressure conditions.3 Only a few of the dynamic tunings of solid-state luminescence of organometallic complexes with high conversion efficiencies have been reported, mainly for porous coordination polymers.4 Recently, Grätzel and Nazeeruddin's group demonstrated an acid-induced cleavage of N∧O and O∧O ancillary ligands from cyclo© 2012 American Chemical Society

metalated Ir(III) complexes, which resulted in the phosphorescence on-to-off switching.5 This was attributed to the use of nonchromophoric ancillary ligands such as picolinic acid and acetylacetone. In this work, we incorporate a highly fluorescent chromophoric dye, 2-(2-hydroxyphenyl)benzothiazole (HBT), as an N∧O ancillary ligand to generate the Ir(III) complex, denoted as iridium bis(2-phenylpyridinato-N,C 2 ′ )(2-(2hydroxyphenyl)benzothiazolato-N,O) (IrHBT, see Figure 1a for the chemical structure). The molecular design of IrHBT is intended to trigger phosphorescence-to-fluorescence switching upon solid-state exposure to the acid or photoacid stimuli, which will provide a novel strategy for multimodal security printing applications. The actual security printing capability was explored in this work by using either a simple phosphorescent paper impregnated with IrHBT or a UV-sensitive polymer film comprising a photoacid generator (PAG) and IrHBT in a PMMA matrix. Received: August 8, 2012 Revised: October 12, 2012 Published: October 19, 2012 15433

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thin film shown in Figure 1a was placed in the middle of a chamber containing shallow HCl(aq) (37%, w/w) for 10 min. A 1H NMR sample was prepared by adding 5 μL of HCl(aq) (37%, w/w) to 1 mL of a DMSO-d6 solution of IrHBT, HBT, or [Ir(ppy)2(μ-Cl)]2 (10 mM). The IrHBT-impregnated paper was fabricated by simply dipping a 70-mm-diameter filter paper (Advantec) into a dilute solution of IrHBT (10 μM in dichloromethane). A steel nib pen with a dilute HCl(aq) solution (10 wt % in water) as an ink was used to handwrite on the IrHBT-impregnated paper. For the preparation of a photopatterned film, a 1,2-dichloroethane solution (2 g) of IrHBT (10 mg), PAG (triphenylsulfonium triflate, 10 mg), and PMMA (180 mg) was prepared. The solution (0.5 mL) was dropped onto a 2 cm × 2 cm glass plate and spin-coated for 60 s at a spin speed of 1000 rpm. The thin film was irradiated with a hand-held UV lamp (254 nm, 3 W) through a photomask (USAF 1951 resolution target, Edmund) for 10 min. Postexposure baking at 100 °C for 60 s was carried out to ensure the complete diffusion of generated photoacid for the high contrast and fine resolution.

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RESULTS AND DISCUSSION IrHBT was synthesized according to a literature method6 (synthesis details in the SI). IrHBT shows long-lived orange phosphorescence in a PMMA matrix with an emission maximum at 586 nm and a photoluminescence lifetime of 2.90 μs. When IrHBT is treated with HCl vapor, the emission changes to green fluorescence with an emission maximum at 514 nm and a photoluminescence lifetime of 1.53 ns (Figure 1a,b and Table 1). The dramatic decrease in the photoTable 1. Photophysical Properties of Filmsa Figure 1. (a) Photograph and (b) photoluminescence spectra of the photoluminescence emission of IrHBT-doped PMMA films under 365 nm excitation (left) before and (right) after treating with HCl vapor. Inset structures indicate the acid-promoted dissociation of HBT. (c) 1 H NMR spectra of IrHBT (10 mM) showing the liberation of the HBT ligand in the absence and presence of HCl (6 equiv). 1H NMR spectra of the HBT ligand (10 mM) and [Ir(ppy)2(μ-Cl)]2 (10 mM) in the presence of HCl (6 equiv) are shown for comparison. 300 MHz, DMSO-d6.

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entry

λem (nm)b

ΦPL (%)c

IrHBT IrHBT + HCl IrHBT + PAGe IrHBT + PAGf HBT

586 514 586 517 513

35.8 6.27 36.6 5.97 20.7

τavd 2.90 1.53 2.84 1.52 2.98

μs ns μs ns ns

a

5 wt % in PMMA. bEmission maxima. cMeasured by an absolute method (experimental details in SI). dObserved at emission maxima. e As-prepared film. f254 nm light irradiated for 10 min with a 3 W hand-held UV lamp, followed by baking at 100 °C for 60 s on a hot plate.

EXPERIMENTAL SECTION

Synthesis. IrHBT was synthesized according to a literature method6 shown in Scheme 1. A chloride-bridged Ir(III) dimer, [(ppy)2Ir(III)(μ-Cl)]2, was prepared by the modified method of Nonoyama,7 which was subsequently chelated with HBT in dichloromethane (DCM). A detailed description of the syntheses and characterizations is provided in the Supporting Information (SI). Sample Preparation. For the acid-induced switching of solid-state luminescence, a 1,2-dichloroethane solution (2 g) of IrHBT (10 mg) and poly(methyl methacrylate) (PMMA) (190 mg) was prepared. The solution (0.5 mL) was dropped onto a 2 cm × 2 cm glass plate and spin-coated for 60 s at a spin speed of 1000 rpm. The thus-prepared

luminescence lifetime suggests the generation of a fluorescent species in the acidic environment. The emission spectrum of the HCl-treated IrHBT film is identical to that of the HBT film (SI, Figures S1 and S2), which confirms that the emission can be assigned to HBT fluorescence and thus indicates the successful acid-induced cleavage of Ir−HBT bonds. To understand the reaction mechanism, we obtained 1H NMR spectra (Figure 1c) and electrospray ionization-mass spectrometry (ESI-MS) data for IrHBT solutions in the absence and

Scheme 1. Synthesis of IrHBT

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presence of HCl. When 5 μL of HCl(aq) (37% w/w) is added to 1 mL of the IrHBT solution (10 mM in DMSO-d6), the 1H NMR signals for IrHBT are distorted and seven new peaks appear (Figure 1c). The characteristic H4 signal from the HBT ligand in IrHBT (8.83 ppm) disappears completely, and signals from HBT (7.00−8.20 ppm) are mixed with signals from [Ir(ppy)2(μ-Cl)]2 (where ppy is 2-phenylpyridinate). The formation of the chloride-bridged Ir(III) dimer upon acidinduced degradation corresponds well with Grätzel and Nazeeruddin’s work on nonchromophoric ligands.5 There are two main peaks in the ESI-MS spectrum of HCl-treated IrHBT at 228.05 and 1071.12, which correspond to [(HBT)(H)]+ and [[Ir(ppy)2(μ-Cl)]2(H)]+, respectively (SI, Figures S5 and S6). No signal for [(IrHBT)(H)]+ was observed, indicating complete conversion. These data support the mechanism that the labile HBT ligand of IrHBT is detached upon treatment with acid.5 Therefore, the photoluminescence changes in IrHBT upon acid treatment can be understood by the elimination of the heavy-atom-induced spin−orbit coupling of HBT from the central Ir(III) core (scheme in Figure 1a).8 The solvent-free solid-state switching ability of IrHBT enables security printing on paper. Phosphorescent paper was prepared simply by dipping a piece of filter paper into a dilute IrHBT solution. The IrHBT-impregnated paper is opaque white under room light and phosphorescent orange under 365 nm UV light. By writing with an HCl-dipped pen, we immediately developed an image with green fluorescence (Figure 2). Although the printed image can be clearly seen as orange and green contrast under 365 nm UV irradiation, it is totally invisible under room light. In addition to the naked-eye luminescent color readout of the inscribed signal, PL decay traces in pristine and printed regions were observed with a focused laser (Figure 2e). (Inconsistency in the lifetime data from an HCl-vapor-treated IrHBT film, security paper, and a photopatterned film originates from two main sources. First, the lifetime of a chromophore can be changed by various environmental factors, such as the polarity of the matrix or the concentration of quenchers such as molecular oxygen. Second, the signal can be perturbed by unwanted interference such as a scattered source beam (in the security paper case) or guided light through a quartz substrate (in the case of the photopatterned film, vide inf ra).) The orange region exhibited long-lived (τav = 0.97 μs) emission characteristic of the phosphorescence from IrHBT. On the contrary, the printed region showed a PL lifetime of 2.61 ns, indicating that the nature of the emission is fluorescence from HBT. By comparing the drastic difference in the lifetimes of the pristine and printed areas, we successfully demonstrated the PL color- and lifetime-based bimodal encryption and readout of data. Security printing of information using light illumination is more practical and beneficial than the wet process of acid-based inscription. To realize the former process, a UV-sensitive phosphorescent PMMA film containing IrHBT and a PAG was fabricated by spin-coating on a quartz substrate. The photophysical properties of pristine and 254 nm UV-treated PMMA composite films containing IrHBT and PAG correspond well to those of pristine and HCl-treated IrHBT films, respectively (Table 1). (In this study, emission from [Ir(ppy)2+] was scarcely detectable because of the extremely low photoluminescence quantum yield (PLQY) of [Ir(ppy)2+] compared to that of HBT under ambient conditions.) To confirm that the same mechanism of HBT elimination is acting, an ESI-MS

Figure 2. Security printing with an acid pen. (a, d) Shown under room light. (b, c) Shown under a 365 nm hand-held UV lamp (3 W). (e) PL decay trace of orange (red squares) and green (blue circles) regions. The black line represents the instrumental response function (IRF).

experiment was carried out for the UV-treated film by redissolving it with DCM. The progression of ESI-MS spectra is undoubtedly attributed to the dissociation of IrHBT, which confirms that photogenerated acid is effectively working for the phosphorescence to fluorescence switching in much the same way as for the wet acid solution (SI, Figure S7). Upon irradiating the film through a photomask with 254 nm UV light, the mask images were successfully transferred to the polymer film as the phosphorescence/fluorescence and orange/green dual emissive patterns (SI, Figure S8). Fine contrast and resolution of the pattern with a line width of ca. 10 μm are clearly achieved (Figure 3a). Because the photopatterned image has an ultraviolet A (UVA, wavelength range of 400−315 nm) readout signal range (SI, Figure S9) and PAG has its sensitivity in the ultraviolet C (UVC, wavelength range of 280−100 nm) region (λabs = 233 nm),9 the nondestructive readout of the signal was realized. When the inscribed photopattern was investigated with fluorescence microscopy, a high-contrast image and spectra of IrHBT phosphorescence and HBT fluorescence were clearly observed (Figure 3a,b). The very characteristic and unmixed photoluminescence spectra of each area (Figure 3b) coincide well with those of the pristine and HCl vapor-treated IrHBT films (Figure 1b). A line pattern of 17.5 μm width for the photopatterned film was observed with a fluorescence lifetime imaging microscopy (FLIM) technique 15435

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Figure 3. (a) Photoluminescence micrograph of the photopatterned film. (b) Unmixed photoluminescence spectra of (green line) masked and (red line) exposed regions of the photopatterned film. (c) FLIM image of the photopatternend film (45 μm × 45 μm). (d) Photoluminescence decay traces of (red squares)masked and (blue circles) exposed regions of the photopatterned film. The black line represents IRF.

Notes

(Figure 3c). A clear line pattern of a masked phosphorescent area (red region in Figure 3c) distinguishes the irradiated fluorescent area (blue region in Figure 3c). With the drastic difference in lifetimes of the phosphorescent area (τav = 1.58 μs) and the fluorescent area (τav = 1.84 ns), the lifetime-based readout of the signal was successfully realized (Figure 3d).

The authors declare no competing financial interest.

CONCLUSIONS In this study, a phosphorescent Ir(III) complex with a chromophoric N∧O ancillary ligand (IrHBT) was successfully demonstrated to show high-contrast phosphorescence-tofluorescence luminescence switching upon acid stimuli in the solid state. The acid-induced cleavage of HBT from IrHBT was explored by ESI-MS and 1H NMR experiments. From the viewpoint of information security, the transformation of the Ir(III) complex between phosphorescent and fluorescent states can be encoded via chemical/irradiation methods. The reconstruction of encrypted information was demonstrated by security printing on the paper impregnated with IrHBT or on the PMMA film containing IrHBT and PAG, which enables the bimodal readout of the photoluminescence color and lifetime. The strategy presented here for the switching of phosphorescence to fluorescence with acid stimuli can provide a general principle for the design of Ir(III) complexes for the multimodal encryption of information.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Experimental details, photographs, PL spectra, PL decay traces, ESI-MS, and absorption spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This research was supported by the Basic Science Research Program (CRI; RIAMIAM0209(0417-20090011)) and the WCU (World Class University) project (R31-2008-00010075-0) through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 15436

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induced degradation of phosphorescent dopants for OLEDs and its application to the synthesis of tris-heteroleptic iridium(III) biscyclometalated complexes. Inorg. Chem. 2012, 51, 215−224. (6) Hao, Y.; Guo, X.; Lei, L.; Yu, J.; Xu, H.; Xu, B. Multicolor emitting from a single component emitter: New iridium(III) complexes with ancillary ligand 2-(2-hydroxyphenyl) benzothiazole. Synth. Met. 2010, 160, 1210−1215. (7) Nonoyama, M. Benzo[H]quinolin-10-Yl-N iridium (III) Complexes. Bull. Chem. Soc. Jpn. 1974, 47, 767−768. (8) Elsässer, T.; Bakker, H. J. Ultrafast Hydrogen Bonding Dynamics and Proton Transfer Processes in the Condensed Phase; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2002. (9) Kirk, R. E.; Othmer, D. F.; Kroschwitz, J. I.; Howe-Grant, M. Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 1991; pp 1−25.

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