Crystallization-Induced Emission Enhancement in a Phosphorus

Jun 12, 2009 - The answer to this question is a firm yes. The. Figure 2. ..... (d) Seo, J.; Chung, J. W.; Jo, E. H.; Park, S. Y. Chem. Commun. 2008,. ...
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Crystallization-Induced Emission Enhancement in a Phosphorus-Containing Heterocyclic Luminogen Lijun Qian,†,‡ Bin Tong,† Jinbo Shen,† Jianbing Shi,† Junge Zhi,ζ Yongqiang Dong,ξ Fan Yang,† Yuping Dong,*,† Jacky W. Y. Lam,§ Yang Liu,§ and Ben Zhong Tang*,§ College of Materials Science and Engineering and College of Science, Beijing Institute of Technology (BIT), 5 South Zhongguancun Street, Beijing 100081, China, Department of Materials Science and Engineering, Beijing Technology and Business UniVersity, Beijing 100037, China, College of Chemistry, Beijing Normal UniVersity, Beijing 100875, China, and Department of Chemistry, The Hong Kong UniVersity of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China ReceiVed: January 22, 2009; ReVised Manuscript ReceiVed: May 10, 2009

Whereas aggregation often quenches luminescence, the emission of a heterocyclic luminogen, 10-[2,5-bis(4pentyloxyphenylcarbonyloxy)phenyl]-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (3), is greatly enhanced by aggregate formation. Crystallization further boosts the emission of 3, turning it from a weak emitter in the solution state to a strong emitter in the crystalline state. The emission of 3 is changed in response to the exposure to vapors of volatile organic compounds (VOCs). The morphology of the thin film of 3 is reversibly and repeatedly modulated between amorphous and crystalline phases by simple fuming-heating and heating-cooling cycles, leading to an emission switching between bright and dark states. The novel attributes of the crystallization-induced emission enhancement, the VOC-responsive emission change, and the morphology-tunable emission switching of 3 could enable it to find applications in an array of technological areas, including chemosensing, optical display, and rewritable information storage. Introduction Luminogenic materials have attracted much attention because of their great potential for high-technology applications, especially in the areas of fluorescence sensing and light-emitting diode fabrication.1,2 Although many luminophores are highly luminescent in dilute solutions, their light emissions are often quenched in the solid state as a result of aggregation of their chromophoric units in the condensed phase.3 The aggregationcaused quenching (ACQ) of light emission is a general phenomenon that must be properly addressed because luminophores are commonly utilized as solid films in their practical applications. The development of luminogens whose films emit more efficiently than their solutions has aroused much interest in recent years. In addition to the silole molecules first reported by our groups in 2001,4,5 a variety of luminogens, including distyrylbenzene, fluorene, pentacene, and pyrene derivatives,6-11 have been found to be stronger emitters in the aggregate state than in the solution state. These molecules can be generally categorized into two groups. In one group, the luminogenic molecules are nonemissive when dissolved in their good solvents but become highly luminescent when aggregated in the solid state, thus behaving exactly opposite to the conventional ACQ luminophores. Because the emission is induced by aggregation, we coined the term aggregation-induced emission (AIE) for this unusual phenomenon.4a In another group, the luminogens are luminescent in the solution state and become more emissive in * Corresponding authors. E-mail: [email protected] [email protected] (B.Z.T.). † College of Materials Science and Engineering, BIT. ‡ Beijing Technology and Business University. ζ College of Science, BIT. ξ Beijing Normal University. § The Hong Kong University of Science & Technology.

(Y.D.),

the aggregate state. Because the light emission is enhanced by aggregate formation, this effect is referred to as aggregationinduced emission enhancement (AIEE).4b Thanks to the enthusiastic efforts of the researchers working in the area, the AIE and AIEE processes are now understood to operate by various mechanisms, including restriction of intramolecular rotation,12 J-aggregate formation, intramolecular planarization, inhibition of photoinduced isomerization and cyclization, and blockage of nonradiative relaxation pathways of the excited states.13 Most of the AIE(E) systems developed so far, however, are based on hydrocarbon aromatic compounds,4-13 and AIE(E) luminogens containing heteroatoms have been much less explored.14,15 Furthermore, the effects of molecular structure and packing arrangement on the AIE(E) processes have rarely been investigated, although the structure-property relationship is of great value in terms of gaining new insights into AIE(E) mechanisms and guiding further research efforts in the development of new AIE(E) luminogens. In this work, we developed the new, heterocyclic AIEE luminogen 3 (Scheme 1), whose light emission is greatly enhanced by aggregate formation. Remarkably, the crystals of 3 emit more efficiently than its amorphous powders, showing a novel phenomenon of crystallization-induced emission enhancement (CIEE). Unlike the previously developed AIEE systems, 3 is an organic luminogen containing an inorganic phosphorus atom. To understand the effect of the oxaphosphaphenanthreneoxide ring on the AIEE and CIEE processes of 3, we designed and prepared a model compound without the heterocyclic unit, 5 (Scheme 2), and compared its emission behaviors with those of 3. In this article, we show that the molecular structure, conformational twisting, structural rigidification, and morphological packing play important roles in the photophysical processes of 3.

10.1021/jp900665x CCC: $40.75  2009 American Chemical Society Published on Web 06/12/2009

Crystallization-Induced Emission Enhancement SCHEME 1: Synthesis of Luminogen 3

SCHEME 2: Preparation of Model Compound 5

J. Phys. Chem. B, Vol. 113, No. 27, 2009 9099 with full-matrix least-squares on F2 using the SHELX-97 program. All non-hydrogen atoms were refined anisotropically. Synthesis and Characterization of 3 and 5. Luminogen 3 and model compound 5 were prepared according to the synthetic routes shown in Schemes 1 and 2, respectively. A typical procedure for the synthesis of 3 is described below. Into a 250 mL one-neck round-bottom flask were added 2.7 g (12 mmol) of 1 and 1 g (3 mmol) of 2 in 30 mL of DCB. After being stirred at 150 °C for 20 h, the mixture was cooled to 100 °C and poured into 400 mL of petroleum ether. The precipitate was filtered, washed with petroleum ether, and dried at 130 °C under a vacuum for 2 h. Model compound 5 was prepared by a similar experimental procedure. Characterization Data for 3. Yield: 50%. IR (KBr), ν (cm-1): 2953, 2870, 1736 (CdO), 1605 (PsPh), 1201 (PdO), 916, 754 (PsOsPh). 1H NMR (400 MHz, DMSO-d6): 8.12-8.14 (2H), 7.95-8.02 (3H), 7.72-7.78 (2H), 7.48-7.56 (3H), 7.37-7.39 (1H), 7.14-7.26 (6H), 6.69-6.71 (2H), 4.09-4.12 (2H), 3.98-4.00 (2H), 1.72-1.77 (4H), 1.36-1.42 (8H), 0.89-0.93 (6H). MS (CI): m/e 704 (M+, calcd 704.25). Characterization Data for 5. Yield: 85%. IR (KBr), ν (cm-1): 2955, 2931, 2872, 2857, 1732 (CdO), 1605, 1508. 1H NMR (400 MHz, CDCl3-d1): 8.13-8.15 (4H), 7.25 (4H), 6.96-6.98 (4H), 4.03-4.06 (4H), 1.81-1.85 (4H), 1.39-1.49 (8H), 0.93-0.97 (6H). MS (CI): m/e 490 (M+, calcd 490.24).

Experimental Section

Results and Discussion

Materials and Instrumentation. 1-Bromopentane, methyl 4-hydroxybenzoate, thionyl chloride, o-dichlorobenzene (DCB), and 1,4-dihydroxybenzene (4) were purchased from Beijing Chemical Regent Co. and used as received without further purifications. 4-(Pentyloxy)benzoyl chloride (1)16 and 10-(2,5dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene10-oxide (2)17 were prepared according to literature methods. 1 H NMR spectra were measured on a Varian 400 spectrometer using deuterated dimethyl sulfoxide (DMSO) as the solvent and tetramethylsilane (TMS) as internal reference. Mass spectra were recorded on a triple quadrupole mass spectrometer (Finnigan TSQ7000) operating in chemical ionization (CI) mode using methane as the carrier gas. Absorption spectra were recorded on a Hitachi U-2800 spectrophotometer. Photoluminescence (PL) spectra were measured on a Perkin-Elmer VARIAN 55 spectrofluorometer. Fluorescence quantum yields (ΦF) of solutions of 3 were estimated by the Demas-Crosby method, using 9,10-diphenylanthracene (ΦF ) 90% in cyclohexane) as the standard.18 Thermal transitions were studied by differential scanning calorimetry (DSC-2100) and thermogravimetric analysis (TGA-2000) at a heating rate of 15 °C/min under nitrogen. Anisotropic optical textures were observed on a Leitz Laborlux12 polarized optical microscope (POM) equipped with a Leitz350 hot stage. Wide-angle X-ray diffraction (XRD) patterns were recorded on a PANalytical X’pert Pro MPD X powder X-ray diffractometer using 1.5418 Å Cu KR wavelength at room temperature (scanning rate, 0.025°/s; scan range, 4.5-30°). Small-angle X-ray diffractograms were measured using PANalytical, Anton Paar SAXSess X-ray scattering system. Single-crystal XRD experiments were performed on a Quantum Design MPMS XL-5 SQUID (superconducting quantum interference device) system equipped with a horizontal rotator sample holder. Data were collected on a Nonius Kappa charge-coupled device (CCD) detector with Mo KR radiation (λ ) 0.71073 Å) at 293 K. Single crystals of 3 were grown from aqueous acetonitrile (AN) solution. The structure was solved by direct methods and refined

Luminogen 3 was readily prepared by the esterification reaction of 4-(pentyloxy)benzoyl chloride (1) with 10-(2,5dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene10-oxide (2) at 150 °C in DCB (Scheme 1). The reaction product was characterized spectroscopically and crystallographically, from which satisfactory analysis data corresponding its expected structure were obtained. Luminogen 3 is completely soluble in common organic solvents such as AN, chloroform, and tetrahydrofuran (THF) but insoluble in water. Whereas the absorption of model compound 5 peaks at 254 nm, the absorption spectrum of 3 comprises a peak at 268 nm and a shoulder at ∼308 nm (Figure S1, Supporting Information).19 The bathochromic shift in the absorption peak of 3 from that of 5 indicates that the former is more conjugated than the latter, as a result of the electronic communication between oxaphosphaphenanthreneoxide and the phenyl rings in 3. Addition of water into the AN solution of 3 changes its absorption spectrum. As can be seen from Figure 1, when the water content in the aqueous AN mixture is e50 vol %, the absorbance of 3 is slightly decreased. At 70% water content, the absorption peak of 3 is bathochromically shifted with a large decrease in intensity. At high water contents (g80%), the absorption peak of 3 is shifted back to the position of its AN solution with a moderate decrease in intensity. The absorption spectra of 3 in the aqueous AN mixtures with g70% water content contain light-scattering tails in the long-wavelength region, suggesting that the molecules of 3 cluster into nanoaggregates in poor solvents.20 Upon photoexcitation, the dilute AN solution of 3 exhibits a PL spectrum with an emission peak at 361 nm (Figure 2). When water is continually added into the AN solution of 3 while the luminogen concentration is kept unchanged at 10 µM, the PL intensity of 3 is gradually decreased when the water content in the aqueous AN mixture is comparatively low (e50%) but greatly increased when the water content is high (>60%). Because water is a nonsolvent of 3, the molecules of 3 must aggregate in aqueous AN mixtures with high water contents,

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Figure 1. Absorption spectra of 3 in acetonitrile (AN) and aqueous AN mixtures with different water contents. Concentration of 3, 10 µM.

in agreement with the observation of the light-scattering tails in the absorption spectra discussed above (cf., Figure 1). Evidently, the emission of 3 is spectacularly boosted by aggregation; in other words, 3 is AIEE-active. Intriguingly, although the PL intensity of 3 is changed dramatically, small variations in its spectral profile and peak position are observed over the whole range of contents of the aqueous AN mixtures. Careful inspection of the PL spectra of 3 in the AN/water mixtures reveals a decrease in the emission intensity when the water content is changed from 80% to 90% (Figure 2B). We speculate that this is due to a morphology change in the nanoaggregates of 3 in aqueous medium. In the AN/water mixtures with low water contents (e.g., 80%), the molecules of 3 can slowly assemble in an ordered fashion to form moreemissive and more-crystalline nanoclusters. However, in mixtures containing very large amounts of water, the molecules of 3 might abruptly agglomerate to form less-emissive and lesscrystalline or even amorphous nanoaggregates. To confirm the crystalline nanoaggregates of 3 are more emissive than their amorphous counterparts, we prepared an amorphous thin film of 3 by heating the sample on a quartz substrate at 160 °C, a temperature well above the melting point of 3 (∼126 °C; cf., Figure S2, Supporting Information).19 Because the DSC thermogram of 3 indicates that it undergoes crystallization transition at ∼102 °C,19 we heated the amorphous film from 80 to 105 °C slowly (at a rate of 3 °C/min) and recorded its PL and UV spectra after the film had cooled to room temperature. The film was heated to the crystallization temperature zone again, and a second set of PL and UV spectra was measured. Such processes were repeated several times, and the results are summarized in Figure 3. With increasing number of the heating cycle, the PL of the film became progressively stronger and red-shifted (Figure 3A). The film treated for four heating cycles emitted at 365 nm, with an intensity more than twice that of the untreated, amorphous film. This suggests that the nanoaggregates formed in the aqueous AN mixtures are neither perfectly crystalline nor completely amorphous (cf., Figure 2). The amorphous and crystalline natures of the films on the quartz substrate before and after the thermal treatment, respectively, are verified by their XRD patterns: the former gives only a weak, broad diffuse halo, whereas the latter displays sharp reflection peaks (Figure S3, Supporting Information).19 The absorption spectral data further confirm that the film of 3 is crystallized after the thermal

Qian et al. treatment (Figure 3B). Although little change is observed in the spectral profile, the intensity is gradually decreased in the absorbing, shorter-wavelength region but increased in the nonabsorbing, long-wavelength region, because of the effect of light scattering by crystallites of growing size.20 The excitation spectrum of the solution of 3 resembles its absorption spectrum (cf., Figures S1 and S4, Supporting Information).19 In the excitation spectrum of the amorphous film of 3, the shoulder at ∼308 nm grows in intensity, in comparison to its main peak in the short-wavelength region. In the excitation spectrum of its crystalline film, the shoulder is evolved into the main peak, which is bathochromically shifted to ∼315 nm. Moreover, the spectrum is more than 3 times stronger in intensity than those of the dilute solution and amorphous film. These spectral data suggest that the crystallization process enhances the electronic communication in the luminogen system, which, in turn, makes the system more photoresponsive. For comparison, we synthesized 5 as a model compound (cf., Scheme 2). Neither its amorphous film nor its crystalline film is emissive (Figure S5, Supporting Information).19 Although 5 is weakly luminescent in the solid state, closer examination reveals that its amorphous film is more emissive than its crystalline counterpart, in agreement with the behaviors of conventional dyes, whose light emissions are commonly weakened or quenched by crystallization. The opposite behavior, that is, the CIEE characteristic, of 3 is therefore evidently due to the attachment of the oxaphosphaphenanthreneoxide pendant to the 1,4-phenylene bis[4-(6-hexyloxy)]benzoate skeleton. To collect more information about the contribution of the oxaphosphaphenanthreneoxide pendant to the photophysical processes of 3, we grew its single crystals through slow crystallization in an AN/water mixture. Crystals of high quality were used for the XRD analysis. The crystal structure of 3 belongs to the monoclinic system with a space group of P2(1)/c (Table S1, Supporting Information).19 It is noteworthy that there exist multiple CArsH · · · O hydrogen bonds21 between the molecules of 3 in the crystal structure, as denoted by the dotted lines in Figure 4. These intermolecular hydrogen bonds tightly hold the molecules of 3 in the crystalline lattice and greatly rigidify their conformations. The CArsH · · · O hydrogen bonds between the hydrogen atoms of the oxaphosphaphenanthreneoxide ring in one molecule of 3 and the oxygen atoms of carbonyl groups in another adjacent molecule of 3 make the heterocyclic rotor difficult to rotate against the phenylene dibenzoate stator. Conformational flexibility associated with molecular motions such as intramolecular rotation is known to nonradiatively deactivate excited states. The conformational stiffening caused by the restricted intramolecular rotation in the crystalline state of 3 blocks the nonradiative relaxation channel and populates its radiative decay pathway.15 This makes its crystals highly luminescent, hence the observed CIEE effect. It now becomes clear that crystallization can enhance the emission of 3. Solvent fuming is known to induce chromophores to crystallize,15c,22 and we thus explored the possibility of manipulating the PL of 3 by vapor fumigation. Vapor-responsive PL systems are of practical value because there is an urgent call for the development of materials and devices that can detect volatile organic compounds (VOCs), given the health hazards posed by exposure to their vapors.23 We coated a thin film of 3 on the inner wall of a quartz cell, at the bottom of which was placed a small container filled with chloroform. The PL of 3 is gradually intensified with exposure time, with a swift increase in the intensity observed at the beginning of the fumigation

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Figure 2. (A) Emission spectra of 3 in AN and AN/water mixtures. (B) Change in the fluorescence quantum yield of 3 with water content in the AN/water mixture. Concentration of 3, 10 µM; excitation wavelength, 275 nm.

Figure 3. Effect of heating on (A) emission and (B) absorption spectra of a thin film of 3. The film was slowly heated to the crystallization temperature zone of 3. λex ) 275 nm.

Figure 4. Multiple aromatic CArsH · · · O hydrogen bonds, denoted by dotted lines in the packing arrangement, rigidify the molecular conformation of 3 in the crystalline state.

process (Figure 5A). The emission of the film fumed for 10 min is 4.4 times stronger than that of the unexposed parent film (Figure 5B). We tested whether a similar effect could be observed when the film was exposed to other VOCs. Vapors of methanol and AN can both induce the film of 3 to luminesce

intensely [Figure S619 (Supporting Information) and Figure 5B]. The PL of 3, however, is hardly affected by fuming with acetone or THF vapor, suggesting that 3 could serve as a selective VOC chemosensor. To verify that the emission enhancement is indeed due to fumigation-induced crystallization,22 we checked the morphological structures of the parent and fumed films by POM and XRD. The untreated parent film was completely dark under POM observation (Figure S7, Supporting Information),19 indicative of its amorphous nature. Birefringent textures, however, were observed after the film was fumigated by the solvent vapors. The fumed films exhibited XRD patterns of sharp refraction peaks (Figure S8, Supporting Information).19 Thus, both the POM and XRD analyses confirmed that the vapor fumigation process transformed the films from the amorphous phase to the crystalline phase. The CIEE effect leads to increments in the PL intensities of films of 3 fumed by vapors of chloroform, acetonitrile, and methanol. The reason why the PL intensities did not change much after the films of 3 had been fumigated by acetone and THF vapors is not clear at present time. One possibility is the entrapment of these solvent molecules in the crystalline lattice. The plasticization effect of the solvent molecules allows intramolecular rotation to occur even in the crystalline phase;24 on the other hand, crystallization tends to restrict the intramolecular rotation. The observed slight changes in the PL intensity after fumigation are probably the result of these two antagonistic effects.25 Amorphous and crystalline films of 3 are less and more emissive, respectively. Can these two states be reversibly switched? The answer to this question is a firm yes. The

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Figure 5. (A) Effect of chloroform vapor on the PL spectrum of a thin film of 3 coated on a quartz plate. (B) Changes in the PL intensity with the time of exposure to solvent vapors. λex ) 275 nm.

Heat-mode phase-change technology has been widely used in rewritable optical media.23 It would be nice if the PL of 3 could be tuned by simple thermal processes of heating and cooling. As described above, the amorphous film of 3 coated on a quartz cell is less emissive. After being annealed at 105 °C for 30 min, the film crystallizes and becomes more emissive (Figure 6B). Further heating to a temperature above 160 °C followed by quick cooling to room temperature amorphizes the film and switches its emission to the less emissive amorphous state. Luminogen 3 is a heterocyclic compound with a high thermal stability, which loses no weight when heated to about 160 °C (Figure S9, Supporting Information).19 Even when 3 is heated to 300 °C, its TGA thermogram records a mere 1% weight loss. The heating-cooling cycle thus causes no harm to 3 and can be repeated many times. In summary, in this work, we have observed AIEE and CIEE effects in 3, a phosphorus-containing heterocyclic compound. The two effects sequentially enhance its light emission: 3 is changed from a weakly luminescent state in the dilute solutions to a more emissive state in the amorphous aggregates (AIEE) and finally to a highly emissive state in the crystalline phase (CIEE), as a result of the progressively fortified restriction of its intramolecular rotation in the different physical phases. The morphological structure of the thin solid film of 3 between amorphous and crystalline phases can be changed by the vapor fuming process, as well as by fuming-heating and heatingcooling cycles, leading to emission switching between bright and dark states. The vapor-responsiveness and thermal tunability of the PL of 3 could enable it to find applications in chemosensors, optical displays, and rewritable optical media. Figure 6. Repeated switching between the less-emissive amorphous state and the more-emissive crystalline state of thin solid films of 3 coated on quartz plates by (A) fuming-heating and (B) heating-cooling cycles. λex ) 275 nm. Images shown in the figure were taken under a polarized optical microscope.

Acknowledgment. This project was partially supported by the National Science Foundation of China (20634020) and the Basic Research Programs of the BIT (BIT-UBF-200504B4213 and BIT-UBF-200504B4215).

fumigated film of 3 was melted by being heated to 160 °C (cf., Figure S2, Supporting Information)19 and then quickly cooled to room temperature. This thermal treatment brought the film back to the amorphous state and accordingly weakened its emission. The film was induced to crystallize by fumigation with vapors of solvents such as methanol, and its PL intensified again (Figure 6A). These fuming-heating cycles change the morphological structure of the film of 3 between amorphous and crystalline phases and thus its light emission between dark and bright states, respectively. The cycle can be repeated many times without fatigue because of the nondestructive nature of this physical process.

Supporting Information Available: Figures showing the absorption spectrum of acetonitrile solutions of 3 and model compound 5; DSC thermograms of 3; XRD patterns of amorphous and crystalline films of 3; excitation spectra of 3 in solution, amorphous, and crystalline states; PL spectra of amorphous and crystalline films of 5; effects of VOC vapors on PL spectra of thin films of 3 coated on quartz plates; POM images of the textures of amorphous and crystalline films of 3; XRD patterns of the films of 3 fumed with VOC vapors; and TGA thermogram of 3. Table giving crystal data and structure refinement parameters for 3. This material is available free of charge via the Internet at http://pubs.acs.org.

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