Article pubs.acs.org/JPCC
Reversible Thermochromism of Aggregation-Induced EmissionActive Benzophenone Azine Based on Polymorph-Dependent Excited-State Intramolecular Proton Transfer Fluorescence Ruirui Wei, Panshu Song, and Aijun Tong* Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *
ABSTRACT: In this work, (2-hydroxy-4-methoxyphenyl)(phenyl)methanone azine (1) was found to exhibit aggregation-induced emission (AIE) and tunable solid fluorescence upon alternate annealing/melting treatments. According to the characterizations by X-ray crystallography, X-ray powder diffraction, and differential scanning calorimetry, the switching between the two different polymorphs was responsible for the tunable solid fluorescence as a consequence of polymorph-dependent excited-state intramolecular proton transfer (ESIPT) fluorescence, while the thermochromism was contributed by the conformational flexibility of rotary phenyl rings. The change in the tightness of packing upon annealing or melting thermal treatments resulted in the emission at different wavelengths. Therefore, polymorph-dependent ESIPT fluorescence could be utilized as a new strategy to develop efficient AIEactive materials in response to external stimuli.
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in the solid states.6c,14 Polymorphism, in which molecules pack in multiple modes,15 offers an opportunity to overcome the challenge. Upon external stimuli, such as heat,3e,6c,14a,16 shear,2c,6c,15d,16b,17 or solvent vapor,3e the molecular packing of materials could be dynamically switched between different polymorph states, and consequently the fluorescence of the materials switches. To date, many organic materials with stimuliresponsive luminescence have been developed and applied as efficient candidates for erasable memories, sensors, and safety materials.18 Hence, we anticipate responsive AIE-active materials as promising candidates to develop organic materials with tunable fluorescence. There are mainly two mechanisms responsible for the switching of fluorescence: (1) molecule packing in solid state transforms between crystalline and amorphous states, and the peripheral hanging phenyl rings adopted a more planar conformation in the latter state, thus leading to redshift of wavelength;3e,5d,19 and (2) π−π overlap is substantially altered in line with the molecular reorganization governed by local dipolar and other secondary bonding interactions, where larger overlap causes delocalization of excited states and induces emissions at longer wavelength.6c,17d Recently, polymorph-dependent fluorescence, which was ascribed to conformational flexibility that was devoted by rotary single bond or flexible side chains, has been reported in compounds with ESIPT feature.20 In this work, polymorphdependent ESIPT fluorescence was utilized as a new strategy to
INTRODUCTION Organic fluorescent materials have been attracting intensive interest because of their potential applications in organic lightemitting diodes (OLED), organic field effect transistors (OFET), fluorescent sensors, etc.1 Traditional fluorescent dyes of large delocalized π-conjugated moieties typically suffered from fluorescence quenching at high concentrations or in aggregated states, and thus could not serve as ideal fluorescent compound. To address this issue, a new class of materials exhibiting aggregation-induced emission (AIE) was developed as efficient emitters.2 Siloes,3 tetraphenylethylene,4 triphenylethylene,5 cyanostilbene,6 triarylamine,7 and their combinations8 have been utilized as building blocks to construct AIE-active materials with a broad range of optical properties. Previously, a series of salicylaldehyde azines (SAAs) have been found to be AIE-active.9 Besides their AIE characteristics, they were also featured in the excited-state intramolecular proton transfer (ESIPT).10 Compounds with ESIPT characteristic usually have high quantum efficiency at concentrated states due to their large Stokes shift, which greatly reduces selfquenching in fluorescence.11 Excellent examples of organic optoelectronics and sensors have been developed by taking advantage of ESIPT.11,12 As a consequence, SAA derivatives that largely benefit from both AIE and ESIPT properties can find their potential applications as promising fluorescent materials. Chemical modification is a frequently used method to construct responsible organic fluorescent materials (especially with tunable emissions) by varying the chemical substitutions.9,13 However, insufficient conversion and irreversibility make it difficult to alter the fluorescence through chemical modifications © 2013 American Chemical Society
Received: November 7, 2012 Revised: January 16, 2013 Published: January 23, 2013 3467
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develop stimuli-responsive AIE-active materials. To validate this strategy, rotary phenyl rings were introduced into the SAA framework to enhance the conformational flexibility. Consequently, this design gave the structure of compound 1 as shown in Scheme 1, which was prepared by the one-step condensation Scheme 1. Molecular Structure of 1
of (2-hydroxy-4-methoxyphenyl)(phenyl)-methanone with hydrazine. Notably, compound 1 was found to exhibit switchable solid fluorescence upon annealing/melting treatments, with large Stokes shift and high fluorescence quantum yield.
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RESULTS AND DISCUSSION Aggregation-Induced Emission of Compound 1 in Water/Ethanol. To test the AIE characteristics of compound 1, the effect of water volume fraction (f w) on the fluorescence was evaluated in water/EtOH mixed media. As shown in Figure 1, 10 μmol L−1 1 displayed nearly no fluorescence in the low water fraction range from 0 to 60% (good solvents), while the fluorescence at 542 nm increased dramatically in solvents of high water fractions from 70% to 90% (poor solvents). The enhanced fluorescence induced by the formation of aggregates in poor solvents is the typical phenomenon for AIE-active compounds. The formation of aggregates was also supported by the clear level-off tail in the absorption spectrum of 1 when the water volume fraction was 90% (Figure 1a).3a,9 In contrast, in a good solvent of 10% water, no such tail was observed because compound 1 was fully dispersed. Their emissions were quite different under the UV illumination: mixtures in 90% water emitted strongly as compared to the diminished fluorescence of that in 10% water (insets in Figure 1b). Therefore, compound 1 exhibited AIE characteristics similar to those of other SAA derivatives, suggesting the two phenyl rings introduced into the SAA framework of 1 did not alter its AIE feature.9 In addition, a Stokes shift as large as 152 nm was observed for 1 as a result of ESIPT.11 The mechanism of the AIE characteristics for compound 1 was proposed as follows: in low water volume fractions, ESIPT process of 1 in solution state was suppressed due to nonradiative way of the excited states, which might be ascribed to the free rotations around N−N and C−C single bonds; however, in high volume fraction, rotations were inhibited because of the closely packed molecules in aggregate state, resulting in strong ESIPT fluorescence. Polymorph-Dependent Fluorescence and Molecular Packing of 1-Crys.(G) and 1-Crys.(YG). Single crystals of 1 with two different polymorphs, 1-Crys.(G) (quantum yield 17% and lifetime 1.24 ns) and 1-Crys.(YG) (quantum yield 13% and lifetime 1.15 ns), were obtained simultaneously during the process of slow evaporation of its concentrated ethyl acetate solution, and larger crystals of 1-Crys.(YG) could be reaped using smaller seeds (Figure 2). Upon the UV illumination, green and yellowish green emissions were observed for 1-Crys.(G) and 1-Crys.(YG), respectively. Their corresponding normalized fluorescence spectra were displayed in Figure 2, with peaks at
Figure 1. (a) Absorption and fluorescence spectra of 10 μmol L−1 1 in “aggregate state” (water/EtOH, 9/1, v/v) and “solution state” (water/ EtOH, 1/9, v/v), λex/λem = 390/542 nm; and (b) effect of water volume fraction (f w) on the fluorescence intensity (at peak wavelength in fluorescence spectra) of 1 in water/EtOH containing 10 mmol L−1 HEPES at pH 7.0. Insets: 10 μmol L−1 1 in 10% and 90% water under illumination at 365 nm.
521 and 540 nm accordingly. To get further insight into the molecular packing of 1-Crys.(G) and 1-Crys.(YG), X-ray crystallographic analysis was carried out, and the resolved structures are shown in Figure 3. First, intramolecular hydrogen bonds from the salicylaldimine, which provided a structural basis for ESIPT process, were observed for both of the two polymorphs. Second, the two salicylaldimine moieties in compound 1 were structurally identical, but they were asymmetric in the crystalline state. This asymmetry was definitely embodied by the different torsion angles (θ) between the phenyl ring and its attached Schiff base ring in each molecule, which were determined as 89.3° and 80.4° for polymorph 1-Crys.(G), and 87.5° and 74.7° for polymorph 1-Crys.(YG). Such a conformational difference brought a significant impact on their molecular packing in two polymorphs. According to the above results, phenyl rings energetically preferred a nearly vertical manner (especially for polymorph 1-Crys.(G)). Thus, the fluorophores of salicylaldimine moieties in the neighbored two layers were separated. As seen in Figure 4 and Table 1, the distance between salicylaldimine moieties in the neighboring two layers was 4.284−4.438 Å for polymorph 1-Crys.(G). However, for polymorph 1-Crys.(YG), this distance was measured as 3.803−3.832 Å, which was slightly shorter than that of 1-Crys. (G). It was caused by the larger torsion angles of phenyl rings in polymorph 1-Crys.(YG), which drew the fluorophores closer. In 3468
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yellowish green fluorescence similar to that of 1-Crys.(YG). Interestingly, once 1-Powd.(YG) was dissolved in THF and after the solvent was thoroughly evaporated under vacuum, it gave a powder residue emitting green fluorescence (denoted as 1Powd.(G)). Normalized solid fluorescence spectra of 1-Powd. (G) and 1-Powd.(YG) were shown in Figure 5, where the peaks of their fluorescence spectra were determined as 519 and 539 nm, respectively. Powder X-ray diffraction patterns of 1-Powd.(G) and 1-Powd.(YG) revealed that they both existed as crystallines rather than amorphous powders (Figure 6). More importantly, the measured XRD pattern of 1-Powd.(G) was found almost the same as the simulated one of polymorph 1-Crys.(G), while 1Powd.(YG) displayed an XRD pattern similar to that of polymorph 1-Crys.(YG). These observations implied that molecular packing in the powder states was the same as that in its corresponding single crystal states. Consequently, bulk solid material of the two different polymorph state, 1-Crys.(G) and 1Crys.(YG), could be facilely obtained through fast recrystallization in THF and one-step condensation, respectively. Switching Between Two Polymorph States in Compound 1 upon Thermal Treatments. Thermal properties of 1-Crys.(G) and 1-Crys.(YG) were then investigated by differential scanning calorimetry (DSC) (Figure 7). In the first heating cycle, 1-Crys.(G) showed a small endothermic peak at 231 °C before the melting point at 236 °C, while only one peak appeared at the melting peak of 236 °C for 1-Crys.(YG) (thermogravimetric analysis indicated that 1 was stable below 250 °C without decomposition; see Figure S1 in the Supporting Information). During the second heating cycle, the DSC curve of 1-Crys.(G) maintained a trend similar to that in the first heating cycle. However, in the case of 1-Crys.(YG), a new peak was observed at 231 °C in addition to its melting peak, which was almost the same as that of 1-Crys.(G). The result implied that heating at melting point would lead to molecular reorganization of 1-Crys.(YG) and transform it into the arrangement of 1-Crys. (G). Furthermore, the small endothermic peak of 1-Crys.(G) emerged at 231 °C was supposed to be a phase transition point, and annealing at this temperature might induce molecular reorganization in polymorph 1-Crys.(G) as well. Powder X-ray diffraction was also employed to monitor the changes caused by thermal treatments. As shown in Figure S2,
Figure 2. Polymorphic single crystals of 1 (1-Crys.(G) and 1-Crys. (YG)) under the illumination at 365 nm and their corresponding normalized fluorescence spectra.
addition, the tighter packing in polymorph 1-Crys.(YG) was further stabilized by the C−H···O (2.556 Å) intermolecular interaction between adjacent layers. On the other hand, for adjacent molecules in the same layer, the van der Waals interaction from C−H···O was stronger in polymorph 1-Crys. (YG) (2.626 Å) than that in 1-Crys.(G) (2.631 and 2.752 Å). Therefore, molecules in polymorph 1-Crys.(YG) packed in a much tighter mode, resulting in the fluorescence in a relative longer wavelength range. Not only in single crystal, but also in its powder state was found the notably polymorph-dependent ESIPT fluorescence for compound 1, which made the fabrication of 1 as organic materials with tunable fluorescence much simpler. The asprepared powder form of 1 (referred as 1-Powd.(YG)) emitted
Figure 3. ORTEP drawing with 50% probability ellipsoids (295 K): (a) 1-Crys.(G); (b) 1-Crys.(YG). 3469
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Figure 4. Molecular packing of adjacent two layers in 1-Crys.(G) (a,b) and 1-Crys.(YG) (c,d) from different perspective of views: the multiple-colored (gray, blue, and red) is the front layer, while the single-colored (green) is the back layer. All distances are in angstroms.
Table 1. Torsion Angles between Phenyl Ring and Its Attached Schiff Base Ring, Distance of C−H···O Interaction (d1, in the same layer; d2, in adjacent layer), and Interplane Distance (d3) in 1-Crys.(G) and 1-Crys.(YG) sample 1-Crys.(G) 1-Crys. (YG)
θ1 (deg)a
θ2 (deg)b
d1 (Å)
89.3 87.5
80.4 74.7
2.631, 2.752 2.626
d2 (Å)
d3 (Å)
2.556
4.284−4.438 3.803−3.832
a
Plane (C8−C9−C10−C11−C12−C13) to plane (N1−C1−C2− C3−O1−H1). bPlane (C22−C23−C24−C25−C26−C27) to plane (N2−C15−C16−C17−O3−H3).
once polymorph 1-Crys.(G) was annealed at 231 °C for 1 h, its XRD pattern was quite different from its original but resembled that of polymorph 1-Crys.(YG), indicating that annealing at this temperature might force phenyl rings to deviate from its energetically preferred vertical conformation (1-Crys.(G)) to a relatively stronger interaction among molecules (1-Crys.(YG)). In contrast, after the polymorph 1-Crys.(YG) was heated at melting point and then cooled to room temperature gradually, its XRD pattern became quite similar to that of polymorph 1-Crys. (G), suggesting that the stronger interaction was destroyed by melting and the phenyl rings recovered to their energetically preferred and nearly vertical conformation. Therefore, packing of 1 could be altered by the annealing/melting process, and the compound could be switched between its two polymorph states. Tunable Fluorescence of Compound 1 upon Thermal Treatments. According to the previously observed polymorphdependent ESIPT fluorescence of 1, switching between different
Figure 5. Solid powders of 1 (1-Powd.(G), treated with THF; 1-Powd. (YG), as prepared) illuminated at 365 nm and their corresponding normalized fluorescence spectra.
polymorph states should lead to tunable fluorescence. Consequently, normalized fluorescence spectra were recorded upon successive melting/annealing process. As indicated in Figure 8, upon repeated thermal cycles, the emission peak of 1 shifted between 522 and 541 nm as expected. In multiple cycles, the tunable fluorescence behavior showed good reproducibility, and 3470
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Figure 6. (a) Measured XRD pattern of powder of 1-Powd.(G) after treatment with THF; (b) simulated XRD patterns calculated from the crystallographic data of 1-Crys.(G); (c) measured XRD pattern of as-prepared solid powder of 1 (1-Powd.(YG)); and (d) simulated XRD patterns calculated from the crystallographic data of 1-Crys.(YG).
Figure 8. Fluorescence switching of compound 1: (a) normalized fluorescence spectra of 1 heated at melting point (236 °C) and annealed at 231 °C, respectively; and (b) peak position versus thermal treating cycle.
Figure 7. Differential scanning calorimetry curves of 1-Crys.(G) and 1Crys.(YG): first heating cycle (a) and second heating cycle (b).
1 was stable against heating at least up to 236 °C. Therefore, the fluorescence of AIE-active compound 1 could be reversibly tuned through the melting/annealing treatments.
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diffraction analysis confirmed the switching between the two polymorph states upon alternate melting/annealing treatments. The thermal cycles induced tunable solid fluorescence with a good stability and reproducibility. In addition to 1, the polymorph-dependent ESIPT fluorescence could be utilized as a new strategy to develop other AIE-active materials with stimuliresponsive fluorescence that can be dynamically controlled by thermal treatment and other external stimuli.
CONCLUSIONS In this article, an AIE-active compound, (2-hydroxy-4methoxyphenyl)(phenyl)-methanone azine (1), was designed and synthesized. Two different types of single crystals, 1-Crys. (G) and 1-Crys.(YG), were obtained with interesting polymorph-dependent ESIPT fluorescence. DSC and powder X-ray 3471
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> 2σ(I)). The final R1 values were 0.1230 (all data). The final wR(F2) values were 0.1463 (all data). CCDC: 909450.
EXPERIMENTAL SECTION Materials and Methods. Unless otherwise mentioned, reagents used in the synthesis were purchased from Alsa Aesar Co., Tianjin, China. All solvents and reagents were of analytical grade and used as received without further purification. Absorption spectra and fluorescence spectra were recorded on a V-550 (JASCO, Japan) UV−vis spectrometer and FP-6500 (JASCO, Japan) spectrofluorimeter, respectively. Quantum yields were determined on FP-6500 equipped with ISF-513 (JASCO, Japan) integrating sphere, and BaSO4 white plate was used as the reference. Lifetime was carried out using LifeSpecRed picosecond lifetime spectrometer (Edinburgh Instruments, England) upon excitation of 372 nm. 1H NMR and 13C NMR spectra were measured using a JNM-ECA300 (JEOL, Japan) spectrometer operated at 300 MHz. Mass spectra were obtained on a LTQ LC-MS (Thermo Fisher, USA) spectrometer with ESI as the ion source. Elemental analyses were conducted using a CE 440 (EAI, USA) elemental analyzer. DSC and TGA curves were recorded on a Q2000 (TA, USA) and Q5000 (TA, USA) instrument, respectively; both were taken under the protection of N2 atmosphere at a scanning speed of 10 °C/min. Reflections of single crystals were collected by Gemini E (Agilent Technologies, England) X-ray single crystal diffractometer at room temperature, and their structures were resolved and analyzed with the assistance of Olex2 software. Powder X-ray diffraction data were acquired on a D8 ADVANCE (Bruker, Germany) Xray diffractometer. Synthesis and Characterization of (2-Hydroxy-4methoxyphenyl)(phenyl)methanone Azine (1). (2-Hydroxy-4-methoxyphenyl)(phenyl)methanone (4.70 g, 21 mmol) was dissolved in absolute ethanol (40 mL), followed by addition of 80% hydrazine hydrate (0.62 mL, 10 mmol), and the mixture was refluxed for 4 days. Precipitates were filtrated under vacuum and washed with absolute ethanol three times, then dried under infrared lamp. Pure product of 1 was obtained as a yellow solid powder (35% yield), and its structure was confirmed by 1H NMR, 13C NMR, and mass spectra. 1H NMR (300 MHz, CDCl3, δ): δ [ppm] = 12.28 (s, 2H, O−H), 7.58 (m, 6H, ArH), 7.31 (m, 4H, ArH), 6.79 (d, J = 8.79 Hz, 2H, ArH), 6.38 (d, J = 2.58 Hz, 2H, ArH), 6.25 (d−d, 3J1(H,H) = 2.58 Hz, 3J2(H,H) = 8.79 Hz, 2H, ArH), 3.74 (s, 6H, −CH3) ppm. 13C NMR (300 MHz, CDCl3, δ): δ [ppm] = 170.1, 163.6, 162.9, 134.8, 133.5, 129.3, 129.0, 127.9, 113.0, 106.9, 101.4, 55.5. ESI mass spectrometry: m/z 451.28 ([M − H]−), 453.21 ([M + H]+); M+ calcd 452.17 ([M]). Anal. Calcd for C28H24N2O4: C, 74.32%; H, 5.35%; N, 6.19%. Found: C, 74.41%; H, 5.31%; N, 6.29%. Crystal Data for 1-Crys.(G). C28H24N2O4, M = 452.49, monoclinic, a = 11.1250(10) Å, b = 21.323(6) Å, c = 10.4804(12) Å, α = 90.00°, β = 107.093(11)°, γ = 90.00°, V = 2376.3(7) Å3, T = 295(2) K, space group P2(1)/c, Z = 4, 10 255 reflections measured, 4666 independent reflections (Rint = 0.0312). The final R1 values were 0.0562 (I > 2σ(I)). The final wR(F2) values were 0.1198 (I > 2σ(I)). The final R1 values were 0.0916 (all data). The final wR(F2) values were 0.1392 (all data). CCDC: 909449. Crystal Data for 1-Crys.(YG). C28H24N2O4, M = 452.49, monoclinic, a = 19.228(4) Å, b = 12.9020(14) Å, c = 21.509(3) Å, α = 90.00°, β = 118.55(2)°, γ = 90.00°, V = 4686.9(16) Å3, T = 295(2) K, space group C2/c, Z = 8, 10 288 reflections measured, 4594 independent reflections (Rint = 0.0659). The final R1 values were 0.0678 (I > 2σ(I)). The final wR(F2) values were 0.1020 (I
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ASSOCIATED CONTENT
S Supporting Information *
Thermal stability of compound 1, powder XRD characterizations of polymorph switching upon thermal treatments, NMR spectra and ESI mass spectrometry of compound 1, and cif files of 1Crys.(G) and 1-Crys.(YG). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-10-62787682. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank Prof. Ruji Wang for refining the crystal structures and the National Natural Science Foundation of China (NSFC, no. 21175079) for financial support.
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REFERENCES
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