Mechanoresponsive Fluorescence of 2-Aminobenzophenone

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Mechanoresponsive Fluorescence of 2-Aminobenzophenone Derivatives Based on Amorphous Phase to Crystalline Transformation with High “Off-On” Contrast Ratio Xiaokun Zheng, Yue Zheng, Lu Peng, Yu Xiang, and Aijun Tong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06920 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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The Journal of Physical Chemistry

Mechanoresponsive Fluorescence of 2-Aminobenzophenone Derivatives Based on Amorphous Phase to Crystalline Transformation with High “Off-on” Contrast Ratio Xiaokun Zheng, Yue Zheng, Lu Peng, Yu Xiang and Aijun Tong* Department of Chemistry, Beijing Key Laboratory for Analytical Methods and Instrumentation, Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Tsinghua University, Beijing 100084, People’ s Republic of China. Fax: +8610-62787682. Email: [email protected]. ABSTRACT: In this paper, mechanoresponsive fluorescent molecules with high “off-on” contrast ratio that undergo molecular packing transformation from amorphous phase to crystalline by scratching is firstly reported. Three 2-aminobenzophenone derivatives 1, 2 and 3, which possess crystallization-induced fluorescence (CIF) characteristics, exhibited bright cyan, blue and green fluorescence colors with “off-on” contrast ratio as high as 175 fold by scratching. The scratching induced transformation from amorphous phase to crystalline of 1, 2 and 3 were directly observed by XRD and SEM studies. In addition, the fluorescence of the scratched sample can readily convert back into the initial non-fluorescent state by fuming in ethanol or heating. Letters with three fluorescence emission colors of 1, 2 and 3 in ethanol solution smeared on a glass surface can be easily written by scratching and erased by fumigation, demonstrating the potential value as reversible mechanoresponsive fluorescent materials.

INTRODUCTION Mechanoresponsive luminescent (MRL) materials1-4, whose photophysical properties can be tuned through alternation of the molecular packing mode by mechanical stimulation, are attracted increasing attention for their potential applications in mechano-sensors, optoelectronic devices, data storage and security papers.5 Most MRL materials are based on organic molecules6-9, organometallic complexes10-15 or organic dyes doped polymers16-18. The molecules generally possess at least two stable states differ in molecular packing modes and conformations of the πconjugated moieties.19 Pre-design with multi-step syntheses and large amounts of the dyes are often necessary to assure high efficiently mechanoresponsive luminescence. Moreover, aggregation-caused quenching (ACQ)20 of πconjugated moieties limited their application in solid with high efficiency. On the other hand, many aggregationinduced emission (AIE)21 molecules, which are highly luminescent in solid or aggregated states but non- or weak luminescent in solution, are found to exhibit MRL behavior. Most reported MRL molecules with AIE properties exhibit two or multi luminescence colors in response to mechanical stimulation.22-28 However, only a few “off-on” MRL molecules with simple molecular structures and high contrast ratio have been reported.29-35 The mechanism of most reported MRL molecules is the molecular packing transition from crystal-to-amorphous36-39 or crystal-to-crystal40-41 in response to mechanical stimulation. However, MRL molecules with amorphous-tocrystalline42-43 transition mechanism upon mechanical stimulation have rare been found. Several approaches, such as thermal annealing44-45, heating46 or fuming47, which can induce the molecular packing transition from amorphous to

crystalline state, have been reported. Sket et.al.42 reported a BF2 complex of dimethoxyphenyl substituted beta-diketone derivative, a compound existing as two polymorphs having different mutual orientations of the two methoxy groups, showed multi chromism including mechanochromism accomponied by rearrangement of the amorphous phase into a more stable crystalline phase. However, to the best of our knowledge, mechanical force caused crystallization of organic molecules has still seldom been reported. In addition, most of the mechanical stimulations are based on crushing48, grinding49-50, shearing51-52 and pressing53-54. Such external force are strong enough to disorder the crystal of the MRL molecules and cause molecular packing transition from crystalline to amorphous state, and often led to low resolution of MRL images.55 In this paper, 2-aminobenzophenone and its derivatives, which were reported as crystallization-induced phosphorescence molecules by Tang et al.56, are found to exhibit MRL property with high “off-on” contrast ratio upon scratching. When scratching the surface of a glass smeared by ethanol solution of 1, 2 or 3 (structures showed in Figure 1), bright cyan, blue or green fluorescence colors with “offon” contrast ratio as high as 175 fold could be generated. In addition, the fluorescence of the scratched sample can readily convert back into their initial non-fluorescent state by fuming in ethanol vapor or heating. Experimental evidence suggested that scratching induced molecular packing transition from amorphous to crystalline and crystallization-induced fluorescence (CIF) with AIE characteristics were the origin of MRL of these simple small organic molecules.

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non-radiative decay of excited state by intramolecular rotation of phenyl rings in solution.58 However, the emission intensity at 483 nm started to increase dramatically when the water volume fraction was over 80%, caused by restriction of intramolecular rotation in aggregated state.59 Compound 2 and 3 displayed similar AIE properties as 1 (Figure S1-2).

Figure 1. (a) Chemical structures of compound 1-3. (b) Photographs of 1, 2 and 3 before and after scratching under a UV lamp at 365 nm.

EXPERIMENTAL SECTION

Materials and instrumentation. All chemicals were purchased from TCI (Shanghai, China) and used as received without further purification. All absorption spectra were measured on a UV visible spectrometer (JASCO V-550, Japan). All fluorescence spectra were recorded with a fluorescence spectrometer (JASCO FP-8600, Japan). The determination of fluorescence quantum yield and lifetime were carried out on a combined steady state and fluorescence lifetime spectrometer (Edinburgh FLS 920, United Kingdom). Quantum yield (Фf) was measured by calibrated integrating sphere. X-ray diffraction (XRD) spectra were recorded using a X-ray diffractometer (Rigaku SmartLab, Janpan). SEM images were taken on a scanning electron microscope (Hitachi SU8010, Japan). IR spectra were carried out on a infrared spectrometer (Bruker Tensor 27, Germany). TGA and DSC traces were measured on a TGA/DSC instrument (Mettler Toledo TGA/DSC 1 Star System, United Kingdom). Preparation of mechanoresponsive fluorescence sample. 5.0×10-2 M stock solution of 1-3 in ethanol were prepared, respectively. 5.0×10-2 M stock solution of 4, 5 in acetone were prepared, respectively. In a typical experiment, mechanoresponsive samples were prepared by smearing 5.0×10-2 M stock solution of 1-5 on glass substrates, subsequently with complete evaporation of ethanol for 1 min. Then the non-fluorescent samples were scratched with pipette tips. The fluorescence spectra were recorded before and after scratching at different time.

Figure 2. Effect of water volume fraction. a) Absorption spectra; b) dynamic light scattering (DLS) analysis (fw = 99%); c) fluorescence spectra, and d) fluorescence intensity change at 483 nm of 1 (200 μM) in water/ethanol mixtures containing 10 mM HEPES buffer at pH 7.0. Excitation was set at 406 nm.

“Off-on” mechanoresponsive fluorescence. Compound 1, 2, and 3 were found to exhibit “off-on” mechanoresponsive fluorescence properties. When non-fluorescent ethanol solution of 1, 2, and 3 were smeared on glass substrates respectively, no emission was observed after evaporation of ethanol. However, after being scratched with a pipette tip, 1, 2, and 3 underwent a gradual fluorescence enhancement over time until strong emission of cyan (475 nm), blue (463 nm) and green (492 nm) were observed, respectively (Figure 3a). Among them, the contrast ratio of mechanoresponsive fluorescence of 1 was as high as 175 fold (the contrast ratio was estimated by the ratio of fluorescence intensities after and before scratching). The time-dependent fluorescence intensity of 1 was recorded after scratching and compared with that without scratching (Figure 3b), indicating that the fluorescence enhancement only occurred under mechanical stimulation. The scratching-induced fluorescent area of 1 did not spread on the glass substrate over time (Figure S3).

RESULTS AND DISCUSSION

The AIE characteristics. The AIE characteristics of 1 were investigated in water/ethanol mixtures with different water volume fractions (fW=0-99%) and the results are shown in Figure 2. As depicted in Figure 2a, in good solvent of ethanol (fW=0%), 1 was well dispersed and showed obvious absorption peak at 238 nm and 391 nm. When the water volume fraction was 99%, the absorption band presented a distinct tail caused by formation of nanoscale particles57 and dynamic light scattering (DLS) analysis of 1 indicated that the average particle diameter was 239 nm (Figure 2b). The fluorescence intensity of 1 was weak when the water volume fraction was less than 80%, due to the

Figure 3. a) Fluorescence spectra of 1, 2 and 3 before and after scratching. Samples of 1, 2 and 3 were prepared by smearing the ethanol solution of 50 mM 1-3 on glass substrates and blow-drying


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completely, respectively. b) Time-dependent fluorescence spectra of 1 before and after scratching.

IR spectra of 1 remained unchanged, further confirming that only physical structural change (molecular stacking mode) took place, rather than chemical reactions activated by mechanical stimulation. 2 and 3 were also found to undergo transformation from the amorphous to crystalline state after scratching (Figure S8). The thermal properties of 1, 2 and 3 were studied by thermalgravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA traces of 1, 2 and 3 showed the compounds were stable up to 200℃ (Figure S9-11a). The first heating scans showed endothermic peaks of melting at 90℃, 110℃ and 98℃ for the crystalline powder of 1, 2 and 3, respectively (Figure S911b). In the first heating-cooling process, the enthalpy change (ΔH) of 1, 2 and 3, respectively, were negative, indicating that the crystalline-amorphous transition was endothermic process and the crystalline states of 1, 2 and 3 were more stable than the amorphous state.

Figure 4. a) Fluorescence spectra and b) fluorescence “off-on” cycles of 1 with scratching-fuming stimulation. c), d) The corresponding fluorescence spectra of 2 and 3. Insets: images were taken under a 365 nm hand held UV lamp.

In addition, similar to many reported mechanoresponsive fluorophores that could revert back to its original state through fuming or heating, the fluorescence of 1 was converted back to the “off” state by fuming in ethanol (Figure 4a) or heating (Figure S4). The “off-on” mechanoresponsive fluorescence cycle of 1 can be repeated for several times (Figure 4b). Similarly, compound 2 and 3 also showed the “off-on” mechanoresponsive fluorescence (Figure 4c-d and S5-6). Amorphous to crystalline transformation upon scratching. To investigate the mechanism of mechanoresponsive fluorescence of 1 after scratching, we further performed the X-ray diffraction (XRD) and SEM study of 1 before and after scratching. As shown in Figure 5a, the powder of compound 1 showed intense and sharp diffraction peaks which is almost identical to the spectrum of the single crystal of 1. However, ethanol solution of 1 on a glass substrate after drying showed only weak and broad signal, which is in accordance with the amorphous state. After scratching, the obvious diffraction peaks could be observed, which corresponded well with the crystalline diffraction peak of 1, indicating molecular packing transformation from amorphous to crystalline state under scratching. SEM analysis (Figure S7, 5c) further confirmed the formation of crystals of 1 with size of 0.2-1.0 μm after scratching. To further exclude the possibility that the crystal formation arisen from solvent evaporation by scratching, the IR spectra of 1 before and after scratching was investigated (Figure 5b). No large broad peak of hydroxyl around 3300 cm-1 was observed for the prepared sample of 1 before scratching, suggesting the complete evaporation of ethanol. Strong peaks at 3300 and 3500 cm-1 were ascribed to the amine group of 1. After scratching, the

Figure 5. a) XRD spectra of 1 before and after scratching, the powder and crystal of 1 were also tested as references. b) IR spectra of 1 before and after scratching. c) SEM images of 1 after scratching with different magnification. Table 1. Optical properties of 1, 2, and 3. compound λex (nm) λem (nm) quantum yield (Фf , %) life time (τ, ns)

crystal before scratching after scratching crystal after scratching

1 406 483 11.74 0.07 9.31 1.76 1.59

2 371 468 2.30 0.02 3.76 1.67 1.52

3 384 497 6.97 0.03 5.48 1.49 1.11

Moreover, as shown in Table 1, the fluorescence lifetimes of 1, 2, and 3 after scratching (τ = 1.59, 1.52, 1.11ns, respectively) are comparable to those in crystalline state (τ = 1.76, 1.67, 1.49 ns, respectively), indicating the mechanoresponsive fluorescence is attributed to the crystalline state, futher indicating the transformation from amophours to crystalline state.



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Figure 6. ORTEP drawing of 1, 2 and 3 with 50% probability ellipsoids and crystal packing patterns of 1(a), 2(b) and 3(c) with the intramolecular and intermolecular hydrogen bonds marked by dotted lines, respectively.

Crystallization-induced fluorescence (CIF) properties. It has been reported by Tang’s group that some benzophenone derivatives showed crystallization-induced phosphorescence (CIP) characteristics.56 To verify that the fluorescence of 1, 2, and 3 are induced by the restriction of intramolecular rotations in their crystalline state, single crystal structure analysis (Figure 6) was performed. As shown in Table 2, in the crystal, the conformation of 1 was locked and stabilized by the strong intramolecular N-H•••O (d=2.05Å) and intermolecular N-H•••O (d=2.17Å) interaction, which cooperatively hampered rotation of phenyl rings and could result in the fluorescence emission. In addition, similar to the case of compound 1, the strong intramolecular and intermolecular hydrogen bonds in the crystal packing arrangements of compound 2 and 3 reinforced the structural rigidification effect, thus blocking the non-radiation decay channels and resulting in the fluorescence emission in the crystal state (Table 2). Interestingly, compound 1-3 all contain an amine group that is essential for the intramolecular and intermolecular hydrogen bonds. Compounds 4 and 5 without the amine group did not show the crystallization-induced fluorescence characteristics under scratching (Figure S13). Table 2. Hydrogen bond lengths (Å) in the crystals of compound 1-3. compound

intramolecular N(1)-H(1A)•••O(1)

intermolecular N(1)-H(1B)•••O(1)

1 2 3

2.05 2.08 2.02

2.17 2.16 2.05

Reversible mechanoresponsive fluorescence writing. Encouraged by the mechanoresponsive fluorescence characteristics of 1, 2 and 3, we further applied them for mechanoresponsive fluorescence writing. As shown in Figure 7, 1, 2 and 3 were coated on glass substrates and showed non-fluorescence. After scratching, the fluorescent letters of “C” “I” “F” in cyan, blue and green were clearly written. The letters were completely erased by fuming in ethanol for 3 minutes. After re-scratching, new letters of “T” “R” “Y” were successfully written and they could be erased again. These results demonstrated that 1, 2 and 3 could be applied for reversible mechanoresponsive fluorescence writing.

Figure 7. Reversible mechanoresponsive fluorescence writing by using 1, 2 and 3, respectively. Fluorescence images were taken under 365nm handheld UV lamp.

CONCLUSION In summary, we discovered 2-aminobenzophenone derivatives 1, 2 and 3, which showed crystallization-induced fluorescence characteristics, exhibited mechanoresponsive fluorescence based on amorphous-to-crystalline transformation with high “off-on” contrast ratio. Interestingly, the mechanoresponsive fluorescence was found to arise from molecular packing transformation from amorphous to crystalline phase under mechanical stimulation, which was contrary to most reported crystalline-to-amorphous transformation mechanism. Moreover, the fluorescence of the scratched sample could readily convert back into the initial non-fluorescent state through fuming in ethanol or heating. 1, 2 and 3 were successfully applied for reversible mechanoresponsive fluorescence writing, demonstrating their potential application in the fields of mechano-sensors and security materials.

ASSOCIATED CONTENT Supporting Information


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The Journal of Physical Chemistry Effect of water volume fraction on the fluorescence and absorption spectra, DLS analyses of 2 and 3; the images of the scratching-induced crystalline area of 1, 2 and 3 with the changing of time and temperature; SEM images of 1 before and after scratching; XRD spectra of 2 and 3; TGA/DSC traces of 1, 2 and 3; fluorescence decay curve and lifetime of 1-3; chemical structures of 4, 5 and their fluorescence spectra under scratching; details of data collection, processing and structure refinement of 1-3. Cif files of compound 1, 2 and 3.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Fax: +86-10-2787682. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are very grateful for the financial support from the National Natural Science Foundation of China (Nos. 21375074, 21390410 and 21621003).

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TOC Graphic


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2-Aminobenzophenone derivatives exhibiting mechanoresponsive fluorescence based on amorphous phase to crystalline transformation with high “off-on” contrast ratio have been successfully applied for reversible mechanoresponsive fluorescence writing. 316x114mm (68 x 73 DPI)


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Mechanoresponsive Fluorescence of 2-Aminobenzophenone

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