Structure, property and mechanism study of fluorenone-based AIE dyes

Dyes and Pigments 129 (2016) 121e128

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Structure, property and mechanism study of fluorenone-based AIE dyes Fan Xu a, Hui Wang a, Xianchao Du a, Wenji Wang a, Dong-En Wang a, Sheng Chen a, Xiang Han a, Na Li a, Mao-Sen Yuan a, b, *, Jinyi Wang a, ** a b

College of Science, Northwest A&F University, Yangling 712100, PR China State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 December 2015 Received in revised form 17 February 2016 Accepted 23 February 2016 Available online 27 February 2016

Aggregation-induced emission (AIE) has attracted considerable interest in recent years. An understanding of the underlying mechanisms for the AIE phenomena is also of great importance. Here, we synthesized four fluorenone derivatives, which all show typical AIE properties. Their enhanced solidstate luminescence showed a 160 nm red-shift (from 380 to 540 nm) in comparison to their luminescence in dilute tetrahydrofuran (THF) solutions. The single-crystal structure and photophysical properties revealed that the bathochromic luminescence is due to the formation of static excimers. To further demonstrate the AIE mechanism of static excimers, rather than restricted intramolecular rotation (RIR), we designed a diphenylfluorenone-diboronic acid adduct, which can react with D-glucose to create structurally rigid oligomers, resulting in the restriction of intramolecular rotations. The spectral change in the D-glucose titration of the adduct indicated that the bathochromic AIE of these fluorenone compounds cannot possibly be caused by RIR and conformational planarization. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Aggregation-induced emission Fluorenone Luminescence mechanism Static excimer Solid-state fluorescence

1. Introduction Organic luminescent materials have attracted significant interest in recent decades, due to their potential applications in the fields of organic electronics [1], optoelectronics [2] and biology [3]. In most cases, organic luminescent materials are required to work in the solid-state rather than in solution [4]. Many organic lightemitting materials have very high fluorescence quantum yields in the solution. However, once they form a solid-state or film, their fluorescence decreases rapidly, and even fully quenches, which is attributed to aggregation-caused quenching (ACQ) [5]. This problem has troubled researchers for a long time. To design and synthesise bright organic solid emitters, many research teams have tried to use physical, chemical and engineering approaches to prevent aggregation quenching processes in order to increase the fluorescence quantum yield [6]. However, these efforts have produced very limited results.

* Corresponding author. College of Science, Northwest A&F University, Yangling 712100, PR China. ** Corresponding author. E-mail addresses: [email protected] (M.-S. Yuan), [email protected] (J. Wang). http://dx.doi.org/10.1016/j.dyepig.2016.02.023 0143-7208/© 2016 Elsevier Ltd. All rights reserved.

In 2001, the group of Tang first reported a type of multi-aryl substituted silole compounds which, almost, do not emit light in organic solution, but exhibit strong luminescence in the aggregation state [7]. This anti-ACQ phenomenon was subsequently defined as aggregation-induced emission (AIE) [8]. In recent years, AIE compounds have caused widespread interest and brought a renaissance to the application of organic luminescent materials [9]. Moreover, by taking advantage of the useful AIE effect, some optoelectronic devices [10], fluorescence sensor systems [11] and optical waveguides [12], with efficient solid-state emissions, have been explored and developed. However, current studies of AIE materials are primarily limited to several classes, such as hexaphenylsilole (HPS) [13], tetraphenylethene (TPE) [14], tetraphenylpyrazine [15], cyano-stilbene [16] and difluoroboron avobenzone [17] compounds. The lack of AIE compounds is the primary limiting factor for practical application of the AIE effect in technological innovations. Therefore, the design and synthesis of new AIE systems that strongly display lighting emission in the solid-state is of great importance. In addition, an understanding of the underlying mechanisms for the AIE phenomena is also crucial in the quest for fundamental knowledge of photophysics and the design of novel AIE molecules. Different AIE systems usually exhibit different AIE working

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processes [18]. The study of AIE mechanisms has become a high profile research area. Up to now, the working principles of AIE have been explored and understood to some extent, and a number of possible mechanisms have been proposed, including restriction of intramolecular rotation (RIR) [19] and intramolecular vibrations [20], conformational planarization [21], relatively loose molecular packing [22], J-aggregate formation [23] and special excimer formation [24]. Among them, the RIR mechanism has been deeply studied and verified by a series of experimental phenomena. In reality, luminescence of most AIE compounds follows the RIR mechanism. But not all AIE situations can be explained by RIR. Although some different mechanisms have been proposed, the studies so far have been woefully insufficient. Therefore, designing experiments and strategies to conduct further research on these mechanisms is necessary. Recently, Tao and our group reported a class of fluorenone compounds, which exhibit not only prominent AIE properties, but also great bathochromically solid-state fluorescence compared to their properties in tetrahydrofuran (THF) solution [24b]. We believed that the bathochromic fluorescent enhancement is due to the formation of static excimers through hydrogen bonds or pep stacking in the solid-state. However, the proposed mechanism has not been completely confirmed. For a more in-depth understanding of the AIE phenomenon and mechanism, in this paper, we designed and synthesized several aryl substituted fluorenone compounds, 1,10 -[(9-oxo-9H-fluorene-2,7-diyl)bis (4,1-phenylene)]bis (ethan-1one) (PDOF), 4,40 -(9-oxo-9H-fluorene-2,7-diyl)dibenzaldehyde (DDOF) and dimethyl 4,40 -(9-oxo-9H-fluorene-2,7-diyl)dibenzoate (NDOF), in which the carbonyl group (ester group) was connected to the peripheral aryl groups (Scheme 1). We consider that due to the electron-withdrawing property, and an exposed oxygen atom, the carbonyl group will facilitate the formation of static excimers. All of these compounds exhibit typical AIE properties and high solid-state fluorescence quantum. Simultaneously, they also show a different spectral character with the fluorenone compound 2,7diphenyl-9H-fluoren-9-one (DPF), which can be regarded as the precursor of the three compounds. In addition, to reveal the mechanism and gain an insight into the optical process, we designed and synthesized a DPF-diboronic acid adduct, [(9H-fluorene-2,7-diyl)bis (4,1-phenylene)]diboronic acid (BDPF), through functionalising the terminal positions of DPF with two boronic acid units. We make the diboronic acid compound BDPF oligomerise with D-glucose (D-Glu) to restrict the intramolecular rotation of the fluorenone molecule [25]. The oligomers exhibit a distinctly different spectrum from those of both the solid-state powder and the THF solution of these fluorenone compounds.

2. Experimental section 2.1. Synthesis and characterizations of the subject compounds Solvents for reactions and spectral measurements were dried and distilled before use. The reagents used for reactions were purchased from J&K Scientific Ltd. 1HNMR spectra were recorded at 25  C on Bruker Avance 500 MHz spectrometer using CDCl3 as solvent. 13CNMR spectra were recorded at 25  C on Bruker Avance 125 MHz spectrometer using CDCl3 as solvent. Element analyses (C, H) were performed using a PE 2400 autoanalyser. Mass spectrometry analyses were performed by a Bruker Biflex III matrix assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometer. Compounds DPF and BDPF were synthesized according to literature method reported by us [24a,26]. 2.1.1. Synthesis of 1,10 -[(9-oxo-9H-fluorene-2,7-diyl)bis(4,1phenylene)]diethanone (PDOF) A mixture of 2,7-dibromo-9H-fluoren-9-one (0.172 g, 0.51 mmol), (4-acetylphenyl)boronic acid (0.25 g, 1.52 mmol), Pd(PPh3)4 (10 mg, 0.01 mmol), toluene (10 mL), ethanol (2.5 mL) and 2 M CH3COOK aqueous solution (1.0 mL) was heated to 80  C with stirring under an argon atmosphere. The yellow solid powder precipitated from the reaction system after the mixture reacted for approximately 24 h at 80  C. After completion, solvent was removed in vacuo and the crude product was purified by column chromatography on silica gel and elution with dichloromethanemethyl alcohol (200:1, v/v). The yield of compound PDOF: 0.158 g, 75%. Melting point (m.p.): 312.3  Ce314.9  C. 1H NMR (CDCl3, 500 MHz, ppm): d 2.71 (s, 6H), 7.72 (d, J ¼ 7.7 Hz, 2H), 7.78 (d, J ¼ 7.8 Hz, 4H), 7.85e7.87 (m, 2H), 8.03 (s, 2H) and 8.11 (d, J ¼ 8.1 Hz, 4H). 13C NMR (CDCl3, 125 MHz, ppm): d 197.6, 144.2, 143.7, 141.2, 136.5, 135.4, 133.7, 129.1, 127.0, 123.3, 121.2 and 26.8. TOF-MS-EI: m/z 416.2 [M]þ. Elemental anal. calcd. for C29H20O3: C, 83.63 and H, 4.84. Found: C, 83.81 and H, 4.71. 2.1.2. Synthesis of 4,40 -(9-oxo-9H-fluorene-2,7-diyl)dibenzaldehyde (DDOF) A mixture of 2,7-dibromo-9H-fluoren-9-one (0.188 g, 0.56 mmol), (4-formylphenyl)boronic acid (0.25 g, 1.67 mmol), Pd(PPh3)4 (10 mg, 0.01 mmol), toluene (10 mL), ethanol (2.5 mL) and 2 M CH3COOK aqueous solution (1 mL) was heated to 80  C with stirring under an argon atmosphere. The yellow solid powder precipitated from the reaction system after the mixture reacted for approximately 24 h at 80  C. After completion, solvent was removed in vacuo and the crude product was purified by column chromatography on silica gel and elution with dichloromethane. The yield of compound DDOF: 0.171 g, 79%. Melting point (m.p.): 232.1  Ce234.3  C. 1H NMR (CDCl3, 500 MHz, ppm): 7.75 (d,

Scheme 1. Molecular structures of compounds DPF, PDOF, DDOF and NDOF.

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J ¼ 7.5 Hz, 2H), 7.85e7.88 (m, 6H), 7.05 (d, J ¼ 8.0 Hz, 6H) and 10.1 (s, 2H). 13C NMR (CDCl3, 125 MHz, ppm): d 193.0, 191.6, 145.5, 143.7, 141.1, 136.0, 135.3, 133.7, 130.2, 127.4, 123.5 and 121.2. TOF-MS-EI: m/z 388.7 [M]þ. Elemental anal. calcd. for C27H16O3: C, 83.49 and H, 4.15. Found: C, 83.47 and H, 4.33.

31G* basis set to optimize the single-molecular ground-state geometry. And the geometry of the NDOF dimer was obtained from its determined X-ray single crystal structure.

2.1.3. Synthesis of dimethyl 4,40 -(9-oxo-9H-fluorene-2,7-diyl) dibenzoate (NDOF) A mixture of 2,7-dibromo-9H-fluoren-9-one (0.156 g, 0.46 mmol), (4-(methoxycarbonyl)phenyl)boronic acid (0.25 g, 1.3 mmol), Pd(PPh3)4 (10 mg, 0.01 mmol), toluene (10 mL), ethanol (2.5 mL) and 2 M CH3COOK aqueous solution (1 mL) was heated to 80  C with stirring under an argon atmosphere. The yellow solid powder precipitated from the reaction system after the mixture reacted for approximately 24 h at 80  C. After completion, solvent was removed in vacuo and the crude product was purified by column chromatography on silica gel and elution with dichloromethane-methyl alcohol (200:1, v/v). The yield of compound NDOF: 0.145 g, 70%. Melting point (m.p.): 294.3  Ce296.5  C. 1 H NMR (CDCl3, 500 MHz, ppm): d 4.01 (s, 6H), 7.71e7.76 (m, 6H), 7.85 (d, J ¼ 7.8 Hz, 2H), 8.02 (s, 2H) and 8.19 (d, J ¼ 8.2 Hz, 2H). 13C NMR (CDCl3, 125 MHz, ppm): d 193.4, 166.9, 144.2, 143.9, 141.3, 135.6, 133.6, 130.3, 129.7, 126.8, 123.3, 121.1, 116.0 and 52.2. TOF-MSEI: m/z 464.5 [M]þ. Elemental anal. calcd. for C29H20O6: C, 74.99 and H, 4.34. Found: C, 74.87 and H, 4.62.

3.1. Synthesis

2.2. Single crystal X-ray diffraction The single crystal of compound NDOF was obtained by the slow diffusion of THF/cyclohexane solutions for several days at room temperature. Since the crystal is stable under ambient condition, the data collection was done without any inert gas protection at room temperature on a Bruker SMART APEX-II CCD area detector using graphite-monochromated Mo Ka radiation (l ¼ 0.71073 Å). Data reduction and integration, together with global unit cell refinements were done by the INTEGRATE program of the APEX2 software. Semi-empirical absorption corrections were applied using the SCALE program for area detector. The structure was solved by direct methods and refined by the full matrix least-squares methods on F2 using SHELX [27]. 2.3. Photophysical properties measurement UVevis absorption spectra for the solutions and the solid-state were recorded with a Shimadzu UV-2550 spectrophotometer. Photoluminescence (PL) spectra were recorded using a Shimadzu RF-5301PC spectrofluorimeter. The fluorescence quantum yield (F) in solution was determined using rhodamine B in ethanol as a reference according to a previously reported method [28]. Quantum yield of the solid-state powder was determined with a PTI C701 calibrated integrating sphere system [29]. Steady-state fluorescence spectra and decay curves were obtained using an Edinburgh FLS920 fluorescence spectrometer equipped with a timecorrelated single photon counting (TCSPC) card. Reconvolution fits of the decay profiles were performed with F900 analysis software to obtain the lifetime values. 2.4. Theoretical calculation In order to understand the spectral behavior and elucidate the influence of the configurational change to the spectra, the orbital energy of the optimized compound NDOF was calculated by using the Gaussian 09 program at the B3LYP Time-Dependent Density Functional Theory (TD-DFT). The geometry of the optimized compound NDOF was obtained by using the B3LYP functional and the 6-

3. Results and discussion

The molecular structures of compounds DPF, PDOF, DDOF and NDOF are outlined in Scheme 1. They were readily synthesized through a conventional Suzuki reaction by reacting 2,7-dibromo9H-fluoren-9-one with the corresponding aryl boric acid. All the new compounds were characterized with 1H-NMR, 13C-NMR, elemental analysis and MALDI/TOF mass spectroscopy. Compound NDOF has been verified by the X-ray single-crystal structure. 3.2. AIE and photophysical properties The photophysical properties of the four compounds DPF, PDOF, DDOF and NDOF have been deeply explored. As shown in Fig. 1a, b and d, in dilute THF solution, all of them exhibit very weak fluorescence, peaked at around 380 nm, with a vibronic feature. According to the research of Zojer [30], in these ketone-containing fluorenone compounds, S0/S1 corresponds to an optically forbidden transition and possesses an nep* character, the energy gap of which is lower than that of the charge-transfer pep* state. This will result in their weak single-molecule luminescence. Different from their dilute THF solutions, their solid powders show very strong fluorescence under 365 nm UV light, with the fluorescence quantum yields F ranging from 47% to 61% (Table 1, F in the solid state is obtained in a calibrated integrating sphere), which show that all of the four fluorenone compounds are highly AIEactive. Besides the fluorescent intensity increase, another remarkable spectral change is that the solid-state powders of the four fluorenone compounds emit bright yellow emission peaked at around 540 nm, which behave a ~160 nm red-shift relative to their diluted THF solutions (Fig. 1b and h). In addition, their concentrated THF solutions also show different spectral characteristics from their diluted THF solutions. Fig. 1e and f display the absorption and normalized PL spectra of the THF solution of the four compounds with the concentration at 100 mM. From the absorption spectra Fig. 1c and e, we can find that the emergence of a new wide absorption band peaked at around 450 nm, suggesting that the molecular aggregation occur in the ground state, which can also be verified by the bathochromic PL spectra (Fig. 1f). Based on the above results, we can deduce that the PL peaks around 380 nm with vibronic features originate from the single-molecule emission and the PL peaks around 540 nm originate from the aggregation emission. Simultaneously, in the process of spectral determination for their THF diluted solution, we also noted that compounds PDOF, DDOF and NDOF, which contain peripheral carbonyl group (ester group), displayed different spectral behavior from compound DPF. We prepared 10 mM THF solutions of the four compounds by diluting their 100 mM THF solutions, then determined the PL properties of the 10 mM THF solutions at once. The emission maxima of compound DPF was located at around 380 nm. However, the emission maxima of the other three compounds were located at around 540 nm, which indicates that the three compounds still keep the aggregation state in that case, and the aggregation state of the three peripheral carbonyl-containing compounds cannot be easily and rapidly dismissed when diluting. This may be because of the existence of the peripheral electron-withdrawing carbonyl group (ester group), which will lead to an acidity rise of the

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Fig. 1. (a) Photos of the THF solutions (10 mM) and solid powders of compounds DPF, PDOF, DDOF and NDOF under natural light and (b) 365 nm UV light. (c) UVevisible absorption spectra and (d) PL spectra of the THF solutions (10 mM) of compounds DPF, PDOF, DDOF and NDOF. (e) UVevisible absorption spectra and (f) normalized PL spectra of the THF solutions (100 mM) of compounds DPF, PDOF, DDOF and NDOF. (g) PL spectral change of the THF solutions (10 mM) of compound DDOF with time. Compounds PDOF and NDOF exhibited similar character to DDOF. (h) Normalized PL spectra of the solid powders of compounds DPF, PDOF, DDOF and NDOF.

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Table 1 Spectroscopic data for compounds DPF, PDOF, DDOF and NDOF. Solution in THFa

DPF PDOF DDOF NDOF a b c d

Solution in THF/watera (80% water)

Crystalline powder

labs (nm)

lem (nm)

Fb

tc(ns)

labs (nm)

lem (nm)

F

lem (nm)

Fd

t(ns)

288,323,337 301,343 304,344 298,328,342

360,375 375,389,542 371,383 366,377

0.01 0.03 0.02 0.02

1.0 1.2 1.9 2.3

286,341,471 299,352,489 296,357,461 301,358,489

534 533 552 532

0.48 0.52 0.44 0.42

534 554 546 546

0.61 0.54 0.47 0.48

18.0 11.7 12.3 16.5

With c ¼ 2.0  105 mol/L. Fluorescence quantum yield was determined using rhodamine B in ethanol as a standard. Fluorescence lifetime. Fluorescence quantum yield in the solid state was obtained using a calibrated integrating sphere.

hydrogen atoms of the benzene rings and fluorenone ring, which will increase the intermolecular interactions through intermolecular hydrogen or CeH…p bonding. However, with increasing time, we found that the green PL peak at 540 nm of DDOF and NDOF progressively vanished after several days, and the blue PL peak at 380 nm gradually boosted, as shown in Fig. 1g. We also noted that the green PL peak of PDOF cannot completely disappear (Fig. 1d), which indicates that PDOF is more likely to aggregate and eventually reaches a balance between the single molecules and the aggregations in THF. To further understand the luminescence properties of compounds DPF, PDOF, DDOF and NDOF, we also measured mixed systems of THF and water (10 mM), and found that not only does increasing the concentration cause luminescence red-shift, but water addition to the THF solutions also can cause this phenomenon. Because water is poor solvent for these four compounds, when the water content increases, it forces molecules to aggregate (Fig. 2a and b). Comparing Figs. 1d and 2b, we can also observe that when water is added to the THF solution, their maximum emission peak position have 160 nm red-shift. At the same time, we can also observe this in Fig. 2b, with the water fraction increasing from 50% to 80%, the PL intensity gradually increases, which can be interpreted by the fact that a high concentration of water will impel the formation of aggregation, i.e., AIE effect. The reason why the emission of the THF/water mixture solutions started to decrease in intensity when the water fraction was raised to >80%, is that the higher water fraction made the aggregation more facile to spontaneous self-assembly and produce precipitate quickly.

3.3. X-ray crystallography and AIE mechanism The

shapes

of

the

greatly

red-shifted,

luminescence spectra of the four fluorenone compounds in THF/ water is obviously different from the majority of spectral characteristics previously reported for AIE dyes, whose enhanced luminescence is caused by restricted intramolecular rotation. In those AIE systems, the luminescence, whether from their organic solutions or aggregated states, their peak positions of PL maxima are not significantly different in the two different states. On the other hand, we have determined the fluorescence lifetime of these four compounds in both diluted THF solution and in solid-state. As shown in Fig. 3, the fluorescence lifetime of these four compounds in THF are much shorter than in the solid-state, which suggested that these compounds experienced different optical decay processes in two different states. Determination of X-ray single-crystal structures is the most effective and powerful tool for acquiring the ground-state molecular structures and revealing the AIE mechanism. The single crystal of NDOF was obtained by slow diffusion of their THF/cyclohexane (1:1 v/v) solutions for several days at room temperature [31]. From the molecular packing of NDOF (Fig. 4a), the obvious characteristic molecular pairs (dimers) can be observed. The two molecules of each dimer arrange in a parallel, but staggered style, and link together by hydrogen bonds with bond lengths of 2.49 and 2.50 Å, respectively (Fig. 4b and c). When one molecule of this dimer becomes excited, this molecule can quickly spread excited energy to another molecule of the dimer through hydrogen bonds, thus forming an excimer. The formation and decay processes of this kind of excimer are different from conventional excimers. Usually, two molecules of conventional excimer combine together after being excited, which then need to rearrange properly or suitably restructure. Structural adjustment is also needed in the decay process, such a process consumes a large part of the excitation energy, which will lead to non-radiation transition and

unstructured

Fig. 2. (a) UVeVis absorption spectra changes of PDOF depending on the water fractions in THF (10 mM). (b) PL spectra changes of PDOF depending on the water fractions in THF. Compounds PDF, DDOF and NDOF exhibited similar character to DDOF (10 mM).

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absorption spectral characteristics of NDOF in dilute (Fig. 1c, ~350 nm) and concentrated (Fig. 1e, ~430 nm) THF solutions. 3.4. Verification of the AIE mechanism

Fig. 3. Fluorescence lifetime profiles of compounds DPF, PDOF, DDOF and NDOF in THF (10 mM) and in the solid-state, respectively.

fluorescence quenching. While in this work, two molecules of the excimer containing hydrogen bonds bonded together very strongly in the ground state, forming a static excimer, no energy is consumed in the formation and decay processes of this excimer, resulting in a fluorescence enhancement. The compounds PDOF, DDOF and NDOF, due to peripheral carbonyl group (ester group), are more convenient to form this static excimer than compound DPF, which is consistent with the spectral results (Fig. 1g). To better understand the spectral behavior and inspect whether the red-shifted solid-state emission of the four compounds is derived from the dimers, we, respectively, calculated the molecular orbital energies of optimized NDOF (the single-molecular groundstate geometry was optimized using the B3LYP functional and 631G* basis set) and the NDOF Dimer (the geometry was directly obtained from its X-ray single crystal structure) using the Gaussian 09 program and time-dependent density functional theory (TDDFT). As shown in Fig. 5, the energy gap of optimized NDOF between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is 3.496 eV (355 nm). When the two NDOF molecules form a dimer via hydrogen bonds, the energy gap of the NDOF Dimer between the HOMO and LUMO is reduced by 0.561 eV (67 nm) compared to that of optimized NDOF. The computed results are in good agreement with the

To further verify the AIE mechanism of the four fluorenone compounds and to prove that their AIE effect cannot possibly be caused by RIR and conformational planarization, we designed and synthesized a diboronic acid compound BDPF (Scheme 2), which can be regarded as an adduct of compound DPF and boronic acid. According to the reference reported by the group of Tang [25], in a carbonate buffer (pH 10.5), the diboronic acid compound BDPF can react with D-Glucose to create structurally rigid oligomers (BDPFGlu), resulting in the restriction of the intramolecular rotations of molecule BDPF. We determined the luminescent properties of compound BDPF in dilute THF and in the solid-state and it exhibited similar spectral characters to DPF with the PL maxima peak located at ~380 nm in THF and ~570 nm in the solid-state. In the D-Glu fluorescence titration of BDPF, with an increase in the amount of D-Glu, neither the 380 nm nor the 570 nm PL peak changed, which indirectly demonstrates that the green solid-state luminescence of these fluorenone compounds cannot possibly be caused by the RIR process. However, a gradual increase in intensity of the 450 nm emission band was observed on the addition of D-Glu to the buffer solution of BDPF, as shown in Fig. 6. We speculate that when BDPF combined with D-Glu to form rigid oligomers, the stereo-hindrance effect urged the two benzene rings of BDPF to coplane with the fluorenone unit, and the 450 nm emission enhancement is derived from the conformational planarization. 4. Conclusion In this study, we designed and synthesized four aryl substituted fluorenone compounds, namely DPF, PDOF, DDOF and NDOF. The four compounds exhibit typical AIE characteristics with high solidstate fluorescence quantum yields. In the process of molecular aggregation, their PL intensity obviously increased and their PL peak position showed an ~160 nm great red-shift (from 380 to 540 nm). We revealed the AIE mechanism through the X-ray single crystal structure of NDOF, combined with the theoretical calculation. In the crystal packing, we found that every two molecules are bound together even in the ground state by intermolecular hydrogen bonds to from a dimer, i.e. static excimers. We believed that the bathochromic fluorescent enhancement is derived from the static

Fig. 4. X-ray crystallographic packing of compound NDOF. (a) Side view of NDOF crystal packing. Top (b) and side (c) view and illustration of the hydrogen bonds bound dimer in NDOF.

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Fig. 5. Molecular orbital diagrams of HOMO and LUMO of optimized NDOF (the geometry was optimized using the B3LYP functional and the 6-31G* basis set) and NDOF Dimer (the geometry was obtained from its X-ray single crystal structure directively) with their relative energy according to TD-DFT calculation.

excimers. In addition, the spectral results showed that the compounds PDOF, DDOF and NDOF, due to peripheral carbonyl group (ester group), are more convenient to form this static excimer than compound DPF. Subsequently, we designed a DPF-diboronic acid adduct BDPF, which can react with D-Glucose to create structurally rigid oligomers, resulting in the restriction of the intramolecular rotations of molecular BDPF. A gradual increase in intensity of the 450 nm emission band was observed on the addition of D-Glu to the buffer solution of BDPF, which indicates that the 540 nm solid-state luminescence of these fluorenone compounds cannot possibly be caused by the RIR process and conformational planarization. Acknowledgment

Scheme 2. Schematic diagram of BDPF reacting with D-glucose to create structurally rigid oligomers.

This study was supported by the National Natural Science Foundation of China (21202132, 21175107 and 21375106), the Fund of Youth Science and Technology Stars by Shaanxi Province (2015KJXX-15), the Ministry of Education of the People's Republic of China (NCET-08-602 0464), the Fundamental Research Funds for the Central Universities (Z109021106 and Z109021303). References

Fig. 6. PL spectra for titrations of BDPF (10 mM) with D-Glucose solution in a carbonate buffer (pH 10.5).

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