More than Restriction of Twisted Intramolecular Charge Transfer

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More than Restriction of Twisted Intramolecular Charge Transfer: Three-Dimensional Expanded #-Shaped Cross-Molecular Packing for Emission Enhancement in Aggregates Yan Qian,† Minmin Cai,† Xinhui Zhou,† Zhiqiang Gao,† Xupeng Wang,† Yuezhi Zhao,† Xiaohong Yan,‡ Wei Wei,† Linghai Xie,† and Wei Huang*,† †

Key Laboratory for Organic Electronics & Information Displays and Institute of Advanced Materials, and ‡School of Opto-Electronic Engineering, Nanjing University of Posts & Telecommunications, Nanjing, 210046, China S Supporting Information *

ABSTRACT: This study developed a new solid-state, highly emissive phenylbenzoxazole-based organic compound, N-(4(benzo[d]oxazol-2-yl)phenyl)-4-tert-butylbenzamide, which exhibited a distinct aggregation-induced enhanced emission. The solid fluorescence efficiency of the newly developed compound was 50.3%, whereas that in THF solution was only 0.22%. The single-crystal analyses revealed that a specific three-dimensional #-shaped cross stacking between molecules was observed in the solid/aggregated state, driven by specific C−H···π interaction and various hydrogen bonds. The expansion of the cross-dipole stacking into the three-dimensional network was believed to be the dominant factor for the emission enhancement in the solids/ aggregates, with respect to the assistant effect of the photoinduced twisted intramolecular charge transfer restriction.

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INTRODUCTION Solid-state, highly emissive organic materials have gained increasing attention due to their potential applications in large-area or flexible display,1 solid-state lighting,2 and organic lasers.3 However, the presence of face-to-face stacking (Haggregation) causing concentration quenching dramatically decreases the luminescence efficiency in the solid state, which may be attributed to the facilitated nonradiative decay induced by strong intermolecular interactions, including exciton coupling and excimer formation. The discovery of the aggregation-induced emission (AIE)4 or aggregation-induced enhanced emission (AIEE)5 has led to increasing studies on newly developed functional materials in recent years for their potential uses in optoelectronic devices,6−8 photomemory,9 logic gates,10 and biosensors.8,11 More importantly, these newly developed functional materials have become one important class of prospective candidates for nondoped organic light-emitting diodes (OLEDs) due to their attractive AIE/AIEE natures.7,12−18 The opt-electronic properties of the materials in such devices, particularly those in solid films, are defined by the intrinsic molecular structure and the © 2012 American Chemical Society

nature of their intermolecular exciton coupling. Therefore, a deep understanding of the effects of both the intrinsic monomer molecular structure and the supramolecular packing on condensed-state photophysical behaviors is required. The most popular AIE/AIEE mechanism at the single molecular level involves the restriction of intramolecular rotation (RIR), which was primarily proposed by Tang and co-workers.19 Other mechanisms involve restriction of intramolecular charge transfer (ICT),20 twisted intramolecular charge transfer (TICT),21,22 or cis−trans isomerization.23 Some types of specific molecular packing at the supramolecular level, such as J-aggregation,5,24 dimer/excimer stacking,18,25 herringbone stacking,26 or even the weakly coupled Haggregation,27 have been helpful in maintaining the emission in the solid state, probably due to some specific exciton coupling. Received: December 20, 2011 Revised: March 28, 2012 Published: April 25, 2012 12187

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Theoretical investigations by Brédas and co-workers revealed that one of the most efficient strategies to prevent any decrease in luminescence efficiency of the chromophores in the condensed state requires adjusting the long axes of adjacent molecules to become perpendicular.28,29 The optical transition to the lowest excited state (S1) in a face-to-face packing (Haggregation) mode is inhibited, and the entire oscillator strength is concentrated on the transition to the second excited state (S2). The absence of any significant oscillator strength in the transition between the ground state (S0) and S1 implies that the luminescence is strongly quenched because radiative decay usually takes place from S1.30 However, the energy splitting between S2 and S1 in a cross-stacking mode is much reduced, thereby facilitating the transfer from S2 to S1. Thus, the luminescence emission is maintained due to the increased oscillator strength of the transition between S0 and S1. However, solid evidence or verification using the crystal data of such cross-dipole stacking that intensifies the solid state emission has not yet been fully elucidated.31−33 Ma and coworkers demonstrated the strong solid-state emission in the distyrylbenzene derivative with one-dimensional cross stacking of the molecules through crystal analysis.31,32 Lahti and coworkers published the two-dimensional cross-dipole stacking of a diphenylenebenzene derivative, providing a basis for improvement of solid-state luminescence.33 However, three-dimensional, expanded, and highly emissive cross-packing supramolecular structures have not been reported yet. Our previous studies22,24,34 revealed that the carboxamide derivative of the excited state intramolecular proton transfer (ESIPT) compound, 2-(2′-hydroxyphenyl)benzoxales (HBX, X = S, O), is AIEE-active, wherein the restriction of the nonradiative TICT in the enol-excited state is important in the emission enhancement of aggregates/solids. In addition, TICT is mainly responsible for the weak emission in solutions as indicated by the dramatic molecular configuration that twists together with the complete charge transfer in the enol-excited state. However, the TICT emission in the long-wavelength region cannot be detected due to overlapping within the large Stokes-shifted keto emission. ESIPT molecules are more stable as enol and keto forms in the ground and excited states, respectively. An extremely fast four-level photophysical cycle (E−E*−K*−K−E) mediated by intramolecular H-bonds occurs immediately after photoexcitation in the ESIPT process (Scheme 1). Therefore, an abnormally large Stokes shift corresponding to the keto-tautomer emission is observed. An intramolecular hydrogen bond is essential in the ESIPT process because the cycle would not occur without it. The current study aims to develop another structurally similar AIEE compound, N-(4-(benzo[d]oxazol-2-yl)phenyl)-4tert-butylbenzamide (OTB), by eliminating the hydroxyl group from the molecular structure. Therefore, understanding the AIEE mechanism is important in constructing a prospective rational design of the series of functional solid-state, highly emissive materials.

Scheme 1. Molecular Structures of the Enol and Keto Forms of 2-(2′-Hydroxyphenyl)benzoxale Compounds, the ESIPT Photophysical Cycle, and the Corresponding Fluorescence Spectra

butylbenzoic acid (0.89 g, 5 mmol) in 30 mL of dry tetrahydrofuran (THF) at 0 °C and then stirred for 3 h until numerous white precipitates emerged. The solid was filtered, and then, the filtrate was concentrated and subjected to silicagel column chromatography (petroleum ether/ethyl acetate ratio = 4:1, Rf = 0.35) to produce 1.65 g of solid and obtain 89% yield. 1H NMR (400 MHz, d-DMSO, 298 K): δ = 10.54 (s, 1 H), 8.19−8.21 (d, J = 8.72 Hz, 2 H), 8.04−8.06 (d, J = 8.73 Hz, 2 H), 7.91−7.93 (d, J = 8.43 Hz, 2 H), 7.77−7.80 (q, J = 3.94 Hz, 2 H), 7.57−7.59 (d, J = 8.39 Hz, 2 H), 7.40−7.42 (t, J = 3.75 Hz, 2 H), 1.33 (s, 9 H); m/z (MALDI-TOF) 371.35 [M + H]+. Anal. Calcd for C24H22N2O2 (370.17): C, 77.81; H, 5.99; N, 7.56. Found: C, 77.96; H, 5.86; N, 7.57. Characterization. The NMR spectrum was recorded on a Bruker AV 400 MHz NMR spectrometer. Mass-spectra were collected using a Microflex MALDI-TOF mass spectrometer (Bruker Daltonics, Germany). UV−vis absorption and photoluminescence spectra were obtained using Shimadzu UV-3600 and RF-5301PC spectrophotometers, respectively. Singlecrystal data were acquired with a Bruker Smart APEX CCD area detector using direct methods. Structure solutions were also obtained using direct methods and were refined through full-matrix least-squares on F2 using SHELXL-97.36 An SEM image was obtained using a Hitachi S-3400 SEM. Lifetimes were measured using an Edinburgh FLSP920 lifetime spectrometer with a 375 nm laser; the fluorescence quantum yields of solids were measured with an integration sphere. The fluorescence quantum yields of the local blue emission of the solution were measured using 9,10-diphenyl anthracene as reference. Theoretical Computation Methods. Geometrical optimization for the ground and excited states was carried out at the B3LYP/6-31G and TD-CAM-B3LYP/6-31G levels, respectively. The TDDFT/B3LYP/6-31g and TDDFT/CAMB3LYP/6-31g calculations of the excitation energies were then performed at the optimized geometries of the ground and

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EXPERIMENTAL METHODS Materials. All the materials were received from Shanghai Chemical Reagents and used without further purification. The solvents, however, were dried and distilled before use. Synthesis of OTB. The synthesis of OTB was carried out following a previously reported procedure.35 A sample of 4(benzo[d]oxazol-2-yl)aniline (BOA) (1.05 g, 5 mmol) and triethylamine (0.71 mL, 5 mmol) were added to 4-tert12188

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excited states, respectively. The long-range corrected functional CAM-B3LYP37 on the TICT excited state was used to obtain excitation energies that correlated well with the experimental results. All the quantum-chemical calculations were performed using the Gaussian09 suite of programs.38

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RESULTS AND DISCUSSION Synthesis and AIEE Investigations. The phenylbenzoxazole-based compound of OTB was synthesized according to a previously published method35 through acylation of BOA with 4-tert-butylbenzoyl chloride. BOA was obtained from the condensation of o-aminophenol with p-aminobenzoic acid in the presence of polyphosphoric acid.39 The synthetic strategy was described in Scheme 2. OTB was soluble in common

Figure 2. Normalized absorption and fluorescence spectra of OTB in THF solution.

Scheme 2. Synthetic Strategy of OTB

organic solvents, including chloroform, acetone, ethanol, THF, acetonitrile, ethyl acetate, and 1,2-dichloromethane, but did not exhibit solubility in water. A significantly enhanced emission in the aggregates/solids was observed. The molecularly dispersed THF solution appeared transparent and produced almost no emission, whereas the nanoparticles or powder exhibited a strong emission (Figure 1). Thus, OTB is AIEE-active.

Figure 3. (a) Optimized molecular configurations of S0 and S1. (b) Frontier molecular orbitals of S1.

complete charge transfer from the benzoxazole unit to the tertbutylphenyl ring occurred in the excited state (Figure 3b), indicating that the TICT excited state is characterized by the complete charge separation, together with a great twisting of the molecular configuration after photoexcitation.40 Consistently, the profiles of the absorption spectra exhibited little change, whereas the 500 nm emission exhibited a gradual bathochromic shift and reduced intensity with increasing solvent polarity (Figure 4). However, the absorption spectra showed almost no change with solvent polarity, indicating that the TICT process occurs only in the excited state. As a result, the photoinduced TICT excited state possessed a larger dipole moment (5.77 D) than the ground state (3.75 D). The emission spectra in the solid state differed greatly from that of the solution; the local emission at 400−450 nm produced the dominant emission band (Figure 5). The spincoated amorphous film exhibited a relatively broad emission around 450 nm, whereas the powder and the crystal showed a relatively blue-shifted sharper emission at approximately 410 nm. A small portion of the long-wavelength emission located at around 507 and 553 nm was observed for the powder and crystal, respectively. However, these long-wavelength emissions cannot be assigned to the TICT emission due to the restriction of twisting the molecular configuration in the solid state, indicating that the long-wavelength emissions probably originated from the excimer CT emission because of the new peaks that appeared within the 410−530 nm range in the excitation spectra not observed in THF solution (Figure S1, Supporting Information), the longer lifetimes of 3−4 ns (Figure S2, Supporting Information), and the high sensitivity of the emission wavelength and intensity to the microenvironment.

Figure 1. Photographs of 5 × 10−6 mol/L OTB dispersed in THF (left) and water (middle) and the powders (right) under UV illumination at 365 nm.

The absorption and emission spectra of OTB in THF solution are shown in Figure 2. The maximum absorption at 325 nm mainly corresponded to the π−π transition caused by the coupling between oxazole and the fused phenyl ring. The fluorescence spectrum revealed the two distinct emissions, particularly at 375 nm and approximately 500 nm emission bands. The former is attributed to the emission of the local excited state, whereas the latter is likely assigned to a TICT emission.22 The tert-butylphenyl ring rotated around the C−N single bond in the imide group, with the dihedral angle of C12−N15−C16-C17 ranging from 26° in S0 to 94° in S1; the dihedral angle between the tert-butylphenyl and the benzoxazole rings ranges from 60° to 84° (Figure 3a). Meanwhile, the 12189

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Figure 4. Absorption (a) and fluorescence spectra (b) of 5 × 10−6 mol/L OTB in various solvents.

The fluorescence quantum efficiency of the local excited state ØLE can be expressed as follows ϕLE =

kF kF + k nr

(1)

where knr is the total nonradiative decay rate constant. In solutions, knr = kIC + kTICT. However, TICT in solids was effectively restricted; thus, knr ≈ kIC. The lifetime of LE*, τLE, is given by 1 τLE = kF + k nr (2) The average lifetime was used to obtain the value of τLE, using the following equation41

Figure 5. Normalized fluorescence spectra of OTB in powder, crystal, and amorphous film.

τLE = =

The excimer does not refer to the conventional face-to-face packed excimer; instead, it specifically refers to the crossstacked excimer. Investigations of the AIEE Mechanism. The measured fluorescence quantum efficiency ØLE in powder was 50.3%, which was 229 times higher than that in THF solution (0.22% for the local emission). The photophysical processes involved in the local excited state of the OTB molecule are shown in Figure 6. The vertical transition from the ground state to the

α1τ12 + α2τ2 2 α1τ1 + α2τ2

(3)

The radiative rate constant kF and the nonradiative rate constant knr in the solution and powder can be calculated by measuring the ØLE and τLE according to eqs 1 to 3. The lifetime consists of a shorter component, 0.08 ns (87%), and a longer component, 1.37 ns (13%), in THF solution, with the average lifetime of 1.01 ns (Figure 7). The emission in

Figure 6. Photophysical scheme of the local excited state of the OTB monomer molecule. Figure 7. OTB fluorescence decay in the THF solution (5 × 10−6 mol/L) and powder.

local excited state (LE*), i.e., the Franck−Condon state, occurred immediately after photoexcitation. The nonradiative intersystem crossing (ISC) decay can be neglected in this work without the heavy atoms possessed in the molecular structure and no phosphoresce at low temperature (77 K). Therefore, three decay ways from LE* were observed, particularly the radiative fluorescence decay, the nonradiative internal conversion to the ground state, and the nonradiative TICT decay to the low-energy TICT excited state.

powder revealed a single exponent decay of 0.98 ns. The calculated knr in the solution and powder was 9.9 × 108 and 5.0 × 108 s−1, respectively. The photophysical parameters were summarized in Table 1. According to our previous discussion, the decrease in knr in the powder is mainly caused by the molecular structural stiffening and restriction of concomitant nonradiative TICT decay.22 12190

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two subunit rings, which is essential for the restriction of the nonradiative TICT and helpful for its AIEE properties. Four OTB molecules per unit cell were observed from the crystal data, exhibiting cross stacking among one another using two kinds of interactions, the C−H···π interaction and the Hbonding with the carbonyl oxygen (Figure 8b). The two molecules were initially cross stacked in edge-to-edge type to form an X-shaped dimer through H-bonding between the tertbutylphenyl hydrogen and the carbonyl oxygen atom, with the distance measured at 2.30 Å. Two X-dimers were then edge-toface connected with each other through a C−H···π bond between the hydrogen of the tert-butyl substituent and the tertbutylphenyl ring of the adjacent molecule. The distance from the hydrogen atom to the central point of the tert-butylphenyl ring measured 2.83 Å, with the angle of the C−H···ring at 86.7°. Two molecules exhibiting the C−H···π interactions adopted an L-shaped aggregation mode for the two X-dimers to form a #-shaped tetramer. The #-aggregations were further expanded to form a 3-D network structure by another type of hydrogen bond between the two #-aggregations formed between the nitrogen atom on the oxazole ring and the neighboring hydrogen atoms of two cross-linked molecules (Figure 8c). The hydrogen atoms originated from the tertbutylphenyl ring, the imide unit, and the central phenyl ring; the distances of the three hydrogen bonds were 2.49, 2.34, and 2.60 Å, respectively. The formation of this 3-D expanded cross packing is shown in Figure 9. Therefore, the 3-D extended cross packing between molecules was cooperatively accounted by the C−H···π interaction and all these various hydrogen bonds. The crossed dihedral angles between the two molecules connected by any of the three interactions, i.e., the C−H···π interaction, H-bonding with the carbonyl oxygen atom, and another H-bonding with the nitrogen atom of the oxazole ring, measured 80°. The lowest optical transition (S1) in a parallel face-to-face aggregation (H-aggregation) is forbidden according to the selection rules; the entire oscillator strength is concentrated in the transition to the second excited state (S2) (Scheme 3). As a result, the absorption of the aggregate is blue-shifted, and its fluorescence is effectively quenched because internal conversion to the lowest (forbidden) excitonic state is much faster than emission, in accordance with Kasha’s rules. Brédas and coworkers suggested that a nonparallel, preferentially perpendicular arrangement of transition dipoles of chromophores can effectively prevent fluorescence quenching because the exciton coupling is much weakened due to the decreased overlap of the wave functions of the adjacent cross stacking molecules.29,31

Table 1. Photophysical Parameters of OTB in the THF Solution and Powder kf (s−1)

knr (s−1)

τLE (ns)

THF

2.2 × 106

9.9 × 108

powder

5.0 × 108

5.0 × 108

0.08 (87%) 1.37 (13%) 0.98 (100%)

(ns)

ØLE

1.01

0.22%

0.98

50.3%

The following equation is considered when kF remained unchanged in the solid state and the AIEE was fully caused by a decrease in knr: (ϕLEP/ϕLETHF) = [(kF + knrTHF)/(kF + knrP)] < (knrTHF/knrP). Thus, if kF remained unchanged in the solid state, the effect of TICT restriction that induces a reduced knr would only result in ØLE enhancement by approximately two times, which is far insufficient for the 229-fold increase in ØLE. As a result, other important factors should contributed to the emission enhancement; kF should not remain constant in both the solid state and solution. On the basis of the calculation, a large enhancement of kF was obtained in the powder by 2 orders of magnitude. The kF in the powder was 5.0 × 108 s−1, whereas that in THF solution was only 2.2 × 106 s−1. The small kF in the monomer is intrinsic and much related to the molecular structure.41 Hence, both the nonradiative TICT process and the very slow fluorescence decay are responsible for the weak emission of the OTB solution. As such, the effect of restriction of TICT leading to a reduced knr acts only as a minor factor in the AIEE characteristics, whereas large enhancement of kF serves as the dominant factor. However, further investigations should be carried out to determine the reason for the enhancement of kF. The crystal data of the OTB fluorophores can help to better understand the supramolecular stacking and the intermolecular interactions in the aggregated/solid state. Single crystals of the OTB were grown from ethyl acetate through slow evaporation. The X-ray crystallographic analysis reveals that the compound crystallizes in monoclinic space group P21/c with the following lattice parameters: a = 14.29 Å; b = 10.87 Å; c = 12.58 Å; and β = 91.84° and z = 4. The molecular configuration in optimized isolated molecules was greatly twisted, with the dihedral angle of 60° between the benzoxazole and the 4-tert-butylphenyl rings. However, the OTB molecular structure in the crystalline state became nearly coplanar (Figure 8a). The dihedral angle between the benzoxazole and the 4-tert-butylphenly rings measured 173°, wherein the structural stiffening of the molecules in aggregates did not allow the free rotation of the C−N bond between the

Figure 8. (a) ORTEP drawing of the OTB with 50% probability ellipsoids, (b) cross stacking via C−H···π interaction and aryl−H···OC hydrogen bond in a unit cell, and (c) the cross stacking via aryl−H···N hydrogen bonds and CON−H···N hydrogen bonds. 12191

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Figure 9. (a) Formation scheme of 3-D expanded cross packing of the OTB molecules and (b) the cross-stacked #-shaped tetramers in the crystal. All the hydrogen atoms are omitted for clarity.

Scheme 3. Energy Splitting of the Two Lowest Excited States in the Monomer, H-Aggregates, and Cross-Stacked Aggregates (X-Aggregates)a

Scheme 4. Transition Energies (Upper) and Related Transition Intensities (Bottom) of the Two Lowest Optical Transitions As a Function of the Dihedral Angle between the Long Molecular Axes29,31

visible region caused by Mie scattering, indicating the growth of the nanoparticles. The absorption ratio in the high-energy region of 250−300 nm increased gradually, and the new absorption at the longer-wavelength region peaked at 360 nm and appeared with gradually increasing intensity. The change in the absorption profiles was consistent with the photophysics in the cross-stacking mode, which will be further discussed later. Meanwhile, the fluorescence intensity increased gradually with aging time, accompanied by the blue-shifted emission with the fine structure gradually appearing, indicating that the nanoparticles grow from the initially amorphous state to a relatively ordered nanostructure. The SEM image shows that most of the OTB molecules easily aggregated into some 500 nm width nanorods with the length of 1−5 μm (Figure 11). The absorption and fluorescence spectra of the OTB nanoparticles dispersed in THF/water mixtures with different water fractions after aging for 1 h were obtained, respectively. The OTB molecules began to aggregate when the water fraction increased to 80%, as indicated by a distinct drop of the absorption intensity (Figure 12a). The change in the absorption profiles of the nanoparticles, compared with the solution, was similar to that in Figure 10b, which is attributed to the cross-dipole stacking between molecules (to be discussed later). Meanwhile, the emission enhancement became prom-

a

The width of the narrows indicates the corresponding transition intensities.

The energy of S2 decreased with the increase in the angle between the long axes, whereas that of S1 increased (Scheme 4). The splitting between the two lowest optical transitions decreases, and a transfer of intensity from S2 to S1 occurs, leading to a significant emission enhancement in solids/ aggregates. Therefore, the AIEE characteristic can be observed in OTB solids with molecules cross stacking between each other. The 3-D cross-dipole stacking is the dominant factor for the AIEE characteristic of the OTB compound due to the reduced energy splitting and a facilitated transfer of intensity between S2 and S128 with respect to the assistant effect of restriction of nonradiative TICT decay.22,42 Both of the factors synergistically resulted in the emission enhancement of the solid/aggregated state. Photophysics in Nanoparticles. The absorption and fluorescence spectra of the OTB nanoparticles dispersed in water/THF mixtures with 5% water fraction at different aging times are shown in Figure 10. The main absorption decreased with increasing time, exhibiting slightly increased tails in the 12192

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Figure 10. Absorption (a), normalized absorption (b), fluorescence (c), and normalized fluorescence spectra (d) of 5 × 10−6 mol/L OTB in water/ THF mixtures with 95% water fraction.

observations were also detected in high polar solvents, such as ethanol and isopropyl alcohol (IPA) (Figure 4b). The increase in water contents may lead to an increase in solvent polarity with the OTB molecules remaining in a molecularly dispersed state with 20%−75% water fraction, which prevents the observation of the TICT emission. The lifetime of the OTB nanoparticles in a THF/water mixture with 5% water fraction was measured at a shorter lifetime component of 0.31 ns (37%) and a longer lifetime component of 1.09 ns (63%), with the average lifetime at 0.94 ns. The fluorescence quantum efficiency of the nanoparticles was 43%. Using eqs 1 and 2, the knr and kF in the THF/water mixture nanoparticles with 5% water fraction were calculated as 6.1 × 108 s−1 and 4.6 × 108 s−1, respectively. The 195-fold enhancement of the relative quantum yield may be induced by the synergistic effects of the cross stacking of long molecular axes (dominant factor) and restriction of photoinduced TICT (minor factor). In addition, the change in the absorption profiles agrees with the photophysics of the cross-dipole stacking. Brédas and coworkers29,31 demonstrated that the splitting between the two lowest optical transitions is decreased in the cross-stacked molecules because of the reduced overlap between their wave functions, and a transfer of intensity occurs from S2 to S1. Therefore, the cross stacking of the dipole moments may lead to an intense blue-shifted absorption (corresponding to the transition to S2) with respect to that of the monomer and the appearance of a weaker red-shifted absorption (or a red tail in the case of highly disordered samples, corresponding to the transition to S1).28 The previous finding agrees with our experimental results (Figure 10b and Figure 12a, inset).

Figure 11. SEM image of OTB nanoparticles dispersed in water/THF mixtures with 95% water fraction.

inent until the water fraction reached 80% (Figure 12b), exhibiting a typical AIEE property. The TICT emission at around 500 nm in nanoparticles could almost not be detected, whereas the local emission at approximately 400 nm became distinct and dominant, indicating that the restriction of TICT in the excited state can contribute to the AIEE characteristic.22 The maximum relative quantum yield of the OTB nanoparticles was achieved with 95% water fraction, which was 195-fold higher than that in THF solution (Figure 12b, inset). The TICT emission around 500 nm also disappeared in the molecularly dispersed solutions with water content of less than 80%, which may be attributed to the solvent effects. Similar 12193

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Figure 12. (a) Absorption and (b) fluorescence spectra of 5 × 10−6 mol/L OTB in water/THF mixtures with different water fractions (vol %) with aging time of 1 h (inset: the relative quantum yield depending on different water fractions).

Science Foundation of China (Grant Nos. 21003076, 21001065, 61076016, 20974046, 61077070, and 61077021), and the Natural Science Foundation of Jiangsu Province (Grant No. BZ2010043) for the financial support.

Meanwhile, the facilitated transfer of intensity between S2 and S1 may result in a bathochromic shift of the emission in the nanoparticles or solids, compared with that of the monomer. An 18 and 35 nm red shift was observed in the nanoparticles and the solids, respectively. The local emission of the THF solution, nanoparticles, and solids peaked at 375, 393, and 410 nm, respectively. The emission of the nanoparticles was about 17 nm blue-shifted compared with that of the single crystal or powders, which may possibly be caused by the less optimal lattice order in the nanoparticles. Meanwhile, the cross-stacked excimer CT emission in the long-wavelength region was not observed in the nanopaticles, probably due to the increased polarity of the microenvironment.

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CONCLUSION In summary, an AIEE-active compound OTB based on phenylbenzoxazole was newly developed. The crystal data revealed that the 3-D expanded cross-molecular stacking was exhibited in the solid/aggregated state, driven by specific C− H···π interaction and various hydrogen bonds. The cross stacking of the dipole moments is considered the dominant factor for the emission enhancement in aggregates/solids due to the reduced energy splitting and a facilitated transfer of intensity between the two lowest excited states, with respect to the assistant effect of restriction of TICT.

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ASSOCIATED CONTENT

S Supporting Information *

The excitation spectra and fluorescence decay of the longwavelength emission in the OTB powder and crystal, calculated results of the ground and excited states, and crystallographic information files (CIF) of the OTB. 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 25 8586 6008. Fax: +86 25 8586 6999. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely express our thanks to the 973 (2009CB930601) and 863 projects (2011AA050526), the National Natural 12194

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