D–A Solid Emitter with Crowded and Remarkably Twisted

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D−A Solid Emitter with Crowded and Remarkably Twisted Conformations Exhibiting Multifunctionality and Multicolor Mechanochromism Yongyang Gong,† Yiren Zhang,† Wang Zhang Yuan,*,† Jing Zhi Sun,‡ and Yongming Zhang*,† †

School of Chemistry and Chemical Engineering, Shanghai Key Lab of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai 200240, China ‡ Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Multifunctional and mechanochromic optoelectronic materials are of significant importance in both academic and technical aspects. In this contribution, a novel D−A conjugate (AN2TPAN), consisting of arylamine and two triphenylacrylonitrile (TPAN) units, with sterically crowded and remarkably twisted conformations, has been carefully designed and synthesized. It shows typical intramolecular charge transfer, aggregation-induced emission, and aggregation-enhanced two-photon absorption characteristics. Moreover, it exhibits more than three emission colors in the solid state upon mechanical grinding. Such multicolor mechanochromism for solid emitters under mild conditions is rarely reported. A multilayer nondoped OLED device of AN2TPAN is fabricated and performs excellently, whose maximal current and external quantum efficiencies are as high as 10.7 cd/A and 3.3%, respectively. The multifunctionality and multicolor mechanochromism of AN2TPAN make it promising in optoelectronic applications.

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INTRODUCTION Over the past decades, fluorescent materials with multifunctionality, such as high luminescent efficiency, good charge transporting capacity, optical waveguiding, and excellent organic light-emitting diode (OLED) or field effect transistor (FET) performances, have drawn increasing attention due to the simplification of troublesome procedures for multiple hightech applications utilizing a single compound.1−7 Luminogens with electron donor (D) and electron acceptor (A) moieties are promising candidates for such purposes due to their intramolecular charge transfer (ICT) characteristic, which endows them with tunable electronic states and thus unique optoelectronic and photovoltaic properties.4,8−12 It is also known that D−A structured luminogens are applicable in twophoton imaging, optical storage, solar cells, bipolar emitters for OLEDs, mechanochromic smart materials, thermally activated delayed fluorescence (TADF), and so on.4,8−19 Materials with the ICT feature, however, normally only offer weak emissions, particularly in the solid states and polar media due to the notorious aggregation-caused quenching (ACQ) problem of orthodox chromophores and the low efficiency of charge transfer emissions.19 Such drawbacks must be overcome because luminogens are often used in the condensed states. Fortunately, Tang’s discovery of the phenomenon of aggregation-induced emission (AIE) provides the possibility to obtain high efficiency D−A emitters, thus paving the way for their potential applications.20−24 Indeed, through decoration © 2014 American Chemical Society

with propeller-like triphenylacrylonitrile (TPAN) units, triphenylamine-cored D−A luminogens with both ICT and AIE characteristics and high solid-state efficiencies were obtained.25 Ma and co-workers also reported a twisting D−A molecule with high efficiencies in both solution and solid states owing to its intercrossed local exciton (LE) and ICT states.26 Moreover, Adachi and co-workers demonstrated that D−A conjugates with proper energy difference between lowest singlet (S1) and lowest triplet (T1) excited states (ΔEST) could exhibit intriguing TADF properties, endowing them with excellent OLED performance.17,18 Recently, smart luminogens displaying reversible solid-state emission upon mechanical stimuli also attracted extensive attention owing to their fundamental importance and potential applications in sensors, photomodulation, security inks, memories, and other optoelectronic devices.27−36 Among these solid emitters, AIE-active mechanochromic luminogens are extremely attractive due to their high solid-state efficiencies.32−36 Luminogens with multicolor mechanochromism are more interesting because of their sensitive responses. Generally, however, the reported mechanochromic materials could only be switched between two emission colors. Development of novel Received: January 17, 2014 Revised: April 29, 2014 Published: May 1, 2014 10998

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tum yields of AN2TPAN in solvents were estimated by using quinine sulfate (ΦF,s = 54% in 0.1 N H2SO4) as a standard, while the solid-state efficiency of the crystal was determined by an integrating sphere. The ground-state geometries were optimized using the density functional with the B3LYP hybrid functional at the basis set level of 6-31G(d). All calculations were performed using the Gaussian 09 package. Single-crystal X-ray diffraction intensity data were collected on a Bruker− Nonices Smart Apex CCD diffractometer with graphitemonochromated Mo Kα radiation. Processing of the intensity data was carried out using the SAINT and SADABS routines, and the structure determination and refinement were conducted using the SHELTL suite of X-ray programs (version 6.10). For both open-aperture Z-scan and 2PA measurements, a mode-locked Ti:sapphire laser was used as the light source. The pulse duration was 100 fs, the repetition frequency was 82 MHz, and the excitation wavelength was 800 nm. The laser beam was focused on the sample by a lens with a focal length of 10 cm. 2PA signals were excited at the focus of the lens and collected by using an Ocean Optics HR4000 Spectrometer. Synthesis of 2,2′-[(Phenylazanediyl)bis(4,1phenylene)]bis(3,3-diphenylacrylonitrile) (AN2TPAN). Into a 50 mL, two-necked, round-bottom flask were placed TPAN−Br (1081 mg, 3.0 mmol), Pd2(dba)3 (57.4 mg, 0.1 mmol), and t-BuONa (384.4 mg, 4.0 mmol). The flask was evacuated under a vacuum and then flushed with dry nitrogen three times. Toluene (30 mL), aniline (93.1 mg, 1.0 mmol), and toluene solution of (t-Bu)3P (2 mL) were injected into the flask, and the mixture was refluxed for 36 h and then cooled to room temperature. The solution was poured into water (50 mL) and extracted with DCM three times. The collected organic layer was washed with water and brine twice and then dried over anhydrous sodium sulfate. After solvent evaporation, the crude product was purified by silica-gel column chromatography using chloroform/hexane (1:5 by volume) as eluent. A yellow solid was obtained in 34% yield (220 mg). 1H NMR (400 MHz, CDCl3, δ): 7.48−7.36 (m, 10H), 7.30−7.27 (m, 2H), 7.25−7.20 (m, J = 1.7 Hz, 5H), 7.12 (t, J = 2.78, 2.04 Hz, 2H), 7.10 (t, J = 1.96, 2.68 Hz, 2H), 6.87−6.85 (m, 6H), 6.86 (t, J = 2.78, 2.04 Hz, 2H), 6.84 (t, J = 1.96, 2.68 Hz, 2H). 13 C NMR (100 MHz, CDCl3, δ): 156.71, 147.24, 146.68, 140.75, 139.60, 130.94, 130.90, 130.18, 130.01, 129.77, 129.21, 129.08, 128.67, 128.51, 125.80, 124.54, 123.16, 120.37 (−CN), 111.35. APCI-MS calcd [M+H]+ m/z for C48H33N3: 651.3, found: 652.7. Preparation of Aggregates for AIE Measurement. A stock solution of samples in THF with a concentration of 200 μM was prepared. An aliquot (1 mL) of the stock solution was transferred to a 10 mL volumetric flask. After an appropriate amount of THF was added, water was added dropwise under vigorous stirring to furnish 20 μM mixtures with different water fractions. The emission measurement of the resultant mixtures was performed immediately. OLED Fabrication. The device was fabricated by the following processes. First, ITO-coated glass substrates were cleaned successively using deionized water, acetone, and isopropanol in an ultrasonic bath and then dried in a drying cabinet followed by pretreatment with oxygen plasma. Then, the organic films of N,N,N′,N′-tetrakis(4-methoxy-phenyl)be nzidine ( MeO− TPD ), 4 ,4′ ,4 ″-t r i (9- ca rb a zo y l ) triphenylamine (TCTA), AN2TPAN, 4,7-diphenyl-1,10-phenanthroline (Bphen), and 8-hydroxyquinolinatolithium (Liq) were deposited by thermal evaporation under a base vacuum of

materials with multicolor mechanochromism, particularly under mild conditions, remains challenging due to the lack of universal design principles. So far, only scattered reports mentioned such multicolor mechanochromic materials.37−39 For example, Kato’s group found a single liquid crystalline luminophore exhibiting tricoloured mechanochromic luminescence at different phases;37 Tian demonstrated an anthracene derivative showing variable emission under normal to extreme pressures up to 7.92 GPa;38 and most recently, Jia presented a pyrene and rhodamine B based fluorophore with tricolored mechanochromism.39 Our previous study indicates that twisted D−A structured AIE luminogens are promising candidates as efficient mechanochromic materials with high contrast.32,33 For these compounds, force-induced conformation planarization will result in red-shifted emission. In this study, we designed and synthesized a new D−A conjugate consisting of arylamine and TPAN units (AN2TPAN, Figure 1), whose conformations are

Figure 1. Structure of AN2TPAN and photographs for the THF solution and single crystals of AN2TPAN. The emission efficiency of AN2TPAN solution (ΦF,s) and crystals (ΦF,c) is denoted in the photographs.

sterically crowded and remarkably twisted. It is found that AN2TPAN exhibits typical ICT and AIE characteristics with high solid-state efficiency, aggregation-enhanced two-photon absorption (2PA) property, and excellent OLED performance. Moreover, it displays distinct multicolor mechanochromism upon manual grinding.

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EXPERIMENTAL SECTION Materials. Sodium tert-butoxide (t-BuONa), sodium hydride (CaH2), and tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3] were obtained from TCI. Tri-tert-butylphosphine [(t-Bu)3P, 0.5 M in toluene] was purchased from Puyang Huicheng Electronic Material Co., Ltd. (China). Benzene, dichloromethane (DCM), and toluene were distilled under normal pressure from CaH2 under nitrogen immediately prior to use. Tetrahydrofuran (THF) was distilled from sodium/ benzophenone under nitrogen before use. Aniline (AN) was dried with CaH2 and distilled under reduced pressure before use. 2-(4-Bromophenyl)-3,3-diphenylacrylonitrile (TPAN−Br) was synthesized according to the reference method.40 Dimethylformamide (DMF) and other commercially available reagents were used without further purification. Instruments. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a Bruker AMX-400 NMR spectrometer in CDCl3 at room temperature; chemical shifts were reported in ppm relative to tetramethylsilane (TMS, δ = 0 ppm). The mass spectrum was recorded on a Varian 500-MS ion trap mass spectrometer with an APCI ion source. UV−vis absorption and emission measurements were performed on a TU-1901 UV−vis spectrophotometer and a PerkinElmer LS 55 luminescence spectrometer, respectively. The emission quan10999

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Scheme 1. Synthetic Route to AN2TPAN

Figure 2. (A) Absorption and (B) emission spectra of AN2TPAN in varying solvents. Concentration = 20 μM; excitation wavelength = 390 nm.

Figure 3. B3LYP/6-31G(d) calculated molecular orbital amplitude plots of (A) HOMO and (B) LUMO levels for AN2TPAN.

about 10−6 Torr. Finally, aluminum (Al) metal was evaporated in another vacuum chamber without breaking the vacuum. The thicknesses of the films were determined by quartz thickness monitors. The active area of the EL device, defined by the overlap of the ITO and the cathode electrode, was 3 mm × 3 mm. Current density−voltage and current efficiency−current density characteristics were measured with a computer controlled Keithley 2400 Source Meter and BM-7A Luminance Colorimeter. The electroluminescence spectrum was measured by a Labsphere CDS-610. All measurements were carried out under air at room temperature without device encapsulation.

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RESULTS AND DISCUSSION AN2TPAN was smoothly obtained by palladium catalyzed Buchwald-Hartwig C−N coupling between AN and TPAN−Br (Scheme 1).41 It shows good solubility in common organic solvents, such as THF, DCM, chloroform, DMF, and acetone. The structure of AN2TPAN was fully characterized by mass and NMR spectra (Figures S1−S3, Supporting Information) as

Figure 4. (A) PL spectra of AN2TPAN in THF and THF/water mixtures. (B) Plots of PL peak intensity vs water fraction ( f w). Concentration = 20 μM; excitation wavelength = 390 nm. The inset graphs are the solution of AN2TPAN in THF (f w = 0) and its suspensions in THF/water mixture with f w = 90% under 365 nm UV light illumination.

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Figure 5. (A) Emission spectrum and (B) open-aperture Z-scan trace of AN2TPAN in 10/90 THF/water mixture excited with an 800 nm laser pulse. The inset panels in part A are the photographs of AN2TPAN in THF and 10/90 THF/water excited with an 800 nm laser pulse. Concentration = 1 mM.

Figure 6. (A) Emission spectra, (B) fluorescent photographs, and (C) XRD patterns of the as prepared solids (aps), ground powders, and heated powders of AN2TPAN. For part A, the excitation wavelength is 390 nm.

Figure 7. (A) ORTEP drawing, (B) intermolecular interactions, and (C) molecular packing of AN2TPAN crystals.

well as single crystal analysis (Table S1, Supporting Information), with satisfactory results obtained. AN2TPAN shows two absorption bands at 293 and 399 nm in THF, which are attributable to the π−π* and ICT transitions, respectively (Figure 2A). The absorption profiles of AN2TPAN vary little in different solvents, suggesting their solvent polarity independent ground state electronic structures,

whereas the emissions exhibit an obvious bathochromic shift (Figure 2B), with emission maxima at 547, 565, 580, and 605 nm in toluene, THF, DCM, and DMF, respectively. The optimized molecular geometries and electron densities of AN2TPAN have been calculated by density functional theory (DFT) in Gaussian 09 at the B3LYP/6-31G(d) level. The electron density of the HOMO state is mainly localized on 11001

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electron-donating triphenylamine, whereas that of the LUMO level is principally localized on electron-withdrawing TPAN units (Figure 3), thus testifying its charge transfer nature. When AN2TPAN is dissolved in organic solvents, no visible emission is observed (Figure S4, Supporting Information). The photoluminescence (PL) quantum yields of AN2TPAN in toluene, THF, DCM, and DMF are as low as 0.22, 0.18, 0.18, and 0.03% with lifetimes (τ) of 1.11, 0.90, 0.83, and 0.78 ns, respectively. However, in crystals, it emits strong green light (520 nm) with quantum efficiency up to 99%, indicating that it is AIE-active (Figure 1). Emission spectra of AN2TPAN in THF and THF/water mixtures were also measured. Water is chosen because it is a typical nonsolvent for AN2TPAN, in which AN2TPAN molecules will be aggregated. When the water fraction (f w) is ≤70%, only weak signals are recorded, since the molecules are still genuinely dissolved in the mixture. However, when f w is increased to 80%, the emission intensity is swiftly boosted due to the molecular aggregation, giving a strong yellow emission at 560 nm (Figure 4). With further addition of water, the emission intensity is continuously strengthened. In the 10/90 THF/water mixture, the emission is remarkably enhanced by ∼98-fold compared to that in THF (Figure 4B), with a much longer τ value of 4.25 ns (Figure S5, Supporting Information). The AIE nature of AN2TPAN also can be seen from the vivid emission contrast of AN2TPAN in THF and 10/90 THF/water mixture. While no visible emission is detected from the former, bright yellow light is observed from the latter (Figure 4B). The efficient solid-state emission and ICT characteristics of AN2TPAN promoted us to explore its potential application as 2PA materials.42,43 When illuminated with an 800 nm

Figure 8. Emission spectra of AN2TPAN absorbed on MC, cotton, and TLC substrates and embedded in PMMA film.

Figure 9. (A) DSC and (B) TGA thermograms of AN2TPAN recorded under nitrogen at scan rates of 10 and 20 °C/min, respectively. The DSC curve is recorded at the first heating circle.

Figure 10. (A) EL spectra of AN2TPAN, (B) luminance−voltage−current density plots, (C) current efficiency and power efficiency vs driving voltage, and (D) external quantum efficiency−voltage plot of the EL device with the configuration of ITO/MeO−TPD (50 nm)/TCTA (10 nm)/ AN2TPAN (20 nm)/Bphen (40 nm)/Liq (1 nm)/Al (100 nm). 11002

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the homogeneous ground powders exhibit a uniform color (Figure 6B), their inhomogeneous admixtures depict two distinct colors (Figure S7B, Supporting Information). To the best of our knowledge, it is the first example that a compound can exhibit more than three emission colors through facile manual grinding, which is highly applicable in sensitive shear and/or pressure sensors.27−31 The single crystal structure of AN2TPAN was obtained to further understand its AIE and mechanochromic behaviors. As shown in Figure 7, it adopts extremely twisted conformations, which makes it highly rotatable in solvents. Such active intramolecular rotations effectively consume the exciton energies, thus making AN2TPAN nonemissive in solutions. Upon aggregation, these intramolecular rotations are highly impeded; meanwhile, no π−π stackings between aromatic rings are formed owing to its propeller-like conformations, thus giving greatly boosted emissions. Specifically, multiple C−H··· H−C (2.390 Å) and C−H···π (2.855 Å) contacts and C−H···N (2.649 Å) hydrogen bonds are present between adjacent molecules in the crystals (Figure 7B), which further rigidify the molecular conformations, thus rendering AN2TPAN highly emissive in the crystalline states. Meanwhile, the presence of large amounts of cavities in the crystals (Figure 7C) makes the molecules highly slideable and compressible. Upon grinding, while intermolecular interactions are destroyed, the molecules are also forced to adopt less twisted conformations, thus giving red-shifted emissions.33 Mechanochromism of AN2TPAN suggests that the emission color of AN2TPAN can be modulated through tuning its conformation. This is further verified by the emission behaviors of AN2TPAN on solid substrate and in polymer matrix. After being immersed in the THF solution of AN2TPAN (20 μM) for several seconds, methyl cellulose (MC), cotton, and TLC plate were quickly taken out. After solvent evaporation, absorbed AN2TPAN molecules on these substrates exhibit different emissions centered at 538, 558, and 579 nm, respectively (Figure 8), which should be ascribed to their varied conformations. However, as a solid solution in poly(methyl methacrylate) (PMMA) film, AN2TPAN shows an even bluer emission (512 nm) compared to that of the crystals (520 nm), indicating that AN2TPAN adopts more twisted conformations in PMMA matrix. It is understandable that AN2TPAN molecules have enough time and activity to adjust their molecular conformations to highly twisted stable states during the drop casting and solvent evaporation processes. Finally, such twisting conformations are further fixed by the formation of rigid PMMA film. However, fast solvent evaporation and AN2TPAN−substrate interaction for other systems may generate kinetically stable conformations with different planarization degrees and thus varying ECLs, therefore resulting in different emissions. These results suggest another facile approach to modulate the solid-state emissions of AN2TPAN. It is noted that these solid-state emissions are also highly efficient due to the AIE nature of AN2TPAN. Such tunable and efficient solid-state emissions of AN2TPAN render it attractive in optoelectronic applications. Thermal properties of AN2TPAN were also investigated. High thermal and morphological stability is greatly desired, since it is critical for the device fabrication and operation. The differential scanning calorimetry (DSC) curve shows that the glass-transition (Tg), crystalline (Tc), and melting (Tm) temperatures of AN2TPAN are 90, 166, and 245 °C (Figure 9A), respectively, suggesting its good morphological stability.

femtosecond laser pulse, the THF solution of AN2TPAN gives no emission, whereas its suspensions in 10/90 THF/water mixture (1 mM) show a bright yellow emission at 547 nm (Figure 5A). This result suggests that AN2TPAN can be excited via a two-photon process and the two-photon excited fluorescence is remarkably intensified by aggregate formation, namely, displaying aggregation-enhanced 2PA characteristics. Figure 5B shows the Z-scan trace of AN2TPAN nanosuspensions, from which the 2PA cross section of the suspensions is calculated as 5782 GM,44 which is among the highest values for AIE luminogens.13−15,35 Such high 2PA cross section of AN2TPAN might be ascribed to its reasonable conjugation and appropriate ICT feature.37 Meanwhile, the sterically crowded and remarkably twisted conformations and high crystalline efficiency of AN2TPAN make it also promising as mechanochromic luminogen with high contrast. Indeed, upon illumination, the as prepared powders that recrystallized from THF/n-hexane give a strong green emission at 524 nm; however, after gentle grinding with a spatula, they show a yellowish green luminescence peaked at 533 nm (G1) (Figure 6A,B). Continuous grinding affords tunable emission colors of light yellow (G2) and yellow (G3) with maxima at 543 and 554 nm (Figure 6A,B), respectively. Apparently, AN2TPAN can yield multiple solid-state emission colors through facile mechanical grinding. Normally, mechanochromic luminogens exhibit only two switchable emission colors, 27−36 and three or more emissions are rarely observed,37−39 particularly for those under mild conditions. When annealed at 160 °C under nitrogen for 10 min or fumed with acetone vapor, the yellow emission of G3 is restored to 524 nm (Figure 6A). The conversion between the green- and yellow-emissive solids as well as their corresponding emission spectra can be repeated many times without fatigue due to the nondestructive nature of the stimuli (Figure S6, Supporting Information). To gain more insight into the mechanism, powder X-ray diffraction (XRD) analysis was conducted. While the as prepared solids exhibit many intense and sharp diffraction peaks, which are indicative of their regular crystalline structure, after grinding, G1 shows rather weak signals. Much weaker (G2) and even no (G3) diffraction peaks are observed with continuous mechanical grinding, indicating their gradually decreased crystalline fractions (Figure 6C). Upon thermal or solvent treatment, however, sharp diffraction peaks emerge again due to the recrystallization of the molecules. Therefore, it is assumed that the emission color is associated with the crystallinity of AN2TPAN powders. For the crystalline as prepared powders, the luminogens adopt a more twisted conformation, which shortens their effective conjugation length (ECL) and hence makes them emit bluer light. On the other hand, for the amorphous solids (G3), the ordered crystalline lattice collapses by mechanical stimuli and the dye molecules are forced to a more planar conformation, which increases the ECL and thus renders them to emit redder PL. For G1 and G2 with different grinding degrees, they are the relatively homogeneous mixtures with different ratios of tiny microcrystals to amorphous solids, whose emissions are actually the overlapped contributions from both crystalline and amorphous components, thus generating varied emission colors with different emission maxima. It is further proved by the emission measurement of the inhomogeneous admixtures of as prepared solids and ground powders, whose emission maximum is gradually red-shifted with increasing amorphous fraction (Figure S7A, Supporting Information). However, while 11003

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The Journal of Physical Chemistry C Meanwhile, thermal gravimetric analysis (TGA) indicates a moderate decomposition temperature (Td, at which a sample loses its 5% weight) of 332 °C for it (Figure 9B). Due to the efficient solid-state emission, reasonable thermal stability, and high morphological stability of AN2TPAN, we studied the electroluminescence (EL) of it by formation of a multilayer nondoped OLED with the device configuration of ITO/MeO−TPD (50 nm)/TCTA (10 nm)/AN2TPAN (20 nm)/Bphen (40 nm)/Liq (1 nm)/Al (100 nm), where MeO− TPD and TCTA are used as hole transporting layers and Bphen and Liq are used as electron transporting layers. The device shows a yellow EL peaked at 560 nm, which agrees well with those of the nanosuspensions (560 nm) and amorphous solids (554 nm) of AN2TPAN, indicating the EL is originated from the amorphous AN2TPAN film. The device performs excellently: it is turned on at 4.8 V, with a maximum luminance (Lmax), current efficiency (CEmax), power efficiency (PEmax), and external quantum efficiency (EQE) of 11430 cd/m2, 10.7 cd/A, 4.9 lm/W, and 3.3%, respectively (Figure 10). Notably, the device also acts well under large current density with high EQE (Figure 10D), which might be ascribed to the bipolar characteristic of AN2TPAN and balanced charge transport of the device. Though the device is not yet optimized, the results clearly demonstrate that AN2TPAN is promising for advanced OLED applications.24

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CONCLUSION In summary, to achieve multifunctional and mechanochromic optoelectronic materials, D−A structured AN2TPAN with sterically crowded and remarkably twisted conformations was designed and synthesized. It exhibits typical ICT and AIE characteristics. On one hand, the AIE nature of AN2TPAN makes it highly emissive in the aggregated states; specifically, in the crystals, effective intermolecular interactions further rigidify the molecular conformations, thus endowing it with a high ΦF,c of 99%. On the other hand, the ICT nature endows it with tunable emissions in various solvents and balanced charge transport in the thin films. Additionally, the AIE and ICT features render AN2TPAN molecules with the aggregationenhanced 2PA characteristics. For the suspensions of AN2TPAN, a large 2PA cross section of 5782 GM is obtained. Moreover, AN2TPAN molecules show tunable multicolor mechanochromism upon mechanical stimuli. Notably, it is the first example that a solid luminogen can display more than three emission colors upon manual grinding. The high solidstate efficiency, efficient EL performance, high 2PA cross section, and tunable multicolor mechanochromism of AN2TPAN make it highly applicable in emerging areas. ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR spectra, mass spectrum, single crystal data of AN2TPAN, and other data. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

The authors thank Yi Zeng for the 2PA measurement. This work was financially supported by the National Science Foundation of China (21104044), the National Basic Research Program of China (973 Program, 2013CB834704), and the Ph.D. Programs Foundation of Ministry of Education of China (20110073120040). W.Z.Y. thanks the Start-up Foundation and SMC-Chenxing Young Scholar Program of Shanghai Jiao Tong University.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 86-21-34202613. *E-mail: [email protected]. Phone: 86-21-34202613. Notes

The authors declare no competing financial interest. 11004

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp500556p | J. Phys. Chem. C 2014, 118, 10998−11005