Published: March 03, 2011

r 2011 American Chemical Society 666 dx.doi.org/10.1021/jz200140c | J. Phys. Chem. Lett. 2011, 2, 666–670

LETTER

pubs.acs.org/JPCL

Multi-Stimuli-Responsive Fluorescence Switching of a Donor-Acceptor π-Conjugated CompoundChuandong Dou, Liang Han, Shanshan Zhao, Hongyu Zhang,* and Yue Wang*

State Key Laboratory of Supramolecular Structure andMaterials, College of Chemistry, Jilin University, Changchun 130012, P. R. China.

bS Supporting Information

Tuning and controlling the photophysical properties of lumi-nescent materials by changing environmental factors has

been intensively studied in terms of fundamental research andpractical applications in the fields of sensing, memories, detec-tion, and display devices.1-9 Particularly, stimuli-responsivefluorescence switching, such as piezochromism, vapochromism,and thermo- and acid-dependent luminescence of smart lumi-nescent materials, has received unprecedented attentions.10-15

Piezochromic luminescence has been obtained by changingmolecular packing modes of solid materials at different phasesthrough grinding and heating treatments.16-21 Vapochromicorganic materials were designed in an effective strategy of tuningweak intermolecular interactions such as π-stacking and hydro-gen bonding.22 Thermo- and acid-dependent luminescent ma-terials also owned pronounced molecular structures, whoseconformations or frontier molecular orbital (FMO) could beaffected by thermo, acid, or base.23 However, reports on multi-stimuli-responsive fluorescence switching of one chromophoreare quite rare,24,25 which is due to the lack of clear guidelines forthe design of molecular structure synchronously possessing allthe features of different smart materials.

Herein, we synthesized an electron donor-acceptor (D-A)structured compound 1 based on the following specific designprinciples. First, the rod-like π-conjugated framework endowsthis molecule with abundant aggregation modes in the solidstates, and the cyano stilbene group can rotate freely, thusresulting in interesting emission properties in solution.26 Second,the aromatic-amine group being responsive to acid stimulus mayinduce significant changes in photophysical properties by dis-rupting the D-A nature.27,28 Finally, the cyano and bulky CF3groups are expected to dramatically affect the molecular packingmodes by hydrogen bonding interactions.29 In the course of

exploring the effects of environmental stimulus on the lumines-cent properties of this fluorophore, we found that the synthesizedorange-red emissive solids exhibited interesting piezochromicfluorescence by the stimuli of grinding and heating, and theground sample showed vapochromic emission upon exposure toorganic vapor at ambient conditions. The solution of thiscompound and its protonated sample at room temperature(298 K) or under frozen conditions (77 K) gave totally differentabsorption and emission properties.

Compound 1 was synthesized by a Knoevenagel reactionunder gentle conditions in good yield (Scheme S1, SupportingInformation). A column chromatography followed by vacuumsublimation gave rise to the pure product, which was fullycharacterized by 1H NMR, mass spectra, element analyses, andfinally X-ray crystal analysis. Upon being excited with UV light,the obtained orange powder and crystals exhibited bright orange-red fluorescence with peaks centered at 587 (Φf = 0.15) and599 nm (Φf = 0.14), respectively, and it was surprisingly changedinto a relatively weak yellow emissive solid (λem = 549 nm,Φf =0.02) after grinding treatment by a spatula or a pestle (Figures 1and 2). The produced ground solid could convert into orangeemissive state upon heating at about 80 �C over 5 min (λem =568 nm,Φf = 0.10). Thus, the phenomenon of the fluorescenceswitched by grinding and heating displays the typical feature ofpiezochromism. Interestingly, the ground sample showed vapo-chromism, i.e., exposing the yellow solid to organic vapor such asCH2Cl2 and CHCl3 could result in the fluorescence changingfrom yellow to orange (Figure 1).

Received: January 29, 2011Accepted: February 22, 2011

ABSTRACT: An electron donor-acceptor structured π-conjugated organiccompound 1 composed of trifluoromethyl-biphenyl and cyano-stilbene-aminewas designed and exhibited multi-stimuli-responsive fluorescence switchingbehaviors. The synthesized solid exhibited piezochromism in that grinding andheating could change the emission colors between orange-red and yellow. Theamorphous 1 also showed interesting vapochromic behavior in that organic vaporcould convert the yellow color into orange. The solution of 1 exhibited nearly nofluorescence at room temperature and intensive yellowish green emission at 77 K,while adding CF3COOH (TFA) resulted in green emissive state at roomtemperature and blue fluorescent state at 77 K.

SECTION: Electron Transport, Optical and Electronic Devices, Hard Matter

667 dx.doi.org/10.1021/jz200140c |J. Phys. Chem. Lett. 2011, 2, 666–670

The Journal of Physical Chemistry Letters LETTER

To gain insight into the mechanism of the present piezo-chromism, several measurements were performed on the ground,unground, and heated samples. The thermal properties of thesesolids were studied by differential scanning calorimentry (DSC).As shown in Figure 3a, the unground sample (sublimatedpowder) displays one endothermic peak at 211 �C, correspond-ing to its melting point. In contrast, the ground sample firstexperiences an exothermic process at relatively lower tempera-ture (about 79 �C) and then melts at 208 �C. The newlyappeared exothermic peak might be ascribed to a phase-to-phasetransition, which suggests that the grinding can convert thethermodynamic phase to a metastable state.30,31 It is notablethat the heated sample only shows an endothermic peak at207 �C, which is quite similar to the melting temperature of theground sample and slightly lower than that of the sublimatedsolid. This is probably because looser and a little more disorderedmolecular packing exists in the heated solid compared with thatin the sublimated powder.

Powder X-ray diffraction (PXRD) pattern of the ungroundsolid clearly shows intensive and sharp reflection peaks(Figure 3b), indicative of a well-ordered microcrystalline-likestructure.32 In contrast, the XRD profile of the ground solid givesno noticeable diffraction, reflecting its amorphous feature in thisstate. The XRD pattern of the heated sample was nearly the sameas that of the unground powder sample, although the diffraction

peaks decreased in intensity, suggesting that the heating treat-ment can convert the amorphous phase to crystalline statethrough molecular repacking. Thus, the piezochromism can beunderstood as a phase transition process: grinding disrupts thesublimated crystalline solid into an amorphous state, in whichmolecular repacking occurred by a simple heating treatment.Fortunately, the orange-red emissive single crystals were ob-tained via slowly evaporating the CH2Cl2 solution of 1, althoughthe crystal quality is not very good. The simulated PXRD patternof the crystal matched well with that of the unground and heatedsamples. Therefore, investigations on the molecular packing ofthe crystal are very helpful for further understanding the presentpiezochromic behavior.

As shown in Figure 3c, the biphenyl-stilbene skeleton is nearlyplanar, while the stilbene-aromatic-amine framework has a tor-sion angle of about 16.1�. The molecules stacked in a face-to-facepacking manner with a slip angle of about 35.4� and an interlayerdistance of about 3.6 Å, consistent with the reported π-πstacking distances (Figure 3d).33 An important structural featureof the molecular packing is that the electron-donating groups ofaromatic-amine and electron-accepting cyano moieties over-lapped between adjacent molecules. The molecules with suchpacking manner formed a one-dimensional (1D) molecular

Figure 1. Molecular structure of compound 1 and stimuli-responsive behaviors of 1 upon grinding, heating, and acid and base vapor treatments.

Figure 3. DSC (a) and PXRD (b) profiles of 1 at different states.Molecular (c) and packing (d) structure in the crystal.

Figure 2. Emission spectra of powder, ground, heated, and crystalsamples.

668 dx.doi.org/10.1021/jz200140c |J. Phys. Chem. Lett. 2011, 2, 666–670

The Journal of Physical Chemistry Letters LETTER

column, which contacted with four other columns by the weakCtN 3 3 3H-C (2.70 Å in distance) and C-F 3 3 3H-C (2.8-2.9 Å in distance) interactions (Figure S1). In the molecularcolumns, the strong intermolecular π-π and charge transferinteractions may induce red-shift fluorescence compared withthe monomer or amorphous states.34,35 The visual observation ofmolecular packing in the crystal implied the importance ofmolecular design, namely, the D-A structured π-conjugatedframework, the cyano, and bulky CF3 groups all played effectiveroles in the stacking process, hence the luminescent properties ofthe bulk sample in various states.

In the absorption spectra, a chloroform solution (1.0� 10-5M)and the ground sample of compound 1 both show a mainabsorption peak at 442 nm together with a weak absorption bandcentered at 282 nm (Figure 4a). Upon heating the groundsample, the absorption band broadened and shifted its maximumpeak to 512 nm, which is identical to that of the ungroundpowder. These spectral changes strongly support the disruptionand healing effects of grinding and heating stimulus, respectively.Compared with the emission spectra of the powder and crystal,the heated sample exhibits around 30-40 nm a blue-shiftemission peak, which may be due to their different molecularstacking processes: the heating is a possible fast molecularrepacking process in which the transition from amorphous stateto crystalline state is incomplete within 10 min; the sublimationand growth of crystal are slow molecular stacking courses bystrong molecular interactions. The sublimation takes over 5 h invacuum conditions, and the growth of a single crystal takes over 1week in the solution state. Long packing time and slow packingrate could make the molecules completely stack in the crystallineface-to-face model and result in red-shift emissions.

Another interesting phenomenon of the present compoundwas its responsive fluorescence behaviors stimulated by tempera-ture, acid, or base (Figures 1 and 4). The chloroform solutionemits nearly unobservable fluorescence at room temperature

(298 K) and intense yellowish green fluorescence under frozenconditions (77 K) with an emission peak centered at 523 nm(Figure 4b). The fluorescence enhancement in the solution andsolid state is similar to the aggregation-induced emission (AIE)phenomenon reported by the Tang and Park groups,respectively.36,37 For the molecule 1, the cyano stilbene grouprotating freely in the solution can be effectively restricted to blockthe nonradiative relaxation pathways under frozen conditions,resulting in the enhancement of monomer fluorescence. Theobserved intense green emission (λem = 529 nm) of 1 in viscoussolvent such as glycerol further supports that this compoundpossesses the AIE feature (Figure S2). In the case of the powderstate, the molecular conformation generates intramolecularplanarization, endowing the solids with intensive red-shiftedemissions.38 After addition of an excess amount of CF3COOH(TFA), the yellow solution became colorless and transparentwith the fluorescence switching into green, and the emissionspectrum give two main fluorescence peaks at 495 and 408 nm.Cooling the solution to 77K gave a bright blue emissive state, andthe emission peak is centered at 403 nm with the peak at 495 nmnearly disappearing. Interestingly, adding triethylamine (NEt3)to this system could restore its initial yellow state, and thesetransitions could be repeated many times. Considering that thearomatic-amine group has a Lewis-basic nitrogen, we believe thatthe protonation and deprotonation were predominant in thetransition process.27 The protonation process was monitored by1H NMR measurements (Figure S3). The molecule 1 in CDCl3exhibited its characteristic NMR signals, and the addition of anexcess of TFA resulted in obvious downfield shift signals,especially that of the protons close to the aromatic amine(protons e and d). These changes could be ascribed to theprotonation effects of TFA on Lewis-basic nitrogen. The pro-tonation can effectively disrupt the D-A nature and then affectthe photophysical property of the entire molecule as reported bythe Bunz group.16,20 In this sense, it is reasonable to observe the

Figure 4. (a) UV/vis absorption spectra of the solution and solids at different states of 1 and (b) emission spectra of the chloroform solution respondingto environmental stimulus. (c) Digital photographs of the stimuli-responsive behaviors of the chloroform solution in NMR tube.

669 dx.doi.org/10.1021/jz200140c |J. Phys. Chem. Lett. 2011, 2, 666–670

The Journal of Physical Chemistry Letters LETTER

changed emission colors and the new absorption peak at 325 nmafter adding TFA. Freezing and protonation could effectivelyrestrict the molecular rotation and inflect the intramolecularD-A nature, respectively, making the solution emit intense bluefluorescence. Additionally, these acid and base stimuli-responsivebehaviors can be performed on the solid (Figure 1). Exposingthe sublimated or heated sample to TFA can result in a blueemissive state, and NEt3 vapor leads the fluorescence to return toorange. The acid and base stimuli-responsive behaviors in solutionand solid may have potential applications in biochemistry andsensor fields.

In conclusion, an electron D-A structured compound 1 wassynthesized, and its luminescent colors can be smartly switchedby various environmental stimulus including mechanical force,organic vapor, thermo, acid, and base. Grinding and heatingtreatments effectively induce the fluorescence changes betweenorange-red and yellow colors by tuning the molecular packing inthe solid states. Upon exposure to TFA vapor, the orange solidsexhibited a phase transition to blue emissive state, and NEt3vapor can restore its original color. Notably, the solution withnearly unobservable fluorescence changes into weak greenemissive state by adding TFA because of the protonation effect,and freezing these two solutions can dramatically enhance theirfluorescence intensity. The molecular design strategy towardmulti-stimuli-responsive fluorescence switching materials may bewidely applicable to the design of other functional materials, andfurther studies along this line are currently in progress in ourlaboratory.

’ASSOCIATED CONTENT

bS Supporting Information. Synthesis and characteriza-tions of compound 1, crystal structures in PDF and cif formats,emission spectrum of 1 in glycerol, as well as the NMR data. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: hongyuzhang@jlu.edu.cn (H.Z.); yuewang@jlu.edu.cn(Y.W.). Fax: þ86-431-85193421. Tel: þ86-431-85168484.

’ACKNOWLEDGMENT

This work was supported by the National Natural ScienceFoundation of China (50733002 and 50903037), theMajor StateBasic Research Development Program (2009CB623600), andthe Program for Changjiang Scholars and Innovative ResearchTeam in University (IRT0422).

’REFERENCES

(1) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T.Organic Chemistry: A Digital Fluorescent Molecular Photoswitch.Nature 2002, 420, 759–760.(2) Toal, S. J.; Jones, K.; Magde, A. D.; Trogler, W. C. Luminescent

Silole Nanoparticles as Chemoselective Sensors for Cr(VI). J. Am. Chem.Soc. 2005, 127, 11661–11665.(3) Lim, S.-J.; An, B.-K.; Jung, S. D.; Chung, M.-A.; Park, S. Y.

Photoswitchable Organic Nanoparticles and a Polymer Film EmployingMultifunctional Molecules with Enhanced Fluorescence Emission andBistable Photochromism. Angew. Chem., Int. Ed. 2004, 43, 6346–6350.

(4) Pauluth, D.; Tarumi, K. Advanced Liquid Crystals for Television.J. Mater. Chem. 2004, 14, 1219–1227.

(5) Che, Y.; Yang, X.; Zang, L. Ultraselective Fluorescent Sensing ofHg2þ through Metal Coordination-Induced Molecular Aggregation.Chem. Commun. 2008, 1413–1415.

(6) Babu, S. S.; Kartha, K. K.; Ajayaghosh, A. Excited State Processes inLinearπ-System-BasedOrganogels. J. Phys. Chem. Lett.2010, 1, 3413–3424.

(7) Srinivasan, S; Babu, P. A; Mahesh, S.; Ajayaghosh, A. ReversibleSelf-Assembly of Entrapped Fluorescent Gelators in Polymerized Styr-ene Gel Matrix: Erasable Thermal Imaging via Recreation of Supramo-lecular Architectures. J. Am. Chem. Soc. 2009, 131, 15122–15123.

(8) Deniz, E.; Sortino, S.; Raymo., F. M. Fluorescence Switchingwith a Photochromic Auxochrome. J. Phys. Chem. Lett. 2010, 1,3506–3509.

(9) Gu., J.; Xie., G.; Zhang., L.; Chen., S.; Lin, Z.; Zhang., Z.; Zhao.,J.; Xie., L.; Tang., C.; Zhao., Y.; et al. Dumbbell-Shaped SpirocyclicAromatic Hydrocarbon to Control Intermolecular π-π Stacking Inter-action for High-Performance Nondoped Deep-Blue Organic Light-Emitting Devices. J. Phys. Chem. Lett. 2010, 1, 2849–2853.

(10) Beyer, M. K.; Clausen-Schaumann, H.Mechanochemistry: TheMechanical Activation of Covalent Bonds. Chem. Rev. 2005, 105,2921–2948.

(11) Balch, A. L. Dynamic Crystals: Visually Detected Mechano-chemical Changes in the Luminescence of Gold and Other Transition-Metal Complexes. Angew. Chem., Int. Ed. 2009, 48, 2641–2644.

(12) Matsushima, R.; Nishimura, N.; Goto, K.; Kohno, Y. Vapo-chromism of Ionic Dyes in Thin Films of Sugar Gels. Bull. Chem. Soc. Jpn.2003, 76, 1279–1283.

(13) Gao, J.; Wang, H.; Wang, L.; Wang, J.; Kong, D.; Yang, Z.Enzyme Promotes the Hydrogelation from a Hydrophobic SmallMolecule. J. Am. Chem. Soc. 2009, 131, 11286–11287.

(14) Chen, J.; Xu, B.; Ouyang, X.; Tang, B.; Cao, Y. Aggregation-Induced Emission of cis,cis-1,2,3,4-Tetraphenylbutadiene from RestrictedIntramolecular Rotation. J. Phys. Chem. A 2004, 108, 7522–7526.

(15) Zucchero, A. J.; Mcgrier, P. L.; Bunz, U. H. F. Cross-ConjugatedCruciform Fluorophores. Acc. Chem. Res. 2010, 43, 397–408.

(16) Sagara, Y.; Mutai, T.; Yoshikawa, I.; Araki, K. Material Designfor Piezochromic Luminescence: Hydrogen-Bond-Directed Assembliesof a Pyrene Derivative. J. Am. Chem. Soc. 2007, 129, 1520–1521.

(17) Kunzelman, J.; Kinami,M.; Crenshaw, B. R.; Protasiewicz, J. D.;Weder, C. Oligo(p-phenylene vinylene)s as a “New” Class of Piezo-chromic Fluorophores. Adv. Mater. 2008, 20, 119–122.

(18) Yoon, S.-J.; Chung, J. W.; Gierschner, J.; Kim, K. S.; Choi, M.-G.;Kim, D.; Park, S. Y. Multistimuli Two-Color Luminescence Switching viaDifferent Slip-Stacking of Highly Fluorescent Molecular Sheets.J. Am. Chem. Soc. 2010, 132, 13675–13683.

(19) Davis, D. A.; Hamilton, A.; Yang., J.; Cremar, L. D.; Gough,D. V.; Potisek, S. L.; Ong,M. T.; Braun, P. V.;Martínez, T. J.;White, S. R.Force-Induced Activation of Covalent Bonds in MechanoresponsivePolymeric Materials. Nat. Chem. 2009, 1, 605–610.

(20) Li, H.; Zhang, X.; Chi, Z.; Xu, B.; Zhou, W.; Liu, S.; Zhang, Y.;Xu, J. New Thermally Stable Piezofluorochromic Aggregation-InducedEmission Compounds. Org. Lett. 2011, 13, 556–559.

(21) Yoon, S.-J.; Park, S. Y. Polymorphic and MechanochromicLuminescence Modulation in the Highly Emissive Dicyanodistyrylben-zene Crystal: Secondary Bonding Interaction in Molecular StackingAssembly. J. Mater. Chem. 2011, 10.1039/c0jm03711g.

(22) Takahashi, E.; Takaya, H.; Naota, T. Dynamic VapochromicBehaviors of Organic Crystals Based on the Open-Close Motions ofS-Shaped Donor-Acceptor Folding Units. Chem.—Eur. J. 2010,16, 4793–4802.

(23) Tolosa, J.; Solntsev, K. M.; Tolbert, L. M.; Bunz, U. H. F.Unsymmetrical Cruciforms. J. Org. Chem. 2010, 75, 523–534.

(24) Dautel, O.; Robitzer, M.; L�ere-Porte, J.-P.; Serein-Spirau, F.;Moreau, J. Self-Organized Ureido Substituted Diacetylenic Organogel.Photopolymerization of One-Dimensional Supramolecular Assembliesto Give Conjugated Nanofibers. J. Am. Chem. Soc. 2006, 128,16213–16223.

670 dx.doi.org/10.1021/jz200140c |J. Phys. Chem. Lett. 2011, 2, 666–670

The Journal of Physical Chemistry Letters LETTER

(25) Suzuki, T.; Shinkai, S.; Sada, K. Supramolecular CrosslinkedLinear Poly(Trimethylene Iminium Trifluorosulfonimide) PolymerGels Sensitive to Light and Thermal Stimuli. Adv. Mater. 2006,18, 1043–1046.(26) An, B.-K.; Lee, D.-S.; Lee, J.-S.; Park, Y.-S.; Song, H.-S.; Park,

S. Y. Strongly Fluorescent Organogel System Comprising Fibrillar Self-Assembly of a Trifluoromethyl-Based Cyanostilbene Derivative. J. Am.Chem. Soc. 2004, 126, 10232–10233.(27) Zucchero, A.; Tolosa, J.; Tolbert, L.; Bunz, U. Bis(40-dibutyla-

minostyryl)benzene: Spectroscopic Behavior upon Protonation orMethylation. Chem.—Eur. J. 2009, 15, 13075–10381.(28) Nishizaka, M.; Mori, T.; Inoue, Y. Conformation Elucidation of

Tethered Donor-Acceptor Binaphthyls from the Anisotropy Factor ofa Charge-Transfer Band. J. Phys. Chem. Lett. 2010, 1, 2402–2405.(29) Zhao, Y.; Gao, H.; Fan, Y.; Zhou, T.; Su, Z.; Liu, Y.; Wang, Y.

Thermally Induced Reversible Phase Transformations Accompanied byEmission Switching Between Different Colors of Two Aromatic-AmineCompounds. Adv. Mater. 2009, 21, 3165–3169.(30) Tsukuda, T.; Kawase,M.;Dairiki, A.;Matsumoto, K.; Tsubomura,

T. Brilliant Reversible Luminescent Mechanochromism of Silver(I)Complexes Containing o-Bis(diphenylphosphino)benzene and Phos-phinesulfide. Chem. Commun. 2010, 46, 1905–1907.(31) Mizuguchi, J.; Tanifuji, N.; Kobayashi, K. Electronic and

Structural Characterization of a Piezochromic Indigoid: 11-(30-Oxodi-hydrobenzothiophen-20-ylidene)cyclopenta[1,2-b:4,3-b0]dibenzothiophene.J. Phys. Chem. B 2003, 107, 12635–12638.(32) Ito, H.; Tomihisa, S.; Naoya, O.; Kitamura, N.; Ishizaka, S.;

Hinatsu, Y.; Wakeshima, M.; Kato, M.; Tsuge, K.; Sawamura, M.Reversible Mechanochromic Luminescence of [(C6F5Au)2(μ-1,4-Diisocyanobenzene)]. J. Am. Chem. Soc. 2008, 130, 10044–10045.(33) Gierschner, J.; Ehni, M.; Egelhaaf, H.; Mili�an Medina, B.;

Beljonne, D.; Benmansour, H.; Bazan, G. Solid-State Optical Propertiesof Linear Polyconjugated Molecules: π-Stack Contra Herringbone.J. Chem. Phys. 2005, 123, 144914.(34) Wakita, J.; Jin, S.; Shin, T. J.; Ree, M.; Ando, S. Analysis of

Molecular Aggregation Structures of Fully Aromatic and SemialiphaticPolyimide Films with Synchrotron Grazing IncidenceWide-Angle X-rayScattering. Macromolecules 2010, 43, 1930–1941.(35) Jenekhe, S.; Osaheni, J. Excimers and Exciplexes of Conjugated

Polymers. Science 1994, 265, 765–768.(36) Luo, J. D.; Xie, Z. L.; Lam, J. W. Y.; Cheng, L.; Chen, H. Y.; Qiu,

C. F.; Kwok, H. S.; Zhan, X.W.; Liu, Y. Q.; Zhu, D. B.; et al. Aggregation-Induced Emission of 1-Methyl-1,2,3,4,5-Pentaphenylsilole. Chem. Com-mun. 2001, 1740–1741.(37) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. Enhanced

Emission and Its Switching in Fluorescent Organic Nanoparticles.J. Am. Chem. Soc. 2002, 124, 14410–14415.(38) Babu, S. S.; Praveen, V. K.; Prasanthkumar, S.; Ajayaghosh, A.

Self-Assembly of Oligo(para-phenylenevinylene)s through Arene-Perfluoroarene Interactions: π Gels with Longitudinally ControlledFiber Growth and Supramolecular Exciplex-Mediated Enhanced Emis-sion. Chem.—Eur. J. 2008, 14, 9577–9584.