regulation of arrangements of pyrene fluorophores via solvates and cocrystals for fluorescence...

Regulation of Arrangements of Pyrene Fluorophores via Solvatesand Cocrystals for Fluorescence ModulationQi Feng,† Mingliang Wang,*,† Baoli Dong,† Jing He,† and Chunxiang Xu*,‡

†School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, P. R. China‡State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, P. R. China

*S Supporting Information

ABSTRACT: 1-Acetyl-3-phenyl-5-(1-pyrenyl)-pyrazoline (APPP) was synthesized and formed four types of different crystalsunder different crystallization conditions: guest-free crystal (I, α polymorph), APPP·chloroform solvate (II), APPP·phenolcocrystal (III), and APPP·acetic acid solvate (IV). Twisted structures and different stacking modes of APPP molecules werefound in the four crystals. Single crystal X-ray analyses revealed pyrene fluorophores adopt an edge-to-face π-stacked arrangementin crystal I, mononer arrangement in crystals II and III, and face-to-face slipped π-stacked arrangement in crystal IV. Anotherphase (β polymorph) was discovered during DSC experiments of crystals II and IV and obtained by desolvation of crystal II.However, solvent-mediated phase transitions revealed β polymorph is a metastable phase at room temperature, and no singlecrystal could be isolated. The optical−physical properties of these solids were investigated. Crystal I shows a broad emission band(λmax = 419 nm) with a red shift of about 40 nm relative to its flurescence in solvents, in which the vibrational features are lessclear. The spectra of crystals II and III and β polymorph are similar to each other, exhibiting the shortest λem (λmax = 399−400nm), and the fluorescence spectra with vibrational features are close to pyrene fluorescence in solvents. In contrast, crystal IVshows structureless and broad emission spectrum with the longest λem (λmax = 452 nm) among these crystals. The difference intheir optical−physical properties is closely related to the different arrangements of pyrene fluorophores. The monomerarrangements of pyrene fluorophores may be responsible for the shortest λem, higher emission quantum yields, and longerlifetimes. For a given organic luminescent material, the strategy based on the solvates and cocrystals can not only tune theoptical−physical properties but also be helpful to find suitable polymorphic phases.

■ INTRODUCTION

Organic luminescent materials have received much attentionrecently due to their promising optoelectronic applications inthe fields of lasers,1,2 sensors,3 and organic light-emittingdiodes.4,5 However, although many dyes emit strongly insolution, they become weak fluorophores in the solid statebecause the molecular aggregation usually promotes theformation of species, such as excimers and exciplexes,6,7

which are detrimental to luminescence. On the other hand,intermolecular interactions, such as face-to-face π−π stacking ofaromatic molecules, also have positive effects on fluorescenceproperties. For example, maximization of the π-orbital overlapcould enhance the charge-transport properties.8−12 Thus, toobtain a material with desired properties remains a challenge.Crystal engineering strategy has received a lot of attention

during the past decade.13,14 For a given substance, formation ofdifferent crystalline forms (such as cocrystals or solvates) canbe considered as an effective approach for regulating molecular

arrangements. Current research in organic fluorescent crystalshas indicated that the intermolecular interactions and molecularstacking modes in the solid state play a key role in the observedbulk luminescent characteristics.15−19 Thus, every differentcrystalline form of the same substance should be considered asa unique material with its own distinct optical−physicalproperties.Pyrene is a flat aromatic molecule which shows excellent

optical−physical properties.20−24 Luminophors containingpyrene fluorophores are usually anticipated to show a highfluorescence quantum yield (ΦF) and thermal stability.Previously, we have reported a pyrene derivative: 1-acetyl-3-(4-methoxyphenyl)-5-(1-pyrenyl)-pyrazoline (AMPP, Scheme1).25 In the crystal, C−H···O hydrogen-bonding interactions

Received: June 3, 2013Revised: August 21, 2013Published: August 22, 2013

Article

pubs.acs.org/crystal

© 2013 American Chemical Society 4418 dx.doi.org/10.1021/cg400853r | Cryst. Growth Des. 2013, 13, 4418−4427

pubs.acs.org/crystal

between methoxyl and other groups were found, which arehelpful for stabilizing the structure and may partly cause theformation of pyrene dimers (face-to-face π−π stacking). In thisstudy, we synthesized a new pyrene derivative, 1-acetyl-3-(phenyl)-5-(1-pyrenyl)-pyrazoline (APPP), which has a similarstructure to AMPP. The APPP molecule exhibits a twistedstructure with the dihedral angle of 60.98° between thepyrazoline and pyrene rings, which is expected to restrict themolecular aggregation and exhibit its monomer emission withhigh fluorescence quantum yield in the solid state.26 However,like many organic luminescent materials, APPP exhibits thestructured emission bands in the range 370−420 nm (λmax =387 nm) in n-hexane solution, which should be assigned topyrene monomer emission. While in the solid state, it shows afew broad emission peaks from 380 to 520 nm (λmax = 419 nm)and displays a greenish-blue fluorescence color. Crystalstructure analysis (crystal I, α polymorph) revealed that C−H···O hydrogen-bonding interactions between the acetyl andother groups lead to the formation of the racemic chain motif inthe structure, and the neighboring chains are linked throughC−H···π interactions between the pyrene rings. In other words,such a red shift of fluorescence in solid-state may be mainlycaused by the edge-to-face π-stacked arrangement of pyrenefluorophores. Herein, in order to explore highly blue-emissiveorganic materials, we attempted to regulate the arrangements ofAPPP molecules by formation of its different solvates andcocrystals. Because phenol and acetic acid usually act ashydrogen bond donors, O−H···O hydrogen bonds would beexpected to be formed with the acetyl group of APPP. Underdifferent crystallization conditions, we obtained three types ofcrystals: APPP·chloroform solvate (II), APPP·phenol cocrystal(III), and APPP·acetic acid solvate (IV). By entrapment of thedifferent organic molecules in the lattice, the stacking modes ofpyrene fluorophores change greatly. The monomer arrange-ments of pyrene fluorophores are found in crystals II and III,while face-to-face slipped π-stacked arrangement is adopted incrystal IV. These different stacked arrangements afford thedifferent fluorescence colors in the solid state, the relationshipbetween the arrangements of pyrene fluorophores and theiroptical−physical properties will be discussed in detail.

Moreover, in this study, the two solvates were found toinstigate further happenings. Another polymorph (β poly-morph) was discovered when we investigated the thermalproperties of crystals II and IV, and the pure phase was isolatedthrough desolvation of crystal II. Although the similarphenomena are relatively few in the previous reports,27−31

the desolvation of the solvates may indeed be a useful methodfor discovering the new polymorphic phase in medicinalchemistry and materials science. The optical−physical proper-ties and stability of this form are also discussed.

■ EXPERIMENTAL SECTIONMaterial Synthesis and Characterization. As shown in Scheme

2, a mixture of acetophenone (1.2 g), 1-pyrenecarboxaldehyde (2.3 g),and 3 M of aqueous sodium hydroxide (6 mL) in ethanol (20 mL) wasstirred at room temperature for 8 h. The resulting solid was filtered toafford 1-phenyl-3-(pyren-1-yl) prop-2-en-1-one (chalcone), which wasused directly without further purification. Then chalcone (1 g) and 3.5g of hydrazine hydrate aqueous solution (80%) were dissolved in 20mL of glacial acetic acid. The mixture was then stirred at 120 °C for 5h, and the resulting solution was cooled to room temperature andpoured into a beaker containing ice water slowly. The crude productwas collected by filtration and recrystallized from ethyl acetate to givepure APPP as a yellow powder: 0.64 g, yield: 60%, mp: 204−206 °C.FT-IR (KBr, cm−1): 3448, 3037, 2358, 1653, 1440, 1414, 1153, 1022,960, 840, 763, 690. 1H NMR (CDCl3, Figure 1S of the SupportingInformation): δ (ppm) 2.57 (s, 3H), 3.25−3.29 (dd, 1H), 4.04−4.10(m, 1H), 6.60−6.64 (dd, 1H), 7.39−7.40 (m, 3H), 7.74−7.77 (m,3H), 7.99−8.04 (m, 3H), 8.10−8.12 (d, 1H), 8.16−8.21 (m, 3H),8.27−8.30 (d, 1H). 13C NMR (CDCl3, Figure 2S of the SupportingInformation): δ (ppm) 21.97, 42.62, 57.54, 121.87, 122.13, 124.71,125.08, 125.14, 125.30, 125.90, 126.44, 126.77, 127.15, 127.26, 127.89,128.53, 130.18, 130.47, 130.71, 131.17, 134.55, 154.17, 168.96.

Crystals’ Preparation. APPP single crystal (I, α polymorph).APPP was dissolved in ethyl acetate in a glass vial. Slow evaporation ofthe solvent at room temperature for 4−5 days yielded the block yellowcrystals.

APPP·chloroform (II) and APPP·acetic acid solvates (IV). APPPwas dissolved in chloroform/petroleum ether (v:v = 1:1) and aceticacid/dichloromethane (v:v = 3:1) mixed solvents in glass vials,respectively. Slow evaporation of the solvents at room temperature for4−5 days yielded the needle, light yellow crystals.

APPP·phenol cocrystal (III). APPP and phenol in an equimolarratio were dissolved in ethyl acetate in a glass vial. Slow evaporation ofthe solvent at room temperature for 4−5 days yielded the needle, lightyellow crystals.

Solvent-Mediated Phase Transitions. Solids (1 g) were addedto 15 mL of solvent (chloroform/petroleum ether (v:v = 4:1) mixedsolvent was 8 mL) with or without stirring at 5−10 °C. The productwere filtered and analyzed by power X-ray diffraction (PXRD) ordifferential scanning calorimetry (DSC) to identify the crystallineforms.

Measurements. PXRD patterns for the solids were recorded usingan 18 KW advance X-ray diffractometer with Cu Kα radiation (λ =1.54056 Å). Single X-ray diffraction data for the four crystals werecollected on a Nonius CAD4 diffractometer with Mo Kα radiation (λ= 0.71073 Å). The structures were solved with direct methods using

Scheme 1. Chemical Structures of Compounds AMPP andAPPP

Scheme 2. Preparation of APPP

Crystal Growth & Design Article

dx.doi.org/10.1021/cg400853r | Cryst. Growth Des. 2013, 13, 4418−44274419

the SHELXS-97 program32 and refined anisotropically using a full-matrix least-squares procedure. All nonhydrogen atoms were refinedanisotropically. Hydrogen atoms were inserted at their calculatedpositions and fixed at their positions, except for those of the OH group(phenol and AcOH), which were freely refined.

1H NMR and 13C NMR spectra were recorded at 303 K on a BrukerAvance 500 MHz NMR spectrometer using CDCl3 as a solvent andTMS as an internal standard. Infrared spectra were obtained with aBruker Tensor 27 FT-IR spectrometer. DSC and thermogravimetricanalysis (TGA) patterns were recorded with a Mettler-Toledo TGA/DSC 1 Thermogravimetric Analyzer with the temperature scannedfrom 50 to 300 °C at 10 °C/min. Hot stage microscopy wasperformed on a LEICA DM750P microscope using a Mettler-ToledoFP82HT hot stage. The data were visualized using the AMC Capturesoftware. UV−vis absorption spectra were recorded on a ShimadzuUV-3600 spectrometer. Fluorescence microscopy images wereobtained on an Olympus BX51 imaging system excited at 365 nm.Fluorescence spectra were obtained on a Horiba FluoroMax 4spectrofluorometer. Solid fluorescent quantum yields were performedusing a Quanta-φ accessory with an excitation wavelength at 320 nm.Fluorescence lifetime measurements for the crystals were undertakenby the time-correlated single-photon counting technique (TCSPC)using a TemPro Fluorescence Lifetime System (Horiba Jobin Yvon)equipped with a NanoLed excitation source of 340 nm.

■ RESULTS AND DISCUSSION

Crystal Structures. Single crystal X-ray diffraction analyseswere performed for the four crystals to determine theirstructures. All crystals II−IV consist of 1:1 molar ratio betweenAPPP and the simple organic molecules (chloroform, phenol,and acetic acid molecules). The asymmetric unit (ASU) ofcrystals II consists of two crystallographically independentAPPP molecules (types A and B), whereas only oneindependent APPP molecule in the ASU of crystals I, III,and IV (their ORTEP plots are presented in Figure 3S of theSupporting Information). Moreover, the four crystals areracemic due to equimolar amounts of R- and S-APPP moleculesin the unit cells. The crystallographic data are presented inTable 1.

Crystal I has a monoclinic system and space group C2/c. TheAPPP molecule shows a twisted structure with the dihedralangle of 60.98° between the pyrazoline and pyrene rings. In thestructure, the enantiomeric molecules associate through the C−H···O hydrogen bonds (Table 2) along the b axis to form the

racemic chain motif (Figure 1a). As shown in Figure 1b, thetwo neighboring chains are linked by C−H···π interactionsbetween the pyrene rings (Table 3), thus the void space(PLATON calculations33 suggested a void space volume of 79.4Å3 in a unit cell) of the unit cell is fairly closed and not capableof including any solvent molecules (Figure 1c).Crystal II has a triclinic system and space group P1. The

dihedral angles between the pyrazoline rings and pyrene ringsof the two crystallographically independent APPP molecules(Figure 2, types A and B are shown in cyan and magenta,respectively) are 75.84° for type A and 71.39° for type B,respectively. As shown in Figure 2a, the one-handed

Table 1. Numerical Details of the Solutions and Refinements of the Four Crystal Structures

crystal I II III IV

formula C27H20N2O C28H21Cl3N2O C33H26N2O2 C29H24N2O3

temperature (K) 293 293 293 293crystal size (mm3) 0.30 × 0.20 × 0.10 0.30 × 0.20 × 0.10 0.20 × 0.10 × 0.10 0.20 × 0.10 × 0.10morphology block needle needle needlecrystal system monoclinic triclinic orthorhombic orthorhombicspace group C2/c Pi Pna21 Pna21a (Å) 30.305(6) 9.3580(19) 28.067(6) 18.567(4)b (Å) 8.0420(16) 13.305(3) 9.837(2) 21.868(4)c (Å) 21.397(4) 20.465(4) 9.1450(18) 5.6420(11)α (deg) 90.00 95.37(3) 90.00 90.00β (deg) 129.08(3) 95.93(3) 90.00 90.00γ (deg) 90.00 100.08(3) 90.00 90.00V (Å3) 4048.0(14) 2479.0(9) 2524.9(9) 2290.8(8)Z 8 4 4 4ρ (calcd)/Mg m−3 1.275 1.361 1.269 1.300θ Range for data collection (deg) 1.73−25.43 1.01−25.38 1.45−25.38 1.44−25.42F(000) 1632 1048 1016 944R1, wR2 [I > 2σ(I)] 0.0642, 0.1317 0.0790, 0.1280 0.0563, 0.0864 0.0530, 0.0562R1, wR2 (all data) 0.1367, 0.1572 0.2022, 0.1595 0.1447, 0.1102 0.1835, 0.0885Goodness-of-fit, S 1.001 1.002 1.010 0.831CCDC 907729 907727 907728 937931

Table 2. Intermolecular Hydrogen Bonds Parameters inCrystals I−IV

crystalD−H(Å)

H···A(Å)

D···A(Å)

∠D−H···A(deg)

IC17−H17A···O1 0.980 2.604 3.502 152.40C27−H27···O1 0.961 2.527 3.485 174.72C23−H23A···O1 0.930 2.550 3.371 147.56IIC45−H45B···O1 0.971 2.457 3.328 149.07C55−H55A···O1 0.980 2.204 3.176 171.62O56−H56A···O2 0.980 2.097 3.045 162.16C18−H18B···O2 0.971 2.632 3.504 149.54IIIO2−H2A···O1 1.016 1.669 2.682 173.80C33−H33A···O1 0.929 2.604 3.271 129.15IVO3−H3A···O1 1.064 1.767 2.692 142.74C27−H27A···O2 0.930 2.360 3.271 166.24



enantiomers of the two types (R−A and R−B or S−A and S−B) associate alternately along the a axis through hydrogenbonds (Table 2) and CO···π and face-to-face π−π

interactions (Table 3) to construct two homochiral helicalchain motifs, in which pyrene rings are widely separated withthe closest centroid distance (dc-dc) of 8.376 Å, suggesting noπ−π interactions between them. The chloroform moleculesreside in the channels through weak C−H···O hydrogen bondsrunning along the α axis (Figure 2, panels b and c).Crystal III forms in the orthorhombic noncentrosymmetric

space group Pna21. The dihedral angle between the pyrazolinering and pyrene ring is 81.16°. APPP is linked with phenolmolecule by hydrogen bonds (Table 2) and C−H···πinteractions (Table 3, Figure 3a, and the APPP and phenolmolecules are shown in gray and brown, respectively). Twohomochiral chains (1 and 2) with mirror symmetry could befound in the two-dimensional (2D) structure (Figure 3b). Eachof the chains is formed by the stacking of one-handedenantiomers through the C−H···π interactions, in whichAPPP molecules are widely separated and no π−π interactionbetween the two pyrene fluorophores (the closest dc−dcbetween two parallel pyrene rings is 9.837 Å). The packing

Figure 1. Structure of crystal I. (a) Racemic chain via hydrogen bonds.(b) The neighboring racemic chains associate through C−H···πinteractions (pyrene rings are displayed in green color and the ball-and-stick model). (c) Arrangement of the racemic chains (projected inthe bc plane). The dotted lines show hydrogen bonds and C−H···πinteractions, respectively. Hydrogen atoms not participating in theinteractions have been omitted for clarity.

Table 3. π−π, C−H···π, and CO···π Interactions inCrystals I−IV

crystal interaction distance (Å)a angle (deg)b

I C2−H2B···pyrene 3.004 114.93II Ph(A)−pyrene(B) 4.011 14.22

Ph(B)−pyrene(A) 3.988 9.68C20O1···pyrene (B) 3.891 66.73C47O2···pyrene (A) 3.813 69.96

III C32−H32A···pyrene 2.864 155.41C33−H33A···pyrene 2.972 142.42C17−H17A···pyrene 2.788 148.23C26O1···pyrene 3.834 66.13

IV C19−H19B···benzene 2.845 176.35C25−H25A···pyrene 2.671 146.55

aThe distances were measured from the center-to-center (c) of thearomatic rings (for π−π interactions) or from the hydrogen or oxygenatoms to the center of the aromatic ring (for C−H···π or CO···πinteractions). bThe angles were measured between the planes of thearomatic rings (for π−π interactions), C−H−c or CO−c (for C−H···π or CO···π interactions).

Figure 2. Structure of crystal II. (a) Homochiral helical chain motifs.(b) 3D packing diagram of crystal II (projected in the bc plane). (c)3D packing diagram of crystal II (side view, projected in the ac plane).The dotted lines show hydrogen bonds and π−π interactions,respectively. Cyan and magenta colored molecules are the twocrystallographically independent molecules. Hydrogen atoms notparticipating in the interactions have been omitted for clarity.



arrangement of the structure is illustrated in Figure 3c, which issimilar to AMPP·phenol cocrystal (Figure 4S of the SupportingInformation).25 However, without the methoxyl group, no C−H···O hydrogen bonding interaction was found between thetwo neighboring homochiral chains.Crystal IV also forms in the orthorhombic noncentrosym-

metric space group Pna21. The dihedral angle between thepyrazoline and pyrene rings is 63.73°. The one-handedenantiomers associate parallel along the c axis to constructtwo homochiral chain motifs, in which the pyrene rings adopt aface-to-face slipped π-stacked arrangement (Figure 4a). Theclosest dc−dc and interplanar separation between the neighbor-ing pyrene moieties are 5.642 and 3.511 Å, respectively. Asshown in Figure 4 (panels b and c) (APPP and acetic acidmolecules are shown in gray and brown, respectively), theacetic acid molecules reside in the channels through the O−H···O hydrogen bonds running along the a axis. In addition,acetic acid molecules also play a bridging role in associating the

neighboring heterochiral APPP molecules through weak C−H···O hydrogen bonds.

Thermal Properties and β Polymorph. The thermalproperties of these crystals were investigated by DSC and TGA.From the profiles (Figure 5), crystal I shows the melting pointat 205 °C, and no phase transition was discovered. Crystal IIIshows a board endothermic peak at 158 °C, which is differentfrom the two starting components, suggesting the formation ofa new phase. In addition, weight loss of 19.33% was found atthe temperature region from 120 to 190 °C in its TGA profile,which corresponds to the removal of phenol molecules fromthe lattice and coincides well with the theoretical weight loss(19.50%) of APPP·phenol cocrystal with the molar ratio of 1:1.Crystal II shows two endothermic peaks at 97 and 185 °C. Inaddition, TGA profile shows the weight loss (22.69%) at the

Figure 3. Structure of crystal III. (a) Hydrogen bonds and the C−H···π interactions between APPP and phenol molecules. (b) The twohomochiral chains (1 and 2) with mirror symmetry (projected in thebc plane). (c) 3D packing diagram of crystal III projected in the acplane. The dotted lines show hydrogen bonds and C−H···πinteractions, respectively. Gray- and brown-colored molecules areAPPP and phenol molecules, respectively. Hydrogen atoms notparticipating in the interactions have been omitted for clarity.

Figure 4. Structure of crystal IV. (a) Homochiral chains constructedby the one-handed APPP enantiomers along the c axis. (b) Hydrogenbonds between the APPP and acetic acid molecules. (c) 3D packingdiagram of crystal IV projected in the ab plane. (d) Top view ofadjacent molecules of APPP along the plane of pyrene stacking. Thedotted lines show hydrogen bonds and π−π interactions, respectively.Gray- and brown-colored molecules are APPP and acetic acidmolecules, respectively. Hydrogen atoms not participating in theinteractions have been omitted for clarity.



temperature region from 80 to 110 °C. Thus, the firstendotherm is ascribed to the removal of chloroform moleculesand the weight loss coincides with the theoretical weight loss(23. 47%) of crystal II with the molar ratio of 1:1. However, thesecond endotherm at 185 °C corresponding to the meltingpoint of the desolvated form indicates a new phase (β form)other than crystal I (α form). In the profile of crystal IV, amajor endotherm was observed at 133 °C, followed by a smallendotherm at 185 °C (sometimes this endothermic peak wassmaller or not observed clearly), which is consistent with thesecond endothermic peak in the DSC pattern of crystal II,

suggesting the same phase also generates from crystal IV duringthe heating process. The weight loss of 13.01% (onsettemperature of 110 °C) in the TGA profile indicates thatacetic acid molecules are removed from the lattice during theprocess, which coincides well with the theoretical weight loss(13.39%) of crystal IV with the molar ratio of 1:1. In order toaccount for the unexpected thermal experiment results ofcrystals II and IV, hot-stage microscopy (HSM) experimentswere performed. From Figure 6, the desolvation of crystal IIbegan as a shadow that appeared on the crystal at thetemperature of 95 °C, then the shadow extended throughout

Figure 5. (a) DSC and (b) TGA profiles of the four crystals and β polymorph.

Figure 6. Photomicrographs of crystal II at various temperatures in the HSM experiment.

Figure 7. Photomicrographs of crystal IV at various temperatures in the HSM experiment. Notice that needle crystals generated from the liquidphase.



the whole crystal at the temperature region from 95 to 100 °C.The morphology of the solid remained unchanged during theprocess, which should be ascribed to desolvation of crystal II.Agreeing with the DSC scans, the opaque solid finally melted at184−188 °C and no transition was observed after that,suggesting the all crystals transform to the β phase afterdesolvation process. From the Figure 7, all crystals of IV meltedat 130−135 °C. However, after melting, the new needle crystalsgenerated from the liquid phase. With the temperatureincreasing, more needle crystals could be observed. Agreeingwith the DSC scans, all the needle crystals melted at 183−186°C again, which could be identified as a β polymorph. Incomparison with crystal II, the generation of β polymorph fromcrystal IV is a melting and recrystallization process. Therelatively small degree of recrystallization from the liquid phasecorresponds to the smaller area of the secondary endothermicpeak in its DSC profile. Thus, the pure β polymorph could noteasily be isolated from the DSC experiment of crystal IV.On the basis of the phenomenon of HSM, the opaque and

needlelike solids were obtained by heating crystal II at 130, 140,and 150 °C, respectively. All the solids showed only oneendothermic peak at 185 °C in the DSC profiles, suggesting thesame phases were yielded by desolvation of crystal II under thedifferent temperatures.Powder X-ray Diffraction (PXRD) Patterns. As shown in

Figure 8, PXRD patterns of the four crystals are in agreement

with that simulated from their single crystal data (Figure 5S ofthe Supporting Information), which can be used for optical−physical properties investigated later. In addition, the βpolymorph shows a different structure relative to the fourcrystals, suggesting different molecular arrangements in thisphase. In summary, DSC and PXRD patterns measured on thesolids can be used to identify the different crystalline forms.Stability and Solvent-Mediated Phase Transitions. As

shown in Figure 9, the solvent-mediated mutual transitionsbetween the three forms (crystals I, II, and β polymorph) wereinvestigated. The powder crystal I was stirred in the mixture ofchloroform/petroleum ether (v:v = 4:1) for 4−5 days, analysisof the product by PXRD (Figure 6S of the SupportingInformation) revealed that it matched with the PXRD patternof crystal II. However, in absence of chloroform, no transitioncould be observed (Table 4). In contrast, rapid conversionsfrom II to I could be observed following the soaking of crystal

II in different polarity solvents (Table 4) for 10−15 h (theproduct was identified by its DSC pattern, Figure 7S of theSupporting Information). During the process, the opaque andneedlelike solid was first noticed after crystal II was immersedin the solvents for 2−3 min (Figure 10), which was identified asthe β form (DSC pattern, Figure 8S of the SupportingInformation). Interestingly, in some low polarity solvents(Table 4), only the β form could be obtained after 2−3 days.However, the slurry of β form in these low polarity solventswould yield crystal I for several hours. As such, this transition islikely promoted strongly by the polarity solvents and physicalperturbation. In accordance with these cases, β polymorph ismetastable at room temperature and prone to convert to the αpolymorph, which may cause inability to produce its singlecrystals by solvent evaporative crystallization experiments(Table 5).

FT-IR Spectroscopy. To investigate the noncovalentinteractions within these crystals, their FT-IR spectra wererecorded (Figure 11). For the carbonyl groups of APPP, thevibrational absorption maximum in crystals I−IV are 1653,1640, 1626, and 1618 cm−1, respectively. Compared withcrystal I, the values in crystals II−IV are systematically shiftedto lower frequencies by approximately 13 to 35 cm−1. Thisindicates that the intermolecular C−H···O and O−H···Ohydrogen-bonding interactions influence the vibrational proper-ties of APPP indeed. Interestingly, for crystals II−IV, thesystematically shifted values to low frequency are consistentwith the fact that CO bond lengths of carbonyl groups(1.195 and 1.216 Å for crystal II and 1.247 and 1.252 Å forcrystals III and IV) increase orderly as a result of hydrogenbonds. In addition, the vibrational absorption maxima at 1716cm−1 in crystal IV should be assigned to the carboxyl group ofacetic acid. In comparison with the four crystals, the βpolymorph shows the higher frequency of 1660 cm−1, which isanother proof of the different arrangements of APPP moleculesin this phase.

Figure 8. PXRD patterns of the four crystals and β polymorph.

Figure 9. Summary of the transitions.

Table 4. Experiments of Solvent-Mediated Phase Transitions

solvents

products of transitionfrom crystal I to β

polymorph

products of transition fromcrystal II to crystal I (without

stirring)

methanol α α

ethanol α α

ethyl acetate α α

acetonitrile α α

acetone α α

n-hexane α β polymorphpetroleum ether α β polymorphpetroleum ether/chloroform (5:1v/v)

II −



Optical-Physical Properties. Diffuse reflectance absorp-tion spectroscopy (Figure 12) and fluorescence emissionspectroscopy (Figure 13) were exploited to investigate theoptical−physical properties of these solids. The absorptionspectra of APPP in solvents are structured (Figure 9S of theSupporting Information) with band maxima only slightly red-shifted compared to that of pyrene. Similarly, the solids (fourcrystals and β polymorph) show broad bands with fine peaks (λ< 365 nm region) arising from pyrene, corresponding to thespectra of APPP in the solvents. However, the bands from 365to 430 nm are also apparent in the spectra, which are not foundin Figure 9S of the Supporting Information and should beascribed to the S0−S1 absorption of pyrene.34 Appearance ofthe well-resolved vibronic structures of S0−S1 absorption maybe ascribed to the intermolecular interactions in the solid state.The fluorescence spectrum of APPP in solvents (Figure 10S

of the Supporting Information) exhibits vibrational peaks at

376, 386, and 395 nm and weak shoulders around 418 and 448nm, corresponding to the pyrene monomer emission. However,in the solid state, APPP (crystal I) shows a somewhat broadfluorescence spectrum with the peaks at 419 and 430 nm andweak shoulders around 399, 458, and 498 nm, which should beassigned to the vibrational structure of pyrene fluorophore. Theemission spectra of crystals II and III are similar to each other.They mainly exhibit two sharp bands around 380−440 nm(two peaks at 400 and 417 nm for crystal II, two peaks at 399and 420 nm crystal III), which are close to the vibrationalstructure of APPP in solvents. However, crystal IV shows astructureless emission profile, which exhibits a broad emissionband with only one peak centered at 452 nm. Irradiating the

Figure 10. Photomicrographs showing the visual changes of transition from crystals II to I.

Table 5. Polymorph Screening of APPP

solvents polymorph

methanol α

ethanol α

ethyl acetate α

dichloromethane α

acetonitrile α

acetone α

n-hexane α

petroleum ether α

petroleum ether with the seeds of β polymorph α

methanol/chloroform (v:v = 1:1) α and crystal IIpetroleum ether/chloroform (v:v = 1:1) crystal II

Figure 11. FT-IR spectra of the four crystals and β polymorph.

Figure 12. Absorption spectra of the solids.

Figure 13. Fluorescence spectra of solids (λex = 365 nm for crystals Iand II, λex = 350 nm for other solids).



crystals with UV light (Figure 14) reveals that they showdifferent fluorescence colors, which can clearly be detectedvisually. These large differences in their spectral characteristicsclearly demonstrate that the optical−physical properties of thecrystals are dependent on the arrangement of pyrenefluorophores. The edge-to-face π-stacking of pyrene fluoro-phores in crystal I (the nearest distances of C−H···C and C−H···center of pyrene rings are 2.904 and 3.004 Å) results in itsbroad emission band, in which the emission maxima shifts tothe longer wavelength region (a red shift of about 40 nmrelative to its fluorescence in solvents), and the vibrationalstructure is less clear. In accordance with the increase in thepeak intensity, such interactions also probably influence thevibrational wave function to overlap well for high vibronicstates.35 Because pyrene fluorophores are situated apart fromeach other and adopt monomer arrangements in crystals II andIII, their emission bands are similar to each other and exhibitthe shortest λem (400 or 399 nm) among these crystals.Moreover, although the spectra of the two crystals are close tothat of APPP in solvents, a slight difference of vibrationalfeatures could still be observed, which may be influenced byintermolecular interactions in the solids, such as C−H···π orπ−π interactions between pyrene and benzene rings. Finally,compared with crystal I, the face-to-face slipped columnstacking of pyrene fluorophores in crystal IV increases their π-orbital overlap, which leads to the largest red-shifted andstructureless emission spectrum. Such π−π interactionsbetween pyrene fluorophores resulting in the red-shiftedemission has been reported previously.36 In correspondenceto the emission of β polymorph, the emission spectrum issimilar to that of crystals II and III. A conclusion could be

drawn that there is no strong π−π interaction between theneighboring pyrene fluorophores, although its crystal structurewas not determined.Emission quantum yields (ΦF) and fluorescence lifetimes

(τF) were measured to obtain further insight into their optical−physical properties. As summarized in Table 6, the ΦF values ofblue-emissive crystals are higher than that of greenish-blue-emissive crystals. The differences in the solid-state ΦF valuesshould be attributed to the different stacking arrangements ofpyrene fluorophores in these crystals. The molecularaggregation usually decreases the fluorescence efficiencies inthe solid state; we supposed that the higher ΦF values ofcrystals II and III should be assigned to the monomerarrangements of pyrene fluorophores in the structures.Interestingly, the trend of their fluorescence lifetimes isanalogous to that of the ΦF values, such that crystals II andIII also exhibit longer fluorescence lifetimes (the correspondingfluorescence decay curves are shown in Figure 12S of theSupporting Information). For our system, the suppression ofπ−π interactions between the pyrene fluorophores maydecrease excited-state nonradiative relaxation. Correspondingto the β polymorph, its ΦF and τF values are close to that ofcrystals II and III and higher than that of crystals I and IV.Thus, we speculated the absence of a strong π−π interactionbetween the neighboring pyrene fluorophores, which agreeswith the conclusion based on its emission spectrum.

■ CONCLUSION

In summary, we demonstrated that tuning the optical−physicalproperties of the pyrene derivative (APPP) could be achieved

Figure 14. Fluorescence microscopy images of the solids under UV light (λex = 365 nm).

Table 6. Optical-Physical Properties Data of Solids

aMaximum wavelengths of diffuse reflectance absorption spectra. bMaximum wavelengths of fluorescence excitation spectra monitored at theirrespective maximum emission peaks (Figure 11S of the Supporting Information). cWavelengths of fluorescence emission spectra excited at 365 nmfor crystals I and II, and 350 nm for other solids. dFluorescent quantum yields excited at 320 nm. eFluorescence lifetimes excited at 340 nm.



by crystallizing into different solvates and cocrystals. Crystalstructures revealed that the arrangements of pyrene fluo-rophores in the four crystals could be classified into three types:edge-to-face π−π stacking (crystal I, α polymorph), monomerarrangement (crystals II and III), and face-to-face π−π stacking(crystal IV). The π-stacked geometries of pyrene fluorophoresare responsible for the larger red-shifted emissions, lower ΦFvalues, and shorter lifetimes. In contrast, suppression of π−πinteractions is crucial for obtaining high blue-emissive solids.Another polymorphic phase (β form) was isolated afterdesolvation of crystal II. However, it could irreversiblytransform to the α form at room temperature, which maycause inability to produce single crystals by solvent evaporativecrystallization experiments. In accordance to its opticalproperties, we supposed that there is no strong π−π interactionbetween pyrene fluorophores in the β form. The strategy basedon the solvates and cocrystals for fluorescence modulation canbe used for other luminescent systems. In addition, theparticular phenomenon of discovering the other polymorphicphase may actually be helpful to find suitable polymorphicphases with desired properties.

■ ASSOCIATED CONTENT*S Supporting Information1H and 13C NMR spectra of APPP; ORTEP plots andsimulated PXRD patterns of crystals I−IV; 3D packing diagramof AMPP·phenol; PXRD, DSC, and TGA patterns of productsof transitions; absorption and emission spectra of APPP invarious solvents; excitation spectra and fluorescence decaycurves for the solids (crystals I−IV and β form); X-raycrystallographic information files (CIFs) for crystals I−IV. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*M.W.: e-mail, [email protected]. C.X.: e-mail,[email protected]; tel, +86 2585092237.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis project gets the supports of the National Basic ResearchProgram of China (Grant 2011CB302004).

■ REFERENCES(1) Schmidtke, J.; Stille, W.; Finkelmann, H.; Kim, S. T. Adv. Mater.2002, 14, 746−749.(2) Gao, F.; Liao, Q.; Xu, Z.; Yue, Y.; Wang, Q.; Zhang, H.; Fu, H.Angew. Chem., Int. Ed. 2010, 49, 732−735.(3) Yan, D. P.; Lu, J.; Ma, J.; Wei, M.; Evans, D. G.; Duan, X. Angew.Chem., Int. Ed. 2011, 50, 720−723.(4) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.;Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas,J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121−128.(5) Strassert, C. A.; Chien, C. H.; Galvez Lopez, M. D.; Kourkoulos,D.; Hertel, D.; Meerholz, K.; Cola, L. D. Angew. Chem., Int. Ed. 2011,50, 946−950.(6) Winnik, F. M. Chem. Rev. 1993, 93, 587−614.(7) Chou, T. C.; Hwa, C. L.; Lin, J. J.; Liao, K. C.; Tseng, J. C. J. Org.Chem. 2005, 70, 9717−9726.(8) Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc.2005, 127, 16866−16881.

(9) Cornil, J.; Beljonne, D.; Calbert, J. P.; Bredas, J. L. Adv. Mater.2001, 13, 1053−1067.(10) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. Rev. 2007, 36,1902−1929.(11) Lehnherr, D.; Murray, A. H.; McDonald, R.; Ferguson, M. J.;Tykwinski, R. R. Chem.Eur. J. 2009, 15, 12580−12584.(12) Chang, Y. C.; Chen, Y. D.; Chen, C. H.; Wen, Y. S.; Lin, J. T.;Chen, H. Y.; Kuo, M. Y.; Chao, I. J. Org. Chem. 2008, 73, 4608−4614.(13) Desiraju, G. R. Angew. Chem., Int. Ed. 1995, 34, 2311−2321.(14) Wuest, J. D. Chem. Commun. 2005, 5830−5837.(15) Mizobe, Y.; Tohnai, N.; Miyata, M.; Hasegawa, Y. Chem.Commun. 2005, 1839−1841.(16) Zhang, G.; Lu, J.; Sabat, M.; Fraser, C. L. J. Am. Chem. Soc. 2010,132, 2160−2162.(17) Davis, R.; Rath, N. P.; Das, S. Chem. Commun. 2004, 74−75.(18) Hisaki, I.; Kometani, E.; Shigemitsu, H.; Saeki, A.; Seki, S.;Tohnai, N.; Miyata, M. Cryst. Growth Des. 2011, 11, 5488−5497.(19) Zhou, T.; Jia, T.; Zhao, S.; Guo, J.; Zhang, H.; Wang, Y. Cryst.Growth Des. 2012, 12, 179−184.(20) Kim, H. M.; Lee, Y. O.; Lim, C. S.; Kim, J. S.; Cho, B. R. J. Org.Chem. 2008, 73, 5127−5130.(21) Kim, H. J.; Hong, J.; Hong, A.; Ham, S.; Lee, J. H.; Kim, J. S.Org. Lett. 2008, 10, 1963−1966.(22) Ji, S. M.; Yang, J.; Yang, Q.; Liu, S. S.; Chen, M. D.; Zhao, J. Z. J.Org. Chem. 2009, 74, 4855−4865.(23) Benniston, A. C.; Harrima, A.; Lawrie, D. J.; Mayeux, A. Phys.Chem. Chem. Phys. 2004, 6, 51−57.(24) Maeda, H.; Maeda, T.; Mizuno, K.; Fujimoto, K.; Shimizu, H.;Inouye, M. Chem.Eur. J. 2006, 12, 824−831.(25) Feng, Q.; Wang, M. L.; Dong, B. L.; Xu, C. X.; Zhao, J.; Zhang,H. J. CrystEngComm 2013, 15, 3623−3629.(26) Hong, Y.; Lam, J.; Tang, B. Chem. Soc. Rev. 2011, 40, 5361−5388.(27) Bhattacharya, S.; Saha, B. K. Cryst. Growth Des. 2013, 13, 606−613.(28) Suitchmezian, V.; Jeß, I.; Nather, C. Int. J. Pharm. 2006, 323,101−109.(29) Landgraf, K. F.; Olbrich, A.; Pauluhn, S.; Emig, P.; Kutscher, B.;Stang, H. Eur. J. Pharm. Biopharm. 1998, 46, 329−337.(30) Surov, O. V.; Voronova, M. I.; Smirnov, P. R.; Mamardashvili,N. Z.; Shaposhnikov, G. P. CrystEngComm 2012, 14, 533−536.(31) Preminger, J. M. R.; Bernstein, J. Cryst. Growth Des. 2005, 5,1343−1349.(32) Sheldrick, G. M. SHELXS97: Programs for Crystal StructureAnalysis; University of Gottingen: Germany, 1997.(33) Spek, A. L. Acta Crystallogr. 2009, D65, 148−155.(34) Shen, Q. J.; Wei, H. Q.; Zou, W. S.; Sun, H. L.; Jin, W. J.CrystEngComm 2012, 14, 1010−1015.(35) Hinoue, T.; Shigenoi, Y.; Sugino, M.; Mizobe, Y.; Hisaki, I.;Miyata, M.; Tohnai, N. Chem.Eur. J. 2012, 18, 4634−4643.(36) Zhao, Z. J.; Chen, S. M.; Lam, J. W. Y.; Wang, Z. M.; Lu, P.;Mahtab, F.; Sung, H. H. Y.; Williams, I. D.; Ma, Y. G.; Kwok, H. S.;Tang, B. Z. J. Mater. Chem. 2011, 21, 7210−7216.



http://pubs.acs.org

http://pubs.acs.org

mailto:[email protected]

mailto:[email protected]

regulation of arrangements of pyrene fluorophores via solvates and cocrystals for fluorescence...

Documents