Sublimation Point Depression of Small-Molecule Semiconductors by

Sublimation Point Depression of Small-Molecule Semiconductors by Sonocrystallization Tu Lee*,†,‡ and Shih Chia Chang† Department of Chemical & Materials Engineering, National Central UniVersity, and Institute of Materials Science & Engineering, National Central UniVersity, 300 Jhong-Da Rd, Jhong-Li City 320, Taiwan, ROC

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2674–2684

ReceiVed December 1, 2008; ReVised Manuscript ReceiVed March 24, 2009

ABSTRACT: Small-molecule semiconductor solids such as pentacene and tris(8-hydroxyquinoline)aluminum(III) (Alq3) were dispersed in a poor solvent, like water, and insonated in a 10 mL scintillation vial with an output frequency of 20 kHz, a voltage of 1500 V, and an optimal induction time for 10 min at -13 °C. Sonocrystallization, at a low bulk solution temperature, gave pentacene powders of a high lattice energy difference value, ∆Elatt, of 2.258 J/g (i.e., 0.6285 kcal/mol) caused by the poor crystallinity of 69% and produced Alq3 powders with only 37 wt % of the stable R-form and 63 wt % of the metastable ε-form mixed with an amorphous phase. Therefore, insonated pentacene and Alq3 powders had depressed sublimation points of 210 and 180 °C, respectively. However, surface energy and impurities had nothing to do with the sublimation point depression. The sublimation point depression of target materials could reduce the heating and cooling duty of the vapor-phase deposition method for the manufacturing of organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and photovoltaic (PV) cells drastically, because the total radiant-heat-transfer rate between heated surfaces is proportional to the fourth power of the absolute temperature according to the Stefan-Boltzmann law. Introduction Organic semiconductors have long remained the focus of intense research for their use in devices such as organic lightemitting diodes (OLEDs),1 organic thin-film transistors (OTFTs),2,3 and photovoltaic (PV) cells.4 It is mainly because their functional properties can be tailor made by molecular engineering and their molecules can be thermally deposited at temperatures low enough on flexible plastic substrates over a large area.5 However, commercialization of this technology calls for further reduction in manufacturing costs, for instance, by low-temperature processing techniques.6 Yet, the poor solubility of most of the organic semiconductors in organic solvents has prevented the direct use of readily available solution-based, thin-film fabrication methods, such as spin-coating and surface-pattern inkjet.5 Overcoming this problem has led to rather onerous chemical approaches in synthesizing soluble precursors7 or substituted derivatives8,9 for the organic semiconductor molecules. Instead of replacing the energy intensive vapor-phase deposition method by solution-based, low-temperature processing techniques, we would suggest keeping the existing deposition techniques10-13 and lowering their heating and cooling duties through the reduction of the evaporation temperature of the target material: the sublimation point depression of the organic semiconductor. Recently, we have reported the success of lowering the sublimation temperature of tris(8-hydroxyquinoline)aluminum(III) (Alq3) from 250 ( 5 to 200 ( 5 °C by cooling recrystallization of Alq3 in N,N-dimethylformamide.14 Again, this solution-based recrystallization method suffers from the prerequisite for the small-molecule semiconductor to have a decent solubility in an organic solvent at 60 °C. But unfortunately, many small-molecule semiconductors, such as pentacene, are insoluble in many organic solvents.5,7 Therefore, the aim of this paper is to demonstrate a robust and one-step process of lowering the sublimation point of small* To whom correspondence should be addressed. Tel: +886-3-422-7151 ext. 34204. Fax: +886-3-425-2296. E-mail: [email protected]. † Department of Chemical & Materials Engineering. ‡ Institute of Materials Science & Engineering.

Figure 1. Molecular structures of (a) pentacene and (b) tris(8hydroxyquinoline) aluminum(III) (Alq3).

molecule semiconductors by insonation even in water without the need of dissolving the small-molecule semiconductors at all! Although sonocrystallization15 has been employed in the solution phase for polymorph induction,16 particle size distribution control,17 nanotechnology,18 cocrystallization,19 and sonochemical fabrication,20 this is the first time for the discussion of this technique to alter the crystal lattice energetics of two model small-molecule semiconductors, pentacene and Alq3 (Figure 1), in an aqueous suspension. Although pentacene was infamous for its poor solubility in organic solvents,5,7 it was renowned for its high field-effect mobility around 3 cm2/(V s) in thin film transistors at room temperature,21 its patterning

10.1021/cg801305r CCC: $40.75  2009 American Chemical Society Published on Web 04/09/2009

Sublimation Point Depression Semiconductors

Crystal Growth & Design, Vol. 9, No. 6, 2009 2675

Figure 2. Polymorphic transformations and processing conditions of pentacene. The “thin film”27 and the “single crystal” phase27 are indicated in blue and red, respectively. Table 1. Unit-Cell Parameters of Five Pentacene Polymorphs pentacene polymorph

a (Å)

b (Å)

c (Å)

R (deg)

β (deg)

γ (deg) Bravais lattices space group

14.1 Å, pentacene-LT (EI), polymorph H 14.37 Å 15.0 Å 15.4 Å 14.5 Å, Campbell, polymorph C, pentacene-HT (EII)

6.266 6.485 5.89 5.77 6.119

7.775 7.407 7.70 7.49 8.058

14.530 14.745 15.7 17.2 14.926

76.475 77.25 77.6 73.5 97.52

87.682 85.72 80.3 75.3 100.19

84.684 80.92 88.4 91.2 94.11

capability,22,23 and its fractional quantum Hall effect24 and superconductivity.25 Pentacene had also been studied extensively for its mechanical behaviors26 and its five different polymorphs which were identified by their layer periodicity, d(001) (i.e., 14.1 Å polymorph27 ) pentacene-LT (EI) polymorph28 ) polymorph H,29 14.37 Å polymorph,30 14.5 Å polymorph28 ) pentacene-HT (EII) polymorph28 ) Campbell’s polymorph31,32 ) polymorph C,29 15.0 Å polymorph,27 and 15.4 Å polymorph27). Their unit cell dimensions and inter-relationships were shown in Table 1 and Figure 2, respectively. On the other hand, Alq3 had also been studied and characterized extensively for its luminescence,33 thermal properties,34 crystallization behaviors,35 and polymorphism (R, β, γ, δ, and ε).36-38 The polymorphic transformations of Alq3 had already been illustrated in detail in ref 14. Materials and Materials Small-Molecule Semiconductors. Deep violet pentacene 14.4 Å polymorphic powder solids (C22H14, ACS grade, purity of 99.9+%, mp 372-374 °C, MW 278.35, Lot 07230EH) and yellowish brown Alq3 R-form powder solids (C27H18AlN3O3, ACS grade, purity of 98%, MW 459.44, Lot 13125MC) were purchased from Sigma-Aldrich (Steinheim, Germany). Solvents. Colorless paraffin oil (analytical specification of Ph. Eur., BP, density (D20/20) 0.827-0.890, dyn. viscosity 110-230 mPa s,

triclinic triclinic triclinic triclinic triclinic

P1 P1 P1 P1 P1

ref 26, 27 27, 28 57 57 28, 31, 32

Lot 73250) was obtained from Riedel-de Hae¨n (Seelze, Germany); colorless anhydrous glycerol (HOCH2CHOHCH2OH, ACS grade, purity of 99.6%, density (g/mL) at 25 °C 1.257, flash point 199 °C, MW 92.10, Lot G28B26) and colorless 1,2,4-trichlorobenzene liquid (C6H3Cl3, ACS grade, purity of 99%, bp 214 °C, mp 16 °C, MW 181.45, Lot E08N63) were purchased from Mallinckrodt Baker, Inc. (Phillipsburg, NJ). Colorless 1,2-dichlorobenzene liquid (C6H4Cl2, ACS grade, purity of 99%, bp 178-180 °C, MW 147.00, Lot 70930) was obtained from Sigma-Aldrich (Steinheim, Germany). Colorless xylene liquid (C6H4(CH3)2, ACS grade, purity of 98.5%, bp 137-144 °C, MW 106.7, Lot 305065) was obtained from Tedia Company (Fairfield, OH). Starch ((C6H10O5)n from potato, purity of e0.7%, Lot 85643) was purchased from Fluka (Duisburg, Germany). In-house distilled water was used for contact angle measurements. Reversible osmosis (RO) water was used for all other experiments clarified by a water purification system (Milli-RO Plus model) bought from Millipore (Billerica, MA). Instrumentation. Hot-Stage Polarized Optical Microscopy (HSPOM). An optical microscope (BX51; Olympus, Tokyo, Japan) equipped with a Linkam hot stage (THMS600; Linkam Scientific Instruments, Surrey, United Kingdom) and a digital camera (Moticam 2000 2.0 M Pixel USB2.0; Motic, Inc., Xiamen, China) was used to take images of sublimed crystals. Data were visualized using Motic Images Plus 2.0 ML (Motic, Inc., Xiamen, China) with a Linkam temperature controller (CI94; Linkam Scientific Instruments, Surrey, United Kingdom) for a hot stage control. Contact thermal microscopy was conducted by heating the sample from room temperature to 175 °C with a rate of 50 °C/min, holding the temperature at 175 °C for 1 min, and then heating to 400 °C with a rate of 5 °C/min.

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Transmission Fourier Transform Infrared (FTIR) Spectroscopy. Transmission FTIR spectroscopy was utilized to measure purity, detect bond formation, and verify chemical identity. Transmission FTIR spectra were recorded on a Perkin-Elmer Spectrum One spectrometer (Perkin-Elmer Instruments LLC, Shelton, CT). The KBr sample disk was scanned with a scan number of 8 from 450 to 4000 cm-1 having a resolution of 2 cm-1. Powder X-ray Diffraction (PXRD). A PXRD diffractogram at 25 °C provided another piece of information for the polymorphism and crystallinity of Alq3 solids. PXRD diffractograms were collected by Bruker D8 Advance (Germany). The source of PXRD was Cu KR (1.542 Å) and the diffractometer was operated at 40 kV and 41 mA. The X-ray was passed through a 1 mm slit and the signal through a 1 mm slit, a nickel filter, and another 0.1 mm slit. The detector type was a scintillation counter. The scanning rate was set at 0.05° 2θ/s ranging from 5° to 35°. The quantity of sample used was around 20-30 mg. Scanning Electron Microscopy (SEM). Pentacene solids were mounted on an aluminum stub with double-sided carbon conductive adhesive tape (product number 16073, Ted Pella Inc., Redding, CA). The solid sample was sputter-coated with 6 nm thick gold film in a Hitachi E-1010 Auto Sputter Coater (Hitachi Ltd., Tokyo, Japan). SEM was carried out using a Hitachi S-3500N (Hitachi Ltd., Tokyo, Japan) instrument equipped with a tungsten filament cathode source. Goldcoated samples were examined with beam energies of 15 kV at a chamber pressure of 10-5 Pa (resolution of ∼3 Å at these voltages). Differential Scanning Calorimetry (DSC). DSC analysis was mainly used to identify the enthalpy of fusion and solid-liquid (melting) temperature. Thermal analytical data of 3-5 mg of samples in perforated aluminum sample pans (60 µL) were collected on a PerkinElmer DSC-7 calorimeter (Perkin-Elmer Instruments LLC, Shelton, CT) with a temperature scanning rate of 10 °C/min from 50 to 450 °C under a constant nitrogen 99.990% purge. The instrument was calibrated with indium and zinc 99.999% having reference temperatures of 156.6 and 419.47 °C, respectively (Perkin-Elmer Instruments LLC, Shelton, CT). Thermal Gravimetric Analysis (TGA). TGA analysis was carried out by TGA 7 (Perkin-Elmer, Norwalk, CT) to monitor sample weight loss as a function of temperature. The heating rate was 10 °C/min ranging from 50° to 300 °C to minimize contamination caused by sublimation. Weight loss was usually associated with solvent evaporation close to the boiling point of a solvent as in the case of solvates or associated with sample decomposition. The open platinum pan and stirrup were washed by ethanol and burned by a spirit lamp to remove all impurities. All samples were heated under a nitrogen atmosphere to avoid oxidization. About 3 mg of sample was placed on the open platinum pan suspended in a heating furnace. Experiments. Solubility Determination. The solubility of pentacene in 1,2-dichlorobenzene at 25, 60, 150, and 200 °C was measured by the gravimetric method, visually assuming the volumes of solvents were the volumes of solution and the volumes of solvents did not change significantly with temperature.14,39 Although the gravimetric method appeared to be rough, its advantages were its robustness and simplicity, without the need of performing any calibration and without any concern of solvate formation. Since pentacene was slightly soluble in 1,2dichlorobenzene and a relatively large volume of solvent was required to dissolve a little amount of pentacene, a small deviation in the solvent volume by human sight should still give a rather accurate result. All measurements were repeated at least three times. Sonocrystallization. About 20 mg of small-molecule semiconductor solids dispersed in 8 mL of water was insonated by an ultransonic cell disruptor (Sonicator 3000; Misonix, Inc., Farmingdale, NY) in a 10 mL scintillation vial. The disruptor had a 22 cm long tip with a diameter of 0.1 cm. Its tip was placed 0.5 cm above the bottom of the scintillation vial and operated with an output frequency of 20 kHz, a voltage of 1500 V, and an induction time of 5, 10, and 30 min. The temperature of the insonation process was maintained by immersing the 10 mL scintillation vial in a bath of coolant of +25 and -13 °C. Grinding. Ground pentacene and Alq3 sample powders were prepared by grinding the purchased powders in an agate mortar and pestle for 45 min. Contact Angle Measurements. All pentacene and Alq3 sample powders were isostatically pressed into disks by a manually operated 15 ton hydraulic press (Specac, Inc., Woodstock, GA) with a pressure of about 7 tons. A 2 µL size droplet of paraffin oil, anhydrous glycerol, and water was deposited separately from a microliter syringe onto the disk surfaces made by the various kinds of powder samples. The contact

Lee and Chang angle of each liquid droplet was then measured by an Olympus SZII Zoom Stereo microscope (Olympus, Tokyo, Japan) equipped with a Sony SSC-DC 50A digital color video camera (Sony Corporation, Tokyo, Japan). Each reported contact angle was the average of three measurements and its deviation was within (2°.

Results and Discussion Unlike the purchased Alq3 R-form powders which could dissolve in nine good solvents,14 such as benzyl alcohol, chloroform, N,N-dimethylformamide (DMF), nitrobenzene, dimethyl sulfoxide (DMSO), N,N-dimethylaniline (DMA), 1,4dioxane, acetone, and tetrahydrofuran (THF) with a solubility of g1 mg/mL at 25 °C, the purchased pentacene 14.1 Å polymorphic powders were only slightly soluble in 1,2dichlorobenzene and 1,2,4-trichlorobenzene31 with a solubility of 0

Figure 6. FTIR spectra of (a) the purchased pentacene, (b) the pentacene powders insonated in 1,2-dichlorobenzene at 25 °C for 10 min, (c) the ground pentacene, (d) the pentacene powders insonated in 1,2dichlorobenzene at -13 °C for 10 min, and (e) the pentacene powders insonated in water at -13 °C for 10 min.

was not caused by the presence of impurities, the decomposition of pentacene powders, or the increase of surface area with high surface energy but was truly due to the alteration of the crystal lattice, all pentacene powder samples were subjected to the analyses of FTIR, PXRD, SEM, contact angle measurements, and DSC. IR bands in Figure 6 remained identical for all samples before and after sonocrystallization or grinding, and no detectable solvate/hydrate formation, impurities, or decomposition was observed. The characteristic C-C stretching vibrations47 at 1634, 1544, 1505, 1447, 1350, and 1299 cm-1, the C-H inplane bending modes47 at 1184, 1164, and 1122 cm-1, the C-H out-of-plane bending modes47 at 995, 900, and 732 cm-1, and the C-C-C in-plane bending mode47 at 826 cm-1 of pentacene were present. PXRD diffractograms of all pentacene powder samples and the calculated PXRD pattern of pentacene for reference were shown in Figure 7. All pentacene samples were pentacene 14.1

(1)

Higher values of Elatt for each sample powder will give larger difference values of the lattice energy, ∆Elatt. We realize that the major contributor to the difference of lattice energy, ∆Elatt, is the difference between the enthalpy of fusion of a powder f , and the enthalpy of fusion of the purchased sample, ∆Hsp f powder, ∆Hpp . f ∆∆Hf ) ∆Hsp - ∆Hfpp ≈ ∆Elatt

(2)

where the enthalpy of fusion, ∆Hf, is a thermal quantity readily measurable by DSC and, therefore, ∆Elatt could be estimated by ∆∆Hf.46 Since the sublimation point measurements of HSPOM in Figure 5 and the PXRD diffractograms in Figure 7 showed that the ground pentacene powders behaved similarly to the pentacene powders insonated in 1,2-dichlorobenzene at 25 °C and the pentacene powders insonated in 1,2-dichlorobenzene at -13 °C for 10 min behaved similarly to the pentacene powders insonated in water at -13 °C for 10 min, we decided to focus only on three significantly different samples for further comparisons in their DSC, contact angle measurements, and SEM analyses. They were the purchased pentacene powders as a control, the ground pentacene powders, and the pentacene powders insonated in water at -13 °C for 10 min.



Figure 7. PXRD diffractograms of (a) the purchased pentacene, (b) the pentacene powders insonated in 1,2-dichlorobenzene at 25 °C for 10 min, (c) the ground pentacene, (d) the pentacene powders insonated in 1,2-dichlorobenzene at -13 °C for 10 min, (e) the pentacene powders insonated in water at -13 °C for 10 min, and (f) the calculated PXRD pattern of pentacene powders.

The areas under the endothermic melting peak at 372 °C of the DSC scans of pentacene powders gave the enthalpy of fusion, ∆Hf, of the purchased pentacene powders, the ground pentacene powders, and the pentacene powders insonated in water at -13 °C of 7.365, 6.073, and 5.107 J/g, respectively (Figure 9). According to eq 2, the ∆Elatt of ground pentacene powders was (7.365 - 6.073) J/g ) 1.292 J/g (i.e., 0.3596 kcal/mol) and the ∆Elatt of pentacene powders insonated in water at -13 °C became (7.365 - 5.107) J/g ) 2.258 J/g (i.e., 0.6285 kcal/mol). Both of the ∆Elatt values fell within the similar window of 0.5-8 kcal/mol for polymorphic transformation caused by bond torsion and intermolecular interactions.50 In other words, the total lattice energy, Elatt, of the pentacene powders insonated in water at -13 °C for 10 min is greater than the total lattice energy, Elatt, of the ground pentacene powders, which is greater than the total lattice energy, Elatt, of the purchased pentacene powders. This trend of Elatt values agreed very well with the

ascending order of their corresponding sublimation points of 210, 240, and 250 °C because14

∆Hs ) Hgas - Hsolid ) RT + ∆Einter + ∆Eintra

(3)

where Hgas and Hsolid are the enthalpy of gas and the enthalpy of solid, respectively, and R is the ideal gas constant. Keeping the enthalpy of sublimation, ∆Hs, and the difference of the intramolecular potential energy, ∆Eintra, constant because of no chemical bond formation or bond breakage, any increase of Elatt can cause an increase in the difference of the intermolecular potential energy, ∆Einter, and that will consequently lower the sublimation temperature, T, in eq 3. Furthermore, the lattice defects and the poor crystallinity of the pentacene sample powders could be estimated by the following equation:51

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crystallinity (%) )

f ∆Hsp

∆Hfpp

× 100%

Lee and Chang

(4)

Therefore, the degrees of crystallinity of the ground pentacene samples and the pentacene powders insonated in water at -13 °C for 10 min were 6.073/7.365% ) 82% and 5.107/7.365% ) 69%, respectively. As expected, insonated pentacene powders had the lowest sublimation point because insonation reduced the degree of crystallinity. To further understand if the contribution from newly created surfaces due to size reduction of particles by grinding or insonation could also lower the sublimation point, a series of contact angle measurements was carried out. It is well-known that the vapor pressure of a particle can be expressed as52

( γr )

P ∝ exp

(5)

where γ is the surface energy and r is the radius very similar to the Freundlich-Ostwald equation for the saturation solubility.53 The surface energy, γ, in eq 5 is the total surface and interfacial free energies (or tensions) existing at the solid-vapor boundary TOT . According to the van (indicated by the subscript SV), γSV TOT consists of two components: an apolar Oss-Good theory, γ or Lifshitz-van der Waals component (indicated by superscript LW) of electrodynamic origin and a polar component caused by Lewis acid-base interactions (indicated by superscript AB).54 TOT LW AB γSV ) γSV + γSV

(6)

and the general contact-angle Young-Dupre equation becomes54

Figure 8. Unit cell of pentacene 14.5 Å polymorph having a dimension of a ) 6.37 Å, b ) 7.53 Å, c ) 15.1 Å, R ) 71.37°, β ) 90.6°, γ ) 87.1° with interlayers of (a) (001), (b) (002), (c) (021), (d) (123), and (e) (152).



LW LW + - + γLV(1 + cos θ) ) 2(√γSV γLV + √γSV γLV + √γSV γLV) (7)

where θ is the contact angle at which a liquid-vapor interface meets the solid surface, the subscript LV indicates the liquid-vapor interface, and the superscripts of + and designate the electron-acceptor and electron-donor components, respectively. γLW SV can be determined first by using apolar liquids + ) γLV ) 0 and such as paraffin oil. For an apolar liquid, γLV LW therefore γLV ) γLV . Equation 7 can be reduced to the form of54 LW γLV(1 + cos θ) ) 2√γSV γLV

(8)

LW Once the γSV value was determined from a single value of the + and γSV averaged contact angles, it was inserted into eq 7. γSV values were calculated using eq 7 when water-glycerol sets 54 were simultaneously solved. The γAB SV in eq 6 was obtained by

AB γSV

) 2√

+ γSV γSV

(9)

Table 3. Contact Angles (deg) of Droplets of Paraffin Oil, Glycerol, and Water on the Surfaces of Pentacene and Alq3 Sample Powder Disks powders

paraffin oil

glycerol

water

purchased pentacene ground pentacene pentacene insonated in water at -13 °C purchased Alq3 ground Alq3 Alq3 insonated in water at -13 °C

15.40 ( 1.11 8.24 ( 0.40 7.54 ( 1.13

71.97 ( 0.68 67.20 ( 2.21 74.03 ( 0.77

73.10 ( 1.11 76.04 ( 1.36 78.90 ( 0.68

13.37 ( 0.90 7.03 ( 0.43 12.59 ( 1.15

52.60 ( 2.87 55.26 ( 2.02 38.33 ( 2.25

55.99 ( 1.71 66.73 ( 1.37 60.03 ( 0.81

Table 4. Surface Free-Energy Component Values of Pentacene and Alq3 Sample Powders (mJ m-2) powders purchased pentacene ground pentacene pentacene insonated in water at -13 °C purchased Alq3 ground Alq3 Alq3 insonated in water at -13 °C

LW γSV

+ γSV

γSV

27.87 0.17 15.97 28.60 0.96 9.41 28.65 0.21 10.47

AB γSV

TOT γSV

3.26 31.13 5.99 34.60 2.99 31.64

28.12 0.66 52.52 11.81 39.93 28.68 3.49 10.78 12.27 40.95 28.20 6.70 11.03 17.19 45.40

TOT γSV

and the was calculated from eq 6. The surface free-energy component values54 of water, glycerol, and paraffin oil were given in Table 2 and all the contactangle measurements and the surface free-energy components of pentacene sample powders were given in Tables 3 and 4, respectively. Although the contribution of Lifshitz-van der Waals components was larger than the one of the acid-base TOT values for the pentacene powders were interactions, all γSV close to each other in the range of 31-35 mJ m-2. Therefore, the surface energy did not contribute too much to the depression of the sublimation point (eq 3) as compared to the alteration of lattice energy in the bulk (eq 5) due to lattice defects or poor crystallinity. In addition, SEM micrographs (not shown) also showed that the extent of particle size reduction from the purchased pentacene powders of 100 µm by either sonocrystallization or grinding only gave the same particle size range of 10 µm. The surface area to the bulk volume ratio was still

Figure 9. DSC scans of (a) the purchased pentacene powders with ∆Hf ) 7.365 J/g, (b) the ground pentacene powders with ∆Hf ) 6.073 J/g, and (c) the pentacene powders insonated in water at -13 °C for 10 min with ∆Hf ) 5.107 J/g. Table 2. Surface Free-Energy Component Values of the Liquids Used (mJ m-2) liquid

γLV

LW γLV

AB γLV

+ γLV

γLV

water glycerol paraffin oil

72.8 64.0 28.9

21.8 34.0 28.9

51.0 30.0 0

25.5 3.92 0

25.5 57.4 0

too low for the surface energy effect to become dominant. Since the γLV values of water and glycerol except for the one of TOT values, the wettabilities paraffin oil were higher than all γSV of water and glycerol with respect to the pentacene powder surfaces were worse than the one of paraffin oil as indicated by the much larger contact angles of the droplets of water and glycerol in general (Table 3). At last, to ensure that our novel method was applicable to other small-molecule semiconductors as well, the same experiments were repeated on Alq3. Instead of using good solvents, bad solvents such as xylene and water were used to prepare Alq3 suspensions for sonocrystallization. Since Alq3 did not cause as much equipment contamination as pentacene upon sublimation, TGA was utilized to determine the sublimation point of Alq3 samples rather than employing HSPOM. TGA scans (Supporting Information) clearly showed that there was no solvate/hydrate formation, and the onset temperatures of the weight loss of the purchased Alq3 powders, the Alq3 powders insonated in water at 25 °C for 10 min, the ground Alq3 solids, the Alq3 solids insonated in xylene at -13 °C for 10 min, and the Alq3 solids insonated in water at -13 °C for 10 min were around >300, 250, 250,180, and 180 °C, respectively. All IR bands of all samples were identical (figure not shown) and the same as the one in ref 14, and no detectable impurity or decomposition was observed. The corresponding PXRD diffractograms of the powder samples were illustrated in Figure 10. The appearance of the diffraction peak at 2θ ) 10.5° for the last two Alq3 powder samples above indicated that they were the high energy crystal form of Alq3 ε-form (Figure 10d,e).38 This might explain why the sublimation points of those two samples with the same sublimation point of 180 °C had a much lower sublimation temperature than the one of the purchased Alq3 R-form powders (Figure 10a)1 and the one of the Alq3 powders insonated in water at 25 °C for 10 min which remained an Alq3 R-form with poor crystallinity (Figure 10b)1 and the one of the ground Alq3 powders which became amorphous due to the significant lowering of all diffraction peak intensities (Figure 10c). Since we prefer water to xylene, we would focus on the Alq3 powders insonated in water at -13 °C for 10 min. Using the diffraction peak 2θ ) 11.2° as the characteristic peak for Alq3 R-form to establish a

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Lee and Chang Table 5. Enthalpies of Fusion of Insonated Pentacene and Alq3 Powders in Water at -13 °C for 5, 10, and 30 min

Figure 10. PXRD diffractograms of (a) the purchased Alq3 powders, (b) the Alq3 powders insonated in water at 25 °C for 10 min, (c) the ground Alq3 solids, (d) the Alq3 solids insonated in xylene at -13 °C for 10 min, and (e) the Alq3 solids insonated in water at -13 °C for 10 min.

Figure 11. DSC scans of (a) the purchased Alq3 powders with ∆Hf ) 112.722 J/g, (b) the ground Alq3 powders with ∆Hf ) 101.540 J/g, and (c) the Alq3 powders insonated in water at -13 °C for 10 min with ∆Hf ) 95.546 J/g.

correlation curve between the diffraction peak intensity and the weight % of Alq3 R-form from the physical mixtures of purchased Alq3 powders and amorphous starch powders, the Alq3 powders insonated in water at -13 °C were determined to contain 37 wt % Alq3 R-form powders and 63 wt % of the mixture of Alq3 ε-form and amorphous powders. DSC scans in Figure 11 further revealed the areas under the endothermic melting peak at 413 °C of the purchased Alq3 powders, of the ground Alq3 powders, and of the Alq3 powders insonated in water at -13 °C for 10 min, giving the enthalpy of fusion, ∆Hf, of 112.722, 101.540, and 95.546 J/g, respectively. Again, according to eq 2, the ∆Elatt of the ground Alq3 powders was (112.722 - 101.540) J/g ) 11.182 J/g (i.e., 3.1125 kcal/mol), and the ∆Elatt of the Alq3 powders insonated in water at -13 °C for 10 min became (112.722 - 95.546) J/g ) 17.176 J/g (i.e., 4.7809 kcal/mol). In other words, the total lattice energy, Elatt, of the Alq3 powders insonated in water at -13 °C for 10 min is greater than the total lattice energy, Elatt, of the

sonocrystallization duration (min)

∆Hf of pentacene (J/g)

∆Hf of Alq3 (J/g)

5 10 30

6.857 5.107 5.153

107.663 95.546 91.206

ground Alq3 powders, which is greater than the total lattice energy, Elatt, of the purchased Alq3 powders. This trend of Elatt values agreed with the ascending order of their corresponding sublimation points of 180, 250, and 300 °C according to eq 3.14 The ∆Elatt values again fell in the window of 0.5-8 kcal/ mol for polymorphic transformation.50 The degree of lattice defects and crystallinity of the ground Alq3 powders and the Alq3 powders insonated in water at -13 °C for 10 min were estimated to be around 90% and 85%, respectively, according to eq 4.50 All of the contact-angle measurements and the surface freeenergy components of the Alq3 sample powders were given in TOT values for the Alq3 Tables 3 and 4, respectively. All γSV powders were close to each other in the range of 40-45 mJ m-2. Similar to the reasons given for the pentacene powders, we believe that the surface energy did not contribute too much to the depression of sublimation point (eq 3) as compared to the alteration of the lattice energy in the bulk (eq 5) due to the formation of Alq3 ε-form in water by sonocrystallization at -13 °C for 10 min. Finally, we also repeated sonocrystallization for two other time intervals of 5 and 30 min to understand the effect of the insonation duration time on the lattice energy, Elatt, for both pentacene and Alq3 powders at -13 °C. Their corresponding Elatt values were quantified through the measurement of the enthalpy of fusion, ∆Hf, on the basis of the endothermic melting peak area in the DSC scan. The results were summarized in Table 5. We could clearly see that 10 min insonation duration time that we used throughout the experiments was long enough to reach an optimal nonequilibrium crystal energetic state for both pentacene and Alq3 powders as reflected by the almost unchanged values of ∆Hf after about 10 min. Conclusions The sublimation points of small-molecule semiconductors such as pentacene and Alq3 could be reduced by 40-150 °C via sonocrystallization even in water at -13 °C. This was made possible through the increase of crystal lattice energy by the introduction of crystal defects, poor crystallinity, or the formation of metastable polymorphs but not so much by the newly created surfaces with high surface energy or the presence of impurities. There were two advantages of this robust solvent-based method: (1) there was no need to use either a good solvent or an organic solvent (a green solvent could be used instead), and (2) this one-step method was ready to be scaled up.15,55 The feasibility of sonocrystallization may also imply that similar kinds of “top down” technological platforms for pharmaceutical development, such as high pressure homogenization56 and milling,52 may also work for making powders of small-molecule semiconductors having exceptionally low sublimation temperatures. Acknowledgment. This work was supported by a research grant from the National Science Council of Taiwan, ROC (NSC 97-2113-M-008-006). The authors wish to thank Ms. Jui-Mei Huang, Ms. Shew-Jen Weng, and Ms. Ching-Tien Lin for their


advice on TGA, PXRD, and SEM, respectively, at the National Central University Precision Instrument Center. Supporting Information Available: TGA scans of (a) the purchased Alq3 powders, (b) the Alq3 powders insonated in water at 25 °C for 10 min, (c) the ground Alq3 solids, (d) the Alq3 solids insonated in xylene at -13 °C for 10 min, and (e) the Alq3 solids insonated in water at -13 °C for 10 min. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG801305R

Sublimation Point Depression of Small-Molecule Semiconductors by

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