Molecularly Smooth Single-Crystalline Films of Thiophene–Phenylene

Feb 21, 2014 - ABSTRACT: Single crystals of thiophene−phenelyne co-oligomers (TPCOs) have previously shown their potential for organic optoelectroni...
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Molecularly Smooth Single-Crystalline Films of Thiophene− Phenylene Co-Oligomers Grown at the Gas−Liquid Interface◆ Valery A. Postnikov,†,○ Yaroslav I. Odarchenko,‡,○ Alexander V. Iovlev,§ Vladimir V. Bruevich,§ Alexander Yu. Pereverzev,∥ Ludmila G. Kudryashova,§ Vladimir V. Sobornov,§ Loïc Vidal,‡ Dmitry Chernyshov,⊥ Yuriy N. Luponosov,@ Oleg V. Borshchev,@ Nikolay M. Surin,@ Sergei A. Ponomarenko,@,∇ Dimitri A. Ivanov,*,‡,# and Dmitry Yu. Paraschuk*,§ †

Physics and Material Science Department, Donbas National Academy of Civil Engineering and Architecture, Derzhavin Str. 2, Makeevka 86123, Ukraine ‡ Institut de Sciences des Matériaux de Mulhouse-IS2M, CNRS UMR 7361, 15 rue Jean Starcky, 68057 Mulhouse, France § Lomonosov Moscow State University, Faculty of Physics and International Laser Center, Leninskie Gory 1, 119991 Moscow, Russian Federation ∥ Lebedev Physical Institute, Russian Academy of Sciences, Leninisky Pr. 53, 119991 Moscow, Russian Federation ⊥ Swiss−Norwegian Beam Lines, ESRF BP-220, 38043 Grenoble, France @ Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, Profsoyuznaya 70, 117393 Moscow, Russian Federation # Faculty of Fundamental Physical and Chemical Engineering, Lomonosov Moscow State University, Leninskie Gory 1/51, 119991 Moscow, Russian Federation ∇ Faculty of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1/3, 119991 Moscow, Russian Federation S Supporting Information *

ABSTRACT: Single crystals of thiophene−phenelyne co-oligomers (TPCOs) have previously shown their potential for organic optoelectronics. Here we report on solution growth of large-area thin single-crystalline films of TPCOs at the gas−liquid interface by using solvent−antisolvent crystallization, isothermal slow solvent evaporation, and isochoric cooling. The studied co-oligomers contain identical conjugated core (5,5′diphyenyl-2,2′-bithiophene) and different terminal substituents, fluorine, trimethylsilyl, or trifluoromethyl. The fabricated films are molecularly smooth over areas larger than 10 × 10 μm2, which is of high importance for organic field-effect devices. The low-defect structure of the TPCO crystals is suggested from the monoexponential kinetics of the PL decay measured in a wide dynamic range (up to four decades) and from low crystal mosaicity assessed by microfocus X-ray diffraction. The TPCO crystal structure is solved using a combination of X-ray and electron diffraction. The terminal substituents affect the crystal structure of TPCOs, bringing about the formation of a noncentrosymmetric crystal lattice with a crystal symmetry Cc for the bulkiest trimethylsilyl terminal groups, which is unusual for linear conjugated oligomers. Comparing the different crystal growth techniques, it is concluded that the solvent−antisolvent crystallization is the most robust for fabrication of single-crystalline TPCOs films. The possible nucleation and crystallization mechanisms operating at the gas−solution interface are discussed.

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INTRODUCTION

various materials studied, co-oligomers containing thiophene− phenylene,4 furan−thiophene,5 and furan−phenylene6 aromatic rings show promising properties for organic optoelectronics. Importantly, single crystalline films show the highest performance in organic field effect transistors (OFETs)7 and OLETs4 because of their unprecedented structural and electronic order. Moreover, single crystals allow observation of intrinsic charge

Recent progress in organic optoelectronics has resulted in commercialization of organic light emitting diodes (OLEDs). Moreover, over the past decade, a novel type of organic lightemitting devices has been developed [i.e., organic light-emitting transistors (OLETs)],1 which can outperform the OLEDs2 and become a promising testbed for organic injection lasers.3 The OLET performance is mainly determined by luminescent and charge transport properties of the active layer(s) so that for efficient single-layer OLETs, the materials should combine high luminescence and ambipolar charge carrier transport. Among © 2014 American Chemical Society

Received: December 16, 2013 Revised: February 18, 2014 Published: February 21, 2014 1726

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Scheme 1. Synthesis of Thiophene−Phenylene Co-Oligomers: (a) 5,5′-Bis(4-trimethylsilyl-phenyl-1-yl)-2,2′-bithiophene TMS−PTTP−TMS, (b) 5,5′-Bis[4-(trifluoromethyl)phenyl-1-yl]-2,2′-bithiophene CF3−PTTP−CF3, and (c) 5,5′-Bis[4fluorophenyl-1-yl]-2,2′-bithiophene F-PTTP-F

method of OFET/OLET single-crystalline films implies fast growth rate, low-defect crystal structure, and smooth surface. The latter is of primary importance for high-performance organic field-effect devices, as the electrical current in them mainly flows within a few near-surface molecular layers. Recently, a promising solvent−antisolvent crystallization (SAC) technique has been proposed for the growth of organic semiconducting single crystals at the gas−solution interface.15,16 As a result, large-area highly luminescent anthracene single crystals were grown,15 and high-performance singlecrystalline inkjet printed OFETs were demonstrated.16 One can expect that large-area low-defect organic semiconducting crystals can be used as a substrate for integrated organic electronic devices similarly as silicon used in inorganic electronics.

transport on an organic semiconductor surface,8 and their interfaces can demonstrate metallic conductivity9 and high photoresponsivity corresponding to an external quantum efficiency of nearly 100%.10 Among organic semiconducting crystals combining the high luminescence and efficient charge transport, the most studied are thiophene−phenylene cooligomers (TPCOs).4 The TPCO single crystals show a high photoluminescence quantum yield11,12 and a charge mobility of ∼1 cm2/(V s).4 The highest charge carriers mobility and photoluminescence quantum yield were reported for the TPCO crystals grown by physical vapor transport (PVT) technique, whereas they can also be grown from solution.13,14 Importantly, efficient solution growth methods of singlecrystalline organic semiconducting films are in great demand for organic electronics as they allow low-cost industrial upscaling. Among various requirements, an efficient growth 1727

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solvent evaporation (ISSE)21 and isochoric cooling (IC)] are given in Section 4 of Supporting Information. Briefly, in the ISSE method, a semiclosed vial with TPCO solution was held at room or higher temperature so that the solvent was evaporating over a period ranging from a few days to one month. In the IC method, a sealed vial filled with a TPCO solution was gradually cooled during a period of up to ten days. As a result, TPCO crystals can be observed at the bottom and surface of the solution. Microscopy. The crystals were examined using optical microscopes Motic BA210 and Leica DM2500 M in transmitted light through crossed polarizers and in reflected light, respectively. The surface of the single-crystal films was investigated with atomic force microscope (AFM) Integra Spectra (NT-MDT) in the tapping mode. The film thickness was measured using AFM or a laser confocal microscope LEXT OLS4000 (Olympus). The AFM images were treated using Gwyddion.22 Optical Absorption and Photoluminescence (PL). The steadystate PL and absorption spectra of solutions were measured with a multifunctional spectrometer ALS01M.23 All measurements were carried out at room temperature on diluted solutions (from 10−5 to 10−6 M) in UV-grade tetrahydrofurane. Emission spectra were registered in the range of 350−700 nm for excitation wavelengths varied in a range of 250−400 nm. The PL excitation wavelength was set at the maximum of the corresponding absorption spectrum. The PL spectra of the crystalline samples were measured using an integrating sphere coupled to a fiber spectrometer S100 (Solar Laser Systems). The PL was excited by a laser diode at a wavelength of 405 nm. The PL kinetics in the films were measured using a confocal microscope MT200 (PicoQuant) by a time-correlated single-photon counting method. The PL excitation wavelength was 376 nm, and the laser pulse repetition rate was 20 MHz. The PL photons were registered with an avalanche photon detector with 4 ps resolution. X-ray Diffraction. Single-crystal diffraction was measured at the Swiss−Norwegian Beamlines of the ESRF. The X-ray patterns were collected with a PILATUS2M pixel area detector. A monochromatic beam at a wavelength of λ = 0.69411 Å was slit-collimated down to 100 × 100 μm2. The sample-to-detector distance and parameters of the detector were calibrated using a LaB6 NIST standard. The detector images were recorded by phi-scans in shutter-free mode with a 0.1 deg angular step. The data were preprocessed by SNBL Tool Box,24 and then by CrysAlis Pro (Agilent Technologies, version 171.36.24). The crystal structures were solved with SHELXS and refined with SHELX.25 Microfocus X-ray Diffraction. The measurements were performed at the ID13 beamline of the ESRF. The monochromatic X-ray beam with the wavelength of 1.0 Å was focused down to 50 nm along both axes using crossed-Fresnel optics. The images were recorded with a FreLon fast CCD camera with a pixel size of 50 μm (not rebinned) and a 16-bit readout. The norm of the scattering vector s (s = 2 sin θ/ λ) was calibrated using the diffraction pattern of corundum. The region of interest was selected with an on-axis optical microscope operated in reflection mode. A beam monitor installed upstream of the sample provided dose-monitoring for online exposure normalization. The free-standing single crystal mounted on a capillary was scanned with the help of an x−y gantry in transmission geometry. The diffraction patterns were collected using a step of 200 nm. The data reduction and analysis, including geometrical and background correction, visualization, and radial, as well as azimuthal integration of the 2D diffractograms, were performed using home-built routines designed in Igor Pro (Wavemetrics Ltd.). Selected-Area Electron Diffraction. SAED experiments were carried out with a Philips CM200 transmission electron microscope operated at 200 keV. Calibration of the electron diffraction patterns was performed using graphite. For the measurements, the TPCO single crystals were deposited on gold TEM-grids with 400 meshes.

In this work, we report on fast growth of large-area highquality single-crystalline TPCO films at the gas−solution interface. Single-crystalline films of a series of TPCOs (Scheme 1), which have identical conjugated cores containing four aromatic rings (5,5′-diphyenyl-2,2′-bithiophene, PTTP) and different terminal substituents such as trifluoromethyl (CF3), trimethylsilyl (TMS), and fluorine (F), were grown and characterized. The synthesis of the two of them, namely 5,5′bis[4-(trifluoromethyl)phenyl-1-yl]-2,2′-bithiophene (CF3− PTTP−CF3)17 and 5,5′-bis(4-trimethylsilyl-phenyl-1-yl)-2,2′bithiophene (TMS−PTTP−TMS),18 was described earlier using less efficient techniques, but the crystalline structure was reported only for the former. The synthesis and properties of 5,5′-bis[4-fluorophenyl-1-yl]-2,2′-bithiophene (F−PTTP− F) are reported for the first time. Apart from affecting the growth conditions, the substituents allow a shift of the frontier molecular orbital energies that are of key importance for achieving the electron or hole charge transport. The CF3− PTTP−CF3 compound exhibits promising properties for applications in OFETs17 and OLEDs19 such as the electron mobility of 0.18 cm2/(V s). Remarkably, the TPCO crystalline films grown at the gas−solution interface can have large domains with molecularly smooth surfaces, which are beneficial for efficient charge transport in the OFET/OLET ultrathin near-surface layer. In addition, in this work, the crystal structure of the TPCO crystals was solved with a combination of X-ray and electron diffraction. From the structural and photoluminescence (PL) kinetic data, one can suggest that the resulting high-quality TPCO structure is comparable to that of vapor-grown TPCO crystals prepared by PVT. The crystallization mechanism at the gas−solution interface will eventually be discussed.

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EXPERIMENTAL SECTION

TPCO Characterization. GPC analysis was performed by means of a Shimadzu LC10AVP series chromatograph (Japan) equipped with an RID-10AVP refractometer and SPD-M10AVP diode matrix as detectors and a Phenomenex column (USA) with a size of 7.8 × 300 mm2 filled with the Phenogel sorbent with a pore size of 500 Å; THF was used as the eluent. Glassware was dried in a drybox at 150 °C for 2 h, assembled while hot, and cooled in an argon stream. For thin layer chromatography, Sorbfil plates were used. In the case of column chromatography, silica gel 60 (Merck) was taken. 1 H NMR spectra were recorded at a Bruker WP-250 SY spectrometer, working at a frequency of 250.13 MHz and utilizing a CDCl3 signal (7.25 ppm) as the internal standard. In the case of 1H NMR spectroscopy, the compounds to be analysed were taken in the form of 1% solutions in CDCl3. The spectra were then processed on the computer using the ACD Labs software. Elemental analysis of C and H elements was carried out using CHN automatic analyzer CE 1106 (Italy). The settling titration using BaCl2 was applied to analyze sulfur. Experimental error is 0.30−0.50%. A spectrophotometry technique was used for the Si analysis, as described in reference 20. Crystal Growth. In the SAC method, the solvent (toluene) and antisolvent (ethanol or isopropanol) were mixed in equal volume parts, and then the TPCO was dissolved in this mixture, assisted by sonication in an ultrasonic bath. The solution was filtered through a PTFE filter with a pore diameter of 0.45 μm. In the next step, a glass beaker with a neat solution was placed in a box containing the antisolvent.15 The box was then closed and placed in an adiabatic environment (Figure S7 of the Supporting Information). The antisolvent in the external box evaporates and condensates onto the oligomer solution; the oligomer solution gradually saturates and, upon reaching the critical saturation, the TPCO crystallizes at its surface. The details of other crystallization methods used [i.e., isothermal slow 1728

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Figure 1. (a) TMS−PTTP−TMS and (b) CF3−PTTP−CF3 single crystals grown by the SAC and IC, respectively.

Table 1. Growth Methods, Conditions, and Maximal Lateral Sizes, d, of the TPCOs Crystals Grown. C Is the Initial Oligomer Concentration, and t Is the Growth Time growth method solvent−antisolvent crystallization (SAC)

isothermal slow solvent evaporation (ISSE)

TPCO

solvent

T (°C)

C (g/L)

t (days)

d (mm)

TMS−PTTP−TMS CF3−PTTP−CF3 F−PTTP−F TMS−PTTP−TMS

toluene−isopropanol/ethanol (1:1) toluene−isopropanol/ethanol (1:1) toluene−ethanol (7:5) toluene hexanea toluene chlorobenzene hexanea toluene toluene toluene toluene toluene

24 24 24 24 24 24 24 24 24 50b 75b 60−25c 60−25c

0.9 0.8 0.4 1.3 0.1 0.9 0.9 0.1 0.4 1.2 1.5 3 2.5

2 2 3 9 8 16 26 >30 4 14 14 10 3

6.3 7.1 4.1 2 1 2 1 0.5 0.2 1 3 5.1 6.2

CF3−PTTP−CF3

F−PTTP−F

isochoric cooling (IC)

TMS−PTTP−TMS CF3−PTTP−CF3

The crystals were observed on the vial bottom. bWith a radiator13 (see Figure S8 of the Supporting Information). cThe initial and final temperatures. aThe statistical dispersion of crystal lateral sizes was from a few tenths of a millimeter to the maximal size, d, in one experiment. The experiment was repeated 3−10 times for each entry.

a

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RESULTS Synthesis. Novel efficient synthetic schemes were elaborated for preparation of 5,5′-bis[4-(trifluoromethyl)phenyl-1yl]-2,2′-bithiophene (CF3−PTTP−CF3) and 5,5′-bis(4-trimethylsilyl-phenyl-1-yl)-2,2′-bithiophene (TMS−PTTP−TMS), while 5,5′-bis[4-fluorophenyl-1-yl]-2,2′-bithiophene (F− PTTP−F) was synthesized for the first time (Scheme 1) using a combination of conventional organic and organometallic reactions. The TPCO with trimethylsilyl end groups TMS−PTTP− TMS was prepared in four steps (Scheme 1a). First, 5,5′dibromo-2,2′-bithiophene (2) was synthesized by bromination of 2,2′-bithiophene (1) in 80% isolated yield, as described in ref 26. Then (4-bromophenyl)(trimethyl)silane (4) was obtained by lithiation of 1,4-dibromobenzene (3) followed by the reaction with trimethylchlorosilane in 91% isolated yield.27 The pinacoline boronic derivative (5) was prepared in 98% isolated yield by lithiation of compound 4 followed by the treatment with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (IPTMDOB). Finally, the thiophene−phenylene oligomer 5,5′-bis(4-trimethylsilylphenyl-1-yl)-2,2′-bitiophene (TMS− PTTP−TMS) was prepared by Suzuki cross-coupling reaction between 5,5′-dibromo-2,2′-bithiophene (2) and trimethyl[4(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]silane (5). Purification of the target oligomer TMS−PTTP−TMS was

made by classical column chromatography on silica gel followed by recrystallization procedure to give pure oligomer TMS− PTTP−TMS in 86% isolated yield, which is more than two times higher as compared to the previously described synthesis of the same oligomer by Kumada cross-coupling reaction.18 The TPCO with trifluoromethyl end groups CF3−PTTP− CF3 was prepared by Suzuki cross-coupling reaction between 5,5′-dibromo-2,2′-bithiophene (2) and 4,4,5,5-tetramethyl-2[4-(trifluoromethyl)phenyl]-1,3,2-dioxaborolane (7) (Scheme 1b). In this case, the reaction yield was 72% that is also higher than those reported earlier.17 The increased yield can be explained under the reaction conditions by the use of more stable compound 7 instead of 4-trifluoromethylphenyleneboronic acid and THF instead of toluene as a solvent. Precursor 7 was synthesized according to ref 26 from 1-bromo-4fluorobenzene (6) in 97% isolated yield. The TPCO with fluoro end groups F−PTTP−F was obtained by lithiation of 2-(4-fluorophenyl)thiophene (8) followed by oxidative coupling reaction in the presence of CuCl2 (Scheme 1c). For the synthesis of compound 8, Kumada coupling of 4-fluorophenylmagnesium bromide to 2-bromothiophene in the presence of Pd(dppf)Cl2, as a catalyst was applied. The purity and molecular structures of all the TPCOs synthesized were confirmed by 1H NMR, GPC spectroscopy, 1729

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Figure 2. Optical micrographs of single crystals of (a and b) TMS−PTTP−TMS, (c and d) CF3−PTTP−CF3, and (e and f) F−PTTP−F grown by the SAC; other micrographs are shown in Figure S9 of the Supporting Information. All the images were recorded in transmitted light except (b) recorded in reflection.

and elemental analysis. The purity of the TPCOs was evaluated as better than 99.9%. More details concerning the TPCO synthesis and characterization can be found in the Supporting Information. Comparison of Crystal Growth Methods. The synthesized TPCOs were crystallized using SAC, ISSE, and IC techniques. The latter approach was recently applied for growth of large-area TPCO crystals at the gas−solution interface.14 Figure 1 presents typical examples of TPCO crystals grown at the solution surface. Table 1 summarizes the growth conditions used and maximal sizes of the TPCO single crystals, which typically had a shape of thin plates. The fastest growth was observed for the SAC method so that the TPCO crystals with CF3- and TMS-terminal groups grew up to 6−7 mm within two days. The resulting crystal thickness ranged from a few hundred nanometers to tens of micrometers, depending on the growth conditions (see Sections 4.2 and 4.3 of the Supporting Information). The size of the largest TPCO single crystals grown by SAC was close to the biggest crystal size reported for the other TPCO compound [2,5-bis(4-biphenylyl)thiophene] using slow IC growth from solution within 20−300 days.14 As follows from Table 1, the ISSE technique provided the lowest crystal growth rates at the solution surface. For TMS−PTTP− TMS and F−PTTP−F, the SAC technique provided larger crystals with about 1 order of magnitude faster growth rate than

the ISSE or IC. The SAC and IC techniques allowed us to grow TMS−PTTP−TMS and CF3−PTTP−CF3 crystals of similar sizes but with a higher growth rate in SAC (Table 1). The growth rate was evaluated from the maximal crystal sizes, and it seems to be underestimated as the larger crystals could be spontaneously cleaved in smaller ones during the growth (Figure 1). Other details are given in Sections 4.2 and 4.3 of the Supporting Information. Crystal Habit and Morphology. Figure 2 shows optical images of (a and b) TMS−PTTP−TMS, (c and d) CF3− PTTP−CF3, and (e and f) F−PTTP−F single crystals grown by the SAC method. Optical images of crystals grown by the ISSE and IC techniques are shown in Figures S10 and S11 of the Supporting Information, respectively. The TMS−PTTP− TMS crystals have a rectangular habit (see also Figures S10a and S11, panels a and c, of the Supporting Information), the CF3−PTTP−CF3 crystals are characterized by a rhomblike shape (see also Figures S9c, S10b, S11d of the Supporting Information), and the F−PTTP−F crystals grow into flat hexagonal plates (Figures S9e and S10c of the Supporting Information), which are often extended along one direction (Figure 2e). On the crystal basal planes facing the solution, one can observe some sparse screw dislocations (Figure S9b of the Supporting Information), as well as pyramids formed due to surface nucleation followed by island growth (Figure 2b). At 1730

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Figure 3. AFM topography images of the top surface of (a) TMS−PTTP−TMS and (b) CF3−PTTP−CF3 films grown by the SAC recorded near the edge and at the central area, correspondingly. Cross sections of the images are shown in Figure S13 of the Supporting Information.

Table 2. Unit Cell Parameters of the Studied TPCOs oligomer

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

symmetry

Za

Herringbone angle (deg)b

tilt with respect to c* (deg)c

TMS-PTTP-TMS CF3−PTTP-CF3 F-PTTP-F

12.25 6.15 5.79

12.25 7.60 7.25

34.69 19.5 35.82

90 90 90

90.55 97.18 94.3

90 90 90

Cc P21/a P21/n

8 2 4

64.9 56.0 54.4

49.3 22.5 3.2

a

Number of molecules per unit cell. bThe herringbone angle was computed from the dihedral angles between the least-squares planes of the nearestneighbor molecules within the molecular array spreading along the ab-plane. cThe tilt angle was measured between the molecular long axis connecting the two carbons at the p-positions of the terminal phenyls and the normal to the ab-plane (c* axis).

Figure 4. (a) a*b-plane projection of the F−PTTP−F unit cell; packing of terminal fluorine atoms in the (b and c) (002) plane. Hydrogens are omitted for clarity.

μm2 (Figure S14 of the Supporting Information). Note that the molecularly flat surface with an area up to 5 × 5 μm2 was reported for distyrylbenzene crystals grown by PVT;28,29 therefore, the solution and vapor growth methods can provide semiconducting organic crystals with a large-area molecularly flat surface. On the other hand, the bottom surface of the solution-grown TPCO crystals is too rough to be studied by AFM. The surface imperfections observed in the optical microscopy images (Figure 2 and Figures S9−S11 of the Supporting Information) are typically a part of the bottom surface. This finding indicates that the interface growth method is capable to provide the molecularly flat single-crystal surfaces of large lateral sizes, which are comparable with the channel length of organic field-effect devices. The molecularly flat organic semiconductor surface over the channel length is in great demand for high-performance OFET/OLET devices. Note that the IC method can also generate TMS−PTTP−TMS and CF3−PTTP−CF3 single-crystals with molecularly flat top surfaces over extended areas (Figure S15 of the Supporting Information). Therefore, one can conclude that molecularly flat TPCO surfaces can be obtained using the different interface growth techniques. Crystalline Structure. To analyze the structure of the TPCO crystals under investigation, single-crystal X-ray diffraction experiments were carried out. Table 2 summarizes the resulting unit cell parameters, while Figures 4−6 show the

the bottom basal surfaces, a well-developed dendritic structure is visible for F−PTTP−F (Figure 2f). By contrast, the top surfaces are less defective than the bottom ones, and the most common morphological features are the growth steps (Figure S9a of the Supporting Information). An increased concentration of growth steps at the periphery of the upper facets is clearly seen for CF3−PTTP−CF3 in Figure 2d. For some crystals of CF3−PTTP−CF3, instability of the growth front is observed so that the otherwise flat crystal growth front breaks down to form isolated lamellae running along the crystal growth direction (Figure S9d of the Supporting Information). From these observations one can suggest that the growth rate is faster at the bottom facet facing the solution than at the top one (see the Discussion section). The AFM study of the single crystalline films revealed molecularly smooth terraces on the top crystal surfaces (i.e., the ones oriented toward the gas phase). For all the TPCO crystals, individual molecular growth steps with a height of ∼2 nm are clearly visible (see Figure S12 of the Supporting Information). The smoothest surface is found in the central area of the top crystal surfaces. Figure 3 shows a near-edge region of (a) TMS−PTTP−TMS and a central region of (b) CF3−PTTP− CF3, exhibiting a height corrugation assigned to a single molecular step. Remarkably, the out-of-edge area of the TMS− PTTP−TMS crystals exhibited the molecularly flat surface with a root-mean-squared roughness of ∼0.05 nm (limited by the instrument) over the areas larger than an AFM scan of 10 × 10 1731

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Figure 5. (a) ac-Plane projection of the CF3−PTTP−CF3 unit cell; (b) view along the long molecular axes. The (002) plane is depicted in red in (a). Hydrogens are omitted for clarity.

Figure 6. (a) Projections of the TMS−PTTP−TMS unit cell along the a*-axis; staggered packing of Si atoms in the (002) plane (b); view along the long molecular axes for one molecular layer showing planarity of the chain and configuration of the (c) terminal TMS groups.

The TMS−PTTP−TMS lattice with the bulkiest trimethylsilane end groups was found to be pseudo-orthorhombic with β = 90.55° (Table 2). The unit cell structure of this compound is very particular. To the best of our knowledge, it is the only example of a conjugated oligomer that crystallizes into a noncentrosymmetric unit cell (symmetry group Cc), which could impart interesting nonlinear optical properties to TMS− PTTP−TMS crystals such as second-harmonic generation and electro-optical effects. The unit cell of TMS−PTTP−TMS contains two layers of molecules strongly inclined with respect to the layer normal (cf. Figure 6a and Table 2). Moreover, the direction of the chain rotates by ca. 90° about the c-axis when one passes from one layer to the next one (Figure 6c). This particular arrangement of molecules gives rise to a noncentrosymmetric unit cell containing eight molecules. The terminal TMS groups pertinent to the neighboring layers adopt a staggered packing (Figure 6b), making the c-parameter become approximately equal to the double of the projection of the long molecular dimension on the c-axis. This situation is similar to that of F−PTTP−F. Interestingly, the tilt of the molecule with respect to the c*axis strongly increases with the volume of the terminal group. In the row F → CF3 → TMS, which corresponds to the increase of the occupied Van-der-Waals volume by about five times, the tilt angle progressively increases from an almost insignificant value (3.2°) to 49.3° (Table 2). Similar behavior

corresponding crystal structures. In the crystalline phase, the studied molecules adopt an almost planar conformation and exhibit herringbone packing typical of oligothiophene and other TPCO compounds.4 The herringbone angle between adjacent molecules in the layers (Table 2) is found to be rather close to that of a single-crystalline quaterthiophene (55.68°)30 as well as to TPCOs such as PPTTPP, PP4TPP, P6TP,31 and C2-PTTPC2.32 The F−PTTP−F compound with the smallest terminal group crystallizes in a unit cell containing four molecules with their long axes perpendicular to the a*b-plane (cf. Figure 4 and Table 2). To optimize the molecular packing in the neighboring layers, some interdigitation of the terminal fluorine atoms takes place in the (002) plane (Figure 4b). The packing constraints make the c-parameter approximately double the long molecular dimension. A similar structure with the same P21/n symmetry was reported by Samulski et al. for the PPTPP oligomer.33 The unit cell of CF3−PTTP−CF3 was reported to have a statistical positional distribution of the CF3 groups.17 Two most probable conformations of the CF3 groups can be identified (Figure 5b). For this crystal, the c parameter is close to the long molecular dimension [i.e., twice smaller than that of the F− PTTP−F unit cell (Table 2)]. This data can be explained by weaker interactions between the neighboring layers in the CF3−PTTP−CF3 structure. As a result, the end-group conformations are less-correlated between the layers. 1732

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Figure 7. TEM images of (a) F−PTTP−F, (b) CF3−PTTP−CF3 crystals, and corresponding SAED patterns of the [001] zone in the insets. (c) Micrographs in crossed polarizers with the growth planes indicated. The dashed lines show the crystal orientation in the dark state.

was observed for the quaterthiophenes where by changing from linear hexyl to branched 2-ethylhexyl side groups, the tilt angle changed from 22 to 58°. 34 Also, for 2-octyldodecyldisubstituted septithiophene, the bulkiness of the end-chain substituent results in a tilt of the molecule in a monolayer by approximately 40° with respect to the film normal.35,36 Although the structure of the studied TPCOs has been resolved, it is important to know the crystallographic growth planes in order to establish the structure−property relationship for the materials investigated. Crystal Growth Planes. The growth planes of the TPCO crystals from fluorine-terminated molecules were established with the help of transmission electron spectroscopy (TEM) and selected-area electron diffraction (SAED) techniques (Figure 7, panels a and b). For both types of crystals, the observed diffraction peaks belong to the [001] diffraction zone. The F− PTTP−F crystals have a hexagonal habit with (020) and (110) facets (Figure 7, panels a and c), whereas CF3−PTTP−CF3 forms rhomblike crystals with (110) facets (Figure 7, panels b and c). In contrast to the F−PTTP−F and CF3−PTTP−CF3 crystals always showing a clear birefringent behavior in crossed polarizers (Figure 7c), some TMS−PTTP−TMS crystals stayed bright in the crossed polarizers, independently of the in-plane rotation angle. Among other reasons, extensive twinning could result in this unusual optical behavior. To reveal possible twins or even polymorphic forms coexisting within isolated single crystals, they were analyzed with the help of microfocus X-ray diffraction. Figure 8 shows the averaged microfocus pattern that can be indexed to the [001] diffraction zone. To analyze the local crystal orientation in detail, a region around the 22̅0 peak was extracted from a sequence of 2D images of a meshscan. This image selection (Figure 8, right panel) shows that the TMS−PTTP−TMS sample is indeed a single crystal, with the mosaicity evaluated from the 2D microfocus scan being less than 1°. From the comparison of the optical micrograph and 2D diffraction patterns, one can deduce that the crystal growth planes are (100) and (010). Photoluminescence Data. Figure 9 presents the photoluminescence (PL) spectra of dilute THF solutions and single crystals of the studied oligomers. The solution spectra of all three TPCO show very similar shapes and are peaked at ∼450 nm. Also, the PL spectra are very close to those of the

Figure 8. Selected 2D-microfocus X-ray diffraction pattern measured on a TMS−PTTP−TMS crystal. Inset: optical micrograph of the corresponding crystal taken with an on-axis optical microscope. Right: results of a microfocus mesh scan covering an area of 2 × 10 μm2 with a step of 200 nm. The panel displays only a small region of the patterns selected around the 22̅0 peak.

unsubstituted TPCO (PTTP) reported earlier.37 A small 10 nm blue shift of the F−PTTP−F PL can be assigned to the far less polarizable fluorine end groups, as compared to the CF3 and TMS groups. This is quite expected as all the TPCO have the same conjugated PTTP core that mainly determines the electronic properties. The optical absorption spectra of the TPCO in solution are also very similar (Figure S16 of the Supporting Information). The PL of the TPCO single crystals is strongly anisotropic as was reported for other TPCO crystals.4 The PL anisotropy is mainly assigned to the waveguiding effect of the TPCO crystals so that the PL leaves the crystal plate mainly from its edges. Because of this, the single-crystal PL spectra shown in Figure 9b were measured by using an integrating sphere. Nevertheless, the PL spectral shape noticeably depended on the crystal size, and we assign this to the PL self-absorption.38 Indeed, the larger the crystal size, the longer the PL path inside the crystal. This results in a partial absorption of the PL blue part, resulting in an apparent PL red shift. The PL self-absorption is exemplified in Figure 9b by the data for large (>1 mm) 1733

dx.doi.org/10.1021/cg401876a | Cryst. Growth Des. 2014, 14, 1726−1737

Crystal Growth & Design

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Figure 9. Photoluminescence spectra of (a) dilute THF solutions and (b) single crystals of the studied TPCOs.

CF3−PTTP−CF3 and TMS−PTTP−TMS crystals and for a small (