Crystal Growth, HOMOâLUMO Engineering, and Charge Transfer

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Crystal Growth, HOMO-LUMO Engineering and Charge Transfer Degree in Perylene-FxTCNQ (x=1, 2, 4) Organic Charge Transfer Binary Compounds Peng Hu, Kezhao Du, Fengxia Wei, Hui Jiang, and Christian Kloc Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01675 • Publication Date (Web): 06 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016

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Crystal Growth & Design

Crystal Growth, HOMO-LUMO Engineering and Charge Transfer Degree in Perylene-FxTCNQ (x=1, 2, 4) Organic Charge Transfer Binary Compounds †

†

†

†

†

Peng Hu, Kezhao Du, Fengxia Wei, Hui Jiang,*, and Christian Kloc*,

† School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore

1 ACS Paragon Plus Environment


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ABSTRACT

The methodologies of searching for novel organic charge transfer binary compounds and large-size crystal growth in the case that only the two starting organic substances are known but the phase diagram is not known, the thermodynamic data of the binary compound are not known and even the existence of new binary compounds is not known, were studied. Centimeter-long crystals of novel perylene-F1TCNQ, perylene-F2TCNQ and peryleneF4TCNQ charge transfer binary compounds are obtained from gas phase. Kinetically lowering the sublimation rate is the key factor for growing large-size charge transfer compound single crystals. Changing the number of fluorine atoms in FxTCNQ results in the variation of the electron affinity, which further changing the HOMO-LUMO of acceptor. Charge transfer degree is increased with increasing of fluorine atoms in the peryleneFxTCNQ system. Therefore, the structure, stoichiometry and the kind of donor and acceptor enable HOMO-LUMO engineering of charge transfer compound and tune the physical properties.


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INTRODUCTION Physical properties of monomolecular organic solids are determined through the molecular formula of building molecules, the crystal structure of solid as well as the occurrence of defects. During the last thirty years, the rational design of organic molecules and improvement of crystals’ purity and quality have allowed to increase the charge mobility in organic semiconductors from the range of 10-5 cm2V-1s-1 to a few tens of cm2V-1s-1.1,

2

However, for further property tuning, additional method, like compounding different type of molecules into new molecular moieties, seems to be needed. The methodology used in inorganic chemistry, especially in semiconducting industry for generation of enormous range of properties by compounding almost all elements from periodic table, can be considered. Firstly, physical properties of elements are modified by doping elements which has higher or lower electronegativity than the host elements. Further property modifications are achievable by compounding different elements into binary or multinary compounds. Binary semiconductors, like II-VI or III-V, can be considered as examples of compound semiconductors where property of compounds is completely different from constituting elements. The similar compounding can be used to organic solids if in place of elements, molecules are used. Two quite different organic molecules may form binary well defined compounds with new structures and properties. Supramolecular chemistry delivers numerous mechanisms for compounding molecules and crystal engineering explains the intermolecular interactions leading to particular structures and compositions. Charge transfer binary compounds are one such family explored in this paper. Unfortunately, it is still difficult to predict or calculate the stoichiometry and structure of new organic binary compounds. For inorganic binary compounds, phase diagrams between 3 ACS Paragon Plus Environment


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elements describe the presence of individual inorganic binary compounds. Therefore, phase diagrams are the first source of information during preparation of particular multi-element compounds. Such binary diagrams have been created during few centuries, accumulated data from numerous laboratories and are available online for almost every combination of elements. However, due to weak intermolecular interactions, large number of polymorphs and deferred phase transitions between thermodynamically stable and metastable phases, the binary phase diagrams between organic molecules are rather rare.

3, 4

Therefore, the binary

compounds occurring in organic phase diagrams are only sporadically known and mostly discovered by trial or errors or predicted by skillful applicability of crystal engineering. Computer modeling simulates a large number of possible crystal structures and stoichiometries. However, due to small differences between Gibbs free energy of possible compounds, the stable phases in particular conditions can only be guess. Additionally, because of large number of organic acceptors and donors, the possible number of binary organic compounds should be orders of magnitude larger than that of inorganic binary compounds. Moreover, the physical properties of already synthesized binary organic compounds are very

intriguing.

superconductivity,6

The

charge strong

transfer

compound

electron-phonon

may

have

interactions,7

high

conductivity,5

ferromagnetism,8

antiferromagnetism,9 optical applications10 and photovoltaic properties.11 Both the theoretical12 and experimental results13-16 indicate that field-effect transistors made from organic charge transfer compounds can display p-, n-, or ambipolar charge transport properties.


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For studies of fundamental processes in devices and intrinsic physical effects, it is convenient to study charge transfer compounds in the form of bulk single crystals.17-19 There have been many reports on the preparation of single crystals made from one type of molecular building block.20-22 However, the crystal growth of charge transfer compounds is more complex than that of monomolecular materials. Rather small crystals are required for X-ray or synchrotron crystal structure determination and, in this way, confirmation of compound existence. Larger crystals are required for electrical or optical property measurements. Even larger crystals are convenient for detailed studies of the anisotropy of the physical properties and the structure-property relationships. Generally, two methods are used to grow single crystals of charge transfer compounds: solution methods and vapor methods.20 The size of charge transfer compound single crystals varies from micrometers

13, 23-28

to millimeters.16,

29, 30

Solution growth methods, such as

solvent evaporation, temperature lowering or anti-solvent methods, use a third component, solvent, that complicates interpretation of the electrical or optical properties and can even form three-component charge transfer compounds in specific conditions. Therefore, gas phase transport in inert gas is the method of choice for the crystallization of very pure crystals, large enough for structure determinations and properties measurements. In our previous work,16 we changed stoichiometry between acceptor and donor. We compounded TCNQ with perylene molecules for synthesizing perylene-TCNQ charge transfer compounds with stoichiometry of 1:1, 2:1 and 3:1. The charge transfer degree increased with increasing of the ratio between donor and acceptor molecules. Another method to regulate the degree of charge transfer is to keep the same donor and change the type of acceptors. It means the HOMO of donor is fixed and the LUMO of acceptor is changed by changing the electron affinity of acceptor. FxTCNQ (x=1, F1TCNQ: 5 ACS Paragon Plus Environment


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2-fluoro-7,7,8,8-tetracyanoquinodimethane;

x=2,

tetracyanoquinodimethane;

F4TCNQ:

x=4,

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F2TCNQ:

2,5,-difluoro-7,7,8,8-

2,3,5,6-tetrafluoro-7,7,8,8-

tetracyanoquinodimethane) are considered to be good acceptors where the electron affinity is changed by the number of fluorine atoms. The chemical structures of FxTCNQ and perylene are shown in Scheme 1, where a, b, c and d in FxTCNQ are the carbon-carbon bond lengths, which is used to calculate the charge transfer degree. In this work, we use the same donor (perylene) and change the electron affinity of acceptor by changing the number of fluorine atoms in FxTCNQ. The energy level31-35 diagram for donor and acceptors is shown in Figure 1. The electron affinity of acceptor is increased by adding more fluorine atoms in FxTCNQ. Therefore, the LUMO of acceptor is closer to the HOMO of donor when more fluorine atoms are present in the FxTCNQ. The existence and phase diagram of any binary compounds between perylene and fluorinated-TCNQ has not been reported until now. The three new charge transfer crystals of perylene-F1TCNQ, perylene-F2TCNQ and perylene-F4TCNQ with centimeter-size were obtained by using single crystals as the starting materials. We found that kinetically lowering sublimation rate is the key factor for growing large-size charge transfer compound single crystals. Charge transfer interaction is affected by the number of fluorine atoms in FxTCNQ and the charge transfer degree is increased due to the increase of fluorine atoms in FxTCNQ. EXPERIMENTAL SECTION Materials and purification. Perylene and F4TCNQ with 99% purity were purchased from Sigma-Aldrich and J&K Chemical of Shanghai, respectively. F1TCNQ and F2TCNQ were purchased from Tokyo Chemical Industry Co., Ltd. (Japan). Perylene were purified by sublimation at 210°C and fluorinated-TCNQ at 220 °C via open system physical vapor transport (PVT) with argon at 1 atm.36 6 ACS Paragon Plus Environment

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Single crystal growth. A two zone semi-transparent furnace was used for crystal growth. The drawing of the apparatus is shown in Figure 2 (j). To grow single crystals, F4TCNQ and perylene with mole ratios of 1:1 were weighed but not mixed. Perylene and F4TCNQ were put in two separated zone of the furnace. The temperatures were set to 210 oC for the furnace zone where parylene was placed and 220 oC for the zone with F4TCNQ. Argon was used as carrier gas with flow rate of 40 mL/min. The total growth procedure lasted ∼8 hours. Small needle-like single crystals below 1 mm in length were grown when grinded commercial F4TCNQ, not purified powder was used. Larger needle-like crystals were grown from commercial, non-grinded F4TCNQ. The largest crystals, more than 1 cm long, were grown when only large crystals formed during purification of F4TCNQ and large crystals formed during purification of perylene were used as starting materials. If pre-synthesized powder of compound Perylene-F4TCNQ was used, higher temperature was required for evaporation, larger number of nucleus was spontaneously formed and rather small crystals were grown. To grow perylene-F1TCNQ and perylene-F2TCNQ single crystals, the same procedure and apparatus were used. The single crystals of F1TCNQ, F2TCNQ and perylene are used as starting materials to grow the large single crystals of the charge transfer compound. Characterization. Sublimation rates were not measured during the crystal growth process but were measured as a function of time in a thermal gravimetric analyzer, TGA. (TA Instruments, model Q500) Platinum TG pans were filled with commercial powders and single crystals of F4TCNQ and perylene for TGA measurement. In TGA experiments, a purge gas of argon with 40 mL/min was used the same as crystal growth procedure. For F4TCNQ powder and crystals, temperature was increased from room temperature to 220 oC with a rate of 20 o

C/min and maintained at 220 oC for 60 minutes. For perylene powder and crystals, the

ramping rate is the same, but the system maintained at 210 oC for 60 mins. The crystal structures of perylene-F1TCNQ and perylene-F2TCNQ and perylene-F4TCNQ single crystals 7 ACS Paragon Plus Environment


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were obtained using a Bruker SMART APEX Π single crystal diffractometer (X-ray radiation, Mo Kα, λ = 0.71073 Å). Exposure times of 10s for perylene-F1TCNQ and perylene-F2TCNQ, and 40s for perylene-F4TCNQ were applied and the frames were integrated with the Bruker SAINT Software package using a narrow-frame algorithm. The integration of the data using a monoclinic unit cell yielded a total of 14484 reflections to a maximum θ angle of 25.56° (0.82 Å resolutions) for perylene-F1TCNQ. And a triclinic unit cell yielded a total of 8624 reflections to a maximum θ angle of 30.60° (0.70 Å resolutions) was applied for peryleneF2TCNQ. Multi-scan absorption correction method has been applied for all the three samples. The data were refined using SHELX for perylene-F1TCNQ and perylene-F2TCNQ, and JANA2006 for perylene-F4TCNQ. Infrared (IR) spectroscopy measurements were taken using a Perkin Elmer Spectrum GX spectrometer with a spectral resolution of 1 cm−1 in a range of 400-4000 cm−1. The single crystals mixed with KBr (Sigma-Aldrich) were completely grounded and then pressed into pellets for IR measurement. The UV-Vis spectrum was collected with a CRAIC 20 Microspectrophotometer on single crystalline specimens. Raman spectra were recorded using a Witec alpha300 SR Confocal Raman Spectroscope with the excitation laser line of 488 nm. RESULTS AND DISCUSSION To grow perylene-FxTCNQ crystals, we used the PVT method, which was similar to the previously described method of growing one-component organic semiconductors.20 The single crystals made from perylene-F4TCNQ charge transfer compounds were grown using either commercial powder or crystals. The optical images of the commercial powder and single crystals of perylene and F4TCNQ are shown in Figure 2 (e and h) and Figure 2 (d and g), respectively. The single crystals of perylene-F4TCNQ charge transfer compound had quite different sizes when using commercial powder or single crystal as the starting materials. The


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crystals showed a maximum length of 4 millimeters with an average length of 2 millimeters when powders were used. Even smaller crystals were formed when grinded powder with a large surface was used, shown in Figure 2 (a and c). Crystals up to 1.5 centimeters long with an average length of 1 centimeter were grown when single crystals were used as the starting materials. The optical image of F1TCNQ, F2TCNQ, perylene and final charge transfer compound single crystals is shown in Figure 3. The perylene-F1TCNQ and perylene-F2TCNQ charge transfer compound single crystals exhibit also near 1 centimeter length when the same method used for perylene-F4TCNQ. It indicates that such method can be successfully applied to other systems. All the three new charge transfer single crystals exhibit needle-like morphology and dark color. However, by using transmission mode of the microscope, the three single crystals display different colors. Perylene-F1TCNQ, perylene-F2TCNQ and perylene-F4TCNQ show green, dark yellow and red color. The optical absorption in peryleneF1TCNQ, perylene-F2TCNQ and perylene-F4TCNQ are measured by a solid UV-Vis spectrum, shown in Figure S1. The single crystal X-ray diffraction data confirmed the existence of stable new binary compounds. Both perylene-F1TCNQ and perylene-F4TCNQ show 1:1 stoichiometry of donor and acceptor, while perylene-F2TCNQ shows 3:2 stoichiometries of perylene and F2TCNQ. (cif files are included in the Supporting Information). All the three perylene-FxTCNQ crystals showed a mixed-stacking structure (Figure 4). In order to show clearly the D-A packing, only one set of possible orientations of perylene-F1TCNQ and perylene-F4TCNQ were shown in Figure 4. In perylene-F1TCNQ, due to the static disordering, F atoms are distributed at two positions - with site occupancy of 73.6% and 26.4%, respectively (Figure S2). In peryleneF2TCNQ, besides the static disorder for F atoms, both perylene and F2TCNQ molecules also



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exhibits disordering that involves two sets of identical packing but possessing different orientations (ratio 0.5:0.5) and they are symmetrically equivalent (Figure S3). In peryleneF1TCNQ and perylene-F4TCNQ, perylene with F1TCNQ or F4TCNQ are face to face and …D-A-D-A-… alternately stacked. The …-D-A-D-A-… stacking direction is along the a-axis (Figure 4b and 4g). However, the structure of perylene-F2TCNQ is a little bit complex. Perylene with F2TCNQ are also face to face stacking. However, the donor-acceptor stacking shows two different directions. One stacking (red dash square in Figure 4c) is along a-axis with …-D-A-D-A-… (Figure 4d) and the adjacent stacking (blue dash square in Figure 4c) is along b-axis with…-D-A-D-D-A-D-… (Figure 4e). The details of crystallographic data of perylene-FxTCNQ were listed in Table 1. The C-H…N hydrogen bonds can be found in perylene-FxTCNQ single crystals (Figure S4) and the lengths of C-H…N hydrogen bonds were listed in Table S1. Furthermore, the halogen…halogen interactions were also shown in Figure S5. The halogen…halogen interactions can be classified as Type I, Type II and quasi Type I/Type II.37, 38 In order to show the halogen…halogen interactions, perylene molecules was not illustrated in Figure S5. Due to the large distance between F…F (7.1649Å) in perylene-F1TCNQ, no obvious halogen…halogen interactions were found. Perylene-F2TCNQ and perylene-F4TCNQ show Type I and quasi Type II halogen…halogen interactions, respectively. Thermogravimetric analysis (TGA) is widely used to determine the vaporization, thermal decomposition,39 sublimation pressures, diffusion coefficients40 and sublimation rate of organics.41, 42 Thus, the sublimation rate under the pressure of 1 atm of argon was determined by TGA. Here, perylene-F4TCNQ is selected as an example to analyze the relationship between the sublimation rate and crystal growth. The TGA results of F4TCNQ single crystals


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at 220 °C are shown in Figure 5. The derivative curve represents the sublimation rate. The climbing portion of the derivative curve was due to heating to the pre-set temperature of the TGA sample chamber. The sublimation rate was constant during the 60 minutes of heating in constant temperature under 1 atm of argon. The sublimation rate of F4TCNQ single crystals was compared with the sublimation rates of commercial F4TCNQ powder and grinded commercial F4TCNQ powder. The sublimation rate of perylene single crystals was compared with the sublimation rate of commercial perylene powder. The results are shown in Figure 6. Perylene single crystals exhibited lower sublimation rate (~ 0.02 mg/min) than perylene powder (~ 0.05 mg/min). The powder form showed higher sublimation not only for perylene but also for F4TCNQ. In addition, the sublimation rate of grinded commercial F4TCNQ powder was much higher that of commercial F4TCNQ powder and single crystals, reaching sublimation rate of 0.1 mg/min. Both for perylene and F4TCNQ, the sublimation rate of the powder was nearly twice that of the single crystals. The smaller particle size exhibited a higher sublimation rate due to larger surface area. The crystal growth can be divided into two steps:43 nuclei formation and the subsequent growth of the individual nuclei. In the nuclei formation process, stable nuclei are formed when the nuclei grow over a certain size.44 Then, the nucleation rate is determined by critical supersaturation.45, 46 Previous crystal growth experiments in an open system in an inert gas flow in 1 atm revealed that very low supersaturation was sufficient for spontaneous nucleation. Even placement of seeds along the entire length of the crystallization tube did not cause seed growth but spontaneous nucleation on the seeds and on the glass tube. Increased crystallization temperature promoted surface diffusion of molecules and enhanced 11 ACS Paragon Plus Environment


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crystallization on the seeds. However, the higher growth temperature caused higher evaporation and therefore a higher sublimation rate, which caused higher supersaturation and spontaneous nucleation. Therefore, many small crystals were formed at low temperature due to low surface diffusion or at high temperature due to high sublimation rate. To obtain large crystals, the temperature needed to be elevated; however, the sublimation rate needed to be reduced. An inert gas atmosphere and reduced evaporation surface led to exactly such conditions. Powder with a large surface evaporates too fast, and many small crystals are formed. Decreasing the evaporation temperature decreases the transport of molecules to the growth zone; however, the temperature of the growth zone also decreases, and many small crystals are formed. A two-component system is even trickier because the stoichiometry in the gas phase is difficult to control in an open system. Experimentally, crystals of individual components and crystals of binary compounds almost always form close to each other at the crystallization zone. In the case of perylene-F4TCNQ, the band gap and the crystal symmetry of every component were different; therefore, it was possible to select binary compounds from F4TCNQ or perylene crystals under optical microscopy. The perylene-F4TCNQ case discussed above can be generalized into unknown charge transfer compounds potentially existing in the acceptor-donor binary system. If both the donor and acceptor sublimed without decomposition, the selection of high growth temperature and low sublimation rate would lead to large crystal formation of binary compounds. In some binary systems, even more compounds with specific stoichiometries may be discovered. In our case, the new charge transfer compound single crystals of perylene-F2TCNQ shows the stoichiometry between perylene and F2TCNQ is 3:2. Another


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example of such case is the crystallization of perylene-TCNQ,16 where compounds with stoichiometries of 1:1, 2:1 and 3:1 were found. The distance between donor and acceptor (D-A inter-planar distance) can be used to evaluate the charge transfer interaction. Table 2 lists the D-A inter-planar distance between perylene and FxTCNQ molecules in the charge transfer crystals. Due to the 3:2 stoichiometries in perylene-F2TCNQ, the donor-acceptor stacking chains go in two directions (Figure 4d and 4e) and the D-A distance is different in these two directions. The average distance is used for comparison. The D-A distance is shorter with the increase of fluorine atoms in FxTCNQ molecules (Figure 7). Charge transfer degree can be estimated from the bond length variation. This method was widely used47-50 in TCNQ based charge transfer compounds and several fluorinated-TCNQ based charge transfer compounds.51-53 Bond lengths a, b, c, and d directly correlate with the charge transfer degree between the donor and acceptor. However, bond b, c and d are more sensitive to the charge transfer process. The c/(b+d) value can be estimated the charge transfer degree:48, 49 DCT=(αCT-α0)/(α-1-α0) where: DCT is the degree of charge transfer, αx=c/(b+d), b, c and d are the bond lengths shown in Scheme 1. The subscripts CT, -1 and 0 refer to charge transfer compound, the anion and the neutral molecular. The bond lengths, c/(b+d) values and predicted charge transfer degree values of perylene-FxTCNQ (x=0, 1, 2, 4) at 100K are listed in Table 3. Charge transfer at room temperature between perylene and FxTCNQ was confirmed by Raman spectrum, shown in Figure 8. For pristine FxTCNQ, the strong C=C in-plane stretching mode (Ag symmetry) with a frequency around 1454±2 cm-1 were observed.

16, 54

This mode is sensitive to the amount of charge on FxTCNQ molecules and it downshift after charge transferred to FxTCNQ.

16, 54, 55

After obtaining electrons from perylene, the peak



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downshift 11 cm-1, 15 cm-1 and 21 cm-1 for perylene-F1TCNQ, perylene-F2TCNQ and perylene-F4TCNQ. The shift indicates the occurrence of charge transfer and downshift values qualitatively confirmed the charge transfer degree increased with the increase of fluorine atoms. Charge transfer degree at room temperature between perylene and FxTCNQ was estimated quantitatively by infrared (IR) spectroscopy, shown in Figure 9. For pristine F1TCNQ, F2TCNQ and F4TCNQ, the C≡N stretching was observed at 2223 cm-1, 2230 cm-1 and 2230 cm-1, which can be assigned to vibrational modes with b1u symmetry.56 For perylene-F1TCNQ, perylene-F2TCNQ and perylene-F4TCNQ, these peaks are at 2221 cm−1, 2225 cm-1 and 2221 cm−1, and are red shifted compared with pristine FxTCNQ, indicative of charge transfer.57 The equation for charge transfer degree calculation by IR spectrum is shown below:52, 58 DCT=2∆ν/ν0(1-ν12/ν02)-1 where: DCT is the degree of charge transfer, ∆ν=ν0-νCT. The ν0,νCT and ν1 represent C≡N stretching modes frequency of the pure FxTCNQ, charge transfer compound and the FxTCNQ anion. The ν0, νCT and ν1 and calculated charge transfer degree values of perylene-FxTCNQ (x=1, 2, 4) at room temperature are listed in Table 4. At low temperature, the intermolecular distances between donor and acceptor are reduced by the lattice spacing decreasing, indicating that interactions become stronger. Therefore, the charge transfer degree at low temperature is larger than at room temperature. Due to the strong electron affinity of fluorine atoms, the charge transfer degree increased with the increase of fluorine atoms in the perylene-FxTCNQ system (Figure 7). CONCLUSIONS To study the degree of charge transfer in the perylene-FxTCNQ system, we grew charge transfer binary compounds and focused on crystal growth in the case that only the two starting organic substances are known, but the phase diagram is not known, the 14 ACS Paragon Plus Environment

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thermodynamic data of the binary compound are not known and even the existence of new binary compounds is not known. We grew three new centimeter-sized charge transfer crystals of perylene-F1TCNQ (1:1 stoichiometry), perylene-F2TCNQ (3:2 stoichiometries) and perylene-F4TCNQ (1:1 stoichiometry) by using single crystals of individual components as the starting materials. We concluded that kinetically lowering of sublimation rate was the key factor for growing large-size single crystals and that the method described could be successfully used to obtain novel unknown binary or multinary structures for the exploration of the physical properties of weakly bonded molecular crystals. The charge transfer degree between fluorinated TCNQ and perylene increased with the number of fluorine atoms in the perylene-FxTCNQ system. It means the stronger acceptors attract more electrons from the same donor. The existence of multiple compounds with stoichiometry different from 1:1 complicates the simple calculation of charge transfer degree from bond length measurements. IR absorption or Raman spectroscopy gives additional information on charge transfer but doesn’t give spatial distributions of transferred charge. Therefore, the structure, stoichiometry and the kind of donor and acceptor enable HOMO-LUMO engineering of charge transfer compound and tune the physical properties. However, the direct prediction of physical properties from structure and charge transfer is far from realization. ASSOCIATED CONTENT Supporting Information. CIF file and checkCif report of perylene-F1TCNQ, peryleneF2TCNQ, perylene-F4TCNQ single crystals. CCDC 1418430 for perylene-F1TCNQ, CCDC 1418429 for perylene-F2TCNQ and CCDC 1406782 for perylene-F4TCNQ. The detailed disorder structures analysis of perylene-FxTCNQ. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION 15 ACS Paragon Plus Environment


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Corresponding Authors *E-mail: [email protected]; *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Authors gratefully thank Singapore Ministry of Education for the support via an AcRT RG125/4 grant. REFERENCES (1)

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Kawase, T.; Shimoda, T.; Ogawa, S., Appl. Phys. Lett. 2007, 90, 102120. (3)

Horst, J. H. t.; Cains, P. W., Cryst. Growth Des. 2008, 8, 2537-2542.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table Caption Table 1 Crystallographic data for perylene-F1TCNQ, perylene-F2TCNQ and peryleneF4TCNQ derived using single crystal X-ray diffraction Table 2 The D-A inter-planar distance between perylene and FxTCNQ (x=0, 1, 2, 4) molecules in the crystals Table 3 The bonds length, c/(b+d) values and predicted charge transfer degree values of perylene-FxTCNQ (x=0, 1, 2, 4) at 100K Table 4 The ν0,νCT and ν1 and calculated charge transfer degree values of perylene-FxTCNQ (x=1, 2, 4) at room temperature



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Page 22 of 38

Table 1

Perylene-F1TCNQ

Perylene-F2TCNQ

Perylene-F4TCNQ

C32H14FN4

C84H40F4N8

C32H12F4N4

Formula weight

473.47

1237.24

528.5

Crystal system

monoclinic

triclinic

triclinic

P 1 21/c 1

P -1

P -1

Lattice parameter a (Å)

7.1649(18)

7.2167(12)

6.9988(7)

Lattice parameter b (Å)

10.894(3)

11.6944(19)

7.2813(7)

Lattice parameter c (Å)

14.446(4)

18.217(3)

11.2715(12)

Lattice parameter α (degree)

90

75.927(4)

104.154(7)

Lattice parameter β (degree)

90.313(7)

82.686(5)

101.861(6)

Lattice parameter γ (degree)

90

72.139(5)

90.291(7)

1127.6(5)

1417.1(4)

544.14(10)

1.395

1.450

1.612

2

1

1

Temperature (K)

100

100

100

µ(Mo Kα)/mm-1

0.090

0.096

0.121

486

636

268

Reflections collected

14484

8624

7899

Independent reflections

2098

8624

2149

Formula

Space group

Cell volume (Å3) Calculated density (g/cm3) Formula units per cell Z

F(000)

R1 (I > 2σ(I))

0.0614

0.0637

0.0390

2

wR(F ) (I > 2σ(I))

0.1094

0.1479

0.1024

R1 (all data)

0.1854

0.1257

0.0533

2

wR(F ) (all data)

0.1494

0.1669

0.1138

GOF

0.968

0.917

1.95


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Table 2

D-A inter-planar distance between perylene and FxTCNQ molecules (Å)

Angles between perylene and FxTCNQ mean planes (o)

Perylene-TCNQ31

a axis:3.346(2)

4.164(6)

Perylene-F1TCNQ

a axis:3.269(1)

2.909(4)

Perylene-F2TCNQ

a axis:3.292(2); b axis: 3.212(1)

0.664(4)

Average: 3.252(2) Perylene-F4TCNQ

a axis:3.218(2)

2.474(3)



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Page 24 of 38

Table 3

a (Å)

b(Å)

c(Å)

d(Å)

c/(b+d)

CT

TCNQ 59

1.346

1.448

1.374

1.441

0.4756

0

TCNQ- 49

1.373

1.432

1.420

1.416

0.4986

1

PeryleneTCNQ 16

1.338(2)

1.441(3)

1.368(3)

1.436(3)

0.4758

0.01±0.07 16

F1TCNQ 60

1.336

1.433

1.370

1.430

0.4785

0

F1TCNQ

1.358(4)

1.422(4)

1.412(5)

1.417(6)

0.4974

1

PeryleneF1TCNQ

1.345(4)

1.441(5)

1.377(4)

1.434(5)

0.4789

0.021±0.061

F2TCNQ 61

1.328

1.443

1.376

1.437

0.4778

0

- 62

F2TCNQ

1.332

1.424

1.412

1.425

0.4956

1

PeryleneF2TCNQ

1.335(2)

1.437(2)

1.384(3)

1.441(3)

0.4809

0.174±0.01

F4TCNQ

1.333(2)

1.436(2)

1.371(8)

1.433(2)

0.4779

0

F4TCNQ

1.356(4)

1.414(4)

1.415(4)

1.430(6)

0.4975

1

PeryleneF4TCNQ

1.344(3)

1.435(2)

1.387(2)

1.435(5)

0.4833

0.303±0.02

- 53

- 63


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Table 4 ν0 (cm-1)

νCT (cm-1)

ν1 (cm-1)

CT

Perylene-F1TCNQ

2223

2221

219853

0.080±0.05

Perylene-F2TCNQ

2230

2225

220053

0.168±0.03

Perylene-F4TCNQ

2230

2221

219456

0.252±0.03



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Scheme 1. The chemical structures of FxTCNQ and perylene. (a, b, c and d in FxTCNQ are represented as carbon-carbon bond length)


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Figure Caption

Figure 1 Energy level diagram for perylene and FxTCNQ. (The HOMO values of F1TCNQ and F2TCNQ are not found in literatures. The HOMO and LUMO values of perylene are obtained from Ref. 35; The HOMO value of TCNQ is obtained from Ref. 36; The LUMO values of TCNQ, F1TCNQ and F2TCNQ are obtained from Ref. 37 and Ref. 38; The HOMO and LUMO values of F4TCNQ are obtained from Ref. 39.)

Figure 2 Optical images of the starting materials and the final charge transfer compound single crystals. a: grinded commercial F4TCNQ powder; b: commercial perylene powder; c: perylene-F4TCNQ charge transfer compound single crystals; d: commercial F4TCNQ powder; e: commercial perylene powder; f: perylene-F4TCNQ charge transfer compound single crystals; g: F4TCNQ single crystals; h: perylene single crystals; i: perylene-F4TCNQ charge transfer compound single crystals; j: single crystal growth apparatus.

Figure 3 Optical images of perylene, F1TCNQ, F2TCNQ single crystals and the final charge transfer compound single crystals. a: F1TCNQ single crystals; b: perylene single crystals; c, perylene-F1TCNQ charge transfer compound single crystals; d: F2TCNQ single crystals; e: perylene single crystals; f: perylene-F2TCNQ charge transfer compound single crystals.

Figure 4 Crystal packing of perylene with fluorinated-TCNQ charge transfer compound single crystals. a: along the (100) direction of perylene-F1TCNQ; b: perylene-F1TCNQ stacking along a-axis; c: along the (100) direction of perylene-F2TCNQ (red dash square indicates perylene-F2TCNQ stacking along a-axis and blue dash square indicates peryleneF2TCNQ stacking along b-axis); d: perylene-F2TCNQ stacking along a-axis with …-D-A-DA-…; e: perylene-F2TCNQ stacking along the b-axis with …-D-A-D-D-A-D-…; f: along the (010) direction of perylene-F4TCNQ; g: perylene-F4TCNQ stacking along a-axis.



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Figure 5 Thermogravimetric measured weight loss and sublimation rate of F4TCNQ single crystals at 1 atm of argon and 220 °C. During the first 10 minutes, the sample was heated from room temperature to 220 °C at a rate of 20 °C/min, and then the temperature was kept constant at 220 °C.

Figure 6 Sublimation rate vs time of perylene at 1 atm of argon and 210 °C (a) and F4TCNQ at 1 atm of argon and 220 °C (b).

Figure 7 Relation between number of fluorine atoms and D-A inter-planar distance, charge transfer degree of perylene-FxTCNQ.

Figure 8 Raman spectra of pristine FxTCNQ (a) and perylene-FxTCNQ (b) at room temperature.

Figure 9 IR spectra of perylene-F1TCNQ (a), perylene-F2TCNQ (b) and perylene-F4TCNQ (c) compared with pristine FxTCNQ at room temperature.


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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Figure 6


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Figure 7



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Figure 8


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Figure 9



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For Table of Contents Use Only

Crystal Growth, HOMO-LUMO Engineering and Charge Transfer Degree in Perylene-FxTCNQ (x=1, 2, 4) Organic Charge Transfer Binary Compounds †

†

†

†

†

Peng Hu, Kezhao Du, Fengxia Wei, Hui Jiang,*, and Christian Kloc*,

HOMO-LUMO engineering and kinetically lowering sublimation rate enable searching novel organic charge transfer binary compounds and growing large-size single crystals in the case that phase diagram and thermodynamic data of binary compound are not known. The charge transfer degree is increased with increasing of fluorine atoms in perylene-FxTCNQ due to the strong electron affinity of fluorine atoms.


Crystal Growth, HOMOâLUMO Engineering, and Charge Transfer

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