Precise growth of low-dimensional pyrene·perylene·TCNQ co-crystals and structure–property related optoelectronic properties

Abstract

A phase-transformation method has been developed for the precise growth of charge-transfer (CT) co-crystals with varied stoichiometries. Perylene·TCNQ (P1T1) microrods, pyrene·perylene·TCNQ (PyPeT) microrods and (perylene)₃·TCNQ (P3T1) microsheets were successfully constructed. The phase transformation can not only happen from P1T1 or P3T to PyPeT, but also from P3T1 to P1T1, which can be controlled by changing the solute concentration. Due to the additional pyrene in PyPeT compared to P1T1, devices based on PyPeT microrods show slightly lower electron mobility and much larger positive threshold than P1T1 microrods. In addition, both of them can be used as phototransistors with a maximum I_on/I_off ratio of ca. 1000.

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Functional organic π-conjugated molecules have attracted considerable interest due to their potential use as building blocks of highly integrated miniaturized devices.^1–6 However, exploiting new π-conjugated molecules often requires relatively complicated molecular design and tedious covalent synthesis. Alternatively, non-covalent synthesis is widely used to produce a number of functional architectures, in which different components are assembled. Charge-transfer (CT) complexes, which contain donor (D) and acceptor (A) molecules, represent one of the most important types of such systems.^7,8 They possess unique properties such as being metallic conducting,^9,10 superconducting,¹¹ ferromagnetic¹² and ambipolar transporting,^13–18 and are therefore ideal building blocks for functional nano/microstructures.

Among the various methods, the solution-based approach is the simplest and most efficient way to obtain the targeted co-crystals with the requirement for device construction and their practical application. However, it is still a big challenge to controllably prepare the co-crystals in solution with defined phase and morphology due to the elusive understanding of their formation mechanism. If the co-crystals are consisted of three components, or formed with varied stoichiometries, the issue becomes more complicated. Until now, the CT complexes are mostly obtained by simply cooling or evaporating a mixture solution containing both D and A molecules at ambient conditions. This strategy fails to control the self-assembly of CT complexes into targeted structures, and the products are mixtures in bulk forms which should be picked by eyes for further study. This has severely hampered the development of CT complexes for both basic research and practical application. Facile methods for controllable preparation of CT complexes with the specific stoichiometry are therefore highly desired as well as deep understanding of their growth process. Recently, we reported the controlled growth of TCNQ·2CuOEP and TCNQ·CuOEP co-crystals, and found the phase transformation phenomenon.¹⁹ Then, Zhu et al. reported a simple solution evaporation method to controllably prepare P1T1 and P3T1 co-crystals.²⁰

For the above purpose, a typical and representative system of CT complex with the varied stoichiometries has been carefully selected for study. The three-component co-crystal pyrene·perylene·TCNQ (PyPeT) is a good choice, which was recently obtained by a solvent evaporation method.²¹ Among them, TCNQ is a typical acceptor molecule widely used in CT complexes, while pyrene and perylene are typical donor molecules. TCNQ can form co-crystals with donor pyrene and perylene individually. Both pyrene–TCNQ and perylene–TCNQ complexes have the varied forms with the diverse stoichiometric ratio of donor and acceptor.^21–23 Therefore, the synthesis of this system also refers to the two-component co-crystals with the varied stoichiometries, which can represent most of the CT complexes with varied molecular packing structures. A solution method developed by this system will be instructive to the precise growth of other CT complexes. In addition, perylene·TCNQ (P1T1) and PyPeT has the similar molecular packing except for an additional pyrene as a guest in PyPeT, which is an excellent example for investigation of structure–property relationship of CT complexes.

In this communication, we report the precise growth of pyrene·perylene·CNQ (PyPeT) microrods, Perylene·TCNQ (P1T1) microrods and (perylene)₃·TCNQ (P3T1) microsheets through a phase-transformation process. The phase transformation mechanism is systematically investigated and further revealed. We find that the threshold concentration of solute for the nucleation of co-crystals with different stoichiometry is different, which is the critical factor for the control of the phase transformation. Moreover, P1T1 and PyPeT microrods were integrated into prototype devices and their charge transport and photoresponse properties were investigated, which demonstrated that the molecular structure could largely affect the intrinsic optoelectronic properties.

In a typical synthesis, 0.2 mL chloroform solution of perylene (8 mM) is added into 0.2 mL chloroform solution of TCNQ (1 mM). Then, 1 mL n-hexane is fast injected into the mixed solution with vigorously shaking, which induces supersaturation by lowering the solubility. Dark blue precipitates will be obtained in 10 min. The scanning electron microscopy (SEM) image (Fig. 1b) reveals that they are microsheets. These microsheets will be disappeared by adding 0.2 mL chloroform solution of TCNQ (3 mM) into the mixed solution after several hours. Dark green precipitates will be remained, which are microrods with rectangular cross-section (Fig. 1c). If 0.2 mL chloroform solution of pyrene (8 mM) was added into the mixed solution, the microsheets will also be converted to microrods after a night. However, these microrods have the rhombic cross-section (Fig. 1d), which are different from the above microrods in Fig. 1c.

Fig. 1 (a) Chemical structures of perylene, pyrene and TCNQ. SEM images of (b) P3T1 microsheets, (c) P1T1 microrods and (d) PyPeT microrods. The insets showed the cross-section features.

Their crystal structures were identified by X-ray diffraction (XRD), which were shown in Fig. 2a. The XRD patterns of the microsheets in Fig. 1b, microrods in Fig. 1c and d can be well indexed to the crystal structure of P3T1, P1T1 and PyPeT respectively (S1, ESI†).^21–23 In the XRD pattern of P3T1 microsheets, only the peaks of (010), (100), (020) and (200) are displayed. The absence of the faces of (00l) implies the microsheets grow along the [001] direction and the enhanced (100) peak indicates the (100) face is parallel with the substrate. However, only the peaks of (011), (020), are displayed in the P1T1 microrods and the peaks of (001), (010), (002) are displayed in the PyPeT microrods. The absence of the faces of (h00) in both patterns indicates a preferred growth along the [100] direction of both P1T1 microrods and PyPeT microrods, i.e., their alternative stack direction. This result is different from the previous work by Zhu and coworkers, which claimed the P1T1 nanowires grow along the [001] direction, i.e., the direction perpendicular to the alternative π–π stacking.²⁰ In addition, no peaks belonging to the other components are found in the XRD patterns, indicating that the final products are co-crystals with the high purity, which can not be achieved by other methods for the preparation of CT co-crystals until now.

Fig. 2 (a) XRD patterns of P3T1 microsheets, P1T1 microrods and PyPeT microrods. (b) Raman spectra of PyPeT microrods, P3T1 microsheets and P1T1 microrods along with the source powders of TCNQ, pyrene and perylene.

To further confirm the molecular structures of the as-obtained products, Raman spectra and ultraviolet visible near-infrared (UV-Vis-NIR) were recorded. Fig. 2b shows the Raman spectra of PyPeT microrods, P3T1 microsheets and P1T1 microrods along with the source powder of pyrene, perylene and TCNQ. The peaks from both perylene and TCNQ can be clearly identified in the spectra of all three forms, implying the existence of perylene and TCNQ molecules. In addition, peaks from pyrene are displayed in the spectra of the PyPeT microrods, indicting the existence of pyrene molecules. Another set of strong evidence to prove the formation of CT complexes is the shift of the TCNQ ν₄ (C=C stretching) frequency. As described previously, there is a linear dependency of TCNQ ν₄ frequencies on the degree of charge transfer.^24,25 The shift of the TCNQ ν₄ frequency, less than 10 cm⁻¹, indicates a weak charge-transfer interaction. In addition, the 1454 cm⁻¹ C=C stretching vibration of TCNQ always splits in the single crystals of both P3T1 and P1T1 forms, but is suppressed in the powder spectra.²⁶ Therefore, the appearance of the double TCNQ ν₄ frequencies indicates that they are single crystals. Fig. S2† shows the UV-Vis-NIR spectra of PyPeT microrods, P3T1 microsheets and P1T1 microrods along with the source powder of pyrene, perylene and TCNQ. Compared to their individual ones, new broad absorption band were observed in the NIR region, which can be assigned to the CT bands.

The synthesis involves a phase transformation process, occurred not only from P3T1 to P1T1, but also from P3T1 to PyPeT. In general, the degree of supersaturation, which relies on the concentration of solute, dominates the nucleation and growth of single-component crystals. This situation becomes more complicated in co-crystal systems, in which the concentrations of each component coupled with each other to determine the supersaturation. When the concentration of perylene is changed to 4 mM (or below) and the TCNQ is changed to 2 mM, the dark green precipitates of P1T1 microrods could be directly obtained in several hours. However, the P3T1 form can not be obtained even increase the concentration of TCNQ. That is to say, the P3T1 form only can be obtained at high concentration of perylene, while the P1T1 form could be obtained at low concentration of perylene, i.e., the threshold concentration of perylene for the nucleation of the P1T1 form is lower than the P3T1 form. Note that the growth rate of the P3T1 form is much faster than the P1T1 form. This is possibly due to the stronger CT interaction in the P3T1 form than that in P1T1 form. Because the mean-plane spacing between donor and acceptor molecules in the mixed stack in P3T1 (3.29 Å) is smaller than in P1T1 (3.44 Å), which promotes intermolecular interaction. Therefore, the P3T1 form will be nucleated rapidly when the concentration of perylene is high and TCNQ is low. The concentration of TCNQ was reduced and can not reach the threshold of P1T1 nucleation when the P3T1 co-crystal was completely formed. When sufficient TCNQ is added in, the nucleation and growth of the P1T1 form is initiated. The P3T1 form will dissolve when the concentration of perylene is reduced to a certain degree. The growth of the P1T1 form will continue until the P3T1 form is completely dissolved. Similarly, the threshold concentrations of perylene and TCNQ for the nucleation of PyPeT co-crystal are lower than P3T1 co-crystals. Thus, when sufficient pyrene is added in the mixed solution with the P3T1 precipitates, the PyPeT co-crystal will nucleate and grow until the P3T1 precipitates were completely dissolved. This analysis is further confirmed by the phase transformation process from P1T1 to PyPeT. The PyPeT co-crystal will be obtained in one day after adding 0.2 mL chloroform solution of pyrene (8 mM) into the above mixed solution with P1T1 precipitates. However, the precipitates of P1T1 microrods were completely disappeared. The whole phase transformation process is shown in Fig. 3 (also in S3†).

Fig. 3 Schematic of the phase transformation process.

Therefore, the growth of other multi-component co-crystals with varied stoichiometries may be precisely controlled by this two-step solution method through a phase transformation process.

In order to have a better understanding of the structure–property relationship of CT complexes, the charge transport and photoresponse properties of P1T1 and PyPeT have been investigated by fabricating their field-effect transistors with bottom gate bottom contact geometry. Fig. 4a shows the schematic diagram of the device with the gold source-drain electrodes. Fig. 4c and e show the typical transfer characteristics of the transistors based on individual P1T1 microrod and PyPeT microrod in the dark and under white light irradiation. In order to obtain the intrinsic property, more than 30 devices were measured for each kind, and these devices with relatively high performance were chosen for the representative ones. It can be seen that both of the devices exhibit n-type transporting characteristics with the threshold voltage of ca. 3 V for P1T1 microrod and 25 V for PyPeT microrod, respectively. The electron mobility calculated from the transfer characteristics are 0.01 and 0.007 cm² V⁻¹ s⁻¹ for P1T1 microrod and PyPeT microrod, respectively. These results indicate that the molecular stacking has much influence on the charge transport. Note that no direct electronic coupling exists between the closest acceptor in the mixed stack. The electronic coupling for electrons may result from the mixing of the frontier orbitals of the two closest TCNQ molecules with the perylene as the bridging molecule by super-exchange effect.²⁷ Although both structures of P1T1 and PyPeT have the similar molecular packing, there is an additional pyrene as a guest inserted between the stacks in PyPeT. Therefore, the electron mobility will be smaller in PyPeT than in P1T1 due to the electron transport in pyrene stack is restricted. In addition, the LUMO level of the PyPeT will shift up compared to P1T1 due to the additional donor molecule of pyrene, whose LUMO level is higher than TCNQ (S4). Therefore, the electron injection barrier for PyPeT will be larger than P1T1. As a result, the gate voltage will be more positive for PyPeT than P1T1 to line up the Fermi level to the conduction band to turn device on. Thus, the devices based on PyPeT exhibit larger positive threshold voltage than P1T1.

Fig. 4 (a) Schematic of device structure. (b) Optical image of the devices based on individual P1T1 microrod (up) and PyPeT microrod (down). Transfer characteristics of the devices based on individual (c) P1T1 microrod (channel length L = 20 μm, channel width W = 1.7 μm) and (e) PyPeT microrod (channel length L = 20 μm, channel width W = 4 μm) measured both in the dark and under white light irradiation (light intensity: 12 mW cm²). Schematic structures of crystal of (d) P1T1 and (f) PyPeT viewed along the c axis. The arrows indicate the charge pathway measured.

CT complexes with the mixed stacks contain the molecular donor–acceptor heterojunction and often display broad absorption bands extended to near-infrared region. Efficient light absorption and excitons generation can be expected in such complexes. However, their photoconductive characteristics were rarely explored.^28,29 Recently, preliminary photoresponse was observed on a CT complex with the segregated stacking.¹⁴ This is different from the present studies of P1T1 microrods and PyPeT microrods, which have the mixed stacking. The current of both devices were significantly increased upon a white light irradiation at a drain voltage bias of 40 V. The light responsivity of the device based on P1T1 microrod is 0.36 A W⁻¹, based on PyPeT is 0.18 A W⁻¹, under a white light illumination of 12 mW cm⁻². Therefore, excitons can be generated and dissociated into free charges and transport along the alternate stacking direction. The Fermi level of the co-crystals will be more close to their LUMO level when the excitons are generated upon irradiation. As a result, the electron injection barrier will be smaller and more electrons will be accumulated in the conducting band, which lead smaller threshold voltage and larger current. Thus, they can be used as phototransistors with the large on/off ratio. The curves of the on/off ratio versus the gate voltage of the co-crystals are shown in Fig. S5,† both of which show the maximum on/off ratio close to 1000. These values are much better than the reported CT complexes with photoresponse property.^20,28,29

Conclusions

In conclusion, we have precisely prepared PyPeT microrods, P1T1 microrods and P3T1 microsheets by a phase-transformation method. The phase transformation can not only happen from phase P3T1 to P1T1, but also from phase P3T1 or P1T1 to PyPeT, which may occur on other CT co-crystals. It was found that the threshold concentration of solute for the nucleation of co-crystals with varied stoichiometry is different, which is the main reason for the phase transformation. Because of the additional pyrene in PyPeT, devices based on PyPeT microrods show smaller electron mobility and larger positive threshold voltage than P1T1, despite the similar molecular stacking in the charge transport direction. In addition, both of them can be used as phototransistors with a maximum I_on/I_off ratio of ca. 1000. This study paves the way to the precise growth of CT complexes with diverse stoichiometries and the target of a rational design of CT complexes with desired properties.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21273272). Dr Peng thanks for the support by the China Postdoctoral Science Foundation (No. 2013M541747) and the Postdoctoral Science Foundation of Jiangsu Province (No. 1302159C).

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure, characterization, schematic of the growth process and curves of on/off ratio versus gate voltage. See DOI: 10.1039/c6ra17200h

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