Surface-Mediated Recrystallization for Highly Conducting Organic

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Surface-Mediated Recrystallization for Highly Conducting Organic Radical Crystal Taeyeon Kwon,†,∥,‡ Jin Young Koo,∥,‡ and Hee Cheul Choi*,†,∥ †

Center for Artificial Low-Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-ro, Nam-Gu, Pohang, 37673, Republic of Korea ∥ Department of Chemistry, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-Gu, Pohang, 37673, Republic of Korea

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ABSTRACT: Electrically conducting cation-charged dihydrophenazinium radical (H2PNZ•+) crystals were formed by a one-pot photoreaction. Furthermore, new secondary crystals were grown directly from the H2PNZ•+ crystal surface through a surface-mediated recrystallization (SMR) process by a secondary photoreaction with water. Depending on the absence or presence of light during the exposure of the H2PNZ•+ single crystal to water, neutral yellow PNZ crystals (SMRY) or densely packed green H2PNZ•+ crystals (SMRG) were formed, respectively, on the surface of the mother H2PNZ•+ crystal. The singlecrystal X-ray analysis reveals that the newly obtained SMRG radical crystal exhibits a shorter intercolumnar distance than the original H2PNZ•+ crystal, resulting in increased electrical conductivity.

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lectroactive organic molecules1 have been actively studied as core components for molecular electronic2−4 and spintronic devices.5−7 Recently, organic charged radicals (anions and cations) have captured broad interest as potential charge carriers that can lead to next-generation electronic devices.8−13 However, although many pioneering studies have been conducted on the synthesis of stable organic charged radical molecules through reactions with oxidants or by photolysis,14,15 there are limitations in device application. The charged radicals are highly reactive under ambient conditions, especially under oxygen and humid environments, and are likely to form dimers upon quenching of charge carrier radical species.16 Furthermore, an important issue in organic radical engineering is persistence in the solid state and the density of electron packing in the system.17 To date, the solidstate isolation of the charged radical crystals has been achieved mostly by (1) employing a modified structure with a πconjugated system, alkyl substituents, and heteroatoms (N, S, Se)18,19 or (2) using weakly coordinated counterions.20−22 In addition, high pressure (higher than gigapascals) has also been introduced to create highly ordered and dense structures by increasing intermolecular radical−radical interactions.23−25 Therefore, the identification of appropriate molecular radicals with higher density and persistence in the solid state remains a major challenge to the application of molecular radicals for future electronic devices. Herein, we report a unique approach to obtain highly conducting organic radical crystals via surface-mediated recrystallization (SMR) using phenazine (PNZ) molecules. SMR is a single-crystal-to-single-crystal transformation process © XXXX American Chemical Society

that can crystallize secondary crystals by sacrificing the mother crystals.26,27 Considering the benefits of the SMR process, we developed an efficient way to create a denser structure without employing high temperature or pressure conditions. First, cation-charged dihydrophenazinium radical (H2PNZ•+) crystals were synthesized by a one-pot photoreaction (Scheme 1a,b). Then, the SMR process was employed to grow highly packed secondary crystals directly from the H2PNZ•+ crystal surface by a secondary photoreaction with water. The singlecrystal X-ray analysis of the H2PNZ•+ crystal before and after the SMR process revealed that the molecular packing distance between the H2PNZ•+ units changed significantly (Scheme 1c). It should be noted that the H2PNZ•+ single crystal showed a reasonable electrical conductivity (3.82 × 10−8 S cm−1), implying that the radical species can participate in electronic conduction in the solid state. Moreover, one of the new secondary radical cation crystals (SMRG) exhibited further increased conductivity (1.16 × 10−3 S cm−1). It is reasonable to conclude that the reduced intercolumnar distance is responsible for the increased electrical conductivity as a result of enhanced charge hopping. PNZ molecules are well-known to promote electron transfer because of their high electron affinity,28−30 and they are expected to form molecular radicals that exhibit persistence and stability in network systems due to their highly πconjugated electronic structure.31−34 Therefore, we selected Received: November 11, 2018 Revised: December 28, 2018 Published: January 2, 2019 A

DOI: 10.1021/acs.cgd.8b01686 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Scheme 1. (a) Reaction of Photoinduced H2PNZ•+ Formation; (b) Photograph of the Photochemical Reaction Setup; (c) Scheme of the H2PNZ•+ Packing Structure before and after the SMR Process

H2PNZ•+ single crystal showed a broad signal in air, with a gyromagnetic factor (g) of 2.008 (Figure 1b). In addition, the ESI-MS of H2PNZ•+ showed two distinct peaks in positive mode; the peaks at m/z 182 and 181 correspond to H2PNZ•+ and its fragment peak resulting from the loss of a proton, respectively (Figure S1). Additionally, the UV−vis spectrum of H2PNZ•+ in distilled CH3OH clearly showed characteristic radical peaks at 500−800 and 445 nm (Figure 1c), corresponding to HOMO → SOMO and HOMO − 1 → SOMO transitions, respectively, as confirmed by time-dependent DFT (TDDFT) calculations. The TDDFT results show that the major transition of H2PNZ•+ has an energy of 1.94 eV, which is consistent with the reference data produced by flash photolysis in trifluoroethanol (Figure S2).36−38 To confirm the stability of the H2PNZ•+ crystals, we performed time-dependent UV−vis measurements in Ar and air. Interestingly, the results showed a salient difference: the H2PNZ•+ solution exposed to Ar for 12 h did not show any spectral change, whereas the sample exposed to air resulted in a clear spectral change (Figure S3). The broad band at 500−800 nm and a sharp peak at 450 nm gradually decreased with increasing absorption at λmax = 362.5 nm, and the peak appeared at the same position as the neutral PNZ molecule’s π → π* transition.39,40 This result implies that the radical cation species converted back to neutral PNZ under ambient conditions. As we believe that such a reversible reaction could be caused by moisture in the air, we conducted control experiments by varying the concentration of water in the H2PNZ•+ solution (Figure S4). As a result, the characteristic radical peaks at 400−800 nm decreased rapidly according to the amount of added water, and the resulting solution immediately examined by ESI-MS exhibited a clear mass peak change (m/z 182 → 181), which is ascribed to the PNZ molecule (Figure S5). Furthermore, additional experiments were conducted to investigate the effect of oxygen on the reoxidation process by measuring the UV−vis spectra according to the amount of oxygen purged into the H2PNZ•+ dissolved in water-free distilled MeOH solvent (Figure S6). As a result, we clearly observed the signature peaks of radical cation (400−800 nm) decreased as the

PNZ as a target molecule to ensure radical stability during the further assembly process. To obtain the large and high-quality PNZ radical crystal, we developed a facile and efficient synthetic procedure consisting of a one-pot photoreaction. The rod-shaped H2PNZ•+ crystals were successfully obtained in a mixture of acetonitrile and 1 M HCl under Hg irradiation (less than 3 h). It is noteworthy that the rod-shaped H2PNZ•+ crystals have not been obtained by a simple reaction.35 The scanning electron microscopy (SEM) and optical microscopy (OM) images clearly show that the as-grown H2PNZ•+ crystals have a uniform morphology with a highly smooth surface (Figure 1a). Furthermore, the presence of the radical species in the H2PNZ•+ single crystals was confirmed by continuous wave electron spin resonance spectroscopy (cw-ESR), electrospray ionization mass spectrometry (ESI-MS), and UV−visNIR spectroscopy. The solid-state ESR spectrum of the

Figure 1. (a) SEM image (left) and OM image (right) of H2PNZ•+ crystals. (b) ESR spectra of H2PNZ•+ crystals (0.22 mg). (c) UV−vis spectra of H2PNZ•+ crystals dissolved in distilled methanol (0.07 mM); the inset shows the radical peak of H2PNZ•+ (0.71 mM). B

DOI: 10.1021/acs.cgd.8b01686 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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size for further analyses (Figure S6). The OM images of SMRG @ H2PNZ•+ and SMRY @ H2PNZ•+ also clearly show that SMRG and SMRY crystals grow from the mother H2PNZ•+ crystal surface (Figure 2b). In addition, we found a two-step reaction mechanism of the SMR process as follows: (1) the dissolution of intercalated chloride ion in the H2PNZ•+ crystal (Figure S7) upon contact with water, resulting in PNZ and hydrochloric acid; (2) secondary crystallization of PNZ depending on the presence or absence of light under acidic conditions, which is the main factor of selective crystallization, as mentioned above. The secondary grown SMRY and SMRG crystals were analyzed by 1H NMR, ESR, and Raman spectroscopy. The 1H NMR and Raman spectra of SMRY clearly showed the formation of PNZ molecules in the SMRY crystals (Figures S8 and S9a), and these results are also consistent with the UV data. In addition, the Raman spectra of SMRG showed characteristic peaks at 1355, 1473, and 1600 cm−1 that correspond to the H2PNZ•+ molecule (Figure S9b).41 Time-dependent ESR measurements suggested the presence of radicals in the SMRG crystals. In the absence of light, the TD-ESR spectrum showed a signal decrease because H2PNZ•+ was converted into the PNZ neutral state. However, we could obtain a continuous signal in the presence of light (Figure S10). On the basis of these experimental results, we confirmed that the recrystallized components consisted of PNZ and H2PNZ•+ species in SMRY and SMRG, respectively. We also investigated the effect of oxygen during the SMR process. By running the SMR process under oxygen (O2saturated water) and inert atmospheres (O2-free water), we confirmed the activation effect of oxygen during the SMR process, as SMRG crystals were obtained in high yield from the O2-saturated water, whereas only a small amount of SMRG crystals were produced from the O2-free water (Figure S12). Therefore, we conclude that the oxygen plays an important role in the SMR process by activating the oxidation reaction during the SMR process, while water simply plays a role as an aqueous solvent medium. Electrical conductivity measurements of radical-containing crystals were performed to evaluate the correlation between the structure and unpaired electrons. Indeed, we observed a significant increase in the conductivity of the SMRG crystal compared to that of the starting H2PNZ•+ crystal (Figure 2c). The H2PNZ•+ single crystal showed an electrical conductivity of 3.82 × 10−8 S cm−1 (Figure S11). This is because the delocalized charge in the H2PNZ•+ crystal has a planar structure, inducing the formation of a stronger intermolecular orbital overlap. However, in the case of SMR G, the conductivity dramatically increased to an average value of 1.16 × 10−3 S cm−1 (Figure S12). The maximum electrical conductivity was 1.69 × 10−3 S cm−1 at room temperature. This value is comparable to that of previously reported stable organic radical devices exhibiting high conductivity in air (10−3 ∼ 500 S cm−1).11,13,25 Moreover, the SMRY crystals showed nonconducting behavior, as expected (Figure S13). Single-crystal X-ray analysis of the H2PNZ•+ crystal was performed before and after exposure to Hg light under humid conditions to reveal the structural contribution to the electrical conductivity. The original H2PNZ•+ crystal was assigned to the C2/m space group and formed three kinds of π−π interactions between H2PNZ•+ molecules with face-to-face distances of 3.314, 3.250, and 3.277 Å along the c-axis (Figure 3a). In addition, two different chloride ions are bound to H2PNZ•+ via hydrogen bonds (Cl1···N1: 3.077 Å, Cl2···N3: 2.899 Å). These

amount of oxygen increased, which indicates that oxygen indeed is the main oxidant for the reoxidation of H2PNZ•+. These control experiments clearly show that H2PNZ•+ crystal has an oxygen responsivity and converts back to the PNZ state in air, whereas it can remain in the radical state for a long time under inert conditions. As the H2PNZ•+ has moisture responsivity, we performed electrical conductivity measurements upon exposure to moisture. Interestingly, during the sample preparation, we observed the SMR phenomenon occurring on the surface of the H2PNZ•+ crystal upon contact with water, resulting in the growth of new needle-shaped secondary crystals. Furthermore, it showed selective growth according to the absence or presence of light (Figure 2a). Newly formed crystals showed

Figure 2. (a) Scheme of secondary crystal growth through SMR under humid conditions. (b) OM images of H2PNZ•+, SMRY @ H2PNZ•+, and SMRG @ H2PNZ•+ crystals. (c) Average electrical conductivity of H2PNZ•+, SMRY, and SMRG crystals.

different colors, yellow (SMRY) and green (SMRG), depending on the secondary photoreaction condition, that is, in the absence or presence, respectively, of a light source during water contact. Thus, the H2PNZ•+ crystals produced SMRY crystals (yellow crystals composed of PNZ species) by the SMR process under dark conditions (SMRY @ H2PNZ•+). In contrast, the H2PNZ•+ crystals produced SMRG crystals (green crystals composed of H2PNZ•+ species) by the SMR process under UV irradiation (SMRG @ H2PNZ•+). It is noteworthy that the formation of SMRY takes longer than 1 day, and relatively small crystals were formed, while the SMRG crystals begin to crystallize within 5 min, resulting in a proper crystal C

DOI: 10.1021/acs.cgd.8b01686 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Accession Codes

CCDC 1863714 and 1863715 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hee Cheul Choi: 0000-0003-1002-1262 Author Contributions ‡

T.K. and J.Y.K. contributed equally.

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Notes

Figure 3. Molecular packing structure of the (a, b) H2PNZ crystal and (c, d) SMRG crystal. Gray, C; blue, N; green, Cl; red, O. Hydrogen atoms are omitted for clarity.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Single-crystal X-ray diffraction measurements were performed at 2D beamline of the Pohang Accelerator Laboratory (PAL), Korea.

bonds are much shorter than the known N···Cl hydrogen bond length (approximately 3.4 Å) and are attributed to the packing structure resulting from interconnecting with H2PNZ•+ to form a nearly perfect 2D sheet perpendicular to [100] (Figure 3b). Moreover, the SMR G crystal showed a cofacial herringbone packing structure assigned to the C2/c space group. It had regular π−π interactions with a distance of 3.356 Å along the a-axis. In addition, it showed a strong Cl1···Cl2 interaction along the c-axis. Compared to the original H2PNZ•+ crystal, the intercolumnar distance between H2PNZ•+ molecules decreased by approximately 6 Å in the SMRG crystal (Figure 3c,d), and the water molecules, which showed a weak intermolecular interaction with PNZ, were replaced by chloride ions, resulting in a strong Cl···Cl chain structure.42,43 On the basis of the single-crystal analysis, we conclude that the enhancement of the electrical conductivity originates mainly from the change in molecular packing, which can induce strong orbital overlap and facile charge transfer. In conclusion, the highly crystalline rod-shaped conducting H2PNZ•+ crystal was successfully obtained by a simple and fast one-pot photoreaction under Hg irradiation. The H2PNZ•+ crystal showed an electrical conductivity of ∼10−8 S cm−1 under ambient conditions and showed secondary crystal growth through the SMR process, resulting in SMRY and SMRG crystals under humid conditions. The nonconducting SMRY crystals (composed of PNZ) were grown on the surface of H2PNZ•+ by contact with water in the absence of light. In the presence of light, the highly conducting (∼10−3 S cm−1) SMRG crystals (composed of H2PNZ•+) were grown within 5 min. Single-crystal X-ray analyses revealed that the intercolumnar distance in the SMRG crystal was shorter than that in the original H2PNZ•+ crystal. The reduced intercolumnar distance is believed to be the main reason for the increased conductivity through improved orbital overlap.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b01686. Experimental methods, crystallographic method, ESI-MS data, UV−vis data, NMR data, Raman data, ESR data, and electrical conductivity data (PDF) D

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DOI: 10.1021/acs.cgd.8b01686 Cryst. Growth Des. XXXX, XXX, XXX−XXX