Shape-controlled synthesis of platinum octaethylporphyrin crystalline aggregates modulated by versatile ionic liquids

Abstract

This work presents a facile approach toward the synthesis of platinum octaethylporphyrin crystalline aggregates with wires, leaves, plates, and four-leaf clover like architectures by exploitation of intriguing ionic liquid-mediated solution self-assembly. Moreover, the well-defined microwires exhibit highly reproducible and sensitive photo-response characteristics.

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In recent years, considerable effort has been focused on the development of effective approaches for the fabrication of functional micro/nanostructures with different morphologies from small organic molecules due to their diversity, tailor-ability, and multi-functionality.^1,2 The enormous potential that porphyrin and its derivatives possess in solar energy conversion, catalysis, photochemistry, sensing and optoelectronic devices has made them favorite building blocks in the emerging field of molecular materials.^3,4 To date, various solution-processed protocols have been exploited to control over the self-assembly of porphyrin, mainly including surfactant-assisted,⁵ template-directing,⁶ and kinetics-driving methods.^7,8 Note that the products prepared by the former two methods often contain an amount of impurities, which require rather tedious post-purification. The latter method without using any additives is of particular interest because the simplicity of fabrication implies minimized processing costs.

However, despite the fact that the molecular aggregation states are strongly depend on the surrounding solvent media,⁹ most studies in this field have hitherto been almost exclusively restricted on traditional molecular solvents as the fluid phase for self-assembly of organic species. Different from the water and organic solvents, room temperature ionic liquids (ILs) generally are a family of unprecedentedly versatile and tunable organic salts through appropriate combination of distinct cations and anions components.¹⁰ They have gained increasing interest as designable solvent with environmentally benign properties such as negligible vapor pressure and favorable chemical and thermal stability, good dissolving ability, thus they are widely used in organic synthesis, catalysis, separation science, electrochemistry.^11,12 However, little attention was paid to fabricate the superstructures of small organic molecules using ILs as promising alternative media. In fact, the previous research has demonstrated the importance of control over assembly pathways, both in terms of aggregates stability as well as dynamics.¹³ In this regard, we envisage that viscous ILs would offer attractive opportunities for facile creation of novel porphyrin aggregates with the controlled morphologies and properties. Meanwhile, the search for alternatives to the damaging volatile solvents has become a high priority because the introduction of cleaner technologies has become a major concern throughout both the industry and academia. Accordingly, the use of ILs can alleviate atmosphere pollution.

Platinum octaethylporphyrin (PtOEP, Fig. S1†) represents one of the important metal complexes and no study has so far been devoted to the optoelectronic performances of one-dimensional (1D) PtOEP assemblies, focusing mainly on the use in electronics and light harvesting systems in the forms of macro-sized bulk phase or amorphous thin films.^14,15 Organic chromophores with 1D architectures have received much interest in the past decade.^16,17 As for these functional systems, a fine balance of the collective interactions of π–π stacking and other non-covalent interactions, entropic contributions and steric constrains are reasonably well established. Herein, we for the first time describe the reliable fabrication of ultralong microwires of PtOEP in bulk quantities by kinetic-driving method, in which ionic liquid [BMIM]BF₄ was used as unique soft material to control the nucleation step. Although the common physical vapor deposition could also produce the short PtOEP nanorods with length less than 5 μm, the elaborate fabrication process and harsh low yield severely limited their practical application.¹⁸ Since some properties of 1D organic materials (such as field emission, long-term waveguide) are extremely affected by the length or the length-to-radius ratio of the assemblies,¹⁹ our new method for ultralong PtOEP microwires is highly desirable. More interestingly, strikingly different other morphologies including microleaves, microplates, and four-leaf clovers like architectures could conveniently produced only by simply varying the molecular structure of ILs under almost all conditions. Our research results strongly indicate this is an alternate synthetic strategy that provides a new platform for preparing various morphologies of PtOEP assemblies without the need for auxiliary shape-regulating surfactants.

In a typical synthesis, 0.5 ml chloroform solution of PtOEP (2 mg ml⁻¹) was carefully injected into 0.5 ml ILs to from a miscible liquid–liquid interface and the mixture solution was then stored without disturbance in the dark for 24 h. The final products were recovered by centrifugation. Fig. 1 shows a set of SEM images of self-assembled architectures of PtOEP formed in different ILs. It can be seen that both the anions (Fig. 1a–c) and the cations of ILs (Fig. 1d–f) play an important role in controlling the morphology of PtOEP assemblies. Microwires with smooth surface are successfully obtained in [BMIM]BF₄. The average diameter of wires was 500–800 nm, and the length is up to 200 μm. By varying the employed cation or anion of ILs, micro/nanoscale leaves, plates, and four-leaf clover-like structures are easily produced in [BMIM]PF₆, [B₃MePy]BF₄, and [BPy]BF₄, respectively. Interesting, only a mixture of several above shapes are obtained in [BMIM]NO₃ or [HOEtMIM]BF₄. Furthermore, in our synthesis, we performed the control experiments by systematically changing (1) the volume ratio of chloroform and [BMIM]BF₄ from 1 : 1 to 1 : 10 and (2) the concentration of PtOEP in chloroform from 0.5 to 4 mg ml⁻¹. As depicted in Fig. 1a and 2, similar wire-shape crystals were all obtained. Moreover, the average length of PtOEP microwires gradually decreased as both the volume of [BMIM]BF₄ and the concentration of PtOEP in the starting solution increased. It was also found that the formation of PtOEP microwires was virtually independent of the sequence of the solvent addition, and thus this soft, solution-based approach should be easily extended to large-scale production of PtOEP microwires.

Fig. 1 SEM images of PtOEP assemblies, which were prepared by injecting chloroform solution of PtOEP into different ionic liquids. (a) [BMIM]BF₄, (b) [BMIM]NO₃, (c) [BMIM]PF₆, (d) [B₃MePy]BF₄, (e) [HOEtMIM]BF₄, and (f) [BPy]BF₄.

Fig. 2 SEM images of PtOEP assemblies prepared by injecting chloroform solution of PtOEP into [BMIM]BF₄. The volume ratio of chloroform and [BMIM]BF₄ was: (a) 5 : 1; (b) 1 : 5; (c) 1 : 10. The concentration of PtOEP was: (d) 4 mg ml⁻¹; (e) 1 mg ml⁻¹; and (f) 0.5 mg ml⁻¹.

The internal structure of the PtOEP microwires was investigated using X-ray diffraction (XRD) (Fig. 3a). All the diffraction peaks could be well-indexed to the triclinic phase.²⁰ Moreover, the diffraction peak of the [1−10] planes of the microwires corresponding to the 2θ = 7.23° is significantly sharp, revealing their preferential growth was along the [001] orientation. This was further confirmed by the well-defined selected area electron diffraction (SAED) pattern of a single PtOEP microwire (Fig. S2†). In addition, the smooth microwire has a single crystalline structure and wire axis is the c axis of the crystal. The Raman, FT-IR and energy dispersive X-ray (EDX) spectrum were measured to analyze the chemical composition of these microwires and the possible structure changes after fabrication. It is seen that the Raman and FT-IR spectrum (Fig. 3b and c) of microwires has similar features to that of PtOEP powder. The peak at 684 cm⁻¹ is attributed to the macrocycle breathing connected with pyrrole stretch (Fig. 3b). Four metal-sensitive IR characteristic bands of microwires are observed at 746, 968, 993, 1231 cm⁻¹, implying that PtOEP does not undergo decomposition or chemical reaction during solution process. In addition, the EDX spectrum of microwires (Fig. S3†) shows only the peaks of C, N and Pt elements, which is consistent with the results of the FT-IR and Raman. X-ray photoelectron spectroscopy (XPS) was used to infer the valence states of Pt in the microwires. The binding energies of 73.1 and 76.2 eV correspond to the 4f_7/2 and 4f_5/2 state of Pt(II) in the Pt–N coordination bonds (Fig. S4†).²¹ The weight percent of Pt was about 26% in the microwires by thermogravimetric analysis (TGA) (Fig. S5†). Therefore, it is concluded that the as-fabricated microwires are purely made of PtOEP molecules without the residual IL. Fig. 3d shows typical UV-vis absorption spectra of PtOEP microwires deposited on quartz and PtOEP monomer in chloroform solution. The absorption of PtOEP solution features an intense band in the near UV region (so-called Soret band) and a weaker Q-band in the 470–550 nm range, which consists of a main S0 → S1 band at 535 nm in addition to a vibronic feature at 500 nm.²² Similar to the monomers, the microwires have three adsorption bands, labelled S, Q1, and Q2. Note that the bands of microwires are broadened, compared with the sharp peaks from monomers. Moreover, the Q bands are red-shifted while the S band is blue-shifted. These changes can be attributed to the H-aggregation formation²³ and highly ordered molecule packing in PtOEP microwires. Thus, the absorption bands of PtOEP microwires actually overlap with the visible-light region.

Fig. 3 (a) XRD patterns, (b) Raman spectra and (c) FTIR spectra of source powder and microwires of PtOEP (d) UV-Vis absorption of PtOEP microwires and monomers in chloroform.

The formation of PtOEP microwires might take place via a crystallization process including the nucleation, oriented assembly and diffusion-limited growth in the course of liquid–liquid interfacial precipitation. First, PtOEP molecules exist as monomer in the dilute chloroform solution. Then, the slow diffusion of the [BMIM]BF₄ into PtOEP solution will drive the precipitation of PtOEP to form nuclei owing to the low solubility of PtOEP in [BMIM]BF₄. In the growth step, the remaining PtOEP molecules would adjust their orientation and growth on the nuclei. Ostwald ripening might also occur to consume small nuclei or crystals for growth. The anisotropic nature of the intermolecular interactions between PtOEP molecules leads to the formation of ordered 1D microwires. The high viscosity of [BMIM]BF₄ (106 mPa s) lead to limited diffusion and offer an ideally low driving force for the slow growth of particles. Furthermore, the viscosity of the mixed solution may be controlled by alternation of the volume content of [BMIM]BF₄ or the initial concentration of PtOEP, allowing the rate of crystal growth to be varied (Fig. 2). In contrast, a direct mixing between the tetrahydrofuran solution of PtOEP and water resulted in the very fast kinetics crystallization process, which invariably produced short nanorods of PtOEP.⁷ The present results indicate that this technically innovative concept based our findings might open a new door toward fabrication of 1D porphyrin-based materials.

The mixed solvent systems based on chloroform and ILs may also provide a promising platform for the synthesis of PtOEP crystalline aggregates with various morphologies due to the intriguing tunability of ILs (Fig. 1). Since the final products induced by drowning-out crystallization all have the high chemical purity (Fig. S6†), we speculate that the diverse morphologies of PtOEP assemblies may be directly related to the different solubility of the PtOEP monomer in ILs used. As reported, the supersaturation of the growth units during the crystal growth can be a key factor to control the shape and surface structures of the aggregates.^24,25 Furthermore, all four pyrrole-ring moieties of each planar PtOEP molecule simultaneously have strong π–π interactions with its neighbors along the three crystallographic directions. Besides, there exist the non-covalent dipolar interactions between the metal cation and π-electrons of the porphyrin ring. Accordingly, the ability of imidazolium or pyridinum cation of the employed ILs to coordinate with platinum metal centre of the PtOEP is different and thus should also influence the whole self-assembly process, as exemplified by various syntheses of metal particles with different morphologies in ILs.^26–28

The single-crystal microwires of PtOEP can be expected to facilitate superior device performance due to the enhanced π–π conjugated morphology and the establishment of charge transport according to the molecular packing orientation. To investigate the photoconductive properties, the as-fabricated PtOEP microwires were directed for the construction of protype photodetector (Fig. S7†). As depicted in Fig. 4a and b, the current of device increases with the bias voltage and behaves two distinct states, a “low” current state in the dark and a “high” current state under the visible light irradiation. The I–V curves are good linear and quasi-symmetric, which proves a fine ohmic contact between the PtOEP microwires and Au electrodes. In the dark, the current was only 0.05 pA at an applied bias of −20 V. However, even at a low incident density of 1.57 mW cm⁻², the photocurrent reaches 4.25 pA, giving an on/off switching ratio of 85. This value can be larger and the current reaches 24 pA under higher incident density of 12.05 mW cm⁻², producing the photocurrent on/off ratio of 480. The photocurrent is sensitive to the intensity of the incident light probably because of the different photo densities from the incident light. In addition, the photoconduction switching is demonstrated to be prompt and reversible by light turning on and off under the intensity of 12.05 mW cm⁻². The response time is ca. 0.8 s from the “off” to “on” state”, and the recovery time is ca. 1.2 s at bias of −20 V (Fig. 4b). The energy gap of the microwires is about 2.1 eV estimated from its absorption spectra and Tauc plot (Fig. 3d and S8†). This gap is narrow enough to permit generation of substantial numbers of charge carriers by white light (Fig. S9†). Hence, the high sensitivity of our microwires-network–based device to light is understandable. It is noted that the prototype device also shows high photocurrent durability and environmental stability. As shown in Fig. 4c, the current in the dark keeps stable in 1000 s, while the photocurrent at −30 V with an illumination intensity of 12.05 mW cm⁻² remains almost constant. More interestingly, these microwires in the fluorescent microscope image exhibit bright red luminescence at crossover points and relatively weaker emission from the body (Fig. 4d). This is a typical feature of an optical waveguide, that is, they are able to absorb the excitation light and propagate the red emission toward tips, which has been observed only in few organic materials.²⁹ The partial overlapping between the absorption and emission spectra is one of the factors to diminish the optical wave-guiding effect.²⁹ The large Stokes shift (>100 nm) and smooth surfaces of the PtOEP microwires with well-ordered molecule arrangement help in minimizing the optical loss, thus contributing to their good optical wave-guiding behaviors (Fig. S10†). The above results suggest that the PtOEP microwires have the promising potential in the field of organometallic-based optoelectronics.

Fig. 4 (a) Current–voltage (I–V) curves of the device based on PtOEP microwires in the dark and under illumination of incident light by varying the power density, 1.57 W cm⁻², 8.28 W cm⁻², 12.05 W cm⁻², respectively. (b) Time-dependent on/off switching of the device base on the PtOEP microwires by switching the light on and off repeatedly and sequentially at a 50 s interval under applied bias of −20 or −30 V. (c) Current measurements of PtOEP microwires in the dark and under constant light exposure for 1000 s. (d) The fluorescence microscope image of PtOEP microwires.

In summary, in view of limitations encountered in conventional media, we have for the first time presented ILs-mediated solution self-assembly for the production of the well-defined single-crystalline PtOEP microwires with smooth surfaces in bulk quantities. This method has a great advantage that the simple procedure allows us prepare controllable morphologies of PtOEP aggregates with desired size or shape and high reproducibility. Notably, these multifunctional microwires had highly sensitive photo-response and active optical waveguide characteristics. We believe that this IL-based powerful synthesis approach opens a new and effective way for the fabrication of porphyrin assemblies with unique optoelectronic properties and is expected to allow access to other organic materials with structural specificity and functional novelty.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21303250 and 61404155) and the Chinese Academy of Sciences.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, chemical structure, EDX spectra, TEM image, SAED pattern, FTIR spectra, UV-vis spectra of micro/nanoscale wires, plates, leaves, and four-leaf clovers. See DOI: 10.1039/c6ra04452b

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