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Triindole-Tris-Alkynyl-Bridged Trinuclear Gold(I) Complexes for Cooperative Supramolecular Self-Assembly and Small-Molecule Solution-Processable Resistive Memories Eugene Yau-Hin Hong and Vivian Wing-Wah Yam* Institute of Molecular Functional Materials [Areas of Excellence Scheme, University Grant Committee (Hong Kong)] and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China S Supporting Information *
ABSTRACT: A novel class of luminescent trinuclear alkynylgold(I) complexes with N-alkyl substituted triindole ligands has been synthesized and characterized. They are found to exhibit rich photophysical and electrochemical properties. The complexes have been demonstrated to display interesting supramolecular assembly with spherical nanostructures in aqueous THF solution through a cooperative growth mechanism. The self-assembly process is shown to be mediated by the π−π stacking interactions and hydrophobic−hydrophobic interactions of the triindole moieties upon solvent modulation. These gold(I) complexes have been employed as active materials in the fabrication of solution-processable resistive memory devices, showing promising binary memory performances with low switching threshold voltages of ca. 1.5 V, high ON/OFF current ratio of up to 105, long retention time of over 104 s, and excellent stability. The present work opens up a new avenue for the future design of versatile organogold(I) complexes that could serve as multifunctional materials. KEYWORDS: gold, self-assembly, supramolecular chemistry, cooperativity, memory devices
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INTRODUCTION
Molecular functional materials based on gold(I) systems have become a rapidly evolving field of research in recent decades and have been gaining increasing impetus for their numerous promising applications such as supramolecular assemblies,14−29 luminescence,3−5,30,31 catalysis32,33 and pharmaceuticals.34−36 The attractiveness of gold(I) complexes stems from their distinct photoluminescence properties as well as the renowned intrinsic propensity toward aurophilic interactions.14−25,37−43 The family of gold(I) alkynyls is in particular one of the most enthralling members in gold(I) systems, thanks to their twocoordinate linear geometry of gold(I) and the linearity of the alkynyl moiety. These privileged features have made alkynylgold(I) complexes ideal candidates as building blocks for supramolecular assembly.23−25,27−29 While there are extensive studies on the functional properties of gold(I) complexes, the explorations of their application in
The design of innovative multifunctional materials is challenging and yet highly desirable and crucial; in fact, those based on small molecules have aroused tremendous interest in materials science because of their attractive features such as synthetic versatility, high flexibility, high purity, and ease of fabrication.1−5 To date, supramolecular assembly represents one of the key areas in the exploration of molecular functional materials. In particular, self-assemblies of organometallic complexes through noncovalent interactions, such as metal− metal interactions, π−π interactions, hydrogen bonding, and hydrophobic−hydrophobic interactions, have been shown to be capable to serve as important building blocks for higherordered functional materials.3−5 The incorporation of transition metal centers with diverse coordination geometry, especially those with square-planar, linear, or other low-dimensional coordination geometry, may provide additional advantages, such as additional metal−metal interactions, ease of extending the π-surface, and unique photophysical properties.2−25 © 2017 American Chemical Society
Received: September 30, 2016 Accepted: December 27, 2016 Published: January 11, 2017 2616
DOI: 10.1021/acsami.6b12404 ACS Appl. Mater. Interfaces 2017, 9, 2616−2624
Research Article
ACS Applied Materials & Interfaces Scheme 1. Synthetic Pathway for Complexes 1−4
Scheme 2. Synthetic Pathway for Complexes 5 and 6
organic electronics are rare.44,45 One particular field of interest in this area is organic memory, which is of topical interest recently due to the rapid development of information technology as well as the limitations faced by the conventional silicon-based memory technologies.46−50 These have urged the development of novel memory materials with advanced device performances. Nevertheless, recent advances on organic memory devices are mainly confined to those based on organic compounds and polymers,46−56 while those based on smallmolecule organometallic complexes are rather underexplored and limited despite their unique electronic features and luminescence properties.59,60 In light of the aforementioned scenarios and as an extension of our continuing interest in small-molecule-based organic memories,57−60 organogold(I) complexes are envisaged to be promising candidates for the exploration of their multifunctional properties and their potential utilization in memory devices. Herein are described the design, synthesis, and characterization of a series of trinuclear alkynylgold(I) complexes with N-alkyl substituted triindole ligands as well as the studies of their cooperative selfassembly behavior upon a variation of solvent composition. More interestingly, these complexes have been employed in the fabrication of solution-processable memory devices with excellent performances, representing the first example of organogold(I) complexes for memory applications.
polymer precursors was achieved by the addition of [Au(tht)Cl] to a suspension of the triindole tris-alkynes and NaOAc in a THF−MeOH solvent system. Complexes 1−4 were synthesized by depolymerization of the corresponding alkynylgold(I) polymers with 3 equiv of 2,6-dimethoxyphenyl isocyanide (Scheme 1), while complexes 5 and 6 were prepared by reacting the chlorogold(I) NHC precursors with the triindole tris-alkyne (Scheme 2) at room temperature under an inert atmosphere. All the complexes are found to be air stable and are soluble in common organic solvents. The identities of the complexes have been confirmed by 1H and 13C{1H} NMR spectroscopies, ESI mass spectrometry, and satisfactory elemental analyses. UV−Vis Absorption Spectroscopy. The electronic absorption spectra of 1−4 feature intense absorptions at ca. 318 and 347 nm and a moderately intense absorption at ca. 374 nm, while 5 and 6 are found to absorb strongly at ca. 323, 345, and 353 nm with extinction coefficients in the order of 104 dm3 mol−1 cm−1. The photophysical data of the complexes are summarized in Table 1, and the selected electronic absorption spectra of complexes 3 and 6 are depicted in Figure 1. The electronic absorption spectra were mainly dominated by ligandcentered transitions and are assigned as a mixture of the π−π* intraligand transitions resulting from the triindole tris-alkynyl core and the peripheral isocyanide or NHC ligands. Luminescence Spectroscopy. Upon excitation at λ > 350 nm, all the gold(I) complexes are found to be emissive in degassed benzene solution at 298 K and exhibit broad, vibronicstructured emission bands. The representative emission spectra of complexes 3 and 6 are shown in Figure 2. Complexes 1−4
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RESULTS AND DISCUSSION Synthesis and Characterization. The N-alkyl substituted triindole ligands were prepared according to a modified literature procedure.61 Formation of the alkynylgold(I) 2617
DOI: 10.1021/acsami.6b12404 ACS Appl. Mater. Interfaces 2017, 9, 2616−2624
Research Article
ACS Applied Materials & Interfaces
longer luminescence lifetimes at 77 K are indicative of a large extent of ligand-centered character in the triplet emissive state. On the other hand, complexes 5 and 6 are shown to have dual, structured emission with peak maxima at ca. 418 nm, 456 nm, and a shoulder at ca. 489 nm. The lower-energy emissions exhibit lifetimes of microseconds and can be quenched on exposure to oxygen (Figure S1). These, together with the large Stokes shift and the nonemissive nature of the peripheral NHC ligands, indicate that the lower-energy emissions are originated from the triplet metal-perturbed π−π* ligand-centered excited states similar to that of complexes 1−4. The intensities of the high-energy emissions at 418 nm are found to be insensitive toward exposure to aerobic condition and have a similar profile as that of the alkynyl ligand (Figure S1), and therefore they are attributed to the singlet metal-perturbed π−π* excited states of alkynyl ligand. The assignments are consistent with that observed in the related gold(I) complexes in the literature.27,62 For emission studies in low-temperature butyronitrile glass at 77 K, all the complexes exhibit structured emission bands. The selected emission spectra of complexes 1 and 5 are shown in Figure S2. Compared with the solution emission, all the complexes show much longer lifetimes in the glass state of millisecond regime. As in benzene solution, the emission bands are assigned as triplet metal-perturbed π−π* excited states of the alkynyl core with predominantly ligand-centered character. The emission spectra of all the complexes show rich vibronic structures with vibrational progressional spacings of ca. 1345 and 2100 cm−1, which agree well with the aromatic CC and CC vibrational modes, respectively. Electrochemical Properties. The electrochemical properties of complexes 1−6 in dichloromethane (0.1 M nBu4NPF6) have been investigated by cyclic voltammetry. The cyclic voltammograms of the complexes generally show no reduction waves but display multiple oxidation waves, in which the second oxidation leads to a quasi-reversible oxidation couple at +1.01 to +1.09 V versus saturated calomel electrode (SCE) and the others give rise to irreversible waves at +0.76 to +1.81 V versus SCE, suggesting that the complexes can be readily oxidized to radical cations and higher cationic species in the potential window of dichloromethane. The electrochemical data are tabulated in Table 2, and the representative cyclic voltammo-
Table 1. Photophysical Data of Complexes 1−6 absorptiona
emission
λmax/nm (εmax/dm3 mol−1 cm−1)
medium (T/K)
1
317 (99580), 346 (92435), 373 (74060)
benzene (298) glass (77)c
2
318 (98015), 347 (91610), 373 (71625)
benzene (298) glass (77)c
3
318 (99000), 347 (91920), 374 (72735)
benzene (298) glass (77)c
4
320 (99675), 348 (95305), 374 (76715)
5
323 (93170), 344 (98435), 353 (87865), 368 (41960)
benzene (298) glass (77)c benzene (298) glass (77)c
6
323 (90135), 345 (99480), 353 (90510), 368 (44000)
complex
benzene (298) glass (77)c
λmax/nm (τ0/μs)
Φlumb
460, 494sh (12.6) 450, 479, 499 (1350) 460, 494sh (12.0) 452, 488, 501 (1300) 460, 493sh (11.0), 453, 485, 501 (1230) 460, 493sh (13.2) 454, 482 (2130) 418 (
