Metal ion-linked multilayers have emerged as a simple and modular means of assembling molecular components on an inorganic substrate. The choice of molecules, based on their excited state energies and redox potentials, has enabled the directional control of energy and electron transfer events for application in electrochromics, solar energy harvesting, molecular rectifiers, photocatalysis, and more. Here, we recount the more than 35-year journey of metal ion-linked multilayers and their transition from planar to mesoporous substrates and from homogeneous to heterogeneous multilayers. This includes showcasing the vast range of components (i.e., substrates, metal ions, and molecules), structural insights, and applications. We also highlight current limitations in our knowledge of and ability to control these systems which must be overcome to realize the full potential of metal ion-linked multilayers. 

Metal Ion-Linked Molecular Multilayers on Inorganic Substrates: Structure and Applications

Tetracyanoquinodimethane (TCNQ) complexes of fluorene, dibenzofuran, dibenzothiophene, terphenyl, dithienothiophene, and carbazole show n-channel transistor properties, whereas that of diaminoterphenyl and phenothiazine exhibits ambipolar characteristics. Ambipolar transport is observed when the highest occupied molecular orbital (HOMO) level of the donor–acceptor complex is located above the ordinary hole transport limit. Since the HOMO of the charge-transfer complex is deeper than that of the component donor owing to the charge-transfer interaction, not all hole-transporting materials show hole transport in the charge-transfer complexes. In addition, the effective (super-exchange) transfer integrals have to be sufficiently large; this is frequently not satisfied when the donor HOMO is practically orthogonal to the acceptor’s lowest unoccupied molecular orbital (LUMO). By applying these criteria to calculations starting from the crystal structures, transistor properties of mixed stack charge-transfer complexes are explained. 

Transistor Properties of Charge-Transfer Complexes─Combined Requirements from Energy Levels and Orbital Symmetry

Organic phototransistors (OPTs) are promising candidates for broadband light detection, offering advantages of spectral tunability, cost-effectiveness, and flexibility. Nonetheless, their practical utility has been impeded by issues such as low exciton dissociation efficiency and persistent photoconductivity. To address this challenge, an electron acceptor (TCNQ) was doped in pentacene as the donor, resulting in the formation of self-organized "embedded" heterojunctions within a single-crystal matrix. This unique organic architecture led to the development of high-performance OPTs, boasting an exceptional photoresponsivity of 1.1 × 104 A W–1 and a specific detectivity of 2.1 × 1013 Jones. Importantly, the device exhibited a fast response time of about 92 μs, maintaining a high photosensitivity exceeding 100, surpassing the capability of detectors constructed from pure organic single crystals. Notably, this "embedded" heterojunction design also enabled the superior infrared photoresponse at 1550 nm by narrowing the band gap. The enhanced performance is a result of the efficient separation of Frenkel excitons and collection of free charges driven by the built-in field at the "embedded" pentacene/TCNQ interface. Furthermore, these OPTs demonstrated versatility in capturing high-resolution images across multiple spectral bands, emphasizing the potential of molecule doping to enhance organic photodetector performance as a straightforward alternative. 

Enabling a High-Sensitivity and Fast-Response Organic Phototransistor Based on an “Embedded” Heterojunction toward Broadband High-Speed Imaging

The investigation of ferroelectric materials is an important aspect of condensed matter physics. Compared to the traditional inorganic ferroelectric ones, organic ferroelectric materials possess unique advantages, including light weight, scalability, flexibility, and solvent treatability. However, the study of organic ferroelectric materials is still in its infancy, especially from the view of supramolecular chemistry; various mechanisms of forming self-assembly and thus diverse building blocks employed are not yet to be further developed. Herein, a pillararene-based charge transfer (CT) self-assembly was produced by utilizing pillar[5]quinone (P5Q) as the electron acceptor and phenothiazine (PTZ) as the electron donor. Compared to the intrinsic P5Q, our fabricated donor–acceptor complex displays particular intriguing features, such as the enhanced second-harmonic generation and ferroelectric responses, benefiting from the CT interaction between P5Q and PTZ. The current work provides a unique strategy for the successful construction of supramolecular ferroelectrics.

Pillararene-based donor–acceptor complex for ferroelectric self-assembled supramolecular materials

Charge transfer (CT) cocrystals, molecular crystals composed of electron donating and accepting species, are being developed for applications in optoelectronics. Here we present optical and electronic characterization of the CT cocrystal phenothiazine–tetracyanoquinodimethane (PTZ–TCNQ). This material has a broad NIR absorption peak with an optical band edge less than 0.6 eV. We used density functional theory calculations to identify the origin of the low energy CT states and changes in the Raman spectra. We also demonstrate the fabrication of long, ribbon-like oriented cocrystals using an evaporative alignment method. Cocrystals grown on Si substrates were fabricated into organic field effect transistors. Despite theoretical predictions of ambipolarity, only electron conduction was observed, with mobilities on the order of 10–4 cm2 V–1 s–1. Measurements of the temperature dependence of the mobility indicated a superexchange mediated hopping mechanism for charge transport, with a characteristic energy scale of 0.19 eV. 

Charge Transport through Superexchange in Phenothiazine–7,7,8,8-Tetracyanoquinodimethane (PTZ–TCNQ) Cocrystal Microribbon FETs Grown Using Evaporative Alignment

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Charge Transport through Superexchange in Phenothiazine–7,7,8,8-Tetracyanoquinodimethane (PTZ–TCNQ) Cocrystal Microribbon FETs Grown Using Evaporative Alignment