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Photoresponsive cages show promise for tunable supramolecular electronics

In a recent study that merges supramolecular chemistry and molecular electronics, a research team has demonstrated how supramolecular porphyrin-based cages can enable tunable photoresponsive charge transport (CT) behaviors in solid-state devices. The findings could pave the way for more versatile and controllable molecular components in optoelectronic applications.

The research is in the journal Angewandte Chemie International Edition.

The collaborative team was led by Professor Cunlan Guo of Wuhan University, Professor Ming Wang of Jilin University, and Professor Haohao Fu of Nankai University.

Molecular electronics has long relied on the structural versatility of small molecules to tune charge transport behaviors. But as demand grows for device components with dynamic responsiveness, supramolecular assemblies are drawing attention for their capacity to fine-tune electron flow via weak, reversible interactions and external stimuli. Among various stimuli, light stands out for its spatial precision and non-invasiveness—qualities ideally suited for next-generation functional devices like molecular switches.

To address the need for light-responsive molecular architectures with robust, predictable properties, the team turned to porphyrins—planar, aromatic macrocycles known for their photostability and strong absorption in the visible range. By coordinating porphyrin derivatives with a bidentate diplatinum(II) motif, they constructed well-defined cage-like supramolecular structures (C-TPyP) featuring rigid pillars and a separation distance of ~18.3 Å between the porphyrin faces.

Electrical junctions built from these supramolecular cages, sandwiched between a self-assembled monolayer (SAM) and an EGaIn top electrode, exhibited distinctly different behaviors under light. While junctions containing monomeric porphyrins showed negligible current changes under 420-nm illumination, those made with supramolecular cages displayed marked photoresponsive charge transport. Notably, the inclusion of metal ions such as Zn²⁺ and Cu²⁺ in the porphyrin cores altered the magnitude of this response, highlighting the tunability of the system.

Supporting these electrical measurements, fluorescence spectroscopy and Kelvin probe force microscopy (KPFM) confirmed that the supramolecular architecture enhanced electron-hole pair separation—an effect directly correlated with observed photoresponsive current behavior.

Further experiments showed that the distance between the porphyrin cages and the gold substrate—modulated by varying the length of the alkanethiol spacer in the SAM—affected the CT efficiency. While increased distances dampened the light-induced response, overly short spacers suppressed molecular behavior, likely due to excessive coupling to the electrode.

This study not only advances the fundamental understanding of charge transport in supramolecular systems but also offers a practical framework for engineering molecular-scale optoelectronic devices. The work underscores how molecular design, assembly precision, and interfacial engineering can converge to create functional electronic elements responsive to environmental cues.