Photochromic Materials and Electron Transfer Mechanisms

Photochromic materials are those that undergo reversible optical and structural transformations in response to external stimuli, most commonly light, heat or mechanical force. Central to their function is the photoinduced or stimulus-driven electron transfer that converts a stable precursor into a coloured radical or charge-separated state and back again. Such electron transfers may occur within a single molecule (intramolecular) or between distinct donor and acceptor units (intermolecular), often aided by supramolecular architectures or extended frameworks. Advances in molecular design have yielded classes of compounds ranging from organic radical semiconductors and viologen derivatives to metal-organic frameworks and coordination polymers. These systems combine reliable switching kinetics, long-lived charge states and tunable absorption across the ultraviolet, visible and near-infrared regions. Applications extend from smart windows and erasable printing to high-contrast optical switching, photodetectors covering the full solar spectrum, pressure- and humidity-sensitive sensors, and molecular magnetic devices exhibiting bistability. Contemporary research seeks to refine control over electron-transfer pathways, improve radical stability under ambient conditions and extend the responsive spectral range, thereby enabling integration into sustainable energy, security technologies and advanced photonic systems.

Research from Nature Portfolio

Recent studies have demonstrated new two-dimensional semiconductors in which π-stacked viologen units intercalated between cyanide-bridged metal layers generate stable radicals upon light or heat activation, extending intrinsic photocurrent response from the ultraviolet well into the shortwave infrared. Another investigation has employed anthracene-dicarboxylate ligands coordinated to manganese(II) to produce photochromic chain complexes; light-induced electron transfer yields diradicals that alter magnetic coupling and drive a remarkable thermal hysteresis of over 170 K, offering a strategy for molecular magnets with wide switching windows.

Photochromic Materials and Electron Transfer Mechanisms publication trend

The graph below shows the total number of articles in photochromic materials and electron transfer mechanisms across all publications each year (not limited to Nature Index journals).

Technical terms

Photochromism: Reversible change in colour or optical properties of a material induced by light irradiation.

Electron transfer: Movement of an electron from a donor to an acceptor species, fundamental to charge separation and radical generation in photochromic systems.

Radical: A chemical species containing an unpaired electron, often responsible for the coloured state in photochromic materials.

Host–guest supramolecule: A complex in which a photoactive guest molecule is encapsulated or bound within a host framework, moderating interactions and stabilising functional states.

Metal–organic framework (MOF): A porous crystalline material composed of metal nodes connected by organic linkers, capable of incorporating photochromic units within its architecture.

Photoconductivity: Increase in electrical conductivity of a material upon exposure to light, arising from photogenerated charge carriers.

Thermal hysteresis: Temperature range over which a material remains in a metastable state after stimulus removal, important for bistable optical or magnetic switching.

References

1. Bottom‐Up Photosynthesis of an Air‐Stable Radical Semiconductor Showing Photoconductivity to Full Solar Spectrum and X‐Ray. Advanced Science (2023).
2. Viologen‐based host–guest supramolecule with tunable intramolecular/intermolecular electron transfer chromism and dynamic fluorescence. Aggregate (2024).
3. Photochromic metal–organic frameworks for inkless and erasable printing. Chemical Science (2016).
4. Piezochromism and hydrochromism through electron transfer: new stories for viologen materials. Chemical Science (2017).
5. Directed self-assembly of viologen-based 2D semiconductors with intrinsic UV–SWIR photoresponse after photo/thermo activation. Nature Communications (2020).
6. Achieving large thermal hysteresis in an anthracene-based manganese(II) complex via photo-induced electron transfer. Nature Communications (2022).