However, the photo-deprotection step requires a metal catalyst and is not reversible, which limits its practical application. Our team recently combined visible light-enabled deprotection of dithioacetal-protected aldehyde moieties with UV light-triggered photo-enol ligation for a dual-colour reaction system 24. Thus, to support the development of advanced two-colour printing systems, there exists a critical need to develop photoresist systems that are strictly gated by two colours of light, ideally enabled by reversible reactive states of two independent chromophores. While this method satisfies criteria (ii), the merocyanine also absorbs light in the UV region, leading to competing initiation 21. The employed system integrates a benzophenone type II photoinitiator into a spiropyran photoswitch, enabling initial excitation (spiropyran to merocyanine) and subsequent benzophenone radical generation at 585 nm and 375 nm, respectively. The team of Hecht recently introduced a dual-colour photoinitiator for a new method of volumetric two-colour 3D printing technology, referred to as xolography 21. To date, there are very few reports on such photochemical systems. The emergence of such dual-wavelength photochemical systems opens up new avenues for novel lithographic techniques 19, 20, 21, and the fabrication of materials with customised micro- or nano-structures 22, 23.ĭual-wavelength photochemistry requires the design of two photoreactive compounds which (i) possess a compatible λ-orthogonal window and (ii) are of reversible nature to avoid competing side reactions. Through this innovation, well-defined control over the photochemical transformations can be attained by regulating the activation and deactivation of reactions with two independent wavelengths of light which serve as gates for a single reaction output. Advances in photochemistry, coupled with modern laser instrumentation, further enables the use of distinct monochromatic wavelengths to conduct highly orthogonal reactions in complex chemical environments, known as λ-orthogonal reactivity 15, 16, 17, 18. The unique spatial and temporal control provided by light enables a range of applications extending from surface patterning 1, 2, 3, 4, 5, to polymer network formation 6, 7, 8, 9, 10, and modulation of biological properties 11, 12, 13, 14. Photochemistry has undergone extensive advancement within the last few decades, now providing a powerful synthetic tool for the design of (macro)molecules with advanced architectures and properties.
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