Abstract Nucleosidic diarylethenes (DAEs) are an emerging class of photochromes but have rarely been used in materials science. Here, we have developed doubly methylated DAEs derived from 2′‐deoxyuridine with high thermal stability and fatigue resistance. These new photoswitches not only outperform their predecessors but also rival classical non‐nucleosidic DAEs. To demonstrate the utility of these new DAEs, we have designed an all‐optical excitonic switch consisting of two oligonucleotides: one strand containing a fluorogenic double‐methylated 2′‐deoxyuridine as a fluorescence donor and the other a tricyclic cytidine (tC) as acceptor, which together form a highly efficient conditional F?rster‐Resonance‐Energy‐Transfer (FRET) pair. The system was operated in liquid and solid phases and showed both strong distance‐ and orientation‐dependent photochromic FRET. The superior ON/OFF contrast was maintained over up to 100 switching cycles, with no detectable fatigue.
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