1,3-Bicyclo[1.1.1]pentanediyl: The Shortest Rigid Linear Connector of Phenylated Photochromic Units and a 1,5-Dimethoxy-9,10-di(phenylethynyl)anthracene Fluorophore

An excess of bis‐1,3‐(4‐iodophenyl)bicyclo[1.1.1]pentane, prepared in 63 % yield by iodination of 1,3‐diphenylbicyclo[1.1.1]pentane, was selectively mono‐coupled with 9‐ethynyl‐1,5‐dimethoxy‐10‐phenylethynylanthracene (26), and subsequently with the zinc derivatives of 1‐(2‐methyl/methoxy‐4‐methyl‐5‐phenylthiophen‐3‐yl)‐2‐(2‐methyl/methoxy‐4‐methylthiophen‐3‐yl)perfluorocyclopentenes (38‐H–41‐H). Regioselective synthesis of the 2‐unsubstituted thiophenes 38‐H–41‐H required intermediate preparation of 2‐trimethylsilyl‐3,5‐dimethyl‐4‐bromothiophene (37) or 2‐trimethylsilyl‐5‐methoxy‐3‐methyl‐4‐bromothiophene (40). Protection of the α‐position of the thiophene ring with a 2‐trimethylsilyl group blocks the rearrangement of the 4‐lithio derivatives into the corresponding 2‐lithiated thiophenes. With the bicyclo[1.1.1]pentane fragment linking the photochromic units 1–3 and 1,5‐dimethoxy‐9,10‐di(phenylethynyl)anthracene as a fluorescent part, quantitative resonance energy transfer between the excited state of the fluorophore (donor) and the closed form of the photochromic units 1–3 (acceptors) was observed. The closed forms of the methoxy‐substituted photochromic units 2 and 3 are less resistant to UV light (313 nm) than the closed form of 1. New 2‐methoxy‐4‐methylthiophene has been used for the preparation of the photochromic compounds that were connected with the fluorophore by the smallest rigid linear bridge (see scheme), which provided quantitative resonance energy transfer between the photochromic and fluorescent units.