Thiophene‐Fused 3‐Methylene‐2,3‐dihydrochalcogenophenes: Fluorescent Dyes Incorporated in a Rigid Dibenzobarrelene Skeleton

Abstract

ABSTRACT3‐Methylene‐2,3‐dihydrothienothiophene and ‐selenophenothiophene derivatives incorporated in a rigid dibenzobarrelene skeleton were synthesized by intramolecular Diels–Alder reaction, and their structures, fluorescent properties, and electrochemistry were elucidated. In 3‐methylene‐2,3‐dihydrothieno[3,2‐b]thiophene derivatives, silyl and phenyl substituents at the terminal position were effective for strong blue fluorescence (λem = 412–455 nm) with high quantum yields (ΦF = 0.65–0.99), whereas the unsubstituted parent compound was weakly fluorescent (λem = 338 nm, ΦF = 0.03). The phenyl‐substituted [3,2‐b]‐derivative showed strong emission also in the solid state (ΦF = 0.70). The corresponding [2,3‐b]‐isomer showed blueshifted absorption (λabs = 342 nm) and emission (λem = 436 nm) maxima with low‐to‐moderate quantum yields in solution (ΦF = 0.24) and in the solid state (ΦF = 0.41). In the case of the selenium congeners (3‐methylene‐2,3‐dihydroselenopheno[3,2‐b]thiophene derivatives), the heavy atom effect due to the selenium was decisive to decrease the quantum yields (ΦF = 0.01–0.06). The oxidation of phenyl‐substituted [3,2‐b]‐derivative yielded the corresponding sulfoxide and sulfone, which showed strong emission at around 440 nm with high quantum yields in dichloromethane (ΦF = 0.91 and 0.98, respectively). A dimer of the phenyl‐substituted [3,2‐b]‐derivative was prepared by oxidative coupling of the lithio derivative. The dimer had weak redshifted emission (λem = 547 nm, ΦF = 0.10) and showed a reversible cyclic voltammogram with E1/2 = 0.35 V versus Fc/Fc+ redox couple.