Synthesis of Fluorene‐Bridged Arylene Vinylene Fluorophores: Effects of End‐Capping Groups on the Optical Properties, Aggregation Induced Emission

Abstract

AbstractWe have synthesized a series of fluorene‐based fluorophores, in which a central fluorene core has been modified with different peripheral arylene vinylene substituents that are able to activate aggregation‐induced emission (AIE) characteristics. 9,9‐Dioctylfluorene doubly end‐capped at the 2,7‐positions with triphenylethene groups, such as 4‐(2,2‐diphenylvinyl)phenyl (F1‐(2,2)‐HTPE) and 4‐(1,2‐diphenylvinyl)phenyl (F1‐(1,2)‐HTPE) were synthesized and compared to the tetraphenylethene analogue (F1‐TPE). Both F1‐(2,2)‐HTPE and F1‐(1,2)‐HTPE glow with a deep blue fluorescence in THF solution with emission maxima (λem) of 426 and 403 nm, respectively. The λem slightly red‐shifts in the solid‐state to 458 nm for F1‐(2,2)‐HTPE and 437 nm for F1‐(1,2)‐HTPE. The fluorescence quantum yields (øF) of F1‐(2,2)‐HTPE (øF=35.1 %) and F1‐(1,2)‐HTPE (øF=26.2%) were found to be higher in solution compared to the near quenching of F1‐TPE (øF=0.1%). Consequently, this results in weaker AIE‐stability of F1‐(2,2)‐HTPE (αAIE=1.5) and F1‐(1,2)‐HTPE (αAIE=1.9) compared to F1‐TPE (αAIE=125), suggesting that four phenyl groups are necessary for efficient AIE‐activity of these fluorene bridged arylene vinylene type materials. In addition, decreasing the steric hindrance around the arylene vinylene moiety by removal of a phenyl ring is another method to decrease the AIE characteristics, in a similar manner to the commonly known "phenyl‐locking". Non‐polar triphenylethenes are poorer AIE materials than their tetraphenylethene analogues. Replacing the hydrogen atom of F1‐(2,2)‐HTPE with a cyano group affords fluorene end‐capped with 2,3,3‐triphenylacrylonitrile (F1‐TPAN), which boosts the AIE‐effect to αAIE=90.5 and red‐shifts the solid‐state emission (λem=528 nm) with near quenching in THF solution (øF=0.12%). X‐ray crystallographic analysis of F1‐TPAN indicates that the introduction of cyano groups can not only diminish the intramolecular steric hindrance in comparison of F1‐TPE, but also improve the molecular cohesion ability via multiple CH···N interactions.