Synthesis and properties of monodisperse oligofluorene-funcfionalized truxenes: Highly fluorescent star-shaped architectures

Abstract

This paper describes the strategy toward novel monodisperse, well-defined, star-shaped oligofluorenes with a central truxene core and from monofluorene to quaterfluorene arms. Introduction of solubilizing n-hexyl groups at both fluorene and truxene moieties results in highly soluble, intrinsically two-dimensional nanosized macromolecules T1-T4. The radius for the largest oligomer of ca. 3.9 nm represents one of the largest known star-shaped conjugated systems. Cyclic voltammetry experiments reveal reversible or quasi-reversible oxidation and reduction processes (Eox = +0.74 to 0.80 V, Ered = -2.66 to 2.80 eV vs Fc/Fc+), demonstrating excellent electrochemical stability toward both p- and n-doping, while the band gaps of the oligomers are quite high (Egcv = 3.20-3.40 eV). Close band gaps of 3.05-3.29 eV have been estimated from the electron absorption spectra. These star-shaped macromolecules demonstrate good thermal stability (up to 400-420°C) and improved glass transition temperatures with an increase in length of the oligofluorene arms (from Tg = 63°C for T1 to 116°C for T4) and show very efficient blue photoluminescence (λPL = 398-422 nm) in both solution (ΦPL = 70-86%) and solid state (ΦPL = 43-60%). Spectroelectrochemical experiments reveal that compounds T1-T4 are stable electrochromic systems which change their color reversibly from colorless in the neutral state (∼340-400 nm) to colored (from red to purple color; ∼500-600 nm) in the oxidized state.