Twisted configuration pyrene derivative: exhibiting pure blue monomer photoluminescence and electrogenerated chemiluminescence emissions in non-aqueous media†
A blue light emitting pyrene derivative, 1-methyl benzoate-pyrene (MB-Py), was synthesized by employing a pyrene moiety as the parent linked with a small electron-donating group, methyl benzoate. Owing to the introduction of methyl benzoate, there is an evident twisted configuration between the methyl benzoate moiety and the central pyrene core with a dihedral angle of 56°, confirmed by density functional theory (DFT) calculations. The basic photophysical, electrochemical and electrogenerated chemiluminescence (ECL) properties of MB-Py together with those of the parent pyrene (Py) and the analogue benzo[α]pyrene (Bz-Py) in MeCN : Bz (v/v = 1 : 1) were extensively characterized for the first time. The electrochemical behavior of MB-Py showed a quasi-reversible one-electron oxidation (E1/2 = +1.11 V vs. SCE) and two reversible closely spaced consecutive one-electron reductions (E1/2,1 = −2.12 V and E1/2,2 = −2.27 V vs. SCE). This redox behavior was confirmed by CV digital simulations and was consistent with the frontier molecular orbitals calculated. It was found that, unlike its parent Py and analogue Bz-Py, MB-Py showed a single-peaked absorption (343 nm) and a blue monomer photoluminescence (PL) emission (428 nm) with a large Stokes shift (85 nm). The electrogenerated radical ions of MB-Py are stable enough to produce an efficient ECL emission (439 nm) through ion annihilation with the S-route. The ECL spectra of MB-Py obtained in both the ion annihilation and coreactant processes are similar to the monomer PL spectrum. This nonplanar twisted conformation pyrene derivative imparts steric hindrance, prevents excimer formation and results in a high PL efficiency and efficient, stable blue ECL monomer emission. Owing to their rigid planar structures, Py and Bz-Py exhibited monomer and excimer blue-violet PL emissions and cyan/green ECL emissions, and could not be used as appropriate blue luminophores.