Insights into the effect of donor-acceptor strength modulation on physical properties of phenoxazine-based imine dyes

Abstract

Novel phenoxazine-based imine dyes having a donor-acceptor-donor (D-A-D) topology were designed, synthesized and thoroughly investigated with regard to their thermotropic, photo-physical and electrochemical behaviour. The hexyl-substituted phenoxazine was selected as donor moiety and diphenyl-1,3,4-oxadiazole or fluorene-containing moieties were employed as electron-acceptor groups, being connected via the imine bond. The supramolecular architecture from solution and in molten state has been investigated by wide angle X-ray diffraction, differential scanning calorimetry and polarized light microscopy. The imines formed distinctive thermotropic mesophases in close relationship with their structure. The molecular rigid rods of the synthesized dyes formed supramolecular layered structures that further combine into worm-like anisotropic primary particles in the condensed phase with a strong impact on the physical properties of the dyes. UV–Vis absorption, photoluminescence, and electrochemical measurements proved the ability of the imine bond to facilitate the intramolecular charge-transfer transition from the donor to the acceptor units. The dyes showed an inverted solvatochromic behaviour, the fluorescence emission being highly sensitive to solvent polarity, yielding blue to green and even orange light in different organic solvents. A change in the relative intensity of the photoluminescence with the excitation wavelength of those phenoxazine-based imine dyes has been detected. The fluorescence quantum yield in solution reached up to 63%, whilst in film state it significantly dropped. By incorporating segments with different electron-accepting capability, the HOMO–LUMO energy gap was modulated from 1.88 to 1.92 eV. The electrochemical features demonstrate that p- and n-type doping to the conducting state of these phenoxazine-based imine materials is achievable. As a consequence of interesting physical properties encountered in these dyes, applications in optoelectronic devices are foreseen.