A new tunable light-emitting and π-stacked hexa-ethyleneglycol naphthalene-bisimide oligomer: synthesis, photophysics and electrochemical properties

Abstract

An oligomer (3) containing flexible hydrophilic hexa(ethylene glycol) and hydrophobic naphthalene-bisimide chromophores has been synthesized by a one-step condensation reaction and its photophysical and electrochemical properties were investigated. 3 was characterized through the data from NMR, IR, UV-vis, GPC, DSC, TGA, elemental analysis and cyclic voltammetry. The average molecular weight (M_w) of 3 was 4430 g mol⁻¹. Intrinsic viscosity was measured as 0.28 dL g⁻¹ in m-cresol at 25 °C. It has high thermal stability (T_d = 325 °C). Interestingly, compound 3 shows excimer-like emission in all kinds of solvents. The band gap energy (E_g), LUMO and HOMO energy values in nonpolar and polar protic solvents were 2.71 eV/3.12 eV, −3.69 eV/−3.88 eV and −6.40 eV/−7.00 eV for 3, respectively. The oligomer showed concentration and solvent dependent fluorescent color tunability. Remarkably, the fluorescent colors of the excimer emissions at 10⁻⁶ M concentration in CHCl₃, DMF and MeOH are light yellow, light blue–yellow and strong blue, respectively, and become more intense at higher concentrations. The excimer emission color in CHCl₃ and DMF is fluorescent yellow and changed to green in MeOH at10⁻⁴ M concentration. 3 shows two reversible reduction steps at −1.103 and −1.457 V (vs.ferrocene/ferrocenium) in nonpolar solvent CH₂Cl₂ and only one at −0.917 V in (50 : 50) CH₃OH–CH₃CN binary solvent mixture with higher reversibility. Strong blue-shifts of emission band were noted in protic solvents, which confirm the existence of a negative solvatochromism probably due to protonation. The strong solvent-dependent photophysical and electrochemical properties, including the large shift of excimer emission maximum reflecting self-assembly mediated through hydrogen bonding and π-stacking interactions, make the oligomer a potential candidate for various photo-sensing applications.