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 (Mw) 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 (Td = 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 CHCl 3 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 CH 2Cl2 and only one at -0.917 V in (50:50) CH 3OH-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. © The Royal Society of Chemistry and Owner Societies 2011.