Abstract
The development and the photophysical behavior of a transparent ion-exchange membrane based on a pH-sensitive polypyridyl ruthenium(II) complex, [(bpy)2RuII(H2bpib)RuII(bpy)2](ClO4)4 (bpy = 2,2′-bipyridine, H2bpib = 1,4-bis([1,10]phenanthroline[5,6-d]-imidazol-2-yl)benzene), are experimentally and theoretically reported. The emission spectra of [(bpy)2RuII(H2bpib)RuII(bpy)2]@Nafion film were observed between pH 2 and pH 11 and showed the highest relative emission intensity at pH 5 (λmaxem = 594.4 nm). The relative emission intensity of the film significantly decreased down to 75% at pH 2 and 11 compared to that of pH 5. The quantum yields (Φ) and lifetimes (τ) showed similar correlations with respect to pH, Φ = 0.13 and τ = 1237 ns at pH 5, and Φ = 0.087 and τ = 1014 ns and Φ = 0.069 and τ = 954 ns at pH 2 and pH 11, respectively. These photophysical data are overall considerably superior to those of the solution, with the radiative- (kr) and non-radiative rate constants (knr) at pH 5 estimated to be kr = 1.06 × 105 s−1 and knr = 7.03 × 105 s−1. Density functional theory calculations suggested the contribution of ligand-to-ligand- and intraligand charge transfer to the imidazolium moiety in Ru-H3bpib species, implying that the positive charge on the H3bpib ligand works as a quencher. The Ru-Hbpib species seems to enhance non-radiative deactivation by reducing the energy of the upper-lying metal-centered excited state. These would be responsible for the pH-dependent “off-on-off” emission behavior.
Highlights
Transparent light-emitting materials are not visible under room light or sunlight, but are able to emit light upon exposure to a specific wavelength of light, e.g., nearultraviolet (UV) light, leading to potential applications in various fields such as lighting, displays, security printing, etc. [1]
UV-vis absorption spectroscopy was employed to determine the amount of molecules introduced into the Nafion membrane
The transparent emissive film [(bpy)2 RuII (H2 bpib)RuII2 ]@Nafion was obtained by soaking the membrane in the solution of the cationic RuII complex, owing to the cation exchange property of Nafion
Summary
Transparent light-emitting materials are not visible under room light or sunlight, but are able to emit light upon exposure to a specific wavelength of light, e.g., nearultraviolet (UV) light, leading to potential applications in various fields such as lighting, displays, security printing, etc. [1]. Among a number of polymeric materials, the cation exchange membrane Nafion possesses excellent durability on chemical and mechanical aspects, and its large ion-exchange capacity allows us to conventionally obtain highly emissive transparent films [8,9,10,11,12,13,14,15,16,17]. Due to the high electronwithdrawing property of the perfluoroalkyl group, proton dissociation occurs readily upon hydration of the Nafion membrane and leads to sulfonate anion (-SO3 − ) formation. This means that Nafion is hydrophilic and has a high affinity for protons. Transparent films with light-emitting, magnetic, or photo-functional properties can be obtained by using ion-exchange membranes such as Nafion without spin coating or casting, which are inevitable parts of constructing composites using conventional polymers [24,25,26,27,28,29,30,31,32]
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