Triplet Energy Migration in Layer-by-Layer Deposited Ultrathin Polymer Films Bearing Tris(2,2‘-bipyridine)ruthenium(II) Moieties

Abstract

Ultrathin polymer films bearing tris(2,2‘-bipyridine)ruthenium(II) (Ru) moieties were fabricated by the layer-by-layer deposition technique. The triplet energy migration among the Ru moieties was investigated by emission spectroscopy for quenching of the phosphorescence of the Ru moiety by a ferrocene moiety in heterostructured layer-by-layer films. The absorbance of the Ru moiety in the films increased linearly as the number of bilayers increased, showing a linear growth of the thickness in a scale of nanometers. The quenching efficiency increased as the concentration of the Ru moiety increased. This increase in the efficiency showed that the triplet excitation energy migrates in the Ru layers. The diffusion of the excitation energy was analyzed quantitatively on the basis of one-dimensional diffusion in a finite thickness. A diffusion coefficient of 2 × 10-5 cm2 s-1 was observed in the layer-by-layer films made of a polycation with 18 mol % of Ru moiety. The root mean square of the migration distance was calculated to be 36 nm, which is much longer than a Perrin radius of 1.5 nm for the direct quenching of the Ru moiety by the ferrocene moiety and comparable to that in singlet energy migration. This efficient triplet energy migration shows that the layer-by-layer films serve as a light-harvesting system.