Exciton Transport in Naphthalene-Doped Poly (Methylmethyl-Acrylate): Spatial and Energetic Disorder

Abstract

A novel method is presented for the study of spatially and/or energetically disordered domains in a photoactivated polymeric material. Long-lived phosphorescence and delayed fluorescence spectra and their decay dynamics are studied in a “plexiglass” sample that is both mechanically improved and optically activated via doping with aromatic molecules (naphthalene, anthracene). The dopants are aggregated in sub-microscopic domains (“cavities”, “pores”). Fractal-like energy transport and exciton recombination (fusion, annihilation) are observed and related to the structural aspects and the energetic disorder. Energy trapping is studied for samples doped heavily with naphthalene and lightly with anthracene. Exciton fusion is studied on samples with naphthalene dopant (and photo-products: excimer, radial, etc.). While the spectra reveal only the high degree of energetic disorder, the decay dynamics characterize the geometric disorder. The naphthalene singlet exciton transport is about three orders of magnitude slower than in crystalline naphthalene and obeys Stern-Volmer kinetics. This corresponds to normal (non-fractal, non-dispersive) diffusion of the excitation (due, in part, to longer-range transitiondipole-transition-dipole interactions). However, the triplet exciton motion and reaction (fusion) clearly show anomalous (dispersive) diffusion and non-classical (fractal-like) recombination kinetics. The effective spectral dimension is about 1.5 over a wide temperature range (77–165K), indicating that the geometrical disorder (“fractal nature of the cavities”) appears to dominate (for the short-range, exchange-like interactions). The triplet exciton transport and kinetics are at least two orders of magnitude slower than in perfectly crystalline naphthalene domains (and all exciton motion is frozen at about 4.2K). Large inhomogeneous fluorescence line-broadening is observed (~100 cm−1), as well as spectral diffusion (due to triplet exciton energy relaxation).