Effects of steric anthracene moieties and keto defects on photophysics and color stability of poly(9,9-di(2-ethylhexyl)fluorene-stat-anthracene) in different local environments

Abstract

UV/vis absorption and photoluminescence (PL) spectroscopy were used to examine the photophysical properties of statistical copolymers, poly(9,9-di(2-ethylhexyl)fluorene-stat-anthracene), with the mole ratio of the anthracene group ranging from 0 to 20%. The incorporation of the anthracene group into the conjugated backbone of the polyfluorene affected the PL pattern and quantum yield of the isolated chain in a dilute solution. An investigation of the photophysical properties in solvent-nonsolvent system revealed the formation of low-energy non-emissive and emissive aggregates. The quantity of these aggregated species depends significantly on the mole ratio of anthracene group in the polymer backbone. The existence of only 5 mol% of anthracene in the copolymer causes significant suppression of the well-known green emission (λ~525 nm). However, a large quantity of non-emissive aggregates indicated by the red-shift peak at ~415 nm in absorption spectra still formed. Increasing the mole ratio of the anthracene group to 20% suppressed both types of aggregates. Measurements of the absorption and PL spectra in the thin films showed consistent results. Although annealing of the homopolymer polyfluorene films at 150 o C in a vacuum oven caused a drastic increase in green emission, only a slight change was detected in the PL spectra of the copolymers. The presence of oxygen during annealing promoted the green emission of the polymer films.