Single Molecule Studies of a 2,7-Bis-(Phenylethenyl)fluorenone: Implications for Green-Emission Bands in Fluorene-Based OLEDs

Abstract

ABSTRACTThe color purity of blue polyfluorene fluorescence emission is often adversely contaminated by green emission bands. The origin of this green emission has been attributed to various factors including on-chain oxidation of fluorene leading to monomeric fluorenone emission and inter/intra-chain interaction between segments of fluorenone units leading to excimer formation on the polyfluorene backbone. We report here emission properties at the bulk and single molecule level of a molecular fluorene derivative and compare them to those of its oxidative fluorenone derivative. Whereas the bulk fluorescence emission of 2,7-bis(3,4,5-trimethoxyphenylethenyl)-9,9-diethyl-9H-fluorene (OFPV) exhibits a blue to blue-green emission at about 480 nm, 2,7-bis(3,4,5-trimethoxyphenylethenyl)-9,9-diethyl-9H-fluorenone (OFOPV) shows red luminescence with a peak centered at about 630 nm. However, the peak position for OFOPV shifts to higher energies (∼540 nm) upon dilution or dispersion in polymer matrices like PMMA or Zeonex. Single molecule measurements of OFOPV show fluorescence spectra dominated by peaks around 540 nm, with a small minority at longer wavelengths that are attributed to emission from dimers or higher aggregates. This distribution indicates that monomeric emission of OFOPV is green, consistent with green emission bands seen in polyfluorenes.