Influence of Keto Groups on the Optical, Electronic, and Electroluminescent Properties of Random Fluorenone-Containing Poly(fluorenylene-vinylene)s

This study deals with the effect that the incorporation of 2,7-fluorenone into the polymer backbone of a poly(fluorenylene-vinylene) exerts on its photophysical properties. The synthesis of a series of random poly(fluorenylene-vinylene)s containing fluorenone at 10, 5, 3, 1% mol/mol ratio with respect to fluorene units (PFVK1-4) was achieved by the Suzuki-Heck reaction cascade between potassium vinyl trifluoroborate and the equivalent quantity of the suitable feed of the corresponding dibromoaryl comonomers in different ratios. The polymers were characterized by H-1 NMR, IR, TGA, DSC, cyclic voltammetry, and UV-vis as well as stationary and time-resolved PL both in solution and in the solid state. In chloroform solution, PFVK1-4 show an emission originating from the PFV backbone while no fluorenone emission could be detected. Moreover, the presence of fluorenone lowers the efficiency quantum yields that inversely follow the fluorenone/fluorene ratio. Conversely, in the solid state, a complete energy transfer occurs and fluorenone acts as the only luminophor even in PFVK1, containing it only in 1% amount respect to fluorene. Consequently, in the solid state PFVK1-4 are all yellow-orange emitters in the solid state. The yellow-orange fluorescence of the obtained polymers in the solid state was compared to the optical behavior of the monodispersed compound 2,7-distyrylfluorenone (DSF). The analysis of the fluorescence decay pathways of the molecules suggests that, differently from DSF, the low-energy emission in the polymers does not originate from a cofacial interaction between fluorenones units. On the contrary, in addition to the fluorenone emission, complex interactions between the fluorenone luminophors and the poly(fluorenylene-vinylene) matrix have to be taken into account for a rationalization of the photophysical properties of these fluorenone-containing polymers in the solid state. Notwithstanding the presence of carbonyl-containing units, usually considered deleterious for the emission properties of poly(arylene-vinylene)s, PFVK1-4 show potential as emitting layers in yellow-orange OLEDs, exhibiting luminances up to 1387 cd/m(2) and current efficiencies as high as 0.15 cd/A.