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 1H 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 yelloworange OLEDs, exhibiting luminances up to 1387 cd/m2 and current efficiencies as high as 0.15 cd/A.