Photophysics of Fluorene Copolymers Containing 1,3,4-Oxadiazole or 1,3,4-Oxadiazole and Carbazole Units

The photophysical properties of a series of 9,9'-dioctylfluorene copolymers containing 1,3,4-oxadiazole units (PF1Ox, PF(2)Ox, PF(3)Ox, and PF(4)Ox) as well as a copolymer containing 9,9-dioctylfluorene, 1,3,4-oxadiazole, and N-octylcarbazole (PFOxCz) have been characterized by fluorescence and time-resolved absorption spectroscopy. It was observed that the photophysical data of PF(2)Ox, PF(3)Ox, and PF40x were almost coincident (lambda(em) = 426 nm, phi(n) = 0.38) and somewhat different from those of PF(1)Ox (lambda(em) = 404 nm, phi(n) = 0.34) and PFOxCz (lambda(em) = 472 nm, phi(n) = 0.39). These differences are attributed to the influence of the largest Ox percentage in the polymer (P1710x) or to the presence of a strong electron-donor carbazole unit (PF0xCz). In transient absorption spectroscopy the presence of triplet excitons (A = 350 and 740 nm, quenched by oxygen), positive polarons (A = 400 and 530 nm, quenched by triethylamine), and photoeject electrons (absorption onset 800 nm, quenched by oxygen) was observed for PF,,Ox (n = 1, 2, 3, 4) with a similar quantum yield for photoejected electrons. PF0xCz exhibits differences in the transient spectrum due to the present carbazole, the polaron absorption maximum appearing at 460 nm and exhibiting a higher efficiency of photoejected electron. Overall the photophysical data obtained indicate that the behavior of PF,Ox and PF0xCz is somewhat different from that of the rest of the fluorene copolymer series (PF(2)Ox, PF(3)Ox, and 131740x) that exhibit almost coincident properties.