Photophysics of Fluorene Copolymers: Control of Fluorescence and Charge Separation by the Presence of Carbazole, Oxadiazole, or Biphenyl Units

Abstract

A set of fluorene copolymers designed to introduce electron donor/acceptor structures in the polymer chain that could eventually produce more efficient charge separation with long lifetimes has been prepared. To validate our approach, herein we report photoluminescence and laser flash photolysis measurements of the series of alternating specially functionalized fluorene copolymers. The copolymers contain 9,9-dioctylfluorene and N-octylcarbazole, oxadiazole, and oxadiazole−octafluorobiphenyl−oxadiazole units. It appears that the copolymer with the longest charge separated state lifetime and the highest charge separation quantum yield of the series is an alternating copolymer of a N-octylcarbazole and trifluorene units in which two fluorenes were attached with hydroxyhexyl side groups and the central fluorene with octyl side group at C-9 positions, respectively. These photophysical data suggest that this polymer a good candidate for photovoltaic cell and polymer light-emitting diode applications.

A Fluorene−Oxadiazole Copolymer for White Light-Emitting Electrochemical Cells

“…8,23 Oxadiazole units have been reported to affect the photophysics properties of polyfluorene-based LEDs. 24 The major color shift of in the EO-PF-DPO LEDs and LECs may be explained by charge trapping: the emission is dominated by the lowest electronic states. Further study is needed to elucidate the color shift.…”

“…White EL from a single emissive polymer has been reported in the literature by (1) covalent bonding of green and red or orange chromophores in a largely blue polymer backbone, (2) electron-trapping effect, and (3) excimer emission . In the case of polyfluorenes, both excimer or keto defects have been used to to achieve white LEDs. , Oxadiazole units have been reported to affect the photophysics properties of polyfluorene-based LEDs . The major color shift of in the EO-PF-DPO LEDs and LECs may be explained by charge trapping: the emission is dominated by the lowest electronic states.…”

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

“…The synthesis of the polymers under study has been reported in our previous publications. , Briefly, the polymerization was done by a ring closing reaction between fluorene−ditetrazole and fluorene−dicarbonyl chloride for PFOx, and the Suzuki reactions of the corresponding oxadiazole containing dibromo-monomers with the diboronic acid monomers of fluorene, difluorene, and carbazole for PF 2 Ox, PF 3 Ox, PF 4 Ox, and PFOxCz, respectively. While PF 1 Ox, PF 3 Ox, PF 4 Ox, and PFOxCz have highly symmetric alternating structures of oxadiazole with fluorene or/and carbazole, the periodicity of PF 2 Ox is slightly low …”

“…The preparation and characterization of the fluorene copolymers has been reported previously. , UV−vis absorption spectra were recorded with a Perkin-Elmer λ35 spectrophotometer. Fluorescence spectra and lifetimes were measured on an Edinburgh Analytical Instruments FL900 spectrophotometer having a Czerny-Turner monochromator in the excitation lamp and detector photomultiplier.…”

“…Conjugated polymers having fluorene units are widely used in polymeric light diodes (PLEDs), , field effect transistors, − and photovoltaic cells. , To improve their chemical stability, efficiency, and ability to form polarons, there is a continuous interest in the synthesis and characterization of new fluorene copolymers incorporating other comonomers. Since charge separation is the common fundamental process lying at the base of most applications, general strategies to improve stability and lifetime of the polarons are based on the synthesis of fluorene copolymers incorporating electron-donor or electron-acceptor units. ,,− Recently, we have reported that fluorene−carbazole copolymers exhibit high efficiency for photoinduced charge separation. , Following this approach, in the present work we have studied the photophysical properties of a series of fluorene/1,3,4-oxadiazole copolymers with the aim of determining the influence of the percentage of 1,3,4-oxadiazole in the conjugated copolymer on the efficiency and lifetime of charge separation. We have also included in our study a copolymer containing alternating units of fluorene/1,3,4-oxadiazole/carbazole that contains electron donor and acceptor units.…”