J. Phys. Chem. C 2010, 114, 14255–14260
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Photophysics of Fluorene Copolymers Containing 1,3,4-Oxadiazole or 1,3,4-Oxadiazole and Carbazole Units ´ lvaro,† Jianfu Ding,*,‡ and Hermenegildo Garcı´a*,† Maykel de Miguel,† Mercedes A Instituto UniVersitario de Tecnologı´a Quı´mica CSIC-UPV and Departamento de Quı´mica, UniVersidad Polite´cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain, and Institute for Chemical Process and EnVironmental Technology, and Institute for Microstructural Science, National Research Council Canada, 1200 Montreal Road, Ottawa, ON, Canada ReceiVed: May 23, 2010; ReVised Manuscript ReceiVed: July 22, 2010
The photophysical properties of a series of 9,9′-dioctylfluorene copolymers containing 1,3,4-oxadiazole units (PF1Ox, PF2Ox, PF3Ox, and PF4Ox) 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 PF2Ox, PF3Ox, and PF4Ox were almost coincident (λem ) 426 nm, φfl ) 0.38) and somewhat different from those of PF1Ox (λem ) 404 nm, φfl ) 0.34) and PFOxCz (λem ) 472 nm, φfl ) 0.39). These differences are attributed to the influence of the largest Ox percentage in the polymer (PF1Ox) or to the presence of a strong electron-donor carbazole unit (PFOxCz). In transient absorption spectroscopy the presence of triplet excitons (λ ) 350 and 740 nm, quenched by oxygen), positive polarons (λ ) 400 and 530 nm, quenched by triethylamine), and photoeject electrons (absorption onset 800 nm, quenched by oxygen) was observed for PFnOx (n ) 1, 2, 3, 4) with a similar quantum yield for photoejected electrons. PFOxCz 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 PF1Ox and PFOxCz is somewhat different from that of the rest of the fluorene copolymer series (PF2Ox, PF3Ox, and PF4Ox) that exhibit almost coincident properties. Introduction Conjugated polymers having fluorene units are widely used in polymeric light diodes (PLEDs),1,2 field effect transistors,3-6 and photovoltaic cells.7,8 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.1,3,9-14 Recently, we have reported that fluorene-carbazole copolymers exhibit high efficiency for photoinduced charge separation.15,16 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. Scheme 1 presents the structure and codes of the conducting copolymers subject to study. We will show below that the percentage of Ox units as well as the combination of Ox and Cz lead to remarkable changes in the lifetimes and efficiency of the photogenerated transients. * To whom correspondence should be addressed. E-mail: hgarcia@ qim.upv.es and
[email protected]. † Universidad Polite´cnica de Valencia. ‡ National Research Council Canada.
SCHEME 1: Chemical Structure of the Fluorene Copolymers under Investigationa
a The terms used to denote the copolymers indicate the monomers forming the structure (F: fluorene; Cz: carbazole; Ox: oxadiazole) and the subscripts indicate the number of these moieties in a repeating unit.
Results and Discussion The synthesis of the polymers under study has been reported in our previous publications.15,17 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 dibromomonomers with the diboronic acid monomers of fluorene, difluorene, and carbazole for PF2Ox, PF3Ox, PF4Ox, and PFOxCz, respectively. While PF1Ox, PF3Ox, PF4Ox, and PFOxCz have highly symmetric alternating structures of oxadiazole with fluorene or/and carbazole, the periodicity of PF2Ox is slightly low.17
10.1021/jp104725u 2010 American Chemical Society Published on Web 08/04/2010
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J. Phys. Chem. C, Vol. 114, No. 33, 2010
de Miguel et al. TABLE 1: Wavelengths of the Maximum Emission Intensity (λem) upon Excitation at the Indicated Wavelengths (λex) and the Corresponding Lifetimes (τfl) and Quantum Yields (Of) for N2-Purged Benzonitrile Solutions of the Fluorene Copolymers under Study copolymer
λex (nm)
λem (nm)
PF1Ox PF2Ox PF3Ox PF4Ox PFOxCz
374, 397 399 382 382 380
426, 454, 484 424 424 424 473
τfl (ns)
1.33 (88%)
