Combined Techniques for the Characterization of Polyfluorene Copolymers and Correlation with their Optical Properties. Filippo Samperi,* ,† Salvatore Battiato, † Concetto Puglisi, † Umberto Giovanella, ‡ Raniero Mendichi, ‡ and Silvia Destri ‡ † Istituto di Chimica e Tecnologia dei Polimeri (ICTP) UOS Catania, CNR, Via Gaifami 18, 95126 Catania Italy ‡ Istituto per lo Studio delle Macromolecole, CNR, Via Bassini 15, 20133 Milano, Italy * S Supporting Information ABSTRACT: New red- and green-emitting copolymers, hereafter core-copolymers, bearing a 4,7-bis(thiophen-2-yl)- benzothiadiazole and a benzothiadiazole residue respectively as bridging core between two identical polymeric arms were synthesized by Suzuki coupling reaction of the dibromine derivative of such chromophores and essentially borolane- ended alternating copolymers [namely P(TPAF)] of triphe- nylammine disubstituted fluorene and dialkylsubstituted fluorene. All polymer samples were characterized by 1 H NMR and in particular by MALDI-TOF MS. MALDI mass spectra allow the identification of many end groups of the initial blue-emitting macromers and therefore of the side reactions occurring during Suzuki polycondensation. The average molar masses were determined by two different SEC apparatus, one calibrated with conventional polystyrene narrow standards and the other with an absolute calibration curve built up by SEC/ MALDI-TOF MS analysis of selected SEC fractions of polydisperse red and green core-copolymers. MALDI mass spectra of these fractions give reliable information on their composition, which combined with their integrated area calculated from the corresponding normalized SEC curves, enable the estimation, for the first time, of the percentage of macromolecules containing the dyes composing the neat core-copolymers. Optical characterization, performed by UV-visible absorption and photoluminescence measurements, of the same SEC fractions gives results in agreement with the different compositions determined by their MALDI mass spectra. 1. INTRODUCTION Conjugated aromatic polymers have been largely used in the last two decades for preparing active layers in optoelectronic devices as organic light-emitting diodes (OLEDs), 1 bulk heterojunction photovoltaic cells 2 and electronic ones like organic field-effect transistors. 3 In particular, in the fabrication of the former group of devices, alternating copolymers are employed and organometallic polycondensations are consid- ered a useful and reliable synthesis method. In the preparation of polymers containing fluorene, benzothiadiazole, and thiophene residues, the Suzuki coupling plays a very important role starting from diboronic acid or diboronic acid ester as 4,4,5,5-tetrametyl-1,3,2-dioxoborolane or 1,3-propane-1,3,2-di- oxoborolane of a monomer and the dibromine or diiodide derivatives of the comonomer. 4-23 For introducing at the same time different emitting dyes in a polymeric skeleton or both electron donor and acceptor side groups more than one dihalide derivative has to be used in a one step synthesis. This strategy, experienced by Chuang et al. 4 for preparing active layers suitable for white-emitting OLEDs (WOLEDs), leads however to a not defined mixture of chemically different polymers due to the different reactivity of dihalide species and also to the intrinsic nature of the Suzuki coupling. Alternatively, we tried another approach to obtain WOLEDs 5 enabling us to determine more precisely the amount of blue, green, and red emitting polymeric species constituting the blend for the device active layer. Two molecular dyes, benzothiadiazole (Btz) as the green emitter and 4,7-bis(thiophen-2-yl)benzothiadiazole (ThBtzTh) as the red emitter, were individually inserted as bridging core between two identical polymeric blue emitting arms. The polymeric arms, as well as the pure blue emitting compound in the blend, consist of a fluorene-based alternating copolymer P(TPAF) made by a triphenylamine disubstituted fluorene unit (TPA) and a dialkyl substituted fluorene (F) one. We have already shown that such an insertion of TPA moieties resulted in reduced interchain interactions, an enhanced hole injection, and increased morphological stability with respect to conventional polyfluorenes (PFs). 6 Moreover, the presence of bulkier side groups contributed to tune resonant energy transfer (RET) process from donor toward the acceptor entities of the polymeric chains, 7 otherwise too efficient to observe the contemporary emission from all the chromophores necessary for the white emission. The blue-emitting copolymer was synthesized by Suzuki polycondensation, while the green Received: October 13, 2011 Revised: January 19, 2012 Published: February 9, 2012 Article pubs.acs.org/Macromolecules © 2012 American Chemical Society 1811 dx.doi.org/10.1021/ma202301j | Macromolecules 2012, 45, 1811-1824