Photophysical properties and quantum chemical studies of poly(2,7-9,9'-dihexylfluorene-dyil)

Abstract

Este trabalho descreve as propriedades fotofísicas (espectros de excitação e de fluorescência, rendimento quântico e tempo de meia-vida de fluorescência) do poli(2,7-9,9'diexil-fluoreno-diil) em soluções diluídas, de quatro solventes (tolueno, tetraidrofurano, clorofórmio e acetato de etila) e no estado sólido. Os espectros de fotoluminescência são característicos da conformação desordenada da cadeia principal, denominada forma α. Espectros eletrônicos de absorção foram simulados para oligômeros de 1 a 11 meros, indicando que o comprimento crítico de conjugação está entre 6 e 7 meros. Os valores teóricos do momento de dipolo mostraram que a conformação helicoidal formada se repete a cada 8 unidades por volta. Mostra-se ainda que no estado sólido ocorrem processos de transferência de energia que levam a uma diminuição do tempo de meia-vida de fluorescência. Com base nos espectros obtidos, pode-se concluir que a fotoluminescência e a eletroluminescência descrita na literatura são provenientes das mesmas unidades emissoras.This work reports the photophysical properties (excitation and fluorescence spectra, fluorescence quantum yield, fluorescence lifetimes) of the poly(2,7-9,9'-dihexylfluorene-dyil) in dilute solutions of four solvents (toluene, tetrahydrofuran, chloroform and ethyl acetate) as well as the properties in solid state. Photoluminescence showed spectra characteristic of disordered α-backbone chain conformation. Simulation of the electronic absorption spectra of oligomers containing 1 to 11 mers showed that the critical conjugation length is between 6 and 7 mers. We also estimated the theoretical dipole moments which indicated that a coil conformation is formed with 8 repeating units per turn. We also showed that some energy transfer process appears in solid state which decreases the emission lifetime. Furthermore, based on luminescent response of the systems herein studied and electroluminescent behavior reported on literature, both photo and electroluminescence emissions arise from the same emissive units.Keywords: polyfluorene, photophysical properties, quantum mechanical calculation, photoluminescence IntroductionPolyfluorene and derivates form a class of conjugated polymers whose photoluminescence emission is strongly dependent on their chemical structure ranging from the violet to red.1-8 Several of them are also electroluminescent materials.1,4-9 The backbone of fluorene-based polymers contains a rigidly planar biphenyl structure which can be functionalized in the C-9 position to enhance the polymer solubility. Nevertheless, this substitution at C-9 induces minor steric effects in the adjacent aromatic rings, 10 leading, in general, to small geometrical disturbance. It is also well known that some polyfluorenes in solid state may form liquid-crystalline phases, with high ordered crystalline phases with thermotropic phase transition from 100 to 170 °C.11 Moreover, the polymer chain may be organized in the solid state undergoing phase structure with different spectral properties: more ordered α-pha...

The effect of complexation with platinum in polyfluorene derivatives: A photo- and electro-luminescence study

“…Apart from the progress in understanding the relations among photo, optical and electrical properties with structure [8,9] and morphology [10], the relevant use of the fluorene moiety in photovoltaic cells has arisen due to its low lying HOMO level, which is a favorable feature for the open circuit voltage of such devices. The pioneering work of Andersson and Inganäs and co-workers [11][12][13] paved the way for other groups [14,15] to optimize polyfluorenes and demonstrate high device performance.…”

Highly fluorescent purine-containing conjugated copolymers with tailored optoelectronic properties

“…16–20,24,25,69–71 These results also show that the increase in sulfur content from P2 F to P3 TFT leads to a decrease in fluorescence quantum yield, from ϕ = 0.61 to ϕ = 0.22, which is consistent with an internal heavy atom effect that facilitates intersystem crossing via spin orbit coupling. 72,73 Nevertheless, P2 F has a higher photoluminescent quantum yield than values reported for homopolymers of poly(alkylfluorene)s in chloroform 74 and a significantly higher quantum yield compared to thiophene-alkylfluorene copolymers, 75 suggesting that the purine unit contributes strongly to the highly fluorescent behavior. The quantum yields of these four purine-containing copolymers are notable, as a benchmark value of ϕ ≥ 0.1 is typically viewed as sufficient for use in organic light-emitting diodes and polymer dots.…”

Energy-Transfer Processes in Donor–Acceptor Poly(fluorenevinylene-alt-4,7-dithienyl-2,1,3-benzothiadiazole)

“…The assignment of red-shifted transitions to macromolecular association is widely spread in the literature on conjugated polymers. The formation of dimers (or higher forms) in the electronic ground- or excited-state − H or J molecular aggregates or still the presence of the so-called β-phase has been reported . The state of aggregation and the macromolecular conformations associated with it can vary with temperature and excitation wavelength, altering the emission profile and making the assignment of the emitting center(s) under each condition very complex.…”