Impact of fluorinated vinylene units on supramolecular organization and optical properties of poly( p-phenylenedifluorovinylene) thin films as a class of blue band gap conjugated polymers (original) (raw)
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Chemistry of Materials, 2005
This contribution reports the combined influences of odd-eVen effects and the specific positioning of alkoxy side chains OR 1) (OC n+10 H 2(n+10)+1) and OR 2) (OC n H 2n+1) (with n) 6, 7, 8, 9) on the phenylene-ethynylene and phenylene-Vinylene segments, respectively, on the optical properties of hybrid polymers P(n+10)/n of general repeating unit:-Ph-CtC-Ph-CtC-Ph-CHdCH-Ph-CHdCH-. For the polymeric materials, visual color impression varies alternatively between orange red (P16/6 and P18/8) and yellow (P17/7 and P19/9) according to the odd and eVen features of the alkoxy side chains, where odd or eVen relates to the total number of sp 3-hybridized atoms within the side chains. This side chain related effect is ascribed to both absorptive and emissive behaviors of the polymers on the basis of photophysical investigations in the bulk. Almost identical thin film absorption spectra were obtained for all four materials; however, the photoluminescence of the odd polymers, P16/6 (λ f) 556 nm) and P18/8 (λ f) 614 nm), was red-shifted relative to that of their eVen counterparts (λ f) 535 nm). Further, the P18/8 maximum at 614 nm can be readily assigned to excimer emission, as evidenced by the largest Stokes shift (5600 cm-1), largest fwhm f-value (3700 cm-1), and the lowest Φ f-value of 24%. The strong π-π interchain interaction in P18/8, due to loose alkoxy side chains packing, does not only favor fluorescence quenching but also enable an effective inter-as well as intra-molecular recombination of the generated positive and negative polarons in electroluminescence, which explains the good EL properties of this polymer irrespective of the solvent used. A voltage-dependent blue shift of the EL spectra of up to 100 nm was observed for P18/8 devices prepared from aromatic solvents. This red to green EL shift as observed with increasing voltage is assigned to conformational changes of the polymer chains with increasing temperature.
Materials, 2010
This review is an overview of our previous work on the synthesis and properties of poly(p-phenylenevinylene)s (PPVs) selectively fluorinated in different positions of the conjugated backbone. Both the synthetic challenges and the effects of functionalization with fluorine atoms on the optical behavior are discussed, highlighting the peculiarities and the interest of this class of conjugated polymers. A general polymerization protocol for PPVs, that is based on the Pd-catalyzed Stille cross-coupling reaction of bis-stannylated vinylene monomers with aromatic bis-halides, has been successfully extended to the synthesis of selectively fluorinated poly(p-phenylenevinylene)s. The properties of a series of these PPVs differing in the number and positions of the fluorine atoms on the conjugated backbone have been studied, even in comparison with the non-fluorinated counterparts. The intriguing optical features of the resulting materials are discussed considering not only the role of the electronic and steric effects induced by the fluorine substituents, but also the impact of the fluorination on the solid state organization and intermolecular interactions.
Journal of Physical Chemistry C, 2008
This study deals with the effect that the incorporation of 2,7-fluorenone into the polymer backbone of a poly(fluorenylene-vinylene) exerts on its photophysical properties. The synthesis of a series of random poly(fluorenylene-vinylene)s containing fluorenone at 10, 5, 3, 1% mol/mol ratio with respect to fluorene units (PFVK1-4) was achieved by the Suzuki-Heck reaction cascade between potassium vinyl trifluoroborate and the equivalent quantity of the suitable feed of the corresponding dibromoaryl comonomers in different ratios. The polymers were characterized by 1 H NMR, IR, TGA, DSC, cyclic voltammetry, and UV-vis as well as stationary and time-resolved PL both in solution and in the solid state. In chloroform solution, PFVK1-4 show an emission originating from the PFV backbone while no fluorenone emission could be detected. Moreover, the presence of fluorenone lowers the efficiency quantum yields that inversely follow the fluorenone/fluorene ratio. Conversely, in the solid state, a complete energy transfer occurs and fluorenone acts as the only luminophor even in PFVK1, containing it only in 1% amount respect to fluorene. Consequently, in the solid state PFVK1-4 are all yellow-orange emitters in the solid state. The yellow-orange fluorescence of the obtained polymers in the solid state was compared to the optical behavior of the monodispersed compound 2,7-distyrylfluorenone (DSF). The analysis of the fluorescence decay pathways of the molecules suggests that, differently from DSF, the low-energy emission in the polymers does not originate from a cofacial interaction between fluorenones units. On the contrary, in addition to the fluorenone emission, complex interactions between the fluorenone luminophors and the poly(fluorenylene-vinylene) matrix have to be taken into account for a rationalization of the photophysical properties of these fluorenone-containing polymers in the solid state. Notwithstanding the presence of carbonyl-containing units, usually considered deleterious for the emission properties of poly(arylene-vinylene)s, PFVK1-4 show potential as emitting layers in yelloworange OLEDs, exhibiting luminances up to 1387 cd/m 2 and current efficiencies as high as 0.15 cd/A.
Synthetic Metals, 2012
Nowadays considerable efforts are devoted to the synthesis of low bandgap conjugated polymers for application in organic polymer solar cells. A large variety of low bandgap polymers are prepared by alternating copolymerization of electron-donating donor and electron-withdrawing acceptor units. The interaction between these two units can reduce the polymer bandgap, increasing the sunlight absorption. Benzothiadiazole is commonly used as acceptor block unit in low bandgap polymers. In this contribution we investigate the supramolecular organization and optical properties of thin films of conjugated polymers consisting of benzothiadiazole and thiophene with electron-withdrawing difluorovinylene, and electron-donating vinylene substituents. Atomic force microscopy and spectroscopic ellipsometry are exploited for the analysis of the morphology and optical transitions, respectively. It is found that F-atoms in the vinylene unit yield a blue-shift of the absorption peaks of 0.2 eV respect to the hydrogenated polymer and an increase in the absorption coefficient of fluorinated polymers, which indicates their potential application as photovoltaic material. The morphology evolution of the conjugated polymers blended with a fullerene derivate ([6,6]-phenyl C 61-butyric acid methyl ester, PCBM) is also investigated by atomic force microscopy.
International Journal of …, 2011
Density functional theory (DFT) and time-dependent DFT (TDDFT) were employed to study ground-state properties, HOMO-LUMO gaps (Δ H-L ), excitation energies (E g ), ionization potentials (IPs), and electron affinities (EA) for PFV-alt-PDONV and PFV-alt-PDIH-PPV having different alternating groups. Excited-state properties were investigated using configuration interaction singles (CISs) while fluorescence energies were calculated using TDDFT. The results show that PFV-alt-PDONV exhibits blueshifted energies for both HOMO-LUMO gaps (Δ H-L ) and excitation energies (E g ) compared with PFV-alt-PDIH-PPV. The predicted IP and EA clearly indicate that PFV-alt-PDIH-PPV has both easier hole creation and electron injection than that of PFV-alt-PDONV. The maximal absorption wavelengths of all polymers are strongly assigned to π → π * transition. The predicted radiative lifetimes of PFV-alt-PDONV and PFV-alt-PDIH-PPV for B3LYP/6-31G(d) are 0.36 and 0.61 ns, respectively, indicating that PFV-alt-PDIH-PPV should have a better performance for long-time emission than that of PFV-alt-PDONV.
Chemistry of …, 2005
A novel conjugated polyfluorene/poly(p-phenylenevinylene) copolymer containing the pendant bis(4alkoxyphenyl) groups in the C-9 position of every alternating fluorene unit has been synthesized and well structurally characterized. The photoluminescence spectrum of this polymer exhibits strong concentration and excitation wavelength dependence in solution. The excited triplet-state maximum of polymer occurs in the region of 460-540 nm with a lifetime of 65.8 µs. This copolymer displays a minor positive nonlinear absorption at the focus of the laser irradiation, suggesting possible reverse saturable absorption. The stable electroluminescent spectrum of the polymer light-emitting diode device based on this copolymer (device configuration, indium-tin oxide/Au/copolymer/LiF/Al) was obtained with a peak wavelength of 515 nm. The bright-green emission observed over the whole active area of the copolymer closely resembles the photoluminescence of the most concentrated solution (0.5 M) used. This suggests that chain stacking in the solid state is responsible for the observed green electroluminescence.
Journal of Polymer Science Part A: Polymer Chemistry, 2006
Poly(9,9-dihexylfluorene-2,7-vinylene-alt-dibenzothiophene-2,8-vinylene) (PS) and poly (9,9-dihexylfluorene-2,7-vinylene-alt-dibenzothiophene-5,5-dioxide-2,8vinylene) (PSO) as well as corresponding model compounds were synthesized by Heck coupling. Both the polymers and model compounds were readily soluble in common organic solvents such as tetrahydrofuran, dichloromethane, chloroform, and toluene. The polymers showed a decomposition temperature at $430 8C and a char yield of about 65% at 800 8C in N 2 . The glass-transition temperatures of the polymers were almost identical (75-77 8C) and higher than those of the model compounds (26-45 8C). All samples absorbed around 390 nm, and their optical band gaps were 2.69-2.85 eV. They behaved as blue-greenish light emitting materials in both solutions and thin films, with photoluminescence emission maxima at 450-483 nm and photoluminescence quantum yields of 0.52-0.72 in solution. Organic light-emitting diodes with an indium tin oxide/ poly(ethylene dioxythiophene):poly(styrene sulfonic acid)/polymer/Mg:Ag/Ag configuration with polymers PS and PSO as emitting layers showed green electroluminescence with maxima at 530 and 540 nm, respectively. V V C 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6790-6800, 2006
Chemistry of Materials, 2004
The microscopic morphology of a series of substituted fluorene-based conjugated polymers and copolymers are analyzed with tapping-mode atomic force microscopy. Different structures are observed depending on the nature of the substituents. Thin deposits of polyfluorenes substituted with linear alkyl groups are made of long fibrils, with lateral dimensions on the order of a few nanometers; polymers with branched alkyl or aromatic substituents form homogeneous, featureless films. To understand how polymer chains pack into these structures, comparisons are made with molecular modeling calculations; the simulation results highlight the dependence between intermolecular π-π interactions and steric hindrance among substituents: linear alkyl substituents allow for a close packing of the conjugated chains into very long, regular π-π stacks, in contrast to the bulkier substituents. A strong correlation is established between the degree of order in the thin deposits and the solid-state photoluminescence spectra; a red shift and the formation of a broad emission band in the green region are observed for deposits showing long-range organization, which is attributed to the formation of aggregates of well-organized, densely packed molecules.