1,721,014 research outputs found
Impact of Precatalyst Activation on Suzuki-Miyaura Catalyst-Transfer Polymerizations: New Mechanistic Scenarios for Pre-transmetalation Events
No full textThe relevance of LnPdX2 precatalyst activation on the Suzuki-Miyaura reaction course was investigated in the case of catalyst-transfer polymerizations. A catalytic study, backed up by theoretical calculations, allowed to ascertain the coexistence of a neutral and an anionic mechanistic pathways in the precatalyst activation, in which the bulky tBu3P external ligand plays a crucial role. The fine-tuning of the catalytic conditions can steer the activation step toward the anionic pathway, leading to the full control over the polymerization course. While providing insights and perspectives into the catalyst-transfer polymerizations, these results uncover unexplored scenarios for the pre-transmetalation events of Suzuki-Miyaura reactions contributing to its full understanding
A Pd(AcO)2/t-Bu3P/K3PO4 catalytic system for the control of Suzuki cross-coupling polymerisation
No full textThe initiation of the Suzuki cross-coupling polymerisation with a fluorene-based AB-type monomer was finely tuned within the chain-growth regime by the suitable control of the species generated by the Pd(AcO)2/t-Bu3P/K3PO4 catalytic system. The prototypical poly(9,9-di-n-octyl-fluorene) was obtained with extremely fast (1 min) polymerisation rates, excellent polydispersities (1.16) and molecular weights dependent on the monomer/catalyst molar ratio, without the need to resort to formal chain-initiators
On the Degradation Process Involving Polyfluorenes and the Factors Governing Their Spectral Stability
No full textThis study deals with an investigation of the spectral stability of differently structured polyfluorenes (PFs), deprived of 9-H defects, embodying 9,9-dialkylfluorene (P1), 9,9-diarylfluorene (P2), or 9,9-diarylfluorene/9,9- dibenzylfluorene units in a 1:1 alternating fashion (P3). Thermal annealing or UV irradiation carried out on films of P1-P3 in air revealed that their typical blue photoluminescence is invariably stained, independently of their 9-substitution, by the appearance of the low-energy band (g-band) pointing out a remarkable effect of light on the degradation process. A more comprehensive picture of the degradation pathway is proposed, including as key step a light-promoted formation of a PF radical cation generated by aerobic oxidation (photoluminescence test) or p-doping (cyclic voltammetry test). The blue emission of P1-P3 could successfully be preserved by dispersing them into a higher band gap matrix, such as polyvinylcarbazole (PVK), indicating a fundamental role of the intermolecular interactions between PF chains in the appearance of the low-energy emission band. Comparison between the optical behavior of suitably prepared PFs containing either fluorenone moieties (PFK) or 9-(bis-methylsulfanyl-methylene)fluorene moieties (PFS) holds regions of planarity within the PF backbone (inducing local intermolecular interactions) and not the fluorenone charge-transfer emission as responsible of the g-band of degraded PFs
A Versatile Synthesis for New 9,10-Bis(4-alkoxyphenyl)-2,7-diiodophenanthrenes: Useful Precursors for Conjugated Polymers
No full textA practical synthesis for 9,10-bis(alkoxyphenyl)-2,7-diiodophenanthrenes is described. 9,10-Bis(4-hexyloxyphenyl)-2,7-diiodophenanthrene was selectively obtained by a one-pot reaction of 10,10-bis-(4-hexyloxy-phenyl)-2,7-diiodo-10H-phenanthren-9-one with 1 equiv of triethylsilane in trifluoroacetic acid. The target molecular architecture was demonstrated to be a useful precursor for obtaining the corresponding poly(phenanthrene) and poly(phenanthrylene-vinylene)
Chain-Growth Versus Step-Growth Mechanisms for the Suzuki-Heck Polymerisation of Fluorenyldibromides with Potassium Vinyl Trifluoroborate
No full textThe mechanism of the Suzuki-Heck (SuHe) polymerisation of 2,7-dibromo-9,9-di(n-dodecyl)fluorene (1) with potassium vinyl trifluoroborate (PVTB) for the synthesis of poly(fluorenylene vinylene)s (PFVs) has been investigated. In the first stage, a palladium-catalysed chain-growth AA/B(C)type polycondensation occurs, as evidenced by the linear trend observed when plotting the molecular weights of the polymer formed against the consumption of the monomer. The chaingrowth stage takes place until complete consumption of 1 and allows one to envisage the alternating addition of PVTB (by a Suzuki step) and 1 (by a Heck step) to the growing chain. Such alternating addition seems to proceed through a peculiar catalyst transfer during which the metal is constantly bound to the growing chain, confirmed by MALDI end-group analysis. On prolonging the reaction after the disappearance of 1, a second stage takes place, leading to higher molecular weight polymers, which are formed by the step-growth condensation of the fragments generated in the first stage. The molecular weights of the final PFVs depend on the PVTB/1 feed ratio. Thus, by using a PVTB/1 molar ratio of 1.1, the final PFV was characterised by an M n of 39600 Da, whereas when using a PVTB/1 molar ratio of 2.0, the final PFV was characterised by an Mn of 13200 Da
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
An engineered co-sensitization system for highly efficient dye solar cells
No full textNovel co-sensitizers have been structurally tailored and implemented in multi-sensitized devices demonstrating synergic efficiency enhancement attributable to improved light-harvesting as well as prevention of charge recombination
New spiro-functionalized poly(fluorene)s: synthesis and properties
No full textThe synthesis of several new spirocycloalkane functionalized polyfluorenes is described. Poly[2,7-(9,9-dioetylfluorene)-alt-2',7'-spiro(cyclohexane-1,9'-fluorene)] (PH), poly[2,7-(9,9-dioctylfluorene)-co-2,7'-spiro(cyclohexane-1,9'-fluorene)] (PF2), poly (2',7'-bis[(9,9-dioctylfluoren-2-yl)]-spiro(cyclohexane-1,9'-fluorene)-7,7-diyl] (PF3), poly[2,7-(9,9-dioctylfluorene)-alt-2',7'-spiro(cyclopropane-1,9'-fluorene)] (PF4) and poly [2,7- (9,9-dioctylfluorene)-co-2',7'-spiro(cyclopropane-1,9'-fluorene)] (PF5) were synthesized by Pd(O) and Ni(O) mediated coupling reactions. All polymers were characterized by NMR, UV-vis, DSC-TGA and PL. Efficient prevention of interchain interactions by the spirocyclohexane moieties guarantees pure blue emission from pristine PF1-3 as revealed by solid state PL spectra. On the other hand, solid state PL spectra of PF4 and PF5 are characterized by an additional emission band in the green region. Thermal annealing on PF1-3 at 150degreesC under nitrogen atmosphere does not cause any substantial changes in their solid state PL spectra, whereas after carrying out the same treatment for 20 h at 110 degreesC under air the polymers show a low-energy emissionbandin the green region which is presumably brought about by chemical modifications
Insight into the Role of Oxidation in the Thermally Induced Green Band in Fluorene‐Based Systems
No full textThe causes of the spectral instability of poly[9,9-dioctylfluoren-2,7-diyl-co-2',7'-spiro(cyclohexane-1,9'-fluorene)] during thermal annealing in air, which leads to a green photoluminescence (PL) emission band, are investigated. The I-green/I-blue ratio evolution (I = intensity) is found to be independent of the amount of monoalkylfluorene defects, despite the fact that their presence might be regarded as a trigger for the radical process leading to polymer degradation in the presence of a trace amount of metal catalyst. Furthermore, the absence of a correlation between the degree of oxidation of the material and the Igreen/Iblue ratio indicates that the spatial disposition of fluorenones formed during the thermal degradation of the material, rather than their amount, is to be strictly related to the I-green/I-blue ratio. The evidenced formation of fluorenone agglomerates, which could be considered the cause for the consistent increase in the Igreen/Iblue ratio during a thermal oxidation of a polyfluorene, confirms that the radical mechanism can also involve dialkylfluorene systems. Finally, the higher resistance to thermal degradation shown by spirocyclohexane fluorene units with respect to dioctylfluorene ones allows the synthesis of new; spectrally stable, fluorene-based copolymers
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