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Synthesis and optical properties of compounds via platinum-catalyzed hydrosilylation of triethynyltriazine and silyl-substituted oligothiophenes
Abstract
New types of starlike compounds and branched polymers with a triazine core and silylbithienyl or silylterthienyl arms were prepared by the use of platinum-catalyzed hydrosilylation. The UV–Vis absorption and fluorescence properties of these molecules have been investigated. In the fluorescence spectra of the silylbithienyl substituted starlike compound, their emission maxima derived from the bithiophene units were observed. In the branched polymer with silylbithienyl units, a peak derived from the interaction between the triazine and bithienyl units was observed at about 500 nm. No significant difference between the starlike compound and branched polymer with silylterthienyl units was observed in their UV–Vis absorption and fluorescence spectra. DFT calculations were performed to understand the photophysical properties of the starlike molecules with silylbithienyl and silylterthienyl arms.
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The surface modification of inorganic materials with organic units is an important process in device preparation. For the modification of TiO2, organocarboxylic acids (RCO2H) are usually used. Carboxylic acids form ester linkages (RCO2Ti) with hydroxyl groups on the TiO2 surface to attach the organic groups on the surface. However, the esterification liberates water as a byproduct, which may contaminate the surface by affecting TiO2 electronic states. In addition, the ester linkages are usually unstable towards hydrolysis, which causes dye detachment and shortens device lifetime. In this review, we summarize our recent studies of the use of polymers composed of disilanylene and π-conjugated units as new modifiers of the TiO2 surface. The TiO2 electrodes modified by those polymers were applied to dye-sensitized solar cells.
Strategies for the design and construction of non-linear, 2D and 3D conjugated macromolecules are presented in this critical review. The materials, termed here as star-shaped structures, feature a core unit which may or may not provide conjugated links between arms that radiate like spokes from a central axle. The arms of the macromolecules consist of linear oligomers or irregular conjugated chains lacking a formal repeat unit. The cores range from simple atoms to single or fused aromatic units and can provide a high level of symmetry to the overall structure. The physical properties of the star-shaped materials can be markedly different to their simple, linear conjugated analogues. These differences are highlighted and we report on anomalies in absorption/emission characteristics, electronic energy levels, thermal properties and morphology of thin films. We provide numerous examples for the application of star-shaped conjugated macromolecules in organic semiconductor devices; a comparison of their device performance with those comprising analogous linear systems provides clear evidence that the star-shaped compounds are an important class of material in organic electronics. Moreover, these structures are monodisperse, well-defined, discrete molecules with 100% synthetic reproducibility, and possess high purity and excellent solubility in common organic solvents. They feature many of the attributes of plastic materials (good film-forming properties, thermal stability, flexibility) and are therefore extremely attractive alternatives to conjugated polymers (210 references).
New types of polymers composed of bithiophene condensed with disilacyclohexadiene rings and benzothiadiazole were prepared by the platinum-catalyzed dehydrogenative bissilylation. UV–Vis absorption and fluorescence properties of these polymers have been investigated. The fluorescence spectra of these polymers showed only the bands ascribed to the benzothiadiazole units even when the bithiophene units were irradiated. For these polymers, photoenergy transfer occurs from donor to acceptor. DFT calculations were performed to understand the photophysical properties of the polymers.
Poly(tetramethylsilarylenesiloxane) derivatives (P1T-P4T, P4TC) that contained thiophene (1T), oligothiophenes such as dithiophene (2T), terthiophene (3T), quaterthiophene (4T), or 2,6-di(2-thienyl)-4,4-diphenylcyclopentadithiophene (4TC) as the main chain components were synthesized via polycondensation reaction of the corresponding disilanol monomers (M1T-M4T, M4TC). The obtained polymers other than P4T that contained quaterthiophene units exhibited the good solubility in common organic solvents, such as toluene, chloroform, tetrahydrofuran and so on. The glass transition temperature (T g ) and melting temperature (T m ) of the obtained polymers regarding P1T-P4T were increased with the extension of the thiophene rings of the monomeric unit as deduced from the results of the differential scanning calorimetry (DSC). On the other hand, the T m was not observed for P4TC that was composed of 2,6-di(2-thienyl)-4,4-diphenylcyclopentadithiophene units. P2T that was composed of dithiophene units exhibited the highest temperature at 5% weight loss (T d5 ) of 467 °C in all present polymers. The bathochromic and hyperchromic effects in the absorption spectra as well as the improvement of fluorescence quantum yield were observed by the introduction of dimethylsilyl group onto the present oligothiophene derivatives. These effects became unremarkable accompanying the elongation of the thiophene rings of the monomeric units; however, 4TC exhibited a more significant change by the introduction of the dimethylsilyl group than the other oligothiophenes derivatives, indicating that the introduction of the cardo-structure was effective in the improvement of the optical properties. M4TC and P4TC exhibited the fluorescence wavelength around 500 nm and the highest fluorescence quantum yield of the present series of poly(tetramethylsilarylenesiloxane) derivatives, i.e., 0.40, plausibly because the free rotation between thiophene rings in oligothiophene inhibited by the cardo-structures and the bulky diphenyl groups at 4-position of cyclopentadithiophene moiety suppressed the nonradiative transition.
New types of molecules, tetrasilylsilanes with the arms composed of oligothienylsilylvinyl units, were prepared by the platinum-catalyzed hydrosilation reactions. UV–Vis absorption and fluorescence properties of these molecules have been investigated. Polymer 9 showed good heat resistance against Td10 (10% weight loss temperature) at 402 °C even in air.
Two-dimensional starlike molecule 7 consisting of benzothiadiazole-thiophenylphenyltriazine groups and pentamethyldisilanyl substituted thienylene units that extend to three directions was synthesized by the reactions of 2,4,6-tris[4-(4-hexyl-5-(7-(3-hexyl-5-iodothiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)-thiophen-2-yl)phenyl]-1,3,5-triazine (5) with the pentamethyldisilanylthienyl substituted stannyl derivative (6). UV-visible absorption and fluorescence properties of starlike molecules 4 and 7 have been investigated in dioxane and in the solid state. The application of TiO2-electrode coated on the surface with the use of 7 to dye-sensitized solar cells has been discussed.
We have synthesized new types of the H-shaped molecules with bis(tert-butyldimethylsilylbithienyl)silyl and bis(tert-butyldimethylsilylterthienyl)silyl units by using Sonogashira coupling reaction. UV–Vis absorption and fluorescence properties of these molecules have been investigated. We have found that the disilanylene group in the H-shaped molecule with bis(tert-butyldimethylsilylbithienyl)silyl units plays an important role in enhancing the quantum efficiency in solution.
Two organosilicon polymers with donor-silicon-acceptor (D-Si-A) repeating units were prepared and their optical properties were investigated. One with bithiophene and diethynylbenzothiadiazole units as the donor and acceptor exhibits clear photoinduced electron transfer even in nonpolar cyclohexane. In contrast, for the other D-Si-A polymer with more expanded π-conjugated units, terthiophene and dithienylbenzothiadiazole, photoenergy transfer occurs efficiently in cyclohexane and electron transfer occurs only in polar benzonitrile. DFT calculations were performed to understand the different photoluminescence properties of these D-Si-A polymers.
The H-shaped molecule, 5,5′-bis{[bis(5′-tert-butyldimethylsilyl)-2,2′-(bithiophen-5-yl)]methylsilyl}-2,2′-bithiophene (3) was prepared by the reaction of 5′-(tert-butyldimethylsilyl)-5-lithio-2,2′-bithiophene with 2-bromo-5-(dichloromethylsilyl)thiophene, followed by treatment of the resulting product (5-bromothiophen-2-yl)bis[5′-(tert-butyldimethylsilyl)-2,2′-bithiophen-5-yl](methyl)silane (1) with compound (2), obtained by the reaction of the lithiated derivative of 1 with 2-isopropyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, in the presence of dichlorobis(triphenylphosphine)palladium, as light yellow solids. Compound 3 showed absorption maximum (位max) at 336 nm with a molar absorption coefficient (蔚) of 113,000 M−1 cm−1 in dioxane. The fluorescence spectrum of 3 excited at 336 nm in dioxane displayed the emission maxima at 377 and 387 nm, with the fluorescence quantum yield of 48%. We also carried out theoretical treatment of compound 3 to elucidate the geometrical and electronic structure, using the Gaussian 03 program package, with the 6-31G and 6-31G(d) basis sets.
Starlike molecules (5) and (7) that have a triazine unit as a core and arms consisting of an alterative arrangement of an Si–Si bond and bithienylene unit were prepared by the Pd-catalyzed reactions of 2,4,6-tris[5-(1,3-dioxa-4,4,5,5-tetramethyl-2-borolanyl)-2-thienyl]triazine with 5-bromothienylpentamethyldisilane and with 1-(5-bromothienyl)-2-bithienyltetramethyldisilane, respectively. In the UV–vis absorption and fluorescence spectra, 5 and 7 showed absorption maxima in a range of 398–399 nm, and revealed higher fluorescence quantum yields than that of 2,4,6-tris(bithienyl)triazine.
UV-visible absorption and fluorescence properties of 14 nano-size star-like compounds, being composed of oligothienylene units bridged by silicon atoms, have been studied in dioxane solution. The absorption and emission maximum wavelengths are found to be dependent on the oligothienylene units. High fluorescence quantum yields and longer lifetimes are observed for the star-like compounds containing bithienylene units, being related to the σ–π interaction and the star-like structure.
The new starlike molecules that have a regular arrangement of an Si-Si bond and bithienylene unit were synthesized by the reaction of tris(chlorodimethylsilyl)methylsilane, which was chosen as a core, with the lithiobithienylene derivatives used for construction of the arms. The starlike molecules showed high fluorescence quantum yields and long lifetimes of the excited state.
The syntheses of two types of starlike molecules with the arms that extend to three and four directions have been reported. The molecules with the arms consisting of a regular alternating arrangement of a silicon–silicon bond and bithienylene unit that extend to three directions were synthesized by the reactions of 1,3,5-tris(chlorodimethylsilyl)benzene, which was chosen as a core, with the lithio[oligo(disilanylenebithienylene)] derivatives. The starlike molecules with extended arms to four directions were prepared by the reaction of 1,2,4,5-tetrakis(fluorodimethylsilyl)benzene used as a core, with lithio[oligo(disilanylenebithienylene)]s. UV–Vis absorption and fluorescence properties of these starlike molecules have been investigated in a dioxane solution. The present molecules showed absorption maxima in a range of 321–337nm, and revealed higher fluorescence quantum yields than that of the corresponding linear polymer, poly[(tetraethyldisilanylene)bithiophene].
The geometric and electronic structures of organosilicon oligomers, nSixL(C=C)y with linear silicon segments and nSixR(C=C)y with cyclic silicon segments, are studied by using the density functional theory. It is found from the calculated results
that the extension of the π-conjugation plays an important role in tuning the geometric and electronic structures. The band
gap tends to be decreased and the hole injection rate tends to be increased with the elongation of the π-conjugated units.
An effective σ-π conjugation occurs between the silanylene moiety and the ethenylene moiety, especially for the polymers with
a rather small π-electron moiety. Interestingly, an even/odd effect of the band gap is found. A PSixR(C=C)y has narrower band gap than a corresponding PSixL(C=C)y. These results are in good agreement with the experimental observations.
The design and synthesis of new materials are two key steps in the advancement of technology. One of the most promising approaches of development of new materials that combine advantages of organic polymers with those of inorganic solids is to devise polymers that have a backbone of inorganic atoms to which are attached organic side groups. Among the best developed examples of ‘inorganic–organic polymers’ are organosilicon polymers. The synthesis of new organosilicon polymers using silyl triflate intermediates is reviewed in this article. Protodephenylation of phenylated polysilanes as well as poly(silylenemethylenes) by triflic acid gave new functionalized compounds. Network polymers were obtained by reductive coupling of silyl triflates with potassium–graphite. Novel poly(silylenealkynes) and poly(silylenearylenes) containing a regular alternating arrangement of silylene groups and organic units were prepared from α,ω-bis[(trifluoromethyl)sulfonyloxysilyl]-substituted compounds and dinucleophiles. Some of the polymers are potential organic precursor for ceramic materials. The correlation between structure and thermolytic behavior is demonstrated on selected examples.
Starlike molecules with arms consisting of a tert-butyldimethylsilyloligothienylenedimethylsilyl unit, MeSi[SiMe2(T)(n)SiMe2(t-Bu)](3) (7-11; n = 2-6, T = thienylene) were prepared by the reactions of tris(chlorodimethylsilyl)methylsilane, which was chosen as a core, with the tert-butyldimethylsilyl-substituted lithiooligothienylenes used for construction of the arms. UV-visible absorption and fluorescence properties of 7-11 have been studied in a dioxane solution. The molecules 7-11 showed high fluorescence quantum yields with respect to the corresponding oligothiophenes. Compounds 7 and 8 revealed longer lifetimes than those of bi- and terthiophene. The results of X-ray crystallographic analysis of the bromo derivative 12 are described. The density functional theoretical calculations were also carried out to compare the geometrical structures and the excitation energies with the available experimental data.
Polycarbosilanes exhibit desirable physiochemical properties such as high thermal stability, elastomeric behavior, diverse functional group tolerance, electronic delocalization, and ease of synthesis. New ceramic precursors, fire‐retardant materials, and flexible conducting polymers have been created using polycarbosilanes. Catalytic hydrosilylation provides a convenient synthetic route to these materials. With its tolerance of many functional groups, catalytic hydrosilylation offers one‐step syntheses of both aliphatic and olefinic polymers and dendrimers whose electronic properties can be tuned selectively.
Copolymerization of conjugated diynes with compounds that contain the silole chromophore offers a route to new luminescent materials that have been applied to the detection of explosives and may be promising organic light‐emitting diode materials. An important feature of the silole materials is the Lewis acidity at silicon, which offers a function not often present in organic polymers.
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Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
[reaction: see text] Synthesis of polyethynyl-substituted aromatic compounds was achieved efficiently by the use of the Negishi cross-coupling reaction, and this method, coupled with the Sonogashira reaction, was applied to the synthesis of differentially substituted hexaethynylbenzenes from chloroiodobenzenes.
- Ohshita