Why does the presence of silicon atoms improve the emission properties of biphenyl derivatives? - Verification of various hypotheses by experiment and theory.

In the course of studying silicon modifications to improve emission properties of commonly used organic compounds, biphenyl with dimethylsilylvinyl groups in the para position (3-Si) was investigated. A comparative study was performed on the exact C-analogue (3-C) and expanded to biphenyl and dimethylbiphenyl to emphasize the general trend observed. Compound 3-Si displayed emission properties clearly different than all of the investigated hydrocarbon compounds, i.e. twice stronger fluorescence (Φf = 0.6) and a 3-times larger radiative rate constant as compared to 3-C in acetonitrile. Searching for the source of the unique emission of 3-Si, singlet and triplet processes were investigated for all of the compounds using steady-state and time-resolved methods, and their principal photophysical parameters are reported. Experimental work was supported by the theoretical predictions obtained using the EOM-CCSD method. The results led to the conclusion that the strong emission of 3-Si must be due to silicon's presence that enhanced intensity borrowing from the strongly allowed S0 → S2 transition and the larger S1 → S0 transition moment.

Synthesis of novel styryl-N-isopropyl-9H-carbazoles for designing trans-conjugated regular silicon hybrid materials.

An efficient synthesis and characterization of several new, conjugated styryl-carbazole compounds functionalized by monovinylsilanes in a stereocontrolled approach is presented. All N-organic derivatives were successfully examined in silylative coupling in the presence of ruthenium-hydride complexes 4 and 5. Furthermore, a novel class of vinylene-arylene linear oligomeric materials with 1,4-bis(dimethylvinylsilyl)naphthalene in the main chain was produced. Both reactions proceed very effectively, stereo- and regioselectively, allowing one to obtain E-isomer derivatives with high isolation yields.

New aryloxybenzylidene ruthenium chelates - synthesis, reactivity and catalytic performance in ROMP.

New phenoxybenzylidene ruthenium chelates were synthesised from the second generation Grubbs catalysts bearing a triphenylphosphine ligand (or its para-substituted analogues) by metathesis exchange with substituted 2-vinylphenols. The complexes behave like a latent catalyst and are characterized by an improved catalytic behaviour as compared to that of the known analogues, i.e., they exhibit high catalytic inactivity in their dormant forms and a profound increase in activity after activation with HCl. The strong electronic influence of substituents in the chelating ligand on the catalytic activity was demonstrated. The catalytic properties were tested in ROMP of cyclooctadien (COD) and a single selected norbornene derivative.

Intramolecular charge transfer photoemission of a silicon-based copolymer containing carbazole and divinylbenzene chromophores. Electron transfer across silicon bridges.

A new copolymer consisting of N-isopropylcarbazole/dimethylsilylene bridge/divinylbenzene units was synthesized and characterized. Dual fluorescence was observed in this copolymer in polar solvents. The absence of the second band at the lower transition energy of the two emission maxima in nonpolar solvents and the quantitative correlation of the lower-energy emission band maxima with solvent polarity indicate that the lower-energy emission band arises from an intramolecular charge transfer (ICT) state. A series of model compounds was synthesized to investigate the source of the charge transfer. It was found that the Si-bridged dyad with a single N-isopropylcarbazole and a single divinylbenzene was the minimum structure necessary to observe dual luminescence. The lack of dual luminescence in low-temperature glasses indicates that the ICT requires a conformation change in the copolymer. Analogous behavior in the Si-bridged dyad suggests that the ICT in the copolymer is across the silicon bridge. Results from time-resolved luminescence measurements with picosecond and subnanosecond excitation were used to support the thesis that twisted charge-transfer states are the likely source of the observed dual luminescence.

Synthesis of new styrylarenes via Suzuki-Miyaura coupling catalysed by highly active, well-defined palladium catalysts.

An efficient synthetic route for well-defined palladium(0) complexes [Pd(η(2)-dba)(PPh3)2] (2), [Pd(η(2)-dba)(PCy3)2] (3) and their crystallographic structures is reported. This is the first crystallographic characterization of palladium complexes coordinated with one dibenzylideneacetone and two phosphines. A highly effective, fully controlled method for selective synthesis of mono- (5-9) and distyrylarenes (10-15) via Suzuki-Miyaura coupling is described.

Kinetics of reversible photoisomerization: determination of the primary quantum yields for the E-Z photoisomerization of silylenephenylenevinylene derivatives.

The kinetics scheme for directly excited, photoreversible reactions is solved exactly under the assumptions of no irreversible side reactions and constant excitation intensity for the duration of the reaction. The advantages of the methodology over the extrapolation-to-zero-time and the back-reaction correction methods are (i) that the quantum yields of both the forward and reverse photoreactions can be obtained starting from either pure reactant or pure product and (ii) the conversion percentage is not limited to a narrow domain in the neighborhood of small conversions. Examples of E-Z photoisomerizations are given to illustrate the fitting procedures required. The results from these examples are compared to the photoisomerization method of extrapolating the empirical quantum yields to zero time and the back-reaction correction. The exact equations are used to justify the extrapolation-to-zero-time method and to establish criteria on extrapolation ranges for the conversion percentage of starting material.

(E)-1,2-Bis{4-[dimeth-yl(vin-yl)silyl]-phenyl}-ethene.

The mol-ecule of the title compound, C(32)H(34)Si(2), is situated about a centre of symmetry. The whole diphenyl-ethene fragment is planar and the C(ar)-Si-C(3) group is rotated by ca 30° with respect to the plane of the benzene ring. The crystal structure is stabilized by some C-H⋯π contacts as well as van der Waals inter-actions.

Synthesis of functionalized vinylgermanes through a new ruthenium-catalyzed coupling reaction.

Vinyl-substituted germanes react stereo- and regioselectively with olefins in the presence of complexes containing Ru-H and Ru-Ge bonds with the formation of functionalized vinylgermanes that cannot be synthesized by olefin cross-metathesis procedures. The reaction opens a new catalytic route for preparation of a class of organogermanes that are potent organometallic reagents for organic synthesis because they show very low toxicity and could replace organotin compounds. The mechanism of this new catalytic route was proven to involve an interesting insertion of the vinylgermane into the Ru-H bond and beta-Ge transfer to the metal with elimination of ethylene and generation of an Ru-Ge bond, followed by insertion of the alkene into the Ru-Ge bond and beta-H transfer to the metal to eliminate the substituted vinylgermane.

Highly stereoselective synthesis, structure, and application of (E)-9-[2-(silyl)ethenyl]-9H-carbazoles.

(E)-N-(Silyl)vinylcarbazole has been easily prepared via a new catalytic route, silylative coupling (SC) of vinylcarbazole with vinyltrisubstituted silanes catalyzed by [RuH(Cl)(CO)(PCy3)2]. X-ray structures of two silylvinylcarbazoles as first N-vinylcarbazole derivatives have been resolved. The Pd-catalyzed Hiyama coupling reaction (also as the tandem reaction with SC) of synthesized (E)-N-(triethoxysilyl)vinylcarbazole with iodobenzene has been performed to afford (E)-N-(phenylvinyl)carbazole with high yield and stereoselectivity.