Synthesis and Properties of Copolymers Having Polyconjugated Blocks in Macromolecules. I. Use of Diazonium Salts in Synthesis of Polyazophenylene and Its Copolymers

Polyazophenylene (PAF) with a system of conjugated double bonds was synthesized from benzidinetetrazonium chloride (BTC). PAF gave an EPR signal and showed anomalous viscosity behavior on dilution of its solutions. Copolymers of PAF and isoprene were synthesized by emulsion polymerization by the Gomberg reaction from BTC and isoprene. The copolymers show high thermal and oxidative stability.     

Kinetic approach to modeling the radical polymerization of butyl acrylate in the presence dibenzyl trithiocarbonate

A mathematical model of the kinetics of the radical polymerization of butyl acrylate in the presence of dibenzyl trithiocarbonate by the reversible addition-fragmentation chain transfer mechanism with cross-termination of radicals and intermediates and quadratic termination of intermediates. The viability of the polymerization mechanism underlying the model and the related mathematical formalism are tested by comparing the calculated and measured values of the average molecular-weight characteristics of resulting poly(butyl acrylate).     

Exponential distance dependence of photoinitiated stepwise electron transfer in donor-bridge-acceptor molecules: implications for wirelike behavior

Donor-bridge-acceptor (D-B-A) systems in which a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN(n)) bridges (n = 1-3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ(+?)-An(-?), and An(-?) acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ(+?)-An(-?)-FN(n)-NI → DMJ(+?)-An-FN(n)(-?)-NI → DMJ(+?)-An-FN(n)-NI(-?). The charge-shift reaction from An(-?) to NI(-?) exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 ?(-1), which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN(n)(-?) radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D(+?)-B(-?)-A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D-B-A molecule.     

Highly active chiral ruthenium catalysts for asymmetric ring-closing olefin metathesis

The synthesis of olefin metathesis catalysts containing chiral, monodentate N-heterocyclic carbenes and their application to asymmetric ring-closing metathesis (ARCM) are reported. These catalysts retain the high levels of reactivity found in the related achiral variants (1a and 1b). Using the parent chiral catalysts 2a and 2b and derivatives that contain steric bulk in the meta positions of the N-bound aryl rings (catalysts 3-5), five- through seven-membered rings were formed in up to 92% ee. The addition of sodium iodide to catalysts 2a-4a (to form 2b-4b in situ) caused a dramatic increase in enantioselectivity for many substrates. Catalyst 5a, which gave high enantiomeric excesses for certain substrates without the addition of NaI, could be used in loadings of < or =1 mol %. Mechanistic explanations for the large sodium iodide effect as well as possible mechanistic pathways leading to the observed products are discussed.     

Exploration of ground and excited electronic states of aromatic and quinoid S,S-dioxide terthiophenes. Complementary systems for enhanced electronic organic materials

We analyze the electronic and molecular structures for the ground and excited electronic states of aromatic terthiophene (3T), the quinodimethane 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (3Q), and isologues with the middle ring S-oxidized (3TO2, 3QO2). These represent extremes of electron rich and deficient ground states, often exhibiting complementary properties. Oxidizing the central sulfur atom affects the molecular structure, electron affinity, and photophysical properties of both pi systems. The consequences for 3T include de-aromatization of the central thiophene, red-shifting of the electronic absorption spectrum, and lowering of the reduction potential. The electron deficient quinoid 3QO2 shows an enhancement of electron affinity from reducing the electron-donor ability of sulfur, and a blue-shifting of its electronic absorption spectrum was seen. Fluorescence emission is quenched in the sulfonated terthiophene, and the contrary effect again would be expected upon sulfonation of a quinoid emitter. Raman vibrational spectroscopy, electrochemistry, and UV-vis and fluorescence spectroscopies are analyzed in conjunction with theoretical calculations.