Tr-conjugated systems

Relative to the rigid tr-conjugated systems, the locahzed diradicals are comph-cated by various possible conformations due to low barriers in rotations of o bonds. 

It should be added that the expansion of the tr-conjugating system and/or the introduction of Se atoms with greater polarizability (as shown in BETS or TTPs) reduce the effective on-site Coulomb energy U. For example, the V values for the TTP-type donors estimated from electrochemical and optical measurements are considerably small. (TTM-TTP)l3 is a 1 1 salt with a highly one-dimensional half-filled band. Although 1 1 salts were believed to be non-metal due to the Coulomb interaction (Mott-insulator), this salt shows metallic behavior down to about 160 K. This system is regarded as a small-f/ case of a one-dimensional conductor with an inherently half-filled band [27]. 

Synthetic principles for bandgap control in Tr-conjugated systems of poly-(thiophene) and related systems with fused thiophene rings 97CRV173. 

The self-rigidification associated with noncovalent S-O intramolecular interactions between adjacent EDOT units has been observed for many EDOT-based tr-conjugated systems [60]. 

Blanchard, P., B. Jousselme, P. Frere, and J. Roncali. 2002. 3- and 3,4-Bis(2-cyanoethylsulfanyl)thio-phenes as building blocks for functionalized thiophene-based tr-conjugated systems. J Org Chem... 

RR CO RR C = NNH-CsFs (rr-p-Tr conjugation system) RNH2 RN = CH-NM62 (p-TT conjugation system)... 

Valence effective hamiltonian (VEH) calculations show that the band gap becomes smaller with decreasing bond length alternation along the C-C backbone in iT-conJugated systems namely, the band gap becomes smaller with increasing quinoid character [72,73]. Therefore, much effort has been made to increase the quinoid character of the Tr-conjugated system at the expense of its aromaticity. 

Delocalization (Section 1.9) Association of an electron with more than one atom. The simplest example is the shared electron pair (covalent) bond. Delocalization is important in conjugated -tr electron systems, where an electron may be associated with several carbon atoms. 

For the ir-conjugated systems as well as the tr-type diradicals, the triplet branched isomers are more stable than the linear ones (e.g., 1 vs 3 5 vs 4 21 vs 22). 

Another dithioether ligand system, whose coordination chemistry and photophysics are currently being developed in our laboratory, is based on a Tr-conjugated 2-azabutadiene array.173 For example, ligand L50 has been coordinated to Re(I) leading to luminescent Re(I) S,N chelate complexes.174 This ligand reacts with Cul to produce the dinuclear complex 50, which is luminescent both in solution and in the solid state (Scheme 27). A recent X-ray diffraction study revealed that the Cu - Cu distance in the rhomboid motif is much beyond that of the related dinuclear compound 48 (2.673(6) vs. 2.7964(6) A). Treatment of [Cu(MeCN)4] PF6] with two equivalents L50 afforded the... 

In conjugate systems, with both a- and re-type orbitals, the tr-re separability assumption is made. That is, it is assumed that the interaction between the a- and re-type electrons may be neglected. Parr, Ellison, and Lykos (1956) have shown that in such a case the total function, 0, may be written as the product... 

Carbonyl groups and conjugated Tr-electron systems are reduced by metals such as Li, Na and K, usually in liquid ammonia solution. Other reactive metals such as zinc and magnesium reduce aldehydes and ketones in the presence of a proton source. 

Mechanism Because the Tr-electron systems of the two functional groups in a,p-unsaturated ketone are conjugated, the radical anion A formed by electron addition from a reducing metal is resonance stabilized. The usual fate of the A is protonation (or other electrophilic bonding) at the P-carbon atom. This creates an enoxy radical B which immediately accepts an electron to form an enolate anion C. Protonation or alkylation of this enolate species then gives a saturated ketone D or E, which may be isolated or further reduced depending on the reaction conditions (Scheme 6.33). 

We therefore start the quantum mechanical treatment of conjugated systems by expressing the total electronic wavefunction in terms of a wavefunction for the tr-eleotrons and a wavefunction for the jr-eloctrons ... 

Unsaturated hydrocarbons have double or triple bonds between carbon atoms. The alkenes have C —C double bonds, described by sp hybridization of the carbon atoms. The alkynes have C —C triple bonds, described by sp hybridization of the carbon atoms. Because bond rotation does not occur readily about a carbon-carbon double bond, many alkenes exist in contrasting isomeric forms, depending on whether bonding groups are on the same (cis) or opposite sides (trans) of the double bond. When two or more double or triple bonds are separated by one single bond, the p orbitals form a conjugated system, in which the de-localized tr orbitals are best described by MO theory. 

The fullerenes, which contain only carbon, are an allotropic form of carbon discovered in 1985. All the fullerenes have even numbers of atoms, with formulas ranging up to C400 and higher. Their tt bonds are conjugated tr electron systems. 

In this contribution we review the connection between the nonlinear optical (NLO) response and the solvatochromic behavior of the important class of organic molecules, namely donor-acceptor -tr-conjugated compounds (D-tr-A). In these compounds, also called push-pull chromophores, an electron-donating group D is conjugated to an electron-acceptor substituent A through a system of the localized tr-bonds (Scheme 1). It is well established that such compounds exhibit the following properties [1-7] ... 

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