Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.

Lewis Acid Adducts of Narrow Band Gap Conjugated Polymers

Abstract: We report on the interaction of Lewis acids with narrow band gap conjugated copolymers containing donor and acceptor units. Examination of the widely used poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b′]dithiophene)-2,6-(diyl-alt-benzo[2,1,3]thiadiazole)-4,7-diyl] (1) shows weaker binding with B(C6F5)3 when compared with a small molecule that contains a cyclopenta-[2,1-b:3,4-b′]dithiophene (CDT) unit flanked by two benzo[2,1,3]thiadiazole (BT) fragments. Studies on model compounds representative of 1, together with a comparison between B(C6F5)3 and BBr3, indicate that the propensity for Lewis acid coordination is decreased because of steric encumbrance surrounding the BT nitrogen sites. These observations led to the design of chromophores that incorporate an acceptor unit with a more basic nitrogen site, namely pyridal[2,1,3]thiadiazole (PT). That this strategy leads to a stronger B−N interaction was demonstrated through the examination of the reaction of B(C6F5)3 with two small molecules bis(4,4-bis(...

Interaction between stacked aryl groups in 1,8-diarylnaphthalenes: Dominance of polar/π over charge-transfer effects

Abstract: Several 1,8-diarylnaphthalenes have been prepared, and the barrier to rotation around the arylhaphthalene bond has been measured. In these molecules steric congestion forces the aryls in a parallel stacked geometry. The barriers to rotation were used to evaluate the strength and to investigate the nature of the interaction between the arenes. The variance of the AG+ for the rotation upon arene substitution with electron donating or electron withdrawing groups indicates that polarh electrostatic effects dominate over charge-transfer effects in determining the arene/arene interaction.

Role of ring torsion angle in polyaniline: Electronic structure and defect states

Abstract: The role of phenyl-ring torsion angle in determining the nature of the ground and charged-defect states of polyaniline is explored. The coupling of the transfer integral between nitrogen atom and phenyl-ring constituents of the polyaniline chain to the dihedral angle of the rings competes with the substantial steric repulsion between adjacent rings in determining the conformation of these systems. The ring conformation of the leucoemeraldine-base (LB) form of polyaniline is described by a novel ring-torsion-angle order parameter. The anharmonicity of the interring steric potential leads to a temperature-dependent mean order parameter, and thus to the prediction of thermochromic effects consistent with experiment. Furthermore, changes in optical spectra accompanying derivatization of the rings can be understood by the response of the substituted polymer to modifications of the steric potential. The existence of two degenerate ring-torsion-angle phases in LB implies that both polaronic and solitonic ring-angle-alternation defect states may be relevant in describing the charged states in polyaniline. As these defects involve substantial changes in ring torsion angle, they are expected to possess large kinetic mass, in agreement with photoinduced absorption experiments on polyaniline. The anticipated Peierls ground state of the oxidized pernigraniline-base form of polyaniline can be regarded in part as a ring-torsion-angle dimerized state; consequently, the charge states of this material are also expected to be massive defects in the ring-rotational order. The importance of ring rotations in other ring-containing electronic polymers, such as poly(paraphenylene sulfide), is discussed.