Structural properties and degree of intramolecular charge transfer of an N-Alkyl indoline-tricyanoquinodimethane system

Article abstract of DOI:10.1246/bcsj.81.869

A series of intramolecular charge-transfer compounds comprised of 1-alkyl-3,3-dimethyl-2-methyleneindoline (I_n, donor, n = alkyl chain length; 1-8, 10, 14, 16, 18, 20, and 22) and 7,8,8-tricyanoquinodimethane (=2-(4′-cyanomethylene-2′,5′-cyclohexadienylidene) malononitrile (3CNQ), acceptor) moieties linked through a π-bond, I _n-3CNQ were prepared, and their structures are discussed. The melting points of the I_n-3CNQ solids decreased with increasing alkyl chain length until reaching a minimum point (95°C) at n = 18, and then began to gradually increase, indicating self-aggregation of the alkyl chains. The I _n-3CNQ derivatives with long alkyl chains (n ≥ 6) produced supercooled liquids displaying conspicuous color changes after melting. Diverse molecular packing patterns were observed in the crystal structures. Most I _n-3CNQ derivatives formed face-to-face and/or side-by-side dimeric motifs to cancel net dipole moments. I₂₀-3CNQ formed a two-dimensional bilayer structure by π-stacking and self-assembly of the alkyl chains. Two types of molecular conformation, which significantly affect the electronic structures, were observed to be dependent on the alkyl chain length. A parameter to evaluate the degree of intramolecular charge transfer, namely bond length ratio, was proposed; it exhibited good agreement with solvatochromic shifts and molecular orbital calculations.