Ethynyl‐Bridged Bis‐quinolinium Dyes: Studies of the Dependence of Different Types of Conjugation on Spectroscopic Properties

Dicationic bis-quinolinium salts possessing different types of conjugation are presented in which the hetareniums are separated by acetylenic or 1,4-diethynylbenzene spacers. Cross-conjugation is induced by interconnections via the 3-positions of the heteroareniums, and conjugation is realized by 2,2/2,4/4,4-interconnections. We also synthesized mixed cross-conjugated/conjugated 2,3/4,3-interconnected species. The different types of conjugation determine the charge distribution according to the rules of resonance which translate into characteristic C-13 NMR as well as UV-Vis-spectroscopic properties, LUMO geometries, and chemical properties. We found that the triple bond's resonance frequencies are sensitive C-13 NMR spectroscopic indicators for the type of conjugation. Combining cross-conjugated and conjugated structure elements of dicationic bis-quinolinium dyes via triple bonds without additional benzene spacer results in considerable bathochromic shifts of the UV-Vis absorption maxima [Delta lambda(max)approximate to 100 nm]. A charge transfer was calculated from the HOMO of the conjugated into the LUMO of the cross-conjugated partial structure.