title: Cyanine-like dyes with large bond-length alternation.
abstract: Herein, the synthesis and properties of alkyne-bridged carbocations, which are analogous in structure to cyanine dyes, are reported. An alkene-bridged dye, linked at the third position of the indole, was also synthesized as a reference compound. These new carbocations are stable under ambient conditions, allowing characterization by UV/Vis and NMR ((1)H and (13)C) spectroscopies. These techniques revealed a large degree of delocalization of the positive charge, similar to a previously reported porphyrin carbocation. The linear and nonlinear optical properties are compared with cyanine dyes and triarylmethyl cations, to investigate the effects of the bond-length alternation and the overall molecular geometry. The value of Re(γ), the real part of the third-order microscopic polarizability, of -1.3×10(-33)  esu for the alkyne-linked cation is comparable to that of a cyanine dye of similar length. Nondegenerate two-photon absorption spectra showed that the alkene-bridged dye exhibited characteristics of cyanines, whereas the alkyne-bridged dye is reminiscent of octupolar chromophores, such as the triarylmethyl carbocation brilliant green. Such attributes were confirmed and rationalized by quantum chemical calculations.
authors.affiliation: Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA, UK.
authors.forename: Karl J
authors.initials: KJ
authors.lastname: Thorley
authors.forename: Joel M
authors.initials: JM
authors.lastname: Hales
authors.forename: Hyeongeu
authors.initials: H
authors.lastname: Kim
authors.forename: Shino
authors.initials: S
authors.lastname: Ohira
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Brédas
authors.forename: Joseph W
authors.initials: JW
authors.lastname: Perry
authors.forename: Harry L
authors.initials: HL
authors.lastname: Anderson
mesh_terms: D000480:Alkynes / Q000138:chemical synthesis* / Q000737:chemistry; D002232:Carbocyanines / Q000138:chemical synthesis* / Q000737:chemistry; D004396:Coloring Agents / Q000138:chemical synthesis* / Q000737:chemistry; D007211:Indoles / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure
chemical_list: D000480:Alkynes; D002232:Carbocyanines; D004396:Coloring Agents; D007211:Indoles
keywords: alkynes; carbocations; cyanines; nonlinear optics