title: Excited-state properties of chiral [4]helicene cations.
abstract: The photophysical properties of a series of helicene cations in various solvents have been investigated using stationary and time-resolved spectroscopy. These compounds fluoresce in the near infrared region with a quantum yield ranging between 2 and 20% and a lifetime between 1 and 12 ns, depending of the solvent. No clear solvent dependence could be recognized except for a decrease of fluorescence quantum yield and lifetime with increasing hydrogen-bond donating ability of the solvent. In water, the helicene cations undergo aggregation. This effect manifests itself by the presence of a slow fluorescence decay component, whose amplitude increases with dye concentration, and by a much slower decay of the polarization anisotropy in water compared to an organic solvent of similar viscosity. However, aggregation has essentially no effect on the stationary fluorescence spectrum, whereas relatively small changes can be seen in the absorption spectrum. Analysis of the dependence of aggregation on the dye concentration reveals that the aggregates are mostly dimers and that the aggregation constant is substantially larger for hetero- than homochiral dimers.
authors.affiliation: Department of Physical Chemistry, University of Geneva, CH-1211, Geneva 4, Switzerland.
authors.forename: Oksana
authors.initials: O
authors.lastname: Kel
authors.forename: Peter
authors.initials: P
authors.lastname: Sherin
authors.forename: Nathalie
authors.initials: N
authors.lastname: Mehanna
authors.forename: Benoît
authors.initials: B
authors.lastname: Laleu
authors.forename: Jérome
authors.initials: J
authors.lastname: Lacour
authors.forename: Eric
authors.initials: E
authors.lastname: Vauthey
mesh_terms: D002412:Cations / Q000737:chemistry; D006860:Hydrogen Bonding; D011083:Polycyclic Compounds / Q000737:chemistry*; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013237:Stereoisomerism
chemical_list: D002412:Cations; D011083:Polycyclic Compounds; D012997:Solvents; C031660:helicenes