title: Unexpected photophysical properties of symmetric indolylmaleimide derivatives.
abstract: Arcyriarubin A and arcyriaflavin A, two strongly emissive and intensely colored natural products containing both two indoles and a maleimide unit, are investigated (in the flavin the two indole moieties are coupled by a cyclization). The photophysical properties of these compounds were studied in several solvents using UV-vis absorption, steady-state and time-resolved emission, nano- and femtosecond transient absorption spectroscopy. Furthermore, the effect of complexation with zinc(II) 1,4,7,11-tetraazacyclododecane on the photophysical properties of these natural products has been investigated. The chemical structures of the compounds would suggest a charge transfer (CT) character in the ground and/or excited states, since indole is a well-known electron donor and maleimide is a good electron acceptor. Their solvatochromic behavior was investigated by using the Kamlet-Taft approach and indicates only a small CT character in the excited state. This is substantiated by the time-resolved spectroscopy and the complexation study. Molecular orbital calculations indicate that there are no electronic transitions in which a large electron density is transferred from one indole unit to the maleimide part. All calculated orbitals show a strong delocalization of the electron density over the whole molecule. These findings corroborate the experimental results. Whereas the two compounds do have a substantial (calculated) ground-state dipole moment (6 D) and show some solvatochromic behavior, they behave more like conjugated aromatic systems than like electron donor-acceptor systems.
authors.affiliation: Molecular Photonic Materials, van't Hoff Institute for Molecular Sciences, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WS Amsterdam, The Netherlands.
authors.forename: Başak Kükrer
authors.initials: BK
authors.lastname: Kaletaş
authors.forename: Christian
authors.initials: C
authors.lastname: Mandl
authors.forename: Gert
authors.initials: G
authors.lastname: van der Zwan
authors.forename: Marianna
authors.initials: M
authors.lastname: Fanti
authors.forename: Francesco
authors.initials: F
authors.lastname: Zerbetto
authors.forename: Luisa
authors.initials: L
authors.lastname: De Cola
authors.forename: Burkhard
authors.initials: B
authors.lastname: König
authors.forename: René M
authors.initials: RM
authors.lastname: Williams
mesh_terms: D007211:Indoles / Q000737:chemistry*; D008301:Maleimides / Q000737:chemistry*; D008745:Methylation; D008958:Models, Molecular; D015394:Molecular Structure; D010777:Photochemistry; D012997:Solvents; D013053:Spectrophotometry; D013237:Stereoisomerism; D013696:Temperature
chemical_list: D007211:Indoles; D008301:Maleimides; D012997:Solvents