Unexpected photophysical properties of symmetric indolylmaleimide derivatives.
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.
Kaletaş
Başak Kükrer
BK
Molecular Photonic Materials, van't Hoff Institute for Molecular Sciences, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WS Amsterdam, The Netherlands.
Mandl
Christian
C
van der Zwan
Gert
G
Fanti
Marianna
M
Zerbetto
Francesco
F
De Cola
Luisa
L
König
Burkhard
B
Williams
René M
RM
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
D007211:Indoles; D008301:Maleimides; D012997:Solvents