Asymmetric Indolylmaleimide Derivatives and Their Complexation with Zinc(II)−Cyclen

The spectroscopic properties of two asymmetric indolylmaleimide derivatives, 4-bromo-3-(1‘H-indol-3‘-yl)maleimide and 4-methyl-3-(1‘H-indol-3‘-yl)maleimide, are investigated. The bromo derivative was crystallized
and its X-ray structure was determined. Both compounds are strongly colored while their separate components
(indole and maleimide) absorb in the UV region only. To understand the ground- and excited-state behavior,
the photophysical properties of the two compounds were studied in detail by steady state and time-resolved
absorption and emission spectroscopy. Their solvatochromic behavior was investigated by using the Kamlet−Taft approach, which indicates some charge transfer (CT) character in the excited state. Nano- and femtosecond
transient absorption spectroscopy was used for the identification and investigation of the CT state. Furthermore,
the effect of the complexation with zinc(II) 1,4,7,11-tetraazacyclododecane (Zn-cyclen) on the photophysical
properties of these two compounds was studied. An enhancement of the fluorescence intensity upon self-assembly (up to 90 times) and high association constants were observed, which illustrate the potential use of
these compounds as luminescent sensors. DFT calculations indicate that HOMO-1 to LUMO excitation is
mainly responsible for the charge transfer character and that this transition changes its character drastically
when Zn-cyclen complexation occurs, thus giving it sensor properties.