The Journey to Fluorescent Bullvalenes: Pitfalls and Prospects

Abstract

Dedicated to Professor Waldemar Adam on the occasion of his 85th birthdayA dinaphthoylmethyl-substituted bullvalene derivative is presented that exhibits excimer fluorescence. It is demonstrated that the fluxional bullvalene core supports the excimer formation of the aroyl substituents, thus offering the opportunity to monitor externally induced structural changes by fluorescence spectroscopy, as exemplarily shown by fluorimetric analysis of host-guest interactions of the diaroylbullvalene with metal cations, which lead to significant changes of the excimer emission. During the synthesis of the bullvalenes, it was discovered coincidentally that the corresponding triplet-sensitized photoreaction also yielded three unknown lumibullvalene isomers. The NMR-spectroscopic analysis of this reaction revealed a sequence and combination of different di-and tri-πmethane rearrangements and sigmatropic rearrangements. More importantly, the static lumibullvalenes proved useful as reference compounds for the evaluation of the emission properties of the fluxional bullvalenes.

Reversible Restrain and Release of the Dynamic Valence Isomerization in a Shape‐shifting Bullvalene by Complex Formation

“…As such an assembly consists of only one bullvalene unit, it has a smaller number of possible isomers and, therefore, a significantly reduced complexity, which in turn should facilitate its analysis. Coincidentally, during our studies on aryl‐substituted bullvalene derivatives, [16] we made accessible a bullvalene‐harmane conjugate 1 that appeared to have the main prerequisites to act as such a chelating ligand (Figure 2). Specifically, the harmane, a ß‐carboline, contains a lateral pyridine unit [17] that is known to complex several transition metal ions.…”

“…SI, Figure S38). Additional 1D-selec-tive TOCSY experiments showed that the isomeric population is dominated by the two most frequently observed isomers [3,6,8,11,16,18] 1 a (60 %) and 1 b (40 %) in CDCl 3 solution at À 50 °C (Figure 2; cf. SI, Figure S40).…”