Exciton chirality in trans-1,2-diamidocyclohexanes: fluorescent chromophores.
N,N'-Carbonyl-bridged dipyrrinones constitute a new class of highly fluorescent chromophores suitable for investigations of stereochemistry and absolute configuration. Xanthoglow (N,N'-carbonylxanthobilirubic acid) diamides of trans-1,2-diaminocyclohexane are strongly fluorescent (phiF=0.37, lambdaem=500 nm, lambdaex=419 nm in CHCl3) but exhibit only weak exciton circular dichroism (CD). In contrast, the diamide of (1R,2R)-diaminocyclohexane from the xanthoglow analogue whose propionic acid has been replaced by benzoic acid (N,N'-carbonyl-8-(4-carboxyphenyl)-3-ethyl-2,7,9-trimethyl-(10H)-dipyrrin-1-one) exhibits even stronger fluorescence (phiF=0.62, lambdaem=495 nm, lambdaex=422 nm in CHCl3) and UV-visible absorption (epsilon=41,600 dm3.mol-1.cm-1 at 424 nm) in organic solvents. Its exciton CD (Deltaepsilon=-13 dm3.mol-1.cm-1, lambda=432 nm; Deltaepsilon=+2 dm3.mol-1.cm-1, lambda=382 nm) correlates with the exciton chirality rule.