Theoretical and Experimental Investigations of Circular Dichroism and Absolute Configuration Determination of Chiral Anthracene Photodimers

Substituted anthracenes photodimerize to stereoisomeric [4 + 4] cyclodimers, some of which are inherently chiral. Recent supramolecular photochirogenic studies enabled the efficient preparation of specific stereoisomers, the absolute configurations of which should reflect the chiral environment of supramolecular host or scaffold employed but have not been determined, hindering detailed mechanistic elucidation and further host/scaffold design. In this study, we performed the combined experimental and state-of-the-art theoretical analyses of the circular dichroism spectra of chiral cyclodimers of 2-anthracenecarboxylic and 2,6-anthracenedicarboxylic acids to reveal the configurational and molecular orbital origin of the Cotton effects observed, and unambiguously determined the absolute configurations of these chiral cyclodimers. The present results allow us to directly correlate the enantiotopic face-selectivity upon photocyclodimerization with the absolute configuration of the cyclodimer derived therefrom and also to precisely elucidate the chiral arrangement of two cyclodimerizing anthracenes.