Fluorescence-Detected Exciton-Coupled Circular Dichroism: Scope and Limitation in Structural Studies of Organic Molecules

The potential of fluorescence-detected circular dichroism (FDCD) to extend the scope of the exciton chirality method is rooted in the increased sensitivity and selectivity of fluorescence as compared with absorbance measurements. In this paper, several practical aspects of FDCD are addressed with model compounds possessing a (1R,2R)-trans-1,2-cyclohexanediol skeleton and two chromophores attached to the diol through ester linkages. We have probed the utility of fluorescent chromophores that have previously been shown to be excellent chromophores for the conventional absorbance-based circular dichroism (CD) exciton chirality method. Certain fluorophores, such as 2-naphthoate or 6-methoxy-2-naphthoate, provide FDCD spectra that are in good agreement with the conventional CD. In other cases, a bischromophoric derivative results in an FDCD spectrum that significantly deviates from the absorbance based data. Here it is shown that the extent of fluorescence polarization, an estimate of sample anisotropy, is directly correlated with the ability to extract a meaningful exciton-coupled FDCD spectrum. If fluorescence polarization is negligible, the solution is isotropic, and the FDCD and conventional CD are in good agreement. Fluorescence lifetime measurements are used to address the origin of solution anisotropy.