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 sensit ivity and selectivity of fluorescence as compared with absorbance measureme nts. In this paper, several practical aspects of FDCD are addressed with mo del compounds possessing a(1R,2R)-trans-1,2-cyclohexansdiol skeleton and tw o 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 d ichroism (CD) exciton chirality method. Certain fluorophores, such as 2-nap hthoate or 6-methoxy-2-naphthoate. provide FDCD spectra that are in good ag reement with the conventional CD. In other cases, a bischromophoric derivat ive results in an FDCD spectrum that significantly deviates from the absorb ance based data. Here it is shown that the extent of fluorescence polarizat ion, an estimate of sample anisotropy, is directly correlated with the abil ity to extract a meaningful exciton-coupled FDCD spectrum. If fluorescence polarization is negligible, the solution is isotropic, and the FDCD and con ventional CD are in good agreement. Fluorescence lifetime measurements are used to address the origin of solution anisotropy.