Multidegree fluorescence–optical rotatory dispersion (F-ORD) from submonolayer interfacial chiral films

Homochiral monomolecular thin films adsorbed to fused silica produced fluorescence–optical rotatory dispersion (F-ORD; i.e., optical rotation of the fluorescence emission relative to the excitation beam) more than seven orders of magnitude greater than anticipated by conventional origins of optical rotatory dispersion (i.e., optical rotation of the excitation beam after passing through a chiral film relative to its initial plane of polarization). Optical rotation of −4.5 ± 0.5° was observed in the principal axis of fluorescence emission relative to that of excitation for submonolayer dip-coated thin films of (S)-naproxen, with the sign of the rotation inverting for comparable films of (R)-naproxen. The chiral-specific F-ORD response was nonreciprocal, inverting in sign upon flipping of the sample orientation (i.e., source-facing versus detector-facing), suggesting selectivity to chiral molecules oriented at the interface. The interface selectivity, chiral selectivity, and large magnitude of the response were in excellent agreement with a fully electric-dipole–allowed orientational mechanism for chiral-specific fluorescence in uniaxial systems. These observations elevate fluorescence as a novel chiral-specific probe with exquisite sensitivity to chirality and interface specificity akin to that normally reserved for even-ordered nonlinear optical interactions, such as second harmonic and sum-frequency spectroscopy.