Coumarins, as a family of molecules, exhibit a wide range of fluorescence emission properties. In many cases, this fluorescence is extremely sensitive to the local environment of the molecule, especially the local polarity and microviscosity. In addition, coumarins show a wide range of size, shape, and hydrophobicity. These properties make them especially useful as fluorescent probes of heterogeneous environments, such as supramolecular host cavities, micelles, polymers and solids. This article will review the use of coumarins to probe such heterogeneous systems using fluorescence spectroscopy.

We report the first use of fluorescence spectroscopy to probe the environment of the cavities that are present in open framework coordination polymers.

The effect of the macrocyclic host compounds cucurbit[n]urils (Qn), with on the fluorescence of the biologically active compound curcumin has been studied. Curcumin, the main constituent of the Indian spice turmeric, is of growing interest because of its wide-ranging pharmaceutical properties. This compound forms strong 2:1 host–guest inclusion complexes with Q6 (the original cucurbituril), with an overall equilibrium constant of (1.9±0.8)×10⁴ M⁻². It is postulated that a Q6 host partially encapsulates each of the two phenyl groups at the ends of the curcumin molecule. The difference in magnitude of the equilibrium constants K ₁ (72±2 M⁻¹) and K ₂ (260±120 M⁻¹) for stepwise encapsulation of the two ends of the curcumin molecule indicates that encapsulation by the first Q6 significantly alters its entire electronic structure, resulting in a more favorable second encapsulation. A very large enhancement of the fluorescence of curcumin results from this complex formation, on the order of 5.0; this is a significant fraction of the polarity sensitivity factor (PSF) of 39 measured for curcumin, that is the ratio of fluorescence intensity in ethanol vs. water. Surprisingly, no such enhancement could be observed in the case of Q7, indicating that the interactions between the guest and the host cavity are not favorable in this case, contrary to expectations. Similarly, no enhancement was observed in the case of Q5, which is not unexpected, because of the extremely small size of the host cavity and portal in this case.

Host–guest inclusion complexes represent an important type of supramolecular structure, one which finds widespread applications in diverse areas including separations science, the food industry, molecular sensors and optical devices. There are several driving forces for the formation of such inclusion complexes in solution; one of the most important is hydrogen bonding between the guest and host molecules. The nature or strength of the hydrogen bonding may change upon electronic excitation of the guest, for example during fluorescence studies or when the inclusion complex is used as an optical sensor. In this Perspective article, the impact of hydrogen bonding between excited state guests and their hosts is examined in detail, in terms of the impact on the formation and stability of such excited state complexes, the effects on guest fluorescence, changes in the stability of ground state guest complexes upon electronic excitation, the application of inclusion complexes as fluorescent sensors and materials, and the use of fluorescence spectroscopy for their study.