Flavonols and 3-hydroxychromones: new applications for a class of environment-sensitive fluorescent molecules
Flavonols are naturally-occuring molecules that exhibit interesting fluorescence properties. Thanks to an excited-state intramolecular proton transfer, to specific interactions with their molecular environment and to ground-state proton transfer reactions, very often a dual or even triple fluorescence (with well-separated emission bands and strong Stokes shifts) is observed. This phenomenon has been largely exploited in the past, and has led to the development of several synthetic 3-hydroxychromones, structurally very similar to flavonols, used as fluorescent probes.
In this feature article, we highlight some recent and innovative developments in the applications of flavonols and 3-hydroxychromones, notably in the fields of fluorescence chemosensors, environment-sensitive fluorophors, bioimaging, and bacteria identification. Such results show the extraordinary versatility of the 3-hydroxychromone structure for the development of various strategies of exploiting the fluorescence of 3-hydroxychromones and flavonols in a large number of analytical and research fields.
Flavonols are a class of flavonoids characterized by the 3-hydroxy-2-phenylchromen-4-one (3-hydroxyflavone) core (see some examples in Figure 1); most of them are present in a wide variety of fruits and vegetables and exhibit several biomedical properties (1). Most flavonols are characterized by a dual fluorescence (in Figure 2 the behaviour of 3-hydroxyflavone, 3HF, is reported). Indeed, electronic transition from the ground state N to the singlet N* (hν1) excited state is followed by the excited-state intramolecular proton transfer (ESIPT) from the 3-OH to the C=O group and leads to the tautomeric excited form T*. This species emits in the ~500-550 nm region (hνT), with an impressive Stokes shift value. The resulting species T undergoes Back Proton Transfer (BPT) to N. However, in polar or protic media, the ESIPT process is hampered, so that fluorescence from the N* state (hνN) takes place competitively.
Here is one of the key reasons for the relevance of flavonols as fluorescent probes: the ratio between the intensity of the N* and T* bands (along ...