ALBERTO MEZZETTI¹*, STEFANO PROTTI²
*Corresponding author
1. Laboratoire de Réactivité de Surface UMR CNRS 7197, Sorbonne Université, Paris, France.
2. PhotoGreen Lab, Department of Chemistry, Pavia, Italy

Abstract

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.

INTRODUCTION

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 (hν_T), 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 (hν_N) 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 ...