Wavelength shifting systems based on flavonols and their metal complexes encapsulated by post-doping in porous SiO 2 xerogel matrices Stefano Protti a,b , Katarzyna Raulin a , Odile Cristini c , Christophe Kinowski c , Sylvia Turrell a , Alberto Mezzetti a,d,⇑ a LASIR UMR CNRS 8516, Bat C5, Université Lille 1, Cité Scientifique, 59655 Villeneuve d’Ascq Cedex, France b Department of Chemistry, University of Pavia, Via Taramelli 10, 27100 Pavia, Italy c PhLAM, UMR CNRS 8523, Bat. P5, Université Lille 1, Cité Scientifique, 59655 Villeneuve d’Ascq Cedex, France d Laboratoire de Photocatalyse et Biohydrogène, SB2SM, URA CNRS 2096, DSV/IBiTecS, Bât 532, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France article info Article history: Available online 13 February 2011 Keywords: 3-Hydroxyflavone Hydroxyflavone metal complexes Wavelength shifter Sol–gel Doping Fluorescent dye abstract SiO 2 xerogels with very low organic and silanol content have been doped with flavonols and flavonol– metal complexes through a post-doping encapsulation procedure. With a final aim of developing wavelength shifters, this work has shown that 3-hydroxyflavone (3HF) and Alð3HFÞ þ 2 -doped xerogels have interesting luminescent properties. Moreover, spectroscopic characterization of these two doped xerogels shows that the encapsulation procedure does not significantly alter the emission properties of the dopant species. For 3HF-doped xerogels, emission stud- ies and Raman analyses suggest that the encapsulated dye is surrounded by an apolar but inorganic nanoenvironment. This study demonstrates the possibility of obtaining organic fluorophor- and metal-complex fluoro- phor-doped xerogels with good optical properties fabricated by a simple soaking and drying post-doping procedure. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Flavonoids are an important class of organic molecules charac- terized by the 2-phenyl-1,4-benzopyrone (flavone) moiety, com- monly found in the vegetable kingdom. Most of them are photo- protective agents in plants [1] and exhibit anti-oxidant properties [2]. Several flavonoids have also found widespread use as analyti- cal reagents for photometric determination of metal cations [3]. 3-Hydroxyflavone (3HF, also known as flavonol) is one of the simplest of the flavonoids and is characterized by interesting photophysical properties which are intrinsically related to the ex- cited state intra-molecular proton transfer (ESIPT) that can occur at the singlet excited state ( 1 N / ) of 3HF. Such a transfer yields the excited tautomer 1 T / , that is characterized by an emission centered at 500–560 nm (hm 00 in Fig. 1), whose position depends on the surrounding environment. Finally, the resulting ground-state tau- tomer T undergoes a non-radiative back proton transfer to give 3HF in its fundamental state (N). Furthermore, a radiative decay from the singlet excited state ( 1 N / ) of 3HF, with a weaker, broad emission centered at around 400–420 nm (hm 0 ) can be also observed. In particular, when either the polarity or the hydrogen bond donating and/or accepting capability of the media increases, there is an associated increase in radiative decay from the 1 N / state [4–6]. This particular photophysical aspect has prompted a long series of studies, making 3HF a model system for the study of in- tra-molecular excited-state proton transfer processes [7–10 and references therein]. Several studies have suggested that 3HF can be used as a molec- ular fluorescent probe [11–17 and references therein], as a laser dye [18] as well as a wavelength shifting device because of its large Stokes shift (Dk  180 nm) [19–22]. In particular, 3HF-based wavelength shifters have been used to improve the sensitivity in the UV range of standard photodetectors [21,22]. For this latter application, the encapsulation of 3HF in SiO 2 matrices appears particularly interesting, because of their radia- tion hardness, chemical durability and high transparency. As the emission properties of 3HF are extremely sensitive to its environ- ment, it is essential to understand the nature of the interactions between the dye and its SiO 2 molecular cage [17,22]. Such an as- pect is crucial for the improvement of fabrication procedures of 3HF-loaded SiO 2 matrices. Furthermore, understanding the inter- action between an encapsulated molecule and its SiO 2 matrix can be very useful in several fields where inclusion of organic mole- cules (dyes, drugs, molecular sensors, etc.) in SiO 2 matrices is being 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.02.010 ⇑ Corresponding author at: Laboratoire de Photocatalyse et Biohydrogène, SB2SM, URA CNRS 2096, DSV/IBiTecS, Bât 532, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France. Tel.: + 33 1 69 08 22 39; fax: + 33 1 69 08 87 17. E-mail addresses: alberto.mezzetti@libero.it, alberto.mezzetti@cea.fr (A. Mezzetti). Journal of Molecular Structure 993 (2011) 485–490 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc