ORIGINAL PAPER Photochromic hybrid sol–gel films containing naphthopyrans Paulo J. Coelho • Luı ´s M. Carvalho • Luis F. F. F. Gonc ¸alves • Carlos J. R. Silva • Ana M. Campos • M. Jesus Gomes Received: 1 March 2010 / Accepted: 15 July 2010 / Published online: 27 July 2010 Ó Springer Science+Business Media, LLC 2010 Abstract Several photochromic benzo- and naphthopy- rans were embedded in hybrid organic–inorganic ureasili- cate based films obtained by a sol–gel process. The resulting films, containing 0.1% (w/w) of the photochromic com- pound, were transparent and flexible and for naphthopyrans they were colourless or presented just a slight colouration. Under UV continuous irradiation the films developed a significant absorption in the visible region that fades in the dark with variable kinetics depending on the naphthopy- ran structure. 2,2-Diphenyl-3H-naphtho[2,1-b]pyran films showed a very fast colouration under UV light giving rise to coloured open forms, containing mainly the TC isomer (98%) along with a very small amount of the more stable TT isomer (2%), that practically returned to the initial absor- bance in 1 min. Keywords Naphthopyran  Sol–gel  Ureasilicate  Organic–inorganic hybrid  Photochromism  UV light  Kinetics 1 Introduction Naphthopyrans are well known molecules that exhibit photochromic properties. When irradiated with continuous near-UV light these uncoloured molecules undergo a pyran-ring opening with the formation of the trans, cis isomer (TC, major product) that, upon isomerization of the double bond, leads to the trans, trans isomer (TT, minor product) (Scheme 1). A photostationary state is usually reached after several minutes of irradiation [1]. When the light source is removed the system returns to the original colourless state, either via a thermal or a photoinduced process. The two photoisomers, although exhibiting similar absorption spectra, show very different thermal stabilities. While the TC isomer rapidly returns to the uncoloured closed form, the TT isomer is thermally more stable and shows a slower fading rate and is therefore responsible for the persistence of a residual colour for several minutes/hours after the removal of the light source. The photochromic naphthopyrans can be divided in two main groups: 3H-naphtho[2,1-b]pyrans and 2H-naph- tho[1,2-b]pyrans (Scheme 1). Usually 2H-naphtho[1,2-b] pyrans give rise to more stable open forms than the cor- responding 3H-naphtho[2,1-b]pyrans and therefore, when the light source is removed, they exhibit slower fading rates. Due to the reversible colour change these compounds found important applications in the manufacture of pho- tochromic ophthalmic lenses and solar protection glasses as well as in the cosmetics and textiles industries [2, 3]. The photochromic properties of these compounds, namely the maximum wavelength of absorption and the fading rates, are very sensitive to structural modifications and most of the recent research in this field has been devoted to the tailoring of their spectral and kinetic prop- erties by the selective introduction of substituents and the P. J. Coelho (&)  L. M. Carvalho Centro de Quı ´mica, Vila Real, Universidade de Tra ´s-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal e-mail: pcoelho@utad.pt L. F. F. F. Gonc ¸alves  C. J. R. Silva  A. M. Campos Centro de Quı ´mica, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal M. Jesus Gomes Centro de Fı ´sica, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal 123 J Sol-Gel Sci Technol (2010) 56:203–211 DOI 10.1007/s10971-010-2295-5