Sub-picosecond transient absorption spectroscopy of substituted photochromic spironaphthoxazine compounds Guy Buntinx a , Olivier Poizat a, * , Sarah Foley a,1 , Michel Sliwa a , Stéphane Aloïse a , Vladimir Lokshin b , André Samat b a Laboratoire de Spectrochimie Infrarouge et Raman (UMR 8516 du CNRS), Centre d’études et de recherches Lasers et Applications (FR 2416 du CNRS), Université de Sciences et Technologies de Lille, Bat C5, 59655 Villeneuve d’Ascq Cedex, France b Centre Interdisciplinaire de Nanoscience de Marseille (UPR 3118), Universités d’Aix-Marseille, Campus de Luminy, Case 901,13288 Marseille Cedex 9, France article info Article history: Received 22 December 2009 Received in revised form 22 April 2010 Accepted 15 May 2010 Available online 24 May 2010 Keywords: Photochromism Spirooxazine Spironaphthoxazine Femtochemistry Time-resolved absorption Transient spectroscopy abstract The photochromic reaction dynamics of spiroindolinenaphthoxazine and its 6 0 CN and 5 0 CHO substituted compounds is investigated in different solvents by femtosecond transient absorption spectroscopy. In addition to the formation of the merocyanine coloured form (ring-opened trans form, OF), another short- lived intermediate species is produced upon photoexcitation, which is not a precursor to the OF product but which is formed in parallel to it via a competing relaxation process. This species is ascribable to either a relaxed s-cis ring-opened isomer on the ground state potential energy surface or to a metastable minimum of the excited S 1 state potential energy surface of the ring-closed form. The observed kinetics suggest that the production of OF (photocoloraton reaction) is controlled by the efficiency of the competing process rather than by an s-cis e trans isomerisation energy barrier. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Photochromic spirooxazine derivatives are the object of considerable efforts of synthesis [1e5] motivated by their wide- spread use in the commercial manufacture of ophthalmic lenses for adjustable sunlight-protection glasses or in other applications requiring the transparency of an object to vary according to surrounding light intensity. They are also promising compounds for a wide range of potential applications in photonics such as memo- ries and switches [6]. Spironaphthoxazine molecules combine indoline and naphthoxazine moieties linked in a perpendicular and electronically uncoupled configuration via a common sp 3 (spiro) carbon (ring-closed form, CF). Accordingly, their absorption spec- trum is roughly the superposition of the UV absorption features of both moieties. The photochromic transformation (Scheme 1) induced by UV irradiation proceeds via the ultrafast cleavage of the spiro carbon-oxygen single bond followed by isomerisation [7e9] and leads to a mixture of coloured, ring-opened merocyanine isomers (open forms, OF) with extended p conjugation, which absorb in the 500e700 nm range. 1 H NMR [10] and resonance Raman [11] measurements, in agreement with theoretical predic- tions [12], have shown that only the two most stable OF isomers, TTC and CTC, are formed. The merocyanine isomers undergo thermal back reaction in the seconds to minutes time domain at room temperature [13]. A major purpose of recent experimental [2e5] and theoretical [14] work on spironaphthoxazines in view of the applications is the design of new structures with well-controlled and tunable optical properties. In this regard, Metelitsa et al. showed that modifying the spironaphthoxazine (SNO) parent molecule by substitution so as to form push-pull type compounds leads to a notable red-shift of the merocyanine absorption but simultaneously reduces the quantum yield of photocoloration, F col (CF/OF reaction yield) [3]. To understand the parameters that control the efficiency of the photochromic process, it is essential to be able to describe in detail, at the molecular level, the photoinduced reaction pathway. The ring-opening reaction mechanism and dynamics have been studied by ultrafast time-resolved spectroscopy for SNO in solution [15e21] and in the microcrystalline powder phase [22e26]. This reaction has also been investigated theoretically by using quantum chemical * Corresponding author. Fax: þ33 3 20 33 63 54. E-mail address: olivier.poizat@univ-lille1.fr (O. Poizat). 1 Present address: Université de Franche-Comté, Laboratoire de Chimie Physique et Rayonnement (UMR CEA E4), 16 route de Gray, 25030, Besançon, France. Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2010.05.009 Dyes and Pigments 89 (2011) 305e312