DOI: 10.1002/adma.200602843 Amplification of the Coloration Efficiency of Photochromic Oxazines** By Massimiliano Tomasulo , Salvatore Sortino,* and Françisco M. Raymo* Photochromic compounds alter reversibly their ability to absorb visible radiations under the influence of optical stimu- lations. [1–5] In fact, their photoinduced transformations can be exploited to switch the color of liquid solutions, rigid polymer matrices or even crystals with optical inputs. Furthermore, these photoinduced processes are generally accompanied by significant structural modifications at the molecular level, which can translate into pronounced changes in the refractive index of the host matrix. Indeed, a diversity of photonic mate- rials and devices have already been designed around the unique absorptive and dispersive effects associated with photochromic compounds. [6–9] Photochromic transformations are commonly based on un- imolecular reactions. [1–5] They generally involve ring closing/ opening steps, cis/trans isomerizations or intramolecular pro- ton transfer. These processes alter the structure of a single chromophoric unit and induce a change in color as a result. In some instances, pairs of photoresponsive chromophores have also been integrated within the same covalent backbone to generate bichromophoric photochromes. [10–27] In the resulting molecular assemblies, the photoisomerizations of the two photochromic units require the absorption of two indepen- dent photons. Furthermore, the photoinduced transformation of one component can affect the spectroscopic response of the other and vice versa. In principle, similar processes can be de- signed to occur on the basis of a single photochemical event. Specifically, the absorption of one photon can be engineered to generate simultaneously two chromophoric units able to absorb in the same region of the visible spectrum. Hence, the coloration efficiency [28] of the resulting bichromophoric photochromes can be significantly greater than that of their monochromophoric counterparts. On the basis of these con- siderations, we have designed a bichromophoric compound based on the photoinduced opening and thermal closing of a [1,3]oxazine ring. In this article, we illustrate our design logic and report the synthesis and photochemical properties of a representative example of this novel class of bichromophoric photochromes. We have recently discovered that the ultraviolet irradiation of either 1a or 2a (Fig. 1) cleaves the C–O bond at the junc- tion of the indole and benzooxazine heterocycles with the opening of the [1,3]oxazine ring. [29–31] In MeCN, this process occurs in less than 6 ns to generate the colored isomers 1b and 2b with quantum yields of 0.03 and 0.1, respectively. These species revert thermally to the original [1,3]oxazines with a lifetime of 25 ns in both instances. The photoinduced opening of the [1,3]oxazine ring converts the 4-nitrophenoxy fragment of 1a and 2a into the 4-nitrophenolate anion of 1b and 2b with the concomitant shift of an absorption band from ca. 315 to 440 nm. Thus, a single chromophore able to absorb in the visible region is produced after the photoinduced reac- tion. The cleavage of the C–O bond, however, generates an indolium cation in addition to the 4-nitrophenolate anion. This transformation brings the group (R in Fig. 1) on the chi- ral center of 1a and 2a in conjugation with the indolium cation of 1b and 2b. In principle, this photoinduced change in conju- gation can be exploited to shift another absorption band from the ultraviolet to the visible region, if the nature of R is ad- justed accordingly. In fact, the ultraviolet excitation of the re- sulting system would generate simultaneously two indepen- dent chromophores both able to absorb in the visible region, leading to an amplification of the coloration efficiency. On the basis of these considerations, we have designed the [1,3]oxazine 3a (Fig. 1), which incorporates a 2-(4-phenylphe- nylene)ethylene group on its chiral center. We have synthe- sized this compound in one step starting from 1a (Fig. 2). Spe- COMMUNICATION 832 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 832–835 – [*] Prof. S. Sortino Dipartimento di Scienze Chimiche, Universitá di Catania 95125 Catania (Italy) E-mail: ssortino@unict.it Prof. F. M. Raymo, Dr. M. Tomasulo Center for Supramolecular Science, Department of Chemistry University of Miami 1301 Memorial Drive, Coral Gables, FL 33146-0431 (USA) E-mail: fraymo@miami.edu [**] We thank the National Science Foundation (CAREER Award CHE- 0237578), the University of Miami and the MIUR (Italy) for financial support. Figure 1. The reversible interconversion of the [1,3]oxazines 1a, 2a, and 3a and the corresponding zwitterions 1b, 2b, and 3b.