Proton-Induced Fluorescence Switching in Novel Naphthalimide-Dansylamide Dyads Sergio Abad, † Marek Kluciar, ‡ Miguel A. Miranda, † and Uwe Pischel* ,‡ Instituto de Tecnologı ´a Quı ´mica, Universidad Polite ´ cnica de Valencia, UPV-CSIC, Av. de los Naranjos s/n, E-46022 Valencia, Spain, and REQUIMTE/Departamento de Quı ´mica, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal upischel@fc.up.pt Received June 15, 2005 Three novel bichromophoric dyads containing dansylamide and 1,8-naphthalimide linked by oligomethylene spacers of varying length were prepared. The fluorescent moiety can be reversibly selected by protonation/deprotonation of the dansyl residue via control of singlet-singlet energy transfer and photoinduced electron transfer, leading to a molecular optical switch with two spectrally distinguished “on” states. The tailored design of photoactive molecular switches showing a defined fluorescence response upon changes in their chemical surrounding, e.g., by addition of protons, has been a topic of considerable interest in recent years. 1 In most cases, the fluorescence of monochromophoric systems has been switched “on” or “off”, by blocking or enabling a transduction mechanism, e.g., photoinduced electron transfer or energy transfer, which allowed communication between the fluorophore and a receptor side. 2 One step ahead of “on-off” or “off-on” switching is the achievement of control over the photoactivity of a certain fluorophore, i.e., the selection between two dif- ferent “on” states, in a multichromophoric system. Few examples for such behavior have been realized by using directional control 3 of energy transfer between two photoactive units by protonation, metal cation complex- ation, temperature change, or electrochemical reduction. This includes an oligophenylenevinylene-phenanthroline dyad 4 as well as ruthenium(II) polypyridine-containing multicomponent systems linked to anthracene, 5 cat- enanes, 6 or osmium(II) polypyridine complexes and cou- marin. 7 Such systems are foreseen to open new perspectives for the realization of artificial functions at the molecular level. For example, the presence or absence of input information, e.g., of chemical nature, can be translated into the photoactivity of either one of the fluorescent output sites, giving rise to spectrally and spatially distinguished optical signals. 5 The latter point is an important one, since the induction of site-selective emis- sion distinguishes this conceptual approach from trivial spectral shifts of fluorescence as a result of changes in the chemical surrounding, e.g., solvent polarity, of a fluorophore. 8 Herein, we report dyads containing two well-known fluorophores, which are widely used in chemosensors and as fluorescent labels: the dansyl (DANS) 9 and 1,8- naphthalimide (NAPIM) fluorophores. 10 As will be shown below, the selection of these two particular chromophores allows controlling of the fluorescent entity in the dyads † Universidad Polite ´cnica de Valencia. ‡ Universidade do Porto. (1) (a) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. 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