Photonic Modulation of Electron Transfer with Switchable Phase Inversion Julien Frey, Gerdenis Kodis, Stephen D. Straight, Thomas A. Moore,* Ana L. Moore,* and Devens Gust* Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, United States * S Supporting Information ABSTRACT: Photochromes may be reversibly photoisomerized between two metastable states and their properties can influence, and be influenced by, other chromophores in the same molecule through energy or electron transfer. In the photochemically active molecular tetrad described here, a porphyrin has been covalently linked to a fullerene electron acceptor, a quinoline-derived dihydroindolizine photochrome, and a dithienylethene photo- chrome. The porphyrin first excited singlet state undergoes photoinduced electron transfer to the fullerene to generate a charge-separated state. The quantum yield of charge separation is modulated by the two photochromes: one isomer of each quenches the porphyrin excited state, reducing the quantum yield of electron transfer to near zero. Interestingly, when the molecule is illuminated with white light, the quantum yield decreases as the white light intensity is increased, generating an out-of-phase response of the quantum yield to white light. However, when the same experiment is performed in the presence of additional, steady-state UV illumination, a phase inversion occurs. The quantum yield of electron transfer now increases with increasing white light intensity. Such effects illustrate emergent complexity in a relatively simple system and could find applications in molecular logic, photochemical labeling and drug delivery, and photoprotection for artificial photosynthetic molecules. The photochemistry leading to this behavior is discussed. ■ INTRODUCTION When two chromophores are present in the same molecule, they can interact with one another. If the interaction is by photoinduced electron transfer, the resulting donor-acceptor systems are of interest as artificial photosynthetic reaction centers, molecular electronic elements, and related devices. When the interaction is by energy transfer, the systems can mimic photosynthetic antenna and photoprotective mecha- nisms, play roles in biomedical science and treatment, etc. When one of the chromophores is a photochrome, the additional element of photoisomerism comes into play. Photochromic molecules are isomerized between two meta- stable forms by light, or light and heat. The isomers have different absorption spectra and other photochemical proper- ties and thus can interact differently with nearby chromophores. A photochrome is a photochemical analogue of a bistable electrical switch, or transistor, and molecules containing photochromes may function as Boolean binary logic devices of many kinds. 1-8 However, an ensemble of photochromic molecules, such as a solution, forms a photostationary state upon illumination in which the ratio of the two isomers is essentially infinitely variable. This property allows the design of analogue molecular devices. For example, we recently reported a molecular photonic analogue of a triode tube, or transistor amplifier, in which irradiation of a solution of the molecules with long- wavelength light of modulated intensity in turn modulated the intensity of shorter-wavelength fluorescence induced by steady- state illumination at a different wavelength. 9 In another example of this effect, we reported an artificial photosynthetic construct that functionally mimics a photoregulatory mechanism found in cyanobacteria. 10 The molecule included a porphyrin-fullerene artificial reaction center that demonstrates photoinduced electron transfer, two antenna chromophores, and a photo- chrome. The quantum yield of photoinduced electron transfer was demonstrated to be inversely related to the intensity of white light excitation. This general behavior is also found in cyanobacteria, where the quantum yield of photosynthetic charge separation decreases as the light intensity increases. Here, we report the synthesis and properties of molecular tetrad 1 (Figure 1), which consists of a porphyrin (P)- fullerene (C 60 ) charge-separation unit linked to both a quinoline-derived dihydroindolizine photochrome (DHI) and a dithienylethene photochrome (DTE). Excitation of the porphyrin moiety initiates photoinduced electron transfer to the fullerene to form a P •+ -C 60 •- charge-separated state. Each photochrome can be independently isomerized between an isomer that has no effect upon P-C 60 photoinduced electron Received: October 29, 2012 Revised: December 10, 2012 Published: December 21, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 607 dx.doi.org/10.1021/jp3106887 | J. Phys. Chem. A 2013, 117, 607-615