We present results from transient absorption spectroscopy on a series of artificial light-harvesting dyads made up of a zinc phthalocyanine (Pc) covalently linked to carotenoids with 9, 10, or 11 conjugated carboncarbon double bonds, referred to as dyads 1, 2, and 3, respectively. We assessed the energy transfer and excited-state deactivation pathways following excitation of the strongly allowed carotenoid S 2 state as a function of the conjugation length. The S 2 state rapidly relaxes to the S* and S 1 states. In all systems we detected a new pathway of energy deactivation within the carotenoid manifold in which the S* state acts as an intermediate state in the S 2 f S 1 internal conversion pathway on a sub-picosecond time scale. In dyad 3, a novel type of collective carotenoid-Pc electronic state is observed that may correspond to a carotenoid excited state(s)-Pc Q exciplex. The exciplex is only observed upon direct carotenoid excitation and is nonfluorescent. In dyad 1, two carotenoid singlet excited states, S 2 and S 1 , contribute to singlet-singlet energy transfer to Pc, making the process very efficient (>90%) while for dyads 2 and 3 the S 1 energy transfer channel is precluded and only S 2 is capable of transferring energy to Pc. In the latter two systems, the lifetime of the first singlet excited state of Pc is dramatically shortened compared to the 9 double-bond dyad and model Pc, indicating that the carotenoid acts as a strong quencher of the phthalocyanine excited-state energy.

The monomer 5-(4-aminophenyl)-10, 20-bis(2,4,6-trimethylphenyl)porphyrin was synthesized and found to electropolymerize on platinum, indium tin oxide, and other electrodes to form a clear, semiconducting film with strong absorption in the visible spectral region. The linear, hole-conducting polymer has a unique structure, with porphyrin units linked to one another through the 5-(4-aminophenyl) nitrogen atom and the carbon atom at the 15-position on the macrocyclic ring. The porphyrin macrocyclic ring is thus an integral part of the linear polymer backbone. The oxidation potential of the film is 0.85 V and the reduction potential is -1.12 V vs SCE. The absorption spectrum of the film resembles that of a monomeric model porphyrin, but with significant peak broadening. Streak camera studies of the fluorescence of the polymer yield a lifetime of 15 ps, indicating strong quenching of the porphyrin first excited singlet state relative to that of the monomer. The properties of the polymer suggest that it may be useful in sensors, catalysts, and solar energy conversion devices.

A triethanolamine-protected silane, 1-(3'-amino)propylsilatrane, was incorporated into the structure of porphyrin- and ruthenium-based dyes and used to link them to transparent semiconductor nanoparticulate metal oxide films. Silatrane reacts with the metal oxide to form strong, covalent silyl ether bonds. In this study, silatrane-functionalized dyes and analogous carboxylate-functionalized dyes were used as visible light sensitizers for porous nanoparticulate SnO(2) photoanodes. The performance of the dyes was compared in photoelectrochemical cells incorporating either non-regenerative or regenerative redox components. The non-regenerative cell used NADH (beta-nicotinamide adenine dinucleotide) as a sacrificial electron donor and Hg(2)SO(4)/Hg as a sacrificial cathode, whereas the regenerative cell used the iodide/triiodide redox couple. Experiments showed that the silyl ether bonding gave the electrodes increased stability toward sensitizer desorption compared to carboxylate surface linkages. Porphyrin-silatrane dyes also demonstrated similar or better performance than their carboxylate analogs in photoelectrochemical cells. The improvement correlates with the results from transient absorbance spectroscopy, which show that the longer linker on the silatrane porphyrins slows charge recombination between oxidized porphyrin and the electrode surface. The improved photoelectrochemical cell efficiency and stability of the silatrane-based dyes compared to carboxylates demonstrate that silatranes are promising agents for bonding organic molecules to metal oxide surfaces.

A molecular "hexad" in which five bis(phenylethynyl)anthracene (BPEA) fluorophores and a dithienylethene photochrome are organized by a central hexaphenylbenzene unit has been prepared. Singlet-singlet energy transfer among the BPEA units occurs on the 0.4 and 60 ps time scales, and when the dithienylethene is in the open form, the BPEA units fluoresce in the 515 nm region with a quantum yield near unity. When the dithienylethene is photoisomerized by UV light to the closed form, which absorbs in the 500-700 nm region, the closed isomer strongly quenches all of the excited singlet states of BPEA via energy transfer, causing the fluorescence quantum yield to drop to near zero. This photochemical behavior permits the hexad to function in a manner analogous to a triode vacuum tube or transistor. When a solution of the hexad is irradiated with steady-state light at 350 nm and with red light (>610 nm) of modulated intensity, the BPEA fluorescence excited by the 350 nm light is modulated accordingly. The fluorescence corresponds to the output of a triode tube or transistor and the modulated red light to the grid signal of the tube or gate voltage of the transistor. Frequency modulation, amplitude modulation, and phase modulation are all observed. The unusual ability to modulate intense, shorter-wavelength fluorescence with longer-wavelength light could be useful for the detection of fluorescence from probe molecules without interference from other emitters in biomolecular or nanotechnological applications.

Zeolites have been identified as promising solid-state host materials for the design of alternative energy systems due to their ability to promote long-lived charge-separated states. Herein we investigate the ability of alkali metal cation-exchanged Y zeolites to mediate the formation of a spatially separated radical cation/radical anion pair by photoinduced electron migration. The chosen system is the electron-transfer reaction between photoexcited trans-anethole and co-incorporated 1,4-dicyanobenzene taking place within the internal cavities of the Y zeolite. The results from the investigation show that varying the alkali metal cation within the Y zeolite framework significantly influences the distance of electron migration and that the electrons travel farthest within dry NaY as compared to the other alkali metal cation-exchanged Y zeolites. The presence of intrazeolite water was also found to impede electron migration within the zeolite matrix.

Green chemistry provides unique opportunities for student engagement through K−12 and community outreach. As a platform for safe, engaging outreach, green chemistry activities allow for hands-on approaches to introducing science and chemistry concepts in informal settings. Green chemistry outreach also is a means for American Chemical Society (ACS) student chapters to earn recognition as a green chemistry student chapter through the ACS student chapter awards. Beyond Benign's College Student Fellows program is highlighted as an example of training college students in green chemistry outreach to promote K−12 and college student engagement. The University of New England (UNE) and Colby College provide examples of college student engagement through different outreach settings. This article outlines the many benefits of utilizing green chemistry activities through K−12 and community outreach, including creating a safe environment for student engagement, creating habits-of-mind for college students, and engaging K−12 students in safer, greener chemistry experiments and activities.

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