Soluble ethyne-linked tetraarylporphyrin arrays that mimic natural light-harvesting complexes by absorbing light and directing excited-state energy have been investigated by static and time-resolved absorption and fluorescence spectroscopies. Of particular interest is the role of the diarylethyne linkers in mediating energy transfer. The major conclusions from this study, which is limited to the examination of arrays containing Zn and free-base (Fb) porphyrins, include the following:  (1) Singlet excited-state energy transfer from the Zn porphyrin to the Fb porphyrin is extremely efficient (95−99%). Competitive electron-transfer reactions are not observed. (2) The rate of energy transfer is slowed up to 4-fold by the addition of groups to the linker that limit the ability of the linker and porphyrin to adopt geometries tending toward coplanarity. Thus, the mechanism of energy transfer predominantly involves through-bond communication via the linker. Consistent with this notion, the measured lifetimes of the Zn porphyrin in the dimers at room temperature yield energy-transfer rates ((88 ps)-1 < k trans < (24 ps)-1) that are significantly faster than those predicted by the Förster (through-space) mechanism ((720 ps)-1). Nevertheless, the electronic communication is weak and the individual porphyrins appear to retain their intrinsic radiative and non-radiative rates upon incorporation into the arrays. (3) Transient absorption data indicate that the energy-transfer rate between two isoenergetic Zn porphyrins in a linear trimeric array terminated by a Fb porphyrin is (52 ± 19 ps)-1 in toluene at room temperature, while the time-resolved fluorescence data suggest that it may be significantly faster. Accordingly, incorporation of multiple isoenergetic pigments in extended linear or two-dimensional arrays will permit efficient overall energy transfer. (4) Medium effects, including variations in solvent polarity, temperature, viscosity, and axial solvent ligation, only very weakly alter (≤2.5-fold) the energy-transfer rates. However, the Fb porphyrin fluorescence in the Zn−Fb dimers is quenched in the polar solvent dimethyl sulfoxide (but not in toluene, castor oil, or acetone), which is attributed to charge-transfer with the neighboring Zn porphyrin following energy transfer. Collectively, the studies demonstrate that extended multiporphyrin arrays can be designed in a rational manner with predictable photophysical features and efficient light-harvesting properties through use of the diarylethyne-linked porphyrin motif.

Polyethylene oxide has been end-tagged with aromatic chromophores (naphthalene, anthracene, pyrene) and the adsorption of these hydrophobically modified polymers at toluene-water and polystyrene-water interfaces characterized. The aredpolymer as deduced from the lowering of the interfacial tension decreases as the end-tag molecular weight increases. These same polymers were adsorbed onto polystyrene latexes, and an estimate of the aredpolymer was made from the increase of the hydrodynamic diameter as a function of adsorbed polymer. The exposure of the chromophore to the aqueous phase in free solution and adsorbed onto latexes was characterized by fluorescence quenching. These results demonstrated hindered access to the aqueous phase when the polymer was adsorbed onto the latex. Fluorescence depolarization measurements also demonstrated a more rotationally hindered chromophore after being adsorbed. Therefore, one can conclude that the hydrophobic end-group is the primary point of adsorption and that the chromophore is partially compartmentalized by the polystyrene matrix

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The excited-state properties of [(L)4MII(dpp)RuII(bpy)2](PF6) n (bpy = 2,2‘-bipyridine; dpp = (2,3-bis(2-pyridyl)pyrazine); (L)4MII = (NH3)4RuII (n = 4) or (CN)4FeII (n = 0)) have been investigated by steady-state and time-resolved picosecond and nanosecond transient luminescence and absorption spectroscopic methods. The [(NH3)4RuII(dpp)RuII(bpy)2]4+* complex is nonemissive in room-temperature H2O solution. Picosecond laser flash photolysis of [(NH3)4RuII(dpp)RuII(bpy)2]4+ reveals the formation of a transient species that decays exponentially (τ = 290 (±80) ps) independent of excitation wavelength. The excited-state behavior of [(NC)4FeII(dpp)RuII(bpy)2]* display a complicated dependence on excitation wavelength. Following 591-nm pulsed laser excitation into the 1[(dπ) (π*) ] ← 1[(dπ) ] metal-to-ligand charge transfer (MLCT) band, a short-lived transient is formed with τ < 80 ps. Laser excitation at 354.7 nm into the 1[(dπ) (π*) ] ← 1[(dπ) ] MLCT transition produces a transient species with τ > 200 ns. One explanation for the wavelength-dependent photophysics of the [(NC)4FeII(dpp)RuII(bpy)2] is that the FeII metal center undergoes a low- to high-spin interconversion following 1[(dπ) (π*) ] ← 1[(dπ) ] excitation, a process not accessible via direct 1[(dπ) (π*) ] ← 1[(dπ) ] excitation.

Photoinduced electron transfer has been investigated for poly(ethy1ene oxide) covalently end-tagged with a single anthracene and pyrene moiety (abbreviated as PEO-An and PEO-Py, respectively) both in homogeneous aqueous solution and adsorbed at the interface of water-soluble latexes, referred to as "microspheres" (abbreviated as $3). The electron-acceptor quencher is a zwitterionic viologen, SPV (4,4'-bipyridinyl-1,l'-bis(propane sulfonate)), that becomes anionic upon reduction. Charge separation following singlet-and tripletexcited-state quenching was found for PEO-An and PEO-Py in both environments. The efficiency of charge separation from the triplet state is high (ca. 0.6-1 .O) and is relatively insensitive to the environment because the back electron-transfer reaction is spin-forbidden. For the singlet state, the efficiency of charge separation is more modest (ca. 0.2-0.3) and is sensitive to changes in environment. In all cases the ion pairs have a long lifetime, in excess of 1 ms. Both fluorescence quenching and charge-transfer-complex formation are used to probe the local environment of the aromatic moiety. We conclude that the chromophores of the PEO-An and PEO-Py are protected either in homogeneous solution or adsorbed onto ,US such that formation of "tight" geminate ion pairs in electron-transfer processes is prevented. The adsorption properties of PEOAn and PEO-Py at H20/pS interfaces are also discussed.