A highly efficient functional mimic of a photosynthetic antenna−reaction center complex has been designed, synthesized, and studied spectroscopically. The antenna, consisting of four covalently linked zinc tetraarylporphyrins, (PZP)3−PZC, has been coupled to a free-base porphyrin−fullerene artificial photosynthetic reaction center, P−C60, to form a (PZP)3−PZC−P−C60 hexad. As revealed by time-resolved absorption and emission studies in 2-methyltetrahydrofuran solution at ambient temperature, excitation of a peripheral zinc porphyrin moiety is followed by singlet−singlet energy transfer to the central zinc porphyrin to give (PZP)3−PZC−P−C60 with a time constant of 50 ps. The excitation is passed on to the free-base porphyrin in 32 ps to produce (PZP)3−PZC−1P−C60, which decays by electron transfer to the fullerene with a time constant of 25 ps. The resulting (PZP)3−PZC−P•+−C60 •- charge-separated state is generated with a quantum yield of 0.98 based on light absorbed by the porphyrin antenna. Direct excitation of the free-base porphyrin moiety or the fullerene also generates this state with a yield near unity. Thermodynamically favorable migration of positive charge into the zinc porphyrin array transforms the initial state into a long-lived ((PZP)3−PZC)•+−P−C60 •- charge-separated state with a time constant of 380 ps. The final charge-separated state, formed in high yield (∼0.90), decays to the ground state with a lifetime of 240 ns. In benzonitrile, the lifetime is 25 μs. A previous hexad, which differs from the current hexad solely in the nature of the free-base porphyrin, gave a charge-separated state with a lower yield (0.69) and a shorter lifetime (1.3 ns). The difference in performance is attributed to differences in electronic composition (a2u versus a1u HOMO), conformation, and oxidation potential (1.05 versus 0.84 V) between the meso-tetraarylporphyrin and the β-octaalkylporphyrin of the current and former hexads, respectively. These results can be explained on the basis of an understanding of factors that affect through-bond energy-transfer and electron-transfer processes. The results demonstrate that it is possible to design and prepare synthetic, porphyrin-based antenna−reaction center complexes that mimic the basic photochemical functions of natural photosynthetic light-harvesting antennas and reaction centers in simple, structurally well-defined constructs.

A photochromic nitrospiropyran moiety (Sp) has been covalently linked to a zinc (PZn) and to a free-base (P(H2)) porphyrin. In the resulting dyads (P(Zn)-Sp(c) and P(H2)-Sp(c)), the porphyrin first excited singlet states are unperturbed by the closed form of the attached spiropyran. Excitation of the spiropyran moiety of either dyad in the near-UV region results in ring opening to a merocyanine form (P-Sp(o)) that absorbs at 600 nm. The open form re-closes thermally in 2-methyltetrahydrofuran with a time constant of 20 s, or following irradiation into the 600 nm band. Excitation of the zinc porphyrin moiety in the merocyanine form of the dyad yields 1PZn-Sp(o). The lifetime of the zinc porphyrin excited state is reduced from its usual value of 1.8 ns to 130 ps by singlet-singlet energy transfer to the merocyanine moiety to give PZn-1Sp(o). The quantum yield of energy transfer is 0.93. Quenching is also observed in the free base dyad, where 1P(H2)-Sp(o) and P(H2)-1Sp(o) exchange singlet excitation energy. This photoswitchable quenching phenomenon provides light-activated control of the porphyrin excited states, and consequently control of any subsequent energy or electron-transfer processes that might be initiated by these excited states in more complex molecular photonic or optoelectronic devices.

Artificial light-harvesting constructs were synthesized by covalently linking two carotenoids to the central silicon atom of a phthalocyanine (Pc) derivative. Triad 1 binds two carotenoids having nine conjugated double bonds, whereas triad 2 binds two carotenoids having 10 carbon-carbon double bonds in conjugation. Fluorescence excitation experiments indicated that, in triad 1 dissolved in n-hexane, the carotenoid to Pc singlet energy transfer efficiency is ca. 92%, whereas in triad 2, it is 30%. Results from ultrafast laser spectroscopy indicate that upon population of the optically allowed S 2 state of the carotenoid the optically forbidden states S 1 and S* are rapidly generated in both triad 1 and triad 2. In triad 1, S 2 , S 1 , and S* all contribute singlet electronic energy to Pc. In triad 2, singlet electronic energy transfer to Pc occurs primarily from the optically allowed S 2 state with little energy transfer to Pc via the S 1 state, and there is no evidence for energy transfer via S*. Instead, in triad 2, we find a multiphased quenching of the Pc singlet excited state on the picosecond and nanosecond time scales. Upon intersystem crossing from the singlet excited state of Pc to the triplet state in triad 1, triplet-triplet energy transfer to either of the carotenoids takes place on a time scale significantly shorter than 5 ns. When dissolved in polar solvents, triads 1 and 2 exhibit lightinduced electron transfer from either of the carotenoid moieties to the excited singlet Pc species with a time constant of about 2 ps. Charge recombination to the singlet ground state occurs in 10 ps in triad 1 and 17 ps in triad 2.

Control of surface states of titanium dioxide nanoparticles using 2-(3,4-dihydroxyphenyl)ethylamine (dopamine) and 3,4-dihydrophenylacetic acid, which act as ligands to the undercoordinated surface sites (carrier traps), is demonstrated by electrochemical techniques. The deepest traps were found to be most reactive and are selectively removed by the addition of the ligands which enhances the kinetics of electron accumulation in the film. Furthermore, a shift in the Fermi level to more positive potentials was detected for electrodes modified with the negatively charged ligand (3,4-dihydrophenylacetic acid) compared to that of electrodes modified with the positively charged ligand (dopamine). The presence of the negative charge on the ligand also contributed to the underpotential of hydrogen evolution on 3,4-dihydrophenylacetic acid-modified electrodes.