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Intramolecular excitation energy transfer has been investigated with bichromophoric compounds in which naphthalene and anthracene as donor (D) and acceptor (A), respectively, are linked through a methylene chain, i.e., 9-anthryl-(CH2)n-1-naphthyl (AnN) with n = 1, 3, and 6 methylene units as spacer. The respective compounds were dispersed in stretched PVA films with the expectation that the methylene chain takes preferred conformations elongated in the direction of stretching polymer films so that the D−A distances are determined by the methylene chain length. From analyses of picosecond time-resolved fluorescence spectra and the fluorescence decay kinetics, it has been found that the energy transfer rates show a systematic change depending on the number of the methylene chain units:  47.6 × 10¹⁰ s^-1 in A1N, 1.96 × 10¹⁰ s^-1 in A3N, and 0.15 × 10¹⁰ s^-1 in A6N at 10 K. These changes in the energy transfer rate are basically consistent with the theoretical estimation based on the Förster's mechanism, with some contribution from through-bond superexchange interaction. The energy transfer rates in nonstretched PMMA films show no systematic change and are faster than those in stretched PVA films, suggesting a large contribution of the folded conformers of D and A. The time-resolved fluorescence spectra of anthracene (A) moiety show an unusual profile in a short time region (<20 ps) in which the 0−0 band is less intense than the 0−1 band. The fluorescence of the anomalous profile was assigned as being due to a resonance emission from higher vibrational levels in S1 populated by the energy transfer process.

Femtosecond time-resolved fluorescence spectra were measured on the carotenoid β-carotene and its analogue β-apo-8‘-carotenal in 2-methyltetrahydrofuran at 295 and 80 K. β-Carotene showed intramolecular vibrational redistribution (IVR) with time constants of 40−80 fs depending on the wavelengths, followed by vibrational relaxation with a time constant of 220 fs. At 80 K, the former process was resolved; however, the latter was not necessarily clear because of large spectral changes. However, β-apo-8‘-carotenal showed relaxation processes different from those of β-carotene; the IVR process was not resolved under both temperature conditions, and at 80 K, the decay constants were prolonged at all wavelengths measured. Changes in the time-resolved fluorescence spectra were discernible but small. These findings indicate that the IVR process of β-apo-8‘-carotenal was very fast, that it was shorter than our time resolution (30 fs), and that the slow relaxation process involved an interaction between a solute and a solvent, especially under the low-temperature condition.

The intramolecular electronic excitation transfer in dithiaanthracenophane (DTA) in THF solution has been investigated by probing the fluorescence anisotropy decay with a femtosecond up-conversion method. In DTA, two anthracene rings are known to be stacked parallel, but with nearly orthogonal orientation. There appeared a damped oscillation of an apparent period of 1.2 ± 0.2 ps and a damping time constant of 1.0 ± 0.1 ps. It has been found that the oscillatory behavior is consistent with the recurrence motion of an excitation between two anthracene moieties, from a theoretical analysis following a manner of Wynne and Hochstrasser (Wynne, K.; Hochstrasser, R. M. J. Raman Spectrosc. 1995, 26, 561). The magnitude of the dipole−dipole energy transfer interaction is estimated to be 40 cm^-1 which is in an acceptable agreement with the experimental value (29 cm^-1) deduced from the oscillation period. In comparison with the similar case of 2,2‘-binaphthyl (BN) reported by Zhu et al. (Zhu, F.; Galli, C.; Hochstrasser, R. M. J. Chem. Phys. 1993, 98, 1042), the electronic dephasing time T2‘ is significantly longer in DTA (1.0 ps) than in BN (0.2 ps). The longer dephasing time in DTA can be explained as arising from a much more fixed and rigid dimeric conformation than in BN.

Distance dependence of the electric field effect on the interlayer photoinduced electron tunneling from cyanine dye to viologen through a fatty acid has been examined using various spacer layers of fatty acid, based on the fluorescence measurements. The magnitude of the field-induced change in tunneling rate is shown to increase monotonically with decreasing the donor−acceptor distance.

Electric field effects on photoinduced electron-transfer processes have been examined in a polymer film for methylene-linked compounds of phenanthrene and N,N-dimethylaniline, based on the measurements of the field effects on fluorescence spectra. Both intra- and intermolecular electron-transfer processes are enhanced by an electric field (F), depending on the methylene chain length, and LE fluorescence emitted from the locally excited state of phenanthrene chromophore is quenched by F. Exciplex fluorescence is also influenced by F. The magnitude of the change in molecular polarizability following fluorescence process has been determined from the Stark shift both for LE fluorescence and for exciplex fluorescence.

A set of four porphyrin trimers (1−4) consisting of an energy-accepting 5,15-diphenylethynyl-substituted Zn(II)-porphyrin core flanked by two energy-donating peripheral Zn(II)-porphyrins have been prepared as a new efficient energy-transfer functional unit. The peripheral porphyrin donor is either a TPP-type Zn(II)-porphyrin for 1 and 2 or a OEP-type Zn(II)-porphyrin for 3 and 4 and the diphenylethynyl substitution axis of the core porphyrin is aligned either orthogonal in 1 and 3 or parallel in 2 and 4 with respect to the long axis of the trimeric arrays. Femtosecond transient absorption spectroscopy and femtosecond up-conversion fluorescence measurement have revealed the very efficient S1−S1 energy-transfer reactions in these porphyrin trimers. The S1−S1 energy transfer is faster in the parallel trimers 2 and 4 than in the orthogonal trimers 1 and 3, reflecting larger electronic coupling in the former pair. The peripheral porphyrin S2-state lifetime is considerably shortened in 1−4, which has been ascribed to S2−S2 energy transfer. Probably the strong Soret-transitions of both the donor and acceptor lead to large Coulombic interactions, thereby rendering S2−S2 energy transfer effective enough to compete with rapid internal conversion to S1-state. These results encourage a new strategy for construction of porphyrin-based supramolecular artificial photosynthetic antenna.

Self-assembled monolayers of ferrocene−porphyrin−C60 triads on gold electrodes were prepared to mimic photosynthetic electron transfer events where efficient conversion of light to chemical energy takes place via the long-lived, charge-separated state with a high quantum yield. Adsorbed amounts of the triads on the gold electrodes, estimated from the charge of the anodic peak of the ferrocene, are comparable to those of the well-ordered porphyrin−alkanethiols and C60−alkanethiols on gold electrodes. The results, together with blocking experiments using a redox probe, indicate that the triad molecules are well-packed with an almost perpendicular orientation on the gold surface. The monolayer thickness obtained using X-ray reflectivity analysis is consistent with the structural model of the monolayer. Photoelectrochemical studies were carried out in a standard three-electrode system using the gold electrodes modified with the self-assembled monolayers of the triads. Stable cathodic photocurrents were observed in the presence of electron carriers such as oxygen and/or methyl viologen in the electrolyte when the modified gold electrodes were illuminated with a monochromic light. A photoinduced multistep electron-transfer mechanism is proposed for the photoelectrochemical cells. Thus, vectorial electron transfer or partial charge transfer occurs from the excited singlet state of the porphyrin to the C60, followed by the successive charge shift from the ferrocene to the porphyrin cation radical, to produce the ferrocene cation radical and the C60 anion radical. The C60 anion radical gives an electron to the counter electrode via the electron carriers in the electrolyte solution, whereas electron transfer takes place from the gold electrode to the ferrocene cation radical, resulting in the recovery of the initial state and the generation of the overall electron flow. The artificial photosynthetic cells show the highest quantum efficiency (20−25%) ever reported for photoinduced multistep electron transfer at monolayer-modified metal electrodes and across artificial membranes using donor−acceptor linked molecules. The result indicates clearly that C60 acts as an excellent electron acceptor as well as an electron mediator in artificial photosynthetic membranes. The molecule-based methodology will provide a new direction for the development of solar energy conversion and molecular devices.

Disulfides, with a systematic series of alkyl spacers containing porphyrins at both terminals, were prepared to investigate the effect of the spacer length on the structure and photoelectrochemical properties of self-assembled monolayers (SAMs) of the porphyrins on a gold electrode. The structure of the SAMs was studied using ultraviolet (UV)−visible absorption spectroscopy in transmission mode, cyclic voltammetry, UV−visible ellipsometry, and fluorescence spectroscopy. These measurements showed that as the length of the spacers increases, the SAMs tend to form highly ordered structures on the gold electrodes. In addition, the structures of the monolayers vary depending on the even and odd number of the methylene spacers (n). From these measurements a porphyrin dimer model is proposed in that the two porphyrins take J-aggregate-like partially stacked structures in the monolayers. Photoelectrochemical studies were carried out in argon-saturated Na2SO4 aqueous solution containing methyl viologen as an electron carrier using the modified gold working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode. The quantum yield increases in a zigzag fashion with an increase in the spacer length up to n = 6 and then starts decreasing slightly as the chain lengths become longer. A plausible explanation for the photocurrent trend comes from the following points:  (i) there are two competitive deactivation pathways for the excited singlet state of the porphyrin dimer, i.e., the quenching by the electrode via energy transfer and by the electron carrier via electron transfer, (ii) the porphyrin aggregation enhances the rate of nonradiative pathway in the excited state, and (iii) the electron transfer rate from the gold electrode to the resulting porphyrin cation radical decreases with an increase of the spacer lengths. These results will provide basic information for the construction of molecular assembly with photoactive function on surface.

Electroabsorption spectra of meso,meso-linked zinc porphyrin arrays doped in a PMMA polymer film have been measured with a field modulation spectroscopy, and the change in electric dipole moment (Δμ) and in molecular polarizability has been evaluated for the transition into each of the exciton splitting components of the Soret band. Δμ following photoexcitation is non-zero in the oligomers, and the magnitude becomes larger with the increasing number of linked molecules, suggesting that photoinduced charge transfer occurs along the linked porphyrin array.

Picosecond time‐resolved fluorescence spectra emitted from intact cells of the green alga Chlorella pyrenoidosahave been measured by means of a new detection technique using a microchannel‐plate photomultiplier. A fluorescence band (F700) was observed at 690–730 nm in the initial time region (0–180 ps), in addition to the well‐known spectrum (F685) of photosystem II (PS II)‐chlorophyll a(Chla) with a peak at 685 nm. F700 decays rapidly with lifetime of 104 ps, while F685 decays much more slowly in bi‐exponential form with lifetimes of 0.64 and 1.7 ns. Appearance of F700 is independent of closure of the reaction center II (RC II). F700 is thus assigned to the fluorescence from PS I‐Chl a, whose decay is governed by a fast energy transfer process from the antenna Chl aof PS I to P700 of RC I.

Exciplex fluorescence in a mixture of N-ethylcarbazole (ECZ) and dimethyl terephthalate (DMTP) doped in a PMMA polymer film, which appears as a result of photoinduced electron transfer from ECZ to DMTP, is quenched by an electric field as the concentration of the electron donor, i.e., ECZ, is increased. Even when the DMTP concentration is increased, the exciplex fluorescence is not quenched by an external electric field as long as the ECZ concentration is low. These concentration dependences indicate that a hole transport occurs very efficiently and that an electron transport does not occur in this photoinduced electron-transfer system. In fact, a photocurrent could be detected in a mixture of ECZ and DMTP in PMMA films only when the concentration of ECZ was high.

Mg-substituted horseradish peroxidases were oxidized by K2IrCl6 or K3Fe(CN)6 to their porphyrin radical form and the 1:1 stoichiometric relationship was confirmed by spectrophotometric, fluorophotometric, and ESR titration methods. The values of E'0 for oxidations of Mg peroxidases A and C were both 0.63 V at pH 6 and depended on pH in the same way as postulated for the Compound I/Compound II couples of the corresponding enzymes. Unlike Zn peroxidase C, Mg peroxidase C was not directly oxidized by H2O2. The oxidation was catalyzed by the native peroxidase. Mg peroxidase C was photooxidized to the radical form faster than Zn peroxidase C, but its oxidation was accomplished by irreversible changes in the porphyrin in the early stage of reaction. The rate of reduction of the oxidized Mg peroxidases in the presence of various electron donors was measured at varying pH values and compared with the rate of Compound I reduction. A role of porphyrin as a site of electron transfer in the peroxidase catalysis was suggested.

The kinetics of iodination and oxidation of hog thyroglobulin were studied with purified hog thyroid peroxidase and the results were compared with the reactions of free tyrosine. From Lineweaver-Burk plots and on the basis of a value of 0.83 for delta epsilon mM at 289 nm/iodine atom incorporated, the rate constant for transfer of an assumed enzyme-bound iodinium cation to thyroglobulin was estimated to be 6.7 X 10(7) and 2.3 X 10(7) M-1 s-1 in native (iodine content = 1.0%) and more iodinated (iodine content = 1.2%) thyroglobulins, respectively. This iodine-transferring reaction was stimulated by iodothyronines, similarly as observed in the reaction with free tyrosine. The iodination of thyroglobulin was inhibited by GSH, the inhibition being competitive with thyroglobulin. Thyroglobulin was oxidized in the presence of a thyroid peroxidase system without giving any appreciable change in absorbance around 300 nm. From stopped flow data, the oxidation was concluded to occur by way of two-electron transfer and the rate constant for the reaction of thyroid peroxidase Compound I with thyroglobulin was estimated to be 1.0 X 10(7) M-1 s-1. The stopped flow kinetic pattern was similar to that observed on the reaction with free tyrosine and monoiodotyrosine. About 6 mol of hydrogen peroxide were consumed per mol of thyroglobulin. Thyroid peroxidase catalyzed thyroglobulin-mediated oxidation of GSH, but lactoperoxidase did not.

Upon photoirradiation under aerobic conditions, the porphyrin prosthetic group in Mg-substituted horseradish peroxidase was oxidized to a mixture of its pi-cation radical and an oxidized product with an absorption band at 448 nm. The 448 nm compound was then converted to a 489 nm compound in the dark and the activation energy for the conversion was 19.3 kcal/mol. About 1 mol of O2 was consumed per mol of the 448 nm compound formed and no O2 consumption was seen in the dark reaction. The substitution of ethyl groups (meso) and hydroxyethyl groups (hemato) for the vinyl groups in protoporphyrin IX did not have an effect on the result. Under anaerobic conditions and in the presence of a suitable electron acceptor, the only photooxidation product of porphyrin was its pi-cation radical. The formation of hydroxyl radicals during irradiation under aerobic conditions was confirmed by the spin-trapping method. The formation of the above two radicals could be followed by ESR spectroscopy separately at a fixed magnetic field which was set to maximize each ESR signal. The rate of hydroxyl radical formation depended linearly on the concentration of Mg peroxidase. The photooxidation of porphyrin was slow and gave nonspecific product(s) when Mg protoporphyrin IX was present in the heme crevice of apomyoglobin or free in solution.