Hiroyoshi Nagae

Calculation of the excitation transfer matrix elements between the S2 or S1 state of carotenoid and the S2 or S1 state of bacteriochlorophyll

Formalism of the excitation transfer matrix element applicable for any multiconfigurational wave functions is made. On the basis of the resultant formulas, the excitation transfer matrix elements between the S2 or S1 state of a carotenoid, neurosporene, and the S2 or S1 state of bacteriochlorophyll a are calculated at various stacked configurations of the two molecules. The results show that the excitation transfer from the carotenoid S1 state to the bacteriochlorophyll S1 state via the Coulomb mechanism including multipole–multipole interactions takes place very efficiently in a speed more rapid than that via the electron‐exchange mechanism. The results also show that the excitation transfer from carotenoid to bacteriochlorophyll occurs directly from the carotenoid S2 state, as well as from the carotenoid S1 state. Furthermore, it is shown that the excitation transfer matrix element due to the electron‐exchange interaction has an oscillatory dependence on the displacement of one molecule from the other w...

A new singlet-excited state of all-trans-spheroidene as detected by resonance-Raman excitation profiles

Abstract Measurements of resonance-Raman excitation profiles of the CC and C–C stretching Raman lines for crystalline all- trans -spheroidene in KBr disc at 77 K identified a new singlet state (17600 cm −1 ) that is located in-between the 2A g − state (14200 cm −1 ) and the 1B u + state (19700 cm −1 ). The particular state is tentatively assigned to the 1B u − state on the basis of the extrapolation of the PPP-MRD-CI calculations for the low-lying singlet states of shorter polyenes [P. Tavan, K. Schulten, J. Chem. Phys. 85 (1986) 6602], and its possible role in mediating the rapid 1B u + to 2A g − internal conversion is discussed.

Structural Transformation Among the Aggregate Forms of Bacteriochlorophyll c as Determined by Electronic-Absorption and NMR Spectroscopies: Dependence on the Stereoisomeric Configuration and on the Bulkiness of the 8-C Side Chain ‡

Transformation among the aggregate forms of bacteriochlorophyll (BChl) c characterized by the wavelength of the Qy absorption, i.e. the dimer (B675), B705, B720 and B745, was traced by electronic‐absorption spectroscopy for each of the isomers including R[E,E], R[P,E], R[I,E], S[P,E] and S[I,E] suspended in the mixtures of methylene chloride and n‐hexane. A combination of NMR spectroscopy determining the structural motifs and calculation of the shift of the Qy absorption reflecting the long‐range transition dipole–transition dipole interactions among the macrocycles in the entire aggregate structures proposed the following models: B705d (B705d′), a linear array of straight (inclined) columns consisting of a pair of the piggyback dimers; B720d and B745d, an assembly of two and five shifted‐inclined columns consisting of more than six piggyback dimers; and B720m and B745m, an assembly of one and two parallel stepwise stacking of ∼30 monomers. Calculations of the steric energies rationalized two different pathways of transformations: the dimer → B705d (B705d′) → B720d → B745d for the R isomers; and the monomer → (B720m) → B745m for the S isomers. Addition of S[I,E] seems to trigger the B745d → B745m transformation of the R isomers.

Effects of Nonpolar and Polar Solvents on the Qx and QY Energies of Bacteriochlorophyll a and Bacteriopheophytin a

The effects of nonpolar and polar solvents on the Qx and Qy energies of bacteriochlorophyll (BChl) a and bacteriopheophytin (BPhe) a were examined by electronic absorption spectroscopy. All of the four different energies exhibited a linear dependence on R(n) = (n2 ‐ l)/(n2+ 2), where n is the refractive index of the solvent, in both nonpolar and polar solvents. The energy of each state of both pigments could be expressed as v = ‐dR(n) + e (in cm‐1) where coefficient d was related to the dispersive interaction between the solute and the solvent molecules. A theory developed by Nagae showed that coefficient d originates from the quantum‐mechanical fluctuation of the multipole moments of the solute, in terms of which the following characteristics of the observed d values were explained: (1) In all of the four cases of the Qx, and Qy energies of both BChl a and BPhe a, the d values for the polar solvents were smaller than those for the non‐polar solvents. (2) In both nonpolar and polar solvents, the d value of BChl a was larger than that of BPhe a in the Qy energy, whereas the d value of BPhe a was larger than that of BChl a in the Qx energy. (3) The d value of the Qx energy was larger than that of the Qy, energy for either case of BChl a or BPhe a.

Excited-State Dynamics of Overlapped Optically-Allowed 1Bu+ and Optically-Forbidden 1Bu- or 3Ag- Vibronic Levels of Carotenoids: Possible Roles in the Light-Harvesting Function

The unique excited-state properties of the overlapped (diabatic) optically-allowed 1Bu+ and the optically-forbidden 1Bu− or 3Ag− vibronic levels close to conical intersection (‘the diabatic pair’) are summarized: Pump-probe spectroscopy after selective excitation with ∼100 fs pulses of all-trans carotenoids (Cars) in nonpolar solvent identified a symmetry selection rule in the diabatic electronic mixing and diabatic internal conversion, i.e., ‘1Bu+-to-1Bu− is allowed but 1Bu+-to-3Ag− is forbidden’. On the other hand, pump-probe spectroscopy after coherent excitation with ∼30 fs of all-trans Cars in THF generated stimulated emission with quantum beat, consisting of the long-lived coherent diabatic cross term and a pair of short-lived incoherent terms.

Dye-Sensitized Solar Cells Based on the Principles and Materials of Photosynthesis: Mechanisms of Suppression and Enhancement of Photocurrent and Conversion Efficiency

Attempts have been made to develop dye-sensitized solar cells based on the principles and materials of photosynthesis: We first tested photosynthetic pigments, carotenoids (Cars), chlorophylls (Chls) and their derivatives, to find sensitizers showing reasonable performance (photocurrent and conversion efficiency). We then tried to introduce the principles of photosynthesis, including electron transfer and energy transfer from Car to Phe a. Also, we tried co-sensitization using the pheophorbide (Phe) a and Chl c2 pair which further enhanced the performance of the component sensitizers as follows: Jsc = 9.0 + 13.8 → 14.0 mA cm−2 and η = 3.4 + 4.6 → 5.4%.

Shifts of the 1A−g→1B+u electronic absorption of carotenoids in nonpolar and polar solvents

The 1B+u energy of carotenoids (spheroidene, neurosporene, and β carotene) exhibits a linear dependence on R(n)=(n2−1)/(n2+2) in both nonpolar and polar solvents: The 1B+u energy is more stabilized in polar solvents in the limit of R(n)→0, and the line for polar solvents has a gentler slope. [As a result, the line for polar solvents crosses the line for nonpolar solvents at R(n)≂0.3.] A theory has been developed, which explains the aforementioned observations as follows. In polar solvents, an electric field is generated by fluctuation of the solvent permanent dipoles, and it affects the rodlike, conjugated chain of the carotenoid in a long spheroidal cavity. The electric field stabilizes the 1B+u energy through the polarization effect and it substantially reduces the dispersive interaction. The higher transition multipoles, rather than the transition dipole of carotenoid, play major roles in the aforementioned interactions between the carotenoid and the solvent molecules.

Stacking of bacteriochlorophyll c macrocycles in chlorosome from Chlorobium limicola as revealed by intermolecular 13C magnetic-dipole correlation, X-ray diffraction, and quadrupole coupling in 25Mg NMR.

The stacking of the bacteriochlorophyll (BChl) c macrocycles and the role of water in forming an aggregate sheet, in chlorosome, were examined by means of (13)C NMR spectroscopy, the measurement of the X-ray diffraction pattern, and (25)Mg NMR spectroscopy. (1) The stacking of the macrocycles, i.e., weakly overlapped dimers forming displaced layers, was selected out of six different kinds of stacking so far identified in the aggregates of isomeric BChl c in solution and in the solid aggregate of an isomeric mixture of BChl c extracted from Chlorobium limicola. The selection was based on the comparison of the intermolecular (13)C...(13)C magnetic-dipole correlations with the nearest-neighbor carbon-to-carbon close contacts simulated for the above six different stackings. It has turned out that the stacking of the macrocycles in chlorosome is basically the same as that in the in vitro solid aggregate. (2) The crucial role of water in stabilizing the aggregate structure in chlorosome was shown by tracing the dehydration processes and by comparison with the solid aggregate using the X-ray diffraction pattern. Possible binding sites of water molecules were located, by structural simulation, based on the particular stacking structure. (3) The dimer-based stacking of the macrocycles was evidenced by (25)Mg NMR spectroscopy, which exhibited a pair of signals showing different quadrupole coupling, due to the presence or absence of a water molecule in the axial position.

Mechanisms of Suppression and Enhancement of Photocurrent/Conversion Efficiency in Dye-Sensitized Solar-Cells Using Carotenoid and Chlorophyll Derivatives as Sensitizers

The mechanisms of suppression and enhancement of photocurrent/conversion efficiency (performance) in dye-sensitized solar cells, using carotenoid and chlorophyll derivatives as sensitizers, were compared systematically. The key factor to enhance the performance was found to be how to minimize interaction among the excited-state dye-sensitizer(s). In a set of retinoic-acid (RA) and carotenoic-acid (CA) sensitizers, having n conjugated double bonds, CA7 gave rise to the highest performance, which was reduced toward RA5 and CA13. The former was ascribed to the generation of triplet and the resultant singlet-triplet annihilation reaction, while the latter, to the intrinsic electron injection efficiency. In a set of shorter polyene sensitizers having different polarizabilities, the one with the highest polarizability (the highest trend of aggregate formation) exhibited the higher performance toward the lower dye concentration and the lower light intensity, contrary to our expectation. This is ascribed to a decrease in the singlet-triplet annihilation reaction. The performance of cosensitization, by a pair of pheophorbide sensitizers without and with the central metal, Mg or Zn, was enhanced by the light absorption (complementary rather than competitive), the transition-dipole moments (orthogonal rather than parallel) and by the pathways of electron injection (energetically independent rather than interactive).

SOLVENT EFFECTS ON TRIPLET-STATE BACTERIOCHLOROPHYLL A AS DETECTED BY TRANSIENT RAMAN SPECTROSCOPY AND THE ENVIRONMENT OF BACTERIOCHLOROPHYLL A IN THE LIGHT-HARVESTING COMPLEX OF RHODOBACTER SPHAEROIDES R26

The frequency of BChl that was bound to the light‐harvesting complex (LHC) of Rhodobacter sphaeroides R26 was found to be 1598 cm‐1, a result which suggests that a pair of BChl molecules form a dimer in the LHC in the T1 state.