Polarization angle dependence of stark absorption spectra of spirilloxanthin bound to the reconstituted LH1 complexes using LH1-subunits isolated from the purple photosynthetic bacterium Rhodospirillum rubrum.

Reconstituted LH1 complexes were prepared using the LH1 subunit-type complexes, isolated from the purple photosynthetic bacterium Rhodospirillum (Rs.) rubrum, and purified all-trans spirilloxanthin. Stark absorption spectra of spirilloxanthin bound to both the native and reconstituted LH1 complexes were compared in different polarization angles (χ) against the external electric field. From the polarization angle dependence of the Stark absorption spectra, two angles were determined in reference to the direction of transition dipole moment (m) of spirilloxanthin: one is the change in polarizability upon photoexcitation (Δα), θ(Δα) and the other is the change in static dipole moment upon photoexcitation (Δµ), θ(Δµ). Despite the symmetric molecular structure of all-trans spirilloxanthin, its Stark absorption spectra show pronounced values of Δµ. This large Δµ values essentially caused by the effect of induced dipole moment through Δα both in the cases for native and reconstituted LH1 complexes. However, slightly different values of θ(Δα) and θ(Δµ) observed for the native LH1 complex suggest that spirilloxanthin is asymmetrically distorted when bound to the native LH1 complex and gives rise to intrinsic Δµ value.

Ultrafast excited state dynamics of fucoxanthin: excitation energy dependent intramolecular charge transfer dynamics.

Carotenoids containing a carbonyl group in conjugation with their polyene backbone are naturally-occurring pigments in marine organisms and are essential to the photosynthetic light-harvesting function in aquatic algae. These carotenoids exhibit spectral characteristics attributed to an intramolecular charge transfer (ICT) state that arise in polar solvents due to the presence of the carbonyl group. Here, we report the spectroscopic properties of the carbonyl carotenoid fucoxanthin in polar (methanol) and nonpolar (cyclohexane) solvents studied by steady-state absorption and femtosecond pump-probe measurements. Transient absorption associated with the optically forbidden S(1) (2(1)A) state and/or the ICT state were observed following one-photon excitation to the optically allowed S(2) (1(1)B) state in methanol. The transient absorption measurements carried out in methanol showed that the ratio of the ICT-to-S(1) state formation increased with decreasing excitation energy. We also showed that the ICT character was clearly visible in the steady-state absorption in methanol based on a Franck-Condon analysis. The results suggest that two spectroscopic forms of fucoxanthin, blue and red, exist in the polar environment.

Femtosecond transient absorption spectroscopic study of a carbonyl-containing carotenoid analogue, 2-(all-trans-retinylidene)-indan-1,3-dione.

The photophysical properties of a carbonyl-containing carotenoid analogue in an s-cis configuration, relative to the conjugated π system, 2-(all-trans-retinylidene)-indan-1,3-dione (C20Ind), were investigated by femtosecond time-resolved spectroscopy in various solvents. The lifetime of the optically forbidden S(1) state of C20Ind becomes long as solvent polarity increases. This trend is completely opposite to the situation of S(1-ICT) dynamics of carbonyl-containing carotenoids, such as peridinin and fucoxanthin. Excitation energy dependence of the transient absorption measurements shows that the transient absorption spectra in nonpolar solvents were originated from two distinct transient species, while those in polar and protic solvents are due to a single transient species. By referring to the results of MNDO-PSDCI (modified neglect of differential overlap with partial single- and double-configuration interaction) calculations, we conclude: (1) in polar and protic solvents, the S(1) state is generated following excitation up to the S(2) state; (2) in nonpolar solvents, however, both the S(1) and the (1)nπ* states are generated; and (3) C20Ind does not generate the S(1-ICT) state, despite the fact that it has two conjugated carbonyl groups.

Deconstructing the excited-state dynamics of ß-carotene in solution.

The femtosecond to nanosecond dynamics of the all-trans ß-carotene carotenoid dissolved in 3-methylpentane is characterized and dissected with excitation-wavelength and temperature-dependent ultrafast dispersed transient absorption signals. The kinetics measured after red-edge (490 nm) and blue-edge (400 nm) excitation were contrasted under fluid solvent (298 K) and rigid glass (77 K) conditions. In all four measured data sets, the S* population kinetics was resolved prompting the development of a modified multicompartment model. The temperature-dependent and excitation wavelength-dependent S* quantum yield is ascribed to a competition of population surmounting a weak (55 cm(-1)) energy barrier on the S(2) state to favor S(1) generation and rapid internal conversion that favors S* generation. When cooled from room temperature to 77 K, the S* decay time scale shifted significantly from 30 to 400 ps, which is ascribed to small-scale structural relaxation with a 115 cm(-1) energy barrier. For the first time under low-energy excitation conditions, the triplet state is observed and confirmed to not originate from S* or S(1), but from S(2). The interconnectivity of the S* and S(1) populations is discussed, and no observed population flow is resolved between S* and S(1). Comparison of samples obtained from different laboratories with different purity levels demonstrates that sample contamination is not the primary origin of the S* state.

Syntheses of C33-, C35-, and C39-peridinin and their spectral characteristics.

Peridinin, a nor-carotenoid, exhibits an exceptionally high energy transfer efficiency to chlorophyll a in photosynthesis in the sea. This efficiency would be related to the unique structure of peridinin. To answer the question of why peridinin possesses the irregular C37 skeleton, we have achieved the synthesis of three peridinin derivatives. Their ultrafast time-resolved optical absorption and Stark spectra measurements have shown the presence of the characteristic intramolecular charge transfer state and the featured electrostatic properties of peridinin.

Syntheses of allene-modified derivatives of peridinin toward elucidation of the effective role of the allene function in high energy transfer efficiencies in photosynthesis.

Peridinin is known as the main light-harvesting pigment in photosynthesis in the sea and exhibits exceptionally high energy transfer efficiencies to chlorophyll a. This energy transfer efficiency is thought to be related to the intricate structure of peridinin, which possesses allene and ylidenbutenolide functions in the polyene backbone. There are, however, no studies on the relationship between the structural features of peridinin and its super ability for energy transfer. We then focused on the subjects of why peridinin possesses a unique allene group and how the allene function plays a role in the exceptionally high energy transfer. Toward elucidation of the exact role of the allene function, we now describe the syntheses of three relatively unstable allene-modified derivatives of peridinin along with the results of the Stark spectroscopy of peridinin and the synthesized peridinin derivatives.

Syntheses, structural characterization and photophysical properties of 4-(2-pyridyl)-1,2,3-triazole rhenium(I) complexes.

Novel chelators, i.e., 4-(2-pyridyl)-1,2,3-triazole derivatives, were synthesized by means of Cu(I)-catalyzed 1,3-dipolar cycloaddition and used to prepare luminescent Re(I) complexes [ReCl(CO)(3)(Bn-pyta)], [ReCl(CO)(3)(AcGlc-pyta)] and [ReCl(CO)(3)(Glc-pyta)] (Bn-pyta = 1-benzyl-4-(2-pyridyl)-1,2,3-triazole, AcGlc-pyta = 2-(4-(2-pyridyl)-1,2,3-triazol-1-yl)ethyl 2,3,4,6-tetra-O-acetyl-beta-d-glucopyranoside, Glc-pyta = 2-(4-(2-pyridyl)-1,2,3-triazol-1-yl)ethyl beta-d-glucopyranoside). X-Ray crystallography of Bn-pyta and Glc-pyta indicated an azocompound-like structure while the 1,2,4-triazole isomer has an azine character. [ReCl(CO)(3)(Bn-pyta)] crystallized in the monoclinic system with space group P2(1)/n. Bn-pyta ligand coordinates with the nitrogen atoms of the 2-pyridyl group and the 3-position of 1,2,3-triazole ring, which is a very similar coordinating fashion to that of the 2,2'-bipyridine derivative. The glucoconjugated Re(I) complexes [ReCl(CO)(3)(AcGlc-pyta)] and [ReCl(CO)(3)(Glc-pyta)] hardly crystallized, and were analyzed by applying extended X-ray absorption fine structure (EXAFS) analysis. The EXAFS analyses suggested that the glucoconjugation at the 1-position of the 1,2,3-triazole makes no influence to the coordinating fashion of 4-(2-pyridyl)-1,2,3-triazole. [ReCl(CO)(3)(Bn-pyta)] showed a blue-shifted maximum absorption (333 nm, 3.97 x 10(3) M(-1) cm(-1)) compared with [ReCl(CO)(3)(bpy)] (371 nm, 3.35 x 10(3) M(-1) cm(-1)). These absorptions were clearly assigned to be the mixed metal-ligand-to-ligand charge transfer (MLLCT) on the basis of time-dependent density functional theory calculation. The luminescence spectrum of [ReCl(CO)(3)(Bn-pyta)] also showed this blue-shifted feature when compared with that of [ReCl(CO)(3)(bpy)]. The luminescence lifetime of [ReCl(CO)(3)(Bn-pyta)] was determined to be 8.90 mus in 2-methyltetrahydrofuran at 77 K, which is longer than that of [ReCl(CO)(3)(bpy)] (3.17 micros). The blue-shifted electronic absorption and elongated luminescence lifetime of [ReCl(CO)(3)(Bn-pyta)] suggested that 4-(2-pyridyl)-1,2,3-triazole functions as an electron-rich bidentate chelator.

Stark Absorption Spectroscopy of Peridinin and Allene-Modified Analogues.

Stark absorption spectra of peridinin (Per) and five allene-modified analogues and their angular dependence as a function of an externally applied electric field were measured in methyl methacrylate polymer at 77K. In all cases, the energetically lowest absorption band has a significant change of static dipole moment upon photoexcitation (Δµ). In particular, Per has the largest value of |Δµ|. The angles between Δµ and the transition dipole moment of all the analogues were determined. It is suggested that the allene group in Per plays a key role as the electron donor in the charge transfer process following photoexcitation. The results of MNDO-PSDCI calculations support this idea.

Sub-micro-second time-resolved absorption spectroscopy of a polar carotenoid analogue, 2-(all-trans-retinylidene)indan-1,3-dione; formation of the dication by direct triplet-excited sensitization.

Sub-micro-second time-resolved difference absorption spectra of a polar carotenoid analogue, 2-(all-trans-retinylidene)indan-1,3-dione (hereafter, we will call RetInd), were recorded in tetrahydrofuran at room temperature upon anthracene-sensitized triplet excitation. In addition to the typical Tn <-- T1 absorption spectrum of anthracene followed by that of RetInd, a novel transient species, which peaked at 670 nm, was detected. The lifetime and the population of the 670 nm species was not affected by the presence of oxygen but was quenched by the cation scavenger, triethylamine. Therefore, we have identified this species as a "cation". The transient 670 nm species was not generated by direct photoexcitation of RetInd in the absence of a triplet sensitizer. Therefore, this species was not generated via the T1 species of RetInd but rather via an "invisible state" of RetInd, which is generated by direct energy or electron transfer from T1 anthracene. This proposed pathway was confirmed by a singular-value decomposition followed by a global fitting analysis. The "cation" of RetInd shows vibrational structure in its absorption spectrum, and its lifetime was determined to be 15 micros. Chemical oxidation of RetInd in 2,2,2-trifluoroethanol (dichloromethane) produced a broad absorption band around 880 (1013) nm, which could be transformed into a shoulder around 640 (675) nm upon addition of increasing amounts of the oxidant, FeCl3. The former absorption band can be assigned to a radical cation, while the latter to a dication. Because of the spectral similarity, the 670 nm species can be assigned to the dication, and the "invisible state" is ascribed to the radical cation of RetInd. This is the first direct evidence for the production of a dication of a biological polyene moiety generated in non-halogenated solution following anthracene-sensitized excitation.