Effect of pi-electron conjugation length on the solvent-dependent S(1) lifetime of peridinin

Stereocontrolled Synthesis of 19'-Deoxyperidinin

The stereocontrolled convergent synthesis of 19'-deoxyperidinin, 2, which might be a useful peridinin analog to understand the ICT characteristics, was efficiently achieved by sequential Pd-catalyzed cross-coupling reactions using bidirectionally extensible conjugated C5 olefin segments. The crucial 5(2H)-ylidenedihydrofuran function of 2 was successfully constructed by the Au-catalyzed regio- and stereoselective 5-exo-dig etherification.

Excited state lifetimes and energies of okenone and chlorobactene, exemplary keto and non-keto aryl carotenoids

Photophysical properties of two typical aryl carotenoids, okenone and chlorobactene, were studied with application of femtosecond and microsecond time-resolved absorption spectroscopies.

Characterization of the intramolecular transfer state of marine carotenoid fucoxanthin by femtosecond pump-probe spectroscopy

Fucoxanthin, containing a carbonyl group in conjugation with its polyene backbone, is a naturally occurring pigment in marine organisms and is essential to the photosynthetic light-harvesting function in brown alga and diatom. Fucoxanthin exhibits optical characteristics attributed to an intramolecular charge transfer (ICT) state that arises in polar environments due to the presence of the carbonyl group. In this study, we report the spectroscopic properties of fucoxanthin in methanol (polar and protic solvent) observed by femtosecond pump-probe measurements in the near-infrared region, where transient absorption associated with the optically allowed S2 (1(1)B u (+) ) state and stimulated emission from the strongly coupled S1/ICT state were observed following one-photon excitation to the S2 state. The results showed that the amplitude of the stimulated emission of the S1/ICT state increased with decreasing excitation energy, demonstrating that the fucoxanthin form associated with the lower energy of the steady-state absorption exhibits stronger ICT character.

Effect of Molecular Symmetry on the Spectra and Dynamics of the Intramolecular Charge Transfer (ICT) state of peridinin

The spectroscopic properties and dynamics of the excited states of two different synthetic analogues of peridinin were investigated as a function of solvent polarity using steady-state absorption, fluorescence, and ultrafast time-resolved optical spectroscopy. The analogues are denoted S-1- and S-2-peridinin and differ from naturally occurring peridinin in the location of the lactone ring and its associated carbonyl group, known to be obligatory for the observation of a solvent dependence of the lifetime of the S(1) state of carotenoids. Relative to peridinin, S-1- and S-2-peridinin have their lactone rings two and four carbons more toward the center of the π-electron system of conjugated carbon-carbon double bonds, respectively. The present experimental results show that as the polarity of the solvent increases, the steady-state spectra of the molecules broaden, and the lowest excited state lifetime of S-1-peridinin changes from ∼155 to ∼17 ps which is similar to the magnitude of the effect reported for peridinin. The solvent-induced change in the lowest excited state lifetime of S-2-peridinin is much smaller and changes only from ∼90 to ∼67 ps as the solvent polarity is increased. These results are interpreted in terms of an intramolecular charge transfer (ICT) state that is formed readily in peridinin and S-1-peridinin, but not in S-2-peridinin. Quantum mechanical computations reveal the critical factors required for the formation of the ICT state and the associated solvent-modulated effects on the spectra and dynamics of these molecules and other carbonyl-containing carotenoids and polyenes. The factors are the magnitude and orientation of the ground- and excited-state dipole moments which must be suitable to generate sufficient mixing of the lowest two excited singlet states.

An intramolecular charge transfer state of carbonyl carotenoids: implications for excited state dynamics of apo-carotenals and retinal

Excited state dynamics of two apo-carotenals, retinal and 12'-apo-β-carotenal, were studied by femtosecond transient absorption spectroscopy. We make use of previous knowledge gathered from studies of various carbonyl carotenoids and suggest that to consistently explain the excited-state dynamics of retinal in polar solvents, it is necessary to include an intermolecular charge transfer (ICT) state in the excited state manifold. Coupling of the ICT state to the A(g)(-) state, which occurs in polar solvents, shortens lifetime of the lowest excited state of 12'-apo-β-carotenal from 180 ps in n-hexane to 7.1 ps in methanol. Comparison with a reference molecule lacking the conjugated carbonyl group, 12'-apo-β-carotene, demonstrates the importance of the carbonyl group; no polarity-induced lifetime change is observed and 12'-apo-β-carotene decays to the ground state in 220 ps regardless of solvent polarity. For retinal, we have confirmed the well-known three-state relaxation scheme in n-hexane. Population of the B(u)(+) state decays in <100 fs to the A(g)(-) state, which is quenched in 440 fs by a low-lying nπ* state that decays with a 33 ps time constant to form the retinal triplet state. In methanol, however, the A(g)(-) state is coupled to the ICT state. This coupling prevents population of the nπ* state, which explains the absence of retinal triplet formation in polar solvents. Instead, the coupled A(g)(-)/ICT state decays in 1.6 ps to the ground state. The A(g)(-)/ICT coupling is also evidenced by stimulated emission, which is a characteristic marker of the ICT state in carbonyl carotenoids.

The intramolecular charge transfer state in carbonyl-containing polyenes and carotenoids

Numerous femtosecond time-resolved optical spectroscopic experiments have reported that the lifetime of the low-lying S(1) state of carbonyl-containing polyenes and carotenoids decreases with increasing solvent polarity. The effect becomes even more pronounced as the number of double bonds in the conjugated π-electron system decreases. The effect has been attributed to an intramolecular charge transfer (ICT) state coupled to S(1), but it is still not clear what the precise molecular nature of this state is, and how it is able to modulate the spectral and dynamic properties of polyenes and carotenoids. In this work, we examine the nature of the ICT state in three substituted polyenes: crocetindial, which contains two terminal, symmetrically substituted carbonyl groups in conjugation with the π-electron system, 8,8'-diapocarotene-8'-ol-8-al, which has one terminal conjugated carbonyl group and one hydroxyl group, and 8,8'-diapocarotene-8,8'-diol, which has two terminal, symmetrically positioned, hydroxyl groups but no carbonyls. Femtosecond time-resolved optical spectroscopic experiments on these molecules reveal that only the asymmetrically substituted 8,8'-diapocarotene-8'-ol-8-al exhibits any substantial effect of solvent on the excited state spectra and dynamics. The data are interpreted using molecular orbital theory which shows that the ICT state develops via mixing of the low-lying S(1) (2(1)A(g)-like) and S(2) (1(1)B(u)-like) excited singlet states to form a resultant state that preferentially evolves in polar solvent and exhibits a very large (∼25 D) dipole moment. Molecular dynamics calculations demonstrate that the features of the ICT state are present in ∼20 fs.

Ultrafast Time-resolved Absorption Spectroscopy of Geometric Isomers of Xanthophylls

This paper presents an ultrafast optical spectroscopic investigation of the excited state energies, lifetimes and spectra of specific geometric isomers of neoxanthin, violaxanthin, lutein, and zeaxanthin. All-trans- and 15,15'-cis-beta-carotene were also examined. The spectroscopy was done on molecules purified by HPLC frozen immediately to inhibit isomerization. The spectra were taken at 77 K to maintain the configurations and to provide better spectral resolution than seen at room temperature. The kinetics reveal that for all of the molecules except neoxanthin, the S(1) state lifetime of the cis-isomers is shorter than that of the all-trans isomers. The S(1) excited state energies of all the isomers were determined by recording S(1) --> S(2) transient absorption spectra. The results obtained in this manner at cryogenic temperatures provide an unprecedented level of precision in the measurement of the S(1) energies of these xanthophylls, which are critical components in light-harvesting pigment-protein complexes of green plants.

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.

Spectroscopic investigation of peridinin analogues having different pi-electron conjugated chain lengths: exploring the nature of the intramolecular charge transfer state

The lifetime of the lowest excited singlet (S(1)) state of peridinin and many other carbonyl-containing carotenoids and polyenes has been reported to depend on the polarity of the solvent. This effect has been attributed to the presence of an intramolecular charge transfer (ICT) state in the manifold of excited states for these molecules. The nature of this ICT state has yet to be elucidated. In the present work, steady-state and ultrafast time-resolved optical spectroscopy have been performed on peridinin and three synthetic analogues, C(33)-peridinin, C(35)-peridinin, and C(39)-peridinin, which have different numbers of conjugated carbon-carbon double bonds. Otherwise, the molecules are structurally similar in that they possess the same functional groups. The trends in the positions of the steady-state and transient spectral profiles for this systematic series of molecules allow an assignment of the spectral features to transitions involving the S(0), S(1), S(2), and ICT states. A kinetics analysis reveals the lifetimes of the excited states and the dynamics of their excited state deactivation pathways. The most striking observation in the data is that the lifetime of the ICT state converges to the same value of 10.0 +/- 2.0 ps in the polar solvent, methanol, for all the peridinin analogues, regardless of the extent of pi-electron conjugation. This suggests that the ICT state is highly localized on the lactone ring, which is a common structural feature in all the molecules. The data further suggest that the S(1) and ICT states behave independently and that the ICT state is populated from both S(1) and S(2), the rate and efficiency from S(1) being dependent on the length of the pi-electron chain of the carotenoid and the solvent polarity.

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.