The excited-state dynamics, luminescence, and redox properties of a series of hexanuclear molybdenum cluster complexes, (nBu4N)2[Mo6X14] and (nBu4N)2[Mo6X8(CF3COO)6] (X = Cl, Br, or I), were investigated. Substitution of the apical halogen ligands for the trifluoroacetate ligands increased the oxidation potentials and induced a blue shift in the absorption and luminescence bands as well as a considerable increase in the luminescence quantum yields for heavy inner ligands. Time-resolved transient absorption measurements showed that the intersystem crossing from the excited singlet states is ultrafast with time constants ranging between <120 fs and 1.68 ps and leads to hot triplet states. The following cooling occurred at a ps time scale and was assigned to electronic redistribution within the emissive triplet state sublevels. The formation of singlet oxygen, O2((1)Δg), suggested earlier on the basis of photooxidation experiments for some complexes, was revised by direct measurements of O2((1)Δg) phosphorescence. We showed the effects of the attached ligands on key physico-chemical and photophysical parameters of the title complexes. The synthesis and structural characterisation of a new cluster complex, (nBu4N)2[Mo6Br8(CF3COO)6], completed the series. Our results demonstrated that the complexes with heavy inner ligands (Br, I) and apical trifluoroacetate ligands were photochemically and electrochemically stable, highly luminescent, and good sensitisers of O2((1)Δg).

Effects of introducing a carbonyl group and its position in the conjugated system of carotenoids were studied by means of femtosecond time-resolved spectroscopy. We have compared four naturally occurring carotenoids with comparable structures, beta-carotene, echinenone, canthaxanthin and rhodoxanthin, which differ in the number and position of conjugated carbonyl group(s). The S(1) lifetime is systematically shorter upon increasing the number of the conjugated C=O groups, yielding 9.3 ps (for beta-carotene, no C=O group), 6.2 ps (echinenone, one C=O group), 4.5 ps (canthaxanthin, two C=O groups), and 1.1 ps (rhodoxanthin, two C=O groups in s-trans configuration). Except for slight polarity-induced broadening of absorption and transient absorption spectra, no other polarity effects, such as shortening of the S(1) lifetimes or transient features attributable to intramolecular charge transfer (ICT) state bands, were observed. The absence of these polarity-induced features is explained as due to the long conjugated chain (no lifetime shortening), and the symmetrical position of the carbonyl groups (no ICT bands). On the other hand, all carotenoids exhibit the characteristic spectral band attributed to the S* state, and for the two longest carotenoids, canthaxanthin and rhodoxanthin, decay of the S* state is markedly longer than that of the S(1) state. Moreover, it is shown that the S* state is preferentially populated for a specific subset of ground state conformations, underlining the importance of carotenoid conformation in S* state formation.

Numerous femtosecond time-resolved optical spectroscopic experiments have reported that the lifetime of the low-lying S1 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 S1, 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 S1 (21Ag-like) and S2 (11Bu-like) excited singlet states to form a resultant state that preferentially evolves in polar solvent and exhibits a very large (~25D) dipole moment. Molecular dynamics calculations demonstrate that the features of the ICT state are present in ~20 fs.

Background:The plant carotenoid zeaxanthin is accumulated under excess light. Results: Zeaxanthin induces a red shift in the carotenoid triplet excited state spectrum and reveals a higher efficiency in controlling chlorophyll triplet formation. Conclusion: Binding of zeaxanthin to specific proteins modulates the yield of dangerous chlorophyll excited states and protects photosynthesis from over-excitation. Significance: Functional dissection of zeaxanthin-dependent photoprotective mechanisms is crucial for understanding how plants avoid photoinhibition.

A quenching mechanism mediated by the orange carotenoid protein (OCP) is one of the ways cyanobacteria protect themselves against photooxidative stress. Here, we present a femtosecond spectroscopic study comparing OCP and RCP (red carotenoid protein) samples binding different carotenoids. We confirmed significant changes in carotenoid configuration upon OCP activation reported by Leverenz et al. (Science 348:1463-1466. doi: 10.1126/science.aaa7234 , 2015) by comparing the transient spectra of OCP and RCP. The most important marker of these changes was the magnitude of the transient signal associated with the carotenoid intramolecular charge-transfer (ICT) state. While OCP with canthaxanthin exhibited a weak ICT signal, it increased significantly for canthaxanthin bound to RCP. On the contrary, a strong ICT signal was recorded in OCP binding echinenone excited at the red edge of the absorption spectrum. Because the carbonyl oxygen responsible for the appearance of the ICT signal is located at the end rings of both carotenoids, the magnitude of the ICT signal can be used to estimate the torsion angles of the end rings. Application of two different excitation wavelengths to study OCP demonstrated that the OCP sample contains two spectroscopically distinct populations, none of which is corresponding to the photoactivated product of OCP.