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The endoperoxides of β-carotene (βCar-EPOs) are regarded as main products of the chemical deactivation of1O2by β-carotene, one of the most important antioxidants, following a concerted singlet-singlet reaction. Here we challenge this view by showing that βCar-EPOs are formed in the absence of1O2in a non-concerted triplet-triplet reaction:3O2 + 3β-carotene → βCar-EPOs, in which3β-carotene manifests a strong biradical character. Thus, the reactivity of β-carotene towards oxygen is governed by its excited triplet state. βCar-EPOs, while being stable in the dark, are photochemically labile, and are a rare example of nonaromatic endoperoxides that release1O2, again not in a concerted reaction. Their light-induced breakdown triggers an avalanche of free radicals, which accounts for the pro-oxidant activity of β-carotene and the puzzling swap from its anti- to pro-oxidant features. Furthermore, we show that βCar-EPOs, and carotenoids in general, weakly sensitize1O2. These findings underlie the key role of the triplet state in determining the chemical and photophysical features of β-carotene. They shake up the prevailing models of carotenoid photophysics, the anti-oxidant functioning of β-carotene, and the role of1O2in chemical signaling in biological photosynthetic systems. βCar-EPOs and their degradation products are not markers of1O2and oxidative stress but of the overproduction of extremely hazardous chlorophyll triplets in photosystems. Hence, the chemical signaling of overexcitation of the photosynthetic apparatus is based on a3chlorophyll-3β-carotene relay, rather than on extremely short-lived1O2.

The electronic structure of the low-energy states of 1,4,5,8-naphthalenetetracarboxy diimide ( 1 ) is investigated in terms of CASSCF method. At this level of approximation the Franck–Condon (FC) parameters were evaluated for the 13 totally symmetric modes of ( 1 ) molecule in its dipole-allowed low-energy 1 1 B 2 u and 1 1 B 3 u electronic states. These parameters are then employed to simulate the vibrational structures of absorption and fluorescence spectra of N , N ′ -dibutyl ( 2 ) and N , N ′ -bis-hydroxypropyl ( 3 ) derivatives of naphthyl diimides measured in the region corresponding to the low-energy 1 1 A g → 1 1 B 2 u and 1 1 A g → 1 1 B 3 u overlapping transitions.

Theoretical analysis of the vibrational magnetic moments for the four T 1u fundamental vibrational states of the C 60 fullerene molecule is reported. An attempt to determine these moments is made in terms of a theory which includes both mechanical and vibronic contributions. At this level the theory leads to magnetic moments which are consistent in sign and relative magnitude with those measured for ν 3 =1183 cm −1 and ν 4 =1430 cm −1 vibrations of C 60 using magnetic vibrational circular dichroism. Predictions for the lower energy vibrations at ν 1 =528 cm −1 and ν 2 =577 cm −1 are also provided. The moments of these latter transitions appear to arise almost totally from the mechanical (ground state) terms in the theory while those of higher energy modes have both vibronic and mechanical contributions. For the ν 4 vibration the absolute magnitude of the combined vibronic and nuclear contributions was found to be larger than found in MVCD experiment. This may be due to the severe approximations implicit in the SCFPPP-CI method used.

The theoretical of magnetic vibrational circular dichroism (MVCD) A terms is reported for the degenerate ν9 and ν10 vibrations of sym-C6H3X3 (X=D, Cl, Br) molecules. An attempt to determine the A1/D0 ratios for A′1→E′(ν9) and A′1→E′(ν10) fundamentals is made in terms of a theory which includes both mechanical and vibronic contributions to MVCD and absorption spectra.

A vibronic theory which includes multimode and multistate effects is developed to study magnetic vibrational circular dichroism (MVCD) and absorption spectra for molecular systems with fourfold symmetry. The MVCD and absorption spectra of the e fundamental transition and its combinations with infrared inactive modes are discussed in some detail to demonstrate the importance of electric and magnetic anharmonicity arising from vibronic coupling. The theory is applied to experimental MVCD spectra for the CuTPP molecule.

The vibronic dimer model is formulated in order to study the absorption and circular dichroism (CD) spectra of (1 R ,2 R )-1,2-bis-( N ′ -cyclohexyl-1 ′ ,4 ′ ,5 ′ ,8 ′ -naphthalenetetracarboxydiimido) cyclohexane (NTD) molecule treated as a dimer with the 1,4,5,8-naphthalenetetracarboxydiimide chromophores. To find out the most stable conformation of the NTD molecule the RHF/STO-3G and the density functional B3LYP/3-21G methods are employed. The results of conformational analysis are shown to be entirely consistent with the experiment. In particular it is shown that the CD and absorption spectra of the NTD dimer observed in the excitation region 25,000–35,000 cm −1 are correctly reproduced by the vibronic dimer model applied with the model parameters directly obtained from CASSCF/5π4n5π * computations. In the debatable region the CD of the NTD dimer results from the 1 1 A g →1 1 B 2 u and 1 1 A g →1 1 B 3 u overlapping transitions in the chromophore. Since that later transition is hidden in the vibronic manifold of the former, the effect of the hidden state on the chiroptical properties of the NTD molecule is discussed in some details.

Chiral properties of peridinin-chlorophyll-protein (PCP) light-harvesting complexes are studied in terms of vibronic dimer theory previously applied to study certain structural aspects of α-crustacyanin pigments. On the base of CD spectra it is shown that the peridinin dimer acts as a chiral group in PCP complexes and its geometrical structure is such that the peridinin monomers cannot be coplanar. Certain observations concerning the energy transfer process in PCP complexes in vivo are also made.

In order to gain control over the assembly and functioning of photosynthetic antenna, we have developed methods to manipulate pigment composition of bacterial LH1 complexes via their reconstitution with modi ed bacteriochlorophylls or carotenoids, major photoactive cofactors of these antennae. In the present work we show how the reconstitution and pigment exchange approach is applied to control structural and functional parameters of LH1 and its subunits. The size of the subunits and the energy of the rst excited singlet state can be controlled via the use of detergent while the thermodynamics of LH1 formation can be modi ed using carotenoids and/or a co-solvent. Carotenoids a ect the e ciency of the intracomplex energy transfer, while the replacement of native bacteriochlorophyll a with its Ni-substituted analog allows one to control the excited state properties of LH1. These results show that LH1 is a very promising model system applicable for the design of bio-inspired device performing solar energy conversion.