Mariusz Pilch

Magnetic circular dichroism (MCD) study of low-energy 1Ag→1T1u transitions in fullerene

Abstract The SCFPPP-CI method has been applied to calculate A 1 / D 0 and B 0 / D 0 MCD characteristics for several 1 A g → 1 T 1u transitions in the C 60 cage isomer. The calculation performed in the full CI-1 scheme have indicated the presence of large magnetic moments in the five low-energy T 1u states. The sign and magnitude of the A 1 / D 0 ratio for the lowest energy transition was found to be in agreement with experiment. The calculated A 1 / D 0 ratio for the most intense transition was found to be at variance with the published interpretation of experimental data. We argue that this may be due to the neglect of a close-lying higher energy state in the moment analysis of the experimental spectra.

Effects of Molecular Symmetry on the Electronic Transitions in Carotenoids.

The aim of this work is the verification of symmetry effects on the electronic absorption spectra of carotenoids. The symmetry breaking in cis-β-carotenes and in carotenoids with nonlinear π-electron system is of virtually no effect on the dark transitions in these pigments, in spite of the loss of the inversion center and evident changes in their electronic structure. In the cis isomers, the S2 state couples with the higher excited states and the extent of this coupling depends on the position of the cis bend. A confrontation of symmetry properties of carotenoids with their electronic absorption and IR and Raman spectra shows that they belong to the C1 or C2 but not the C2h symmetry group, as commonly assumed. In these realistic symmetries all the electronic transitions are symmetry-allowed and the absence of some transitions, such as the dark S0 → S1 transition, must have another physical origin. Most likely it is a severe deformation of the carotenoid molecule in the S1 state, unachievable directly from the ground state, which means that the Franck-Condon factors for a vertical S0 → S1 transition are negligible because the final state is massively displaced along the vibrational coordinates. The implications of our findings have an impact on the understanding of the photophysics and functioning of carotenoids.

Tuning the Thermodynamics of Association of Transmembrane Helices

Modular photosynthetic LH1 complex is applied as a model system to investigate the thermodynamics of a self-assembling membrane protein and the effects of cosolvents and cofactor (carotenoid) on the process. Native chromophores of LH1, bacteriochlorophyll, and carotenoid are excellent intrinsic spectroscopic reporter molecules. Their presence allows us to follow the association of transmembrane helices of LH1, without the use of any external markers, by electronic absorption/emission and circular dichroism. Furthermore, the assembly correctness can be monitored by the intracomplex energy transfer. Both the cosolvent and carotenoid markedly affect DeltaH degrees and DeltaS degrees associated with the complex formation in detergent, but the driving force of the process remains almost constant due to an efficient enthalpy-entropy compensation in the system. In the absence of cosolvent and cofactor, the energy of interactions between transmembrane helices in LH1 equals -580 kJ/mol. DeltaH degrees drastically increases upon the addition of acetone (-1160 kJ/mol) and carotenoid (-1900 kJ/mol), whereas DeltaS degrees lowers from +1.5 kJ/mol.K to -0.4 kJ/mol.K and to -2.6 kJ/mol.K, respectively. The stabilization of the ensemble by cofactor seems to be due to the pi-pi stacking of aromatic residues of LH1 polypeptides with the carotenoid pi-electron system. The cosolvent, lowering the medium permittivity and thus enhancing helix-helix interactions, has an ordering effect on the system (DeltaS degrees<0). This effect of cosolvent on DeltaH degrees and DeltaS degrees of association of transmembrane helices is relevant for crystallization of membrane proteins, as it explains in thermodynamic terms the action of amphiphiles used for crystallization of membrane proteins in the micellar phase.

Molecular symmetry determines the mechanism of a very efficient ultrafast excitation-to-heat conversion in Ni-substituted chlorophylls.

In the Ni-substituted chlorophylls, an ultrafast (<60 fs) deactivation channel is created, which is not present in Ni-porphyrins. This observation prompted us to investigate in detail the mechanism of excitation-to-heat conversion in Ni-substituted chlorophylls, experimentally, using time-resolved laser-induced optoacoustic spectroscopy, and theoretically, using group theory approach. The Ni-substituted chlorophylls show exceptional photostability and the optoacoustic measurements confirm the prompt and very efficient (100%) excitation-into-heat conversion in these complexes. Considering their excellent spectral properties and the loss-free excitation-into-heat conversion they are likely to become a new class of versatile photocalorimetric references. The curious features of the Ni-substituted chlorophylls originate from the symmetry of a ligand field created in the central cavity. The central N-Ni(2+) bonds, formed via the donation of two electrons from each of the sp(2) orbitals of two central nitrogens to an empty [Formula: see text] hybrid centered on Ni(2+), have a considerable covalent character. The extreme rate of excited state relaxation is then not due to a ladder of the metal centered d-states, often invoked in metalloporphyrins, but seems to result from a peculiar topology of the potential energy surface (a saddle-shaped crossing) due to the covalent character of the N-Ni(2+) bonds. This is confirmed by a strong 0→0 character of electronic transitions in these complexes indicating a similarity of their equilibrium geometries in the ground (S(0)) and the excited states (both Q(X) and Q(Y)). The excitation energy is very efficiently converted into molecular vibrations and dissipated as heat, involving the central Ni(2+). These Ni-substituted pigments pose a fine exemplification of symmetry control over properties of excited states of transition metal complexes.

The absorption and fluorescence study of 1,4,5,8-naphthalenetetracarboxy diimides in their low-energy 11B2u and 11B3u electronic states. The Franck–Condon analysis in terms of CASSCF method

Abstract 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.

High-pressure and theoretical studies reveal significant differences in the electronic structure and bonding of magnesium, zinc, and nickel ions in metalloporphyrinoids.

High pressure in combination with optical spectroscopy was used to gain insights into the interactions between Mg(2+), Zn(2+), and Ni(2+) ions and macrocyclic ligands of porphyrinoid type. In parallel, the central metal ion-macrocycle bonding was investigated using theoretical approaches. The symmetry properties of the orbitals participating in this bonding were analyzed, and pigment geometries and pressure/ligation effects were computed within DFT. Bacteriopheophytin a was applied as both a model chelator and a highly specific spectroscopic probe. The analysis of solvent and pressure effects on the spectral properties of the model Mg(2+), Zn(2+), and Ni(2+) complexes with bacteriopheophytin a shows that various chemical bonds are formed in the central pocket, depending on the valence configuration of the central metal ion. In addition, the character of this bonding depends on symmetry of the macrocyclic system. Since in most cases it is not coordinative bonding, these results challenge the conventional view of metal ion bonding in such complexes. In (labile) complexes with the main group metals, the metal ion-macrocycle interaction is mostly electrostatic. Significantly, water molecules are not preferred as a second axial ligand in such complexes, mainly due to the entropic constraints. The metal ions with a closed d shell may form (stable) complexes with the macrocycle via classical coordination bonds, engaging their p and s orbitals. Transition metals, due to the unfilled d shell, do form much more stable complexes, because of strong bonding via both coordination and covalent interactions. These conclusions are confirmed by DFT computations and theoretical considerations, which altogether provide the basis to propose a consistent and general mechanism of how the central metal ion and its interactions with the core nitrogens govern the physicochemical properties of metalloporphyrinoids.

The circular dichroism (CD) and absorption studies of 1,4,5,8-naphthalene tetracarboxydiimide dimer in terms of vibronic coupling theory

Abstract 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.

Circular dichroism (CD) study of peridinin–chlorophyll a protein (PCP) complexes from marine dinoflagellate algae The tetramer approach

The absorption and circular dichroism spectra of PCP (peridinin–chlorophyll a protein) light-harvesting complexes from Glenodiniumsp. and Gonyaulaxpolyedra are studied in terms of vibronic coupling theory. Analysis based on a tetramer model allows the determination of the relative positions of peridinin molecules in the PCP complexes studied. The tetrameric arrangementof four peridinins around the chlorophyll a is suggested to favour the efficient transfer of energy from the peridinin to thechlorophyll. The simple arguments presented suggest that the energy transfer from peridin to chlorophyll might occur from theΨ1± states of the peridinin tetramer to the Bx, and By states of the chlorophyll a.

Theoretical analysis of magnetic vibrational circular dichroism for 1,3,5-trihalobenzene derivatives

Abstract 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.

Zeeman effect in T1u symmetry vibrational states of C60 fullerene

Abstract 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.