Methodological CASPT2 study of the valence excited states of an iron-porphyrin complex

The singlet valence excited states of an iron-porphyrin-pyrazine-carbonyl complex are investigated up to the Soret band (about 3 eV) using multi-state complete active space with perturbation at the second order (MS-CASPT2). This complex is a model for the active site of carboxy-hemoglobin/myoglobin. The spectrum of the excited states is rather dense, comprising states of different nature: d→π* transitions, d→d states, π→π* excitations of the porphyrin, and doubly excited states involving simultaneous intra-porphyrin π→π* and d→d transitions. Specific features of the MS-CASPT2 method are investigated. The effect of varying the number of roots in the state average calculation is quantified as well as the consequence of targeted modifications of the active space. The effect of inclusion of standard ionization potential-electron affinity (IPEA) shift in the perturbation treatment is also investigated.     

Photochemistry and photophysics of the endoperoxide of mesodiphenylhelianthrene: a contribution to the localization of the S1(π*σ*) state of aromatic endoperoxides

The endoperoxide of mesodiphenylhelianthrene MDHPO has been studied in detail with respect to fluorescence and photo-induced rearrangement. MDHPO proved to be non-fluorescent, although its absorption spectrum is dominated at the low energy side by a strong ππ* band with a maximum at 429.5 nm. Irradiation of that band effects rearrangement to the corresponding diepoxide MDHDO, a reaction typical for S(1)(π*σ*) excited endoperoxides (EPOs). The absorption spectrum of the product MDHDO is blue shifted by only 3.5 nm. MDHDO has the same extended planar aromatic system like its precursor MDHPO, but MDHDO fluoresces strongly. These results set the excitation energy of the S(1)(π*σ*) state of MDHPO to ≤23?000 cm(-1), which is considered to be a generally realistic value of the S(1)(π*σ*) state energy of aromatic EPOs. The main reaction of S(1)(π*σ*) excited MDHPO is, however, chemical deactivation to ground state MDHPO via an oxygen biradical. The sequence of O-O bond opening and closing is the general way of repopulation of the S(0) state of aromatic EPOs from S(1)(π*σ*) excited states.     

Singlet oxygen production by pyrano and furano 1,4-naphthoquinones in non-aqueous medium

The influence of ring size on the photobehaviour of condensed 1,4-naphthoquinone systems, such as pyrano- and furano-derivatives (1 and 2, respectively) has been investigated. The absorption spectra for both families of naphthoquinones reveal clear differences; in the case of 2 they extend to longer wavelengths. A solvatochromic red shift in polar solvents is consistent with the π,π* character of the S(0)→ S(1) electronic transition in all cases. Theoretical (B3LYP) analysis of the HOMO and LUMO Kohn-Sham molecular orbitals of the S(0) state indicates that they are π and π* in nature, consistent with the experimental observation. A systematic study on the efficiency of singlet oxygen generation by these 1,4-naphthoquinones is presented, and values larger than 0.7 were found in every case. In accordance with these results, laser flash photolysis of deoxygenated acetonitrile solutions led to the formation of detectable triplet transient species with absorptions at 390 and 450 nm (1) and at 370 nm (2), with φ(ISC) close to 1. Additionally, the calculated energies for the T(1) states relative to the S(0) states at UB3LYP/6-311++G** are ca. 47 kcal mol(-1) for 1 and 43 kcal mol(-1) for 2. A comparison of the geometrical parameters for the S(0) and T(1) states reveals a marked difference with respect to the arrangement of the exocyclic phenyl ring whilst a comparison of electronic parameters revealed the change from a quinone structure to a di-dehydroquinone diradical structure.     

Synthesis, structure, photoluminescence and theoretical studies of an In(III) complex with 2-(2′-hydroxylphenyl)benzoxazole

The synthesis and characterization of [In(pbx)₃] (1) (Hpbx = 2-(2′-hydroxylphenyl)benzoxazole) are presented. The ground and low lying excited electronic states in 1 are studied using density functional theory level (DFT). The optimized geometry is compared to the experimentally observed structure. Time-dependent density functional theory level (TDDFT) is employed to investigate the excited singlet states. The calculated energies of the low lying singlet states in 1 are in considerable agreement with the experimental data. All the low lying transitions are categorized as π → π∗ ligand-to-ligand charge transfer transitions (LLCT) in nature. The emissive state of 1 is assigned as a singlet metal-perturbed π → π∗ ligand-to-ligand charge transfer transition (LLCT).     

On the photophysics of metallophthalocyanine-based photothermal sensitizers: synergism between theory and experiment

A comprehensive understanding of the factors governing the efficiency of metallophthalocyanine-based photothermal sensitizers requires the knowledge of their excited-state dynamics. This can only be properly gained when the nature and energy of the excited states (often spectroscopically silent) lying between the photogenerated state and the ground state are known. Here the excited state deactivation mechanism of two very promising metallophthalocyanine-based photothermal sensitizers, NiPc(OBu)(8) and NiNc(OBu)(8), is reviewed. It is shown that time dependent density functional theory (TDDFT) methods are capable to provide reliable information on the nature and energies of the low-lying excited states along the relaxation pathways. TDDFT calculations and ultrafast experiments consistently show that benzoannulation of the Pc ring modifies the photodeactivation mechanism of the photogenerated S(1)(pi,pi*) state by inducing substantial changes in the relative energies of the excited states lying between the S(1)(pi,pi*) state and the ground state.     

Probing the geometries,relative stabilities,and electronic properties of neutral and anionic AgnSm (n + m ≤ 7) clusters

The vertical excitation energies of 3,4-dicyano-6-methoxy and 3,4-dicyano-6,7-dimethoxy carbostyril have been computed with different approximations for the time-dependent density functional theory (TD-DFT) procedure and with different implementations of the continuum solvation model COSMO. Different DFT functionals were tested in TD-DFT and Tamm-Dancoff approximations (TDA) for the excitation energies in the gas phase. TDA-B3LYP showed the best agreement with the experimental data. Then TDA-B3LYP computations were performed combined with the COSMO model of solvation comparing a linear response (LR) and a post-configuration interaction (CI) implementation of the fast solvent reorganization. The post-CI solvent model overestimates the π→π* transitions and strongly underestimates the n→π* transition. The TDA approximation in combination with the linear response implementation of the COSMO solvation model perfectly computes the experimental results. TDA-LR is the most reliable method for the computation of the vertical excitation energies in a solvent. Comparison with explicit solvent calculations shows there is only a minor effect on the energies of the electronic interaction of the solute with the solvent.     

Ab initio calculations of molecular properties of low–lying electronic states of 2–cyclopenten–1–one – link with biological activity

Geometries, vibrational frequencies, vertical and adiabatic excitation energies, dipole moments and dipole polarizabilities of the ground and the three lowest electronic excited states, S(1)(n, π (*)), T(1)(n, π (*)), and T(2)(π, π (*)) of the 2-cyclopenten-1-one molecule (2CP) were calculated at the CCSD and CCSD(T) levels of approximation. Our results indicate that two triplets T(1)(n, π (*)) and T(2)(π, π (*)) are lying very close each to other, while the singlet S(1)(n, π (*)) is well above them. There are dramatic changes in dipole moments for (n, π (*)) excited states in respect to the ground state. On the other hand the T(2)(π, π (*)) state has a similar dipole moment as the ground state. These changes can be interpreted within the MO picture using electrostatic potential maps and changes in model IR spectra. Our CCSD(T) dipole moment data for the ground state and almost isoenergetic triplets T(1)(n, π (*)) and T(2)(π, π (*)) are 1.469?a.u., 0.551?a.u., and 1.124?a.u., respectively. Dipole polarizabilities of investigated excited states are much less affected by electron excitations than dipole moments. These are the first dipole moment and polarizability data of 2CP in the literature. The changes of molecular properties upon excitation to S(1)(n, π (*)) and T(1)(n, π (*)) correlate with the experimental data on the biological activity of 2CP related to the α, β-unsaturated carbonyl group.     

Towards elucidating the energy of the first excited singlet state of xanthophyll cycle pigments by X-ray absorption spectroscopy

The first excited singlet state (S(1)) of carotenoids (also termed 2A(g)(-)) plays a key role in photosynthetic excitation energy transfer due to its close proximity to the S(1) (Q(y)) level of chlorophylls. The determination of carotenoid 2A(g)(-) energies by optical techniques is difficult; transitions from the ground state (S(0), 1A(g)(-)) to the 2A(g)(-) state are forbidden ("optically dark") due to parity (g <-- //--> g) as well as pseudo-parity selection rules (- <-- //--> -). Of particular interest are S(1) energies of the so-called xanthophyll-cycle pigments (violaxanthin, antheraxanthin and zeaxanthin) due to their involvement in photoprotection in plants. Previous determinations of S(1) energies of violaxanthin and zeaxanthin by different spectroscopic techniques vary considerably. Here we present an alternative approach towards elucidation of the optically dark states of xanthophylls by near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The indication of at least one pi* energy level (about 0.5 eV below the lowest 1B(u)(+) vibronic sublevel) has been found for zeaxanthin. Present limitations and future improvements of NEXAFS to study optically dark states of carotenoids are discussed. NEXAFS combined with simultaneous optical pumping will further aid the investigation of these otherwise hardly accessible states.     

Photophysics of pyrene-labelled compounds of biophysical interest.

The effects of the chemical constitution and structure of the substituent on the excited state dynamics of several model fluorescent pyrene-labelled molecules of biophysical interest have been examined. Nine new 1-substituted pyrenyl compounds, Py-NH-CO-C2H5, Py-NH-CO-Leu-Boc, Py-CH2-NH-CO-C2H5, Py-CH2-NH-CO-Leu-Boc, Py-CO-NH-C3H7, Py-CO-NH-Leu-OMe, Py-CH2-CO-NH-C3H7, Py-CH2-CO-NH-Leu-OMe and Py-C3H6-CO-NH-Leu-OMe, have been synthesized and their electronic spectra, fluorescence quantum yields and excited state lifetimes measured. These data have been used to calculate the radiative, kr, and non-radiative decay constants of their S1 states and the values of these constants correlated with the structures of the tethers. Non-radiative S1 decay rates (mainly intersystem crossing to T1) vary in parallel with the radiative rates so that the excited state lifetimes and radiative rate constants change considerably with the structure of the substituent whereas the quantum yields of fluorescence do not. An excellent correlation between [epsilon]max of the S1-S0 transition and either kr or the excited state lifetime is observed as long as no additional intermolecular or intramolecular excited state decay process of significant rate competes with the 'normal' radiative and non-radiative (ISC) decay processes of the pyrenyl chromophore. This correlation may have predictive value. Rates of bimolecular quenching of the S1 states of these molecules by molecular oxygen have been measured. The quenching process is diffusion-controlled with a spin statistical factor of 1, indicating that the S1-T1 electronic energy spacings of all the derivatives exceed the O2(1Deltag-3Sigmag-) electronic excitation energy of ca. 1 eV.     

Photoexcited triplet states of new UV absorbers, cinnamic acid 2-methylphenyl esters

Phosphorescence spectra of nonphosphorescent or very weakly phosphorescent new UV absorbers, 2-methylphenyl cinnamate (MePC), 2-methylphenyl 4-methoxycinnamate (MePMC) and 2-methylphenyl 4-ethoxycinnamate (MePEC) have been observed by using external heavy atom effects of ethyl iodide in ethanol at 77 K. The lowest excited triplet (T(1)) energies of these new UV absorbers are lower than those of a widely used UV-A absorber, 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM), in both keto and enol forms. The intermolecular triplet-triplet energy transfer from photolabile BM-DBM to MePMC was observed by measuring the time-resolved phosphorescence spectra. Electron paramagnetic resonance spectra have been observed for the T(1) states of these new UV absorbers in ethanol at 77 K by using benzophenone as a triplet sensitizer. The observed T(1) lifetimes, zero-field splitting (ZFS) parameters and molecular orbital calculations of the ZFS parameters suggest that T(1) states of these new UV absorbers posses mainly (3)ππ* character. The deactivation processes of the lowest excited singlet (S(1)) states are predominantly fluorescence and internal conversion to the ground (G) states in MePMC and MePEC, while the main deactivation process of the S(1) state of MePC is internal conversion to the G state. The molar absorption coefficients of MePMC and MePEC in the UV-A and UV-B regions are larger than that of most widely used UV-B absorber, octyl methoxycinnamate.     

Elucidating the role of the proximal cysteine hydrogen-bonding network in ferric cytochrome P450cam and corresponding mutants using magnetic circular dichroism spectroscopy

Although extensive research has been performed on various cytochrome P450s, especially Cyt P450cam, there is much to be learned about the mechanism of how its functional unit, a heme b ligated by an axial cysteine, is finely tuned for catalysis by its second coordination sphere. Here we study how the hydrogen-bonding network affects the proximal cysteine and the Fe-S(Cys) bond in ferric Cyt P450cam. This is accomplished using low-temperature magnetic circular dichroism (MCD) spectroscopy on wild-type (wt) Cyt P450cam and on the mutants Q360P (pure ferric high-spin at low temperature) and L358P where the "Cys pocket" has been altered (by removing amino acids involved in the hydrogen-bonding network), and Y96W (pure ferric low-spin). The MCD spectrum of Q360P reveals fourteen electronic transitions between 15200 and 31050 cm(-1). Variable-temperature variable-field (VTVH) saturation curves were used to determine the polarizations of these electronic transitions with respect to in-plane (xy) and out-of-plane (z) polarization relative to the heme. The polarizations, oscillator strengths, and TD-DFT calculations were then used to assign the observed electronic transitions. In the lower energy region, prominent bands at 15909 and 16919 cm(-1) correspond to porphyrin (P) → Fe charge transfer (CT) transitions. The band at 17881 cm(-1) has distinct sulfur S(π) → Fe CT contributions. The Q band is observed as a pseudo A-term (derivative shape) at 18604 and 19539 cm(-1). In the case of the Soret band, the negative component of the expected pseudo A-term is split into two features due to mixing with another π → π* and potentially a P → Fe CT excited state. The resulting three features are observed at 23731, 24859, and 25618 cm(-1). Most importantly, the broad, prominent band at 28570 cm(-1) is assigned to the S(σ) → Fe CT transition, whose intensity is generated through a multitude of CT transitions with strong iron character. For wt, Q360P, and L358P, this band occurs at 28724, 28570, and 28620 cm(-1), respectively. The small shift of this feature upon altering the hydrogen bonds to the proximal cysteine indicates that the role of the Cys pocket is not primarily for electronic fine-tuning of the sulfur donor strength but is more for stabilizing the proximal thiolate against external reactants (NO, O(2), H(3)O(+)), and for properly positioning cysteine to coordinate to the iron center. This aspect is discussed in detail.     

Interpretation of hypochromic and hyperchromic intensity changes in the Raman spectra of polypeptides and polynucleotides undergoing transition.

The hyperchromic and hypochromic changes in the intensity of the amide-I and amide-III lines of polypeptides and certain ring vibrations of the bases of polynucleotides are shown to be related to similar changes in the lower energy uv absorption bands. The selection rules strictly limit the pairs of excited electronic states that can contribute to the elements of the polarizability matrix. An energy-dependent term in this equation weights the contribution of the pairs of electronic transitions in favor of those involving the lower energy transitions. For both polypeptides and polynucleotides, there is a large hypochromic inensity change in the first π → π* exciton band upon the coil-to-helix transition. Through the selection rules, certain conformationally sensitive Raman lines are shown to derive their intensity predominantly from this band and hence also display hypochromism. Again, through an application of the selection rules, certain Raman lines can be demonstrated to depend predominantly for their intensity upon the n → π* transition, and consequently have the opposite hyperchromic intensity change upon the same conformational transition.     

The very early events following photoexcitation of carotenoids

The recent availability of laser pulses with 10-20 fs duration, tunable throughout the visible and near infrared wavelengths, has facilitated the investigation, with unprecedented temporal resolution, into the very early events of energy relaxation in carotenoids [Science 298 (2002) 2395; Synth. Metals 139 (2003) 893]. This has enabled us to clearly demonstrate the existence of an additional intermediate state, Sx, lying between the S2 (1(1)Bu+) and S1 (2(1)Ag-) states. In addition, by applying time-resolved stimulated Raman spectroscopy with femtosecond time resolution, it has also been shown that vibrational relaxation in electronic excited states plays an important role in these interconversions. In this mini-review, we describe briefly the current understanding of Sx and the other intermediate excited states that can be formed by relaxation from S2, mainly focusing attention on the above two topics. Emphasis is also placed on some of the major remaining unsolved issues in carotenoid photochemistry.     

Phototautomerism of the GC base pair of DNA

Electronic spectra and ground and excited state electronic structures of normal G and rare tautomeric G1z.sbnd;C1 base pairs as well as of the individual rare tautomeric bases (purines and pyrimidines) have been studied using the VE-PPP molecular orbital method. The nature and consequences of the lowest energy purine-localized and purine to pyrimidine charge transfer type π?π1 singlet excitations of the base pairs have been investigated. The results indicate that in these excited states, particularly in the charge transfer excited state, the probability for the GC base pair to change over to G1C1 would be larger than in the ground state. The likeliness of the relevance of results obtained experimentally by other workers from the study of a model system to the GC base pair is discussed.     

Circular dichroism of axially chiral methaqualone, 3-Aryl-2-mercapto- and 3-aryl-2-alkylthio-4(3H)-quinazolinones: conformational dependence of CD and assignment of absolute configuration

Rotational strengths calculated on the basis of quantum-mechanically obtained minimum energy geometries were used to determine the absolute configurations of axially chiral 3-aryl-4(3H)-quinazolinones from the sign of the observed Cotton effects (CEs). For the spectral data, CNDO/S calculations were used; for the geometries, ab initio (RHF/6-31G) and semiempirical (AM1) theories were used. Oscillator and rotational strengths of all excited states down to 200 nm were compared to experimental absorption and circular dichroism (CD) data. It was found that the sign of the ¹L_b Cotton effects obtained for the enantiomers of methaqualone and derivatives of 3-aryl-2-alkylthio-4(3H)-quinazolinones can be correlated unambiguously with the absolute configuration. Furthermore, the sign of the Cotton effect of the π-π* transition of the thiocarbonyl chromophore of 3-aryl-2-mercapto-4(3H)-quinazolinones is suitable for a successful stereochemical correlation. Chirality 10:253–261, 1998. © 1998 Wiley-Liss, Inc.     

Infrared and electronic absorption spectra of n-butyronitrile and its ions using Møller Plesset method

We report theoretical infrared and electronic absorption spectra of gauche and anti conformers of n-butyronitrile, their ions and 2-methylpropanenitrile isomer of n-butyronitrile. The coupled cluster theory (CCSD) and second order Møller-Plesset perturbation (MP2) theory with TZVP basis set are used for the study. Vibrational frequencies of gauche and anti conformers of neutral n-butyronitrile at MP2/TZVP and CCSD/TZVP levels are in agreement with the experimental determinations. Rotational and distortion constants are also in good agreement with the available experimental values. Time dependent density functional theory is used to study the electronic absorption spectra of gauche and anti conformers, their ions and an isomer of butyronitrile. All the electronic transitions of gauche and anti conformers of neutral n-butyronitrile and 2-methylpropanenitrile are σ→σ* transitions whereas ions of n-butyronitrile show both σ→σ* as well as π→π* transitions in vacuum UV, far UV and visible regions. This study helps in detection of neutral gauche and anti conformer and their ions in interstellar medium.     

An experimental and theoretical study of the electronic spectra of tetraethylammonium [bis(1,3-dithiole-2-thione-4,5-dithiolato)zincate(II)], [NEt4]2[Zn(dmit)2], and tetraethylammonium [bis(1,3-dithiole-2-one-4,5-dithiolato)zincate(II)], [NEt4]2[Zn(dmio)2]

Metal-dmit complexes and related compounds have been the object of intense study in the last decade. Despite such efforts on the study of its structural properties, very few attempts have been made to the spectroscopic study of these metal complexes. Experimental reports of its infrared, Raman and UV–Vis spectra present the main spectroscopic features, however, many details of the electronic structure have still to be fully investigated and inconsistent assignments are found in the literature. This work presents a detailed analysis of the UV–Vis spectra of the zinc-dmit, [Zn(dmit)₂]⁻², and the zinc-dmio complex, [Zn(dmio)₂]⁻². The experimental spectrum was deconvoluted and analysed with several theoretical methodologies including ab initio CI calculations, ab initio TD and zindo semi-empirical methods. The results confirm the multi-configuration nature of several excited states and the calculated results were concordant for several transitions. The results lead to a new assignment of the 457 nm band in the [Zn(dmit)₂]⁻² as π(pS_m) → π*CS band. In the metal-dmio, the sulfur substitution by oxygen results in a larger HOMO–LUMO gap and a change in the nature of the frontier orbitals. As the first transition we found, for the dmit compound, a high-intensity π → π*CS while for the zinc-dmio, a low-intensity π → σ*C–S transition.     

Studying excited states of proteins by NMR spectroscopy.

Protein structure is inherently dynamic, with function often predicated on excursions from low to higher energy conformations. For example, X-ray studies of a cavity mutant of T4 lysozyme, L99A, show that the cavity is sterically inaccessible to ligand, yet the protein is able to bind substituted benzenes rapidly. We have used novel relaxation dispersion NMR techniques to kinetically and thermodynamically characterize a transition between a highly populated (97%, 25 degrees C) ground state conformation and an excited state that is 2.0 kcal mol(-1) higher in free energy. A temperature-dependent study of the rates of interconversion between ground and excited states allows the separation of the free energy change into enthalpic (Delta H = 7.1 kcal mol(-1)) and entropic (T Delta S = 5.1 kcal mol(-1), 25 degrees C) components. The residues involved cluster about the cavity, providing evidence that the excited state facilitates ligand entry.     

Towards elucidating the energy of the first excited singlet state of xanthophyll cycle pigments by X-ray absorption spectroscopy

The first excited singlet state (S₁) of carotenoids (also termed 2A_g⁻) plays a key role in photosynthetic excitation energy transfer due to its close proximity to the S₁ (Q_y) level of chlorophylls. The determination of carotenoid 2A_g⁻ energies by optical techniques is difficult; transitions from the ground state (S₀, 1A_g⁻) to the 2A_g⁻ state are forbidden (“optically dark”) due to parity (g ← //→ g) as well as pseudo-parity selection rules (− ← //→ −). Of particular interest are S₁ energies of the so-called xanthophyll-cycle pigments (violaxanthin, antheraxanthin and zeaxanthin) due to their involvement in photoprotection in plants. Previous determinations of S₁ energies of violaxanthin and zeaxanthin by different spectroscopic techniques vary considerably. Here we present an alternative approach towards elucidation of the optically dark states of xanthophylls by near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The indication of at least one π* energy level (about 0.5 eV below the lowest 1B_u⁺ vibronic sublevel) has been found for zeaxanthin. Present limitations and future improvements of NEXAFS to study optically dark states of carotenoids are discussed. NEXAFS combined with simultaneous optical pumping will further aid the investigation of these otherwise hardly accessible states.     

Time dependent – density functional theory characterization of organic dyes for dye-sensitized solar cells

We aim at providing better insight into the parameters that govern the intramolecular charge transfer (ICT) and photo-injection processes in dyes for dye-sensitised solar cells (DSSC). Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations are utilized to study the geometry, electronic structure, electrostatic potential (ESP) and absorption spectrum, for a representative donor-π bridge-acceptor (D–π–A) dye for DSSC. The coplanar geometry of the dye (D1) facilitates strong conjugation and considerable delocalization originating the π CT interaction from donor to acceptor orbitals and the hyper-conjugative interactions involving Rydberg states. A model simulating the adsorption of the dye on the TiO₂ surface is utilized to estimate binding energies. The effect of fluorine substituents in the π-spacer on the quantum efficiency of DSSCs was investigated. Gibb’s free energy values, redox potentials, excited state lifetime, non-linear optical properties (NLO) and driving forces for D1 and its fluorinated derivatives were computed.