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Resonance Raman scattering (RRS) spectra of the complexes of 5,10,15,20-tetrakis[4-(N-methylpyridyl)]porphyrin with Co(II) and Cu(II) (CoIITMpyP4 and CuTMpyP4) in various solvents and in complexes with DNA are studied. Under nanosecond pulsed excitation, additional lines have been found in the RRS spectra of CoIITMpyP4 in a dimethylformamide (DMF) solution containing formic acid as an impurity and in a complex with DNA. At the same time, such lines are absent in the spectra of CoIITMpyP4 in pure DMF, dimethyl sulfoxide, water, and alcohols under the same excitation conditions. To interpret the experimental data, we have calculated the structure and vibrations of the solvate complexes of CoIITMpyP4 and CuTMpyP4 with water, methanol, and formic acid in the ground and in excited states. Based on the obtained data, additional lines in the Raman spectra are assigned to the dd excited state corresponding to the d(z2) → d(x2 – y2) transition, the lifetime of which increases with the formation of complexes of CoIITMpyP4 with formic acid. According to the calculation results, this correlates with a decrease in rate constant kic of the internal conversion due to an increase in energy gap ΔE between the ground and dd states of CoIITMpyP4. A decrease in the kic value in the case of binding of CoIITMpyP4 and CuTMpyP4 to DNA is caused by the additional interaction of the extra-ligand water molecule with one of its bases directly or via several water molecules immobilized on DNA surface. This complicates the conformational rearrangement of the water molecule in the dd excited state, which gives rise to an increase in the ΔE value.

The study of the excited states and photophysical characteristics of β-nitro-tetraphenylporphyrin (TPP–NO2) has been carried out using resonance Raman scattering (RRS) spectroscopy and methods of the density functional theory. The appearance of new lines, the intensity of which depends on the composition of the matrix and excitation wavelength, has been found in the TPP–NO2 RRS spectra in the low-temperature matrix. The calculation of the vibrational states of TPP–NO2 allowed the linking of the additional lines with the asymmetric vibrations of the nitro group and valence C–C vibrations of the phenyl ring (Ph1) that was nearest to it. The activation of these modes is related to the specific features of the TPP–NO2 geometry in the charge transfer (CT) state from Ph1 to the porphyrin macrocycle. It has been concluded on the basis of the analysis of the data of the study of the RRS spectra and the results of calculations that use the СAM-B3LYP and wB97XD functionals that the CT states do not play a significant role in the TPP–NO2 fluorescence quenching, as previously assumed. The fluorescence quenching owes to strengthening channels of internal and inter-conversion by reducing the energy gaps ΔE(S1 – T1) and ΔE(S1 – S0) as well as increasing the spin-orbit coupling between the S1 and T1 states. It has been shown that TPP–NO2 is characterized by conformational heterogeneity both in the ground and in the excited states, which explains the absence of the monoexponentiality of fluorescence decay kinetics.

A molecular simulation of the solvent effect on radiative rate constant k r of singlet oxygen is carried out. This study included a search for the most probable conformations of the complexes of molecules of singlet oxygen and ten solvents and calculation of dipole moments M of transitions a1Δ g –b1Σ + (M a–b ) and a1Δ g –X3Σ - (M a–X ) of the oxygen molecule for them. Averaging of M a–b by conformations, taking into account the probability of their formation for complexes without atoms with a large atomic number (Cl, S), yields values that, as a rule, correlate well with the behavior of k r in the experiment. Taking into account the possibility of decreasing the distance (compared to equilibrium) between molecules in a collision complex at room temperature made it possible to achieve satisfactory agreement of the calculated and experimental data also for complexes with CCl4, C2Cl4, and CS2. The obtained data indicate that a number of factors affect k r . The correlation of k r with molecular polarizability in a number of cases is due, on the one hand, to its effect on the strength of dispersion interactions in the complex and, on the other hand, to the fact that it to some extent reflects the position of the upper filled orbitals of the solvent molecule. Both factors affect the degree of mixing of the π orbitals of the singlet oxygen molecule with the orbitals of the solvent molecule, which, as was found earlier, facilitates the activation of the a1Δ g –b1Σ + transition and the borrowing of its intensity by the a1Δ g –X3Σ - transition.

Quantum-chemical calculations of the structure in the ground and lower singlet excited states and the vibrations (in the ground state) of special pair P of photosynthetic reaction center of purple bacteria (RCPb) Rhodobacter Sphaeroides, consisting of two bacteriochlorophyll molecules PA and PB, have been carried out. It is shown that excitation of the special pair is followed by fast relaxation dynamics, accompanied by the transformation of the initial P* state into the P A δ+ P B δ- state (δ ~ 0.5) with charge separation. This behavior is due to the presence of several nonplanar vibrations with participation of the acetyl group of macrocycle PВ in the nuclear wave packet on the potential surface of the P* state; these vibrations facilitate destabilization of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the macrocycle PA and formation of the P A δ+ P B δ- state. The structural transformations in the P* state are due to its linking character in the contact region of the acetyl group-containing pyrrole rings of PA and PB. The transition from the P* state to specifically the P A δ+ P B δ- state is related to the fact that the acetyl group PA is involved in the intermolecular hydrogen bond with amino acid residue HisL168; for this reason, this group and the pyrrole ring linked with it can hardly participate in structural transformations. The electronic matrix element Н 12 of the electron transfer from the special pair in the P A δ+ P B δ- state to a molecule of accessory bacteriochlorophyll ВА greatly exceeds that for the transfer to ВB. This circumstance and the fact that the P A δ+ P B δ- state is energetically more favorable than the P* state facilitate the preferred directionality of the electron transfer in RCPb Rhodobacter Sphaeroides with participation of the cofactors located in its subunit L.

We have experimentally studied the spectral-luminescent and photophysical properties, as well as interaction processes with molecular oxygen, of eclipsed β-alkyl-substituted porphyrins para-methoxyphenyl rings of which in opposite meso-positions of the macrocycle are linked with each other in the ortho-position with the (-OCH2)-Ph-(CH2O-) bridge. Using methods of the density functional theory, we have calculated the structures of these porphyrins in the ground and lowest triplet states, electronic transitions and vibrational states, and matrix elements of the direct spin-orbit interaction. It has been found that the lifetime of the T1 state of the investigated compounds is noticeably shortened due to the enhancement of the internal conversion. This enhancement is caused by the conformational dynamics, which promotes increasing Franck-Condon factors and the spin-orbit interaction between the T1 and S0 states. Nevertheless, this does not lead to a considerable decrease in the singlet-oxygen formation quantum yield. We have calculated the structures of a porphin dimer and possible dimeric forms of examined porphyrins. It has been shown that distortion of the porphyrin macrocycle according to type of ruffling weakly affects the binding energy of the porphyrin macrocycles in the dimer, this effect being unable to prevent aggregation. At the same time, the screening of the macrocycle by the phenyl ring lowers the probability of formation of oligomeric structures, which, in view of the determined photophysical properties of investigated compounds, may be used in photochemical applications that require elevated concentrations of porphyrins.

Quantum-chemical calculations are performed to analyze the factors affecting rate constant k T of quenching of the lowest triplet state by molecular oxygen and quantum yield γΔ of singlet oxygen generation in chemical dimers of porphyrins bound by phenyl spacers at one of the meso positions (OEP)2-Ph, (TPP)2, and their Zn complexes. It is established that, for both types of dimers, the triplet excitation is localized on one of the macrocycles. The steric hindrance of macrocycles at the site of the phenyl ring of (OEP)2-Ph, (ZnOEP)2-Ph, and their monomeric analogues OEP-Ph and ZnOEP-Ph facilitates its rotation by 90° in the triplet state. The lowest triplet state energy in this (U) conformation is lower than 7800 cm−1, which makes impossible electronic excitation energy transfer to molecular oxygen. The potential barrier of transformation to the U conformation is considerably lower for dimers than for monomers. Because of this, the rate of conformational transformations for dimers is higher and some of the (OEP)2-Ph and (ZnOEP)2-Ph molecules have time to transform into the new U conformation before diffusion collision with O2 molecules in solution. This leads to a noticeable decrease in γΔ in accordance with experimental data. It is shown that the behavior of k T in the series of the studied dimers, their monomeric analogues, and relative compounds corresponds to the model of dipole-dipole electronic excitation energy transfer 1(3M⋯3Σ g − ) → 1(1M0⋯1Δg) in collisional complexes.

Structural factors that provide localization of excited states and determine the properties of primary donor and acceptor of electron in the reaction center of photosystem II (PSII RC) are studied. The results of calculations using stationary and time-dependent density functional theory indicate an important role of protein environments of chlorophylls PA, PB, BA, and BB and pheophytins HA and HB in the area with a radius of no greater than ≤10 A in the formation of excitonic states of PSII RC. When the neighboring elements are taken into account, the wavelength of long-wavelength Q y transition of chlorophyll molecules is varied by about 10 nm. The effect is less developed for pheophytin molecules (Δλ ≅ 2 nm). The following elements strongly affect energy of the transition: HisA198 and HisD197 amino-acid residues that serve as ligands of magnesium atoms affect PA and PB, respectively; MetA183 affects PA; MetA172 and MetD198 affect BA; water molecules that are located above the planes of the BA and BB macrocycles form H bonds with carbonyl groups; and phytol chains of PA and PB affect BA, BB, HA, and HB. The analysis of excitonic states, mutual positions of molecular orbitals of electron donors and acceptors, and matrix elements of electron transfer reaction shows that (i) charge separation between BA and HA and PB and BA is possible in the active A branch of cofactors of PSII RC and (ii) electron transfer is blocked at the BB - HB fragment in inactive B branch of PSII RC.

We have studied the nature and unusual photophysical properties of triplet states of a series of sterically hindered porphyrins (meso-phenyl-substituted derivatives of octamethylporphyrin (PS-OMP)) using quantum-chemical calculations of the parameters that determine the probability of deactivation of the T1 state and modeling the T-T absorption spectra. We show that a decrease in the lifetime of the T1 state of PS-OMP is related with the enhancement of the channel of nonradiative deactivation (T1 → S0), which occurs (i) due to the conformational lability in the T1 state, as a result of which the energy gap T1-S0 considerably decreases, and (ii) because of an increase in the matrix element of the spin-orbit interaction due to a change in the hybridization of atoms of the macrocycle as a consequence of its nonplanar distortion. The value of the vibronically induced matrix element of the spin-orbit interaction between the S0 and T1 states of PS-OMP depends weakly on the type of the conformation and the value and the character of distortion of the porphyrin macrocycle. The conformational lability of PS-OMP clearly manifests itself in the spectra and kinetics of the T-T absorption of these compounds at room temperature and determines the nonmonoexponential character of the phosphorescence decay kinetics in frozen matrices. Using porphin as an example, we have shown that nonplanar distortions of the macrocycle facilitate a decrease in the phosphorescence rate constant at 77 K, which is caused, on the one hand, by an increase in the energy gaps T1-Sn and S0-Tn and, on the other hand, by an increase in the wavelength of the transition T1 → S0.

Using quantum-chemical methods, we have studied the role played by water molecules W-A and W-B that are bound by hydrogen bonds to accessory bacteriochlorophyll molecules B A and B B in the process of primary charge separation in the reaction center of Rhodobacter Sphaeroides. We have found that the occurrence of a rotational mode of the W-A molecule at 32 cm−1 and/or its harmonics in stimulated emission of an electron donor P* and the dynamics of population of the states P+B − and P+H − may be related to the structural heterogeneity of the reaction center and the existence of a conformation in which the W-A molecule is predominantly involved in one hydrogen bond (with BA). Based on the calculated redox potentials B A and P, it has been shown that the appearance of the W-A molecule in the reaction center reduces the energy of the P+B − state by ∼600 cm−1. This is somewhat smaller than the influence of the amino-acid residue TyrM210 (∼870 cm−1) and correlates well with a substantial decrease in the electron transfer rate in mutant forms of reaction centers GM203L (which do not contain W-A molecules) and YM210F (in which TyrM210 is replaced with Phe). The data obtained allow us to suggest that rotation of the water molecule with a fixed position of its H atom that is involved in a hydrogen bond with the keto carbonyl group of B A is initiated due to the charge separation between the halves of special pair P and the formation of the state P + P − . The large effect of this rotation on the kinetics of population of the states P+B − and P+H − after the excitation of P is quite consistent with its influence on the energy of the state P+B − .

Particular features and quenching mechanisms of exciton luminescence of water-soluble nanocomposites that are formed as a result of the interaction of surface charged semiconductor quantum dots (QDs) CdSe/ZnS (dCdSe = 2.8 nm) and cationic porphyrins (H2TMPyrP4+ and ZnTMPyrP4+) have been studied theoretically and experimentally. It has been found that, in CdSe/ZnS⊕Porphyrin conjugates, there occurs long-range inductive resonance electronic excitation energy transfer from surface modified (with thioglycolic or mercaptoundecanoic acid) QDs to porphyrins, which is accompanied by quenching of the exciton luminescence of QDs and an increase in the fluorescence intensity of porphyrin. It has been shown that, when mercaptoundecanoic acid is used as a QD shell, the QD luminescence quenching efficiency by porphyrins follows the Forster-Galanin theory and depends on the overlap integral between the CdSe/ZnS luminescence band and the absorption spectra of free-base porphyrin H2TMPyrP4+ and its metal complex ZnTMPyrP4+. It has been revealed that, as the QDs ↔ Zn-porphyrin intercenter distance decreases from 39.1 (mercaptoundecanoic acid) to 30.1), a considerable QD luminescence quenching is observed; however, the energy transfer efficiency substantially decreases, from 55% in the former case to 23% in the latter one. Based on the spectral-luminescent data and quantum-chemical calculations, it has been found that the chemical change of H2TMPyrP4+ in the structure of the complex with CdSe/ZnS QDs passivated by thioglycolic or mercaptoundecanoic acid is caused by the formation of a metal complex ZnTMPyrP4+. Based on calculations of the redox-potentials, it has been concluded that the low luminescence quantum yield of CdSe/ZnS QDs passivated by residues of mercaptocarboxylic acids S−(CH2)nCOO− and its dependence on the number of CH2 groups are related to the possibility of photoinduced electron transfer from the HOMO of passivating molecules to QDs (QD* ⇐ S−(CH2)nCOO− hole transfer). It has been shown that the quenching of the exciton luminescence of QDs in heterogeneous structures CdSe/ZnS(thioglycolic acid)⊕ZnTMPyrP4+, which is complementary to the energy transfer, can be caused by the photoinduced electron transfer that involves the participation of the LUMO of the ZnTMPyrP4+ molecule (QD* ⇒ ZnTMPyrP4+).

Using methods of density functional theory, we calculate the structure and simulate the electronic absorption spectra of the anion-radical of bacteriochlorophyll and its form protonated at the keto carbonyl group in a solution and in the photosynthetic reaction center of Rhodobacter Sphaeroides. We show that, in both cases, the best agreement between the calculated and experimental data is achieved under the assumption that the anion-radical form of bacteriochlorophyll is formed. An experimentally observed large difference between the spectral properties of the reduced form of bacteriochlorophyll in vitro and in vivo is related to peculiarities of the environment of bacteriochlorophyll in the native system and with the fact that, in this case, the initial nuclear configuration of B A has no time to completely relax in accordance with a new electronic configuration during the lifetime of the state P + B − . Taking into account the data obtained in this work, we believe that the recently proposed model of coupling of the primary charge separation in the photosynthetic reaction center of Rhodobacter Sphaeroides with the double proton transfer seems to be unlikely.

The structure and vibrational states of complexes of Co- and Ni-porphine monoanions with Na+, K+, and Rb+ cations are examined on the basis of density functional theory calculations. It is found that anion×cation pairs exist in a conformation with the cation situated along the axis connecting opposite carbon atoms in meso-positions. Interaction with the counter-ion assists localization of a major portion of the excess of charge on the metal and not on the macrocycle as in the case of free anions. The formation of anion–cation pairs, in spite of a decrease in the anion charge and its localization on the metal, is accompanied by larger structural rearrangements due to the asymmetric localization of the counter-ion and its influence on the π-electron density distribution. The possibility of forming sandwich-type dimers of anion–cation pairs with D2h-symmetry is demonstrated. It is shown that such a model for anionic forms of Co- and Ni-porphine, as compared with those for free anions and anion–cation pairs, is in better accordance with data of IR and resonance Raman spectroscopy.

Using the density functional theory, vibrations of different model forms of the chlorophyll a molecule in the ground and triplet states have been calculated. The assignment of the experimental difference IR spectrum corresponding to the formation of the triplet state of the photosynthetic reaction center of photosystem II has been proposed on this basis. It has been shown that molecules of accessory chlorophyll BA and BB located between the special pair and HA and HB pheophytin molecules can be involved in the intermolecular hydrogen bond with the water molecule. The energy of this interaction in the triplet state of molecules for BA is larger on 6 kcal/mol. This allows us to relate this pigment to the location of the triplet excitation at the reaction center of photosystem II.

The directional self-assembly of nanosized, structurally organized pentads that include five tetrapyrrole macrocycles and are based on the two-point coordination interaction of two covalently bound dimers of Zn porphyrins (homo-and heterodimers) with molecules of either the free base or the Cu complex of tetrametapyridyl-substituted porphyrin extra ligand is implemented in methylcyclohexane at 295 K. Using the method of the density functional theory (DFT) in the B3LYP/6-31g(d) approximation, the geometry of the pentad is fully optimized and the main factors that determine its redox properties are determined. The energies of the lowest excited states of the pentad are calculated by the ZINDO/S method, and it is shown that the occurrence of identical molecules in the system facilitates the formation of excitonic states with different contributions from the charge-transfer component. The directional energy transfer and the photoinduced electron transfer, which leads to the formation of a low-lying charge-transfer state (CT state), are studied and the rate constants of these two processes are determined (kET ∼ 1011 s−1 and kPET ≈ (1.8−6.0) × 109 s−1, respectively). The strong effect of the temperature of the solvent on the efficiency of relaxation processes in pentad complexes under study is revealed and studied. Roles played by the low-lying CT state and d-π exchange effects (the Cu-contained pentad) in fluorescence quenching of pentad complexes are determined.

The structure and vibrations of neutral porphine metal complexes (Me-P, Me = Co, Ni, Cu) and their d-anionic forms with an additional electron localized in vacant $$ d_x^2 -_y^2 - $$ and $$ d_z^2 $$ -orbitals are compared based on calculations by a DFT method. It is shown that such electron population causes a significant increase of the electronic charge on the macrocycle rather than on the Me atom and is accompanied by a considerable redistribution of π- and σ-electron densities (ρπ ρσ). A predominant gain of ρπ (0.49e) is found for the monoanion of Co-P (Co-P–, $$ d_z^2 $$ -monoanion); of ρσ (0.6e), for Ni-P–( $$ d_x^2 -_y^2 - $$ monoanion). These features are reflected in both the structure of the anions and the behavior of their vibrational frequencies. The greatest frequency shifts among IR active modes when populating the $$ d_z^2 $$ - and $$ d_x^2 -_y^2 - $$ orbitals occur for out-of-plane vibrations (>30 cm–1) and in-plane modes (34–46 cm–1) involving MeN- and CαCm-bonds, respectively. Abnormally large frequency lowering is found for B1g-type modes (active in the resonance Raman spectrum) involving mainly CαCm-, CβCβ-, CαCβ-, and MeN-bonds. This is related to a change in the dπ-eg interaction strength during such vibrations that contributes to a decrease in the corresponding force constants.

The directional self-assembly of nanosized, structurally organized triads is implemented in methylcyclohexane at 295 K, which is based on the two-point extra coordination of nonsymmetric, covalently bound heterodimers of Zn porphyrins to bipyridyl-substituted porphyrin free-base extra ligands. Based on experimental data and theoretical calculations, the structural organization is determined and information on the energetics of electronic interactions of components is obtained and the rate constants of the directional energy transfer (kET ∼ 1011 s−1) and photoinduced electron transfer (kPET ∼ 2.7 × 109 s−1) are determined. The effects of the orientation of interacting macrocycles, the intercenter distances and the solvent temperature on the efficiency of relaxation processes in the triad is investigated.

The structure, solvation energies, and electronic absorption spectra of products corresponding to the second stage of reduction of Zn porphin (Zn-P) with metallic sodium are calculated using methods of density functional theory. Calculated solvation energies of free and bound ions make it possible to assume that, in commonly used aprotic solvents, reduction products form [2Na+ · Zn-P2−] anion-cation contact complexes. The relative energies of the four stable isomers [2Na+ · Zn-P2−] detected in the gas phase and solvents are considerably different. The arrangement of Na+ on different sides of the macrocycle and along the NN axis favors the interaction with the solvent and stabilization in the solution. This conclusion is consistent with the data of calculations of electronic absorption spectra and of modeling of the spectral manifestations of the electron photodetachment from dianions of Zn-tetraphenylporphyrin (Zn-TPP) in frozen matrices. The behavior of the calculated transitions in the series of neutral and charged forms of Zn-P and Zn-TPP is well consistent with experimental data. Despite the fact that the spectral properties of the neutral and dianionic forms of Zn-P and Zn-TPP in the visible range are similar, the nature of their excited states is different.

Based on the calculation of frequencies of the normal vibrations of Zn complexes of octaalkylporphyrins (Zn-OAPs), Zn porphin (Zn-P), and their meso-deuterated derivatives by the method of density functional theory (B3LYP/6–31G(d, B3LYP/6–311+G(d)), a new interpretation of C m H(D) out-of-plane deformation vibrations of porphyrins is proposed. It is shown that the vibration patterns of these modes are characterized by high contributions (>90%) of the coordinates of displacement out of the macrocycle plane of the H(D) atoms in meso positions. At the same time, a small change in the vibration pattern of such a mode in the d 4 isotopomer (∼5%) is accompanied by a large change in its intensity. The geometry of free π anionic forms of Zn-OAPs and Zn-P and their complexes with cations Na+ and K+ is optimized, and the normal vibrations of these compounds are calculated. A comparison with experimental IR spectra of solid samples shows that anion-cation complexes form under these conditions. The interaction with cations lowers the symmetry of the nuclear skeleton of reduced forms of Zn-OAPs, considerably decreases their charge, and markedly affects the structure of this skeleton and its vibrations.

The behavior of the bands of the NH stretching vibrations of sterically distorted porphyrins whose macrocycles are undergoing different types of deformation (saddling, elongation, waving, etc.) is studied in CHCl3 and CCl4 solutions. The geometry of sterically distorted porphyrins and the frequencies of their normal vibrations are calculated in terms of the density functional theory. The relative strength of intramolecular hydrogen bonds is estimated in terms of the theory of natural (localized) orbitals. The frequency of the stretching NH vibrations of porphyrins with saddling, elongation, or waving deformation of their macrocycles is shown to be inversely proportional to the energy of stabilization E (2), arising due to the charge transfer from the orbitals of unshared electron pairs of the nitrogen atoms of the pyrrolenyne rings to the antibonding σ*NH orbitals. It is shown that this frequency can serve as a measure of the strength of intramolecular hydrogen bonds. A specific feature of the ruffling deformation of the porphyrin macrocycle is that the strengthening of the hydrogen bonds (due to a decrease in the size of the internal porphyrin window) is accompanied by a shortening of the NH bonds, whereas, depending on the magnitude of the ruffling, the frequency of the stretching NH vibrations decreases or increases. This phenomenon is associated with the fact that the shortening of the NH bond facilitates a decrease in its effective van der Waals radius and weakening of destabilizing interactions of the σNH orbitals.

Using the DFT Becke–Lee–Yang–Parr exchange‐correlation three‐parametric functional (B3LYP), we have calculated the geometry, the electron density distribution, and the vibration frequencies for pyrrole, porphin, and their 13 analogs with a regularly varying structure. It is shown that the determining role in the change in the frequencies of stretching and out‐of‐plane deformation vibrations of porphin compared to pyrrole is played by intramolecular hydrogen bonds at the porphyrin macroring center. We have separately considered and compared with the data for porphin complexes of pyrrole with different intermolecular hydrogen bonds: with an aromatic ring (NH···π cluster formed by the pyrrole self‐aggregation in the liquid state) and an unshared electron pair (NH···OH2‐ and NH···NC4H4 clusters). The degree of participation of the NH group in the hydrogen bonds increases in the series NH···π cluster, porphin, NH···OH2‐ and NH···NC4H4 clusters.

The influence of anion-cation interactions on the structure and electronic absorption spectra of cationic 5,10,15,20-tetrakis-(4-N-methylpyridyl)porphyrin and anionic 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin has been investigated by absorption spectroscopy and molecular modeling. It is shown that the shifts of bands in the electronic absorption spectra appearing when passing from aqueous solutions to solutions in organic solvents are due to the interaction of ionized peripheral substituents of the porphyrin macrocycle with counterions. The bathochromic shift of the Q x transition is due to the influence of counterions on the energy characteristics of the molecule itself, primarily, as a result of Coulomb interactions changing the relative position of the orbitals and the distribution of electron density on them. The hypsochromic shift of the Q y transition is a consequence of the increase in the dihedral angle between the plane of the porphyrin macrocycle and the aromatic rings with ionized groups due to the decrease in the π-electron interaction between them under the action of counterions.

The electron transfer process in bacterial reaction centers is investigated. It is found that the side indole group Trp M252 in Rb sphaeroides (Trp M250 in Rps viridis) assists tunneling electron transfer by a distance of ∼14A from pheophytin to quinone.

Spectroscopic manifestations of the intramolecular hydrogen bond (IHB) in ordinary and sterically hindered porphyrins were investigated by the methods of IR spectroscopy and molecular modeling. It is shown that the IHB plays a part in the formation of the νNH and γNH vibrations of porphyrins. By changing the type and value of the distortion of the macroring at the expense of steric effects of interaction of volumetric substituents, the parameters and spectroscopic manifestations of the IHB can be varied in a significantly larger range as compared with ordinary tetrapyrroles having nearly quadratic and planar macrorings.

We present a brief review of the results of an investigation into the electronic structure of π- and M-anions of tetrapyrrole compounds by the methods of molecular spectroscopy, namely, of electronic absorption and picosecond spectroscopy and also of IR, RRS-, EPR-, NMR1N- and NMR13S-spectroscopy. Experimental data are compared with the results of quantum-chemical calculations.

The theoretical analysis of the electronic excitation energy transfer in the complexes between water soluble CdSe/ZnS quantum dot (QD) and 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin Zn complex (ZnTMPyrP) has been performed. Quantum chemical methods have been applied to check a possibility of electron transfer processes in this system. The quantum yield and the lifetime of the QDs luminescence in this system are shown to be determined not only by excitation energy transfer but additionally by photoinduced electron transfer from QD to porphyrin and from passivating moieties of mercaptocarboxylic acids S(CH2)nCOO to QD.

It has been found that relatively small changes in conditions of crystallization in the synthesis process of μ-oxo dimers can lead to quite different products with dimeric or monomeric structures. Molecular structures of these products, discussed on the basis of Fe-octaethylporphyrin (FeOEP) complexes, have been studied using IR, NMR, EPR, resonance Raman (RR), mass and Mossbauer spectroscopies. The use of chloroform with admixture of alcohols (methanol, deuteromethanol and ethanol) in place of pure chloroform for crystallization of the eluate obtained during synthesis of μ-oxo dimers leads to the formation of alkoxy complexes instead of the μ-oxo dimers. It has been also established that dissolution of the μ-oxo dimers in chloroform-ethanol or chloroform-methanol mixtures leads to conversion of the μ-oxo dimers into the alkoxy complexes after evaporation of the solvents. Methoxy and ethoxy ligands in Fe(OEP)OCH 3 and Fe(OEP)OC 2 H 5 complexes can exchange the positions during addition of ethanol to the former and methanol to the latter complex in solution. The possibility of generation of doubly bridged structures of the type: OEP  Fe R R Fe  OEP where R is OCH 3 or OC 2 H 5 is discussed.

Spectroscopic manifestations of the intramolecular hydrogen bond (IHB) in ordinary and sterically hindered porphyrins were investigated by the methods of IR spectroscopy and molecular modeling. It is shown that the IHB plays a part in the formation of the νNH and γNH vibrations of porphyrins. By changing the type and value of the distortion of the macroring at the expense of steric effects of interaction of volumetric substituents, the parameters and spectroscopic manifestations of the IHB can be varied in a significantly larger range as compared with ordinary tetrapyrroles having nearly quadratic and planar macrorings.

On the basis of an investigation of IR spectra, it has been found that trianions of Ni octaethylporphin, when there is a change in their phase state in the series from solution to film, are capable of entering into a reaction of partial anion-cation recombination, resulting in the formation of dianions with two additional electrons located on the porphyrin macroring, rather than one electron, as is the case when the dianions are obtained directly. Conditions have been found for stabilization of the trianions of Ni-octaethylporphin in the solid phase and their electronic configuration has been established; this includes the positioning of two additional electrons in π-orbitals of the macroring and one additional electron in the \(d_{x^2 - y^2 }\) orbital of the central metal atom.

On the basis of an investigation of IR spectra, electronic configurations have been established for the monoanions of Ni and Co octaethylporphin and the dianion of Ni octaethylporphin in the solid phase; characteristic features have been revealed in the effects on the IR spectra of metalloporphyrins from occupancy of their vacant $${\text{d}}_{{\text{z}}^{\text{2}} }$$ and $$d_{x^2 - y^2 }$$ orbitals by additional electrons. It has been shown that interaction of the anions with alkali metal countercations may lead (depending on the electronic configuration of the anion and the type of cation) to activation of bands of composite vibrations in the IR spectrum. It has been found that the monoanions of Ni octaethylporphin are capable of existing in d- and π-electronic configurations, depending on the phase state (solution or film) and the crystal modification in the solid phase.

It is well known that methanol and ethanol at low concentrations associate with ferrihemoglobin and ferrimyoglobin to form complexes in which the alcohol groups are bound directly to the ferric ions. The nature of the complexes [heme iron-alcohol ligand] and their relations to the function of hemoprotein molecules have not been satisfactorily explained so far. Octaethylporphynatoiron complexes (Fe-OEP), taking into account their molecular structure, are often used as model systems for studies of heme properties. Ogura et. al., while investigating the photoreduction mechanism of Fe(III)-OEP complex with 2- methylimidazole in dichloromethane (CH2Cl2) solution by resonance Raman and absorption spectroscopies, have found that a trace amount of alcohol present in CH2Cl2 as a stabilizer can initiate the photoreduction process. An unexpected finding in this study was the observation that laser irradiation into the CT band at 585 nm didn't induce the photoreduction. Recently we observed the same photoreduction process, when the starting materials were alkoxocomplexes of Fe-OEP [Fe(OR)-OEP, were R equals CH3, C2H5]. This paper contains new data regarding the structure, spectroscopic, photochemical and photophysical properties of the photoactive products, generated in the reactions of Fe(III)-porphyrins with alcohols.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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The transformation of metalloporphyrin molecules into their mono- and di-π-anions during contact of the metalloporphyrin solution with the Na and K metals has been examined by electronic absorption spectroscopy, ESR, 1 H and 13 C NMR, IR and Raman spectroscopy. The charge distribution is calculated by the CNDO/2 method. It is shown that the generation of anions is accompanied by characteristic changes in all the spectra and in the charge distribution. Hyperfine splitting in the ESR spectra of anion radicals of zinc complexes of monoaza- and diazaetioporphyrins is detected. ESR spectra are simulated on the basis of McConnel and Chesnut's relation and unpaired electron distribution. In 1 H NMR spectra of dianions large upfield shifts of proton signals under the influence of paramagnetic ring current are observed. The 13 C chemical shifts of neutral porphyrins correlate with the charge distribution, while those of the corresponding dianions are determined by changes in the mean average excitation energy, paramagnetic ring current, and electron density. The majority of the bands of IR and Raman spectra undergo a low-frequency shift on passing to anions. A comprehensive analysis reveals a qualitative picture of changes in specific bonds of a molecule and the electronic structure as a whole. These data are confirmed by quantum-chemical calculation of the electron density in dianions.

The paper covers two aspects: 1) regularities of manifestation of metalloporphyrin reduction by alkali metals (Na, K) in vibrational spectra (IR, resonance Raman spectra) and a possibility of determining the electron structure of ionization products using these data; 2) the usage of spectra variations in the series starting molecule-monoanion-dianion for matching up the bands in the spectra of deuterized modificationes, choosing the model of the force field, and determining the form of normal vibrations.