A. M. Shul'ga

Electronic structure of metalloporphyrin π-anions

Abstract 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 Chesnuts 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.

Relaxation processes in self-assembling triads based on porphyrin Zn-heterodimers

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.

Self-assembling effects and mechanisms of interchromophore interactions in porphyrin pentads

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.

Effect of extra liganding on spectral and kinetic characteristics and on quenching of excited states of multiporphyrin complexes by molecular oxygen

Based on studies of spectral and kinetic parameters of dimers of Zn porphyrins and of multiporphyrin self-assembling complexes formed on their basis, we find that extra liganding of dimers by pyridine inappreciably lowers the energy of the triplet level E(T1). We show that, in this case, the nonradiative deactivation T1 ↝ S0 of the electronic excitation energy of dimers Zn porphyrins increases not only due to increasing Franck-Condon factor. We discuss mechanisms of the quenching action of an extra ligand related to an accepting role played by high-frequency overtones of vibrations of extra ligand molecules, to an enhancement of the spin-orbit interaction due to energy lowering of σπ* states, and to out-of-plane distortion of dimers caused by the displacement of the Zn2+ ion out of the plane of the tetrapyrrole macrocycle. Quenching of triplet states of extra liganded dimers of Zn porphyrins by molecular oxygen in liquid solutions at 295 K depends on the character of donor-acceptor interactions with pyridine and rigidity of a linking molecular fragment. We find that the rate constants of oxygen quenching of the excited electronic states S1 and T1 of multiporphyrin complexes depend on their structure and composition, as well as steric hindrances, created by dimers (screening effects) for contact interactions of a π-conjugated system of the free base (extra ligand) with molecular oxygen. Screening effects of extra ligands by dimer molecules of Zn porphyrins, which reduce the oxygen quenching rate constants kS and kT, are found to barely affect the singlet-oxygen generation quantum yield γΔ.

Interpretation of vibration spectra of metallochlorins

The infrared spectra of Zn-chlorin and Zn-7,8-diethylchlorin, the fine structure of their fluorescence, and the vibrational spectra of Cu-chlorin are interpreted on the basis of normal-mode analysis. The effects of deuteration on the spectral shift of the chlorins is determined. Quantum-chemical calculations are presented which determine the electron density distribution and transitions between ground and vibrational states.

The Electronic Structure of Transient Species in the Photochemical Reduction of Porphyrins

Studies on the electronic structure of π-radical anions and π-dianions of Zn porphyrins, Zn chlorins, π-anions of Zn phlorins, and porphodimethenes by electronic absorption spectroscopy, luminescence, picosecond spectroscopy, ESR, NMR, IR, and Raman spectroscopy are briefly surveyed. The experimental data are compared with the results of quantum-chemical calculations. The electronic structure of M-anions with an excess charge on the central ion of a transition metal (Fe, Ni, Co, and Pd) is discussed.

Spectroscopy of the anions of tetrapyrrole compounds

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.

Manifestation of vibronic intensity borrowing in fluorescence spectra of metalloporphyrins

In selective laser excitation and at T=4.2 K, fine-structure spectra of the fluorescence of the complexes of Mg, Zn, In, and TiO with octaethylporphyrin are obtained. It is shown that the fine-structure fluorescence spectra of these compounds are formed to a large aegree by ribronic interaction between quasi-degenerate S1 and S2 electron states. An increase in the relative intensities of a number of negatively polarized fluorescence lines in displacement of the excitation line from a longwave edge of the band of an S0→S1 transition in the shortwave direction is revealed.

Analysis of the conformation composition of ethane-bisporphyrin solutions from absorption spectra in the region of the soret band

It is found that in absorption spectra of porphyrin dimers linked by a flexible bridge, the shape of the Soret band strongly depends on the solvent, which is explained by differences in the conformation composition of the dimers studied. The conformation composition was analyzed with the deconvolution method, which made it possible to resolve and sharpen all components of the complex contour, of the Soret band. Using different solvents, we studied changes in the behavior of every elementary band with a model of exciton (dipole-dipole) interaction and found that in all the solutions this dimer occurred in the form of face-to-face and side-to-side conformers. The relative content of face-to-face conformation markedly increased with the polarity of the solvent.

Mechanisms of interchromophore interactions and relaxation of electron excitation energy in covalently bound porphyrin dimers

The results presented in the work and the conclusions made on their basis show that the directed synthesis of covalently bound dimers of porphyrins and chlorins with required structural and energy properties of their components allows determination and investigation of real concrete mechanisms determining deactivation of electron excitation energy in these systems and, consequently, control of the interaction of this kind of object with molecular oxygen. The results obtained can be used in practice for seeking effective quenchers (as well as sensitizers) of singlet oxygen for inhibition of its destructive role in biological systems and for protecting materials from oxidation. Finally, the authors have shown recently that a combination of the methods of chemical synthesis with the principles of supermolecular chemistry is promising in forming self-organizing trimer and pentamer complexes with effective energy and electron transfer in solutions.