539.19Quantum-chemical calculations of excited electronic states of porphin (H 2 P) and Mg-porphin (MgP) have been carried out in the framework of the INDO/S method with varying off-diagonal matrix elements of the one-electron Hamiltonian H µν and electron-electron interaction integrals γ µν as functions of the internuclear distance. It has been found that a simultaneous increase in the π-type overlap factor for H µν and decrease in the γ µν integrals, as compared with γ µν NM calculated by the Nishimoto-Mataga formula, make it possible to reproduce the positions of the Q-and B-transitions in the experimental absorption spectrum with a precision of 300 cm -1 . In this case, the N x -transition intensity of the H 2 P molecule is halved, which means that only two B-transitions should be related to the Soret band as in the four-orbital model. Using the proposed parameterization (INDO/Sm), electronic spectra have been calculated for a number of porphyrin compounds including chlorine, bacteriochlorin, tetrabenzoporphin, and tetraazaporphin. The results obtained agree with the experimental data to an accuracy of 300-700 cm -1 whereas the accuracy of the standard INDO/S calculation is no better than 3000 cm -1 .Introduction. We took the following approach for quantum-chemical calculations of a series of porphyrin compounds [1][2][3][4][5]. The geometric structure of the ground state of the molecules was determined by the limited and unlimited Hartree-Fock method (LHF and UHF) with Hamiltonian AM1 [6]. The geometry found from the optimization was used as starting data in calculations of excited states by the CNDO/S method [7,8]. The results of such an approach indicate that the characteristics of the electronic spectra of the examined series (primarily the shifts of the levels) relative to the prototype of the porphyrin class, porphin (H 2 P), reproduce very accurately the experimental data.Moreover, a comparison of the calculated and experimental data for H 2 P itself (Table 1) shows that there are not only several insufficiencies typical of the CNDO/S method but also those noted earlier more than once [19][20][21].1) The energy of levels 1 1 B 3u (Q x ) and 1 1 B 2u (Q y ) is systematically lowered with an uncertainty δE Q ~3000 cm -1 . The energy of levels 2 1 B 3u (B x ) and 2 1 B 2u (B y ), on the other hand, is systematically raised. The oscillator strength of transitions G → B x and G → B y (G is the ground state 1 1 A g ) is raised by four times. It must be noted that use of the random phase approximation [22] instead of overlap of singly excited configurations and the INDO/S Hamiltonian [23,24] instead of the CNDO/S ** Hamiltonian improves the characteristics of the B-states. However, the energy of the Q-states becomes even lower.2) The configurational composition of a pair of Q-states and a pair of B-states is determined mainly by the electron configurations of the four-orbital model [19][20][21]. However, the increased energies of the a u b 3g and b 1u b 2g configurations results in their contribution to the ...

UDC 535.37We have investigated the spectral characteristics, quantum yield ϕ F , lifetime τ F , and polarization of fluorescence of NH-isomers of dibenzobarrelene disubstituted tetraazaisobacteriochlorin at 293 and 77 K. The following information has been obtained from low-temperature measurements: 1) the weak shoulder on the edge of the long-wavelength 0-0 absorption band disappears at 77 K and, therefore, can be assigned to the cis-NH-isomer [the principal component of the tautomeric mixture is the trans-isomer a (b)]; 2) the low value of ϕ F at room temperature is due to temperature-viscosity quenching whereas ϕ F at 77 K is close to 1 and τ F = 5.3 nsec; 3) the energy of the weak electronic transition G → Q 2 and the interval ∆E Q 2 Q 1 ≈ 2750 cm -1 have been determined. Quantum-chemical calculations found that replacing the methine bridges by nitrogen stabilizes the non-aromatic trans-isomer a (b) whereas the non-planarity of the macrocycle and the alternation of bond lengths increase for the cis-isomers. The calculated electronic absorption spectra of the a isomer reproduce rather well the experimental data. It is shown that the four-orbital model is applicable for the lower excited states Q 1 and Q 2 for all NH-isomers.

Cross cyclotetramerization of trans-2,3-diphenylbutanedinitrile with 1,2,5-thia(selena)diazole-3,4-dicarbonitriles or phthalodinitrile in the presence of magnesium butoxide gave mixtures of Mg(II) porphyrazine complexes which were treated with trifluoroacetic acid to isolate unsymmetrical hexaphenyl-substituted 1,2,5-thia(selena)diazolo-and benzo-fused porphyrazines together with diphenyltribenzoporphyrazine. Their 1H NMR and electronic absorption spectra (in the UV and visible regions) were recorded. The effect of benzene and heteroring fusion on the electronic and steric structure and spectral properties of porphyrazine derivatives was studied in terms of the molecular orbital perturbation theory and semiempirical quantum-chemical calculations (AM1, PM3, ZINDO/S, CNDO/S)

The effect of the addition of phenyl groups to pyrrole rings of tetraazachlorins, a new class of tet rapyrroles, on the photophysical properties and electronic structure of the molecules has been investigated by a complex of experimental and theoretical methods. Characteristics of fluorescence at 293 and 77 K have been determined for phenyl substituted tetraazachlorins. The objects of this study include unsubstituted tet raazaporphine. The introduction of phenyl groups affords a marked increase in the fluorescence quantum yield. For tetraazaporphine and phenyl substituted tetraazachlorins, fluorescence buildup occurs as the tem perature is decreased from 293 to 77 K, but to a lesser extent than for tetraazachlorins having no phenyl groups, which were earlier studied by the authors. The fluorescence buildup mechanism is discussed. The sin glet oxygen generation quantum yield has been determined for the tetrapyrroles examined. This characteristic increases upon tetrapyrrole is phenylation. The electronic structure and absorption spectra of unsubstituted porphine and chlorin, tetraazaporphine, tetraazachlorin, octaphenyltetraazaporphine, and tetramethyl hexaphenyltetraazachlorin have been calculated by the INDO/Sm method (original modification of the INDO/S method) with molecular geometry optimization using DFT. The results of the quantum chemical calculation of the absorption spectra are in good agreement with experimental data for transitions to the low est excited electronic states Q x (S 1 ) and Q y (S 2 ).

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ev UDC 539.19Using the restricted and unrestricted Hartree-Fock method (RHF and UHF) with the AM1 Hamiltonian, we have calculated the geometric structure of the NH-isomers of isobacteriochlorin and its meso-cyano derivative. We have used the CNDO/S method for all the considered structures to calculate the transitions to excited electronic states. We have analyzed the effect of taking into account electron correlation on the results of the calculations of the chemical and geometric structure and the electronic spectra. For the a isomer with the imino hydrogens opposite each other, we obtained a planar structure corresponding to literature x-ray diffraction data. For the aromatic c isomer and other isomers with the imino hydrogens adjacent to each other, the calculations lead to a substantial nonplanarity of the macrocycle (defined mainly by the tilt of the pyrrole rings) and also deviation of the NH bond from the plane of the pyrrole ring. The ground-state energies of the a and c isomers have close values, which generally explains the observed NH tautomerism. We calculated the dipole moments of the isomers in the ground state and the excited state. The results of the calculation of the characteristics of the excited states with UHF geometry on the whole seem to be preferred over those with RHF geometry. We show that only for the aromatic c isomer can we describe the electronic absorption spectrum within a four-orbital model.

Spectral and luminescent properties of a new phthalocyanine analog, the magnesium complex of diphenyl substituted mono (1,4-diazepino)tribenzoporphyrazine, have been studied at 293 and 77 K. The results have been explained on the basis of quantum-chemical calculations for both this molecule and its simpler analogs with respect to 6H/1H-isomerism of the 1,4-diazepine ring. The small difference between the ground-state energies for the 6H and 1H isomers makes tautomerism between them possible. It is shown that the electronic transitions with intramolecular charge transfer from the diazepine fragment to the macrocycle have relatively low energy, especially for the 1H isomers. These states can be of considerable importance for photophysical and photochemical processes. A comparison of the theoretical and experimental electronic absorption spectra has confirmed that the diazepine ring in the substituted Mg-diazepinoporphyrazine is present in the 6H-form. It is established that substitution of one benzene ring by a diazepine fragment leads to quenching of fluorescence, decreasing its quantum yield to 27% (for Mg-phthalocyanine 76%).Introduction. The synthesis and investigation of phthalocyanine analogs, including those with rings other than benzene, are currently of great interest in the chemistry of porphyrins and phthalocyanines. Such compounds are expected to possess fundamentally different physicochemical properties and to be promising for creating new materials with given properties.Octaphenyl derivatives of porphyrazine (tetraazaporphine) with four annelated 7-membered diazepine rings, both the free bases and the metal complexes, have been synthesized and characterized [1,2]. We use for these the abbreviations MPADz 4 Ph 8 (M = 2H, Mg, Zn), where PA is porphyrazine; Dz, diazepine; and Ph, phenyl.