Quantum-chemical study of nh tautomerism in porphin

Calculations of the hypersurface of the adiabatic potential which determines the movement of NH protons in the centre of the porphin molecule are carried out by the CNDO/2 method. Simultaneous displacements of two NH protons and the displacement of only one proton are considered. The barrier heights for NH tautomerism in these two cases are evaluated, as well as the frequencies of NH vibrational modes for a porphin molecule of D_2h symmetry. The data obtained indicate that shifts of the central protons from one nitrogen atom to another occur almost independently of each other, and the degenerate rearrangement A → B proceeds in a stepwise way, via the intermediate isomer C with adjacent disposition of NH protons (see Fig. 1). The role of tunnelling through the barrier in this process is emphasized. The electronic absorption spectra of the two porphin isomers of D_2h and C_2v symmetry are calculated using the CNDO/S method.     

Computational study of thioflavin T torsional relaxation in the excited state

Quantum-chemical calculations of the Thioflavin T (ThT) molecule in the ground S0 and first excited singlet S1 states were carried out. It has been established that ThT in the ground state has a noticeable nonplanar conformation: the torsion angle phi between the benzthiazole and the dimethylaminobenzene rings has been found to be approximately 37 degrees. The energy barriers of the intramolecular rotation appearing at phi = 0 and 90 degrees are quite low: semiempirical AM1 and PM3 methods predict values approximately 700 cm-1 and ab initio methods approximately 1000-2000 cm(-1). The INDO/S calculations of vertical transitions to the S1(abs) excited state have revealed that energy ES1(abs) is minimal for the twisted conformation with phi = 90 degrees and that the intramolecular charge-transfer takes place upon the ThT fragments' rotation from phi = 0 to 90 degrees. Ab initio CIS/RHF calculations were performed to find optimal geometries in the excited S1 state for a series of conformers having fixed phi values. The CIS calculations have predicted a minimum of the S1 state energy at phi approximately 21 degrees; however, the energy values are 1.5 times overestimated in comparison to experimental data. Excited state energy dependence on the torsion angle phi, obtained by the INDO/S method, reveals that ES1(fluor) is minimal at phi = approximately 80-100 degrees, and a plateau is clearly observed for torsion angles ranging from 20 to 50 degrees. On the basis of the calculation results, the following scheme of photophysical processes in the excited S1 state of the ThT is suggested. According to the model, a twisted internal charge-transfer (TICT) process takes place for the ThT molecule in the excited singlet state, resulting in a transition from the fluorescent locally excited (LE) state to the nonfluorescent TICT state, accompanied by torsion angle phi growth from 37 to 90 degrees. The TICT process effectively competes with radiative transition from the LE state and is responsible for significant quenching of the ThT fluorescence in low-viscosity solvents. For viscous solvents or when the ThT molecule is located in a rather rigid microenvironment, for example, when it is bound to amyloid fibrils, internal rotation in the dye molecule is blocked due to steric hindrance, which results in suppression of the LE --> TICT quenching process and in a high quantum yield of fluorescence.     

Erratum to: “The Fluorescence and Electronic Structure of Phenyl-Substituted Tetraazachlorin Molecules” [Optics and Spectroscopy 117, 722 (2014)]

Optics and Spectroscopy - should read “porphyrazine” instead of “porphyrazin”; should read “intersystem crossing” instead of “intercombinational...     

The fluorescence and electronic structure of phenyl-substituted tetraazachlorin molecules

The effect of the addition of phenyl groups to pyrrole rings of tetraazachlorins, a new class of tetrapyrroles, 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 tetraazaporphine. 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 temperature 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 singlet 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 tetramethylhexaphenyltetraazachlorin 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 lowest excited electronic states Q _x (S ₁) and Q _y (S ₂).     

Two-photon absorption of tetraphenylporphin free base

We study two-photon absorption (TPA) spectra for toluene solution of 5,10,15,20-tetraphenyl-21H,23H-porphin (H₂TPP) in B and Q bands regions and find the maximum TPA cross-section values of 25 and 1-6 GM in laser wavelength ranges, 730-790 and 1100-1400 nm, correspondingly. In the 730-790 nm range the spectrum is attributed to parity allowed two-photon transition into g parity state, positioned nearby B state. Much lower TPA cross-section of ∼1 GM is measured for the transition into pure electronic Q state and it is due to the contribution of only those low-symmetry H₂TPP confomers, where mutual orientation of the porphyrin plane and the four phenyl rings lifts the center of inversion. The intermediate values of TPA cross-section of ∼6 GM are observed for the transition into vibronic Q states and are explained by TPA-allowed transition into vibronic states, which can occur even for totally centrosymmetrical molecules. Measurement of two-photon polarization ratio, Ω=σ_circ/σ_lin, shows that for parity allowed g→g transition Ω =1.05±0.05 and for transitions into electronic and vibronic Q-states, Ω=0.62±0.06 and 0.79±0.1 (depending on wavelength), respectively. Quantum-chemical calculations of both u and g parity excited energy levels of H₂TPP molecule are performed by CNDO/S method and the results are in good agreement with the experimental data.     

Solution of the Inverse Problem for a Complex Vibronic Analogue of the Fermi Resonance on the Basis of Plane Jacobi Rotations

Optics and Spectroscopy - Based on algebraic methods, an exact solution is found to the inverse problem for a complex vibronic analogue of the Fermi resonance, which consists in determining from...     

Calculation of electronic spectra of porphin and its derivatives by a modified INDO/S method

Quantum-chemical calculations of excited electronic states of porphin (H₂P) 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 {ie027-01} and electron-electron interaction integrals {ie027-02} as functions of the internuclear distance. It has been found that a simultaneous increase in the π-type overlap factor for {ie027-03} and decrease in the {ie027-04} integrals, as compared with {ie027-05} 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 H₂P 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. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 28–35, January–February, 2008.     

Calculations of derivatives of porphin and porphyrazine with an annulated five-membered heteroaromatic ring using a modified INDO method

Quantum-chemical calculations for series of porphin (H₂P) and porphyrazine (H₂PA) derivatives whose molecules contain a 3,4-annulated five-membered heteroaromatic ring of pyrrole, furan, and thiophene as well as 1,2,5-triazole, 1,2,5-oxadiazole, and 1,2,5-thiadiazole have been carried out by the AM1 and INDO/Sm methods (m means “modified parametrization”). As follows from the unrestricted Hartree-Fock AM1 calculations, the ground state energy of the a isomer is lower than that of the b isomers for all compounds, the a and b isomers being NH isomers with the additional five-membered ring fused to a pyrrolenine or pyrrole ring, respectively. Condensation of the five-membered ring for the b isomers of the H₂P derivatives is shown to extend the main conjugation path from 18-membered to 21-membered cyclopolyene. However, the 18-membered cyclopolyene for the a isomers of the H₂P derivatives and 16-membered cyclopolyene for both isomers of the H₂PA derivatives prove to be isolated from the five-membered ring carrying six π-electrons. Based on INDO/Sm computations of electronic absorption spectra of the H₂P and H₂PA derivatives, it is shown that the Q_x level of the a isomers is sensitive to the nature of the annulated ring whereas the Q_y level is practically not shifted. On the contrary, the Q_x level of the b isomers is relatively insensitive to the nature of the annulated ring while the Q_y level decreases monotonously in the H₂P and H₂PA series, being the first excited state level for the majority of the derivatives (X is along the NH-HN axis; Y, perpendicular to it). The electronic absorption spectra at the border of the visible and near-UV regions are determined mainly by two high-intensity G → B_x and G → B_y transitions for the H₂P derivatives. However, four intense G → B_x, G → B_y, G → N_x, and G → N_y transitions are characteristic of the spectra of the H₂PA derivatives. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 606–622, September–October, 2008.     

Phenyl substituted Mg porphyrazines: The effect of annulation of a chalcogen-containing heterocycle on the spectral-luminescent properties

We have performed complex experimental and theoretical investigations of the spectral-luminescent properties and electronic structure of new phthalocyanine analogs, Mg octaphenylporphyrazine and its derivatives with an annulated thiadiazole or selenadiazole ring instead of two phenyl groups. Fluorescence characteristics have been determined at 293 and 77 K: emission, excitation, and fluorescence polarization spectra; fluorescence quantum yield ?? _F , and lifetime ?? _F . Annulation of a five-membered chalcogen-containing heterocycle leads to splitting of the long-wavelength absorption band Q(0-0) and to the bathochromic shift of its longest wavelength component Q _x (0-0), which increase upon passage from S to Se. At the same time, the fluorescence quantum yield ?? _F and lifetime ?? _F decrease, which is related to the intramolecular heavy-atom effect. The geometric structure of the ground state of the Mg porphyrazine molecules has been determined based on the density functional theory (DFT), and excited electronic states have been calculated with modified parametrization of the INDO/S method, INDO/Sm. Semiquantitatively, the calculated level positions of the lowest Q states and spectral shifts of Mg octaphenylporphyrazine and S-derivative agree with experimental data. For the range of the Soret band, calculated transition energies and their intensity distributions substantially depend on the dihedral angle ?? between a phenyl ring and porphyrazine macrocycle. We show that, based on calculations at the angle ?? = 60°, bands in the observed absorption spectra can be assigned with an accuracy of ??2000 cm^?1.