Andrei Chernook

Formation and optical properties of self-organized pentameric porphyrin arrays

Abstract Principles of formation, electronic absorption and fluorescence spectra are reported for self-organized pentameric arrays of tetrapyrrolic macrocycles. In these arrays two molecules of Zn-porphyrin dimers, Zn(II 1,4-bis[5-(10,15,20-tri-p-hexylphenylporphyrinyl)]-benzene ((ZnHTPP)2) are bound via one molecule of a tetrapyridyl-substituted free base of porphyrin or tetrahydroporphyrin. The process of self-assembly is based on the twofold coordination of the central Zn ions in the dimer with the nitrogen atoms of the pyridyl rings in the free base which is strong enough to make the complexes stable at room temperature. The formation of the complexes can be followed by changes in the absorption bands of (ZnHTPP)2 characteristic of an axial extra-ligation of Zn-porphyrins with pyridine or pyridyl-substituted compounds. The spectral behavior of the free bases in the pentads is determined by a non-planar distortion of their macrocycle caused by the two-point binding with the dimers. The fluorescence intensity of the Zn-porphyrin dimer decreases essentially upon complexation with the tetrapyridyl-substituted free bases. This quenching effect is assigned to a singlet-singlet energy transfer from the complexed Zn-porphyrin dimers to the free base subunit in the pentad.

Spectral peculiarities of NH-tautomerism in isocycle-containing porphyrins and their covalently linked dimers

Abstract It has been found experimentally that a whole class of isocycle-containing porphyrins, including synthetic and natural objects and their covalently linked dimers, shows NH-tautomerism which manifests itself in isotropic solutions in normal electronic spectra both at 77 K and higher temperatures (up to 500 K).

NH tautomerism and visible absorption spectra of porphyrins with asymmetrical substitution: Oscillator model and MO calculations

Abstract On the basis of experiments the oscillator model has been established for individual NH tautomers of porphyrins with asymmetrical substitution. CNDO/2 calculations explain the inversion of Q x (0,0) and Q y (0.0) electronic transition intensities in NH tautomers as a consequence of the inversion of LUMO coefficients c 1 and c 2 for fixed x and y molecular oxcillators.

Spectroscopy of interchromophoric interactions in self-organized porphyrin and chlorin complexes

Abstract Spectral-luminescent properties of multimolecular complexes (triads and pentads, complexation constants range from 5·10 6 to 5·10 7 M −1 ) formed by two-fold coordination of dipyridyl and tetra-pyridyl substituted porphyrin or related molecules with Zn-porphyrin and Zn-chlorin dimers having various spacers between macrocycles (-CH 2 -CH 2 - or phenyl ring) have been studied in methylcyclohexane solutions in a temperature range from 140 to 360 K. The red shift of Zn-dimer electronic Q- and B-bands ( Δv ≤550 cm −1 ) upon complexation with pyridyl containing ligands is explained in terms of extra-ligation which influences the relative position for HOMOs a 1u and a 2u according to the four-orbital model. The splitting of Zn-dimer B-bands ( Δ E≥600 cm −1 ) as well as the significant transformation of these bands in trimeric and pentameric complexes (redistribution of the absorption band intensities in Soret region and appearance of complicated splitted spectral manifolds Δv ≤1900 cm −1 ) is connected with excitonic interactions of strong B-transitions of π-conjugated macrocycles included in the complex. Observed experimental splittings are compared with theoretical values calculated using the point dipole approximation and a computer-simulated geometry of the complexes under investigation. It has been observed that mutual influence of π-electronic macrocycles in the complexes leads to the red shift of porphyrin extra-ligand free base Q-bands ( Δv ≈120 cm −1 ) and is accompanied by quenching a fluorescence of certain components.

Energy transfer in ethane-bisporphyrin dimers studied by fluorescence line narrowing and spectral hole burning

Abstract The quasi-line fluorescence excitation spectrum of 1,2-bis (2,3,7,8,12,13,17,18-octaethyl-21 H ,23 H -porphino) ethane at 4.8 K consists of two subbands with the splitting mean value of 51cm −1 , that are ascribed to the donor and the acceptor half of the homodimer. The donors fluorescence is quenched by an efficient energy transfer to the acceptor. The energy transfer rate of 10 11 s −1 , determined by spectral hole burning, has been compared with the calculated value and a conclusion of nonconsistency with the Forster energy transfer mechanism has been drawn.

Fluorescence line narrowing study of cyclopentaneporphyrin chemical dimers at 4.2 K

Abstract The monomers of Zn-cyclopentanporphyrins (ZnOEP-cycle and ZnOEP-cycleCH 2 ) and their chemical dimers covalently linked via isocycles (Zn-cyclodimers) have been studied by the method of fluorescence line narrowing (FLN) in tetrahydrofurane-toluene (3:1) glassy matrixes at 4.2 K. Well-resolved fluorescence spectra upon laser excitation into the S 0 → S 1 absorption band have been observed. In contrast to this, it has been shown that the excitation into the region of S 0 → S 2 absorption band leads to FLN disappearance for individual monomers. On the basis of this effect positions of the S 0 → S 2 electronic transitions in the energy scale have been determined. The normal coordinate treatment of FLN spectra of ZnOEP-cycleCH 2 and those for the Zn-cyclodimers permitted us to determine the normal modes which are connected with the formation of the dimeric species. The weak interaction of Q-transitions of donor ( D , ZnOEP-cycle) and acceptor (A, ZnOEP-cycleCH 2 ) subunits in Zn-cyclodimers manifests itself in the strong fluorescence quenching of D . Under excitation into the S 0 → S 1 transition of D the FLN spectra of the dimers (belonging to their A subunits) were not observed. These facts are connected with the effective non-radiative singlet-singlet energy transfer in conditions of essential spectra inhomogeneity.

Dynamics of self-organized porphyrin aggregates

Supramolecular ensembles stable at room temperature (complexation constant and activation energy range from 5 (DOT) 106 M-1 to 5 (DOT) 107 M-1 and from 0.5 to 1.0 eV correspondingly) containing up to five macrocyclic fragments have been constructed using two-fold ligation of Zn-porphyrin and Zn-chlorin chemical dimers by pyridyl substituted porphyrin or related molecules. Spectral, photophysical and thermodynamic properties of triadic and pentadic arrays have been studied in a temperature range from 140 to 360 K. Kinetic behavior of the complexes was investigated using a fluorescent picosecond laser setup ((Delta) t approximately equals 30 ps) with 2-D (wavelength-lifetime) registration. Observed spectral properties are explained in terms of extra-ligation (red shift of all electronic bands <EQ 550 cm-1)) and excitonic splitting ((Delta) E < 1900 cm-1). Nonradiative for- and backward excitation energy transfer (K > 1010 c-1), electron transfer and d-(pi) interactions are discussed as the main paths of electronic excitation deactivation in the complexes.

Spectroscopic and SEM investigations of porous silicon doped with dyes

Results of photoluminescence (PL) and scanning electron microscopy (SEM) studies of porous silicon (PS) layers doped with stilbene 1 (St1) and kiton red 620 (Kt) dyes before and after aging are reported. The PS layers impregnated with St1 and Kt emit in different spectral regions. An extent of the layer oxidation before the dye impregnation has been shown to give a possibility of tuning the PS emission range. The revealed effects might be applied in optoelectronic technology.

Quantum-Chemical and Experimental Study of Electron-Excited State Properties and Ligation Effects in Dimeric and Trimeric Tetrapyrrole Compounds

Porphyrins, chlorophylls and their various covalently linked dimers play a number of important roles in biological systems and up to now are used as a basis for considerable biomimetic systems. One of the most important questions for chemical dimers of tetrapyrrolic compounds is the problem of the correlation of the architecture of these complexes with their absorption spectra properties. In this report we present results of semiempirical quantum mechanical configuration interaction calculations (with CNDO/S parametrization) which have been carried out on a series of M-porphyrin monomers (MgOEP, MgOEP-meso CH3, MgTPP), their covalently linked dimers with various spacers (-CH2-CH2- bond for (MgOEP)2 and phenyl ring for (MgTPP)2) as well as of corresponding complexes with pyridine. In calculations the usual geometry for monomelic compounds has been used, the geometry of chemical dimers has been created on the basis of NMR 1H data as well as of the computer simulation (HyperChem). Also we report some preliminary results for triadic complexes which are formed by two-fold coordination of (MgTPP)2 with di-pyridyl containing porphyrin free base as an extra-ligand. The results of theoretical calculations are compared with experimental data obtained for Zn-complexes of corresponding compounds mentioned above.

Picosecond Study of Energy and Electron Transfer Processes in Self-Organized Porphyrin Complexes

Natural and synthetic complexes of tetrapyrrolic compounds are known to be used in wide areas of investigations and applications: vectorial energy and electron transfer1, 2, photochemical molecular devices3, light-harvesting and -transforming systems4, sensitizers for photodynamic therapy5. The main aim of our study is focused on the constriction of supramolecular systems containing more than two π-conjugated porphyrin-like macrocycles and investigation of electronic excitation energy deactivation processes in such arrays. With respect to studies performed so far2, 6, 7 in our case porphyrin or chlorin subunits are connected in two fundamentally different ways8: i) by chemical spacers of various nature (-CH2-CH2- bond or phenyl ring) that link together monomeric compounds in Zn-porphyrin and Zn-chlorin chemical dimers and, additionally ii) by self-assembling non-covalent binding interactions of these dimers with pyridyl-mesosubstituted tetrapyrroles (two-fold extra-ligation effect with “mix and match” principle) which lead to the formation of trimeric and pentameric complexes stable in methylcyclohexane solutions at 295 K. More than 40 combinations of interacting moeties have been obtained and investigated. Computer simulated (HyperChem) structures of some systems under consideration are presented on Fig. 1.