E. I. Zenkevich

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.

Photophysical properties of self-aggregated porphyrin: semiconductor nanoassemblies

Colloidal semiconductor nanocrystals from CdSe show photoluminescence quenching via titration with porphyrin derivatives. This quenching is an indication of the formation of nanoassemblies via surface attachment of pyridyl linker groups. As a consequence of the complex formation, dynamic and/or static interactions between QD and porphyrins are induced. Quenching efficiencies depend critically on sample stability, temperature, solvent, and electronic properties of the porphyrins. In order to optimize photoinduced dynamic processes these parameters have to be under control.

Energy and charge transfer dynamics in self-organized multimolecular arrays

Abstract The main kinetic parameters of the nonradiative singlet-singlet energy transfer and the charge separation have been studied for well-defined supramolecular arrays with controllable number and photophysical properties of subunits in methylcyclohexane in the temperature range of 77–300 K (cw, picosecond time-resolved fluorescence spectroscopy and femtosecond pump-probe kinetic data). The arrays formation is based on covalently linked Zn-porphyrin dimers with or without electron acceptors (quinone, anthraquinone, pyromellitimide) and the noncovalent self-assembling with pyridyl substituted tetrapyrrolic extra-ligands (porphyrins, chlorins and tetrahydroporphyrins, fluorinated tetraphenylporphyrins). It has been found that the competition between the energy transfer (within ⩽ 10 ps) and charge transfer (within 300 fs–700 ps) processes in the systems under consideration depends on the structure, spectral and redox properties of interacting subunits and may be driven by the distance, temperature and solvent polarity. The possible mechanisms of the excited S-states quenching in the complexes (including ‘superexchange’ model) are discussed.

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.

Competition between electron transfer and energy migration in self-assembled porphyrin triads

Abstract The photoinduced electron transfer (ET) and the energy migration (EM) processes have been studied in liquid solutions and polymeric (PMMA) films for the triads consisting of the Zn-octaethylporphyrin chemical dimer (the energy and electron donor, D ) and dipyridyl substituted tetrapyrrole extra-ligands (porphyrins, chlorin, tetrahydroporphyrin) as the acceptors, A . On the basis of the time correlated single photon counting technique and femtosecond pump-probe spectroscopy, it has been shown that D fluorescence quenching with time constant ranging from 1.7 to 10 ps is due to competing EM and ET processes from the dimer to A s. In addition, the fluorescence decay time shortening (by ∼1.3–1.6 times in toluene at 293 K) is observed for electron accepting extra-ligands in the triads. The acceptor fluorescence quenching is hard dependent on the mutual spatial arrangement of the triad subunits, but becomes stronger upon the solvent polarity increase (addition of acetone to toluene solutions) as well as the temperature lowering (from 278 to 221 K). The possible reasons and mechanisms of the non-radiative deactivation of locally excited S 1 -states in the triads are discussed taking into account a close lying charge-separated state. The obtained experimental data are analyzed using the reduced density matrix formalism in the frame of Haken–Strobl–Reineker approach. This model includes EM and ET processes as well as the dephasing of coherence between the excited electronic states of the triad.

Electron transfer in porphyrin multimolecular self-organized nanostructures

Abstract On the base of of covalent and non-covalent bonds nanoscale self-assembling multiporphyrin arrays with well-defined geometry, the controllable number of interacting components and their spectral and photophysical properties were formed. The deactivation of excited singlet and triplet states was studied using steady-state, time-resolved picosecond fluorescence (Δ½≈30 ps) and femtosecond pump-probe (Δ½≈280 fs) spectroscopy in solvents of various polarity at 77–300 K. It has been found that the competition between the non-radiative energy transfer (within ≤10 ps) and charge transfer (within 300 fs - 700 ps) processes in the systems depends on the structure, spectral and redox properties of interacting subunits and may be driven by the distance, temperature and solvent polarity. The possible pathways and mechanisms of the electron transfer in the systems of various types are discussed (Marcus theory for the “normal” region and the non-adiabatic case., the “superexchange” mechanism).

Radiationless intermolecular energy transfer with participation of acceptor excited triplet states

Abstract Radiationless energy transfer between like and unlike molecules has been experimentally studied under conditions where acceptor molecules have been excited to the triplet state Homogeneous singlet-triplet-triplet migration has been discovered in highlyconcentrated chlorophyll “a” and pheophytin “a” solutions in castor oil at 183 K by measuring the variation of pigment relative quantum yields of fluorescence and triplet state formation as a function of exciting pulse intensity. Heterogeneous single-triplet-triplet energy transfer has been observed in solid solutions of different complex organic molecules (perylene + phenanthrene, Na-fluorescein+chlorophyll “a”, pyrene+Mg-phthalocyanine) as the fluorescent donor state quenching in the presence of acceptor triplet-excited molecules. Primary emphasis is placed on a direct observation of the effect of energy transfer on the excited-state lifetime of the donor. The benzophenone phosphorescence quenching (shortening of phosphorescence lifetime) in the presence of Mg-mesoporphyrin triplet molecules has been found to be caused by the heterogeneous triplet-triplet-triplet energy transfer. Good agreement of the theoretical and experimental results permits us to conclude that all types of observed transfer processes are described by the Forster-Galanin theory for dipole-dipole radiationless energy transfer with no additional assumptions.