Alexander M. Shulga

Photophysical and photochemical properties of potential porphyrin and chlorin photosensitizers for PDT

Structural and optical properties as well as photophysical and photochemical parameters (excited S1 and T1 state lifetimes at 77 K and in the presence of O2 in solution at 293 K; efficiencies of singlet oxygen, 1Δg, generation) are presented for porphyrins and chlorins with potential for the PDT of cancer: chlorin p6 and its trimethyl ester, chlorin e6 and its Na3 and K3 salts, purpurin-18 and its monomethylester, 5,10,15,20-tetrakis(3-methoxyphenyl)porphyrin (TPPM), 5,10,15,20-tetrakis(2,4-difluoro-3-methoxyphenyl)porphyrin (TPPMF) and GaTTP in different solvents (ethanol, toluene, pyridine and buffer pH 7.4) at 77–300 K. It has been shown that for monomeric chlorin e6, chlorin p6 and its derivatives the photophysical parameters are similar, as follows: fluorescence lifetimes τs in the presence of oxygen are 3.2–4.5 ns at 293 K; fluorescence quantum yields φ1 vary from 0.1 to 0.2 depending on the solvent; phosphorescence quantum yields φ1 are of t order 10−5; T1 state lifetimes τT = 1.5–2.0 ms at 77 K and 250–390 ns at 293 K in the presence of O2. By use of the direct kinetic measurement of singlet oxygen emission at 1.27 μm on laserexcitation the quantum yields of 1Δg generation by chlorins have been measured: φΔ = 0.35−0.68. In this case values of φ1 and φΔ depend strongly on the solve probably because of the formation of aggregates. For TPPM, TPPMF and Ga-TPP the φΔ values measured are higher (0.87–0.98) and are explained by the higher intersystem crossing S1 → T1 quantum yields.

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.

Interaction of multiporphyrin systems with molecular oxygen in liquid solutions: extra-ligation and screening effects

Abstract Steady-state and time-resolved studies indicate that for a sequence of porphyrin or chlorin chemical dimers Zn-cyclodimer→(ZnOEP) 2 Ph→(ZnOEP) 2 →(ZnHTPP) 2 →(ZnOEChl) 2 with relative lowering of excited S 1 - and T 1 -states, the extra-ligation by pyridine (PYR) does not influence essentially on fluorescence parameters but leads to an increase of T 1 -states non-radiative decay (the most pronounced for dimers with higher lying T 1 -levels). For pyridinated dimers at 293 K T 1 -states quenching by molecular oxygen depends on the spacer flexibility and donor–acceptor interactions with PYR. In self-assembled triads and pentads energy and electron transfer (within a few ps) takes place from Zn-dimers to pyridyl substituted porphyrin extra-ligand, H 2 P, followed by the effective population of H 2 P T 1 -state. For these systems, bimolecular constants of H 2 P T 1 -states quenching by O 2 decrease by 1.4–1.8 times with respect to those found for individual monomeric porphyrins. This effect is explained by the screening action of a strongly quenched Zn–porphyrin dimer subunit limiting the access of oxygen molecule to the excited extra-ligand.

Electronic spectra and symmetry of metalloporphyrins in low-temperature rare gas and nitrogen matrices

Abstract The electronic absorption and luminescence spectra of zinc, magnesium and free base porphine have been measured in low temperature nitrogen, argon, krypton and xenon matrices. The absorption spectra of the three compounds show similar multiplet structures, characteristic of each matrix. Analogies are discussed between the splitting of spectral lines in free base porphine, due to phototautomerization, and the crystal field induced lifting of the degeneracy of the Jahn-Teller unstable S 1 state in metalloporphyrins. The similarity between the absorption spectral pattern observed in free base porphine and metalloporphyrins indicates that the symmetry distortion in the latter occurs along the N-N axis. The magnitude of the crystal field splitting is correlated with the nature of the matrix.

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).

Self-assembled nanoscale photomimetic models: structure and related dynamics

Abstract Using static and time-resolved measurements, dynamics of non-radiative relaxation processes have been studied in self-assembled porphyrin triads of various geometry, containing the main biomimetic components, Zn–porphyrin dimers, free-base extra-ligands (porphyrin, chlorin or tetrahydroporphyrin), and electron acceptors A (quinone or pyromellitimide). The strong quenching of the dimer fluorescence is due to energy and sequential electron transfer (ET) processes to the extra-ligand (∼0.9–1.7 ps), which are faster than a slower ET (34–135 ps) from the dimer to covalently linked A in toluene at 293 K. The extra-ligand S 1 -state decay ( τ S =940–2670 ps) is governed by competing processes: a bridge (dimer) mediated long-range ( r DA =18–24 A) superexchange ET to an acceptor, and photoinduced hole transfer from the excited extra-ligand to the dimer followed by possible superexchange ET steps to low-lying charge transfer states of the triads. The subsequent ET steps dimer→monomer→A taking place in the triads, mimic the sequence of primary ET reactions in photosynthetic reaction centers in vivo.

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.

Structure and excited state properties of multiporphyrin arrays formed by supramolecular design

Structurally defined nanoscale self-assembled multiporphyrin arrays of variable geo-metry and composition (up to eight tetrapyrrole macrocycles) have been formed via two-fold extra-ligation in solutions at 77-293 K. The array formation is based on non-covalent binding interactions of the phenyl bridged Zn octaethylporphyrin chemical dimers or trimers, (ZnOEP)2Ph or (ZnOEP)3Ph2, with di- and tetrapyridyl substituted tetrapyrrole extra-ligands (porphyrin, pentafluorophenyl substituted porphyrin, Cu porphyrin, tetrahydroporphyrin). Using steady-state and time-resolved measurements, spectral properties as well as pathways and dynamics of non-radiative relaxation processes (energy migration, photoinduced electron transfer, exchange d-π effects, realized in nano-femtosecond time scale) have been studied in these complexes upon variation of the composition, mutual geometry, redox and photophysical properties of interacting subunits as well as on the tempera-ture and polarity of surrounding.

Spectroscopic and photophysical properties of covalent ether-bonded porphyrin-chlorin heterodimers

Abstract Comprehensive experimental and theoretical studies of structural, spectral and photophysical properties of the porphyrin-chlorin heterodimers 1 and 2 with a simple ether bond between macrocycles, P-O-Chl, characterised by different geometries, have been provided in toluene, tetrahydrofurane and a mixture of diethyl ester: petroleum ether: isopropanol 5:5:2 in a temperature range 77–293 K. In absorption porphyrin and chlorin subunits of the P-O-Chl heterodimers act as discrete entities rather than one large, delocalized π-electron system. The weak interaction of π-conjugated P and Chl macrocycles lead to small red shifts (by 2–5 nm) of electronic spectra of the dimers, but does not change the main photophysical parameters of Chl subunit in the P-O-Chl heterodimers 1 and 2 in a temperature range 77–293 K. Upon the optical excitation of covalently coupled π macrocycles of P-O-Chl heterodimers in solvents of various polarity, the P and Chl subunits do not interact via charge transfer mechanism, because the energies of charge-transfer (CT) states are essentially higher than the locally excited S1-states of interacting macrocycles. The strong porphyrin fluorescence quenching in P-O-Chl system is caused by the effective singlet-singlet nonradiative energy transfer to chlorin subunit with probabilities of fda = 1.35 × 1011 − 6.8 × 1010s−1 depending on temperature. This process takes place without quantum losses and occurs after the complete vibrational relaxation within the lifetime of the donor S1 excited state. The dynamics of singlet-singlet nonradiative energy transfer in a whole temperature range and the main experimental findings for the P-O-Chl heterodimer 2 are appropriately described by the Forster-Galanin inductive-resonant mechanism without any additional assumptions.