E. I. Sagun

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.

Photoinduced relaxation processes in complexes based on semiconductor CdSe nanocrystals and organic molecules

Possible pathways and mechanisms of photoinduced relaxation processes in CdSe and CdSe/ZnS nanocrystals that are surface-passivated (as a result of two-point interactions) by nitrogen-containing ligands of different nature (pyridyl-substituted porphyrin molecules and their derivatives, 2,2′-dipyridyl, and 1,10-phenanthroline) are studied in toluene at 295 K by the methods of steady-state and time-resolved spectroscopy. In nanocrystal-organic ligand composites, a high luminescence-quenching efficiency of nanocrystals by molecules of tetrapyrrole compounds compared to 2,2′-dipyridyl, 1,10-shenanthroline, and pyridine can be associated with the electronic properties of a π conjugated macrocycle and anchor groups. The fundamental role that mesomeric effects and the partial overlap of HOMOs and LUMOs of porphyrin and meso-pyridyl rings play in the enhancement of nonradiative recombination of charges in a surface interface layer is substantiated.

Relaxation processes with participation of excited S1 and T1 states of spatially distorted meso-phenyl-substituted octamethylporphyrins

We have performed comparative analysis of the spectral properties and photophysical parameters of spatially distorted octamethylporphyrins (OMPs) upon varying the number (from one to four) and position of bulky meso-phenyl substituents in liquid solutions at 293 K and in solid media at 77 K. It has been substantiated that, for the series of studied compounds, which are characterized by a high degree of nonplanar distortions, considerable changes in the spectral-kinetic properties of singlet and triplet states at 293 K in liquid solutions are determined by the dynamic relaxation of the tetrapyrrole macrocycle that is caused by steric interactions of meso-phenyls with neighboring bulky groups β-CH3. The decay of T1 states of mono- and di-meso-phenyl-substituted (trans) OMP molecules in liquid solutions has a biexponential character, which is indicative of conformational rearrangements of the macrocycle in the T1 state. We are the first to measure the phosphorescence of spatially distorted metal-free OMP molecules in solid solutions at 77 K. It has been sub-stantiated that the decrease in the phosphorescence lifetime and quantum yield observed in the series of studied compounds with increasing number of meso-phenyl substituents at 77 K is related to a gradual increase in the probability of the nonradiative intersystem crossing, caused by the lowering of the energy of the T1 level. We have revealed that spatially distorted tetrapyrrole macrocycles in rigid media at 77 K in the ground state are represented by two conformations. It has been found that the oxygen quenching rate constants of excited S1 and T1 electronic states of planar and spatially distorted porphyrin molecules are close to each other and depend weakly on the character distortion of the macrocycle. We have shown that the singlet oxygen generation quantum yield (γδ) of studied compounds depends on the number of meso-phenyl substituents in the OMP molecule and is determined by the character of the conformational dynamics of the tetrapyrrole macrocycle at 293 K.

Photoinduced relaxation processes in self-assembling complexes from CdSe/ZnS water-soluble nanocrystals and cationic porphyrins

Particular features and quenching mechanisms of exciton luminescence of water-soluble nanocomposites that are formed as a result of the interaction of surface charged semiconductor quantum dots (QDs) CdSe/ZnS (dCdSe = 2.8 nm) and cationic porphyrins (H2TMPyrP4+ and ZnTMPyrP4+) have been studied theoretically and experimentally. It has been found that, in CdSe/ZnS⊕Porphyrin conjugates, there occurs long-range inductive resonance electronic excitation energy transfer from surface modified (with thioglycolic or mercaptoundecanoic acid) QDs to porphyrins, which is accompanied by quenching of the exciton luminescence of QDs and an increase in the fluorescence intensity of porphyrin. It has been shown that, when mercaptoundecanoic acid is used as a QD shell, the QD luminescence quenching efficiency by porphyrins follows the Förster-Galanin theory and depends on the overlap integral between the CdSe/ZnS luminescence band and the absorption spectra of free-base porphyrin H2TMPyrP4+ and its metal complex ZnTMPyrP4+. It has been revealed that, as the QDs ↔ Zn-porphyrin intercenter distance decreases from 39.1 (mercaptoundecanoic acid) to 30.1), a considerable QD luminescence quenching is observed; however, the energy transfer efficiency substantially decreases, from 55% in the former case to 23% in the latter one. Based on the spectral-luminescent data and quantum-chemical calculations, it has been found that the chemical change of H2TMPyrP4+ in the structure of the complex with CdSe/ZnS QDs passivated by thioglycolic or mercaptoundecanoic acid is caused by the formation of a metal complex ZnTMPyrP4+. Based on calculations of the redox-potentials, it has been concluded that the low luminescence quantum yield of CdSe/ZnS QDs passivated by residues of mercaptocarboxylic acids S−(CH2)nCOO− and its dependence on the number of CH2 groups are related to the possibility of photoinduced electron transfer from the HOMO of passivating molecules to QDs (QD* ⇐ S−(CH2)nCOO− hole transfer). It has been shown that the quenching of the exciton luminescence of QDs in heterogeneous structures CdSe/ZnS(thioglycolic acid)⊕ZnTMPyrP4+, which is complementary to the energy transfer, can be caused by the photoinduced electron transfer that involves the participation of the LUMO of the ZnTMPyrP4+ molecule (QD* ⇒ ZnTMPyrP4+).

Photoinduced electron transfer in meso-nitrophenyl-substituted porphyrins and their chemical dimers

It is shown that steric interactions of volume substituents in the β-positions of pyrrole rings and the nitro group in mono-and di-meso-phenyl-substituted of octaethylporphyrins and their chemical dimers containing the electron-acceptor NO2 group in the ortho-position of the phenyl ring at 295 K favor the direct overlap of molecular orbitals of the interacting subunits, resulting in the efficient quenching of fluorescence due to the direct electron transfer from the S1 level to the lower-lying state via the “through-space” mechanism. The electron transfer in these compounds in nonpolar media (the rate constant ketS=(3.2–9.5)×109 s−1 is nonadiabatic, whereas in strongly polar solvents (ketS=2×1011 s−1) the adiabatic effects can be manifested. In compounds containing the NO2 group in meta-or para-positions of the phenyl ring, the nonadiabatic electron transfer from the S1 level occurs less efficiently both in polar [ketS=(0.2–5)×1010 s−1] and nonpolar media [ketS=(0.1–1.0)×107 s−1]. In this case, the electron transfer involves molecular orbitals of phenyl (the “through-bond” mechanism), and its efficiency depends on the orbital electron density in the meta-and para-positions of the phenyl ring. Based on the experimental data obtained and analysis of the electron transfer within the framework of the Marcus theory, the energy scheme of relaxation processes of the electronic energy in the compounds under study involving charge transfer states is suggested.

Relaxation processes in self-assembling triads based on porphyrin Zn-heterodimers

The directional self-assembly of nanosized, structurally organized triads is implemented in methylcyclohexane at 295 K, which is based on the two-point extra coordination of nonsymmetric, covalently bound heterodimers of Zn porphyrins to bipyridyl-substituted porphyrin free-base extra ligands. Based on experimental data and theoretical calculations, the structural organization is determined and information on the energetics of electronic interactions of components is obtained and the rate constants of the directional energy transfer (kET ∼ 1011 s−1) and photoinduced electron transfer (kPET ∼ 2.7 × 109 s−1) are determined. The effects of the orientation of interacting macrocycles, the intercenter distances and the solvent temperature on the efficiency of relaxation processes in the triad is investigated.

Self-assembling effects and mechanisms of interchromophore interactions in porphyrin pentads

The directional self-assembly of nanosized, structurally organized pentads that include five tetrapyrrole macrocycles and are based on the two-point coordination interaction of two covalently bound dimers of Zn porphyrins (homo-and heterodimers) with molecules of either the free base or the Cu complex of tetrametapyridyl-substituted porphyrin extra ligand is implemented in methylcyclohexane at 295 K. Using the method of the density functional theory (DFT) in the B3LYP/6-31g(d) approximation, the geometry of the pentad is fully optimized and the main factors that determine its redox properties are determined. The energies of the lowest excited states of the pentad are calculated by the ZINDO/S method, and it is shown that the occurrence of identical molecules in the system facilitates the formation of excitonic states with different contributions from the charge-transfer component. The directional energy transfer and the photoinduced electron transfer, which leads to the formation of a low-lying charge-transfer state (CT state), are studied and the rate constants of these two processes are determined (kET ∼ 1011 s−1 and kPET ≈ (1.8−6.0) × 109 s−1, respectively). The strong effect of the temperature of the solvent on the efficiency of relaxation processes in pentad complexes under study is revealed and studied. Roles played by the low-lying CT state and d-π exchange effects (the Cu-contained pentad) in fluorescence quenching of pentad complexes are determined.

The influence of electronic and structural factors on the photoinduced electron transfer in sterically strained meso-nitrophenyl-substituted porphyrins

The electronic and structural factors affecting the efficiency of the photoinduced electron transfer in meso-nitrophenyl-substituted octaethylporphyrins are theoretically analyzed by semiempirical methods of quantum chemistry. It is shown that the experimental differences between the rate constants of electron transfer associated with the change in the position of the nitro group in the meso-phenyl ring (ortho, meta, or para positions) are determined by such factors as torsional vibrations of the phenyl ring around the single C1-Cm bond, electronic properties of the phenyl group, rotations of the nitro group around the single C-N bond, and out-ofplane deformations of the porphyrin macrocycle. It is ascertained that the matrix elements of electronic interactions and the energies of excited charge-transfer states depend substantially both on the position of the nitro group in the meso-phenyl ring and on intramolecular vibrations of the phenyl and the nitro groups. In nonpolar media, the fluorescence quenching in the compounds under study occurs mainly due to the admixture of chargetransfer excitations to the lowest S1 state of porphyrin. Upon increasing polarity, the main channel of deactivation of excited singlet states is direct photoinduced electron transfer either through space (the ortho position) or through a bond involving the participation of orbitals of the phenyl spacer (the meta and para positions).

Dynamic relaxation of the triplet states of mono- and dimesoaryl derivatives of porphyrins and their chemical dimers

For the first time it is discovered that the mono- and dimesoaryl substitution in porphyrins (free bases and their Zn-complexes) as well as the formation of chemical dimers with a phenyl spacer are manifested in the abrupt reduction (300–500 times less) of the lifetime of T1-states at 295 K and are not accompanied by enhancement of the nonradiative deactivation of the S1-state. These effects are associated with the nonlinear dynamic deformations of the π-conjugate macrocycle in the T1-state due to torsional oscillations of a phenyl ring about a single C–C-bond in sterically complicated porphyrins.

Conformational dynamics and spin-orbit interactions in a series of sterically hindered porphyrins in the lowest triplet state

We have studied the nature and unusual photophysical properties of triplet states of a series of sterically hindered porphyrins (meso-phenyl-substituted derivatives of octamethylporphyrin (PS-OMP)) using quantum-chemical calculations of the parameters that determine the probability of deactivation of the T1 state and modeling the T-T absorption spectra. We show that a decrease in the lifetime of the T1 state of PS-OMP is related with the enhancement of the channel of nonradiative deactivation (T1 → S0), which occurs (i) due to the conformational lability in the T1 state, as a result of which the energy gap T1-S0 considerably decreases, and (ii) because of an increase in the matrix element of the spin-orbit interaction due to a change in the hybridization of atoms of the macrocycle as a consequence of its nonplanar distortion. The value of the vibronically induced matrix element of the spin-orbit interaction between the S0 and T1 states of PS-OMP depends weakly on the type of the conformation and the value and the character of distortion of the porphyrin macrocycle. The conformational lability of PS-OMP clearly manifests itself in the spectra and kinetics of the T-T absorption of these compounds at room temperature and determines the nonmonoexponential character of the phosphorescence decay kinetics in frozen matrices. Using porphin as an example, we have shown that nonplanar distortions of the macrocycle facilitate a decrease in the phosphorescence rate constant at 77 K, which is caused, on the one hand, by an increase in the energy gaps T1-Sn and S0-Tn and, on the other hand, by an increase in the wavelength of the transition T1 → S0.