Pathways for photoinduced electron transfer in meso-nitro-phenyl-octaethylporphyrins and their chemical dimers

The photophysical properties of meso-nitro-phenyl-octaethylporphyrins and their dimers with electron-accepting NO₂ groups in the para-, meta- and ortho-positions of the phenyl ring were studied. For the ortho-NO₂ case in deaerated toluene at 295 K, strong fluorescence quenching is caused by the intramolecular electron transfer from the porphyrin S₁ state in the absence of phenyl ring librations around the single C–C bond (‘normal’ region, non-adiabatic case). T₁ state lifetime shortening for the same compounds is explained by thermally activated transitions to upper-lying charge-transfer states of the radical ion pair as well as by the rise of the intersystem crossing T₁ → S₀ rate constants caused by T₁ states mixing with charge-transfer states. © 1999 Elsevier Science B.V. All rights reserved

Laser confocal and spatially-resolved fluorescence spectroscopy of porphyrin distribution on plasma deposited polymer films

Laser confocal microscopy and spatially-resolved fluorescence spectroscopy have been used for the study of the interaction of di(p-aminophenyl)etioporphyrin (DAPEP) and tetra(p-aminophenyl)porphyrin (TAPP) molecules with the surface of thin (7-15 μm) polypropylene films treated by 0.5 M KCl solution and activated by air non-equilibrium plasma at the normal atmospheric pressure. Confocal microscopy data (using laser scanning microscope LSM 510, Carl Zeiss) show that after treatment the polymer surface becomes inhomogeneous, and porphyrin moieties are randomly distributed on both film surfaces with a penetration depth of ~1 μm. On the basis of spatially resolved fluorescence measurements (using a home-built confocal microscope with a time resolution up to 100 ms/frame and high spatial resolution) it has been found that fluorescence spectra of individual spots correspond to monomeric species. It means that spatially closed few porphyrin molecules in the spot are not aggregated and do not interact significantly

Steric interactions influence on electron transfer efficiency in meso-nitrophenylporphyrins and their chemical dimers

Steric interactions upon mono- and di-meso-phenyl substitution in octaethylporphyrins (OEP) and their chemical dimers with the phenyl spacer manifests itself in the dramatical T₁-state lifetimes shortening at 293 K (from ms down to μs in degassed toluene solutions) without any influence on spectral-kinetic parameters of S₀- and S₁-states. This effect is explained by non-planar dynamic conformations in excited T₁-states caused by the phenyl ring torsional librations around a single C-C bond. For meso-ortho-nitrophenyl substituted OEPs the S₁-state quenching is caused by the direct “through space” electron transfer to low-lying CT state while for the corresponding Pd-complexes the direct electron transfer takes place from the locally excited T₁-state. For the last two cases steric interactions provide the optimal geometry with high electronic coupling between porphyrin macrocycle and nitro-group

Manifestation of nonplanarity effects and charge-transfer interactions in spectral and kinetic properties of triplet states of sterically strained octaethylporphyrins

Properties of the triplet states of octaethylporphyrins with the steric hindrance (free bases and Pd complexes) are studied by the methods of stationary and kinetic spectroscopy in the temperature range from 77 to 293 K. The mono-mesophenyl substitution results in a decrease in the quantum yield and shortening of the phosphorescence lifetime of Pd complexes by 250–3500 times in degassed toluene at 293 K. The phosphorescence quenching is caused by nonplanar dynamic conformations of the porphyrin macrocycle in the T₁ state, which also lead to the appearance of new bands at λ ~ 1000 nm in the T—T absorption spectra. As the number of meso-phenyls (Pd-octaetyltetraphenylporphyrin) increases, the quantum yield of phosphorescence further decreases (<10⁻⁵) at 293 K, the lifetime of the T₁ state shortens (<50 ns), and the efficiency of the singlet oxygen generation abruptly decreases (<0.01). The intense bathochromic emission of this compound at 705 nm with a lifetime of 1 ms at 77 K is assigned to the phosphorescence of a nonplanar conformation. Upon meso-orthonitrophenyl substitution, the quenching of phosphorescence of Pd complexes (by more than 10⁴ times at 293 K) is caused by direct nonadiabatic photoinduced electron transfer from the T₁ state to the nearest charge-transfer state with the probability k_et^T = (1.5-4.0) × 10⁶ s⁻¹. The induced absorption of ortho-nitro derivatives in the region between 110 and 1400 nm is caused by mixing of pure ππ⃰ states with charge-transfer states. © 2001 MAIK “Nauka/Interperiodica”

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

Steady-state and time-resolved studies indicate that for a sequence of porphyrin or chlorin chemical dimers Zn-cyclodimer → (ZnOEP)₂Ph → (ZnOEP)₂ → (ZnHTPP)₂ → (ZnOEChl)₂ with relative lowering of excited S₁- and T₁-states, the extra-ligation by pyridine (PYR) does not influence essentially on fluorescence parameters but leads to an increase of T₁-states non-radiative decay (the most pronounced for dimers with higher lying T₁-levels). For pyridinated dimers at 293 K T₁-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, H₂P, followed by the effective population of H₂P T₁-state. For these systems, bimolecular constants of H₂P T₁-states quenching by O₂ 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. © 2002 Elsevier Science B.V. All rights reserved

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

Wavelength dependence of the efficiency of singlet oxygen generation upon photoexcitation of photosensitizers

The dependence of the efficiency of singlet oxygen (1Δg) generation upon excitation of photosensitizer at different wavelength was observed for several derivatives of palladium porphyrin in carbon tetrachloride. The efficiency of singlet oxygen generation upon excitation in a blue region of the spectrum (Soret band) exceeds by several times the efficiency at excitation in the red spectral region (Q band). The effect of enhancement of singlet oxygen generation upon CW photoexcitation to Soret band of photosensitizer may be explained by influence of high laying triplet states of a donor molecule on the triplet-triplet energy transfer

Unusual dynamic relaxation of triplet-excited meso-phenyl-substituted porphyrins and their chemical dimers at room temperatures

It has been found that for mono- and di-meso-phenyl-substituted octaethylporphyrin (OEPs), etioporphyrins, corresponding Zn complexes and OEP chemical dimers (with a meso-phenyl spacer) a drastic shortening of triplet lifetimes from a ms to μs timescale in deaerated toluene solutions at 295K takes place without any changes of the spectral-kinetic parameters of the S₀ and S₁ states. The observed effects are connected with torsion librations of the phenyl ring around a single C–C bond in sterically encumbered porphyrins leading to non-planar dynamic distorted conformations in excited T₁ states accompanied by an enhancement of the T₁-state nonradiative deactivation pathways

Pathways for photoinduced electron transfer in meso-nitro-phenyl-octaethylporphyrins and their chemical dimers

The photophysical properties of meso-nitro-phenyl-octaethylporphyrins and their dimers with electron-accepting NO₂ groups in the para-, meta- and ortho-positions of the phenyl ring were studied. For the ortho-NO₂ case in deaerated toluene at 295 K, strong fluorescence quenching is caused by the intramolecular electron transfer from the porphyrin S₁ state in the absence of phenyl ring librations around the single C–C bond (‘normal’ region, non-adiabatic case). T₁ state lifetime shortening for the same compounds is explained by thermally activated transitions to upper-lying charge-transfer states of the radical ion pair as well as by the rise of the intersystem crossing T₁ → S₀ rate constants caused by T₁ states mixing with charge-transfer states. © 1999 Elsevier Science B.V. All rights reserved