Fluorescence line narrowing study of cyclopentaneporphyrin chemical dimers at 4.2 K

The monomers of Zn-cyclopentanporphyrins (ZnOEP-cycle and ZnOEP-cycle=CH2) 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 S₀→S₁ absorption band have been observed. In contrast to this, it has been shown that the excitation into the region of S₀→S₂ absorption band leads to FLN disappearance for individual monomers. On the basis of this effect positions of the S₀→S₂ electronic transitions in the energy scale have been determined. The normal coordinate treatment of FLN spectra of ZnOEP-cycle=CH2 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=CH2) subunits in Zn-cyclodimers manifests itself in the strong fluorescence quenching of D. Under excitation into the S₀→S₁ 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

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

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”

Fluorescence line narrowing study of cyclopentaneporphyrin chemical dimers at 4.2 K

The monomers of Zn-cyclopentanporphyrins (ZnOEP-cycle and ZnOEP-cycle=CH2) 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 S₀→S₁ absorption band have been observed. In contrast to this, it has been shown that the excitation into the region of S₀→S₂ absorption band leads to FLN disappearance for individual monomers. On the basis of this effect positions of the S₀→S₂ electronic transitions in the energy scale have been determined. The normal coordinate treatment of FLN spectra of ZnOEP-cycle=CH2 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=CH2) subunits in Zn-cyclodimers manifests itself in the strong fluorescence quenching of D. Under excitation into the S₀→S₁ 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

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

Temperature dependent steric hindrance effects in triplet state relaxation of meso-phenyl-substituted Pd-octaethylporphyrins

The analysis of spectral-kinetic results of transient absorption and phosphorescence experiments has been carried out in a wide temperature range (80-293K) for a series of Pd-octaethylporphyrin derivatives (PdOEP) with increasing number of bulky meso-phenyl substituents. In order to elucidate the influence of steric hindrance interactions, as well as porphyrin macrocycle non-planarity on photophysical characteristics and relaxation pathways of triplet states, the following parameters were compared: i) T-state positions on the energy scale, ii) phosphorescence lifetimes and T1 -Tn transient absorption spectra, iii) phosphorescence quantum efficiencies, iv) activation energies of temperature-dependent phosphorescence rate constants for planar PdOEP and non-planar PdOEP-meso(Ph)n molecules