A. M. Shul’ga

Formation of intersystem crossing transitions in Pd(II) and Pt(II) porphyrins: Nonplanar distortions of the macrocycle and charge transfer states

We found new bands in the absorption spectra of Pt(II) and Pd(II) complexes of octaethylporphyrin and tetraphenylporphyrin that differ in the nature, number, and position of their side substituents. The bands are observed at 295 K in the range 570–690 nm and are attributed to spin-forbidden transitions from the ground S0 state to the excited T1 and T2 triplet states (the internal heavy atom effect). We determined the frequency distribution, number, and nature of these transitions, as well as their extinction coefficients (ɛ = 6.0–210.0 M−1 cm−1), using computer decomposition of complex contours into Gaussian components and additional data obtained from the phosphorescence and phosphorescence excitation spectra of these complexes (295–77 K). In comparison to Pd complexes of porphyrins with planar macrocycles, nonplanar distortions of the tetrapyrrole macrocycle in the ground S0 state of the sterically hindered PdOETPP molecule cause a bathochromic shift of the bands of the electronic spin-forbidden S0 → T1 and S0 → T2 transitions, as well as an increase in their extinction coefficients. For the PdOEP-Ph(o-NO2) molecule, which contains the electron acceptor nitro group, an absorption band attributed to an electronic transition from the ground state S0 to a charge transfer state (λmax = 905 nm, ɛ = 10.0 M−1 cm−1) is observed at 295 K.

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.

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 ketT=(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 ππ* states with charge-transfer states.

Spectral manifestations of nonplanarity effects in Pd complexes of porphyrins

It is found on the basis of kinetic spectral studies in nonane, dodecane, and vitreous solutions at 77 K (phosphorescence spectra, polarized phosphorescence, decay kinetics, and phosphorescence excitation spectra) that symmetric and nonsymmetric Pd-porphyrins are characterized by the presence of two noninteracting spectrally different long-and short-wavelength forms in the ground state. The existence of the long-wavelength form is associated with the displacement of the central Pd ion from the macrocycle plane, which leads to the formation of the nonplanar “dome” conformation of the porphyrin ligand. In the case of a nonsymmetrically substituted Pd-tetramethyl-diethylporphyrin molecule, the nonplanar conformation of the π-conjugated macrocycle is characterized by the splitting of the triplet state into two components (T1 and T2, Δν=90 cm−1 at 77 K). Both narrow components, which have the same decay, form a dual phosphorescence of the long-wavelength form of this compound, which is caused by efficient thermal exchange between T1 and T2 levels in the course of deactivation to the ground state.

Fine-structure spectra of metal complexes of porphin in tetrahydrofuran at low temperatures: Manifestation of nonplanarity effects

The fine-structure fluorescence spectra of Mg and Zn porphins in solid tetrahydrofuran matrices at the liquid helium temperature are recorded for the first time. The fluorescence spectra of Mg porphin molecules deposited from the gas phase on a sapphire substrate simultaneously with tetrahydrofuran molecules are measured. Based on the data obtained from the fluorescence and fluorescence excitation spectra, it is ascertained that there are two noninteracting spectrally different long-and short-wavelength forms of metal complexes of porphins in the ground state. In the case of Mg porphin, the spectral gap for these two forms at 4.2 K amounts to 330 cm−1. The short-wavelength form is attributed to the nonplanar saddle conformation of a porphin molecule, while the long-wavelength form is associated with the nonplanar domed conformation, in which the central metal ion is displaced out of the plane of the porphyrin ligand. The fine-structure fluorescence spectra of both forms of Mg and Zn porphins are measured in tetrahydrofuran at 4.2 K upon selective laser excitation. The frequencies of the normal vibrations in the ground electronic state are determined and the reasons for the differences in the vibrational frequencies of the forms are discussed.

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.

Spectral manifestations of the splitting of the lowest triplet levels in Pd complexes of nonsymmetric porphyrins

Based on the study of the phosphorescence and phosphorescence excitation spectra of Pd(II) tetramethylporphyrin (PdTMP) and Pd(II) tetramethyldiethylporphyrin (PdTMDEP) in solutions in 2-methyltetrahydrofuran and dodecane in the temperature range 77–283 K, the occurrence of the splitting of the lowest degenerate singlet (S1) and triplet (T1, T2) levels of porphyrin molecules is established. In the absorption of molecules of the compounds studied, two components, S1 and S2, are revealed in the range of allowed long-wavelength Q(0,0) transitions (530–550 nm) and four components, T1–T4, are observed in the range of spin-forbidden intersystem crossing transitions S0 → Tn (560–670 nm), with all the triplet levels T1-T4 being located below the S1 level on the energy scale. It is shown that an increase in the degree of deformation of the porphyrin macrocycle caused by steric interactions between β alkyls and hydrogen meso atoms is accompanied by an increase in the splitting in the system of singlet levels δE(S2–S1) from 120 cm−1 for PdTMDEP to 215 cm−1 for PdTMP, as well as by an increase in the splitting in the system of triplet levels δE(T2–T1) from 190 cm−1 for PdTMDEP to 250 cm−1 for PdTMP.

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.

Picosecond dynamics of photoinduced electron transfer involving singlet and triplet states in nitroporphyrin molecules

The dynamics of relaxation processes in meso-ortho-nitrophenyl-substituted octaethylporphyrins OEP-Ph(o-NO2) and PdOEP-Ph(o-NO2) occurring with the participation of the S1 and T1 states is studied in polar (dimethylformamide) and nonpolar (toluene) media at 295 and 77 K using pico-and nanosecond laser kinetic spectroscopy. It is shown that, at 295 K, the steric interactions between β-alkyl substituents in porphyrins and ortho-nitro groups of meso-phenyl in these compounds create optimal conditions for overlapping of molecular orbitals of the donor and the acceptor, which ensures an efficient photoinduced electron transfer (PET). It is ascertained that, in free-base OEP-Ph(o-NO2), the PET occurs only via the S1 state of porphyrin in time intervals of 40 ps (dimethylformamide) and 125 ps (toluene), whereas the competing intersystem crossing S1 ⇝ T1 is unlikely. In the case of the metal complex PdOEP-Ph(o-NO2), the PET involves both S1 and T1 states. In the PdOEP-Ph(o-NO2) molecule, direct PET from the T1 state to the charge-transfer state also occurs in the picosecond range (20 and 46 ps for dimethylformamide and toluene, respectively, at 295 K), with the rate constants being 3–5 times smaller in comparison with the corresponding values found for the PET occurring via the S1 state of this compound. For both compounds, the experimentally observed long-lived component in the decay kinetics of the induced T1-Tnabsorption (250–700 ns) is due to the recombination processes in ion-radical pairs whose lifetime decreases with an increase in the polarity of the medium and is almost independent of the presence of molecular oxygen in the solution. The PET is completely absent at 77 K in the compounds under investigation when they are dissolved in rigid solutions.