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9 July 2007 Laser induced picosecond electron transfer with participation of excited states for porphyrins covalently linked with electron acceptor
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Abstract
The dynamics and pathways of relaxation processes in meso-ortho-nitrophenyl substituted octaethylporphyrins OEP-OEPPh(o-NO2) and PdOEP-Ph(o-NO2) occurring with the participation of the S1 and T1 states was studied in polar Ph((dimethylformamide) and nonpolar (toluene) solvents at 295 and 77 K using pico- and nanosecond laser kinetic spectroscopy. At 295 K, steric interactions between bulky β-alkyl substituents and ortho-nitro groups of meso-phenyl in these compounds create optimal conditions for overlapping of molecular orbitals of the porphyrin macrocycle (donor) and NO2-group (acceptor), thus leading to an efficient photoinduced electron transfer (PET). For free-base OEP-Ph(o-NO2), PET occurs only via the porphyrin S1 state within 40 ps (dimethylformamide) and 125 ps (toluene), whereas the competing intersystem crossing S1 ~~>T1 is low probable. For metallocomplex PdOEP-Ph(o-NO2), PET involves both S1 and T1 states. In the last case, the direct PET from the T1 state to CT state also occurs within picosecond range (20 and 46 ps for dimethylformamide and toluene, respectively, at 295 K). Rate constants for PET with participation of T1 states are by 3-5 times smaller with respect to those found for PET occurring via the S1 state. For both compounds, the observed long-lived component (250-700 ns) in decays of the transient T1-Tn absorption is due to the recombination processes of radical-ion pairs whose lifetime decreases with an increase of the surrounding polarity and is hardly dependent on the presence of molecular oxygen in the solution. For both compounds, PET is completely absent in rigid solutions at 77 K.
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Evgenii I. Sagun, Eduard I. Zenkevich, Valentin N. Knyukshto, Alexander M. Shulga, and Sergei A. Tikhomirov "Laser induced picosecond electron transfer with participation of excited states for porphyrins covalently linked with electron acceptor", Proc. SPIE 6727, ICONO 2007: Nonlinear Laser Spectroscopy and High-Precision Measurements; and Fundamentals of Laser Chemistry and Biophotonics, 67272U (9 July 2007); https://doi.org/10.1117/12.752504
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