TY - JOUR
T1 - Intramolecular and intermolecular photoinduced electron transfer in isomeric mesoporphyrin nitrobenzyl esters
T2 - Structure and solvent effects
AU - Fajari, Lluis
AU - Fors, Pere
AU - Lang, Kamil
AU - Nonell, Santiago
AU - Trull, Francesc R.
N1 - Funding Information:
This work was part of the CICYT research programme MAT91-0901-C03-01. Financial support to P.F. and K.L. from the Spanish Ministerio de Educaci6n y Ciencia is acknowledged. S.N. thanks the Max-Planck-Gesellschaft for an Otto Hahn fellowship. Credit should be given to Dr. J. DeGraziano and Professors D. Gust and T.A. Moore (Center for the Study of Early Events in Photosynthesis, Arizona State University) for their assistance with the time-resolved fluo-
PY - 1996/1/25
Y1 - 1996/1/25
N2 - Photoinduced intramolecular electron transfer was studied in a series of flexible dyads consisting of the isomeric ortho-, meta-and para-nitrobenzyl monoesters and diesters of mesoporphyrin existing preferentially in folded conformations. This process was compared with the intermolecular electron transfer between mesoporphyrin dimethyl ester and the 2-, 3-and 4-nitrobenzyl acetates. In both cases, electron transfer occurs from the porphyrin first excited singlet state, and the rate constant is lower in the ortho isomer, although intermolecular electron transfer is less isomer selective. The selectivity in the intermolecular case can be related to the isomer redox potentials, but in the dyads an ortho effect must also be taken into account; this effect produces a deviation from coplanarity between the nitro group and the phenyl ring. Electron paramagnetic resonance (EPR) in a low temperature matrix permits the charge transfer product to be detected. The process does not lead to the production of long-lived species, as shown by optoacoustic calorimetry. The efficiency of photoinduced electron transfer strongly depends on the ability of the solvent to stabilize the radical ion pair; this process occurs in CH3OH, CH3CN and CH2Cl2, but is inhibited in toluene and benzene. In the last two solvents, singlet oxygen is formed instead via energy transfer from the triplet state, with the same quantum yield as for the nitrobenzyl-free porphyrin.
AB - Photoinduced intramolecular electron transfer was studied in a series of flexible dyads consisting of the isomeric ortho-, meta-and para-nitrobenzyl monoesters and diesters of mesoporphyrin existing preferentially in folded conformations. This process was compared with the intermolecular electron transfer between mesoporphyrin dimethyl ester and the 2-, 3-and 4-nitrobenzyl acetates. In both cases, electron transfer occurs from the porphyrin first excited singlet state, and the rate constant is lower in the ortho isomer, although intermolecular electron transfer is less isomer selective. The selectivity in the intermolecular case can be related to the isomer redox potentials, but in the dyads an ortho effect must also be taken into account; this effect produces a deviation from coplanarity between the nitro group and the phenyl ring. Electron paramagnetic resonance (EPR) in a low temperature matrix permits the charge transfer product to be detected. The process does not lead to the production of long-lived species, as shown by optoacoustic calorimetry. The efficiency of photoinduced electron transfer strongly depends on the ability of the solvent to stabilize the radical ion pair; this process occurs in CH3OH, CH3CN and CH2Cl2, but is inhibited in toluene and benzene. In the last two solvents, singlet oxygen is formed instead via energy transfer from the triplet state, with the same quantum yield as for the nitrobenzyl-free porphyrin.
KW - Donor-acceptor dyads
KW - Intramolecular
KW - Photoinduced electron transfer
KW - Porphyrins
UR - http://www.scopus.com/inward/record.url?scp=0030600668&partnerID=8YFLogxK
U2 - 10.1016/1010-6030(95)04192-3
DO - 10.1016/1010-6030(95)04192-3
M3 - Article
AN - SCOPUS:0030600668
SN - 1010-6030
VL - 93
SP - 119
EP - 128
JO - Journal of Photochemistry and Photobiology A: Chemistry
JF - Journal of Photochemistry and Photobiology A: Chemistry
IS - 2-3
ER -