Enhanced Electron-Transfer Properties of Cofacial Porphyrin Dimers through p-p Interactions (original) (raw)
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Photoinduced electron transfer in meso-nitrophenyl-substituted porphyrins and their chemical dimers
Optics and Spectroscopy, 2000
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 NO 2 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 S 1 level to the lower-lying state via the "through-space" mechanism. The elec
The Journal of Physical Chemistry C, 2010
phyrin) afforded a porphyrin triad, Sn(DPP)(H 2 F 16 DPPCOO) 2 (1), in which the Sn(DPP) unit is linked with the two H 2 F 16 DPPCOOunits by strong coordination bonds. The H 2 F 16 DPPCOOunit of Sn(DPP)-(H 2 F 16 DPPCOO) 2 was diprotonated by the reaction with trifluoroacetic acid (CF 3 COOH) to afford a robust electron acceptor-donor-acceptor porphyrin triad, Sn(DPP){(H 4 F 16 DPPCOO)(CF 3 COO) 2 } 2 (2), in which the Sn(DPP) unit and the H 4 F 16 DPP 2+ COO-(H 4 F 16 DPPCOO +) unit act as an electron donor and an acceptor, respectively. The photodynamics of 1 was examined by femtosecond laser flash photolysis measurements in PhCN to reveal that the energy transfer occurs from the singlet excited state of the Sn(DPP) unit to the H 2 F 16 DPPCOOunit to generate the singlet excited state of H 2 F 16 DPPCOO-. In contrast to the case of 1, the transient absorption spectra of 2 that contains the diprotonated form (H 4 F 16 DPPCOO +), observed by femtosecond laser flash photolysis, clearly indicated the occurrence of fast electron transfer from the singlet excited state of the Sn(DPP) unit to the H 4 F 16 DPPCOO + unit. The resulting singlet electron-transfer (ET) state composed of Sn(DPP) •+ and H 4 F 16 DPPCOO • decays to the ground state with the rate constant of 1.4 × 10 10 s-1 in competition with generation of the triplet ET state, which was also detected by the nanosecond transient absorption spectroscopy. The lifetime of the triplet ET state (50 µs) was much longer than that of the singlet ET state (71 ps) due to the spin-forbidden character of the back electron-transfer process.
Israel Journal of Chemistry, 1989
The spin density distributions in the cation radicals of various covalently linked porphyrin dimers have been studied by liquid phase ESR and ENDOR methods to find out whether these systems show intramolecular electron delocalization. Such a delocalization is known to occur in the bacteriochlorophyll dimer ("special pair") in the photosynthetic reaction center. The dimers that were studied in this work were derived from zinc mesotetratolylporphyrin (ZnTTP) and linked at the ortho or para positions of one phenyl ring with varying bridge lengths. IH and 14N hyperfine coupling constants could be measured for the dimer cation radicals and compared with those of monomeric ZnTTP as well as ZnTTP derivatives that carry alkoxy or hydroxy substituents to mimic the bridges of the dimers. By comparing the hyperfme data of the monomer and dimer cation radicals it is concluded that in the present dimers the unpaired electron is localized on one porphyrin unit.
Journal of Porphyrins and Phthalocyanines, 2001
Porphyrins and related compounds are basic moieties which upon photoexcitation produce paramagnetic transients important to many processes in biology, material science and light–energy conversion. This short review demonstrates the application of time-resolved EPR spectroscopy to two processes in which the photoexcited singlet and/or triplet are involved: (1) intramolecular electron transfer in photoexcited donor–acceptor systems embedded in liquid crystals, where the porphyrins are the electron donors attached to different types of acceptors; and (2) intermolecular magnetic interactions between photoexcited porphyrin triplets and free radicals. In both systems the electron spin plays an important part with regards to the route of the magnetic interactions involved.
Chemphyschem, 2008
Two compounds containing a porphyrin dimer and a perylene tetracarboxylic diimide (PDI) linked by phenyl (1) or ethylene groups (2) are prepared. The photophysical properties of these two compounds are investigated by steady state electronic absorption and fluorescence spectra and lifetime measurements. The ground state absorption spectra reveal intense interactions between the porphyrin units within the porphyrin dimer, but no interactions between the porphyirn dimer and PDI. The fluorescence spectra suggest efficient energy transfer from PDI to porphyrin accompanied by less efficient electron transfer from porphyrin to PDI. The energy transfer is not affected by the dimeric structure of porphyrin or the linkage between the porphyrin dimer and PDI. However, the electron transfer from porphyrin to PDI is significantly affected by either the linkage between the donor and the acceptor or the polarity of the solvents. The dimeric structure of the porphyrin units in these compounds significantly promotes electron transfer in nonpolar, but not in polar solvents.
The Journal of Physical Chemistry A, 1998
Two closely related, rigidly linked porphyrin-naphthoquinone dyads have been prepared and studied using time-resolved fluorescence and absorption methods. Dyad 1, whose quinone carbonyl groups are relatively close to the porphyrin macrocycle, exhibits photoinduced electron-transfer rate constants that are virtually independent of solvent dielectric constant and temperature within the range 77-295 K. Dyad 2, which has a similar donor-acceptor linkage but whose quinone carbonyl groups are ∼2 Å farther from the porphyrin, features photoinduced electron-transfer rate constants that decrease with decreasing solvent dielectric constant. Electron transfer in this molecule ceases at low temperatures. Photoinduced electron transfer in dyad 2 exhibits the usual dependence on free energy change and solvent reorganization observed in many similar porphyrinquinone systems. The behavior of 1 may be attributed at least in part to the smaller separation of the porphyrin radical cation and the quinone radical anion, which leads to nearly barrierless electron transfer and makes transfer less susceptible to effects due to changes in solvent dielectric properties and temperature. Charge recombination rates in the dyads are substantially slower than charge separation rates, unlike those of many porphyrin-quinone systems. This suggests that these molecules might be useful as components of more complex molecular devices. S1089-5639(98)00068-1 CCC: $15.00
Multistep photoinduced electron transfer in self-organised nanoscale porphyrin triads
Physica E: Low-dimensional Systems and Nanostructures, 2002
Well-deÿned structurally organised porphyrin triads of a controlled geometry and nanoscale size have been formed in liquid solutions using the combination of a covalent approach and non-covalent self-assembly. The triads contain zinc-octaethylporphyrin chemical dimer, (ZnOEP) 2Ph, with covalently linked electron acceptors (p-benzoquinone, Q or pyromellitimide, Pim), and additional dipyridyl-substituted tetrapyrrole extra-ligands. Steady-state, picosecond uorescence (t 1=2 ≈ 75 ps) and femtosecond pump-probe (1=2 ≈ 280 fs) data show that non-radiative deactivation of the dimer S1-states (S ¡ 1 ps) is due to both the S-S energy transfer (ZnOEP)2Ph→extra-ligand and the sequential photoinduced electron transfer (ZnOEP)2Ph→Q (or Pim) at rDA = 10:8 A. The additional decay shortening of the extra-ligand S1-states by 3-6 times (toluene, 293 K) is attributed to the increased "superexchange" mediated long distant (rDA ≈ 18-21 A) one-step electron transfer extra-ligand→Q (or Pim).
Chemical Physics Letters, 1999
The photophysical properties of meso-nitro-phenyl-octaethylporphyrins and their dimers with electron-accepting NO 2 groups in the para-, meta-and ortho-positions of the phenyl ring were studied. For the ortho-NO case in deaerated toluene 2 at 295 K, strong fluorescence quenching is caused by the intramolecular electron transfer from the porphyrin S state in the 1 Ž. absence of phenyl ring librations around the single CC bond 'normal' region, non-adiabatic case. T state lifetime 1 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 T \ S rate constants caused by T states mixing with 1 0 1 charge-transfer states.