Photoinduced electron transfer in a symmetrical diporphyrin?fullerene triad (original) (raw)
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Photoinduced Electron Transfer of Dialkynyldisilane-Linked Zinc Porphyrin–[60]Fullerene Dyad
Bulletin of the Chemical Society of Japan, 2006
A zinc porphyrin-fullerene dyad with a disilane as a-conjugated linker has been newly synthesized to evaluate the electron transfer ability of the oligosilane chain. Its photoinduced processes have been studied using the timeresolved fluorescence and absorption measurements. Photoexcitation of the dyad causes the energy and/or electron transfer from the excited singlet state of the ZnP to C 60 moiety in polar solvents. The charge separation takes place as a final step in the excited-state process to yield the radical-ion pair with a radical cation on the zinc porphyrin and a radical anion on the fullerene, similar to other porphyrin-fullerene dyads. Its lifetime has been estimated to be 0.43-0.52 ms on the basis of the decay rate of the fullerene radical anion in polar solvents.
Photoinduced Electron Transfer in a Hexaphenylbenzene-based Self-assembled Porphyrin-fullerene Triad
Photochemistry and Photobiology, 2007
A hexaphenylbenzene-based zinc porphyrin dyad forms a 1:1 complex with a fullerene bearing two pyridyl groups via coordination of the pyridyl nitrogens with the zinc atoms. The fullerene is symmetrically located between the two zinc porphyrins. The binding constant for the complex is 7.3 · 10 4 M )1 in 1,2-difluorobenzene. Photoinduced electron transfer from a porphyrin first excited singlet state to the fullerene occurs with a time constant of 3 ps, and the resulting charge-separated state has a lifetime of 230 ps. This self-assembled construct should form a basis for the construction of more elaborate model photosynthetic antenna-reaction center systems.
Synthesis and photophysics of new types of fullerene-porphyrin dyads
Carbon, 2000
The synthesis and photophysical properties of three types of porphyrin-fullerene (P-C ) dyads are presented: (a) 60 conformationally flexible dyads with polyether linkers, (b) rigid dyads with steroid linkers, and (c) a parachute-shaped dyad in which the flat porphyrin and spherical fullerene chromophores are in very close proximity. The course of events following photoexcitation of these dyads and (in some cases) corresponding porphyrin-zinc complexes has been probed through studies of fluorescence quenching, measurements of fluorescence lifetimes and quantum yields for singlet molecular oxygen formation, and transient absorption spectroscopy. These data provide insight into the dynamic competition between intramolecular energy transfer and electron transfer in these dyads, and, for the case of the parachute-shaped dyad, the kinetics of back-electron transfer.
Journal of the American Chemical Society, 2010
A novel porphyrin tripod (TPZn 3) was synthesized via "click chemistry". Three porphyrin moieties of TPZn 3 are geometrically close and linked by a flexible linker. The electron-transfer oxidation of TPZn 3 results in intramolecular π-dimer formation between porphyrin moieties as indicated by electrochemical, vis-NIR, and ESR measurements. The cyclic voltammogram of TPZn 3 exhibited stepwise one-electron oxidation processes of three porphyrin moieties in the range from 0.58 to 0.73 V (vs SCE in CH 2 Cl 2). When TPZn 3 was oxidized by tris(2,2′-bipyridyl)-ruthenium(III) ([Ru(bpy) 3 ] 3+), the oxidized species (TPZn 3) n+ (0 < n e 3) exhibited a charge resonance band in the NIR region due to the π-dimer formation between porphyrin moieties. A supramolecular electron donor-acceptor system was also constructed using TPZn 3. The flexible conformation of TPZn 3 makes it possible to capture a fullerene derivative containing a pyridine moiety (PyC 60) inside the cavity by π-π interactions as well as the coordination bond between Zn 2+ and the pyridine moiety. The formation of a 1:1 supramolecular complex of TPZn 3 with PyC 60 (TPZn 3-PyC 60) was indicated in the UV-vis and 1 H NMR spectra in nonpolar solvents. The association constant of TPZn 3 with PyC 60 (1.1 × 10 5 M-1 in toluene) is much larger as compared with those of the corresponding monomer (MPZn) and dimer porphyrin (DPZn 2). The dynamics of photoinduced electron transfer from the singlet excited state of TPZn 3 to PyC 60 was examined by laser flash photolysis measurements. The efficient intracomplex photoinduced electron transfer in TPZn 3-PyC 60 occurred in nonpolar solvents, resulting from the π-π interactions between the porphyrin and fullerene moieties, together with intramolecular π-bond formation between the porphyrin radical cation and the neutral porphyrin in TPZn 3 •+ .
J. Am. Chem. …, 2004
As part of a continuing investigation of the topological control of intramolecular electron transfer (ET) in donor-acceptor systems, a symmetrical parachute-shaped octaethylporphyrin-fullerene dyad has been synthesized. A symmetrical strap, attached to ortho positions of phenyl groups at opposing meso positions of the porphyrin, was linked to [60]-fullerene in the final step of the synthesis. The dyad structures were confirmed by 1 H, 13 C, and 3 He NMR, and MALDI-TOF mass spectra. The free-base and Zn-containing dyads were subjected to extensive spectroscopic, electrochemical and photophysical studies. UV-vis spectra of the dyads are superimposable on the sum of the spectra of appropriate model systems, indicating that there is no significant ground-state electronic interaction between the component chromophores. Molecular modeling studies reveal that the lowest energy conformation of the dyad is not the C2v symmetrical structure, but rather one in which the porphyrin moves over to the side of the fullerene sphere, bringing the two π-systems into close proximity, which enhances van der Waals attractive forces. To account for the NMR data, it is proposed that the dyad is conformationally mobile at room temperature, with the porphyrin swinging back and forth from one side of the fullerene to the other. The extensive fluorescence quenching in both the free base and Zn dyads is associated with an extremely rapid photoinduced electron-transfer process, kET ≈ 10 11 s -1 , generating porphyrin radical cations and C60 radical anions, detected by transient absorption spectroscopy. Back electron transfer (BET) is slower than charge separation by up to 2 orders of magnitude in these systems. The BET rate is slower in nonpolar than in polar solvents, indicating that BET occurs in the Marcus inverted region, where the rate decreases as the thermodynamic driving force for BET increases. Transient absorption and singlet molecular oxygen sensitization data show that fullerene triplets are formed only with the free base dyad in toluene, where triplet formation from the charge-separated state is competitive with decay to the ground state. The photophysical properties of the P-C 60 dyads with parachute topology are very similar to those of structurally related rigid π-stacked P-C60 dyads, with the exception that there is no detectable charge-transfer absorption in the parachute systems, attributed to their conformational flexibility. It is concluded that charge separation in these hybrid systems occurs through space in unsymmetrical conformations, where the center-to-center distance between the component π-systems is minimized. Analysis of the BET data using Marcus theory gives reorganization energies for these systems between 0.6 and 0.8 eV and electronic coupling matrix elements between 4.8 and 5.6 cm -1 .
Photoinduced electron transfer in a β,β′-pyrrolic fused ferrocene–(zinc porphyrin)–fullerene
Physical Chemistry Chemical Physics, 2007
A donor-acceptor linked triad with a short spacer (Fc-ZnP-C 60 ) 1 was designed and synthesised to attain the longest charge-separation lifetime, 630 ms, ever reported for triads at room temperature. The ferrocene electron donor and fullerene electron acceptor of triad 1 are attached to imidazole rings fused to opposite b,b 0 -pyrrolic positions of the zinc porphyrin. After excitation of the porphyrin, electron transfer to C 60 occurs within 230 ps, followed by hole transfer to ferrocene after 500 ps to produce the long-lived charge-separated state.
Photoinduced electron transfer in π-extended tetrathiafulvalene–porphyrin–fullerene triad molecules
Journal of Materials Chemistry, 2002
Two molecular triads consisting of a porphyrin (P) covalently linked to a fullerene electron acceptor (C 60 ) and a p-extended tetrathiafulvalene electron donor (TTF) have been synthesized. Time resolved spectroscopic investigations of the triad featuring a free base porphyrin moiety (TTF-P 2H -C 60 ) show that in 2-methyltetrahydrofuran solution, excitation of the porphyrin leads to formation of a TTF-P 2H ? 1 -C 60 ? 2 charge-separated state in 25 ps. Electron transfer from the TTF generates a final TTF ? 1 -P 2H -C 60 ? 2 state with an overall yield of 0.87. This species decays to the ground state in 1.07 ms. Similar experiments on the zinc analog, TTF-P Zn -C 60 , show formation of TTF-P Zn ? 1 -C 60 ? 2 in 1.5 ps, followed by generation of TTF ? 1 -P Zn -C 60 ? 2 with a yield of 0.09. This charge-separated state also decays to the ground state in 1.07 ms. Comparison of these results with those for previously reported triads with different donor moieties reveals differences in electron transfer rate constants that can be qualitatively understood in the framework of the Marcus-Hush electron transfer formalism.
The Journal of Physical Chemistry A, 2012
Photoinduced charge transfer in a doublelinked zinc porphyrin−fullerene dyad is studied. When the dyad is excited at the absorption band of the charge-transfer complex (780 nm), an intramolecular exciplex is formed, followed by the complete charge separated (CCS) state. By analyzing the results obtained from time-resolved transient absorption and emission decay measurements in a range of solvents with different polarities, we derived a dependence between the observable lifetimes and internal parameters controlling the reaction rate constants based on the semiquantum Marcus electron-transfer theory. The critical value of the solvent polarity was found to be ε r ≈ 6.5: in solvents with higher dielectric constants, the energy of the CCS state is lower than that of the exciplex and the relaxation takes place via the CCS state predominantly, whereas in solvents with lower polarities the energy of the CCS state is higher and the exciplex relaxes directly to the ground state. In solvents with moderate polarities the exciplex and the CCS state are in equilibrium and cannot be separated spectroscopically. The degree of the charge shift in the exciplex relative to that in the CCS state was estimated to be 0.55 ± 0.02. The electronic coupling matrix elements for the charge recombination process and for the direct relaxation of the exciplex to the ground state were found to be 0.012 ± 0.001 and 0.245 ± 0.022 eV, respectively.
2014
Two novel donor-bridge-acceptor arrays (ZnP-nTV-C 60 ) with zinc porphyrin (ZnP) and fullerene (C 60 ), covalently connected by oligo(thienylenevinylene) (nTV) molecular wires (n = 3 and 8; 1a,b), have been prepared in a multistep convergent manner. The influence of the nTV-length on the electrochemical and electronic properties of the ZnP-nTV-C 60 triads has been revealed. Interestingly, an efficient photoinduced electron transfer process occurs in both triads with formation of intermediate radical-ion pairs (namely, ZnP + -nTV-C 60 À and ZnP-nTV + -C 60 À ) as confirmed by the nanosecond transient absorption measurements in the visible and NIR regions. In polar and nonpolar solvents, the rate constants of charge-separation processes (k CS ) via 1 ZnP*-nTV-C 60 were found to decrease from ca. 1.2 Â 10 10 s À1 for n = 3 (R DA = 20 Å) to (5-7) Â 10 9 s À1 for n = 8 (R DA = 60 Å) on the basis of fluorescence lifetime measurements of the ZnP moiety. From these data, together with those previously obtained ones for n = 4 in the related ZnP-nTV-C 60 systems, a low attenuation coefficient was evaluated for the nTV molecular wires.