Elucidating the Ultrafast Dynamics of Photoinduced Charge Separation in Metalloporphyrin-Fullerene Dyads Across the Electromagnetic Spectrum

Metalloporphyrins are prominent building blocks in the synthetic toolbox of advanced photodriven molecular devices. When the central ion is paramagnetic, the relaxation pathways within the manifold of excited states are highly intricate so that unravelling the intramolecular energy and electron transfer processes is usually a very complex task. This fact is critically hampering the development of applications based on the enhanced coupling offered by the electronic exchange interaction. In this work, the dynamics of charge separation in a copper porphyrin-fullerene are studied with several complementary spectroscopic tools across the electromagnetic spectrum (from near-infrared to X-ray wavelengths), each of them providing specific diagnostics. Correlating the various rates clearly demonstrates that the lifetime of the photoinduced charge-separated state exceeds by about 10-fold that of the isolated photoexcited CuII porphyrin. As revealed by the spectral modifications in the XANES region, this stabilization is accompanied by a transient change in covalency around the CuII center, which is induced by an enhanced interaction with the C60 moiety. This experimental finding is further confirmed by state-of-the art calculations using DFT and TD-DFT including dispersion effects that explain the electrostatic and structural origins of this interaction, as the CuIIP cation becomes ruffled and approaches closer to the fullerene in the charge-separated state. From a methodological point of view, these results exemplify the potential of multielectron excitation features in transient X-ray spectra as future diagnostics of subfemtosecond electronic dynamics. From a practical point of view, this work is paving the way for elucidating out-of-equilibrium electron transfer events coupled to magnetic interaction processes on their intrinsic time-scales

Bridging-type changes facilitate successive oxidation steps at about 1 V in two binuclear manganese complexes - implications for photosynthetic water-oxidation.

The redox behavior of two synthetic manganese complexes illustrates a mechanistic aspect of importance for light-driven water oxidation in Photosystem II (PSII) and design of biomimetic systems (artificial photosynthesis). The coupling between changes in oxidation state and structural changes was investigated for two binuclear manganese complexes (1 and 2), which differ in the set of first sphere ligands to Mn (N(3)O(3) in 1, N(2)O(4) in 2). Both complexes were studied by electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) in three oxidation states which had been previously prepared either electro- or photochemically. The following bridging-type changes are suggested. In 1: Mn(II)-(mu-OR)(mu-OCO)(2)-Mn(II)Mn(II)-(mu-OR)(mu-OCO)(2)-Mn(III)-->Mn(III)-(mu-OR)(mu-OCO)(mu-O)-Mn(III). In 2: Mn(II)-(mu-OR)(mu-OCO)(2)-Mn(III)Mn(III)-(mu-OR)(mu-OCO)(2)-Mn(III)-->Mn(III)-(mu-OR)(mu-OCO)(mu-O)-Mn(IV). In both complexes, the first one-electron oxidation proceeds without bridging-type change, but involves a redox-potential increase by 0.5-1V. The second one-electron oxidation likely is coupled to mu-oxo-bridge (or mu-OH) formation which seems to counteract a further potential increase. In both complexes, mu-O(H) bridge formation is associated with a redox transition proceeding at approximately 1V, but the mu-O(H) bridge is observed at the Mn(2)(III,III) level in 1 and at the Mn(III,IV) level in 2, demonstrating modulation of the redox behavior by the terminal ligands. It is proposed that also in PSII bridging-type changes facilitate successive oxidation steps at approximately the same potential

Bridging-type changes facilitate successive oxidation steps at about 1 V in two binuclear manganese complexes - implications for photosynthetic water-oxidation

The redox behavior of two synthetic manganese complexes illustrates a mechanistic aspect of importance for light-driven water oxidation in Photosystem 11 (PSII) and design of biomimetic systems (artificial photosynthesis). The coupling between changes in oxidation state and structural changes was investigated for two binuclear manganese complexes (1 and 2), which differ in the set of first sphere ligands to Mn (N3O3 in 1, N2O4 in 2). Both complexes were studied by electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) in three oxidation states which had been previously prepared either electro- or photochemically. The following bridging-type changes are suggested. In 1: Mn-II-(mu-OR)(mu-OCO)(2)-Mn-II double left right arrow Mn-II-(mu-OR)(mu-OCO)(2)-Mn-III double right arrow Mn-III-(mu-OR)(mu-OCO)-(mu-O)-Mn-IV. In 2: Mn-II-(mu-OR)(mu-OCO)(2)-Mn-III double left right arrow Mn-III-(mu-OR)(mu-OCO)(2)-Mn-III double right arrow Mn-III-(mu-OR)([mu-OCO)(mu-O)-Mn-IV. In both complexes, the first one-electron oxidation proceeds without bridging-type change, but involves a redox-potential increase by 0.5-1 V. The second one-electron oxidation likely is coupled to mu-oxo-bridge (or mu-OH) formation which seems to counteract a further potential increase. In both complexes, mu-O(H) bridge formation is associated with a redox transition proceeding at similar to 1 V, but the mu-O(H) bridge is observed at the Mn-2(III,III) level in I and at the Mn-III,Mn-IV level in 2, demonstrating modulation of the redox behavior by the terminal ligands. It is proposed that also in PSII bridging-type changes facilitate successive oxidation steps at approximately the same potential. (c) 2006 Elsevier Inc. All rights reserved

Tetraaminoperylenes: their efficient synthesis and physical properties

Trimethylsilylation of 1,8-diaminonaphthalene gave 1,8-bis(trimethylsilylamino)naphthalene (1a), which was in turn lithiated with two molar equivalents of n-butyllithium to give the tris(thf)-solvated dilithium diamide 1,8- {(Me 3SiN)Li(thf)} 2C 10H 6(thf) (2a). Metal exchange of 2a with TlCl was carried out in two steps, via the previously characterized mixed-metal amide 1-{(Me 3SiN)Li(thf) 2}-8-{(Me 3SiN)Tl}- C 10H 6, to give the dithallium diamide 1,8-{(Me 3SiN)Tl} 2C 10H 6 (3a). Thermolysis of 3a cleanly gave a 1:1 mixture of the 4,9-bis(trimethylsilylamino)peryle- nequinone-3,10-bis(trimethylsilylimine) (4a) and 1a. By this route, a whole series of silylated homologues of 4a was obtained in good yields, while the same method proved to be inefficient for the synthesis of the alkyl-substituted analogues. Compound 4a and its tert-butyldimethylsilyl derivative 4d were reduced with sodium amalgam to give, after protonation, the corresponding 3,4,9,10-tetraaminoperylenes 7a and 7d. Cyclic voltammetry showed two reversible, closely spaced reduction waves (E red1 = - 1.39, E red2 = - 1.59 V versus SCE) corresponding to this conversion. The perylenes 7a and 7d are thought to be the primary products in the reaction cascade leading to the perylene derivatives, involving the thermal demetalation of the thallium amides, possibly via Tl 11-Tl 11 intermedi- ates, first to give 7a and its analogues. The final oxidation of the tetraaminoperylenes by one molar equivalent of 3a and analogous thallium amides gave the quinoidal derivatives such as 4a and 4d, a step that could be studied by direct reaction of the isolated species. The UV/ Vis absorption spectra of the 4,9-bis(silylamino)perylenequinone-3,10-bis(silylimines) are characterized by a long-wavelength absorption band with a pronounced vibrational structure (λ max = 639 nm, lg ε = 4.53) attributed to a �* � and a �* � n absorption band at 454 nm (lg ε 4.83), along with intense absorption in the UV region. A weak red emission with a rather low quantum yield (Φ n = 0.001, λ max = 660 nm) is observed upon irradiation of a sample; the lifetime of the emission is only 66 ps. The low emission quantum yield is attributed to the *� � n transition of the amino perylene, which induces strong spin-orbit coupling, leading to a large triplet yield. The triplet state was probed by transient absorption spectroscopy and found to have a lifetime of 200 ns in air, and 1100 ns in argonflushed solution. Treatment of 4a with a stoichiometric amount of KF in methanol/water under phase-transfer conditions (with the cryptand C 222) gave an almost quantitative yield of the parent compound 4,9-diaminoperylenequinone-3,10-diimine (8). Treatment of 8 with two molar equivalents of the ruthenium complex Ru(bpy) 2(acetone) 2(PF 6) 2, generated in situ, yielded the blue dinuclear ruthenium complex (bpy) 4Ru 2{μ 2-N,N�:N�, N�-4,9-(NH 2) 2-3,10-(NH) 2 C 20H 8]}](PF 6) 4 (9), the redox properties of which were studied by cyclic voltammetry. The difference in the potentials of the two one-electron redox steps (225 mV) indicates strong coupling of the metal centers through the 4,9-diaminoperylenquinone-3,10-dimine bridging ligand and corresponds to a comproportionation constant K c of 6.3 x 10 3. The UV/Vis absorption spectrum of the mixed valent form, which is stable in air, has a characteristic intervalence charge-transfer (IVCT) band in the near infrared at 930 nm (lg ε = 3.95), from which an electronic coupling parameter J of 760 cm -1 could be estimated, placing compound 9 at the border-line between the class II and class III cases in the Robin-Day classification

Photo-induced oxidation of a dinuclear Mn(2)(II,II) complex to the Mn(2)(III,IV) state by inter- and intramolecular electron transfer to Ru(III)tris-bipyridine.

To model the structural and functional parts of the water oxidizing complex in Photosystem II, a dimeric manganese(II,II) complex (1) was linked to a ruthenium(II)tris-bipyridine (Ru(II)(bpy)(3)) complex via a substituted L-tyrosine, to form the trinuclear complex 2 [J. Inorg. Biochem. 78 (2000) 15]. Flash photolysis of 1 and Ru(II)(bpy)(3) in aqueous solution, in the presence of an electron acceptor, resulted in the stepwise extraction of three electrons by Ru(III)(bpy)(3) from the Mn(2)(II,II) dimer, which then attained the Mn(2)(III,IV) oxidation state. In a similar experiment with compound 2, the dinuclear Mn complex reduced the photo-oxidized Ru moiety via intramolecular electron transfer on each photochemical event. From EPR it was seen that 2 also reached the Mn(2)(III,IV) state. Our data indicate that oxidation from the Mn(2)(II,II) state proceeds stepwise via intermediate formation of Mn(2)(II,III) and Mn(2)(III,III). In the presence of water, cyclic voltammetry showed an additional anodic peak beyond Mn(2)(II,III/III,III) oxidation which was significantly lower than in neat acetonitrile. Assuming that this peak is due to oxidation to Mn(2)(III,IV), this suggests that water is essential for the formation of the Mn(2)(III,IV) oxidation state. Compound 2 is a structural mimic of the water oxidizing complex, in that it links a Mn complex via a tyrosine to a highly oxidizing photosensitizer. Complex 2 also mimics mechanistic aspects of Photosystem II, in that the electron transfer to the photosensitizer is fast and results in several electron extractions from the Mn moiety