Unexpected Change in Charge Transfer Behavior in a Cobalt(II) Porphyrin−Fullerene Conjugate That Stabilizes Radical Ion Pair States

Abstract

Two cobalt(II) porphyrin-C(60) malonate-linked conjugates, the mono-connected Co1 and the bis-connected trans-2 isomer Co3, have been synthesized for the first time either by direct cyclopropanation with the precursor malonate Co4 or by metalation of the bisadduct H(2)3. For the investigation of the interaction between the porphyrin donor and fullerene acceptor within these dyads, electrochemical and photophysical investigations have been carried out. Compared to Zn3 and trans-2 bisadduct 7, the first reduction of the fullerene moiety within Co3 becomes easier (40 mV in dichloromethane and 20 mV in benzonitrile), indicating significant interactions between the pi-system of the fullerene and the d-orbitals of the central Co atom. Compared to the Co complexes 9, Co4, and Co1, the first oxidation of Co3 is considerably shifted to more positive potentials, if benzonitrile instead of dichloromethane is used as solvent. At the same time, the oxidation is no longer centered on the Co(II) center but on the porphyrin macrocycle, as corroborated by spectroelectrochemistry. A similar solvent dependence was observed in transient absorption spectroscopic measurements. In toluene, benzonitrile and anisole photoinduced electron transfer within Co3 leads to the formation of a charge-separated state Co(II)P.+ -C(60).- with a lifetime of 560 +/- 20 ns in benzonitrile, whereas in other solvents such as THF, nitrobenzene, ortho-diclorobenzene, and tert-butylbenzene the formation of a Co(III)P-C(60).- as transient was detected, which is, however, short-lived (860 +/- 40 ps in THF) and exhibits charge recombination dynamics that are in the Marcus inverted region. Particularly important is the fact that the electronic coupling (V) in Co(III)P-C(60).- is 18 cm(-1) substantially smaller than the V value of 313 cm(-1) in ZnP.+ -C(60).- .

A New Type of Dendritic [60]Fullerene–Metallo‐tetraphenylporphyrin Diads (M = Zn, Co)

“…This is a typical phenomenon which is commonly observed during the formation of (porphyrinato)Co complexes. [15] In order to obtain a clean sample of 11 with Co II exclusively, the mixture of 10 was allowed to react with a saturated aqueous solution of Na 2 S 2 O 4 . Compounds 8, 9 and 11 were completely characterized by 1 H NMR (Figure 4), 13 C NMR, UV/Vis, IR spectroscopy, mass spectrometry, and elemental analysis.…”

“…This can be seen, for example, in the UV/ Vis spectrum which shows an asymmetrical Soret band with low extinction coefficients, compared to the Zn complex 24 ( Figure 5). [15] The 13 2 -carbon atoms in the range between δ = 140 and 145 ppm. The lack of the malonate proton signal in the 1 H NMR spectrum is an evidence for the successful Bingel [21] reaction as well.…”

“…This behaviour is in agreement with that of related compounds reported in the literature. [15] However, using the mixture 25 as starting material allows for the clean formation of either pure Co II or Co III complexes (Scheme 3). The reduction of 25 with a saturated aqueous Na 2 S 2 O 4 solution leads to the paramagnetic cobalt(II) diad 26.…”

“…[8] On the other hand electronic communication between C 60 and a tetraphenylporphyrin addend is very pronounced if monoadducts and bisadducts such as 3-5 ( Figure 2) are investigated. [4, [14][15] We and other groups have recently synthesized a broad variety of such diads and systematically investigated their electronic and photophysical properties. [4,[8][9][10][11][12][13] These diads are able to undergo photoinduced electron transfer with the porphyrin serving as donor and the fullerene as acceptor.…”

“…[4] For example, in contrast to diads 3 and 5 the very close metal-fullerene contact (2.71 Å) in the rigid diad 4 disfavours the oxidation of Co II to Co III . [14][15] We now wish to present the development of a new type of porphyrin-fullerene-dendrimer hybrids of the general formula 6 (Figure 3), where one addend only, namely a malonate involving a tetraphenylporphyrin and a dendritic terminus, is attached to C 60 . In monoadducts of C 60 the characteristic fullerene properties are largely retained.…”