Energy and Electron Transfer in β-Alkynyl-Linked Porphyrin−[60]Fullerene Dyads

Abstract

Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which pi-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H(2)TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C(60) dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C(60) triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP(*+)/C(60)(*-) charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP(*+)/C(60)(*-) in PhCN was approximately 40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k(CS)) to rates for charge recombination (k(CR)), k(CS)/k(CR), is quite small, approximately 7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C(60) to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k(CS)/k(CR) approximately equal to 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k(CR) but not on k(CS). On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the beta-linked dyads.

Selective Triplet‐State Formation during Charge Recombination in a Fullerene/Bodipy Molecular Dyad (Bodipy=Borondipyrromethene)

“…There have been numerous such systems studied in recent years, [17] mostly using the fullerene as an electron acceptor. Alternative systems have used the fullerene as a triplet reservoir [18] due to its triplet level being situated at low energy. In fact, the triplet energy of C 60 is quite well defined despite its extremely poor phosphorescence yield in a low temperature glass.…”

Buckyballs

“…Very fast electron transfer for both CS and CR has been reported for compound 59, with two acetylene bonds connecting the C 60 unit with the β-position of ZnP (k CS > 1 Â 10 11 s À1 , k CR ¼ 2.5 Â 10 10 s À1 ) [244]. In contrast, for compounds 60, in which the polyacetylene bridge is connected through a phenyl ring in the meso position of the porphyrin, CS and CR are in the regions of 7.5 AE 2.4 Â 10 9 s À1 and 1.6 AE 0.2 Â 10 6 s À1 , with a β value of 0.06 Å À1 in PhCN [245].…”

Light‐Harvesting Materials for Organic Electronics

“…9.23). A representative example is that by Vail et al [69] where a ZnP unit is covalently linked to the C 60 through a polyacetylene moiety (compounds 30 and 31). Depending on the way of binding between the bridge and the donor unit, either through the -position of ZnP or through a phenyl ring in the meso position of the porphyrin [70], a great variation in the electron transfer for charge separation (CS) and charge recombination (CR) were reported, with values of k CS = 1 × 10 11 s −1 , k CR = 2.5 × 10 10 s −1 (for compound 30) and 7.5 ± 2.4 × 10 9 s −1 and 1.6 ± 0.2 × 10 6 s −1 , [1], (for compound 31).…”