Precise control of intramolecular charge-transport: the interplay of distance and conformational effects.

A new series of donor-bridge-acceptor (D-B-A) compounds consisting of π-conjugated oligofluorene (oFL) bridges between a ferrocene (Fc) electron-donor and a fullerene (C60 ) electron-acceptor have been synthesized. In addition to varying the length of the bridge (i.e., mono- and bi-fluorene derivatives), four different ways of linking ferrocene to the bridge have been examined. The Fc moiety is linked to oFL: 1) directly without any spacer, 2) by an ethynyl linkage, 3) by a vinylene linkage, and 4) by a p-phenylene unit. The electronic interactions between the electroactive species have been characterized by cyclic voltammetry, absorption, fluorescence, and transient absorption spectroscopy in combination with quantum chemical calculations. The calculations reveal exceptionally close energy-matching between the Fc and the oFL units, which results in strong electronic-coupling. Hence, intramolecular charge-transfer may easily occur upon exciting either the oFLs or Fcs. Photoexcitation of Fc-oFL-C60 conjugates results in transient radical-ion-pair states. The mode of linkage of the Fc and FL bridge has a profound effect on the photophysical properties. Whereas intramolecular charge-separation is found to occur rather independently of the distance, the linker between Fc and oFL acts (at least in oFL) as a bottleneck and significantly impacts the intramolecular charge-separation rates, resulting in beta values between ßCS 0.08 and 0.19 Å(-1). In contrast, charge recombination depends strongly on the electron-donor-acceptor distance, but not at all on the linker. A value of ßCR (0.35±0.01 Å(-1)) was found for all the systems studied. Oligofluorenes prove, therefore, to be excellent bridges for probing how small structural variations affect charge transport in D-B-A systems.

Control over charge transfer through molecular wires by temperature and chemical structure modifications.

A series of electron donor-acceptor arrays containing π-conjugated oligofluorenes (oFL) of variable length between a zinc porphyrin (ZnP) as electron donor and fullerene (C(60)) as electron acceptor have been prepared by following a convergent synthesis. The electronic interactions between the electroactive species were determined by cyclic voltammetry, UV-visible, fluorescence, and femto/nanosecond transient absorption spectroscopy. Our studies clearly confirm that, although the C(60) units are connected to the ZnP donor through π-conjugated oFL frameworks, no significant electronic interactions prevail in the ground state. Theoretical calculations predict that a long-range electron transfer occurs primarily due to a maximized π-conjugated pathway from the donor to the acceptor. Photoexcitation of ZnP-oFL(n)-C(60) results in transient absorption maxima at 715 and 1010 nm, which are unambiguously attributed to the photolytically generated radical ion pair state, [ZnP(•+)-oFL(n)-C(60)(•-)], with lifetimes in the microsecond time regime. Temperature-dependent photophysical experiments have shown that the charge-transfer mechanism is controllable by temperature. Both charge separation and charge recombination processes give rise to a molecular wire behavior of the oFL moiety with an attenuation factor (ß) of 0.097 Å(-1). The correlation ß to the connection pattern between the ZnP donor and the oFL linker revealed that even small alterations of the linker π-electron system break the homogeneous π-conjugation pattern, leading to higher values of ß.

Electron donor-acceptor interactions in regioselectively synthesized exTTF(2)-C(70)(CF(3))(10) dyads.

The decakis(trifluoromethyl)fullerene C(1)-C(70)(CF(3))(10), in which the CF(3) groups are arranged on a para(7)-meta-para ribbon of C(6)(CF(3))(2) edge-sharing hexagons, and which has now been prepared in quantities of hundreds of milligrams, was reacted under standard Bingel-Hirsch conditions with a bis-pi-extended tetrathiafulvalene (exTTF) malonate derivative to afford a single exTTF(2)-C(70)(CF(3))(10) regioisomer in 80 % yield based on consumed starting material. The highly soluble hybrid was thoroughly characterized by using 1D (1)H, (13)C, and (19)F NMR, 2D NMR, and UV/Vis spectroscopy; matrix-assisted laser desorption ionization (MALDI) mass spectrometry; and electrochemistry. The cyclic voltammogram of the exTTF(2)-C(70)(CF(3))(10) dyad revealed an irreversible second reduction process, which is indicative of a typical retro-Bingel reaction; whereas the usual phenomenon of exTTF inverted potentials (E1ox>E2ox), resulting in a single, two-electron oxidation process, was also observed. Steady-state and time-resolved photolytic techniques demonstrated that the C(1)-C(70)(CF(3))(10) singlet excited state is subject to a rapid electron-transfer quenching. The resulting charge-separated states were identified by transient absorption spectroscopy, and radical pair lifetimes of the order of 300 ps in toluene were determined. The exTTF(2)-C(70)(CF(3))(10) dyad represents the first example of exploitation of the highly soluble trifluoromethylated fullerenes for the construction of systems able to mimic the photosynthetic process, and is therefore of interest in the search for new materials for photovoltaic applications.

Dendritic porphyrin-fullerene conjugates: efficient light-harvesting and charge-transfer events.

A novel dendritic C(60)-H(2)P-(ZnP)(3) (P=porphyrin) conjugate gives rise to the successful mimicry of the primary events in photosynthesis, that is, light harvesting, unidirectional energy transfer, charge transfer, and charge-shift reactions. Owing, however, to the flexibility of the linkers that connect the C(60), H(2)P, and ZnP units, the outcome depends strongly on the rigidity/viscosity of the environment. In an agar matrix or Triton X-100, time-resolved transient absorption spectroscopic analysis and fluorescence-lifetime measurements confirm the following sequence. Initially, light harvesting is seen by the peripheral C(60)-H(2)P- *(ZnP)(3) conjugate. Once photoexcited, a unidirectional energy transfer funnels the singlet excited-state energy to H(2)P to form C(60)-*(H(2)P)-(ZnP)(3), which powers an intramolecular charge transfer that oxidizes the photoexcited H(2)P and reduces the adjacent C(60) species. In the correspondingly formed (C(60))(*-)-(H(2)P)(*+)-(ZnP)(3) conjugate, an intramolecular charge-shift reaction generates (C(60))(*-)-H(2)P-(ZnP)(3) (.+), in which the radical cation resides on one of the three ZnP moieties, and for which lifetimes of up to 460 ns are found. On the other hand, investigations in organic media (i.e., toluene, THF, and benzonitrile) reveal a short cut, that is, the peripheral ZnP unit reacts directly with C(60) to form (C(60))(*-)-H(2)P-(ZnP)(3) (*+). Substantial configurational rearrangements- placing ZnP and C(60) in proximity to each other-are, however, necessary to ensure the required through space interactions (i.e., close approach). Consequently, the lifetime of (C(60))(*-)-H(2)P-(ZnP)(3) (*+) is as short as 100 ps in benzonitrile.