Long-lived long-distance photochemically induced spin-polarized charge separation in β,β′-pyrrolic fused ferrocene-porphyrin-fullerene systems

Abstract

The exceptionally long lived charge separation previously observed in a b,b 0 -pyrrolic-fused ferroceneporphyrin-fullerene triad (lifetime 630 ms) and related porphyrin-fullerene dyad (lifetime 260 ms) is attributed to the production of triplet charge-separated states. Such molecular excited-state spin polarization maintained over distances of up to 23A is unprecedented and offers many technological applications. Electronic absorption and emission spectra, femtosecond and nanosecond time-resolved transient absorption spectra, and cyclic voltammograms of two triads and four dyads are measured and analyzed to yield rate constants, donor-acceptor couplings, free-energy changes, and reorganization energies for charge-separation and charge-recombination processes. Production of long-lived intramolecular triplet states is confirmed by electron-paramagnetic resonance spectra at 77-223 K, as is retention of spin polarization in p-conjugated ferrocenium ions. The observed rate constants were either first predicted (singlet manifold) or later confirmed (triplet manifold) by a priori semiclassical kinetics calculations for all conceivable photochemical processes, parameterized using densityfunctional theory and complete-active-space self-consistent-field calculations. Identified are both a ps-timescale process attributed to singlet recombination and a ms-timescale process attributed to triplet recombination.

The Importance of Motions that Accompany Those Occurring Along the Reaction Coordinate

“…Subsequent timeresolved spectroscopic experiments found the anticipated singlet process and measured all of the rate constants for both it and the 1 ms process. [60] Many new experiments and calculations then confirmed that the 1 ms process is of very low quantum yield and occurs on the triplet manifold, despite the very rapid spin depolarization normally associated with the ferrocene cation. [60] The final observed kinetics scheme is shown in Fig.…”

“…[60] Many new experiments and calculations then confirmed that the 1 ms process is of very low quantum yield and occurs on the triplet manifold, despite the very rapid spin depolarization normally associated with the ferrocene cation. [60] The final observed kinetics scheme is shown in Fig. 4, indicating how the various photochemical charge-separation and charge-recombination processes operate.…”

“…Instead, the reaction coordinate is dominated by a collective fluctuation in the coordinates of the surrounding solvent. These effects may be modelled quantitatively for both the CreutzTaube ion [59] and general systems relevant to solar-energy conversion and organic conductivity [60] using non-equilibrium solvation methods that derive from Marcus' original treatment. [18] Indeed, any reaction that involves changing charge distributions along the reaction coordinate will be sensitive to environment, providing for example a key mechanism allowing protein structure to catalyze reactions in enzymes.…”

“…The differences between the observed and calculated couplings are of an order of magnitude, which is unusually large, but this is still small enough to be useful; the observed data is also somewhat uncertain owing to the unknown accuracy of the approximations used to interpret the observed rate constants. [60] In total 17 vibrational modes were used in these calculations, taking the most active modes describing the change in the geometries of the ferrocene, porphyrin, and fullerene structural components accompanying charge transfer to/from these groups. Density functional theory (DFT) was used for most calculations but complete-active-space self-consistent-field calculations were used to obtain the spinorbit allowed triplet charge-recombination couplings.…”

Nanocarbons as Electron Donors and Acceptors in Photoinduced Electron-Transfer Reactions

“…45 Because the reorganization energies of self-exchange electrontransfer reactions of fullerenes are very small, 46-48 a large number of electron donor-acceptor conjugates of fullerenes used as electron acceptors have so far been reported to mimic the multistep photoinduced charge-separation processes with slow charge recombination in the photosynthetic reaction center. [49][50][51][52][53][54][55][56][57][58][59][60][61] The longest charge-separated state (1.6 s) was attained by photoexcitation of a ferrocene-zinc porphyrinzinc porphyrin-fullerene tetrad (Fc-ZnP-ZnP-C 60 ) tetrad in Figure 1. 61 The lifetime of the charge-separated state (Fc + -ZnP-ZnP-C 60…”