The realization of highly efficient photoinduced charge separation across the pi-stacked base pairs in duplex DNA remains elusive. The low efficiencies (<5%) typically observed for charge separation over a dozen or more base pairs are a consequence of slow charge transport and rapid charge recombination. We report here a significant (5-fold or greater) enhancement in the efficiency of charge separation in diblock purine oligomers consisting of two or three adenines followed by several guanines, when compared to oligomers consisting of a single purine or alternating base sequences. This approach to wire-like behavior is attributed to both slower charge recombination and faster charge transport once the charge reaches the G-block in these diblock systems.

The binding energy and electronic coupling of perylenediimide (PDI) pi-stacked dimers were calculated using M06-2X/6-31++G** as a function of stacking geometry. Due to shallow minima in the potential energy surface, electronic coupling can vary by over an order of magnitude among energetically accessible geometries. The coupling was then determined for 20 PDI derivatives with various substitutions at the imide region, and several were identified as the most promising candidates for organic thin film transistors (OTFTs). This strategy of side-by-side comparison of binding energy and electronic coupling may prove useful for other pi-stacked OTFTs such as pentacene and poly(thiophene) derivatives.

Oxidation-state-specific dynamics at the Fe M 2,3 -edge are measured on the sub-100 fs time scale using tabletop high-harmonic extreme ultraviolet spectroscopy. Transient absorption spectroscopy of α-Fe 2 O 3 thin films after 400 nm excitation reveals distinct changes in the shape and position of the 3p → valence absorption peak at ∼57 eV due to a ligand-to-metal charge transfer from O to Fe. Semiempirical ligand field multiplet calculations of the spectra of the initial Fe 3+ and photoinduced Fe 2+ state confirm this assignment and exclude the alternative d−d excitation. The Fe 2+ state decays to a long-lived trap state in 240 fs. This work establishes the ability of time-resolved extreme ultraviolet spectroscopy to measure ultrafast charge-transfer processes in condensed-phase systems.

Triplet energy transfer (TT), a key process in molecular and organic electronics, generally occurs by either strongly distance-dependent single-step tunneling or weakly distance-dependent multistep hopping. We have synthesized a series of pi-stacked molecules consisting of a benzophenone donor, one to three fluorene bridges, and a naphthalene acceptor, and studied the rate of TT from benzophenone to naphthalene across the fluorene bridge using femtosecond transient absorption spectroscopy. We show that the dominant TT mechanism switches from tunneling to wire-like hopping between bridge lengths 1 and 2. The crossover observed for TT can be determined by direct observation of the bridge-occupied state.

Photochemical electron donor-acceptor triads having an aminopyrene primary donor (APy) and a p-diaminobenzene secondary donor (DAB) attached to either one or both imide nitrogen atoms of a perylene-3,4:9,10-bis(dicarboximide) (PDI) electron acceptor were prepared to give DAB-APy-PDI and DAB-APy-PDI-APy-DAB. In toluene, both triads are monomeric, but in methylcyclohexane, they self-assemble into ordered helical heptamers and hexamers, respectively, in which the PDI molecules are pi-stacked in a columnar fashion, as evidenced by small- and wide-angle X-ray scattering. Photoexcitation of these supramolecular assemblies results in rapid formation of DAB(+*)-PDI(-*) spin-polarized radical ion pairs having spin-spin dipolar interactions, which show that the average distance between the two radical ions is much larger in the assemblies (31 A) than it is in their monomeric building blocks (23 A). This work demonstrates that electron hopping through the pi-stacked PDI molecules is fast enough to compete effectively with charge recombination (40 ns) in these systems, making these materials of interest as photoactive assemblies for artificial photosynthesis and organic photovoltaics.