Raanan Carmieli

A Radically Configurable Six-State Compound

Radically Organic Metals such as manganese are relatively stable over a wide range of oxidation states. In contrast, purely organic compounds are rarely susceptible to incremental addition or removal of electrons without accompanying fragmentation or coupling reactions. Barnes et al. (p. 429; see the Perspective by Benniston) report a catenane (a compound comprising interlocked rings) in which the topological structure stabilizes six different states that successively differ by the presence or absence of one or two electrons in the framework. The hepta-oxidized state proved remarkably resilient to oxygen exposure. An interlocked-rings topology stabilizes a wide range of collective oxidation states in a metal-free organic compound. [Also see Perspective by Benniston] Most organic radicals possess short lifetimes and quickly undergo dimerization or oxidation. Here, we report on the synthesis by radical templation of a class of air- and water-stable organic radicals, trapped within a homo[2]catenane composed of two rigid and fixed cyclobis(paraquat-p-phenylene) rings. The highly energetic octacationic homo[2]catenane, which is capable of accepting up to eight electrons, can be configured reversibly, both chemically and electrochemically, between each one of six experimentally accessible redox states (0, 2+, 4+, 6+, 7+, and 8+) from within the total of nine states evaluated by quantum mechanical methods. All six of the observable redox states have been identified by electrochemical techniques, three (4+, 6+, and 7+) have been characterized by x-ray crystallography, four (4+, 6+, 7+, and 8+) by electron paramagnetic resonance spectroscopy, one (7+) by superconducting quantum interference device magnetometry, and one (8+) by nuclear magnetic resonance spectroscopy.

Electron Sharing and Anion–π Recognition in Molecular Triangular Prisms

Stacking on a full belly: Triangular molecular prisms display electron sharing among their triangularly arranged naphthalenediimide (NDI) redox centers. Their electron-deficient cavities encapsulate linear triiodide anions, leading to the formation of supramolecular helices in the solid state. Chirality transfer is observed from the six chiral centers of the filled prisms to the single-handed helices.

Photoinitiated Charge Transport through π-Stacked Electron Conduits in Supramolecular Ordered Assemblies of Donor−Acceptor Triads

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.

Charge-Transfer and Spin Dynamics in DNA Hairpin Conjugates with Perylenediimide as a Base-Pair Surrogate

A perylenediimide chromophore (P) was incorporated into DNA hairpins as a base-pair surrogate to prevent the self-aggregation of P that is typical when it is used as the hairpin linker. The photoinduced charge-transfer and spin dynamics of these hairpins were studied using femtosecond transient absorption spectroscopy and time-resolved EPR spectroscopy (TREPR). P is a photooxidant that is sufficiently powerful to quantitatively inject holes into adjacent adenine (A) and guanine (G) nucleobases. The charge-transfer dynamics observed following hole injection from P into the A-tract of the DNA hairpins is consistent with formation of a polaron involving an estimated 3-4 A bases. Trapping of the (A 3-4) (+*) polaron by a G base at the opposite end of the A-tract from P is competitive with charge recombination of the polaron and P (-*) only at short P-G distances. In a hairpin having 3 A-T base pairs between P and G ( 4G), the radical ion pair that results from trapping of the hole by G is spin-correlated and displays TREPR spectra at 295 and 85 K that are consistent with its formation from (1*)P by the radical-pair intersystem crossing mechanism. Charge recombination is spin-selective and produces (3*)P, which at 85 K exhibits a spin-polarized TREPR spectrum that is diagnostic of its origin from the spin-correlated radical ion pair. Interestingly, in a hairpin having no G bases ( 0G), TREPR spectra at 85 K revealed a spin-correlated radical pair with a dipolar interaction identical to that of 4G, implying that the A-base in the fourth A-T base pair away from the P chromophore serves as a hole trap. Our data suggest that hole injection and transport in these hairpins is completely dominated by polaron generation and movement to a trap site rather than by superexchange. On the other hand, the barrier for charge injection from G (+*) back onto the A-T base pairs is strongly activated, so charge recombination from G (or even A trap sites at 85 K) most likely proceeds by a superexchange mechanism.

Magnetic field-induced switching of the radical-pair intersystem crossing mechanism in a donor-bridge-acceptor molecule for artificial photosynthesis

A covalent, fixed-distance donor-bridge-acceptor (D-B-A) molecule was synthesized that upon photoexcitation undergoes ultrafast charge separation to yield a radical ion pair (RP) in which the spin-spin exchange interaction (2J) between the two radicals is sufficiently large to result in preferential RP intersystem crossing to the highest-energy RP eigenstate (T(+1)) at the 350 mT magnetic field characteristic of X-band (9.5 GHz) EPR spectroscopy. This behavior is unprecedented in covalent D-B-A molecules, and is evidenced by the time-resolved EPR (TREPR) spectrum at X-band of (3*)D-B-A derived from RP recombination, which shows all six canonical EPR transitions polarized in emission (e,e,e,e,e,e). In contrast, when the RP is photogenerated in a 3400 mT magnetic field, the TREPR triplet spectrum at W-band (94 GHz) of (3*)D-B-A displays the (a,e,e,a,a,e) polarization pattern characteristic of a weakly coupled RP precursor, similar to that observed in photosynthetic reaction center proteins, and indicates a switch to selective population of the lower-energy T(0) eigenstate.

Time-resolved EPR studies of charge recombination and triplet-state formation within donor-bridge-acceptor molecules having wire-like oligofluorene bridges.

Spin-selective charge recombination of photogenerated radical ion pairs within a series of donor-bridge-acceptor (D-B-A) molecules, where D = phenothiazine (PTZ), B = oligo(2,7-fluorenyl), and A = perylene-3,4:9,10-bis(dicarboximide) (PDI), PTZ-FL(n)-PDI, where n = 1-4 (compounds 1-4), is studied using time-resolved electron paramagnetic resonance (TREPR) spectroscopy in which the microwave source is either continuous-wave or pulsed. Radical ion pair TREPR spectra are observed for 3 and 4 at 90-294 K, while the neutral triplet state of PDI ((3)*PDI) is observed at 90-294 K for 2-4 and at 90 K for 1. (3)*PDI is produced by three mechanisms, as elucidated by its zero-field splitting parameters and spin polarization pattern. The mechanisms are spin-orbit-induced intersystem crossing (SO-ISC) in PDI aggregates, direct spin-orbit charge-transfer intersystem crossing (SOCT) from the singlet radical pair within 1, and radical pair intersystem crossing (RP-ISC) as a result of S-T(0) mixing of the radical ion pair states in 2-4. The temperature dependence of the spin-spin exchange interaction (2J) shows a dramatic decrease at low temperatures, indicating that the electronic coupling between the radical ions decreases due to an increase in the average fluorene-fluorene dihedral angle at low temperatures. The charge recombination rates for 3 and 4 decrease at low temperature, but that for 2 is almost temperature-independent. These results strongly suggest that the dominant mechanism of charge recombination for n >or= 3 is incoherent thermal hopping, which results in wire-like charge transfer.

Direct Measurement of Photoinduced Charge Separation Distances in Donor-Acceptor Systems for Artificial Photosynthesis Using OOP-ESEEM

The distance over which two photogenerated charges are separated in electron donor-acceptor systems for artificial photosynthesis depends on the structure of the system, while the lifetime of the charge separation and, ultimately, its ability to carry out useful redox chemistry depend on the electronic coupling between the oxidized donor and reduced acceptor. The radical ions produced by charge separation are frequently delocalized over the pi systems of the final oxidized donor and reduced acceptor, so that there is often significant uncertainty as to the average distance between the separated charges, especially in low dielectric constant media, where the Coulomb attraction of the ions may be significant and the charge distribution of the ions may be distorted, so that the average distance between them may be shorter than that implied by their chemical structures. The charge separation distances between photogenerated radical ions in three donor-acceptor molecules having different donor-acceptor distances were measured directly from their dipolar spin-spin interactions using out-of-phase electron spin echo envelope modulation (OOP-ESEEM). The measured distances in toluene at 85 K compare favorably to the calculated distances between the centroids of the spin distributions of the radical ions within the radical ion pairs. These results show that despite the intrinsically nonpolar nature of medium, the spin (and charge) distributions of the RPs are not significantly distorted by Coulomb attraction over these long distances. This study shows that OOP-ESEEM is well-suited for probing the detailed structural features of charge-separated intermediates that are essential to understanding how to design molecular structures that prolong and control charge separation for artificial photosynthesis.

Gated Electron Sharing Within Dynamic Naphthalene Diimide-Based Oligorotaxanes†

The controlled self-assembly of well-defined and spatially ordered π-systems has attracted considerable interest because of their potential applications in organic electronics. An important contemporary pursuit relates to the investigation of charge transport across noncovalently coupled components in a stepwise fashion. Dynamic oligorotaxanes, prepared by template-directed methods, provide a scaffold for directing the construction of monodisperse one-dimensional assemblies in which the functional units communicate electronically through-space by way of π-orbital interactions. Reported herein is a series of oligorotaxanes containing one, two, three and four naphthalene diimide (NDI) redox-active units, which have been shown by cyclic voltammetry, and by EPR and ENDOR spectroscopies, to share electrons across the NDI stacks. Thermally driven motions between the neighboring NDI units in the oligorotaxanes influence the passage of electrons through the NDI stacks in a manner reminiscent of the conformationally gated charge transfer observed in DNA.

Excited State, Charge Transfer, and Spin Dynamics in DNA Hairpin Conjugates with Perylenediimide Hairpin Linkers †

A series of short DNA hairpins (nG) using perylene-3,4:9,10-bis(dicarboximide) (PDI) as the hairpin linker was synthesized in which the distance between the PDI and a guanine-cytosine (G-C) base pair is systematically varied by changing the number (n - 1) of adenine-thymine (A-T) base pairs between them. Due to the relatively large hydrophobic surface of PDI, the nG hairpins dimerize in buffer solutions. The photophysics and photochemistry of these hairpins were investigated using femtosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Photoexcitation of the self-assembled PDI dimer within each nG hairpin results in subpicosecond formation of its lower exciton state ((1*)PDI(2)) followed by formation of an excimer-like state ((1*X)PDI(2)) with tau = 10-28 ps. Both of these states are lower in energy than (1*)PDI, so that neither can oxidize A, C, and T. Electron transfer from G to (1*)PDI(2) is faster than formation of (1*X)PDI(2) only for 1G. Electron transfer from G to (1*X)PDI(2) for 2G-8G, occurs by the superexchange mechanism and, thus, becomes exponentially less efficient as the G-PDI(2) distance increases. Nevertheless, TREPR studies show that photoexcitation of 2G and 4G produce spin-correlated radical ion pairs having electron spin polarization patterns indicating that a low yield of charge separation proceeds from (1*X)PDI(2) by the radical pair intersystem crossing (RP-ISC) mechanism to initially yield a singlet radical ion pair. The strong spin-polarization of the radical ion pairs makes it possible to observe them, even though their concentration is low. As expected, the hairpin lacking G (0G) and that having the longest G-PDI(2) distance (8G) display no TREPR radical ion pair signals. Hairpins 0G, 2G, 4G, and 8G all exhibit triplet EPR spectra at 85 K. Simulations of the spectra show that (3*)PDI is produced mainly by a spin-orbit-induced intersystem crossing mechanism, while the spectra of 2G and 4G have 5% and 21% contributions, respectively, from (3*)PDI produced by charge recombination of radical ion pairs that originate from RP-ISC. These low percentages of RP-ISC derived (3*)PDI result mainly from the low yield of radical ion pairs in 2G and 4G.

Mechanical bonds and topological effects in radical dimer stabilization

While mechanical bonding stabilizes tetrathiafulvalene (TTF) radical dimers, the question arises: what role does topology play in catenanes containing TTF units? Here, we report how topology, together with mechanical bonding, in isomeric [3]- and doubly interlocked [2]catenanes controls the formation of TTF radical dimers within their structural frameworks, including a ring-in-ring complex (formed between an organoplatinum square and a {2+2} macrocyclic polyether containing two 1,5-dioxynaphthalene (DNP) and two TTF units) that is topologically isomeric with the doubly interlocked [2]catenane. The separate TTF units in the two {1+1} macrocycles (each containing also one DNP unit) of the isomeric [3]catenane exhibit slightly different redox properties compared with those in the {2+2} macrocycle present in the [2]catenane, while comparison with its topological isomer reveals substantially different redox behavior. Although the stabilities of the mixed-valence (TTF2)(•+) dimers are similar in the two catenanes, the radical cationic (TTF(•+))2 dimer in the [2]catenane occurs only fleetingly compared with its prominent existence in the [3]catenane, while both dimers are absent altogether in the ring-in-ring complex. The electrochemical behavior of these three radically configurable isomers demonstrates that a fundamental relationship exists between topology and redox properties.