Thea M. Wilson

Framework reduction and alkali-metal doping of a triply catenating metal-organic framework enhances and then diminishes H2 uptake.

A permanently microporous metal-organic framework compound with the formula Zn(2)(NDC)(2)(diPyTz) (NDC = 2,6-naphthalenedicarboxylate, diPyTz = di-3,6-(4-pyridyl)-1,2,4,5-tetrazine) has been synthesized. The compound, which features a triply catenating, pillared-paddlewheel structure, was designed to be easily chemically reduced (diPyTz sites) by appropriate channel permeants. Reduction was achieved by using the naphthalenide anion, with the accompanying metal cation (Li(+), Na(+) or K(+)) serving to dope the compound in extraframework fashion. H(2) uptake at 1 atm and 77 K increases from 1.12 wt % for the neutral material to 1.45, 1.60, and 1.51 wt % for the Li(+)-, Na(+)-, and K(+)-doped materials, respectively. The isosteric heats of adsorption are similar for all four versions of the material despite the large uptake enhancements for the reduced versions. Nitrogen isotherms were also measured in order to provide insight into the mechanisms of uptake enhancement. The primary mechanism is believed to be dopant-facilitated displacement of catenated frameworks by sorbed H(2). More extensive cation doping decreases the H(2) loading.

Intramolecular Energy Transfer within Butadiyne-Linked Chlorophyll and Porphyrin Dimer-Faced, Self-Assembled Prisms

The synthesis and photophysical properties of butadiyne-linked chlorophyll and porphyrin dimers in toluene solution and in several self-assembled prismatic structures are described. The butadiyne linkage between the 20-positions of the macrocycles results in new electronic transitions polarized along the long axes of the dimers. These transitions greatly increase the ability of these dimers to absorb the solar spectrum over a broad wavelength range. Femtosecond transient absorption spectroscopy reveals the relative rate of rotation of the macrocycles around the butadiyne bond joining them. Following addition of 3-fold symmetric, metal-coordinating ligands, both macrocyclic dimers self-assemble into prismatic structures in which the dimers comprise the faces of the prisms. These structures were confirmed by small-angle X-ray scattering experiments in solution using a synchrotron source. Photoexcitation of the prismatic assemblies reveals that efficient, through-space energy transfer occurs between the macrocyclic dimers within the prisms. The distance dependence of energy transfer between the faces of the prisms was observed by varying the size of the prismatic assemblies through the use of 3-fold symmetric ligands having arms with different lengths. These results show that self-assembly of discrete macrocyclic prisms provides a useful new strategy for controlling singlet exciton flow in antenna systems for artificial photosynthesis and solar cell applications.

Photophysics and Redox Properties of Rylene Imide and Diimide Dyes Alkylated Ortho to the Imide Groups

Ruthenium-catalyzed C-H bond activation was used to directly attach phenethyl groups derived from styrene to positions ortho to the imide groups in a variety of rylene imides and diimides including naphthalene-1,8-dicarboximide (NMI), naphthalene-1,4:5,8-bis(dicarboximide) (NI), perylene-3,4-dicarboximide (PMI), perylene-3,4:9,10-bis(dicarboximide) (PDI), and terrylene-3,4:11,12-bis(dicarboximide) (TDI). The monoimides were dialkylated, while the diimides were tetraalkylated, with the exception of NI, which could only be dialkylated due to steric hindrance. The absorption, fluorescence, transient absorption spectra, and lowest excited singlet state lifetimes of these chromophores, with the exception of NI, are nearly identical to those of their unsubstituted parent chromophores. The reduction potentials of the dialkylated chromophores are approximately 100 mV more negative and oxidation potentials are approximately 40 mV less positive than those of the parent compounds, while the corresponding potentials of the tetraalkylated compounds are approximately 200 mV more negative and approximately 100 mV less positive than those of their parent compounds, respectively. Continuous wave electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) data on the radical anion of PDI reveals spin density on the perylene-core protons as well as on the beta-protons of the phenethyl groups. The phenethyl groups enhance the otherwise poor solubility of the bis(dicarboximide) chromophores and only weakly perturb the photophysical and redox properties of the parent molecules, rendering these derivatives and related molecules of significant interest to solar energy conversion.

Intersystem Crossing Mediated by Photoinduced Intramolecular Charge Transfer: Julolidine−Anthracene Molecules with Perpendicular π Systems

Time-resolved electron paramagnetic resonance studies show that the primary mechanism of triplet formation following photoexcitation of julolidine-anthracene molecules linked by a single bond and having perpendicular pi systems is a spin-orbit, charge-transfer intersystem crossing mechanism (SOCT-ISC). This mechanism depends on the degree of charge transfer from julolidine to anthracene, the dihedral angle (theta1) between their pi systems, and the magnitude of the electronic coupling between julolidine and anthracene. We compare 4-(9-anthracenyl)-julolidine with the more sterically encumbered 4-(9-anthracenyl)-3,5-dimethyljulolidine and find that fixing theta1 congruent with 90 degrees serves to enhance SOCT-ISC by increasing the change in orbital angular momentum accompanying charge transfer. Given that the requirements for the SOCT-ISC mechanism are quite general, we expect it to occur in a variety of electron donor-acceptor systems.

Toward an n-type molecular wire: electron hopping within linearly linked perylenediimide oligomers.

A series of linearly linked perylenediimide (PDI) dimers and trimers were synthesized in which the PDI pi systems are nearly orthogonal. These oligomers and several model compounds were singly reduced, and intramolecular electron hopping between the PDI molecules was probed by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy. When the functional groups attached to the ends of the oligomers were chosen to make each PDI molecule electronically equivalent, the single electron hops between the PDI molecules with rates that significantly exceed 10(7) s(-1). Rapid electron hopping between pairs of PDI molecules having orthogonal pi systems is unexpected and may expand the possible design motifs for organic electronic materials based on PDI.

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.

Electron hopping among cofacially stacked perylenediimides assembled by using DNA hairpins

The self-assembly of redox-active molecules into ordered arrays capable of rapid, long-distance charge transport is important for the development of functional nanomaterials for organic electronics. In this regard, DNA shows great promise as a structural scaffold for the helical arrangement of chromophores and other (semi)conducting materials. Base substitutions and modifications, sugar modifications, and noncovalent interactions have all been used for the construction of such DNA-based structures. Perylenediimides (PDIs), which have the advantages of strong absorptivity, high fluorescence quantum yields, high photochemical and thermal stability, strong hydrophobic p–p stacking interactions, and semiconducting properties, have been incorporated in a variety of structures. Recently, Wagner and Wagenknecht reported the preparation of a PDI derivative, P (Figure 1), which is readily incorporated into an oligonucleotide and serves as a base-pair surrogate when located opposite an abasic site in a duplex structure. The incorporation of P in opposite complementary oligonucleotides has been shown to result in the formation of stable duplexes in which the P units are located in a zipperlike fashion within the hydrophobic interior of the resulting duplex. 17] The stacking of P units within the duplex resulted in an excimer-like state following photoexcitation. We report herein the results of our investigation of intramolecular electron hopping within a series of synthetic DNA hairpins 1–4 (Figure 1). These hairpins possess compact 3’-CCA loop regions connecting poly(T)–poly(A) stems containing a single P moiety located opposite an abasic site (1), two P moieties located opposite abasic sites and attached either to the same strand (in 2s) or to opposite strands (in 2o), or three or four P moieties positioned adjacent to one another but on opposite strands in a zipperlike fashion (in 3 and 4). The EPR spectra of the singly reduced duplexes were consistent with electron hopping between two sites in both dimers (the hairpins containing two P moieties) and the trimer 3, and among three sites in the tetramer 4. Herein, we discuss the origin and implications of partial electron sharing. Oligonucleotides containing P were synthesized by the method of Wagner and Wagenknecht; the CCA linker in hairpins 1–4 has been employed previously in the synthesis of stable minihairpins. 17, 18] The characterization of 1–4, including mass spectrometry and circular dichroism (CD), is described in the Supporting Information. An intensity reversal was observed in the UV/Vis absorption spectra for the 0!0 and 0!1 transitions in 2–4 with respect to those of 1 (Figure 2). This result indicates that the p-stacked P chromophores are exciton-coupled. Moreover, the A0!0/A0!1 ratio for the vibronic bands decreased as the number of P units increased, and there was a notable difference between dimers 2s and 2o, presumably as a result of the conformational changes imposed by the attachment of the two P units to either one strand or opposite strands within the duplex. Upon the partial chemical reduction of P with sodium dithionite, peaks corresponding to the radical anion appeared at 727, 815, and 980 nm, whereas the vibronic progression in the visible region decreased in intensity (see Figure S1 in the Supporting Information). The Figure 1. DNA hairpin structures.

Challenges in Distinguishing Superexchange and Hopping Mechanisms of Intramolecular Charge Transfer through Fluorene Oligomers

The temperature dependence of intramolecular charge separation in a series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene FL(n) (n = 1-4) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidation of PTZ via the FL(n) bridge. In toluene, the temperature dependence of the charge separation rate constants for PTZ-FL(n)-PDI, (n = 1-4) is relatively weak and is successfully described by the semiclassical Marcus equation. The activation energies for charge separation suggest that bridge charge carrier injection is not the rate limiting step. The difficulty of using temperature and length dependence to differentiate hopping and superexchange is discussed, with difficulties in the latter topic explored via an extension of a kinetic model proposed by Bixon and Jortner.

Rapid intramolecular hole hopping in meso-meso and meta-phenylene linked linear and cyclic multiporphyrin arrays

A series of zinc porphyrin arrays comprised of a meso-meso linked porphyrin dimer, a meta-phenylene linked dimer, gable-like tetramers consisting of the meso-meso linked dimers bridged via a meta-phenylene linker, and a dodecameric ring composed of this alternating dimeric pattern were singly oxidized, and intramolecular hole hopping between the porphyrin moieties was probed using electron paramagnetic resonance (EPR) spectroscopy. Electron nuclear double resonance (ENDOR) spectroscopy was also used to probe hole hopping within the dimers. Rapid hole hopping occurs between both porphyrins within both dimers and among three porphyrins of the tetramers with rates >10(7) s(-1) at 290 K. Additionally, the hole hops among 8-12 porphyrins in the dodecameric ring with a rate that is >10(7) s(-1) at 290 K, but hopping is slow at 180 K. These results show that hole hopping is rapid even though the meta-phenyl bridges and direct meso-meso linkages do not provide optimal electronic coupling between the porphyrins within these multiporphyrin arrays. This greatly expands the scope of possible structures that can be employed to tailor the design of long distance charge transport systems.

Excitonic coupling in linear and trefoil trimer perylenediimide molecules probed by single-molecule spectroscopy.

Perylenediimide (PDI) molecules are promising building blocks for photophysical studies of electronic interactions within multichromophore arrays. Such PDI arrays are important materials for fabrication of molecular nanodevices such as organic light-emitting diodes, organic semiconductors, and biosensors because of their high photostability, chemical and physical inertness, electron affinity, and high tinctorial strength over the entire visible spectrum. In this work, PDIs have been organized into linear (L3) and trefoil (T3) trimer molecules and investigated by single-molecule fluorescence microscopy to probe the relationship between molecular structures and interchromophoric electronic interactions. We found a broad distribution of coupling strengths in both L3 and T3 and hence strong/weak coupling between PDI units by monitoring spectral peak shifts in single-molecule fluorescence spectra upon sequential photobleaching of each constituent chromophore. In addition, we used a wide-field defocused imaging technique to resolve heterogeneities in molecular structures of L3 and T3 embedded in a PMMA polymer matrix. A systematic comparison between the two sets of experimental results allowed us to infer the correlation between intermolecular interactions and molecular structures. Our results show control of the PDI intermolecular interactions using suitable multichromophoric structures.