[show abstract][hide abstract] ABSTRACT: 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.
Angewandte Chemie International Edition 11/2013; · 13.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: A series of d(8) transition-metal (Pt(II) and Pd(II)) coordination complexes incorporating phosphine-functionalized aminoazobenzene derivatives as hemilabile phosphino-amine (P,N) ligands were synthesized and studied as model weak-link approach (WLA) photoresponsive constructs. The optical and photochemical properties of these complexes were found to be highly influenced by various tunable parameters in WLA systems, which include type of metal, coordination mode, type of ancillary ligand, solvent, and outer-sphere counteranions. In dichloromethane, reversible chelation and partial displacement of the P,N coordinating moieties allow for toggling between aminoazobenzene- or pseudostilbene- and azobenzene-type derivatives. The reversible switching between electronic states of azobenzene can be controlled through either addition or extraction of chloride counterions and is readily visualized in the separation between π-π* and n-π* bands in the complexes' electronic spectra. In acetonitrile solution, the WLA variables inherent to semiopen complexes have a significant impact on the half-lives of the corresponding cis isomers, allowing one to tune their half-lives from 20 to 21000 s, while maintaining photoisomerization behaviors with visible light. Therefore, one can significantly increase the thermal stability of a cis-aminoazobenzene derivative to the extent that single crystals for X-ray diffraction analysis can be grown for the first time, uncovering an unprecedented edge-to-face arrangement of the phenyl rings in the cis isomer. Overall, the azobenzene-functionalized model complexes shed light on the design parameters relevant for photocontrolled WLA molecular switches, as well as offer new ways of tuning the properties of azobenzene-based, photoresponsive materials.
Journal of the American Chemical Society 11/2013; · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Motor molecules present in nature convert energy inputs, such as a chemical fuel or incident photons of light, into directed motion and force biochemical systems away from thermal equilibrium. The ability, not only to control relative movements of components in molecules, but also and to drive their components preferentially in one direction relative to each other using versatile stimuli, is one of the keys to future technological applications. Herein, we describe a wholly synthetic, small-molecule system which, under the influence of chemical reagents, electrical potential, or visible light, undergoes unidirectional relative translational motion. Altering the redox state of a cyclobis(paraquat-p-phenylene) ring simultaneously (i) inverts the relative heights of kinetic barriers presented by the two termini - one a neutral 2-isopropylphenyl group and the other a positively charged 3,5-dimethylpyridinium unit - of a constitutionally asymmetric dumbbell, which can impair threading/dethreading of a pseudorotaxane, and (ii) controls the ring's affinity for a 1,5-dioxynaphthalene binding site located at the dumbbell's central core. The formation and subsequent dissociation of the pseudorotaxane by passage of the ring over the neutral and positively charged termini of the dumbbell component in one, and only one, direction relatively defined has been demonstrated by (i) spectroscopic (1H NMR and UV/vis) means and cyclic voltammetry, as well as with (ii) DFT calculations and by (iii) comparison with control compounds in the shape of constitutionally symmetrical pseudorotaxanes, one with two positively charged and the other with two neutral ends. Operation of the system relies solely on reversible, yet stable, noncovalent bonding interactions. Moreover, in the presence of a photosensitizer, visible light energy is the only fuel source that is needed to drive the unidirectional molecular translation, making it feasible to repeat the operation numerous times without the buildup of byproducts.
Journal of the American Chemical Society 10/2013; · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Multi-electron acceptors are essential components for artificial photosynthetic systems that must deliver multiple electrons to catalysts for solar fuels applications. The recently developed box-like cyclophane incorporating two extended viologen units joined end-to-end by two paraphenylene linkers - namely, ExBox4+ - has a potential to be integrated into light-driven systems on account of its ability to complex with π-electron rich guests such as perylene, which has been utilized to great extent in many light-harvesting applications. Photo-driven electron transfer to ExBox4+ has not previously been investigated, however, and so its properties, following photoreduction, are largely unknown. Here, we investigate the structure and energetics of the various accessible oxidation states of ExBox4+ using a combination of spectroscopy and computation. In particular, we examine photo-initiated electron transfer from perylene bound within ExBox4+ (ExBox4+perylene) using visible and near-infrared femtosecond transient absorption (fsTA) spectroscopy. The structure and conformational relaxation dynamics of ExBox3+perylene+ are observed with femtosecond stimulated Raman spectroscopy (FSRS). From the fsTA and FSRS spectra, we observe that the central paraphenylene spacer in one of the extended viologen units on one side of the cyclophane becomes more coplanar with its neighboring pyridinium units over the first ~5 ps after photoreduction. When the steady-state structure of chemically-generated ExBox2+ is also investigated using Raman spectroscopy, it is found to have the central paraphenylene rings in both of its extended viologen units rotated to be more coplanar with their neighboring pyridinium units, further underscoring the importance of this subunit in the stabilization of the reduced states of ExBox4+.
The Journal of Physical Chemistry A 10/2013; · 2.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: Two covalent perylene-3,4:9,10-bis(dicarboximide) (PDI) dimers in which the PDI molecules are attached to a xanthene (Xan) scaffold in which the long axes of the two π-πstacked PDI molecules are slipped by 4.3 Å and 7.9 Å were prepared. These dimers are designed to mimic J-aggregates and provide insights into the photophysics of triplet state formation in PDI aggregates that target organic electronics. Using ultrafast transient absorption and stimulated Raman spectroscopy, the mechanism of (3*)PDI formation was found to depend strongly on a competition between the rate of Xan(•+)-PDI(•-) formation involving the spacer group and the rate of excimer-like state formation. Which mechanism is favored depends on the degree of electronic coupling between the two PDI molecules and/or solvent polarity. Singlet exciton fission to produce (3*)PDI does not compete kinetically with these processes. The excimer-like state decays relatively slow with τ = 28 ns to produce (3*)PDI, while charge recombination of Xan(•+)-PDI(•-) yields (3*)PDI more than an order of magnitude faster. The perpendicular orientation between the π-orbitals of PDI and the Xan bridge provides a large enough orbital angular momentum change to greatly increase the intersystem crossing rate via Xan(•+)-PDI(•-) →(3*)PDI charge recombination. These results highlight the importance of understanding inter-chromophore electronic coupling in a wide range of geometries as well as the active role that molecular spacers can play in the photophysics of covalent models for self-assembled chromophore aggregates.
The Journal of Physical Chemistry A 09/2013; · 2.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: The crystal structure of N,N-di-(n-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide), 1, obtained by X-ray diffraction reveals that 1 has a nearly planar perylene core, and π-π stacks at a 3.5 Å interplanar distance in well-separated slip-stacked columns. Theory predicts that slip-stacked, π-πstacked structures should enhance inter-chromophore electronic coupling and thus favor singlet exciton fission. Photoexcitation of vapor-deposited polycrystalline 188 nm thick films of 1 results in a 140 ± 20% yield of triplet excitons ((3*))1 in τSF = 180 ± 10 ps. These results illustrate a design strategy for producing perylenediimide and related rylene derivatives that have the optimized inter-chromophore electronic interactions which promote high-yield singlet exciton fission for potentially enhancing organic solar cell performance and charge separation in systems for artificial photosynthesis.
Journal of the American Chemical Society 09/2013; · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report femtosecond stimulated Raman spectroscopy measurements of lattice dynamics in semiconductor nanocrystals and characterize longitudinal optical (LO) phonon production during confinement-enhanced, ultrafast intraband relaxation. Stimulated Raman signals from unexcited CdSe nanocrystals produce a spectral shape similar to spontaneous Raman signals. Upon photoexcitation, stimulated Raman amplitude decreases owing to experimentally resolved ultrafast phonon generation rates within the lattice. We find a ∼600 fs, particle-size-independent depletion time attributed to hole cooling, evidence of LO-to-acoustic down-conversion, and LO phonon mode softening.
[show abstract][hide abstract] ABSTRACT: We report here a potassium-induced guanine-quadruplex as a supramolecular platform for controlled assembly of electron donor-acceptor systems. A monodisperse, C4-symmetric octamer of a guanine-perylene-3,4,9,10-bis(di-carboximide) conjugate (GPDI) was prepared in tetrahydrofuran. The two layers of cyclic guanine tetramers have the same direction of rotation, and the PDI moiety between each layer adopts a nearly eclipsed relationship (H-aggregation) revealed by small- and wide-angle X-ray scattering, NMR spectroscopy, and steady-state UV-Vis absorption. Following photoexcitation of the PDI moiety in the quadruplex, charge separation occurs in tau = 98 ± 12 ps to give G+•-PDI-• that recombines in tau = 1.2 ± 0.2 ns, which is >100 times longer than that in the monomeric GPDI dyad. The transient absorption spectrum of G+•-PDI-• within the GPDI-quadruplex suggests the formation of a radical anion delocalized over the neighboring PDI units, and this result is consistent with the more favorable electrochemical reduction potential for PDIs in the quadruplex relative to the monomer.
Journal of the American Chemical Society 08/2013; · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Ordered multi-spin assemblies are required for developing solid-state molecule-based spintronics. A linear donor-chromophore-acceptor (D-C-A) molecule was covalently attached inside the 150 nm diam. nanopores of an anodic aluminum oxide (AAO) membrane. Photoexcitation of D-C-A in a 343 mT magnetic field results in sub-nanosecond, two-step electron transfer to yield the spin-correlated radical ion pair (SCRP) (1)(D(+)˙-C-A(-)˙), which then undergoes radical pair intersystem crossing (RP-ISC) to yield (3)(D(+)˙-C-A(-)˙). RP-ISC results in S-T0 mixing to selectively populate the coherent superposition states |S'〉 and |T'〉. Microwave-induced transitions between these states and the unpopulated |T+1〉 and |T-1〉 states result in spin-polarized time-resolved EPR (TREPR) spectra. The dependence of the electron spin polarization (ESP) phase of the TREPR spectra on the orientation of the AAO membrane pores relative to the externally applied magnetic field is used to determine the overall orientation of the SCRPs within the pores at room temperature.
Chemical Communications 08/2013; · 6.38 Impact Factor
[show abstract][hide abstract] ABSTRACT: DNA hairpins having both a tethered anthraquinone (Aq) end-capping group and a perylenediimide (PDI) base surrogate were synthesized, wherein Aq and PDI are each separated from a G-C base pair hole trap by A-T and I-C base pairs (G = guanine, A = adenine, T= thymine, C = cytosine, I = inosine). Selective photoexcitation of PDI at 532 nm generates a singlet radical ion pair (RP), (1)(G(+•)-PDI(-•)), while selective photoexcitation of Aq at 355 nm generates the corresponding triplet RP, (3)(G(+•)-Aq(-•)). Subsequent radical pair intersystem crossing within these spin-correlated RPs leads to mixed spin states that exhibit spin-polarized, time-resolved EPR spectra in which the singlet- and triplet-initiated RPs have opposite phases. These results demonstrate that a carefully designed DNA hairpin can serve as a photodriven molecular spin switch based on wavelength-selective formation of the singlet or triplet RP without significant competition from undesired energy transfer processes.
Journal of the American Chemical Society 07/2013; · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: A new family of paramagnetic microporous polymeric organic frameworks (POFs) has been developed through the co-polymerization of terephthalaldehyde and pyrrole, indole, and carbazole respectively. These POFs are functionalized with stable unpaired electrons and electron paramagnetic resonance 10 spectroscopy and pulsed ENDOR spectroscopy confirms their existence. The free radicals are produced in the early steps of polymerization and are responsible for their optical properties and chemical adsorption properties of the POFs. The optical band – gap of these materials ranges from 0.8 to 1.6 eV. The new POFs have spherical morphology and exhibit surface areas up to 777 m 2 g -1 (Pyr-POF-1). Furthermore, Pyr-POF-1 displays uptake of CO 2 14% wt and C 2 H 6 9% wt. at 273 K/1 bar and H 2 1.2% wt at 77 K/1 15 bar. The pyrrole based material (Pyr-POF-2) shows high capacity for CO 2 , 10 % wt, at ambient conditions (298 K/1 bar) and isosteric heat of CO 2 adsorption of ~ 34 kJmol -1 . The preparation of this new POF family is based on a very simple synthetic pathway and highlights the significance of terephthaladehyde as a precursor for the synthesis of low cost functional porous polymers.
Journal of Materials Chemistry 07/2013; · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Periodic dielectric structures are typically integrated with a planar waveguide to create photonic band-edge modes for feedback in one-dimensional distributed feedback lasers and two-dimensional photonic-crystal lasers. Although photonic band-edge lasers are widely used in optics and biological applications, drawbacks include low modulation speeds and diffraction-limited mode confinement. In contrast, plasmonic nanolasers can support ultrafast dynamics and ultrasmall mode volumes. However, because of the large momentum mismatch between their nanolocalized lasing fields and free-space light, they suffer from large radiative losses and lack beam directionality. Here, we report lasing action from band-edge lattice plasmons in arrays of plasmonic nanocavities in a homogeneous dielectric environment. We find that optically pumped, two-dimensional arrays of plasmonic Au or Ag nanoparticles surrounded by an organic gain medium show directional beam emission (divergence angle <1.5° and linewidth <1.3 nm) characteristic of lasing action in the far-field, and behave as arrays of nanoscale light sources in the near-field. Using a semi-quantum electromagnetic approach to simulate the active optical responses, we show that lasing is achieved through stimulated energy transfer from the gain to the band-edge lattice plasmons in the deep subwavelength vicinity of the individual nanoparticles. Using femtosecond-transient absorption spectroscopy, we verified that lattice plasmons in plasmonic nanoparticle arrays could reach a 200-fold enhancement of the spontaneous emission rate of the dye because of their large local density of optical states.
[show abstract][hide abstract] ABSTRACT: A series of donor-chromophore-acceptor-stable radical (D-C-A-R•) molecules having well-defined molecular structures were synthesized to study the factors affecting electron spin polarization transfer from the photogenerated D+•-C-A-• spin-correlated radical pair (RP) to the stable radical R•. Theory suggests that the magnitude of this transfer depends on the spin-spin exchange interaction (2JDA) of D+•-C-A-•. Yet, the generality of this prediction has never been demonstrated. In the D-C-A-R• molecules described here, D = 4-methoxyaniline (MeOAn), 2,3-dihydro-1,4-benzodioxin-6-amine (DioxAn), or benzobisdioxole aniline (BDXAn), C = 4-aminonaphthalene-1,8-dicarboximide, and A = naphthalene-1,8:4,5-bis(dicarboximide) (1A,B-3A,B) or pyromellitimide (4A,B-6A,B). The terminal imide of the acceptors is functionalized with either a hydrocarbon (1A-6A) or a 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (R•) (1B-6B). Photoexcitation of C with 416 nm laser pulses results in two-step charge separation to yield D+•-C-A-•-(R•). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy using continuous (CW) microwaves at both 295 and 85 K and pulsed microwaves at 85 K (electron spin echo detection) are used to probe the initial formation of the spin-polarized RP and the subsequent polarization of the attached R• radical. The TREPR spectra show that |2JDA| for D+•-C-A-• decreases in the order MeOAn+• > DioxAn+• > BDXAn+• as a result of their spin density distributions, while the spin-spin dipolar interaction (dDA) remains nearly constant. Given this systematic variation in |2JDA|, electron spin echo detected EPR spectra of 1B-6B at 85 K show that the magnitude of the spin polarization transferred from the RP to R• depends on |2JDA|.
The Journal of Physical Chemistry A 05/2013; · 2.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: Using visible and near-infrared transient absorption spectroscopy to track distinct excited state, cation, and anion signals, we report a detailed kinetic analysis of photoinitiated multi-step charge separation and ultrafast charge transfer induced dissociation in a self-assembled donor–bridge–acceptor–cobaloxime triad. The donor–bridge–acceptor ligand consists of a perylene chromophore linked via a xylene bridge to a pyridyl-substituted 1,8-naphthalimide electron acceptor. Coordination of the ligand to the catalyst [Co(dmgBF2)2(L)2], where dmgBF2 = (difluoroboryl)dimethylglyoximato and L = water or a solvent molecule, yields a donor–bridge–acceptor–catalyst triad assembly. Photoexcitation with 416 nm laser pulses generates the perylene S1 excited state. Subsequent electron transfer from perylene to the acceptor occurs in τ = 9.0 ± 0.1 ps followed by electron transfer to the catalyst in τ = 6 ± 1 ps. Of the charge-separated state population formed, 79 ± 1% undergoes charge recombination to either the singlet ground state (τ = 0.8 ± 0.1 ns) or the perylene triplet state (τ = 4.3 ± 0.1 ns). Co(I)-pyridyl bond dissociation with τ = 2.4 ± 0.2 ns competes with intramolecular charge recombination resulting in a 21 ± 1% yield of dissociated oxidized photosensitizer and reduced catalyst. Subsequent diffusional charge recombination occurs with k = (1.8 ± 0.2) × 1010 M−1 s−1. This detailed analysis of the electron transfer and dissociation dynamics of an integrated photosensitizer–catalyst system will inform the rational design of novel molecular assemblies that efficiently absorb photons, transfer electrons, and catalyze fuel-forming reactions.
Energy & Environmental Science 05/2013; 6(6):1917-1928. · 11.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: Through a combination of protecting groups, postsynthesis deprotection, and postsynthesis metallation, a homogeneously inaccessible, single-site vanadyl-(monocatecholate) moiety can be incorporated into the dipyridyl struts of a Zn-based, pillared paddlewheel MOF. The resulting MOF, which has large pores, exhibits catalytic activity in the benzylic oxidation of tetralin in the presence of tert-butylhydroperoxide. ■ INTRODUCTION As a class of porous and crystalline coordination polymers, metal-organic frameworks (MOFs) 1−4 are highly promising catalyst scaffolds due to their uniform and well-defined structures and tailorable micropore environments. 5,6 Because MOFs are microporous materials comprising metal nodes linked together by organic struts, both opportunistic and designed catalysis have been demonstrated at unsaturated metal nodes, at organocatalyst-or metal-complex-tethered organic linkers, as well as through catalysts physically encapsulated by the micropores. 5−11 While the majority of catalytically active MOFs reported to date contain active metal centers or complexes, these motifs are often pre-existing components of the secondary building units (SBUs) of the MOFs, 12−14 heterogenized homogeneous catalytic metal complexes, 15−24 or encapsulated metal complexes 25−27 or clusters. 8,28 Examples of MOFs featuring unique metal coordination environments that are inaccessible in solution (or otherwise) remain rare; 29,30 however, their investigation offers unique opportunities for the development of MOF-based catalysts that can access novel activity or mechanistic pathways. We and others have previously demonstrated that the single-site activity of homogeneous metalloporphyrin, 18,21,22,31−33 chiral metallosalen, 15,17,20 and metalloBINOL 16,23,24 catalysts, along with their chemo-and enantioselectivities, 6,11 can be integrated with the shape-and size-selectivity of the MOF environments. Specifically, we have shown that dipyridyl-functionalized analogues of homogeneous porphyrin 18,22,31 and salen 15,17 complexes can be synthesized and readily deployed as struts in permanently microporous Zn-based pillared-paddle-wheel MOFs that are catalytically active and can be modified postsynthetically. 19,22 Herein, we extend this MOF scaffold to display a novel vanadyl(monocatecholate) motif that is catalytically active in a single-site fashion. Unlike metallo-porphyrins and metallosalens, this monocatecholate motif is inaccessible in solution because the low-steric coordination environment of the catechol ligand tends to overwhelmingly favor coordinatively saturated bis-and tris-chelate binding modes. 34 We hypothesize that the pillared paddlewheel MOF scaffold, constructed from orthogonal carboxylate and dipyridyl struts, would be ideal platforms for spatially isolating catechol-functionalized struts, which can then be postsynthetically metallated to achieve well-defined metal motifs with unsatu-rated (or labile) coordinative sites capable of novel catalytic behaviors. 35,36 To achieve MOFs with large pores and apertures, we employ a series of protected catechol-function-alized dipyridyl strut L1 in combination with the catenation-suppressing 37 dibromotetratopic ligand L2. The resulting MOFs can then be deprotected postsynthetically and metallated with vanadium(IV) ions to afford our desired catalytically active MOF materials. To prevent opportunistic catalysis by metal nodes, 38 we select redox-inert Zn ions as the structural metal ions. Because catechol groups readily chelate to Zn ions during crystal growth to form bis-or tris-catecholate homogeneous complexes 34 or amorphous coordination networks, 39 we started our MOF synthesis with protected catechol struts L1. We note that previous attempts, by our groups and others, 30 to grow
[show abstract][hide abstract] ABSTRACT: Small- and wide-angle X-ray scattering (SAXS/WAXS) and DOSY 1H NMR reveal that a zinc methyl pyrochlorophyllide a (Chl) derivative with a 3-pyridyl ligand at the Chl 20-position self-assembles in toluene solution into a cyclic trimer having a radius of 8.2 ± 0.1 Å. Femtosecond transient absorption spectroscopy shows that singlet–singlet annihilation occurs within the cyclic trimer as a result of energy transfer between the Chls that occurs with an (11 ± 1 ps)−1 rate constant, which agrees with the predictions of Förster theory, even though the Chl–Chl distances are comparable to the length of the Chl transition dipole.
Chemical Physics Letters 03/2013; 556(01, 2013). · 2.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: Biology sustains itself by converting solar energy in a series of reactions between light harvesting components, electron transfer pathways, and redox-active centers. As an artificial system mimicking such solar energy conversion, porous chalcogenide aerogels (chalcogels) encompass the above components into a common architecture. We present here the ability to tune the redox properties of chalcogel frameworks containing biological Fe(4)S(4) clusters. We have investigated the effects of [Sn(n)S(2n+2)](4-) linking blocks ([SnS(4)](4-), [Sn(2)S(6)](4-), [Sn(4)S(10)](4-)) on the electrochemical and electrocatalytic properties of the chalcogels, as well as on the photophysical properties of incorporated light-harvesting dyes, tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)(3)(2+)). The various thiostannate linking blocks do not alter significantly the chalcogel surface area (90-310 m(2)/g) or the local environment around the Fe(4)S(4) clusters as indicated by (57)Fe Mössbauer spectroscopy. However, the varying charge density of the linking blocks greatly affects the reduction potential of the Fe(4)S(4) cluster and the electronic interaction between the clusters. We find that when the Fe(4)S(4) clusters are bridged with the adamantane [Sn(4)S(10)](4-) linking blocks, the electrochemical reduction of CS(2) and the photochemical production of hydrogen are enhanced. The ability to tune the redox properties of biomimetic chalcogels presents a novel avenue to control the function of multifunctional chalcogels for a wide range of electrochemical or photochemical processes relevant to solar fuels.
Journal of the American Chemical Society 01/2013; · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Molecular reorganization around the core of C 60 has been achieved by electron transfer centered on p-dimerizable viologen subunits located in a restricted region of space. Fullerene C 60 hexaadducts, featuring 12 viologen subunits, have been prepared by using copper-mediated Huisgen 1,3-dipolar cycloaddition of azides with alkynes. Detailed electrochemical studies, supported by UV-Vis and EPR spectroscopic analyses, demonstrate that the linkers bearing the viologen subunits attached to specific positions around the all-carbon sphere, allow the formation of six intramolecular p-dimers. Theoretical calculations reveal that the close proximity of the orbitals of the viologen subunits attached to the C 60 facilitate the p-dimerization of the bis-radical species. These investigations support the fact that the motion of discrete peripheral groups oriented around the all-carbon sphere of C 60 can be controlled electrochemically using noncovalent reversible interactions.
[show abstract][hide abstract] ABSTRACT: Two X-shaped, cruciform electron Donor2-Acceptor-Acceptor'2 (D2-A-A'2) molecules, 1 and 2, in which D = zinc 5-phenyl-10,15,20-tripentylporphyrin (ZnTPnP) or zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP), respectively, A = pyromellitimide (PI), and A' = perylene-3,4:9,10-bis(dicarboximide) (PDI), were prepared to study self-assembly motifs that promote photoinitiated charge separation followed by electron and hole transport through π-stacked donors and acceptors. PDI secondary electron acceptors were chosen because of their propensity to form self-ordered, π-stacked assemblies in solution, while the ZnTPnP and ZnTPP donors were selected to test the effect of peripheral substituent steric interactions on the π-stacking characteristics of the cruciforms. Small- and wide-angle X-ray scattering measurements in toluene solution reveal that 1 assembles into a π-stacked structure having an average of 5 ± 1 molecules, when  ≅ 10-5 M, while 2 remains monomeric. Photoexcitation of the π-stacked structure of 1 results in formation of ZnTPnP+•-PI-PDI-• in τCS1 = 0.3 ps, which is nearly 100-fold faster than the formation of ZnTPnP+•-PI-• in a model system lacking the PDI acceptor. The data are consistent with a self-assembled structure for 1 in which the majority of the intermolecular interactions have the ZnTPnP donor of one monomer cofacially π-stacked with the PDI acceptor of a neighboring monomer in a crisscrossed fashion. In contrast, 2 remains monomeric in toluene, so that photoexcitation of ZnTPP results in the charge separation reaction sequence: 1*ZnTPP-PI-PDI -> ZnTPP+•-PI-•-PDI -> ZnTPP+•-PI-PDI-•, where τCS1 = 33 ps and τCS2 = 239 ps. The perpendicular orientation of ZnTPnP and ZnTPP relative to PDI in 1 and 2 is designed to decrease the porphyrin-PDI distance without greatly decreasing the overall number of bonds linking them. This serves to decrease the Coulomb energy penalty required to produce D+•-PI-PDI-• relative to the corresponding linear D-PI-PDI array, while retaining the weak electronic coupling necessary to achieve long-lived charge separation, as evidenced by τCR = 23.5 ns for ZnTPP+•-PI-PDI-•.
The Journal of Physical Chemistry B 01/2013; · 3.61 Impact Factor