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.

Photophysical properties of an alkyne-bridged bis(zinc porphyrin)-perylene bis(dicarboximide) derivative.

We report the synthesis, electrochemistry, and photophysical properties of a new donor-acceptor-donor molecule in which the meso carbon atoms of two zinc porphyrin (POR) units are linked through ethynylene bridges to the 1,7-positions of a central perylene-3,4:9,10-bis(dicarboximide) (PDI). In contrast to previously studied systems incorporating POR and PDI groups, this alkyne-based derivative shows evidence of through-bond electronic coupling in the ground state; the new chromophore exhibits absorption features similar to those of its constituent parts as well as lower energy features (at wavelengths up to ca. 1000 nm), presumably arising from donor-acceptor interactions. Transient absorption measurements show that excitation at several visible and near-IR wavelengths results in the formation of an excited-state species with a lifetime of 290 ps in 1% (v/v) pyridine in toluene. The absorption spectrum of this species resembles the sum of the spectra for the chemically generated radical cation and radical anion of the chromophore. The chromophore shows moderate two-photon absorption cross sections (2000-7000 GM) at photon wavelengths close to the onset of its low-energy one-photon absorption feature.

Synthesis and photophysical properties of donor- and acceptor-substituted 1,7-bis(arylalkynyl)perylene-3,4:9,10-bis(dicarboximide)s.

A series of 1,7-bis(arylethynyl)-N,N'-bis[2,6-diisopropylphenyl]perylene-3,4:9,10-bis(dicarboximide)s has been obtained from Sonogashira coupling of the 1,7-dibromoperylene-3,4:9,10-bis(dicarboximide) with p-substituted phenylacetylenes in which the p-substituents include pi-donors (dialkylamino, diarylamino, p-(diarylamino)phenyl, alkoxy) and pi-acceptors (diarylboryl, p-(diarylboryl)phenyl). The bis(arylethynyl)-substituted chromophores all show two reversible molecular reductions and are all slightly more readily reduced than unsubstituted perylene-3,4:9,10-bis(dicarboximide)s with the electrochemical potentials being rather insensitive to the pi-donor or acceptor nature of the aryl group. The amine derivatives also show reversible molecular oxidations. The UV-vis spectra of the chromophores with alkoxy and boryl substituents show red-shifted absorptions relative to unsubstituted perylene diimides with discernible vibronic structure. In contrast, the lowest energy absorptions of the amino derivatives are broad and structureless, suggesting donor-to-acceptor charge-transfer character. Transient absorption spectra for the amine derivatives were interpreted in terms of ultrafast charge separation, followed by charge recombination on a time scale of ca. 80-2000 ps. Two compounds were also synthesized in which an additional stronger, but more weakly coupled, donor group is linked by a nonconjugated bridge to the p-amine donor, to investigate the effect on the charge recombination lifetimes; however, the lifetimes of the charge-separated states, ca. 150 and 1000 ps, were within the range observed for the simple amine systems. Finally, the two-photon absorption properties of three bis(arylethynyl)-substituted derivatives were investigated, along with those of 1,7-di(pyrrolidin-1-yl)-N,N'-bis[2,6-diisopropylphenyl]perylene-3,4:9,10-bis(dicarboximide). As with other perylene-3,4:9,10-bis(dicarboximide)s and related species, strong two-photon absorption (>1000 GM) was observed for three of these species close to the one-photon absorption edge; however, an additional feature (100-500 GM) was also observed at longer wavelength. An example with (p-aminophenyl)ethylnyl substituents showed a qualitatively different two-photon spectrum with a cross-section >500 GM being observed over a broad wavelength range.

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.

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.

Fast energy transfer within a self-assembled cyclic porphyrin tetramer.

The structure of a cyclic self-assembled tetramer of an asymmetric meso-ethynylpyridyl-functionalized Zn(II)-porphyrin was established by solution-phase X-ray scattering and diffraction; femtosecond transient absorption and anisotropy spectroscopies were used to (a) observe rapid energy transfer (3.8 ps(-1)) between porphyrin subunits and (b) establish that the transfer occurs between adjacent units.

Controlling energy and charge transfer in linear chlorophyll dimers.

A series of linkers constructed from combinations of phenyl and ethynyl groups is shown to permit ultrafast energy transfer between two chlorophylls, while allowing control over radical cation migration between them.

Electron-transfer dynamics of photosynthetic reaction centers in thermoresponsive soft materials.

Poly(ethylene glycol)-grafted, lipid-based, thermoresponsive, soft nanostructures are shown to serve as scaffolding into which reconstituted integral membrane proteins, such as the bacterial photosynthetic reaction centers (RCs) can be stabilized, and their packing arrangement, and hence photophysical properties, can be controlled. The self-assembled nanostructures exist in two distinct states: a liquid-crystalline gel phase at temperatures above 21 degrees C and a non-birefringent, reduced viscosity state at lower temperatures. Characterization of the effect of protein introduction on the mesoscopic structure of the materials by 31P NMR and small-angle X-ray scattering shows that the expanded lamellar structure of the protein-free material is retained. At reduced temperatures, however, the aggregate structure is found to convert from a two-dimensional normal hexagonal structure to a three-dimensional cubic phase upon introduction of the RCs. Structural and functional characteristics of the RCs were determined by ground-state and femtosecond transient absorption spectroscopy. Time-resolved results indicate that the kinetics of primary electron transfer for the RCs in the low-viscosity cold phase of the self-assembled nanostructures are identical to those observed in a detergent-solubilized state in buffered aqueous solutions (approximately 4 ps) over a wide range of protein concentrations and experimental conditions. This is also true for RCs held within the lamellar gel phase at low protein concentrations and at short sample storage times. In contrast are kinetics from samples that are prepared with high RC concentrations and stored for several hours, which display additional kinetic components with extended electron-transfer times (approximately 10-12 ps). This observation is tentatively attributed to energy transfer between RCs that have laterally (in-plane) organized within the lipid bilayers of the lamellar gel phase prior to charge separation. These results not only demonstrate the use of soft nanostructures as a matrix in which to stabilize and organize membrane proteins but also suggest the possibility of using them to control the interactions between proteins and thus to tune their collective optical/electronic properties.

Low-temperature frequency domain study of excitation energy transfer in ethynyl-linked chlorophyll trefoils and aggregates.

Using hole-burning spectroscopy, we show that excitation energy transfer (EET) time in ethynyl-linked chlorophyll trefoil (ChlT1) monomer is very fast (∼2.5 ps) at liquid helium temperature. This is consistent with data obtained by femtosecond transient spectroscopy experiments performed at room temperature, in which an EET time of 1.8 ps was observed (Kelley, R. F. et al. Angew. Chem. Int. Ed. 2006, 45, 7979). This finding further supports the importance of through-bond electronic coupling at low temperature. In addition, we show that ChlT1 (even at very low concentrations) in methyl tetrahydrofuran-ethanol glass (1:200 v/v; T ∼ 5 K) forms different types of aggregates. It is demonstrated that the relative distribution of various types of aggregates (whose possible structures are briefly discussed) depends on the cooling rate and matrix composition. For example, the EET time in two types of ChlT1-based aggregates is slower by a factor of ∼5-7 with respect to that observed for ChlT1 monomer. This indicates that ChlT1 aggregates can retain ultrafast energy transfer properties similar to those observed in natural photosynthetic antennas. It is anticipated that such building blocks could be utilized in future photovoltaic devices.

Photoinitiated charge transport in supramolecular assemblies of a 1,7,N,N'-tetrakis(zinc porphyrin)-perylene-3,4:9,10-bis(dicarboximide).

We report the synthesis and photophysical characterization of a multichromophore array, (Z3PN)4PDI, consisting of four zinc 5-phenyl-10,15,20-tri(n-pentyl)porphyrins (Z3PN) attached to the 1,7,N,N'-positions of perylene-3,4:9,10-bis(dicarboximide) (PDI). The dynamics of energy and charge transport within this system were compared to those of two model compounds, N,N'-(Z3PN)2PDI and 1,7-(Z3PN)2PDI. The symmetry of the lowest unoccupied and highest occupied molecular orbitals of PDI results in significantly different electronic couplings between Z3PN and PDI when they are connected at the 1,7-positions vs the N,N'-positions of PDI. This results in two distinct pathways for electron transfer in (Z3PN)4PDI. Using a combination of metal-ligand binding with the bidentate ligand 1,4-diazabicyclo[2.2.2.]octane (DABCO) and pi-pi stacking, (Z3PN4)PDI forms a supramolecular assembly, [[(Z3PN)4PDI]2-DABCO4]2, in toluene solution. The structure of this hierarchical assembly is characterized with the use of solution-phase X-ray scattering techniques and demonstrates both efficient light harvesting and facile charge separation and transport using multiple pathways.

Photoinduced charge separation in self-assembled cofacial pentamers of zinc-5,10,15,20-tetrakis(perylenediimide)porphyrin.

A bichromophoric electron donor-acceptor molecule composed of a zinc tetraphenylporphyrin (ZnTPP) surrounded by four perylene-3,4:9,10-bis(dicarboximide)(PDI) chromophores (ZnTPP-PDI(4)) was synthesized. The properties of this molecule were compared to a reference molecule having ZnTPP covalently bound to a single PDI (ZnTPP-PDI). In toluene, ZnTPP-PDI(4) self-assembles into monodisperse aggregates of five molecules arranged in a columnar stack, (ZnTPP-PDI(4))(5). The monodisperse nature of this assembly contrasts sharply with previously reported ZnTPP-PDI(4) derivatives having 1,7-bis(3,5-di-t-butylphenoxy) groups (ZnTPP-PPDI(4)). The size and structure of this assembly in solution was determined by small angle X-ray scattering (SAXS) using a high flux synchrotron X-ray source. The ZnTPP-PDI reference molecule does not aggregate. Femtosecond transient absorption spectroscopy shows that laser excitation of both ZnTPP-PDI and (ZnTPP-PDI(4))(5) results in quantitative formation of ZnTPP(+*)-PDI(-*) radical ion pairs in a few picoseconds. The transient absorption spectra of (ZnTPP-PDI(4))(5) suggest that the PDI(-*) radicals interact strongly with adjacent PDI molecules within the columnar stack. Charge recombination occurs more slowly within (ZnTPP-PDI(4))(5)(tau= 4.8 ns) than it does in ZnTPP-PDI (tau= 3.0 ns) producing mostly ground state as well as a modest yield of the lowest triplet state of PDI ((3*)PDI). Formation of (3*)PDI occurs by rapid spin-orbit induced intersystem crossing (SO-ISC) directly from the singlet radical ion pair as evidenced by the electron spin polarization pattern exhibited by its time-resolved electron paramagnetic resonance spectrum.

Intramolecular electron transfer through the 20-position of a chlorophyll a derivative: an unexpectedly efficient conduit for charge transport.

Suzuki cross-coupling reactions have afforded 20-phenyl-substituted Chlorophyll a derivatives (ZCPh) in good yields and significant quantities from readily available Chl a. A series of donor-acceptor dyads was synthesized in which naphthalene-1,8:4,5-bis(dicarboximide) or either of two perylene-3,4:9,10-bis(dicarboximide) electron acceptors is attached to the para position of the 20-phenyl group. Comparisons with the analogous dyads based on a zinc 5,10,15-tri(n-pentyl)-20-phenylporphyrin donor show that, for a given acceptor and solvent, the rates of photoinduced charge separation and recombination as well as the calculated electronic coupling matrix elements, V, for these reactions differ by less than a factor of 2. However, EPR and ENDOR spectroscopy corroborated by DFT calculations show that the highest occupied MO of ZCPh+* has little spin (charge) density at the 20-carbon atom, whereas Z3PnPh+* has significant spin (charge) density there, implying that V, and therefore the electron-transfer rates, should differ significantly for these two macrocyclic donors. DFT calculations on ZCPh+* and Z3PnPh+*, with two -0.5 charges located where the nearest carbonyl oxygen atoms of the acceptor would reside in the donor-acceptor dyads, show that the presence of the negative charges significantly shifts the charge density of both ZCPh+* and Z3PnPh+* from the macrocycle onto the phenyl rings. Thus, the presence of adjacent covalently linked radical anions at a fixed location relative to each of these radical cations results in nearly identical electronic coupling matrix elements for electron transfer and therefore very similar rates.

Electron hopping in pi-stacked covalent and self-assembled perylene diimides observed by ENDOR spectroscopy.

We have carried out room-temperature, solution-phase electron paramagnetic resonance and electron-nuclear double resonance studies on a series of radical anions based upon perylene-3,4:9,10-bis(dicarboximide) (PDI). The following systems were studied: two PDI monomers, a covalent, cofacial dimer, and two covalent trefoil-PDI3 molecules, one of which self-assembles into pi-stacked dimers. Full sharing of the unpaired electron in the covalent and self-assembled dimers is revealed by a halving of the hyperfine coupling constants in these species, relative to those of the monomers. These results and the electronic absorption spectra show that electron hopping on a >107 Hz time scale occurs between a reduced and neutral chromophoric pair.

Electron donor-bridge-acceptor molecules with bridging nitronyl nitroxide radicals: influence of a third spin on charge- and spin-transfer dynamics.

Appending a stable radical to the bridge molecule in a donor-bridge-acceptor system (D-B-A) is potentially an important way to control charge- and spin-transfer dynamics through D-B-A. We have attached a nitronyl nitroxide (NN*) stable radical to a D-B-A system having well-defined distances between the components: MeOAn-6ANI-Ph(NN*)-NI, where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, Ph = phenyl, and NI = naphthalene-1,8:4,5-bis(dicarboximide). MeOAn-6ANI, NN*, and NI are attached to the 1, 3, and 5 positions of the Ph bridge. Using both time-resolved optical and EPR spectroscopy, we show that NN* influences the spin dynamics of the photogenerated triradical states (2,4)(MeOAn(+)*-6ANI-Ph(NN*)-NI(-)*), resulting in slower charge recombination within the triradical compared to the corresponding biradical lacking NN*. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn(+)(*) and NI(-)(*) is not altered by the presence of NN*, which only accelerates radical pair intersystem crossing. Charge recombination within the triradical results in the formation of (2,4)(MeOAn-6ANI-Ph(NN*)-(3)NI), in which NN* is strongly spin-polarized. Normally, the spin dynamics of correlated radical pairs do not produce a net spin polarization; however, net spin polarization appears on NN* with the same time constant as describes the photogenerated radical ion pair decay. This effect is attributed to antiferromagnetic coupling between NN* and the local triplet state (3)NI, which is populated following charge recombination. This requires an effective switch in the spin basis set between the triradical and the three-spin charge recombination product having both NN* and (3)NI present.

Conformationally gated switching between superexchange and hopping within oligo-p-phenylene-based molecular wires.

We observe well-defined regions of superexchange and thermally activated hopping in the temperature dependence of charge recombination (CR) in a series of donor-bridge-acceptor (D-B-A) systems, where D = phenothiazine (PTZ), B = p-phenylene (Ph(n)), n = 1-4, and A = perylene-3,4:9,10-bis(dicarboximide) (PDI). A fit to the thermally activated CR rates of the n = 3 and n = 4 compounds yields activation barriers of 1290 and 2030 cm(-1), respectively, which match closely with theoretically predicted and experimentally observed barriers for the planarization of terphenyl and quaterphenyl. Negative activation of CR in the temperature regions dominated by superexchange charge transport is the result of a fast conformational equilibrium that increasingly depopulates the reactive state for CR as temperature is increased. The temperature dependence of the effective donor-acceptor superexchange coupling, V(DA), measured using magnetic field effects on the efficiency of the charge recombination process, shows that CR occurs out of the conformation with lower V(DA) via the energetically favored triplet pathway.

Wire-like charge transport at near constant bridge energy through fluorene oligomers.

The study of photoinitiated electron transfer in donor-bridge-acceptor molecules has helped elucidate the distance dependence of electron transfer rates and behavior of various electron transfer mechanisms. In all reported cases, the energies of the bridge electronic states involved in the electron transfer change dramatically as the length of the bridge is varied. We report here, in contrast, an instance in which the length of the bridge, and therefore the distance over which the electron is transferred, can be varied without significantly changing the energies of the relevant bridge states. A series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene (FL(n)) 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 rate constants for both charge separation and recombination as well as the energy levels of the relevant FL(n)(+.) bridge states for n = 1-4 are only weakly distance dependent. After the initial photo-generation of (1)(PTZ(+.)-FL(n)-PDI(-.)), radical pair intersystem crossing results in formation of (3)(PTZ(+.)-FL(n)-PDI(-.)) that recombines to give (3.)PDI. The dependence of the (3.)PDI yield on an applied magnetic field shows a resonance, which gives the singlet-triplet splitting, 2J, of the radical ion pair. The magnitude of 2J directly monitors the contribution of coherent charge transfer (superexchange) to the overall electron transfer rate. These data show that charge recombination through FL(n) is dominated by incoherent hopping at long distances.