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.     

Time-resolved EPR studies of photogenerated radical ion pairs separated by p-phenylene oligomers and of triplet states resulting from charge recombination

Photoexcitation of a series of donor-bridge-acceptor (D-B-A) systems, where D = phenothiazine (PTZ), B = p-phenylene (Phn), n = 1-5, and A= perylene-3,4:9,10-bis(dicarboximide) (PDI) results in rapid electron transfer to produce 1(PTZ+*-Phn-PDI-*). Time-resolved EPR (TREPR) studies of the photogenerated radical pairs (RPs) show that above 150 K, when n = 2-5, the radical pair-intersystem crossing mechanism (RP-ISC) produces spin-correlated radical ion pairs having electron spin polarization patterns indicating that the spin-spin exchange interaction in the radical ion pair is positive, 2J > 0, and is temperature dependent. This temperature dependence is most likely due to structural changes of the p-phenylene bridge. Charge recombination in the RPs generates PTZ-Phn-3*PDI, which exhibits a spin-polarized signal similar to that observed in photosynthetic reaction-center proteins and some biomimetic systems. At temperatures below 150 K and/or at shorter donor-acceptor distances, e.g., when n = 1, PTZ-Phn-3*PDI is also formed from a competitive spin-orbit-intersystem crossing (SO-ISC) mechanism that is a result of direct charge recombination: 1(PTZ+*-Phn-PDI-*) --> PTZ-Phn-3*PDI. This SO-ISC mechanism requires the initial RP intermediate and depends strongly on the orientation of the molecular orbitals involved in the charge recombination as well as the magnitude of 2J.     

Enhanced Electron Transfer Ability via Coordination in Block Copolymer/Porphyrin/Fullerene Micelle

Inspired by structures of antenna-reaction centers in photosynthesis,the complex micelle was prepared from zinc tetra-phenyl porphyrin (ZnTPP),fullerene derivative (PyC60) and poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-bPCL).The core-shell structure made the hydrophobic donor-acceptor system work in aqueous.In micellar core,coordination interaction occurred between ZnTPP and PyC60 molecules which ensured the enhanced energy migration from the donor to the acceptor.The enhanced interaction between porphyrin and fullerene was confirmed by absorption,steady-state fluorescence and transient fluorescence.The generation of singlet oxygen and superoxide radical was detected by iodide method and reduction of nitro blue tetrazolium,respectively,which confirmed that electron transfer reaction in the complex micellar core occurred.Moreover,the complex micelle exhibited effective electron transfer performance in photodebromination of 2,3-dibromo-3-phenylpropionic acid.The complex micellar structure endowed the donor-acceptor system with improved stability under irradiation.This strategy could be helpful for designing new electron transfer platform and artificial photosynthetic system.     

Making a molecular wire: charge and spin transport through para-phenylene oligomers

Functional molecular wires are essential for the development of molecular electronics. Charge transport through molecules occurs primarily by means of two mechanisms, coherent superexchange and incoherent charge hopping. Rates of charge transport through molecules in which superexchange dominates decrease approximately exponentially with distance, which precludes using these molecules as effective molecular wires. In contrast, charge transport rates through molecules in which incoherent charge hopping prevails should display nearly distance independent, wirelike behavior. We are now able to determine how each mechanism contributes to the overall charge transport characteristics of a donor-bridge-acceptor (D-B-A) system, where D = phenothiazine (PTZ), B = p-oligophenylene, and A = perylene-3,4:9,10-bis(dicarboximide) (PDI), by measuring the interaction between two unpaired spins within the system's charge separated state via magnetic field effects on the yield of radical pair and triplet recombination product.     

Excited-state symmetry breaking in cofacial and linear dimers of a green perylenediimide chlorophyll analogue leading to ultrafast charge separation

Photoexcitation of chromophoric dimers constrained to a symmetric pi-stacked geometry by their molecular structure usually produces excimers independent of solvent polarity, while dimers with edge-to-edge perpendicular pi systems undergo excited-state symmetry breaking in highly polar solvents leading to intradimer charge separation. We present direct evidence for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 1,7-bis(pyrrolidin-1'-yl)-perylene-3,4:9,10-bis(dicarboximide) (5PDI) in the low polarity solvent toluene to produce a radical ion pair quantitatively. This dimer, cof-5PDI2, was synthesized by attaching two 5PDI chromophores via imide groups to a xanthene spacer. For comparison, a linear symmetric dimer, lin-5PDI2, was prepared in which the 5PDI chromophores are linked end-to-end via a N-N single bond between their imides. The edge-to-edge pi systems of the 5PDI chromophores within lin-5PDI2 are perpendicular to one another. Ground-state absorption spectra of both 5PDI dimers show exciton coupling, which is consistent with the orientation of the 5PDI chromophores relative to one another. Ultrafast transient absorption spectroscopy following excitation of the dimers with 700 nm, 100 fs laser pulses shows that quantitative intradimer electron transfer occurs in cof-5PDI2 in toluene with tau = 0.17 ps followed by charge recombination to the ground state with tau = 222 ps. Similar measurements on lin-5PDI2 reveal that photoinduced electron transfer does not occur in toluene, but occurs in more polar solvents such as 2-methyltetrahydrofuran, wherein tau = 55 ps for charge separation and tau = 99 ps for charge recombination. Excited-state symmetry breaking in 5PDI dimers provides new routes to biomimetic charge separation and storage assemblies that can be more easily prepared and modified than those based on multiple tetrapyrrole macrocycles.     

Electron transport in silver-semiconductor nanocomposite films exhibiting multicolor photochromism

In the multicolor photochromism of TiO2 nanoporous films loaded with photocatalytically deposited Ag nanoparticles, visible light-induced electron transfer from Ag to oxygen molecules plays an essential role. Here we examined the effect of TiO2 on the electron transfer. We found that not only photocatalytically deposited Ag, but also electrodeposited Ag and commercially available Ag nanoparticles in a nanoporous TiO2 film exhibit the multicolor photochromism. The electrodeposited Ag exhibits the multicolor photochromism also in a nanoporous ZnO film, but not in nanoporous indium-tin oxide (ITO) and SiO2 matrices. Photoelectrochemical measurements for the Ag-TiO2 nanocomposite elucidated that some of the photo-excited electrons on Ag are transferred to oxygen molecules via TiO2 and non-excited Ag. Thus, an n-type semiconductor plays an important role in the charge separation between the excited electrons and Ag+. Non-excited Ag on TiO2 also plays an important role in the charge separation and/or catalysis of oxygen reduction. Replacement of the non-excited Ag with Pt accelerated the electron transport from the photo-excited Ag to oxygen molecules and the photochromic behavior.     

Self-doped n-type small molecular electron transport materials for high-performance organic solar cells

Two naphthalene diimide (NDI) and perylene diimide (PDI) based n-type water/alcohol soluble small molecules (NFN and PFP) are designed and utilized as electron transport layers (ETLs) for organic solar cells (OSCs). NFN and PFP are synthesized by using Sonogashira coupling from alkynyl modified fluorene with mono-bromo substituted NDI and PDI. Density functional theory study results of NFN and PFP show that they possess excellent planarity due to the employment of triple bonds as connection units. Moreover, it was shown by electron paramagnetic resonance study that both NFN and PFP possess obvious self-doping behaviors, which may effectively enhance their charge transporting capability as ETLs in OSCs. Power conversion efficiencies of 8.59% and 9.80% can be achieved for OSCs with NFN and PFP as ETLs, respectively. The higher power conversion efficiency (PCE) of PFP based photovoltaic device is originated from the stronger doping property and higher mobility of PFP.     

Surface Architectures Built around Perylenediimide Stacks

Simple stacks of perylenediimides (PDIs) grown directly on solid surfaces are an intriguing starting point for the construction of multicomponent architectures because their intrinsic activity is already very high. The ability of PDI stacks to efficiently generate photocurrent originates from the strong absorption of visible light and the efficient transport of both electrons and holes after generation with light. The objective of this study was to explore whether or not the excellent performance of these remarkably simple single‐channel photosystems could be further improved in more sophisticated multicomponent architectures. We report that the directional construction of strings of anions or cations along the PDI stacks does not significantly improve their activity; that is, the intrinsic activity of PDI stacks is too high to yield ion‐gated photosystems. The directional construction of electron‐ and hole‐transporting stacks of naphthalenediimides (NDIs) and oligothiophenes along the central PDI stack did not improve photocurrent generation under standard conditions either. However, the activity of double‐channel photosystems increased with increasing thickness, whereas increasing charge recombination with single‐channel PDI stacks resulted in decreasing activity with increasing length. Most efficient long‐distance charge transport was found with double‐channel photosystems composed of PDIs and NDIs. This finding suggests that over long distances, PDI stacks transport holes better than electrons, at least under the present conditions. Triple‐channel photosystems built around PDI stacks with oligothiophenes and triphenylamines were less active, presumably because hole mobility in the added channels was inferior to that in the original PDI stacks, thus promoting charge recombination.     

Assembling a Phthalocyanine and Perylenediimide Donor–Acceptor Hybrid through a Platinum(II) Diacetylide Linker

In a novel electron‐donor–acceptor conjugate, phthalocyanine (Pc) and perylenediimide (PDI) are connected through a trans‐platinum(II) diacetylide linker to yield Pc‐Pt‐PDI 1 . In the ground state, the presence of Pt^II disrupts the electronic communication between the two electroactive components, as revealed by UV/Vis spectroscopy and electrochemical studies. The photophysical behavior of 1 is compared with that of the corresponding Pc‐PDI electron‐donor–acceptor conjugate 2 in terms of charge separation and charge recombination. The insertion of Pt^II between Pc and PDI impacts the results in a longer‐lived Pc ^{. +}/PDI ^{. ?} radical ion‐pair state. In addition, the intermediately formed Pc triplet excited state is formed with higher quantum yields in 1 than in 2 .     

Competition between singlet fission and charge separation in solution-processed blend films of 6,13-bis(triisopropylsilylethynyl)pentacene with sterically-encumbered perylene-3,4:9,10-bis(dicarboximide)s

The photophysics and morphology of thin films of N,N-bis(2,6-diisopropylphenyl)perylene-3,4:9,10-bis(dicarboximide) (1) and the 1,7-diphenyl (2) and 1,7-bis(3,5-di-tert-butylphenyl) (3) derivatives blended with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) were studied for their potential use as photoactive layers in organic photovoltaic (OPV) devices. Increasing the steric bulk of the 1,7-substituents of the perylene-3,4:9,10-bis(dicarboximide) (PDI) impedes aggregation in the solid state. Film characterization data using both atomic force microscopy and X-ray diffraction showed that decreasing the PDI aggregation by increasing the steric bulk in the order 1 < 2 < 3 correlates with a decrease in the density/size of crystalline TIPS-Pn domains. Transient absorption spectroscopy was performed on ~100 nm solution-processed TIPS-Pn:PDI blend films to characterize the charge separation dynamics. These results showed that selective excitation of the TIPS-Pn results in competition between ultrafast singlet fission ((1*)TIPS-Pn + TIPS-Pn → 2 (3*)TIPS-Pn) and charge transfer from (1*)TIPS-Pn to PDIs 1-3. As the blend films become more homogeneous across the series TIPS-Pn:PDI 1 → 2 → 3, charge separation becomes competitive with singlet fission. Ultrafast charge separation forms the geminate radical ion pair state (1)(TIPS-Pn(+?)-PDI(-?)) that undergoes radical pair intersystem crossing to form (3)(TIPS-Pn(+?)-PDI(-?)), which then undergoes charge recombination to yield either (3*)PDI or (3*)TIPS-Pn. Energy transfer from (3*)PDI to TIPS-Pn also yields (3*)TIPS-Pn. These results show that multiple pathways produce the (3*)TIPS-Pn state, so that OPV design strategies based on this system must utilize this triplet state for charge separation.     

Enhanced electron transfer ability <Emphasis Type="Italic">via</Emphasis> coordination in block copolymer/porphyrin/fullerene micelle

Inspired by structures of antenna-reaction centers in photosynthesis, the complex micelle was prepared from zinc tetra-phenyl porphyrin (ZnTPP), fullerene derivative (PyC₆₀) and poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-b-PCL). The core-shell structure made the hydrophobic donor-acceptor system work in aqueous. In micellar core, coordination interaction occurred between ZnTPP and PyC₆₀ molecules which ensured the enhanced energy migration from the donor to the acceptor. The enhanced interaction between porphyrin and fullerene was confirmed by absorption, steady-state fluorescence and transient fluorescence. The generation of singlet oxygen and superoxide radical was detected by iodide method and reduction of nitro blue tetrazolium, respectively, which confirmed that electron transfer reaction in the complex micellar core occurred. Moreover, the complex micelle exhibited effective electron transfer performance in photodebromination of 2,3-dibromo-3-phenylpropionic acid. The complex micellar structure endowed the donor-acceptor system with improved stability under irradiation. This strategy could be helpful for designing new electron transfer platform and artificial photosynthetic system.     

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.     

Solar‐Powered Organic Semiconductor–Bacteria Biohybrids for CO2 Reduction into Acetic Acid

An organic semiconductor–bacteria biohybrid photosynthetic system is used to efficiently realize CO₂ reduction to produce acetic acid with the non‐photosynthetic bacteria Moorella thermoacetica. Perylene diimide derivative (PDI) and poly(fluorene‐co‐phenylene) (PFP) were coated on the bacteria surface as photosensitizers to form a p‐n heterojunction (PFP/PDI) layer, affording higher hole/electron separation efficiency. The π‐conjugated semiconductors possess excellent light‐harvesting ability and biocompatibility, and the cationic side chains of organic semiconductors could intercalate into cell membranes, ensuring efficient electron transfer to bacteria. Moorella thermoacetica can thus harvest photoexcited electrons from the PFP/PDI heterojunction, driving the Wood–Ljungdahl pathway to synthesize acetic acid from CO₂ under illumination. The efficiency of this organic biohybrid is about 1.6 %, which is comparable to those of reported inorganic biohybrid systems.     

“Organic Metals”—The Intermolecular Migration of Aromaticity

The electrical conductivity of a wide variety of organic materials is reviewed and a comparison made between those materials which transport charge efficiently and those that do not. This comparison reveals that the most efficient conductors contain molecules (a) whose radical ions form a new aromatic sextet upon one-electron oxidation or reduction and (b) whose aromaticity can migrate by mixed-valence interaction. The electrical data are presented in light of the chemistry of these materials and the importance of chemical bonding interactions in the electron transport process. Several new examples are discussed and directions for further research explored.     

Supramolecular Zinc Phthalocyanine–Imidazolyl Perylenediimide Dyad and Triad: Synthesis,Complexation, and Photophysical Studies

Two new supramolecular architectures based on zinc phthalocyanine (Pc) and imidazolyl‐substituted perylenediimide (PDI), ZnPc/DImPDI/ZnPc 1 and ZnPc/ImPDI 2 , have been prepared. A strong electron‐donor, 8 , which contained eight tert‐octylphenoxy groups was synthesized to ensure high solubility, thereby reducing aggregation in solution and providing σ‐donor features while avoiding regioisomeric mixtures. Also, PDI units were functionalized with tert‐octylphenoxy groups at the bay positions, which provide solubility to avoid aggregation in solution, together with one and two imidazole moieties in the amide position, 6 and 4 , respectively, to be able to strongly coordinate with the ZnPc complex. Supramolecular complexation studies by ¹H NMR spectroscopy and ESI‐MS demonstrate a high coordinative binding constant between imidazole‐substituted 4 or 6 and 8 . The same results were confirmed by UV/Vis and fluorescence titration studies. UV/Vis titration studies revealed the formation of a 1:1 complex ZnPc/ImPDI 2 for the systems 8 and 6 and a 2:1 complex ZnPc/DImPDI/ZnPc 1 for the interaction of 8 and 4 . The binding constant in both cases was determined to be on the order of 10⁵ M ⁻¹. Femtosecond laser flash photolysis measurements provided a direct proof of the charge‐separated state within both supramolecular assemblies by observing the transient absorption band at 820 nm due to the zinc phthalocyanine radical cation. The lifetimes of charge‐separated states are (9.8±3) ns for triad 1 and (3±1) ns for dyad 2 . As far as we know, this is the first time that a radical ion pair has been detected in a supramolecular assembled ZnPc–PDI system and has obtained the longest lifetime of a charge‐separated state published for ZnPc–PDI assemblies.     

Solar-Powered Organic Semiconductor–Bacteria Biohybrids for CO2 Reduction into Acetic Acid

An organic semiconductor–bacteria biohybrid photosynthetic system is used to efficiently realize CO₂ reduction to produce acetic acid with the non-photosynthetic bacteria Moorella thermoacetica. Perylene diimide derivative (PDI) and poly(fluorene-co-phenylene) (PFP) were coated on the bacteria surface as photosensitizers to form a p-n heterojunction (PFP/PDI) layer, affording higher hole/electron separation efficiency. The π-conjugated semiconductors possess excellent light-harvesting ability and biocompatibility, and the cationic side chains of organic semiconductors could intercalate into cell membranes, ensuring efficient electron transfer to bacteria. Moorella thermoacetica can thus harvest photoexcited electrons from the PFP/PDI heterojunction, driving the Wood–Ljungdahl pathway to synthesize acetic acid from CO₂ under illumination. The efficiency of this organic biohybrid is about 1.6 %, which is comparable to those of reported inorganic biohybrid systems.     

Photocurrent Generation through Charge‐Transfer Processes in Noncovalent Perylenediimide/DNA Complexes

The charge‐transfer process in noncovalent perylenediimide (PDI)/DNA complexes has been investigated by using nanosecond laser flash photolysis (LFP) and photocurrent measurements. The PDI/DNA complexes were prepared by inclusion of cationic PDI molecules into the artificial cavities created inside DNA. The LFP experiments showed that placement of the PDI chromophore at a specific site and included within the base stack of DNA led to the efficient generation of a charge‐separated state with a long lifetime by photoexcitation. When two PDI chromophores were separately placed at different positions in DNA, the yield of the charge‐separated state with a long lifetime was dependent upon the number of A–T base pairs between the PDIs, which was explained by electron hopping from one PDI to another. Photocurrent generation of the DNA‐modified electrodes with the complex was also dependent upon the arrangement of the PDI chromophores. A good correlation was obtained between observed charge separation and photocurrent generation on the PDI/DNA‐modified electrodes, which demonstrated the importance of the defined arrangement and assembly of organic chromophores in DNA for efficient charge separation and transfer in multichromophore arrays.     

A first principles study on organic molecule encapsulated boron nitride nanotubes

The electronic structures of boron nitride nanotubes (BNNTs) doped with organic molecules are investigated using density functional theory. An electrophilic molecule introduces acceptor states in the wide gap of BNNT close to the valence band edge, which makes the doped system a p-type semiconductor. However, with typical nucleophilic organic molecules encapsulation, only deep occupied molecular states but no shallow donor states are observed. There is a significant electron transfer from a BNNT to an electrophilic molecule, while the charge transfer between a nucleophilic molecule and a BNNT is negligible. When both electrophilic and nucleophilic molecules are encapsulated in the same BNNT, a large charge transfer between the two kinds of molecules occurs. The resulting small energy gap can strongly modify the transport and optical properties of the system.     

Light harvesting zinc naphthalocyanine-perylenediimide supramolecular dyads: long-lived charge-separated states in nonpolar media

Photoinduced electron-transfer dynamics of self-assembled donor-acceptor dyads formed by axial coordination of zinc naphthalocyanine, ZnNc, and perylenediimide (PDI) bearing either pyridine (py) or imidazole (im) coordinating ligands were investigated. The PDIim unit was functionalized with tert-octylphenoxy groups at the bay positions, which avoid aggregation providing solubility, to examine the effect of the bulky substituents at the bay positions on the rates of electron-transfer reactions. The combination between zinc naphthalocyanine and perylenediimide entities absorbs light over a wide region of the visible and near infrared (NIR) spectrum. The binding constants of the self-assembled ZnNc:PDIpy (1) and ZnNc:PDIim (2) in toluene were found to be 2.40 × 10(4) and 1.10 × 10(5) M(-1), respectively, from the steady-state absorption and emission measurements, indicating formation of moderately stable complexes. The geometric and electronic calculations by using an ab initio B3LYP/6-311G method showed the majority of the highest occupied frontier molecular orbital (HOMO) on the zinc naphthalocyanine entity, while the lowest unoccupied molecular orbital (LUMO) was on the perylenediimide entities, suggesting that the charge-separated states of the supramolecular dyads are ZnNc˙(+):PDI˙(-). The electrochemical results suggest the exothermic charge-separation process via the singlet states of both ZnNc and PDI entities in nonpolar toluene. Upon coordination of perylenediimide to ZnNc, the main quenching pathway involved charge separation via the singlet-excited states of ZnNc and PDIs. Clear evidence of the intramolecular electron transfer from the singlet-excited state of ZnNc to PDI within the supramolecular dyads in toluene was monitored by the femtosecond laser photolysis by observing the characteristic absorption band of the PDI radical anion (PDI˙(-)) and the ZnNc radical cation (ZnNc˙(+)) in the visible and NIR regions. The rate constants of charge-separation (k(CS)) processes of the self-assembled dyads 1 and 2 were determined to be 4.05 × 10(10) and 1.20 × 10(9) s(-1), respectively. The rate constant of charge recombination (k(CR)) and the lifetime of charge-separated states (τ(CS)) of dyad 1 were determined to be 2.34 × 10(8) s(-1) and 4.30 ns, respectively. Interestingly, a slower charge recombination (2.20 × 10(7) s(-1)) and a longer lifetime of the charge separated state (45 ns) were observed in dyad 2 in nonpolar toluene by utilizing the nanosecond transient measurements. The absorption in a wide section of the solar spectrum and the high charge-separation/charge-recombination ratio suggest the usefulness of the self-assembled zinc naphthalocyanine-perylenediimide dyads as good photosynthetic models.     

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.