Linked Donor-acceptor Systems Research Articles

Cover Feature: Quadrupolar Ultrafast Charge Transfer in Diaminoazobenzene‐Bridged Perylenediimide Triads (Chem. Eur. J. 13/2022)

The transfer of charge in covalently linked donor–acceptor systems is highly important in many areas of chemistry, artificial photosynthesis, semiconductor devices, and solar cells. Intramolecular charge transfer occurs when a fraction of the charge is transferred between the entities after photo-excitation of the electron donor or acceptor entity. In this study, molecular triads comprising two electron-deficient perylenediimides are covalently linked to a bridging electron donor, diamino-azobenzene, to give strong ground-state polarization resulting in wide-band optical coverage. Using fluorescence spectro-electrochemistry, charge-transfer reversal in these triads is also seen. Further, the formation of ultrafast quadrupolar charge transfer upon photoexcitation is demonstrated. More information can be found in the Research Article by F. Fernández-Lázaro, F. D′Souza, et al. (DOI: 10.1002/chem.202104574).

Distinct Crystalline Aromatic Structural Motifs: Identification, Classification, and Implications.

Spatial noncovalent helical organization of nucleobases in DNA and radial organization of chromophores in natural light-harvesting systems are fascinating yet enigmatic. Understanding the numerous weak interactions that drive the formation of elegant supramolecular architectures in native natural systems and developing bioinspired design strategies have seen a surge of interest in recent decades. Self-assembly of functional chromophores in the crystalline phase is a definitive strategy to identify novel molecule-molecule interactions, in particular, atom-atom interactions, and to understand the synergistic nature of noncovalent interactions that stabilizes the supramolecular organization. This Account narrates our recent efforts in developing desirable supramolecular motifs employing weak interaction-based strategies and our observation of deviations from the common motifs chartered in aromatic systems. Modulation of long-range aromatic interactions through chemical modifications (acylation, benzoylation, haloacylation, and alkylation of chromophores) to attain a preferred stacking (herringbone, lamellar, or columnar) is presented. Particular attention has been given to attaining lamellar or columnar packing possessing potential interchromophoric electronic coupling mediated high charge mobility. Supramolecular arrangements of noncovalently or covalently associated donor-acceptor systems that open up additional possibilities of packing modes (segregated, mixed etc.) are explored. Our persistent efforts yielded distinct twisted-segregated and alternate distichous stacks for the nonparallel covalently linked donor-acceptor systems that favor a long-lived photoinduced charge-separated state. We further move on to discuss the unconventional packing motifs that were identified recently. The highly sought-after Greek cross (+) stacking of chromophores in crystalline phase and an elegant crystalline radial arrangement of chromophores are examined. The Greek cross (+) stacked architecture exhibits monomer-like emission characteristics owing to the absence of exciton coupling across the orthogonally stacked chromophores. Crystalline helical chromophore assembly is yet another emerging motif with far-reaching applications in domains ranging from asymmetric catalysis to chiral smart materials and has been accounted here by citing certain phenomenal examples from literature. Thus, this Account demonstrates that identifying and classifying new structural motifs based on topological aspects, such as interchromophoric orientation (cross) and extended chromophore arrangement in the crystal lattice (radial, helical, etc.), are crucial since such fundamental characteristics dictate the properties emerging out of the corresponding motifs. Encouraged from ours and others' works, we propose the addition of new aromatic supramolecular structural motifs, namely, cross-stacked, helical, and radial arrangements, in order to expand the classification. We believe that identifying new emergent property-based supramolecular motifs and investigating the methods to achieve the desired motif will eventually have implications in fundamental crystal engineering, supramolecular chemistry, and biomimetic design of functional materials.

Mono‐ and Bis(tetrathiafulvalene)‐1,3,5‐Triazines as Covalently Linked Donor–Acceptor Systems: Structural, Spectroscopic, and Theoretical Investigations

Reaction of 2,4,6-trichloro-1,3,5-triazine with lithiated tetrathiafulvalene (TTF) in stoichiometric conditions, followed by treatment with sodium methanolate, provides mono- and bis(TTF)-triazines as new covalently linked (multi)donor-acceptor systems. Single-crystal X-ray analyses reveal planar structures for both compounds, with formation of peculiar segregated donor and acceptor stacks for the mono(TTF)-triazine compound, while mixed TTF-triazine stacks establish in the case of the bis(TTF) derivative. Cyclic voltammetry measurements show reversible oxidation of the TTF units, at rather low potential, with no splitting of the oxidation waves in the case of the dimeric TTF, whereas irreversible reduction of the triazine core is observed. Intramolecular charge transfer is experimentally evidenced through solution electronic absorption spectroscopy. Time-dependent DFT calculations allow the assignment of the charge transfer band to singlet transitions from the HOMO of the donor(s) to the LUMO of the acceptor. Solution EPR measurements correlated with theoretical calculations were performed in order to characterize the oxidized species. In both cases the spectra show very stable radical species and contain a triplet of doublet pattern, in agreement with the coupling of the unpaired electron with the three TTF protons. The dication of the bis(TTF)-triazine is paramagnetic, but no spin-spin exchange interaction could be detected.

Intramolecular Electron and Energy Transfer in an Axial ZnP−Pyridylfullerene Complex As Studied by X- and W-Band Time-Resolved EPR Spectroscopy

Light-driven electron transfer (ET) and energy transfer (EnT) in a self-assembled via axial coordination Zn-porphyrin-pyridylfullerene (ZnP-PyrF) complex were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) and 95 GHz (W-band). The studies over a wide temperature range were carried out in media of different polarity, including isotropic toluene and tetrahydrofuran (THF), and anisotropic nematic liquid crystals (LCs), E-7 and ZLI-4389. At low temperatures (frozen matrices), photoexcitation of the ZnP donor results mainly in singlet-singlet EnT to the pyridine-appended fullerene acceptor. In fluid phases ET is the dominant process. Specifically, in isotropic solvents the generated radical pairs (RPs) are long-lived, with lifetimes exceeding that observed for covalently linked donor-acceptor systems. It is concluded that in liquid phases of both polar and nonpolar solvents the separation of the tightly bound complex into the more loosely bound structure slows down the back ET (BET) process. Photoexcitation of the donor in fluid phases of LCs does not result in the creation of the long-lived RPs, since the ordered LC matrix hinders the separation of the complex constituents. As a result, fast intramolecular BET takes place in the tightly bound complex. Contrarily to the behavior of covalently linked donor-acceptor systems in different LCs, the polarity of the LC matrix affects the ET process. Moreover, in contrast to covalently linked D-s-A systems, utilization of LCs for the coordinatively linked D-s-A complexes does not reduce the ET rates significantly.

Electric field effects on fluorescence quenching due to electron transfer. II. Linked donor–acceptor systems

A theoretical model has been proposed and applied to explain recent experimental results concerning the external electric field effect on fluorescence quenching due to intramolecular electron transfer in donor–acceptor pairs linked by a spacer. These results show that for some systems with a short length of a spacer and for a very low concentration of donor–acceptor pairs, the fluorescence quenching of the photoexcited donor is reduced by the electric field instead of being enhanced. The model predicts that the reducing effect of the external electric field on the fluorescence quenching alternates with the enhancing effect when the standard free energy change or the donor–acceptor separation is varied. For systems with a high concentration of the donor–acceptor pairs the resultant effect of the field is determined additionally by competition between the intramolecular and intermolecular electron transfers.

Time-Resolved Fluorescence and Solvatochromy of Directly Linked Pyrene−DMA Derivatives in Alcoholic Solution

Picosecond transient absorption spectroscopy and picosecond spectro-streak fluorescence measurements have been carried out in order to understand the complex photophysical dynamics of directly linked donor-acceptor systems and the solvation dynamics in longer-chain alcohols. With these techniques, the fast solvent response to optically generated charge transfer (CT) species can be followed. Studies were performed on covalently linked donor-acceptor systems of the type pyrene with derivatives of N,N-dimethylaniline: dimethyl-(4pyren-1-yl-phenyl)-amine, dimethyl-(3-methyl-4-pyren-1-yl-phenyl)-amine and (3,5-dimethyl-4-pyren-1-ylphenyl)-dimethylamine. The results are interpreted with particular emphasis on solvation dependent electronic restructuring of the CT species. We suggest a mechanism where the adiabatic electronic restructuring coincides with the coupling to the torsional degrees of freedom of the solvent.

Photoinduced Charge Migration in the Picosecond Regime for Thianthrene-Linked Acridinium Ions

The thianthrene ring system has a novel redox chemistry that has been the subject of study for three decades. The sulfur heterocycle undergoes a reversible on-electron oxidation (E{sub 1/2} = 1.23 vs SCE, CH{sub 3}CN), yielding a radical cation species that is stable enough to be isolated for independent investigation; magnetic resonance studies on the cation have included ESR, ENDOR, and CIDEP measurements. Photoinduced electron transfer involving thianthrene (TH) as an electron donor has been studied, including a report of trapping of the radical cation. As part of a study of linked donor-acceptor systems that employ the acridinium ion as electron acceptor, the authors have prepared the first linked thianthrenes appropriate for investigation of intramolecular electron transfer involving TH as an electron donor. Principal findings include the observation of very fast forward and back electron transfer (charge shift, CSH) in the picosecond time domain involving TH and acridinium subunits.

分子認識を介した生体内電子移動反応のモデル系構築 ユビキノンとチトクロムｃの合成レセプター

Electron transfer (ET) reaction in biological systems plays an important role in respiratory oxidative phosphorylation and photosynthesis. In the respiratory system, two electron carriers, ubiquinone and cytochrome c, control the overall reaction of ET process via specific interaction with membrane-bound oxidoreductases. The present review describes two topics of the synthetic modeling of noncovalently linked donor-acceptor systems using porphyrin unit; (1) preparation and characterization of ubiquinone receptors by use of a functional porphyrin, which shows the photoinduced ET reaction within the porphyrin-quinone complex formed by hydrogen bonding interaction. (2) construction of binding sites on the surface of myoglobin and mimicking of protein-protein complexation. In this review, I wish to report the suitable model systems to evaluate the intermolecular ET of the electron carriers via molecular recognition.

Intramolecular “Through-Bond” and “Through-Space” Electron Transfer Pathways in Covalently Linked Porphyrin-Quinone Molecules

Abstract In order to understand electron transfer (ET) pathways through bonds and through space, a porphyrin-spacer-benzoquinone molecule, where the spacer is spiro[4.4]nonane and a phenyl group is inserted between the space of the redox pair, was prepared. On the basis of MM2 calculation, the edge-to-edge distances between the phenyl and the porphyrin or quinone rings are in the range of 2.5—4.2 Å. Therefore, it was expected that there are two possible intramolecular ET pathways in the compound, i.e., through bond and through space. The ET rates for charge separation process (kcs) were obtained on the basis of fluorescence lifetime. The kcs value of the above compound is almost identical with that of a reference compound in which no inserted phenyl ring is present. Therefore, it was concluded that the inserted π-system between the redox pair is not used as a stepping stone in ET pathways; in other words, ET in the present molecules takes place in a through-bond mechanism. To compete with fast ET with through-bond pathways in covalently linked donor–acceptor systems where the two ET pathways are possible, through-space ET may function under quite limited conditions.

Photophysics of linked donor—acceptor systems: through-space and through-bond interactions

Possible competition between through-σ-bond and through-space interaction on the one hand and between through-σ-and through-π-bond interaction on the other has been studied in flexibly and rigidly linked donor—acceptor systems. In a first part, pyrenyl-substituted donor—acceptor compounds were studied, where a permethylated oligosilane chain acts as a flexible spacer. The possible involvement of a sigma delocalized oligosilane chain in the electron transfer process is studied. Intramolecular electron transfer was observed from a permethylated hexasilane chain towards pyrene in the excited state. Through-space ground-state interactions were observed for the donor—acceptor substituted hexamethyl trisilane chain. In order to study the electron transfer process via through-σ-and through-π-bond interactions, rigid trichromophoric compounds in an A 1—D—A 2 configuration were studied. One of the acceptors is directly bound to the donor moiety, while the other acceptor is separated from the donor by a rigid spacer of three sigma bonds. The study of these compounds by means of absorption, stationary and time-resolved emission spectroscopy reveals a competition between the two possible electron transfer directions in one of the trichromophoric compounds.

Synthesis of rigidly linked donor–acceptor systems designed to test the effect of orbital symmetry on the dynamics of long-range intramolecular electron transfer processes

The synthesis of the novel rigid bichromophoric systems, 3, 4, 6, together with the monochromophoric system, 5, is described. Such systems are characterized in terms of each possessing a polynorbornyl bridge, one end of which is terminally fused to a bicyclo[2.2.2]octene unit. Deacetalization of the dimethoxyacetal, 7, with formic acid gave the ketone, 9, which readily underwent thermal decarbonylation to give the diene 11. Diels–Alder reaction of 11 with dicyanoacetylene or dimethylacetylene dicarboxylate readily gave the adducts, 13 and 15, respectively, resulting from endo-attack by the dienophile on the diene. Catalytic hydrogenation of 13 and 15 gave 3 and 6 respectively. In a similar manner, 4 and 5 were synthesized starting from the dimethoxyacetals, 8 and 19a, respectively. Single crystal X-ray structures of 3 and 6 were determined and are consistent with the structures shown.

A correlation between the rates of photoinduced long-range intramolecular electron transfer in rigidly linked donor-acceptor systems and computed π,π and π *,π * splitting energies in structurally related dienes

It has been found that the rates of photoinduced intramolecular electron transfer in a series of rigidly linked donor (dimethoxynaphthalene)-acceptor (dicyanovinyl) systems, in which the length and configuration of the hydrocarbon bridge are varied, correlate well with the computed Koopmans' theorem STO-3G π *,π * splitting energies in a structurally related series of dienes. It is proposed that the origin of this correlation lies in a through-bond coupling mechanism which obtains between the π * LUMOs of the chromophores and the bridge σ and σ * orbitals in both series of molecules.

Synthesis and crystal structures of three rigidly linked donor–acceptor systems designed to test the effect of bridge configuration on the dynamics of long-range intramolecular electron transfer processes

The synthesis and single crystal X-ray structures of three rigid bichromophoric systems are described, namely the dicyanovinyl derivatives of 7,12-dimethoxydodecahydro-1,4:6,13-dimethanopentacen-15-one, 3, 8,13-dimethoxyhexadecahydro-1,4:5,16:6,17:7, 14-tetramethanohexacen-17-one, 4 and 6,11-dimethoxy-4b, 12b-dimethyldodecahydro-4,13;5,12-dimethano-2H-indeno-[5′,6′:3′,4′]cyclo-buta[1′,2′:3,4]cyclobuta[1,2-b]anthracen-2-one 5. Compounds 3 and 4 were synthesized from dimethanonaphthacene 8via Diels–Alder reaction with tetrachlorodimethoxycyclopentadiene 9(in the case of 3) and via successive Diels–Alder reactions, firstly with hexachlorocyclopentadiene, 14, and then with 9, in the case of 4. The synthesis of 5 was achieved through ring expansion of the dichloroketene [2 + 2] cycloadduct formed from 21. Differences in the rates of photoinduced intramolecular electron transfer in 3–5, compared with those for 1 and 2, are rationalized in terms of the differing configurations of the hydrocarbon bridges in these systems, as revealed by X-ray crystallography.

Through-bond modulation of intramolecular electron transfer in rigidly linked donor-acceptor systems

Photoinduced electron transfer rates are reported for a pair of rigid bichromophoric molecules, 1(8) and 3(8), in a variety of solvents. It was found that intramolecular electron transfer in 1(8) is up to five times faster than in 3(8). X-ray crystallography revealed that the cyclopentyl ring attached to the dicyanoethylene group is “flatter” in 3(8) compared to 1(8), and this results in diminished through-bond coupling in 3(8) compared to 1(8). This conclusion is reinforced by the observation that the calculated through-bond π, π and π * interaction energies in dienes 7(6) and 9(6) are larger than those in the dienes 8(6) and 10(6), respectively.

Strong effects of the bridge configuration on photoinduced charge separation in rigidly linked donor-acceptor systems

Photoinduced electron-transfer rates are reported for two pairs of rigid bichromophoric molecules 1(6)/ 2(6) and 1(8)/ 2(8). In the first pair electron donor and acceptor are separated by six, in the second pair by eight, carbon—carbon σ bonds. While these σ bonds provide an all-trans coupling path in 1(6) and 1(8), that path contains s-cis elements in 2(6) and 2(8), which - as shown by X-ray structure data and by spectroscopic evidence - leads to a slight decrease in the effective, spatial donor-acceptor separation. Nevertheless, photoinduced electron transfer in each of the “stretched” compounds is about one order of magnitude faster than in the corresponding “bent” compound. This remarkable effect is interpreted as resulting from the unique ability of an all-trans array of σ bonds to mediate electronic through-bond interaction (TBI). Interestingly the solvent dependence of the rate of photoinduced electron transfer is significantly larger in the “bent” systems, thus indicating that superexchange via solvent molecules becomes competitive with TBI if an all-trans array is not available.

Roles of organized molecular assemblies in artificial photosyntheses

As has been verified in biological photosynthesis, organized molecular assemblies (OMAs) are capable of controlling photochemical energy conversion. Charge separation of photoactivated species has bee attempted using type I OMAs such as micelles, microemulsions, bilayer membranes and polysoaps. The charge separation is enhanced by molecular diffusion which is aided by local electric field and segregation effects due to microenvironmental compartmentalization. The transport of electrons across the membrane wall of vesicles has been studied in an effort to achieve up-hill energy conversion. The attempts were successful in principle, but in practice considerable improvement of the membrane is required. However, new synthetic amphiphiles have been developed which form bilayers, and stable membranes with variable thicknesses (of he order of 50 Å - 10 μm) have become available. Geometric structures of the membrane may also be modified easily. Both energy transfer and electron transport systems can be constructed using the new bilayer membranes with appropriately immobilized elements. Linked donor—acceptor systems, as represented by porphyrin—quinone, may serve as good photoreaction centres when they are installed on these type II OMAs with immobilized elements. The development of type III OMAs is required for efficient coupling of elementary processes in artificial photo-synthesis. The construction of charge pools in OMAs seems to be one of the solutions, as has been demonstrated in several examples such as the cooperative activation of two photoredox systems by the use of OMAs with electron pools.