Dipolar Photosystems: Engineering Oriented Push–Pull Components into Double‐ and Triple‐Channel Surface Architectures

Push–pull aromatics are not popular as optoelectronic materials because their supramolecular organization is difficult to control. However, recent progress with synthetic methods has suggested that the directional integration of push–pull components into multicomponent photosystems should become possible. In this study, we report the design, synthesis, and evaluation of double‐ or triple‐channel architectures that contain π stacks with push–pull components in parallel or mixed orientation. Moreover, the parallel push–pull stacks were uniformly oriented with regard to co‐axial stacks, either with inward or outward oriented push–pull dipoles. Hole‐transporting (p) aminoperylenemonoimides (APIs) and aminonaphthalimides (ANIs) are explored for ordered push–pull stacks. For the co‐axial electron‐transporting (n) stacks, naphthalenediimides (NDIs) are used. In double‐channel photosystems, mixed push–pull stacks are overall less active than parallel push–pull stacks. The orientation of the parallel push–pull stacks with regard to the co‐axial NDI stacks has little influence on activity. In triple‐channel photosystems, outward‐directed dipoles in bridging stacks between peripheral p and central n channels show higher activity than inward‐directed dipolar stacks. Higher activities in response to direct irradiation of outward‐directed parallel stacks reveal the occurrence of quite remarkable optical gating.     

Controlling Electron Rebound within Four‐Base π‐Stacks in Z‐DNA by Changing the Sugar Moiety from Deoxy‐ to Ribonucleotide

Charge transfer through DNA is of great interest because of the potential of DNA to be a building block for nanoelectronic sensors and devices. The photochemical reaction of 5‐halouracil has been used for probing charge‐transfer processes along DNA. We previously reported on unique charge transfer following photochemical reaction of 5‐bromouracil within four‐base π‐stacks in Z‐DNA. In this study, we incorporated a guanosine instead of a deoxyguanosine into Z‐DNA, and found that electron transfer occurs in a different mechanism through four‐base π‐stacks.     

Self‐Assembly of Ureido‐Pyrimidinone Dimers into One‐Dimensional Stacks by Lateral Hydrogen Bonding

Ureido‐pyrimidinone (UPy) dimers substituted with an additional urea functionality self‐assemble into one‐dimensional stacks in various solvents through lateral non‐covalent interactions. ¹H NMR and DOSY studies in CDCl₃ suggest the formation of short stacks (<10), whereas temperature‐dependent circular dichroism (CD) studies on chiral UPy dimers in heptane show the formation of much larger helical stacks. Analysis of the concentration‐dependent evolution of chemical shift in CDCl₃ and the temperature‐dependent CD effect in heptane suggest that this self‐assembly process follows an isodesmic pathway in both solvents. The length of the aggregates is influenced by substituents attached to the urea functionality. In sharp contrast, UPy dimers carrying an additional urethane group do not self‐assemble into ordered stacks, as is evident from the absence of a CD effect in heptane and the concentration‐independent chemical shift of the alkylidene proton of the pyrimidinone ring in CDCl₃.     

1‐Benzyl‐3,3‐dichloro‐1H‐indol‐2(3H)‐one

The title compound, C₁₅H₁₁Cl₂NO, was synthesized from N‐­benzyl­isatin. The compound crystallizes as stacks of mol­ecules running down the c axis. Mol­ecules within each of these stacks inter­act with each other through π–π and C—H⋯π inter­actions, and inter­act with neighbouring stacks through C—H⋯O inter­actions.     

Pyridine‐4‐carb­aldehyde azine

In the crystal structure of the title compound, C₁₂H₁₀N₄, the pyridine ring makes a dihedral angle of 1.12 (9)° with the mean plane of the complete almost planar and crystallographically centrosymmetric mol­ecule. There are stacks of parallel mol­ecules along the a‐axis direction, with alternate stacks having a herring‐bone arrangement relative to each other and an interplanar spacing of 3.551 Å.     

Discrete π‐Stacks from Self‐Assembled Perylenediimide Analogues

The formation of well‐defined finite‐sized aggregates represents an attractive goal in supramolecular chemistry. In particular, construction of discrete π‐stacked dye assemblies remains a challenge. Reported here is the design and synthesis of a novel type of discrete π‐stacked aggregate from two comparable perylenediimide (PDI) dyads ( PEP and PBP ). The criss‐cross PEP ‐ PBP dimers in solution and ( PBP ‐ PEP )‐( PEP ‐ PBP ) tetramers in the solid state are well elucidated using single‐crystal X‐ray diffraction, dynamic light scattering, and diffusion‐ordered NMR spectroscopy. Extensive π–π stacking between the PDI units of PEP and PBP as well as repulsive interactions of swallow‐tailed alkyl substituents are responsible for the selective formation of discrete dimer and tetramer stacks. Our results reveal a new approach to preparing discrete π stacks that are appealing for making assemblies with well‐defined optoelectronic properties.     

Charge Transfer Induces Formation of Stimuli‐Responsive,Chiral, Cohesive Vesicles‐on‐a‐String that Eventually Turn into a Hydrogel

A charge transfer (CT) mediated two‐component, multistimuli responsive supergelation involving a L ‐histidine‐appended pyrenyl derivative (PyHisOMe) as a donor and an asymmetric bolaamphiphilic naphthalene‐diimide (Asym‐NDI) derivative as an acceptor in a 2:1 mixture of H₂O/MeOH was investigated. Asym‐NDI alone self‐assembled into pH‐responsive vesicular nanostructures in water. Excellent selectivity in CT gel formation was achieved in terms of choosing amino acid appended pyrenyl donor scaffolds. Circular dichroism and morphological studies suggested formation of chiral, interconnected vesicular assemblies resembling “pearls‐on‐a‐string” from these CT mixed stacks. XRD studies revealed the formation of monolayer lipid membranes from these CT mixed stacks that eventually led to the formation of individual vesicles. Strong cohesive forces among the interconnected vesicles originate from the protrusion of the oxyethylene chains from the surfaces of the chiral vesicles.     

Effect of External Electric Field on H‐Bonding and π‐Stacking Interactions in Guanine Aggregates

The structure and electronic properties of guanine oligomers and π stacks of guanine quartets (G‐quartets) with circulene are investigated under an external field through first‐principles calculations. An electric field induces nonplanarity in the guanine aggregates and also leads to an increase in the H‐bond distances. The calculations reveal that the binding energy of the circulenes with G‐quartets increases on application of an electric field along the stacking direction. The HOMO–LUMO gap decreases substantially under the influence of an external field. The contribution of a simple dipole–dipole interaction to the stability of the stacked system is also analyzed. The electric field along the perpendicular axis increases the dipole moments of the guanine dimer, trimer, and quartet. Such an increase in the dipole moment facilitates stacking with circulenes. The stability of G‐quartet–circulene π stacks depends on the phase of the dipole moment (in‐phase or out‐of‐phase) induced by an external electric field. The stability of stacks of bowl‐shaped circulenes with G‐quartets depends on the direction of the applied field.     

Control of H‐ and J‐Type π Stacking by Peripheral Alkyl Chains and Self‐Sorting Phenomena in Perylene Bisimide Homo‐ and Heteroaggregates

The synthesis, self‐assembly, and gelation ability of a series of organogelators based on perylene bisimide (PBI) dyes containing amide groups at imide positions are reported. The synergetic effect of intermolecular hydrogen bonding among the amide functionalities and π–π stacking between the PBI units directs the formation of the self‐assembled structure in solution, which beyond a certain concentration results in gelation. Effects of different peripheral alkyl substituents on the self‐assembly were studied by solvent‐ and temperature‐dependent UV‐visible and circular dichroism (CD) spectroscopy. PBI derivatives containing linear alkyl side chains in the periphery formed H‐type π stacks and red gels, whereas by introducing branched alkyl chains the formation of J‐type π stacks and green gels could be achieved. Sterically demanding substituents, in particular, the 2‐ethylhexyl group completely suppressed the π stacking. Coaggregation studies with H‐ and J‐aggregating chromophores revealed the formation of solely H‐type π stacks containing both precursor molecules at a lower mole fraction of J‐aggregating chromophore. Beyond a critical composition of the two chromophores, mixed H‐aggregate and J‐aggregate were formed simultaneously, which points to a self‐sorting process. The versatility of the gelators is strongly dependent on the length and nature of the peripheral alkyl substituents. CD spectroscopic studies revealed a preferential helicity of the aggregates of PBI building blocks bearing chiral side chains. Even for achiral PBI derivatives, the utilization of chiral solvents such as (R)‐ or (S)‐limonene was effective in preferential population of one‐handed helical fibers. AFM studies revealed the formation of helical fibers from all the present PBI gelators, irrespective of the presence of chiral or achiral side chains. Furthermore, vortex flow was found to be effective in macroscopic orientation of the aggregates as evidenced from the origin of CD signals from aggregates of achiral PBI molecules.     

4‐Iodoquinoline

Nearly planar molecules of the title compound, C₉H₆IN, are packed in inclined stacks along the short crystallographic b axis and molecules in adjacent stacks are packed to form antiparallel zigzag chains. Short intermolecular N...I contacts [3.131 (3) Å] are observed between molecules in adjacent stacks. A network of C—H...π hydrogen bonds [2.821 (5) and 3.083 (3) Å] between molecules in adjacent stacks is also present. These motif‐generating interactions, including the weak C—H...π interactions, are of relevance in crystal engineering and design.     

Individually Addressable Patterned Multilayer Microchambers for Site‐Specific Release‐On‐Demand

Patterned arrays of light‐responsive microchambers are suggested as candidates for site‐specific release of chemicals in small and precisely defined quantities on demand. A composite film is made of poly(allylammonium)‐poly(styrene sulfonate) multilayers and gold nanoparticles incorporated between subsequent stacks of polyelectrolytes. The film shaped as microchambers is loaded with colloid particles or oil‐soluble molecules. The microchambers are sealed onto a glass slide precoated with an adhesive poly(diallyldimethylammonium)‐poly(styrene sulfonate) multilayer film. A focused laser beam is used for remote addressing the individual microchambers and site‐specific release of the loaded cargo.     

Synthesis and Characterization of Planar Five‐Ring‐Fused Dithiophene‐dione

A series of new organic semiconductors based on s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione was successfully synthesized and characterized. The electron withdrawing carbonyl group lowers the LUMO energy levels, leading to increased electronegativities, which is beneficial for high photo‐stability in air. The n‐alkyl substituted compounds, 1c and 1d , crystallize with the rigid coplanar systems packed into slipped face‐to‐face π‐stacks. Interestingly, 1c and 1d also show liquid crystalline behaviors, which give highly ordered molecular packing over large area.     

Synergistically Directed Assembly of Aromatic Stacks Based Metal‐Organic Frameworks by Donor‐Acceptor and Coordination Interactions

A synergistically directed assembly approach to distinctive metal‐organic frameworks utilizing both donor‐acceptor (D‐A) interaction from aromatic systems and coordination interactions is presented. Based on such an approach, the coronene‐tpt (tpt = 2,4,6‐tri(4‐pyridyl)‐1,3,5‐triazine) stacks based coronene‐MOF‐1 — 4 have been successfully fabricated. Their structural discrepancies with coronene‐ absent control products, 1′ — 4′ , illustrate clearly the significance of coronene‐tpt based D‐A interactions in these architectures. All these coronene‐MOFs contain varied coronene‐tpt stacks as organic secondary building blocks (SBUs), which are closely interrelated with the coordination based framework structures. Moreover, porous coronene‐MOF‐1 and ‐2 exhibit high physicochemical stability and significant light hydrocarbons storage and separation performances.     

3‐Methylamido‐3′,4′‐ethyl­enedithiotetra­thia­fulvalene­(+) potassium tetra­isothio­cyanato­mercury(II)

In the title compound, (C₁₀H₉NOS₆)K[Hg(SCN)₄] or (EDT–TTF–CONHMe)K[Hg(SCN)₄)], fully oxidized organic (EDT–TTF–CONHMe)^+· radical cations form quasi‐one‐dimensional stacks running along the monoclinic 2₁ axis and alternating along the crystallographic [101] direction with inorganic anion stacks made from mixed K⁺–[Hg(SCN)₄]²⁻ ribbons. For each anion, three essentially collinear SCN ligands inter­act with the K⁺ ions via short N⋯K contacts, while the terminal N atom of the fourth SCN group is engaged in a number of hydrogen‐bond contacts with the –CH, –NH and –CH₂ hydrogen‐bond donors of the amide function. Radical cations are dimerized along the stacks and the crystal conductivity is activated.     

Silanol Mediated π‐Assembly of Extended N‐Heterocycles

Self‐assembly of silanetriols with 4,4′‐bipyridine affords infinite columns of cofacially arranged π‐stacks as evidenced by X‐ray data. Variation of the silanetriol reveals a subtle balance between hydrogen bonding and π interactions, which, by proper choice of the substituent, may foster either dominant hydrogen bonding or dominant π interaction.     

Long‐Wavelength Fluorescence from 2‐Aminopurine‐Nucleobase Dimers in DNA

When 2‐aminopurine (2AP) is substituted for adenine in DNA, it is widely accepted that its fluorescence spectrum is essentially unchanged from that of the free fluorophore. We show that 2AP in DNA exhibits long‐wavelength emission and excitation bands, in addition to the familiar short‐wavelength spectra, as a result of formation of a ground‐state heterodimer with an adjacent, π‐stacked, natural base. The observation of dual emission from 2AP in a variety of oligodeoxynucleotide duplexes and single strands demonstrates the generality of this phenomenon. The photophysical and conformational properties of the long‐wavelength‐emitting 2AP‐nucleobase dimer are examined. Analogous long‐wavelength fluorescence is seen when 2AP π‐stacks with aromatic amino acid sidechains in the active sites of methyltransferase enzymes during DNA nucleotide flipping.     

Extended O‐Doped Polycyclic Aromatic Hydrocarbons

The synthesis of O‐doped benzorylenes, in which peripheral carbon atoms have been replaced by oxygen atoms, has been achieved for the first time. This includes key high‐yielding ring‐closure steps which, through intramolecular C?O bond formation, allow stepwise planarization of oligonaphthalenes. Single‐crystal X‐ray diffraction showed that the tetraoxa derivative forms remarkable face‐to‐face π–π stacks in the solid state, a favorable solid‐state arrangement for organic electronics.     

Crystal‐Packing Trends for a Series of 6,9,12,15,18‐Pentaaryl‐1‐hydro[60]fullerenes

The relationship between the size of the substituents of aryl groups in a series of fifteen 6,9,12,15,18‐pentaaryl‐1‐hydro[60]fullerenes and the solid‐state structures and packing motifs of these compounds has been analyzed. Pentaarylfullerenes have a characteristic “badminton shuttlecock” shape that causes several derivatives to crystallize into columnar stacks. However, many pentaarylfullerenes form non‐stacked structures with, for example, dimeric, layered, diamondoid, or feather‐in‐cavity relationships between molecules. Computational modeling gave a qualitative estimate of the best shape match between the ball and socket surfaces of each pentaarylfullerene. The best match was for pentaarylfullerenes with large, spherically shaped para‐substituents on the aryl groups. The series of pentaarylfullerenes was characterized by single‐crystal X‐ray diffraction. A total of 34 crystal structures were obtained as various solvates and were categorized by their packing motifs.     

trans‐1‐Cyano‐2‐(2‐methoxy­phenyl)‐1‐nitro­ethyl­ene

The title compound, C₁₀H₈N₂O₃, has been prepared by condensation of 2‐methoxy­benz­aldehyde and nitro­aceto­nitrile in ethanol at room temperature. Its investigation has been undertaken as a part of search for new nonlinear optical compounds. The π‐conjugated mol­ecule is almost planar. Molecules in the crystal are packed in stacks with antiparallel molecular orientation and slightly alternating distances between mean molecular planes.     

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.