End-to-End Bent Perylene Bisimide Cyclophanes by Double Sulfur Extrusion.

Bending inherently planar π-cores consisting of only six-membered rings has traditionally been challenging because a powerful transformation is required to compensate for the significant strain energy associated with bending. Herein, we demonstrate that sulfur extrusion can achieve substantial molecular bending of a perylene structure to form a substructure of a Vögtle belt, a proposed yet hitherto elusive carbon nanotube fragment. Bent perylene bisimide (PBI) derivatives were synthesized through a double-sulfur-extrusion reaction from the corresponding sulfur-containing V-shaped precursors with an internal alkyl tether. The effect of bending the inherently planar PBI core, which is a recent topic of interest for the design of advanced organic electronic and optoelectronic materials, was investigated systematically. Increasing the curvature leads to a red shift in the absorption and emission spectra, while the fluorescence quantum yields remain high. This stands in contrast with the nonemissive features of previously reported nonplanar PBI derivatives based on conjugative tethers. Detailed photophysical measurements indicated that the increasing curvature with shorter alkyl tethers (i) slightly facilitates intersystem crossing and (ii) significantly suppresses the internal conversion in the excited state of the present bent PBI derivatives. The latter characteristics originate from the restricted dynamic motion associated with the charge-transfer (CT) character between the core chromophores and the N-aryl units.

Excimer Formation Driven by Excited-State Structural Relaxation in a Covalent Aminonaphthalimide Dimer.

Strongly coupled excimer formation from interchromophoric charge transfer driven by the ultrafast excited-state structural dynamics of a 5,5'-linked 4-amino-1,8-naphthalimide covalent homodimer was investigated by ultrafast transient spectroscopy and chemical calculations. Theoretical calculations indicate that the structural relaxation associated with the dihedral motion leads to significantly enhanced interchromophoric charge transfer (CT) coupling, which favors the formation of an excimer-like symmetry-broken CT state. The formation and relaxation dynamics of the excimer state in the dimer are identified via ultrafast transient absorption and fluorescence spectroscopy. The structural relaxation following the photoexcitation occurs in tens of picoseconds and stabilizes the dimer to the strongly coupled excimer state. The highly polar solvents further stabilize the excimer state and enhance the CT character, which enable efficient electron and excitation energy transport in covalent molecular aggregates.

Diazazethrene bisimide: a strongly electron-accepting π-system synthesized via the incorporation of both imide substituents and imine-type nitrogen atoms into zethrene.

The development of highly electron-accepting π-systems is a fundamentally challenging issue despite their potential applications as high-performance n-type organic semiconductors, organic rechargeable batteries, and stable redox-active organocatalysts. Herein, we demonstrate that the incorporation of both imide substituents and imine-type nitrogen atoms into zethrene affords the strongly electron-accepting π-system diazazethrene bisimide (DAZBI). DAZBI has a low-lying LUMO (-4.3 eV vs. vacuum) and is readily reduced by the weak reductant l-ascorbic acid to afford the corresponding dihydro species. The injection of two electrons into DAZBI provides the corresponding dianion. These reduced species display remarkable stability, even under ambient conditions, and an intense red fluorescence. A DAZBI dimer, which was also synthesized, effectively accommodated four electrons upon electron injection.

Symmetry-breaking charge separation in a nitrogen-bridged naphthalene monoimide dimer.

The photophysical properties of 4-aminonaphthalene-1,8-imide-based derivatives, bis-ANI, consisting of two naphthalimide (NI) units linked by a butylamine bridge and its monomer ANI have been intensively investigated by steady-state and transient spectroscopy combined with quantum chemical calculations. The excited state relaxation dynamics of the two molecules are studied in three solvents of varying polarity - from hexane to tetrahydrofuran to acetone. A strong reduction in the fluorescence quantum yields and larger red shifts of the emission spectra are observed when going from the monomer ANI to dimer bis-ANI with increasing solvent polarity. It is found that the presence of the central amino linker in bis-ANI facilitates the formation of an asymmetric CS state between the ANI and NI moieties in bis-ANI, where NI˙- is the dominant radical anion unit after CS, evidenced by the femtosecond transient absorption measurements and spectroelectrochemistry in polar solvents. Femtosecond transient absorption spectra and quantum chemical calculations reveal the conformational change after the formation of the symmetry-breaking charge separation (SBCS) state upon photoexcitation, while a near-orthogonal structure in the excited state of bis-ANI retards charge recombination. In addition, it is evidenced that the rate of SBCS can be tuned by changing the different polar solvents.

Acridino[2,1,9,8-klmna]acridine Bisimides: An Electron-Deficient π-System for Robust Radical Anions and n-Type Organic Semiconductors.

We report the synthesis and properties of acridino[2,1,9,8-klmna]acridine bisimide (AABI), a nitrogen-doped anthanthrene with two imide functionalities. AABI exhibits excellent electron affinity as evident by its low-lying LUMO level (-4.1 eV vs. vacuum). Single-electron reduction of one AABI derivative afforded the corresponding radical anion, which was stable under ambient conditions. Photoconductivity measurements suggest that the intrinsic electron mobility of an N-phenethyl AABI derivative obeys a band-transport model. Accordingly, an electron mobility of 0.90 cm2 V-1 s-1 was attained with the corresponding single-crystal organic field-effect transistor (OFET) device. The vacuum-deposited OFET device consisting of a polycrystalline sample exhibited high electron mobility of up to 0.27 cm2 V-1 s-1 even in air. This study demonstrates that dual incorporation of both imide substituents and imine-type nitrogen atoms is an effective strategy to create novel electron-deficient π-systems.

Non-Planar Perylene Bisimide Analogues with Inserted Carbonyl and Methylene Subunits.

The synthesis and properties of dinaphthotropone bisimide (DNTrBI) and dinaphthocycloheptatriene bisimide (DNCHepBI) are described. Their molecular design is conceptually based on the insertion of a carbon atom into a perylene bisimide (PBI) core. These molecules adopt non-planar structures due to the presence of a seven-membered ring. The PBI derivative into which a carbonyl group was inserted (DNTrBI) immediately underwent nonradiative decay and/or intersystem crossing in its excited state. The PBI derivative into which a methylene group was inserted (DNCHepBI) was susceptible to deprotonation on account of the two electron-withdrawing naphthalene monoimide units. Subsequent aerobic oxidation resulted in the formation of a C-C bond at the central methylene unit, thus affording a σ-dimer. The formation of this C-C bond is dynamically redox-active, i.e., electron injection into the σ-dimer almost quantitatively regenerated the deprotonated DNCHepBI.

Aggregation-Induced Emission of Nitrogen-Bridged Naphthalene Monoimide Dimers.

We have prepared four 4-aminonaphthalene monoimide derivatives and examined the aggregation-induced emission (AIE) properties. A nitrogen-bridged dimer is AIE-active and exhibits bright green emission with a high quantum yield in the solid state. The X-ray diffraction analysis suggests that key to the bright luminescence is the favorable crystal packing dominated by CH/π interaction. The late-stage cyanation of the dimer tuned its AIE color from green to orange.