Nonalternant Nanographenes Containing N-Centered Cyclopenta[ef]heptalene and Aza[7]Helicene Units.

Introducing helical subunits into negatively curved π-systems has a significant effect on both the molecular geometry and photophysical properties; however, the synthesis of these helical π-systems embedded with nonbenzenoid subunits remains challenging due to the high strain deriving from both the curvature and helix. Here, we report a family of nonalternant nanographenes containing a nitrogen (N)-doped cyclopenta[ef]heptalene unit. Among them, CPH-2 and CPH-3 can be viewed as hybrids of benzoannulated cyclopenta[ef]heptalene and aza[7]helicene. The crystal structures revealed a saddle geometry for CPH-1, a saddle-helix hybrid for CPH-2, and a twist-helix hybrid for CPH-3. Experimental measurements and theoretical calculations indicate that the saddle moieties in CPHs undergo flexible conformational changes at room temperature, while the aza[7]helicene subunit exhibits a dramatically increased racemization energy barrier (78.2 kcal mol-1 for CPH-2, 143.2 kcal mol-1 for CPH-3). The combination of the nitrogen lone electron pairs of the N-doped cyclopenta[ef]heptalene unit with the twisted helix fragments results in rich photophysics with distinctive fluorescence and phosphorescence in CPH-1 and CPH-2 and the similar energy fluorescence and phosphorescence in CPH-3. Both enantiopure CPH-2 and CPH-3 display distinct circular dichroism (CD) signals in the UV-vis range. Notably, compared to the reported fully π-extended helical nanographenes, CPH-3 exhibits excellent chiroptical properties with a |gabs| value of 1.0 × 10-2 and a |glum| value of 7.0 × 10-3; these values are among the highest for helical nanographenes.

Dynamic Timing Control over Multicolor Molecular Emission by Temporal Chemical Locking.

Dynamic control over molecular emission, especially in a time-dependent manner, holds great promise for the development of smart luminescent materials. Here we report a series of dynamic multicolor fluorescent systems based on the time-encoded locking and unlocking of individual vibrational emissive units. The intramolecular cyclization reaction driven by adding chemical fuel acts as a chemical lock to decrease the conformational freedom of the emissive units, thus varying the fluorescence wavelength, while the resulting chemically locked state can be automatically unlocked by the hydrolysis reaction with water molecules. The dynamic molecular system can be driven by adding chemical fuels for multiple times. The emission wavelength and lifetime of the locking states can be readily controlled by elaborating the molecular structures, indicating this strategy as a robust and versatile way to modulate multi-color molecular emission in a time-encoded manner.

Synthesis and Characterization of Oxygen-Embedded Quinoidal Pentacene and Nonacene.

Extension of oxygen-embedded polycyclic aromatic hydrocarbons (PAHs), in particular with a defined topology, is synthetically challenging primarily because of limited regio-specific methods and poor solubility of PAHs. We reported herein an efficient way to construct quinoidal pentacenes and nonacenes with regular zigzag O-inserted edges. These O-embedded backbones composed of benzene, pyranyl, and cyclohexa-1,4-diene moieties provided access to a new class of longitudinally conjugated acenes with superior stability. Their structures, confirmed by single-crystal XRD analysis, indicated that they possessed rich hydrogen/halogen-bonding interactions, which likely contribute to the strengthened aggregation. In contrast to many other O-annulated PAHs generally displaying short-absorption wavelengths due to partially interrupted electron communication, the O-embedded quinoidal acene exhibited highly red-shifted absorptions (up to ∼699 nm) and narrowed energy gaps (down to ∼1.5 eV). As with more O-containing rings and quinoidal subunits in the backbone, the conjugation size was enlarged, and the molar absorption coefficients (ε) of the λmax significantly increased further, in particular, a noticeable lower-energy peak at ∼790 nm for O-doping nonacenes N1-OH/OMe. By the thin-film-based organic field-effect transistor measurements, the relatively ordered O-doping pentacene P1-OMe possessed a hole transporting efficiency (µh) of 0.00406 cm-2 V-1 s-1 in in-air fabricated devices, while O-pentacene P1-PFB with two perfluorobutoxyl substituents witnessed an improved µh up to 0.0152 cm-2 V-1 s-1. In addition, one- or two-electron oxidation of O-pentacene/nonacene generated the corresponding radical cations or dications, in which electronic properties were dependent on the number of O-containing six-membered rings and quinoidal subunits. The study provided insights into the relationships between molecule structures and optoelectronic properties for the unique class of O-embedded PAHs.

Toward helical-shaped diradicaloids: cyclobutenyl o-quinodimethane-bridged indeno[1,2-b]fluorenes.

Open-shell diradicaloids have been intensively investigated recently due to their attractive and unique optical, electronic and magnetic properties. However, open-shell diradicaloids with non-planar conjugated frameworks are less studied, especially helical-shaped π systems. In this work, we synthesized tetraindeno-based singlet biradicals in a helical geometry, bridged by a curved cyclobutenyl o-quinodimethane molecule. Large diradical characters and extremely small singlet-triplet energy gaps, as well as unusual half-field transitions (forbidden ΔMs = 2) were revealed for these 3D diradicals.

9-Ethynylfluoroenyl Radicals: Regioselective Dimerization and Post Ring-Cyclization Reactions.

9-Ethynylfluorenyl radical derivatives were readily prepared in situ and underwent simultaneous intermolecular coupling reactions. Interestingly, the dimerization process took place in either a head-to-tail or a head-to-head mode between the acetylenic or the allenic resonance forms dependent on the terminal substituents, which could be well explained by their different spin distribution and steric hindrance effects. The structures of the products were confirmed by X-ray crystallographic and other spectroscopic analyses. It was also found that the newly generated dipropinyl dimers underwent a rearrangement and ring-cyclization reaction at room temperature, eventually giving unique difluorenylidene cyclobutene derivatives.

Multifaceted Regioregular Oligo(thieno[3,4-b]thiophene)s Enabled by Tunable Quinoidization and Reduced Energy Band Gap.

Thiophene-based materials have occupied a crucial position in the development of organic electronics. However, the energy band gaps of oligo- and polythiophenes are difficult to modulate without resorting to push-pull electronic effects. We describe herein a new series of monodisperse oligo(thieno[3,4-b]thiophene) derivatives with well-defined regioregular structures synthesized efficiently by direct C-H arylation. These compounds show a unique palette of colors and amphoteric redox properties with widely tunable energy band gaps. The capacity to stabilize both cations and anions results in both anodic and cathodic electrochromism. Under excitation, these compounds can produce photoionized states able to interconvert into neutral triplet or form these through singlet exciton fission or intersystem crossing. These features arise from a progressive increase in quinoidization on a fully planar platform making the largest effective conjugation length among hetero-oligomers. Oligo(thieno[3,4-b]thiophene)s might represent the more distinctive family of oligothiophenes of this decade.

Photophysical Properties of Intramolecular Charge Transfer in a Tribranched Donor-π-Acceptor Chromophore.

The photophysical properties of intramolecular charge transfer (ICT) in a novel tribranched donor-π-acceptor chromophore, triphenoxazine-2,4,6-triphenyl-1,3,5-triazine (tri-PXZ-TRZ), with thermally activated delayed fluorescence character was investigated in different aprotic solvents by steady-state spectroscopy and femtosecond and nanosecond transient absorption spectroscopy measurements. Increasing the solvent polarity led to a significant increase in the Stokes shift. The large Stokes shift in highly polar solvents was attributed to ICT properties upon excitation; this resulted in a strong interaction between the tri-PXZ-TRZ molecule and the surrounding solvent, which led to a strong solvation process. Quantum-chemical calculations and changes in the dipole moment showed that this compound has a large degree of ICT. Furthermore, an apolar environment helped to preserve the symmetry of tri-PXZ-TRZ and to enhance its emission efficiency. The femtosecond and nanosecond transient absorption spectroscopy results indicated that the excited-state dynamics of this push-pull molecule were strongly influenced by solvent polarity through the formation of a solvent-stabilized ICT state. The excited-state relaxation mechanism of tri-PXZ-TRZ was proposed by performing target model analysis on the femtosecond transient absorption spectra. In addition, the delayed fluorescence of tri-PXZ-TRZ was significantly modulated by a potential competition between solvation and intersystem crossing processes.