Pushing the Length Limit of Dihydrodiboraacenes: Synthesis and Characterizations of Boron‐Embedded Heptacene and Nonacene

Abstract

Boron‐embedded heteroacenes (boraacenes) have attracted enormous interest in organic chemistry and materials science. However, extending the skeleton of boraacenes to higher acenes (N≥6) is synthetically challenging because of their limited stability under ambient conditions. Herein, we report the synthesis of boron‐embedded heptacene (DBH) and nonacene (DBN) as the hitherto longest boraacenes. The former is highly stable (even after 240 h in tetrahydrofuran), while the latter is air‐sensitive with the half‐life (t1/2) of 11.8 min. The structures of both compounds are verified by single‐crystal X‐ray diffraction, revealing a linear backbone with an antiaromatic C4B2 core. Photophysical characterizations associated with theoretical calculations indicate that both compounds exhibit highly efficient anti‐Kasha emissions. Remarkably, the air‐stable DBH manifests an ultrahigh photoluminescence quantum yield (PLQY) of 98±2 % and can be chemically reduced to its radical anion and dianion states, implying the value of boron‐doped higher acenes as novel functional materials.

[4]Triangulenes Modified by Three Oxygen-Boron-Oxygen (OBO) Units: Synthesis, Characterizations, and Anti-Kasha Emissions

“…Normally, photoluminescence of organic molecules originates from only the lowest excited state of a given spin multiplicity and is irrespective of the excitation wavelength, which is also known as Kasha’s rule . The significant overlap between the weak absorption band over 400 nm and fluorescence exhibits the possibility of anti-Kasha emissions (Figure c), which is also in agreement with the forbidden S 0 → S 1 and S 0 → S 2 transitions calculated by TD-DFT (Section S7 in SI). , To further investigate the anti-Kasha emission behaviors of 1-C5 , 1-C10 , and 1-Mes , the excitation-wavelength-dependent fluorescence and phosphorescence spectra (Figure b,d and Section S5 in SI) and emission-wavelength-dependent excitation spectra (Figure c and Section S5 in SI) were exploited. − Different solvents were tested to exclude the influence of solvents on the anti-Kasha emission phenomenon (Figures S26–S34 in SI). As shown in Figure b, the right shoulder intensity at 410 nm of the emission of 1-Mes was increased when the excitation wavelength was changed from 350 to 370 nm.…”

“…43 The significant overlap between the weak absorption band over 400 nm and fluorescence exhibits the possibility of anti-Kasha emissions (Figure 3c), which is also in agreement with the forbidden S 0 → S 1 and S 0 → S 2 transitions calculated by TD-DFT (Section S7 in SI). 44,45 To further investigate the anti-Kasha emission behaviors of 1-C5, 1-C10, and 1-Mes, the excitationwavelength-dependent fluorescence and phosphorescence spectra (Figure 4b,d and Section S5 in SI) and emissionwavelength-dependent excitation spectra (Figure 4c and Section S5 in SI) were exploited. 45−47 Different solvents were tested to exclude the influence of solvents on the anti-…”

“…38,48 Therefore, temperature-dependent fluorescence measurement was used to validate the thermal repopulation process, in which excitons in lower excited states could be thermally repopulated to the higher ones if the energy gaps are small (Figure 4a). 44,49 Luckily, the thermal repopulation was evidenced by notable fluorescence changes at different temperatures (Figure S36 in SI) in PMMA film. The anti-Kasha emission behaviors of compounds 1-C5 and 1-C10 were also observed (Figures S20, S21, S23, and S24 in SI).…”

“…However, we found that the wavelength differences of the emission bands from upper excited states are quite small, which is also in agreement with the small energy gaps between different excited states from TD-DFT calculations. This is different from the classical anti-Kasha emission, which usually requires a large energy gap between the S 2 and S 1 states to weaken the internal conversion (IC) rate. , Therefore, temperature-dependent fluorescence measurement was used to validate the thermal repopulation process, in which excitons in lower excited states could be thermally repopulated to the higher ones if the energy gaps are small (Figure a). , Luckily, the thermal repopulation was evidenced by notable fluorescence changes at different temperatures (Figure S36 in SI) in PMMA film. The anti-Kasha emission behaviors of compounds 1-C5 and 1-C10 were also observed (Figures S20, S21, S23, and S24 in SI).…”

Boron-Doped Pentacenes: Isolation of Crystalline 5,12- and 5,7-Diboratapentacene Dianions

“…Despite these promising results, diboraacenes (DBAs) beyond 9,10-diboraanthracenes are underexplored, and the overwhelming majority of structures contain both boron atoms in the same ring. There are few examples of π-extended DBAs, ,, but none of them contain an aromatized acene motif. Overall, synthetic routes to DBAs with boron atoms separated into different rings are almost nonexistent, and it has been noted that such targets are synthetically challenging to achieve .…”