Stable Radical Cations and Their π‑Dimers Prepared from Ethylene- and Propylene-3,4-dioxythiophene Co-oligomers: Combined Experimental and Theoretical Investigations

Co-oligomers composed of two 3,4-ethylene­dioxy­thiophene (EDOT) units and two or three 3,4-propylene­dioxy­thiophene (ProDOT) units, i.e., <b>2E2P</b>_<b>Et</b> and <b>2E3P</b>_<b>Et</b>, were newly synthesized together with the ProDOT trimer <b>3P</b>_<b>Me</b>. On the basis of cyclic voltammetry, the gaps between the first and second oxidation potentials (Δ<i>E</i>^1–2) of <b>2E2P</b>_<b>Et</b> and <b>2E3P</b>_<b>Et</b> were found to be larger than that of the previously synthesized ProDOT tetramer <b>4P</b>_<b>Hex</b>. These co-oligomers gave the fairly stable radical cations <b>2E2P</b>_<b>Et</b>^•+ and <b>2E3P</b>_<b>Et</b>^•+ by chemical oxidation with AgSbF₆. The disproportionation of <b>2E2P</b>_<b>Et</b>^•+ and <b>2E3P</b>_<b>Et</b>^•+ into neutral and dicationic species, which was observed for <b>4P</b>_<b>Hex</b>^•+, was inhibited in accord with the larger Δ<i>E</i>^1–2. Additionally, the formation of the π-dimers (<b>3P</b>_<b>Me</b>)₂²⁺, (<b>2E2P</b>_<b>Et</b>)₂²⁺, and (<b>2E3P</b>_<b>E</b>)₂²⁺ was clearly observed in dichloromethane solution at low temperatures with UV–vis–NIR spectroscopy. Furthermore, the π-dimerization enthalpies of <b>2E2P</b>_<b>Et</b>^•+ and <b>2E3P</b>_<b>Et</b>^•+ were greater than that of <b>3P</b>_<b>Me</b>^•+, suggesting the formation of fully π-contacted structures. The structures of the π-dimers were optimized at the B97D3 method, and the calculated absorption spectra of the π-dimers obtained using TD-DFT methods were in reasonable agreement with the observed ones, supporting the reliability of the calculated structures

Antiaromaticity of Planar Bisdehydro[12]- and Tetrakisdehydro[16]annulenes Fused with Dithieno[3,4‑<i>b</i>:3′,4′‑<i>d</i>]thiophenes

Thermally stable bisdehydro[12]- and tetrakisdehydro[16]­annulenes with planar structures were successfully synthesized by fusion with two dithieno­[3,4-<i>b</i>:3′,4′-<i>d</i>]­thiophene units. The planarized [4n]­annulene cores induced substantial antiaromaticity. However, the HOMO–LUMO gap slightly increased as the π-core expanded from the planar cyclooctatetraene. This phenomenon, in contrast to that of typical π-conjugated systems, was attributed to the decrease in antiaromaticity of the [4n]­annulene cores. Both TD–HF and GIAO–HF calculations supported this conclusion

Donor–Acceptor Segregated Paracyclophanes Composed of Naphthobipyrrole and Stacked Fluoroarenes

The expeditious synthesis of donor–acceptor segregated paracyclophanes has been achieved by a selective S_NAr reaction of hexafluorobenzene with <i>o</i>-dipyrrolylbenzenes and subsequent cyclodehydrogenation. An orthogonally arranged D–A segregated structure was confirmed by X-ray crystallography. The combined results of DFT calculations and absorption spectra revealed the charge transfer (CT) nature from the naphthobipyrrole (donor) to the stacked fluoroarene moiety (acceptor)

Effect of Substituents on the Structure, Stability, and π‑Dimerization of Dithienylpyrrole Radical Cations

A series of 2,5-di­(2-thienyl)-<i>N</i>-methylpyrrole derivatives <b>1a</b>–<b>1d</b> with methylthio end-capping groups and electron-donating substituents at the 3-position of the thiophene rings was synthesized, and the effects of the substituents on the structure, stability, and π-dimerization ability of the radical cation were investigated using UV–vis–NIR and electron spin resonance spectra and density functional theory (DFT) calculations. Among the electron-donating methyl, methoxy, and methylthio substituents, the methoxy derivative <b>1c</b> gave the most stable radical cation, which persisted in dichloromethane at room temperature under nitrogen for several hours without any apparent decomposition. In addition, <b>1c</b>^•+ had the largest π-dimerization enthalpy among <b>1a</b>^•+–<b>1d</b>^•+. DFT calculations with the M06-2X method revealed that methyl and methylthio derivatives <b>1b</b>^•+ and <b>1d</b>^•+ as well as <b>1c</b>^•+ adopt a cis–cis conformation, in contrast to the trans–trans conformer of unsubstituted <b>1a</b>^•+, while the π-dimers of all of these compounds were shown to have a cis–cis conformation. On the basis of further detailed analyses, the preformed cis–cis conformation and the weaker intramolecular and intermolecular steric repulsions were considered to explain why <b>1c</b>^•+ has the largest π-dimerization enthalpy

Dithieno[3,4‑<i>b</i>:3′,4′‑<i>d</i>]thiophene-Annelated Antiaromatic Planar Cyclooctatetraene with Olefinic Protons

The design and synthesis of a new planar cyclooctatetraene (COT) with protons directly connected to the COT ring was attained by monoannelation with dithieno[3,4-<i>b</i>:3′,4′-<i>d</i>]thiophene. The planar structure of the COT core was unambiguously confirmed by X-ray crystallography. The magnetic antiaromaticity of the COT core was found to be higher than that of the previously synthesized planar COTs with olefinic protons, according to the results of ¹H NMR and absorption spectra as well as NICS calculations

Donor–Acceptor Segregated Paracyclophanes Composed of Naphthobipyrrole and Stacked Fluoroarenes

The expeditious synthesis of donor–acceptor segregated paracyclophanes has been achieved by a selective S_NAr reaction of hexafluorobenzene with <i>o</i>-dipyrrolylbenzenes and subsequent cyclodehydrogenation. An orthogonally arranged D–A segregated structure was confirmed by X-ray crystallography. The combined results of DFT calculations and absorption spectra revealed the charge transfer (CT) nature from the naphthobipyrrole (donor) to the stacked fluoroarene moiety (acceptor)

Dithieno[3,4‑<i>b</i>:3′,4′‑<i>d</i>]thiophene-Annelated Antiaromatic Planar Cyclooctatetraene with Olefinic Protons

The design and synthesis of a new planar cyclooctatetraene (COT) with protons directly connected to the COT ring was attained by monoannelation with dithieno[3,4-<i>b</i>:3′,4′-<i>d</i>]thiophene. The planar structure of the COT core was unambiguously confirmed by X-ray crystallography. The magnetic antiaromaticity of the COT core was found to be higher than that of the previously synthesized planar COTs with olefinic protons, according to the results of ¹H NMR and absorption spectra as well as NICS calculations

Pyrrole-Fused Azacoronene Family: The Influence of Replacement with Dialkoxybenzenes on the Optical and Electronic Properties in Neutral and Oxidized States

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene <b>3b</b> and triazacoronene <b>4a</b> were elucidated. The photophysical properties for neutral compounds <b>1</b>–<b>4</b> were investigated using steady-state UV–vis absorption/emission spectroscopy and time-resolved spectroscopy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence and phosphorescence allowed us to estimate the small S₁–T₁ energy gap (Δ<i>E</i>_S–T) to be 0.35 eV (<b>1a</b>), 0.26 eV (<b>2a</b>), and 0.36 eV (<b>4a</b>). Similar to the case of previously reported hexapyrrolohexaazacoronene <b>1</b> (HPHAC), electrochemical oxidation revealed up to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of azacoronene π-systems were examined using UV–vis–NIR absorption, ESR, and NMR spectroscopies for the chemically generated radical cations and dications. Combined with the theoretical calculations, the experimental results clearly demonstrated that the replacement of pyrrole rings with dialkoxybenzene plays a critical role in the electronic communication, where resonance structures significantly contribute to the thermodynamic stability of the cationic charges/spins and determine the spin multiplicities. For HPHAC <b>1</b> and pentaazacoronene <b>2</b>, the overall aromaticity predicted for closed-shell dications <b>1</b>^<b>2+</b> and <b>2</b>^<b>2+</b> was primarily based on the theoretical calculations, and the open-shell singlet biradical or triplet character was anticipated for tetraazacoronene <b>3</b>^<b>2+</b> and triazacoronene <b>4</b>^<b>2+</b> with the aid of theoretical calculations. These polycyclic aromatic hydrocarbons (PAHs) represent the first series of nitrogen-containing PAHs that can be multiply oxidized

Pyrrole-Fused Azacoronene Family: The Influence of Replacement with Dialkoxybenzenes on the Optical and Electronic Properties in Neutral and Oxidized States

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene <b>3b</b> and triazacoronene <b>4a</b> were elucidated. The photophysical properties for neutral compounds <b>1</b>–<b>4</b> were investigated using steady-state UV–vis absorption/emission spectroscopy and time-resolved spectroscopy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence and phosphorescence allowed us to estimate the small S₁–T₁ energy gap (Δ<i>E</i>_S–T) to be 0.35 eV (<b>1a</b>), 0.26 eV (<b>2a</b>), and 0.36 eV (<b>4a</b>). Similar to the case of previously reported hexapyrrolohexaazacoronene <b>1</b> (HPHAC), electrochemical oxidation revealed up to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of azacoronene π-systems were examined using UV–vis–NIR absorption, ESR, and NMR spectroscopies for the chemically generated radical cations and dications. Combined with the theoretical calculations, the experimental results clearly demonstrated that the replacement of pyrrole rings with dialkoxybenzene plays a critical role in the electronic communication, where resonance structures significantly contribute to the thermodynamic stability of the cationic charges/spins and determine the spin multiplicities. For HPHAC <b>1</b> and pentaazacoronene <b>2</b>, the overall aromaticity predicted for closed-shell dications <b>1</b>^<b>2+</b> and <b>2</b>^<b>2+</b> was primarily based on the theoretical calculations, and the open-shell singlet biradical or triplet character was anticipated for tetraazacoronene <b>3</b>^<b>2+</b> and triazacoronene <b>4</b>^<b>2+</b> with the aid of theoretical calculations. These polycyclic aromatic hydrocarbons (PAHs) represent the first series of nitrogen-containing PAHs that can be multiply oxidized

Pyrrole-Fused Azacoronene Family: The Influence of Replacement with Dialkoxybenzenes on the Optical and Electronic Properties in Neutral and Oxidized States

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene <b>3b</b> and triazacoronene <b>4a</b> were elucidated. The photophysical properties for neutral compounds <b>1</b>–<b>4</b> were investigated using steady-state UV–vis absorption/emission spectroscopy and time-resolved spectroscopy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence and phosphorescence allowed us to estimate the small S₁–T₁ energy gap (Δ<i>E</i>_S–T) to be 0.35 eV (<b>1a</b>), 0.26 eV (<b>2a</b>), and 0.36 eV (<b>4a</b>). Similar to the case of previously reported hexapyrrolohexaazacoronene <b>1</b> (HPHAC), electrochemical oxidation revealed up to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of azacoronene π-systems were examined using UV–vis–NIR absorption, ESR, and NMR spectroscopies for the chemically generated radical cations and dications. Combined with the theoretical calculations, the experimental results clearly demonstrated that the replacement of pyrrole rings with dialkoxybenzene plays a critical role in the electronic communication, where resonance structures significantly contribute to the thermodynamic stability of the cationic charges/spins and determine the spin multiplicities. For HPHAC <b>1</b> and pentaazacoronene <b>2</b>, the overall aromaticity predicted for closed-shell dications <b>1</b>^<b>2+</b> and <b>2</b>^<b>2+</b> was primarily based on the theoretical calculations, and the open-shell singlet biradical or triplet character was anticipated for tetraazacoronene <b>3</b>^<b>2+</b> and triazacoronene <b>4</b>^<b>2+</b> with the aid of theoretical calculations. These polycyclic aromatic hydrocarbons (PAHs) represent the first series of nitrogen-containing PAHs that can be multiply oxidized