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

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

Additive Electron Pathway and Nonadditive Molecular Conductance by Using a Multipodal Bridging Compound

We designed and synthesized a new quadrivial anchoring unit <b>4-TEB</b>, to construct a stable single-molecule junction with gold electrodes, which should have equivalent conducting electron pathways between two electrodes. The conductances of single-molecule junctions comprising <b>4-TEB</b> and its bidirectional counterpart <b>2-TEB</b> were determined to be 2.7 × 10^–4<i>G</i>₀ (2<i>e</i>²/<i>h</i>) and 5.0 × 10^–5<i>G</i>₀, respectively, by using scanning tunneling microscope break junction (STM-BJ) techniques. The single <b>4-TEB</b> molecule junction had higher stability and conductivity compared to those of the single <b>2-TEB</b> molecule junction. Although the number of electron pathways from/to the electrode to/from the molecule was additive using the equivalent multianchoring, the conductance of the single-molecule junction was not additive. From first-principles electronic transport calculations, the mechanism for the new quadrivial <b>4-TEB</b> single-molecule junction involved an overlap resonance effect to the HOMO conducting orbital, giving rise to tunneling. Using fixed nanogap electrodes, we constructed stable molecular junctions of <b>4-TEB</b> and observed symmetric peaks in the derivative of the conductance–voltage (<i>G–V</i>) curves, which were assigned to electron transport through the HOMO on the basis of theoretical calculations