π-Stacked Ion Pairs: Tightly Associated Charged Porphyrins in Ordered Arrangement Enabling Radical-Pair Formation

Abstract

π-Electronic ion pairs are of interest for fabricating electronic materials that use intermolecular interactions based on electrostatic and dispersion forces, defined as i π− i π interactions, to provide dimension-controlled assemblies. Porphyrin ions, whose charge is delocalized in the core units, are suitable for ordered arrangement and assemblies by ion pairing. Herein, charged porphyrins were found to form solid-state assemblies and solution-state stacked ion pairs according to the peripheral electron-donating groups (EDGs) and electron-withdrawing groups (EWGs). The concentration-dependent 1 H NMR signal shifts of a porphyrin ion pair, comprising a meso-EWG cation and a meso-EWG anion, provided a hetero-dimerization constant of 2.8 × 10 5 M −1 in CD 2 Cl 2 at 20 °C. In the ion pair of a meso-EWG cation and a meso-EDG anion, the electron transfer in the steady and excited states according to solvent polarity and photoexcitation, respectively, produced the radical pairs. The electron spin resonance analysis in frozen toluene revealed the formation of a heterodiradical in a closely stacked structure by the antiferromagnetic dipolar interaction and temperature-dependent spin transfer behavior.

Substituent‐Dependent Photoexcitation Processes of π‐Stacked Ion Pairs

“…Charged porphyrins, with charge in the core units, are suitable components of π-sips. Porphyrin-Au III complexes, such as 1 a + and 1 b + , [9,10] containing a tricationic d 8 metal ion are candidate π-electronic cations, whereas deprotonated meso-hydroxyporphyrins, such as 2 a À and 2 b À , [9,11,12] behave as counter π-electronic anions (Figure 1b). Various ion pairs have been formed from these charged porphyrins as π-sips in solution and solid-state assemblies.…”

“…Various ion pairs have been formed from these charged porphyrins as π-sips in solution and solid-state assemblies. [9,12] In particular, the ion pair comprising cation 1 b + and anion 2 a À , activated by electron-withdrawing and electron-donating substituents, respectively, underwent electron transfer in the ground state to form the corresponding π-stacked radical pair (π-srp). [9] On the other hand, porphyrin radical pairs were also produced from 1 b + -2 a À and 1 b + -2 b À by photoexcitation of the activated and less-activated π-sips by electron transfer (Figure 1c).…”

“…[9,12] In particular, the ion pair comprising cation 1 b + and anion 2 a À , activated by electron-withdrawing and electron-donating substituents, respectively, underwent electron transfer in the ground state to form the corresponding π-stacked radical pair (π-srp). [9] On the other hand, porphyrin radical pairs were also produced from 1 b + -2 a À and 1 b + -2 b À by photoexcitation of the activated and less-activated π-sips by electron transfer (Figure 1c). However, the details of the effects of substituents on the excited-state behavior of π-sips have not been fully elucidated.…”

Anion-Responsive Boron-Spiro-Centered π-Electronic Systems That Form Ion-Pairing Assemblies

“…Further modifications at boron moieties have also been conducted, as seen for the catecholate complexes [e.g., 2a and 2b (Figure b)], which show less emissive properties probably caused by the intramolecular electron transfer between the core π-units and the aryl moieties around boron . The electronic states and assembly behavior of orthogonally arranged π-electronic units based on π-electronic anions can be tuned by appropriate countercations, exhibiting fascinating electronic properties . To investigate the effects of the orthogonally introduced π-electronic moieties on the physical properties and assembled structures, more π-extended systems, such as naphthalenediolates comprising the smallest acene, have been examined in this study (Figure c).…”

“…6 The electronic states and assembly behavior of orthogonally arranged π-electronic units based on π-electronic anions can be tuned by appropriate countercations, exhibiting fascinating electronic properties. 7 To investigate the effects of the orthogonally introduced π-electronic moieties on the physical properties and assembled structures, more π-extended systems, such as naphthalenediolates comprising the smallest acene, have been examined in this study (Figure 1c). Properties and assemblies can also be modulated by the substituting positions of the π-extended systems.…”