Surface-Mediated Recrystallization for Highly Conducting Organic Radical Crystal

Abstract

Electrically conducting cation-charged dihydrophenazinium radical (H 2 PNZ •+ ) crystals were formed by a one-pot photoreaction. Furthermore, new secondary crystals were grown directly from the H 2 PNZ •+ crystal surface through a surface-mediated recrystallization (SMR) process by a secondary photoreaction with water. Depending on the absence or presence of light during the exposure of the H 2 PNZ •+ single crystal to water, neutral yellow PNZ crystals (SMR Y ) or densely packed green H 2 PNZ •+ crystals (SMR G ) were formed, respectively, on the surface of the mother H 2 PNZ •+ crystal. The single-crystal X-ray analysis reveals that the newly obtained SMR G radical crystal exhibits a shorter intercolumnar distance than the original H 2 PNZ •+ crystal, resulting in increased electrical conductivity.

Phase Change-Driven Radical Population and Electrical Conductivity Increases of Organic Radical Crystals by Mild Heating

“…In addition, a broad peak appears from 800 to 1400 nm before heating, which is not shown in the solution phase. 23 According to refs 34 and 35, NIR absorption is caused by intermolecular overlaps between molecules in a crystal. This peak is remarkably reduced after heating, which is well matched with increasing the intermolecular distance in surface X-ray diffraction (SXRD) data.…”

“…According to ref , the H 2 PNZ · + crystals were synthesized by a photoreduction method in acidic conditions. The final crystal was confirmed to have the same crystal structure as in the reference.…”

“…The increased number of radicals means more carriers in the new band, and there are more chances to hop into the conduction band. , The radicals generally are produced by the redox process and kept by avoiding dimerization. , Although the modulation of electrical conductivity by the population change of available radicals has been suggested from many charge-transfer complex systems, also known as spin-Peierls or Peierls instability, it has been rarely measured especially at near room temperature because Peierls instability generally evolves at very low temperature. , To overcome this temperature limit issue, it needs to develop molecular systems that may behave like spin-Peierls at a higher temperature. Recently, we have reported the air-stable phenazine radical crystal that has dihydrophenazine radical molecules stacked together, which shows a structural similarity to spin-Peierls . While many organic radical crystals exhibit Peierls instability by showing low conductivity at low temperature from highly conducting systems, the dihydrophenazine radical crystal provides an opportunity to overcome the Peierls instability as the intermolecular distance of the stacked phenazine radical molecules can be modulated upon external stimuli, like heat.…”

“…21,22 To overcome this temperature limit issue, it needs to develop molecular systems that may behave like spin-Peierls at a higher temperature. Recently, we have reported the air-stable phenazine radical crystal that has dihydrophenazine radical molecules stacked together, 23 which shows a structural similarity to spin-Peierls. 24 systems, the dihydrophenazine radical crystal provides an opportunity to overcome the Peierls instability as the intermolecular distance of the stacked phenazine radical molecules can be modulated upon external stimuli, like heat.…”

Highly Conducting and Flexible Radical Crystals

“…[13] Meanwhile, air-stable radical crystals have been found for organic semiconductors, metals, or even superconductors. [14][15][16][17][18][19][20] The flexibility of organic crystals has been suggested to be induced by synergetic effect of rigid molecular layer consisting of van der Waals interaction and weak intermolecular interactions, such as hydrogen bond or CÀH•••p. [8,9,[21][22][23] It is expected to be further improved by the presence of molecules acting as a mechanical ball bearing capable of hydrogen bonding in the structure.…”