Pancakes under Pressure: A Case Study on Isostructural Dithia- and Diselenadiazolyl Radical Dimers

• Published in: Inorganic chemistry 2019-03, Vol.58 (5), p.3550-3557
• Main Authors: Yong, Wenjun, Lekin, Kristina, Bauer, Robert P. C, Tse, John S, Desgreniers, Serge, Secco, Richard A, Hirao, Naohisa, Oakley, Richard T
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.9b00142

The isostructural dimers of the 1,4-phenylene-bridged bis-1,2,3,5-dithia- and bis-1,2,3,5-diselenadiazolyl diradicals 1,4-S/Se are small band gap semiconductors. The response of their molecular and solid state electronic structures to pressure has been explored over the range 0–10 GPa. The crystal structures, which consist of cofacially aligned (pancake) π-dimers packed into herringbone arrays, experience a continuous, near-isotropic compression. While the intramolecular covalent E–E (E = S/Se) bonds remain relatively unchanged with pressurization, the intradimer E···E separations are significantly shortened. Molecular and band electronic structure calculations using density functional theory methods indicate that compression of the π-dimers leads to a widening of the gap ΔE between the highest occupied and lowest unoccupied molecular orbitals of the dimer, an effect that offsets the expected decrease in the valence-to-conduction band gap E g occasioned by pressure-induced spreading of the valence and conduction bands. Consistent with the predicted consequences of this competition between intra- and interdimer interactions, variable temperature high pressure conductivity measurements reveal at best an order-of-magnitude increase in conductivity with pressure for the two compounds over the pressure range 0–10 GPa. While a small reduction in the thermal activation energy E act with increasing pressure is observed, extrapolation of the rate of decrease suggests a projected onset of metallization (E act ≈ 0) in excess of 20 GPa.