ja502753t_si_006.cif (13.58 kB)
Heat, Pressure and Light-Induced Interconversion of Bisdithiazolyl Radicals and Dimers
dataset
posted on 2015-12-17, 02:25 authored by Kristina Lekin, Hoa Phan, Stephen
M. Winter, Joanne W. L. Wong, Alicea A. Leitch, Dominique Laniel, Wenjun Yong, Richard A. Secco, John S. Tse, Serge Desgreniers, Paul A. Dube, Michael Shatruk, Richard T. OakleyThe
heterocyclic bisdithiazolyl radical 1b (R1 = Me, R2 = F) crystallizes in two phases. The
α-phase, space group P21/n, contains two radicals in the asymmetric unit, both of
which adopt slipped π-stack structures. The β-phase, space
group P21/c, consists
of cross-braced π-stacked arrays of dimers in which the radicals
are linked laterally by hypervalent 4-center 6-electron S···S–S···S
σ-bonds. Variable-temperature magnetic susceptibility measurements
on α-1b indicate Curie–Weiss behavior (with
Θ = −14.9 K), while the dimer phase β-1b is diamagnetic, showing no indication of thermal dissociation below
400 K. High-pressure crystallographic measurements indicate that the
cross-braced π-stacked arrays of dimers undergo a wine-rack
compression, but the dimer remains intact up to 8 GPa (at ambient
temperature). The resistance of β-1b to dissociate
under pressure, also observed in its conductivity versus pressure
profile, is in marked contrast to the behavior of the related dimer
β-1a (R1 = Et, R2 = F), which
readily dissociates into a pair of radicals at 0.8 GPa. The different
response of the two dimers to pressure has been rationalized in terms
of differences in their linear compressibilities occasioned by changes
in the degree of cross-bracing of the π-stacks. Dissociation
of both dimers can be effected by irradiation with visible (λ
= 650 nm) light; the transformation has been monitored by optical
spectroscopy, magnetic susceptibility measurements, and single crystal
X-ray diffraction. The photoinduced radical pairs persist up to temperatures
of 150 K (β-1b) and 242 K (β-1a) before reverting to the dimer state. Variable-temperature optical
measurements on β-1b and β-1a have afforded Arrhenius activation energies of 8.3 and 19.6 kcal
mol–1, respectively, for the radical-to-dimer reconversion.
DFT and CAS-SCF calculations have been used to probe the ground and
excited electronic state structures of the dimer and radical pair.
The results support the interpretation that the ground-state interconversion
of the dimer and radical forms of β-1a and β-1b is symmetry forbidden, while the photochemical transformation
is symmetry allowed.