  The nanostructure of two novel sulfur containing dimer materials has been
investigated experimentally by hard and by resonant tender X-ray scattering
techniques. On cooling the dimers through the nematic to twist-bend nematic
(N-NTB) phase transition, the correlation length associated with short-range
positional order drops, while the heliconical orientational order becomes more
correlated. The heliconical pitch shows a stronger temperature dependence near
the N-NTB transition than observed in previously studied dimers, such as the
CBnCB series of compounds. We explain both this strong variation and the
dependence of the heliconical pitch on the length of the spacer connecting the
monomer units by taking into account a temperature dependent molecular bend and
intermolecular overlap. and. The heliconical structure is observed even in the
upper 3-4{\deg}C range of the smectic phase that forms just below the NTB
state. The coexistence of smectic layering and the heliconical order indicates
a SmCTB -type phase where the rigid units of the dimers are tilted with respect
to the layer normal in order to accommodate the bent conformation of the
dimers, but the tilt direction rotates along the heliconical axis. This is
potentially similar to the SmCTB phase reported by Abberley et al (Nat. Commun.
2018, 9, 228) below a SmA phase.
