Self-Assembled Pd(II) Barrels as Containers for Transient Merocyanine Form and Reverse Thermochromism of Spiropyran

Self-assembly of a cis-blocked Pd­(II) 90°
ditopic acceptor [cis-(tmeda)­Pd­(NO3)2] (M) with a tetradentate donor L1 [benzene-1,4-di­(4-terpyridine)] in 2:1
molar ratio yielded two isometric molecular barrels MB1 and MB3 in DMSO [tmeda = N,N,N′N′-tetramethylethane-1,2-diamine].
Exclusive formation of the symmetrical tetrafacial barrel (MB1) was achieved when the self-assembly was performed in aqueous medium.
The presence of a large confined cavity makes MB1 a potential
molecular container. Spiropyran (SP) compounds exist
in stable closed spiro form in visible light and convert to transient
open merocyanine (MC) form upon irradiation with UV-light
or upon strong heating. The transient MC form readily
converts to the stable closed SP form in visible light. MB1 has been employed as a safe container to store the planar
and unstable merocyanine isomers (MC1/2)
of different spiropyran molecules (SP1/2) [SP1/2 = 6-bromo-spiropyran and 6-nitrospiropyran]
for several days. The transient MC forms (MC1 and MC2) were found to be stable inside the molecular
container MB1 under visible light and even in the presence
of different stimuli such as heat and UV light for a long time. Such
stabilization of MC forms inside the confined cavity
of MB1 is noteworthy. This phenomenon was generalized
by utilizing a carbazole-based molecular barrel (MB2)
as a host, which also showed a similar stabilization of transient MC form in visible light at room temperature. Moreover, reverse
thermochromism was observed as a result of heating of the MC1
⊂ MB2 complex, which de-encapsulates the guest in the
form of SP1 to give a colorless solution. Moreover, both
the host molecules (MB1, MB2) were capable
of stabilizing transient MC2 even in the solid state.
Such stabilization of transient MC forms in the solid
state and transformation of SP forms to MC forms in the solid state in the presence of molecular barrel are
remarkable, and these properties have been employed in developing
a magic ink.