Modulation
of Electronics and Thermal Stabilities
of Photochromic Phosphino–Aminoazobenzene Derivatives in Weak-Link
Approach Coordination Complexes
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Abstract
A series of d<sup>8</sup> transition-metal
(Pt(II) and Pd(II))
coordination complexes incorporating phosphine-functionalized aminoazobenzene
derivatives as hemilabile phosphino–amine (P,N) ligands were
synthesized and studied as model weak-link approach (WLA) photoresponsive
constructs. The optical and photochemical properties of these complexes
were found to be highly influenced by various tunable parameters in
WLA systems, which include type of metal, coordination mode, type
of ancillary ligand, solvent, and outer-sphere counteranions. In dichloromethane,
reversible chelation and partial displacement of the P,N coordinating
moieties allow for toggling between aminoazobenzene- or pseudostilbene-
and azobenzene-type derivatives. The reversible switching between
electronic states of azobenzene can be controlled through either addition
or extraction of chloride counterions and is readily visualized in
the separation between π–π* and n−π*
bands in the complexes’ electronic spectra. In acetonitrile
solution, the WLA variables inherent to semiopen complexes have a
significant impact on the half-lives of the corresponding <i>cis</i> isomers, allowing one to tune their half-lives from
20 to 21000 s, while maintaining photoisomerization behaviors with
visible light. Therefore, one can significantly increase the thermal
stability of a <i>cis</i>-aminoazobenzene derivative to
the extent that single crystals for X-ray diffraction analysis can
be grown for the first time, uncovering an unprecedented edge-to-face
arrangement of the phenyl rings in the <i>cis</i> isomer.
Overall, the azobenzene-functionalized model complexes shed light
on the design parameters relevant for photocontrolled WLA molecular
switches, as well as offer new ways of tuning the properties of azobenzene-based,
photoresponsive materials