A Molecular Chameleon: Reversible pH- and Cation-Induced Control of the Optical Properties of Phthalocyanine-Based Complexes in the Visible and Near-Infrared Spectral Ranges

A series
of novel nonperipherally substituted tetra-15-crown-5-dibutoxyoxanthrenocyanines
(H2, Mg, Zn), acting as chameleons with the unique properties
of switchable absorption and emission in the near-infrared (NIR) spectral
range have been synthesized and characterized by X-ray diffraction.
The attachment of 15-crown-5-α-dibutoxyoxanthreno moieties to
phthalocyanine is responsible for the high solubility of the resulting
molecules and the red shift of the Q band to the NIR region and offers
a unique possibility for postsynthetic modification of the optical
properties of the molecules. Both aggregation of phthalocyanine and
its participation in an acid–base equilibrium strongly alter
their optical properties. For example, the absorption of complexes
can be reversibly tuned from 686 up to 1028 nm because of the cation-induced
formation of supramolecular dimers or subsequent protonation of meso-N atoms orf macrocycle, in contrast to peripherally
substituted tetra-15-crown-5-phthalocyanines without oxanthrene moieties.
The reversibility of these processes can be controlled by the addition
of [2.2.2]­cryptand or amines. All investigated compounds exhibit fluorescence
with moderate quantum yield, which can also be switched between the
ON and OFF states by the action of similar agents.