Unprecedented Strong Panchromic Absorption from Proton-Switchable Iridium (III) Azoimidazolate Complexes

Two new heteroleptic iridium(III) complexes
bearing an aryldiazoimidazole ligand are reported. These
complexes differ structurally with respect to the protonation
state of the imidazole ring, but can be independently accessed by varying the synthetic conditions. Their structures
have been unequivocally confirmed by X-ray crystal structure analysis, with surprising differences in the structural
parameters of the two complexes. The strongly absorbing
nature of the free diazoimidazole ligand is enhanced in
these iridium complexes, with the protonated cationic
complex demonstrating extraordinarily strong panchromic
absorption up to 700 nm. The absorption profile of the deprotonated neutral complex is blueshifted by about 100 nm
and thus the interconversion between the two complexes as
a function of the acidity/basicity of the environment can be
readily monitored by absorption spectroscopy. Theoretical
calculations revealed the origins of these markedly different
absorption properties. Finally, the protonated analogue has
been targeted as an acceptor material for organic photovoltaic (OPV) applications, and preliminary results are reported.