Photophysical and Structural Properties of Cyanoruthenate Complexes of Hexaazatriphenylene

The tritopic bridging ligand hexaazatriphenylene (HAT) has been used to prepare the mono-,
di-, and trinuclear cyanoruthenate complexes [Ru(CN)4(HAT)]2- ([1]2-), [{Ru(CN)4}2(μ2-HAT)]4- ([2]4-), and
[{Ru(CN)4}3(μ3-HAT)]6- ([3]6-). These complexes are of interest both for their photophysical properties
and ability to act as sensitizers, associated with strong MLCT absorptions; and their structural properties,
with up to 12 externally directed cyanide ligands at a single “node” for preparation of coordination networks.
The complexes are strongly solvatochromic, with broad and intense MLCT absorption manifolds arising
from the presence of low-lying π* orbitals on the HAT ligand, as confirmed by DFT calculations; in aprotic
solvents [3]6- is a panchromatic absorber of visible light. Although nonluminescent in fluid solution, the
lowest MLCT excited states have lifetimes in D2O of tens of nanoseconds and could be detected by time-resolved IR spectrosocopy. For dinuclear [2]4- and trinuclear [3]6- the TRIR spectra are indicative of
asymmetric MLCT excited states containing distinct Ru(III) and Ru(II) centers on the IR time scale. The
complexes show red 3MLCT luminescence as solids and in EtOH/MeOH glass at 77 K. Ln(III) salts of
[1]2-, [2]4-, and [3]6- form infinite coordination networks based on Ru−CN−Ln bridges with a range of
one-, two-, and three-dimensional polymeric structures. In the Yb(III) and Nd(III) salts of [3]6- the complex
anion forms an 8-connected node. Whereas all of the Gd(III) salts show strong 3MLCT luminescence in
the solid state, the Ru-based emission in the Nd(III) and Yb(III) analogues is substantially quenched by Ru
→ Ln photoinduced energy transfer, which results in sensitized near-infrared luminescence from Yb(III)
and Nd(III).