Exploiting exciton coupling of ligand radical intervalence charge transfer transitions to tune NIR absorption

Abstract

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region.

Exciton Coupling in Redox‐Active Salen based Self‐Assembled Metallacycles

“…The extent of the blue-shift depends on the distance and angular orientation between the transition dipole moments in the metallacycles, [3 a] 2* + and [(3 b) 2 ] 8* + , in comparison to [4] * + . [59,63,70,71] For the metallacycle [3 a] 2* + , where the two salen units are cofacially aligned and 3.84 Å apart (NiÀ Ni distance for 3 a in the solid state), a 400 cm À 1 blue-shift vs. [4] * + is observed, consistent with the exciton model (Figures 1 and 7). This compares favourably to a previously reported Ni salen dimer complex, exhibiting a similar shift (330 cm À 1 ) with two salen units cofacially aligned and 3.98 Å apart.…”

“…[58] Utilizing salen-type ligands (salen = N 2 O 2 bis(Schiff-base)-bis(phenolate)) in the supramolecular self-assemblies introduces mixed-valence species upon oxidation that can exhibit low energy ligand radical intervalence charge transfer (IVCT) bands in the NIR. [59,[60][61][62][63] Metal complexes with redox-active ligands as part of the supramolecular assemblies and frameworks provide an opportunity for reactivity tuning and development of materials applications. [64][65][66][67][68][69] Over five decades ago, Kasha described the relationship between molecular packing and photophysical properties in terms of spectral shifts, which are based on the alignment of transition moment dipoles and exciton coupling.…”

“…Our group has recently reported on a series of dimers with different orientations of the oxidized Ni salen ligand radical chromophores. [59,63] For example, a cofacial alignment in a Ni salen dimer showing a Ni•••Ni distance of 3.98 Å in the neutral form results in a blue-shifted IVCT band of double intensity upon oxidation with respect to the oxidized monomeric analogue. [59] However, a Pd macrocycle containing four Ni salen units arranged into a square showed no exciton effect upon oxidation, due to the extended distance and 90°angle between the salen units.…”

“…[59,63] For example, a cofacial alignment in a Ni salen dimer showing a Ni•••Ni distance of 3.98 Å in the neutral form results in a blue-shifted IVCT band of double intensity upon oxidation with respect to the oxidized monomeric analogue. [59] However, a Pd macrocycle containing four Ni salen units arranged into a square showed no exciton effect upon oxidation, due to the extended distance and 90°angle between the salen units. [72] Aside from our recent report on the coordination-driven self-assembly of the metallacycle containing Ni(Salen) pPy as a linker, [72] and characterization of its redox properties, the use of salen complexes in self-assembly is limited, [73,74,75] and the redox chemistry of the assembled macrocycles is virtually unexplored.…”

A tetranuclear nickel cluster isolated in multiple high-valent states

“…salen) derivatives are known redoxactive ligands and experience distinct site-specific alterations in bond lengths at either Ni-N/O bonds, or within the ligand multiple bonds upon oxidation. [38][39][40][41] In conclusion, a novel tetranuclear Ni species isolated in multiple high-valent states, including an all Ni(III) state, is reported and supported by spectroscopic, crystallographic, magnetometric, and computational data. We are currently investigating potential applications of this cluster in mediating multi-electron transformations at small molecules of energy importance.…”