Charge Transfer Chromophores Derived from 3d-Row Transition Metal Complexes

Abstract

A series of new charge transfer (CT) chromophores of “α-diimine-MII-catecholate” type (where M is 3d-row transition metals—Cu, Ni, Co) were derived from 4,4′-di-tert-butyl-2,2′-bipyridyl and 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) in accordance with three modified synthetic approaches, which provide high yields of products. A square-planar molecular structure is inherent for monomeric [CuII(3,6-Cat)(bipytBu)]∙THF (1) and NiII(3,6-Cat)(bipytBu) (2) chromophores, while dimeric complex [CoII(3,6-Cat)(bipytBu)]2∙toluene (3) units two substantially distorted heteroleptic D-MII-A (where D, M, A are donor, metal and acceptor, respectively) parts through a donation of oxygen atoms from catecholate dianions. Chromophores 1–3 undergo an effective photoinduced intramolecular charge transfer (λ = 500–715 nm, extinction coefficient up to 104 M−1·cm−1) with a concomitant generation of a less polar excited species, the energy of which is a finely sensitive towards solvent polarity, ensuring a pronounced negative solvatochromic effect. Special attention was paid to energetic characteristics for CT and interacting HOMO/LUMO orbitals that were explored by a synergy of UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT study. The current work sheds light on the dependence of CT peculiarities on the nature of metal centers from various groups of the periodic law. Moreover, the “α-diimine-MII-catecholate” CT chromophores on the base of “late” transition elements with differences in d-level’s electronic structure were compared for the first time.

Thermal Behavior of the “α‐Diimine‐NiII‐Catecholate” Chromophores

“…It should be mentioned, that chromophores 1 – 3 are synthesized and investigated earlier by us ( 1 , [20a] 2 , [20b] 3 [20c] ) from the standpoint of their molecular structure (single crystal X‐ray diffraction method), spectral behavior (UV‐vis‐NIR spectroscopy), and the energies of frontier HOMO/LUMO orbitals (DFT calculations). Herein we report the thermal behavior of Ni II complexes 1 – 3 as potential candidates for testing in multilayer optoelectronic devices with “evaporation‐deposition” technology for the layer formation.…”

“…Incorporation of bulky non‐conjugated substituents into the diimine ligand drastically changed the crystal packing of chromophores 2 and 3 against the layered structure of complex 1 : a “herringbone‐like” molecular distribution (the most common situation for LL′CT complexes) was realized in single crystals of 2 [20b] and 3 [20c] . Remarkably, the T‐motif of compound 2 contains two crystallographically independent molecules A and B , which are paired in a displaced “head‐to‐tail” pattern due to the short intermetallic Ni⋅⋅⋅Ni contacts (3.635 Å) and intermolecular interactions “bipyridine – chelate cycles of catecholate” [20b] . Since the unit cells of complex 2 are represented by the aforementioned molecular pairs, the Hirshfeld surface for molecule B was calculated as half of the dimer (Figure 6, view A).…”

Exploring the Role of Transition Metal Complexes in Artistic Coloration through a Bottom-Up Scientific Approach

“…The unique quality of transition metals that gives them the ability to produce these bright, visible wavelengths of light derives from their unfilled valence d-orbitals and ligand interactions, resulting in d-d orbital splitting and/or charge transfer [3,13,19]. The energetic interaction of visible light with the electrons in the d-orbitals of the central metal ion and at times with those of the ligands, as well as other variables that will be discussed, determine the magnitudes of the wavelengths that the gaps between such orbitals (Figure 1b,c) [20][21][22]. These magnitudes are influenced by several factors including the electronic structure of the center metal ions, geometry of the molecular structure, particle size, coordination number, and ligand field strength.…”

Donor‐Acceptor Tin(IV) Complexes with α‐Diimine and Catecholate Ligands

“…This geometry corresponds to derivatives having d 8 configuration, for example, such derivatives as metal complexes of transition metals Ni, Pt or Pd [5b,e,f,7a,e,8] . Much less frequently, complexes of other transition metals such as copper [10] or cobalt [11] have been investigated as donor‐acceptor chromophores of this type.…”