Deep-Red Luminescence from Platinum(II) Complexes of N^N^–^N-Amido Ligands with Benzannulated N-Heterocyclic Donor Arms

Abstract

A synthetic methodology for accessing narrow-band, deep-red phosphorescence from mononuclear Pt(II) complexes is presented. These charge-neutral complexes have the general structure (N^N -^N )PtCl, in which the Pt(II) centers are supported by benzannulated diarylamido ligand scaffolds bearing substituted quinolinyl and/or phenanthridinyl arms. Emission maxima ranging from 683 to 745 nm are observed with lifetimes spanning from 850 to 4500 ns. In contrast to the corresponding proligands, benzannulation is found to counter-intuitively but markedly blue-shift emission from metal complexes with differing degrees of ligand benzannulation but similar substitution patterns. This effect can be further tuned by incorporation of electron-releasing (Me, tBu) or electron-withdrawing (CF 3 ) substituents in either the phenanthridine 2-position or quinoline 6-position. Compared with symmetric bis(quinoline) and bis(phenanthridine) architectures, "mixed" ligands incorporating one quinoline and one phenanthridine unit present a degree of charge transfer between the N-heterocyclic arms that is more pronounced in the proligands than in the Pt(II) complexes. The impact of benzannulation and ring-substitution on the structure and photophysical properties of both the proligands and their deep-red emitting Pt(II) complexes is discussed.

In Pursuit of Panchromatic Absorption in Metal Coordination Complexes: Experimental Delineation of the HOMO Inversion Model Using Pseudo-Octahedral Complexes of Diarylamido Ligands

“…In agreement with ground-state DFT calculations, the π*-orbitals of the phenanthridine arms serve as the acceptor orbitals for transitions at lower energies (λ > 460 nm). Higher energy excitations (λ < 460 nm) populate orbitals that are largely localized on the quinoline arms of the ligand, consistent with phenanthridine offering more energetically accessible acceptor orbitals . Altogether, the majority of absorptions observed in the visible region of the spectra of [( t Bu L ) 2 Co] + , ( t Bu L ) 2 Ni, and [( t Bu L ) 2 Ga] + are mainly comprised of ligand-to-ligand charge-transfer character (LLCT) consistent with the experimentally observed strong ε values.…”

Pseudo-octahedral nickel(ii) complexes of strongly absorbing benzannulated pincer-type amido ligands: ligand-based redox and non-Aufbau electronic behaviour

“…15 We have investigated the incorporation of benzannulated phenanthridine (3,4-benzoquinoline) moieties in diarylamido ligands. 16 In addition to producing Pt MCCs that combine strong absorption and red-shied phosphorescence, 17,18 neutral Fe-based MCCs comprised of two such ligands show strong panchromatic absorption and nanosecond charge-transfer lifetimes in their neutral forms, along with a rich redox prole. 19 Similarly strong but narrower absorption bands can also be seen with isoelectronic Co(III) analogues, as well as congeners of redox-silent Zn(II) and Ga(III) ions.…”

Brightly Luminescent Platinum Complexes of N∧C–∧N Ligands Forming Six-Membered Chelate Rings: Offsetting Deleterious Ring Size Effects Using Site-Selective Benzannulation

“…The absorption spectra of the Pt­(II) complexes are shown in Figure together with that of Pt­(dqb)­Cl, grouped to aid comparison, with data compiled in Table . The spectra of the complexes are qualitatively similar to one another, with high energy bands (300–350 nm) consistent with the presence of benzannulated phenanthridinyl ligand arms with energetically accessible, vacant π* orbitals . Compared with related complexes of N ∧ N –∧ N -type diarylamido pincer ligands bearing phenanthridinyl donors, which present strong, defined absorption bands at ∼500 nm and are deep red in color, the lowest energy features of the N ∧ C –∧ N -supported complexes appear as shoulders at ∼400 nm, consistent with the yellow color of the complexes.…”

“…Despite prevailing expectations, site-selective benzannulation induces a significant shift of emission to higher energy, decreasing the observed Stokes’ shift through heightened molecular rigidity owed to the phenanthridinyl units’ enhanced ability to resist molecular reorganization in their excited states compared to quinolinyl congeners . Substituents in the 2-position of the phenanthridinyl arms can further modulate luminescence, contingent on the substituent’s electron donor/acceptor character . Curious as to the ability of ligand benzannulation to potentially offset the deleterious effect of increased chelate-ring size in cyclometalated Pt­(II) emitters by impacting radiative/nonradiative decay rates, we decided to investigate the use of site-selective benzannulation to construct cyclometalated pincer-type ligands that combine a central aryl donor with flanking, 6-membered chelate-forming, phenanthridinyl arms.…”