Conformational Locking of the Geometry in Photoluminescent Cyclometalated N^C^N Ni(II) Complexes

Abstract

In our research aimed at replacing precious transition metals like platinum with abundant base metals such as nickel for efficient triplet emitters, we synthesized and studied Ni(II) complexes [Ni(L^NHR)Cl]. These complexes containing the N^C^N cyclometalating dipyridyl-phenide ligand, equipped with pending H-bonding amine groups (NH(C₆H₅) (L^NHPh) and NH(C₆H₅CH₂), ClL^NHBn). Molecular structures determined from experimental X-ray diffractometry and density functional theory (DFT) calculations in the ground state showed marked deviation of the Cl^- coligand (ancillary ligand) from the ideal planar coordination, with τ₄ values of 0.35 and 0.33, respectively, along with hydrogen bonding interactions of the ligand NH function with the Cl^- coligand. The complexes exhibit long-wavelength absorption bands at approximately 425 nm in solution, with the experimental spectra being accurately reproduced through time-dependent density functional theory (TD-DFT) calculations. Vibrationally structured emission profiles and steady-state photoluminescence quantum yields of 30% for [Ni(L^NHPh)Cl] and 40% for [Ni(L^NHBn)Cl] (along with dual excited state lifetimes in the ns and in the ms range) were found in frozen 2-methyl-tetrahydrofuran (2MeTHF) glassy matrices at 77 K. Furthermore, within a poly(methyl methacrylate) matrix, the complexes showed emission bands centered at around 550 nm within a temperature range from 6 K to 300 K with lifetimes similar to 77 K. Based on TD-DFT potential scans along the metal-ligand (Ni-N) coordinate, we found that in a rigid environment that restricts the geometry to the Franck-Condon region, either the triplet T₅ or the singlet S₄ state could contribute to the photoluminescence.

Keywords: DFT calculations; cyclometalation; electrochemistry; nickel(II); photoluminescence.

Scheme 1

The previously reported complexes [Ni(dpb)(carbazolate)] and [Ni(dpb)X] (Hdpb = 1,3-bis(2-pyridyl)benzene), and the herein reported Ni(II) complexes [Ni(L^NHPh)Cl] and [Ni(L^NHBn)Cl].

Figure 1

Molecular structures of the complexes [Ni(L^NHR)Cl] (N^C^N = L^NHPh, top left and L^NHBn, top right) from X-ray diffraction studies on single crystals (top, 50% displacement ellipsoids) and their side-on views featuring the deviation τ from the square planar coordination (bottom).

Figure 2

Selected experimental redox potentials (bars) and HOMO–LUMO gaps (ΔE_exp.) from cyclic voltammetry along with DFT-calculated orbital compositions and energies of the HOMO and LUMO and the corresponding HOMO–LUMO gap (ΔE_calc.) for the Ni complexes. HOMO = highest occupied molecular orbital; LUMO = lowest unoccupied molecular orbital.

Figure 3

UV-vis absorption spectra of the complexes [Ni(N^C^N)Cl] (N^C^N = L^NHPh (a) and L^NHBn (b) in THF solution at 298 K with calculated time-dependent DFT (TD-DFT) transitions (spectrum as line and sticks for individual transitions, in red).

Figure 4

Photoluminescence excitation (red, λ_em = 525 nm) and emission (black, λ_ex = 350 nm) spectra of the complexes [Ni(N^C^N)Cl] at 77 K in a glassy matrix of frozen 2-methyltetrahydrofuran (2MeTHF). (a): [Ni(L^NHPh)Cl]; (b): [Ni(L^NHBn)Cl].

Figure 5

Photoluminescence spectra of the complexes [Ni(L^NHPh)Cl] (a) and [Ni(L^NHBn)Cl] (b) (λ_ex = 350 nm) at different temperatures within poly(methyl methacrylat) (PMMA) films (2.5%).

Figure 6

Ground and excited state energies for fixed Ni–N distances of the central [Ni(N^C^N)Cl] unit from time-dependent DFT (TD-DFT) calculations using the Tamm–Dancoff approximation (TDA) [59], in vacuum for both substituents, Ph (top) and Bn (bottom). Elongation of the Ni–N bonds promotes non-radiative transitions from T₁–T₃ to the S₀ ground state due to conical intersections, whereas the S₁ state should be emissive. In a rigid environment, where the Ni–N distance is kept at ca. 2.0 Å, luminescence could also occur from the T₅ and S₄ states.