Pt(II) Complexes with Tetradentate C^N*N^C Luminophores: From Supramolecular Interactions to Temperature-Sensing Materials with Memory and Optical Readouts

Abstract

A series of four regioisomeric Pt(II) complexes (PtL^a-n and PtL^b-n) bearing tetradentate luminophores as dianionic ligands were synthesized. Hence, both classes of cyclometallating chelators were decorated with three n-hexyl (n = 6) or n-dodecyl (n = 12) chains. The new compounds were unambiguously characterized by means of multiple NMR spectroscopies and mass spectrometry. Steady-state and time-resolved photoluminescence spectroscopy as well quantum chemical calculations show that the effect of the regioisomerism on the emission colour and on the deactivation rate constants can be correlated with the participation of the Pt atom on the excited state. The thermal properties of the complexes were studied by DSC, POM and temperature-dependent steady-state photoluminescence spectroscopy. Three of the four complexes (PtL^a-12, PtL^b-6 and PtL^b-12) present an intriguing thermochromism resulting from the responsive metal-metal interactions involving adjacent monomeric units. Each material has different transition temperatures and memory capabilities, which can be tuned at the intermolecular level. Hence, dipole-dipole interactions between the luminophores and disruption of the crystalline packing by the alkyl groups are responsible for the final properties of the resulting materials.

Keywords: Pt(II) complex; organometallic compounds; phosphorescent complexes; thermochromism.

Scheme 1

Synthesis of the complexes. (i) NaH/DMF; (ii) alkylbromide, NaH /DMF; (iii) 4-hydroxyphenylboronic acid, K₂CO₃, Pd(PPh₃)₄/THF; (iv) 3-hydroxyphenylboronic acid, K₂CO₃, Pd(PPh₃)₄/THF; (v) alkylbromide, K₂CO₃/butanone; (vi) K₂PtCl₄/acetic acid.

Figure 1

Absorption (left) and emission spectra (λ_exc = 350 nm, right) of Pt^a-6 (green) and Pt^b-6 (orange) in fluid DCM solution at r.t. (solid lines), in 2-MeTHF glassy matrices at 77 K (dashed lines) and in PMMA films at r.t. (dotted lines).

Figure 2

Molecular orbitals dominating the T₁ state of PtL^a-1 and PtL^b-1 at the optimized T₁ geometry (left). Characterization of the emissive T₁ state by correlated electron–hole pair analysis (right).

Figure 3

Molecular orbitals dominating the description of the T₁ state for the dimers of PtL^a-1 A (left) and PtL^b-1 B (right) at the optimized T₁ geometry (left). Calculated (solid lines) and experimental (dashed lines) emission spectra of the aggregates (right).

Figure 4

(a) Emission spectra of solid PtL^a-12 upon heating (λ_exc = 470 nm). (b) Emission intensity at 512 nm and 661 nm as a function of the temperature. (c) X-ray diffraction pattern of PtL^a-12 before and after the phase transition. Inset: detail of the X-ray diffraction pattern after the phase transition. (d) Photographs of solid PtL^a-12 at different temperatures under UV-light excitation, λ_exc = 365 nm.

Figure 5

Emission spectra of PtL^a-12 (red), PtL^b-12 (green) and PtL^b-6 (orange) after melting and 48 h storage under ambient conditions (left). Photographs of the complexes PtL^a-12 (A), PtL^b-12 (B) and PtL^b-6 (C) after melting and 48 h storage under ambient conditions as observed under room light and under UV-light irradiation, λ_exc = 365 nm (right).