Self-assembly and phosphorescence of ionic<italic> </italic>Pd(II) <italic>N</italic>-heterocyclic allenylidene complexes in nonpolar solvents

Abstract

<p indent=0mm>In sharp contrast with the rich phosphorescent Pt(II) complexes reported in the literatures, phosphorescent Pd(II) complexes in room temperature solutions are scarce, in spite of having the same d⁸ electronic configuration and square-planar coordination mode. Due to their low-lying d-d transition state, phosphorescence origin of the Pd(II) complexes are usually ligand-based, rather than much metal participation. Recently, it has been proved to be an effective strategy to construct self-assembled aggregates via metallophilic Pd···Pd interactions for Pd(II) complexes to achieve metal-metal-to-ligand charge transfer (MMLCT) and corresponding phosphorescent emission in room temperature fluid state. Herein, a cationic <italic>N</italic>-heterocyclic allenylidene (NHA) Pd(II) complex <bold>3</bold>·PF₆ has been synthesized via a facile two-step transmetalation reaction of NHA precursor 2-ethynyl-1,3-dimethyl-1H-imidazol-3-ium hexafluorophosphate (<bold>1</bold>·PF₆) and a pincer type [(C^N^N)PdCl] (HC^N^N = 6-phenyl-4-(3,4,5-tris(dodecyloxy)phenyl)-2,2′-bipyridine) <bold>2</bold> with long alkyl chains in the presence of excessive amount of Ag₂O. It is stable towards air, moisture and common organic solvents. The purity of the cationic NHA Pd(II) complex <bold>3</bold>·PF₆ was characterized via NMR, HR-MS. And IR spectra revealed its resonance structure between zwitterionic acetylide and <italic>N</italic>-heterocyclic allenylidene to some extent. Solid state complex <bold>3</bold>·PF₆ showed orange emission with microsecond lifetime at both room temperature and <sc>77 K.</sc> Temperature-dependent PXRD and TGA/DSC analysis demonstrated the changes of complex <bold>3</bold>·PF₆ from an amorphous phase to a stable crystalline one upon raising the temperature. Due to the lipophilic property of the long alkyl chains, ionic complex <bold>3</bold>·PF₆ can be soluble in both polar (such as dimethyl sulfoxide (DMSO), CH₃CN, and dichloromethane) and nonpolar (such as cyclohexane, methylcyclohexane, and decalin) organic solvent, despite its ionic nature. Complex <bold>3</bold>·PF₆ in degassed dilute DCM solution showed rather weak photoluminescence with peak maximum at <sc>562 nm</sc> (QY < 1%, <italic>τ</italic>: <sc>17 ns).</sc> By increasing the volumetric ratio of cyclohexane which lowered solvent polarity, UV-vis absorption spectra of <bold>3</bold>·PF₆ in mixed solvent of DCM/cyclohexane showed a distinct low-energy absorption band with peak maximum at approximately <sc>500 nm,</sc> and the corresponding phosphorescence emerged. The NMR spectra recorded in mixed solvent deuterium substituted DCM/cyclohexane also showed shifted ¹H chemical-shift and broaden peaks by increasing the volumetric ratio of cyclohexane, which revealed the deeper degree of aggregation of <bold>3</bold>·PF₆ by decreasing the solvent polarity. Meanwhile, enhancement of red-shifted photoluminescence reached its largest absolute QY of 35.4% with emission peak maximum at <sc>612 nm</sc> and lifetime of <sc>730 ns</sc> in pure cyclohexane. The photophysical data recorded for <bold>3</bold>·PF₆ aggregates in other nonpolar solvents (such as toluene and decalin) showed similar broad and featureless phosphorescent emission band with peak maximum at around <sc>620 nm.</sc> The origin of the phosphorescence for <bold>3</bold>·PF₆ aggregates in nonpolar solvents can be ascribed to MMLCT enabled by the metallophilic Pd···Pd interactions. To further explore the self-assembly mode of <bold>3</bold>·PF₆ in nonpolar solvents, the corresponding temperature-dependent UV-vis absorption spectra have been recorded. The distinct low-energy absorption bands represented the aggregation degrees of the <bold>3</bold>·PF₆ monomer in the fluid state. The plot of variable-temperature UV-vis absorbance at the distinct low-energy band (peak maximum at <sc>504 nm)</sc> against temperature well-fits the nucleation-elongation model for the self-assembly of <bold>3</bold>·PF₆ in nonpolar solvent (such as methylcyclohexane and decalin). In conclusion, we have synthesized an ionic Pd(II) <italic>N</italic>-heterocyclic allenylidene complex with lipophilic long alkyl chains and manipulated its self-assembly behavior in nonpolar solvents by controlling the polarity and temperature. Corresponding MMLCT phosphorescence was also recorded and correlated with metallophilic Pd···Pd interactions in the aggregates.