Exploring Metal-NHC Complexes for Optoelectronic Applications: Photophysics and Tunable Emission Properties

N-heterocyclic carbene metal complexes have gained considerable attention in the last decade due to their versatility, ease of synthesis, and wide variety of applications including antitumor agents, catalysts, liquid crystalline materials, organometallic polymers, photoactive materials, and organic light-emitting diodes (OLEDs).

In this report, we first evaluated the heavy atom effect on a model platinum acetylide complexes featuring N-heterocyclic carbenes as chromophores and evaluated photo physical properties. Two new Pt (II) acetylide complexes with the structure (ICy)2PtR2 (where ICy = 1,3-bis-(cyclohexyl) imidazol-2-ylidene and R = 4-bromo phenylacetylene, IPt-MB; 3,5-dibromo phenylacetylene, IPt-DB;) were synthesized. Both complexes showed the blue emission in both solution and solid-state but differ significantly in terms of lifetime and quantum yield. In comparison to the model complex, IPt-MB showed poor quantum yield and short lifetime decay, whereas IPt-DB showed several-fold higher quantum yield and longer lifetime decay. Next, we investigate a series of Pt(II) acetylide complexes containing N-heterocyclic carbenes with extended conjugation, featuring phenanthrene, pyrene, and fluorene rings, to investigate their corresponding influences on the photophysical properties. In IPt-Phe-PE1 and Au-Phe-PE1, the introduction of phenanthrene chromophore led to the phosphorescence emission observed from both phenylacetylene and phenanthrene triplet states, whereas Au-Phe-PE2 exhibited the phosphorescence emission primarily originating from the PE2 ligand. In contrast, BPt-Fl-PE1 demonstrated a broad phosphorescence emission which is primarily attributed to the charge transfer between phenylacetylene and oligo-fluorene whereas IPt-Pyrene-PE1 exhibited originating from the triplet state of the pyrene chromophore. Throughout these investigations, we observed the significant role of phenylacetylene ligand in inducing phosphorescence in these complexes.

Lastly, a nitroxide-mediated polymerization (NMP) reaction was carried out to synthesize a series of random co-polymer by mixing a styrene monomer with a controlled amount of p-(trimethylsilyl ethynyl)styrene. A successful iClick reaction was carried out between Poly-10-Au-PPh3 and PPh3AuN3 to obtain Poly-10-Au2-PPh3.