Photoluminescence of self-assembled Ag(i) and Au(i) N-heterocyclic carbene complexes. Interplay the aurophilic, hydrogen bonding and hydrophobic interactions

Abstract

N-Hexadecanyl-N′-(2-hydroxyhexadecanyl)imidazol-2-ylidene (denoted as C₁₆,C₁₆(2-OH)-imy), in which one of the alkyl chains was anchored with a hydroxyl group at 2-position, was used to synthesize silver and gold N-heterocyclic carbene (NHC) complexes. The silver complex [(Ag(C₁₆,C₁₆(2-OH)-imy)₂)₂][Ag₂Cl₄] (1) shows liquid crystalline behavior and gold complexes [Au(C₁₆,C₁₆(2-OH)-imy)Cl] (2), [Au(C₁₆,C₁₆(2-OH)-imy)₂][BF₄] (3) and [Au(C₁₆,C₁₆(2-OH)-imy)₂][PF₆] (4) are soft materials. Compounds 1 and 2 are emissive, arising from metal–metal interactions. Compound 2 shows different emission colors depending on whether the sample is prepared as a crystal, soft material, or thin film. The relative strengths of aurophilic, hydrogen bonding and chain–chain hydrophobic interactions in the different states are investigated by powder X-ray diffraction, and Raman and IR spectroscopies. Results show that the one with a stronger Au–Au interaction displays a longer emission wavelength and has weaker O–H hydrogen bonding and hydrophobic interactions.