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.