Homoleptic Alkynylpyridinium Au(I) Complexes as Organometallic 'D-π-A' Chromophores

A series of terminal acetylenes bearing methylpyridinium acceptor group attached to the alkynyl unit with different π-conjugated aromatic linker have been synthetized. These alkynylpyridinium salts are efficient 'push-pull' chromophores demonstrating bright UV-vis fluorescence with quantum yields up to 70 %. The homoleptic bis-alkynyl Au(I) complexes based on these alkynylpyridinium ligands demonstrate complicated photophysical behavior including dual emission in solution. The variation of the linker allows one to tune the intrasystem charge transfer and thus to change the electronic and photophysical properties of the organogold 'D-π-A' system. This study demonstrates that absolute and relative intensities of the bands in emission spectra and their energies are responsive to solvent system and anion nature even in case of weakly coordinative anions. TDDFT calculations show that the transitions related to emission of the complex cations are strongly associated with the hybrid MLCT/ILCT charge transfer, thus demonstrating that the complex molecule acts as a unified 'D-π-A' system.