Significant differences between solid state and solution photochemistry and photophysics of mesogenic organometallic gold complexes

Abstract

Five new gold complexes 4-ROC6H4C≡CAuN≡CC6H4-4-OR′ (R/R′ = CH3/C9H19 (C1N9 ), C15H31/C9H19 (C15N9 ), C6H13/C15H31 (C6N15 ), C9H19/C15H31 (C9N15 ), C12H25/C15H31 (C12N15 )) were synthesized and characterized (1H and 13C NMR, IR, Raman spectroscopy, and high resolution mass spectrometry). Their organized smectic phases were investigated by TGA, DSC, powder XRD, and polarized light optical microscopy, and the solids are found to have crystalline and amorphous domains. No evidence for Au•••Au interactions was observed. The steady state and time-resolved absorption and emission properties at 298 and 77 K were examined, and surprisingly, the excited lifetime of the triplet excited state in the solid state is extremely short-lived (<100 ps) in comparison with the microsecond time scale recorded for the solution and at 77 K. The photosensitization of 1O2 was observed in solution but not in the solid state. The nature of the singlet (ligand-to-ligand charge transfer) and triplet (ethynyl/intraligand ππ*) excited states were assessed using DFT and TD-DFT computations. The thermal and UV-photochemical formation of gold nanoparticles were performed in solution (slow) and in the solid state (faster). The thermally generated nanoparticles are found to be larger (2–20 nm; TEM) and exhibit well-defined shapes, whereas the photochemically generated ones are smaller (1–10 nm) and show ill-defined shapes.