Contrasting photophysical properties of rhenium(i) tricarbonyl complexes having carbazole groups attached to the polypyridine ligand

Abstract

In the present work, new rhenium(I) polypyridyl compounds, fac-[Re(L)(CO)₃(cbz₂phen)]^+/0 where cbz₂phen = 4,7-di(9H-carbazol-9-yl)-1,10-phenanthroline and L = Cl⁻, pyridine (py) or 2-aminomethylpyridine (ampy) were synthesized and characterized by ¹H NMR, UV-Vis and IR spectroscopy combined with theoretical calculations using time-dependent density functional theory (TD-DFT). Their photophysical properties were investigated by steady state and time-resolved emission spectroscopy. These compounds show a strong and broad absorption band around 350–500 nm that, also by TD-DFT, corresponds to the carbazol → phenanthroline intraligand charge transfer transition, ¹ILCT_cbz2phen, with some contribution of the Re(I) → phenanthroline metal-to-ligand charge transfer transition, ¹MLCT_{Re→cbz2phen}. In contrast to typical Re(I) polypyridyl complexes, cbz₂phen-based Re(I) compounds exhibit two emission maxima in CH₃CN solution and relatively low emission quantum yields, 10⁻³–10⁻². Solution phase time-resolved photoluminescence and excited state quenching experiments provided meaningful information on the presence of multiple emitter states after light excitation, which were identified as an ¹ILCT_cbz2phen excited state deactivation at higher energies and a long-lived phosphorescence attributed to the ³MLCT_{Re→cbz2phen} excited state. When embedded into a PMMA matrix, the radiative decay from the singlet state is inhibited and the contribution of both ³MLCT and ³ILCT_cbz2phen to the luminescence is observed. The photophysics of these Re(I) compounds reported herein provide new insights into the understanding of substitutional groups on the polypyridyl ligands that are relevant to practical and fundamental development of photo-induced molecular devices.