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

In the present work, new rhenium( i ) polypyridyl compounds, fac -[Re(L)(CO) 3 (cbz 2 phen)] +/0 where cbz 2 phen = 4,7-di(9 H -carbazol-9-yl)-1,10-phenanthroline and L = Cl − , pyridine (py) or 2-aminomethylpyridine (ampy) were synthesized and characterized by 1 H 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, 1 ILCT cbz 2 phen , with some contribution of the Re( i ) → phenanthroline metal-to-ligand charge transfer transition, 1 MLCT Re→cbz 2 phen . In contrast to typical Re( i ) polypyridyl complexes, cbz 2 phen-based Re( i ) compounds exhibit two emission maxima in CH 3 CN 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 1 ILCT cbz 2 phen excited state deactivation at higher energies and a long-lived phosphorescence attributed to the 3 MLCT Re→cbz 2 phen excited state. When embedded into a PMMA matrix, the radiative decay from the singlet state is inhibited and the contribution of both 3 MLCT and 3 ILCT cbz 2 phen 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. The fac -[Re(CO) 3 (cbz 2 phen)(L)] 0/+1 complexes showed a remarkable presence of the ILCT cbz 2 phen fluorescence in addition to the usually observed 3 MLCT Re→cbz 2 phen . In PMMA films the emission is completely turned into a triplet excited state manifold.