Ground- and excited-state properties of Re(I) carbonyl complexes-Effect of triimine ligand core and appended heteroaromatic groups

In this work, a series of six rhenium(I) complexes bearing 2,2 ':6 ',2 ''-terpyridine (terpy), 2,6-di(thiazol-2-yl)pyridine (dtpy), and 2,6-di(pyrazin-2-yl)pyridine (dppy) with appended quinolin-2-yl and N-ethylcarbazol-3-yl groups were prepared and spectroscopically investigated to evaluate the photophysical consequences of both the trisheterocyclic core (terpy, dtpy and dppy) and the heterocyclic substituent. The [ReCl(CO)3(L-n-kappa N-2)] complexes are regarded as ideal candidates for getting structure-property relationships, while terpy-like framework represents an excellent structural backbone for structural modifications. The replacement of the peripheral pyridine rings of 2,2 ':6 ',2 ''-terpyridine by thiazoles and pyrazines resulted in a significant red-shift of the absorption and emission of [ReCl(CO)(3)(L-n-kappa N-2)] due to stabilization of the ligand-centred LUMO orbital. Both quinoline and Nethylcarbazole are extended pi-conjugation organic chromophores, but they differ in electron-donating abilities. The low-energy absorption band of Re(I) complexes with the triimine ligands bearing quinolin-2-yl group was contributed by the metal-to-ligand charge-transfer (MLCT) electronic transitions. The introduction of electrondonating N-ethylcarbazol-3-yl substituent into the triimine acceptor core resulted in the change of the character of the HOMO of Re(I) complexes and a significant increase of molar absorption coefficients of the longwavelength absorption, which was assigned to a combination of (MLCT)-M-1 and (ILCT)-I-1 (intraligand chargetransfer) transitions. Regardless of the appended heteroaromatic group, the emitting excited state of Re(I) terpy-based complexes was demonstrated to have predominant (MLCT)-M-3 character, as evidenced by comprehensive studies including static and time-resolved emission spectroscopy along with ultrafast transient absorption measurements. The diodes with Re(I) complexes dispersed molecularly in a PVK:PBD matrix were emissive and effects of the complex structure on colour of emitted light and its intensity was pronounced.

Automated summary

In this work, a series of rhenium(I) complexes with different substituents were synthesized and their photophysical properties were studied, with the substitution resulting in a significant red-shift in absorption and emission, contributing to the stabilization of the ligand-centred LUMO orbital.