Influence of substituents in 1,10-phenanthroline on the structural and photophysical properties of W(CO)₄(1,10-phenanthroline-type) complexes

In this work, methyl and phenyl substituents were introduced in the 2,9- and 4,7-positions of the phen ligand in order to study its impact on the structural and photophysical properties of [W(CO)₄(phen-type)] complexes [phen-type = 4,7-dimethyl-1,10-phenanthroline (4,7-DMPhen); 4,7-diphenyl-1,10-phenanthroline (BPhen), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)]. Crystallographic analysis of these complexes allows to investigate the correlation between the geometrical parameters with their photophysical properties measured by steady-state and time-resolved spectroscopies. The (OC)_eq–W–(CO)_eq angle is the most significantly influenced geometrical parameter by the substituents, showing values of 92.5°, 90.2°, and 81.2° for [W(CO)₄(4,7-DMPhen)], [W(CO)₄(BPhen)], and [W(CO)₄(BCP)], respectively. The smallest angle observed for [W(CO)₄(BCP)] is attributed to a steric repulsion exerted by the methyl groups to the equatorial carbonyls. Remarkably, a smaller OC_eq–W–CO_eq angle favors the High-Energy (HE) emission band, as evidenced in the photoluminescence (PL) spectra of [W(CO)₄(BCP)]. Conversely, a greater angle favors the Low-Energy (LE) emission band, as observed in the PL spectra of [W(CO)₄(4,7-DMPhen)] and [W(CO)₄(BPhen)]. The PL lifetime of the LE emission band becomes shorter when decreasing the (OC)_eq–W–(CO)_eq angle. Furthermore, the absorption feature was affected by the 2,9-dimethyl substituents, showing greater contribution from the high-energy shoulder on the visible absorption band. This study shows that the perturbation of the (OC)_eq–W–(CO)_eq angle induced by the substituents in the phen ligand influences on the photophysical properties of [W(CO)₄(phen-type)] complexes.