Investigating Light Emission Efficiency of W(CO)_6 Complexes: Analysis with bpy Ligand Variations*

Transition-metal complexes (TMCs) are a pivotal class of compounds with promising applications in optoelectronics, solar energy conversion, and the biomedical domain. In this study, we present photoluminescence data for six W(CO)_6 complexes. We selected six models of these complexes to shed light on the photoluminescence (PL) mechanism inherent to transition-metal complexes. By melding the ab initio electronic structure with a time-dependent density matrix approach, we deciphered the photo-induced, time-variant excited-state dynamics. Our findings emphasized the phonon-induced relaxation of the photoexcited state in W(CO)_6. Utilizing the dissipative Redfield equation of motion, we determined the dissipative excited state dynamics of electronic degrees of freedom, taking nonadiabatic couplings as parameters. Our analyses revealed that the rate of electron relaxation outpaces that of the hole. We further integrated the dissipative excited-state dynamics with radiative recombination channels to predict the PL spectrum. The bidentate ligands we examined consist of a pyridine and an imidazole moiety, conjoined by a C–C bond. Notably, these complexes exhibited heightened phosphorescence from the metal-to-ligand charge transfer (MLCT) excited state, marked by quantum yields. Upon initial photoexcitation, an electron transitioned from an occupied state (represented in blue) to an unoccupied state (depicted in orange). Over time, the populations of the electron/hole states underwent changes, exhibiting an intermediate lifetime and a distinct charge distribution pattern. In essence, our study accentuates the swifter relaxation dynamics of electrons relative to holes during MLCT charge transfers. A pivotal observation from our results is the formation of two bonds of W(CO)_6 spanning an emission range from UV to the infrared spectrum.