Harvesting luminescence via harnessing the photophysical properties of transition metal complexes

The main goal of this review is to provide systematic elucidation of the correlation between structural characters and the photophysical properties of a series of heavy transition metal complexes. Depending on types of metal ions, chromophoric and ancillary ligands, several intriguing cases encountered in our recent studies will be exemplified as prototypes to shed light on their excited-state relaxation pathways. Particular attention is paid to: (i) the intersystem crossing and/or radiative decay rates versus contribution of the metal dπ orbital, (ii) crucial factors that facilitate the radiationless deactivation, such as the metal-centred dd transition, resulting in weakness of the metal–ligand bond, and other transitions weakening the specific bonds and flexible structural framework that induces the low-frequency vibrational deactivation, (iii) intra-ligand versus inter-ligand charge transfer affecting the photophysical properties; that is, an issue of current interest regarding whether to treat the whole transition metal complex as a single entity or as several distinctive chromophores separated by the core metal ion. We then formulate a discussion from the standpoint of fundamental photophysical theory. The results, together with modern computational approaches for supplementary support, allow us to make adequate comparison with respect to classic organic fluorescence counterparts. Many similarities can be identified between organic fluorophores and late transition metal based phosphors; nevertheless, certain distinctions can also be extracted. We then conclude this review by providing guidelines on how to harvest the emission via suppressing the weighting of radiationless deactivation routes. However, for transition metal complexes, quantitative assessment of radiationless deactivation and hence the accurate prediction of emission efficiency is still a long term goal to be attained.