ligand to ligand charge transfer (LLCT) transition

https://doi.org/10.1351/goldbook.L03519
An electronic transition of a metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer between two ligands has occurred.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2252 (https://doi.org/10.1351/pac199668122223)