triplet-triplet energy transfer 

Acronym: TTET
https://doi.org/10.1351/goldbook.T06506
Energy transfer from an electronically excited triplet donor to produce an electronically excited acceptor in its @T06503@.
Source:
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2282 [Terms] [Paper]