Intramolecular singlet fission in a face-to-face stacked tetracene trimer.

A covalently linked tetracene trimer with a "face-to-face" stacked structure was prepared and its molecular structure is characterized by H NMR, MALDI-TOF mass spectroscopy, and elemental analysis. The minimized molecular structure reveals that three tetracene subunits within this trimer adopt a partially "face-to-face" stacked configuration. Its absorption spectrum differs significantly from that of the monomeric and dimeric counterparts in solution due to the strong ground state interactions between the neighboring tetracene subunits. Its fluorescence is almost quenched completely. An ultrafast fluorescence decay component is observed in its fluorescence dynamics, indicating the presence of an ultrafast nonradiative decay channel in the trimer. The nonradiative channel is proved to be intramolecular singlet fission (iSF) by femto-second transient absorption studies. Different from the strongly coupled triplet state observed in the corresponding dimer, weakly coupled triplet states can be formed in this trimer. The triplet quantum yield of trimer 4 can reach up to 126% in solution.