Singlet Fission in a Covalently Linked Cofacial Alkynyltetracene Dimer

Singlet
fission is a process in which a singlet exciton converts
into two triplet excitons. To investigate this phenomenon, we synthesized
two covalently linked 5-ethynyl-tetracene (ET) dimers with differing
degrees of intertetracene overlap: BET-X, with large, cofacial overlap
of tetracene π-orbitals, and BET-B, with twisted arrangement
between tetracenes exhibits less overlap between the tetracene π-orbitals.
The two compounds were crystallographically characterized and studied
by absorption and emission spectroscopy in solution, in PMMA and neat
thin films. The results show that singlet fission occurs within 1
ps in an amorphous thin film of BET-B with high efficiency (triplet
yield: 154%). In solution and the PMMA matrix the S1 of
BET-B relaxes to a correlated triplet pair 1(T1T1) on a time scale of 2 ps, which decays to the ground
state without forming separated triplets, suggesting that triplet
energy transfer from 1(T1T1) to a
nearby chromophore is essential for producing free triplets. In support
of this hypothesis, selective excitation of BET-B doped into a thin
film of diphenyltetracene (DPT) leads to formation of the 1(T1T1) state of BET-B, followed by generation
of both DPT and BET-B triplets. For the structurally cofacial BET-X,
an intermediate forms in <180 fs and returns to the ground state
more rapidly than BET-B. First-principles calculations predict a 2
orders of magnitude faster rate of singlet fission to the 1(T1T1) state in BET-B relative to that of crystalline
tetracene, attributing the rate increase to greater coupling between
the S1 and 1(T1T1) states
and favorable energetics for formation of the separated triplets.