Singlet Exciton Fission in Thin Films of <i>tert</i>-Butyl-Substituted
Terrylenes
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Abstract
Two
terrylene chromophores, 2,5,10,13-tetra(<i>tert</i>-butyl)terrylene
(<b>1</b>) and 2,5-di(<i>tert</i>-butyl)terrylene
(<b>2</b>), were synthesized and studied to
determine their singlet exciton fission (SF) efficiencies. Compound <b>1</b> crystallizes in one-dimensional stacks, whereas <b>2</b> packs in a slip-stacked, herringbone pattern of dimers motif. Strongly
quenched fluorescence and rapid singlet exciton decay dynamics are
observed in vapor-deposited thin films of <b>1</b> and <b>2</b>. Phosphorescence measurements on thin films of <b>1</b> and <b>2</b> show that SF is only 70 meV endoergic for these
chromophores. Femtosecond transient absorption experiments using low
laser fluences on these films reveal rapid triplet exciton formation
for both <b>1</b> (τ = 120 ± 10 ps) and <b>2</b> (τ = 320 ± 20 ps) that depends strongly on film crystallinity.
The transient absorption data are consistent with formation of an
excimer state prior to SF. Triplet exciton yield measurements indicate
nearly quantitative SF in thin films of both chromophores in highly
crystalline solvent-vapor-annealed films: 170 ± 20% for <b>1</b> and 200 ± 30% for <b>2</b>. These results show
that significantly different crystal morphologies of the same chromophore
can both result in high-efficiency SF provided that the energetics
are favorable