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Long-Lived Triplet Excited States of Bent-Shaped Pentacene Dimers by Intramolecular Singlet Fission
Intramolecular
singlet fission (ISF) is a promising photophysical
process to construct more efficient light energy conversion systems
as one excited singlet state converts into two excited triplet states.
Herein we synthesized and evaluated bent-shaped pentacene dimers as
a prototype of ISF to reveal intrinsic characters of triplet states
(e.g., lifetimes of triplet excited states). In this study, <i>meta-</i>phenylene-bridged TIPS-pentacene dimer (PcD-3Ph) and
2,2′-bipheynyl bridged TIPS-pentacene dimer (PcD-Biph) were
newly synthesized as bent-shaped dimers. In the steady-state spectroscopy,
absorption and emission bands of these dimers were fully characterized,
suggesting the appropriate degree of electronic coupling between pentacene
moieties in these dimers. In addition, the electrochemical measurements
were also performed to check the electronic interaction between two
pentacene moieties. Whereas the successive two oxidation peaks owing
to the delocalization were observed in a directly linked-pentacene
dimer (PcD) by a single bond, the cyclic voltammograms in PcD-Biph
and PcD-3Ph implied the weaker interaction compared to that of <i>p-</i>phenylene-bridged TIPS-pentacene dimer (PcD-4Ph) and PcD.
The femtosecond and nanosecond transient absorption spectra clearly
revealed the slower ISF process in bent-shaped pentacene dimers (PcD-Biph
and PcD-3Ph), more notably, the slower relaxation of the excited triplet
states in PcD-Biph and PcD-3Ph. Namely, the quantum yields of triplet
states (Φ<sub>T</sub>) by ISF approximately remain constant
(ca. 180–200%) in all dimer systems, whereas the lifetimes
of the triplet excited states became much longer (up to 360 ns) in
PcD-Biph as compared to PcD-4Ph (15 ns). Additionally, the lifetimes
of the corresponding triplet states in PcD-Biph and PcD-3Ph were sufficiently
affected by solvent viscosity. In particular, the lifetimes of PcD-Biph
triplet state in THF/paraffin (1.0 μs) increased up to approximately
three times as compared to that in THF (360 ns), whereas those of
PcD-4Ph were quite similar in both solvent