Intramolecular Excimer Formation between 3,6-Di(thiophen-2-yl)pyrrolo[3,4‑<i>c</i>]pyrrole-1,4(2<i>H</i>,5<i>H</i>)‑dione
Chromophoric Groups Linked by a Flexible Alkyl Spacer
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Abstract
Bichromophoric
molecules containing two 3,6-di(thiophen-2-yl)pyrrolo[3,4-<i>c</i>]pyrrole-1,4(2<i>H</i>,5<i>H</i>)-dione
(DPP) moieties linked via aliphatic spacers of different length are
synthesized. Optical absorption spectroscopy indicates that the molecules
adopt an extended conformation in solution. Fluorescence spectroscopy
shows that photons are emitted from the locally excited singlet state
in an extended conformation. In sufficiently polar solvents, quenching
of fluorescence occurs and fluorescence quantum yield (Φ<sub>F</sub>) and fluorescence lifetime (τ<sub>F</sub>) measurements
indicate formation of an intramolecular excimer as the quenching mechanism.
The redox potentials of the molecules and the solvent polarity dependence
of the quenching are consistent with significant charge-transfer character
of the excimer state. Photoinduced absorption measurements show enhanced
intersystem crossing to the triplet state in polar solvents. Results
indicate that in donor–acceptor π-conjugated materials
involving the DPP moiety, excimer-like interchain polaron pair excited
states could play an important role in the photophysics because of
their close proximity in energy to the lowest singlet excited state
