Developing Quinoidal Fluorophores with Unusually Strong Red/Near-Infrared Emission

Despite
the dominant position of aromatic fluorophores, we report
herein the design and synthesis of quinoidal fluorophores based on
rarely emissive quinoidal bithiophene. Quinoidal bitheno­[3,4-b]­thiophene, QBTT-C6, consisting of cruciform-fused
(E)-1,2-bis­(5-hexylthiophen-2-yl)­ethene and quinoidal
bithiophene, shows a fluorescence quantum yield of 8.5%, 25-fold higher
than that of the parent quinoidal QBT chromophore, but
its maximum emission is at similar wavelengths. QBTT-Ar’s featuring intramolecular charge transfer can further shift
the maximum emission into the near-infrared region. The intramolecular
charge transfer is programmably enhanced by tuning the substituents
on the aryl groups from the electron-withdrawing trifluoromethyl to
the electron-donating methoxy groups. Unexpectedly, a positive relationship
between intramolecular charge transfer and fluorescence quantum yield
is observed; as a result, QBTT-FL gives an unprecedentedly
high fluorescence quantum yield of up to 53.1% for quinoidal oligothiophenes.
With detailed photophysical and theoretical investigations, we demonstrate
that the nonradiative intersystem crossing (S1 →
T2) is significantly restrained in QBTT-Ar’s, which can be attributed to the faster reverse intersystem
crossing (T2 → S1) characteristic of
a small activation energy. This work reveals the possibility for developing
red/near-infrared fluorophores from the less explored quinoidal molecules
because of their intrinsically narrow bandgaps.