Aggregation Enhancement on
Two-Photon Optical Properties
of AIE-Active D‑TPE‑A Molecules
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Abstract
We present an aggregation enhancement
in two-photon-excited fluorescence
(TPEF) of about 2 orders of magnitude in a series of novel noncentrosymmetric
D-π-A molecules. Detailed analysis shows that their TPEF is
enhanced concomitantly with both fluorescence quantum efficiency and
two-photon absorptivity. These D-π-A molecules use tetraphenylethylene
(TPE) as the π-bridge, giving them aggregation-induced emission
characteristics. These branched or butterfly-configured molecules
demonstrate two-photon absorption (TPA) transition bands similar to
those in their linear absorption. The molecular TPA cross sections
in the aggregation environment reach around 50–130 GM, and
peak within the available wavelength range of a Ti:Sapphire femtosecond
oscillator, which makes the molecules quite favorable for bioimaging
applications. We observe that two-photon absorptivity increases progressively
with the addition of more donor and acceptor moieties on the TPE backbone,
which is presumably attributed to the improved conjugation length
and enhanced intramolecular charge transfer, and hence better delocalization
of π-electrons