General Design Strategy to Precisely Control the Emission of Fluorophores via a Twisted Intramolecular Charge Transfer (TICT) Process

Fluorogenic probes for bioimaging have become essential tools for
life science and medicine, and the key to their development is a precise
understanding of the mechanisms available for fluorescence off/on
control, such as photoinduced electron transfer (PeT) and Förster
resonance energy transfer (FRET). Here we establish a new molecular
design strategy to rationally develop activatable fluorescent probes,
which exhibit a fluorescence off/on change in response to target biomolecules,
by controlling the twisted intramolecular charge transfer (TICT) process.
This approach was developed on the basis of a thorough investigation
of the fluorescence quenching mechanism of N-phenyl
rhodamine dyes (commercially available as the QSY series) by means
of time-dependent density functional theory (TD-DFT) calculations
and photophysical evaluation of their derivatives. To illustrate and
validate this TICT-based design strategy, we employed it to develop
practical fluorogenic probes for HaloTag and SNAP-tag. We further
show that the TICT-controlled fluorescence off/on mechanism is generalizable
by synthesizing a Si–rhodamine-based fluorogenic probe for
HaloTag, thus providing a palette of chemical dyes that spans the
visible and near-infrared range.