Fluorescent Environment-Sensitive Dyes as Reporters of Biomolecular Interactions - Progress in Molecular Biology and Translational Science - page 46

present very high solvatochromism. 18 We should also mention polymethine
dyes displaying near-IR emission and solvent-dependence of their
lifetime, 48 as well as BODIPY derivatives, capable of switching on-off
their fluorescence in response to solvent polarity changes. 49 Although
these fluorophores have not yet been applied for biomolecular studies,
they appear as attractive building blocks for future high-performance
polarity-sensitive labels of biomolecules.

3. TWO-BAND SOLVATOCHROMIC DYES BASED
ON ESIPT
An alternative mechanism of solvent sensitivity can be realized
by utilizing the so-called ESIPT. Particularly interesting ESIPT dyes are
3-hydroxychromones (3HCs), presenting dual emission originating from
the normal excited state (N*) and the ESIPT tautomer (T*) ( Fig. 2.5 ). 50
The pathway for ESIPT in 3HCs is provided by the intramolecular H-bond
through a five-membered cycle, which is much weaker than the six-
membered cycle presented by other ESIPT systems. Therefore, it can be
easily perturbed by H-bonding interactions, thus modulating the dual
emission of 3HCs.

- H

O H

O

N *

O -

T *

ESIPT

O

O O

N

N

+ +

h n abs h n N * h n T *

+

H

O H

O

O

-

N T

O

O

O

N

N

Figure 2.5 Photophysical cycle of a 3HC derivative 4 0 -( N , N -diethylamino)-3-
hydroxyflavone. On electronic excitation (N!N*), a charge transfer from the 4 0 -
dialkylamino group to the 4-carbonyl takes place followed by an ESIPT process
(N*

T transition, the proton remains at the 4-carbonyl group, producing
a zwitterionic T state that rapidly converts into the stable N state.

! T*). After T* !