Fluorescent Environment-Sensitive Dyes as Reporters of Biomolecular Interactions - Progress in Molecular Biology and Translational Science - page 47

O H O H

O O

O O

O

3HC-A 3HC-B

N

Polarity

Toluene EtOAc CH 2 CI 2 CH 3 CN CH 3 CN EtOH MeOH H 2 O

1.0 1.0

N*
band

T*
band

N*
band

T*
band

0.8 0.8

3HC-A 3HC-B

0.6 0.6

0.4 0.4

0.2 0.2

0.0 0.0

450 500 550

Wavelength (nm)

600 650 350 400 450 500 550

Wavelength (nm)

600 650

Figure 2.6 Chemical structure of 3HC dyes and their solvent-dependent dual emission.
Reproduced from Ref. 7 .

Among the 3HCs developed so far, two derivatives are of particular
interest: 4 0 -(dialkylamino)-3-hydroxyflavone (3HC-A) and 2-(2-furyl)-3-
hydroxychromone (3HC-B) ( Fig. 2.6 ). Owing to the 4 0 -dialkylamino
group, the N* excited state of 3HC-A exhibits a large dipole moment,
where the electronic charge is transferred from the dialkylamino group to
the chromone moiety ( Fig. 2.5 ). 51 In contrast, the ESIPT product T* state
exhibits much lower charge separation and, thus, lower dipole moment.
Therefore, only the N* state shows a significant shift to the red on increase
in solvent polarity ( Fig. 2.6 ). This red shift is accompanied by an increase in
the relative intensity of the N* band, because this state becomes energetically
more favorable than the T* state. 52 Therefore, the intensity ratio of the N*
and T* bands, that is, N*/T*, is an important indicator of solvent polarity. 51
However, 3HC-A shows dual emission only in the range of low-polar and
polar aprotic solvents ( Fig. 2.6 ). In polar protic solvents, including water, the
ESIPT is efficiently inhibited so that the T* emission is no more observed. 51
Therefore, this dye could be applied for probing biological environments of