3.1 Solvated discoloration properties
The dye A were successfully obtained by condensing aldehydes 6-(dibutylamino)benzo[b]thiophene-2-carbaldehyde 3 with a strong TCF electron acceptor in ethanol. The chemical structures of all the intermediates and dye A were characterized by 1H NMR, 13C NMR and MALDI-TOF.
The absorption and emission spectra of dye A in several solvents with different polarities are shown in Fig. 1. The solvent-dependent absorption and fluorescence emission λmax, abs and λmax, em and ET (30) of the dye in various solvents are listed in Table 1. As the solvent polarity increased, a bathochromic shift observed. This is the positive solvatochromism (Fig. 1a). The UV-Vis absorption peaks of the dye A showed a shift with the solvent polarity, which extended from 618 nm in toluene to 643 nm in chloroform. The emission peaks of dye A also showed shifts with solvent polarity, which extended form 690 nm in toluene to 746 nm in methanol. The fluorescence intensity is relatively low in the long wavelength direction. This is because the smaller the energy gap, the more serious the nonradiative transition. These results demonstrated that a strong π-π* transition with charge transfer exists in the dye A. The intramolecular charge transfer (ICT) is from benzo[b]thiophene moiety donor to the TCF acceptor is strongly enhanced upon excitation from the phenomenon of obvious bathochromic shift of the fluorescence peaks in polar solvents. The positive solvatochromism is because the dye A has larger dipole moment in the excited state than in the ground state. The above phenomenon indicates that the dye exhibits strong solvatochromic property. As the polarity of the solvent increases, the dipole moment of the molecule changes greatly after excitation, hence color change of solvent. The dipole action of the excited state in a highly polar solvent is more stable than the dipole action of the ground state in a small polar or non-polar solvent. In the solvents of different polarities, as the dipole moment of the dye increases, the positive solvatochromism is observed, and the redshift of the absorption peak and the emission peak appear (Fig. 1). However, in the chloroform and DMF, the emission peak delivers blueshift, and the dipole moment decreases in the electronic transition, resulting in negative solvatochromism. The reason may be that as the solvent polarity increases, the ground state of the dye changes. It is more stable than the excited state.
Table 1
λmax, abs, λmax, em and ET (30) of solvent-dependent absorption and fluorescence emission of the molecule in various solvents
Solvent
|
λmax, abs (nm)
|
λmax, ema (nm)
|
ET(30)(kcal/mol)
|
Toluene
|
618
|
690
|
33.9
|
THF
|
624
|
731
|
37.4
|
Chloroform
|
659
|
689
|
39.1
|
Acetone
|
629
|
732
|
42.2
|
DMF
|
657
|
672
|
43.8
|
Acetonitrile
|
634
|
736
|
45.6
|
Ethanol
|
640
|
745
|
51.9
|
Methanol
|
643
|
746
|
55.4
|
a Excitation wavelength is the maximum absorption wavelength in each solvent. |
Since the photophysical properties of D-π-A in solution depend on the polarity of solvent, we studied the maximum absorption and fluorescence of dye A in different polar solvents depending on the polarity of solvent. In Fig. 2, the dependence of the absorption and emission peaks of dye A on ET (30) solvent polarity parameter can be fitted to almost linear function. As the solvent polarity parameter increased, the bathochromic shift exhibited. This phenomenon indicated that dye A represented positive solvatochromism, where the ground state is less polar than the excited state. The charge delocalization from the electron-donating benzo[b]thiophene moiety to the electron-accepting the tricyanofuran (TCF) may be attributed to the polar ground state and ICT properties of the molecule, which leads to the red shift of the maximum absorption [29, 30].
As shown in Fig. 2, the fitting slope (2.05) in b is greater than the fitting slope in a (1.15), which indicates that the solvatochromism effect in emission is stronger than the solvatochromism in absorption. Fluorescence has stronger solvent polarity dependence since the relaxation of the originally formed excited state leads to a large amount of charge redistribution, the ICT excited state has a larger dipole moment than the ground state [31–33].
Figure 3 is a photograph of the color change of dye in the solution of the above eight solvents. It can be observed that as the polarity of the solvent changes, the color of the solution also undergoes obvious change. Furthermore, the color of the dye A in CHCl3 solution changes most obviously.
3.2 Theoretical calculation and electrochemical properties of the dye
DFT calculations have been carried out on dye A using B3LYP/6-311G* geometries by means of Gaussian 03 (G03). Figure 4 showed the electron distribution of the HOMO and LUMO energy level of the dye. It indicates that the electron density is uniformly distributed along the donor and π-bridge moiety at the HOMO state. While at the LUMO state, the electrons were shifted to the π-bridge and acceptor moiety due to the intramolecular charge transfer. The HOMO–LUMO energy gap of the dye A obtained from DFT calculations are summarized in Table 2. The HOMO–LUMO energy gap was used to understand the charge transfer interaction occurring in a chromophore molecule.
Table 2
Calculated properties and electrochemical properties of the dye
HOMO/eV
|
LUMO/eV
|
Ege/eV
|
Eox/V
|
Ered/V
|
Ege/eV
|
-5.67
|
-3.31
|
2.36
|
0.85
|
-0.66
|
1.51
|
The electrochemical reduction/oxidation behavior of the dye A was studied by cyclic voltammetry (CV). As shown in Fig. 5, the dye A showed an oxidation reversible wave with half-wave potentials E1/2= 0.5(Eox + Ered) at about 0.85 V (vs. Ag/AgCl), while an irreversible reduction wave with potential at about 0.66 V corresponding to the donor and acceptor group, respectively.
3.2 pH-induced switching in absorption and fluorescence spectra with color change of solution
The dye was dissolved in a solution of 1×10− 5 mol/L with DMSO as the solvent, and acid (CF3COOH)/base ((Bu)4N(OH)) was added to the solution dropwise. The interaction in solutions after adding acid and base was studied by spectrophotometer and change of solution color. Figure 6 shows the UV-Vis absorption and fluorescence spectral changes of the dye in an acid/base DMSO solution.
In the Fig. 6(a), the absorption intensity at 650 nm was significantly reduced after adding base to the solution of the dye. The color changed from blue to yellow, as shown in Fig. 7(a) and Fig. 7(b). The absorption intensity at 650 nm was also reduced by adding acid to the solution in Fig. 6(a), but much higher than the solution adding base (yellow) as shown in Fig. 7(c). After the solution was added with the base, the fluorescence spectrum also showed the same result as the UV-Vis spectrum in Fig. 6(b). When the solution was added with the acid, the peak of fluorescence spectrum decreased slightly, but the peak of fluorescence spectrum adding the base almost disappeared.
Then, upon addition of acid or base to the dye A DMSO solution in Fig. 7, the yellow solution or the light green solution of dye A became gradually blue and recovered completely back to the initial state, as shown in Fig. 8. The chromotropic behavior of solvents can be explained by the extreme resonance structure: zwitterions [13]. The phenomenon indicates that when DMSO is used as a solvent, addition of acid or base into the dye A solution is reversible. It may be that H+ or OH− induces the formation of zwitterion by the addition of acid/base, and the structure of the molecule is restored by adding base/acid. The comparison test proves that DMSO does not react with the molecule, CF3COOH and Bu4N(OH).
The positive solvatochromism for dye A showed that the charge transfer direction is from benzo[b]thiophene moiety to TCF, which indicated that the molecular ground state is neutral structure [34]. When the dye A in DMSO is added the base or acid, the zwitterionic was formed. The chemical structures of zwitterionic and neutral were as shown in Fig. 9. The above reversible process with acid/base trigger may be due to the mutual transformation of neutral and zwitterionic structures.