3.1 Spectral Properties of MSPPP
3.1.1 Absorption
The absorption spectra of 1-(4-methylsulfonyl phenyl)-3-(4-N, N dimethyl (amino phenyl)-2-propen-1-one in acetone were recorded for a wide range of concentrations from 0.65mM to 6.5mM. It showed an absorption band at 405 nm. By the increasing concentration, there was no new absorption peak in the longer wavelength region of the absorption spectra, and the absorption profile remained unchanged. However, the optical density of the band 405 nm increased monotonically when the concentration was increased see (Fig. 2).
3.1.2 Fluorescence of MSPPP
The fluorescence spectra of MSPPP in acetone, for the different concentrations 0.65–6.5 mM, were investigated. The results showed only one band at 515 nm, the spectra profile did not change irrespective of concentration for this dye. This indicates that the absence of excimer or exciplex for these dye solutions over all the concentrations used, as illustrated in Fig. 3. Moreover, the intensity decreased with an increasing the concentration of the dye.
The above results have been compared with the spectral properties of 3-(4-(dimethyl amino) phenyl)-1-phenyl-(2E)-propen-1-one (DAPPP) as a reference dye [3]. the absorption and fluorescence spectra of DAPPP in acetone for different concentrations ranging from 0.65 to 6.5mM. It showed an absorption band at 405 nm. By the increasing concentration, there was no new absorption peak observed, whereas the fluorescence spectra of DAPPP in acetone under an excitation wavelength of 355 nm for different concentrations (0.65 to 6.5mM). The fluorescence spectra had one band, at 525nm. The difference between MSPPP and DAPPP in their chemical structures is a methyl group attached to para-position to ring A. Here the methyl group is acting as electron donors by inductive effect. The addition of methyl group to DAPPP dye leads to a decrease in the fluorescence wavelength by 10 nm to blue shift, whereas no significant change in the absorption spectra is expected.
To investigate the impact of the solvents on the absorption and fluorescence spectra, the MSPPP was dissolved in different organic solvents with different dielectric constants. The concentration of these solutions was fixed to be 0.65mM. It could be seen that the dielectric constant of the solvents plays an important role in the absorption and emission bands, as showing in Fig. 4 and Fig. 5.
3.2 Optical band‑gap
The optical energy band gaps (Eg) of MSPPP were calculated by using Tauc's relation [10].
$$\left(\alpha hʋ\right)=A(hʋ-{E}_{g}{)}^{\frac{3}{2}}$$ 1
where, h is Planck's constant and ʋ is the frequency of incident photons, A is constant, \({h}_{v}\) is the photon energy, Eg is the optical band gap of MSPPP. The energy band gaps of MSPPP were estimated by extrapolating the linear part of the graph of \({\alpha hv}^{\frac{2}{3}}\) against (hν) until the hν axis was intersected (see Fig. 6). The value of the energy gap of MSPPP obtained by this method is 3.12 eV.
3.3 Stokes’ shift
MSPPP was dissolved in various organic solvents that have different dielectric constants. The concentration was kept at 0.65mM. It was observed that there are very small changes in the absorption and fluorescence spectra, the only difference being a small shift in the peaks of the absorption and the fluorescence wavelengths. The variation of the Stokes shift as a function of the dipole factor of the solvent, as defined by Lippert and Mataga et al. [11] was shown in “Fig. 7 and Fig. 8”. It can be noticed that MSPPP in the solution undergoes considerable changes in the electron delocalization and turn strongly polar in the excited state than in the ground state. The Stokes shift has a linear variation with the dipole factor, which is written as:
where Df is dipole factor, νa and νf are the absorption and fluorescence peaks in wave numbers respectively, ε is the dielectric constant and n is the solvents refractive index. µe is the dipole moment of the solute in the excited state. µg is representing the dipole moment of the solute the ground states, respectively, and a is the radius of the solvent cage around the solute.
This dipole factor is a measure of dipole-dipole interaction between the solvents and the solute. It could be seen that MSPPP is slightly less polar than DAPPP. These results indicate that all these dyes display great variations in the dipole moment in the excited state.
3.4 Quantum yield of fluorescence
The fluorescence quantum yield (ΦF) of Rhodamine 6G in ethanol is 0.94 and it is used as a reference. The ΦF of MSPPP was calculated using the following equation [3].
where the indices S and R refer to the sample and reference, respectively, and the integral over I represent the area under the fluorescence spectrum. A is the optical density, and n is the refractive index of the solvents. The results of the ΦF of the solvated MSPPP are shown in Table 1.
Table 1
The optical properties of MSPPP in different solvents
Solvent
|
Dielectric constant ε
|
MSPPP λmax [nm]
|
Stocks shift
cm− 1
|
ɸF %
|
Absorption
|
Fluorescence
|
Benzene
|
2.23
|
406
|
472.38
|
3490
|
13
|
Toluene
|
2.60
|
404
|
477
|
3810
|
11
|
Chloroform
|
4.79
|
418
|
507.5
|
4260
|
89
|
Acetic acid
|
6.15
|
427
|
511.7
|
3870
|
20
|
(THF)
|
7.55
|
403
|
502.8
|
4930
|
86
|
Acetone
|
20.5
|
405
|
515
|
5350
|
66
|
Ethanol
|
24.2
|
419
|
533
|
4990
|
44
|
Methanol
|
32.5
|
420
|
532.5
|
5380
|
22
|
DMF
|
36.7
|
415
|
527.5
|
5140
|
96
|
Acetonitrile
|
37.5
|
410
|
528.5
|
5470
|
40
|
The fluorescence spectra of MSPPP in different organic solvents were recorded. The concentration was kept at 0.65mM. The fluorescence spectra were recorded at the excitation wavelength of 355 nm. The results showed that the quantum yield depends on the solvent and the chemical structure of the dye. The results obtained were compared with the quantum yield of DAPPP [3] and displayed in Table 2.
Table 2
quantum yields of fluorescence (ΦF) for MSPPPP and DAPPP
Solvent
|
Dielectric constant\(\epsilon\)
|
\({\text{ᶲ}}_{\text{F}}\)MSPPP
|
\({\text{ᶲ}}_{\text{F}}\)DAPPP
|
Benzene
|
2.23
|
0.13
|
0.26
|
Toluene
|
2.60
|
0.11
|
0.22
|
Chloroform
|
4.79
|
0.89
|
0.66
|
Acetic acid
|
6.15
|
0.20
|
0.05
|
Tetrahydrofuran (THF)
|
7.55
|
0.86
|
0.98
|
Acetone
|
20.5
|
0.66
|
0.85
|
Ethanol
|
24.2
|
0.44
|
0.35
|
Methanol
|
32.5
|
0.22
|
0.02
|
Dimethylformamide (DMF)
|
36.7
|
0.96
|
0.93
|
Acetonitrile
|
37.5
|
0.40
|
0.66
|
3.5 Amplified spontaneous emission (ASE)
To study the ASE properties of MSPPP under high power laser excitation; MSPPP was dissolved in acetone, the concentration fixed at 1 mM. This solution was transversely excited with a UV laser at 355 nm. At a pump power of 3 mJ, the ASE spectrum was noted. This was the minimum concentration and pump power excitation for MSPPP to produce an ASE spectrum at 520 nm with a full width at half maximum (FWHM) of 5 nm as shown in Fig. 9. The results obtained were compared with that DAPPP under the same operating conditions. Focusing our attention on MSPPP and DAPPP molecular Structures, these almost have the same chemical structure with different substitutions in the fourth position. Whereas we have a methyl group in the fourth position of MSPPP, this difference has shifted the ASE of MSPPP10 nm to the blue region regarding DAPPP.
Table 3 displayed the ASE spectra of MSPPP dissolved in different solvents under identical conditions; the concentration and pump power were kept at 6.5mM and 9 mJ for each solution. The ASE in acetic acid, methanol, benzene, and toluene was not detected even at high pump power energy and concentration. In toluene and benzene, this may be due to the lowest solubility of the MSPPP for these solvents. The absence of the ASE spectra in acetic acid may be due to the protonation of the N-dimethylamino group of MSPPP with responsibility for their photo properties. Here the methanol could play a similar role by deactivation of the lone pair of N-dimethylamino group by hydrogen bonding; this slightly appears in ethanol which gives poor ASE. Figure 11 showed that the DAPPP produced a high intense ASE at 530.7 nm.
Table 3
Solvent
|
Dielectric constant\(\epsilon\)
|
\({?}_{\text{m}\text{a}\text{x}}\)MSPPP
|
Benzene
|
2.23
|
…….
|
Toluene
|
2.60
|
…….
|
Chloroform
|
4.79
|
513
|
Acetic acid
|
6.15
|
……….
|
Tetrahydrofuran (THF)
|
7.55
|
511
|
Acetone
|
20.5
|
522.5
|
Ethanol
|
24.2
|
548
|
Methanol
|
32.5
|
……
|
Dimethylformamide (DMF)
|
36.7
|
541
|
Acetonitrile
|
37.5
|
544
|
The results obtained were compared with the ASE of DAPPP under identical conditions [3] and displayed in Table 4. For DAPPP it was found that when the dielectric constant increases the emission wavelength is increasingly red shifted. In addition, The ASE in acetic acid, methanol, benzene, and toluene was not detected even at high pump power energy and concentration. In toluene and benzene, this may be due to the lowest solubility of the DAPPP for these solvents. Here we find that MSPPP and DAPPP may be agreed in the absence of the ASE spectra in acetic acid may be due to the protonation of the N-dimethylamino group of MSPPP with responsibility for their photo properties. Here the methanol could play a similar role by deactivation of the lone pair of N-dimethylamino group by hydrogen bonding; this slightly appears in ethanol which gives poor ASE.
Table 4
The ASE spectra of MSPPP and DAPPP in different solvents
Solvent
|
Dielectric constant\(\epsilon\)
|
\({?}_{\text{m}\text{a}\text{x}}\)MSPPP
|
\({?}_{\text{m}\text{a}\text{x}}\)DAPPP
|
Benzene
|
2.23
|
…….
|
………
|
Toluene
|
2.60
|
…….
|
………
|
Chloroform
|
4.79
|
513
|
524
|
Acetic acid
|
6.15
|
……….
|
…….
|
Tetrahydrofuran (THF)
|
7.55
|
511
|
516
|
Acetone
|
20.5
|
522.5
|
532
|
Ethanol
|
24.2
|
548
|
542
|
Methanol
|
32.5
|
……
|
…….
|
Dimethylformamide (DMF)
|
36.7
|
541
|
548
|
Acetonitrile
|
37.5
|
544
|
547
|
The ASE intensities variation of MSPPP dissolved as a function of the concentration is shown in Fig. 10. The concentrations were taken from 2 to 6mM. The solvents were DMF, acetone, and THF and the pump power was 9 mJ. It was found that, as the concentration increased, the intensity of the ASE increases for each solution. It was seen that the ASE did not reach saturation even at high concentrations.