## Effect of solvent on linear absorption property of natural pigment

The linear optical properties of AB 32 dye in different solvents were studied by UV-Visible spectrophotometer between 400 nm and 800 nm. The solvent polarity has an important factor that affects the linear optical properties of AB 32 dye. The UV-visible absorption spectra of the dye sample dissolved in various polar solvents such like methanol, acetone, ethanol, water, DMF and DMSO is shown in Fig. 3. From Fig. 3, the resonance absorption range was observed between the absorption peak of AB 32 dye in all solvents and the used laser source. Also, the absorption peak is varying with respect to the polarity of the solvent. The UV-visible absorption spectrum of the sample encompasses the entire visible region and the absorption shift towards a red region by increasing the solvent polarity is the result of positive solvatochromism and the corresponding shift is called bathochromic shift [27]. Bathochromic shift is the result of π-π* transition where the excited states is more polarized than the ground state [28]. The spectral features and linear absorption coefficient of the AB 32 dye is presented in Table 1.

Table 1

Linear and spectral parameters of polar solvents

Solvent | Linear refractive index (n0) | Dielectric constant (ε) | Hydrogen bond donor (α) | Hydrogen bond acceptor (β) | Polarizability (π*) | Linear absorption coefficient (α0/cm) |

Methanol | 1.329 | 32.7 | 0.98 | 0.66 | 0.60 | 6.01 |

Acetone | 1.358 | 20.7 | 0.08 | 0.48 | 0.62 | 0.90 |

Ethanol | 1.361 | 24.50 | 0.86 | 0.75 | 0.52 | 0.57 |

Water | 1.330 | 78.40 | 1.17 | 0.47 | 1.09 | 6.18 |

DMF | 1.430 | 38.00 | 0.00 | 0.69 | 0.88 | 6.07 |

DMSO | 1.479 | 46.68 | 0.00 | 0.76 | 1.00 | 4.64 |

### Effect Of Solvent On Third-order Nlo Features Of Azo Dye

The OA and CA techniques are used to find the third-order NLO susceptibility (χ(3)) of the title compound. In CA technique, an aperture with suitable opening is placed before the detector; so that only centre part of the Gaussian beam alone entered into the detector. In the case of OA method, a converging lens is used to receive the beam transmittance which is placed before the detector instead of an aperture. The nonlinear index of refraction and nonlinear coefficient of absorption the sample is determined from CA and OA technique which is directly related to real part and imaginary part of χ(3) respectively. Figure 4 (a-f) shows the OA result of AB 32 dye in methanol, acetone, ethanol, water, DMF and DMSO at 0.01 mM concentration. In Fig. 4 (a-f), the NLA curve of the sample shows both negative and positive nonlinear absorption due to SA and RSA property of the dye sample. The transmittance graph of AB 32 dye dissolved in water, DMF and DMSO shows RSA character, while the sample displays SA features in methanol, acetone and ethanol. SA ascends from high light intensities at focus and therefore the photon absorption significantly increasing before attaining to the ground state. Conversely, AB 32 dye is dissolved in water, DMF and DMSO displays RSA, due to strong interaction between the light intensity and the sample at the focus. The excited state absorption cross-section is larger than ground state is the consequence of RSA. In OA technique; the nonlinear absorption transmittance is given by,

$$T \left(z, s=1\right)=\sum _{m=0}^{\infty }\frac{{\left[-{q}_{o}\left(z\right)\right]}^{m}}{{\left[m+1\right]}^{\frac{3}{2}}}, for \left|{q}_{o}\left(0\right)\right|<1 \left(1\right)$$

where

where Leff = effective length of the sample and Zo = sample diffraction length. The nonlinear absorption coefficient (β) is given by,

$$\beta =\frac{2\surd 2\varDelta T}{{I}_{0}{L}_{eff}}\left(\frac{cm}{W}\right) \left(3\right)$$

Closed aperture method is used to find the sign and magnitude of nonlinear refractive index of the sample. The nonlinear refraction from CA technique comprises the influence of nonlinear absorption. Therefore, the pure part of nonlinear refraction is attained by dividing CA data from corresponding OA data. Figure 5 (a-f) shows the pure nonlinear refraction curve of AB 32 dye in polar solvents. A pre focal peak followed by post focal valley transmittance feature from the curve is the result of self-defocusing. Self-defocusing is ascending from thermal nonlinearity which arises from absorption of used light source. The CW laser irradiation produces a temperature variation inside the sample, which leads to thermal lensing. If the temperature of the sample increases, the index of refraction becomes negative and the sample serves as a defocusing lens. The origin of nonlinear refraction in materials may be electronic, molecular, electrostatic or thermal nonlinearity [19]. In organic samples thermal nonlinearity is the predominant mechanism which is confirmed by peak-valley separation. The peak-valley separation is 1.7 times the Rayleigh length is the clear indication of thermal nonlinearity [19]. The normalized transmittance of the dye sample is given by,

$$T\left(z\right)=1-\varDelta {\varnothing }_{o}\frac{4X}{({X}^{2}+1)({X}^{2}+9)} \left(4\right)$$

where X = Z/Z0.

The nonlinear index of refraction (n2) is calculated by using the relation

$${n}_{2} =\frac{\varDelta {\varnothing }_{0}\lambda }{2\pi {I}_{0}{L}_{eff}}\left(\frac{{m}^{2}}{W}\right) \left(5\right)$$

where \(\varDelta {\varnothing }_{0}\)= On-axis phase shift, λ = Wavelength of the light source and I0 = Intensity of the light beam at the focus.

The measured value of nonlinear refractive index of AB 32 dye in different solvents is tabulated in Table 2. The real and imaginary components of χ(3) is given by,

Table 2

Third-order NLO characteristics of acid black 32 dye in polar solvents

Solvent | n2 X 10–7 (cm2/W) | β X 10–2 (cm/W) | Re (χ3) X 10–6 (esu) | Im (χ3) X 10–7 (esu) | χ(3) X 10–6 (esu) |

Methanol | – 4.38 | – 1.06 | – 1.41 | – 1.73 | 1.42 |

Acetone | – 2.06 | – 0.75 | – 0.69 | – 1.78 | 0.71 |

Ethanol | – 2.96 | – 1.25 | – 1.00 | – 2.14 | 1.02 |

Water | – 4.29 | 0.68 | – 1.39 | 1.12 | 1.39 |

DMF | – 2.64 | 1.73 | – 0.99 | 3.27 | 1.04 |

DMSO | – 2.70 | 0.92 | – 1.08 | 1.86 | 1.09 |

$$Re\left[{\chi }^{\left(3\right)}\right]\left(esu\right)=\frac{{\epsilon }_{0}{c}^{2}{n}_{0}^{2}}{{10}^{4}\pi }{n}_{2}\left(\frac{c{m}^{2}}{W}\right) \left(6\right)$$

$$Im\left[{\chi }^{\left(3\right)}\right]\left(esu\right)=\frac{{\epsilon }_{0}{c}^{2}{n}_{0}^{2}\lambda }{{10}^{2}4{\pi }^{2}}\beta \left(\frac{cm}{W}\right) \left(7\right)$$

where c = velocity of light in vacuum and ε0 = vacuum permittivity. The third-order NLO susceptibility of AB 32 dye is given by,

$${\chi }^{\left(3\right)}=\sqrt{{\left(Re\right({\chi }^{3})}^{2}+{\left(Im\right({\chi }^{3})}^{2}} \left(esu\right) \left(8\right)$$

The measured value of third-order NLO susceptibility χ(3) of AB 32 dye is presented in Table 2. It is noted from Table 2 that, the dye sample exhibits large nonlinear optical susceptibility in methanol than other polar solvents.

The spectral features of the solute molecules due to the solvent effect can arise from either specific (dielectric enrichment) or non-specific (hydrogen bonding) solute-solvent interactions. The solvent effect on the solute molecules was determined by solvent polarity scale or solvatochromism. The solvent environment plays a major role between solute and solvent interaction and it influences the third-order NLO characteristics of the materials [27]. Solvent parameters such as solvent hydrogen bond donor, solvent hydrogen bond acceptor and polarizability are the major spectral factors that affecting the third-order NLO properties of the sample. The nonlinear absorption coefficient of the dye sample exhibits both saturable and reverse saturable absorption behavior due to difference in polarizability of the solvent. AB 32 dye shows RSA character in high polar solvents such as water, DMF, DMSO and conversely the dye sample exhibits SA property in low polar solvents like methanol, acetone and ethanol.

Different kinds of solute-solvent interactions are involved and solvent polarity parameters alone cannot be used to describe the solvent effects. Because the influence of solvent on third-order NLO characteristics of materials are more complicated. Therefore for a precise investigation, Kamlet-Abboud-Taft solvent polarity scale which comprises of specific and nonspecific interaction contributions is used. The solvent dependent NLO properties of the sample is derived by using the relation

$$A={A}_{o}+a\alpha +b\beta +s\pi \left(9\right)$$

where A is the solvent dependent parameter

Ao is the regression value of the solute molecules

a,b,s are the regression coefficient

The obtained results from the above equation are tabulated in Table 3. From Table 3, a good relationship was observed between multi-parameter scale and third-order NLO features of the AB 32 dye. Solvent hydrogen bond donor (α), Solvent hydrogen bond acceptor (β) and solvent polarizability (π) with various contributions have played an important role in the third-order NLO properties of AB 32 dye. For better comparison, the obtained data was transformed into contribution percentage and it is shown in Table 4. From Table 4, the solvent hydrogen bond acceptor has dominant contribution on third-order NLO refractive index of AB 32 dye. Similarly, the nonlinear absorption coefficient of AB 32 dye was greatly influenced by solvent hydrogen bond acceptor and solvent polarizability. The solvent hydrogen bond donor is also contributed and played a minor role and therefore it is suggested that both specific and nonspecific interactions are involved between solvent and solute molecule.

Table 3

Regression fit to solvent polarity scale

Multi-parameter scale | A0 | a | b | s |

n2 | 8.78 X 10–8 | 1.59 X 10–7 | 8.43 X 10–8 | 1.2 X 10–7 |

Β | 2.25 X 10–3 | 1.34 X 10–3 | 1.63 X 10–2 | 1.61 X 10–3 |

Table 4

Percentage contribution for n2and β

Multi-parameter scale | Pα (%) | Pβ (%) | Pπ (%) |

n2 | 43 | 23 | 33 |

Β | 70 | 85 | 83 |