**3. External fields effect on electronic and vibrational spectroscopic properties**

#### **3.1. Temperature dependence on the optical properties of β-Car**

There are many researches on electronic and vibrational properties on polyene. Even though the main goal of the extensive studies on polyene properties is to develop opticelectronic devices working in room temperature, studies focusing on temperature dependence of photophysical properties can provide valuable information about the electronic and vibrational characters of conjugated polyene materials. It is essential to have a profound knowledge on excited and ground states of β-Car in order to understand the mechanism of the photoprotective and antioxidative function of carotenoids. It is known that the steady-state absorption and photoluminescence spectra of inorganic materials usually red-shift with the increasing temperature [33, 35]. Extensive temperature dependence researches on conjugated polymers can provide valuable information for studying the β-Car optical response in low temperature. The spectral shift of conjugated polymers can attribute to the thermal conformational changes, which is mainly due to the change of their effective conjugation length. Using the relationship between the zero-phonon and vibrational peak intensities, we can obtain the Huang-Rhys factor. This is an effective parameter which can describe the electron-phonon coupling of the polymer material in different temperature. Given that the unexpectedly high Raman scattering activity of CC bond length vibrations is due to the extended π-conjugation giving a strong electron-phonon coupling, the π-electron delocalization and the electron-phonon coupling should show dependence on temperature.

External Field Effect on Electronic and Vibrational Properties of Carotenoids http://dx.doi.org/10.5772/intechopen.78593 49

**Figure 3.** Absorption spectra of β-Car dissolved in cyclohexanol in different temperatures from 68 to 25°C.

coupled with C=C torsional modes. The overtone 2*v*<sup>1</sup>

**3.1. Temperature dependence on the optical properties of β-Car**

bination *v*<sup>1</sup> + *v*<sup>2</sup>

background has been subtracted.

48 Progress in Carotenoid Research

**properties**

on temperature.

(2310 cm−1), 2*v*<sup>2</sup>

(2675 cm−1) are also appeared in the resonance Raman spectrum.

**Figure 2.** Raman spectrum of β-Car dissolved in carbon disulfide recorded at the room temperature. The fluorescence

There are many researches on electronic and vibrational properties on polyene. Even though the main goal of the extensive studies on polyene properties is to develop opticelectronic devices working in room temperature, studies focusing on temperature dependence of photophysical properties can provide valuable information about the electronic and vibrational characters of conjugated polyene materials. It is essential to have a profound knowledge on excited and ground states of β-Car in order to understand the mechanism of the photoprotective and antioxidative function of carotenoids. It is known that the steady-state absorption and photoluminescence spectra of inorganic materials usually red-shift with the increasing temperature [33, 35]. Extensive temperature dependence researches on conjugated polymers can provide valuable information for studying the β-Car optical response in low temperature. The spectral shift of conjugated polymers can attribute to the thermal conformational changes, which is mainly due to the change of their effective conjugation length. Using the relationship between the zero-phonon and vibrational peak intensities, we can obtain the Huang-Rhys factor. This is an effective parameter which can describe the electron-phonon coupling of the polymer material in different temperature. Given that the unexpectedly high Raman scattering activity of CC bond length vibrations is due to the extended π-conjugation giving a strong electron-phonon coupling, the π-electron delocalization and the electron-phonon coupling should show dependence

**3. External fields effect on electronic and vibrational spectroscopic** 

(3040 cm−1), and com-

We measured the absorption of β-Car dissolved in cyclohexanol in the temperature range from 68 to 25°C with an interval of 5°C. As shown in **Figure 3**, with the decreasing of the temperature, the thermal motion, the entropy of the molecule decreases and the thermal disorder of the β-Car molecule decreases. At the same time, the degree of order of the molecular structure increases, the molecule becomes straight, so that the π electron delocalization expands. The π-π\* transition energy narrows which leads to the observed absorption redshift, which is different from the absorption properties of inorganic materials. With the narrowing of the π-π\* gap, the influence on the CC vibrational bands from electronic transition becomes stronger. Correspondingly, we recorded the resonance Raman spectra in the same temperature range in order to examine the influence from the narrowing π-π\* gap. **Figure 4** shows the resonance Raman spectra of β-Car molecule dissolved in the cyclohexanol in different temperature from 60 to 20°C with the 514.5-nm laser excitation. The bands' frequency

**Figure 4.** Resonance Raman spectra from 68 to 26°C of β-Car dissolved in cyclohexanol. The laser excitation wavelength is 514.5 nm.

**Figure 5.** Temperature dependence of RSCSs from (a) C–C and (b) C=C vibrations.

present little shift in this small temperature range, which is consistent with the work by [24]. However, the intensity variation from the resonance Raman scattering results is relatively large and more obvious. Taking the advantage of the Dudik equation, we can calculate the C=C and C▬C modes Raman scattering cross section (RSCS). From **Figure 5**, with decreasing temperature, the RSCS declines, which means that the π electron modulation on CC vibration is strengthened. Finally, the observed Raman intensity enhances.

In order to get an evaluation on electron-phonon coupling strength, we use the method reported by [36] to calculate the electron-phonon coupling constants and their temperature dependence which can also reflect the effective conjugation length. The equations can be expressed in the simplified form as follows:

$$\mathbf{S} = \frac{I\_{\rm no}}{I\_{\rm no}} \,\tag{1}$$

(ω<sup>1</sup> = 1155 cm−1, ω<sup>2</sup> = 1520 cm−1). With the *I*

**3.2. Pressure dependence on the optical properties of β-Car**

Raman intensity enhancement.

1 , *I*

**Figure 6.** Temperature dependence on (a) C▬C and (b) C=C electron-phonon coupling coefficient.

2, and ω<sup>1</sup>

could be calculated. Combining this result with the value of HR factor from absorption spectrum, it is then allowed to derive the electron-phonon coupling constant for the dissolved β-Car molecule. The results are shown in **Figure 6**, which indicate that the modulation on CC modes becomes stronger and the electron-phonon coupling is enhanced. As a result, the CC modes show

Pressure dependence of absorption and resonance Raman scattering of β-Car was pioneered by [37] in the 1980s. Their research firstly showed that the absorption from β-Car under pressure presented a large red-shift and a strong broadening of the vibronic peaks. This work suggested that the assumption of linear coupling of pressure to configurational coordinate needed to be modified and the potential energy curves showed pressure-induced shift. Better information about this was reported later by their works on resonance Raman scattering of β-Car. However, they concluded that the configurational coordinate models and solvent models could not sufficiently explain the large shift in the electronic spectrum of β-Car under pressure [38]. The work by Ref. [39] studied the solvent effect and pressure effect on β-Car dissolved in n-hexane and carbon disulfide. It was concluded that the spectral response from β-Car should due to the environmental effect rather than structural distortion and considered high-pressure

technology as a potential way in exploring the biological functions of carotenoids.

shortened, and the π-electron delocalization is blocked and thus the energy increases.

The absorption and resonance Raman spectra of β-Car molecules at pressures of 0.04–0.60 GPa in carbon disulfide were measured. The experimental result (**Figure 7**) is that as the pressure increases, the absorption spectrum red-shifts and the reason should be the fact that the molecules are compressed and the π-electron energy gap decreases under pressure. The CC bond length is

, ω<sup>2</sup>

obtained by Raman spectra, the *V*<sup>1</sup>

External Field Effect on Electronic and Vibrational Properties of Carotenoids

http://dx.doi.org/10.5772/intechopen.78593

/ *V*<sup>2</sup>

51

$$\mathbf{S} = \frac{V\_1^2}{\omega\_1^2} + \frac{V\_2^2}{\omega\_2^{2\prime}} \tag{2}$$

$$\frac{I\_1}{I\_2} = \frac{\left(\frac{V\_1^\*}{\omega\_1^\*}\right)}{\left(\frac{V\_2^\*}{\omega\_2^\*}\right)}\tag{3}$$

where S is the Huang-Rhys factor, *I <sup>10</sup>* and *I <sup>00</sup>* can be extracted from absorption spectrum, which, respectively, represents the zero-phonon line intensity and the first vibrational peak intensity. The *V*1 and *V*<sup>2</sup> are the electron-phonon coupling constants, which lead to the broadening of absorption band and resonant enhancing of the two totally symmetric phonons observed in Raman scattering

**Figure 6.** Temperature dependence on (a) C▬C and (b) C=C electron-phonon coupling coefficient.

(ω<sup>1</sup> = 1155 cm−1, ω<sup>2</sup> = 1520 cm−1). With the *I* 1 , *I* 2, and ω<sup>1</sup> , ω<sup>2</sup> obtained by Raman spectra, the *V*<sup>1</sup> / *V*<sup>2</sup> could be calculated. Combining this result with the value of HR factor from absorption spectrum, it is then allowed to derive the electron-phonon coupling constant for the dissolved β-Car molecule.

The results are shown in **Figure 6**, which indicate that the modulation on CC modes becomes stronger and the electron-phonon coupling is enhanced. As a result, the CC modes show Raman intensity enhancement.

#### **3.2. Pressure dependence on the optical properties of β-Car**

present little shift in this small temperature range, which is consistent with the work by [24]. However, the intensity variation from the resonance Raman scattering results is relatively large and more obvious. Taking the advantage of the Dudik equation, we can calculate the C=C and C▬C modes Raman scattering cross section (RSCS). From **Figure 5**, with decreasing temperature, the RSCS declines, which means that the π electron modulation on CC vibration

In order to get an evaluation on electron-phonon coupling strength, we use the method reported by [36] to calculate the electron-phonon coupling constants and their temperature dependence which can also reflect the effective conjugation length. The equations can be

> \_\_10 *I* 00

2 \_\_\_ *ω*1 2 + *V*2 2 \_\_\_ *ω*2

> ( *V*2 2 \_\_\_ *ω*2 <sup>2</sup>)

respectively, represents the zero-phonon line intensity and the first vibrational peak intensity. The

band and resonant enhancing of the two totally symmetric phonons observed in Raman scattering

are the electron-phonon coupling constants, which lead to the broadening of absorption

\_\_1 *I* 2 = ( *V*1 2 \_\_\_ *ω*1 <sup>2</sup>) \_\_\_\_\_

*<sup>10</sup>* and *I*

, (1)

2, (2)

, (3)

*<sup>00</sup>* can be extracted from absorption spectrum, which,

is strengthened. Finally, the observed Raman intensity enhances.

**Figure 5.** Temperature dependence of RSCSs from (a) C–C and (b) C=C vibrations.

expressed in the simplified form as follows:

50 Progress in Carotenoid Research

<sup>S</sup> <sup>=</sup> *<sup>I</sup>*

<sup>S</sup> <sup>=</sup> *<sup>V</sup>*<sup>1</sup>

*<sup>I</sup>*

where S is the Huang-Rhys factor, *I*

*V*1 and *V*<sup>2</sup> Pressure dependence of absorption and resonance Raman scattering of β-Car was pioneered by [37] in the 1980s. Their research firstly showed that the absorption from β-Car under pressure presented a large red-shift and a strong broadening of the vibronic peaks. This work suggested that the assumption of linear coupling of pressure to configurational coordinate needed to be modified and the potential energy curves showed pressure-induced shift. Better information about this was reported later by their works on resonance Raman scattering of β-Car. However, they concluded that the configurational coordinate models and solvent models could not sufficiently explain the large shift in the electronic spectrum of β-Car under pressure [38]. The work by Ref. [39] studied the solvent effect and pressure effect on β-Car dissolved in n-hexane and carbon disulfide. It was concluded that the spectral response from β-Car should due to the environmental effect rather than structural distortion and considered high-pressure technology as a potential way in exploring the biological functions of carotenoids.

The absorption and resonance Raman spectra of β-Car molecules at pressures of 0.04–0.60 GPa in carbon disulfide were measured. The experimental result (**Figure 7**) is that as the pressure increases, the absorption spectrum red-shifts and the reason should be the fact that the molecules are compressed and the π-electron energy gap decreases under pressure. The CC bond length is shortened, and the π-electron delocalization is blocked and thus the energy increases.

**Figure 7.** Absorption spectra from β-Car dissolved in carbon disulfide under pressure from 0.04 to 0.60 GPa.

formation of the resonant Raman spectrum based on the linear polyene molecule is the result of the vibrational modulation process of the π-electron energy gap on the CC atom, which is the result of electron-phonon coupling. Similarly, according to **Figures 7** and **8**, using theory reported by [36], it is calculated that as the pressure increases, the electron-phonon coupling coefficient increases (**Figure 9**), that is, the electron-phonon coupling strength decreases. The π-electron energy gap weakens the modulation of the CC bond vibration. It is thus able to conclude that as the pressure increases, the β-Car molecule is compressed, the energy of the system increases, and the π-electron energy gap decreases. Therefore, the π-electron modu-

 and *V2 .*

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53

lates the CC vibration, the Raman spectrum blue-shifts, and the intensity decreases.

The polarizability, polarity, density, and dielectric constant of the solvent will affect the system energy and energy gap of the π electron and thus influence the π-electron energy gap modulation on CC vibration [40]. The absorption spectra and resonance Raman spectra of β-Car in 10 different polarizability solvents were measured. As the polarizability increases,

This is because when the β-Car molecule is in the vibrational ground state, two π electrons in C=C are in the π bond orbital, and there is no polarity. When the π-π\* transition occurs, π electrons are in the π-bonded orbitals and the π\* anti-bond orbits, respectively, and lead to generate polarities. The excited state polarity is stronger, so that the energy drop is greater than the ground-state energy. Therefore, the difference between the π electron energies becomes smaller. At the same time, as the polarizability increases, the concentration of the solution density increases, the space of the β-Car molecule swing decreases, and the order of the molecular structure increases. In this context, the effective conjugate length increases.

**3.3. Solvent effect on the optical properties of β-Car**

**Figure 9.** Pressure dependence on electron-phonon coupling coefficient *V1*

the absorption spectra of β-Car red-shifts (**Figure 10**).

**Figure 8.** Raman spectra of C▬C (1155 cm−1) and C=C (1520 cm−1) bond in different pressures.

It is evident from **Figure 8** that as the pressure increases, the Raman spectra blue-shift and the intensity of the Raman spectra decreases with increasing pressure. Obviously, the absorption spectra and Raman spectra changes are different very much from the effects of temperature on β-Car molecules. When the temperature is lowered, the absorption spectrum and the Raman spectrum are red-shifted, and the spectral intensity is increased. This is due to the π-electron energy gap that enhances the modulation of the CC bond vibration. Electronphonon coupling enhancement is perfectly explained by theories such as "effective conjugate length" and "coherent weakly damped vibration" of linear polyene molecules. When under pressure, the absorption spectrum of β-Car is red-shifted, the Raman spectrum is blue-shifted, and the Raman intensity is reduced. The above theory cannot be explained satisfactorily. The

**Figure 9.** Pressure dependence on electron-phonon coupling coefficient *V1* and *V2 .*

formation of the resonant Raman spectrum based on the linear polyene molecule is the result of the vibrational modulation process of the π-electron energy gap on the CC atom, which is the result of electron-phonon coupling. Similarly, according to **Figures 7** and **8**, using theory reported by [36], it is calculated that as the pressure increases, the electron-phonon coupling coefficient increases (**Figure 9**), that is, the electron-phonon coupling strength decreases. The π-electron energy gap weakens the modulation of the CC bond vibration. It is thus able to conclude that as the pressure increases, the β-Car molecule is compressed, the energy of the system increases, and the π-electron energy gap decreases. Therefore, the π-electron modulates the CC vibration, the Raman spectrum blue-shifts, and the intensity decreases.

#### **3.3. Solvent effect on the optical properties of β-Car**

It is evident from **Figure 8** that as the pressure increases, the Raman spectra blue-shift and the intensity of the Raman spectra decreases with increasing pressure. Obviously, the absorption spectra and Raman spectra changes are different very much from the effects of temperature on β-Car molecules. When the temperature is lowered, the absorption spectrum and the Raman spectrum are red-shifted, and the spectral intensity is increased. This is due to the π-electron energy gap that enhances the modulation of the CC bond vibration. Electronphonon coupling enhancement is perfectly explained by theories such as "effective conjugate length" and "coherent weakly damped vibration" of linear polyene molecules. When under pressure, the absorption spectrum of β-Car is red-shifted, the Raman spectrum is blue-shifted, and the Raman intensity is reduced. The above theory cannot be explained satisfactorily. The

**Figure 8.** Raman spectra of C▬C (1155 cm−1) and C=C (1520 cm−1) bond in different pressures.

**Figure 7.** Absorption spectra from β-Car dissolved in carbon disulfide under pressure from 0.04 to 0.60 GPa.

52 Progress in Carotenoid Research

The polarizability, polarity, density, and dielectric constant of the solvent will affect the system energy and energy gap of the π electron and thus influence the π-electron energy gap modulation on CC vibration [40]. The absorption spectra and resonance Raman spectra of β-Car in 10 different polarizability solvents were measured. As the polarizability increases, the absorption spectra of β-Car red-shifts (**Figure 10**).

This is because when the β-Car molecule is in the vibrational ground state, two π electrons in C=C are in the π bond orbital, and there is no polarity. When the π-π\* transition occurs, π electrons are in the π-bonded orbitals and the π\* anti-bond orbits, respectively, and lead to generate polarities. The excited state polarity is stronger, so that the energy drop is greater than the ground-state energy. Therefore, the difference between the π electron energies becomes smaller. At the same time, as the polarizability increases, the concentration of the solution density increases, the space of the β-Car molecule swing decreases, and the order of the molecular structure increases. In this context, the effective conjugate length increases.

**Figure 10.** Absorption spectra of β-Car in (a) polar solvents and (B) non-polar solvents.

The reduction of the π-electron energy gap causes red-shift of the electron absorption peak. **Figure 11** shows the resonant Raman spectra of measured C▬C, C=C modes of β-Car molecules. Extracting data from **Figure 11**, it can be calculated that as the solvent polarizability increases, the Raman scattering cross section of the CC bond increases. The increase in the polarizability and the increase in the molecule induced dipole moment.

According to **Figures 10** and **11**, using the method of calculating electron-phonon coupling constant by [36], the variation law of electron-phonon coupling coefficient with solvent polarizability is obtained (**Figure 12**). It is able to conclude that as the polarizability increases, the π-electron energy gap enhances the modulation of the CC bond vibration, the electron-phonon coupling coefficient decreases, the electron-phonon coupling increases, the Raman activity increases, and the Raman cross section increases. It is clear that as the solvent polarizability increases, the π-electron energy gap increases the modulation of the CC bond vibration.
