**3.4. Phase transition on the optical properties of β-Car**

Investigation on β-Car solution in different phases should give more information on its electronic and vibrational states. Physical properties of carotenoids in a condensed phase were pioneered by Hashimoto et al., whose study included a wide range of researches on different types of films and their optical properties. Not surprisingly, their work provides valuable information on electronic and vibrational dynamics of optically forbidden 21 Ag -Ag transition. Their observation of soliton-like excitations gave a new method in studying the physical properties of carotenoids and related their optical properties to the one-dimensional conducting polymers. The physical properties of β-Car in solid phase and liquid phase are quite different due to the polyene structure and the solvent effect. These two effects jointly lead to variation on optical response when the solution is in different phases.

**Figures 13** and **14** show the absorption and resonance Raman spectra of β-Car dissolved in cyclohexanol at 65–22°C. As the temperature decreases, the solution undergoes liquid-solid phase change at 20°C. From **Figure 13**, we can find that with a decreasing temperature, the

**Figure 12.** The relationship between electron-phonon coupling coefficient and polarizability in (a) non-polar and (b)

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**Figure 11.** Raman spectra of β-Car in (a) non-polar solvents and (B) polar solvents.

polar solvents.

External Field Effect on Electronic and Vibrational Properties of Carotenoids http://dx.doi.org/10.5772/intechopen.78593 55

**Figure 11.** Raman spectra of β-Car in (a) non-polar solvents and (B) 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

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.

Investigation on β-Car solution in different phases should give more information on its electronic and vibrational states. Physical properties of carotenoids in a condensed phase were pioneered by Hashimoto et al., whose study included a wide range of researches on different types of films and their optical properties. Not surprisingly, their work provides valuable

Their observation of soliton-like excitations gave a new method in studying the physical properties of carotenoids and related their optical properties to the one-dimensional conducting polymers. The physical properties of β-Car in solid phase and liquid phase are quite different due to the polyene structure and the solvent effect. These two effects jointly lead to variation

Ag -Ag

transition.

information on electronic and vibrational dynamics of optically forbidden 21

polarizability and the increase in the molecule induced dipole moment.

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

54 Progress in Carotenoid Research

**3.4. Phase transition on the optical properties of β-Car**

on optical response when the solution is in different phases.

**Figure 12.** The relationship between electron-phonon coupling coefficient and polarizability in (a) non-polar and (b) polar solvents.

**Figures 13** and **14** show the absorption and resonance Raman spectra of β-Car dissolved in cyclohexanol at 65–22°C. As the temperature decreases, the solution undergoes liquid-solid phase change at 20°C. From **Figure 13**, we can find that with a decreasing temperature, the

**Figure 13.** Absorption spectra of β-Car dissolved in cyclohexanol in different temperatures.

With the decrease of temperature, the CC modes red-shift and their intensity enhance. The shift magnitude becomes larger after the solution phase transition, which is consistent with the absorption results. In the solid phase, there is no Brownian motion, the molecular density increases, and the movement of the β-Car molecule is prevented. Increasing the molecular structure of β-Car in an orderly manner reduces the energy of the system, the π-electron energy gap is greatly affected by temperature, the modulation of CC bond vibration is enhanced, the coupling of electron-phonon is enhanced, and the Raman-active mode red-shift is accelerated. The spectral intensity increases. According to the relationship between energy

**Figure 15.** Temperature dependence of β-Car molecular electron-phonon coupling coefficient λ (A) before and (B) after

relationship between temperature and coupling coefficient can be calculated [41]. **Figure 15** shows the relationship between the electron-phonon coupling coefficient and the temperature

Resonance Raman spectroscopy is one of the main spectral technologies for the study of linear polyene molecules. Raman spectroscopy is the result of π-electron energy gap modulation of CC atoms. External fields such as temperature, pressure, solvent effect, and phase transition have influence on the degree of ordering of polyene molecules, the effective conjugate length, and the degree of electron delocalization, and the physics behind the phenomenon of the influence by different external fields on these characteristics are not the same. Changing the system energy, the absorption spectrum would show a red-shift or a blue-shift, and the

~exp.(1/2λ), as well as λ and ω1, ω2, 2λ~(ω<sup>1</sup>

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 ω<sup>2</sup> ω<sup>3</sup> )2 , the

gap Eg

phase transition.

**4. Conclusion**

and the coupling coefficient E<sup>g</sup>

before and after the phase change.

**Figure 14.** Raman spectra of C▬C and C=C vibration of β-Car dissolved in cyclohexanol in different temperatures.

absorption spectra of β-Car molecules red-shift when the solution is in both liquid and solid phases. The solid phase (R = 2.24 nm/°C) absorption spectral red-shift is greater than that of liquid phase (R = 0.23 nm/°C). This result shows that the electron-phonon coupling of β-Car in solid phase has a more sensitive temperature dependence compared to the liquid phase. From **Figure 14**, the resonance Raman spectra show a similar changing tendency upon temperature.

**Figure 15.** Temperature dependence of β-Car molecular electron-phonon coupling coefficient λ (A) before and (B) after phase transition.

With the decrease of temperature, the CC modes red-shift and their intensity enhance. The shift magnitude becomes larger after the solution phase transition, which is consistent with the absorption results. In the solid phase, there is no Brownian motion, the molecular density increases, and the movement of the β-Car molecule is prevented. Increasing the molecular structure of β-Car in an orderly manner reduces the energy of the system, the π-electron energy gap is greatly affected by temperature, the modulation of CC bond vibration is enhanced, the coupling of electron-phonon is enhanced, and the Raman-active mode red-shift is accelerated. The spectral intensity increases. According to the relationship between energy gap Eg and the coupling coefficient E<sup>g</sup> ~exp.(1/2λ), as well as λ and ω1, ω2, 2λ~(ω<sup>1</sup> ω<sup>2</sup> ω<sup>3</sup> )2 , the relationship between temperature and coupling coefficient can be calculated [41]. **Figure 15** shows the relationship between the electron-phonon coupling coefficient and the temperature before and after the phase change.

#### **4. Conclusion**

**Figure 14.** Raman spectra of C▬C and C=C vibration of β-Car dissolved in cyclohexanol in different temperatures.

**Figure 13.** Absorption spectra of β-Car dissolved in cyclohexanol in different temperatures.

56 Progress in Carotenoid Research

absorption spectra of β-Car molecules red-shift when the solution is in both liquid and solid phases. The solid phase (R = 2.24 nm/°C) absorption spectral red-shift is greater than that of liquid phase (R = 0.23 nm/°C). This result shows that the electron-phonon coupling of β-Car in solid phase has a more sensitive temperature dependence compared to the liquid phase. From **Figure 14**, the resonance Raman spectra show a similar changing tendency upon temperature. Resonance Raman spectroscopy is one of the main spectral technologies for the study of linear polyene molecules. Raman spectroscopy is the result of π-electron energy gap modulation of CC atoms. External fields such as temperature, pressure, solvent effect, and phase transition have influence on the degree of ordering of polyene molecules, the effective conjugate length, and the degree of electron delocalization, and the physics behind the phenomenon of the influence by different external fields on these characteristics are not the same. Changing the system energy, the absorption spectrum would show a red-shift or a blue-shift, and the corresponding electron-phonon coupling increases or decreases. Finally, different modulation modes are generated for the C–C and C=C vibration. In general, when the system energy decreases, the π electron energy gap is reduced. With a decreasing energy gap, the modulation is enhanced, and the electron-phonon coupling is strengthened. Raman spectra will redshift and their intensity is enhanced and vice versa.

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