**2. Electronic and vibrational properties of carotenoids**

photochemistry including energy transfer, electron transfer, radical pair recombination observed in natural photosynthetic reaction centers, and antennas can be mimicked [2, 3]. They also have a protective function against the drastic change of the environment, that is, the dissipation of excess energy. Molecular properties of carotenoids are sufficiently studied using multiple methods. Understanding the intermolecular interactions is one of the primary objectives. Correlating the intermolecular interactions with physical properties in a condensed phase will help in interpreting the structure dependence of carotenoids in different environments [3, 4]. The all-trans-β-carotene (β-Car), with 11 conjugated double bonds, has unique biochemical and optical properties and is considered to be a model in studying conjugated polyenes. Conjugated polyenes are electroluminescent materials that are well studied because of their significance in both physics and chemistry [5–9]. This fact, combined with their functional properties of semiconductor and their solubility in common organic solvent, makes the conjugated polyenes a new class of electronic plastic materials with potential applications in electro-optical and photovoltaic devices. Many organic thin films have been fabricated and their physical properties have been examined in order to develop fine nonlinear optical devices. Carotenoids, although due to their poor structural stability, can be a good model to investigate because of their large nonlinear optical susceptibilities [9–12]. Different types of β-Car thin-films were fabricated, and their optical absorption and resonance Raman spectroscopy were examined. Photoinduced and time-resolved absorption spectroscopy reveal that the infrared absorption bands may correspond to the recombination of the soliton-like excitations in β-Car single crystals [13–16]. Spectroscopic properties and dynamics of the excited singlet states have been investigated in different liquids or polymers as a function of refractive index [17], static permittivity [18], temperature [19], pressure [20], and external electric field [21–23]. Although the solvated carotenoids have been extensively studied, the temperature dependence of Raman-active modes of carotenoids remained unknown until the work by reference [24]. The results suggested that the spectral properties of CC-stretching modes are very sensitive to the temperature, which was similar to that of polyconjugated molecules. In addition, they revealed that the temperature effect and vibronic coupling together contributed to the observed Raman mode shifts. The position of v1 Raman band versus conjugation chain length was revealed by combining the use of electronic absorption and Raman spectroscopy [25]. Carbon disulfide and pyridine have been chosen as solvent because of their polarizability, which are close to that of membrane lipids and proteins [26]. Observations based on different laser excitation of β-Car provided strong evidence that the enhancement of the Raman bands should be explained by electron-phonon coupling mechanism [27]. Another model, named coherent weak-damping CC vibration model, also indicated that the unusual strong intensity enhancement of polyene's overtone bands should be contributed by π-electronphonon coupling [28]. Recent resonance Raman spectroscopic study in vivo concluded that the use of a high polarizability solvent was necessary to mimic the electrostatic environment in vivo. However, more experiments are needed on excited-state and ground-state dynamics dependence on environmental effect. Therefore, the external effects on electronic absorption and Raman scattering should be investigated and electron-phonon coupling strength in different external fields should also be examined. In this review, temperature, pressure, solvent polarizability, and phase transition effect on CC vibration modes and electron-phonon coupling are studied. The results reveal that the electron-phonon coupling can be deduced by

46 Progress in Carotenoid Research

β-Car has nine CC double bonds (C=C) and single bonds (C–C) and is a typical model of π-electron system. The work in early 1970s by references [29, 30] proposed that the lowestlying excited state S1 (21 Ag − ) was silent in absorption spectrum. The most likely reason is that the Franck-Condon factors for a vertical transition are negligible since the final state is massively displaced [31]. The strong absorption of β-Car happened from π-π\* transition, which is also called S0 (11 Ag − )-S2 (21 Bu + ) transition. The excited S<sup>2</sup> state decays by internal transition to S1 , and itself decays to ground state S0 in the same way. Recently, other "dark" states S\* have been proposed for relaxation pathways in several computational chemistry [32]. The S<sup>0</sup> –S2 transition displays a characteristic three peak structure by promoting a single electron from its ground state to the lowest allowed excited state which are termed 0–0, 0–1, and 0–2 (**Figure 1**). Resonance Raman scattering is ideally vibrational technique for observation on carotenoids electronic ground state. Four main bands are observed in resonance Raman spectrum. **Figure 2** shows the resonance Raman spectrum from β-Car, four main fundamental bands are labeled from *v*<sup>1</sup> to *v*<sup>4</sup> . The most intense band is *v1* from C=C vibration, which depends strongly on π-electron conjugation and molecular configuration. The *v2* bands arise from C–C bands stretching coupled with C–H in-plane bending modes [34]. The *v*<sup>3</sup> band around 1000 cm−1 is assigned as in-plane rocking vibrations of the methyl groups and *v*<sup>4</sup> band near 960 cm−1 arises from C–H out-of-plane wagging motions

**Figure 1.** Absorption of β-Car dissolved in cyclohexanol. Line a represents the experimental curve and B is the fitting curve. Components C, D, and E are Gaussian deconvolution of the experimental curve (see Ref. [35]).

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

coupled with C=C torsional modes. The overtone 2*v*<sup>1</sup> (2310 cm−1), 2*v*<sup>2</sup> (3040 cm−1), and combination *v*<sup>1</sup> + *v*<sup>2</sup> (2675 cm−1) are also appeared in the resonance Raman spectrum.

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

External Field Effect on Electronic and Vibrational Properties of Carotenoids

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

49

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