**1. Introduction**

Carotenoid pigments are found in organisms from many phyla and in all wild-type photosynthetic bacteria, algae, and higher plants examined to date [1, 2]. They are important components of biomimetic photosynthetic constructs where much of the photophysics and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 using the electronic band gap and Raman intensity. Different mechanisms are introduced to

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 lowest-

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,

states S\* have been proposed for relaxation pathways in several computational chemistry

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,

C=C vibration, which depends strongly on π-electron conjugation and molecular configu-

**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]).

) transition. The excited S<sup>2</sup>

transition displays a characteristic three peak structure by promoting a

to *v*<sup>4</sup>

bands arise from C–C bands stretching coupled with C–H in-plane bending

band around 1000 cm−1 is assigned as in-plane rocking vibrations of the

band near 960 cm−1 arises from C–H out-of-plane wagging motions

) was silent in absorption spectrum. The most likely reason is

External Field Effect on Electronic and Vibrational Properties of Carotenoids

state decays by internal

from

47

in the same way. Recently, other "dark"

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

. The most intense band is *v1*

explain the external field dependence on β-Car electronic and vibrational properties.

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

, and itself decays to ground state S0

lying excited state S1

which is also called S0

–S2

transition to S1

[32]. The S<sup>0</sup>

ration. The *v2*

modes [34]. The *v*<sup>3</sup>

methyl groups and *v*<sup>4</sup>

(21 Ag −

> (11 Ag − )-S2 (21 Bu +

four main fundamental bands are labeled from *v*<sup>1</sup>

using the electronic band gap and Raman intensity. Different mechanisms are introduced to explain the external field dependence on β-Car electronic and vibrational properties.
