A Study on the Optoelectronics Parameters of Natural Dyes Extracted From Beetroot, Cabbage, Walnut and Henna For Potential Applications in Organic Electronics

In this work, the optoelectronic parameters of natural dyes extracted from beetroot, red cabbage, walnut leaves, and henna are determined and investigated. These include optical energy gap ( E g ), extinction coefficient ( k ), refractive index ( n ), dielectric constant ( 𝜀 𝑟 ) and optical conductivity ( 𝜎 𝑟 ) . Results showed that cabbage dye exhibited a maximum values of 𝑛 = 6.08, 𝜀 𝑟 = 37.18 and 𝜎 𝑟 = 13.92 × 10 −3 𝑆/𝑐𝑚 , while henna dye presented the lowest values of 𝑛 = 1.19, 𝜀 𝑟 = 1.5 and 𝜎 𝑟 = 0.039 × 10 −3 𝑆/𝑐𝑚 compared to the optoelectronic parameters of the other studied samples. The optical absorption in the dyes were found to obey a direct allowed transition between the molecular energy levels. The results presented here can be of a special importance for some potential applications in organic electronics such as organic photovoltaics and sensors.


Introduction
Nowadays, the use of organic dyes has become the focus of interest in both academia and industry owing to the unique features offered by the dyes including cost-effectiveness, flexibility, electronic tuneability and light weight properties [1][2][3]. Furthermore, organic dyes show higher absorption coefficient and stronger exciton generation compared to those of the inorganic materials [4,5].
Commercial dyes are commonly used in various applications, but they suffer from high cost, supply shortage, heavy metal toxicity and challenging synthesis, which ultimately limit their broad usage compared to the natural dyes. Alternatively, it is possible to overcome these limitations by utilizing natural dyes [5,6].
Dyes are organic compounds which are capable of well absorbing light and retaining their intrinsic color because of the presence of chromophore [7]. Betalains are vacuolar nitrogenous plant pigments having a core structure known as betalamic acid [8]. The best-known source of betalains can be beetroot and cacti [9]. Anthocyanins was also found in a large group of plants which contain red-blue pigments, which is water soluble and pH sensitive [10]. Red cabbage is one of the main sources of anthocyanins and are chemically glycoside moieties of anthocyanidins derived from the flavylium [11]. In addition to different natural pigments, brown dye and red-orange dye were found in juglone (black Walnuts) and lawsone (henna plant), respectively [12,13]. A review of literature showed that organic dyes have been successfully utilized in photovoltaic devices [14][15][16][17][18][19][20].
Additionally, organic dyes can be of a special interest for the coloration technology and various organic electronics applications. However, before these materials are highly employed for viable applications, a broad investigation on their optoelectronic properties is necessary. Therefore, in the current research work, different natural dyes are extracted through a green method and their optoelectronic parameters are comprehensively investigated. The natural dyes were extracted from beetroot, red cabbage, walnut leaf, and henna using specified chemical solvents. This can be a unique detailed study which is performed to determine the most important parameters of the dyes including optical energy gap, refractive index, dielectric constant, and optical conductivity. The results are primarily important when these natural dyes are considered for the application of organic electronics.

Materials and methods
The Beetroots, Red Cabbages and Henna were purchased in the local markets, while Walnut leaves were collected in a local farm. The raw materials were cut into small pieces and crushed by a grinder followed by drying process to produce very fine powders. To extract the dyes, ethanol, distilled water and a combination of ethanol-HCl were used. Table 1 shows the designation of the extracted samples and the used solvents for this purpose. In a first approach, 100 ml distilled water was put into a specified beaker then boiled on a hotplate for 2 minutes. Later on, the beakers were transferred, and 50 g of each sample was inserted into the boiled water, separately. The prepared solutions of sample B1 and C1 were left for 2 hours and then filtered and stored in the refrigerator for 24 hours. In the second approach, 100 ml of ethanol was mixed with HCl (0.1) with the ratio of 99:1, then 50 g of samples B2 and C2 was put into the beakers and left for 2 hours. Afterwards, filtration process was performed, and the extracted solutions were stored in the refrigerator for 24 hours. In the third approach, 200 g of the mashed materials of sample B3 and sample C3 was separately put into a beaker containing distilled water as a solvent, then the beaker was transferred into a hot bath and agitated at 60℃ for 6 hours. After filtration process, the prepared solutions were put into the oven at 50℃ until they became a paste. Later, 0.1 g of the paste was diluted using 10 ml of distilled water. Finally, a Soxhlet extractor was used for the extraction of dyes with ethanol solvent. The same procedure of preparation, filtration and extraction were performed for the other samples.
To extract the dye from walnut leaves, 150 g of its powder was immersed in 500 ml of ethanol and left for 24 hours in a hot bath at 60℃. The extracted samples are shown in Figure 1.   Where t is the thickness of the cuvette (q1 cm) and A is the absorbance. For the beetroot extract, the absorption spectrum indicated the presence of betalain chromophore [10], as showed multiple peaks of absorption around 475, 525 and 575 nm due to the presence of mixed pigments. These pigments are yellow betaxanthins (indicaxanthin), red-purple betacyanin (betanin) and betanidin respectively [22]. It was reported that the batlaian dyes can be affected by temperature and pH [23][24][25]. Figure 2(a) shows the absorption coefficient spectrum of beetroot dye which was extracted by different methods. Results showed that all samples are strongly absorbent in the ultraviolet region, which can be ascribed to the presence of red-violate betalain group [24]. However, different absorption peaks in the visible region have been observed. Sample B2 showed absorption peaks at 486 and 549 nm, which suggested that the extracts contain both betaxanthins and betanidin, and one peak in B3 sample was observed in the visible region at 417 nm, due to the presence of indicaxanthin. However, no peaks were observed in the visible region of samples B1 and B4.

Results and discussion
The absorption spectra of the solutions from red cabbage are shown in Figure 2(b). One can notice that all the samples show a strong absorption in the UV region, which is well matched to the absorption characteristic of anthocyanin dye. This strong absorption in the UV region is due to the conjugated aromatic phenolic double bonds in anthocyanin [26]. Sample C1 and C2 have almost the same absorption peaks around 560 nm, indicating the presence of anthocyanidin [27]. The peaks between 360 nm to 470 nm are belong to a non-acylated pigment [28], which are observed in all samples except C2.  Henna. It can be seen that both dyes exhibited similar spectrum due to their comparable chemical structures. The broad absorption in UV region is referred to the presence of intermolecular hydrogen bonds from benzoquinone and electron delocalization of benzoquinone converts a quinone ring. In addition, the UV absorption points the existence of naphthoquinone in the form of hydrogen bonds with carbonyl [29]. However, the absorption in visible region (blue and red light) are corresponding to the mixture of chlorophyll [30], these peaks were also observed for both extracted dyes. assign the nature of absorption transitions is highly efficient [31,32] despite its seldom use by the researchers [33][34][35][36][37][38]. The Tauc's equation can be represented as follows [39]: where is an energy-independent constant, is the energy gap, and the value of s determine the type and nature of transition [40] = 1 2 for direct allowed transition , 3 2 for direct forbidden transitions, respectively, and this exponent value equals to 2 for the indirect allowed transitions  , which indicated the presence of a direct allowed transition between the intermolecular energy bands of the dyes.
Afterwards, the precise value of the energy gap was determined by Tauc's equation through plotting ( ℎ ) 2 as a function of (ℎ ) and taking the extrapolation of linear portion at ( ℎ ) 2 = 0.
The energy gap location of the samples is depicted in Figure 4 (a-d)and the determined values of are shown in Table 2. Results showed that energy gap for each dyes extracted from beetroot and red cabbage are different because they are pH and temperature sensitive. Noticeably, the dyes extracted from henna and walnut processed a very close value of energy gap due to similarity in their structure.    were calculated from the absorbance data using the following equations [21]: Where R is the reflectance and is the absorption coefficient. They were calculated using the  Table 2. It was observed from Table 2 that the value of n is changed with respect to the method of extraction and the solvent used for extraction. The red cabbage dyes experienced a high value of refractive index (C3), while henna has the lowest value of n.
Furthermore, Figure 5 (e-h) shows the extinction coefficient (k) as a function of wavelength for all the dye samples. Based on equation 5, it can be seen that the variation in k is almost similar to the corresponding absorption coefficient, as shown in Figure 2 (a-d). The variation of (k) represent the loss of incident electromagnetic wave because of scattering and absorption per unit thickness in the medium [33]. For most of the studies dyes, the maximum values of (k) was observed to be in the UV region.   the material can be polarized due to the influence of electromagnetic field, while the imaginary part shows the optical loss and is described by equation 8. The optical loss takes place because of the time lag of /2 between the electric field of the incident photon and the component of electronic or atomic polarization in the optical frequency range [41].
Noteworthy, the real part of dielectric constant resembles the refractive index due to small value of k. However, the imaginary part is mainly dependent on the absorption coefficient (see Equation 7and 8). The variation in real dielectric constant for all the extracted dyes is shown in Figure 6(ad). It was seen that the peak observed in the UV region is belong to B2 sample, while other samples show their characteristic peaks at different wavelengths. It has been reported that the higher value of real dielectric constant indicates the presence of interaction between electron and incident photon in the extracted dyes [35]. It worth to note that the mixed dye of anthocyanin and betalin dye has a broad peak ranging from 400 to 600 nm ( Figure 6-c), whereas walnut leaves and henna has a sharp peak around 500 and 700 nm. The values of real dielectric constants are listed in Table   2, which were determined from the intersection of extrapolation of the plateau region to y-axis.
Dyes extracted from red cabbage, especially C3 presented the maximum value of 1 , while dyes extracted from henna has lowest value. One can notice that the value of 1 for the extracted dyes with water solvent is greater compared to those extracted with ethanol and HCl solvent. This might be due to the polar nature of water. In addition to dielectric constant, Figure 6 (e-h) shows the imaginary dielectric constant of all the extracted dyes as a function of wavelength. It can be concluded that the variation of imaginary part of dielectric constant is related to the absorption coefficient of the dyes and their mixtures, as shown in Figure 6-g with a broad peak across UV and visible region. Furthermore, the dissipation factor (tan ) was calculated from equation 8 by dividing the imaginary part to the real part of dielectric function [31]. The spectra of dissipation factors for all samples were presented in Figure 7(a-d), wherein each dye presented different characteristic peaks regardless of whether extracted from beetroot or red cabbage has different peak value and it's depend the type due to the effect of solvents and extraction methods. The anthocyanin dye, which was extracted from red cabbage with distilled water as a solvent (C3), has maximum peak value in the range of 400 to 500 nm compared to that of betalain dye extracted from beetroot with the same solvent and extraction method. This is ascribed to the rate of absorption of anthocyanin which is greater than that of the betalain dye, whereas their mixture showed a broad peak in the UV region. However, walnut leave and henna showed a broad peak expanded to 500 nm and a sharp peak near 700 nm.     To determine the electronic characteristics of a material, optical conductivity is a powerful tool to be used, which describes the response of electron to the absorbed photon. Since the optical conductivity is a complex variable, it is derived from optical dielectric constant [36]: * = + … … … … . (10) Where and are the real and imaginary part of complex optical conductivity respectively, is the angular frequency, and is the permittivity of free space. One can conclude from equation 10 that the real optical conductivity is related to the imaginary optical dielectric constant and hence depends on the absorption coefficient. Thus, increasing the magnitude of real optical conductivity is directly related to the transport response of electrons which are excited by the absorbed photon energy [42]. In contrast, the imaginary optical conductivity is related to the real optical dielectric constant hence represents the interaction between electromagnetic wave and electrons in the form of polarization. The spectra of real and imaginary optical conductivity as shown in Figure 8 and the value of real optical conductivity are listed in Table 2. It was concluded that anthocyanin dye (C3) has the maximum value, which is = 13.92 × 10 −3 / , while the extracted dye from henna has the lowest value of = 0.039 × 10 −3 / . In addition, it was seen from the spectra of real conductivity that both betalain and anthocyanin dyes extracted by distilled water as a solvent (B3&C3) have a broad peak over UV and visible region. The henna and walnut leaves have a broad peak starting from UV region until 500 nm in visible region and a sharp peak at near IR region. Figure 8 (e-h) shows the imaginary spectra of the optical conductivity and they are similar to real optical dielectric constant spectra. Noticeably, the combination of two dyes together has led to a breaded photo-absorption in the UV region, indicating that molecular interactions between their moieties are mostly taken place in the high energy levels.  Walnuts leaves and Henna.

Conclusions
Natural dyes were successfully extracted from beetroot, red cabbage, walnut and henna using a facile approach and were broadly characterized. A broad spectrum of optoelectronics parameters of the samples such as absorption coefficient, optical band gap, dispersion parameters, and optical conductivity was investigated. The UV results showed that dyes extracted from beetroot and red cabbage are btalain and anthocyanin derivatives, respectively. However, dyes in henna and walnut have almost similar spectra due to their similar aromatic conjugation double bonds. The cabbage dye showed maximum value of refractive index, real optical dielectric constant, real optical conductivity, while henna showed the minimum value for these parameters. By taking derivatives of Tauc's equation, the type of electronic transitions was estimated to be direct allowed transition for all the dyes.