Natural Light Harvesting Materials: A Study on Optoelectronic Properties of Potential Natural Dyes

Natural dyes are being highlighted by research and industry trends; the majority of plant species offer abundant sources of colouring compounds. They can be harvested using affordable technology and used in cutting-edge applications. Hence, an examination of the optoelectronic characteristics of Carissa caranda fruit dye has been looked into in this paper. Anthocyanins present in Carissa caranda were extracted by solvent extraction of the fruits with ethanol and methanol solvents, maintaining pH at 7 under room temperature. UV-visible spectroscopy analyzed the dye extracts, and the optoelectronic properties such as optical energy band gap, extinction coefficient, and refractive index, are studied. UV-Vis investigations revealed large absorption peaks in the visible area and obvious band gaps. Carissa caranda natural dye had the lowest direct bandgap of 2.98eV and an indirect bandgap of 1.93eV for ethanolic extraction. It was found that the optical absorption in the dyes obeyed both direct and indirect transitions between the molecular energy levels. FTIR spectroscopy has been used to confirm the composition of the natural dyes. The findings here may be particularly significant for organic electronics applications, including O-LEDs and sensors.


Introduction
Natural colours produced from flora are safe since they are non-toxic, biodegradable, and noncarcinogenic [1].They act as an alternative dye that is better for the environment and does not cause pollution or wastewater problems [2].Colours can be extracted from a wide variety of plant, animal, and insect sources.Over 500 different plant species have been used to create natural dyes in India [3].Due to the special characteristics these materials offer, such as flexibility, optoelectronic tuneability, and cost-effectiveness, the application of organic dyes has recently attracted attention from both industry and academia [4][5][6].Since the dawn of time, plant sources have provided a plethora of coloured dyes, many of which are highly fluorescent [7].Therefore, materials with light-sensitive optical properties, such as nonlinear optical materials, have garnered increased interest.Carissa carandas is a species of flowering shrub in the family Apocyanaceae, commonly known as Karonda.[11].The carissa fruit has been a highly potent cosmeceutical agent [8,9].karonda leaves shows Antioxidant activity and DNA damage inhibition, anti-inflammatory properties [10,11], organoleptic qualities [12].Karonda fruits are employed in the Catalytic Application of Silver Nanoparticles [13], textile dyeing [14], the food industry, confectionaries [15], and nutraceutical supplements [16].However, the optoelectronic properties of the flower extracts have not yet been studied, though.Due to these factors, two different solvents were used in the current study to extract the dye from Carissa caranda fruits.The UV-visible and FTIR spectroscopy have been used to analyzed.Exploring the feasibility of these natural dyes for prospective use in organic electronics, in particular, is made relevant by the results that have been described.

Experimental 2.1 Materials and methods
The Carissa caranda fruits of consistent dark red colour having no physical damage were collected from the lush campus of Bangalore University.The fruits were thoroughly cleaned with distilled water, then the seeds were removed and the fruits were taken for the extraction of the colour.The extraction solvents ethanol, and methanol, were of AR grade and purchased from S.D. Fine chemicals.An FTIR (Fourier transform infrared) spectrophotometer (Bruker alpha I ATR FTIR), and ocean optics UV-Vis-NIR spectrometer (USB4J00022) are used to evaluate the dye extract.

Extraction of natural dye
It was established from the literature that ethanol, because of its increased solubility, was the optimum solvent for color extraction [15].We have extracted dye in two different solvents Ethanol and Methanol, by employing the same extraction procedure (Figure 1).Investigations into the activation and inactivation of dye components with absorption in both the solvent media were conducted For the karonda dye extraction, 10g of deseeded fruits were pounded into a paste in a mortar.The pulp was then extracted and transferred to a beaker containing 100 mL of methanol.The mixture was then allowed to rest for 24 hours in a dark room to improve the dye extraction.After filtration, further analysis was carried out.

The UV-Vis absorption spectroscopy
The most prominent characterization technique to measure the absorbance of a material is UV-visible spectrophotometry.Furthermore, bandgap and other optical properties of the material can be investigated.From the absorbance spectra, the electronic transitions and band structure pertaining to that material can be calculated [17].Figure 2(a) shows the absorption spectra of the Karonda dyes extracted in ethanol and methanol solvents.The primary anthocyanin pigment in karonda fruit dye was identified by HPLC analysis as cyanidin 3-O-glucoside [18].(1) It is calculated using the values of the absorbance where A is the absorbance and t is the cuvette thickness.Although there is a slight shift in the peak positions, the dye extracts in both solvents exhibit a comparable absorbance nature, demonstrating broad absorption in the UV area.The UV absorbance peaks at 301 and 347 nm in methanolic extracts and at 315 and 348 nm in ethanolic extracts are related to the Quercetin pigments of the anthocyanin group, which correlate to the π to π * transition.Similar visible peaks may also be seen in cyanidin-3-O-glucoside [18] pigments of the anthocyanin, which are found at 526 and 539 nm and correlate to the n to π* transition.According to earlier research, the quercetin pigment exhibits a UV absorbance peak at 301 (nm) and a cyanidin-3-O-glucoside peak at 535 (nm), respectively.However, we saw somewhat different peaks than those previously reported due to the variable climatic circumstances in which plants are cultivated.
The inset graph in Figure 2(a) shows the transmittance spectra of the karonda dye extracts in both solvents.Transmittance is very low in the UV-visible regions.Anisotropic behavior is seen in both the dye samples.Similarly, the inset graph in Figure 2(b) presents the Extinction coefficient spectra which tell us about how the energy is lost by the incident electromagnetic wave as a result of scattering and absorption per unit thickness of the medium.The band gap energy Eg, is dependent on the photon energy hv, and the absorption coefficient, is determined by Tauc's relation [19].
Where αo is an energy-independent constant, Eg is the energy gap, and n shows the nature of the optical transition.Using the linear region as an extrapolation to the X-axis, as illustrated in the below figures 3(a) & 3(b), the optical bandgap Eg is computed.The methanolic and ethanolic extracts' direct energy gap of 3.18eV and 2.98eV and indirect band gap, respectively, were estimated to be 2.02eV and 1.93eV respectively.Using the linear region as an extrapolation to the X-axis, as illustrated in the below figures 3(a) & 3(b), the optical bandgap Eg is computed.The methanolic and ethanolic extracts' direct energy gap of 3.18eV and 2.98eV and indirect band gap, respectively, were estimated to be 2.02eV and 1.93eV respectively.The mixture of pigments in the karonda dye extracts is what causes both bandgaps to exist.The dye extracted from the ethanol solvents shows less direct and indirect bandgaps compared to the methanolic dye extract.The refractive index (n) at a specific wavelength can be determined according to Swanepoel's formula [20].
N = (2s ( Here, TM and Tm stand for the upper and lower envelop transmittances, respectively, at a given wavelength.S= 1.51 is wavelength-dependent and can be computed using the transmittance spectra.The dye's refractive indices, 1.5138 for methanol and 1.5160 for ethanol are hardly different from one another.The components of each dye's refractive index showed higher values for photon energy at the visible wavelength ranges.Significant linear behaviour was seen between 600 and 1100 nm, whereas both the dyes displayed divergences between 200 and 600 nm [21].

FTIR (Fourier Transform Infrared) Spectroscopy
Figure 5 presents the FTIR spectra of the methanolic and ethanolic extract of karonda ye in the wavenumber region of 4000 to 400 cm -1 .This spectrum consists of various functional groups with intense absorption bonds along the wavenumber range.The absorption range and corresponding class of compounds for both dye extracts are shown in Table 1.The absorbance peaks in the spectra are the results of anthocyanins and their derivatives and of phenolic groups.The phenolic compounds of both methanolic and ethanolic extracts are the reason for bands at 2840 to 3331cm -1 responsible for the C-H stretching of alkanes and alkynes [22].The small shoulder bumps around 1640 to 1087cm -1 are due to the C=C stretching of alkanes of anthocyanin derivatives.1420 to 1330cm -1 bands were because of O-H bending or vibrations of alcohols.1087 to 1124cm -1 bands were corresponding to strong C-O stretching of secondary alcohols.600 to 860cm -1 band range of FTIR spectra is due to the presence of C=C bending of alkenes and C-Cl bonds of halogens.Hence, FTIR spectroscopy yields partial confirmation of the chemical components present in the karonda dye extracts.
Figure 2(b) displays the dye's absorption coefficient over the 200-1200 nm wavelength range.

Figure 1 .
Figure 1.Pictorial representation of Karonda dye extraction with methanol and ethanol solvents

Figure 4 (
Figure 4(b).Refractive index spectra of karonda dye as a function of wavelength.