Optical properties of natural dyes : prospect of application in dye sensitized solar cells ( DSSCs ) and organic light emitting diodes ( OLEDs )

Optical properties (absorbance, fluorescence, and transmittance) of the natural dyes extracted from flower, leaf, bark and rhizome of some plant species available in Nepal have been explored, with the prospect of application in dye-sensitized solar cells (DSSCs) and organic light emitting diode (OLED) employing UV-Vis., Fluorescence and FTIR Spectroscopy. The extraction process was carried out using solvents with a varying polarity index of ca. 0 to 10: cyclohexane (CH), dichloromethane (DCM), acetone, ethanol (EtOH), methanol (MeOH) and double distilled water (DDW). The absorbance was found to depend on the nature of solvent used; the dye samples, A. vasica Nees (in DDW, acetone, DCM and CH), N. arbor-tristis L (in MeOH and CH), U. dioica L (in EtOH, acetone, DCM and cyclohexane), O. wightiana Wall (in acetone and CH), A. vulgaris L (in DCM and cyclohexane), C. coccinea Wall (in DCM), R. anthopogon D.Don (in DCM) and W. fruticosa L (in CH), showed good absorbance in the visible-region and blue shift in the absorbance maxima was noticed with increase of the polarity index of solvents. Some plant extracts showed intense fluorescence emission in the visible region, which hints at their prospect of application in the OLED devices.


Introduction
Nepal, with its diverse flora, is a storehouse of economically important plant species which can have high potential applications in drug synthesis, textile dye, and cosmetics.Traditionally many of the plant species such as Acacia nilotica Linn., Alnus glutinosa Linn., Althaea rosea Cav., Curcuma longa Linn., Lawsonia alba Linn.and Woodfordia fruticosa Kurz have been utilized to extract natural dyes (Gokhale et al., 2004) in these regards.Also, as natural dyes are colorants they also have prospects of employing as a light absorber (sensitizer) in DSSC and also as a fluorescence emitter in OLED: In DSSCs, these materials are considered promising for utilizing as a sensitizer instead of less environmentally friendly organo-metallic complexes (e.g., Ruthenium dye).In this connection, some of the dyes used as sensitizer are cyanin, anthocyanin, tannin, chlorophyll, betalains, carotenoids (Gao et al., 2000;Sirimanne et al., 2006;Fernando et al., 2008;Calogero et al., 2010).
The first DSSC, which belong to thin film solar cells, was realized with a chlorophyll sensitized zinc oxide (ZnO) electrode in 1972 and is considered to be a promising means toward harvesting solar energy in low cost (EIER, 2006).In particular, it comprises a thin film of a compound semiconductor with a wide bandgap (mostly titanium oxide (TiO 2 ) or zinc oxide (ZnO)) on top of which a dye is coated for photosensitization.This kind of photo-cell is also known as the "Grätzelcell'', which was originally co-invented in 1988 by Brian O'Regan and Michael Grätzel (O'Regan and Gratzel, 1991;Nazerruddin et al., 1993;Grätzel, 2003).In DSSCs, when a dye molecule is photo-excited, the electron jumps from HOMO to LUMO level of dye.If the LUMO of dye matches with the conduction band of the compound semiconductor electron hops to the later material, reach to load and eventually recombines with the ionized dye molecules with the aid of electrolyte.
With TiO 2 -based dye-sensitized solar cells, a maximum of 14% efficiency has been achieved using standard ruthenium polypyridyl complexes as a sensitizer in the laboratory condition (Nazeeruddin et al., 2005;Qin and Peng, 2012).Further improvement in photovoltaic properties and durability of these kinds of cells by utilizing an appropriate sensitizer would certainly facilitate widespread utilization of this technology.
There are several studies that employ the natural dyes as a sensitizer (Hao et al., 2006;Chang and Lo, 2010;Susanti et al., 2014) but the downside is that they have low efficiency.This problem can be circumvented by identifying natural dyes which increase the performance efficiency.For example, Susanti et al., 2014, has reported the performance of photo-cells fabricated with ZnO and tamarillo fruit as a sensitizing dye.Similarly, the extracts from spinach and ipomoea were used as good natural sensitizers for dye sensitized solar cells (Chang et al., 2010).Higher incident photon to current conversion efficiency (IPCE), which is proportional to the absorption coefficient of the dye, over the visible and near IR region of the solar spectrum, is one of the necessary conditions to achieve enhanced solar cells performance.Therefore, this study has focused on identifying the natural dyes which show good absorbance over the visible and near-IR regions.

Materials and methods
Extracts of natural dye from fifteen plant species (flower, leaf, bark, and rhizome) were obtained with the solvent extraction method using both the polar and nonpolar solvents; double distilled water (DDW), methanol (MeOH), ethanol (EtOH), acetone, dichloromethane (DCM) and cyclohexane (CH).The plant samples collected for extraction of natural dyes were: 1.The absorbance and fluorescence of all the dye samples were measured on the Genesis-10 UV-Visible spectrophotometer and F93 Fluoro-Spectrophotometer in the wavelength of 300-900 nm.To identify probable chromophore groups present in a given natural dye, IR spectra were recorded with FT-IR spectroscopy (Shimadzu Corporation, IRPrestige21).

Results and discussion
The extracts obtained from a total of 15 plant samples by using cold solvent extraction method were re -dissolved in the same solvent in which they were originally extracted.The solutions were then used for investigation of their optical properties, in particular, absorbance, fluorescence, and transmittance (IR spectroscopy).The observed results are summarized in the following subsections.wightiana Wall, A. vasica Nees (Flow., in CH) was found to exhibit better absorption in the UV-Vis spectrum of electromagnetic radiation.Remaining natural dye samples showed a monotonic decrease in the absorption for all the solvents studied, from wavelength (λ), 310 nm to 900 nm.In addition, when the polarity of the solvent increases, blue shift in the major peaks of each absorbance spectrum was noticed (Figure 2, a-f).This may be due to the effect of polarity of the solvent (as expected) or it may be due to the extraction of different molecule(s) with unique absorbance which was absent in that particular solvent.

Absorption of natural dye
The absorption maxima for dye extracts in the wavelength range ca.400-500 nm may be attributed to absorption due to xanthophyll, flavone, carotene, and rhein molecules, respectively (Zhou et al., 2011).Absorbance maxima observed at wavelength λ = 411 nm and λ = 665 nm (in cyclohexane) indicates the presence of either chlorophyll, anthocyanin or carotene or mixture of these compounds in the extract (i.e.these peaks may be due to absorption by individual species or may be attributed to the superposition of absorption peaks) since the natural dye generally contains either anthocyanin, chlorophyll carotenoids or mixture of them which have absorption peak at wavelengths 420 nm and 560 nm (Sari and Sunardi, 2011).

Fluorescence of plant extracts (natural dyes)
Figure 3 demonstrates fluorescence spectra, after excitation with 340 nm, as a function of emitted radiation wavelength for all the 15 natural dye extracts in various solvents.Fluorescence spectra indicated that emission heavily depends on the solvent, just as absorbance did.Almost all of the natural dyes extract exhibited fluorescence spectra at the excitation wavelengths of ca.400 nm (onset) -650 nm (offset) and the emission wavelengths of 640 nm -700 nm, except for few dyes.It was very interesting to note that for a particular sample in the same solvent, fluorescent maxima with respect to absorbance maxima shifted by about 25 nm.For example, the absorbance maximum (λmax) of the dye sample A. vasica Nees that appeared at about 400 nm (see in Figure 2(a)), is found to occur at 425 nm in the fluorescence spectrum (see in Figure 3(a)).
Again, a careful observation of Figure 3 (a-e) indicated that when polarity of the solvent was increased, the major peak of the fluorescence spectrum shifted towards blue side: for instance, the fluorescence peak of sample N. arbor-tristis L. (in cyclohexane, see in Figure 3 (e)) that appeared at ca.525 nm found to occur at ca. 500 nm in distilled water (see in Figure 3(a)).Also, the Figure 3(f) represents the fluorescence spectra of some of the selected dye samples which showed good absorbance and emission in the visible or in the near-IR region of the solar spectrum.The selective samples and solvents used are B. aristata DC, N. arbortristis L, B. ceiba L (in DDW); Artemisia vulgaris L, J. humile L (in MeOH); J. humile L, C. coccinea Wall, W. fruticosa L (in EtOH) and N. arbor-tristis L, R. anthopogon D.Don (in CH).Thus, the prevalence of emissions in broad wavelengths range in between 400 nm to 800 nm (except only a small window at about 725 nm), of the natural dye samples may be useful for synthesizing organic light emitting diodes.

IR spectroscopy of natural dye
IR spectra recorded from natural dye samples; A. vasica, A. vulgaris, B. artista, U. dioica, J. humile, R. arborium, O. wightiana, N. arbortritis, C. coccinea, A. vasica flower and R. anthopogon, extracted in methanol, are presented in Figure 4(a-c) along with probable functional groups present in each sample.All the samples analyzed showed broad peak around 3300 cm -1 which is characteristic of stretching vibration of O-H bond.All three samples in Figure (4a) showed IR active in the peak positions at ca. 3326 cm -1 and ca.2928 cm-1 which may be assigned to the bonded -OH and C-H stretching modes, respectively.Also, A. vasica and U. dioica showed IR active in the peak position at ca.1628 cm -1 , which may be assigned to the bonded C=O stretching.Other probable functional groups are indicated in the figures themselves (see in Figure 4(b)).Similarly, J. humile, R. arborium, and O. wightiana showed IR active in the peak position at ca. 3301 cm -1 and 1607 cm -1 which may be assigned to the bonded -OH group and bending -NH group, respectively.All the three samples presented in Figure 4(c); showed IR active in the peak positions at ca. 3357 cm -1 and 1028 cm -1 which may be assigned to the bonded -OH and ester C-O modes, respectively.

Conclusion
From the UV-Vis-, fluorescence-and IRspectroscopic investigation of natural dyes, it was observed that all the 15 samples exhibited a broad absorbance peak from wavelength λ, ca.300 nm (onset) -ca.500 nm (offset) and another additional sharp peak at ~650nm with FWHM of ca. 15 nm.Moreover, among all the analyzed samples, A. vasica Nees, N. arbor-tristis L, U. dioica L, A. vasica Nees, C. coccinea Wall and O. wightiana Wall, A. vasica Nees found to exhibit fairly better absorption both in terms of intensity and broader wavelength range in the visible spectrum.Also, major peaks in absorbance spectrum shifted towards blue side FULL PAPER Figure 4(a).IR spectra recorded from natural dye samples; A. vasica, A. vulgaris, B. artista and U. dioica extracted in methanol.All the three samples showed IR active in the peak positions at ca. 3326 cm -1 and 2928 cm -1 which may be assigned to the bonded -OH and C-H stretching modes, respectively.Also, U. dioica and A. vasica showed IR active in the peak position at ca.1628 cm -1 , which may be assigned to the bonded C=O stretching.Other probable functional groups are shown in the figure itself.
Figure 4 (b).IR spectra recorded from natural dye samples; J. humile, R. arborium, O. wightiana and N. arbortritis extracted in methanol.All the four samples showed IR active in the peak positions at ca. 2933 cm -1 which may be assigned to the C-H stretching modes.Similarly, J. humile, R. arborium and O. wightiana showed IR active in the peak position at ca. 3301 cm -1 and 1607 cm -1 which may be assigned to the bonded -OH group and bending -NH group respectively.Other probable functional groups are shown in the figure itself.FULL PAPER when the polarity of the solvent increased.This may be due to the effect of the polarity of the solvent or it may be due to the extraction of different molecule (s) with unique absorbance which was (were) absent in a particular solvent.
Absorbance peak observed at wavelength λ = 411 nm and λ = 665 nm (in cyclohexane) indicated the presence of either chlorophyll, anthocyanin or carotene or mixture of these compounds in the extract.These peaks may due to absorption by individual species or the resulting peaks may be attributed to the superposition of the individual absorption peaks.Since the natural dye generally contains either anthocyanin, chlorophyll carotenoids or mixture of them which have absorption peaks at wavelengths 420 nm and 560 nm, exact identification of contributing species was not possible in our analysis of crude extract.
In summary, the prevalence of both the absorption and emissions in broad wavelength range from 400 nm to 800 nm (except only a small window at about 725 nm), by some selective samples, particularly by B. aristata DC, N. arbor-tristis L, B. ceiba L; A. vulgaris L, J. humile L; C. coccinea Wall, W. fruticosa L and N. arbor-tristis L, R. anthopogon D.Don, indicated that these samples may be useful for synthesizing OLEDs.

Figure 2
Figure 2 (a-f).Absorbance as a function of radiation wavelength for natural dye samples extracted in different solvents

Figure 4
Figure4 (c).IR spectra recorded from natural dye samples; C. coccinea, A. vasica flower and R. anthopogon extracted in methanol.All the three samples showed IR active in the peak positions at ca. 3357 cm -1 and 1028 cm -1 which may be assigned to the bonded -OH and ester C-O modes, respectively.Also, A. vasica flower and R. anthopogon showed IR active in the peak position at ca.2929 cm -1 , which may be assigned to the -CH stretching.Other probable functional groups are shown in the figure itself.
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