DETERMINATION OF GROUND AND EXCITED STATES DIPOLE MOMENTS OF EOSIN B DYE BY SOLVATOCHROMIC SHIFT METHOD

 

We live in a colourful world, the preparation and use of dyestuffs is one of the oldest human activities. With the advent of the industrial revolution, chemistry began to play a prominent role in the improvement of the existing colorants and their applications. Among the groups of organic dyes, the eosin family of dyes constitutes a widely used set of fluorescent stains in modern biology and histology. They are commonly used as probe for the investigation of many chemically important systems, and have been studied extensively both experimentally and (Zerkerhamidin et at.,2015). effect of solvent on the absorption and fluorescence characteristics of organic compounds has been a subject of extensive research. Excitation of a molecule by photon causes a redistribution of charges leading to conformational changes in the excited state. This can result in increase or decrease of dipole moment of the excited state as compared to ground state. The dipole moment of an electronically excited state of a molecule is important property that provides information on the electronic and geometrical structure of the molecule in short –lived state. Knowledge of the excited- state dipole moment of electronically excited molecules is quite useful in designing non-linear optical materials, elucidating the nature of the excited states and in determining the course of photochemical transformation. (Joshi et al.,2012).

Chemicals can be analysed both quantitatively and qualitatively through a number of different analytial methods, but one big area of analysis is by using spectroscopy. (Soul et al., 2019)

Spectroscopy is the study of the absorption and emission of light and other radiation by matter. It involves the splitting of light (or more precisely electromagnetic radiation) into its constituent wavelengths (a spectrum), which is done in much the same way as a prism splits light into a rainbow of colours. In fact, old style spectroscopy was carried out using a prism and photographic plates.

Modern spectroscopy uses diffraction grating to disperse light, which is then projected onto CCDs (charge-coupled devices), similar to those used in digital cameras. The 2D spectra are easily extracted from this digital format and manipulated to produce 1D spectra that contain an impressive amount of useful data.

Recently, the definition of spectroscopy has been expanded to also include the study of the interactions between particles such as electrons, protons, and ions, as well as their interaction with other particles as a function of their collision energy(Chu et al.,2023).

analysis has been crucial in the development of the most fundamental theories in physics, including quantum mechanics, the special and general theories of relativity, and quantum electrodynamics. Spectroscopy, as applied to high-energy collisions, has been a key tool in developing scientific understanding not only of the electromagnetic force but also of the strong and weak nuclear forces(Chu et al.,2019).

Spectroscopy methods can be categorized depending on the types of radiation, interaction between the energy and the material, the type of material and the applications the technique is used for. There are many different types of spectroscopy, but the most common types used for chemical analysis include the following; 

In AAS atoms absorb ultraviolet or visible light to transition to higher levels of energy. AAS quantifies the amount of absorption of ground state atoms in the gaseous state. AAS is commonly used in the detection of metals 

In AES, atoms are excited from the heat of a flame, plasma, arc or spark to emit light. AES used the intensity of light emitted to determine the quantity of an element in a sample.

In atomic fluorescence spectroscopy, it is a beam of light that excites the analysts, causing them to emit light. The fluorescence from a sample is then analysed using a fluorimeter, and it is commonly used to analyse organic compounds.

Nuclear magnetic resonance (NMR) uses resonance spectroscopy and nuclear spin states for spectroscopic analysis. All nuclei have a nuclear spin, and the spin behaviour of the nucleus of every atom depends on its intramolecular environment and the external applied field.

When nuclei of a particular element are in different chemical environments within the same molecule, there will be varied magnetic field strengths experienced due to shielding and de-shielding of electrons close by, causing different resonant frequencies and defines the chemical shift values.

Spin-spin coupling takes into account that the spin states of one nucleus affect the magnetic field that is experienced by neighbouring nuclei, via intervening bonds. Spin-spin coupling causes absorption peaks of each group of nuclei to be split into a number of components.

There are multiple types of NMR analyses, which are hydrogen NMR, carbon 13 NMR, DEPT 90 and DEPT 135 NMR. The NMR spectrum of a compound shows the resonance signals that are emitted by the atomic nuclei present in a sample, and these can be used to identify the structure of a compound.

Infrared (IR) analyses compounds using the infrared spectrum, which can be split into near IR, mid-IR and far IR. Near IR has the greatest energy and can penetrate a sample much deeper than mid or far IR, but due to this, it is also the least sensitive. Infrared spectroscopy is not as sensitive as UV/Vis spectroscopy due to the energies involved in the vibration of atoms being smaller than the energies of the transitions.

IR uses the principle that molecules vibrate, with bonds stretching and bending, when they absorb infrared radiation. IR spectroscopy works by passing a beam of IR light through a sample, and for an IR detectable transition, the molecules of the sample must undergo dipole moment change during vibration. When the frequency of the IR is the same as the vibrational frequency of the bonds, absorption occurs and a spectrum can be recorded.

Raman spectroscopy is similar to IR in that it is a vibrational spectroscopy technique, but it uses inelastic scattering. The spectrum of Raman spectroscopy shows a scattered Rayleigh line and the Stoke and anti-Stoke lines, which is different from the irregular absorbance lines of IR.

Raman spectroscopy works by the detection of inelastic scattering, also known as Raman scattering, of monochromatic light from a laser in the visible, near-infrared or ultraviolet range. For a transition to be Raman active, there must be a change in the polarizability of the molecule during the vibration and the electron cloud must experience a positional change.

The technique provides a molecular fingerprint of the chemical composition and structures of samples, but Raman scattering gives inherently weak signals. Techniques such as Surface Enhanced Raman Spectroscopy (SERS) have been developed to enhance sensitivity when using Raman spectroscopy.

In spectroscopy, a spectrometer is used to measure the intensity of light as a function of wavelength or frequency. The spectrometer can be design for different region of the electromagnetic spectrum, including ultraviolet (UV), visible (VIS), infrared (IR), or even X-ray and gamma-ray regions.

Until the 1850s virtually all dyes were obtained from natural sources, most commonly from vegetables, such as plants, trees, and lichens, with a few from insects. Solid evidence that dyeing methods are more than 4,000 years old has been provided by dyed fabrics found in Egyptian tombs. Ancient hieroglyphs describe extraction and application of natural dyes. Countless attempts have been made to extract dyes from brightly coloured plants and flowers; yet only a dozen or so natural dyes found widespread use. Undoubtedly most attempts failed because most natural dyes are not highly stable and occur as components of complex mixtures, the successful separation of which would be unlikely by the crude methods employed in ancient times. Nevertheless, studies of these dyes in the 1800s provided a base for development of synthetic dyes, which dominated the market by 1900.

However, dye is a substance used to impart colour to textiles, paper, leather, and other materials such that the colouring is not readily altered by washing, heat, light, or other factors to which the material is likely to be exposed. Dyes differ from pigments, which are finely ground solids dispersed in a liquid, such as paint or ink, or blended with other materials. Most dyes are organic compounds (i.e., they contain carbon), whereas pigments may be inorganic compounds (i.e., they do not contain carbon) or organic compounds. Pigments generally give brighter colours and may be dyes that are insoluble in the medium employed(Abrahat et al., 2019).

There are several ways to classify dyes. For example, they may be classified by fiber type, such as dyes for nylon, dyes for cotton, dyes for polyester, and so on. Dyes may also be classified by their method of application to the substrate. Such a classification would include direct dyes, reactive dyes, vat dyes, disperse dyes, azoic dyes, and several more types of dyes. (Gregory et al., 1990)

Eosin is a class of fluorescent red dye. It is an artificial derivative of fluorescein consisting of two closely related compounds, Eosin Y and Eosin B. Eosin B is a dibromodinitro derivate of fluorescein and has a faint bluish cast. Eosin B performs equally well as Eosin Y, and sometimes can give a more brilliant red color.  Eosin B can be used to stain cytoplasm, red blood cells, collagen, and muscle fibers for histological examination. It is most often used as a counterstain to hematoxylin in H&E staining.(Lai et al., 2012)

 

Figure 1. structure of Eosin B

Alcohols are widely used as a solvent. They are relatively safe and can be used to dissolve many organic compounds that are insoluble in water. They used, for example, in many perfumes and cosmetics. (Rijavec et al.,2007) alcohols used in this work include; methanol, ethanol, propanol, butanol, and hexanol.

Solvatochromism is a reversible change of the absorption or emission spectrum of a material that is induced by the action of solvents. The colour change is the consequence of the absorption maximum shift, which occurs due to differences between the solvation energy of the initial and excited state in various solvents. The excited state, which is more polar than the initial state, is more stable in more polar solvents. Such systems require lower energy for excitement, which leads to bathochromic shift of the absorption spectrum. This phenomenon is called positive solvatochromism. The less polar excited state than the initial state produces a counter-effect and a hypsochromic shift of the absorption maximum. This phenomenon is called negative solvatochromism. The majority of solvatochromic materials are metal complexes. (Edward et al.,2007).

The study of the determination of ground and excited state dipole moments of eosin B by solvatochromic shift method is Important as it provide analytical information on the ground and excited state dipole moment of eosin B

The aim of the study is to determine the ground and excited states dipole moments of Eosin B by solvatochromic shift method.

The aim of this project will be achieved through the following objectives;