COL_2014_NOV_article17

Excerpt: In case the solute has an appreciable dipole and no net charge, the ground (or relaxed excited) state solvation results largely from the dipole-dipole interactions between the solvent and the solute

Introduction

A PHYSICALLY measurable property of any chemical entity is influenced by the environment in which the chemical entity is situated. It has long been known that absorption spectra of chemical compounds may be influenced by the surrounding medium. Particularly in colorants solvents can bring about a change in the position, intensity and shape of the absorption bands. It may be said in simple terms that color of a solution of colorant depends on the nature of the solvent as well as the nature of the colorant. This phenomenon was later referred to as Solvatochromism by Hantsch.

Today, however, the term solvatochromism has wider connotation.

Solvatochromism in colorants can thus be defined as the pronounced ability of a chemical substance to change its color due to a change in the polarity of the medium. The change in color is due to change in position and sometimes intensity of an electronic absorption or emission band. Positive solvatochromism corresponds to bathochromic shift with increasing solvent polarity, and negative solvatochromism corresponds to hypsochromic shift with increasing solvent polarity. The sign of the solvatochromism depends on the difference in dipole moment of the molecule of the colorant between its ground state and excited state.

Physicochemical explanation

In physicochemical terms solvatochromism can be partly described as changes in electronic-charge distribution induced by solvents with different dielectric constants and solvation characteristics. A precise understanding of local charge densities for both the ground and the excited state of colorants, therefore, helps a sufficient deal to rationalize these multifaceted phenomena. In other words, we may say that the phenomenon of solvatochromism arises from a change in the electronic structure and distribution of charge of the excited state as compared with the ground state. If the excited state is more polar than the ground state, it will be better stabilized by polar solvation and its energy lowers so that the transition will occur at longer wavelength, i.e. there will be a bathochromic shift ("red" shift) with increasing solvent polarity resulting in positive solvatochromism. Sometimes polar solvents stabilize the ground state of the dipolar colorant molecules relative to the excited state. The energy difference between ground state and the excited state becomes larger with increasing solvent Queries and Responses: author.colourage@gmail.com polarity. The increase of the transition energy results in a hypsochromic shift which leads to a negative solvatochromism. It is thus obvious that solvatochromism is caused by differential solvation of the ground and first excited states of the light-absorbing molecule. In this explanation the first excited state is understood as the so- called Franck-Condon excited state with the solvation pattern present in the ground state.

An example of compound exhibiting positive solvatochromism is 4, 4'-bis(dimethylamino) fuchsone (1) which is orange in non-polar toluene, red in slightly polar acetone, and red-violet in more polar methanol. Examples of colorants exhibiting negative solvatochromism are 2-(4'- hydroxystyryl)-N-methyl quinolinium betaine (2), and 4-(4'- hydroxystyryl)-N-methyl pyrydinium iodide (3). The compound (2) is ink-blue in non-polar chloroform and blood-red in polar water. The compound (3) is violet in n- butanol, red in 1-propanol, orange in methanol, and yellow in water.

According to Franck-Condon principle the nuclei of the absorbing entity (i.e. absorbing molecule + salvation shell) do not appreciably alter their positions during an electronic transition. This is because of the fact that the time required for a molecule to get electronically excited is much shorter than that required to execute vibrations or rotations. It follows, therefore that the first excited state of a molecule in solution and the ground state have the same solvation pattern (such an excited state is called Franck-Condon excited state), and the ground state corresponds to an equilibrium ground state.

In a situation where the lifetime of the excited molecule is sufficiently large enough, then reorientation of the solvent molecules, according to the new excited situation takes place and a relaxed excited state with a solvent shell in equilibrium

with this state comes into vogue. From this equilibrium


Professor Dr. N. Sekar is the head of the Dyestuff Technology Department of Institute of Chemical Technology. He has published over 80 research papers and 200 feature and explanatory articles. He is a chartered colourist and Fellow of Society of Dyers and Colourists (UK). Dr. Sekar has recently been selected as the Fellow of Maharashtra Academy of Sciences for his contributions to chemical

sciences. He is a Hon Advisor to COLOURAGE.


excited state the phenomenon of luminescence emanates. By analogy, there is a Franck-Condon ground state after emission with the solvation pattern of the equilibrium excited state, which persists briefly until the solvent molecules reorganize to the equilibrium ground state. Such a differential solvation of these two states is responsible for the solvent influence on emission of fluorescence spectra. The term solvatochromism includes the solvent dependence of the position of emission bands in fluorescence spectra. The above phenomenon is variously referred to as solvatofluorism or fluorosolvatochromism.

The solvatochromism exhibited by a colorant depends upon the following.

  1. Chemical structure of the chromophore
  2. Physical properties of he colorant
  3. Physical properties of the solvent molecules All the above factors determine the strength if the intermolecular solvent-solute interactions in the equilibrium ground state and the Frank-Condon excited state. It can be largely generalized that the colorant molecule with a large change in their permanent dipole moment upon excitation exhibits a strong solvatochromism. In case the dipole moment of the solute colorant increases during the electronic transition a positive solvatochromism occurs. In case of a decrease of the solute dipole moment upon excitation, a negative solvatochromism is usually observed. Another important factor which determines the sign of solvatochromism is the ability of a solvent to donate or to accept hydrogen bonds to solvent molecules or from solvent molecules in its ground as well as Frank-Condon excited state.

Several theories have been developed to explain solvatochromism and the phenomenon suggests innovative application to the colorant molecule.

Dipole moments in the ground and excited states

The interaction between solvent and solute molecules can change the geometry, the electronic structure, and the dipole moment of a solute, absorption spectra as well as the emission (fluorescence) band positions of solvent-sensitive dyes. The interaction will vary with the polarity of the medium. The Franck-Condon principle says that optical absorption is a vertical excitation process, in which the electronic distribution in both the solute and solvent is altered, while the nuclear coordinates of the solute, as well as the solvent molecules are unchanged. In the solution phase the solute and solvent molecules will normally reorient after excitation, and new solvation equilibrium in the excited state will be established. Then emission or fluorescence will originate from this equilibrium state. Again, this is also a vertical electronic transition process.

In case the solute has an appreciable dipole and no net charge, the ground (or relaxed excited) state solvation results largely from the dipole-dipole interactions between the solvent and the solute; there is an oriented solvent cage around the dipolar solute, leading to a net stabilization between the ground (or relaxed excited) state solute and the solvent molecules. Normally, if the solute dipole moment in the excited state is larger than that in the ground state, the excited state is better stabilized relative to the ground state. With increasing solvent polarity, there will be a red shift for both the absorption and emission bands. On the other hand, a blue shift will occur if the solute dipole moment in the ground state is larger than that in the excited state. 

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