Introduction:

      Chromophores are portions of molecules that are responsible for its color, because of either conjugated pi bond systems or metal complexes.  The properties of these systems allow the molecules to absorb and release photons with wavelengths that lie within the visible spectrum (around 400-750nm).  In conjugated chromophores, the photon absorbed causes the electron to transition between energy levels created by the extended pi bonds.  Examples of conjugated chromophores include retinal (a compound present in the retina used to detect colors), some food colorings and fabric dyes, as well as lycopene and β-carotene (compounds found commonly in fruits and vegetables).  The color from metal complex chromophores arises from the splitting of d-orbitals caused by binding transition

 metals to ligands.  Examples of these chromophores include chlorophyll (used by plants in photosynthesis), hemoglobin and hemocyanin (present in blood samples), and colorful minerals like amethyst and malachite.

     A quantum mechanical system can only take only certain discrete values of energy. Those discrete values are called energy levels. This term is commonly used to describe the energy levels of electron

s in atoms or molecules. Ther

efore, since the system can only take discrete values of energy, it is said that the energy spectrum of the system is quantized. Electrons in atoms and/or molecules can change energy levels by either emitting or absorbing a photon. However, in order for that to happen the energy of the photon must be equal to the energy difference between the two energy levels. The lowest possible energy level is called the ground state and if the electron occupies a higher energy level, it is said to be in an excited state. Moreover, an electron can be excited to a higher energy level by absorbing a photon that has energy equal to the energy difference between the two levels, and the movement of an electron to a higher energy state is termed electron excitation. Specifically, in respect to chromophore, photons (light) from the visible spectrum that hit it, can be absorbed by electrons that get excite

d from their ground states to higher energy levels.

Molecular Orbitals

     Many chromophores are conjugated systems: a system of delocalized electrons due to unhybridized p-orbitals.  This is typically seen in alternating single-double bond systems

 but can also be observed in systems with double bonds next to a single

 bond neighboring lone-pair p-orbital electrons.  Essentially, conjugation is the overlap of p-orbitals over a single bond.  This allows pi electrons to be delocalized over all of the parallel p-orbi

tals so that they do not “belong” to any one orbital, but are shared by a group of atoms.  This tends to increase the stability of the molecule as a whole.

Particle in a box

      The particle in a box model is often used to describe the wave functions that are solutions to the Schrodinger equation.  It is used as a method of differentiating between classical mechanics and quantum mechanics.  More specifically, it is used to illustrated the idea that energy levels are quan

tized and not discrete.  From this, an equation describing the allowed energy levels of wavefunctions can be derived.  

The 1D particle in a box model can be extended

 to model the energy of conjugated systems.  In this scenario, the length of the box L is analogous to the size of the conjugated system and n refers to the highest occupied molecular orbital. To predict what wavelengths of energy will be absorbed by a conjugated system, we must remember that absorbing of energy means the excitation of an electron from the HOMO to the LUMO.  Thus, we can calculate the transition as ∆E using the above equation. By making the appropriate substitutions into the equation for energy, it is evident that a more conjugated molecule (longer molecule, larger L) absorbs radiation of lower energy (i.e longer wavelength).

Visible Light spectrum and Complimentary Color

     Color originates in light. As we already know visible light is part of the electromagnetic spectrum. Sunlight, as our light source, is colorless. In reality, a rainbow shows the fact that all the colors of the spectrum are present in white light. Our eye can usually detect the range of wavelength, or the light spectrum from about 400 nanometers (violet) to about 700 nanometers (red). We perceive this range of light wavelengths as a smoothly varying rainbow of colors, otherwise known as the visual spectrum.As illustrated in the diagram

on the top right, light goes from the source (the sun here) to the object (the apple), and finally to the detector (the eye and brain). All the" invisible" colors of sunlight shine on the apple as well.The surface of a red apple absorbs all the colored light rays, except for those corresponding to red, which is reflected to the human eye. The eye receives this reflected red light and

sends a message to the brain.

     In the opposite case, when we can use the quantum model (particle in the box) to predict the range of wavelength of absorption, we can measure the absorption by the visible light

 spectrum detector to obtain various of useful information. 

And we can use the complimentary color to predict the color observed by our sight. A complementary color is a color that is on the opposite side of the color wheel (shown on the right), which is used to predict the visible color of theanalyte.

     For instance, Bromocresol Green (BCG) is an organic molecule that is used as a pH indicator and as a tracking dye for DNA agarose gel electrophorsis. According to the diagram shown below, the acidic form of BCG has the maximum absorption is around 450 nanometer. From figure 2, this wavelength corresponds to blue. Thus, from the color wheel, we can see the color

on the opposite side of blue is orange, which is the color observed in the experiment. Using the same logic, we can predict the basic form of BCG has the color blue or blue green.

Applications:

     Chromophores have many applications as dyes and colorings in many different industries including clothing, food processing, art, and design.  Different colors will be produced depending on the chemical structure of the chromophore.  For example, using the particle-in-a-box model, adding one additional double bond to a conjugated double bond system will change the length of the “box” and therefore alter the wavelength of light required to excite electrons and hence change the color observed.  Other chromophores, such as chlorophyll, are key components to biological processes.  The conjugated double bonds of chlorophyll allow the molecules to absorb light in the visible spectrum and convert the photons of energy absorbed into chemical energy through photosynthesis.  Without the conjugated systems, the wavelengths required to excite electrons would be less abundant.  Because they would be higher in energy, they could destroy many chemical bonds or alter protein structures rather than provide a viable source of energy for plants.

Resources

http://www.colormatters.com/seecolor.html

http://www.webexhibits.org/causesofcolor/1.html

http://en.wikipedia.org/wiki/Bromocresol_green

Chemistry 260 Lectures