Topic: Color and theories of Color

Name: SAIMA LATIFRoll No. 36 Institute Of Chemistry PU Lahore

COLOR

‘‘An attribute of things that results from the light they reflect, transmit, or emit in so far as this light causes a visual sensation that depends on its wavelengths’’.

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Chromatics

The science of color is called chromatics, colorimetry, or simply color science.

In Chemistry, it is called ‘Color Chemistry’.

Types of Color

Primary Color

Secondary Color

Tertiary Color

Law of Color Mixing

• In 1861, the James Clerk Maxwell (Additive color) red, green and blue to produce other colors.

• In 1868, Ducos du Hauron, (Subtractive color/ Complementary color mixing) yellow, magenta and cyan create black.

Electro-magnetic Spectrum

• The visible region ranges from 380 nm to 780 nm.

Interaction with light mater

Due to energy difference between excited state and ground state According to Plank’s Equation:

E = hv

E = Energy difference b/w two states V= frequency of absorbed lighth= Plank’s Constant

Further modify:

E = hc/λ

c= Velocity of light

Complementary Colors

Examples

Compound

Number of C=C

in conjugated

system

Main colour

absorbed

Colour

compound

appears

Vitamin A 5 Violet Yellow

β-carotene 11 Blue Orange

Lycopene 11 Green Red

OH

CH3CH3CH3H3C

CH3

CH3CH3CH3H3C

CH3

H3C

H3CCH3CH3CH3

Vitamin A

Beta-carotene

Lycopene

Causes of Color

• Excitation(e.g vapour lamp, neon signs), vibration, rotation

• Transition metal compound (d-orbital involve)

• Color in organic compounds due to π-π*transitions

• Organic molecules that are coloured contain delocalised electrons spread over a number of atoms. That is called conjugated system.

HH

HH

HH

Colorant

• This term is used for both type of coloring materials e.g pigment and dye. These distinguished on the basis of the solubility:

Pigment: insoluble in water and most of the solvents

Dyes: Soluble in water and most of the solvents

• Classification of colorants: • Chemical classification

• Azo, carbonyl, anthraquinones, phthalocyanin et.c

• On the basis of electronic excitation • Donor-Aceptor polyene

• Cyanin or n-π* chromogen

Attributes of Color

• Hue (Shade)

• Strength/ intensity of color

• Brightness of color

Hue (Shade)

• Hue of dye can be determined by absorbed λ of light e.g

• Bathochromic Shift:

is a change of spectral band position in the absorption, or emission spectrum of a molecule to a longer wavelength

• Hypsochromic Shift:is a change of spectral band position in the absorption, or emission spectrum of a molecule to a shorter wavelength

Intensity of Color determined by using Beer-Lambert Law:

• Most absorbing materials satisfy the Beer-Lambert Law. Intensity of color of dye is given by ɛ.

A = ɛcl

• A= Absorbance

• C= Concentration

• l= Path length

• ɛ=molar extinction coefficient

Brightness of Color

• It is characterized by the UV- spectrum and shape of absorption spectrum.

• Spectrum of red surface e.g (Low reflectance and hight absorption)

• Red (600-700 nm)

• Green (500-600 nm)

• Blue (400- 500 nm)

Theories OF Color

Theories of Color

Classical theory

Witt’s Theory

(Chromophore-Auxochrome

Theory)

Bury’s Theory of Color

TautomericTheory

(Baeyer’s Theory)

Armstrong’s Theory

(Quinonoid Theory)

Modern theory

VBT

(Resonance Theory)

MOT

Classical theories of Color

Witt’s Theory

Chromophore‐Auxochrome theory: In 1876, Witt put forward a theory according to which the color of a substance is due to the presence of an unsaturated group known as chromophores(Greek chroma‐color, and phores‐bearing). The important chromophores are: • ‐C = C ‐• ‐C = N ‐• ‐C = O ‐• ‐N = N ‐• ‐NO2

• ‐Quinoid rings

Auxochromes: responsible for the darkening of colorExamples:• ‐OH or ‐NH2

• ‐‐NR2

Types of Chromophore

• Single chromophore is sufficient to impart color to the compound.

• Example: ‐NO, ‐NO2, ‐N=N, =N=N‐N, o,p‐quinonoid etc.

• More than one chromophore is required to impart the color, e.g. >C=O, >C=C< etc.

• Example: Acetone and diketone

Diketone

Dilthey and witzinger’s Theory

• Dilthey and witzinger Refined the witt's theory

• ‘‘Stated the chromophores are EWG and auxochrome are EDGThey are linked through the conjugated system’’.

• By this why donor-acceptor chromogen was born.

Armstrong’s Theory

Quinonoid theory: Armstrong in 1885 suggested that all coloring matters have quinonoid structures, and thus believed if a compound have quinonoid form, in a structure it is colored, otherwise it is colorless.

Example:

• benzene and benzoquinones

Baeyer’s Theory

• In 1907, Baeyer proposed the ‘‘theory of tautomerism’’(Baeyer’s Theory) which states that there is a rapid oscillation between two tautomeric forms of a compound.

• Example: Doebner’s violet dye (1) (1a), with a rapid flipping of the chloride ion from one amino group to another (see Figure 1).

unsubstitued diaminotriphenyl methane dye is Called Doebner's violet.

Figure 1: Movement of atom

Watson’s Theory

• In 1914, Watson was proposed ‘‘Tautomerism is the basic requirement for colored molecule and that must have conjugated chain in a quinonoid form in all the possible tautomers’’.

• The theories of Armstrong and Watson became invalid after the discovery of dyes without a quinonoid structure.

Bury’s theory

• In 1935, Bury was highlighted the relationship between the color of a dye and resonance. Bury’s theory ‘color is due to the involvement of a chromogen in resonance in the molecule’.

• Baeyer’s idea that the color of Doebner’s violet arose from the oscillation of atoms was disproved by Bury, who proposed that it was the electrons that moved and not the atoms (see Figure 2).

Figure 2: Movement of electrons

Modern theories of Color

Valence Bond Theory or (Resonance Theory)

Two assumptions apply the valence-bond approach to color and constitution, which are:

The ground electronic state of the dye resembles the most stable resonance form.

The first excited state of the dye resembles the less stable, charge-separated form.

Postulates

• Chromophores have the π‐electrons which moved from ground state to excited state by the absorption of radiation, thus producing the color.

• Auxochromes are groups: • Increase resonance by interacting the unshared pair of electrons on

nitrogen or oxygen atoms of the auxochromes with the π electrons of the aromatic ring.

• This increase in resonance increases the intensity of absorption of light and also shifts the absorption band to longer wavelength.

• The dipole moment changes due to oscillation of electron pairs.

VBT

• Resonance theory explains the • Relation of the color and

• The symmetry of the molecule

• Transition dipole of the molecule

because

High canonical str.s ∝ high color intensity ∝ high bathochromic shift

Example: 1

• EDG and EWG will effect its bathochromic shift and λmax.

Canonical forms

Example: 2

• p-nitro dyes have λmax 486 nm but o-nitro dye have λmax 462 nm

Because:

• In ortho, nitro group is out of plane ‘‘so rotation is required’’

λmax of o-nitro = 462 nm

Example:3

• Five membered heterocyclic rings have maximum bathochrmic shift as compare to six membered ring.

Because • Less steric interaction • Coplanar Geometric arrangements • Less steric congestion • 126ᵒ bond angle

λmax = 608 nm

λmax = 614 nm

Molecular Orbital Theory

• Molecular orbitals generates by the overlapping of atomic orbital.

• End-to-end overlapping form ….. σ-bond

• Sideways overlapping form ……. π bond

The σ bonding orbital is the highest occupied molecular orbital

(HOMO), and the lowest unoccupied molecular orbital (LUMO)

is the σ* anti-bonding orbital.

While these organic compounds do absorb light, the energy

transitions involved in promoting an electron from σ to σ* are

very large.

Organic compounds that contain only σ bonds are colourless.

Incre

asin

g e

nerg

y

σ bonding orbital

π bonding orbital

non-bonding orbital

π* anti-bonding orbital

σ* anti-bonding orbital

These absorptions

correspond to the UV part

of the spectrum.

Molecular Orbital Theory

Possible Allows transitions are:

σ σ* (UV- region)

π π* (appear in visible region)

n σ* (saturated molecule + heteroatom)

n π* (Unsaturated molecule + heteroatom)

Molecular Orbital Theory

Incre

asin

g e

nerg

y

σ bonding orbital

π bonding orbital

non-bonding orbital

π* anti-bonding orbital

σ* anti-bonding orbital

Hückel Molecular Orbital (HMO) Method

• Method use for the calculation on conjugated molecules.

• HMO method used to find:

• Bond resonating energies (ẞ )

• Pi-electron charge density (Q)

• Pi-bond order (P)

• Degree of pi-overlap of atomic orbitals

• This method is good to calculate the electronic transition energy of Linear polyenes

Aromatic hydrocarbons.

• PPP-MO approach is best over the HMO because of • Inter-electronic interaction energies

• Along with molecular geometery

• This Method is suitable for the treatment of large molecules

• Handle the hetero-atomic species

• Help to find • Magnitude of dipole moment

• lamda max.

Pariser, Parr and Pople (PPP) Molecular Orbital Method

PPP-MO Method

THANKS