Absorption shifts (Chromophores and Auxophores Concept)

Chromophores and Auxophores Concept

1. CHROMOPHORE CONCEPT PRESENTED BY; D.MAHENDRA Dept of Pharmaceutical Analysis& Quality Assurance Mail:- [email protected]

2. CHROMOPHORE It is a Greek word . Chroma = “color” & phoros = “bearer” Defined as any isolated covalently bonded group that shows a characteristic absorption of Electromagnetic radiation in the UV or visible region. Compound containing chromophore is CHROMOGEN Eg: C=C, C=O, NO 2

3. TYPES OF CHROMOPHORES INDEPENDENT CHROMOPHORES If one chromophore is required to impart colour Eg: Azo group –N=N-, Nitroso group –NO- DEPENDENT CHROMOPHORES If more than one chromophore is required to impart colour Eg: Acetone having one ketone group is colorless whereas diacetyl having two ketone groups is yellow.

4. Groups λmax C – C 1350 C = C 1900 C = O 1900 2800 O – H 1850 NO 2 2800 C 6 H5( PHENYL) 1950 2500 SIMPLE CHROMOPHORIC GROUPS

5. Auxochrome A saturated/ unsaturated group with non bonding electrons when attached to chromophore altering both wavelength as well as intensity of absorption. Eg: OH, NH 2 , NHR, COOH, CN, Cl etc..

6. Two types : Basic/positive auxochromic groups Effective in acid solutions Eg: OH, OR, NHR etc. Acidic/negative auxochromic groups Effective in alkaline solutions Eg: NO, CO, CN etc.

7. CHANGES IN POSITION & INTENSITY OF ABSORPTION For isolated chromophore groups such as –C=C- absorption takes place at far UV which cannot be easily studied but the position & intensity can be modified: bathochromic hypsochromic hypochromic 200 wavelength 800 absorbance hyperchromic λ max

8. BATHOCHROMIC SHIFT/ RED SHIFT Shift of absorption maximum towards longer wavelength Due to presence of groups like OH, NH 2 etc . (auxochromes) Eg: ethylene shows absorption at 170nm whereas 1,3-butadiene shows at 217nm.

9. HYPSOCHROMIC SHIFT / BLUE SHIFT Shift of absorption maximum towards shorter wavelength May be caused by removal of conjugation Eg : In Aniline, absorption takes place at 280nm . In acidic solutions, absorption takes place at 200nm

10. HYPERCHROMIC SHIFT Increase in intensity of absorption By introduction of ‘auxochrome’ Eg : introduction of methyl group in position 2 of pyridine increases intensity of absorpion from 2750 to 3560 HYPOCHROMIC SHIFT Decrease in intensity of absorption Eg : introduction of methyl group in position 2 of biphenyl group causes hypochromic effect because of distortion

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