This document discusses chromophores and how solvents affect absorption spectra. It defines a chromophore as a covalently bonded group that absorbs UV or visible radiation. Chromophores are classified as independent or dependent based on the number needed to impart color. Absorption maxima can shift to longer (bathochromic) or shorter (hypsochromic) wavelengths due to auxochromes or solvent changes. Solvent polarity also affects absorption based on the type of electronic transition involved. Temperature and solvent interactions determine the fineness of absorption bands.

1. CHROMOPHORE CONCEPT
AND
SOLVENT EFFECTS
1
Presented by;
SWEETY GEORGE
1st year M PHARM

2.  It is defined as the any isolated covalently
bonded group that shows a characteristic
absorption in the uv or visible region.
 Term chromophore was orginally used for
unsaturated groups of atoms which were
essential for colour.
CHROMOPHORE

3. Two Types
 INDEPENDENT CHROMOPHORE:
If only one chromophore is suffient to
impart colour in a compound then that chromophore is
refered as independent chromophore.
Eg: Azo group(N=N)
• DEPENDENT CHROMOPHORE
If more than one chromophore is required
to impart colour in the chromogen then that is refered as
dependent chromophore.
Eg:CH2=CH2 IS colourless.
;

4.  Chromophore in which the group contains ∏ electrons
and they under go n-> ∏* transition such
chromophore are ethylenes,acetylenes.
 chromophore which contain both ∏ electrons and n
electrons.such chromophores undergo two types of
transition.∏->∏*,n->∏*.
 Eg:carbonyls,nitrates.

5. Auxochrome
 The functional group with non-bonding electrons
that does not absorb radiation in near UV region but
when attached to a chromophore alters the
wavelength & intensity of absorption.
TWO TYPES
1. Basic or positive auxochromic groups(NH2,OH)
2. Acid or negative auxochromic groups(NO2,CO)

7. • When absorption maxima (λmax) of a
compound shifts to longer wavelength, it is
known as bathochromic shift or red shift.
• The effect is due to presence of an
auxochrome or by the change of solvent.
• Bathochromic Shift (Red Shift)1

8. • When absorption maxima (λmax) of a
compound shifts to shorter wavelength, it is
known as hypsochromic shift or blue shift.
• The effect is due to presence of an group
causes removal of conjugation or by the
change of solvent.
• Hypsochromic Shift (Blue Shift)2

9. • When absorption intensity (ε) of a
compound is increased, it is known as
hyperchromic shift.
• The shift is due to the change in structure of
a molecule and medium.
• Hyperchromic Effect3

10. • When absorption intensity (ε) of a
compound is decreased, it is known as
hypochromic shift.
• Hypochromic Effect4

11. Wavelength ( λ )
Absorbance(A)
Shifts and Effects
Hyperchromic shift
Hypochromic shift
Red
shift
Blue
shift
λmax

12. EFFECT OF SOLVENT
 The position and intensity of an absorption band may
shift when the spectrum is recorded in different
solvents.
 A dilute sample solution is preffered for analysis.
 Most commonly used solvent is 95% ethanol.It is best
solvent as it is cheap,transparent down to 210nm.

13. 1. n ->∏* transition.
Absorption band moves to shorter
wavelength by increasing the polarity of the
solvent.Ground state is more polar as compared to
the excited state.
Eg:absorption maximum of acetone is at 279nm in
hexane as compared to 264nm in water.
 ∏ ->∏* transition.
Absorption band moves to longer
wavelength by increasing the polarity of the solvent.

14. Choice of solvent
 It should not itself absorb radiations in the region
under investigations.
 It should be less polar so that it has minimum
interaction with the solute molecule.

15. Effect of solvent polarity on the
various types of bands
 K-band
Due to conjugated enes & enones are affected differently
by changing the polarity of the solvent.
K bands due to conjugated dienes are not affected by
changing the polarity of the solvent, while these bands
due to enones shows a red shift by increasing the
polarity of solvent.
 R band
The absorption shifts to shorter wavelength (blue shift)
with increasing polarity of solvent.

16.  B band
The position as well as the intensity of the band is not
shifted by increasing the polarity of the solvent. But
the heterocyclic aromatic compound, a marked
hyperchromic shift (increase in €max ) is observed by
increasing the polarity of the solvent.

17. EFFECT OF TEMPERATURE AND SOLVENT
ON THE FINENESS OF ABSORPTION BAND
 Vibrational & rotational states depends on
temperature.
“As temp decreased, vibrational & rotational state
of molecule is lowered.”
ie, when the absorption of light occurs at a lower
temp,smaller distribution of excited state results. It
produces finer structure in the absorption band than
what noticed at higher temp.

18.  Effect of solvent also affects the fineness of absorption
bands in the uv spectruum.
If the dielectic constantof the solvent is high,there
will be stronger solute-solvent interaction. Due to
this,vibrational and rotational energy states of a
molecule increase and thus fineness of absorption
band falls.