Presentation contains various examples of n to π* chromogens and their behavior in different conditions. Its related to subject called Color Chemistry.

1. The Imino Group
 Azines
 Effect of extending conjugations
 Application of Imino chromogens
 Comparisons
 Application in dyes

2. Azines
 Obtained by replacing one or more ‘C’ atoms by nitrogen in the
benzenoid hydrocarbons.
 Special type of Imino compound where the C-N bond order resembles
that of C-C bond (0.6).
 But lone pair characteristics of N remain essentially same.
 Substitution of one C by N gives poorly resolved n->pi* band and
introduction of second N displaces this band to longer wavelengths.

3. Effect of extending conjugations
 Extending conjugations of azines generally
unsuccessful in providing visible n->pi* bands because
of the masking effect of pi->pi* bands.

4. Application of Imino chromogens
Quinone imine dyes: These are a group of dyes that are
derivatives of para- Quinone imine (I) or para- Quinone diimine
(II), in which the hydrogen atom of an imine group is replaced by
an aromatic residue containing an NH2 or OH– group.

5.  They are used as intermediate products in the production of sulphur dyes
and are also commonly used in color photography and in the production
of pencils, as well as for dyeing paper and fur. In addition,
they are used as chemical indicators.
Reference: Stepanov, B. I. Vvedenie v khimiiu i tekhnologiiu organicheskikh
krasitelei. Moscow, 1971.
 Sunscreen: Pigment: Scytonemin-3a-imine
The UV/Vis spectrum had maxima at 237, 366, 437 and 564 nm.
Reference: Cidya S. Grant, J.W. Louda, Organic Geochemistry
(Volume 65, December 2013, Pages 29–36)

6. OLED (Organic Light Emitting Diodes):
 An Aromatic Imine Group Enhances the Electro-Luminescence
Efficiency and Color Purity of Blue Emitter for Organic Light Emitting
Diodes.
 DPBP-EPY has the same structure with DPBP-EIF, one of blue-light-
emitting materials, except their cores, in which the former has two
imine groups, but the latter has only carbon-containing groups.
 The maximum values in the UV-visible and PL spectra of DPBP-EPY
were about 19–46 nm red-shifted from those of DPBP-EIF due to the
electron-withdrawing effect of the imine groups whether in solution or
in solid.

7.  In addition, in case of DPBP-EPY where imine group is substituted,
LUMO level of DPBP-EPY decreased while HOMO level did not show
any significant change.
 It was shown that DPBP-EPY emitted purer and deeper blue-light
with CIE coordinate (0.157, 0.131) than DPBP-EIF with CIE coordinate
is (0.179, 0.191).
Reference: Park, Youngil; Kim, Beomjin; Lee, Ki-Ho; Lee, Ji-Hoon; Oh, Se-
Young; Park, Jongwook, Journal of Nanoscience and
Nanotechnology( Volume 12, Number 5, May 2012, pp.
4325-4329(5))

8. Pb2+ Indicator:
 Imine-bridged tetrathiafulvalene-π-pyridine derivatives,
2NTTF, 3NTTF and 4NTTF on the addition of the micromolar
concentration of Pb2+ to the solution, display remarkable
changes in solution color, 1H NMR spectra and electrochemical
properties.
 The nitrogen atom of the imine-bridge in 2NTTF not only serves as
a π-conjugated spacer but also participates in the sensing and
coordinating properties.
Reference: JingJing Wang, XiaoJun Wang, Yan Geng, ChenHo
Tung, LiZhu Wu, Science in China Series B: Chemistry
(June 2009, Volume 52, Issue 6, pp 765-770 ).

9. Absorption of Imino Group:
 Expectation: Imino group to lie at higher energies than corresponding
orbital of the carbonyl group as N is less electronegative than O. We
expect Bathochromic Shift.
 Lone pair on N orbital is sp2 while O has pure p orbital. Due to
increase in s character, n orbital is lowered in energy. Hence
Hypsochromic Shift is observed.
 Thus, Imino groups always absorb at shorter wavelength than their
Carbonyl counterparts.

10. Comparison of nπ* transitions of
Imino & Carbonyl groups
COMPOUND MAX. WAVELENGTH SOLVENT
CH3CHO 293 nm hexane
CH3N=C(CH3)2 244 nm cyclohexane
CH3COCH3 279 nm hexane
252 nm cyclohexane
N
B
u

11.  Here, distinct n  π* bands in the visible region are never
observed distinctly.
 Loss of planarity arises from the preferred rotation about the
N-aryl which enables the N lone pair electron to be
conjugated with the benzene ring.
 Though n π* is obscured by π  π* , the transition is found to
be 360 nm.
NON - PLANAR PLANAR
BENZYLIDENE
ANILINE
AZOBENZENE

12. Application In Dyes:
 Quinoneimine Dyes- Indamines, Indoaniline,
Indophenol depending on whether both electron
donors are Amino group, Amino group and hydroxy
group or both Hydroxy group.
 The three heterocyclic types are named after their
basic structure :
4.22- AZINES
4.33- OXAZINES
4.44- THIAZINES

13. Colour Photography :
 It is an example of Oxidative Coupling.
 The Oxidation Reagent is the Silver Ion.
 The reaction is catalysted by metallic silver.
 An electrophilic reagent is formed (4.25) which attacks an
aromatic compound containing an electron donor in an
electrophilic aromatic substitution. Further oxidation finally
converts the product of substitution into the indoaniline type
dye.

14. The Nitroso Group
 Introduction
 Behavior as a n→π* Chromogen
 Effect of substituent on absorbance wavelength
 Effect of solvent on absorbance wavelength
 Derivatives of nitroso group
 Dyes based on nitroso group

15. Introduction
 Nitroso refers to a functional group in organic chemistry which has
the -N=O group attached to an organic moiety.
 Nitroso groups can be categorized as
C- nitroso compounds i.e. nitrosoalkanes ; R-N=O,
S- nitroso compounds i.e. nitrosothiols ; R-S-N=O,
N- nitroso compounds i.e. nitrosamines ; RR’N−N=O,
O- nitroso compounds i.e. alkyl nitrites ; R-O-N=O
 The nitrogen is trigonal planar with a bond angle of 125°.
 Nitroso groups are strongly electron withdrawing.

16. Behavior as a n→π* Chromogen
 Nitroso group behave as due to n→π* transition of lone pair of
Oxygen atom.
 According to Perturbational theory, the replacement of carbon by
nitrogen in the carbonyl group will lower the energy of the π*
orbital, thus giving a bathochromic shift of the n→π* band with
respect to carbonyl group.
 Also nitroso can be said to be derived from azo group(−N=N−) by
replacing one of the nitrogen by more electronegative atom
oxygen, which also suggest lowering of the energy of the π* orbital,
thus giving bathochromic shift of the n→π* band with respect to
carbonyl group.
 Having established that the π* orbital energy will be lower than that
of either the carbonyl or azo chromogens, it remains to ascertain
the relative position of n orbital.

17.  In nitroso group adjacent nitrogen and oxygen atoms can overlap
and interact to give two new orbitals. Molecular orbital energy
calculations indicate that the higher energy orbital from the two i.e.
the one involved in the lower energy transition, is essentially
localized on oxygen.
 Thus we have a situation in which the energy of the relevant
n orbital is higher than in the carbonyl group.
 These various effects result in an appreciable narrowing of the gap
between the n and π* orbitals and the nitroso group shows an
n→π* band at unusually long wavelengths, in the region of
700 nm.
 Even the simple nitrosoalkanes are blue-green in colour. But the
absorption intensity is low as comparable to carbonyl group.

18.  As we can energy difference between n and π* orbital in nitroso group
less than carbonyl group and thus gives a strong Bathochromic shift
and thus absorbs at around 700 nm.

19. Nitroso compound Carbonyl compound
 The n→π* bands of several nitroso compounds along with respective
carbonyl compounds are given in the table :-
1-nitrosobutane
λmax = 630 nm
Butanal
λmax = 180 nm
Nitrosobenzene
λmax = 750 nm
Benzaldehyde
λmax = 247 nm
Phenylbenzaldehyde
λmax = 428 nm
Phenylnitrosomethane
λmax = 678 nm

20. Effect of substituents
 Aryl substituents have only a small effect on the position of nitrosobenzene
peak, electron donating group exerting a small Hypsochromic effect.
Compound Wavelength (nm)
750
745
Nitrosobenzene
p-methyl nitrosobenzene
 The marginal effect of substituents suggest that the π* orbital is largely localized
on nitroso group.

21.  But p-N,N-dimethylaminonitrosobenzene shows largest displacement from 750 nm
to 707 nm.
400 nm
Yellow colour
707 nm
Bright green
colour
 Thus the solid is bright green in colour but appears yellow in dilute solutions.

22. Effect of solvent on absorbance wavelength
 The n→π* bands of nitroso compound show the unusual Hypsochromic shift in
polar solvents. For e.g.
736 nm in
Hexane
707 nm in1,2-dichloro
ethane
553 nm in
water
Hypsochromic
shift

23. Derivatives of nitroso group
 The nitrites, nitrosamines and thionitrites are analogous to the esters, amides
and thioesters of carbonyl group.
 Theses substituents are both σ-withdrawing and π-donating, and thus they
will increase the energy gap between the n and π* orbitals of the nitroso
group. Thus gives Hypsochromic shift of n→π* band of nitroso. So, they
absorb at shorter wavelengths than the nitrosoalkanes or nitrosobenzene.
 The nitrites and nitrosamines do not show n→π* bands in visible region, but
because of tailing of their near U.V. bands, they are pale yellow coloured.
 Sulphur has weaker σ-withdrawing effect than Oxygen and Nitrogen, and
consequently the n orbital of the nitroso group is not lowered in energy to
the same extent. The thionitrites therefore tend to absorb at longer
wavelengths than the nitrites and nitrosamines and are red to violet in
colour.

24. Compound Wavelength (nm)
361
357
550
570
n butyl nitrite
Ethyl thionitrite
Trifluoromethyl thionitrite

25.  The nitro group may be formally derived by co-ordinating the nitrogen lone pair
electrons of the nitroso group to a second oxygen atom.
 The introduction of oxygen will lower the energy of the n orbital level by its
σ-withdrawing effect and raise the energy of π* orbital by π-donating effect.
 Thus nitro group shows n→π* band at much shorter wavelength than the nitroso
group.
Nitroso compound Nitro compound
Trimethyl nitrosomethane
λmax = 665 nm
2 methyl 2 nitro propane
λmax = 262 nm

26. Dyes based on nitroso group
 The most important nitroso compounds are derivatives of 1-nitroso-2-naphthol.
 The complex of this dye with divalent iron is a green pigment; it is resistant to
the action of light and heat and is commonly used for dyeing rubbers, in the
production of wallpaper and pencils , and in the paint and varnish industry.

27.  Nitroso derivative of 6-hydroxy-indazole gives deep green shade with excellent
fastness properties.

28. The Thionitroso Group
 Thionitroso group as a n→π* Chromogen
 Examples

29.  The Thionitroso group simply means the replacement of oxygen by
Sulphur in the Nitroso group.
 Middleton was the first to isolate a Sulphur analogue of nitrosamines
i.e. N-Thionitroso dimethyl amine.
 This compound is a deep purple crystalline solid and the colour
changes depending upon the polarity of the solvent.

30.  Thus in cyclohexane which is a less polar solvent the n to π*
transition takes place at 587 nm while in ethanol at 533 nm.
 The nitrosamine corresponding to the thionitrosamine absorbs at about
370 nm in a polar solvent and thus the sulphur Aston produces a very high
bathochromic shift.

31.  This is because of the reason that sulphur is less electronegative than
oxygen and thus the energy of the sulphur non-bonding orbitals is
considerably high.
 Due to this reason there is a decrease in the n to π* transition
causing a bathochromic shift.
 It is likely that the thionitrosamine absorb in the infrared region
because of the bathochromic shift relative to nitroso group and
these compounds are probably colorless.