The Imino Group Azines

Effect of extending conjugations

Application of Imino chromogens

Comparisons

Application in dyes

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.

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.

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.

They are used as intermediate products in the production of sulphur dyes

and are also commonly used in color photography and in the productionof 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)

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.

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))

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 ).

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.

Comparison of nπ* transitions of

Imino & Carbonyl groupsCOMPOUND MAX. WAVELENGTH SOLVENT

CH3CHO 293 nm hexane

CH3N=C(CH3)2 244 nm cyclohexane

CH3COCH3 279 nm hexane

252 nm cyclohexane

N

B

u

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

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

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.

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

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.

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.

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.

As we can energy difference between n and π* orbital in nitroso group

less than carbonyl group and thus gives a strong Bathochromic shiftand thus absorbs at around 700 nm.

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 nmButanal

λmax = 180 nm

Nitrosobenzene

λmax = 750 nm

Benzaldehyde

λmax = 247 nm

Phenylbenzaldehyde

λmax = 428 nm

Phenylnitrosomethane

λmax = 678 nm

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.

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.

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

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.

Compound Wavelength (nm)

361

357

550

570

n butyl nitrite

Ethyl thionitrite

Trifluoromethyl thionitrite

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

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.

Nitroso derivative of 6-hydroxy-indazole gives deep green shade with excellent

fastness properties.

The Thionitroso Group

Thionitroso group as a n→π* Chromogen Examples

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.

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.

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.