Dyes and solvatachromism Flashcards
1
Q
Dyes
A
- λ of light absorbed depends on chemical structure
- Molecules of coloured organic compounds contain two parts:
1. An aromatic ring or a fused ring system with extensive electron delocalisation e.g.
look at ppt
- An extensive conjugated double bond system containing unsaturated groups, known as chromophores, e.g
azo carbonyl nitroso ethylene nitrito
look at ppt
2
Q
The intensity of colour can be increased by
A
- substituents containing lone pairs of electrons to the aryl ring ‘auxchromes’
look at ppt
– the entire structure of the colorant is called the chromogen
3
Q
approximate relative strengths of bonding between a dye and a fabric
A
look at ppt
4
Q
Classification of colorants
A
- The Colour Index International, is a comprehensive list of known commercial dyes and pigments and is updated regularly.
- Each colorant is given a Colour Index (C.I.) Name and Number e.g. C.I Acid Red 37
- Classification is by chemical structure and by method of application
- 25 structural classes according to chemical type.
- Most important are:
- azo dyes, b) anthraquinone dyes, c) phthalocyanines
5
Q
Azo dyes
A
- Azobenzene is the chromophore of azo dyes.
- Colour can be modified and intensity of the colour increased by varying the auxochromes
look at ppt
- The molecular structures of some azo dyes showing the auxochrome
6
Q
Anthraquinone dyes
A
- Based on anthracene
look at ppt
7
Q
Phthalocyanines
A
Phthalocyanines are made up of four
molecules of isoindole:
coordinate with metal atoms.
The most important, (2% of all colorants), are the copper phthalocyanines, used for their brilliant blue and green colours e.g. C.I. Direct Blue 86:
Sulphonic acid group helps water solubility
8
Q
Classification of colorants by methods of application
A
- To obtain the required shade the dyer makes mixtures of dyes and must ensure that these are compatible.
Dye transfer from solution to fibre is controlled by: - pH of the dyebath (for acid and basic dyes)
- an electrolyte (a solution of sodium sulfate or chloride)
- the temperature (within the range of ambient to 400 K)
- dispersing agents (surfactants) that produce a stable aqueous dispersion of dyes if they are of low solubility
9
Q
AUXILIARIES for TEXTILES
A
- • Antifoams e.g. silicones
- • Levelling and anti-migration agents
- • Wetting and deaerating agents
- • Lubricants and anticrease
- • Dispersing and sequestering agents
- • Detergent and washing agents e.g. non-ionic or anionic detergents
- • Antistatic agents
- • Dye Fixative
- • Corrosion inhibitors
- • Stabilizer ,pH buffer
- • Dye fixative stripper
- • Fire retardant
- • Printing auxiliaries
- • Oil and water repellents
- • Catalysts
- • Odour masking
- • Moisture management agents
- • Anti-slip agents
- • Elasticity control agents
look on ppt
10
Q
Example of direct dye for dying cotton
A
- Linear and planar structure binding to cellulosic chains in the fibre, via intermolecular (including hydrogen) bonding.
look at ppt
11
Q
Reactive dyes
A
- fast dyes that would covalently bond to fabrics rather than by weak intermolecular forces.
- During the 1950’s, chemists at ICI modified the structure of azo dyes by adding reactive groups (trichlorotriazane) to combine with the amino groups of proteins in wool.
- dye molecules reacted with both the amine and hydroxyl groups on cotton fibres under alkaline conditions (not so good for wool as it dissolves in alkali) with the elimination of molecules of hydrogen chloride
12
Q
basic dyes
A
- Earliest synthetic dye e.g. Mauveine
- The chromophore is present as a cation.
- Mauveine is a mixture of four related aromatic compounds differing in number and placement of methyl groups. There are now 12 related structures.
- ~100 basic (cationic) dyes whose colours span reds, yellows and blues, with bright strong shades.
- many are triarylcarbonium ions
- e.g. C.I. Basic Green 4 (Malachite Green)
13
Q
disperse dyes (group 1)-
A
- Hydrophobic and almost insoluble in water. Affinity for hydrophobic fibres, e.g.polyesters, and are applied as very fine dispersions in water. Mostly azobenzene compounds and can give colours across the spectrum. Some are anthraquinone.
- smallest dye molecules among all dyes
- Contain nitro, amine, hydroxyl substituents.
14
Q
Pigments
A
- used in the coloration of paints, printing inks, ceramics and plastics.
- finely divided solid essentially insoluble.
- The chromophores used in pigments are usually the same as those used in dyes but the pigments are large molecules and do not have solubilising groups.
- They contain groups that form intermolecular bonds that help to reduce solubilities.
- The larger the molecule, the more opaque the pigment.
- Organic pigments generally produce a higher intensity and brightness of colour than inorganic pigments such as chrome yellow (lead(II) chromate(VI)).
- pigments have wavelength-selective absorption.
- differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.
- inorganic pigments e.g vermilion (mercury sulfide) or cadmium yellow (cadmium sulfide)
- absorb light by transferring an electron from the negative ion (S2−) to the positive ion (Hg2+ or Cd2+).
- “charge-transfer” complexes with broad absorption bands that subtract most of the colors of the incident white light.
15
Q
azo chemistry organic pgiments
A
- yellow, orange and red shade areas e.g. a monoazo pigment
- Copper phthalocyanines provide the majority of blue and green pigments. Structurally complex but inexpensive to make e.g.
ppt
16
Q
optical brighteners
A
- organic additives that absorb UV light and then transmit it in the blue range
- work via a fluorescent mechanism
- make whites appear ‘brighter’ to the human eye (by emitting more visible light than that which shines on it)
- mask yellowing in lacquers, paints, inks, plastics, photo-processing solutions and fibers.
17
Q
Laundry detergent fluorescing under uv light
A
- Fluorescent emission is a short-lived period of light emission by a fluorophore, unlike phosphorescence, which is long-lived
- “whitening” effect; they make yellow/orange materials look less so, by compensating the deficit in blue and purple light reflected by the material, with the blue and purple optical emission of the fluorophore
18
Q
The most common classes of compounds with this property are the stilbenes (fluoresce)
A
- Used in laundry detergents and cosmetics
- 90 optical brighteners produced commercially, only a few are used.
- Fade on prolonged exposure to UV due to formation of optically inactive cis-stilbenes
- Degraded by O2 in air like most dyes
- Blue light emission boosted by addition of polyols e.g. PVA
- Excess brightener gives greening due to other emissions
19
Q
Sigma bond
A
- A covalent bond resulting from the formation of a molecular orbital by the end-to-end overlap of atomic orbitals, denoted by the symbol σ.
look at ppt
20
Q
pi bond
A
- A covalent bond resulting from the formation of a molecular orbital by side-to-side overlap of atomic orbitals along a plane perpendicular to a line connecting the nuclei of the atoms, denoted by the symbol π.
look at ppt
- Non bonding- when there is no overlap of orbitals on adjacent atoms.
21
Q
Electronic (UV) spectroscopy
A
- Light absorbed – electron excited to higher molecular orbital
- Typical for unconjugated organic molecules
* below 150 nm (vacuum UV)
n , below 200 nm (quartz UV)
n * 200-400nm (quartz UV)
look at ppt
22
Q
Definitions
A
- Chromophores
- Part of molecule responsible for absorption
- Auxochromes
- Groups that modify absorption of neighboring chromophores
- Often have lone pairs, e.g. –OH, -OR, -NR2, -halogen
- Bathochromic shift: towards longer wavelength
- Hypsochromic shift: towards shorter wavelength
- Hyperchromic effect: increase in peak absorbance
- Hypochromic effect: decrease in peak absorbance
23
Q
Effect of conjugation lamda max
A
- The n→π* transition for methyl vinyl ketone is at 324 nm, and the π→π* transition is at 219 nm.
- Both λmax values are at longer wavelengths than the corresponding λmax values of acetone because methyl vinyl ketone has two conjugated double bonds
look at ppt
24
Q
Conjugation and energy
A
- Conjugation raises the energy of the HOMO and lowers the energy of the LUMO, so less energy is required for an electronic transition in a conjugated system than in a nonconjugated system
- The more conjugated double bonds there are in a compound, the less energy is required for the electronic transition.
- Both the λmax and the molar absorptivity increase as the number of conjugated double bonds increases
- λmax of a compound can be used to predict the number of conjugated double bonds in the compound.
look at ppt
25
Q
What happens if the compound has enough conjugated double bonds
A
- it will absorb visible light (λmax > 400 nm) and the compound will be colored.
- Carotene, a precursor of vitamin A, is an orange substance found in carrots, apricots, and sweet potatoes.
- Lycopene is red and is found in tomatoes, watermelon, and pink grapefruit.
26
Q
Increased conjugation leads to
A
- longer absorption wavelengths
- Butadiene absorbs at 170nm whereas conjugated polymer lycopene absorbs at 470nm
27
Q
Effect of sorption of dye on compound
A
- Extensive conjugation of dye sorbed to albumin yields large bathochromic shift to longer λ
look at ppt
28
Q
Auxochromes-
A
- Auxochromes with lone pairs often lead to increased delocalization (and conjugation) and therefore a bathochromic shift
look at ppt
29
Q
Solvent pH- effect on phenol spectrum
A
- Increasing pH shifts equilibrium to right
- More non-bonding electrons in phenoxide ion
higher extinction coefficient
greater delocalization bathochromic shift
look at ppt
30
Q
Solvent pH:effect on aniline spectrum
A
- Decreasing pH shifts equilibrium to right
- No non-bonding electrons in anilinium ion
lower extinction coefficient
less delocalization hypsochromic shift
look at ppt
31
Q
Polarity effects of the solvent
A
- Change in colour of a chemical substance due to solvent polarity “solvatochromic effect”
- Ground and excited states of a molecule have different dipole moments - a change in solvent polarity will stabilise these states differently changing energy gap
- Gives info on the interactions of solute and solvent molecule
- -ve solvatochromism is a hypsochromic (blue shift) with increasing solvent polarity e.g 4-(4′-hydroxystyryl)-N-methylpyridinium iodide, which is red in 1-propanol, orange in methanol, and yellow in water.
- +ve solvatochromism is a bathochromic (red shift) with increasing solvent polarity e.g. 4,4’-bis(dimethylamino)fushone
- Orange in nonpolar toluene, red in slightly polar acetone and red-violet in more polar methanol
32
Q
Effect of solvent polarity on tyrosine spectrum
A
- Solid line: solv = water
- Dashed line: 80% water and 20% ethylene glycol
look at ppt
33
Q
Uses of solvatochromism
A
- Sensors
- Molecular electronics for construction of molecular switches
- Detection of explosives in a new technique using carbon nanotubes
- Light frequency of emitted by nanotubes changes when exposed to explosive molecules even a single molecule of the nitro aromatic explosive TNT
- Developed at MIT by Strano
- One layer thick carbon nanotube coated in protein fragments found in bee venom which bind to certain explosives like TNT
- This shifts the λ of the fluorescent light given off by the nanotube
- Easy and sensitive detection but needs a concentrator for commercial use
- Currently using ion spectrometers as cheap and reliable ( detect charged particles)
34
Q
Whats a molecular switch
A
- a molecule that can be reversibly shifted between two or more stable states
- The oldest forms of synthetic molecular switches are pH indicators, which display distinct colors as a function of pH.
35
Q
Molecular switch in context
A
- the dielectric constant and hydrogen bonding capacity are the most important properties of the solvent.
- With various solvents there is a different effect on the electronic ground state and excited state of the solute, so that the size of energy gap between them changes as the solvent changes.
- This is reflected in the absorption or emission spectrum of the solute as differences in the position, intensity, and shape of the spectroscopic bands. When the spectroscopic band occurs in the visible part of the spectrum solvatochromism is observed as a change of colour.
- Solvatochromism can in principle be used in sensors and in molecular electronics for construction of molecular switches
36
Q
Reichardts dye dissolved in different solvents
A
look at ppt
