dyes and solvatochromism Flashcards
dyes
• λ of light absorbed depends on chemical structure
molecules of coloured organic compounds contain two parts
- An aromatic ring or a fused ring system with extensive electron delocalisation
- An extensive conjugated double bond system containing unsaturated groups, known as chromophores,
how can the intensity of colour be increased
by substituents containing lone pairs of electrons to the aryl ring “auxochromes
what is the entire structure of the colourant called
chromogen
see pp for
approximate relative strengths of bonding between a dye and a fabric
classification of colourants
- The Colour Index International, produced by the Society of Dyers and Colourists, in Bradford, 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.
what are the 3 most important structural classes according to chemical type
azo dyes
anthraquinone dyes
phthalocyanines
azo dyes
• Azobenzene is the chromophore of azo dyes.
• Colour can be modified and intensity of the colour increased by varying the auxochromes
see pp for diagrams
anthraquinone dyes
• Based on anthracene
phthalocyanines
• 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:
classification of colourants by methods of application
• 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
fabric dying machine
dyestuff and auxiliaries can be injected into the machine by the high pressure-dosing pump
auxiliaries for textiles
- 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
see pp for
important dye and fibre table
direct dye for dying cotton
linear and planar structure binding to cellulosic chains in the fibre, via intermolecular bonding
reactive dyes
- 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
basic dyes
- Earliest synthetic dye e.g. Mauveine
- Discovered by William Henry Perkin at age of 18
- 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)
disperse dyes group 1
- 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.
pigments
• used in the coloration of paints, printing inks, ceramics and plastics.
• finely divided solid essentially insoluble.
•
• pigments have wavelength-selective absorption.
• differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.
optical brighteners
- 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.
laundry detergent fluorescing under UV light
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
sigma bond
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.
see pp for diagram
Pi bond
• 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.
see pp for diagram
non-bonding
• when there is no overlap of orbitals on adjacent atoms.
electronic UV spectroscopy
• Light absorbed – electron excited to higher molecular orbital
see pp for diagram
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 shorted wavlength
hyperchromic effect
increase in peak absorbance
hypochromic effect
decrease in peak absorbance
effect of conjugation on wavelength max
- 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
- 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.
- If a compound has enough conjugated double bonds, 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.
- Increased conjugation leads to longer absorption wavelengths
- Butadiene absorbs at 170nm whereas conjugated polymer lycopene absorbs at 470nm
effect of sorption of dye on compound
• Extensive conjugation of dye sorbed to albumin yields large bathochromic shift to longer λ
auxochromes
• Auxochromes with lone pairs often lead to increased delocalization (and conjugation) and therefore a bathochromic shift
solvent pH: effect on phenol spectrum
• Increasing pH shifts equilibrium to right
• More non-bonding electrons in phenoxide ion
higher extinction coefficient
greater delocalization bathochromic shift
solvent pH: effect on aniline spectrum
• Decreasing pH shifts equilibrium to right
• No non-bonding electrons in anilinium ion
lower extinction coefficient
less delocalization hypsochromic shift
(l,e)=(280,1430)
polarity effects of the solvent
- 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
effect of solvent polarity on tyrosine spectrum
• Solid line: solv = water
• Dashed line: 80% water and 20% ethylene glycol
see pp for graph
uses of solvachromism
• 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)
molecular switch
- 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.
- In this context, 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.
see pp for
Reichardt’s dye
chromophores in pigments
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 and inorganic pigments
Organic pigments generally produce a higher intensity and brightness of colour than inorganic pigments such as chrome yellow (lead(II) chromate(VI)).
inorganic pigments
- 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.
organic pigments
Many organic pigments are based on azo chemistry and dominate the 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
fluorescent emission
- 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
- The most common classes of compounds with this property are the stilbenes, e.g., 4,4′-diamino-2,2′-stilbenedisulfonic acid
-ve solvatochromism
-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
+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
