18.9 What chemicals change colour with heat, light, or pressure?
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
18.9 What chemicals change colour with heat, light, or pressure?
Compounds that visibly and reversibly change colour when subjected to a
change in their environment are known as chromogenic materials. There are
four major categories - electrochromic, photochromic, piezochromic, and
thermochromic, all of which are extensively discussed in a recent, well
referenced, monograph in Kirk Othmer [8].
Electrochromic materials exhibit a change in light transmittance or
reflectance induced by direct current at potentials of approximately one
volt. The change usually is an oxidation-reduction reaction, using either
inorganic or organic compounds, and the colour change can occur at either
the anode or the cathode - which are usually thin films. There are two major
classes, the ion-insertion/extraction type - such as tungsten trioxide, and
the noninsertion group - such as the viologens, a family of halides of
quaternary bases derived from 4,4'-bipyridinium. One viologen example is
1,1'-diheptyl-4,4'-bipyridinium bromide [6159-05-3], which changes from
clear to bluish purple. The most common application of viologens has been
the electrochromic interior rearview mirrors available for cars since 1988.
These utilise a substituted viologen as the cathode colouring material, with
a compound like phenylene diamine as the anode colouring electrochromic
material. The mechanism details, along with a description of the ingenious
control system, are described in a recent comprehensive review of
electrochromic materials [9].
Photochromic materials undergo a reversible change in light absorption that
is induced by electromagnetic radiation, however most common applications
involve reversible changes in colour or transparency on exposure to visible
or ultraviolet light. This is often seen as a change in the visible spectrum
( 400 - 700 nm ), and can be rapid or very slow. There are two major classes
of photochromic materials, inorganic and organic.
Examples of the inorganic type are the silver halides, which are suspensions
of fine ( 10-20 nm ) silver halide crystals dispersed throughout a glass that
has been slowly cooled. An alternative technique involves diffusion of the
silver halide into the surface of the glass. The cuprous ion can catalyse
both the photochromic darking and thermal fading reactions, and the colour
can be shifted from grey to brown by the addition of gold or palladium -
which may be added to the glass in trace amounts. The most popular current
application for glass containing silver halide is for prescription eyewear.
The organic photochromic systems can be subdivided according to the type of
reaction. Geometric isomerism can result in different optical properties,
eg azobenzene ( C12H10N2 [103-33-3] ) undergoes photoisomerization, and the
cis form [1080-16-6] has higher absorbance than the trans form [17082-12-1].
Cycloaddition can produce photochromism, such as the reversible formation of
the colourless 4b,12b,endoperoxide ( C28H14O4 [74292-77-6] ) from the red
parent compound dibenzo(a,j)perylene-8,16-dione ( C28H14O2 [5737-94-0] ).
Dissociation, either heterolytic ( photolysis of triphenylmethyl chloride
[76-83-5] ), or homolytic ( photolysis of bis(2,4,5-triphenylimidazole
[63245-02-3] to form a red-purple free radical ), may also produce
photochromism.
UV can excite polycyclic aromatics, such as 1,2,5,6-dibenzacridene ( C21H13N
[226-36-8] ), to their triplet state, which has a different absorption
spectrum. Viologens may undergo redox reactions and exhibit photochromic
behaviour when crystalline and subjected to UV. The most popular photochromic
materials utilise reversible electrocyclic reactions, and are often indolino
spiropyrans and indolino spiroxazines, however the mechanism also covers
fulgide, stilbene, and dihydroindolizine examples. Details and structures
are provided in the Kirk Othmer monograph [8], and the Journal of Chemical
Education has published descriptions and preparation techniques for both
inorganic [10] and organic photochromic compounds and sunglasses [11].
Piezochromic materials change colour as they are compressed. There are three
common types:- organic molecules ( such as N-salicylidene-2-chloroaniline
[3172-42-7] ), metal cluster compounds ( such as the octahalodirhenates,
(Re2X8)2-, where X=Cl,Br,I ), and copper (II) organic complexes with
compounds like ethylene diamine. They are still being researched, and
interested readers should investigate the references in the Kirk Othmer
monograph [8].
Thermochromic materials reversibly change colour as their temperature is
changed. There are a very large number of systems, but one common example
of thermochromic transitions in metal complexes is the transition between
the blue tetrahedral and pink octahedral coordinations of cobalt (II) when
cobalt chloride is added to anhydrous ethanol and the temperature changed.
Examples of thermochromic transitions in inorganic compounds include
Ag2HgI4 [12344-40-0] and VO2, and several inorganic sulfides also have large
changes occurring in the infra-red range, and are being considered for IR
imaging applications.
There are thousands of organic thermochromic compounds, with well known
examples including di-beta-naphthospiropyran [178-10-9] ( thermally-induced
heterolytic bond cleavage resulting in ring opening), poly(xylylviologen
dibromide [38815-69-9] ( charge transfer interactions resulting in hydration-
dehydration changes ), and ETCD polydiacetylene [63809-82-5] ( thermally-
induced transitions in the unsaturated backbone resulting in rearranged side
groups ). Information on photochromism in organic and polymeric compounds is
available in published reviews [12,13].
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