.f. Soc. Cosmetic Chemists 18 245-264 (1967) ¸ 1967 Society of Cosmetic Chemists of Great Britain The action of light matters on colouring K. McLAREN* Presented at the Symposium on "Colour", organised by the Society of Cosmetic Chemists of Great Britain in Eastbourne, Sussex, on 27th April 1966. Synopsis--An account is given of the fundamental mechanisms which occur when colouring matters absorb light. Whilst this usually results in photodegradation of organic colouring matters, in very few cases have the ensuing chemical reactions been elucidated but those that are understood are described. The practical method of determining fading rates, light fastness, is described in some detail and mention is also made of the other photochemical reaction which occurs with certain coloured textiles, tendering of the fibre. INTRODUCTION Colouring matters owe their definitive characteristic to their ability to absorb visible light selectively so that the light reflected from (or trans- mitted by) them appears coloured. Light is a form of energy and amolecule which has absorbed a photon is said to be in an excited state this term is very apt because it implies that the molecule is more reactive than it is in its normal condition which is called the "ground state." The study of light absorption and the very large number of mechanisms which may follow is the science of photochemistry which only began to advanc6 during the last 25 years. Considerable progress in the understanding of the basic steps in photochemistry could only be made following progress in spectroscopy and it accelerated rapidly following the introduction in 1950 of a technique for studying species whose lifetimes are measured in micro-seconds, the elegant flash photolysis technique of Porter (1). It is not immediately applicable to the technically important photochemical problem of how dyes fade, however, for reasons described in 1952 by Dr. D. A. Clibbens, then at the Shirley Institute, in stylish prose:-- "Although the fundamental scientist may begin by thinking about the behaviour of dyed textiles exposed to air and daylight, he will probably * Dyestuffs Division, Imperial Chemical Industries Ltd., Manchester, 9. 245

246 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS conclude by working with materials that are not textile fibres, that are not dyes, of which air is not an important environment and that are illuminated by some source of light other than the sun. He hopes that the study of his simple model will establish general laws that can be applied to more complicated systems. This is the method of fundamental research- a very powerful weapon, aimed, however, at a very long-range target." In complete contrast to fundamental research has been the widespread empirical method of collecting observations about the behaviour of dyed textiles on exposure to daylight when the nature of the materials and the conditions of exposure are completely beyond the control of the observer and often unknown to him. Such observations have been made over hundreds of years: Plutarch has written that Alexander the Great found among the treasures in the palace of the King of Persia large quantities of cloth dyed purple which had retained its "beautiful brilliancy" even though it was 180 years old (2). The same Alexander equipped his soldiers with a photochemical clock- pieces of chemically treated cloth worn on the arm which changed according to the length of time they were exposed to the sun- known irreverently to historians as "Alexander's Rag Time- band" ($). In 1689, Cole (4) noted that Tyrian Purple when applied to cloth in a colourless leuco form "will presently appear of a pleasant green colour and if placed in the sun it will change to a deep green and in a few minutes this will alter to a blue from that in a little time more it will be purplish red after which in an hour or two (supposing the sun is still shining) it will be a very deep purple red beyond which the sun can do no more." In 1814 Bancroft (5) carried out extensive investigations and discovered that the nature of the substrate had a marked effect on the rate of fading of a given dye and concluded that fading is sometimes due to oxidation and sometimes to reduction- a remarkable conclusion at the time as it is now known to have been perfectly correct. In 1857 Chevreul (6), the head of the dyeing department of the Gobelin tapestry works, noted that dyes did not fade in vacuo, also now known to be essentially true and possibly of technological importance in this age of space travel! The first synthetic dyes (1856-1900) were very fugitive though this defect does not appear to have been realized by dyers and printers. James Morton (7), whose firm had been making furnishing fabrics since the time of the discovery of Perkin's Mauve, has told vividly how, in 1902, he saw some of their tapestries in Liberty's shop window in Regent Street which "had changed so radically that I scarcely recognized my own handiwork.