Mary Virginia Orna, O.S.U. College of New Rochelle New Rochelle. New York 10801
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The Chemical Origins of Color
It was not until Newton's experiments in the 17th century that a firm theoretical founda tion regarding the nature of color was laid.
Color is a property of materials that has been an integral part of human experience in every age and civilization. It has caused man to wonder about its origin (I) and experiment in its production. Historically, the use of color was chiefly an art which developed slowly into an organized hody of knowledge so that by the Fifth Century, B.C., the Greeks were writing treatises on color harmony, perspective, and the preparation of niements. Thev also succeeded in exnandine the artist's paiette to include-white lead, red lead, &d veriilion. I t was left to the practical, aggressive Roman businessman to commercialize color usage by the manufacture and distribution of "mass-produced" colored items. This was the beginning of a more advanced, hut strictly empirical, color technology, and it was not until Newton's experiments in the Seventeenth Century that a firm theoretical foundation regarding the nature of color was laid ( 2 ) . Today, color science plays a major role in business, science and industry. I t is one of the few disciplines that cuts across the boundaries of art, biology, physics, psychology, chemistry, geology, mineralogy, and many other fields. There is hardly an object or a substance in nature that is not colored, and virtually every commercially marketed item today is either deliberately colored or de-colored (3). Chemists have always had a great interest in color. As early as 1909. N. Bierrum remarked that one of the most invariant properties of a given chemical species is its color, that is, its absorption spectrum, and he attributed the colors observed largely to the ligands in the first coordination sphere of metal complexes (4).Since that time, many a student in introductnry chemistry has monitored a chemical change by observing a color change. Virtually every quantitative analysis laboratory manual includes analysis by permanganate and dichromate redox titrations; the color changes of bromcresol green, methyl red, and crystal violet are universally used to detect the endpoints of particular t w e s of titrations: EDTA com~lexeswith certain metals yield intense blues and reds; the fdrmation of a deep hlue precipitate of Fe3[Fe(CN)& upon reaction with halide is used to determine the place of the Fe2+-Fe" couple in a potential series of the halogens, and many a freshman has seen the dramatic transformation of pale blue aqueous copper(I1) sulfate to a deep royal hlue upon the addition of ammonia. Colored compounds are all around us and we do not hesitate to utilize their properties for specific purposes. But how often do we bother to classify the compounds of color for our students or attempt to explain the nature of color? In my own experience, we often refer to the fact of color, but only rarely and sketchily do we look into the fundamental reasons for its occurrence. I would like to suggest that it may he very worthwhile to pay more attention to color and its chemistry than we have in the past. With the movement toward more descriptive chemistry in chemical education, color provides a perfect link between an easily observed and described property and an underlying theory. With the rise of s~ectroscoov .. in the undereraduate curriculum, visible spectroscopy provides a very familiar 478 1 Journal of Chemlcal Education
starting place and frame of reference within which one can introduce the other spectroscopies. Color lends itself to the inclusion of interesting, enriching, and exciting topics in introductory and advanced courses. I t could even he the pivot for a course for nonscience majors, or a c o m e for intermediate or advanced chemistry majors, or part of a special topics course or seminar. It is a topic that has stimulated the imaginations of artists and poets over the centuries; perhaps it can stimulate chemical imaginations to a greater degree than it has in the oast. Finallv. i t is a verv-nractical tonic with manv . applications to indus&ial problems and needs. Color usage extends to l i e h t i n ~oroblems in work areas. streets. airstrios. theatres andplac& of public assembly; it'is a fundamenial part of the dye, pigments, printing, textile, plastics, photographic, and entertainment industries; color coding of lights, sims, tools, electrical wirine and utilities conduits are Dart of o;r everyday lives. Color thus provides a most appropriate interface between academia and industry. I t is the purpose of this paper to discuss the topic of cnlor science with emphasis on the main classes of compounds which exhibit color in order to provide students with a basic introduction to color science and chemical educators with some ideas to include in their respective chemical curricula. The Nature of Color Color is fundamentallv a suhiective phenomenon. I t is the result of a stimulus received hy the eye and interpreted by the brain, and no one can explain the nroduction of color without taking three factors intdaccount:the light source, the object i t illuminates, and the eye and brain which receive and perceive the color. 1) The Light Source. Electromagnetic radiation has been characterized quite elegantly by the Einstein-Planck relationship, E = hv, where E is the energy of the individual photon, h is the Planck constant and u is the frequency of the radiation in sec-'. A convenient value for h is 4.136 X lo-'" eV-sec so that all energies may he rumputed directly in elertron volts. Every source of illumination emitb photons of a ranee of enereies. The intensitv of the radiation mav varv ~~"with wavelength to yield a spectral power distribution curve such as that shown in Fieure l a for a tunesten source. Since the distribution of radiant power varies from source to source, or from time to time in the same source, i t is important to specify
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Flgure 1 a. Specwal power dlsw~butloncurve-mcandescent light b. Reflsctance cuve of m o d l t y q oblecr, c, homulus facolor cuve (oofen pacewed as red).
Recent research indicates that three-color information is somehow processed in the retina and encoded in two-color, on-offsignals which find their way to higher visual centers. the tvne .. of illumination under which an ohiect is viewed. A sour(.e which emiw energy continuously over the limited resnmse ranee of the human eve. from about 380 to 720 nm, and with appreciable intensities&all wavelengths is perceived by the eve as white and is therefore descrihed as "white" light. On the other hand, a sodium arc lamp exhibits a spectrum of discrete lines with its most intense line a t 590 nm and is perceived as "yellow." Dispersion of white light with a prism or grating yields the familiar spectral colors, ranging from red a t around 700 nm to violet a t around 400 nm, a t various viewing positions. 2) The Object. Every illuminated object modifies the light which falls upon it in several of the followingways: reflection, transmission, absorption, scattering, dispersion, interference, and diffraction. Such modification gives rise to what we perceive as black, white, and colored ohjects which may he transparent, translucent, or opaque. Although light modification by an ohject is a very complex phenomenon, the process of greatest interest to the chemist is selective absorption which vields a characteristic transmission or reflectance curve for an absorbing species. I t is this curve superimposed on the spectral power distribution curve of Figure l a which provides the stimulus curve for the human eye or other suitahle detector as illustrated in Figure l b (5). 3) The Detector. A variety of color detectors is available for study, and perhaps the most familiar is the human eye itself, together with its response areas in the nervous system and the brain. Although this detector has been the subject of much study, it is still not clear how it works, hut all other visible light detector systems devised by man have tried to duplicate its results in one wav or another. The most imiortant structure in the human eye for the nercention . of color is the retina. which contains the cone cells responsible for color vision (6).One of the earliest theories of color vision was that of Thomas Young (1802) elaborated upon by H. von Helmholtz around 1852. The combined YoungHelmholtz theory postulates that since the retina responds in a t least three different ways to different colors, there must he three different kinds of r e c e ~ t o neach , of which is sensitive to a particular portion of the visible spectrum (7).It took more than a century to obtain the cone-specific spectrophotometric data that showed that there are indeed three types of cones, each of which contains one of three light-sensitive pigments (a,!?).The existence of this three-color, three-receptor physiological system is consistent with the three-primary postulate of Young and Helmholtz. However, the fact that virtually every color test given has induced subjects t o name four instead of three unique primaries gave rise to the opponentprocess theory of Hering (lo), which holds that yellow must he counted as a primary color along with red, green, and blue. This theory takes into account the complementarity of redgreen and yellow-blue and assumes that these four colors, together with black and white, form three pairs of unique sensory qualities which are mutually exclusive or "opponent" to one another. Recent research indicates that three-color information is somehow nrocessed in the retina and encoded into two-color, on-off signals which eventually find their way to the hieher visual centers via a nathwav about which we still know very little (11). I t is nossihle. without much knowledee of the mechanism of cdo;sensiti&y, to measure the relatiGe responsivity of the eve to various waveleneths of visible light. The result is a spectral response curve, and a complete description of the color stimulating the eye would then involve a combination
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of this response curve with the stimulus curve of Figure lb. If the stimulus contains all the wavelengths of the visible spectrum, the eye perceives white light, hut when only some of the wavelengths are nresent. the eve ~erceivescolor. For example, if the &en wavelengths are a6&rhed hy a modifying ohiect. the eve sees the remaining wavelenrrths, that is, the hlie-rkd combination we call magenta, t h e complement to green. Color Modification I t is obvious from this discussion that the only color-producine factor over which the chemist has anv control is the modif;ing ohject. Although most colorimetric measurements are made with reference to a articular standard illuminant and are described by a built-in "standard observer" response curve (specified by the International Commission on Illumination in 1931), the chemist ultimately is responsible only for the reflectance or transmittance curve of the modifying object since, once it leaves his hands, i t may he viewed under any light source and by any observer or detector. T o be sure, the chemist may he required to design a modifying object for observation under specified illumination conditions. Chemists mav wish to modifv colors or observe colormodifying comp&nds for a variet; of reasons. On the practical side, color matching and color formulation are very important in many different industries. From a more theoretical standpoint, spectroscopists are interested in all the electronic processes a material can undergo, and these extend throughout the ultraviolet as well as the visible rezion of the swctrum. Spectrascopic data can yield a great deal of information ahout how structural changes affect the energy spacings in molecules, and once a theoretical framework for color modification of molecules is laid, the wheel comes full circle when the industrial chemist utilizes theory to obtain the results he wants. However, the mast valid reason of all is still scientific curiwity: chemists are basically people who want to know why. In order to examine the reasons for the existence of color in material ohjects, we must constantly refer to the principles of quantum mechanics which state that only certain discrete energy levels are permitted for electrons in bound states. As a consequence, there are well-defined energy differences between these allowed levels, and when electromagnetic radiation interacts with an ohject, only those wavelengths whose energies correspond exactly to the energy level differences in the ohject will be absorbed. The absorbed energy is utilized to excite a soecies from one electronic level to another. and it is these electronic transitionsthat give rise torolor and color modification in atoms, molecules and crystals. If, in the absence of any elertromagnetic radiation, all the electrons in a molecule are in their lowest availahle enerev levels, the molecule is said to be in its ground state. ~ h s o r ~ t i of o nsuitable enereies in the form of electromagnetic radiation can promote the species to excited states. All atoms, molecules andcrystals whether colored or colorless, exhibit this hehavior since there is no difference in principle between electronic transitions giving rise to absorption in the visible and ultraviolet regions. The appearance of color, caused hy absorption over a restricted wavelenrrth range, is determined by the sensitivity of the eye. But color is not connected with anyone special feature of molecular structure (12). If the energy spacings in a molecule are large, that is, greater than ahout 3.2 eV, photons in the ultraviolet are required to excite i t from the &ound state t o an excited state. But. if the molecule can be modified structurally so that its dnergy ~
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Volume 55, Number 8, August 1978 / 479
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Accordina r * transitions and r - to the MO model, n transitions may occur in the visible region. spacings decrease, we will observe a shift in the absorption maximum to longer wavelengths corresponding to the lower energies required for electronic excitation. This is called a bathochromic shift. It is a necessary, hut not sufficient, condition for those materials which exhihit color to have sets of electronic energy levels separated by no less than about 1.7 eV (720 nm) and no more than 3.2 eV (380 nm). and thus ahsorl; in the "isible region Because vibiational A d rotational enerev levels are suoerimoosed on the electronic levels in a m o l e h e , photons at a number of wavelengths on both sides of the ~rincioalahsorotion band are alsoabsorbed. eivine rise t o theabroad bands iharacteristic of ultraviolet and visible soectra. The major electronic processes in materials which give rise to color t ~ yselective absorption mav he classified as follows 1) oreanic conmounds ~. transitions in coniuested ,2) intern,olrrular charge transfer transitions 3) intrarndeednr c h a r g ~trnmfrr transltionr 4) crystal field transitions 5) band transitions
Transltlons in Con/ugaied Organic Compounds In 1876,O. N. Witt (13)proposed that in order for an organic compound to exhihit color, it must contain an nnsaturated group called a chromophore. Some common chromo-C=C-, A==, 4 s H s . However, phores are -N=N-, their presence does not mean that a molecule will necessarily possess a color. Witt also proposed that the presence of other groups, called auxochromes, such as -OH, -NH2, and -NHR, served to strengthen and deepen the color of a molecule. A molecule containing a chromophore but not an auxochrome is called a chromoeen. Addition of auxochromes or accumulation of chromophores can lead to the development of color in a chromoeen. The basis for Witt's emoirical observations and thosiof others can be seen in three models formulated using the principles of quantum mechanics and described below. The Molecular Orbital Model The three types of valence electrons giving rise to electronic transitions in organic molecules are those involved in single bond ( a )and double bond (u) formation, and non-bonded (n) electrons associated with heteroatoms. When the constituent atoms of a molecule are a t the equilibrium distances characteristic of the stable molecule, the atomic orbitals can he linearly combined to form molerular orhitals. The total wave function of the molecule is taken an a romhination of these molecular orhitals. Those rnolerular nrhital4 where the dectron density is gvatrst hetween the nuclei are termed hondina orbitals and have energies lower than the contrihutingatomic orbitals. Those molecular orhitals with very small electron densities between the nuclei are termed antibonding orbitals, designated by a "*." Saturated molecules. which can onlv undereo a a* transitions, require large excitation energies and atworb in the far ultraviolet. Nun-honded electrons a r t less riehtlv held and thus n n* transitions require far less energy for molecular excitation and eenerallv occur in the visible reeion. Since n-orbital overlap is not as great as in a-bonding, u a* transition energies take on values intermediate to the other two twerr and occur in the near ultraviolet and visihle regions. Extended r-coniugation leads ton bathochrwnicshift in rhe n +* absorptibnmaxima. For example, ethene contains the -M- chromophore but is colorless because there is a 7.52
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480 1 Journal of Chemical Education
r*
eV energy gap between its most closely spaced energy levels and i t absorbs in the far ultraviolet. Addition of more -C=Cchromophores leads to energy levels tbat are more closely spaced. Ultimately, as in the case of B-carotene, containing 11 such chromophores (see formula below), the energy gap is only around 2.5 eV, and an absorption maximum occurs in the blue region of the visible spectrum.
An example of the effect of the presence of heteroatoms can be seen in comparing stilbene, a colorless compound, with ambenzene, which is orange (3.14.15).
Molecular orbital theory can predict the intensities of absorption hands for simple molecules.
We will briefly examine the nature of each of these types of transitions and look a t some examples.
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The Valence Bond Model
This model is very familiar to chemists in its semi-intuitive qualitative extension known as resonance theory, where the hathochromic shift we obsewed above as aresult of extended conjugation is treated in terms of an increasing number of contrihutine structures with similar relat,ive stabilities. Valence hond 'theory is very concerned with the contributions made t o snectra bv ionic states and can also oredict the expected lockion of"absorption hands.
The Free Electron Madel This is a modification of the MO theory which singles out one or several MO's for semi-quantitative treatment. This model is particularly applicable t o conjugated u-electron systems, and since most dyes and organir pigmentn fall into this category, it is quite an appropriate method. This model assumes that the conjugated r-electrons are in a well of constant notential ener& whose houndaries are rouehlv a little longe; than the leng% of the carhon chain, and tgat ihe electrons are free to move within the system. These simplifying assumptions reduce the problem t o solving the Schrodinger equation for a particle in a one-dimensional box, and this leads t o the calculation of the theoretical absorption wavelength by A = 8mcL2/(n 1)h. where m is the mass of the electron. c is the veloci&of light; L is the length of the potential energy well, n is the number of conjugated T-electrons and h is the Planck constant. This model has yielded calculated values very close to the observed values for several series of dyes
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(16).
In(ermolecular Charge Transfer Transitions There is a whole group of organic compounds for which the intramolecular model discussed above must be modified. For examole. a 1:l mixture of auinone and hvdroauinone in an . . alcohblic solution yields bdautiful dark green crystals of a material tbat could onlv be formed from one molecule of quinone reacting with one molecule of hydroquinone to form a weakly bound complex somewhat like the following
