DESIGN OF COSMETIC SUN SCREENS 105 The instability of the conjugated polyolefins toward oxidation and polymerization makes them unsuitable for use as sun screens. However, the aromatic compounds constitute a very stable series of conjugated double bond structures. The conjugated system of alternating single and double bonds permit resonance structures to contribute largely to the molecular structure. For benzene there are a large number of possibilities shown on bottom of page 104. For benzene, Xmax. for the polar structures occm-s at 180 mt• and at 200 mu, in the far ultraviolet. As the length of the resonating structures is increased, again there is an increase in Xmax.. This may be accomplished by such structures as styrene. Styrene shows maximum absorption at 244 © •CH CH• •_ ,] :CH--CHa The addition of certain other groups to the benzene ring also results in increasing resonance possibilities. Thus the substitution of an amino group for a hydrogen also has a bath©chromic effect. Aniline has its absorption maximum at 230 rest. : N !q._, NH2 I II .. Further lengthening of the chromophore results in materials useful as cosmetic sun screens. The introduction of a carboxylic group para to the amino group in aniline shifts the absorption maximum to 290.0 mt•. : NH2 ½NH• I II c c /% /% O OH ©: O OH In this case, the contribution of the indicated transition is not a maximum because of the competing contributions of other, more probable structures:

106 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS : NH• H: NH• I I [ I c c //% //% 0 OH 0 0: © This ionization prevents the desired transition by bonding the unshared electron pair of the nitrogen atom in the ammonium ion, so that the quin- oidal resonance structure cannot be formed. Thus, PABA is a relatively poor sun screen, and the absorption coefficient is low. However, when ionization is prevented by esterification of the carboxylic group, the quinoidal resonance is no longer impeded. The major electronic transition in glyceryl-p-aminobenzoate is shown in the following scheme. : NH= •NHz I c c //% O OCH=--CHOH--CHzOH ©:O OCH= CHOH CH=OH Here the absorption maximum is at 297.5 m•, which corresponds exactly to the erythematogenic maximum of sunlight. To compare effectiveness, more than a 15 per cent concentration of PABA is required to equal the screening efficiency of 2 per cent glyceryl-p-aminobenzoate. The relative positions of the polar groupings in the molecule greatly affect the energy and efficiency relationships. A change from the para to the ortho isomer in aminobenzoate esters results in a decrease in the chromophore cross-sectional area. Because of this the ortho isomers (anthranilates) are much weaker absorbers than the corresponding p- aminobenzoates. When the meta-aminobenzoate is tested, it is found that the peak absorption has shifted to lower wavelengths, and that the material is a very weak absorber. This is a consequence of the structure: electronic shifts involving both polar groups are not readily possible in " o/NH,, I [•///NH= .--.'• • c c c O OR O OR ©:O OR