104 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS /.C. :C k •- •C : : C( • /C: ..C• This type of transition requires about 170 kcal./gm.-mole for a single olefinic group, and corresponds to an absorption at 170 m/•, in the far ultraviolet. At this point the emax. iS high and the transition probability is about 1.0. As the unsaturated structures increase in length, i.e., there are greater numbers of unsaturated bonds in the molecule, the ionic and homopolar resonance structures contribute more and more to both the ground state and excited states of the molecule. This results in the lowering of the potential energy levels of both the ground and the excited states with respect to the unconjugated system. But since the contribution of res- onance structures is greater in the excited than in the ground state, the energy level of the former is lowered more than that of the latter. The transition energy therefore is reduced and the absorption bands are dis- placed to longer wavelengths (bathochromic shift). e ß ß e C C•C C • C=C C•C • --C--C----C--C-- ß . .. As the number of conjugated double bonds in the resonating system in- creases, this shift of em=x. to longer wavelengths continues, as shown in Table 1. TABLE 1--ABSORPTION MAXIMA OF CONJUGATED UNSATURATES OF THE SERIES (--C=C )n kmax. ½max. Color 1 170.0 15,000 Colorless 2 217.0 21,000 Colorless 3 258.0 35,000 Colorless 4 310.0 42,000 Pale Yellow 5 328.0 51,000 Yellow (Vitamin A) 11 472.0 170,000 Red 15 528.0 150,000 Violet 0 Kekul• structures 0:0 • :0• Dipolaf structures Homopolar structures

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: