When organic molecules are coloured, this is typically due to conjugation. Conjugation refers to the path available to electron movement created through neighbouring pi orbitals with delocalised electrons, such as the alternate double bonds in aromatic rings. Electrons can move through the connected pi orbitals, after first being excited by ambient electromagnetic radiation. The longer the available path/conjugation, the higher the energy of light absorbed by the electrons. Thus, conjugation can cause the colour of the molecule to vary depending on the energy of the light being absorbed. The parent molecule (shown below) is yellow in colour due to the conjugation between the alternate double bonds. Generally speaking, conjugation can only exist in molecules that are planar – i.e. they exist in a flat conformation. In the case of the above structure (1), there is the potential to increase the conjugation by forcing the central section to remain planar by placing a hydrocarbon tether/ring at that site (shown below). Figure 1 - Ball-and-stick models of the geometry-optimized structures of 1, 5-7 showing the change in overall planarity caused by the additional hydrocarbon ring. (1) Considering the need for planarity over the molecule, rank the systems 5, 6 and 7 from the least to the most conjugated. 5 is Blank 1 Question 8[select: , the most conjugated, neither most nor least conjugated, least conjugated] . 6 is Blank 2 Question 8[select: , the most conjugated, neither most nor least conjugated, least conjugated] . 7 is Blank 3 Question 8[select: , the most conjugated, neither most nor least conjugated, least conjugated] . (1) Huck, L. A.; Leigh, W. J. Journal of Chemical Education 2010, 87 (12), 1384-1387.多重下拉选择题