The Occurrence of Hypsochromic Shifts on Alkyl Substitution: Structure and Color of Methylated Derivatives of Azulene

While two methylated derivatives of azulene show the usual bathochromic shift with respect to azulene itself, the remaining three isomers show an unusual displacement of the visible spectrum toward shorter wave-lengths. No steric effects can be claimed to account for these last results which are considered as representing genuine hypsochromic shifts brought about by alkyl substitution and due to the particularities of the structure of azulene and its derivatives. It is then shown by molecular orbital method calculations that Mulliken's N→V theory of color accounts satisfactorily for the observed facts. In the methylated derivatives showing a bathochromic shift, the calculated excitation energy of the N→VI transition appears to be smaller than the corresponding energy in azulene itself, while the reverse is true for the compounds exhibiting a hypsochromic shift. The general conditions necessary for the occurence of hypsochromic shifts upon alkylation and a few predictions of such shifts in other hydrocarbons are given.