Photochromism in spiroindolinonaphthoxazine dyes: Effects of alkyl and ester substituents on photochromic properties

The impact of substituents on the photochromism of some spirooxazines has been explored. A series of photochromic spiroindolino[2,1-b][1,4]naphthoxazine dyes was prepared with varying alkyl substitution patterns in the 3- to 6-positions of the indoline system. It was found that placing methyl functions onto the phenyl ring produced bathochromic shifts in λmax of the photomerocyanine as expected, but contrary to literature indications lowered photocoloration half-life. However, replacing the 3,3-dimethyl pattern with 3-ethyl-3-methyl substitution led to more intense photochromism: half-life increased without affecting λmax. In contrast, introduction of a cyclohexyl ring to give a 3,3-pentamethylene fragment and substitution at the 5′-position of the naphthoxazine moiety with either ester or hydroxymethyl groups significantly increased the speed of fade in general contrast to literature reports for analogous dye series. Similar effects were also observed concerning alkyl substitution in a smaller synthesised set of photochromic [1,2-b][1,4]oxazines.