Improving the photochromic properties of indolo[2,1-b][1,3]benzoxazines with phenylic substituents

• Photochromic properties of 10 new indolo-benzoxazine derivatives were investigated.
• Group I compounds have phenylic substituents in the 4th position of indole ring.
• Group II compounds have phenylic substituents in the o-position of phenolic ring.
• Group I compounds exhibit a peculiar induced absorption band in the near-IR.
• Group I compounds excel group II compounds in quantum yield and fatigue resistance.

This work presents a series of new photochromic compounds of indolo[2,1-b][1,3]benzoxazine family. Photochromic systems based on the opening and closing of [1,3]oxazine ring are among the fastest photochromic switches, completing the full “on-off” cycle in less than few tens of nanoseconds. Furthermore, indolo[2,1-b][1,3]benzoxazines are distinguished for their excellent fatigue resistance as well as substantial quantum yield of photochromic transformation, both important for practical applications. Photochromic properties can be selectively altered by introducing structural modifications to the parent compound, namely, by varying substituents attached to certain positions. We aimed to enhance the photochromic performance of indolo[2,1-b][1,3]benzoxazines by modifying the parent molecule with phenylic substituents. In this article we introduce 10 new compounds, which are classified into two groups according to the attachment site of the phenylic substituent: group I compounds have modifications in the 5th position of the indole ring, group II–in the 4th position of the indolo[2,1-b][1,3]benzoxazine molecule. The investigation of photochromic properties of the compounds in acetonitrile solutions revealed the outstanding parameters of the new molecules. Group I compounds have an exceptional feature in the induced absorption spectra – an additional band in the nIR, which is observed for the first time in the compounds of indolo[2,1-b][1,3]benzoxazine family. On the other hand, all group II compounds display enhanced switching speeds; indeed, the fastest relaxation time value ever reported for similar compounds was measured in this group. In general, group I compounds were found to be superior to both parent molecule and group II compounds in both photostability and quantum yield, thus demonstrating the potential of molecular design for development and realization of photochromic compounds with desired properties.

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