Electronic Effects of Substituents on Indole Nitrogen on the Photochromic Properties of Indolylfulgides

The attempt to synthesize N-p-tolylsulfonylindolylfulgide by Stobbe condensation was unsuccessful because hydrolysis of the Stobbe-condensation product eliminated the sulfonyl moiety to give, after several steps, the N-unsubstituted fulgide. Pd(II)-catalyzed carbonylation of 2-butyne-1,4-diol derivative bearing the N-p-tolylsulfonylindole, however, afforded the desired N-p-tolylsulfonylindolylfulgide. A comparison of absorption spectroscopic and photochromic properties of these fulgides with those of the known N-methylindolylfulgide gave the following results. The electron-withdrawing substituent on nitrogen (i) shortened the absorption maximum of the colored form, (ii) suppressed thermal E–Z isomerization, and (iii) enlarged quantum yields of photochromic reactions.