preparation of novel mono phthalides is disclosed . specifically , these compounds are chromogenic monophthalides of the formula ## str1 ## wherein a is as hereinafter defined and selected from moieties of the type ## str2 ## wherein l is an indole moiety as hereinafter defined ; wherein b is a moiety of the type ## str3 ## as hereinafter defined . the process disclosed comprises condensing indolylethylene with a keto acid or its derivative and an electron acceptor in an organic solvent .