the chromogenic compounds of this invention have the following general formula : ## str5 ## wherein y 3 is independently selected from dialkylamino including symmetrical and unsymmetrical alkyl ( c 1 - c 8 ), alkylcycloalkylamino , dicycloalkylamino , alkylarylamino , diarylamino , dialkoxyalkylamino , ## str7 ## wherein each of y 1 , y 2 , y 4 - y 15 is the same as y 3 or independently selected from alkyl ( c 1 - c 8 ), alkoxy ( c 1 - c 8 ) or halogen ; wherein r is independently selected from alkyl ( c 1 - c 8 ), alkoxyalkyl , aryl ( substituted or unsubstituted ); wherein z is independently selected from ch 2 , o , s , so 2 or nr . wherein each r a is independently selected from alkyl ( c 1 - c 8 ) and hydrogen ; wherein each n is an integer selected from 0 to four ; wherein each of x 1 - x 4 is independently selected from hydrogen , halogen , alkyl ( c 1 - c 8 ), alkoxy ( c 1 - c 8 ), dialkylamino including symmetrical and unsymmetrical alkyl ( c 1 - c 8 ), alkylcycloalkylamino , dicycloalkylamino , alkylarylamino , diarylamino , ## str9 ## wherein each l 1 and l 2 is the same or different and is each independently selected from indole moieties ( j1 ) through ( j4 ) ( l 1 need not be the same as l 2 ) ## str10 ## wherein in ( j1 ) through ( j4 ) above each of r 5 , r 6 , r 13 , r 14 , r 21 , r 22 , r 29 and r 30 need not be the same and is each independently selected from hydrogen , alkyl ( c 1 - c 8 ), cycloalkyl , alkoxyalkyl , aroxyalkyl , substituted or unsubstituted aryl such as phenyl , naphthyl or heterocyclyl ; each of r 1 , r 2 , r 3 , r 4 , r 7 , r 8 , r 9 , r 10 , r 11 , r 12 , r 15 , r 16 , r 17 , r 18 , r 19 , r 20 , r 23 , r 24 , r 25 , r 26 , r 27 and r 28 need not be the same and is each independently selected from hydrogen , alkyl ( c 1 - c 8 ), cycloalkyl , substituted or unsubstituted aryl , halogen , alkoxy ( c 1 - c 8 ), aroxy , cycloalkoxy , dialkylamino including symmetrical and unsymmetrical alkyl ( c 1 - c 8 ), alkylcycloalkylamino , dicycloalkylamino , alkylarylamino , diarylamino , ## str11 ## mono ( indolylethylenyl ) phthalides ( i ) were prepared by condensing indolylethylenes ( ii ) with keto acids or their derivatives ( iii ) using condensing agents ( iv ) [ e . g . acid anhydrides , acid chlorides and lewis acids ] in an organic solvent . preferred examples of acid anhydrides and acid chlorides are acetic anhydride , propionic anhydride and acetyl chloride . preferred lewis acids are zinc chloride , boron trifluoride etherate , zinc chloride / phosphoryl chloride and zinc chloride / thionyl chloride . since keto acids or their derivatives ( iii ) undergo ring - chain tautomerism , they contain at least two reactive centers in either open or ring structure . ring isomers can form derivatives not only from cyclic but also from acyclic structure , depending on the nature of reagents , temperature , solvent and substitution on ( iii ). on the other hand , open or cyclic isomers may yield cyclic derivatives . ## str12 ## in the following examples , general procedures for preparing certain mono ( indolylethylenyl ) phthalides of structure ( i ) are described ; and the examples are not intended to be exhaustive and the moieties , as previously defined , are all eligible for use in any combination in preparing the compounds . unless otherwise noted , all measurements , percentages and parts are by weight and in the metric system . in the following examples , general procedures for preparing certain mono ( indolylethylenyl ) phthalides of structure ( i ) are described ; and the examples are not intended to be exhaustive and the moieties , as previously defined , are all eligible for use in any combination in preparing the compounds . unless otherwise noted , all measurements , percentages and parts are by weight and in the metric system . in table 1 , &# 34 ; sh &# 34 ; refers to a shoulder in the absorption spectra . as an illustration , in fig1 there is a shoulder at 570 nm . 2 -( 4 - diethylaminobenzoyl ) benzoic acid ( 3 . 0 g , 0 . 01 mole ) and 1 , 1 - bis ( 1 - ethyl - 5 - methoxy - 2 - methylindole - 3 - yl ) ethylene ( 4 . 0 g , 0 . 01 mole ) in 1 , 2 - dichloroethane ( 20 ml ) and acetic anhydride ( 20 ml ) were heated at 100 ° c . ( oil bath temperature ) for 4 hours . the reaction mixture was cooled to room temperature ; treated with ice , toluene and aqueous sodium hydroxide ( 10 %); stirred at 60 ° c . for 30 minutes ; toluene layer separated and the aqueous layer extracted twice with toluene . the toluene extracts were combined , washed twice with hot water , dried and concentrated . the residue was chromatographed on silica gel using toluene and toluene : acetone :: 4 : 1 as eluants . fractions containing the blue band were collected , combined and concentrated . the residue was recrystallized from 1 , 2 - dichloroethane / methanol . the product was obtained as a white solid , m . p . : 217 °- 219 ° c . ; yield : 5 . 5 g ( 81 %). a solution of this product gives a blue color to paper coated with a phenolic resin , with reflectance minima at 570 ( shoulder ) and 703 nm ; and a royal blue color to paper coated with silton clay , with reflectance minima as a broad band from 550 to 750 nm . the calculated analysis for c 44 h 47 n 3 o 4 , the title compound , is c , 77 . 50 %; h , 6 . 95 %; n , 6 . 16 %; and o , 9 . 39 %. found on analysis : c , 77 . 04 %; h , 7 . 12 %; and n , 5 . 97 %. 2 -( 4 - diethylaminobenzoyl ) benzoic acid ( 3 . 0 g , 0 . 01 mole ) and 1 , 1 - bis ( 1 - ethyl - 2 - methylindole - 3 - yl ) ethylene ( 3 . 4 g , 0 . 01 mole ) in 1 , 2 - dichloroethane ( 20 ml ) and boron trifluoride etherate ( 2 ml , 2 . 3 g , 0 . 016 mole ) were heated ( oil bath temperature 100 ° c .) with exclusion of moisture . after 10 hours , the reaction mixture was cooled to room temperature , stirred with dilute ammonium hydroxide and toluene for 10 minutes at 60 ° c . and the organic layer separated . the organic layer was washed with hot water , dried and concentrated and the resulting residue chromatographed on silica gel using toluene and toluene : acetone :: 4 : 1 as eluants . fractions containing the blue band were collected , combined and concentrated and the residue was further purified by medium pressure liquid chromatography on silica gel . after recrystallization from toluene / hexane , the product was obtained as a pale yellow solid , m . p . : 124 °- 126 ° c . yield : 4 . 5 g ( 72 %). a solution of the product gives a blue color to paper coated with a phenolic resin , with reflectance minima at 570 ( shoulder ) and 710 nm ; and a royal blue color to paper coated with silton clay , with reflectance minima as a broad band from 550 to 750 nm . the calculated analysis for c 42 h 43 n 3 o 2 , the title compound , is c , 81 . 13 %; h , 6 . 97 %; n , 6 . 76 %; and o , 5 . 15 %. found on analysis : c , 80 . 81 %; h , 7 . 27 %; and n , 6 . 80 %. 2 -( 4 - diethylaminobenzoyl )- 5 - dimethylaminobenzoic acid ( 1 . 9 g , 5 . 5 mmole ) and 1 -( 5 - chloro - 2 , 7 - dimethyl - 1 - ethylindole - 3 - yl )- 1 -( 1 - ethyl - 2 , 5 , 7 - trimethylindole - 3 - yl ) ethylene ( 2 . 3 g , 5 . 5 mmole ) were mixed with glacial acetic acid ( 25 ml ) and concentrated sulfuric acid ( 0 . 6 g , 5 . 9 mmole ) and the reaction mixture was stirred at 40 ° c . for 24 hours with exclusion of moisture . then , the reaction mixture was cooled to room temperature and poured into excess aqueous sodium hydroxide ( 10 %) and toluene ( 200 ml ). after stirring at 40 ° c . for 30 minutes , the toluene layer was separated , washed with hot water , dried and concentrated . the residue was purified by column chromatography ( silica gel ), followed by medium pressure liquid chromatography on silica gel . after recrystallization from toluene / methanol the product was obtained as a bluish white solid , m . p . : 190 °- 193 ° c . yield : 1 . 3 g ( 32 %). a solution of this product gives a blue color to paper coated with a phenolic resin , with reflectance minima at 540 ( shoulder ) and 719 nm ; and a blue color to paper coated with silton clay , with reflectance minima at 647 and 700 ( shoulder ) nm . the calculated analysis for c 47 h 53 n 4 o 2 cl , the title compound , is c , 76 . 14 %; h , 7 . 21 %; n , 7 . 56 %; o , 4 . 32 %; and cl , 4 . 78 %. found on analysis : c , 75 . 77 %; h , 7 . 16 %; n , 7 . 63 %; and cl , 4 . 93 %. 3 - acetoxy - 3 -( 4 - dimethylaminophenyl )- 4 , 5 , 6 , 7 - tetrachlorophthalide ( 4 . 5 g , 0 . 01 mole ), 1 , 1 - bis ( 1 - ethyl - 2 - methylindole - 3 - yl ) ethylene ( 3 . 4 g , 0 . 01 mole ) and zinc chloride ( 1 . 4 g , 0 . 01 mole ) in 1 , 2 - dichloroethane ( 40 ml ) were refluxed with stirring in a moisture free atmosphere . after 10 hours , the reaction mixture was worked up as described in example 1 . the crude product was heated in hexane and filtered . yield : 3 . 2 g ( 44 %), pale yellow solid , m . p . : 211 °- 213 ° c . a solution of the product gives a bluish green color to paper coated with a phenolic resin , with reflectance minima at 600 ( shoulder ) and 719 nm ; and a royal blue color to paper coated with silton clay , with reflectance minima at 600 and 720 ( shoulder ) nm . the calculated analysis for c 40 h 35 n 3 o 2 cl 4 , the title compound , is c , 65 . 67 %; h , 4 . 82 %; n , 5 . 74 %; o , 4 . 37 %; and cl , 19 . 39 %. found on analysis : c , 65 . 60 %; h , 5 . 10 %; n , 5 . 58 %; and cl , 19 . 66 %. the principles , preferred embodiments and modes of operation of the present invention have been described in the foregoing specification . the invention which is intended to be protected herein , however , is not to be construed as limited to the particular forms disclosed , since these are to be regarded as illustrative rather than restrictive . variations and changes can be made by those skilled in the art without departing from the spirit and scope of the invention . table 1__________________________________________________________________________reflectance minima and color of mono ( indolylethylenyl )- phthalides on resin - coated and silton - coated papers reflectance minima ( nm )* and color onentrycompound resin - coated silton - coated__________________________________________________________________________ ## str13 ## 570 ( sh ) 703 blue 550 - 750 broad band royal blue ## str14 ## 570 ( sh ) 710 blue 550 - 750 broad band royal blue ## str15 ## 540 ( sh ) 719 blue 647 700 ( sh ) blue ## str16 ## 570 682 825 ( sh ) bluish green 572 700 ( sh ) purple ## str17 ## 565 702 greyish blue 570 698 blue ## str18 ## 600 ( sh ) 719 green 600 720 ( sh ) blue ## str19 ## 620 ( sh ) 713 bluish green 600 690 ( sh ) blue ## str20 ## 610 ( sh ) 690 ( sh ) 762 bluish green 620 ( sh ) 700 ( sh ) 758 pale green ## str21 ## 590 ( sh ) 740 pale blue 600 ( sh ) 741 greenish blue10 . ## str22 ## 610 ( sh ) 720 green 600 ( sh ) 696 green__________________________________________________________________________ * only the reflectance minima above 500 nm are reported . sh = shoulder , = phenyl