Cationic dyes of the general formula ##STR1## where A and A' may be identical or different and are each a monovalent radical of a dye from the series comprising the azo or quinophthalone dyes, or A and A' independently of one another are each one of the radicals of the formulae ##STR2## R is C.sub.1 -C.sub.6 -alkyl, benzyl or aryl, R.sup.1 is hydrogen, C.sub.1 -C.sub.4 -alkyl or aryl, D' and D" are respectively a monovalent and a divalent radical of a dye from the series comprising the triphenylmethane or xanthene dyes, n is an integer .gtoreq.1 and m is an integer .gtoreq.1, the sum (n+m) being .ltoreq.10 on average, are soluble in dilute acids, and, when they are used for wet-end dyeing of paper, give bleeding-fast, bleachable dyeings and a colorless effluent.

The present invention relates to cationic dyes which contain 
bismethyleneimidazole groups, and to their use. 
Cationic dyes which contain methyleneimidazole groups have been proposed 
(U.S. patent application Ser. No. 233,479), the dyes being obtained by 
reacting a neutral dye with an imidazole and formaldehyde. These dyes, 
when they are used for wet-end dyeing of paper, give bleeding-fast dyeings 
coupled with low contamination of the effluent. 
It is an object of the present invention to provide cationic dyes which are 
superior to those described in the above patent application in respect of 
bleeding-fastness on paper, and performance in wet-end dyeing of paper. 
We have found that this object is achieved by cationic dyes of the general 
formula 
##STR3## 
where A and A' may be identical or different and are each a monovalent 
radical of a dye from the series comprising the azo or quinophthalone 
dyes, or A and A' independently of one another are each one of the 
radicals of the formulae 
##STR4## 
R is C.sub.1 -C.sub.6 -alkyl, benzyl or aryl, R.sup.1 is hydrogen, C.sub.1 
-C.sub.4 -alkyl or aryl, D' and D" are respectively a monovalent and a 
divalent radical of a dye from the series comprising the triphenylmethane 
or xanthene dyes, n is an integer .gtoreq.1 and m is an integer .gtoreq.1, 
the sum (n+m) being .ltoreq.10 on average. 
The substituents D'--CH.sub.2 --, --D"--CH.sub.2 -- and R-- are bonded to 
carbon atoms of the imidazole ring, and, furthermore, the --CH.sub.2 
groups of D'--CH.sub.2 -- and --CH.sub.2 --D"-- are bonded to carbon atoms 
of the aromatic rings of D"-- and --D"-- respectively. 
Preferred dyes I are those in which A and A' are each a quinophthalone 
radical or D' and D" are each the radical of a triphenylmethane or 
xanthene dye, the sum (m+n) being .ltoreq.10 on average, preferably 
.ltoreq.8. 
Compared with the conventional dyes, the dyes I, when they are used for 
wet-end dyeing of paper, give bleeding-fast dyeings and a colorless 
effluent. They are from slightly to readily soluble in dilute acids, eg. 
5-30% strength by weight acetic acid, propionic acid, methoxyacetic acid 
or methanesulfonic acid. 
The dyes I are obtained by condensing an appropriate dye D'H 
(.dbd.D"H.sub.2) (IV) with a bifunctional imidazole, preferably a 
bishydroxymethylimidazole V, in the presence of an acidic agent; 
##STR5## 
In formula V, R.sup.1 is hydrogen, C.sub.1 -C.sub.4 -alkyl or aryl, 2 of 
the radicals R are --CH.sub.2 --OH and the remaining radical R is C.sub.1 
-C.sub.6 -alkyl, aryl, eg. phenyl, or benzyl. 
Preferred imidazoles V are those derived from 2-C.sub.1 -C.sub.6 
-alkylimidazoles, eg. 2-methylimidazole or 2-ethylimidazole, and from 
2-arylimidazoles, eg. 2-phenylimidazole. 
The molar ratio of IV to V is as a rule from 1:0.8 to 1:2.0, preferably 
from 1:1 to 1:1.2. 
The reaction is carried out in general at from 0 to 50, preferably from 5 
to 20, .degree.C. The reaction time is as a rule from 2 to 20 hours, and 
is chosen so that the resulting dye I is still readily soluble in dilute 
acids. The reaction time depends in particular on dye IV. Thus, for 
example, a readily soluble dye is obtained if C.I. Solvent Dye, C.I. No. 
45,195, is stirred with the imidazole V in 96% strength by weight sulfuric 
acid for 6 hours at from 20.degree. to 25.degree. C. 
If a dye which contains more than one radical (ring) capable of reacting 
with the hydroxymethyl groups in V is used as IV, eg. the xanthene dye 
C.I. No. 45,195, reaction with the bifunctional imidazole V gives a 
product I which contains more than 2 dye radicals, ie. n and/or m&gt;1. In 
this case, if the reaction time is excessively long, the products are 
polymeric dyes which are only slightly soluble, if at all, in dilute acid. 
As a rule, dyes where (n+m) .ltoreq.10, in particular .ltoreq.8, are still 
readily soluble, and those where (m+n) .ltoreq.6 are particularly readily 
soluble. 
As a consequence of the synthesis, suitable terminal groups for the 
polymeric dyes of the formula I are --OH and --D'. 
The optimum conditions for the reaction of a particular dye may be readily 
determined by a simple series of experiments. 
Acidic agents which are suitable for the process and which may 
simultaneously serve as the reaction medium are, for example, 80-100% 
strength by weight sulfuric acid or phosphoric acid, with or without 
agents which split off water, eg. sulfur trioxide (oleum), phosphorus 
pentoxide, metaphosphoric acid, polyphosphoric acid, acetic anhydride, 
benzenesulfonic acid or toluenesulfonic acid. A preferred acidic medium 
which may simultaneously serve as the reaction medium is 96-100% strength 
by weight sulfuric acid, not more than 12% strength by weight oleum or a 
mixture of 85% strength phosphoric acid and phosphorus pentoxide. 
The reaction mixture is discharged onto water or ice/water, and the dye is 
precipitated as the free base by adding a basic agent, eg. sodium 
hydroxide solution, potassium hydroxide solution or ammonia, and is 
separated off by filtration and dried, or processed further in the form of 
the water-moist press cake. 
Particularly preferred dyes are those of the formulae 
##STR6## 
where R' and R" are each C.sub.1 -C.sub.3 -alkyl, p is from 3 to 6 on 
average, E' is --OH and E is --H or 
2-phenyl-4-hydroxymethyl-4-methyleneimidazole.

The Examples which follow illustrate the invention. Parts and percentages 
are by weight. 
EXAMPLE 1 
31 parts of C.I. Solvent Dye No. 45,195 and 15.2 parts of 
2-phenyl-4,5-dihydroxymethylimidazole are added successively to 270 parts 
of 96% strength sulfuric acid at 0.degree.-5.degree. C. in the course of 3 
hours. Thereafter, the reaction mixture is stirred for 4 hours at from 
20.degree. to 25.degree. C. (the starting materials are no longer 
detectable in the thin layer chromatogram), and then discharged onto 1,000 
parts of ice-water. After the resulting mixture has been neutralized with 
ammonia, the product is filtered off under suction and dried at 60.degree. 
C. under reduced pressure. 
Yield: 47 parts of a dye which probably has the formula 
##STR7## 
The molar ratio of dye to imidazole is 5:6. 
Analysis: N Calculated: 8.28%; N Found: 8.2%; p.apprxeq.5. 
The dye is insoluble in organic solvents, such as methanol, ethanol of 
chlorobenzene, but readily soluble in dilute acids, such as acetic acid, 
propionic acid, methoxyacetic acid or methanesulfonic acid. 
.lambda..sub.max =522 nm (dilute acetic acid). 
EXAMPLE 2 
The procedure described in Example 1 is followed, except that the dye of 
the formula 
##STR8## 
is used instead of C.I. Solvent Dye No. 45,195. 
Yield: 48 parts of a dye which probably has the formula 
##STR9## 
The molar ratio of dye to imidazole is 5:6. 
Analysis: N Calculated: 7.98%; N Found: 8.0%; p.apprxeq.5. 
.lambda..sub.max (in 10% strength acetic acid): 524 nm. 
EXAMPLE 3 
10.2 parts (0.05 mole) of 2-phenyl-4,5-dihydroxymethylimidazole and 28.3 
parts (0.05 mole) of the dye of the formula 
##STR10## 
are introduced successively into 150 parts of concentrated sulfuric acid 
at from 0.degree. to 5.degree. C. in the course of 3 hours. The reaction 
mixture is allowed to warm up to room temperature, stirred for a further 3 
hours at from 20.degree. to 25.degree. C., discharged onto 2,000 parts of 
ice, and then neutralized with 450 parts of 50% strength sodium hydroxide 
solution. The precipitated dye is filtered off under suction, washed, and 
dried at 70.degree. C. under reduced pressure. 
Yield: 31 parts of a dye which probably has the formula 
##STR11## 
The molar ratio of starting dye to imidazole is 5:5. 
p.apprxeq.5. 
.lambda..sub.max =524 nm (in 10% strength acetic acid). 
EXAMPLE 4 
25.8 parts of Solvent Blue 23, C.I. No. 42,760 and 15.3 parts of 
2-phenyl-4,5-dihydroxymethylimidazole are introduced successively into 270 
parts of concentrated sulfuric acid at from 0.degree. to 5.degree. C. The 
mixture is allowed to warm up to 20.degree.-25.degree. C., stirred at this 
temperature for 10 hours, discharged onto 1,000 parts of ice-water and 
then neutralized with ammonia. The dye is filtered off under suction and 
dried at 60.degree. C. under reduced pressure. 
Yield: 38 parts of a dye which probably has the formula 
##STR12## 
The molar ratio of starting dye to imidazole is 3:4. 
Analysis: N Calculated: 10.56%; N Found: 11.2%; p.apprxeq.3. 
.lambda..sub.max (in 90% strength acetic acid): 593 nm. 
EXAMPLE 5 
The procedure described in Example 1 is followed, except that 9 parts of 
2-methyl-4,5-dihydroxymethylimidazole are used instead of 
2-phenyl-4,5-dihydroxymethylimidazole. 
Yield: 33 parts of a dye which probably has the formula 
##STR13## 
The molar ratio of starting dye to imidazole is 5:6. 
Analysis: N Calculated: 9.19%; N Found: 8.9%; p.apprxeq.5. 
.lambda..sub.max : 524 nm (in 40% strength acetic acid). 
EXAMPLE 6 
16.1 parts of 5,6-benzoquinophthalone and 15.2 parts of 
2-phenyl-3,4-dihydroxymethylimidazole are introduced successively into 270 
parts of concentrated sulfuric acid at from 0.degree. to 5.degree. C. The 
reaction mixture is allowed to warm up to 20.degree.-25.degree. C., 
stirred at this temperature for 10 hours, discharged onto 1,000 parts of 
ice-water, and then neutralized. Thereafter, the precipitate is filtered 
off under suction and dried at 60.degree. C. under reduced pressure, 
giving 32 parts of the dye 
##STR14## 
Analysis: N Calculated: 6.88%; N Found: 8.6%. 
.lambda..sub.max (in 50% strength acetic acid): 422 nm. 
USE EXAMPLE 
2 g of a 10% strength solution of the dye from Example 1 in acetic acid are 
added to a suspension of 50 g of bleached sulfite pulp in 2 l of water, 
and the mixture is stirred gently for 15 minutes at from 20.degree. to 
25.degree. C., and then diluted with water to a solids content of 0.2%. 
This suspension is used to produce paper sheets in a laboratory 
sheet-forming apparatus, and the sheets are dried for 5 minutes at 
100.degree. C. Sheets which have been dyed red are obtained and the 
effluent from the dyeing is completely colorless. The bleeding-fastnesses 
in accordance with DIN 53,991 when various test liquids are used are rated 
at 5, ie. no bleeding. The dyeing can be bleached with hypochlorite. 
Similar results are obtained when the dyes obtained as described in 
Examples 2, 3, 4, 5 and 6 are used.