Squaraine dyes

A compound of the formula:

• ##STR39## wherein:
• R.sub.1 and R.sub.4 are independently selected from the group consisting of lower alkyl (1 to 4 carbon atoms);
• R.sub.2 and R.sub.3 are independently selected from the group consisting of:
• (a) lower alkyl (1 to 4 carbon atoms),
• (b) alkyl of from 5 to 20 carbon atoms, and ##STR40## wherein R.sub.16 is hydroxyl, of lower alkoxy (1 to 4 carbon atoms), or --NH.sub.2 NH.sub.2 and salts thereof, or --NH(CH.sub.2).sub.c NH.sub.2 and salts thereof, c is 1 to 5, or ##STR41## d is 1 to 5, or ##STR42## b is 1 to 5, or --NH(CH.sub.2).sub.t --S--S--CH.sub.3 wherein t is 1 to 5, or ##STR43## wherein R.sub.29 is ##STR44## wherein R.sub.30 is lower alkyl (1 to 4 carbon atoms), or ##STR45## wherein k is 1 to 5, or lower alkoxy (1 to 4 carbon atoms), or ##STR46## and salts thereof, or dioleoylphosphotidylethanolamine, or --NH(CH.sub.2).sub.n NHR.sub.17 wherein R.sub.17 is ##STR47## wherein R.sub.18 is lower alkyl (1 to 4 carbon atoms);
• --(CH.sub.2).sub.r NH.sub.2 wherein r is 1 to 5;
• hydrogen including acid salts thereof;
• ##STR48## and n is 1 to 5, preferably 2 to 3, --NH(CH.sub.2).sub.p--R.sub.19 wherein R.sub.19 is ##STR49## wherein q is 1 to 5;
• --NH.sub.2 ;
• phenyl, amino substituted phenyl;
• and p is 1 to 15, preferably 2 to 12, and
• R.sub.5 and R.sub.6 are independently selected from the group consisting of hydrogen, hydroxyl, lower alkoxyl (1 to 4 carbon atoms), alkoxyl of from 5 to 20 carbon atoms, ##STR50## wherein R.sub.20 is hydrogen or lower alkoxyl (1 to 4 carbon atoms);
• with the proviso that only one of R.sub.2 or R.sub.3 is lower alkyl (1 to 4 carbon atoms) when R.sub.5 and R.sub.6 are both hydroxyl or both hydrogen.