Heat bleachable antihalation composition, elements containing same and method of use

A photothermographic element comprising a support having thereon a photothermographic layer, and further comprising, on or in said support, an antihalation composition comprising:

• (a) a formazan dye that absorbs at from about 400 to about 850 nm, and
• (b) at least one hexaarylbiimidazole that consists essentially of an oxidative arylimidazole dimer of a compound represented by either structure Ia or Ib:
• ##STR11## wherein R and R' are independently hydrogen, an alkyl group of 1 to 12 carbon atoms, an alkoxy group of 1 to 8 carbon atoms, amino, a cycloalkyl group having 5 to 7 carbon atoms in the ring, or an electron-rich heterocyclic group having 5 to 7 atoms in the ring, m and n are independently integers of 0 to 5, provided that at least one of R and R' is said alkoxy group, X and X' are independently oxy or imino, Z is alkylene of 1 or 2 carbon atoms, and R.sup.1 and R.sup.2 are independently defined the same as R and R' except that neither R.sup.1 nor R.sup.2 is hydrogen,
• said antihalation composition being bleachable when subjected to a temperature of from 80.degree. to 150.degree. C. for from 1 to 60 seconds.

A method for providing an image comprising subjecting an imagewise exposed photothermographic element to heat at from about 80.degree. to 150.degree. C. for from 1 to about 60 seconds,

• said element comprising a support having thereon a photothermographic layer, and further comprising on or in said support, an antihalation composition comprising:
• (a) a metallized formazan dye complex that absorbs at from 500 to 850 nm, said complex comprising a formazan dye and a metal that has a log (K) with said dye of less than or equal to about 25,
• (b) at least one hexaarylbiimidazole that consists essentially of an oxidative arylimidazole dimer of a compound represented by either structure Ia or Ib:
• ##STR12## wherein R and R' are independently hydrogen, an alkyl group of 1 to 12 carbon atoms, an alkoxy group of 1 to 8 carbon atoms, amino, a cycloalkyl group having 5 to 7 carbon atoms in the ring, or an electron-rich heterocyclic group having 5 to 7 atoms in the ring, and m and n are independently integers of 0 to 5, provided that at least one of R and R' is said alkoxy group, X and X' are independently oxy or imino, Z is alkylene of 1 or 2 carbon atoms, and R.sup.1 and R.sup.2 are independently defined the same as for R and R' except that neither R.sup.1 and R.sup.2 is hydrogen, and
• (c) at least one organic carboxylic acid having a pKa of from 3 to 6,
• said antihalation composition being bleachable when subjected to said heating conditions.