Sensitized onium salts

Claim 1

An onium salt capable of generating acid upon exposure to actinic radiation having the following structure: (R.sub.1).sub.a (R.sub.2).sub.b (R.sub.3).sub.c Q.sup.+ --A--M.sup.+ X.sup.- --B--X'.sup.- where: Q is S; R 1 , R 2 and R 3 are independently aromatic groups having 6-12 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; or aliphatic groups having 1-12 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; or aralkyl groups having 7-18 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; M + is a cationic organic group having the structure: Q'.sup.+ (R.sub.4).sub.d (R.sub.5).sub.e (R.sub.6).sub.f where Q' is S; R 4 , R 5 , and R 6 are independently aromatic groups having 6-12 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; or aliphatic groups having 1-12 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; or aralkyl groups having 7-18 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; f=0; d and e are 0, 1 or 2 such that d+e=2 when R 4 and R 5 are monovalent groups; and d+e=1 when R 4 or R 5 is a divalent group; A is a divalent group selected from the group of hindered alkylene groups having 4-12 carbon atoms; aromatic groups having 6-18 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; and aralkyl groups having 7-24 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups; B is a divalent aromatic sensitizer which absorbs radiation having a wavelength longer than 300 nm and is capable of transferring an electron to Q, x - and X' - are anionic groups independently selected from the group consisting of sulfate, sulfonate, sulfite phosphate, phosphite and carboxylate; and when c=0; a and b=0, 1, or 2 such that a+b=2 when R 1 and R 2 are monovalent groups; and a+b=1 when R 1 or R 2 is a divalent group; wherein A provides a spatial separation between Q and M, and B provides a spatial separation between X and X', such that the spatial separation provided by A between Q and M is substantially the same as the spatial separation provided by B between X and X'.