The invention provides novel compounds having the general formula: wherein X1 is N or N+O-, and one of X2, X3 and X4 is N or N+-O- and the remainder of X2, X3 and X4 is C. R2, R3, R4, R5, R6. A, B and Y are as described herein. Additionally compositions compounds of Formula I and methods of use are further described herein.