. . "where at least one bond between adjacent ring atoms is a double bond, and one or more of positions 1 to 6 are optionally a heteroatom selected from N for position 1 and any double bond position and otherwise S, O or NH for positions 2 to 6, provided that not more than two positions are S or O, and further provided that R5 including R6 comprises only one N if R4 comprises an N, no N is present if R" . .