Bromothymol Blue is a chemical indicator for pH in a water solution. It is an anionic dye. It is yellow when acidic, green in a narrow range when neutral, and blue when basic. It is sold in its dark green powder form, and used in a water solution.
Its full chemical name is 4,4'-(3H-2,1 Benzoxathiol-3-ylidene) bis 2 bromo-3-methyl-6-(1-methylethyl)phenol S,S-dioxide. As you can see, there are three benzene rings and a thionyl group (SO3) bonded together by a single carbon atom, so that the sulfur is attached to the plain benzene ring. Two of the rings are aryl rings. More precisely they are 2-bromo 3-methyl 6-(1-methyl ethyl) 3-phenyl rings. That is, the groups are: methyl, bromide, hydroxide and methyl ethyl (a carbon atom with two methyls and one hydrogen). Its molecular weight is 624.38 g mol-1.
It is hazardous, because it is an irritant and harmful if swallowed. When swallowed, induce vomiting immetiadely or at least give milk. No further safety data is available. Similar dyes have not caused permanent eye injury, but BTB itself has not been researched. No LD50 test has been done. Someone should go and poison cute animals with this.
(Stop whining. I am completely heartless towards animals.) | 
