Optical brighteners are synthetic chemicals that make fabrics appear to glow in the presence of ultraviolet light. They don't have anything to do with getting things clean -- they're only added to detergents to make laundry appear brighter and whiter than it really is.
The most common optical brighteners used to achieve this affect are called disodium diaminostilbene disulfonate and disodium distyrylbiphenyl disulfonate. Don't ask me to pronounce them and for goodness' sake never attempt to spell them yourself without a trained professional present. Just look for them on product labels if you want to avoid optical brighteners.
That said, most manufacturers don't actually list these chemicals by name. Instead, they just use the generic term "optical brightener(s)" on their ingredients panel -- if the ingredients are listed at all. But some companies will list the chemicals, and you should be prepared.