You asked about what chemicals. I'm not expert, other people here could be more informative, but for starters there are the HCA group of chemicals (some of which are good! or harmless!).
But others are carcinogen. when you cook meat at high temperature it makes HCAs (heterocyclic amines) and probably some of the bad ones included.
I don't know much about it. FWIW here's a wikipedia article:
http://en.wikipedia.org/wiki/Heterocyclic_amine
It shows a picture of one of the nice HCAs, actually a vitamin. But it also shows a picture of some slightly charred BBQ and says no don't eat this.
The people who did the pancreatic cancer study say if you barbecue CUT OFF the charred part and just eat the rest. You could do that with the chicken thighs if just the bottom was burned. they also suggest, if you want, microwave the meat first to get it almost cooked, then pour off the melted fat and juice and stuff and take the almost cooked meat out to the pit and grill it a bit just for the good bbq taste, you know with the sauce and all that. sounds complicated, not in the primitive spirit of real guys at the smoking altar, but that's what I saw they were suggesting.