Not that I'm aware of. Most poisons kill relatively quickly, and the ones that take longer are usually just as if not even more painful. Poison is an awful way to go, most accounts of famous people ctbing with poison (socrates) have been romanticised. Even poisons that are well-know for killing quickly (ie cyanide) are likely to be extremely painful for the few minutes it takes to finish you off.
I am not aware of any painless method that does not involve either (A) massive trauma and gore (jumping, firearms etc.) or (b) extensive planning (charcoal, inert gas). The few methods that meet the criteria of painless and dignified have been banned in most countries, as if they were readily accessible, people would just ctb all the time.