IT doesnt need to be pure
Yeah, at this point the question is really what's your intention with that extraction. Because really you'd have to go ahead and get rid of tannin and other polyphenols and all that other phytopharmaceutical gunk in there to arrive at a few mgs of reasonably "pure" levels of caffeine for hours of work.
At the same time you can just go get yourself a kilogram bucket of pharmaceutical grade pure caffeine synthesized from urea for 40 bucks on ebay.