No, at least not from what I've seen before ever. My dad drinks a ridiculous amount of diet pop, and he is an extremely smart person with a high IQ. However, if you drink a lot of it, since it's pop, you probably won't stay very skinny, and I've heard it's pretty bad for your heart. So if you drink a lot of it, I'd be a little more worried about your heart than your brain, and that's really only if you drink it like everyday.
However I don't think it contains aspartame though I may be wrong. That I'm pretty sure if consumed in large amounts frequently too, can cause brain cancer to possibly develop.
Basically, don't drink a lot of any kind of pop