Should D-Phenylalanine (DPA) be taken on an empty stomach, away from food, or how would you recommend that I take it? And is it better to take it in the morning or evening, or it doesn't really matter?

I would take it on an empty stomach. That’s what I always did. Amino acids are better absorbed when taken on an empty stomach away from food. If you take it with a meal, it competes with other amino acids in your food for absorption.  

And it doesn’t really matter too much when you take it. But my recommendation would be to take it earlier in the day. Because it might increase your energy and motivation.