Just do it and tell us your results. You raise the question and you answer it. That's the way how the science works.

What Yurii said. Don't forget your positive control.

Frozen plant tissues are comparable to fresh plant tissues in showing antioxidant activity. Antioxidant activity is due to secondary metabolites, and these are not degraded upon freezing. In my opinion frozen plant tissues show similar antioxidant activity and can be used further. for more info see Article Antioxidant Properties of Fresh and Frozen Peels of Citrus Species

I agree to the answer given by Mishra. It is also good to do the antioxidant test of fresh leaves and frozen samples and compare.