Why is my rhododendron's leaves drooping, curling and browning?

I planted the yellow rhododendron a few months ago and it's been growing fine until a few days ago. Here in the northeast it's been around 100 degrees and two days ago noticed the plant was drooping excessively, so I watered it. However, it never really perked up; the next day I watered it again and the same result! The plant has some shade and the soil has some clay.