My central air unit is exposed to direct sun light all day...

I wouldn't plant anything too close as it may interfere with the vents. You can build a simple wooden structure --even using wood pallets to help shield it from the sun. Be careful --as there is a possibility that "critters" may nest in this new protected area.

I get the feeling you think this will improve the efficiency of the air conditioning system, but all of the cooling action happens at the condenser coil located at fan/duct/furnace module of your central air system. I don't think shading the compressor will improve your air conditioner efficiency. But I don't think it would hurt to have a roof over it, especially to keep rain off of the compressor. It would probably last a bit longer being protected from the elements.

Thank you