Connecting iMac to Macbook pro for screen display

I'm trying to use my iMac early 2009 as a display screen for my Macbook pro mid-2015. My iMac has a mini display port and my Macbook has a thunderbolt 2 port. I've tried to connect the two with a thunderbolt 2 cable and a mini display port cable but neither work. My iMac is using El Capitan and my Macbook is using Mojave. Can anyone tell me how to get it to work?