TDM iMac not working

So i bought Apple Thunderbolt to Thunderbolt Cable today. I wanted to use iMac 27inch Mid2011 as an external display for MacBook Pro 15inch Mid2012.

I booted MacBook first and then iMac. Plugged the Thunderbolt cable in both of them but nothing happened on iMac. I kept pressing Cmd+ f2 but it just won't go to Target Display Mode. Am i doing something wrong?

One more thing Target Disk Mode is working fine.