Help with use of pgrep or pkill commands with full name options.

I trying to use pgrep or pkill utility commands to find processes but using
the full path names, by example, find /usr/bin/bash (package procps-ng):

Then, if I use pgrep -f /usr/bin/bash, don't find nothing.

Using pkill -f /usr/bin/bash don't get the task.

If I use pidof /usr/bin/bash, this works.

How do I get it to do use pgrep, pkill, etc?

Regards,
Cesar Jorge