In FAA land logging PIC time if appropriately rated and sole manipulator of controls (even if you're the FO) is technically allowed.
That being said pretty much all decent 121 carriers won't recognize any time logged as PIC while being an FO at a 121 carrier. They'll only recognize PIC time logged as aircraft captain. It's not hard for them to work out either as the ALPA seniority lists are available to most majors recruiting dept.