The tape-measure shot is the second-longest at Great American Ball Park since it opened in 2003, trailing only Adam Dunn's 535-foot shot to right-center in 2004.
GABP isn't the first ballpark unable to hold Francisco, who said somebody told him the ball hit a tree outside the stadium.
"I hit a couple out of the ballpark at (Class A) Dayton," he recalled. "It was just a good pitch to hit. I don't know if they got the ball or not, but they put a branch up there."
Cubs manager Mike Quade was surprised to hear the public address announcer announce the distance before Francisco's next at-bat.
"I can't remember the P.A. guy announcing how far a ball went," Quade said. "How do they measure that?"
The homer was Francisco's third of the year for the Reds and the longest in Major League Baseball this season, according to ESPN's Home Run Tracker.
"Francisco is a powerful young man," manager Dusty Baker said.
"That was the longest one I've seen in a while. When Francisco gets a good pitch to hit, he is as powerful as anyone. You'd rather have a young hitter who is more aggressive. You can calm them down. The more he plays, the better he is at getting good pitches to hit."