Arsenal signed Chilean striker Alexis Sanchez from Barcelona that day in 2014 for an undisclosed fee.
Sanchez, then 25, swapped Camp Nou for Emirates Stadium for £35m after agreeing a five-year deal.
The Gunners faced stiff competition to land Sanchez, who had also been targeted by Premier League rivals Liverpool and Italian champions Juventus.
Sanchez said: “I’m so happy to join a club that has a great manager, a fantastic squad of players and huge support around the world.
“I can’t wait to play in the Premier League and the Champions League.”
Sanchez scored 47 goals in 141 appearances for Barca and showed his skills at the World Cup in Brazil, scoring twice to help Chile reach the last 16, where they lost on penalties to Brazil.
His first three seasons in north London were highly successful, scoring 72 goals in all competitions for the Gunners.
But after a planned £60million move to Manchester City collapsed on transfer deadline day in the summer of 2017, Sanchez’s form plummeted.
The Chilean had hoped to team up with former Barca boss Pep Guardiola at City, but the agreed transfer depended on Arsenal guaranteeing a replacement and, when they failed to sign Thomas Lemar, the agreement was cancelled.
The following January, Sanchez moved to Manchester United in a swap deal for midfielder Henrikh Mkhitaryan after Gunners manager Arsene Wenger admitted uncertainty over the club’s future striker had affected the team spirit.
Sanchez scored 80 goals in all competitions in total for Arsenal and was involved in 121, more than any other Gunners player during his time at the Emirates.