In a bowl, combine the pineapple juice, soy sauce, brown sugar, ketchup, chicken broth, minced garlic, ground ginger, sesame oil, and ground black pepper. Stir until the brown sugar is fully dissolved.
If using, add the optional crushed red pepper flakes.
Place the chicken thighs in a resealable plastic bag or shallow dish. Pour the marinade over the chicken, making sure each piece is well coated.
Seal the bag or cover the dish and refrigerate for at least 2 hours, up to overnight for best results.
Preheat the grill to medium-high heat.
Remove the chicken from the marinade and grill for 6-8 minutes per side, or until fully cooked.
If using, grill the optional fresh pineapple slices for 2-3 minutes per side.
Serve the grilled chicken hot, optionally garnished with grilled pineapple slices.