Slice chicken lengthwise into 2 to 3 equal portions, about 1 1/2-inch wide. Combine 1 Tbs. brown sugar, 1 tsp. minced ginger, juice from half an orange, 2 Tbs. sesame oil, 1 Tbs. soy sauce, and scallion whites in a shallow bowl; add chicken and turn to coat. Let marinate in the refrigerator until ready to grill, or up to 4 hours.