A series of new coelenterazine analogs with varying substituents at the C-6 position of the imidazopyrazinone core have been designed and synthesized for the extension of bioluminescence substrates. Some of them display excellent bioluminescence properties compared to DeepBlueC™ or native coelenterazine with both in vitro and in vivo biological evaluations, thus placing these derivatives among the most ideal substrates for Renilla bioluminescence applications.