Any animal with an eye has photoreceptors, which are essentially light-sensitive neurons. Green algae have channelrhodopsin, which are ion channels that open and close in response to light. Clearly, the nervous system and its molecular components have evolved to use light effectively.
I'm wondering if light can act as a neurotransmitter in the classical sense; that is, the presynaptic cell releases a photon, which travels across the synaptic cleft and interacts with a receptor, depolarizing the postsynaptic cell. Light can be generated in vivo through chemiluminescence, and as with channelrhodopsin, different proteins can activate in response to different wavelengths or intensities. I suspect this would be much faster than traditional chemical signaling.