A diffusion molecular channel is a channel with memory, as molecules released into the medium hit the receptors after a random delay. Coding over the diffusion channel is performed by choosing the type, intensity, or the released time of molecules diffused in the environment over time. To avoid intersymbol interference (ISI), molecules of the same type should be released at time instances that are sufficiently far apart. This ensures that molecules of a previous transmission are faded in the environment, before molecules of the same type are reused for signaling. In this paper, we consider ISI-free time-slotted modulation schemes. The maximum reliable transmission rate for these modulations is given by the constrained coding capacity of the graph that represents the permissible transmission sequences. However, achieving the constrained coding capacity requires long blocklengths and delays at the decoder, making it impractical for simple nanomachines. The main contribution of this paper is to consider modulations with small delay (short blocklength) and show that they get very close to constrained coding capacity.