Disclaimer: Potentially more of a physics question, but this area is muddy.
I'm trying to find 720 nm LEDs but they're proving to be very uncommon. In fact, there seems to be a gap between 660 nm up to ~730 nm. I've only been able to find one supplier of 720 nm LEDs (data sheet here). These LEDs also tend to be very low powered compared to their counterparts in the nearby range. For example, a typical 690 nm LED outputs 690 mW, whereas the 720 nm LED only ouputs 170 mW, and that's the strongest one I could find (which is no longer carried).
My gut feeling says this has to do with the material being used in the LED (in this case GaAlAs). Perhaps it's very difficult to dope the material in such a way that it emits 720 nm light? I've already noticed that these LEDs are prone to burning out. Is the material just not very good for this purpose? Hopefully someone with more insight into this topic could shed some light (heh) on the situation.