The book 'Ignition!' tells the history of propellant research and has this to say about ozone from page 112 available here
For it has its drawbacks. The least of these is that it's at least as
toxic as fluorine. (People who speak of the invigorating odor of ozone
have never met a real concentration of it!) Much more important is
the fact that it's unstable — murderously so. At the slightest provocation
and sometimes for no apparent reason, it may revert explosively
to oxygen. And this reversion is catalyzed by water, chlorine, metal
oxides, alkalis —and by, apparently, certain substances which have
not been identified. Compared to ozone, hydrogen peroxide has the
sensitivity of a heavyweight wrestler.
Since pure ozone was so lethal, work was concentrated on solutions
of ozone in oxygen, which could be expected to be less dangerous.
Further reading of that chapter will explain more the issues with dealing with ozone. One of the major issues was that when mixed with O2 to increase stability and reduce toxicity at 75% O2 and 25% O3, it tended to separate from O2 which has a lower boiling point (at 90K compared to ozone at 161K) in effect distilling into higher and higher concentrations of ozone, so we are back to the more volatile, more toxic product. (see page 113 of Ignition for this info)
In conclusion, from page 114
For ozone still explodes. Some investigators believe that the explosions
are initiated by traces of organic peroxides in the stuff,
which come from traces, say, of oil in the oxygen it was made of.
Other workers are convinced that it's just the nature of ozone to
explode, and still others are sure that original sin has something to do
with it. So although ozone research has been continuing in a desultory
fashion, there are very few true believers left, who are still convinced
that ozone will somehow, someday, come into its own. Im not one
of them.
