I was just chasing down a reference I had remembered in which it was claimed that picric acid (2,4,6,-trinitrophenol) has been used as an energy-boosting additive in racing gasolines. So, no doubt, has everything else under the sun. Picric acid is a high explosie used as a shell-filling in WW I and earlier. Like many early explosives, in the presence of water, and in contact with certain metals that promoted catalytic reactions, it could form "undesirably sensitive explosive compounds." It was normal to varnish the explosives cavity inside steel shells to discourage such catalytic contact. Sounds like dandy stuff.
Why not simply use an oxygen-bearing compound such as nitromethane or nitrous oxide? Well, when you add oxygen-smuggling stuff like that, you must also correct the fuel delivered to realize the extra power. When you add explosives, you have a turn-key operation; no mixture correction necessary. Of course, explosives in gasoline are quite often powerful pro-knocks. Indeed, amyl nitrate -- another explosive -- is commonly used as a cetane improver (in effect, a knock-promoter) in Diesel fuel, in which instant light-off is desirable.
In the excellent little volume "Molecules", by P.W. Atkins, he describes the explosive TNT (2,4,6 - trinitrotoluene) as "an assembly of carbon atoms that are on the brink of oxidation; its oxygen atoms have only to be nudged into slightly different locations for them to be able to swoop down on the carbon and hydrogen atoms of the benzenelike ring and carry them off as carbon dioxide and water,leaving the nitrogen atoms to fall together and move off as gaseous nitrogen.
This nice description gives us the essentials of a monopropellant. In high school we used to daydream about running engines on a slurry of pentolite. Marvelous stuff, with a detonation velocity of 8100 meters per second, pentaerythritol tetranitrate was invented in 1891. It became visually famous when Harold Edgerton (the E of E, G, & G) made a widely-circulated photograph of a cube of the stuff exploding, with mathematical-looking plumes of gas flaring from its corners.
I am fascinated by hydrazine. This is strange stuff, with a structure that is hard to put one's finger upon, and the ability to be entirely rearranged or even completely taken to pieces by the electric fields on the surfaces of certain metals. The rocket fuel book says it is stabilized by mixture with toluene and aniline, and by golly that's just what was found with hydrazine in a recently-analyzed sample of "racing gasoline."
Tons of this stuff are used for preparation of boiler feedwater -- because it is so avid in its pursuit of oxygen, that great enemy of boilers. Upon either thermal or catalytic decomposition, hydrazine evolves ammonia, nitrogen, and hydrogen. Although its chemical formula is simple enough - N2H4 - its electronic structure is frightening to behold; I can't even meaningfully read the stuff about it in the rocket-fuel book. The result is that the molecule looks like many things, rather than just obediently sitting still like those smug arrangements of colored balls and shiny rods we played with chemistry.
The attraction of hydrazine is that it can act as a combustion accelerator. In these times of ghastly four-stroke combustion chambers, that is an attractive quality indeed. Yet after the scare stories from the 1960s, when a number of racers inadvertently blew themselves up by mixing highly-concentrated nitromethane/alcohol brews with 1.5% hydrazine, who would want to travel that path? Still, curiosity pads in on silent feet, like a black cat in a dark room.