Popcorn has the distinction of having been composed specifically for the Moog; as far as I know it was the first piece of music composed for a synthesiser, as opposed to a reinterpretation of a piece originally composed for traditional instruments.
As a purely electronic piece, it's predated by Silver Apples of the Moon, by Morton Subotnik [1], the first electronic album to be produced in the record industry.
I had the pleasure of taking a class by Mort at NYU and learned a lot from his philosophy of music. He provided the musical philosophy behind the design of Don Buchla's eponymous electronic sound platform [2]. Mort is responsible for the Buchla's lack of a familiar 12-key per octave keyboard. Mort felt that as long as they were synthesizing sound from scratch, there was no reason to box themselves into traditional scales, which result from the harmonic nature of sounds produced by resonant instruments (like winds and strings).
By contrast, Bob Moog created a synthesizer that is very much within the tradition of classical music. I imagine that this continuity, exemplified by Switched On Bach, is probably why the Moog is the far more famous instrument today. It's a much easier on ramp for musicians used to traditional instruments.
Scott was a genius because he realised that you could not only make sound electronically, you could build hardware that would compose music. Everyone else is still playing catch-up with that idea.
There was also a fully electronic score by Bebe and Louis Barron for a film called Forbidden Planet.
Moog basically made the technology self-contained and sort-of affordable. With a Moog you had a complete keyboard synthesizer in a box, and you didn't need a room for it.
My favourite Moog music is the classical albums made by Isao Tomita in the 70s. I don't think anyone else has come close to the inventiveness, poetry, and sheer programming genius in the sound design.
