quaternary ammonium compounds
quinarenes
quinhydrones
quinomethanes
quinomethides/quinone methides
quinones
quinone diazides
quinonimines/quinone imines
quinonoximes
radicals
radical anions
radical cations
radical ions
radical pairs
reductones
Reissert compounds
retinoids
retro
rhodamine dyes
ribonucleic acids (RNA)
ribonucleotides
rotaxanes
rotenoids