The Preparation and Photophysical Properties of 3-Substituted 4-Azafluorenones

A method for the preparation of 3-substituted azafluorenones is presented. Condensation of ninhydrin with thiomethylamidrazone gives a triazine from which 3-thiomethyl-4-azafluorenone is produced after Diels-Alder cycloaddition with norbornadiene followed by two retro-Diels-Alder cycloreversions. Oxidation of the sulfide to the sulfone allows for nucleophilic substitution at the 3-position. Using different amidrazones can give other substituents at the 3-position directly. The photophysical properties of the azafluorenones are characterized and compared with computational calculations. Compounds with a substituent bearing an n-donor group show significant fluorescence. The n-donor group is prominent in the HOMOs in these systems, whereas in the compounds with weak emission, it is not. The El Sayed rules for intersystem crossing do not explain the emission intensity ordering of these compounds.