Learning how to fine-tune diradical properties by structure refinement

The literature has seen a large increase in the number of new carbon-based organic diradicals/diradicaloids in recent years. While a plethora of new and exciting structures have been created, there seemingly is a gap in knowledge of what fundamental electronic parameters are in play and thus how to rationally manipulate said parameters to "fine tune" the resultant diradical properties. Since 2014, the Haley group has been exploring methods to systematically alter the diradical character and the singlet-triplet energy gap in said class of molecules. Our entrance into organic diradicals began with the pi-expansion of the benzene core of indeno[1,2-b]fluorene up to the anthracene core of diindenoanthracene (DIAn). DIAn possessed moderate diradical character (y =0.62) with a surprising level of stability (more than 2 months in solution). From this molecular blueprint for producing stable diradicals, the Haley lab has investigated how to fine tune diradical properties via structural changes in two key positions: (a) the length of the acene core and (b) thoughtful exchange of the outer arenes. With this strategy at our disposal, we can make large scale changes to the diradical character index and singlet-triplet energy gap through changing the core length, and these properties can be further fine-tuned in a series of closely related diradicals by careful exchange of the outer arenes utilizing the straightforward methods described in this mini-review.