Theoretical and Experimental Investigations of Stable Arylfluorene-Based Radical-Type Mechanophores

Radical-type mechanophores (RMs) can undergo homolytic cleavage of their central C-C bonds upon exposure to mechanical forces, which affords radical species. Understanding the characteristics of these radical species allows bespoke mechanoresponsive materials to be designed and developed. The thermal stability of the central C-C bonds and the oxygen tolerance of the generated radical species are crucial characteristics that determine the functions and applicability of such RM-containing mechanoresponsive materials. In this paper, we report the synthesis and characterization of two series of arylfluorene-based RM derivatives, that is, 9,9'-bis(5-methyl-2-pyridyl)-9,9'-bifluorene (BPyF) and 9,9'-bis(4,6-diphenyl-2-triazyl)-9,9'-bifluorene (BTAF). BPyF and BTAF derivatives were synthesized without generating any peroxides initially, albeit that BPyF slowly converted to the corresponding peroxide in solution. DFT calculations revealed the importance of the thermodynamic stability and the values of the α-SOMO levels of the corresponding radical species for their thermal stability and oxygen tolerance. Furthermore, the mechanochromism of BTAF was demonstrated by ball-milling a BTAF-centered polymer, which was synthesized by atom-transfer radical polymerization (ATRP).