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

Abstract

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).

A rational design strategy of radical-type mechanophores with thermal tolerance

“…SOMO levels are often used to evaluate the reactivity of molecules; therefore, they can be considered as a good indicator for the reactivity of RMs toward oxygen molecules. 54 In this study, the SOMO levels clearly show a negative correlation with the σ p values within the series of radicals that share the same skeleton, reflecting the substituent effect at the para -position ( Fig. 6 ).…”

“…In contrast to the complicated synthetic protocols for previously reported RMs such as DABBF, TASN, and DFSN, 48–51 the BiACA structures provided significantly more accessible protocols to the desired electron-deficient RMs, allowing a systematic investigation of the substituent effect. All BiACAs were obtained without peroxidized products, which is indicative of low-lying singly occupied molecular orbital (SOMO) levels 52–54 and slow C–C bond cleavage under ambient conditions ( vide infra ). We conducted variable-temperature electron paramagnetic resonance (EPR) spectroscopic measurements to determine the bond dissociation ratio (BDR) of the central C–C bond of the RMs and their dissociation constant ( K d ) at 100 °C (Table S1†).…”

Mechanochemical Degradation of Biopolymers

“…Selective cleavage of Se-Se bonds is difficult with classical reactions because of their similarity to S-S bonds. Sonication of diselenidebonded polymers can cleave the Se-Se bond, regenerating selenoproteins [222][223][224][225].…”