Mechanochromic Luminescence from 9‐Phenylfluorene Derivatives: A Synergy between Molecular Polarity and Steric Hindrance

Comprehensive Summary Mechanochromic luminescent (MCL) compounds, possessing emission‐color‐change feature under mechanical stimulus, are attracting much attention due to their potential applications in optical recording, chemical sensor and information encryption. However, most of the current reported MCL compounds are donor‐π‐acceptor (D‐π‐A) structure, which are unfavorable to achieve spatial conformational regulation towards property modification compared with those possessing spatial D‐A stacking structure. Herein, we propose a synergistic strategy, namely simultaneously regulating molecular polarity and steric hindrance, to construct a series of D‐A type compounds for MCL property modification. Through adopting 9‐phenylfluorene as the molecular skeleton to interrupt π‐conjugation and using N ‐ethylcarbazole as electron donor together with 4,6‐diphenyl‐1,3,5‐triazine as acceptor to regulate molecular polarity, we successfully unlock the MCL property in D‐A type target compounds. Notably, after 9‐phenylfluorene is introduced into N ‐ethylcarbazole position for increasing the steric hindrance, more obvious MCL behavior with a large emission red‐shift of 38 nm is achieved. Moreover, the target 9‐phenylfluorene derivatives are evaluated towards potential application in information encryption. It is expected that our study provides an inspiring strategy to develop new MCL materials for multifunctional application.