Tailoring Emission Color Shifts in Mechanofluorochromic-Active AIE Systems of Carbazole-Based D−π–A Conjugates: Impact of π Spacer Unit Variants

The expanding field of organic electronics has led to the emergence of donor−π–acceptor (D−π–A) systems as promising candidates for various optoelectronic applications. Recent advancements in the design of organic fluorophores allow precise modulation of their optical properties in the solution and solid states, broadening their utility. Here, we synthesize and characterize two series of D−π–A systems based on carbazole–cyanostilbene compounds (3a–c and 4a–c), which feature differences in π-spacer groups and substitution on the acceptor cyanostilbene units. These compounds exhibit intramolecular charge transfer (ICT) characteristics, with 3c possessing a phenyl spacer and −CF3 substitution displaying notable solvatochromism and polarity-enhanced emission (PEE). Additionally, both series show aggregation-induced emission (AIE), with emission color shifts controlled by spacer groups. These compounds demonstrate diverse emissive behaviors, including locally excited (LE), twisted intramolecular charge transfer (TICT), and π–π stacked aggregated states, contributing to the AIE phenomenon. Compounds with phenyl spacers (3a–c) exhibit higher solid-state quantum yields than those of thiophene (4a–c). Furthermore, the impact of spacer groups and acceptor substitutions on the solid-state emissive properties and mechanofluorochromism (MFC) are highlighted. Compounds with phenyl spacers and substitution (3a, 3c) show a blue-shifted MFC, while those with thiophene (4a, 4c) display a red-shifted MFC. These findings offer insights into the development of advanced materials with tunable emission properties and high thermal stability for optoelectronic applications.