Design and synthesis of donor-acceptor functionalized stimuli-responsive materials

Abstract

In recent years, the development of luminescent materials which show response to different external stimuli (temperature, pressure, electricity, pH, polarity etc.) has gained significant attention considering their utilization in applications such as organic light emitting diodes (OLEDs), information devices, security inks, fluorescent probes, fluorescent sensors, reversible switches, pressure sensors, bioimaging and other optoelectronic devices. Stimuli-responsive materials are new-generation materials which can be constructed by using variety of organic molecules, metal-complexes, metal organic frameworks and polymeric materials. Mechanochromic materials are stimuli-responsive materials which display reversible changes in their emission on application of mechanical stimuli such as grinding, rubbing, shearing or scratching. In general, majority of the practical applications of stimuli-responsive materials are in the solid state; hence significant amount of solid-state emission is crucial for the design of these materials. The common organic fluorophores such as perylene have been employed in optoelectronic and material devices but their use in the solid-state application is limited due to aggregation caused quenching (ACQ). The organic fluorophores which are often highly emissive in solution state, exhibit ACQ characteristics i.e. non-radiative decay of the excited state as an outcome of the extensive π– π stacking between the closely lying molecules in the aggregated state. The concept of aggregation-induced emission (AIE) established by Tang et al. is beneficial in resolving ACQ and furnishing organic fluorophores with efficient solid-state emission. The AIE luminogens or AIEgens possess a non-planar propeller type structure consisting of rotors attached to a stater that hinder the π–π stacking interactions between the molecules on aggregation by the restriction of intramolecular rotations (RIR) resulting in enhanced emission on aggregation or in solid state. The classic examples of AIEgens are tetraphenylethylene (TPE), hexaphenylbenzene (HPB), 9,10-di((E)- styryl)anthracene (DSA) and their functionalized derivatives (Figure 1.).