Aggregation-induced phosphorescence enhancement (AIPE) based on transition metal complexes—An overview

• The design principles for AIPE probes based on d-block arena are highlighted.
• Efficient techniques to generate AIPE on transition metal complexes are described.
• In-depth mechanism and rationale for AIPE are explained.
• Various applications on AIPE probes are demonstrated.

Aggregation-induced emission (enhancement) (AIE(E)) is an extraordinary phenomenon in the field of photochemistry and offers a new platform for researchers to investigate the light-emitting process from nanoaggregates. Non- or weakly emissive molecules are induced to emit efficiently through nanoaggregate formation, which are referred to as aggregation-induced emission (AIE) and aggregation-induced emission enhancement (AIEE), respectively. A large number of AIE/AIEE active organic molecules have now been developed and reviews on them appeared. Comparatively little attempt is made on the AIEE activity based on organometallic and coordination complexes. In the case of most of the transition metal complexes intersystem crossing from the excited S1 to T1 state is close to unity and predominant emission process is phosphorescence. This review concentrates on the design, synthesis, and photophysical behavior and important applications of aggregation-induced phosphorescence enhancement (AIPE)-active molecules based on transition metal complexes. Researchers have incorporated a rotating or isomerizable or long alkyl chain unit in the ligand or cyclometalated ligand, to generate a family of AIPE active members. Several transition metal complexes including Re(I), Ir(III), Pt(II), Au(I), Zn(II) and Cu(I) have been demonstrated to exhibit a significant AIPE phenomenon in the presence of appropriate stimuli. The phosphorescence intensity and quantum yield could be enhanced by adding poor solvents to induce nanoaggregate formation to restrict the intramolecular rotation or isomerization. The characteristics of highly phosphorescent aggregates differentiate them from conventional chromophores and make them ideal candidates for high-tech applications in the field of chemosensors, bioprobes, stimuli-responsive nanomaterials and optoelectronic materials, etc.

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