Aggregation-induced emission (AIE) is another photophysical phenomenon associated with chromophore aggregation. The concept of AIE has been introduced by a group of researchers in 2001. In the AIE process, non-emissive luminogens are induced to emit by the aggregate formation. The luminogens exhibiting AIE attributes are termed AIEgens, by analogue with the term of 'mesogens' for the molecules showing mesomorphism under suitable conditions.