Aggregation-induced
emission (AIE), usually referring to the phenomenon
in which molecules emit more strongly in the aggregate state than
in the solution state, is intriguing and promising in various optoelectronic
and biosensing applications. In this Perspective, the basic principles
that can lead to AIE and experimental evidence to reveal the AIE mechanism
of tetraphenyl ethylene (TPE)-type molecules are discussed. AIE is
the consequence of two factors: (1) the fast energy dissipation by
crossing a conical intersection (CI) in solutions but not in solids
results in low luminescence efficiencies in the solutions, and (2)
the weak intermolecular coupling and thus slow intermolecular energy/charge
transfers in the AIE solids effectively prevent quenching and result
in relatively high luminescence efficiencies. The key to AIE is that
the luminescence efficiency is tuned by controlling molecules to cross
or not to cross a CI by changing the phase of molecules. How fast
a molecule can cross a CI is dependent on the energy barrier of isomerization,
which can be tuned in many ways, including mechanical or electrical
stimuli, in addition to changing phases. Barrier-dependent crossing
CI also results in a very important consequence: excitation-wavelength-dependent
fluorescence yield within one electronic excited state, an anti-Vavilov’s
rule phenomenon. In principle, there can be an alternative way to
tune luminescence efficiency by manipulating the formation of CIs
instead of crossing or not crossing them. This approach relies on
the fact that the electronic ground state and the excited state have
many different properties, e.g., dipole moment. By tuning the environment,
e.g., dielectric constant, to favor or disfavor one state, one may
be able to lift or lower the potential surface of one state so that
the potential surfaces of two states can vary between intersected
and not contacted.