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The origin of aggregation-induced enhanced emission (AIEE) is investigated considering cyano-substituted 1,2-bis(pyridylphenyl)ethene (CNPPE) as an example. On the basis of ONIOM calculations using time-dependent density functional theory (TD-DFT), we found that pseudo-degeneracy of excimers formed in solid phase plays an important role in the appearance of AIEE. The electron density difference delocalized over molecules gives rise to small diagonal vibronic coupling constants (VCCs), which suggests that the internal conversion is more suppressed in the solid phase than in the solution phase. The reduction of the off-diagonal VCCs owing to the packing effect is elucidated by vibronic coupling density (VCD) analysis. Pseudo-degeneracy enables fluorescence from the high singlet excited states against Kasha's rule because the electron density difference and the overlap density between the excited states vanish. A Hubbard model of a pseudo-degenerate electronic system is constructed to explain the vanishing mechanism. We propose the following design principle for AIEE: a candidate molecule for AIEE should have pseudo-degenerate adiabatic electronic states in the aggregation phases originating from excimer formation.

Photochromic molecules exhibit photoresponsive variations in their physical properties. For example, energy level, π-conjugation, dipole moment, ionic state and steric conformation can be changed reversibly with alternate UV-visible light illumination. Photochromism has already been proposed for application to various optoelectronic devices. However, practical progress on such applications only became evident after the recent noteworthy developments on organic field-effect transistors (OFETs). This article reviews various applications of such photochromism in organic electronics. First, the photoswitching properties of electrical currents passing through single photochromic molecules are briefly overviewed. Second, effective applications of photochromic reactions in two-dimensional thin film-based diodes are introduced. These studies provided the basis for subsequent developments in OFETs. Finally, recent advances regarding photochromism in OFETs are emphasized as realistic optoelectronic applications.

Either co-milling or simple mixing of benzoquinone (BQ) and diol crystals of varied melting point temperature produces adduct crystals through very slow and gradual molecular diffusion processes in the solid state. Their formation process was compared and discussed based on the crystal structures of the starting diols and the diol–BQ adducts.