Controlling the optical properties of boron subphthalocyanines and their analogues

Boron subphthalocyanines (SubPcs) are cone shaped π-conjugated molecules comprised of three azomethine-bridged isoindole units and a central boron atom with an axial substituent. These molecules are particularly interesting for their optical properties offering potential applications for organic photovoltaics, organic light-emitting diodes, photodynamic therapy, and fluorescence imaging. In this review, we summarize how absorption and fluorescence properties can be finely tuned by substituent groups at either the periphery of the SubPc or at the axial position at the central boron atom. By suitable functionalization, fluorescence can for example be controlled by acid/base stimuli or by light/heat stimuli causing isomerization of an appended photo/thermoswitch. Moreover, optical properties can be tuned by contraction or expansion of the π-conjugated core. Key synthetic protocols for functionalization at peripheral and axial positions are also covered.