A Photo-Responsive Charge-Assisted Hydrogen-Bonded Organic Network with Ultra-Stable Viologen Radicals

Hydrogen-bonded organic networks(HONs)have attracted intense research interest due to their mild synthesis conditions,good solvent processability and strong reproducibility.Most HONs are constructed by electrically neutral organic ligands through intermo-lecular interactions such as weak hydrogen bonding,π-π stacking.But there are a few charge-assisted hydrogen-bonding networks(CAHONs)constructed by oppositely charged organic components.Under hydrothermal conditions,we successfully synthesized a 3D CAHON(H2CV)(H2BTEC)(1)constructed by H2CV(CV=N,N'-4,4'-bipyridine dipropionate)and H4BTEC(H4BTEC=1,2,4,5-benzene-tetracarboxylic acid)through multiple charge-assisted hydrogen bonding.Upon soft X-ray,ultraviolet light or sunlight irradiation un-der ambient conditions,1 quickly changes from colorless to blue(1P),and kinetic calculations show that its photochromism meets first-order dynamics.Interestingly,the colored crystals 1P with long-lived colored radicals will not fade after being placed for more than two months at room temperature in air,owing to the fact that the photogenerated viologen radicals can be stabilized by the abundant hydrogen bonds,the coplanar pyridinium rings and the slow electron-hole recombination.Moreover,1 is a typical direct band gap semiconductor,evidenced by the calculated band gap of 3.24 eV,in agreement with the experimental value of 3.40 eV.In view of its strong colored stability,1 is designed for distinctive inkless printing.