Metal-induced dimensionality tuning in a series of bipyrimidine-based ligands: a tool to enhance two-photon absorption

We report the synthesis, the photophysical and the two-photon absorption (2PA) properties of a series of octupolar bipyrimidine-based ligands incorporating N-substituted amines as terminal donor groups. The effect of replacing phenylvinylene pi-conjugated linkers by fluorenylvinylene ones was also investigated. The linear absorption spectrum of these compounds is dominated by an intensive charge transfer band which is sensitive to N-substitution and the pi-bridge nature. The excitation anisotropy spectrum indicates that this band encompasses multiple S-0 -> S-n transitions, whose occurrence is well rationalized on the basis of the Frenkel exciton model. The 2PA spectrum also corroborates the presence of several electronic transitions. In apolar or moderately polar medium, excited ligands mainly deactivate through a highly emissive intramolecular charge transfer (ICT) state localized within a single branch of the chromophore. In highly polar medium, the solvent-induced stabilization of the low emissive twisted intramolecular charge transfer (TICT) state leads to a severe quenching of the fluorescence. The same mechanism is observed upon complexation with Zn2+. According to single-crystal X-ray analyses, metal-induced planarization of the bipyrimidine chelating site was observed for the short length ligands. Such a dimensional change from D-2d to D-2h symmetry leads to a decrease of 2PA cross-sections with respect to the free ligands. A divergent effect is observed for the complex with the long length ligand since the three-dimensional structure is maintained which induces a sizeable increase of the 2PA cross-section with a maximum value of up to 2000 GM.