Zn2+ responsive two-photon fluorescent probes based on branch structure: a computational investigation

A series of zinc ion fluorescent probes on the basis of multi-branched ligands were investigated in theory. The three-branched ligand TPPA (N,N,N′,N′-tetraphenyl-p-phenylenediamine) has better three-dimensional spatial localisation, which can detect zinc at the parts per million level. The complex coordinated with Zn²⁺ can show a significant improvement in two-photon absorption (TPA) cross-section in the near-infrared (NIR) excitation region. The calculated results reveal that the stability and sensitivity of Zn²⁺ complexes will be enhanced by increasing the number of branches. The selectivity of double phenyl-p-phenylenediamine (DPPA) ligand to Zn²⁺ will be better compared to Cd²⁺. With regard to the studied ligands single phenyl-p-phenylenediamine (SPPA), two connected single phenyl-p-phenylenediamine (2CSPPA), DPPA and TPPA, λ^EM_max shows a red-shift and ?^EM gets stronger upon the addition of Zn²⁺. Most of the molecules exhibit TPA peaks in the NIR region. The theoretical investigations demonstrate that DPPA-Zn²⁺ shows good TPA activity at a telecommunication wavelength.