Metal–Organic Layers with an Enhanced Two-Photon Absorption Cross-Section and Up-Converted Emission

Two-photon absorption (TPA) of many chromophores can be enhanced by building them into the structure of metal–organic frameworks (MOFs). However, light scattering of MOFs defocuses the light beam and thus decreases the nonlinear optical effects like TPA. Nanoscale two-dimensional (2D) metal–organic layers (MOLs) are 2D analogues of MOFs with low light scattering. In this work, we obtained a MOL containing tri­(4-carboxylic-biphenyl)-4′-amine (H3TCBPA) that exhibited a TPA cross-section of 13 000 ± 2000 GM, which is ∼7 times that of the H3TCBPA ligand in homogeneous solution (1850 ± 200 GM). The MOL also gave bright up-converted fluorescence thanks to efficient TPA and a high fluorescence quantum yield. The theoretical calculation showed that a large dipole moment of the excited state due to electron redistribution between the ligands and the metal-connecting nodes in the MOL is the reason for the TPA enhancement. This work illustrates MOLs as 2D materials with enhanced TPA for generating bright upconverted fluorescence.