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 (H(3)TCBPA) that exhibited a TPA cross-section of 13 000 +/- 2000 GM, which is similar to 7 times that of the H(3)TCBPA 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.