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.