Interfacial Synthesis of Two-Dimensional Porphyrin Polymer Films with Large Optical Nonlinearity

Two-dimensional polymers (2DPs) and their layer-stacked 2D covalent organic frameworks have recently emerged as nonlinear optical (NLO) materials for potential applications in optics. However, the chemistry for designing third-order NLO 2DP films with large nonlinear absorption coefficient (beta) has remained a mystery. Herein, three highly crystalline porphyrin-integrated 2D polyimines (named as 2DPI-Zn-Azo, 2DPI-2H-Azo, and 2DPI-Zn), which are homogeneous films showing large lateral areas over cm2, uniform transparency, and thickness of tens of nanometers are reported. Particularly, the 2DPI-Zn-Azo film comprising zinc porphyrin and -N=N- displays a large saturable absorption under 532 nm and the highest beta (-1.88 x 105 cm GW-1) among the three 2D polyimines, that is also 2-5 orders of magnitude higher than the state-of-art performance of photoactive small molecules, porphyrin-integrated 2DPs, and inorganic 2D materials. Control experiments in combination with theoretical calculation discover that the embedding of metal centers and -N=N- results in highly delocalized pi-electrons and narrow bandgap in 2DPI-Zn-Azo, which enables fast transfer of the photogenerated electrons after the light-excited charge separation, thus boosting the NLO performance. This work opens up a new path for the construction of highly efficient third-order NLO film materials, and pushes the development of 2DPs for optics and optoelectronics.