Azulene-porphyrin based covalent organic frameworks for applications in second order nonlinear optics

Abstract

The feasible structural manipulation, extended conjugation networks and good chemical stability of covalent organic frameworks (COFs) render them suitable as promising nonlinear optical (NLO) materials. However, the high symmetry of COFs diminishes their second order NLO properties. By combining the polarity of azulene with the good π-conjugation of porphyrin, a series of two-dimensional azulene-porphyrin based COFs with various types of connections (C=C, C=N, N=N and B=N bonds) are designed in the present work. The N=N and C=N connected azulene-porphyrin based COF exhibits a strong second harmonic generation (SHG) response, and the coordination of Zn further enhances the SHG responses. The mechanism of such enhancement by introducing heteroatoms and metals is scrutinized. The microscopic understanding of the correlation between the electronic structures-NLO properties facilitates the design and experimental exploration of novel COFs with exceptional NLO performance.