Advanced functional materials: fully conjugated covalent organic frameworks with octapole structure -- new two-photon fluorescence materials with high quantum efficiency and high absorption cross section

As a star material in new materials science covalent organic frameworks (COFS) have many advantages such as low density high specific surface area diverse structure easy regulation of pore structure easy functional modification. They are favored by researchers in the fields of gas storage / separation catalysis sensing energy storage optoelectricity. A series of two-dimensional or three-dimensional COFS linked by borate ester bond imine bond achieve single photon fluorescence by introducing aggregation induced luminescent groups or non planar units changing the stacking mode. Although some breakthroughs have been made in recent years there are still great challenges in solid-state fluorescence quantum efficiency luminescence stability. At present most of the research on luminous COFS only focuses on the linear optics in the ultraviolet visible region while the nonlinear optical properties (such as two-photon fluorescence) of COFS in the near-infrared region are rarely concerned although two-photon fluorescence has the advantages of low photobleaching high three-dimensional resolution strong penetrability. Generally speaking two-photon absorption (TPA) cross section is an important performance index of

. For small organic molecules researchers found that dipole compounds with donor – π – receptor (D – π – a) structure quadrupole compounds with D – π – D or a – π – a structure octapole compounds with multi branched structure (A3 – (D – nucleus) or D3 – (a – nucleus)) tend to show larger absorption cross sections. In addition previous studies have shown that conjugation length strength of receptor donor molecular height symmetry molecular planarity also affect the size of TPA cross section. Compared with one-dimensional linear molecules of dipole quadrupole compounds octapolar compounds with two-dimensional planarity tend to show more effective intramolecular charge transfer achieve larger absorption cross section.

In view of this Professor Yu Dingshan Professor Chen Xudong of Sun Yat sen University proposed to introduce octapolar structure (such as (A3 – (d-core)) into the highly regular 2D COFS framework maintaining the high symmetry of the octupole structure extending its conjugation length which opens up a new way for the design of two-photon fluorescence materials exps the application potential of COFS materials. In this work triphenylbenzene benzene were used as the core connecting units three different connecting bonds (imine borate ester cyano substituted carbon carbon double bond) were combined to construct the material platform of COFS with similar three branched structure but different connecting bonds the structure-activity relationship between molecular structure optical properties was explored. The results show that unlike borate ester imine bonded COFS cyano substituted carbon carbon double bond COFS not only form an effective octapole structure fully conjugated skeleton but also have strong electron withdrawing groups extended conjugation length showing a large TPA cross section (800 nm Two photon fluorescence with high solid state fluorescence quantum efficiency (27.2%) is superior to the corresponding model molecule (104gm) most of the reported two-photon materials. The laboratory has proved that the cyano substituted carbon carbon double bond COFS coated on the surface of blue light-emitting diodes (LEDs) can achieve low color temperature (3538 K) warm white LED light CIE is (0.39 0.37) it can be used as information storage encryption material to hide information in visible light read information in ultraviolet or near-infrared light. The researchers of

believe that this research can add color to the application of COFS provide new research ideas for the design of COFS based nonlinear optical materials. The non-linear optical materials based on COFS will shine in the fields of optical information storage two-photon imaging up conversion laser optical limiting.

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