King Fai Li

Helicity multiplexed broadband metasurface holograms

Metasurfaces are engineered interfaces that contain a thin layer of plasmonic or dielectric nanostructures capable of manipulating light in a desirable manner. Advances in metasurfaces have led to various practical applications ranging from lensing to holography. Metasurface holograms that can be switched by the polarization state of incident light have been demonstrated for achieving polarization multiplexed functionalities. However, practical application of these devices has been limited by their capability for achieving high efficiency and high image quality. Here we experimentally demonstrate a helicity multiplexed metasurface hologram with high efficiency and good image fidelity over a broad range of frequencies. The metasurface hologram features the combination of two sets of hologram patterns operating with opposite incident helicities. Two symmetrically distributed off-axis images are interchangeable by controlling the helicity of the input light. The demonstrated helicity multiplexed metasurface hologram with its high performance opens avenues for future applications with functionality switchable optical devices.

Cyanines as new fluorescent probes for DNA detection and two-photon excited bioimaging

A series of cyanine fluorophores based on fused aromatics as an electron donor for DNA sensing and two-photon bioimaging were synthesized, among which the carbazole-based biscyanine exhibits high sensitivity and efficiency as a fluorescent light-up probe for dsDNA, which shows selective binding toward the AT-rich regions. The synergetic effect of the bischromophoric skeleton gives a several-fold enhancement in a two-photon absorption cross-section as well as a 25- to 100-fold enhancement in two-photon excited fluorescence upon dsDNA binding.

Exceptionally Strong Multiphoton-Excited Blue Photoluminescence and Lasing from Ladder-Type Oligo(p-phenylene)s

We report the synthesis and investigation of multiphoton absorption properties of a novel series of diphenylamino-end-capped ladder-type oligo(p-phenylene)s which exhibit greatly enhanced and efficient multiphoton (from two- to five-photon) upconverted blue photoluminescence with which the record-high intrinsic three-photon absorption cross-section of 4.56 × 10(-76) cm(6) s(2) in the femtosecond regime has been obtained. Exceptionally efficient two- to five-photon-excited lasing in the blue region has also been demonstrated in which the highest two-photon-excited lasing efficiency of 0.34% has been achieved.

Ruthenium(II) Complex Incorporated UiO-67 Metal–Organic Framework Nanoparticles for Enhanced Two-Photon Fluorescence Imaging and Photodynamic Cancer Therapy

Ruthenium(II) tris(bipyridyl) cationic complex (Ru(bpy)32+) incorporated UiO-67 (Universitetet i Oslo) nanoscale metal-organic frameworks (NMOFs) with an average diameter of ∼92 nm were developed as theranostic nanoplatform for in vitro two-photon fluorescence imaging and photodynamic therapy. After incorporation into porous UiO-67 nanoparticles, the quantum yield, luminescence lifetime, and two-photon fluorescence intensity of Ru(bpy)32+ guest molecules were much improved owing to the steric confinement effect of MOF pores. Benefiting from these merits, the as-synthesized nanoparticles managed to be internalized by A549 cells while providing excellent red fluorescence in cytoplasm upon excitation with 880 nm irradiation. Photodynamic therapeutic application of the Ru(bpy)32+-incorporated UiO-67 NMOFs was investigated in vitro. The Ru(bpy)32+-incorporated UiO-67 NMOFs exhibited good biocompatibility without irradiation while having good cell-killing rates upon irradiation. In view of these facts, the developed Ru(bpy)32+-incorporated NMOFs give a new potential pathway to achieve enhanced two-photon fluorescence imaging and photodynamic therapy.

Highly Efficient Multiphoton‐Absorbing Quadrupolar Oligomers for Frequency Upconversion

Two series of quadrupolar diphenylamino-endcapped oligofluorenes, PhN-OF(n)-NPh (n=2-5) and PhN-OF(n)-TAZ-OF(n)-NPh (n=1-4), which have an electron-withdrawing 1,2,4-triazole (TAZ) moiety as central core, with D-π-A-π-D structural motif (D=donor, A=acceptor), have been synthesized by palladium-catalyzed Suzuki cross-coupling of 9,9-dibutyl-7-(diphenylamino)-2-fluorenylboronic acid and the corresponding (1,2,4-triazole-based) aryl halide as key step. On pumping with infrared femtosecond lasers, these oligomers showed very strong multiphoton-excited blue photoluminescence. These D-π-D and D-π-A-π-D quadrupolar oligofluorenes exhibit superior three-photon absorption properties compared to the respective D-π-A counterparts with a highest three-photon absorption cross-section (σ(3)) of up to 2.72×10(-77) cm(6) s(2) . Despite the comparable linear and multiphoton absorption properties of the two types of quadrupolar oligomers PhN-OF(n)-NPh and PhN-OF(n)-TAZ-OF(n)-NPh, only the former exhibit remarkably intense and highly efficient multiphoton-excited frequency-upconverted deep blue lasing, which gives rise to record high lasing efficiency of 0.097% and very narrow of full width at half-maximum of the lasing spectra. Our findings suggest that quadrupolar-type molecules/oligomers are superior for multiphoton excited frequency upconverted lasing to their dipolar counterparts and also provide important guidelines to design highly efficient three-photon absorption molecules for photoluminescence and lasing applications.

Fluorene‐Based π‐Conjugated Oligomers for Efficient Three‐Photon Excited Photoluminescence and Lasing

A novel series of diphenylamino- and 1,2,4-triazole-end-capped, fluorene-based, pi-conjugated oligomers that includes extended oligofluorenes and oligothienylfluorenes has been synthesized by means of the palladium-catalyzed Suzuki cross-coupling of 9,9-dibutyl-7-(diphenylamino)-2-fluorenylboronic acid and the corresponding 1,2,4,-triazole-based aryl halide as a key step. It was demonstrated that efficient two- and three-photon excited photoluminescence and lasing in the blue region are obtained by pumping near-infrared femtosecond lasers on these materials. Although the absorption and emission maxima of the highly fluorescent and extended oligofluorenes reach a saturation limit, there exists an effective conjugation length for an optimum three-photon absorption cross section in the homologous oligofluorene series. On the other hand, the multiphoton excited emission spectrum and lasing wavelength can easily be modified or tuned by an incorporation of thienyl unit(s) into the fluorene-based pi-conjugated core with which exceptionally large three-photon absorption cross sections up to 3.59 x 10(-77) cm6 s2 in the femtosecond regime have been obtained, thereby highlighting the potential of this series of photonic materials. The optimized full width at half-maximum of the cavityless three-photon upconverted blue lasing spectra are sharply narrowed to approximately 6 nm with an efficiency of up to 0.013%.

Efficient Organic Light-Emitting Diode through Triplet Exciton Reharvesting by Employing Blended Electron Donor and Acceptor as the Emissive Layer.

A blended bimolecular exciplex formation was demonstrated between two individual donor and acceptor molecules, which are tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and 2,4,6-tris(3-(pyridin-3-yl)biphenyl-3-yl)-1,3,5-triazine (Tm3PyBPZ). The photoluminescence spectrum of the exciplex in the solid state showed an emission with a peak around 514 nm (∼2.49 eV). By applying this exciplex as an emitting layer, a highly efficient all-fluorescent organic lighting emitting diode with maximum efficiencies of 13.1% and 53.4 lm/W can be realized under an extremely low turn-on voltage of only 2.4 V. The thermally activated delayed fluorescence (TADF) process is believed to be responsible for the excellent device performance.

Self-Assembly of Electron Donor–Acceptor-Based Carbazole Derivatives: Novel Fluorescent Organic Nanoprobes for Both One- and Two-Photon Cellular Imaging

In this study, we report fluorescent organic nanoprobes with intense blue, green, and orange-red emissions prepared by self-assembling three carbazole derivatives into nanorods/nanoparticles. The three compounds consist of two or four electron-donating carbazole groups linked to a central dicyanobenzene electron acceptor. Steric hindrance from the carbazole groups leads to noncoplanar 3D molecular structures favorable to fluorescence in the solid state, while the donor-acceptor structures endow the molecules with good two-photon excited emission properties. The fluorescent organic nanoprobes exhibit good water dispersibility, low cytotoxicity, superior resistance against photodegradation and photobleaching. Both one- and two-photon fluorescent imaging were shown in the A549 cell line. Two-photon fluorescence imaging with the fluorescent probes was demonstrated to be more effective in visualizing and distinguishing cellular details compared to conventional one-photon fluorescence imaging.

Bioorthogonal Labeling, Bioimaging, and Photocytotoxicity Studies of Phosphorescent Ruthenium(II) Polypyridine Dibenzocyclooctyne Complexes

The synthesis, characterization, photophysics, lipophilicity, and cellular properties of new phosphorescent ruthenium(II) polypyridine complexes functionalized with a dibenzocyclooctyne (DIBO) or amine moiety [Ru(N^N)2 (L)](PF6 )2 are reported (L=4-(13-N-(3,4:7,8-dibenzocyclooctyne-5-oxycarbonyl) amino-4,7,10-trioxa-tridecanyl-aminocarbonyl-oxy-methyl)-4-methyl-2,2-bipyridine bpy-DIBO, N^N=2,2-bipyridine bpy (1u2009a), 1,10-phenanthroline phen (2u2009a); L=4-(13-amino-4,7,10-trioxa-tridecanylaminocarbonyl-oxy-methyl)-4-methyl-2,2-bipyridine bpy-NH2 , N^N=bpy (1u2009b), phen (2u2009b)). The strain-promoted alkyne-azide cycloaddition (SPAAC) reaction of the DIBO complexes 1u2009a and 2u2009a with benzyl azide were studied. Also, the DIBO complexes 1u2009a and 2u2009a can selectively label N-azidoglycans located on the surface of CHO-K1 and A549 cells that were pretreated with 1,3,4,6-tetra-O-acetyl-N-azidoacetyl-D-mannosamine (Ac4 ManNAz). Additionally, the intracellular trafficking and localization of these biomolecules were monitored using laser-scanning confocal microscopy. Interestingly, the biolabeling and cellular uptake efficiency of the DIBO complexes 1u2009a and 2u2009a were cell-line dependent, as revealed by flow cytometry and ICP-MS. Furthermore, the complexes showed good biocompatibility toward the Ac4 ManNAz-pretreated cells in the dark, but exhibited photoinduced cytotoxicity due to the generation of singlet oxygen.

Efficient two- to five-photon excited violet emission of calix[4]arene-based multiple donor–accepter assembly

Remarkably strong multiphoton, from two- to five-photon, upconverted violet fluorescence is first observed on a calix[4]arene-based multi-dipolar assembly which is strongly enhanced as compared to that of the corresponding dipolar counterparts.