Research ArticleMATERIALS SCIENCE

Continuous color tuning of single-fluorophore emission via polymerization-mediated through-space charge transfer

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Science Advances  07 Apr 2021:
Vol. 7, no. 15, eabd1794
DOI: 10.1126/sciadv.abd1794
  • Fig. 1 Schematic illustration of color-tunable solid-state emission from a single fluorophore by polymerization-mediated TSCT.

    Top row: Making chain end groups CT donors through a chain-end transformation shifts emission color from blue to yellow. Left column: Multicolor emission (blue to green) due to donor monomer–dependent TSCT. Right column: Multicolor emission (yellow to blue) due to D-A distance–dependent TSCT. M1, M2, and M3 represent the styrenic monomers with increasing electron-donating ability, and E represents transformed chain end group. The background colors represent tunable fluorescence emission colors from the polymers in aggregate/solid state. The black arrows represent both intrachain and interchain TSCT.

  • Fig. 2 Chemical structures and photophysical properties of functionalized NDI fluorophore and NDI polymers.

    (A) Chemical structures of NDI-diBr and NDI polymers and representation of ATRP-mediated TSCT. The curved arrow represents both intrachain and interchain TSCT. (B) PL spectra of NDI-diBr in DMF/H2O mixtures. Inset: Photographs under UV lamp (365 nm) of NDI-diBr in powder form, as well as dissolved to a concentration of 0.1 mM in DMF/H2O mixtures with increasing water fraction [fw = 0 to 95% (v/v)]. a.u., arbitrary units. (C) PL spectra of NDI-diBr in various aromatic solvents. Inset: Photographs under UV lamp (365 nm) of NDI-diBr in various solvents with 1.5 mM. (D) Photographs of powders and thin films of polymers made from different monomers. (E) PL spectra and (F) CIE 1931 chromaticity diagram of NDI-PSCl-1, NDI-PS-1, NDI-PSmMe-1, and NDI-PSpMe-1 thin films. (G to J) PL spectra of the four polymers in DMF and DMF/H2O (20/80, v/v). Inset: Photographs of polymers in DMF or DMF/H2O under UV lamp (365 nm). The measuring concentration of NDI-diBr or NDI polymers was 5.0 μM in (B) and (G) to (J) and 1.5 mM for (C). The excitation wavelength was 370 nm. Note that curves of NDI-PSCl-1 and NDI-PSpMe-1 in (E) were smoothed by the Savitzky-Golay method. Photo credit: Suiying Ye, ETH Zurich.

  • Fig. 3 NDI polymers with high end-group fidelity (>90%) before and after debromination and their photophysical properties.

    (A) Structural illustration of end-group transformation–induced TSCT of the debrominated polymers (NDI-PS-d, NDI-PSCl-d, NDI-PSmMe-d, and NDI-PSpMe-d). The curved arrows represent both intrachain and interchain TSCT. (B, E, H, and K) PL spectra of thin films produced with (B) NDI-PS-1, (E) NDI-PSCl-1, (H) NDI-PSmMe-1, and (K) NDI-PSpMe-1 (DP 22, 18, 20, and 19, respectively) before and after debromination at various conversions. Inset: Photographs under UV lamp (365 nm) of polymers in powder form with increasing debromination levels from left to right. The excitation wavelength was 370 nm. (C, F, I, and L) CIE 1931 chromaticity diagram of thin films from (C) NDI-PS-1, (F) NDI-PSCl-1, (I) NDI-PSmMe-1, and (L) NDI-PSpMe-1 before and after debromination at various conversions. (D, G, J, and M) Maximum emission wavelength (λem) of thin films made of NDI polymers with increasing molar masses, before and after debromination at various conversions. Note that curves of (E) and (K) were smoothed by the Savitzky-Golay method. Photo credit: Suiying Ye, ETH Zurich.

  • Fig. 4 Atomistic understanding of the TSCT mechanism in the debrominated oligomers (NDI-PMn-d).

    (A) Comparison of spatial distribution and band diagrams of ground-state HOMO/LUMO levels for the original NDI-PMn oligomers with n = 8—including NDI-PSCl8, NDI-PS8, NDI-PSmMe8, and NDI-PSpMe8—and (B) their debrominated counterparts NDI-PMn-d calculated using the CAM-B3LYP hybrid functional (50). By varying the monomer substituents, the HOMO-LUMO energy gap ranks by PSCl > PS > PSmMe > PSpMe, agreeing with the experimental trend. Moreover, after debromination, the HOMO levels change from backbone phenyl groups to the vinyl benzene end groups, resulting in a chain length–dependent emission red shift. (C) Schematic energy diagrams for the neat NDI core (left), the original NDI-PMn (middle), and the debrominated NDI-PMn-d oligomers (right), respectively.

  • Fig. 5 Mechanistic investigations using theoretical calculations.

    (A and B) NDI-PS2 and NDI-PS2-d originating NTOs and corresponding wavelengths for the first excited state of molecular conformations from the top three populated clusters in DMF/H2O (20/80, v/v) from MD simulations. (C) Snapshots from MD simulations of NDI-PS2 and NDI-PS2-d aggregates. (D) Dimer configuration, orbital, and corresponding wavelength of NDI-PS2-d aggregate. In all cases, transitions were to the LUMO, which was identical in spatial distribution to the LUMO of the NDI core.

  • Fig. 6 Photophysical properties of NDI-PS polymers with different DP values and low end-group fidelity (<60%).

    (A) Photographs under UV lamp (365 nm) of powders and thin films produced with polymers synthesized by ATRP with polymerization times >16 hours and DP values ranging from 3 to 70. (B) GPC spectra of polymers with DP 18 to 55 (polymers with DP 3 and 14 are not shown here because of the short retention time on GPC). (C) PL spectra and (E) CIE 1931 chromaticity diagram of thin films produced from NDI-PS with different molar masses (the polymer with DP 22 is not shown because of debromination conversion lower than 30%). (D) Relationship between film emission wavelength and DP of the polymers. The excitation wavelength was 370 nm. (F) Schematic illustration of the photolithography and photographs before and after UV irradiation of polymer thin films that contain BAPO as the photoinitiator and were produced with NDI-PS-1d polymers (debrominated NDI-PS-1 at conversion 84%). Glass substrate: 24 mm by 24 mm. Photo credit: Suiying Ye, ETH Zurich.

Supplementary Materials

  • Supplementary Materials

    Continuous color tuning of single-fluorophore emission via polymerization-mediated through-space charge transfer

    Suiying Ye, Tian Tian, Andrew J. Christofferson, Sofia Erikson, Jakub Jagielski, Zhi Luo, Sudhir Kumar, Chih-Jen Shih, Jean-Christophe Leroux, Yinyin Bao

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    • Supplementary Materials and Methods
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