Novel Highly Luminescent Amine-Functionalized Bridged Silsesquioxanes

Abstract

Amine-functionalized bridged silsesquioxanes (BSs) were synthesized from bis[(3-trimethoxysilyl)propyl] amine via a solvent-mediated route. BS-1 and BS-2 were obtained at neutral pH with sub- and stoichiometric amounts of water, respectively, and high tetrahydrofuran content. BS-3 was prepared with hyperstoichiometric water concentration, high tetrahydrofuran content, and hydrochloric acid. BS-4 was synthesized with hyperstoichiometric water concentration, high ethanol content, and sodium hydroxide. BS-1 and BS-2 were produced as transparent films, whereas BS-3 and BS-4 formed white powders. Face-to-face stacking of flat or folded lamellae yielded quasi-hydrophobic platelets with emission quantum yields of 0.05 ± 0.01 (BS-1 and BS-2) or superhydrophilic onion-like nanoparticles with exciting emission quantum yields of 0.38 ± 0.03 (BS-3) and 0.33 ± 0.04 (BS-4), respectively. The latter two values are the largest ever reported for amine-functionalized siloxane-based hybrids lacking aromatic groups. Fast Grotthus proton hopping between = [Formula: see text]/ = NH groups (BS-3) and = N^-/ = NH groups (BS-4), promoted by H⁺ and OH^- ions, respectively, and aided by short amine-amine contacts provided by the onion-like morphology, account for this unique optical behavior.

Keywords: bis[(3-trimethoxysilyl)propyl]amine; luminescence; morphology; sol-gel chemistry; solvent-assisted structuring.

Figure 1

Structure of the BTMSPA precursor.

Figure 2

¹³C CP/MAS (A) and ²⁹Si MAS (B) NMR spectra of the BS-1 (black line), BS-2 (gray line), BS-3 (red line), and BS-4 (blue line) hybrids.

Figure 3

SWAXS spectra of the BS-1 (black line), BS-2 (gray line), BS-3 (red line), and BS-4 (blue line) hybrids.

Figure 4

SEM images of the BS-1 (A,B), BS-2 (C,D), BS-3 (E,F), and BS-4 (G,H) hybrids.

Figure 5

TEM images of the BS-1 (A), BS-2 (B), BS-3 (C,D), and BS-4 (E,F) hybrids.

Figure 6

TGA curves of the BS-1 (black line), BS-2 (gray line), BS-3 (red line), and BS-4 (blue line) hybrids.

Figure 7

Time-dependence of the contact angle of the BS-1 (black line), BS-2 (gray line), BS-3 (red line), and BS-4 (blue line) hybrids. Insets: HR-SEM images of BS-1 and BS-4.

Figure 8

Mechanistic scenario for the synthesis of the BS-1 (top) and BS-4 (bottom) hybrids.

Figure 9

Excitation (A) and emission (B) spectra monitored around 445 nm and excited at 360 nm, respectively, of the BS-1 (black line), BS-2 (magenta line), BS-3 (red line), and BS-4 (blue line) hybrids. Excitation (C) and emission (D) spectra of BS-4 monitored and excited at distinct wavelengths.