Energy-Transfer-Induced Multiexcitation and Enhanced Emission of Hyperbranched Polysiloxane

Abstract

Fluorescent hyperbranched polysiloxane (HBPSi) has attracted increasing attention due to its good biocompatibility. However, its emission mechanism remains an open question. Unfortunately, the excitation spectra of HBPSi are rarely systematically investigated and show a narrow excitation band, which hinders the emission mechanism study. Herein, we synthesized a series of novel HBPSi containing L-glutamic acid (HBPSi-GA). Surprisingly, these polymers have four excitation peaks and two emission peaks, which are caused by the energy transfer from free functional groups to heterogeneous electron delocalizations in different clusters. Meanwhile, the fluorescence and biocompatibility of HBPSi-GA are significantly improved with increasing L-glutamic acid. Furthermore, HBPSi-GA exhibits dual stimuli-responsive fluorescence to temperature and Fe 3+ as well as potential application in cell imaging. This research possesses important guidance to develop multiexcitation unconventional fluorescent polymers.

Oleic Acid Constructed Supramolecular Hyperbranched Polysiloxane with Enhanced Fluorescence and Excellent Drug Delivery Ability

“…An obvious red-shift of the absorption peak at 230 nm is observed with increasing concentration, which might be caused by the gradually aggregate of the free functional groups. [15] Furthermore, in the absorbance spectra of HBPSi-OA2, peaks at 300 and 360 nm are much stronger than that of HBPSi, meaning that oleic acid promotes the formation of electron delocalizations efficiently.…”

“…Based on the UV-vis absorbance results, 260 nm UV light causes the n-𝜎* transition of free functional groups, however, the light from electron delocalizations (360 and 440 nm) is generated, therefore, the energy absorbed by the free functional groups might transfer to the electron delocalizations via fluorescence resonance energy transfer (FRET) (Figure S7, Supporting Information). [15,20,21] At the same time, 300 nm excitation induces the n-𝜋* transition of smaller electron delocalizations, however, emission from larger electron delocalizations is observed, thus, some energy absorbed by smaller electron delocalizations probably transfers to larger electron delocalizations by FRET. [15,20,21] The fluorescence changes of HBPSi and HBPSi-OA2 followed concentration (Figures S7-S10, Supporting Information) also demonstrate the FRET process.…”

“…In the UV-vis absorbance spectra of HBPSi and HBPSi-OA2 (Figure 1), four absorbance bands are observed at about 230, 260, 300, and 360 nm, which are caused by the n-𝜎* transition of free functional groups (hydroxyl, amines, and ether groups), n-𝜎* transition of ─Si(O) 3 functional groups, n-𝜋* transition of electron delocalizations in small clusters of electron-rich groups (named smaller electron delocalizations), and n-𝜋* transition of electron delocalizations in large clusters of electron-rich groups (named larger electron delocalizations), respectively. [14,15] In the clusters, the electron-rich atoms are so close that their electron clouds overlap and form different electron delocalizations. An obvious red-shift of the absorption peak at 230 nm is observed with increasing concentration, which might be caused by the gradually aggregate of the free functional groups.…”

“…Emission at 320, 360 nm is probably ascribed to the formed smaller electron delocalizations, while, emission at 440 nm might be caused by the formed larger electron delocalizations. [15,19] Under excitation of 300 nm, three emission peaks at 320, 360, and 440 nm are generated. However, there is only one emission at about 440 nm under excitation of 360 nm.…”

“…[10,11] Our group first found hyperbranched polysiloxanes can emit blue light under UV light. [7,12,13] To enhance the fluorescence, our group introduced electron-rich molecules/functional groups, such as polyether amine, [12] betacyclodextrin, [14] l-glutamic acid, [15] carbonyl and carboxyl groups, [16] into HBPSi, which can enhance aggregate state and electronic communication, leading to improved quantum yield (QY) and fluorescence intensity. It is well known that the morphology of molecules could be further adjusted by changing the hydrophilic-hydrophobic effect.…”