Photoexcitation-controlled self-recoverable molecular aggregation for flicker phosphorescence

Abstract

Chemical systems with external control capability and self-recoverability are promising since they can avoid additional chemical or energy imposition during the working process. However, it remains challenging to employ such a nonequilibrium method for the engineering of optoelectronic function and for visualization. Here, we report a functional molecule that can undergo intense conformational regulation upon photoexcitation. It enables a dynamical change in hydrophobicity and a follow-up molecular aggregation in aqueous media, accordingly leading to an aggregation-induced phosphorescence (AIP) behavior. This successive performance is self-recoverable, allowing a rapid (second-scale cycle) and long-standing (>10³ cycles) flicker ability under rhythmical control of the AIP. Compared with traditional bidirectional manipulations, such monodirectional photocontrol with spontaneous reset profoundly enhances the operability while mostly avoiding possible side reactions and fatigue accumulation. Furthermore, this material can serve as a type of luminescent probe for dynamically strengthening visualization in bioimaging.

Keywords: aggregation-induced phosphorescence; conformational regulation; luminescent probe; photoexcitation; self-recoverable.

Fig. 1.

Schematic illustration of the photoexcitation-controlled AIP. (A) Chemical structure of the related compounds. (B) The proposed conformational change upon photoexcitation of compound 3. The dihedral torsion as well as the related atoms were defined alongside the structure. (C) The successive action of the photoactivation and self-recovery cycle of the AIP behavior of compound 3 in aqueous media, based on the conformational interconversion accompanied by a long-lifetime dark state. ISC, intersystem crossing.

Fig. 2.

Theoretical study of the conformational change of compound 3 upon photoexcitation. (A) Calculated GS potential energy surface, where the black solid circle indicates the equilibrium geometry. (B) Absorption spectrum calculated based on the GS equilibrium geometry. (C) ES1 potential energy surface. (D) Scan of the ES1 oscillator strength.

Fig. 3.

Photoactivation and self-recovery of the phosphorescence property of compound 3 in water (1 × 10⁻⁵ M) under irradiation by a commercial UV lamp (365 nm, 4 W). Continuous enhancement of the (A) PL spectra and (B) absorption spectra with prolonged irradiation duration. (C) PL lifetime at 505 nm after irradiation. (D) Image of photocontrolled phosphorescence on and off with different cycles. The light source was turned on for 1 min and off for 1 min during each cycle.

Fig. 4.

Nanoscale study (DLS results and TEM images of compound 3 in water) and computational data for confirming an AIP process. DLS results (A) before and (C) after irradiation for 5 min. TEM image (B) before and (D) after irradiation for 5 min. (Scale bars: 100 nm.) Equilibrium geometry of the (E and F) GS and (G and H) ES1. The van der Waals surface has been used in E and G, and a water molecule has been shown to guide the eyes. The triangles indicate the distances between corresponding hydrogen atoms.

Fig. 5.

Dynamic emission of compound 3 for imaging. (A) Confocal microphotographs (FITC channel) of HeLa cells incubated with compound 3 upon rhythmical irradiation at 405 nm. (B) Corresponding brightness intensity readout curves along the line highlighted in A. (C) Continuous enhancement of luminescence of the cell solution dyed with compound 3 with prolonged irradiation time in a visualized patterning experiment.