The influence of the molecular packing on the room temperature phosphorescence of purely organic luminogens

Abstract

Organic luminogens with persistent room temperature phosphorescence (RTP) have attracted great attention for their wide applications in optoelectronic devices and bioimaging. However, these materials are still very scarce, partially due to the unclear mechanism and lack of designing guidelines. Herein we develop seven 10-phenyl-10H-phenothiazine-5,5-dioxide-based derivatives, reveal their different RTP properties and underlying mechanism, and exploit their potential imaging applications. Coupled with the preliminary theoretical calculations, it is found that strong π-π interactions in solid state can promote the persistent RTP. Particularly, CS-CF₃ shows the unique photo-induced phosphorescence in response to the changes in molecular packing, further confirming the key influence of the molecular packing on the RTP property. Furthermore, CS-F with its long RTP lifetime could be utilized for real-time excitation-free phosphorescent imaging in living mice. Thus, our study paves the way for the development of persistent RTP materials, in both the practical applications and the inherent mechanism.

Fig. 1

The room temperature phosphorescence (RTP) behavior of the six target compounds. The molecular structures of the six compounds of CS-CH₃O, CS-CH₃, CS-H, CS-Br, CS-Cl, and CS-F, and their corresponding phosphorescence lifetimes in crystals at room temperature (RT). The photographs were taken at different times, before and after turning off the 365 nm UV irritation under ambient conditions

Fig. 2

The phosphorescence spectra and corresponding time-resolved decay curves for the six compounds. a The normalized room temperature phosphorescence spectra of CS-CH₃O, CS-CH₃, CS-H, CS-Br, CS-Cl, and CS-F in crystal state. b Time-resolved PL-decay curves for their room temperature phosphorescence in crystal state. c The normalized low temperature (77 K) phosphorescence spectra in dichloromethane solution, concentration: 10⁻⁵ M. d Time-resolved PL-decay curves for their low temperature phosphorescence in dichloromethane solution, concentration: 10⁻⁵ M. e The proposed mechanism for organic persistent RTP: the strong π–π interaction could decrease the radiative transition (k_P) and non-radiative transition (k_TS) from T₁ to S₀ state, thus achieving the persistent room temperature phosphorescence

Fig. 3

Single-crystal structures of the six target compounds. Entire and local packing modes of the crystals for CS-CH₃O, CS-CH₃, CS-H, CS-Br, CS-Cl, and CS-F: the local packing pictures were selected from the parts in cycles of corresponding entire ones, in which the centroid–centroid distances and the dihedral angles of the involved phenyl rings are listed and the phenyl rings involved in the stronger π–π interactions in entire are labeled by pink color

Fig. 4

The influence of π–π interactions on the electron redistribution and RTP behavior. a The orbital interaction of π–π stacking in excited state: the electrons would redistribute in new orbitals for the π–π interactions, then three electrons are stabilized and one electron is destabilized, and thus the net stabilization is two electrons. b Depiction of the electrostatic model of substituent effects on π–π interactions from Hunter–Sanders model: electron-withdrawing substituents could enhance π–π interactions by decreasing the π-electron density of the substituted π-system and relieving the π–π repulsion, while electron-donating substituents hinder π–π stacking through the opposite mechanism. c Difference electrostatic potential (ESP) analysis of isolated CS-CH₃O, CS-CH₃ (a), CS-H, CS-Br, CS-Cl, and CS-F. The potential energy range is −0.015 to 0.015 H q^-1 for all surfaces shown, red indicates areas with dense electron density, yellow for normal, while blue areas suggest less electron density

Fig. 5

The photo-induced phosphorescence property of CS-CF₃. a The room temperature phosphorescence spectra of CS-CF₃ crystal before and after 365 nm UV irradiation for 3 min. b The UV–visible spectra of the CS-CF₃ crystal before and after UV irradiation for 5 min. c Time-resolved PL-decay curves for the room temperature phosphorescence of CS-CF₃ crystal under 5 min 365 nm UV irradiation and after 1 or 2 or 4 h of standing at 77 K or 298 K. d The crystal structures of coupled CS-CF₃ before and after UV irradiation for 5 min. e Double security protection applications by using three kinds of components of CS-CF₃, CS-F and (4-methoxyphenyl)(phenyl)methanone. Under 365 nm UV irradiation, it presents blue pattern 8; then switching off the UV light suddenly, it turns to green pattern 7; if the UV irradiation could be kept for about 5 min, it would appear green pattern 9 after turning off the UV light

Fig. 6

In vivo real-time excitation-free phosphorescent imaging of lymph nodes. a Top-down approach to synthesize the water-soluble nanoparticles of CS-F: the crystal was added to the aqueous solution of PEG-b-PPG-b-PEG (F127) under continuous sonication, then the aqueous solution was filtered through a 0.22 µm polyvinylidene fluoride (PVDF) (Millipore). b Ultralong phosphorescence and fluorescence imaging of lymph node in living mice 1 h after the intradermal injection of CS-F nanoparticles into the forepaw of mice