Near-Infrared-Excitable Organic Ultralong Phosphorescence through Multiphoton Absorption

Abstract

Organic ultralong room-temperature phosphorescence (OURTP) with a long-lived triplet excited state up to several seconds has triggered widespread research interests, but most OURTP materials are excited by only ultraviolet (UV) or blue light owing to their unique stabilized triplet- and solid-state emission feature. Here, we demonstrate that near-infrared- (NIR-) excitable OURTP molecules can be rationally designed by implanting intra/intermolecular charge transfer (CT) characteristics into H-aggregation to stimulate the efficient nonlinear multiphoton absorption (MPA). The resultant upconverted MPA-OURTP show ultralong lifetimes over 0.42 s and a phosphorescence quantum yield of ~37% under both UV and NIR light irradiation. Empowered by the extraordinary MPA-OURTP, novel applications including two-photon bioimaging, visual laser power detection and excitation, and lifetime multiplexing encryption devices were successfully realized. These discoveries illustrate not only a delicate design map for the construction of NIR-excitable OURTP materials but also insightful guidance for exploring OURTP-based nonlinear optoelectronic properties and applications.

Figure 1

Molecular design strategy of MPA-OURTP materials. (a) Exciton transformation pathways of one-photon- (OPA), two-photon- (TPA), and three-photon- (3PA) triggered OURTP. The ground state (S₀) molecule can be excited to S_n by absorption of one, two, or three photons and then fall to S₁ through IC for fluorescence (Fl.). The triplet excited state (T_n) can be populated from S₁ via ISC, and the radiative decay of the lowest T_n (T₁) leads to phosphorescence (Phos.), while by further stabilization for T_n^∗, OURTP is produced. (b) Design of MPA-OURTP molecules in a D-A-D architecture with strong and abundant in-plane (dashed black) and interlayer (red line) intermolecular interactions in crystal. (c, d) Schematic drawing of (c) MPA-OURTP molecules using a planar π-conjugation donor and a difluoroboron β-diketonate acceptor with synergistic effects of intramolecular CT (ICT) and intermolecular space CT (SCT) for MPA and (d) the molecular structure of the designed model compound of CzPAB.

Figure 2

Photoluminescence properties of CzPAB powder under ambient conditions. (a) SSPL (blue) and OURTP (red) spectra excited by 365 nm UV light and 720 and 800 nm NIR lasers. Insets show the corresponding photographs on excitation (left) and removal (right) of the illumination light. (b) Excitation-OURTP emission mapping with a delay time of 25 ms. (c) SSPL spectra under different strengths of 720 (top) and 800 nm (bottom) NIR lasers with logarithmic plots of the integrated emission intensity versus the laser powers (insets). (d) OURTP lifetime decay profile at 530 nm excited by an 800 nm laser. (e) Transient emission decay image excited by 365 nm UV light.

Figure 3

Theoretical and single-crystal analyses of CzPAB. (a) NTO analyses on S₀ → S₁ and S₀ → T₁ excitations and orbital overlap extents (I_S and I_T) at single molecular (SM) and dimer states. (b) CT amount (q) of SM and dimers extracted from the single crystal at the S₀ state. (c) SSPL and phosphorescent (delay 5 ms) spectra in toluene (top) and powder (bottom) at 77 K. (d) TD-DFT-calculated excited state energy levels and the SOC constants between S₁ and T_n. (e) Molecular arrangement in single crystal with various intermolecular interactions (left) and representative molecular packing (top right) for H-aggregation (bottom right).

Figure 4

Proposed mechanism of MPA-OURTP and photophysical properties and aggregation structures of DCzB and DPAB powders. (a) Mechanisms of OURTP and MPA-OURTP. (b) Design principles of MPA-OURTP molecules by integrating intra- and intermolecular CT characters into H-aggregations in a quadrupolar D-A-D skeleton. (c, d) SSPL (blue) and OURTP (pink) spectra of the (c) DCzB and (d) DPAB powders excited by 365 nm UV light and 800 nm NIR laser. Insets show the corresponding molecular structures. (e) The logarithmic plots of the integrated emission intensity versus the 800 nm laser power. (f) OURTP lifetime decay profiles at 475, 495, and 525 nm for DCzB (top) and at 400 and 458 nm for DPAB (bottom) excited by 365 nm UV light. (g) Molecular arrangements in single crystals of DCzB (left) and DPAB (middle) with representative molecular packing structures.

Figure 5

Applications of MPA-OURTP materials. (a) Schematic drawing of the bottom-up strategy to prepare CzPAB nanoparticles. (b) Particle size distribution revealed by dynamic light scattering. The inset is a transmission electron microscope image. The scale bar is 200 nm. (c) OURTP lifetime decay profile at 530 nm of CzPAB nanoparticles excited by 365 nm UV light under ambient conditions. (d) Two-photon confocal laser scanning microscopy imaging of HeLa cells stained with CzPAB nanoparticles after incubation for 6 h. The cellular images were captured by collecting the luminescence from 500 to 750 nm under the excitation of 800 nm NIR laser. The scale bar is 10 μm. (e) The setup for the visual NIR laser power detector. (f) Power-dependent OURTP images of CzPAB powder from the video recorded after turning off the 800 nm NIR laser. (g) Evolution mapping of grayscale (G) values of the OURTP images under different laser off-time. (h) Design and (i) demonstration of the excitation and lifetime multiplexing encryption device. The scale bar is 1 cm.