Excitation-Dependent Quadruple-Level Emission from an Isolated Molecule for Dynamic Information Encryption

Abstract

Stimuli-responsive single-molecule multi-emission materials have long attracted considerable attention due to their great potential in non-phase-separated smart luminescence. Here, a new strategy is demonstrated for manipulating electron transfer based on donor-acceptor decoupling to regulate energy levels, aiming to achieve excitation-dependent (Ex-De) single-molecule emission with switchable multiple fluorescence and phosphorescence. The synthesized 10-phenyl-10H,13'H-spiro[acridine 9,6'-pentacen]-13'-one (ACRSP) exhibits anti-Kasha quadruple-level emission and opposite Ex-De afterglow in different environments. The high-energy emission bands of multi-fluorescence in solution respond to excitation, whereas in poly(methyl methacrylate) (PMMA), phosphorescence-fluorescence multi-emission causes Ex-De to appear in the low-energy emission band. Experimental and computational results indicate that exciton spin ratios and emissive state compositions vary with excitation modes, leading to dual Ex-De behavior from three fluorescence and one phosphorescence emissions. Donor-acceptor decoupling separates locally excited (LE) and charge transfer (CT) states, while triplet level inversion enables Ex-De behavior and room-temperature phosphorescence (RTP) coexistence (τ = 770.54 ms). By tuning the excitation mode of ACRSP, we achieve Ex-De long afterglow emission from an isolated molecule, enabling time-resolved and excitation-responsive multi-dimensional information encryption. This work offers design guidelines for purely organic Ex-De systems and paves the way for next-generation single-molecule responsive luminophores.

Keywords: NEVPT2; anti‐kasha emission; excitation‐dependent emission; multi‐dimensional information encryption; room‐temperature phosphorescence.

Figure 1

Schematic illustration of Ex‐De quadruple‐level emission. a) Molecular structure of ACRSP and its donor‐acceptor excitation modes. b) Jablonski diagram illustrating quadruple‐level emission of ACRSP. c) Multistep electron transfer and fluorescence‐phosphorescence generation pathways under excitation‐mode separation, originating from donor excited charge transfer (DECT) and acceptor excited charge transfer (AECT) processes. d) Schematic illustration of dual‐phase Ex‐De afterglow behavior.

Figure 2

Single‐crystal structure and vertical excitation modes of ACRSP. a–c) Single crystal structure of ACRSP highlighting key bond angles a) hydrogen bonds b) and π‐π stacking c). d) Absorption spectra of ACRSA and ACRSP measured at 10 µm concentration in toluene. e) Excitation modes and associated molecular orbitals of ACRSP, represented by donor and acceptor fragments.

Figure 3

Photophysical properties of ACRSP. a,b) Emission spectra (10 µm) of ACRSP excited at 320nm a) and 380nm b) wavelengths in Tol, Diox, o‐DCB, DCM, DMSO. c) Emission delay curve (10 µm) of the CT emission peak of ACRSP before and after degassing in DMF. d,e) Emission spectra (10 µm) of ACRSP excited at 320nm (d) and 380nm (e) wavelengths in Acetone and DMF. f) The first NOCV pairs of ACRSP in DMF and acetone (with isovalue = 0.0002 a.u.) g) CIE chromaticity diagram of ACRSP at multiple excitation wavelengths in o‐DCB and photos of ACRSP solutions (5 µm) in different solvents under 365 nm irradiation. Tol: toluene, Diox: 1,4‐dioxane, o‐DCB: 1,2‐dichlorobenzene, DCM: dichloromethane, DMSO: dimethyl sulfoxide, DMF: N,N‐Dimethylformamide, Ace: Acetone.

Figure 4

Thermodynamics and kinetics analyses of the AECT_nπ* pathway. a) The minimum energy profile (MEP) of ACRSP for the AECT_nπ* processes, the structural evolution along the relaxation path, and its key bond parameters are provided. b) The calculated non‐radiative rate constants for AECT_nπ* were shown in both singlet and triplet states, along with the corresponding electronic coupling values (NAC and SOC), reorganization energy (λ), electronic energy gap (ΔE), and Huang‐Rhys (HR) factor, within the framework of the MLJ theory. The computational results were obtained at the NEVPT2/def2‐TZVP//IRC/CASSCF(10e/10o) level in gas and solvents. SSC: Singlet‐Singlet Crossing, STC: Singlet‐Triplet Crossing, and RET: Restricted Electron Transfer.

Figure 5

Photophysical properties of ACRSP doped in PMMA and BA matrix. a–c) Schematic diagram of the single‐molecule behavior of ACRSP under solvent host a), PMMA host b), and BA matrix host c). d) Triple state energy level and transition property of ACRSA and ACRSP at T₁ state minimum point calculated at the NEVPT2/def2‐TZVP//CASSCF(10e/10o) level. e) Fluorescence spectra of ACRSA‐0.1%‐PMMA film at 365 nm excitation. f) Phosphorescence and fluorescence spectra of ACRSP‐0.1%‐PMMA film at 365 nm excitation, with 1 ms delay for phosphorescence. g) The emission delay curve of the RTP of ACRSP‐0.1%‐PMMA. h) The emission delay curve of the RTP of ACRSP‐0.1%‐BA. i) Photos of ACRSP‐0.1%‐BA before and after turning off the UV 365 nm light.

Figure 6

Excitation‐dependent properties of the ACRSP‐0.1%‐PMMA. a–c) Luminescence spectra a) contour map b) and CIE chromaticity diagram c) of ACRSA‐0.1%‐PMMA. d, Proposed mechanism of ACRSP under PMMA host.

Figure 7

Multi‐dimensional codes for advanced dynamic information encryptions by ACRSP‐0.1%‐PMMA and ACRSA‐0.1%‐PMMA. a–c). Photos of chemical structure, numbers, and butterfly image by ACRSP‐0.1%‐PMMA and ACRSA‐0.1%‐PMMA under daylight, 310 nm, and 365 nm UV light before and after activation, and after removal of the UV light. d, Schematic diagram of mask printing project and multilayer programmable printing based on ACRSP‐0.1%‐PMMA.