Fast, efficient, narrowband room-temperature phosphorescence from metal-free 1,2-diketones: rational design and the mechanism

Abstract

We report metal-free organic 1,2-diketones that exhibit fast and highly efficient room-temperature phosphorescence (RTP) with high colour purity under various conditions, including solutions. RTP quantum yields reached 38.2% in solution under Ar, 54% in a polymer matrix in air, and 50% in crystalline solids in air. Moreover, the narrowband RTP consistently dominated the steady-state emission, regardless of the molecular environment. Detailed mechanistic studies using ultrafast spectroscopy, single-crystal X-ray structure analysis, and theoretical calculations revealed picosecond intersystem crossing (ISC) followed by RTP from a planar conformation. Notably, the phosphorescence rate constant k _p was unambiguously established as ∼5000 s^-1, which is comparable to that of platinum porphyrins (representative heavy-metal phosphor). This inherently large k _p enabled the high-efficiency RTP across diverse molecular environments, thus complementing the streamlined persistent RTP approach. The mechanism behind the photofunction has been elucidated as follows: (1) the large k _p is due to efficient intensity borrowing of the T₁ state from the bright S₃ state, (2) the rapid ISC occurs from the S₁ to the T₃ state because these states are nearly isoenergetic and have a considerable spin-orbit coupling, and (3) the narrowband emission results from the minimal geometry change between the T₁ and S₀ states. Such mechanistic understanding based on molecular orbitals, as well as the structure-RTP property relationship study, highlighted design principles embodied by the diketone planar conformer. The fast RTP strategy enables development of organic phosphors with emissions independent of environmental conditions, thereby offering alternatives to precious-metal based phosphors.

Fig. 1. Comparison of representative metal-free organic phosphors (blue filled circles), thiocarbonyls (blue open circles), and 1 (orange diamonds) in solution. (a) Molecular structures of 1. (b) Room-temperature phosphorescence quantum yields (Φ_p) vs. lifetimes (τ_p); (c) full-width-at-half-maxima (FWHM) vs. Φ_p. Blue and orange broken lines in (b) represent k_p = 100 and 5000 s⁻¹, respectively, assuming unity intersystem crossing quantum yields.

Fig. 2. (a) Steady-state photoluminescence spectra of 1a in cyclohexane (4.4 × 10⁻⁶ M, excited at 368 nm). (b) Photograph of solutions under 365 nm excitation. (c) fs transient absorption spectra of 1a in cyclohexane excited at 355 nm. (d and e) Selected results from the global analysis based on a sequential model; (d) evolution-associated spectra and (e) corresponding concentration kinetics. Coherent artefact signals are omitted for clarity. (f) Schematic summary of the excited-state dynamics in 1a.

Fig. 3. (a) Steady-state photoluminescence (PL) spectra of 1a–1c in cyclohexane (10–4.4 × 10⁻⁶ M, excited at 368 nm) and (b) CIE1931 chromaticity diagram for their PL in cyclohexane. (c) Photophysical properties of 1a–1c in cyclohexane under Ar (10–4.4 × 10⁻⁶ M, excited at 368 nm). Φ_p, RTP quantum yields; FWHM, full-width-at-half-maxima; λ_em, emission maxima; τ_p, lifetimes; k_p and k_nr, phosphorescence and nonradiative rate constants. (d and g) Steady-state PL spectra of (d) 1a–1c@PMMA (5 wt%, excited at 355–368 nm) and (g) a 1c crystal in cyclohexane (1.0 × 10⁻⁵ M, excited at 368 nm). (e and h) Photographs of (e) 1b@PMMA and (h) a 1c crystal under 365 nm excitation. (f) Φ_p in air and the FWHM of 1b-doped polymer films (5 wt%, excited at 350–375 nm). (i) ORTEP drawing of the crystal structure of 1c. Thermal ellipsoids are set at the 50% probability level.

Fig. 4. Energy diagram of 1a in the S₁-optimized trans-planar (TP) geometry. Energies and the spin–orbit coupling matrix element between the S₁ and T₃ states 〈S₁|H_SO|T₃〉 were calculated at the TDA/uCAM-B3LYP-D3/6-311G(d) level of theory.

Fig. 5. (a) Energy diagram of 1a in the T₁-optimized trans-planar (TP) geometry depicting the principal intensity-borrowing. Energies, the transition dipole moment μ_S3–S0, and the spin–orbit coupling matrix element 〈S₃|H_SO|T₁〉 were calculated at the TDA/(u)CAM-B3LYP-D3/6-311G(d) level. (b) Principal natural transition orbitals (NTOs) for S₃–S₀ (left) and T₁–S₀ (right) transitions.

Fig. 6. (a) Chemical structures of 1a, 2a, and 3a, and their Φ_p, estimated k_p, steady-state photoluminescence spectra, and the corresponding FWHM in cyclohexane (1.0 × 10⁻⁵ M, 2.6 × 10⁻³ M, and 1.0 × 10⁻⁴ M, respectively) excited at 368 nm under Ar. k_p was derived as k_p = Φ_p/τ_p, assuming ϕ_ISC = 1. (b) Calculated factors of k_p for 1a–3a. (c) Franck–Condon analysis of the emission spectra of 1a (left) and 2a (right). Huang–Rhys (HR) factors were 0.51 and 2.22, respectively. (d) Bond-elongation ratio between the optimized geometries in the S₀ and T₁ states of 1a and 2a. (e) Natural transition orbitals of the T₁ states for 1a and 2a.