First, fundamental luminescent properties of the model material 1.8-mDTAZ-PhtCz were characterized. It exhibits absorption peaks at 330-345 nm, and with emission peak at 425 nm in degassed toluene (Fig. 1a). Temperature-dependent, time-resolved PL spectra show that multiple emissive components including PF, TADF and RTP from the model material. The temperature-dependent transient PL reveals that TADF decreases when cooling from 292 K to 252 K, while phosphorescence gradually dominates below 232 K. Only long-lived phosphorescence remains at 77 K (Fig. 1c), reflecting the competitive relationship between TADF and phosphorescence. Single-crystal structure indicates the presence of intermolecular hydrogen bonds (2.48-2.54 Å) and π-π stacking (Fig. 1d), with a twisted donor-acceptor configuration that effectively inhibits molecular motion and non-radiative relaxation. The crystal shows afterglow up to 42 s after the cease of UV excitation (Fig. 1e).
Direct experimental evidence for the existence of the second triplet state (T₂) was obtained via nanosecond transient absorption (ns-TA) spectroscopy combined with multi-dimensional data analysis (Fig. 2a). Forward Evolving Factor Analysis (EFA) revealed that the ns-TA signal comprises three principal components (Fig. 2b), corresponding to three excited states with distinct lifetimes. Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) fitting further identified Component 2 as the singlet state S₁, while Component 1 and Component 3—exhibiting slow microsecond-scale decay—were assigned to two triplet states (Fig. 2c and 2d). These triplet states possess similar spectral energies but markedly different dynamical behaviors. The measured kinetics were successfully described by a three-component model within the investigated time window (Fig. 2e). Global analysis based on a parallel decay model yielded lifetimes of 15.2 ± 0.3 ns (S₁), 2.1 ± 0.1 μs (T₂), and 8.2 ± 1.1 μs (T₁). The fourfold difference in lifetime between T₂ and T₁, coupled with their similar decay-associated difference spectra (DADS, Fig. 2f), indicates their proximity in energy.
The excited-state energy levels and electron density distributions of 1.8‑mDTAZ‑PhtCz were simulated using the Restricted Open-shell Kohn-Sham (ROKS) method with the LC‑ωPBE08 functional. The calculated S₁, T₂, and T₁ energies are 2.978 eV, 2.953 eV, and 2.912 eV, respectively, indicating the presence of higher-lying triplet T₂ between S₁ and T₁ (Fig. 3a). The simulated S₁ energy deviates by only 0.007 eV from the experimental value (2.985 eV), while the T₁ energy (2.912 eV) also aligns with experiment (2.945 eV). As shown in Fig. 3b, the electron density distribution analysis reveals that S₁ exhibits dominant charge-transfer (CT) character, with holes localized on the carbazole donor and electrons on the phenyl‑triazine acceptor. In contrast, both T₁ and T₂ feature mixed local excitation (LE) and CT characteristics. Notably, in T₂, the hole density extends across both donor and acceptor moieties, while electron density remains concentrated on the acceptor. This spatial overlap provides the necessary condition for efficient reverse intersystem crossing (rISC) from T₂ to S₁.
The multi-channel emission dynamics were fully reconstructed, and the four-level model was validated through experimental transient PL decay and theoretical fitting. The decay exhibits three distinct stages over nine orders of magnitude: PF (ns), TADF (µs), and RTP (ms) (Fig. 4a). Only the four-level model that includes bimolecular annihilation (S₁–S₁, S₁–T₂, T₁–T₁) accurately fits the complete decay, while the model without annihilation deviates at long times (ms), thus confirming annihilation’s critical role at high triplet concentrations (Fig. 4b). Simulations of exciton density evolution quantified the dynamics: PF arises from S₁ radiative decay (lifetime ~8 ns); TADF (10⁻⁷–10⁻⁵ s) is attributed to rISC from T₂ to S₁; and RTP (10⁻⁴–10 s) results from slow radiative decay of T₁ (lifetime ~0.75 s).
Based on the multi-excited state energy transfer characteristics of the model material, a full-visible spectrum multi-color emission system was constructed through Förster Resonance Energy Transfer (FRET). The energy transfer schematic shows (Fig. 5a) that the multi-excited state energies (S₁, T₂, T₁) of 1.8-mDTAZ-PhtCz can be efficiently transferred to conventional fluorescent acceptors. The 200 μs delayed emission spectra of blue acceptor TBPe (2 wt% doping), green acceptor TTPA (8 wt% doping), yellow acceptor SYPPV (2 wt% doping), and red acceptor DCJTB (8 wt% doping) all show clear acceptor characteristic peaks (Fig. 5b). Time-resolved microscopy demonstrates that patterned letters “D, B, O, C” in doped films exhibit color-specific delayed emission still observable 1.6 s after UV turn-off (Fig. 5c).
Through molecular structure optimization, the derivative 1.8‑pDTAZ‑PhtCz achieves efficient PF and RTP dual emission. Extending the conjugation between donor and acceptor finely tunes the excited-state energy landscape, leading to a red-shifted UV‑Vis absorption onset (400 nm), a PF peak at 430 nm (compared to 425 nm for the parent compound, Fig. 6a), and an RTP peak at 523 nm (Fig. 6b). Transient PL measurements give a PF lifetime of 5.2 ns (Fig. 6c) and an RTP lifetime of up to 118.7 ms (Fig. 6d). ns‑TA spectroscopy combined with evolving factor analysis (EFA) confirms that the derivative retains the three excited states S₁, T₁, and T₂ (Figs. 6e, f). However, the S₁–T₁ energy gap (Δ E ST ) increases to 0.3 eV, which completely suppresses TADF, leaving only PF and RTP. Performance evaluation reveals an RTP quantum yield of 33.6%, higher than that of typical pure organic RTP materials (<10%).
Simultaneous delayed fluorescence and phosphorescence in organic luminescent material employing multiple excited states