1. Fundamental Luminescent Properties of 1.8-mDTAZ-PhtCz

• Absorption & Emission:
  • Absorption peaks at 330-345 nm.
  • Emission peak at 425 nm in degassed toluene.
• Temperature-Dependent Photoluminescence:
  • Coexistence of prompt fluorescence (PF), thermally activated delayed fluorescence (TADF), and room temperature phosphorescence (RTP).
  • TADF intensity decreases from room temperature (292 K) down to 252 K, while phosphorescence (phosphorescent emission) dominates at temperatures below 232 K. Phosphorescence remains at 77 K, suggesting long-lived triplet emission at low temperature.
• Crystal Structure:
  • Shows intermolecular hydrogen bonding and π-π stacking contributing to molecular rigidity and suppression of non-radiative decay.
  • Donor-acceptor twist helps restrict molecular motion, enhancing emission efficiency.
• Afterglow:
  • Ultralong afterglow (phosphorescence) visible up to 42 seconds after turning off UV excitation.

2. Identification of the Second Triplet State (T₂) and Excited-State Dynamics

• Nanosecond Transient Absorption (ns-TA):
  • Reveals three principal excited states with distinct lifetimes: S₁ (singlet), T₂ (second triplet), and T₁ (lowest triplet).
  • Lifetimes: S₁ ≈ 15.2 ns, T₂ ≈ 2.1 μs, T₁ ≈ 8.2 μs.
  • T₂ and T₁ have similar spectral energies but notably different decay times, indicating T₂ lies energetically between S₁ and T₁.
• Spectral and kinetic analysis support a three-state model describing the decay dynamics during photoluminescence.

3. Theoretical Simulation of Excited States

• ROKS Method with LC-ωPBE08 Functional:
  • Energy order: S₁ (2.978 eV), T₂ (2.953 eV), T₁ (2.912 eV).
  • Good agreement between calculated and experimental energies.
• Electron Density Distribution:
  • S₁ state exhibits strong charge transfer (CT) with holes localized on the donor (carbazole) and electrons on the acceptor (phenyl-triazine).
  • T₁ and T₂ states show mixed local excitation (LE) and CT characteristics.
  • T₂ state’s spatial overlap in hole and electron density facilitates efficient reverse intersystem crossing (rISC) from T₂ back to S₁, critical for TADF.

4. Multi-Channel Emission Dynamics Model

• Four-Level Model Incorporating Bimolecular Annihilation:
  • Emission decay stages span nanoseconds (PF), microseconds (TADF), to milliseconds (RTP).
  • Models including exciton-exciton annihilation (S₁–S₁, S₁–T₂, T₁–T₁) fit the data best, especially for long-time decay tails.
• Excitonic Processes:
  • PF: Radiative decay of S₁ (~8 ns lifetime).
  • TADF: rISC from T₂ to S₁ (~10⁻⁷–10⁻⁵ s timescale).
  • RTP: Radiative decay of T₁ (~0.75 s lifetime), leading to extended phosphorescence.

5. Application: Multi-Color Emission via Förster Resonance Energy Transfer (FRET)

• Energy Transfer to Fluorescent Acceptors:
  • The multi-state excited system transfers energy efficiently from S₁, T₂, and T₁ to doped acceptors: blue (TBPe), green (TTPA), yellow (SYPPV), and red (DCJTB).
  • Resulting acceptor emission delayed for up to 1.6 s after UV excitation is turned off.
• Patterned films demonstrate ultralong persistent multi-color emission, useful for advanced display and anti-counterfeiting applications.

6. Derivative 1.8-pDTAZ-PhtCz: Dual PF and RTP Emission

• Structural Modification:
  • Increased conjugation between donor and acceptor causes a redshift in absorption/emission.
• Spectral Shifts:
  • Absorption onset at 400 nm, fluorescence peak at 430 nm, RTP peak at 523 nm.
• Transient Lifetimes:
  • PF lifetime ~5.2 ns, RTP lifetime extended to 118.7 ms.
• ns-TA Spectroscopy:
  • Still shows S₁, T₂, and T₁ but increased singlet-triplet gap (ΔEST) of 0.3 eV suppresses TADF.
• High RTP Quantum Yield:
  • Achieves 33.6%, significantly higher than typical organic RTP materials.

Summary

The study achieves a detailed understanding of the photophysical processes in a new donor-acceptor organic emitter:

• Identifies a second triplet state (T₂) that plays a vital role in enabling efficient TADF through rISC.
• Demonstrates complex interplay between PF, TADF, and RTP emissions controlled by temperature and molecular design.
• Employs advanced spectroscopic and computational tools to fully elucidate excited state dynamics.
• Leverages multi-excited state energy transfer to produce a full visible-spectrum, multi-color delayed emission system.
• Tailors molecular structure (derivative 1.8-pDTAZ-PhtCz) to optimize dual PF and RTP emission with high efficiency and long lifetimes conducive to practical applications.

If you need insights on a specific graph panel or want explanations of mechanisms, energy transfer, or kinetic modeling details, feel free to ask!