Cyclization-Promoted Ultralong Low-Temperature Phosphorescence via Boosting Intersystem Crossing

Area of Science:

• Materials Science
• Organic Chemistry
• Photophysics

Background:

• Ultralong organic phosphorescence is crucial for advanced optical materials and devices.
• Conventional methods rely on heavy atoms or carbonyl groups, complicating synthesis.
• Efficient intersystem crossing (ISC) and suppressed nonradiative decay are key for long phosphorescence lifetimes.

Purpose of the Study:

• To develop a novel strategy for achieving ultralong organic phosphorescence.
• To enhance intersystem crossing (ISC) and rigidification in organic molecules.
• To explore cyclization as a method to promote phosphorescence without heavy atoms or carbonyl groups.

Main Methods:

• Synthesized and studied N-butyl carbazole and its linearly conjugated tetramer.
• Investigated the effect of cyclization on carbazole units.
• Measured phosphorescence lifetime (τp) and efficiency at 77 K.
• Analyzed the energy gap between singlet and triplet states.

Main Results:

• N-butyl carbazole showed a short phosphorescence lifetime (1.45 ms) and low efficiency.
• Linear conjugation of four carbazole units increased the lifetime to 2.24 s.
• Cyclization dramatically decreased the singlet-triplet energy gap to 0.04 eV, enhancing ISC.
• Cyclization also increased molecular rigidity, suppressing nonradiative decay and yielding a lifetime of 3.41 s.

Conclusions:

• Cyclization is an effective strategy to promote ultralong organic phosphorescence.
• This method enhances ISC and rigidification, leading to high phosphorescence efficiency and prolonged lifetimes.
• The developed approach offers a promising route for designing heavy-atom-free ultralong phosphorescent organic materials.