Polymorph-Dependent Photophysics of Blue-Emitting Brominated Organic Crystals

Area of Science:

• Organic Chemistry
• Materials Science
• Photophysics

Background:

• Nonmetal organic molecules typically exhibit inefficient blue phosphorescence.
• Challenges include weak intersystem crossing and nonradiative decay of triplet states.
• Halogenation and molecular packing are crucial for efficient phosphorescence.

Purpose of the Study:

• To investigate efficient room-temperature phosphorescence (RTP) in organic crystals.
• To explore the role of bromine atoms and molecular packing in blue phosphorescence.
• To analyze the distinct photophysical properties of polymorphic forms of Br8.

Main Methods:

• Synthesis and characterization of 1,4-bis-(bromomethyl)-2,5-bis-(octyloxy)-benzene (Br8).
• Analysis of two polymorphs (Br8-H and Br8-J) in crystal and thin-film forms.
• Photophysical measurements including quantum yield, emission spectra, and lifetimes.

Main Results:

• Br8-J polymorph (head-to-tail packing) shows higher quantum yields (up to 38%) than Br8-H (face-to-face packing, up to 2%).
• Both polymorphs exhibit blue emission (420-470 nm) and RTP with lifetimes of ~325 μs.
• Br8-J demonstrates efficient solid-state phosphorescence, attributed to optimal intermolecular halide bonding and heavy atom effect.

Conclusions:

• Controlled molecular packing, specifically head-to-tail arrangement, is key to achieving efficient blue RTP in organic crystals.
• Bromine incorporation and specific crystal packing significantly enhance phosphorescence efficiency.
• Findings highlight the potential of halogenated organic molecules for solid-state light-emitting applications.