Triarylborane-Based Helical Donor-Acceptor Compounds: Synthesis, Photophysical, and Electronic Properties

Area of Science:

• Organic Chemistry
• Materials Science
• Photophysics

Background:

• Helicene derivatives are explored for their unique chiroptical and electronic properties.
• Tuning photophysical properties of organic molecules is crucial for developing advanced materials.
• Donor-acceptor substituted polycyclic aromatic hydrocarbons offer tunable optoelectronic characteristics.

Purpose of the Study:

• To synthesize and characterize novel donor-acceptor substituted helicene derivatives.
• To investigate the electrochemical and photophysical properties of these helicenes experimentally and theoretically.
• To compare the photophysical properties of substituted helicenes with their unsubstituted counterparts.

Main Methods:

• Synthesis of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthren-4-amine (1) and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthren-8-amine (2).
• Experimental studies including electrochemical and photophysical measurements (fluorescence quantum yields).
• Theoretical calculations to understand electronic transitions (HOMO→LUMO) and excited-state dynamics.

Main Results:

• Synthesized and characterized two novel helicene derivatives, 1 and 2.
• Observed moderate fluorescence quantum yields in THF (0.48 for 1, 0.61 for 2), exceeding unsubstituted helicenes.
• Reported significantly higher fluorescence quantum yields in the solid state (1.00 for 1, 0.55 for 2) attributed to restricted molecular motion.
• Identified S1 ←S0 transitions as predominantly HOMO→LUMO.
• Observed a decrease in the interplanar angle of the [5]helicene core in 2 upon UV excitation.

Conclusions:

• Donor and acceptor substitution enhances fluorescence quantum yields in helicene derivatives.
• Solid-state emission is significantly improved compared to solution-phase emission due to suppressed molecular motion.
• The observed structural changes in the excited state of [5]helicene 2 provide insights into excited-state dynamics and potential applications in responsive materials.