Understanding the Structural Evolution of Single Conjugated Polymer Chain Conformers

Area of Science:

• Materials Science
• Polymer Chemistry
• Spectroscopy

Background:

• Single molecule photoluminescence (PL) spectroscopy is crucial for understanding conjugated polymer photophysics.
• Previous studies focused on polarization and excitation intensity, neglecting PL spectral line shape information.
• Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) exhibits distinct emissive conformers.

Purpose of the Study:

• To investigate the information contained within PL spectral line shapes of MEH-PPV.
• To explain the vibronic progression intervals observed in MEH-PPV conformers using the "Missing Mode Effect" (MIME).
• To correlate spectral line shapes with the intra-chain order and conformational characteristics of MEH-PPV.

Main Methods:

• Analysis of single molecule PL spectra of MEH-PPV at dilute levels.
• Application of a model based on the "Missing Mode Effect" (MIME).
• Correlation of observed vibronic progression intervals with specific skeletal vibrational modes, particularly the C-H wag.

Main Results:

• MEH-PPV conformers show characteristic MIME progression intervals that do not match ground state Raman frequencies.
• The 'blue' emitting form (~18,200 cm⁻¹) has intervals of ~1200–1350 cm⁻¹, while the 'red' form (~17,100 cm⁻¹) has intervals of ~1350–1450 cm⁻¹.
• The Raman-active C-H wag (~966 cm⁻¹) significantly influences the MIME interval, with lower intra-chain order leading to smaller intervals.

Conclusions:

• PL spectral line shapes, explained by MIME, provide insights into the conformational heterogeneity of conjugated polymers.
• Intra-chain order (planarity) is a key factor differentiating emissive conformers of MEH-PPV.
• This approach offers a simpler method to study polymer conformation without specialized low-temperature or single-molecule Raman setups.