Photoinduced ICT vs. excited rotamer intercoversion in two quadrupolar polyaromatic N-methylpyridinium cations

Area of Science:

• Photochemistry
• Physical Chemistry
• Materials Science

Background:

• Investigating excited state dynamics is crucial for understanding photo-induced behavior in push-pull organic molecules.
• N-methylpyridinium cations with polyaromatic donors represent a class of compounds with potential applications in optoelectronics.

Purpose of the Study:

• To fully investigate the excited state dynamics of two quadrupolar polyaromatic N-methylpyridinium cations.
• To elucidate the distinct photo-induced behaviors and excited state fates of naphthyl and pyrenyl substituted derivatives.

Main Methods:

• Femtosecond-resolved fluorescence up-conversion and transient absorption spectroscopy.
• Density Functional Theory (DFT) quantum mechanical calculations.
• Temperature-dependent stationary measurements.

Main Results:

• Both molecules exhibit charge transfer upon absorption, confirmed by solvatochromism.
• The pyrenyl derivative shows dominant intramolecular charge transfer, while the naphthyl derivative exhibits rotamer interconversion.
• Excited state processes are primarily controlled by solvent viscosity, not polarity.

Conclusions:

• The excited state dynamics of these N-methylpyridinium cations are highly dependent on the specific polyaromatic donor groups.
• Solvent viscosity plays a critical role in controlling the timescale of excited state processes, from inertial solvation to tens of picoseconds.