Vibronic Coupling Drives the Ultrafast Internal Conversion in a Functionalized Free-Base Porphyrin.

Area of Science:

• Photochemistry
• Molecular Spectroscopy
• Quantum Chemistry

Background:

• Internal conversion (IC) is a crucial radiationless decay pathway in polyatomic molecules.
• Molecular vibrations are theoretically predicted to play a significant role in facilitating IC between excited electronic states.
• Ultrafast experimental techniques are essential for probing the structure-function relationships governing these dynamics.

Purpose of the Study:

• To elucidate the specific vibrational modes that drive the internal conversion process in the Q band of a functionalized porphyrin.
• To understand the interplay between molecular vibrations and electronic states in ultrafast energy transfer.
• To explore how porphyrin functionalization impacts the efficiency of internal conversion.

Main Methods:

• Ultrafast multidimensional spectroscopies were employed to observe the dynamics.
• Theoretical modeling was used to complement experimental observations.
• Coherent wave packet analysis was utilized to identify key vibrational modes.

Main Results:

• A 60 fs Q-Q internal conversion process was observed.
• The IC is driven by the synergistic action of multiple high-frequency vibrational modes.
• A 1510 cm⁻¹ mode was identified as the primary tuning mode, optimizing the geometry for energy surface crossing.
• Short-lived vibrations (1200-1400 cm⁻¹) were found to promote IC within approximately 60 fs.
• A 1350 cm⁻¹ coupling mode was identified as responsible for vibronic mixing within the Q states.

Conclusions:

• Porphyrin-core functionalization can effectively modulate internal conversion rates.
• The identified vibrational modes provide critical insights into the mechanism of IC in porphyrins.
• These findings open new possibilities for designing functionalized porphyrins for applications in photocatalysis and optoelectronics.