Symmetry-Breaking Enhanced Herzberg-Teller Effect with Brominated Polyacenes

Area of Science:

• Quantum Chemistry
• Spectroscopy
• Molecular Physics

Background:

• Molecular symmetry dictates selection rules for electronic transitions.
• Franck-Condon (FC), Franck-Condon/Herzberg-Teller (FC/HT) interference, and Herzberg-Teller (HT) coupling are key factors in vibrationally resolved electronic transitions.
• The nuclear dependence of electronic wave functions is critical in these processes.

Purpose of the Study:

• Investigate the impact of molecular symmetry on light absorption spectra of polyacenes.
• Analyze how monobromination affects symmetry and vibronic coupling.
• Compare the accuracy of FC approximation versus HT theory for spectral line shapes.

Main Methods:

• Studied highly symmetric tetracene, pentacene, and hexacene, and their monobrominated derivatives.
• Employed theoretical calculations projecting vibrational normal modes onto irreducible representations.
• Performed numerical simulations to validate theoretical correlations.

Main Results:

• Monobromination lowers molecular symmetry, enabling greater contributions from vibrational modes to FC/HT interference and HT coupling.
• Deduced a linear relationship between FC/HT intensity and polyacene size, and a quadratic dependence for HT intensity.
• HT theory improved agreement with experimental line shapes by ~20% compared to the FC approximation.

Conclusions:

• Symmetry breaking via monobromination is essential for understanding vibronic coupling in polyacenes.
• Herzberg-Teller theory is imperative for accurately describing spectra, especially in low-symmetry systems where FC intensity can be weak.
• This work highlights the fundamental role of vibronic coupling in chemical, biological, and photophysical processes.