The temperature dependence of vibronic lineshapes: linear electron-phonon coupling

Area of Science:

• Physical Chemistry
• Theoretical Chemistry
• Spectroscopy

Background:

• Vibronic transitions in dye molecules are crucial for understanding their optical properties.
• Electron-phonon coupling significantly influences these transitions, especially at different temperatures.
• Existing models like the Huang-Rhys approximation have limitations.

Purpose of the Study:

• To develop a comprehensive theory for calculating vibronic transition lineshapes in dye molecules.
• To incorporate the effects of linear electron-phonon coupling, including Dushinsky effects and temperature dependence.
• To provide a theoretical framework that goes beyond current approximations.

Main Methods:

• A second-order cumulant expansion was used to derive lineshape expressions.
• The theory was compared with generalized Redfield theory and the spin-boson model.
• Analysis included both pure electron-phonon coupling and bilinear coupling.
• Vibronic density of states and spectral densities were investigated.

Main Results:

• The derived expressions accurately predict absorption and emission lineshapes.
• The theory successfully incorporates Dushinsky effects and temperature dependence.
• Bilinear coupling leads to spectra with zero-phonon lines and phonon-side bands.
• Low-frequency spectral density behavior dictates dominant spectral features.

Conclusions:

• The developed theory offers a more accurate and versatile approach to studying vibronic transitions.
• It provides insights into the temperature dependence and complex coupling mechanisms in dye molecules.
• The findings are relevant for designing and understanding light-harvesting and light-emitting materials.