Optical spectra of molecular aggregates and crystals: testing approximation schemes

Area of Science:

• Physical Chemistry
• Spectroscopy
• Condensed Matter Physics

Background:

• Exciton delocalization and molecular vibrations significantly impact molecular aggregate optical spectra.
• The exciton-vibration interplay presents a complex, non-adiabatic problem requiring approximation schemes.

Purpose of the Study:

• To assess the reliability of approximation schemes for optical spectra in molecular aggregates.
• To investigate the accuracy of the Heitler-London approximation (exciton model) for spectral band positions, shapes, and intensities.

Main Methods:

• Exploiting translational symmetry for numerically exact solutions of the Hamiltonian in large systems.
• Focusing on systems with large intermolecular distances and dipole-dipole interactions.
• Analyzing symmetric molecules with negligible permanent multipolar moments.

Main Results:

• The exciton model provides good approximations for exciton band positions and reasonable approximations for bandshapes.
• The exciton model fails to accurately predict spectral intensities, underestimating them in J-aggregates and overestimating in H-aggregates.
• An exact sum-rule validates these findings regarding intensity prediction errors.

Conclusions:

• The exciton model, despite its limitations in intensity prediction, remains valuable for understanding exciton band positions and shapes.
• Further investigation into approximation schemes for vibrational basis reduction is warranted.