Choosing a Functional for Computing Absorption and Fluorescence Band Shapes with TD-DFT

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Spectroscopy

Background:

• Accurate simulation of molecular spectra is crucial for understanding electronic transitions.
• Conjugated molecules exhibit complex absorption and emission properties influenced by molecular structure and environment.
• Time-Dependent Density Functional Theory (TD-DFT) is a widely used method for predicting excited-state properties.

Purpose of the Study:

• To evaluate the performance of various hybrid functionals within TD-DFT for simulating absorption and emission band shapes of conjugated molecules.
• To assess the impact of diffuse atomic orbitals and bulk solvent effects on spectral simulations.
• To compare theoretical predictions with experimental data for a set of 20 representative conjugated compounds.

Main Methods:

• Simulations performed using Time-Dependent Density Functional Theory (TD-DFT) with diffuse atomic orbitals.
• Inclusion of bulk solvent effects in the theoretical model.
• Assessment of six hybrid functionals: B3LYP, PBE0, M06, M06-2X, CAM-B3LYP, and LC-PBE.
• Evaluation of basis set and integration grid effects.

Main Results:

• Most functionals, except LC-PBE, successfully reproduced experimental absorption and fluorescence band shapes.
• Average errors for emission spectra were larger than for absorption spectra.
• B3LYP functional exhibited the smallest mean absolute deviation.
• M06-2X offered a good balance, accurately predicting 0-0 energies and band shapes.
• Mean absolute deviations were approximately 100 cm⁻¹ for absorption and 250 cm⁻¹ for emission peak positions.
• Relative intensities were reproduced with approximately 10-15% accuracy.

Conclusions:

• TD-DFT, particularly with functionals like B3LYP and M06-2X, provides reliable predictions for absorption and emission spectra of conjugated systems.
• The choice of functional and computational parameters impacts the accuracy of spectral simulations.
• Further refinement of theoretical models can improve the prediction of excited-state properties for complex molecules.