Thioindigo dyes: highly accurate visible spectra with TD-DFT

Area of Science:

• Computational Chemistry
• Spectroscopy
• Organic Dyes

Background:

• Thioindigo dyes are important organic chromophores.
• Understanding their structure-property relationships is crucial for dye design.

Purpose of the Study:

• To computationally evaluate the structure and visible spectra of thioindigo dyes and derivatives.
• To investigate the influence of solvent, isomerization, and substitution on dye color.

Main Methods:

• Time-Dependent Density Functional Theory (TD-PBE0) with a PBE0/6-311G(d,p)//PBE0/6-311G(d,p) approach.
• Polarizable Continuum Model (PCM) for bulk solvent effects.
• Analysis of 170 thioindigo dye cases, including derivatives and related molecules.

Main Results:

• Achieved a mean unsigned error of 6.9 nm (0.03 eV) in predicting maximum absorption wavelengths (λmax).
• Errors are an order of magnitude smaller than previous studies on indigoids.
• Established a linear correlation between central double bond length and λmax.
• Identified that cis conformers have higher excitation energies due to a less stabilized Lowest Unoccupied Molecular Orbital (LUMO).

Conclusions:

• The computational approach provides highly accurate predictions of thioindigo dye colors.
• Solvent effects, trans-cis isomerization, and chemical substitutions significantly impact dye spectra.
• Central double bond length is a key predictor of thioindigo color.