Relation of molecular structure to Franck–Condon bands in the visible-light absorption spectra of symmetric cationic cyanine dyes

• A Franck–Condon computation models the solution spectra of seven cyanine dyes.
• Vibrational modes with the greatest Franck–Condon involvement are identified.
• Two subsets of the modes determine vibronic band position and absorptivity.
• Vibronic effects may be assigned to specific structural features in the dyes.

A Franck–Condon (FC) model is used to study the solution-phase absorbance spectra of a series of seven symmetric cyanine dyes having between 22 and 77 atoms. Electronic transition energies were obtained from routine visible-light absorbance and fluorescence emission spectra. Harmonic normal modes were computed using density functional theory (DFT) and a polarizable continuum solvent model (PCM), with frequencies corrected using measured mid-infrared spectra. The model predicts the relative energies of the two major vibronic bands to within 5% and 11%, respectively, and also reproduces structure-specific differences in vibronic band shapes. The bands themselves result from excitation of two distinct subsets of normal modes, one with frequencies between 150 and 625 cm−1, and the other between 850 and 1480 cm−1. Vibronic transitions excite symmetric in-plane bending of the polymethine chain, in-plane bends of the polymethine and aromatic C–H bonds, torsions and deformations of N-alkyl substituents, and in the case of the indocyanines, in-plane deformations of the indole rings. For two dyes, the model predicts vibronic coupling into symmetry-breaking torsions associated with trans–cis photoisomerization.

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