Relation of molecular structure to FranckCondon bands in the visiblelight absorption spectra of symmetric cationic cyanine dyesMarch 1, 2015
In this work, a Franck Condon (FC) model is used to study the solutionphase absorbance spectra of a series of seven symmetric cyanine dyes having between 22 and 77 atoms. Electronic transition energies were obtained from routine visiblelight 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 midinfrared spectra. The model predicts the relative energies of the two major vibronic bands to within 5% and 11%, respectively, and also reproduces structurespecific 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 cm1, and the other between 850 and 1480 cm1. Vibronic transitions excite symmetric inplane bending of the polymethine chain, inplane bends of the polymethine and aromatic CH bonds, torsions and deformations of Nalkyl substituents, and in the case of the indocyanines, inplane deformations of the indole rings. For two dyes, the model predicts vibronic coupling into symmetrybreaking torsions associated with transcis photoisomerization.
Coauthor and Biola graduate Katrina Lin narrates a brief overview of this work:
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