title: Relation of molecular structure to Franck-Condon bands in the visible-light absorption spectra of symmetric cationic cyanine dyes.
abstract: 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 625cm(-1), and the other between 850 and 1480cm(-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.
authors.affiliation: Department of Chemistry, Physics, and Engineering, Biola University, 13800 Biola Avenue, La Mirada, CA 90639, USA. Electronic address: katrina.lin@biola.edu.
authors.forename: Katrina Tao Hua
authors.initials: KT
authors.lastname: Lin
authors.affiliation: Department of Chemistry, Physics, and Engineering, Biola University, 13800 Biola Avenue, La Mirada, CA 90639, USA. Electronic address: john.silzel@biola.edu.
authors.forename: John W
authors.initials: JW
authors.lastname: Silzel
mesh_terms: D002232:Carbocyanines / Q000737:chemistry*; D002412:Cations / Q000737:chemistry*; D004396:Coloring Agents / Q000737:chemistry*; D004583:Electrons; D008027:Light; D008958:Models, Molecular; D011789:Quantum Theory; D013053:Spectrophotometry
chemical_list: D002232:Carbocyanines; D002412:Cations; D004396:Coloring Agents
keywords: Absorbance; Cyanine; FCClasses; Franck–Condon; TDDFT