Relationship between the Molecular Structure of Cyanine Dyes and the Vibrational Fine Structure of their Electronic Absorption Spectra

Abstract

Electronic absorption spectra of symmetrical cyanine dyes show vibronic sub-bands, attributed to the symmetric C-C valence vibration of the polymethine chain in the electronic excited state. Displacements in the equilibrium configuration between electronic ground and excited states of cyanine dyes lead to longer C-C bonds in the excited state. Additionally, in the electronic ground state, a small degree of bond localisation always remains in the chain depending on the different heterocyclic terminal groups. Our investigations suggest that we can use (3)J(H,H) coupling constants in the polymethine chain to characterise the bond localisation within the chain. Based on these values and the Franck-Condon principle, the intensity distribution among the vibrational sub-bands can be explained.

Laser-Limited Signatures of Quantum Coherence

“…The linear absorption spectrum of the monomer subunits (see Supplementary Fig. 1) features a pronounced vibronic progression due to strong coupling of the S 0 -S 1 electronic transition to a C-C stretching mode in the polymethine chain [28]. In the dimer compound, electronic interactions cause a delocalization of the vibronic excitations over the two monomers, leading to polaronic Frenkel excitons .…”

Aromatic/Proaromatic Donors in 2‐Dicyanomethylenethiazole Merocyanines: From Neutral to Strongly Zwitterionic Nonlinear Optical Chromophores

“…Except for 3, all the studied compounds present structured absorption bands in their spectra in CH 2 Cl 2 , their absorption curves displaying a definite shoulder or a second band on the high-frequency side, which can be attributed to a vibronic transition. This assignment is based on the energy spacing between these subbands (1100 AE 100 cm À1 ), corresponding to the totally symmetric C À C valence vibration of the polymethine chain in the electronic excited state (S 1 ), [56] and on the variation of the spectral shape in different solvents, that is, an increase in the lower energy band at the expense of the higher energy band on increasing the polarity of the solvent [57] (see Figure S3 in the Supporting Information). Moreover, the concentration-dependent UV/Vis spectra of 2 b in the lowpolar solvent dioxane ( Figure S4 in the Supporting Information) allow us to rule out the formation of H-aggregates as the origin of the higher energy band.…”

Relationship between the Molecular Structure of Merocyanine Dyes and the Vibrational Fine Structure of Their Electronic Absorption Spectra

“…First, one has to analyze the solvent influences on the structure of the absorption band. The electronic absorption spectra of cyanines and some merocyanines show prominent short-wavelength subbands, attributed to the totally symmetric C = C valence vibration of the chain in S 1 (see reference [13] and references therein). After light absorption the electron in the antibonding orbital weakens the bonds in S 1 , and, therefore, the equilibrium bond lengths in the electronic excited state R e (S 1 ) are larger in comparison to those in the ground state R e (S 0 ).…”

“…[13] To test our theoretical understanding further, it is important to establish whether the electronic structures of merocyanines can be adequately described with two limiting forms.…”