Beschreibung:
<jats:title>Abstract</jats:title><jats:p><jats:bold>Vibronic sub‐bands</jats:bold> in the electronic absorption spectra of symmetrical cyanine dyes (see picture) are attributed to the symmetric CC valence vibration of the polymethine chain in the excited state. The <jats:sup>3</jats:sup><jats:italic>J</jats:italic>(H,H) coupling constants in the polymethine chain can be used to characterize the bond localization within the chain in the ground state and thus to explain the intensity distribution of the sub‐bands.<jats:boxed-text content-type="graphic" position="anchor"><jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" mimetype="image/gif" position="anchor" specific-use="enlarged-web-image" xlink:href="graphic/mcontent.gif"><jats:alt-text>magnified image</jats:alt-text></jats:graphic></jats:boxed-text></jats:p><jats:p><jats:italic>Electronic absorption spectra of symmetrical cyanine dyes show vibronic sub‐bands, attributed to the symmetric CC 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 CC 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</jats:italic> <jats:sup><jats:italic>3</jats:italic></jats:sup>J<jats:italic>(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.</jats:italic></jats:p>