This article provides an overview of the photophysical behavior diversity of polymethine chromophores which are
ubiquitous in biological imaging and material sciences. One major challenge in this class of chromophore is to correlate
the chemical structure to the observed optical properties, especially when symmetry-breaking phenomena occur. With
the constant concern for rationalization of their spectroscopy, we propose an extended classification of polymethine dyes
based on their ground state electronic configuration using three limit forms namely: cyanine, dipole and bis-dipole. The
chemical modifications of the dye and the influence of exogenous parameters can promote dramatic spectroscopic
changes that can be correlated to significant electronic reorganization between the three-abovementioned forms. The
deep understanding of such phenomena should allow to identify, predict and take advantage of the versatile electronic
structure of polymethines.
The development of near-infrared dyes for third order nonlinear optical applications and particularly for optical power limiting at telecommunications wavelengths triggered the revival of old dyes like polymethine dyes featuring an odd number of carbon atoms. Currently numerous research endeavor is focused on the understanding of their spectroscopy in solution and in the solid state in close relationship with their electronic structure. In this context, the dramatic effect of the counter ion on the heptamethine optical properties was highlighted depending on the dissociating character of the media. The unusual consideration of this parameter allows us to explore the so-called “cyanine limit” and to finely tune the heptamethine absorption in view of future use as dopant in a material for optical limiting applications.