On-the-fly Femtosecond Action Spectroscopy of Charged Cyanine Dyes: Electronic Structure versus Geometry.

Understanding optical properties of molecular dyes is compulsory to drive progresses in molecular photonics. This requires a fundamental comprehension of the role of electronic structure, geometry and interactions with the environment in order to guide molecular engineering strategies. In this context, we studied charged cyanine dyes molecules in the gas phase with controlled micro-environment to unravel the origin of the spectral tuning of this class of molecules. This was performed using a new approach combining femtosecond multiple photon action spectroscopy of on-the-fly mass-selected molecular ions and high level quantum calculations. While arguments based on molecular geometry are often used to design new polymethine dyes, we provide experimental evidence that electronic structure is of prior importance and hence, the decisive criterion as suggested by recent theoretical investigations.