Structural, photophysical and lasing properties of pyrromethene dyes

The molecular structure and the photophysical properties of pyrromethene-BF2 (PM) dyes are studied with the aim of finding the best structural and environmental conditions which optimize the laser performance of these dyes. To this end, UV-Vis absorption and fluorescence spectra and fluorescence decay curves of several PM dyes are registered in a multitude of solvents with different physicochemical properties and in polymeric solid matrices. Quantum mechanical calculations at different levels are also applied in order to explain the photophysical behaviour of these dyes. The studied pyrromethenes incorporate alkyl (methyl, ethyl and tert-butyl), sulfonate, cyano, and acetoxy- and methacryloyloxy-polymethylene groups in different positions of the chromophore. In the last case, the presence of the polymerizable acryloyl group facilitates the covalent linkage of the chromophore to a polymeric chain, of special technological interest in the development of tunable dye lasers in the solid state. From the experimental results and the theoretical calculations, we discuss different mechanisms of internal conversion for PM dyes, such as the loss in the planarity of the chromophore, the electron flow through the delocalized pi-system, the vibrational coupling and the formation of an intramolecular charge transfer state. The present work demonstrates the good correlation between the photophysical and the lasing properties of PM dyes with different structural (substituents) and environmental (solvents and polymers) conditions.