NonlinearEmission of Quinolizinium-Based Dyes withApplication in Fluorescence Lifetime Imaging.

Chargedmolecules based on the quinolizinum cation have potentialapplications as labels in fluorescence imaging in biological mediaunder nonlinear excitation. A systematic study of the linear and nonlinearphotophysics of derivatives of the quinolizinum cation substitutedby either dimethylaniline or methoxyphenyl electron donors is performed.The effects of donor strength, conjugation length, and symmetry inthe two-photon emission efficiency are analyzed in detail. The bestperforming nonlinear fluorophore, with two-photon absorption crosssections of 1140 GM and an emission quantum yield of 0.22, is characterizedby a symmetric D-π-A+-π-D architecture basedon the methoxyphenyl substituent. Application of this molecule asa fluorescent marker in optical microscopy of living cells revealedthat, under favorable conditions, the fluorophore can be localizedin the cytoplasmatic compartment of the cell, staining vesicular shapeorganelles. At higher dye concentrations and longer staining times,the fluorophore can also penetrate into the nucleus. The nonlinearlyexcited fluorescence lifetime imaging shows that the fluorophore lifetimeis sensitive to its location in the different cell compartments. Usingfluorescence lifetime microscopy, a multicolor map of the cell isdrafted with a single dye. [ABSTRACT FROM AUTHOR]