π-Extended Four-Coordinate Organoboron N,C-Chelates as Two-Photon Absorbing Chromophores.

Four-coordinate N,C-chelate organoboron dyes with alkynyl spacers were synthesized by Heck alkynylation. These dyes are π-extended analogues of the recently reported class of four-coordinate borylated arylisoquinolines (BAI). Depending on the electron-donor substitution, they feature an intramolecular charge-transfer (ICT) character in the excited state. This translates into pronounced apparent Stokes shifts (up to 8500 cm-1) and a solvatofluorochromic behavior. In general, the observed emission quantum yields are high in nonpolar media (ΦF ca. 0.5-0.6). For the dye with the most pronounced ICT rather high emission quantum yields (ΦF ca. 0.4) are observed for emissions with maxima longer than 600 nm in solvents of moderate polarity. The π-extended dyes show interesting two-photon absorption (TPA) properties, maintaining high cross sections (up to 60 GM) in the near-infrared wavelength window (>900 nm). One of the dyes was designed as dimeric chromophore, integrating the acceptor-π-acceptor (A-π-A) format. This alternative design showed no ICT behavior but led to the observation of high two-photon-absorption (TPA) cross sections (ca. 220 GM at 700 nm). All investigated dyes show pronounced photostability, providing added value to this structural and photofunctional extension of the BAI dye platform.