Abstract
The arylation of readily available benzo[b]quinolizinium-9-boronic acid with carbocyclic or heterocyclic bromoarenes in the presence of a palladium catalyst [Pd(dppf)Cl2˙CH2Cl2 or Pd(PPh3)2Cl2] gives the corresponding cationic biaryl products in yields of 15-81%. All aryl-substituted benzo[b]quinolizinium (acridizinium) derivatives exhibit a long-wavelength absorption maximum between 409 and 422 nm in water, which change marginally in different solvents (Δλ <10 nm). As the only exception, the 9-(N,N-dimethylaminophenyl)benzo[b]quinolizinium exhibits pronounced solvatochromic behavior (H2O: λabs = 422 nm; MeCN: λabs = 474 nm; CH2Cl2: λabs = 507 nm) due to the strong donor-acceptor interplay in the cationic chromophore. Depending on the donor strength of the aryl substituent, the benzo[b]quinolizinium derivatives exhibit fluorescence bands in water with long-wavelength maxima between λfl = 452 nm (R = phenyl) and 529 nm (R = 4-methoxyphenyl). The non-fluorescent 9-(N,N-dimethylaminophenyl)benzo[b]quinolizinium may be employed as a pH-sensitive light-up probe (pH 2.8-4.8), since its emission intensity increases by a factor of 130 upon protonation.
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