Synthesis of conformationally stable 1,8-diarylnaphthalenes: Development of new photoluminescent sensors for ion-selective recognition

Highly constrained 1, 8-diarylnaphthalenes exhibiting stability to isomerization have been prepared utilizing two consecutive CuO-promoted Stille cross-couplings of 1,8-dibromonaphthalene and 4-alkyl-9trimethylstannylacridines. Screening of Pd catalysts Pd(PPh3)(4), PdCl(2)dppf, or Pd-2(dba)(3)/t-Bu3P and bases such as Cy2NMe, t-BuOK, K3PO4, and Cs2CO3 in DIME or DIMF revealed superior results of Stille over Suzuki coupling with acridylboronic acids or pinacolate derivatives. The meso syn- and C-2-symmetric anti-isomers of 1,8-bis(4,4'-dimethyl-9,9'-diacridyl)naphthalene, 2, and 1,8-bis(4,4'-diisopropyl-9,9'-diacridyl)naphthalene, 3, did not show any sign of syn/anti-interconversion after heating to 180 degreesC for 24 h. Using the Eyring equation, we calculated the Gibbs standard activation energy for isomerization, DeltaGdegrees(double dagger), to be higher than 180 kJ/mol. PM3 calculations of 2 and 3 suggest a highly congested structure exhibiting two parallel acridyl moieties perpendicular to the naphthalene ring. UV and fluorescence spectroscopy studies of 2 and 3 revealed remarkable quantum yields of these blue and green light emitters. Fluorescence titration experiments with the syn-isomer of 2 showed highly efficient quenching by Cu(II) ions, whereas almost no quenching effects were observed with Cu(l) and Zn(II) salts. The striking difference in fluorescence quenching was attributed to significant photoincluced electron transfer, resulting in nonradiative relaxation of excited Cu(II)-syn-2. Stern-Volmer plots of syn-2 in the presence Of CuCl2 showed a sigmoidal quenching curve indicating cooperative recognition, whereas a linear response was observed with CuCl and ZnCl2. Fluorescence experiments in the presence of various amounts of CuCl, CuBr, and Cu(ACN)(4)BF4 proved that the quenching is cation selective and independent of the nature of counteranions.