Carbon Nanohoops: Excited Singlet and Triplet Behavior of Aza[8]CPP and 1,15-Diaza[8]CPP

The excited state properties of two nitrogen-doped cyclo-paraphenylene molecules, or carbon nanohoops, have been studied using steady-state and time-resolved absorption and emission spectroscopies. Quantum yield of fluorescence (Phi(f) = 0.11 and 0.13) and intersystem crossing (Phi(isc) = 0.45 and 0.32) were determined for aza[8]CPP and 1,15-diaza[8]CPP, respectively. We also present the proton transfer reaction between trifluoroacetic acid and the nitrogen-doped nanohoops, which resulted in significant modifications to the steady-state absorption and emission spectra as well as the triplet-triplet absorption spectra. From fluorescence quenching data we determine the equilibrium constant for the proton transfer reaction between aza[8]CPP (K-eq = 1.39 x 10(-3)) and 1,15-diaza[8]CPP (K-eq = 2.79 x 10(-3)) confirming that 1,15-diaza[8]CPP is twice as likely to be protonated at a particular concentration of trifluoroacetic acid.