The vibrational fine structure shows maxima at 535, 580, and 625 nm for both AdTc and mAdTc2 in toluene. These are redshifted to 550, 592, and 641 nm for PhTc and to 555, 597, and 643 nm for mPhTc2 as a result of extended πconjugation. Under ambient conditions, PhTc and AdTc fluoresce strongly with quantum yields in the range of 70%, regardless of solvent polarity (Table ). In contrast the fluorescence quantum yields of mPhTc2 and mAdTc2 are quenched to 6.9 and 15.6% in toluene, respectively, and to 21.3 and 19.6% in benzonitrile, respectively.