Size and Shape dependent Photoluminescence Properties of Diamondoids and Cyclohexane

LOT-QuantumDesign GmbH. Im Tiefen See 58. D-64293 Darmstadt. +49 6151 8806 0. info@lot-qd.de. www.lot-qd.com/ccd Abstract We present photoluminescence spectra of a new class of carbon nanocrystals called diamondoids, and of cyclohexane. For this study, specific isomers of the five smallest members of these perfectly size and shape selectable molecular diamonds have been brought in to the gas phase and irradiated with synchrotron radiation. All the investigated compounds show intrinsic photoluminescence in the ultra violet spectral region. The spectra are dominated by vibrational fine structure. The origin of these progressions is discussed with the assistance of quantum chemical calculations. The first excited state of adamantane is a molecular orbital with lowered symmetry due to geometrical changes that occur upon excitation. The luminescence of adamantane can be attributed to a transition from this delocalized first excited state into the ground state. Our data indicate that these geometrical changes of the excited state structure is not limited to adamantane and accounts for the luminescence with loosened symmetry selection rules observed in all the other diamondoids.