Size and shape dependent photoluminescence and excited state decay rates of diamondoids

Abstract

We present photoluminescence spectra and excited state decay rates of a series of diamondoids, which represent molecular structural analogues to hydrogen-passivated bulk diamond. Specific isomers of the five smallest diamondoids (adamantane-pentamantane) have been brought into the gas phase and irradiated with synchrotron radiation. All investigated compounds show intrinsic photoluminescence in the ultraviolet spectral region. The emission spectra exhibit pronounced vibrational fine structure which is analyzed using quantum chemical calculations. We show that the geometrical relaxation of the first excited state of adamantane, exhibiting Rydberg character, leads to the loss of T d symmetry. The luminescence of adamantane is attributed to a transition from the delocalized first excited state into different vibrational modes of the electronic ground state. Similar geometrical changes of the excited state structure have also been identified in the other investigated diamondoids. The excited state decay rates show a clear dependence on the size of the diamondoid, but are independent of the particle geometry, further indicating a loss of particle symmetry upon electronic excitation.

Laser-induced fluorescence of free diamondoid molecules

“…25,29,45 In a previous study, we have already shown that based on the calculation of vibrationally resolved FranckCondon spectra in the frame of the TDDFT method, which includes the structural displacements upon changing the electronic state, the experimental emission spectrum of adamantane can be accurately reproduced. 28 Here, we have further exploited this approach and have obtained a very good agreement of the calculated vibrationally resolved spectra with their measured high-resolution counterparts for a large class of higher diamondoids. A thorough Franck-Condon analysis, including Herzberg-Teller coupling if necessary, has provided a detailed understanding of the form and composition of the spectral features.…”

“…The frequencies closely match the results of the Raman measurements reported by Filik et al 17 Previous calculations for adamantane have shown that the tetrahedral symmetry exhibited in the electronic ground state is reduced in the electronically excited state. 28 Similarly, for triamantane, a change in the molecular geometry after excitation is observed, and the calculated excited state structure shows no symmetry. 15,24 The absorption spectrum thus shows a smooth onset with the optical gap value of 6.10 eV.…”

“…Since the highest attainable photon energy of the used laser system is E6.05 eV, the examined samples are restricted to diamondoids with excitation energies in this range and lower (triamantane and its higher homologues). Since the laser provides much higher fluence than the synchrotron radiation used in previous work, 28 the luminescence spectra could be recorded with much higher resolution and lower excitation energies. This allows a detailed comparison with theory and the accurate vibrational analysis of the spectra.…”

“…The fluorescence spectra of crystalline diamondoids are found to be structureless with maxima around 295 nm, independent of their size and shape. 27 In a recent study, we presented size-and shape-dependent photoluminescence and excited state lifetimes for several diamondoids in the gas phase recorded using synchrotron radiation, 28 enabling us to resolve vibrational structure for several diamondoid molecules. However, due to the comparatively low fluence of the synchrotron light, excitation at the optical gaps was not always possible and prevented a resolution of the fine structure in some cases.…”

Shape-Engineering of Self-Assembled Organic Single Microcrystal as Optical Microresonator for laser Applications

“…The shape and the size of these micro/nanostructures precisely lie at the heart of the physical and chemical properties of the functional micro/nanomaterials10111213141516. In recent decades, tremendous efforts have been made towards the development of synthetic procedures for inorganic compounds, such as Ag, Pd, and ZnO171819.…”