UV resonance Raman analysis of trishomocubane and diamondoid dimers

Abstract

We present resonance Raman measurements of crystalline trishomocubane and diamantane dimers containing a C=C double bond. Raman spectra were recorded with excitation energies between 2.33 eV and 5.42 eV. The strongest enhancement is observed for the C=C stretch vibration and a bending mode involving the two carbon atoms of the C=C bond, corresponding to the B2g wagging mode of ethylene. This is associated with the localization of the π-HOMO and LUMO and the elongation of the C=C bond length and a pyramidalization of the two sp(2)-hybridized carbon atoms at the optical excitation. The observed Raman resonance energies of the trishomocubane and diamantane dimers are significantly lower than the HOMO-LUMO gaps of the corresponding unmodified diamondoids.

Vibrationally resolved optical spectra of modified diamondoids obtained from time-dependent correlation function methods

“…The most striking features are recurrences after about 22 fs in both cases, which translate into a vibrational level spacing of about 1520 cm À1 (or about 0.19 eV) -in good correspondence to the softened CQC vibration, also known from experiment. 11 The slightly more pronounced recurrence features in AdaQDia compared to AdaQAda, explain the slightly more pronounced vibrational features in the spectra of the former.…”

“…Specifically, we consider AdaQAda, AdaQDia, DiaQDia and AdaQDiaQAda, which are experimentally known. 11 DiaQDia exists as two geometric isomers, E and Z, depending on the geometry around the CQC bond connecting the two diamantane units and has been studied with analogous theoretical models in ref. 18.…”

“…The strong reduction of the optical gap in CQC-connected dimers by the here observed order of magnitude is very consistent with experimental findings. 11 To illustrate effects of the vibronic finestructure on spectra in detail, we show in Fig. 2 absorption, emission and resonance Raman spectra for AdaQAda and AdaQDia, obtained by using (TD-)B3LYP/TZVP/IMDHOFAD and the S 0 and S 1 states.…”

“…10 Vibrations involving excited states are also important for the interpretation of resonance Raman spectra, another important tool to characterize diamondoids. 11 In general, vibronic spectra are powerful tools to unravel details of optical properties of diamondoids and the focus of this paper.…”

“…Several of these (and related) molecules have been synthesized by Schreiner and co-workers. 11,13 For example, DiaQDia which exists as E and Z stereoisomers has been the subject of recent physico-chemical 11 and theoretical 18 characterization. In DiaQDia, a greatly reduced HOMO-LUMO gap is found compared to pristine diamondoids, as well as a strongly enhanced CQC vibration in resonance Raman.…”