UV resonance Raman analysis of trishomocubane and diamondoid dimers Reinhard Meinke, Robert Richter, Andrea Merli, Andrey A. Fokin, Tetyana V. Koso, Vladimir N. Rodionov, Peter R. Schreiner, Christian Thomsen, and Janina Maultzsch Citation: The Journal of Chemical Physics 140, 034309 (2014); doi: 10.1063/1.4861758 View online: http://dx.doi.org/10.1063/1.4861758 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Second-order many-body perturbation study of ice Ih J. Chem. Phys. 137, 204505 (2012); 10.1063/1.4767898 Pressure-induced metallization and resonant Raman scattering in Zn 1 x Mn x Te J. Appl. Phys. 104, 013503 (2008); 10.1063/1.2949707 Solvent effects on resonant first hyperpolarizabilities and Raman and hyper-Raman spectra of DANS and a water-soluble analog J. Chem. Phys. 125, 054506 (2006); 10.1063/1.2227028 Resonance Raman analysis of nonlinear solvent dynamics: Betaine-30 in ethanol J. Chem. Phys. 121, 11195 (2004); 10.1063/1.1809591 Solvation state selective excitation in resonance Raman spectroscopy. II. Theoretical calculation J. Chem. Phys. 109, 9084 (1998); 10.1063/1.477464
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THE JOURNAL OF CHEMICAL PHYSICS 140, 034309 (2014)
UV resonance Raman analysis of trishomocubane and diamondoid dimers Reinhard Meinke,1,a) Robert Richter,2 Andrea Merli,2 Andrey A. Fokin,3,4 Tetyana V. Koso,3 Vladimir N. Rodionov,4 Peter R. Schreiner,3 Christian Thomsen,1 and Janina Maultzsch1 1
Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany 3 Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany 4 Department of Organic Chemistry, Kiev Polytechnic Institute, pr. Pobedy 37, 03056 Kiev, Ukraine 2
(Received 22 October 2013; accepted 27 December 2013; published online 16 January 2014) 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 sp2 -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. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4861758] I. INTRODUCTION
Diamondoids are fascinating nanoparticles that have many properties in common with diamond, such as mechanical hardness, field emitting properties, or optical transparency.1–4 They are well-defined, size-selected cagelike hydrocarbons, stable at ambient conditions, and they form van-der-Waals crystal structures.5 Diamandoid derivatives and related cage-like hydrocarbons like trishomocubanes are used in medical applications,6, 7 but they are also foreseen as building blocks in nanoscale field emission, electronic or optoelectronic devices.4, 8 The optical gaps in small diamondoids, depending on size and shape, are typically above 6 eV.9, 10 For some applications, modifications of the optical gap or the electronic structure in general are desired. Such modifications can be achieved, among others, by covalent functionalization, substitution, or dimer formation.2, 10–13 Unsaturated moieties of coupled diamondoids can be considered as models for sp2 -defects of diamond surfaces and macroscopic diamond. In particular, diamondoid dimers with sp2 -hybridized carbon forming double bonds are expected to have significantly lower HOMO-LUMO gaps (highest occupied molecular orbital–lowest unoccupied molecular orbital) than the individual constituents, as the HOMO and LUMO are likely to be localized at the C=C bonds. In this work we analyze the optical gaps of trishomocubane and diamantane dimers by resonance Raman spectroscopy. We observe resonances around 5.1 eV, which are lower in energy than the optical gap of diamantane (6.40 eV).9, 14 Furthermore, the stretch vibration of the C=C double bond is selectively enhanced when excited in resonance in all investigated samples. This hints to an elongation of the C=C bond in the excited state, which in turn is consistent with the expected localization of the HOMO and LUMO a) Electronic mail: [email protected]
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at the double bond. Finally, resonance Raman experiments of diamantane dimers in the gas phase show higher resonance energy than in the crystalline state.
II. EXPERIMENTAL AND THEORETICAL DETAILS
We investigated samples in solid state (small crystals or powder) by resonance Raman spectroscopy with excitation energies from the visible to the deep UV range. Raman spectra were recorded using a triple monochromator system (Horiba Jobin Yvon T64000). Different laser sources were used: an Ar-ion laser (Coherent Innova 90C FreD) in fundamental and second harmonic generation (SHG), a HeCd laser, and a tunable Ti:Sa laser system with SHG and third harmonic generation pumped by a pulsed frequency doubled Nd:YAG laser. The Raman spectrum at 2.33 eV (532 nm) was recorded with a frequency doubled Nd:YAG laser in continuous wave operation. All measurements were done in backscattering geometry, using a 100× objective for the visible and a 20× objective for the UV, at room temperature. The intensities are normalized to the Raman signal of CaF2 . For analyzing the Raman resonance profiles, the peak area is taken as intensity. The spectral resolution is 3–5 cm−1 in the UV and 1–2 cm−1 in the visible range. We observed that the samples in solid state were damaged by deep UV illumination. Because the damage occurred even at laser power below 0.1 mW, the integration time was kept short (1–10 s) and the laser was scanned over the sample surface during the measurement with scanning speed of around 10 μm per second. This results in an intensity error increase to about 50% in the UV range. Yet, the intensity increase due to resonance excitation in the UV is above this error. Additionally, we observed a clear polarization dependence of the Raman signal for some of the crystals. By scanning over an
140, 034309-1
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ensemble of different orientations, we obtain an average value for the intensity. Resonance Raman measurements in the gas phase were recorded in 90◦ scattering geometry using a Shamrock SR 303i-B spectrometer combined with an Andor iDus DU420ABU2o CCD camera. The light source is an optical parametric oscillator (Continuum Panther EX OPO) pumped by a nanosecond Nd:YAG laser (Continuum Powerlite DLS) and focused by a lens (f = 116 mm). The pulse energy was 0.16 mJ at 10 Hz repetition rate. Measurements were performed in the range from 4.94 eV (251 nm) to 6.03 eV (206 nm) at a sample temperature of 160 ◦ C. The investigated materials are a trishomocubane dimer and two diamantane dimers. The trishomocubane dimer [Fig. 2(b)], Ci -trans-Cs -8-trishomocubylideneCs -8-trishomocubane,15 can also be seen as a dimer.16 The pentacyclo[5.4.0.02, 6 .03, 10 .05, 9 ]undecane diamantane dimers [Fig. 4(b)] are the isomers syn- and anti-3-diamantylidene-3-diamantane. The dimers are formed by a C=C double bond. For further details, the reader is referred to Refs. 5 and 13. We performed density functional theory (DFT) computations of the vibrational modes and the HOMO and LUMO, using the M06-2X17 functional and the double-ζ basis set cc-pVDZ18 as implemented in Gaussian 0919 with default settings for self consistent field (SCF) convergence criteria and cutoffs. Schlegel’s algorithm20 with the geometry optimization method using an energy-represented direct inversion in the iterative subspace algorithm (GEDIIS)21 was used. Self-consistent cycles with differences in total energies lower than 10−6 a.u. between two iterations were performed.
III. RESULTS AND DISCUSSION
Figure 1 shows Raman spectra of the trishomocubane dimer in the range from 600 cm−1 to 1800 cm−1 at 2.33 eV (532 nm, off-resonance) and 5.19 eV (239 nm) excitation energy (close to resonance). The CCC bend,
J. Chem. Phys. 140, 034309 (2014) TABLE I. Normal mode frequencies in cm−1 calculated at M06-2X/ccpVDZ level of theory. Trishomocubane dimer is abbreviated with thc=thc and diamantylidene-diamantane with dia=dia.
Sample thc=thc thc=thc thc=thc thc=thc syn-dia=dia syn-dia=dia anti-dia=dia anti-dia=dia
Mode type CCC bend CH2 twist/CH wag CH2 rock/CH wag C=C stretch CCC-bend C=C stretch CCC-bend C=C stretch
Experimental frequency 708 1098 1110 1710 716 1659 715 1662
Calculated frequency 706 1120 1145 1827 744 1761 742 1765
CC stretch (≈600–900 cm−1 ), CH wag, CH2 twist, rock (≈900–1350 cm−1 ), and scissors modes (1440–1460 cm−1 ) can be seen. The mode at 1710 cm−1 is the C=C stretch vibration, indicative of the double bonded dimers. Overall, different vibrational modes are differently enhanced at 5.19 eV (239 nm) excitation energy; some modes are not enhanced. Two modes show a particularly strong enhancement, the C=C stretch vibration at 1710 cm−1 and a mode at 708 cm−1 . The latter is a bending mode, where the two carbon atoms of the C=C bond become pyramidalized in opposite directions. Both vibrations are shown schematically in the inset. Table I shows the experimental frequencies of the investigated vibrational Raman modes compared to the calculated values. In Fig. 2(a), we show the Raman spectrum of the C=C vibration at different excitation energies and the corresponding resonance profile in Fig. 2(c). The maximum of the resonance profile is at 5.11 ± 0.03 eV. For comparison, Fig. 3 shows the resonance Raman profiles of the bending mode at 708 cm−1 and exemplary two other modes with significantly weaker (1098 cm−1 ) or not visible enhancement (1110 cm−1 ). These two modes are indicated with asterisks in Fig. 1. While the resonance maximum of the mode at 1098 cm−1 is at the same energy as for the C=C stretch vibration, the resonance
FIG. 1. Raman spectra of the trishomocubane dimer excited off-resonant at 2.33 eV (532 nm, black curve) and close to resonance at 5.19 eV (239 nm, red curve). The spectra are normalized in intensity to the Raman peak of CaF2 . The intensity of the off-resonant spectrum was multiplied by a factor 40 and slightly vertically offset for better comparison. Molecules are shown without hydrogens for clarity. Blue arrows at the C atoms schematically indicate the vibrational modes. The resonance profiles of the three peaks marked by asterisks are shown in Fig. 3.
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J. Chem. Phys. 140, 034309 (2014) TABLE II. Intensity ratios near resonance of four vibrational modes in the trishomocubane dimer: intensities at the resonance maximum Ires (5.11 eV or 5.16 eV) compared to the intensities at 2.33 eV (532 nm).
Mode type CCC bend CH2 twist/CH wag CH2 rock/CH wag C=C stretch
FIG. 2. (a) Raman spectra of the trishomocubane dimer with excitation energies (given next to the spectra) between 2.33 eV (532 nm) and 5.42 eV (229 nm). The C=C stretch mode at 1710 cm−1 is shown. Spectra are normalized to the 321 cm−1 mode peak intensity of CaF2 and vertically offset for clarity. (b) Structure of the trishomocubane dimer with Ci symmetry. The structure was determined by X-ray crystal structure analysis.5, 15 (c) Intensity profile of the C=C stretch vibration of the trishomocubane dimer with intensity maximum at 5.11 ± 0.03 eV.
maximum of the CCC bending mode at 708 cm−1 seems to be at a slightly higher energy at 5.16 ± 0.03 eV. In Table II, we compare the intensity increase of the mentioned four modes at their resonance maxima (5.11 eV and 5.16 eV) with respect to their intensity at 2.33 eV excitation energy. A similarly selective enhancement of the C=C stretch vibration is also observed in diamondoid dimers containing C=C bonds. For example, we show in Fig. 4(a) the C=C stretch region for two different diamantane dimers, syn-diamantylidene-diamantane and anti-diamantylidenediamantane; Fig. 4(b) shows their X-ray crystal structures.15 For both diamantane dimers, a strong enhancement is observed in the deep UV region, with resonance maxima at 5.10±0.20 eV for the syn-isomer and 5.20±0.05 eV for the anti-isomer. In general, in the UV the intensities for the diamantane dimers are by about one order of magnitude lower than for the trishomocubane dimer. An enhancement of the
FIG. 3. Raman resonance profile of the trishomocubane dimer between 2.33 eV (532 nm) and 5.42 eV (229 nm) for three vibrational modes: twist/wag mode at 1098 cm−1 , rock/wag mode at 1110 cm−1 , and the CCC bending mode at 708 cm−1 . The C=C stretch mode at 1710 cm−1 is enhanced by about a factor of three more relative to the twist/wag mode at 1098 cm−1 .
Raman shift (cm−1 ) 708 1098 1110 1710
Intensity ratio Ires /I2.33 eV 270 120 ≈1 540
bending mode at around 710 cm−1 , as for the trishomocubane dimer, was also observed for the diamantane dimers. In contrast to covalently bound solids, the geometrical structure of molecules can change during the transition from the ground state to an excited state. Resonance Raman spectroscopy provides information about the electronic structures of molecules and their geometry in the excited states. In general, the resonance Raman enhancement of a vibrational mode is strong if the molecule in the corresponding excited state is distorted along this vibrational mode.22, 23 For example, if a transition causes an elongation of a certain bond, the corresponding stretch vibration is enhanced; if a bond is twisted, the corresponding torsional vibration is enhanced accordingly. One of the most investigated molecules with respect to excited state geometries is ethylene. In the ground state it displays planar D2h symmetry. In the first excited state, the symmetry changes to D2d (with small deviations due to possible pyramidalization).24–26 For higher excitations, the carbon atoms become significantly pyramidalized in ethylene. In our case, because the HOMO and the LUMO are localized at the C=C bond of double bonded trishomocubane and diamantane dimers, we expect an elongation of the C=C bond in the excited state compared to the ground state.
FIG. 4. (a) Raman spectra of two diamantane dimers (top: syn-isomer; bottom: anti-isomer) off-resonant at 2.41 eV (514 nm, green, lower curves) and close to resonance at 5.08 eV (244 nm) and 5.19 eV (239 nm, blue, upper curves, intensities × 0.1 for better visibility) with the C=C stretch vibration at 1659 cm−1 (top) and 1662 cm−1 (bottom). The spectra are normalized in intensity to the CaF2 Raman peak. (b) X-ray structures of the syn-isomer (top) and the anti-isomer (bottom). The structures were determined by X-ray crystal structure analysis.5, 15
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FIG. 5. HOMO and LUMO of the trishomocubane dimer. The ethylene-like π -orbitals localized around the C=C bond are clearly seen. The isovalue for the electron density is 0.06 electrons per Å3 .
Figure 5 shows the computed HOMO and LUMO for the trishomocubane dimer in the ground state geometry. The computations show the localization of the HOMO and LUMO around the C=C bond; they can be seen as ethylene-like π orbitals. These molecular orbitals are responsible for the first electronic transition, which, in principle, can be observed in the absorption spectrum or in the resonance Raman profile. As the center of the analyzed samples is similar to the ethylene structure and due to the fact that the HOMO and LUMO are localized at the C=C bond (Fig. 5), we can draw some analogies. Because of the localization of the HOMO and LUMO, we expect the strongest resonance enhancement related to geometrical changes of the C=C part of the molecule. Indeed, the selective enhancement of the 1710 cm−1 C=C stretch vibration can be taken as evidence for the elongation of the C=C bond in the excited state. This has been recently predicted by time-dependent density functional theory calculations of diamantane dimers.27 In the resonance Raman spectra of ethylene, a strong overtone of the torsional mode appears.26 This enhancement of the overtone of the torsional mode is taken as a proof that the molecule has a twisted structure in the excited state, i.e., the D2h symmetry of ethylene in the ground state changes to D2d symmetry in the excited state.24–26 The equivalent torsional overtone mode of the trishomocubane dimer is expected to appear at around 140 cm−1 according to our calculations, as the torsional mode at around 70 cm−1 is not Raman active. However, we do not observe such a mode, which implies that there is no significant torsion during the transition. This fact possibly arises from the extra stabilization of the planar form of the trishomocubane dimer due to favorable intramolecular attractive interactions between the hydrogens that surround the central C=C bond [Fig. 2(b)]. Those contact distances are close to the optimal value of 2.3 Å previously observed in hydrocarbon van der Waals clusters. Recently we have found that such interactions stabilize hydrocarbon dimers that are connected with single C–C bonds.28, 29 In the diamantane dimers, on the other hand, the intramolecular distances between the hydrogens surrounding the central C=C bond are only about 2.0 Å.5 These short dis-
J. Chem. Phys. 140, 034309 (2014)
FIG. 6. Resonance Raman profile of the C=C stretch vibration of the trishomocubane dimer in the gas phase up to 6.03 eV (206 nm, upper limit of energy range of the laser). The inset shows Raman spectra of the trishomocubane dimer at different excitation energies. The second-order Raman peak at around 3400 cm−1 and the third-order peak at 5100 cm−1 of the C=C stretch vibration are visible. Spectra are vertically offset for clarity.
tances suggest that due to repulsive Van der Waals forces the molecular structure might be twisted in the excited state, in addition to the C=C bond elongation. However, the experimental results are not clear about this question. The secondorder Raman modes of the twisting modes are predicted at 66 cm−1 and 90 cm−1 for the anti- and syn-isomer, respectively. These frequencies are superimposed by translational and librational modes of the molecular crystal, which appear in the region between 20 cm−1 and 100 cm−1 . In fact, for the anti-isomer we observe a strong enhancement in the UV of a mode at 60 cm−1 , which may be assigned to the overtone of the twisting mode, however, the assignment is not unambiguous at the present stage. Preliminary Raman spectroscopy results from the trishomocubane dimer in the gas phase show a slightly different resonance behavior (Fig. 6). Here the resonance enhancement of the C=C stretch vibration appears at significantly higher excitation energy: we observe a continuous increase of the Raman intensity up to 6.03 eV (206 nm). Also higher-order Raman modes of the C=C stretch vibration (Fig. 6) are observable. For other vibrational modes, the intensity is too weak and the spectral resolution is not sufficient for analysis. Further experimental and theoretical investigations are needed for understanding the apparent differences between solid-state and gas-phase Raman spectra of the investigated compounds. IV. CONCLUSIONS
We presented resonance Raman measurements of double bonded trishomocubane and diamantane dimers in the solid state. The characteristic stretch vibration of the C=C double bond at 1710 cm−1 shows the strongest enhancement of all Raman modes with resonance maxima between 5.1 and 5.2 eV for the three different compounds. This selective resonance enhancement is assigned to an elongation of the C=C bond in the excited state, consistent with a localization of the HOMO and LUMO at the double bond. A second selectively enhanced Raman mode is the CCC bending
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vibration at 708 cm−1 (resonance maximum at 5.16 eV in the trishomocubane dimer), which suggests a pyramidalization of the two carbon atoms of the C=C bond. On the other hand, twisting of the C=C bond in the excited state is found to be negligible in the trishomocubane dimer, based on the absence of the torsional mode overtone in the Raman spectra. Although twisting can be expected for the diamantane dimers, we do not observe clear evidence for this in the spectra, due to spectral overlap with translational and librational modes of the molecular crystals. The Raman resonance maxima observed in the molecular crystals of the trishomocubane and diamantane dimers are at lower energies than in the gas phase; as expected they are also lower than the optical absorption energy of isolated diamantane. ACKNOWLEDGMENTS
We thank K. Wendt (University of Mainz) for developing the second and third harmonic generation Ti:Sa laser system. We thank S. Banerjee and P. Saalfrank (University of Potsdam) for sharing the results of Ref. 27 prior to publication. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under Grant Nos. MA4079/6-1 (R.M., C.T., and J.M.) and MO719/10-1 (R.R. and A.M.) within the Forschergruppe FOR1282 and by the European Research Council (ERC) under Grant No. 259286-CCCAN (J.M.). 1 J. E. P. Dahl,
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THE JOURNAL OF CHEMICAL PHYSICS 140, 034309 (2014)
UV resonance Raman analysis of trishomocubane and diamondoid dimers Reinhard Meinke,1,a) Robert Richter,2 Andrea Merli,2 Andrey A. Fokin,3,4 Tetyana V. Koso,3 Vladimir N. Rodionov,4 Peter R. Schreiner,3 Christian Thomsen,1 and Janina Maultzsch1 1
Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany 3 Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany 4 Department of Organic Chemistry, Kiev Polytechnic Institute, pr. Pobedy 37, 03056 Kiev, Ukraine 2
(Received 22 October 2013; accepted 27 December 2013; published online 16 January 2014) 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 sp2 -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. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4861758] I. INTRODUCTION
Diamondoids are fascinating nanoparticles that have many properties in common with diamond, such as mechanical hardness, field emitting properties, or optical transparency.1–4 They are well-defined, size-selected cagelike hydrocarbons, stable at ambient conditions, and they form van-der-Waals crystal structures.5 Diamandoid derivatives and related cage-like hydrocarbons like trishomocubanes are used in medical applications,6, 7 but they are also foreseen as building blocks in nanoscale field emission, electronic or optoelectronic devices.4, 8 The optical gaps in small diamondoids, depending on size and shape, are typically above 6 eV.9, 10 For some applications, modifications of the optical gap or the electronic structure in general are desired. Such modifications can be achieved, among others, by covalent functionalization, substitution, or dimer formation.2, 10–13 Unsaturated moieties of coupled diamondoids can be considered as models for sp2 -defects of diamond surfaces and macroscopic diamond. In particular, diamondoid dimers with sp2 -hybridized carbon forming double bonds are expected to have significantly lower HOMO-LUMO gaps (highest occupied molecular orbital–lowest unoccupied molecular orbital) than the individual constituents, as the HOMO and LUMO are likely to be localized at the C=C bonds. In this work we analyze the optical gaps of trishomocubane and diamantane dimers by resonance Raman spectroscopy. We observe resonances around 5.1 eV, which are lower in energy than the optical gap of diamantane (6.40 eV).9, 14 Furthermore, the stretch vibration of the C=C double bond is selectively enhanced when excited in resonance in all investigated samples. This hints to an elongation of the C=C bond in the excited state, which in turn is consistent with the expected localization of the HOMO and LUMO a) Electronic mail: [email protected]
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at the double bond. Finally, resonance Raman experiments of diamantane dimers in the gas phase show higher resonance energy than in the crystalline state.
II. EXPERIMENTAL AND THEORETICAL DETAILS
We investigated samples in solid state (small crystals or powder) by resonance Raman spectroscopy with excitation energies from the visible to the deep UV range. Raman spectra were recorded using a triple monochromator system (Horiba Jobin Yvon T64000). Different laser sources were used: an Ar-ion laser (Coherent Innova 90C FreD) in fundamental and second harmonic generation (SHG), a HeCd laser, and a tunable Ti:Sa laser system with SHG and third harmonic generation pumped by a pulsed frequency doubled Nd:YAG laser. The Raman spectrum at 2.33 eV (532 nm) was recorded with a frequency doubled Nd:YAG laser in continuous wave operation. All measurements were done in backscattering geometry, using a 100× objective for the visible and a 20× objective for the UV, at room temperature. The intensities are normalized to the Raman signal of CaF2 . For analyzing the Raman resonance profiles, the peak area is taken as intensity. The spectral resolution is 3–5 cm−1 in the UV and 1–2 cm−1 in the visible range. We observed that the samples in solid state were damaged by deep UV illumination. Because the damage occurred even at laser power below 0.1 mW, the integration time was kept short (1–10 s) and the laser was scanned over the sample surface during the measurement with scanning speed of around 10 μm per second. This results in an intensity error increase to about 50% in the UV range. Yet, the intensity increase due to resonance excitation in the UV is above this error. Additionally, we observed a clear polarization dependence of the Raman signal for some of the crystals. By scanning over an
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ensemble of different orientations, we obtain an average value for the intensity. Resonance Raman measurements in the gas phase were recorded in 90◦ scattering geometry using a Shamrock SR 303i-B spectrometer combined with an Andor iDus DU420ABU2o CCD camera. The light source is an optical parametric oscillator (Continuum Panther EX OPO) pumped by a nanosecond Nd:YAG laser (Continuum Powerlite DLS) and focused by a lens (f = 116 mm). The pulse energy was 0.16 mJ at 10 Hz repetition rate. Measurements were performed in the range from 4.94 eV (251 nm) to 6.03 eV (206 nm) at a sample temperature of 160 ◦ C. The investigated materials are a trishomocubane dimer and two diamantane dimers. The trishomocubane dimer [Fig. 2(b)], Ci -trans-Cs -8-trishomocubylideneCs -8-trishomocubane,15 can also be seen as a dimer.16 The pentacyclo[5.4.0.02, 6 .03, 10 .05, 9 ]undecane diamantane dimers [Fig. 4(b)] are the isomers syn- and anti-3-diamantylidene-3-diamantane. The dimers are formed by a C=C double bond. For further details, the reader is referred to Refs. 5 and 13. We performed density functional theory (DFT) computations of the vibrational modes and the HOMO and LUMO, using the M06-2X17 functional and the double-ζ basis set cc-pVDZ18 as implemented in Gaussian 0919 with default settings for self consistent field (SCF) convergence criteria and cutoffs. Schlegel’s algorithm20 with the geometry optimization method using an energy-represented direct inversion in the iterative subspace algorithm (GEDIIS)21 was used. Self-consistent cycles with differences in total energies lower than 10−6 a.u. between two iterations were performed.
III. RESULTS AND DISCUSSION
Figure 1 shows Raman spectra of the trishomocubane dimer in the range from 600 cm−1 to 1800 cm−1 at 2.33 eV (532 nm, off-resonance) and 5.19 eV (239 nm) excitation energy (close to resonance). The CCC bend,
J. Chem. Phys. 140, 034309 (2014) TABLE I. Normal mode frequencies in cm−1 calculated at M06-2X/ccpVDZ level of theory. Trishomocubane dimer is abbreviated with thc=thc and diamantylidene-diamantane with dia=dia.
Sample thc=thc thc=thc thc=thc thc=thc syn-dia=dia syn-dia=dia anti-dia=dia anti-dia=dia
Mode type CCC bend CH2 twist/CH wag CH2 rock/CH wag C=C stretch CCC-bend C=C stretch CCC-bend C=C stretch
Experimental frequency 708 1098 1110 1710 716 1659 715 1662
Calculated frequency 706 1120 1145 1827 744 1761 742 1765
CC stretch (≈600–900 cm−1 ), CH wag, CH2 twist, rock (≈900–1350 cm−1 ), and scissors modes (1440–1460 cm−1 ) can be seen. The mode at 1710 cm−1 is the C=C stretch vibration, indicative of the double bonded dimers. Overall, different vibrational modes are differently enhanced at 5.19 eV (239 nm) excitation energy; some modes are not enhanced. Two modes show a particularly strong enhancement, the C=C stretch vibration at 1710 cm−1 and a mode at 708 cm−1 . The latter is a bending mode, where the two carbon atoms of the C=C bond become pyramidalized in opposite directions. Both vibrations are shown schematically in the inset. Table I shows the experimental frequencies of the investigated vibrational Raman modes compared to the calculated values. In Fig. 2(a), we show the Raman spectrum of the C=C vibration at different excitation energies and the corresponding resonance profile in Fig. 2(c). The maximum of the resonance profile is at 5.11 ± 0.03 eV. For comparison, Fig. 3 shows the resonance Raman profiles of the bending mode at 708 cm−1 and exemplary two other modes with significantly weaker (1098 cm−1 ) or not visible enhancement (1110 cm−1 ). These two modes are indicated with asterisks in Fig. 1. While the resonance maximum of the mode at 1098 cm−1 is at the same energy as for the C=C stretch vibration, the resonance
FIG. 1. Raman spectra of the trishomocubane dimer excited off-resonant at 2.33 eV (532 nm, black curve) and close to resonance at 5.19 eV (239 nm, red curve). The spectra are normalized in intensity to the Raman peak of CaF2 . The intensity of the off-resonant spectrum was multiplied by a factor 40 and slightly vertically offset for better comparison. Molecules are shown without hydrogens for clarity. Blue arrows at the C atoms schematically indicate the vibrational modes. The resonance profiles of the three peaks marked by asterisks are shown in Fig. 3.
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J. Chem. Phys. 140, 034309 (2014) TABLE II. Intensity ratios near resonance of four vibrational modes in the trishomocubane dimer: intensities at the resonance maximum Ires (5.11 eV or 5.16 eV) compared to the intensities at 2.33 eV (532 nm).
Mode type CCC bend CH2 twist/CH wag CH2 rock/CH wag C=C stretch
FIG. 2. (a) Raman spectra of the trishomocubane dimer with excitation energies (given next to the spectra) between 2.33 eV (532 nm) and 5.42 eV (229 nm). The C=C stretch mode at 1710 cm−1 is shown. Spectra are normalized to the 321 cm−1 mode peak intensity of CaF2 and vertically offset for clarity. (b) Structure of the trishomocubane dimer with Ci symmetry. The structure was determined by X-ray crystal structure analysis.5, 15 (c) Intensity profile of the C=C stretch vibration of the trishomocubane dimer with intensity maximum at 5.11 ± 0.03 eV.
maximum of the CCC bending mode at 708 cm−1 seems to be at a slightly higher energy at 5.16 ± 0.03 eV. In Table II, we compare the intensity increase of the mentioned four modes at their resonance maxima (5.11 eV and 5.16 eV) with respect to their intensity at 2.33 eV excitation energy. A similarly selective enhancement of the C=C stretch vibration is also observed in diamondoid dimers containing C=C bonds. For example, we show in Fig. 4(a) the C=C stretch region for two different diamantane dimers, syn-diamantylidene-diamantane and anti-diamantylidenediamantane; Fig. 4(b) shows their X-ray crystal structures.15 For both diamantane dimers, a strong enhancement is observed in the deep UV region, with resonance maxima at 5.10±0.20 eV for the syn-isomer and 5.20±0.05 eV for the anti-isomer. In general, in the UV the intensities for the diamantane dimers are by about one order of magnitude lower than for the trishomocubane dimer. An enhancement of the
FIG. 3. Raman resonance profile of the trishomocubane dimer between 2.33 eV (532 nm) and 5.42 eV (229 nm) for three vibrational modes: twist/wag mode at 1098 cm−1 , rock/wag mode at 1110 cm−1 , and the CCC bending mode at 708 cm−1 . The C=C stretch mode at 1710 cm−1 is enhanced by about a factor of three more relative to the twist/wag mode at 1098 cm−1 .
Raman shift (cm−1 ) 708 1098 1110 1710
Intensity ratio Ires /I2.33 eV 270 120 ≈1 540
bending mode at around 710 cm−1 , as for the trishomocubane dimer, was also observed for the diamantane dimers. In contrast to covalently bound solids, the geometrical structure of molecules can change during the transition from the ground state to an excited state. Resonance Raman spectroscopy provides information about the electronic structures of molecules and their geometry in the excited states. In general, the resonance Raman enhancement of a vibrational mode is strong if the molecule in the corresponding excited state is distorted along this vibrational mode.22, 23 For example, if a transition causes an elongation of a certain bond, the corresponding stretch vibration is enhanced; if a bond is twisted, the corresponding torsional vibration is enhanced accordingly. One of the most investigated molecules with respect to excited state geometries is ethylene. In the ground state it displays planar D2h symmetry. In the first excited state, the symmetry changes to D2d (with small deviations due to possible pyramidalization).24–26 For higher excitations, the carbon atoms become significantly pyramidalized in ethylene. In our case, because the HOMO and the LUMO are localized at the C=C bond of double bonded trishomocubane and diamantane dimers, we expect an elongation of the C=C bond in the excited state compared to the ground state.
FIG. 4. (a) Raman spectra of two diamantane dimers (top: syn-isomer; bottom: anti-isomer) off-resonant at 2.41 eV (514 nm, green, lower curves) and close to resonance at 5.08 eV (244 nm) and 5.19 eV (239 nm, blue, upper curves, intensities × 0.1 for better visibility) with the C=C stretch vibration at 1659 cm−1 (top) and 1662 cm−1 (bottom). The spectra are normalized in intensity to the CaF2 Raman peak. (b) X-ray structures of the syn-isomer (top) and the anti-isomer (bottom). The structures were determined by X-ray crystal structure analysis.5, 15
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FIG. 5. HOMO and LUMO of the trishomocubane dimer. The ethylene-like π -orbitals localized around the C=C bond are clearly seen. The isovalue for the electron density is 0.06 electrons per Å3 .
Figure 5 shows the computed HOMO and LUMO for the trishomocubane dimer in the ground state geometry. The computations show the localization of the HOMO and LUMO around the C=C bond; they can be seen as ethylene-like π orbitals. These molecular orbitals are responsible for the first electronic transition, which, in principle, can be observed in the absorption spectrum or in the resonance Raman profile. As the center of the analyzed samples is similar to the ethylene structure and due to the fact that the HOMO and LUMO are localized at the C=C bond (Fig. 5), we can draw some analogies. Because of the localization of the HOMO and LUMO, we expect the strongest resonance enhancement related to geometrical changes of the C=C part of the molecule. Indeed, the selective enhancement of the 1710 cm−1 C=C stretch vibration can be taken as evidence for the elongation of the C=C bond in the excited state. This has been recently predicted by time-dependent density functional theory calculations of diamantane dimers.27 In the resonance Raman spectra of ethylene, a strong overtone of the torsional mode appears.26 This enhancement of the overtone of the torsional mode is taken as a proof that the molecule has a twisted structure in the excited state, i.e., the D2h symmetry of ethylene in the ground state changes to D2d symmetry in the excited state.24–26 The equivalent torsional overtone mode of the trishomocubane dimer is expected to appear at around 140 cm−1 according to our calculations, as the torsional mode at around 70 cm−1 is not Raman active. However, we do not observe such a mode, which implies that there is no significant torsion during the transition. This fact possibly arises from the extra stabilization of the planar form of the trishomocubane dimer due to favorable intramolecular attractive interactions between the hydrogens that surround the central C=C bond [Fig. 2(b)]. Those contact distances are close to the optimal value of 2.3 Å previously observed in hydrocarbon van der Waals clusters. Recently we have found that such interactions stabilize hydrocarbon dimers that are connected with single C–C bonds.28, 29 In the diamantane dimers, on the other hand, the intramolecular distances between the hydrogens surrounding the central C=C bond are only about 2.0 Å.5 These short dis-
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FIG. 6. Resonance Raman profile of the C=C stretch vibration of the trishomocubane dimer in the gas phase up to 6.03 eV (206 nm, upper limit of energy range of the laser). The inset shows Raman spectra of the trishomocubane dimer at different excitation energies. The second-order Raman peak at around 3400 cm−1 and the third-order peak at 5100 cm−1 of the C=C stretch vibration are visible. Spectra are vertically offset for clarity.
tances suggest that due to repulsive Van der Waals forces the molecular structure might be twisted in the excited state, in addition to the C=C bond elongation. However, the experimental results are not clear about this question. The secondorder Raman modes of the twisting modes are predicted at 66 cm−1 and 90 cm−1 for the anti- and syn-isomer, respectively. These frequencies are superimposed by translational and librational modes of the molecular crystal, which appear in the region between 20 cm−1 and 100 cm−1 . In fact, for the anti-isomer we observe a strong enhancement in the UV of a mode at 60 cm−1 , which may be assigned to the overtone of the twisting mode, however, the assignment is not unambiguous at the present stage. Preliminary Raman spectroscopy results from the trishomocubane dimer in the gas phase show a slightly different resonance behavior (Fig. 6). Here the resonance enhancement of the C=C stretch vibration appears at significantly higher excitation energy: we observe a continuous increase of the Raman intensity up to 6.03 eV (206 nm). Also higher-order Raman modes of the C=C stretch vibration (Fig. 6) are observable. For other vibrational modes, the intensity is too weak and the spectral resolution is not sufficient for analysis. Further experimental and theoretical investigations are needed for understanding the apparent differences between solid-state and gas-phase Raman spectra of the investigated compounds. IV. CONCLUSIONS
We presented resonance Raman measurements of double bonded trishomocubane and diamantane dimers in the solid state. The characteristic stretch vibration of the C=C double bond at 1710 cm−1 shows the strongest enhancement of all Raman modes with resonance maxima between 5.1 and 5.2 eV for the three different compounds. This selective resonance enhancement is assigned to an elongation of the C=C bond in the excited state, consistent with a localization of the HOMO and LUMO at the double bond. A second selectively enhanced Raman mode is the CCC bending
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vibration at 708 cm−1 (resonance maximum at 5.16 eV in the trishomocubane dimer), which suggests a pyramidalization of the two carbon atoms of the C=C bond. On the other hand, twisting of the C=C bond in the excited state is found to be negligible in the trishomocubane dimer, based on the absence of the torsional mode overtone in the Raman spectra. Although twisting can be expected for the diamantane dimers, we do not observe clear evidence for this in the spectra, due to spectral overlap with translational and librational modes of the molecular crystals. The Raman resonance maxima observed in the molecular crystals of the trishomocubane and diamantane dimers are at lower energies than in the gas phase; as expected they are also lower than the optical absorption energy of isolated diamantane. ACKNOWLEDGMENTS
We thank K. Wendt (University of Mainz) for developing the second and third harmonic generation Ti:Sa laser system. We thank S. Banerjee and P. Saalfrank (University of Potsdam) for sharing the results of Ref. 27 prior to publication. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under Grant Nos. MA4079/6-1 (R.M., C.T., and J.M.) and MO719/10-1 (R.R. and A.M.) within the Forschergruppe FOR1282 and by the European Research Council (ERC) under Grant No. 259286-CCCAN (J.M.). 1 J. E. P. Dahl,
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