Plattenspieler trans-Orbitale

Struktur

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Orbitale

STS-Spektrum

LUMOs

HOMOs

Orbitalenergien

231     L+9     -0.65   A
230     L+8     -0.7    A
229     L+7     -0.74   A
228     L+6     -0.98   A
227     L+5     -1.15   A
226     L+4     -1.17   A
225     L+3     -1.5    A
224     L+2     -2.46   A
223     L+1     -2.49   A
222     LUMO    -2.85   A
221     HOMO    -5.33   A
220     H-1     -5.46   A
219     H-2     -6.16   A
218     H-3     -6.19   A
217     H-4     -6.39   A
216     H-5     -6.53   A
215     H-6     -6.76   A
214     H-7     -6.79   A
213     H-8     -6.87   A
212     H-9     -6.96   A
211     H-10    -6.99   A

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Interpretation of STS-spectra

According to our DFT-calculations (B3LYP/6-311+G*) the n and 𝜋 orbitals of the diazapyridine unit differ in the cis and trans configuration, similar to the parent azobenzene. The highest occupied orbital with coefficients located at the azopyridine unit (n type orbital) is considerably higher in energy for the cis as compared to the trans isomer (E_cis=-6.19 eV(MO=219)), (E_trans=-6.53 eV(MO=216)).

We therefore identify the signal at -1 eV in the STS spectrum of the cis isomer as the lone pair orbital mainly located at the azo-group of the azopyridine unit (MO=219). The azo group of the cis isomer is the highest part of the molecule with respect to the surface and its lone pair orbitals are pointing away from the surface and thus are exposed to the STM tip which gives rise to a large signal.

The peak at about -1.7 eV in the spectrum of the trans isomer and at -1.6 eV in the cis isomer can be assigned to the 𝜋^* orbitals of the azopyridine units, which are the LUMOs in our DFT calculations (E_cis=-2.58 eV(MO=222)), (E_trans=-2.85 eV(MO=222)).

The lower energy of the trans LUMO as compared to the cis thus is also in agreement with the STS spectrum.