Light‐Induced Switching of Tunable Single‐Molecule Junctions

A major goal of molecular electronics is the development and implementation of devices such as single‐molecular switches. Here, measurements are presented that show the controlled in situ switching of diarylethene molecules from their nonconductive to conductive state in contact to gold nanoelectrodes via controlled light irradiation. Both the conductance and the quantum yield for switching of these molecules are within a range making the molecules suitable for actual devices. The conductance of the molecular junctions in the opened and closed states is characterized and the molecular level E 0, which dominates the current transport in the closed state, and its level broadening Γ are identified. The obtained results show a clear light‐induced ring forming isomerization of the single‐molecule junctions. Electron withdrawing side‐groups lead to a reduction of conductance, but do not influence the efficiency of the switching mechanism. Quantum chemical calculations of the light‐induced switching processes correlate these observations with the fundamentally different low‐lying electronic states of the opened and closed forms and their comparably small modification by electron‐withdrawing substituents. This full characterization of a molecular switch operated in a molecular junction is an important step toward the development of real molecular electronics devices.

. a) -c): normalized histograms of the molecular level E 0 for single, assembled, closed SMWs with -H and -CF 3 side-groups that were dissolved in toluene (1on t , 2on t ) as well as mesitylene (1on m ). (d): results summarized in one plot. The values were obtained from the single level transport model applied to 1990 (1on t ), 260 (2on t ) and 196 (1on m ) IVcharacteristics. Figure S2. IV-curves of single SMW junction with -H (a) and -CF 3 (b) side-groups. The blue points correspond to experimental data, the red curve to the fit from the single level model. The values for Γ, E 0 and T illustrate the expected behavior for the different side-groups in an excellent way. The slight asymmetry in the IV-characteristic of the SMWs with the CF3 sidegroups (right panel) indicates the development of two peaks in Figure 2 in the main paper. Figure S3. Normalized histograms of the symmetry factor α for assembled (a, b) and in situ switched (c, d) SMWs with -H (1on t ) and -CF 3 (2on t ) side-groups, given by the ratio of the scattering rates Γ L and Γ R . We divided the smaller of the two values by the bigger such that α adopts values between 0 and 1. The latter indicates is a highly symmetric molecule gold junction. The values of the assembled SMWs were extracted from 1990 (1on t ) and 260 (2on t ), the values of the in situ switched SMWs from 301 (1on t ) and 196 (2on t ) IV-curves. Figure S4. Normalized histograms of conductance traces of toluene (a, 87 traces) and mesitylene (b, 570 traces) without dissolved molecules. Figure S5. Normalized histograms of conductance traces taken from SMWs with -H (a, 296 traces) and -CF 3 (b, 117 traces) side-groups that were assembled in the conductive state. The two pronounced plateaus between G ≈ 10 -3 -10 -5 G 0 for the -H side-groups may arise from the thiol linkers that create two ways of molecule gold junctions. This effect may be suppressed for the SMW with -CF 3 side-groups due to their lower conductance and a different conformation.

Computational details
For the quantum chemical investigations of the present rather large molecules the semiempirical program package MOPAC in the PM7 parameterization was chosen. [4] The approach was evaluated by comparison with literature values from full ab-initio CAS(10,10) and CAS(12,12) calculations. [5] The energies and states involved in the ring opening and closure mechanism of the central difurylcyclopentene moiety agree within 0.1 eV, which is sufficiently accurate for the present qualitative study.
In order to calculate the excitation frequencies of the ring-opened off-state and the ring-closed on-state the geometries of 1on, 1off, 2on, and 2off were optimized at the multi-reference level, including all 2p-based valence orbitals in the configuration interaction ansatz. All four molecules are stable ground state structures of about 32 Å distance between the two S atoms; the ring-opened forms are more stable than the ring-closed ones by 0.12 eV (1) and 0.29 eV (2). A rotation of the side-groups around the bond between the furyl group and the central cyclopentene unit is energetically unfavorable for all structures by on average 0.1 eV per rotated bond. All four molecules exhibit a singlet ground state with single HOMO and LUMO levels, which span the whole molecule in the case of the ring-closed forms. In contrast, for the ring-opened forms doubly degenerate HOMO and LUMO levels are obtained, which are each confined to only one side of the SMW. In very good qualitative agreement with the experiment these results indicate that only the ring-closed forms 1on and 2on exhibit molecular states suitable for ballistic transport, whereas the aromatic system is broken in two parts for the ring-opened switches.
The single-reference HOMO-LUMO gaps of the ring-opened forms are larger than the ones of the ring-closed forms by 0.7 eV (1) and 0.5 eV (2), and the gaps of the substituted compounds 2on and 2off are smaller than the ones of 1on and 1off by 0.3 eV and 0.5 eV, respectively.
These findings indicate that the substitution of the phenyl rings with -CF 3 substituents changes neither the localization pattern nor the general orbital splitting. Due to their electron-withdrawing character the -CF 3 groups, however, modify the aromatic nature of the frontier orbitals: the reduced HOMO-LUMO gaps of compounds 2 indicate that the alignment with the Fermi level of the lead and also the coupling strength of compounds 1 and 2 will differ.
The first singlet excitation energies obtained from full CI calculations are in very good agreement with experimental observations for the molecules in toluene solution. [6] Visible light can excite the ring-closed forms at about 530 nm (1on) and 550 nm (2on), whereas UV light of about 400 nm and 420 nm is required to excite the ring-opened forms 1off and 2off.
This reflects the delocalization of the frontier orbitals of the ring-closed switches 1on and 2on and the confinement of the aromatic system to subunits of the ring-opened molecules 1off and 2off. Figure S6 shows the energies of the ground state and the first two excited singlet states as a function of the C-C atom distance at the central part of the switch, which is chosen here as reaction coordinate. Apart from this reaction coordinate all degrees of freedom were fully optimized at the CI level. The transition between ring-closed and ring-opened states occurs in a conrotatory manner [7] and involves an energy barrier of 2. There, the optimum sulfur-to-sulfur distance shrinks by about 2 Å with respect to both reactant and product. Along with the arguments given in (4) for a reduced quantum yield of the ring opening reaction due to the presence of an energy barrier and several conical intersections on the first excited energy surface this geometry change may be another reason for the low propensity of the molecule to switch back to the opened form in situ.