One-Way Optoelectronic Switching of Photochromic Molecules on Gold

We investigate photochromic molecular switches that are self-assembled on gold. We use two experimental techniques; namely, the mechanically controllable break-junction technique to measure electronic transport, and UV/Vis spectroscopy to measure absorption. We observe switching of the molecules from the conducting to the insulating state when illuminated with visible light ($\lambda =546\mathrm{n}\mathrm{m}$), in spite of the gold surface plasmon absorption present around this wavelength. However, we fail to observe the reverse process which should occur upon illumination with UV light ($\lambda =313\mathrm{n}\mathrm{m}$). We attribute this to quenching of the excited state of the molecule in the open form by the presence of gold.

Figure 1

Photochromic molecular switch between two Au contacts in closed state (a) and open state (b). By exposing the molecule to light of wavelength in the range $500\mathrm{n}\mathrm{m}<{\lambda }_1<700\mathrm{n}\mathrm{m}$, the molecule will switch from (a) to (b). In solution the protected molecule can be switched back to the (a) state by exposing it to UV light with wavelength in the range $300\mathrm{n}\mathrm{m}<{\lambda }_2<400\mathrm{n}\mathrm{m}$. However, this is not possible if the molecules are connected to gold.

Figure 2

Mechanically controllable break-junction (MCBJ) technique. Top: Scanning electron micrograph of the free standing gold bridge. Bottom: Layout of the technique (not to scale). The whole device is $2\times 2{\mathrm{m}\mathrm{m}}^2$. In reality the polyimide insulating layer is $\sim 1000$ times thinner than the phosphor bronze.

Figure 3

MCBJ results. (a) Typical IV of the connected molecule in the closed form and (b) resistance versus time. At $t=0$ a lamp is turned on ($\lambda =546\mathrm{n}\mathrm{m}$). After approximately 20 s a clear jump is observed (1 V bias). (c) Typical IV of the molecule after switching. The line is a fit to the Stratton formula (for details see text).

Figure 4

Absorption spectra of the molecular switches. (a) Molecular switches in toluene. Solid line: closed form; dashed line: open form. The switching process in solution shows excellent reversibility. The decay half time for opening is a few minutes [8]. (b) Closed form self-assembled on gold colloids. Solid line: before irradiation with $\lambda =546\mathrm{n}\mathrm{m}$; dashed line: after one day of irradiation. From the curves taken in between we get a decay half-time of 40 min. (c) Open form self-assembled on gold. Solid line: before irradiation with $\lambda =313\mathrm{n}\mathrm{m}$; dashed line: after two days of irradiation. Since the optical density is proportional to concentration, the absolute values of the $y$ axes in (b) and (c) are different.

Figure 5

Potential energy curves of the molecular switch along the switching coordinate $q$ (black lines: optically allowed states; gray lines: optically forbidden excited states). The vacuum level is set at 0 eV. The switching process is initiated by an excitation to the first excited state ($T_c,T_o$). For a molecule in solution, the evolution of the switching process is indicated with dashed arrows. For a molecule connected to gold, however, the potential curve of the first excited state comes very close to the Au Fermi level (at $0.22<q<0.25\mathrm{n}\mathrm{m}$), resulting in an efficient mixing of these respective states. When the open molecule goes to the first excited state ($T_o$), the closing process sets in, i.e., the excited state potential curve is followed towards lower $q$ (solid arrow). However, when $0.22<q<0.25\mathrm{n}\mathrm{m}$, the interaction with the gold makes the excited molecule relax back to the ground state (at the right side of the local maximum), and hence back to the open form. As a consequence there is no switching from the open to the closed form.