Effect of the Molecular Polarizability of SAMs on the Work Function Modification of Gold: Closed‐ versus Open‐Shell Donor–Acceptor SAMs

Abstract

Charge injection barriers at metal/organic interfaces can be tuned by modifying the work function of metallic electrodes using self‐assembled monolayers (SAMs) of polar molecules. An interesting example of polar molecules is offered by donor–acceptor (D–A) dyads based on ferrocene (Fc) as electron‐donor unit and either a polychlorotriphenylmethyl radical or a polychlorotriphenylmethane as electron‐acceptor units, connected by a π‐conjugated vinylene bridge. The D–A radical exhibits high chemical and thermal stability and presents different electronic, optical, and magnetic properties with respect to the closed‐shell form. The magnitude of the shift in the charge injection barriers for these two D–A systems is estimated by means of surface potential measurements performed by Kelvin probe force microscopy. The experimental data are compared with density functional theory calculations, which evidence the importance of the molecular dipole moments and polarizabilities to understand the experimental values. In order to achieve high work function shifts of metals upon SAM formation, the molecules forming the SAM have to exhibit both a high permanent dipole moment and a low polarizability along the direction normal to the substrate. In presence of polarizable molecules, the work function shifts can be enhanced by reducing the intermolecular interactions; by using mixed SAMs with active molecules embedded into a passive matrix.

Reversible switching of the Au(111) work function by near infrared irradiation with a bistable SAM based on a radical donor–acceptor dyad

“…5c and d). In our previous work, 22 we demonstrated that the polarizability of radical molecules in SAM S2 is higher than in SAM S1, meaning that the surface dipole in S2 is more sensible to the changes in the surrounding charge density. Consequently, the negative shift induced by the thermal motion is higher in the case of the radical SAM S2 (B285 meV) than in the non-radical SAM S1 (B145 meV).…”

“…The reported values of Df, as obtained by KPFM measurements using an in situ reference surface, were 50 meV and 22 meV for the non-radical S1 and radical S2 SAMs, respectively. 22 We demonstrated that the radical form has a larger polarizability than the non-radical molecules, pointing to larger depolarization effects in the former case due to its higher charge-transfer character. This implies that the permanent dipole of a given molecule (m i ) is modified to a stronger extent in S2 due the charge redistribution that takes places upon interacting with the electric field generated by the dipoles of the neighbouring molecules within the SAM, thus rationalizing the lower WF values of S2 with respect to S1.…”

“…31 This long-range correlated functional has been chosen due to problems of standard hybrid functionals to describe charge-transfer states. 32,33 As it has been done in our previous paper, 22 the orientation of the molecules in the gas phase has been set to match experimental data: heights for S1 and S2 of d S1 = 2.27 nm and d S2 = 2.23 nm from photoemission spectroscopy, respectively. 21 The zwitterionic ground state (D + -A À ) has been achieved by applying an electric field E = 0.3 V Å À1 to the optimized neutral (D-A) geometries in the direction of the Fc-PTM vector.…”

“…The evolution of the surface potential with the NIR light in the radical S2 SAM has been further analysed by means of Time Dependent-Density Functional Theory (TD-DFT) calculations. To do so, we have used the same geometries of the isolated molecules as in our previous studies, 22 as guided by experimental measurements (see Experimental section). In particular, we have calculated the normal dipole component in the isolated molecules (m z ) for the neutral radical (D-A) and excited zwitterionic state (D + -A À ) 0 , while the effect of changes in the charge density induced by the neighbouring molecules has been taken into account by the normal component of the molecular polarizability (a zz ).…”

“…18 Finally, while spiropyran-based SAMs yielded theoretical WF differences equal to 230 meV, 19 Ultraviolet Photoelectron Spectroscopy (UPS) measurements showed no changes in the WF upon photoisomerization for these SAMs which turn from a neutral (closed-SP) to a zwitterionic form (open-SP) because the molecular dipoles lie parallel to the metal surface instead of perpendicular to be relevant. 20 Recently, we reported two SAMs, 21,22 S1 and S2, on Au(111) based on two donor-acceptor (D-A) dyads consisting on a ferrocene (Fc) unit acting as an electron donor (D), and either a PTM radical (where the central C atom of the PTM unit presents an unpaired electron) or a non-radical aHPTM unit (with the central C atom hydrogenated), as electron accepting (A) groups (see Fig. 1).…”