Erik R. McNellis

Azobenzene at coinage metal surfaces: Role of dispersive van der Waals interactions

We use different semiempirical dispersion correction schemes to assess the role of long-range van der Waals interactions in the adsorption of the prototypical molecular switch azobenzene

Structure and excitonic coupling in self-assembled monolayers of azobenzene-functionalized alkanethiols.

({\text{C}}_{6}{\text{H}}_{5}{\text{-N}}_{2}{\text{-C}}_{6}{\text{H}}_{5})

Tuning Electron Transfer Rates through Molecular Bridges in Quantum Dot Sensitized Oxides

at the coinage metal surfaces Cu(111), Ag(111), and Au(111). Compared to preceding density-functional theory results employing a semilocal exchange and correlation functional we obtain partly sizable changes in the computed adsorption geometry and energetics. The discomforting scatter in the results provided by the different schemes is largely attributed to the unknown form of the damping function in the semiempirical correction expression. Using the congeneric problem of the adsorption of benzene as a vehicle to connection with experiment, we cautiously conclude that the account of dispersive interactions at the metal surfaces provided by the various schemes is in the right ballpark, with the more recent, general schemes likely to overbind.

Stabilizing a molecular switch at solid surfaces: A density functional theory study of azobenzene at Cu(111), Ag(111), and Au(111)

Optical properties and the geometric structure of self-assembled monolayers of azobenzene-functionalized alkanethiols have been investigated by UV/visible and near edge X-ray absorption fine structure spectroscopy in combination with density-functional theory. By attaching a trifluoro-methyl end group to the chromophore both the molecular tilt and twist angle of the azobenzene moiety are accessible. Based on this detailed structural analysis the energetic shifts observed in optical reflection spectroscopy can be qualitatively described within an extended dipole model. This substantiates sizable excitonic coupling among the azobenzene chromophores as an important mechanism that hinders trans to cis isomerization in densely packed self-assembled monolayers.

Coverage- and Temperature-Controlled Isomerization of an Imine Derivative on Au(111)

Photoinduced electron transfer processes from semiconductor quantum dots (QDs) molecularly bridged to a mesoporous oxide phase are quantitatively surveyed using optical pump-terahertz probe spectroscopy. We control electron transfer rates in donor-bridge-acceptor systems by tuning the electronic coupling strength through the use of n-methylene (SH-[CH2]n-COOH) and n-phenylene (SH-[C6H4](n)-COOH) molecular bridges. Our results show that electron transfer occurs as a nonresonant quantum tunneling process with characteristic decay rates of β(n) = 0.94 ± 0.08 and β(n) = 1.25 per methylene and phenylene group, respectively, in quantitative agreement with reported conductance measurements through single molecules and self-assembled monolayers. For a given QD donor-oxide acceptor separation distance, the aromatic n-phenylene based bridges allow faster electron transfer processes when compared with n-methylene based ones. Implications of these results for QD sensitized solar cell design are discussed.

Tuning the effective spin-orbit coupling in molecular semiconductors

We present a density-functional theory trend study addressing the binding of the trans-cis conformationalswitch azobenzene C6H5-N= N-C6H5 at three coinage-metal surfaces. From the reported detailed energetic-, geometric-, and electronic-structure data we conclude that the governing factor for the molecule-surface interaction is a competition between covalent bonding of the central azo -N= N- bridge on the one hand and the surface interaction of the two closed-shell phenyl -C6H5 rings on the other. With respect to this factor the cis conformer exhibits a more favorable gas-phase geometric structure and is thus more stabilized at the studied surfaces. With the overall binding still rather weak the relative stability of the two isomers is thereby reduced at Ag111 and Au111. This is significantly different at Cu111, where the cis bonding is strong enough to even reverse the gas-phase energetic order at the level of the employed semilocal electronic exchange and correlation xc functional. While this actual reversal may well be affected by the deficiencies due to the approximate xc treatment, we critically discuss that the rationalization of the general effect of the surface on the metastable molecular states is quite robust. This should equally hold for the presented analysis of recent tip-manipulation and photoexcitation isomerization experiments from the view point of the derived bonding mechanism.

Structure and energetics of azobenzene on Ag(111): benchmarking semiempirical dispersion correction approaches.

The isomerization behavior of photochromic molecular switches is strongly influenced by adsorption on metal surfaces. For (E)-3,5-di-tert-butyl-N-(3,5-di-tert-butylbenzylidene)aniline (abbreviated as TBI for tetra-tert-butyl imine), it is found that a layer adsorbed on Au(111) can undergo an isomerization from the trans to the cis and back to the trans configuration when continuously increasing the sample temperature and accordingly decreasing the sample coverage. The conformation and adsorption geometry of TBI are determined from near-edge X-ray absorption fine structure measurements in agreement with density functional theory calculations taking into account the van der Waals interaction between adsorbate and metal surface. The coverage- and temperature-controlled conformational transitions are reversible and are driven by the higher packing density of the less stable cis-isomer in combination with the low thermal activation barrier of the trans- to cis-isomerization typical for imine derivatives. This unexpected scenario is corroborated by thermal desorption and vibrational spectroscopy as well as scanning tunneling microscopy.

Azobenzene-functionalized alkanethiols in self-assembled monolayers on gold

The control of spins and spin to charge conversion in organics requires understanding the molecular spin-orbit coupling (SOC), and a means to tune its strength. However, quantifying SOC strengths indirectly through spin relaxation effects has proven difficult due to competing relaxation mechanisms. Here we present a systematic study of the g-tensor shift in molecular semiconductors and link it directly to the SOC strength in a series of high-mobility molecular semiconductors with strong potential for future devices. The results demonstrate a rich variability of the molecular g-shifts with the effective SOC, depending on subtle aspects of molecular composition and structure. We correlate the above g-shifts to spin-lattice relaxation times over four orders of magnitude, from 200 to 0.15 μs, for isolated molecules in solution and relate our findings for isolated molecules in solution to the spin relaxation mechanisms that are likely to be relevant in solid state systems.