The single-molecule conductance of hydrogen-bonded and alkane systems are compared in this theoretical investigation. The results indicate that for short chains, the H-bonded molecules exhibit larger conductance than the alkanes. Although earlier experimental investigations attributed this observation to a large density of states (DOS) corresponding to an occupied molecular orbital below the Fermi energy, the current work indicates the presence of a Fano resonance in the transmission function in the vicinity of the Fermi energy. The inclusion of this observation is essential in understanding the behavior of these systems. We also address the characteristics of the H-bond for transport and provide an explanation for the presence of a turnover regime wherein the conductance of the alkanes becomes larger than the H-bonded systems. Incidentally, this feature cannot be explained using a simple DOS argument.

We explore a connection between the static molecular polarizability and the molecular conductance that arises naturally in the description of electrified molecular interfaces and that has recently been explored experimentally. We have tested this idea by using measured conductance of few different experimental design motifs for molecular junctions and relating them to the molecular polarizability. Our results show that for a family of structurally connected molecules the conductance decreases as the molecular polarizability increases. Within the limitations of our model, this striking result is consistent with the physically intuitive picture that a molecule in a junction behaves as a dielectric that is polarized by the applied bias, hence creating an interfacial barrier that hinders tunneling. The use of the polarizability as a descriptor of molecular conductance offers significant conceptual and practical advantages over a picture based on molecular orbitals. To further illustrate the plausibility of this idea, we have used Simmons’ tunneling model that incorporates image charge and dielectric effects to describe transport through a barrier that represents the molecular junction. In such a model, the barrier height depends on the effective dielectric constant of the electrode–molecule–electrode junction, which in turn can be approximately expressed in terms of the molecular polarizability via the classical Clausius–Mossotti relation. Despite the simplicity of our model, it sheds light on a hitherto neglected connection between molecular polarizability and conductance and paves the way for further experimental, conceptual, and theoretical developments.

Interfacial charge transfer has been an area of intense interest because of its relevance in molecular electronics, dye-sensitized solar cells, surface-enhanced Raman scattering (SERS), and photocatalysis. Although the chemical natures of both the contact and the linker have been shown to play important roles in determining the properties of hybrid dye/molecule–metal oxide complexes, little is known about the nature of the charge-transfer pathways. In this work, we explore in detail the idea that Raman enhancement and charge transfer are intimately related. To this end, we analyze the vibrational modes of molecules exhibiting the maximum enhancement of the Raman activities when they are adsorbed on semiconducting metal oxide nanoparticles. Our analysis of the potential energy distributions of these modes in the hybrid complexes indicates the significant involvement of bending and torsional modes of atoms deep within the metal oxide nanoparticle. Whereas the individual contribution of each of these oxide bending and torsional modes is very small (∼1%), their cumulative contribution (∼20–35%) is substantial. We found that the observed Raman enhancement can be correlated to changes in the magnitude of the atomic polarizabilities. More importantly, we note that there is a direct correlation between the observed Raman enhancement and the electron-transfer rates across the molecule–metal oxide interface. Although the current work is a step in our attempts to find a propensity rule connecting Raman enhancement and charge transfer through preferential modes, the involvement of the low-frequency torsional modes of the metal oxide implies that modes involving both the molecule and atoms deep inside the nanoparticle could be responsible for the bulk of charge transfer. The results of the current work are also relevant in understanding the nature of charge-transfer pathways in dye-sensitized solar cells and photoinduced catalysis. The identification of vibrational modes involved in enhancement of the Raman response could lead to interesting insights into interfacial energy transfer and thermoelectric effects in nanosystems.

Salicylate and salicylic acid derivatives act as electron donors via charge-transfer complexes when adsorbed on semiconducting surfaces. When photoexcited, charge is injected into the conduction band directly from their highest occupied molecular orbital (HOMO) without needing mediation by the lowest unoccupied molecular orbital (LUMO). In this study, we successfully induce the chemical participation of carbon dioxide in a charge transfer state using 3-aminosalicylic acid (3ASA). We determine the geometry of CO2 using a combination of ultraviolet–visible spectroscopy (UV–vis), surface enhanced Raman scattering (SERS), 13C NMR, and electron paramagnetic resonance (EPR). We find CO2 binds on Ti sites in a carbonate form and discern via EPR a surface Ti-centered radical in the vicinity of CO2, suggesting successful charge transfer from the sensitizer to the neighboring site of CO2. This study opens the possibility of analyzing the structural and electronic properties of the anchoring sites for CO2 on semiconducting surfaces and proposes a set of tools and experiments to do so.Keywords: catechol; charge-transfer; CO2 activation; SERS; TiO2;

Semiconducting oxide nanoparticles have proven to be excellent in detecting extremely low-concentrations of molecules through surface-enhanced Raman scattering (SERS) effects. While the enhancement of the Raman activities arises from a large increase in polarizability due to charge transfer from the molecule to the semiconducting nanoparticle, little is known about how the oxide composition, nanoparticle size, solvent, or pH affects the observed Raman activities. In the current study, we examine these effects by carrying out extensive computational investigations of semiconducting TiO2, SnO2 and Fe2O3 nanoparticles and their complexes with both catechol and dopamine. An increase in the size of the oxide cluster or a decrease in the pH of the system under observation leads to enhanced Raman activities; the variation of the activities in different solvents is very much dependent on the nature of the vibrational modes. The marked increase in the Raman activities of molecules adsorbed on SnO2 or Fe2O3 over that of molecules adsorbed on TiO2 seems to indicate that these oxide nanoparticles would be useful substrates for SERS sensors. Our results also indicate that the Raman activities of some of the TiO2 modes are magnified upon adsorption of molecules, which concurs with some very recent experimental observations. All these results are consistent with a recently proposed theoretical model of SERS on semiconducting substrates. Further, this work has implications on the development of molecular sensing, dye-sensitized solar cells, and photocatalysis.

The reported observation of SERS on semiconductors has confirmed the feasibility of distinguishing the charge-transfer mechanism from the electromagnetic one responsible for the enhancement of the signal in metal nanoparticles. Experimental investigation of the well characterized dopamine−TiO2 system revealed an unexpected dependence on coverage and size. We propose here a theoretical model applicable to SERS on semiconducting substrates that explains this remarkable behavior. The model is based on a competition mechanism arising from the formation of an electron gas in the conduction band of the semiconductor due to the photoexcitation of a charge-transfer complex. Taking into account the two competing effects, a linear increase in the Raman intensity arising from increasing coverage and a quenching effect due to the photon absorption by the electron gas, provides excellent agreement between our model and the experiment for 5 nm nanoparticles. Discrepancies for the case of 2 nm nanoparticles are attributed to quantum confinement, an effect that is investigated elsewhere.