The goal of this study is to shed light on the charge-transfer (CT) mechanism of surface-enhanced Raman scattering (SERS) by considering the properties of CT excited states. The calculations have been done by means of an excited-state gradient approximation for a pyridine molecule interacting with a silver cluster, and provided a satisfactory improvement in comparison to previous work. The effect of electrode potential on the SERS-CT spectra has been modelled theoretically by applying an external electric field for selected CT transitions and the enhancement of the ν and ν modes and a decline in the intensity of the ν mode under a negative electric field (which is directed toward the cluster) have been observed. These results match well with the experimental studies and also explain the effect of electrode potentials on the patterns of spectra, as experimental evidence of the CT mechanism. Finally, this study demonstrated that the excited state vector gradient can be used as a distinguishing factor to explain the SERS selection rules.

The intrinsic challenge in the elucidation of charge-transfer surface-enhanced Raman scattering (SERS) has inspired the present study. It is believed that changing the surface may serve as illustrative evidence for studying the influence of the chemical nature and electronic structure of the substrate on the resonance and nonresonance chemical mechanism of SERS. With the aim of investigating the important parameters which are effective on the ground- and excited-state properties, in this work we have focused on changing the composition of the substrate. Therefore, 6- and 20-atom pure as well as bimetallic silver and gold clusters have been used as models, and the adsorption of pyridine on these clusters has been studied. It has been found that through the nonresonant chemical mechanism, the Au adsorption site becomes favorable. Therefore, the static chemical enhancement of the Au binding site is more than that of Ag, and it is related to the greater binding energy of gold. To determine the effect of the surface on the resonance chemical mechanism, we have calculated the relative intensity of the pyridine–metal cluster on the charge-transfer (CT) resonance condition. The relative intensities of simulated spectra match well with the available experimental results and suggest that changing the surface, which reveals the trend by applying negative potential on a given surface, could be explained by variation of the effective charge of the cluster. These calculations also show the importance of variation of the excited-state vector gradient and dimensionless displacement for different surfaces and their effects on the selection rules. An illuminating insight about the absolute SERS-CT intensity has been provided that shows the higher average scattering cross sections in the order of 105 for binding through Ag in comparison to 103 for binding through Au. In addition, the enhancement factor for the silver binding site has been obtained as 104 in comparison to that of gold, which is 102. This factor is the intermediate value of 103 for alloys, which confirms the idea that alloying improves the enhancement factor of gold.

The prospect of challenges in reproducing and interpretation of resonance Raman properties of molecules interacting with metal clusters has prompted the present research initiative. Resonance Raman spectra based on the time-dependent gradient approximation are examined in the framework of density functional theory using different methods for representing the exchange-correlation functional. In this work the performance of different XC functionals in the prediction of ground state properties, excitation state energies, and gradients are compared and discussed. Resonance Raman properties based on time-dependent gradient approximation for the strongly low-lying charge transfer states are calculated and compared for different methods. We draw the following conclusions: (1) for calculating the binding energy and ground state geometry, dispersion-corrected functionals give the best performance in comparison to ab initio calculations, (2) GGA and meta GGA functionals give good accuracy in calculating vibrational frequencies, (3) excited state energies determined by hybrid and range-separated hybrid functionals are in good agreement with EOM-CCSD calculations, and (4) in calculating resonance Raman properties GGA functionals give good and reasonable performance in comparison to the experiment; however, calculating the excited state gradient by using the hybrid functional on the hessian of GGA improves the results of the hybrid functional significantly. Finally, we conclude that the agreement of charge-transfer surface enhanced resonance Raman spectra with experiment is improved significantly by using the excited state gradient approximation.

In the present work, a series of aryl‐cycloplatinated(II) complexes with general formula [Pt(C^N)(Ar)(κ1‐dppm)], 1, [C^N = 7,8‐benzoquinolinyl (bzq) or 2‐phenylpyridinyl (ppy); Ar = C6F5 or p‐MeC6H4, dppm = 1,1‐bis(diphenylphosphanyl)methane] was employed in the reaction with AuCl(SMe2) in order to generate heterobimetallic PtII‐AuI complexes, [Pt(C^N)(Ar)(µ‐dppm)Au(Cl)], 2, featuring a dppm bridge between the metal centers. The expectation was to induce metallophilic character into the excited state and to reduce non‐radiative deactivation pathways of the dangling auxiliary κ1‐dppm ligand through molecular motions, to improve the photophysical properties. After characterization of the new complexes by means of NMR spectroscopy and X‐ray crystallography technique, the photophysical properties of all the complexes were investigated by UV/Vis and photoluminescence spectroscopy. Both of the monometallic complexes and heterobimetallic products have shown to be luminescent in different states and temperature conditions. However, by addition of AuI, the impact on the photophysics of the heterobimetallic products in relation to the precursors with dangling dppm is minimal, a finding which can be attributed to the absence of a PtII‐AuI bond in these compounds. Indeed, the character of the excited states of the monomer PtII complexes and their corresponding bimetallic PtII‐AuI ones are similar, as confirmed by density functional theory (DFT) and time resolved DFT (TD‐DFT) calculations. The cytotoxic activities of the compounds along with that of [ClAu(µ‐dppm)AuCl] were evaluated against human breast cancer (MCF‐7), human lung cancer (A549), human ovarian cancer (SKOV3) and non‐tumorigenic epithelial breast (MCF‐10A) cell lines. The highest activity was found for the heterometallic Pt‐Au species, suggesting a cooperative effect of both metallic fragments. The most cytotoxic compound, i.e. [Pt(bzq)(p‐MeC6H4)(µ‐dppm)Au(Cl)], 2b, effectively causes cell death in MCF‐7 cancer cell line by inducing apoptosis. Fluorescence microscopy experiments for 2a were performed.

We appreciate Aranda's comments on our recent work entitled ''Elucidation of charge-transfer SERS selection rules by considering the excited state properties and the role of electrode potential''. We would also like to thank the editor of Physical Chemistry Chemical Physics for giving us an opportunity to specify more details of our work in this reply. An important part of our article concerns the role of the electrode potential in charge-transfer SERS spectra and we would like to first address the questions that Aranda et al. posed about our labeling.