Molecular design of photoactive acenes for organic photovoltaics

Exploring the Adsorption Mechanism of Tetracene on Ag(110) by STM and Dispersion-Corrected DFT

Self-assembled strategy has been proven to be a promising vista in constructing organized low-dimensional nanostructures with molecular precision and versatile functionalities on solid surfaces. Herein, we investigate by a combination of scanning tunneling microscopy (STM) and dispersion-corrected density functional theory (DFT), the adsorption of tetracene molecules on the silver substrate and the mechanism mediating the self-assembly on Ag(110). As expected, ordered domain is formed on Ag(110) after adsorption with adjacent molecules being imaged with alternating bright or dim pattern regularly. While such behavior has been assigned previously to the difference of molecular adsorption height, herein, it is possible to investigate essentially the mechanism leading to the periodic alternation of brightness and dimness for tetracene adsorbed on Ag(110) thanks to the consideration of Van der Waals (vdW) dispersion force. It is demonstrated that the adsorption height in fact is same for both bright and dim molecules, while the adsorption site and the corresponding interfacial charge transfer play an important role in the formation of such pattern. Our report reveals that vdW dispersion interaction is crucial to appropriately describe the adsorption of tetracene on the silver substrate, and the formation of delicate molecular architectures on metal surfaces might also offers a promising approach towards molecular electronics.

Excited-State Studies of Polyacenes: A Comparative Picture Using EOMCCSD, CR-EOMCCSD(T), Range-Separated (LR/RT)-TDDFT, TD-PM3, and TD-ZINDO

The low-lying excited states (La and Lb) of polyacenes from naphthalene to heptacene (N = 2-7) are studied using various time-dependent computational approaches. We perform high-level excited-state calculations using equation of motion coupled cluster with singles and doubles (EOMCCSD) and completely renormalized equation of motion coupled cluster with singles, doubles, and perturbative triples (CR-EOMCCSD(T)) and use these results to evaluate the performance of various range-separated exchange-correlation functionals within linear-response (LR) and real-time (RT) time-dependent density functional theories (TDDFT). As has been reported recently, we find that the range-separated family of functionals addresses the well-documented TDDFT failures in describing these low-lying singlet excited states to a large extent and are as about as accurate as results from EOMCCSD on average. Real-time TDDFT visualization shows that the excited state charged densities are consistent with the predictions of the perimeter free electron orbital (PFEO) model. This corresponds to particle-on-a-ring confinement, which leads to the well-known red-shift of the excitations with acene length. We also use time-dependent semiempirical methods like TD-PM3 and TD-ZINDO, which are capable of handling very large systems. Once reparametrized to match the CR-EOMCCSD(T) results, TD-ZINDO becomes roughly as accurate as range-separated TDDFT, which opens the door to modeling systems such as large molecular assemblies.

Sequestration and selective oxidation of carbon monoxide on graphene edges

The versatility of carbon nanostructures makes them attractive as possible catalytic materials, as they can be synthesized in various shapes and chemically modified by doping, functionalization, and the creation of defects in the nanostructure. Recent research has shown how the properties of carbon nanostructures can be exploited to enhance the yield of chemical reactions such as the thermal decomposition of water (Kostov et al 2005 Phys. Rev. Lett. 95) and the dissociation of methane into carbon and hydrogen (Huang et al 2008 J. Chem. Phys. at press). In this work, we consider the carbon-mediated partial sequestration and selective oxidation of carbon monoxide (CO), both in the presence and absence of hydrogen. Using first-principles calculations we study several reactions of CO with carbon nanostructures, where the active sites can be regenerated by the deposition of carbon decomposed from the reactant (CO) to make the reactions self-sustained. Using statistical mechanics, we also study the conditions under which the conversion of CO to graphene and carbon dioxide is thermodynamically favorable, both in the presence and in the absence of hydrogen. These results are a first step toward the development of processes for the carbon-mediated partial sequestration and selective oxidation of CO in a hydrogen atmosphere.