Molecular Ion Formation by Photoinduced Electron Transfer at the Tetracyanoquinodimethane/Au(111) Interface

Optically induced processes in organic materials are essential for light harvesting, switching, and sensor technologies. Here we studied the electronic properties of the tetracyanoquinodimethane­(TCNQ)/Au(111) interface by using two-photon photoemission spectroscopy. For this interface we demonstrated the lack of charge-transfer interactions, but we found a significant increase in the sample work function due to UV-light illumination, while the electronic structure of the TCNQ-derived states remain unaffected. Thereby the work function of the interface can be tuned over a wide range via the photon dose. We assigned this to a photoinduced metal-to-molecule electron transfer creating negative ions. The electrons are bound by a small potential barrier. Thus thermal activation reverses the process resulting in the original work function value. The presented photoinduced charge transfer at the TCNQ/Au(111) interface can be used for continuous work function tuning across the substrate’s work function, which can be applied in device-adapted hole-injection layers or organic UV-light sensors

Hot Excitons Increase the Donor/Acceptor Charge Transfer Yield

Understanding the photoinduced ultrafast charge transfer (CT) dynamics across the donor/acceptor interface is a prerequisite for optimizing the performance of organic photovoltaic devices. Time-resolved second harmonic generation, an interface-sensitive probe with femtosecond temporal resolution, is applied to investigate the well-defined single heterojunction C₆₀/P3HT. The de-excitation of hot singlet excitons in the conduction bands of the polymer into localized excitonic states is observed. In the presence of the electron acceptor, the ultrafast population of a CT state is identified as the dominating relaxation channel. Interestingly, the charge transfer yield correlates with the excitation wavelength and rises with increasing excess energy

Optically Induced Inter- and Intrafacial Electron Transfer Probed by Two-Photon Photoemission: Electronic States of Sexithiophene on Au(111)

Using two-photon photoemission spectroscopy, we investigated the electronic structure of the organic semiconductor α-sexithiophene (6T) adsorbed on Au(111). Beside the quantitative determination of the energetic position of electronic states originating from the highest occupied molecular orbitals (HOMO and HOMO-1) and the lowest unoccupied molecular orbitals (LUMO and LUMO+1), a localized exciton state that possesses a binding energy of 0.9 eV has been identified. Whereas the creation of the exciton is the result of an intramolecular excitation involving a HOMO−LUMO transition, the transient population of the LUMO and LUMO+1 follow from an optically induced charge transfer from the metallic substrate to the molecule. The present study provides important parameters such as the energetic position of the transport level and the exciton binding energy, which are needed to understand the physics in organic-molecules-based optoelectronic devices

Tracking and Removing Br during the On-Surface Synthesis of a Graphene Nanoribbon

The fabrication of graphene nanoribbons (GNRs) requires a high degree of precision due to the sensitivity of the electronic structure on the edge shape. Using Br-substituted molecular precursors, this atomic precision can be achieved in a thermally induced two-step reaction following Br dissociation on a Au(111) surface. Using DFT, we find evidence that the Br atoms are bound to the intermediate polyanthrylene chains. We employ temperature-programmed desorption to demonstrate the associative desorption of HBr and molecular hydrogen during the final cyclodehydrogenation step of the reaction. Both processes are found to have similar activation barriers. Furthermore, we are able to remove Br atoms from the polyanthrylene chains by providing molecular hydrogen. The subsequent formation of GNR via a cyclodehydrogenation demonstrates that Br does not influence this part of the overall reaction

Photoisomerization of an Azobenzene on the Bi(111) Surface

Modifying surface-bound molecular switches by adding side groups is an established concept for restoration of functionality which a molecule possesses in solution and which is often quenched upon adsorption. Instead of decoupling the photochromic unit from the substrate, we follow a different approach, namely treating the complete molecule–substrate system. We use photoelectron spectroscopies to determine the energetic positions of the frontier orbitals of di-<i>m</i>-cyanoazobenzene on Bi(111) and to elucidate the isomerization mechanism which is stimulated by a substrate-mediated electron transfer process

Reversible Photoswitching of the Interfacial Nonlinear Optical Response

Incorporating photochromic molecules into organic/inorganic hybrid materials may lead to photoresponsive systems. In such systems, the second-order nonlinear properties can be controlled via external stimulation with light at an appropriate wavelength. By creating photochromic molecular switches containing self-assembled monolayers on Si(111), we can demonstrate efficient reversible switching, which is accompanied by a pronounced modulation of the nonlinear optical (NLO) response of the system. The concept of utilizing functionalized photoswitchable Si surfaces could be a way for the generation of two-dimensional NLO switching materials, which are promising for applications in photonic and optoelectronic devices

Influence of Core Substitution on the Electronic Structure of Benzobisthiadiazoles

Benzobisthiadiazoles (BBTs) are promising organic semiconductors for applications in field effect transistors and solar cells since they possess a strong electron-accepting characteristic. Thereby, the electronic structure of organic/metal interfaces and within thin films is essential for the performance of organic electronic devices. Here, we study the structural and electronic properties of two BBTs, with different core substitution patterns, a phenyl (BBT-Ph) and a thiophene (BBT-Th) derivative adsorbed on Au(111) using vibrational and electronic high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. In the mono- and multilayer, both BBTs adopt a planar adsorption geometry with the molecular backbone, as well as the phenyl and thiophene side groups are oriented parallel to the gold substrate. The energies of the lowest excited electronic singlet states (S) and the first triplet state (T1) are determined. The optical gap (S0 → S1 transition) is found to be 2.2 eV for BBT-Ph and 1.6 eV for BBT-Th. The energy of T1 is identified to be 1.2 eV in BBT-Ph and in the case of BBT-Th 0.7 eV. Thus, both the optical gap size as well as the T1 energy are drastically reduced in BBT-Th compared to BBT-Ph. Based on our quantum chemical calculations, this is attributed to the electron-rich nature of the five-membered thiophene rings in conjunction with their preference for planar geometries. Variation of the substitution pattern in BBTs opens an opportunity for tailoring their electronic properties