Electronic structure and excited state dynamics in a dicyanovinyl-substituted oligothiophene on Au(111)

Abstract

Dicyanovinyl (DCV)-substituted oligothiophenes are promising donor materials in vacuum-processed small-molecule organic solar cells. Here, we studied the structural and the electronic properties of DCV-dimethyl-pentathiophene (DCV5T-Me2) adsorbed on Au(111) from submonolayer to multilayer coverages. Using a multi-technique experimental approach (low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and two-photon photoemission (2PPE) spectroscopy), we determined the energetic position of several affinity levels as well as ionization potentials originating from the lowest unoccupied molecular orbitals (LUMO) and the highest occupied molecular orbitals (HOMO), evidencing a transport gap of 1.4 eV. Proof of an excitonic state was found to be a spectroscopic feature located at 0.6 eV below the LUMO affinity level. With increasing coverage photoemission from excitonic states gains importance. We were able to track the dynamics of several electronically excited states of multilayers by means of femtosecond time-resolved 2PPE. We resolved an intriguing relaxation dynamics involving four processes, ranging from sub-picosecond (ps) to several hundred ps time spans. These show a tendency to increase with increasing coverage. The present study provides important parameters such as energetic positions of transport levels as well as lifetimes of electronically excited states, which are essential for designing organic-molecule-based optoelectronic devices.

Electronic structure of an iron porphyrin derivative on Au(1 1 1)

“…6, we summarize the level alignments with respect to the vacuum level (E vac ). As discussed previously [28,29,50], one has to consider that electronic states determined by STS are transport states because electron tunneling into unoccupied molecular states results in the formation of negative ion resonances. On the other hand, tunneling out of occupied molecular states leads to the creation of positive ion resonances.…”

Direct Imaging of the Induced‐Fit Effect in Molecular Self‐Assembly

“…This technique has been widely utilized to identify and characterize the structural and electronic details of individual molecules, including their conformation, adsorption geometry, bond order, and charge distribution . Although noncovalent bonds cannot be directly resolved in AFM images, several studies have demonstrated that the bonding structure can be precisely determined from the atomically resolved molecular arrangement . However, a detailed study of changes in geometric structure when the molecule's environment is altered by intermolecular interactions has not been addressed with AFM experimentally until now.…”

“…[26] Although noncovalent bonds cannot be directly resolved in AFM images, [27] several studies have demonstrated that the bonding structure can be precisely determined from the atomically resolved molecular arrangement. [28][29][30][31][32][33] However, a detailed study of changes in geometric structure when the molecule's environment is altered by intermolecular interactions has not been addressed with AFM experimentally until now.Here, we report that intermolecular interactions steer the molecules into assemblies, where intermolecular interactions Molecular recognition is a crucial driving force for molecular self-assembly. In many cases molecules arrange in the lowest energy configuration following a lock-and-key principle.…”

“…[26] Although noncovalent bonds cannot be directly resolved in AFM images, [27] several studies have demonstrated that the bonding structure can be precisely determined from the atomically resolved molecular arrangement. [28][29][30][31][32][33] However, a detailed study of changes in geometric structure when the molecule's environment is altered by intermolecular interactions has not been addressed with AFM experimentally until now.…”

“…DCV6T is a flexible specie because of its long thiophene backbone and the absence of side groups. Due to this flexibility, DCV6T is expected to be an ideal candidate to exhibit the induced‐fit effect by adapting its shape to its local environment. In our experiment, we found evidence of this effect in the simultaneous self‐assembly pattern of DCV6T in islands and chains, each with the molecules lying in different molecular conformations.…”