Photoluminescence studies of oligothiophene self-assembled monolayers at low excitation energy

Polymorphism in self-assembled terphenylthiolate monolayers on Au(111)

Self-assembled monolayers (SAMs) of terphenylthiol (TPT) on gold Au(111) substrates exhibit well-ordered structures over large scales if they are annealed in ethanol at 40 °C after their formation. Using high-resolution STM, two distinct, ordered phases could be observed. The simpler phase, designated as α-phase, consists of closely packed molecules in the well-known (2√3 × √3)R30° structure. It could be demonstrated that under less suitable imaging conditions this phase can be mistaken as the hexagonal (√3 × √3)R30°, which resolves a discrepancy in between previous reports. The second phase is characterized by a stripe pattern with a periodicity of 2.0 nm and can be described by a point-on-line incommensurate (4 × n) ([Formula: see text]) lattice with n close to 8. This β-phase contains four pairs of terphenylthiolate molecules, which might be held together by either disulfide bonds or the recently discussed S-Au-S motif, and is thus 35% ± 15% less densely packed than the α-phase. The coexistence of these phases explains the variability of spectroscopic results obtained in the past for terphenylthiolate layers, since their relative proportion determines the average thicknesses/tilt angles found in these studies.

Stacking of oligo- and polythiophene cations in solution: Surface tension and dielectric saturation

The stacking of positively charged (or doped) terthiophene oligomers and quaterthiophene polymers in solution is investigated applying a recently developed unified electrostatic and cavitation model for first-principles calculations in a continuum solvent. The thermodynamic and structural patterns of the dimerization are explored in different solvents, and the distinctive roles of polarity and surface tension are characterized and analyzed. Interestingly, we discover a saturation in the stabilization effect of the dielectric screening that takes place at rather small values of epsilon(0). Moreover, we address the interactions in trimers of terthiophene cations, with the aim of generalizing the results obtained for the dimers to the case of higher-order stacks and nanoaggregates.