Light emission from the M-type enantiomer of a helicene derivative (2,13-bis(hydroxymethyl)[7]-thiaheterohelicene) adsorbed on the clean Au(111) and the C₆₀-covered Au(111) surfaces were investigated by tunneling-current-induced light-emission technique. Plasmon-originated light emission was observed on the helicence/Au(111) surface and it was strongly suppressed on the area where the helicene molecules were adsorbed at the edges of the Au(111) terraces. To avoid luminescence quenching of excited helicene molecules and to suppress strong plasmon light emission from the Au(111) surface, C₆₀ layers were used as decoupling buffer layers between helicene molecules and the Au(111) surface. Helicene molecules were adsorbed preferentially on the Au(111) surface rather than on the C₆₀ buffer layers due to the small interaction of the molecules and C₆₀ islands. This fact motivated us to deposit a multilayer of helicene molecules onto the C₆₀ layers grown on the Au(111) surface, leading to the fact that the helicene/C₆₀ multilayer showed strong luminescence with the molecules character. We consider that such strong light emission from the multilayer of helicene molecules has a plasmon origin strongly modulated by the molecular electronic states of (M)-[7]TH-diol molecules.

Triplet excited states in organic molecules are generally optically inactive due to spin-forbidden transitions to the singlet ground state. Recent studies have extensively investigated triplet states to circumvent limitations imposed by spin statistics in organic light-emitting diodes. Room-temperature electrophosphorescence and electrofluorescence have previously been observed in thienyl-substituted phenazines without heavy metals; however, phosphorescence was absent when spin density was localized on the thiophene units, despite the expected heavy-atom effect from the sulfur atom. In this study, we investigate the spin dynamics of thiophene-fused π-systems lacking phenazine units to enable the effective utilization of triplet states in thiophene-containing π-systems. This investigation reveals delayed emission in the excited dimers of fused thiophenes, likely associated with triplet–triplet annihilation. Optical and EPR measurements, together with quantum chemical calculations, indicate that the fused thiophenes form head-to-tail dimers in both the ground and excited states.

We investigated the adsorption and aggregation properties of 2,13-bis(hydroxymethyl)-[7]thiaheterohelicene ([7]TH-diol) on the Ag(111) surface by scanning tunneling microscopy (STM) and molecular dynamics (MD) simulation. STM observation revealed that both racemic and enantiopure [7]TH-diol formed apparently similar “zigzag” chain structures. To elucidate the molecular arrangements in these structures, MD simulation successfully differentiates the formation mechanisms of these structures, demonstrating that hetero-chiral chains are stabilized primarily by van der Waals forces, whereas homo-chiral chains are stabilized through hydrogen bonding. The formation of homo-chiral chains is driven by the alignment of hydroxymethyl groups between the neighboring molecules, whereas the steric hindrance of helical skeletons affects chain growth. These findings highlight the critical role of inter-molecular interactions—particularly hydrogen bonding—in the self-assembly of helicene molecules.

The racemic mixture of [7]thiaheterohelicene is a chiral π-conjugated system in which heteroatom substitution modulates intermolecular interactions and coupling to metal surfaces. Here, we investigate the coverage-dependent self-assembly and electronic structure of [7]thiaheterohelicene on Au(111) using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), X-ray photoelectron spectroscopy (XPS), and ultraviolet–visible spectroscopy (UV–Vis). Stepwise thermal deposition produces a sequence of supramolecular phases, evolving from isolated molecules and small aggregates at low coverage to extended, densely packed molecular overlayers at high coverage. STM reveals that adsorption is influenced by the Au(111)-(22 × √3) herringbone reconstruction, which acts as an anisotropic template for molecular stabilization, diffusion, and nucleation while remaining largely preserved beneath the assemblies. XPS confirms that rac-[7]TH molecules remain intact and adsorb via weak, site-dependent physisorption without C–S bond cleavage and reveal distinct sulfur environments associated with different adsorption environments on the surface. STS measurements of the compact molecular layer yield an apparent quasiparticle gap of ~4.2 eV, which is larger than the 3.1 eV optical gap measured in solution due to the different nature of tunneling and optical excitations as well as substrate-induced screening effects.