Optical Spectroscopy of Isolated and Aggregate Hexabenzocoronene Derivatives:  A Study of Self-Assembling Molecular Nanowires

Abstract

The low, medium, and high concentration luminescence and luminescence-excitation spectra for alkyl substituted hexa-peri-hexabenzocoronene (HBC-C 8,2 ) and hexa(4-n-dodecylphenyl) substituted hexa-perihexabenzocoronene (HBC-PhC 12 ) are presented. A study of the concentration dependence of the optical properties of these self-assembling molecular nanowires, in the low to medium concentration regime, associates the spectrum at ∼10 -13 M with the isolated molecule and indicates that previously published spectra of HBC's by others were the product of aggregation phenomena. The insertion of an exo-phenyl group between the HBC core and the alkyl side chains, as in HBC-PhC 12 , was found not to extend the conjugation but did increase the inhomogeneous broadening of the isolated molecule luminescence. The continued presence of HBC-PhC 12 isolated molecules, at high concentration, implies that HBC-C 8,2 aggregates are thermodynamically more stable than HBC-PhC 12 aggregates. This is further supported by the calculated values for the Coulombic ground-state binding energy (W), the Coulombic excited state-ground-state interaction energy (W′) and the resonance interaction energy (β) for the aggregates of both derivatives (HBC-C 8,2 : W ≈ 0 eV, W′ ≈ 0.16 eV, and β ) 0.27 ( 0.02 eV; HBC-PhC 12 : W ≈ 0 eV, W′ ≈ 0 eV, and β ) 0.23 ( 0.02 eV). In the excited state, the exo-phenyl groups of HBC-PhC 12 were found to enhance the aggregate intermolecular vibronic coupling despite sterically hindering the full aggregation in the ground state. In conclusion, the spectroscopic properties of both derivatives were found to be very sensitive to aggregation at low concentration and strongly correlated to the observed macroscopic physical properties.

Self-assembled graphitic nanotubes with one-handed helical arrays of a chiral amphiphilic molecular graphene

“…For example, a MeTHF solution of (S)-2 (3 mg͞ml) at 50°C showed an absorption band centered at 360 nm, characteristic of highly dispersed HBCs. When the solution was allowed to cool to 20°C, the resultant suspension displayed a decrease in intensity of this absorption band and an appearance of red-shifted absorption bands at 398 and 421 nm, characteristic of -stacked HBC units (30,31). The hot solution of (S)-2, when rapidly cooled to 20°C, showed a time-dependent spectral change with an isosbestic point at 382 nm, which subsided within 30 min (Fig.…”

About Supramolecular Assemblies of π-Conjugated Systems

“…277 The additional aromatic interactions in 122 and the out-of-plane orientation of the peripheral phenyl rings induces a helical orientation of the HBC cores giving a helical crystalline phase with a higher persistence length. 278 Spin-cast films of 122 revealed arrays of uniform parallel nanoribbons with lengths of 300 nm as studied by AFM ( Figure 42). Slow evaporation of the solvent resulted in long, isolated regular ribbons 3.8 nm in height and 21 nm in width representing parallel single columns with the columnar axis oriented parallel to the substrate.…”

Chiralitätsverstärkung in dynamischen supramolekularen Aggregaten

“…Erschwerend wirkt auch, dass die HBC-Moleküle in Lösung leicht um die Säulenachse rotieren. [71] Daher werden zusätzlich spezifische Wechselwirkungen benötigt, um die relative Anordnung der Moleküle festzulegen, wie es beispielsweise in den Komplexen von carboxysubstituierten HBCs mit Poly(l-lysin) der Fall ist, die in der Mesophase eine helikale Überstruktur einnehmen. [72] Bisher wurde eine Chiralitätsverstärkung nur für amphiphile HBC-Derivate beobachtet (Abbildung 8 a).…”