Optical spectroscopy of isolated and aggregate hexabenzocoronene derivatives: A study of self-assembling molecular nanowires

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-peri-hexabenzocoronene (HBC-PhC₁₂) 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-PhC₁₂, was found not to extend the conjugation but did increase the inhomogeneous broadening of the isolated molecule luminescence. The continued presence of HBC-PhC₁₂ isolated molecules, at high concentration, implies that HBC-C_8,2 aggregates are thermodynamically more stable than HBC-PhC₁₂ 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-PhC₁₂: W ≈ 0 eV, W' ≈ 0 eV, and β = 0.23 ± 0.02 eV). In the excited state, the exo-phenyl groups of HBC-PhC₁₂ 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.