The low, medium and high concentration luminescence and luminescence-excitation spectra for alkyl substituted hexa-peri-hexabenzocoronene (HBC-C8,2) 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 single 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 single molecule luminescence. The continued presence of HBC-PhC12 single molecules, at high concentration, implies that HBC-C8,2 aggregates are thermodynamically more stable than HBC-PhC12 aggregates. 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.
The vibronic structure of the luminescence and luminescence-excitation spectra of alkyl substituted hexa-peri-hexabenzocoronene (HBC-C8,2) and hexa(4-n-dodecylphenyl) substituted hexa-peri-hexabenzocoronene (HBC-PhC12) are described and explained in terms of the collective behavior of molecules in a molecular nanowire structure. The low concentration species of HBC-C8,2(10-13 M) was found to have a homogeneouly broadeend emission expected for an isolated molecule. At medium (10-8 M) and high (10-6 M) concentration the HBC moelcules aggregate into nanowires and a change in the vibronic and electronic structure is observed. The addition of exo-phenyl groups, as in the case of HBC-PhC12, was found to increase the configurational coordinate displacement in the photo-exicted state by increasing the intermolecular vibronic coupling. These result coroelate well with the observed reduced photo-luminescence efficiency of the HBC-PhC12 nanowires.