CCL: About virtual orbitals in oligomeric pi-conjugated system

 Sent to CCL by: "Daniel  Morales Salazar"
 [danielmoralessalazar91[*]gmail.com]
 Dear all,
 I am performing calculations on a pi-conjugated monomer system (83 atoms
 of which 43 are hydrogens) with a Donor-Acceptor-Donor type of electronic
 structure. At the B3LYP/6-311G(d,p) level of theory the structural and
 optical parameters compared well with experimental X-ray data and UV-Vis-
 NIR spectroscopy. In the monomer case the HOMO density is located on the
 Donor fragments whereas the LUMO density is found on the acceptor units,
 naturally.
 I started modelling oligomeric systems, up to the tetramer. In this
 latter one, for example, I noticed that the LUMO, LUMO+1 LUMO+2 LUMO+3
 were localized on different acceptor units. The same was true for the
 dimer (LUMO, LUMO+1) and trimer (LUMO, LUMO+1,LUMO+2). At first sight ,
 these orbital densities made me think that the role of the acceptor was
 "lost" upon increasing the conjugation length/size.
 However, when I looked at the energies of these frontier virtual
 orbitals, the differences on the dimer (LUMO to LUMO+1), trimer (LUMO to
 LUMO+2) and tetramer (i.e. LUMO to LUMO+3) are very low. Less than 0.1 eV
 in the tetramer system.
 Since these energy differences are so low, can I consider all of these
 virtual orbitals (i.e. LUMO to LUMO+3 in the tetramer model) to be an
 intrinsic part of the molecular electronic structure of the system; that
 is of course, understanding the fact that they are unoccupied orbitals,
 and what I mean by intrinsic is that either three of them may be
 accessible and/or act as acceptors. For example if a donor of the right
 symmetry or energy approaches the molecular motif; or if there is an
 intramolecular charge transfer with the right symmetry. Therefore, one
 could conclude that the role of the acceptor fragments remained intact in
 the oligomers as compared the monomers.
  Is this interpretation a valid one?
 I would like some confirmation as I have read the virtual orbitals
 obtained in DFT have no physical significance (apart from the HOMO, and
 perhaps LUMO).
 Thank you very much for your help and I hope the question was somehow
 clear.
 Sincerely,
 Daniel Morales Salazar