Molecular orbital assistance in the design of intramolecular and photoinduced electron transfer systems
Conventional DFT and TDDFT implementations are not very successful when applied to charge-transfer states. For photoinduced electron transfer the excited absorbing state is in resonance with the charge-transfer state
A great deal of insight can be gained for charge-transfer systems from simple molecular orbital energy diagrams of the separate donor and acceptor moieties and this can be of great value for nano-hybrids where quantitative information on the charge-transfer states is still an impossible task.
Example1: a PET sensor for OP nerve agents
In terms of orbital diagrams the photoinduced charge transfer (or electron transfer) is indicated by the existence of degeneracy between the D→D* and D→A* excitations: (I. D. Petsalakis et al. J. Mol. Struct. (THEOCHEM) 867 (2008) 64
Figure 2: Orbital levels for the tertiary amine-pyrene sensor
A theoretical approach has been proposed for the design of donor-acceptor ICT systems and for photoinduced electron transfer (PET) hybrids of fullerene, based on orbital level diagrams of the separate donor and acceptor moieties. , (I. D. Petsalakis and G. Theodorakopoulos, Chemical Physics Letters 525–526 (2012) 105–109)
Minimization of the HOMO-LUMO gap in ICT systems translates to a requirement for near degeneracy of the HOMO of the donor and LUMO of the acceptor
Figure 3. Orbital level diagram for the TCBD-TTF model system
Similarly, near degeneracy of the LUMO of the donor and LUMO of the acceptor indicates the possibility of PET in the combined hybrid
Figure4: Orbital energies of CN-substituted pyrene and comparison with the levels of C60 fullerene
