A comparative study of one and two dimensional π-conjugated systems

• Electronic structure of 2D systems depends on the connection density.
• 2D systems with high connection density can possess high spin ground states.
• 2D systems with high connection density can possess high spin ground states.
• 2D conjugated systems have higher ionization potentials and smaller band gap.
• Reorganization energies for both 1D and 2D systems drop linearly with 1/n.

The comparative study of one (1D) and two dimensional (2D) π-conjugated systems has been carried out at D3 dispersion corrected B3LYP and restricted active space (RAS) levels of theory using oligomer approach. Two sets of systems differeing by the connection density have been studied. The first one is poly-m-phenylene (1D-PMP) and the corresponding 2D analogue, porous graphene (2D-PMP). The second one is trans-polyacetylene (1D-PA) and graphene nanoflakes(2D-PA). Both 1D- and 2D-PMP have closed shell singlet ground state. 2D-PMP systems have higher ionization potential (IP), electron affinity (EA) and lower band gap (Eg) than 1D-PMP of the similar size. The ground state of 1D-PA is a single reference singlet while the nature of 2D-PA ground state depends strongly on the size. The small members of 2D-PA series have singlet ground states (single- or multireferencial). The larger members have single reference high spin ground states with number of unpaired electrons increasing with their size and reaching 12 for the largest member of 2D-PA series according to both B3LYP and RAS methods. Similar to PMP, Eg decreases with size more rapidly for 2D-PA than for 1D-PA and ionization potentials are higher for 2D-PA of the similar size.Unlike 1D-PMP and 2D-PMP the electron affinities of 1D-PA are consistently higher then these of 2D-PA.