Theoretical investigation of the static (hyper)polarizabilities and reorganization energy of 4,5-dicyanoimidazole chromophore and derivatives containing benzene rings and a saturated bridge

- (Hyper)polarizabilities behavior is agreement with the Valence-Bond Charge-Transfer Model.
- The reorganization energy for electron transport and hole suggesting a good charge transport balance performance.
- (Hyper)polarizability and reorganization have dependences with donor/acceptor groups and size of π-conjugated chain.

Calculations at HF, DFT (CAM-B3LYP) and MP2 level were carried out with a 6-31+G(d,p) basis set to estimate the polarizability α, the first β and second hyperpolarizability γ and reorganization energy λ of 4,5-dicyanoimidazole chromophore 1 and derivatives formed by adding a benzene ring 2, a π bond containing bridge plus a benzene ring (styryl) 3, and a biphenyl 4. Three different donors electron groups D were attached at the para position of the rings or to the chromophore. Our results showed that the 〈α〉, βtotal and 〈γ〉 values increased as D: H < OCH3 < N(CH3)2. These properties were enhanced when the intra-molecular charge transference was increased due to the substitution of double bonds by aromatic rings in the bridge or by increasing the donor strength of the D group. The increase in the (hyper)polarizability was accompanied by a decrease in the HOMO-LUMO energy gap in agreement with the Valence-Bond Charge-Transfer and sum-over-states models. The reorganization energy data showed that this parameter for electron (λe) and hole transport (λh) for the substituted chromophore (D: OCH3), molecule 3 (D: H and OCH3) and 4 (D: H) are very close to each other. Then, all them should have good charge transport balance performance. Molecule 2 (D: H) showed the smallest λe and higher λh and can be used as electron transfer material, while molecule 3[D: N(CH3)2] showed a small λh, and can be employed as hole transfer material.

For 4,5-dicyanoimidazole molecules studied, the increase in the value of the (hyper)polarizabilities as accompanied by a decrease in the HOMO-LUMO energy difference is agreement with the Valence-Bond Charge-Transfer Model and sum-over-states (SOS) expressions. All the properties showed a linear increased according to the sequences 3 > 4 > 2 > 1 when the HOMO-LUMO gap decreased. Therefore, the biphenyl π-conjugated system enhance both, the electronic delocalization between de donor group and the 4,5-dicyanoimidazole acceptor moiety (HOMO-LUMO gap decreased), and de NLO properties.