Theoretical study of spectroscopic properties of insulated molecular wires formed by substituted oligothiophenes and cross-linked α-cyclodextrin

Abstract

The inclusion compound formed between cross-linked α-cyclodextrin dimer and substituted oligothiophene, was investigated using density functional theory (DFT). Energy gap, spectroscopy (IR, UV–vis, 13C NMR, and 1H NMR) and first hyperpolarizability data were analyzed for the free species and inclusion compound, pp-PT@(αCD–αCD). The semiconducting property of the included pp-PT was not substantially affected on inclusion, with the energy gap increasing by only 10% after interaction with αCD–αCD. On the other hand, the nonlinear optical (NLO) response was significantly decreased, with the first hyperpolarizability, β, predicted to be just more than 60% lower for the [2]rotaxane than for free pp-PT, but still having considerable magnitude. This was explained by the two-state model based on the charge-transfer contribution to the electronic transitions. The sensitivity of electronic spectra might also be useful for the inclusion complex characterization. The IR spectrum was slightly sensitive to the host–guest interaction and the calculated 13C NMR and 1H NMR chemical shifts for the pp-PT guest showed appreciable variations of 5–10 and 1–1.5 ppm, respectively, and so can be used for the characterization of inclusion compounds. We concluded that the formation of inclusion complexes with CDs, seems indeed very promising and the use of encapsulating conducting material should be experimentally pursued.