title: Theoretical study on oligothiophene N-succinimidyl esters: size and push-pull effects.
abstract: We report a theoretical study on the optical properties of bithiophene and terthiophene N-succinimidyl esters, which have been functionalized with a methylsulfanyl group in the alpha or the beta positions. Time-dependent density functional theory (TD-DFT) and approximate coupled-cluster singles and doubles with the resolution of identity technique (RI-CC2) calculations have been performed in the ground and excited states. The RI-CC2 results for absorption and fluorescence energies are in better qualitative agreement with experiments, whereas TD-DFT does not correctly describe the higher energy part of the absorption spectra of beta-substituted bithiophenes, due to the presence of charge-transfer states. Systems functionalized at the alpha position show a large red-shift of the main absorption and fluorescence band and a larger Stokes-shift compared to the unsubstituted species. These effects are in most cases less pronounced for the beta-substituted structures. In particular, we found that the Stokes-shift of the alpha-substituted structures is larger than the one of the beta-substituted species due to a more planar orientation of the methylsulfanyl group with respect to the neighbouring thiophene in the excited state.
authors.affiliation: National Nanotechnology Laboratory of CNR-INFM, Distretto Tecnologico ISUFI, Università del Salento, Via per Arnesano, I-73100, Lecce, Italy. manuelpiacenza@gmail.com
authors.forename: M
authors.initials: M
authors.lastname: Piacenza
authors.forename: M
authors.initials: M
authors.lastname: Zambianchi
authors.forename: G
authors.initials: G
authors.lastname: Barbarella
authors.forename: G
authors.initials: G
authors.lastname: Gigli
authors.forename: F
authors.initials: F
authors.lastname: Della Sala