Structural and Electronic Properties of Poly(9,9-dialkylfluorene)-Based Alternating Copolymers in Solution: An NMR Spectroscopy and Density Functional Theory Study

Abstract

The structural and electronic properties of three alternating poly(9,9-dialkylfluorene) copolymers, poly[2,7-(9,9-bis(octyl)-fluorene)-alt-benzothiadiazole] (F8BT), poly[2,7-(9,9-bis(2′-ethylhexyl)-fluorene)-alt-thiophene S,S-dioxide] (PFTSO2), and poly[2,7-(9,9-bis(2′-ethylhexyl)-fluorene)-alt-1,4-phenylene] (PFP), containing, respectively, benzothiadiazole (BT), thiophene S,S-dioxide, and phenylene groups, have been investigated and compared to those of the homopolymer poly[2,7-(9,9-bis(2′-ethylhexyl)-fluorene)] (PF2/6). The NMR spectra and corresponding shielding tensors of the 1H and 13C nuclei have been studied in solution and are interpreted by density functional theory (DFT) with complete geometry optimization using the B3LYP functional. Particular emphasis is placed on the conformational behavior and electronic properties in the electronic ground state. In addition, time-dependent DFT is applied to obtain detailed insight into the character of selected excited states. A new TDDFT interpretation is presented for optical absorption spectra of singlet and triplet states that have previously been reported for these fluorene-based conjugated copolymers using photoexcitation and pulse radiolysis-energy transfer studies. As well as providing detailed assignment of excited states, the results show that the triplet excitation is slightly more localized than S1 excitation, in agreement with experimental observations. The DFT analysis provides a link between structure and NMR, optical, electronic properties, allowing optimization of the potential of such polymers for photovoltaic and electroluminescence applications.