Electronic Structure and Optical Properties of an Alternated Fluorene–Benzothiadiazole Copolymer: Interplay between Experimental and Theoretical Data

Abstract

The donor-acceptor copolymer containing benzothiadiazole (electron acceptor), linked to functionalized fluorene (electron donor), [poly[9,9-bis(3'-(tert-butyl propanoate))fluorene-co-4,7-(2,1,3-benzothiadiazole)] (LaPPS40), was synthesized through the Suzuki route. The polymer was characterized by scanning electron microscopy, gel permeation chromatography, NMR, thermal analysis, cyclic voltammetry, X-ray photoelectron spectroscopy, UV-vis spectrometry, and photophysical measurements. Theoretical calculations (density functional theory and semiempirical methodologies) used to simulate the geometry of some oligomers and the dipole moments of molecular orbitals involved were in excellent agreement with experimental results. Using such data, the higher energy absorption band was attributed to the π-π* (S(0) → S(4)) transition of the fluorene units and the lower lying band was attributed to the intramolecular (ICT) (S(0) → S(1)) charge transfer between acceptor (benzothiadiazole) and donor groups (fluorene) (D-A structure). The ICT character of this band was confirmed by its solvatochromic properties using solvents with different dielectric properties, and this behavior could be well described by the Lippert-Mataga equation. To explain the solvatochromic behavior, both the magnitude and orientation of the dipole moments in the electronic ground state and in the excited state were analyzed using the theoretical data. According to these data, the change in magnitude of the dipole moments was very small for both transitions but the spatial orientation changed remarkably for the lower energy band ascribed to the ICT band.

Femtosecond Two-Photon Absorption Spectroscopy of Poly(fluorene) Derivatives Containing Benzoselenadiazole and Benzothiadiazole

“…Such spectra present two absorption bands between 370–500 nm and 270–340 nm (lowest energy band—LB) for PFeBT; 390–540 nm and 270–340 nm (higher energy band—HB) for PFeBSe. According to [25], these two bands can be majorly ascribed to transitions from HOMO → LUMO + 3 and to the HOMO → LUMO (HOMO means highest occupied molecular orbital, and LUMO means lowest unoccupied molecular orbital). The last one is responsible for the charge transfer that occurs from the fluorene to the benzoselenadiazole and benzothiadiazole units.…”

“…The synthesis of PFeBT and PFeBSe (Figure 4) was performed according to the procedure reported in [25], which briefly consists of adding 0.31 mmol 4,7-dibromo-2,1,3-benzothiadiazole (or 4,7-dibromo-2,1,3-benzoselenadiazole), 0.31 mmol 2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9bis (3-(tert-butyl propanoate)) fluorene, 8 mg of Pd(Ph3P)4, and 3.7 mmol of potassium carbonate to a 25 mL flask under an Ar atmosphere. Then, a mixture of water and toluene was added to the flask, and Ar was bubbled into the system for 20 min.…”

Interplay among electronic characteristics, morphology and device efficiency in three fluorene alternated copolymers

“…This fact indicates that the transition of lowest energy H ! L, theoretically assigned in 511 nm, can be assigned to a charge transfer (ICT) state, between the F and Bz moieties, typical of DA structures [17,31], redshifted of about 66 nm in relation to experimental data. The other transition at 342 nm, related to H !…”

“…The UV-vis absorption spectrum in diluted CHCl 3 solution of P (FBz) exhibited two major absorption peaks around 320 and 445 nm, attributable to the p-p* transition of the main chain and to the formation of an intrachain charge transfer complex (ICT) between the donor (fluorene) and the acceptor (benzothiadiazole), respectively. This type of structure has been widely explored in the literature [17,18] and is typical of alternated copolymers whose components have different electro-affinities. The enhanced electron-accepting ability of the benzothiadiazole moiety is further illustrated in the Theoretical Calculations section.…”

Photophysical behavior of block copolymers containing EDOT, thiophene, and benzodiathiazole units linked to fluorene

“…In addition, they present a favorable tendency to form well‐ordered structures, due to the strong interaction between the heteroatoms and/or π–π interactions . Depending on the eletroaffinity of the vicinal group, benzodiathiazole can form an intrachain charge transfer complex (ICT), characterized by a dual absorption profile . Due to their electronic properties, all the conjugated units chosen have been successfully employed to synthesize electrochromic polymers …”