Tuning the Light Absorption of Donor–Acceptor Conjugated Polymers: Effects of Side Chains and ‘Spacer' Units in Thieno[3,4-b]pyrazine-Fluorene Copolymers

Abstract

Four donor–acceptor (D-A) copolymers of 2,3-difunctionalized thieno[3,4-b]pyrazine (TP) and 9,9-dioctylfluorene were prepared in order to illustrate the ability to tune the electronic and optical properties of TP-based D-A polymers by the choice of side chains on the TP unit. In addition, analogous polymers were prepared with and without thiophene spacer units in order to allow discussion of their effect on the material properties. This combination of effects allowed tuning of the band gap over the range of 1.6–2.1 eV. Full optical and electrochemical characterization is reported for all materials, as well as initial photovoltaic device data for representative materials.

Minimizing Polymer Band Gap via Donor‐Acceptor Frameworks: Poly(dithieno[3,2‐b:2′,3′‐d]pyrrole‐alt‐thieno[3,4‐b]pyrazine)s as Illustrative Examples of Challenges and Misconceptions

“…Here, however, this is perhaps expected as the strong electron‐donating nature of the alkoxy side chains result in significant destabilization of the LUMO, which could move the reduction to more negative potentials such that it would now lie outside the solvent window. Such a significant shift to more negative potentials is also consistent with other previously reported TP polymers containing alkoxy side chains …”

“…In contrast, the alkoxy‐derived P3 exhibits a significant blue‐shifted absorbance in both solution and the solid‐state. While this may be partially due to the lower molecular weight of P3 , such a blue shift is also consistent with the trends observed for other alkoxy‐functionalized TP materials, in which the electron‐donating alkoxy groups destabilize the polymer LUMO to a greater extent than any analogous shifts in the polymer HOMO, thus resulting in an increase in the corresponding band gap . Extrapolation of the low‐energy edge of the thin film spectrum of P3 gives an E g value of ca.…”

“…As with P2 , P3 exhibited good solubility, but provided lower molecular weight materials (M n =3600, PDI=1.7). As such, the reduction in molecular weight does not seem to be a limit of solubility and the M n value here is similar to other polymers of dialkoxy TP units (i. e. M n =2700–4800) . The lower molecular weights typical for these materials is attributed to reduced reactivity of the alkoxy derivative to oxidative addition, as the strong electron‐donating groups of 3 c should significantly reduce its rate of oxidative addition …”

“…The view of equivalent mixing of the HOMO and LUMO levels is just one of a number of assumptions and misconceptions that have developed in applications of the D‐A model, the most basic of which is that monomeric units act exclusively as donors, acceptors, or neutral ′spacers′ (often viewed as playing no significant role in the respective E g ) . At odds with the common view regarding the singular nature of monomeric building blocks, however, have been reports revealing that a class of commonly applied acceptors, thieno[3,4‐ b ]pyrazines (TP, Figure ), also exhibit properties of very strong donors equivalent to that of 3,4‐ethylenedioxythiophene (EDOT) . As TP monomers act simultaneously as both strong acceptors and strong donors, such units have been given the designation of ambipolar units .…”

International Year of Light and Light-Based Technologies

“…[19] Tim Bender and co-workers describe novel light-absorbing arylsubstituted boron subphthalocyanines. [20] Seth Rasmussen and colleagues report the influence of side chains and spacer units in light-absorbing thieno [3,4-b]pyrazine-flourene copolymers, [21] and David Lewis et al describe the effect of fullerene content in polymers. [22] Kenneth Ghiggino and his team report a new lightabsorbing copolymer based on benzodithiophene and [3,4-c] pyrrole-4,6-dione.…”