Article pubs.acs.org/JPCC
Polyfulvenes: Polymers with “Handles” That Enable Extensive Electronic Structure Tuning Christian Dahlstrand,† Burkhard O. Jahn,† Anton Grigoriev,‡ Sébastien Villaume,†,§ Rajeev Ahuja,‡ and Henrik Ottosson*,† †
Department of ChemistryBMC, Uppsala University, Box 576, 751 23 Uppsala, Sweden Department of Physics and Astronomy, Materials Theory Division, Uppsala University, Box 530, 751 21 Uppsala, Sweden
‡
S Supporting Information *
ABSTRACT: The fundamental electronic structure properties of substituted poly(penta)fulvenes and pentafulvene-based polymers are analyzed through qualitative molecular orbital (MO) theory combined with calculations at the B3LYP and HSE06 hybrid density functional theory (DFT) levels. We argue that the pentafulvene monomer unit has a unique character because electron density in the exocyclic CC double bond can be polarized into and out of the five-membered ring, a feature that is not available to other more commonly used monomers. It is investigated how the energy gaps between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively), as approximate band gaps, are influenced by exocyclic substitution, introduction of linker groups, benzannulation, and ring substitution. In particular, the exocyclic positions of the fulvene act as “handles” by which the electronic structure of the polymer can be tuned between the quinoid and fulvenoid valence bond isomers; electron-withdrawing exocyclic substituents lead to polyfulvenes in the quinoid form while those with electron-donating substituents prefer the fulvenoid. Taken together, the HOMO−LUMO gaps of polyfulvenes can be tuned extensively, varying in ranges 0.77−2.44 eV (B3LYP) and 0.35−2.00 eV (HSE06) suggesting that they are a class of polymers with highly interesting, yet nearly unexplored, properties.
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INTRODUCTION There is a growing demand for organic polymers with band gaps below 1 eV because these could find applications in polymer-based bulk heterojunction solar cells where they would harvest near-infrared solar irradiation.1−6 The common design approach to such low band gap organic materials is nowadays the combination of donor and acceptor units into donor− acceptor copolymers.7−11 The combination of different monomers with alternating quinoid or aromatic structures into copolymers is an alternative method to achieve such polymers,12 and an extensive quantum chemical survey of such quinoid−aromatic copolymers recently put focus on this approach.13 Yet, it is also established that low band gaps can be achieved in homopolymers by destabilizing the ground state toward the aromatic ↔ quinoid level crossing.14,15 Herein, we report on an approach to low band gap homopolymers in which the monomers have structural moieties that act as “handles” and by which the electronic properties of the polymer can be tuned predictably and extensively toward the aromatic ↔ quinoid level crossing through a choice of substituents. Pentafulvene (Figure 1), or shortly fulvene, as a core structure could represent such a monomer because the lowest excitation energies as well as their ionization energies and electron affinities could be varied predictably through choice of exocyclic and/or endocyclic substituents.16−20 The rational variation in the first excitation energies, which is a © 2015 American Chemical Society
Figure 1. Structure of the (penta)fulvene together with its highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) calculated at the B3LYP/6-31G(d) level.
result of the reversal in the electron-count rule for aromaticity in the lowest ππ* excited states (T1 and S1) when compared to the ground state (S0) (Baird’s vs Hückel’s rule),21−25 can be useful for design of low band gap homopolymers. Indeed, fulvenes act as “aromatic chameleons” because they can adapt to the reversal in the electron-count rule for aromaticity through shift of the electron density in the exocyclic CC double bond into or out of the five-membered ring.26 Here, it can also be noted that 6,6-dicyanofulvenes have interesting electrochemical properties,20,27−30 and it has been found that they are useful as n-type additives in bulk heterojunction solar cells.31 Indeed, 6,6-dicyanofulvene has been labeled an “old Received: August 18, 2015 Revised: October 24, 2015 Published: October 28, 2015 25726
DOI: 10.1021/acs.jpcc.5b08042 J. Phys. Chem. C 2015, 119, 25726−25737
Article
The Journal of Physical Chemistry C dog” which can be taught “new tricks”,30 and it should be interesting to incorporate this unit into polymers. The aim of the present study is to investigate to what extent the substituents at the fulvene monomers affect the electronic properties of a polyfulvene polymer. Polyfulvenes have previously been examined,32−38 but to the best of our knowledge this is the first time the effect of substitution is thoroughly investigated while the polymers are allowed to be nonplanar, a crucial factor for proper description of the electronic structure of polyfulvenes. We focused the presently reported study to polyfulvenes in which the fulvene monomers are linked via their 1- and 4-positions. Fulvene is valence isoelectronic with pyrrole, thiophene, and selenophene. However, instead of a ring heteroatom which contributes two pπ electrons to the five-membered ring, the fulvene unit has an exocyclic CC double bond with two pπ electrons which can be added into or taken away from the ring. Indeed, the exocyclic as well as the ring positions in fulvene act as handles for tuning of the electronic structure.16,39 This feature, enabled through the fulvene core, is not available in any of the cyclic monomers commonly found in conducting polymers because π-electron density cannot be pushed into or pushed out of these rings. It should therefore be important to explore if and how the properties of substituted fulvenes are transformed when the fulvene is incorporated into a polymeric structure. An in-depth understanding of these factors could allow for completely new polymers for organic electronics. Polyfulvenes are not the only polymer type that has been overlooked. Polyfurans have been outside the focus for long, yet, they were recently synthesized and reported by Bendikov and co-workers, and found to possess remarkable properties.40 Thus, expanding the toolbox for polymer design could lead to polymers with highly interesting features. Taken together, fulvenes should offer unique possibilities in polymer research because of their handles which potentially can be used for band gap tuning. The general findings may also extend to monomer classes outside the fulvenes because we recently showed that siloles and cyclopentadienes are valence isolobal with fulvenes as they can be described as cross-hyperconjugated aromatic chameleons.20 Thus, their electronic and optical properties can be tuned through substituents similarly to those of fulvenes. The findings reported herein should thereby also be applicable to other polymer classes than polyfulvenes. The design of polymers with predictable and adjustable band gaps and band profiles is of great importance for many areas of applications.41−47 One major goal is to achieve polymers with very low band gaps (
