2,2′‐Biphospholes: Building Blocks for Tuning the HOMO–LUMO Gap of π‐Systems Using Covalent Bonding and Metal Coordination

Abstract

Organic p-conjugated systems are useful lightweight semiconducting materials for electronics. [1] The control of their HOMO-LUMO energy gap (E g ) is one of the key issues in the optimization of these materials for applications (organic light-emitting devices (OLEDs), organic field-effect transistors (OFETs), photovoltaic cells, etc.). [2] The E g of an isolated conjugated system (A; Figure 1) is a function of several structural factors including the bond-length alternation (E BLA ), aromatic character of the building blocks (E res ), electronic nature of the lateral substituents (E sub ), and the torsion angle between consecutive subunits (E q ) ( Figure 1). [2] Among all these structural factors, only the torsion angle q can potentially be varied in a continuous way to allow progressive switching from a highly delocalized low-gap system (q = 08) to a non-delocalized high-gap system (q = 908). However, this structural parameter is probably the most difficult factor to tune experimentally. In fact, the torsion angle can be fixed to 08 (fully planar systems) by rigidification through either the introduction of a covalent bond between the elemental building blocks (ladder-type molecules) or by metal coordination (for example 2,2'bipyridines chelated by metal ions). [3] However, to date no general strategy that allows tuning of this torsion angle over a large range is known.Herein, we show that exploiting the properties of the 2,2'biphosphole unit B (modification of the substituents on P, or metal coordination; Figure 2) [4][5][6] allows a unique combined covalent/metal coordination approach for an on-demand tuning of the torsion angle q between the two conjugated P heteroles. This simple and straightforward molecular engineering, which is not possible with other p-building blocks (2,2'-bipyridine, 2,2'-bithiophene, 2,2'-bipyrrole, 2,2'-bisilole, etc.), allows an unprecedented fine control of the energy gap, E g , of the p-systems A (Figure 1) through two orthogonal approaches. Lastly, the impact of this tuning for optoelectronic applications is demonstrated with the use of 2,2'biphosphole-based small molecules as emitting materials for OLEDs, including white organic light-emitting devices (WOLEDs).The target 2,2'-biphosphole oligomers 2 a-c and 3 b,c (Scheme 1) were prepared from 1-aryl-3,4-dimethylphospholes 1 a-c using a synthetic strategy discovered by Mathey et al., [5] and then later developed by Gouygou et al. for Pbridged derivatives. [6] The newly prepared compounds 2 b and 3 b,c were isolated in approximately 50 % overall yield. The 31 P NMR spectroscopy shows that they exist as a mixture of stereoisomers, as already observed for related biphosphole Figure 1. Structural factors determining the band gap of p-conjugated systems.Figure 2. Covalent and metal coordination approaches to tune the torsion angles (q) within the 2,2'-biphosphole building blocks.

Phosphorus Centers of Different Hybridization in Phosphaalkene‐Substituted Phospholes

“…For a first indication of whether it would be possible to perform selective chemistry on either of the two phosphorus heteroatoms, PPAs 5 a – c were subjected to oxidative conditions by using an excess amount of sulfur in CH 2 Cl 2 . Although such oxidation by sulfur is well established for phospholes[ 9 , 10d , h ] it has previously been shown that phosphaalkenes can also be oxidized by sulfur to thiaphosphiranes. [ 20 ] However, procedures for phosphaalkene oxidation usually suggest the addition of base to accelerate the reaction, although sulfurization is also described to proceed without a base but under extended reaction times.…”

Redox‐Induced Reversible PP Bond Formation to Generate an Organometallic σ4λ4‐1,2‐Biphosphane Dication

“…[2] In principle, the capacity to change the properties of a phosphorus center actively during a process creates further important potential, such as on-off switching in catalysis, [3] or profound gating of the properties of electronic materials. [4] Studies of how phosphorus centers can be efficiently modulated using ferrocenes, [5] cobaltocenes, [6] TTF derivatives, [7] or other redox-active groups have appeared; however, these redox-active groups are often spatially close to the phosphorus center, so the electronic outcome of the redox change can be conflated with significant steric effects. This is obviously undesirable if geometrical properties need to be maintained, so the possibility of modulating the properties of a phosphorus atom by a molecular wire [8] is attractive.…”

Cascade Reactions Forming Highly Substituted, Conjugated Phospholes and 1,2‐Oxaphospholes

“…[18] It has recently been demonstrated that by altering the torsion angle of biphospholes the communication between the two phosphole rings can be tuned, and thus used to engineer the HOMO-LUMO gap. [19] Owing to the rigidity of the ethylene bridge between the two phosphole units in 4, p conjugation is well mediated and the lowest energy absorption maximum is observed at 445 nm (e = 7300 m À1 cm À1 ). The rigidity imposed by the ethylene spacer, as well as its contributions to the overall p system, gives rise to a 50 nm red shift compared to a 5,5'-diphenylsubstituted 2,2'-biphosphole.…”