Comparative Study on the Structural, Optical, and Electrochemical Properties of Bithiophene‐Fused Benzo[c]phospholes

Abstract

Three types of bithiophene-fused benzo[c]phospholes were successfully prepared by Ti(II)-mediated cyclization of the corresponding dialkynylated bithiophene derivatives as a key step. Each sigma(3)-phosphorus center of the benzo[c]phosphole subunits was readily transformed into sigma(4)-phosphorus center by Au coordination or oxygenation. In addition, the bithiophene subunit was functionalized at the alpha,alpha'-carbon atoms by Pd-catalyzed cross-coupling reactions with heteroarylmetals and by an S(N)Ar reaction with hexafluorobenzene. The experimentally observed results (NMR spectroscopy, X-ray analysis, UV/Vis absorption/fluorescence spectroscopy, and cyclic/differential-pulse voltammetry) have revealed that the structural, optical, and electrochemical properties of the bithiophene-fused benzo[c]phospholes vary considerably depending on the pi-conjugation modes at the bithiophene subunits and the substituents of the heterocyclopentadiene components. The appropriately ring-annulated sigma(3)-P derivatives and sigma(4)-P-AuCl complexes were found to emit fluorescence in the orange-red region, and the sigma(4)-P-oxo derivatives proved to undergo reversible one-electron reduction at -1.4 to -1.8 V (vs ferrocene/ferrocenium). These results indicate that the bithiophene-fused benzo[c]phospholes possess narrow HOMO-LUMO gaps and low-lying LUMOs, which was confirmed by density functional theory calculations of their model compounds. The time-of-flight measurement of an ITO/benzo[c]phosphole/Al device showed that the electron mobility in the P-oxo derivative is one-order higher than that in Alq(3) at low electric fields. The present study demonstrates that the arene-fused benzo[c]phosphole skeleton could be a highly promising platform for the construction of a new class of phosphole-based optoelectrochemical materials.

Synthesis and Structure–Property Relationships of 2,2′‐Bis(benzo[b]phosphole) and 2,2′‐Benzo[b]phosphole–Benzo[b]heterole Hybrid π Systems

“…As shown in Figure 4, both 5 and 6 are mostly coplanar with torsion angles at the inter-ring linkages of 175.73(11)8 for 5 and 177.27 (14)8 for 6. In addition, the relatively short interring C À C bond lengths (1.4458(17) for 5 and 1.442 (2) for 6) are responsible for the effective conjugation between the benzo [b]phosphole and the benzo [b]heterole p systems. It is worth noting that both compounds, 5 and 6, are racemic crystals, in which the (R) and (S) stereoisomers make densely p-p stacked pairs in a head-to-tail orientation (Figure S1 Table 2.…”

“…[1] Recently, many efforts have been devoted to the development of arene-fused phosphole p systems for use in materials chemistry. [2] In particular, benzo [b]phos-A C H T U N G T R E N N U N G pholes [3] and related compounds [4][5][6][7] have been receiving increasing attention because they possess rigid and extended p skeletons, which would be beneficial in the molecular design of phosphole-based sensitizers, fluorophores, and semiconductors. The benzo [b]phosphole skeleton contains two functionalization sites, that is, a-and b-carbon atoms, on one side of the phosphole unit.…”

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

“…[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). [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. However, this structural parameter is probably the most difficult factor to tune experimentally.…”

“…[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). [3] However, to date no general strategy that allows tuning of this torsion angle over a large range is known. However, this structural parameter is probably the most difficult factor to tune experimentally.…”

“…[3] However, to date no general strategy that allows tuning of this torsion angle over a large range is known. However, this structural parameter is probably the most difficult factor to tune experimentally.…”