Effects of heterole spacers on the structural, optical, and electrochemical properties of 2,5-bis(1,5-diphenylphosphol-2-yl)heteroles

Abstract

2,5-Bis(1,5-diphenylphosphol-2-yl)he-terole derivatives were prepared via Pd-catalyzed Stille-type cross-coupling reactions of α-(tributylstannyl) phosphole or α-iodophosphole with the corresponding 2,5-difunctionalized heteroles (pyrrole, furan, and thiophene). X-ray crystallographic analyses of three σ4-P derivatives have elucidated that the coplanar phosphole-heterole-phosphole π-networks are constructed in both anti-anti and syn-anti conformations. In the case of the pyrrole-linked σ4-P derivatives, there is a weak hydrogen-bonding interaction between NH and P=X (X = O, S) groups. The optical and electrochemical properties of the σ3-P and σ4-P=O derivatives were revealed experimentally by means of UV-vis absorption/emission spectroscopy and cyclic/differential pulse voltammetry. In all the heterole-linked derivatives, the P-oxidation lowers LUMO levels more efficiently than HOMO levels and, as a result, narrows HOMO-LUMO gaps. In each series (σ3-P or σ4-P=O), the lowest π-π* transition energies of the phosphole-heterole-phosphole π-systems were found to decrease in the following order: thiophene > furan > pyrrole, and their electrochemical oxidation and reduction potentials shift to the negative side in the same order. These results basically reflect the differences in ionization potential and electron affinity of the central heterole spacers. Density functional theory calculations on the model compounds supported the experimentally observed results. The HOMO of the molecule resides on the conjugated polyene network of the π-system, whereas the LUMO is strongly located on the phosphole subunits. The present comparative study demonstrates, for the first time, that the intriguing electronic structures and the fundamental properties of the 2,5-bis(1,5-diphenylphosphol-2-yl)heteroles can be finely tuned by simple selection of the appropriate heterole spacer. © 2011 Wiley Periodicals, Inc.