Two series of 2,5-dipyridyl- and 2,5-dithienylphosphole derivatives containing sigma3- or sigma4-P atoms were prepared, and their optical (UV/Vis absorption, fluorescence spectra) and electrochemical properties were systematically evaluated. These physical properties depend mainly on the natures of the 2,5-substituents and of the phosphorus moiety, and they revealed that these compounds contain extended pi-conjugated systems. Structure-property relationships were established on the basis of these experimental data and ab initio calculations on the parent molecules. The limited aromatic character and low-lying LUMO of the phosphole ring appear to be crucial for achieving a highly delocalised pi system. Electrooxidation of 2,5-dithienylphosphole derivatives affords electroactive films with low optical band gaps. As observed for the corresponding monomers, the optical and electrochemical properties of the polymers can be varied over a wide range by modifying the nature of the phosphorus moiety.

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Heterophospholes 1231 D. Six-Membered Rings 1233 1. Phosphinine (3) 1233 2. Azaphosphinines 1235 3. Di-and Triphosphinines 1235 4. Hexaphosphinine (P 6 ) 1236 E. Azaphosphaullazine (4,9b-Diaza-2phosphacyclopenta[c,d]phenalene) (16) 1236 IV. "B"-and "C"-Type Bonding, Phosphorus Lone Pair in Delocalized Aromatic Systems 1236 A. Four-Membered Rings 1236 B. Five-Membered Rings 1237 1. Phosphole (4) 1237 2. 3H-1,3-Azaphospholes 1238 3. Polyphospholes 1238 C. Six-Membered Rings 1239 1. Cyclic Phosphinocarbenes 1239 2. Cyclic Phosphinoboranes 1240 D. Nine-Membered Rings 1240 E. Fused Systems 1240 V. "D"-Type Bonding 1241 A. Comparison of the Bonding in H 3 PdCH 2 and HPdCH 2 1241 B. Four-Membered Rings 1241 C. λ 5 -Phosphinine (33), 1λ 5 ,3λ 5 ,5λ 5 -Triphosphinine 1242 VI. Outlook 1243 VII. Acknowledgments 1243 VIII. References 1243