Oligo phosphole s

When considering the synthesis of phospholes, one has to forget most of the classical and powerful methods employed for the preparation of thiophenes and pyrroles. For example, Paal-Knorr condensation, direct ortho-lithiation, halogenation with NBS or I2/Hg2+ and Vilsmeier-Haack formylation are not operative in phosphole chemistry. Likewise, no chemical or electrochemical oxidative polymerization 

It can be concluded that the size of the family of linear oligo(phosphole)s is somewhat limited to bi- and tetramers to date, although longer derivatives are potentially accessible via the efficient synthetic routes developed by Mathe/s group. The synthesis of other oligo (phosphole) s and the elucidation of their photo-physical and electrochemical properties are still needed in order to establish reliable structure-property relationships. 

4 Design of Ji-Conjugated Systems Using Organophosphorus Building Blocks 

The above-mentioned examples nicely illustrate the diversity of structures offered through the use of l,l -diphosphole building blocks. Importantly, it has been shown that these moieties exhibit g-jt conjugation, a type of electronic interaction that is still relatively rare. Together, these results show that this type of P-based moiety is a promising elementary unit for the engineering of conjugated systems. 

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Phospholes can readily be prepared on a large scale and are known with a vast range of substituants [6, 16b,c]. However, the synthesis of oligo(phosphole)s analogous to (C) (Fig. 1) is a real synthetic challenge since the low aromatic character of phosphole prevents the functionalisation of the P-Ca,a carbon atoms via electrophilic substitution and inhibits their preparation using electropolymerisation. As a consequence, no poly(phosphole)s have yet been reported, although bi- and tetra-phospholes have been prepared by stepwise routes (Scheme 2). 

The discovery that the oxidative coupling of 2-lithiophosphole, obtained from (7a), led to biphosphole (8a) (Scheme 2) was a breakthrough [19], opening the way to oligo(phosphole)s. This very efficient methodology has been... 

In conclusion, a variety of linear or cyclic oligo(phospholes)s and their derivatives are accessible via a set of efficient synthetic strategies. The potential of these compounds as advanced 71-conjugated systems is broadened by the presence of reactive trivalent P-centres, which allow a range of additional chemical modifications to be achieved. However, elucidation of structure-property relationships for these derivatives is still needed. 

Unfortunately, the optical and electrochemical properties of these linear phosphole oligomers have not yet been elucidated, precluding an interesting comparison with related oligo(pyrrole)s. However, oligo(phosphole)s 9 (R = CH3), 11 and 14 (Scheme 4.3) have been characterized by X-ray diffraction studies, which does give some insight into the properties of these systems. 

Table TDDFT(B3LYP) calculated excitation energies (eV) for oligo(cyclopentadiene)s, oligo(pyrrole)s, oligo(phosphole)s (Ph) ollgo(thlophene)s (Th)n, and the corresponding polymers (DFT HOMO-LUMO gaps in parentheses)...

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