Solution spectroscopic properties of polyDCHD-HS: a novel highly soluble polydiacetylene

The solution properties of a novel soluble polydiacetylene, poly[1,6-bis(3,6-dihexadecyl-9H-carbazol-9-yl)hexa- 2,4-diyne] (polyDCHD-HS), have been investigated by using UV–Vis absorption, fluorescence, and FT-Raman spectroscopies. The polymer gives rise to stable, brilliant red solutions in aromatic solvents and in chloroform at room temperature with no evidence of the colour transition to yellow that usually takes place by increasing the temperature with other soluble polydiacetylenes. A particular role appears to be played by benzene–polyDCHD-HS interactions, because in this solvent a very narrow and intense excitonic peak is observed at room temperature in the electronic spectrum. Interestingly, these solutions show also a well-resolved fluorescence spectrum, with a very small Stokes shift and a rather high quantum yield (≅102). Furthermore, a very efficient transfer of the excitation energy from the carbazolyl substituents to the conjugated backbone is found to take place. On account of the observation that by increasing the temperature of the benzene solutions up to 75 C the excitonic feature in both the absorption and the emission spectra undergoes fully reversible broadening and intensity reduction accompanied by the increase of the Stokes shift, without any dramatic colour change, it is likely that the rates of torsional motions in the polymer chains are thermally activated without modifying the average conjugation length of the polymer chains. FT-Raman measurements in benzene solutions in the same temperature range have confirmed this conclusion. The photophysical properties of polyDCHD-HS in benzene around 60 C are quite similar to those observed in other aromatic solvents at room temperature. By heating up the red solutions in toluene or in chlorobenzene, no spectral shift is observed but only a limited increase of the absorption band broadening. All these findings strongly suggest that by the appropriate choice of the substituents it is possible to obtain stable solutions of the red form of polydiacetylenes.