Effect of alkyl side-chain length and solvent on the luminescent characteristics of poly(3-n-alkylthiophene)

In poly(3-n-alkylthiophene) (P3AT), the length of alkyl side chain as well as the solvent used for film casting affected the conformation and eventually the luminescent characteristics of the polymers. When the length of alkyl side chain was shorter than 12 carbon atoms, the electronic absorption and PL spectra of P3AT film showed red shift as the length of the alkyl side chain increased. This is because, when the length of the alkyl side chain is not too long, solvation is promoted with increase in the length of the alkyl side chain, leading to expanded chain structures with longer π-conjugation lengths. Conversely, when the length of the alkyl side chain was longer than 12 carbon atoms, the electronic absorption and PL spectra of P3AT film showed blue shift as the length of the alkyl side chain increased. This is because, when the length of the alkyl side chain is too long, the steric hindrance becomes predominant leading to highly distorted chain conformation with shorter π-conjugation lengths. The PL intensity of the film cast from THF was higher by an order of magnitude than that of the film cast from chloroform, toluene, or p-xylene. In the P3AT films cast from a poor solvent such as THF, the polymer chains are highly distorted leading to high PL intensity due to exciton localization. In the EL spectra of P3AT films cast from various solvents, λ_max shifted as much as 140 nm depending upon the solvent characteristics. This result demonstrates that the color of EL can be varied from blue to orange simply by casting the same polymer from a different solvent.