Poly(3-hexylthiophene-2.5-diyl): Evidence of different polymer chain conformations in the solid state from a combined study of regioregularity control and Raman spectroscopy

Abstract

In this work a study is reported on the Raman and UV–vis absorption spectra of recently synthesized poly (3-hexylthiophene-2,5-diyl) (P3HT) samples whose degree of head-to-tail (H-T) regioregularity has been, on purpose, modified in the range 98%-64% by a controlled insertion of head-to-head (H-H) coupled bithiophene monomers. The materials under study show the strongest Raman scattering line near 1450 cm−1, when recorded in off-resonance conditions. Based on the Effective Conjugation Coordinate theory we learn that this scattering originates from the collective CC stretching mode along the polymer backbone (Я mode). A curve fitting analysis reveals that this line is the result of the convolution of three strong components at 1445 cm−1, 1450 cm−1 and near 1480 cm−1 with different relative intensities as function of the sample regioregularity. These lines are assigned to polymer chains which take up different conformations namely: i) co-planar sequences of thiophene units, labelled as hairy-A, which originate the crystalline domains of form-1, ii) conformationally slightly distorted sequences of thiophene units, labelled as hairy-B, and iii) highly distorted/disordered sequences of thiophene units which originate the amorphous phase. Also the UV–vis absorption spectra recorded on thin films of the same materials can be interpreted as the convolution of the absorptions originated from the polymers chains in the three different conformations as in i), ii), and iii). With multi-wavelength resonance Raman spectroscopy is thus possible to enhance selectively the scattering of each type of chain. From the Raman data recorded in off-resonance a straightforward method to obtain the relative amount of the above structures is proposed, showing that solid rr-P3HT at room temperature is composed approximately by 50% of hairy-A chains and 50% of hairy-B chains with a negligible contribution by the amorphous phase. The information derived from this study are of great help for the understanding of the conformational structure in the solid state of the technologically relevant parent molecule rr-3PHT.