Effect of molecular mass on supramolecular organisation of poly(4,4″-dioctyl-2,2′:5′,2″-terthiophene)

Abstract

The effect of the chain length on the type and extent of the 2D supramolecular organization in poly(4,4''-dioctyl-2,2':5',2''-terthiophene) (PDOTT) monomolecular layers deposited on highly oriented pyrolytic graphite (HOPG) is studied by scanning tunneling microscopy (STM) and analyzed in terms of molecular modeling. The strictly monodispersed fractions of increasing molecular mass used in this study were obtained by chromatographic fractionation of the crude product of 4,4''-dioctyl-2,2':5',2''-terthiophene oxidative polymerization. STM investigations of PDOTT layers, deposited on HOPG from poly- and monodispersed fractions, show that polydispersity can be considered as a key factor seriously limiting supramolecular ordering. This is a consequence of significant differences in the type of supramolecular order observed for molecules of different chain length. It has been demonstrated that shorter molecules (consisting of 6 and 9 thiophene units) form well-defined two-dimensional islands, while the interactions between longer molecules (consisting of 12 and 15 thiophene units) become anisotropic. Consequently, for higher molecular mass fractions, the supramolecular organization is one-dimensional and consists of more or less separated rows of ordered macromolecules. In this case an increase of the chain length leads to amplification of the intermolecular interactions proceeding via interdigitation of the alkyl substituents of adjacent molecules. Polydispersed fractions show much less ordered organization because of the incompatibility of the supramolecular structures of molecules of different molecular masses. This finding is of crucial importance for the application of polythiophene derivatives in organic and molecular electronics since ordered supramolecular organization constitutes the condition sine qua non of good electrical transport properties.

Nanoscale Inhomogeneity of Conducting‐Polymer‐Based Materials

“…Jaroch et al [22] used STM to study the effect of molecular weight (MW) and polydispersity index on the local inhomogeneity of yet another regio-regular polythiophene, poly(4,4 00 -dioctyl-2,2 0 :5 0 ,2 00 -terthiophene) (PDOTT). This polymer has a reduced sidechain density as compared to regio-regular P3HT and therefore is able to show even better packing and excellent charge-carrier mobilities, generally higher than that found in regioregular P3HT.…”

“…Eventually, this paradox was related to the difference in inhomogeneity and the nanoscale structural organization of materials with low and high MW and has become one of the manifestations of the importance of the inhomogeneity factor in establishing the performance of organic materials. Very recently, the importance of a related factor, the polydispersity index (PI), which characterizes the range of MWs found in a given polymer sample, has started to be recognized [22,27,40,41].…”

“…In addition to the average MW, the importance of another factor, the polydispersity index (PI), has started to be recognized [22,27,40,41]. The PI characterizes the range of MWs that are present in a polymer sample.…”

“…One way of achieving this is to prepare a polymer in which not every thiophene ring bears a side chain. Two examples of such polymers are poly(4,4 00 -dioctyl-2,2 0 :5 0 ,2 00 -terthiophene) (PDOTT) [22] and poly(3,3 000 -dialkylquaterthiophene)s (PQT) [23][24][25][26][27], which have two alkyl-substituted thiophene rings flanking one (PDOTT) or two (PQT) nonsubstituted rings. These polymers show excellent charge-carrier mobilities that are generally higher than those found with regio-regular P3HT.…”

Self-Assembly of Alkyl-Substituted Oligothiophenes on MoS2: A Joint Experimental/Theoretical Study

“…15−21 In this context, several STM studies have shown the possibility of adsorption of thiophene derivatives on HOPG 20,22−34 or of polythiopenes. 35,36 Among those studies, several have reported that some oligothiophene derivatives form ordered monolayers at the liquid−graphite interface, such as alkylated oligothiophenes, 24,26 alkylated oligothiophenes with halogen substituents, 28,32 or carboxylated oligothiophenes. 20,29 These studies showed that the formation of the physisorbed monolayer strongly depends on the structure and the length of the oligothiophene and the presence of halogen substituents.…”