Isomerization effect on the heat capacities and phase behavior of oligophenyls isomers series

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Highlights

  • Isomerization effect on the heat capacity of the polyphenyls isomers series.

  • Isomerization effect on solid–liquid equilibrium, enthalpies and entropies of fusion.

  • Why higher melting temperature for the para than for the ortho/meta series?

  • Thermal Behavior of the polyphenyls isomers series.

Abstract

In this work an analysis of the effect of ortho, meta and para isomerization and the number of phenyl rings on the heat capacity and phase behavior of oligophenyls, n(Ph), from 3 up to 5 is presented. The phase behavior from 298.15 K to the melting temperature was analysed by differential scanning calorimetry (DSC) and the temperature, enthalpies and entropies of fusion along the oligophenyls series were determined. With the exception of terphenyl isomers, the meta oligophenyls series show lower melting temperatures and higher entropies of fusion. Heat capacities, at T = 298.15 K, were determined for the ortho and meta quaterphenyl and p-quinquephenyl oligomers by means of high precision drop calorimetry. The heat capacities of the ortho and meta isomers are systematically lower than those of the corresponding para isomers. It was found that the subtle isomerization effect on the heat capacity is partially related to the ring rotation restrictions in their supramolecular structure and increases with the increase of the number of phenyl rings.

Introduction

Polyphenyls and their derivatives are a family of organic compounds on which extensive studies have been performed due to its potential functionalities as electrical and heat conduction materials or optical devices [1], [2], [3], [4], [5], [6], [7]. These molecules exhibit a great variety of thermophysical properties and supramolecular structures due to the possible conformations of phenyl rings and the effect of substitutes in the mainframe structure. The para-polyphenyls adopt a rigid-rod structure with non-planar or a pseudo-planar supramolecular conformation depending on the temperature. [8], [9], [10].Their extended n-conjugation makes them good candidates as active components in devices such as light emitting diodes or field-effect transistors [6], [11], [12], [13], [14], [15], [16]. These materials have a high thermal stability and, in some cases, show a stable nematic phase transition [17], [18], [19], [20], [21], [22], [23], [24], [25]. The ortho and meta structural isomeric series have some additional conformational constrains that contributes to a higher differentiation on their thermal properties. Due to the meta-polyphenyls angular arrangement the phenyl rings do not have a rigid molecular shape [26], [27], [28]. Thus, for these molecules, rotation between the phenyl rings is facilitated in the liquid and gas phase [7]. The ortho-polyphenyls was shown to adopt a helical supramolecular structure with π–π stacking interactions [29].

The development of organic synthesis, especially the Suzuki–Miyaura cross coupling [30], [31], [32], [33], has resulted in the ability to produce and study more versatile and diverse organic molecules [34], [35], [36], [37], [38]. Since the oligophenyls presented in this work are non-functionalized organic compounds, the systematic study of their thermophysical properties allows for the understanding and interpretation of the influence of isomerism and aromatic interactions on these parameters. In a previous work by Santos et al. [39], the results of heat capacity measurements on some members of the oligophenyls series (ortho, meta, para-terphenyl and para-quaterphenyl) were presented and discussed. A subtle isomerization effect on the heat capacity was observed for the terphenyl isomers suggesting that the observed lower heat capacity of the ortho and meta isomers could be partially related to the ring rotation restrictions in their supramolecular structure [39].

This work aims the study of the effect of isomerization and number of phenyl on the heat capacities and phase behavior of the extended oligophenyls isomers series. The studied compounds are the ortho, meta and para oligophenyls series with number of phenyl rings, n(Ph), from 3 up to 5. Figure 1 depicts the polyphenyls studied in this work, and table 1 lists the abbreviation used and their IUPAC names as well as information concerning the purification and assigned purity of the studied samples.

Section snippets

Experimental

The relative atomic masses used in this work were those recommended by the IUPAC Commission in 2009 [40]. The reported uncertainties are twice the standard deviation of the mean.

Heat capacities of the oligophenyls at T = 298.15 K

The molar heat capacities at T = (298.15 ± 0.02) K, Cp,m°, the average masses, msample, and the total number of drop experiments, Ndrops, for each compound are presented in table 2. The sample masses (for calibration and experiments) were corrected for the buoyancy effect. The reported uncertainty is twice the standard deviation of the mean and includes the calibration uncertainty.

Table 3 lists the average molar and specific heat capacity for the para-, meta-, ortho-oligophenyls series at T = 298.15 K

Acknowledgements

Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support to Centro de Investigação em Química, University of Porto. Ana S.M.C. Rodrigues and Marisa A.A. Rocha acknowledge the financial support from FCT and the European Social Fund (ESF) under the Community Support Framework (CSF) for the award of a Research Grants SFRH/BD/81261/2011 and SFRH/BD/60513/2009, respectively.

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