Effect of Confined Hindrance in Polyphenylbenzenes - ACS Publications

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The Effect of Confined Hindrance in Polyphenylbenzenes Carlos F. R. A. C. Lima, Ana S. M. C. Rodrigues, and Luis M. N. B. F. Santos J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b00579 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017

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The Journal of Physical Chemistry

The Effect of Confined Hindrance in Polyphenylbenzenes Carlos F. R. A. C. Lima,*,†,§,‡ Ana S. M. C. Rodrigues, †,‡ and Luís M. N. B. F. Santos*,† †

CIQUP, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do

Porto, Porto, Portugal. E-mail: [email protected], [email protected] §

Department of Chemistry & QOPNA, University of Aveiro, Aveiro, Portugal.

Abstract

A comprehensive thermodynamic study of the whole ortho-polyphenylbenzenes series, from biphenyl (n=1) to hexaphenylbenzene (n=6), is presented. Combustion calorimetry and phase equilibria measurements for 1,2,3,4-tetraphenylbenzene (n=4) and pentaphenylbenzene (n=5), together with literature data, were used to understand and quantify the constraint effect of orthosubstitution on the molecular energetics and phase stability of polyaromatic compounds. All the derived thermodynamic properties (enthalpy of sublimation, entropy of sublimation, and gas phase molecular energetics) show a marked trend shift at n=4 to n=5, which is related with the change of the degree of molecular flexibility after 1,2,3,4-tetraphenylbenzene (n=4). The greater intramolecular constraint in the more crowded members of the series (n=5 and n=6) leads to a

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significant change in the molecular properties and cohesive energy. The trend shift in the molecular properties is related with the decrease of molecular flexibility, which leads to lower molecular entropy and destabilization of the intramolecular interaction potential due to the increased hindrance in a confined molecular space.

Introduction Polyphenylbenzenes and their derivatives are polyaromatic compounds with interesting thermal, electronic and optical properties.1-8 These compounds have several technological applications in the fields of organic semiconductors (OSCs), catalysis, thermal fluids, and biochemistry.1-8 In these compounds van der Waals dispersion forces are the main component of inter and intramolecular interactions, and thus they are very important for defining the preferred molecular and supramolecular structures and energetic stability. A previous work has demonstrated how phenyl substituents distributed around the central benzene ring can induce some interesting structural changes at the supramolecular and molecular levels.9 The results have shown that the cohesive energy in the crystal phase is reduced by ortho- bounded phenyl groups, which compromise the intermolecular interactions; a situation that is particularly evident in the extreme case of hexaphenylbenzene (n=6). The entropic differentiation in this family of compounds was found to be ruled by molecular symmetry and the hindered phenyl-phenyl (Ph-Ph) internal rotation. The presence of substituents in a molecule is often a relevant factor altering its physicalchemical properties, like, for example, reactivity, energetics, and conformational equilibria. Substituents influence molecular properties in two ways: a) electronic effects, which arise from


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the orbital overlap between the substituent and the rest of the molecule; b) stereochemical effects, which arise from the possible existence of short intramolecular contacts. The phenyl substituent has intrinsically low electron donating and withdrawing power, both by inductive and resonance effects, as reflected in its low σmeta and σpara Hammett constants.10 Moreover, in the ortho- position the phenyl rings are forced, due to steric constraints, to deviate significantly from coplanarity, which further reduces electronic interactions within the molecule.11-14 Hence, in the particular case of ortho-polyphenylbenzenes stereochemistry dominates over electronic effects.

Figure 1. Schematic structure of the ortho-polyphenylbenzenes series (from n=1 to n=6).

Herein, we report the experimental measurement of the thermodynamic properties for some key members of the polyphenylbenzenes series, 1,2,3,4-tetraphenylbenzene (n=4) and pentaphenylbenzene (n=5), in order to comprehensively evaluate the structural and energetic features across the whole ortho- series (in this work the ortho- series is understood as the series


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of compounds that result from the successive addition of phenyl subsituents to an ortho- position of the benzene ring, as illustrated in figure 1). The experimental results achieved in this work, together with those obtained previously for this series of compounds,9 allow investigating the effect of the successive addition of ortho-phenyl rings around benzene on the chemistry of benzene derivatives. The ortho-polyphenylbenzenes considered herein are shown in figure 1 and are abbreviated as: ortho-terphenyl (12dPhB), 1,2,3-triphenylbenzene (123tPhB), 1,2,3,4tetraphenylbenzene (1234TPhB), pentaphenylbenzene (PPhB), and hexaphenylbenzene (HPhB).

Experimental section Purification and characterization of compounds The 1234TPhB (CAS number 1487-12-3) and PPhB (CAS number 18631-82-8) compounds were obtained commercially from Sigma-Aldrich (with stated purity > 99%), and purified by repeated sublimation under reduced pressure. The purity of the samples was checked by gas chromatography (HP 4890 equipped with a cross-linked, 5% diphenyl and 95% dimethylpolysiloxane column), showing a %(m/m) purity of 99.8% for 1234TPhB and of 99.9% for PPhB. The relative atomic masses used were those recommended by IUPAC in 2007.15

Combustion calorimetry ° The standard molar enthalpies of combustion, ∆ , at T = 298.15 K, for the compounds studied

were measured using an isoperibol mini-bomb combustion calorimeter, described in detail elsewhere.16 The mini-bomb is made of stainless steel with 0.46 cm wall thickness and 18.2 cm3


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of internal volume. The internal fittings located on the head of the mini-bomb (electrodes, crucible support and sheet) are all made of platinum. The program LABTERMO was used to compute the corrected adiabatic temperature change, ∆Tad.17 The energy equivalent of the calorimeter, ε(calor) / J·K-1 = {1945.08 ± 0.28 (0.014 %)}, was obtained from calibration experiments with benzoic acid (Calorimetric Standard NIST 39j) – the detailed results are presented in the supporting information. The densities of 1234TPhB and PPhB were taken from the crystallographic data as 1.247 g·cm-3 and 1.20 g·cm-3, respectively.18-21 The values of (∂u/∂p)T at T = 298.15 K, were assumed to be 0.2 J·g-1·MPa-1 – the corresponding energetic correction usually leads to negligible errors in the final combustion results.22,23 Standard state corrections were calculated for the initial and final states by the procedures given by Hubbard et al. and by Good and Scott.24,25

Knudsen effusion with quartz crystal microbalance The vapour pressures of 1234TPhB and PPhB as a function of temperature were measured by the combined Knudsen/Quartz crystal effusion apparatus developed in our laboratory.26 This technique is based on the simultaneous gravimetric and quartz crystal microbalance mass loss detection, enabling the use of a temperature-step methodology, and having the advantages of small sample sizes and effusion times and the possibility of achieving temperatures up to 650 K. Like a typical Knudsen effusion experiment, the system is kept at high vacuum, enabling free effusion of the vapor from the Knudsen cell, which is kept in an oven at a fixed temperature. The experimental temperature (and pressure) ranges for the compounds studied were: 411 – 433 K (0.11 – 1.03 Pa) for 1234TPhB and 447 – 467 K (0.11 – 0.71 Pa) for PPhB.


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Results and discussion Thermodynamic results The energetics of the solid 1234TPhB and PPhB were evaluated by high precision combustion ° calorimetry. Table 1 lists the derived standard molar energies of combustion, ∆ (cr), the ° standard molar enthalpies of combustion, ∆ (cr), and the standard molar enthalpies of ° ° ° (cr), of the crystalline solids. To derive ∆ (cr) from ∆ (cr), the standard formation, ∆

molar enthalpies of formation of H2O(l) and CO2(g) at T = 298.15 K, –(285.830 ± 0.042) and – (393.51 ± 0.13) kJ·mol-1, respectively, were used.27 In accordance with the normal ° thermochemical practice, the combined expanded uncertainty assigned to ∆ (cr) is twice the

overall standard deviation of the mean and include the uncertainties in calibration and in the ° auxiliary quantities used; the uncertainty in ∆ (cr) is the combined standard uncertainty. The

detailed results are presented as supporting information.

° Table 1. Standard (p0 = 105 Pa) molar energies of combustion, ∆ (cr), standard molar ° ° (cr) and standard molar enthalpies of formation, ∆ (cr), in the enthalpies of combustion, ∆

crystalline state, at T = 298.15 K, for the compounds studied. ° ∆ (cr) / kJ·mol-1

° ∆ (cr) / kJ·mol-1

° ∆ (cr) / kJ·mol-1

1234TPhB

−15266.3 ± 6.1

−15279.9 ± 6.1

330.5 ± 7.3

PPhB

−18292.3 ± 7.7

−18308.5 ± 7.7

426.3 ± 9.0


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The solid-gas equilibrium was evaluated by measuring the vapour pressures (0.1 to 1.0 Pa) of the two compounds in temperature intervals of about 20 K using the Knudsen effusion methodology. Table 2 lists the derived standard (po = 105 Pa) molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, for the two compounds. In this work, we assume

° ∆ , = −(35 ± 8) J·K-1·mol-1, which is the typical value for polyphenylbenzenes.9,28 The

° values of ∆ , reported in previous works for several similar polyphenylbenzenes do not

change significantly with the number and position of phenyl substituents,9,28 within the ° experimental error generally associated to , measurements and estimations by group

additivity and computational methods. The detailed sublimation results and procedures for the

° ° ° determination of the thermodynamic functions of sublimation (∆ , ∆ and ∆ ) are

presented in the supporting information. Table 3 lists the relevant thermodynamic data for the ortho-polyphenylbenzenes. The standard molar enthalpies of formation in the gas phase,

° ° ° 298.15K. ∆ (g), at T = 298.15 K, were calculated as: ∆ cr, 298.15K + ∆

Table 2. Standard (p0 = 105 Pa) molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, for the compounds studied.

° ∆ / kJ.mol-1

° ∆ / J·K-1.mol-1

° ∆ / kJ.mol-1

1234TPhB 154.1 ± 1.9

262.4 ± 4.8

75.9 ± 2.4

170.2 ± 1.3

268.6 ± 3.4

90.1 ± 1.6

PPhB


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Table 3. Selected standard (po = 105 Pa) molar thermodynamic properties, at T = 298.15 K, for the ortho-polyphenylbenzenes.

a

n(Ph)a

° ∆ / kJ·mol-1

° ∆ / J·K-1·mol-1

° ∆ (g) / kJ·mol-1

Benzene29,30

0

(44.7 ± 0.2)b

140.6b

82.9 ± 0.9

Biphenyl31

1

81.5 ± 0.2

180.3 ± 0.5

182.0 ± 0.7

12dPhB28

2

103.0 ± 0.4

210.6 ± 1.3

285.5 ± 3.6

123tPhB32

3

134.1 ± 1.1

245.1 ± 3.2

376.7 ± 5.3

1234TPhB

4

154.1 ± 1.9

262.4 ± 4.8

484.6 ± 7.5

PPhB

5

170.2 ± 1.3

268.6 ± 3.4

596.5 ± 9.1

HPhB9

6

175.5 ± 2.1

245.4 ± 5.2

695.6 ± 8.3

n(Ph) is the number of phenyl substituents in the central benzene ring. b Derived hypothetical

values: they refer to conditions of temperature and pressure at which benzene is a liquid;

° ° derivation of ∆ is described in reference 29 and ∆ was obtained from extrapolation of

the data reported in reference 30.

Sublimation equilibrium

° , with the total number of phenyl Figure 2 plots the trend in the enthalpies of sublimation, ∆

° substituents, n(Ph), for the ortho-polyphenylbenzenes series. Along the series ∆ , increases


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° asymptotically to a plateau at n=5. A slight decrease of ∆ can already be noticed for n=4;

the lower cohesive energy of 1234TPhB is also evidenced from comparison with its isomer

° 1,2,4,5-tetraphenylbenzene, for which ∆ is slightly higher (161.4 ± 1.6 kJ·mol−1).9 This

° lowering of ∆ , more significant for n>4, results from the decrease of the available molecular

surface for intermolecular interactions with the progressive insertion of phenyl substituents around bezene. The availability of aromatic π-faces that are able to establish optimal intermolecular contacts (π···π and C−H···π) is significantly reduced above n=4, thus attenuating

° the increase in ∆ , as schematicaly represented in figure 3. These thermodynamic results

follow the conclusions of the previous study of Wuest and coworkers,20 where the authors have shown that in non-planar molecules with many phenyl substituents, like PPhB and HPhB, the faces of the substituting rings and of the central benzene are highly obstructed, resulting in small numbers of intermolecular C−H···π interactions.

° Figure 2. Plot of ∆ , at T = 298.15 K, as a function of the number of phenyl substituents in

benzene, n(Ph), for the ortho-polyphenylbenzenes.


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Figure 3. Schematic representation of available molecular surface for direct intermolecular interactions – note how the geometry of PPhB effectively precludes much π-face direct contact with other PPhB molecules.

The effect of ortho-substitution is even more pronounced in the entropy of sublimation,

° , with HPhB presenting an abnormally low value, as depicted in figure 4. The trend in ∆

° ° ° ∆ is quite similar to that observed for ∆ − a slight decrease in ∆ at n=4 is followed

° by a more pronounced decrease for n>4. The diluted increase in ∆ after n=3 is consistent

with the greater clustering of bulky ortho-substiuents around the benzene core. As more phenyls are present their internal rotational motion becomes increasingly more hindered, which results in

° lower S0(g) and consequently lower ∆ . This effect becomes more pronounced in PPhB and

° HPhB, and contribute for their relatively low ∆ . As previously stated by Lima et al., the high

molecular symmetry of HPhB (D6d symmetry point group) also contributes for decreasing

° 9 ∆ . This effect is less significant in PPhB and the other members of the ortho- series due to

their lower symmetry.


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° Figure 4. Plot of ∆ , at T = 298.15 K, as a function of the number of phenyl substituents in

benzene, n(Ph), for the ortho-polyphenylbenzenes.

° This analysis follows the semi-empirical model of ∆ for general

polyphenylbenzenes, as proposed in the previous work of Lima et al..9 The detailed description and results of this model are presented in the supporting information. This model considers the

° following contributions for estimation of ∆ : a) molecular symmetry; b) hindered rotational

profile of each substituent. For hindered rotation it considers three types of rotors in polyphenylbenzenes: the less hindered meta-/para- phenyls with no ortho- neighbours, the phenyls with one ortho- relation, and the most hindered phenyls in the middle of two orthorings.9 For most polyphenylbenzenes this model provides good estimations (< 4 J·K−1·mol−1).

° However, it predicts significantly lower ∆ for 123tPhB and 1234TPhB, which probably

arises from an incomplete description of the phenyl vibrational motions. The phenyl internal


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rotation in these compounds shall be less hindered than predicted by the model, which suggests significant vibrational coupling involving the internal rotation of various rings. For PPhB and

° ° HPhB the model yields accurate estimations of ∆ (∆ can be adequately predicted by

considering the internal rotation of each phenyl ring individually). In these cases the environment around benzene is too bulky for substituents to effectively couple internal rotational motions.

° ° Figure 5 and table 4 present the results of comparing ∆ and ∆ of two

polyphenylbenzenes with their napthalene analogues. The values obtained show that, while for

° R1 all systems are thermodynamically similar, for R2 there is a clear differentiation in ∆

° ° ° and ∆ . These homodesmotic schemes indicate that ∆ and ∆ for 1234TPhB are

° significantly higher than predicted by direct comparison with its analogue. The value of ∆∆

for R2 agrees with the previous observation that phenyl internal rotation in 1234TPhB (and also in 123tPhB) is less hindered than expected – in the naphthalene analogue the presence of the additional fused ring increases the rotational barrier of this vibrational motion. The results of

° ∆∆ indicate that 1234TPhB is able to maximize intermolecular interactions more effectively

than its naphthalene analogue – despite being a smaller non-polar hydrocarbon it has similar

° 11 ∆ .


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Figure 5. Homodesmotic reaction schemes for comparing polyphenylbenzenes with their naphthalene analogues.

Table 4. Enthalpy and entropy changes associated with the homodesmotic reaction schemes presented in figure 5.a

a

° ° ∆∆ / kJ.mol-1 ∆∆ / J·K-1.mol-1

° ∆ (g) / kJ.mol-1

R1

−1.7 ± 0.7

3.7 ± 1.9

−1.7 ± 2.9

R2

−27.9 ± 2.1

−43.0 ± 5.2

11.6 ± 7.9

The values for the naphthalene derivatives are presented in the supporting information and were

° ° products ° reactants taken from the literature;11,29,33,34 ∆∆ ∑ ∆ $ ∑ ∆ and

° ° g ° g, ° g, similarly for ∆∆ ; ∆ ∑ ∆ products $ ∑ ∆ reactants.


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Gas phase molecular energetics ° Figure 6 presents the plots of the enthalpies of formation in the gas, ∆ (g), and crystal, ° ∆ (cr), phases against n(Ph) for the ortho-polyphenylbenzenes series. Although the large ° ° magnitude of the values of ∆ (g) and ∆ (cr) dilutes the energetic differentiation across the

series, it is possible to observe the significant destabilization in PPhB and HPhB in both crystal

° and gas phases; more notorious in the crystal due to the contribution of the low ∆ .

° ° Figure 6. Plots of ∆ (g) = f {n(Ph)} (top) and ∆ (cr) = f {n(Ph)} (bottom).


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The molecular destabilization of the ortho- compounds is already noticed for n(Ph) = 4, ° as translated by the direct comparison of ∆ (g) between the two isomers 1234TPhB (484.6 ±

7.5, this work) and 1,2,4,5-tetraphenylbenzene (473.2 ± 6.1, from reference 9). The homodesmotic reaction scheme presented in figure 7, concerning the breaking of the phenylphenyl bonds in the ortho-polyphenylbenzenes to yield biphenyl, is a good model to evaluate the total interaction enthalpy between the phenyl substituents in a given polyphenylbenzene. The ° ° experimental results obtained for ∆)* (g) are presented in table 5. The values of ∆ (cr) for

all the compounds under study were determined by combustion calorimetry. Thus, in the ° calculation of the uncertainties in ∆)* (g), the contributions of the individual uncertainties of

CO2(g) and H2O(l) cancel out, and can be ignored in this comparative analisys.

Figure 7. Homodesmotic reaction scheme used for the evaluation of the total substituent interaction enthalpy in ortho-polyphenylbenzenes. Figure adapted from reference 9.


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° Table 5. Experimental values of ∆)* (g), at T = 298.15 K, for the individual homodesmotic

reactions presented in figure 7. n

° ∆)* (g) / kJ.mol-1

12dPhB9

2

−4.4 ± 3.0

123tPhB9

3

3.5 ± 4.6

1234TPhB

4

−5.3 ± 6.2

PPhB

5

−18.1 ± 7.4

HPhB9

6

−18.1 ± 6.7

° For n

Effect of Confined Hindrance in Polyphenylbenzenes - ACS Publications

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