Effect of Self-Association on the Phase Stability of ... - ACS Publications

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Effect of Self-Association on the Phase Stability of Triphenylamine Derivatives Carlos F. R. A. C. Lima,*,†,‡ José C. S. Costa,† André Melo,§ Hilário R. Tavares,‡ Artur M. S. Silva,‡ and Luıś M. N. B. F. Santos*,† †

CIQ, Departamento de Quı ́mica e Bioquı ́mica, Faculdade de Ciências da Universidade do Porto, P-4169-007 Porto, Portugal Department of Chemistry & QOPNA, University of Aveiro, P-3810-193 Aveiro, Portugal § LAQV-REQUIMTE, Departamento de Quı ́mica e Bioquı ́mica, Faculdade de Ciências da Universidade do Porto, P-4169-007 Porto, Portugal ‡

S Supporting Information *

ABSTRACT: The self-association equilibrium, i.e. formation of noncovalent dimers, in two triphenylamine derivatives, TPD (N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine) and mMTDAB (1,3,5-tris[(3-methylphenyl)phenylamino]benzene), in solution was evaluated by 1H NMR spectroscopy. The gas-phase energetics of the respective dimerization processes was explored by computational quantum chemistry. The results indicate that self-association is significantly more extensive in TPB than in TDAB. It is proposed that this fact helps to explain why TPB presents a stability higher than expected in the liquid phase, which is reflected in a lower melting temperature, a less volatile liquid, and possibly a higher tendency to form a glass. These results highlight the influence of self-association on the phase equilibria and thermodynamic properties of pure organic substances.

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INTRODUCTION Hole transport materials (HTMs) have been widely studied and functionalized as the hole transport layer in organic lightemitting diodes (OLEDs).1−10 One of the key challenges for developing high-performance OLEDs is the design of HTMs that present good thermal, chemical, and long-term stability, high efficiency of hole injection, and good hole mobility.6−10 Triphenylamine (TPA) derivatives have been recognized as important constituents of organic electronic devices, due to the excellent hole transport capabilities of their manufactured thin films. The good performance of TPA derivatives as HTMs is attributed to their thermal stability in the amorphous phase, relatively high hole mobility, and low ionization potentials. In this context much effort has been made to understand the relationship between their molecular and supramolecular structure and energetics and transport properties.11−18 This work explores the influence of subtle phenomena at the molecular level on the thermodynamic properties of pure substances. It focuses on understanding the effect of selfassociation, i.e. formation of noncovalent dimers, on the relative phase stability of organic compounds, in particular those with technological relevance, such as organic semiconductors (OSCs) with application in molecular electronics. Knowing and understanding the thermodynamic properties of these compounds is of prime importance for their rational and successful application as OSCs. In a recent work we have shown that self-association in oligothiophenes can significantly influence their thermodynamic properties: namely, increasing © 2015 American Chemical Society

the stability of the liquid phase and consequently contributing to a decrease of the melting temperature.19 Self-association can also promote some degree of organization and structuration at the nanoscale in apparently isotropic media. Nanostructuration in the liquid phase has already been reported in other systems, namely ionic liquids and aliphatic and aromatic hydrocarbons.20−24 Herein, a similar study was carried out for the TPA derivatives shown in Figure 1. The main objective was to explain why, according to previously reported thermodynamic results, pure liquid TPB shows an unexpected stabilization in comparison to TDAB.25 For this purpose the self-association equilibrium constants, Kass, were evaluated by 1H NMR spectroscopy in CDCl3. Since the solubility of TPB and TDAB in common organic solvents is insufficient for this NMR study, the more soluble analogues TPD and mMTDAB were used, in an attempt to reproduce the relative tendency of TPB and TDAB to self-associate. The experimental results were supported by the calculation of interaction energies for the gas phase dimers of TPB and TDAB by computational quantum chemistry methods. Some studies on the thermochemistry of triphenylamines and related compounds (e.g., triphenylalkanes and derivatives) can be found in the literature;25−27 however, the data are still scarce and insufficient for an adequate understanding of Received: February 2, 2015 Revised: May 25, 2015 Published: June 2, 2015 6676

DOI: 10.1021/acs.jpca.5b01079 J. Phys. Chem. A 2015, 119, 6676−6682

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The Journal of Physical Chemistry A

resolution of 0.01 mg; (2) immediately before each NMR spectrum was recorded, the respective sample was dissolved in the NMR tube by adding 0.600 mL of CDCl3 with a micropipet (P1000). By adoption of this methodology, no sign of mMTDAB decomposition was detected in the NMR spectra. The concentrations of both compounds ranged from 0.02 to 0.3 mol dm−3 and were chosen to ensure a significantly large experimental interval with a balanced distribution of data points. In order to attain greater variation in chemical shift, Δδ, with concentration, high molar concentrations were preferred, taking into account the solubility limit of the compounds at T = 295 K in CDCl3 and the reproducibility of the experimental conditions for both compounds. Care was taken in sample handling to avoid solvent evaporation and/or solute precipitation to any significant extent. The self-association equilibrium constants, Kass, were determined from a nonlinear fitting of the chemical shift and concentration data and assuming the equilibrium

Figure 1. Triphenylamine derivatives studied in this work and the adopted acronyms.

2A(soln) ⇌ A 2(soln)

(1)

where A represents a given triphenylamine derivative. For each measured concentration the chemical shift, δcalc, was calculated as

structural/energetic relationships in this type of compounds. Therefore, the present work also intends to contribute to a deeper understanding of the factors ruling molecular and supramolecular energetics in triphenylamine derivatives.

δcalc = p(A) ·δcalc(A) + p(A 2) ·δcalc(A 2)

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(2)

where δcalc(A) and δcalc(A2) are the calculated chemical shifts of pure monomer A and dimer A2, respectively, and p(A) and p(A2) are their relative equilibrium populations, calculated as p(A) = [A]/([A] + [A2]) and p(A2) = 1 − p(A), where the equilibrium concentrations [A] and [A2] were computed by considering the expression of Keq for eq 1 and the equation of mass balance [A]T = 2[A2] + [A], where [A]T is the total initial concentration of the A sample. In this way the fitting parameters were Kass, δcalc(A), and δcalc(A2). The fitting was applied to each well-resolved aromatic peak of the compounds so as to minimize the quantity ∑(δcalc − δobs)2, where δobs is the observed chemical shift at each concentration, and Kass was taken as the average of all concordant values. Computational Details. All quantum chemical calculations were performed using the Gaussian 09 software package.28 The calculation of the electronic interaction energies, ΔintEm, corrected for BSSE by the counterpoise method,29,30 for the gas phase dimers of TPB and TDAB were performed at the M06-2X/6-31+G(d,p)//M06-2X/cc-pVDZ level of theory (geometry optimization using M06-2X/cc-pVDZ, without correction for BSSE, followed by a BSSE single-point energy calculation using M06-2X/6-31+G(d,p)).31 The use of the M06-2X functional ensures that dispersive interactions, which are essential for describing the energetics of the dimers studied, are taken into account.31 In order to account for the electronic geometry distortion energy of dimerization, ΔdistEm, the geometry optimizations of the respective TPB and TDAB isolated monomers were carried out at the M06-2X/631+G(d,p) level of theory. High-Resolution Scanning Electron Microscopy. Thin films of TPB and TDAB were obtained by vacuum deposition using the combined Knüdsen/quartz crystal effusion apparatus described in detail by Santos et al.32 The compounds were deposited onto the respective surface (ITO and/or gold) by effusion from a Knüdsen cell, which was kept at T = 500 K for TPB and at T = 508 K for TDAB, while the target surface was kept at ambient temperature (T ≈ 298 K) for ITO and at T ≈ 263 K for gold; the effusion time was 180 min. The topographic

EXPERIMENTAL SECTION 1 H NMR Dilution Experiments. The compounds TPD (N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine) and mMTDAB (1,3,5-tris[(3-methylphenyl)phenylamino]benzene) were purchased from Sigma-Aldrich and purified by recrystallization and sublimation under reduced pressure, as previously reported.25 The purity of the samples was confirmed by gas chromatography, showing a %(m/m) purity higher than 99% in both cases. The 1H NMR spectra in CDCl3, at T = 295 K, were recorded on a Bruker Avance 300 spectrometer (300.13 MHz) for various sample concentrations using TMS as the reference. The dilution experiments for TPD were carried out in the following way: (1) three samples of the compound were prepared in separate flasks and weighed in a Mettler AE163 balance with a resolution of 0.01 mg; (2) CDCl3 was added to each sample with a micropipette (P1000), ensuring that all the compound dissolved; (3) an aliquot of each sample was poured into the respective NMR tube with a micropipet; (4) the NMR spectrum was acquired; (5) the sample was diluted directly in the NMR tube by adding a known amount of pure CDCl3 with a micropipet, and the NMR spectrum was recorded again; (6) step 5 was repeated for various sample concentrations. The experimental procedure for mMTDAB was slightly different, because this compound was found to decompose significantly when it was dissolved in CDCl3 and exposed to sunlight (a strongly colored purple solution results from direct sunlight irradiation in seconds). In fact, in the 1H NMR spectrum of this purple solution the disappearance of the three-proton singlet at δ ∼6.4 ppm and the appearance of a new broad peak at δ ∼6.2 ppm (integrating to slightly less than three protons) was observed, while the rest of the spectrum remained unchanged, which suggests the presence of NH groups in the decomposition product. In order to avoid significant mMTDAB decomposition during the NMR dissolution experiments, these were carried out as follows: (1) 15 samples of different amounts of the compound were directly prepared in separate NMR tubes and weighed on a Mettler AE163 balance with a 6677


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The Journal of Physical Chemistry A images of the thin films were obtained by high-resolution scanning electron microscopy with X-ray microanalysis and backscattered electron diffraction pattern analysis, with a FEI Quanta400FEG/EDAX Genesis X4M instrument at 15 kV in low-vacuum mode at the CEMUP (Centro de Materiais da Universidade do Porto). Images were acquired using a secondary (SE) detector.

Table 1. Self-Association Equilibrium Constants, Kass, and Standard Molar Gibbs Energies, ΔassG°m, in CDCl3, at T = 295 K, for the Two Triphenylamine Derivatives Studied, As Measured by 1H NMR Spectroscopy

RESULTS AND DISCUSSION Self-Association in Solution. Figure 2 presents the concentration dependence of chemical shifts for TPD in

a

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compound

Kass

ΔassG°m/kJ mol−1

TPD mMTDABa

0.36 ± 0.03

2.5 ± 0.2

Kass could not be calculated for mMTDAB from the 1H NMR spectral datathe linear Δδ = f(concentration) dependence (absence of any significant curvature) indicates that Kass is negligible.

comparison between Kass for TPD and mMTDAB is more accurate, since both measurements were conducted under similar experimental conditions. Moreover, the polynomial fittings of Δδ = f(concentration) for all of the well-defined peaks of TPD (Figure 2) have r2 > 0.99, increasing the confidence of the derived value of Kass. As noted before, the Kass value could only be evaluated by 1H NMR for the more soluble derivatives TPD and mMTDAB (TPB and TDAB were found not to be soluble enough in order to produce reliable NMR data). However, the −CH3 substituents in TPD and mMTDAB should not affect self-association significantly, at least in what concerns the relative comparison between Kass for TPB/TDAB and TPD/mMTDAB. The contribution of the −CH3 groups to the intermolecular interactions is expected to be of minor importance in comparison to the main driving force for selfassociationthe aromatic interactions between the TPA derivatives (as can be anticipated from Figure 3, the −CH3 groups can be easily positioned without compromising the interaction). Hence, it can be assumed that the same trend in self-association holds for the less soluble TPB and TDAB. Therefore, these results indicate that self-association in CDCl3 is more extensive in TPD (or TPB) than in mMTDAB (or TDAB). Self-Association in the Gas Phase. The M06-2X/ccpVDZ optimized geometries for the gas phase TPB and TDAB dimers are shown in Figure 3. The computational search of the potential energy surfaces of the dimers, respecting the relative orientation of the two monomers, indicated the existence of two stable conformers for the TPB dimer, while for TDAB only one such conformation was found (the optimizations were carried out from various different initial geometries, and the structures shown in Figure 3 were found to be the minimum energy configurations). The structures obtained indicate that the two monomers are oriented in a way that favors the establishment of intermolecular aromatic interactions between the phenyl rings. With the exception of TPB (dimer 2) the monomers overlap significantly in the dimer. The BSSE corrected electronic interaction energies, ΔintEm, for the dimers of TPB and TDAB, calculated at the M06-2X/631+G(d,p)//M06-2X/cc-pVDZ level of theory, are presented in Table 2. The interaction energies, at T = 0 K, are uncorrected for ZPE (zero-point energy) and thermal contributions to enthalpy and entropythese contributions are expected to be similar for TPB and TDAB (not so different molecular mass and similar bonding structure), and thus they can be neglected for qualitative purposes (relative comparisons between interaction energies). To the best of our knowledge, experimental values for the enthalpies of formation in the condensed phase and/or gas phase for the triphenylamine derivatives studied herein are not available in the literature. Hence, at the present time we cannot effectively compare

Figure 2. Concentration dependence of chemical shifts in CDCl3, at T = 295 K, for the five well-defined peaks of TPD: (■) H1, r2 = 0.997; (●) H2, r2 = 0.997; (○) H3, r2 = 0.996; (+) H4, r2 = 0.995; (□) CH3, r2 = 0.997. [A]T denotes the total concentration of TPD in the sample.

CDCl3. The chemical shift data are presented as Δδ = δ(ci) − δ(cmax), where δ(ci) and δ(cmax) are, respectively, the observed chemical shifts at the given sample concentration ci and at the highest concentration studied cmax. The observed behavior is consistent with the formation of stacked dimers in solutionas the concentration of TPD increases, the self-association equilibrium is shifted in the direction of forming more dimer and the observed signals shift upfield.19,33 The Δδ = f(concentration) correlation showed a significantly more pronounced curvature for TPD than for mMTDAB, evidencing that, in contrast to TPD, mMTDAB does not self-associate to any significant extent in CDCl3 solution. For this reason it was only possible to derive a reliable value for the self-association equilibrium constant, Kass, for TPD. The detailed experimental NMR results are presented as Supporting Information. The derived Kass value for TPD is presented in Table 1, alongside the standard molar Gibbs energy of self-association, calculated as ΔassG°m = −RT ln(Kass), where T = 295 K. The Kass value obtained for TPD is relatively small and does not ensure that the experimental errors (derived from sample concentration, variations in the reference peaks, other contributions to Δδ, etc.) are made virtually irrelevant.34 However, although the individual results of Kass can be significantly influenced by experimental errors, the qualitative 6678


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31+G(d,p) level) of, respectively, the monomer with the geometry it adopts in the dimer and the optimized isolated monomer. The computational results obtained indicate that intermolecular interactions are significantly stronger in TDABΔintEm is higher (more negative) when excluding the contribution of distortion energy (the molecules have the same geometry as isolated monomers and in the dimers). However, when ΔdistEm is taken into account, the order in ΔintEm is invertedas can be observed, this contribution is quite substantial, particularly in TDAB. Likewise, while the intermolecular interactions in TPB (dimer 1) are significantly stronger than those in TPB (dimer 2), which is in agreement with the fact that the monomers of TPB are significantly more overlapped in dimer 1, a more pronounced geometry adjustment of the monomers is required in dimer 1. This is translated into a higher ΔdistEm value, and the net effect yields a similar ΔintEm value. According to the computational values of ΔintEm obtained, a (more negative) ΔassG°m value significantly lower than that measured by 1H NMR in CDCl3 (Table 1) could be expected. However, the effect of solvation is certainly enough to account for this discrepancy, for both enthalpic and entropic reasons. The computational results indicate that, in the gas phase, selfassociation is energetically/enthalpically more favored in TPB than in TDAB. Hence, both computational and experimental 1 H NMR results agree that TPB/TPD show greater tendency to self-associate than TDAB/mMTDAB. Self-Association and Phase Equilibria of Triphenylamine Derivatives. Assuming that self-association in the gas phase and in solution is more extensive in TPB than in TDAB, the same trend can be extrapolated, at least qualitatively, to the pure liquid phase of these substances. The greater or smaller tendency to self-associate will be reflected in a greater or smaller tendency of the molecules to form organized regions in the liquid, consisting of transient dimers or larger selfassociated aggregates. On average, this brings up some heterogeneity and structuration at the nanoscale of the otherwise purely isotropic liquid and influences its thermodynamic stability.19−24 For instance, a higher tendency to selfassociate in solution can be reflected in an average state of the pure liquid phase that has a more significant statistical contribution from locally ordered structures (e.g., dimers) and a weaker character of a purely randomized liquid. A recent work by Costa et al. on the phase change thermodynamics of some TPA derivatives, including TPB, TPD, TDAB, and mMTDAB, indicated a significant but unexpected thermodynamic differentiation between TPB and TDAB.25 Those results are summarized in Table 3, and they clearly show a discrepancy between the sublimation and fusion

Figure 3. M06-2X/cc-pVDZ optimized geometries for the gas phase dimers of TPB and TDAB.

Table 2. Electronic Interaction Energies, ΔintEm (Corrected for BSSE and Uncorrected for ZPE), Calculated at the M062X/6-31+G(d,p)//M06-2X/cc-pVDZ Level of Theory for the Gas Phase Dimers of TPB and TDAB ΔintEm/kJ mol−1 compound

excluding ΔdistEma

including ΔdistEmb

TPB (dimer 1) TPB (dimer 2) TDAB

−84 −66 −113

−58 −55 −47

a

Computational results excluding the contribution of the geometry distortion energy, ΔdistEm, of the two monomers associated with the formation of the dimer. bComputational results including ΔdistEm for both monomers.

experimental and computational results about molecular energetics of triphenylamines. The geometries of the isolated monomers were obtained at the M06-2X/6-31+G(d,p) level and are in agreement with the reported X-ray crystallographic structures.35,36 When these geometries are compared with those that the monomers adopt in the dimer complex, one can also observe that the monomers significantly distort their geometry in order to maximize intermolecular interactions. This fact suggests that distortion energy may have a significant contribution for the selfassociation process. Table 2 presents the ΔintEm values both excluding and including the contribution from geometry distortion energy, which was calculated as Δ dist E m = Em,monomer(dimer) − Em,momomer, where Em,monomer(dimer) and Em,monomer are the calculated energies (at the M06-2X/6-

Table 3. Standard Molar Enthalpies and Entropies of Fusion and Sublimation, at T = 298.15 K, for the Compounds Studieda ΔH°m(T = 298.15K)/kJ mol−1 compound TPB TPD TDAB mMTDAB a

6679

fusion

sublimation

± ± ± ±

198.5 ± 2.0

35.9 33.2 50.7 42.4

2.1 1.4 2.3 1.6

200.8 ± 2.0

ΔS°m(T = 298.15K)/J K−1 mol−1 fusion

sublimation

± ± ± ±

294.0 ± 5.2

65.2 71.7 89.5 89.1

5.3 3.9 5.7 4.2

298.1 ± 5.2

Values taken from the work of Costa et al.25 DOI: 10.1021/acs.jpca.5b01079 J. Phys. Chem. A 2015, 119, 6676−6682

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cancel out in the phase equilibria differential analysis, and for this purpose it is safe to assume equal H(g) and S(g) values. From inspection of Figure 4 it is clear that TPB, relative to TDAB, has an extra factor of stabilization that is only exhibited in the liquid phase. The effect of this factor is to lower H°m(l) (stabilizing the liquid) but to decrease S°m(l) (destabilizing it). However, and as evidenced by the lowering of Tm in TPB, the net effect is to increase the stability of the liquidlower G°m(l)or else this outcome would simply not occur spontaneously. The higher tendency of TPB for self-association can nicely fit to these observations. A higher fraction of associated molecules exist in liquid TPB. Moreover, a higher tendency to selfassociate can also be reflected on a greater propensity of the molecules in the liquid to establish specific intermolecular interactions. This contributes for increasing the cohesive energy of the liquid and lowers H°m(l) (and hence ΔfusH°m), but it comes with an associated entropic penalty deriving from the greater number of self-associated molecules and the higher degree of organization that stronger and more specific interactions generally imply (enthalpy/entropy compensation), thus lowering S°m(l) (and hence ΔfusS°m). The net effect is to decrease G°m(l) and thus increase the temperature range at which the most stable phase of the pure compound is the liquid. It is interesting to note that TPB exibits a greater ΔvapH°m than TDAB, despite having lower molecular mass and fewer functional groups. The greater tendency of TPB to selfassociate is also in accordance with this fact, since it can be related with a higher propensity of TPB to establish stronger intermolecular interactions in the liquid. There is no sublimation data for TPD and mMTDAB, but a comparison of their fusion results (see Table 3) suggests that they may follow the same tendency of TPB and TDAB, with the difference being that the presence of the methyl groups disturbs crystal packing and destabilizes the solid, which results in lower Tm values for TPD (442.2 K) and mMTDAB (455.8 K).25 Again, the more extensive self-association in TPD may also contribute for its lower Tm when compared to mMTDAB. It is interesting to note that, on comparison of the selfassociation results for the triphenylamines studied herein and the oligothiophenes,19 greater thermodynamic differentiation in the liquid phase between interrelated compounds occurs for those presenting greater differences in Kass (in this case TPB and TDAB). This further supports that self-association significantly influences the thermodynamic stability of the liquid phase and hence leads to observable changes in phase equilibria. The influence of self-association may also be noted in glass transitions and respective glass-transition temperatures, Tg. The reason why no Tg was observed for TDAB yet, while for TPB Tg = 350.0 ± 1.5 K,25,37 can also be related with the lower tendency of TDAB to self-associate in the liquid. Because a lower tendency to self-associate lowers the stability of the liquid, it is expected that the glass (which is in many ways similar to a frozen liquid) of such a compound will also not benefit from that additional stabilization, and thus its formation will be less favored thermodynamically relative to the formation of the crystalline solid. Intermolecular interactions in the glass are always weaker than those in the crystalline solid; thus, formation of a glass comes always with a cost in cohesive energy. When self-association is significant, this energetic penalty can be lowered and the formation of the glass becomes

results of TPB and TDABwhile the standard molar enthalpies, ΔsubH°m, and entropies, ΔsubS°m, of sublimation are similar for both compounds, the standard molar enthalpies, ΔfusH°m, and entropies, ΔfusS°m, of fusion are markedly different. This indicates that, in a relative thermodynamic sense, the solid and gas phases of TPB and TDAB are similar but some factor is granting an additional source of differentiation between their liquid phases. Both solids have similar volatility, but TPB (Tm = 504.6 K) clearly presents a lower melting temperature, Tm, than TDAB (Tm = 526.3 K).25 This can also be observed in the derived standard molar Gibbs energies of sublimation, ΔsubG°m, and vaporization, ΔvapG°m while the ΔsubG°m values are similar for TPB (110.9 ± 2.5) and TDAB (111.9 ± 2.5), the ΔvapG°m value for TPB (94.5 ± 3.6) is considerably higher than that for TDAB (87.9 ± 3.8) (values in kJ mol−1).25 This is in accordance with the greater stability, and hence lower volatility, of liquid TPB. In order to better visualize this differentiation in the liquid phase, Figure 4 presents the relative enthalpic and entropic phase diagrams of TPB and TDAB. Herein it is relevant to say that most factors that can differentiate between H(g) and S(g) in these compounds are intramolecular in nature and are also reflected to a great extent in the relative stabilities of the solid and liquid phases. Hence, their contributions are expected to

Figure 4. Enthalpy and entropy diagrams for the fusion and sublimation of TDAB (left) and TPB (right). The values report ΔsubH°m and ΔfusH°m (kJ mol−1) and ΔsubS°m and ΔfusS°m (J K−1 mol−1), at T = 298.15 K.25 6680


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substances and in some cases lead to important energetic differentiation between similar compounds.

a more probable route. The higher tendency of TDAB to crystallize can be nicely illustrated by the SEM images presented in Figure 5 of TPB and TDAB thin films obtained

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ASSOCIATED CONTENT

* Supporting Information S

Text, figures, and tables giving detailed results of the 1H NMR dilution experiments and computational calculations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b01079.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for C.F.R.A.C.L.: [email protected]. *E-mail for L.M.N.B.F.S.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Fundaçaõ para a Ciência e Tecnologia (FCT), Lisbon, Portugal, and the European Social Fund (ESF) for financial support to the CIQ, University of Porto (Projects: PEst-C/QUI/UI0081/2011, FCUP-CIQ-UP-NORTE-070124-FEDER-000065, PTDC/AAC-AMB/121161/2010, UID/QUI/50006/2013), and to the Organic Chemistry Research Unit (Project: PEst-C/QUI/UI0062/2013). C.F.R.A.C.L. and J.C.S.C. also thank the FCT and the European Social Fund (ESF) under the third Community Support Framework (CSF) for the award of the Research Grants SFRH/BPD/77972/2011 and SFRH/BD/74367/2010, respectively.

Figure 5. Topographic images obtained by SEM of TPB and TDAB thin films deposited onto gold and ITO surfaces. Images were acquired by using a secondary electron detector (SE).

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by vacuum deposition onto gold and ITO surfaces. More detailed results on the morphology of TPA thin films can be found elsewhere.25 Using the same deposition conditions it can be observed that, while TPB forms amorphous films, TDAB forms well-defined crystals. Moreover, it can be observed that TDAB forms crystals on both surfaces, which were also kept at different temperatures (T ≈ 263 K for gold and T ≈ 298 K for ITO) during deposition.

REFERENCES

(1) Zhao, X.; Zhan, X. Electron Transporting Semiconducting Polymers in Organic Electronics. Chem. Soc. Rev. 2011, 40, 3728− 3743. (2) Figueira-Duarte, T. M.; Müllen, K. Pyrene-Based Materials for Organic Electronics. Chem. Rev. 2011, 111, 7260−7314. (3) Tao, Y.; Yang, C.; Qin, J. Organic Host Materials for Phosphorescent Organic Light-Emitting Diodes. Chem. Soc. Rev. 2011, 40, 2943−2970. (4) Huang, J.; Su, J.-H.; Tian, H. The Development of Anthracene Derivatives for Organic Light-Emitting Diodes. J. Mater. Chem. 2012, 22, 10977−10989. (5) Costa, J. C. S.; Lima, C. F. R. A. C.; Santos, L. M. N. B. F. Electron Transport Materials for Organic Light-Emitting Diodes: Understanding the Crystal and Molecular Stability of the Tris(8hydroxyquinolines) of Al, Ga, and. J. Phys. Chem. C 2014, 118, 21762− 21769. (6) Shirota, Y.; Okumoto, K.; Inada, H. Thermally Stable Organic Light-Emitting Diodes Using New Families of Hole-Transporting Amorphous Molecular Materials. Synth. Met. 2000, 111, 387−391. (7) Kato, T.; Mori, T.; Mizutani, T. Effect of Fabrication Conditions on Photoluminescence and Absorption of Hole Transport Materials. Thin Solid Films 2001, 393, 109−113. (8) Koene, B. E.; Loy, D. E.; Thompson, M. E. Asymmetric Triaryldiamines as Thermally Stable Hole Transporting Layers for Organic Light-Emitting Devices. Chem. Mater. 1998, 10, 2235−2250. (9) Tang, C. W.; Vanslyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913−915. (10) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-Emitting Diodes Based on Conjugated Polymers. Nature 1990, 347, 539−541. (11) Cocherel, N.; Leriche, P.; Ripaud, E.; Gallego-Planas, N.; Frère, P.; Roncali, J. Electropolymerization of Triphenylamine−Dithiafulvene

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CONCLUSIONS In summary, by a combined experimental 1H NMR and computational study, it was found that TPB has a higher tendency than TDAB to self-associate, i.e. form noncovalent dimers, in solution, in the gas phase, and, by analogy, in the liquid phase. It was proposed that the more extensive selfassociation in TPB contributes significantly to the stabilization of the liquid phase, which results in a lower melting temperature and lower volatility. Self-association increases the stability of the liquid phase because it increases the cohesive energy of the liquid by promoting the establishment of more specific, and hence stronger, intermolecular interactionsthis decreases H°m(l) and thus ΔfusH°m. However, this comes with an associated, but not as important (or else self-association would not be observed), entropic penaltythis decreases S°m(l) and thus ΔfusS°m. The effect of self-association on phase stability is thus to stabilize the liquid and alter the melting and vaporization properties of the pure substance. Moreover, it was suggested that an increased tendency to self-associate can also contribute to an increased tendency of a substance to form a glass. This work evidences how self-association can have a noticeable influence on the thermodynamic properties of pure 6681


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