Article pubs.acs.org/JPCA
Infrared Investigations of 4‑Hydroxycyanobenzene Single Crystals E. Capria,† L. Benevoli,† A. Perucchi,† B. Fraboni,‡ M. Tessarolo,§ Stefano Lupi,∥ and A. Fraleoni-Morgera*,† †
Organic Optoelectronics Laboratory, Sincrotrone Trieste SCpA, SS 14, km 163.5, 34149 Basovizza (TS), Italy Department of Physics and Astronomy, University of Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy § Eurotech SpA, V. Fratelli Solari 3/a, 33020 Amaro (UD), Italy ∥ CNR-IOM and Department of Physics, Università di Roma “La Sapienza”, Piazzale Aldo Moro 2, 00185 Roma, Italy ‡
S Supporting Information *
ABSTRACT: 4-Hydroxycyanobenzene (4HCB) single crystals (SCs) and polycrystals (PCs) have been analyzed by means of both unpolarized and linearly polarized (LP) infrared (IR) beams. Most of the signals found at room temperature (298 K) were assigned to welldefined vibrational modes. Using an LP-IR beam and keeping the beam polarization aligned with either the a or the b crystal axis, anisotropic spectra of SCs were also attributed. The differences between the LP and unpolarized spectra of SCs are discussed in view of spatially anisotropic vibronic couplings between the benzenic π electrons and the molecular functional groups (FGs), with reference to the overall lattice arrangement and the polarizability of the FGs. In addition, signals suggesting the low-concentration presence of tautomers within the crystal were detected. LP-IR measurements of SCs in the temperature range between 298 and 120 K are also reported and discussed, with particular reference to the hydrogen-bonding-related functional groups of 4HCB, allowing the assignment of OH bending signals that were otherwise not clearly attributable and the inference of an anisotropic shrinking of the crystals. Overall, the presented results show that LP-IR spectroscopy is a valuable tool for noncontact, nondestructive characterization of organic semiconducting single crystals.
I. INTRODUCTION Organic semiconducting single crystals (OSSCs) are considered model systems for probing and investigating charge transport and electronic properties of organic semiconductors,1−5 thanks to their low density of defects, absence of grain boundaries or different crystalline phases, as well as their overall ordered (though often complex) structural arrangement. However, the available data on the subject show some variability, mainly attributed to problems with crystal purity.6,7 Another interesting feature of OSSCs is their established anisotropic transport behavior, which is related to their intrinsic structural asymmetry and is considered to be a signature of good electronic quality.1 To date, however, general correlations between the structural features and the anisotropic electronic properties in OSSCs are not completely clear, despite the considerable impact that such knowledge would have on the possibility of rationally designing organic semiconductors for practical applications.8−10 This lack of information is mainly due to the complex spatial arrangement of the asymmetric molecules in the crystal lattices and to the mentioned limited reproducibility in the electronic behavior of the crystals. In this view, wide-band-gap (4.2 eV) crystals grown from solution, based on the dipolar 4-hydroxycyanobenzene (4HCB) molecule, evidenced well-reproducible three-dimensional and two-dimensional anisotropic electronic properties,11−13 suggesting their use as model systems for exploring structure− electronic property correlations in OSSCs. The characterization of OSSCs can be carried out using infrared (IR) analysis, which is sensitive to the relative position of the functional groups in © 2013 American Chemical Society
single molecules and to intermolecular interactions. In particular, linearly polarized infrared (LP-IR) spectroscopy can provide a wealth of very detailed information on the properties of organic crystals, allowing even the discrimination between different polymorphs.14,15 Thanks to this sensitivity, LP-IR spectroscopy has already been exploited for probing crystalline charge-transfer organic conductors, evidencing an actual two-dimensional anisotropy of their electronic structure,16−18 although surprisingly, to the best of our knowledge, until recently, only one attempt of exploiting LP-IR spectroscopy for OSSCs was reported.19 We hence approached the LPIR analysis of 4HCB single crystals, showing that this technique allows the determination of the orientation of the main crystallographic axes with no need for X-ray diffraction (XRD) (provided that this characterization has been carried out on the crystal at least once), the identification of the displacement of the transition moment vector upon electrical polarization, and anisotropic electrical polarizability.20 To gain further information on the correlations existing between the 4HCB crystal structure and its electronic properties, a more detailed and accurate picture of its general LP-IR features is needed. Therefore, we report here an IR investigation of 4HCB polycrystals (PCs) and single crystals (SCs) that significantly extends and deepens the work aimed at the determination of the IR features of 4HCB crystals already reported in ref 20. In Received: May 22, 2013 Revised: July 4, 2013 Published: July 5, 2013 6781
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III. RESULTS AND DISCUSSION III.A. Room-Temperature Analysis. III.A.1. General IR Features of 4HCB Polycrystals and Single Crystals. The IR analysis of 4HCB PCs was carried out with an unpolarized beam, whereas that of SCs was conducted by means of both unpolarized and linearly polarized (LP) beams. Most of the observed signals were attributed on the basis of established general IR attributions21,22 and previous specific work performed on 4HCB molecules in solution by other researchers.23,24 When some disagreement between the two cited sources was found, we privileged the available specific literature on 4HCB with respect to the general IR assignments. In view of the rather consistent work recently carried out on these systems11−13,20,25,26 and forthcoming reports on the subject, we believe that it is useful to report here the actual spectral shapes and features of the 4HCB crystals (Figure 1), whereas relatively detailed assignments are reported in Table S1 of the Supporting Information. In general, we observed that, under unpolarized IR light, the spectrum of a 4HCB single crystal (Figure 1, cyan curve) is not dramatically different from
particular, most of the signals found for PCs and SCs at room temperature are assigned to precise vibrational modes. For SCs, a further refinement of the data is carried out using an LP-IR beam, aligned with the two main crystallographic axes, as well as at various intermediate angles. The collected spectral data are discussed and interpreted in terms of functional group assignments and physicochemical phenomena such as polarizations, solid-state tautomerisms, hydrogen-bonding-related structural features, and vibronic couplings between the benzenic π electrons and the CN and OH functional groups. The relations of these parameters to the molecular arrangement in the crystal lattice are also, when possible, considered. Finally, LP-IR measurements in the temperature range between 298 and 120 K are reported, together with more signal assignments enabled by the low-temperature probing, and the behavior in selected spectral ranges signaling anisotropic thermal shrinking is discussed and commented.
II. EXPERIMENTAL SECTION 4-Hydroxycyanobenzene (4HCB) was purchased from Fluka (≥97%), and ethylic ether (≥99.5%) was obtained from Aldrich. Raw 4HCB crystals were purified as explained elsewhere,25 whereas ethylic ether was used without further treatment. To allow transmission measurements, very thin single crystals (50−150 μm thick) of 4HCB were grown on silicon wafers with low IR absorption (about 50%). 4HCB single crystals were grown from a starting 4 mg/mL solution in ethyl ether. The solution was prepared and then filtered and poured into a beaker containing silicon chips. The beaker was then covered with aluminum foil and left in a thermostatic room at 6 °C to slow the evaporation. In 48 h, the solvent evaporated, leaving 4HCB crystals on the silicon surface that were used with no further treatment. Infrared measurements on polycrystals (obtained after depositing a 4HCB solution in CHCl3 onto a clean KBr window) were performed on a Bruker Tensor 27 instrument, whereas those on single crystals were carried out with a Bruker IFS66v spectrometer, coupled to a Hyperion 2000 infrared microscope equipped with a liquid-N2-cooled HgCdTe detector. In both cases, spectra were collected at 2 cm−1 resolution with 256-scan data sets. Linearly polarized (LP) IR spectra were collected by inserting a wire-grid polarizer along the incoming beam path, keeping the light polarization vector perpendicular to the c axis of the 4HCB elementary cell, at different angles with respect to the a and b axes. To correlate the IR polarization angle with the crystallographic direction, the axes of the crystals were previously identified by means of current−voltage measurements, as described in ref 20. Low-temperature measurements were performed by inserting the sample into a homemade thermostatic system equipped with a cool plate. Temperature control resulted from the balance between the heat subtracted from the plate by a constant flow of liquid nitrogen and the heat supplied to it using a thermoresistor driven by a power supply. Measurements at 279 K were performed by substituting the liquid nitrogen with a constant flow of a 50/50 ethylene glycol/water solution, pumped through the cold plate with a Grant C2G\GP-200 refrigerated circulating bath. All measurements with this experimental setup were carried out under a nitrogen gas atmosphere to avoid humidity condensation.
Figure 1. Infrared spectra of 4HCB polycrystals and single crystals: orange, unpolarized IR spectrum of polycrystals; blue, unpolarized IR spectrum of single crystals; red, LP-IR spectrum of single crystals, E∥a; green, LP-IR spectrum of single crystals, E∥b. For the LP-IR spectra of single crystals, the gray plots were obtained at beam polarizations intermediate between E∥a and E∥b. In the insets, selected spectral zones of the LP-IR spectra of single crystals are shown. Panels I and II present the spectral ranges of 650−1650 and 1650−3500 cm−1, respectively. 6782
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visible. This feature is particularly useful for assessing the crystallographic orientation of the 4HCB crystals by simple LPIR measurements, without the need for XRD measurements, as previously reported.20 The very strong peak for CN stretching is visible at 2234 cm−1 (Q2). In unpolarized IR spectra of PCs and SCs and LP-IR spectra of SCs, this signal is blue-shifted (15 cm−1) from that obtained in dilute solutions,21 in line with the known behavior of proton-accepting functional groups (FGs) with increasing hydrogen-bonding strength.27,28 For LPIR-probed SCs, a small difference (about 4 cm−1, higher than the instrumental resolution of 2 cm−1) was noticed between the E∥a and E∥b polarizations, whereas the overall signal intensity increased by about 17% on moving from E∥a to E∥b. Finally, the broad, very intense peak centered around 3290 cm−1 (U) due to the OH bond stretching is considerably red-shifted in PCs and unpolarized SCs with respect to the corresponding signal obtained from the spectrum in solution (about 300 cm−1),23 again in line with the known features of OH groups involved in hydrogen-bond interactions. Under LP-IR probing, an appreciable maximum shift (about 13 cm−1) was found between E∥a and E∥b (Figure 1, panel II, label U). In general, it is interesting to notice that, at room temperature, the most evident spectral variations in terms of LP-IR-originated dichroism were related to benzenic ring vibrations, a finding we attribute to the larger spatial extent of the benzenic π orbitals with respect to the OH and CN ones, and to the intrinsic asymmetry of the benzenic ring, which allows anisotropic interactions of its π electrons with the incoming polarized IR radiation, depending on their mutual orientations. III.A.2. Analysis of the LP-IR Behavior of CN and OH Stretching Vibrations. It is well-established that IR absorption occurs when a chemical species presents a variation in its dipole moment upon reaching a vibrational excited state. In turn, this is caused by the interaction between E, the electric field vector of the incoming light beam, and M, the transition moment vector of the molecule (i.e., the variation of the dipole moment with the change in normal coordinates upon photoexcitation). It is useful to recall that E is usually unpolarized and, as such, interacts with matter in an isotropic fashion, unless the matter is intrinsically anisotropic. On the other hand, a linearly polarized electric field E cane be used to probe an intrinsically anisotropic sample, delivering interactions strongly dependent on the angle between E and M. The probe of these interactions can give interesting information on the nature and properties of the considered sample. Finally, it is important to recall also that M is different from μ0, the ground state dipole moment, and that M might have a different direction from μ0.22 Under reasonable assumptions (described in detail in the Supporting Information), the absorbance of LP-IR light due to the vibrational transition of a certain functional group is proportional to
that of a PC (Figure 1, orange curve). This is in line with the structural nature of PCs, which differ from SCs only in having several randomly oriented crystalline domains (with sizes ranging from a few hundred micrometers to a few millimeters). The few observable differences (see, for example, the 650−730 cm−1 zone in Figure 1, panel I) can be attributed, at least partially, to the different conditions under which the two types of spectra were recorded (see the Experimental Section). On the other hand, LP-IR spectra of 4HCB SCs having polarization directions aligned with the crystallographic axes present several points of interest, especially when compared with the corresponding unpolarized SC and PC spectra. In general, in the LP-IR spectra of SCs, a clear and marked anisotropy of absorption (in terms of spectral shapes, peaks intensities, and positions) along the two main crystallographic axes a and b was observed at any polarization angle (gray curves between LP E∥a and E∥b spectra in Figure 1), and maxima and minima of the spectra were always found corresponding to beam polarization parallel to either the a or b axis (E∥a or E∥b, respectively). For example, referring to Figure 1, panel I (inset a), it is possible to see that, for multiplet A, changing polarization from E∥b (green curve) to E∥a (red curve) results in the peak at 671 cm−1 (A1) disappearing and leaving the place to another marked signal (A2) at 693 cm−1. Peak A3 remains little affected by the polarization change (indeed, it is clearly visible for the unpolarized spectra of both PCs and SCs as well). A triplet of very weak peaks (C) between 947 and 966 cm−1, associated with CH and ring deformations and predicted theoretically (but not observed in solution) by Binev,23 is visible in the unpolarized IR spectrum of the SC and in the LPIR spectrum for E∥a only, but not for E∥b. The LP-IR peak at 1106 cm−1 (D), attributed to CH and benzenic ring deformation and to CC stretching, increases in amplitude when the polarization is changed from E∥b (green curve) to E∥a (red curve), but does not shift dramatically. A smaller peak at 1124 cm−1(E), visible only for E∥a, is assigned to the CH deformation and to CC stretching. Peaks E and F (1167 cm−1, attributed to a mix of different vibrations; see Table S1 of the Supporting Information for details) exhibit marked variations in intensity (but not in shape) on passing from E∥b to E∥a, with the first increasing by a factor of almost 4 and the second decreasing by a similar magnitude. The positions of the multiplet H (1208−1251 cm−1), attributed to CH and ArOH deformations and partially to CC stretching, appear to be insensitive to the beam polarization, whereas its intensity is a minimum for E∥a and tends to increase toward a maximum for E∥b. The same appears to be true for the multiplet J (1330−1400 cm−1), assigned to CH deformation, where again the maximum peak intensity is found for E∥b, and a definite increase of the J3 peak with respect to E∥a is noticed. The intensity of multiplet K (1410−1470 cm−1, CH deformation) tends to decrease at E∥b, with peak K1 (see inset b, panel II) almost disappearing. Quadruplet N (between 1602 and 1616 cm−1, attributed to CC stretching and ring deformations) shows one of the most remarkable dichroic responses of the LP-IR spectrum of the SC, presenting one strong peak (N2 and N3, overlapped so as to exhibit only one peak; see Figure 1, panel II, inset c) at about 1611 cm−1 at E∥b. When the beam polarization was moved toward E∥a, the intensity of this signal decreased, and concurrently, two peaks of almost half its amplitude arose at 1602 and 1616 cm−1 (N1 and N4, respectively), until, at E∥a, only these two peaks were
∑ (M·E)2 = ∑ (|M||E|)2 cos2 β
(1)
where β is the angle between M and E and the summation is carried out over all the possible transition moments. In our case, it was possible to limit the degree of complexity of the experimental system by positioning the platelet-like crystals so as to have the LP-IR light-propagation vector k parallel to the c axis of the crystalline unit cell (i.e., normal to the surface defined by the a and b axes − Λ plane−; see Figure 2a for reference). With this experimental setup, the electric field 6783
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Figure 2. (a) Three-dimensional sketch showing the vectors, axes, and planes used in the described models. a, b, and c are the main 4HCB crystal axes referred to the unit cell; the other parameters are defined throughout the text. (b) Spatial arrangement of the two independent 4HCB molecules in the unit cell of a single crystal. The C1, N1, and C2, N2 labels identify the carbon and nitrogen atoms belonging to the two independent molecules of the unit cell labeled 1 and 2, respectively, whereas the H′1 and H′2 labels identify the hydrogen atoms of the nearest neighbor establishing a hydrogen bond with the corresponding molecules. The misalignment angles ζn (n = 1 or 2) between the CN and OH groups establishing the hydrogen bonds are highlighted for both independent molecules. (c−f) Dependences of the (c,d) integrated IR absorbance and (e,f) peak position for (c,e) νOH and (d,f) νCN as values found experimentally (red squares) or determined on the basis of the negligible-misalignment model (orange triangles/curve) and the significant-misalignment model (brown diamonds/curve). See text for details on the models.
vector (Eab) is always contained in the Λ plane. Defining αE as the polarization angle between Eab and the crystal axis a (see Figure 2a), we analyzed the LP-IR absorbance spectra of νCN and νOH (around 2235 and 3290 cm−1, respectively) in a 4HCB SC for different polarization angles (αE = 0° corresponds to E∥a, whereas αE = 90° corresponds to E∥b). For the νOH and νCN vibrations, the integrated absorbance ( (αE)ω0 = (1/cL)∫ ω0 A(ω;αE)ω0 dω (where c is the molar concentration and L is the optical path) as a function of αE is reported in panels c and d, respectively, of Figure 2 (shown in both graphs with red squares). These integrated absorbances were then fitted with the following expression derived from eq 1 (see the Supporting Information) 2m
((αE)ω0F = s + D ∑
Lj(ω) =
(3)
is a Lorentzian function associated with the jth independent molecule, which depends on a) ω1, i.e. the resonance wavelength associated to the first molecule; b) Δωj, i.e. the frequency shift respect to ω1 associated to each other independent molecule; c) σj, i.e. the Lorentzian scale parameter. Because the 4HCB crystal unit cell contains two independent molecules (see Figure 2b), to account for the Davydov splitting effect,29 every peak in the experimental IR spectra was fitted by two distinct and arbitrarily chosen Lorentzian curves, each associated with one independent molecule. The association was made by computing the projection of μ0/|μ0| along axis a, Xj = a·(μ0/|μ0|)j. The independent molecule presenting the higher value of Xj was assumed to be the one with the larger IR absorption when E∥a, and vice versa for E∥b. The calculation was carried out on the basis of the two further hypotheses of i) μ0 being parallel to the stretching axis of the considered chemical group and ii) a negligible misalignment between the directions of M and the corresponding μ0. Therefore, the most red- (blue-) shifted components for the OH (CN) peak shift were assigned to the independent molecule presenting the
sin 2 θj(αE) cos2 γj(αE)
j=1
∫ω0 Lj(ω) dω
⎧ ⎫ σj 1⎪ 2⎪ ⎬ ⎨ + σ j 2 ⎪ π⎪ ⎭ ⎩ [ω − (ω1 + Δωj)]
(2)
where s is a constant accounting for baseline correction, D is an empirical constant, θj is the angle between M and c (see Figure 2a), m is the number of independent molecules in the crystal unit cell, γj is the angle between Eab and the projection of M on the plane Λ (M sin θ). Finally 6784
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Figure 3. Sketch of the two possible tautomers and resonance formulas (A, molecular; B,C: charge-separated) of 4HCB. The red dotted lines indicate hydrogen bonds established between neighboring molecules. For simplicity of visualization, the COH segment in these structures has been drawn as linear, although, in reality, it should have an angle of about 116° (from X-ray measurements12,30). [See also the Supporting Information (Table S2) for a complete table of atom coordinates of the 4HCB single crystal.]
larger Xj. The fitting was applied by adjusting the parameters of eq 2 to optimize the shape of the peak with respect to the experimental IR spectra (see the Supporting Information for details). The fit of the OH stretching peak for this negligiblemisalignment model was found to be in a satisfactorily good agreement with the experimental data (reported in Figure 2c,e as red squares) for both the integrated absorbance (Figure 2c, orange triangles) and the maximum absorption wavenumber (Figure 2e, orange triangles). On the other hand, for νCN, the fittings obtained with this model, for both the integrated absorbance (Figure 2d, orange triangles) and the peak maximum position (Figure 2f, orange triangles), were not very consistent with the measured values. Therefore, both the OH and CN stretching peaks were refitted assuming that some misalignment between M and μ0 can occur, with the M−μ0 angles varied until a reasonable agreement between the experimental and calculated data was found for both vibrations. When M was assumed to be parallel to the direction of the CN····HO hydrogen bond, the fit for the OH vibration satisfactorily resembled the experimental trend for the integrated absorbance (although the absolute values were not in good agreement; see Figure 2c, brown diamonds, fit, compared to red squares, experimental data) and compares rather well with the experimental peak maximum (Figure 2e, brown diamonds, fit, compared to red squares, experimental data). Under the same conditions, the CN stretching provided a more than satisfactory agreement between the experimental and calculated values for both the integrated absorbances (Figure 2d, brown diamonds versus red squares) and the peak maximum (Figure 2d, brown diamonds versus red squares), indicating that the vibrational excited state of CN has a transition moment aligned with the hydrogen bond (which, according to the 4HCB crystal structure determined from the already published XRD measurements,12,30 indeed results in being misaligned with respect to the main 4HCB molecular axis by 6.9° and 9.3°, depending on the considered molecule; see Figure 2b, angles ξ1 and ξ2, and Table S2 of the Supporting Information), rather than with the CN bond axis. The found misalignment is attributed to the electrostatic force generated by the proton involved in the hydrogen bond, resulting in a non-negligible M−μ0 solid angle. The fact that the significantmisalignment model is able to better interpret the νCN than the νOH properties is, in turn, explained by considering the highly polarizable (and, hence, easily subject to electrostatic attraction/repulsion) nature of the π electrons of the cyano group with respect to those of the oxidrilic ones, whereas the surprisingly small spectral shift of νCN along the two E directions compared to that of νOH is attributed to the difference in bond strengths of the two groups. III.A.3. Spectroscopic Hints of Tautomerism in 4HCB Crystals. Another interesting analysis of the IR spectra of 4HCB crystals can be made by considering that the peculiar
donor−acceptor dipolar structure of 4HCB is based on the presence of a hydroxylic group (electron donor) and a cyano group (electron acceptor). Compounds containing a nitrogen atom and hydroxylic groups have been reported to present tautomerism, that is, the simultaneous presence of two (or more) different chemical species, in chemical equilibrium, usually on an intramolecular basis.31,32 IR spectroscopy can reveal tautomerism by providing evidence of signals associated with each different chemical structure present in the considered compound. In our case, the characteristic features due to the CN and OH moieties of the 4HCB molecule are well distinguishable, as expected, but signals not attributable to the functional groups of 4HCB are also present. In particular, a weak, though evident, signal at 2181 cm−1 (Figure 1, labeled as Q1) is visible, independently from the beam polarization or from the single or polycrystalline nature of the considered sample. Interestingly, Binev experimentally found a peak at 2187 cm−1 for the 4HCB oxyanion, and he attributed it, supported by ab initio calculations, to the stretching of a CN group, weakened by the formation of the molecular oxyanion with respect to its analogue in the neutral form of 4HCB.21 It is then reasonable to assign the 2181 cm−1 signal to a weakened CN bond. Another feature that was difficult to attribute is the doublet found at 3173 and 3199 cm−1, in a spectral region where neither CH nor OH stretchings are usually reported. Nonetheless, the doublet is clearly visible in the spectra of both PCs and SCs, under both unpolarized and polarized beams (Figure 1, panel II, peaks labeled as T). It is known that, for organic crystals, NH stretching can appear in the 3050−3200 cm−1 range,33,34 but the only possibility to have NH stretching vibrations in 4HCB crystals is to consider some degree of tautomerism of the molecule, as sketched in Figure 3. The 3173 and 3199 cm−1 signals can then be assigned to a small amount of a charge-separated tautomer of 4HCB (Figure 4, structural formula C), originating from a proton exchange between the hydrogen-bond-forming groups OH and NC belonging to two neighboring molecules. In addition to the mentioned two classes of signals (the weakened CN stretching and the NH stretching), the tautomer should present a strengthened (because of the augmented charge density occurring on the oxygen atom) CO bond. The corresponding signals, according to an already cited study by Binev, fall around 1524 and 1382 cm−1 (stretching of the C O− bond belonging to the 4HCB oxyanion).23 Unfortunately, in these spectral ranges, the 4HCB crystals already present a number of other contributions (see Figure 1), which hinders the possibility of precisely identifying the said strengthened CO stretchings. Despite this problem, considering that we observed two of three classes of characteristic signals and that we did not observe the third one because of spectral zone overlapping with other stronger peaks (and not because of an observable absence of it), we tentatively attribute the discussed 6785
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Figure 4. Temperature evolution of selected portions (I, II, and III) of the LP-IR spectra of a 4HCB single crystal at (a) E∥a and (b) E∥b.
developing a doublet) and to 724 cm−1 for E∥b (see panel I of Figure 4a,b). Therefore, we attribute these signals to the δOH (out-of-plane) vibration. On the same grounds, the signals found at 298 K at 1223 and 1232 cm−1 for E∥a and at 1222 and 1230 cm−1 for E∥b, which showed limited but observable blue shifts upon cooling, were associated with the OH in-plane bending. From Figure 4a,b (panel II), it is possible to observe significant variations in the OH stretching peak shape for both axes (although the phenomenon is more marked for the E∥a spectrum). These variations were found to include both the peak maximum wavenumber (ωmax) and its area and appeared to be rather sensitive to the temperature. In particular, a plot of ωmax versus T for E∥a and E∥b (Figure 5b, green curve for E∥b and red curve for E∥a) shows that, at (relatively) high temperatures, no significant changes occur, whereas a rather sudden red shift is noticed upon a temperature threshold, which is higher for E∥b (around 200 K) than for E∥a (around 150 K). The most marked shifts with respect to the room-temperature maxima have always been observed at the lowest temperatures we were able to reach (120 K), suggesting that even lower temperatures could result in more marked shifts. On the basis of the general infrared behavior of the hydrogen bond,27 these shifts can be associated with an increasing hydrogen-bond strength upon decreasing temperature, which, in turn, implies a decreased distance between the H-bondforming moieties. Therefore, the different dependences of the νOH maxima shifts for E∥a and E∥b indicate an anisotropic thermal contraction of 4HCB crystals, a phenomenon already reported in the past for organic crystals. 39,40 Further investigations of this latter observation are ongoing. To evidence the variations of the spectral shape, for both the E∥a and E∥b ν OH peaks, we calculated the integral contributions for the right and left sides of the peak, defined as
features as belonging to the 4HCB tautomers described in the structural formulas sketched in Figure 3, present in very small amounts (given the weakness of the observed signals). This attribution is also supported by the fact that organic crystals presenting tautomerism are well-known (although most of the literature dealing with this subject reports intramolecular proton transfers,32,35 and only a few examples of actual heterotautomeric crystals are known36,37). To further confirm this hypothesis, more experimental work is ongoing. From an application point of view, it is worth observing that this kind of tautomerism could have an active role in the electrical transport properties of 4HCB SCs, because proton-coupled electron transfer (PCET) is an established mechanism for charge transfer in several electronically active molecular systems.38 III.B. Low-Temperature Analysis. LP-IR investigations (with E∥a and E∥b only) of SCs were carried out at temperatures ranging from 298 to 120 K. The temperature decrease allowed for the detection of evident shape changes with respect to the corresponding spectra taken at room temperature, as well as non-negligible signal shifts in some cases. In particular, going from room temperature to 120 K, some doublets arising from singlet peaks and a red shift of about 15 cm−1 for the OH stretching bands (under both polarizations) were noticed (Figure 4a,b). It is known that, upon an increase of the hydrogen-bonding strength, the OH stretching and bending signals experience a marked red shift and a limited blue shift, respectively.27 Therefore, the rather pronounced red shift observed for the OH stretching band at 120 K with respect to its roomtemperature counterpart indicates that the temperature decrease causes an increase in the hydrogen-bond strength. It is then possible to exploit these low-temperature data to discern the signals deriving from the bending of the hydroxylic group from other vibrations occurring in the same spectral zone, considering that the OH bending signal should blue shift with decreasing temperature more appreciably than other kinds of vibrations. On this basis, we observed that the signals found in the 298 K spectra at 716 and 720 cm−1 for E∥a and E∥b, respectively, shifted at 120 K to 720 and 725 cm−1 for E∥a (i.e.,
iR =
∫ω
ωi
c
A(ω) dω
iL =
∫ω
ωc
0
A(ω) dω
(4)
where ω0 and ωi are the lower and upper limits, respectively, of the frequency integration interval, ωc = (ωi − ω0)/2, and ε is 6786
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Figure 5. Behavior of the νOH peak with decreasing temperature. The red and green curves are referred to axes a and b, respectively. (a) Shift in the νOH maximum (ωmax) as a function of the sample temperature. (b) Visual description of the parameters used for assessing the νOH signal asymmetry, shown for a model peak (see text for further details). (c) Variations in the peak asymmetry (indicated by the parameter Δi = iL − iR, where iR and iL represent the integrated absorbance values of the right and left parts of the peak, respectively) with temperature.
identified in the spectra of both PCs and SCs were tentatively assigned to the presence of a small amount of charge-separated tautomers of the 4HCB molecule within the crystal lattice. LPIR measurements in the temperature range between 298 and 120 K allowed for the assignment of a few peaks whose origin was not clear in the room-temperature spectra. Moreover, the same low-temperature measurements evidenced an anisotropic shrinking of the crystals, as well as an existing extended vibronic coupling (which might have some role in the transport properties of the crystal) between the benzenic ring and the two functional groups. This latter point highlights the usefulness of infrared observations for investigating organic semiconductors.
the peak eccentricity (see Figure 5b). Upon cooling, as is visible in Figure 5c, for E∥a, Δi increased rather monotonically and was always positive; that is, for any temperature, the integrated absorbance of the νOH signal at higher wavenumbers (iL) was more intense than that at lower wavenumbers (iR). Although Δi was not found to be monotonic for E∥b, its general value was always negative, marking an evident difference with respect to E∥a. Different behaviors of the νOH signal upon temperature decrease for E∥a and E∥b was noticed as well for benzoic acid crystals by Flakus et al., who attributed this finding to the ability of the easily polarizable π electrons of the phenyl ring to establish a non-negligible vibronic coupling with the electrons involved in the hydrogen bond.41 Therefore, also considering the nondramatic structural difference between benzoic acid and 4HCB, we conclude that, for 4HCB SCs, the same phenomenon is present and the observed features are due to a higher polarizability or/and a higher density of π electrons along the a axis rather than along the b axis , which is indeed in line with the known anisotropic mobility features of these crystals.12,13 This vibronic coupling could also play some role in the reported transport properties of 4HCB single crystals,11−13 which are remarkable with respect to the very small dimensions of the 4HCB molecule. Interestingly, the whole LP-IR spectrum of 4HCB SCs presents rather evident differences for E∥a and E∥b, even at room temperature (see Figure 1), in any part of the spectrum (i.e., for features associated indifferently with the benzenic ring or to its substituents), further supporting the view that an extended vibronic coupling is present in the molecule. The possibility to observe this enhanced electronic polarizability by means of a traditionally vibrational-devoted technique such as infrared spectroscopy is rather intriguing, because it allows a widely diffused, easily accessible, and nondestructive characterization tool to be used to investigate properties linked to π electrons belonging to organic semiconductors.
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ASSOCIATED CONTENT
S Supporting Information *
Complete attributions of 4HCB poly- and single-crystal IR vibrations with respect to almost all visible signals for the unpolarized IR spectrum of polycrystals, unpolarized IR spectrum of a single crystal, linearly polarized IR spectra (E∥a and E∥b) of single crystals at 298 K, and linearly polarized IR spectra (E∥a and E∥b) of single crystals at 120 K (Table S1); spatial coordinates of the atoms forming the 4HCB singlecrystal lattice for each of the two independent molecules of the unit cell, with a reference figure evidencing the atomic numbering of the two molecules (Table S2); and detailed description of the procedure used in section III.A.2 for fitting the behavior of the CN and OH stretching vibrations. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +39-0403758810. E-mail: alessandro.fraleoni@elettra. trieste.it.
IV. CONCLUSIONS IR spectra of 4HCB polycrystals and single crystals, under both unpolarized and linearly polarized beams, have been described, and most of the signals found at room and low temperature (298 and 120 K, respectively) have been assigned to precise vibrational modes. For SCs, LP-IR investigations carried out with E∥a and E∥b provided markedly different responses, and the most prominent and/or significant differences between the LP-IR and unpolarized spectra of the crystals were discussed. A higher polarizability of the cyano group with respect to the oxidrilic one was inferred from calculations carried out on data deriving from LP-IR measurements. Weak but clear signals
Notes
The authors declare no competing financial interest.
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REFERENCES
(1) De Boer, R. W. I.; Gershenson, M. E.; Morpurgo, A. F.; Podzorov, V. Organic Single-Crystal Field-Effect Transistors. Phys. Status Solidi a 2004, 201, 1302−1331. (2) Braga, D.; Horowitz, G. High-Performance Organic Field-Effect Transistors. Adv. Mater. 2009, 21, 1473−1486. (3) Podzorov, V.; Menard, E.; Borissov, A.; Kiryukhin, V.; Rogers, J. A.; Gershenson, M. E. Intrinsic Charge Transport on the Surface of Organic Semiconductors. Phys. Rev. Lett. 2004, 93, 086602.
6787
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The Journal of Physical Chemistry A
Article
(4) Podzorov, V.; Menard, E.; Rogers, J. A.; Gershenson, M. E. Hall Effect in the Accumulation Layers on the Surface of Organic Semiconductors. Phys. Rev. Lett. 2005, 95, 226601. (5) Nan, G.; Yang, X.; Wang, L.; Shuai, Z.; Zhao, Y. Nuclear Tunneling Effects of Charge Transport in Rubrene, Tetracene, and Pentacene. Phys. Rev. B 2009, 79, 115203. (6) Zeis, R.; Besnard, C.; Siegrist, T.; Schlockermann, C.; Chi, X.; Kloc, C. Field Effect Studies on Rubrene and Impurities of Rubrene. Chem. Mater. 2006, 18, 244−248. (7) Jurchescu, O. D.; Baas, J.; Palstraa, T. T. M. Effect of Impurities on the Mobility of Single Crystal Pentacene. Appl. Phys. Lett. 2004, 84, 3061−3063. (8) Facchetti, A. Semiconductors for Organic Transistors. Mater. Today 2007, 10, 28−37. (9) Anthony, J. E. The Larger Acenes: Versatile Organic Semiconductors. Angew. Chem., Int. Ed. 2008, 47, 452−483. (10) McCulloch, I.; Heeney, M.; Chabinyc, M. L.; DeLongchamp, D.; Kline, R. J.; Cölle, M.; Duffy, W.; Fischer, D.; Gundlach, D.; Hamadani, B.; Hamilton, R.; Richter, L.; Salleo, A.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Zhang, W. Semiconducting Thienothiophene Copolymers: Design, Synthesis, Morphology, and Performance in Thin-Film Organic Transistors. Adv. Mater. 2009, 21, 1091−1109. (11) Fraboni, B.; Fraleoni-Morgera, A.; Cavallini, A. ThreeDimensional Anisotropic Density of States Distribution and Intrinsic-Like Mobility in Organic Single Crystals. Org. Electron. 2011, 11, 10−15. (12) Fraboni, B.; Femoni, C.; Mencarelli, I.; Setti, L.; Di Pietro, R.; Cavallini, A.; Fraleoni-Morgera, A. Solution-Grown, Macroscopic Organic Single Crystals Exhibiting Three-Dimensional Anisotropic Charge-Transport Properties. Adv. Mater. 2009, 21, 1835−1839. (13) Fraboni, B.; DiPietro, R.; Castaldini, A.; Cavallini, A.; FraleoniMorgera, A.; Setti, L.; Mencarelli, I.; Femoni, C. Anisotropic Charge Transport in Organic Single Crystals Based on Dipolar Molecules. Org. Electron. 2008, 9, 974−978. (14) Moynihan, H. A.; O’Hare, I. P. Spectroscopic Characterization of the Monoclinic and Orthorhombic Forms of Paracetamol. Int. J. Pharm. 2002, 247, 179−185. (15) Hanai, K.; Kuwae, A.; Takai, T.; Senda, H.; Kunimoto, K.-K. A Comparative Vibrational and NMR Study of cis-Cinnamic Acid Polymorphs and trans-Cinnamic Acid. Spectrochim. Acta A 2001, 57, 513−519. (16) Bright, A. A.; Garito, A. F.; Heeger, A. J. Optical Conductivity Studies in a One-Dimensional Organic Metal: Tetrathiofulvalene Tetracyanoquinodimethan (TTF) (TCNQ). Phys. Rev. B 1974, 10, 1328−1342. (17) Grant, P. M.; Greene, R. L.; Wrighton, G. C.; Castro, G. Temperature Dependence of the Near-Infrared Optical Properties of Tetrathiofulvalinium Tetracyanoquinodimethane (TTF-TCNQ). Phys. Rev. Lett. 1973, 31, 1311−1314. (18) Tanner, D. B.; Cummings, K. D. Far-Infrared Study of the Charge Density Wave in Tetrathiofulvalene Tetracyanoquinodimethane (TTF-TCNQ). Phys. Rev. Lett. 1981, 47, 597−600. (19) Li, Z. Q.; Podzorov, V.; Sai, N.; Martin, M. C.; Gershenson, M. E.; Di Ventra, M.; Basov, D. N. Light Quasiparticles Dominate Electronic Transport in Molecular Crystal Field-Effect Transistors. Phys. Rev. Lett. 2007, 99, 016403. (20) Fraleoni-Morgera, A.; Tessarolo, M.; Perucchi, A.; Baldassarre, L.; Lupi, S.; Fraboni, B. Polarized Infrared Studies on Charge Transport in 4-Hydroxycyanobenzene Single Crystals. J. Phys. Chem. C 2012, 116, 2563−2569. (21) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.; Methuen & Co.: London, 1958. (22) Chalmers, J. M., Griffiths, P. R., Eds. Handbook of Vibrational Spectroscopy; Wiley & Sons: Chichester, U.K., 2002; Vol. 1, pp 128− 132, 693−745. (23) Binev, Y. I. Ab Initio MO and Experimental Studies on the Vibrational Spectra and Structure of 4-Hydroxybenzonitrile and of Its Anion. J. Mol. Struct. 2001, 535, 93−101.
(24) Georgieva, M. K.; Angelova, P. N.; Binev, I. G. The Conversion of 3- and 4-Hydroxybenzonitriles (m- and p-Cyanophenols) into Oxyanions, Followed by IR Spectra, ab Initio and Density Functional Calculations. J. Mol. Struct. 2004, 692, 23−35. (25) Fraleoni-Morgera, A.; Benevoli, L.; Fraboni, B. Solution Growth of Single Crystals of 4-Hydroxycyanobenzene (4HCB) Suitable for Electronic Applications. J. Cryst. Growth 2010, 312, 3466−3472. (26) Fraboni, B.; Ciavatti, A.; Merlo, F.; Pasquini, L.; Cavallini, A.; Quaranta, A.; Bonfiglio, A.; Fraleoni-Morgera, A. Organic Semiconducting Single Crystals as Next Generation of Low-Cost, RoomTemperature Electrical X-ray Detectors. Adv. Mater. 2012, 24, 2289− 2293. (27) Chalmers, J. M., Griffiths, P. R., Eds. Handbook of Vibrational Spectroscopy; Wiley & Sons: Chichester, U.K., 2002; Vol. 3, pp 1919− 1934. (28) Jacobs, J.; Willner, H.; Gottfried, P. Fourier Transform Infrared Spectra of the Complexes XCN···HF (X = F, CF3, SF5) Isolated in an Argon Matrix. J. Phys. Chem. 1992, 96, 5793−5796. (29) Davydov, A. S. Theory of Molecular Excitons; McGraw-Hill: New York, 1962. (30) Higashi, T.; Osaki, K. p-Cyanophenol. Acta Crystallogr. B 1977, 33, 607−609. (31) Ertan, N.; Gürkan, P. Synthesis and Properties of Some Azo Pyridone Dyes and Their Cu(II) Complexes. Dyes Pigments 1997, 33, 137−147. (32) Filarowski, A.; Głowiaka, T.; Koll, A. Strengthening of the Intramolecular O···H···N Hydrogen Bonds in Schiff Bases as a Result of Steric Repulsion. J. Mol. Struct. 1999, 484, 75−89. (33) Parry, G. S. The Crystal Structure of Uracil. Acta Crystallogr. 1954, 7, 313−320. (34) Newman, R.; Badger, R. M. Infrared Spectra of Cyanuric Acid and Deutero Cyanuric Acid. J. Am. Chem. Soc. 1952, 74, 3545−3548. (35) Gilli, P.; Bertolasi, V.; Pretto, L.; Lyčka, A.; Gilli, G. The Nature of Solid-State N−H···O/O−H···N Tautomeric Competition in Resonant Systems. Intramolecular Proton Transfer in Low-Barrier Hydrogen Bonds Formed by the ···OC−CN−NH··· ···HO−CC− NN··· Ketohydrazone−Azoenol System. A Variable-Temperature Xray Crystallographic and DFT Computational Study. J. Am. Chem. Soc. 2002, 124, 13554−13567. (36) McConnell, J. F.; Sharma, B. D.; Marsh, R. E. Co-crystallization of Two Tautomers: Crystal Structure of Isocytosine. Nature 1964, 203, 399−400. (37) Steiner, T.; Koellner, G. Coexistence of Both Histidine Tautomers in the Solid State and Stabilisation of the Unfavourable Nδ−H Form by Intramolecular Hydrogen Bonding: Crystalline L-HisGly Hemihydrate. Chem. Commun. 1997, 1207−1208. (38) Cukier, R. I.; Nocera, D. G. Proton-Coupled Electron Transfer. Annu. Rev. Phys. Chem. 1998, 49, 337−369. (39) Miyazaki, A.; Enoki, T.; Uekusa, H.; Ohashi, Y.; Saito, G. Phase Transition of γ-(BEDT-TTF)3(HSO4)2. Phys. Rev. B 1997, 55, 6847− 6855. (40) Haas, S.; Batlogg, B.; Besnard, C.; Schiltz, M.; Kloc, C.; Siegrist, T. Large Uniaxial Negative Thermal Expansion in Pentacene Due to Steric Hindrance. Phys. Rev. B 2007, 76, 205203. (41) Flakus, H. T.; Chelmecki, M. Infrared Spectra of the Hydrogen Bond in Benzoic Acid Crystals: Temperature and Polarization Effects. Spectrochim. Acta A 2002, 58, 179−196.
6788
dx.doi.org/10.1021/jp405058h | J. Phys. Chem. A 2013, 117, 6781−6788
