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Spectroscopic Tracking of Schiff Base Compounds’ Hydrogen Bonding Reorganization Associated with Solid-to-Solid Phase Transition Satomi Hara,† Hirohiko Houjou,*,† Isao Yoshikawa,† Hiroyasu Sato,‡ Akihito Yamano,‡ Yukiko Namatame,‡ Toshiki Mutai,† and Koji Araki† †

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan

‡

S Supporting Information *

ABSTRACT: A series of 2,6-dihydroxynaphthalene-1-methylidene alkylamines whose alkyl chain lengths ranged from 9 to 12 was spectroscopically examined. Transmission ultraviolet− visible absorption microspectroscopy revealed that the spectra of solid thin-films of the crystalline samples showed two distinct profiles depending on polymorphs as well as on alkyl chain length. We concluded that these spectral changes occurred not because of conventional intramolecular proton transfer but because of the molecules’ interactions with an external proton source, that is, the intermolecular proton transfer. The spectral changes were accompanied by changes in the intermolecular hydrogen bonding network. When a crystal of a sample compound was heated, its spectrum changed dramatically before the crystal underwent a solid-to-solid phase transition to another polymorph. We concluded that these spectral changes indicated strengthening of intermolecular hydrogen bonding or intermolecular proton transfer, which would have triggered a drastic change in the hydrogen bonding network structure.

1. INTRODUCTION Chemical and physical properties of crystalline materials are often influenced by polymorphism, that is, the distinct packing patterns of molecular constituents.1−3 Several studies have focused on switching molecular functions in the solid state by controlling the molecular packing, which has attributes of reversibility and bistability.4−8 The polymorphism associated with alteration of the interatomic connectivity (e.g., prototropy), referred to as desmotropism, is an important topic in the field of crystal engineering.9,10 Thorough understanding of the mechanism of this type of solid-state prototropy will provide insights for applications in medicinal11,12 and dyestuff chemistry.13,14 Prototropic isomerization of salicylaldehyde Schiff bases has been extensively studied in relation to their thermochromic and photochromic properties.15−19 More specifically, the change in these compounds’ color is conventionally attributed to the change in abundance ratio of enol-imine (OH) to keto-enamine (NH) forms, each of which is represented by a resonance hybrid of covalent and ionic canonical structures (Figure 1). Additionally, the absorption spectra of salicylaldehyde Schiff bases change in response to a given molecular environment in solution,20−25 in the solid state,26−30 or in other phases.31−33 However, the origin of spectral changes associated with polymorphism (i.e., crystallochromism) has been relatively less clarified as compared to that for solvatochromism, even after the discovery that a certain type of intermolecular hydrogen bonding (HB) increases preference for the NH© 2014 American Chemical Society

Figure 1. Possible canonical structures of 2,6-dihydroxynaphthalene-1aldehyde alkylamine Schiff bases (DNSB-m).

form.34,35 In this report, we provide insight into the relationship between the crystal structure and electronic spectra of a series of Schiff base compounds that we studied using a microscopic transmission absorption method. We found crystal packing structures associated with distinct HB network patterns had Received: June 25, 2014 Revised: July 28, 2014 Published: July 29, 2014 6979

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solution-phase spectra. These data disagree with our recent report, in which the peaks corresponding to the OH-form were recognizable for analogous Schiff base compounds, although the solid thin-film spectra are less resolved than the solutionphase spectra in that report.38 Conventional reflectance spectroscopy utilizing the Kubelka−Munk method, which provides data for shorter wavelengths, was employed to obtain solid-state absorption spectra, but we still could not find any bands corresponding to the OH-form. Additionally, this method yielded even worse resolution of spectra, to the extent that we could not differentiate between type-I and -II profiles (Supporting Information Figure S4). We found that the addition of acid caused a spectral change quite similar to the change from type-II to type-I.37 Figure 3

slight but crucial differences in their electronic spectra, undetectable by conventional diffuse reflectance measurements.

2. RESULTS AND DISCUSSION We found that the solid thin-film ultraviolet−visible (UV−vis) absorption spectra of 2,6-dihydroxynaphthalene-1-aldehyde alkylamine Schiff bases36 (DNSB-m, Figure 1) vary depending on the length (m) of their alkyl chains. Furthermore, two distinct crystals of DNSB-10, that is, prisms obtained from THF−methanol and needles obtained from 2-propanol, yielded different spectra: the former crystal exhibited an absorption band with a maximum at 416 nm and a shoulder around 459 nm (Figure 2i, hereafter referred to as type-I), whereas the

Figure 2. Solid thin-film UV−vis absorption spectra (left) and IR spectra (right) of DNSB-10 samples i−iv, which have various crystallization histories (see text).

Figure 3. Absorption spectra of a solid thin-film of DNSB-9: sample as prepared (solid line), after HCl gas was flowed over the sample (dashed-dotted line), and after subsequent flowing of triethylamine gas over the sample (dotted line).

latter exhibited a band with bimodal maxima at 436 and 462 nm (Figure 2ii, type-II). The IR spectra of these two polymorphs yielded completely different sets of peaks, especially in the region of C−C bond stretching (1200−1700 cm−1), suggesting a drastic reconstruction of conjugated double bonds.26,37 The UV−vis and IR spectra of the other compounds each were assigned to one of these two profile types (Supporting Information, Figure S1). The needles of DNSB-10 underwent a phase transition to another crystalline state at around 120 °C and then melted at 137 °C, the melting point of the prisms (Supporting Information Figure S2). After the needles were heated at 120 °C for 10 min, the sample showed an absorption spectrum quite similar to that of the prisms, and thus we assigned the sample as type-I (Figure 2iii). After this solid was collected and recrystallized from 2-propanol, the subsequent spectrum (Figure 2iv) was substantially similar to that of the original needles (Figure 2ii). The corresponding IR spectra for these samples were perfectly synchronized to the changes in absorption spectra (Figure 2, right). These results clearly show that the observed spectral changes were associated with a phase transition between the two polymorphs. Comparison of the UV−vis absorption spectra of solutionphase samples (Supporting Information Figure S3) suggested that the type-II profile corresponds to the NH-form that prevails in polar solvents. In contrast, we did not find any absorption bands that corresponded to the type-I profile in the

shows the spectra of a solid thin-film of DMSB-9 before (solid line) and after (dashed-dotted line) HCl gas was flowed over the sample. A transition from a type-II to type-I spectral profile is observed. Subsequent addition of triethylamine gas resulted in recovery of the type-II spectral profile (dotted line). A similar change in spectral profile was also observed for an ethanolic solution of Schiff bases (a representative example of such spectra is shown in Supporting Information Figure S5). These results suggest that the spectral change takes place reversibly by means of protonation and deprotonation of the Schiff base compound. The lengths of the C−O and C−N bonds obtained by X-ray crystal analysis of DNSB-m suggest that all the crystals examined in this study were composed of molecules in NHform (Supporting Information Table S1).34,35,38 The relatively low HOMA (harmonic oscillator approximation of aromaticity) indices39−42 (0.4−0.6, Supporting Information Table S1) observed for the six-member-ring attached to the imine group indicate a moderate loss of aromaticity, further suggesting the prevalence of NH-form in these samples. For all the crystals, the 2-O---6-O distance ranged between 2.57 and 2.68 Å, indicating the formation of a HB network involving the 2-O and 6-OH groups. The structure of the HB network was categorized into two types: one consisting of a zigzag line of naphthalene rings with the alkyl chains protruding in alternate 6980

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directions (referred to as “alternate,” Figure 4a), and the other consisting of a straight line of naphthalene rings with the alkyl

conjugated system. We presumed that alteration of the conjugated system, which was associated with intermolecular proton transfer from the 6-OH group to the 2-O group (Scheme 1), would have changed the electronic state of the molecule and thus given rise to the two distinct phases observed in this study. Scheme 1. Presumptive Intermolecular Proton Transfer Associated with the Transition between α- and β-Phases

Figure 4. Crystal structures of DNSB-10 with different HB networks: (a) “alternate” and (b) “bunting” types, denoted as α- and β-phases, respectively.

Next we investigated whether the two processes, that is, the change in spectral profile and the change in molecular packing, occurred simultaneously. First, we examined α-DNSB-10 needles obtained from 2-propanol. During heating from 27 to 120 °C, the spectral profile changed rapidly from type-II to type-I as the temperature increased from 80 to 90 °C (Supporting Information Figure S6). The spectral profile observed after heating was quite similar to that obtained by flowing HCl gas over the sample (Figure 3, dashed line), both in shape and in intensity. This spectral change implies that the molecule adopted an electronic state characteristic of the bunting HB network, while the actual HB network remained alternate. The mismatch of the protonation state and HB network order that occurred between 90 and 120 °C may have triggered the phase transition from α- to β-phase. A diagram that shows the relationships between the changes in phase, spectral profile, and HB network for DNSB-10 is shown in Figure 5. We propose that the present molecular system works as a bistable switch in which the molecular internal energy relaxes by the alteration of molecular arrangement. Regrettably, the absorption spectra of DNSB-10 were not of good quality at temperatures higher than 91 °C, owing to considerable shrinking of the solid thin-film. We next examined

chains extending parallel to each other, like a flowing bunting (“bunting,” Figure 4b). As evidenced by the symmetry of the molecular arrangement, the presence of a 21-axis indicated that the HB network for a given sample was alternate (Table 1). Table 1 m

space group

packing factora

HB network

spectral profile

9 (unheated) 9 (after melting) 10 (needles) 10 (prisms) 11 12

P21/c P-1 P21/c C2/c P21/n C2/c

0.715 0.708 0.719 0.701 0.731 0.706

alternate bunting alternate bunting alternate bunting

II I II I II I

a

Calculated by the method described in ref 36.

The packing factors36 estimated for the alternate HB network are slightly higher (0.71−0.73) compared to those (0.70−0.71) for the Bunting network. For all crystals examined in this study, the type-I and -II spectral profiles corresponded to the bunting and alternate HB networks, respectively (Table 1). Herein we refer to crystals with alternate and bunting HB networks as αand β-phases, respectively. The UV−vis spectra and X-ray structural analysis suggest that both the α- and the β-phases are in NH-form, although it is also possible that some intermediate state between quinoid and ionic characters is stabilized by intermolecular HB.43,44 At present, we can presume that the difference between the type-I and -II spectral profiles represents distinct states associated with the protonation or deprotonation of the hydroxy arylaldimine moiety. Since the relative contributions of ionic and covalent structures in a resonance hybrid are significantly influenced by the local polarity of their molecular environment, it is likely that the packing factor characteristic of the alternate and bunting HB networks critically regulates the reorganization of the π-

Figure 5. Relationship between phase transitions, changes in spectral profile, and changes in the HB network of DNSB-10. 6981

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drastic change between 80 and 91 °C, indicating that the changes in UV−vis and IR spectra occur simultaneously (Supporting Information Figure S11). We concluded from these spectral changes that during the temperature increase from 80 to 115 °C, the molecule adopted a bunting HB network while the actual HB network remained alternate. The mismatch of the protonation state and HB network again may have triggered the phase transition from α- to β-phase, as was observed for DNSB-10. Consequently, our present results demonstrate a generalizable mechanism for spectroscopic changes associated with solid-to-solid phase transition. In this paper, however, we confined our study to the recognition of four crystallographically distinct states and two spectroscopically distinct states of this compound. Further understanding of this phenomenon is expected if the crystal structures of individual states are clarified.

the compound α-DNSB-9, obtained as yellow prisms (P21/c) from a solution of THF−petroleum ether. This compound exhibited a solid−solid phase transition at 115 °C before melting at 150 °C (Supporting Information Figure S7). Because the thermal behavior of DNSB-9 was more complicated than that of DNSB-10 (Supporting Information Figure S2), we examined the change in crystal structure induced by the phase transition by means of simultaneous differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements. During the heating process, the XRD pattern drastically changed at around 115 °C. This phase, referred to as β1, melted at 150 °C. When the melt was cooled to 89 °C, the XRD pattern changed again (Supporting Information Figure S8). This phase, referred to as β2, underwent further phase transition as the sample was cooled to 27 °C: an exothermic peak appeared, indicating the presence of another crystalline phase. XRD patterns acquired throughout the heating and subsequent melting processes are shown with the corresponding thermal analysis chart in Supporting Information Figure S9. Although the quality of the crystal was not sufficiently good for satisfactory R values, X-ray structural analysis revealed that this crystal has a bunting HB network; hence, we denote it as β3-DNSB-9. The obtained β3-DNSB-9 crystal underwent phase transition among its β2, β3, and liquid phases. As expected, the solid thin-film absorption spectra of β3-DNSB-9 showed a typeI profile (Supporting Information Figure S1) and changed only slightly when HCl gas was flowed over the sample. The spectral profile of β2-DNSB-9, which was obtained by heating β3DNSB-9 to 120 °C, was moderately similar to that of β3-DNSB9, although the shoulder around 460 nm was weaker and another shoulder around 390 nm was newly observed in the β2DNSB-9 spectrum (Supporting Information Figure S10). We examined the changes in the spectra of α-DNSB-9 during heating from 25 to 115 °C. As shown in Figure 6, the spectral profile changed rapidly from type-II to type-I as the specimen temperature increased from 75 to 85 °C. Above 89 °C, the spectra converged to a profile quite similar to that of β3-DNSB9 (Supporting Information Figure S10). Similarly, IR spectra acquired at various temperatures from 27 to 131 °C showed a

3. CONCLUDING REMARKS In summary, we have discovered new crystalline molecular systems that afford distinct electronic spectra tightly associated with the alteration of the HB network structure. A drastic change in electronic state, which was probably related to reorganization of the π-conjugated system, occurred with increasing temperature in advance of the solid-to-solid phase transition. Our system serves as a good model for examining the relationship between the electronic state of Schiff base compounds and their molecular environment, especially as the environment relates to intermolecular HB. In addition, the results elicited a fundamental question about the change in electronic spectra, which cannot be explained from the conventional viewpoint of intramolecular prototropic equilibrium between enol-imine and keto-enamine forms. Further studies using a broader variety of analogous compounds and theoretical approaches are in progress. 4. EXPERIMENTAL SECTION Solid thin-film UV−vis absorption spectra were measured for samples smeared onto glass slides, with a sample thickness that yielded a maximum absorbance of about 1. The specimen was placed on the stage of a conventional optical microscope equipped with a UV-transmitting objective lens and a portable fiber optic spectrophotometer. Light from a xenon lamp was guided into the condenser lens through a quartz optical fiber and was focused on the specimen at a selected area of about 50 μmϕ. Powder XRD and DSC data were simultaneously collected with a Rigaku SmartLab diffractometer, λ(Cu Kα) = 1.5418 Å, equipped with a Rigaku X-ray DSC attachment. The XRD scan rate was 40°/min, and the sampling step was 0.02°. Samples were heated or cooled at a rate of 5 °C/min under nitrogen at a flow rate of 50 mL/min. For X-ray diffraction of single crystals, data were collected on a diffractometer, λ(Cu−Kα) = 1.5418 Å (for α-DNSB-10, βDNSB-10, α-DNSB-12), and a diffractometer, λ(Mo−Kα) = 0.71075 Å (for α-DNSB-9, β3-DNSB-9, α-DNSB-11). The structure was solved by direct methods using SHELXS-201345 or SIR201146 and refined by the full-matrix least-squares method using SHELXL-2013.45 Crystallographic data have been deposited with Cambridge Crystallographic Data Centre: Deposition numbers CCDC-1006228 to 1006233 to compounds α-DNSB-9, β3-DNSB-9, α-DNSB-10, β-DNSB-10, αDNSB-11, β-DNSB-12. Copies of the data can be obtained free

Figure 6. Solid thin-film UV−vis absorption spectra of DNSB-9 measured at various temperatures. Inset: Ratio of absorbances at 463 and 425 nm as a function of temperature. The solid line is shown for visual clarity. 6982

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ACKNOWLEDGMENTS A part of this work (X-ray single crystallographic structure analysis) was conducted in Research Hub for Advanced Nano Characterization, the University of Tokyo, supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; fax: +44 1223 336033; e-mail: [email protected]). α-DNSB-9. C20H27NO2, yellow plate, Mw = 313.44, monoclinic, a = 5.5610(2), b = 17.1532(7), c = 18.8156(8), β = 90.495(6)°, V = 1794.73(13) Å3, Dcalcd = 1.160 g/cm3, T = 295(2) K, space group P21/c (#14), Z = 4, μ(Mo Kα) = 0.74 cm−1, 17 178 reflections measured and 4083 unique (2θmax = 55.0°, Rint = 0.025) which were used in all calculations. R = 0.049, wR = 0.159. β3-DNSB-9. C20H27NO2, yellow plate, Mw = 313.44, triclinic, a = 7.518(4), b = 9.451(4), c = 13.973(7), α = 101.667(13), β = 104.184(14), γ = 102.178(12)°, V = 906.4(8) Å3, Dcalcd = 1.148 g/cm3, T = 296(2) K, space group P-1 (#2), Z = 4, μ(Mo Kα) = 0.73 cm−1, 4254 reflections measured and 1887 unique (2θmax = 41.6°, Rint = 0.1933) which were used in all calculations. R = 0.110, wR = 0.367. α-DNSB-10. C21H29NO2, yellow needle, Mw = 327.45, monoclinic, a = 5.546(1), b = 17.391(1), c = 19.510(1), β = 90.800(1)°, V = 1881.6(4) Å3, Dcalcd = 1.156 g/cm3, T = 298(2) K, space group P21/c (#14), Z = 4, μ(Cu Kα) = 5.71 cm−1, 18 050 reflections measured and 3646 unique (2θmax = 146.5°, Rint = 0.055) which were used in all calculations. R = 0.088, wR = 0.229. β-DNSB-10. C21H29NO2, yellow prism, Mw = 327.45, monoclinic, a = 16.997(1), b = 8.114(1), c = 27.974(1), β = 91.596(1)°, V = 3856.5(5) Å3, Dcalcd = 1.128 g/cm3, T = 298(2) K, space group C2/c (#15), Z = 8, μ(Cu Kα) = 5.58 cm−1, 19 151 reflections measured and 3743 unique (2θmax = 146.5°, Rint = 0.069) which were used in all calculations. R = 0.071, wR = 0.185. α-DNSB-11. C22H31NO2, yellow prism, Mw = 341.49, monoclinic, a = 9.089(4), b = 17.412(8), c = 12.318(5), β = 95.570(7)°, V = 1943.2(15) Å3, Dcalcd = 1.167 g/cm3, T = 295(2) K, space group P21/n (#14), Z = 4, μ(Mo Kα) = 0.76 cm−1, 13 053 reflections measured and 3525 unique (2θmax = 55.0°, Rint = 0.0527) which were used in all calculations. R = 0.057, wR = 0.169. β-DNSB-12. C23H33NO2, yellow plate, Mw = 355.50, monoclinic, a = 16.974(1), b = 8.145(1), c = 30.645(1), β = 94.488(1)°, V = 4223.8(6) Å3, Dcalcd = 1.118 g/cm3, T = 298(2) K, space group C2/c (#15), Z = 8, μ(Cu Kα) = 5.44 cm−1, 20 533 reflections measured and 4034 unique (2θmax = 146.2°, Rint = 0.059) which were used in all calculations. R = 0.080, wR = 0.208.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

UV−vis absorption spectra in solution and in solid thin-film; DSC data, XRD-DSC, temperature controlled IR spectra; geometrical parameters obtained from crystal structures. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81(3)5452-6367. Fax: +81(3)5452-6366. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6983

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dx.doi.org/10.1021/jp5063034 | J. Phys. Chem. A 2014, 118, 6979−6984