J O U R N A L O F C H E M I S T R Y Materials 5-Amino-3-imino-1,2,6,7-tetracyano-3H-pyrrolizine: characterization of the solvent-free solid phase and interaction with ammonia and water Vincenzo Fares,*a Alberto Flamini,*†a Donatella Capitanib and Roberto Rellac aIstituto di Chimica deiMateriali del CNR, Area della Ricerca di Roma, PO Box 10, 00016 Monterotondo Stazione, Roma, Italy bIstituto di Strutturistica Chimica del CNR, Area della Ricerca di Roma, PO Box 10, 00016 Monterotondo Stazione, Roma, Italy cIstituto per lo Studio di NuoviMateriali per l’Elettronica del CNR, V ia Arnesano, 73100 L ecce, Italy The 5-amino-3-imino-1,2,6,7-tetracyano-3H-pyrrolizine (LH, C11H3N7), previously characterized as the 251 1-chloronaphthalene adduct, has been further investigated as a solvent-free solid phase.Strong intermolecular interactions take place in this phase, as revealed by the optical spectra of evaporated LH thin films (lmax=615 and 570 nm) compared to the optical spectrum of LH in solution (lmax=580 nm). 13C NMR spectra also support the occurrence of intermolecular attractive CN group interactions in the solid state. X-Ray diVraction patterns indicate that the controlled sublimation process of LH (Tsubl=200 °C, 10-6 mmHg) leads to films composed of highly oriented crystallites, with two main sets of diVracting planes parallel to the film surface.The refractive index of LH as an evaporated thin film has also been determined in the 400–800 nm spectral range (n=1–2). LH interacts with ammonia and/or water in the gas phase.In the first case the acid–base reaction (LH+NH3PL¾·NH4+) occurs. The resulting L¾ anion (L¾OC11H2N7-) is the 2-(5-amino-3,4-dicyano-2H-pyrrol-2-ylidene)-1,1,2-tricyanoethanide (A, lmax=525 nm) or the isomer 1,2,6,7-tetracyano-3,5-dihydro-3,5-diiminopyrrolizinide (B, lmax=680 nm), depending on the relative amount of water to ammonia in the gas phase. This reaction is driven by the hydrogen bonding of NH4+ to B and/or to water.In the second case a fast proton scrambling occurs. We have recently synthesized and structurally characterized the title pyrrolizine (LH, Scheme 1) as the 251 1-chloronaphthalene adduct: 2LH·NAPH.1 Our interest in LH is mainly due to its chemico-physical properties: (i ) it is a planar, intensely coloured molecule [in tetraydrofuran (THF): lmax= 580 nm, e580=20 000 dm3 mol-1 cm-1], (ii ) it can develop attractive potentials in the solid state through several mechanisms such as p-orbital overlap, hydrogen bonding, dipolar CN group interactions, (iii ) it forms mono- and/or bis-pyrrolizinato metal complexes, (iv) it can be deposited under vacuum as thin Scheme 1 Non-systematic numbering is used for the NMR assignments films and (v) in this state it is a semiconductor of low conductivity.In research aimed at addressing its possible use in technological applications, by analogy with molecular materials based on evaporated dyes and/or semiconductors,2 LH It was recrystallized from acetone–water (151) and then dried has been further characterized both in solution and in the in an oven at 60 °C in air for 4 h.From thermal gravimetric solid state. Moreover, we investigated whether LH has any and diVerential analyses, performed with a Du Pont 950 recognition properties for selective detection of species of apparatus, it did not show any weight loss nor heat exchange environmental interest in air. Herein the results of these studies up to 250 °C. Its density (d) was measured (1.45 g cm-3) by are reported.suspending the powder in a suitable mixture of solvents. Evaporated films were deposited on glass plates in an Edwards Auto 306 vacuum coater as described previously.1 LH can be Experimental fully deuterated on exposure to D2O vapor in a dry-box, as Materials proved by the infrared spectra (Fig. 11; see later). Spin coated films were deposited from a THF solution with a Convac LH was prepared from NaL¾ according to the previously spinner model 1001.Film thicknesses were measured using an reported procedure.1 Alpha-Step stylus profilometer. Scanning electron microscopy (SEM) images were recorded on a JEOL SM6100 instrument. Powder X-ray diVraction (XRPD) X-Ray diVraction data, both for powder samples and for polycrystalline thin films, were collected on a Seifert XRD3000 two-circle automated diVractometer at room temperature using Cu-Ka radiation with a graphite monochromator.The step scanning technique, with steps of 2h=0.025° and a stepping time of 10 s, was used over the range 5°2h50 °. † E-mail: flamini@nserv.icmat.mlib.cnr.it J. Mater. Chem., 1998, 8(5), 1139–1144 1139Optical and infrared spectroscopic measurements Optical spectra were recorded on a Cary 5 spectrometer.Dried and freshly distilled solvents were used for the solution spectra. The spectra of the films were measured by inserting the glass slide supporting the film vertically across the light beam into a 10 mm square quartz cuvette. An uncoated glass slide was placed in the reference compartment. In addition, reflectivity measurements in the 400–800 nm spectral range were carried out using the integrating sphere accessory of the spectrophotometer. IR spectra were recorded on a Perkin Elmer 16F PC FTIR spectrometer as Nujol mulls. 13C NMR measurements Solid state 13C CP-MAS NMR spectra at 50.13 MHz were recorded on a Bruker AC-200 spectrometer, equipped with an HP amplifier, 1 H 200 MHz, 120 W cw, and with a pulse amplifier, M3205.The spin rate of the sample was 8 kHz. The p/2 pulse width was 3.1 ms, the contact time for the crosspolarization experiment was 4 ms and the relaxation delay was 10 s. 13C spectra were obtained with 512 words in the time domain, zero filled and Fourier transformed with a size of 1 K. Fig. 1 XRD patterns of diVerent samples of LH: (a) evaporated film, All the solution experiments were performed on a Bruker (b) solvent-free microcrystalline powder and (c) microcrystalline 2LH·NAPH AMX-600 spectrometer.High resolution 13C NMR spectra at 50.9 MHz were obtained with broad band proton decoupling performed with a GARP sequence.3 Acquisition and relaxation 3.17(100), 2.76(32). (ii ) Microcrystalline 2LH·NAPH, of known delay were chosen according to the Ernst relationship4 in order structure.1 From the corresponding XRPD [Fig. 1(c)] it shows to maximize the signal to noise ratio for the long relaxation a much higher degree of crystallinity in this case, the FWHM of quaternary C atoms. Spectra were obtained with 16 K words ranging from 0.12 to 0.90 °. (iii ) Microcrystalline powder, in the time domain and Fourier transformed on a size of 8 K.obtained by accurate grinding of LH evaporated films. Its diVraction pattern is identical with that of powder (i ), so Electrical resistivity measurements and sensor property studies proving that the films deposited by sublimation have the same crystal structure as the source powder. (iv) LH deposited under Electrical measurements were performed on our samples in vacuum as thin films.Several samples of diVerent thickness order to test the electrical sensing properties of the active layer. (50–800 nm) have been examined. Their XRD patterns are all To this end, alumina substrates were first prepared by thermal identical: the one relating to a 400 nm thin film is reported in evaporation and deposition of a patterned microelectrode [Fig. 1(a)]. As only two peaks are present, at 5.21 and 3.17 A ° , array consisting of interdigitated pairs of gold fingers about replacing the set of peaks from the same films once ground 40 nm thick. The dc resistance of the various samples was (see point iii ), we must infer that the controlled sublimation measured by an electrometer, Keithley model 617.The average process leads to thin films constituted of highly oriented resistivity of the samples of typical dimension crystallites, with two main sets of diVracting planes parallel to 1×1×1.5×10-5 cm3 measured in a flux of dry air was about the film surface, with interplanar distances typical for face-to- 1.7×106 V cm. The eVect of diVerent toxic gases on electrical face p-interacting conjugated systems, so suggesting a depos- conductivity was measured in a dynamic pressure system ition process leading to face-to-face LH units.implemented in our laboratory where dry air at ambient pressure was used as the carrier and reference gas. The gas Optical spectra concentration was varied by using a MKS Instrument mass flow controller, model 647. Evaporated thin films deposited on glass showed well resolved optical spectra.From these spectra, after normalization to the Results and Discussion concentration of LH in the solvent-free solid phase (C=6.2 M) and to the film thickness, the molar extinction coeYcient (e) For our objectives, one of the most relevant properties of LH of LH in the film vs. wavelength can be calculated. On is that it sublimes without decomposition aVording thicknesscomparison with the corresponding values of LH in solution controlled thin films.Thus, the following discussion deals (Fig. 2) a remarkable feature appears. The single band of LH mainly with diVerent sets of experimental data for LH in the in solution (lmax=580 nm) is replaced in the solid state by a solvent-free solid phase, either as a powder or thin films.When double band (lmax=615 and 570 nm) of approximately the appropriate, a comparison with the corresponding data for same total area and with two associated components of the 2LH·NAPH and of LH in solution is made. same intensity. Clearly, such spectral variations originate in solid-state intermolecular interactions. In this regard, either of X-Ray diVraction patterns the following mechanisms could be in operation, depending on the crystal structure: the well-known Davydov-type XRD measurements were made on several samples.(i ) Solventfree microcrystalline powder was obtained as described in the coupling5 or a charge-transfer between adjacent molecules, which occurs widely in polycyano-substituted molecules.6 Experimental. The corresponding XRPD pattern [Fig. 1(b)] shows a series of broad peaks characterized by FWHM values Occasionally, some evaporated thin films, as well as the spincoated films, showed optical spectra quite diVerent from that in the range 0.30–1.20°; this indicates a poorly crystalline structure. Consequently, any attempt to indicize the spectrum just discussed. In Fig. 3 the spectra in question, derived from evaporated LH films of the same thickness, are reported: failed.The main peaks (interplanar distances, in A ° ) and their relative intensities (in parentheses), are as follows: 9.04(9), spectrum 1 exhibits more than the two bands expected for exciton splitting within the crystal as usually observed (spec- 8.26(39), 5.57(61), 5.21(28), 4.54(21), 4.02(21), 3.50(8), 1140 J.Mater. Chem., 1998, 8(5), 1139–1144Fig. 2 Normalized optical spectra of LH: (~) as evaporated film and (-- -- -) in THF solution Fig. 4 SEM micrographs of the LH evaporated films, whose optical spectra are reported in Fig. 3 Fig. 3 Optical spectra of two diVerent LH evaporated films of the same thickness (150 nm) trum 2). In conjunction with the SEM images of the source films (1¾ and 2¾ of Fig. 4), this could be explained by the diVerent film morphology, which may imply a larger amorphous content in 1¾, than in 2¾. In turn, 2¾ clearly shows a layered Fig. 5 Refractive index (n) vs. wavelength for a typical evaporated LH crust probably of crystalline structure. thin film The refractive index n of the as-deposited LH thin films vs. wavelength was also calculated from a computer fit of both transmission T ( l ) and reflection R( l ) measurements based on 13C NMR spectra a model that considers a parallel sided isotropic and absorbing film between transparent media of indexes n0 (air) and n2 Before examining these data, let us consider the tautomerism between the two energetically equivalent LH configurations (a (glass substrate), the latter being assumed to be very thick with respect to the wavelength.7 Fig. 5 shows the refractive and b in Scheme 1), which would directly influence the NMR spectra. Previously we ascertained that such tautomerism exists index n of a typical evaporated LH thin film in the 400–800 nm spectral range. As can be seen, the refractive index n of the only in solution. Accordingly, it was found that the NMH proton exchange at room temperature is fast in THF solution film changes slowly between 1 and 2.J. Mater. Chem., 1998, 8(5), 1139–1144 1141as revealed by the 13C NMR spectrum of LH in this solvent, to zero in solution. The resonances of these carbons are shifted to high field either due to intermolecular attractive CN group while it does not occur in the solid state for the 1-chloronaphthalene adduct, as proved by the successful refinement of the interactions9 or by the magnetic anisotropy of a neighboring CN group.10 molecular structure of LH from the single crystal X-ray analysis of 2LH·NAPH. These findings indicate that the tautomerism in question requires an intermolecular exchange mechanism.Sensing properties of LH The current 13C NMR data, as it will be seen, substantiate the In view of the utilization of LH for detecting species of supposed mechanism and in addition indicate that the NMH environmental interest in the air, we probed LH films, deposited proton exchange even in solution can be slowed down by a on interdigitated electrodes, as conductimetric sensors.No suitable solvent such as dimethoxyethane (DME).Thus, LH significant variations in their electrical conductivity were in DME exhibits eleven 13C NMR resonances (Fig. 6). The observed on exposure to NH3, NO, CO or H2 gas up to assignments, presented in Table 1, are based on the correlation 100 ppm in dinitrogen. However, see the following section. between the observed intensity of the resonance with the relaxation time of the carbon atom originating from the Interaction with ammonia resonance itself.8 That is, the nearer the carbon atom is to an eYcient relaxation center, i.e.a proton, the higher the intensity The interaction of LH with NH3 occurs only in the presence of the associated resonance. The NMR spectrum of the solid of water and it depends markedly on the experimental conshows a clear correspondence with the solution spectrum, for ditions.We selected and present here three typical examples. the most intense peaks. Two extra broad resonances appear (1) When NH3 (500 ppm in N2) is slightly humidified, by in the solid spectrum, absent in the solution spectrum, at passing over liquid water before interacting with LH, the limiting high field (110 ppm). We tentatively assigned these spectral changes occurring (Fig. 7) are not fully reversible and, resonances to the nitrile carbon atoms, whose resonance is on comparison with the results of the subsequent experiments broadened by the 14N quadrupole, which in turn is averaged (2 and 3), they indicate the formation of two species in the film, A and B (lmax=525 and 680 nm, respectively). (2) Only B is formed when experiment (1) is carried out immediately after exposing the film to a water-free ammonia stream.In this case, the film will sense a smaller amount of water relative to the ammonia content in the gas phase. The consequent modifi- cation occurring is rapid and irreversible (Fig. 8). In (3), only A is formed when LH comes into contact, in air, with the saturated vapor of a concentrated aqueous ammonia solution (35 wt%).In this case the interaction is fully reversible, so that LH is reformed from A in the absence of ammonia in air with an associated negligible hysteresis (Fig. 9). On the basis of our previous studies on LH and of the wellknown solvatation eVects on the basicity of NH3,11 the results of the above experiments can be reasonably interpreted as follows.A and B are simply L¾ isomers, resulting from the same reaction LH+NH3�NH4+·L¾. LH is a weak acid and it reacts with NH3 provided that the resulting NH4+ is Fig. 6 13C NMR spectra of LH as solid sample (top) and in DME solution (bottom) Table 1 13C solution and CP-MAS NMR spectral data of LHa d Assignmentb Solution Solid Assignmentb C1, C7 110.8, 112.35 0 C1, C7, C3 C2, C6 126.6, 152.15 107, 92 C8, C9, C10, C11 C8, C9, C10, C11 114.8, 114.5, 113.1, 112.7 C3, C5 111.8, 123.2 125 C2, C5 Fig. 7 Spectral changes with time (every 2 min) undergone by a thin C4 154.9 150 C4, C6 film of LH (thickness 150 nm) on exposure to NH3 (500 ppm in N2) humidified by passing over liquid water. The arrows indicate the aSee Experimental for details of data collection.bSee Scheme 1 for the numbering. direction of the spectral changes. 1142 J. Mater. Chem., 1998, 8(5), 1139–1144metal [e.g. ZnII], is stabilized by hydrogen bonding to NH4+. At the end of experiment (1), NH4+ will be solvated partly as NH4+·4H2O lying close to A as the counteranion and partly as B·NH4+·2H2O. For further support of this explanation, we report (Fig. 10) the normalized solution optical spectra of AsPh4·L¾ and (acac)ZnL¾ (acac=acetylacetonato), both structurally characterized,12,13 as representative examples of the isomers A and B, respectively. Note that LH, due to the absence of mesomeric resonance, exhibits a less intense spectrum relative to A and B both in solution and in the solid state. Finally, as implicated by these findings, we infer that LH could be used as a specific optical sensor for ammonia in air under rigorously humidity controlled conditions.Such a sensor would be highly desirable for practical use, given that the existing ones lack selectivity as they are based on pH indicator dyes.14 In our laboratory, several trials are in progress to this end. Fig. 8 Spectral changes with time (every 1 min) undergone by a thin film of LH (thickness 150 nm) on exposure to NH3 as described in Fig. 7. In this case, the film was previously exposed to a water-free ammonia stream immediately before the interaction with humidified ammonia (see text). The arrows indicate the direction of the spectral changes. Fig. 10 Normalized solution optical spectra of (- -- --) AsPh4·L¾ in THF, (~) LH in THF and (A) (acac)ZnL¾ in toluene Fig. 9 Spectral changes with time (every 20 min) undergone by a thin film of LH (thickness 170 nm) after exposure to the saturated vapor of a concentrated aqueous ammonia solution (35 wt%). The arrows indicate the direction of the spectral changes. (--) Spectrum of the sample prior to exposition to NH3–H2O vapor. eYciently solvated. This may likely occur with the intervening four hydrogen bonds.So, NH4+ forms four hydrogen bonds with four H2O molecules if there is enough water available in the system then L¾ adopts the most stable configuration A as in experiment (3). Alternatively, NH4+ forms two hydrogen bonds with two H2O molecules and two others with the imino groups of isomer B as in the experiment (2) if there is little water available. We note that in the latter case, B itself, which so far Fig. 11 Infrared transmission spectra of (~) LH and (- -- --) LD both as Nujol mulls was known to exist only when coordinated to a late transition J. Mater. Chem., 1998, 8(5), 1139–1144 11433 A. J. Shaka, P. B. Baker and R. Freeman, J. Magn. Reson. 1985, Interaction with water 64, 547. 4 R. R. Ernst, G. Bodenhauesen and A.Wokaun, Principles of LH in the solid state interacts with water vapor, causing fast Nuclear Magnetic Resonance in One and Two Dimensions, and quantitative scrambling protons, despite its lack of aYnity Clarendon Press, Oxford, 1987, ch. 4, p. 125. for water. This property is remarkable and in the appropriate 5 M. Pope and C. E. Swenberg, Electronic processes in organic crysconditions can be exploited for detecting water in air by means tals, Oxford University Press, New York, 1982, p. 59. of infrared spectroscopy. For istance, LH can be easily fully 6 (a) M. Bonamico, V. Fares, A. Flamini, A. M. Giuliani and P. Imperatori, J. Chem. Soc., Perkin T rans. 2, 1988, 1447; (b) deuterated (see Experimental). In the 3500–3000 cm-1 IR M. Bonamico, V. Fares, A.Flamini and P. Imperatori, J. Chem. spectrum of the deuterated species, say LD, this region lacks Soc., Perkin T rans. 2, 1990, 121; (c) M. Bonamico, V. Fares, any absorptions (Fig. 11). On exposure to air, the strong NMH A. Flamini, P. Imperatori and N. Poli, J. Chem. Soc., Perkin T rans. stretching vibration infrared-active absorptions appear in this 2, 1990, 1359; (d) V. Fares, A.Flamini and N. Poli, J Chem. Res., spectral region as a consequence of the transformation 1995, (S) 494; (M) 3054. 7 O. S. Heavens in Physics of thin Films, ed. G. Hass and R. E. Thun, LD�LH by water vapor. In no case is water (H2O or D2 O) Academic Press, New York, 1964, p. 193. trapped in the sample, thus avoiding the additional compli- 8 E. Breitmaier and W. Voelter, 13C NMR Spectroscopy, 2nd edn., cation of discriminating between OMH(D) and NMH(D) Verlag Chemie, New York, 1978, p. 111. absorptions in evaluating IR sensor applications. It is also 9 J. B. Stothers, Carbon-13 NMR Spectroscopy, Academic Press, found, as expected, that this transformation is chemically fully New York, 1972, p. 226 10 J. A. Polple, J. Chem. Phys., 1956, 24, 1111. reversible. 11 F. E. Condon, J. Am. Chem. Soc., 1965, 87, 4481. We thank the Progetto Strategico ‘Materiali Innovativi’ of 12 V. Fares, A. Flamini and N. Poli, J Chem. Res., 1995, (S) 228; CNR for partial financial support, Mr Claudio Veroli for (M) 1501. technical assistance during the X-ray work and Dr A. 13 V. Fares and A. Flamini, to be published. 14 (a) C. Preininger, G. J. Mohr, I. Klimant and O. S. Wolfbeis, Anal. Capobianchi for help in the experiments with ammonia. Chim. Acta, 1996, 334, 113; (b) A. Mills, L. Wild and Q. Chang, Mikrochim. Acta, 1995, 121, 225; (c) R. Klein and E. Voges, Fresenius J. Anal. Chem., 1994, 349, 394; (d) A. Sansubrino and M. Mascini, Biosens. Bioelectron., 1994, 9, 207; (e) Y. Sadaoka, Y. Sakai and M. Yamada, J. Mater. Chem., 1993, 3, 877; ( f ) References Y. Sadaoka, Y. Sakai and Y. Murata, Talanta, 1992, 39, 1675; (g) 1 V. Fares, A. Flamini and N. Poli, J. Am. Chem. Soc., 1995, 117, P. C� ag¡lar and R. Narayanaswamy, Analyst, 1987, 112, 1285; (h) J. F. Giuliani, H.Wohltjen and N. L. Jarvis, Opt. L ett., 1983, 8, 54. 11 580. 2 H. Bo� ttcher, T. Fritz and J. D. Wright, J. Mater. Chem., 1993, 3, 1187. Paper 7/06942A; Received 25th September, 1997 1144 J. Mater. Chem., 1998, 8(5), 1139&ndas