Langmuir 1996,11,2705-2712
2705
Langmuir-Blodgett Films of Triazolehemiporphyrazines: Evidence for Molecular Organization S. Pfeiffer, C. Mingotaud," C. Garrigou-Lagrange, and P. Delhaes Centre de Recherche Paul Pascal, CNRS, Avenue A. Schweitzer, 33600 Pessac, France
A. Sastre and T. Torres Departamento de Quimica Organica (C-l), Universidad Autbnoma de Madrid, 28049 Madrid, Spain Received November 22, 1994. I n Final Form: March 31, 1995@ I
Triazolehemiporphyrazine derivatives bearing crown ether macrocycles have been investigated in Langmuir and Langmuir-Blodgett films. Using infrared dichroism and ESR experimentsand after careful assignments of infrared bands, the organization of these molecules within the LB multilayer has been characterized. These compounds form stacks of macrocycles and are tilted with respect to the normal of the substrate. These aggregates of about 50 molecules are partially oriented in the plane of the layers, showing that some ordering occurs during the transfer of the monolayer onto a solid substrate. This so-called stack-of-cardsconfiguration is a first step to organize the crown ether groups in order to form ionic channels in LB films.
Introduction Well-ordered and defect-free Langmuir-Blodgett films (LB) of porphyrins1 and phthalocyanines2 are of growing interest because of their potential uses in molecular electronics and optoelectronics. The LB technique for depositing thin organic films has been widely used with amphiphilic phthalocyanines3 where a high degree of molecular order has been achieved. For example, the supramolecular organization in two-dimensional networks could be applied to molecular re~ognition.~Another possibility is the formation of ionic channels based on wellordered phthalocyanines with suitable substituents such as crown ethers5 which are known to selectively form complexes with alkali-metal ions. These channels could be obtained only if some in-plane order is achieved within the plane of the layer. Such in-plane organization has been already reported for different compounds including phthalocyanines and derivatives6 and has been explained by flow orientation during the transfer p r o ~ e s s . For ~ example, Wegner et al. recently reported on rodlike
* To whom correspondence
should be addressed. Abstract published in Advance ACS Abstracts, June 1, 1995. (1)(a) Chou, H.; Chen, C. C.; Stork, K. F.; Bohn, P. W.; Suslick, K. S. J.Phys. Chem. 1994,98,383.(b) Bosoni, F.; Ricciardi, G.; Lelj, F.; Martini, G. Thin Solid Films 1994,243,335. (2)(a)Snow, A. W.; Barger, W. J. InPhthalocyanines, Propertiesand Applications; Leznoff, C. C.; Lever, A. B. P., Eds.; VCH: NewYork, 1989;Vol. 1, p 341.(b) Poynter, R. H.; Cook, M. J.; Chesters, M. A.; Slater, D. A,; McCurdo, J.;Welford, K. Thin Solid Films 1994,243,346. (3)(a) Ulman, A. Ultrathin organic films, 1st ed.; Academic Press: San Diego, 1991;pp 167-176. (b) Mingotaud, A. F.; Mingotaud, C.; Patterson, L. K. Handbook ofMonolayers, 1st ed.; Academic Press: San DieFo. 1993:DD 1114-1210. Lefe, D.; Porteu, F.; Balog, P.; Roulliay, M.; Zalczer, G.; Palacin, S. Langmuir 1993,9,150. (5)(a) Sielcken, 0. E.; Schram, J.; Nolte, R. J. M.; Schoonman, J.; Drenth, W. J. J. Chem. Sac. Chem. Commun. 1988,108.(b) Simon, J.; Engel, M. K, Soulie, C. New J. Chem. 1992,16, 287. (6)(a) Kalina, D. W.; Crane, S. W. Thin Solid Films 1986,134,109. (b) Ogawa, K.; Yonehara, H.; Maekawa, E. Thin Solid Films 1992, 2101211,535.(c) Cook, J.C.; McMurdo, J.;Miles, D. A.; Poynter, R. H.; Simmons, J. M.; Haslam, S. D.; Richardson, R. M.; Welford, K. J.Mater. Chem. 1994,4,1205.(d) Fu, Y.; Forman, M.; Leznoff, C. C.; Lever, A. B. J. Phys. Chem. 1994,98,8985.(e) P. A. Albouy J.Phys. Chem. 1994, 98,8543. (7)(a) Sugi,M.; Minari, N.; Ikegami, K.; Kuroda, S.-I.;Saito, IC; Saito, M. Thin Solid Films 1989,178,157. (b)Tabe,Y.;Sugi, M.;Ikegami, K.; Kuroda, S.-I.;Saito, K.; Saito, M. Thin Solid Films 1992,2101211, 32. @
(z)
macromolecules suitable for the LB technique based on peripherally substituted phthalocyaninato polysiloxaness and having a high in-plane orientation. One other promising series of compound is the hemiporphyrazine~~ which are highly conjugated, nonaromatic macrocycles having several structural features in common with porphyrins and phthalocyanines. Recently, electricallo and nonlinear optical" properties of triazolehemiporphyrazines12have been reported, and new hemiporphyrazinato polygerman~xanesl~ have been used to form LB films with in-plane order. In this paper, we describe the properties of crown-ethersubstituted triazolehemiporphyrazines in Langmuir and LB films. Building ionic channels require high organization and close packing of the set of molecules constituting the column. It is why we carefully analyzed the orientation and the aggregation state of such compound within the LB films. The orientation has been evaluated using IR linear dichroism, while ESR experiments indicate the existence of aggregated strands of these macrocycles. The assignment of the IR absorption bands of the title compoundsto their corresponding vibrational modes which has been necessary in order to analyze the IR dichroism and to calculate the molecular orientation in the films is given in the Appendix.
Synthesis and Experimental Methods The free-base triazolehemiporphyrazine 1-Hz(Figure 1) was synthesized as regio isomer mixture by reaction of (8) Sauer, T.; Amdt, T.; Batchelder, D. N.; Kalachev, A. K.; Wegner, G. Thin Solid Films 1990,187,357. (9)(a)Attanasio, D.; Collamati, I.; Cervone, E. Inorg. Chem. 1983, 22,3281.(b)Agostinelli, E.; Attanasio, D.; Collamati, I.;Fares, V. Inorg. Chem. 1984,23,1162. (10)Fernlndez-Llzaro, F.; Rodriguez-Morgade, S.;Torres, T. Synth. Met. 1994,62,281. (11)(a)Diaz-Garcia, M. A.; Ledoux, I.;Fernlndez-Lbzaro, F.; Sastre, A.; Torres, T.; Agullb-L6pez, F.; Zyss, J. J. Phys. Chem. 1994,98,4495. (b) Diaz-Garcia, M. A.; Ledoux, I.; Fernlndez-Llzaro, F.; Sastre, A.; Torres, T.; Agull6-L6pez, F.; Zyss, J. Nonlinear Opt., in press. (12)Fernlndez-Llzaro, F.; de Mendoza, J.; Mo, 0.; R6driguezMoreade. S.: Torres, T.: Ylnez, M.; Elguero, J. J. Chem. SOC.,Perkin Tra&. 2'1989,797.' (13)Ferencz. A.: Ries. R.:, Wemer. G. Angew. - Chem., Int. Ed. Engl. 1993,32,1184.' '
0743-746319512411-2705$09.0010 0 1995 American Chemical Society
Pfeiffer et al.
2706 Langmuir, Vol. 11, No. 7, 1995
ESP 300 E system working a t 10 GHz (X-band) which is equipped with a liquid 4He accessory. UV-vis spectra were taken with a Perkin-Elmer 330 spectrophotometer. IR experiments have been conducted with a computer-controlled Nicolet 750 interferometer equipped with a KRS-5metal grazing polarizer. IR spectra of some reference compounds (see Figure 1)were obtained with a Philips PU 9716 spectrophotometer. IR spectra of 1 LB films deposited onto CaFz (wavenumbers range down to 900 cm-l) and ZnSe (down to 600 cm-') have been obtained. The IR spectra of LB films deposited onto CaF2thus exhibit additional bands due to behenic acid (vc-0 a t 1700 cm-' and increased Y C H ~and ~ C peaks) H ~ and are otherwise identical with spectra obtained from ZnSe substrates. We compared the LB film spectra of 1 with those of related reference compounds, namely, triazolehemiporphyrazines bearing only one type of substituents (alkyl chains or crown ethers, respectively). In the Appendix, we discuss their assignment by comparing with phthaloc y a n i n e ~ , ~triazoles,20 '-~~ ortho-disubstitutedZ1and 1,2,4,5tetrasubstituted benzenes,22 and 1,4,7,10,13-pentaoxacyclopentadecane crown ethers ( 1 5 - c r o ~ n - 5 ) . ~ ~ ) ~ ~ Infrared Linear Dichroism. To determine the orientation of the molecules in LB films, a powerful method to be employed is linear dichroism in infrared spectrosHN-N copy.25,26The absorption of a polarized IR beam is 3 wilh M=Hz,Cu or Co proportional to OliEY where pi is the transition dipole moment of the vibration being studied and E is the local Figure 1. Molecular structures of the triazolehemiporphyraIR electric field. By variation of the orientation of the zines 1-3. electric field, it is possible to determine the orientation of pi. Using polarized light, one obtains three different IR 4,5-(1,4,7,10,13-pentaoxatridecamethylen)-l,3-diiminoisoindole14with the corresponding l-dodecyl-3,5-diamino-l,2,4- spectra. In the first two cases the incident light is parallel to the substrate normal while E is either parallel or triazole following established procedures.11J5 Metalated perpendicular to the transfer direction t;in the third case, triazolehemiporphyrazines (Figure 1)1 4 0 (M = Co(II)), E is polarized parallel t o t whereas the IR beam is incident l - N i (M = Ni(II)), and l-Cu(M = Cu(I1)) were prepared a t an angle of 60"to the substrate normal. The in-plane as previously reported15 while the metalation was moniand out-of-plane dichroic ratios a and /3 for each band are tored by IR spectroscopy. Spreading solvent was chlodefined as roform (HPLC grade from Prolabo). Solutions (ea. 1mmol L-l) of all compounds 1 were kept at - 18"C under nitrogen in-plane dichroic ratio a = AI/AII in order to avoid possible oxidation especially of the cobalt complex and solvent evaporation. out-of-plane dichroic ratio p(60") = A,,(60")/Al,(O") Langmuir films were obtained by spreading solutions of 1 onto water with a specific resistance higher than 18 where A is the absorption of the IR band. The ratios a MQ em produced by a Millipore ultrapure water system. and /3 are related to the Euler angles o and 4 (see Figure The compression isotherm curves were recorded using a 2) describing the orientation of the transition dipole rectangular homemade Teflon trough, working at 20 "C moment pi through the following equations:26 and under a continuous nitrogen flow saturated with water. The surface pressure was measured by a Wilhelmy P, = (cos 2 w ) = (1- a)/(l a) (1) balance using a probe made of platinized platinum. The compression of the Largmuir film is done with a constant speed of typically 1.5 A2/(moleculem i d . This speed was (cos2 #)/(sin2 4) = (1/2 ~,/2)~(p,n,,n~,n,,i,r) (2) low enough to avoid nonequilibrium effects. For transfer of monolayers as LB films, a commercially available where ( ) is the average over all possible orientations and ATEMETA trough16 also with a Wilhelmy balance was F a function depending on refractive indexes nl of air, n2 used. Here, isotherms were obtained in the stepwise of the LB film, and 72.3 of the substrate. The refraction compression mode where the surface pressure lT is angle r is related to the angle i through the Snellincreased in steps of usually 2 mN/m. The monolayers have been transferred onto ZnSe and (17)Shurvell, H. F.;Pinzuti, L. Can. J. Chem. 1966,44,125. (18)Kobayashi, T.; Furokawa, F.; Uyeda, N.; Suito, E. Spectrochim. CaF2 substrates for IR and UV-vis spectroscopy and onto Acta 1970,26A,1305. quartz plates for ESR experiments. CaF2 substrates are (19)Sidorov, A. N.; Kotlyar, I. P. Opt. Spectrosc. 1961,11, 92. precoated with three layers of behenic acid in order to (20)SaidiIdrissi, M.;Senechal,M.; Sauvaitre, H.; Ganigou-Lagrange, ensure optimum transfer ratios. Typically 50 layers were C. Can. J. Chem. 1983,61, 2133. (21)Brigodiot, M.; Lebas, J. M. J. Chim. Phys. 1965,62, 347. transferred onto each substrate side for the experiments. (22)Garrigou-Lagrange, C.; Le Calv6, N.; Vignalou, C. J . Chim. Phys. ESR experiments have been conducted with a Bruker 1966,63,1454. (23)Takeuchi, H.;Arai, T.;Harada, I. J. Mol. Struct. 1986,146,197. F1ZH25
N- N
+
+
(14)Sielcken, 0.E.; van de Kuil, L. A.; Drenth, W.; Schoonman, J.; Nolte, R. J. M. J. Am. Chem. SOC.1990,112,3086. (15)FernLndez-Lbzaro, F. ;Sastre,A,; Torres, T. J. Chem. SOC.,Chem. Commun., in press. (16)ATEMETA, 35 Bd Anatole France, 93200 St Denis, France.
(24)Ahsen, V.; Yilmazer, E.; Ertas, M.; Bekaroglu, 0.J . Chem. SOC., Dalton Trans. 1988,401. (25)Breton, J.;Michel-Villaz, M.; Paillotin, G.; Vandevyver, M. Thin Solid Films 1972,13, 351. (26)Chollet, P. A. Thin Solid Films 1983,99,197.
LB Films
Langmuir, Vol. 11, No. 7, 1995 2707
of Triazolehemiporphyrazines N
.
VU
h
e z e
Y
40 50
30 20-
1070
A
75
80
85
90
95
100
95
100
Molecular area (@)
(direction of transfer)
Figure.2. Angles w and 4 definingthe orientationof a transition dipole moment pi of the molecule with respect to the substrate system OLMN; the transfer direction t is parallel to the L axis.
................................. R N-N
-
I'
X
Figure 3. Symmetry of compounds 1 and definition of the molecular axes X,Y,and 2.The 2 axis is the normal to the molecular X Y plane.
Descartes relation. Approximations made in deducing these equations are that the LB film thickness is small with respect to the IR wavelength and that the IR absorptions are weak. The order parameter PZdescribes the average orientation of the transition dipoles within the plane of the film: Pz = 0 for all dipoles having random distribution in the substrate plane. PZ= 1for all dipoles perfectly oriented in the transfer direction. PZ= -1for all dipoles perfectly oriented in the direction perpendicular to the transfer. The molecular axes XYZ of the title compounds 1 are defined in Figure 3 , Z being the normal to the molecular X Y plane. The symmetry of the macrocycle in its planar conformation is assumed to be D 2 h . Introducing the crown ether groups with unknown conformation as well as the alkyl chains reduces the symmetry to C2"or C 2 h depending on the relative position of the two alkyl chains (Figure 3). The vibrational modes and electronic transitions are nondegenerate because it is possible to distinguish transition dipole moments parallel &) to the molecular X axis from those perpendicular 01,) to X. In the framework of group theory, the above symmetries have been used for the assignment of the IR absorption bands which gives the direction ofthe transition dipole moments within the molecule (cf. Appendix). The dichroic ratios of these absorptions together with eqs 1and 2 allow one to deduce the molecular orientation with respect to the substrate (laboratory frame).
Results and Discussion Langmuir Films. The isotherms of the four compounds on pure water are given in Figure 4. The compounds form stable monolayers with collapse pressures ranging between 41 and 50 mNlm and corresponding
70
75
80 85 90 Molecular area (dit)
Figure 4. Compression isotherms at room temperature for (a, top) 1-H2 (1)and 140(m) and (b, bottem)for 1-Cu(1) and 1-Ni
(m). molecular areas of 71-78 k.The onset of the isotherms on compressing the four compounds 1 occurs between 90 and 100 &. No first-order phase transition is clearly observed a t room temperature. Although the 1-Hs isotherm shows a n inflection point a t moderate pressure, the shape of the isotherms is not modified dramatically with the exchange of the central metal. The small changes in the compressibility and areas of these isotherms could be induced perhaps by slight differences in the interactions between macrocycles depending on the nature of the central metal. From CPK model calculations, the molecular dimensions without the alkyl chains are obtained as length 27.8 A (along the X axis, see Figure 31, width 10.2 8, (Yaxis), and thickness (given by the crown ether group) 5.4 A. Using these values, the orientation of the molecules at the surface can be estimated. A flat arrangement on the nitrogen-water interface giving a n estimated molecular area A > 284 as well as a longedge-on orientation with A > 150 A2 can be ruled out, whereas the orientation with the short edge ofthe molecule (Le., one crown ether group a t the water surface) gives an estimated area ofA > 55 A2. This last value is compatible with the molecular area observed in the isotherms. Because in this edge-on conformation the crown ether group is in contact with water, it can be tested if there is a change of the molecular arrangement when alkali-metal chlorides are added t o the water subphase. Crown ethers are known to selectively interact with alkali ions.27 In case of 15-crown-5, a Na+ ion fits into the crown ether ring forming a complex, while K+ induces a sandwichlike cluster between two crown ethers. The influence of K+ cations dissolved in the subphase onto the 1-Cu isotherm has been investigated by varying the salt concentration over 4 orders of magnitude. The molecular area a t the collapse pressure increases less than 4% a s compared to
&
(27) Sielcken, 0. E.; van Tilborg, M. M.; Roks, M. F. M.; Hendriks, R.; Drenth, W.; N o h , R. J. M. J.Am. Chem. SOC.1987,109, 4261.
2708 Langmuir, Vol. 11, No. 7, 1995
Pfeiffer et al.
Table 1. Assignment of Vibrational Transitions and Orientation of Dipole Moments in 1 angles of orientation (deg)
dichroic ratios v (cm-l)
a
B
assignment
direction of trans moment
6.z
bY
e
1-Hz 3373 3304 1656 1502 1225 1062
0.39 0.33 2.81 0.63 4.60 2.86
1.06 1.09 1.17 1.21 1.57 1.27
VNH
1564 1509 1493 1219 1077
2.63 0.87 0.55 4.26 4.05
1.58 1.26 1.06 1.30 1.37
VC-N,
VNH VC-N.,. Y19b bsuand triazole
vWC 'C-Naza
X X Y X Y Y
47 45
av
45
57 44 54 54 55
66
1-cu Y X X Y Y
triazole v19b blu vWC bz, VC-N,
av
48 45 48 46
58 56 54
66
1-co 1578 1512 1491 1218 1074
5.55 0.44 0.62 3.47 3.84
1.53 0.99 0.99 0.93 1.09
Y X X Y Y
vC=N., triazole v19b blu "WC bzu vC-NaEa
av
57 49 51 50
65 61 61
54
1-Ni 1587 1520 1493 1219 1077
4.92 0.60 0.55 4.26 4.05
1.52 1.03 1.06 1.30 1.37
VC-N.,
triazole V19b bs.
vPC bz. VC-N.,.
Y X X Y Y av
pure water, and only for the highest concentration (1 mol L-l), the surface pressure starts building up a t a higher molecular area of about 115 A2. The surface pressure a t the collapse point does not depend on the ion concentration. The influence of cation size onto the isotherm has been studied using Li+, Na+, K+, and Cs+ dissolved as their chlorides in 0.1 molar concentration. The isotherms obtained differ no more than 7%in the collapse area. It is therefore concluded that a complexation or a specific interaction between the crown ethers and Na+ or K+ ions can be ruled out for the monolayers and that the small changes of the isotherms are due to modification of the ionic strength of the subphase. This is in agreement with spectroscopic data taken from LB films which were transferred from a 0.1 mol L-l KCl subphase. The IR bands due to the crown ether moiety of 1-Cu do not shift or change their intensity a s compared to bands in LB film spectra from pure water. One cannot rule out that adsorption of ions takes place in the monolayer a t very low surface pressure. In such case and to explain the experimetal results, one should suppose that adsorbed ions should be continuously expelled from the interface to the subphase during compression. Anyway, the interaction between the crown ether groups and the ions does not seem to be high enough to alter the molecular arrangement and the behavior of such compounds at the nitrogen-water interface. Langmuir-Blodgett Films. The transfers have been performed a t room temperature and for a surface pressure of 20 mN/m for l-Hz, 1-Ni, and 1-Cu and 30 d i m for 1 4 0 . These transfer pressures were choosen in order to obtain the highest transfer ratios. The dipping speed was typically 1 c d m n . The Y-typetransfer with transfer ratios between 0.6 and 0.7 leads to optically defect-free multilayers. Studies of the molecular orientation within the LB films could be done using dichroism experiments either in the UV-visible range or in the IR range. Even if these films present indeed some UV-visible dichroism, we used
56 49 48 48
58 56 57
60
mainly the IR technique to characterize the LB films because UV-visible dichroism allows one to evaluate only the orientation of the macrocycle and not of each part of the molecule (macrocycle,alkyl chains, crown ether, etc.). Infrared Linear Dichroism of the LB Films. A comparison of the IR spectra of all four compounds 1 in KBr pellets and in LB films shows that all absorptions of the KBr pellet samples are present in the LB film spectra for each compound 1 and that the LB films present sharper absorption bands. It can be concluded from this comparison that the compounds 1 are stable a t the waternitrogen interface. Pronounced in-plane and out-of-plane dichroism has been observed for most of the absorption bands of the four title compounds 1 in their LB films (see Figure 5). The in-plane organization indicates that some ordering occurs during the transfer onto solid substrate. One could then suppose that compounds 1 form aggregates at the nitrogen-water interface, aggregates which could be oriented during the buildup of the multilayers. Moreover, the in-plane dichroic ratio of two particular strong absorptions associated with the porphyrazine ring (1569 and 1347 cm-I for 1-Cu) is constant after depositing 10 or even fewer layers. A differential calculation omitting the absorption of the first two layers which are least organized shows that the dichroic ratio for the additional (n - 2) layers is independent of the layer number. Furthermore, the in-plane and out-of-plane dichroic ratios for most of their IR absorptions are qualitatively consistent and quantitatively similar (Table 1). This indicates a similar orientation of the macrocycles in all four compounds. The numerical values of the dichroic ratios are not exactly the same for all vibrational modes of the same symmetry (e.g., V1gb b3- and V ~ O Cb3u, see Table 1) within a given molecule. This may be due not only to experimental uncertainty but also to the intrinsic fact that the transition dipole moments are not exactly parallel (or perpendicular) to the main axis X of the molecule. In other words, the initially assumed D2h, Czu,and C2h symmetries are not
LB Films of Triazolehemiporphyrazines
- O 002' 1 -0 C
e
E perpendicular to the dipping
Wavenumben (an-I )
Figure 6. In-plane infrared dichroism of 50 layers of 1-Hz deposited onto CaF2 substrate. 4
f
N
Figure 6. Orientation of the molecular plane of 1 in the LB films.
strictly valid for the molecules in the LB films. However, it is possible to quantitatively analyze the in-plane and out-of-plane dichroic ratios and to deduce the molecular orientation starting from the following observation. The strong in-plane dichroism (corresponding to PZ= 0.5) of the vw vibrations in 1-Hz (at 3373 and 3304 cm-l, see Figure 6) is clear evidence for the orientation of the macrocycle. More generally, all bands associated with transition dipoles parallel to the X axis (respectively Y axis) have a corresponding order parameter PZpositive (respectively negative). This indicates that the X axis of the molecules lies preferentially parallel to the transfer direction. Using this result, we ascertain that the other absorptions which are already assigned and which also have in-plane dichroic ratios CL < 1(i.e., P2 > 0)correspond to vibrational modes with their transition dipole moment parallel to the molecular X axis. For example, we find in all four coumpounds 1 that the VC-N,,, and VC-N,,, modes, both with a < 1,correspond to the collective bzu modes of the four CN aza double (respectively single) bonds of the macrocycle with pi parallel to X. On the basis of several clearly assigned absorptions with pronounced dichroism given in Table 1, the corresponding mean angles between molecular and laboratory reference systems have been calculated for all compounds 1 and are also given in Table 1. Due to the nondegeneracy of the vibrational modes, transition dipole moments pi are distinguished into px and py which are perpendicular to each other and which are positioned by the angles w and #x (respectively o and #y) in the laboratory trihedral OLMN (see Figure 2). The orientation of the molecular plane X Y (approximately given by px and p y ) is described
Langmuir, Vol. 11, No. 7, 1995 2709 through their normal Z which makes an angle 8 with the substrate normal. This angle 8 is deduced from the values of & and $y. As one can see in Table 1, the angles of all the derivatives 1 are very close, indicating a similar organization of these compounds within the LB film. In average over all these series, & is found a t 47" ( f 3 " )and +y at 57" (f3"). The average tilt angle 8 of the normal to the molecular plane with respect to the substrate normal is then found around 62" (f6"). The combination of this results with the presence of aggregates of compound 1 within the film-as expected after observing in-plane dichroism and as demonstrated by ESR experiments (see below)-leads to the so-called stack-of-cards model for the molecular organization in the LB film (see Figure 6). This kind of stacking has already been described for different derivatives and in particular for macrocycle compounds such as porphyrins.28 The low dichroism for the symmetric and asymmetric CH2 stretching modes and the fact that these IR bands are a superposition of crown ether and alkyl chain CH2 groups does not allow a conclusion about a preferred orientation of the alkyl chains in the LB films. However, a pronounced dichroism is observed for the vcoc vibrations a t 1133 and 938 cm-' ( a% 0.8,,8 % 1.5)of the crown ethers in all four compounds 1. Although their orientations cannot be evaluated due to their pi direction being unknown, it is important to note that the crown ethers which are rigidly connected to the oriented core of the molecule also exhibit a preferred arrangement in the LB films. Summarizing these results clearly the molecules 1 are oriented in the LB films. The orientation of the central macrocycle extends to the benzenic moiety and comprises the crown ether substituents as well. A comparison of the four compounds 1 shows little difference between the free-base molecule 1-Hz and the metal complexes. It appears that there is a rearrangement process during the transfer of the monolayers with the edge-on arrangement onto the substrate where the molecules in the LB film are more tilted. The comparably low values of the transfer ratios are easily explained by this rearrangement process. To confirm the organization of these LB films, X-ray diffraction studies are currently under progress. ESR on LB Films. To obtain some information on the aggregation state of the derivatives 1 within the LB film, one can use the ESR technique. Within the triazolehemiporphyrazine series 1,the only member to possess a spin s = V2 is 1-Cu. For this compound or some other derivatives, it is well-known that the ESR spectra present characteristics which have to be associated with the axial symmetry of the monomer ligand field and also the coupling between the copper It appears indeed that the structure and shape of the resonance line furnish information about the type of aggregation^.^^ (1)On monomer species in a dilute medium, a superhyperfine structure is found due to both interactions between the electronic and the nuclear spins of the paramagnetic center (I6scU = 3/2)and the neighboring atoms, i.e., the nitrogens (IN = 1). (2) On dimers, because of the proximity of two Cu2+ ions, the observed ESR signal is drastically modified. A (28) Shick, G. A,; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. SOC.1989,I l l , 1344. (29) (a)Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.;Sugi, M.; King, L. G.; Crossley, M. J. J. Phys. Chem. 1993,97, 12862.(b) Vandevyver,M.; Barraud,A.;Ruaudel-Teixier,A.; Maillard,P.;Gianotti, C. J. Colloid Interface Sci. 1982,85, 571. (30) Porteu, F.;Palacin,S.;Ruaudel-Teixier,A.; Barraud,A. J. Phys. Chem. 1991,95,7438.
Pfeifferet al.
2710 Langmuir, Vol. 11, No. 7, 1995
1
I
T: 295K
Table 2. Infrared Spectra of Triazolehemiporphyraeines 2'
Hz 3360w 3296 3060w 2950sh 2920vs 2848s -2001
'
2000
I
I
I
3000
1 T: 10K T : 10K
I
H
I
(90UM)
half field
400
0
: 400
1660vs 1650sh 1485s 1470m 1434w 1370m
co
3060w 2950sh 2915vs 2845s 1610w 1600vw 1570sh 1560vs
3060w 2950sh 2920vs 2848s
1507s 1463M 1440sh 1363s
1510M 1468M 1425w 1378sh 1360m
-400
1
Q
200-
-d
0-200
1
'Oo0
cu
-
-400-
Figure 7. ESR signal of a 50 layers LB film of 1-Cu (a, top) at room temperature (reference DPPH)and (b, bottom) at 10 K. The inset in (b) shows the half-field signal.
1346w 1330vw 1308M 1280sh
765s 750vw 723m 683s
2950sh 2920vs 2850s 1630w 1600vs 1590sh 1535sh 1520M 1470m 1445sh 1378M
assignments VNH VCH aromatic VaaCHa VasCH2 VsCH2
ring 6CHl
ring (triazole?) 6NH?
1313s 1287m 1267w 1 2 6 0 ~ 1250w 1213vw 1215w 1200m 1200w 1 1 8 5 ~ 1167w 1147vw 1140w 1133w 1127w 1100s llOOm 1073m 1054m 1057w 1040sh 1033vw 1003vw lOO8w 950vw 953vw 89Ow 855vw 790w 780m
1578vs
Ni
885w 787w 780m 770w 758s 720m 700s
1330w 1317m 1310M 1290m 1290w 1 2 8 0 ~ 1272vw 1260vw 1250vw 1215vw 1200w 1200w 1175w 1172vw
ring
CH2?
1120m 1lOOM 1080m 1045w
1130vw 1103M 1080vw 1062w
1005vw 950vw 914w 885vw
1006vw 950vw
780m
783w
760s 733sh 730M 700M
760s
ring
722w 700M
TCHl
885vw 875w
r ring
vs = very strong, s = strong, M = mean strong, m = mean, w = weak, vw = very weak, sh = shoulder.
Figure 8. Variation of the spin susceptibility xp with temperature: ( 0 ) total susceptibility and ( x ) its paramagnetic component.The inset shows the linear dependence of the spin susceptibility on the inverse of temperature.
dipolar couplingis introduced which on one side suppresses the hyperfine interactions and on the other side induces a thermally accessible triplet state: the detection of a new signal a t half-field (g % 4)is the signature of Cu2+ dimers. (3) On multimers, if a n aggregation of macrocycles is occurring, they will behave as one-dimensional antifer-
romagnetic Heisenberg magnetic system.31 The ESR line will be broadened, exhibiting the behavior of a spin chain, and besides the half-field signal will disappear. We have carried out ESR measurements on LB films of 1-Cu (50 layers on each side of a quartz plate). The main result as shown on Figure 7a is the presence of a broad line a t g = 2.050 which is characteristic of intermolecular interactions because no hyperfine structure is detected. Besides we do not detect any anisotropic behavior when the sample is rotated inside the resonance cavity. The temperature dependence of the ESR signal has been recorded down to liquid helium temperature. As shown in Figure 7b, a unique signal is detected a t low temperature together with the appearance of a very weak g % 4 line below 50 K(see inset in Figure 7b). This result compared with previous data30 indicates that the dimer concentration inside the multilayers could be very low and that we are rather in the presence of multimers. Looking a t the T dependence of the spin susceptibility (Figure 8), we can decompose it into two terms: one is the Curie tail which is very large a t low temperature (see (31)Eastman,M.P.;Horng,M.-L.;Freiha,B.;Sheu,K. W.Liq.Cryst. 1987,2, 223.
LB Films of Triazolehemiporphyrazines
Langmuir, Vol. 12, No. 7, 1995 2711
Table 3. Infrared Spectra of Triazolehemiporphyrazines
Table 4. Infrared Spectra of Triazolehemiporphyrazines
3
1 in KBr
2H 3290s 3150m 3040m 2880sh 2800sh 1720w 1650sh 1635vs 1600s 1560m 1498s 1477M 1450m 1400w 1366sh 1341s 1290vs 1250vw 1217s 1173w 1124s 1106w 1078w 990m 940m 890m 850m 838m 810m 785w 761M 730w 720sh 680m a
co
assignments
H2
3060vw 2910M 2860
3060vw 2900mbr
VNH central free W H triazole YNH triazole, VCH ammatic vCH2 VCHz W H bound
1628sh 1550s
1640sh 1570vs
3370 w 3303 s 3077 w 2954 sh 2923 vs 2871 sh 2852 s 1609 m
3075w 2953 sh 2924vs 2871 sh 2853s 1610m
1660 vs
1569 vs
3076w 2952 sh 2922s 2871 sh 2853M 1621w 1606 w 1580 vs
1501 vs 1501 vs 1501 vs 1466 m 1452 m 1384 sh
1509M
1511s
1590 vs 1536 w 1520 s
1490m 1465sh 1449m 1378 sh 1360 sh 1347 vs 1283m 1235vw 1218w 1136M 1093vw 1082 sh 1070m 1060 sh 1046 sh 1007vw
1490M 1465111 1451111 1384 sh
1492 M 1466 m 1452 m 1382 sh
1344 vs 1277M 1233w 1217w 1134M 1096sh
1352 vs 1280 s 1233 w 1217 m 1132 s
central ring ring
1073m 1060 sh
VC-N,?
1016sh
1077 M 1059 w 1045 sh 1008 sh
978 w 937 m 909vw 864m
978 w 938 m 912vw 861m
977 m 937 M 910 w 862 m
vc-c? vcoc
818vw
820vw
818 vw
789 w 762M 722 m 595 w
791 w 764M 718 m 598 w
793 w 762 M 714 m 598 w
cu
1490s 1460sh 1450s 1345vs 1288vs 1250w 1213M
1494v
ring
1450m 1420vw 1360sh 1330s 1280M
BCHz
ring triazole ring
1212m
vK'C bi:
1124s 1106w 1060sbr 978w 938M 910sh 870w 850sh
1130s lll0w 1065sbr 99ow 940m 910sh 865w
780sh 761m
795w 767m
723vw
720w 655m
vcoc
ring rCHz
r ring
Cf. text.
inset of Figure 8) and the other obtained after subtraction of the previous one is approximately a quasi constant paramagnetism. From the low temperature Curie constant (ca. 3 x emu CGS mol-l), we can evaluate the concentration of localized spin centers which is less than 1%. If we assume the presence of linear chains of porphyrazines, only the one with a n odd number of molecules inside the stack could give rise to a Curie term: this is a finite size effect which is specific of a 1D magnetic system.32 Then, the previous result implies that the stacks contain in average 50 macrocycles. Besides for a magnetic chain, we could observe in the case of the 1D antiferromagnetic Heisenberg system a weak maximum in the susceptibility corrected by the Curie term, its maximum being sensitive to the value of the exchange integral.33 However, it is impossible here to evaluate such a n exchange integral because of the lack of experimental accuracy.
Conclusion Using IR linear dichroism and ESR experiments, it is shown that crown ether substituted triazolehemiporphyrazines have a preferred molecular orientation in their LB films. While the macrocycles are tilted with respect to the substrate they aggregate into elongated strands comprising about 50 molecules. Thus our work provides the first step toward a supramolecular architecture of ionic channels built from crown ether substituted macrocycles. Experiments on similar triazole-containing macrocycles with three crown ether substituents are underway. (32) Clark, W. G.; Hammann, J.; Sanny, J.; Tippie, L. C. Lecture Notes i n Physics; Springer-Verlag: Berlin, 1978; Vol. 96, p 255 (33) Carlin, R. L. In Mugnetochemistry; Springer-Verlag: Berlin, 1986; Chapter V, p 70.
1354 vs 1283 vs 1235 sh 1223 M 1131 s 1095 w 1084 sh 1058 m 1047 sh 1006 w 995 w 978 m 935 m 891 w 862 m 842 sh 818 vw 805 vw 784 w 763 M 673 M
cu
co
Ni
3076 w 2952 sh 2923 vs 2870 sh 2852 s 1629 m
assignment VNH VNH VCH aromatic vas CHs vas CH2 vs CHI va CHz
ring? VC-Nam
triazole BNH v19b bBu BCH2 chains 8CHz crown ethers
v W C bz.
vcoc
VOOC
b3"
ring rC=N
Acknowledgment. This work was supported by the EC (HCM-ERBCHRXCT940558)and the CICYT, Spain (MAT-93-0075). Appendix Assignment of the IR Absorption Bands. Assignment of the IR absorption bands of the triazolehemiporphyrazine 1 was achieved by comparison of the IR spectra of different relative compounds. The chemical structures of these hemiporphyrazine derivatives are given in Figure 1. The analyses of the IR spectra are given below and lead to the final assignment of 1 given in the Table 4. TriazolehemiporphyrazinesSubstitutedwith Two Aliphatic Chains: 2. Four molecules were studied, the nonmetalated one and three metal complexes with Cu, Co, and Ni. Interpretations that we propose are given in Table 2. To the NH bonds of the free base molecule 2-H2 correspond a strong absorption band a t 3296 cm-' and a weak one at 3360 cm-l. The CH stretching modes of the aliphatic chains are observed as strong and narrow bands, around 2850 and 2920 cm-l, with a shoulder around 2950 cm-l. The aromatic V C H vibrations appear as very weak absorptions around 3060 cm-l. A strong absorption at 1650 cm-l is present on the spectrum of the hydrogenated derivative 2-H2, whereas the analogous absorption is observed around 1600 cm-l in free-base or metalated phthalocyanines a s d assigned to a YC=C v i b r a t i ~ n . ~To ',~~ explain the difference between nonmetalated and free-
Pfeiffer et al.
2712 Langmuir, Vol. 11, No. 7, 1995 base molecule in the case of the triazolehemiporphyrazines, we propose to assign this 1650 cm-l absorption to the VC-N,,, mode. The strong hypsochromic effect of this band for 2-Hz compared to phthalocyanines could be induced by a deviation from a planar conformation of the central ring more pronounced in the case of 2-Hz than for the more rigid and more symmetric phthalocyanine ring. Thereby, mesomerism is modified a s well as the bond character of the C-N-C aza bridge. The C=N bond should have more double-bondcharacter in compound 2-Hz than in phthalocyanines and the C-N bond a more singlebond character. The corresponding VC-N,,, mode is then expected a t a smaller wavenumber than in the planar metal complexes. Indeed, we localize it a t 1054 cm-l in the hydrogenated compound and between 1070 and 1080 cm-l in the metal complexes. Some vibrations of the central cycle were around 1490, 1370 (may be more specifically due to the triazole rings), 1310, 760, and 700 cm-l. The two last ones or, a t least, the latter one, are associated to modes of the ring with a transition dipole moment perpendicular to the macrocycle plane. The relatively strong band around 720 cm-l may be due to a rocking vibration of the CH2 groups. The B C H ~ vibration is localiz6d around 1470 cm-l while the absorption around 1100 cm-l is related to a BCH mode of the ortho-disubstituted benzenic ring.21 Molecules Substituted by 16-Crown-6 Crown Ethers: 3. We compared again spectra of the free base molecule 3-Hz with those of copper and cobalt complexes 3-Cuand 3.c0, whereas the nickel complexwas discarded, due to a n incomplete complexation. The proposed assignments are reported in Table 3.
On the spectrum of the hydrogenated compound, we see the central v, stretching mode around 3290 cm-'. The absorptions around 3130,3020,2890, and 2800 cm-l are probably due to the triazolic NH groups. For the metal complexes, we observe a broad absorption with shoulders around 3400 cm-', attributable to VNH and broad bands around 2860 and 2920 cm-l which probably correspond to V C H of ~ the crown ethers and to bound NH. The VC-N., and the ring modes of 3 are located a t almost the same position as in the molecules 2. For the tetrasubstituted benzene moieties of 3, one observes two VNC modes a t 1210 and 860 cm-l. Comparing with the tetrasubstituted benzene referencez2 and assuming the D2h symmetry for the molecule, these bands could be assigned respectively to the V ~ O Cmodes bzu and b3u. The transition dipole moments of these collective modes are then parallel (respectively perpendicular) to the molecular Xaxis in case ofb2, (respectively b3J modes. Bands around 1130 and 940 cm-' can correspond to vcoc vibrations of the crown ethers, while the band a t 720 cm-l can be assigned to a rocking mode rCH2 for the crown ether CHZ groups. Their wavenumbers seem to be similar as in aliphatic chains. TetrasubstitutedTriazolehemiporphyrazines: 1. To assign the absorption spectra of the title compounds, we compare spectra of compounds 2 and 3. We assume that the symmetry of these compounds is close to the D2h symmetry. The b3u and bzu notations are the same as before. The proposed assignments of 1 are given in Table 4. LA9409261
