J. Phys. Chem. 1993,97, 9181-9186
9181
Mono- and Multilayer Films of Discotic Metal-( Alkylthio)tetraazaporphyrins Francesca Bonosi,' Giampaolo Ricciardi? Francesco Lelj,* and Giacomo Martini'*+ Dipartimento di Chimica, Universitb di Firenze, 501 21 Firenze, Italy, and Dipartimento di Chimica, Universitb della Basilicata. 851 00 Potenza, Italy Received: February 22, 1993; In Final Form: May 3, 1993
We report on theuseofacopper compoundof 2,3,7,8,12,13,17,18-octakis(octylthio)-5,10,15,20-tetraazaporphyrin for the fabrication of a spreading monolayer and Langmuir-Blodgett films. This compound, as a free ligand or metal substituted, gives mesogenic material with a discotic, columnar structure. Its structural and magnetic properties have been characterized by optical and ESR spectroscopies. The spreading monolayer was not homogeneous and use of a typical film-forming amphiphile, such as stearic acid, was necessary for obtaining stable, reproducible surface films, which were easily transferred onto quartz plates treated with dimethyldichlorosilane. The properties of the film were also analyzed with both UV-vis and ESR spectroscopies. The film did not show orientation order, and the discotic material maintained a columnar aggregate structure.
Introduction
Porphyrins and metalloporphyrinsare largely known for their wide biological distribution and for their thermal, chemical, and mechanicalstability.' In addition, transition metal complexes of porphyrins substituted with long alkyl chains have been shown to form liquid crystals2-*with structural features similar to those ones reported for metal-alkyl phthalocyanine^.^^ These compounds decompose at temperatures as high as 56&570 K, and they give thermotropic columnar liquid crystals, i.e. discotic mesophases which are stable in a wide range of temperatures depending on the chain length and on the metal ion.376 The results obtained with Cu(11)-, Cd(I1)-, Zn(I1)-, and Pd(I1)-porphyrin complexesindicatea marked influenceon the mesogenic properties due to the nature of the metal and to the length of the alkyl chainsS4 A number of new discotic metallomesogens of 2,3,7,8,12, 13,17,18-octakis(alkylthio)-5,10,15,20-tetraazaporphyrin(TAP) R
R
have recently been prepared and characterized by differential scanning calorimetry (DSC), optical spectroscopy, and X-ray diffraction.3 As other metal derivatives of polyazamacrocycles, these compounds are expected to be sensitive to environmental gases, such as NO, NO2, Clz, Iz.~C-~~ In this work we tried to fabricate, with liquid crystal compounds formed as columnar aggregates, thin films as simply spread monolayers and as Langmuir-Blodgett (LB) films, either of pure compounds or mixed with a known film-formingdiluent such as stearicacid. This would allow us to obtain materials with unusual reactive and conductive mono-and/or bidimensional properties. This has been done in the past by Cook et al.13-lS with a copper compound of unsymmetrically substituted phthalocyanine. However, to our best knowledge, none of the liquid crystal polyazamacrocycles used until now for molecular thin film fabrication have shown the typical liquid-crystal-phasetransition in the film. Thus any new information obtained in this field seems to be of interest. We used the well-characterizedfree base (M = Hz, HzRs-TAP) Universita di Firenze.
and the copper (M = Cu, CURS-TAP) derivatives of the TAP (R = S(CHZ).ICHJ)ring, which are discotic mesogens.3 CURSTAP was further characterized by ESR spectroscopy, either as pure compound or as diluted in the diamagnetic Ni analogue compound. The electron spin resonance (ESR)technique has also been used, together with optical spectroscopy, for the film characterization. Experimental Section Materials. All materials used in this work were reagent grade. Steric acid and CdC12 were supplied by Sigma. KHC03 was supplied by Merck. The metal-TAPS used in this work were prepared and purified accordingto the reaction schemes reported in refs 3 and 5 , which represent a modification of the procedure used by Schramm and Hoffman16for the synthesis of the octakis(methy1thio)porphyrin compounds. The characterization of the compounds was mainly made by elemental analysis and NMR spectra.3 A 1/ 100 (metal atom/metal atom) sample of Cu/NiRg-TAP was prepared from the appropriate amounts of chloroform solutions of the copper and nickel compounds. The resulting mixturewas evaporated to dryness under stirring, and the resulting solid was taken without further treatment. Techniques. The spreading isotherms at 293 K of pure and mixed films were prepared with the aid of the Lauda FW1 film balance, using a discontinuous compression, with a waiting time of 90 s between two successive steps. The compression+xpansion+xmpression cycles at 293 K were determined with the Lauda F W 2 film balance with a continuous compression at 5 mm/min. LB deposition was performed with the KSV LBSOOO ALT apparatus by using the following experimental conditions:
CuRg-TAP 201 1 SA/CuRrTAP 221/10/1 SA/H*RkTAP/ CUR*-TAP
%app,
no.of layers 1 198
mN/m 15 18
mmjmin
380
21
1
Vdipp,
2
1
For pure Cu(I1) compound, a low transfer ratio (0.4)was obtained. For mixed LB films, transfer ratios near unity were always obtained. In order to obtain better deposition hydrophobic quartz slides were used. Quartz plates were prepared by overnightimmersion of accurately cleaned hydrophilic quartz plates in a 10% (v/v) solution of dimethyldichlorosilanein 1,1,1-trichloroethane, kept in an oven at 483 K for about 1 h and then rinsed with acetone,
0022-3654/93/2097-918 1%04.00/0 0 1993 American Chemical Society
9182 The Journal of Physical Chemistry, Vol. 97, No. 36, 1993
Bonosi et al.
TABLE I: Temperature (K) of the Solid-Liquid Crystal and Liquid Crystal-Isotropic Phase Transitions of the Mesogenic Compounds Used in this Work. compound Ts-LC TLC-~W HzRs-TAPO 354.8 b CURS-TAP“ 340.9e 425.1 NiRs-TAP 340.9 392.1 From ref 3. Mesomorphysm is not present. Polymorphism.
100 G 4
CuRa-TAP powder 140
i
0
no
0
60/,,J , , , , , , , , , , , ( , , ,,,, ~
20 250
350
450
550
TEMPERATURE (K)
Figure 1. Temperature dependenceof the ESR line width of CURS-TAP powder.
chloroform, and twice-distilled water. Twice-distilled water, purified with a MilliQ water system (Millipore) up to a resistivity greater than 18 M k m , was used for the subphase preparation. The electronic spectra were recorded with a Perkin-Elmer Lambda 5 UV-vis spectrometer; s- and p-polarized light was obtained with the aid of two Polaroid sheet polarizers in the 350800-nm range. The ESR spectra were recorded using the a Bruker 200D ESR spectrometer operating at the X band. Temperature was varied with the Bruker 100/700 ST accessory. Results and Discussion Propertiesof the Pure Compounds. Both free base and metalsubstituted tetraazaporphyrins studied in this work were mesogenic compounds that give discotic liquid-crystal phases with a hexagonal arrangement of the columns, as revealed by X-ray diffraction?+sColumn diameters of 23.6 A and packing periodicity of 4.1 A have been reported for both Ni-and Cu-TAPderivatives, with a mean tilting angle between the column axis and the normal to the molecular plane of 30°. Table I reports the phase transition temperatures of the pure Ni- and Cu-compounds. The phase transition temperatures of the 1/100 Cu/Ni compound were the same as for the pure Ni compound. ESR Spectra. The ESR spectra of CURB-TAPstudied in this work were typical of copper-porphyrin complexes either in liquid solutions or in the solid state.” A l e 3 mol/L solution of CURBTAP in CHCl3 gave a broad four-line absorption without a complete resolution of the Cu (I= 3/2) hyperfine structure. The measured averaged values of ( g ) and (A(Cu)) were 2.077 and 9.8 mT, respectively. Partial resolution of the third and fourth Cu hyperfine lines allowed measurement of a 14N (I = 1) superhyperfine splitting (shfs) constant of 1.63 mT, on the basis of the coupling of the copper unpaired electron with four almost equivalent I4N nuclei of the TAP macrocycle. The ESR absorption from magnetically undiluted CURB-TAP was a single, narrow line with a linewidth that typically depended on the temperature (Figure 1). In the investigated temperature range the line width was mainly governed by Heisenberg spin
0
63cu
A
6%
b)
Figure 2. (a) Overall ESR spectrum at 306 K of CuRgTAP diluted in thecorrespondingdiamagnetic Nicompound (dilution lllO0 molar ratio). (b) Low field components of the parallel hyperfinepattern of the spectrum a.
exchange. The width markedly increased in the ranges of phase transition temperatures, namely, 333-343 K for T ~ Land C 420440 K for T L C - ~respectively. , As suggested by Andre et a1.’ in other mesogenic discotic material, the line width increase was likely due to a modification of the exchange interaction because of changes of the mean molecular distances or to their random distribution. A contribution to the line width from dipolar coupling has been reported by the same authors as a width decrease at temperatures slightly below T ~ Land C attributed to thermal expansion. This effect was not observed in our case. In order to get more details on the ESR parameter temperature dependence, we studied CURB-TAPdiluted (1/100 molar ratio) with the correspondingdiamagnetic, isomorphous Ni compound. Figure 2 shows the room temperature ESR spectrum of this solid mixture. This was a typical first derivativeESRpowder spectrum from well-isolated paramagnetic centers dispersed in a diamagnetic matrix. Both 63Cu (I = 3/2) and 6sCu ( I = 3/2) isotope hyperfine couplings were resolved from the low-field hyperfine structure. The 14Nsuperhyperfinestructure was resolved on both parallel and perpendicular components. All the relevant magnetic parameters have been calculated according to the procedure reported in ref 18, and they were as follows: T=308K g,, = 2.148
A(%I)
g, = 2.038
= 22.6 mT
B(Cu) = 3.4 mT
A ( 6 3 C ~=)21.2 mT A(N) = 1.78 mT B(N) = 1.55 mT T=353K 811 = 2.145 g, = 2.040
A(Cu) = 20.8 mT A(N) = 1.74 mT
B(Cu) = 3.2 mT B(N) = 1.54 m
Discotic Metal-(Alky1thio)tetraazaporphyrins
The Journal of Physical Chemistry, Vol. 97, No. 36, 1993 9183
T=353K
160
j,,r"l"no 330
200
T=483K I I I ( I 1 I I I I I I I I I I I I I ( 1 I I I ( I 1 I 1 I I I I
3dO
430
400
530
TEMPERATURE (K) Figure 3. Temperature dependenceof the A(Cu) parallel splitting with increasing and decreasing temperature. These values were in a good agreement with those reported for similar c o m p ~ u n d s . ~ J ~ The J " ~ data ~ agreed for coordination of four equivalent nitrogens around Cu(I1) as observed with several tetraazamacrocycle ligand~.1~-~1 The ESR first-derivative powder spectrum was observed well above T ~ Land C also in the temperature range of existence of the isotropic phase. The main feature of the temperature dependence of this absorption was a progressive decrease of the Cu parallel hyperfine splitting with increasing temperature and a corresponding increase of the spectral overall line width. No resolution of the 63Cuand 65Curesolutions was observed at a temperature higher than 340-350 K. Figure 3 shows the temperature dependence of All as a function of increasing and decreasing temperature. Two main observations could be made from this behavior: (a) The line width trend did not show any clear singularity corresponding to the two transition temperatures observed by DSC of this sample. (b) A marked hysteresis existed in the temperature range 400500 K, this agreed with the hysteresis behavior also observed in the DSC patterns. Finally, a narrow, unstructured signal appeared at g = 2.004 for T > 480 K and gained intensity with increasing temperature, becoming the dominant absorption at T > 500 K. When the sample was cooled to room temperature, the intensity of the g = 2.004 signal decreased, although it was again apparent (Figure 4). Surprisingly enough the room temperature spectrum from cooled sample no longer showed any 63Cu-65Cuhyperfine pattern resolution. The ,411 decrease with temperature was due to a progressive increaseof the motional frequency toward the motional narrowing region for which the typical four line isotropicspectrum is obtained as in a fluid solution. In the reported range of temperature, the motion of the complex was not fast enough to completely average the magnetic anisotropies. Thus slow motion conditions were responsible of the spectra at 483 K, and this could also explain the observed broadening of the nitrogen superhyperfine lines. The overall ESR behavior of CuRs-TAP described above was very similar to that reported by Andre et aL7 for the mesogenic compound octakis[(dodecyloxy)methyl]phthalocyanine. By using a copper impurity in the metal-free compound, these authors find line broadening at T > TSLCwith a partial loss of the 14N shfs. The g = 2.004 signal was due to the presence of the tetraazamacrocycle. ESR absorptions due to free radicals have been often reported for diamagnetic and paramagnetic porphyrins and phthalocyanine~~J3.2~ and attributed to charge-transfer interactions between the macrocycle and molecular oxygen.23
100 Q c--l
Figure 4. ESR spectra of the 1/100 Cu/NiRgTAP compound at temperatures below TSU:(a), intermediate between TSLCand T L G ~ (b), and above T L G (c). ~ Spectrum d was rccorded at the same temperature as spectrum a after cooling the sample previously heated at T = 513 K.
CUrq-TAP. 4 . 6 . l O t CHCL molutlon
?
s cd
Y
-Sr/CUrq-?AP
eo/i. LB mal
(2 layer@) !'I ! I I ,
Figure 5. Electronic spectra of CURBTAPin CHCI, solution (concentration, 4.8 X IW mol/L) (dashed line) and as thin LB film in isotropic light (full line).
The formation of a free radical with g = 2.002 has also been reported in Cu- and VO-polyazamacrocycle complexes after oxidation with 12.7325-27 Electronic Spectra. The electronic spectrum of CuRs-TAP in a 4 X 10-6 mol/L CHCl3 solution (Figure 5 and Table 11) showed the typical Soret and Q bands of the tetraazamacrocycle ring whose assignment is described in the literature.28 The decrease of the height ratio QJQe has been used in the past as an indication of aggregation of the macrocycles in solution. We found just a small increase (from 2.6 to 2.9) in this ratio by decreasing the compound concentration from 4 X 10-6 down to
The Journal of Physical Chemistry, Vol. 97, No. 36, 1993
9184
TABLE 11: Intensity Ratios of the Q. and Q~,Electronic Transitions of C a - T A P in chloroform Soluhon at Mfferent Concentrations and in Thin Films electronic transitions and attribution chloroform solution thin film A, nm attribution polarization A, nm AA, nm 348 Soret band, A A* (XY) 350 2
-
n-#
500
492
*-A*
(XY)
623
10
(XY)
723
50
10 15 20 25 30
Q-band, Q(0,O)
673
*-A*
concentration, mol/L
QJQ8
2 x 10-5 4.8 X 1 W 1v7
LB film of 20/1 SA/CuRs-TAP (2 layers) LB film of pure CuRgTAP (1 layer)
absorbance ratios
2.7 (A673/A613) 2.6 (A673/A613) 2.9 (A673/A613) 1.8 (&3/A623) 1.5 (A700/A630)
- - SA SA/H2R TAP/CuR.TAP ----- C~R,TA$
, , , ,,
\
221/10/1
t
!
t
!
\ \
0
, I 3 I S 1~~~
10
20
SO
40
SURFACE AREA
5#
60
(A /molecule)
70
TABLE IIk Surface Area Occupied by the Single Components in a Spreading Monolayer of the 221/10/1 (Atomic Ratios) SA/H&-TAP/Ca-TAP Mixture as a Function of the Surface Pressure. surface area, Az/moIecuIe surface pressure, SA/H&-TAP/ mN/m SAb CuRgTAF CuRs-TAPd 5
-8
Q-band, Q(1-O)
613
Bonosi et al.
I-
80
Figure 6. 293 K spreading isotherms of pure CuRgTAP (- -), of the
221/10/1 SA/HzRrTAP/CuRs-TAP mixture (--), and of SA (-), on a water subphase containing 1 W mol/L Cd2+and 5 X 10-5 mol/L KHCO3.
le7 mol/L, thus indicating a small, appreciable aggregation at the highest concentrations used. Monolayers at the Air-Water Interface. Before considering the possibility of Langmuir-Blodgett film formation onto a solid support, we shortly report on the features of the monolayers obtained by spreading the TAP derivatives at the air-water interface. Compounds with alkyl-substituted azamacrocycles, such as phthalocyanines, porphyrins, and tetraazaannulenes, are known to give in many cases stable monolayers at the air-water interface. Depending on the chemical structure, two limiting arrangements of the molecular plane at the water surface are possible: (i) side-on (flat); (ii) edge-on (perpendicular) arrangeWeak core-core interactions and the Occurrence of polar groups in the core region favor a flat arrangement of the molecules. On the other hand, strong interactions between the cores and the lack of polar interactions in the core region can lead to an edge-on orientation with a columnar packing of the molecules. Surface positions of the molecular plane intermediate between the above two arrangements have been reported. In the present case, the amphiphilic character of the TAP macrocycle,greatly reduced because of the presence of the highly polarizable sulfur atoms in the eight side chains, and the scarce water solubility of the whole molecule did not result in the formation of homogeneous, stable monolayers. The spreading behavior of the pure Cu-compound was characterized by the formation of microcrystallites at the water-air interface. Figure 6 shows the 293 K spreadingisotherm of CuRgTAP on a subphase containing 10-6 mol/L CdCl2 and 5 X 10-5 mol/L KHC03. Experimental runs carried out with different surface concen-
23.6 22.8 22.0 21.5 21.4 21.2
26.7 24.6 22.6 21.7 21.3 21.1
89.7 60.6 34.6 25.1 20.3 18.1
Water subphase: Cd*+, 1 W mol/L; KHCO3, 5 X 10-5 mol/L. Temperature, 293 K. bSurface area of SA determined from the experimental isotherm of stearic acid. cSurface area of this unit determined from the experimental isotherm of the mixture. d Surface area of the single Cu component calculated from the mixture isotherm. trations of the amphiphile gave well-defined and reproducible results of the surface pressure. However the higher surface pressures u were only found for very low area/molecule values, which were too small also for the closest packing of the molecules in the monolayer. The experimental limiting area, Ao, was 38 A2/molecule, well below the estimated values for flat and edgeon arrangements, which were -240 and -97 A2, respectively. This meant that CURS-TAPwas not able to form a true monolayer at the air-water interface. The above results indicated the surface formation of columnar aggregates, which started to interact as a result of lateral compression. This gave rise to a dependence of the surface pressure on the surface concentration of molecules. Similar suggestions have been given for copper-substituted compounds of unsymmetrically substituted phthalocyanine~.~3-~~ When the sample was in mixture with stearic acid and H2RSTAP (SA/H~R~-TAP/CUR~-TAP = 221/ 10/ I), the spreading isotherm showed a marked slope decrease at about 7 mN/m, as followed by a slope increase at about 20 mN/m, with a limiting area value of 21.9 A2/molecule, which is determined by the polar head of the stearic acid matrix (solid line in the Figure 6). Analogous plateaus have been reported in many cases of polyazamacrocycles ~ r f a c t a n t s , 3and ~ ~they ~ ~ -have ~ been typically interpreted as proof of changes in the surface arrangement of the macrocycle plane. This could also be the explanation of the isotherm of our mixture, even if the pure CuRg-TAP isotherm did not show that feature. Table I11 reports the calculated area values at different u for CURS-TAP in the above mixture. At u < utraM, the calculated A0 = 89.7 A2/molecule was in a good agreement for an edge-onorientation of the macromolecule plane. At u > 7 mN/m, the occupied areas were greatly reduced, thus indicating a surface transition of the macrocycle toward a less ordered state, characterized by a superposition of columnar assemblies, with the formation of disordered domains. Figure 7 shows the u / A isotherms of the 221/10/1 mixture in compression-expansion~mpression cycles, with the first compression up to 5 mN/m (that is below utraM), 21 mN/m, and 32 mN/m. An appreciable hysteresis was always observed, indicating a not completely reversible behavior of the mixed monolayer. Furthermore, the second compression isotherm was different in all three cases from the normal one in the 0-20 mN/m surface pressure range; that is the pressure range that was affected by the presence of the TAP ring. At u > 20 mN/m the second compression isotherm perfectly reproduced the normal one, which was mainly determined by the presence of the SA matrix. Lanpuir-Blodgett Films. The LB film formation was very difficult when hydrophilic quartz was used. In this case very low transfer ratios were obtained in the deposition. The results improved with supports treated with dimethyldichlorosilane as reported in the experimental section. In no case, however, the formation of the LB film with more than one layer of pure CuRS-
The Journal of Physical Chemistry, Vol. 97, No. 36, 1993 9185
Discotic Metal-(Alky1thio)tetraazaporphyrins
.’
shift of the Q branch (particularly for the Qnband). This was in line with the results previously reported for porphyrins from fluid to solid phases.4143 In addition, the marked decrease of the height ratio R, = Qa/@ (1.8 for the mixture and 1.5 for the pure compound, not reported in the figure) with respect to the value of CURB-TAPin chloroform solution was a clear argument in favor of the surface aggregation of the macrocycles, with a progressive increaseoftheintermolecularinteractions. The strong increase of the Q.- Qp frequency differencecould not beattributed to variations in the chemical environment. All the above arguments led to the conclusion that the 623 nm band, which we called QBband, is a new band due to the formation of surface aggregates. The same film sample was also investigated with polarized light by using the light polarization plane, E, parallel (Ep)or perpendicular (E,) to the incidence plane of radiation and at an incidence angle of the light beam with respect to the plane normal, i = 0”. No dependence of the UV-vis spectra on the orientation of the electric field vector with respect to the dipping direction was observed. This might occur because (i) the plane of the porphyrin ring was parallel to the substrate surface (flat orientation) or (ii) the film was almost completely disordered on the substrate surface (random distribution of columnar domains). The ESR spectra together with all the previous observations strongly favored hypothesis ii. ESR Spectra. LB films (up to 200 layers) of the 20/1 SA/ CuRgTAP mixture gave invariantlya single, exchange-narrowed line with a line width AH = 5.0-5.1 mT at room temperature without any appreciable dependence on the orientation of the film plane with respect to the magnetic field direction. This ruled out either orderedmagneticarrangementor low dimensional magnetism, which should give both width and field position dependence of the line.The increase of the AH observed in the film with respect to the values observed in the solid phase meant increased mean Cu-Cu distances as arisen from dilution of the paramagnets due to the SA presence and from increased disorder, which was also suggested by the electronic spectra. The possibilityof formationof a LB film in which CubTAPoccurred as a mesophase could be excluded since a marked orientation dependence of the line width was expected but did not occur. When the LB film was built up with CURB-TAPdiluted in HzRs-TAP, the corresponding ESR spectrum showed a partial overlapping of resolved I4Nshfs structure on a broad (AH 40 mT) unstructured signal (Figure 8). Neither absorptions show orientation dependence. A small fraction of CuReTAP molecules in the diamagneticdiluentwas responsible for the partially resolved
a
1 ;
n=
I “/m
+
q -
\
% El o l
\
‘. , \\
b
n
I 4
= 32 d / m
-
I
-
2600
- 7
2700
2800
2900
3000
3100
3200
3300
3400
3600
3600
MAGNETIC FELD IGurs)
Figure 8. ESR spectrum of the LB film (380 layers) formed with the 221/10/1 stearic acid/HzRrTAP/CuRrTAP mixture. T = 293 K.
9186 The Journal of Physical Chemistry, Vol. 97, No. 36, 1993
signal, with the larger fraction of the paramagnetic components near enough to each other to give the broad spectrum. The actual spin concentration responsible for the structured signal could not be valued from the ESR signal intensity only, because of the very large uncertainty of this kind of measurement. These data once more supported the hypothesis that a LB film was formed with a partial dilution of CuRg-TAPin HzRg-TAP, which was however largely disordered although in the presence of a typically filmforming amphiphilic such as SA.
Conclusion The use of mesogenic, discotic materials with liquid crystal properties, such as copper and nickel compounds of octakis(octylthio)tetraazaporphyrin, for the preparation of homogeneous and stable mono- and multilayers was fruitful only when these compounds were diluted in a classical amphiphilic matrix. However the thin films did not show any appreciable orientation order with the desired properties of a liquid crystal. Nevertheless the results obtained in this work seemed to be encouraging enough and further work is in progress with differently substituted polyazamacrocycles,such as, for instance, TAP substituted with alkyl chains containing polar groups which are expected to give lower plane-to-plane interactions and favor a flat orientation at the air-water interfa~e.~’ Efforts will also be made to use triggers in the dilution mixtures, as has been done by Azumi et a1.,48who employed n-alkanes to change the orientation of dye molecules in LB films.
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