268
Langmuir 1993,9, 268-272
Thermodynamic and Spectral Properties of Monolayers and Langmuir-Blodgett Films of the Nickel Compound of an Amphiphilic Tetraazaannulene Derivative Francesca Bonosi,f Francesco Lely,* Giampaolo Ricciardi,* Mawizio Romanelli,?and Giacomo Martini'tt Dipartimento di Chimica, Universith di Firenze, 50121 Firenze, Italy, and Dipartimento di Chimica, Universith della Basilicata, 85100 Potenza, Italy Received March 13,1992. In Final Form: July 8,1992 The properties of the monolayer of the nickel compound with the amphiphilic, macrocyclic ligand 6,13-bis(he.adecanoyl)-6,7,12,14-tetramethyldibenzo[b,iltetraza[l4lannulene, Ni(dhtmdbTM1, onto a water eubphaee surface has been studied from the spreading isotherm. The beat conditione for the transference of the monolayersas Langmuir-Blodgett (LB)film on a solid support have been found. From optical spectra and transference ratios, the LB film was regular and homogeneous. The use of isotropic and polarized light allowed us to establishthat a structuraltransition from a probableedge-on arrangement of the macrocycle at the &/water interface to an almost flat arrangement occurred during the thin, multilayer film build up.
Introduction A large amount of literature exists on the me of polyazamacrocycles,such asporphyrins, phthalocyanines, and their derivatives, to build up thin filmsto be employed as semiconductors, molecular metals, biosensors, and environmentals e n ~ o r s . ~This - ~ ~pushed scientiststo search for polyazamacrocycles other than porphyrins and phthalocyanines and to study their physical and chemical properties. The derivatives of dibenzo[b,iI[1,4,8,1lltetraaza[l41annulene (dbTAA) belong to a class of substances whose chemical, electrical, and magnetic properties undergo remarkable changes on exposure to various volatile compounds.14-18 Whereas the R = H cycle is planar with CW symmetry (or Dw, depending on M = 2H or metal ion), when R = CH3 strong intramolecular steric interUnivereite di Firenze. Universite d e b Basilicata. (1)Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-assembly, Academic Press: New York, 1991. (2) Roberta, G., Ed. Longmuir-Blodgett Films; Plenum Press: New York, 1990. (3) Mercer-Smith, J. A.; Whitten, D. G. J. Am. Chem. SOC.1979,101, 6260. (4) Bull, R.A.; Bulkowski, J. E.J. Colloid Interface Sci. 1985,92,1. (5) Flbrsheimer, M.; MBhwald, H. Thin Solid Films 1988,169, 115. (6) Gust, D.; Moore, T.A.; Luttrull, D. K.; DeGraziano, J. M.; Boldt, N. J.; Van der Auweraer, M.; Schryver, F. C. Langmuir, 1991, 7,1483. (7) KO, W. H.; Fu, C. W.; Wang, H. Y.; Batzel, D. A,; Kenney, M. E.; Lando, J. B. Sens. Mater. 1990,2,39. (8)Wang, H. Y.; KO, W. H.; Batzel, D. A.; Kenney, M. E.;Lando, J. B. Sens. Actuators 1990, B1,138. (9) Simon, J.; Andre, J. J. Molecular Semiconductors; SpringerVerlag: New York, 1985. (10) Barger, W. R.;Wohltjen, H.; Snow, A. W.; Lint, J.; Jarvis, N. L. Fundamentala and Applications of Chemical Sensors; Schuetze, D., Hammerle, R., Eda.;ACS Symposium Series 309; American Chemical Society: Washington, DC,19s6; p 167. (11)Pace, M. D.; Barger, W. R.; Snow, A. W. J. Magn. Reson. 1987, 76,73. (12) Lecomte,C.; Baudin, C.; Berleur, F.; Ruaudel-Teixies,A.; Barraud, A. Thin Solid Films 1986,133,103. (13) W e g " , A.; Hunzinger, M.; Tieke, B. J. Chem. SOC.,Chem. Commun. 1989, 1179. (14) Coedken, V. L.; Molin-Case, J.; Wang, Y.J. Chem. SOC.,Chem. Commun. 1978.373. (15) &taka, K.; Nakamura, H.; Hashimoto, M. Inorg. Chim.Acta 1984, 8 3. .,l. 67 . -... (16) W e b , M. C.; Gordon, G.C.; Goedken, V. L. J. Am. Chem. SOC. 1979,101,857. (17) Hunziker, M.; Hilti, B.; Rihs, G.Helo. Chim. Acta, 1981,64,82. (18) Coedken, V. L.; Peng, S. H.; Molin-Norris, J. A,; Park, Y. J. Am. Chem. SOC.1976,98,8391. f
R' = CHs(CHn)qr-CO
actions of the methylgroups with the benzene rings induce marked deviation from planarity.lo In this case the symmetry decreases from D a to CZVwith M = metal ion and from CW to CZ with M = 2H. A saddle-shaped structure is proposed for the tetramethylmacrocyclewhose noticeable chemical reactivity is due to the uncomplete electron delocalization through the whole 14-membered ring.20 Metal complexeswith dbTAA ligands receive increasing attention due to the close similarities to porphyrin and phthalocyanine systema.13~2~They are therefore also considered as mimeting structures of the prosthetic group of severalmetalloproteins.18 Moreover, iodine dopinghas been shown a viable method for the preparation of highly conducting charge transfer aalta with a good stability.13J7~22*23 Langmuil-Blodgett films of semiconducting copper and nickel derivatives of alkylated dbTAA have also been prepared, although homogeneous deposition of the Ni(I1) compounds is obtained only when in mixture with cadmium ara~hidate.132~ For both filmsan improved in-plane conductivity is found after exposure to IZ or IS-. This work describes the preparation and the spectral features of the nickel(I1) derivative of the 6,13-bis( h e . a d ~ o y l ) - 6 , 7 , 1 2 , 1 4 t e ~ e ~ l ~ ~ ~ [ [14lannulene, Ni(dhtmdbTAA1. The properties of ita monolayers at the airlwater interface and of ita LB f i i (19) Wang, Y.; Pew, 5.M.; Lee, J. L.; Chuang, M. C.; Thnng, C. P.; Wang, C. J. J. Chm. Chem. Soc. (Taipei) lBSZ, 2!J, 217. (20) Curtis, N. F. In Coordirwtion Chemistry of Macrocyclic Compounds; Nelson, G.A., Ed.; Plenum P r m : New York, 1979;p 281. (21) Tieke, B.; W e g " , A. Thin Solid Films 1989,179,109. (22) Lin, LA.; Marks, T.J.; Kannewurf, C. R.;Lyding, J. W.; McClure, M. S.; Ratejack, M. T.;Whang, T.4. J. Chem. Soc. 1980,964. (23) Wegmann, A.; Hunziker, M.; Tieke,B. Eur. Pat. Appl. EP 541,201 (Cl.B05D1/20) 8 Nov 1989.
0743-7463/93/2409-0268904.00/0Q 1993 American Chemical Society
Langmuir, Vol. 9, No. I , 1993 269
Elms of Ni Amphiphilic Tetraazaannulene Derivatives
- - CECl, Cart
rolution
5.7.10% film
.,.,....- LB monolayer
T= 15OC 1.12
izol R
f
0.56
Ol,,~,,,,,,,,,,,,,,,,,,~,,,,,,,,,,, 0
60
40
120
SURFACE AREA (A*/molec)
Figun, 1. S reading isothermat 15OC of a 1.16 mmoVL solution of Ni(dhtmibTAA) in CHCl3.
0.28
j V Q.
1.05 0.00
T= 15OC
'\
260
I
400
t
8
8
I
800
I
,
,
800
A (nm)
4
4
0.85
Figure 3. UV-vis spectra of 5.7 X mmoVL of Ni(dhtmdbTAA)in CHCb (a),of the cast f i b on a quartz slide (b), and of a single layer deposited on a quartz slide (c) at a dipping pressure of 25 "em-' and with a dipping speed of 2 mm*min-l. Once the desired r was reached, the deposition was carried out immediately. All of the spectra were obtained with isotropic light and with an incidence angle i = 0'. The absorbance ecale refers to the fluid solution only.
4 - 5mN/m 15 mN/m - - 25 mN/m
.- - - - - 0.75
-0
1
40
1
80
I'
Dipping direction
120
TIME (min) Figure 2. Time stability of the Ni(dhtmdbTAA1monolayer at the aidwater interface expressed as the variation of the ratio between the area at time t and time zero against time.
are also investigated. The study of molecular order in mono-and muhilayers was mainly approached in terms of positionand orientationof the self-organizingmolecules with respect to the solid surface onto which they were deposited. The analysis was carried out on the basis of the spreading isotherm and of optical spectra.
Experimental &tion Materials. All chemicale necessary for the synthesis were of reagent grade and used without further purification, unless otherwise stated. Ni(tmdbTAA) was synthesized accordingto the procedure of Coedken and Web.% Ni(CH3COO)~4H~0 (4.0 g, 16.0 mmol) was refluxed in 100mL of ethanol with 1,2-diaminobeneene (3.47 g, 32.0 "01) for 3 h under Nz.After addition of pentane-2,4dione (3.2 g, 32.0 mmol), the gas flux was stopped and the reflux was maintained for 24 h. The green brownish solution was refriserated overnight and fiitered. The resulting violet-purple precipitate,after accuratewaehingwithwater and cold methanol, was carefully dried over PzOS.The compound was purified by column chromatography(2 X 20 cm)on neutral alumina (Fluka) and wing CH&& as eluant. Yield (baaed on Ni): 70%. Anal. Calcd for CnH~N4Ni:C, 66.87; H, 6.62; N, 13.97. Found C, 66.80; H, 6.48; N, 13.98. UV-vis data in CHzCl2, E, cm-l (log e): 17 180 (3.76); 24 700 (3.88); Soret band, 25 640 (4.20); 30 300 (3.80); 37 730 (3.42). lH N M R data (Bruker 300 MHz, CDCls, (CHASi as internal standard): 6 = 2.10 s (12 H), 4.86 s (2 HI, 6.56476 m (8 H, aromatic rings). (24) Goedken, V. L.;Weire,
M.C . Inorg. Synth.
1980,20,115.
Figure 4. Sketch of the relationship among incidence angle of the polariied light and the geometry of the film layers. n is the normal to the substrate; k indicates the light propagation direction; E,and E,.are the electric field vectors, perpendicular and parallel to the mcident plane, respectively. Ni(dhtmdbTAA1 was prepared by a modification of the procedure described by Eilmes= by reaction of Ni(tmdbTAA) and hexadecanoyl chloridein the preae.nce of ( 0 8 in refluxing toluene and N2 atmoephere. One gram (2.60 m o l ) of the purified and carefully dried Ni(tmdbTAA) was refluxed, under Ar,with 1.40 g (5.0 mmol) of freshly distilled CH3(CH2)&OCl and 0.6 g (5.0 m o l ) of (CZHS)&J in dry toluene for 24 h at 120-130 OC. After cooling at room temperature, the dark green suepension was kept at -20 OC until a graygreen solid separated. The precipitate was fdtered and dried in vacuum and washed several times with warm water (-60 "C) until complete removal of (C2H&NHC1was reached. The solid was then dried over PZOS and purified by column chromatography (2 X 20 cm)on neutral alumina and wing CH2C12 as eluant. Further pdication, involving the removal of pereieting tracea of unreactedC H ~ C H ~ I C COC1, was obtained b y evaporation of this impurity under high vacuum (10-4 Torr) at 120 OC. Yield (baaed on Ni(tmdbTAA), 20%. Anal. Calcd for C&N,OzNi: C, 73.87; H, 9.41; N, 6.58; Ni, 6.68. Found C, 73.38; H, 9.10; N, 6.42; Ni, 6.60 (detarmined by atomic absorption technique). 1H NMR data: 6 = 6.62 m (8 H, aromatic rings); 2.72 t (4 H, -CH2CO-); 2.01 s (12 H, -N==CCH3); 1.30 s (56 H, 4H2- of the alkyl chains);0.89 t (6 H, terminal CH3- of the chains). (25) Eilmee, J. Polyhedron 1986,4,943.
270 hngmuir, Vol. 9, No. 1, 1993 compound Ni(dbTAA) in CHzC12
Ni (tmdbTAA) in CH2Clz
Ni(dhtmdbTAA1 in CHC13
Ni(dhtmdbTAA)
cast f i
is00
Table I. Optical Data of Metal Compoundr of the TAA Macrocycle Elog e transitions spectroscopic excited state 504 19 840 3.93
hmu (nm)
469 426 402 356 306 274
586 394 374 333 323 267 589 430 sh 394 325 sh 268 610
4M sh 344 sh 268
Ni(dhtmdbTAA1 LB f i ,1 layer
io00
Bonoei et al.
600 440s 395 262
21 300 23 500 24 875 28 100 32 700
36500 17 100 25 400 26 740 30 030 30960 37 450 16 980 23 250 25 400 37 770 31 300 16 400 20 660 29 070 37 300 16 670 22 730 25 300 38 170
ref
4.01 4.86 4.62 3.96 4.32 4.52 3.80 4.37 4.19 3.82 2.79 4.36 3.56 3.92 4.24 3.94 4.35
Methodr. A chloroform (Merck, purity >W%)solution of Ni(dhtmdbTAA1 (1.16 mmol/L) was used for the isotherm determination. Twice-dietilled water, p d i e d with the MilliQ water-eyetem (Mipore) witha resistivitygreater than 18Mn.cm waa ueed aethe eubphase. The epreading isotherme were obtained with the aid of the KSV LB5OOO apparatue at a continuoue compression rate of 5 mm/min, with a hydrophilic (Delrin) movable barrier. The subphase temperature wae kept constant at 15 "C with a Haake D8 thermostat. Langmuirslodgett filmswere depoeited at a Surface pressure r = 25 "em-' on either hydrophilic or hydrophobic silanized quartz platee, at a dippingspeedof 2 " in. LB f h depoeition was ale0 performed at r = 5 mN-m-' on a hydrophilic quartz slide in order to determine the effect of molecular packing on the opticalpropertiea of the LB assemblies. The transfer ratioe were 0.85-1 for the upstroke and 0.6-0.7 for the downstroke on hydrophilic quartz,whereae higher transfer ratim were obtained with ailanhad quartz platm. The observed transfer ratio values were only indicative of homogeneous transfer without having any stoichiometric meaning about the amount of transferred material. Hydrophilic quartz elides were accurately cleaned with hot chromic acid. Silanized quartz platee for LB film depoeition were prepared by immersing overnight hydrophilic quartz platee in a 10% mlution of dimethyldichlmilane in l,l,l-trichlom ethane, kept in oven at 110 "C for about 1h and then carefully rinaed with acetsne, chloroform, and twice-distilled water. After the eilanization procedure the substrata were not wetted by the water. The optical spectra were registered with a Perkin-Elmer Lambda 5 UV-via epectrometer; e- and p-polarized light was obtained with the aid of two Polaroid sheet polarizere in the 360-800 nm range. The coated plate wae located in the normal sample position,and the uncoated one was located in the reference position. The incidence angle of the light, i,with respect to the normal to the plate wae either 0" or 46".
Rerults and Mscursion Moaolayen at the Air/Water Interface. Figure 1 shows the 15OC surfacepreeeure/areai a o t h m of a CHC& solution of Ni(dhtmdbTAA) over a pure water subphase. The monolayer was quite stable at the water-air interface with a marked decrease of the T slope in the region of premure 10-16 "em-l. Above 16"em-1 the monolayer was a liquid expanded film with (C,-I)- = 81.7 mN.m-l, = 34 ""-1, and collapse area A,u collapse pressure
= 36A2/molecule. W of these values were in a fairly good agreement with those reported by Wegmann et al.19 for spreading monolayers of copper bis(dodecy1thiodibenzo[14]TAA). ThelimitingareaA0,i.e.theareapermolecule extrapolatedat the highest r elope before collapse, was 48 A2, which agreed for an edgaon orientation of the TAA plane with respect to the water surface with the hydrocarbonchainsale0perpendicularlyoriented. From simple molecular modeling (Orbit Molecular Building Syetem, Cochranes of Oxford, Oxford, UK) of the macrocycle under saddle conformation an occupied area of 59 A2/ molecule resulted for an edge-on arrangement whereas 190-200 A2 were required for an arrangement of the TAA plane parallel to the water surface with small differenin the area value depending on the poaitiona of the -CHs and 40group. T h e preaence of hydrophilic carbonyl group could be responsible for the obeerved plateau In the lower r value region the macrocycleaeeumeda slightly tilted orientation owing to the interaction of the c--O groups with the water surface. Thh arrangement was progressively lost toward the edge-on orientation with increasing the film condensation,which ale0 corresponded to a more ordered arrangement of the CIS hydrocarbon taile. To our knowledge, a plateau-like portion of the isothermhae never been observed in the isothernu of TAA derivatives. Examples of a plateau in the spreading iaotherma of amphiphilic polyazamacrocyclea have been reported.*= For instance, odakie(decy1oxy)phthalocyanine moleculess exhibit a flat arrangement at the air/ water interface at low preeaure (A 400-160 A), b e i i then squeezed up to a vertical orientation at high T . A phase change haa been suggested for copper(II)octalris(dodeooxymethy1)phthalocyanine arising from steric arrangement of the alkylozy groups.= In other caees, stacking of metal phthalocyanine molecules over one another at the &/water interface haa been hypothbefore the usual edge-on conf'iiation takes place.2@+1
-
~
~
(26) Dent, N.;Grundy,M. J.;Richudron,R.M.;Rooer, S.J.; McKeown, N. B.;Cook, M.J. J. Chim. Phya. (Paris) lB88,86,1005.
(27) Cook,M. J.; Dum, A. J.; Daniel, M.F.;Hut,R.C. 0.;Richudron, R. M.;Row, S.J. Thin SolidFilm 1988,164, S96. (28)Kalina, D.W.; Crane, S.W.Thin Solid Film 1986,109,184.
Films of Ni Amphiphilic Tetraazaannulene Derivatives
Langmuir, Vol. 9, No, 1,1993 271
0.20 0.20
-0.15
i
1
A A
la)
LB film (17 layers:
1
= 395 n m = 600 nm
m
Y
*
w
go.10
d
0.00
1
0
I
/
5
10
15
20
Number of layers
Figure 5. Variation of the absorbance of the transitions at 396 nm and at 600 nm as a function of the Ni(dhtmdbTAA) layers deposited on a quartz slide.
0.00
f
I
,
Our data pushed toward the edge-on orientation at high
(29)Mukhopadhyay, S.; Ray, A. K.;Hogarth, C. A. J . Mater. Sci.: Mater. Electron. 1990, I , 110. (30)Fujiki, M.; Tabei, H. Longmuir 1988,4,320. (31) Ogawa, K.; Kinoehita, SA.;Yonehara, H.;Nakahara,H.;Fukuda, K.J. Chem. SOC., Chem. Commun. 1988, 477. (32)Brooks,J. H.; Alexander,A. E. In Retardation ofEuaporation By Monolayers; La Mer, V. K.,Ed.;Academic Press: New York, 1962; p 245. (33) Bailey, C. L.; Bereman, R. D.; Rillema, D. P.; Nowak, R. Znorg. Chem. 1984,23,3956.
I
,
,
I
550
750
A (nm)
r,although we could not exclude the possibility that some
stackingof the TAA macrocyclesoccurred at the air-water interface. Direct optical spectroscopy could give more details on this point. The stability with time of the Ni(dhtmdbTAA) was checked at different r values (Figure 2). The stability is expreaeed by the ratio R, = A(t)/A(O)at constant surface pressure, with A(t) and A(0) being the aredmolecule at the time t and zero, respectively. Significant deviations of R,from unity resulted only at the highest ?r values after time lapses longer than 2 h (e.g. R,= 0.88 at r = 25 mN9m-l after 2 h). The shape of R,against t was always a concave shape. This excluded the monolayer collapse as the determining process for the loss of film area with time, which gives a convex shape.32 The concavity of the relaxation curve was simply due to small molecular rearrangement. Langmuir-Blodgett Films. The experimental conditions discussed above for the monolayers suggested a reliable transferance from the aidliquid interfaceto a solid substrate even at relatively high surface pressures (r = 25 mN-m-1). This was done with good transfer ratiosfor both down- and upstrokes with dipping rate of 2 mm/min. The goodness, the reproducibility of the film, and the macrocycle arrangementwere checked by opticalspectroscopy. The electronic spectra of the Ni(dhtmdbTAA) were investigatedin the range 200-800 nm under isotropiclight and in the range 350-800 nm under polarized light. Figure 3 shows the UV-vis absorption of a 5.7 X 1W2 mmol/L solution of the compound in CHCh. As reported for other Ni(I1) complexes with TAAs,~~ our compound exhibited two relatively intense absorptions in the visible region (A = 589 and 430 nm, respectively) and other intense absorptions in the UV region of the spectrum (A = 394, 325, and 268 nm), in a very good agreement with the Ni(11)compound of tmdbTAAwithout the two long alkanoyl chains.33 The general feature of the spectrum was also
I
I
350
0.25
LB film (17 layers) 0.20
n
A .--. 0.00
I
I
I
I
~
I
I
272 Lmgmuir, Vol. 9, No.1, 1993 ulation of the excited states, both a and @ transitions are mainly I-.ligand centered in natureaMThe 426-nm peak is due to the 4ba 3a,, transition (Soret or B band), which ale0 involves an electronic rearrangement in the r ligand syatem. By passing from Ni(dbTAA) to Ni(dhtmdbTAA),only small spectra changes were observed, namely a 30-40 nm blue shift,in the Soretand @ transitions, whereas a more significant 85-nmred-shiftwas given by the a transition. This could be essentially due to the symmetrydecrease from DW of the unmethylated cycle to C% of the methylated and Cl6-methylatedones. The band assignement did not change with the 589-, 430-, and 394nm transitions being the a, 8, and Soret transitions, respectively. The excited state which originated from the Soret transition was Bz Cy-polarized)and the excited states that originated from the a and @transitionswere both B1 (x-polarized). The optical spectra of the cast film and of a single layer deposited on a quartz slide are also reported in Figure 3. Both spectra were obtained with isotropic light at an incidence angle of the light beam with respect to the plate normal, i = Oo (Figure 4). The main absorption bands were wider than those in fluid phase, with a 10-15 nm red shift of the lower energy absorptions (A = 600 and 442 nm, respectively, see Table I) as reported for porphyrins from fluid to solid phase and in LB films.*38 The shoulder at 325 nm (30 800 cm-l) was no longer observed because of the band broadness and of the extensive light scattering. In Figure 5 the absorbances at 600 nm and at 395 nm are plotted as a function of the number of deposited layers. The good linear relationship indicated that the coverage of consecutive layers was even and consistent with the formation of a good, reproducible LB film.
-
(36)Goutarman,M.In The Porphyrins, Vol.III: Physical Chemistry, Part A; Dolphin, D.,Ed.; Academic Prese: New York, 1978;p 1. (36)Weigh, J. W. J. Mol. Spectrosc. 1967,I, 216. (37)B o b , J. S.;Goubrman, M.; Howell, B.B. J. Lomin. 1976,10, 295. (38)Schmehl, R. H.; Shaw, G.L.; Whitten, D.G.Chem. Phys. Lett. 1978,58,4,549.
Bonosi et al. Details on the type of macrocycle ordering within monolayers of the LB film were obtained from a consideration of the polarization effects on the optical spectra, because of their dependenceon the anieotropicorientation of the transition moment (Figure 6). With i = Oo, no absorbance changes were observed with the light polarization plane, E,parallel (E,,)or perpendicular (&,) to the plane of incidence for either the 395- or the 6OO-nmpeaks. In contrast, dichroic ratios P = AJA, > 1 were observed with i = 45O, almost independently of the number of the deposited layers. No deviation of the dichroic ratio from unity was observed for the cast film for both i = Oo and 45O. The absorbance independence of the light polarization at i = Oo and ita dependence at i = 46O led to the conclusion that there was a high ordering degree with an almost flat arrangement of the TAA rings with the IC and y molecular axes at an angle of about 45O with respect to the dipping direction. Finally, the optical spectra did not depend on the type of substrate over which the LB film was deposited and the same molecular order and packing were obtained when the monolayer was transferred at r = 5 "em-l.
Conclusion The introduction of two cl6 chains at symmetrical positions of the tmdbTAA ring rendered the nickel compound well suitable to form spreading monolayers stable enough to be regularly and homogeneously overlapped as LB films on quartz plates. During the LB building up, a structural transition took place from an edgesnarraneementattheair/waterinte~a~toanalm~t flat arrangement when deposited on a solid support. The possible use of the thin films described in this paper as sensor for environmental gases is under investigation. Acknowledgment. Thanks are due to the Italian Comiglio Nazionale delle Ricerche (CNR) and to the Italian MiniiteroDell'UniversitA e d e b Ricerca Scientfica (MURST) for the financial support.
