J. Phys. Chem. 1995,99, 17593-17605
17593
Diskotic Multiyne Langmuir-Blodgett Films. 1. Structural X-ray and Optical Microscopy Investigation Radoslav Ionov* Institute of Applied Physics, Technical University, BG-I 156 Sofa, Bulgaria
Angelina Angelova Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., BI. 21, BG-1113 Sofa, Bulgaria Received: August 23, 1994; In Final Form: April 4, 1995@
The type of the diskotic phases in monolayers, Langmuir-Blodgett (LB) films and bulk liquid-crystalline state is considered as resulting from steric factors and intermolecular interactions. The role of the dipoledipole and n-n interactions for the induction of various diskotic phases in spread and deposited thin layers is emphasized on the basis of the recent achievements in structural modification of diskotic films. The monolayer organization of a novel disklike mesogenic multiyne amphiphile is characterized by means of surface pressure/area and surface potentidarea isotherms. The structure and the morphology of the deposited LB films are investigated by means of X-ray diffraction and optical microscopy. Depending on the packing density of the layers, characterized by the average transfer ratios, three types of films structures are established-crystalline, teared, and smectic nematic, DSN. The DSNdiskotic phase, experimentally induced in the thin films by means of the LB technique, exhibits a higher degree of molecular order than the nematic phase of the bulk disk-shaped amphiphile. Models are presented for the "edge-on" nematic in-plane molecular arrangement of the spread diskotic monolayers and the smectic nematic, &-like, multilayer structure of the LB films deposited at transfer ratios close to unity. Unification of the notations of the diskotic phases is proposed.
Introduction
-b
Liquid crystals (LC) have been a subject of great theoretical and practical interest'-5 since 1888. The attention has mainly been paid to rodlike mesogenes, while the interest in investigation of disklike LC has increased considerably since their discovery6-s in 1977. Three main LC phases have been established: nematic, smectic, and columnar. Each of them may exhibit different modifications. The experimental methods for an induction of LC phase transitions have been well developed for rodlike mole~ules.~-'' Recently, theoretical models predicting the LC phase diagrams on the basis of the shape of the molecules have been created.'*-I5 However, only a few review article^'^.'^ have been devoted to the investigations of the possible diskotic LC phases. Chargetransfer interactions have mainly been utilized during the last for an induction of highly ordered diskotic LC phases. Experiments on the modification of the phase state of the diskotic mesogenes in thin Langmuir-Blodgett (LB) films have been reported as ell.^^-^^ However, an induction of a higher ordered diskotic phase in the films of compounds exhibiting a low ordered bulk LC state has not been achieved by means of the LB technique yet. The molecular organization of the LB films of disk-shaped mesogenes depends not only on the intermolecularinteractions typical for the classical amphiphiles but also on the "core-core" interactions. These interactions, specific for the disk-shaped molecules, include steric and n-n interactions. In the present work, the monolayer organization and the structure of the LB films of a novel disk-like multiyne amphiphile (Figure 1, compound 4b in Table 1) are investigated.
* Author for correspondence. @Abstractpublished in Advance ACS Abstracts, May 15, 1995. 0022-365419512099- 17593$09.0010
3.02 nm
___I(
T
P
2.25 nm
'. .. 8X 4b Figure 1. Chemical structure and molecular geometry of the investigated disklike amphiphile pentakis((4-pentylpheny1)ethynyl)phenoxy-
undecanoic acid (compound 4b). The dipole moment, p, is determined by the polarization of the aromatic core.
In part 1, the monolayers of 4b are characterized at the liquid gas interface by means of surface pressurelarea and surface potentidarea isotherms. Small-angle X-ray diffraction and optical microscopy are applied for a characterization of the transferred LB films. The effect of both the LB technique and the molecular peculiarities of 4b on the induction of new diskotic structures is discussed. In part 2, the optical investigation of 0 1995 American Chemical Society
17594 J. Phys. Chem., Vol. 99, No. 49, I995
Ionov and Angelova
TABLE 1: Characteristic Parameters and Bulk Phase Transitions Temperatures of the Diskotic Compounds 1-7 (Figure 2)o compound n X P (nm) Rb (nm) phase transition temp ("C) ref 1 2a
7
2b 3a 3b 4a 4b 4c 4d 4e 4f Sa 5b
9
R7
1.09 0.82 0.82 0.82 0.82 0.82 0.82 0.82 0.82 0.56 0.56 0.56 0.56 0.40 0.40
(CHd60H
0.40
2.32 1.92 1.93 1.51 1.51 0.82 0.82 0.82 0.82 1.23 1.37 1.78 2.33 1.21 1.48 1.21 1.8-2.8
7 5
0
5
(CH&COOH
R4 Rq (CH2)sCOOH
Ra 4 5 8 12 5 7 5 1-12
Sd 6a
6b 6c 7 a
2.16 2.19
8
5c
Nb
1.20 1.09
8
Rs
1.06
C 121.3 N D 174.8 I C 60.3 ND 112.6 D,, Dro 136.6 Dho 200 I C 68.2 N D 109.9 I c 71.5 N D 96.5 C 107.1 N D 190.6 I C 86.4 Nb 109.9 I C 84.0 ND 90.9 I non-liquid crystal non-liquid crystal non-liquid crystal non-liquid crystal c < rOOm Dho 277 I C < room Dho265 I C < room Dh, 200 I c < rOOm Dho 288 I C68 4 , 1 2 3 I c64 Dho 89 I C 35 D,, 66 I C 74.3 DSN154.1 I
32 32 32 30,3 1 31 34 58 18 18 58 18 41 41 41 41.26 42 61 24,28 36,37
r - hard core radius; R - extended state molecular radius; R, = CnHzn+l. Phase state: C - crystalline, I - isotropic, ND - nematic diskotic, - nematic biaxial, Dsc - diskotic smectic columnar, DSN- diskotic smectic nematic, D, - diskotic ordered rectangular columnar, Dh, -
diskotic ordered hexagonal columnar. Estimated according to refs 53 and 55.
0
r R.
3
2
1
9
b
4
OR. &OR.
5
6
7
Figure 2. Chemical structures of the disklike derivatives of triphenylene-centered (l),naphthalene-centered (2) and benzene-centered (3) hexakis((4-alkylphenyl)ethynyl)arenes, pentakis((4-alkylpheny1)ethyny1)phenyl ethers (4), 2,4,6-tris(3,4-alkoxyphenyl)pyryliumtetrafluoroborates (5), triphenylenes (6), bis[ 1,3-bis@-n-alkoxyphenyl)propane1,3-dionato]copper(II) (7).
the same diskotic LB films by means of UV-vis absorption and fluorescence spectroscopy is presented. The results obtained are rationalized in view of the effect of specific intermolecular interactions on the structure of the thin diskotic LB films. To our knowledge, the role of the dipole-dipole interactions for the type of the LC phases formed has not been discussed for disklike molecules so far.
Drd
Figure 3. Schematic molecular arrangement in the diskotic LC phases with increasing degree of order: NO, nematic diskotic (DN); N,, nematic diskotic columnar (DKc);DSN,diskotic smectic nematic; DSC,diskotic smectic columnar; Drd, diskotic rectangular disordered columnar; Dhar diskotic hexagonal ordered columnar.
Relationship between the Molecular Peculiarities and Interactions and the Type of the Diskotic LC Phases The molecules forming diskotic LC usually consist of a hard Molecular interactions include ionic interaction^;'^.^^ dipolecore with or without flexible wings (Figure 2). The following dipole interaction^,^-' I charge-transfer interactions,18-22and diskotic LC phases, with an increasing degree of the molecular hydrogen bonding.@ Steric factors are related to the shape and order, have been reported (Figure 3): nematic d i s k o t i ~ ~ . ~ ~the - ~ geometrical ~ peculiarities of the mesogenic molecules.'-5 (ND);nematic b i a ~ i a l(Nb); ~ ~ .nematic ~ ~ co1umnari8,i9,32,35 (Nc); Steric Effects. The theoretical w0rks,'~3'~ based on the shape of ideal disks, have predicted a nematic diskotic LC phase at a diskotic smectic n o n c o l ~ m n a r(nematic, ~ ~ ~ ~ ~ D s N ) ; diskotic smectic c0lumnar'~3*~*~* (Dsc);disordered or ordered rectangulow density of the disks and a hexagonal columnar phase-at a (Drd, Dro),and hexagona15*32.39.4',42 (ad, columnar. higher one. In special cases, a novel cubatic diskotic LC phase The type of the LC phases depends on the intermolecular (a disordered state of mutually perpendicular small columns of interactions and the steric peculiarities of the molecules. equal length-to-width dimension) has been predicted.I5 Often, lar32539340
a0)
J. Phys. Chem., Vol. 99, No. 49, 1995 17595
Diskotic Multiyne Langmuir-Blodgett Films. 1 the reak disk-shaped molecules are not ideal hard platelets. Their specific molecular features and intermolecular interactions may increase additionally the variety of the diskotic LC phases formed. A smectic diskotic LC phase has been p r e d i ~ t e d ’as ~ .well. ~~ Recently, it has been experimentally proven for a smectic columnar Smectic noncolumnar diskotic structures (with a nematic arrangement of the molecules in the layers have been also studied. However, the disposition of the disks in the layer planes has not been evidenced yet.I7 The notations of the diskotic smectic liquid-crystalline phases are still under In this study, we shall adopt the notations DSCand DSNfor the diskotic smectic columnar and the diskotic smectic nematic phase, respecti~ely.~~ The diskotic molecules can be arranged in columns in an ordered or in a disordered state. The planes of the molecules can form different angles with the columnar axis.17929It is worth mentioning that the stacking of the disk-shaped molecules into columns is not due to some special attractive forces. The molecular geometry plays an essential role at a higher density of the disks and the columns are formed as a result of the excluded-volume e f f e ~ t . ’ ~ .In ’ ~ special cases$9 the n-n interactions could favor a columnar arrangement. The most ordered columnar phase of a highest packing density is the Dho phase. In addition to the important role of the shape of the disks, other steric rules have been established to influence the diskotic LC phases: (i) the larger the core diameter, the more probable is the ND phase.Is The largest core diameter reported so far32 is 2.4 nm; (ii) the specific nature and the length of the wing groups can affect the type of the diskotic LC phases;s0 (iii) the geometrical asymmetry of the m o l e c ~ l e s ’(e.g., ~ ~ ’ ~a tail sticking out of the disk diameter) or the molecular tilt within the columns5’ may disturb the arrangement of the disks and alter the type of the LC phase. Effect of the Intermolecular Interactions. n-n Interactions. The large x-electron system of the cores of the diskshaped molecules (Figure 2) determines the essential role of the n-n interactions for the formation of diskotic LC phases. According to the recently developed theory,52in special cases only,49the n-n interactions favor a “face-to-face” stacking of the disks (Le., a columnar arrangement). In most of the cases, the repulsion of the n-electron systems favours a nonparallel or “offset” geometry which prevents the columnar arrangement. This implies that the columnar diskotic LC phases, observed experimentally in the cases of n-n electron repulsion, are result of the prevalence of the excluded-volume (steric) effect. The geometry of the n-n interactions (Le., the mutual position of the disks) is determined by electrostatic intermolecular interaction^.^^ However, the net value of the n-n interaction energy is dominated by its van der Waals contribution which is proportional to the area of the n-electron overlap. The larger the n-electron system the greater the role of the n-x interactions. Therefore, for large cores (without positive net charge), the n-n repulsion in a “face-to-face” geometry will favor a nematic arrangement of the disks. This can explain the established preference for a formation of a ND phase on increase of the core diameter.I8 The n-n interactions affecting the optical properties of the diskotic mesogenes and the inferences proceeding from the electrostatic theory of the n-n interactions for the diskotic phases will be considered in part P9 Charge-Transfer Interactions. Induction of diskotic LC phases, differing from those existing in the bulk state of the materials, has been achieved r e ~ e n t l y on ~ ~the ~ ‘ basis ~ ~ ~of~ charge-transfer interactions. Charge-transfer complexes have
b8
C
Dbo
0
0 I
-/ \ .
I / Dbo
\
/
Ds
Dao
I \
/ \
Dw DSN ND
Figure 4. Top: assembly of disk-shaped molecules with (a) antiparallel
“face-to-face” and head-to-tail “edge-by-edge’’dipole configuration; (b) parallel “face-to-face” and head-to-head “edge-by-edge”configuration of the dipoles; (c) diskotic hexagonal columnar ordered phase, &, with dipole configuration of minimum energy. Bottom: dipole configurations of “side-on’’ (left) and “edge-on’’ (middle and right) arranged disk-shaped amphiphiles at the aidwater interface and corresponding diskotic LC phases in the deposited LB films. been prepared by mixing the diskotic (donor) molecules with appropriate acceptor molecules. The charge-transfer mechanism involves strong interactions between the nearest-neighbor molecules. It can be considered as a powerful microtechnique capable to create the most ordered and dense diskotic phase, even in non-liquid-crystalline compounds.I8 Dipole-Dipole Interactions. The effect of the dipole-dipole interactions on the type of the diskotic LC phases has not been intensively studied. The asymmetrical attachment of a strongly electronegative atom (e.g., oxygen) to the strongly polarizable aromatic core of the disks (Figure 2) will result in a charge asymmetry, thus defining a dipole moment, p. Such asymmetry causes dipole-dipole interactions between the diskotic molecules, and it could be responsible for the observation of a Nb phase.34 To illustrate the possible effect of the dipole interactions on the type of the diskotic LC phases, we shall consider, for simplicity, molecules interacting only through their permanent dipoles which are directed in the disks plane (Figure 4, top). Induction effects will not be invoked since they are of a second order. The most stable configuration (of minimum energy) of four densely packed dipoles is shown schematically on Figure 4a. The most unstable one is given on Figure 4b. The disk-shaped molecules possess good rotational mobility in a liquid phase. Due to the torque, they will rotate and the dipole configuration of lowest energy may be achieved. “Faceto-face’’ antiparallel dipole attraction favors the columnar ordering of the molecules. Thus, the ordered LC phases can be stabilized additionally by the attractive dipole-dipole forces. Such an example is the most ordered hexagonal columnar, Dho, phase (Figure 4c). Obviously, every reduction of the rotational mobility of the discs will hamper the “face-to-face” antiparallel dipole arrangement. The resulting repulsion of the “face-toface” parallel oriented dipoles would favor a noncolumnar (nematic) structure. Langmuir-Blodgett Technique. The LB technique53appears an alternative approach to the charge-transfer bulk mixing mechanism for affecting the molecular order in the diskotic LC. It has the advantage for a macroscopic variation of the monolayers density by means of an extemal force. However,
a0,
Ionov and Angelova
17596 J. Phys. Chem., Vol. 99, No. 49, 1995 the molecular arrangement in the spread and the deposited films is determined by the microscopic intra- and interlayer molecular interactions. This interactions can strongly be influenced by the physicochemical factors involved in the LB experiment (pH, counterion content, and concentration of the aqueous subphase, deposition velocity, transfer pressure, temperature, etc.). Generally, on a proper selection of these parameters, a variety of different thin film structures can be realized.53 Thus, a diskotic (No) structure differing from that in the bulk (Dh,) LC state has been o b ~ e r v e d . ~In~principle, .~~ not only a modification of the bulk LC phases but also an induction of new diskotic LC phases in thin mono- and multilayers is feasible to occur. Langmuir-Blodgett Technique and Dipole-Dipole Interactions. The behavior of the disk-shaped amphiphiles at the air/ water interface is govemed by various types of intermolecular forces (e.g., ionic, steric, dipole, dispersion van der Waals, hydrophobic interactions). The hydrophobic-hydrophilic balance determines the orientation of the molecule^^^.^^ in the spread monolayers-“side-on’’ or “edge-on” (Figure 4,bottom). For “side-on” arrangement, the rotational mobility of the polarized diskotic molecules is rather good, and the torque will orient them in a configuration of minimum energy. The Y-type LB transfer of such a monolayer onto a solid support would create hexagonal columnar, Dh,, or smectic (sandwich), D,, structures suggested by Laschewsky.I6 These structures will be additionally stabilized by the antiparallel “face-to-face” and parallel “edge-by-edge” dipole attraction. The molecules arranged “edge-on” at the aidwater interface may possess good rotational mobility around the axis normal to the water surface. The most stable dipole configuration can be achieved for dipole moments oriented parallel to the water surface. A columnar arrangement of the disks would be favored. Hence, the hexagonal, Dh,, and the smectic columnar, DSC, structures in the LB films will be additionally stabilized by the dipole attraction. When the molecular dipole moments are oriented perpendicularly to the water surface, the dipole repulsion of the “faceto-face” parallel oriented dipoles would favor a nematic arrangement of the disks in the monolayers. The Y-type LB transfer of such monolayers will result in the most energetically unstable dipole configuration. Hence, a smectic nematic, D S N , or a nematic diskotic, ND,phase would be most probably formed in the LB films. Mixing of molecules of different magnitude or different orientation of the dipole moments with respect to the water surface could provide further possibilities for influencing the type of the diskotic LC phases in monolayers and LB films. A diskotic monolayer arrangement of mosaic structure is presented in Figure 5 for two kinds of “edge-on” oriented molecules mixed in an appropriate molar ratio. These molecules are assumed to have different antiparallel dipole moments which are perpendicular to the water surface. It is intriguing whether this hypothetical structure can be realized experimentally and whether the Y-type LB transfer of such mosaic monolayers may produce nematic mosaic, &M, or smectic mosaic, D S Mdiskotic , LB films (Figure 5, bottom). As it was mentioned above, another hypothetical diskotic LC phase is the cubatic one. It has been predicted in the computer ~imulation’~ by a variation of the lateral compression of the disks. To date, this phase has not been proved experimentally. The LB technique appears suitable for an experimental verification of the 2D existence of the cubatic phase because it allows the monolayer density to be changed in a wide region (similarly to the computer simulations). The square symmetry, recently
/
DNM
\
DSM
Figure 5. Mosaic structural (top) consisting of two types of diskshaped molecules with opposite orientations and unequal magnitudes of the dipole moments. Hypothetical mosaic molecular arrangement in mixed diskotic monolayers at the aidwater interface (middle) and possible nematic mosaic, DNM, and smectic mosaic, DSM,structures in deposited LB films (bottom).
observed in LB bilayers,54 is probably the first experimental evidence of the 2D analogue of the cubatic diskotic phase. Molecular Peculiarities of the Investigated Compound 4b. The chemical structure and the geometrical shape of the 4b molecule in an extended state is shown on Figure 1. The molecular dimensions are obtained from the reported bond lengths and bond angles of similar compound^.^^ A length of 0.138 nm was taken for each CH2 unit53 in the hydrocarbon wings and the tail. The estimated diameters of the molecule and its hard core are about 3.02 and 1.64nm, respectively. The molecule is disklike, since one section of the disk is cut off and a flexible hydrocarbon tail is attached in the open triangular segment. The cut-off section reduces the dimension of the disklike moiety to about 2.25 nm (Figure 1). The flexible wings and the tail may change their orientation under external influence. The tail sticks out about 0.4 nm of the disk diameter in the extended state configuration. The flexibility of the hydrocarbon chain and the C-0-C angle55of about 120” do not restrict the tail orientation in the disk plane. The strongly polarizable multiaromatic core and the attachment of an oxygen atom (from the alkoxy group) to the central benzene ring defines a charge asymmetry, and hence a dipole moment, p, of the molecule (Figure 1). The phenolic oxygen atoms are electron d ~ n a t i n g , and ~ ~ .hence ~ ~ the aromatic core appears a n-electron-rich system. Usually, the attachment of an alkoxy group to one of the benzene rings induces permanent dipole moments of magnitude from 1.0 to about 2.5 D.57 The value of p for the compound 4b could be even higher because of the larger dimension of the aromatic core. Molecular Peculiarities and Diskotic LC Phases of Similar Compounds. A. Bulk State. Table 1 summarizes the information of relevance for the recently reported mesogenic compounds 1-7 (Figure 2). These compounds have been studied in series of works by the groups of Praefcke et a1.18.31.32,34,s8 (compounds 1-4), Barraud, Markovitsi et a1.25-27.4’(compounds 5 and 6), Ringsdorf et a1.’9.23.24,28.42,59 (compounds 6), and Ohta et a1.36,37 (compounds 7). The LC phases of the compounds 1 and 2 are stable in a wide temperature region. Highly ordered D,, and Dho phases
J. Phys. Chem., Vol. 99, No. 49, 1995 17597
Diskotic Multiyne Langmuir-Blodgett Films. 1 have been induced by mixing of these compounds with molecules of permanent dipole moment.32 This has proven experimentally the significant role of the permanent-induced dipole interactions for the structure organization of the diskotic mesogenes. The symmetrical compounds 3 and the asymmetrical compounds 4a and 4b exhibit a nematic phase. The compounds 4c-f, which do not possess hydrocarbon wings, are not LC. Obviously, the wings stabilize the diskotic LC phases. The effect of the tail, as an order-disturbing element, is very important for the structure of these compounds. In the extended state of the molecules, the tail sticks about 0.4 nm out of the disk diameter for the compound 4b and about 0.7-1.2 nm for the compounds 4d-f. Charge-transfer interactions between the molecules with wings (4a, 4b as donors) and 2,4,7-trinitrofluorenone (TNF) (as an acceptor) have resulted in a formation of a a0 phase. For the non-liquid-crystalline compounds 4c-f (without wings), the charge-transfer interactions’* have produced a Dho phase only when the tial does not stick out of the disk (compound 4c). The parameter of the hexagonal lattice found in the later case is ah = 1.65 nm. The tail in the compounds 4 originates nearly from the dick centre and it has a diameter@‘ of about 0.497 nm. This tail dimension does not prevent the close packing of the donor and the acceptor molecules into columns with very small intracolumnar distanceI8 (0.352 nm). The reason for this is that the TNF molecules are without tails and their dimension is smaller than that of the disk core. Despite the close-packed structure of the charge-transfer complexes, the tails remain in a disordered state with a mean lateral distance of 0.49 nm. When the tail sticks considerably out of the disk diameter, even the strong charge-transfer interactions cannot arrange the molecules into a hexagonal columnar structure. Instead, a nematic columnar phase has been observed for the compound 4f, the mean intercolumnar distance being about 1.62 nm. The ionic compounds 5 do not possess a tail attached to the central oxygen atom. This charged center determines an asymmetrical distribution of the charge of the aromatic core, and hence a dipole moment. The molecules of 5 are in a LC state at room temperature and they form a very stable helicoidal Dho phase. The compounds 6 are characterized by a face-centered orthorhombic unit cell in a crystalline state.6’$62A specific peculiarity of the compound 6c are the two tails sticking about 0.3-0.4 nm out of its disk diameter. A smectic columnar LC phase has been obtained for this material after its cooling from an isotropic ~ t a t e , instead ~ ~ . ~ of ~ the Dho phase42%61.62 for the compounds 6a and 6b. A Dho phase has been obtained for 6c only when strong charge-transfer interactions with TNF have taken place. A Dho phase has not been formedI9 for the symmetric compound 6a in the case of charge-transfer interactions with an alkyl (C16H33)-substituted TNF derivative. The asymmetry introduced by the long alkyl chain has induced a N , phase. The smectic phase, DSN,of the compound 7 is not a columnar one. The transition temperatures given in Table 1 refer to R = C12H25.~~ The experimentally determined dimensions of the lamellas increase proportionally to the length of the hydrocarbon wings. To our knowledge, interdigitation of the wings has not been reported for the bulk state of the above compounds. Such effect has not been established even for the compound 7, characterized with considerable space between the four wings. B. Lungmuir-Blodgett Films. Diskotic LB films (Table 2)
TABLE 2: Monolayer Characteristics, LB Deposition Parameters, and Diskotic Phases of Amphiphilic Mesogenes in Bulk State and in Thin LB Films“ LC phases in A , Zt compound (mN/m) (nm2) (mN/m) ~c
4b
5d 6c
28 40 52
0.85 0.90 0.70
25 30 50
a
-
bulk LBfilm
0.86 N D - DSN 1.00 Dho- NO 1.00 Dsc- Dsc
ref this study 25,26 24,28
nC,monolayer collapse pressure; nt,constant transfer pressure; A,,, area per molecule at n,; a,average transer ratio. Diskotic LC phases: NO, nematic diskotic; DSN,diskotic smectic nematic; DSC,diskotic smectic columnar; Dho. diskotic ordered hexagonal columnar.
have been reported23-28s63for only a few of the mesogenic compounds listed in Table 1. The LB deposition has been performed at high constant surface pressures, zt, resulting in transfer ratios close to unity. On the basis of the data for the molecular radii (Table 1) and the areas per molecule (Table 2), an “edge-on” orientation, with hydrocarbon wings sticking up, can be inferred for these compounds in the spread monolayers. To date, the application of the LB technique for a preparation of thin diskotic LC films has resulted mainly in the following effects: (i) induction of a same diskotic phase in the LB films (in a solid-28or liquid-~tate~~ temperature region) as in the bulk LC state: e.g., a smectic columnar phasez8is characteristic for the compound 6c both in the bulk and in the thin films. Correspondingly, a Dho phase24has been formed in the mixtures of 6c and TNF; (ii) transition from a Dho bulk LC phase of the compound 5d to a low ordered nematic LC phase in the LB films. The induced transition, Dho No, could be related to both: (a) the “edge-on” molecular arrangement in the monolayers at the airlwater interface with a definite orientation of the disks (determined by the location of the hydrophilic and the hydrophobic groups in the molecules); (b) the ionic and dipole repulsion between the parallel molecular dipoles oriented normally to the water surface (Figure 4, bottom). The common peculiarities of the films of the diskotic mesogenic compounds considered can be summarized as follows: (a) the hydrophobic wings stick up in the spread monolayers; (b) the interdigitation of the wings is considerably hampered or vanished; (c) the wings are in a disordered state with an average lateral distance of about 0.45-0.50 nm; (d) the distorting elements (e.g., tails sticking out) induce low ordered LC phases; (e) more dense and higher ordered LC phases are obtained usually at small core radii; (f) the strong charge-transfer interactions induce usually higher ordered LC phases; (g) the asymmetrical charge distribution within the cores may influence the diskotic LC phases through dipole-dipole interactions.
-
Experimental Section The monolayer investivations and the Langmuir-Blodgett deposition were performed with a KSV-5000LB film equipment as described in ref 63. A single-compartment Teflon trough with symmetrically moving hydrophilic barriers and a platinum Wilhelmy plate for a determination of the surface pressure were used. Monolayers surface potential was measured by means of a vibrating condenser, operating with the same film balance. A platinum counter electrode was used. All chemicals were of the same high purity as previously reported.63 Chloroform (Merck, for spectroscopy) was used as spreading solvent of 4b. The aqueous salt subsolutions were prepared from MilliQ filtered water. Their pH values were controlled by adding NaHC03, NaOH, or HNO3 (all of AnalaR grade). The temperature of the aqueous subphase was 20.0 f 0.5 “C. The
Ionov and Angelova
17598 J. Phys. Chem., Vol. 99, No. 49, 1995
\
z
E
W
F3
25F 20
15
-
rn rn
Fa
TABLE 3: Deposition Conditions for a Preparation of the Diskotic LB Films of 41p sample group notation Me pH p ( % ) n,(mN/m) aA/aR(a) I A15 Ba2+ 3.5 0 10 0.39/0.29(0.34) 5.6 0 8 0.55/0.27 (0.41) 47 P1 5.6 0 10 0.57/0.33 (0.45) AM Q1 Ba2+ 7.0 50 12/30 0.80/1.00 I1 G3 1 5.6 0 10 0.76/0.28(0.52) 0 10 0.73/0.33(0.53) S16 5.6 Q18 Ba2+ 7.0 50 10 0.70/0.46 (0.58) 70 10 0.82/0.42(0.62) G14 PbZ+ 4.1 G2 CdZ+ 5.6 0 10 0.78/0.54(0.66) S21 Ba2+ 9.9 100 15 0.70/0.70 (0.70) I11 Q13 Ba2+ 10.0 100 15 0.73/0.71(0.72) 25 0.86/0.80 (0.83) 414 Ba2+ 10.0 100 25 0.94/0.78 (0.86) G10 Ba2+ 10.6 100 S26 Ba2+ 10.0 100 25 0.89/0.83(0.86) Ba2+ 10.5 100 25 0.95/0.77 (0.86) P18
10-
O"
0.6
0.8
1.0
1.2
1
0.3
1.4
Area ( n m y m o l e c ) Figure 6. Surface pressure/area ( d A ) , surface potentiavarea (AVIA), and surface dipole momendarea (&A) isotherms of unionized monoM BaBr2 subphase of pH 3.5. In the layers of 4b, spread on 2 x acidic pH region between 5.6, the isotherms are not affected by the presence of counterions in the aqueous subphase.
a Solid substrates: A - gold; Q - quartz; S - silicon; G - glass, P - polystyrene. Me - metal counterion in the aqueous subphase of concentration 2 x M; p - percent of metal counterion binding to the monolayer carboxylic groups (according to reference 75); nt transfer pressure; a A . aR- average transfer ratios on dipping and on aR)/2. AM - alternating withdrawal, respectively; a = ( a A multilayer of 4b and barium arachidate.
+
Results
:
20
VI
I !
t, 0.8 1.0
0.8 1.0
0.8 1.0
Area (nm2/molec) Figure 7. Surface pressure/area isotherms of diskotic monolayers of 4b on 2 x M BaBr2subphase of pH 7.0 (l), 8.3 (2), and 10.0 (3). synthesis and the purification of the compound 4b have been described in recent publication^.'^^^',^^.^^.^^ LB deposition was performed at various constant surface pressures, nt, determined by the monolayer isotherms. In most of the cases, the transfer process was limited by the low collapse pressures of the diskotic films (see, e.g., Figures 6 and 7 below, and Figure 2 of ref 63). All substrates used were hydrophobized, and a deposition velocity of 0.14 cm/s was employed. Usually, 20 deposition strokes were conducted for a preparation of the diskotic LB films. An alternating LB multilayer (Ql) was prepared by subsequent deposition of barium arachidate and 4b monolayers (Table 3). Fifteen periods were deposited. A cast film of the diskotic material was obtained by deposition of 20 p L of chloroform solution (1 x M) of 4b onto a quartz substrate. A Siemens 500 D X-ray diffractometer, allowing small-angle X-ray scattering measurements of a better precision than in the previous was employed. The experimental equipment has been described p r e v i o u ~ l y . ~Optical ~ - ~ ~ microphotographs of the LB films were obtained by means of an Olympus BH-2 microscope in a reflection mode. The microscope was equipped with an optical cable and a Panasonic color CCTV camera (Model WVCL 310-G). The maximum magnification of the system was about 3400 times.68
1. Characterization of the Spread Monolayers of 4b. A systematical study of the surface pressure/area isotherms of the diskotic monolayers has been performed in the previous work.63 Specific dependencies of the monolayers collapse pressures on the subsolution pH has been established for the different counterions used (Pb2+,Cd2+, Ba2+, Na+). Figure 6 presents the surface pressure/area and the surface potentiahea isotherms of a unionized monolayer of 4b. From these data, the normal component of the surface dipole moment per molecule, p l , is estimated on the basis of the Helmholtz eq~ation:~~.~~ pL = roAVA
(1)
where AV is the monolayer surface potential measured, A is the average area per molecule, and €0 is the permittivity of vacuum. This calculation assumes the permittivity of the monolayer to be unity. The variation of p l on monolayer compression is also plotted on Figure 6. It is seen from Figure 6 that the diskotic monolayers with undissociated carboxylic groups (at subsolution pH values less than 5.6) are characterized by a low collapse pressure, n,,of only 12.4 mN/m. The n,values in this region are not affected by the presence of metal cations in the subphase. The mean molecular area, A, determined at a surface pressure of 10 mN/ m, is about 0.85 nm2. The corresponding value of the surface dipole moment, p1, is about 0.7 D/molecule. This comparatively large value of p l results from several contribution^,^^-^' associated, e.g., with the hydrophobic and the hydrophilic regions of the monolayer in direction normal to the surface, with the hydration shell of the hydrophilic groups of the latter, etc. In the case of monolayer ionisation, the total surfaq potential measured includes also a contribution from the electrostatic double layer potential of the films, yk,. The effect of the divalent cations on the AV/A isotherms of diskotic monolayers will be reported elsewhere.72 Figure 7 shows typical behavior of xlA isotherms of ionized diskotic monolayers of 4b. The subphase in this case contains Ba2+ counterions. Due to the dissociation of the monolayers carboxylic groups and their interaction with the counterions,
J. Phys. Chem., Vol. 99, No. 49, 1995 17599
Diskotic Multiyne Langmuir-Blodgett Films. 1 the collapse pressure of the films increases on increase of the subsolution pH. This provides a possibility for LB deposition at higher surface pressures. Maximum rc, values of about 30 mN/m are found for subphase pH values above 10.0. Although polyvalent ions are known to cause a contraction of the spread monolayers, one observes a slight expansion of the diskotic monolayers on increase of the subphase pH. The mean area, A, of the completely dissociated monolayers, interacting with Ba2+ cations, increases to about 0.91 nm2/molecule at n = 10 mN/m. 2. Characterization of the Deposited Diskotic Films of 4b. Table 3 summarizes the most important deposition parameters of the investigated diskotic LB films. The samples are divided into three groups according to their different structure and packing density of the 4b molecules. In this classification, the main parameter is the average transfer ratio, a. It will be shown below that a is responsible for the difference in the structure organization of the diskotic films. In the case of pure water subphase and salt subsolutions of pH below 5.6, a is always lower than 0.6. The deposition process is limited to low transfer pressures due to the low collapse pressure of the monolayers. The alternating multilayer film (Ql) has a specific position in this classification because of the strongly reduced packing density of 4b in a direction normally to the film surface. However, in the lateral direction of the alternating LB film, the high transfer ratio on dipping, a l ,determines a considerable in-plane packing density of 4b. In principle, the transfer ratios increase on the increase of the deposition pre~sure.'~The mean transfer coefficients are close to unity for strongly alkaline Ba2+-containing subphases where the deposition was performed at a highest nt(Table 3). Figures 8 and 9 show typical small-angle X-ray diffraction patterns of the three groups diskotic LB films considered. The notations of the samples in these Figures are the same as in Table 3. The corresponding optical microphotographs are presented on Figures 10 and 11. The following features are observed: A . Discotic Films of the First Group. Dendritic films:These films are obtained at low transfer ratios, usually being less than about 0.5 (Table 3). In spite of these low transfer ratios, the X-ray pattems of the samples (Figure 8a) show a Bragg peak at 28 = 5.68". Its position is the same for all LB samples (A15, Q7, Pl). This peak corresponds to a repeated spacing d = 1.55 nm. Optical microphotographs (Figure 10a,b) illustrate a crystalline dendritic surface morphology of the layers. The dendritic structure is well defined for the films deposited with the lowest transfer ratio on gold (sample A15). The dendrites cover effectively about 90% of the substrate surface (Figure loa). Single dendrites are observed only near the edges of the films. With a slight increase of a (sample Pl), the dendrites begin to aggregate, they become thicker and cover effectively smaller substrate area. The morphological features of the same Q7 are similar to those of P1 (Figure lob). Castfilm: The cast film is obtained after a fast evaporation of the spread chloroform solution of 4b on quartz. A sharp Bragg peak at the same position, 28 = 5.68", is 0bserved.h the X-ray diffraction pattern (Figure 8b). The film is visualized in the optical microphotographs as a dense core of small crystals (Figure 1Oc) surrounded by satellite rings of crystallized diskotic material (Figure 10d). AZtemating LB multilayer (AM):This film (Ql) exhibits eight Bragg peaks in the X-ray pattern (Figure 8c). They determine two periods of 5.19 and 5.59 nm in the alternating multilayer structure. The crystalline Bragg peak (d = 1.55 nm) is also registered in the X-ray pattern. This sample reveals a densely
&,
6
4
,
2 n
,
,
,
,
v)
a 0 W
'2
cy
I
0 7
x
8
x +a
.a
4
C
Q
+a
-C 500
400
300 200 1 oc
0
2
4
6
8
10
12
14
2 Theta (deg.) Figure 8. Small-angleX-ray diffraction pattems representative for the diskotic LB films of the first group investigated samples (Table 3); (a) dendritic films (A1.5, Q7) obtained at low transfer ratios; (b) cast film of 4b; (c) alternating LB film of 4b and barium arachidate (Ql). c denotes the X-ray peak of the crystalline diskotic material.
packed surface morphology (Figure 1la). However, it contains a number of small gaps and looks quite rough. E . Diskotic Films of the Second Group. These are films characterized by intermediate transfer ratios (Table 3). The X-ray diffractograms of the samples of this group (Figure 9a) exhibit a shoulder corresponding to a bilayer periodicity of 4.45 nm. The crystalline peak at 28 = 5.68" is absent in the patterns. The surface morphology of these teared films (Figure 1lb,c) is more rough as compared to the first group films. Holes and protrusions are observed instead of aggregates and crystalline dendntes. The protrusions size (white areas) increases in the order of the increase of a (G31 G14 G2). C . Diskotic Films of the Third Group. All films in this group are prepared with average transfer ratios higher than 0.7. One Bragg peak at 28 = 1.98" accompanied by a few Kiessig fringes (fringes of equal inclination) are recorded in the small-angle X-ray diffractogram of the sample Q13 (Figure 9b). New morphological elements, as compared to the other two groups of samples, appear in the optical microphotography of the sample 413. Small smooth liquidlike areas are seen on Figure 1Id. The dimension of these areas increases considerably on increase of the transfer ratio to about 0.83 in the sample 414 (Figure lle). Correspondingly, one Bragg peak and a larger number of Kiessig fringes are observed in the X-ray spectra of 414 (not shown). Further increase of the transfer ratios in the samples G10 and S26 results in optically completely different, very smooth films (Figure 1If). Not only portions but the entire substrate surface of these samples are covered by a film of a new structure. The two Bragg peaks and the largest number of well defined Kiessig fringes observed (Figure 9b) are indicative for very smooth film surfaces. The period of the obtained
- -
17600 J. Phys. Chem., Vol. 99, No. 49, 1995
Ionov and Angelova angle transmission X-ray diffraction measurements of the samples S26 show one broad peak corresponding to a Bragg spacing of 0.37 nm and absence of peaks in the low-angle region.
Discussion
>r + .v)
C 600
1,
Q)
e
C
400
-
200
-
01s
1, 1
9 2
x20
G10
3
4
5
2 Theta (deg.) Figure 9. Typical small-angle X-raydiffraction patterns of the diskotic LB films of (a) second group samples (G31,S21) and (b) third group samples (413, G10). The pattern of the sample G10 is representative also for the sample S26 (Table 3).
structure is 4.45 nm. The crystalline peak at 5.68’ is not registered in the X-ray patterns of this group samples. The large
Monolayers of 4b at the Airwater Interface. The experimentally observed areas per molecule (Figure 6 and 7) determine an “edge-on” orientation of the 4b molecules at the aidwater interface. The hydrocarbon wings tend to orient normally to the water surface and the shape of the molecule is slightly changed due to the hydrophobic effect (Figure 12, top). The molecule contains two hydrophilic parts (Figure 1). Usually, the hydrophilic groups of the long-chain o-terminated bifunctional monolayers are in contact with the aqueous p h a ~ e . ~Here, ~ . ~the carboxylic group of 4b contacts the water surface and the hydrophilic alkoxy group is bonded to the hydrophobic core of the molecule. The degree of dipping of the molecules into the aqueous subphase (maximum to the central oxygen atom) depends on the hydrophilic-hydrophobic balance and the monolayer compression. From the areas per molecule in the compressed monolayers, A = 0.85 nm2, a “face-to-face” distance between the 4b molecules of about 0.326 nm can be estimated taking into account the core diameter of 1.64 nm and the cross-sectional aream (0.185 nm2) of the hydrocarbon chains determining a thickness of the two short wings of 0.97 nm. This value demonstrates a very close packing of the molecules on the water surface. In the densely packed monolayers, the tails may bury into the open triangular segments of the neighboring molecules. The tails stick to the benzene rings and in this way reduce considerably the effective area of the hydrophobic part immersed
Figure 10. Optical microphotographs of the first group investigated diskotic films of 4b: dendritic films A15 (a) and PI (b), crystalline core (c), and satellite rings (d) of the cast diskotic film. The bar corresponds to 10 pm.
J. Phys. Chem., Vol. 99, No. 49, 1995 17601
Diskotic Multiyne Langmuir-Blodgett Films. 1
I
I
,
L-
a I
Figure 11. Optical microphotographs of the alternating LB multilayer Q1 (a) and the LB films of the second and third group samples (Table 3): G31 (b), G2 (c), 413 (d), 414 (e), G10 (f). The arrow shows the direction of increasing of the average transfer ratios, a. The bar corresponds to 10 pm.
into the aqueous subphase (screening effect). The tails, originating from the central benzene ring, have a diameter of about 0.5 nm (estimated from the area per fatty acidsm molecules of 0.194 nm2). Therefore, the columnar ordering of 4b with a coincidence of the molecular centres will result in a intermolecular “face-to-face” distance of about 0.5 nm. This value is larger than the experimentally observed. The molecules can stick closer in the highly compressed diskotic monolayers if there exist an “edge-by-edge” shift74 of the positions of the molecular centers which is larger than the tail diameter. Such shift is favored by the repulsion of the parallel molecular dipoles of 4b oriented normally to the water surface (Figure 4, right bottom). Hence, the columnar ordering of the molecules on the water surface is considerably disturbed and a nematic arrangement prevails in the spread monolayers (Figure 12). This arrangement is supported by the observed red shift of the
absorption band49which is typical for J aggregate formation in the monolayers. The nematic noncolumnar organisation of the diskotic monolayers at the &/water interface results both from the geometrical peculiarities of the 4b molecules and from their in-plane dipoledipole interactions (Figure 4, bottom). The positive values of the monolayer surface potential A V (Figure 6) illustrate that the normal surface dipole moment, p1, is directed from the water surface toward the upper hydrophobic portion of the monolayer which contacts the air. Due to the opposite direction of the molecular dipole moment, related to the alkoxy oxygen atom (Figure l), these A V values are smaller in comparison to those for monolayers of methyl group-terminated long-chain molec u l e ~ . ~ ~However, -~’ the mean normal surface dipole moment per molecule, p1, shows essential values resulting from several contribution^.^^-^' Since the theoretical models for an inter-
Ionov and Angelova
17602 J. Phys. Chem., Vol. 99, No. 49, 1995
Figure 12. "Edge-on" orientation of the 4b molecules at the aidwater interface (top) and nematic in-plane molecular arrangement of the spread diskotic monolayers (bottom). The orientation of the flexible wings alters the molecular shape on Figure 1.
pretation of the surface potential measurements of diskotic monolayers have not been developed yet, an attempt for a more will be presented el~ewhere.'~ detailed analysis of Structure of the LB Films of 4b. A . Films of rhe First Group. Films deposited at low transfer ratios: The low transfer coefficients of this group samples, a, indicate that the structure of the films on a solid support differs from that on the water surface. In principle, the structure of the deposited LB layers is not simply related to that on the water surface76even at a = 1. Only one X-ray peak at 28 = 5.68" was recorded in the small-angle X-ray diffractograms for all films of this group (Figure 8a). It is like that of the 4e films deposited under similar condition^.'^ The possible origins of this peak are discussed below. (i) Hexagonal columnar packing of the molecules as assumed in ref 77 for the films of the compound 4e. The position of the observed X-ray peak corresponds to a Bragg spacing d = 1.55 nm and may determine a hexagonal lattice of a h = 2d x 3-"* = 1.79 nm. This value is possible if one assumes complete interdigitationof the wings of the disklike molecules, both laterally and normally. Such an interdigitation is slightly probable to occur in the films deposited with low packing density (low a). Absence of an interdigitationhas been evidenced26 even for diskotic films deposited at transfer ratios close to 1. Therefore, the presence of a hexagonal packing is questionable. The hexagonal columnar packing is the most dense and ordered state for the diskotic molecules. The hexagonal unit cell in a direction normal to the substrate requires a presence of a minimum three columnar layers with columns laying parallel to the substrate. The films consisting of 20 monolayers deposited with transfer coefficients of 0.34 show nearly a complete coverage of the entire substrate surface (Figure loa). The amount of the deposited diskotic material corresponds to about two or three hexagonal lattices normally to the substrate. However, the width and the intensity of the X-ray peak indicate a presence of a higher number of unit cells: The half-width at half-maximum (hwhm) of the peak for the sample A15 is d = 0.0018 A-l. Assuming a position correlation function of an Omstein-Zemike type,78exp(-r@), a correlation length 5 = 55.5 nm is estimated. Therefore, the practical position correlation of the unit cells exist over this distance. This corresponds to about 18 unit cells of a hexagonal packing and requires simultaneously about 36 deposited monolayers to cover effectively the substrate surface. Since this is not the case, another structure of reduced density should be present in the dendritic layers.
The theoretical investigations for ideal disk-shaped mole c u l e ~ ' ~have . ' ~ predicted an isotropic or a nematic arrangement at a low molecular density. The compound 4b exhibits only a nematic phase in bulk which is stable in a narrow temperature region (Table 1). Its molecular peculiarities, the disturbing effect of the tail, and the repulsion of the parallel oriented 4b dipoles in the spread monolayers have a distorting effect on the hexagonal columnar arrangement. The observation of the same X-ray peak in the pattem of the alternating LB film (Figure 8c), where a hexagonal packing does not take place, strongly supports another origin of this peak. (ii) Crystalline state. The structure organization of the dendritic layers prepared at low transfer ratios could be similar to that obtained by means of the cast film method. This possibility was tested by studying a cast film of 4b on a quartz substrate. Small crystallites are observed in the optical microphotographs (Figure 10c,d). The X-ray diffraction also c o n f i i s the crystalline state of the cast film. The position of the X-ray peak (Figure 8b) is the same as for the dendritic layers (Figure 8a). The single X-ray peak can be assigned to unit cells with different structures. Usually, similar diskotic compounds exhibit orthorhombic or monoclinic ce11s.28,55f"A 020 peak79-81of a orthorhombic unit cell (probably body centred) with a dimension of 3.1 nm corresponds to the experimentally observed X-ray peak position. This unit cell fits well to the dimensions of the 4b molecule (Figure 1). The period established in a direction normal to the substrate indicates an absence of an interdigitation of the flexible wings and less-dense molecular packing as compared to the hexagonal columnar state. For the same amount of deposited diskotic material on the same substrate area, the less-dense packed orthorhombic organization will form thicker films than the hexagonal one. Correspondingly, the correlation length will be larger in the case of an orthorhombic structure since there will be a bigger number of unit cells per unit area in a normal direction to the substrate. The correlation length estimated for the cast film, = 252 nm, corresponds to about 81 unit cells. The dendritic morphology observed with the diskotic films of this group (Figure loa) is typical for the crystalline state.82 The position of the X-ray peaks (Figure 8a) coincides with that for the crystalline cast film. The orthorhombic crystalline structure explains better the correlation length and the intensity of the peaks of the dendritic film A15. The correlation length estimated for the sample Q7, 6 = 88.3 nm, determines a position correlation of about 28 unit cells. This is possible if aggregation of the material occurs. The aggregation is indeed evidenced by the optical microscopy (Figure lob). Therefore, the structural organization of the LB films deposited at low transfer ratios corresponds to the crystalline state of aggregated bulk diskotic material. The crystallization is facilitated by the lower density of the layers under the reduced extemal pressure. Alternating LB multilayer: The 4b monolayers in the alternating LB film Q1 are separated by means of densely packed barium arachidate layers. An interdigitation between the adjacent arachidate and 4b monolayers is not probable. The intralayer density of the 4b monolayers is expected to be close to that of the third-group samples due to the nearly the same transfer ratio of 0.8 (Table 3). The eight Bragg peaks observed in the X-ray pattem (Figure 8c) determine a good periodicity of the alternating multilayer. The splitting of the high order peaks is typical for a mosaic s t r u ~ t u r e . ~ Two ~ - ~ periods ~ ~ ~ ' of 5.19 and 5.59 nm are superimposed indicating a coexistence of two type of domains in the multilayer structure. Assuming the same thickness of 2.81 nm for the barium arachidate monolayers as in its multilayers
e
J. Phys. Chem., Vol. 99, No. 49, 1995 17603
Diskotic Multiyne Langmuir-Blodgett Films. 1 at pH 7, one obtains thicknesses of 2.39 and 2.78 nm for the 4b monolayers in the two types of domains. Figure 1la shows rough surface morphology of the altemating film with many irregularities and gaps. A number of holes were found in the optical microphotographs and the atomic force microscopy images of the single barium arachidate multilayer as well. Near these gaps, the density of the layers is lower and a small part of the 4b molecules may crystallize in the free space. This crystallization is evidenced by the observation of the weak X-ray peak at 26 = 5.68”. The layer thickness of 2.39 nm, estimated for the first type of domains in the altemating multilayer, is close to the 4b monolayer thickness in the third-group structures (see below). The second domain thickness of 2.78 nm can be explained by formation of mixed areas of 4b and barium arachidate molecules in the 4b sublayers near the gaps of the film. The mixed domains are probably formed by a penetrations3 of the barium arachidate molecules from the upper layers into the vacated places of the crystallized 4b molecules. The thickness of the mixed domains may vary with the molar ratio of the compon e n t ~ . ~ ~ ~ ~ ~ Therefore, the good periodicity of the altemating multilayer in a normal direction to the substrate is accompanied by an inhomogeneous in-plane structure of the monolayers. Two types of domains coexist in the diskotic monolayers together with small crystallites of 4b near the gaps. This results in the observation of rough surface morphology of the altemating film and the lack of Kiessig fringes in the small-angle X-ray diffractogram (Figure 8c). B. Films of the Second Group (Intermediate Transfer Ratios). The lack of the X-ray peak at 5.68” (Figure 9a) and the absence of a dendritic morphology (Figure llb,c) is evidence that the crystalline structure of the first-group films is replaced by a new one in this group samples. Both, the films transferred from divalent salt subsolutions and from pure water subphase belong to the second-group samples. Therefore, the transfer ratio, rather than the presence of metal cations, is the major parameter determining this new structural arrangement. The increased average transfer ratios (Table 3) lead to higher density of the deposited diskotic layers. This prevents the crystallization and requires other structure organization of the films. The optical microphotographs (Figure 1lb,c) show teared, inhomogeneous noncrystalline layers with holes and protrusions. The shoulder observed in the X-ray diffractogram (Figure 9a) corresponds to a bilayer period of 4.45 nm. Despite the tearing of the film into small pieces, it indicates the onset of the formation of a new structure which is completely expressed at the maximum transfer ratios (third group samples). Therefore, under the conditions of intermediate transfer ratios, the crystalline structure is replaced by an intermediate one in which structural rudiments of a new order appear. C. Films of the Third Group (TransferRatios Close to Unity). Under defined external conditions, the molecular organization of the diskotic films corresponds to a structure of minimum energy. On changing the external conditions, a new structure can appear after overcoming the energetical threshold for the existence of the old structure. It is possible that the structure of the diskotic layers on a solid support becomes closer to that on the water surface on increase of the transfer ratios. The presence of a new structural organization in the LB films 413 of the third group is indicated by the clearly defined Bragg peak, determining a bilayer periodicity of 4.45 nm, and the observation of a few Kiessig fringes (Figure 9b). New features are established also in the surface morphology of the sample (Figure lld). The increase of the transfer ratios in samples 414 and
G10 is related to an improvement of the film homogeneity, integrity, and smoothness (Figure 1le$). The completely new diskotic structure, found with the films of the third group, exhibits a bilayer periodicity of 4.45 nm normal to the substrate. It fits very well to the total film thickness estimated for the sample G10 from the positions of the Kiessig fringess6 (Figure 9b). The determined monolayer thickness of 2.225 nm is less than the molecular diameter, but it fits to the dimension of the disklike model of the molecule in an extended state (Figure 1). From these values, a small molecular tilt of about 9” of the disc plane with respect to the surface normal can be evaluated. The observation of only two Bragg peaks, despite the very high electron density of the incorporated barium ions, indicates the existence of fluctuations of the position of the barium ions in the clearly defined 4b bilayers of the multilayers. If these fluctuations were related to the interdigitation of the discs of the adjacent monolayers, this would increase the surface roughness and reduce the number of Kiessig fringes. The absence of an interdigitation normal to the substrate and the wriggle of the tails into the triangular segments of the neighboring 4b molecules explain simultaneously both the fluctuations of the positions of the barium ions and the large number of Kiessig fringes observed with the samples of the third group (Figure 9b). This model can explain also the larger bilayer period of 6.3 nm determineds7 for uranyl ions-containing 4b LB films: the linear uranyl counterions are of large dimension and can prevent the tails to wriggle into the open neighboring triangular segments. This could be an explanation for the smaller areas per molecule (0.70 nm2) obtained only for monolayers spread on uranyl acetate subphase. The transmission low-angle X-ray measurements of the sample S26 did not show any indication for a columnar ordering of the disklike molecules. The large-angle X-ray peak found at 0.37 nm corresponds to a nematic in-plane arrangement of the discs. The absence of a columnar ordering is primarily determined by the molecular peculiarities of the 4b molecules, related to the presence of the tail as a disturbing element,I8 as well as by the dipole repulsion of the parallelly oriented dipoles (Figure 4). The established red shift of the absorption and fluorescence bands,49which is typical for J aggregates, gives an evidence for an “offset face-to-face” in-plane arrangement of the disk planes in the deposited layers. Characteristic for the structural organisation of the third group diskotic samples are (i) an “edge-on” arrangement of the 4b molecules on a solid support, (ii) well-defined layers normally to the substrate, and (iii) a nematic ordering of the molecules in the layer planes. Figure 13 shows schematically the LB multilayer structure achieved at transfer ratios close to unity. The peculiarities of this structure are typical for the smectic liquid crystalline phase. Therefore, a structural arrangement of a higher order, close to a smectic nematic DSNphase, was induced by means of the LB deposition technique with disklike molecules possessing a nematic phase in the bulk state. This arrangement is stabilized by the intermolecular interactions in the films: while the bilayer periodicity normal to the substrate is determined predominantly by ionic interactions, the offset in-plane arrangement of the disklike molecules is a result of dipole-dipole interactions which can be considered as a special case of the n-n interaction^.^^
Conclusion The amphiphilic 4b molecules are “edge-on” oriented at the aidwater interface. In the compressed diskotic monolayers, their arrangement is nematic and the tails wriggle into the open
17604 J. Phys. Chem., Vol. 99, No. 49, 1995
DSN
Cr
DSN
- a
I
0.5
0
1
Figure 13. Smectic nematic DSN-like arrangement of the disklike 4b molecules induced in the LB multilayers deposited at transfer ratios close to unity. The arrow shows the phase transition of the molecular organisation of the diskotic films from a crystalline (Cr) to DsN-like on increase of the average transfer ratios, a.
triangular segments of the adjacent molecules. The nematic ordering is due to both the presence of tails and the dipole repulsion of the parallelly iriented normal surface dipoles. The LB transfer of the densely packed diskotic monolayers on a solid support results, at transfer ratios close to 1, in a homogeneous smecticlike multilayer structure of the films (Figure 13). The reduction of the transfer ratios at lower deposition pressures leads to a tearing of the films. The teared film structure is formed of coexisting small domains of a bilayer periodicity and disordered areas. Further reduction of the deposition pressure and the average transfer ratios favors the free crystallization of the less dense films. A body-centered orthorhombic unit cell is energetically preferred in the crystallized dendritic films. The results show that it is possible with the disklike 4b amphiphile to induce by means of the LB technique a higher ordered, smectic diskotic nematic “DSN solid”, structure arrangement than in the bulk diskotic nematic, ND, phase. Terminology Remark. The terminology of the diskotic liquid-crystalline phases is still under disc~ssion.’~ The structural organisation of the LB films of diskotic compounds may resemble some of the diskotic liquid-crystalline phases. The LB films deposited at room temperature are usually in a solid state. Hence, an additional indication is necessary in the notation of the diskotic phase formed in the LB films, e.g., “Dholike” or
‘‘&a
solid”.
Toward the aim to introduce a symmetry in the notations of the diskotic LC phases, we would like to propose a discussion for a change of the notation of the diskotic nematic phase from N D into DN and of the diskotic nematic columnar phase from Nc into DNC. The capital letter, D , in these notation is related to the shape of the molecules. Therefore, it should be present in the notations as for all other diskotic phases.
References and Notes (1) De Gennes, P. G. The Physics of Liquid Crystals; Clarendon Press: Oxford, 1974. (2) Demus, D.; Richter, L. Textures of Liquid Ctystals: Verlag Chemie: Weinheim, 1978.
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