~~~~ Langmuir-Blodgett multilayers of six compact porphyrin amphiphiles' Colin L. Honeybourne and Kevin J. Barrel1 Department of Chemical and Physical Sciences, The University of the West of England, Frenchay Campus, Bristol, UK BS16 1QY By a modification of the Jackson-MacDonald condensation between two symmetrically substituted dipyrrylmethanes, we have synthesised two porphyrin free-base amphiphiles and four metalloporphyrin amphiphiles all of which serve as models for the naturally derived product mesoporphyrin IX dimethyl ester. These compounds have different substituents along their hydrophobic edge to those found in mesoporphyrin IX, which has the sequence Me, Et, Me, Et. Our materials have the sequences (Me), or (Et)4 thus making the hydrophobic edge slightly larger, or slightly smaller than that contained in mesoporphyrin IX.Upon an aqueous subphase, Langmuir films yere obtained at pressures in the range 14-50 mN dm3 mol-', with, at full compression, molecular areas of the order of 65 A2 and angles-of-tilt of the macrocyclic plane to the subphase of some 80". LAXRD data were taken on Langmuir-Blodgett multilayer films of the (Et), compounds which proved to have the threshold minimum magnitude of hydrophobic edge for multilayer formation, with Y-type bilayers being given by the zinc and silver complexes. In research upon Langmuir-Blodgett thin films of porphyrins Tredgold and co-worker~l-~ have utilised the naturally derived mesoporphyrin IX (and related diesters, diols and metal com- plexes) whereas Honeybourne and co-worker~~-~ have syn- thesised a range of models for mesoporphyrin IX.In the latter, emphasis has been placed upon varying the size of the hydro- phobic edge (rings C and D in Fig. 1) for use in a range of studies such as ferrochelatase kinetic^,^ concentration depen- dent NMR' and the formation of Langmuir films and Langmuir-Blodgett multilayers.6 Mesoporphyrin IX has a hydrophobic edge consisting of R1=R3=methyl and R2=R4=ethyl. Models for mesoporphyrin IX include4.' R' =R2=R3=R4=H, methyl or ethyl and R1 =R2 =propyl, R2=R3=ethyl.6 In this work we report the synthesis, character- isation and thin film deposition of the all-methyl and all-ethyl variants mentioned above. Koga et uL7 have very recently reported the aggregation of mesoporphyrin IX in mixed ?Presented at the Second International Conference on Materials Chemistry, MC', University of Kent at Canterbury, 17-21 July 1995.R' R2 Langmuir-Blodgett films with arachidic acid. In contrast to these compact porphyrins, which lack the long alkyl chains characteristic of the molecules usually employed in the forma- tion of Langmuir-Blodgett films, a number of groups of workers are using the readily-synthesised meso-substituted porphyrins in which meso-substitution at the four CHC bridges by aryl groups has taken place. At least one of these four aryl groups carries a long-chain alkyl s~bstituent.~-'~These molecules include sulfonamidoporphyrins8 and nitro-phenyl-amid~phenylporphyrins.~Other workers have used either the tetra-4-pyridyl meso-substituted porphyrin free base (in conjunction with a novel inter-layer spacing technique)" or the new stellular pentaporphyrins with a,w-dioxyalkyl (C-3, C-4) spacer groups.12 Chou et uL9 have remarked that, in the absence of steric constraints from multiple aliphatic chains, porphyrin-porphy- rin stacking permits close packing of the porphyrin rings.However, without any aliphatic chains, the Langmuir films will be very rigid and their flow properties will be adverse as the substrate breaks and re-breaks the surface, giving either no multilayers, or multilayers of poor quality. We seek to 43 R3 R4 M Fig. 1 Structure and labelling of new porphyrins and metalloporphyrins J.Muter. Chem., 1996, 6(3), 323-329 323 identify the minimum size of hydrophobic edge that allows Langmuir-Blodgett multilayer deposition of a model for the free base of mesoporphynn IX dimethyl ester and its complexes without recourse to the use of additives" (such as arachidic acid, mesitylene, etc) so that the packing density of the porphynn nngs is maximised within a layer and the inter-layer spacing is minimised (thereby facilitating the Mott hopping of charge carriers between layers) Experimental Synthetic procedures Porphyrin free base diesters. The final stages of the complex and lengthy synthetic method, the full details of which will be submitted elsewhere (and are available from the author by 11 request) are shown in Scheme 1 In this paper R1=R2=R3= R4=either methyl or ethyl The key reaction was the MacDonald conden~ation'~ between two dipyrrylmethanes, one being a dialdehyde (7) that exhibits the valuable property of long-term chemical stability (at least 10 years when kept in the dark at <4 "C) The second dipyrrylmethane that featured in the condensation reaction was a relatively unstable di- carboxylic acid (8) formed by de-esterification (under dry nitrogen) of a benzyl diester (9) The dicarboxylic acid (8) was utilised as soon as possible without purification of the initial product The success of the condensation reaction, to produce a diprotonated porphodimethene (10) was indicated by the development of a deep-red burgundy colouration, at which stage sufficient sodium acetate was added to inhibit any further chemical reactions induced by the high acidity of the medium Rapid oxidation by atmospheric oxygen produced the porphy- Scheme 1 Flow chart of final stages of the synthesis of porphynn free bases (Jackson-McDonald condensation) 324 J Muter Chem , 1996, 6(3), 323-329 rin diacid (11) which was easily converted into the methyl diester which was then extracted into chloroform.Purification was achieved by utilisation of an alumina column and recrystal- lisation from methanol. The complete reaction sequence depicted in Scheme 1 proceeded with 10-15% overall yield. Zinc complexes. Quantitative yields were given by main- taining a mixture of equimolar quantities of zinc acetate hydrate and the porphyrin free base at reflux for lOmin, utilising a solvent consisting of a 9 :1 (v/v) mixture of chloro- form and methanol.Silver complexes. The inorganic starting material was a salt containing monovalent silver, as either the acetate or the nitrate. During the course of the reaction between equimolar quantities of silver salt and porphyrin free base, at elevated temperature in acetic acid [tetraethyl compound (6)] or dimethylformamide [hexamethyl compound (3)], the silver is oxidised'' to the divalent state, with the complexes of Ag" being produced in virtually quantitative yield. Confirmation of structures by spectroscopy The absorption spectra in the UV and visible regions were consistent with those expected either from porphyrin free bases (1and 4) exhibiting an etio-type distribution of relative intensit- ies or from their corresponding divalent metal complexes.The complexation process increases the symmetry of the chromo- phore from D,h to D&. Thus, the vibrational interactions due to the two N-H bonds cause four absorption bands (Qk, Qlx, QOy, Q1,) to occur in the visible region, which are reduced to two bands upon complexation (Fig. 2). The N-H stretching 0.0 t0.0 0. -0.6 .0.4 0. -0.2 0. .o.o 400 500 600 700 0.8, ,2.4 400 500 600 700 A/nm Fig. 2 (a)Absorption spectra of LB multilayers of compound 4 (4, 8, 18, 28 and 38 layers). (b) Absorption spectra of compound 6 as a 40-layer LB film (-) and a solution (1.4 x lop5 mol dmP3) in chloroform (---).frequency (a weak band at 3330 cm-') and the high-field NH proton NMR signal (ca. 6 -4 at higher concentrations) also disappear (as expected) upon successful completion of the complexation reaction. Porphyrins and metalloporphyrins display a large diamag- netic ring current which moves the internal (NH) protons to high applied fields, and the peripheral protons (especially the four bridging CH protons) to low applied field. The tendency of porphyrins to aggregate is so strong that one molecule in a dimer is influenced by the ring current of its paired neighbour. The free bases in this work exhibited large concentration- dependent effects similar in kind to our earlier published results' for R1=R2=R3=R4=H.Thus the results given in Table 1have been obtained by extrapolating our proton NMR data to zero concentration. Molecular masses, corresponding to molecular structural formulae were confirmed using the molecular ion peak pro- duced by electron impact16 mass spectrometry (Hewlett- Packard 5995), and supported by CHN microanalytical data. Preparation of thin films Sample preparation. An appropriately small quantity of each porphyrin was weighed, using a Cahn Electronic Microbalance, and dissolved in Aristar-grade chloroform to give an accurately Itknown concentration in the region of 1mg ~rn-~. was essential to use these solutions as soon as possible, storing them in tightly sealed containers in the dark at a temperature of ,<4"C.Substrate preparation. The substrates chosen were glass microscope slides (Richardson), optically flat quartz (UQG Ltd) and silicon wafer (University of Bristol and Plessey, Caswell). The surfaces of these substrates were rendered hydro- phobic as described later. The surfaces of glass or quartz slides were first treated with chromic acid and then washed thoroughly with dust-free water (Millipore) followed by washing with HPLC-grade isopropyl alcohol (IPA). They were then further cleaned by the rinsing action of IPA using a specially designed Soxhlet extractor to facilitate cleaning a large number of substrates at one time. After drying in dust-free air, the substrates were rinsed with a 2% (v/v) solution of dichlorodimethylsilane in l,l,l-trichloro- ethane, using the same bulk Soxhlet technique. The substrates were then washed with water (Millipore) to remove any residual hydrochloric acid.The hydrophobic nature of the substrates was checked by pulling them through the surface of pure water. A completely dry surface indicated that the substrates were uniformly hydrophobic. The preparation of hydrophobic silicon surfaces was carried out on a small scale, with each sample being treated individu- ally. The slice of silicon wafer was cleaned in a Soxhlet Table 1 Proton NMR chemical shifts (6, ppm) of the new porphyrin free bases 1 and 4 extrapolated to zero concentration Me, hydrophobic edge 1 Et, hydrophobic edge 4 NH -3.41 -3.38 -CH2CH3 - 1.92 -CH2CH2C02CH3 3.34 3.30 -CH, 3.60 3.66 -(CH2)2C02CH3-CH2CH3 3.65 - 3.66 4.10 -CH2CH2C02CH3 4.37 4.44 HI% 9.88 10.11 HB 9.94 10.11 HY H6 9.94 10.01 10.11 10.11 -CH3 3.50 J.Mater. Chem., 1996, 6(3), 323-329 325 extractor with high-purity chloroform, and then by HPLC- grade IPA After drying in a stream of dust-free air, the sample was subjected to further cleaning in an ultrasonic bath contain- ing pure water (Millipore) The surface of the water was removed by suction (to avoid contamination) The sample was then removed from the water, and emerged completely dry if the cleaning process had been successful Using plastic utensils, the cleaned silicon was chemically etched in 5% hydrofluoric acid for approximately 25 s Copious rinsing with pure water then produced a totally hydrophobic silicon substrate without the use of hydrophobizing chemical agents The hydrophobic nature of freshly etched silicon has been a point of contention because silicon forms a new oxide layer by atmosphenc oxidation of the freshly etched surface, thereby rendering it hydrophilic To overcome the effect of this oxide layer, hydrophobizing silylating agents are usually employed However, we wish to emphasise that the formation of the oxide layer on freshly etched silicon proceeds sufficiently slowly to permit the use of this matenal as a hydrophobic substrate Using the technique of X-ray photoelectron spectroscopy (XPS), the extent of oxidation approximately 10 min after chemical etching was determined The spectrum obtained of the chemical shift of the Si(2p) electrons contains only a single signal (from elemental silicon) The extent of oxidation slowly increases over a period of 11 days as shown by the gradual appearance of a much smaller peak corresponding to the chemical shift of Si(2p) electrons in silicon dioxide The Langmuir-Blodgett trough.The apparatus used was supplied and designed by Nima Technology Ltd It consisted of a circular boro-silicate glass trough, with a tnangular-sector barner constructed of polytetrafluoroethylene Control of the equipment was by Nima Technology software implemented on a BBC microcomputer Changes in the surface pressure (surface tension) of a floating Langmuir monolayer were monitored by a feedback loop consisting of a linear voltage differential transducer and a Wilhelmy plate comprising a piece of filter paper 10mm in width The temperature of the subphase was maintained by a thermostatically controlled water bath, and the pH monitored by a pH probe The dipper mechanism was equipped with controls for selecting dipping speed and length of dipping stroke Between each experiment, the trough and barrier were scrupulously cleaned This was carried out by drying, rinsing with chloroform and then with IPA, followed by drying in dust-free air Finally, the equipment was thoroughly rinsed with pure water (Millipore) To ensure that contamination by air-borne particles was kept to a minimum, the equipment was housed in a microbio- logical cabinet (Hepaire) This latter was fitted with a fan such that a positive internal air-pressure could be maintained, with air being drawn through a microbiological filter Preparation of Langmuir and Langmuir-Blodgett films. The subphase for the spreading of floating monolayers (1 e Langmuir layers) consisted of pure water (Milli-Q grade) at 20°C The pH was 5 8 due to the buffering action of dissolved carbon dioxide The porphynn molecules were spread from a solution in chloroform onto the clean surface of the subphase The solution was added dropwise in close proximity to the surface of the subphase Sufficient time was allowed for each drop to evaporate before adding the next drop Usually 25 mm3 of solution, added using a graduated syringe, was adequate The molecules were then compressed between the fixed and moveable barriers the rate of movement of the latter reduced the surface area by 100 cm2 min-' Good quality pressure-area (PA) isotherms were produced during the formation of Langmuir monolayers The stability of the Langmuir monolayers at a predetermined surface press- 326 J Muter Chem ,1996,6(3), 323-329 ure was monitored with respect to time a dipping pressure of 35 mN m-' was thereby found to be the most suitable To obtain LB films, layers were transferred consecutively on to solid substrates at dipping speeds rangmg from 5 to 10mm min-l Multilayer formation proceeded via a modified Y-type deposition with material being deposited on to the solid substrate on both the up-stroke and the down-stroke Deposition was executed without intervening pauses for the films to drain because the films emerged completely dry when pulled from the aqueous subphase Low-angle X-ray diffraction (LAXRD).The application of LAXRD to the study of the structure of LB films has been described by Tredgold l7 The X-ray measurements were taken on a Philips Diffractometer PW1700 (by courtesy of Rolls Royce, Bristol) The quasi-chromatic source was a chromium target X-ray tube equipped with a vanadium filter Divergence and scatter slits were used that possessed angular apertures of 025" The divergence slit ensured that small angles could be scanned without saturation of the signal by the X-ray beam A receiving slit (02 mm) defined the width of the reflected beam The sample was rotated by 8 and the counter tube was rotated by 28 To obtain maximum resolution, a 0 25", 10 s step scan was used The substrate used for these measurements was hydrophobic silicon the LB film facing the moveable barrier was chosen because this was of higher visual quality Results of Thin Layer Formation Pressure-area (PA) isotherms of Langmuir monolayers Hexamethylporphyrin compounds.Fig 3 shows a PA iso- therm for the silver complex (3) on a subphase of pure water The plot was obtained by compressing the monolayer at 100cm2 min- I, whilst monitoring surface pressure and area This is a very good quality isotherm showing the solid, liquid and gas phases The semicondensed or liquid phase occurs in the region of 0-14 mN dm3 mol-' The condensed or solid phase lies in the region of 14-50 mN dm3 mol-' The region above 50 mN dm3 mol-' represents the onset of collapse within the porphyrin monolayer If the condensed (solid) portion of the isotherm is extrapolated to zero pressure, the moiecular area for this porphyrin molecule is found to be 68 A2 This value indicates that the molecules were orientated at 20" to the normal to the subphase, with the methyl ester groups in the subphase and the porphyrin nuclei projecting above the subphase If the porphyrin nngs had been lying parallel to th? water surface then a molecular area in the region of 238 A2 would have been expected These conclusions 70.01 .o area/A2 molecule-1 Fig.3 Pressure-area (PA) isotherm of a floating Langmuir monolayer of compound 3 (see Fig 1) on a subphase of pure water 40.0 80.0 120.0 wed2 molecule-' Fig. 4 Pressure-area (PA) isotherms of a floating Langmuir monolayer of compound 3 (see Fig. 1) after a number of compressions and decompressions were arrived at with the aid of a space filling model of the porphyrin. Fig.4 demonstrates the reproducible nature of this PA isotherm after several expansions and compressions. This characteristic indicated that the film was robust and not subject to collapse. The reproducibility of these plots confirmed that the monolayer was a true Langmuir film and not a system of aggregated molecules that formed crystalline islands on the surface of the subphase.In addition, it was also concluded that the monolayer was contamination free due to the uniform- ity of the isotherm. The free base hexamethylporphyrin derivative (1) also produced reproducible isotherms on a subphase of water with the three phases of solid, liquid and gasebeing clearly visible. A molecular area of approximately 63 A2 indicates that the porphyrin planes subtend an angle of ca. 18" to the normal to the subphase. The phase transitions occurred at different surface pressure values to those given by the compound 3. The semicondensed region was generally between the values of 0 and 7.5 mN dm3 mol-', whereas the condensed portion was in the range of 7.5-22 mN dm3 mol-'.Above this range collapse occurred. Changing the compression rate from 100 to 50 cm2 min-' had no effect on the position of these phases. The zinc complex (2) did not form the same reproducible isotherms that were described in the previous two cases. The first compression of the monolayer on a subphase of water produced a PA curve with distinctive phases of solid, liquid and gas. However, after two or three further expansions and compressions the isotherms only exhibited a solid and gas phase. This effect was unchanged whether the monolayer was on a surface of pure water or aqueous cadmium chloride. This behaviour may be attributed to aggregation of the molecules within the porphyrin monolayer. On the first compression the molecules were free to move and align themselves relative to one another.However, eventually the molecules aggregated together such that when the film was expanded the molecules were unable to respread. This was observed as a loss of the liquid or semicondensed region in the isotherm. This effect was further substantiated by utilisation of an experiment similar to that performed by Baker et ul.'* This was carried out by modifying the spreading mixture to chloro- form and mesitylene, in the ratios of 4: 1 respectively. The mesitylene was less volatile than the chloroform and therefore evaporated at a slower rate. This provided the required 'lubri- cation' of the molecule such that reproducible isotherms could be obtained (Fig. 5). Tetraethylporphyrin compounds.These compounds (the free- base, 4, the zinc complex, 5, and the silver complex, 6) all 25.0 50.0 75.0 100.0 area/A2molecule-' Fig. 5 Pressure-area (PA) isotherms for a floating Langmuir mono- layer of compound 2 (see Fig. 1) on a subphase of cadmium dichloride (2.5 x lop4mol dm3) utilising a chloroform-mesitylene mixture as the spreading phase produced PA isotherms of high quality on the aqueous sub- phase. The isotherms displayed the characteristic features of 'solid', 'liquid' and 'gaseous' phases, and became reproducible after only two compression-decompression cycles; the zinc compound did not exhibit behaviour similar to that of its hexamethyl counterpart (2). Angles subtended to the normal to the subphase of 18-22' were exhibited in all cases. The collapse pressure for all three compounds occurred at 45 mN dm3 mol-l. Deposition of Langmuir-Blodgett multilayers Hexamethylporphyrin compounds. The zinc (2) and silver hexamethylporphyrins (3)produced stable floating monolayers at surface pressures of 30-40 mN dm3 mol-' on subphases of aqueous cadmium chloride and pure water.The free base derivative (1) also demonstrated stability but at the lower surface pressure of 15 mN dm3 mol- '. Molecular area us. time plots indicate that there was negligible area loss after a time period of approximately 10 min in all three cases. Once stability had been achieved, multilayer formation was attempted using substrates of hydrophobic glass and quartz. Monolayer and multilayer deposition on to these substrates did not take place.In all three cases, glass or quartz was passed through the monolayer at a rate of 5-10 mm min-', and turbulence could be seen at the three-phase contact point (air/film/subphase). The film appeared to attach to the substrate and then break away sending a shock wave through the monolayer. As a result little or no deposition took place. When the same experiment was carried out using hydrophilic quartz, a monolayer could be deposited by drawing the substrate up through the surface. High deposition ratios were recorded (90-100%) for the zinc and silver derivatives. The free base porphyrin also deposited a monolayer in the same way but only gave 65% coverage. Attempts to produce multi- layer films proved fruitless. The same turbulence effect took place similar to that previously described. Drainage times of between 1 and 2 h were employed for the monolayers but this had no effect in furthering the number of layers deposited.This lack of deposition was first believed to be a function of the substrate. However, the alteration of substrate prep- aration conditions and the use of hydrophilic and hydrophobic solids discounted this theory. The rigidity of the porphyrin monolayers was deemed to be the main cause of the lack of deposition because, even when a spreading mixture of chloro- form and mesitylene (4: 1) was used (such that the molecules could flow to a greater extent due to the lubricating effect of the mesitylene) no multilayer deposition took place.J. Muter. Chem., 1996, 6(3), 323-329 327 Tetraethylporphyrin compounds. The tetraethylporphyrin compounds all produced stable floating monolayers, from which multilayer films could be produced, using monolayers on the subphase surfaces of water or aqueous cadmium chlonde The optimum dipping pressures were determined from PA isotherms and the area-time plots For the zinc, free base and silver tetraethylporphyrins these were found to be 40, 35 and 30 mN dm3 mol-' respectively Multilayers were then deposited on to hydrophobic glass, quartz and silicon sub- strates at dipping rates of 5-10 mm m1n-l Although the extent of deposition was slightly greater on the up-stroke than on the down-stroke in the case of the metalloporphyrins (5,6), both gave good-quality Y-type films However, in contrast, the free base gave a mode of deposition intermediate between Y-type and X-type The down-stroke gave an average of 61% deposition whereas the upstroke gave 97% deposition This discrepancy between the deposition ratios may be explained by the poor flow qualities of the Langmuir mono- layer of compound 4 It was noticed that a slightly inferior film was obtained for the substrate surface adjacent to the fixed barrier A visually perfect film was obtained on the surface facing the movable barrier This phenomenon was further investigated by a simple experiment Fine talcum powder was sprinkled over the surface of a condensed porphyrin monolayer A glass substrate was then passed through the surface of the subphase It was observed that the powder moved to the substrate surface nearest the movable barner with great ease The flow of powder to the reverse side of the substrate appeared to be hindered This was very noticeable in the small area between the substrate edge and the outer perimeter fixed barrier This observation demon- strated the need for slow speeds for the deposition of this compound, so that recovery of uniformity within the floating Langmuir monolayer can take place A similar experiment was carried out using stearic acid In this case the powder flowed freely around the edges of the substrate giving an even distribution across the surface during deposition This experiment demonstrated how the shape of a molecule can affect the rheology of a floating monolayer The stearic acid molecules are long and slender and therefore move freely under compression However, the porphyrins are bulky molecules and therefore would not be expected to flow so efficiently It is clear that the hydrophobic edge in the tetraethylporphy- rin free base (4) is not quite large enough to give high quality multilayers by Y-type deposition, although reasonable quality multilayers are obtained for the zinc and silver complexes of 4 The hydrophobic edge in the hexamethyl compounds (1-3) is of inadequate size for any type of multilayer deposition on a solid substrate Characterisationof Langmuir-Blodgett multilayers The satisfactory quality" of the LB films obtained by multi- layer deposition on hydrophobic glass was confirmed by the linearity of the plot of absorbance against number of layers The principal optical absorption band (the Soret band) was used to obtain these results In accord with the work of Luk," the Soret band was broadened by solid-state interactions from the 'full width at half height' value obtained from solution spectra A further change, characteristic of solid state interactions, is the red shift of the bands in the 500-600 nm region If diffraction maxima occur in the LAXRD of LB films, then it is possible to obtain a value for the d spacing because of the ordering perpendicular to the surface of the substrate In the case of films that deposit in the Y-type mode, the d spacing will provide an estimate of the thickness of the bilayer If the 328 J Mater Chem, 1996,6(3), 323-329 4.001 I 3.504 I 1.50 2e/degrees Fig.6 LAXRD plot for a Langmuir-Blodgett multilayer (60 layers) of compound 5 (see Fig 1) on a fresh, chemically etched silicon wafer ( Plessey-Caswell), using a Phillips Diffractometer PW 1700 (Rolls Royce, Bristol) number of layers is known then the thickness of the film can be calculated The diffraction data from the tetraethylporphyrin free base (4) was very noisy, although some evidence of long- range order could be seen However, a multilayer of 60 layers of the zinc compound (5) gave good first and second prder peaks (Fig 6), corresponding to a bilayer spacing of 34 A and a tilt-angle of 31" to the normal to the substrate Conclusions We have shown that the R1 =R2=R3=R4 =methyl compound (1) and its zinc and silver complexes (2 and 3 respectively) do not have a sufficiently large hydrophobic edge to permit the formation of Langmuir-Blodgett multilayers due to the poor flow properties of the Langmuir monolayers These compounds (1-3) have a hydrophobic edge smaller than that in the naturally derived analogue mesoporphyrin IX dimethyl ester The R1=R2=R3=R4= ethyl compound (4) and its zinc and silver complexes (5 and 6 respectively), all of which have a hydrophobic edge slightly larger than mesoporphyrin IX dimethyl ester, give Langmuir-Blodgett multilayers by Y-type deposition (for 5 and 6) or by a mode of deposition intermedi- ate between Y-type and X-type for the free base (4) The Langmuir monolayer of 4 has been shown to exhibit inferior flow properties when compared with compounds 5 and 6 In common with the work of Tredg~ld,'~ we have found that the use of metalloporphyrins (of Zn and Ag) improves the prospect for Y-type deposition Tredgold's work also suggests that further improvements in the quality of multilayers would be obtained by utilising the diol analogues of 5 and 6 [say, 5 (diol) and 6 (diol)] The use of 5 and 6 or 5 (diol) and 6 (diol) would yield a higher density of packing of porphyrin moieties within a layer, bilayers that were more compact and a reduced repeat-spacing between bilayers than the heavily substituted l3systems currently being used by many workers If a Langmuir-Blodgett multilayer of a pure free base porphyrin is required, then recourse must be taken to the R1 = R4=propyl, R2 =R3=ethyl analogue (see Fig 1) reported else- where,6 if a high packing density of porphyrin moieties is required References 1 R Jones, R H Tredgold and P Hodge, Thrn Solid Films, 1983, 99,25 2 R H Tredgold, A J Vickers, A Hoorfar, P Hodge and E Khoshdel, J Phys D Appl Phys ,1985,18,1139 3 R.H. 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Photochem. Photobiol. A, 1995,87, 115. 14 G. P. Arsenhault, E. Bullock and S. F. MacDonald, J. Am. Chem. SOC.,1960,82,4384. 15 G.D. Dorraigh, J. R. Miller and F. M. Huennekens, J. Am. Chem. Soc., 1959,73,4315. 16 A. H. Jackson, G. W. Kenner and K. M. Smith, Tetrahedron, 1965, 21,2913. 17 R. H. Tredgold, Rep. Prog. Phys., 1987,50, 1609. 18 S. Baker, M. C. Petty, G. G. Roberts and M. V. Twigg, Thin Solid Films, 1983,99, 53. 19 S. Y. Luk, F. R. Mayers and J. 0.Williams, Thin Solid Films, 1988, 157, 69. Paper 5/05612H; Received 23rd August 1995 J. Mater. Chem., 1996, 6(3), 323-329 329