Interfacial and film-formation behaviour of photoactive ruii-bipyridyl-based metallosurfactants and a rare example of a monolayer-based logic-gate approach

Article abstract of DOI:10.1002/cplu.201200207

Three derivatives of tris(bipyridyl)-ruthenium(II) complexes with different alkyl-chain lengths (nC18H37 (1), nC14H29 (2) and nC10H21 (3)) were synthesised. All these complexes behaved as an amphiphile and their surface properties were studied at the air-

Full text of DOI:10.1002/cplu.201200207

DOI: 10.1002/cplu.201200207
Interfacial and Film-Formation Behaviour of Photoactive
RuII–Bipyridyl-Based Metallosurfactants and a Rare
Example of a Monolayer-Based Logic-Gate Approach
Prasenjit Mahato,[a] Sukdeb Saha,[a] Sipra Choudhury,*[b] and Amitava Das*[a]
Three derivatives of tris(bipyridyl)–ruthenium(II) complexes
with different alkyl-chain lengths (nC18H37 (1), nC14H29 (2) and
nC10H21 (3)) were synthesised. All these complexes behaved as
an amphiphile and their surface properties were studied at the
air–water interface by measuring surface pressure–area (P–A)
isotherms. The surface morphology of the resulting films at the
air–water interface was also studied by using Brewster angle
microscopy. Mean molecular areas of these complexes were
measured from the P–A isotherms, which were approximately
200 ꢀ2, thereby indicating a parallel arrangement of the Ru–bi-
pyridyl moiety of the complexes. Mono- and multilayer Lang-
muir–Blodgett (LB) films were formed on different solid surfa-
ces with transfer ratios close to one. Similarities in the absorp-
tion and fluorescence spectra of these amphiphiles in solution
as well as in LB films deposited on a quartz surface confirmed
the successful transfer of these films onto the substrates. The
latter provided information about the arrangements of metal-
losurfactant molecules within the LB films. The two-dimension-
al concentrations of these films were calculated from the Lam-
bert–Beer law as well as from the P–A isotherm, which con-
firmed regular and reproducible transfer of the complex mono-
layers from the air–water interface onto the quartz surface. The
surface morphology of these films on various substrates was
characterised by atomic force microscopy. Furthermore, by oxi-
dising the monolayer of complex 3, a one-input sequential
logic gate was constructed.
Introduction
The development of thin films and surface engineering are in-
creasingly being acknowledged as major research interests for
the generation of new molecular electronic and photonic ma-
terials.[1] The fundamental understanding of molecular interac-
tions and orientations in such films or surfaces are essential for
developing device-quality mono- and multilayers.[2] Generally
followed procedures for fabrication of thin-film devices are
vacuum deposition,[3] spin coating,[4] self-assembly[5,6a] and
Langmuir–Blodgett (LB) techniques.[6,7] Among these tech-
niques, the generation of mono- or multilayers by using the LB
film technique has a special significance for developing ultra-
thin films of appropriately functionalised molecules with con-
trolled thickness and well-defined molecular orientation,[8] as
these have potential applications as biosensors,[9] gas sen-
sors[10] and for other high-end technological purposes.[11] The
LB technique has been effectively used to construct organised
systems for efficient energy and electron transfer.[12] Metal-con-
taining soft materials like metallopolymers,[13] metallomeso-
gens[14] and metallosurfactants[15] take advantage of the coop-
erativity between transition metals and organic scaffolds to
build up organised supramolecular architectures with unique
geometric, redox and magnetic properties. For this, such
metal-containing soft materials are to be organised in highly
ordered assemblies and transferred onto surfaces. Among the
metallosurfactants studied in LB films, ruthenium(II)–bipyridyl-
based amphiphilic complexes have received considerable at-
tention,[16] owing to their rich photophysical and electrochemi-
cal properties,[17] diversity of coordination forms,[18] high ther-
mal, chemical and photophysical stability as well as their solu-
bility in common solvents. LB films of different ruthenium com-
plexes have successfully been used in making nonlinear optical
(NLO) materials,[19] light-emitting diodes (LED)[20] and high-per-
formance sensors.[21]
To date, enormous efforts have been devoted to the con-
struction of a molecular logic gate, which is an important
device for information processing and computation at the mo-
lecular level.[22] At present there are several molecules reported
that can perform functions of various basic and complex logic
operations, but in most cases the performance has been limit-
ed to the solution phase,[22] which remains far from practical
for applications in information technology.
For constructing solid-state molecular logic devices, the im-
mobilisation of molecules onto solid substrates is essential.
This has led to an enormous opportunity for the development
of a hybrid system with a molecule fabricated on a solid sur-
face that is capable of executing Boolean operations under the
influence of certain external stimulation(s).[23] The Langmuir–
[a] P. Mahato, S. Saha, Dr. A. Das
Analytical Discipline and Centralized Instrument Facility
Central Salt and Marine Chemicals Research Institute (CSIR)
Bhavnagar, 364002, Gujarat (India)
Fax: (+91)278-2656762
E-mail: amitava@csmcri.org
[b] Dr. S. Choudhury
Chemistry Division, Bhabha Atomic Research Center
Trombay, Mumbai, 400085 (India)
E-mail: sipra@barc.gov.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201200207.
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Interfacial and Film-Formation Behaviour
Blodgett (LB) technique is one of most powerful techniques for
building up nanoassemblies onto solid supports.
The chain length on the n-alkyl substituent was varied system-
atically in L3 or L4 or L5 and thus in complex 1, 2 and 3 from
C18 to C10, respectively, with an aim to tune the amphiphilicity
of these photoactive metallosurfactants. Our earlier study re-
veals that 1 tends to form vesicles in a polar medium and re-
verse vesicles in an apolar medium.[24] The aggregation behav-
iour and Langmuir monolayer formation of these complexes
(1, 2 and 3) at the air–water interface were studied by surface
pressure versus area (P versus A) isotherms and Brewster
angle microscopy (BAM). The metallosurfactant was
initially dissolved in a volatile organic solvent like
chloroform. Chloroform is immiscible with water and
spreads on water surfaces. Thus, a solution of the re-
spective RuII–polypyridyl complex in chloroform was
allowed to spread on the water surface in a Langmuir
trough. As the chloroform evaporates, the amphi-
philes form a monolayer over the water surface in
which the polar RuII centre, which is hydrophilic, re-
mains attached to the water surface, whereas the
lipophilic tail points upwards. As the barriers of the
trough were compressed, the surface tension (g) at
the air–water interface in the presence of the amphi-
philic species was expected to decrease relative to
that of the bare air–water interface (g0 =72 mNmꢀ1
at 258C), which results in an increase in P (=g0ꢀg).
Compression isotherms are plots of surface pressure
More recently, we have reported that a [Ru(bpy)3]2+-based
(bpy=2,2’-bipyridyl derivative) symmetrical amphiphilic com-
plex (1) containing three C18H37 chains at equilateral positions
generates interesting structures that differ in nature, size and
shape depending on the polarity of the solvent environ-
ment.[24] In the present study, we report the synthesis of two
new analogous complexes (2 and 3 in Scheme 1) having vary-
(P, in mNmꢀ1) versus mean molecular area (A, ex-
pressed in ꢀ2) and provide fundamental information
concerning the two-dimensional molecular organisa-
tion of the monolayer at the air–water interface, col-
lapse pressures (Pc), limiting areas per molecule
Scheme 1. Synthetic methodology that was adopted for the synthesis of complexes 1, 2
and 3. 1) nBuLi, diisopropylamine, dry THF, 08C, 24 h; 2) POCl3/pyridine, 08C, 8 h; 3) HCl,
2008C, 5 h; 4) CnH2n+1, DMF, K2CO3, KI, 908C, 48 h; 5) RuCl3, ethanol/dioxan (1:1 v/v),
858C, 24 h.
ing alkyl-chain lengths. We have also studied various aggregat-
ed structures and films that were developed from complexes
1, 2 and 3 under certain conditions. These metallosurfactants
were found to form a stable Langmuir monolayer at the air–
water interface. The morphology of the domains formed and
the real-time changes of the film during compression as well
as the nature of the film after collapse of the monolayer at the
air–water interface were evaluated using Brewster angle mi-
croscopy (BAM). The morphologies of these monolayer films
on different solid substrates were studied through atomic
force microscopy (AFM). Further spectral responses on oxida-
tion of the monolayer of 3 on a glass plate could be used for
demonstrating a one-input sequential logic gate. To the best
of our knowledge, this is the first study on a metallosurfactant-
based LB monolayer acting as a logic device.
(Alim) and the average area of the molecule at the collapse of
the monolayer (Ac). Simultaneously, Brewster angle microscopy
evaluates film homogeneity, domain and agglomerate forma-
tion upon passing vertically polarised light through media pos-
sessing different refractive indexes.
Figure 1 shows the P–A isotherms for complexes 1, 2 and 3
and suggests that the three complexes are surface active. The
P–A isotherm for metallosurfactant 1 reveals that interactions
Results and Discussion
Figure 1. Langmuir–Blodgett isotherms obtained for complexes 1, 2 and 3.
Amphiphilic properties
Tris(2,2’-bipyridyl)–ruthenium(II) complexes are known to be
soluble in a polar solvent like water when used as the dichlor-
ide salt. This led us to synthesise [Ru(L)3]Cl2 (complexes 1, 2
and 3; L is L3 or L4 or L5; see Scheme 1) with the cationic RuII
centre as the hydrophilic head group and the n-alkyl chain
substituted in Lx (x=3, 4 or 5) as the hydrophobic tail group.
between the molecules at the air–water interface start at
200 ꢀ2. Furthermore, two distinctly different slopes are ob-
served in the isotherm. For the initial region, until surface pres-
sure reaches approximately 15 mNmꢀ1, the increase in surface
pressure (P) with the change in mean moleculer area (A) is
not sharp. This state is called the liquid-expanded state. How-
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S. Choudhury, A. Das et al.
ever, after the surface pressure reaches approximately
20 mNmꢀ1, a steep rise in the P–A isotherm is observed
(Figure 1), which could be ascribed to the formation of the
liquid-compressed state.[25] At this situation, P rises steeply
without remarkable change in A [ꢀ2]. Figure 1 further reveals
that the isotherm of complex 1 shows a sudden drop in sur-
face pressure after 51 mNmꢀ1 with an average area per mole-
cule at a collapse of 125 ꢀ2 (Ac). This is termed the collapse
area and could be determined by extrapolating the steepest
slope of the P versus A curve (before the collapse) to zero
pressure (i.e., P=0).[11] This collapse shows the signature of
a constant pressure collapse.[26]
beam at the Brewster angle.[28] When the angle of incidence of
this beam is at the Brewster angle, the reflected intensity is
a minimum for an air–water interface, which has a transition
region where the refractive index changes smoothly from one
value to another. The reflected intensity at this angle is strong-
ly dependent on the interfacial properties, mostly when a mon-
olayer is involved in the interface. By using BAM, the homoge-
neity of the film, and agglomerates and domains in films at
the air–water interface could be recognised.[29] Reflected light
is a function of the orientation of the molecules in monolayer
domains. BAM images were recorded at different surface pres-
sures during the compression experiments for assessing the
changes and domain formation at the air–water interface.
Selected BAM images of the air–water interface during com-
pression experiments after spreading solutions of complexes 1,
2 and 3 in chloroform on pure water are shown in Figure 2. Ini-
tially the blank water surface was focused in such a way that
almost no reflection of p-polarised light came from the bare
air–water interface and appeared black. After spreading any of
these three complexes on the air–water interface, non-uniform
domains with various morphology were formed, which were
brighter than the black water background. Initially at zero sur-
face pressure (P=0 mNmꢀ1) for complex 1, the surface
showed brighter domains of irregular shapes that indicated
scattered Langmuir monolayer formation on water (dark back-
ground; Figure 2). Initially with the increase in surface pressure,
movement of the domains was found to increase and it was
difficult to focus all the domains available in the CCD camera
at a time; however, a more uniform monolayer film was
formed at Pꢁ1.5 mNmꢀ1 as brighter domains came closer to
each other. With further compression, at approximately P
ꢁ6 mNmꢀ1, a homogeneous surface with random multiple
ring-shaped bright circles were observed.[30] These rings are
called Newton circles and are essentially multilayer granules
that are formed by ejection of matter owing to the localised
oscillation and reflect the thermodynamic instability of the film
(Figure 2a).[30] With further compression, the movement of
these bright Newton circles became faster, which caused diffi-
culties in focusing the camera on all the circles available at
one time. With further lateral compression, a greater number
of Newton circles over a smaller area was evident. For Pꢂ
20 mNmꢀ1, the density of these rings was even higher and
they combined to form a brighter film (Figure 2a). After further
The results in Figure 1 suggest that individual molecules of
complex 2 start interacting at the air–water interface at an
average molecular area of 196 ꢀ2 (Table 1). The nature of the
Table 1. Mean molecular area and collapse pressures obtained from
Langmuir–Blodgett (LB) isotherms for complexes 1, 2 and 3.
Complex
Mean molecular area [ꢀ2]
Collapse pressure [mNmꢀ1
]
1
2
3
200
196
193
51
48
48
isotherm of this complex is similar to that of 1, the only differ-
ence is the lower slope of the P–A isotherm curve. The iso-
therm shows a high collapse pressure of 48 mNmꢀ1 with a con-
stant pressure collapse similar to that of complex 1 and with
a mean area at a collapse of 106 ꢀ2 (Ac). Higher collapse pres-
sures for complexes 1 and 2 indicate the formation of a con-
densed and stable monolayer at the air–water interface. For
complex 3, a mean molecular area of 193 ꢀ2 is evident
(Figure 1), while the surface pressure is found to increase
sharply until an inflection point is reached at around
32 mNmꢀ1. This indicates a mesophasic change from the
liquid-expanded state to the liquid-compressed state and the
monolayer collapses at 48 mNmꢀ1. Nearly analogous curves
were obtained when the compression–expansion cycle was re-
peated. No irreversible change occurred during the second
compression process. All three complexes showed constant-
pressure collapse and followed the Ries mechanism of folding,
bending and breaking into multilayers.[27] The large mean mo-
lecular areas (ca. 200 ꢀ2 per molecule) of these three metallo-
surfactants suggest a parallel orientation of these complexes
with respect to the air–water interface with alkyl chains point-
ing upward. These alkyl chains serve as the hydrophobic part
and the central tris(2,2’-bpy)–ruthenium(II) moiety as the hy-
drophilic part of the amphiphilic species in the resulting
film.[8d]
compression with Pꢂ30 mNmꢀ1
, the homogeneous film
became more compact and the brightness of the film was en-
hanced (Figure 2a). However, after the collapse pressure, the
monolayer film collapsed and a rough and non-homogeneous
film was observed (Figure 2a).
For complex 2, similar changes at the air–water interface
were observed (Figure 2b) as lateral compression was applied.
At Pꢁ1.5 mNmꢀ1
, almost a continuous monolayer was
formed and further compression led to the formation of spora-
dic Newton rings. However, the size of these Newton rings was
larger than those of complex 1. At Pꢁ20 mNmꢀ1, the abun-
dance of the rings was enhanced and they were more densely
Brewster angle microscopic studies
Brewster angle microscopy (BAM) is a technique that allows
the in situ study of thin films at the gas–liquid or solid–gas in-
terfaces. BAM is based on the study of the reflected light
coming from an interface illuminated by a p-polarised laser
packed (Figure 2b). At even higher surface pressure
(
ꢁ50 mNmꢀ1), a rough film appeared with dark (monolayer)
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Interfacial and Film-Formation Behaviour
[Ru(L)3(H2O)Cl]+ and [Ru(L)3(H2O)2]2+, at the air/water interface.
Each of these species should exhibit distinct dipole moments
that increase molecular motion. Thus, the film stabilises itself
by the formation of domains that lead to a decrease in mobili-
ty.[32] Complex 3 with the shortest chain length of the three
complexes was also studied. Figure 2c reveals that this com-
plex forms almost a uniform monolayer throughout the com-
pression with very few Newton rings at higher pressure. At
higher lateral compression, brighter film formation is evident
(Figure 2c), which signifies a more compact film formation
with increase in surface pressure.
Atomic force microscopic studies
Monolayer films of three amphiphilic complexes (1, 2 and 3)
were deposited on a hydrophilic surface (glass) at different sur-
face pressures. The morphology of these films was analysed by
atomic force microscopy (AFM). Figure 3A and B show the
Figure 3. AFM images of the transferred film of complex 1 that was deposit-
ed at a surface pressure of A) 10 (one monolayer) and B) 30 mNmꢀ1 (one
monolayer) on glass; AFM images of the transferred film of complex 2 that
was deposited at a surface pressure of C) 10 and D) 30 mNmꢀ1 (one mono-
layer) on glass; E) AFM images of the transferred film of complex 3 deposit-
ed at a surface pressure of 30 mNmꢀ1 (one monolayer) on glass.
Figure 2. Selected Brewster angle micrographs for complexes A) 1, B) 2 and
C) 3 obtaind at different surface pressures.
and bright (multilayer) patches, which were attributed to the
collapse of the monolayer and the formation of a multilayer.
Thus, on further increase in lateral compression with P>
20 mNmꢀ1, the interspatial distance between the resulting do-
mains was found to be narrower and eventually led to a col-
lapse of the monolayer at P=50 mNmꢀ1 with the resulting
change in morphologies. According to Vollhardt and Wiede-
mann,[31] the growth of these structures occurs due to crowd-
ing of the domains from supersaturation on the surface of the
surrounding phase. Again, it can also be argued that [Ru(L)3]Cl2
(L is L1, L2 or L3) may remain in a dynamic equilibrium with its
monohalogenated and completely solvated analogues, namely,
AFM images of the monolayer of complex 1 deposited onto
a glass surface at surface pressures of 10 and 30 mNmꢀ1, re-
spectively. Formation of a more compact monolayer film was
observed at higher surface pressure (Pꢁ30 mNmꢀ1). This is in
accordance with the P–A isotherm of complex 1 (Figure 1);
the P–A isotherm of complex 1 is steeper at P=30 mNmꢀ1
than at P=10 mNmꢀ1. Thus, results obtained from each of
these techniques are in good agreement. Figure 3C and D
show the AFM images of monolayers of complex 2 transferred
at 10 and 30 mNmꢀ1 surface pressures, respectively. The AFM
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S. Choudhury, A. Das et al.
image of the film, transferred at P=10 mNmꢀ1, shows mono-
layer film formation with many bright spots (Figure 3C); where-
as the film transferred at P=30 mNmꢀ1 appeared more com-
pact and contains a greater number of bright spots (Fig-
ure 3D). Figure 3E shows the AFM image of a monolayer of
complex 3 transferred onto the hydrophilic glass surface at
P=30 mNmꢀ1. A smooth homogeneous surface indicates the
formation of a compact homogeneous monolayer. These AFM
results of the transferred monolayer on the solid surfaces can
be correlated well with the BAM images (Figure 2) at the corre-
sponding surface pressure, which represents the morphology
of the monolayer formed at the air–water interface. BAM
images (Figure 2) reveal that bright Newton rings were formed
for complexes 1 and 2 at lower surface pressure (Figure 2A
and B). The appearance of these Newton rings were found to
increase with the increase in the surface pressure (P) and after
a certain pressure the Newton rings were so close that they
formed a homogeneous film (Figure 2A and B). The AFM
Figure 4. A) Electronic absorption spectra recorded with 3 (bottom), 5, 7, 9
and 11 (top) layers of deposition of complex 1 on a quartz substrate. B) Plot
of absorbances at 299, 366 and 486 nm for varying number of deposition
layers on quartz surfaces.
Table 2. Absorption maxima for the 1MLCT band for complexes 1, 2 and
3 as a solution in chloroform and as a LB film.
1
Complex
Absorbance maxima [nm] of MLCT band
Solution
Film
image of complex 1 shows a homogeneous film at 30 mNmꢀ1
,
1
2
3
474
471
470
495
487
487
which is also observed in the BAM images. The homogeneous
nature of the film could be attributed to the narrower interspa-
tial distances between the greater number of Newton rings
formed at the high surface pressure. The AFM image of the
transferred film of complex 1 reveals a greater number of
bright spots for the film transferred at a surface pressure of
30 mNmꢀ1 than that transferred at 10 mNmꢀ1, which agrees
well with the morphology observed in the BAM images for the
corresponding surface pressures (Figure 2). A similar increase
in the density of bright spots in the AFM image of the trans-
ferred film of complex 2 with an increase in surface pressure is
also evident in Figure 3 and is in good agreement with the
greater number of Newton rings observed in the BAM images
at 30 mNmꢀ1 than that at 10 mNmꢀ1 (Figure 2). At the surface
pressure of 30 mNmꢀ1, the AFM image of the transferred film
of 3 shows homogeneous compact morphology, which is also
evident in the corresponding BAM image at a comparable sur-
face pressure (Figure 2).
MLCT band in the LB films, relative to that in solution, is pre-
sumably as a result of the change of the microenvironment in
film compared to that in solution.
1
A similar redshift for the MLCT band was also observed for
complexes 2 and 3 (Table 2). This redshift can be ascribed to
the aggregate nature of the molecules in the solid LB film.[34]
Alternative explanations include the change in the refractive
index going from solution to the solid material.[35a] Such a red-
shift in the electronic spectrum was reported earlier for RuII–
polypyridyl complexes[36] as well as other organic molecules.[36]
The absorption spectra of the quartz plates with deposition
of different multilayers (3 to 11) of complex 1 were recorded
(Figure 4A) and the respective absorbance for each multilayer
at 299 (ligand-centred p–p* band), 366 (intraligand charge-
transfer band) and 486 nm (1MLCT band) was plotted (Fig-
ure 4B). The data in Figure 4B reveals that the absorbances at
these three wavelengths follow a linear relationship with the
layer number. This linear dependence indicates that in all cases
the Langmuir monolayer was transferred in a regular and re-
producible manner with each dipping cycle.[37]
The LB films of these three complexes (complexes 1, 2 and
3), deposited on quartz surfaces, were further studied by ab-
sorption and fluorescence spectroscopy. Multilayer Y-type LB
films of 1, 2 and 3 were transferred onto hydrophilic quartz
plates to record the electronic absorption spectra. Electronic
spectra of a solution of 1 in chloroform showed three peaks at
297 (6.5ꢂ104 mꢀ1 cmꢀ1), 366 (6.8ꢂ104 mꢀ1 cmꢀ1) and 474 nm
(3.88ꢂ104 mꢀ1 cmꢀ1; Figure S7 in the Supporting Information).
The high-energy band at 297 nm is predominantly as a result
of a ligand-centred p–p* transition, whereas the absorption at
366 nm is predominantly a result of a intraligand charge-trans-
fer transition.[33] The absorption band at 474 nm primarily
arises as a result of a metal-to-ligand (Rudp!Lp*) charge-trans-
fer transition (1MLCT).[33] In the case of a LB film, the respective
absorption bands appeared at 299, 366 and 495 nm (Figure 4),
respectively. This indicates that lmax for the high-energy transi-
tion bands remained almost unchanged, whereas an apprecia-
Similar phenomena were also observed for complexes 2 and
3 (Figure S8 and S9 in the Supporting Information). From the
linear plot of absorbance versus number of layers of these
metallosurfactants, the surface concentration of the LB film
can be derived by using a modified expression [Eq. (1)] of the
Lambert–Beer law for two-dimensional concentration:[37,38]
G ¼ D=1000e
ð1Þ
in which G, D and e represent the surface concentration
(molcmꢀ2), absorbance per layer (AU/layer) and molar extinc-
tion coefficient (dm3 molꢀ1 cmꢀ1), respectively, at a particular
wavelength. D can be calculated from the slope of a linear plot
1
ble redshift (ꢁ21 nm) was observed for the MLCT band. Fur-
thermore, this band was found to be broader for the LB film
than for that in solution (Figure S7). The broadening of the
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Interfacial and Film-Formation Behaviour
of absorbance versus the number of layers for a given com-
plex. The surface concentration at the air–water interface can
also be obtained from the limiting surface area (A0 in ꢀ2),
which is determined by the extrapolation of the line to zero
surface pressure in the P–A isotherm following Equation (2).[38]
Table 4. Emission spectral data of complexes 1, 2 and 3 in solution
(chloroform) and the nine-monolayer LB film of 1 (deposited on quartz)
using lex =490 nm as the excitation wavelength.
Complex
Emission maxima
in solution [nm]
Emission maxima
of LB film [nm]
1
2
3
680
680
678
659
666
665
G ¼ 1016=A0N
ð2Þ
in which N is Avogadro’s number. The multilayer LB film-form-
ing properties of the complexes are summarised in Table 3.
deposition, electrodeposition and so forth. By using the LB
film-deposition technique, we could easily form a monolayer
of complex 3 on a glass surface that could be used for execut-
ing a chemical-input-based solid-state logic-gate operation.
The absorption spectrum of a monolayer of complex 3 on
a glass substrate was recorded. This showed an absorption
maximum at 487 nm. Since the glass surface is supposed to be
polar, polar RuII centres of the RuII–polypyridyl-based surfactant
remained in contact with the glass surface, whereas the hydro-
phobic tails were pointing upwards (Figure 5C). On treatment
with an aqueous solution of Ce4+ (as ammonium ceric nitrate),
Ce4+ ions could diffuse inside the monolayer and oxidise the
RuII centre of the RuII–polypyridyl complex to RuIII. This was ex-
Table 3. Multilayer LB film-forming properties for complexes 1, 2 and 3.
Complex
Absorbance [layer]
Surface concentration [molcmꢀ2
]
calculated by the
Lambert–Beer law
calculated from A0
1
2
3
8.9ꢂ10ꢀ3
8.0ꢂ10ꢀ3
8.74ꢂ10ꢀ3
1.31ꢂ10ꢀ10
1.26ꢂ10ꢀ10
1.3ꢂ10ꢀ10
0.83ꢂ10ꢀ10
0.79ꢂ10ꢀ10
0.8ꢂ10ꢀ10
The surface concentrations in a LB layer as well as at the air–
water interface calculated following both methods [Eqs. (1)
and (2)] are consistent with each other. This further validates
that the monolayers of the LB film of the respective complex
are regularly transferred from the air–water interface onto the
quartz substrates.
1
pected to bleach the RuII[p]!bpy[p*]-based MLCT transition
and lead to an overall decrease in the absorbance at approxi-
mately 487 nm. Partial oxidation of the RuII centres present in
the monolayer led to a partial bleaching of this absorption
band (Figure 5A).
Emission spectral data of the three complexes in solution
and as a LB film are summarised in Table 4. In general, the
emission spectra of solutions of the complexes in chloroform
and as a LB film show a broad emission band in the region
659–680 nm (Figure S10 in the Supporting Information). This
emission band is attributed to the 3MLCT-based excited
state.[39] The emission maxima of the LB film on the quartz sur-
face was found to be blueshifted relative to those in solution
and are attributed to the phenomenon of luminescence rigido-
Figure 5A reveals only partial bleaching of the band at
487 nm on treatment with Ce4+. This suggests that only a par-
tial oxidation of the RuII-based monolayer by Ce4+ could be
achieved, possibly as a result of effective shielding of the
[Ru(bpy)3]2+ moiety by hydrophobic long chains. The presence
or absence of an arbitrary chemical input was defined as a logi-
cal 1 or 0, respectively. The output was dependent on the
formal oxidation state of the system, which was monitored by
UV-visible spectroscopy. The threshold value was set at 5.0ꢂ
3
chromism.[40] In the fluid state, the MLCT state of the chromo-
phore is stabilised by reorganisation of the solvent
dipoles around the chromophore dipole, whereas in
a film no such solvent stabilisation is possible. The
3
net result is the destabilisation of the MLCT excited
state in the rigid environment of the film and a shift
of the emission band to higher energies.
Logic-gate behaviour of a monolayer based on
complex 3
Recenly, van der Boom and co-workers have report-
ed that a silica surface functionalised with an OsII–
polypyridyl complex could demonstrate the function
of a one-input sequential logic gate.[23a] However, the
LB film-deposition technique provides uniform ar-
Figure 5. A) Absorption spectra of a Ru2+/Ru3+-based LB monolayer of complex 3
(Ru2+ =red line, Ru3+ =blue line, baseline=black line). The absorption intensities at
487 nm were used as an output (0 or 1). B) Truth table with one chemical input (Ce4+
rangements of molecules with controlled thickness
and well-defined molecular orientation that could
not be achieved in cast films, and other thin-film
techniques like spin coating, dip coating, vacuum
)
with different current states (Ru2+/Ru3+). C) Sequential logic circuit based on the optical
output with four combinations of one input.
ChemPlusChem 2012, 77, 1096 – 1105
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1101

S. Choudhury, A. Das et al.
10ꢀ2; output values above and below this threshold value
were defined as 1 and 0, respectively (Figure 5A). Thus, a one-
input sequential system was designed with Ce4+ ions in an
aqueous solution as the input.[23a] The four possible combina-
tions were demonstrated with the same monolayer (Figure 5B
and C). In the absence of any Ce4+ ions, the monolayer was in
state 1 (Ru2+ complex existed), which was then changed to
state 0 (predominant Ru3+ complex; Figure 5B). Since the cur-
rent state is variable, the output becomes dependent on the
previous input of the logic gate. Thus, based on the optical re-
sponses of the monolayer of 3 a sequential logic circuit could
be constructed. Therefore, our results on molecular logic oper-
ations agreed well with the logic function demonstrated earlier
using a silica-surface-grafted OsII–polypyridyl complex.[23a] The
ease in developing LB films and thus the photoactive surface
for executing such sequential circuit operations has signifi-
cance in developing systems for practical applications.
quential logic-gate operation. These results further reveal that
a [Ru(bpy)3]-based monolayer in the presence of suitable
input(s) can speak the complex language of information tech-
nology.
Experimental Section
Materials and reagents
Analytical- and reagent-grade solvents and compounds were used
for studies unless mentioned otherwise and were used as received.
4,4’-Dimethyl-2,2’-bipyridyl, 4-methoxybenzaldehyde, 1-bromoocta-
decane, 1-bromotetradecane, 1-bromodecane, butyllithium, diiso-
propylamine and ruthenium trichloride were procured from
Sigma–Aldrich, whereas all other chemicals were purchased from
S.D. Fine Chemicals (India). Nanopure water with a resistivity of
17.5–18 MWcmꢀ1 was used for LB film studies. L1 and L2 were syn-
thesised following previously reported procedures.[41]
Conclusion
Here, we have reported the synthesis of a series of symmetrical
[Ru(bpy’)3]-based (bpy’=4-methyl-4’-(4-(alkoxystyryl)-2,2’-bipyr-
idine) metallosurfactants with three different chain lengths:
nC18H37 (1), nC14H29 (2) and nC10H21 (3). All three complexes
form a stable monolayer at the air–water interface, which is
evident from the surface pressure–area (P–A) isotherm meas-
urements as well as from Brewster angle microscopy. From the
P–A isotherm the average area per molecule was obtained. No
significant dependence of the average area per molecule with
variation in the alkyl-chain length of the metallosurfactants
was observed. This indicates that the hydrophobic long-chain
tails are pointed upwards from the [Ru(bpy’)3]2+ head group
with respect to the air–water interface. The change in slope in
the P–A isotherm of these complexes indicates the mesopha-
sic changes from the liquid-expanded state to the liquid-com-
pressed state during the compression of the monolayer. Again,
high collapse pressures of these isotherms indicate the forma-
tion of homogeneous and compact monolayer film formation
for the complexes. From Brewster angle microscopy, various
domains formed by these metallosurfactants at low surface
pressure as well as the formation of a compact monolayer
with gradual increase in surface pressure were observed. These
LB monolayers could be transferred successfully on the solid
surfaces like glass, mica and quartz by a vertical dipping
method at different surface pressures. The morphology of the
transferred films was characterised by AFM and correlated with
the surface morphology at the air–water interface studied by
BAM. Absorption and emission spectra of the transferred films
on the quartz surface were recorded, which support the uni-
form and regular multilayer formation on quartz surfaces. The
surface concentration of the transferred film as well as at the
air–water interface was calculated (Lambert–Beer law modified
for two-dimensional concentration [Eq. (1)] and limiting surface
area [Eq. (2)]) and both data agree with each other. More im-
portantly, we have shown that these amphiphilic RuII com-
plexes could be used for developing a LB film that is capable
of demonstrating the novel concept of optical one-input se-
Instrumentation
Microanalyses (C, H, N) were performed using a Perkin–Elmer 4100
elemental analyzer. IR spectra were recorded as KBr pellets using
a Perkin–Elmer GX 2000 spectrometer. UV-visible spectra of the
complexes in solution were obtained by using a Cary 500 scan UV-
Vis-NIR spectrometer. UV-visible spectra of the LB films were re-
corded on a Chemito UV 2600 spectrophotometer. Fluorescence
spectra were recorded with a HORIBA JOBIN YVON spectrophotom-
1
eter. H NMR spectra were recorded on a Bruker 500 MHz FT NMR
(Advance-DPX 500) spectrometer at room temperature (258C). Tet-
ramethylsilane (TMS) was used as an internal standard for all 1H
NMR spectroscopy studies. ESIMS measurements were carried out
on a Waters QTof-Micro instrument.
Compression isotherms and deposition and LB films
Pressure–area isotherm measurements were carried out by using
a computer-controlled KSV 5000 Langmuir double-barrier Teflon
trough. The surface pressure was measured with a platinum Wilhel-
my plate microbalance. The compression rate (the barrier speed)
used was 5 mmminꢀ1 at 258C. Any impurities from the surface of
the freshly poured aqueous subphase were removed by vacuum
using a suction pump, after the compression of the barriers.
Spreading solutions for complexes 1, 2 and 3 with known concen-
tration (0.9–1.0 mgmLꢀ1) were prepared in HPLC-grade chloroform.
A known quantity (typically 80–90 mL) of this solution was then al-
lowed to spread on the clean aqueous subphase with a Hamilton
microsyringe. The system was allowed to equilibrate for approxi-
mately 20 min before monolayer compression. At least three inde-
pendent measurements were carried out for each experiment to
ensure reproducibility of the measurements. By using the conven-
tional vertical dipping method, LB films were transferred onto sur-
faces like quartz and glass. The glass and quartz slides were kept
overnight in chromic acid, which was then rinsed thoroughly with
Milli-Q water and immersed in an ultrasonic bath of chloroform for
10 min for appropriate cleaning. Then these slides were finally
dried. The transfer ratios were 0.93 in the upward stroke and 0.78
in the downward stroke.
1102
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ChemPlusChem 2012, 77, 1096 – 1105

Interfacial and Film-Formation Behaviour
yield of the purified compound (L4; yield was calculated based on
the starting compounds) was found to be 65% (546 mg,
1.13 mmol). 1H NMR (500 MHz, CDCl3, TMS): d=8.61 (d, 3J(H,H)=
Brewster angle microscopy
The morphology of the Langmuir films at the air–water interface
was observed by a Brewster angle microscope. BAM images of the
monolayer were recorded using a nanofilm_ep3bam with a polar-
ised Nd:YAG laser (50 mW, 532.0 nm), and a CCD camera (768ꢂ572
pixel) was used for recording images. The compression rate was
2 mmminꢀ1. The field of view was 487ꢂ387 microns and the lateral
resolution was about 2 mm. The film was examined at different
stages of the compression process. The length scales of the
images were corrected for the angle of incidence of the incident
laser beam. Images presented are typically 300ꢂ300 mm2 in area.
The Brewster angle (ꢁ53.18) was maintained between the incident
p-polarised light of 532 nm and the bare air–water interface. At
this stage, a negligible amount of light was reflected from the air–
water interface towards the CCD camera, so the whole surface ap-
peared black. Upon spreading the amphiphilic material at the air–
water interface the refractive index of the interface was changed
and a little portion (10ꢀ6 times) of the incident light was reflected,
which was captured by the CCD camera.[42] The nature of the mon-
olayer formed was studied based on the reflected light from the
interface.
3
5.5 Hz, 1H; 6-bpy), 8.57 (d, J(H,H)=5 Hz, 1H; 6’-bpy), 8.48 (s, 1H;
3
3-bpy), 8.26 (s, 1H; 3’-bpy), 7.5 (d, J(H,H)=8.5 Hz, 2H; 3,5-phenyl),
7.41 (d, 3J(H,H)=16 Hz, 1H; vinyl), 7.34 (dd, 3J(H,H)1 =5.5 Hz,
3
3
3J(H,H)2 =2 Hz, 1H; 5-bpy), 7.16 (dd, J(H,H)1 =5 Hz, J(H,H)2 =1 Hz,
1H; 5’-bpy), 6.98 (d, 3J(H,H)=16 Hz, 1H; vinyl), 6.92 (d, 3J(H,H)=
9 Hz, 2H; 2,6-phenyl), 3.99 (t, 2H; OꢀCH2), 2.45 (s, 3H; bpyꢀCH3),
1.82–1.77 (m, 2H; longchain(ꢀCH2)), 1.46–1.43 (m, 2H; longchain(ꢀ
CH2)), 1.26 (b, 20H; (ꢀCH2)10), 0.88 ppm (t, 3H; longchain(ꢀCH3));
ESIMS (+ve mode): m/z (%): 485.48 (100) [M+H+]; elemental analy-
sis calcd (%) for C33H44N2O: C 81.77, H 9.15, N 5.78, found: C 81.7,
H 9.11, N 5.8.
Ligand L5: The synthesis and purification procedures adopted for
L5 were similar to those mentioned for L4 with a necessary change
in one of the reactants, namely, n-alkyl bromide. 1-Bromodecane
(0.32 mL, 1.736 mmol) was used for this reaction instead of 1-bro-
motetradecane. The isolated yield of compound L5 (yield was cal-
culated based on the starting compounds) was evaluated as 63%
(421 mg, 0.98 mmol). 1H NMR (500 MHz, CDCl3, TMS): d=8.61 (d,
3
3J(H,H)=5.5 Hz, 1H; 6-bpy), 8.57 (d, J(H,H)=5 Hz, 1H; 6’-bpy), 8.48
3
(s, 1H; 3-bpy), 8.26 (s, 1H; 3’-bpy), 7.5 (d, J(H,H)=8.5 Hz, 2H; 3,5-
3
3
Atomic force microscopy
phenyl), 7.41 (d, J(H,H)=16.5 Hz, 1H; vinyl), 7.35 (d, J(H,H)=5 Hz,
3
3
1H; 5-bpy), 7.16 (d, J(H,H)=4.5 Hz, 1H; 5’-bpy), 6.98 (d, J(H,H)=
16 Hz, 1H; vinyl), 6.92 (d, J(H,H)=8.5 Hz, 2H; 2,6-phenyl), 3.99 (t,
AFM studies were carried out under ambient conditions using
a scanning probe microscope NT-MDT (Ntegra Aura, Moscow) in
semicontact mode using a rectangular cantilever of Si3N4.
3
2H; OꢀCH2), 2.46 (s, 3H; bpyꢀCH3), 1.82–1.77 (m, 2H; longchainꢀ
CH2), 1.48–1.43 (m, 2H; longchainꢀCH2), 1.28 (b, 12H; (ꢀCH2)6),
0.88 ppm (t, 3H; longchainꢀCH3); ESIMS (+ve mode): m/z (%):
429.33 (100) [M+H+]; elemental analysis calcd (%) for C29H36N2O: C
81.27, H 8.47, N 6.54; found: C 81.2, H 8.43, N, 6.5.
Synthesis
Ligand L3: Ligand L3 was synthesised following one of our recently
published procedures[24] and was characterised using standard ana-
lytical techniques. The yield for pure L3 was evaluated based on
the reactant used and was found to be 73% (476 mg, 0.89 mmol).
1H NMR (500 MHz, CDCl3, 258C, TMS): d=8.61 (d, 3J(H,H)=5 Hz,
1H; 6-bpy), 8.57 (d, 3J(H,H)=4.5 Hz, 1H; 6’-bpy), 8.48 (s, 1H; 3’-
bpy), 8.25 (s, 1H; 3-bpy), 7.49 (d, 3J(H,H)=8 Hz, 2H; 3,5-phenyl),
7.40 (d, 3J(H,H)=16 Hz, 1H; vinyl), 7.34 (d, 3J(H,H)=4 Hz, 1H; 5-
Complex 1: The synthesis of 1 was reported earlier and we adopt-
ed a similar methodology for the present study.[24] The isolated
yield of the 1 after purification by column chromatography was
(yield was calculated based on the starting compounds) was 25%
(141 mg, 0.078 mmol). 1H NMR (500 MHz, CDCl3, TMS): d=9.13–
3
9.09 (m, 6H; 6,6’-bpy), 7.99 (d, J(H,H)=17 Hz, 3H; vinyl), 7.65 (d,
3J(H,H)=8 Hz, 6H; 3,5-phenyl), 7.59–7.5 (m, 6H; 3,3’-bpy), 7.38–
7.37 (m, 3H; 5-bpy), 7.22–7.20 (m, 3H; 5’-bpy), 7.09 (d, 3J(H,H)=
3
3
bpy), 7.15 (d, J(H,H)=3.5 Hz, 1H; 5’-bpy), 6.98 (d, J(H,H)=16, 1H;
3
3
16.5 Hz, 3H; vinyl), 6.92 (d, J(H,H)=8.5 Hz, 6H; 2,6-phenyl), 3.98 (t,
vinyl), 6.91 (d, J(H,H)=8 Hz, 2H; 2,6-phenyl), 3.98 (t, 2H; OꢀCH2),
6H; OꢀCH2), 2.65 (s, 9H; bpyꢀCH3), 1.78 (b, 6H; longchainꢀCH2),
1.46–1.44 (m, 6H; longchainꢀCH2), 1.26 (b, 84H; longchainꢀ
(CH2)14), 0.88 ppm (t, 9H; longchainꢀCH3); ESIMS (+ve mode):
2.45 (s, 3H; bpyꢀCH3), 1.8–1.78 (m, 2H; longchain(ꢀCH2)), 1.46–1.44
(m, 2H; longchain(ꢀCH2)), 1.26 (b, 28H; (ꢀCH2)14), 0.88 ppm (t, 3H;
longchain(ꢀCH3)); ESIMS (+ve mode): m/z (%): 541.59 (100) [M+H+
]; elemental analysis calcd (%) for C37H52N2O: C 82.17, H 9.69, N
5.18; found: C 82.1, H 9.64, N 5.15.
1
1= m/z (%): 861.65 (15) = [M2+]; elemental analysis calcd (%) for
2
2
C110H156Cl2N6O3Ru: C 74.25, H 8.67, N 4.72; found: C 74.16, H 8.81,
N 4.59.
Ligand L4: The methodology used for the synthesis of L4 was simi-
lar to that for L3. Ligand L2 (500 mg, 1.74 mmol) was dissolved in
dry DMF (ꢁ60 mL) in a two-neck round-bottomed flask under the
positive pressure of N2 gas. Finely ground pre-dried K2CO3 (365 mg,
2.64 mmol) was added to this with rapid stirring. 1-Bromotetrade-
cane (0.52 mL, 1.74 mmol) was added in a dropwise manner using
a syringe and the solution colour was found to change to dark
yellow. To this resulting reaction mixture, finely ground pre-dried
KI (439.25, 2.64 mmol in powder form) was added. Then the solu-
tion temperature was raised to 908C and was stirred for 48 h. Next,
the reaction solution was allowed to attain room temperature
(258C) and was filtered. The filtrate was collected and the solid res-
idue was washed thoroughly with CH2Cl2. This CH2Cl2 washing was
added to the filtrate and was evaporated to dryness through using
a rotary evaporator to isolate the crude product. Then the crude
product was purified by gravity chromatography using silica gel as
the stationary phase and CH2Cl2 as the mobile phase. The isolated
Complex 2: This complex was prepared by following the proce-
dure that was adopted for 1. Ligand L4 (220 mg, 0.454 mmol) was
dissolved in ethanol/dioxan (1:1, v/v; 25 mL) mixed solvent
medium. To this, RuCl3·xH2O (39.46 mg, 0.151 mmol) was added
under an inert atmosphere and was heated at reflux for 24 h with
continuous stirring. Then the reaction mixture was cooled to room
temperature and was evaporated to dryness under reduced pres-
sure. The crude solid was subjected to chromatography on alumina
grade III using acetonitrile as the eluent. The major fraction was
isolated and the solvent was removed to isolate the desired com-
pound in a pure form (23% calculated based on the starting com-
1
pounds; 170 mg, 0.104 mmol). H NMR (500 MHz, CDCl3, TMS): d=
9.0 (b, 3H; 6-bpy), 8.9 (b, 3H; 6’-bpy), 7.83 (d, J=16 Hz, 3H; vinyl),
7.62 (d, J=6.5 Hz, 6H; 3,5-phenyl), 7.57–7.54 (m, 6H; 3,3’-bpy),
7.51 (b, 3H; 5-bpy), 7.32 (b, 3H; 5’-bpy), 7.16 (d, J=17 Hz, 3H;
vinyl), 6.96 (d, J=7.5 Hz, 6H; 2,6-phenyl), 4.0 (t, 6H; OꢀCH2), 2.61
ChemPlusChem 2012, 77, 1096 – 1105
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1103

S. Choudhury, A. Das et al.
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(s, 9H; bpyꢀCH3), 1.79–1.75 (m, 6H; longchainꢀCH2), 1.46–1.42 (m,
6H; longchainꢀCH2), 1.25 (b, 60H; longchainꢀ(CH2)10), 0.87 ppm (t,
1
9H; longchainꢀCH3); ESIMS (+ve mode): = m/z (%): 777.88 (100)
2
1= [M2+]; elemental analysis calcd (%) for C99H132Cl2N6O3Ru: C 73.12,
2
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H 8.18, N 5.17; found: C 73.0, H 8.2, N 5.13.
Complex 3: This complex was prepared following a procedure that
was adopted for complex 2. Ligand L5 (200 mg, 0.467 mmol) was
used instead of L4 for the synthesis of this complex. The crude
product was purified by gravity chromatography using an Al2O3
grade III column using a acetonitrile/chloroform (8:2, v/v) solvent
mixture as the eluent. The yield of the desired complex in the pure
form was found to be 25% (170 mg, 0.117 mmol) calculated based
on the starting compounds used for the reaction. 1H NMR
3
(500 MHz, CDCl3, TMS): d=9.06 (b, 6H; 6,6’-bpy), 7.96 (d, J(H,H)=
3
16 Hz, 3H; vinyl), 7.64 (d, J(H,H)=8 Hz, 6H; 3,5-phenyl), 7.65–7.51
(m, 6H; 3,3’-bpy), 7.38–7.37 (m, 3H; 5-bpy), 7.21 (b, 3H; 5’-bpy),
3
3
7.09 (d, J(H,H)=16 Hz, 3H; vinyl), 6.92 (d, J(H,H)=8.5 Hz, 6H; 2,6-
phenyl), 3.98 (t, 6H; OꢀCH2), 2.65 (s, 9H; bpyꢀCH3), 1.79–1.76 (m,
6H; longchainꢀCH2), 1.45–1.42 (m, 6H; longchainꢀCH2), 1.29 (br,
36H; longchainꢀ(CH2)6), 0.88 ppm (t, 9H; longchainꢀCH3); ESIMS
(+ve mode): 1= m/z (%): 693.86 (30) 1= [M2+]; elemental analysis
2
2
calcd (%) for C87H108Cl2N6O3Ru: C 71.68, H 7.47, N 5.76; found: C
71.53, H 7.5, N 5.73.
Acknowledgements
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A.D. acknowledges DSTand CSIR for financial support. P.M. and
S.S. acknowledge CSIR for research fellowship.
Keywords: Langmuir–Blodgett films
monolayers · surfactants · thin films
·
logic gates
·
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Received: August 10, 2012
Revised: September 21, 2012
Published online on October 30, 2012
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