Alkylamine sensing using langmuir-blodgett films of n-alkyl-N-phenylamide- substituted zinc porphyrins

Article abstract of DOI:10.1021/jp803577d

Two porphyrin compounds, zinc(II) 5,10,15,20-tetrakis(3,5,5-trimethyl-N- phenylhexanamide)porphyrin and zinc(II) 5,10,15,20-tetrakis(2,2-dimethyl-N- phenylpropanamide)porphyrin, have been investigated as possible candidates for the detection of alkylamines. UV-visible spectroscopy has shown that their solution absorption spectra are significantly modified upon interaction with a range of organic analytes, including acetic acid, butanone, ethylacetate, hexanethiol, octanal, octanol, alkylamines, and trimethylphosphite. Large spectral changes are observed for the family of alkylamines as a result of the specific affinity between zinc and the amine moiety. Langmuir-Blodgett (LB) films of the porphyrins have been fabricated in order to assess their solid-state sensing capability toward amines. The surface pressure-area (a??-A) isotherms reveal a clear three-phase Langmuir film behavior and show that these monolayer films may be compressed to a relatively high surface pressure (a??40-50 mN m^-1)- The isotherm data alongside molecular modeling suggest a relatively flat orientation of the porphyrin rings of both compounds: that is, a mutually parallel alignment of the plane of the porphyrin ring and that of the water surface. LB films deposited at 15 mN m^-1 have been exposed to alkylamine vapor (carried by N₂). A red shift and increase in intensity of the Soret band absorbance is observed which can be reversed by flowing pure N₂ over the gently heated sample (60?°C) after exposure. Primary amines were expected to invoke the greatest sensing response due to (i) their larger association constants with these porphyrins compared to secondary and tertiary amines and (ii) the ease of diffusion of amines which is expected to follow the order primary > secondary > tertiary due to the steric hindrance arising from the bulky secondary and tertiary amines. However, the magnitude of the absorbance change is largest for exposure to the secondary amines, dipropylamine and dibutylamine, for both porphyrins, compared to primary and tertiary amines. This trend follows that observed when the amines were added to solutions of the porphyrins. The rate of response of the porphyrin LB films falls as the molecular weight of the diffusing alkylamine increases. Furthermore, a greater rate of response is observed for the phenylhexanamide porphyrin compared to the phenylpropanamide porphyrin due to its lower molecular density within the LB film and therefore more porous structure. ? 2008 American Chemical Society.

Full text of DOI:10.1021/jp803577d

11278
J. Phys. Chem. B 2008, 112, 11278–11283
Alkylamine Sensing Using Langmuir-Blodgett Films of n-Alkyl-N-phenylamide-Substituted
Zinc Porphyrins
S. Brittle,† T. H. Richardson,*,† A. D. F. Dunbar,† S. Turega,‡ and C. A. Hunter‡
Department of Physics and Astronomy, Hicks Building, UniVersity of Sheffield, Hounsfield Road,
Sheffield S3 7RH, U.K., and Department of Chemistry, Dainton Building, UniVersity of Sheffield,
Brook Hill, Sheffield S3 7HF, U.K.
ReceiVed: April 24, 2008; ReVised Manuscript ReceiVed: June 12, 2008
Two porphyrin compounds, zinc(II) 5,10,15,20-tetrakis(3,5,5-trimethyl-N-phenylhexanamide)porphyrin and
zinc(II) 5,10,15,20-tetrakis(2,2-dimethyl-N-phenylpropanamide)porphyrin, have been investigated as possible
candidates for the detection of alkylamines. UV-visible spectroscopy has shown that their solution absorption
spectra are significantly modified upon interaction with a range of organic analytes, including acetic acid,
butanone, ethylacetate, hexanethiol, octanal, octanol, alkylamines, and trimethylphosphite. Large spectral
changes are observed for the family of alkylamines as a result of the specific affinity between zinc and the
amine moiety. Langmuir-Blodgett (LB) films of the porphyrins have been fabricated in order to assess their
solid-state sensing capability toward amines. The surface pressure-area (Π-A) isotherms reveal a clear three-
phase Langmuir film behavior and show that these monolayer films may be compressed to a relatively high
surface pressure (∼40-50 mN m-1). The isotherm data alongside molecular modeling suggest a relatively
flat orientation of the porphyrin rings of both compounds: that is, a mutually parallel alignment of the plane
of the porphyrin ring and that of the water surface. LB films deposited at 15 mN m-1 have been exposed to
alkylamine vapor (carried by N2). A red shift and increase in intensity of the Soret band absorbance is observed
which can be reversed by flowing pure N2 over the gently heated sample (60 °C) after exposure. Primary
amines were expected to invoke the greatest sensing response due to (i) their larger association constants
with these porphyrins compared to secondary and tertiary amines and (ii) the ease of diffusion of amines
which is expected to follow the order primary > secondary > tertiary due to the steric hindrance arising from
the bulky secondary and tertiary amines. However, the magnitude of the absorbance change is largest for
exposure to the secondary amines, dipropylamine and dibutylamine, for both porphyrins, compared to primary
and tertiary amines. This trend follows that observed when the amines were added to solutions of the porphyrins.
The rate of response of the porphyrin LB films falls as the molecular weight of the diffusing alkylamine
increases. Furthermore, a greater rate of response is observed for the phenylhexanamide porphyrin compared
to the phenylpropanamide porphyrin due to its lower molecular density within the LB film and therefore
more porous structure.
Introduction
urine, and semen. Amine detection is of commercial interest
because of toxicity and the potential health hazard to people
working in the numerous industries which use amines. For
example aliphatic amines are used in the production of dyes,
emulsifiers, stabilizers, and corrosion inhibitors. The multiple
amine content of some foods indicates its degree of spoilage,
especially in fish and meats.9
Porphyrin molecules possess highly conjugated π-electron
systems which give rise to their characteristic UV-visible
absorption spectra and hence their intense color.1,2 Recently,
they have been exploited as sensor materials for both inorganic3
and organic4 vapors since their spectra can be modified as a
result of the binding interaction between the porphyrin ring and
the analyte vapor molecule. To achieve a relatively fast response
(approximately seconds), ultrathin films are required that offer
a large surface area to volume ratio. The Langmuir-Blodgett
technique5,6 is an effective means of achieving this and is
significantly faster than the commonly used layer-by-layer self-
assembly methods.7,8 In this work, two zinc substituted por-
phyrins, 5,10,15,20-tetrakis(3,5,5-trimethyl-N-phenylhexana-
mide)porphinatozinc(II) (1) and 5,10,15,20-tetrakis(2,2-dimethyl-
N-phenylpropanamide)porphinatozinc(II) (4) were studied for
potential use as amine sensing materials. The amine scent is
very common and is found in odors from fish, rotting flesh,
Experimental Section
Materials. Compound 1 (Figure 1) and the corresponding
tert-butyl derivative, 4, used in this study were synthesized by
coupling 5,10,15,20-tetrakis(3-aminophenyl)-21H,23H-porphy-
rin,10 2, with isononyl chloride to provide porphyrin 5 in 50%
yield and trimethylacetyl chloride to provide, 5,10,15,20-
tetrakis(2,2-dimethyl-N-phenylpropanamide)-21H,23H-porphy-
rin, 3, in 89% yield. Porphyrin 5 was then metalated with zinc
acetate in 50% yield to give porphyrin 1, and 3 was metalated
to give zinc porphyrin, 4, in 24% yield.
Synthesis. 5,10,15,20-Tetrakis(3,5,5-trimethyl-N-phenylhex-
anamide)-21H,23H-porphine, 5. To a mixture of 2 (0.325 g,
0.482 mmol) and triethylamine (1.63 mL, 11.6 mmol) in
tetrahydrofuran (THF, 20 mL) under nitrogen at room temper-
* To whom correspondence should be addressed. Phone/Fax: +44 114
2224280. E-mail: t.richardson@shef.ac.uk.
† Department of Physics and Astronomy.
‡ Department of Chemistry.
10.1021/jp803577d CCC: $40.75
2008 American Chemical Society
Published on Web 08/19/2008

Alkylamine Sensing Using Zinc Porphyrin LB Films
J. Phys. Chem. B, Vol. 112, No. 36, 2008 11279
and the resulting blue gum was purfied on deactivated alumina
(80 g) eluting with DCM. The product 1 was isolated as a purple
colored solid, 0.159 g (50%). Mp ) 195 °C (decomp.). 1H NMR
(250 MHz, CDCl3, CD3OD, 2%): δH 8.89 (2H, s), 8.76 (1H,
s), 8.14-8.19 (1H, m), 7.86-8.00 (2H, m), 7.57 (1H, s),
2.28-2.38 (2H, m), 2.04-2.014 (1H, m), 1.02-1.28 (2H, m),
0.96 (3H, d, J ) 5), 0.83 (9H, s). MS (El+): m/z (%) 1299
(100) [M+]. HRMS (El+). Calcd for C80H96N8O464Zn: 1296.6846.
Found: 1296.6835. UV/vis (DMSO) [λmax (nm) (ꢀ (mol-1
cm-1))] 421 (4.6 × 105), 549 (1.6 × 104), 595 (4.7 × 103).
Zinc(II) 5,10,15,20-Tetrakis(2,2-dimethyl-N-phenylpropana-
mide)porphyrin, 4. A mixture of 3 (0.200 g, 0.198 mmol) and
zinc acetate (0.727 g, 3.96 mmol) in 95:5 THF:DCM (200 mL)
was refluxed under nitrogen for 48 h. The solution was
concentrated under reduced pressure, and the resulting blue gum
was purified by column chromatography on deactivated alumina
(60 g) eluting with DCM:MeOH 99:1. The product 4 was
isolated as a purple solid, 0.052 g (24%). Mp ) 250 °C
(decomp.). 1H NMR (250 MHz, CDCl3, CD3OD, 2%): δH 8.96
(1H, s), 8.94 (2H, s), 8.58 (1H, s), 8.20 (1H, d, J ) 8), 7.94
(1H, d, J ) 8), 7.71 (1H, dd, J ) 6, J ) 6), 1.27 (9H, s). 13C
NMR (100 MHz, CD3SOCD3): δC 176.6, 149.9, 143.6, 137.9,
131.6, 129.7, 126.5, 120.4, 118.9, 39.5, 26.9. MS (ES+): m/z
(%) 1012 (30), 1074 (100) [M + H+]. HRMS (ES+). Calcd
for C64H65N8O464Zn: 1073.4420. Found: 1073.4425 [M + H+].
UV/vis (DMSO) [λmax (nm) (ꢀ (mol-1 L cm-1) 431 (1.9 × 105),
562 (4.9 × 104), 601 (2.7 × 103).
Figure 1. Synthetic route to zinc porphyrins 1 and 4.
ature, isononanoyl chloride (0.366 mL, 1.93 mmol) was added,
and the solution was stirred for 18 h. The solution was
concentrated under reduced pressure and the resulting blue gum
dessolved in DCM (250 mL), washed with 10% NaHCO2 (aq),
(2 × 200 mL), brine (200 mL), and condensed under reduced
pressure. The crude product was purfied on silica (30 g) eluting
with DCM-5% methanol. The product 5 was isolated as a
purple colored solid, 0.297 g (50%). Mp ) 250 °C (decomp.).
1H NMR (250 MHz, CD3SOCD3): δH 10.25 (1H, s), 8.89 (2H,
s), 8.50 (1H, d, J ) 10), 8.08 (1H, s), 7.92 (1H, d, J ) 8), 7.74
(1H, dd, J ) 8, J ) 8), 2.2-2.39 (2H, m), 2.19 (1H, s),
1.06-1.36 (2H, m), 0.98 (3H, d, J ) 5), 0.88 (9H, s), -2.96
(0.5H, s). MS (El+): m/z (%) 675 (100), 1236 (10), [M + H+].
HRMS (El+). Calcd for C80H99N8O4: 1235.7789. Found:
1235.7825. UV/vis (DMSO) [λmax (nm) (ꢀ (mol-1 cm-1))] 423
(5.1 × 105), 516 (1.8 × 104), 551 (7.9 × 103), 590 (5.3 × 103),
645 (4.2 × 103).
Experimental Methods. Solution UV-Visible Spectroscopy.
UV-visible titrations were carried out by preparing a 10 mL
sample of the porphyrin 1 at known concentration (1-10 µM)
in spectroscopic grade toluene. A 2 mL aliquot of this solution
was removed, and a UV-visible spectrum was recorded using
a Peltier thermostat set at 298 K. A 2 mL solution of pyridine
ligand (5-2000 µM) was prepared using the porphyrin solution
so that the concentration of porphyrin remained constant
throughout the titration. Aliquots of pyridine solution were added
successively to the cell containing the porphyrin solution, and
the UV-visible spectrum was recorded after each addition.
Changes in absorbance for the Soret band of the porphyrin were
fit to a 1:1 binding isotherm in Microsoft Excel to obtain the
association constant. Each titration was repeated at least twice,
and the experimental error is quoted as twice the standard
deviation at a precision of one significant figure.
5,10,15,20-Tetrakis(2,2-dimethyl-N-phenylpropanamide)-
21H,23H-porphyrin, 3. A mixture of 2 (0.200 g, 0.296 mmol)
and triethylamine (0.360 g, 3.56 mmol) in 10:1 THF:DCM
(250 mL) was stirred under nitrogen at room temperature.
Trimethylacetyl chloride (0.146 mL, 1.19 mmol) was added,
and the solution was stirred for 3 days. The precipitate was
collected and washed 3 times with DCM (3 × 50 mL), giving
3, which was isolated as a purple solid, 0.265 g (89%). Mp )
Solutions of each of the porphyrins were prepared using
chloroform as the solvent. 2 mL quantities of each of these
porphyrin solutions were placed in quartz cuvettes, and initial
absorption spectra of the solutions were recorded using an Ocean
Optics USB2000 spectrometer with a Mikropack Mini D2
UV-vis-IR light source. The absorbance spectra were recorded
in the range of 350-680 nm with respect to a reference taken
of chloroform. Subsequently, aliquots of 100 µL of the analyte
to be investigated were added to the porphyrin solution which
was then stirred. After each addition, the exposed absorption
spectrum was recorded. This process was repeated for both zinc
porphyrins with each of the nine different analytes. The
absorbance spectrum of each analyte in chloroform was also
recorded to confirm they are transparent in the wavelength range
of interest.
1
220 °C (decomp.). H NMR (250 MHz, CD3SOCD3): δH 9.59
(1H, s), 8.91 (2H, s), 8.59 (1H, d, J ) 6), 8.18 (1H, d, J ) 8),
7.92 (1H, d, J ) 8), 7.74 (1H, dd, J ) 8, J ) 8), 1.27 (9H, s),
-2.94 (0.5H, s). 13C NMR (100 MHz, CD3SOCD3): δC 177.4,
141.8, 138.5,130.0, 127.5, 126.8,120.4, 120.1, 27.7. MS (FAB+):
m/z (%) 1011.6 (100) [M + H+]. HRMS (FAB+). Calcd for
C64H67N8O4: 1011.528528. Found: 1011.531530. UV/vis (DMSO)
[λmax (nm) (ꢀ (mol-1 L cm-1))] 423 (7.7 × 105), 516 (2.9 ×
104), 550 (1.3 × 104), 591 (8.6 × 103), 647 (8.2 × 103).
Langmuir Isotherms. Chloroform solutions of 1 and 4 were
produced to a concentration of ∼0.2 mg/mL. The porphyrin
solution was added dropwise onto the subphase surface of a
Nima Langmuir trough (Nima model 611), using a 250 µL
Hamilton syringe. The subphase of pure water was supplied by
an Elga Purelab water system, providing pure water with
Zinc(II) 5,10,15,20-Tetrakis(3,5,5-trimethyl-N-phenylhex-
anamide)porphine, 1. A mixture of 5 (0.300 g, 0.243 mmol)
and zinc acetate (0.890 g, 4.85 mmol) in chloroform (20 mL)/
MeOH (30 mL) was stirred under nitrogen at room temperature
for 1.5 h The solution was concentrated under reduced pressure,

11280 J. Phys. Chem. B, Vol. 112, No. 36, 2008
Brittle et al.
TABLE 1: Principal Absorption Wavelengths and Molar
Extinction Coefficients for 1 and 4
porphorin
parameter
1
λ (nm)
424 (Soret)
185615
424 (Soret)
183254
551 (Q-band)
11764
551 (Q-band)
13530
ꢀ (mol-1 L cm-1
λ (nm)
)
)
4
ꢀ (mol-1 L cm-1
resistivity >15 MΩ cm. A sufficient time period (5-10 min)
was allowed for the cholorform to evaporate from the subphase
surface. The Teflon barriers of the Nima Langmuir trough were
then closed slowly at a speed of 50 cm2 min-1 in order to
compress the floating porphyrin Langmuir film. The surface
pressure was measured during the compression using a Wil-
helmy plate (filter paper) and monitored using Nima software.
The area per molecule was plotted against surface pressure to
obtain the Langmuir isotherm.
Figure 2. Difference spectra after the addition of several analytes to
a chloroform solution of 1.
LB Film Fabrication. To produce LB films of 1 and 4, a
Langmuir layer was prepared in the same way as described
above. The barriers of the Nima trough were then closed until
the desired deposition pressure was obtained; this surface
pressure was then maintained using the Langmuir trough
feedback mechanism. The zinc porphyrin was then transferred
onto a clean, hydrophobic (via silanization), glass substrate using
the LB technique using a deposition arm speed of 10 mm min-1
and a surface pressure of 15 mN m-1. Samples consisting of 20
Langmuir layers were produced.
Vapor DeliWery. LB films of 1 and 4 were exposed to amine
vapors using a specially designed gas testing chamber compris-
ing two Tylan FC-260 mass flow controllers, which deliver a
mixture of N2 and amine vapor for the exposure stage, followed
by pure N2 for the recovery stage. The delivery of these gases
was controlled and automated using a program developed using
Labview software. The liquid amine bottled samples were kept
in iced water during exposure. N2 flowed over the surface of
the amine liquid, transporting the gas mixture to the sample
chamber. The LB film samples were positioned on a Peltier
heating device containing an aperture in its center to allow
transmission UV-visible spectroscopy to be conducted in situ.
The Peltier heater was used to heat the films to 60 °C in order
to accelerate desorption and hence allow the films to recover.
LB film spectra over the range of 350-850 nm (tungsten light
source) were monitored continually during exposure and
recovery using a World Precision Instruments “Spectromate”
spectrometer.
Figure 3. Difference spectra after the addition of several analytes to
a chloroform solution of zinc porphyrin 4.
Results and Discussion
Table 1 shows the principal absorption wavelengths and
extinction coefficients for solutions (chloroform solvent) of 1
and 4. The very similar values confirm that the alkyl chain of
the amide group plays little or no role in determining the
oscillator strength of the porphyrin transitions. The addition of
analytes (in liquid form) to porphyrins in solution offers an
insight into the interaction between the nonaggregated, discrete
porphyrin molecule and the analyte species to be bound.
Figure 2 shows the difference spectra of a chloroform solution
of 1 for the range of nine analytes. Each difference spectrum is
formed by subtracting the spectrum obtained prior to exposure
to any analyte liquid from that measured after the addition of
each analyte (saturation). Clearly, the response is unique for
each analyte and is particularly strong (large absorbance change)
for propylamine and octanol. Figure 3 shows the difference
spectra corresponding to the exposure of a solution (chloroform)
of 4 to the nine analytes. Comparison with Figure 2 reveals
that the two porphyrins behave similarly.
Figure 4. Difference spectra after the addition of alkylamines to a
chloroform solution of porphyrin 1: (a) effect of propylamine, dipro-
pylamine, and tripropylamine, and (b) effect of butylamine, dibutylamin,
and tributylamine.
Since amines have yielded the largest change in both zinc
porphyrin spectra, their sensitivity in solution and solid state
(LB films) was investigated using six alkylamines comprising
two examples of primary, secondary, and tertiary amines. Figure
4 shows the difference spectra for the n-butylamines (Figure
4a) and the n-propylamines (Figure 4b) when added to a solution
of porphyrin 1. The largest absorbance change is observed for
dibutylamine and dipropylamine.
The Langmuir isotherms of 1 and 4 are shown in Figure 5.
Each isotherm clearly shows three separate phases.

Alkylamine Sensing Using Zinc Porphyrin LB Films
J. Phys. Chem. B, Vol. 112, No. 36, 2008 11281
Figure 5. Surface pressure-area isotherms of 1 and 4.
Figure 7. Absorbance dynamics for a 20 layer LB film of 4 upon
exposure to dipropylamine.
Figure 6. Spectra of 20 layer sample of 4 during exposure and recovery
cycles to dipropylamine.
However, the takeoff area (the area per molecule at which
the surface pressure Π becomes >0 mN m-1) is significantly
larger for 1 (730 ( 10 Å2) compared to 4 (260 ( 10 Å2). The
middle of the three phases (liquid phase) is more expanded for
1 than 4. Both of these differences are attributed to the length
of the alkyl chain within the amide functional group, which is
longer for 1 (hexanamide) than 4 (propanamide). The longer
chain results in increased steric hindrance for the packing of
molecules of 4 compared to 1. At higher surface pressure, the
solid phases are closer together for the two materials,
although the area per molecule for 1 is appreciably larger
(425 ( 20 Å2 by extrapolation of the solid phase) than for 4
(240 ( 20 Å2 by extrapolation). Corey-Pauling-Koltun
(CPK) modeling indicates that molecules of 1 could indeed
be oriented in a completely flat orientation (that is, the
porphyrin ring could be oriented parallel to the plane of the
water surface). The modeled cross-sectional area is 420 (
40 Å2, which is in close agreement with the measured value.
The molecular orientation of 4 may be slightly tilted with
respect to the water surface since the modeled value of 360
( 30 Å2 is slightly greater than the measured projected area
per molecule on the water surface. An alternative interpreta-
tion may be that a small degree of aggregation between
molecules of 4 exists within the Langmuir film.
Figure 8. Histogram of the absorbance changes of the two porphyrin
films when exposed to each of the six amine vapors.
The maximum absorption changes in the LB spectra of 1 and
4 occur not on the Soret band peak itself but on either side of
this peak. By identifying the wavelength that changes the most
during exposure to amine vapors, it is possible to maximize
the signal response of the zinc porphyrin amine sensor and to
monitor this signal as a function of time. The dipropylamine
vapor is delivered to the sample for 600 s during a single
exposure cycle, until a recovery stage in which clean nitrogen
gas is delivered to the sample for a further 600 s. The four
different wavelengths that are tracked in this case are the original
Soret band (435 nm), the wavelengths for which maximum
absorbance change is observed (424 and 443 nm), and a
wavelength outside the absorption spectrum for reference
purposes (750 nm).
Figure 7 shows the absorbance values at these four wave-
lengths plotted as a function of time. Upon exposure to the
amine vapor, dramatic changes in absorbance are observed. At
the Soret band (435 nm) a small increase in absorbance is
detected, but at either side of the Soret band (424 or 443 nm),
absorbance changes of around 20-30% are observed. It is clear
from Figure 7 that the exposure- recovery cycle is reproducible
and that, over the four complete cycles shown, little or no
degradation in response is observed.
Figure 6 shows the effect of dipropylamine vapor (carried
by N2) on a 20 layer LB film of 4. Upon exposure, the Soret
band absorbance at 434 nm shifts to 443 nm and its intensity
increases by ∼10%; this is accompanied by a Q-band shift from
560 to 567 nm. Although these changes are very small, the
World Precision Instruments spectrophotometer (Spectromate)
is capable of monitoring such changes reliably.
The plot in Figure 8 represents an example of the data
collected by measuring the response of 20 layer LB films of
each porphyrin to six different organic amines, namely, pro-

11282 J. Phys. Chem. B, Vol. 112, No. 36, 2008
Brittle et al.
Figure 10. Histogram of the t90 value of the two porphyrin films when
exposed to each of the six amine vapors.
Figure 9. Vapor pressures (at 0 °C) for alkyl (methyl, propyl, and
butyl) amines, dialkylamines, and trialkylamines.
series follows the trend butylamine (MW ) 73) > dibuty-
lamine (MW ) 129) > tributylamine (MW ) 185). Thus, if
concentration were the dominating factor in determining the
sensor response, the primary amines would be expected to
show by far the greatest response.
The ease of diffusion of amines through a relatively dense
LB matrix is expected to follow the order primary > secondary
> tertiary due to the steric hindrance arising from the bulky
secondary and tertiary amines. The time taken for 90% of the
total change in absorbance (saturation) to occur is known as t90
and is a parameter often used in the sensor community to
quantify the response rate. The t90 values, shown in Figure 10,
indicate that the rate of response is diffusion-limited and the
time taken for a porphyrin-amine binding pair to form increases
as the molecular weight of the amine increases. This relation is
pylamine, dipropylamine, tripropylamine, butylamine, dibuty-
lamine, and tributylamine. These amines have been chosen since
there are two examples of primary, secondary, and tertiary
amines, and they are all volatile liquids at 0 °C, the temperature
at which they are maintained prior to vapor delivery into the
sample chamber. Figure 8 depicts the average absorbance change
(over four response-recovery cycles) at 443 nm for the exposure
of each porphyrin to each of the six amines.
The response of the 20 layer LB film of 4 is greater than that
of film 1 for each corresponding amine. This is because the
smaller area per molecule (Figure 5) of 4 compared to 1 means
that the LB film of 4 possesses a higher molecular density and
therefore a greater density of available active sites for interaction
with amine molecules. The two secondary amines, dipropy-
lamine and dibutylamine, induce a larger response, in both of
the porphyrins, compared to the primary or tertiary amines. This
trend agrees with the response measured in solution (Figure 4)
and has been observed previously in a related system.11 It was
anticipated that primary amines would produce the greatest
sensing response due to their larger solution binding constants
with these porphyrins compared to secondary and tertiary
amines. In this work, it was found that, for tert-butylamine,
not linear but empirically follows a trend in which t90
)
bM∼1.4-1.8 in which b is a constant and M is the molecular
weight of the diffusing amine molecule.12 Figure 10 also shows
that the t90 value is greater for amine diffusion through 4 than
1. The lower area per molecule of 4 compared to 1 results in
an LB matrix through which diffusion would be expected to be
more difficult due to the increased molecular density. The only
exception to this trend is for the diffusion of propylamine
through 1, which takes longer than expected to react.
K ) 4100 ( 100 M-1; for diethylamine, K ) 810 ( 100 M-1
;
and for triethylamine, K ) 9.3 ( 1.0 M-1. However, the
association constant is indicative of the fraction of binding pairs
(porphyrin-amine) rather than the strength of the effect each
bound amine has on the absorption spectrum. Our research data
presented in Figure 4 and Figure 8 show that the secondary
amines produce a larger absorbance change than primary or
tertiary amines. We hypothesize that the binding of secondary
amines by these porphyrins results in a strongly asymmetric
porphyrin ring distortion compared to that resulting from the
binding of tertiary amines. Primary amines are expected also
to induce an asymmetric ring distortion, but this will be less
pronounced than for secondary amines as a result of the presence
of only one alkyl group (primary) rather than two in secondary
amines.
Additionally, it must be noted that the liquid amines are held
at 0 °C prior to delivery into the sample chamber. Since their
vapor pressures are different, the concentrations delivered to
the test chamber will not be the same. Figure 9 shows the vapor
pressures for a series of primary, secondary, and tertiary amines
including some of those tested here. The vapor pressures for
the propylamine series follows the trend propylamine (mo-
lecular weight, MW ) 59) > dipropylamine (MW ) 101)
> tripropylamine (MW ) 143), and that for the butylamine
Conclusions
Electronic spectroscopy has shown that the UV-visible
absorptionspectraoftwoporphyrincompounds,zinc(II)5,10,15,20-
tetrakis(3,5,5-trimethyl-N-phenylhexanamide)porphyrin and
zinc(II) 5,10,15,20-tetrakis(2,2-dimethyl-N-phenylpropanamide)-
porphyrin, are significantly modified when exposed to a range
of organic analytes including acetic acid, butanone, ethylacetate,
hexanethiol, octanal, octanol, alkylamines, and trimethylphos-
phite. In this work, their large spectral response to a family of
alkylamines, which occurs due to the specific affinity between
zinc and the amine moiety, has been investigated. The addition
of n-butylamines and n-propylamines to solutions of 1 results
in the greatest absorbance changes occurring for the two
secondary amines, dipropylamine and dibutylamine. Surface
pressure-area isotherms yield a clear three-phase Langmuir film
behavior and show that these floating films may be compressed
to a relatively high surface pressure (∼40-50 mN m-1). A red
shift and decrease in intensity of the Soret band absorbance is
observed during exposure of LB films of porphyrins 1 and 4 to
alkylamine vapor (carried by N2) which is completely reversible.
The greatest absorbance change occurs for both porphyrins (in
both solution and solid states) during exposure to the secondary

Alkylamine Sensing Using Zinc Porphyrin LB Films
J. Phys. Chem. B, Vol. 112, No. 36, 2008 11283
References and Notes
amines, dipropylamine and dibutylamine, compared to primary
and tertiary amines. This is an unexpected result since primary
amines exhibit the largest association constants when compared
to secondary and tertiary amines and are also expected to diffuse
more rapidly through an organic matrix. The t90 value (the time
taken for 90% of the total change in absorbance (saturation) to
occur) of the porphyrin LB films increases as the molecular
weight of the diffusing alkylamine increases. Moreover, higher
response rates are observed for the phenylhexanamide porphyrin
than the phenylpropanamide porphyrin because of its lower
molecular density within the LB film and therefore more porous
structure. Future work will focus on investigating the depen-
dence on the optical response of porphyrin LB films to different
concentrations of secondary amines in order to determine the
detection limit and saturation point of these materials.
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Acknowledgment. The authors of this paper thank the
EPSRC for the financial support they have provided to support
this research (Grant GR/S96845/01). We also thank William
Barford for useful discussion.
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