Dynamics within a single molecular layer. Aggregation, relaxation, and the absence of motion

Article abstract of DOI:10.1021/ja962021s

We report on the transient and steady-state optical responses of the chromophore 2,2′-bithiophene-5,5′-diylbis(phosphonic acid) (BDP) incorporated within a single zirconium-phosphonate layer as a function of chromophore density. While the dilute solution optical response of BDP reveals no anomalous behavior, its characteristics are substantially different when confined within a monolayer. We vary the concentrations of layer constituents to determine the extent of interaction between BDP moieties within a single monolayer. We observe limited initial aggregation of BDP, the extent of which is determined largely by the conditions under which the monolayer is formed. Over time, the fractional contribution of BDP aggregates to the total optical response decreases to a limiting value, implicating surface adsorption site density as the dominant factor in determining the morphology of the organobis(phosphonate) layer. Motional relaxation measurements of BDP within the layer show that the chromophores are immobile on the hundreds-of-picoseconds time scale of our experiments.

Full text of DOI:10.1021/ja962021s

12788
J. Am. Chem. Soc. 1996, 118, 12788-12795
Dynamics within a Single Molecular Layer. Aggregation,
Relaxation, and the Absence of Motion
J. C. Horne and G. J. Blanchard*
Contribution from the Department of Chemistry, Michigan State UniVersity,
East Lansing, Michigan 48824
X
ReceiVed June 17, 1996
Abstract: We report on the transient and steady-state optical responses of the chromophore 2,2′-bithiophene-5,5′-
diylbis(phosphonic acid) (BDP) incorporated within a single zirconium-phosphonate layer as a function of chromophore
density. While the dilute solution optical response of BDP reveals no anomalous behavior, its characteristics are
substantially different when confined within a monolayer. We vary the concentrations of layer constituents to determine
the extent of interaction between BDP moieties within a single monolayer. We observe limited initial aggregation
of BDP, the extent of which is determined largely by the conditions under which the monolayer is formed. Over
time, the fractional contribution of BDP aggregates to the total optical response decreases to a limiting value, implicating
surface adsorption site density as the dominant factor in determining the morphology of the organobis(phosphonate)
layer. Motional relaxation measurements of BDP within the layer show that the chromophores are immobile on the
hundreds-of-picoseconds time scale of our experiments.
5-28
29-31
nonlinear optics,7,19,20
Phosphonic acids
Introduction
modification,2
electronic device,
3
2-37
and molecular recognition applications.
Organized molecular assemblies have found use in numerous
chemical and physical applications, such as device patterning,
nonlinear optics, and tribology.
materials have characteristically well-defined structures of
controlled composition and, with a small amount of material,
they offer a widely tunable range of chemical, electrical, and
optical properties. Metal-phosphonate (MP) organic multilayer
structures have been investigated extensively8 and, like other
self-assembling monolayers, show potential for use in surface
form strong, sparingly soluble complexes with metal ions, giving
them significant advantages over many self-assembled mono-
layer (SAM) systems, such as the thiol/gold SAMs, which have
been shown to be labile.
SAMs in ease of synthesis, which generally involves immersion
of the functionalized substrate into a solution of the appropriate
1-7
This class of interfacial
3
8-40
MP structures are comparable to
(
R,ω)-organobis(phosphonate). MP multilayers are versatile in
-24
a chemical sense because the identity of individual layers can
be controlled selectively as the structure is assembled and, in
this way, chemical or electrical potential, as well as optical
properties, can be built into the system in three dimensions rather
than two. Langmuir-Blodgett films can also be assembled as
*
Author to whom correspondence should be addressed.
Abstract published in AdVance ACS Abstracts, December 1, 1996.
X
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S0002-7863(96)02021-5 CCC: $12.00 © 1996 American Chemical Society

Dynamics within a Single Molecular Layer
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12789
Scheme 1. Structures of Bis(phosphonates): (a)
determined primarily by the silanized site density of the surface
and not simply by the chromophore density.
2
1
,2′-Bithiophene-5,5′-diylbis(phosphonic acid) (BDP) and (b)
,6-Hexanediylbis(phosphonic acid) (HBPA)
Experimental Section
Synthesis of 2,2′-Bithiophene-5,5′-diylbis(phosphonic acid) (BDP).
BDP was synthesized by modification of a published procedure on a
longer (R,ω)-bisphosphonated thiophene oligomer.18 All reagents were
purchased from Aldrich Chemical Co. and used as received. Tetrahy-
2 2
drofuran (THF) was distilled over CaH and then sodium, under N
atmosphere. 2,2′-Bithiophene (6.01 mmol) was dissolved in 200 mL
of THF (under Ar) and cooled to -40 °C. An excess of n-butyllithium
(25 mmol, 2.5 M in hexanes) was added to this solution, and the mixture
was stirred for 90 min. A second solution of n-butyllithium (0.5 mmol)
and bis(dimethylamino)phosphochloridate (25 mmol) in 20 mL of THF
was added to the first via canula. The reaction solution was allowed
to warm slowly to room temperature and was stirred for 3 days. The
organics were extracted from water with ether, and the remaining
aqueous layer was extracted twice with CH Cl . The combined organics
were reduced in volume by rotary evaporation to about 1.5 g of a dark
red oil. The tetraamide, a pale yellow solid which exhibited blue
fluorescence on UV excitation, was collected chromatographically on
silica plates (Fisher Scientific) using a mobile phase of 5EtOH:3MeOH:
ordered multilayer structures, but it is a relatively delicate
process and, once formed, the films are not robust due to the
weak nature of the associations between layers. For these
reasons, MP multilayers are robust, nearly ideal model systems
for the examination of energy relaxation both within and
between individual molecular layers. This same structural motif
holds significant technological promise for optical signal
processing and information storage applications.
2
2
2
THF. The purified product was added to 100 mL of dioxane and
The connection between these apparently disparate uses for
MP systems can be reconciled as follows. The density of optical
information that can be stored within a single molecular layer
is limited, and the spatial resolution or “bit size” available with
optical approaches is determined by the diffraction limit for the
optics used and the wavelengths of reading and writing light.
One way to enhance the information density attainable within
a specific volume is through the addition of a third dimension,
but such an approach can also cause complications in control
over material properties and basic physical processes. The
ability to store information in a material optically depends on
making a specific, stable modification to the optical response
of a given layer of the material. The presence of other optically
active constituents within the same layer or in neighboring layers
creates the possibility of intra- or interlayer energy exchange
processes, which serve to place limits on the utility of these
materials for information storage applications.
dissolved at 80 °C. After the resulting mixture was cooled to room
temperature, 50 mL each of water and concentrated HCl was added,
and the solution was refluxed overnight. The solvent was partially
removed, and a dark green-yellow solid was recovered in ca. 10% yield.
1
Product identity was verified by H NMR (DMSO-d
6
): δ 7.3-7.5 (m).
Synthesis of 1,6-Hexanediylbis(phosphonic acid) (HBPA). All
chemicals were purchased from Aldrich Chemical Co. Triethyl
phosphite was dried over sodium and vacuum distilled. HBPA was
prepared by the Michaelis-Arbuzov reaction of 1,6-dibromohexane
(
32 mmol) with excess triethyl phosphite (117 mmol).22 The mixture
was refluxed under Ar for 5 h to allow evolution of ethyl bromide.
Excess triethyl phosphite was removed by vacuum distillation. Con-
centrated HCl (50 mL) was added to the colorless solution, which was
then refluxed overnight. The resulting clear brown solution was cooled
to room temperature. Upon cooling, a white precipitate (57% yield)
formed which was collected by vacuum filtration and washed with
1
acetonitrile. H NMR (DMSO-d
6
): δ 1.2-1.6 (m).
Metal-Phosphonate Multilayer Synthesis. Silica substrates, cut
from quartz slides, were cleaned by immersion in piranha solution
We consider the interlayer and intralayer relaxation processes
of MP assemblies separately because of the synthetic control
we can achieve over these structures. In this initial work, we
consider intralayer effects from a fundamental perspective using
2 2 2 4
O :3H SO
) for 15 min,42 rinsed with distilled water, hydrolyzed
(
1H
in 2 M HCl for 5 min, rinsed with distilled water, and dried under a N
stream. The substrate surface was silanized by refluxing in 1% v/v
3-aminopropyl)trimethoxysilane (Petrarch) in anhydrous octane under
2
(
1
8
2
1
,2′-bithiophene-5,5′-diylbis(phosphonic acid) (BDP, Scheme
) as the chromophore. Thiophenes are well understood and,
an Ar atmosphere for 10 min, followed by thorough rinsing with
4
1
reagent grade n-hexane. This amine primer layer was derivatized to
the phosphonate by reaction with 100 mM POCl (Aldrich) and 100
3
when phosphonated, form stable, relatively ordered layers
because of their comparatively rigid structure, which prevents
them from attaching to the surface at two points. To gain the
information of interest here, we vary the concentration of the
chromophore within a zirconium-phosphonate (ZP) monolayer.
An optically inactive component can be used either for spacing
between chromophore layers or for dilution of the chromophore
within layers. In these experiments, we dilute the chromophore
within a single layer using 1,6-hexanediylbis(phosphonic acid).
This spacer molecule, in its all-trans form, is approximately
the same length as the chromophore, so that the thickness of
the monolayer is uniform (Scheme 1). We discuss below the
extent of synthetic control we have over the composition of an
individual layer, the steady-state optical response of BDP, and
how it is related to the measured dynamical relaxation processes
operating within the layer. Our data indicate that both ag-
gregated and nonaggregated forms of the chromophore exist
within a monolayer and that the relative amount of each is
mM 2,4,6-collidine (Aldrich) in anhydrous acetonitrile under Ar for 1
h and rinsed with solvent grade acetonitrile. The phosphonic acid
surface was zirconated overnight with a 5 mM solution of ZrOCl
2
in
6
0% EtOH/H O. Subsequent layers were added by sequential reaction
2
with bis(phosphonate) (1.25 mM in 95% EtOH, 3 h at 55 °C) and
zirconium (5 mM in 60% EtOH, 30 min) solutions, with extensive
rinsing (H O) between each step. The pH of the solutions was
2
maintained between 2 and 4.
Steady-State Optical Spectroscopy. The absorption spectra of BDP
monolayers were measured using a Hitachi U-4001 UV-visible
spectrophotometer. The samples were held vertically in place at 45°
with respect to the incident beam and were collected with 5 nm spectral
resolution. The emission spectra of the monolayers and solutions were
collected using a Hitachi F-4500 fluorescence spectrophotometer.
Monolayer samples were excited at 320 nm through a 10 nm slit. The
emission bandpass was adjusted according to spectral intensity. The
samples were held vertically, at 45° with respect to the excitation beam
propagation in such a way as to minimize reflection of excitation
radiation into the emission collection optics.
(
41) Horne, J. C.; Blanchard, G. J.; LeGoff, E. J. Am. Chem. Soc. 1995,
(42) Caution! Piranha solution is extremely corrosiVe and is a potent
oxidizer.
1
17, 9551.

12790 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Horne and Blanchard
increments, and semiempirical optimization was performed until the
lowest energy conformation for the fixed interring bond torsion was
attained. The heat of formation and S T S transition energy were
0 1
calculated for each geometrically optimized conformation.
Results and Discussion
We are concerned with understanding the optical response
of (R,ω)-bisphosphonated chromophores, specifically oligomeric
thiophene derivatives, as constituents of metal-phosphonate
multilayer structures. As discussed above, the structural
regularity attainable in these systems allows for the selective
examination of intermolecular interactions between layers and
within layers. Our initial work in this area focuses on the
simplest case, that of a single layer containing a variable
chromophore density. We want to discern, ultimately, how
intermolecular interactions and photophysical processes within
one layer will affect the optical response of, and relaxation
dynamics within, more complex layered structures.
There are several factors that can, in principle, contribute to
the optical response of this system. One factor is the stoichi-
ometry of the monolayer deposition, which we can determine
using UV-visible absorption spectrometry. The spontaneous
emission response of the monolayer is also a rich source of
information on how the components of the monolayer are
assembled and how they interact. Fluorescence lifetime and
reorientation dynamics data obtained using transient emission
spectroscopy provide insight into the local environment(s) of
the chromophores within a monolayer. Semiempirical calcula-
tions performed on the chromophore aid in our interpretation
of the experimental data and thus our understanding of mono-
layer properties. We consider in the following sections each
of these aspects in the characterization of mixed ZP monolayers.
Stoichiometry of Monolayer Formation. Little is known
about the competition between different bis(phosphonate)s for
active metal ion sites. There is some evidence that the formation
of different mixed monolayers does not follow the stoichiometry
of the solution from which they are formed precisely,50 and thus
we need to determine whether or not such is the case for the
BDP/HBPA system. In addition to complications arising from
competition for binding sites, the stoichiometry of a multicom-
ponent deposition solution may not be reflective of the resulting
monolayer concentration if solution phase aggregation of one
or more of the constituents is occurring. For these reasons, it
is important to establish a direct means of calibration for this
particular system. To study the deposition stoichiometry of
mixed monolayers relative to the solution from which they are
formed, we varied the proportions of the chromophore (BDP)
and the optically inactive component (HBPA) in ethanol solution
and compared these data to absorbance measurements of the
resulting monolayers. The total concentration of bisphospho-
nates (BDP + HBPA) in the deposition solutions was 1.25 mM,
with the concentration of BDP fixed in increments from 1.25
mM (100%) to 0.0125 mM (1%). The absorption spectra of
the resulting monolayers are shown in Figure 2. These data
are background subtracted using a separate primed and zircon-
ated substrate as the background (Figure 2 inset). This
background spectrum is identical to that of a pure HPBA
monolayer. The raw spectra exhibit a slight sample-to-sample
variation in absorbance baseline, which can be attributed to slight
absorptive differences between individual quartz substrates and
positioning of the sample in the instrument. To correct for this
baseline variation, the absorbances reported in Figure 3 are the
differences between the absorbance at λmax and the interpolated
baseline. A linear regression fits the data well, indicating that
Figure 1. Example of excited state population decay (b) and
instrument response function (0) for BDP in ethanol.
Time-Correlated Single Photon Counting Spectroscopy. The
spectrometer we used for the fluorescence dynamics measurements of
BDP solutions and monolayers has been described in detail before,43
and we present here a brief overview of the system. The light pulses
used to excite the sample are generated with a cavity-dumped,
synchronously pumped dye laser (Coherent 702-2) excited by the second
harmonic of the output of a mode-locked CW Nd:YAG laser (Quant-
ronix 416). All samples were excited at 320 nm (Kiton Red, Exciton;
3
LiIO Type I SHG). Monolayer samples were held approximately
horizontally, with 5° tilts away from horizontal in two directions:
toward the excitation beam and toward the detector. Fluorescence from
the sample was imaged through a reflecting microscope objective.
Lifetimes were collected across the emission bands at 54.7° with respect
to the excitation polarization for solutions and without polarization bias
for monolayers. Fluorescence was collected at 390 nm (5 nm FWHM
bandwidth for solution, 20 nm for monolayers) with polarizations of
0
° and 90° for rotational diffusion dynamics measurements on all
samples. A representative lifetime decay and instrument response
function (∼35 ps FWHM) are shown in Figure 1.
Data Analysis. The lifetimes we report here were fit to sums of
exponentials using Microcal Origin software and are reported as the
values and uncertainties of the fit to individual decays. Rotational
diffusion information for solution measurements is reported as the
average of six data sets with their associated 95% confidence limit
uncertainty. For monolayers, eight pairs of alternating parallel and
perpendicular scans were used to produce an average anisotropy
function. R(0) was determined to (0.01 by regression of data at times
after the instrument response, and several data sets were averaged for
each sample.
Calculations. Semiempirical calculations44-48 were performed using
Hyperchem Release 4.0 (Hypercube, Inc.) on an IBM compatible PC
(
Gateway 2000 P5-120). The PM3 parametrization, used for these
calculations, is a modification of the AM1 parametrization that treats
molecules containing heteroatoms, such as sulfur, more accurately than
previous parametrizations. An initial optimization of the structure was
4
9
performed using a molecular mechanics routine (MM+) followed by
geometry optimization at the semiempirical level using an SCF
algorithm. The torsion of the 2,5′ BDP σ bond was set at 10°
(
43) DeWitt, L.; Blanchard, G. J.; LeGoff, E.; Benz, M. E.; Liao, J. H.;
Kanatzidis, M. G. J. Am. Chem. Soc. 1993, 115, 12158.
44) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J.
Am. Chem. Soc. 1985, 107, 3902.
(
(45) Dewar, M. J. S.; Dieter, K. M. J. Am. Chem. Soc. 1986, 108, 8075.
(46) Stewart, J. J. P. Comput.-Aided Mol. Des. 1990, 4, 1.
(47) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899.
(48) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4907.
(49) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127.
(50) Rabolt, J. F. Private communication.

Dynamics within a Single Molecular Layer
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12791
Table 1. Comparison of Fractional BDP Concentrations in
Deposition Solution and in the Resulting Monolayer, Determined
According to Absorbance (see text for a discussion)
%BDP in deposition solution
%BDP in monolayer
1
00
100
58
34
29
26
5
1
0
0
5
1
curvilinear. From these data we can establish a working
relationship between solution and layer composition that is
straightforward. For monolayer formation where BDP is the
only phosphonate-containing constituent in the deposition
solution, we assert that a limiting layer of the chromophore is
present, and we know experimentally that a pure HBPA
monolayer represents a zero-absorbance condition. We further
assert that Beer’s Law holds for this system so that the
absorbance of monolayers formed from mixed solutions is
indicative of the chromophore concentration. For high fractional
chromophore coverage, a direct correspondence between solu-
tion and monolayer composition is recovered, but for lower
fractional coverage, we find significant preference for adsorption
of BDP compared to HBPA (Table 1). This is not an
unexpected result. For many ionic association processes, the
formation constant of the complex depends sensitively on the
identity of the species involved, and there is no reason to expect
that not to be the case here. This result is observed, at least in
part, because there is a ∼25-fold stoichiometric excess of BDP
Figure 2. Background corrected absorbance spectra of BDP mono-
layers: (s) 100%, (- - -) 58%, (‚ ‚ ‚) 34%, (- ‚ -) 26%. Inset: Back-
ground spectrum.
1
5
relative to the number of substrate active sites (∼1.5 × 10 )
for the 1% BDP deposition solution. This ratio correlates
remarkably well with the observed monolayer concentration for
this solvent system. We expect that it would be possible to
achieve lower fractional coverages of the SiOx surface by using
smaller absolute amounts of BDP in the deposition solution,
and we recognize that the relative efficiency of adsorption of
both species will be determined by the solvent system.
The monolayer emission intensities varied with each sample
but did not follow any trend, suggesting that the quantum
efficiency of the monolayer samples is related to some factor(s)
in addition to the amount of chromophore present. Part of the
variation, again, may be due to inconsistency in the positioning
of the sample. The relatively complicated behavior of the
emission response for this system is also not a surprising result,
and the details of this complexity contain useful chemical
information as we will discuss below.
Monolayer Composition. The dilute solution behavior of
BDP is typical of a simple chromophore. Its absorption and
emission spectra have well-defined bands, and it exhibits a
single-exponential fluorescence intensity decay in time. With
monolayer samples, a double-exponential decay is observed,
with a fast component that is the same as the solution lifetime,
∼200 ps, and a slow component, ∼1 ns, as shown in Figure 4.
This decay functionality is observed for all monolayer samples,
and the time constants recovered for each component are the
same over the entire concentration range studied, as shown in
Figure 5a,b. We observe that, as we collect lifetime data at
several wavelengths within the BDP emission band, the form
of the decay changes, as indicated Figure 6. Fitting these data
recovers the same two time constants, but the fractional
contribution of each decay component varies as a function of
the wavelength at which the data are collected. Specifically,
the relative contribution of the longer lifetime increases on the
red side of the emission band for all samples (see Figure 7a,b).
Such behavior has been observed previously for different
chromophores imbedded in ZP layers, although its origin was
Figure 3. Dependence of monolayer absorbance on concentration of
BDP in deposition solution. Solid line: Beer’s Law predicted depen-
dence, Dashed line: regression of experimental data.
there is a direct relationship between the deposition stoichiom-
etry and the fractional concentrations of the components present
in the deposition solution. These data reveal that the cor-
respondence between solution composition and monolayer
composition is not one-to-one. Beer’s law requires there to be
no residual sample absorption at [BDP] ) 0, and the experi-
mental background corrected data produce a linear regression
to a measurable absorbance. Clearly for [BDP] ) 0, ABDP )
0
, and the regression to a non-zero intercept implies that
monolayer composition is offset from solution composition. In
other words, for low fractional BDP concentrations (<1%), the
relationship between solution and monolayer composition is

12792 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Horne and Blanchard
Figure 4. Representative fluorescence lifetimes of (b) solution and
O) monolayer.
(
Figure 6. Fluorescence lifetime decays of BDP monolayers at (9)
3
90 nm, (O) 475 nm, (b) 525 nm, and (0) 575 nm.
Figure 5. (a) Fluorescence lifetimes of BDP monolayers and solution
at 400 nm. The data were fit to the function f(t) ) A
1
exp(-t/τ
1
) + A
2
Figure 7. Emission wavelength dependence of (a) lifetimes and (b)
normalized fast exponential prefactors for 29% BDP monolayer.
exp(-t/τ2). (b) Normalized prefactors of fast exponential decay
component for data taken at 400 nm.
not isolated from the substrate, indicating that the presence of
the second lifetime component is not the result of a single
chromophore interacting with different silica sites on the same
surface. Another possibility is that we are observing excitation
transport, or “hopping”, within the layer. If this were the case,
the lifetimes would change as a function of chromophore
concentration within the monolayer. More concentrated chro-
mophore layers would exhibit longer lifetimes, or a wider
distribution of lifetimes, because the exciton would “hop” from
one chromophore to the next until it found a trap site. It is
likely that such a process would not lead to a double-exponential
decay functionality. Thus these complex monolayer decay
properties cannot be accounted for by either chromophore-
substrate interactions or simple excitation transport.
not determined.12 One explanation for the existence of two
lifetime components in these monolayers is the interaction of
the chromophore with active hydroxyl sites on the silica surface.
Wang and Harris found a similar lifetime behavior for pyrene
on silica and determined that it was due to the existence of two
distinguishable active silanol sites on the surface. To explore
the possible role of chromophore-substrate interactions, we
synthesized a multilayer structure with a ∼26% BDP layer
spaced away from the SiOx surface by three HBPA layers. This
structural motif prevents the chromophore from interacting with
the surface. The same lifetime measurements were made, and
the results were identical to those of the monolayers that were
51
(51) Wang, H.; Harris, J. M. J. Phys. Chem. 1995, 99, 16999.

Dynamics within a Single Molecular Layer
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12793
Figure 9. Calculated rotational energy barriers for BDP: (2) S
1) S
0
and
(
1
.
The ZP lattice has been shown to cause disorder in layers
because the spacing it enforces is larger than the molecular area
Figure 8. Emission spectra of fresh samples (solid lines) and aged (2
months) samples (dashed lines) for (a) aggregate-like and (b) monomer-
like samples. The vertical lines are guides for the eye to the band
maxima.
9
,55
of most organic moieties.
This means that the spacing
between individual chromophores may be larger than the
intermolecular distance that is optimum for aggregate formation,
so that, even if the chromophores are deposited onto the surface
as pairs, they are likely to separate over time, giving rise to the
spectral “annealing” we observe. This finding is fully consistent
with our FTIR data on HBPA monolayers on SiOx (not shown),
indicating some structural heterogeneity within the layer. Also,
we detect an apparent photodegradation of the chromophores
within the layer (Figure 8a) which may indicate that the
aggregates, as well as the monomer, are being destroyed,
resulting in a sample with a higher proportion of monomer and
a spectrum reflecting that change.
We expect that, on the basis of the ionic nature of the metal-
phosphonate bond(s), surface dissociation and diffusion will play
a role in achieving the annealed condition for these layers. Our
linear response data, however, do not provide significant insight
into the role of diffusion for this system. If bound BDP
aggregation was mediated by surface diffusion, we would
observe aggregate-like absorption spectra in proportion to the
surface concentration of the BDP for the annealed layers.
Instead, we observe that the stable long-term condition for these
films is in the form of monomeric chromophores, regardless of
the initial BDP concentration. While the implications of these
data remain to be explored more completely, it is clear that, for
this system, surface diffusion is not the only process at work in
determining the absorption properties of these films.
We consider next the possibility that we are observing
emission from an aggregate or excimer moiety that forms at
short intermolecular spacings, such as are found in a monolayer.
Aggregation has been observed for several molecules, such as
pyrene, which is known to form excimers in concentrated
52
solutions and at solid interfaces. The steady-state emission
spectra of the monolayer samples are consonant with this
explanation, as we find two distinct classes of spectra, one which
has been observed at all chromophore concentrations and is
similar to the dilute solution spectrum and one seen for
chromophore monolayer concentrations above 34% (see Figure
8
a,b). This latter feature, which we designate as the aggregate
spectrum, is broader and red shifted by about 50 nm from the
dilute solution, or monomer, spectrum. The intensity of the
monomer emission spectrum is several times that of the
aggregate spectrum, although quantitative intensity comparisons
are difficult. We observe both monomer and aggregate peaks
in the 100% BDP (1.25 mM) deposition solution, implying that
the resulting monolayer will be comprised of both monomer
and aggregate, with the ratio of the two forms being determined,
at least initially, by their deposition kinetics. We note that,
occasionally, a monolayer synthesized from concentrated BDP
solution yields a monomer-like spectrum, which we believe is
due to incomplete monolayer formation.
The existence of two measurably different spectra based on
two chromophore species can be understood from a structural
perspective with the aid of semiempirical calculations. We
calculated the interring rotational energy barriers for both the
ground state and the first singlet excited state of BDP, as shown
in Figure 9. The S0 potential energy surface is essentially
independent of thiophene interring torsion angle, with rotational
barriers of less than 1 kcal/mol. However, the S1 surface has
The monolayer aggregate spectra anneal over time to more
monomer-like spectra, but samples that were characterized
initially by a monomer-like emission response stayed the same
except for diminution of intensity, as seen in Figure 8. There
is evidence in the literature that the packing density of the
organic portions of the ZP structures is mediated by the lateral
13,53,54
spacing of the inorganic interlayer linking functionalities.
1
2 kcal/mol barriers at 90° and 270° (rings perpendicular). The
(
52) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Molecules; Academic Press: New York, 1971.
53) Schilling, M. L.; Katz, H. E.; Stein, S. M.; Shane, S. F.; Wilson,
(54) Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25,
(
420.
(55) Bent, S. F.; Schilling, M. L.; Wilson, W. L.; Katz, H. E.; Harris, A.
L. Chem. Mater. 1994, 6, 122.
W. L.; Buratto, S.; Ungashe, S. B.; Taylor, G. N.; Putvinski, T. M.; Chidsey,
C. E. D. Langmuir 1993, 9, 2156.

12794 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Horne and Blanchard
energetic minima occur at 0°, the anti conformer, and 180°,
the syn conformer, where the molecule can adopt a quinoid-
like S1 resonance structure. In a system where there is freedom
of motion, such as in solution, the conformational distribution
of the chromophore is unbiased with respect to interring rotation
prior to excitation. Upon excitation, the conformational dis-
tribution of BDP is biased toward the S1 minima, where the
molecules take on the planar, quinoidal structure. Planar BDP
chromophore pairs are more likely to aggregate than nonplanar
species if they are in close proximity, by a dipole coupling
mechanism. In a monolayer, we expect different behavior
because the chromophores are not able to rotate freely (Vide
infra). In the monomer limit within a monolayer, chromophores
are spaced such that the probability of intermolecular interactions
between chromophores is low, so the chromophores are excited
from an average of all conformations, resulting in an emission
spectrum with a distribution of energies skewed to high energies
by S1 emission contributions from twisted conformers. In more
concentrated monolayers, chromophores are likely to be close
enough to allow interaction which would serve to fix the
molecules in more planar conformations and result in the lower
energy spectrum. In addition, aggregation is expected to yield
5
2
a spectral red shift, as is seen for the pyrene excimer.
Rotational Dynamics. The rotational dynamics of the
chromophores within a monolayer are an information-rich
component of the optical response. Molecular reorientation
Figure 10. Reorientation data for a 26% BDP monolayer. (a) Raw
transient signal intensities for emission polarized parallel (upper) and
perpendicular (lower) to excitation pulse polarization. Included is the
instrumental response function. (b) Induced orientational anisotropy
function derived from the data presented in panel a and regressed fit.
measurements are well established as sensitive indicators of
intermolecular interactions.5
6-61
Most rotational diffusion
measurements are performed on probe molecules in solution,
where the orientational distribution of the chromophore can be
completely random, while for a (partial or complete) monolayer
of the same molecule, we expect its motional freedom to be
constrained both by surface attachment and by interactions with
neighboring molecules. For such a system, the hindered rotor
polynomial. For the hindered rotor model, it is important to
consider two limiting cases, t ) 0 and t ) ∞. At t ) 0, where
the second term in eq 2 is unity, the information available from
the measurement is determined by the optical properties of the
probe molecule and thus we can determine δ directly. For BDP
in solution we measure R(0) ) 0.24, yielding δ ) 31° ( 2°. At
t ) ∞, any rerandomization of the chromophores will be
complete and the information available from the measurement
is related only to the average tilt angle of the chromophore
ensemble within the layer according to the hindered rotor
6
2,63
model is appropriate for interpreting anisotropy data.
In
this model, the surface-bound probe molecule is confined to a
cone of rotation, where the base of the cone is defined by the
attachment point of the probe to the surface and the cone
semiangle is designated as θ0. The optical and geometric
properties of the tethered probe molecule also affect the form
of the experimental signal. We select a nonrandom orientational
distribution of chromophores within the monolayer with a laser
pulse and monitor the change in emission intensity as a function
of time following excitation. This measurement is made for
emission signals polarized parallel and perpendicular to the
excitation polarization, and these data are combined to produce
the induced orientational anisotropy function, R(t), (Figure 10):
6
3
model.
2
R(∞) ) 2/5P (cos θ ) P (cos θ )〈P (cos θ)〉
(3)
2
ex
2
em
2
In eq 3, θex is the angle of the excited transition moment with
respect to the cone center axis, θem is the emitting transition
moment angle with respect to this same axis, and θ is the
average tilt angle of the chromophore within the cone. The
terms for θex and θem are, together, representative of the angle
δ, and for simplicity, we take θex ) 0 and θem ) δ. From
these quantities we can extract the average tilt angle, θ, from
the experimental data.
I (t) - I (t)
|
⊥
R(t) )
(1)
I (t) + 2I (t)
|
⊥
The function R(t) is related to the properties of the probe
6
3
molecule through
In the hindered rotor model, there can also be a measurable
relaxation associated with the randomization of chromophore
orientations within the confining cone, with the time constant
of this relaxation being related to the motion of the chromophore
about its tether and the semiangle of the cone. It is evident
from the experimental data presented in Figure 10 that we do
not detect a transient relaxation of the chromophore within the
monolayer, once contributions from the instrumental response
function are accounted for. This finding is important in and of
itself because it implies the rigidity of these systems on a
nanosecond time scale, and from the infinite time response, we
can derive useful information on the average tilt of the
chromophore in the cone.
R(t) ) 2/5P (cos δ)〈P [µ(0)‚µ*(t)]〉
(2)
2
2
where δ is the angle between µ and µ*, the excited and emitting
transition moments and P2 is the second-order Legendre
(
(
(
(
(
(
(
(
56) Jiang, Y.; Blanchard, G. J. J. Phys. Chem. 1994, 98, 6436.
57) Blanchard, G. J. J. Phys. Chem. 1991, 95, 5293.
58) Blanchard, G. J. J. Phys. Chem. 1989, 93, 4315.
59) Blanchard, G. J. Anal. Chem. 1989, 61, 2394.
60) Blanchard, G. J. J. Phys. Chem. 1988, 92, 6303.
61) Blanchard, G. J.; Cihal, C. A. J. Phys. Chem. 1988, 92, 5950.
62) Szabo, A. J. Chem. Phys. 1984, 81, 150.
63) Lipari, G.; Szabo, A. Biophys. J. 1980, 30, 489.

Dynamics within a Single Molecular Layer
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12795
Table 2. Dependence of Infinite Time Anisotropy on Fractional
Composition of Monolayer
within the layer is initially monomer. Upon excitation, BDP
relaxes to its S1 conformational minima, where it is planar, and
if other planar chromophores are in close proximity, they will
aggregate. The second possibility is that the chromophore is
prealigned as aggregates in solution and is deposited onto the
surface as such. We found no evidence of motion in the
dynamical data, likely ruling out the possibility that aggregate
formation in BDP monolayers is driven by optical excitation.
We note also that, if aggregation were mediated by excitation
of BDP, then the measured population relaxation dynamics
would be more complex than the double-exponential decay
functionality we recover, and we would expect a chromophore
concentration dependence, which we fail to observe. We believe
that the deposition of preformed aggregates is more likely, based
on both the dynamics data and the observation of monomer and
aggregate bands in concentrated BDP deposition solution
spectra.
%BDP
R(∞)
1
00
0.06 ( 0.01
0.05 ( 0.01
0.07 ( 0.01
0.05 ( 0.01
5
3
2
8
4
6
It is fair to question whether the absence of a decay in R(t)
is reflective of the absence of dynamics or the presence of
unresolved, fast motion. We expect that the motion of BDP in
a low-viscosity solvent will place a qualitative lower bound on
the time constant for any such motion in the monolayer. For
BDP in 95% ethanol, the reorientation time we measure, τOR
)
299 ( 41 ps, is on the same order of magnitude as the
monomer lifetime. This reorientation time is significantly
slower than would be predicted by the Debye-Stokes-Einstein
6
4
65
model in the stick limit (60 ps), and we attribute this
difference to partial deprotonation of the terminal phosphonate
moieties. Ionic charge will give rise to strong association with
the solvent, slowing rotation of the chromophore. In a mono-
layer, the dynamics are expected to be mediated by attachment
of the chromophore to the substrate as well as by dipole-dipole
interactions between molecules and by the proximity of its
neighbors. We expect that, if the chromophore were to exhibit
a dynamical response within the monolayer, it would occur on
a time scale at least similar to that seen for BDP in ethanol, if
not longer. We believe that the absence of a measurable decay
in R(t) is indicative of a highly rigid environment. We recover
the same value for the steady-state anisotropy for all monolayer
concentrations studied, as shown in Table 2, another indication
that the chromophores are unable to randomize on the time scale
of the BDP excited-state lifetime. From the monolayer data,
R(∞) ) 0.06 ( 0.01, yielding θ ) 35° ( 2°. This value for
the tilt angle is similar to that recovered for alkanediylbis-
Conclusions
Our initial investigation of the optical response of a chro-
mophore-containing ZP monolayer indicates that these as-
semblies are ideal for the selective examination of interlayer
and intralayer energy relaxation phenomena. We have dem-
onstrated control over the concentration of chromophore in the
layer, which is limited by the higher affinity of BDP for the
metal binding sites. Some degree of aggregation occurs in the
BDP monolayers, as verified by the presence of the two species
detected with both the steady-state and fluorescence lifetime
spectroscopies. The presence of aggregated chromophore does
affect the population relaxation dynamics within the monolayer,
but in a way that can be monitored. The time constants for the
fluorescence decays remain concentration independent, ∼200
ps for the monomer and ∼1 ns for the aggregate, although the
contribution of each species’ lifetime varies across the emission
band. The rotational motion data are constant within the
chromophore concentration range 26%-100% BDP. We find
a steady-state anisotropy of ∼0.06, consistent with the restric-
tions of monolayer attachments and interactions, from which
we determine an average tilt angle of 35°. The absence of
rotational motion demonstrates that the chromophores are in a
highly rigid environment on the hundreds-of-picoseconds time-
scale of our experiments. This finding will simplify the
interpretation of results on interlayer transport within multilayer
structures because the chromophores should be even more
restricted in an environment with attachments at both phospho-
nate functionalities.
(
phosphonate)s14 but, for the BDP chromophores, indicates that
organization within the layer is not determined fully by the
structure of the (presumably all-anti) chromophore, which would
suggest a tilt angle of ∼17°. It appears that there is either a
contribution to the measured tilt angle of BDP from the metal-
phosphonate coordination or from intermolecular interactions
between the organic functionalities of the bis(phosphonate)
species. Unfortunately, we do not have sufficient knowledge
of the surface roughness or active site spacing to resolve this
question, especially in light of the several possible coordination
4
+ 66
numbers of Zr .
We can use the rotational dynamics results to clarify some
ambiguity about the origin of the aggregates. One explanation
for the presence of aggregate is that all of the chromophore
Acknowledgment. We are grateful to the National Science
Foundation for support of this work through Grant CHE 95-
0
8763. We thank C. J. Ruud for his help in the preparation of
(
64) Debye, P. Polar Molecules; Chemical Catalog Co.: New York,
1
929.
the bisphosphonated compounds.
(
(
65) Hu, C. M.; Zwanzig, R. J. Chem. Phys. 1974, 60, 4354.
66) Fay, R. C.; Pinnavaia, T. J. Inorg. Chem. 1968, 7, 508.
JA962021S