Effects of temperature and Alkyl groups of poly(Alkyl methacrylate)s on inter- and intramolecular interactions of excited singlet states of pyrenyl guest molecules

Article abstract of DOI:10.1021/jp105461r

Temperature-induced changes in the static and dynamic characteristics of the fluorescence from pyrene and N,N-dimethyl-3-(pyren-1-yl)propan-1-amine (PyC3NMe2) have been used to determine the locations and mobilities of these probes in the anisotropic environments provided by films of 5 poly(alkyl methacrylate) (PAMA) polymers in which alkyl is ethyl, butyl, isobutyl, cyclohexyl, and hexadecyl. Whereas emission from pyrene reports on the polarity of the guest sites and the ability of molecules to diffuse translationally between sites, emission from PyC3NMe2 yields information about the fluidity and the shape of the guest sites. Data have been obtained from 20 to >400 K, a range that spans the onsets of several relaxation processes in the hosts. Those data indicate that the pyrenyl groups reside near to ester functionalities in most of the PAMAs, although the distance from them (and the main chains) depends upon the bulkiness of the alkyl groups. Among the most important conclusions derived from this research is that the rates of segmental relaxation phenomena near the probe molecules-and not free volume, as was concluded previously from fluorescence measurements in polyethylene films-are the dominant contributors to the fluorescence changes. Of practical importance, changes in those rates have permitted the onset temperatures of many of the relaxation phenomena occurring in the vicinity of the probes to be located.

Full text of DOI:10.1021/jp105461r

J. Phys. Chem. B 2010, 114, 12221–12233
12221
Effects of Temperature and Alkyl Groups of Poly(Alkyl methacrylate)s on Inter- and
Intramolecular Interactions of Excited Singlet States of Pyrenyl Guest Molecules
Shibu Abraham,† Teresa D. Z. Atvars,‡ and Richard G. Weiss*,†
Department of Chemistry, Georgetown UniVersity, Washington, D.C. 20057-1227, and Instituto de Qu´ımica,
UniVersidade Estadual de Campinas, Caixa Postal 6154, Campinas, SP, Brazil
ReceiVed: June 14, 2010; ReVised Manuscript ReceiVed: August 22, 2010
Temperature-induced changes in the static and dynamic characteristics of the fluorescence from pyrene and
N,N-dimethyl-3-(pyren-1-yl)propan-1-amine (PyC3NMe2) have been used to determine the locations and
mobilities of these probes in the anisotropic environments provided by films of 5 poly(alkyl methacrylate)
(PAMA) polymers in which alkyl is ethyl, butyl, isobutyl, cyclohexyl, and hexadecyl. Whereas emission
from pyrene reports on the polarity of the guest sites and the ability of molecules to diffuse translationally
between sites, emission from PyC3NMe2 yields information about the fluidity and the shape of the guest
sites. Data have been obtained from 20 to >400 K, a range that spans the onsets of several relaxation processes
in the hosts. Those data indicate that the pyrenyl groups reside near to ester functionalities in most of the
PAMAs, although the distance from them (and the main chains) depends upon the bulkiness of the alkyl
groups. Among the most important conclusions derived from this research is that the rates of segmental
relaxation phenomena near the probe moleculessand not free volume, as was concluded previously from
fluorescence measurements in polyethylene filmssare the dominant contributors to the fluorescence changes.
Of practical importance, changes in those rates have permitted the onset temperatures of many of the relaxation
phenomena occurring in the vicinity of the probes to be located.
1. Introduction
ments that constitute their cages, but slow relaxation of these
chains can impede translational, rotational, and conformational
Polymers contain “voids” as a result of packing constraints
on their chains.1 These spaces facilitate chain segmental motions
and diffusion and, in some cases, aid in forming “templates”
that affect the reactivity of a guest molecule.2 In addition, the
reactions or spectroscopic responses of environmentally sensitive
guest molecules can be used to probe the microstructural features
of polymer matrixes3 and, especially, the sites in which the
guests reside.4-6 For example, fluorescence from pyrene (PyH)
and its derivatives is sensitive to temperature, polarity, and the
presence of various types of excited state quenchers,7 and the
relaxations of polymer chains can have a marked influence on
the luminescence of PyH molecules. Thus, the intensity ratios
of the third most energetic and the most energetic vibronic bands
in the fluorescence spectrum of PyH (I3/I1) and decay constants
of its excited singlet state have been shown to depend upon the
exact composition of ethylene-co-vinyl acetate copolymers as
well as to identify olefinic segments as the loci for preferential
attachment of the PyH molecules.8 In addition, the amount of
void space surrounding a guest molecule during the course of
its photoreactions in constraining polymer matrixes can lead to
products (or intermediates) that are totally different from those
formed in solution state reactions.9
motions of guest molecules so that they occur on time scales
that are longer than necessary for reactants to move to
products.11 In this regard, chain relaxation processes, especially
in the glassy states of neat polymers, such as a poly(alkyl
methacrylate)(PAMA),canbeveryslowandlocallyanisotropic.12,13
For example, PAMAs with long n-alkyl groups have nanoseg-
regated regions enriched with either low polarity alkyl or higher
polarity ester groups14 that provide potentially very different
environments to guest molecules.
The rates of relaxation of polymer segments for rotation of
the ester (-COOR) side groups (ꢀ-relaxation) and movements
of the backbone chains (R-relaxation) in several PAMAs are
available over wide temperature ranges.15 As the length of an
alkyl side group (R) increases, neighboring backbone chains
are pushed further apart, causing “internal plasticization” and
decreasing the barriers to movement of the chains.15 Therefore,
the activation energy of the R-relaxation process decreases with
increasing alkyl chain length, whereas that of the ꢀ-relaxation
decreases to a negligible extent:15 the activation energy for
R-relaxation decreases from ∼419 kJ mol-1 for poly(methyl
methacrylate) to 205 kJ mol-1 for poly(n-propyl methacrylate);
the corresponding values for ꢀ-relaxation in both polymers are
Because the lifetimes of the “cages” in which guest reactions
reside in isotropic solvents are very short (i.e., comparable to
the times needed for the substrates to change to product
structures), substantial cage-escape of substrate fragments and
minimal spatial templating by the cage are expected.10 In
polymer matrixes, the guest molecules template the conforma-
tions and orientations of the neighboring polymer chain seg-
∼88 kJ mol-1 15,16
.
Here, we investigate the dependence of the optical properties
of two luminescent guest molecules, PyH and N,N-dimethyl-
3-(pyren-1-yl)propan-1-amine (PyC3NMe2), on the length and
size of alkyl side chains of five different PAMA hosts above
and below their glass or melting transition temperatures. Both
intermolecular excimer formation (i.e., static from ground-state
aggregation or dynamic from translational diffusion) and
intramolecular exciplex formation (i.e., from conformational
changes) have been monitored. From correlations of the static
* E-mail: weissr@georgetown.edu.
† Georgetown University.
‡ Universidade Estadual de Campinas.
10.1021/jp105461r  2010 American Chemical Society
Published on Web 09/08/2010

12222 J. Phys. Chem. B, Vol. 114, No. 38, 2010
Abraham et al.
TABLE 1: Phase Transition Temperatures, Densities, And
Average Molecular Weights of the PAMAs Employed in
This Study
(Aldrich, 99%) was recrystallized twice from ethanol (mp
425-426 K (lit.19,20 mp 425-426 K)).
2.2. Syntheses. PyC3NMe2 was synthesized as shown in
Scheme 1.21 Details of experimental procedures and character-
izations are described in the Supporting Information. PyC3NMe2
was stored as its hydrochloride salt, N,N-dimethyl-3-(pyren-1-
yl)propyl-1-ammonium chloride hydrate: mp 456-472 K (dec).
1H NMR (CDCl3, 400 MHz), δ 2.46-2.50 (m, 2H), 2.71-2.72
(m, 6H), 3.02-3.05 (m, 2H), 3.50-3.53 (t, 2H, J ) 7 Hz),
7.86-7.88 (d, 1H, J ) 7.6), 8.0-8.19 (m, 8H). Elemental
analysis: calcd for C21H22ClN·H2O: C, 73.78%; H, 7.08%; N,
4.10%. Found: C, 74.12%; H, 7.29%; N, 4.10%. In a typical
procedure to regenerate PyC3NMe2, N,N-dimethyl-3-(pyren-
1-yl)propan-1-ammonium chloride hydrate (2 mg) was stirred
with 10 mL of diethyl ether and 5 mL of 30% aqueous NaOH
for 5 min under a nitrogen atmosphere. The mixture was
extracted with diethyl ether (3 × 20 mL), and the combined
extracts were washed twice with 20 mL of water and dried over
anhydrous Na2SO4. The volatile liquid was concentrated under
reduced pressure in a rotary evaporator to obtain PyC3NMe2
phase
transition
densityb,c
mol wtc
(Mw)
polymer
temperaturea (K)
(g cc-1
)
PCHMA
PEMA
PIBMA
PBMA
368
342
322
290
281
1.10
1.11
1.05
1.07
0.87
65 000
350 000
70 000
180 000
200 000
PHDMA
a Glass transition temperatures (Tg) of the PAMA, except
PHDMA, for which the melting temperature of crystallized side
chains (Tm) is given (vide infra), from DSC measurements. b At 298
K. c Data from suppliers.
and dynamic fluorescence properties of the two probe
molecules, we have discerned how the structures of the alkyl
chains and phases of PAMA influence intimate solute-
matrix interactions. Specifically, we have examined transla-
tional and microdiffusional motions of PyH molecules and
conformational preferences and labilities of PyC3NMe2
molecules within the PAMA matrixes using steady-state and
time-resolved fluorescence. We find that hole-free volumes
(which are related to the sizes of the cavities in which the
probe molecules reside) of the PAMAs are much less
important than the rates of relaxation of their various chain
segments (which are related to the rates of size and shape
changes of the guest cavities17) in mediating the fluorescence
properties of the probe molecules. This conclusion was
unexpected because changes in the fluorescence of different
probe molecules indicate that hole-free volumes are the more
important factor in polyethylene matrixes.4,18
1
(∼99% pure by GC analysis) as a viscous liquid. H NMR
(CDCl3, 400 MHz): δ 1.99-2.07 (m, 2H), 2.27 (s, 6H),
2.42-2.45 (t, 2H, J ) 7.2 Hz), 3.36-3.40 (t, 2H, J ) 7.8 Hz),
7.87-7.89 (d, 1H, J ) 8), 7.96-8.17 (m, 7H), 8.29-8.31 (d,
1H, J ) 9.6). MS-EI: m/z observed, 288; calculated, 287.4 (M+,
C21H21N).
2.3. Instrumentation. UV/vis absorption spectra were re-
corded on a Varian UV-visible (Cary 300 Bio) spectropho-
tometer. Steady-state excitation and emission spectra were
obtained on a Photon Technology International Fluorometer
(SYS 2459) with a Quantumwest temperature controller and
an Omega temperature probe. The probe was placed in decane
liquid that filled the cuvette in which the sample container, a 7
mm (width) × 4 mm (length) × (0.4 or 3 mm (i.d.)) flattened
Pyrex capillary (Vitro Dynamics, Inc.) was placed. Spectra were
recorded front-face at an angle of ∼45° with respect to the
incident beam, and the emission was collected at 90° with
respect to the excitation source. Fluorescence measurements of
PyH in the PAMAs were carried out at 20-420 K at intervals
of 10° using a SPEX 0.5 m spectrograph. Excitation was with
the 325 nm line of a Kimmon model IK helium-cadmium laser.
Fluorescence rise and decay histograms were obtained with an
Edinburgh Analytical Instruments time-correlated single photon
counter (model FL900) using H2 as the lamp gas. An “instrument
response function” was determined using Ludox as scatterer.
Data were collected in 1023 channels. Deconvolution was
performed by nonlinear least-squares routines that minimize ꢁ2
using software supplied by Edinburgh. Fits were considered
acceptable when ꢁ2 e 1.3, and residual plots exhibited no
systematic deviations from zero. Differential scanning calori-
metric measurements were carried out on a TA Instruments
differential scanning calorimeter (Q 200) under a nitrogen
atmosphere with a heating/cooling rate of 10 °C/min. Glass
2. Experimental Section
2.1. Reagents/Materials. Poly(butyl methacrylate) (PBMA),
poly(hexadecyl methacrylate) (PHDMA), poly(ethyl methacry-
late) (PEMA), and poly(cyclohexyl methacrylate) (PCHMA)
were purchased from Scientific Polymer Products, Inc., and
poly(isobutyl methacrylate) (PIBMA) was from Aldrich. They
were purified as described in the Supporting Information. Some
characteristics of these polymers are included in Table 1. PyH
SCHEME 1

Interactions of Pyrenyl Guest Molecules
J. Phys. Chem. B, Vol. 114, No. 38, 2010 12223
transition temperatures were determined using Universal Analy-
sis 2000 software supplied by TA Instruments from the onset
of intersection of the first and second tangents of the baseline
displacements in thermograms.
2.4. Preparation of Doped Films. Doped films, 0.2-0.8 mm
thick, of all PAMAs except PHDMA were prepared by
dissolving PyH or PyC3NMe2 and a polymer in dichlo-
romethane or chloroform and pouring the solution onto a Teflon
plate. The film was dried by evaporation at room temperature,
and the surfaces were washed with methanol. Doped PBMA,
PIBMA, PCHMA, and PEMA films were heated to their glass
transition temperatures and cut to the desired sizes.
Doped films of PHDMA were prepared by placing dichlo-
romethane solutions of it and PyH or PyC3NMe2 into flattened
Pyrex cuvettes (∼0.4 mm thickness). The solvent was evapo-
rated by warming the solutions on a water bath (∼345 K), with
a drying tube filled with anhydrous CaSO4 connected to the
cuvette and then in a desiccator at ∼0.2 Torr for 6-12 h
(dynamic vacuum) and 12-24 h (static vacuum).
Figure 1. (A) Absorption (solid line) and excitation (λem 400 nm,
dashed line) spectra and (B) emission (λex 340 nm) spectra of PyH in
PHDMA (9 × 10-5 mol/(kg PHDMA)) at 293 K.
Concentrations of dopant molecules in the solvent-free films
(∼10-4 mol/(kg of polymer)) were calculated from absorption
spectra using Beer’s law and film thicknesses that were averages
of at least five different positions.22 The films were flame-sealed
in flattened glass capillaries after being degassed at least 5 times
by freeze-pump-thaw cycles at <10-5 Torr in a mercury-free
vacuum line.
Figure 2. I3/I1 ratios from ∼10-4 mol PyH/(kg PAMA) (λex 325 nm)
recorded from low to high temperatures: PEMA (3), PBMA (0),
PHDMA (4), PIBMA (f), and PCHMA (O). Arrows refer to melting
or glass transition temperatures measured by differential scanning
calorimetry.
3. Results
3.1. Melting or Glass Transitions in PAMAs. Although
main chain tacticity and molecular weight influence the nature
and temperature of the glass or melting transitions in PAMAs,
the type of alkyl group attached to the carboxy moieties is the
most important factor when the average main chain lengths are
in the range of the polymers described in Table 1.23 Increasing
the length of alkyl side chains decreases the glass transition
temperatures of PAMAs.24,25 For example, when the alkyl chain
length increased from ethyl (PEMA) to butyl (PBMA), the glass
transition temperature decreased by 52°. On the other hand,
replacing a linear n-alkyl side chain by a branched isomeric
one is known to increase the glass transition temperature:15 the
glass transition of PIBMA is 32° higher than that of PBMA,
and the glass transition temperature of PCHMA, 368 K, is much
higher than those observed for the n-alkyl derivatives. When
the n-alkyl side chain is g12 carbon atoms in length, the
PAMAs form crystallized regions that undergo a first-order
melting transitions:23 in PHDMA, Tm ) 281 K, and the
crystalline hexadecyl chains form microdomains that separate
the noncrystalline main chains.15
3.2. PyH-Doped PAMAs. 3.2.1. Absorption and Emission
Studies of Doped Polymers. Films of the PAMAs are transparent
above 300 nm so that the structured π-π* absorption bands
from PyH and PyC3NMe2 at 300-350 nm and fluorescence at
360-450 nm (or to >450 nm for excimers or exciplexes) were
clearly discernible (Figure 1). The excitation spectra were similar
in shape and position to the corresponding absorption spectra
but were offset or broadened slightly in some cases. These
differences may result from the nonuniformity of sites in which
the guest molecules reside or to slight differences between the
electronic characteristics of isolated and aggregated pyrenyl
groups.3
molecules may experience more or less polar environments,
depending upon their locations within the polymer matrixes.
Preferential confinement of guest molecules to a specific type
of environment can be expected in these polymers, especially
in their glassy states. Regardless, guest molecules are excluded
from the nanocrystalline regions of hexadecyl chains in
PHDMA2,26 so that PyH and PyC3NMe2 reside in regions of
higher polarity, on average, below Tm than above it. Movement
of guest molecules between and within sites is facilitated in
films of the PAMAs above their Tg’s as a result of more rapid
relaxation processes. The same is true for PHDMA above its
Tm because of increased rates of polymer relaxation and,
additionally, the ability of the guests to access the regions where
the hexadecyl chains are melted. Thus, steady-state fluorescence
measurements and I3/I1 intensity ratios are indicative of the
ensemble averages of sites in which guest molecules reside in
the PAMAs.
The 0-0 electronic transition associated with the I1 band from
PyH is more strongly coupled than the I2 or I3 band to vibronic
aspects of the medium;27 the I1 band is the most sensitive, and
I3 is the least sensitive to solvent polarity. For that reason, only
I3/I1 ratios in the PAMA matrixes are discussed in detail here.
Both temperature and solvent polarity can affect I3/I1 ratios.7,28
However, the changes induced by temperature alone are smooth
and continuous; abrupt excursions of I3/I1 values over small
temperature increments indicate large changes in the probe
environment and, thus, the onset or cessation of a relaxation
process in the polymer film.
The I3/I1 ratios from PyH in the PAMAs did not change
appreciably at temperatures below ∼100 K (Figure 2). Similar
effects were observed for I2/I1 ratios in PEMA, PBMA, and
PCHMA; the intensities of the I2 band from PyH in PHDMA,
and PIBMA were too weak to allow precise I2/I1 ratios to be
calculated (Figure S1 of Supporting Information). Above 100
K, I3/I1 increased slightly up to the glass transition (or melting)
3.2.2. I3/I1 Ratios and Changes in Fluorescence Intensities
as a Function of Temperature. As mentioned, PAMAs consist
of more polar ester groups and less polar alkyl groups that tend
to aggregate into nanosegregated spaces. Therefore, guest

12224 J. Phys. Chem. B, Vol. 114, No. 38, 2010
Abraham et al.
Figure 3. Effect of temperature on (A) the emission spectra (λex 325 nm) of ∼10-4 mol PyH/(kg PBMA): black line, 20 K; red line, 120 K; green
line, 180 K; blue line, 210 K; cyan line, 250 K; magenta line, 280 K; and orange line, 420 K; and (B) the intensity of the 366 nm fluorescence band
of ∼10-4 mol PyH/(kg PAMA): PHDMA, 4; PBMA, 0; PIBMA, f; PEMA, 3; and PCHMA, O. The inset in A shows the isoemissive point
region within the rectangle. Arrows in B refer to bulk melting or glass transition temperatures from differential scanning calorimetry.
temperatures. For example, I3/I1 from 8 × 10-5 mol PyH/(kg
PBMA) increased from 0.75 at 120 K to only 0.89 at 270 K
(near the glass transition temperature, 290 K) but rose to 0.96
at 290 K and 1.82 at 420 K. The lack of an exact correspondence
between the I3/I1 inflection temperature and Tg (or Tm) can be
explained by how the two phenomena are measured: I3/I1 ratios
are monitors of local environments where PyH molecules reside;
glass or melting transitions are bulk properties of the polymers
that are almost impervious to local disturbances caused by the
very low PyH concentrations employed here.
Similar increases in I3/I1 were observed in the other glass
forming PAMAs as temperature was increased (Figure 2). In
heated films of PHDMA, a different behavior was observed:
I3/I1 increased abruptly near 290 K and then decreased at a
slightly higher temperature. We suspect that this excursion is
related to the onset of relaxation processes in the PHDMA
chains. The general trend of the I3/I1 ratios, to increase slightly
below the Tg (or Tm) temperatures and then to increase rapidly
above them, is indicative of an average environment that is more
polar below the glass (or melting) transition temperature (i.e.,
PyH molecules reside closer to the ester groups). This conclusion
is supported by observations in other media that I3/I1 ratios are
less sensitive to temperature in low polarity environments than
in more polar ones.28
As with the I3/I1 ratios, the overall PyH fluorescence
intensities in the PAMAs do not change with temperature in a
regular fashion. Instead, small but perceptible slope changes
are observed that, in many cases, can be associated with the
onsets on heating (or cessations on cooling) of host relaxation
processes. Although the overall intensities decrease with
increasing temperature, a small band centered at 366 nm and
to the blue of the 0-0 (I1) emission band, first increases (above
∼100 K in all of the PAMAs, except PHDMA, for which the
break commences at lower temperature) and then decreases with
increasing temperature (Figure 3); rotations of side chains (or
A 365 nm hot band from emission of PyH in polyethylenes
(PEs) has been attributed to guest molecules located in the
interfacial regions (i.e., between melted and crystalline do-
mains).3 In PAMAs, the presence of an isoemissive point at
368 nm (or at 369 nm in PCHMA) for PyH indicates that the
increased population of vibronically excited excited singlet states
is at the expense of excited singlets in their zeroth vibronic level.
The difference in maxima between the hot band and 0-0 band
for PyH in the PAMAs, ∼485 cm-1 (0.06 eV), is similar to the
energies of vibrations of methylene chains; thus, the intensity
changes at 366 nm may be related to vibronic coupling between
PyH singlet states and alkyl groups of the PAMA that constitute
the cavity walls.35
3.2.3. Monomeric and Excimeric PyH Emissions in PAMA
Films. As expected, increasing the PyH concentration at one
temperature increased the intensity of excimer emission (cen-
tered near 480 nm) relative to that of the monomeric emissions
(Figure 4). Excimer emission was observed at much lower
concentrations in both solid and melted states of PHDMA than
in the other polymers. The excitation maxima of excimer and
monomer are not affected by PyH concentration in PHDMA,
although the spectra were broader at higher concentrations. (see
Figure S2 of the Supporting Information). As mentioned,
spectral differences observed in PHDMA at different PyH
concentrations may result from inhomogeneities (i.e., different
site types) in the polymer matrix.3
The independence of the shapes of the absorption and
excitation spectra to PyH concentration below ∼10-3 mol/(kg
PAMA) as well as to emission wavelength for the excitation
spectra are suggestive of very little ground state aggregation
and a dominance of dynamically formed excimers in PHDMA.
Moreover, the peak-to-valley (P-V) ratios7b for PyH excitation
spectra at a concentration of 1.5 × 10-3 mol/(kg PHDMA) were
similar when acquired at emission wavelengths of 480 (exci-
meric emission) and 380 nm (monomeric emission). At much
lower concentrations (<10-3 mol/(kg PHDMA)), the excitation
spectra collected at emission wavelengths g450 nm were too
weak to make realistic P-V ratio comparisons. Consistent with
the presence of ground state PyH aggregates in the other
PAMAs,36 the absorption and excitation spectra were red-shifted
with respect to those in PHDMA. These results indicate that
the presence of long alkyl chains in PHDMA, even below its
Tm, allow dynamic processes leading to excimers, whereas the
shorter or branched chains in the other more rigid PAMAs
decrease diffusion within the excited singlet lifetimes of the
PyH molecules. Thus, most of the excimer-like emission
emanates from excitation of ground state aggregates in glass
forming PAMAs.
γ-relaxations) are known to commence near 100 K (Tγ
)
102-125 K29). Thus, the increase in the 366 nm band intensity
may be related to enhanced electron-phonon coupling,3 whereas
the subsequent decrease is probably a consequence of enhanced
competing matrix-related decay processes (i.e., main-chain
mobility associated with R-relaxations that accompany transi-
tions from the glassy to melted states;30,31 vide infra) becoming
dominant. The observed decrease in the intensity of the 366
nm band from 40 to 80 K in PHDMA may be due to increased
mobility of short segments of flexible alkyl side chains.
Hot bands in emission have been observed in a number of
organic lumophores in which upper vibronic levels are populated
or excited state energies are split by coupling with a host.32-34

Interactions of Pyrenyl Guest Molecules
J. Phys. Chem. B, Vol. 114, No. 38, 2010 12225
Figure 4. Intensity-normalized (at 376 nm) emission spectra at different concentrations of PyH in PAMA films at 293 K (λex 340 nm): (A) 9 ×
10-5 (s) and 1.5 × 10-3 (- - -) mol/(kg PHDMA; (B) 10-4 (s), 9.6 × 10-3 (- - -), and 0.215 ( · · · · ) mol/(kg PBMA).
Figure 5. Emission spectra (λex 340 nm) of PyH in PAMA films: (A) 1.5 × 10-3 mol/(kg PHDMA) (s, 263 K; - - -, 293 K; · · · · , 323 K;
-·-·-, 343 K) and (B) 0.215 mol/(kg PBMA) (s, 263 K; - - -, 298 K; · · · · , 331 K; -·-·-, 341 K).
Reabsorption of the emitted light (and significant aggregation)
at 0.215 mol PyH/(kg PBMA) is evidenced by a relative
decrease in the intensity of the I1 band (Figure 4B) and distortion
of the emission spectrum. Therefore, no attempt has been made
to analyze data from this sample quantitatively. However,
additional analysis of these results, based upon dynamic
measurements, will be presented later.
Above Tm in PHDMA films, the intensity of the red-shifted
excimer emission was considerably reduced relative to the
monomer emission (Figure 5A). This comportment suggests a
decrease in intermolecular interactions among guest molecules
Figure 6. Emission spectra (λex 345 nm) of 10-4 mol PyC3NMe2/(kg
PIBMA) at 293 K (s), 327 K (- - -), and 360 K ( · · · · ).
at the higher temperatures or increased dissociation of the
excimers.37 In PBMA at temperatures both below and above
the glass transition, no excimer emission was observed when
PyH was ∼10-3 mol/(kg PBMA); at an exceedingly high PyH
intensities were much lower than those in isotropic solvents of
concentration, 0.215 mol/kg, excimeric emission was observed,
similar polarity, such as ethyl acetate.40 These lower ratios are
and its intensity increased with increasing temperature (Figure
attributed to the very high microviscosities in the films that
5B) as a result of ground state aggregation and the need for
inhibit the motions of the propylene spacer that are necessary
1
much less motion for a PyH to encounter a ground state PyH
to bring the 1-pyrenyl and dimethylamino groups into the correct
molecule. In PBMA films at this very high PyH concentration,
there was a relative increase in excimer fluorescence as the
temperature was increased and an isoemissive point at 428 nm.
exciplex orientation.20,41,42
In PIBMA and PCHMA, the emission spectrum of PyC3NMe2
consisted of 1-pyrenyl peaks at 379, 399, and 420 nm, and a
The preference for monomer f excimer emissions at higher
broad exciplex emission centered near 450 nm (Figures 6 and
temperatures is consistent with increased mobility or reorienta-
S3 of the Supporting Information). The intensities of the
tions of the PBMA chains. Above the glass transition temper-
monomeric and exciplex emissions decreased and increased,
respectively, as temperature was increased (Figure S4 of the
Supporting Information). These changes were accompanied by
an isoemissive point at 429 nm (Figure 6) and were reproducible
when the temperature was increased or decreased (Figures S5
of the Supporting Information). In the PAMAs with the
unbranched alkyl side chains, PHDMA, PBMA, and PEMA,
the exciplex emission intensities were small and changed very
little with increasing temperature (Figure S6 of the Supporting
Information). We attribute these differences in the exciplex
ature, the mobility of chains may facilitate excimer formation
by bringing PyH molecules into the appropriate proximity and
orientation. Similar temperature-induced orientational changes
have been reported for methyl 4-(1-pyrenyl)butanoate doped
into polystyrene matrixes.12
3.3. PyC3NMe2 in PAMA Films. 3.3.1. Absorption and
Static Emission Studies. PyC3NMe2 is known to form intramo-
lecular exciplexes whose emission intensity depends upon
factors that include solvent polarity and temperature.38,39 In all
of the PAMA films, the ratios of monomer to exciplex emission

12226 J. Phys. Chem. B, Vol. 114, No. 38, 2010
Abraham et al.
TABLE 2: Decay Components (τi), Their Percentages (in parentheses), and Preexponentials (Ai) from Analyses of Histograms
from ∼10-4 mol/(kg PAMA) Films at 298 Ka
lumophore
PyHb,c
polymer
λ
em, nm
τ1, ns
A1
τ2, ns
A2
ꢁ2
PEMA
380
380
380
380
380
380
450
450
380
450
450
380
450
450
380
450
450
380
450
450
310
318
329
333
1.2
1.2
1.1
1.2
1.2
1.2
1.2
1.3
1.3
1.3
1.3
1.1
1.1
1.1
1.1
1.2
1.2
1.2
1.2
1.1
PCHMA
PIBMA
PBMA
PHDMA
PEMA
366
PyC3NMe2c,d,e
184 (93)
191 (84)
184 (87)
208 (93)
202 (89)
208 (88)
186 (92)
166 (79)
186 (73)
200 (75)
175 (61)
200 (47)
190 (80)
182 (80)
190 (75)
0.566
0.096
0.102
0.350
0.095
0.091
0.116
0.023
0.020
0.210
0.092
0.063
0.213
0.096
0.087
50 (7)
51 (8)
36 (5)
77 (7)
24 (7)
0.158
0.032
0.031
0.070
0.062
0.060
0.024
0.018
0.017
0.122
0.139
0.140
0.109
0.054
0.061
PBMA
30 (8)
56 (6)
PHDMA
PIBMA
PCHMA
33 (12)
51 (17)
115 (25)
64 (34)
85 (44)
91 (20)
56 (14)
66 (18)
b
a PHDMA and PBMA are in their melted states at this temperature, and the other PAMAs are in their glassy states. λex ) 340 nm;
percentage of one decay component was >98%; the second minor component is not included. c A e 1 ns component (presumably from
d
scattered excitation radiation) is not included. λex ) 345 nm. e 450 nm decays were fitted by either allowing all components to change or
keeping constant the τ1 component at the 380 nm excitation value (shown in italics) while optimizing the total curve fit.
emission to conformational preferences of PyC3NMe2 and to
variations in the rates of the side-chain relaxations in PAMAs
(vide infra).
(ΦM) is expressed by eq 1, where kM is the rate constant for
radiative decay and is independent of temperature within the
range explored here,19 and kiM is the rate constant for the sum
of internal conversion and intersystem crossing. Because ΦM
is directly proportional to the total intensity of monomer
fluorescence (IM), the quantum yield and intensity ratios are
equal at two different temperatures when the sample position
and instrumental parameters are unchanged. Under these condi-
tions, eq 2 applies; see Supporting Information for a derivation.
3.4. Dynamic Emission Studies. At 298 K, time-correlated
single-photon counting histograms from films of ∼10-4 mol
PyH/(kg PAMA) were monoexponential decays (>98%) with
lifetimes of >300 ns (Table 2) whose magnitudes parallel the
expected PAMA polarities: faster decays were observed in the
higher polarity PEMA (310 ns) than in the lower polarity
PHDMA (366 ns). At higher PyH concentrations (∼0.2 mol/
(kg PAMA)) in all of the PAMAs except PHDMA, the lifetimes
remained near 300 ns (Table S1 of the Supporting Information).
In PHDMA, the decay constants were 155 and 366 ns in 1.5 ×
10-3 and ∼10-4 mol PyH/kg, respectively (Table S2 and Figure
S7 of the Supporting Information). These results are consistent
with much more rapid diffusion of PyH in PHDMA (above Tm)
than in the other PAMAs (vide ante).
kM
ΦM
)
(1)
(2)
k
+ kiM
)
(
M
I0
IF
Ea
ln
- 1 ) constant -
(
)
RT
Films of ∼10-4 mol PyC3NMe2/(kg PAMA) yielded decay
histograms that were biexponential, with the major component
(>75%) having a 186-200 ns decay constant (Table 2). The
shorter decay component may be from excitation of ground state
molecules, which are in a conformation close to that necessary
to form an exciplex.6 Because decay constants from histograms
monitored at longer emission wavelengths did not possess a
negative pre-exponential factor (Table 2), it is likely that
intramolecular formation of exciplexes is occurring from
partially folded conformers on a very rapid time scale (<1 ns)
that is unresolvable with our instrumentation. Consistent with
this hypothesis, the amount of the shorter decay is higher in
the glassy states of the two polymers with the branched alkyl
groups, PIBMA and PCHMA.
In eq 2, I0 and IF are areas under the fluorescence curves at
very low temperatures (where kiM is constant) and at a higher
temperature, respectively. In the present study, integrated
fluorescence areas (from emission spectra of intensity versus
wavenumber) are plotted as a function of temperature from 20
to 420 K; I0 was estimated from the average area of fluorescence
intensities over a range of temperatures from 20 K to a higher
temperature at which the area remained almost constant. If, at
least in part, changes in kiM are a consequence of coupling
between the vibronic modes of the PyH excited singlet state
and relaxation modes of nearby polymer chains, the energy of
activation (Ea) in eq 2 includes the contributions from relaxations
of polymer chains near the lumophores.
4.2. Dynamic and Static Formation of PyH Excimers.
Because the dissociation of [PyH + PyH] should be very slow
in the PAMA polymer films, the florescence intensity ratios for
excimer and monomer emissions, I480/IM, under photostationary
state conditions are related to the rate constants in Scheme 2
by eq 3;43 the second term on the right side of the equation
needs to be included only when excitation of the ground state
4. Kinetic and Spectral Analyses
4.1. Monomer Fluorescence. Changes in monomer fluo-
rescence intensity at j10-4 mol PyH/(kg PAMA) (i.e., where
no excimeric emission could be detected) were used to determine
the activation energies for nonradiative processes. Under these
conditions, the quantum yield for monomer fluorescence of PyH

Interactions of Pyrenyl Guest Molecules
J. Phys. Chem. B, Vol. 114, No. 38, 2010 12227
SCHEME 3
SCHEME 2: Kinetic Steps for Formation of PyH
1
Excimer, PyD, via Static and Dynamic Pathwaysa
a PyH + PyH is the ground state dimer, and “R” is the fraction of
light absorbed by non-aggregated PyH molecules.
SCHEME 4:46
influence significantly the rates of intramolecular exciplex
formation.38,45 Exciplex formation by PyC3NMe2 requires the
lone-pair orbital on nitrogen to reside over and be approximately
perpendicular to the π-orbitals of the pyrenyl group.19
Figure 7. Arrhenius plots of the ratio of excimer to monomer
fluorescence intensities of 1.5 × 10-3 mol PyH/(kg PHDMA)) (λex 325
nm). Activation energies (E′a) are calculated for temperature ranges
below and above Tm (indicated by the vertical dashed line).
For purposes of analysis, we assume that the two principal
conformers of PyC3NMe2 (A and B in Scheme 3), present in
mole fractions, fA and fB,38,46 have similar extinction coefficients.
aggregates contributes to the emission intensity at 480 nm; see
the Supporting Information for details.
1
The excited singlet state of A, A, becomes the exciplex (1E)
after a shape change that involves two C-C bond rotations and
1
1
overcoming a small energy barrier; to become E, B must
undergo a larger shape change (that may or may not include
formation of the 1A conformation) unless inversion about
nitrogen or rotation around the H2C-N bond occurs very rapidly
with respect to C-C rotations. The steps associated with
interconversions among these conformers and the formation of
their exciplex-like conformation (E) are shown in Scheme 4.
In low-viscosity liquid solutions, PyC3NMe2 exists mostly
in its A conformer and interconverts with the B conformer on
time scales of ∼10 ns.38 In glassy and melted PAMA films,
fluorescence decays of the locally excited pyrenyl singlet states
of PyC3NMe2 are biexponential (Table 2), suggesting the
presence of more than one kinetically indistinguishable family
of PyC3NMe2 conformers. In addition, in the glassy states of
the more rigid PIBMAsand especially in PCHMA films (and
probably in the glassy and even melted states of the other
polymers)sdissociation of the exciplex is expected to be very
slow (i.e., k-A or k-B , k4). Although, C-N rotation or nitrogen
inversion in acyclic amines have similar energy barriers in
solution (<30 kJ mol-1);47-49 inversion (being less volume
demanding) is more facile in polymer cavities.48 On these bases,
a mixture of A and B conformers appears to be present in the
PAMA films. Thus, if kA ) kB ) k, quantum yields of monomer
(ΦM) and exciplex (ΦE) emissions (or corresponding fluores-
cence intensities, IM and IE) can be related to the rate constants
in Scheme 4 by eq 4.38,46
I480
IM
R2kDFkD[PyH]
+ kiD + k-D + k-′ D
≈
+
kM
k
(
)
DF
1
[
]
R(1 - R)kDFkD′ PyH + PyH
(3)
1
[
]
kM
k
+ kiD + k-D + k-′ D PyH
(
)
DF
No evidence for ground state aggregation is observed in
j10-4 mol PyH/(kg PAMA) films at all temperatures examined
here. Thus, in these films, the second term on the right side of
eq 3 can be neglected because all of the excimer formation is
presumed to be dynamic (i.e., no [PyH + PyH] is present) and
the activation energy of excimer formation (E′) can be obtained
a
from the slope of an Arrhenius plot of I480/IM, provided radiative
the decay constants (kM and kDF) are temperature-independent
and kiD and k-D are smaller than kDF.44
In PBMA, PEMA, PIBMA, and PCHMA, a considerable
amount of PyH ground state aggregation was indicated by an
instantaneous component in excimer rise times (Table S1 of
the Supporting Information) at higher PyH concentrations
necessary to observe its emission. Thus, the simplified form of
eq 3 can be applied only to PHDMA, in which dynamic excimer
formation with a growing-in excimer component (Table S2 of
the Supporting Information) is observed at a relatively low
concentration, 1.5 × 10-3 mol PyH/(kg PHDMA). An Arrhenius
plot of excimer-monomer emission in PHDMA is shown in
Figure 7, along with the activation energies (E′) calculated in
two different temperature regimes. The values suggest similar
deactivation pathways of PyH in both the solid and melted
states of PHDMA.
a
ΦE
kEk
IE
1
)
=
(4)
ΦM
kM(kE + kiE)
IM
4.3. Intramolecular Exciplex Formation from PyC3NMe2.
Motions of the propylene chain of PyC3NMe2, especially those
emanating from its two principal conformations, are known to
Furthermore, assuming the same temperature dependence for
kE and kM,38,44 the difference between the activation energies

12228 J. Phys. Chem. B, Vol. 114, No. 38, 2010
Abraham et al.
alkyl side chains increase the average distance between neigh-
boring main chains, facilitate the formation of voids,15 and allow
short-range translational mobility, even in the solid state. The
side groups in the glassy states of PIBMA and PCHMA restrict
guest mobility,52 but their cavities are sufficiently flexible to
permit short-range conformational changes at higher tempera-
tures (i.e., especially in the rubbery states).
Hole free-volume can be a useful parameter to understand
the structural and dynamic properties guest molecules in some
polymer matrixes,6,53 such as those afforded by PEs.4 In these
polymers, alkyl chains are folded into crystalline regions with
interfacial regions at their borders and more disordered chains
in amorphous regions.54 Guest molecules are excluded from
entering the crystalline regions, but do divide themselves
between the interfacial and amorphous regions.2 The largest
mean void volume in any of the PAMAs investigated here as
determined by positron annihilation spectroscopy, 213 Å3, is
smaller than the van der Waals volume of even a PyH molecule,
322 Å3.22,55 Thus, the polymer components that make up the
cavity walls are expected to be in close contact with the pyrenyl
guest molecules.
Figure 8. Arrhenius plot of the ratio of exciplex (IE) to monomer (IM)
fluorescence intensities for 1 × 10-4 mol PyC3NMe2/(kg PIBMA) (λex
) 345 nm). Activation energies (E′a′e) are calculated for temperature
ranges below and above Tg (indicated by the vertical dashed line).
(E′′) from exciplex formation (E′′) and the apparent activation
a
ae
energy contribution from radiative and nonradiative components
of the exciplex (∆Ee) can be obtained from Arrhenius plots using
eq 4 (see the Supporting Information); E′a′e and ∆Ee are two
temperature-dependent variables in eq 4.38,44 An Arrhenius plot
of the exciplex-monomer fluorescence ratio for PyC3NMe2
in PIBMA is shown in Figure 8.
However, the results presented here indicate that microdif-
fusion and the fluorescence properties of guest molecules in
the PAMAs depend much less on hole free Volume than on chain
segment relaxation rates, and those rates are very dependent
upon the nature of the PAMA side chains. We find no direct
correlation between the photophysical properties of our probes
and void free volumes, although a strong correlation exists
between optical properties of the guests and the microstructural
features of the polymers. If the photophysical properties of the
guest molecules were affected only by free volume changes,
similar trends should have been observed in the five PAMAs
because their void volumes always increase with increasing
temperature. Our temperature-dependent positronium lifetime
annihilation spectroscopy measurements56 and those by others57
show a gradual increase in free volume with increasing
temperature in four of the 5 PAMAs; there is no discernible
slope change in the free volumes in the temperature range from
below to above Tm (or Tg). Because they do not show such a
slope discontinuity, although there is a noticeable correlation
between polymer chain relaxation parameters and the fluores-
cence properties of both PyH and PyC3NMe2 (vide infra), we
discount free volume changes as being the primary cause for
the excursions in our fluorescence measurements. As will be
shown, the large, abrupt changes in fluorescence properties, most
pronounced in the temperature ranges encompassing Tm (or Tg),
appear to be caused by coupling with polymer chain-segment
interactions (i.e., the structure type and mobility (or relaxation)
of polymer chains in PAMAs, as well as temperature).
5.1. Information Derived from PyH Fluorescence.
5.1.1. From Monomer Intensities. At 20 K, the shapes of both
the guest molecules and polymer chains are virtually “frozen”,
and the high fluorescence intensities can be attributed to very
little vibronic coupling between the guest molecules and the
polymer matrixes. Except in PIBMA, PEMA, and PCHMA, the
fluorescence intensity of PyH is not significantly affected up to
∼100 K in the PAMAs (Figures 9 and S8 of the Supporting
Information). Consistent with the assumption that matrix-guest
interactions are minimal in this range of temperatures, the
relative intensity of the emissive hot band from PyH does not
change up to ∼100 K in three of the PAMAs; in PIBMA and
PHDMA, initial decreases are observed, probably as a conse-
quence of small movements of the alkyl groups (δ-relaxations).58
For example, rotations of the ester methyl group in PEMA occur
5. Discussion
For the fluorescence from any probe to report on changes in
its host properties, it must be located in the proximity of where
the changes are occurring, and it must interact with the host
matrix, perturbing the emission in some way. In neat anisotropic
polymers, such as the PAMAs investigated here, there may be
more than one type of guest site, and changes may occur to the
polymer matrix at locations that are far removed from the guest
sites or at them. Thus, although it is not always possible to
discern host transitions or relaxations from the emission of a
probe molecule, it is sometimes possible to detect subtle changes
in the host matrix that are very difficult to detect using bulk
measurements. Like any other technique, fluorescence from
guest molecules is useful only when its limitations are kept in
mind.
The two probes used in the present study are designed to
report on the size, shape, and nature of the cavity walls of the
occupied guest sites and their changes with time: PyH can
provide a measure of translational diffusion and rates of
segmental chain motions in its local environment; conforma-
tional motions of PyC3NMe2 can sense the lability of the walls
of the occupied sites and, therefore, the rates of segmental chain
motions. In addition, the length and shape of the alkoxycarbonyl
side groups in PAMAs are known to influence the structural
and conformational order of the main chains,50,51 and our studies
demonstrate that the nature and mobility of the side chains affect
the fluorescence properties of the guest molecules. Furthermore,
the temperature dependence of fluorescence of the pyrenyl
probes suggests that the five PAMAs can be broadly classified
into three categories: (1) those with linear, short side groups
(PBMA and PEMA); (2) that with a long n-alkyl group
(PHDMA); and (3) those with a more rigid, branched alkyl
group (PCHMA and PIBMA).
The small side chains of PEMA and PBMA attenuate
movements of segments of the main chains and increase the
activation energies for R-relaxation.15 As a result, guest mol-
ecules are less mobile than in the comparable phases of the
other PAMAs at comparable temperatures. In PHDMA, the long

Interactions of Pyrenyl Guest Molecules
J. Phys. Chem. B, Vol. 114, No. 38, 2010 12229
Figure 9. Effect of temperature on the total area of fluorescence intensity (from 360 to 470 nm, but calculated from integration of fluorescence
intensity vs wavenumber plots; λex ) 325 nm) and activation energy plots from ∼10-4 mol PyH/(kg PAMA): (A, B) PEMA, (C, D) PHDMA, and
(E, F) PIBMA. Relaxation temperatures indicated in the graphs with arrows are based upon slope changes. Temperature regions used to calculate
the activation energies are delineated with red lines. Dashed lines represent glass transition or melting temperatures. I0 values are taken as the
average intensities at 20-40 K in PEMA, 20-50 K in PHDMA, and 20-30 K in PIBMA. Slope changes in the integrated intensity curves were
reproducible at least within the limits of the symbol sizes in films of the same type (see Figure S9 in Supporting Information); they are not due to
instrumental artifacts or specific placement of the samples in the spectrometer.
at ∼40 K,15 and rotations of isopropyl groups in PIBMA
commence at ∼60 K.59 Care must be taken in assigning the
origin of small excursions in fluorescence intensities, especially
at lower temperatures. For example, the small (but reproducible)
perturbations in the fluorescence intensity of PyH near 40 K in
PEMA (Figure 9A) may be a result of rotations of the methyl
group or to a heretofore unidentified relaxation process. In either
case, our techniques are incapable of identifying its origin.
Substantial decreases in the fluorescence intensities of PyH
occur in the PAMAs above ∼100 K as a result of increased
guest-matrix interactions, attributed to the onset of polymer
chain relaxations that can be identified as excursions in the
slopes of intensity-temperature plots. For PEMA, two lower-
onset temperatures are noted: the one at 100 K is associated
with small rotations of the lateral segments, and the one at 150
K is attributed to rotations of the ester groups.15 The 150 K
excursion agrees with the literature value for a γ-relaxation
(Tγ).29,60 For the other acrylates with longer and larger lateral
groups, these processes are observed at 130 K for PHDMA,
140 K for PIBMA, 150 K for PBMA, and 160 K for
PCHMA.29,58,60 The small changes in the fluorescence intensities
of PyH in PAMAs at the onsets of γ-relaxations are not
surprising, given the large differences between the rates of
γ-relaxations and vibronic modes associated with pyrenyl
excited singlets.
As mentioned, the rates of radiationless decay and, therefore,
the fluorescence quantum yield and intensity of PyH molecules
can change when fundamental vibrations and harmonics of the
excited singlet state are coupled with vibrational modes of the
PAMAs.61 PyH in its B1u electronic state has vibrations in
the range 498-3102 cm-1,3 and vibrational modes of chain
segments of PAMAs are in the range ∼700-3000 cm-1 35,62
.
In
this regard, the temperature dependence of IR spectra of
polyethylenes has shown that low-energy vibrations (<2700
cm-1) are less populated at low temperatures.3 Similarly, the
absence of energy-matched “electron-phonon pairs” may be
responsible for inefficient quenching of PyH excited singlet
states at the onset of γ-relaxations in PAMAs.
However, strong coupling between pyrenyl excited singlets
and surrounding polymer chains at higher temperatures is
evidenced by larger decreases in the fluorescence intensity at
the onsets of ꢀ- and R-relaxations. Obviously, chain mobility
and translational diffusion of guest molecules increase with
increasing temperature and become significantly larger above

12230 J. Phys. Chem. B, Vol. 114, No. 38, 2010
Abraham et al.
TABLE 3: Temperature Ranges of Increasing Hot-Band (366 nm) Intensity (∆Thot band, K) and Tg (and Tm) Values from
Spectra Recorded from Low to High Temperatures on ∼10-4 mol PyH/(kg PAMA) Filmsa
γ (Ea)
ꢀ (Ea)
R (Ea)
b
polymer
PEMA
∆Thot band
present work
lit.
present work
lit.
present work
lit.
Tg(DSC)
120-300
120-282
100-281
120-310
110-260
150c
159d
(2.9)
150c
118-14129,60
260c
269d
(11.7)
240c
179-225
(12-25)29
330c
294-305
(45-77)29
342
322d
(27.3)
290c
(2-6)29,60
PBMA
102-12529
152-208
(15-32)29
245-275
(62-67)29
290
281e
368
322
280d
(30.8)
280c
(3.6)
130c
(4-7)
(6.4)
PHDMA
PCHMA
PIBMA
220c
215d
(1.8)
160c
(8.7)
(17.8)
330c
260c
∼200-30060
256d
324d
(5.0)
140c
(12.1)
210-240c
233d
(30.1)
∼150-20060
330c
322d
(2.6)
(10.5)
(19.7)
a The onset temperatures of the relaxation temperatures and activation energies (Ea (kJ mol-1) in parentheses) within the temperature regions
specified in Figure 9 are included with available literature values. b Intensities were nearly constant below the lower temperature and decreased
above the higher temperature in each range. c From plots of integrated fluorescence areas versus temperature. d From Arrhenius plots. e Tm.
the glass transition temperatures. Closer to the glass transition
(i.e., R-relaxation) or melting temperatures, the significant
increases in main chain motions15,30,31 lead to large decreases
in the fluorescence intensities.
with the other PAMAs indicate that a significant fraction of
the PyH molecules reside in sites that are near the polymer main
chains.
To the extent that the activation energies (Ea in eq 2)
determined from the Arrhenius plots of PyH in the PAMAs
(Figure 9) are affected by quenching induced by coupling with
chain relaxations, they should correlate with the energies
required for the latter. However, they are not expected to be
the same magnitude because more than one process can
influence coupling of PyH singlet modes within one temperature
range, and a fraction of the PyH molecules may be in sites far
from where the potentially strongest coupling might occur. Apart
from changes in chain mobility, electronic properties of the
medium, such as its dielectric and index of refraction,26,65 are
also affected by temperature and, therefore, will influence the
Both ꢀ- and R-relaxations involve movements of larger chain
segments than those of the γ-relaxations,58 and in some cases,
they are not well resolved in our fluorescence plots.63 In general,
ꢀ-relaxtions (involving the side groups) and R-relaxations
(involving the main chain) in acrylates are strongly coupled
processes.52 Consequently, the high temperature part of the
ꢀ-relaxation dominated temperature range can be strongly
overlapped by the lower temperature part of the R-relaxation
temperature range.64 This indicates that motions of smaller chain
segments are weakly coupled to the singlet excited states of
pyrenyl in acrylates. Thus, the small amplitude motions induce
less nonradiative deactivation than the motions of larger
segments when the pyrenyl groups are equidistant from both.
1
dynamics of the PyH states, but they are expected to change
smoothly with temperature.
For example, the Ea obtained from the fluorescence intensity
measurements of a film of ∼10-4 mol PyH/(kg PEMA) is ∼0.6
kJ mol-1 for the temperature range 110-140 K. Only γ-relax-
ations are known to operate for PEMA in this temperature
region.15 Fluorescence intensity changes were larger at higher
temperatures, and Ea values increased to 2.9 kJ mol-1 for the
PyH-doped PEMA in the range 160-260 K. The Ea values from
PyH fluorescence in PBMA, PIBMA, and PHDMA in their
γ-relaxation regions are also very low, e5 kJ mol-1 (Table 3),
and the values for PEMA and PBMA are lower than those
determined from measurements with smaller fluorophores, such
as xanthone and benzophenone,29 probably as a result of the
less effective interactions between the segmental motions of the
polymer chains and the PyH molecules in the PAMA cavities.
Ea values of PyH fluorescence in PEMA in the 270-330 K
range, associated with ꢀ-relaxations,29,60 and above 340 K, from
R- relaxations, are 11.7 and 27.3 kJ mol-1, respectively (Figure
9), and are larger than the Ea from the slope in the γ-relaxation
region. However, they, too, are much lower than the activation
energies obtained for neat PEMA from bulk dielectric or NMR
measurements,15,66 although the trend toward increasing activa-
tion energies in the higher temperature ranges is consistent with
the changes in the activation energies for PAMA relaxations
from NMR studies.66
Above the glass (melting) transitions, the emission intensities
decreased monoexponentially with temperature, as expected for
a lumophore in a liquid-like medium. Therefore, the temperature
of the glass transition in the proximity of the pyrenyl groups
was determined spectroscopically by the onset of the exponential
decrease of the fluorescence intensity. For example, the ꢀ-re-
laxation of PEMA has a well-defined onset temperature at Tꢀ
) 260 K according to measurements of the integrated fluores-
cence area (Figure 9). The area under the fluorescence curve
decreases faster above Tꢀ, and only a smooth change of slope
can be observed at the R-relaxation, estimated from the
beginning of exponential decay to be at 330 K; the bulk glass
transition temperature, determined by DSC, is (as expected) at
a slightly higher temperature, 342 K. The difference between
these Tꢀ and TR values and those reported in the literature,
294-305 K,29,60 are consistent with local perturbations caused
by the PyH probes (Table 2).
With the exception of PCHMA, for which TR by fluorescence
was 320 K and Tg ) 368 K, TR from spectroscopic measure-
ments and glass transition temperatures from DSC thermograms
are similar. For PCHMA, the difficulty to define the onsets of
the ꢀ- and R-relaxations using fluorescence may result from
the bulkier cyclohexyl side groups excluding PyH molecules
from the immediate vicinity of the main chains. These results

Interactions of Pyrenyl Guest Molecules
J. Phys. Chem. B, Vol. 114, No. 38, 2010 12231
Another probe of the local environment is the temperature
dependence of the I3/I1 fluorescence band ratios of PyH
molecules. The effects of temperature on I3/I1 and on the total
fluorescence intensities of PyH in PAMAs follow similar trends:
I3/I1 ratios increased slightly above Tγ and more so above Tꢀ
and TR. For example, the I3/I1 ratio in ∼10-4 mol PyH/(kg
PEMA) is ∼0.7 at Tγ, increased slightly to ∼0.9 at TR, and
increased more above TR (to 2.1 at 390 K; Figure 2). The
temperature dependence of the PyH I3/I1 ratios in the PAMAs
and their similarity to the I3/I1 ratios at 277 K in butyl acetate28
(∼0.8 and ∼1 in the PAMAs) suggest that the average local
environment experienced by the PyH molecules in the PAMAs
is near the ester groups and, therefore, near the main polymer
chains.
the microcrystallites of PHDMA melt, the volume that the PyH
molecules can occupy expands (increasing the average separa-
tion between PyH molecules), but the microviscosity decreases,
as well. The observed decreases in excimer fluorescence
intensity above the melting point in PHDMA indicate that the
“dilution effect” is more important than the increased rates of
diffusion. Even for concentrations somewhat higher than the
∼10-3 mol PyH/(kg PAMA) employed for PHDMA, no static
or dynamic excimer emission was detected in the glassy or
rubbery states of the other PAMAs. Thus, we conclude that the
PyH molecules are better dispersed in these matrixes than in
PHDMA and that their local viscosities are significantly higher.
Only at much higher PyH concentrations could excimer emis-
sions be detected (Table S1 of the Supporting Information), and
the lack of a negative pre-exponential (or its appearance only
at elevated temperatures) in analyses of our TCSPC histograms
for PyH fluorescence leads us to conclude that these excimers
are formed predominantly by excitation of ground state aggregates.
It is known that the interaction energies of carbonyl groups
in acrylates and polycyclic aromatic hydrocarbons are similar
to those of π-π interactions between aromatic groups:67,68 the
calculated binding energy for formaldehyde-benzene and
benzene-benzene (plane-to-edge) interactions are 1.86 and 1.78
kcal mol-1, respectively.69 Similar interaction energies are
expected in PAMAs if PyH molecules are near the methacrylate
carbonyl groups. Although the magnitude of similar interaction
energies of carbonyl groups with excited singlet states of
aromatic molecules are unknown, it is reasonable to assume
that they will be stronger in the more-polarizable excited singlet
states than in the ground state. If the interactions are spatially
specific within the PAMAs, any change in the orientation or
The activation energy for dynamic excimer formation (E′,
a
Scheme 2) in 1.5 × 10-3 mol PyH/(kg PHDMA) increased
slightly from 10 kJ mol-1 in the temperature region below TR
to 11 kJ mol-1 above it (Figure 7). These values are close to
those found in low-viscosity solvents (e.g., PyH in ethyl
acetate43). They support our contentions that the melted polymer
chains in PHDMA are very mobile and that the average distance
between PyH molecules below TR is smaller than would be
expected in a normal “solution”.
1
relaxations of the side chains would affect PyH-carbonyl
5.2. Information Derived from PyC3NMe2. PyC3NMe2
is a probe of the “stiffness” of the cavity walls in the PAMAs.
Its dimethyl amino group permits intramolecular exciplex
formation if the appropriate conformation can be achieved
during the excited singlet lifetime of the pyrenyl moiety. As
mentioned in the Results section, the initial conformations of
the PyC3NMe2 molecules determine whether intramolecular
exciplex formation is static and the ease of its formation if it is
dynamic. In an attractive hypothesis, the stiffer chains in PIBMA
and PCHMA (based on the activation energies for R-relaxations
in the glassy state: 188 kJ mol-1 for PIBMA and 222 kJ mol-1
for PCHMA15) may force the amino donor group of PyC3NMe2
to reside closer to the pyrenyl group, on average (and, thereby,
make easier dynamic exciplex formation), than in the other
PAMAs. Increasing temperature would accelerate exciplex
formation further by softening the cavity walls.
interactions (and thus, the fluorescence properties). In the glassy
(or solid) states of the PAMAs, these interactions would have
a significant impact on translational diffusion of PyH molecules.
The observed temperature dependence of the fluorescence
intensity and I3/I1 ratio may be related strongly to variations of
these matrix-guest interactions, as mediated by changes in chain
relaxation types and rates. This appears to be the case for the
intensity fluctuations of the hot band at 366 nm, where its
increased intensity within the ꢀ- and γ-relaxation regions can
be explained by increasing electron-phonon coupling. At
temperatures close to the glass transition or melting temperatures
(R-relaxations), the observed decreases in the hot-band intensity
are consistent with diminished carbonyl-aromatic interactions.30,31
5.1.2. From Excimer Fluorescence Data. The dynamics of
excimer formation or decay can be used to probe microdiffusion
of PyH in the PAMAs. Because the local viscosity in the
PAMAs is very high (and translational diffusional rates are low),
dynamic excimer formation can take place only over short
distances. When low concentrations and rigid cavity walls in
PAMAs keep the pyrenyl moieties isolated from one another,
no dynamic or static excimer emission occurs. In this work,
static excimer emission was not used to probe chain dynamics
because it pertains to PyH solubility rather than its diffusion in
the PAMAs. Only in PHDMA could significant dynamic
excimer formation be confirmed by TCSPC experiments: there
was a protracted rise component to the histograms monitored
at the excimer emission wavelengths (Figure S7 of the Sup-
porting Information) and the ratio of pre-exponentials for the
decay constants was -1 (Table S2 of the Supporting Informa-
tion).
NMR studies have also shown that side group mobility
depends upon the nature of alkyl chain and temperature in
PAMAs.52,70 For example, the rates of ꢀ-relaxations that involve
180° flips of the ester groups remain almost constant in
poly(methyl methacrylate) from room temperature to 370 K63
but increase with increasing temperature in PEMA, PIBMA,
and PCHMA.52,70 Combining these NMR results with the
photophysical data reported here, we believe that side group
rotations modify the cavity walls and, thus, the conformations
of the PyC3NMe2 molecules. Thus, in the less rigid PAMAs,
the PyC3NMe2 molecules may be able to adopt less “con-
strained” conformations that are less prone to fold into the
exciplex geometry. This hypothesis is consistent with the
temperature dependence of PyH fluorescence in the PAMAs,
in which a strong correlation between side-chain mobility and
guest fluorescence intensity was observed. The temperature-
induced conformational changes are evident in the larger relative
amounts of exciplex emission from PyC3NMe2 in the glassy
and rubbery states of PIBMA than in PHDMA, PEMA, or
Below TR, PyH molecules are excluded from regions in
PHDMA where alkyl side-chains are crystalline. Thus, the
effectiVe PyH concentrations are higher than those calculated
from the total polymer volume, and the (statistically) enhanced
proximity of PyH molecules in the amorphous parts facilitates
PBMA. The lower activation energy for exciplex formation (E′′,
a
1
dynamic excimer formation during the PyH lifetimes. When
eq 4) in glassy PIBMA, 13.4 kJ mol-1, than in its rubbery state,

12232 J. Phys. Chem. B, Vol. 114, No. 38, 2010
Abraham et al.
16.9 kJ mol-1, also supports our contention that the matrixes
of PHDMA, PEMA, or PBMA favor conformations of
PyC3NMe2 that are less conducive to exciplex formation and
attenuate motions from those conformations to the exciplex
geometry.
films are also provided. This material is available free of charge
via the Internet at http://pubs.acs.org.
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