'Remote' adiabatic photoinduced deprotonation and aggregate formation of amphiphilic N-alkyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride salts

Article abstract of DOI:10.1021/ja208674s

The absorption and emission properties of a series of amphiphilic N-alkyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride salts were investigated in solvents of different polarities and over a wide concentration range. For example, at 10^-5 M concentrations in tetrahydrofuran (THF), salts with at least one N-H bond exhibited broad, structureless emissions even though time-correlated single photon counting (TCSPC) experiments indicated negligible static or dynamic intermolecular interactions. Salts with a butylene spacer or lacking an N-H bond showed no discernible structureless emission; their emission spectra were dominated by the normal monomeric fluorescence of a pyrenyl group and the TCSPC histograms could be interpreted on the basis of intramolecular photophysics. The broad, structureless emission is attributed to an unprecedented, rapid, adiabatic proton-transfer to the medium, followed by the formation of an intramolecular exciplex consisting of amine and pyrenyl groups. The proposed mechanism involves excitation of a ground-state conformer of the salts in which the ammonium group sits over the pyrenyl ring due to electrostatic stabilization. At higher concentrations, with longer N-alkyl groups, or in selected solvents, electronic excitation of the salts led to dynamic and static excimeric emissions. For example, whereas the emission spectrum of 10^-3 M N-hexyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride in THF consisted of comparable amounts of monomeric and excimeric emission, the emission from 10^-5 M N-dodecyl-N-methyl-3-(pyren-1-yl) propan-1-ammonium chloride in 1:9 (v:v) ethanol/water solutions was dominated by excimeric emission, and discrete particles near micrometer size were discernible from confocal microscopy and dynamic light scattering experiments. Comparison of the static and dynamic emission characteristics of the particles and of the neat solid of N-dodecyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride indicate that molecular packing in the microparticles and in the single crystal are very similar if not the same. It is suggested that other examples of the adiabatic proton transfer found in the dilute concentration regime with the pyrenyl salts may be occurring in very different systems, such as in proteins where conformational constraints hold ammonium groups over aromatic rings of peptide units.

Full text of DOI:10.1021/ja208674s

ARTICLE
pubs.acs.org/JACS
‘Remote’ Adiabatic Photoinduced Deprotonation and Aggregate
Formation of Amphiphilic N-Alkyl-N-methyl-3-(pyren-1-yl)propan-1-
ammonium Chloride Salts
Shibu Abraham and Richard G. Weiss*
Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, United States
S Supporting Information
b
ABSTRACT: The absorption and emission properties of a
series of amphiphilic N-alkyl-N-methyl-3-(pyren-1-yl)propan-
1-ammonium chloride salts were investigated in solvents of
different polarities and over a wide concentration range. For
example, at 10À5 M concentrations in tetrahydrofuran (THF),
salts with at least one NÀH bond exhibited broad, structureless
emissions even though time-correlated single photon counting
(TCSPC) experiments indicated negligible static or dynamic
intermolecular interactions. Salts with a butylene spacer or
lacking an NÀH bond showed no discernible structureless emission; their emission spectra were dominated by the normal
monomeric fluorescence of a pyrenyl group and the TCSPC histograms could be interpreted on the basis of intramolecular
photophysics. The broad, structureless emission is attributed to an unprecedented, rapid, adiabatic proton-transfer to the medium,
followed by the formation of an intramolecular exciplex consisting of amine and pyrenyl groups. The proposed mechanism involves
excitation of a ground-state conformer of the salts in which the ammonium group sits over the pyrenyl ring due to electrostatic
stabilization. At higher concentrations, with longer N-alkyl groups, or in selected solvents, electronic excitation of the salts led to
dynamic and static excimeric emissions. For example, whereas the emission spectrum of 10À3 M N-hexyl-N-methyl-3-(pyren-1-
yl)propan-1-ammonium chloride in THF consisted of comparable amounts of monomeric and excimeric emission, the emission
from 10À5 M N-dodecyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride in 1:9 (v:v) ethanol/water solutions was
dominated by excimeric emission, and discrete particles near micrometer size were discernible from confocal microscopy and
dynamic light scattering experiments. Comparison of the static and dynamic emission characteristics of the particles and of the neat
solid of N-dodecyl-N-methyl-3-(pyren-1-yl)propan-1-ammonium chloride indicate that molecular packing in the microparticles and
in the single crystal are very similar if not the same. It is suggested that other examples of the adiabatic proton transfer found in the
dilute concentration regime with the pyrenyl salts may be occurring in very different systems, such as in proteins where
conformational constraints hold ammonium groups over aromatic rings of peptide units.
’ INTRODUCTION
ammonium chloride salts (PNn, where n is the length of the N-
alkyl group) are driven by hydrogen-bonding, London dispersion,
and electrostatic interactions, as well as by packing constraints
imposed by molecular shape and solvent polarity.4 Additionally,
electronically excited pyrene derivatives may interact with
ground-state molecules to form excimers and exciplexes which
can be detected easily by static and dynamic fluorescence measure-
ments.5 Thus, when pyrenyl and amino groups are linked by short,
saturated spacers, such as ethylene or propylene chains, in α-(pyren-
1-yl)alkan-ω-amines, intramolecular exciplexes can be formed; virtually
no exciplex fluorescence is observed when the linkers are butylene or
longer spacers,6 although excimers and intermolecular exciplexes can be
formed with spacers of virtually any length.7
The balance of interaction forces, both stabilizing and desta-
bilizing, among amphiphilic molecules and between the solvent and
the amphiphiles determines the degree and type of their aggrega-
tion.1 Crudely, the amphiphileÀamphiphile interactions can be
separated into those affecting the hydrophilic and hydrophobic
parts of each molecule, and they can lead to distinct ordering
within the aggregates; even in the absence of aggregation, solventÀ
amphiphile interactions can affect the preferred conformations
of the amphiphiles. Spectroscopic and electron microscopy tech-
niques have been shown to be effective tools to probe both the
conformational and aggregate interactions at the molecular level.2,3
When the amphiphile contains a luminescing group, emission
spectroscopy (and especially fluorescence) can be a very sensitive
tool to probe the conformational preferences and the nature of
the aggregates. Here, we have attached the lipophilic lumophore,
pyrene, to ammonium cationic head groups through α,ω-substitu-
tion of alkane linkers (Chart 1). Conformational preferences and
aggregation in these N-alkyl-N-methyl-3-(pyren-1-yl)propan-1-
Here, we have used the static and dynamic fluorescence pro-
perties of the salts in Chart 1 to probe the dependence of mole-
cular conformation and aggregation on salt concentration and
Received: September 14, 2011
Published: November 03, 2011
r
2011 American Chemical Society
19250
dx.doi.org/10.1021/ja208674s J. Am. Chem. Soc. 2011, 133, 19250–19256
|

Journal of the American Chemical Society
ARTICLE
Chart 1. Structures of Amphiphilic Pyrenyl Ammonium Salts
Investigated
Figure 1. Aromatic (a and c) and aliphatic (b and d) regions of NMR
spectra of PN6 and its precursor amine, respectively, in CDCl3 at 25 °C.
structure (N.B., length of the spacer and the length of the N-alkyl
group), the polarity of the solvent, and the ability of the solvent to
act as a proton acceptor. With regard to the latter, at very low
concentrations, where no ground-state aggregation is evident,
we find that those salts with a propylene spacer and at least one
NÀH bond can undergo a very efficient and fast, unpreceden-
ted (to our knowledge)8 adiabatic, proton-transfer within their
excited singlet states, leading to intramolecular exciplex emissions
characteristic of the corresponding N-alkyl-N-methyl-3-(pyren-
1-yl)propan-1-amines.9 Such ‘remote’ proton transfers are de-
pendent on the ground-state conformations of the molecules.
They compete with ‘normal’ pyrenyl emission from isolated
molecules and excimer-like emission from excitation of ground-
state aggregates and dynamically formed excited state pairs.
Figure 2. Normalized (at 380 nm) emission spectraofPN6 (λex 345 nm)
at 20 °C: 1 Â 10À3 M (solid line) and 3 Â 10À5 M (dashed line) in
acetonitrile; 2 Â 10À5 M in CH2Cl2 (dotted line).
’ EXPERIMENTAL SECTION
washing with a solvent in which the amine was soluble but the salt
was not (see Supporting Information). Complete transformation
of the amines to their ammonium salts was confirmed by
elemental analyses, mass spectra, and NMR spectroscopy. The
NMR peaks of the methylene protons in the spacers and carbon
atoms near the ammonium centers of the salts were shifted
downfield with respect to those of the amines. An example using
PN6 is shown in Figure 1. In addition, HCl gas was bubbled for
30 min into a solution of PN1 in tetrahydrofuran (THF). The
THF was removed on a rotary evaporator and the residue was
washed with water and dried under vacuum for 24 h. Within the
limits of experimental error, the photophysical properties of the
dried residue were the same as those of the original PN1.
At 10À5 M concentrations, solutions of the ammonium salts
exhibited structured pyrenyl UVÀvis absorption spectra. For
example, PN6 showed a vibronic progression of peaks at 342,
326, and 312 nm in acetonitrile, CH2Cl2, or ethanol that is
characteristic of πÀπ* transitions for unaggregated pyrenyl
groups.5a Additional evidence for the isolated nature of the
molecules in very dilute solutions of these solvents is found in
the fluorescence spectra which give no indication of an excimeric
emission (a broad, unstructured band centered ∼485 nm in
concentrated solutions of the salts; vide infra); only the classic,
structured emission of a pyrenyl group, with peaks at 379, 400,
and 422 nm, could be detected (Figure 2).
Syntheses and characterization details for the molecules in Chart 1
are included in the Supporting Information file. UVÀvis absorption
spectra were recorded on a Perkin-Elmer Lambda-6 spectrophotometer.
Steady-state excitation and emission spectra were obtained on a Photon
Technology International Fluorimeter (SYS 2459); all emission spectra
were normalized at the I1 band (380 nm). Confocal images were recor-
ded on a Zeiss LSM510META microscope with 364 nm excitation. One
or two drops of each solution were placed on a Petri dish and covered
with a lid immediately before measurement.
Solutions of the pyrenyl derivatives in organic solvents for single-photon
counting were flame-sealed after degassing at least 5 times at <10À5 Torr by
freeze (liquid nitrogen)-pump-thaw cycles in flattened glass capillaries (0.4,
3, or 8 mm thickness; Vitro Dynamics). Before photophysical measure-
ments were made, nitrogen was bubbled for 10 min through solutions
containing water and KBr pellets were purged with nitrogen for 15 min.
Fluorescence rise and decay histograms were collected on an Edinburgh
Analytical Instruments FL900 single-photon counting system 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 χ2 using
software supplied by Edinburgh. Fits were considered acceptable when χ2 e
1.3 and residual plots exhibited no systematic deviations from zero. Single-
crystal X-ray studies were carried out using a Siemens SMART 1000
diffractometer with an APEXII CCD detector (Bruker-AXS) and Mo Kα
(0.71073 Å) radiation. The crystal structures were solved by methods
using SHELX-97 and XSEED software.10,11 The program Mercury
(version 2.3) was used to open the CIF file and generate the figures.
However, the emission spectra of the PNn at concentrations
near 10À5 M in THF consisted of a broad emission band
centered at 510 nm (Figure 3), like the emission observed for the
intramolecular exciplexes of the precursors of the PNn (Figure S1),
the N-alkyl-N-methyl-3-(pyren-1-yl)propan-1-amines! No such
emission was observed at comparable concentrations in ethanol,
CH2Cl2, or acetonitrile. The possibility that the 510 nm emissions
’ RESULTS AND DISCUSSION
Syntheses of the ammonium salts were accomplished by
continuous bubbling of anhydrous hydrogen chloride through
solutions of the corresponding amines, followed by repeated
19251
dx.doi.org/10.1021/ja208674s |J. Am. Chem. Soc. 2011, 133, 19250–19256

Journal of the American Chemical Society
ARTICLE
substituent has 3 hydrogen atoms on nitrogen, was discernible
(but weak) while no exciplex emission could be detected from
PNQ12, another salt with a propylene spacer whose ammonium
substituent is quaternary (i.e., lacking a hydrogen atom on
nitrogen) (Figure 4).
The first vibronic emission band (I1 at ∼378À380 nm) of the
pyrenyl salts in THF was strong and sharp, and the third vibronic
band (I3 at ∼390 nm) was very weak as well as poorly resolved in
several cases. Although the I1/I3 peak-height ratio of unsubstitu-
ted pyrene is very sensitive to solvent polarity, those of alkylated
pyrenes are much less so.16 Regardless, the magnitudes of the
I1/I3 ratios of PN1, PN4, and PNQ12 (Figure 4), 2À3, and those
of the PNn (Figure 3 and Figure S1), 3À4, indicate that the local
pyrenyl environments experienced by the PNn in THF are more
polar than those of PN1, PN4, and PNQ12, suggesting that the
two sets of salts may exist in different average conformations
(vide infra).
Figure 3. Normalized (at 379 nm) emission spectra (λex 345 nm) at
20 °C of PN6 in THF at 1 Â 10À3 M (solid line), 3.8 Â 10À4 M (dotted
line), 3.8 Â 10À5 M (dash-dotted line), and 2 Â 10À5 M (dashed line).
A mechanism which is proposed to explain the role of solvent
polarity on the photoinduced exciplex formation from the PNn at
low concentrations is shown in Scheme 1. Density functional
theory calculations at the M06/6-31G* level identified two dif-
ferent families of energy-minimized conformers for the PN1
cation, corresponding to the ammonium and pyrenyl groups
being near (B, i.e., with a bent propylene linker) or far (O, i.e.,
with an extended propylene linker) (Scheme 1).17 Although our
discussion in this part will focus on the fate of the B conformers
upon photoexcitation, a complete understanding of the role of
the B and O conformers on the optical properties of ammonium
salts requires studies at different concentrations and in different
solvents. Salts such as the PNn exist preferentially in extended
conformations (O) in high polarity solvents which can stabilize
the charged centers of both the ammonium cation and chloride
anion;18 in lower polarity solvents, the positively charged center
of nitrogen must rely more heavily on the π-electrons of the
pyrenyl moiety for added stabilization,19 and this requires that
the propylene chains be bent (into an exciplex-like geometry,
conformation B) when the interactions are intramolecular. If the
electrostatic stabilizations of the ammonium cationic center by
lone pairs of electrons on oxygen in THF (and related solvents)
and by the π-electrons of the pyrenyl ring in the ground state are
competitive, a significant fraction of the PNn molecules will exist
in the bent conformation B. The experimental data indicate that
this is the case. Furthermore, the presence of exciplex emission
from salts with 1 or 3 hydrogen atoms attached to the ammonium
nitrogen and its absence from the salt with no hydrogen atoms on
nitrogen, PNQ12, is a clear indication that deprotonation of the
ammonium group (leading to an amine group) during the exci-
ted singlet-state lifetime of the pyrenyl moiety is responsible for
the exciplex emission. For deprotonation to occur on a short time
scale, both the thermodynamic driving force for proton transfer
must be favorable and the energy barrier to it must be very low.8
When completely dissolved in organic solvents such as CH2Cl2,
acetonitrile, or ethanol at e10À5 M concentrations, these salts
fluoresce with decay times ∼200 ns, indicative of emission from
isolated excited singlets in their O conformations.20 Excitation of
the isolated molecules can lead to dynamic and static (i.e., from
ground-state aggregates) excimer emission with decay times
typically <100 ns21 at higher concentrations. As mentioned, at
concentrations up to 10À3 M in these solvents, the shapes of the
absorption spectra from PN6 were unaltered. However, the emis-
sion spectra contained a new, broad band with a maximum at
485 nm that is characteristic of an excimer (Figure 2).5 Data from
Figure 4. Emission spectra (λex 345 nm) of 4 Â 10À6 M PN (solid line),
4 Â 10À5 M PNQ12 (dashed line), and 4 Â 10À5 M P4N (dotted line)
in THF at 20 °C.
are from ground-state dissociation of the salt is extremely
unlikely because the known pKa of triethylammonium perchlo-
rate in THF is 14.07 (and Ka = 2.2 Â 10À14).12 If (as is
reasonable) the PNn salts have pKa values near that of triethyl-
ammonium perchlorate, the amount of free amine would be
<10À19 M at 10À5 M PNn concentrations; ground-state dis-
sociation is negligible and the amount of amine present in THF is
far too small to be detected by our fluorimeter. Thus, the 510 nm
band is attributed to emission from an intramolecular exciplex.
The aggregation of other amphiphilic pyrenyl molecules has
been investigated13 and analogous behavior is expected for the
pyrenyl salts in Chart 1. For example, as the concentration of
PN6 in THF was increased, the relative intensity of the 510 nm
peak (with respect to the monomeric emission) decreased and
blue-shifted to ∼485 nm (Figure 3). However, the very low
intensity of the broad emission peak at the intermediate con-
centration, 3.8 Â 10À5 M, indicates that at least one other distinct
species besides the excimeric and exciplex precursors must be
present as the PN6 begin to aggregate.14
The ‘Low Concentration’ Regime. The origin of the exciplex-
like emissions from the PNn at very low concentrations was
investigated in several additional ways. The fluorescence from
very dilute solutions of P4N, a salt with a butylene spacer
between the pyrenyl and dimethylammonium groups, was found
to contain almost no exciplex-like emission (Figure 4). This result
was anticipated based upon the known dependence of fluorescence
spectra from alkanes with α-aryl and ω-dimethylamino substituents
on spacer length.6,15 Also, exciplex emission from very dilute solu-
tions of PN, a salt with a propylene spacer whose ammonium
19252
dx.doi.org/10.1021/ja208674s |J. Am. Chem. Soc. 2011, 133, 19250–19256

Journal of the American Chemical Society
ARTICLE
Scheme 1. Proposed Mechanism for Excited-State Deprotonation of Isolated PNn Molecules in THF and Other Solvents in
Which Exciplex-Like Emission Was Found
time-correlated single-photon counting (TCSPC) experiments
show that excimer formation at this concentration is almost ex-
clusively dynamic: when monitored at 380 nm (i.e., isolated emitt-
ing pyrenyl singlets), a decay time of 89 ns (98%) was found; the
excimer emission monitored at 510 nm exhibited a delayed
growth with a 48 ns rise time (and negative pre-exponential term)
and a decay component of 89 ns, matching the monomeric
emission (Figure S2); the ratio of the pre-exponential terms for
the two time constants of the excimer emission was À1. Thus, in
these more polar solvents, ground-state aggregation is virtually
absent at PN6 concentrations up to 10À3 M.
The emission spectra of the PN1, PN6, and PN12 salts in THF
at ∼10À3 M concentrations were similar to those observed in
acetonitrile or ethanol; they consisted of monomer peaks and a
broad excimer band with a maximum at ca. 485 nm. The decay
time of the monomer emission of 3.8 Â 10À4 M PN6 in THF
monitored at 380 nm was 169 ns (98%), and two additional
components with 50À60 and 2À3 ns rise times (i.e., negative
pre-exponentials) were observed in histograms collected at
510 nm (Table S1). In the 510 nm histograms, the importance
of the long-lived decay component decreased and a new compo-
nent, having a 21À32 ns decay time, appeared with decreasing
concentration; note data at 10À5 M. These data suggest that the
environment experienced by excited singlets of some of the PN6
molecules changes drastically between 3.8 Â 10À4 and 10À5 M
concentrations despite the fact that the shape of the emission
spectra do not. The decrease in the decay time and in the
importance of the longer-lived decay component supports our
hypothesis that the PNn prefer to adopt the more constrained B
conformation at lower concentrations in solvents like THF. The
emission spectra of PN1 and PN12 (Figure S3 and Table S2) at
very low concentrations in THF also exhibit a broad emission
band at 510 nm and a 3À4 ns short-lived decay components that
are similar to the exciplex emission observed for the amine. The
multiexponential fits, with more than one short-lived component,
suggest that the exciplex emission involves more than two species
in THF.
studies also support our attribution of the 510 nm emissions from
the PNn salts to exciplexes of the corresponding N-alkyl-N-
methyl-3-(pyren-1-yl)propan-1-amines.6 The absence of a protra-
cted rise time for the 510 nm emissions suggests that deprotona-
tion of the ammonium salts within the excited singlet states of the
pyrenyl moieties is an adiabatic process, leading directly to the
excited singlet-state of the amines in their intramolecular exciplex
geometry (i.e., deprotonation occurs from conformer B).
In general, conjugated aromatic amines and phenols are more
acidic in their S1 states than in their ground states, while aromatic
heterocyclic compounds are more basic,22 and electronic excita-
tion is known to decrease the pKa of ammonium salts which are
conjugated with aromatic groups.23 However, because the aro-
matic and ammonium parts of the PNn are not conjugated, a
different source of the enhanced acidity of the excited singlet-
states of the PNn in conformer B, the greater polarizability of the
aromatic π-electrons, must be responsible.
This explanation can account for only a part of how and why
the excited singlet-states of the amine exciplexes can be formed
adiabatically. The transformation of ammonium salts to amines
can be envisioned to occur by more than one pathway. An excited
aromatic group has been suggested to transfer an electron intra-
molecularly to an antibonding orbital of an ammonium group in
tryptamine, creating a putative aromatic cation-radical and a neutral,
radical-centered nitrogen.24 Dissociation of these species, leading
to loss of a hydrogen atom from nitrogen, is proposed to occur
via π,σ* states.24a,b No exciplex emission has been detected
during this process. Because excitation of the PNn in THF does
produce a great deal of exciplex emission and there is no indi-
cation of chemical loss of PNn upon irradiation (which would be
expected if free radicals were involved), this mechanism does not
seem viable here.
An alternative pathway, involving loss of a proton from the
ammonium group to the medium or chloride,25 is proposed
instead. Starting with excitation of the pyrenyl moiety in con-
formation B, a state analogous to (but not the same as) the π,σ*
singlets could be formed. The proximity of the pyrenyl and
ammonium groups would facilitate orbital mixing and electron
exchange, allowing the rapid26 adiabatic formation of the pyrenyl-
amine exciplexes (and proton loss). Note that the dipole moment
In P4N and PNQ12, salts with no exciplex-like emission, no
negative pre-exponential component was observed in THF when
emission was monitored at 450 nm (Table S3). These dynamic
19253
dx.doi.org/10.1021/ja208674s |J. Am. Chem. Soc. 2011, 133, 19250–19256

Journal of the American Chemical Society
ARTICLE
between the pyrenyl and ammonium groups is predicted to
change direction along the reaction pathway leading to proton
loss. In highly polar media, where conformer O is favored in
both the ground and excited states, the intramolecular inter-
actions necessary for strong orbital mixing between the two
groups are absent and proton loss from the ammonium group
does not occur. Even if the conversion of the O to B conformer
is favored thermodynamically, it is unlikely that the necessary
movement of solvent molecules (which may include their tran-
slational diffusion along with the ammonium center in addition
to their very fast relaxation about the excited state) and chain
bending of the propylene spacer could occur within the excited
singlet lifetime of the pyrenyl moieties; exciplex emission is not
observed in highly polar solvents.16
the PNn, the onset of aggregation depended on the length of the
N-alkyl chain and the polarity of the solvent (vide ante). Con-
sistent with the attribution of the red-shift to aggregation of PN12
molecules, the emissionspectrum at10À4 M in 1/9 (v/v) ethanol/
water was dominated by a broad band centered at 485 nm (i.e.,
very little monomer emission was evident; Figure 5A). From
confocal microscopy, the diameters of the aggregates are calcu-
lated to be ∼0.8À1.1 μm (Figure 5B).27
Clearly, solvent polarity plays an important role in the critical
concentration at which aggregation (as indicated experimentally
by the excimeric emission at ∼485 nm) commences. Also, as the
length of the alkyl substituent on the ammonium nitrogen of the
PNn increases, the ability of high polarity media to solvate
effectively the salts and keep them separated decreases. This
leads to the aggregation of 10À4 M PN12 noted in 1:9 (v/v)
ethanol/water. Even at 10À5 M concentrations in the same
solvent mixture, PN12 still exhibited a red-shifted absorption
spectrum and a broad emission band centered at 485 nm that is
characteristic of aggregation. The histograms from this sample
could be fitted well to two decays (Table S4) with 53 ns as the
major component (92À93%) and 13À19 ns as the minor one
(7À8%). The absence of a growing-in component indicates
ground-state aggregation rather than dynamic excimer formation.
The sizes of the particles in this sample were smaller, ∼0.4 μm
(Figures S5 and S6B from dynamic light scattering and confocal
microscopy, respectively), than those found at 10À4 M. We estimate
the number of PN12 molecules in the average-sized particles at 10À5
and 10À4 M PN12to be ca. 106 and 107, respectively (see Supporting
Information).
The ‘High Concentration’ Regime. Although the appear-
ances of the absorption spectra of 10À5 M PN1 and PN6 were
unaltered when various amounts of water were added to the mis-
cible organic solvents, the absorption spectra of PN12 were red-
shifted and broadened (Figure S4A), indicating aggregation. In
When the concentrations were increased to ∼6 Â 10À4 M,
the shapes of the absorption and emission spectra of PN12 in
freshly prepared samples were unchanged, as were the absorption
(Figure S4B) and emission spectra of PN1 and PN6. However,
the PNn appeared to precipitate slowly: the intensities of the
absorbances and emissions decreased with time although their
shapes remained the same; at g10À3 M PNn, the presence of
precipitated material was visible to the eye. The effects of em-
ploying smaller amounts of water to ethanol solutions of 10À4 M
PNn were not investigated.
Figure 5. (A) Emission spectrum (λex 345 nm) of 10À4 M PN12 in 1/9
(v/v) ethanol/water and (B) its confocal microscopic image.
For comparison purposes, the absorption and emission spec-
tra of PN12 were measured in the solid state as finely dispersed
particles in a KBr pellet. The absorption spectrum was much
broader and red-shifted (peak maxima at 349, 332, and 318 nm)
in the pellet compared to that in solution (Figure 6A). The
excitation spectrum was also broad and red-shifted, with a
maximum at 350 nm (Figure S7). The emission spectrum was
almost exclusively excimeric; it consisted of a broad band
Figure 6. (A) Absorption and (B) emission (λex 345 nm) spectra of
0.01 mol % PN12 in a KBr pellet.
Figure 7. Crystal packing arrangement of PN12 molecules in two adjacent layers.
19254
dx.doi.org/10.1021/ja208674s |J. Am. Chem. Soc. 2011, 133, 19250–19256

Journal of the American Chemical Society
ARTICLE
centered at ∼480 nm (Figure 6B) which is very similar to the
emission for the 1/9 (v/v) ethanol/water solution in Figure 5A.
The decay constants from the solid (Table S4), 51À52 ns
(38À40%) and 16À18 ns (40À44%), and the lack of a growth
component, are remarkably similar to the fit in the solution
as well.
solvent and in the solid (neat) state suggests similar molecular
packing arrangements. From a single-crystal X-ray analysis of the
latter, pyrenyl groups are within ∼3 Å in adjacent layers. Thus,
small motions, perhaps facilitated by excitation, can lead to
excimeric species.
Although the research here identifies the gross nature of the
excited state processes that occur in the individual PNn and their
aggregates, how the balance between them can be altered by
structural modifications of the PNn or changes in solvent and
concentration, many very important details concerning these
processes remain to be revealed. We believe that a large range of
other aromatic amphiphiles with other types of ‘remote’ groups
capable of donating a proton also undergo analogous adiabatic
proton losses in their excited states. In the future, we intend to
investigate the rate of proton loss from the PNn and rate of
exciplex formation of the corresponding amines, the influences of
temperature29 and different anions and anion concentrations
(added as inorganic salts) on these processes. Also, we will better
calibrate the conformational changes between B and O and the
excimer/exciplex interplay by ‘titrating’ the emission carefully as
a function of PNn concentration, N-alkyl chain length, and
solvent polarity.30
Insights into the basis for the absorption and emission data in
the pellet (and, by extrapolation, in the solution aggregates) were
achieved when the molecular packing arrangement of PN12 was
determined from a single-crystal X-ray analysis (Table S5 and
Figures S8ÀS10). The extended molecular packing diagram
shows extensive hydrogen-bonding interactions among ammo-
nium head groups which lead to a head-to-tail orientation of the
two PN12 molecules in a unit cell and close contacts among pairs
of pyrenyl groups (Figure 7). Although pyrenyl groups of
molecules in adjacent layers are tilted with respect to each other,
small motions initiated by electronic excitation can bring them
into geometries amenable to excimer formation. The closest con-
tact between pyrenyl groups is ∼3 Å (Figure 7 and Figure S10).
In addition, a hydrogen atom on the central methylene of one
molecule is within 2.84 Å of a carbon atom of the pyrenyl moiety
in a neighboring molecule. Small changes in the orientation of
these two parts upon electronic excitation may aid the move-
ment of the pyrenyl groups into an excimeric geometry as well.
Regardless, the packing arrangement of the pyrenyl groups in the
aggregates detected by confocal microscopy and dynamic light
scattering and that found in the crystal must be very similar.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis and instrumentation
b
details; additional spectra and images; details of PN12 single
crystals and fluorescence lifetime data. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ CONCLUSIONS
The absorption and emission spectra have provided evidence
for the nature of inter/intra-molecular interactions of the pyrenyl
amphiphiles in Chart 1. At relatively high concentrations of the
PNn and in poor Lewis base solvents, only characteristic mono-
meric and excimeric emissions of pyrenyl excited singlet states
are detected. However, in better Lewis base solvents, such as
THF, photoexcitation of the PNn leads to rapid, adiabatic proton
transfer and exciplex emission from the corresponding N-alkyl-
N-methyl-3-(pyren-1-yl)propan-1-amine (i.e., the PNn from
which a proton has been lost). The data indicate that the ability
of the adiabatic process to occur depends on the ability of the
solvent (or chloride anion) to accept a proton as well as its ability
to promote the formation of a ground-state conformer of the
PNn which places the ammonium cation center over the pyrenyl
π-electron cloud. We conjecture that excitation of PNn con-
formers in which the ammonium center is held far from the
pyrenyl π-electron cloud does not lead to proton transfer and
exciplex emission by the corresponding amines because emission
from the isolated pyrenyl excited singlet states is faster than the
rate at which the molecules can adopt an appropriately bent con-
formation. Although exciplex emission from excitation of similar
ammonium salts has been not reported, we suggest that analogous
processes involving ‘remote’ proton transfers may occur in some
proteins and other conformationally constrained systems.28
The length of the N-alkyl group of the PNn and solvent pola-
rity are important as well in determining the balance between
monomeric and aggregates. For example, aggregates of PN12
(i.e., the PNn with the longest N-alkyl chain and, therefore, the
most hydrophobic PNn) are formed even at l0À5 M concentra-
tions in a very polar solvent such as 1/9 (v/v) ethanol/water.
Comparison of the static and dynamic excimeric emission char-
acteristics of PN12 in the micrometer-range particles in the polar
’ AUTHOR INFORMATION
Corresponding Author
weissr@georgetown.edu
’ ACKNOWLEDGMENT
We thank the National Science Foundation for its support of
this research. We are grateful to Drs. Michael Kruhlak and Ferenc
Horkay of the National Institutes of Health for providing the
confocal microscope and for technical assistance, to Prof. Daniel
L. Blair of the Georgetown Physics Department for assis-
tance with the dynamic light scattering measurements, to Prof.
K. Travis Holman and Mr. Akil Joseph for help in obtaining the
X-ray diffraction data, and to Prof. Miklos Kertesz and Mr. Brad
Slepetz for help with the DFT calculations.
’ REFERENCES
(1) (a) Israelachvili, J. N. Intermolecular and Surface forces: With
Applications to Colloidal and Biological Systems Academic Press Ltd.:
London, 1985. (b) Whitten, D. G.; Chen, L.; Geiger, H. C.; Perlstein, J.;
Song, X. J. Phys. Chem. B 1998, 102, 10098.
(2) (a) Fuhrhop, J.-H.; Wang, T. Chem. Rev. 2004, 104, 2901. (b)
Hamley, I. W. Introduction to Soft Matter: Polymers, Colloids Amphiphiles;
John Wiley: West Sussex, England, 2000. (c) Shimizu, T.; Masuda, M.;
Minamikawa, H. Chem. Rev. 2005, 105, 1401.
(3) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (b)
Holmberg, K.; J€onsson, B.; Kronberg, B.; Lindman, B. Surfactants and
Polymers in Aqueous Solution; John Wiley: West Sussex, England, 2003.
(4) (a) Meyer, E. E.; Rosenberg, K. J.; Israelachvili, J. Proc. Natl.
Acad. Sci. U.S.A. 2006, 103, 15739. (b) Okano, L. T.; Quina, F. H.;
Seoud, O. A. E. Langmuir 2000, 16, 3119. (c) Belman, N.; Israelachvili,
J. N.; Li, Y.; Safinya, C. R.; Bernstein, J.; Golan, Y. J. Am. Chem. Soc. 2009,
19255
dx.doi.org/10.1021/ja208674s |J. Am. Chem. Soc. 2011, 133, 19250–19256

Journal of the American Chemical Society
ARTICLE
131, 9107. (d) Bales, B. L.; Zana, R. J. Phys. Chem. B 2002, 106, 1926. (e)
Kumar, S.; Bansal, D.; Kabir-ud, D. Langmuir 1999, 15, 4960.
(5) (a) Winnik, F. M. Chem. Rev. 1993, 93, 587. (b) de Greef,
T. F. A.; Ercolani, G.; Ligthart, G. B. W. L.; Meijer, E. W.; Sijbesma, R. P.
J. Am. Chem. Soc. 2008, 130, 13755.
(23) Wehry, E. L.; Robes, L. B. Spectrochim. Acta 1965, 21, 1976.
(24) (a) Grꢁegoire, G.; Jouvet, C.; Dedonder, C.; Sobolewski, A. L.
Chem. Phys. 2006, 324, 398. (b) Kang, H.; Dedonder-Lardeux, C.;
Jouvet, C.; Grꢁegoire, G; Desfranc-ois, C.; Schermann, J.-P.; Barat, M.;
Fayeton, J. A. J. Phys. Chem. A 2005, 109, 2417. (c) Kang, H.; Jouvet, C.;
Dedonder-Lardeux, C.; Martrenchard, S.; Charriꢂere, C.; Grꢁegoire, G.;
Desfranc-ois, C.; Schermann, J. P.; Barat, M.; Fayeton, J. A. J. Chem. Phys.
2005, 122, 084307. (d) Lucas, B.; Barat, M.; Fayeton, J. A.; Perot, M.;
Jouvet, C.; Grꢁegoire, G.; Nielsen, S. B. J. Chem. Phys. 2008, 128, 164302.
(25) Presiado, I.; Gepshtein, R.; Erez, Y.; Huppert, D. J. Phys. Chem.
A 2011, 115, 7591.
(6) Swinnen, A. M.; Van der Auweraer, M.; De Schryver, F. C.;
Nakatani, K.; Okada, T.; Mataga, N. J. Am. Chem. Soc. 1987, 109, 321.
(7) (a) Sulak, K.; Wolszczak, M.; Chittofrati, A.; Szajdzinska-Pietek,
E. J. Phys. Chem. B 2004, 109, 799. (b) Hrdlovic, P.; Chmela, S.
J. Photoch. Photobiol., A 1998, 118, 137. (c) Hrdloviꢀc, P.; Horinovꢁa, L.;
ꢀ
Chmela, S.; Can., J. Chem. 1995, 73, 1948.
(8) When an ammonium group possesses at least one N—H bond
and is attached directly to an aromatic group, deprotonation can be
enhanced in the electronically excited singlet state. (a) Morimoito, A.;
Yatsuhashi, T.; Shimada, T.; Biczꢁak, L.; Tryk, D. A.; Inoue, H. J. Phys.
Chem. A 2001, 105, 10488. (b) Solntsev, K. M.; Huppert, D.; Tolbert,
L. M.; Agmon, N. J. Am. Chem. Soc. 1998, 120, 7981. (c) Domcke, W.;
Sobolewski, A. L. In Continuum Solvation Models in Chemical Physics:
From Theory to Applications; Mennucci, B.; Cammi, R., Eds.; John Wiley:
West Sussex, England, 2007. (d) Tolbert, L. M.; Solntsev, K. M. Acc.
Chem. Res. 2002, 35, 19.
(9) These processes are different from some reported indirect
proton transfer processes in which intermolecular energy transfer is
followed by deprotonation of an electronically excited acceptor that is
attached directly to an ammonium group. See, for example:Sytnik, A.;
Litvinyuk, I. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 12959.
(26) (a) Mohammed, O. F.; Pines, D.; Nibbering, E. T. J.; Pines, E.
Angew. Chem., Int. Ed. 2007, 46, 1458. (b) Mohammed, O. F.; Pines, D.;
Dreyer, J.; Pines, E.; Nibbering, E. T. J. Science 2005, 310, 5745.
(27) Insufficient scattered light reached the detector in dynamic
light scattering experiments to obtain good correlation curves at this
concentration.
(28) (a) Hsieh, C.-C.; Jiang, C.-M.; Chou, P.-T. In Hydrogen Bonding
and Transfer in the Excited State; Han, K.-L., Zhao, G.-J., Eds.; John
Wiley: West Sussex, England, 2011. (b) Pless, S. A.; Hanek, A. P.; Price,
K. L.; Lynch, J. W.; Lester, H. A.; Dougherty, D. A.; Lummis, S. C. R. Mol.
Pharmacol. 2011, 79, 742. (c) Gallivan, J. P.; Dougherty, D. A. Proc. Natl.
Acad. Sci. U.S.A. 1999, 96, 9459.
(29) Cohen, B; Huppert, D. J. Phys. Chem. A 2001, 105, 2980.
(30) (a) Presiado, I.; Erez, Y.; Huppert, D. J. Phys. Chem. A 2010,
114, 9471. (b) Siu, H.; Prazeres, T; J., V.; Duhamel, J. Macromolecules
2005, 38, 2865.
(10) (a) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112. (b)
Sheldrick, G. Acta Crystallogr., Sect. A 1990, 46, 467.
(11) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189.
(12) Garrido, G.; Koort, E.; Rꢂafols, C.; Bosch, E.; Rodima, T.; Leito,
I.; Rosꢂes, M. J. Org. Chem. 2006, 71, 9062.
(13) (a) Vorobyova, O.; Lau, W.; Winnik, M. A. Langmuir 2001,
17, 1357. (b) Tsujii, K.; Sunamoto, J.; Nome, F.; Fendler, J. H. J. Phys.
Chem. 1978, 82, 423.
(14) Unfortunately, the data in hand do not provide sufficient
information to identify the nature of the additional species. However,
knowledge that monomeric PN6 dominate at very low concentrations
and large aggregates form at higher concentrations leads us to conjecture
that at least some of the emission at 3.8 Â 10À5 M PN6 in THF may be
from excitation of isolated ground state dimers.
(15) Vanderauwera, P.; DeSchryver, F. C.; Weller, A.; Winnik, M. A.;
Zachariasse, K. A. J. Phys. Chem. 1984, 88, 2964.
(16) (a) Abraham, S.; Atvars, T. D. Z.; Weiss, R. G. J . Phys. Chem. B
2010, 114, 12221. (b) Powell, J. R.; Pandey, S.; Miller, B. J.; Acree, W. E.;
Hansen, P. E.; Fetzer, J. C. J. Lumin. 1996, 69, 27.
(17) Because the calculations do not include a chloride anion or
solvent molecules to stabilize the positive charge on nitrogen, the B
conformers are estimated to be ∼12 kcal molÀ1 more stable than the O
ones; the π-electrons of pyrenyl offer the only major stabilization of the
charge. Our attempts to calculate the process in Scheme 1, starting with
the ground state of conformer B of PN1 and proceeding to the pyrenyl-
amine exciplex, have failed thus far. We attribute the lack of success to
our inability to find energy convergence when more than one THF
molecule and a chloride anion are present.
(18) (a) Kauffman, J. F.; Khajehpour, M.; Saleh, N. J. Phys. Chem. A
2004, 108, 3675. (b) Scherer, T.; van Stokkum, I. H. M.; Brouwer, A. M.;
Verhoeven, J. W. J. Phys. Chem. 1994, 98, 10539.
(19) (a) Morikubo, N.; Fukuda, Y.; Ohtake, K.; Shinya, N.; Kiga, D.;
Sakamoto, K.; Asanuma, M.; Hirota, H.; Yokoyama, S.; Hoshino, T.
J. Am. Chem. Soc. 2006, 128, 13184. (b) Petersen, F. N. R.; Jensen, M. Ø.;
Nielsen, C. H. Biophys. J. 2005, 89, 3985. (c) Duan, G.; Smith, V. H.;
Weaver, D. F. J. Phys. Chem. A 2000, 104, 4521.
(20) Zimerman, O. E.; Weiss, R. G. J. Phys. Chem. A 1998, 102, 5364.
(21) (a) Tsujii, K.; Sunamoto, J.; Nome, F.; Fendler, J. H. J. Phys.
Chem. 1978, 82, 423. (b) Yip, J.; Duhamel, J.; Bahun, G. J.; Adronov, A.
J. Phys. Chem. B 2010, 114, 10254.
(22) Kosower, E. M.; Huppert, D. Annu. Rev. Phys. Chem. 1986, 37,
127.
19256
dx.doi.org/10.1021/ja208674s |J. Am. Chem. Soc. 2011, 133, 19250–19256