Conformationally dynamic π-conjugation: Probing structure - Property relationships of fluorescent tris(N-salicylideneaniline)s

Article abstract of DOI:10.1021/jp2079583

We recently reported the design and synthesis of a series of conformationally dynamic chromophores that are built on the C ₃-symmetric tris(N-salicylideneaniline) platform. This system utilizes cooperative structural folding - unfolding motions

Full text of DOI:10.1021/jp2079583

ARTICLE
pubs.acs.org/JPCA
Conformationally Dynamic π-Conjugation: Probing
StructureꢀProperty Relationships of Fluorescent
Tris(N-salicylideneaniline)s
Mario Vieweger, Xuan Jiang, Young-Kwan Lim, Junyong Jo, Dongwhan Lee,* and Bogdan Dragnea*
Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States
S Supporting Information
b
ABSTRACT: We recently reported the design and synthesis of
a series of conformationally dynamic chromophores that are
built on the C3-symmetric tris(N-salicylideneaniline) platform.
This system utilizes cooperative structural foldingꢀunfolding
motions for fluorescence switching, which is driven by the
assembly and disassembly of hydrogen bonds between the rigid
core and rotatable peripheral part of the molecule. Here, we
report detailed time-resolved spectroscopic studies to investi-
gate the structureꢀproperty relationships of a series of func-
tionalized tris(N-salicylideneaniline)s. Time-resolved fluor-
escence decay spectroscopy was applied to determine the main relaxation mechanisms of these π-extended fluorophores, and to
address the effects of hydrogen bonding, steric constraints, and extension of the π-conjugation on their relaxation dynamics. Our
results agree well with the conformational switching model that was previously suggested from steady-state experiments. Notably,
extension of the π-conjugation from peripheral aryl groups resulted in the stabilization of the excited states, as evidenced by longer
lifetimes and lower nonradiative decay constants. As a consequence, an increase in the fluorescence quantum yields was observed,
which could be explained by the suppression of the torsional motions about the CꢀN bonds from an overall increase in the quinoid
character of the excited states. A combination of time-resolved and steady-state techniques also revealed intermolecular interactions
through πꢀπ stacking at higher concentrations, which provide additional de-excitation pathways that become more pronounced in
solid samples.
’ INTRODUCTION
pathways supported by either hyperbranched or ring-fused 2-D
structural skeletons plays a critical role by reducing conforma-
tional disorder and preventing thermal dissipation of energy.9
Evaluation and elaboration of structureꢀproperty relationships
in 2-D structural settings, however, require access to structural
motifs that allow for facile and systematic modification of rel-
evant geometric and electronic parameters.
We recently reported the synthesis and characterization of
a series of C3-symmetric π-conjugation that utilize cooperative
structural foldingꢀunfolding motions for fluorescence switching
(Scheme 1).8,10ꢀ15 These fluorophores have three ortho-sub-
stituted aryl groups that are attached directly to the 3-fold
symmetric tris(N-salicylideneamine) core {C6O3(CHNH)3},16,17
which mimics the shape of triphenylene.18 As shown in Scheme 1,
An increasing number of charge transporters and light emit-
ters are built with organic molecules.1 The practical utility of such
plastic electronics derives from the ability to manipulate the
optical and electronic properties of well-defined synthetic sys-
tems by either covalent or noncovalent structural modifications
of their π-conjugated backbone.2ꢀ4 Within this context, research
elsewhere has investigated the relationship between conforma-
tional dynamics and photophysical properties of one-dimen-
sional (1-D) π-conjugation.5 Specifically, it has been shown that
conformational restriction and enforced coplanarity of 1-D con-
jugation lead to a decrease in the HOMOꢀLUMO gap and en-
hancement in fluorescence efficiency.6
Studies on the structureꢀproperty relationships of two-
dimensional (2-D) conjugation should complement such efforts
by providing additional data sets to be tested and interpreted
using models developed for simpler 1-D systems.7 For 2-D sys-
tems, the added dimensionality of electron delocalization at excited
states clearly impacts the photophysical properties of π-conjuga-
tion beyond the confinement of linear skeleton.8 Understand-
ably, directional transfer of excited-state energies in light-harvesting
systems has been one of the major driving forces in such re-
search.3 The shape-persistent nature of the rigid π-conjugation
the OꢀH O hydrogen-bonding contacts between the alcohol
3 3 3
groups at the “wingtips” of the aryl rings and the ketone oxygen
atoms of the molecular core stabilize the “folded” conformer A.
In addition to flattening the entire structure to define an extended
π-conjugation, such intramolecular hydrogen-bonding network
Received: August 18, 2011
Revised:
October 17, 2011
Published: October 17, 2011
r
2011 American Chemical Society
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suppresses internal torsional motions of A to make it fluorescent.
provide useful structureꢀproperty relationships that will guide
rational structure design in such directions. Toward this objec-
tive, weinitiated comparative spectroscopicstudies on compounds
1ꢀ10 listed in Figures 1 and 2.
Loss of such OꢀH O contacts results in the “unfolded” and
3 3 3
nonemissive conformer B (Scheme 1) having freely rotating
CꢀN bonds. Here, the lack of structure-rigidifying hydrogen-
bonding interactions facilitates nonradiative decay of the excited
states through internal torsional motions. This simple mechan-
istic model was supported by the loss of fluorescence intensity in
the presence of hydrogen-bonding DMSO solvents or Fꢀ anions,
The series 1ꢀ3 (Figure 1) were investigated specifically to
address the role of hydrogen bonding and steric constraints on
the stability of the excited states. The series 4ꢀ10 (Figure 2)
were studied to delineate the effects of extended π-conjugation
on both the energy window and the efficiency of light emission.
We applied time-resolved spectroscopy to determine the origins
and kinetics of the main relaxation mechanisms of these func-
tionalized tris(N-salicylideneaniline)s 1ꢀ10. Individual contri-
butions of these excited-state decay processes on the commonly
measured steady-state properties, such as fluorescence spectra
and emission quantum yields, were evaluated across the series,
the details of which are presented in this work.
which effectively compete with the intramolecular OꢀH
O
3 3 3
hydrogen bonds to unfold the molecule.10 This fluorescence
onꢀoff switching scheme, which is based on the hydrogen
bonding between the rotatable perimeter and the rigid core of
the fluorophores, was exploited subsequently for a turn-on sig-
naling of fluoride ions in solution12 and signal amplification
through orientation-dependent fluorescence resonance energy
transfer (FRET).13
The use of functionalized tris(N-salicylideneaniline)s as sti-
muli-responsive molecular switches and sensors should benefit
significantly from (i) maximizing changes in the emission inten-
sity (ΔI) upon structural foldingꢀunfolding, and (ii) modulating
spectral windows of photoexcitation and emission through
structural modifications of the π-conjugation. Detailed spectro-
scopic studies on a homologous set of molecules should thus
’ EXPERIMENTAL SECTION
General Considerations. All reagents were obtained from
commercial suppliers and used as received unless otherwise
noted. Diisopropylamine was degassed by freezeꢀpumpꢀthaw
cycles (ꢁ3) and stored under nitrogen. The compounds 1,3,5-
triformylphloroglucinol16a and 2-iodo-4-tert-butyl-aniline19 were
prepared according to literature procedures. The syntheses of
1,10 2,12 and 4ꢀ1011 have previously been reported. All air-
sensitive manipulations were carried out under nitrogen atmosphere
in an M.Braun glovebox or by standard Schlenk-line techniques.
Scheme 1. Chemical Structure of a Functionalized Tris-
(N-salicylideneaniline) and a Cartoon-Style Representation
of Its “Folded” Conformation (A) through a Cyclic Array of
Intramolecular Hydrogen Bonds; Loss of These Key OꢀH
1
Physical Measurements. H NMR and 13C NMR spectra
3 3 3
OꢀH O Contacts Leads to Structural “Unfolding” (B)
3 3 3
were recorded on a 400 MHz Varian Inova NMR spectrometer
or a 300 MHz Varian Gemini 2000 NMR spectrometer. Chemi-
cal shifts were reported versus tetramethylsilane and referenced
to the residual solvent peaks. High-resolution chemical ioniza-
tion (CI) and electrospray ionization (ESI) mass spectra were
obtained on a Thermo Electron Corp. MAT 95XP-Trap using
CH4 as CI reagent. FT-IR spectra were recorded on a Nicolet
510P FT-IR spectrometer with EZ OMNIC ESP software.
UVꢀvis spectra were recorded on a Perkin-Elmer Lambda 19
UV/vis/near-IR spectrometer or a Varian Cary 100 Bio UVꢀvi-
sible spectrophotometer. Steady-state fluorescence spectra were
recorded on a Perkin-Elmer LS50B luminescence spectrometer
or a Photon Technology International QM-4-CW spectrofluo-
rometer with FeliX32 software. Measurements were performed
under temperature control and purging with dry nitrogen for
temperatures below 15 ꢀC. Fluorescence measurements were per-
formed with an excitation at 380 nm, and concentrations were chosen
with absorbances below 0.1 to reduce the inner filter effect. The fluo-
rescence spectra were integrated and the quantum yields obtained
according to eq 1 using Coumarin 30 (Q = 0.67 in MeCN solution)
with Freely Rotating CꢀN Bonds between the C3-Symmetric
Molecular Core and Peripheral Aryl Groups
Figure 1. Chemical structures of tris(N-salicylideneaniline) derivatives 1ꢀ3.
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Figure 2. Chemical structures of π-extended tris(N-salicylideneaniline) derivatives 4ꢀ10.
as a reference:20,21
!
ꢀ
ꢁ
Gradf
ηf2
η2st
Qf ¼ Qst
ð1Þ
Gradst
The indices “f” and “st” stand for the fluorophore and the
quantum yield standard (=reference molecule), respectively. Q
represents the emission quantum yield, Grad is the gradient of
integrated fluorescence intensity versus absorbance at λ = 380 nm,
and η is the index of refraction of the solvent.
Lifetime Measurements. Samples were excited by a mode-
locked Ti:S laser (Mira 900-F, Coherent Inc., CA) with a rep-
etition rate of 76 MHz and a pulse duration of less than 200 fs. To
allow for two-photon absorption to occur, the 800 nm output was
focused onto the sample. Fluorescence photons were detected
orthogonal to the incident beam on a fast photomultiplier tube
(PMA 165-P, PicoQuant GmbH, Germany), preamplified, and
collected by time-correlated single photon counting (TCSPC)
electronics (TimeHarp200, PicoQuant, GmbH, Germany) for
processing (Figure 3).
A short pass filter and a Glan-Taylor polarizer at magic angle
position (54.7ꢀ) were placed in the detection path to ensure
that only fluorescent photons were detected and to avoid rota-
tional diffusion artifacts. Temperature control was achieved with
a FLASH 200 fluorescence cuvette holder (Quantum Northwest
Inc., Seattle) under dry-gas purging. The wavelength and band-
pass selections were obtained using a H10 VIS monochromator
(HORIBA Jobin Yvon Inc., NJ). The monochromator features a
holographic grating with a linear dispersion of 1200 g/mm, which
corresponds to a bandpass of 8 nm/mm slit width. Because of
intensity and concentration considerations, a bandpass of 16 nm
was chosen and measurements were obtained at 5 nm intervals
throughout the fluorescence spectrum of the sample.
Figure 3. Experimental setup for time-correlated single photon count-
ing (TCSPC).
Fluorescence decay curves were fitted to a multiexponen-
tial decay model (eq 2) using global fluorescence decay data
analysis software (FluoFit 4.2, PicoQuant, GmbH, Germany).22ꢀ29
The goodness of the fits was evaluated according to their
reduced χ2-values,26,27 the normal deviate of the reduced
χ2-value (Zχ2),26 the plot of the weighted residuals,26,30 and
the autocorrelation function26,27 of the weighted residuals.
Only the fits with the lowest number of excited-state life-
times that gave satisfactory fitting results were used for
further analysis.
n
αieð ꢀ t=τ Þ
ð2Þ
i
IðtÞ ¼
∑
i¼1
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where I(t) is the recorded intensity at time t, αi is the pre-
exponential factor or amplitude of decay component i, τi is
the lifetime, and t is the measurement duration.
The lifetimes and their corresponding pre-exponential factors
thus obtained were then used to calculate weighing factors for
lifetime specific contributions to the average lifetimes and steady-
state properties. These weighing factors are the fractional ampli-
tude (ai) and the fractional intensity (fi):
Figure 4. Deviation of the peripheral aryl rings from the tris(N-
salicylideneanilinle) core as measured by the dihedral angle ϕ.
αi
ai ¼
ð3Þ
n
αi
∑
i¼1
Z
∞
IiðtÞ dt
IðtÞ dt
αiτi
0
Z
fi ¼
¼
ð4Þ
∞
n
αiτi
∑
0
i¼1
The fractional amplitude ai is the fraction of the integral under
the decay curve with which component i contributes to the total
detected fluorescence intensity (∑ni=l αi = 1 and therefore ai = αi).
The fractional intensity fi takes into account the lifetime and
therefore the average duration of the lifetime-specific excitationꢀ
emission cycles (∑ni=lfi = 1).
The average lifetimes are defined as the sum over the products
of weighing factor and lifetime for i = n contributing decay
components:
Figure 5. (a) Electronic absorption and (b) emission spectra of 1ꢀ3 in
CH2Cl2 at T = 25 ꢀC.
(s, 9H), 0.22 (s, 9H). 13C NMR (100 MHz, CDCl3): δ 145.9, 140.5,
128.6, 127.2, 114.0, 107.2, 102.4, 98.9, 33.8, 31.3, 0.14. FT-IR
(thin film on NaCl, cmꢀ1): 3477, 3380, 2960, 2903, 2868, 2147,
1726, 119, 1500, 1463, 1408, 1363, 1304, 1278, 1250, 1159, 928,
844, 760, 698, 653, 629, 492. HRMS (CI) calcd for C15H23NSi
[M]+ 245.1594, found 245.1593.
n
2,4,6-Tris((4-tert-butyl-2-((trimethylsilyl)ethynyl)phenyl-
amino)methylene)cyclohexane-1,3,5-trione (3). A solution of
1,3,5-triformylphloroglucinol (40 mg, 0.19 mmol) and 4-tert-
butyl-2-((trimethylsilyl)ethynyl)aniline (0.271 g, 1.10 mmol) in
anhydrous EtOH (10 mL) was heated at 90 ꢀC under nitrogen
for 12 h. With progress of the reaction, the mixture became hete-
rogeneous. The product was isolated by filtration, washed with
EtOH, and dried in vacuo to furnish 3 as a yellow powder (0.15 g,
0.17 mmol, 90%). 1H NMR (400 MHz, CDCl3): δ 13.68 (d, J =
12.9 Hz, 3H), 8.79 (d, J = 12.9 Hz, 3H), 7.52 (d, J = 1.8 Hz, 1H),
7.41 (dd, J = 8.4, 1.8 Hz, 1H), 7.31 (d, J = 8.4 Hz, 1H), 1.31
(s, 9H), 0.46 (s, 9H). 13C NMR (100 MHz, CDCl3): δ 185.1,
147.4, 147.1, 139.2, 129.7, 127.3, 113.4, 113.1, 107.6, 102.3, 99.9,
34.5, 31.2, 0.06. FT-IR (thin film on NaCl, cmꢀ1): 2960, 2901, 2868,
2154, 1614, 1594, 1573, 1444, 1347, 1286, 1268, 1239, 1034, 986,
927, 844, 815, 760, 634, 491. HRMS (CI) calcd for C54H69N3O9-
Si3 [M]+ 891.4641, found 891.4624.
αiτi
∑
n
i¼1
Æτæa ¼
¼
αiτi
ð5Þ
∑
n
i¼1
αi
∑
i¼1
n
αiτ2
∑
i¼1
n
i
n
Æτæf ¼
¼
fiτi
ð6Þ
∑
i¼1
αiτi
∑
i¼1
Consistently, the average lifetimes are referred to as ampli-
tude-averaged (Æτæa) and intensity-averaged (Æτæf). The latter
represents the average period of time the chromophore stays
in the excited state, while the former turns out to be the
parameter of choice for most quantitative analysis due to its
relation to the steady-state fluorescence spectrum.31 For de-
tails of decay analysis and lifetime fitting results, see the
Supporting Information.
’ RESULTS AND DISCUSSION
4-tert-Butyl-2-((trimethylsilyl)ethynyl)aniline. In a glove-
box, PdCl2(PPh3)2 (30 mg, 43 μmol) and CuI (16 mg, 84 μmol)
were loaded into a 50 mL tube equipped with a Teflon-lined
screw cap. The tube was sealed, removed from the glovebox, and
placed under nitrogen atmosphere. Portions of 2-iodo-4-tert-
butylaniline (0.38 g, 1.4 mmol), trimethylsilylacetylene (0.30 g,
Conformational Dynamics in Solution. The design of 1ꢀ3
takes into account variations in the number of hydrogen bonds
and the degree of steric constraints, which should impact confor-
mational dynamics of the molecules in solution (Scheme 1). As
we reported previously,10,12 both 1 and 2 have symmetry-rein-
i
3.0 mmol), and a mixture of Pr2NH/THF (10 mL, 1:1, v/v)
forced OhydroxylꢀH OhydroxylꢀH Oketo (for 1) or Ohydroxylꢀ
3 3 3 3 3 3
were delivered under nitrogen atmosphere. The reaction mixture
was stirred at room temperature for 12 h, and filtered through a
pack of Celite. The filtrate was concentrated under reduced
pressure, and the residual brown material was purified by flash
column chromatography on SiO2 (hexanes:EtOAc = 10:1, v/v)
H
Oketo (for 2) hydrogen-bonding interactions between the
3 3 3
peripheral aryl rings and the tris(N-salicylideneaniline) core
(Figure 1), which function as a conformational lock to effectively
flatten the molecule (Scheme 1). A close comparison of the X-ray
structures reveals a larger dihedral angle (see Figure 4) for 1
(20.9ꢀ) relative to 2 (10.9ꢀ11.9ꢀ), which apparently reflects an
increased steric congestion between the ethynyl-extended wing-
tip groups in 1 that are brought in close proximity upon structural
1
to afford a yellow oil (0.274 g, 1.12 mmol, 79%). H NMR
(300 MHz, CDCl3): δ 7.32 (d, J = 1.8 Hz, 1H), 7.16 (dd, J = 8.4,
1.8 Hz, 1H), 6.64 (d, J = 8.4 Hz, 1H), 4.10 (br s, 2H), 1.23
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Figure 6. Fluorescence decay profile and instrument response function (IRF) for (a) 1, (b) 2, and (c) 3 in CH2Cl2 at T = 20 ꢀC. The quality of the fits is
indicated by the low χ2 values and the random distribution of weighted residuals, which are shown below for each plot.
Table 1. Lifetime Measurements of 1ꢀ3 from Single Deconvolution Fitting
compound
τ1 (ns)
τ2 (ns)
τ3 (ns)
f1
f2
f3
Æτæint (ns)
1
2
3
0.209 ( 0.002
0.295 ( 0.003
0.110 ( 0.002
0.592 ( 0.017
0.719 ( 0.001
0.678 ( 0.099
1.84 ( 0.13
1.74 ( 0.04
0.786 ( 0.003
0.092 ( 0.013
0.988 ( 0.002
0.185 ( 0.002
0.860 ( 0.012
0.012 ( 0.002
0.029 ( 0.002
0.048 ( 0.001
0.327 ( 0.004
0.729 ( 0.012
0.116 ( 0.002
Table 2. Radiative and Nonradiative Decay Constants of 1ꢀ3
at T = 20 ꢀC
decay pathways involved.
kr
Qf ¼
¼ τ kr
ð7Þ
ð8Þ
compound knr (nsꢀ1) kr (nsꢀ1
)
Qf
Æτæamp (ns) Stokes shift (nm)
3
kr þ knr
1
2
3
3.85
1.39
8.88
0.241 0.059 0.245 ( 0.002
0.149 0.097 0.651 ( 0.006
0.144 0.016 0.111 ( 0.002
30
18
23
1
τ ¼
kr þ knr
Using the instrument setups (Figure 3) and procedures
described in the Experimental Section, we recorded and analyzed
fluorescence decay of 1ꢀ3 by TCSPC. While fitting with two
exponential decay components provided reasonable results for 3
(Figure 6c), an additional third decay component was required to
fit the lifetime decay curves of 1 and 2 (Figure 6a and b).
A summary of the individual lifetimes and fractional intensities
obtained from time-resolved studies is provided in Table 1. Here,
lifetimes were assigned to three domains, τ1 (0.1ꢀ0.3 ns), τ2
(0.45ꢀ0.8 ns), and τ3 (1.4ꢀ1.9 ns), the physical meaning of
which became our immediate interest. According to the frac-
tional intensities (f1ꢀf3), compound 1 has significant contribu-
tions from both τ1 (79%) and τ2 (18%), and a trace amount of
τ3 (3%). On the other hand, 2 shows small amounts of τ1 (9%)
and τ3 (5%) components but a significant contribution from
τ2 (86%). In 3, τ1 (99%) prevails and τ3 could not be detected
within the sensitivity of the setup.
The results from the deconvolutions were then used to
calculate amplitude-averaged lifetimes, as well as radiative and
nonradiative rate constants listed in Table 2. While no significant
differences were observed in the radiative rate constants for 2
(0.149 nsꢀ1) and 3 (0.144 nsꢀ1), which share an essentially
identical π-skeleton, 1 having an additional ethynyl unit on each
aniline fragment shows a slightly higher kr of 0.241 nsꢀ1. In
contrast to the radiative rate constants, the nonradiative rate con-
stants differ significantly across the series 1ꢀ3. While the range of
radiative rate constants spans less than a factor of 2, the lack of
hydrogen bonding results in a more than 6-fold increase in the
nonradiative rate constants when a comparison is made between
2 and 3.
folding (Scheme 1). For comparative studies, compound 3 was
prepared as a reference system that does not have any hydrogen
bonds between the core and peripheral aromatics.
In CH2Cl2 at 25 ꢀC, 2 displays intense (ε = 93 000 Mꢀ1
cmꢀ1) visible absorptions at λmax,abs = 419 and 440 nm
(Figure 5), which are slightly red-shifted with respect to those
of 1 (λmax,abs = 414 and 432 nm). Following excitation at
λ = 340 nm, 2 emits at λmax,em = 458 nm with a small Stokes shift
(Δλ = 18 nm), which is smaller than that of 1 (Δλ = 30 nm)
and presumably reflects the more rigid molecular structure. In
support of this notion, the fluorescence quantum yield (=Qf)
of 2 is 9.7%, which is significantly higher than that (Qf = 4.2%)
of 1 under similar conditions. While 3 shows absorption and
emission features comparable to those of 2, its fluorescence
quantum yield of Qf = 1.6% is more than 6 times lower than
that of 2.
Our initial experiments thus targeted a better understand-
ing of the relationship between molecular structure, optical
properties, and dynamic behavior of 1ꢀ3 in solution. For this
purpose, time-resolved fluorescence spectroscopy was ap-
plied. As described by eqs 7 and 8, Qf corresponds to the
probability that the excited state is deactivated by a radiative
rather than a nonradiative pathway, with rate constants of kr
and knr, respectively. This relationship allows for a quantita-
tive analysis of the competing processes and the stability of the
excited states of the molecules.32,33 While most chromophores
have only one radiative pathway, several competing nonradia-
tive pathways may exist. To account for such possibilities, knr
is defined as the sum of the rate constants of all nonradiative
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Figure 7. Steady-state fluorescence spectra (black) of (a) 1, (b) 2, and (c) 3, comprised of contributions from DAS of different lifetimes τ1 (red),
τ2 (green), and τ3 (blue) in CH2Cl2 at T = 25 ꢀC.
Figure 8. Concentration-dependent changes in the relative intensities (f1, f2, and f3) of lifetime components (τ1, τ2, and τ3) plotted for (a) 1, (b) 2,
and (c) 3.
A significantly decreased fluorescence quantum yield of 3 is
thus a consequence of an increase in knr, rather than a decrease
in kr. This finding quantitatively substantiates the intuitive fluor-
escence switching model shown in Scheme 1, in which the loss
of structural rigidity opens additional nonradiative channels for
de-excitation of B, in a situation similar to 3. Consistent with this
model, the smallest knr of 1.39 nsꢀ1 in the series (Table 2) is
associated with 2 having the highest Qf of 9.7%. To better under-
stand the nature of nonradiative decay pathways that dictate the
emissive properties of these molecules, we decided to investigate
the origins of the individual lifetime components of 1ꢀ3.
Intermolecular Interactions in Solution. A dynamic inter-
conversion between the planar A and nonplanar B conformer
(Scheme 1) was previously investigated by variable-temperature
(VT) 1H NMR spectroscopy on solution samples.15 While most
fluorophores operate by a single emitting state, the involvement
of at least two different conformations of tris(N-salicylideneaniline)s
in solution immediately raises the question of how many emissive
species actually contribute to the experimentally observed fluor-
escence spectra. To address this point, we monitored the wave-
length dependence of the decay response across the entire emission
spectrum. Using the steady-state emission intensity of the sample,
Fss(λ), as a reference, spectra associated with the individual fluor-
escence decay lifetimes (DAS(λ, τi)) were reconstructed using
eq 9:34
measurements were thus taken in increments of 5 nm with a
bandwidth of 8 nm. The decay profiles thus obtained were ana-
lyzed globally, the results of which are displayed in Figure 7: the
steady-state fluorescence spectra (Fss) are in black; the decay-
associated spectra are in red (τ1), green (τ2), and blue (τ3).
Despite limitations in the spectral resolution of the current
setup, the decay-associated spectra (DAS) clearly revealed the
emergence of two emission maxima for 1 and 2. The main
emission with peak intensity at λ = 456ꢀ464 nm corresponds to
τ1 and τ2, whereas the emission of τ3 appears red-shifted at λ =
490ꢀ540 nm. The emission maxima of τ1 and τ2 are within two
data points of the DAS, which makes it less straightforward to
interpret whether they originate from distinctive species or
minor changes in the conformation of the same chromophore.
The emission associated with the τ3 component, however, is
markedly red-shifted from those of τ1 and τ2 (see Figure 7b, for
example) and is associated with the longest lifetime (Table 1).
These properties suggest a moreextended electronic structure that
typically evolves from intermolecular interactions of π-conjugated
molecules in solution.35
If intermolecular πꢀπ interaction is indeed responsible for
the emitting species with τ3, its solution population would show
concentration dependence. We thus carried out TCSPC experi-
ments on solution samples with a concentration range of 50ꢀ500
μM to examine the involvement of such solution “aggregates”.
To ensure that the changes in fractional intensities directly reflect
the changes in the composition of the excited state, measure-
ments were performed over the same amount of fluorescent
photons. For 1 and 2, the red-shifted component with the
lifetime τ3 shows an increase in fractional intensities toward
higher concentrations (Figure 8). This increase amounts to 1.9 (
0.7% for 1 and 10.9 ( 0.9% for 2. In contrast, no significant
αiðλÞ
DASðλ, τiÞ ¼
FssðλÞ
ð9Þ
αiðλÞτi
∑
To collect a sufficient amount of photons (10 000 cts) in a
reasonable amount of time (<10 min), a compromise was needed
between the resolution and measurement time. A total of 18
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Table 3. Lifetime Measurements of Solid Samples of 1 and 2
compound
τ2 (ns)
τ3 (ns)
f2
f3
Æτæint
1
2
0.459 ( 0.006
0.555 ( 0.009
1.31 ( 0.02
1.83 ( 0.03
0.732 ( 0.058
0.598 ( 0.027
0.268 ( 0.058
0.402 ( 0.027
0.686 ( 0.049
1.066 ( 0.034
Figure 9. Concentration-dependent (1ꢀ50 mM) changes in the (a) OhydroxylꢀH, (b) NenamineꢀH, and (c) CvinylꢀH proton resonances of 1 (b) and 2
(9) in CDCl3 at T = 25 ꢀC.
changes in the intensities could be detected for 3 (0.4 ( 0.2%).
This concentration dependence of the fractional intensities
suggests that the red-shifts in the DAS are due to intermolecular
interactions that lead to aggregation in solution, presumably via
arrangement presumably allows for a more favorable intermole-
cular interaction, which is reflected in the higher fractional inten-
sity of the τ3 component in 2.
The solution dynamics leading to aggregation was probed in-
dependently by concentration-dependent 1H NMR spectroscopy.
The chemical shifts of the OꢀH protons in 1 and 2 were moni-
tored in CDCl3 over the concentration range of 1ꢀ50 mM at
25 ꢀC. As shown in Figure 9, the OꢀH proton resonances of 1
show negligible change from 5.24 to 5.25 ppm. The chemical
shifts of other protons, such as those associated with the enamine
NꢀH, vinyl, and aromatic CꢀH protons, also remain essentially
unchanged. In contrast, the OꢀH protons of 2 show significant
downfield shift from 5.20 to 5.49 ppm with an increase in concen-
tration (Figure 9a), whereas other protons shift to upfield. For
example, the NꢀH protons shift from 13.95 to 13.55 ppm and
the CꢀH protons from 8.59 to 8.80 ppm (Figure 9b and c).
These observations suggest that an intermolecular association is
indeed occurring for 2 in solution, leading to stacking and aggre-
gation at higher concentrations.
stacking of rigid flat molecules assisted by OꢀH O contacts
3 3 3
between individual molecules (vide infra). These findings are in
accordance with the observation of intermolecular hydrogen
bonds in the X-ray structure of 2,12 which might occur for mole-
cules in solution. The lack of such hydrogen-bonding capabilities
also explains why no stacking (and therefore no τ3 component)
could be detected for 3.
To correlate the dynamics in solution with molecular struc-
tures and to determine if such stacking interactions occur for
molecules in the ground state or in the excited state, solid-state
samples were prepared by dropcasting 1 or 2 on glass substrates.
TCSPC experiments on these samples revealed good fits for two
decay components for 1 and 2 (Table 3). The lifetimes are on the
order of 0.45ꢀ0.56 and 1.3ꢀ1.9 ns, which overlap fairly well with
the lifetimes τ2 and τ3 of solution samples. The corresponding
fractional intensities show a significant increase in the longest
decay component with 26.8 ( 5.8% for 1 and 40.2 ( 2.7% for 2,
as compared to 1.9 ( 0.7% and 10.9 ( 0.9% obtained for the
corresponding solution samples at the high concentration end
(500 μM). The third, shortest lifetime component could not be
detected, which may be due to the suppression of nonradiative
decay pathways due to a dense packing of the molecules within
the solid film.
A higher degree of stacking found for 2 (40%) relative to 1
(27%) in the solid state, as deduced from the fractional intensities
(Table 3), indicates that its molecular geometry prefers inter-
molecular interactions. One structural parameter to evaluate the
overall planarity of the molecule, as well as the degree of conju-
gation, is the dihedral angle ϕ between peripheral aryl rings and
the core unit (Figure 4). Steric interactions between two neigh-
boring propargyl alcohol groups, three pairs in total (Scheme 1
and Figure 1), result in a relatively large dihedral angle of 20.9ꢀ in
1.10 The absence of such steric congestion allows 2 to fully relax
and orient more properly in the plane of the π-system, which
results in smaller dihedral angles of 10.9ꢀ11.9ꢀ.12 This planar
In sum, the time-dependent spectral decay and the concentra-
1
tion dependence of TCSPC and H NMR data are consistent
with the existence of aggregation in solution, which results in a
red-shifted emission and elongated lifetime of τ3. We thus con-
clude that τ3 results from a concentration-dependent intermole-
cular process rather than conformational switching of discrete
molecular species (Scheme 1).
Photophysical Consequences of Hydrogen Bonding and
Steric Constraints. Findings described in the previous section
suggest that contribution of τ3 can be suppressed by lowering
sample concentrations. Under such conditions, the excited states
associated with τ1 and τ2 could be studied with minimal compli-
cations from intermolecular processes. As the DAS did not allow
a clear distinction of the components associated with τ1 and τ2,
the nature of the de-excitation pathways was explored by study-
ing their temperature dependence. Previous time-resolved studies
on simple N-salicylideneaniline derivatives identified two decay
components with lifetimes that are similar to those observed for
1ꢀ3.36 These lifetimes were explained by invoking an isomeriza-
tion between the cis-keto* and twist-keto* forms of the molecule
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Figure 10. Temperature-dependent changes in the fractional intensities of (a) 1, (b) 2, and (c) 3.
Figure 11. Lifetime-associated contributions of the decay components to the total emission quantum yield of (a) 1, (b) 2, and (c) 3 as a function of
temperature.
in the excited state. The feasibility of similar processes at the
tris(N-salicylideneaniline) core prompted us to investigate the
effects of temperature on the fractional intensities of the lifetime
components. Subsequent measurements were thus conducted for
solution samples of 1ꢀ3 (25 μM) in CHCl3 from ꢀ25 to +50 ꢀC
(Figure 10).
As shown in Figure 10b, the composition of the excited state
hardly changes for 2 (85ꢀ95% of τ2) within the temperature
range of ꢀ25 to +50 ꢀC. In contrast, 3 shows an “inversion” in the
fractional intensities as a function of temperature (Figure 10c),
which results in the decrease of f2 from ca. 70% at ꢀ25 ꢀC to
<10% above 10 ꢀC with concomitant increase of f1. Under similar
conditions, 1 also shows a depletion of the state associated with
τ2 with increasing temperature (from 80% at ꢀ25 ꢀC to 20% at
50 ꢀC), although it slopes less steeply and still contributes signi-
ficantly (50%) to the excited state at 10 ꢀC (Figure 10a). At low
temperatures, the state associated with the lifetime τ2 thus be-
comes dominant for all three systems and shows the trend of
f2 (2) > f2 (1) > f2 (3).
The fractional intensities plotted in Figure 10 show the average
population of the excited states associatedwithindividuallifetimes.
As the lifetimes change with temperature due to the activation
barriers of nonradiative relaxations, so do the populations of the
excited states. The nature of the relaxation mechanisms, along with
their activation barriers and dependence on temperature, influ-
ences therate atwhichthe population of theexcited stateschanges.
The competition between the radiative and nonradiative processes
for the relaxation of the molecule results in a lower fluorescence
efficiency at a higher temperature and vice versa.
originates from an inversion in the populations of the excited
states or two independent processes with very different tempera-
ture dependences, an absolute measurement was needed. For
this purpose, the fractional amplitudes and the steady-state
emission quantum yields were used to calculate the lifetime-
associated contributions to the total fluorescence quantum yield
(eqs 3 and 7). The fractional amplitudes represent the relative
contribution that the individual lifetimes have on the total amount
of photons recorded. This dependence on the total fluorescence
intensity allows calculation of the contribution of individual life-
times to the emission quantum yield of the molecules. An absolute
measure of the components contributing to the steady-state fluo-
rescence is shown in Figure 11, which compares fractional ampli-
tudes of individual lifetimes relative to the total amount of photons
recorded.
The lifetime dependence of the emission quantum yields
plotted in Figure 11 shows the influence that the composition
of the excited state has on the fluorescence spectra. Overall, the
quantum yields decrease with increasing temperature. This is a
typical behavior, which reflects higher rate constants of non-
radiative decay at elevated temperatures. As shown in Figure 11,
both τ1 and τ2 contribute significantly to the total quantum yield
at low temperatures. At ꢀ10 ꢀC, they contribute about equally in
the case of 1 and 3, whereas τ1 contributes 4 times as much to the
fluorescence intensity of 2 as τ2. Toward the high temperature
end, however, τ2 loses its contribution. For 1 and 3, the contri-
bution from τ1 remains quite steady until it reaches a “critical
temperature”, beyond which it starts to decrease as well. This
critical temperature is 20ꢀ25 ꢀC for 1, and 5ꢀ10 ꢀC for 3.
According to our analysis of Figure 11, the steady-state emis-
sion quantum yields of tris(N-salicylideneaniline)s are dictated
To determine whether the inversion in the fractional inten-
sities for 1 and 3 as a function of temperature (Figure 10a and c)
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Table 4. Lifetime and Decay Rate Constants of Photo-excited 4ꢀ10 in CHCl3 at 25 ꢀC
compound
τ1 (ns)
τ2 (ns)
τ3 (ns)
a1
a2
a3
Æτæamp (ns)
Qf
knr (nsꢀ1
)
kr (nsꢀ1
)
4
0.198
0.186
0.220
0.281
0.206
0.220
0.309
0.364
0.559
0.896
1.06
0.559
0.163
0.130
0.156
0.147
0.156
0.236
0.441
0.837
0.870
0.844
0.853
0.845
0.720
0.218
0.403
0.629
0.674
0.478
0.572
0.610
0.053
0.12
0.16
0.12
0.13
0.19
0.12
4.34
2.18
1.34
1.31
1.82
1.42
1.44
0.243
0.298
0.254
0.178
0.272
0.332
0.197
5
6
7
8
0.661
0.760
0.962
9
10
2.68
0.045
by an interplay between two different decay components that
have significant temperature dependence. The low temperature
range below the critical temperature (vide supra) is dominated
by τ2, where contributions from τ1 remain fairly constant. On the
other hand, the high temperature range above the critical tem-
perature is dominated by τ1, where τ2 has essentially vanished.
For 2, the critical temperature across which this switching occurs
seems to lie above the experimentally accessible temperature
window. This might be the reason that τ2 still has significant
contribution to the emission at T = 50 ꢀC (Figure 11b).
we investigated the structureꢀproperty relationships of tris-
(N-salicylideneaniline)s 5ꢀ10 having radially disposed π-conjuga-
tions that are extended from the C3-symmetric core of the parent
system 4 (Figure 2). Across the series 4ꢀ10, an identical set of
structure-rigidifying hydrogen-bonding network is maintained so
that our research focus can be placed on the effects of extended
π-conjugation on the emitting states.
As we previously reported,11 compounds 4ꢀ10 show intense
(ε > 75 000 Mꢀ1cmꢀ1) longer wavelength (λabs,max = 415ꢀ
475 nm) absorptionsandblue emissions(λem,max = 453ꢀ480 nm).
Relatively small Stokes shifts (Δλ = 24ꢀ35 nm) displayed by
these molecules are consistent with the rigid nature of their
extended π-system and small structural rearrangements upon
photoexcitation and de-excitation.32,33,39 In general, compounds
5ꢀ10 having extended π-conjugation show red-shifted bands in
both absorption and emission relative to those of the reference
system 4.11 This trend apparently has its origin in the decrease
of the HOMOꢀLUMO gaps by an effective expansion of the
π-conjugation, as can be deduced from their chemical structures.
The emission efficiency of tris(N-salicylideneaniline)s also
depends on peripheral π-extension (Table 4). As compared to
the benchmark molecule 4 (Qf = 5.3%), the fluorescence quan-
tum yields of ethynylene-extended 8 (Qf = 13%) and 9 (Qf =
19%) show ca. 2-fold and 4-fold enhancement, respectively.
Installation of additional ethynylphenyl branches as in 10, how-
ever, decreased the emission efficiency to Qf = 12%. A similar
trend was observed along the series 4 f 6 f 7, with an initial
enhancement (Qf = 16% for 6) followed by a decrease (Qf = 12%
for 7) with increasing conjugation. According to eq 7, an increase
in Qf results either from a longer lifetime or from a larger radiative
decay rate kr. We thus measured fluorescence decay profiles to
track the origins of such behavior.
The temperature dependence of the fluorescence lifetime
(Figure 10) and the contribution of different decay components
to the overall emission quantum yields (Figure 11) suggest that
nonradiative decay pathways are available for the excited state
associated with τ2. The contributions of τ2 across the series 1ꢀ3
also concur with the steady-state fluorescence efficiency, which
follows the trend of 2 > 1 > 3. The molecular basis of this non-
radiative decay was previously postulated to be structural un-
folding through loss of hydrogen bonds (Scheme 1) and thermal
relaxation of the excited state through internal torsional motions.
An intriguing observation from Figure 11 is an essentially con-
stant (ca. 3%) contribution of the τ1 component to the emission
quantum yields of 1ꢀ3, which implicates that this de-excitation
pathway is not influenced by hydrogen bonding or steric con-
straints associated with peripheral aryl groups. It is therefore
most likely related to processes occurring at the {C6O3(CHNH)3}
core, such as a cis-keto* to twist-keto* isomerization of the
N-salicylideneaniline fragments.36 The detailed molecular mechan-
ism responsible for this phenomenon is yet to be elucidated.
Temperature-dependent studies described in this section
confirmed that τ2 is the main relaxation process, which is also
responsible for the structure-dependent changes in the fluores-
cence quantum yields observed for 1ꢀ3. The dependence of
τ2 on hydrogen bonding and steric constraints further suggests
that the emission efficiency of tris(N-salicylideneaniline) fluor-
ophores should be enhanced by suppressing bond twisting about
the CarylꢀNenamine bonds. One synthetic approach to achieve
this goal is installing π-conjugated chemical functionalites at the
para-positions of the aryl groups so that the excited state could
readily acquire quinoid character with higher rotational barrier of
the CarylꢀNenamine linkages.37,38 The following section describes
structureꢀproperty relationships of such π-extended tris(N-sali-
cylideneaniline)s investigated by time-resolved spectroscopic
techniques.
To suppress potential aggregation in solution, time-resolved
experiments were performed on samples at low concentrations.
With the exception of 10, the emission from π-extended tris-
(N-salicylideneaniline)s could be fitted with two decay components
(Table 4), which are of the same order as τ1 and τ2 in 1ꢀ3. In
general, the lifetime tends to increase with extension of the
peripheral π-conjugation, with 0.90ꢀ1.06 and 0.66ꢀ0.96 ns for
τ2 in the vinylene- and ethynylene-extended series, respectively,
as compared to 0.36 ns for the reference molecule 4. The
amplitudes also show an almost 2-fold increase in the contribu-
tion of τ2 to the fluorescence spectrum, with 84ꢀ87% in 5ꢀ9
relative to 44% in 4. These findings suggest that the increase in
the emission quantum yields might be due to an enhanced con-
formational stability of the excited states of π-extended mole-
cules, which leads to longer lifetime.
Photophysical Consequences of Extending π-Conjuga-
tion. The TCSPC studies on 1ꢀ3 have established that mol-
ecules sharing an essentially identical tris(N-salicylideneaniline)
core have similar kr values but different knr parameters (Table 2),
which collectively result in markedly different fluorescence emis-
sion efficiencies. To test the general applicability of this model,
We subsequently determined radiative and nonradiative decay
rate constants of 4ꢀ10 and compared them to those of 1ꢀ3. As
anticipated from their essentially superimposable π-conjugation,
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Figure 12. Structure-dependent changes in the kr (ꢀ) and knr (- - -).
4 has a kr value similar to that of 1 and only a slightly higher
knr value. The π-extended molecules 5ꢀ10 show comparable
radiative decay rate constants of kr = 0.172ꢀ0.332 nsꢀ1, which
decrease slightly with extension of the π-conjugation as shown in
Figure 12. On the other hand, a large decrease in the nonradiative
decay constant knr was observed with increasing π-conjugation
(Figure 12).
Because the torsional motions of aryl rings surrounding the
tris(N-salicylideneamine) {C6O3(CHNH)3} core constitute the
main nonradiative decay pathway (Scheme 1), a decrease in knr
with elongated π-conjugation presumably reflects suppressed
CarylꢀNenamine bond twisting motions resulting from an in-
creased contribution of the quinoid character.37,38,40 Therefore,
a net enhancement in fluorescence quantum yield observed in
π-extended tris(N-salicylideneaniline)s is not so much from an
increase in kr as from a decrease in knr. We note, however, that
the knr value essentially levels off at certain points (see changes in
knr upon moving from 6 to 7 in Figure 12a; also from 9 to 10 in
Figure 12b), and further structural extension beyond these points
has an adverse effect on the emission efficiency because kr drops
slightly while knr remains essentially constant.
resulted in red-shifts in absorption and emission spectra, along
with enhanced emission quantum yields. Fluorescence decay
measurements on these π-extended tris(N-salicylideneaniline)s
5ꢀ10 revealed two main decay lifetimes as in simpler systems
1ꢀ4. Extension of the π-conjugation apparently results in more
stable excited states due to a decrease in the nonradiative decay
rate, while the radiative decay rate remains essentially constant
across the series.
Prevailing paradigms in π-conjugated organic molecules focus
primarily on strategies to rigidify the structural backbone to pro-
mote intimate electronic communication between neighboring
units. Challenging this “static” view, we have devised chemically
intuitive and operationally simple means to trigger bond twisting
motions between neighboring π-fragments, which translate di-
rectly to changes in emission properties of tris(N-salicylideneaniline)s.
Findings described in this work have significantly enhanced our
fundamental understanding of the de-excitation pathways of these
conformationally dynamic fluorophores and laid a solid ground-
workfor their rational structuralevolution for potential applications
in light-harvesting, chemical sensing, and molecular switching.
’ ASSOCIATED CONTENT
’ SUMMARY AND OUTLOOK
S
Supporting Information. Comparison of decay analysis,
b
We have investigated structureꢀproperty relationships of
a series of dynamic fluorophores that are built around a
C3-symmetric tris(N-salicylideneaniline) core. In solution, these
molecules undergo reversible switching motions that intercon-
vert the emissive folded and the nonemissive unfolded confor-
mer. To address the role of hydrogen bonds and steric con-
straints in this process, a combination of time-resolved and
steady-state studies was applied on model systems 1ꢀ3. Our
concentration- and temperature-dependent TCSPC studies have
established (i) good correlation between nonradiative decay rate
and fluorescence quantum yield, and (ii) contribution of three
distinct decay components τ1, τ2, and τ3 in the de-excitation
process. While the structure-independent and temperature-in-
sensitive τ1 component has a constant contribution to the total
quantum yield, the main relaxation process τ2 changes dramati-
cally as a function of temperature and depends strongly on the
presence of hydrogen bonding and steric interactions of periph-
eral aryl groups involved in conformational switching. In addi-
tion, the concentration-dependent τ3 component implicated
interchromophore interactions at higher concentrations, which
was independently confirmed by 1H NMR studies.
lifetime fitting results for decay-associated spectra, lifetime fitting
results for concentration dependence, lifetime fitting results for
temperature dependence, and lifetime fitting results for tempera-
ture-dependent lifetime-associated contributions to the quantum
yields of 1ꢀ3. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: dongwhan@indiana.edu (D.L.); dragnea@indiana.edu
(B.D.).
’ ACKNOWLEDGMENT
This work was supported by the National Science Foundation
(CAREER CHE 0547251) and the U.S. Army Research Office
(W911NF-07-1-0533). We further acknowledge support from
the National Institutes of Health (grant GM081029) and the
Indiana METACyt Initiative of Indiana University, funded in part
through a major grant from the Lilly Endowment, Inc. M.V. is
grateful for a College of Arts and Sciences Dissertation Year
Research Fellowship from Indiana University.
Installation of either vinylene- or ethynylene-bridged π-
conjugations onto the 3-fold symmetric core of these molecules
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