Dynamics of the higher lying excited states of cyanine dyes. An ultrafast fluorescence study

Article abstract of DOI:10.1021/jp400278t

The electronic relaxation dynamics of the second singlet excited states of several cyanine dyes was studied through the femtosecond fluorescence up-conversion technique. Our interest in these molecules comes from the potential applications of systems with upper excited singlet states with a long lifetime, which can include electron and energy transfer from the higher lying singlets after one- or two-photon absorption. We studied three series of cyanines with 4-quinolyl, 2-quinolyl, or benzothiazolyl type end groups, each with varying sp² carbon conjugation lengths in the methinic bridge. The dynamics after electronic excitation to singlet states above the fluorescent state vary significantly as a function of cyanine structure and conjugation length. In particular, for the 4-quinolyl series the cyanine with an intermediate conjugation length (three methinic carbons) has the slowest S ₂ decays with lifetimes of 5.4 ps in ethanol and 6.6 ps in ethylene glycol. On the other hand, we observed that the 2-quinolyl family has S ₂ decay times in the subpicosecond range independent of the conjugation length between the end groups. The slowest internal conversion was observed for the benzothiazolyl type cyanine with five methinic carbons, with an S₂ lifetime of 17.3 ps in ethanol. For the planar cyanines of this study we observed for the first time a clear systematic trend in the S ₂ decay times which closely follow the energy gap law. It was also demonstrated that a slow S₂ decay is as well observed upon excitation through degenerate two-photon absorption with near-IR pulses. The present study isolates the most important variables for the design of cyanines with long S₂ lifetimes.

Full text of DOI:10.1021/jp400278t

Article
pubs.acs.org/JPCB
Dynamics of the Higher Lying Excited States of Cyanine Dyes. An
Ultrafast Fluorescence Study
Cesar A. Guarin, Juan. P. Villabona-Monsalve, Rafael Lopez-Arteaga, and Jorge Peon*
́
Instituto de Química, Universidad Nacional Auton
́ ́ ́
oma de Mexico, Circuito Exterior, Ciudad Universitaria, Mexico, 04510, D.F.,
Mexico
́
*
S Supporting Information
ABSTRACT: The electronic relaxation dynamics of the
second singlet excited states of several cyanine dyes was
studied through the femtosecond fluorescence up-conversion
technique. Our interest in these molecules comes from the
potential applications of systems with upper excited singlet
states with a long lifetime, which can include electron and
energy transfer from the higher lying singlets after one- or two-
photon absorption. We studied three series of cyanines with 4-
quinolyl, 2-quinolyl, or benzothiazolyl type end groups, each
2
with varying sp carbon conjugation lengths in the methinic
bridge. The dynamics after electronic excitation to singlet states above the fluorescent state vary significantly as a function of
cyanine structure and conjugation length. In particular, for the 4-quinolyl series the cyanine with an intermediate conjugation
length (three methinic carbons) has the slowest S decays with lifetimes of 5.4 ps in ethanol and 6.6 ps in ethylene glycol. On the
2
other hand, we observed that the 2-quinolyl family has S decay times in the subpicosecond range independent of the conjugation
2
length between the end groups. The slowest internal conversion was observed for the benzothiazolyl type cyanine with five
methinic carbons, with an S lifetime of 17.3 ps in ethanol. For the planar cyanines of this study we observed for the first time a
2
clear systematic trend in the S decay times which closely follow the energy gap law. It was also demonstrated that a slow S
2
2
decay is as well observed upon excitation through degenerate two-photon absorption with near-IR pulses. The present study
isolates the most important variables for the design of cyanines with long S lifetimes.
2
INTRODUCTION
however, to this date no direct and accurate measurement of
the higher lying singlet lifetimes or a systematic study about the
factors that govern the upper state dynamics in different
cyanines has been made.
In this contribution we report on direct measurements of the
lifetimes of the second excited singlet states of three cyanine
families each with varying conjugation lengths. We studied
these series in order to determine the structural features that
■
Cyanines are polymethinic dyes with a positive charge
delocalized through a chain of sp carbons with amino end
2
groups. Because of their extended conjugation, these systems
have found multiple applications including optical storage and
1
2−4
processing, imaging of biological samples,
light-energy9
5
,6
7
8
conversion, nonlinear optics, optical limiting, sensitization,
etc. The electronic structure of cyanines is determined by an
odd number of conjugated p orbitals and an even number of π
contribute to a slow internal conversion from S . We focused
on the photophysics of cyanines with the following end groups:
z
2
10,11
electrons associated with them.
The resulting nodal
structure of the π and π* orbitals in these systems makes the
first singlet excited state be significantly stabilized in
4
-quinolyl, 2-quinolyl, and benzothiazolyl. The structures of
these cyanines are summarized in Scheme 1 together with the
abbreviations used herein and in previous studies.
comparison with the S state. Spectroscopically, the electronic
2
structure of these dyes produces an intense S −S transition in
0
1
Molecules with a long-lived upper excited state could have
significant applications since the energy available through the
population of the upper singlets can be significantly larger than
that of the S state (around 3 eV for the S state in certain
the visible or near-IR region and much less intense S −S (n ≥
0
n
11
2) transitions in the near-UV region.
Since the S −S energy gap in some cyanines can be as large
2
1
1
2
as 1 eV, the coupling between these states is significantly
12
cyanines against 1.5−2 eV for the S state). This has been
1
reduced in comparison with typical chromophores. Besides
the energy relations, it has been pointed out that due to their
size, a kind of structural inertia in these molecules redounds in
slow structural relaxation which may imply that the energy
proven to redound in interesting electron transfer schemes in
1
3,17−20
different molecular systems.
Also, the long S lifetimes
2
13
surface of the upper states is explored slowly after excitation.
The slow S −S internal conversion observed in certain cyanine
Received: January 9, 2013
Revised: May 14, 2013
2
1
1
4−16
dyes
can be explained by the above-mentioned features;
©
XXXX American Chemical Society
A
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Scheme 1. Molecular Structure and Abbreviations for the
Cyanines of This Study (Abbreviations Used in Previous
Publications Are Indicated in Parentheses)
conversion. Our results also show for the first time clear
differences about the upper state relaxation times of the
different cyanine series and the dependence on their conjugated
skeleton size. As we show, the S lifetimes can vary by more
2
than 2 orders of magnitude depending on the system size and
the kind of end groups. In order to determine the feasibility of
producing a long-lived upper state through two-photon
excitation, we also include experiments that form the upper
singlets by degenerate two photon excitation in the near-IR
(
800 nm), demonstrating that the long-lived S states may also
2
be formed by nonlinear excitation.
EXPERIMENTAL SECTION
■
Materials. The cyanines in Scheme 1 were purchased from
Aldrich and were used after extensive recrystallization from
methanol and the solvent mixture dichloromethane−ether,
until large high-purity crystal samples were obtained. After the
purification stage, the samples were kept in dark conditions
before the laser studies. HPLC quality solvents from Aldrich
were used.
Synthesis of 2-(2-(2-Chloro-3-[2-(3-ethyl-3H-benzo-
thiazol-2-ylidene)ethylidene]cyclohex-1-enyl)(vinyl)-3-
ethylbenzothiazol-3-ium Iodide (Cl-TC7). This dye (Cl-
TC7) was synthesized from a solution of 1.6 g of 3-ethyl-2-
methylbenzothiazolium iodide, 0.75 g of N-[(3-(anilino-
methylene)-2-chloro-1-cyclohexen-1-yl)methylene]aniline
monohydrochloride, and 0.47 g of sodium acetate in 100 mL of
absolute ethanol. The mixture was heated under reflux for 20
min. After cooling overnight at −5 °C, the resulting crystals
a
b
c
d
e
Reference 45. Reference 16. Reference 43. Reference 55. Reference
f
4
1. Reference 37.
29
in certain cyanines are related to applications like optical
limiting.
were washed with water, cold ethanol, and diethyl ether.
1
6,21,22
Green crystals were obtained after recrystallization from
methanol. Yield 70%; mp 269 °C; λmax (ethanol) = 798.5 nm
The NMR spectra was compared with the one reported by Li et
Cyanine dyes have an additional aspect that makes them
interesting in regards to their upper states: Because of the
symmetry relations between their ground, first excited, and
upper excited states, it has been shown extensively that these
compounds have considerably large two-photon absorption
cross sections for near-IR wavelengths (for example, for C2v
symmetry, S : A , S : B , S : A for a near-resonant two-photon
process with an intermediate state).
relations go in addition to an intense one-photon allowed first
transition with a sharp onset in the near-IR which augments
their nonlinear absorption for photons near the S edge.
Such ease to form the upper electronic states through two-
photon absorption, together with the relatively long lifetime of
these states, opens the possibility of interesting applications in
which the cyanines may transfer energy from their upper states
to other chromophores after two-photon excitation of the
cyanine. Such a concept of a two-photon photosensitizer or
antenna requires as a first step the kind of characterization of
the present studies to understand and predict which type of
30
al.
Optical Spectroscopy. The steady state absorption spectra
were acquired in a Cary-50 (Varian) spectrophotometer, and
for the emission spectra, a Cary Eclipse (Varian) fluorimeter
was used. The emission and excitation spectra were corrected
for the instrumental response. All experiments were performed
at room temperature (20 ± 1 °C) under aerated conditions.
For the emission measurements from the upper states, the
absorption at the excitation wavelength was kept below 0.05,
except for the 2QC series, 4QC1, and TC3 for which higher
concentrations were necessary in order to observe the emission
from the higher lying states. For these cyanines, the absorbance
at the excitation wavelength was of 0.15.
0
1
1
1
2
1
2
3−26
These symmetry
2
4,26−28
1
The femtosecond fluorescence up-conversion setup has been
31−34
described previously.
It is based on a regeneratively
amplified, 1 kHz Ti:sapphire laser centered at 800 nm,
producing a 0.7 W pulse train of 150 fs in duration. The
second or third harmonic was used for excitation and was
obtained by sum frequency mixing in 0.5 mm β-BBO crystals.
The samples were studied in 1 mm flow cells, and the
fluorescence was collected with a pair of parabolic mirrors and
refocused to the up-conversion β-BBO crystal where it was
crossed with ∼1 mW of the 800 nm fundamental beam split
cyanines may have a long-lived S state.
The experiments presented herein determine the dynamics
of the upper states by exciting the compounds in the UV region
2
(
266 or 400 nm), which correspond to direct excitations to the
second singlet S or higher lying singlets S (n > 2). We
2
n
followed the upper state dynamics by time-resolving the
transient spontaneous emission from the second singlet excited
states using the femtosecond fluorescence up-conversion
technique. Additionally, we monitored the population growth
of the first singlet excited state (forming from internal
conversion) by resolving the emission from the S fluorescent
state. These experiments were able to observe for the first time
in a direct way the one to one kinetics of the S −S internal
previously. The sum frequency signal was collected with a CaF
2
lens and focused into a double 10 cm monochromator (Oriel)
and detected with a photomultiplier tube. The excitation beam
was modulated at 1/3 of the laser repetition rate with a phase-
locked chopper, so that the up-conversion signal could be
detected with a lock-in amplifier (Standford Research Systems).
The polarization of the excitation pulses was adjusted with wave
1
2
1
B
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The Journal of Physical Chemistry B
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plates for magic angle conditions with respect to the up-
conversion crystal (type I sum frequency process, detecting the
vertical component of the fluorescence intensity). The
instrument response functions (IRF) for the up-conversion
experiments were determined to be Gaussian with a full width
at half-maximum of 450 fs for 400 nm excitation and 500 fs for
31−33
266 nm excitation.
From the inspection of exponential
functions convoluted with these instrumental response
functions and considering the signal-to-noise ratio of our
experiments, the shortest lifetimes that can be measured with
this setup have time constants of ∼40% of the respective IRF.
Some of the emission decay times in this contribution are
reported as being below this instrumental limit. For all up-
conversion experiments, solvent-only traces were taken back-to-
back and with identical alignment to the solution measure-
ments to ensure the absence of signals near t = 0 due to
scattering from the solvent.
Figure 3. Absorption (main graph) and emission (inset) spectra of
cyanines TC3 (black), TC5 (blue), and TC7 (red) in ethanol solution.
with the transitions at higher energies. This redounds changes
in the S −S gap as the polymethinic chain increases. The
2
1
RESULTS AND DISCUSSION
Steady State Spectroscopy. Figures 1−3 show the steady
state absorption and emission spectra of the molecules of
■
quantification and dynamical effects of this observation are
presented in a latter paragraph.
Figure 4 shows emission spectra in the visible range, in the
region between the first and second absorption transitions for
Figure 1. Absorption (main graph) and emission (inset) spectra of
cyanines 2QC1 (black), 2QC3 (blue), and 2QC5 (red) in ethanol
solution.
Figure 4. Corrected emission spectra from the S2 state (red),
absorption (blue), and corrected excitation spectra (black, insets) for
4
QC3 (A, λexc = 400 nm, λdet = 460 nm) and 4QC5 (B, λexc = 450 nm,
λdet = 540 nm).
4
QC3 and 4QC5. As can be seen, a clear emission is detected
with peaks at 460 nm for 4QC3 and 523 nm for 4QC5. The
respective emission spectra for 4QC1, TC3, and the 2QC series
are included in the Supporting Information. From the position
of the emission bands in Figure 4 and the excitation spectra
Figure 2. Absorption (main graph) and emission (inset) spectra of
cyanines 4QC1 (black), 4QC3 (blue), and 4QC5 (red) ethanol
solution.
(
see below), such fluorescence signals can be readily assigned to
the radiative decay of the S states of the respective
2
1
4,16
Scheme 1 in ethanol solution. For the three cyanine series, it
can be seen that the first and higher transitions undergo
systematic shifts when the conjugation length is changed.
However, the S −S absorptions undergo somewhat different
cyanines.
Similarly, for the benzothiazolyl type cyanines
(TC series), the fluorescence spectra in the 400−600 nm
region show clear emission peaks with maxima at 429 nm for
TC5 and 494 nm for TC7 as shown in Figure 5. As in the case
of the 2QC series, for the shortest cyanine TC3, weak S2
0
1
shifts as the polymethinic length is increased in comparison
C
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The Journal of Physical Chemistry B
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Figure 5. Corrected emission spectra from the S2 state (red),
absorption (blue), and corrected excitation spectra (black, insets) for
TC5 (A, λexc = 330 nm, λdet = 450 nm) and TC7 (B, λexc = 365 nm, λdet
=
490 nm).
emission is observed only with somewhat more concentrated
solutions with an absorbance of 0.15 at the excitation
wavelength (see Supporting Information). The quantum yields
for the S emissions of all cyanines were determined to be Φ
2
fluor
−3
<
10 from comparisons with the emissions of coumarin 153
and coumarin 343.
The excitation spectra with detection near the S emission
Figure 6. Femtosecond fluorescence up-conversion results with
detection of the S emission for 2QC1 (A, λ = 266 nm), 2QC3
2
2
exc
maxima for 4QC3, 4QC5, TC5, and TC7 are shown in the
respective insets of Figures 4 and 5. As can be seen, the
absorption features for the UV regions of the four molecules
are reproduced, indicating that the emission does not come
from an impurity. From the normalization of the excitation
spectra to the absorption spectra at the respective longest
wavelength vibroelectronic peaks (near the blue edge of the
emissions, see insets), it is clear that the quantum yield for S2
emission is nearly constant as the excitation wavelength was
scanned through the band systems in the spectral regions
depicted in the insets. This indicates that, considering that the
multiple vibroelectronic absorption peaks on the blue side of
(
B, λexc = 400 nm), and 2QC5 (C, λexc = 400 nm). Insets: time-
resolved fluorescence from the S1 state (blue). The detection
wavelengths (λfluo) are indicated. The gray lines are the solvent-only
scans.
in the steady state spectra. For this series, independent of the
polymethinic chain length, the full decay of this state takes
place within 1 ps, and the traces were described as a single-
exponential decay with a time constant of τ < 0.2 ps convoluted
with our IRF (τ < 0.3 ps for 2QC3). The insets of Figure 6
show the time-resolved emission from the “fluorescent” first
singlet excited states of the 2QC cyanines. Consistent with the
ultrafast S decays, the S emissions show an instrument limited
the S emission correspond to a series of different electronic
2
states S (n ≥ 2), the quantum yield for the formation of the
n
transient emissive S state is above ∼90%, implying a regular
2
2
1
highly efficient S −S internal conversion. This is further
rise (τrise < 0.3 ps), followed by a single- or double-exponential
decay. The S decays in these and other cyanines have been
extensive studied by several groups.
n
2
studied through femtosecond experiments with different
excitation wavelengths shown below.
1
35−45
The relatively short
Time-Resolved Fluorescence. Femtosecond fluorescence
up-conversion traces for the nine cyanines of Scheme 1 are
included in Figures 6−8, and the results of their analysis are
summarized in Table 1. The main graphs in these figures
correspond to the time-resolution of the spontaneous emission
fluorescent state lifetimes in these systems is due to rapid
fluctuations in the polymethinic chain leading to efficient
isomerizations and/or internal conversion to the electronic
ground state with the intermediacy of S −S conical
1
0
3
5,36,38,42,43,46
intersections.
Since the focus of this study are
from the S states. Figure 6 shows the data for the 2QC series.
the S dynamics, the S time-resolved emission traces were
2
2
1
For the 2QC3 and 2QC5 cyanines, an excitation wavelength of
acquired to verify the consistency of the kinetics of the
400 nm was used, while 266 nm excitation was necessary to
population of the S fluorescent state as the S emission is
1
2
excite the upper singlets in 2QC1. As can be seen, using the up-
conversion technique, ultrafast decaying transients in the S2
emission region were well resolved despite the weak emissions
depleted. In all cases, the S lifetimes are consistent with the
1
35,36,38,42,43,46
respective previous studies,
while the S decays are
2
reported herein for the first time with femtosecond resolution.
D
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Figure 7. Femtosecond fluorescence up-conversion results with
Figure 8. Femtosecond fluorescence up-conversion results with
detection of the S emission for TC3 (A, λ = 266 nm), TC5 (B,
detection of the S emission for 4QC1 (A, λ = 266 nm), 4QC3
2
exc
2
exc
(
B, λexc = 400 nm), and 4QC5 (C, λexc = 400 nm). Insets: time-
λexc = 266 nm), and TC7 (C, λexc = 400 nm). Insets: time-resolved
fluorescence from the S state (blue) and from the S state for TC7
resolved fluorescence from the S state (blue) and from the S state for
1
2
1
2
4
QC3 (B) and 4QC5 (C) with 266 nm excitation (red). The
(C) with 266 nm excitation (red). The detection wavelengths (λfluo
)
detection wavelengths (λfluo) are indicated. The gray lines are the
solvent-only scans.
are indicated. The gray lines are the solvent-only scans.
The up-conversion results for the 4QC series are included in
Figure 7. As can be seen, for this series there are important
differences in comparison with the 2QC series, and there are
also important changes in the dynamics as the polymethinic
to the 400 nm excitation show that even when different upper
states are formed, the molecules nearly instantaneously form
the second excited state where the population is retained for
several picoseconds, giving rise to spontaneous emission signals
from this state before it undergoes further internal conversion.
chain length is increased: the S emission trace of the shortest
2
cyanine, 4QC1, is similar to the respective results of the 2QC
series. That is, there is an ultrafast decay of the emission signal
It should be noted that the amplitude of τ changes from 0.2 to
1
in the visible region with an S lifetime of τ < 0.4 ps (detected
0.46 when changing the excitation wavelength from 400 to 266
nm. We consider that this is an indication that the first time
constant is related to the early evolution of the system in the S2
potential energy surface and that when the excitation energy is
increased by 1.56 eV, this results in the formation of a
population of molecules formed from internal conversion from
2
at 420 nm, between the first and second absorption bands).
Correspondingly, we observed an instrument limited rise of the
S1 fluorescence (660 nm), followed by its decay with a time
constant of τ = 1.8 ps.
For the 4QC3 cyanine, the S fluorescence signal shows a
2
much slower decay (see Figure 7B). This signal was followed at
the higher lying states (S , n > 2). Apparently, this results in a
larger fraction of the systems evolving through a faster channel
n
480 nm and shows a biexponential decay with time constants of
τ = 1.0 ps and τ = 5.4 ps. Such slow internal conversion and
most likely related to the relaxation within the S state (∼1 ps).
1
2
2
the steady state detection of the S emission are clear
Overall, the biexponential nature of the emission decays in this
zone is probably due to spectral modulations associated with
the electronic and vibrational relaxation, and possibly, also from
the fact that a fraction of the population may relax faster into
2
12
exceptions to Kasha’s rule. The slow (picoseconds) decay
of the S state was also observed when the system was excited
2
to an upper state S with 266 nm light instead of 400 nm light.
n
This is shown in the red trace in the inset of Figure 7B. In this
case, we observed a double-exponential decay with time
constants of τ = 0.9 and τ = 5.3 ps. The similar time scales
the S statepossibly a sign of structural inhomogeneity in the
1
cyanine population. In any case, these experiments show that
for 4QC3 there is a transient S population following the
1
2
2
E
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Table 1. Parameters for the Decay of the S and S States for the Cyanines of Scheme 1 in Ethanol Solutions
2
1
cyanine
state detected
λfluo [nm]
τ1 [ps]
a1
τ2 [ps]
a2
τ3 [ps]
a3
a
2
2
2
2
2
2
4
4
4
4
4
4
4
4
QC1
QC1
QC3
QC3
QC5
QC5
QC1
QC1
QC3
QC3
QC3
QC5
QC5
QC5
S2
385
570
520
625
500
745
420
660
480
480
745
530
525
880
430
620
455
715
500
500
<0.2
0.6 ± 0.02
<0.3
1
a
S1
0.96
1
5.1 ± 0.3
11.9 ± 0.2
14.3 ± 0.6
0.04
1
b
S2
a
S1
b
S2
<0.2
1
a
S1
1
a
S2
<0.4
<0.4
1
a
S1
−0.42
0.20
1.8 ± 0.1
5.4 ± 0.3
5.3 ± 0.5
7.5 ± 0.9
2.9 ± 0.1
3.1 ± 0.1
10.0 ± 1.0
1.5 ± 0.07
2.2 ± 0.04
17.3 ± 0.1
17.2 ± 3.0
4.3 ± 0.1
4.6 ± 0.2
3.4 ± 0.2
0.58
0.80
0.54
0.26
0.16
0.47
0.29
1
b
S2
1.0 ± 0.05
a
S2
0.9 ± 0.3
2.8 ± 0.2
0.5 ± 0.04
0.46
a
S1
−0.50
0.84
57 ± 1
0.24
b
S2
a
S2
<0.4
0.53
a
S1
3.0 ± 0.3
−0.50
106 ± 8
155 ± 3
562 ± 15
0.21
0.51
0.50
a
TC3
TC3
TC5
TC5
TC7
TC7
TC7
S2
a
S1
−0.49
0.51
−0.24
0.47
0.78
−0.38
a
S2
1.9 ± 0.06
2.0 ± 0.4
<0.3
0.8 ± 0.1
<0.2
0.49
−0.26
0.53
a
S1
b
S2
a
S2
0.22
a
S1
870
−0.15
845 ± 18
0.47
a
b
Excitation wavelength (λ ): 266 nm. λ = 400 nm.
exc
exc
formation of the upper states with a lifetime of several
picoseconds.
4QC series, the fastest dynamics are seen for the shortest
cyanine TC3 which has a fast 1.5 ps single-exponential S decay
2
The time-resolved S emission of 4QC3 at 745 nm is also
and a 2.2 ps growth of the S state as shown in the inset of
1
1
included in the inset of Figure 7B (blue), where a clear slow rise
is observed consistent with a slow accumulation of the S1
population. The first excited singlet fluorescence rise and decay
Figure 8A. The TC cyanine of intermediate size: TC5 actually
shows by far the slowest S decay of all the cyanines, with time
2
constants of τ = 1.9 ps and τ = 17.3 ps. The corresponding
1
2
were described with multiexponential terms with τ = 2.8 ps
slow S accumulation was also verified and is included in the
1
1
(
rise), τ = 7.5 ps (decay), and τ = 57 ps (decay, lifetime). We
inset of Figure 8B. The 17.3 ps we observed for the second
component is consistent with a previous report on the lifetime
2
3
notice that the 2.8 ps growth is not identical to the 5.4 ps decay
observed for the S emission. This could be due to the fact that
of the S state of this cyanine, determined through transient
absorption with picoseconds resolution. Similarly to the 4QC
2
2
16
the S state also undergoes rapid vibrational relaxation, or it
1
may correspond to a kind of average between the 1 and 5.4 ps
series, the longest cyanine (here TC7) shows S decays in an
2
components observed for the S emission. In any case, some
intermediate time scale in comparison with TC3 (fastest) and
2
spectral evolution is expected as the S state is populated by
TC5 (slowest). This is summarized in Table 1.
1
internal conversion. Such spectral evolution can also be related
to the relaxation of the solvent shells around the electron
density distribution of the fluorescent state (ethanol, with
solvent relaxation components in the range from subpico-
seconds to more than 10 ps). All these relaxation events are
likely to produce modulations in the intensity of the S1
emission at the detection wavelength. It is clear however that
The overall tendencies of the S states in the three cyanine
2
series can be easily understood according to the following
considerations: the structures of two shortest quinolyl cyanines
2QC1 and 4QC1 have been studied by X-ray crystalography.
These compounds have been shown to be highly distorted from
planarity due to sterical hindrance between the aromatic end
4
4,47−50
groups.
It has been established previously that such
the accumulation of the S population is not instantaneous
distortion from planarity makes these systems have fast and
1
given its formation due to internal conversion from a somewhat
efficient photoisomerization channels upon electronic excita-
3
7−39,42,43,51−54
long-lived S state.
tion.
More specifically, it has been shown that
2
The 4QC5 molecule shows the same general of behavior as
the nonplanar geometries of 2QC1 and 4QC1 allow a fast and
predirected evolution in the potential energy surfaces toward
conical intersections which drives the loss of population of the
S states. It is easy to translate this behavior to the S dynamics
4
QC3; however, the overall time for the S −S conversion is
2
1
slightly faster, having time constants of τ = 0.5 ps and τ = 2.9
1
2
ps for the second excited singlet decay. For 4QC5 the S signal
1
1
2
accumulation is again consistent with a slow population growth
as shown in the inset of Figure 7C. The excitation with 266 nm
light is again included in the inset with similar observations as
in the 4QC3 molecule. In summary, for the 4QC series, the
slowest S −S conversion was observed for the medium size
observed herein for 2QC1 and 4QC1: their nonplanar
geometries are associated with rapid evolution in the S2
surface, moving to geometries with a large S −S coupling
2
1
and thereby defining short S lifetimes.
2
The rest of the cyanines of this study are planar according to
2
1
2
4,35,44,47,49,55−57
cyanine 4QC3 with a somewhat faster process for 4QC5 and
ultrafast decay for the shortest cyanine 4QC1.
previous studies.
The planar cyanines, 2QC3−
5, 4QC3−5, and the TC series actually show a clear systematic
trend in relation to their S lifetimes as elaborated next: the
The up-conversion results for the TC family are shown in
Figure 8. Again, in congruence with the observations for the
2
energy gap between the first and second singlet excited states
F
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The Journal of Physical Chemistry B
Article
can be estimated for most of these compounds using the steady
state emission observed from the S state. The S −S gap was
2
2
1
thus determined as the difference between the energy of the S2
emission and that of the regular S fluorescence (notice that the
1
absorption spectra are significantly crowded with vibroelec-
tronic peaks in this region, making it uncertain to establish the
S2 energy from these spectra). The ordering of the planar
cyanines of our study with respect to their energy gap (S −S )
2
1
is the following in order of decreasing gap: TC5, 4QC3, TC7,
Cl-TC7, 4QC5, TC3, 2QC3, and 2QC5 (see below).
The log of the S decay rates are plotted as a function of the
2
S −S energy gaps for these cyanines in Figure 9. As can be
2
1
Figure 10. Time-resolved fluorescence results for 4QC3 in different
solvents (ethylene glycol (EtGly) and ethanol (EtOH). The detection
wavelength (λfluo) is indicated. The gray lines are the solvent-only
signal detected at this λfluo
.
Table 2. Parameters for the Decay of the S State for Diverse
2
Cyanines in Ethylene Glycol Solutions
cyanine
λfluo [nm]
τ1 [ps]
a1
τ2 [ps]
a2
a
TC5
435
500
480
2.1 ± 0.1
0.52
0.52
0.84
1
17.5 ± 0.9
5.0 ± 0.3
6.6 ± 0.3
0.48
0.48
0.16
b
TC7
<0.3
1.6 ± 0.07
<0.2
b
4
QC3
QC3
b
2
500
a
b
Figure 9. Experimental S −S internal conversion rate constants
λexc = 266 nm. λ = 400 nm.
2
1
exc
(
logarithmic scale) as a function of the S −S energy gap for the
2
1
cyanines of this study. The continuous line shows the linear trend for
the data with a correlation coefficient of r = 0.95.
1
.0 and 5.4 ps in ethanol to 1.6 and 6.6 ps in ethylene glycol,
while for TC5, they go from 1.9 and 17.3 ps in ethanol to 2.1
and 17.5 ps in ethylene glycol. Results for TC7 and 2QC3 in
ethylene glycol are also included in Table 2.
seen, a nice linear trend is observed with a correlation
coefficient of r = 0.95, consistent with the second excited
singlet having lifetimes governed by the energy gap law with
The overall change in the S decays (considering the longer
2
component in the decay) is between 1% and 22%, showing
some effect of the viscosity in the S lifetimes. This effect is thus
related to a reduction of the rate of geometry changes in the S
surface after excitation.
has an effect on the S lifetimes, from the clear verification of
the energy gap law in Figure 9, it is clear that the S −S energy
gaps are much clearer predictors of the S lifetimes.
5
8,59
2
respect to the S state.
1
2
1
4,26
⎛
kIC = C exp⎜−γ
⎝
⎞
ΔE2−1
hωm
While the solvent viscosity certainly
⎟
⎠
2
2
1
where C is a pre-exponential factor that measures the electronic
2
coupling matrix element, ΔE
is the energy difference
Since the objective of our study was to determine which kind
2
−1
between the S and the S states, hω is the energy of the
of cyanines may have long S lifetimes, for electron and energy
2
1
m
2
vibrational mode in the acceptor state, and γ is related to the
degree of displacement of the potential energy surfaces between
the two states. From the large similarity of the cyanine’s bridges
and previously studied polyenic molecules, the relevant
acceptor mode can be postulated to be the CC stretching
transfer from S applications, we have measured the S decays
2
2
of the heptamethine cyanine chlorinated at the meso position
shown in Figure 11 (Cl-TC7). This compound is interesting
from two points of view. First, it contains a labile chloride in the
meso position, from which the cyanine can be easily
functionalized to include electron and/or energy acceptors in
future molecular designs for one- and two-photon sensitiza-
5
9
mode. It should be noticed that the trend shown in Figure 9
involves different types of cyanines with different conjugation
sizes and different end groups. To our knowledge, this is the
first time the energy gap law is recognized for the S −S
1
3,17,60
tion.
In the second place, Cl-TC7 contains a rigidized
middle section thanks to the six-membered cycle that joins
carbons 3′ and 5′, limiting their relative mobility. The steady
state and fluorescence up-conversion results for this compound
are included in Figure 11. The time constants for the
biexponential decay were determined to be <0.4 and 3.1 ps.
These times are comparable and even slightly faster than those
of the analogous compound TC7 which does not possess the
middle cyclic system. Given the similarities between the
dynamics of these two compounds, it can be concluded that
2
1
internal conversion in cyanines. Such observation should serve
as the primary variable in the design of cyanines with long S2
lifetimes, which, as mentioned previously, are of interest in
1
3,17,19,60,61
energy or electron transfer applications.
We have additionally tested how an increase of the viscosity
of the solvent media may affect the S lifetimes. In order to
2
keep a similar hydrogen bonding environment to the ethanol
solvent (viscosity: 1.2 mPa·s), we made up-conversion
measurements in ethylene glycol at room temperature
the S decays are not strongly influenced by the geometry
2
(
1
viscosity: 16 mPa·s). The results are summarized in Figure
restrictions imparted at the central region in the Cl-TC7
0 and Table 2. As can be seen, in the more viscous ethylene
cyanine. On the other hand, the S lifetime in this compound
2
glycol solutions, the S decays show clear but not major
differences in comparison with the ethanol solutions. In
particular, for the 4QC3 cyanine, the time constants go from
follows the aforementioned energy gap law in Figure 9 and
remains in the picoseconds regime; a time scale large enough
for energy or electron transfer events to donor molecules which
2
G
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The Journal of Physical Chemistry B
Article
Table 3. Parameters for the Decay of the S and S States of Cl-TC7 in Ethanol Solutions
2
1
state detected
λfluo [nm]
τ1[ps]
a1
τ2 [ps]
a2
τ3 [ps]
a3
b
S2
500
530
870
<0.4
<0.3
2.7 ± 0.02
0.74
0.58
3.1 ± 0.2
3.1 ± 0.4
6.0 ± 0.2
0.26
0.42
0.28
a
S2
b
S1
−0.50
720 ± 20
0.22
a
b
λexc = 266 nm. λ = 400 nm.
exc
Table 4. Parameters for the Fluorescence Decay of the S2
State upon Two-Photon Excitation at 800 nm for Three
Cyanines in Ethanol Solutions
cyanine
λfluo [nm]
τ1 [ps]
<0.3
a1
τ2 [ps]
a2
2
4
QC3
QC3
460
480
435
1
1.0 ± 0.07
0.6 ± 0.06
0.85
0.71
5.1 ± 0.9
15.6 ± 2
0.15
0.29
TC5
Figure 12. Time-resolved fluorescence S emission from two-photon
2
excitation at 800 nm for 2QC3, 4QC3, and TC5. The detection
wavelengths (λfluo) are indicated. The gray lines are the solvent-only
scans.
to the method in the Supporting Information. The resulting
values for 800 nm excitation are 321 GM for TC5 and 187 GM
for 4QC3, confirming that these compounds have significant
two-photon cross sections at the excitation wavelength.
The up-conversion experiments in Figure 12 are among the
first ones to time-resolve emission with femtosecond resolution
after two-photon excitation and the first ones to detect upper
26
state emissions after two-photon excitation. Because of the
difficulty of these experiments and the position of the S1
absorptions, here we only studied the two cyanines with the
slowest S decays plus 2QC3 as a representative member of the
2
2
2
Q series. As can be seen, the S dynamics from compounds
QC3, 4QC3, and TC5 in these experiments are similar to
2
those obtained with direct single-photon excitation with 400 or
Figure 11. (A) Absorption (blue), corrected emission (red, λexc = 375
266 nm light. The time constants for the 4QC3 system are 1.0
nm), and excitation (black, inset, λdet = 530 nm) spectra of the S state
and 5.1 ps. For 2QC3 the S lifetime remained below 0.3 ps,
2
2
of Cl-TC7 (structure shown). (B) Detection of the S state emission
and for TC5 the time constants are 0.6 and 15.6 ps. The
experiments of Figure 12 demonstrate the feasibility of forming
a relatively long-lived (picoseconds) upper excited state with
the absorption of two photons at 800 nm, potentially allowing
for electron or energy transfer events from S2 after the
nonlinear excitation.
2
with femtosecond fluorescence up-conversion (red, λexc = 400 nm).
Inset: time-resolved fluorescence from the S (blue) and from the S2
1
(
red) states (λexc = 266 nm). The detection wavelengths (λfluo) are
indicated. The gray lines are the solvent-only scans.
can be bonded at a very near distance as substituents at the
meso position.
CONCLUSIONS
■
Finally, we have performed several experiments where the
cyanines were excited by degenerate two-photon absorption
with 800 nm pulses. The results for 2QC5, 4QC3, and TC5 are
included in Figure 12. The excitation of these molecules with
two photons is an efficient process since, as has been
We present the first detailed characterization of the dynamics of
the upper excited states in cyanines with femtosecond
resolution. It was observed that the shortest cyanines 2QC1
and 4QC1 have ultrafast (<0.4 ps) S2 lifetimes, with a
correspondingly fast population of their S states. For these
short cyanines, the rapid dynamics are associated with the fact
1
2
3,25,26,62−67
established previously,
the excited states in the
respective energy region have large cross sections for this
nonlinear excitation. Such is granted by the symmetry relations
of the states, taking the first singlet as an intermediate out of
resonance transition with a sharp onset, ideal for the 800 nm
two-photon excitation (see refs 22−28 and 62−67 for detailed
descriptions of the nonlinear optical properties of cyanines and
related compounds). For cyanines 4QC3 and TC5, we have
measured the two-photon absorption cross sections according
that their ground state geometry is nonplanar due to steric
4
7,48,50
hindrance between the end groups.
This distortion
predisposes the systems to rapid decays in the upper states
given that the resulting wave packet evolves directly to regions
44
of high S −S coupling.
2
1
On the other hand, the rest of the cyanines of our study are
planar and, for the three series 2QC, 4QC, and TC, the energy
gap rule is strictly followed. From the S −S gaps, it results that
2
1
H
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The Journal of Physical Chemistry B
Article
for the 4QC and TC cyanines the medium size molecules
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(
4QC3 and TC5) have the longest S lifetimes within their
2
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one with the overall slowest S decay of all the compounds of
2
4
(
this study, with a lifetime component of 17.3 ps in ethanol.
The influence of the solvent viscosity was tested through
comparisons between ethanol and ethylene glycol solutions.
Although differences among these two solvents are observed,
the upper state lifetimes do not show dramatic changes upon
the increment of viscosity from 1.2 mPa·s in ethanol to 16
mPa·s in ethylene glycol. Though certainly there is an influence
of viscosity, the above-mentioned trend which follows the
7
(
Photovoltaic Properties of New Cyanine-Naphthalimide Dyads
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̈
esch, F.; Hany, R. On the Use of cyanine Dyes as Low-bandgap
energy gap rule implies that it is the relative energies of S and
2
S1 energies what most closely determines the upper state
2
(
dynamics. Additionally, we verified that the S states can have
2
lifetimes in the picoseconds region, independently of whether
Y.-L.; Lu, J.-M. Third-Order Nonlinear Optical Properties of
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near direct S excitation is made (400 nm), an upper singlet
2
state is formed (S with n > 2 formation with 266 nm light), or
n
degenerate two-photon excitation is made with 800 nm
femtosecond pulses.
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The present characterization isolates for the first time several
of the variables that determine the lifetime of the upper singlets
in symmetric cyanines. Transfer of energy and/or electrons
from such upper states has been considered of potential
relevance in photosensitization and photovoltaic applications.
Our experiments set the design variables for such systems,
pointing to the energy gap law as the most relevant factor
determining the upper state dynamics for the planar cyanines,
́
(9) Bazylinska, U.; Pietkiewicz, J.; Saczko, J.; Nattich-Rak, M.;
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2
internal conversion to the first singlet excited state.
S2 lifetimes of many picoseconds may allow for efficient
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(
13) Bouit, P.-A.; Spaenig, F.; Kuzmanich, G.; Krokos, E.; Oelsner,
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ASSOCIATED CONTENT
Supporting Information
S emission spectra for 2QC1, 2QC3, 2QC5, 4QC1, and TC3
in ethanol solutions, experimental data for the energy gap and
■
*
S
2
(
14) Kasatani, K.; Sato, H. Viscosity-Dependent Decay Dynamics of
the S State of Cyanine Dyes with 3, 5, and 7 Methine Units by
2
S −S internal conversion rate, experimental procedures for the
2
1
Picosecond Fluorescence Lifetime Measurements. Bull. Chem. Soc. Jpn.
1996, 69, 3455−3460.
determination of the two-photon absorption coefficients of
TC5 and 4QC3, up-conversion results at additional wave-
lengths, and full refs 23, 28, and 67. This material is available
free of charge via the Internet at http://pubs.acs.org.
̌ ́ ́
(15) Rehak, V.; Novak, A.; Titz, M. S2 → S0 Fluorescence of
Cryptocyanine Solutions. Chem. Phys. Lett. 1977, 52, 39−42.
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(
Limiting and Picosecond Relaxation of Carbocyanines Upper
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AUTHOR INFORMATION
Corresponding Author
E-mail jpeon@servidor.unam.mx (J.P.).
Notes
■
*
(
17) Kesti, T.; Tkachenko, N.; Yamada, H.; Imahori, H.; Fukuzumi,
S.; Lemmetyinen, H. C70vs. C60 in Zinc Porphyrin-Fullerene Dyads:
Prolonged Charge Separation and Ultrafast Energy Transfer from the
Second Excited Singlet State of Porphyrin. Photochem. Photobiol. Sci.
2003, 2, 251−258.
The authors declare no competing financial interest.
(18) Mataga, N.; Chosrowjan, H.; Shibata, Y.; Yoshida, N.; Osuka, A.;
Kikuzawa, T.; Okada, T. First Unequivocal Observation of the Whole
Bell-Shaped Energy Gap Law in Intramolecular Charge Separation
ACKNOWLEDGMENTS
For financial support we thank CONACyT Grant 178541 and
PAPIIT-UNAM Grant IN 204211.
■
from S Excited State of Directly Linked Porphyrin−Imide Dyads and
2
Its Solvent-Polarity Dependencies. J. Am. Chem. Soc. 2001, 123,
1
(
2422−12423.
19) LeGourrier
Sun, L.; Hammarstro
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