Synthesis and photophysical properties of a highly fluorescent azo derivative

Article abstract of DOI:10.1039/b902018g

A highly fluorescent and photochromic azo derivative has been obtained by linking in a covalent manner a red-emitting squaraine unit to an azo dye. The bifunctional molecule exhibits uncoupled chromophores in the ground state allowing for the specific add

Full text of DOI:10.1039/b902018g

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis and photophysical properties of a highly fluorescent
azo derivative
´
Li-Hong Liu, Keitaro Nakatani and Elena Ishow*
Received (in Montpellier, France) 30th January 2009, Accepted 27th March 2009
First published as an Advance Article on the web 16th April 2009
DOI: 10.1039/b902018g
A highly fluorescent and photochromic azo derivative has been obtained by linking in a covalent
manner a red-emitting squaraine unit to an azo dye. The bifunctional molecule exhibits uncoupled
chromophores in the ground state allowing for the specific addressing of the azo
photoisomerization and squaraine emission. Photosensitization of the squaraine unit by exciting
the azo moiety in its UV absorption band has been obtained. Such an unusual energy transfer has
been modulated by changing the solvent from chloroform to toluene. This led to a dramatic
enhancement of the emission quantum yield Ff (from 0.35 up to 0.71) without affecting the
photoisomerization process. Strong competitive p-stacking interactions between toluene and
squaraine have been put forward to interpret the emission changes.
very efficient emission quenching of neighboring fluorophores.10
Introduction
Biologists have benefited from this effect to follow the
dynamics of molecular beacons or the pairing of two DNA
complementary strands containing an azo and emissive units
by monitoring the emission changes as a function of the
azo–fluorophore distance.11 Recently several groups reported
the formation of fluorescent azo derivatives in solution12 or in
the solid state upon aggregation.13 Nevertheless fluorescence
appeared at the expense of the photoisomerization process
which was totally suppressed.
The concomitant association of photochromic and fluorescent
entities has been boosted in the very recent years by the
promising development of high density optical memories,1
photoswitchable light emitting organic diodes,2 and probe
photoactivation for live-cell imaging.3 The working principle
of these photonic devices relies on the modulation of the
fluorophore emission intensity by the photochromic reaction
which advantageously occurs in a reversible and contact-free
manner in less than a few picoseconds.4 In his pioneering
work, Irie et al. devised a bifunctional system made of a
diarylethene unit linked through an alkyl spacer to a fluor-
escent moiety.5 This latter was carefully chosen so that its
radiative excited state could deactivate the closed form
To circumvent the antinomic properties of fluorescence and
photoisomerization in an azo-containing molecule, we have
earlier proposed an approach involving the introduction of a
fluorophore emitting at low energy, far from the azo absorp-
tion band.14 We came up with a squaraine unit known for
its remarkable emission properties characterized by a high
emission quantum yield (Ff 40.7), a sharp and narrow
emission spectrum peaking in the red region (lem 4640 nm)
and a large photostability, hence its multifold use for applica-
tions spanning from photonic materials to ion detection.15 Its
attachment to the Disperse-Red 1, a push–pull azo compound,
was accomplished by using an aminoethyl linker whose
saturated character ensured partial electronic disconnection
between the two functional entities. A short spacer was
preferred over longer alkyl chains to restrain deleterious
folding which is likely to lead to intramolecular p–p stacking
of the azo and squaraine backbones and form a radiationless
species.16 The bifunctional compounds we synthesized
following these guidelines did fluoresce and photoisomerize.
They suffer however from an undesirable photoinduced
electron transfer (PET) in the excited state from the nitrogen
atom of DR1, hence the incorporated squaraine unit under-
went a large decrease in Ff down to 0.1.
following a Forster energy transfer mechanism the efficiency
¨
of which was governed by the spectral overlap between the
fluorophore emission band and the absorption band of the
closed photoisomer. Since then, numerous studies have been
performed on diarylethene,6 spirooxazine7 and spiropyrane8
derivatives aiming at improving the quenching mechanism
efficiency and reducing the risks of retrocyclization upon light
reabsorption by the closed forms. Curiously little attention has
been paid to the combination of fluorophores with azo
derivatives. These latter photochromes actually exhibit fasci-
nating properties as potential photoactivable nanomotors due
to their photoorientation and photomigration abilities when
subjected to polarized light.9 Attaching a fluorescent tail to an
azo unit would open new horizons on the reversible nano-
structuration of fluorescent patterns and optical manipulation
of fluorescent biolabels at the molecular level.
Such an association is however facing a picosecond photo-
isomerization process of the azo unit which mostly leads to the
We want to show herein that replacing the nitrogen atom
with oxygen allowed us to notably improve the emission
properties of the bifunctional molecule while the azo unit
keeps photoisomerizing in solution. The incorporation of the
less electron-donating oxygen atom led not only to a decrease
PPSM-UMR CNRS 8531, ENS Cachan, PRES-UniverSud Paris,
´
61 Avenue du President Wilson, Cachan Cedex, 94 325, France.
E-mail: elena.ishow@ens-cachan.fr; Fax: 33-1-4740-2454;
Tel: 33-1-1-4740-7660
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in the PET phenomenon but also to the blue shift of the azo
absorption maximum, which limits the risks of energy transfer
from the squaraine to the azo unit. Influence of the solvent
onto the emission properties has been evidenced, leading to a
considerable fluorescence enhancement through competitive
p–p interactions with the squaraine unit without affecting the
azo photoisomerization process.
(d, J = 8.8 Hz, 2 H), 6.71 (t, J = 7.3 Hz, 1 H), 4.23 (t, J =
6.2 Hz, 2 H), 3.78 (t, J = 6.2 Hz, 2 H), 3.50 (q, J = 7.0 Hz 2 H),
1.22 (t, J = 7.0 Hz, 3 H) ppm; 13C NMR (75 MHz, CDCl3):
d 162.3, 155.9, 148.2, 147.4, 146.9, 129.4, 125.5, 124.6, 123.0,
116.2, 114.8, 111.8, 65.9, 49.5, 45.7, 12.2 ppm; HRMS
(MALDI-TOF): m/z (%) [M + H]+ calc. for C22H23N4O3:
391.1772, found: 391.1770 (100); lmax(chloroform, E-isomer)/nm
375 (log e 4.45).
Experimental
4-[2-(N0-{40-[4-(Dibutylaminophenyl)squaraine]phenyl}-N0-ethyl-
amino)ethyloxy]-40-nitroazobenzene 5 (AzoO-bsq)
Materials and synthesis
1H and 13C NMR spectra were recorded on a Bruker 300 MHz
spectrometer, and chemical shifts d were reported in ppm
relative to TMS and referenced to the residual solvent. Mass
Compound 3 (0.19 g, 0.5 mmol) was stirred overnight
with 1-(p-dibutylaminophenyl)-2-hydroxycyclobuten-3,4-dione
4 (0.18 g, 0.6 mmol) in the presence of tributylorthoformate
(2 mL) in refluxing propan-2-ol (10 mL) under argon. After
cooling down and storage at 0 1C for 12 h, the precipitate was
filtered off and washed with methanol to yield a dark blue
solid. Purification on column chromatography (silica gel,
toluene–ethyl acetate 2 : 8) provided compound 5 as a fine
dark blue powder (98 mg, 29%); mp 184.5 1C; 1H NMR
(300 MHz, CDCl3): d 8.39 (d, J = 8.8 Hz, 2 H), 8.38 (d, J =
9.2 Hz, 2 H), 8.36 (d, J = 9.2 Hz, 2 H), 7.98 (d, J = 8.8 Hz,
2 H), 7.96 (d, J = 9.2 Hz, 2 H), 7.02 (d, J = 9.2 Hz, 2 H), 6.84
(d, J = 9.2 Hz, 2 H), 6.73 (d, J = 9.5 Hz, 2 H), 4.31 (t, J =
5.3 Hz, 2 H), 3.94 (t, J = 5.2 Hz, 2 H), 3.67 (q, J = 7.0 Hz, 2 H),
3.44 (t, J = 7.7 Hz, 4 H), 1.65–1.60 (m, 4 H), 1.44–1.01
(m, 4H), 1.32 (t, J = 7.0 Hz, 3 H), 0.99 (d, J = 7.4 Hz, 6 H)
ppm; 13C NMR (75 MHz, CDCl3): d 187.3, 183.2, 161.7,
155.9, 154.0, 152.6, 148.3, 147.2, 133.7, 132.8, 125.6, 124.6,
123.1, 120.3, 119.5, 114.8, 112.4, 112.2, 65.7, 51.2, 49.6, 46.4,
29.5, 20.1, 13.7, 12.3 ppm; HRMS (MALDI-TOF): m/z (%)
[M + H]+ calc for C40H44N5O5: 674.3342, found: 674.3322
(100); lmax(chloroform, E-isomer)/nm 375 (log e 4.47), 636 (5.49).
spectra were obtained with
a MALDI-TOF Perseptive
Biosystems instrument. Bis[4-(dibutylamino)phenyl]squaraine
bsq,17 4-hydroxy-40-nitroazobenzene 118 and 1-(p-dibutyl-
aminophenyl)-2-hydroxycyclobuten-3,4-dione 419 were synthe-
sized according to literature procedures.
N-(2-Tosyloxyethyl)-N-ethylaniline 2
A mixture of N-ethyl-N-hydroxyethylaniline (0.83 g, 5 mmol)
and freshly distilled triethylamine (1.0 mL, 7.5 mmol)
dissolved in anhydrous dichloromethane (50 mL) was cooled
to 0–5 1C under argon. Tosylate chloride (1.40 g, 7.5 mmol)
was added portionwise over 30 min. The mixture was kept
stirring under argon at 0–5 1C for 6 h and poured into an
ice–water mixture (50 mL). The crude product was extracted
with dichloromethane. The combined organic layers were
washed with brine and dried over anhydrous magnesium
sulfate. Removal of the solvent under vacuum and purification
by silica gel column chromatography using a 2 : 1 mixture of
petroleum ether–ethyl acetate as the eluent gave 2 as a color-
less oil (0.71 g, yield 44%). 1H NMR (300 MHz, CDCl3):
d 7.76 (d, J = 8.3 Hz, 2 H), 7.29 (d, J = 8.7 Hz, 2 H), 7.18
(dd, J = 7.3 Hz, 7.8 Hz, 2 H), 6.68 (dd, J = 6.9 Hz, 7.3 Hz,
1 H), 6.56 (d, J = 8.0 Hz, 2 H), 4.15 (t, J = 6.5 Hz, 2 H), 3.58
(t, J = 6.5 Hz, 2 H), 3.32 (q, J = 7.0 Hz, 2 H), 2.43 (s, 3 H),
1.11 (t, J = 6.9 Hz, 3 H) ppm; 13C NMR (75 MHz, CDCl3):
d 146.9, 145.0, 132.7, 129.9, 129.4, 127.9, 116.5, 111.8, 67.0,
48.9, 45.5, 21.7, 12.2 ppm; HRMS (MALDI-TOF): m/z (%)
[M + H+] calc. for C17H22NO3S 320.1320, found 320.1309
(100); Elemental analysis, Found: C, 63.91; H, 6.89; N, 4.42.
Calc. for C17H22NO3S: C, 63.92; H, 6.63; N, 4.39%.
Measurements
UV-Vis spectra were recorded on a Varian Model Cary 5 E
spectrophotometer. Melting points were measured by using a
Perkin Elmer Pyris Diamond Differential Scanning Calori-
meter. Corrected emission spectra were obtained using a Spex
Fluorolog 1681 spectrofluorometer. Fluorescence quantum
yields were determined from solution of bis[4-(dibutylamino)-
phenyl]squaraine absorbing equally at the excitation wave-
length. Fluorescence intensity decays were measured by the
time-correlated single-photon counting method with a pico-
second laser excitation at 495 nm provided by a Spectra-
Physics setup composed of a titanium-sapphire Tsunami Laser
pumped by an argon ion laser, and doubling LBO crystals.
Light pulses were selected by an optoacoustic crystal at a
repetition rate of 4 MHz. Fluorescence photons were detected
at 630 nm through a monochromator by a Hamamatsu MCP
photomultiplier R3809U connected to a constant-fraction
discriminator. Pulse deconvolution was performed from the
time profile of the exciting pulse recorded under the same
conditions by using a Ludox solution. Photoisomerization
kinetics was recorded on magnetically stirred solutions using
a continuous Hg–Xe lamp (Hamamatsu, LC8-06) equipped
with an optical fiber and a 366 nm interferential filter
(5.9 mW cmꢁ2) as a pump source while simultaneous probing
4-[2-(N-Ethyl-N-phenylamino)ethyloxy]-40-nitroazobenzene 3
A mixture of compound 2 (0.7 g, 2.2. mmol), 4-hydroxy-40-
nitroazobenzene 1 (0.49 g, 2 mmol) and potassium carbonate
(0.42 g, 3 mmol) in anhydrous dimethylformamide (4 mL) was
stirred under argon at 100 1C for 24 h. After cooling to room
temperature, the reaction mixture was poured under stirring
into a 1 mol Lꢁ1 hydrochloric acid solution (20 mL) and the
solid was filtered off. The residue was washed with water to
neutrality, and recrystallized from ethyl acetate–acetonitrile to
give compound 3 as shiny orange–red flakes (0.66 g, 77%); mp
1
113 1C; H NMR (300 MHz, CDCl3): d 8.36 (d, J = 8.8 Hz,
2 H), 7.98 (d, J = 9.2 Hz, 2 H), 7.95 (d, J = 9.2 Hz, 2 H), 7.25
(dd, J = 7.3, 8.8 Hz, 2 H), 7.02 (d, J = 9 Hz, 2 H), 6.75
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was performed with a continuous Xe lamp (450 W) and a CCD
camera coupled with a spectrometer (Princeton Instruments).
Computations
TD-DFT calculations were conducted using the Becke’s three-
parameter hybrid functional and the correlation functional of
Lee, Yang and Parr (B3LYP)20 with a 6-31G(d) basis set21 as
implemented in the GAUSSIAN 03 package.22 Illustrations
were obtained with GaussView 3.0.
Results and discussion
Fig. 1 UV-vis absorption spectra of the bifunctional compound
azoO-bsq and model compounds bsq and azoO in chloroform solution.
Synthetic strategy and absorption properties
The synthesis of the bifunctional compound 5 (azoO-bsq)
proceeded after a three-step sequence allowing us to also
generate the fluorescent bsq and photochromic azoO model
compounds (Scheme 1).
visible at 633 nm. These bands have been ascribed to
transitions located on the azo and the squaraine units
respectively from comparison with the absorption spectra of
the model compounds (Fig. 1).
Model compounds are mandatory to value the spectroscopic
properties of the photoactive units once they are covalently
connected in azoO-bsq. 4-Hydroxy-40-nitroazobenzene 1 was
first synthesized through the azo coupling reaction between
phenol and 4-nitroaniline in hydrochloric acid solution. The
azo compound 1 was reacted further with the previously
tosylated N-ethyl-N-(2-hydroxyethyl)phenylamine 2 following
a nucleophilic substitution in anhydrous dimethylformamide in
the presence of potassium carbonate to give a red compound 3
in 77% yield. Finally, the azo compound 3, serving also as the
model photochrome azoO, was coupled to the dibutylsquarate
4, prepared after a three-step procedure, in refluxing propan-2-
ol with tributylorthoformate. The bifunctional azoO-bsq was
obtained as a dark blue powder in 29% yield. Following the
same reaction conditions, the fluorescent model compound
bis[4-(dibutylamino)phenyl]squaraine bsq was generated from
dibutylsquarate and N,N-dibutylphenylamine and obtained as
a shiny gold–green powder.
Since the squaraine band is ca. ten times as high as the azo
one absorbing in the UV, the solution appears bright blue
(Fig. 2).
These maxima correspond almost exactly in energy to those
of the separate photochrome azoO and fluorophore bsq
models absorbing at 373 and 640 nm, respectively. Moreover
the sum in a ratio 1 : 1 of the molar absorption coefficients of
the model compounds nearly corresponds to the values
established for azoO-bsq. We can thus conclude that the azo
and squaraine units do not significantly interact in the ground
state. Replacing a dialkylamino moiety with an alkoxy one has
led to a 100 nm hypsochromic shift of the azo band due to a
strong decrease in the push–pull strength. This allowed us to
photoaddress each unit independently and investigate the
emission and photochromic properties of the whole.
The emission properties were first investigated in dilute
chloroform solution such that the absorbance of the squaraine
unit was always kept lower than 0.1 to minimize the risks
of emission reabsorption and intermolecular aggregation.
Excitation of azoO-bsq in the squaraine-located band at
590 nm led to a strong and narrow red emission centered at
648 nm (Fig. 3). This fluorescence was found to be slightly
blue-shifted with regard to bsq emitting at 653 nm.
UV-vis absorption and emission properties
The UV-vis absorption spectrum of azoO-bsq in chloroform
exhibits two bands peaking in the UV at 373 nm and in the
When the excitation was performed in the azo UV band of
azoO-bsq around 375 nm, the same red emission was observed.
Its magnitude was twice as large as that obtained for a solution
of bsq of equal concentration, and excited at the same wave-
length. Two isoabsorbing wavelengths in the azo and
squaraine bands, namely at 377 and 590 nm were considered.
Scheme 1 Synthetic pathway to azoO-bsq. Reagents and conditions:
(i) tosylate chloride, triethylamine, CH2Cl2, 0 1C, 6 h; (ii) 4-hydroxy-
40-nitroazobenzene 1, dimethylformamide, K2CO3, 100 1C, 24 h;
(iii) tributylorthoformate, dibutylsquarate 4, propan-2-ol, 83 1C, 12 h.
Fig. 2 AzoO, azoO-bsq and bsq in chloroform solution: left: absorption,
right: emission; lexc = 547 nm (Hg–Xe lamp).
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Fig. 3 Emission spectra of bsq and azoO-bsq in chloroform solution.
Excitation was performed at 590 nm (bold line) and 377 nm (dashed
line).
After subtracting the UV-excited squaraine contribution, the
squaraine photosensitization by the azo moiety was assessed
to represent 25% of the intensity obtained upon excitation at
590 nm. The azo participation in the emission process was
confirmed by recording an excitation spectrum at 660 nm.
Two bands, characteristic of the azo and squaraine absorption
ranges, were obtained. The intensity of the azo band was
significantly reduced to 60% of the azo absorption band in
accord with the partial photosensitization effect reported
above (Fig. 4).
Fig. 5 DFT-computed molecular orbitals of azoO-bsq in the gas
phase with the correlation functional B3LYP and a 6-31G(d) basis set.
insight into the two energy transfer mechanisms which respec-
tively depend on the molecular orbital overlap (Dexter) and
The fluorescence quantum yield ff was valued to 0.35 after
excitation at 590 nm, which is significantly larger than that of
the amino analogue azoN-bsq (ff = 0.1). The emission
enhancement in azoO-bsq was reasonably attributed to the
reduced electron-donating character of the ethoxy group,
which is less prone to photoinduced electron transfer than
the amino moiety in azoN-bsq.
the transition dipole moments (Forster) attached to the azo
¨
and squaraine units. The computations provided two distinct
p–p* transitions specifically located on the azo (HOMOꢁ2 -
LUMO+1) and squaraine parts (HOMO - LUMO) (Fig. 5).
These transitions matched fairly well both the azo- and
squaraine-centered bands of the experimental absorption
spectrum. The involved frontier MOs present a lateral
p-overlap, which is a pre-requisite for a Dexter energy transfer
to occur between the azo and squaraine units. As for the
When chloroform was replaced with toluene, a dramatic
enhancement of ff up to 0.71 was observed for azoO-bsq
(up to 0.29 for azoN-bsq). In contrast, the emission quantum
yield of bsq only increased from 0.92 to 1.00. This enhance-
Forster energy transfer, the orientation of the transition dipole
¨
ment was accompanied by
a notable decrease of the
moments related to the azo and squaraine moieties appears to
be quite favorable. The calculations showed that their vectors
point along the main axis of the corresponding chromophore,
and form a 161 angle in the minimized elongated geometry.
However, the donor emission and acceptor absorption spectra
should display substantial spectral overlap, not found in our
systems. By comparison with bsq, the squaraine absorption in
azoO-bsq at 375 nm has been inferred to be six times as low
squaraine’s photosensitization effect by exciting the azo unit,
down to 16% of the signal obtained after direct excitation in
the squaraine-based transition. At this step, both a Dexter
and/or Forster energy transfer processes could be envisaged to
¨
explain the observed photosensitization effect. This latter is all
the more surprising since the azo photoisomerization reaction
occurs in the picosecond time range and can efficiently
deactivate close emitters.
as the azo one. If a Forster energy transfer operates, the
¨
Therefore we conducted TD-DFT calculations on azoO-bsq
in the gas phase (B3LYP/6-31G(d)) to gain a qualitative
oscillator strength of the squaraine acceptor should be large
enough to compensate the weak spectral overlap between
the donor (azo) and the acceptor (squaraine) moieties as
well as the subpicosecond radiative decay of the azo S2 and
S1 excited states.23 We indeed found by computations an
oscillator strength f of about 1.47 for the squaraine-based
HOMO - LUMO transition in agreement with the values
reported in literature. Finally, the considerable changes caused
by toluene onto the emission properties of azoO-bsq could be
interpreted as a strong solvation effect of the p-conjugated
backbone, producing remote units and ‘‘electronic insulation’’
of the squaraine. Consequently, the emission of the squaraine
increases while its photosensitization by exciting the azo unit
diminishes. These observations allow us to suggest that the
energy transfer from the azo to the squaraine units may rather
Fig. 4 Normalized excitation (lem = 660 nm) and absorption spectra
of azoO-bsq in chloroform solution.
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Table 1 UV-vis spectroscopic data (absorption, emission) of com-
pounds azoO, azoO-bsq, azoN-bsq and bsq, rate constants of the back
thermal reaction Z–E and minimum conversion yields for the photo-
chromic azo compounds in toluene solution
and azo units, this ratio fell to 2.5 in acetonitrile. Simulta-
neously the half-bandwidth of the squaraine-located transition
broadened from 27 nm (30 nm) to 34 nm when going from
toluene (chloroform) to acetonitrile. These spectral observa-
tions evidence the fact that toluene and chloroform limit the
risks of squaraine aggregation at high concentration by
exerting disruptive p–p interactions. Moreover, toluene
displays moderate polarity, known to considerably influence
the back thermal reaction of the photogenerated Z-isomers,
and has largely been used for other azo derivatives, which
facilitates kinetics comparisons.
lmaxabs/nm lmaxem/nm ffa tfb/ns 10ꢁ4k/sꢁ1 Zf (%)
azoO
azoO-bsq 375
632
azoN-bsq 463
635
bsq
375
—
644
—
—
3.2d
0.71 0.26c 4.1d
1.96
0.29 0.70c 115e
67
14
—
29
—
—
—
649
647
1.24
1.00 2.31
—
—
636
a
b
lexc = 590 nm with bsq as a standard.
Major component of the biexponential lifetime decay: 99.5%
l
exc = 495 nm, lem = 630 nm.
c
Photoisomerization was carried out with a continuous
Hg–Xe lamp source equipped with a 366 nm bandpass filter.
Absorption changes were probed in situ by using a low-power
white-light source. Solutions of azoO-bsq and azoO with
identical absorbance at the azo-centered band maximum were
prepared to permit straightforward comparison between both
compounds and discard artifacts due to distinct light absorp-
tion or concentration effects. The depletion of the E-isomer
absorption band was neatly observed at 375 nm while a new
weak band, characterizing the formation of the Z-isomer,
appeared at 482 and 463 nm for azoO-bsq and azoO, respec-
tively (Fig. 6). Two isosbestic points were observed, the first
one being formed around 330 nm (327 and 331 nm for
azoO-bsq and azoO, respectively) and the second one in the
visible at 425 nm for azoO-bsq and 444 nm for azoO. This
indicates that the azo units in the E- and Z-isomers behave in a
similar way for both the bifunctional and the azo compounds,
hence the azoO-bsq species exists in a more probable
monomeric form.
d
(azoO-bsq), 89% (azoN-bsq). Measured in isoabsorbing toluene
e
solutions at 366 nm and kinetics recorded at 375 nm. Kinetics recorded
f
at 460 nm after excitation at 488 nm. Assuming no absorption from the
Z photoisomers.
follow a dipole–dipole coupling mechanism. However, the
flexibility of the ethyl linker allows both units to freely rotate
in solution, leading to a decrease in the dipole–dipole
coupling. In the same way, the spatial overlap can severely
drop and so reduce the efficiency of the through-bond
electron-exchange mechanism. Therefore both the Dexter
and the Forster mechanisms may operate, each depending
¨
on the experimental conditions involved.24
Finally, measurements of the lifetime were conducted at
l
exc = 495 nm to avoid excitation in the azo band and risks of
photoisomerization. Emission was recorded at lexc = 630 nm
by using a monochromator to avoid detection of solvent-
squaraine excited states emitting beyond 660 nm.25 In chloro-
form and toluene solutions, the fluorescence decay could be
modeled by a biexponential lifetime. The major component
was found at t1 = 0.68 ns (93%) in chloroform, and evolved
to t1 = 1.96 ns (99.5%) in toluene solution, close to the 2.31 ns
monoexponential lifetime measured for bsq (Table 1). The
absence of fluorescent impurities was carefully checked at
this stage. Furthermore, the synthetic pathway adopted to
generate the asymmetric squaraine rules out the accumulation
of other squaraine byproducts. The longer decay observed in
toluene agrees with the enhanced emission quantum yield
reported above. A reasonable explanation for the presence
of two lifetimes for azoO-bsq and their solvent-dependency
may rely on the flexible ethyl spacer. The molecule can adopt a
variety of conformations in solution enabling remote positions
of the squaraine unit from the azo one (hence the long lifetime
especially in toluene) as well as close vicinity of the azo and
squaraine units to form a short-lived excited p–p adduct.
The back-thermal relaxation kinetics for both compounds
was determined at the depletion minimum at 375 nm once the
photostationary state was reached. The differential absor-
bance time-evolution could be modeled by a monoexponential
law in accord with the first-order Z - E thermal reaction in a
fluid and isotropic medium, [E] = [E]0 + {[E]P ꢁ [E]0}exp(ꢁkt)
where k designates the reaction rate constant and [E]0 and [E]P
the concentrations of the E-isomer before irradiation and at
the photostationary state, respectively (Fig. 7). AzoO-bsq
was found to relax slightly faster than azoO with k around
4.1 ꢂ 10ꢁ4 against 3.2 ꢂ 10ꢁ4 sꢁ1 respectively. This process is
almost two orders of magnitude slower than that for azoN-bsq
as expected from the less polar character of the NQN bond in
azoO-bsq.27 The similarity between the rate constants proves
Photochromic properties
Measurements of the azo photochromic behavior require
solutions of concentration above 1 ꢂ 10ꢁ5 mol Lꢁ1 to ensure
adequate detection sensitivity. This concentration range is
likely to give rise to squaraine dimerization characterized by
strong association constants around 105 molꢁ1 L.26 Choice of
the solvent was therefore crucial to provide an accurate picture
of the photochromic properties of the ‘‘isolated’’ molecule.
Whereas toluene and chloroform solutions gave the same
absorption ratio of 10 between the maxima of the squaraine
Fig. 6 Absorption changes of 1 ꢂ 10ꢁ5 mol Lꢁ1 solutions of azoO-bsq
and azoO in toluene after irradiation at 366 nm (5.9 mW cmꢁ2).
Recorded at time for A: azoO-bsq: (a) 0 s, (b) 5 s, (c) 30 s, (d) 60 s,
(e) 120 s, and for B: azoO: (a) 0 s, (b) 5 s, (c) 20 s, (d) 40 s, (e) 60 s.
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moiety and move away from it. Temperature-dependent
experiments and transient absorption spectroscopy measure-
ments to characterize the energy transfer dynamics are cur-
rently under study. They will be coupled with single molecule
fluorescence investigations to track the azo unit’s motion
during the photoisomerization process.
References
Fig. 7 Kinetics of the back thermal relaxation of 1 ꢂ 10ꢁ5 mol Lꢁ1
solutions of azoO-bsq and azoO in toluene after irradiation at 366 nm
(5.9 mW cmꢁ2). Absorption changes followed at 373 nm.
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1408 | New J. Chem., 2009, 33, 1402–1408