Red-edge-wavelength finely-tunable laser action from new BODIPY dyes

Article abstract of DOI:10.1039/b925561c

New BODIPY dyes with two 4-formylphenyl, 4-(2,2-dimethoxycarbonylvinyl) phenyl and 4-(2,2-dicyanovinyl)phenyl groups at the 3- and 5-positions have been successfully designed and synthesized via palladium-catalyzed coupling reaction or Knoevenagel-type co

Full text of DOI:10.1039/b925561c

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Red-edge-wavelength finely-tunable laser action from new BODIPY
dyesw
M. J. Ortiz,*a I. Garcia-Moreno,b A. R. Agarrabeitia,a G. Duran-Sampedro,a
b
c
d
d
´
˜
A. Costela, R. Sastre, F. Lopez Arbeloa, J. Banuelos Prieto and
d
´
I. Lopez Arbeloa
Received 4th December 2009, Accepted 24th March 2010
First published as an Advance Article on the web 25th May 2010
DOI: 10.1039/b925561c
New BODIPY dyes with two 4-formylphenyl, 4-(2,2-dimethoxycarbonylvinyl)phenyl and
4-(2,2-dicyanovinyl)phenyl groups at the 3- and 5-positions have been successfully designed
and synthesized via palladium-catalyzed coupling reaction or Knoevenagel-type condensations.
Structural modification of the BODIPY core via conjugation-extending residues significantly
affects the spectroscopy and photophysical properties of the BODIPY fluorophore. These
substituents cause the largest bathochromic shift in both absorption and emission spectra,
which are shifted toward the red compared to its 4-phenylsubstituted analogue. Additionally,
the fluorescence quantum yields and the Stokes shifts are also significantly higher than the
corresponding phenyl-substituted dye. New BODIPY dyes have a high laser photostability,
superior to that of commercial dyes with laser emission in the same spectral region, such as
Perylene Red and Rhodamine 640. The substitution introduced in these derivatives allows to
obtain tunable laser emission with a bandwidth of 0.15 cmÀ1 and a tuning range of up to 50 nm.
So with these three dyes it is possible to cover the spectral range 590–680 nm in a continuous
way and with stable laser emission and small linewidth.
The photophysical properties of these dyes can be modulated
1. Introduction
to some extent incorporating the adequate substitution in the
molecular structure of the parent BODIPY chromophore.
Thus, the design of long-wavelength BODIPY dyes has
especially attracted attention in the last years. Red-shifts of
the optical spectra can be realized by substitution with aryl,
vinyl, styryl, ethynylphenyl, fused aryl groups at the core or
inserting heteroatoms at different positions of the chromo-
phore (aza-BODIPY).1a,c,2
The development of new fluorescent BODIPY dyes has become
a booming area of research due to the potential applications of
these dyes, for their use as sensors in biology and in clinical
diagnosis, photosensitizers for photodynamic therapy, laser
generators, waveguides, manufacture of light emitting diodes
(OLED), photovoltaic cells and electroluminescent devices
including also the usual conventional applications of organic
dyes.1 All these and other emerging applications are conditioned
by the emission wavelength, quantum yield and stability of the
dyes under the working conditions required for each specific
application.
Recently, the synthesis of 3,5-dichloro-4,4-difluoro-8-(4-tolyl)-
4-bora-3a,4a-diaza-s-indacene (1) has been described.3 In the
present work, we have carried out the synthesis, photophysical
and lasing characterization of new 8-tolyl-substituted bora-
diazaindacenes (2–4) with absorption and emission spectra
shifted towards longer wavelengths by extending the conjuga-
tion of the BODIPY core at the 3,5-positions (see Fig. 1).
The photophysical properties of these compounds have
been compared with other 8-tolyl-substituted BODIPYs, and
in addition, the behaviour of commercial dyes with laser
emission in the same spectral region, such as Rhodamine
In particular, BODIPYs are one of the most used laser dyes
family as active media of tunable dye lasers in the green–
yellow visible region of the electromagnetic spectrum due to
their high lasing efficiency and photostability, high fluores-
cence quantum yield, low rate constant of intersystem crossing
and large molar absorption coefficient.1a,c
a Departamento de Quimica Organica I, Facultad de Ciencias
Quimicas, Universidad Complutense, 28040-Madrid, Spain.
E-mail: mjortiz@quim.ucm.es
b Instituto de Quimica-Fisica ‘‘Rocasolano’’ (IQFR), CSIC,
Serrano 119, 28006-Madrid, Spain
c Instituto de Ciencia y Tecnologia de Polimeros (ICTP), CSIC,
Juan de la Cierva 3, 28006-Madrid, Spain
d Departamento de Quimica Fisica, UPV-EHU, Apartado 644,
48080 Bilbao, Spain
w Electronic supplementary information (ESI) available: Synthesis,
characterization and electron density maps of dyes. See DOI: 10.1039/
b925561c
Fig. 1 Chemical structures of the BODIPY dyes 1–4.
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640 and Perylene Red was analyzed under the same pumping
conditions. The present results suggest that the inclusion of
para substituted phenyl groups at 3 and 5 positions of the
8-tolyl-BODIPY gives rise to an extended red emission, charac-
terized by higher fluorescence capacity and a larger Stokes shift
than other red-sensitive dyes. All these factors contribute to the
laser behaviour of these new red emitting BODIPY, which lase
with good efficiency and higher photostability than other
commercial dyes lasing in the same spectral region.
were obtained on a LECO CHNS-932 apparatus at the
Universidad Complutense de Madrid analysis services and
were within 0.4% of the theoretical values.
2.4 Photophysical properties
The photophysical properties were registered in 3 Â 10À6
M
solutions in different solvents, prepared by adding the corres-
ponding solvent to the residue from the adequate amount of a
ca. 10À3 M stock solution in acetone, after vacuum evapora-
tion of the solvent. UV-Vis absorption and fluorescence
spectra were recorded on a Cary 4E spectrophotometer and
on a SPEX Fluorolog 3–22 spectrofluorimeter, respectively.
Fluorescence quantum yields (f) were evaluated from corrected
spectra, using a ca. 10À6 M solution of the commercial PM650
dye in ethanol (f = 0.10)4 as reference. Radiative decay
curves were registered by the time correlated single-photon
counting technique (Edinburgh Instruments, model FL920).
Fluorescence emission was monitored at the maximum emission
wavelength after excitation at 470 nm by means of a diode
laser (PicoQuant, model LDH470) with 150 ps FWHM pulses.
The fluorescence lifetime (t) was obtained after the deconvolution
of the instrumental response signal from the recorded decay
curves by means of an iterative method. The goodness of the
exponential fit was controlled by statistical parameters
(chi-square, Durbin-Watson and the analysis of the residuals).
The rate constant of radiative (kfl) and non-radiative (knr)
deactivations were calculated by means of: kfl = f/t and
knr = (1-f)/t.
2. Experimental
2.1 Materials
Starting materials and reagents used in the preparation of
BODIPY dyes 1–4 are commercially available unless the
synthesis is described. The solvents were dried and distilled,
before use.
The commercial laser dye Rhodamine 640 (laser grade,
Exciton) was used as received with a purity >99% (checked
by spectroscopic and chromatographic methods). Perylene
Red (BASF Lumogen Red 305) was first used as received with
a purity of only 90% (checked by spectroscopic and chromato-
graphic methods). Then, the dye purity was improved up to
99% through flash column chromatography based on silica
with hexane–ethyl acetate 95/5 v/v proportion as eluent, to
remove the N-methyl-2-pyrrolidone, identified by 1H-NMR as
the principal impurity. Solvents for laser studies were of
spectroscopic grade (Merck, Aldrich or Sigma) and were used
without purification.
The ground state geometry was optimized using the B3LYP
method and the double valence basis set (6-31G) implemented
in the Gaussian 03 software. The energy minimization process
was performed without any geometrical restrictions, and was
considered to be adequately concluded when the analysis of the
vibrational frequencies did not give any negative frequency.
2.2 Synthesis of BODIPY 1-4
BODIPYs 2–4 were synthesized from 3,5-dichloro-4,4-di-
fluoro-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene (1).3 Compound
1 was synthesized by a method previously described.3 Their
spectroscopic data are in agreement with those previously
described (1H NMR spectra is shown in the ESI, page 5).w
2.5 Laser experiments
2.3 Characterization of the new dyes
Liquid solutions of dyes were contained in 1 cm optical-path
quartz cells that were carefully sealed to avoid solvent
evaporation during experiments. The liquid cells of the newly
synthesized red-edge dyes were transversely pumped at 532 nm,
with 5 mJ pulseÀ1, 6 ns FWHM pulses from a frequency-
doubled Q-switched Nd:YAG laser (Monocrom OPL-10) at a
repetition rate of up to 10 Hz, and at 568 nm, with 5 mJ, 10 ns
FWHM pulses from a Nd-YAG-pumped dye laser (Spectron
SL800 with an ethyl acetate solution of PM567). The exciting
pulses were line-focused onto the lateral flat surface of the
solid samples, providing pump fluences on the active medium
of 180 mJ cmÀ2. The oscillation cavity (2 cm length) consisted
of a 90% reflectivity aluminium mirror, with the end face of
the sample as output coupler.
Spectral data of the known compounds were in accordance
with the literature data. Flash column chromatography was
1
performed using silica gel Merck 60 (230–400 mesh). H and
13C NMR spectra were recorded using a Bruker Avance-DPX-
300 spectrometer (300 MHz for 1H and 75 MHz for 13C)
1
and a Bruker Avance-AV-500 spectrometer (500 MHz for H
and 125 MHz for 13C). All spectra were recorded in CDCl3.
1H chemical shifts are reported in ppm relative to tetramethyl-
silane (d = 0.00 ppm), using the residual solvent signal as
internal reference. 13C chemical shifts are reported in ppm with
CDCl3 (d = 77.67 ppm) as the internal standard. Chemical
shift multiplicities are reported as s = singlet, d = doublet,
t = triplet, q = quartet and m = multiplet. IR spectra
(in cmÀ1) were recorded in a Bruker Tensor-27-FTIR spectro-
photometer. Melting points were determined in open capillaries
and are uncorrected. Mass spectra were registered by electron
impact at 70 eV in a VGI2-250 spectrometer. High resolution
mass spectra were determined by electro spray ionization in
the positive mode (ESIw) in a Accurate-Mass Q-TOF LC/MS
6520 (Agilent Technologies). Combustion analyses (C, H, N)
For pumping at 532 nm, the concentrations of new
BODIPY 2, 3 and 4 were 9 Â 10À4 M, 8 Â 10À4 M and
3 Â 10À3 M, respectively, leading to solutions with an optical
density of ca. 16. For pumping at 568 nm, the concentrations
of 2, 3 and 4 dyes were 9 Â 10À4 M, 4 Â 10À4 M and 2 Â 10À3 M,
respectively, matching the optical density selected for irradia-
tion at 532 nm.
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The photostability of each dye was evaluated by irradiating
under lasing conditions at 532 nm 10 mL of a solution in ethyl
acetate. The solutions were contained in a cylindrical Pyrex tube
(1-cm height, 1-mm internal diameter) carefully sealed to avoid
solvent evaporation during the experiments. Monitoring of
sample photolysis was carried out by recording the laser-
induced fluorescence emission, excited transversally to the
capillary with the same pump pulses from the Nd:YAG laser
used for producing dye laser emission, as a function of the
pump pulses at 10 Hz repetition rate. The fluorescence emission
was monitored perpendicular to the exciting beam, collected by
an optical fiber, and imaged onto the input slit of a mono-
chromator (Acton Research corporation) and detected with a
charge-coupled device (CCD) (SpectruMM:GS128B). The
fluorescence emission was recorded by feeding the signal to
the boxcar (Stanford Research, model 250) to be integrated
before being digitized and processed by a computer. Each
experience was repeated at least three times. The estimated
error of the energy measurements was 10% and the experi-
mental error in the photostabilty measurements was estimated
to be on the order of 7%. Details of the experimental setup can
be found elsewhere.5,6
Narrow-linewidth laser emission and tuning ranges of dye
solutions were obtained by placing the samples in a home-
made Shoshan-type oscillator7 consisting of full-reflecting
aluminium back and tuning mirrors, and a 2400 lines mmÀ1
holographic grating in grazing incidence, with outcoupling via
the grating zero order. Wavelength tuning was accomplished
by rotation of the tuning mirror. Tuning mirror and grating
(both from Optometrics) were 5 cm wide and the angle of
incidence on the grating was 88.51. Laser linewidth was
measured with a Fabry-Perot etalon (IC Optical Systems)
with a free spectral range of 15.9 GHz.
Scheme 1 Synthetic routes of derivatives 2–4 from 1.
presence of b-alanine and acetic acid led to the formation of
4 (21%).
3.2 Photophysical properties
The new BODIPYs 2–4 present absorption and fluorescence
bands at around lab E 550–575 nm and lfl E 595–625 nm (see
Fig. 2), shifted toward the red spectral region with respect to
other alkyl-substituted BODIPY dyes with absorption and
fluorescence bands at the green/yellow spectral region.8 The
absorption and fluorescence bands of the new dyes are well
separated from each other, leading to a relatively large Stokes
shift (DnSt E 1200–1800 cmÀ1), about 3 times higher than that
of typical alkyl-BODIPY derivatives. Fluorescent dyes with
large Stokes shifts are of special interest in photonics because
the loss of emission intensity by reabsorption effects, one of
the main factors contributing to the losses in the resonator
3. Results and discussion
3.1 Synthesis
BODIPY fluorophore can be easily functionalized at the
3- and 5-positions with two 4-formylphenyl groups by
palladium-catalyzed coupling reaction of the 3,5-dichloro-
4,4-difluoro-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene (1)3 using
the Suzuki reaction. In these conditions, BODIPY derivative 2
was obtained (see Scheme 1). Compound 2 was transformed in
the dimethoxycarbonylvinyl and dicyanovinyl-substituted
fluorophores 3 and 4, respectively, by Knoevenagel condensa-
tion (see Scheme 1). Suzuki coupling for the formation of
aryl-substituted dye 2 was carried out under microwave or
ultrasonic irradiation. Thus, 1 was treated with 2.5 equivalents
of 4-formylphenylboronic acid under microwave irradiation
for 30 min at 150 1C with a power of 200 W affording the
diarylated derivative 2 (46%). When the reaction was run
under ultrasonic irradiation at 75 1C with a power of 720 W,
longer time (15 h) and 6 equivalents of boronic acid were
required, affording 2 in 50% yield. Subsequently, BODIPY 3
was obtained with a 47% yield, from diaryl-substituted dye 2
with dimethyl malonate in the presence of TiCl4. Similarly, the
Knoevenagel reaction of compound 2 and malononitrile in the
cavity of
a dye laser, is reduced. The new dyes are
also characterized by broad spectral bands, mainly for dye 4
(fwhm E 2200 cmÀ1).
Quantum mechanical calculations predict that the S0–S1
transition of these new BODIPYs, responsible for these Vis
absorption and emission bands, involves the HOMO and
LUMO molecular orbitals. The electronic density of these
orbitals is illustrated in Fig. 3 for the specific case of 4 (see ESI
page 12 for similar electronic clouds obtained to compound 2
and 3).w As can be observed, the electronic p-system is not
only localized in the BODIPY core but is also extended
through the aromatic ring of the substituents at the 3- and
5-position, mainly in the excited state. This high delocalization
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Fig. 2 Absorption (normalized to emission spectrum), fluorescence spectra (scaled by their fluorescence quantum yield), and fluorescence decay
curves of dyes 2, 3 and 4 in trifluoroethanol.
in the electronic p-system should be responsible for the large
spectral red shifts observed for these dyes.
more prominent for dyes 3 and 4 than for dye 2 (see Fig. 2),
probably because of the presence of vinyl groups in the former
dyes and stronger electron withdrawing units at the para-phenyl
position (methoxycarbonyl or cyano versus formaldehyde).
Fluorescence decay curves of 2–4 are analyzed as mono-
exponential decays (see Fig. 2), and the fluorescence lifetimes
are evaluated from the corresponding slopes. The photo-
physical parameters of BODIPYs 2–4 in several solvents are
summarized in Table 1. The fluorescence quantum yield and
lifetime (f E 0.40–0.65, and t E 3.5–5.0 ns, respectively) of
these compounds are relatively high, in spite of the red
emission of these dyes. In fact, systems with low S1–S0 energy
have low fluorescent capacities because the non-radiative
deactivation processes are favoured via a rapid internal con-
version mechanism.11 The increase in the internal conversion is
probably the most critical issue to develop successfully efficient
red emitting dyes. However, present red-emitting BODIPY
dyes are characterized by a relative low non-radiative rate
constant (knr o 1.5 Â 108 sÀ1) and a relative high fluorescence
Many of the reported red-emitting BODIPYs are achieved
by the incorporation of delocalized substituents, bearing
sometimes electron donor groups, usually at 3 and 5 positions
of the chromophoric core.9 The resonant interaction between
such functional groups and the p-system of the BODIPY core
provides a merocyanine like chromophore where electronic
density could be shifted from the donor substituent to the
BODIPY core leading to the formation of a charge transfer
state, which quenches efficiently the fluorescent emission
mainly in polar media.10 In our case, the presence of electron
acceptor groups at 3 and 5 positions causes a strong red shift,
however such charge transfer states are not formed. In fact,
and as discussed below, both the fluorescence quantum yield
and the lifetime of dyes 2, 3, and 4 are nearly solvent
independent. Therefore, the cyanine like delocalization typical
of BODIPY is kept, as shown in Fig. 3, where the electronic
density is extended through the whole molecule.
The optimized geometry in the ground state shows a nearly
planar BODIPY core, with the phenyl unit at the meso
8-position twisted 511 and the aromatic rings at 3,5 positions
twisted around 301, with respect to the dipyrromethene plane.
Upon excitation, the electronic distribution is extended to the
vinyl group at para position of the phenyl substituents at 3 and
5 positions. Consequently the fluorescence band is more
extended into the red, explaining the above commented large
Stokes shift of these compounds. These red displacements are
rate constant (kfl E 1.3 Â 108
s
À1, in spite of the low
absorption probability, molar absorption emax E 1–3 Â
104 MÀ1 cmÀ1). Moreover, it has previously been probed that
these dyes with a pendant phenyl group at the meso 8-position
can enhance the internal conversion process via a rota-
tional motion of the phenyl group, reducing the fluorescence
efficiency.2b,12 In the present cases, such a mechanism seems to
have a minor effect, probably due to the high delocalization of
the chromophoric p-system through the aromatic substituents
at 3 and 5 positions.
In general, compound 4, bearing cyano groups, presents the
lowest fluorescent efficiency while dyes 2 and 3 show the highest
f value. The presence of electron withdrawing cyano groups
induces a decrease in the kfl value, which is correlated with a
lower absorption probability in BODIPY 4. A more proper
assignation of a decrease in the fluorescence probability should
be made on the basis of the kfl/n,3 since the Einstein sponta-
neous emission is proportional to the fluorescence frequency.
Again, it is confirmed that dye 4 bearing a cyano group has a
lower fluorescence emission probability. Although this reduc-
tion in the fluorescence ability will reduce the lasing efficiency of
this compound, its high Stokes shift would decrease the losses in
the resonator cavity favouring its laser action.
Fig. 3 Electronic density maps of the HOMO and LUMO orbitals
for dye 4.
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Table 1 Photophysical properties of dyes 2, 3 and 4 in apolar
(c-hexane), polar (acetone and ethyl acetate) and polar/protic solvents
(ethanol, methanol and F3-ethanol); absorption (lab) and fluorescence (lfl)
wavelength (Æ0.5 nm), molar absorption (emax, Æ 0.1 Â 104 MÀ1 cmÀ1),
fluorescence quantum yield (f Æ 0.05) and lifetime (t, Æ 0.05 ns), radiative
(kfl, 108 sÀ1) and non-radiative (knr, 108 sÀ1) rate constants, and Stokes
5 positions of BODIPY core. The p-electronic delocalization
through these groups induces a double bond character to
the linking BODIPY-phenyl bonds, reducing the rotational
motion of these aryl groups. Moreover, this extended p-system
also reduces the electronic density at the meso 8-carbon of the
BODIPY-core, mainly in the excited state (see Fig. 3), decreasing
the effect of the 8-aryl rotation on the internal conversion
of these dyes. The fluorescence bands of the new dyes 2, 3
and 4 are more extensively red-shifted with a higher fluores-
cence quantum yield because the presence of adequate
para-substituents at the 3,5-phenyl groups favours the delocali-
zation of the p-system through these moieties, increasing their
fluorescence ability.
shift (DnSt, cmÀ1
)
Solvent
lab
emax lfl
f
t
kfl
knr
DnSt
Dye 2
F3-ethanol
Methanol
Ethanol
Acetone
Ethyl acetate 558.0 3.0
c-Hexanea
Dye 3
551.5 3.0
557.0 3.1
559.0 3.1
558.5 3.1
595.5 0.58 4.54 1.28 0.92 1365
598.0 0.46 3.65 1.26 1.48 1225
600.5 0.53 3.91 1.35 1.20 1250
599.5 0.51 3.48 1.46 1.41 1225
599.0 0.57 3.67 1.55 1.17 1240
597.5 0.54 3.68 1.46 1.25 1080
561.0 3.3
Therefore, the present red-emitting BODIPY dyes show an
important fluorescence capacity in the red part of the visible
region and are promising candidates to be used as active media
for developing an efficient red-dye laser.
F3-ethanol
Methanol
Ethanol
562.5 1.8
570.0 1.9
573.5 1.9
572.5 1.9
613.5 0.59 4.98 1.18 0.82 1470
618.5 0.60 4.56 1.31 0.88 1380
621.0 0.62 4.67 1.33 0.81 1330
620.0 0.61 4.61 1.32 0.84 1345
620.0 0.64 4.56 1.40 0.79 1355
618.0 0.56 4.39 1.27 1.00 1190
Acetone
Ethyl acetate 572.0 1.9
c-Hexanea
Dye 4
575.5 1.8
3.3 Lasing properties
F3-ethanol
Methanol
Ethanol
Acetone
Ethyl acetate 563.0 0.6
c-Hexanea
558.0 0.6
564.0 0.7
566.0 0.6
565.5 0.6
622.0 0.42 4.80 0.87 1.21 1845
623.0 0.41 4.53 0.90 1.30 1675
625.0 0.47 4.59 1.02 1.15 1665
625.0 0.44 4.80 0.92 1.16 1690
617.5 0.54 4.66 1.16 0.98 1570
622.5 0.53 4.63 1.14 1.01 1420
An ethyl acetate solution of derivative 2 transversely pumped
at 532 nm with 5 mJ pulseÀ1 in a simple plane–plane non-
tunable resonator emits broad-linewidth laser light at 615 nm,
with an efficiency (Eff, percentage of the excitation energy
converted into laser emission) of 14%. The extension of the
conjugation at 3,5-BODIPY positions, red-shifts effectively
the wavelength of the laser emission up to 37 nm, although
with a significant decrease in the laser efficiency of dyes 3 and 4
(see Table 2). The laser-pump of the new dyes at a wavelength
(568 nm) near their absorption maxima improves the laser
efficiency which increases for a factor of up to 5 with respect to
that registered under excitation at 532 nm (see Table 2).
The lasing characteristics observed in concentrated solu-
tions of the new BODIPY dyes show good correlation with
their photophysical properties in dilute solution. The presence
of 3,5-dimethoxycarbonylvinyl- and dicyanovinyl substituents
induced spectral red-shifts in the absorption and fluorescent
bands similar to those observed in the lasing emission. A slight
decrease in the kfl value of dye 4 with respect to dye 2 (by a
factor of ca 1.4) could suggest a diminution in the laser action,
although not so significant reduction of the laser efficiency.
The actual effect of the solvent on the dye laser emissions
was analyzed in solutions of apolar, polar nonprotic, and
polar protic solvents at the dye concentrations that optimized
the corresponding laser efficiency of each derivative. The
results are summarized in Table 2. The low solubility of these
new dyes in some common solvents prevents attaining the
concentrated solutions required for laser experiments and,
consequently, the correlation with the photophysical proper-
ties. However, some interesting tendencies can be derived from
these experimental results: higher laser efficiencies of dye 2 are
reached for polar aprotic solvents such as ethyl acetate
and acetone. However, the very polar protic nature of the
solvents (2,2,2-trifluoroethanol and methanol) improved the
laser efficiencies of dyes 3 and 4 with respect to the values
registered in apolar media. As previously discussed for com-
mercial BODIPY dyes,14 the behaviour of the new BODIPY
derivatives in different solvents confirms the difficulty in
understanding and predicting laser action on the basis of the
572.0 0.4
a
Not fully soluble.
The inclusion of p-substituted-phenyl groups at 3,5 positions
results in more polar BODIPY dyes reducing their solubility in
apolar media (i.e. c-hexane). The solvent effect on the photo-
physics of these new dyes (see Table 1) is the typical one observed
in many alkyl-BODIPY dyes,8 for instance: hypsochromic
shifts of the spectral bands with the solvent polarity/acidity;
a nearly solvent independent fluorescence quantum yield, and
a progressive increase of the fluorescence lifetime and Stokes
shift with the solvent’s acidity and polarity. These BODIPYs
have low solubility in apolar media. Anyway, their photophysics
in such media (c-hexane, dioxane, dietilether and THF) was also
registered (data in c-hexane included in Table 1, data in the rest
of solvents not shown) for comparison. The results are consis-
tent with the above commented evolutions with the solvent
properties, corroborating the absence of any quenching charge
transfer state in polar media and the maintenance of the cyanine
like electronic p-system delocalization, in spite of the incorpora-
tion of electron withdrawing groups (aldehide, ester and cyano).
The photophysical behaviour of the new red emitting dyes 2,
3 and 4 is compared with other structurally related 8-tolyl-
BODIPYs found in the literature,13 (see Fig. 4). The unsubsti-
tuted 8-tolyl-BODIPYs present a very low fluorescence quantum
yield (see Fig. 4), attributed to an important internal conver-
sion deactivation via the rotational motion of the pendant
8-aryl group.2b,12 Besides, red-emitting dyes present low fluores-
cence capacity because the low S0–S1 energy gap favours the
internal conversion processes.11 However, the incorporation
of 3,5-phenyl groups in 8-tolyl-BODIPY not only leads to a
large red shift, but also to a progressive enhancement of the
fluorescence quantum yield (see Fig. 4). Experimental results
suggest that the internal conversion of 8-tolyl-BODIPYs is
drastically reduced when aryl groups are incorporated at 3 and
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Fig. 4 Chemical structures of 8-tolyl-BODIPYs susbtituted at 3 and 5 positions from the literature13 and compounds 2–4 synthetized in this work.
Table 2 Maximum wavelength of the laser emission (ll) and lasing efficiency (Eff) of the new BODIPY dyes in several solvents under excitation at
532 nm and, within parentheses, at 568 nm
Dye
Data
THF
EtOAc
Acetone
CH2Cl2
F3-EtOH
MeOH
2a
ll/nm
Eff (%)
ll/nm
Eff (%)
ll/nm
Eff (%)
619 (620)
11 (16)
636 (632)
2 (8)
646 (659)
2 (5)
615 (615)
14 (20)
632 (628)
2 (10)
648 (654)
4 (8)
615 (613)
14 (20)
617
10
3b
4c
625
6
655 (657)
5 (11)
628 (626)
11 (17)
648
652
3
3
a
b
c
Dye concentrations at 532 nm: 9 Â 10À4 M and at 568 nm: 9 Â 10À4 M. Dye concentrations at 532 nm: 8 Â 10À4 M and at 568 nm: 4 Â 10À4 M. Dye
concentrations at 532 nm: 3 Â 10À3 M and at 568 nm: 2 Â 10À3 M.
polarity/polarizability or H-bond donor/acceptor ability or both
liquid media, at least for dyes with a complex molecular structure.
The photostability (evolution of the emission with the
number of pump pulses at 10 Hz) of each dye was evaluated
by irradiating under lasing conditions 10 mL of a solution in
ethyl acetate contained in capillary tubes carefully sealed to
avoid solvent evaporation during experiments. The concentra-
tions of laser dyes were adjusted so that the laser action was
optimum in all cases. Although the optical quality of the
capillary prevents laser emission from the dyes, information
about their photostabilities can be obtained by monitoring the
decrease in laser-induced fluorescence intensity perpendicular
to the exciting beam, under transversal excitation of the
capillary, as a function of the number of pump pulses. The
results obtained from the dyes are plotted in Fig. 5.
In order to put the present results in proper perspective, the
lasing parameters of two well-known dyes lasing at the same
wavelengths as the dyes studied herein were also measured in
liquid solutions at 532 nm under similar conditions. Perylene
Red, used as received, in ethyl acetate solution (5 Â 10À4 M)
lases at 614 nm with an efficiency of 20%, while after purifica-
tion the conversion efficiency of this dye increases to 26%.
Rhodamine 640 (1 Â 10À3 M) lases in the same spectral region
with higher efficiency (40%) but exhibiting a dual laser emis-
sion, at 620 and 650 nm, due to reabsorption/reemission
phenomena and inhomogeneous spectral broadening.15 For
comparison, the lasing photostabilities obtained under the
above described experimental conditions for commercial dyes
Perylene Red and Rhodamine 640 are also included in Fig. 5.
The commercial dyes exhibit lasing efficiencies higher than
those obtained with the new BODIPY derivatives described
here, although with lower photostabilities: the initial fluores-
cence emission of Rhodamine 640 drops to 45% of its initial
value after 60 000 pump pulses while the purified Perylene Red
maintains a 70% of its initial emission after 100 000 pump
pulses.
Under 532 nm pumping, the new derivatives 3 and 4 result
in being highly photostable, with the emission dropping by less
than 20 and 10%, respectively, with respect to the initial value
after 100 000 pump pulses, while the derivative 2 experiences a
faster photodegradation process, losing up to 40% of its
fluorescence emission.
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Fig. 6 Laser tunability of new 2, 3, and 4 BDP dyes in ethyl acetate
solutions. The solid lines in the tuning curves represent guidelines for
the eyes.
be extended to other dyes of this family, with practical
applications in optical and sensing fields.
Acknowledgements
This research was financed by the Spanish MICINN (Project
MAT2007-65778-C02-01 and -02 and Project CTQ2008-02820)
and Universidad Complutense of Madrid (Project GR58/08).
We acknowledge support provided by the Centro de Resonancia
Magnetica of the Universidad Complutense de Madrid.
Fig. 5 Normalized laser-induced fluorescence emission as a function
of the number of pump pulses for the following dye solutions: (A) new
BDP dyes 2, 3 and 4 in ethyl acetate solution; and (B) Per-Red in ethyl
acetate and Rh640 in ethanol. Pump laser wavelength, energy and
repetition rate: 532 nm, 5.5 mJ pulseÀ1 and 10 Hz.
References
1 (a) A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891–4932;
(b) R. Ziessel, G. Ulrich and A. Harriman, New J. Chem., 2007, 31,
496–501; (c) G. Ulrich, R. Ziessel and A. Harriman, Angew. Chem.,
Int. Ed., 2008, 47, 1184–1201; (d) S. Ela-Erten, M. D. Yilmaz,
B. Icli, I. Dede, S. Icli and E. U. Akkaya, Org. Lett., 2008, 10,
3299–3302; (e) S. Ozlem and E. U. Akkaya, J. Am. Chem. Soc.,
2009, 131, 48–49; (f) R. Ziessel, G. Ulrich, A. Harriman, M. A.
H. Alamiry, B. Stewart and P. Retailleau, Chem.–Eur. J., 2009, 15,
1359–1369.
2 (a) A. Burghart, H. Kim, M. B. Welch, L. H. Thoresen,
J. Reibenspies and K. Burgess, J. Org. Chem., 1999, 64,
7813–7819; (b) J. Chen, A. Burghart, A. Derecskei-Kovacsand
and K. Burgess, J. Org. Chem., 2000, 65, 2900–2906; (c) B. Turfan
and E. U. Akkaya, Org. Lett., 2002, 4, 2857–2859; (d) A. Coskun
and E. U. Akkaya, J. Am. Chem. Soc., 2005, 127, 10464–10465;
(e) For a recent review, see: A. B. Descalzo, H. J. Xu, Z. Shen and
K. Rurack, Ann. N. Y. Acad. Sci., 2008, 1130, 164–171.
The tuning capability, one of the most important features of
laser dyes, was determined for the new BODIPY dyes by
placing their liquid solutions in the grazing-incidence grating
tunable resonator. Tunable laser emission with a linewidth of
the order of 0.15 cmÀ1 and a tuning range of up to 50 nm was
thus recorded, and the spectral region 590–680 nm can be
continuously covered with narrow-linewidth and stable laser
radiation by using the three dyes (see Fig. 6).
4. Conclusions
3 W. Qin, T. Rohand, M. Baruah, A. Stefan, M. Van der Auweraer,
W. Dehaen and N. Boens, Chem. Phys. Lett., 2006, 420, 562–568.
We have successfully designed and synthesized new BODIPY
dyes to achieve large spectral red-shifts with particularly useful
photophysical properties. The position of the emission band
can be modulated by the type of substituent attached to the
BODIPY core. All the red emitting dyes presented in this work
are characterized by a high fluorescence capacity since the
coupling between the electronic clouds of the pyrromethene
core and the 3,5-aryl groups reduces the rotational motion of
the phenyl groups. Besides, the new dyes exhibit large Stokes
shift, which could also improve their fluorescence ability from
a technological point of view. The new BODIPY dyes lase with
good efficiency and high photostability, allowing a wavelength
finely tunable over a wide range (590–680 nm) with a narrow-
linewidth. Considering the easy synthetic build-up, the wide
variety of possible substituents and the large number of
described BODIPY laser dyes, this powerful approach could
4 F. Lo
I. Lopez Arbeloa, ChemPhysChem, 2004, 5, 1762–1771.
5 A. Costela, I. Garcıa-Moreno, D. del Agua, O. Garcı
R. Sastre, J. Appl. Phys., 2007, 101, 073110–073121.
pez Arbeloa, J. Banuelos, V. Martınez, T. Arbeloa and
´ ´
´
´
´
a and
6 A. Costela, I. Garcia-Moreno, C. Gomez, R. Sastre, F. Amat-Guerri,
M. Liras, F. Lopez Arbeloa, J. Banuelos Prieto and I. Lopez Arbeloa,
J. Phys. Chem. A, 2002, 106, 7736–7742.
7 I. Soshan, N. N. Danon and U. J. Oppenheim, Appl. Phys., 1977,
48, 2295–2299.
8 F. Lo
I. Lopez Arbeloa, Int. Rev. Phys. Chem., 2005, 24, 339–374.
pez Arbeloa, J. Banuelos, V. Martınez, T. Arbeloa and
´ ´
´
9 (a) Z. Dost, S. Atilgan and E. U. Akkaya, Tetrahedron, 2006, 62,
8484–8488; (b) W. Qin, T. Rohand, W. Dehaen, J. N. Clifford,
K. Driesen, D. Beljonne, B. V. Averbeke, M. V. Auweraer and
N. Boens, J. Phys. Chem. A, 2007, 111, 8588–8697;
(c) K. Umezawa, Y. Nakamura, H. Makino, D. Citterio and
K. Suzuki, J. Am. Chem. Soc., 2008, 130, 1550–1551;
(d) D. Zhang, Y. Wen, Y. Xiao, G. Yu, Y. Liu and X. Qian,
Chem. Commun., 2008, 4777–4779; (e) B. Ventura, G. Marconi,
ꢀc
This journal is the Owner Societies 2010
7810 | Phys. Chem. Chem. Phys., 2010, 12, 7804–7811

View Article Online
M. Broring, R. Kruger and L. Flamigni, New J. Chem., 2009, 33,
¨
¨
428–438.
12 (a) S. Badre
´
, V. Monnier, R. Me
´
allet-Renault, C. Dumas-Verdes,
E. Y. Schmidt, A. I. Mikhaleva, G. Levi, A. Ibanez,
B. A. Trofimov and R. B. Pansu, J. Photochem. Photobiol., A,
2006, 183, 238–246; (b) Q. Zheng, G. Xu and P. N. Prasad,
Chem.–Eur. J., 2008, 14, 5812–5819; (c) M. Alvarez, A. Costela,
10 (a) L. G. S. Brooker, J. Am. Chem. Soc., 1965, 87, 937–938; (b) L.
G. S. Brooker, A. C. Craig, D. W. Heseltine, P. W. Jenkins and
L. L. Lincoln, J. Am. Chem. Soc., 1965, 87, 2443–2450;
(c) J. Fabian and H. Hartmann, Light Absorption of Organic
Colorants, Springer-Verlag, Berlin, 1980; (d) N. Tyutyulkov,
J. Fabian, A. Melhorn, F. Dietz and A. Tadjer, Polymethine Dyes:
Strcuture and Properties, St. Kliment Ohridski University Press,
Sofia, 1991.
I. Garcı
Arbeloa, J. Banuelos and I. Lo
Sci., 2008, 7, 802–813.
´
a-Moreno, F. Amat-Guerri, M. Liras, R. Sastre, F. Lo
´
pez
´
pez Arbeloa, Photochem. Photobiol.
13 (a) T. Rohand, W. Qin, N. Boens and W. Dehaen, Eur. J. Org.
Chem., 2006, 4658–4663; (b) A. Cui, X. Peng, J. Fan, X. Chen,
Y. Wu and B. Guo, J. Photochem. Photobiol., A, 2007, 186, 85–92.
11 (a) P. O. Andersson, S. M. Bachilo, R.-L. Chen and T. Gillbro,
J. Phys. Chem., 1995, 99, 16199–16209; (b) C. E. Carvalho,
I. M. Brinn, A. V. Pinto and M. C. Pinto, J. Photochem. Photobiol.,
14 J. Banuelos Prieto, F. Lo
T. Lopez Arbeloa and I. Lo
108, 5503–5508.
15 I. Garcıa-Moreno, A. Costela, M. Pintado-Sierra, V. Martı
R. Sastre, J. Phys. Chem. B, 2009, 113, 10611–10618.
´
pez Arbeloa, V. Martı
nez Martınez,
´ ´
´
´
pez Arbeloa, J. Phys. Chem. A, 2004,
A, 2000, 136, 25–33; (c) J. Banuelos, F. Lo
V. Martınez, T. Arbeloa, F. Amat-Guerri, M. Liras and I. Lo
Arbeloa, Chem. Phys. Lett., 2004, 385, 29–35.
´
pez Arbeloa,
´
´
pez
´
´
n and
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Phys. Chem. Chem. Phys., 2010, 12, 7804–7811 | 7811