First highly efficient and photostable e and Ca derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) as dye lasers in the liquid phase, thin films, and solid-state rods

Article abstract of DOI:10.1002/chem.201303579

A new library of E- and C-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives has been synthesized through a straightforward protocol from commercially available BODIPY complexes, and a systematic study of the photophysical properties and lase

Full text of DOI:10.1002/chem.201303579

DOI: 10.1002/chem.201303579
Full Paper
&
Laser Chemistry
First Highly Efficient and Photostable E and C Derivatives of
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) as Dye Lasers
in the Liquid Phase, Thin Films, and Solid-State Rods
Gonzalo Duran-Sampedro,[a] Ixone Esnal,[c] Antonia R. Agarrabeitia,[a] Jorge BaÇuelos Prieto,[c]
Luis Cerdꢀn,[b] Inmaculada Garcꢁa-Moreno,*[b] Angel Costela,[b] IÇigo Lopez-Arbeloa,[c] and
Marꢁa J. Ortiz*[a]
Abstract: A new library of E- and C-4,4-difluoro-4-bora-3a,4a-
diaza-s-indacene (BODIPY) derivatives has been synthesized
through a straightforward protocol from commercially avail-
able BODIPY complexes, and a systematic study of the pho-
tophysical properties and laser behavior related to the elec-
tronic properties of the B-substituent group (alkynyl, cyano,
vinyl, aryl, and alkyl) has been carried out. The replacement
of fluorine atoms by electron-withdrawing groups enhances
the fluorescence response of the dye, whereas electron-
donor groups diminish the fluorescence efficiency. As a con-
sequence, these compounds exhibit enhanced laser action
with respect to their parent dyes, both in liquid solution and
in the solid phase, with lasing efficiencies under transversal
pumping up to 73% in liquid solution and 53% in a solid
matrix. The new dyes also showed enhanced photostability.
In a solid matrix, the derivative of commercial dye PM597
that incorporated cyano groups at the boron center exhibit-
ed a very high lasing stability, with the laser emission re-
maining at the initial level after 100000 pump pulses in the
same position of the sample at a 10 Hz repetition rate. Dis-
tributed feedback laser emission was demonstrated with or-
ganic films that incorporated parent dye PM597 and its
cyano derivative. The films were deposited onto quartz sub-
strates engraved with appropriate periodical structures. The
C derivative exhibited a laser threshold lower than that of
the parent dye as well as lasing intensities up to three
orders of magnitude higher.
Introduction
Among the numerous classes of highly fluorescent dyes
used as the active media of tunable lasers, the family of di-
fluoroboron dipyrromethene complexes, or BODIPYs, is one
with the highest potential.[2] Their development has acquired
great importance in the last two decades owing to the excel-
lent and easily modulated photophysical properties of these
fluorophores.[3] As a consequence, they are also intensively ap-
plied as fluorescent probes in biological systems, photosensi-
tizers for photodynamic therapy, and as materials for incorpo-
ration into electroluminescent devices.[2a,4]
Tunable lasers are an important tool in a wide variety of fields
such as spectroscopy, photochemistry, material diagnosis, med-
icine, or integrated optical devices. Organic dye molecules
offer clear advantages over other commercially available multi-
wavelength laser sources such as low cost, ample spectral cov-
erage over the visible region of the electromagnetic spectrum,
and high conversion efficiency.[1]
The incorporation of appropriate functional groups into the
chromophore could lead to new photophysical processes or
large spectral shifts; therefore, depending on the desired appli-
cation, one should carefully choose the kind of substituents
and the position in which they will be incorporated into the
BODIPY. A strategy that causes a substantial increase of the po-
tential applications of the chromophore BODIPY is the substi-
tution of fluorine atoms.[3a–c] Thus, a wide variety of new
BODIPYs have been synthesized by means of fluorine displace-
ment by alkyl or aryl groups (C-BODIPYs), ethynyl groups (E-
BODIPYs), and alkoxy or aryloxy groups (O-BODIPYs).[5] The in-
troduction of these functional groups leads to an increase in
the Stokes shift as well as to the improved chemical and pho-
tochemical stability of these fluorophores, thereby opening the
way to a new class of highly luminescent dyes.
[a] G. Duran-Sampedro, Prof. A. R. Agarrabeitia, Prof. M. J. Ortiz
Departamento de Quimica Organica I
Facultad de Ciencias Quimicas, Universidad Complutense
28040 Madrid (Spain)
Fax: (+34)913944103
E-mail: mjortiz@quim.ucm.es
[b] Dr. L. Cerdꢀn, Prof. I. Garcꢁa-Moreno, Prof. A. Costela
Instituto de Quimica-Fisica “Rocasolano” (IQFR), CSIC
Serrano 119, 28006 Madrid (Spain)
Fax: (+34)915642431
E-mail: i.garcia-moreno@iqfr.csic.es
[c] I. Esnal, Dr. J. BaÇuelos Prieto, Prof. I. Lopez-Arbeloa
Departamento de Quimica Fisica, UPV-EHU
Apartado 644, 48080 Bilbao (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303579.
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Despite
the
considerable
volume of work on the synthesis
and applications of this type of
fluorophores, there are few re-
ports on their optical and lasing
properties.[5c,d] In this field, our
research group has described
the synthesis and characteriza-
tion of the emission properties
of a set of O-BODIPYs in which
two carboxylate groups are con-
nected to the boron center in
place of the fluorine atoms. It
was shown that these dyes are
highly fluorescent and exhibit
enhanced laser action with re-
spect to their F-BODIPY ana-
logues, both in liquid solution
and the solid phase.[5c]
Considering these results, in
the present work we have
Scheme 1. Molecular structures of commercial dyes and their analogues 1–7 synthesized in this work.
synthesized
a new library of
E- and C-BODIPY derivatives
(named 1–7 in Scheme 1) from the commercially available
BODIPYs and we have carried out a systematic study of their
photophysical properties and laser behavior related to the
electronic properties of the B-substituent group (alkynyl,
cyano, vinyl, aryl, and alkyl). As far as we know, there is only
one precedent on lasing performance of E-BODIPYs by Ray
et al. in which it is revealed that this substitution at the boron
center does not significantly improve the laser efficiency but
enhances the photostability owing to their lower reaction rates
with 1O2 and lower 1O2 generation capacity.[5d] However, the
new BODIPY derivatives synthesized herein through a straight-
forward protocol exhibit lasing efficiencies up to four times
higher than those recorded for the corresponding commercial
dyes with high photostability in the liquid phase, the bulk
solid state, and in thin films.
The reaction of PM597 with mono-TMS-protected 1,4-di-
ethynylbenzene 8 gave E-BODIPY 3a in good yield. The remov-
al of the TMS protection yielded a mixture of dideprotected
derivative 3b (80%) and monodeprotected derivative 3c
(15%), which were separated by means of flash chromatogra-
phy on silica (Scheme 2).
Replacement of the fluorine atoms by cyano groups (com-
pounds 4a–d) was carried out by the reaction of BODIPY with
TMSCN in the presence of AlCl3 as Lewis acid (compounds 4a
Results and Discussion
BODIPY dyes 1–7 were successfully obtained from commercial-
ly available dyes PM546, PM567, PM580, PM597, PM597-8C9,
PM605, and PM650 with organolithium, Grignard reagents, or
trimethylsilyl cyanide (TMSCN) as nucleophilic agents depend-
ing upon the desired change in the boron atom. BODIPY deriv-
atives 1b,[6] 2b,[6] and 6[7] were synthesized by the methods
previously described. Dyes 1a–e were prepared in good yield
by the introduction of two trimethylsilylacetylene units by
using an excess amount of the corresponding organolithium
derivative, generated in situ from trimethylsilylacetylene and n-
butyllithium. No monosubstituted derivatives were formed
under the reaction conditions. Removal of the trimethylsilyl
groups using an excess amount of NaOH afforded 2a–e as the
major products. Only compound 2a was obtained in addition
to a small percentage of the monoprotected derivative.
Scheme 2. Synthesis of BODIPYs 3a–c.
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and 4c) or in the absence of AlCl3 (compounds 4b and 4d) in
82 to 85% yield, except for 4d, which was obtained in 25%
isolated yield.
infer rigidity onto the whole structure. Indeed, opposite results
(high bathochromic shifts and reduced fluorescence quantum
yields) have been reported for acetylene groups directly at-
tached to the core at the 2- and 6-positions.[9]
Furthermore, the use of an excess amount of Grignard re-
agents (vinylmagnesium bromide or ethylmagnesium bromide)
led to rapid substitution of the fluorine atoms and allowed the
preparation of compounds 5 and 7, respectively, in moderate
yield.
Nevertheless, the electronic character of the group anchored
to the boron atom plays a key role in the fluorescence perfor-
mance of the resulting E- and C-BODIPY (Figure 1). Such an
effect can be easily discussed by considering the set of deriva-
tives from PM567. The acetylene group behaves like a weak
electron acceptor (Hammet parameter sp+ =0.18). A further in-
crease in such behavior through the presence of cyano groups
(sp+ =0.66) results in an improvement in the fluorescence be-
havior (see compounds 4a–4d in their respective series in
Figure 1). Such an enhancement is especially noticeable in
PM650 since the fluorescence quantum yield increases from
0.15 to 0.29. It is possible that the two cyano groups linked to
the boron atom counteract the charge separation induced by
the meso-cyano substitution to hamper the population of the
ICT state. However, a withdrawing character that is too strong
could damage the fluorescence performance, which is what
happens in O-BODIPYs that bear nitro groups (sp+ =0.74)
owing to the activation of ICT processes.[5c]
Photophysical properties
The commercial F-BODIPYs that bear linear alkyl groups
(PM546, PM567, and PM580) set themselves apart by their
high fluorescence performance (approaching 100% in Figure 1
and Table S1 in the Supporting Information), although the fluo-
rescence capacity of the BODIPYs depends on the functionali-
By contrast, the presence of electron-donor groups, such as
phenyl rings (sp+ =À0.18), leads to a decrease in the fluores-
cence quantum yield and lifetime (Figure 1). Moreover, this
compound (6) exhibits a high Stokes shift (1350 versus
550 cmÀ1 in PM567) as a result of a bathochromic shift of the
fluorescence band (Figure 2). Such a trend suggests a high
geometrical rearrangement upon excitation, which enables the
energy relaxation and the consequent loss of fluorescence
emission. In addition, the optimized excited-state geometry
points to the high steric hindrance induced by the two rings
attached to the boron (Figure S1 in the Supporting Informa-
tion). As a result, the two phenyl groups are placed orthogo-
nally and the chromophore is distorted (deviation from planari-
ty up to 258). This lack of planarity enhances the internal con-
version and explains the recorded lower fluorescence perfor-
mance for compound 6.
Figure 1. Fluorescence quantum yield in ethyl acetate of the commercial
F-BODIPYs (black) and their corresponding E-BODIPYs bearing acetylene
(2a–e, grey, striped if phenyl is also grafted, 3b), acetylene TMS (1a, 1c–e,
light grey, striped upon addition of a phenyl, 3a), and C-BODIPYs bearing
a cyano (4a–d, white), vinyl (5, sparse stripe), ethyl (7, dense stripe), and
phenyl group (6, grid).
zation of the core. Thus, the presence of methylenacetoxy at
the meso position (PM605) reduces the radiative deactivation
probability owing to its electron-withdrawing character, where-
as the tert-butyl groups (PM597 and PM597-8C9) enhance the
nonradiative pathways owing to the geometrical distortion in-
duced by the sterical hindrance to accommodate such bulky
groups. The most noteworthy case is the appearance of
a quenching intramolecular charge transfer (ICT) state by the
grafting of the cyano at the central position (PM650).[8]
Similar trends are observed with the unsaturation degree of
the alkyl chains attached to the boron atom. A change from
acetylene (compound 2b) to vinyl (5) and to ethyl (7) implies
a reversal of the electronic behavior from electron donor to ac-
ceptor (sp+ =0.18, À0.16, and À0.30, respectively). According-
ly, the fluorescence quantum yield and lifetime decrease in the
same fashion (Figures 1 and 2). In fact, in the C-BODIPY that
bears ethyl (compound 7), the Stokes shift is quite high
(1115 cmÀ1) as a consequence of the shift of the fluorescence
band towards lower energies. Again, the optimized excited-
state geometry confirms the geometrical distortion (up to 238;
Figure S2 in the Supporting Information) to be the main path-
way for nonradiative deactivation.
The replacement of fluorine atoms in the above BODIPYs by
acetylenes (2a–e) does not have a great impact on the photo-
physics of the chromophore (Figure 1). The fluorescence quan-
tum yields of the resulting E-BODIPYs remain high (set of deriv-
atives from PM546, PM567, and PM580), or at least similar to
their corresponding counterpart (PM597). Moreover, the addi-
tion of trimethylsilyl (1a, 1c, and 1d), aryl (3b), or both (3a)
onto the triple bond has a minor effect. These results are rea-
sonable keeping in mind that the boron atom does not take
part in the delocalized p system but acts as a bridge unit to
The quantum mechanical calculations (Table 1) reproduce
the experimental features well. Indeed, the absorption and
fluorescence spectra signatures are satisfactorily predicted the-
oretically. Both compounds 6 and 7 stand out through an
emission shifted to lower energies and a higher Stokes shift as
result of an important change in the geometry upon excita-
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through the whole chromophore and hence the aromaticity of
the p system (evaluated by the bond-length alternation (BLA)
parameter;[10] data included in Figure S3 in the Supporting In-
formation).
The presence of acetylene groups leads to a higher positive
charge in the boron atom and a slightly lower charge alterna-
tion through the core, with little influence on the aromaticity
(similar BLA and fluorescence capacity to their corresponding
reference F-BODIPYs). An increase in the electron-donor ability
of the substituent (cyano groups) implies a strong rearrange-
ment of the charge distribution, which results in a more aro-
matic chromophore, as reflected in the BLA parameter (i.e.,
0.023 for compound 4a), which is the lowest among all the
BODIPYs reported herein. Accordingly, the derivatives with
a cyano group at the boron atom (4a–4d) yield the highest
fluorescence capacity. However, a progressive increase in the
electron-donor ability of the substituent (from acetylene to
ethyl) results in less aromatic dyes (BLA=0.027, 0.029, and
0.031 for compounds 2b, 5, and 7, respectively), which is in
agreement with the registered lower fluorescence efficiencies.
Especially remarkable is the asymmetric disposition of the
phenyl rings in compound 6. This should be a consequence of
the steric hindrance between their ortho-hydrogen and the
methyl groups in the 3- and 5-positions Figure S1 in the Sup-
porting Information). As a result, one of the rings acquires
a high positive charge (Figure S3 in the Supporting Informa-
tion) and leads to a less aromatic dye (BLA=0.028), thereby re-
ducing the fluorescence capacity.
Whereas the lengthening of the alkyl chain (from ethyl to
butyl) at the 2- and 6-positions of PM567 to yield PM580 has
a minor effect on the photophysical properties, important
changes are noticed when the aliphatic chain is enlarged (from
one methylene to nine) at the 8-position of PM597 to yield
PM597-8C9 (Figure 1 and Table S1 in the Supporting Informa-
tion). The photophysical properties of PM597 were controlled
by the steric hindrance induced by the tert-butyl groups,
which leads to a loss of planarity in the chromophore. As con-
sequence, the nonradiative rate constant (from 0.28ꢃ108 sÀ1 in
PM567 to 1.23ꢃ108 sÀ1 in PM597) and the Stokes shift (from
550 to 1330 cmÀ1, respectively) increased significantly. These
trends seem to be more evident upon grafting a linear chain
at the 8-position, because the nonradiative rate (knr) enhances
up to 7.9ꢃ108 sÀ1, thereby reducing the fluorescence quantum
yield and lifetime (0.10 and 1.14 ns). In addition, the Stokes
shift increases up to 2330 cmÀ1 as a result of a stronger batho-
chromic shift in the fluorescence band (placed at 595 nm, also
theoretically predicted in Table 1).
Figure 2. a) Absorption and normalized fluorescence (bold) spectra in ethyl
acetate of PM567 derivatives bearing acetylene (2b) and phenyl (6) groups.
b) Fluorescence decay curves of PM567 derivatives bearing vinyl (5) and
ethyl (7) groups.
Table 1. Theoretical simulation (TD-B3LYP) of the absorption and fluores-
cence transition of representative C-BODIPYs: absorption (DEab) and fluo-
rescence (DEfl) energy gap, and Stokes shift (DnSt).
DEab [eV]
DEfl [eV]
DnSt [cmÀ1
]
PM567
2b
6
5
2.91
2.94
2.92
2.95
2.99
2.87
2.84
2.87
2.89
2.77
2.88
2.83
2.73
2.62
380
445
1210
580
1265
1170
1795
7
PM597
PM597-8C9
tion. Such theoretically suggested loss of planarity correlates
well with the evolution of the nonradiative rate constant
(Table S1 in the Supporting Information).
The optimized geometries show that the linear chain in
PM597-8C9 adopts a linear disposition, far away from the inda-
cene core (Figure S1 in the Supporting Information). However,
its presence seems to enhance the steric hindrance and a dras-
tic geometrical rearrangement takes place after excitation. As
a result, the chromophore geometry is highly distorted and
a deviation of planarity up to 308 is predicted. Overall, such
loss of planarity implies lower aromaticity, but surprisingly, the
opposite happens in the compounds that bear 2,6-tert-butyl.
In fact, the BLA parameters suggest that these compounds
To address the opposite influence of the electron donor
(harmful to the fluorescence capacity) and acceptor (slightly
beneficial) groups, which are attached to boron in the E- and
C-BODIPYs, we simulated the charge-density distribution in
some selected compounds (Figure S3 in the Supporting Infor-
mation). In a previous study on O-BODIPYs, we concluded that
although the boron atom does not take part in the delocalized
p system, its substitution determines the charge distribution
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Table 2. Lasing efficiencies of commercial BODIPY dyes and their C-BODIPY derivatives in different solvents at the dye concentrations that optimize their
laser action of solutions in ethyl acetate (lexc: pumping wavelength).
lexc =355 nm
lexc =532 nm
Solvent
PM546 1a 2a PM567 1b 2b 4a 5
6
7
PM580 1c 2c PM597 1d 2d 3a 3b 4b PM597 1e 2e PM605 4c PM650 4d PM650
-8C9
ÀFÀCN
EtOAc
Acetone
MeOH
EtOH
F3ÀEtOH
CH2Cl2
THF
23
14
58 52
56 49
48
36
34
36
67 60 68 54 60 30
65 57 65 45 56 25
57
57
51
65 63
63 62
61 57
53
50
54
51
56
70 63 73 68 73
67 62 71 65 70
63 59 69 63 68
65 61 69 60 67
63 57 70 62 65
55
52
47
62 59
59 56
59 54
55
57
55
56
51
68
69
66
67
61
35
31
12
45
40
30
36
30
22
58 54 61
52
18
58 50
62 56 63 44 54 27
27
38
28
50
59
61 59
66 64
50
60 56
(PM597 and PM597-8C9) become more aromatic as the excit-
ed-state distortion increases (BLA of 0.022 and 0.019, respec-
tively). These evolutions have been previously reported and ex-
plained in terms of the “loose-bolt” or Brunings–Corwin effect,
in which the lower planarity by constrained geometries implies
a lower resonance energy, thereby placing the excited and
ground states energetically closer.[11] Therefore, the strong
bending of the chromophore in the excited state should be
the reason for the important shift of the emission band to-
wards higher energies and, at the same time, the reason for
the reduction of the fluorescence efficiency in those com-
pounds that bear tert-butyl groups at the 2- and 6-positions
(PM597 and mainly PM597-8C9). In these dyes, further replace-
ment of the fluorine atoms by acetylene (1e, 2e) slightly amel-
iorates the fluorescence capacity, in line with the previously
discussed E- and C-BODIPYs.
es, which become increasingly important as the dye concentra-
tion rises. The optimum value of the dye concentration thus
determined was then used to prepare dye solutions in
a number of polar protic and polar aprotic solvents to analyze
the effect of the nature of the solvent on the laser properties
of the different derivatives. The results obtained are collected
in Table 2.
The observed lasing efficiencies within the different families
of E- and C-BODIPYs correlate well with their photophysical
properties, determined in more diluted solutions (Tables S1
and S2 in the Supporting Information): higher lasing efficiency
corresponds in general to a lower nonradiative rate constant.
The few exceptions to this general rule (i.e., in the PM580
family) can be understood in terms of the effect of the much
higher dye concentration in the solutions utilized in the laser
experiments.
The lasing properties of the new dyes follow a dependence
on the solvent character similar to that exhibited by the com-
mercial ones, with the lasing efficiencies being higher in polar
aprotic solvents than in polar protic ones. It can be seen in
Table 2 that, with the exception of dye 7, in all cases the lasing
efficiency of the derivatives is higher than that of the commer-
cial parent dye. The highest lasing efficiencies were obtained
with the derivatives that incorporate cyano groups (com-
pounds 4a–d), which is in agreement with their photophysical
behavior discussed above. The improvement in their fluores-
cence capacity was owing to the action of the cyano group
counteracting the charge separation as a result of the in-
creased electron-donor ability of the cyano substituent. The
effect of the unsaturation degree of the alkyl chains attached
to the boron atom, discussed in detail above, is also clearly in-
dicated by the lasing data. Thus, the change from acetylene
(compound 2b) to vinyl (5) and to ethyl (7), which produces
a reversal of the electronic behavior from electron donor to ac-
ceptor as well as an increased geometrical distortion in the ex-
cited-state geometry of those compounds, takes place in
accord with a decrease in the lasing efficiency following the se-
quence Eff(2b)>Eff(5)>Eff(7) (Table 2).
Lasing properties
Liquid phase
According to their absorption properties, the lasing properties
of PM546 and its derivatives 1a and 2a were studied under
pumping at 355 nm, whereas all the other dyes studied here
were pumped at 532 nm. The dye concentrations in the lasing
studies are in the millimolar range, as required by our experi-
mental conditions of transversal excitation and strong focusing
of the incoming pump radiation. In this way, the pump radia-
tion is totally absorbed within the first millimeter of the solu-
tion to obtain an emitted laser beam with a near-circular cross-
section.
To optimize the laser action, for the different dyes we first
analyzed the dependence of their laser emission on dye con-
centration in ethyl acetate. The results obtained by varying the
dye concentrations over the range 0.3 to 10 mm, depending
on the dye, while keeping all the other experimental parame-
ters constant, are collected in Table S2 in the Supporting Infor-
mation. In all cases the dyes followed the typical behavior,
with the lasing efficiency (defined as the ratio between the
energy of the dye laser output and the pump energy incident
on the sample surface) first increasing with dye concentration
until a maximum value is reached. Increasing the dye concen-
tration beyond this point results in a decrease in the lasing effi-
ciency that can be related to reabsorption/reemission process-
An important parameter for any practical application of dye
lasers is their lasing photostability under repeated pumping.
Table 3 collects the data on the decrease in the laser-induced
fluorescence intensity under transversal excitation of a capillary
that contains dye solutions in ethyl acetate (see the Experi-
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Table 3. Intensity of the laser-induced fluorescence emission (I) after 100000 pump pulses for dyes in ethyl acetate solution.
lexc =355 nm
lexc =532 nm
PM546 1a 2a PM567 1b 2b 4a
6
7
PM580 1c 2c PM597 1d 2d 3a 3b 4b PM597 1e 2e PM605 4c PM650 4d
-8C9
I [%][a]
60
92 76
17
55 70 100 100 50
48
84 70
85
100 94 65 72 100 30[b] 100 100 20[c] 100
80
75
[a] I [%]=100 (I/I0), with I0 being the initial intensity. [b] After 60000 pump pulses. [c] After 40000 pump pulses.
mental Section) after a given number (n) of pump pulses for
Table 4. Lasing properties of dyes incorporated into solid PMMA matri-
both the commercial dyes and their derivatives synthesized in
the present work. Dye PM546 and its derivatives 1a and 2a
were pumped at 355 nm and at 5 Hz repetition rate. All the
other dyes were pumped at 532 nm and at a 10 Hz repetition
rate. The pump energy was 5 mJ in all cases. Dyes 4a–c, which
exhibited the highest lasing efficiencies and the best photo-
physical behavior, also were the most photostable, with their
laser-induced fluorescence emission remaining at the initial
level after 100000 pump pulses. This stability is significantly
higher than that of the corresponding commercial dyes
(PM567, PM597, and PM605). Compound 4d, which is the de-
rivative of the meso-cyano-substituted dye PM650, exhibits
lower photostability than 4a–c, thereby reflecting the presence
of the ICT state in PM650 with its harmful effects on laser per-
formance and stability.[8]
ces.[a]
Dye
Eff [%]
ll [nm]
In [%]
n
PM567
4a
PM597
4b
PM605
4c
34
49
36
53
37
51
566
568
585
588
595
597
30
96
85
100
0
100000
100000
100000
100000
65000
80
100000
[a] Eff: lasing efficiency; ll: peak laser emission wavelength; In [%]: intensi-
ty of the laser output after n pump pulses in the same position of the
sample; In [%]=100ꢃ(In/I0), with I0 being the initial intensity.
grade, so that higher efficiencies are to be expected with laser-
grade surfaces.
To assess the photostability of the new materials, we fol-
lowed the evolution of their laser emission under repeated
pumping in the same position of the sample at a repetition
rate of 10 Hz. All three derivatives demonstrated a much
higher photostability than the parent dyes, with dye 4b, which
has a laser emission that remains at the initial level after
100000 pump pulses in the same position of the sample, ex-
hibiting the best behavior once again. These results show that
the newly developed C-BODIPYs are excellent candidates to
achieve SSDLs with greatly improved performance.
Bulk solid state
The excellent laser performance exhibited by the new dyes in
liquid solution led us to explore their behavior as photonic ma-
terials, either in bulk as solid-state dye lasers (SSDLs) or incor-
porated into thin-film structures. First, we analyzed the laser
behavior of the dyes incorporated into bulk solid samples cast
in a cylindrical shape, thereby forming rods 10 mm in diameter
and 10 mm in length (for more details, see the Supporting In-
formation). The dyes chosen to perform this study were 4a–4c
because of their excellent lasing properties in liquid solution,
and their behavior as SSDLs was compared with the corre-
sponding parent dyes PM567, PM597, and PM605, respectively.
As matrix material we chose poly(methylmethacrylate) (PMMA)
because it mimics ethyl acetate, the solvent in which those
dyes produced the highest lasing efficiencies (Table 2). In addi-
tion, PMMA has very high transparency, high laser damage
threshold, very good chemical compatibility with the chosen
dyes, and, most importantly from a practical point of view, the
final production costs would be low owing to its cheapness.
The dye concentration in the matrix was that which optimized
the lasing efficiency in ethyl acetate (Table S2 in the Support-
ing Information).
Thin films
As a proof of concept, we also assessed the laser properties of
the new BODIPYs operated as dye-doped polymer thin-film
lasers, since over the last few years there has been a growing
interest in the development of these types of devices because
of their potential applications as coherent light sources suita-
ble for integration in spectroscopic and sensing devices.[12]
Given the outstanding laser properties shown by derivative
4b both in liquid and the solid state, we decided to choose
this dye and its parent dye PM597 as reference to develop the
thin-film laser. The host matrix was PMMA. Thin films approxi-
mately 800–850 nm thick were prepared from solution by solv-
ing adequate amounts of dye and PMMA in chloroform, and
the resulting solutions were spin-coated onto quartz sub-
strates. The complete details on the sample preparation and
evaluation can be found in the Supporting Information.
Table 4 contains the lasing performance of the selected
dyes. The lasing efficiencies of the C-BODIPYs enhanced signifi-
cantly those obtained with the corresponding commercial
parent dyes, and correlate with those obtained in liquid solu-
tion, with the efficiency of dye 4b being the highest. These ef-
ficiencies are lower than those obtained in liquid solution,
probably owing to the fact that the finishing of the surface of
the solid samples relevant to the laser operation was not laser-
To obtain sufficient pump absorption and gain in the thin
films, dye concentrations were selected to render an absorb-
ance of 0.08 at the pump wavelength (532 nm). This means
concentrations of 19 and 23 mm for PM597 and derivative 4b,
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respectively. Such high concentrations are prone to give rise to
self-quenching[13] (mirrored as band displacements and reduc-
tion in fluorescence quantum yield and extinction coefficients)
and aggregate formation[11] (appearance of new absorption
bands). To assess the extent of both phenomena, the absorp-
tion and emission spectra of both dyes were collected
(Figure 3). Even at these high dye concentrations, the absorp-
length (lDFB) in DFB lasers is given by the Bragg condition
m·lDFB ꢀ2neff·L, in which m is the diffraction order in which
the device operates, neff is the effective refractive index experi-
enced by the propagating mode, and L is the period of the re-
fractive index modulation (Figure 3b, inset). By working with
the second-order m=2, lDFB ꢀneff·L, the second-order diffrac-
tion provides in-plane feedback, and the laser light is diffracted
out of the surface of the film perpendicular to the plane of the
waveguide through first-order diffraction (Figure 3b, inset). The
fundamental transverse electric (TE0) propagating mode in
waveguides approximately 800–850 nm thick with a refractive
index of 1.4900 (PMMA), deposited onto a quartz substrate
(n=1.456), experiences an effective refractive index neff =
1.472.[15] By taking into account that the resonant wavelength
must be in the emission window of the selected dyes (lDFB
ꢀ590 nm), a substrate with modulation period L=400 nm
was used. For the present experiments, the quartz substrate
was scribed by e-beam lithography[16] with a squared grating
(Figure 3b, inset) of period L=400 nm, a duty cycle of 50%,
and a depth of 100 nm. These waveguides might sustain
a second propagating mode, the fundamental transverse mag-
netic (TM0) mode, which does not participate in the laser pro-
cess. We have excited the samples with the pump polarization
parallel to the grating grooves so that the transition dipole
moment of the emitting molecules and the grating wave
vector (which is perpendicular to the grooves and parallel to
the structure) fully overlap. This way the laser threshold of the
TE0 mode is maximally reduced.[17]
By pumping the devices well above the threshold (Ipump
=
130 kWcmÀ2), DFB laser emission with a linewidth of approxi-
mately 0.2 nm centered at 587.6 (PM597) and 588.5 nm (deriv-
ative 4b) was obtained (Figure 3a), which is in agreement with
the Bragg resonant wavelength expected for the chosen corru-
gated substrate and film thickness. The difference in the emis-
sion wavelengths is consistent with the slight changes in the
sample thicknesses (ꢀ Æ50 nm) owing to the deposition pro-
cess, which modifies the effective refractive index neff and, in
turn, the resonant wavelength.
Figure 3. a) Absorption (dashed line), fluorescence (lexc =500 nm, dotted
line), and laser spectra (solid line) of derivative 4b (23 mm) doped in PMMA.
b) DFB output intensity as a function of pump intensity from thin films of
commercial PM597 19 mm (filled circles) and derivative 4b (23 mm; hollow
circles) doped in PMMA. Inset: sketch of DFB laser. The arrows represent in-
plane feedback owing to second-order diffraction (solid arrows) and out-
coupled laser emission owing to first-order diffraction (dashed arrow).
Figure 3b shows the dependence of the DFB laser intensity
on the pump intensity (light/light curve) for the samples
doped with PM597 (filled circles) and derivative 4b (hollow cir-
cles). From this plot one can determine the laser thresholds,
which are the points in which there is a change in slope in the
light/light curve. From the data in Figure 3b it can be seen
that derivative 4b not only has a lower DFB laser threshold
than the parent dye (20 versus 55 kWcmÀ2) but presents
tion and fluorescence spectra are redshifted by 2.5 and 0.5 nm,
respectively, with respect to the ones registered with the
dilute solutions, whereas the band shapes are unchanged (i.e.,
there are no new spectral features; compare Figures 2a and
3a). Nevertheless, the extinction coefficients of PM597 and de-
rivative 4b are reduced from 7.7 and 6.7ꢃ104 mÀ1 cmÀ1 (2ꢃ
10À6 m in EtOAc) to approximately 6.2 and 5.2ꢃ104 mÀ1 cmÀ1
(ꢀ20 mm in PMMA), respectively. The fluorescence quantum
yield was not measured for the thin films, so it cannot be com-
pared with the one in dilute solutions, but it will be arguably
lower in the thin films. The previous facts indicate that dyes
PM597 and derivative 4b will suffer from a certain amount of
self-quenching (but no aggregation) in the thin films, but we
will see that it is not enough to prevent laser action from
taking place.
a
threefold enhancement in the output intensity when
pumped well above threshold (Ipump =130 kWcmÀ2), which is in
agreement with the results obtained in bulk and liquid media.
Conclusion
We have successfully designed and synthesized new E- and C-
BODIPYs through a straightforward protocol from the commer-
cially available BODIPYs. The experimental trends of the fluo-
rescence efficiencies with the electron-donor/-acceptor proper-
ties of the susbtituents (Hammet parameter) at the boron
To obtain resonant oscillation within the thin film, we made
use of distributed feedback (DFB) owing to Bragg scattering in
periodic structures (Figure 3b, inset).[14] The emission wave-
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Duarte), CRC, Boca Ratꢄn, 2009; d) I. D. W. Samuel, G. A. Turnbull, Chem.
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atom are properly rationalized by means of theoretical calcula-
tions in terms of the charge distribution and aromaticity (BLA)
and excited-state geometry. Accordingly, the replacement of
fluorine atoms by electron-acceptor moieties improves the
fluorescence performance of commercial BODIPYs, whereas
the electron-donor ones are harmful and lead to distorted geo-
metries that enhance the nonradiative relaxation pathways.
The newly synthesized E- and C-BODIPYs exhibit enhanced
laser action with respect to their parent dyes, both in liquid
and in the solid phase, with the lasing properties correlating
well with their photophysical behavior. The highest lasing effi-
ciencies, up to 73% in the liquid phase and up to 53% in solid
matrix under demanding transversal pumping conditions, were
obtained with the derivatives that incorporated cyano groups
at the boron center. These same derivatives also proved to be
the most photostable, with stabilities significantly higher than
that of the corresponding commercial dyes. Particularly rele-
vant was the behavior of dye 4b in solid matrix, which exhibit-
ed a very high lasing stability, with the laser emission remain-
ing at the initial level after 100000 pump pulses in the same
position of the sample at 10 Hz repetition rate.
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DFB laser emission was demonstrated with organic films
that incorporated parent dye PM597 and its derivative 4b, de-
posited onto quartz substrates engraved with appropriate peri-
odical structures. Derivative 4b exhibited a laser threshold
lower than that of the parent dye as well as lasing intensities
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These results show that the newly developed E- and
C-BODIPYs are excellent candidates to achieve SSDLs with
much improved performance.
Experimental Section
BODIPY dyes PM546, PM567, PM580, PM597, P597-8C9, PM605,
and PM650 were purchased from Exciton and used as received
(purity >99%). Experimental procedures, characterization, and
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lations, preparation of laser samples, and methods followed to ana-
lyze the photophysical and laser properties in the liquid and solid
phase are described in detail in the Supporting Information.
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This work was supported by the Ministerio de Economia y
Competitividad (projects MAT2010-20646-C04-01, MAT2010-
20646-C04-02, and MAT2010-20646-C04-04) and Gobierno
Vasco (project IT339-10), which is also thanked by I.E. for a con-
tract (SPE12UN140 and SPE13UN). G.D.-S thanks the MICINN
for a predoctoral scholarship (FPI, cofinanced by Fondo Social
Europeo).
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Keywords: donor–acceptor systems · dyes/pigments · laser
chemistry · photophysics · polymers
Received: September 10, 2013
Revised: November 18, 2013
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Published online on January 22, 2014
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