New analogues of the BODIPY Dye PM597: Photophysical and lasing properties in liquid solutions and in solid polymeric matrices

Article abstract of DOI:10.1021/jp902734m

New tailormade BODIPY dyes have been synthesized by a simple protocol to reach wavelength finely tunable laser action from 540 to 625 nm while maintaining highly efficient and photostable laser emission. The new dyes are analogues of the commercial dye PM

Full text of DOI:10.1021/jp902734m

8118
J. Phys. Chem. A 2009, 113, 8118–8124
New Analogues of the BODIPY Dye PM597: Photophysical and Lasing Properties in Liquid
Solutions and in Solid Polymeric Matrices
A. Costela,† I. Garc´ıa-Moreno,*,† M. Pintado-Sierra,† F. Amat-Guerri,‡ R. Sastre,§ M. Liras,|
F. Lo´pez Arbeloa,⊥ J. Ban˜uelos Prieto,⊥ and I. Lo´pez Arbeloa⊥
Instituto de Qu´ımica-F´ısica Rocasolano, Serrano 119, 28006 Madrid, Spain, Instituto de Qu´ımica Orga´nica
General, CSIC, Juan de la CierVa 3, 28006 Madrid, Spain, Instituto de Ciencia y Tecnolog´ıa de Pol´ımeros,
Juan de la CierVa 3, 28006 Madrid, Spain, UniVersidad Miguel Herna´ndez, Ferrocarril s/n, Edificio
TorreVaillo, 03202 Elche, Alicante, Spain, and Departamento de Qu´ımica-F´ısica, UPV-EHU,
Apartado 644, 48080 Bilbao, Spain
ReceiVed: March 26, 2009; ReVised Manuscript ReceiVed: May 22, 2009
New tailormade BODIPY dyes have been synthesized by a simple protocol to reach wavelength finely tunable
laser action from 540 to 625 nm while maintaining highly efficient and photostable laser emission. The new
dyes are analogues of the commercial dye PM597 with the eight position free (PTH8) or substituted by the
groups acetoxymethyl (PTAlk) or p-acetoxymethylphenyl (PTAr). The photophysical properties strongly depend
on the geometrical distortion from planarity of the indacene core generated by the presence of the bulky
2,6-di-tert-butyl groups and the eight substituent. In both liquid and polymeric solid solutions, lasing efficiencies
of up to 63 and 48%, respectively, were observed under transversal pumping at 532 nm with high
photostabilities. In the case of PTAlk incorporated into silicon-containing solid organic matrices, the laser
emission remained at 92% of its initial intensity value after 100 000 pumping pulses in the same position of
the sample at 30 Hz repetition rate. The laser action of the new dyes enhances that of the parent dye PM597
and outperforms the lasing behavior of dyes considered to be benchmarks over the green-yellow to red spectral
region.
1. Introduction
Modern biotechnological, optical, and electronic applications
require new fluorophores with predetermined properties such
as high photostability, high fluorescence quantum yield, large
Stokes shift, optimized absorption profile, and suitable anchoring
groups so as to meet the demands for more sensitive analytical
protocols, sensors, and light-emitting devices.1,2 4,4-Difluoro-
4-bora-3a,4a-diaza-s-indacene (BODIPY or BPD) dyes are
currently some of the most used chromophores as active media
for dye lasers because of their high lasing efficiency, low
intersystem crossing probability, and good thermal, chemical,
and photochemical stability.3-5 Several attempts have been made
to shift the emission of BDPs over the visible spectral region,
especially to the red, by substituting the chromophore core with
an aryl group at the eight (or meso) position or with aryl, styryl,
or ethynylphenyl groups at the 3,5-positions, or by introducing
a nitrogen atom at the eight position (8-aza-BDPs), by fusing
aromatic rings and so on, always trying to increase the
delocalization of the electronic π-system.5 These chemical
modifications can also induce new photophysical processes, such
as photoinduced electron transfer, allowing the application of
BDP dyes as molecular sensors in antenna systems or in light
harvesting arrays.6-10
Figure 1. Structure of the laser dyes PM597 and PM567.
substitution patterns, over the last few years we have studied
the effect of changing the 8-methyl group of the dye PM567
(Figure 1) by other groups while maintaining the rest of the
substituents.11 The presence of an acetoxy or a methacryloyloxy
group separated from the eight position by a polymethylene or
a poly-p-phenylene chain does not significantly modify the
photophysics of the chromophore, with regard to that of the
parent dye PM567, especially if the linear polymethylene chain
has five or more methylene groups.11b,d The new dyes, when
dissolved in a variety of organic solvents, lased more efficiently
than PM567 and, under continuous UV irradiation, demonstrated
improved photostabilty.11a,b In addition, when these analogues
of PM567 were either dissolved in poly(methyl methacrylate)
(PMMA) or covalently bound to the same polymer, they lased
more efficiently than the parent dye PM567 incorporated into
PMMA.11c,d
Looking for shifting this behavior to the red region of the
spectrum, a number of analogues of 2,6-di-tert-butyl-1,3,5,7,8-
pentamethyl-BDP, a laser dye commercially known as PM597
(Figure 1), have been synthesized following the above-described
strategy. The fluorescence and lasing bands of PM597 are
bathochromically shifted with respect to those of PM567.12
Despite having a lower fluorescence quantum yield than PM567,
PM597 exhibits high lasing efficiencies because its very high
To modulate to some extent the photophysical and lasing
properties of BDP dyes by the introduction of appropriate
* Corresponding author. E-mail: i.garcia-moreno@iqfr.csic.es. Tel: +34
91 5619400, ext. 1202. Fax: + 34 91 5642431.
† Instituto de Qu´ımica-F´ısica Rocasolano.
‡ Instituto de Qu´ımica Organica General.
§ Instituto de Ciencia y Tecnolog´ıa de Pol´ımeros.
| Universidad Miguel Herna´ndez.
⊥ UPV-EHU.
10.1021/jp902734m CCC: $40.75  2009 American Chemical Society
Published on Web 06/19/2009

Analogues of PM597 As Laser Dyes
J. Phys. Chem. A, Vol. 113, No. 28, 2009 8119
SCHEME 1: Reagents and Conditionsa
received. Other reagents were from commercial sources and
were used as received.
2.2. Preparation of Solid Polymeric Samples. Solid ma-
trices of PMMA or of the copolymers of MMA with HEMA or
with TMSPMA, incorporating PTAr, PTH8, PTAlk, or PM597
as true solutions, and the solid copolymer of PTArMA with
PMMA were prepared essentially as described elsewhere.11c An
adequate amount of the dye was dissolved in pure MMA or in
its mixtures with HEMA or TMSPMA, and the resulting
mixtures were polymerized by free radical bulk polymerization,
yielding the materials named dye/PMMA, dye/COP(MMA-
HEMA), dye/COP(MMA-TMSMA) (all true solutions), and
COP(PTArMA-MMA) (dye copolymer). The solid samples
were cast in a cylindrical shape, forming rods of 10 mm diameter
and 10 mm length. A cut was made parallel to the axis of the
cylinder to obtain a lateral flat surface of ca. 6 × 10 mm. We
prepared this surface and the ends of the laser rods for lasing
experiments by using a grinding and polishing machine (Phoenix
Beta 4000, Bu¨ehler) until optical-grade finishing. The planar
grinding stage was carried out with Texmet 1000 sand paper
using as abrasive 6 µm diamonds suspended in mineral oil. The
final polishing stage was made with a G-Tuch Microcloth, using
a cloth disk Mastertex with 1 µm diamonds in mineral oil.
2.3. Photophysical Properties. The photophysical properties
were registered in 2 × 10-6 M solutions in different solvents,
prepared by adding the corresponding solvent to the residue
from the adequate amount of a ca. 10-3 M stock solution in
acetone after vacuum evaporation of the solvent. UV-vis
absorption and fluorescence spectra were recorded on a Cary
4E spectrophotometer and on a SPEX Fluorolog 3-22 spectrof-
luorimeter, respectively. Fluorescence quantum yields (ΦF) were
evaluated from refractive-index-corrected spectra, using as a
reference a 10-6 M solution of PM597 in methanol (ΦF )
0.48).12 Radiative decay curves were registered with the time-
correlated single-photon counting technique (Edinburgh Instru-
ments, model FL920). Fluorescence emission was monitored
at the maximum emission wavelength after excitation at 410
nm by means of a diode laser (PicoQuant LDH410) with 150
ps fwhm pulses, 10 MHz repetition rate, and a power supply of
0.65 mW. The fluorescence lifetime (τ) was obtained from the
slope after the deconvolution of the instrumental response.
a (a) 1:2 molar ratio 2:1, CH2Cl2, Ar, reflux, 5 h, then r.t., Et3N, 30
min, then BF3 ·OEt2, 2 h, 23%; (b) PTAr: AcONa, Et3N, DMF, Ar, 40
°C, 24 h, 23%; PTArMA: methacrylic acid, K2CO3, Et3N, DMF, Ar,
40 °C, 24 h, 20%; (c) 1:4 molar ratio 1:1, CH2Cl2, Ar, r.t., 12 h, then
Et3N, 5 min, then BF3.OEt2, 1 h.
Figure 2. Structure of the monomers.
Stokes shift reduces the losses in the resonator cavity due to
reabsorption/reemission phenomena.12 Besides, PM597 is also
characterized by its high photostability, rendering efficient laser
signal after thousands of pumping pulses.13
In the present work, we have synthesized and characterized
new BDP dyes with the same substituents as PM597 in positions
one to seven but containing at position eight the groups
p-acetoxymethylphenyl (PTAr), p-methacryloyloxymethylphenyl
(PTArMA), hydrogen (PTH8), or acetoxymethyl (PTAlk)
(Scheme 1). PTAr is a model compound of the monomer
PTArMA. PTArMA has been covalently bound to the polymeric
chain of PMMA, rendering the solid copolymer COP(MMA-
PTArMA). This copolymer provides additional channels for the
dissipation of the excess absorbed energy; consequently, it was
expected that the bound chromophore could show increased laser
photostabilty.11c,d The analogue PTH8 was also studied to gain
deeper insight into the geometrical distortion from planarity of
the substituted indacene core induced by the 2,6-di-tert-butyl
and 1,7-dimethyl groups. The photophysical and lasing proper-
ties of the new analogues have been systematically analyzed in
air-equilibrated liquid solutions in several solvents as well as
in solid solutions of PMMA and in copolymers of MMA with
2-hydroxyethyl methacrylate (HEMA) or with the silylated
monomer 3-(trimethoxysilyl)propyl methacrylate (TMSPMA)
(Figure 2).11e,f
Quantum mechanical calculations were carried out in the
Gaussian 03 software.14 Ground state (S0) geometry was
optimized with the B3LYP method, whereas the first excited
singlet (S1) geometry was obtained by means of the CIS method.
In both cases, the double valence basis set was used. The spectral
transitions were simulated with the time-dependent (TD-B3LYP)
method: absorption via the Franck-Condon transition from S0
and fluorescence via the vertical transition from S1.
2.4. Laser Evaluation. For laser experiments, liquid solu-
tions of the dyes were contained in 1 cm optical-path quartz
cells that were carefully sealed to avoid solvent evaporation
during the experiments. Both the liquid-containing cells and the
solid polymeric samples were transversely pumped at 532 nm
with 5.5 mJ, 6 ns fwhm pulses from a frequency-doubled
Q-switched Nd/YAG laser (Monocrom OPL-10) at a repetition
rate of 10 Hz. Some samples were pumped at 30 Hz with 5.5
mJ, 10 ns fwhm pulses from a diode pumped, frequency
doubled, Q-switched Nd:YAG laser (Monocrom HALAZEN
532-12). The parallelism of the sample faces was controlled by
illuminating the samples with the beam of a He-Ne laser along
the axis of the cylinder and checking out the overlap of the
reflections from both faces. Details of the experimental systems
can be found elsewhere.11c
2. Experimental Methods
2.1. Materials. The dyes PM597, PM567, and sulfor-
hodamine B (all >99% purity, laser grade, from Exciton) were
used as received; its purity was confirmed by spectroscopic and
chromatographic methods. All commercial solvents used in
photophysical and laser experiments were of the highest purity
available and were used without further purification; those used
in synthetic work were purified by standard methods. The
monomer MMA (Merck) was successively washed with 5%
aqueous NaOH and water, dried over Na2SO4, and distilled
under reduced pressure. HEMA (Aldrich) was distilled under
reduced pressure before use. TMSPMA (Aldrich) was used as

8120 J. Phys. Chem. A, Vol. 113, No. 28, 2009
Costela et al.
We obtained narrow-line-width laser emission and tuning
ranges of liquid solutions of the dyes by placing the samples in
a homemade Shoshan-type oscillator11e consisting of full-
reflecting aluminum back and tuning mirrors and a 2400 lines
mm-1 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 88.5°. Laser line width was measured with
a Fabry-Perot etalon (IC Optical Systems) with a free spectral
range of 15.9 GHz.
3. Results and Discussion
3.1. Synthesis. The dye PTAr was synthesized in two steps
(Scheme 1): (1) The reaction of 3-tert-butyl-2,4-dimethylpyrrole
(1)15 with p-chloromethylbenzoyl chloride (2) yielded a sub-
stituted dipyrromethene that, after reacting in situ with boron
trifluoride diethyl etherate in the presence of triethylamine, under
conditions formerly used for the synthesis of similar dyes11d
produced the corresponding 8-p-chloromethylphenyl-BDP dye
3. (2) Intermediate 3 was converted to PTAr by reaction with
sodium acetate in dimethylformamide (DMF).11d PTArMA was
similarly synthesized by the reaction of intermediate 3 with
methacrylic acid in the presence of potassium carbonate in DMF.
PTAlk was obtained from the corresponding dipyrromethene
(synthesized from 1 and 2-acetoxyacetyl-4-tert-butyl-3,5-dim-
ethylpyrrole (4)), as before. PTH8 was isolated in the same
reaction, although with very low yield (2%). The alternative
synthesis of PTAlk by the reaction between 1 and acetoxyacetyl
choride in toluene, under the conditions used for the synthesis
of 3, gave rise to PTH8 with higher yield (14%), although only
traces (ca. 1%) of PTAlk were detected. For more details of
these syntheses, see the Supporting Information.
Figure 3. (A) Absorption and (B) fluorescence (normalized to their
fluorescence quantum yield values) spectra of (A) PTH8, (B) PTAlk,
(C) PTAr, and the parent dye PM597 (dashed line). 2 × 10-6
M
solutions in cyclohexane.
3.2. Photophysical Properties. The fluorescence bands of
PTAr, PTAlk, and PM597 are broad (Figure 3), whereas PTH8
shows a band with the typical vibrational structure of most BDP
dyes. The structureless emission bands can be ascribed to
geometrical changes of the indacene core, mainly in the S1
state.12 Fluorescence decay curves of all of these analogues are
monoexponential, with lifetimes in the range of 1.09-5.58 ns
(Table 1). In PM597, quantum mechanical calculations indicate
that the presence of the bulky tert-butyl groups at positions two
and six induces a loss of planarity of the indacene core, with
dihedral angles of the pyrrole units of ca. 3° in the S0 state12
(Figure 4, see also Table S1 in the Supporting Information).
The loss of planarity is even more important in the S1 state,
with a dihedral angle of ca. 10°, explaining the important Stokes
shift experimentally observed for this dye (ca. 1400 cm-1).12,16
Quantum mechanical calculations at the DFT level for these
analogues suggest that the disruption from the planarity of the
pyrrole rings depends on the eight substituent. Indeed, a similar
distorted geometry is theoretically obtained for PTAr and PTAlk,
whereas a nearly planar structure is predicted for PTH8, without
eight substituent, with a dihedral angle of the pyrrole rings of
ca. 0.5° in its S0 state and totally planar in the S1 state (Figure
4). The S1 planar structure of PTH8 leads to the observed
fluorescence band with vibrational structure and with a short
Stokes shift (190 cm-1), a value much lower than the shift
observed in the parent dye PM597.
TABLE 1: Photophysical Properties of PTAr, PTH8, and
PTAlk and of the Parent Dye PM597, all 2 × 10-6 M in
Cyclohexane, and Predicted Properties in Gas Phasea
data
PTAr PTH8 PTAlk PM597
experimental λabs ((0.1)/nm
530.5 533.0 555.0 529.0
εmax ((0.1)/104 M-1 cm-1
8.2
7.6
7.5
8.1
f ((0.01)
λflu ((0.2)/nm
∆νSt/cm-1
ΦF ((0.03)
τ ((0.05)/ns
kfl/108 s-1
knr/108 s-1
∆Eabs/eV
f
0.51
0.43
0.50
0.53
547.5 538.5 586.5
580 190 960
0.16 0.86 0.16
571.0
1395
0.32
1.32
1.21
6.36
2.86
0.46
5.58
1.54
0.25
2.89
0.51
2.88
1.82
1.09
1.46
7.70
2.71
0.46
3.91
0.81
1.74
2.89
0.52
2.72
1.61
predicted
∆Eflu/eV
kfl/108 s-1
a Absorption (λabs) and fluorescence (λflu) wavelength maxima,
molar absorption coefficient (εmax), oscillator strength (f), Stokes
shift (νSt), fluorescence quantum yield (ΦF), lifetime (τ), radiative
(kfl), and nonradiative (knr) deactivation rate constants and energy
gaps (∆Eabs and ∆Eflu).
groups, quantum mechanical calculations suggest that the
p-phenylene ring is twisted ca. 77° with respect to the inda-
cene core. Such a disposition minimizes the overlap between
the electronic π-clouds of both aromatic systems, avoiding the
formation of new absorption and fluorescence bands. It is likely
that the excitation of this distorted geometry does not modify
the geometry of the indacene core, explaining the relatively low
Stokes shift that is experimentally observed for PTAr (580
In PTAr, the deviation from the planarity in the S0 state
(dihedral angle of ca. 7°) is larger than that deduced for PM597
because of the additional steric hindrance between the p-
phenylene ring at position eight and the methyl groups at
positions one and seven. Indeed, and because of these methyl

Analogues of PM597 As Laser Dyes
J. Phys. Chem. A, Vol. 113, No. 28, 2009 8121
Figure 4. Theoretically optimized ground (S0) and excited (S1) state
geometry of PM597 and PTH8.
cm-1). PTAlk, with an 8-acetoxymethyl group, shows an
intermediate behavior with a moderate Stokes shift (ca. 960
cm-1).
In PTAlk, the 8-acetoxymethyl group induces a bathrochromic
shift of the absorption and fluorescence bands with respect to
PM597 (Figure 3). Similar results were previously observed for
related eight-substituted analogues of PM567,11b and they can
be ascribed to the electron-withdrawing effect of the acetoxym-
ethyl group that stabilizes the high electronic density at position
eight of the LUMO state with respect to the HOMO state.11b
When the acetoxymethyl group is separated from the indacene
core by a p-phenylene group, as in PTAr, this electron-
withdrawing effect is drastically reduced,17 and, consequently,
only minimum spectral shifts, with respect to PM597, are
observed in this dye (Figure 3). In this case, and as mentioned
above, the phenylene ring is disposed nearly perpendicular to
the indacene core, avoiding the overlapping of their respective
electronic clouds.17
Figure 5. Lasing efficiencies in ethanol of PTAlk (black bars) and
PTAr (ray bars) (upper figure) and laser emission spectra of PTAlk in
ethanol as a function of the dye concentration.
The fluorescence quantum yield and lifetime also show a
marked change with the eight substituent (Table 1). Indeed,
PTAr and PTAlk present lower fluorescence quantum yields
and lifetimes than their parent dye PM597, mainly because of
an increase in the nonradiative deactivation rate constant. The
opposite behavior is observed for PTH8 with much higher values
for the fluorescence quantum yield and the lifetime. In previous
work on the dye PM567 and its analogues,11b,d it was observed
that the inclusion of 8-acetoxymethyl or 8-p-acetoxymethylphe-
nyl groups induce a decrease in the fluorescence quantum yield.
In the case of the 8-acetoxymethyl analogue, this behavior was
ascribed mainly to a decrease in the radiative rate constant due
to the electron-withdrawing effect of the acetoxy group together
with an increase in the nonradiative rate constant. In the case
of the 8-p-acetoxymethylphenyl analogue, the observed increase
in the internal conversion processes could be explained as the
result of rotation/vibration of the pendant p-phenylene group.
Similar explanations can be applied to the PM597 analogues.
In the case of PTH8, the high fluorescence quantum yield (0.86,
Table 1) and lifetime (5.58 ns) values are typical for most BDP
dyes and can also be explained in terms of geometrical
parameters. The absence of any substituent at the eight position
in PTH8 drastically reduces the effect of the tert-butyl groups
at the two and six positions on the planarity of the indacene
core, favoring the radiative deactivation and reducing the internal
conversion processes.
protic solvents (methanol) is similar to that exhibited in the
nonpolar solvent cyclohexane, it is difficult to choose the best
media for achieving high lasing efficiencies.
3.3. Lasing Properties. 3.3.1. In Liquid Phase. The de-
pendence of the laser action of each PM597 analogue on the
dye concentration was analyzed in ethanol solutions with optical
densities (1 cm path length) in the range of 1.5-35.0 while
keeping constant all other experimental parameters. To assess
the effect of the eight substituent on the laser operation, the
lasing properties of solutions of PM597 were similarly studied.
As an example, the lasing efficiencies and peak wavelengths
of the emission of PTAr and PTAlk as a function of the dye
concentration are shown in Figure 5. As expected, the lasing
efficiency of all analyzed solutions increased significantly with
the dye concentration, reaching a maximum value for solutions
with an optical density of ca. 15. The highest lasing efficiency
was reached with the solutions with concentrations 5 × 10-4
M for PTAr and PTAlk and 6 × 10-4 M for PTH8 and PM597.
From this point on, further increases in dye concentration result
in a decrease in lasing efficiency, with the laser emission being
shifted to lower energies as the concentration increases. This
dependence must be related to reabsorption/reemission phe-
nomena on the laser emission intensity and could also be the
origin of the decrease in the lasing efficiency observed in more
concentrated solutions.18 Possible aggregation can be discarded
because the shape of the absorption bands does not depend on
the dye concentration.
The solvent effect on the photophysical properties of the
PM597 analogues (Supporting Information) is typical for most
BDP dyes: spectral shifts (ca. 5 nm) to lower energies in polar/
protic media and a low influence of the medium on the
fluorescence quantum yield and Stokes shift. Taking into account
the fact that the photophysics of the new analogues in polar
The actual effect of the solvent in the dye lasing emission
was analyzed in solutions with concentrations set at the values
that optimize the lasing action in ethanol. In all selected solvents,

8122 J. Phys. Chem. A, Vol. 113, No. 28, 2009
Costela et al.
TABLE 3: Laser Parameters of PM597 and Their
TABLE 2: Maximum Wavelength of the Laser Emission
(λlaser) and Energy Conversion Lasing Efficiency (eff) of the
New Analogues of PM597 and of the Parent Dye PM597 in
Several Solvents
Analogues Incorporated into PMMA, PHEMA, and into
Copolymers of MMA with HEMA or TMSPMAa,b
tuning
eff/% λlaser/nm I/% range/nm
dye
PTAr
PTArMA PMMA
PTH8
solid medium
PMMA
ethyl
dye
data CF3CH2OH MeOH EtOH acetone acetate cyclohexane
46
38
48
47
39
35
31
48
42
40
35
31
559
559
562
565
63 540-580
PTAra
λ
laser/nm
eff/%
laser/nm
eff/%
laser/nm
eff/%
laser/nm
eff/%
560
50
561
50
612
42
583
58
561
52
560
60
609
45
583
56
561
60
562
58
607
40
584
55
562
50
561
61
608
51
584
54
562
55
560
63
608
52
583
59
565
35
563
59
615
44
595
50
61
PMMA
85 540-585
88
PTH8b
λ
COP(MMA-HEMA 7:3)
COP(MMA-HEMA 5:5)
COP(MMA-HEMA 3:7)
PHEMA
566 100
PTAlka
PM597b
λ
567
568
606
605
606
609
609
65
37
λ
PTAlk
PMMA
98 585-625
COP(MMA-HEMA 7:3)
COP(MMA-HEMA 5:5)
COP(MMA-HEMA 3:7)
PHEMA
98
70
45
30
a Dye concentration: 5 × 10-4 M. b Dye concentration: 6 × 10-4 M.
the dyes emit laser light with high efficiency (Table 2) similar
to or even higher than that exhibited by the parent dye PM597
and other BDP dyes when pumped under identical experimental
conditions,11a,b with the lasing bands shifting toward lower
energies as the solvent polarity decreases.
COP(MMA-TMSPMA 9:1) 42
COP(MMA-TMSPMA 7:3) 42
606 100
607 100
PM597 PMMA
52
580
88 555-580
a As defined in Table 2; I: intensity of the dye laser output after
100 000 pump pulses in the same position of the sample, referred to
the initial intensity I0, I ) 100(I/I0), at 10 Hz repetition rate and 5.5
Good correlations between the photophysical properties
(Table S2 in the Supporting Information) and the lasing
characteristics (Table 2) of the new dyes can be observed. The
maximum wavelengths of the lasing emissions and those of the
fluorescence emissions follow the same solvent and eight-
substituent dependence. In all of the selected solvents, the lowest
fluorescence quantum yields were recorded for PTAlk, which
are related to a lower lasing efficiency of this dye. However,
the low fluorescence quantum yield of PTAlk could, to some
extent, be compensated by its high Stokes shift, reducing the
losses at the resonator cavity by reabsorption/reemission effects
and giving rise to relatively high lasing efficiencies, similar to
those registered for the other BDP analogues. PTH8 shows both
the highest ΦF and the lowest knr values (Table 1), which are
nearly solvent independent, and, consequently, exhibits some
of the highest lasing efficiencies (Table 2). In addition, the lasing
efficiencies of PM597 and its analogue PTAr in cyclohexane
solution follow the same behavior as that exhibited by their
photophysical parameters: this apolar solvent impairs the lasing
efficiency, with respect to the values registered in polar nonprotic
and polar protic solvents.
3.3.2. In Solid State. The experiments were carried out
using samples with the dye concentration producing the
highest lasing efficiency in ethanol solution: (5 to 6) × 10-4
M, depending on the dye. MMA was chosen as the main
monomeric component of the formulations because this ester
mimics ethyl acetate, a solvent where all studied dyes give
rise to high lasing efficiencies. First, the dyes were incor-
porated as true solutions into the solid homopolymer PMMA
to establish the effect of the eight substituent on their lasing
action when pumped under the experimental conditions
described above. To put the present results into proper
perspective, the lasing parameters of PM597 in PMMA were
also measured under the same conditions (Table 3).
mJ pulse-1 b Same parameters for PTArMA covalently bound to
.
PMMA are also included.
We studied the lasing stabilities of the dyes in PMMA
solutions by following the evolution of the laser output as a
function of the number of pump pulses in the same position of
the sample at 10 Hz repetition rate. The photophysical data of
the dyes in different solvents, commented above, provide useful
information on the potential of the solid materials as laser media.
Both the lasing efficiency and the photostability in solid solution
are higher when the fluorescence quantum yield and the Stokes
shift are higher and when the nonradiative rate constant is lower.
Therefore, the shift to higher energies of the PTAr lasing band
with respect to that of PM597 significantly reduces the Stokes
shifts, increases the reabsorption/reemission effects, and drasti-
cally impairs the photostability.
Contrary to the behavior previously observed in other BDP
dyes,11d the covalent bonding of the monomeric dye PTArMA
to PMMA chains does improves neither the lasing efficiency
nor the photostability of the chromophore with respect to the
model dye PTAr dissolved in PMMA. It is very likely that the
extension and probability of the reabsorption/reemission effects
are favored by the low Stokes shift and could counteract the
improvement in the dissipation of the excess of absorbed energy
not converted into emission, provided by the covalent linkage
of the dye to the polymeric chain. According to this argument,
a higher Stokes shift should result in a higher photostability, as
is seen in the case of PTAlk/PMMA if compared with PTAr/
PMMA. Notwithstanding, PTH8, which in cyclohexane solution
presents the lowest Stokes shift but the highest fluorescence
quantum yield and the lowest nonradiative constant (Table 1),
exhibits in PMMA a high photostability, reflecting the difficulty
of understanding at the present time the detailed mechanism of
the emission process of these dyes in the solid state.
Broad-band and efficient laser emission, with beam diver-
gence of ca. 5 mrad and pulse duration of ca. 5 ns fwhm, was
registered from all the solid solutions. No significant differences
were observed in the wavelength of the maximum laser emission
of each dye between their liquid and solid solutions. The lasing
efficiencies of the solid materials, in the range of 31-48%, are
somewhat lower than those of the corresponding ethyl acetate
solutions. In this regard, it has to be taken into account that the
finishing of the surface of the solid samples relevant to the laser
operation was not laser-grade so that even higher lasing
efficiencies are to be expected with laser-grade surfaces.
To study the influence of the polymeric medium polarity on
the laser action of the dyes, we also prepared solid solutions of
the analogues PTH8 and PTAlk in copolymers of MMA with
different volumetric proportions of the more polar monomer
HEMA (Table 3). It is seen that an increase in the polarity of
the medium results in a slight hypsochromic shift of the laser
emissions. Lasing efficiencies and photostabilities are similar
or slightly lower than those obtained with the same dye dissolved
in the homopolymer PMMA. High proportions of HEMA impair

Analogues of PM597 As Laser Dyes
J. Phys. Chem. A, Vol. 113, No. 28, 2009 8123
Figure 6. Normalized laser output as a function of the number of pump
pulses at 30 Hz repetition rate in the same position of the sample for
PTAlk incorporated into (A) PMMA, (B) COP(MMA-TMSPMA 9:1),
and (C) COP(MMA-TMSPMA 7:3).
the laser action of PTH8 and, more significantly, that of PTAlk,
following in some way the same dependence of the photophysi-
cal properties of these dyes on the liquid solvent polarity.
Previous studies had revealed that the laser action of BDP dyes
in polymeric matrices was greatly enhanced by properly
incorporating monomers with silicon atoms in their structures.18
Therefore, trying to improve the laser action of the new dyes
in polymeric solid materials further, PTAlk, the dye with the
highest photostability in pure PMMA, was incorporated as true
solutions into copolymers of MMA with TMSPMA in volu-
metric proportions in the range of 10-30%. Taking into account
the fact that TMSPMA has one silicon atom per mole, the
concentration of silicon atoms in the solid copolymers varies
from 1% in COP(MMA-TMSPMA 9:1) to 4% in COP(MMA-
TMSPMA 7:3). The laser performance of PTAlk in the silicon-
containing organic matrices under 10 Hz pumping repetition
rate was similar to that obtained in pure PMMA homopolymer.
The accumulation of heat into polymeric solid-state dye lasers
increases significantly with the pumping repetition rate, resulting
in a decrease in the lasing stability.19 Consequently, we pro-
ceeded to pump at a 30 Hz repetition rate some of the PTAlk-
doped solid materials that previously exhibited good photosta-
bility when pumped at 10 Hz (Figure 6). Under 30 Hz, PTAlk
showed a steady decrease in the laser output with the number
of pump pulses, which is faster in the sample with lower silicon
content. The best overall laser performance was obtained with
PTAlk incorporated as solid solution into COP(MMA-TM-
SPMA 7:3), where the lasing efficiency was 42% and the laser
emission was highly stable, remaining at 90% of its initial laser
output intensity after 100 000 pump pulses at a 30 Hz repetition
rate in the same position of the sample. The lasing photostability
increases with the content of the silicon-modified monomer in
the matrix because of the improved thermal conductivity of the
copolymers with respect to the homopolymer PMMA.19
Figure 7. Modulated laser tunability of PTAr, PM597 and PTAlk in
ethanol. Similar results are reached in solid polymeric solutions.
with an efficiency of 32%, but the laser emission disappeared
after just 70 000 pump pulses. Consequently, the new analogues
of PM597 incorporated into polymeric matrices outperform the
laser action of dyes considered to be benchmarks over the
spectral region from the green-yellow to the red.
We determined the tuning capability of the new dyes, one of
the most important features of laser dyes, by placing their solid
and liquid solutions in a grazing-incidence grating tunable
resonator. Tunable laser emission with line width on the order
of 0.15 cm-1 and tuning range of up to 50 nm was recorded
(Table 3, Figure 7), so that the spectral region 540-625 nm
can be continuously covered with narrow-line-width and stable
laser radiation by using these dyes.
4. Conclusions
We have designed and synthesized tailormade BDP dyes for
applications that require absorption and emission in specific
regions of the visible near-IR spectral range. The new dyes are
analogues of the commercial laser dye PM597 with 8-hydrogen
(PTH8), 8-acetoxymethyl (PTAlk), or 8-p-acetoxymethylphenyl
(PTAr) groups instead of the 8-methyl group. The different
eight-substituents allowed us to analyze the influence of the
geometrical distortion of the indacene core, induced by the
presence of the 2,6-di-tert-butyl and 1,7-dimethyl groups, on
the corresponding photophysical and lasing properties.
To put the present results into proper perspective, the lasing
parameters of PM567 and sulforhodamine B, two well-known
dyes that lase at the same wavelengths as the herein studied
dyes, were also measured in solid solutions under comparative
conditions. PM567 in PMMA lased at 562 nm with an efficiency
of 39% and with low photostability, with only 30% of its initial
emission intensity remaining after 100 000 pump pulses at 10
Hz. Likewise, sulforhodamine B incorporated into the ho-
mopolymer PHEMA (the low solubility of this dye in PMMA
precludes its incorporation into this polymer) lased at 604 nm
In liquid solutions, the photophysical parameters depend on
the nature of the solvent in the same way as that previously
observed in other BDP dyes: hypsochromic shifts of the
absorption and fluorescence bands in polar/protic media and a
low dependence of the fluorescence quantum yield and Stokes
shift on the polarity of the media. Good correlations between
the photophysical properties of the new dyes in diluted solutions
and the lasing characteristics in more concentrated solutions have
been observed. The new BDP dyes show high lasing emission
efficiencies that are similar to or even higher than those exhibited

8124 J. Phys. Chem. A, Vol. 113, No. 28, 2009
Costela et al.
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The laser action of the new BDP dyes outperforms that of
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Acknowledgment. This work was supported by projects
MAT2007-65778-C02-01 and -02 of the Spanish Ministerio de
Investigacio´n, Ciencia e Innovacio´n (MICIN). M.P.-S. thanks
MICIN for a predoctoral scholarship (cofinanced by Fondo
Social Europeo). M.L. was a recipient of a Juan de la Cierva
contract (MICIN).
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Supporting Information Available: Synthesis and charac-
terization of compounds, dihedral angles of the optimized
geometries of PTAlk and PTAr, and photophysical properties
of the studied dyes in liquid solution. This material is available
free of charge via the Internet at http://pubs.acs.org.
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