Design and development of a new pyrromethene dye with improved photostability and lasing efficiency: Theoretical rationalization of photophysical and photochemical properties

Article abstract of DOI:10.1021/jo702346s

(Chemical Equation Presented) In an attempt to develop a photostable and efficient pyrromethene compound for use in liquid dye lasers, three congeners of the commercially available pyrromethene 567 (PM567) laser dye were synthesized and their photophysical properties, lasing efficiencies, and photochemical stabilities were studied. In general the presence of an aryl group at C-8 of the pyrromethene chromophore increased the photostability. One of the congeners possessing a C-8 trimethoxyphenyl moiety showed significantly improved lasing parameters than PM567. Compared to PM567, the photochemical stability of the new dye was 2-fold, while it showed an equivalent lasing efficiency to that of PM567 at a significantly lower concentration. The increased photostability of these new dye molecules could be explained by theoretical calculation on their capacity to generate singlet oxygen (¹O₂) and probability of reaction with ¹O₂. Our calculations were in agreement with the experimental results and indicated that a systematic design of new derivatives of pyrromethene chromophore might lead to improved laser dye molecules.

Full text of DOI:10.1021/jo702346s

Design and Development of a New Pyrromethene Dye with
Improved Photostability and Lasing Efficiency: Theoretical
Rationalization of Photophysical and Photochemical Properties
Soumyaditya Mula,† Alok K. Ray,‡ Manas Banerjee,§ Tandrima Chaudhuri,§
Kamalesh Dasgupta,‡ and Subrata Chattopadhyay*,†
Bio-Organic DiVision, Laser and Plasma Technology DiVision, Bhabha Atomic Research Centre, Mumbai -
400085, India, and Chemistry Department, Burdwan UniVersity, Burdwan - 713104, India
schatt@barc.goV.in
ReceiVed October 31, 2007
In an attempt to develop a photostable and efficient pyrromethene compound for use in liquid dye lasers,
three congeners of the commercially available pyrromethene 567 (PM567) laser dye were synthesized
and their photophysical properties, lasing efficiencies, and photochemical stabilities were studied. In
general the presence of an aryl group at C-8 of the pyrromethene chromophore increased the photostability.
One of the congeners possessing a C-8 trimethoxyphenyl moiety showed significantly improved lasing
parameters than PM567. Compared to PM567, the photochemical stability of the new dye was 2-fold,
while it showed an equivalent lasing efficiency to that of PM567 at a significantly lower concentration.
The increased photostability of these new dye molecules could be explained by theoretical calculation
on their capacity to generate singlet oxygen (1O2) and probability of reaction with 1O2. Our calculations
were in agreement with the experimental results and indicated that a systematic design of new derivatives
of pyrromethene chromophore might lead to improved laser dye molecules.
Introduction
Dipyrromethene-BF2 (PM) fluorophores (4,4-difluoro-4-bora-
3a,4a-diaza-s-indacene) (Figure 1) are highly fluorescent dyes
and used for such diverse applications as biolabels,1a artificial
light harvesters,1b sensitizers for solar cells,1c fluorescent
sensors,1d-k molecular photonic wires,1l,m and electron-transfer
reagents.1n Further, these dyes are also very useful for laser
applications since they have a low intersystem crossing (ISC)
rate and low triplet excitation coefficients over the laser spectral
region2a-f and often possess a triplet-triplet absorption coef-
ficient about one-fifth that of the rhodamine dyes.3a,b These are
ionic dyes exhibiting good solubility in many organic solvents
and even in methyl methacrylate (MMA) that is useful for solid-
state dye laser applications.4a-d Some of the PM dyes outperform
the widely used laser dye, rhodamine 6G (Rh6G), considered
FIGURE 1. Chemical structures of the PM dyes 1-4.
as the benchmark in lasing efficiency and photochemical
stability. The two major inherent deficiencies of the PM dyes
are their small Stokes’ shift and photochemical instability. The
former reduces the lasing efficiency of the PM dyes due to the
ground state absorption (GSA). The problem has been partially
* Fax: 91-22-5505151.
† Bio-Organic Division, Bhabha Atomic Research Centre.
‡ Laser and Plasma Technology Division, Bhabha Atomic Research Centre.
§ Burdwan University.
10.1021/jo702346s CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/27/2008
2146
J. Org. Chem. 2008, 73, 2146-2154

New Pyrromethene Dye with ImproVed Photostability
SCHEME 1a
a (i) (a) NaNO2/CH3CO2H/15 °C/overnight; (b) Acetylacetone/Zn/60 °C/1 h, (ii) KOH/ethylene glycol/160 °C/4 h, (iii) LiAlH4/THF/∆/4.5 h, (iv) (a)
RCHO/4-toluenesulphonic acid/CH2Cl2/25 °C/24 h; (b) DDQ/4 h; (c) Et3N/1 h; (d) BF3‚Et2O/25 °C/4 h.
resolved by incorporating simple or bicyclic rigid aryl substit-
uents at the C-3 center or the boron atom of the PM moiety.5a-d
Although the lasing action of the PM dyes has been the primary
focus of research,6a more effort is needed to enhance their
photostability, especially for commercial applications. The
photodegradation of the PM dyes is mediated by their photo-
chemical reaction with singlet oxygen.6b,c Some solid-state dye
laser materials containing dispersed PM dyes, developed with
the primary aim of improving the laser energy output,4b-e also
showed better photostability compared to liquid dye lasers.
Possibly, polymerization of the suitable monomer containing
the dispersed dyes provides the polymeric solid matrix free of
oxygen, increasing the photochemical half-lives of dyes.
However, the photochemical degradation continues to be a
hurdle in the long-term operation of the PM-based liquid dye
lasers, especially for high power and high-repetition rate
operation.7a,b
It was envisaged that suitable modifications of the substitution
pattern of the PM dyes might provide a photostable laser dye
with better spectral and lasing properties. Earlier, similar
approaches have been adopted to synthesize a host of analogues
of the most extensively used laser dye, PM567 (1), by
incorporating different substituents at C-8 and/or C-2 + C-6
positions of the pyrromethene moiety.2a,6c,d Among these,
PM650 (with a C-8 CN) showed a ∼50 times higher half-life
than 1.6c In contrast, the effect of alteration of the alkyl
substituents at the pyrrole rings was much less.6d Most
importantly, the fluoresecence quantum yields of these analogues
were too low to be used as laser dyes.2a,6c It was felt that a
systematic study on this aspect to improve the photochemical
stabilities of the PM dyes in solution is lacking. The primary
aim of the investigation was to develop a photostable and
efficient laser dye by rational design. To this end, we considered
1 as the model reference dye, synthesized its three congeners
2-4, studied their lasing property and photostability, and
rationalized the results in terms of their photophysical and
photochemical attributes as well as theoretical calculations.
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Kumar, S.; Mayekar, N. V.; Sinha, S.; Kundu, S.; Chattopadhyay, S.;
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Molecular Photochemistry; University Science Books: CA, 1991; p 588.
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B. D.; Harriman, A.; Ziessel, R. J. Am. Chem. Soc. 2006, 128 10231-
10239.
Results and Discussion
It is now well established6b-d that the triplet state of the
excited PM dye molecules generates singlet oxygen (1O2), which
reacts primarily at the C-7′-C-8 double bond of the dye
molecules. This produces an unstable peroxo compound,
eventually leading to the breakdown of the dye structure. The
involvement of the triplet state of the dyes in the process was
revealed from their fast decomposition in the presence of a triplet
sensitizer, like benzophenone (BP).6c Likewise, the enhanced
operating life of the deoxygenated dye solutions,4d and also in
the presence of 1 wt % of singlet oxygen quenchers, such as
Tin770, TBP,6d and especially DABCO,3b,6b confirmed the
1
participation of O2 in the dye degradation.
Thus, the photostability of the PM dyes might be increased
by reducing or eliminating their reaction with 1O2 and/or
reducing generation of 1O2. It was anticipated that steric
(6) (a) Rahn, M. D.; King, T. A.; Gorman, A. A.; Hamblett, I. Appl.
Opt. 1997, 36, 5862-5871. (b) Rahn, M. D.; King, T. A. Appl. Opt. 1995,
34, 8260-8271. (c) Jones, G. H.; Klueva, O.; Kumar, S.; Pacheco, D. Solid
State Lasers SPIE 2001, 24, 4267-4271. (d) Jones, G., II; Kumar, S.;
Klueva, O.; Pacheco, D. J. Phys. Chem. A 2003, 107, 8429-8434.
(7) (a) Assor, Y.; Burshtein, Z.; Rosenwaks, S. Appl. Opt. 1998, 37,
4914-4920. (b) Ray, A. K.; Kundu, S.; Sasikumar, S.; Rao, C. S.; Mula,
S.; Sinha, S.; Dasgupta, K. Appl. Phys. B 2007, 87, 483-488.
J. Org. Chem, Vol. 73, No. 6, 2008 2147

Mula et al.
FIGURE 2. Absorption and fluorescence spectra of the PM dyes 1-4 in ethanol. Thick and thin lines illustrate absorption and fluorescence
spectra, respectively. The spectra of different dyes are represented as (a) dye 1, (b) dye 2, (c) dye 3, and (d) dye 4.
TABLE 1. Photophysical Parameters of the Dyes 1-4 in Different
crowding at the C-8 position of the PM moiety would retard
the reaction with O2. With this aim, we synthesized three
Solvents
1
λabs
((0.2 nm)
λem
((0.3 nm)
PM567 congeners, 2-4 (Scheme 1), following a standard
protocol. Given that the pentamethyl-2,6-diethyl PM moiety is
known to show remarkably low T-T absorption,3b we expected
that a structural change at C-8 would not alter the lasing
characteristics of the PM dyes, but might increase their
photostability.
a
dye
solvent
Φfl
dye 1
methanol
ethyl acetate
TFE
517.0
517.0
518.0
518.0
522.0
522.0
521.0
523.1
522.0
522.0
522.0
524.4
527.0
527.0
528.0
528.0
533.0
533.0
533.1
534.2
535.1
535.6
534.5
536.3
535.4
533.6
535.6
535.4
535.5
535.4
536.1
536.5
0.91b
0.80b
0.97b
0.84b
0.89
0.79
0.95
0.78
0.88
0.77
0.92
0.76
1.03
0.86
1.09
0.85
ethanol
dye 2
dye 3
dye 4
methanol
ethyl acetate
TFE
Each of these compounds possesses a sterically bulky aryl
group at the C-8 position of the PM moiety. Among these, the
dyes 2 and 3 are known,8 while 4 is reported for the first time.
It is worth mentioning that the conversion of 7 to 8 can be
carried out more efficiently with LiAlH4 rather than the Wolff-
Kishner reduction, generally used for this purpose. In keeping,
compound 8 turned into a thick blackish liquid. Hence we used
it immediately for the subsequent steps, carried out in one pot.
Photophysical Characteristics. The photophysical param-
eters (longest wavelength absorption maxima (λabs), emission
maxima (λem), and fluorescence quantum yields (Φfl)) of the
dyes 2-4 relative to those of dye 1 in different polar protic
and aprotic solvents are presented in Table 1. Although the
absorption and fluorescence spectra of the dyes 1-4 were
recorded in different solvents, only those obtained in ethanol
are shown in Figure 2.
The PM dyes 1-4 in ethanol had intense S0 to S1 absorption
bands at 518-528 nm, each with a small fwhm that is
characteristic of the cyanine type of chromophores. Such dyes
are found to be of interest as optimizing chromophores for
organic photorefractive materials. The spectral properties of the
BODIPY chromophore had an exceptionally weak influence
from the solvent polarity. The shapes of the absorption spectra
of the dyes 2-4 were similar to that of 1, but the maxima were
bathochromically shifted by 5-8 nm. Among the dyes, com-
pound 4 showed sharper and stronger absorption and emission
spectra, reflecting a significantly high polarizability of the dye
chromophore due to the increased π electron cloud.
ethanol
methanol
ethyl acetate
TFE
ethanol
methanol
ethyl acetate
TFE
ethanol
a The fluorescence of the dyes 1-4 were measured at O.D. 0.1 at
excitation wavelength 490 nm. The fluorescence quantum yields of the dyes
2-4 refer relative to that of the dye 1. bThe reported8 data of 1 are presented
for comparison.
The fluorescence spectra of the dyes were also identical
throughout the series and entirely consistent with fluorescence
from the PM subunit.9 The spectra were almost mirror images
of the respective absorption spectra, with minimum energy loss.
However, compared to the λabs, the bathochromic shifts in the
fluorescence maxima of 2-4 from that of 1 were less (1-3
nm). These results confirmed the insignificant effect of the
introduction of the 8-aryl group on the photophysical properties
of the PM dyes.9 Compared to the dye 1, the relative
fluorescence quantum yields (Φfl) of 2 and 3 were slightly less
in all the solvents, while that of 4 was significantly more. The
dyes 2-4 showed maximum Φfl in trifluoroethanol (TFE) in
which the dye 1 also had maximum fluorescence quantum yield.
The dye 4 showed almost the same fluorescence quantum yield
as that of 1, although it was reduced in 2 and 3.
(8) Lopez, A. F.; Lopez, A. T.; Lopez, A. I.; Garcia-Moreno, I.; Costella,
A.; Sastre, R.; Amat-Guerri, F. Chem. Phys. Lett. 1999, 299, 315-321.
(9) Lo’pez Arbeloa, F.; Ban˜uelos, J.; Martinez, V.; Arbeloa, T.; Lo’pez,
Arbeloa, I. Int. ReV. Phys. Chem. 2005, 24, 339-374.
2148 J. Org. Chem., Vol. 73, No. 6, 2008

New Pyrromethene Dye with ImproVed Photostability
TABLE 2. Photophysical and Lasing Parameters of the Dyes 1-4 and Rh6G in Ethanol
λabs
((0.2 nm)
ꢀmax × 10-4
λfl
τ
% η
kr × 10-8
a
b
e
dye
(M-1 cm-1
)
((0.3 nm)
Φfl
((0.05 ns)c
((1.0)d
(s-1
)
dye 1
dye 2
dye 3
dye 4
Rh6G
518.0
523.1
524.4
528.0
530.0
7.1
5.8
6.3
10.4
9.7
534.2
536.3
535.4
536.5
556.0
0.84
0.78
0.76
0.85
0.95
6.59
5.84
5.51
6.98
4.50
47.4
36.8
37.2
45.7
24.6
1.27
1.34
1.38
1.22
2.11
a Extinction coefficients at respective λmax. bFluorescence quantum yield. cFluorescence lifetime. dMaximum lasing efficiency at optimized concentrations.
eRadiative rate.
FIGURE 4. Broad band lasing efficiency of the PM dyes 1-4 and
Rh6G in ethanol, obtained at their respective optimized concentrations.
The concentrations of the dyes 1-4 and Rh6G were 0.82, 0.71, 0.74,
0.30, and 0.17 mM, respectively.
FIGURE 3. Concentration-dependent broad band lasing efficiency of
the PM dyes in ethanol.
their smaller fluorescence quantum yields and radiative rates
(kr), all the PM dyes showed substantially higher lasing
efficiency than Rh6G. The higher laser efficiencies of the PM
Some other photophysical parameters of the dyes 1-4 as well
dyes may be attributed to their smaller excited-state (singlet-
as rhodamine 6G (Rh6G) in ethanol, which are relevant for their
singlet, triplet-triplet) and ground-state absorption cross sections
lasing property, are included in Table 2. Comparison of the
at the laser wavelengths.10 This is beneficial for power scaling
results of the reference dye 1 and the dyes 2-4 revealed
in the master oscillator-power-amplifier design of pulsed dye
interesting features. Incorporation of a phenyl substitution at
lasers. Due to steric reasons, the C-8 aryl group in the dyes
C-8 (dye 2) reduced the ꢀmax compared to that of 1, which was
2-4 would be placed perpendicular to the plane of the PM core
compensated by gradual introduction of a +R group (OMe).
(Vide infra). Further, free rotation of the aryl group would be
Thus, the ꢀmax of the dye 3 with one OMe group was more
severely constrained especially in 4 due to the 2,6-methoxyl
than that of 2, while the ꢀmax of 4 possessing three OMe groups
exceeded even that of both PM567 and Rh6G. Overall,
parameters such as a higher ꢀmax and a comparable Φfl of 4
Vis-a`-Vis 1 favored its good lasing efficiency at a lower
concentration, compared to 1. The radiative rates (kr) of the dyes,
groups in it. Such a molecular constraint is well-known to
enhance fluorescence and, in turn, the lasing efficiency as
observed in our studies.
The spatial profile of the lasing output for each dye was
circular at the respective maximum lasing efficiency. This would
ensure uniform propagation of the laser light over a longer
distance, which is beneficial for different types of laser
calculated using the standard expression kr ) Φfl/τ revealed a
maximum value (2.11 × 108) for Rh6G, while those for the
PM dyes 1-4 were much less ((1.22-1.38) × 108).
applications. One of the most striking observations was that the
Lasing Characteristics. The concentration dependent lasing
dye 4 (0.30 mM) showed almost equal lasing efficiency as that
efficiencies of the PM dyes in ethanol (Figure 3) followed an
expected pattern, initially increasing with the dye concentration
of 1 (0.82 mM). Besides being beneficial from the point of view
of economy, use of a smaller concentration of 4 would also
and reaching a maximum before decreasing again. Maximum
reduce the GSA losses at the laser frequencies, improving its
lasing efficiency values (η) of the dyes at their respective
optimum dye concentrations are shown in Figure 4 and also
listed in Table 2.
The lasing efficiency of 1-4 followed the same pattern as
that observed with their respective Φfl and ꢀmax values. This
suggested that incorporation of the C-8 aryl group had a similar
effect on their respective lasing efficiency. Thus, while the dye
lasing performance. Given that the ratio of the ꢀmax values of
the dyes 4 and 1 was ∼1.5, the former would have 1.5 times
more absorption at a comparable concentration. This partially
explains the better laser efficiency of 4 at a lower concentration.
In addition, a probable difference in the excited and ground
states absorption cross-sections of the 1 and 4 at their respective
lasing wavelengths might also contribute to this.
2 showed less efficiency compared to 1, introduction of one
OMe group (in 3) partially restored the efficiency, which was
nearly restored to that of 1 in 4 (three OMe groups). In spite of
(10) Liang, F.; Zeng, H.; Sun, Z. J. Opt. Soc. Am. B 2001, 18, 1841-
1845.
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Mula et al.
TABLE 3. Quantum Yield of Photodegradation of the PM Dyes
TABLE 4. Net Charges (Mulliken) on Different Atoms of the Dyes
1-4 at Their Respective Optimized Ground State Geometries,
Calculated by ab Initio Hartree-Fock (6-31G**)
1-4a
concn (M) Φ1
Φ2
Φ1 -1 of 2-4/ Φ2 -1 of 2-4/
pd
pd
pd
pd
dye
× 104
× 104 × 106
Φ1 -1 of 1
Φ2 -1 of 1
dye
C-1
C-2
C-3
N-3′
B-4
C-8
C-8′
C-9
pd
pd
dye 1
dye 2
dye 3
dye 4
1.4
1.6
1.5
1.2
1.166 16.30
dye 1 +0.03 -0.15 +0.35 -0.80 +1.13 +0.08 +0.18 -0.35
dye 2 +0.03 -0.16 +0.36 -0.80 +1.13 +0.02 +0.21 -0.06
dye 3 +0.03 -0.16 +0.37 -0.81 +1.15 +0.02 +0.21 -0.09
dye 4 +0.05 -0.17 +0.37 -0.82 +1.16 +0.05 +0.22 -0.20
0.383
0.427
0.585
2.73
5.04
9.11
3.04
2.73
1.99
5.97
3.23
1.79
a Different concentrations of the dyes were used to maintain the O.D.
values of their solution at ∼1 at their respective λmax
.
TABLE 5. Calculated Absorption Parameters of 1-4 Using CIS
and TDDFT Methods
CIS
TDDFT
Photostability Characteristics. The quantum yields of
photodegradation of the dyes 1-4 at different concentrations,
measured in air-equilibrated ethanol solution without or with
Es0fs1
(eV)
λabs
(nm)
Es0fs1
(eV)
λabs
(nm)
dye
f
f
100 mM DABCO (Φ1 and Φ2pd, respectively) are shown in
pd
dye 1
dye 2
dye 3
dye 4
3.434
3.400
3.378
3.338
361.0
364.7
367.0
371.4
0.9666
0.9685
0.9357
0.8788
2.815
2.774
2.774
2.769
440.0
447.0
447.0
447.6
0.2627
0.2088
0.2105
0.2375
Table 3. The enhancement in photostability (Φpd-1) of the PM
dyes 2-4 in ethanol, compared to that of 1 under the above
conditions, are shown in columns 5 and 6, respectively, of the
same table.
All the dyes 2-4 showed higher photostability than 1 (Table
3, columns 5 and 6), which was more in the presence of the
singlet oxygen quencher, DABCO. The effect of DABCO
further confirmed the role of O2 in the photodegradation of
the PM dyes.
The additional common photodegraded product (RT 6.73 min,
M+ 304) was assigned the structure 11 and produced by direct
scission of the C-8 substituents (Figure 5). Formation of
additional products 12a (M+ 94), 12b (M+ 124), and 12c (M+
184) from 2-4, respectively, which account for the C-8 aromatic
substituents also confirmed the direct photolysis of the dyes.
Theoretical Interpretations. The PM dyes 1-4 possess a
compact chromophore with large values of the integrated
absorption, which can be used to explore the electronic structure
of BODIPY by means of semiempirical calculations.11 In our
studies we analyzed the absorption characteristics and reactivity
of the PM dyes 1-4 with 1O2 by extensive theoretical
calculations to explain their lasing efficiency and photostability.
The ab initio MO calculations were carried out using the better
features of both Spartan’02 (Unix), Wavefunction, Inc. (USA),
2001 and Gaussian 03 (Unix), Gaussview-3, Gaussian, Inc.
(USA), 2003 softwares. The geometries of the dyes were
optimized in their respective ground states by the ab initio
Hartree-Fock method and 6-31G** basis set.
Analysis of the Mulliken charge distribution (Table 4) over
the atoms of the PM chromophore unit did not show much
variation in the electron densities at different atoms, except at
C-9. Changing of the C-8 methyl substituent of 1 to the phenyl
group led to an abrupt decrease in the electron density at C-9
in 2. This was, however, compensated by a gradual introduction
of the electron donating OMe groups in the dyes 3 and 4.
The change in electron density at C-9 appeared to have a
correlation with the change in broad band lasing efficiency of
the PM dyes. The boron atom plays a crucial role in bridging
the two pyrromethene units in the PM dyes, ensuring extended
conjugation that is essential for their lasing property. The Lewis
acidity of boron coupled with its size compatibility with the
pyrrole nitrogen atoms is responsible for this. Evidently, a higher
electron density at C-9 in 4, as found in our theoretical
calculations, would facilitate the extended conjugation explain-
ing its superior lasing efficiency.
1
Among the dyes, 2 showed the highest photostability, which
was gradually reduced on increasing the numbers of the OMe
substituents in the phenyl ring (dyes 3 and 4). However, even
then, the photostability of 4 was double that of 1, under
irradiation by a 532 nm pump. The effect of DABCO (100 mM)
on the photostablities of the dyes was different, being most
pronounced for 2 followed by 3. With 4, DABCO had a
marginal effect, suggesting the formation of an insignificant
1
amount of O2 during its photoexcitation. In separate experi-
ments, we have also confirmed the dye degradation by HPTLC
analyses of the individual dyes after irradiation with the 532
nm pump for 20 min (see Supporting Information, Table S1).
To confirm that the 1O2 or other active oxygen species (produced
during photoexcitaion of the PM dyes) reacts primarily at the
olefinic site and not at the aryl moiety at C-8, we also measured
the reduction potentials of the compounds 1 and 4 using cyclic
voltammetry (see Supporting Information, Figure S2 and Figure
S3). The lower reduction potential of the dye 4 (-1.236 V)
than that of the dye 1 (-1.363 V) confirmed that the photo-
oxidation of the PM dyes occurs overwhelmingly at the olefin
function. We did not observe any change in shape of the longest
wavelength absorption band (S0-S1) of the dye solutions, except
for the decrease in peak heights, during photoexposure. This
was expected considering that the degradation of the dyes at
the olefinic site produces nonfluorescent products, which would
not interfere with the spectral measurements of the dye solutions.
Characterization of Photodegradation Products. The HPLC
and LCMS analyses of the photodegradation products of the
PM dyes revealed that the process followed an identical pathway
irrespective of the nature of the C-8 substituent in them. All
the dyes gave three major common products as revealed from
HPLC (see Supporting Information, Figure S4 and S5). One of
the products (RT 7.02 min, M+ 187) was identified as 9, while
the other (RT 7.21 min, M+ 324) was attributed to 10. Both
The absorption parameters such as excitation energies (Es0fs1)
and oscillator strength (f) of the dyes were calculated at their
respective ground state geometries with the help of a singles
excitation method, CIS. The trend of bathochromic shifts in the
absorption maxima of 1-4, observed by experiments, was also
1
these products are formed by 1,2-cycloaddition of O2 at the
C-8 olefinic site of the PM dye structure followed by suitable
cleavage as shown in Figure 5. This is consistent with our
electrochemical results of the dyes and also that observed
earlier.6c
(11) Bergstro¨m, F.; Mikhalyov, I.; Ha¨gglo¨f, P.; Wortmann, R.; Ny, T.;
Johansson, L. B.-Å. J. Am. Chem. Soc. 2002, 124, 196-204.
2150 J. Org. Chem., Vol. 73, No. 6, 2008

New Pyrromethene Dye with ImproVed Photostability
FIGURE 5. Photodegradation products of the PM dyes 1-4.
TABLE 6. Ab Initio Calculated Molecular Parameters of the Dyes 1-4, 11, and 13
molecular
paramaters
dye 1
dye 2
dye 3
dye 4
11
13
E-HOMO (eV)
E-LUMO (eV)
band gap(eV)
µ0 (debye)
-6.8022
+1.0171
+7.8193
+4.5544
+4.5750
-6.7374
+1.0005
+7.7379
+4.8522
+4.9276
-6.7158
+0.9988
+7.7145
+5.8935
+5.9608
-6.7123
+0.9468
+7.6590
+5.0115
+5.0260
-6.8092
+0.9172
+7.7264
+4.0618
+4.0875
-8.2127
+4.7577
+12.9703
+0.0000
+0.0017
µ (S1, debye)
indicated by the CIS calculation (from 361 to 371 nm) (Table
5). The coefficients of leading configurations (HOMO f LUMO
values for all the dyes ∼0.9) indicated that, in all the dyes, the
CIS excited states arise from HOMO f LUMO transitions.
Compared to the experimental results, calculation by the CIS
method furnished shorter λmax values. However, this is due to
the shortcoming of the theoretical method, as reported earlier.12
A better program for calculating the transition energies is
the TDDFT method, employing an LSDA exchange-correlation
potential within the DFT model. Hence this was also used taking
the same basis set to calculate the transition energies. Among
the various available potentials and hybrid functionals (LSDA,
B3PW91, B3LYP, MPW1PW91, PBE1PBE), the LSDA per-
formed best. The blue-shifted absorptions (by CIS) were
corrected to a large extent but not completely by this program
(Table 5). The effect of solvent molecules was not considered
in the calculations, which might partially explain the differences
between the calculated and experimental values. The ZINDO
method, which is more commendable for this purpose, could
not be availed because it lacks parameters for boron within
Gaussian03.
Our calculations of the molecular parameters revealed a
minimum change of the dipole moments of the So and S1 states
of all the dyes explaining13a,b the weak solvatochromism. The
calculated dipole moments of the dyes 2-4 both in the ground
(µ0), excited singlet (µS1) states were more than that of 1 (Table
6), suggesting better polarizability of the chromophore and
associated higher lasing efficiency of the former dyes, as
(12) Stratmann, R. E.; Scuseria, G. E. J. Chem. Phys. 1998, 109, 8218-
8224.
(13) (a) Reichhardt, W. Solvents and solVents effects in organic chemistry;
VCH: Weinheim, 1988. (b) Liptay, W. Dipole Moments and Polarizabilities
of Molecules in Excited Electronic State; Lim, E. C., Ed.; Academic Press:
New York, 1974; Vol. 1, pp 129-229.
J. Org. Chem, Vol. 73, No. 6, 2008 2151

Mula et al.
1
3
TABLE 7. Ab Initio Calculated HOMO and LUMO Energies in O2 and O2
RHF-1O2
UHF-3O2
molecular orbital
HOMO
LUMO
1.3328
R-HOMO
-14.564
R-LUMO
â-HOMO
-16.2102
â-LUMO
Hartree-Fock ab initio
(6-31G**) orbital energy
eigenvalues (eV)
-12.2715
13.0781
3.5947
FIGURE 6. Chemical structures of the PM dyes 11 and 13.
observed by experiments. Admittedly, the experimental results
did not exactly match with the theoretical predictions but
provided a rationale to explain the better lasing efficiency of
the dye 4.
FIGURE 7. (a) XY and (b) XZ views of the energy minimized
structure of 4, as revealed from the Gaussian-03 calculation.
As a representative example, the geometry of the ground state
of 4, derived from ab initio calculations, is illustrated in Figure
7.
1
Considering the role of O2 in the photodegradation of the
PM dyes, the increase in their photostability can be attributed
to their reduced ability to react and/or generate 1O2. The
reactivity of the dyes toward 1O2 can be governed by steric and/
or electronic factors. Comparison of the HOMO and LUMO
The steric energies of the PM dyes, estimated from the
molecular mechanics MM2 calculation (Table 8), also revealed
maximum steric energy of 4, which was >10 kcal/mol than
that of 1. Due to the presence of the planer unsubstituted phenyl
group in 2, the steric interaction between the C-8 substituent
and the C-1 and C-7 methyl groups of the PM molecule is
expected to be minimum accounting for its lowest steric energy.
However, even then the H-2 and/or H-6 atoms of the C-8 phenyl
ring would cover both the faces of the reaction site (double
1
energies of the dyes (Table 6) and O2 (Table 7) revealed that
the reaction between the HOMO of dyes and LUMO of 1O2 is
more favored than that between the LUMO of the dyes and
HOMO of 1O2. Further, the band gap between HOMO-LUMO
of the dyes gradually decreased along the sequence, dye 1 to
dye 4 (Table 6). Thus, considering the electronic factor alone,
the theoretical calculation predicted the relative reaction prob-
1
bond) of the dye molecules, hindering their reaction with O2.
1
Overall, from a steric perspective the relative probability of the
ability of O2 with the dyes as 1 < 2< 3 < 4.
1
reaction with O2 would be dye 1 > dye 2 ≈ dye 3 . dye 4.
Regarding the photodecomposition of the compound 4, the
relative reaction probability of its constituent segments, the PM
core (11) and the aryl moiety (13) (Figure 6) with 1O2 was also
estimated. The energy gap LUMO (1O2) - HOMO (13) is
significantly higher (1.4035 eV) than that of LUMO (1O2) -
HOMO (11) (Table 6). This clearly revealed that the photo-
oxidation of 4 would be exclusively at the C-8 olefin function.
The theoretical calculated relative reaction probability of 1O2
with the dyes was in contrast to the experimental results and
indicated that the reaction probability might be controlled more
significantly by the steric parameters. Calculations based on
conformational analysis of the dye chromophores 2-4 revealed
that the plane containing the aryl groups to be perpendicular
with respect to the PM ring, suggesting an insignificant influence
of the electronic π-π resonance interactions between the C-8
aryl group and the PM moiety. This was also corroborated by
our experimental results on the photophysical properties of the
PM dyes. The structural disposition could be due to the
geometrical restrictions imposed by the C-1 and C-7 methyl
groups. It was found that the active double bond site of the PM
moiety in 2 and 3 is spatially crowded from both the sides by
the H-2 and/or H-6 atoms of the aryl groups in the dyes. The
crowding would be more severe in the case of 4, possessing
the bulkier OMe group in both these positions of the phenyl
moiety, reducing the probability of its reaction with 1O2 further.
This more or less correlated with our experimental results, which
showed the photodegradation trend as dye 1 > dye 4 > dye 3
≈ dye 2. The comparative lower stability of 4 Vis-a`-Vis that of
2 and 3 indicates a greater contribution of the electronic factor
for the former, as anticipated. Evidently, the electronic factor
contributes significantly with compound 4. This was expected
considering the higher calculated electron densities at its C-8
and C-9 positions (Table 4).
1
With regard to the generation of O2 by the PM dyes, the
calculated triplet excitation energies (ET) revealed maximum
(25.48 kcal/mol) and minimum (22.16 kcal/mol) values, re-
spectively, for the dyes 1 and 4 (Table 9). It is known that
molecular oxygen has a ground triplet state (3Σ) and an excited
1
singlet state comprising two very low-lying states (1∆ and Σ)
with T-S excitation energies as 22.50 kcal/mol and 37.5 kcal/
mol,14 respectively. Thus, sensitizers possessing excitation
energies ∼23 kcal/mol may be quenched via energy transfer to
1
1
produce O2 in its ∆ state according to:
Sens* + 3O2(3Σ) f Sens + 1O2(1∆)
1
In consideration of the ET values of the dyes, the yield of O2
generation would gradually fall from dye 1 to dye 4. It is
apparent that PM567 can very well generate 1O2, which, in turn,
2152 J. Org. Chem., Vol. 73, No. 6, 2008

New Pyrromethene Dye with ImproVed Photostability
TABLE 8. Steric Energies of the PM Dyes 1-4
(5.3 g) were added, and the mixture was stirred at 60 °C for 1 h.
The mixture was brought to room temperature and extracted with
CHCl3. Concentration of the extract in vacuo followed by crystal-
lization from CHCl3 gave 6 as a white solid (6.35 g, 80%). Mp:
dye
1
2
3
4
steric energies
(kcal/mol)
19.3909
13.0432
18.5264
29.9860
1
142 °C (lit.15 mp: 143-144 °C); IR: 3302, 1678, 1646 cm-1; H
NMR: δ 1.36 (t, J ) 7.1 Hz, 3H), 2.43 (s, 3H), 2.51 (s, 3H), 2.57
(s, 3H), 4.34 (q, J ) 7.1 Hz, 2H), 9.41 (broad s, 1H, NH); 13C
NMR: δ 12.5, 14.1, 14.6, 30.9, 60.2, 117.8, 123.1, 129.3, 138.9,
162.0, 195.4; MS (m/z): 209 (M+).
TABLE 9. Ab Initio Calculated Energies of the Singlet/RHF and
Triplet/UHF States
energy of 1S0,
RHF (au)
energy of 3T1,
UHF (au)
ET ) 3T1 - 1S0
(kcal/mol)
2,4-Dimethyl-3-acetyl-1H-pyrrole (7). A mixture of 6 (3.0 g,
0.014 mol) and KOH (1.5 g, 0.027 mol) in ethylene glycol (10
mL) was heated at 160 °C for 4 h. After cooling, the mixture was
extracted with CHCl3, and the extract was washed with water and
brine and dried. Removal of solvent gave pure 7 as a white solid
dye
E
E
dye 1
dye 2
dye 3
dye 4
-1028.883 301 35
-1219.400 211 71
-1333.280 880 29
-1561.044 900 43
-1028.842 699 36
-1219.364 167 78
-1333.244 973 94
-1561.009 581 82
25.48
22.62
22.53
22.16
1
(1.86 g, 97%). Mp: 96 °C; IR: 3300, 1680 cm-1; H NMR: δ
2.26 (s, 3H), 2.43 (s, 3H), 2.50 (s, 3H), 6.36 (s, 1H), 8.52 (broad
s 1H, NH); 13C NMR: δ 13.7, 15.2, 30.7, 115.0, 120.5, 120.7,
136.2, 195.9; MS (m/z): 137 (M+). Anal. Calcd for C8H11NO: C,
70.04; H, 8.08; N, 10.21. Found: C, 70.10; H, 8.02; N, 10.17.
2,4-Dimethyl-3-ethyl-1H-pyrrole (8). To a stirred suspension
of LiAlH4 (0.333 g, 8.76 mmol) in dry THF (20 mL) was added 7
(1.0 g, 7.30 mmol) in THF (10 mL). The mixture was refluxed for
4.5 h and brought to room temperature, and the excess hydride
was decomposed with aqueous saturated Na2SO4. The mixture was
extracted with chloroform, and the extract was concentrated in
vacuo. The residue was subjected to column chromatography
(neutral Al2O3, hexane-EtOAc) furnishing 8 as a brown liquid
(0.673 g, 75%). IR: 3377, 1689, 1644 cm-1; 1H NMR: δ 1.23 (t,
3H, J ) 7.4 Hz), 2.18 (s, 3H), 2.28 (s, 3H), 2.54 (q, J ) 7.4 Hz,
2H), 6.48 (s, 1H), 7.51 (broad s, 1H, NH); 13C NMR: δ 10.1, 10.9,
15.5, 17.3, 112.8, 117.4, 120.1, 123.1; MS (m/z): 123 (M+).
2,6-Diethyl-4,4-difluoro-1,3,5,7-tetramethyl-8-phenyl-4-bora-
3a,4a-diaza-s-indecene (2). A mixture of 8 (0.450 g, 3.7 mmol),
benzaldehyde (0.196 g, 1.85 mmol), and p-toluenesulphonic acid
(0.015 g) in dry CH2Cl2 was stirred at room temperature for 24 h.
DDQ (0.420 mg, 1.85 mmol) was added to the resulting deep red
solution, and stirring continued for 4 h. The mixture was treated
with triethylamine (0.5 mL) and stirred for another hour. Finally,
BF3‚Et2O (0.762 mL, 4.6 mmol) was added to the mixture in
portions during 4 h, and the solution stirred at room temperature
overnight. The resulting dark mixture was washed with aqueous
saturated NaHCO3, water, and brine and dried. Removal of solvent
in vacuo followed by column chromatography of the residue (silica
gel, hexane-EtOAc) furnished 2 (0.175 g, 24.9%), which was
recrystallized from hexane to afford orange needles. Mp: 184-
can react with the 1S0 state of the dye leading to its degradation.
Since the ET value of 4 was marginally less than that of
molecular oxygen, it would generate a minimum amount of 1O2,
although the possibility of 1O2 generation by 4 cannot be
completely excluded. Thus, the relative 1O2 generation capacities
of the dyes would be dye 1 > dye 2 > dye 3 > dye 4. The
marginal effect of DABCO in the photodegradation of 4 was
consistent with calculated values.
Overall, the better photostability of the PM dyes 2-4 Vis-a`-
Vis 1 can be attributed to the combined effects of lesser capacity
1
1
to generate O2 and probability to react with the O2. Among
the controlling factors for the photostability of the dyes, the
contribution of steric hindrance outweighs the electronic factor
favoring lower reactivities of the dyes 2-4, especially dye 4
1
with O2. In addition, the relative triplet state energies of the
dyes and excitation energy of 1O2 predict formation of a
1
minimum amount of O2 with 4. All these factors ultimately
make the dye 4 two times more photostable than dye 1.
Conclusions
Overall, we have developed a new pyrromethene laser dye 4
with improved lasing characteristics and photochemical stability
by a rational design. The synthesis is based on replacing the
C-8 methyl group with a suitable aryl group following a
convenient protocol. Considering the prohibitively high cost5a
of the PM dyes, the ease of preparation of the new dyes would
be very attractive for a wider availability and application.
Incorporation of a C-8 aryl group increased the photostability
of the PM molecules. It was shown that the associated reduction
in fluorescence and lasing efficiencies can be efficiently
modulated by introducing suitable electron donating groups. Our
theoretical calculations on the electron densities, energy levels
(HOMO, LUMO), and steric energies of the dye, as well as,
singlet oxygen generation capacity provided a rationale to
explain most of the experimental observations. The study
provided possible future directions to improve the design of
highly efficient PM dye molecules in terms of higher photo-
chemical stability for their applications in liquid dye lasers.
1
185 °C (lit.1g mp 185-186 °C); H NMR: δ 0.97 (t, J ) 7.6 Hz,
6H), 1.26 (s, 6H), 2.27 (q, J ) 7.6 Hz, 4H), 2.52 (s, 6H), 7.25-
7.29 (m, 2H), 7.45-7.48 (m, 3H); 13C NMR: δ 11.6, 12.5, 14.6,
17.1, 128.3, 128.7, 129.0, 130.8, 132.7, 135.8, 138.4, 140.2, 153.7;
MS (m/z): 380 (M+).
2,6-Diethyl-4,4-difluoro-1,3,5,7-tetramethyl-8-(4′-methoxyphe-
nyl)-4-bora-3a,4a-diaza-s-indecene (3) and 2,6-Diethyl-4,4-dif-
luoro-1,3,5,7-tetramethyl-8-(2′,4′,6′-trimethoxyphenyl)-4-bora-
3a,4a-diaza-s-indecene (4). Following the same procedure, the dyes
3 (0.094 g, 28%) and 4 (0.85 g, 22%) were prepared from 8 using
4-methoxybenzaldehyde and 2,4,6-trimethoxybenzaldehyde, re-
spectively.
3.10 Mp: 192 °C; 1H NMR: δ 0.97 (t, J ) 7.4 Hz, 6H), 1.32 (s,
6H), 2.29 (q, J ) 7.4 Hz, 4H), 2.52 (s, 6H), 3.87(s, 3H), 7.0 (d, J
) 8.6 Hz, 2H), 7.14 (d, J ) 8.6 Hz, 2H); 13CNMR: δ 11.8, 12.5,
14.6, 17.1, 55.3, 114.4, 127.9, 129.5, 131.2, 132.6, 138.4, 153.5,
160.0; MS (m/z): 410 (M+).
Experimental Section
1
Ethyl 4-Acetyl-3,5-dimethyl-1H-pyrrole-2-carboxylate (6). To
a stirred cooled (0 °C) solution of ethyl acetoacetate (4.9 g, 0.038
mol) in acetic acid (10 mL) was added a cold solution of sodium
nitrite (2.8 g, 0.041 mol) in H2O (10 mL). After stirring the mixture
for 24 h at 25 °C, acetyl acetone (3.8 g, 0.038 mol) and zinc dust
4. Mp: 157 °C; H NMR: δ 1.05 (t, 6H, J ) 7.6 Hz), 1.57 (s,
6H), 2.15 (s, 6H), 2.37 (q, 4H, J ) 7.6 Hz), 2.48 (s, 6H), 3.76 (s,
3H), 6.08 (s, 1H), 6.93 (s, 1H); 13CNMR: δ 9.4, 12.5, 14.6, 17.3,
55.3, 92.9, 118.5, 131.6, 132.4, 136.6, 154.7,161.5; MS (m/z): 470
(M+).
(14) Reference 4d, p 354.
(15) Shrout, D. P.; Lightner, D. A. Synthesis 1990, 1062-1065.
J. Org. Chem, Vol. 73, No. 6, 2008 2153

Mula et al.
Anal. Calcd for C26H33BF2N2O3: C, 66.39; H, 7.07; N, 5.96.
Found: C, 66.44; H, 7.14; N, 5.91.
Photostability Studies. The rate of photodegradation of the dyes
in ethanol was experimentally measured under irradiation from the
532 nm output of a pulsed Nd:YAG laser. The measurements were
carried out using 2 mL of the dye solutions in the absence4b of
lasing conditions, by removing the cavity mirrors. The pump beam
was directly incident on the dye solution, taken in the same dye
cell, without any focusing lens. In each case, the dye concentration
was adjusted such that the entire pump energy (∼6 mJ/pulse) was
absorbed by the dye solution over the irradiation period (∼1.5 h).
The change in absorbances of the dye solutions were periodically
measured using an absorption spectrophotometer to monitor the
progressive decrease in the dye concentration. The average power
of the incident pump radiation was measured by a power meter,
and quantum yields of dye photodegradation16 were calculated.
Photodegradation Products Analysis. 2 mL of ethanol solution
of each dye were irradiated by the 532 nm output of a pulsed Nd:
YAG laser (pump energy ∼6 mJ/pulse) in the absence4b of lasing
conditions, by removing the cavity mirrors. The pump beam was
directly incident on the dye solution, taken in the same dye cell,
without any focusing lens. The dye solutions were irradiated until
all the dye molecules degraded. Then HPLC and LCMS were done
with photodegradated dye solutions using a C18 column (250 mm
× 4.6 mm, 5µ) and an acetonitrile-H2O (80:20) mixture as solvent
at a flow rate of 0.5 mL/min. The products were detected at 254
nm.
Photophysical Studies. The absorption and emission spectra of
the dyes (∼10-6 M) in various solvents were measured using a 1
cm quartz cuvette. The fluorescence quantum yields (Φfl) of the
dyes 2-4 relative to that of the reference dye 18 and the molar
extinction coefficients (ꢀmax) were determined in ethanol. The
excited state (S1) lifetimes of 2-4 in ethanol were determined by
the time-resolved fluorescence measurements, carried out using an
LED based time-correlated single-photon-counting (TCSPC) spec-
trometer. The fluorescence decays were measured with a 490 nm
LED (1 MHz) excitation source and a TBX4 detection module
coupled with a special Hamamatsu PMT. The instrument response
function was 1.2 ns at fwhm. Following deconvolution analysis of
the fluorescence decays, the time resolution of the present setup
was ∼50 ps. All the measurements were carried out at ambient
room temperature (298 ( 1 K) using a microprocessor based
temperature controller.
Lasing Studies. The comparative lasing efficiencies of the dye
solutions were measured using a broad-band dye laser setup that
was transversely pumped by the second harmonic (at 532 nm) of
a Q-switched Nd:YAG laser, at a repetition rate of 10 Hz with
∼5.8 mJ pulse energy and ∼6 ns fwhm pulses. A schematic of the
dye laser set up is shown as Supporting Information (Figure S1).
To reduce the pulse to pulse fluctuations, the Nd:YAG laser was
operated at a higher pulse energy (∼100 mJ) by adjusting the
Q-switch timing appropriately. The pump energy was attenuated
by a combination of a half-wave and a polarizer plate. The pump
beam was divided into two parts by a 50% beam splitter. One part
was routed to the dye cell by a mirror, while the other part was
routed to a power meter for measuring the average power. The
individual dye solutions were taken in a 1 cm quartz dye cell (optical
path ∼6 mm along the dye laser axis) that was carefully sealed by
a stopcock to avoid solvent evaporation during experiments. The
dye cell was placed between a high reflectivity (>99%) mirror and
an output coupler (4% reflectivity, wedge angle ∼1°), and the dye
solutions were continuously stirred by a Teflon coated magnetic
bar to avoid local heating due to the pump beam. The dye cells
were suitably tilted along the laser axis to avoid dye cell window
lasing along the resonator axis. The pump beam (diameter ∼6 mm)
was focused by a cylindrical lens (focal length ∼15 mm) to form
a line image in the dye solutions. The broad band lasing efficiency
of each dye solution was determined as a function of the dye
concentrations, keeping other parameters, including alignment of
the cavity, unchanged. For this, different concentrations of the dyes
2-4, and the reference dyes, 1 and Rh6G in ethanol, were used
keeping the optical density at 1-5 at the pump wavelength (532
nm).
Electrochemical Studies. Cyclic voltammetry studies were
carried out in deoxygenated ethanol containing 0.1 M tetraethy-
lammonium iodide as the supporting electrolyte. The reduction
potentials of the compounds 1 and 4 were measured against a
standard Calomel electrode as reference at a scan rate of 100 mV
s-1
.
Acknowledgment. We thank A. K. Satpati of ACD, BARC
for his help in electrochemistry experiments.
Supporting Information Available: 1H and 13C NMR spectra
of the intermediates and dyes. Schematic diagram of the Nd:YAG
laser setup used for the lasing experiments. Cyclic voltammogram
of dye 1 and 4. HPLC profile of the photodegradated products of
the dye 1 and 4. HPTLC data for photodegradation of the dyes
and configuration coefficients of the dyes (S1 state) as calculated
by the TDDFT method. Computational Data. These materials are
available free of charge via the Internet at http://pubs.acs.org.
JO702346S
(16) Sinha, S.; Ray, A. K.; Kundu, S.; Sasikumar, S.; Nair, S. K. S.;
Dasgupta, K. Appl. Phys. B 2001, 72, 617-621.
2154 J. Org. Chem., Vol. 73, No. 6, 2008