Chlorinated BODIPYs: Surprisingly efficient and highly photostable laser dyes

FULL PAPER

DOI: 10.1002/ejoc.201200946

Chlorinated BODIPYs: Surprisingly Efficient and Highly Photostable Laser

Dyes

Gonzalo Duran-Sampedro,^[a]Antonia R. Agarrabeitia,^[b]Inmaculada Garcia-Moreno,*^[a]

Angel Costela,^[a]Jorge Bañuelos,^[c]Teresa Arbeloa,^[c]Iñigo López Arbeloa,^[c]

Jose Luis Chiara,*^[d]and María J. Ortiz*^[b]

Keywords: Dyes/Pigments / Laser dyes / Photophysics / Fluorescence / Halogenation

A series of mono- to hexachlorinated BODIPY dyes have

been prepared in good to excellent yields through the use of

N-chlorosuccinimide as an inexpensive halogenating rea-

gent. This library of chlorinated dyes allowed analysis in de-

tail, from the experimental and theoretical points of view, of

the dependency of the photophysical and optical properties

of the dyes on the number and positions of the chlorine sub-

stituents on their BODIPY cores. Quantum mechanical calcu-

lations predict the regioselectivity of the halogenation reac-

tion and explain why some positions are less prone to chlori-

nation. The new chlorinated BODIPYs exhibit enhanced la-

ser action with respect to their non-halogenated analogues,

both in liquid solution and in the solid phase. In addition,

chlorination is a facile and essentially costless protocol for

overcoming important shortcomings exhibited by commer-

cially available BODIPYs, which should favor their practical

applications in optical and sensing fields.

the manufacture of waveguides,^[5]light-emitting diodes

(OLEDs),^[6]photovoltaic cells,^[7]and electroluminescent de-

vices,^[6]in addition to the usual conventional applications

of organic dyes. These and other emerging uses are condi-

tioned by the emission wavelength, quantum yield, and sta-

bility of a given dye under the working conditions needed

for each specific application; these can be particularly de-

manding, as is often the case with the new imaging tech-

niques developed in optical microscopy that demand high-

intensity laser irradiation.

The photophysical properties of a BODIPY dye are

highly dependent on the substitution pattern of the ind-

acene core, which allows the design of new dyes with im-

proved optical properties ranging from the blue to the red

region of the spectrum by appropriate selection of the elec-

tronic properties and positions of the substituents.^[1]Core-

halogenated BODIPYs are highly versatile starting materi-

als for the modulation and optimization of these properties,

facilitating the introduction of chemical diversity in the in-

dacene nucleus through nucleophilic substitution of halo-

gen^[8]or through metal-catalyzed cross-coupling reac-

tions.^{[2e,3m,4m,8l–8m,8o–8q,9]}The chloro-BODIPYs have been

the derivatives most widely used for this purpose, allowing

the introduction of a wide range of C-, N-, O-, S-, Se-, and

Te-based substituents.^{[4k,8a–8n,8p,9b,9e,9g,9s]}Bromo- and iodo-

BODIPYs have attracted interest in their own right as pho-

tosensitizers for solar hydrogen production^[10]and as probes

for PDT.^[3]Iodine or bromine incorporation into the

BODIPY core enhances intersystem crossing to the triplet

excited state upon irradiation, as required for efficient sing-

let oxygen generation in PDT.

Introduction

In the past year alone, over 500 articles have been pub-

lished on the synthesis and applications of 4-bora-3a,4a-

diaza-s-indacene dyes, known as BODIPYs. The growing

interest in these versatile fluorophores is due to their favor-

able spectroscopic properties, characterized by high absorp-

tion coefficients, high fluorescence quantum yields, high

photostabilities, and low sensitivities to medium effects.^[1]

Currently, the development of new BODIPY dyes is driven

by their potential applications as sensors in biology and in

clinical diagnosis,^[1g,2]as photosensitizers for photodynamic

therapy (PDT),^[3]and as laser generators,^[1f,4]as well as for

[a] Instituto Química-Física “Rocasolano”, CSIC,

Serrano 119, 28006 Madrid, Spain

Fax: +34-915642431

E-mail: i.garcia-moreno@iqfr.csic.es

Homepage: http://ww w .iqfr.csic.es/ql/Web_QL/

Laser_material.htm

[b] Departamento de Química Orgánica I, Facultad de Ciencias

Químicas, Universidad Complutense,

28040, Madrid, Spain

Fax: +34-913944103

E-mail: mjortiz@quim.ucm.es

Homepage: http://ww w .ucm.es/info/quimorga/index_ENG.htm

[c] Departamento de Química Física, Universidad del País

Vasco-EHU, Facultad de Ciencias y Tecnología,

Apartado 644, 48080 Bilbao, Spain

[d] Instituto de Química Orgánica General, CSIC,

Juan de la Cierva 3, 28006 Madrid, Spain

Fax: +34-915644853

E-mail: jl.chiara@iqog.csic.es

Homepage: http://ww w .iqog.csic.es/iqog/investigador/jose-luis-

chiara-romero

Supporting information for this article is available on the

WWW under http://dx.doi.org/10.1002/ejoc.201200946.

Eur. J. Org. Chem. 2012, 6335–6350

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FULL PAPER

Conversely, in spite of the wide synthetic use of chloro-

The 2-mono-, 2,6-di-, 2,3,5-tri-, and 2,3,5,6-tretra-

BODIPYs, their photophysical properties have scarcely chlorinated derivatives 2, 3, 5, and 6, respectively, were pre-

been studied, possibly due to the expectation that chlorine pared by direct halogenation of BODIPY 1 with increasing

substituents should impact negatively on both stability and amounts of N-chlorosuccinimide (NCS) as halogenating

fluorescence, owing to their high chemical reactivity and to reagent (Scheme 2). Treatment of 1^[16]with NCS (2 equiv.)

the heavy atom effect.^[11]Chlorine has, however, been re- in THF at room temperature thus gave the 2-chloro-

ported to increase fluorescence in certain cases,^[12]including BODIPY 2. No other monochlorinated derivatives were

some very recent examples of BODIPY dyes.^{[8d,8j,8m,9g,13]}formed under the reaction conditions. Increasing the

Accordingly, in this work we have synthesized a series of amount of NCS to 3 equiv. afforded the 2,6-dichloro-

new BODIPY dyes bearing one to six chlorine atoms at BODIPY 3 as major product (85%). Although it was pos-

different positions on the boradiazaindacene core sible to obtain the 2,3,5-trichloro derivative 5 by further

(Scheme 1). This library of chlorinated BODIPYs allowed increasing the amount of NCS to 4 equiv., the yield was low

us to perform a detailed analysis, both from experimental (32%), due to the concomitant formation of other poly-

and from theoretical points of view, of the dependency of chlorinated products, including the 2,3,6-trichloro deriva-

the emitting properties of the dye on the number and posi- tive, that could not be isolated pure. When 10 equiv. of NCS

tions of chlorine substituents on the BODIPY core. In the were used, the 2,3,5,6-tetrachloro-BODIPY 6 was obtained

following section we describe the preparation of four dif- in 78% isolated yield. Use of larger amounts of NCS gave

ferent series of chlorinated BODIPY dyes together with a complex mixtures of polychlorinated products.

theoretical study of their structures and charge distribu-

tions, and finally we discuss the photophysical properties

and laser behavior of the new dyes.

Scheme 2. Synthesis of BODIPYs 2, 3, 5, and 6.

For the preparation of 3,5-dichloro BODIPY 4, we used

the synthetic route described by Dehaen and Boens,^[8b]

starting from the corresponding dipyrromethane derivative

18.^[17]A similar strategy was employed for the synthesis of

the hexachloro derivative 7 (Scheme 3), because this com-

pound could not be isolated by direct halogenation of the

Scheme 1. Core-chlorinated BODIPY dyes studied in this work and

BODIPY core, as mentioned above. Hexachlorination of

18, requiring the use of a large excess (14 equiv.) of NCS at

room temperature, was followed by oxidation of the chlor-

inated dipyrromethane with DDQ and subsequent treat-

ment with BF₃·OEt₂in the presence of triethylamine to af-

ford 7 in 30% overall yield.

starting BODIPYs (1, 8, 11, 15) used for their synthesis.

Results and Discussion

Synthesis

Direct halogenation of BODIPY 11^[4l]gave a complex

For the synthesis of the new chloro-BODIPYs we fol- mixture of products that was very difficult to purify. In this

lowed the three general strategies previously described for case, halogenation of the pyrrole precursor was more ap-

other halo-BODIPYs by electrophilic halogenation: a) di- propriate (Scheme 4). 2-Acetylpyrrole was treated either

rect halogenation of the unsubstituted BODIPY with 1.1 or with 3 equiv. of NCS in THF at room tempera-

dye,^{[3c–3k,3m–3n,8o–8q,9c–9d,9f,9h,9o–9r,10,13a,14]}b) halogenation ture to give 2-acetyl-5-chloropyrrole (19)^[8m,9e,18]or a mix-

of the dipyrromethane intermediate,^{[2e,8b,8e,8k,8n,9g,9l,9n,9t,15]}ture of 2-acetyl-3,5-dichloropyrrole (20) and 5-acetyl-2,3-

and c) halogenation of the pyrrole precursors.^{[8j,8m,8p,9a,9e,9n]}dichloropyrrole (21),^[18b]respectively, in moderate yields.

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Eur. J. Org. Chem. 2012, 6335–6350

Efficient and Highly Photostable Laser Dyes

Subsequent treatment of these pyrroles with 3-ethyl-2,4-di-

methylpyrrole and phosphorus oxychloride, followed by

treatment with BF₃·OEt₂in the presence of triethylamine,

yielded the chlorinated BODIPY dyes 12 (52%), 13 (54%),

and 14 (48%).

2-Chloro-BODIPY 9 and 2,6-dichloro-BODIPY 10 were

obtained in good yields (62–65%) by direct chlorination of

dye 8^[4k]with appropriate amounts of NCS (Scheme 5). A

similar procedure was used for the preparation of com-

pounds 16 (73%) and 17 (67%) starting from commercially

available dye 15 (PM546, Scheme 5).

Theoretical Study

We calculated the charge distributions (CHelpg) of the

new BODIPYs with the objective of understanding the re-

gioselectivities of the electrophilic halogenation reactions

(Table 1 and Figures S1–S4 in the Supporting Information).

The charge distribution map of, for instance, the parent

non-halogenated compound 1 indicates that the BODIPY

chromophore is most susceptible to electrophilic attack by

chlorine (Table 1) at the 2- and the 6-positions, which have

the highest negative charge, consistently with experimental

observations in the synthesis of mono- and dichlorinated

dyes 2 and 3.

Scheme 3. Synthesis of hexachlorinated BODIPY 7.

Table 1. Potential electrostatic map (red denotes negative and blue

positive charge) of compound 1 and CHelpg charge distributions

for the chromophoric carbon atoms susceptible to electrophylic at-

tack by chlorine in the ground states of its chlorinated derivatives

2–7. The full data, including the excited states, are listed in Fig-

ure S1 in the Supporting Information.

Scheme 4. Chlorination of 2-acetylpyrrole to generate BODIPY

dyes 12, 13, and 14.

Dye BODIPY carbon atoms

1

2

3

5

6

7

1

2

3

4

5

6

7

–0.109

–0.107

–0.135

–0.081

–0.078

–0.112

0.084

–0.180

–0.181

0.055

–0.201

–0.181

0.043

0.041

0.046

0.061

0.200

0.189

0.171

0.190

0.041

0.036

0.039

0.197

0.168

0.173

0.171

–0.179

0.064

0.061

–0.198

0.055

0.040

0.028

–0.111

–0.150

–0.131

–0.082

–0.129

–0.114

0.066

0.004

Upon substitution, the highly electronegative chlorine

atom perturbs the charge distribution in the BODIPY

framework, inducing a positive charge at the site of substi-

tution and a negative charge at the adjacent carbon. As a

Scheme 5. Synthesis of BODIPYs 9, 10, 16, and 17.

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FULL PAPER

result, the positions most susceptible to further electrophilic

The optimized geometries predict that the 8-tolyl ring

substitution are now expected to be carbons 1 and 7 in pref- will be twisted with respect to the nearly planar indacene

erence to 3 and 5, according to their calculated charges. core, hampering electronic coupling between their elec-

However, the opposite order of reactivity is observed for tronic clouds. In all of the derivatives 2–6, with free posi-

the chlorination reaction, probably as a consequence of the tions (1 and 7) adjacent to the aryl group, its rotation angle

steric hindrance introduced by the tolyl substituent on the is about 55°, whereas the incorporation of chlorine at those

flanking CH carbons 1 and 7. Similar regioselectivity orders positions (compound 7) induces enough steric hindrance to

have been observed for stepwise brominations^[8o]and iodin- twist the phenyl ring to a perpendicular disposition (89°)

ations^[3n]of the same or closely related 8-arylated with respect to the chromophoric plane. These theoretical

BODIPYs. In any case, use of large excesses of the haloge- gas-phase predictions are in good agreement with the exper-

nating reagents allows substitution at the 1- and the 7-posi- imentally obtained (X-ray diffraction, solid state) geome-

tions.^[8o]

tries depicted in Figure 1. Indeed, the measured rotation

angles are about 45° for compounds 5 and 6, and 86° for

derivative 7. This concordance indicates that the calculated

geometries are accurate.

Photophysical and Lasing Properties

Photophysics of Chlorinated 8-Tolyl-BODIPYs

The photophysical properties of this set of derivatives

(Table 2) are governed by the presence of the tolyl unit at

the meso position. In parent compound 1, the absence of

substituents at both ortho positions of the phenyl ring and/

or the corresponding adjacent positions of the chromo-

phoric system allows the unrestricted rotation of the aryl

group.^[19,20]Accordingly, the probability of internal conver-

sion processes is greatly increased due to vibrational cou-

pling, inducing drastic decreases both in fluorescence capa-

bility and in fluorescence lifetime, with respect to those ex-

hibited by the fully unsubstituted BODIPY (φ ≈ 0.90 and τ

≈ 7–8 ns).^[4o]In addition, the absence of the steric hindrance

that would be required to constrain the phenyl ring in a

perpendicular disposition could enable interaction with the

BODIPY chromophore, distorting its planarity and further

contributing to the nonradiative deactivation pro-

cesses.^[21,22]

Monochlorination of 1 at the 2-position and dichlorina-

tion at the 2- and 6-positions (dyes 2 and 3, respectively)

shift the spectral bands to lower energies and reduce the

Figure 1. X-ray structures of BODIPY dyes 5–7.

Table 2. Photophysical properties of 8-tolyl-BODIPY (1) and its chlorinated derivatives 2, 3, and 5–7 in a common solvent (cyclohexane).

The biexponential deconvolutions at the emission maxima are listed together with the amplitude-averaged lifetimes (ϽτϾ). The rate

constants are calculated from these last values. The full photophysical data in several media are collected in Table S1 in the Supporting

Information. The rate constants were calculated from the main lifetimes.

Dye

λ_ab

[nm]

ε_max(f)

m

λ_fl

[nm]

φ

τ

[ps]

ϽτϾ

[ps]

Ͻk_flϾ

Ͻk_nrϾ

[10^–4

^–1cm^–1]

[10^–8s^–1]

1

2

3

5

6

7

500.5

518.5

538.0

530.0

546.5

537.0

6.9 (0.42)

516.0

534.5

555.0

542.0

557.5

548.5

0.036

0.093

0.153

0.56

340

1.05

1.06

0.97

1.44

1.16

1.84

28.3

10.4

5.38

1.13

1.37

2.25

2285 (≈ 0 %)

845 (98%)

2255 (2%)

1320 (90%)

3860 (10%)

2120 (12%)

4110 (88%)

1180 (22%)

4720 (78%)

1500 (10%)

2550 (90%)

6.5 (0.42)

1.4 (0.11)

10.5 (0.50)

8.3 (0.54)

9.5 (0.46)

873

1574

3871

3941

2445

0.46

0.45

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Efficient and Highly Photostable Laser Dyes

absorption probabilities, particularly in the latter case. Such bonds (e.g., by C–Cl bonds) should favor fluorescence emis-

trends are attributable to the electron-withdrawing effect of sion.^[24]However, such effects are rather small (i.e., Ͻ10%)

the electronegative chlorine atom, which removes electron and cannot explain the dependence of the fluorescence ca-

density from the BODIPY core. The fluorescence quantum pacity on the position of the chlorine substituent in the

yields and lifetimes of the new dyes increase significantly BODIPY core. Charge transfer phenomena can also be

with chlorination (Figure 2) as a consequence of the in- ruled out because the photophysical properties are not very

duced drastic reduction in nonradiative deactivation pro- sensitive to the environment polarity (see Table S1 in the

cesses (Table 2 and Table S1 in the Supporting Infor- Supporting Information, in which the photophysical prop-

mation). In fact, the nonradiative rate constant decreases erties of the new dyes are analyzed as a function of the

by about 50% for each chlorine atom incorporated on the nature of the solvent). Moreover, electronic interaction be-

BODIPY core. Moreover, further chlorination at the 3- and tween the phenyl ring and the chromophore as a conse-

5-positions to afford the tri- and tetrachlorinated deriva- quence of the number of chlorine substituents can be dis-

tives 5 and 6 gives rise to a further and significant modifica- counted because the associated twist angles are almost the

tion of the photophysical behavior, revealing that halogena- same in the different derivatives (around 55°, except in the

tion gives entirely different results depending on the posi- case of the hexachlorinated compound 7, in which this an-

tions at which the chlorine atoms are incorporated. Chlori- gle is 89°) and also in the different simulated solvents. The

nation at the 3- and 5-positions thus leads to an important chlorination effect should therefore be related to its influ-

increase in the absorption probability (ε of dye 6 is six times ence on the free motion of the 8-tolyl group, which is the

higher than that of 3), fluorescence quantum yield (φ of dye key factor in the photophysics of this set of derivatives. A

6 is three times higher than that of 3), and fluorescence closer inspection of Table S1 in the Supporting Information

lifetime, once more due to a drastic decrease in the nonradi- reveals that the relationship between the fluorescence quan-

ative deactivation processes (Table 2 and Figure 2). In fact, tum yield and the degree of chlorination and anchoring po-

the increases in the fluorescence capacities of dyes 5 and sition on the core is mainly governed by the probability of

6 cannot be associated with increases in the radiative rate the nonradiative processes (internal conversion). The meso

constants, but with decreases in the nonradiative pro- position in compound 1 is characterized by a high elec-

cesses – Ͻk_nrϾ – varying from ca. 5ϫ10⁸s^–1in dichlorin- tronic density in the LUMO state and, consequently, it

ated 3 to ca. 1.2ϫ10⁸s^–1in tri- and tetrachlorinated 5 and should be very sensitive to substituent effects.^[4a]The re-

6 (Table 2). Similar photophysical properties were reported moval of electronic density from the central position bear-

by Boens et al. for compound 4, with chlorine substituents ing the tolyl substituent by the electronegative chlorine sub-

at the 3- and 5-positions.^[23]The same trends hold true for stituents should thus be beneficial for the fluorescence

the fully chlorinated derivative 7, although the spectral capabilities of the chlorinated derivatives because it should

bands are in this case no longer shifted to lower energies, significantly reduce the deleterious influence of the phenyl

in spite of the larger number of chlorine atoms on the free rotation on their photophysics. In this way, the chlor-

BODIPY core.

inated derivatives show behavior similar to that previously

To provide an acceptable explanation for the dependence observed for long-wavelength 8-tolyl-BODIPY dyes, in

of the photophysical properties of these BODIPY deriva- which electron density withdrawal from the chromophore

tives on the numbers and positions of chlorine atoms, some core by extended π-conjugation through 3- and 5-aromatic

considerations are to the point. Drexhage points out that substituents significantly reduced the influence of the free

C–H bond vibrations in the chromophore favor internal motion of the 8-tolyl group and led to highly efficient red-

conversion processes and that the replacement of such emitting BODIPYs.^[4k]Although this effect is already

Figure 2. a) Fluorescence spectra of 8-tolyl-BODIPY (1) and its mono- (2), di- (3), tri- (5), tetra- (6), and hexachlorinated (7) derivatives,

scaled by their fluorescence capabilities in cyclohexane, on excitation at the vibrational shoulder at lower energies. b) Corresponding

fluorescence decay curves of these dyes.

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achieved on incorporation of chlorine atoms at the 2- and BODIPY carbon atoms further separated from the chlorin-

6-positions of the BODIPY core (compounds 2 and 3), it ated one. As would be expected, the charge rearrangements

becomes more relevant in dyes 4–7, in which the 3- and 5- induced by chlorination increase with the number of chlo-

positions are also chlorinated.

rine atoms, always being more significant in the excited

This behavior is also confirmed by the biexponential states, in which the electronic motion is favored. In ad-

character acquired by the fluorescence decay curves (moni- dition, substitution at the terminal positions of the delocal-

tored at the maximum of the emission spectrum) with chlor- ized π-systems (the 3- and 5-positions) favors electronic de-

ination. Whereas the fluorescence decay of the parent dye localization, especially in the excited states (Figure 3 and

1 is essentially a monoexponential curve characterized by Supporting Information, Figure S1), efficiently removing

a very short lifetime (Ͻ500 ps), monochlorination (dye 2) the electronic density from the meso positions (in S₁from

already induces a biexponential decay, lengthening the short 0.135 in compound 1 to 0.270 in tetrachlorinated 6) and,

lifetime (ca. 800 ps) and giving rise to a longer one (ca. 2– consequently, significantly reducing the deleterious effect of

3 ns). The contributions and durations of this component the 8-tolyl motion on the photophysics. This “push-pull”

increase progressively with the number of chlorine atoms: effect^[3n,16]causes the significant increases observed both in

from ca. 10% and 3.8 ns, respectively, in dye 3 up to ca. the absorptions and in the fluorescence probabilities of dyes

80% and 4.7 ns, respectively, in tetrachlorinated dye 6. This 5–7. Such charge rearrangement upon excitation should be

biexponential deconvolution was confirmed at different the reason for the lack of correlation between the molar

emission wavelengths, covering the whole fluorescence spec- absorptions and the radiative deactivation rate constants.

trum (see Table S2 in the Supporting Information). If the

decay curves are monitored at the edges of the emission

spectrum, the contributions of the short lifetimes are in-

creased. Such biexponential behavior suggests the presence

of two interconverting excited states. According to pre-

viously reported theoretical modeling of the excited states

of 8-aryl-substituted BODIPYs, the phenyl ring can adopt

two different conformations in the exited state: twisted and

coplanar with respect to the BODIPY core, due to its unre-

stricted rotation.^[21,22]Unfavorable steric interactions in the

coplanar conformer cause an out-of-plane distortion of the

indacene framework, this being a non-fluorescence state.

The short (Ͻ1 ns) and the long (ca. 4 ns) components of

these lifetimes should hence be assigned to fluorescence

strongly reduced by the free motion of the 8-tolyl group,

which makes electronic coupling between the phenyl group

and the BODIPY core feasible, and to the normal emission

of the BODIPY dye, because it reaches values typical of

Figure 3. Potential electrostatic maps (red denotes negative and

blue positive) together with the CHelpg charge distributions in the

excited states for tetra- (6) and hexachlorinated (7) dyes. The whole

charge distributions in ground and excited states are listed in Fig-

ure S1 in the Supporting Information.

At this point, it should be noted that, according to theo-

these chromophores, respectively.^[4a]It seems that the emis- retical calculations, chlorination does not imply a signifi-

sion bands of the coplanar states (poorly fluorescent) cant modification in the dihedral angle of the 8-phenyl

should be broader than those of the twisted states (mainly group with respect to the BODIPY plane (Figure 3, 55°).

responsible for the fluorescence signal), explaining why the Therefore, the smaller influence of the free motion of the

contributions of the short lifetimes to the decay curves are phenyl group on the photophysics of the chlorinated deriva-

higher at the edges of the emission spectra. Similar trends tives is only due to the significant reductions in the elec-

have previously been reported for the decay curves of ana- tronic charge density at the meso positions. On the other

logues bearing iodine atoms.^[3n]In fact, the fluorescence hand, the incorporation of chlorine atoms at the 1- and 7-

capabilities of the chlorinated derivatives increase simulta- positions of the BODIPY framework, adjacent to the tolyl

neously with the contributions and lengthenings of this sec- group, has the opposite effect, concentrating electronic

ond components in the corresponding fluorescence decay charge around the meso positions and enhancing the influ-

curves (Figure 2), in accordance with lower populations or ence of aryl rotation. Indeed, a direct comparison of the

influence of the coplanar quenching excited states.

charge distributions of compounds 7 and 6 indicates that

The simulated charge distributions (CHelpg) of all these the electronic charge at the meso position is less positive in

chlorinated derivatives confirm the above hypothesis (Fig- the former (0.195 and 0.248, respectively, Figure 3). How-

ure S1 in the Supporting Information). In general, the pres- ever, in the case of dye 7, the steric hindrance induced by

ence of the highly electronegative chlorine atom induces a the chlorine atoms alters the orientation of the phenyl ring

charge redistribution in the BODIPY core, in which the car- with respect to the indacene plane, and places it nearly per-

bon bonded to chlorine acquires a strong positive charge pendicular (Figures 1 and 3, 86° from X-ray and 89° from

while the adjacent ones become more negatively charged theoretical calculations, respectively). In this arrangement,

and, consequently, the positive charges of the neighboring the phenyl free motion is hindered and dye 7 exhibits a fluo-

carbons are increased. Such effects are softened at the rescence capability similar to that of dye 6.

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Efficient and Highly Photostable Laser Dyes

Photophysics of Chlorinated Red-Emitting-BODIPY Dyes

ties of dyes 9 and 10 with respect to the non-chlorinated

dye 8. Moreover, the further shifts of the spectral bands to

lower energies upon chlorination imply that their excited

and ground states are energetically closer and that, accord-

ingly, the internal conversion processes are enhanced in

terms of the energy gap law, which works well for dyes emit-

ting in the red part of the visible.^[25–28]

The above hypothesis about the electron-withdrawing ef-

fect of chlorine on the free rotation of the 8-aryl group was

confirmed by analysis of the influence of chlorination on

the photophysics of the red-emitting BODIPY

8

(Scheme 5Ͻxshr5Ͼ), with the 3- and 5-positions substi-

tuted with p-formylphenyl groups.^[4k]In this derivative 8,

which also bears a tolyl group at the meso position, the

deleterious influence of this aryl free motion on the fluores-

cence emission is minimized by the delocalization of the π-

system through the aromatic substituents at the 3- and 5-

positions.^[4k]Consequently, the chromophoric electronic

density is removed from the meso position, explaining its

high fluorescence quantum yield in relation to that of par-

ent compound 1 (see the charge distributions in Figures S1

and S2 in the Supporting Information). Dye 8 was mono-

(compound 9) and dichlorinated (compound 10) at the 2-

and 6-positions (Scheme 5), as predicted by quantum me-

chanical calculations, which revealed that these positions

have the highest negative charges in the chromophore and

are hence the most susceptible to electrophilic attack (Fig-

ure S2 in the Supporting Information). The corresponding

photophysical data in a common solvent are collected in

Table S3 in the Supporting Information.

Photophysics of Chlorinated Alkyl-BODIPY Dyes

To gain further insights into the effect of chlorination on

the photophysics of BODIPY dyes, we studied cases of

alkyl-substituted BODIPYs. We selected the previously re-

ported compound 11^[4l](Scheme 1) and the well-known

commercial dye PM546 (15, Scheme 5).^[29]The correspond-

ing photophysical data in a common solvent are collected

in Table 3.

Table 3. Photophysical properties of compounds 11 and 15 and of

their mono- and dichlorinated derivatives (12–14 and 16–17,

respectively) in cyclohexane. The full photophysical data in all the

solvents studied are included in Tables S4 and S5, respectively, in

the Supporting Information.

Dye λ_ab

[nm]

ε_max(f)

[10^–4

λ_fl

φ

τ

k_fl

k_nr

m

^–1cm^–1] [nm]

[ns] [10^–8s^–1] [10^–8s^–1]

Figure 4 shows that chlorination of dye 8 induces further

shifts of its bands to the red, as well as a decrease in the

absorption probability in the dihalogenated derivative (from

3.3ϫ10⁴m^–1cm^–1in 8 to 1.1ϫ10⁴m^–1cm^–1in 10). More-

11

12

13

14

504.0 3.3 (0.22)

515.0 0.96 5.46 1.75

520.5 0.78 5.07 1.54

526.0 0.57 3.95 1.44

531.0 0.83 5.29 1.57

512.0 0.91 5.23 1.74

525.5 0.91 5.54 1.64

542.5 0.87 5.65 1.54

0.07

0.43

1.09

0.32

0.17

0.16

0.23

510.5 5.1 (0.33)

504.0 3.3 (0.27)

521.0 3.3 (0.24)

over, the fluorescence quantum yields also decrease (from 15 499.5 9.7 (0.50)

16

17

512.0 5.5 (0.26)

527.0 7.1 (0.34)

0.54 to 0.42 upon the first halogenation to 0.39 after attach-

ment of the second chlorine atom), through reductions in

the radiative deactivation rate constants and increases in

the internal conversion probabilities (Table S3 in the Sup-

In this set of derivatives the fluorescence decay curves

porting Information). In the previous set of derivatives 1– are satisfactorily analyzed as monoexponentials, thus cor-

7, the chlorine electron-withdrawing effect enhances the roborating the influence of the aryl ring on the appearance

fluorescence emission by decreasing the internal conversion of second exponentials for the derivatives 1–7 discussed

associated with the 8-tolyl motion. In the case of com- above. The reference compound 11 is characterized by an

pounds 8–10, however, such a nonradiative pathway is al- asymmetric substitution pattern of the pyrroles, one being

ready counteracted by the aromatic substitution at the 3- fully alkylated and the other fully unsubstituted. The incor-

and 5-positions, so the high electronegativity of the chlorine poration of chlorine at the 5-position (compound 12) in-

atom decreases the π-delocalization of the BODIPY creases the absorption probability, in accordance with the

through the 3- and 5-aromatic substituents, causing the ob- push-pull effect postulated above, but decreases the fluores-

served reductions in the absorption and emission probabili- cence capability of the dye. Both the reference dye 11 and

Figure 4. a) Absorption and b) fluorescence spectra of compound 8 and of its mono- (9) and dichlorinated (10) derivatives scaled by their

molar absorptions and fluorescence abilities, respectively, in cyclohexane.

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Figure 5. a) Fluorescence spectra of compound 11 and of its mono- (compound 12) and dichlorinated derivatives 13 and 14, scaled by

their fluorescence capabilities in cyclohexane. b) Corresponding fluorescence decay curves of these dyes.

its monochlorinated derivative 12 exhibit only slight sensi- lasing efficiency, and the lower the nonradiative rate con-

tivities to solvent (see Table S4 in the Supporting Infor- stant, the higher the lasing photostability.

mation), with the fluorescence quantum yields decreasing

Although the parent dye 1 does not lase, its chlorinated

only slightly on going from cyclohexane (0.96 for 11 and derivatives exhibit unexpected but highly efficient and pho-

0.78 for 12) to 2,2,2-trifluoroethanol (0.75 for 11 and 0.55 tostable laser emission centered at ca. 575 nm when placed

for 12). This could be attributable to the asymmetric substi- in a simple plane-plane nontunable resonator with a pump

tution patterns of the pyrroles, with the alkylated ones be- threshold energy of 0.8 mJ, a beam divergence of 5 mrad,

having as electron donors and the unsubstituted or haloge- and a pulse duration of 8 ns full-width at half maximum

nated ones as electron acceptors, giving rise to asymmetric (FWHM). The dependence of the laser action of these new

charge distributions between the pairs of pyrrole rings in chlorinated derivatives on the corresponding dye concentra-

the BODIPY frameworks that enhance nonradiative deacti- tions was analyzed in ethyl acetate solutions by varying the

vation processes mainly in polar media.

optical densities from 8 to 30, while keeping all other exper-

Further halogenation produces entirely opposite results imental parameters constant. As an example, the results ob-

depending on the positions at which the chlorine is at- tained for di- and trichlorinated dyes 3 and 5 are shown in

tached. Whereas chlorination at the 5- and 6-positions Figure S5 in the Supporting Information.

(compound 14) causes a larger shift to lower energies and

Using the dye concentrations that optimized the laser

a slight recovery of the fluorescence capability (Figure 5), efficiency of each derivative, we analyzed the dependence of

chlorination at the 5- and 7-positions (compound 13) in- their lasing properties both on the number and on the posi-

duces just the opposite effect (i.e., a shift to higher energies tions of the chlorine atoms attached to the chromophore.

and a further reduction in the fluorescence capability). Such The laser efficiencies increase significantly with increasing

opposed tendencies are mainly related to nonradiative pro- chlorination of the BODIPY core, from 30% in monochlor-

cesses (Table 3), because the asymmetry in the pyrrole inated dye 2 up to 60% in hexachlorinated dye 7 (Table 4).

charge distribution mentioned above (Figure S3) is more Consistently with the photophysical properties, chlorination

pronounced upon substitution at the 7-position than at the at the 3- and 5-positions (dye 4) enhances the laser effi-

6-position.

ciency with respect to that observed for dye 3, which is also

In the case of dye PM546 (15), monochlorination (com- a dichlorinated derivative but with different regiochemistry

pound 16) keeps the typical fluorescence quantum yield of (substituted at the 2- and 6-positions).^[23]

the BODIPY chromophore, with values very close to unity

The experimentally observable solvent effects on the dye

(Table 3 and Supporting Information, Table S5). Dichlori- laser action were analyzed in solutions of polar aprotic and

nation (compound 17) produces more pronounced electron polar protic solvents. No apolar solvents such as cyclohex-

removal from the chromophore and a consequent slight de- ane were used, due to the low solubilities of these new dyes,

crease in the fluorescence ability, in good agreement with which prevented the production of solutions with the high

previous results reported for BODIPYs bearing electron ac- dye concentrations required for laser operation under the

ceptor moieties.^[30]As discussed above, the 2- and 6-posi- pumping conditions selected in this work. Once again, pho-

tions are not very sensitive to substituent effects (Figure S4 tophysical and lasing properties are correlated: the higher

in the Supporting Information), so, apart from the expected the fluorescence quantum yields, the higher the lasing effi-

spectral shift to lower energies, the optimal fluorescence be- ciencies. The laser actions of these chlorinated dyes in polar

havior of this BODIPY is preserved upon chlorination.

solvents such as acetone and methanol are thus less efficient

than those registered in ethyl acetate. The p-tolyl chlorin-

ated derivatives lase with high efficiencies, independently

of the nature of the solvent (Table 4). These efficiencies are

Lasing Properties

The lasing behavior of the p-tolyl chlorinated dyes 2–7 quite surprising in view of the low fluorescence quantum

shows good correlation with their photophysical properties: yields exhibited by these dyes (e.g., dye 2 lases with an effi-

the higher the fluorescence quantum yield, the higher the ciency of 28% in methanol in spite of having a fluorescent

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Efficient and Highly Photostable Laser Dyes

Table 4. Lasing properties^[a]of the new chlorinated derivatives of dyes, maintaining their initial laser outputs without sign of

the p-tolyl BODIPY dye 1 in several solvents upon excitation at

532 nm.

degradation after 100000 pumping pulses at 10 Hz repeti-

tion rate even under the drastic pumping conditions se-

Dye

2^[b]

Data

EtOAc

MeOH

Acetone CH₂Cl₂

lected in this work. Once again, this laser behavior can be

explained well in terms of the photophysics, because chlor-

ination decreases the nonradiative rate constants signifi-

cantly (Table 2) and, consequently, increases the laser life-

times.

λ_l[nm]

Eff [%]

I [%]

λ_l[nm]

Eff [%]

I [%]

λ_l[nm]

Eff [%]

I [%]

λ_l[nm]

Eff [%]

I [%]

λ_l[nm]

Eff [%]

I [%]

λ_l[nm]

Eff [%]

I [%]

572

30

90

575

44

95

572

48

100

573

58

100

575

57

100

571

60

571

28

569

17

575

28

3^[c]

4^[d]

5^[e]

6^[f]

7^[g]

574

38

575

29

580

40

570

42

568

37

574

44

570

49

567

48

569

53

570

46

568

51

574

47

569

49

568

52

569

51

100

[a] Eff: energy conversion efficiency. λ_l: peak wavelength of the laser

emission. I: intensity of the dye laser output after 100000 pump

pulses with respect to its initial intensity I₀. I(%) = (I/I₀)ϫ100,

at 10 Hz repetition rate. [b] Dye concentration: 5ϫ10^–3m. [c] Dye

concentration: 9ϫ10^–4m. [d] Dye concentration: 9ϫ10^–4m.

[e] Dye concentration: 2ϫ10^–3m. [f] Dye concentration: 5ϫ10^–4m.

[g] Dye concentration: 8ϫ10^–4m.

Figure 6. Normalized laser-induced fluorescence emissions as a

function of the number of pump pulses for the dyes 1 (᭹), 2 (᭺),

and 6 (᭡) in ethyl acetate solutions. Pump laser wavelength, energy,

and repetition rate: 532 nm, 5.5 mJ per pulse, and 10 Hz.

quantum yield of only 0.044). This behavior might be re-

lated to the short fluorescence lifetimes observed in the

This enhancement of the laser behavior is also observed

chlorinated derivatives, which lead to radiative rate con- for all the other chlorinated BODIPY dyes 9 and 10 (de-

stants similar to those observed in other BODIPY rived from 8), 12–14 (derived from 11), and 16 and 17 (de-

dyes.^[4k,4l]

rived from 15), when pumped under identical experimental

The lasing photostabilities of these dyes in ethyl acetate conditions (Table 5). Dye 10, for instance, exhibits a laser

solutions were also analyzed by the protocol described in efficiency of 30% in ethyl acetate solution, which is a two-

the Exp. Section. To compare the effect of chlorination on fold increase with respect to its non-chlorinated parent dye

this laser parameter properly, the photostability of the par- 8. The effect of chlorination on the laser actions of dyes 11

ent dye 1 was also analyzed, because although it does not and 15 is outstanding. The low absorptions of parent dyes

lase, it was possible to follow the evolution of its laser-in- 11 and 15 at 532 nm preclude pumping at this standard

duced fluorescence as a function of the number of pump wavelength. In fact, dye 11 has to be pumped at its maxi-

pulses. The results are reported in Table 4 and, for purposes mum absorption wavelength (500 nm), increasing the tech-

of clarity, the actual patterns of the fluorescence emissions nological complexity of the experimental setup and there-

of dyes 1, 2, and 6 under pumping at a repetition rate of fore limiting its applicability. The commercial dye PM546

10 Hz are plotted in Figure 6. Once again, chlorination en- (15) has to be pumped at a blue-shifted wavelength

hances the performances of these dyes: even monochlorin- (355 nm), which is another standard wavelength used to in-

ated 2 is more photostable than the parent compound 1, duce laser emission from dye molecules. An advantage of

and the photostabilities increase with the number of chlorination of these dyes is the red-shifting of their absorp-

chlorine substituents in the BODIPY core. The tri-, tetra-, tion bands, allowing efficient pumping of the chlorinated

or hexachlorinated derivatives are thus highly photostable derivatives at 532 nm, in the case of dye 11, and even at

Table 5. Lasing properties of the new chlorinated BODIPY dyes in ethyl acetate solution pumped at 532 nm. For purposes of comparison,

the corresponding data for the non-chlorinated dyes 8, 11, and 15, evaluated under identical experimental conditions, are also included.

Dye 11 was pumped at 500 nm and dyes 15 and 16 were pumped at 355 nm (data in parentheses).

Laser data Dyes

8^[a]

9^[b]

10^[c]

11^[d]

12^[e]

13^[e]

14^[e]

15^[f]

16^[f(g)]

17^[h]

Eff [%]

λ_l[nm]

I [%]

14

615

50

28

612

85

30

618

100

(34)

(530)

(80)

38

561

100

46

566

100

36

562

100

(23)

(541)

(60)

40 (30)

558 (556) 562

90 100

51

[a] Dye concentration: 1ϫ10^–3m. [b] 8ϫ10^–4m. [c] 1ϫ10^–3m. [d] 6ϫ10^–4m. [e] 15ϫ10^–3m. [f] 25ϫ10^–3m. [g] 3ϫ10^–3m. [h] 3.5ϫ10^–3m.

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I. Garcia-Moreno, J. L. Chiara, M. J. Ortiz et al.

FULL PAPER

both standard wavelengths (355 and 532 nm) in the case tion of the exciting laser beam, the relative orientation of

of dye 15. The monochlorinated derivative 12 thus lases at the transition moments in the dye molecule for the pumping

561 nm with an efficiency of 38%, which increases to 46% and laser transitions, and the rotational diffusion-relaxation

for dichlorinated 13. Similarly, monochlorinated 16 exhibits time. Matching the polarization of the pump laser to the

a highly efficient laser emission in ethyl acetate when preferred polarization of the gain medium can enhance the

pumped either in the UV or in the visible spectral regions. photon efficiency of the system. In contrast, if the applied

Whereas PM546 lases at 541 nm with an efficiency of 23% field is not properly polarized or oriented with respect to

when pumped at 355 nm, its monochlorinated derivative 16 the molecular dipole moment, the observed response will

lases at 556 nm with an efficiency of 30% at the same be reduced. To this end, the modules and orientations of

pumping wavelength, which increases to 40% upon pump- the dipole moments of these molecules in the ground and

ing at 532 nm.

excited states and the corresponding transition moments of

Similar enhancement of the laser performances of dyes absorption and emission were calculated (see Figures S6–S9

8, 11, and 15 is also observed when the lasing properties of in the Supporting Information). The calculations showed

their chlorinated derivatives are analyzed as functions of that the transition moments of absorption and emission of

the solvent (see Table S6 in the Supporting Information). In the dye molecules studied are parallel to each other for exci-

the same way, the photostabilities of these dyes are signifi- tation in the visible region of the spectrum because the

cantly improved by chlorination, the effect on the behavior same electronic transition is involved. The dye laser emis-

of dye 8 being particularly remarkable; the laser output of sions were then measured as a function of the relative orien-

8 has decreased by 50% after 100000 pump pulses, but the tation between the polarization of the pumping radiation

presence of chlorine atoms is enough to maintain the laser and that of the dye molecules. The pump laser was horizon-

emission at 100% of its initial value under the same experi- tally or vertically polarized by placing a combination of po-

mental conditions.

larizer and λ/2 plate in the optical path. It was found that

Contrary to the behavior previously observed for the the tetrachlorinated dye 6, which has a high dipole moment

chlorinated derivatives of parent dye 1, the laser data for (11.230 D) totally aligned along (or parallel to) the trans-

chlorinated derivatives of 8, 11, and 15 do not correlate versal axis of the molecule, leads to a nonpolarized and

with their photophysical properties. In these dyes, the pres- highly efficient laser emission, regardless of the pump radia-

ence of chlorine induces significant increases in the laser tion being horizontally or vertically polarized.

actions (efficiencies and photostabilities) although, as dis-

With regard to dye 8, its monochlorination to give 9 al-

cussed above, it has a deleterious effect on the photophysi- lows a better alignment of the molecular dipole moment

cal properties (Tables S3–S5 in the Supporting Information) along the transversal axis, reducing the twist angle of the

with the fluorescence quantum yields decreasing and the dipole moment with respect to the axis induced by the p-

nonradiative rate constants increasing in relation to those formylphenyl substituent groups, which are disposed sym-

registered for the non-chlorinated parent dyes.

metrically, but slightly rotated with respect to the BODIPY

A careful analysis of the results recorded with all the core (Figure S8 in the Supporting Information) For this

chlorinated BODIPY derivatives synthesized in this work reason, dye 9 exhibits a nonpolarized laser emission with

suggests important questions about the influence of chlo- efficiency higher than that of its parent dye 8 and with a

rine on the laser action of these dyes related to the following higher fluorescence quantum yield.

facts: a) a dye, such as tetrachlorinated 6, with a fluores-

The influence of polarization on the laser actions of

cence quantum yield in ethyl acetate as low as 0.39, exhibits chlorinated dyes 12, 14, and 16 is more drastic, with dipole

a laser efficiency as high as 57%, b) a monochlorinated dye moments much lower and more rotated with respect to the

such as 12, with a fluorescence quantum yield of 0.64, transversal axis than those exhibited by the other chlorin-

which is higher than that of 6, exhibits a much lower laser ated derivatives discussed previously (Figure S8 in the Sup-

efficiency (38%), c) monochlorinated laser dye 16, with the porting Information). Consequently, their laser emissions

highest fluorescence quantum yield (0.94), is not the most are highly polarized and, although efficient, are not as high

efficient laser dye of the series, and d) the good correlations as could be expected for dyes with the highest fluorescence

established between the laser action and the photophysical quantum yields (0.78, 0.83, and 0.94, respectively).

properties of the chlorinated dyes derived from the parent

compound 1 are not adhered to by the chlorinated dyes inated dyes in the liquid phase led us to analyze the laser

derived from compounds 8, 11, and 15. behavior of some of them when incorporated into solid-

The encouraging results registered with these new chlor-

To gain deeper insight into the dependence of the laser state matrices. The experiments were carried out with sam-

behavior on chlorine substitution, we considered the effect ples with those dye concentrations that had the highest las-

of the polarization of the pump laser beam on the laser ing efficiencies in liquid solutions. MMA (methyl meth-

emissions of these new dyes, because the presence of an ele- acrylate) was chosen as the main monomeric component of

ment as electronegative as chlorine should induce high di- the formulations because this ester mimics ethyl acetate, a

pole moments, the orientations of which would depend solvent in which the studied dyes showed high lasing effi-

strongly on the number and positions of chlorine atoms on ciencies. Broad-band and efficient laser emission, with

the BODIPY core. In the case of a dye molecule, the influ- beam divergence of ca. 5 mrad and pulse duration of ca.

ence of polarization is determined mainly by the polariza- 5 ns FWHM, was registered for dyes 6 and 10 incorporated

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Efficient and Highly Photostable Laser Dyes

as true solutions into the solid homopolymer PMMA the pump laser, which significantly enhances the photon

(Table 6). No significant differences were observed in the efficiencies of the systems. In addition, chlorination is a fac-

wavelengths of the maximum laser emissions of each dye ile and essentially costless protocol for overcoming impor-

between their liquid and solid solutions. The dye-doped so- tant shortcomings exhibited by some commercial BODI-

lid matrices exhibit high photostabilities, analyzed by moni- PYs, such as low absorptions at standard laser pumping

toring the evolution of the laser outputs as a function of wavelengths and/or low photostabilities. In view of the easy

the number of pump pulses in the same position of the sam- synthetic assembly, and the large number of described

ple at a 10 Hz repetition rate. The lasing efficiencies of the BODIPY laser dyes, we are confident that this powerful

solid materials (28% for 6 and 30% for 10) are lower than approach can be extended to other dyes of this family, fur-

those of the corresponding liquid solutions. Surface fin- thering their practical application in optical and sensing

ishing of the solid samples in these experiments was not fields.^[31]

laser-grade, and higher lasing efficiencies would be expected

with improved polishing.

Experimental Section

Table 6. Laser properties of chlorinated BODIPY dyes 6 and 10 in

General: Starting materials and reagents used in the preparation

of BODIPYs are commercially available unless their synthesis is

described. The solvents were dried and distilled before use. Spectral

data for the known compounds were in accordance with the litera-

ture data. Flash column chromatography was performed with silica

PMMA pumped at 532 nm.

Dye

[Dye] (m)

Eff [%]

λ_l[nm]

I [%]

6

10

5.0ϫ10^–4

28

30

565

614

100

95

1.1ϫ10^–3

gel (Merck 60, 230–400 mesh). H and ¹³C NMR spectra were re-

1

corded with a Bruker Avance DPX 300 spectrometer (300 MHz for

¹H and 75 MHz for ¹³C) and a Bruker Avance III spectrometer

(700 MHz for ¹H and 176 MHz for ¹³C). All spectra were recorded

in CDCl₃. ¹H chemical shifts are reported in ppm relative to tet-

ramethylsilane (δ = 0.00 ppm) with use of the residual solvent sig-

nal as the internal reference. ¹³C chemical shifts are reported in

ppm with CDCl₃(δ = 77.67 ppm) as the internal standard. IR spec-

tra (cm^–1) were recorded with a Bruker Tensor 27 FTIR spectro-

photometer. Melting points were determined in open capillaries

and are uncorrected. High-resolution mass spectra were deter-

mined by ESI with a FTMS Bruker APEX Q IV instrument and

by EI with a Thermofisher MAT 95 XP.

Conclusions

We have prepared a series of new mono- to hexachlorin-

ated BODIPY dyes through three general synthetic strate-

gies based on regioselective electrophilic chlorination with

NCS as an inexpensive halogenating reagent. We have dem-

onstrated that it is possible to control the degree of chlor-

ination of the BODIPY core, which allows the synthesis of

mono-, di-, and polychlorinated derivatives.

Quantum mechanical calculations can predict the posi-

tions at which chlorine will be incorporated and explain

why some positions are less accessible to chlorination.

Moreover, the positions at which the halogen is attached

modulate the photophysical properties of the resulting

BODIPYs. The fluorescence capacity of 8-tolyl-BODIPY

or similar derivatives characterized by the presence of an

aryl ring with unrestrained rotation can be tuned by the

incorporation of appropriate substituents. In particular, the

introduction of electron acceptor chlorine substituents at

the 3- and 5-positions greatly increases the fluorescence

capability of this compound. From a photophysical point

of view, the chlorination of other BODIPYs leads to small

reductions in fluorescence ability, but high fluorescence

quantum yields are still obtained, making the use of these

compounds as laser dyes possible.

The new chlorinated BODIPYs exhibit enhanced laser

actions with respect to their non-halogenated analogues,

both in liquid solution and in the solid phase. In fact, under

demanding transversal pumping conditions, the presence of

chlorine substituents allows lasing efficiencies as high as

60% to be reached, with high photostabilities. The surpris-

ingly efficient laser actions from these newly chlorinated

BODIPYs can be directly related to the improvement in-

duced by chlorine in their photophysical properties and/or

in their molecular dipole moments, increasing the modules

BODIPY dyes 1,^[16]4,^[8b,23]8,^[4k]and 11^[4l]were synthesized by the

methods described previously. BODIPY 15 was purchased from

Lasing, S.A. and used as received.

General Procedure for the Chlorination of BODIPY Core and Acet-

ylpyrrole: NCS in dry THF was slowly added at room temp. to a

solution of starting material in dry THF and the reaction mixture

was stirred for 12–72 h under argon. The solvent was removed and

the mixture was diluted with EtOAc, washed with aqueous HCl

(10%) and H₂O, dried with MgSO₄, filtered, and concentrated to

dryness. The halogenated compounds were purified by flash

chromatography on silica gel.

General Procedure for the Synthesis of BODIPYs 12–14: POCl₃

(1.1 equiv.) was added to a solution of chlorinated 2-acetylpyrrole

(1 equiv.) in CHCl₃, and the mixture was stirred for 30 min at room

temp. 3-Ethyl-2,4-dimethylpyrrole (1 equiv.) in CHCl₃was then

added and the resulting solution was stirred for 12 h at room temp.

Triethylamine (1 equiv.) was added, followed by BF₃·Et₂O

(1 equiv.), and the mixture was stirred for 3–4 h before being

quenched with aqueous HCl (10%) and extracted with CH₂Cl₂.

The organic extracts were washed with water, dried with MgSO₄,

and filtered, and the solvents were evaporated to dryness. The dyes

were isolated and purified by flash chromatography on silica gel.

2-Chloro-4,4-difluoro-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene (2):

BODIPY 1 (50 mg, 0.18 mmol) in dry THF (20 mL) and NCS

(47 mg, 0.36 mmol) in dry THF (5 mL) were allowed to react for

12 h. Flash chromatography with hexane/EtOAc (98:2) afforded 2

(45 mg, 80%) as a green-orange solid; m.p. 171.8–172.0 °C. ¹H

and changing the orientations to match the polarization of NMR (700 MHz, CDCl₃): δ = 7.92 (s, 1 H, 5-H), 7.68 (s, 1 H, 3-

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6345

I. Garcia-Moreno, J. L. Chiara, M. J. Ortiz et al.

H), 7.39 (d, J = 8.4 Hz, 2 H, 2Ј-H), 7.28 (d, J = 8.4 Hz, 2 H, 3Ј- with H₂O, dried with MgSO₄, and filtered, and the solvents were

FULL PAPER

H), 6.95 (d, J = 4.2 Hz, 1 H, 7-H), 6.74 (s, 1 H, 1-H), 6.52 (d, J =

evaporated to dryness. Purification by flash chromatography on sil-

ica gel with hexane/EtOAc (99:1) as eluent yielded 7 (245 mg, 30%)

4.2 Hz, 1 H, 6-H), 2.41 (s, 3 H, CH₃) ppm. ¹³C NMR (176 MHz,

CDCl₃): δ = 147.8 (C-8), 145.8 (CH-5), 142.0 (C-4Ј), 139.7 (CH-3), as a maroon solid; m.p. 275.1–276.0 °C. ¹H NMR (700 MHz,

135.4 (C-7a), 133.2 (C-8a), 133.1 (CH-7), 130.6 (2ϫCH-2Ј), 130.5

(C-1Ј), 129.4 (2ϫCH-3Ј), 127.3 (CH-1), 121.5 (C-Cl), 119.3 (CH- H, 3Ј-H), 2.40 (s, 3 H, CH₃) ppm. ¹³C NMR (176 MHz, CDCl₃):

6), 21.5 (CH ) ppm. IR (neat): ν = 1548, 1405, 1364, 1259, 1105, δ = 143.7 (C-8), 142.5 (C-4Ј) 140.7 (C3-Cl and C5-Cl), 132.9 (C-7a

985 cm^–1. HRMS (ESI⁺): calcd. for [C₁₆H₁₂BClF₂N₂+ Na]⁺and C-8a), 130.1 (2 ϫ CH-2Ј), 127.7 (2 ϫ CH-3Ј), 127.6 (C-1Ј),

CDCl₃): δ = 7.26 (d, J = 8.4 Hz, 2 H, 2Ј-H), 7.06 (d, J = 8.4 Hz, 2

˜

3

339.0648; found 339.0628.

126.3 (C1-Cl and C7-Cl), 121.4 (C2-Cl and C6-Cl), 21.6

(CH ) ppm. IR (neat): ν = 1552, 1383, 1246, 1108 cm^–1. HRMS

˜

3

2,6-Dichloro-4,4-difluoro-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene

(3): BODIPY 1 (40 mg, 0.14 mmol) in dry THF (15 mL) and NCS

(56 mg, 0.42 mmol) in dry THF (5 mL) were allowed to react for

24 h. Flash chromatography with hexane/EtOAc (98:2) afforded 3

(39 mg, 85%) as an orange solid; m.p. 196.3–197.0 °C. ¹H NMR

(700 MHz, CDCl₃): δ = 7.73 (s, 2 H, 3-H, and 5-H), 7.37 (d, J =

8.4 Hz, 2 H, 2Ј-H), 7.29 (d, J = 8.4 Hz, 2 H, 3Ј-H), 6.79 (s, 2 H, 1-

H, and 7-H), 2.41 (s, 3 H, CH₃) ppm. ¹³C NMR (176 MHz,

CDCl₃): δ = 148.0 (C-8), 142.5 (C-4Ј), 141.8 and 141.7 (CH-3 and

CH-5), 133.6 (C-7a and C-8a), 130.6 (2 ϫCH-2Ј), 130.1 (C-1Ј),

129.6 (2ϫCH-3Ј), 128.6 and 128.5 (CH-1 and CH-7), 122.6 (C2-

(ESI^–): calcd. for [C₁₆H₇BCl₆F₂N₂+ CH₃OH – H]⁺518.8964;

found 518.8926.

2-Chloro-4,4-difluoro-3,5-bis(4-formylphenyl)-8-(4-tolyl)-4-bora-

3a,4a-diaza-s-indacene (9): BODIPY 8 (80 mg, 0.16 mmol) in dry

THF (15 mL) and NCS (43 mg, 0.32 mmol) in dry THF (10 mL)

were allowed to react for 48 h. Flash chromatography with hexane/

EtOAc (9:1) afforded 9 (55 mg, 65%) as a red solid; m.p. 182.4–

182.9 °C. ¹H NMR (700 MHz, CDCl₃): δ = 9.99 (s, 1 H, CHO),

9.95 (s, 1 H, CHO), 7.91 (d, J = 8.4 Hz, 2 H, 4-formylphenyl), 7.90

(d, J = 8.4 Hz, 2 H, 4-formylphenyl), 7.83 (d, J = 8.4 Hz, 2 H, 4-

formylphenyl), 7.79 (d, J = 8.4 Hz, 2 H, 4-formylphenyl), 7.44 (d,

J = 8.4 Hz, 2 H, 4-tolyl), 7.32 (d, J = 8.4 Hz, 2 H, 4-tolyl), 6.99 (d,

J = 4.2 Hz, 1 H, 7-H), 6.87 (s, 1 H, 1-H), 6.66 (d, J = 4.2 Hz, 1 H,

6-H), 2.44 (s, 3 H, CH₃) ppm. ¹³C NMR (176 MHz, CDCl₃): δ =

191.9 (CHO), 191.7 (CHO), 158.9 (C), 151.3 (C), 146.2 (C), 141.8

(C), 137.5 (C), 137.4 (2 C), 136.7 (C), 136.6 (C), 135.5 (C), 133.1

(CH), 131.1 (2ϫCH), 131.0 (C), 130.7 (2ϫCH), 130.0 (2ϫCH),

129.6 (2ϫCH), 129.4 (2ϫCH), 129.2 (2ϫCH), 127.8 (CH), 122.1

Cl and C6-Cl), 21.6 (CH ) ppm. IR (neat): ν = 1550, 1484, 1358,

˜

3

1261, 1112 cm^–1. HRMS (ESI⁺): calcd. for [C₁₆H₁₁BCl₂F₂N₂

+

Na]⁺373.0258; found 373.0187.

2,3,5-Trichloro-4,4-difluoro-8-(4-tolyl)-4-bora-3a,4a-diaza-s-

indacene (5): BODIPY 1 (90 mg, 0.32 mmol) in dry THF (15 mL)

and NCS (171 mg, 1.28 mmol) in dry THF (10 mL) were allowed

to react for 72 h. Flash chromatography with hexane/EtOAc (98:2)

afforded 5 (40 mg, 32%) as an orange-red solid, together with an

inseparable mixture of chlorinated products; m.p. 194.3–195.0 °C.

¹H NMR (700 MHz, CDCl₃): δ = 7.31 (d, J = 8.4 Hz, 2 H, 2Ј-H),

7.27 (d, J = 8.4 Hz, 2 H, 3Ј-H), 6.86 (d, J = 4.2 Hz, 1 H, 7-H), 6.74

(s, 1 H, 1-H), 6.41 (d, J = 4.2 Hz, 1 H, 6-H), 2.41 (s, 3 H,

CH₃) ppm. ¹³C NMR (176 MHz, CDCl₃): δ = 146.6 (C-8), 144.3

(C5-Cl), 142.0 (C-4Ј), 140.6 (C3-Cl), 134.1 (C-7a), 132.8 (CH-7),

131.1 (C-8a), 130.5 (2ϫCH-2Ј), 129.5 (2ϫCH-3Ј), 129.2 (C-1Ј),

127.2 (CH-1), 120.6 (C2-Cl), 119.6 (CH-6), 21.5 (CH₃) ppm. IR

(C-Cl), 121.9 (CH) ppm. IR (neat): ν = 2926, 2860, 1702, 1542,

˜

1260, 1073, 800 cm^–1. HRMS (EI): calcd. for C₃₀H₂₀BClF₂N₂O₂

524.1272; found 524.1280.

2,6-Dichloro-4,4-difluoro-3,5-bis(4-formylphenyl)-8-(4-tolyl)-4-bora-

3a,4a-diaza-s-indacene (10): BODIPY 8 (80 mg, 0.16 mmol) in dry

THF (15 mL) and NCS (85 mg, 0.64 mmol) in dry THF (10 mL)

were heated at reflux for 3 h. Flash chromatography with hexane/

EtOAc (9:1) afforded 10 (55 mg, 62%) as a red solid; m.p. 211.3–

211.8 °C. ¹H NMR (700 MHz, CDCl₃): δ = 9.98 (s, 2 H, 2ϫCHO),

7.87 (d, J = 7.7 Hz, 4 H, 3Ј-H), 7.75 (d, J = 7.7 Hz, 4 H, 2Ј-H),

7.44 (d, J = 8.4 Hz, 2 H, 2ЈЈ-H), 7.34 (d, J = 8.4 Hz, 2 H, 3ЈЈ-H),

6.93 (s, 2 H, 1-H and 7-H), 2.45 (s, 3 H, CH₃) ppm. ¹³C NMR

(176 MHz, CDCl₃): δ = 191.8 (2ϫCHO), 153.2 (C-3 and C-5),

146.4 (C-8), 142.2 (C-4ЈЈ), 136.8 (C-4Ј), 134.9 (C-1Ј), 133.4 (C-7a

and C-8a), 130.9 (4 CH-2Ј), 130.6 (2ϫCH-2ЈЈ), 130.3 (C-1ЈЈ), 129.6

(2ϫCH-3ЈЈ), 129.2 (4 CH-3Ј), 129.1 (CH-1 and CH-7), 123.0 (C2-

(neat): ν = 1554, 1387, 1258, 1107, 800 cm^–1. HRMS (ESI⁺): calcd.

˜

for [C₁₆H₁₀BCl₃F₂N₂+ Na]⁺406.9865; found 406.9834.

2,3,5,6-Tetrachloro-4,4-difluoro-8-(4-tolyl)-4-bora-3a,4a-diaza-s-

indacene (6): BODIPY 1 (65 mg, 0.23 mmol) in dry THF (20 mL)

and NCS (308 mg, 2.3 mmol) in dry THF (10 mL) were allowed to

react for 48 h. Flash chromatography with hexane/EtOAc (98:2)

afforded 6 (75 mg, 78 %) as an orange-red solid; m.p. 231.0–

1

231.8 °C. H NMR (700 MHz, CDCl₃): δ = 7.30 (d, J = 8.4 Hz, 2

Cl and C6-Cl), 21.6 (CH ) ppm. IR (neat): ν = 2924, 2856, 1700,

˜

3

H, 2Ј-H), 7.28 (d, J = 8.4 Hz, 2 H, 3Ј-H), 6.79 (s, 2 H, 1-H and 7-

H), 2.41 (s, 3 H, CH₃) ppm. ¹³C NMR (176 MHz, CDCl₃): δ =

144.4 (C-8), 142.6 (C3-Cl and C5-Cl), 142.4 (C-4Ј), 131.4 (C-7a

and C-8a), 130.5 (2 ϫ CH-2Ј), 129.6 (2 ϫ CH-3Ј), 128.9 (C-1Ј),

128.2 (CH-1 and CH-7), 121.6 (C2-Cl and C6-Cl), 21.5 (CH₃) ppm.

1540, 1268, 1219, 1144, 1070, 800, 768 cm^–1. HRMS (ESI^–): calcd.

for [C₃₀H₁₉BCl₂F₂N₂O₂+ CH₃OH – H]⁺589.1169; found

589.1234.

Chlorination of Acetylpyrrole: 2-Acetylpyrrole (200 mg, 1.83 mmol)

in dry THF (30 mL) and NCS (269 mg, 2.01 mmol) in dry THF

(30 mL) were allowed to react for 24 h at room temp. Flash

chromatography with hexane/EtOAc (95:5) afforded 2-acetyl-5-

chloropyrrole (19,^[9e,18]110 mg, 42%) as a colorless solid.

IR (neat): ν = 1549, 1384, 1245, 1106 cm^–1. HRMS (EI): calcd. for

˜

C₁₆H₉BCl₄F₂N₂417.9579; found 417.9583.

1,2,3,5,6,7-Hexachloro-4,4-difluoro-8-(4-tolyl)-4-bora-3a,4a-diaza-

s-indacene (7): NCS (3.38 g, 23.5 mmol) in dry THF (15 mL) was

added slowly to a stirred solution of 5-(4-tolyl)dipyrromethane^[17]

(18, 400 mg, 1.69 mmol) in dry THF (30 mL), and the reaction

mixture was heated at reflux under argon for 24 h. The solvent was

removed, and DDQ (422 mg, 1.86 mmol) was added to the solution

of the intermediate hexachlorodipyrromethane generated above in

CH₂Cl₂(50 mL). The mixture was stirred at room temp. for 1 h and

triethylamine (1.5 mL, 8.4 mmol) was then added. After 10 min,

BF₃·Et₂O (2 mL, 13.5 mmol) was added and the resulting solution

was stirred for 24 h at room temp. The crude mixture was washed

2-Acetylpyrrole (200 mg, 1.83 mmol) in dry THF (30 mL) and

NCS (734 mg, 5.5 mmol) in dry THF (50 mL) were allowed to react

for 72 h. Flash chromatography with hexane/EtOAc (95:5) afforded

2-acetyl-3,5-dichloropyrrole (20, 141 mg, 44%) as a colorless solid

and 5-acetyl-2,3-dichloropyrrole (21,^[18b]125 mg, 39%) as a color-

less solid.

2-Acetyl-3,5-dichloropyrrole (20): ¹H NMR (300 MHz, CDCl₃): δ

= 10.09 (br. s, 1 H, NH), 6.07 (d, J = 3.0 Hz, 1 H, 3-H), 2.51 (s, 3

H, CH₃) ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 186.7 (CO), 127.6

6346

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Eur. J. Org. Chem. 2012, 6335–6350

Efficient and Highly Photostable Laser Dyes

(C-2), 122.5 (C5-Cl), 119.7 (C3-Cl), 110.4 (CH), 28.1 (CH₃) ppm. afforded 16 (50 mg, 73%) as an orange solid; m.p. 259.4–259.7 °C.

IR (neat): ν = 3218, 1643, 1446, 1400, 761 cm^–1. HRMS (EI): calcd.

for C₆H₅Cl₂NO 176.9749; found 176.9751.

¹H NMR (700 MHz, CDCl₃): δ = 6.03 (s, 1 H, 6-H), 2.53 (s, 3 H,

CH₃-C8), 2.46 (s, 6 H, CH₃-C3 and CH₃-C5), 2.36 (s, 3 H, CH₃-

C7), 2.34 (s, 3 H, CH₃-C1) ppm. ¹³C NMR (176 MHz, CDCl₃): δ

= 156.0 (C-5), 148.3 (C-3), 142.8 (C-7), 141.7 (C-8), 134.1 (C-1),

132.7 (C-7a), 129.8 (C-8a), 122.1 (CH-6), 121.3 (C-Cl), 17.5 (CH₃-

C7), 16.7 (CH₃-C8), 14.6 (CH₃-C5), 14.4 (CH₃-C1), 12.2 (CH₃-

˜

5-Acetyl-2,3-dichloropyrrole (21): ¹H NMR (300 MHz, CDCl₃): δ

= 10.81 (br. s, 1 H, NH), 6.76 (d, J = 2.7 Hz, 1 H, 4-H), 2.51 (s, 3

H, CH₃) ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 186.3 (CO), 128.0

(C-5), 120.5 (C2-Cl), 115.5 (CH), 110.5 (C3-Cl), 23.9 (CH₃) ppm.

C3) ppm. IR (neat): ν = 1560, 1365, 1268, 1110 cm^–1. HRMS

˜

IR (neat): ν = 3218, 1643, 1447, 1399, 768 cm^–1. HRMS (EI): calcd.

˜

(ESI⁺): calcd. for [C₁₄H₁₆BClF₂N₂+ Na]⁺319.0961; found

319.0980.

for C₆H₅Cl₂NO 176.9749; found 176.9750.

5-Chloro-2-ethyl-4,4-difluoro-1,3,8-trimethyl-4-bora-3a,4a-diaza-

s-indacene (12): 2-Acetyl-5-chloropyrrole (19,^[9e,18]91 mg,

0.63 mmol) in CHCl₃(10 mL), POCl₃(0.06 mL, 0.7 mmol), 3-

ethyl-2,4-dimethylpyrrole (82 mg, 0.63 mmol) in CHCl₃(10 mL),

triethylamine (0.08 mL, 0.63 mmol), and BF₃·Et₂O (0.08 mL,

0.63 mmol) were allowed to react by the general procedure de-

scribed above. Flash chromatography with hexane/CHCl₃(5:5) af-

forded 12 (98 mg, 52%) as an orange solid; m.p. 171.2–171.7 °C.

¹H NMR (700 MHz, CDCl₃): δ = 6.90 (d, J = 4.2 Hz, 1 H, 7-H),

6.20 (d, J = 4.2 Hz, 1 H, 6-H), 2.51 (s, 3 H, CH₃), 2.44 (s, 3 H,

CH₃), 2.35 (q, J = 7.7 Hz, 2 H, CH₂), 2.26 (s, 3 H, CH₃), 0.99 (t,

J = 7.7 Hz, 3 H, CH₃CH₂) ppm. ¹³C NMR (176 MHz, CDCl₃): δ

= 161.1 (C), 140.7 (C), 138.6 (C), 135.7 (2 C), 134.0 (C), 133.1 (C),

122.8 (CH), 114.5 (CH), 17.1 (CH₂), 15.9 (CH₃), 14.5 (CH₃), 14.1

2,6-Dichloro-4,4-difluoro-1,3,5,7,8-pentamethyl-4-bora-3a,4a-di-

aza-s-indacene (17): BODIPY 15 (50 mg, 0.19 mmol) in dry THF

(15 mL) and NCS (76 mg, 0.57 mmol) in dry THF (10 mL) were

allowed to react for 24 h. Flash chromatography with hexane/

EtOAc (98:2) afforded 17 (42 mg, 67%) as an orange solid; m.p.

261.1–261.6 °C. ¹H NMR (700 MHz, CDCl₃): δ = 2.56 (s, 3 H,

CH₃-C8), 2.48 (s, 6 H, CH₃-C3 and CH₃-C5), 2.35 (s, 3 H, CH₃-

C1 and CH₃-C7) ppm. ¹³C NMR (176 MHz, CDCl₃): δ = 150.7

(C-3 and C-5), 142.2 (C-8), 135.8 (C-1 and C-7), 130.4 (C-7a and

C-8a), 122.4 (2ϫC-Cl), 17.1 (CH₃-C8), 14.6 (CH₃-C1 and CH₃-

C7), 12.3 (CH -C3 and CH -C5) ppm. IR (neat): ν = 1560, 1362,

˜

3

1270, 1111 cm^–1. HRMS (EI): calcd. for C₁₄H₁₅BCl₂F₂N₂

330.0668; found 330.0668.

(CH ), 13.1 (CH ) ppm. IR (neat): ν = 1577, 1403, 1212, 1101,

˜

3

Preparation of Solid Polymeric Samples: Solid matrices of PMMA

incorporating some of the new fluorinated BDP dyes as true solu-

tions were prepared essentially as described elsewhere.^[4d]An appro-

priate amount of the dye was dissolved in pure MMA, which was

polymerized to yield the materials named dye/PMMA. 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. 6ϫ 10 mm.

This surface and the ends of the laser rods were prepared for lasing

experiments with a grinding and polishing machine (Phoenix

Beta 4000, Büehler) until optical-grade finishing. The planar grind-

ing stage was carried out with a Texmet 1000 sand paper and 6 μm

diamonds suspended in mineral oil as abrasive. The final polishing

stage was carried out with a G-Tuch Microcloth and a cloth disk

Mastertex with 1 μm diamonds in mineral oil.

771 cm^–1. HRMS (EI): calcd. for C₁₄H₁₆BClF₂N₂296.1060; found

296.1064.

5,7-Dichloro-2-ethyl-4,4-difluoro-1,3,8-trimethyl-4-bora-3a,4a-di-

aza-s-indacene (13): 2-Acetyl-3,5-dichloropyrrole (20, 90 mg,

0.50 mmol) in CHCl₃(10 mL), POCl₃(0.05 mL, 0.55 mmol), 3-

ethyl-2,4-dimethylpyrrole (91 mg, 0.50 mmol) in CHCl₃(10 mL),

triethylamine (0.06 mL, 0.50 mmol), and BF₃·Et₂O (0.06 mL,

0.50 mmol) were allowed to react by the general procedure de-

scribed above. Flash chromatography with hexane/EtOAc (98:2) af-

forded 13 (90 mg, 54%) as an orange solid; m.p. 193.1–193.6 °C.

¹H NMR (700 MHz, CDCl₃): δ = 6.21 (s, 1 H, 6-H), 2.72 (s, 3 H,

CH₃), 2.51 (s, 3 H, CH₃), 2.36 (q, J = 7.7 Hz, 2 H, CH₂), 2.31 (s,

3 H, CH₃), 1.00 (t, J = 7.7 Hz, 3 H, CH₃CH₂) ppm. ¹³C NMR

(176 MHz, CDCl₃): δ = 162.2 (C), 141.7 (C), 139.8 (C), 137.0 (2

C), 135.0 (C), 133.2 (C), 127.2 (C), 126.4 (C), 115.6 (CH), 17.2

(CH₂), 16.3 (CH₃), 14.9 (CH₃), 14.4 (CH₃), 13.2 (CH₃) ppm. IR

Photophysical Properties: Solvents for laser and photophysical

studies were of spectroscopic grade (Merck, Aldrich, or Sigma) and

were used without purification; those used in synthetic work were

purified by standard methods. Other reagents were from commer-

cial sources, and used as received. The commercial laser dyes Pyrro-

methene 546 (Exciton) was used as received with a purity Ͼ99%

(checked by spectroscopic and chromatographic methods). The

monomer MMA (Merck) was successively washed with aqueous

NaOH (5%) and water, dried with Na₂SO₄, and distilled under

reduced pressure. Other reagents were from commercial sources

and were used as received. The photophysical properties were de-

termined in diluted solutions (around 2ϫ10^–6m), prepared by add-

ing the corresponding solvent (spectroscopic grade) to the residue

from the appropriate amount of a concentrated stock solution in

acetone, after vacuum evaporation of this solvent. UV/Vis absorp-

tion and fluorescence spectra were recorded with a Varian model

CARY 4E spectrophotometer and a SPEX Fluorolog 3–22 spectro-

fluorimeter, respectively. Fluorescence quantum yields (φ) were de-

termined by use of diluted dye solutions of suitable commercial

BODIPYs (PM567 and PM650, φ = 0.84 and 0.10, respectively).^[4a]

Radiative decay curves were registered by the time-correlated sin-

(neat): ν = 1568, 1417, 1375, 1194, 1084, 1031, 803 cm^–1. HRMS

˜

(EI): calcd. for C₁₄H₁₅BCl₂F₂N₂330.0668; found 330.0669.

5,6-Dichloro-2-ethyl-4,4-difluoro-1,3,8-trimethyl-4-bora-3a,4a-di-

aza-s-indacene (14): 5-Acetyl-2,3-dichloropyrrole (21, 90 mg,

0.50 mmol) in CHCl₃(10 mL), POCl₃(0.05 mL, 0.55 mmol), 3-

ethyl-2,4-dimethylpyrrole (91 mg, 0.50 mmol) in CHCl₃(10 mL),

triethylamine (0.06 mL, 0.50 mmol) and BF₃·Et₂O (0.06 mL,

0.50 mmol) were allowed to react by the general procedure de-

scribed above. Flash chromatography with hexane/EtOAc (95:5) af-

1

forded 14 (80 mg, 48%) as an orange solid; m.p. 187.3–187.8. H

NMR (700 MHz, CDCl₃): δ = 6.83 (s, 1 H, 7-H), 2.53 (s, 3 H,

CH₃), 2.41 (s, 3 H, CH₃), 2.36 (q, J = 7.7 Hz, 2 H, CH₂), 2.28 (s,

3 H, CH₃), 1.00 (t, J = 7.7 Hz, 3 H, CH₃CH₂) ppm. ¹³C NMR

(176 MHz, CDCl₃): δ = 163.7 (C), 141.8 (C), 137.8 (C), 136.6 (2

C), 134.7 (C), 131.7 (C), 130.7 (C), 119.5 (CH), 116.2 (C), 17.1

(CH₂), 15.8 (CH₃), 14.4 (CH₃), 14.2 (CH₃), 13.3 (CH₃) ppm. IR

(neat): ν = 1579, 1404, 1206, 1147, 1093, 1022, 801 cm^–1. HRMS

˜

(EI): calcd. for C₁₄H₁₅BCl₂F₂N₂330.0668; found 330.0670.

2-Chloro-4,4-difluoro-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-

indacene (16): BODIPY 15 (60 mg, 0.23 mmol) in dry THF (15 mL) gle-photon-counting technique (Edinburgh Instruments, model

and NCS (37 mg, 0.28 mmol) in dry THF (5 mL) were allowed FL920, fitted with a Hamamatsu C4878 microchannel plate detec-

to react for 48 h. Flash chromatography with hexane/CHCl₃(7:3)

tor), with picosecond time-resolution. Fluorescence emission was

Eur. J. Org. Chem. 2012, 6335–6350

www.eurjoc.org

6347

I. Garcia-Moreno, J. L. Chiara, M. J. Ortiz et al.

FULL PAPER

monitored at the maximum emission wavelength after excitation at

470 nm and 530 nm with a diode laser (PicoQuant, model LDH470

and LDH530) with 150 ps FWHM pulses. The fluorescence life-

time (τ) was obtained after deconvolution of the instrumental re-

sponse signal from the recorded decay curves by an iterative

method. The goodness of the exponential fit was controlled by sta-

tistical parameters (chi-square, Durbin–Watson, and analysis of re-

siduals). The radiative (k_fl) and nonradiative (k_nr) rate constants

were calculated from the fluorescence quantum yield and lifetime;

k_fl= φ/τ and k_nr= (1 – φ)/τ. In the case of biexponential deconvol-

utions the amplitude-averaged lifetime – ϽτϾ – was calculated as

the sum of the products of each lifetime and their contibutions

divided by 100. The corresponding rate constants were calculated

from this average lifetime (Ͻk_flϾ and Ͻk_nrϾ).

CCDC-882023 (for 5), -882022 (for 6), and -882021 (for 7) contain

the supplementary crystallographic data for this paper. These data

can be obtained free of charge from The Cambridge Crystallo-

graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Supporting Information (see footnote on the first page of this arti-

cle): copies of ¹H and ¹³C NMR spectra of all new compounds,

full photophysical and laser data for compounds 1–17 in different

solvents, quantum mechanical simulations of their charge distribu-

tions and dipole moment orientations, and additional figures and

tables mentioned in the article.

Acknowledgments

Laser Experiments: Liquid solutions of dyes were placed in 1 cm

optical path quartz cells carefully sealed to avoid solvent evapora-

tion during experiments. The solutions of the newly synthesized

dyes, as well as the solid samples, were transversely pumped at dif-

ferent wavelengths matching the maximum absorptions of the cor-

responding dyes: at 355 nm, with 5 mJ per pulse, 8 ns FWHM

pulses from the third-harmonic of a Q-switched Nd:YAG laser

(Spectron SL282G) and 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. The exciting pulses were line-

focused onto the cell, providing pump fluences on the active me-

dium of 110 mJcm^–2. The oscillation cavity (2 cm length) consisted

of a 90% reflectivity aluminum mirror, with the lateral face of the

cell as output coupler.

This work was supported by the Ministerio de Economia y Compe-

titividad (MAT2010-20646-C04-01, MAT2010-20646-C04-02,

MAT2010-20646-C04-03, MAT2010-20646-C04-04, and TRACE

2009-0144), the Comunidad de Madrid and the European Social

Fund (S2009/PPQ-1634 “AVANCAT”), and the Gobierno Vasco

(project number IT339-10). The SGI/IZO-SGIker UPV-EHU is

gratefully thanked for allocation of computational resources. G. D.

thanks Ministerio de Economía y Competitividad for an FPI fel-

lowship.

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Received: July 17, 2012

Published Online: October 4, 2012

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Chlorinated BODIPYs: Surprisingly efficient and highly photostable laser dyes

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