Congeners of Pyrromethene-567 Dye: Perspectives from Synthesis, Photophysics, Photostability, Laser, and TD-DFT Theory

Article abstract of DOI:10.1021/acs.joc.5b00654

In an attempt to develop photostable and efficient BODIPY (PM) dyes for use in liquid dye lasers, three new congeners of widely used laser dye, PM567, were synthesized and their photophysical properties in various organic solvents, laser performances, and

Full text of DOI:10.1021/acs.joc.5b00654

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Article
Congeners of Pyrromethene-567 dye: Perspectives from
synthesis, photophysics, photostability, laser and TD-DFT theory
Kishor G Thorat, Priyadarshani Kamble, Ramnath Mallah, Alok K. Ray, and Nagaiyan Sekar
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b00654 • Publication Date (Web): 22 May 2015
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Congeners of Pyrromethene-567 dye: Perspectives from synthesis, photophysics,
photostability, laser and TD-DFT theory
Kishor G. Thorat,[1] Priyadarshani Kamble,[2] Ramnath Mallah, [1] Alok K. Ray*[2] and N.
Sekar*[1]
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[1]Dyestuff Technology Department. Institute of Chemical Technology, Matunga, Mumbai.
[2]Laser & Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai.
*Corresponding Author
alokray@barc.gov.in; nethi.sekar@gmail.com
ABSTRACT: In an attempt to develop photostable and efficient BODIPY (PM)
dyes for use in liquid dye lasers, three new congeners of widely used laser dye,
PM567, were synthesized and their photophysical properties in various organic
solvents, laser performances and photostabilities in selected solvent, 1, 4-dioxane,
have been investigated using a frequency doubled Q-switched (10 Hz) Nd: YAG
laser at 532 nm. The results of photostability study in non-polar 1, 4-dioxane
revealed the remarkable enhancement in stability of the novel dyes compared to
that of PM567 as well as improved laser performances. Cyclic voltammetry study
strongly supports the observed enhancement in photostability of the novel dyes
compared to that of PM567. The observed properties of the novel dyes in relation
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to that of PM567 have been rationalized by extensive use of DFT and TD-DFT
using B3LYP/6-31G(d) method of theory.
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INTRODUCTION
BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes exhibit high fluorescence
quantum yields1a-b and can be used for such diverse applications as bio-labels,2 sensitizers in
solar cells,3 fluorescent sensors,4a-f molecular photonic wires5 and electron transfer reagents.6
Over the past decades the BODIPY dyes received plenty of attention and same is reflected in
a large no of publications.4e-f, 7a-i They exhibit low intersystem crossing (ISC) rates and low
triplet extinction coefficients over the laser spectral region1a, 5, 8a-e and hence are very useful
in laser applications.9 BODIPY (also called Pyrromethene, PM) dyes exhibit good solubility
in many organic solvents even in methyl methacrylate (MMA) which is useful for solid-state
dye laser applications.10a-d In spite of excellent photophysical and laser properties of the PM
dyes they are not yet the reliable and sustainable laser dyes for high average power dye laser
applications due to rapid photochemical degradation in normally used alcohol solvents,
particularly sensitive to photo-chemical reactions with dissolved oxygen.11 The problem has
partially been addressed by doing modifications either at the meso position, C-3 and/or C-5
center or at the boron atom of the pyrromethene moiety.12a-d
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Figure 1: Chemical structures of the BODIPY dyes under study.
It was envisaged that the photo stabilities and lasing activities of PM dyes can be tuned by
suitable modifications of the substitution pattern on the PM core.9, 12d Earlier investigators have
synthesized a host of analogues of the well-known commercial PM laser dye, PM567, by
incorporating different substituents at C-8 and/or C-2 + C-6 positions (refer Fig. 1 for general
structure of PM dye) of the PM moiety.8a,13 Among these, PM650 (with CN at C-8 ) showed
about fifty times higher photostability than PM567 but the fluorescence quantum yields of
these analogues were too low to be used as laser dyes. The effect of alteration of the alkyl
substituents at the pyrrole rings was found to be much less on their photochemical stabilities.8a,
13, 14
It is now well established that under lasing conditions the dye in its triplet state is mainly
responsible for the generation of singlet oxygen, the involvement of triplet state of the
pyrromethene dye in the generation of singlet oxygen has been confirmed from their higher
decomposition rates in presence of benzophenone, a triplet sensitizer.14 The enhanced photo
stability of the PM dyes in deoxygenated dye solutions, and also in the presence of 1 wt% of
singlet oxygen quenchers like Tin770, TBP, and DABCO confirmed the involvement of 1O2 in
the degradation of dye molecules and thereby decreasing the lasing efficiency over the period
of longer operating time.9 Further, singlet oxygen (1O2) reacts at C7’-C8 double bond of the
dye molecule which produces an unstable peroxo compound leading to the breakdown of the
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dye structure.9,15 Therefore, the photo stability of the PM dyes might be increased by reducing
the generation of 1O2 and/or their rate of reaction with 1O2.
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It was felt that if one judiciously changes the substituents at either C8 or C2 + C6,
possibility of obtaining photostable PM dyes will be more compared to the commercially
available known dye PM567. As far as high-average power applications of PM based liquid
dye lasers are concerned one needs to focus more on their photochemical stability in addition
to high conversion efficiency. The primary aim of the investigation was to develop a few
congeners of PM567 with enhanced photostability while maintaining high dye laser efficiency
by rational design.
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To this end we have synthesized the three congeners of well-known commercially available
laser dye, PM567 with 2 and 6-position modified using the pyrrole modified at the 3-position,
benzyl group in place of ethyl. Also it was expected that the change in ethyl to benzyl groups
at 2 and 6-postions of the PM core could alter the electrochemical properties of the PM dye to
some extent leaving photophysical properties unaltered. We have studied photophysical
properties in various organic solvents, laser efficiency and photostability properties in a
selected solvent 1, 4-dioxane of the novel PM dyes relative to that of PM567. The cyclic
voltammetry study has been performed to get the idea about the electronic changes over the
PM core and the results are correlated with the observed photostabilities of the corresponding
novel BODIPY dyes relative to PM567. Also DFT and TD-DFT calculations using B3LYP/6-
31G(d) level of theory have been employed in order to get more insight on molecular structures
and observed photophysical, photostability and laser properties of the novel BODIPY dyes.
The present modifications produce the positive effect on photostability and improve the laser
performances of the studied dyes relative to that of widely used laser dye, PM567.9, 15 Also
maximum molar extinction coefficients of new laser dyes compared to that of PM567,10b are
found to be improved by about two folds on account of modifications at 2 and 6-postions of
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the BODIPY dyes. Thus, a lower concentration of new dyes is required for laser applications.
Overall the present modifications are found to be very useful in improving laser performances
and photostabilities of the BODIPY laser dyes.
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RESULTS AND DISCUSSION
Synthesis: Three novel pyrromethene laser dyes 2-4 (Figure 1) have been synthesized from
the pyrrole in which there is benzyl substituent at 3-position instead of ethyl in kryptopyrrole,15
used in the synthesis of well-known laser dye, PM567.9, 15
The benzyl pyrrole intermediate (9) (Scheme1 ) was obtained very easily after simple work up
from its corresponding salt of carboxylic acid at room temperature as a pure solid and can be
stored in refrigerator for a few weeks. The benzyl pyrrole intermediate (9) (though reported in
by Treibs, et al16 in1954) and its analogues17a-b received negligible attention as far as synthesis
and properties of BODIPY dyes. This could be because phenyl group is not in direct
conjugation with the heterocyclic pyrrole ring and would expect to produce negligible effect
on photophysical properties of typical BODIPY dyes. But for the BODIPY as a laser dye
emitting in the green-yellow region of electromagnetic spectrum, even little changes in its
electronic properties could produce beneficial impact particularly on its photostability
properties as a result of suitable modification over the PM core.9, 15 Therefore, we decided to
use benzyl pyrrole intermediate in place of commonly used kryptopyrrole for the synthesis of
new BODIPY laser dyes.
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Scheme 1: Synthesis of the pyrrole intermediate, 9
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The dyes 2 and 3 were synthesized using the benzyl pyrrole (9) and the corresponding acid
chlorides (Scheme 2).
Scheme 2: Synthesis of the dyes 2 and 3
Also we have synthesized the dye 4 by using benzyl pyrrole-2-aldehyde (10) with the reported
protocol18 (Scheme 3).
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Scheme 3: Synthesis of the dye 4
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Photophysical Characteristics. The measured photophysical properties of the dyes 1-4 such
as longest wavelength absorption maxima (λabs), emission maxima (λem), fluorescence
quantum yields (Φfl) in solvents of various polarities and fluorescence life times (τ), and
calculated radiative decay (kr) and non-radiative decay (knr) rates in selected solvents are
presented in Table 1. The absorption and fluorescence spectra of the dyes 1−4 were recorded
in various organic solvents (Table 1) but only those obtained in 1, 4-dioxane are shown in
Figures 2 and 3, respectively.
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3
1.0
4
0.8
0.6
0.4
0.2
0.0
390
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510
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570
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Wavelength (nm)
Figure 2: Normalized absorption spectra of the dyes 1-4 in 1, 4-dioxane.
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Wavelength (nm)
Figure 3: Normalized emission spectra of the dyes 1-4 in 1, 4-dioxane. Excitation wavelength was 490 nm.
The PM dyes 1-4 in different organic solvents had intense S0→S1 absorption bands in the
range of 518−530 nm, each with a small fwhm. Table 1 reveals that the change in solvent
polarities have no or very weak influence on the absorption and emission maxima of the dyes
2-4. In different organic solvents the dye 2, as a result of modifications at 2 and 6-positions,
relative to the dye 1 showed, absorption maxima shifted hypsochromically by 1-3 nm and
emission maxima shifted bathochromically by 2-3 nm, resulting increased Stokes shift by 3-6
nm (Table 1). The shapes of the absorption and emission spectra of the dyes 2 and 4 were
similar to that of 1 and like typical BODIPYs they are mirror images of each other. From
Figures 2 and 3 and Table 1 it can be seen that the shapes of the absorption and emission spectra
of the dye 2 are very similar to that of the dye 1 with no considerable change in fwhm. The
absorption and emission spectra of the dye 2 was observed to sharpen a little by 1-2 nm
compared to that of the dye 1. The dye 2 showed good fluorescence quantum yields ranging
from 0.78-0.97 in various organic solvents (Table 1). Compared to the dye 1, the dye 2 gave
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slightly decreased fluorescence life times with minimum of 4.88 ns in 1, 4-dioxane and
maximum of 6.01 ns in n-heptane (Table 1).
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Table 1: Photophysical parameters of the dyes 1-4 in various solvents.
fwhm[b]
fwhm[d]
nm nm
Δν [e]
cm-1
[a]
[c]
λabs
nm
λem
nm
εmax/104[f]
τ[h]
ns
kr/108[i] knr/108[j]
[g]
Solvent
фfl
M-1cm-1
s-1
s-1
nm
cm-1
nm
Dye 1
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Ethanol
1,4-dioxane
n-Heptane
518
519
523
25.3
25.0
21.5
957.2
941.8
802.3
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533
534
41.5 1425.2
40.9 1399.8
40.4 1381.1
15
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Dye 2
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Dye 3
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Dye 4
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9
543.3
506.1
393.9
8.1
8.4
9.1
0.84[k] 6.19
0.83[k] 5.48
1.36
1.52
1.63
0.258
0.310
0.033
0.98
6.03
ACN
DMF
514
517
518
516
517
516
516
521
518
520
26.3 1006.3
26.7 1009.7
27.2 1027.4
25.8
26.3
25.5
25.4
24.5
25.0
22.0
536
539
540
536
536
536
536
537
535
536
39.7 1381.4
37.2 1288.3
42.5 1455.7
40.2 1396.9
39.7 1369.0
41.0 1419.5
40.1 1385.9
36.7 1350.9
39.7 1365.3
38.3 1313.4
799
789
787
723
686
723
723
572
613
574
13.4
12.2
13.0
11.2
11.7
13.8
10.2
10.1
11.6
10.5
0.82
0.88
0.89
0.78
0.80
0.86
0.86
0.80
0.87
0.97
n.d.
n.d.
n.d.
n.d.
5.39
n.d.
n.d.
n.d.
4.88
6.01
n.d.
n.d.
n.d.
n.d.
1.48
n.d.
n.d.
n.d.
1.78
1.61
n.d.
n.d.
n.d.
n.d.
0.371
n.d.
n.d.
n.d.
0.266
0.049
DMSO
MeOH
EtOH
Acetone
EtOAc
CHCl3
981.6
996.8
972.1
966.9
913.4
941.6
821.1
1,4-dioxane
n-Heptane
ACN
DMF
520
523
524
521
522
521
521
525
523
523
24.3
24.1
24.3
23.6
23.7
23.1
23.0
22.9
22.8
21.5
905.9
886.1
893.3
878.0
877.1
857.3
851.4
838.7
839.5
791.8
534
537
538
535
535
535
535
536
537
537
26.4
26.7
26.9
26.1
26.0
25.1
25.4
26.2
25.8
24.7
922.9
923.4
924.6
812.1
907.9
874.4
887.5
901.2
893.0
859.9
504
498
497
502
465
502
502
391
498
498
12.8
12.1
12.2
11.6
11.4
12.7
11.6
12.5
12.2
10.6
0.71
0.74
0.89
0.81
0.74
0.74
0.79
0.77
0.80
0.87
n.d.
n.d.
n.d.
n.d.
4.73
n.d.
n.d.
n.d.
4.17
4.42
n.d.
n.d.
n.d.
n.d.
1.56
n.d.
n.d.
n.d.
1.92
1.97
n.d.
n.d.
n.d.
n.d.
0.549
n.d.
n.d.
n.d.
0.479
0.294
DMSO
MeOH
EtOH
Acetone
EtOAc
CHCl3
1,4-dioxane
n-Heptane
ACN
DMF
524
526
527
526
527
525
526
532
528
530
27.9 1037.5
28.0 1030.0
28.3 1037.9
27.1
27.5 1012.5
26.7
26.0
23.5
25.6
17.8
537
540
541
538
536
538
535
540
537
536
21.9
24.0
24.0
21.5
23.6
22.7
23.1
22.6
23.1
19.7
764.3
828.1
825.4
749.6
814.9
789.0
799.0
769.6
794.8
680.6
462
493
491
424
319
460
320
279
317
211
10.0
9.6
10.0
10.5
9.7
11.3
11.0
12.1
11.4
9.9
0.74
0.86
0.89
0.79
0.83
0.84
0.85
0.85
0.86
0.96
n.d.
n.d.
n.d.
n.d.
6.0
n.d.
n.d.
n.d.
5.8
n.d.
n.d.
n.d.
n.d.
1.38
n.d.
n.d.
n.d.
1.48
n.d.
n.d.
n.d.
n.d.
n.d.
0.283
n.d.
n.d.
n.d.
0.241
n.d.
DMSO
MeOH
EtOH
Acetone
EtOAc
CHCl3
999.8
987.1
970.0
845.1
936.1
639.0
13
9
8
9
6
1,4-dioxane
n-Heptane
n.d.
[a]Wavelength at absorption maximum; [b]fwhm of longest wavelength absorption spectral band; [c]Wavelength at emission
maximum; [d]fwhm of emission spectral band; [e]Stokes shift; [f]Molar extinction coefficient at absorption maximum;
[g]Fluorescence quantum yield calculated using PM567 in ethanol (фfl=0.84) as standard.[h]Fluorescence life time; [i]Radiative
rate constant; [j]Non-radiative rate constant; [k]Values taken from the reference;10b n.d.= not determined.
The dye 3, in which there is a phenyl group at meso position along with benzyl groups at 2
and -6 positions (Figure 1), showed narrow absorption and emission peaks with 1-4 nm red
shift compared to the dye 1. From Table 1 it can be seen that change in solvent polarity had
little influence on shapes and positions of absorption and emission bands. The dye 3 compared
to the dye 2 in different organic solvents show sharper absorption and emission bands with
slightly lower fluorescence quantum yields (ranging from 0.71-0.89) (Table 1). Table 1 reveals
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the decrease in fluorescence life times of the dye 3 in different solvents compared to the dyes
1 and 2.
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The dye 4, in which there is benzyl groups at 2 and 6-positions and no substituent at meso
positon (Figure 1), show sharper absorption and emission bands compared to that of the dyes
1 and 2 with a red shift of 7-9 nm and 2-3 nm in absorption and emission, respectively, relative
to that of the dye 1 resulting in to the decrease in Stokes shift. The dye 4 shows good
fluorescence quantum yields ranging from 0.74 to 0.96 in different solvents and have
comparable fluorescence life time with that of the dye 1 (Table 1). We have observed a
considerable enhancement in the molar extinction coefficients (εmax) at absorption maxima of
the novel PM dyes 2-4 (Table 1) in solvents of different polarities.
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Laser Characteristics. Interestingly, compared to ethanol we observed surprisingly high
photostability of the well-known laser dye 1 (Table 2) in 1, 4-dioxane. Therefore 1, 4-dioxane
solutions of the dye 1 and also that of the dyes 2-4 were comparatively evaluated for laser
performances using frequency doubled (532 nm) output of a pulsed Nd:YAG laser as an
excitation source. The results of the dye concentration dependent broad band lasing studies of 1,
4-dioxane solutions of the PM dyes 1-4 are presented in Table 2 and shown in Figures 4 and 5.
The lasing efficiency values of the dyes 1-4 in 1, 4-dioxane (Figure 4) followed the expected
pattern that is initially increasing with dye concentration, reaching a maximum value followed
by decrease at higher concentration. The maximum broad band lasing efficiency values (η) of
the dyes 1-4 at their respective optimum dye concentrations are shown in Figure 5 and also listed
in Table 2. Lasing efficiency of the dyes 1-4 followed the similar pattern as that observed for
their respective fluorescence quantum yields (фfl) and molar extinction coefficient (εmax) values.
This suggests that incorporation of benzyl groups at 2 and 6-positions, similar to photophysical
properties, had no or very little effect on respective lasing profiles of the dyes 2-4. For each dye
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(1-4) laser, the spatial profile of lasing output was almost circular at the respective maximum
lasing efficiency values.12d From the Table 2 it can be seen that the dye 1 shows almost same
lasing efficiency in 1, 4-dioxane compared to that in ethanol but the concentration required to
obtain maximum lasing efficiency in 1, 4-dioxane (0.92 mM) is slightly higher than that that in
ethanol (0.83 mM). But interestingly the dyes 2 (0.35 mM) and 4 (0.59 mM) showed similar
maximum lasing efficiency in 1, 4-dioxane using much lower concentration as that of the dye 1
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(0.83 mM).
Table 2.Lasing characteristics of the PM dyes 1-4[a] in 1, 4-dioxane
Φpd-1 of 1 in
Dioxane/ Φpd-1 of 1
in EtOH
Φpd-1 of 2-4 in
Conc. [b]
(mM)
ɳ[c]
[%]
-1[e]
Dye
Φpd
Dioxane / Φpd-1 of 1
in Dioxane
0.83
0.92
0.59
0.51
0.35
45.2[d]
45.0
44.5
38.2
42.6
1.7 x 103[f]
3.2 x 104
9.2 x 104
11.7 x 104
7.9 x 104
-
-
18.82
1
-
-
-
2.88
3.66
2.47
2
3
4
[a]Different concentrations of the dyes were used to maintain the O. D. values of their solution at ~2.5 (1 mm cell)
at 532 nm for photostability (Φpd-1) determination (Error: ±1%); [b]Concentration of the dye required to obtain
maximum lasing efficiency; [c]Broad band lasing efficiency in 1, 4-dioxane (Error: ±1%); [d]Broad band lasing
efficiency of the dye 1 in ethanol (Error: ±1%); [e]Photostability in 1, 4-dioxane (Error: ±1%); [f]Photostability of
9
the dye 1 in ethanol taken from our earlier work.
As expected the dye 3 (0.51 mM) showed slightly lower maximum lasing efficiency (38.2%)
compare to that of dye 1, 2 and 4, which can be attributed to free rotation of phenyl group at
meso position leading to the increased loss of excitation energy by non-radiative decay process
(Table 1). The dyes 2-4 in 1, 4-dioxane produces the maximum lasing efficiency at relatively
lower concentrations as that of the dye 1. Beneficially, it would reduce the losses due to the
ground state absorption (GSA) at high power dye amplifier operations, improving their laser
performances.12d The dyes 2 and 4 have almost same εmax (Table 1), but the required
concentration to obtain the maximum lasing efficiency in case of the dye 4 is far less than that
of the dye 2 which can be attributed to the red shifted absorption maxima of the dye 4 (528 nm),
appearing closer to the pump beam wavelength at 532 nm compared to that of the dye 2 (518
nm).
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Dye 1 EtOH
Dye 1 Dioxane
Dye 2 Dioxane
Dye 3 Dioxane
Dye 4 Dioxane
48
44
40
36
32
28
24
20
16
12
8
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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
Concentration (mM)
Figure 4. Concentration-dependent broad band lasing efficiency of the PM dyes 1-4 in 1, 4-dioxane, determined by
532 nm irradiation with a Q-switched Nd:YAG laser.
1. Dye 1
2. Dye 2
3. Dye 3
50
4. Dye 4
4
45
40
35
30
25
20
15
10
5
45
40
35
30
25
20
15
10
5
0
0
1
2
3
4
Figure 5. Broad-band lasing efficiencies (%ɳ) and photo stabilities (Φpd-1) of the BODIPY dyes 1–4 in 1, 4-
dioxane, determined by pumping with 532 nm radiation of a Q-switched (10 Hz) Nd:YAG laser.
Photostability characteristics: The quantum yield of photodegradation (Φpd) or
photostabilities (Φpd-1) of the dyes 1-4 in air-equilibrated 1, 4-dioxane solutions were measured
under non-lasing conditions and are enlisted in Table 2, column 4, and also shown in Figure 5.
The enhancement in Φpd-1 value of the dye 1 in 1, 4-dioxane compared to that in ethanol is
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shown in column 5 and the enhancement in Φpd-1 of the dyes 2-4 in 1, 4-dioxane compared to
that of the dye 1 in 1, 4-dioxane are shown in column 6 of Table 2. The reason for selecting 1,
4-dioxane solvent for detail laser study is that the PM dyes 2-4 as well as dye 1 showed a large
enhancement in photostability in 1, 4-dioxane than that in commonly used ethanol.
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Among the dyes 2-4, the dye 3 has the highest photostability followed by the dye 2 and
then 3. In 1, 4-dioxane the dyes 2 to 4 are found to be more photostable by 2.88, 3.66 and 2.47
times, respectively, than that of dye 1 (Table 2). The photostability results revealed that the
benzyl groups at 2 and 6-positions, even though not in conjugation with the PM core, induced
a positive effect on photostability. The negative inductive effect (-I) of the benzyl groups may
be responsible to reduce the electron density on the PM core to some extent.
Cyclic voltammetry: Cyclic voltammetry analysis of the BODIPY dyes 1-4 showed a
reversible peak in each case in the anodic portion of the cyclic voltamograms (Figure 6), which
was assigned to one-electron oxidation of the BODIPY unit.19 From the figure 6 it can be seen
that the incorporation of the benzyl groups at 2 and 6-postions of the PM chromophore have a
0
0
positive effect on the oxidation potentials (Eox). The dyes 2 (Eox =1.054 V), 3 (Eox =1.083 V)
0
and 4 (Eox =1.047 V) possess higher oxidation potentials as compared to that of the dye 1
0
(Eox =0.997 V), by 57 mV, 86 mV and 50 mV respectively. The above CV data suggested the
trend in probability of oxidation of the dyes increases in the order 3<2<4<1, which agrees well
with the trend in observed laser photo-stabilities of the dyes (1<4<2<3). To obtain further
insight on structure-property relationship of these dye molecules we have carried out DFT and
TD-DFT calculations, which are discussed in next sections.
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1
2
3
4
[Fc/Fc+]
3x10-5
2x10-5
[BODIPY/BODIPY+]
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1x10-5
0
-1x10-5
-2x10-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
E (V) against Fc/Fc+
Figure 6: Cyclic voltamograms of the BODIPY dyes 1-4 in acetonitrile at room temperature.
Theoretical Interpretations:
The BODIPY dyes 1-4 possess a compact chromophore with high molar extinction coefficients,
which can be used to explore the electronic structure of BODIPY chromophores in various
solvents by means of DFT and TD-DFT calculations using B3LYP/6-31G(d) level of theory. In
our studies we have analyzed the absorption and emission characteristics of the PM dyes 1-4 and
correlated them with the photophysical and photostability properties.
Geometrical parameters. The dyes 1-4 were optimized in their S0 and S1 states with the help
of DFT and TD-DFT using B3LYP/6-31G(d) functional for all the atoms in vacuum and
various organic solvents, but results obtained in 1, 4-dioxane are presented in Table 3. The
BODIPY unit was found to be perfectly planer in ground state optimized geometries of dyes
1-4 with very little or negligible displacement (0.6-1.1o) of boron atom which revealed an
effective overlap of p-orbitals. In the dye 3, meso (C-8) phenyl substituent oriented
orthogonally with the BODIPY plane with little twist making an angle of 92.3o in the S0 state,
suggesting no electronic interactions with the PM core.
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Table 3:Selected Bond lengths, Bond angles and Torsional angles of the dyes 1-4, optimized in 1, 4-dioxane
media using B3LYP/6-31G(d) level of theory, (bond lengths are in Å, dihedral angles are in degree o)
Dye 1
Dye 2
Dye 3
Dye 4
Atom no.
GS[a]
ES[b]
GS
ES
GS
ES
GS
ES
7
Bond lengths
8
9
C1-C2
C2-C3
1.399
1.417
1.348
1.552
1.397
1.409
1.509
1.506
1.540
1.412
1.399
1.418
1.345
1.554
1.396
1.411
1.509
1.507
1.526
1.408
1.425
1.350
1.544
1.402
1.428
1.505
1.502
1.526
1.400
1.416
1.349
1.553
1.397
1.404
1.496
1.507
1.527
1.408
1.425
1.349
1.543
1.402
1.429
1.486
1.502
1.527
1.400
1.419
1.352
1.562
1.395
1.389
-
1.401
1.425
1.350
1.554
1.401
1.392
-
1.415
1.350
1.543
1.403
1.424
1.505
1.499
1.545
C3-N3’
N3’-B4
B4-F4’
C7’-C8
C8-C9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
C6-C10
C10-C11
1.506
1.526
1.503
1.526
Bond angles
C3-N3’-B4
N3’-B4-F4’
F4’-B4-F4’’
125.2
109.8
109.9
124.9
109.9
109.1
125.1
109.9
110.1
124.9
109.9
109.2
Dihedral angles
178.5
-
125.4
109.7
110.1
124.3
110.3
109.2
126.8
109.7
110.3
125.6
110.1
109.3
C2-C3-N3’-B4
C7’-C8-C9-C12
C6-C10- C11-C11’
179.4
-
-
179.9
-
-
178.7
179.8
92.3
34.7
174.5
108.4
40.9
179.7
-
36.4
179.8
-
42.1
35.2
42.4
[a]Computed geometrical parameters of molecules in ground state; [b]Computed geometrical parameters of
molecules in excited state
The phenyl rings of 2 and 6-substituents in case of the dyes 2-3 appear to make an angle of
35.2, 34.7 and 36.4o, respectively, in S0 state. Shortening of the C8-C9 bond by 0.01 Å is
observed in the case of the dye 3. A considerable decrease in the length of bond between C10-
C11 is observed upon changing groups at 2 and 6-positions of the PM core from ethyl (the dye
1) to benzyl (the dyes 2-4), from 1.540 Å for the dye 1 to 1.526, 1.527 and 1.527 Å for the dyes
2-4, respectively, in S0 state.
The S1 optimized states of the dyes 1-4 revealed that there is no remarkable change in the
geometry from that of the respective S0 states except in the case of the dye 3 where boron atom
was found to be displaced more from the PM core by 5.3o and the meso phenyl from its
orthogonal arrangement displaced more by 16.1o (Table 3).
Mulliken Charges. The analysis of Mulliken charge distribution of the dyes 2-4 in solvent 1,
4-dioxane (Table 4 and Figures S1-S4), relative to that of the dye 1 over the atoms of the PM
core (C-2, C-7’, C-8, C-9, and C-10), showed considerable change in the electron densities.
Changing the C-8 methyl substituent to the phenyl (dye 3) led to a little increase in the electron
density at C-7, but abrupt decrease in the electron density at C-8 is observed.
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Table 4: Mulliken Charges (e.s.u.) on different selected atoms of the dyes 1-4 at their respective optimized
ground state geometries, calculated by B3LYP/6-31G (d)level of theory in 1, 4-dioxane.
Dye
1
C-1
C-2
C-3
N-3’
B-4
C-7’
0.182
0.182
0.215
0.248
C-8
C-9
-0.013
-0.010
0.023
-
C-10
0.087
0.087
0.088
0.088
0.020
0.011
0.008
0.005
0.315
0.327
0.333
0.329
-0.576
-0.578
-0.582
-0.586
0.693
0.693
0.695
0.697
0.116
0.118
-0.027
-0.056
-0.055
-0.124
-0.122
-0.120
2
3
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31
32
33
34
35
36
37
38
39
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4
A similar trend is observed in the dye 4, hydrogen at meso position, but the change in electron
density at C-7’ and C-8 is more pronounced.
Also when the ethyl substituents of the dye 1 at 2 and 6-positions were changed to benzyl
groups in the dyes 2-4, the abrupt decrease in the electron density at C-2 and C-10 are observed.
From this it is clear that though the phenyl rings in benzyl groups are not in conjugation with
BODIPY chromophore, still can alter the electronic properties of the PM moiety leaving
positive impact on its photostability.
When compared the Mulliken charges in S0 states of the dyes 1-4 with their respective S1 states,
there is no or negligible change in the electron densities over the various atoms of PM moiety
(Table S1) which is consistent with experimental observations of small Stokes shift of the PM
dyes.
Calculated absorptions and emissions. The ground state optimized structures of dyes in
solvents of various polarities were subjected to TD first twenty states calculations20 in order to
get vertical excitations using B3LYP/6-31G(d) method. This led to more understanding of the
absorption properties of the synthesized PM dyes and the results are summarized in Tables 5
and 6.
Table 5: Calculated absorption parameters of the PM dyes 1-4 in ethanol, 1, 4-dioxane and n-heptane
using TD-DFT (B3LYP/6-31G(d)) method
Ethanol
1, 4-dioxane
n-Heptane
[a]
[b]
Dye
ES0→S1
(eV)
λab
(nm)
ES0→S1
(eV)
λab
(nm)
ES0→S1
(eV)
λab
f[c]
f
f
(nm)
438.9
440.7
444.7
441.4
1
2
3
4
2.8247
2.8133
2.7882
2.8088
0.5933
0.7023
0.6881
0.7036
2.7524
2.7950
2.7778
2.7968
450.5
443.6
446.3
443.3
0.5823
0.7384
0.7257
0.7462
2.8100
2.8002
2.7832
2.8026
441.2
442.8
445.5
442.4
0.6202
0.7331
0.7195
0.7399
[a]Energy required for the S0→S1transition; [b]Experimental maximum absorption wavelength; [c]Oscillator strength
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The results of DFT and TD-DFT also suggest no or negligible influence of the change in solvent
polarity on the absorption of the PM dyes (Table 6). The absorption maxima in all different
solvents are centered around 440-447 nm with 14.5 to 16.2% deviations from the experimental
values of absorption maximum. In all the solvents for the dyes 2-4, the vertical excitations are
associated with the HOMO→LUMO transitions and strong oscillator strengths ranging from
0.68 to 0.74. For comparison we have also performed the same calculations for dye 1 (Table
5) in selected solvents and got similar observations like the dyes 2-3.
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13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
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56
57
58
59
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Table 6: TD-DFT absorption and emission parameters of the dyes 2-4 in various solvents.
TD-B3LYP/6-31G(d)-Absorptions
TD-B3LYP/6-31G(d)-Emissions
[a]
[f]
λabs
λem
[b]
[g]
D[d]
(%)
Major
λem
D[i]
Solvent
Major
λabs
f[c]
f[h]
(nm)
(nm)
(nm)
contribution[e]
(nm)
(%)
Contribution
Dye 2
ACN
DMF
514
517
518
516
517
516
516
521
518
520
440.2
442.6
442.2
439.7
440.7
440.7
441.4
443.6
443.6
442.8
0.6970
0.7163
0.7129
0.6935
0.7023
0.7026
0.7149
0.7332
0.7384
0.7331
14.4
14.4
14.6
14.8
14.8
14.6
14.5
14.9
14.4
14.9
H→L(94.77)
H→L(95.31)
H→L(95.26)
H→L(94.67)
H→L(94.92)
H→L(94.91)
H→L(95.20)
H→L(96.64)
H→L(95.76)
H→L(95.62)
536
539
540
536
536
536
536
537
535
536
536.0
539.0
540.0
536.0
536.0
536.0
536.0
537.0
535.0
536.0
0.6258
0.6436
0.6412
0.6222
0.6289
0.6281
0.6236
0.6353
0.6097
0.5931
13.8
13.8
14.0
13.8
13.6
13.6
13.2
12.9
12.0
12.1
L→H(94.99)
L→H(95.49)
L→H(95.43)
L→H(94.89)
L→H(95.08)
L→H(95.06)
L→H(94.92)
L→H(95.25)
L→H(94.45)
L→H(93.91)
DMSO
MeOH
EtOH
Acetone
EtOAc
CHCl3
1,4-Dioxane
Heptane
Dye 3
ACN
DMF
520
523
524
521
522
521
521
525
523
523
444.2
446.6
446.5
443.7
444.7
444.6
444.7
447.1
446.3
445.5
0.6827
0.7033
0.6985
0.6789
0.6881
0.6882
0.7028
0.7197
0.7257
0.7195
14.6
14.6
14.8
14.8
14.8
14.7
14.7
14.8
14.7
14.8
H→L(93.75)
H→L(94.41)
H→L(94.29)
H→L(93.63)
H→L(93.92)
H→L(93.92)
H→L(94.31)
H→L(94.83)
H→L(94.91)
H→L(94.71)
534
537
538
535
535
535
535
536
537
537
463.6
466.0
465.5
463.2
464.3
464.4
466.9
469.6
473.5
474.2
0.5995
0.6187
0.6165
0.5955
0.6022
0.6007
0.5913
0.6031
0.5659
0.5440
13.2
13.2
13.5
13.4
13.2
13.2
12.7
12.4
11.8
11.7
L→H(93.47)
L→H(94.11)
L→H(94.03)
L→H(93.34)
L→H(93.58)
L→H(93.54)
L→H(93.35)
L→H(93.78)
L→H(92.66)
L→H(91.85)
DMSO
MeOH
EtOH
Acetone
EtOAc
CHCl3
1,4-Dioxane
Heptane
Dye 4
ACN
DMF
524
526
527
526
527
525
526
532
528
530
440.9
443.3
443.0
440.5
441.4
441.3
441.6
442.7
443.3
442.4
0.6975
0.7206
0.7167
0.6932
0.7036
0.7041
0.7183
0.7402
0.7462
0.7399
15.9
15.7
15.9
16.3
16.2
15.9
16.0
16.8
16.0
16.5
H→L(91.73)
H→L(92.58)
H→L(92.44)
H→L(91.57)
H→L(91.44)
H→L(91.95)
H→L(92.34)
H→L(93.08)
H→L(93.16)
H→L(92.92)
537
540
541
538
536
538
535
540
537
536
460.4
462.9
462.4
460.0
461.5
461.1
463.5
466.5
473.4
479.0
0.6274
0.6479
0.6457
0.6233
0.6620
0.6316
0.6185
0.6282
0.5450
0.4697
14.3
14.3
14.5
14.5
13.9
14.3
13.4
13.6
11.8
10.6
L→H(92.52)
L→H(93.27)
L→H(93.18)
L→H(92.36)
L→H(93.98)
L→H(92.59)
L→H(92.22)
L→H (92.60)
L→H(89.68)
L→H(86.53)
DMSO
MeOH
EtOH
Acetone
EtOAc
CHCl3
1,4-Dioxane
Heptane
[a]Experimental maximum absorption wavelength; [b]TD-B3LYP/6-31G(d) absorption; [c]Oscillator strength of vertical
excitation; [d]Deviation from the observed absorption maximum; [e]Major electronic transition; [f]Experimental maximum
emisson wavelength; [g]TD-B3LYP/6-31G(d) emission; [h]Oscillator strength of vertical emission; [i]Deviation from the
observed emission maximum; H=HOMO; L=LUMO.
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From the increased oscillator strengths of the dyes 2-4 in various solvents compared to that of
the dye 1 (Table 5) it can be seen that TD-DFT also account for the enhanced molar extinction
coefficients of the dyes 2-4.
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The minimum energy S1, B3LYP/6-31G(d), optimized structures were further subjected to the
TD ten states energy calculations20 in order to get emissions and the results are summarized in
Table 6. From the calculated emissions it can be seen that like vertical excitations, there is
negligible influence of changes in solvent polarity on the emission of novel PM dyes 2-4. The
TD-DFT emission maxima calculated for the dye 2 in various solvents are associated with
LUMO→HOMO transition and centered around 462.8 to 471.0 nm with oscillator strength of
the order of 0.59-0.64 (Table 6). Similarly the TD-DFT emissions for the dye 3 in various
solvents are associated with LUMO→HOMO transition and centered around 463.5-474.3 nm
with oscillator strengths in the range of 0.59-0.60 (Table 6).
Dipole moments. The TD-DFT calculated dipole moments of the dyes 1-4 in solvents of
different polarities are summarized in Table 7. The calculations revealed a minimum change
in dipole moments of the S0 and S1 states of the dyes 1-4 in the solvents of various polarities
supporting the weak solvatochromism. The calculated dipole moments of the dyes 2-3, both in
the ground (µ0) and the excited singlet (µ1) states, were higher compared to that of the dye 1,
suggesting better polarizability of the dyes 2 and 3.
Table 7: Dipole moments of the PM dyes 1-4 in various solvents using TD-DFT (B3LYP/6-31G(d)) method
Dye 1
Dye 2
Dye 3
Dye 4
[a]
[b]
Solvent
µ0
µ1
µ0
D
µ1
D
µ0
D
µ1
D
µ0
D
µ1
D
D
D
DMSO
DMF
ACN
MeOH
EtOH
Acetone
EtOAc
CHCl3
Dioxane
Heptane
n.d.
n.d.
n.d.
n.d.
5.7137
n.d.
n.d.
n.d.
4.6850
4.5632
n.d.
n.d.
n.d.
n.d.
5.8945
n.d.
n.d.
n.d.
4.8960
4.7791
5.8223
5.8014
5.7971
5.7872
5.7517
5.7207
5.3401
5.2149
4.6871
4.5640
5.9851
5.9649
5.9607
5.9511
5.9167
5.8866
5.5127
5.3924
4.8876
4.7738
5.9592
5.9440
5.9406
5.9334
5.9058
5.8815
5.5755
5.4730
5.0186
4.9075
6.1635
6.1472
6.1439
6.1362
6.1084
6.0840
5.7833
5.6908
5.2572
5.1566
5.1088
5.0902
5.0863
5.0775
5.0461
5.0201
4.6761
4.5655
4.1036
3.9966
5.1860
5.1675
5.1636
5.1494
5.0801
5.0839
4.7620
4.6589
4.2771
4.2431
[a]Dipole moment in ground state (S0); [b]Dipole moment in excited state (S1); n.d.= not determined
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However, as there is no substituent at meso position in case of the dye 4, the dipole moments
of the dye 4 in S0 and S1 are less compared to that of the dye 1 suggesting poor polarizability
of the dye 4 relative to that of the dyes 1-3. With the decrease in solvent polarity from DMSO
to n-heptane the dipole moments of the S0 and S1 states of the dyes 1-4 are found to be
decreasing.
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Molecular Orbital Energies. Energy levels of the frontier molecular orbitals especially
HOMO-1, HOMO, LUMO and LUMO+1 and their spatial distributions can give the idea about
behavior of the dye molecules in terms of photophysical properties, lasing properties and
photostabilities. The frontier molecular orbitals of the dyes 1-4 in 1, 4-dioxane, calculated by
B3LYP/6-31G(d) level of theory, are shown in Figure 7 and HOMO and LUMO energies and
the band gaps of the dyes 1-4 in ethanol, 1, 4-dioxane and n-heptane and that of the dyes 2-4
in solvents of different polarities are summarized in Table 8 and Table S2, respectively and
also shown in Figures 8-10.
From Figure 7 it can be seen that in the HOMO of the dye 1, the electron density is localized
on the BODIPY core but completely away from N-3, N-3’ and B-4 but in the LUMO of the
dye 1, it can be seen that the electron density is distributed almost uniformly over the whole
chromophore. From this it is confirmed that the HOMO of the dye can easily react with the in
situ generated singlet oxygen (1O2) upon laser excitation, because the C7’-C8 double bond has
a maximum electron density in the HOMO compared to the LUMO (Figure 7).
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Figure 7: Frontier molecular orbitals of dyes 1-4 in 1, 4-dioxane, calculated by B3LYP/6-31G(d)level of theory
(The orbital diagrams are plotted with the contour value of 0.02 a. u.)
Table 8: Energy of HOMO, LUMO and band gaps of the PM dyes 1-4 in ethanol, 1, 4-dioxane and n-
heptane using TD-DFT (B3LYP/6-31G(d)) method
Ethanol
EL
1,4-dioxane
EL
n-Heptane
EL
[a]
[b]
Dye
EH
eV
EH-EL
eV
EH
eV
EH-EL
eV
EH
eV
EH-EL
eV
2.974
2.982
2.936
2.953
eV
eV
eV
1
2
3
4
-5.361
-5.432
-5.456
-5.469
-2.362
-2.429
-2.510
-2.503
2.999
3.003
2.946
2.965
-5.246
-5.334
-5.332
-5.374
-2.269
-2.349
-2.395
-2.420
2.977
2.984
2.937
2.954
-5.235
-5.324
-5.319
-5.365
-2.261
-2.342
-2.383
-2.412
[a]Energy of highest occupied molecular orbital (HOMO); [b]Energy of lowest unoccupied molecular orbital (LUMO).
In case of the dyes 2-4 the electron density in HOMO and LUMO shows similar observations
like that of the dye 1 confirming again the involvement of HOMO of the dye 2-4 in the
photochemical reaction with single oxygen (1O2). Figures 8-10 and Table S2 reveal that among
the dyes 2-4 the order of the band gap (HOMO→LUMO) is 2>4>3 and for each of them the
band gap is found to be decreasing with the solvent polarity (dimethyl sulfoxide to n-heptane).
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Figure 8: Energy gaps between HOMO and LUMO of the dye 2 in various solvents.
Figure 9: Energy gaps between HOMO and LUMO of the dye 3 in various solvents.
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Figure 10: Energy gaps between HOMO and LUMO of the dye 4 in various solvents.
CONCLUSION
We have designed and synthesized three congeners of the widely used BODIPY laser dye
PM567 by modifications at the 2 and 6-positions/or at both the 2 and 6-positions and the meso
position. Remarkably enhanced photostability and lower concnetration of dye requirement to
get comparable lasing output in 1, 4-dioxane relative to that of PM567 are the positive effects
of the PM dyes, 2 and 4. From superior laser properties and enhanced photostabilities relative
to that of PM567, it may be envisaged that the dyes 2 and 4 can be used efficiently as laser
dyes in green-yellow region. The photophysical and laser properties of the synthesized PM
dyes experimentally and structural, photophysical and electronic properties by means of DFT
and TD-DFT are extensively studied. These findings will be helpful for the future design and
synthesis of efficient and photostable BODIPY laser dyes.
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EXPERIMENTAL SECTION
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General Methods. Laser grade Pyrromethene 567 was procured and used without any
further purification. The purity of the dye was found to be >99%, as determined with
spectroscopy and chromatographic methods. All the chemicals and spectroscopy grade
solvents were obtained from the local suppliers unless otherwise mentioned and used as such
without further purifications. All the common chemicals were of analytical grade. The solvents
were purified by standard procedures. All the reactions were monitored by TLC (thin-layer
chromatography) with detection by UV light. The FT-IR spectra were recorded on a FT/IR
spectrometer. 1H NMR, 13C NMR, 11B NMR and 19F NMR spectra were recorded on 500 MHz
instrument using TMS as an internal standard. HRMS analysis were done using QTOF LC/MS
spectrometer, Mass spectra were recorded on mass spectrometer.
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Photophysical studies: The absorption spectra were recorded on UV-vis spectrometer, and
the corresponding maximum molar extinction coefficients (Ɛmax) were determined. The
emission spectra were recorded on fluorescence spectrophotometer using 10 mm cuvette with
2.5 nm slit width at 490 nm excitation wavelength. The relative fluorescence quantum yields
(Φfl) of the dyes 2-4 were determined using the dye 1 as a standard. The excited-state (S1)
lifetimes of the dyes 1-4 were determined by time resolved fluorescence measurements carried
out with an LED-based time-correlated single photon-counting (TCSPC) spectrometer. The
fluorescence decays were measured with a 390 nm LED (1 MHz) excitation source and a TBX4
detection module coupled to a special PMT instrument. Following deconvolution analysis of
the fluorescence decays, the time resolution of the present setup was around 50 ps. All the
measurements were carried out at ambient temperature (298±1 K), maintained with a
microprocessor based temperature controller.
Broad Band Laser study: The lasing studies of the dyes 1–4 in 1, 4-dioxane were carried
out by using a constructed broad-band dye laser setup (a dye cell of width ~10 mm placed
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between a high reflectivity (~100%) mirror and wedged output coupler (~4% reflectivity)),
transversely pumped by the second harmonic (at 532 nm) output of a Q-switched pulsed
Nd:YAG laser at a repetition rate of 10 Hz with a pulse energy of around 4.2 mJ and fwhm of
5–7 ns. All the laser data for the dyes in 1, 4-dioxane were measured by using the same dye
laser setup, which is schematically shown in Figure S5 in the supporting information. The
pump and the dye laser powers were measured with the same power meter. To determine the
lasing efficiency at various concentrations of 1, 4-dioxane solutions of the dyes 1-4, initially
dye solution of higher concentration was tested in each case and then stepwise diluted. The
absorbance of each dye solutions was measured from which the dye concentration was
calculated by using the formula, A=ɛbc, where A is absorbance; ɛ is molar extinction
coefficient at absorption maxima; b is path length (0.1 cm); and c is the concentration.
Photostability study: The quantum yield of the photodegradation (Φpd) of the dye is
defined as the probability of the decomposition of the dye molecules by the absorbed pump
photons. The photostability is expressed as the inverse of the Φpd value. A known quantity of
the dye solution (2.5 mL) in a dye laser cuvette was exposed to a pump energy of 6.7-7.2 mJ
at 532 nm. The concentration of the dye solution was chosen such that the pump beam was
totally absorbed within the dye solution in the cuvette during the excitation of 3-4 h. The
solution was constantly stirred with a teflon-coated magnetic stirrer to avoid local heating. The
number of photodegraded dye molecules in the exposed volume of the dye solutions was
quantitatively estimated from the absorbance at the corresponding λmax before and after photo
exposure for a set period of time. The reflection loss of the pump beam on incident surfaces of
the dye cell was determined by calculating the absorbed cumulative pump photons.
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Electrochemical studies: Cyclic voltammetry was performed at 25 C in deoxygenated
acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) and a
solute concentration of 0.6-1 mM. The glassy carbon (2 mm diameter) was used as working
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electrode, platinum wire as counter electrode and standard calomel electrode as reference
electrode with ferrocene (~1 mM) as an internal standard. The redox potentials were
standardized with ferrocene (Fc) as the internal reference and referenced to SCE by taking E0
(Fc/Fc+) = +0.405 V versus SCE. All waves were monoelectronic.
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Computational details: All the computations were performed using of the Gaussian 09
program package.21 The ground state (S0) geometry of the dyes was optimized using DFT22
method. The functional used was B3LYP, (the B3LYP combines Becke’s three parameter
exchange functional (B3)23 with the nonlocal correlation functional by Lee, Yang and Parr
(LYP).24 The basis set used in both, DFT and TD-DFT methods for all the atoms was 6-31G(d).
In order to verify whether the optimised structures have minimum energy, frequency
computations were performed at the same level of theory. The vertical excitation energies and
oscillator strengths were obtained for the 20 lowest S0→S1 transitions at the optimized ground
state equilibrium geometries by using the Time Dependent Density Functional Theory (TD-
DFT) using the same hybrid functional and basis set.20, 25a-g To obtain their minimum energy
geometries (which correspond to the emissive state) the low-lying first singlet excited states
(S1) of the dyes were relaxed using the TD-DFT. The emission wavelengths and oscillator
strengths were obtained for the 10 lowest S1→S0 transitions at the optimized excited state
equilibrium geometries by using the Time Dependent Density Functional Theory (TD-DFT)
using the same hybrid functional and basis set.20 The frequency computations were also carried
out at the same level of theory on the optimised geometry of the first excited state of the dyes.
All the computations in the various solvents media were carried out using the Self-Consistent
Reaction Field (SCRF) under the Polarisable Continuum Model (PCM).26a-b The electronic
absorption spectra, including wavelengths, oscillators strengths, and main configuration
assignment, were systematically investigated using TD-DFT with PCM model on the basis of
the optimized ground structures.
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Synthesis of 3-Benzylpentane-2, 4-dione (6): To a stirred solution of acetyl acetone (5 g,
0.0499 mol) in acetone (25 mL) at 0 °C was added sodium carbonate (6.86 g, 0.0648 mol) in
portions and the mixture was allowed to stir for 15 min. Then to this was added drop wise a
solution of benzyl bromide (8.96 g, 0.0524 mol) in acetone (20 mL) for 15 min.The resulting
mixture was allowed to stir at reflux temperature for 24 h. The cooled reaction mixture was
filtered to remove solid sodium carbonate, filtrate was concentrated under vacuum and the pale
yellow liquid obtained was allowed to stand for a 5-6 hours. The off white solid formed was
discarded by decantation and obtained product (6), pale yellow liquid, was used as such without
any further purification for next step. Crude yield: 8.37 g, (88%).
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Synthesis of Ethyl 4-benzyl-3,5-dimethyl-1H-pyrrole-2-carboxylate (7): To a solution
of ethyl acetoacetate (3.42 g, 0.0263 mol) in acetic acid (12 mL) at 0°C was added a cold
solution of sodium nitrite (2.2 g, 0.0315 mol) in water (10 mL). After stirring the mixture for
12 h at 15-20 °C, the compound 6 (5 g, 0.0263 mol) in one portion was added followed by a
portion wise addition of zinc dust (3.46 g, 0.0526 mol) maintaining the temperature below
15 °C. After completion of addition, zinc was allowed to react at room temperature with
vigorous stirring for 15-20 min and then the mixture was heated at 80 °C for1.5 h. It was
allowed to attain room temperature then poured on to the ice cold water (~500 mL). The
precipitated off white solid was isolated by filtration, dried and recrystallized from chloroform
gave 7 as white solid. Yield: 6.12 g, 90.5%; Mp:127 °C; 1H NMR; (500 MHz; CDCl3, TMS):
δ 1.35 (t, J=7 Hz, 3H), 2.18 (s, 3H), 2.22 (s, 3H), 3.76 (s, 2H), 4.3 (q, J=7.5 Hz, 2H) 7.09-7.17
13
(m, 3H), 7.25 (t, J=7.5 Hz, 2H), 8.63 (s, 1H); C NMR; (500 MHz; CDCl3, TMS): δ 10.8,
11.9, 14.6, 29.8, 59.7, 117.0, 120.3, 125.7, 128.0, 128.3, 130.1, 141.2, 161.6; MS (ESI): m/z
calcd for (M+H)+ C16H20NO2 258.1; found 257.9
Synthesis of Sodium 4-benzyl-3, 5-dimethyl-1H-pyrrole-2-carboxylate (8): To a
solution of 7 (5 g, 0.0194 mol) in ethanol (20 mL) was added sodium hydroxide (1.55 g, 0.0388
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mol) followed by ~0.5 mL water. The resulting mixture was allowed to stir at reflux
temperature for 5 h. After completion of reaction, confirmed by TLC, the reaction mixture
allowed to cool to r.t. The excess ethanol was removed under vacuum and white solid product
was isolated after filtration, dried and washed with little ice cold water followed by 5% ethyl
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acetate in pet ether yielded 8 in the pure form as white solid (4.62 g, 94.6%); H NMR (500
MHz; D2O, TMS): δ 2.03 (s, 3H), 2.04 (s, 3H), 3.56 (s, 2H), 7.04-7.08 (m, 3H), 7.18 (t, J=7.5
13
Hz, 2H); C NMR (500 MHz; CDCl3, TMS): δ 11.1, 11.3, 31.1, 118.0, 120.3, 124.2, 124.3,
125.8, 128.4, 128.5, 142.8, 167.7; HRMS (ESI): m/z calcd for (M-23)+ C14H14NO2 228.1030;
-
found 228.0155
Synthesis of 3-Benzyl-2, 4-dimethyl-1H-pyrrole (9): To the compound 8 (4 g, 0.0159
0
mol) in 50 mL round bottom flask was added trifluoroacetic acid (8 mL) at 20 C under N2
atmosphere and stirred for 5 min. Then to this was added 20 mL chloroform and immediately
washed with cold water followed by twice with saturated sodium bicarbonate solution. Then
the organic layer was dried on sodium sulphate and solvent was evaporated which upon cooling
furnish 9 as off white to light brown solid (1.8 g, 61%) which was stored in refrigerator and
used for next step without further purification.
1H NMR (500 MHz; CDCl3, TMS): δ 1.96 (s, 3H), 2.18 (s, 3H), 3.78 (s, 2H), 6.44 (s, 1H), 7.16
(d, J=7.5 Hz, 3H), 7.20-7.24 (m, 2H), 7.61 (s, 1H); 13C NMR (500 MHz; CDCl3, TMS): δ 10.6,
11.5, 30.2, 113.0, 125.3, 125.4, 126.4, 128.1, 128.2, 128.2, 142.3; IR: cm-1 3415, 1604, 1490,
1452, 1351, 1226, 1155; MS (ESI): m/z calcd for (M+H)+ C13H16N 186.1; found 186.3.
Synthesis of 4-Benzyl-3, 5-dimethyl-1H-pyrrole-2-carbaldehyde (10): The compound
10 was synthesized from their corresponding precursors following the procedure reported in
reference;18 Yield=90.6%; H NMR (500 MHz; CDCl3, TMS): δ 2.22 (s, 3H), 2.24 (s, 3H),
1
3.78 (s, 2H), 7.11 (d, J=7.5 Hz, 2H), 7.18 (t, J=7.5 Hz, 1H), 7.27 (t, J=7.5 Hz, 2H), 9.45 (s,
1H), 9.80 (bs, 1H); 13C NMR(500 MHz; CDCl3, TMS): δ 9.0, 12.1, 29.6, 126.0, 126.4, 126.5,
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127.8, 127.9, 128.0, 128.5, 128.6, 173.9; HRMS (ESI): m/z calcd for (M+H)+ C14H16NO
214.1234; found 214.1218.
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Synthesis of 2, 6-Dibenzyl-4, 4-difluoro-1, 3, 5, 7, 8-pentamethyl-4-bora-3a, 4a-diaza-
s-indecene (2): The compound 9 (200 mg, 1.08 mmol) was dissolved in 100 mL dry
dichloromethane and the mixture was degassed for 10 minutes using dry nitrogen. Then to this
was added 38.5 µL (0.54 mmol) of acetyl chloride under N2 atmosphere. The resulting mixture
was allowed to stir at room temperature for 4 hours. Then the dichloromethane was removed
under vacuum to dryness and the obtained dark red solid was washed twice with n-hexane (15
mL x 2). The dried solid was again dissolved in fresh dichloromethane (100 mL) followed by
drop wise addition 2.5 mL of triethyl amine. The reaction mass was allowed to stir for 10 min.
at room temperature and then to this was added 3 mL of BF3:OEt2 drop wise over 5 min. under
N2 atmosphere at r.t. The resulting reaction mixture was stirred for 1 hour at room temperature
then washed with water followed by 1 N NaOH ice cold solution. The organic layer was
separated, dried over sodium sulphate and concentrated under vacuum and obtained dark red
colored solid was then subjected to silica gel column chromatography using 0.5 to 1% ethyl
acetate in hexane yielded 88 mg dark orange colored solid. Yield= 18.4%; 1H NMR (500 MHz,
CDCl3, TMS): δ 2.34 (s, 6H), 2.47 (s, 6H), 2.65 (s, 3H), 3.81 (s, 4H), 7.11 (d, J=7.5 Hz, 4H),
7.19 (t, J=7.5 Hz, 2H), 7.26-7.29 (m, 4H); 13C NMR (500 MHz, CDCl3, TMS): δ 12.8, 18.9,
17.1, 29.6, 126.0, 127.9, 128.5, 128.8, 131.9, 137.8, 140.0, 140.5, 152.9; 19F NMR (500 MHz,
CDCl3, TMS): δ -145.87 (q, J=35.5 Hz). HRMS (ESI): m/z calcd for (M+H)+ C28H30BF2N2
443.2472; found 443.2451.
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Synthesis of 2, 6-Dibenzyl-4, 4-difluoro-1, 3, 5, 7-tetramethyl-8-phenyl-4-bora-3a, 4a-
diaza-s-indecene (3): Following the similar procedure as described for the dye 2, the dye 3 (96
mg) was prepared from compound 9 (200 mg, 1.08 mmol) and benzoyl chloride (38.5 µL, 0.54
mmol).Yield= 17.6%; 1H NMR(500 MHz, CDCl3, TMS): δ 1.29 (s, 6H), 2.50 (s, 6H), 3.70 (s,
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4H), 7.06 (dd, J=7.5 Hz and J=1 Hz, 4H), 7.16 (t, J=7.5 Hz, 2H), 7.23-7.29 (m, 4H), 7.30-7.32
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(m, 2H), 7.45-7.48 (m, 3H); C NMR (500 MHz, CDCl3, TMS): δ 12.1, 12.9, 29.6, 126.0,
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128.0, 128.2, 128.4, 128.9, 129.1, 129.3, 131.0, 135.5, 139.8, 139.9, 140.9, 154.7; B NMR
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(500 MHz, CDCl3, TMS): δ 0.84 (t, J=103 Hz); 19F NMR (500 MHz, CDCl3, TMS): δ -145.54
(q, J=35 Hz); HRMS (ESI): m/z calcd for (M+H)+ C33H32BF2N2 505.2628; found 505.2631
Synthesis of 2, 6-Dibenzyl-4, 4-difluoro-1, 3, 5, 7-tetramethyl-8H-4-bora-3a, 4a-diaza-
s-indecene (4): The dyes 4 was synthesized following the procedure reported in reference.18
1
Yield=86.4%; H NMR(500 MHz, CDCl3, TMS): δ 2.17 (s, 6H), 2.45 (s, 6H), 3.78 (s, 4H),
7.05 (s, 1H), 7.12 (d, J=7.5 Hz, 4H), 7.19 (t, J=7.5 Hz, 2H), 7.26-7.29 (m, 4H); 13C NMR (500
MHz, CDCl3, TMS): δ 9.8, 12.9, 29.9, 119.2, 126.1, 128.0, 128.2, 128.5, 132.6, 138.1, 139.7,
155.7; 11B NMR (500 MHz, CDCl3, TMS): δ 0.9 (t, J=103.5 Hz); 19F NMR (500 MHz, CDCl3,
TMS): δ -146.06 (q, J=35 Hz).HRMS (ESI): m/z calcd for (M+H)+ C27H28BF2N2 429.2315;
found 429.2314
ACKNOWLEDGEMENTS
Authors thank Mr. Amol Patil and Prof. B. M Bhange, ICT for their help in electrochemistry
experiments. Also we are thankful to Dr. V. Sudarshan, Chemistry Division, BARC for his
help in fluorescence lifetime decay measurements. One of the authors (KGT) is thankful to the
Council of Scientific and Insdustrial Research (CSIR), India for Junior and Senior Research
Fellowships. RM is thankful to the UGC-CAS in Green Technology for the award of JRF and
SRF under SAP.
Supporting Information Available: Mulliken charge density table, optimized geometry of
the dyes, and schematic representation of the broad band dye laser setup. Plausible reaction
1
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mechanisms of the dyes with singlet oxygen and DFT and TD-DFT data. H and C NMR,
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