Hexa boron-dipyrromethene cyclotriphosphazenes: Synthesis, crystal structure, and photophysical properties

Article abstract of DOI:10.1021/ic1016092

We have synthesized four examples of a cyclotriphosphazene ring appended with six boron-dipyrromethene dyes N₃P₃(BODIPY) ₆by adopting two different methods. In method I, 1 equiv of N ₃P₃Cl₆

Full text of DOI:10.1021/ic1016092

10606 Inorg. Chem. 2010, 49, 10606–10616
DOI: 10.1021/ic1016092
Hexa Boron-Dipyrromethene Cyclotriphosphazenes: Synthesis, Crystal Structure,
and Photophysical Properties
M. Rajeswara Rao,† R. Bolligarla,‡ Ray J. Butcher,§ and M. Ravikanth*,†
†Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India,
‡School of Chemistry, University of Hyderabad, Hyderabad 500 046, India, and §Department of Chemistry,
Howard University, Washington, D.C. 20059, United States
Received August 9, 2010
We have synthesized four examples of a cyclotriphosphazene ring appended with six boron-dipyrromethene dyes
N3P3(BODIPY)6 by adopting two different methods. In method I, 1 equiv of N3P3Cl6 was treated with 6 equiv of
meso-(o- or m- or p-hydroxyphenyl)boron-dipyrromethene in tetrahydrofuran (THF) in the presence of cesium
carbonate. This afforded N3P3(BODIPY)6 in yields ranging from 80 to 90%. In method II, we first prepared hexakis-
(p-formylphenoxy)cyclotriphosphazene N3P3(CHO)6 by treating 1 equiv of N3P3Cl6 with 6 equiv of 4-hydroxybenzal-
dehyde in the presence of cesium carbonate in THF. In the second step, N3P3(CHO)6 was condensed with excess of
pyrrole in the presence of catalytic amount of trifluoroacetic acid (TFA) in CH2Cl2 at room temperature and afforded
hexakis(p-phenoxy dipyrromethane)cyclotriphosphazene. In the last step, the hexakis(p-phenoxy dipyrrometh-
ane)cyclotriphosphazene was first oxidized with 6 equiv of DDQ in CH2Cl2 at room temperature for 1 h followed by
neutralization with triethylamine and further reaction with excess BF3 Et2O afforded the target N3P3(BODIPY)6 in 16%
3
yield. The route II was used only for the synthesis of one target compound whereas the route I was used for the
synthesis of all four target compounds. The four compounds were characterized by mass, NMR, absorption,
electrochemical, and fluorescence techniques. The crystal structure solved for one of the compounds revealed that the
P3N3 ring is slightly puckered and the six substituents were not interacting with each other and attained pseudo-axial
and pseudo-equatorial positions. The photophysical studies in five different solvents indicated that the compounds
exhibit large Stokes’ shifts unlike reference monomeric BODIPYs indicating that the compounds are promising for
fluorescence bioassays. The quantum yields and lifetimes of compounds 1-4 depends on the type of BODIPY unit
attached to the cyclotriphosphazene ring.
Introduction
toward chromatography on silica and alumina.1 BODIPY
dyes have been used extensively as labeling reagents, fluorescent
switches, chemosensors, and as laser dyes.1 Multi-BODIPY
dyes containing more than one BF2-dipyrrin complex have
recently attracted considerable attention owing to their novel
features and also their potential use as functional dyes in bio-
medical and material applications.2 Porphyrins,2a anthracenes,2b
pyrenes,2c truxene,2d and so forth have been used as scaffolds
to build multi-BODIPY systems. However, most BODIPY
dyes have the fatal disadvantage that their Stokes’ shifts are
less than 20 nm. A small Stokes’ shift can cause self-quenching
and measurement error by excitation light and scattered
light. Both of these can decrease the detection sensitivity to
a great extent. Therefore, the BODIPY dyes with a larger
Stokes’ shift are very promising for fluorescence bioassays.
Hexachlorocyclotriphosphazene is a robust inorganic ring
containing six labile groups that can be substituted with the
desired groups. This feature has been exploited by several
researchers and used to prepare a variety of hexa-substituted
cyclotriphosphazenes.3 Specially, the cyclotriphosphazenes
Among the fluorescent dyes available, the boron-
dipyrromethene (BODIPY) dyes are becoming increasingly
popular because the BODIPY dyes have remarkable chara-
cteristics such as high absorption coefficients, high fluores-
cence yields, long excited state lifetimes, and good solubility
in organic solvents with excellent photostability and amenability
*To whom correspondence should be addressed. E-mail: ravikanth@
chem.iitb.ac.in.
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pubs.acs.org/IC
Published on Web 10/14/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10607
Chart 1
containing coordinating groups such as pyrazoles,4 hydra-
zones,5 hydrazine motifs,6 pyridines,7 and so forth have been
synthesized and studied extensively for their metal complex-
ing properties. The redox active groups such as ferrocenes
have also been substituted on cyclotriphosphazene and ex-
plored for their redox properties.8 Interestingly, the reports
on cyclotriphosphazene containing fluorescent groups are
very few. Majoral and co-workers9 synthesized a series of
phosphorus dendrimers containing fluorophores using cyclo-
triphosphazene as platform, and their photophysical studies
indicated that this kind of compound would find applications
in devices such as organic light emitting diodes and organic
nanodots and also for the elucidation of biological mechan-
isms and for medical imaging. We recently prepared cyclo-
triphosphazene appended with six porphyrins in high yield
under simple reaction conditions.10 Porphyrins are tetrapyr-
rolic aromatic macrocycles present at the active site of several
biomolecules and well-known for their metal coordination
properties. However, porphyrins are moderately fluorescent,
and their fluorescence yields are much lower than many other
commercially available aromatic fluorescent compounds
such as pyrene, anthracene, fluorenene, rhodamine, and so
forth. In this paper, weusedBODIPYdye as fluorophore and
synthesized four novel hexa BODIPY appended cyclotriphos-
phazenes, hexa(boron-dipyrromethene)cyclotriphosphazenes
N3P3(BODIPY)6 1-4 (Chart 1) under mild reaction con-
ditions. The spectral, electrochemical, and photophysical
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10608 Inorganic Chemistry, Vol. 49, No. 22, 2010
Scheme 1. Synthesis of BODIPY 5
Rao et al.
0.5 ppm for compounds 5-7 and at 1.0 ppm for com-
pound 8. The more downfield shift of compound 8 in 11B
NMR is due to the presence of electron donating alkyl
groups at β-pyrrole carbons.
To prepare the target compounds 1-4, we first at-
tempted to prepare hexa(BODIPY)cyclotriphosphazene
3 by two different routes. Initially, we synthesized the
compound 3 by following the method10 which we used
earlier for the synthesis of hexaporphyrinyl substituted
cyclotriphosphazene (Route I) (Scheme 2). The com-
pound 3 was prepared by reacting 1 equiv of N3P3Cl6 with
Chart 2
6
equiv of N,N0-difluoroboryl-5-(p-hydroxyphenyl)
dipyrrin13 7 in the presence of 12 equiv of cesium carbo-
nate in tetrahydrofuran (THF) at 0 °C and continued
stirring at 50 °C for 6 h. The progress of the reaction was
monitored by thin layer chromatography (TLC) analysis
which initially showed the spots corresponding to par-
tially substituted products which disappeared as the
reaction progressed and showed one single spot corre-
sponding to the compound 3. The crude reaction mixture
was subjected to column chromatography and afforded
pure compound 3 as red powder in 88% yield. Alter-
nately, the compound 3 was also prepared by following
route II in three steps (Scheme 3). In the first step,
hexakis(p-formylphenoxy)cyclotriphosphazene N3P3(CHO)6
9 was synthesized14 by treating 1 equiv of N3P3Cl6 with 6
equiv of p-hydroxybenzaldehyde in the presence of ce-
sium carbonate in THF at 0 °C and stirred additionally
for 48 h at 50 °C. The crude compound was purified by
column chromatography and afforded desired aldehyde 9
as a white solid in 92% yield. The molecular ion peak at
861.9 in the matrix-assisted laser desorption/ionization
time-of-flight (MALDI-TOF) mass spectrum confirmed
the identity of the compound 9. The compound 9 was
properties were investigated and in one case the single crystal
structure was determined. The photophysical studies of com-
pounds 1-4 studied in different solvents indicated that the
compounds exhibit large Stokes’ shifts compared to their
corresponding reference monomers and thus may find appli-
cations as fluorescent labels in biological experiments.
Results and Discussion
Synthesis and Characterization. The required boron-
dipyrromethene precursors containing hydroxyphenyl
functional group 5-8 were synthesized by following the
standard protocol11 as shown for the BODIPY dye 5 in
Scheme 1. In the first step, the corresponding dipyrro-
methanes were prepared by condensing 1 equiv of appro-
priate aldehyde with 25 equiv of pyrrole in the presence of
1
further confirmed by 31P and H spectroscopy. In 31P
0.1 equiv of BF3 OEt2 for 10 min at room temperature.12
3
NMR, the compound 9 showed only one signal at 5.31
ppm supporting the hexasubstitution on the cyclotripho-
sphazene ring. In 1H NMR, a singlet at 9.94 ppm corres-
ponding to an aldehyde proton was present. In the
second step, 1 equiv of compound 9 was condensed
with 240 equiv of pyrrole in the presence of 0.6 equiv of
trifluoroacetic acid (TFA) in CH2Cl212 at room temperature
for 30 min followed by standard workup, and quick flash
column chromatographic purification afforded hexakis-
(p-phenoxydipyrromethane)cyclotriphosphazene 10 as
a brownish solid in 62% yield. The compound 10 was
The crude compounds were purified by flash column
chromatography and yielded the corresponding dipyrro-
methanes as white solids. The dipyrromethanes were then
subjected to a two step one pot reaction by oxidizing the
dipyrromethanes in the first step with DDQ and reacted
in second step with BF3 OEt2 at room temperature. Col-
3
umn chromatographic purification on silica yielded the
functionalized BODIPY dyes 5-8 (Chart 2) as bright
fluorescent green powders in 33-42% yields. The BODIPYs
5-8 were confirmed by the molecular ion peak in mass
spectra and characterized by various spectroscopic
techniques. The 1H NMR spectra of BODIPYs 5-8 were
slightly different from each other because of the presence
of hydroxyl group at different position on meso-phenyl
group. However, there are some major differences were
noted in their 19F and 11B NMR spectra of 5-8. In 19F
NMR, the compounds 6-8 exhibited a typical quartet at
-145 ppm but compound 5 showed a complicated two
sets of doublet of quartets in the -144 to -146 ppm
region (Supporting Information). This is due to the ortho-
substitution causing inequivalence in the chemical envir-
onment for two fluoride ions. The fluoride ion first
couples with boron (I = 3/2) and gives a quartet which
then couples with the second fluoride ion to give a doublet
of a quartet. In 11B NMR, a typical triplet was observed at
1
characterized by 31P and H spectroscopic techniques.
In 31P NMR, compound 10 showed a singlet at 7.04 ppm
which was upfield shifted by 2.2 ppm compared with
compound 9. In 1H NMR, the 36 pyrrole protons of six
meso-aryl dipyrromethane units appeared as sets of three
signals at 5.81, 6.09, and 6.82 ppm and 12 pyrrole -NH
protons appeared as one broad singlet at 7.79 ppm. The
six meso protons appeared as a singlet at 5.30 ppm
(Supporting Information). In the last step, the compound
10 was first oxidized with 6 equiv of DDQ in CH2Cl2 at
room temperature for 1 h followed by neutralization with
triethylamine, and further reaction with excess BF3 Et2O
afforded the crude product. The crude product was
3
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€
€
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Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10609
Scheme 2. Synthesis of Compound 3 by Route I
Scheme 3. Synthesis of Compound 3 by Route II
subjected to column chromatographic purification and
afforded the desired pure compound 3 as orange solid in
16% yield. Since the route I is more convenient and
involves less steps, the other hexa BODIPY appended
cyclotriphosphazenes 1, 2, and 4 were prepared by follow-
ing route I. Coupling of N3P3Cl6 with 6 equiv of corre-
sponding meso-hydroxyphenyl BODIPY 5, 6, and 8 in the
presence of Cs2CO3 in THF for 6 h at room temperature

10610 Inorganic Chemistry, Vol. 49, No. 22, 2010
Rao et al.
Figure 1. 1H NMR spectrum of compound 3 recorded in CDCl3. The 31P, 11B, and 19F NMR spectra of compound 3 recorded in CDCl3 were shown as
inset (a), (b), and (c), respectively.
followed by alumina column chromatography yielded red
solids of hexa BODIPY appended cyclotriphosphgazenes
1, 2, and 4 respectively in 80-90% yields. In all these
reactions, initially we noticed the formation of partially
substituted products which slowly converted to hexa
BODIPY substituted cyclotriphosphazenes as the reac-
tion progressed. It is noted that the compounds 1 and 2
were formed only at room temperature conditions, and
any heating of the reaction mixture even to 50 °C resulted
in decomposition. The compounds 1-4 were freely solu-
ble in common organic solvents. The compounds 1-4
were confirmed by the appropriate molecular ion peak in
ES-MS mass spectra (Supporting Information). The
compounds 1-4 were characterized by 31P, 11B, 19F and
1H NMR spectroscopy as shown for compound 3 in
1H NMR compared with their corresponding monomeric
BODIPYs 5 and 6 respectively (Supporting Information).
In compounds 1 and 2, the number of signals were more
and appeared at upfield compared with their correspond-
ing monomeric BODIPYs. This supports an interaction
among BODIPY units in compounds 1 and 2. Thus, the
NMR studies supported that the six BODIPY units in
compounds 1 and 2 interact with each other whereas in
compounds 3 and 4, the BODIPY units behave indepen-
dently and do not interact with each other.
Crystallographic Studies. The single crystal of com-
pound 1 was obtained from deuterated chloroform in
an NMR tube on slow evaporation over a period of two
weeks, and the complex crystallized with four molecule
of CDCl3 in the unit cell (not shown). The compound
1 crystallized in monoclinic with a P1 space group. Figure 2a
shows the X-ray structure of 1 (CCDC-786046) and the
selected parameters, bond lengths, and bond angles are
summarized in Table 1. For clarity, simplified structures
of 1 are also shown in Figures 2b and 2c. As clear from the
structure, the six BODIPY pendant groups are arranged
in a symmetric manner about the cyclotriphosphazene
ring, and each side of the ring contains three BODIPY
pendant groups (Figure 2c). The P3N3 ring is slightly
puckered in a chair conformation. The mean deviation of
the ring atoms from their mean plane (rms) is 0.088 A.
Because of puckering of P3N3 ring, the six substituents
can be considered as pseudo-equatorial and pseudo-axial
which can be clearly seen in the deviation of the oxygen
atoms from the P3N3 ring. Thus O1a (þ1.01°), O2b
(-0.84°), and O2c (þ0.97°) are pseudo-equatorial
and O2a (-1.41°), O1b (þ1.50°), and O1c (-1.40°) are
pseudo-axial. The P-N distance, P-N-P angles, and
N-P-N angles are very close to previously observed
hexasubstituted cyclotriphosphazenes. The average P-N
bond length in P3N3 ring is 1.58 A while the average
N-P-N and P-N-P bond angles are 117.5° and 121.2°
respectively. The average P-O bond length is 1.58 A
while the average O-P-O bond angle is 99.2°. Each
BODIPY framework consisting two pyrrole rings and
1
Figure 1. We also recorded H-1H COSY spectrum for
compound 1 to arrive at the structures of these com-
pounds (Supporting Information). The compounds 1-4
showed one singlet in 5.1-6.8 ppm region in 31P NMR
(Figure 1a) supporting the hexa substitution of the cyclo-
triphosphazene ring. In 11B NMR, the compounds 1-3
showed a triplet at ∼0.5 ppm (Figure 1b) and compound 4
at 1.0 ppm. These chemical shifts in 11B NMR spectra of
compounds 1-4 are almost identical with their corre-
sponding monomeric BODIPYs supporting the magnet-
ically equivalent environment for all six boron atoms in
compounds 1-4. Similarly, in 19F NMR, a quartet signal
at -145 ppm (Figure 1c) was observed for compounds
2-4 which is in agreement with their correspond-
ing monomeric BODIPYs. However, like monomeric
BODIPY 5, the corresponding hexa BODIPY substituted
cyclotriphosphazene 1 also showed two sets of doublet
of quartets in the -145 ppm region in 19F NMR. The
most interesting observations were made in the 1H NMR
spectra of compounds 1-4. Compounds 3 and 4 exhib-
1
ited a simple H NMR (Figure 1) spectra which closely
matched with their corresponding monomeric BODIPYs
7 and 8 respectively. This indicated that the BODIPY
units are not influencing each others in compounds 3 and
4. However, compounds 1 and 2 exhibited quite different

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10611
Figure 2. (a) Crystal structure of compound 1. Hydrogen atoms and solvent molecules have been omitted for clarity. Simplified crystal structure of
compound 1 showing only (b) two BODIPYs on one “P” and (c) one BODIPY on each “P” of P3N3 ring.
Table 1. Selected Bond Lengths [A] and Bond Angles [deg] for compound 1
the meso-aryl ring and the plane defining various dipyrrin
atoms in compound 1 is in the range 60-81° which is
bond length (deg)
bond angle (A)
almost similar to the reported monomeric BODIPYs.15
Absorption and Electrochemical Studies. The absorp-
tion properties of compounds 1-4 were studied in five
different solvents, and the data are presented in Table 2.
The comparison of absorption spectra of compounds
1-4 recorded in chloroform is shown in Figure 3 and
the comparison of compound 3 with its monomer 7
recorded in chloroform at the same concentration is
presented as an inset in Figure 3. The inspection of data
in Table 2 and the Figure 3 reveal the following: (1)
compounds 1-4 showed typical BODIPY absorption
features15 with a strong band at the 500-530 nm region
corresponding to S0fS1 transition with a vibronic transi-
tion on the higher energy side as a shoulder and an ill-
defined, weak band corresponding to the S0fS2 transi-
tion at about 420 nm in all solvents. The absorption
spectra exhibited 8-10 nm blue shift when solvent is
P1-N1
1.583(4)
1.574(4)
1.586(4)
1.580(4)
1.578(4)
1.585(4)
O1A-P1-O2A 100.6(2)
P1-N3
P2-N1
P2-N2
P3-N2
P3-N3
O1B-P2-O2B
98.7(2)
O1C-P3-O2C 98.3(2)
N1-P2-N2
N1-P1-N3
N2-P3-N3
P1-N1-P2
P1-N3-P3
P2-N2-P3
N1-B-N2
N1-B-F1
N1-B-F2
117.1(2)
118.0(2)
117.5(2)
121.2(2)
120.6(2)
121.8(2)
105.8(4)-106.4(5)
109.8(5)-110.1(5)
109.5(5)-111.0(5)
110.1(5)-111.5(5)
109.5(5)-110.1(5)
109.4(4)-109.9(4)
P1-O1A 1.579(3)
P1-O2A 1.597(3)
P2-O1B 1.577(3)
P2-O2B 1.589(3)
P3-O1C 1.581(3)
P3-O2C 1.590(3)
B-F1
B-F2
B-N1
B-N2
N1-C8
1.383(7)-1.390(6) N2-B-F1
1.376(7)-1.392(7) N2-B-F2
1.542(7)-1.552(8) F1-B-F2
1.531(7)-1.557(6)
1.386(6)-1.398(6)
N1-C11 1.338(8)-1.345(6)
C7-C8
C2-C7
1.380(6)-1.400(6)
1.491(6)-1.495(6)
the central six membered ring containing boron atom is
essentially planar like its monomeric BODIPY.15 The two
fluorine atoms are equidistant above and below the plane
of the pyrrole moieties. The six dihedral angles between
(15) Kee, H. L.; Kirmaier, C.; Yu, L.; Thamyongkit, P.; Youngblood,
W. J.; Calder, M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W. R.;
Birge, R. R.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2005, 109, 20433–
20443.

10612 Inorganic Chemistry, Vol. 49, No. 22, 2010
Rao et al.
Table 2. Photophysical Properties of BODIPYs 1-4 in Different Solvents
compound
solvent
λabs(nm)
λem(nm)
FWHMabs (cm-1
)
Δνst(cm-1
)
ε
φ
τ(ns)
kr 108 s-1
knr 109 s-1
C6H6
CHCl3
THF
MeOH
MeCN
C6H6
CHCl3
THF
MeOH
MeCN
C6H6
CHCl3
THF
MeOH
MeCN
C6H6
CHCl3
THF
MeOH
MeCN
516
514
517
513
506
510
509
506
504
503
508
505
503
501
499
530
531
526
525
524
536
534
534
532
531
540
534
535
530
528
564
530,553
528,557
523
522
546
2371
2575
2308
2683
2500
1901
1925
1929
1984
1888
1683
1764
1865
2034
1844
1191
1512
1191
1230
1289
723
728
615
696
930
1089
919
1071
973
941
1954
930,1718
941,1927
839
882
552
517
561
529
531
4.41
4.48
4.49
4.45
4.43
4.56
4.61
4.57
4.45
4.51
4.49
4.55
4.53
4.28
4.48
4.43
4.48
4.39
4.36
4.39
0.100
0.103
0.056
0.009
0.005
0.057
0.062
0.040
0.003
0.003
0.042
0.039
0.027
0.003
0.003
0.518
0.627
0.534
0.270
0.228
1.16
1.28
1.33
0.53
0.20
0.77
0.72
0.83
0.30
0.28
0.60
0.49
0.43
0.14
0.14
4.51
5.48
5.16
3.04
2.34
0.86
0.80
0.42
0.17
0.25
0.74
0.86
0.48
0.10
0.10
0.70
0.79
0.62
0.21
0.21
1.14
1.14
1.03
0.88
0.97
0.77
0.70
0.70
1.86
4.97
1 22
1.30
1.15
3.32
3.56
1.59
1.96
2.26
7.12
7.12
0.10
0.07
0.09
0.24
0.33
1
2
3
4
546
542
540
539
Figure 3. Comparison of normalized absorption spectra of compounds
1-4 recorded in chloroform. Inset shows the comparison of absorption
spectra of compound 3 along with its monomer 7 recorded in chloroform
using concentration of 8 ꢀ 10-6 M.
Figure 4. Comparison of reduction waves of cyclic voltammograms of
compounds 3 and 4 recorded in CH2Cl2 containing 0.1 M TBAP as
supporting electrolyte recorded at 50 mV s-1 scan speed.
changed from benzene or chloroform to acetonitrile,
which is consistent with the general behavior of mono-
meric BODIPY chromophores;16 (2) The compounds
1-4 exhibited slight red shifts (up to 10 nm) with
approximately three times intensity enhancement com-
pared with their respective monomers. (3) The S0fS1
transition in compounds 1-4 is generally broad with
fwhm ∼1500-2500 cm-1 compared with their corre-
sponding monomeric BODIPYs 5-8. However, among
compounds 1-4, the S0fS1 transition in compounds 2-4
is relatively sharp with fwhm ∼1500-1900 cm-1 whereas
it is quite broad in 1 with fwhm ∼2500 cm-1. This may be
related to the type of linking of BODIPY units to the
cyclotriphosphazene ring. In compound 1, the BODIPY
units are linked through its ortho position of meso-phenyl
group causing steric crowding resulting in broadening of
the absorption band. The solvent effect on absorption
properties studied for compounds 1-4 indicated that the
effects are similar to those of corresponding monomers16
and no unusual effects were observed in any solvent. The
electrochemical properties of compounds 1-4 were de-
termined by cyclic voltammetry at a scan rate of 50 mV/s
using tetrabutylammonium perchlorate as supporting
electrolyte. A comparison of reduction waves of com-
pounds 3 and 4 is shown in Figure 4, and the redox
potential data is presented in Table 3. All four com-
pounds showed one reversible reduction and one ill-
defined oxidation except for compound 4 which showed
one reversible oxidation. The compound 4 exhibited
easier oxidation and difficult reduction compared with
compounds 1-3 which is due to the electron rich BODIPY
core.
Fluorescence Studies. The steady state fluorescence
studies were carried out on compounds 1-4 in different
solvents, and the comparison of normalized steady state
fluorescence spectra of compounds 1-4 recorded in
chloroform using excitation wavelength of 488 nm is pre-
sented in Figure 5. The fluorescence studies of compounds
(16) (a) Costela, A.; Garcıa-Moreno, I.; Gomez, C.; Sastre, R.; Amat-
´
ꢁ
~
ꢁ
Guerri, F.; Liras, M.; Lopez Arbeloa, F.; Banuelos Prieto, J.; Lopez Arbeloa,
~
ꢁ
I. J. Phys. Chem. A 2002, 106, 7736–7742. (b) Banuelos Prieto, J.; Lopez
ꢁ
Arbeloa, F.; Martínez Martínez, V.; Arbeloa Lopez, T.; Amat-Guerri, F.; Liras, M.;
ꢁ
ꢁ
Lopez Arbeloa, I. Chem. Phys. Lett. 2004, 385, 29–35. (c) Lopez Arbeloa, F.;
~
ꢁ
Banuelos Prieto, J.; Martínez Martínez, V.; Arbeloa Lopez, T.; Lopez Arbeloa, I.
ChemPhysChem 2004, 5, 1762–1771.

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10613
Table 3. Electrochemical Redox Data [V] of 1-4 in CH2Cl2 Containing 0.1 M
TBAP as the Supporting Electrolyte
compound
Eox(V)
Ered(V)
1
2
3
4
1.59a
1.61a
1.87a
1.12
-0.79
-0.71
-0.72
-1.25
a Irreversible oxidation waves.
Figure 7. Fluorescence decay profile and weighted, residual, distribu-
tion fit of 4 in five different solvents. The excitation wavelength used was
406 nm.
core; (5) Stokes’ shift which is the difference between
the maximum of the lowest-energy absorption band and
the maximum of the emission band was found to be larger
for compounds 1-4 compared with their respective
monomers 5-8. The overlay of absorption and emission
spectra of compound 3 is shown in Figure 6, and its inset
shows it for the corresponding reference monomer 7. It is
clear from Figure 6 that the compound 3 exhibits 1718
cm-1 nm Stokes’ shift compared with 618 cm-1 Stokes’
shift16 observed for compound 7. This supports a geo-
metric displacement of the excited state of compounds
1-4 and the maximum for 3 with respect to the ground
state; Thus, the larger Stokes’ shifts observed for com-
pounds 1-4 are advantageous for their potential applica-
tions in direct multicolor labeling experiments. (6) the
quantum yields of compounds 1-4 in a solvent like
chloroform are in the range of 0.04-0.6, and compound
4 is the most fluorescent compound among all; the
quantum yield decreases in solvents of increasing polar-
ity, and compound 4 exhibited decent quantum yields in
all solvents compared to compounds 1-3. Furthermore,
the quantum yields of compounds 1-4 are in the range of
their respective reference monomers indicating that the
fluorophores in compounds 1-4 behave independently.
Similar observations were made by Ainscough and co-
workers7e in their recent report on cyclotriphosphazene
appended with Ru(II) and Re(I) bipyridyl complexes.
To explore the fluorescence dynamics of compounds
1-4, the fluorescence decay profiles in different solvents
were collected. The fluorescence decays of compounds
1-4 were fitted to a single exponential in all solvents, and
the fluorescence decay traces of compound 4 collected at
respective emission maxima in different solvents are pre-
sented in Figure 7. The singlet state lifetimes τ depended
on the type of BODIPY dye connected to cyclotriphos-
phazene, and the lifetimes are in the range of 0.5 to 5 ns.
The compound 4 which showed maximum quantum
yields exhibited the highest singlet state lifetime. The
singlet state lifetime is also solvent dependent like the
quantum yield and lower in solvents with high polarity.
Using quantum yield and singlet state lifetime, we calcu-
lated radiative (kr) and nonradiative (knr) rate constants
(Table 2). The kr and knr data are in agreement with the
quantum yield and lifetime data, and maximum kr and
minimum knr were observed for compound 4, the most
fluorescent compound among the compounds studied
Figure 5. Comparison of normalized emission spectra of compounds
1-4 recorded in chloroform.
Figure 6. Normalized absorption (solid line) and emission spectra
(dashed line) of 3 recorded in benzene. The inset shows absorption
(solid line) and emission spectra (dashed line) of corresponding monomer
7 recorded in benzene. The concentrations used were 8 ꢀ 10-6 M.
1-4 reveal the following: (1) the compounds 1, 2, and 4
showed one single fluorescence band in all solvents
whereas the compound 3 showed either two banded
fluorescence spectrum or a broad fluorescence band with
a shoulder in all solvents; (2) the fluorescence band was
blue-shifted on increasing solvent polarity for com-
pounds 1-4; (3) the bandwidth of the fluorescence
depended on the type of BODIPY connected to the
cyclotriphosphazene ring; compound 4 showed sharp
emission and compound 3 showed very broad emission;
(4) the fluorescence band of compound 4 was maximum
red-shifted compared with compounds 1-3 because of the
presence of electron donating substituents on BODIPY

10614 Inorganic Chemistry, Vol. 49, No. 22, 2010
Rao et al.
were recorded with a Varian spectrometer operating at 96.3
MHz. Tetramethylsilane (TMS) was used as an external refer-
1
ence for recording H (of residual proton; δ = 7.26 ppm) and
13C (δ = 77.0 ppm) spectra in CDCl3. Absorption and steady-
state fluorescence spectra were obtained with a Perkin-Elmer
Lambda-35 spectrometer and a PC1 Photon Counting Spectro-
fluorimeter (ISS, U.S.A.). Fluorescence spectra were recorded
at 25 °C in a 1 cm quartz fluorescence cuvette. The fluorescence
quantum yields (Φf) were estimated from the emission and
absorption spectra by the comparative method at the excitation
wavelength of 488 nm using Rhodamine 6G (Φf = 0.88)17 as the
standard. The time-resolved fluorescence decay measurements
were carried out at the magic angle using a picosecond- diode-
laser-based, time-correlated, single-photon-counting (TCSPC)
fluorescence spectrometer from IBH, U.K. All the decays were
fitted to a single exponential. Cyclic voltammetric (CV) and
differential pulse voltammetric (DPV) studies were carried out
with an electrochemical system utilizing a three-electrode con-
figuration consisting of a glassy carbon (working) electrode,
platinum wire (auxiliary) electrode, and a saturated calomel
(reference) electrode. The experiments were performed in dry
CH2Cl2 using 0.1 M TBAP as the supporting electrolyte. Half-
wave potentials were measured using DPV and also calculated
manually by taking the average of the cathodic and anodic
peak potentials. MALDI-TOF spectra were obtained with an
Axima- CFR spectrometer, manufactured by Kratos Analyti-
cals. The ES-MS spectra were recorded with a Q-Tof micro mass
spectrometer.
The single crystals of compound 1 (CCDC-786046) was
obtained from deuterated chloroform in NMR tube on slow
evaporation over a period of two weeks. Crystal data for com-
pound 1 was measured at 100(2) K on a Bruker SMART APEX
CCD area detector system [λ(Mo-KR) = 0.71073 A], graphite
monochromator, 2400 frames were recorded with an ω scan
width of 0.3°, each for 8 s, crystal-detector distance 60 mm,
collimator 0.5 mm. Data reduction by SAINTPLUS (Software
for the CCD Detector System, Bruker Analytical X-ray Systems
Inc., Madison, WI, 1998). Structure solution for the compound
3 was obtained using direct methods (SHELXS-97)18 and
refined using full-matrix least-squares methods on F2 using
SHELXL-97.19
Figure 8. Color response of compounds 1-4 under a UV lamp.
here. Furthermore, we also observed clear changes in color
under a UV lamp as well as the naked eye (Supporting
Information). The chloroform solutions of compounds 1-4
under a UV lamp is shown in Figure 8. As clearly noted
from Figure 8 compound 4 is bright green fluorescent
compared with others.
Conclusions
In conclusion, we have described the synthesis of four
cyclophosphazene appended with six boron-dipyrromethenes
N3P3(BODIPY)6 1-4 by following two different routes.
Although route I was straightforward to synthesize the de-
sired compound, the route II gives an idea about the robust-
ness of the cyclotriphosphazene ring toward various reaction
and purification conditions used here. The NMR studies
indicated that the BODIPY units are interacting with each
other in solution although the crystal structure solved for
one of the compounds showed no interaction between the
BODIPY units in the solid state. The absorption and fluor-
escence properties studied in different solvents indicated that
the properties are sensitive to the type of BODIPY units
connected to the cyclophosphazene ring. The compounds
exhibited large Stokes’ shifts with comparable quantum yields
compared to their respective reference boron-dipyrromethene
monomers. This kind of cyclophosphazene appended with
fluorophores may have potential applications as organic light
emitting diodes.
General Procedure for the Preparation of meso-(hydroxyphenyl)-
dipyrromethane. Pyrrole (25 equiv) and the hydroxy aldehyde
(1.0 equiv) were added to a dry round-bottomed flask and
degassed with a stream of nitrogen for 5 min. TFA (0.10 equiv)
was then added, and the solution was stirred under nitrogen at
room temperature for 10 min and then quenched with 0.1 M
NaOH. Ethyl acetate was then added. The organic phase was
washed with water and dried (Na2SO4), and the solvent removed
under vacuum to afford orange oil. The resulting crude product
was purified by silica gel column chromatography to give pure
meso-(hydroxyphenyl)dipyrromethane as a crystalline white
solid.
General Procedure for the Synthesis of meso-(hydroxyphenyl)-
BODIPYs 5-8. A sample of meso-(o/m/p-hydroxyphenyl)-
dipyrromethane (0.84 mmol) is dissolved in dichloromethane
and oxidized with DDQ (1 mmol) at room temperature under
stirring for 1 h. Triethylamine (33 mmol) was added, and the
Experimental Section
General Information. Chemicals. THF and benzene were
dried with sodium benzophenone ketyl, and CHCl3, CH3OH,
and MeCN were dried with calcium hydride and distilled prior
solution stirred for an additional 10 min. BF3 Et2O (42 mmol)
was then added, and continued stirring at room temperature for
additional 1 h. TLC analysis indicated the formation of expected
3
to use. BF3 OEt2 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
3
(DDQ) were obtained from Spectrochem (India) and used as
obtained. All other chemicals used for the synthesis were reagent-
grade unless otherwise specified. Column chromatography was
performed on silica (60-120 mesh).
(17) Qin, W.; Leen, V.; Rohand, T.; Dehaen, W.; Dedecker, P.; Van der
Auweraer, M.; Robeyns, K.; Van Meervelt, L.; Beljonne, D.; Van Averbeke,
B.; Clifford, J. N.; Driesen, K.; Binnemans, K.; Boens, N. J. Phys. Chem. A
2009, 113, 439–447.
Instrumentation. 1H NMR spectra (δ values, ppm) were
recorded using Varian VXR 300 and 400 MHz spectrometers.
13C NMR spectra were recorded with a Varian spectrometer
operating at 100.6 MHz. 19F NMR spectra were recorded with a
Varian spectrometer operating at 282.2 MHz. 11B NMR spectra
(18) Sheldrick, G. M. SHELXS-97: Program for crystal structure solution;
€
€
University of Gottingen: Gottingen, Germany, 1997.
(19) Sheldrick, G. M. SHELXL-97: Program for crystal structure refine-
€
€
ment; University of Gottingen: Gottingen, Germany, 1997.

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10615
compound as an orange-red spot. The reaction mixture was then
diluted with CH2Cl2 and washed thoroughly with 0.1 M NaOH
solution and water. The organic layers were combined, dried
over Na2SO4, filtered, and solvent was removed on a rotary
evaporator under vacuum. The resulting crude product was
purified by column chromatography on silica gel, using petro-
leum ether/ethyl acetate (85:15) to afford the expected meso-
(hydroxyphenyl)BODIPYs 5-8 as orange solids.
4,4-Difluoro-8-(o-hydroxyphenyl)-4-bora-3a,4a-diaza-s-inda-
cene (5). 33% yield. 1H NMR (400 MHz, CDCl3, δ in ppm): 6.53
(d, 3J = 3.6 Hz, 2H; py), 6.93 (d, 3J = 3.9 Hz, 2H; py), 7.03-7.07
(m, 2H; Ar), 7.28 (d, 3J = 7.3 Hz, 1H; Ar), 7.44 (t, 3J = 7.2 Hz,
1H; Ar), 7.94 (s, 2H, py); 19F NMR (282.2 MHz, CDCl3, δ in
ppm): -144.6 [dq, J(B, F), J(F, F)], -145.6 [dq, J(B, F), J(F, F)];
11B NMR (96.3 MHz, CDCl3, δ in ppm): 0.53 [t, J(B,F)]; 13C
NMR (100 MHz, CDCl3, δ in ppm): 117.0, 119.0, 120.1, 120.4,
131.6, 135.2, 143.0, 144.8, 153.6; ES-MS: (C15H11BF2N2O) 265
[Mþ - F].
4,4-Difluoro-8-(m-hydroxyphenyl)-4-bora-3a,4a-diaza-s-indacene
(6). 37% yield. 1H NMR (400 MHz, CDCl3, δ in ppm):
6.52-6.53 (m, 2H; py), 6.95 (d, 3J = 4.0 Hz, 2H; py), 6.99 (s,
1H; Ar), 7.04-7.06 (m, 1H; Ar), 7.09 (d, 3J = 7.6 Hz, 1H; Ar),
7.37 (t, 3J = 7.9 Hz, 1H; Ar), 7.93 (s, 2H, py); 19F NMR (282.2
MHz, CDCl3, δ in ppm): -144.6 [q, J(B, F)]; 11B NMR (96.3
MHz, CDCl3, δ in ppm): 0.54 [t, J(B,F)]; 13C NMR (100 MHz,
CDCl3, δ in ppm):117.5, 118.0, 118.7, 123.2, 129.9, 131.8,
135.0, 135.2, 144.3, 147.0, 155.7; ES-MS: (C15H11BF2N2O)
265 [Mþ1]þ.
afforded pure 9 as a white solid in 92% yield (23.5 g).
1
mp >300 °C; H NMR (400 MHz, CDCl3, δ in ppm) 7.16
(d, 3J (H, H) = 8.5 Hz, 12H; Ar), 7.75 (d, 3J (H, H) = 8.5 Hz,
12H; Ar), 9.94 (s, 6H; -CH0); 31P{1H} NMR (161.8 MHz,
CDCl3, δ in ppm): 5.34 (s); ES-MS: (C42H30N3O12P3) m/z
(%): 862.9 [Mþ].
Hexakis(meso-phenyldipyrromethane)cyclotriphosphazene (10).
A mixture of 9 (0.5 g, 0.58 mmol) and pyrrole (6.0 mL, 86
mmol) in a 100 mL flask was degassed by bubbling argon for 10
min. Trifluoroacetic acid (0.026 mL, 0.34 mmol) was added to
initiate the reaction, and the mixture was stirred at room
temperature for 20 min. The reaction mixture was then diluted
with ethyl acetate (50 mL) and washed with 0.1 M NaOH
(2 ꢀ 100 mL). The organic layer was dried over anhydrous
Na2SO4. The solvent and excess pyrrole was removed on a
rotary evaporator under reduced pressure. The resulting crude
viscous liquid was purified by column chromatography, and
the desired compound 10 was collected as a brown solid using
dichloromethane/methanol (98:2) in 62% yield (0.55 g). mp
120-121 °C; 1H NMR (400 MHz, CDCl3, δ in ppm): 5.30 (s,
6H, -CH), 5.81 (s, 12H, Py), 6.10 (m, 12H), 6.60 (m, 12H), 6.82
(d, 3J (H, H) = 8.5 Hz, 12H; Ar), 6.94 (d, 3J (H, H) = 8.5 Hz,
12H; Ar), 7.79 (s, 12H); 31P{1H} NMR (161.8 MHz, CDCl3, δ
in ppm): 7.04 (s).
Hexakis[4,4-difluoro-8-(p-phenoxy)-4-bora-3a,4a-diaza inda-
cene]cyclotriphosphazene (3). In the final step, compound 10
(500 mg, 0.32 mmol) was taken in CH2Cl2 (30 mL) and oxidized
with a solution of DDQ (437 mg, 1.9 mmol) in CH2Cl2 (30 mL)
at room temperature for 1 h. Triethylamine (10.71 mL, 76
mmol) was added to the solution, and the mixture was stirred for
10 min. BF3 Et2O (12.01 mL, 96 mmol) was then added to the
reaction mixture, and stirring was continued at room tempera-
ture for additional 1 h. The reaction mixture was diluted with
CH2Cl2 and washed thoroughly with 0.1 M NaOH (2 ꢀ 100
mL). The combined organic extracts were dried over Na2SO4,
filtered, and solvent was removed to afford crude product. The
crude product was purified by silica gel column chromatogra-
phy using CH2Cl2 and afforded pure compound 3 as red solid in
16% yield (93 mg).
1,2,6,7-Tetraethyl-4,4-difluoro-8-(p-hydroxyphenyl)-3,5-dimethyl-
4-bora-3a,4a-diaza-s-indacene (8). 42% yield. 1H NMR (400
MHz, CDCl3, δ in ppm): 0.67-0.70 (t, 3J = 7.3 Hz, 6H; -CH3),
1.01-1.05 (t, 3J = 7.3 Hz, 6H; -CH3), 1.60-1.66 (q, 3J = 7.3
Hz, 4H; -CH2), 2.27-2.33 (q, 3J = 7.3 Hz, 4H; -CH2), 2.53 (s,
6H; -CH3), 6.89 (d, 3J = 8.2 Hz, 2H; Ar); 7.25 (d, 3J = 8.2 Hz,
2H; Ar); 19F NMR (282.2 MHz, CDCl3, δ in ppm): -145.5 [q,
J(B, F)]; 11B NMR (96.3 MHz, CDCl3, δ in ppm): 1.08 [t, J(B,
F)]; ES-MS: (C25H31BF2N2O) 405.2 [Mþ - F].
Hexakis[4,4-difluoro-8-(p-phenoxy)-4-bora-3a,4a-diaza-s-
indacene]cyclotriphosphazene (3). Route-I. A sample of 7 (18.6 mg,
0.06 mmol) and cesium carbonate (42 mg, 0.13 mmol) was added
to a hexachlorocyclotriphosphazene (4 mg, 0.01 mmol) in THF at
0 °C. The reaction was brought to room temperature and stirred
for 6 h at 50 °C. The progress of the reaction was monitored with
TLC analysis at regular intervals. The reaction was stopped after
complete disappearance of 7 and appearance of a new spot cor-
responding to the required compound 3as judged by TLC analysis.
The reaction mixture was concentrated on a rotary evaporator
under reduced pressure, and the resulting crude compound was
subjected to silica gel column chromatography. The desired com-
pound was collected as the first band using dichloromethane. The
solvent was removed under reduced pressure and afforded 3 as a
red solid in 88% yield (18.5 mg); mp 180-181 °C; 1H NMR (400
MHz, CDCl3, δ in ppm): 6.43 (m, 12H), 6.71 (m, 12H), 7.31 (d, 3J
(H, H) = 8.5 Hz, 12H; Ar), 7.51 (d, 3J (H, H) = 8.5 Hz, 12H; Ar),
7.90 (s, 12H); 31P{1H} NMR (161.8 MHz, CDCl3, δ in ppm): 6.36
(s); 19F NMR (282.2 MHz, CDCl3, δ in ppm): -145.1 [q, J(B, F)];
11B NMR (96.3 MHz, CDCl3, δ in ppm): 0.36 [t, J(B,F)]; 13C
NMR (100 MHz, CDCl3, δ in ppm): 118.9, 121.0, 130.9, 131.0,
131.5, 132.1, 134.6, 144.9; ES-MS: (C90H60B6N15O6P3): 1815.17
[Mþ - F].
Hexakis[4,4-difluoro-8-(o-phenoxy)-4-bora-3a,4a-diaza inda-
cene]cyclotriphosphazene (1). The compound 1 was prepared by
following route I
80% Yield; 1H NMR (400 MHz, CDCl3, δ in ppm): 6.30 (s,
12H; py), 6.47 (d, 3J = 3.0 Hz, 12H; py), 6.55 (d, 3J = 8.2 Hz,
6H; Ar), 6.77 (t, 3J = 8.2 Hz, 6H; Ar), 6.98 (t, 3J = 7.3 Hz, 6H;
3
Ar), 7.15 (d, J = 7.3 Hz, 6H; Ar), 7.82 (s, 12H, py); 31P{1H}
NMR (161.8 MHz, CDCl3, δ in ppm): 5.17 (s); 19F NMR (282.2
MHz, CDCl3, δ in ppm): -144.4 [dq, J(B, F), J(F, F)], -145.0
[dq, J(B, F), J(F, F)]; 11B NMR (96.3 MHz, CDCl3, δ in ppm):
0.40 [t, J(B,F)]; 13C NMR (100 MHz, CDCl3, δ in ppm): 118.6,
122.9, 123.2, 131.1, 131.6, 132.0, 133.3, 135.2, 144.5, 150.4; ES-
MS: (C90H60B6F12N15O6P3) 1855.2 [MþNa]þ
Hexakis[4,4-difluoro-8-(m-phenoxy)-4-bora-3a,4a-diaza inda-
cene]cyclotriphosphazene (2). The compound 2 was prepared by
following route I
85% Yield; 1H NMR (400 MHz, CDCl3, δ in ppm):
3
6.33-6.34 (m, 12H; py), 6.63 (d, J = 3.7 Hz, 12H; py), 7.12
(d, 3J = 8.0 Hz, 12H; Ar), 7.18 (s, 6H; Ar), 7.23 (m, 6H; Ar), 7.87
(s, 12H, py); 31P{1H} NMR (161.8 MHz, CDCl3, δ in ppm): 6.80
(s); 19F NMR (282.2 MHz, CDCl3, δ in ppm): -144.5 [q, J(B,
F)]; 11B NMR (96.3 MHz, CDCl3, δ in ppm): 0.40 [t, J(B,F)]; 13
C
Route-II. Hexakis(4-formylphenoxy)cyclotriphosphazene (9).
A sample of 4-hydroxybenzaldehyde (22.1 g, 180.5 mmol) was
added to a mixture of P3N3Cl6 (10.3 g, 29.74 mmol) and K2CO3
(50.0 g, 361.76 mmol) in THF (250 mL) at 0 °C. The reaction
mixture was then stirred at 50 °C for 48 h. The solvent was
removed under vacuum. The residue was extracted with CH2Cl2,
and the solvent was removed on a rotary evaporator to afford
the crude compound. The crude compound was subjected to
silica gel column chromatography using dichloromethane and
NMR (100 MHz, CDCl3, δ in ppm): 118.8, 122.7, 122.9, 127.9,
130.0, 131.2, 134.4, 135.3, 144.8, 149.9; ES-MS: (C90H60B6F12-
N15O6P3) 1814.6 [Mþ-F]
Hexakis[1,2,6,7-tetraethyl-4,4-difluoro-8-(p-phenoxy)-3,5-
dimethyl-4-bora-3a,4a-diaza-s-indacene]cyclotriphosphazene (4).
The compound 4 was prepared by following route I
83% Yield; 1H NMR (400 MHz, CDCl3, δ in ppm): 0.67-0.70
3
3
(t, J = 7.3 Hz, 6H; -CH3), 1.01-1.05 (t, J = 7.3 Hz, 6H;

10616 Inorganic Chemistry, Vol. 49, No. 22, 2010
Rao et al.
-CH3), 1.60-1.66 (q, 3J = 7.3 Hz, 4H; -CH2), 2.27-2.33 (q,
3J = 7.3 Hz, 4H; -CH2), 2.53 (s, 6H; -CH3), 6.89 (d, 3J = 8.2
Hz, 2H; Ar); 7.25 (d, 3J = 8.2 Hz, 2H; Ar); 31P{1H} NMR (161.8
MHz, CDCl3, δ in ppm): 5.61 (s); 19F NMR (282.2 MHz,
CDCl3, δ in ppm): -146.1 [q, J(B, F)]; 11B NMR (96.3 MHz,
CDCl3, δ in ppm): 1.07 [t, J(B,F)]; ES-MS: (C150H180B6F12N15-
O6P3) 2697.4 [MþNa]þ
Acknowledgment. We thank CSIR and DST for financial
support. M.R.R. thanks CSIR, New Delhi for a fellowship.
Supporting Information Available: Crystallographic data in
CIF format. Further details are given in Figures 1-47. This
material is available free of charge via the Internet at http://
pubs.acs.org.