Nitro and amino BODIPYS: Crucial substituents to modulate their photonic behavior

Article abstract of DOI:10.1039/c2ra22916a

The present work deals with the synthesis and photophysical, quantum mechanical, and lasing characterization of novel BODIPYs bearing amino and nitro groups at different positions in the core. The results emphasize the relevant role on the photophysical and lasing properties, not only of the attached functionality but also of the position in which is grafted, as well as the molecular structure of the indacene core. A wide part of the visible spectrum can be covered by the insertion of an amino group at position 3 (red shift) or 8 (blue shift). Furthermore, the electron withdrawing character of the nitro substituent induces intramolecular charge transfer processes, the efficiency of which depends on the position of the nitro group on the BODIPY core. All these experimental findings can be rationalized with the help of quantum mechanical calculations.

Full text of DOI:10.1039/c2ra22916a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Nitro and amino BODIPYS: crucial substituents to
modulate their photonic behavior3
Cite this: RSC Advances, 2013, 3,
1547
a
a
a
b
´
Ixone Esnal, Jorge Ban˜uelos,* In˜igo Lopez Arbeloa, Angel Costela,
b
c
c
´
Inmaculada Garcia-Moreno, Miguel Garzon, Antonia R. Agarrabeitia
and Mar´ıa Jose Ortiz
c
´
The present work deals with the synthesis and photophysical, quantum mechanical, and lasing
characterization of novel BODIPYs bearing amino and nitro groups at different positions in the core.
The results emphasize the relevant role on the photophysical and lasing properties, not only of the
attached functionality but also of the position in which is grafted, as well as the molecular structure of the
indacene core. A wide part of the visible spectrum can be covered by the insertion of an amino group at
position 3 (red shift) or 8 (blue shift). Furthermore, the electron withdrawing character of the nitro
substituent induces intramolecular charge transfer processes, the efficiency of which depends on the
position of the nitro group on the BODIPY core. All these experimental findings can be rationalized with
the help of quantum mechanical calculations.
Received 15th November 2012,
Accepted 16th November 2012
DOI: 10.1039/c2ra22916a
www.rsc.org/advances
Post-modification of the BODIPY core is a convenient
method for the facile functionalization of these dyes. The
Introduction
The family of difluoroboradiazaindacenes, known by the trade
name BODIPY, is currently considered as comprising some of
the most useful and versatile fluorophores. The numerous
applications of these compounds (biotechnology, electronics,
etc.) require the design of new fluorophores with properties
such as high stability, high fluorescence quantum yields, large
Stokes shifts, and suitable absorption profiles, as well as
incorporating substituents that optimize the function of these
compounds in the different applications.1 The incorporation
of appropriate functional groups onto the chromophore could
lead to new photophysical processes or large spectral shifts.
Thus, the type of substituents and the position in which they
are attached to the chromophore allow obtainment of, for
example, a highly fluorescence dye in the red part of the visible
spectrum or a system in which fluorescence properties are
sensitive to a certain property of the surrounding environ-
ment. Therefore, one should choose carefully the kind of
substituents and the position in which they will be incorpo-
rated to BODIPY, depending on the desired application.1
extensive literature available on BODIPY dyes shows that
substitution at all positions has been investigated.1–9 Several
synthetic strategies have been used for this purpose, such as
Knoevenagel-type condensation1a–c,g,2 and oxidative formyla-
tion of methyl-substituted BODIPYs,3 nucleophilic substitu-
tions1a–c,g,4 and palladium coupling reactions1a,5 on
halogenated BODIPYs, direct hydrogen substitution,6 the
Liebeskind–Srogl reaction7 and electrophilic substitution
¨
reactions.1a,8 Furthermore, the replacement of fluoride atoms
by carbon and oxygen nucleophiles or by other halogen atoms
has been shown to be an important advance for the
introduction of functionality.1a–c,2c,k,8b,9
Functionalization at the 2,6-positions from 1,3,5,7-tetra-
methyl substituted BODIPYs can be achieved by electrophilic
substitution reactions.1a,8 Thus, halogenation at these posi-
tions has been extensively studied.5f,8b–f However, the nitration
of BODIPYs has not attracted so much research interest as
demonstrated by the few related publications in the literature.
As we know, there is an example of nitration in the 2,6-
positions of the commercial BODIPY core named PM546 (A),10
and there are several Japanese patents with two dinitro
derivatives (B11 and C12), but neither their syntheses nor their
spectroscopic data have been described (Fig. 1). Moreover,
electrophilic substitution reactions in pyrrole-unsubstituted
BODIPYs have attracted relatively little research interest until
very recently4h,8g–j and, specifically, there is no study on the
nitration reaction in this type of compound. In contrast, there
are numerous examples of BODIPYs with nitro groups but not
a
´
´
´
Departamento de Quımica Fısica, Universidad Paıs Vasco (UPV-EHU), Aptdo. 644,
48080, Bilbao, Spain
bDepartamento de Sistemas de Baja Dimensionalidad, Superficies y Materia
Condensada, Instituto Quimica-Fisica ‘‘Rocasolano’’, CSIC, Serrano 119, 28006
Madrid, Spain
c
´
´
´
Departamento de Quımica Organica I, Facultad de Ciencias Quımicas, Universidad
Complutense, 28040, Madrid, Spain
Electronic Supplementary Information (ESI) available: details of the synthesis
3
route, NMR spectra, photophysical data. See DOI: 10.1039/c2ra22916a
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 1547–1556 | 1547

View Article Online
Paper
RSC Advances
Furthermore, the present work provides new insight into
the effect of the molecular structure on the photophysical
properties of the resulting chromophore. The nitration of
BODIPYs induces new photophysical processes such as charge
transfer phenomena, the influence on the optical properties of
which is strongly dependent on the type of substituent, as well
as the position where it is anchored to the chromophore.
Therefore, the correct selection of functional groups and the
most convenient position to graft them to the core allows the
design of tailor-made BODIPY for the desired application,
ranging from BODIPY covering a wide part of the visible part
of the spectrum (amino substitution) to fluorophores sensitive
to the environmental properties (nitro substitution). Moreover,
these derivatives should be excellent scaffolds to further
functionalization, leading to a wide battery of available
structures based on BODIPY.
Fig. 1 Chemical structures for 2,6-dinitro-1,3,5,7-substituted BODIPYs (A, B and
C), and 8-dinitroaryl BODIPY (D).
Results and discussion
Synthesis
Nitration of the BODIPY core has been carried out by
electrophilic substitution reaction. Thus, the reaction of
BODIPY dye 114 with 2 equiv. HNO3 in Ac2O at 0 uC gave a
mixture of 6-nitro derivative 3 (11%) and 5-nitro derivative 4
(37%), which were separated via flash chromatography on
silica. A similar result was obtained from pyrrole-unsubsti-
tuted BODIPY 215 with 4 equiv. HNO3 in Ac2O, yielding 2-nitro
derivative 5 (6%) and 3-nitro derivative 6 (36%). No other nitro
BODIPY was formed under these reaction conditions.
Increasing the amount of HNO3 gave a complex mixture of
nitro products.
directly attached to the BODIPY core (D),1a,13 and most of
them are used as precursors of amino groups (Fig. 1).
In this work we report the synthesis of new nitro derivatives
of two different BODIPY scaffolds, by electrophilic substitution
and subsequent reduction to the corresponding amino
derivatives (Fig. 2). This library of new dyes allows us to
analyze in detail the dependence of their optical properties on
the nature and position of the substituents linked to a
symmetric and asymmetric BODIPY core.
Due to the low yields obtained from b-nitro derivatives 3 and
5, we carried out the independent synthesis of these
compounds from nitro pyrrole precursors (Scheme 1). Thus,
the reaction of 2-acetyl-4-nitro-1H-pyrrole (10)16 and 3-ethyl-
2,4-dimethyl-1H-pyrrole with phosphorus oxychloride in
CHCl3, followed by treatment with BF3?OEt2 in the presence
of triethylamine, gave a moderate yield of BODIPY 3. For the
preparation of BODIPY 5, we carried out the nitration of
(1H-pyrrol-2-yl) (4-tolyl) methanone (11),17 yielding a mixture
of (5-nitro-1H-pyrrol-2-yl) (4-tolyl) methanone (12) and (4-nitro-
1H-pyrrol-2-yl) (4-tolyl) methanone (13), which were separated
via flash chromatography on silica. The condensation of
compound 13 and pyrrole with phosphorus oxychloride,
followed by treatment with BF3?OEt2 in the presence of
triethylamine allowed us to isolate BODIPY 5, but, as in the
direct nitration of BODIPY 2, with a very low yield (Scheme 1).
In addition, this compound was very unstable and its
photophysical study was not possible. The amino BODIPYs 7
and 8 were readily obtained by reduction of the nitro to the
amino group in the presence of molecular hydrogen and a
solid catalyst with a quantitative good yield. However, BODIPY
9 could not be isolated because it decomposes quickly.
Photophysical properties
The photophysical properties of the parent compound 1 have
Fig. 2 Starting BODIPYs (1, 2) and core-nitro and amino BODIPY dyes (3–9)
been previously analyzed as a function of the solvent nature.14
studied in this work.
1548 | RSC Adv., 2013, 3, 1547–1556
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
reference dye 1 (Table 1). In fact, the electron releasing
capacity of the amino at position 5 is not strong enough to
induce a charge transfer state, which usually drastically
quenches the fluorescence emission of BODIPYs. Theoretical
simulations do not suggest an electronic density shift from the
amino to the BODIPY core upon excitation, and the frontier
orbital contour maps keep the normal features of the
fluorophore, but extended to the amino group.
The inclusion of the same group at position 6 (compound 9)
gave rise to a very unstable product, with fast degradation,
losing the spectral bands typical of BODIPY. The electronic
coupling of the amino at such a position could give rise to the
formation of an emanine–imine tautomeric equilibrium
(Scheme S1, ESI ) destroying the delocalized p-system of the
3
chromophore. Therefore, entirely different results were
obtained by changing the position in which the amino group
was attached. Such a key role of the substitution position on
the optical properties is reinforced by previous results reported
by a BODIPY bearing just an amino group in the meso
position.7e,19 While the presence of this group in position 5 led
to a slight bathochromic shift of the spectral bands, the
substitution at position 8 induced a large blue shift (Fig. 3,
right).
The reason for such opposite behavior can be understood
through the theoretical simulation of the electronic distribu-
tion in the frontier orbitals involved in the corresponding
spectral transition. With independence of the substitution, the
electronic density at position 5 of the BODIPY core is nearly
similar in the HOMO and LUMO states (Fig. 3). Thus, the
5-amino substituent only increased the delocalization of the
p-system, red-shifting the spectral bands. However, the
electronic density at position 8 of the BODIPY core increases
significantly upon excitation from the HOMO to the LUMO
state, highlighting the meso position as being very sensitive to
substituent effects. Then, the electron donor amine linked at
the 8-position raised up the energy of the LUMO state, leaving
the HOMO state unaltered,19 and, consequently, leading to an
increase of the energy gap, which is reflected in the blue-shift
of the spectral bands.
Nitro substitution. Following an opposite behavior to that
previously described for amino substitution, the presence of a
nitro group at position 5 (compound 4) as well as at 6
(compound 3) gave rise to a hypsochromic shift of the
absorption band due to its electron withdrawing character
(Fig. 4), which removes part of the electronic density
delocalized through the BODIPY core. Nonetheless, the most
striking feature induced by nitro substitution is a drastic
change in the shape of both the absorption and fluorescence
spectra. Indeed, the absorption bands seem to be split into two
transitions, behavior that becomes clearer in fluorescence,
where very broad emission bands were registered.
Scheme 1 Independent synthesis of BODIPYs 3 and 5.
Its photophysics were conditioned by the asymmetric sub-
stitution pattern, which reduces the absorption probability,
while its fluorescence efficiency remained high, close to 100%
in apolar solvents (Table 1). In addition, the asymmetric
charge distribution in the pyrroles, induced by the substitu-
tion pattern, increased the non-radiative deactivation pathway
and led to a moderate fluorescence quenching in polar media
(around 25%), not reflected in the fluorescence lifetime. This
behavior was a consequence of a concomitant decline in the
radiative rate constant (parallel to the decrease in the molar
absorption) and an enhancement of the non-radiative deacti-
vation rate constants with the solvent polarity, which further
favored the charge separation between the pyrrols.
Amino substitution. With respect to the parent dye 1, the
presence of an amino group at position 5 (compound 7) red-
shifts the spectral bands (around 20 nm), owing to the electron
donor ability of the amine, which enhances the delocalized
electronic density in the chromophoric core (Table 1 and
Fig. 3, left). Similar red shifts have been reported for BODIPYs
bearing amino groups at positions 3 and 5.18 The 5-amino
substituent increases the asymmetry in the charge distribution
with the consequent decrease in the absorption probability of
the new derivative 7. Alternatively the fluorescence quantum
yield remains high in apolar media, whereas it significantly
decreases in polar media, following the same dependence on
the solvent polarity to that previously observed for the
Table 1 Photophysical properties of compound 1 and its 5-amino derivative (7)
in apolar (cyclohexane) and polar media (methanol). The full photophysical data
in other solvents are collected in Table S1 (ESI3)
Taking into account the strong electron-withdrawing char-
acter of the substituent, one could wonder that this photo-
physical behavior is related to the presence of an
intramolecular charge transfer (ICT) state, active even in the
ground state. However, the influence of the ICT on the spectral
bands is not well defined, since absorption transitions
attributed to ICT have been reported as red-shifted and as
blue-shifted, with respect to the locally excited (LE) state.20
lab (nm)
emax (M21 cm21
)
lfl (nm)
Q
t (ns)
1
c-Hexane
Methanol
7
c-Hexane
Methanol
504.0
495.5
33000
25000
515.0
512.5
0.96
0.76
5.46
5.78
522.0
510.0
19000
9000
538.0
534.0
0.97
0.74
4.44
4.54
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 1547–1556 | 1549

View Article Online
Paper
RSC Advances
Fig. 3 (Left) Absorption and fluorescence spectra of the alkylated BODIPY 1 (a) and its 5-amino substituted derivative 7 (b). (Right) For comparison purposes, the
corresponding spectral bands of the fully unsubstituted BODIPY dye (named as BDP) and its 8-amino derivative ((c) and (d), respectively)7c,19 are included. For the sake
of clear understanding of the opposite spectral shifts induced by the amino group depending on the grafting position, the HOMO and LUMO contour maps of the
BODIPY core are also included.
More clearly perceptible is the dependence of the ICT states on
the solvent nature, since they are stabilized in polar media due
to their high dipole moment. Thus, their spectral bands are
very sensitive to the solvent polarity and usually show large
Stokes shifts in polar solvents.
change in the vibrational structure of the bands induced by
the nitro group, because of the following features: (a) the
vibrational resolution appears clearer in apolar media; (b)
there is only one emitting entity, since the spectral bands and
the decay curves are independent of the excitation and
emission wavelengths; (c) the solvatochromism of the spectra
However, the spectra of these nitro derivatives are char-
acterized by small Stokes shifts and low solvatochromism.
Indeed, the presence of such a possible second band becomes
less evident in polar media, whereas the spectral transitions
involving ICT states usually appear clearer, as the polarity of
the solvent increases. Moreover, the fluorescence spectra and
fluorescence decay curves of these nitro-substituted BODIPYs
do not change with the excitation wavelength (or emission
wavelength in the later case), and also their excitation spectra
perfectly match the absorption ones, regardless of the
emission wavelength. All this evidence allows rejection of the
hypothesis of the presence of an ICT state as the origin of the
apparent spectral splitting, which could be more related to the
is quite low (see Fig. S1 in the ESI ), and (d) theoretical
3
simulation of the absorption spectrum only predicts one
transition in the visible region.
Regarding the fluorescence properties, the presence of the
nitro group reduced the fluorescence efficiency and the
lifetime of the BODIPY, as shown in Table 2 and Fig. 5. Such
a decrease was more evident in polar media, where the
fluorescence decay is faster. This trend is more dramatic in
compound 4, because in the most polar media the fluores-
cence ability becomes negligible and the corresponding decay
curve has to be deconvoluted as biexponential, with a
dominating short lifetime (around 50 ps in methanol). The
marked sensitivity of the fluorescence quantum yield and
decay curve on the solvent polarity can be related to the
presence of an extra non-radiative deactivation pathway in
these media.
Taking into account the strong electron-withdrawing ability
of the nitro group, the reason for the fluorescence quenching
should be an ICT state, built up from the LE state. The
presence of an ICT has been discussed before to explain the
shape of the spectra. Although such a hypothesis was
discarded as an explanation of the spectral splitting, it does
not mean that the ICT is not taking place in the excited state.
From the above discussion, it is clear that the ICT state is not
formed in the ground state. Nonetheless, it can be populated
in the excited state, but with absence of self-emission, which
allows the maintenance of the vibrational resolution of the
absorption spectrum in the fluorescence one. Therefore, the
formation of an ICT state upon excitation in the nitro
derivatives 3 and 4 explains the decrease in fluorescence
ability. Generally, the ICT states are characterized by a poor or
negligible emission, since they are placed at lower energies
than the LE state and non-radiative deactivation is more
Fig. 4 Absorption and fluorescence spectra of compound 1 (a) and its nitro
derivatives 3 (b) and 4 (c).
1550 | RSC Adv., 2013, 3, 1547–1556
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
Table 2 Photophysical properties of reference compound 1 and its nitro derivatives (3 and 4) in cyclohexane and methanol. The full photophysical data in several
solvents are listed in Table S2 (ESI3)
lab (nm)
emax (M21 cm21
)
lfl (nm)
Q
t (ns)
kfl (1028 s21
)
knr (1028 s21
)
c-Hexane
1
3
504.0
458.0
476.0
474.0
33000
24000
24000
25000
515.0
509.0
530.0
530.5
556.0
0.96
0.53
5.46
3.13
1.75
1.70
0.07
1.49
4
0.17
1.53
1.11
5.42
Methanol
1
3
495.5
446.5
25000
21000
512.5
509.5
526.5
530.5
0.76
0.17
5.78
1.54
1.31
1.10
0.41
5.39
4
462.5
20000
0.005
0.05 (91%)
1.73 (9%)
—
—
feasible. In fact, in some derivatives, the ICT emission is
weakly detected in non-polar media. The ICT stabilization in
polar media implies a loss of its fluorescence emission,
because the consequent red-shift favors the non-radiative
deactivation from the ICT state.20a Moreover, the charge
transfer could be as feasible as a ‘‘dark’’ charge separation
(CS) state could be populated from the ICT state. In this state
the charge transfer is so high that the charges are fixed and the
electronic delocalization is stopped, hence contributing
further to the loss of fluorescence emission.
The fluorescence quenching ability of the ICT state induced
by the nitro group depends on the chromophoric position at
which it is attached. In fact, its inclusion at position 5 led to a
more drastic decrease in the fluorescence efficiency and
lifetime than at position 6. Generally, in BODIPYs the dipole
moment is located along the transversal axis with the positive
charge density placed around the central position 8 and the
negative charge density at the fluorine atoms (Fig. 6). Thus, the
charge separation should be more effective upon incorpora-
tion of an electron acceptor group (nitro) at the bottom part of
the BODIPY (position 5, compound 4) than at the rest of the
positions (i.e., position 6, compound 3). In fact, the presence
of the nitro substituent significantly increases the theoretically
calculated dipole moment of the molecule, from 6.6 D of the
parent compound 1 to 11.6 D for derivative 3 and up to 12.8 D
for compound 4 (Fig. 6). These results indicate that the
quenching effect of the ICT is more important in the derivative
4 where, in polar media, the fluorescence almost disappears
(,0.01). Moreover, in this last derivative the decay curve
becomes biexponential, with a main component characterized
by a very fast lifetime (around 50 ps), which is attributed to the
quenching of the fluorescence LE state by the ICT state, and a
residual contribution of a lifetime around 1.5–2 ns, resem-
bling that of the LE state. In more polar solvents such as 2,2,2-
trifluoroethanol, the fluorescence quantum yield decreases,
making an accurate deconvolution of the corresponding decay
curve impossible.
Consequently, the nitro derivatives described herein work as
polarity sensors due to the ICT state. They do not represent
good laser dyes but, apart from being excellent starting points
for further functionalization, they are very sensitive to the
environmental polarity.
Amino and nitro derivatives of 8-tolyl BODIPY 2. The
photophysics of the reference compound 2 are controlled by
the presence of the phenyl ring in the meso position. The
absence of substituents at adjacent positions enables free
rotation of the aryl group. Such a motion greatly increased the
internal conversion process, owing to vibrational couplings,
causing a drastic decrease of the BODIPY fluorescence ability
(Q , 0.05 and t , 400 ps; see Table S3, ESI ). Moreover,
3
Fig. 5 (A) Fluorescence decay curve of compound 1 (grey) and its nitro derivatives 3 (b) and 4 (a) in cyclohexane. (B) Decay curves of compound 2 as a function of the
solvent; cyclohexane (a), acetone (b) and F3-ethanol (c).
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 1547–1556 | 1551

View Article Online
Paper
RSC Advances
The vibrational resolution of the spectral bands upon
nitration of the reference 8-tolyl-BODIPY (compound 6) was
not so marked as previously observed in the nitro derivatives
of compound 1. In this case, the spectral bands are similar to
that shown by BODIPY dyes (see Fig. S2, ESI ), although they
3
became broader in polar media. Once more, these features
confirm that the nitro group tends to alter the vibrational
resolution of the electronic transitions, and reinforce the
above interpretation of the absence of spectral bands from any
ICT state for derivative 6, as well for 3 and 4.
Surprisingly, whereas the nitration of alkyl BODIPY was very
harmful to the fluorescence efficiency, the nitration of the
8-tolyl-BODIPY at position 5 (compound 6) ameliorates the
fluorescence ability of the chromophore (Fig. 8). The electron
withdrawing character of the nitro group removes electronic
density from the chromophore, including from the central
position. Thus, the effect of the aryl free motion is reduced
and the fluorescence efficiency is somewhat recovered.
The essential role of the electron character of the sub-
stituent in the fluorescence ability of derivatives bearing bulky
groups with free motion should be noted. Taking into account
the fact that the internal conversion processes have been
associated with the structural rigidity/flexibility, the electron
donor moieties (i.e. amine) increase the negative influence of
such rotational freedom on the fluorescence while the electron
acceptor ones (i.e. nitro) counteract this non-radiative path-
way. To this aim, the most effective positions to be substituted
are 3 and 5 (beginning and end of the delocalized p-system of
the BODIPY).8j,15
Fig. 6 Electrostatic potential mapped onto the electronic density for compound
1 and its nitro derivatives 3 and 4. The corresponding calculated dipole
moments are also included.
interaction between the phenyl and indacene electronic clouds
could not be discarded and could further take part in the non-
radiative deactivation processes.21
While the presence of an amine at position 5 in asymmetric
compound 1 (dye 7) led to a bathochromic spectral shift and
kept the high fluorescence ability of the BODIPY, the presence
of such a group in the 8-tolyl-BODIPY core (compound 8) gave
completely opposite results: a slight hypsochromic shift and a
lower fluorescence efficiency (Q , 0.03 and t , 150 ps; see
Table S3, ESI ). Therefore, in this case, the electron releasing
3
ability of the amine substituent increases the non-radiative
deactivation processes. This donor resonant effect should
increase the electronic charge at the BODIPY core, and hence,
the deleterious effect in fluorescence of the phenyl free motion
should be more important than in the above amino derivatives
without such an 8-aryl group. The molecular orbitals of
compound 8, depicted in Fig. 7, show that transition from
the HOMO to the LUMO state implies a shift of electronic
density from the amino-BODIPY core to the phenyl ring.
Considering the electron donor character of the amine and the
ability of the phenyl group to behave as donor or acceptor
depending on its substitution, the additional presence of a
quenching ICT state should be operative. Indeed, in the most
acidic media (2,2,2-trifluoroethanol), where the amine can be
protonated and its electron donor ability should be lower, the
highest fluorescence quantum yield is achieved, in concor-
dance with a lower formation probability of the ICT state.
Following a similar behavior to that previous described and
discussed for the nitro derivatives 3 and 4, in the case of
compound 6, the electron withdrawing effect of the nitro
group was so high that, in the more polar environments, the
mentioned ICT state is more stabilized and effectively
quenches its fluorescence emission (Fig. 8). In fact, the
fluorescence quantum yield of dye 6 in methanol clearly
Fig. 8 Evolution of the fluorescence quantum yield of reference compound 2
(triangles) and its nitro derivative 6 (circles) in several solvents ordered by their
polarity.
Fig. 7 HOMO and LUMO contour maps of compound 8.
1552 | RSC Adv., 2013, 3, 1547–1556
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
drops, to values as low as those of the reference compound 2,
and, simultaneously, the lifetime becomes shorter (around 500
reabsorption/re-emission processes. The growing importance
of reabsorption/re-emission processes with dye concentration
is also reflected in the progressive red-shift of the wavelength
of the laser emission.
In Table 3 are also presented the efficiency and peak
wavelength of the laser emission of compound 7 in polar
aprotic and polar protic solvents at the determined optimum
dye concentration. The lasing efficiencies correlate well with
the dye photophysical properties in diluted solutions (Table
ps in methanol, Table S3, ESI ). It should be mentioned that
3
the decay curve of the nitro-derivative 6 is described as
monoexponential even in polar media, while the nitration of
compound 1 at the same position 5 (compound 4) gave rise to
a biexponential decay in polar solvents. Such behaviour should
be related to the inherent high internal conversion probability
induced by the aryl free motion in the derivatives of compound
2, which reduces the influence of the ICT process in the
fluorescence decay of dye 6.
S1, ESI ): the higher the fluorescence quantum yield, the
3
higher the lasing efficiency, with the lowest efficiency being
obtained in polar protic solvents. In this regard, it should be
indicated than no laser emission was observed when the
solvent was 2,2,2-trifluoroethanol.
Due to the hypsochromic shift of the absorption bands in
the nitro derivatives, their lasing properties were first studied
under pumping at 355 nm, which is the usual pump
wavelength for dyes emitting in the blue spectral region.
Once again, although the highest fluorescence quantum yields
of the nitro derivatives 3 and 4 were obtained in apolar solvent
cyclohexane, these compounds could not be dissolved in this
solvent at the high concentrations needed for laser operation
under the transversal pumping arrangement.
Lasing properties
The position of the absorption bands of the derivatives with
amino substitution allows for studying their lasing properties
under pumping at 532 nm. Under our experimental conditions
(transversal excitation and strong focusing of the incoming
pump radiation, see experimental section), the concentration
of the dyes in the different solvents for the lasing studies
should be in the millimolar range, so that the incoming pump
radiation penetrates the sample (i.e. is absorbed) to a depth
similar to that of the thickness of the pump stripe at the input
face of the cell onto which the pumping radiation is focused
(about 0.3 mm). This gives rise to an emitted beam with a
near-circular cross-section and optimizes the lasing efficiency
(ratio between the energy of the dye laser output and the pump
energy incident on the sample surface).
Compound 9, where the amino group was at position 6, was
very unstable, as discussed above, when dealing with its
photophysical properties, and its lasing behavior could not be
characterized. Thus, only the lasing properties of compound 7,
with the amino group at position 5, could be studied. Although
the highest fluorescence quantum yield of this compound was
Compound 4 did not exhibit laser action, even under our
drastic pumping conditions, which was to be expected taking
into account the low fluorescence efficiency of this dye in all
solvents (see Table S2, ESI ). Compound 3 did exhibit laser
3
action, albeit with efficiencies of less than 10% under the 355
nm pumping (Table 4). From Table S2, ESI, and Fig. 3, it is
3
seen that although the absorption bands of the nitro
derivatives are blue shifted, away from the typical pump
wavelength of 532 nm, the shift is not enough to move the
bands as far away as 355 nm, so that absorption at this
wavelength is rather low. Higher lasing efficiency would be
expected if the nitro derivatives are pumped at or near the
maximum of their absorption bands. Thus, we used a
Coumarin 460-based dye laser, working at 450 nm (see
experimental section), to pump compound 3 at the peak of
its absorption band. As can be seen in Table 4, under these
conditions the lasing efficiency of this compound in ethyl
acetate doubled to 18%. This lasing efficiency is still lower
than those obtained with the amino derivatives (Table 3), as is
to be expected given the lower fluorescence quantum yields of
obtained in cyclohexane (Table S1, ESI ), it could not be
3
dissolved in this apolar solvent at the concentrations required
to obtain laser emission. Thus, the dependence of the laser
emission on the concentration of the solvent was studied in
ethyl acetate, which was the solvent where compound 7
exhibits the next highest quantum yield. It was found (Table 3)
that the optimum concentration was 1.4 6 1023 M, rendering
a lasing efficiency of 29%. Lower and higher concentrations
resulted in decreased lasing efficiency. This behavior of the
emission efficiency is typical of lasing dyes. The lasing
efficiency first increases with dye concentration up to a point
from where further increases in concentration result in a
decrease in the lasing efficiency, due to the increase in
Table 4 Maximum wavelength of the laser emission (lla) and lasing efficiency
(Eff) of compound 3 as a function of the concentration in ethyl acetate and in
different solvents at a given concentration
Table 3 Maximum wavelength of the laser emission (lla) and lasing efficiency
(Eff) of compound 7 as a function of the concentration in ethyl acetate and in
different solvents at a given concentration. Pump wavelength was 532 nm
a
Solvent
lpump (nm)
c (mM)
lla (nm)
Eff (%)
Ethyl acetate
355
355
355
450
450
450
450
450
2.2
4.5
6.7
2.2
4.5
6.7
4.5
4.5
534
539
540
537
538
540
534
539
9
7
4
11
18
16
8
Solvent
c (mM)
lla (nm)
Eff (%)
Ethyl acetate
0.8
1.4
1.9
2.8
1.4
1.4
1.4
563
564
568
568
558
573
576
24
29
20
14
12
20
23
Ethyl acetate
Acetone
Methanol
Methanol
Acetone
DCM
5
a
lpump: pump wavelength.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 1547–1556 | 1553

View Article Online
Paper
RSC Advances
compound 3 with respect to those of compound 7 (Tables S1
Characterization of the new dyes
and S2, ESI ). When compound 3 is dissolved in acetone or
3
Spectral data of the known compounds were in accordance
with the literature data. Flash column chromatography was
performed using silica gel Merck 60 (230–400 mesh). H and
methanol its lasing efficiency decreases, in good correlation
with the photophysical properties (lower fluorescence quan-
tum yields in these solvents than in ethyl acetate).
The 8-tolyl BODIPYs, derivatives of compound 2 with amino
or nitro groups at position 4 (compounds 8 and 6, respec-
tively), have very low fluorescence quantum yields (Table S3,
1
13C NMR spectra were recorded with a Bruker Avance-DPX-300
spectrometer (300 MHz for 1H and 75 MHz for 13C) and a
Bruker Avance III spectrometer (700 MHz for 1H and 176 MHz
for 13C). Chemical shift multiplicities are reported as s =
singlet, d = doublet, t = triplet, q = quartet and m = multiplet.
IR spectra (in cm21) were recorded in a Bruker Tensor-27-FTIR
spectrophotometer. Melting points were determined in open
capillaries and are uncorrected. High resolution mass spectra
were determined by ESI (electrospray ionization) in a FTMS
ESI ) in all solvents and, correspondingly, their laser emission
3
was very inefficient. Under 355 nm pumping and at the
optimum concentration of 2 6 1023 M, compound 8 lased at
550 nm with an efficiency of just 1%, whereas compound 6,
which has somewhat higher fluorescence quantum yields,
emitted laser radiation at 563 nm with an efficiency of 4%.
Bruker APEX
Q IV and by EI (electron impact) in a
Thermofisher MAT 95 XP (see ESI ).
3
Photophysical properties
Conclusion
The photophysical properties were registered in dilute solu-
tions (around
1026 M), prepared by adding the
The present work highlights the essential role of the nature
and position of the substituent, as well as the molecular
structure of the indacene core, in the optical properties of
BODIPYs. In this sense, the same functional group may lead to
entirely opposite results, just by varying the position in which
it is incorporated to the chromophore and/or the molecular
structure of the core. The amine group at position 3 led to a
modest red shift, but the same group at position 8 led to a
large hypsochromic shift. On the other hand, the strong
withdrawing character of the nitro group induced intramole-
cular charge transfer processes in the excited state, where the
quenching effect over the fluorescence emission depends
strongly on the position in which it is added to the indacene
core. In spite of this, laser action has been demonstrated in
both amine and nitro derivatives, with lasing properties
exhibiting good correlation with the photophysical properties.
Furthermore, the amino and nitro derivatives described
herein are excellent precursors, from a synthetic point of view,
to design BODIPY derivatives with different functionalizations.
2
6
corresponding solvent to the residue from the adequate
amount of concentrated stock solution in acetone, after
vacuum evaporation of this solvent. UV-Vis absorption and
fluorescence spectra were recorded on a Varian model CARY
4E spectrophotometer and a SPEX Fluorolog 3-22 spectro-
fluorimeter, respectively. The fluorescence spectra were
corrected from the wavelength dependence of the detector
sensibility. The fluorescence quantum yield (Q) was obtained
using a methanolic solution of PM567 as a reference (Qr =0.91).
Radiative decay curves were registered with the time correlated
single-photon counting technique (Edinburgh Instruments,
model FL920) using
a
microchannel plate detector
(Hamamatsu C4878), with picosecond time resolution (y20
ps). Fluorescence emission was monitored at the maximum
emission wavelength after excitation at 410, 440 and 470 nm
by means of a diode laser (PicoQuant, model LDH410, 440 and
470, respectively) with 150 ps FWHM pulses. The fluorescence
lifetime (t) was obtained from the slope after the deconvolu-
tion of the instrumental response signal from the recorded
decay curves by means of an iterative method. The closeness of
the exponential fit was controlled by statistical parameters
(chi-square, Durbin–Watson and the analysis of the residuals).
The radiative (kfl) and non-radiative deactivation (knr) rate
constants were calculated by means of: kfl = Q/t and knr = (1 2
Q)/t, respectively.
Experimental section
Materials
Starting materials and reagents used in the preparation of
BODIPYs are commercially available. The solvents were dried
and distilled before use.
Laser experiments
Liquid solutions of dyes were contained in 1 cm optical-path
length cells, which were carefully sealed to avoid solvent
evaporation during the experiments. The dye solutions were
transversely pumped with nanosecond pulses either at 532 nm
(6 ns FWHM and 5.5 mJ/pulse) or at 355 nm (8 ns FWHM and
3.5 mJ/pulse). The source of the 532 nm pulses was a
frequency-doubled Q-switched Nd:YAG laser (Monocrom
OPL-10). The 355 nm pulses were the third harmonic of a
Q-switched Nd:YAG laser (Spectron SL282G). In both cases the
excitation pulses were line-focused onto the cell, providing
pump fluences on the active medium of 180 mJ cm22 and 110
mJ cm22 for 532 nm and 355 nm pumping, respectively. The
Synthesis of BODIPY 1–9
BODIPY dyes 114 and 215 were synthesized by the methods
previously described. These compounds were transformed in
the corresponding nitro BODYPYs 3–6 by treatment with
HNO3/Ac2O. By means of a catalytic hydrogenation over Pd on
charcoal, the nitro derivatives 4 and 6 can be easily reduced to
the corresponding amino BODIPYs 7 and 8. The syntheses and
spectroscopic data for all compounds are described in the
ESI.
3
1554 | RSC Adv., 2013, 3, 1547–1556
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
E. Bahaidarah, A. Harriman and R. Ziessel, J. Am. Chem.
Soc., 2012, 134, 6100–6103; (m) C. W. Evans, M. J. Latter,
D. Ho, S. A. M. A. Peerzade, T. D. Clemons, M. Fitzgerald, S.
A. Dunlop and K. S. Iyer, New J. Chem., 2012, 36, 1457–1462.
3 A. Haefele, C. Zedde, P. Retailleau, G. Ulrich and R. Ziessel,
Org. Lett., 2010, 12, 1672–1675.
oscillation cavity (2 cm in length) consisted of a 90%
reflectivity back aluminum mirror and the lateral face of the
cell as an output coupler. Experiments were also performed at
a pump wavelength of 450 nm, with 12 ns, 3 mJ pulses from a
dye laser (ethanolic solution of Coumarin 460 pumped with
the Spectron SL282G 355 nm pulses). The energy of dye and
pump laser pulses was measured with GenTec ED-100A and
ED-200 pyroelectric energy meters. The spectral characteristics
of the laser emission were determined by collecting a fraction
of the emission by an optical fiber attached to the input slit of
4 (a) T. Rohand, M. Baruah, W. Qin, N. Boens and
W. Dehaen, Chem. Commun., 2006, 266–268; (b) L. Li,
B. Nguyen and K. Burgess, Bioorg. Med. Chem. Lett., 2008,
¨
18, 3112–3116; (c) O. Dilek and S. L. Bane, Tetrahedron Lett.,
˜
2008, 49, 1413–1416; (d) E. Fron, E. Coutino-Gonzalez,
a
spectrograph/monochromator (SpectraPro-300i, Acton
L. Pandey, M. Sliwa, M. Van der Auweraer, F. C. De
Schryver, J. Thomas, Z. Dong, V. Leen, M. Smet, W. Dehaen
and T. Vosch, New J. Chem., 2009, 33, 1490–1496; (e)
W. Qin, V. Leen, W. Dehaen, J. Cui, C. Xu, X. Tang, W. Liu,
T. Rohand, D. Beljonne, B. Van Averbeke, J. N. Clifford,
K. Driesen, K. Binnemans, M. Van der Auweraer and
N. Boens, J. Phys. Chem. C, 2009, 113, 11731–11740; (f)
L. Jiao, C. Yu, M. Liu, Y. Wu, K. Cong, T. Meng, Y. Wang
and E. Hao, J. Org. Chem., 2010, 75, 6035–6038; (g) L. Jiao,
C. Yu, T. Uppal, M. Liu, Y. Li, Y. Zhou, E. Hao, X. Hu and
M. G. H. Vicente, Org. Biomol. Chem., 2010, 8, 2517–2519;
(h) V. Leen, T. Leemans, N. Boens and W. Dehaen, Eur. J.
Org. Chem., 2011, 23, 4386–4396; (i) O. Dilek and S. L. Bane,
J. Fluoresc., 2011, 21, 347–354; (j) M. Zhang, E. Hao, J. Zhou,
C. Yu, G. Bai, F. Wang and L. Jiao, Org. Biomol. Chem., 2012,
10, 2139–2145; (k) X. Li, S. Huang and Y. Hu, Org. Biomol.
Chem., 2012, 10, 2369–2372; (l) G. Ulrich, A. Haefele,
P. Retailleau and R. Ziessel, J. Org. Chem., 2012, 77,
5036–5048.
5 (a) T. Rohand, W. Qin, N. Boens and W. Dehaen, Eur. J. Org.
Chem., 2006, 4658–4663; (b) W. Qin, T. Rohand,
W. Dehaen, J. N. Clifford, K. Driesen, D. Beljonne, B. Van
Averbeke, M. Van der Auweraer and N. Boens, J. Phys.
Chem. A, 2007, 111, 8588–8597; (c) S. Rihn, P. Retailleau,
N. Bugsaliewicz, A. De Nicola and R. Ziessel, Tetrahedron
Lett., 2009, 50, 7008–7013; (d) R. Guliyev, A. Coskun and E.
U. Akkaya, J. Am. Chem. Soc., 2009, 131, 9007–9013; (e) O.
A. Bozdemir, Y. Cakmak, F. Sozmen, T. Ozdemir,
A. Siemiarczuk and E. U. Akkaya, Chem.–Eur. J., 2010, 16,
6346–6351; (f) M. J. Ortiz, I. Garcia-Moreno, A.
R. Agarrabeitia, G. Duran-Sampedro, A. Costela, R. Sastre,
Research Corporation) equipped with a charge-coupled device
(CCD)/SpectruMM:GS 128B).
Acknowledgements
This work was supported by Projects MAT2010-20646-C04-01,
-02 and -04 and TRACE 2009-0144 of the Spanish Ministerio de
´
Economıa y Competitividad (MINECO). Ixone Esnal thanks the
Gobierno Vasco for predoctoral contracts (S-PE10UN65,
SPE11UN064, SPE12UN140 and IT339-10)
References
1 (a) A. Loudet and K. Burgess, Chem. Rev., 2007, 107,
4891–4932; (b) R. Ziessel, G. Ulrich and A. Harriman, New J.
Chem., 2007, 31, 496–501; (c) G. Ulrich, R. Ziessel and
A. Harriman, Angew. Chem., Int. Ed., 2008, 47, 1184–1201;
˜
(d) F. L. Arbeloa, J. Banuelos, V. Martinez, T. Arbeloa and I.
L. Arbeloa, Trends Phys. Chem., 2008, 13, 101–122; (e) A.
C. Benniston and G. Copley, Phys. Chem. Chem. Phys., 2009,
11, 4124–4131; (f) M. Benstead, G. H. Mehl and R. W. Boyle,
Tetrahedron, 2011, 67, 3573–3601; (g) N. Boens, V. Leen and
W. Dehaen, Chem. Soc. Rev., 2012, 41, 1130–1172.
2 (a) K. Rurack, M. Kollmannsberger and J. Daub, Angew.
Chem., Int. Ed., 2001, 40, 385–387; (b) O. Buyukcakir, O.
A. Bozdemir, S. Kolemen, S. Erbas and E. U. Akkaya, Org.
Lett., 2009, 11, 4644–4647; (c) O. A. Bozdemir, R. Guliyev,
O. Buyukcakir, S. Selcuk, S. Kolemen, G. Gulseren,
T. Nalbantoglu, H. Boyaci and E. U. Akkaya, J. Am. Chem.
Soc., 2010, 132, 8029–8036; (d) T. Bura, P. Retailleau and
R. Ziessel, Angew. Chem., Int. Ed., 2010, 49, 6659–6663; (e)
S. Kolemen, O. A. Bozdemir, Y. Cakmak, G. Barin, S. Erten-
Ela, M. Marszalek, J.-H. Yum, S. M. Zakeeruddin, M.
´
˜
´
F. Lopez Arbeloa, J. Banuelos Prieto and I. Lopez Arbeloa,
Phys. Chem. Chem. Phys., 2010, 12, 7804–7811; (g)
C. Thivierge, J. Han, R. M. Jenkins and K. Burgess, J. Org.
Chem., 2011, 76, 5219–5228; (h) T. K. Khan and
M. Ravikant, Tetrahedron, 2011, 67, 5816–5824; (i) Y. Wu,
X. Ma, J. Jiao, Y. Cheng and C. Zhu, Synlett, 2012, 23,
778–782.
¨
K. Nazeeruddin, M. Gratzel and E. U. Akkaya, Chem. Sci.,
2011, 2, 949–954; (f) T. Bura, P. Retailleau, G. Ulrich and
R. Ziessel, J. Org. Chem., 2011, 76, 1109–1117; (g) O.
A. Bozdemir, S. Erbas-Cakmak, O. O. Ekiz, A. Dana and E.
U. Akkaya, Angew. Chem., Int. Ed., 2011, 50, 1–7; (h) H. He,
P.-C. Lo, S.-L. Yeung, W.-P. Fong and D. K. P. Ng, Chem.
Commun., 2011, 47, 4748–4750; (i) T. Bura, D. Hablot and
R. Ziessel, Tetrahedron Lett., 2011, 52, 2370–2374; (j)
N. Boens, W. Qin, M. Baruah, W. M. De Borggraeve,
A. Filarowski, N. Smisdom, M. Ameloot, L. Crovetto, E.
M. Talavera and J. M. Alvarez-Pez, Chem.–Eur. J., 2011, 17,
10924–10934; (k) S. Zhu, J. Zhang, G. Vegesna, A. Tiwari, F.-
T. Luo, M. Zeller, R. Luck, H. Li, S. Green and H. Liu, RSC
6 (a) C. Thivierge, R. Bandichhor and K. Burgess, Org. Lett.,
2007, 9, 2135–2138; (b) J. Chen, M. Mizumura,
H. Shinokubo and A. Osuka, Chem.–Eur. J., 2009, 15,
´
5942–5949; (c) V. Leen, V. Zaragozı Gonzalvo, W.
M. Deborggraeve, N. Boens and W. Dehaen, Chem.
Commun., 2010, 46, 4908–4910; (d) V. Leen, M. Van der
Auweraer, N. Boens and W. Dehaen, Org. Lett., 2011, 13,
1470–1473; (e) B. Verbelen, V. Leen, L. Wang, N. Boens and
W. Dehaen, Chem. Commun., 2012, 48, 9129–9131.
7 (a) T. V. Goud, A. Tutar and J.-F. Biellmann, Tetrahedron,
˜
2006, 62, 5084–5091; (b) E. Pena-Cabrera, A. Aguilar-
´
´
´
Adv., 2012, 2, 404–407; (l) J.-H. Olivier, J. Barbera,
Aguilar, M. Gonzalez-Domınguez, E. Lager, R. Zamudio-
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 1547–1556 | 1555

View Article Online
Paper
RSC Advances
´
´
Vazquez, J. Godoy-Vargas and F. Villanueva-Garcıa, Org.
S. Cameron and A. Thompson, Chem. Commun., 2012, 48,
1003–1005.
Lett., 2007, 9, 3985–3988; (c) J. Han, O. Gonzalez, A. Aguilar-
˜
Aguilar, E. Pena-Cabrera and K. Burgess, Org. Biomol.
10 M. Shah, K. Thangaraj, M.-L. Soong, L. T. Wolford, J.
H. Boyer, R. Politzer and T. G. Pavlopoulos, Heteroat.
Chem., 1990, 1, 389–399.
Chem., 2009, 7, 34–36; (d) E. Lager, J. Liu, A. Aguilar-
Aguilar, B. Z. Tang and E. Pena-Cabrera, J. Org. Chem.,
˜
´
´
´
2009, 74, 2053–2058; (e) C. F. A. Gomez-Duran, I. Garcıa-
11 T. Suzuki, T. Tanaka, I. Higashiguchi and A. Oda, JP Patent
11176572, 1999.
´
˜
Moreno, A. Costela, V. Martın, R. Sastre, J. Banuelos,
´
´
˜
F. Lopez Arbeloa, I. Lopez Arbeloa and E. Pena-Cabrera,
12 K. Takuma, T. Misawa, K. Sugimoto, T. Nishimoto,
H. Tsukahara and K. Takuma, JP Patent 10273504, 1998.
13 T. Tachikawa, N. Wang, S. Yamashita, S.-C. Cui and
T. Majima, Angew. Chem., Int. Ed., 2010, 49, 8593–8597.
Chem. Commun., 2010, 46, 5103–5105; (f) I. J. Arroyo, R. Hu,
B. Z. Tang, F. I. Lopez and E. Pena-Cabrera, Tetrahedron,
2011, 67, 7244–7250.
´
˜
˜
8 (a) L. Li, J. Han, B. Nguyen and K. Burgess, J. Org. Chem.,
2008, 73, 1963–1970; (b) L. Bonardi, G. Ulrich and
R. Ziessel, Org. Lett., 2008, 10, 2183–2186; (c) O.
14 J. Banuelos Prieto, A. R. Agarrabeitia, I. Garcia-Moreno,
I. Lopez-Arbeloa, A. Costela, L. Infantes, M. E. Perez-Ojeda,
M. Palacios-Cuesta and M. J. Ortiz, Chem.–Eur. J., 2010, 16,
14094–14105.
¨
A. Bozdemir, O. Bu¨yu¨kcakir and E. U. Akkaya, Chem.–Eur.
J., 2009, 15, 3830–3838; (d) K. Krumova and G. Cosa, J. Am.
Chem. Soc., 2010, 132, 17560–17569; (e) S. Rihn, M. Erdem,
A. De Nicola, P. Retailleau and R. Ziessel, Org. Lett., 2011,
13, 1916–1919; (f) R. P. Sabatini, T. M. McCormick,
T. Lazarides, K. C. Wilson, R. Eisenberg and D.
W. McCamant, J. Phys. Chem. Lett., 2011, 2, 223–227; (g)
C. Yu, L. Jiao, H. Yin, J. Zhou, W. Pang, Y. Wu, Z. Wang,
G. Yang and E. Hao, Eur. J. Org. Chem., 2011, 5460–5468; (h)
L. Jiao, W. Pang, J. Zhou, Y. Wei, X. Mu, G. Bai and E. Hao,
J. Org. Chem., 2011, 76, 9988–9996; (i) Y. Hayashi,
S. Yamaguchi, W. Y. Cha, D. Kim and H. Shinokubo, Org.
Lett., 2011, 13, 2992–2995; (j) M. J. Ortiz, A. R. Agarrabeitia,
15 A. Cui, X. Peng, J. Fan, X. Chen, Y. Wu and B. Guo, J.
Photochem. Photobiol., A, 2007, 186, 85–92.
16 H. Oda, T. Hanami, T. Iwashita, M. Kojima, M. Itoh and
Y. Hayashizaki, Tetrahedron, 2007, 63, 12747–12753.
17 A. R. Katritzky, K. Suzuki, S. K. Singh and H.-Y. He, J. Org.
Chem., 2003, 68, 5720–5723.
18 W. Qin, V. Leen, T. Rohand, W. Dehaen, P. Dedecker,
M. Van der Auweraer, K. Robeyns, L. Van Meervelt,
D. Beljonne, B. Van Averbeke, J. N. Clifford, K. Driesen,
K. Binnemans and N. Boens, J. Phys. Chem. A, 2009, 113,
439–447.
˜
´
´
19 J. Banuelos, V. Martin, C. F. A. Gomez-Duran, I. J. Arroyo-
˜
´ ˜ ´
Cordoba, E. Pena-Cabrera, I. Garcıa-Moreno, A. Costela, M.
G. Duran-Sampedro, J. Banuelos Prieto, T. Arbeloa Lopez,
´
´
´
W. A. Massad, H. A. Montejano, N. A. Garcıa and I. Lopez
Arbeloa, Tetrahedron, 2012, 68, 1153–1162.
E. Perez-Ojeda, T. Arbeloa and I. Lopez Arbeloa, Chem.–Eur.
J., 2011, 17, 7261–7270.
9 (a) A. Harriman, L. Mallon and R. Ziessel, Chem.–Eur. J.,
2008, 14, 11461–11473; (b) T. W. Hundnall, T.-P. Lin and F.
20 (a) M. Kollmannsberger, K. Rurack, U. Resch-Genger and
J. Daub, J. Phys. Chem. A, 1998, 102, 10211–10220; (b) Z.
R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev.,
2003, 103, 3899–4032; (c) W. Qin, M. Baruah, M. Van der
Auweraer, F. C. De Schryver and N. Boens, J. Phys. Chem. A,
2005, 109, 7371–7384.
21 H. L. Kee, C. Kirmaier, L. Yu, P. Thamyongkit, W.
J. Youngblood, M. E. Calder, L. Ramos, B. C. Noll, D.
F. Bocian, W. E. Scheidt, R. R. Birge, J. S. Lindsey and
D. Holten, J. Phys. Chem. B, 2005, 109, 20433–20443.
¨
P. Gabbaı, J. Fluorine Chem., 2010, 131, 1182–1186; (c) J.-
H. Olivier, A. Haefele, P. Retailleau and R. Ziessel, Org.
Lett., 2010, 12, 408–411; (d) C. A. Wijesinghe, M. E. El-
Khouly, J. D. Blakemore, M. E. Zandler, S. Fukuzumi and
F. D’Souza, Chem. Commun., 2010, 46, 3301–3303; (e)
T. Bura and R. Ziessel, Org. Lett., 2011, 13, 3072–3075; (f)
X.-D. Jiang, J. Zhang, T. Furuyama and W. Zhao, Org. Lett.,
2012, 14, 248–251; (g) T. Lundrigan, S. M. Crawford, T.
1556 | RSC Adv., 2013, 3, 1547–1556
This journal is ß The Royal Society of Chemistry 2013