Ethynyl-boron subphthalocyanines displaying efficient cascade energy transfer and large stokes shifts

Article abstract of DOI:10.1002/anie.200803131

A new class of luminescent dyes incorporates aromatic residues attached by an ethynyl link to the boron center of a subphthalocyanine (SubPc; see picture). Efficient energy transfer from energydonor subunits to the SubPc produces very large Stokes shifts,

Full text of DOI:10.1002/anie.200803131

Communications
DOI: 10.1002/anie.200803131
Subphthalocyanines
Ethynyl–Boron Subphthalocyanines Displaying Efficient Cascade
Energy Transfer and Large Stokes Shifts**
Franck Camerel, Gilles Ulrich, Pascal Retailleau, and Raymond Ziessel*
The demand for “smart” fluorophores is ever increasing, not
only for use in rapid, high-throughput procedures in biomed-
ical analysis, but also in photodynamic cancer therapy and
chemidosimetry, as well as for broader applications in
electroluminescence, nonlinear optics, and laser technology.
Of the many different dyes considered as fluorophores, those
based on cyanine, pyrene, perylene, porphyrin, and indacene
skeletons are the most promising.[1,2] As another class of
potentially interesting compounds, the phthalocyanines (Pcs)
and subphthalocyanines (SubPcs), a constricted form of Pcs,
suffer the drawbacks of generally poor solubility and small
Stokes shifts (Dl ꢀ 250 cmÀ1) when excited in the low-energy
absorption band.[3,4] Their solubility has, however, been
enhanced by modifying the organic core with paraffin
chains, which also allows these molecules to be organized at
the supramolecular level in liquid crystals[5] and Langmuir–
Blodgett films.[6] The subphthalocyanines, which have a
boron(III) atom coordinated within the macrocycle, have
been functionalized at the boron atom with alkoxy, silyloxy,
and phenoxy groups,[4] and, in one instance, with a phenyl
group.[7] Such species, incidentally, may be chiral when the
dinitrile precursor used in the conventional syntheses does
not have C2v symmetry.[8]
units in some of the substituents used, can be switched off by
the protonation of these units.
Replacement of the chlorido ligand on the boron atom in
the starting material Cl-SubPc[9] is readily achieved by
reaction with the corresponding Grignard reagents (ethynyl-
tolyl 1, ethynylphenyliodo 2, ethynylgallate 3, or ethynyl(di-n-
butyl)aminophenyl 4) in hot THF. The introduction of the
iodophenyl substituent in 2 is particularly useful because of
Herein we describe a rational method for increasing both
the solubility and Stokes shifts of SubPcs, and for tuning their
fluorescence properties, by introducing functionalised alkynyl
substituents at the boron atom. This strategy enables the
addition of various functionalities without perturbing the
bowl-shaped structure and the intriguing spectroscopic prop-
erties of the SubPcs. We have prepared a number of these E-
SubPcs (E = ethynyl) with the objectives of enhancing
solubility by introducing gallate or truxene substituents and
improving their properties as fluorophores by introducing
pyrene or truxene substituents, which not only increase
absorption but also efficiently transfer their absorbed
energy to the SubPc fluorophore. Fluorescence quenching,
which potentially arises from the presence of di-n-butylamino
[*] Dr. F. Camerel, Dr. G. Ulrich, Dr. R. Ziessel
Laboratoire de Chimie MolØculaire, ECPM-CNRS
25 rue Becquerel, 67087 StrasbourgCedex 02 (France)
E-mail: ziessel@chimie.u-strasbg.fr
Dr. P. Retailleau
Laboratoire de Cristallochimie, ICSN-CNRS, Bât 27
1 avenue de la Terrasse, 91198 Gif-sur-Yvette, Cedex (France)
Figure 1. a) ORTEP view of compound 1 showingthermal ellipsoids at
the 50% probability level. Hydrogen atoms have been omitted for
clarity. Selected data B1–N2 1.504(3), B1–N4 1.499(5), B1–N6
1.502(4), B1–C1 1.586(4) Š and N2-B1-N4 103.1(2), N4-B1-N6
103.0(2), N6-B1-N2 103.4(2), N2-B1-C1 115.8(2), N4-B1-C1 115.2(2),
N6-B1-C1 114.6(2)8. b) View showingthe crystal packingin the
ac plane.
[**] This work was supported by the Centre National de la Recherche
Scientifique and the Minist›re de la Recherche et des Nouvelles
Technologies. We are indebted to Prof. J. Harrowfield for his
comments on the manuscript.
Supportinginformation for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803131.
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the ease of subsequent Pd0-catalyzed cross-coupling reactions
with ethynyltruxene (and other) derivatives in benzene/
triethylamine (TEA) at 608C. The highly soluble E-SubPcs
produced in all these reactions are easily purified by flash
chromatography and conveniently characterized by NMR
spectroscopy and mass spectrometry. The absence of aggre-
gates is proved by the well-defined NMR spectra of these
The boron atom of 4 displays an almost-regular tetrahe-
dral geometry (Figure 2a). In contrast to 1, a slight tilt of 12.28
between the B1–C1 and the C1–N7 axes can be observed,
which shows that the ethynyl–nBu2N subunit does not bind
exactly along the C3 axis of the SubPc fragment. The lattice of
compound 4 can be described as being composed of molecular
layers stacked along the c direction. In the ab layer, segrega-
tion between the subphthalocyanine fragments and the
ethynyl(di-n-butyl)aminophenyl fragments can be clearly
observed. The phenyl groups weakly interact (4.24 Š) in a
head-to-tail fashion to form dimers along the a axis whereas
short contacts (3.379 Š) between the isoindole of SubPc
fragments stabilize the edifice in the b direction. The distance
between neighboring boron atoms is 8.862(17) Š. The molec-
ular organization defines infinite channels parallel to the
a direction which accommodate disordered solvent mole-
cules. Unlike fluoro-SubPcs,[13] the molecular shape of com-
pounds 2 and 4 (inverted umbrella) prevents columnar
stacking of the molecules.
The absorption spectra of all compounds show a strong
Soret band near 300 nm (e ꢀ 30000mÀ1 cmÀ1) and an intense
Q band around 565 nm (e ꢀ 50000–89000mÀ1 cmÀ1), as for
simple SubPcs bearing axial chlorido ligands.[14] The axial
substituents have only a very small effect on the position of
the Q band (labs = 567 nm for 1–3 and labs = 573 nm for the Cl-
SubPc precursor). The electronic absorption spectra of 3 and
6 reflect the presence of the electronically independent SubPc
and ethynylgallate (lmax at 271 and 304 nm) or ethynylpyrene
subunits (lmax at 386 and 414 nm) without noticeable absorp-
tion shifts, which is consistent with the absence of orbital
overlap between the SubPc organic core and the axially
connected nuclei (Figure 3).
1
compounds. The H NMR spectra display two multiplets in
the aromatic region with a diagnostic AA’BB’ system for the
isoindole core, and the 11B NMR spectra show a typical singlet
around À17 ppm.[4a] Although trigonal alkynylborane deriv-
atives are well documented,[10] tetrahedral species are
rare[11,12] and, to the best of our knowledge, the synthesis of
E-SubPcs has not been reported to date.
Chloride substitution at the vertex of the boron tetrahe-
dron by ethynyltolyl (1) and ethynyl(di-n-butyl)aminophenyl
(4) was unambiguously confirmed by X-ray crystal structure
analysis. The molecular unit discernable within the crystalline
lattice of 1 shows coordination of the tetrahedral N3C to the
boron atom as well as the effective threefold symmetry of the
SubPc subunit (Figure 1a). Projection of the crystalline lattice
along the a axis shows that the molecular array must at least in
part be determined by interactions between parallel aromatic
entities. In particular, in the ac sheet (Figure 1b), oppositely
oriented SubPc units (B–B 11.241 Š) do appear to be
involved in true “face-to-face” p stacking with the formation
of dimers through interactions of the tolyl rings (3.52Š).
Interestingly, the shortest B–B distance (8.667 Š) in the
lattice occurs for two facing SubPc molecules located
diagonally across the “voids” defined by the interlocking
sheets of the view along a. In this centrosymmetric pair, the
shortest interatomic contacts (3.411 Š) are between a bridg-
ing N atom of one SubPc unit and an aromatic C atom of the
other.
For all the new compounds, excitation in the low energy
absorption band around 567 nm lead to a strong emission at
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Figure 3. Absorption, emission and excitation spectra in dichlorome-
thane of a) 3 and b) 6.
Figure 2. a) ORTEP view of compound 4 showingthermal ellipsoids at
Table 1: Spectroscopic data (298 K) for SubPc derivatives.[a]
the 50% probability level. Hydrogen atoms have been omitted for
clarity. Selected data: B1–N2 1.496(9), B1–N4 1.508(11), B1–N6
1.487(9), B1–C1 1.565(10) Š and N2-B1-N4 102.5(5), N4-B1-N6
102.1(5), N6-B1-N2 103.6(5), N2-B1-C1 114.6(5), N4-B1-C1 118.3(5),
N6-B1-C1 113.9 (5)8. b) Views showingthe crystal packingin the
ab plane.
[b]
[c]
Cmpd labs
e
lF
FF tF
kr[c]
knr
Stokes
[nm] [mÀ1 cmÀ1
]
[nm] [%] [ns] [107 sÀ1
]
[108 sÀ1
] shift
[cmÀ1
]
1
2
3
4
567 53300
567 88000
567 65600
566 76000
575 15
575 17
575 12
2.8 5.36
3.3 5.15
2.9 4.13
0.4 0.25
1.4 1.21
2.6 6.15
3.03
2.51
2.24
24.8
5.90
3.23
3.35
–
245
245
245
456
456
214
12000
–
245
6760
–
581
1
575 nm, with quantum yields in the range 12–23% (Table 1)
and Stokes shifts of 245 cmÀ1, which are comparable with
those of related SubPcs.[15] The fluorescence spectra show
excellent mirror symmetry with the lowest energy absorption
transition, which confirms that these transitions are weakly
polarized and originate from the same state, as is typical of
singlet emitters. Furthermore, the fluorescence decay profiles
can be fitted to a single exponential decay, with fluorescence
lifetimes of about 2ns, which is also typical of a singlet excited
state. For dyes 5 and 6, excitation into the absorption band of
the truxene and pyrene residues (at 340 and 414 nm respec-
tively) leads principally to the characteristic emission of the
SubPc fragment at 575 nm as well as a very weak emission
signal from the polycyclic fragments (f= 0.1% residual
fluorescence at 380 nm for compound 5 and f= 2% residual
fluorescence at 477 nm for compound 6; see Table 1 for the
4+H+
–
–
581 17
575 16
575 13
5
568 89400
340 68500
–
5.00
–
380 0.1 n.d.
6
567 59000
414 0.665
575 23
575 18
3.0 6.00
2.73
2.56
–
–
7.66
–
477
2
n.d.
A
B
324 21500
308 41400
410 36700
387 37700
380 32
17.8 0.18
38.2
–
484 85
2.6 3.27
57.7
–
[a] Determined in degassed dichloromethane solution. n.d.=not deter-
mined. [b] Determined usingrhodamine 6G ( f =0.86 in methanol[20]
)
F
and cresyl violet (fF =0.50 in ethanol[20]) as reference. All fF values were
corrected for changes in refractive index. [c] Calculated using the
followingequations: kr =FF/tF, knr =(1ÀFF)/tF, assumingthat the
emittingstate is produced with unit quantum efficiency.
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fluorescence of the reference compounds). The fluorescence
excitation spectra perfectly match the absorption spectra,
confirming a very efficient cascade energy transfer from the
truxene or pyrene residues to the SubPc emitter (Figure 3).
An energy transfer efficiency of > 96% can be derived from
the quantum yield measurements. For the pyrene derivative 6,
the more favorable spectral overlap is responsible for the
almost quantitative energy transfer. The virtual Stokes shifts
increase from 250 cmÀ1 in Cl-SubPc to 6800 cmÀ1 for 5 and
12000 cmÀ1 for 6.[15]
When strongly electron-donating groups such as nBu2N
are linked to the SubPc by an ethynylphenyl unit (compound
4), marked quenching is observed (94% of the initial
fluorescence). The calculated driving force for photoinduced
electron transfer from the nBu2N fragment to the SubPc*
(excited state) is DG = À0.31 eV, calculated by using the
Rehm–Weller equation,[16] with the assumption that the
electrostatic factor is negligible and that the emitting Q-
band level lies at 2.22 eV. The redox potentials required for
these calculations are given in Table 2. Addition of excess
HClO4 in CH2Cl2 restores the emission intensity, giving a
quantum yield of 17% and a corresponding increase of kr
from 0.25 ” 107 sÀ1 to 1.2” 10 7 sÀ1.
Stepwise addition of HClO4 in dichloromethane progres-
sively enhances the fluorescence intensity of 4 (Figure 4). The
changes in the absorption spectra upon addition of the acid
showed a well-defined isosbestic point that is indicative of a
single equilibrium step. This was analyzed using SPECFIT[17]
gave log b = (4.3 Æ 0.1), a value similar to analogous dialkyl-
Figure 4. Titration of 4 with HClO4 in CH2Cl2 in nBu4NPF6 (0.01m)
a) by absorption and b) by fluorescence lexc =500 nm.
Table 2: Electrochemical data and optical HOMO–LUMO gaps.[a]
Cmpd
Redox potentials [V]
versus SCE (DEp [mV])
HOMO–LUMO gap [eV]
amino derivatives.[18] Protonation of the nBu2N fragment
inhibits the photoinduced electron transfer (PET) process
and the initial fluorescence is restored. Several protonation/
deprotonation cycles were achieved, with no noticeable
decomposition of compound 4.
Eox
Ered
CV
UV/Vis[b]
2.15
1
2
3
4
+0.91 (60)
À1.09 (70)
À1.64 (80)
À1.08 (70)
À1.62 (ir.)
À1.11 (70)
À1.71 (ir.)
À1.14 (70)
À1.68 (80)
–
À1.10 (60)
À1.64 (ir.)
À1.18 (60)
À1.61 (60)
À1.77 (ir.)
À1.82 (ir.)
À1.33 (80)
À1.80 (ir.)
2.00
+0.91 (60)
+0.94 (70)
1.99
2.05
1.91
2.15
2.15
2.13
The axial grafting of aryl residues to the boron atom
facilitates the oxidation of the SubPc to SubPc+C by 140 mV
compared to the starting Cl-SubPc, whereas the reduction of
the dye to the radical anion SubPcÀC is cathodically shifted by
90 mV in most cases (Table 2).[19] Interestingly, the SubPc2À
dianion is also observed around À1.64 V but is not observed
with the Cl-SubPc. The experimentally determined electro-
chemical HOMO–LUMO (HOMO = highest occupied
molecular orbital, LUMO = lowest unoccupied molecular
orbital) gaps are consistent with the gaps determined by
optical spectroscopy (Table 2). As would be expected by
grafting pyrene and truxene fragments, additional redox
processes are present, in particular the irreversible oxidations
of pyrene at + 1.20 V and truxene at + 1.45 V. These results
reflect trends found for related boron-substituted ethynyl
derivatives, for which similar shifts of both redox processes
have been observed.[12b]
+0.99 (ir.)
+0.77 (ir.)
+0.99 (ir.)
+0.96 (ir)
+1.45 (ir.)
+0.96 (ir.)
+1.20 (ir.)
4+H+[c]
5
–
2.06
2.13
2.15
6
2.14
2.15
A
B
+1.35 (ir.)
+1.16 (ir.)
–
–
–
–
[a] Potentials determined by cyclic voltammetry in deoxygenated CH2Cl2
solution, containing nBu4NPF6 (0.1m), at a solute concentration of ca.
1.5 mm and at 208C. Potentials were standardized versus ferrocene (Fc)
as internal reference and converted to the saturated calomel electrode
(SCE) scale assumingthat E1/2 (Fc/Fc+)=+0.38 V (DEp=60 mV) vs.
SCE. Error in half-wave potentials is Æ10 mV. Irreversible processes are
indicated as ir. All reversible redox steps result from one-electron
processes. [b] Calculated from the Q band transition. [c] In the presence
of CF3COOH vapor. The cathodic processes are masked by the reduction
of the protons.
In compound 4, the nBu2N fragment is oxidized at
+ 0.77 V. The addition of trace amounts of CF3COOH
during the cyclic voltammetry clearly resulted in the disap-
pearance of the wave at + 0.77 V (versus SCE), whereas the
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[3] M. Hanack, H. Heckmann, R. Polley, Methoden Org.Chem.
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potential (Table 2).
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In summary, we have developed a general method for the
engineering of axially modified 14-p-conjugated SubPcs. The
use of alkynylaryl residues is an elegant means to simulta-
neously import solubility, polarizability, and strong electronic
absorption. By grafting pyrene or truxene groups, efficient
energy transfer to the SubPc occurs, which increases the
virtual Stokes shifts to 6800 cmÀ1 for pyrene and to
12000 cmÀ1 for truxene substitution. Replacement of B Cl
À
À
by B C ꢁ CAr does not significantly perturb the HOMO–
LUMO band gap (Eg ꢀ 2.05 eV) but induces a translation to
the anodic zone by approximately 100 mV. Grafting strong
nBu2N electron-donating groups to the SubPc quenches its
fluorescence by photoinduced electron transfer, however,
stepwise protonation restores the fluorescence and this allows
determination of the local pH. The chemistry at the boron
atom of SubPc provides a methodology that is well suited to
the construction of more complicated architectures bearing
electron or hole transporting modules for application in light-
emitting devices. The large Stokes shifts and the acid-
triggered “switching on” of emission suggest that the new
dyes might be useful as acid-sensing or acid-activated
fluorophores.
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[15] We define here a virtual Stokes Shift that correspond to the
energy difference between the energy input and the energy
output keeping in mind that by definition a real Stokes shift
corresponds to the energy difference belonging to the same
excited state.
[16] D. Rehm, A. Weller, Isr.J.Chem. 1970, 8, 259.
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Received: June 30, 2008
Published online: October 9, 2008
Keywords: alkynes · cross-coupling· energy conversion ·
.
luminescence · subphthalocyanines
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