A highly efficient sensor molecule emitting in the near infrared (NIR): 3,5-distyryl-8-(p-dimethylaminophenyl)-difluoroboradiaza-s-indacene

Article abstract of DOI:10.1039/b007379m

The spectroscopic properties and the photophysical behaviour of difluoroboradiaza-s-indacene 1, especially designed for the near infrared (NIR) spectral region and equipped with a p-dimethylaminophenyl group at the meso-position, were studied by steady-st

Full text of DOI:10.1039/b007379m

View Online / Journal Homepage / Table of Contents for this issue
A highly efficient sensor molecule emitting in the near
infrared (NIR): 3,5-distyryl-8-(p-dimethylaminophenyl)-
diÑuoroboradiaza-s-indacene
Knut Rurack,*a Matthias Kollmannsbergerb and Jorg Daub*b
a Dept. I.3902, Federal Institute for Materials Research and T esting (BAM), Richard-W illstaetter
Str. 11, D-12489 Berlin, Germany. E-mail: knut.rurack=bam.de
b Institute of Organic Chemistry, University of Regensburg, D-93040 Regensburg, Germany.
E-mail: joerg.daub=chemie.uni-regensburg.de
Received (in Cambridge, UK) 12th September 2000, Accepted 5th October 2000
First published as an Advance Article on the web 8th December 2000
The spectroscopic properties and the photophysical behaviour of diÑuoroboradiaza-s-indacene 1, especially
designed for the near infrared (NIR) spectral region and equipped with a p-dimethylaminophenyl group at the
meso-position, were studied by steady-state and time-resolved optical spectroscopy. Solvent-dependent
measurements revealed that for 1, excited state deactivation is governed by population of a non-emissive charge
transfer excited state (1CT) as the solvent polarity increases, whereas reference compound 2 shows strong
Ñuorescence from a locally excited state (1LE) in all the solvents employed. Accordingly, protonation of 1
completely suppresses the quenching excited state charge transfer process and leads to strong enhancement of
Ñuorescence in the NIR, distinguishing 1 as a very sensitive Ñuorescent sensor molecule for pH or solvent acidity in
this favourable wavelength region.
nisms and Ñuorescence switching behavior of the symmetri-
cally styryl-substituted BDP dye 1.
1
Introduction
DiÑuoroboradiaza-s-indacenes (boronÈdipyrromethene dyes,
BDPs) are a class of highly rigidized, polymethine-like Ñuores-
cent dyes that have found widespread application as laser
dyes, as well as in biological research, e.g. for protein or DNA
labelling.1 They present many advantageous photonic proper-
ties, such as high extinction coefficients, high Ñuorescence
quantum yields and good photostability, and can be excited at
wavelengths P500 nm.2 Lately, the functionalization of BDP
dyes has received much attention because of the need for Ñuo-
rescent labels emitting in di†erent wavelength regions and
especially in the near infrared (NIR). Wavelength tuning of the
BDP chromophore can be accomplished by attaching auxo-
chromic substituents to the 3,5-positions.3 Using aromatic
groups3ahc or heterocyclic moieties,3d,e it is possible to shift
both absorption and emission bands by 100È200 nm to the
red. However, up to now, none of these advanced BDP dyes
has been equipped with a third functional group¤ which, in a
certain environment, can drastically inÑuence the spectro-
scopic properties of the chromophore by activation of an
intramolecular photophysical process, such as, for instance,
charge (CT) or electron (ET) transfer. Obviously, such an
approach would be highly suitable for photonic sensing or
switching applications in the NIR.4,5
2
Results and discussion
2.1 Spectroscopic properties and photophysical behaviour in
non-acidic media
The absorption and Ñuorescence properties of the reference
compound 2 are essentially solvent independent. The absorp-
tion spectrum (Fig. 1b) presents the typical BDP features,2,6
showing the narrow and intense (e \ 104 000 M~1 cm~1 in
acetonitrile) structured S ^ S transition with a maximum at
630 nm which is red-shifted by 130 nm as compared to con-
ventional BDP dyes. The Ñuorescence spectrum, a mirror-
image of the absorption spectrum, is only slightly
Stokes-shifted, high Ñuorescence quantum yields are found
and transÈcis isomerisation is not observed.
1
0
1 shows an additional, broad and structureless band in the
As an extension of our work on charge transfer-active
boronÈdipyrromethene dyes,6 we have embarked on the
development of NIR-emitting BDP Ñuorosensors containing
electron-donating amino groups, thus enabling us to detect
with a high degree of sensitivity free (solvated) protons in this
advantageous wavelength region. Here, we report the synthe-
sis (Scheme 1), spectroscopic properties, photophysical mecha-
¤ A third group, because all BDP dyes of this type synthesized so far
are symmetrically substituted at both the 3- and 5-positions.
Scheme 1 Synthetic scheme for the dyes investigated.
DOI: 10.1039/b007379m
New J. Chem., 2001, 25, 289È292
289
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

View Online
originating from CT Ñuorescence is found in the emission
spectrum. Whereas the radiative rate constant, with similar
values to 2, does not change, the quenching reaction is accom-
panied by a drastic increase in the non-radiative rate constant.
Obviously, the stabilization of the CT state opens up a relax-
ation pathway for the LE state that proceeds entirely via non-
radiative processes.
2.2 Proton-induced e†ects
Upon protonation, the dimethylamino group changes from an
electron donor to an acceptor in such a way that any CT
interaction is completely suppressed. In the absorption spec-
trum, the CT band disappears and the LE absorption is
shifted to longer wavelengths (Table 1, Fig. 2). The latter can
be explained by the fact that, whereas the HOMO of the BDP
chromophore has a node at the bridgehead carbon, a large
coefficient is found for the LUMO at that position and con-
version of the 8-donor to an 8-acceptor lowers the transition
energy.° Consequently, binding of a proton also inhibits the
fast excited state intramolecular quenching reaction, resulting
in spectrosopic features which are very similar to those of 2 in
the respective solvents (Table 1). Therefore, 1, which is essen-
tially non-Ñuorescent in a highly polar environment, becomes
strongly Ñuorescent in the presence of protons.
Fig. 1 Absorption and Ñuorescence spectra of 1 (a) and 2 (b) in ace-
595 nm). The
emission spectra in acetonitrile, identical in shape, are omitted for
better clarity. The thin lines in the top graph represent the CT absorp-
tion bands as derived from a decomposition procedure given in the
Experimental.
tonitrile (É É É) and diethyl ether (ÈÈ) at 298 K (j
exc
The pK of 1, determined as 2.32, lies in the usual range of
a
values reported for other 8-dimethylaminophenyl-substituted
BDP8 or related donorÈacceptor-substituted biaryl dyes, such
as p-anthracen-9-yldimethylaminobenzene (pK \ 2.5 in
absorption spectrum (Fig. 1a and data in Table 1) which
a
ethanolÈwater 4 : 1)9 or 4-dimethylamino-4@-cyano-biphenyl
largely overlaps with the low energy S ^ S transition
1
0
(pK \ 2.35 in ethanolÈwater 1 : 1)5.
located on the BDP chromophore. In accordance with studies
on other biaryls7 which do not carry sterically demanding
substituents in direct neighborhood of the interannular bond,”
this band can be attributed to a charge transfer transition
a
Comparing the Ñuorescence intensity and lifetime data of 1
and 1ÈH` in Table 1, this styryl-substituted BDP dye is dis-
(1CT ^ S ) involving the amino group and the BDP chromo-
0
phore. In contrast to the S ^ S and S ^ S (at 350 nm)
1
0
2
0
transitions leading to pÈp* locally excited (LE) states of the
BDP chromophore, this band shows a strong positive solva-
tochromism commensurate with the stabilization of the CT
state with increasing solvent polarity.
The emission behavior of 1 is strongly solvent-dependent
(Table 1). In non-polar solvents the emission is strong and the
shape of the Ñuorescence band is similar to that of 2, i.e. emis-
sion only occurs from the BDP LE state. In more polar sol-
vents the characteristic BDP Ñuorescence is drastically
quenched (by a factor of O2 ] 104) with both Ñuorescence
quantum yield and lifetime decreasing at the same time (Table
1). No red-shifted, broad and solvent polarity-dependent band
Fig. 2 Absorption and emission spectra of 1 (ÈÈ) and 1ÈH` (É É É) in
acetonitrile at 298 K (c \ 6.5 ] 10~7 M, c \ 1 ] 10~4 M).
1
H`
” Ground state geometry optimization of 1 by the AM1 method
(AMPAC 5.0, Semichem, Inc., 1994) revealed a twist angle between
the anilino and the BDP fragment of ca. 50¡, comparable to twist
angles found for related biaryls.7
° Calculations on the electronic structure of the BDP core were per-
formed using the AM1 method with VAMP 6.5. (G. Rauhut, A. Alex,
J. Chandrasekhar, T. Steinke, W. Sauer, B. Beck, M. Hutter, P.
Gedeck and T. Clark, Erlangen, 1996).
Table 1 Spectroscopic data for 1, 2 and 1ÈH` in di†erent solvents at 298 Ka
Solventb
j
c/nm
jCT c/nm
j
/nm
U
q /ps
k d/108 s~1
k d/108 s~1
abs
abs
em
f
f
f
nr
1
MeCN
THF
620
626
622
623
628
633
628
630
634
637
545
522
513
510
È
È
È
È
È
È
636
637
631
631
642
646
641
639
652
656
4 ] 10~4
0.017
0.75
0.92
0.84
0.83
0.81
0.70
0.75
\3
[1.3
1.4
1.7
1.9
1.7
1.8
1.7
1.4
1.6
1.5
[3330
1
42
80
1
Et O
4340
4870
4860
4560
4830
4840
4820
4390
0.6
0.2
0.3
0.4
0.4
0.6
0.5
0.7
2
1
Hex
2
MeCN
THF
2
2
Et O
2
2
Hex
1ÈH`
1ÈH`
MeCN
THF
0.67
a Experimental conditions: c \ 1 ] 10~6 M, j \ 595 nm for steady-state, 595 and 327 nm for time-resolved Ñuorescence measurements, see
dye
exc
Experimental. b Hex \ n-hexane. c Global maximum and maximum of the CT absorption band, for the Ðtting procedure, see Experimental.
d k \ U /q , k \ (1 [ U )/q .
f
f
f
nr
f
f
290
New J. Chem., 2001, 25, 289È292

View Online
d 6.82 (d, 2 H, J \ 4.5), 6.94 (d, 2 H, J \ 4.5), 7.29È7.55 (m,
tinguished as a very powerful NIR Ñuorosensor showing
enhancement factors of ca. 2000. Analytical applications
would include, for instance the detection of traces of free
(solvated) protons in polar solvents or the monitoring of pH
by employing, e.g. time-resolved Ñuorometry with pulsed laser
diodes in a simple single excitation/single emission wavelength
operation mode.
15H), 7.35 (d, 2 H, J \ 16.5), 7.80 (d, 2 H, J \ 16.5 Hz);
HRMS (EI, 70 eV) m/z: calc. for C
found 472.1921 (D \ 0.2 ppm).
H
N BF 472.1922,
31 23
2
2
4.3 Steady-state absorption and Ñuorescence spectroscopy
UV/Vis-spectra were recorded on Bruins Instruments
a
Omega 10 absorption spectrometer and steady-state emission
spectra on a Spectronics Instruments 8100 spectroÑuorometer.
The Ñuorescence experiments, the determination of the rela-
3
Conclusion
The studies presented reveal that the unique features of the
diÑuoroboradiaza-s-indacene chromophore, such as high
molar extinction coefficients and Ñuorescence quantum yields,
are maintained when shifting the absorption and emission to
the red by elongation of the conjugated electron system, i.e. by
substitution at the 3- and the 5-positions. Additionally, intro-
ducing a strongly electron donating aminophenyl group to the
meso-position induces a quenching intramolecular charge
transfer process in the excited state. With this advanced
tive Ñuorescence quantum yields (U ) and the correction of the
f
Ñuorescence spectra were carried out as described in ref. 6c, d.
Cresyl Violet in methanol (U \ 0.54 ^ 0.03)11 and Rho-
f
damine 101 in ethanol (U \ 1.00 ^ 0.02)12 were used as Ñuo-
f
rescence standards. The uncertainties of the measurement
were determined to ^5% (for U [ 0.5), ^20% (for 1 in THF)
f
and ^30% (for 1 in acetonitrile), respectively.
4.4 Time-resolved Ñuorescence spectroscopy
chromophoric design,
““switching onÏÏ of the Ñuorescence can be achieved in the
NIR.
a
very efficient proton-triggered
Fluorescence lifetimes (q ) were measured employing a unique
f
laser impulse Ñuorometer with ps time resolution. The sample
was excited with a synchronously pumped dye laser (Spectra
Physics Model 3500 and Model 2760; laser dye: Rhodamine
6G from Lambda Physik) pumped by the 1.2 W single-line
output (514 nm) of a mode-locked argon ion laser (Spectra
Physics Model 2040E-15S and Model 451, FWHM 100 ps)
providing pulses with 1.6 ps duration at a repetition rate of 82
MHz. For excitation in the S ^ S band, the third harmonic
4
Experimental
4.1 Materials
All the solvents employed for the spectroscopic measurements
were of spectroscopic grade and purchased from Aldrich. Per-
chloric acid (70%, Suprapur, Merck)/hydrochloric acid (30%,
Suprapur, Merck) were used for the protonation experiments
in organic/mixed aqueous solvents. Doubly distilled water
(pH 6.39) was provided by the Laboratory for Trace Elemen-
tal Analysis, BAM, Berlin.
2
0
output [sum frequency of the ground wave and the second
harmonic (LBO crystal), generated in a BBO crystal] of a
regenerative
mode-locked
argon
ion
laser-pumped
Ti : sapphire laser was used.13 For further details on the
experimental setup, the recording of the Ñuorescence decay
proÐles and data analysis, see ref. 6c, d. The experimental
accuracy was determined to ^3 ps and temporal calibration
of the experimental setup was checked with Pinacyanol in
ethanol (q \ 13 ps ^ 1 ps),14 Rose Bengal in methanol (q \
4.2 Synthesis
f
f
0.50 ns ^ 0.02 ns)12 and Cresyl Violet in methanol (q \ 3.15
f
1: 200 mg (1.2 mmol) 2-styrylpyrrole10 and 320 mg (2.4 mmol,
4-fold excess) 4-dimethylaminobenzaldehyde were stirred in 50
ml CH Cl containing two drops of triÑuoroacetic acid for 5 h
under a N -atmosphere. 260 mg (1.2 mmol) DDQ was added
and stirring was continued for 15 min, followed by the addi-
ns ^ 0.1 ns).15 The accuracy of the Ðt of the single decays, as
judged by reduced chi-squared (s2), the autocorrelation func-
R
tion C( j) of the residuals and the Durbin-Watson parameter
2
2
2
(DW), was always acceptable yielding values of s2 \ 1.2 and
R
DW [ 1.8, respectively.
tion of 3 ml N-ethyldiisopropylamine and 3 ml BF É Et O.
3
2
After stirring for another 30 min, the reaction mixture was
washed with water, dried and the solvent evaporated. Column
chromatography of the residue (SiO , CH Cl Èpetroleum
4.5 Fit of the absorption spectra of 1
After conversion to the energy (wavenumber) scale, the struc-
tured BDP-localized absorption band is modeled from the
main and the two sub bands of 2 in the respective solvents
(employing Gaussian functions). Then, a fourth component
(Gaussian-type) is included to describe the CT band.
2
2 2
ether 1 : 1, R \ 0.8, blue band) followed by recrystallisation
f
from CHCl Èhexane yields 1 as a purple microcrystalline
3
powder.
Yield: 46 mg (0.09 mmol, 15%); mp: 335È338 ¡C; IR (KBr:
v (cm~1): 1604, 1528 (C2N), 1123 (BÈF); 1H NMR (250 MHz,
CDCl ) d 3.09 (s, 6 H), 6.80 (d, 2 H, J \ 9.1), 6.95 (s, 4 H),
4.6 Determination of pK
3
a
7.25È7.45 (m, 8 H), 7.48 (d, 2 H, J \ 9.1), 7.66 (m, 4 H), 7.81 (d,
The pK (^0.1) of 1 was derived from Ñuorometric titrations
2 H, J \ 16.5 Hz); HRMS (EI, 70 eV) m/z: calc. for
a
(N \ 3, excitation at the isosbestic point at 630 nm, emission
C
H
N BF 515.2344, found 515.2344 (D \ 0.0 ppm).
33 28
3
2
monitored at 656 nm) in an ethanolÈwater 1 : 1 (v/v) mixture5
2: 338 mg (2 mmol) 2-styrylpyrrole10 and 106 mg (1 mmol)
by employing the HendersonÈHasselbalch equation pK \
benzaldehyde were stirred in 50 ml CH Cl containing two
drops of triÑuoroacetic acid for 5 h under a N -atmosphere.
227 mg (1 mmol) DDQ was added and stirring was continued
a
2
2
pH [ log(Imax [ I )/(I [ Imin).
F
F
F
F
2
for 15 min, followed by the addition of
3 ml N-
Acknowledgement
We gratefully acknowledge Ðnancial support by the Deutsche
Forschungsgemeinschaft (Da 92/24-1).
ethyldiisopropylamine and 3 ml BF É Et O. After stirring for
3
2
another 30 min, the reaction mixture was washed with water,
dried and the solvent evaporated. Column chromatography of
the residue (SiO , CH Cl Èpetroleum ether 1 : 1, R \ 0.8, red
2
2
2
f
Ñuorescing band) followed by recrystallisation from
References
CHCl Èhexane yields 2 as purple needles.
3
Yield: 40 mg (0.08 mmol, 8%); mp: 273È276 ¡C; IR (KBr: v
1
R. P. Haugland, Handbook of Fluorescent Probes and Research
Chemicals, 6th edn., Molecular Probes, Eugene, OR, 1996.
(cm~1) 1544 (C2N), 1131 (BÈF); 1H NMR (250 MHz, CDCl )
3
New J. Chem., 2001, 25, 289È292
291

View Online
Huber, O. S. Wolfbeis and J. Daub, Chem. Commun., 1997, 1717;
(c) M. Kollmannsberger, K. Rurack, U. Resch-Genger and J.
Daub, J. Phys. Chem. A, 1998, 102, 10211; (d) K. Rurack, M.
Kollmannsberger, U. Resch-Genger and J. Daub, J. Am. Chem.
Soc., 2000, 122, 968.
J. Herbich and A. Kapturkiewicz, J. Am. Chem. Soc., 1998, 120,
1014; M. Maus, W. Rettig, S. Depaemelaere, A. Onkelinx, F. C.
De Schryver and K. Iwai, Chem. Phys. L ett., 1998, 292, 115; M.
Maus, W. Rettig, D. Bonafoux and R. Lapouyade, J. Phys. Chem.
A, 1999, 103, 3388.
T. Werner, C. Huber, S. Heinl, M. Kollmannsberger, J. Daub and
O. S. Wolfbeis, FreseniusÏ J. Anal. Chem., 1997, 359, 150.
H. Shizuka, T. Ogiwara and E. Kimura, J. Phys. Chem., 1985, 89,
4302.
2
3
E. Vos de Wal, J. Pardoen, J. A. van Koeveringe and J. Lugten-
burg, Recl. T rav. Chim. Pays-Bas, 1977, 96, 306; J. Karolin,
L. B.-A. Johansson, L. Strandberg and T. Ny, J. Am. Chem. Soc.,
1994, 116, 7801.
(a) L. H. Thoresen, H. Kim, M. B. Welch, A. Burghart and K.
Burgess, Synlett, 1998, 1276; (b) H. Kim, A. Burghart, M. B.
Welch, J. Reibenspies and K. Burgess, Chem. Commun., 1999,
1889; (c) A. Burghart, H. Kim, M. B. Welch, L. H. Thoresen, J.
Reibenspies, K. Burgess, F. Bergstrom and L. B.-A. Johansson, J.
Org. Chem., 1999, 64, 7813; (d) J. Chen, J. Reibenspies, A.
Derecskei-Kovacs and K. Burgess, Chem. Commun., 1999, 2501;
(e) J. Chen, A. Burghart, A. Derecskei-Kovacs and K. Burgess, J.
Org. Chem., 2000, 65, 2900.
For recent reviews, see: A. P. de Silva, H. Q. N. Gunaratne, T.
Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Rademacher
and T. E. Rice, Chem. Rev., 1997, 97, 1515; A. P. de Silva and
A. J. M. Huxley, in Applied Fluorescence in Chemistry, Biology
and Medicine, ed. W. Rettig, B. Strehmel, S. Schrader and H.
Seifert, Springer, Berlin, 1999, p. 179; and various contributions
to Near-Infrared Dyes for High T echnology Applications, ed. S.
Dahne, U. Resch-Genger and O. S. Wolfbeis, Kluwer Academic,
Dordrecht, 1998.
7
8
9
4
10 W. Hinz, R. A. Jones and T. Anderson, Synthesis, 1996, 8, 620.
11 D. Magde, J. H. Brannon, T. L. Cremers and J. Olmsted III, J.
Phys. Chem., 1979, 83, 696.
12 D. F. Eaton, Pure Appl. Chem., 1988, 60, 1107.
13 U. Resch and K. Rurack, Proc. SPIEÈInt. Soc. Opt. Eng., 1997,
3105, 96.
14 N. Boens, N. Tamai, I. Yamazaki and T. Yamazaki, Photochem.
Photobiol., 1990, 52, 911.
15 W. R. Ware, M. Pratinidhi and R. K. Bauer, Rev. Sci. Instrum.,
1983, 54, 1148.
5
6
For a more general introduction to Ñuorescent pH probes, see:
M. Maus and K. Rurack, New J. Chem., 2000, 24, 677.
(a) M. Kollmannsberger, T. Gareis, S. Heinl, J. Breu and J. Daub,
Angew. Chem., Int. Ed. Engl., 1997, 36, 1333; (b) T. Gareis, C.
292
New J. Chem., 2001, 25, 289È292