A substituted BF2-chelated tetraarylazadipyrromethene as an intrinsic dual chemosensor in the 650-850 nm spectral range

Article abstract of DOI:10.1039/b713020a

The synthesis and spectral analysis of a new class of long wavelength intrinsic fluorosensor is reported. Chemosensor performance reveals large off/on fluorescence intensity responses to acid analyte with low response to microenvironment polarity. Applica

Full text of DOI:10.1039/b713020a

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PAPER
www.rsc.org/njc | New Journal of Chemistry
A substituted BF2-chelated tetraarylazadipyrromethene as an intrinsic
dual chemosensor in the 650–850 nm spectral range
John Killoran,a Shane O. McDonnell,a John F. Gallagherb and Donal F. O’Shea*a
Received (in Durham, UK) 24th August 2007, Accepted 13th November 2007
First published as an Advance Article on the web 20th November 2007
DOI: 10.1039/b713020a
The synthesis and spectral analysis of a new class of long wavelength intrinsic fluorosensor is
reported. Chemosensor performance reveals large off/on fluorescence intensity responses to acid
analyte with low response to microenvironment polarity. Application to ratiometric fluorescence/
UV-visible analysis is outlined.
Herein, we report the synthesis and chemosensor character-
Introduction
istics of a double amine receptor substitution pattern (Fig. 1
Ar1 and Ar2 = p-(CH3)2NC6H4). Based upon the results of an
The development of organic chromophores with spectral
properties in the near-infrared (NIR) and visible red spectral
earlier model system, it was envisaged that substituting the
regions continues to attract a sustained research interest. New
parent chromophore molecule with two intrinsically connected
chromophore scaffolds with controllable photophysical pro-
amine donors would produce a more advanced molecular
perties in the 650–900 nm spectral region offer potential for
assembly which could generate pronounced photophysical
exploitation in a diverse range of material and biological
changes as a result of a substrate recognition event by the
applications, such as optical data storage,1 photoconductors,2
amine substituents.12 It was anticipated that this electron
electrochromic devices,3 chemosensors,4 immunoassay labels
donor–acceptor double amine receptor–chromophore design
and bioconjugated probes,5 and in vitro and in vivo imaging
would undergo pronounced spectral changes due to variance
agents.6 Specifically, NIR dyes are suitable as in vitro and
in the internal charge transfer (ICT) properties of the system in
in vivo biomedical diagnostic agents due to efficient penetra-
response to acid analyte. A study of the analyte responsive
tion of light through tissue and the low auto-fluorescence of
photophysics of this system would provide the framework for
endogenous chromophores in this spectral region.7 Despite the
more elaborate receptors to detect other analytes.13
optical benefits of utilising this spectral region for analytical
techniques, there is a surprising scarcity of organic compounds
which have the desired absorption and emission properties. In
spite of their poor photostability and lengthy synthetic routes,
the cyanine dyes have to date been the most widely utilised
class for applications in this spectral region.8
Results and discussion
Synthesis
The synthesis of our target compound 1a was achieved in a
routine four step route from acetophenone and 4-dimethyl-
aminobenzaldehyde.14 Condensation of the aldehyde with
acetophenone gave the a,b-unsaturated ketone 2 which
following conjugate addition of nitromethane provided the
1,3-diaryl-4-nitrobutan-1-one 3 in 87% yield after recrystalliza-
tion (Scheme 1).
We are currently interested in devising new chemosensors
based upon the BF2-chelated-azadipyrromethenes, which are
readily synthesised, show high photostability, and are amen-
able to peripheral functionalisation to engender higher-order
function.
The BF2 chelated-tetraarylazadipyrromethene 1 and related
structural analogues have recently been reported as a class of
chromophore with high absorption extinction coefficients
(70–80 Â 103 MÀ1 cmÀ1) and fluorescence quantum yields
(0.23–0.36) between 650 and 750 nm (Fig. 1).9 We have also
shown that with selective positioning of prototype amine
receptors this class can be utilized as a visible red and/or
NIR chemosensor operating by either a photoinduced electron
transfer (PET) or a charge transfer (CT) mechanism.10
Subsequently this fluorophore class has been successfully
adapted as chemosensors for analytes such as saxitoxin and
mercury(II).11
Generation of the tetraarylazadipyrromethene
4 was
achieved by heating 3 with ammonium acetate in ethanol for
48 h during which time the product precipitated from solution.
Filtration of the precipitate from the crude reaction mixture
gave the pure product as a dark blue solid in 45% yield. A
higher yield could be obtained if butanol was used as solvent
(50%) but silica gel chromatography was required for purifi-
cation. Compound 4 was converted to our targeted structure
a Centre for Synthesis and Chemical Biology, School of Chemistry and
Chemical Biology, University College Dublin, Belfield, Dublin 4,
Ireland. E-mail: donal.f.oshea@ucd.ie; Tel: +353-(0)1-7162425
b School of Chemical Sciences, Dublin City University, Dublin 9,
Ireland
Fig. 1 BF2-chelated tetraarylazadipyrromethenes.
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Fig. 3 Compound 1a (molecule A) with displacement ellipsoids
depicted at the 30% probability level. Weak intramolecular interac-
tions are present involving N1A, F1A/F2A and neighbouring
aromatic C–H groups (as noted in the text).
Scheme 1
1a by reaction with boron trifluoride diethyletherate and
diisopropylethylamine in dichloromethane for 24 h. An iso-
lated purified yield of 53% was obtained following chromato-
graphy on silica gel.
only significant intermolecular interactions involve the rela-
tively weak C–HÁ Á ÁN/F and C–HÁ Á Áp(arene) interactions of
which two are noteworthy namely C42B–H42BÁ Á ÁCg1
[H42BÁ Á ÁCg1 is 2.79 A, angle of 1511, C42BÁ Á ÁCg1 is
X-Ray crystallographic study
3.649(6) A, Cg1
=
centroid C11A–C16A] and
C47B–H47EÁ Á ÁCg2i [H47EÁ Á ÁCg2i is 2.82 A, angle of 1371,
C47BÁ Á ÁCg2i is 3.604(6) A, Cg2 = centroid C31B–C36B,
symmetry operation i = 1 À x, Ày, 1 À z] (Fig. 2). These
weak interactions and packing forces combine to enforce
subtle differences between molecules A and B in the solid state.
The interactions in 1a mainly comprise intramolecular aro-
matic C–HÁ Á ÁF involving the BF2 moiety. The interactions for
molecule A are H16AÁ Á ÁF1A at 2.16 A (C16AÁ Á ÁF1A is
3.046(6) A, angle of 1521) and H32AÁ Á ÁF2A at 2.15 A
(C32AÁ Á ÁF2A is 3.006(7) A, angle of 1501): molecule B is
similar with H16BÁ Á ÁF1B 2.17 A (C16BÁ Á ÁF1B is 3.020(5) A,
angle of 1501) and H32BÁ Á ÁF2B 2.20 A (C32BÁ Á ÁF2B is
3.047(6) A, angle of 1481) (Fig. 3).
The crystal and molecular structure of 1a have been deter-
mined by single-crystal X-ray diffraction both at 294 and
120 K: only the latter results will be discussed. Compound
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1a crystallizes in the triclinic system, space group (P1, no. 2)
with two molecules (A and B) in the asymmetric unit and
though broadly similar in conformation there are quite distinct
differences in the bond and torsion angles (Fig. 2 and 3). The
The NMe2 groups are representative with a separation of ca.
0.6 A for the NÁ Á ÁN non-bonded distances of 7.617(7) and
7.041(6) A in molecules A and B, respectively. There are no
discernible differences in the internal bond lengths between
molecules A and B primarily due to the data quality: the esds
are large and preclude meaningful one-for-one comparisons.
However, there are distinct differences in atom separation
distances in the molecules as a result of subtle conformational
differences. In molecule A, the C3AÁ Á ÁN24A and
C8AÁ Á ÁN44A distances are 5.682(8) and 5.649(7) A [5.666(8),
5.646(7) A in B], but this is not significant for elongation or
shortening along the C–C6H4–NMe2 groups in either mole-
cule. A more distinct difference is the BÁ Á ÁN distances for
B1AÁ Á ÁN24A/N44A being 9.215(9)/9.209(8) A (similar to
B1BÁ Á ÁN44B, 9.190(8) A) but longer by 0.08 A in comparison
to B1BÁ Á ÁN24B at 9.128(9) A. Therefore, molecules A and B
differ subtly and examination of the NÁ Á ÁN distances between
Fig. 2 A view of molecules A and B of 1a with atoms drawn as their
van der Waals spheres highlighting the C–HÁ Á Áp(arene) interactions
between A and B.
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the para-disubstituted C–C6H4–NMe2 rings highlights these
subtleties with N24AÁ Á ÁN44A 7.617(7) A and N24BÁ Á ÁN44B
7.041(6) A, indicating an opening up or bending of these
groups relative to one another (the N24Á Á ÁN1Á Á ÁN44 angles
in A, B are 70.04(7), 65.39(7)1). The distortion can also be
observed in the torsion angles where the N24Á Á ÁC3Á Á Á;
C8Á Á ÁN44 angles for A, B differ by 101 (À0.83(13)1 in A and
9.20(14)1 in B).
The three central fused-rings of 1a (comprising 12 atoms, 4
of which are common to the five/six-membered fused-ring,
discounting the F atoms) are essentially co-planar (though
with a slight ‘ruffling’) with the maximum deviation for any
non-hydrogen atom in either of the two five-membered C4N or
the six-membered BC2N3 rings in molecules A, B is for C1A,
C3A by 0.020(3) A in C4N and 0.026(3) A for C1B in the
BC2N3 core in B (Fig. 4).
larger pyramidalisation at the N atom (greater NÁ Á ÁC3 plane
distance). Therefore, the resulting and decreasing ability of the
lone pair of electrons on the N atom to participate in
conjugation with the aromatic C6 group is observed through
(a) longer C–N bond lengths, (b) increased twisting from co-
planarity and (c) increased N atom pyramidalisation. These
effects at the C6–NMe2 chromophore can have a profound
effect on the photophysical properties of a system in the solid
state and in the solution if there is a measure of steric influence
restricting rotation of the NMe2 and other pendant groups.
Spectroscopic study
Aqueous solutions of 1a, suitable for pH titrations, were made
using the emulsifier Cremophor EL (CrEL), a commonly used
non-ionic surfactant exploited as a formulating agent for
poorly water-soluble pharmaceuticals.15 Examination of the
excited state response of aqueous formulated solutions of 1a to
varying acidic conditions were carried out. We were pleased to
observe virtually complete quenching of the fluorescence in the
pH range where the amine receptor remained unprotonated
(Fig. 5, red trace). Upon protonation, the fluorescence spec-
trum showed a strong proton induced fluorescence enhance-
ment with a wavelength of maximum fluorescence at 685 nm
(Fig. 5, green trace). A sigmoidal plot of pH vs. fluorescence
intensity predicted an apparent pKa of less than 1.0 (Fig. 5,
inset). This value is indicative of a strong coupling of the
receptor and fluorophore sub-units, though it should be noted
that pKa values in a micellar microenvironment are often
lower than might be anticipated. Significantly, the enhance-
ment of fluorescence intensity was greater than 250-fold
between the chemosensor off and on positions. The co-planar-
ity of the receptors and the fluorophore of chemosensor 1a
facilitates a donor–acceptor CT interaction between the anili-
no lone pairs of electrons and the fluorophore in the unpro-
tonated state, which gives rise to highly efficient quenching of
the fluorescence emission. This is further endorsed by analysis
of the absorption spectrum of 1a. Unprotonated 1a displays
strong CT characteristics with three bands between 500 and
The range of interplanar angles between the five- and six-
membered [C4N/BC2N3] rings is from 4.6(3) to 6.4(3)1, high-
lighting the essentially co-planar nature of the 12 atoms in the
central fused-ring moieties.9 The C6H5 (phenyl) ring orienta-
tions differ with angles of 17.4(3)1 (A) and 13.9(2)1 (B) for the
[C11–C16]/[C31–C36] planes, while the NMe2 substituted C6
ring planes are oriented at 10.5(3)1 (A) and 9.4(2)1 (B).
The four NMe2 (molecules A and B) groups differ consider-
ably, both in their twist angles with respect to the C6 rings, but
also due to pyramidalisation effects at the N atoms. A con-
venient measure of pyramidalisation is the distance from the N
atom to the C3, [CMe/Carom/CMe], plane (this is 0.0 A when the
N and 3C atoms are co-planar). In 1a, the range is from
0.003(5) A (for N44BÁ Á ÁC3) to 0.137(6) A (for N24AÁ Á ÁC3),
indicating a range of geometries within the NMe2 groups. A
Cambridge Structural Database (CSD) analysis for
C–C6–NMe2 moieties in a range of 300+ molecules shows a
direct correlation between increasing C–NMe bond length
(increasing single bond character), increasing C6/NMe2 twist
angle (from 01) and pyramidalisation at the N atom (increas-
ing NÁ Á ÁC3 plane distance).12 In 1a, the longest C–NMe
distance is 1.384(7)
A
for C24A–N24A [range of
1.353(7)–1.373(6) A for the three remaining C–N distances
involving N44A, N24B, N44B]: a corresponding twist of
14.0(7)1 at N24A [2.1(5)–4.3(4)1 at N44A, N24B, N44B], and
a N24AÁ Á ÁC3 distance of 0.137(6) A [0.003(5)–0.065(6) A for
N44A, N24B, N44BÁ Á ÁC3]. In 1a our results follow the ob-
served trend observed in the CSD analysis for C–C6–NMe2
systems described above. Increasing single bond character in
the C–N bond (C–N lengthening) correlates well with in-
creased twisting away from C6/NMe2 co-planarity and with
Fig. 5 pH responsive fluorescence spectra of 1a (excitation 630 nm,
1 Â 10À6 M) in H2O/CrEL, INaCl = 150 mmol LÀ1. The pH values in
order of decreasing intensity are 0.25, 0.40, 0.55, 0.65, 0.80, 0.95, 1.15,
1.45, 1.90 and 5.45. Inset shows sigmoidal plot predicting an apparent
pKa value of o1.
Fig. 4 A view along the BÁ Á ÁN axis of the central core non-hydrogen
atoms in 1a (molecule A) with the hydrogen atoms and peripheral C/N
atoms removed for clarity.
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Fig. 8 Ratiometric pH profile of 1a (1 Â 10À6 M) for increase of
fluorescence emission at 685 nm (K experimental; — sigmoidal fit)
and decrease of absorbance band 3 at 789 nm (m experimental; —
sigmoidal fit) for 1a.
Fig. 6 pH responsive absorbance spectra of 1a (1 Â 10À6 M) in H2O/
CrEL, INaCl = 150 mmol LÀ1. The pH values in order of decreasing
intensity of band 3 are 5.45, 1.90, 1.45, 1.15, 0.95, 0.80, 0.65, 0.55,
0.40 and 0.25.
The solvent effects on the steady state fluorescence proper-
ties of 1a and 1a-2H+ are shown in Fig. 9. It is remarkable to
note that in all the organic solvents studied, polar and non-
polar, the fluorescence of 1a is almost completely quenched at
the respective emission wavelengths of 1a-2H+. This is in
contrast to other aniline substituted fluorosensors including
the aniline substituted 4,4-difluoro-4-bora-3a,4a-diaza-s-inda-
cenes (BODIPY class) which exhibit pronounced solvent depen-
dent fluorescence quantum yields with high yields recorded in
non-polar solvents.17 In the less polar solvents cyclohexane,
chloroform and toluene the unprotonated fluorosensor shows
a low intensity long wavelength emission ranging from 795 to
822 nm, attributed to emission from a CT species. The fluores-
cence spectra of 1a-2H+ (TFA added to solution of 1a) show
some solvent polarity dependency with positions of maximum
fluorescence varying from 677 nm in ethanol to 692 nm in
toluene (Table 1). Full widths at half maximum (FWHM) in all
solvents are narrow ranging from 39 to 43 nm. All solvents show
excellent fluorescent enhancement factors (FEF) with the un-
protonated fluorosensor in the more polar solvents showing
complete quenching of fluorescence.
900 nm (Fig. 6, red trace). Computational deconvolution of
bands 1 and 2 gave maxima at 586 and 620 nm, respectively
(Fig. 7, inset). At pH values greater than 2, no spectral shifts are
demonstrated by 1a. At pH values o2, bands 1 and 3 can be
clearly seen to progressively reduce in intensity and a new band
centred at 665 nm for 1a-2H+ appears (Fig. 6, green trace).
A plot of pH vs. intensity of absorbance bands 1 and 3
clearly demonstrates the double protonation event occurring
as a consequence of the two amine receptors. Two sigmoidal
response curves were calculated for band 1 and 3 (red and
blue, respectively) predicting an apparent pKa of 1.4 for the
first protonation and o1 for the second (Fig. 7) As the pKa of
the second protonation was similar to that observed in the
fluorescence titration this would indicate that it is only upon
the second protonation that the fluorescence of 1a is re-
established. This is clearly demonstrated in a ratiometric plot
of disappearance of the absorption band at 789 nm vs.
emergence of fluorescence intensity giving an intersect at the
pKa value (Fig. 8).16
The absorption spectrum of 1a in organic solvents exhibited
three distinct bands, as was observed for the aqueous
Fig. 7 pH profiles for disappearance of band 1 at 586 nm (red circles,
experimental; line, sigmoidal fit) and band 3 at 789 nm (blue triangles,
experimental; line, sigmoidal fit) of 1a (1 Â 10À6 M). Inset shows
resolved component peaks of the absorbance spectrum generated with
PeakFit software.
Fig. 9 Fluorescence spectra of 1a and 1a-2H+ (TFA added) (excita-
tion 630 nm; 5 Â 10À7 M) in acetonitrile (pink), chloroform (yellow),
cyclohexane (red), ethanol (blue) and toluene (green).
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Table 1 Spectroscopic fluorescence data for 1a-2H+
Table 2 Spectroscopic absorbance data for 1a
Solventa
lmax/nm
FWHM/nm
FEF
Solventa
Band 1/nm
Band 2/nm
Band 3/nm
Cyclohexane
Toluene
Chloroform
Ethanol
Acetonitrile
680
692
684
677
679
39
43
41
40
40
180
240
4250
4250
4250
Cyclohexane
Toluene
Chloroform
Ethanol
576
574
572
569
572
613
625
625
631
648
744
763
770
772
799
Acetonitrile
a
a
Room temperature.
Room temperature.
formulated solutions. The shortest wavelength band was more
distinct in polar solvents such as acetonitrile than cyclohexane
(Fig. 10). The absorbance bands 2 and 3 show a strong positive
solvatochromism in proportion to increasing solvent polarity,
bathochromically shifting to 648 and 799 nm in acetonitrile
when compared to 613 and 744 nm in cyclohexane (Table 2).
Specifically band 3 exhibited the strongest positive solvato-
chromism with increasing solvent polarity, which is a common
characteristic of CT compounds.
are quoted in d (ppm) and coupling constants in Hz. Melting
points were determined on a Reichert Thermovar melting
point platform. Mass spectral analyses were performed on a
Micromass Quattro Micro. IR spectra were recorded on a
Mattson Instruments Galaxy series FT-IR 3000 spectometer.
Absorption spectra were recorded on a Varian Cary 50 UV-
visible spectrophotometer. Fluorescence spectra were recorded
on a Cary Eclipse spectrofluorometer. Elemental analyses were
carried out at the microanalytical laboratory, University
College Dublin.
Conclusions
3-(4-Dimethylaminophenyl)-1-phenylpropenone (2). 4-Di-
methylaminobenzaldehyde (10.0 g, 67 mmol), 1-phenyletha-
none (8.05 g, 67 mmol) and three NaOH pellets were dissolved
in methanol (100 mL) and stirred at room temperature until a
heavy precipitate formed (24 h). The precipitate was isolated
by filtration and washed with cold methanol to give the
product without any further purification, as a bright yellow
solid of 2 (14.07 g, 84%), mp 110–112 1C. 1H NMR (300 MHz,
CDCl3) d 7.98–8.02 (m, 2H), 7.79 (d, J = 15.5 Hz, 1H),
7.43–7.56 (m, 5H), 7.32 (d, J = 15.5 Hz, 1H), 6.64–6.69 (m,
2H), 3.00 (s, 6H). 13C NMR (75 MHz, CDCl3) d 190.8, 152.3,
146.0, 139.3, 132.3, 130.6, 128.7, 128.5, 122.9, 117.1, 112.0,
40.33. IR (KBr disc): 1649 cmÀ1. ES-MS: m/z [M + H]+
252.1. Anal. Calc. for C17H17NO: C, 81.24; H, 6.82; N, 5.57.
Found: C, 80.95; H, 6.82; N, 5.49%.
In conclusion, we have outlined a facile synthesis to a new
class of ratiometric fluorometric and colourimetric chemosen-
sor with potential to be exploited and adapted to suit a diverse
range of analytical, imaging and material applications. Speci-
fically the low pH range in which sensor response occurs
indicates that this platform would be suitable for detection
of other analytes at physiological pH. Chemosensor perfor-
mance is excellent with large off/on fluorescence intensity
responses and low microenvironment polarity effects. In addi-
tion the described synthetic route would readily allow for
analogues functionalised with other substrate specific recep-
tors to be generated.
Experimental
3-(4-Dimethylaminophenyl)-4-nitro-1-phenylbutan-1-one (3).
3-(4-Dimethylaminophenyl)-1-phenylpropenone 2 (7.0 g, 28.0
mmol) was dissolved in anhydrous methanol (75 mL), nitro-
methane (8.5 g, 140 mmol) and diethylamine (10.22 g, 140
mmol) were added, and the reaction was heated under reflux
for 24 h. The reaction mixture was allowed to cool to room
temperature, acidified with 1 M hydrochloric acid and the
resulting precipitate was isolated by filtration. Recrystalliza-
tion from methanol gave the product 3 as a yellow solid (7.6 g,
87%), mp 113–115 1C. 1H NMR (300 MHz, CDCl3) d
7.88–7.92 (m, 2H), 7.51–7.57 (m, 1H), 7.40–7.46 (m, 2H),
7.09–7.14 (m, 2H), 6.63–6.68 (m, 2H), 4.76 (dd, J = 12.3,
6.8 Hz, 1H), 4.61 (dd, J = 12.3, 7.9 Hz, 1H), 4.11 (m, 1H),
3.44 (dd, J = 6.1, 17.0 Hz), 3.36 (dd, J = 7.6, 17.0 Hz), 2.89 (s,
6H). 13C NMR (75 MHz, CDCl3) d 197.5, 150.3, 136.8, 133.6,
128.9, 128.3, 128.3, 126.6, 113.0, 80.2, 42.0, 40.6, 38.8. IR (KBr
disc): 1680, 1546 cmÀ1. ES-MS: m/z [M + H]+ 313.2. Anal.
Calc. for C18H20N2: C, 69.21; H, 6.45; N, 8.97. Found: C,
69.40; H, 6.39; N, 8.83%.
General
1H NMR spectra were recorded on a Varian FT spectrometer
at 300 MHz and 13C NMR spectra at 75 MHz, in CDCl3 with
tetramethylsilane as the internal standard. All chemical shifts
Fig. 10 Absorbance spectra of 1a (5 Â 10À6 M) in acetonitrile (pink),
chloroform (yellow), cyclohexane (red), ethanol (blue) and toluene
(green).
[3-(4-Dimethylaminophenyl)-5-phenyl-1H-pyrrol-2-yl][3-(4-di-
methylaminophenyl)-5-phenylpyrrol-2-ylidene]amine (4).
A
100 mL round-bottomed flask was charged with 3-(4-di-
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methylaminophenyl)-4-nitro-1-phenylbutan-1-one 3 (1.0 g,
3.21 mmol), ammonium acetate (8.7 g, 112 mmol) and ethanol
(20 mL) and heated under reflux for 48 hours. During the
course of the reaction, the product precipitated from the
reaction mixture. The reaction was cooled to room tempera-
ture, filtered and the isolated solid washed with cold ethanol
(2 Â 10 mL) to yield the product 4 as a blue–black solid (0.40
g, 46%), mp 248–250 1C. 1H NMR (CDCl3) d 8.04 (d, J = 9.0
Hz, 4H), 7.94 (d, J = 7.0 Hz, 4H), 7.40–7.53 (m, 6H), 7.05
(s, 2H), 6.78 (d, J = 9.0 Hz, 4H), 3.04 (s, 6H), (NH not
observed). 13C NMR (CDCl3) d 154.8, 150.4, 149.8, 143.1,
132.9, 130.5, 129.7, 129.2, 126.6, 122.9, 112.2, 111.9, 40.6. IR
(KBr disc): 3462, 1607 cmÀ1. lmax (CHCl3): 623 nm. HRMS:
calc. for C36H34N5 [M + H]+: 536.2814, found: 536.2806.
Anal. Calc. for C36H33N5: C, 80.72; H, 6.21; N, 13.07. Found:
C, 80.44; H, 6.14; N, 13.10%.
lected on an Enraf-Nonius CCD diffractometer at 120 K for
comparison purposes. A lower R-factor with improved lower
esds were obtained for a detailed comparison of the two
independent molecules (A and B) in the asymmetric unit. Brief
details of the 294 K dataset of 1a are listed and all further
discussion refers to the low-temperature 120 K dataset of 1a.
All non-hydrogen atoms were refined using anisotropic dis-
placement parameters and hydrogen atoms were treated as
riding atoms using the SHELXL97 defaults at the appropriate
temperatures (294 and 120 K).
Crystal data for 1a at 294 K. C36BF2N5, M = 583.48,
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triclinic, space group P1, a = 10.372(10), b = 14.250(13), c
= 21.168(15) A, a = 106.19(8), b = 91.03(11), g = 92.06(9)1,
V = 3001(5), Z = 4, m = 0.086 mmÀ1; 1492 reflections with
I 4 2s(I), R(I 4 2s(I)) = 0.1541, wR(I 4 2s(I)) = 0.3428.
BF2-chelated-[3-(4-dimethylaminophenyl)-5-phenyl-1H-pyrrol-
Crystal data for 1a at 120 K. C36BF2N5, M = 583.48,
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triclinic, space group P1, a = 10.2169(7), b = 14.0769(8),
2-yl][3-(4-dimethylaminophenyl)-5-phenylpyrrol-2-ylidene]amine
(1a).
[3-(4-Dimethylaminophenyl)-5-phenyl-1H-pyrrol-2-yl]-
c = 21.0039(15) A, a = 105.548(2), b = 91.046(2), g =
92.138(5)1, V = 2907.0(3), Z = 4, m = 0.088 mmÀ1; 3569
[3-(4-dimethylaminophenyl)-5-phenylpyrrol-2-ylidene]amine 4
(0.1 g, 0.186 mmol) was dissolved in dry CH2Cl2 (50 mL),
treated with diisopropylethylamine (0.14 mL, 0.78 mmol) and
boron trifluoride diethyl etherate (0.14 mL, 1.4 mmol), and
stirred at room temperature under nitrogen for 24 h. The
mixture was washed with water (2 Â 50 mL), and organic layer
was dried over sodium sulfate and evaporated to dryness.
Purification by column chromatography on silica gel eluting
with CH2Cl2–hexane (3 : 1) gave the product 1a as a metallic
brown solid (0.057 g, 53%), mp 258–261 1C. 1H NMR (CDCl3)
d 8.07 (dd, J = 2.0, 7.0 Hz, 4H), 7.98–8.01 (m, 4H), 7.41–7.46
(m, 6H), 6.81 (s, 2H), 6.76 (dd, J = 2.0, 7.0 Hz, 4H), 3.07 (s,
12H). 13C NMR (CDCl3) d 158.1, 151.3, 145.8, 144.0, 132.7,
131.1, 130.2, 129.5, 128.5, 121.5, 115.4, 112.2, 40.4. IR (KBr
disc): 1603, 1487 cmÀ1. EI-MS: m/z 583. HRMS: calc. for
C36H33BF2N5 [M + H]+: 584.2797, found: 584.2813. Anal.
Calc. for C36H32BF2N5: C, 74.10; H, 5.53; N, 12.00. Found: C,
72.52; H, 5.55; N, 11.56%. Crystals were grown by the slow
evaporation of a chloroform solution.
reflections with
I 4 2s(I), R(I 4 2s(I)) = 0.0862,
wR(I 4 2s(I)) = 0.1415.
CCDC reference numbers 646558 (294 K) and 646559
(120 K). For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b713020a
Formulation
Compound 1a (0.005 mmol) was dissolved in THF (1 mL) and
Cremophor EL (0.1 mL) added. The mixture was sonicated for
30 min followed by removal of the THF under reduced
pressure. The resulting blue oil was dissolved in 25 mL of
saline solution (2.19 g NaCl in 250 mL water) and filtered
through an Acrodisc 25 mm syringe filter (with 0.2 mm HT
Tuffryn membrane). Final concentration was checked by
UV-visible spectral analysis.
UV-visible and fluorescence procedures
X-Ray data collection, structure solution and refinement
Solvents used were spectrophotometric grade chloroform,
which was distilled over potassium carbonate prior to use,
spectrophotometric grade acetonitrile and HPLC grade cyclo-
hexane. UV-visible spectra were determined from a 1 cm path
quartz cell at room temperature. Baseline corrected UV-visible
spectra were collected between 250 and 1100 nm. Fluorescence
spectra were determined at room temperature from a 1 cm path
quartz cell with excitation and emission slit widths of 5 nm.
FEF = fluorescence enhancement factor (IFmax/IFmin).
Data were collected on a Siemens-Bruker P4 diffractometer for
1a at room temperature (294 K) and processed using the
XSCANS suite of programs.18 A low-temperature study was
also subsequently undertaken of 1a at 120 K using an Enraf-
Nonius diffractometer at the University of Southampton
(ESPRC service) and data were processed using the SMART
suite of programs.19 Compound 1a crystallizes in the triclinic
ꢀ
system (P1, no. 2) with two molecules (A/B) in the asymmetric
unit. Solution and refinement was undertaken using
SHELXS97 and SHELXL9720 and the graphics generated
with PLATON.21
Acknowledgements
No disorder is present in 1a but an initial study of a very
weakly diffracting crystal at room temperature (294 K) pro-
vided only gross conformation details and resulted in a high
R-factor: however, the geometry looks reasonable, though
with geometric data having rather high esds (s.u.’s). Better
quality crystals of 1a were subsequently obtained via several
recrystallization experiments and data were subsequently col-
This work was funded under the Program for Research in
Third-Level Institutions administered by the HEA. S. O.
McD. thanks the Irish Research Council for Science, Engi-
neering and Technology for a studentship. J. F. G. thanks
Dublin City University for the purchase of a Siemens P4
diffractometer and computer system. Thanks to Dr D. Rai
of the CSCB Mass Spectrometry Centre for mass analyses and
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008
488 | New J. Chem., 2008, 32, 483–489

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ESPRC service (University of Southampton) for collecting a
low temperature dataset.
Org. Biomol. Chem., 2006, 4, 776. For CT examples, see: ; (c) S. O.
McDonnell and D. F. O’Shea, Org. Lett., 2006, 8, 3493.
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J. S. Pharr, J. Org. Chem., 2007, 72, 2187; (b) A. Coskun,
M. Deniz Yilmaz and E. U. Akkaya, Org. Lett., 2007, 9,
607.
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