BORAZANs: Tunable fluorophores based on 2-(pyrazolyl)aniline chelates of diphenylboron

Article abstract of DOI:10.1021/jo0705420

(Figure Presented) The reaction between 2-pyrazolyl-4-X-anilines, H(pzAn^X), (X = para-OMe (L1), Me (L2), H (L3), Cl (L4), CO ₂Et (L5), CF₃ (L6), CN (L7)) and triphenylboron in boiling toluene affords the respective, highly emissive N,N′-boron chelate complexes, BPh₂(pzAn^X) (X = para-OMe (1), Me (2), H (3), Cl (4), CO₂Et (5), CF₃ (6), CN (7)) in high yield. The structural, electrochemical, and photophysical properties of the new boron complexes can be fine-tuned by varying the electron-withdrawing or -donating power of the para-aniline substituent (delineated by the substituent's Hammett parameter). Those complexes with electron-withdrawing para-aniline substituents such as CO₂Et (5), CF₃ (6), and CN (7) have more planar chelate rings, more 'quinoidal' disortion in the aniline rings, greater chemical stability, higher oxidation potentials, and more intense (φ_F = 0.81 for 7 in toluene), higher-energy (blue) fluorescent emission compared to those with electron-donating substituents. Thus, for 1 the oxidation potential is 0.53 V versus Ag/AgCl (compared to 1.12 V for 7), and the emission is tuned to the yellow-green but at an expense in terms of lower quantum yields (φ_F = 0.07 for 1 in toluene) and increased chemical reactivity. Density functional calculations (B3LYP/6-31G*) on PM3 energy-minimized structures of the ligands and boron complexes reproduced experimentally observed data and trends and provided further insight into the nature of the electronic transitions.

Full text of DOI:10.1021/jo0705420

BORAZANs: Tunable Fluorophores Based on 2-(Pyrazolyl)aniline
Chelates of Diphenylboron
Brendan J. Liddle, Rosalice M. Silva, Tyler J. Morin, Felipe P. Macedo, Ruchi Shukla,
Sergey V. Lindeman, and James R. Gardinier*
Department of Chemistry, Marquette UniVersity, Milwaukee, Wisconsin 53201-1881
james.gardinier@marquette.edu
ReceiVed March 15, 2007
X
The reaction between 2-pyrazolyl-4-X-anilines, H(pzAn ), (X ) para-OMe (L1), Me (L2), H (L3), Cl
(
L4), CO
highly emissive N,N′-boron chelate complexes, BPh
CO Et (5), CF (6), CN (7)) in high yield. The structural, electrochemical, and photophysical properties
2 3
Et (L5), CF (L6), CN (L7)) and triphenylboron in boiling toluene affords the respective,
X
2
(pzAn ) (X ) para-OMe (1), Me (2), H (3), Cl (4),
2
3
of the new boron complexes can be fine-tuned by varying the electron-withdrawing or -donating power
of the para-aniline substituent (delineated by the substituent’s Hammett parameter). Those complexes
with electron-withdrawing para-aniline substituents such as CO
planar chelate rings, more ‘quinoidal’ disortion in the aniline rings, greater chemical stability, higher
oxidation potentials, and more intense (φ ) 0.81 for 7 in toluene), higher-energy (blue) fluorescent
emission compared to those with electron-donating substituents. Thus, for 1 the oxidation potential is
.53 V versus Ag/AgCl (compared to 1.12 V for 7), and the emission is tuned to the yellow-green but
at an expense in terms of lower quantum yields (φ ) 0.07 for 1 in toluene) and increased chemical
2 3
Et (5), CF (6), and CN (7) have more
F
0
F
reactivity. Density functional calculations (B3LYP/6-31G*) on PM3 energy-minimized structures of the
ligands and boron complexes reproduced experimentally observed data and trends and provided further
insight into the nature of the electronic transitions.
Introduction
of fluorescence have been exploited in many useful applications
3
4
such as in laser dyes, in sensors and switches, in biological
The design of electroactive luminophores is of fundamental
and practical interest for a range of important applications from
5
imaging and assaying, and even in light-harvesting and
6
photovoltaics. The successful use of these boron compounds
1
OLED devices to immunoassaying. Particularly noteworthy are
in such a wide variety of applications has incited research
the electroactive boron-dipyrrin (or BODIPY)2 compounds
whose intense narrow absorption bands and high quantum yields
(2) (a) Haugland, R. P.; Kang, H. C. U.S. Patent 4,774,339, 1988. (b)
Haugland, R. P. Handbook of Fluorescent Probes and Research Chemicals,
6th ed.; Molecular Probes: Eugene, OR, 1996. (c) Treibs, A.; Kreuzer,
F.-H. Liebigs Ann. Chem. 1968, 718, 208.
(3) Boyer, J. H.; Haag, A. M.; Sathyamoorthi, G.; Soong, M. L.;
Thangaraj, K.; Pavlopoulos, T. G. Heteroat. Chem. 1993, 4, 39.
*
Author to whom correspondence should be addressed. Phone: 414-288-
533. Fax: 414-288-7066.
1) Valeur, B. Molecular Fluorescence, Principles and Applications;
Wiley-VCH: New York, 2002.
3
(
1
0.1021/jo0705420 CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/28/2007
J. Org. Chem. 2007, 72, 5637-5646
5637

Liddle et al.
SCHEME 1. Preparation of BORAZANs
interest into the synthesis and study of other photoactive N,N-
As the first part in a series, we report our findings concerning
a new class of highly emissive (tunable from blue to yellow-
green) boron fluorophores, the BORAZANs (boron azo-
anilines), that are obtained from the reaction between triphe-
nylboron and various 2-(pyrazolyl)anilines. For simplicity, we
will use shorthand notation when referring to the ligands and
BORAZAN complexes. The pyrazolyl-aniline ligands can be
represented by the formula H(pz An ) where pz ) pyrazolyl,
An ) aniline, y and x represent substitution on the correspond-
ing component; the unsubstituted parent compound has no
superscripts (y or x) and an un-numbered superscript refers to
substitution at the 4-position of the respective fragment.
7
a-g
7h,i
and related
N,O-chelates of boron. Ideally, any new
boron-containing fluorescent dyes would be easily pre-
pared, be stable, and have electronic properties with broad-
spectrum tunability that would open the door for potential new
applications.
We were intrigued by the recent reports of high yield
syntheses of 2-(pyrazolyl)aniline derivatives by copper-catalyzed
y
x
8
cross-coupling reactions, as these compounds could be envi-
sioned as N,N-chelate ligands to boron or other Lewis acids.
In fact, the coordination chemistry of a few 2-(pyrazolyl)aniline
9
derivatives has sporadically been reported, but little is known
about the electronic properties of the ligands or the complexes
thereof. We have recently discovered that the 2-(pyrazolyl)-
anilines can serve as an interesting new class of fluorescent
Results and Discussion
‘
turn-on’ sensing ligands giving intense emission, both in
Synthesis. The desired BORAZAN dyes were prepared as
8a
solution and the solid state, on reaction with certain Lewis acids.
outlined in Scheme 1. A minor modification of Buchwald’s
8
b-e
and Taillefer’s
copper-catalyzed N-arylation reaction be-
(4) (a) Desvergne, J.-P., Czarnik, A. W., Eds. Chemosensors of Ion and
10,11
tween pyrazole and 2-bromoanilines
was exploited in the
Molecule Recognition; Kluwer: Dordrecht, the Netherlands, 1997. (b) Beer,
G.; Niederalt, C.; Grimme, S.; Daub, J. Angew. Chem., Int. Ed. 2000, 39,
current work. In our hands the use of xylenes as a solvent gave
the highest yields of product, although the exact mechanism
for enhanced yields is unclear. When the reaction was performed
either neat or in refluxing DMF, we encountered extensive
decomposition and difficulties in product separation. When
toluene rather than xylenes was used (over the same time
period), lower yields were obtained likely due to lower reaction
temperatures. It is noted that in cases where only modest yields
of L1-L7 were obtained (using xylenes or toluene), the reaction
simply appeared incomplete (TLC), with no identifiable side
products. The ensuing reaction of the appropriate 2-pyrazolyl-
aniline ligand with triphenylboron proceeds to completion to
give the desired BORAZAN within a few hours in refluxing
toluene. If the reaction is performed in hexanes, the reaction is
incomplete, giving, at first, a precipitate with distinct bright cyan
luminescence (presumably a simple Lewis acid-base adduct,
Ph3B‚[(µ-NH2)-p-(X)C6H3(pz)]) and then only a mixture on
further heating. The higher reflux temperature of toluene
compared to hexanes is apparently necessary to drive the
reaction to completion in a reasonable time period.
3
252. (c) Kollmannsberger, M.; Rurack, K.; Resch-Genger, U.; Daub, J. J.
Phys. Chem. A 1998, 102, 10211. (d) Kollmannsberger, M.; Gareis, T.;
Heinl, S.; Breu, J.; Daub, J. Angew. Chem., Int. Ed. 1997, 36, 1333.
(
5) For example: Zeng, L.; Miller, E. W.; Pralle, A.; Isacoff, E. Y.;
Chang. C. J. J. Am. Chem. Soc. 2006, 128, 10.
6) (a) Hattori, S.; Ohkubo, K.; Urano, Y.; Sunahara, H.; Nagano, T.;
Wada, Y.; Tkachenko, N. V.; Lemmetyinen, H.; Fukuzumi, S. J. Phys.
Chem. B 2005, 109, 15368. (b) Imahori, H.; Norieda, H.; Yamada, H.;
Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc.
(
2
6
001, 123, 100. (c) Wagner, R. W.; Lindsey, J. S. Pure Appl. Chem. 1996,
8, 1373.
(7) For example: (a) McDonnell, S. O.; O’ Shea, D. F. Org. Lett. 2006,
8
(16), 3493. (b) Chen, T.-R.; Chien, R.-H.; Jan, M.-S.; Yeh, A.; Chen,
J.-D. J. Organomet. Chem. 2006, 691, 799. (c) Liu, Q. D.; Mudadu, M. S.;
Thummel, R.; Tao, Y.; Wang, S. AdV. Funct. Mater. 2005, 15 (1), 143. (d)
Chen, H.-Y.; Chi, Y.; Liu, C.-S.; Yu, J.-K.; Cheng, Y.-M.; Chen, K.-S.;
Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Carty, A. J.; Yeh, S.-J.; Chen, C.-T.
AdV. Funct. Mater. 2005, 15, 567. (e) Park, N. G.; Lee, J. E.; Park, Y. H.;
Kim, Y. S. Synth. Met. 2004, 145, 279. (f) Kaplan, G. M.; Frolov, A. N.;
Rtishchev, N. I.; El’tsov, A. V. Zhur. Organ. Khim. 1991, 27, 872. (g)
Kaplan, G. M.; Frolov, A. N.; Rtishchev, N. I.; El’tsov, A. V.; Ponomareva,
T. K. Zhur. Obsch. Khim. 1991, 61, 1810. (h) Cui, Y.; Liu, Q.-D.; Bai, D.
R.; Jia, W.-L.; Tao, Y.; Wang, S. Inorg. Chem. 2005, 44, 601. (i) Qin, Y.;
Kiburu, Y.; Shah, S.; J a¨ kle, F. Org. Lett. 2006, 8, 5227.
(8) (a) Antilla, J. C.; Baskin, J. M.; Barder, T. E.; Buchwald, S. L. J.
The BORAZAN dyes exhibit a range of air-stability depend-
ing on both the nature of the para-aniline substituent and the
physical state. Thus, the air stability of the dyes (toward
Org. Chem. 2004, 69, 5578. (b) Cristau, H.-J.; Cellier, P. P.; Spindler, J.-
F.; Taillefer, M. Eur. J. Org. Chem. 2004, 695-709. (c) Taillefer, M.; Xia,
N.; Ouali, A. Angew. Chem., Int. Ed. 2007, 46, 934. (e) Christau, H.-J.;
Cellier, P. P.; Spindler, J.-F.; Taillefer, M. Chem. Eur. J 2004, 10, 5607.
(
f) See, also, Lindley, J. M.; McRobbie, I. M.; Meth-Cohn, O.; Suschitzky,
H. J. Chem. Soc., Perkin Trans. 1 1980, 4, 982.
9) (a) Mukherjee, A.; Subramanyam, U.; Puranik, V. G.; Mohandas,
(10) (a) Tidwell, J. H.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116,
11797. (b) Tobe, Y.; Utsumi, N.; Kawabata, K.; Nagano, A.; Adachi, K.;
Araki, S.; Sonoda, M.; Hirose, K.; Naemura, K. J. Am. Chem. Soc. 2002,
124, 5350.
(11) Kelley, W. S.; Monack, L.; Rogge, P. T.; Schwartz, R. N.; Varimbi,
S. R.; Walter, R. I. Liebigs Ann. Chem. 1971, 744, 129.
(
Sarkar, A. Eur. J. Inorg. Chem. 2005, 1254. (b) Piers, W. E.; Bourke, S.
C.; Conroy, K. D. Angew. Chem., Int. Ed. 2005, 44, 5016. (c) Saha, N.;
Saha, A.; Chaudhuri, S.; Mak, T. C. W.; Banerjee, T.; Roychoudhury, P.
Polyhedron 1992, 11, 2341.
5638 J. Org. Chem., Vol. 72, No. 15, 2007

Boron Diazoanilines
hydrolysis both in the solid state and in solution) is significantly
greater for derivatives with electron-withdrawing para-aniline
substituents (CO2Et, CF3, CN) compared to derivatives with
more electron-donating substituents (OMe, Me). For the usual
kinetic reasons, the solids are more resistant to (air) hydrolysis
than when in solution. The BORAZANs 5-7 with more
electron-withdrawing groups can be stored as solids at atmo-
spheric conditions for weeks to months before detectable
hydrolysis products (notably the free ligand) can be observed
by NMR, but the remaining derivatives begin decomposing over
the period of days to weeks. Thus, the dyes are best stored as
solids in a desiccator or in a drybox. The BORAZANs are
soluble in most organic solvents except hexanes and lower
alkanes, in which cases they are only slightly soluble. When
protected from atmospheric moisture and when using dried,
distilled solvents, solutions of BORAZANs are indefinitely
stable. However, when hydrocarbon solutions are exposed to
atmospheric conditions, BORAZANs 5-7 appear stable whereas
1
-4 slowly degrade by hydrolysis over the period of days to
weeks. The rate of decomposition is greatly accelerated to a
period of hours for 1-4 in Lewis basic solvents, owing to the
greater hygroscopic nature of these solvents compared to
hydrocarbons. Again, the rate of solvolysis increases with
increasing electron-donating character of the aniline’s para-
CF3
substituent. For example, Ph2B(pzAn ) (6) appears quite stable
(
retaining its fluorescence) over a period of weeks after
OMe
dissolution in absolute ethanol; however, both Ph2B(pzAn
(
)
1) and Ph2B(pzAnMe) (2) decompose within several minutes
FIGURE 1. Molecular structures of (a) H(pzAnMe) (L2) and (b) its
Me
after dissolution. Chlorinated solvents must be rigorously treated
to eliminate the presence of trace HCl which is also deleterious.
Solid-State Structure. The molecular structures of the free
2
BORAZAN derivative Ph B(pzAn ) (2) with atom labeling. Ellipsoids
are shown at the 50% probability level.
Me
substituents, inferred by both the decrease in the N(3)-C(9)
bond distance (increasing double-bond character) [1.390, 1.388,
1.365, 1.354 Å for X ) OMe, Me, CF3, CN, respectively], and
the average distance for C(4)-C(5) and C(7)-C(8) bonds
ligand H(pzAn ), L2 and its BORAZAN derivative Ph2B-
Me
(
pzAn ) (2) are found in Figure 1. Three other BORAZANs
X
Ph2B(pzAn ) [X ) MeO (1), CF3 (6), and CN (7)] are
structurally very similar to 2, and their structures are provided
in the Supporting Information. Certain intramolecular geometric
features of the structurally characterized compounds that are
pertinent to the ensuing discussion, especially concerning the
origin of the electronic tunability of the fluorescent dyes, are
summarized in Table 1. Also, the lessons learned from a
thorough examination of the structures may help in the future
[
1.390, 1.388, 1.383, 1.374 Å for X ) OMe, Me, CF3, CN,
respectively] (see Figure 1 for atom labeling). There is also a
concomitant increase in planarity of the three-coordinate aniline
nitrogen, evident from the sum of the angles about N(3) of
3
(
3
43.8°, 344.2°, 350.8°, 353.2° for 1, 2, 6, and 7, respectively
for an ‘ideal’ trigonal-pyramidal nitrogen the sum would be
28.5° while in planar nitrogen the sum would be 360°). Thus,
‘molecular design’ of other Lewis acid chelate complexes of
the electron-withdrawing groups appear to reduce the stereo-
chemical influence of the aniline nitrogen lone pair. Perhaps as
a consequence of these structural influences, the degree of
chelate ring-puckering, best measured by the normal vector
originating from boron extending to the mean plane of the C6N
framework of the aniline moiety (0 Å for a planar chelate ring),
decreases with increasing electron-withdrawing power of para-
aniline substituents (0.82, 0.81, 0.76, and 0.63 Å for 1, 2, 6,
and 7, respectively). Alternate descriptors of ring-puckering
the 2-(pyrazolyl)aniline scaffold. In the solid state, all of the
BORAZANs possess only C1 symmetry due to distorted
tetrahedral boron and puckered (half-chair type) chelate rings
that distinguish ‘axial’ and ‘equatorial’ phenyl groups on boron.
X
Throughout the series Ph2B(pzAn ) [X ) MeO (1), Me (2),
CF3 (6), CN(7)], the average B-N and B-C bond distances
are rather unvarying, with the average B-N distance (1.57 Å)
being 0.05 Å shorter than the average B-C distance (1.62 Å).
For each BORAZAN, two inequivalent B-N and B-C bond
distances can be differentiated. As can be seen from Table 1,
the B-N(anilino) bonds are ca. 0.06 Å shorter than the B-N(pz)
formal dative bonds while the ‘equatorial’ B-C bonds are about
(such as the dihedral angle between the mean plane of the aniline
group and the plane defined by pyrazolyl-nitrogen, boron, and
aniline-nitrogen atoms) give identical trends, but are complicated
by other structural distortions, as described below.
0
.01 Å shorter than the ‘axial’ bonds. It can also be seen that
within the BORAZAN series there are structural trends that
Further inspection of the structural ramifications of complex-
ation shows several other features of relevance for understanding
the properties of the current BORAZAN systems. By comparing
the pzAn framework in the structures of L2 and 2, the most
obvious and important difference is the increase in coplanarity
between the pyrazolyl and aniline rings of the latter. While
coplanarity could be measured using the dihedral angle between
mean planes of the pyrazolyl and aniline rings (13.2° in 2 versus
1
2
correlate well with the primitive Hammett parameters, σp, of
the para-aniline substituents, as might be expected. For instance,
the ‘quinoidal’ distortion of the aniline unit appears to increase
with increasing electron-withdrawing character of the para-X
(12) Isaacs, N. Physical Organic Chemistry, 2nd ed.; Longman Scientific
and Technical: Essex, 1995; Chapter 4.
J. Org. Chem, Vol. 72, No. 15, 2007 5639

Liddle et al.
TABLE 1. Selected Bond Distances and Angles for H(pzAnMe) (L2), Ph2B(pzAnOMe) (1), Ph2B(pzAnMe) (2), Ph2B(pzAnCF3) (6), and
CN
a
Ph2B(pzAn ) (7)
compound
H(pzAnMe) (L2)
Ph2B(pzAnOMe) (1)
Ph2B(pzAnMe) (2)
Ph2B(pzAnCF3) (6)
Ph2B(pzAnCN) (7)
B-N(1), pz (Å)
B-N(3), An (Å)
B-C (ax) (Å)
B-C (eq) (Å)
HN-C(1) (Å)
Σ∠ ’s about N(3)
C(4)-C(9) (Å)
C(8)-C(9) (Å)
C(4)-C(5) (Å)
C(7)-C(8) (Å)
C(5)-C(6) (Å)
C(6)-C(7) (Å)
C(4)-N(2) (Å)
-
1.597
1.538
1.629
1.613
1.390
343.76
1.410
1.392
1.391
1.388
1.385
1.390
1.424
2.438
0.819
36.17
45.03
166.35
14.17
1.20
1.601
1.537
1.626
1.611
1.388
344.22
1.401
1.405
1.394
1.381
1.388
1.393
1.426
2.438
0.810
35.72
44.80
167.02
13.63
0.72
1.604
1.537
1.621
1.615
1.365
350.84
1.408
1.406
1.386
1.380
1.386
1.388
1.428
2.439
0.762
33.61
43.86
167.92
14.73
3.05
1.605
1.538
1.630
1.624
1.354
353.08
1.422
1.411
1.378
1.370
1.387
1.403
1.423
2.449
0.630
27.72
37.12
170.49
12.23
2.55
-
-
-
1.386
344.73
1.405
1.401
1.392
1.387
1.394
1.397
1.429
2.923
-
N(1)‚‚‚N(3) (Å)
b
⊥
(
(
B‚‚‚(An) (Å)
c
N2BN3) -(An) (deg)
N1BN3) -(An) (deg)
-
-
127.91
52.76
2.00
51.36
d
C(5)C(4)-N(2)N(1) (deg)
C(9)C(4)-N(2)N(1) (deg)
N(2)C(4)-C(9)N(3) (deg)
mpl(pz)-(An) (deg)
13.78
13.19
14.62
13.51
a
See Figure 1 for atom labeling. Also: pz ) pyrazolyl; An ) Aniline; (An) ) mean plane of C6N aniline ring; mpl(pz) ) mean plane of pyrazolyl ring.
b
Distance of normal vector between mean aniline plane and boron. c Angle between plane defined by N(2), B(1), N(3) and (An). d Angle between plane
defined by N(1), B(1), N(3) and (An).
BORAZANs, pyrazolyl ring-twisting becomes less pronounced
more coplanar with the aniline ring) along the series. Thus,
the torsion angle increases in the following order: 166.4° (X
(
)
)
OMe) < 167.0° (X ) Me) < 167.9° (X ) CF3) < 170.5 (X
CN). Another possible torsion angle that could describe
pyrazolyl ring-twisting, C(9)C(4)-N(2)N(1) (where a torsion
of 0° would constitute coplanarity), is a less desirable descriptor
since this angle is directly modified by another commonly
observed distortion derived from out-of-plane (‘oop’) bending
of the aniline ring (top right of Figure 2). The N(2)C(4)-C(9)N-
(3) torsion angle of any value other than the ideal 0° denotes
out-of-plane (‘oop’) distortion in the ligand framework, occur-
ring for either or both pyrazolyl or aniline-nitrogen moieties.
This torsion varies between 0.72° for 2 and 3.05° for 7 but with
no obvious trend. The ‘oop’ distortion is clearly a minor but
significant contributor to the pyrazolyl-aniline dihedral angle.
Interestingly if the ‘oop’ torsion angle is subtracted from the
alternate pyrazolyl ring-twisting torsion, C(9)C(4)-N(2)N(1),
or from the dihedral angle of mean pyrazolyl and aniline planes
of a given compound, the collection of resultant angles
corroborates the trend that increasing electron-withdrawing
power of the para-aniline substituent increases the coplanarity
of the nitrogen-containing rings in the BORAZAN series.
Finally, it should be noted that the distance between the aniline
nitrogen and the remote pyrazolyl nitrogen, N(3)‚‚‚N(1) in
Figure 1, which is a crude measure of the ‘bite’ of the chelate
remains fairly constant at about 2.44 Å in the BORAZAN series.
The minor increase in this distance observed for 7 compared to
FIGURE 2. Common distortions in the 2-(pyrazolyl)aniline ligand
framework.
1
, for instance, is likely due to the slightly shorter C(9)-N(3)
5
1.4° in L2), there is no clear trend correlating this angle with
bond and the minor geometric changes associated with the
slightly greater degree of coplanarity in the ligand system.
other structural features in the BORAZAN series 1, 2, 6, and 7
because this dihedral is the sum of several possible ligand
distortions, shown in Figure 2. The major contributor to the
dihedral angle is pyrazolyl ring-twisting (bottom of Figure 2),
described best by the C(5)C(4)-N(2)N(1) torsion angle (see
Figure 1 for atom labeling) which is ideally 180° for coplanarity.
As anticipated from the other structural features such as the
decrease in chelate ring-puckering with increasing electron-
withdrawing capabilities of the para-X substituents in the
NMR. The NMR spectral data for the BORAZANs in various
solvents indicate that the compounds are stereochemically
nonrigid in solution. As discussed above, the solid-state
structures have only C1 symmetry where the phenyl groups on
boron are symmetrically nonequivalent and can be labeled as
either ‘axial’ or ‘equatorial’. However, the room temperature
1
H NMR spectra gave only one set of resonances for boron-
5640 J. Org. Chem., Vol. 72, No. 15, 2007

Boron Diazoanilines
phenyl rings rather than two sets (axial, equatorial) as would
be expected if the solid-state structures persisted in solution.
Thus, in solution, a dynamic process exchanges the sym-
metrically nonequivalent phenyl groups, giving rise to an
apparently more symmetrical species (an average structure with
1
1
presumably overall Cs symmetry). Also, since the B NMR
spectra of all the BORAZANs showed only one set of relatively
narrow signals (w1/2 ∼ 350 Hz) near 0 ppm (relative to BF3‚
OEt2), consistent with four-coordinate boron (resonances for
three-coordinate boron typically occur more downfield above
1
3
about +25 ppm), we tentatively assign this dynamic process
as being due to a low-energy ring flipping process (with amine
inversion). The dynamic process was still at the fast exchange
limit even at -80 °C in toluene-d8. Finally, in the series of
ligands L1-L7 or the BORAZANs 1-7, there was a decent
correlation between the primitive Hammett parameter, σp, of
the aniline’s para-X substituent and the chemical shift of the
NH resonance (Supporting Information); the more electron-
withdrawing the substituent (i.e., the more positive σp), the more
downfield (deshielded) is the NH resonance. This trend is also
anticipated from the previous structural discussion regarding
the stereochemical activity of the aniline-nitrogen lone pair in
the BORAZANs.
Computational Studies. Time-dependent density functional
calculations (B3LYP/6-31G*, SPARTAN06)14 performed on
PM3 energy-minimized structures of the free H(pzAn ) ligands,
FIGURE 3. Frontier orbitals of H(pzAnMe), L2, and its corresponding
BORAZAN, 2, obtained from density functional (B3LYP/6-31G*)
calculations.
X
L1-L7, and BORAZAN dyes, 1-7, were investigated in order
to lend insight into the nature of the electrochemical and
photophysical properties of the compounds. The frontier orbitals
for a representative ligand, L2, and the corresponding BORA-
ZAN, 2, are given in Figure 3, and complete results for all
compounds can be found in the Supporting Information. For
each compound, the orbitals from HOMO(-5) to LUMO(+5)
and the lowest-energy excitations (that correspond to the three-
lowest energy absorption bands) were calculated (Supporting
Information). A comparison of the calculated energy levels for
each series of compounds is given in Figure 4 while the
calculated wavelength and oscillator strength of the lowest-
energy transition for each compound are given in Table 3. For
all L1-L7 and 1-7, the HOMO is mainly the nonbonding
representation of the aniline-centered π-system, encompassing
the aniline’s nitrogen-centered lone pair. There is also a
significant (π-antibonding) contribution from the para-aniline
substituent’s orbitals to the HOMO, in cases except L3 and 3
where π-interactions (of an H substituent) are impossible.
Inspired by the seminal work of Kasha and Rawls on the
photophysics of aniline derivatives,15 it is convenient to refer
to the HOMO (and other frontier orbitals containing significant
contributions from the conjugated aniline lone pair) as a πL (π-
lone-pair) orbital to distinguish it from a pure π orbital since
C-N bond rotation, or other deviations from nonplanarity, could
give rise to more nonbonding character of the nitrogen lone
pair and drastically change the photophysics of the molecule.
This πL distinction is also important since any electronic
transitions involving such orbitals have characteristics (ꢀ,
oscillator strengths, excited-state lifetimes, solvatochromism,
etc.) that are between that expected for either a pure n-π* or
1
5b
π-π* transition. When comparing the relative energy of the
X
HOMO of a given H(pzAn ) ligand with that for its corre-
sponding BORAZAN (Figure 4), the latter is destabilized, owing
to an additional antibonding π-interaction between the boron-
bound carbons and the aniline nitrogen’s p-orbital (best viewed
in the diagrams found in the Supporting Information) that is
not present in the former. Also from Figure 4 it is seen that, for
a given series of ligands or complexes, there is a stabilization
of the HOMO with an increase in electron-withdrawing character
of the aniline’s para-substituent. The LUMO of each compound
is π* in character and spans the π-systems of both the aniline
and the pyrazolyl rings with only a small contribution from the
aniline nitrogen’s conjugated p-orbital. Relative to the HOMO,
there is only very small energy stabilization with increasing
electron-withdrawing character of the para-aniline substituent
across the series of either ligands or BORAZANs; the dif-
ferential energy stabilization is the origin of the tunability of
the absorption/emission processes in the series of dyes. For the
ligands L1-L7, the HOMO(-5) to HOMO(-1) orbitals are
π-bonding, the virtual orbitals LUMO (+1) to LUMO(+3) are
π* antibonding, while the LUMO(+4) and LUMO(+5) are σ*
antibonding. In the BORAZAN’s, 1-7, new states arise due to
the contributions from the diphenylboron moiety. Thus, four
new π-orbitals located mainly on the boron-phenyls, HOMO-
(-1) to HOMO(-4), appear before the next-lowest pzAn-based
πL-orbital, HOMO(-5). The corresponding four new π* orbitals
include contributions from both the pzAn and diphenylboryl
moieties and constitute the LUMO(+2) to LUMO(+5).
(
13) (a) Phillips, W. D.; Miller, H. C.; Muetterties, E. L. J. Am. Chem.
Soc. 1959, 81, 4496. (b) Beachley, O. T., Jr. J. Am. Chem. Soc. 1970, 92,
372. (c) Wrackmeyer, B.; N o¨ th, H. Chem. Ber. 1977, 110, 1086.
5
The calculated absorption spectrum (Supporting Information)
for each ligand and each BORAZAN qualitatively reproduced
the experimentally observed spectrum in terms of both the
number and intensity of expected bands, as well as trends within
each series of compounds, but obvious differences in the
(
14) Shao, Y.; et. al. Phys. Chem. Chem. Phys. 2006, 8, 3172.
(15) (a) Kasha, M.; Rawls, R. Photochem. Photobiol. 1968, 7 (6), 561.
(
b) As these author’s point out, an n-π* transition in hindered aromatic
amines or in N-heterocycles refers to electronic transitions from the lone
pairs occupying orbitals orthogonal to the π-system (as found in pyridine,
for instance) to the appropriate π-system orbital.
J. Org. Chem, Vol. 72, No. 15, 2007 5641

Liddle et al.
FIGURE 4. Relative energies of frontier orbitals of the ligands, L1-L7 (left), and BORAZANs, 1-7 (right) obtained from DFT (B3LYP/6-31G*)
calculations.
TABLE 2. Summary of Electrochemical Data for H(pzAn)
Ligands and Diphenyl-BORAZAN Derivatives (See Supporting
Information for voltammograms)a
compound
oxid. (V)
red. (V)a
OMe
H(pzAn ), L1
0.75, 1.29
0.87, 1.11
1.09
1.01, 1.16
1.17, 1.31
1.23, 1.49
1.31
0.53
0.69
0.79
0.84
Me
H(pzAn ), L2
-
H(pzAn), L3
-
Cl
H(pzAn ), L4
-
CO2Et
H(pzAn
), L5
-2.56
-
CF3
H(pzAn ), L6
CN
H(pzAn ), L7
-2.62
-2.59
-2.59
-2.67
-2.41
-2.40
-2.39
-2.42
OMe
Ph2B(pzAn ), 1
Ph2B(pzAnMe), 2
Ph2B(pzAn), 3
Cl
Ph2B(pzAn ), 4
CO2Et
Ph2B(pzAn
), 5
1.07
1.04
1.12
FIGURE 5. Cyclic voltammograms (100 mV/s) of CH
3
CN solutions
Ph B(pzAnCF3), 6
2
Me
Me
CN
of H(pzAn ), L2 (blue, bottom), and Ph
with NBu PF as supporting electrolyte.
2
B(pzAn ), 2 (black, top),
Ph B(pzAn ), 7
2
4
6
a
Versus Ag/AgCl scan rate of 100 mV/s in CH3CN with NBu4(PF)6 as
absolute energies of bands (owing, in part, to deficiency in the
solvation model) account for deviations from the experimentally
observed results (vide infra). The calculated spectrum for each
ligand consists of three bands while that of each BORAZAN
consists of four bands. For the ligands, extensive mixing of states
occurs. The lowest-energy excitation band was found to be
mainly πL-π* due to the HOMO-LUMO transition with some
contribution from an energetically accessible πL-πL* transition
supporting electrolyte.
versus a pzAn ligand and a decrease in oxidation potential with
increasing electron-donating character of the para-aniline sub-
stituent, both explained by the destabilization of the HOMO as
predicted by calculations. Representative voltammograms for
Me
Me
H(pzAn ), L2, and Ph2B(pzAn ), 2, are given in Figure 5
while a summary of the data for the remaining compounds is
provided in Table 2. Each ligand, L1-L7, undergoes an
irreversible oxidation above about 0.8 V (depending on sub-
stituent), and the corresponding voltammogram is reminiscent
of ECE-type behavior where the electrochemically generated
species (which is yet to be identified) typically has a reversible
oxidation. It should be noted that the oxidation of para-
substituted aniline derivatives is known to be irreversible, giving
mixtures of electrochemically active species (head-to-tail, tail-
to-tail, and head-to-head dimers (both hydrazoaryls and diaz-
oaryls) and other species such as diarylamines) whose compo-
sition depends on the experimental conditions and nature of
[HOMO(-1)-LUMO(+1)]. Similarly, the remaining two bands
were extensively mixed but were mainly π-π* in character
with minor contributions from the HOMO-LUMO(+N) (N )
1-3). The BORAZAN spectra were better behaved. The lowest
energy excitation was due to the pure HOMO-LUMO transi-
tion, as might be anticipated. The next higher-energy excitation
band was calculated to be π(boron phenyl)-π*(pzAn) in
character and is composed of four overlapping HOMO(-N)-
LUMO (N ) 1-4) transitions. The third highest energy
excitation band originates from HOMO-LUMO(+N) (N )
1
-4) transitions and is therefore πL-π* in character and had a
1
7
much higher intensity than the lowest-energy bands. Presumably,
the highest energy band would mainly be π-π* in character
because of the numerous closely spaced HOMO(-N)-LUMO-
substituents. The oxidation of a given BORAZAN complex
is typically 150-300 mV energetically more favorable than that
(16) (a) Galus, Z.; Adams, R. N. J. Phys. Chem. 1963, 67, 862. (b) Bacon,
(
+(N+1)) (N ) 1-4) transitions involving the boron phenyl
groups.
Voltammetry. Since the aniline moiety is a well-known
J.; Adams, R. N. J. Am. Chem. Soc. 1968, 90, 6596. (c) Hand, R. L.; Nelsen,
R. F. J. Am. Chem. Soc. 1974, 96, 850. (d) MacDiarmid, A. G.; Epstein, A.
J. Faraday Discuss. Chem. Soc. 1989, 88, 317. (e) MacDiarmid, A. G.
Angew. Chem., Int. Ed. 2001, 40, 2581. (f) Heeger, A. J. Angew. Chem.,
Int. Ed. 2001, 40, 2591.
1
6
electron donor, the electrochemistry of CH3CN and CH2Cl2
solutions of the ligands and BORAZANs were examined by
cyclic voltammetry. The results of the electrochemical studies
showed a decrease in oxidation potential for the BORAZAN
(17) (a) Sharma, L. R.; Manchanda, A. K.; Singh, G.; Verma Ranjit, S.
Electrochim. Acta 1982, 27 (2), 223. (b) Daniels, D. G. H.; Naylor, F. T.;
Saunders, B. C. J. Chem. Soc. 1951, 3433.
5642 J. Org. Chem., Vol. 72, No. 15, 2007

Boron Diazoanilines
TABLE 3. Selected Experimental and Calculated (TD-DFT B3LYP/6-31G*, SPARTAN06) Photophysical Properties of Ligands and
BORAZAN Complexes
experimental
calculated
λmaxa
(nm)
ꢀ
a
Es0-0 b
(nm, eV)
λemc,
(nm)
λemd
(nm)
τFe
(ns)
λem,a
(nm)
λexc
(nm)
oscillator
strength, f
1
d
a
compd
(L mol - cm-1
)
φF
φF
L1
L2
L3
L4
L5
L6
L7
1
2
3
4
5
322
311
303
317
280
305
304
386
373
364
375
358
358
366
3300
4100
3400
4700
18000
3700
3700
4900
4400
7000
6600
6500
4700
4800
357, 3.48
342, 3.63
334, 3.71
346, 3.59
316, 3.93
335, 3.70
340, 3.65
436, 2.83
418, 2.98
409, 3.03
419, 2.96
403, 3.08
405, 3.12
398, 3.06
-
-
-
-
-
-
-
-
-
318
301
268
304
294
291
296
432
413
408
417
400
405
400
0.1736
0.1879
0.1106
0.1830
0.1624
0.1836
0.1637
0.0755
0.0818
0.0734
0.0832
0.0844
0.0759
0.0856
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
371
-
-
-
-
0.01
-
-
-
-
-
-
-
367
522
488
476
480
467
468
452
0.007
0.07
0.58
0.65
0.69
0.70
0.66
0.81
-
-
478
466
466
468
447
440
447
6.0
7.5
6.3
8.8
10.5
15.9
10.5
533
508
484
483
468
476
468
0.002
0.03
0.17
0.51
0.43
0.49
0.74
6
7
a
CH3CN. b Low-energy onset of absorption. c Most intense band, solid. d Most intense band, toluene. e Degassed (Ar), CH2Cl2.
of the corresponding free ligand and appears to be quasirevers-
ible (only in the case of 1, does the oxidation appear reversible).
As mentioned previously, for each series of ligands, L1-L7,
or BORAZANs, 1-7, the oxidation becomes more favorable
with increasing electron-donating ability of the aniline’s para-
substituent. Finally, a highly irreversible reduction (cathodic)
wave is also observed in the voltammogram of each BORAZAN
that is not typically present in that of the corresponding free
ligand. This process is more favorable for derivatives with
electron-withdrawing substituents occurring at ca. -2.4 V (vs
Ag/AgCl in acetonitrile; see Table 2 and Supporting Informa-
tion) compared to that for derivatives with electron-donating
substituents at -2.6 V, but the reduction potential is rather
FIGURE 6. Electronic spectra of H(pzAnCF3), L6 (blue, absorption),
invariant with regard to the type of electron-withdrawing or
CF3
2
and of Ph B(pzAn ), 6 (solid black line, absorption; dotted black line,
-donating substituent. These results are consistent with the DFT
3
emission), in CH CN.
calculations which indicated a stabilization of the LUMO of a
given BORAZAN relative to the H(pzAn) but only a slight
stabilization with electron-withdrawing character of the para-
aniline substituent across a given series of compounds.
Electronic Spectra. The photophysical data for the ligands
L1-L7 and BORAZAN complexes 1-7 are collected in Table
Supporting Information) giving rise to overlapping π -π *
L
L
transitions and also possibly fortuitous overlap with the n-π*
and other transitions involving the ethoxycarbonyl group. The
low-energy absorption band of each BORAZAN exhibits slight
negative solvatochromism, with a very small hypsochromic shift
with increasing solvent polarizability (e10 nm range over the
series; toluene, CH2Cl2, THF, EtOH, CH3CN, DMF). There is
a decent correlation (Supporting Information) between the onset
of the lowest-energy absorption band (corresponding to the
3
and in the Supporting Information. The absorption spectrum
CF3
of a representative free ligand, H(pzAn ), L6, along with the
absorption and emission spectra for the corresponding BORA-
CF3
ZAN, Ph2B(pzAn ), 6 (each in CH3CN), are given in Figure
X
6
. The absorption spectrum of each H(pzAn ) ligand consists
0
-0
of three bands; one high-intensity, high-energy band at ca. 230
minimum HOMO-LUMO gap, Es ) and the Hammett
1
2
nm (ꢀ ≈ 26 000), a second less intense band at ca. 250 nm
parameter, σR, which indicates a significant resonance
effect is operative in this transition, a trend that is also suggested
from the density functional calculations. The more electron-
rich para-aniline substituents and those with greater de-
grees of π-conjugation give rise to more red-shifted absorp-
tion bands. The extension of this lowest-energy absorption band
into the violet region of the spectrum accounts for the pale
yellow color of the BORAZAN derivatives 1-4 with electron-
donating groups; BORAZANs 5-7 and all ligands, L1-L7,
are colorless.
(ꢀ ≈ 8600) (in some cases this band occurs as a shoulder to the
high-energy band), and a low-energy, low-intensity band above
about 300 nm (ꢀ ≈ 4500). With the BORAZANs, each band
appears to undergo a bathochromic shift, and a new high-energy
band appears as a shoulder near 200 nm, presumably for the
π-π* transitions involving the boron-phenyl groups. The
apparent bathochromic shifts of the absorption bands in the
BORAZANs, compared to those in the free ligand, were
predicted from the calculations. Also, since the number of bands
and their intensities correlate well with the calculated spectra,
the calculated assignments for electronic transitions (addressed
earlier) appear reasonable. It should be noted that in the case
of L5, the rather large extinction coefficient for the first band
is likely due to the near degeneracy of the LUMO and LUMO-
Irradiation of the BORAZANs (either in the solid state or in
hydrocarbon or halocarbon solution) with UV light results in
intense emission that varies from blue for derivatives with
electron-withdrawing para-aniline substituents to yellow-green
for electron-donating substituents (Figure 7). With the exception
of L5 and L7, which are only weakly emissive, the free ligands
(
+1) (in fact, slight energetic inversion; see Figure 4 and
J. Org. Chem, Vol. 72, No. 15, 2007 5643

Liddle et al.
FIGURE 7. Overlay of normalized emission spectra of 1-7 obtained
in toluene solution.
FIGURE 9. Lippert-Mataga plot of Stokes shift versus solvent
orientational polarizability parameter for a representative BORAZAN,
CN
2
Ph B(pzAn ) (7).
using Gaussian fits with full-width-at-half-maximums between
0 and 80 nm (typical of organic luminophores) revealed three
4
components comprising the fluorescence band of each BORA-
ZAN (in the same solvent); the high-energy component is
variably found between 440 and 460 nm (depending on
substitution), the next lower-energy component is found between
465 and 480 nm, and the lowest-energy band is found between
B(pzAnCN) 7 in the
FIGURE 8. Normalized emission spectra of Ph
2
solid state (dashed line) and in various solvents (solid lines).
4
90 and 505 nm. This fine structure may be vibronic in origin
since apparent energy differences between the three components
of the emission band match well with IR frequencies involving
-
1
in-plane N-H and C-H(pzAn) bending near 1300 cm and
those for coupled B-N stretches and oop-C-H (BPh) bending
are nonluminescent. The fluorescence radiative de-excitation
pathway was confirmed from the measured excited-state life-
times which were between 6 and 16 ns. As seen in Table 3, the
fluorescence quantum yields of the BORAZANs decrease with
increasing electron-donating power of the para-aniline substit-
-
1 20
motions near 700 cm . When investigating the solvent
dependence of the emission, care is needed to avoid inadvert-
ently overestimating the Stokes shift in 3-7, since the relative
intensities of the components of the emission bands change with
solvent polarity (Figure 8); the high-energy band predominates
in solvents of low polarity while the medium-energy band
becomes more intense at the expense of the former in more
polar solvents. Each component of the fluorescent emission
band, however, only undergoes a very slight bathochromic shift
with increasingly polar solvents. In fact, by examining the high-
energy component of the emission band by Lippert-Mataga
CN
uents (φF ranging from 0.81 for Ph2B(pzAn ) (7) to 0.07 for
OMe
Ph2B(pzAn ) (1) in toluene). Such behavior may be antici-
pated from the energy gap law that predicts increased nonra-
diative decay rates for lower-energy transitions.18 Interestingly,
the fluorescence quantum yields also show a significant
dependence on solvent polarity, decreasing with increasing
solvent polarity (φF is 0.002 for 1 and 0.74 for 7 in CH3CN).
As seen in Table 3, the solvent-dependent drop-off in fluorescent
quantum yields is more significant in the cases of 1-3 (with
more electron-donating para-aniline substituents) than for 4-7.
We tentatively attribute this behavior to the greater chemical
2
1
plots of the Stokes shift versus solvent orientational polariz-
ability parameter for each BORAZAN (shown for 7 in Figure
), slopes on the order of 1100 cm for each BORAZAN are
-
1
9
obtained which are about 5-10% of the value found for other
systems that exhibit significant intramolecular charge-transfer.22
This low value is expected in the current system since the extent
of intramolecular charge-transfer associated with an πL-π*
reactivity of the former which, by interaction with solvent, may
enhance losses predicted by the energy gap law;18 a separate
report by our group will further elaborate on this behavior. For
a given BORAZAN, the solution emission is red-shifted relative
to the solid-state emission (Figure 8) owing to solvent relax-
19
ation, and the emission bands are relatively broad (ω1/2 ) 70-
0 nm) and are featureless for 1 and 2 but those for 3-7 show
detectable fine structure. Deconvolution of the latter spectra
(20) (a) Williams, V.; Hofstadter, R.; Herman, R. C. J. Chem. Phys. 1939,
7
, 802. (b) Ginsburg, N.; Matsen, F. A. J. Chem. Phys. 1945, 13 (5), 167.
c) Weyssenhoff, Kraus, F. J. Chem. Phys. 1971, 54 (6), 2387.
21) (a) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956,
8
(
(
2
9, 465. (b) Lippert, E. Z. Naturforsch. Teil A 1955, 10, 541.
(22) (a) Mishra, A.; Behera, B. G.; Krishna, M. M. G.; Periasamy, N. J.
(18) (a) Caspar, J. V.; Sullivan, B. P.; Kober, E. M.; Meyer, T. J. Chem.
Phys. Lett. 1982, 91 (2), 91. (b) Penner, A. P.; Siebrand, W.; Zgierski, M.
Z. J. Chem. Phys. 1978, 69 (12), 5496. (c) Englman, R.; Jortner, J. Mol.
Phys. 1970, 18 (2), 145.
Luminescence 2001, 92, 175. (b) Mutai, T.; Cheon, J.-D.; Arita, S.; Araki,
K. J. Chem. Soc., Perkin Trans. 2001, 2, 1045. (c) Krebs, F. C.; Spanggaard,
H. J. Org. Chem. 2002, 67, 7185. (d) Strehmel, B.; Sarker, A. M.; Malpert.
J. H.; Strehmel, V.; Seifert, H.; Neckars, D. C. J. Am. Chem. Soc. 1999,
121, 1226.
(19) Jager, W. F.; Volkers, A. A.; Neckars, D. C. Macromolecules 1995,
2
8, 8153.
5644 J. Org. Chem., Vol. 72, No. 15, 2007

Boron Diazoanilines
transition should indeed be small due to the significant, but
incomplete, overlap of the HOMO and LUMO, as the calcula-
tions suggest.
H
4
were added to facilitate workup. The mixture was extracted with
three 100 mL portions of CH Cl which were combined and dried
over MgSO and filtered. The solvent was removed by rotary
2
2
4
evaporation to leave oily residues. The residues were purified by
column chromatography by using 4:1 hexanes:ethyl acetate as the
Summary. A new class of highly emissive fluorescent dyes
based on the diphenylboron N,N′-chelate complexes of 2-(pyra-
zolyl)anilines has been prepared and extensively characterized
by multiple methods. The results of X-ray crystallographic
studies on the L2 and its boron complex 2 revealed that chelation
to the Lewis acidic diphenylboron moiety brings the pyrazolyl
and aniline rings closer to coplanarity than found in the free
ligand. Moreover, comparisons of intramolecular geometries in
eluent for all cases (R
methyl derivative, for which methylene chloride was the eluent (R
0.4, SiO plate). After column chromatography, all products
were initially isolated as oils but could be crystallized (except
for the CF derivative) by cooling supersaturated hexanes
f 2
ca. 0.4, SiO plate), except the trifluoro-
f
)
2
3
solutions (heated at reflux) to room temperature over the
course of several hours. The trifluoromethyl derivative is a color-
less oil at room temperature but crystallizes in a refrigerator.
The amount of bromoaniline and pyrazole, the yields, and the
characterization data for a representative ligand L1 is found
below while details for L2-L7 can be found in the Supporting
Information.
1
, 2, 6, and 7, which differ only in the para-aniline substituent,
demonstrate greater ‘quinoidal’ distortion and an increased
coplanarity between pyrazolyl and aniline rings with increasing
electron-withdrawing character of the para-aniline substituent.
It was also found that the electronic properties of the fluorescent
dyes could be varied in a predictable manner by changing the
electron-donating or withdrawing character of the para-aniline
substituent. For instance, the color of emission could be tuned
from blue to yellow-green by increasing the substituent’s
electron-donating power. This change comes at a small expense
in terms of fluorescent quantum yield and stability toward
protonolysis by hydrogen-donating solvents (alcohols, water),
with the more electron-donating derivatives being most reactive
and least luminous. The results of density functional calculations
H(pzAnOMe), L1. With 0.856 g (4.24 mmol) 2-bromo-4-
methoxy-aniline and 0.346 g (5.08 mmol) pyrazole, 0.601 g (75%
yield) of L1 was obtained as colorless needles. Mp, 44-46 °C.
Anal. Calcd (obsd) for C10
H
6
11
N
3
O, C, 63.48 (63.07); H, 5.86 (5.66);
) δ 7.63 (d, J ) 2 Hz, 1H), 7.20 (d,
J ) 1 Hz, 1H), 6.67 (s, 1H), 6.52 (d, 1H), 6.32 (d, 1H), 6.07 (dd,
1
22.21 (21.95). H NMR (C
D
6
H
1
J ) 2, 1 Hz, 1H), 4.16 (br s, 2H), 3.26 (s, 3H). H NMR (CDCl
7.75 (d, J ) 1 Hz, 1H), 7.73 (d, J ) 2 Hz, 1H), 6.80 (d, J ) 8
Hz, 1H), 6.79 (s, 1H), 6.70 (d, J ) 8 Hz 1H), 6.46 (dd, J ) 2,1
3
)
δ
H
13
Hz, 1H), 4.22 (br s, 2H), 3.78 (s, 3H). C NMR (C
37.8, 131.4, 129.7, 126.4, 122.0, 116.0, 106.4, 55.4. C NMR
CDCl ) δ 150.3, 140.8, 137.8, 131.4, 129.7, 126.4, 122.0, 116.0,
06.4, 55.4. UV-vis (CH
6 6 C
D ) δ 150.3,
(B3LYP/6-31G*) qualitatively reproduced experimental obser-
13
1
(
1
vations and provided insight into the πL-π* nature of the
emission (where πL indicates the special character of the
π-system of the HOMO); the HOMO was found to be the
nonbonding representation of the aniline-centered π-system that
includes the nitrogen lone-pair while the LUMO extended over
the π-system of both the pyrazolyl and aniline moieties.
According to the calculations, the tunability of this first
generation of BORAZAN dyes originates from the greater
destabilization of the HOMO relative to the LUMO on increas-
ing the electron-donating character of the para-aniline substitu-
ent. With the exception of 3, the aniline’s para-substituent is
involved in an antibonding π-interaction (not possible for 3)
that destabilizes the HOMO to varying extent. Further desta-
bilization of the HOMO arises due to an antibonding interaction
between the boron-bound carbons and the p-orbital containing
the aniline nitrogen lone-pair, which becomes more pronounced
with increasing electron-donating character of the para-aniline
substituent.
The ready availability of starting materials with nearly
unlimited substitution patterns combined with the relative ease
of dye synthesis render these and other pyrazolyl-aniline chelates
very attractive potential luminophores for further investigation.
Forthcoming reports from this group will demonstrate that
simple structural modifications allow for a means to extend the
emission range into the red and for providing a strategy to
improve dye stability toward solvolysis.
3
C
-1
-1
3
CN) λmax, nm (ꢀ, M cm ) 200 (21,-
-
1
000), 229 (16,000), 322 (3,300). IR (KBr pellet, cm ): 3743 νN-H
str, 3421, 3350, 3135, 3118, 3001, 2962, 2935, 2833, 1628 νN-H
wag, 1595, 1518, 1452, 1396, 1340, 1290, 1261, 1227, 1180,
in-plane
1111, 1045, 953, 866, 812, 758, 625 νN-H oop wag
.
General Procedure for Syntheses of Diphenylboron Deriva-
tives, 1-7. Under nitrogen, an equimolar mixture of triphenylboron
and the desired pyrazolyl aniline in 20-30 mL toluene were heated
at reflux overnight. After cooling, solvent was removed by vacuum
distillation to leave a glassy residue. Next, 25 mL of dried hexanes
were added, and the mixture was heated under nitrogen with stirring
to leave the desired compound as a powder. After cooling to room
temperature, the mixture was separated by cannula filtration. The
insoluble component was dried under vacuum. Additional crops
of the BORAZAN could be obtained from the hexane-soluble
components after concentration and cooling to give a precipitate
that is collected by filtration and dried, as above. The quantities of
reagents, isolated yields of powder, and characterization data of
crystals (the characterization data of the powders and crystals were
identical) are given below for 1 while the data for 2-7 are found
in the Supporting Information.
Ph B(pzAnOMe), 1. A mixture of 0.59 g (3.1 mmol) L1 and 0.75
2
g (3.1 mmol) of BPh
3
afforded 0.93 g (85%) of 1 as a yellow solid.
O: C,
4.81 (74.36); H, 5.71 (5.81); N, 11.90 (11.69). H NMR (C
Mp, 191-193 °C, dec Anal. Calcd. (obsd.) for C22
H20BN
3
1
7
6 6
D )
δ
H
7.52 (d, J ) 8 Hz, 4H), 7.33 (dd, J ) 8, 8 Hz, 4H), 7.23 (t, J
8 Hz, 2H), 6.99 (d, J ) 2 Hz, 1H), 6.57 (d, J ) 2 Hz, 1H), 6.52
)
(
(
(
dd, J ) 8, 8 Hz, 1H), 6.35 (d, J ) 8 Hz, 1H), 6.22 (s, 1H), 5.48
Experimental Section
13
dd, J ) 2, 2 Hz, 1H), 3.78 (br s, 1H), 3.21 (s, 3H). C NMR
150.0, 135.1, 134.5, 134.0, 131.1, 126.7, 126.1, 121.7,
C
6
D
6
) δ
19.1, 115.7, 106.5, 55.5. B NMR (C
491 Hz). UV-vis (CH
C
General Procedure for Ligand Syntheses (L1-L7). A mixture
of the desired bromoaniline (1 equiv), pyrazole (1.2 equiv), K -
2
11
1
6
D
6
) δ
B
0.5 ppm (ω1/2 )
-
1
-1
CO
3
(2.1 equiv), 20 mol % N,N′-dimethylethylenediamine, and
3
CN) λmax, nm (ꢀ, M cm ): 197 (46,-
-1
5-10 mL of p-xylenes were degassed by three freeze/pump/thaw
000), 242 (21,000), 292 (3,800), 386 (4900). IR (KBr pellet, cm ):
3734 νN-H str, 3419 br, 3396, 3147, 3130, 3118, 3086, 3064, 3020,
3008, 2966, 2947, 2928, 2832, 1627 νN-H in-plane wag, 1518, 1458,
1433, 1385, 1365, 1342, 1316, 1292, 1271, 1238, 1215, 1190, 1141,
1138, 1097, 1045, 999, 985, 933, 926, 887, 870, 845, 810, 781,
760, 742, 706, 665, 648, 619 νN-H oop wag, 598, 561, 540.
cycles. Under a nitrogen blanket, 5 mol % CuI was added, and the
resulting mixture was subject to two more freeze/pump/thaw cycles.
The mixture was heated under nitrogen at reflux for 36 h (until
starting materials were no longer detected by TLC). After cooling
2
to room temperature, 100 mL of H O and a few crystals of EDTA-
J. Org. Chem, Vol. 72, No. 15, 2007 5645

Liddle et al.
Acknowledgment. J.R.G. thanks Marquette University, the
Petroleum Research Fund (74371-G), and an NSF instrumenta-
tion grant (CHE-0521323) for support.
of frontier orbitals, Cartesian coordinates for modeled compounds,
tables of data from TD-DFT calculations, Hammett plots, crystal-
lographic information files (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
Supporting Information Available: General experimental
methods, spectral and electrochemical characterization data, figures
JO0705420
5646 J. Org. Chem., Vol. 72, No. 15, 2007