Molecular motion and performance enhancement of BORAZAN fluorescent dyes

Article abstract of DOI:10.1002/ejic.200800806

The preparation of three 2,6-dipyrazolyl-4-X-anilines, H(pz ₂An^X) (X = p-CF₃, Cl, tBu) using CuI-catalyzed amination is described. Subsequent reactions of H(pz₂An^X) with triphenylboron proceeds with b

Full text of DOI:10.1002/ejic.200800806

FULL PAPER
DOI: 10.1002/ejic.200800806
Molecular Motion and Performance Enhancement of BORAZAN Fluorescent
Dyes
Tyler J. Morin,[a] Sergey V. Lindeman,[a] and James R. Gardinier*[a]
Keywords: Chelates / Boron / Electrochemistry / UV/Vis spectroscopy / Fluorescence / Dyes/pigments / Density functional
calculations
The preparation of three 2,6-dipyrazolyl-4-X-anilines,
tional calculations (B3LYP/6-31G*). The di-pyrazolyl deriva-
H(pz2AnX) (X = p-CF3, Cl, tBu) using CuI-catalyzed amin- tives exhibit greater stability toward solvolysis and higher
ation is described. Subsequent reactions of H(pz2AnX) with
triphenylboron proceeds with benzene elimination to give
the corresponding Ph2B(pz2AnX) compounds in high yields.
The Ph2B(pz2AnX) are more highly emissive in the solid state
than the previously reported BORAZAN fluorophores,
Ph2B(pzAnX), their monopyrazolyl counterparts. As with the
Ph2B(pzAnX), the color of emission in Ph2B(pz2AnX) can be
tuned simply by varying the para-aniline substituent where
the emission of Ph2B(pz2AnX) is red-shifted relative to the
corresponding Ph2B(pzAnX) derivatives. The electronic prop-
erties were studied by cyclic voltammetry and electronic ab-
sorption/emission spectroscopy as well as by density func-
photoluminescent quantum yields (despite the red-shift in
emission) compared to their monopyrazolyl counterparts pre-
sumably due to kinetic stabilization of the chromophore im-
parted by the second pyrazolyl ligand. For Ph2B(pz2AnX),
evidence for intramolecular motion of the diphenylboryl moi-
ety traversing both pyrazolyl groups was detected by vari-
able temperature 1H NMR spectroscopy. The rate increases
with increasing electron-donor abilities of the para-aniline
substituent.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
ene) = 0.07] and readily decomposed in protic media. The
quantum yields of emission were also greatly reduced in
aprotic Lewis-basic solvents compared to hydrocarbon sol-
vents. We conjectured that dynamic solution processes may
be related to the low quantum yield of emission and the
high reactivity of BORAZANS toward Lewis bases. That
is, the integrity of the dye would be compromised if boron-
pyrazolyl ring dissociation occurred; resulting in lower
quantum yields and the resulting three-coordinate boron
may provide a pathway for degradation. In order to test this
hypothesis, we have modified the dye structure by putting
an additional pyrazolyl ring at the 6-position of the aniline
(Figure 2). Such a substitution was designed with the intent
of kinetically stabilizing the dye by effectively doubling the
rate of boron-pyrazolyl bond formation should boron-pyr-
Introduction
There is a longstanding interest in developing new
brightly-emitting fluorophores for fundamental studies and
for a variety of applications, from sensors and display tech-
nology to biomedical imaging.[1] Boron species such as
BODIPYTM[2] and other N,N-[3] and N,O- chelates[4] have
been particularly well-suited for such studies owing to their
favourable synthetic protocol, stability, and photophysical
properties. We recently reported[5] on a series of colour
tuneable fluorescent dyes based on the diphenylboron com-
plexes of (2-pyrazolyl)-4-X-anilines, or BORAZANs (Fig-
ure 1) of the type Ph2B(pzAnX) where pz is a pyrazolyl, An
is aniline and superscript X is the para substituent of the
aniline ring. The emission colour, intensity, and reactivity
of the fluorescent dyes could be tuned in a regular way by
varying the para-aniline substituent. For instance, the cyano
em
derivative gave the most intense blue emission [
λ
(tolu-
max
ene) = 452nm,ΦF(toluene) = 0.81] and was the most stable
toward solvolysis with protonated solvents such as water
or alcohols. The methoxy derivative gave much less intense
em
yellow-green emission [
λ
(toluene) = 522 nm, ΦF (tolu-
max
[a] Department of Chemistry, Marquette University,
Milwaukee, WI 53201-1881, USA
Fax: +1-414-288-7066
E-mail: james.gardinier@marquette.edu
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Figure 1. The BORAZAN dye framework and representative solid-
state structure of Ph2B(pzAnCF3) from ref.[1]
104
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 104–110

Performance Enhancement of BORAZAN Fluorescent Dyes
azolyl dissociation occur in solution. Moreover, with such a
substitution pattern any dynamic processes would be easily
detected by NMR, as the pyrazolyl rings would be magneti-
cally inequivalent in a static structure, such as in Figure 2,
but would be equivalent if exchange occurred. We report
here on the successful implementation of this structural
modification for enhancing the performance of BORAZAN
dyes. Moreover, the new ligand may be attractive for the
future development of mono- and dimetallic transition
metal systems, and the current study provides a basis for
evaluating the potential photo- and electrochemical “non-
innocence” of this scaffold.
Scheme 1. Preparation of BORAZAN dyes.
Figure 2. Representation of the 2nd-generation Ph2B(pz2AnX) com-
pounds.
over time even in the solid state. As further testament to
the improved stability of the dyes, the dipyrazolyl deriva-
tives 2a–2c can survive flash chromatography on silica gel, a
procedure that immediately annihilates the monopyrazolyl
BORAZANs 1a–1c.
Results and Discussion
A summary of the preparative routes to the various li-
The molecular structures of the free ligand H(pz2AntBu
)
gands and BORAZAN dyes is given in Scheme 1. The syn- (L2c) and four BORAZAN derivatives Ph2B(pzAntBu) (1c),
theses of the first-generation H(pzAnX) ligands [X = CF3 and Ph2B(pz2AnX) (X = CF3, Cl, tBu; 2a–2c) have been
(L1a), Cl (L1b), tBu (L1c)] and BORAZAN dyes determined by single-crystal X-ray diffraction. The struc-
Ph2B(pzAnX) [X = CF3 (1a), Cl (1b), tBu (1c)] follows the tures of H(pz2AntBu) (L2c) and its BORAZAN derivative
literature method (Scheme 1, a),[5] where L1c and 1c repre- Ph2B(pz2AntBu) (2c) is given in Figure 3 while all other
sent new derivatives. The latter were prepared by monobro- structures, a Table of metrical parameters, and full struc-
mination of p-tert-butylaniline followed by copper-cata- tural discussion are provided in the Supporting Infor-
lyzed amination by the methodology of Buchwald[6] and mation. The structure of L1c (Figure 3, a) reveals a pyrami-
Taillefer[7] to afford L1c which was subsequently converted dal amino nitrogen (sum of angles around N1 = 341.7°)
to 1c in high yield by reaction with triphenylboron in tolu- with hydrogen atoms directed below one face of the aniline
ene. The second-generation ligands H(pz2AnX) [X = CF3 ring. The pyrazolyl rings are also directed below the same
(L2a), Cl (L2b), tBu (L2c)] and BORAZAN dyes face with an average dihedral of 43° with respect to the
Ph2B(pz2AnX) [X = CF3 (2a), Cl (2b), tBu (2c)] were pre- central aniline ring to give relatively long and presumably
pared by an analogous route (Scheme 1, b) after first di- weak intramolecular hydrogen bonding interactions
brominating the appropriate aniline.[8] The subsequent cop- N1H1b···N12 (2.15 Å, 133°) and N1H1a···N22 (2.20 Å,
per-catalyzed amination of the 2,6-dibromo-4-X-aniline re- 133°). In 2c, the boron-bound pyrazolyl is closer to copla-
quired longer reaction times to afford the desired narity with respect to the central aniline ring (dihedral of
H(pz2AnX) in reasonable yield. The ensuing reaction with 14.1°) whereas the other pyrazolyl is further from coplanar-
triphenylboron proceeded smoothly to afford high yields of ity with a dihedral of 49.3° A feature in 2c that is common
the desired 2a–2c with the elimination of benzene.[9]
to all other structurally characterized BORAZAN dyes is
The BORAZAN dyes are insoluble in hexanes, 1a–1c are C1 (rather than Cs) symmetry due to puckering of the six-
soluble in benzene whereas 2a–2c (based on pz2AnX) are member chelate ring into a pseudo half-chair conformation
modestly soluble, and all are soluble in toluene, halocar- with a distorted tetrahedral boron residing above (0.42 Å)
bons, and polar Lewis-basic solvents (THF, CH3CN, the remaining atoms of the chelate ring As with previously
DMF). The compound Ph2B(pzAntBu) (1c) rapidly decom- reported members of the BORAZANs, the average B–N
poses (over a period of minutes) in alcohols, as found distance (1.57 Å) is 0.05 Å shorter than the average B–C
previously for Ph2B(pzAnX) (X
= Me, MeO), but distance (1.62 Å) regardless of the substitution pattern
Ph2B(pz2AnX) [X = CF3 (2a), Cl (2b), tBu (2c)] persist over along the dye framework. Also, the B–C bond of the axial
days in this type of solvent. In addition, the second-genera- phenyl is detectably longer (1.62–1.64 Å) than that of the
tion BORAZAN dyes 2a–2c appear indefinitely air stable equatorial phenyl (1.61–1.62 Å), presumably due to the
in the solid or in solution, in stark contrast to the first gen- anti-bonding interaction involving the anilne moiety (see
eration counterparts 1a–1c which decompose by hydrolysis HOMO in top of Figure 5).
Eur. J. Inorg. Chem. 2009, 104–110
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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105

T. J. Morin, S. V. Lindeman, J. R. Gardinier
FULL PAPER
spectra for 2c and other derivatives acquired at different
temperatures are collected in the Supporting Information.
At –80 °C there are two resonances for 4-pyrazolyl hydro-
gen atoms at 5.94 and at δ = 5.36 ppm. Comparisons of
chemical shifts of the 4-pyrazolyl hydrogen resonances in
Ph2B(pz2AntBu) (2c) with those of the related free ligands
H(pzAntBu) (L1c) (δH = 6.11) and H(pz2AntBu) (L2c) (δH
=
6.12) and the monopyrazolyl BORAZAN derivative
Ph2B(pzAntBu) (1c) (δH = 5.53) establish that the lower field
resonance is due to the free pyrazolyl while the higher field
resonance is for the boron-bound pyrazolyl. On warming
to 0 °C there is a constant downfield shift in both reso-
nances. Above 0 °C the resonances broaden and coalesce
at 30 °C, and above the coalescence temperature the single
exchange-averaged resonance sharpens. Similar characteris-
tics are shared in the spectra of 2a and 2b. The activation
barriers for (in hence, the rate of) exchange varies in a regu-
lar way with the electron-donating character of the para-
aniline substituent, as summarized in Table 1. The more
electron-donating substituent gives rise to lower activation
barriers and faster rates of exchange. As the activation bar-
rier for exchange falls in line with the expected bond
strength of a B–N dative interaction,[10] the mechanism for
pyrazolyl exchange, presumably involves pyrazolyl bond
dissociation. In this context, electron-withdrawing para-ani-
line substituents render the aniline nitrogen electron-de-
ficient and the nitrogen lone pair is less able to stabilize
three-coordinate boron by conjugation with the empty p-
orbital on boron. The effect is to induce more Lewis-acidic
boron and, hence, more stable boron-pyrazolyl bonds.
Figure 3. Molecular structure of (A) H(pz2AntBu) (L2c), two views
shown. (B) left: Ph2B(pz2AntBu) (2c) with atom labelling and most
hydrogen atoms removed for clarity. right: alternate views with hy-
drogen atoms added. Ellipsoids are shown at the 50% probability
level.
The solution NMR spectra of all BORAZANs [either
Ph2B(pzAnX) (1a–1c) or Ph2B(pz2AnX) (2a–2c)] clearly
show dynamic processes occur such that their solution
structures appear to have higher than the C1 symmetry indi-
cated from either solid state structural or theoretical studies
(vide infra). For instance, only one set of resonances for
phenyl ring hydrogen atoms is observed for all the
BORAZANs even though, based on the solid-state struc-
tures, two sets are expected (one set each for “axial” and
“equatorial” phenyls). We previously attributed this obser-
vation to a low energy ring-flipping process, that was fast
on the NMR time-scale even at –80 °C in [D8]toluene, a
process we still favour after considering the results of vari-
able temperature NMR studies on the Ph2B(pz2AnX) series
of compounds which showed only one set of phenyl hydro-
Figure 4. The 4-pyrazolyl region of the 1H NMR spectra for a [D8]-
toluene solution of Ph2B(pz2AntBu) acquired at various tempera-
tures.
Density functional calculations (B3LYP/6-31G*,
gen resonances at –80 °C. With the Ph2B(pz2AnX) series (X SPARTAN06)[12] were performed on the free ligands
= CF3, Cl, tBu), variable temperature studies also indicate H(pzAntBu) (L1c), H(pz2AnX) (L2a–L2c), and the boron
that exchange occurs rendering the boron-bound and “free” derivatives, Ph2B(pznAnX) (n = 1, 2; X = CF3, Cl, tBu),
pyrazolyl rings inequivalent at low temperatures but equiva- 1a–1c and 2a–2c, respectively) using ab initio (HF/321-G)
lent at higher temperatures. As a representative example, a energy-minimized structures in an effort to rationalize the
1
portion of the H NMR spectra of Ph2B(pz2AntBu) (2c) in rather unexpected electronic properties of the BORAZAN
the region showing the resonance(s) for the hydrogen(s) at dyes. The geometry optimization showed good correlation
the 4-position of the pyrazolyl at various temperatures is with the solid-state structures in both bond lengths and
shown in Figure 4 (X = tBu). More complete 1H NMR angles, especially showing chelate ring-puckering as found
106
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 104–110

Performance Enhancement of BORAZAN Fluorescent Dyes
Table 1. Activation parameters for pyrazolyl exchange in the H2N-aryl moiety about C–N bond (or other distortions)
Ph2B(pz2AnX) (X = CF3, Cl, tBu).
change the photophysics of the molecule by affording more
Compound
Tc [K] ∆ν [Hz][a] kc [s–1 [b]
]
∆G‡ [kcal/mol][c]
nonbonding character to the nitrogen lone pair. The
LUMO of each compound is π* in character and spans the
pi-systems of both the aniline and the pyrazolyl rings with
only a small contribution from the conjugated p-orbital of
the aniline nitrogen. For the H(pznAnX) (n = 1,2) ligands,
the HOMO(–4) to HOMO(–1) orbitals are π-bonding, the
virtual orbitals LUMO (+1) to LUMO(+3) are π* anti-
bonding, while the LUMO(+4) and higher are σ* anti-
bonding. In the Ph2B(pznAnX) (n = 1,2), orbital contri-
butions from the diphenylboron moiety distinguish the
BORAZANs from the ligands. The HOMO(–1) to
HOMO(–4), are essentially four linear combinations of bo-
ron-phenyl π-orbitals; the next-lowest pzAn-based πL-or-
bital is HOMO(–5). The corresponding four π* orbitals in-
clude contributions from both the pzAn and diphenylboryl
moieties and constitute the virtual orbitals LUMO(+1) to
LUMO(+5).
The relative energies of the frontier orbitals for the twelve
compounds in the bottom of Figure 5 (and Supporting In-
formation) reveals a number of trends that correlate well
with experimental data. When comparing the relative en-
ergy of the HOMO of a given H(pzAnX) ligand with that
for its corresponding BORAZAN (Figure 5), the latter is
destabilized owing to an antibonding pi-interaction be-
tween the boron-bound carbon atoms and the p-orbital of
the aniline nitrogen, which is not present in the former.
There is a stabilization of the HOMO with an increase in
electron-withdrawing character of the para substituent of
the aniline, rendering the aniline a poorer donor, as ex-
pected. Contrary to initial expectations based solely on in-
ductive effects, substitution of the ortho hydrogen with a
pyrazolyl results in a small stabilization of the HOMO that
arises due to increased conjugation with the pi-system of
the new pyrazolyl group. This increase in conjugation for
the dipyrazolyl system causes a significant stabilization of
the LUMO, which results in an overall smaller HOMO/
LUMO energy gap compared to the monopyrazolyl sys-
tems. As with the monopyrazolyl systems the HOMO is
destabilized to a greater extent than the LUMO is stabilized
on changing para-aniline substituents, which provides an
additional basis for tuning the electronic properties of the
BORAZAN dyes.
Ph2B(pz2AnCF3
)
343
318
303
132
152
232
586.46
675.32
1030.75
15.8
14.5
13.6
Ph2B(pz2AnCl
)
Ph2B(pz2AntBu
)
[a] Chemical shift difference in the absence of exchange. [b] Rate
constant at coalescence temperature kc = π∆ν(2)–1/2. [c] ∆G‡
4.57(Tc)[10.32 + log(Tc/kc)] as in reference.[11]
=
in the solid state. A representative set of frontier orbitals
(HOMO and LUMO) for 2a is given in the top of Figure 5
while a comparison of energy levels of a more extensive set
of frontier orbitals [LUMO(+4) to HOMO(–4)] for all the
ligands, H(pznAnX) (n = 1, 2; X = CF3, Cl, tBu) and their
corresponding BORAZANs Ph2B(pznAnX) (n = 1, 2) are
given in in the bottom of Figure 5. Complete molecular or-
bital diagrams are provided in the Supporting Information.
For all, the HOMO is mainly the nonbonding representa-
tion of the aniline-centred pi-system, encompassing the ni-
trogen-centered lone pair of the aniline.
Figure 5. Top: HOMO and LUMO for representative complex 2a.
Bottom: comparison of the energy levels of frontier orbitals
LUMO(+4) to HOMO(–4) for H(pznAnX) (n = 1, 2) and corre-
sponding BORAZANs Ph2B(pznAnX) (n = 1, 2) where X = CF3,
Cl, and tBu (left to right) for each compound type from density
functional calculations (B3LYP/6-31G*). Vertical scale represents
relative energy. Gas-phase HOMO–LUMO gap energy is ∆E.
Electrochemistry
As aniline derivatives are well known electron donors,[14]
There is also a significant (pi-antibonding) contribution and BORAZAN derivatives were previously found to be
from the orbitals of the para-aniline substituent to the electroactive,[5] the electrochemistry of CH3CN solutions of
HOMO. Following the convention established by from the the new ligands and BORAZANs were examined by cyclic
seminal work of Kasha and Rawls on the photophysics of voltammetry. The electrochemical data are collected in
aniline derivatives,[13] it is convenient to refer to the HOMO Table 2 while voltammograms for the ligands and com-
(and other frontier orbitals containing significant contri- plexes are found in the Supporting Information. Each of
butions from the conjugated aniline lone pair) as a πL (pi- the compounds exhibited an irreversible oxidation in the
lone-pair) to provide a distinction from a pure π orbital. range of 0.7–1.4 V (vs. Ag/AgCl), where the reported poten-
Deviations from aniline nonplanarity such as twisting of tials are those for the anodic wave observed at a scan rate
Eur. J. Inorg. Chem. 2009, 104–110
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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107

T. J. Morin, S. V. Lindeman, J. R. Gardinier
Table 2. Electronic properties of various pyrazolylaniline ligands and diphenylboryl complexes.
FULL PAPER
Compound
E1/2 (V vs. Ag/AgCl)[a]
Oxidation[b] Reduction[c]
Absorption[d]
Emission[d]
λmax (nm)
Ref.
em
λmax (ε, –1 cm–1), CH2Cl2
ΦF (C7H8, CH3CN)
[2]
[2]
[e]
[e]
[e]
[e]
[2]
[e]
[e]
[e]
[e]
[e]
H(pzAnCF3), L1a
H(pzAnCl), L1b
1.23
–
–
–
234 (21,000), 259 (8,600), 305 (3,700)
233 (24,000), 255 (9,600), 317 (4,700)
234 (17,825), 251sh (7,252), 305 (3,755)
237 (24,971), 259sh (7,135), 310 (5,049)
240 (24,814), 263sh (5,109), 329 (4,369)
229 (26,464), 262sh (6,935), 315 (5,433)
248 (31,000), 288 (13,000), 358 (4,700)
249 (32,000), 289 (7,800), 375 (6,600)
245 (32,251), 371 (4,853)
–
–
–
–
–
–
–
–
–
–
–
1.01
0.88
1.41
1.24
0.99
1.04
0.84
0.74
1.13
1.10
0.86
H(pzAntBu), L1c
H(pz2AnCF3), L2a
H(pz2AnCl), L2b
H(pz2AntBu), L2c
Ph2B(pzAnCF3), 1a
Ph2B(pzAnCl), 1b
Ph2B(pzAntBu), 1c
Ph2B(pz2AnCF3), 2a
Ph2B(pz2AnCl), 2b
Ph2B(pz2AntBu), 2c
–2.60
–2.50
–
–2.39
–2.41
–2.64
–2.43
–2.58
–2.65
–
468
481
495
474
493
502
0.66, 0.49
0.44, 0.23
0.33, 0.03
0.75, 0.55
0.53, 0.31
0.46, 0.16
252 (30,016), 292 (6,500), 372 (6,359)
251 (26,768); 289sh (3,528); 388 (5,460)
247 (28,505), 380 (5,706)
[a] Scan rate of 100 mV/s in CH3CN with NBu4(PF)6 as supporting electrolyte. [b] Anodic peak potential. [c] Cathodic peak potential.
[d] 295 K, CH2Cl2. [e] This work.
of 0.100 V/s; the cathodic wave is either absent or noticeably to the high-energy band),and a low-energy, low-intensity
less intense than expected. For each series of ligands and band for the πL–π* (HOMO–LUMO) transition above
BORAZANs, the oxidation becomes more favourable with about 300 nm (ε ≈ 3000). As indicated from the calculations,
increasing electron donating character of the para-aniline the energies of all the absorption bands of the dipyrazolyl
substituent, in accord with previous findings and with the derivatives are red-shifted with respect to the monopyrazo-
calculations that showed a destabilization of the HOMO for lyl derivatives. With the BORAZANs, each band undergoes
such substitution. Thus, for the BPh2(pz2AnX), the oxi- both hyper- and bathochromic shifts and a new high energy
dation potentials increase along the series X = tBu (Epa
0.86 V) Ͻ X = Cl (Epa = 1.10 V) Ͻ X = CF3 (Epa = 1.13 ron-phenyl groups) appears as a shoulder near 200 nm.
V). Also in agreement with calculations and earlier results, The ligands are not emissive under irradiation with UV-
=
band (presumably for the π–π* transitions involving the bo-
the oxidations of the BORAZAN complexes are more light but the BORAZANs exhibit intense emission (either
favourable than those for the free ligands because there is a in the solid state or in hydrocarbon or halocarbon solution)
destabilization of the HOMO brought about by antibond- that varies from blue for derivatives with electron-with-
ing interactions with the σ-orbitals of the boron-bound car- drawing trifluoromethyl para-aniline substituents to green
bon atoms. The dipyrazolyl derivatives (both free ligands for the tert-butyl derivatives (Figure 6). Previous excited-
and BORAZAN complexes) are more difficult to oxidize state lifetime measurements of Ph2B(pzAnX) (X = CN, CF3,
than the monopyrazolyl analogues. Thus, the oxidation po- CO2Et, Cl, Me, OMe) established the fluorescent nature
tentials for the series Ph2B(pz2AnX) [X = CF3 (1.13 V), Cl (ns lifetimes) of emission. As with the previously reported
(1.10 V), tBu (0.86 V)] are higher than the corresponding monopyrazolyl derivatives, the emission of the di-pyrazolyl
potentials for the Ph2B(pzAnX) series [X = CF3, (1.04 V), derivatives Ph2B(pz2AnX) (X = CF3, Cl, tBu) exhibit a reg-
Cl (0.84 V), tBu (0.74 V)]. While this result is surprising on ular red-shift of emission with increasing electron-donating
first inspection considering the expected inductive effects of character of the para-aniline substituent. Thus, the emission
em
replacing hydrogen with a more electron-donating pyrazo- maximum of Ph2B(pz2AnX) occurs at λmax 474, 493, and
lyl, this trend was correctly predicted by the calculations, 502 nm for X = CF3 (2a), Cl (2b), and tBu (2c), respectively.
which showed that the origin is due to stabilization of the The quantum yields of emission diminish regularly along
HOMO via conjugation with the pi-orbitals of the second the series X = CF3, Cl, tBu (0.75, 0.53, 0.46 respectively) in
pyrazolyl.
accord with the energy gap law which delineates that quan-
tum yields for emission will decrease with lower energy
emission.[15] After considering the implications of the en-
ergy gap law, there is a remarkable improvement in the
Electronic Spectra
The electronic (absorption/emission) spectra of the newly quantum yields of emission of 2a–2c vs. the corresponding
prepared H(pznAnX) ligands and Ph2B(pznAnX) (n = 1,2; X monopyrazolyl derivatives 1a–1c. That is, despite the fact
= CF3, Cl, tBu) compounds parallel those of the previously that di-pyrazolyl derivatives 2a–2c exhibit red-shifted emis-
reported monopyrazolyl (n = 1; X = CF3, Cl) derivatives. sion compared to 1a–1c (Supporting Information), the for-
A summary of the electronic properties are collected in mer enjoy a 10–20% increase in fluorescence quantum yield
Table 2 and complete spectra are found in the Supporting (Table 2) with respect to the latter. We tentatively attribute
Information. The electronic absorption spectrum of each the improvement in quantum yield to the kinetic stabiliza-
H(pznAnX) (n = 1,2) ligand consists of three bands for π- tion of the dye framework brought about by the additional
π* transitions; one high-intensity, high-energy band at ca. pyrazolyl (increasing the amount of chelated boron) which
230 nm (ε ≈ 20,000), a second less intense band at ca. 250 is supported by the observation that the Stokes shift
(ε ≈ 7000) nm (in some cases this band occurs as a shoulder (6000Ϯ500 cm–1) in toluene is slightly smaller for the di-
108
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Eur. J. Inorg. Chem. 2009, 104–110

Performance Enhancement of BORAZAN Fluorescent Dyes
pyrazolyl derivatives compared to that observed for the brought about by the additional pyrazolyl group that could
monopyrazolyl derivatives (Stokes shift 6400Ϯ500 cm–1). favour an increased rate of boron-pyrazolyl bond formation
Moreover, the fluorescent quantum yields for 2a–2c in should dissociation occur. In fact, such dissociation was
CH3CN are lower than in toluene but are higher than for easily detected by variable-temperature NMR spectroscopic
analogous 1a–1c in the Lewis-base solvent. As with the pre- studies of toluene solutions of C1-symmetric 2a–2c since the
viously reported derivatives, the efficacy of fluorescence resonances for “free” and boron-bound pyrazolyl groups
quenching of 2a–2c in acetonitrile most greatly affects de- undergo exchange. The rates of boron-pyrazolyl dissoci-
rivatives with electron-rich para-aniline substituents [with ation decrease (hence, the stability of the chelate ring in-
weaker (or less inert) boron-pyrazolyl dative bonds]. Aceto- creases) in the order tBu Ͼ Cl Ͼ CF3, an order that may
nitrile could be envisioned to coordinate the Lewis-acidic be indicative of the anticipated (in)ability of the aniline to
boron centers in the dyes, compromising the integrity of the stabilize a three-coordinate boron by conjugation between
chelate ring and destroying the chromophore. The bis(pyr- the aniline lone pair and the empty p-orbital on boron. We
azolyl)aniline ligand scaffolds appear to offer a greater re- are further probing the reaction chemistry of these new li-
sistance to such a degenerative process compared to mono- gands towards transition metals and for the construction of
(pyrazolyl)aniline derivatives.
supramolecular assemblies.
CCDC-698323 (for Ph2B(pzAntBu) 1c), -698324 (for H(pz2AntBu
)
L2c), -698325 (for Ph2B(pz2AnCF3) 2a), -698326 (for Ph2B(pz2AnCl)
2b), -698327 (for Ph2B(pz2AntBu) 2c), and -698328 (for (Ph2B)2-
(pz2AntBu) 3c) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Supporting Information (see also the footnote on the first page of
this article): Complete experimental details. Details of X-ray crys-
tallographic studies, tables of X-ray data, molecular and supra-
molecular structural discussion, NMR spectra, cyclic voltammog-
rams, UV/Vis and emission spectra, details of computational stud-
ies, comparison of results from different basis sets, frontier orbital
diagrams.
Figure 6. Overlay of normalized emission spectra of Ph2B(pz2AnX)
where (X = CF3, 2a left beaded line; X = Cl, 2b centre dashed line;
X = tBu, 2c right solid line).
Acknowledgments
J. R. G. thanks Marquette University, the Petroleum Research
Fund (74371-G), and the National Science Foundation (NSF)
(CHE-0521323) for support.
Conclusions
Three examples of 2,6-dipyrazolylanilines H(pznAnX) (X
= CF3, Cl, tBu) have been prepared by exploiting copper-
catalyzed amination reactions between pyrazole and 2,6-di-
bromoaniline. The reaction chemistry of these derivatives
with triphenylboron afforded Ph2B(pz2AnX) [X = CF3 (2a),
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Received: August 12, 2008
Published Online: November 25, 2008
[9] It was possible to isolate a few crystals of a side product (Ph2B)2-
(µ-pz2AntBu) 3c (see structure in the Supporting Information).
110
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