Structurally tunable 3-cyanoformazanate boron difluoride dyes

Article abstract of DOI:10.1002/chem.201404297

The straightforward synthesis of a series of 3-cyanoformazanate boron difluoride dyes is reported. Phenyl, 4-methoxyphenyl and 4-cyanophenyl N-substituted derivatives were isolated and characterized by single-crystal X-ray crystallography, cyclic voltammetry, and UV/Vis spectroscopy. The compounds were demonstrated to possess tunable, substituent-dependent absorption, emission, and electrochemical properties, which were rationalized through electronic structure calculations. Simple but bright: A novel class of boron difluoride dyes based on 3-cyanoformazanate ligands is reported. The dyes were synthesized by straightforward synthetic methods and shown to possess readily tunable and substituent-dependent absorption, emission, and electrochemical properties. The trends observed in experimental results were confirmed and rationalized by using electronic structure calculations.

Full text of DOI:10.1002/chem.201404297

DOI: 10.1002/chem.201404297
Communication
&
Boron Difluoride Dyes
Structurally Tunable 3-Cyanoformazanate Boron Difluoride Dyes
Stephanie M. Barbon, Pauline A. Reinkeluers, Jacquelyn T. Price, Viktor N. Staroverov, and
Joe B. Gilroy*[a]
BF2 complexes of dipyrrin ligands (BODIPYs) 4 are by far the
best known of these chelates due to their high fluorescence
Abstract: The straightforward synthesis of a series of 3-cy-
quantum yields, exceptional stability, and the ease of optical
anoformazanate boron difluoride dyes is reported. Phenyl,
tuning through structural variation. BODIPYs have been used
4-methoxyphenyl and 4-cyanophenyl N-substituted deriv-
extensively as sensors, organic light-emitting diodes, and fluo-
atives were isolated and characterized by single-crystal X-
rescent imaging agents. For the latter application, fluorescent
ray crystallography, cyclic voltammetry, and UV/Vis spec-
dyes should have high absorption coefficients, high fluores-
troscopy. The compounds were demonstrated to possess
cence quantum yields, large Stokes shifts (SS), tunable emis-
tunable, substituent-dependent absorption, emission, and
sion and absorption spectra, and exceptional stability under
electrochemical properties, which were rationalized
a wide range of conditions.
through electronic structure calculations.
Formazans, which can exist as ‘closed’, ‘open’, or ‘linear’ geo-
metric isomers,[9] 5a–c, have long been used as dyes in the
textile industry[10] and as colorimetric indicators of cell activi-
ty.[11] However, with the exception of a few recent examples,
Boron complexes of chelating nitrogen-donor ligands have
their coordination chemistry remains largely undeveloped.[12,13]
been widely studied as functional materials with structurally
tunable properties.[1,2] The most common of these complexes[3]
Herein, we report the synthesis and characterization of BF2
are produced via the coordination of boron difluoride [BF2]+
complexes of 3-cyanoformazanate ligands and demonstrate
fragments with bidentate, monoanionic nitrogen-donor ligands
such as anilido-pyridines,[4] indigo-N,N’-diarylamines,[5] b-diketi-
minates,[6] and dipyrrins,[7,8] resulting in structures 1–4, respec-
tively. These complexes are generally redox-active, highly ab-
sorbing and emissive, and chemically stable under diverse con-
ditions.
their readily tunable optical and electronic properties and po-
tential utility as functional materials.
3-Cyanoformazans 6a–c, which exist in the ‘open’ form due
to the presence of a linear cyano substituent at the 3-position,
were synthesized according to a published procedure (see Fig-
ure S1 and S2 in the Supporting Information).[9] BF2 complexes
7a–c were produced by reacting 3-cyanoformazans with
excess boron trifluoride diethyl etherate and triethylamine in
refluxing toluene (Scheme 1, Figure S3–S8). It is noteworthy
that the entire synthesis of 7a–c involves only inexpensive and
[a] S. M. Barbon, P. A. Reinkeluers, Dr. J. T. Price, Prof. Dr. V. N. Staroverov,
Prof. Dr. J. B. Gilroy
Department of Chemistry and the Centre for Advanced Materials and Bio-
materials Research (CAMBR)
The University of Western Ontario
1151 Richmond St. N., London, Ontario, N6A 5B7 (Canada)
E-mail: joe.gilroy@uwo.ca
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404297.
Scheme 1. Synthesis of 3-cyanoformazanate BF2 dyes 7a–c.
Chem. Eur. J. 2014, 20, 11340 – 11344
11340
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
commercially available reagents, consists of two straightfor-
ward high-yield steps, and requires only a few hours of hands-
on bench time.
with the boron atom displaced from the N4 plane by 0.143 ꢁ.
The second conformer, conformer B, is shaped like a dragonfly
and has the boron atom displaced from the N4 plane by
0.376 ꢁ. The two conformers were found by electronic struc-
ture calculations to have very close energies (see below).
BF2 dyes 7a–c have diagnostic 11B and 19F NMR spectra, each
exhibiting a 1:2:1 triplet in their 11B NMR spectrum (7a: d=
ꢀ0.8 ppm, 7b: d=ꢀ0.9 ppm, and 7c: d=ꢀ0.7 ppm) and
The aryl substituents of BF2 complexes 7a, 7b, and 7c (con-
former A) are moderately twisted with respect to the formaza-
nate backbone, with the angle between the N4 plane and the
plane defined by the N-aryl substituents ranging from 4.7 to
25.48. For comparison, the same angles in conformer B of 7c
are 36.6–37.68. Similar BF2 complexes of other monoanionic, bi-
dentate N-donor ligands exhibit significantly larger angles be-
tween the N-aryl substituents and the ligand backbones in-
volved. For example, the aryl substituents in anilido-pyridine
BF2 complex 1 (Ar=phenyl) is twisted by 778,[4b] while the aryl
substituents in b-diketiminate BF2 complexes are almost per-
pendicular to the ligand backbone, with twisting of 76–888.[6]
Table 2 summarizes the optical characterization data for BF2
complexes 7a–c collected in THF, CH2Cl2, and toluene solu-
tions. Each complex shows strong, substituent-dependent ab-
sorption of visible light due to p!p* transitions with lmax
values increasing from 502 nm (7a) to 515 nm (7b) to 572 nm
(7c) in toluene (Figure 2, S11–S13). We note that the UV/Vis
and emission spectra of 7b (R=CN) are red-shifted relative to
a
1:1:1:1 quartet in their 19F NMR spectrum (7a: d=
ꢀ133.6 ppm, 7b: d=ꢀ129.6 ppm, and 7c: d=ꢀ135.4 ppm).
The appearance of these signals was accompanied by the loss
1
of the characteristic formazan NH signals in the H NMR spec-
tra. Dyes 7a–c are air- and moisture-stable in the solid state
and in solution. The solid-state structures of BF2 complexes
7a–c were determined by single-crystal X-ray diffraction
(Figure 1, S9, S10, Table 1, S1). The solid-state structures show
Figure 1. Solid-state structures of the two conformers of 7c. Thermal ellip-
soids are shown at 50% probability and hydrogen atoms are removed for
clarity.
Figure 2. UV/Vis absorption (left) and emission (right) spectra of 7a–c record-
ed in 10ꢀ5 m degassed toluene solutions.
that the boron atoms in complexes 7a–c are four-coordinate
and adopt a slightly distorted tetrahedral geometry. Each com-
plex has a delocalized formazanate backbone, with all CꢀN
and NꢀN bonds approximately half way between the standard
single and double bonds for the respective atoms involved.[14]
The boron atoms are displaced slightly from the N4 plane of
the formazanate backbone, by 0.192 ꢁ in 7a and by 0.025 ꢁ in
7b. The unit cell of the crystalline structure of 7c contains two
molecular units in distinct conformations (Figure 1). Conformer
A is almost flat and has a geometry very similar to 7a and 7b,
7a (R=H), somewhat counterintuitively (see below). The 3-cya-
noformazan ligands 6a–c are non-emissive across a broad con-
centration range in THF, CH2Cl2, and toluene. Under similar
conditions, BF2 complexes 7a–c are highly emissive in the visi-
ble region, with lem of 586 nm (F=0.15, SS=84 nm), 598 nm
(F=0.14, SS=83 nm), and 656 nm (F=0.77, SS=84 nm) ob-
served in toluene for 7a–c, respectively. For comparison, a BF2
Table 1. Selected bond lengths [ꢁ] and angles [8] for BF2 complexes 7a–c, determined by X-ray diffraction.
7a (R=H)[a]
7b (R=CN)
7c (R=OMe)
Conformer A
Conformer B
N1–N2, N3–N4
C1–N2, C1–N4
N1–B1, N3–B1
N1-N2-C1, N3-N4-C1
N2-C1-N4
1.2900(15), 1.2948(15)
1.3408(17), 1.3379(17)
1.5748(18), 1.5771(17)
117.14(11), 117.30(11)
129.33(12)
1.3017(15), 1.2945(15)
1.3289(17), 1.3382(17)
1.5714(18), 1.5817(18)
117.28(11), 117.76(11)
129.41(11)
1.307(2), 1.304(2)
1.340(3), 1.335(3)
1.558(3), 1.567(3)
116.75(18), 116.90(18)
130.0(2)
1.306(2), 1.302(2)
1.335(3), 1.340(3)
1.563(3), 1.563(3)
116.21(17), 116.21(17)
129.32(18)
N1-B1-N3
105.55(10)
105.65(10)
106.85(17)
104.75(16)
[a] The second molecule in the unit cell of 7a has a very similar geometry and is not reported here.
Chem. Eur. J. 2014, 20, 11340 – 11344
www.chemeurj.org
11341
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Table 2. Solution characterization data for BF2 formazanate complexes 7a–c.
Solvent
lmax [nm]
e [Mꢀ1 cmꢀ1
]
lem [nm]
Stokes shift
[nm]
Stokes shift
Quantum
yield [F][a]
E8red1 [V][b]
E8red2 [V][b]
[cmꢀ1
]
THF
CH2Cl2
toluene
489
491
502
25400
34600
30400
585
584
586
96
93
84
3356
3243
2855
0.05
0.09
0.15
7a (R=H)
ꢀ0.53
ꢀ1.68
THF
CH2Cl2
toluene
497
499
515
22600
22400
35000
590
589
598
93
90
83
3172
3062
2695
0.12
0.17
0.14
7b (R=CN)
7c (R=OMe)
ꢀ0.21
ꢀ0.68
ꢀ1.25
ꢀ1.82
THF
CH2Cl2
toluene
556
558
572
33400
35300
42700
662
661
656
106
103
84
2880
2793
2239
0.46
0.65
0.77
[a] Quantum yields were measured according to a previously published method[18] using ruthenium tris(bipyridine) hexafluorophosphate as a relative stan-
dard,[19] and corrected for detector non-linearity (Figure S15). [b] Cyclic voltammetry experiments were conducted in acetonitrile containing 1 mm analyte
and 0.1m tetrabutylammonium hexafluorophosphate at a scan rate of 100 mVsꢀ1. All voltammograms were referenced internally against the ferrocene/fer-
rocenium redox couple.
complex of a cyano-substituted pyridomethene ligand closely
related to 7a exhibits a smaller Stokes shift (SS=6 nm) and
a quantum yield of 0.19.[3b]
To rationalize the trends in the UV/Vis spectra and cyclic vol-
tammograms of 7a–c, we calculated the molecular orbitals
and the lowest electronic excitation energies of these com-
pounds with density functional theory (DFT) methods. All cal-
culations were carried out with the Gaussian 09 program[15]
using the M06 functional[16] and the 6-311+G* basis set, in
vacuum and in the presence of toluene as a solvent. The sol-
vent was simulated implicitly using the self-consistent reaction
field (SCRF) method with the polarizable continuum model.
The M06 functional was chosen because it has the best per-
formance in time-dependent DFT calculations of valence excit-
ed states among 24 common density-functional approxima-
tions.[17]
The effect of electron-withdrawing and electron-donating
groups at the N-aryl substituents was investigated by cyclic
voltammetry (Table 2). Each complex exhibited two reversible
reduction waves (Figure 3) but only 7c gave rise to an irrever-
Because the molecules of 7a–c appear to be relatively
planar in the crystalline phase, we first optimized their geome-
tries under the constraint of C2v symmetry, but none of the C2v
structures turned out to be a minimum on the potential
energy surface. Unconstrained geometry optimization yielded
structures of Cs symmetry (shaped like dragonflies) which were
confirmed by vibrational analysis to be true minima. The Cs
structures are more stable than the corresponding C2v struc-
tures by only 3–5 kJmolꢀ1, meaning that the planar conforma-
tions are readily accessible (see Table S2). The gas-phase and
SCRF calculations for C2v and Cs structures led to the same con-
clusions, so we will discuss here only the implicitly solvated
structures, while the gas-phase results are relegated to the
Supporting Information (Table S3).
Figure 3. Cyclic voltammograms of 7a–c recorded at 100 mVsꢀ1 in a 1 mm
acetonitrile solution containing 0.1m tetrabutylammonium hexafluorophos-
phate.
sible oxidation event within the electrochemical window of
acetonitrile (see Figure S14). The reduction potentials followed
the expected trend based on the para-substituent introduced
at the aryl rings. BF2 complex 7a (R=H) is reduced to a radical
anion and then a dianion at ꢀ0.53 V and ꢀ1.68 V against the
ferrocene/ferrocenium redox couple, while 7b (R=CN) was re-
duced at ꢀ0.21 V and ꢀ1.25 V and 7c (R=OMe) was reduced
at ꢀ0.68 V and ꢀ1.82 V. Previous reports of the electrochemis-
try of main-group and transition-metal complexes of formaza-
nate ligands have demonstrated reversible reduction to the
radical anion[12a,c,d] and dianion[13] forms. In this case, BF2 com-
plexes 7a–c also exhibit a second reversible one-electron re-
duction to the dianion.
To understand why the electron-withdrawing CN group red-
shifts (not blue-shifts) the UV absorption band for 7b (R=CN)
relative to 7a (R=H), we note that the HOMO and LUMO of
compounds 7a–c are p-conjugated (see Figure 4, S16–S18). Re-
Figure 4. HOMO and LUMO for 7c (C2v).
Chem. Eur. J. 2014, 20, 11340 – 11344
www.chemeurj.org
11342
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[1] D. Frath, J. Massue, G. Ulrich, R. Ziessel, Angew. Chem. 2014, 126, 2322–
2342; Angew. Chem. Int. Ed. 2014, 53, 2290–2310.
placement of an H atom (7a) with a CꢁN group (7b) increases
the spatial extent of p-conjugation in the molecule and there-
fore lowers the energy of the p!p* transition. The same
effect is observed for monosubstituted benzenes, PhꢀR, where
the energy of the p!p* transition as a function of substituent
also decreases in the order R=H> R=CN> R=OMe (Fig-
ure S19).
[2] For example: a) F. Cheng, F. Jꢂkle, Chem. Commun. 2010, 46, 3717–
3719; b) G. M. Fischer, E. Daltrozzo, A. Zumbusch, Angew. Chem. 2011,
123, 1442–1445; Angew. Chem. Int. Ed. 2011, 50, 1406–1409; c) E. Firin-
ci, J. I. Bates, I. M. Riddlestone, N. Phillips, S. Aldridge, Chem. Commun.
2013, 49, 1509–1511; d) J.-S. Lu, S.-B. Ko, N. R. Walters, Y. Kang, F. Sau-
riol, S. Wang, Angew. Chem. 2013, 125, 4642–4646; Angew. Chem. Int.
Ed. 2013, 52, 4544–4548; e) D. Frath, A. Poirel, G. Ulrich, A. De Nicola, R.
Ziessel, Chem. Commun. 2013, 49, 4908–4910.
[3] For example: a) T. W. Hudnall, F. P. Gabba¨ı, Chem. Commun. 2008, 4596–
4597; b) Y. Kubota, T. Tsuzuki, K. Funabiki, M. Ebihara, M. Matsui, Org.
Lett. 2010, 12, 4010–4013; c) S. Hapuarachchige, G. MontaÇo, C.
Ramesh, D. Rodriguez, L. H. Henson, C. C. Williams, S. Kadavakkollu, D. L.
Johnson, C. B. Shuster, J. B. Arterburn, J. Am. Chem. Soc. 2011, 133,
6780–6790; d) Y. Yang, R. P. Hughes, I. Aprahamian, J. Am. Chem. Soc.
2012, 134, 15221–15224; e) Y. Yang, X. Su, C. N. Carroll, I. Aprahamian,
Chem. Sci. 2012, 3, 610–613; f) W. Li, W. Lin, J. Wang, X. Guan, Org. Lett.
2013, 15, 1768–1771; g) I.-S. Tamgho, A. Hasheminasab, J. T. Engle, V. N.
Nemykin, C. J. Ziegler, J. Am. Chem. Soc. 2014, 136, 5623–
5626.
In contrast to the UV/Vis absorption spectra, the cyclic vol-
tammograms of 7a–c should correlate not with electronic exci-
tation energies but with the electron-acceptor capacity of the
substituents, which is characterized by the LUMO energies. Ex-
perimentally, the peak–current potentials decrease in the order
7b (R=CN)> 7a (R=H)> 7c (R=OMe) (Table 2). This trend is
fully consistent with the calculated LUMO energies of the sub-
stituted complexes (Table 3). Thus, electronic structure calcula-
tions support all experimental observations.
[4] a) Y. Ren, X. Liu, W. Gao, H. Xia, L. Ye, Y. Mu, Eur. J. Inorg.
Chem. 2007, 1808–1814; b) J. F. Araneda, W. E. Piers, B.
Table 3. Lowest electronic excitations and HOMO/LUMO energies of the substituted
BF2 complexes in toluene solution calculated at the M06/6-311+G* level of theory
using the SCRF method.
Heyne, M. Parvez, R. McDonald, Angew. Chem. 2011, 123,
12422–12425; Angew. Chem. Int. Ed. 2011, 50, 12214–
12217.
[5] a) G. Nawn, K. M. Waldie, S. R. Oakley, B. D. Peters, D.
Mandel, B. O. Patrick, R. McDonad, R. G. Hicks, Inorg. Chem.
2011, 50, 9826–9837; b) G. Nawn, S. R. Oakley, M. B. Majew-
ski, R. McDonald, B. O. Patrick, R. G. Hicks, Chem. Sci. 2013,
4, 612–621.
[6] a) F. P. Macedo, C. Gwengo, S. V. Lindeman, M. D. Smith, J. R.
Gardinier, Eur. J. Inorg. Chem. 2008, 3200–3211; b) S. M.
Barbon, V. N. Staroverov, P. D. Boyle, J. B. Gilroy, Dalton
Trans. 2014, 43, 240–250.
[7] For recent reviews: a) A. Loudet, K. Burgess, Chem. Rev.
2007, 107, 4891–4932; b) G. Ulrich, R. Ziessel, A. Harriman,
Angew. Chem. 2008, 120, 1202–1219; Angew. Chem. Int. Ed.
2008, 47, 1184–1201; c) N. Boens, V. Leen, W. Dehaen,
Chem. Soc. Rev. 2012, 41, 1130–1172.
Lowest excitation energy[a] eHOMO eLUMO HOMO–LUMO
[eV]
[nm]
[eV]
[eV]
gap [eV]
7a (R=H)
2.39
518
526
584
ꢀ6.94 ꢀ3.89 3.05
ꢀ7.49 ꢀ4.48 3.01
ꢀ6.34 ꢀ3.63 2.71
C2v structures 7b (R=CN) 2.36
7c (R=OMe) 2.12
7a (R=H)
2.65
468
483
538
ꢀ7.12 ꢀ3.76 3.36
ꢀ7.62 ꢀ4.36 3.26
ꢀ6.47 ꢀ3.54 2.93
Cs structures 7b (R=CN) 2.57
7c (R=OMe) 2.30
[a] Computed using time-dependent DFT. The first excited states of the C2v and Cs
structures have B2 and A’’ symmetry, respectively.
[8] For example: a) T. Lazarides, T. M. McCormick, K. C. Wilson, S.
Lee, D. W. McCamant, R. Eisenberg, J. Am. Chem. Soc. 2010,
132, 350–364; b) O. Altan Bozdemir, S. Erbas-Cakmak, O. O. Ekiz, A.
Dana, E. U. Akkaya, Angew. Chem. 2011, 123, 11099–11104; Angew.
Chem. Int. Ed. 2011, 50, 10907–10912; c) J. C. T. Carlson, L. G. Meimetis,
S. A. Hilderbrand, R. Weissleder, Angew. Chem. 2013, 125, 7055–7058;
Angew. Chem. Int. Ed. 2013, 52, 6917–6920; d) C. Zhang, J. Zhao, S. Wu,
Z. Wang, W. Wu, J. Ma, S. Guo, L. Huang, J. Am. Chem. Soc. 2013, 135,
10566–10578.
In conclusion, we have described the straightforward synthe-
sis of a series of 3-cyanoformazanate BF2 dyes exhibiting varia-
ble redox, absorption, and emission properties, which can be
tuned through simple changes of N-aryl substituents. Based on
these factors, we envision this new class of dyes to be of sig-
nificant interest in the optical imaging and functional materials
fields. Efforts to synthesize water-soluble analogs of 3-cyano-
formazanate dyes and to explore the potential utility of 3-cya-
noformazanate BF2 dyes in materials-based applications are
currently underway.
[9] J. B. Gilroy, P. O. Otieno, M. J. Ferguson, R. McDonald, R. G. Hicks, Inorg.
Chem. 2008, 47, 1279–1286.
[10] For example: a) Y. Gçk, Dyes Pigm. 1989, 11, 101–107; b) M. Szymczyk,
A. El-Shafei, H. S. Freeman, Dyes Pigm. 2007, 72, 8–15.
[11] For example: a) C. J. Goodwin, S. J. Holt, S. Downes, N. J. Marshall, J. Im-
munol. Methods 1995, 179, 95–103; b) W. M. Frederiks, J. van Marle, C.
van Oven, B. Comin-Anduix, M. Cascante, J. Histochem. Cytochem. 2006,
54, 47–52.
Acknowledgements
[12] For example: a) J. B. Gilroy, M. J. Ferguson, R. McDonald, B. O. Patrick,
R. G. Hicks, Chem. Commun. 2007, 126–128; b) J. B. Gilroy, B. O. Patrick,
R. McDonald, R. G. Hicks, Inorg. Chem. 2008, 47, 1287–1294; c) S. Hong,
L. M. R. Hill, A. K. Gupta, B. D. Naab, J. B. Gilroy, R. G. Hicks, C. J. Cramer,
W. B. Tolman, Inorg. Chem. 2009, 48, 4514–4523; d) M.-C. Chang, T.
Dann, D. P. Day, M. Lutz, G. G. Wildgoose, E. Otten, Angew. Chem. 2014,
126, 4202–4206; Angew. Chem. Int. Ed. 2014, 53, 4118–4122.
[13] For a transmetallation route to triaryl formazante BF2 complexes, see:
M.-C. Chang, E. Otten, Chem. Commun. 2014, 50, 7431.
This work was supported by the Natural Sciences and Engi-
neering Research Council of Canada (NSERC) Discovery Grants
(J. B. G. and V. N. S.), Canada Graduate Scholarships program
(S. M. B.), and by The University of Western Ontario. We thank
Prof. Elizabeth R. Gillies for access to instruments in her lab
and Prof. Mark Workentin for helpful discussions.
[14] CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 2012.
[15] Gaussian 09, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Men-
nucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian,
A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara,
Keywords: boron
electrochemistry · emission spectroscopy · X-ray diffraction
·
density functional calculations
·
Chem. Eur. J. 2014, 20, 11340 – 11344
www.chemeurj.org
11343
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Ko-
bayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyen-
gar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B.
Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg,
S. Dapprich, A. D. Daniels, ꢃ, Farkas, J. B. Foresman, J. V. Ortiz, J. Cio-
slowski, D. J. Fox. Gaussian, Inc., Wallingford CT, 2009.
[16] Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 2008, 120, 215–241.
[17] S. S. Leang, F. Zahariev, M. S. Gordon, J. Chem. Phys. 2012, 136, 104101.
[18] S. Fery-Forgues, D. Lavabre, J. Chem. Educ. 1999, 76, 1260–1264.
[19] K. Suzuki, A. Kobayashi, S. Kaneko, K. Takehira, T. Yoshihara, H. Ishida, Y.
Shiina, S. Oishi, S. Tobita, Phys. Chem. Chem. Phys. 2009, 11, 9850–9860.
Received: July 8, 2014
Published online on July 25, 2014
Chem. Eur. J. 2014, 20, 11340 – 11344
www.chemeurj.org
11344
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim