Selective bromination of perylene diimides under mild conditions

Article abstract of DOI:10.1021/jo070367n

(Chemical Equation Presented) A novel method for the bromination of perylene diimides, PDI (1), under mild conditions is reported. Variation of the reaction conditions allows mono- and dibromination of PDIs to afford 2 and 3 (these can be separated throug

Full text of DOI:10.1021/jo070367n

Selective Bromination of Perylene Diimides under Mild Conditions
†
‡
†
‡
Paramasivan Rajasingh, Revital Cohen, Elijah Shirman, Linda J. W. Shimon, and
,
†
Boris Rybtchinski*
Department of Organic Chemistry and Chemical Research Support Unit, Weizmann Institute of Science,
RehoVot 76100, Israel
boris.rybtchinski@weizmann.ac.il
ReceiVed February 22, 2007
A novel method for the bromination of perylene diimides, PDI (1), under mild conditions is reported.
Variation of the reaction conditions allows mono- and dibromination of PDIs to afford 2 and 3 (these
can be separated through standard procedures) or exclusive dibromination to afford 3. Pure 1,7 regioisomers
are obtained through repetitive crystallization. The structure of 1,7-3b was elucidated by a single-crystal
X-ray analysis. The facility of the bromination reaction, which decreases in the order 1a > 1b > 1c,
depends on PDI aggregation propensities. Monobrominated PDIs were utilized for the syntheses of novel
unsymmetrical piperidinyl (4a and 4b) and trimethylsilylethynyl derivatives (5a and 5b). Computational
studies (DFT) on imide substituent rotation in PDIs reveal that in the case of bulky groups there is a
restricted rotation leading to isomers, in agreement with our experimental results. An aromatic core twist
in PDIs bearing one and two bromine substituents was also investigated by DFT.
artificial photosynthetic systems.1
6-23
Importantly, photophysical
Introduction
and redox properties of PDIs can be conveniently modified
through substitution in the aromatic core at the positions 1, 6,
Perylene diimides (PDIs) are outstandingly versatile organic
chromophores.1 They demonstrate exceptional thermal and
photochemical stability, strongly absorb visible light, and show
(7) Jones, B. A.; Ahrens, M. J.; Yoon, M. H.; Facchetti, A.; Marks, T.
2-4
J.; Wasielewski, M. R. Angew. Chem., Int. Ed. 2004, 43, 6363-6366.
high fluorescence quantum yields.
PDIs have been utilized
(8) Yoo, B.; Jung, T.; Basu, D.; Dodabalapur, A.; Jones, B. A.; Facchetti,
1
5-8
9,10
as industrial dyes, electronic materials,
sensors,
photo-
A.; Wasielewski, M. R.; Marks, T. J. Appl. Phys. Lett. 2006, 88, 082104/1.
(9) Zang, L.; Liu, R.; Holman, M. W.; Nguyen, K. T.; Adams, D. M. J.
Am. Chem. Soc. 2002, 124, 10640-10641.
1
1-15
voltaics,
and building blocks for light-harvesting and
(10) Wang, W.; Wan, W.; Zhou, H.-H.; Niu, S.; Li, A. D. Q. J. Am.
*
Author to whom correspondence should be addressed. Phone: 972-8-
9
342172. Fax: 972-8-9344142.
Chem. Soc. 2003, 125, 5248-5249.
†
Department of Organic Chemistry.
Chemical Research Support Unit.
(11) Schmidt-Mende, L.; Fechtenk o¨ tter, A.; M u¨ llen, K.; Moons, E.;
Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-1122.
(12) Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J.; Grimsdale, A. C.;
M u¨ llen, K.; MacKenzie, J. D.; Silva, C.; Friend, R. H. J. Am. Chem. Soc.
2003, 125, 437-443.
‡
(
(
(
(
(
1) Zollinger, H. Color Chemistry, 3rd ed.; VCH: Weinheim, 2003.
2) W u¨ rthner, F. Chem. Commun. 2004, 1564-1579.
3) Langhals, H. HelV. Chim. Acta 2005, 88, 1309-1343.
4) Wasielewski, M. R. J. Org. Chem. 2006, 71, 5051-5066.
5) Struijk, C. W.; Sieval, A. B.; Dakhorst, J. E. J.; van Dijk, M.; Kimkes,
(13) Yakimov, A.; Forrest, S. R. Appl. Phys. Lett. 2002, 80, 1667-1669.
(14) Breeze, A. J.; Salomon, A.; Ginley, D. S.; Gregg, B. A.; Tillmann,
H.; Horhold, H. H. Appl. Phys. Lett. 2002, 81, 3085-3087.
(15) Shin, W. S.; Jeong, H.-H.; Kim, M.-K.; Jin, S.-H.; Kim, M.-R.;
Lee, J.-K.; Leec, J. W.; Gal, Y.-S. J. Mater. Chem. 2006, 16, 384-390.
(16) Qu, J.; Pschirer, N. G.; Liu, D.; Stefan, A.; Schryver, F. C. D.;
M u¨ llen, K. Chem. Eur. J. 2004, 10, 528-537.
P.; Koehorst, R. B. M.; Donker, H.; Schaafsma, T. J.; Picken, S. J.; van de
Craats, A. M.; Warman, J. M.; Zuilhof, H.; Sudh o¨ lter, E. J. R. J. Am. Chem.
Soc. 2000, 122, 11057-11066.
(
6) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14,
9
9-117.
1
0.1021/jo070367n CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/30/2007
J. Org. Chem. 2007, 72, 5973-5979
5973

Rajasingh et al.
SCHEME 1. Bromination of 1a-c
2
26
7
, and 12 (bay region). Substitutions at bay positions and
tyl, ethylpropyl, and cyclohexyl substituents (compounds 1a,
2
7
28
expansion of the PDI core are usually carried out starting from
the halogenated derivatives, particularly brominated PDIs.
1b, and 1c, respectively, see Scheme 1). The solubility of
the compounds decreases in the order 1a > 1b > 1c, reflecting
the decrease of the imide substituent bulk and the increase of
2
,4
3
These are almost exclusively synthesized through bromination
of perylene dianhydride (PDA) in concentrated H2SO4 upon
heating, followed by imidation with amines.24 Usually this
bromination procedure affords a mixture of di-, tri-, and
tetrabrominated PDIs. The dibromoperylene diimides contain
,7 (major) and 1,6 (minor) regioisomers. Recently, purification
of the 1,7 regioisomer by repetitive recrystalization has been
reported. Among the brominated perylene diimides, 1,7-
dibrominated PDIs are the most widely used as starting materials
for a broad variety of PDI derivatives.2,4
Herein we report on PDI bromination methodology that
employs mild conditions (organic solvent, room temperature)
resulting in facile formation of mono- and dibrominated perylene
diimides. The selectivity of the reaction toward exclusive
dibromination can be controlled through variation of the reaction
conditions. To the best of our knowledge, our method is the
first reported procedure allowing exclusive dibromination of
PDIs. Computational studies on the imide group rotation and
PDI aromatic core twist are also reported, providing insight into
the inherent dynamics of PDI molecules.
PDI aggregation propensity, which is controlled by π-π
2-4
stacking.
Notably, compound 1a has been reported to
demonstrate very interesting solid-state properties (strong,
2
6
anisotropic fluorescence, small Stokes shifts); hence, bromi-
nation of 1a would be highly desirable and can lead to novel
photonic materials.
1
2
5
High resolution H and 13C NMR spectra of compound 1a26
(starting material in bromination reaction) suggested that two
very similar isomers in a 1:1 ratio are present, see Figure S1
(Supporting Information). This phenomenon was addressed by
computational studies (see below), revealing that the isomers
are due to restricted rotation (kinetic barrier of ∼32 kcal/mol)
of the bulky dimethylpentyl group around the N-C bond; see
Figure 3 for the structures of the rotational isomers. Restricted
1
rotation of bulky PDI imide substituents (2,5-di-tert-butylphe-
nyls) leading to two isomers has been reported.2
9-31
All
dimethylpentyl PDI derivatives described below contain two
rotational isomers in 1:1 ratio. Clear observation of the two
isomers in the NMR spectra of dimethylpentyl PDIs was
possible only using 500 MHz NMR.
In order to accomplish bromination, dimethylpentyl PDI,
compound 1a, was stirred with excess of bromine (68 equiv)
in dichloromethane at 22-24 °C for 2 days (Scheme 1),
resulting in dibrominated PDI 3a (26%), monobrominated PDI
2a (57%), and unreacted starting material 1a (15%). Compounds
2a and 3a were separated by column chromatography. They
were characterized by multinuclear NMR spectroscopy, mass
spectroscopy, elemental analysis, UV-vis and fluorescence
spectroscopy, and electrochemistry; see Table 1. Mass spec-
troscopy and elemental analysis of 2a indicated that it is a
monobrominated derivative, while NMR spectra showed that
two very similar isomers in a 1:1 ratio were present (both of
them corresponded to the PDI brominated in the bay region)
because of restricted rotation of the bulky dimethylpentyl group
Results and Discussion
Our approach to bromination of PDIs was based on the
following observation. When we performed literature procedures
to obtain PDI derivatives functionalized in the bay region, the
reactions appeared to be significantly more facile in the case of
PDIs bearing bulkier imide substituents, apparently because of
the reduced aggregation. Thus, we chose for our studies on
bromination three known PDI derivatives bearing dimethylpen-
2
(
17) W u¨ rthner, F.; You, C. C.; Saha-M o¨ ller, C. R. Chem. Soc. ReV. 2004,
3, 133-146.
18) Sautter, A.; Kaletas, B. K.; Schmid, D. G.; Dobrawa, R.; Zimine,
3
(
M.; Jung, G.; van Stokkum, I. H. M.; De Cola, L.; Williams, R. M.;
W u¨ rthner, F. J. Am. Chem. Soc. 2005, 127, 6719-6729.
1
3
(
19) You, C. C.; Hippius, C.; Grune, M.; W u¨ rthner, F. Chem. Eur. J.
around the N-C bond. Notably, in the C NMR spectrum of
2
006, 12, 7510-7519.
20) Li, X. Y.; Sinks, L. E.; Rybtchinski, B.; Wasielewski, M. R. J. Am.
Chem. Soc. 2004, 126, 10810-10811.
21) Addicott, C.; Oesterling, I.; Yamamoto, T.; M u¨ llen, K.; Stang, P.
J. J. Org. Chem. 2005, 70, 797-801.
22) R o¨ ger, C.; M u¨ ller, M. G.; Lysetska, M.; Miloslavina, Y.; Holzwarth,
A. R.; W u¨ rthner, F. J. Am. Chem. Soc. 2006, 128, 6542-6543.
23) Rybtchinski, B.; Sinks, L. E.; Wasielewski, M. R. J. Am. Chem.
Soc. 2004, 126, 12268-12269.
24) B o¨ hm, A.; Arms, H.; Henning, G.; Blaschka, P. In Ger. Offen.;
BASF A.-G., Germany). DE 19547209, 1997.
25) W u¨ rthner, F.; Stepanenko, V.; Chen, Z.; Saha-M o¨ ller, C. R.; Kocher,
N.; Stalke, D. J. Org. Chem. 2004, 69, 7933-7939.
2
a the carbonyl groups give rise to eight signals of equal
(
(
(26) Langhals, H.; Krotz, O.; Polborn, K.; Mayer, P. Angew. Chem., Int.
Ed. 2005, 44, 2427-2428.
(
(27) Demmig, S.; Langhals, H. Chem. Ber. 1988, 121, 225-30.
(28) Locklin, J.; Li, D.; Mannsfeld, S. C. B.; Borkent, E.-J.; Meng, H.;
Advincula, R.; Bao, Z. Chem. Mater. 2005, 17, 3366-3374.
(29) Langhals, H. Chem. Ber. 1985, 118, 4641-4645.
(30) Ebeid, E. Z. M.; El-Daly, S. A.; Langhals, H. J. Phys. Chem. 1988,
92, 4565-4568.
(
(
(
(
(31) Prathapan, S.; Yang, S. I.; Seth, J.; Miller, M. A.; Bocian, D. F.;
Holten, D.; Lindsey, J. S. J. Phys. Chem. B 2001, 105, 8237-8248.
5974 J. Org. Chem., Vol. 72, No. 16, 2007

SelectiVe Bromination of Perylene Diimides
TABLE 1. Absorption and Emission Properties of Compounds
2
a,b-5a,b in Chloroform, and Redox Potentials in Dichloromethane
a
(in V vs SCE)
absorption
λmax (ꢀ)
emission
λem (λex)
b
c
b
b
1E1/2RED 2E1/2RED E1/2OX
compound
Φ
2
2
1
1
4
4
5
5
a
b
,7-3a
,7-3b
a
b
a
b
524 (74669) 538 (488) 0.92 -0.59
524 (61578) 537 (488) 0.88 -0.59
526 (49825) 545 (490) 0.82 -0.52
526 (45474) 545 (490) 0.81 -0.54
602 (18481) 727 (600) 0.20 -0.66
601 (18009) 726 (600) 0.19 -0.74
537 (66690) 547 (493) 0.76 -0.61
537 (60348) 546 (493) 0.76 -0.60
-0.80
-0.79
-0.73
-0.76
-0.86
-0.93
-0.81
-0.80
1.06
1.06
FIGURE 1. ORTEP drawing of structure of 1,7-3b (left, 50%
probability, hydrogens are omitted for clarity) and view along the N-N
axis showing twisted perylene backbone (right, 50% probability, alkyl
groups at imide nitrogens and all hydrogens are omitted for clarity).
There is a minor disorder in one of the ethylpropyl groups. Selected
bond lengths (Å): Br1-C1, 1.905(4); C1-C2, 1.402(5); C2-C3,
a
All measurements were performed at room temperature, oxidation
2
potentials of 2, 3, and 5 are high as typical of PDI systems, beyond the
limits allowed by our solvent/electrolyte system. In nm. In M cm
b
c
-1
-1
.
1
.375(6); C3-C13, 1.486(5); C13-N1, 1.395(6); C13-O2, 1.225(6);
C3-C20, 1.406(6); C20-C21, 1.423(5); C21-C22, 1.434(5); C22-
C1, 1.403(5); C22-C34, 1.468(5).
intensity (Figure S2) instead of four signals; the latter would
be expected for a compound with facile rotation of the imide
substituents.
SCHEME 2. Monosubstituted PDI Derivatives
1
The 500 MHz H NMR spectra of 3a showed the presence
of 1,7-3a and 1,6-3a regioisomers25 (in a 5:1 ratio, respectively,
Figure S3), which could not be separated using column
chromatography. The major 1,7-3a regioisomer can be purified
by repetitive recrystallization from a dichloromethane/hexane
mixture (v/v, 1:1), similar to the procedure reported by W u¨ rthner
25
et al. The recrystallization process was monitored by 500 MHz
1
H NMR spectroscopy (Figure S3). After three successive
recrystallizations, the pure regioisomer 1,7-3a was obtained as
1
13
indicated by the H (Figure S3b) and C NMR spectra (Figure
S4b). In the mother liquor, the isomer 1,6-3a is a major species
(
Figures S3 and S4).
Interestingly, exclusive dibromination can be also achieved:
dicyclohexyl-1,7-dibromo PDI.25 Overall, the structure of 1,7-
reflux of a dichloromethane solution of 1a with bromine (68
equiv) for 24 h results in exclusive formation of dibrominated
compound 3a (yield 92%, 1,7 to 1,6 isomer ratio 3:1). The
regioisomers can be separated using recrystallization as de-
scribed above.
3b is similar to the reported crystal structure of N,N′-dicyclohexyl-
2
5
1,7-dibromo PDI.
2
8
Compound 1c does not undergo bromination at room
temperature. It was brominated upon reflux in dichloromethane
for 4 days to give exclusiVely dibrominated derivative 3c (85%
yield, mixture of 1,7 and 1,6 regioisomers in 4:1 ratio). The
regioisomer separation procedure for 3c has been previously
Bromination of ethylpropyl PDI 1b27 (Scheme 1) was
performed using conditions similar to those employed for 1a.
The monobromo PDI (2b) and dibromo PDI (3b) were separated
by column chromatography, and pure 1,7-3b regioisomer (Figure
S5) was obtained after three repetitive recrystallizations from a
dichloromethane/hexane mixture (v/v, 1:1). Compounds 2b and
2
5
reported. Harsher conditions had to be employed for bromi-
nation of 1c due to its higher aggregation propensity and lower
3
solubility in comparison with 1a and 1b. Under these condi-
tions, the monobrominated derivative most probably reacts
further with bromine, resulting in exclusive dibromination.
We observe that the facility of the bromination reactions (as
exemplified by the reaction times and temperatures) decreases
in the order 1a > 1b > 1c. This trend supports the notion that
the reactivity of PDIs depends on their aggregation (which also
influences their solubility properties). Notably, while bromina-
tion of 1a is very facile at room temperature, compound 1c
cannot be brominated at ambient temperatures.
3
b were fully characterized; see Table 1. Exclusive formation
of dibrominated derivative 3b takes place upon reflux in
dichloromethane with bromine for 2 days (89% yield, 1,7 to
1
,6 isomer ratio 3:1). Our computational studies (see below)
indicate that the rotation of an ethylpropyl group around the
N-C bond occurs with a barrier of 17 kcal/mol. The time scale
of this rotation (∼300 ms) is comparable to the NMR time scale,
in agreement with our experimental observations: at room
temperature no rotational isomers were observed, whereas
certain broadening of NMR signals is observed for 2a and 3a.
While H NMR is well resolved at room temperature, showing
only very slight broadening of several signals, C NMR was
measured at 50 °C to eliminate signal broadening.
While the disubstitution chemistry at 1 and 7 positions of
2
PDIs is well developed, unsymmetrical monosubstitied PDI
1
33-37
derivatives are relatively rare.
As monobrominated PDIs
13
(2a,b) are readily available through our procedure, they were
(32) Selected crystallographic data for 1,7-3b: Crystal system: triclinic,
The structure of 1,7-3b was further elucidated by X-ray
3
2
space group: P-1, unit cell dimensions: a ) 10.5940(2) Å, b ) 11.2600-
crystallography. Single crystals of 1,7-3b suitable for X-ray
analysis were grown from dichloromethane/hexane solution. The
central six-membered ring is twisted (Figure 1) with dihedral
angles of 23.3° and 21° associated with bay area carbon atoms
C1-C22-C34-C12 and C6-C23-C33-C7, respectively,
these values being similar to the core twist angle of 24° in N,N′-
(
3) Å, c ) 14.3580(3) Å, R ) 80.2930(10)°, â ) 68.7930(13)°, γ ) 89.137-
(13)°, V 1571.93(6) Å3, Z ) 2, density (calcd) 1.545 mg/m3, R indices [I
> 2σ(I)]: R1 ) 0.0517, wR2 ) 0.1422, R indices (all data): R1 ) 0.0608,
wR2 ) 0.1477.
(
33) Langhals, H.; Kirner, S. Eur. J. Org. Chem. 2000, 365-380.
(
34) Li, Y.; Li, H.; Li, Y.; Liu, H.; Wang, S.; He, X.; Wang, N.; Zhu,
D. Org. Lett. 2005, 7, 4835-4838.
J. Org. Chem, Vol. 72, No. 16, 2007 5975

Rajasingh et al.
FIGURE 2. Imide substituent rotation in PDIs (1a-c).
FIGURE 4. Core twist in monobromo-PDI, 2a.
2
documented; it is most probably due to an interplay of several
factors that result in similar HOMO-LUMO gaps for the
brominated and nonsubstituted PDIs. Monopiperidinyl PDI
derivatives 4a,b show absorption and emission that are less red-
shifted than those observed for disubstituted piperidinyl deriva-
tives (absorption maximum at 680 nm, emission at 770 nm)38
as expected for lower electron-donating ability of one piperidine
group vs two such groups. The emission quantum yields (0.2)
are identical for 4a,b and the 1,7-dipiperidinyl PDI derivative.38
Compounds 5a,b show slightly red-shifted absorption and
fluorescence, which is apparently due to extended conjugation
involving acetylene groups.
Computational Studies. Imide substituent rotation and PDI
ring twist (in the case of brominated derivatives) represent
essential properties of the systems described above and can lead
to isomer formation. These PDI characteristics are also of
fundamental interest, since they describe the inherent dynamics
of PDI molecules. To gain insight into these phenomena, we
performed DFT studies on imide rotation and ring twist in PDI
systems.
FIGURE 3. Computed structures for rotational isomers and the
transition states for imide group rotation. Hydrogens (except for
NCHR
2
) are omitted for clarity.
DFT calculations at the B3LYP/cc-pVDZ//B3LYP/D95V
level of theory were performed for the PDI derivatives used in
our studies (Figure 2). For each compound we have located
two isomers: exo, where the NCHR2 hydrogen atoms point in
opposite directions, and endo, where they point in the same
direction (Figure 3).
used for further functionalization at the bay position to give
novel unsymmetrical derivatives; see Scheme 2 and Table 1.
The nucleophilic substitution of mono bromoperylene diimide
2a,b) with piperidine3 (Scheme 2) afforded the correspond-
8,39
(
ing monopiperidinyl diimides 4a,b. Sonogashira cross-coupling
afforded ethynyl derivatives 5a,b (Scheme 2).
Photophysical and redox properties of monobrominated PDI
derivatives 2a,b are similar to those of dibrominated 3a,b. The
properties of the latter are very similar to the reported photo-
physical and redox characteristics of 1,7-dibromo-PDIs. The
absorption and emission maxima of mono- and dibrominated
PDIs are almost identical to those of nonsubstituted PDIs,
despite the PDI core twist in the brominated compounds and
electron-withdrawing nature of Br groups. This effect is well
For each isomer couple we have located the transition state
3
for the rotation around the N-C(sp ) bond (Figure 3). The
rotation free energy kinetic barriers around the N-CHR2 bonds
correspond well to the steric bulk of the imide substituents: 12.7
kcal/mol for 2R ) Cy, 17.0 kcal/mol for R ) Et, and 31.6 kcal/
2
i
mol for R ) Pr. These findings show that the rotation around
2
the N-C bond for 2R ) Cy and R ) Et can occur at room
temperature, while the rotation around the N-C bond for R )
i
Pr is restricted because of the larger steric bulk of the
dimethylpentyl group, in agreement with our experimental
(
35) Li, Y.; Li, Y.; Li, J.; Li, C.; Liu, X.; Yuan, M.; Liu, H.; Wang, S.
Chem. Eur. J. 2006, 12, 8378-8385.
36) Li, Y.; Wang, N.; He, X.; Wang, S.; Liu, H.; Li, Y.; Li, X.; Zhuang,
J.; Zhu, D. Tetrahedron 2005, 61, 1563-1569.
37) Xiao, S.; El-Khouly, M. E.; Li, Y.; Gan, Z.; Liu, H.; Jiang, L.; Araki,
observations. The rotation around the N-C bond for R ) Et
(rotation time of ca. 300 ms as estimated from Eyring equation)
(
is somewhat comparable to the NMR time scale, which is in
1
13
(
agreement with slight broadening of the H and C NMR
signals of 2b-5b. In the case of brominated PDI derivatives
the rotational barriers are similar to the nonbrominated mol-
Y.; Ito, O.; Zhu, D. J. Phys. Chem. B 2005, 109, 3658-3667.
(
(
38) Fan, L.; Xu, Y.; Tian, H. Tetrahedron Lett. 2005, 46, 4443-4447.
39) Zhao, Y.; Wasielewski, M. R. Tetrahedron Lett. 1999, 40, 7047-
i
7
050.
ecules. Thus, for monobrominated PDI with R ) Pr, the N-C
5976 J. Org. Chem., Vol. 72, No. 16, 2007

SelectiVe Bromination of Perylene Diimides
core twist. Bulky substituents such as dimethylpentyl show
restricted rotation (barrier of 32 kcal/mol) resulting in observa-
tion of two rotational isomers. Smaller groups, such as cyclo-
hexyl (rotation barrier 12.7 kcal/mol) and ethylpropyl (17 kcal/
mol) show rotation at room temperature, which is more facile
in the case of the less bulky cyclohexyl group. The aromatic
core twist is facile for both mono- and dibromo-PDI derivatives,
the kinetic barrier for the twist being 2 and 6 kcal/mol,
respectively. Thus, while imide substituent rotation can result
in isomers, the PDI core twist is fast at ambient conditions and
should not result in isomer formation, in agreement with our
experimental observations.
Experimental Section
Bromination of N,N′-Bis(2,4-dimethylpent-3-yl)perylene-3,4:
2
6
9
,10-tetracarboxylic Diimide (1a). A mixture of 1a (1 g, 1.7
mmol) and bromine (18.66 g, 0.116 mol) in 60 mL of dichlo-
romethane was stirred at 22-24 °C in a closed round-bottom
flask for 2 days. The excess of bromine was removed by air
bubbling, and the solvent was removed under vacuum. The crude
product was purified by silica gel column chromatography with
chloroform as an eluent. The first band was collected to afford
dibromo perylene diimide (mixture of 1,7-3a and 1,6-3a in 5:1
ratio) as an orange solid (330 mg, 26%). The second band
afforded 1-bromo N,N′-bis(2,4-dimethylpent-3-yl)perylene diimide
FIGURE 5. Core twist in dibromo-PDI, 1,7-3a.
rotation barrier was found to be 32.8 kcal/mol, and for the 1,7-
i
dibrominated PDI with R ) Pr the barrier for rotation is 32.5
kcal/mol.
In order to address the motion associated with the PDI core
twist, we have calculated the structure of the PDI-Br (where
i
R ) Pr) compound (2a, Figure 4). In this case we have found
the PDI skeleton to be twisted, as expected for the congested
2a (650 mg, 57%) as an orange solid. The third band gave the
2
bay region. The free energy barrier for the PDI aromatic core
unreacted perylene diimide 1a (150 mg, 15%). The regioisomers,
1,7-3a and 1,6-3a, could not be separated by column chromato-
graphy.
Regioisomerically pure 1,7-dibromoperylene diimide (1,7-3a)
was obtained by repetitive crystallization. The mixure (330 mg)
of 1,7-3a and 1,6-3a (5:1) was crystallized from 50 mL of
twisting (Figure 4) was found to be rather low, 2 kcal/mol;
therefore, core twist is expected to be facile. Accordingly, no
additional isomers are expected to be observed as a result of
PDI bromination. Similar results were obtained for the PDI
derivative with two Br atoms (compound 3a). The core twist
barrier for this case was found to require a slightly higher energy
of 6 kcal/mol (Figure 5). It should be noted that the aromatic
core structure of the calculated dibromo-PDI (3a) corresponds
well to the X-ray structure (3b), with the similar core twist
2 2
CH Cl :hexane (v/v, 1:1) mixture at room temperature for 5 days.
The crystallization was repeated at the same conditions for
two more times to yield 180 mg (55%) of pure 1,7-3a as an orange
solid.
1
-Bromo-N,N′-bis(2,4-dimethylpent-3-yl)perylene-3,4:9,10-tet-
(dihedral angles associated with the bay area in calculated 3a
1
racarboxylic Diimide (2a). H NMR: δ 9.8 (m, 2H), 8.96 (s, 1H),
.93 (s, 1H), 8.6-8.75 (m, 10H), 4.75 (m, 4H), 2.72 (m, 8H), 1.13
(d, 12H, J ) 6.5 Hz), 1.12 (d, 12H, J ) 7.0 Hz), 0.95 (d, 12H, J
are 21.1° and 21.6°; these angles are 23.3° and 21° in the X-ray
structure of 3b).
8
)
1
1
7.0 Hz), 0.94 (d, 12H, J ) 6.5 Hz). 13C { H} NMR: δ 165.3,
1
64.94, 164.89, 164.13, 164.09, 163.81, 163.77, 162.9, 139.6, 138.9,
34.0, 133.94, 133.91, 133.88, 133.63, 133.60, 133.58, 133.57,
Conclusions
Brominated perylene diimides, typically used as starting
materials in PDI chemistry, are usually synthesized by bromi-
nation of perylene dianhydride upon heating in sulfuric acid,
followed by imidization. Our study has shown that reaction of
PDI with bromine under mild condition results in facile
bromination to afford mono- and dibrominated perylene diim-
ides. The variation of reaction conditions also allows bromi-
nation of PDIs to exclusively afford dibrominated derivatives
in good yields (both 1,7 and 1,6 regioisomers are formed; the
major 1,7 isomer can be purified by recrystallization). To the
best of our knowledge, the known procedures for PDI bromi-
nation do not result in exclusive dibromination, but in mixtures
133.5, 131.6, 131.1, 130.9, 130.4, 128.92, 128.91, 128.72, 128.71,
128.11, 128.09, 127.0, 124.03, 124.02, 123.9, 123.84, 123.78,
123.75, 123.6, 123.5, 123.41, 123.39, 123.3, 123.2, 123.01, 123.00,
1
6
2
λ
22.98, 122.96, 122.9, 122.7, 122.6, 120.97, 120.95, 120.9, 65.41,
5.35, 65.20, 65.19, 29.11, 29.10, 29.06, 29.0, 21.8, 21.7, 20.6,
+
0.51, 20.50. MS 664.50 [M ] (calcd 664.19). UV/vis (CHCl
3
):
-1
-1
max/nm (ꢀ/M cm ) ) 524 (74 669), 488 (48 630), 458 (18 465).
): λmax ) 538 nm, fluorescence quantum yield
) 0.92. Anal. Calcd for C38 37BrN : C, 68.57; H, 5.60; N,
.21. Found: C, 68.43; H, 5.61; N, 4.33.
,7-Dibromo-N,N′-bis(2,4-dimethylpent-3-yl)perylene-3,4:9,-
Fluorescence (CHCl
3
Φ
4
f
H
2 4
O
1
1
10-tetracarboxylic Diimide (1,7-3a). H NMR: δ 9.52 (d, 1H, J
) 8.0 Hz), 9.50 (d, 1H, J ) 8.0 Hz), 8.95 (s, 1H), 8.92 (s, 1H),
8.73 (d, 1H, J ) 8.0 Hz), 8.68 (d, 1H, J ) 8.0 Hz), 4.75 (two
2
of brominated PDI derivatives.
triplets merged t, 2H, J ) 8.5 Hz), 2.72 (m, 4H), 1.12 (d, 12H, J
Readily available monobrominated PDIs (2a and 2b) were
utilized to synthesize novel unsymmetrical derivatives, exem-
plifying the potential chemical diversity that can be achieved.
Since the brominated PDI derivatives can be functionalized
through a wide variety of available methods, our study provides
a convenient synthetic route to mono- and disubstituted PDI
derivatives.
1
)
7.0 Hz), 0.94 (d, 12H, J ) 7.0 Hz). 13C { H} NMR: δ 164.7,
1
1
1
2
64.2, 163.5, 163.0, 138.5, 137.9, 132.90, 132.88, 132.84, 132.77,
32.7, 130.6, 129.9, 129.3, 128.5, 127.1, 123.6, 123.24, 123.22,
23.02, 123.00, 122.64, 122.62, 120.8, 65.44, 65.36, 29.05, 29.02,
1.7, 20.5. MS 742.38 [M ] (calcd 742.10). UV/vis (CHCl
+
3
): λmax
/
-1
-1
nm (ꢀ/M cm ) ) 526 (49 825), 490 (33 702), 463 (13 458).
Fluorescence (CHCl ): λmax ) 545 nm, fluorescence quantum yield
Φ ) 0.82. Anal.Calcd for C H Br N O : C, 61.30; H, 4.87; N,
3
Employing DFT calculations, we have addressed the rotation
of imide substituents around the N-C bond and PDI aromatic
f
38 36
2
2
4
3.76. Found: C, 61.35; H, 4.89; N, 3.80.
J. Org. Chem, Vol. 72, No. 16, 2007 5977

Rajasingh et al.
Bromination of N,N′-Bis(ethylpropyl)perylene-3,4:9,10-tet-
racarboxylic Diimide (1b). A mixture of 1b27 (2 g, 3.77 mmol)
and bromine (31.1 g, 0.194 mol) in 100 mL of dichloromethane
was stirred at 22-24 °C in a closed round-bottom flask for 4 days.
The excess of bromine was removed by air bubbling, and the solvent
was removed under vacuum. The crude product was purified by
silica gel column chromatography with chloroform as an eluent.
The first band was collected to afford dibromoperylene diimide
134.03, 134.01, 133.98, 132.0, 131.5, 131.2, 130.8, 129.4, 129.2,
129.04, 129.02, 128.5, 127.1, 126.10, 126.08, 125.38, 125.36, 125.3,
124.7, 124.0, 123.9, 123.69, 123.67, 123.4, 123.32, 123.26, 123.2,
122.93, 122.91, 122.77, 122.76, 122.69, 122.67, 122.65, 122.6,
122.31, 122.29, 121.72, 121.69, 121.54, 121.51, 121.08, 121.05,
65.2, 65.04, 64.95, 64.9, 53.0, 29.7, 29.14, 29.09, 25.8, 23.7, 21.9,
+
21.84, 21.77, 20.68, 20.66, 20.61, 20.60. MS 669.71 [M ] (calcd
-
1
-1
669.36). UV/vis (CHCl
447 (14 425). Fluorescence (CHCl
quantum yield Φ ) 0.2. Anal. Calcd for C43
H, 7.07; N, 6.27. Found: C, 77.11; H, 7.11; N, 6.08.
3
): λmax/nm (ꢀ/M cm ) ) 602 (18 481),
): λmax ) 727 nm, fluorescence
: C, 77.10;
(
red solid, 650 mg, 25%) as a mixture of regioisomers (1,7-3b:
,6-3b ) 5:1). The second band yielded 1-bromo-N,N′-bis-
ethylpropyl)perylene diimide 2b (1.15 g, 50%) as a red solid. The
3
1
(
f
47 3 4
H N O
third band gave the unreacted perylene diimide (500 mg, 25%).
The regioisomeric dibromoperylene diimides 1,7-3b and 1,6-3b
could not be separated by column chromatography.
Regioisomerically pure 1,7-dibromoperylene diimide (1,7-3b)
was obtained by repetitive crystallization. The mixure (200 mg) of
1-Piperidinyl-N,N′-bis(ethylpropyl)perylene-3,4:9,10-tetracar-
boxylic Diimide (4b). Compound 2b (100 mg, 0.164 mmol) was
reacted with piperidine following the procedure analogous to the
1
synthesis of 4a to afford 4b (97 mg, 96%) as a green solid. H
NMR (400 MHz, CDCl , TMS): δ 9.86 (d, 1H, J ) 8.4 Hz), 8.60
3
1
,7-3b and 1,6-3b (5:1) was crystallized from 50 mL of CH
2
Cl
2
:
(m, 6H), 5.10 (m, 2H), 3.50 (m, 2H), 2.97 (m, 2H), 2.30 (m, 4H),
hexane (v/v, 1:1) mixture at room temperature for 5 days. The
crystallization was repeated at the same conditions for two more
times to yield 80 mg (40%) of pure 1,7-3b as a red solid.
1.82-2.0 (m, 10H), 0.95 (two triplets merged, 12H, J ) 7.5 Hz).
13
C NMR (100 MHz, 323K): δ 164.4, 164.3, 164.2, 164.1, 152.7,
135.8, 134.8, 134.1, 131.4, 131.1, 131.0, 130.72, 130.66, 129.5,
129.1, 128.7, 128.4, 127.2, 125.8, 125.5, 124.91, 124.87, 123.9,
123.6, 123.4, 123.3, 123.2, 123.0, 121.7, 121.5, 121.3, 121.2, 57.8,
57.6, 53.0, 29.5, 25.9, 25.1, 23.7, 11.2, 11.1. MS 613.63 (calcd
1
-Bromo-N,N′-bis(ethylpropyl)perylene-3,4:9,10-tetracarbox-
1
ylic Diimide (2b). H NMR: δ 9.78 (d, 1H, J ) 8.5 Hz), 8.92 (s,
1
H), 8.69 (m, 3H), 8.62 (d, 1H, J ) 8.5 Hz), 8.61 (d, 1H, J ) 8.0
-
1
-1
Hz), 5.07 (m, 2H), 2.27 (m, 4H), 1.95 (m, 4H), 0.94 (t, 6H, J )
613.29). UV/vis (CHCl
447 (14 076). Fluorescence (CHCl
quantum yield Φ ) 0.19. Anal. Calcd for C39
H, 6.40; N, 6.85. Found: C, 76.43; H, 6.52; N, 6.59.
3
): λmax/nm (ꢀ/M cm ) ) 601 (18 009),
): λmax ) 726 nm, fluorescence
: C, 76.32;
7
1
1
5
6
4
.5 Hz), 0.93 (t, 6H, J ) 7.5 Hz). 13C { H} NMR (323K): δ 164.1,
63.8, 163.7, 162.9, 139.0, 133.9, 133.6, 133.5, 130.9, 130.4, 129.0,
28.8, 128.2, 128.0, 127.1, 124.0, 123.9, 123.5, 123.2, 122.8, 120.8,
1
3
f
39 3 4
H N O
+
8.1, 57.9, 25.04, 24.99, 11.08, 11.06. MS 608.36 [M ] (calcd
1-Trimethylsilylacetylene-N,N′-bis(2,4-dimethylpent-3-yl)-
perylene-3,4:9,10-tetracarboxylic Diimide (5a). In a glove box
filled with dry nitrogen compound 2a (100 mg, 0.15 mmol), Pd-
(PPh ) (17 mg, 0.015 mmol), trimethylsilyl acetylene (22 mg, 0.22
-
1
-1
08.13). UV/vis (CHCl
88 (40 027), 458 (15 254). Fluorescence (CHCl
) 0.84. Anal. Calcd for C34
: C, 67.00; H, 4.80; N, 4.60. Found: C, 66.81; H, 4.90; N,
3
): λmax/nm (ꢀ/M cm ) ) 524 (61 578),
3
): λmax ) 537
nm, fluorescence quantum yield Φ
BrN
.53.
,7-Dibromo-N,N′-bis(ethylpropyl)perylene-3,4:9,10-tetracar-
f
29
H -
3
4
2
O
4
mmol), and 2 mL of diisopropylamine were mixed in a vial
equipped with a magnetic stirrer, and then CuI (1.5 mg, 0.008
mmol) was added to the mixture. The mixture was allowed to stirr
at room temperature overnight. The solvent was removed under
vacuum, and the resulting mixture was subjected to silica gel
chromatography using chloroform as an eluent to yield 98 mg (96%)
4
1
1
boxylic Diimide (1,7-3b). H NMR: δ 9.50 (d, 2H, J ) 8.0 Hz),
8
1
.9 (s, 2H), 8.7 (d, 2H, J ) 8.0 Hz), 5.06 (m, 2H), 2.25 (m, 4H),
.94 (m, 4H), 0.92 (t, 12H, J ) 7.5 Hz). 13C { H} NMR (323K):
1
1
δ 163.7, 163.1, 138.3, 138.0, 133.0, 132.8, 130.2, 129.9, 129.5,
of 5a as a red solid. H NMR: δ 10.42 (two doublets, 2H, J ) 8.5
1
28.6, 128.5, 127.4, 123.7, 123.3, 120.8, 58.2, 25.2, 11.3, 11.2.
Hz), 8.82 (s, 1H), 8.80 (s, 1H), 8.40 (m, 10H), 4.78 (m, 4H), 2.77
+
-1
MS 686.28 [M ] (calcd 686.04). UV/vis (CHCl
3
): λmax/nm (ꢀ/M
cm ) ) 526 (45 474), 490 (30 652), 458 (12 208). Fluorescence
CHCl ): λmax ) 545 nm, fluorescence quantum yield Φ ) 0.81.
Anal. Calcd for C34 : C, 59.32; H, 4.10; N, 4.07.
(m, 8H), 1.14 (two doublets merged, 24H, J ) 6.5 Hz), 0.96 (two
-
1
13
1
doublets merged, 24H, J ) 6.5 Hz), 0.42 (s, 9H). C { H} NMR:
δ 165.3, 165.1, 164.6, 164.2, 164.00, 163.97, 163.5, 139.4, 138.7,
134.7, 134.6, 134.54, 134.52, 134.33, 134.32, 134.30, 134.28, 134.0,
133.94, 133.90, 133.87, 131.8, 131.53, 131.46, 131.1, 130.80,
130.76, 129.08, 129.07, 128.63, 128.61, 127.23, 127.19, 127.15,
126.7, 124.04, 124.02, 123.83, 123.80, 123.50, 123.45, 123.43,
123.41, 123.21, 123.18, 123.1, 123.03, 123.01, 122.99, 122.88,
122.85, 122.6, 122.5, 121.98, 121.96, 120.0, 119.93, 119.90, 107.21,
107.15, 107.1, 105.9, 65.27, 65.25, 65.19, 65.18, 29.12, 29.10,
29.05, 29.0, 21.80, 21.76, 21.7, 20.58, 20.56, 20.51, 20.49, -0.40.
(
3
f
2 2 4
H28Br N O
Found: C, 59.23; H, 4.16; N, 4.03.
Exclusive Formation of Dibrominated Perylene Diimide
Derivatives. A mixture of perylene diimide 1a (50 mg, 0.085 mmol)
and bromine (0.935 g, 5.8 mmol) in 3 mL of dichloromethane was
heated to 50 °C in a closed vial (equipped with a Teflon liner) for
1
day. The excess of bromine was removed by air bubbling, and
the solvent was removed under vacuum. Silica gel column
chromatography using chloroform as an eluent afforded 3a (1,7-
+
-1
MS 682.47 [M ] (calcd 682.32). UV/vis (CHCl
3
): λmax/nm (ꢀ/M
cm ) ) 537 (66 690), 499 (39 606), 467 (14 050). Fluorescence
(CHCl ): λmax ) 547 nm, fluorescence quantum yield Φ ) 0.76.
Anal. Calcd for C43 Si: C, 75.63; H, 6.79; N, 4.10. Found:
C, 75.99; H, 6.98; N, 3.82.
-
1
3a:1,6-3a ) 3:1) as an orange solid (58 mg, 92%).
An analogous procedure applied to 1b (50 mg, 0.094 mmol) and
c (50 mg, 0.09 mmol) with heating for 2 days and 4 days,
3
f
1
46 2 4
H N O
respectively, afforded 3a (1,7-3b:1,6-3b ) 3:1) as a red solid (57
mg, 89%) and 3c (1,7-3c:1,6-3c ) 4:1) as a red solid (55 mg, 85%).
1-Trimethylsilylacetylene-N,N′-bis(ethylpropyl)perylene-3,4:
9,10-tetracarboxylic Diimide (5b). Compound 5b was obtained
as a red solid in 97% yield using procedure analogous to the one
1-Piperidinyl-N,N′-bis(2,4-dimethylpent-3-yl)perylene-3,4:9,-
1
0-tetracarboxylic Diimide (4a). Compound 2a (100 mg, 0.15
1
mmol) was dissolved in 10 mL of piperidine. The solution was
heated to 60 °C under dry nitrogen for 5 h with stirring. Excess of
piperidine was removed under vacuum, and the residue was
subjected to column chromatography on silica gel using chloroform
used for the synthesis of 5a. H NMR: δ 10.15 (d, 1H, J ) 8.0
Hz), 8. 32 - 8.63 (m, 6H), 5.06 (m, 2H), 2.27 (m, 4H), 1.98 (m,
4H), 0.98 (m, 12H), 0.45 (s, 9H). 13C NMR (323K): δ 164.3, 164.1,
163.6, 138.9, 134.8, 134.7, 134.4, 134.1, 131.3, 131.0, 130.9, 129.4,
128.9, 127.5, 127.39, 127.35, 127.0, 124.2, 124.0, 123.6, 123.4,
1
as an eluent to afford 4a (96 mg, 96%) as a green solid. H NMR:
δ 9.87 (m, 1H), 8.63 (m, 6H), 4.77 (m, 2H), 3.52 (d, 2H, J ) 11.5
123.0, 122.7, 120.2, 107.3, 106.3, 58.2, 58.1, 25.3, 25.2, 11.30,
+
Hz), 2.97 (t, 2H, J ) 11.0 Hz), 2.72 (m, 4H), 1.90 (m, 6H), 1.12
11.26, -0.34. MS 626.70 [M ] (calcd 626.26) (UV/vis (CHCl
3
):
-1
-1
(
two doublets merged, 12H, J ) 6.5 Hz), 0.95 (two doublets
λ
max/nm (ꢀ/M cm ) ) 537 (60 348), 499 (35 877), 467 (12 743),
433 (5 562). Fluorescence (CHCl ): λmax ) 546 nm, fluorescence
quantum yield Φ ) 0.76. Anal. Calcd for C39 Si: C, 74.73;
H, 6.11; N, 4.47. Found: C, 74.45; H, 6.27; N, 4.10.
13
1
merged, 12H, J ) 6.5 Hz). C { H} NMR: δ 165.5, 165.4, 165,
3
3
, 165.2, 164.4, 164.3, 164.2, 164.1, 152.69, 152.86, 152.83, 136.02,
36.00, 135.95, 135.9, 134.87, 134.85, 134.81, 134.79, 134.1,
f
38 2 4
H N O
1
5978 J. Org. Chem., Vol. 72, No. 16, 2007

SelectiVe Bromination of Perylene Diimides
Computational Methods. All calculations were carried out using
Density Functional Theory as implemented in Gaussian 03 pro-
gram.40 Geometry optimizations for minima were carried out using
the standard Schlegel algorithm4 in redundant internal coordinates
(75,302) grid, or when necessary for conversion with the “ultrafine”
grid, i.e., a pruned (99 590) grid.
45,46
The B3LYP
exchange correlation functional (three-parameter
1,42
hybrid density functional method) was employed together with two
until in the neighborhood of the solution and then continued using
basis sets. The first, D95V, is the Huzinaga-Dunning double-ú
analytical second derivatives.43 Optimizations for transition states
basis47 set and the second, cc-pVDZ, is the Dunning correlation-
48
consistent polarized valence double-ú basis set. Geometry opti-
mizations were carried out using the former basis set while the
energetics of the reaction were calculated at these geometries with
the latter basis set, to increase the accuracy of the results. Zero-
point and RRHO (rigid rotor-harmonic oscillator) thermal correc-
tions were obtained from the unscaled computed frequencies.
were carried out with an initial guess for the transition state being
generated from manual manipulation of the geometry using
MOLDEN.44 Geometries were optimized using the default pruned
(40) Gaussian 03; Revision C.02; Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J., J.
A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.;
Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda,
R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski,
V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D.
K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui,
Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople,
J. A. Gaussian, Inc., Wallingford CT, 2004.
Acknowledgment. This work was supported by Israel
Science Foundation and a research grant from Sir Harry
Djanogly, CBE. E.S. acknowledges a doctoral fellowship from
the Feinberg Graduate School of the Weizmann Institute of
Science. B.R. holds the Abraham and Jennie Fialkow Career
Development Chair. We thank Dr. Galina Golubkov, Dr. Haim
Weissman, and Ms. Alona Ustinov for valuable discussions.
Supporting Information Available: X-ray crystallographic data
of 1,7-3b (CIF), xyz coordinates of computed structures, and NMR
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
(
41) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. J. Comput.
JO070367N
Chem. 1996, 17, 49.
(
42) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214.
(43) Stratmann, R. E.; Burant, J. C.; Scuseria, G. E.; Frisch, M. J. J.
(46) Stevens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623.
(47) Dunning, T. H., Jr.; Hay, P. J. Modern Theoretical Chemistry;
Plenum: New York, 1976; Vol. 3.
Chem. Phys. 1997, 106, 10175.
44) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000,
4, 123-129.
45) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(
1
(
(48) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007-1023.
J. Org. Chem, Vol. 72, No. 16, 2007 5979