π-Stacking modulates the luminescence of (dppz)Ni(Mes)Br (dppz = dipyrido[3,2-a:2′,3′-c]phenazine, Mes = 2,4,6-trimethylphenyl)

Article abstract of DOI:10.1002/zaac.200700082

The red colour of the novel organonickel complex [(dppz)Ni(Mes)Br] (dppz = dipyrido[3,2-a:2′,3′-c]phenazine, Mes = 2,4,6-trimethylphenyl) originates from long-wavelength MLCT/L′LCT charge transfer bands. However, luminescence in dilute solution comes presumably from the ³π- π* (phenazine) excited state. The red-shifted emission exhibited in concentrated solutions is assigned to dimers. In the solid state emission is quenched. The crystal structure reveals two different types of π-π stacking along the crystallographic a axis.

Full text of DOI:10.1002/zaac.200700082

DOI: 10.1002/zaac.200700082
π-Stacking Modulates the Luminescence of [(dppz)Ni(Mes)Br] (dppz ؍
dipyrido[3,2-a:2Ј,3Ј-c]phenazine, Mes ؍ 2,4,6-trimethylphenyl)
a,
a
a
b
´
Axel Klein *, Natascha Hurkes , Andre Kaiser , and Wolfram Wielandt
a Köln/Germany, Universität zu Köln, Institut für Anorganische Chemie
b Graz/Austria, Karl-Franzens-Universität, Institut für Chemie
Received February 15th, 2007.
Professor Dieter Naumann zum 65. Geburtstag gewidmet
Abstract. The red colour of the novel organonickel complex
[(dppz)Ni(Mes)Br] (dppz dipyrido[3,2-a:2Ј,3Ј-c]phenazine,
Mes ϭ 2,4,6-trimethylphenyl) originates from long-wavelength
MLCT/LЈLCT charge transfer bands. However, luminescence in di-
lute solution comes presumably from the 3π-π* (phenazine) excited
state. The red-shifted emission exhibited in concentrated solutions
is assigned to dimers. In the solid state emission is quenched. The
crystal structure reveals two different types of π-π stacking along
the crystallographic a axis.
ϭ
Keywords: Nickel; N-ligands; Luminescence; Structure elucidation;
Stacking interactions
1 Introduction
Organometallic nickel complexes with α-diimine ligands
like 2,2Ј-bipyridine (bpy), 1,10-phenanthroline (phen) or di-
azabutadienes (R-DAB) have gained an enormous interest
in the last decade. This is mainly due to their success as
effective catalysts in olefin polymerisation or olefin/CO co-
polymerisation, [1] as well as their role in electrocatalytic
CϪC and CϪP coupling reactions [2, 3]. Detailed investi-
gation of photophysical properties of organonickel com-
plexes with α-diimine ligands however have been scarce
[4Ϫ7] until Yamamoto et al. reported and discussed the
optical spectra of some alkyl and aryl complexes with
2,2Ј-bipyridine (bpy) [8]. We have recently synthesized a
number of organonickel complexes [(N∧N)Ni(Mes)Br] or
[(N∧N)Ni(Mes)2] (Mes ϭ 2,4,6-trimethylphenyl ϭ mesityl)
with various diimine ligands and investigated their struc-
tures and reactivity towards ligand exchange reactions [9,
10] using X-ray diffraction and absorption spectroscopy. We
also studied their photophysics and photochemistry [9, 11]
by a combination of multiple spectroscopy and quantum
chemical calculations and recently their electrochemical be-
haviour [3, 12]. These studies have revealed that the lowest
Scheme 1 The complex [(dppz)Ni(Mes)Br] with numbering of the
dppz ligand
unoccupied molecular orbitals were mainly localised in the
diimine π* levels, whereas the highest occupied molecular
orbitals (HOMO) are formed by varying contributions of
nickel(d) orbitals and co-ligand orbitals (aryl and/or hal-
ide). Luminescence from the corresponding 3MLCT/LЈLCT
excited state was not observed.
In view of these results we have extended our studies to
the novel complex [(dppz)Ni(Mes)Br] (dppz ϭ dipyrido[3,2-
a:2Ј,3Ј-c]phenazine, see Scheme 1). Dppz has a number of
very interesting properties. i) It has a largely extended π-
system in contrast to bpy or phen. Recently it was shown
that the use of dppz in catalysts of the type [(N∧N)2Pd]2ϩ
instead of e.g. phen leads to largely accelerated reactivity in
CO/styrene copolymerisation [13]. ii) Dppz possesses three
very close lying lowest unoccupied molecular orbitals
(LUMOs) with very different character (Scheme 2). In the
free ligand b1(phenazine) forms the LUMO, at higher en-
ergy follows the b1(bpy) (SLUMO) and finally the a2(bpy)
orbital (TLUMO). As a consequence of the small energy
differences between these three and the marked differences
in the local electron density distribution [e.g. b1(bpy) attains
far higher electron density on the chelate binding N atoms
than b1(phenazine)] the sequence of these orbitals and thus
the photophysical or redox properties can easily be switched
* Prof. Dr. Axel Klein
Institut für Anorganische Chemie
der Universität zu Köln
Greinstraße 6, D-50939 Köln
Phone: ϩ49 221 4704006; FAX: ϩ49 221 4704899
e-mail: axel.klein@uni-koeln.de
Supporting information for this article is available on the
WWW under http://www.wiley-vch.de/home/zaac or from the
author
Z. Anorg. Allg. Chem. 2007, 633, 1659Ϫ1665
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1659

A. Klein, N. Hurkes, A. Kaiser, W. Wielandt
Scheme 2 Schematic representation of the three lowest unoccup-
ied molecular orbitals
by metal coordination, changes in co-ligand coordination,
or by electron transfer [14Ϫ17], as well as by solvation or
protonation of the phenazine nitrogen atoms [18, 19]. iii)
Furthermore dppz is well known to intercalate DNA. Tran-
sition metal complexes of dppz with rhenium, ruthenium,
rhodium, platinum, copper or nickel have been widely stud-
ied with respect to luminescence monitoring of DNA bind-
ing (“light-switch effect”) or photoinduced DNA cleavage
[19Ϫ22].
In this contribution we present the synthesis of the novel
organometallic complex [(dppz)Ni(Mes)Br], its analytical
data and crystal (and molecular) structure, together with a
preliminary investigation of its interesting photophysical
properties.
Fig.
1
Crystal structure of [(dppz)Ni(Mes)Br] showing the
stacking along the a axis. H atoms are omitted for clarity.
2.2 Crystal structure of [(dppz)Ni(Mes)Br]
From a concentrated solution of the complex in acetone we
obtained suitable single crystals for XRD. The structure
was solved in the monoclinic space group P21/n (for details
see Table 1 and Experimental Part). The exceptionally high
R(int) value is probably caused by fair crystal quality com-
bined with insufficient absorption correction. None the less,
the overall good quality of the structure solution and refine-
ment justifies a detailed discussion of the structure. From
the crystal structure (see Fig. 1) the π-π-stacking of the
dppz ligands along the crystallographic a axis is obvious.
The molecules exhibit their dppz ligands oriented back-to-
back thus preventing any Ni···Ni interactions. A closer view
(Fig. 2) reveals that there are two alternating packing mo-
tives. One (called P) is characterized by a graphite-like
stacking, a small offset of the two molecular axes and the
two mesityl co-ligands bending towards each other, thus
forming kind of pairs. The other motive is called the neigh-
bouring interaction (N) and exhibits a different stacking
motive, a larger offset of the molecular axes and the two
mesityl co-ligands bending away. Interestingly the in-
2 Results and Discussion
2.1 Preparation
The complex [(dppz)Ni(Mes)Br] was prepared from trans-
[(PPh3)2Ni(Mes)Br] [9] and dppz [23] as recently described
for bpy and related α-diimine ligands [9, 11] in excellent
yield (97 %) as an air-stable bright red microcrystalline ma-
terial. The complex is soluble and stable in common organic
solvents. However, in acetonitrile and methanol it un-
dergoes rapid cleavage of the bromide co-ligand. This be-
haviour has also been observed for the bpy derivative [10].
Table 1 Selected crystallographic and structural data for [(dppz)Ni(Mes)Br]
formula / formula weight
crystal system
space group
C27H21BrN4Ni / 540.10
monoclinic
P21/n
782.00(16) pm
1574.0(3) pm
1938.0(4) pm
distances (in pm)
Ni(1)ϪC(1)
Ni(1)ϪN(1)
Ni(1)ϪN(2)
Ni(1)ϪBr(1)
189.3(8)
193.2(6)
198.2(6)
229.57(16)
145.2(10)
cell
a
b
c
C(29)ϪC(30)
β
101.01(3)°
angles (in °)
Volume (in 106 pm3) / Z
ρCalc. (in g·cmϪ3) / F(000)
µ (in mmϪ1) / Temp. (in K)
Θ range for data collection
Index ranges
2.3415(8) / 4
C(1)ϪNi(1)ϪN(1)
C(1)ϪNi(1)ϪN(2)
N(1)ϪNi(1)ϪN(2)
C(1)ϪNi(1)ϪBr(1)
N(1)ϪNi(1)ϪBr(1)
N(2)ϪNi(1)ϪBr(1)
dihedral angles (in °)
NNNi/CBrNi
92.7(3)
174.0(3)
83.1(3)
89.8(2)
176.2(2)
94.64(18)
1.532 / 1096
2.558 / 293(2)
2.50 to 28.21°
Ϫ10 Յ h Յ 10, Ϫ20 Յ k Յ 19,
Ϫ23 Յ l Յ 23
22360 / 5292
0.2194 / 91.6 %
5292 /301 / 0
0.803
Reflections coll. / unique
R(int) / Compl. to Θmax
Data / param. / restraints
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
5.3
90.5
1.3
NNNiCBr/Mes
py(1)/py(2)
Σof ang. around Ni
R1 ϭ 0.0631, wR2 ϭ 0.1129
R1 ϭ 0.2129, wR2 ϭ 0.1549
0.371 and Ϫ0.713
360.24
∆ρmax/min (in 106 e·nmϪ3
)
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 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2007, 1659Ϫ1665

π-Stacking Modulates the Luminescence of [(dppz)Ni(Mes)Br]
Fig. 3 Molecular structure of [(dppz)Ni(Mes)Br] showing the
atomic numbering scheme. The probability ellipsoids are drawn at
the 30 % probability level, H atoms are omitted for clarity.
Fig. 2 Stacking of [(dppz)Ni(Mes)Br] molecules along the a axis
(left) forming pairs with the stacking pair motive P (middle) or the
neighbouring motive N (right)
including cases in which the vectors of the two co-planar
dppz ligands exhibit orientations between head-to-tail
(180°) and perpendicular (90°) e.g. 105° in [(dppz)-
Ru(NO)(C5Me5)](OTf)2 [21f] or alternating 180° and 60°
[(dppz)CuCl(µ-η2,η2-terephthalate)CuCl(dppz)] [28]. On
the other hand in the binuclear complex cis-[Rh2(µ-
O2CCH3)2(dppz)2](CH3COO)2 [21i] the two dppz ligands
are forced by the RhϪRh bond towards an almost eclipsed
arrangement with the result, that the shortest interplanar
distance of 349.4(7) pm is markedly lengthened, due to the
unfavourable π-π interaction [29]. This means that in the
stacked systems a distance of 340 pm denotes a small but
marked positive π-π interaction.
Furthermore it is impossible to give a firm rule for the
formation (or non-formation) of extended systems vs. di-
meric units or vs. no stacking at all. Other intermolecular
contacts than the described stacking were not found in the
crystal structure of [(dppz)Ni(Mes)Br].
The molecular structure of the complex is shown in Fig-
ure 3. The molecule shows a planar coordination sphere
around the nickel atom (sum of angles 360.24°) and the
mesityl co-ligand is oriented almost perpendicular to the
coordination plane (90.5°). The same has been found for
the bpy-derivative [(bpy)Ni(Mes)Br] (bpy ϭ 2,2Ј-bipyri-
dine) [9]. Also other binding parameters are very alike. As
expected, the dppz ligand is entirely planar. The two pyri-
dine moieties show only a marginal py/py tilt angle which
is probably caused by the optimisation of the chelate bite
angle NϪNiϪN (83.1(3)°) a value which has also been ob-
served for related systems [9Ϫ11].
terplanar distances are both the same (339 pm) within the
experimental error, making our assignment to pairs (P) and
neighbours (N) somewhat arbitrary.
It is not unexpected that dppz exhibits π-stacking in tran-
sition metal complexes since the extended π-system is ide-
ally suited for this. However in most of the compounds
structurally characterised so far, only dimer formation in
the solid state is observed. Some of these dimeric units
pack in one of the two head-to-tail alternatives observed
for [(dppz)Ni(Mes)Br], e.g. the
P
type dimers in
[(dppz)Pt(Mes)2] [24] or in [(dppz)Ru(Me6C6)Cl](CF3SO3)
[21e] or the N type stacking in [(dppzc)Zn(STol)2] (dppzc ϭ
11,12-(15crown5)-dppz) [14b]. However there are many
more alternatives including head-to-tail stacking without
any offset as in [(dppz)Cu(PPh3)I] [25] or [(CNdppz)Ru-
(bpy)2](PF6)2 (CNdppz ϭ 11,12-dicyanodppz) [14c].
Extended stacking (beyond dimers) is found in the com-
plex [(dppz)CuCl(dppz)]·(dppz), where the complex mol-
ecule exhibits an alternate stacking with co-crystallized
dppz molecules along the crystallographic a axis [21o]. Two
different pairs are also found in the extended π-stacks of
[(dppz)Re(CO)3(py)](CF3SO3) [21a], in which N type pairs
alternate with pairs of almost perpendicular arrangement.
Perpendicular means that the two dppz ligands are packed
in a coplanar fashion but the orientation of the dppz ligand
vectors is about 90° which allows distinctly smaller overlap
compared to the head-to-tail (180°) or head-to-head (0°)
orientation. The hexacoordinate [(dppz)Ir(bpy)2](PF6)·
0.5acetone [26] forms stacks in which P type pairs (180°)
alternate with perpendicular arrangements (90°). The CuII
complex [(dppz)Cu(ONO2)(H2O)] [27] also exhibits ex-
tended head-to-tail stacking along the crystallographic a
axis and again two different types of stacking pairs are
formed. One reminds to the P pairs in [(dppz)Ni(Mes)Br]
the others show a huge offset of the two dppz molecular
axes, allowing only the overlap of each one pyridyl unit with
the back end of the phenazine core. When going through
all these stacked structures, we found that the interplanar
distance lies in almost any case between 340 and 350 pm.
The above described “well aligned” dimeric structural units
exhibit only slightly closer contacts than the other examples
2.3 Absorption and emission spectroscopy
Transition metal complexes of dppz have been found to ex-
hibit interesting photophysical properties due to the mul-
3
3
tiple excited states available: π-π*(phenazine), π-π*(bpy)
3
or MLCT. In our case the stacking of the molecules in the
solid might give rise to additional features as excited dimers
or concentration dependent quenching caused by dimeris-
ation or oligomerisation.
Z. Anorg. Allg. Chem. 2007, 1659Ϫ1665
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1661

A. Klein, N. Hurkes, A. Kaiser, W. Wielandt
Emission from dilute solutions
Irradiating the solution into the long-wavelength absorp-
tion band at around 500 nm does not lead to luminescence.
3
The assumed MLCT excited state is not emissive under
these conditions, which is in line with investigations of re-
lated α-diimine complexes [(N∧N)Ni(Mes)Br] (N∧N ϭ bpy
and related α-diimines) [11]. But stands in contrast to corre-
sponding Re [14d, 16, 21b], Ru [14d-g, 18, 19b] or Pt com-
3
plexes [2, 1l], for which MLCT emission is observed. Ir-
radiation into the band system at 370 nm leads to intense
emission centred at 530 nm. The band is structured (see
Table 3) and maximum emission intensity is observed upon
excitation at 400 nm. This energy lies on the border between
the broad and partly structured long-wavelength band sys-
tem and the intense and sharp bands observed in the UV
region. The free ligand dppz is reported to exhibit double
emission in CH2Cl2 solution at 457 nm from the 3π-π*(bpy)
Fig. 4 Absorption spectrum of [(dppz)Ni(Mes)Br] in CH2Cl2
(left). Emission spectrum obtained upon irradiation with λex
400 nm (right) and the excitation spectrum corresponding to emis-
sion at 570 nm (dashed).
ϭ
3
excited state and at 538 nm from the π-π*(phenazine) ex-
cited states, thus creating a structured band centred at
466 nm with shoulders at 457 and 538 nm. The excitation
maximum lies at 400 nm [17a]. The excitation maximum
does not correspond to the main absorption band at
380 nm but to its long-wavelength tail, thus supporting the
assignment of a 3π-π*(phenazine) excited state. For the
dppz complex [(dppz)NiMesBr] the spectra look very simi-
lar, however the entire emission band system seems to be
red shifted with the maximum at 535 nm and shoulders at
510 and 570 nm. The emission quantum yield was deter-
mined to 3·10Ϫ3 which approaches almost the value for the
free ligand (3.2·10Ϫ3). It seems that the character of the
emission is retained while the energy is shifted by the pres-
ence of the metal and the co-ligands. For the dicationic
complex [(dppz)Ni(bpy)2]2ϩ the emission spectra also looks
similar, but their energy is blue-shifted (maximum at
480 nm) [22b]. However, the measurements were performed
in aqueous media and the complex intercalated into DNA,
thus preventing an exact comparison of the absorption
energies.
a)
Table 2 Absorption and emission data of [(dppz)Ni(Mes)Br]
compound
/ condition
absorption λ emission λ (λex
)
excitation λ (λem
)
b)
c)
[ε]
[quantum yield Φ]
[(dppz)NiMesBr]
/ in CH2Cl2
497 [4150]
not detected (500)
372sh, 393,
370 [12660] 508sh, 530, 569sh (400) 408sh (530)
conc. ca. 3·10Ϫ5 M 325 [12190] [3·10Ϫ3
]
272 [41690]
[(dppz)NiMesBr]
/ in CH2Cl2
497, 370,
325, 272
not detected (500)
368sh, 398,
505sh, 532, 572sh (400) 407sh (572)
conc. ca. 6·10Ϫ4
[(dppz)NiMesBr]
/ in CH2Cl2
M
[3.8·10Ϫ4
]
497, 370,
325, 272
612 (400) [5.1·10Ϫ5
618 (440)
]
403 (610)
conc. ca. 2·10Ϫ3
M
631 (500)
a) Absorption, emission (excitation given in parentheses) and excitation (cor-
responding emission given in parentheses) maxima λ (in nm). b) Extinction
coefficient ε (in MϪ1cmϪ1). c) Determined with [(bpy)3Ru]2ϩ in MeCN (Φ ϭ
0.062) as actinometer [32] at 298 K.
Absorption spectroscopy
The absorption spectrum of [(dppz)Ni(Mes)Br] in CH2Cl2
solution is shown in Figure 4 together with the emission
spectrum obtained upon irradiation with λex ϭ 400 nm and
the corresponding excitation spectrum for the observed em-
ission at 570 nm. Essential absorption and emission data of
the complex are collected in Table 2.
In view of detailed studies of the photophysics of the
dppz ligand [17] and a number of transition metals com-
plexes containing Re [14d, 16, 21a, b], Ru [14d-g, 18, 19b],
Pt [2, 1l, 30], Cu [14b, 21m] or Zn [14b] we can assign the
long-wavelength absorption band at around 500 nm to a
mainly Ni(d)-π*(bpy) charge transfer (MLCT) probably
with marked contributions from the Mes co-ligand
(LЈLCT) [11]. This is supported by the strong negative sol-
vatochromism of this band (467 nm [21410 cmϪ1] in DMF
compared to 519 nm [19270 cmϪ1] in toluene). The sharp
and intense band systems at 370, 325 and 272 nm (the latter
is not shown in Fig. 4) are due to ligand-centred π-π* tran-
sitions.
Emission from concentrate solutions
In highly concentrate solutions of [(dppz)NiMesBr] in
CH2Cl2 irradiation at 400 nm leads to a broad emission
centred at 610 nm (Fig. 5). The emission quantum yield is
markedly reduced to 5·10Ϫ5. In this case also irradiation at
λex > 400 nm results in emission (see Table 2). However the
quantum yields decrease dramatically upon going from
λex ϭ 400 to 500 nm. Upon dilution (Fig. 6) the emission
spectra gradually change with the 610 nm band vanishing
and the above described band at 530 nm is reappearing.
We suppose that the long-wavelength emission originates
from excited dimers (or oligomers) in the concentrate solu-
tions. The question arises if excitation into the long-wave-
length charge transfer band centred at 500 nm might lead
to some excitation transfer into this emitting state. A closer
inspection of the excitation band centred at 400 nm reveals
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Z. Anorg. Allg. Chem. 2007, 1659Ϫ1665

π-Stacking Modulates the Luminescence of [(dppz)Ni(Mes)Br]
Conclusion
The novel complex [(dppz)Ni(Mes)Br] shows luminescence
at ambient temperature in solution but not in the solid. We
assume that the character of the emission is the same as for
3
the free ligand (mainly from the π-π*(phenazine) excited
state) while the energy is shifted by the presence of the me-
tal and the co-ligands. In concentrate solutions a weak red-
shifted emission is assigned to dimers or oligomers. This is
in line with the supposed quenching in the solid state by
packing of the molecules. The crystal structure (extended
stacking along the a axis) supports this assumption. Future
investigation will seek to clarify the character of the long-
wavelength emission. Also we want investigate the lumi-
nescence lifetimes, possible quenching reactions, the behav-
iour at low-temperature and potential applications of this
complex.
Fig. 5 Emission spectra of [(dppz)Ni(Mes)Br] in CH2Cl2 at va-
rious excitation wavelength ranging from 400 nm (upper spectrum)
to 500 nm (lowest spectrum) in steps of 10 nm.
Experimental Part
Preparation of [(dppz)Ni(Mes)Br]: Trans-[(PPh3)2Ni(Mes)Br] [9]
(704 mg, 0.9 mmol) and dppz [23] (226 mg, 0.8 mmol) were dis-
solved in 200 mL abs. toluene and stirred for 24 h at ambient tem-
perature. After evaporation of all volatiles and washing with n-pen-
tane (to remove PPh3) the residue was recrystallised from CH2Cl2/
n-heptane (1:1). We obtained the compound in 97 % yield (420 mg,
0.78 mmol, referred to the dppz ligand) as air-stable bright red
microcrystalline material. Anal. Calc. for C27H21BrN4Ni (MW:
540.10 g molϪ1): C, 60.04; H, 3.92; N, 10.37. Found: C, 60.01; H,
3.90; N, 10.31 %.
1H NMR (CD2Cl2) δ ϭ 9.76 (dd, 1H, 3J ϭ 5.16 Hz, 4J ϭ 1.49 Hz, H8); 9.68
(dd, 1H, 3J ϭ 8.12 Hz, 4J ϭ 1.51 Hz, H10); 9.64 (dd, 1H, 3J ϭ 7.87 Hz, 4J ϭ
1.57 Hz, H1); 8.41 (m, 2H, H15,H18); 8.034 (m, 2H, H16,H17); 8.029 (dd,
1H, 3J(av.) ϭ 5.32 Hz, H9); 7.63 (dd, 1H, 3J ϭ 7.97 and 5.41 Hz, H2); 7.58
(dd, 1H, 3J ϭ 5.51 Hz, 4J ϭ1.53 Hz, H3); 6.61 (s, 2H, MesH); 3.15 (s, 6H,
o-Me); 2.30 (s, 3H, p-Me). 13C NMR (CD2Cl2) δ ϭ 152.4 (C8); 151.5 (C3);
149.4 (C5); 146.4 (C5); 142.7 (C14,C19); 139.6 (C14); 139.3 (C21); 134.5
(C10); 133.7 (C1); 131.7 (C16,C17); 129.5 (C15,C18); 128.5 (C22); 127.7
(C11); 126.6 (C2); 126.4 (C9); 125.9 (Mes); 25.1 (o-Me); 20.0 (p-Me). Num-
bering as in Scheme 1.
Instrumentation. Elemental analysis was obtained using a Henatech
CHNS EuroEA 3000 analyzer. NMR spectra were recorded on a
Bruker Avance 400 spectrometer (1H: 400,13 MHz, 13C:
100,61 MHz) using a triple resonance 1H,19F,BB inverse probe
Fig. 6 Emission spectra of [(dppz)Ni(Mes)Br] in CH2Cl2 (λex
400 nm). Increasing emission intensity goes along with increasing
concentrations.
ϭ
1
head. The unambiguous assignment of the H and 13C resonances
was obtained from 1H NOESY, 1H COSY, gradient selected 1H,
13C HSQC and HMBC experiments. All 2D NMR experiments
were performed using standard pulse sequences from the Bruker
pulse program library. Chemical shifts were relative to TMS. UV/
VIS absorption spectra were recorded on Varian Cary 05E or
Cary50 Scan spectrophotometers. Emission spectra were recorded
on a Spex FluoroMax-3 spectrophotometer. The quantum yields
were determined using [(bpy)3Ru]2ϩ in MeCN solution (Φ ϭ 0.062)
as actinometer [32].
that there is some residual intensity down to 480 nm (Fig.
4). Thus we don’t believe that such excitation transfer from
the charge transfer state plays a role, although at the present
stage we cannot rule out this possibility entirely.
Emission from the solid
Crystal structure analysis for [(dppz)Ni(Mes)Br]. The measurement
was performed at 293(2) K using graphite-monochromatised Mo-
Kα radiation (λ ϭ 71.073 pm) on a IPDS II (STOE and Cie.). The
structure was solved by direct methods (SHELXS-97) [33] and re-
fined by full-matrix least-squares techniques against F2 (SHELXL-
97) [34]. The non-hydrogen atoms were refined with anisotropic
displacement parameters without any constraints. The hydrogen
Irradiation of a microcrystalline powder of [(dppz)Ni-
MesBr] at 400 nm (or at other wavelengths) does not give
any luminescence. Also a carefully ground sample with KBr
did not show any luminescence. We suspect that the lumi-
nescence observed in fluid solution is quenched in the solid
state due to the stacking of the molecules in the crystal.
Z. Anorg. Allg. Chem. 2007, 1659Ϫ1665
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1663

A. Klein, N. Hurkes, A. Kaiser, W. Wielandt
atoms were included by using appropriate riding models. Further
details are listed in Table 1. Full structural information has been
deposited with the Cambridge Crystallographic Data Centre, No.
CCDC 635136. Copies of the data can be obtained, free of charge,
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: ϩ44-1223 336033 or e-mail: deposit@ccdc.cam.ac.uk)
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Acknowledgment. We thank Dr. Ingo Pantenburg, University of
Cologne, for collecting the crystal data.
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