Novel phenanthroline ligands and their kinetically locked copper(I) complexes with unexpected photophysical properties

Article abstract of DOI:10.1021/ic051828v

The new, sterically encumbered phenanthroline ligands 1a,b, both characterized by the presence of bulky aryl substituents (3,5-di-tert-butyl-4- methoxyphenyl, 2,4,6-trimethylphenyl) in the 2,9-position, were prepared along with their homoleptic [Cu(1a,b)<

Full text of DOI:10.1021/ic051828v

Inorg. Chem. 2006, 45, 2061−2067
Novel Phenanthroline Ligands and Their Kinetically Locked Copper(I)
Complexes with Unexpected Photophysical Properties
Venkateshwarlu Kalsani and Michael Schmittel*
Center of Micro and Nanochemistry and Engineering, Organische Chemie I,
UniVersita¨t Siegen, Adolf-Reichwein-Strasse, D-57068 Siegen, Germany
Andrea Listorti, Gianluca Accorsi, and Nicola Armaroli*
Molecular Photoscience Group, Istituto per la Sintesi Organica e la FotoreattiVita`,
Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy
Received October 22, 2005
The new, sterically encumbered phenanthroline ligands 1a,b, both characterized by the presence of bulky aryl
substituents (3,5-di-tert-butyl-4-methoxyphenyl, 2,4,6-trimethylphenyl) in the 2,9-position, were prepared along with
their homoleptic [Cu(1a,b)2]+ and heteroleptic complexes [Cu(1a,b)(phen)]+ (phen
) parent 1,10-phenanthroline).
Due to the pronounced steric shielding, particularly effective in ligand 1a, the formation of the homoleptic complex
[Cu(1a)2]+ becomes very slow (5 days). Once formed, the homoleptic complexes [Cu(1a,b)2]+ do not exchange
ligands even with phen added in excess because they are kinetically locked due to the large tert-butylphenyl
substituents at the phenanthroline unit. The electronic absorption spectra of the homoleptic complexes [Cu(1a)2]+
and [Cu(1b)2]+ evidence a strongly different ground state geometry of the two compounds, the former being
substantially more distorted. This trend is also observed in the excited-state geometry, as derived by emission
spectra and lifetimes in CH2Cl2 solution. The less distorted [Cu(1b)2]+, compared to [Cu(1a)2]+, is characterized by
a 15- and over 100-fold stronger emission at 298 and 77 K, respectively. Noticeably, the excited-state lifetime of
[Cu(1a)2]+ in solution is unaffected by the presence of molecular oxygen and only slightly shortened in nucleophilic
solvents. This unusual behavior supports the idea of a complex characterized by a “locked” coordination environment.
Introduction
ambient acid concentrations.3,7 However, by far, the most
fruitful field of exploitation of these ligands is coordination
chemistry, which has received a major impetus through the
pioneering work of Sauvage and Dietrich-Buchecker et al.
constructing topologically spectacular architectures such as
catenates, knots, rotaxanes, etc.8-10 Some of these coordina-
tion compounds were even designed to perform motions at
the molecular level via chemical, photochemical, or elec-
trochemical stimulation.11-15 More recently, further develop-
1,10-Phenanthroline and its substituted derivatives repre-
sent a widely investigated class of chelating agents.1 Some
intrinsic properties of phenanthroline ligands (e.g. structural
rigidity and luminescence) make them attractive as analytical
probes, e.g. in proton and cation sensing2-5 or DNA
intercalation and groove binding.6 Interestingly, suitably
engineered phenanthroline ligands can even operate as
rudimentary molecular machines simply by varying the
* To whom correspondence should be addressed. E-mail:
schmittel@chemie.uni-siegen.de (M.S.), armaroli@isof.cnr.it (N.A.).
(1) Sammes, P. G.; Yahioglu, G. Chem. Soc. ReV. 1994, 23, 327-334.
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Buchecker, C. O.; Kern, J. M. J. Chem. Soc., Faraday Trans. 1992,
88, 553-556.
(3) Armaroli, N. Photochem. Photobiol. Sci. 2003, 2, 73-87.
(4) Aragoni, M. C.; Arca, M.; Demartin, F.; Devillanova, F. A.; Isaia, F.;
Garau, A.; Lippolis, V.; Jalali, F.; Papke, U.; Shamsipur, M.; Tei, L.;
Yari, A.; Verani, G. Inorg. Chem. 2002, 41, 6623-6632.
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F.; Valtancoli, B. Eur. J. Inorg. Chem. 1999, 1911-1918.
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1386-1387.
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Buchecker, C. O.; Kern, J. M.; Bailal, A. J. Chem. Soc., Dalton Trans.
1993, 3241-3247.
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(9) Sauvage, J.-P., Dietrich-Buchecker (Eds.), C. O. Molecular Catenanes,
Rotaxanes and Knots. A Journey through the World of Molecular
Topology; Wiley-VCH: Weinheim, Germany, 1999.
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10.1021/ic051828v CCC: $33.50
Published on Web 01/17/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 5, 2006 2061

Kalsani et al.
ments in the supramolecular chemistry of these systems have
been achieved by some of us via the assembly of nanoscale
racks,16 grids,17 boxes,18 and baskets19 based on heteroleptic
Cu(I)-bis(phenanthroline) complexes, which are potentially
interesting as photoactive units.20-22 Control of the sophis-
ticated heteroleptic architectures had been achieved by the
use of the HETPHEN (HETeroleptic bis(PHENanthroline))
concept.23 This approach employs bulky aryl substituents at
the bis(imine) coordination site to control the metal com-
plexation equilibrium.
Figure 1. Light excitation into the lowest MLCT excited state(s) of Cu-
(I)-bis(phenanthroline)s leads to a flattening distortion of the coordination
environment, thus exposing the metal center to nucleophilic attack by
external molecules such as solvent or counterions.
Tetrahedral Cu(I)-bis(phenanthroline) complexes (here to
+
after indicated as [Cu(NN)2 ]) are characterized by a rela-
+
Also excited-state characteristics of [Cu(NN)2 ] (e.g.
tively long-lived metal-to-ligand-charge-transfer (MLCT)
emission in the vis spectral region, and in recent years, they
have emerged as potential alternatives to the very popular
Ru(II)-polypyridine complexes24-27 in view of practical
applications (e.g. light harvesting and solar energy conversion
devices), thanks to lower price, larger abundance, and smaller
environmental impact.22 The MLCT absorption bands of [Cu-
emission spectra and excited-state lifetimes) strongly depend
on the substitution pattern of the phenanthroline ligands.22
The effects on the luminescence intensity and lifetime can
be attributed to a distortion occurring in the MLCT excited
state, since the metal center changes its formal oxidation state
from Cu(I) to Cu(II), thus assuming a more flattened
coordination geometry (Figure 1).30 In the flattened structure
a fifth coordination site is made available at the newly formed
d9 ion,31 allowing for attack by nucleophilic species such as
solvent molecules and counterions. The resultant pentaco-
ordinated excited complexes (exciplexes) deactivate via
nonemissive deactivation paths.22 This deactivation mecha-
nism via the formation of an intermediate species, hypoth-
esized a long time ago,32,33 has been recently demonstrated
experimentally via ultrafast spectroscopic techniques, such
as time-resolved X-ray31,34-36 and electronic absorption37,38
spectroscopies.
+
(NN)2 ] cover a wide spectral range (380-700 nm), as a
result of an envelope of at least three different electronic
transitions.22,28 The corresponding spectral intensities are
strictly related to the symmetry of the complex that itself is
affected by the distortion from the tetrahedral geometry. The
distortion is largely dictated by the position and chemical
nature of the substituents on the chelating ligand.22 For
instance, complexes of 2,9-diarylphenanthroline are charac-
terized by π-stacking interactions between the aryl groups
of one ligand and the phenanthroline moiety of the other
ligand,29 which brings about a strongly distorted ground-
state tetrahedral geometry (D2 symmetry).20,22 This explains
the very different MLCT absorption profile characterizing
complexes of 2,9-diarylphenanthroline ligands compared to
those of 2,9-dialkylphenanthroline type.22
To disfavor the formation of nonemissive pentacoordinated
+
exciplexes of [Cu(NN)2 ] described in Figure 1, the strategy
is 2-fold: (i) choose bulky substituents R at the phenanthro-
line ligands that shield the metal site from intermolecular
attack;39,40 (ii) work in poor donor solvents.22 The latter
constraint has practically limited the solvent choice to CH2-
Cl2 since in other media (methanol, acetonitrile, acetone)
(11) Livoreil, A.; Sauvage, J. P.; Armaroli, N.; Balzani, V.; Flamigni, L.;
Ventura, B. J. Am. Chem. Soc. 1997, 119, 12114-12124.
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(13) Raehm, L.; Sauvage, J. P. In Molecular Machines and Motors; 2001;
Vol. 99, pp 55-78.
+
there is virtually no luminescence from [Cu(NN)2 ].
In an attempt to further elucidate the role of the ligand
structure in defining the photophysical properties of [Cu-
(14) Dietrich-Buchecker, C.; Jimenez-Molero, M. C.; Sartor, V.; Sauvage,
J. P. Pure Appl. Chem. 2003, 75, 1383-1393.
(15) Collin, J. P.; Sauvage, J. P. Chem. Lett. 2005, 34, 742-747.
(16) Kalsani, V.; Bodenstedt, H.; Fenske, D.; Schmittel, M. Eur. J. Inorg.
Chem. 2005, 1841-1849.
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Scaltrito, D. V.; Meyer, G. J. J. Am. Chem. Soc. 2002, 124, 10861-
10867.
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2004, 490-491.
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Chem. Commun. 2002, 2566-2567.
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Coord. Chem. ReV. 2000, 208, 243-266.
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125, 7022-7034.
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(24) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277.
(25) Sauvage, J. P.; Collin, J. P.; Chambron, J. C.; Guillerez, S.; Coudret,
C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. ReV.
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(28) Everly, R. M.; McMillin, D. R. J. Phys. Chem. 1991, 95, 9071-9075.
(29) Meyer, M.; Albrecht-Gary, A. M.; Dietrich-Buchecker, C. O.; Sauvage,
J. P. Inorg. Chem. 1999, 38, 2279-2287.
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S.; Wu, G.; Gembicky, M.; Novozhilova, I. V. J. Am. Chem. Soc.
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D. R. Inorg. Chem. 1999, 38, 4388-4392.
2062 Inorganic Chemistry, Vol. 45, No. 5, 2006

Phenanthroline Ligands and Their Cu(I) Complexes
Scheme 1. Synthesis of Ligands 1a,b and Structure of Ligand 5
Figure 2. ESI MS of [Cu(1a)2]+ and its isotopic distribution (dark,
experimental; red, calculated). The signal of 1-H+ is due to residual traces
of acid in the instrument.
ground-state electronic properties for [Cu(1a)2]+, compared
+
to analogous homoleptic [Cu(NN)2 ]complexes.
[Cu(MeCN)4]PF6
1a
1b
5
8 [Cu(1a)2]+
8 [Cu(1b)2]+
8 [Cu(5)2]+
(1)
(2)
(3)
25 °C/∼5 d
[Cu(MeCN)4]PF6
immediately
[Cu(MeCN)4]PF6
immediately
The structures of [Cu(1a)2]+ and [Cu(1b)2]+ were con-
+
(NN)2 ] complexes, we present herein the synthesis of two
1
firmed by H NMR, 13C NMR, ESI MS, and elemental
novel complexes of this family, which turn out to exhibit
quite interesting kinetic and photophysical properties.
analysis. The 1H NMR spectrum of [Cu(1a)2]+ shows a single
sharp set of signals with a spectral pattern being typical for
copper(I) bis(phenanthroline) complexes.23,44 It reveals an
upfield shift of 0.64 ppm for the phenyl protons (from δ
8.12 to 7.48 ppm). Equally, the methoxy and tert-butyl
protons experience significant upfield shifts (from δ 3.77 to
3.20 and 1.55 to 0.90 ppm, respectively). All assignments
are confirmed by COSY. The 13C NMR shows a single set
of 13 expected signals for [Cu(1a)2]+, further supporting the
suggested structure. ESI MS (electrospray ionization mass
spectroscopy) provides conclusive evidence for the complex
structure exhibiting a singly charged signal at m/z ) 1296.1
assigned to [Cu(1a)2]+ after the loss of hexafluorophosphate
as the counterion (Figure 2). No other signals could be
detected up to 4000 Da. It is noteworthy to mention that
under ESI MS conditions copper(I)-bis(phenanthroline)
complexes normally undergo extensive fragmentation through
loss of ligands (>30%).45 In contrast, [Cu(1a)2]+ and [Cu-
(1b)2]+ do not show any significant fragmentation (<5%)
suggesting an excellent kinetic stability.
Results and Discussion
Synthesis and Kinetic Studies. Phenanthrolines 1a,b are
prepared according to known procedures (Scheme 1).41 The
aryllithium compound 2-Li, obtained from 2 (2 equiv) and
n-BuLi (2.1 equiv), is added to the parent phenanthroline
(phen) affording 3 in 71% yield after oxidative rearomati-
zation. Following the same procedure, ligand 1a is prepared
from 3 and 2-Li, while 1b is formed from 3 and 4-Li. Ligand
5 is well-known from the literature.2,30,42,43
The new copper(I) complexes [Cu(1a)2]+ and [Cu(1b)2]+
are obtained quantitatively by treatment of 1a,b, respectively,
with [Cu(CH3CN)4]PF6 (1:0.5 equiv) in CH2Cl2 at ambient
temperature (eqs 1-3), as was the known [Cu(5)2]+.30,43 It
is important to note that the formation of [Cu(1a)2]+ is very
slow (25 °C/5 d), whereas [Cu(1b)2]+ and [Cu(5)2]+ form
immediately. Complex [Cu(1a)2]+ exhibits a magenta color,
whereas [Cu(1b)2]+ and [Cu(5)2]+ show a dark red hue, the
color expected for a typical bis(phenanthroline)copper(I)
complex. These preliminary observations suggest peculiar
The dynamics of ligand binding in [Cu(1a,b)2]+ and for
comparison in [Cu(5)2]+ is probed by adding the parent 1,-
10-phenanthroline (phen) in various amounts (1:0.5-10
equiv). In [Cu(5)2]+, similar to other homoleptic complex-
(41) Schmittel, M.; Michel, C.; Wiegrefe, A.; Kalsani, V. Synthesis 2001,
1561-1567.
(42) Klemens, F. K.; Palmer, C. E. A.; Rolland, S. M.; Fanwick, P. E.;
McMillin, D. R.; Sauvage, J. P. New J. Chem. 1990, 14, 129-133.
(43) Dietrich-Buchecker, C. O.; Marnot, P. A.; Sauvage, J. P.; Kirchhoff,
J. R.; McMillin, D. R. J. Chem. Soc., Chem. Commun. 1983, 513-
515.
(44) Schmittel, M.; Lu¨ning, U.; Meder, M.; Ganz, A.; Michel, C.; Herderich,
M. Heterocycl. Commun. 1997, 3, 493-498.
(45) Schmittel, M.; Kishore, R. S. K. Org. Lett. 2004, 6, 1923-1926.
Inorganic Chemistry, Vol. 45, No. 5, 2006 2063

Kalsani et al.
Table 1. Formation of Homoleptic and Heteroleptic Cu(I)-Bis(phenanthroline) Complexes with 1aa
expt no.
sequential additn
obsd species by ESI MS
I
II
III
IV
[Cu(1a)2]+ + phen (0.5-10 equiv)
[Cu(phen)2]+ + 1a (0.25-1.0 equiv)
1a + phen + Cu(I)
only [Cu(1a)2]+ and phen detected (analyzed after 15 min and after 15 d)
mixed complex [Cu(1a)(phen)]+ formed (detected after 15 min)
mixed complex [Cu(1a)(phen)]+ formed (detected after 15 min)
[Cu(phen)]+ + 1a (0.25-1 equiv)
while [Cu(1a)(phen)]+ forms instantly, [Cu(1a)2]+ is only detected after
longer times; [Cu(1a)2]+ increases with time (10% to 60% over a period of 18 h)
a There is a time delay of about 15 min until the ESI measurement.
es,16,46 ligand exchange is instantaneous, as indicated by 1H
NMR and ESI MS through the emergence of new signals
corresponding to [Cu(5)(phen)]+ and [Cu(phen)2]+.
With [Cu(1a,b)2]+, however, the situation is drastically
different. As indicated by ESI MS and 1H NMR, not a single
trace of the heteroleptic complex [Cu(1a,b)(phen)]+ is
detectable at RT (room temperature) over a period of 15 days
from a mixture of phen (0.5 equiv) and [Cu(1a,b)2]+ (1
equiv). Even upon addition of an excess of phen (10 equiv),
no signals of [Cu(1a)(phen)]+ are detected. The same result
is obtained after refluxing the reaction mixture for 1 week
in dichloromethane or acetonitrile, pointing to a marked
kinetic inertness of [Cu(1a)2]+.
Interestingly, the heteroleptic [Cu(1a)(phen)]+ complex is
cleanly obtained when we add ligand 1a to [Cu(phen)2]+ or
[Cu(phen)]+ (Table 1, experiments II and III, observed over
several days at RT). In experiment IV both [Cu(1a)(phen)]+
and [Cu(1a)2]+ are formed, with the amount of [Cu(1a)2]+
increasing over time. These results indicate that once the
[Cu(1a)2]+ complex is formed it is kinetically “locked” due
to the large substituents at the 2,9-positions of the phenan-
throline. To the best of our knowledge this is the first Cu(I)-
bis(phenanthroline) complex that does not exchange with
added 2,9-unsubstituted phenanthrolines thanks to a peculiar
kinetic inertness.
the second ligand 1a, a process that is clearly accompanied
by large steric repulsions. As a consequence, the association
process [Cu(1a)]+ + 1a f [Cu(1a)2]+ becomes rather slow
(about 5 days at RT for completion), while the reverse
process must be even slower by many orders of magnitude
(kdiss ) kassK-1). Practically, the slow dissociation process
means that the complex is kinetically locked. Unfortunately,
all attempts to obtain the solid-state structure of [Cu(1a)2]+
have been unsuccessful.
Photophysical Properties. Electronic Absorption Spec-
tra. The absorption spectra of [Cu(1a)2]+ and [Cu(1b)2]+
are depicted in Figure 4. In the UV region, where the ligand-
centered (π-π*) transitions of the phenanthroline ligands
are dominant,22 the difference recorded for the two complexes
are mainly attributable to the difference in the phenyl
fragments, in particular the chemical nature and the position
of their substituents.22 The absorption bands in the vis spectral
region are ascribed to an envelope of MLCT electronic
transitions.20-22
The complex [Cu(1a)2]+ exhibits a quite peculiar MLCT
absorption profile if compared to previously investigated Cu-
(I)-bis(phenanthroline) complexes with aryl substituents in
the 2 and 9 ligand positions.22 In a number of complexes
with such ligands reported so far a pronounced maximum
has been found in the range 440-470 nm, with molar ex-
tinction coefficients between 3000 and 5000 M-1 cm-1.40,48-50
On the contrary, the most prominent absorption band of [Cu-
(1a)2]+ is the one centered at 560 nm, while the band peaked
at 444 nm appears as just a weak shoulder. This is a rather
unusual behavior for Cu(I) homoleptic complexes with
To understand the marked kinetic behavior of [Cu(1a)2]+,
we calculated its energy minimized structure by ZINDO/1
semiempirical self-consistent field (SCF) calculations47 (Fig-
ure 3). From the structure it appears that the 3,5-di-tert-
butylphenyl groups at the phenanthroline are responsible for
the kinetic locking of the ligands at the Cu(I) ion. Both in
the association and dissociation process, the tert-butyl groups
of one ligand 1a will have to slip by the tert-butyl groups of
Figure 4. Absorption and (inset) luminescence spectra of [Cu(1a)2]+ (full
line) and [Cu(1b)2]+ (dashed line) in CH2Cl2 at 298 K. The region above
400 nm is multiplied by a factor of 10. Luminescence spectra are taken at
identical absorbance values (0.20) and upon excitation at 550 and 422 nm
for [Cu(1a)2]+ and [Cu(1b)2]+, respectively.
Figure 3. Model structure of [Cu(1a)2]+ optimized by ZINDO/1 semiem-
pirical self-consistent field (SCF) calculations.
2064 Inorganic Chemistry, Vol. 45, No. 5, 2006

Phenanthroline Ligands and Their Cu(I) Complexes
Table 2. Luminescence Properties of [Cu(1a)2]+ and [Cu(1b)2]+ in
CH2Cl2
[Cu(1a)2]+ is a very poor luminophore (Table 2) and
exhibits a relatively short lifetime of only 87 ns, comparable
to that of [Cu(dmp)2]+ (73 ns, dmp ) 2,9-dimethyl-1,10-
phenanthroline).52 The short lifetime of [Cu(dmp)2]+ is
related to the small size of the methyl substituents on the
phenanthroline ligand, which cannot contrast effectively
excited-state geometric distortion and subsequent nucleo-
philic attack (Figure 1). Clearly, in our present case, the small
size of the substituent cannot be invoked to rationalize the
weak luminescence signal. This can be explained, instead,
assuming that the strong ground-state geometric distortion,
derived from the analysis of the absorption spectrum (see
above), is kept in the excited state. The distorted [Cu(1a)2]+
complex emits at relatively low energy and due to the energy-
gap law50,53 nonradiative deactivations, always largely pre-
vailing over radiative transitions in Cu(I)-phenanthrolines,
are further strengthened. At 77 K the MLCT emission band
of [Cu(1a)2]+ is hardly distinguishable over the instrumental
noise. The substantial decrease of luminescence observed
on passing to low-temperature rigid matrix is in line with
the behavior of virtually all the Cu(I)-diphenylphenanthro-
line complexes investigated to date. This behavior has been
rationalized assuming that emission in [Cu(NN)2]+ stems
from a low-lying poorly emitting MLCT triplet (3MLCT),
whose population is depleted at higher temperatures as a
consequence of a thermal equilibrium with an upper lying
singlet (1MLCT) exhibiting stronger luminescence and
mainly responsible for emission at room temperature (two-
level model).50,54
Despite its small emission yield [Cu(1a)2]+ allows an
evaluation of the importance of bulky phenanthroline ligands
to prevent interactions of the Cu(I) ion with external
molecules and, thus, exciplex deactivation (Figure 1). The
emission yield and MLCT lifetime of [Cu(1a)2]+ is insensi-
tive to the presence of O2 in solution (Table 2), and to the
best of our knowledge, this is an unprecedented finding for
Cu(I)-phenanthrolines. Moreover [Cu(1a)2]+ exhibits MLCT
luminescence also in solvents with a more nucleophilic
character than CH2Cl2, such as THF, DMF, and MeOH. In
the latter case the lifetime is 62 ns and the luminescence
quantum yield is 7.0 × 10-5, to be compared with 87 ns
and 1.0 × 10-4 in CH2Cl2. This result is particularly
interesting because most Cu(I)-phenanthrolines lack any
luminescence in MeOH32 and confirms that [Cu(1a)2]+
possesses an extremely rigid and protected coordination
environment, giving support to the notion of a locked
complex (see above).
298 K
77 K
max,a nm
b
compd
λ
max,a nm
104Φem
τ,c ns
λ
τ,c µs
[Cu(1a)2]+
[Cu(1b)2]+
704 (750)
660 (690)
1 (1)
15 (18)
87 (87)
266 (285)
d
d
2.9
673 (685)
a Emission maxima from spectra uncorrected and (in parentheses)
corrected for the photomultiplier response. b Emission quantum yields in
air-equilibrated and (in parentheses) deoxygenated solutions. c Excited-state
lifetimes in air-equilibrated and (in parentheses) deoxygenated solutions.
d Very weak signal, hardly distinguishable from instrumental noise.
symmetrically substituted phenylphenanthrolines, which has
to be related to the presence of the cumbersome tert-butyl
groups on the ligands, that strongly affect the ground-state
coordination geometry of the complex and, accordingly, have
an effect on electronic transition probabilities.22 Notably, the
MLCT absorption spectral shape of [Cu(1a)2]+ is similar to
that of a Cu(I)-catenate complex made of two interlocked
27-membered ring containing a phenanthroline moiety.51 This
suggests that the above-mentioned locked structure of [Cu-
(1a)2]+ is rather similar, in terms of ground-state geometric
distortion, to that of the catenate complex. This latter
compound is characterized by a low-symmetry strongly
distorted tetrahedral geometry, and its absorption spectrum
is scarcely affected by temperature decrease,51 which also
supports the view of a fairly rigid coordination environment.
The electronic absorption spectrum of [Cu(1b)2]+ is
substantially different from that of [Cu(1a)2]+ (Figure 4). In
particular, a dramatic decrease of the lowest energy MLCT
bands is observed. This effect, normally observed in Cu(I)
complexes of 2,9-dialkylphenanthroline-type ligands, has
been related to negligible distortions from the D2d tetrahedral
symmetry.22 In the present case, in analogy with Miller et
al.,49 the reduced distortion is most likely driven by the -CH3
groups present in one phenyl unit, ortho to the C atom
connected to the phenanthroline fragment. Such methyl
groups prevent extensive ππ* stacking interactions between
their phenyl ring and the phenanthroline of the other ligand,
which is typical for these compounds.22,29 Thus, it is
confirmed that little variations in ligand structures result in
dramatic effect on the ground-state geometry of Cu(I)-
phenanthrolines. This is reflected in the electronic absorption
profiles of [Cu(1a)2]+ and [Cu(1b)2]+ which, in CH2Cl2
solution, turn out to be magenta and deep red, respectively.
Emission Spectra. The luminescence data and lifetimes
of [Cu(1a)2]+ and [Cu(1b)2]+ at 298 and 77 K in CH2Cl2
are collected in Table 2. The two metal complexes show a
broad luminescence band centered around 700 nm, attribut-
able to deactivation of MLCT excited states (Figure 4).21,22
The MLCT emission spectrum of [Cu(1b)2] has an
emission maximum at 690 nm and is much more intense
than the corresponding spectrum of [Cu(1a)2]+ (Table 2).
The emission band of [Cu(1b)2] is substantially blue shifted,
suggesting a relatively higher lying MLCT state, less affected
by nonradiative deactivations50 (energy-gap law). Clearly,
(46) Colton, R.; James, B. D.; Potter, I. D.; Traeger, J. C. Inorg. Chem.
1993, 32, 2626-2629.
(47) Molecular structure optimized by ZINDO/1 semiempirical self-
consistent field (SCF) calculations, with parameters according to
HyperChem 7.52 (Hypercube, Inc.)
(48) Miller, M. T.; Karpishin, T. B. Inorg. Chem. 1999, 38, 5246-5249.
(49) Miller, M. T.; Gantzel, P. K.; Karpishin, T. B. Inorg. Chem. 1999,
38, 8, 3414-3422.
(52) Vo¨gtle, F.; Lu¨er, I.; Balzani, V.; Armaroli, N. Angew. Chem., Int. Ed.
Engl. 1991, 30, 1333-1336.
(53) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145-164.
(54) Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K. Inorg. Chem.
2003, 42, 6366-6378.
(50) Felder, D.; Nierengarten, J. F.; Barigelletti, F.; Ventura, B.; Armaroli,
N. J. Am. Chem. Soc. 2001, 123, 6291-6299.
(51) Gushurst, A. K. I.; McMillin, D. R.; Dietrich-Buchecker, C. O.;
Sauvage, J. P. Inorg. Chem. 1989, 28, 4070-4072.
Inorganic Chemistry, Vol. 45, No. 5, 2006 2065

Kalsani et al.
the higher lying MLCT levels of this compound are the result
of a less distorted excited-state geometry compared to [Cu-
(1a)2]+; a similar trend has been observed for the ground
state (see above). Unlike the case of [Cu(1a)2]+, the lifetime
of [Cu(1b)2]+ is sensitive to the presence of molecular
oxygen, with a lifetime of 266 and 285 ns in air equilibrated
and oxygen-free solutions, respectively. However the effect
is small if compared to similar simpler compounds such as
[Cu(dtp)2]+ (dtp ) 2,9-ditolyl-1,10-phenanthroline), whose
lifetimes are 139 and 237 ns under the same conditions.52
Also some effect of solvent nucleophilicity is found for [Cu-
(1b)2]+, but nevertheless the compound is emissive in MeOH
with a respectable lifetime of 182 ns. These results demon-
strate that also the coordination environment of [Cu(1b)2]+
is well protected from external contacts, though not locked
as much as [Cu(1a)2]+.
At 77 K [Cu(1b)2]+ exhibits a very strong emission band,
as only rarely observed for [Cu(NN)2]+,50 which tend to
become very poor emitters in low-temperature rigid matrixes.
The bright orange luminescence observable under these
conditions, though a quantitative measure in a rigid matrix
is difficult, can be roughly estimated at least 100 times more
intense than that of [Cu(1a)2]+ and comparable to that of
[Cu(dhp)2]+ (dhp ) bis(2,9-hexyl-1,10-phenanthroline)) re-
ported earlier.50
So far, to the best of our knowledge, only Cu(I) complexes
of (some) alkylphenanthroline ligands have turned out to be
highly luminescent at 77 K,50 and this is likely to be the
first example where such a behavior is found for compounds
of phenylphenanthroline-type ligands. The shape of the
absorption spectrum and the position of the emission band
of [Cu(1b)2]+ are similar to those of alkylphenanthroline
Cu(I) complexes, pointing to little distortion from the ground-
state induced by the specific substituent onto the chelating
agents (see above). These findings and the low-temperature
behavior of [Cu(1b)2]+ support our previous hypothesis50 that
the occurrence of strong luminescence at 77 K in Cu(I)-
phenanthrolines depends on electronic (i.e. alkyl vs phenyl
substituents) and structural (i.e. distortion from the ground
state) factors, with the latter likely to play a predominant
role.
phenylphenanthroline ligands strongly emitting at 77 K; this
confirms that structural rather than electronic factors mainly
dictate the luminescence performances of these compounds
in rigid matrix.
The results presented here give further insight into the
rationalization of the photophysical properties of phenan-
throline ligands and related [Cu(NN)2]+ complexes and
provide valuable information for understanding the behavior
of more sophisticated architectures including such molecular
motifs as key building blocks.19,45
Experimental Section
1H NMR and 13C NMR were measured on a Bruker AC 400
(400 MHz) unless specified otherwise. ESI MS spectra were
measured on a LCQ Deca Thermo Quest instrument. Typically,
each time 25 scans were accumulated for one spectrum. Phenan-
throlines 1a,b and 3 were prepared according to known procedures
1
(Scheme 1).41 All compounds were characterized by H and 13C
NMR, ESI MS, IR, and elemental analysis.
2,9-Bis(3,5-di-tert-butyl-4-methoxyphenyl)[1,10]phenanthro-
1
line (1a): 78%; mp > 230 °C; H NMR (CD2Cl2, 400 MHz) δ
8.31 (d, 2H, J ) 8.3 Hz, phen), 8.12 (s, 4H, phenyl), 8.04 (d, 2H,
J ) 8.3 Hz, phen), 7.81 (s, 2H, phen), 3.77 (s, 6H, methoxy), 1.55
(s, 36, t-Bu); 13C NMR (CD2Cl2, 50 MHz) δ 161.0, 158.1, 146.3,
144.0, 136.7, 134.8, 127.7, 126.3, 125.9, 120.9 (arom), 64.4
(methoxy), 35.9, 32.0 (aliph); IR (KBr) ν 3387, 2960, 2867, 1623,
1607, 1590, 1498, 1466, 1408, 1392, 1359, 1313, 1259, 1225, 1115,
1006, 889, 854, 804, 738, 648; ESI MS m/z (%) 1296.1, [M +
H]+. Anal. Calcd for C42H52N2O2: C, 81.78; H, 8.50; N, 4.54.
Found: C, 81.71; H, 8.89; N, 4.42.
2-(3,5-Di-tert-butyl-4-methoxyphenyl)-9-(2,4,6-trimethylphenyl)-
1
[1,10]phenanthroline (1b): 63%; mp 182 °C; H NMR (CDCl3,
400 MHz) δ 8.30 (d, J ) 8.0 Hz, 1H, phen), 8.28 (d, J ) 8.0 Hz,
1H, phen), 8.15 (s, 2H, phenyl), 8.04 (d, J ) 8.4 Hz, 1H, phen),
7.83 (m, 2H, phen), 7.63 (d, J ) 8.0 Hz, 1H, phen), 7.01 (s, 2H,
phenyl), 3.75 (s, 3H, methoxy), 2.39 (s, 3H, benzyl), 2.32 (s, 6H,
benzyl), 1.54 (s, 18H, t-Bu); 13C NMR (CD2Cl2, 50 MHz) δ 161.5,
160.0, 158.5, 146.7, 146.3, 144.3, 138.3, 137.8, 136.9, 136.8, 135.8,
134.9, 129.0, 127.6, 127.4, 126.9, 126.4, 126.1, 125.6, 120.8, 64.9,
36.4, 32.5, 21.6, 21.4; IR (KBr) ν 3417, 2960, 2916, 2867, 1620,
1587, 1540, 1495, 1479, 1410, 1394, 1356, 1313, 1223, 1115, 1099,
1007, 887, 859, 751, 729, 631, 583; ESI MS m/z (%) 517.5 (100),
[M + H]+. Anal. Calcd for C36H40N2O: C, 83.68; H, 7.80; N, 5.42.
Found: C, 83.22; H, 8.24; N, 5.15.
Conclusions
2-(3,5-Di-tert-butyl-4-methoxyphenyl)[1,10]phenanthroline] (3):
71%; mp 156 °C; H NMR (CDCl3, 400 MHz) δ 9.24 (dd, J )
In this paper we present the synthesis of two novel
phenanthroline ligands (1a,b) and the preparation of the
corresponding homoleptic Cu(I)-bis(phenanthroline) com-
plexes [Cu(1a)2]+ and [Cu(1b)2]+, whose photophysical
properties turn out to be extremely different from each other
due to dramatic variations in the ground- and excited-state
geometries. Due to geometric constraints brought about by
the bulky tert-butyl ligands on the phenyl rings, the homo-
leptic phenanthroline complexes [Cu(1a,b)2]+ are kinetically
locked and do not exchange with added ligands. In agreement
with the calculated structure, the results of kinetic and
luminescence studies suggest that [Cu(1a)2]+ has an ex-
tremely blocked and shielded coordination environment.
Finally, [Cu(1b)2]+ turns out to be, to the best of our
knowledge, the first example of a [Cu(NN)2]+ complex with
1
4.3, 1.8 Hz, 1H, phen), 8.33 (d, J ) 8.3 Hz, 1H, phen), 8.29 (dd,
J ) 8.1, 1.8 Hz, 1H, phen), 8.21 (s, 2H, phenyl), 8.11 (d, J ) 8.3
Hz, 1H, phen), 7.84 (d, J ) 8.6 Hz, 1H, phen), 7.79 (d, J ) 8.6
Hz, 1H, phen), 7.67 (dd, J ) 8.1, 4.3 Hz, 1H, phen), 3.82 (s, 3H,
methoxy), 1,61 (s, 18H, aliph); 13C NMR (CDCl3, 100 MHz) δ
162.0, 159.9, 151.2, 151.1, 147.0, 144.9, 137.6, 136.9, 135.7, 129.9,
128.1, 127.6, 127.3, 126.9, 123.8, 122.2 (arom), 65.5 (methoxy),
36.9, 33.1 (alkyl); IR (KBr) ν 3395, 2959, 1618, 1589, 1547, 1508,
1489, 1445, 1409, 1362, 1226, 1117, 1009, 779, 746, 717, 627;
ESI MS m/z (%) 399.4 (100), [M + H]+. Anal. Calcd for
C27H30N2O: C, 81.37; H, 7.59; N, 7.03. Found: C, 81.08; H, 7.86;
N, 6.82.
General Procedure for the Copper Complex Formation.
Homoleptic complexes were prepared by reacting 1a or 1b with
2066 Inorganic Chemistry, Vol. 45, No. 5, 2006

Phenanthroline Ligands and Their Cu(I) Complexes
[Cu(MeCN)4]PF6 (1:0.5 equiv, respectively) in dichloromethane.55
All complexes were analyzed in solution by NMR and ESI MS
without any further purification. However, prior to elemental
analysis the copper complexes were recrystallized using dichlo-
romethane and/or diethyl ether. In some cases, the solvents showed
up in the elemental analysis.
149.1, 147.9, 146.9, 144.1, 143.7, 143.4, 143.1, 140.8, 139.0, 137.3,
136.5, 136.1, 135.4, 134.4, 133.9, 129.0, 128.0, 126.8, 126.5, 125.8,
124.9, 117.6, 116.7 (arom), 63.4 (methoxy), 35.1, 31.0 (3C) (aliph);
IR (KBr) ν 3423, 2965, 1618, 1583, 1509, 1459, 1411, 1223, 1111,
1003, 840, 729, 557, 541; ESI MS m/z (%) 759.4 (100), [M - PF6]+.
Anal. Calcd for [CuC48H48N4O‚PF6]: C, 63.67; H, 5.34; N, 6.19.
Found: C, 63.34; H, 5.53; N, 5.85.
1
[Cu(1a)2]+: H NMR (CD2Cl2, 400 MHz) δ 8.38 (d, J ) 8.5
Hz, 4H, phen), 7.98 (s, 4H, phen), 7.85 (d, J ) 8.5 Hz, 4H, phen),
7.48 (s, 8H, phenyl), 3.20 (s, 12H, methoxy), 0.90 (s, 72H, t-Bu);
13C NMR (CD2Cl2, 100 MHz) δ 161.9, 157.6, 144.1, 144.0, 136.7,
133.6, 128.8, 126.9, 126.3, 126.1 (arom), 63.6 (methoxy), 35.3,
31.7 (aliph); IR (KBr) ν 3393, 2961, 1582, 1504, 1451, 1409, 1391,
1349, 1314, 1113, 1004, 862, 841, 753, 557, 526; ESI MS m/z (%)
1296.1 (100), (M - PF6]+. Anal. Calcd for CuC84H104N4O4‚PF6:
C, 69.95; H, 7.27; N, 3.88. Found: C, 69.76; H, 7.58; N, 3.69.
Photophysical Measurements. Absorption spectra were re-
corded with a Perkin-Elmer λ40 spectrophotometer. Emission
spectra were obtained with an Edinburgh FLS920 spectrometer
(continuous 450 W Xe lamp), equipped with a Peltier-cooled
Hamamatsu R928 photomultiplier tube (185-850 nm) or a
Hamamatsu R5509-72 supercooled photomultiplier tube (193 K,
800-1700 nm range). Corrected spectra were obtained via a
calibration curve supplied by the instrument manufacturer. Emission
quantum yields were determined according to the approach
described by Demas and Crosby56 using air-equilibrated [Os-
(phen)3]2+ in acetonitrile (Φem ) 0.005)57 as standard. O2 removal
from CH2Cl2 solutions was accomplished by bubbling argon for
10 min. Emission lifetimes were determined with the time-correlated
single photon counting technique using an Edinburgh FLS920
spectrometer equipped with a laser diode head as excitation source
(1 MHz repetition rate, λexc ) 407 or 637 nm, 200 ps time resolution
upon deconvolution) and an Hamamatsu R928 PMT as detector.
All measurements were carried out in spectroscopy grade solvents,
used without further purification.
1
[Cu(1b)2]+: H NMR (CD2Cl2, 200 MHz) δ 8.54 (d, J ) 8.3
Hz, 2H, phen), 8.50 (d, J ) 8.3 Hz, 2H, phen), 8.15 (d, J ) 8.3
Hz, 2H, phen), 8.05-8.12 (m, 6H, phen), 7.47 (s, 2H, phenyl),
7.44 (s, 2H, phenyl), 6.29 (s, 2H, MesH), 5.74 (s, 2H, MesH), 3.41
(s, 6H, methoxy), 2.12 (s, 6H, benzyl), 2.09 (s, 12, benzyl), 0.99
(s, 36H, t-Bu); ESI MS m/z (%) 1095.7(100), [M - PF6]+. Anal.
Calcd for CuC72H80N4O2‚PF6‚0.5CH2Cl2: C, 67.80; H, 6.36; N,
4.36. Found: C, 67.62; H, 6.49; N, 4.33.
[Cu(1a)(phen)]+: 1H NMR (CD2Cl2, 200 MHz) δ 8.66 (d, J )
8.1 Hz, 2H, phen), 8.47 (d, J ) 4.3 Hz, 2H, phen), 8.34 (d, J )
8.1 Hz, 2H, phen), 8.16 (s, 2H, phen), 8.04 (d, J ) 8.4 Hz, 2H,
phen), 7.91 (s, 2H, phen), 7.63 (dd, J ) 8.1, 4.3 Hz, 2H, phen),
7.27 (s, 4H, phenyl), 2.98 (s, 6H, methoxy), 0.82 (s, 32H, t-Bu);
13C NMR (CD2Cl2, 50 MHz) δ 160.2, 158.9, 148.1, 143.9, 143.5,
142.9, 137.0, 136.1, 134.8, 129.4, 128.0, 126.4, 126.3, 126.1, 125.9,
125.1 (arom), 64.1 (methoxy), 35.1, 31.0 (aliph); IR (KBr) ν 3404,
2922, 1586, 1508, 1459, 1408, 1356, 1224, 1115, 1006, 838, 726,
557; ESI MS m/z (%) 859.5 (100), [M - PF6]+. Anal. Calcd for
[CuC54H60O2‚PF6‚CH2Cl2]: C, 60.58; H, 5.73; N, 5.14. Found: C,
60.51; H, 5.88; N, 5.03.
Acknowledgment. We acknowledge financial support
from the DFG (Schm 647-12 and 647-13), the Fonds der
Chemischen Industrie, and the CNR (commessa PM-P04-
ISTM-C1/PM-P04-ISOF-M5, Componenti Molecolari e Su-
pramolecolari o Macromolecolari con Proprieta` Fotoniche
ed Optoelettroniche). We thank Ravuri S. K. Kishore for
experimental help and Dr. Andrea Barbieri for HyperChem
calculations.
[Cu(1b)(phen)]+: 1H NMR (CD2Cl2, 200 MHz) δ 8.64 (d, J )
8.4 Hz, 2H, phen), 8.46 (m, 3H), 8.17 (m, 4H), 7.92 (s, 2H, phen),
7.70 (m, 3H), 7.45 (s, 2H, phenyl), 5.71 (s, 2H, MesH), 2.80 (s,
3H, methoxy), 1.65 (s, 3H, benzyl), 1.57 (s, 6H, benzyl), 0.83 (s,
18H, t-Bu); 13C NMR (CD2Cl2, 50 MHz) δ 160.6, 159.5, 158.5,
IC051828V
(56) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024.
(57) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys.
Chem. 1986, 90, 3722-3734.
(55) Kubas, G. J. Inorg. Synth. 1979, 19, 90-92.
Inorganic Chemistry, Vol. 45, No. 5, 2006 2067