Luminescence studies of copper(I)-containing [2]pseudorotaxanes

Article abstract of DOI:10.1139/V10-093

This report describes photoluminescence studies of copper-containing [2]pseudorotaxanes that mimic elements of functioning molecular machines. Excitation with visible light induces a formal oxidation of the metal center and simulates an actuation process. In all four [2]pseudorotaxanes studied, the ring ligand is the same, but the thread ligand is variable, namely 2,9-di(anisol-4-yl)-1,10-phenanthroline (dap), 6,6′-di(anisol-4-yl)-2, 2′-bipyridine (o-dabipy), 5,5′-di(anisol-4-yl)-2,2′-bipyridine (m-dabipy), or 8,8′-di(anisol-4-yl)-3,3′-bi-isoquinoline (dabiiq). The absorbance bandshapes suggest that aryl substituents extending from the core ligands engage in stacking interactions and induce a partially flattened structure in the ground state. More severe flattening occurs in the excited state and precludes the observation of emission if inter-ligand steric forces do not limit the distortion. Thus, the [2]pseudorotaxanes containing dap or o-dabipy as the thread ligand exhibit uncorrected emission maxima at around 720 nm in room-temperature dichloromethane, while the less constrained analogues, containing dabiiq or m-dabipy, are not emissive in fluid solution and barely exhibit a signal in rigid media. In dichloromethane, the luminescence quantum yields of the dap-and o-dabipy-containing systems are 6 × 10^-4 and 4 × 10^-4, and the excited-state lifetimes are 98 ns and 90 ns, respectively.

Full text of DOI:10.1139/V10-093

98
Luminescence studies of copper(I)-containing
[2]pseudorotaxanes
Breeze N. Briggs, Fabien Durola, David R. McMillin, and Jean-Pierre Sauvage
Abstract: This report describes photoluminescence studies of copper-containing [2]pseudorotaxanes that mimic elements
of functioning molecular machines. Excitation with visible light induces a formal oxidation of the metal center and simu-
lates an actuation process. In all four [2]pseudorotaxanes studied, the ring ligand is the same, but the thread ligand is vari-
able, namely 2,9-di(anisol-4-yl)-1,10-phenanthroline (dap), 6,6’-di(anisol-4-yl)-2,2’-bipyridine (o-dabipy), 5,5’-di(anisol-4-
yl)-2,2’-bipyridine (m-dabipy), or 8,8’-di(anisol-4-yl)-3,3’-bi-isoquinoline (dabiiq). The absorbance bandshapes suggest that
aryl substituents extending from the core ligands engage in stacking interactions and induce a partially flattened structure
in the ground state. More severe flattening occurs in the excited state and precludes the observation of emission if inter-
ligand steric forces do not limit the distortion. Thus, the [2]pseudorotaxanes containing dap or o-dabipy as the thread
ligand exhibit uncorrected emission maxima at around 720 nm in room-temperature dichloromethane, while the less con-
strained analogues, containing dabiiq or m-dabipy, are not emissive in fluid solution and barely exhibit a signal in rigid
media. In dichloromethane, the luminescence quantum yields of the dap- and o-dabipy-containing systems are 6 Â 10–4
and 4 Â 10–4, and the excited-state lifetimes are 98 ns and 90 ns, respectively.
Key words: copper(I), luminescence, rotaxane, charge transfer.
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Resume : On a effectue des etudes de photoluminescence de [2]pseudorotaxanes contenant d cuivre et qui imitent les ele-
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ments de machines moleculaires operationnelles. L’excitation a l’aide de lumiere visible induit une oxydation formelle du
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centre metallique et simule un processus d’activation. Dans les quatre [2]pseudorotaxanes etudies, le noyau du ligand est
ˆ
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le meme, mais le ligand commun est variable, soit la 2,9-di(anisol-4-yl)-1,10-phenanthroline (dap), la 6,6’-di(anisol-4-yl)-
´
2,2’-bipyridine (o-dabipy), la 5,5’-di(anisol-4-yl)-2,2’-bipyridine (m-dabipy) et la 8,8’-di(anisol-4-yl)-3,3’-biisoquinoleine
`
´
(dabiiq). Les formes des bandes d’absorption suggerent que les substituants aryles qui s’orientent vers l’exterieur des li-
`
gands de coeur donnent lieu a des interactions d’empilement et qu’ils induisent une structure partiellement aplatie dans
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l’etat fondamental. Un aplatissement plus important se produit dans l’etat excite et il empeche l’observation de l’emission
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si les forces steriques inter-ligand ne limitent pas la distorsion. Ainsi, les [2]pseudorotaxanes qui contiennent du « dap »
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ou du « o-dibipy » comme ligand commun donnent lieu a des maxima d’emission non corrigee autour de 720 nm, a la
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temperature ambiante, dans le dichloromethane, alors que les analogues avec moins de contraintes, contenant du « dabiiq »
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ou du « m-dapiry » ne donnent pas lieu a de l’emission en solution fluide et ils presentent a peine un signal en milieu so-
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lide. Dans la dichloromethane, les rendements quantiques de luminescence des systemes contenant du « dab » ou du
« o-dabipy » sont de 6 Â 10–4 et 4 Â 10–4 et les temps de vie des etats excites sont respectivement de 98 et 90 ns.
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Mots-cles : cuivre(I), luminescence, rotaxane, transfert de charge.
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[Traduit par la Redaction]
motion in response to an external input.2–4 For example, a
copper-containing [2]rotaxane with 1,10-phenanthroline and
2,2’:6’,6@-terpyridine stations in the ring ligand is capable of
displaying rotary motion, as recently demonstrated by
Raehm et al.5 Electrochemical actuation is possible because
the copper(I) and copper(II) oxidation states characteristi-
cally prefer different coordination numbers. Thus, the thermo-
dynamically favored copper(I) form is four-coordinate and
Introduction
Copper(I) is a useful templating agent for the synthesis of
molecules exhibiting interesting topologies.1 An example
is a two-component molecular assembly known as a
[2]rotaxane, which threads together like an axle through a
wheel. Elaborate versions can function as molecular machines,
i.e., molecular device prototypes that execute mechanical
Received 6 April 2010. Accepted 17 June 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 25 November
2010.
This article is part of a Special Issue dedicated to Professor J. C. Scaiano.
B.N. Briggs and D.R. McMillin.2 Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907-2084, USA.
F. Durola.1 Centre de Recherche Paul Pascal, UPR 8641 du CNRS, 115 avenue Schweitzer, 33600 Pessac, France.
´
´
J. Sauvage. Laboratoire de Chimie Organo-Minerale, Institut de Chimie, LC3 UMR 7177 du CNRS, Universite de Strasbourg, 4 rue
Blaise Pascal, 67070 Strasbourg CEDEX, France.
1Corresponding author (e-mail: durola@crpp-bordeaux.cnrs.fr).
2Corresponding author (e-mail: mcmillin@purdue.edu).
Can. J. Chem. 89: 98–103 (2011)
doi:10.1139/V10-093
Published by NRC Research Press

Briggs et al.
99
Fig. 1. Copper-containing [2]pseudorotaxanes studied along with
entails complexation to the phenanthroline embedded in the
ring component. Upon oxidation of the copper center, the
metal prefers to be five-coordinate, hence the ring ligand
pirouettes about the thread and coordinates to the metal via
the terpyridine moiety.5 Regeneration of the phenanthroline-
bound form occurs with reduction back to copper(I).
m-30 the ring ligand for each.
Photoluminescence studies are potentially relevant to the
dynamics of the system because population of the emitting
metal-to-ligand charge-transfer (MLCT) excited state entails
formal oxidation of the metal center.6–9 In general, two im-
+
portant structural changes occur in a Cu(NN)2 system after
absorption to the Franck–Condon state. (Herein, NN desig-
nates a diimine ligand such as 1,10-phenanthroline or 2,2’-
bipyridine). First, the excited state undergoes a flattening
distortion,7–11 which is complete in less than a picosec-
ond.12,13 Subsequently, at least in donor media, expansion
of the coordination number and solvent-induced exciplex
quenching occurs on a nanosecond time scale.12,13 There
are, however, few reports about luminescence studies in-
volving copper(I)-containing [2]rotaxanes and related sys-
tems.14 Most of the work focuses on systems designed to
produce charge separation. Armaroli et al.15 have, for exam-
ple, studied a copper(I)-containing [2]rotaxane that incorpo-
rated a bis(1,10-phenanthroline) ligand environment for the
metal center as well as bulky C60 groups as termini at both
ends of the thread ligand. They find that photoexcitation in-
duces net electron transfer from the copper center to a C60
and virtually complete quenching of the CT emission from
the copper center as well as the fluorescence from the C60
unit.15 Using a related structure, which included a zinc por-
phyrin as well, Li et al. have detected MLCT luminescence
from the bis(1,10-phenanthroline)copper center, but the life-
time is very short due to oxidative quenching by C60 accept-
ors.16
was obtained. The crude product was thoroughly washed
with water to remove sodium carbonate. The product was
then washed five times with CH2Cl2 to remove starting ma-
terial and soluble byproducts. The title product was thus ob-
tained pure as a white solid (54 mg, 46% yield).
1H NMR (DMSO-d6, 300 MHz): d (ppm) = 9.00 (d, 2H, J =
2.3 Hz), 8.45 (d, 2H, J = 8.3 Hz), 7.93 (dd, 2H, J = 8.3,
2.3 Hz), 7.79 (d, 4H, J = 8.8 Hz), 7.10 (d, 4H, J = 8.8 Hz),
3.84 (s, 6H).
For convenience, the emission studies described below
utilize [2]pseudorotaxanes, which lack the bulky ‘‘stopper’’
groups that prevent unthreading in a true [2]rotaxane. Figure
1 shows the structure of the m-30 ring ligand, which all systems
have in common. The thread component is any one of four dif-
ferent ligands also depicted in Fig. 1, namely 2,9-di(anisol-4-
yl)-1,10-phenanthroline (dap), 6,6’-di(anisol-4-yl)-2,2’-bipyridine
(o-dabipy), 5,5’-di(anisol-4-yl)-2,2’-bipyridine (m-dabipy), or
8,8’-di(anisol-4-yl)-3,3’-bi-isoquinoline (dabiiq).
Threaded complexes: Cu(m-30)(NN)+ PF6
–
In a typical procedure, Cu(CH3CN)4PF6 (1 equiv.) in de-
gassed CH3CN (0.5 mL per mg of complex) was added to a
stirred degassed solution of macrocycle m-30 (1 equiv.) in
CH2Cl2 (0.3 mL per mg of m-30). A deep orange color ap-
peared instantly. A short while later, a solution in CH2Cl2
(0.3 mL per mg ligand) of 1 equiv. of the diimine ligand
(NN, respectively, dap, o-dabipy, m-dabipy, or dabiiq) was
added at room temperature, and the solution immediately
turned dark red. After the solution was stirred for 2 h under
argon at room temperature, the solvents were removed under
vacuum. The dark red solid thus obtained was dissolved
again in a minimum of degassed CH3CN. As a precaution,
the solution was filtered, after which evaporation of the fil-
trate yielded the copper complex quasi-quantitatively.
Experimental
Synthesis
Previously published procedures yielded the coordinating
macrocycle m-3017 and the ligands dap,18 o-dabipy,19 and
dabiiq.20
5,5’-Dianisyl-2,2’-bipyridine (m-dabipy)
The 5,5’-dibromo-2,2’-bipyridine precursor21 (150 mg,
0.32 mmol), 4-methoxyphenyl boronic acid (106 mg,
0.704 mmol), and Na2CO3 (675 mg, 6.4 mmol) were placed
in a two-necked round-bottom flask under argon. Then, tol-
uene (30 mL), deionized water (10 mL), and ethanol (5 mL)
were added. The solution was degassed and stirred under ar-
gon, then Pd(PPh3)4 (37 mg, 0.032 mmol) was added. The
mixture was then degassed again and stirred at 90 8C for
16 h. Solvents were evaporated and a poorly soluble product
Cu(m-30)(dap)+ PF6
–
1H NMR (CD3CN, 300 MHz): d (ppm) = 8.65 (d, 2H, J =
8.4 Hz), 8.55 (d, 2H, J = 8.4 Hz), 8.25 (s, 2H), 8.09 (s, 2H),
7.92 (d, 2H, J = 8.4 Hz), 7.88 (d, 2H, J = 8.4 Hz), 7.55 (d,
4H, J = 8.7 Hz), 7.34 (d, 4H, J = 8.7 Hz), 6.09 (d, 4H, J =
8.8 Hz), 6.01 (d, 4H, J = 8.8 Hz), 3.82 (s, 4H), 3.73–3.70
(m, 4H), 3.60–3.57 (m, 4H), 3.55–3.51 (m, 4H), 3.46 (s,
Published by NRC Research Press

100
Can. J. Chem. Vol. 89, 2011
6H), 3.44–3.41 (m, 4H). ES-MS m/z = 1021.3218 (calcd.
1021.3238 for C60H54CuN4O8 ).
Fig. 2. Absorbance spectra of dap, o-dabipy, dabiiq, and m-dabipy
complexes in dichloromethane at room temperature.
+
Cu(m-30)(o-dabipy)+ PF6
–
1H NMR (CD3CN, 300 MHz): d (ppm) = 8.50 (d, 2H, J =
8.4 Hz), 8.27 (d, 2H, J = 8.0 Hz), 8.10 (t, 2H, J = 8.0 Hz),
8.02 (s, 2H), 7.90 (d, 2H, J = 8.4 Hz), 7.56 (d, 2H, J =
8.0 Hz), 7.54 (d, 4H, J = 8.8 Hz), 7.39 (d, 4H, J = 8.8 Hz),
6.44 (d, 4H, J = 8.8 Hz), 6.00 (d, 4H, J = 8.8 Hz), 4.02–3.99
(m, 4H), 3.76 (s, 4H), 3.74 (br, 8H), 3.73–3.70 (m, 4H), 3.36
(s, 6H). ES-MS m/z = 997.3212 (calcd. 997.3238 for
+
C58H54CuN4O8 ).
Cu(m-30)(m-dabipy)+ PF6
–
1H NMR (CD3CN, 300 MHz): d (ppm) = 8.74 (d, 2H, J =
8.4 Hz), 8.29–8.16 (m, 10H), 7.55 (d, 8H, J = 8.6 Hz), 7.06
(d, 4H, J = 8.6 Hz), 6.27 (d, 4H, J = 8.6 Hz), 3.85 (s, 6H),
3.82–3.79 (m, 4H), 3.78 (s, 4H), 3.77–3.74 (m, 4H), 3.69–
3.66 (m, 4H), 3.65–3.62 (m, 4H). ES-MS m/z = 997.3207
+
(calcd. 997.3238 for C58H54CuN4O8 ).
Cu(m-30)(dabiiq)+ PF6
–
1H NMR (CD3CN, 300 MHz): d (ppm) = 9.04 (s, 2H),
8.69 (d, 2H, J = 8.4 Hz), 8.63 (s, 2H), 8.18 (d, 2H, J =
8.3 Hz), 8.14 (s, 2H), 8.10 (d, 2H, J = 8.4 Hz), 7.93 (dd,
2H, J = 8.2, 7.2 Hz), 7.60 (d, 2H, J = 7.1 Hz), 7.49 (d, 4H,
J = 8.8 Hz), 7.26 (d, 4H, J = 8.8 Hz), 6.92 (d, 4H, J =
8.8 Hz), 6.15 (d, 4H, J = 8.8 Hz), 3.87 (s, 4H), 3.77 (s,
6H), 3.76–3.73 (m, 4H), 3.65–3.62 (m, 4H), 3.58–3.55 (m,
4H), 3.48–3.44 (m, 4H). ES-MS m/z = 1097.3534 (calcd.
+
1097.3551 for C66H58CuN4O8 ).
Methods
ratus used for the lifetime measurements is in the litera-
ture.23 The NMR spectrometer was a Bruker AVANCE 300
(300 MHz). The mass spectrometer was a Bruker MicroTOF
(ES-MS).
In the emission studies involving solid-state or frozen-
glass samples, a 620 nm long-pass filter was in place to re-
ject scattered light. The excitation beam also passed through
a notch filter centered at 450 nm. For excitation and emis-
sion, the slit settings were 10 nm and 20 nm, respectively.
The glassing solvent was a 4:1 mixture of ethanol:methanol,
and the sample holder was a 3 mm i.d. quartz tube im-
mersed in a finger dewar filled with liquid nitrogen. The
same assembly without cryogen also allowed for measure-
ments from solid-state samples. For frozen glass and solid-
state emission measurements, the blank was either an empty
capillary tube or one loaded with a non-emissive, solid-state
copper(I) compound. The procedure for deoxygenation of
dichloromethane entailed a sequence of freeze–pump–thaw
cycles. For lifetime measurements, a combination of 620
and 700 nm long pass filters prevented scattered light from
impinging on the detector. The setup also included a 450 nm
notch filter in the laser beam. The laser dye was Coumarin
440 in ethanol at a concentration of 6.1 Â 10–3 mol/L. The
method of Parker and Rees yielded estimates of emission
quantum yields.22 Residual solvent protons (1H NMR:
CD3CN: 1.96 ppm, DMSO-d6: 2.50 ppm) served for refer-
encing NMR signals.
Results
The solvent employed for most of the solution work was
dichloromethane (DCM) wherein the four copper complexes
exhibit the visible absorption spectra depicted in Fig. 2. In
each case, a strong, variably structured absorption occurs in
the vicinity of 450 nm along side a broad shoulder that ap-
pears at ~580 nm. The shoulder is especially prominent in
the spectra of the o-dabipy and m-dabipy complexes. De-
spite similar absorption throughout the series, only the dap
and o-dabipy complexes exhibit a detectable emission signal
in room-temperature solution. The emission is broadband in
both cases and maximizes between 710 nm and 720 nm
(Table 1); see Fig. 3 for data pertaining to the o-dabipy
complex. In deoxygenated DCM at room temperature, the
luminescence quantum yields are 6 Â 10–4 and 4 Â 10–4 for
the dap and o-dabipy complexes, respectively. The corre-
sponding emission lifetimes are 98 ns and 90 ns. In an alco-
hol glass at 77 K, the same two complexes exhibit an
emission maximum at about 740 nm, and the lifetime of the
emission signal extends considerably to 990 ns for the dap
complex and 870 ns for the o-dabipy analogue. In the frozen
glass, the dabiiq and m-dabipy complexes also emit, though
very weakly, and exhibit lifetimes of 185 ns and 155 ns, re-
Instrumentation
The spectrophotometer for the absorbance measurements
was a Varian Cary 300 Bio unit. A Varian Cary Eclipse unit
yielded photoluminescence data. A description of the appa-
Published by NRC Research Press

Briggs et al.
101
Table 1. Emission data for Cu(m-30)(NN)+ systems.
la nm (t, ns)
Complex
298 K in DCMb
77 K in EtOH/MeOH 298 K in solid state
Cu(m-30)(dap)+
Cu(m-30)(o-dabipy)+
Cu(m-30)(m-dabipy)+
Cu(m-30)(dabiiq)+
718 (98)
745 (990)
740 (870)
760 (155)
760 (185)
720 (270)
730 (150)
770 (—c)
770 (—c)
715 (90)
c
—
c
—
aFrom uncorrected spectra.
bQuantum yields: dap complex 6.4 Â 10–4; o-dabipy complex 4.0 Â 10–4.
cNot resolved.
Fig. 3. Uncorrected emission spectra of the hexafluorophosphate
salt of Cu(m-30)(o-dabipy)+ measured in room-temperature dichlor-
omethane (thin), the solid state (thick  1/2), as well as a 77 K al-
cohol glass (-D- Â 1/6), and from the corresponding salt of Cu(m-
30)(m-dabipy)+ in an alcohol glass at 77 K (-&-). Emission inten-
sities are arbitrary.
Fig. 4. Uncorrected emission spectra measured from solid samples
of [Cu(m-30)(dap)]PF6, [Cu(m-30)(o-dabipy)]PF6, [Cu(m-30)(m-
dabipy)]PF6, and [Cu(m-30)(dabiiq)]PF6 at room temperature.
wavelength (730 nm), and that may account for the smaller
increase in the emission lifetime (to 150 ns). Figure 4 shows
that the dabiiq and m-dabipy complexes both exhibit weak
emissions, centered at ~770 nm in the solid state. Unfortu-
nately, neither signal is strong enough to permit a lifetime
measurement.
spectively. See Fig. 3 for the 77 K spectrum of the [2]pseu-
dorotaxane with m-dabipy as the thread ligand. The emissions
from the dap and o-dabipy complexes are both much more
intense and durable enough to persist in the weakly basic
solvent acetonitrile at room temperature, although the life-
times contract to 50 ns and 30 ns, respectively.
Discussion
CT absorption and emission
The emission results are qualitatively similar in the solid
state; see Fig. 4. In the case of the dap complex, the solid-
state emission spectrum virtually coincides with that ob-
tained in room-temperature, fluid solution, but the lifetime
extends to 270 ns. On the other hand, the emission maxi-
mum for the o-dabipy complex shifts to slightly longer
Previous work has shown that the MLCT excitation that
dominates the visible spectrum of a Cu(NN)2 complex orig-
inates in the dp orbitals of the metal that have nonzero
overlap with empty p*(NN) acceptor orbitals of the li-
gand.24,25 The presence of ligands with chemically distinct
p systems in the thread and ring components may introduce
+
Published by NRC Research Press

102
Can. J. Chem. Vol. 89, 2011
+
Fig. 5. Schematic view of bonding overlap formed by the half-
occupied ds orbital of a copper(II) center and the donor orbitals of
two a,a-diimine ligands in the limit of a planar coordination geo-
metry.
plex (90 ns in DCM). Normally, Cu(NN)2 complexes based
on 2,2’-bipyridine ligands have relatively short-lived excited
states, e.g., 34 ns for Cu(6,6’-diphenyl-4,4’-dimethyl-2,2-bi-
pyridine)2 . For comparison, the corresponding excited-
+ 33
state lifetime is 310 ns for Cu(2,9-diphenyl-4,7-dimethyl-
+ 37
1,10-phenanthroline)2 . Ligand-dependent distortions may
account for the difference in lifetimes of the two controls,
especially torsional relaxation about the 2,2’ bond of the bi-
pyridine ligand. In each case, bond length changes also oc-
cur in the excited state due to population of a p* orbital of
the ligand. Bond length changes are apt to be greater in a
bipyridine complex because the p* orbital delocalizes over
fewer atoms than is the case with a phenanthroline moiety.
One possibility is that the structural changes are less impor-
tant in the o-dabipy-containing [2]pseudorotaxane because
the CT excitation actually localizes on the m-30 ring ligand.
In line with this possibility, the emission occurs at about the
same energy whether the thread ligand is dap or o-dabipy.
The m-30 ring ligand is no panacea, however, because the
a broadening of the visible spectrum. Moreover, the should-
ers that appear at l ~575 nm in Fig. 2 are characteristic of
+
Cu(NN)2 systems in which at least one of the ligands has
+
bis-thread complex Cu(dap)2 exhibits a longer excited-state
an aryl substituent positioned ortho to one or both of the
imine nitrogens.26,27 The shoulders result from two distinct
ground-state distortions that destroy idealized D2d symmetry
and split the dp orbital energies.28–30 One is a ‘‘rocking’’
distortion that imparts a pyramidal coordination geometry,
probably promoted by the formation of a stacking interac-
tion involving an aryl substituent on one ligand and the aro-
matic core of the opposite ligand.29 The second distortion
entails a tetragonal flattening of the N4 donor set about the
copper center.30 For the latter distortion, the driving force is
presumably the mesomeric interaction(s) that results (result)
when the aryl substituent(s) adopts (adopt) smaller dihedral
angle(s) relative to the mean plane of the NN ligand.30 Sig-
nificant flattening is possible with a very modest driving
force because the major opposing force derives from interli-
gand steric interactions when the central ion is a d10 closed-
shell metal center. The same interactions account for notable
differences in the formation constants of Cu(NN)+ and
lifetime (260 ns26) than the dap-containing [2]pseudoro-
taxane. Vibrationally assisted radiationless decay may be
+
less efficient for Cu(dap)2 if the absence of a ring ligand
permits better nesting of the ground- and excited-state po-
tential energy surfaces.
Moreover, the m-30 ligand is not sufficiently structure-
preserving to permit a photoluminescence signal if fluid sol-
ution from the [2]pseudorotaxane containing either m-dabipy
or dabiiq as the thread ligand. In both cases, the anisole sub-
stituents of the thread ligand are evidently far enough re-
moved from the opposite m-30 ligand that appreciable
flattening occurs in the excited state and quenches the emis-
sion. Even in the solid state, where packing forces can come
into play and help preserve structure,11 the emissions from
the m-dabipy and dabiiq complexes are very weak and max-
imize at relatively long wavelengths.
In summary, absorbance bandshapes reveal that all of the
[2]pseudorotaxanes studied adopt low symmetry, partially
flattened structures due to intra- and inter-ligand interactions
involving bulky anisole substituents. Sterically active ligand
substituents must be present to inhibit a flattening distortion
in the excited state that quenches the photoluminescence.
However, the steric interactions may produce inter-ligand
‘‘friction’’ that affects the mechanical response time. In fact,
the [2]pseudorotaxanes that contain either the relatively
open dabiiq or m-dabipy ligand (and exhibit faster mechani-
cal motion38) are non-luminescent in room-temperature di-
chloromethane solution, whereas the dap and o-dabipy
analogues exhibit emissions that maximize at ~720 nm and
decay with time constants of 98 ns and 90 ns, respectively.
The lifetime for the Cu(m-30)(o-dabipy)+ system is more
than twice what one would expect for a simple bis-bipyridine
analogue. At 77 K in a frozen glass, the decay times for the
emissions from Cu(m-30)(dap)+ and Cu(m-30)(o-dabipy)+
extend to almost a microsecond, whereas in room-temperature
acetonitrile solution, the same complexes undergo solvent-
induced exciplex quenching on a time scale of just under a
microsecond, according to lifetime data.
+
Cu(NN)2 systems with aryl- as opposed to alkyl-substituted
NN ligands.31
Further structural changes occur in the MLCT excited
state due to the d-shell vacancy created, another conse-
quence of which is the shift in the X-ray absorption edge of
the copper center.12 Nominally containing copper(II), the
thermally equilibrated excited state becomes even more flat-
tened, as a consequence of a second-order Jahn–Teller ef-
fect.11 In more descriptive terms, additional flattening
occurs because it leads to improved overlap amongst sigma-
donor orbitals of the ligand and the half-occupied d orbital
of copper in the CT excited state (Fig. 5). Indeed, the same
kind of flattening is evident in the structures of oxidized
Cu(NN)2 analogues.32,33 When the flattening is severe, as
2+
+
in the case of Cu(phen)2 , the excited-state potential energy
surface may impinge on the ground-state surface. The emis-
sion yield then becomes vanishingly small due to efficient
non-radiative decay, even in frozen solution.34,35 Indeed, for
+
a Cu(NN)2 system to exhibit emission in solution, sterically
active groups must be present adjacent to the donor nitro-
gens to limit the extent of flattening.7,9,36
Within the [2]pseudorotaxane series studied herein, the m-
30 ring ligand is a key structural element that clearly has a
salutary effect on the emission lifetime of the o-dabipy com-
Acknowledgements
The authors acknowledge support from the National Sci-
Published by NRC Research Press

Briggs et al.
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