Luminescence enhancement and tuning via multiple cooperative supramolecular interactions in an ion-paired multinuclear complex

Article abstract of DOI:10.1039/c1cc11161b

[(3,5-(CF₃)₂Pz)(AgL)₂]⁺[Ag ₅(3,5-(CF₃)₂Pz)₆(CH ₃CN)]^- (L = 2-(N,N-diethylanilino-4-yl)-4,6-bis(3,5- dimethylpyrazol-1-yl)-1,3,5-triazine) shows bright and tunable emissions influenced by its supramolecular structure. Columnar stacks are assembled via cooperative interactions that include Ag^I...Ag^I argentophilic bonding, π...π stacking and Ag^I...π interactions.

Full text of DOI:10.1039/c1cc11161b

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COMMUNICATION
Luminescence enhancement and tuning via multiple cooperative
supramolecular interactions in an ion-paired multinuclear complexw
a
Chi Yang,a Oussama Elbjeirami,z Chammi S. Palehepitiya Gamage,b H. V. Rasika Diasb
and Mohammad A. Omary*a
Received 26th February 2011, Accepted 11th April 2011
DOI: 10.1039/c1cc11161b
[(3,5-(CF3)2Pz)(AgL)2]+[Ag5(3,5-(CF3)2Pz)6(CH3CN)]ꢀ (L = 2-
(N,N-diethylanilino-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,
5-triazine) shows bright and tunable emissions influenced by its
supramolecular structure. Columnar stacks are assembled via
cooperative interactions that include AgIꢁ ꢁ ꢁAgI argentophilic
bonding, pꢁ ꢁ ꢁp stacking and AgIꢁ ꢁ ꢁp interactions.
Metalꢁꢁꢁmetal, pꢁꢁꢁp, and metalꢁꢁꢁp supramolecular interactions
play crucial roles in transport and storage of charge and energy,
both in natural systems and synthetic materials.1 Examples
include the cofacial porphyrin dimer in the light-harvesting
antennae of green plants and photosynthetic bacteria,2 the
stacked base pairs of duplex DNA and RNA in natural
systems,3 and the p stacking array of conjugated polymers in
materials science.4 Many cofacial molecular materials possess
high hole and charge mobility, rendering them significant in
optoelectronic devices.5–7
Scheme 1 Synthesis and chemical structure of 1.
Argentophilic (AgIꢁꢁꢁAgI) bonding and AgIꢁꢁꢁarene (metalꢁꢁꢁp)
supramolecular interactions are common motifs in silver(I)
coordination compounds that promise unique photonic, electro-
conductive, sensing, and catalytic applications.8–12 We are
interested in constructing luminophores with multiple attractive
interactions (e.g. metallophilic, pꢁ ꢁ ꢁp, and Mꢁ ꢁ ꢁp contacts) in a
single molecule. Such combinations are expected to impart
fascinating luminescence and unipolar or ambipolar electrical
conductivity based on the cooperativity and electronic communi-
cation between the aforementioned interactions. Here we
report the synthesis and remarkable structural and lumines-
cence properties of a new ion-paired cluster Ag(I) anilino-
bis(pyrazolyl)-triazine complex 1 (Scheme 1). Ambipolar ligands
such as L convey remarkable photophysical properties (e.g., L is
a superior antenna for Eu(III) sensitized phosphorescence via
unique singlet energy transfer).13 This work is the first among
our ongoing efforts to expand the coordination chemistry of
such chromophoric ligands to transition metal centers with
expected improvement in photophysical properties vs. non-
fluorescent ligands such as polypyridyls.
The synthesis of 1 (Scheme 1) is based on reaction of L with
the cyclic trinuclear silver(I) complex [(3,5-(CF3)2Pz)Ag]3.14
We have shown that such cyclic trimers form mononuclear or
dinuclear analogues upon reaction with other ligands.15 Hereby,
reaction of [(3,5-(CF3)2Pz)Ag]3 in CH2Cl2 with chromophore L
in CH3CN affords compound 1 as orange crystals in 68% yield.y
The crystal structure of 1 shows a dinuclear cation (Fig. 1a)
and pentanuclear anion (Fig. 1b).z The cation comprises two
4-coordinate AgI centers by coordination of one N atom from
bridging [3,5-(CF3)2Pz]ꢀ (avg. Ag–N = 2.168 A) and three
N atoms from the chelating L molecule; the two planar
[AgL] units adopt a cofacial arrangement of L chromophores
(Fig. 1a). This arrangement is facilitated by an intramolecular
argentophilic bond (3.089(9) A) that is further supported by the
nearly perpendicular bridging [3,5-(CF3)2Pz]ꢀ ligand to the
AgL plane. The AgꢁꢁꢁAg distance in the cation of 1 is similar to
that observed for Ag(I)-terpyridine complexes (3.005–3.142 A).16
The two planar L chromophores in the cation are in parallel
planes to one another but are partially staggered with a torsion
angle of 53.341 about the Ag1–Ag2 axis. The cofacial dimer
cations of 1 are stacked through strong AgIꢁ ꢁ ꢁp interactions
(2.861 and 3.007 A) and slightly-offset head-to-tail pꢁ ꢁ ꢁp
stacking interactions (3.296 A) to furnish a supramolecular
a Department of Chemistry, University of North Texas, Denton,
Texas 76203, USA. E-mail: omary@unt.edu
b Department of Chemistry and Biochemistry,
The University of Texas at Arlington, Arlington, Texas 76019,
USA. E-mail: dias@uta.edu
w CCDC 640177. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1cc11161b
z Present address: Department of Chemistry, King Fahd University of
Petroleum & Minerals Dhahran, Kingdom of Saudi Arabia.
c
7434 Chem. Commun., 2011, 47, 7434–7436
This journal is The Royal Society of Chemistry 2011

Scheme 2 Proposed conformational rearrangement of the binuclear
cation. Toggling between form A and B leads to on/off switching of
intramolecular interactions and affects the luminescence signal, which
can be further modified upon supramolecular association (Fig. 1c and d).
interactions (MIꢁ ꢁ ꢁMI metallophilic, pꢁ ꢁ ꢁp, and metalꢁ ꢁ ꢁp) in a
single molecule. Thus, although the free ligand L is only weakly
luminescent in polar solvents, 1 shows brilliant visible emissions
in such solutions or the solid state. The photophysical data
below are strongly governed by the supramolecular interactions
of the different structural moieties in 1, as illustrated in
Scheme 2. The conformation of 1 could be rather flexible, akin
to what we found in other Ag(I) azolate frameworks.20 Thus,
equilibrium between conformations A and B (Scheme 2), as well
as association of the latter can take place—depending on
factors such as solvent, concentration or temperature. In dilute
solutions, for example, the monomer conformation A proved
dominant, wherein the intramolecular AgIꢁ ꢁ ꢁAgI and pꢁ ꢁ ꢁp
interactions are off. In concentrated solutions, the intra-
molecular dimer B is dominant as the intramolecular AgIꢁ ꢁ ꢁAgI
and pꢁ ꢁ ꢁp interactions are turned on. Obviously the p-dimer
further aggregates as observed in the solid state (see Fig. 1c) and
could be present in highly concentrated solutions at low
temperatures. As shown in Fig. 2, the fluid-solution emission
spectrum of 1 consists of two broad emission bands centered at
460 nm (t = 1.0 ns) and 560 nm (t = 2.5 ns), assignable to
fluorescence of forms A and B, respectively. The higher-energy
band is similar in profile to the fluorescence of the coordinated
ligand L.13 The intensity ratio of the two emission bands is
strongly concentration dependent such that the higher-energy
band is dominant in dilute solutions while the lower-energy
band is dominant at higher concentrations (Fig. 2). These
spectral observations, therefore, substantiate the model in
Scheme 2 such that concentration-dependent transformation
between conformations A and B take place in solution.
Fig. 1 ORTEP plots of the cation [(3,5-(CF3)2Pz)(AgL)2]+ (a), the
anion [Ag5(3,5-(CF3)2Pz)6(CH3CN)]ꢀ (b), and the cation packing
columns showing the pꢁꢁꢁp and Agꢁꢁꢁp interactions (c) in crystals of 1.
Trace (d) shows a space-filing view of the stacked columns. Hydrogen
atoms, CF3, and CH3 groups are omitted for clarity.
structure consisting of one-dimensional columns running
along the a-axis (Fig. 1c and d). The strong AgIꢁ ꢁ ꢁp inter-
actions obviously stabilize the formation of supramolecular
stacks by counteracting electrostatic repulsion between cations
due to the attractive nature of cationꢁ ꢁ ꢁp interactions. One of
the silver(I) atoms in the cation exhibits a two positional
disorder with occupations of 0.67 and 0.33 for Ag2 and Ag20,
respectively (Fig. 1c). At the Ag2 position, the AgIꢁ ꢁ ꢁAgI
attraction is strong whereas the AgIꢁ ꢁ ꢁp interaction is strong
at the Ag20 position. Such disorders have been suggested to
minimize the stacking energy and possibly assist in charge
separation and energy storage in d10 columnar stacks.17
In the pentanuclear cluster anion, five [3,5-(CF3)2Pz]ꢀ
ligands coordinate to five AgI atoms in a m2-Z1,Z1 coordination
mode while another [3,5-(CF3)2Pz]ꢀ ligand serves as an asymmetric
m3-Z1,Z1,Z1 bridge with two N atoms coordinating to three
AgI atoms (Ag7–N30 2.088(5) A, Ag4–N29 2.535(5) A,
Ag3–N29 2.513(5) A, and Ag3–N29–Ag4 89.861). The five
Ag atoms in the pentanuclear cluster can be regarded as a
nearly square-pyramidal pentagon, wherein Ag7 occupies the
apical site while the remaining four Ag atoms are at the
equatorial plane. The shortest argentophilic distance within
the anion is Ag4ꢁ ꢁ ꢁAg5 (3.222 A). We note that such penta-
nuclear complexes are quite rare among multinuclear metal
clusters with diverse structures and geometries.18,19
The extended stacking of chromophores in the solid state
usually leads to fluorescence quenching, although Ag–Ag
interaction based on significant photoluminescence changes
in different aggregate states is known.11b However, due to the
heavy atom effect of silver, which induces spin–orbit coupling,
it is anticipated that intersystem crossing could be particularly
efficient at low temperatures in the solid state so as to sensitize
the phosphorescence from the aggregate of the coordinated
chromophore L. Indeed, complex 1 exhibits multiple bright
emission colors found to be phosphorescence (Fig. 2, bottom).
Two bright emission bands, green and orange-red, are exhibited
by the crystal, and can be isolated or mixed, at low temperatures.
Among the two unstructured major emissions, the lower-energy
The molecule L has been demonstrated to be a bright blue
emitter based on its charge transfer character.13 Compound 1
offers several strategies to fine- and coarse-tune both the
emission colors and intensities based on multiple supramolecular
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7434–7436 7435

In summary, we have demonstrated remarkable structural
and photophysical properties for a novel silver(I) supramolecular
compound consisting of a binuclear luminophoric cation
ion-paired by a pentanuclear cluster. Cooperative supra-
molecular interactions in the cationic stacks combined with
Ag heavy-atom effects imparted fascinating luminescence
across the UV/Vis region.
Support by the National Science Foundation (CHE-0911690
to M.A.O.) and the Robert A. Welch Foundation (B-1542 to
M.A.O. and Y-1289 to H.V.R.D.) is gratefully acknowledged.
C. Y. dedicates this manuscript to Prof. Yansheng Yang.
Notes and references
y 1: a solution of a mixture of L (41.6 mg, 0.1 mmol) in MeCN (5 mL)
and [(3,5-(CF3)2Pz)Ag]3 (186.4 mg, 0.2 mmol) in CH2Cl2 (20 mL) was
stirred at 60 1C for 0.5 h. The resulting yellow precipitate was collected
by filtration and washed with Et2O and cold CH2Cl2. Orange single
crystals of 1 were obtained by vapour diffusion of CH2Cl2 into MeCN
solution. Mp 263–265 1C (Dec.); elemental analysis: calcd (%) for
C83H66Ag7F42N31: C 32.68, H 2.18, N 14.23; found (%): C 32.88,
H 2.09, N 14.31. 1H NMR (CDCl3, RT): d 1.20 (t, 12H, J = 4 Hz,
NCH2CH3), 1.91 (s, 12H, CH3), 2.01 (s, 3H, CH3CN), 2.67 (s, 12H,
CH3), 3.41 (q, J = 4 Hz, 8H, NCH2CH3), 5.93 (s, 2H, 3,5-(CH3)2Pz-H),
6.00 (s, 1H, 3,5-(CF3)2Pz-H of cation), 6.54 (d, J = 4 Hz, 4H, Ph-H),
6.78 (s, 6H, 3,5-(CF3)2Pz-H of anion), 7.98 (d, J = 4 Hz, 4H, Ph-H).
z Crystal data for 1: C83H66F42N31Ag7, FW = 3050.76, monoclinic,
P21/a, a = 13.2664(5) A, b = 38.8979(15) A, c = 20.6041(8) A, b =
102.919(1)1, V = 10363.3(1) A3, Z = 4, T = 100 K, Dc = 1.924 g cmꢀ3
R1 = 0.0424, wR2 = 0.0916, GOF = 1.179. CCDC 640177.
;
Fig. 2 Top: emission spectra of 1 in MeCN: a: 2.5 ꢂ 10ꢀ7, b: 1.2 ꢂ 10ꢀ6
,
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,
and h: 2.6 ꢂ 10ꢀ3 mol Lꢀ1. Bottom: crystal emission (lexc = 330 nm)
´
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band with a maximum at B600 nm dominates at higher
temperatures, whereas the higher-energy emission with B525 nm
maximum dominates at lower temperatures. These two over-
lapping emissions can be distinguished by their respective
lifetimes at lower temperatures, nearly seconds for the green
emission (B0.4 s at 77 K) and milliseconds for the orange
emission (B96 ms at 77 K). Visually, irradiation of solid 1
with a hand-held UV lamp at near liquid nitrogen tempera-
tures results in bright orange emission that turns into green
when the UV illumination is turned off, consistent with the
aforementioned lifetime magnitudes. The green phosphorescence
band is present in the Gd(III) complex of L, rendering an
assignment to triplet emission from coordinated L.13 The
orange phosphorescence is more broad and brighter than the
green band, suggesting triplet emission from aggregated stacks
of the coordinated L ligand via Agꢁ ꢁ ꢁAg, pꢁ ꢁ ꢁp, and Agꢁ ꢁ ꢁp
interactions. Consistent with this assignment is the observation
that the intensities and lifetimes of the two triplet emission
bands decrease concomitantly upon heating. Finally, the overall
spectral data suggest that the light harvesting and emission
processes in 1 are primarily localized on the AgL cations,
whereas the pentanuclear anions simply act as counterions
from a photophysics standpoint. This is evidenced by the fact
that the fluorous metal–organic framework FMOF-1,20 which
comprises a similar [AgI(m-NN)]n high-nuclearity cluster to the
anionic cluster in 1, is not luminescent even at 77 K.
c
7436 Chem. Commun., 2011, 47, 7434–7436
This journal is The Royal Society of Chemistry 2011