Angewandte
Communications
Chemie
Mechanochromic Materials
Bismuth-Based Coordination Polymers with Efficient Aggregation-
Induced Phosphorescence and Reversible Mechanochromic
Luminescence
Abstract: Two bismuth coordination polymers (CPs), (TBA)-
(aggregation-caused quenching, ACQ). However, in some
cases, the restriction of intramolecular motion (RIM) that is
observed upon aggregation can also have the opposite effect,
namely an increase in the emission efficiency as a result of the
blocking of rotations and vibrations that are responsible for
non-radiative pathways in non-rigid environments: This
process is called AIE (aggregation-induced emission) or
[
BiBr (bp4mo)] (TBA = tetrabutylammonium) and [BiBr -
4 3
(
bp4mo) ], which are based on the rarely used simple ditopic
2
ligand N-oxide-4,4’-bipyridine (bp4mo), show mechanochro-
mic luminescence (MCL). High solid-state phosphorescence
quantum yields of up to 85% were determined for (TBA)-
[
BiBr (bp4mo)] (l = 540 nm). Thorough investigations of
4 em
[
3–7]
the luminescence properties combined with DFT and TDDFT
calculations revealed that the emission is due to aggregation-
induced phosphorescence (AIP). Upon grinding, both samples
became amorphous, and their luminescence changed from
yellow to orange and red, respectively. Heating or exposure to
water vapor led to the recovery of the initial luminescence.
These materials are the first examples of mechanochromic
phosphors based on bismuth(III).
AIP (aggregation-induced phosphorescence).
Such mate-
rials have received much attention in the past decade because
they have great potentialities for MCL compared to non-AIE
[8–17]
materials.
MCL compounds are smart materials whose
emission color changes when a mechanical force such as
grinding is applied. Some of these materials can also recover
their initial emission upon exposure to vapors or heating.
[
9,13]
Most MCL materials are AIE organic compounds
(mainly
[
11]
[12]
pyrene
and anthracene
derivatives) whose changes in
T
he search for solid-state luminescent materials and materi-
emission are usually accompanied by the amorphization of
the sample, which is due to their conformational flexibility
and the facile modification of weak intermolecular interac-
tions (p–p, weak hydrogen bonding) under pressure. Metal
complexes are another interesting class of materials that may
als with tunable luminescence, such as those exhibiting
mechanochromic luminescence (MCL), is of great interest
owing to their enormous potential in optical and optoelec-
tronic devices, such as lighting and displays, memory devices,
and sensors. The use of luminogens based on various
complexes and coordination polymers (CPs) is very promising
[
8]
II[14]
exhibit MCL. Most of these complexes are based on Pt
III [15]
I
II
I
I
and Ir , whereas some based on Au , Zn , Ag , Cu , and
[
1]
III
[8,16,17]
in fields such as lighting technology or chemical sensors
Al have also been reported.
[
2]
(
porous CPs). A disadvantage of molecular luminogens,
In contrast to the well-known, commercially available
including CPs, can be the reduction of the emission efficiency
upon aggregation as a result of concentration quenching
ditopic ligand N,N’-dioxide-4,4’-bipyridine (bp4do), which is
[
18]
extensively used in the CP field, N-oxide-4,4’-bipyridine
[19]
(
bp4mo) has rarely been used. To the best of our knowl-
edge, no bismuth complexes or CPs based on bp4mo or bp4do
have been reported thus far, whereas we recently showed the
ability of bipyridinium derivatives, including one pyridyl-N-
oxide and N-oxide-2,2’-bipyridine, to bind bismuth ions,
[
*] Dr. O. Toma, M. Allain, Prof. N. Mercier
MOLTECH-Anjou, UMR-CNRS 6200, University of Angers
2
Bd Lavoisier, 49045 Angers (France)
E-mail: nicolas.mercier@univ-angers.fr
[
20]
giving AIE complexes. Moreover, bismuth ions and halides
have a great tendency to form halobismuthate anions,
Dr. C. Botta
Istituto per lo Studio delle Macromolecole (ISMAC), CNR
Via Corti 12, 20133 Milano (Italy)
E-mail: c.botta@ismac.cnr.it
[
21]
particularly with bipyridinium derivatives.
Herein, we
report two luminescent CP materials obtained with the
Dr. A. Forni
BiBr /bp4mo system: (TBA)[BiBr (bp4mo)] (1; TBA = tet-
3
4
Istituto di Scienze e Tecnologie Molecolari
Universitꢀ degli Studi di Milano
via C. Golgi 19, 20133 Milano (Italy)
E-mail: a.forni@istm.cnr.it
rabutylammonium) and [BiBr (bp4mo) ] (2) show AIP with
3 2
quantum yields (QYs) of 85% (1) and 15% (2). Furthermore,
both materials exhibit MCL, which is unprecedented for
bismuth(III)-based compounds.
Dr. F. Meinardi
CP 1 was obtained as yellow needle-like crystals by the
slow evaporation of a mixture of bp4mo·2H O, BiBr , and
Dipartimento di Scienza dei Materiali
Universitꢀ degli Studi di Milano Bicocca
via Cozzi 55, 20125 Milano (Italy)
2
3
(TBA)Br, whereas yellow crystals of 2 were prepared by
a
solvothermal method from mixture of BiBr3,
a
bp4mo·2H O, and (TBA)Br in MeCN at 758C. The asym-
2
metric unit of the structure of 1 includes one bp4mo molecule
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!