Combined effects of ion-pairing on multi-emissive properties of benzimidazolium salts

Article abstract of DOI:10.1039/d1tc00503k

The possibility to control luminescence properties of purely organic emitting salts is receiving an ever-growing interest. A full understanding of the manifold effects of counterions on the photophysical properties of charged luminophors deserves thorough

Full text of DOI:10.1039/d1tc00503k

Journal of
Materials Chemistry C
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Combined effects of ion-pairing on multi-emissive
properties of benzimidazolium salts†
Cite this: J. Mater. Chem. C, 2021,
, 4182
9
a
b
a
b
Gabriele Di Carlo, * Alessandra Forni,* Paola Moretti, Daniele Marinotto,
c
a
a
a
Chiara Botta, Maddalena Pizzotti, Francesca Tessore
and Elena Cariati
The possibility to control luminescence properties of purely organic emitting salts is receiving an ever-
growing interest. A full understanding of the manifold effects of counterions on the photophysical
properties of charged luminophors deserves thorough investigation. Here, we disclose the impact of
different anions, i.e. triflate, iodide and nitrate, on the multiple emissions of a new class of ionic
luminophors, based on 1,3-dimethyl-2-(p-tolyl)-1H-benzo[d]imidazol-3-ium, 1. The specific ion-pairing
plays a key role in the emissive properties of solids, depending on both the electronegativity of the
anion and its distance from the cation. Three main effects emerge from strong ion-pairing: (i)
fluorescence quenching, (ii) phosphorescence enhancement and iii) efficient direct population of the
triplet-state. As a result, a weak interaction (1-OTf) guarantees intense fluorescence (QY 4 50%), while a
Received 1st February 2021,
Accepted 26th February 2021
strong pairing (1-NO
phosphorescence (RTP) enhancement. The anion-controlling strategy also enables long-lasting RTP
288 ms) in 1-OTf and ultrabright phosphorescence (QY 4 70%) in 1-NO
Á0.5EtOH, under direct triplet-
excitation at 350 nm.
3
Á0.5EtOH and 1-IÁ0.5CH
2 2
Cl )
promotes molecular room temperature
DOI: 10.1039/d1tc00503k
(
3
rsc.li/materials-c
Variation of halides, as counterions, has been explored as a
viable strategy to manipulate the photophysical properties of
Introduction
1
9,20
To date purely organic compounds exhibiting room temperature cationic emitters.
luminescence in the solid state have made impressive strides emitting salts may positively affect their crystal packing,
as advanced optical materials for a wide range of potential resulting in enhanced emissive properties by reducing non-
On one hand, the counter-ion of light-
2
1
1–3
22
applications. The profitable properties, such as low-cost starting radiative decay channels. On the other hand, electronic
materials, rationally tailored molecular structures and good proces- coupling within ionic pairs is expected to influence the com-
4
–7
sability under mild conditions, make them very attractive. Mole- pound’s photophysical properties. Stronger external heavy
2,8,9
10,11
12
23
cular design,
crystal packing engineering,
co-crystallization
atom effect (EHE) has been observed in halogen-substituted
13,14
and heavy atom effect
are some viable strategies explored in the N-alkylcarbazole by varying the linker length and thus the
last decade to boost the phosphorescence of light-emitting solids. halogen–luminophore distance. However, the impact of ion
The switch between fluorescence and phosphorescence emissions separation on organic salts showing an unchanged ionic cou-
has also been precisely regulated either through external stimuli, ple, to the best of our knowledge, has never been reported to
15,16
such as mechanical forces and pH variation,
design.
or molecular date. Thus, the role of specific counterions requires immediate
attention in order to develop a strategy to control the optical
17
Very recently, a growing interest has emerged in designing properties of charged luminophores. This will open new ways to
organic salts as solid emitters with tunable photophysical design advanced purely organic room temperature phos-
1
8
6–9
properties by varying both cation and anion constituents.
phorescent (RTP) materials with tunable emissions by simply
varying the counterions.
a
In this work we report on the preparation and characteriza-
tion of a new class of ionic luminogens, namely benzimidazolium
salts, 1, with triflate, nitrate and iodide counterions featuring
different sizes and electronic properties. Four crystal structures
are isolated depending on the relative anion and crystallization
solvent producing two diverse nitrate crystals which differ in their
ionic distances. The resulting compounds exhibit multiple emis-
sions in the solid-state, spanning from fluorescence (FL) to
Department of Chemistry, University of Milan and INSTM Research Unit,
Via C. Golgi 19, 20133 Milan, Italy. E-mail: gabriele.dicarlo@unimi.it
b
CNR-SCITEC, Istituto di Scienze e Tecnologie Chimiche ‘‘G. Natta’’,
Via C. Golgi 19, 20133 Milan, Italy. E-mail: alessandra.forni@scitec.cnr.it
c
CNR-SCITEC, Istituto di Scienze e Tecnologie Chimiche ‘‘G. Natta’’,
Via A. Corti 12, 20133 Milano, Italy
†
Electronic supplementary information (ESI) available. CCDC 2018732–2018735.
For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/d1tc00503k
1
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M 3
molecular phosphorescence (PH ) and supramolecular phos- (F equal to 77.5, 52.6 and 53.6% for 1-OTf, 1-NO and 1-I
phorescence (PH ). Both the distance and the nature of counterions respectively). At 77 K (Fig. 1 right), alongside the FL, a very
S
are found to have an impact on the photophysical behavior by weak structured phosphorescence emerges at about 450 nm
affecting both molecular and supramolecular features of 1-salts. (tav 4 2 s, see Table 1 and Fig. S5, ESI†).
Spectroscopic studies along with theoretical investigations and
A combination of photophysical and NMR studies on diluted
structural analysis shed light on the role of ion-pairing, evidencing solutions of 1-salts in a low polar solvent has been performed in
a three-fold impact on the emissive behavior of 1-salts: FL order to study the ion pairing effect. NMR studies of the three
quenching, PH
triplet-states.
M
enhancing and direct excitation triggering of salts confirm an effect of the counterion moving from polar
DMSO-d (Fig. S1, ESI†) to non-polar CDCl solutions (Fig. S2,
ESI†) as also reported for p-twisted benzimidazolium
6
3
2
5
1
chromophores. In DMSO-d6 all the H-NMR signals of the
Results and discussion
three salts are superimposable, while in CDCl the chemical
3
Synthetic procedures
shifts of hydrogens, adjacent to the imidazolium ring (CH –N
3
and Hc), are affected by the nature of the counterion, in
agreement with a tighter ion pairing in the solvent with a lower
dielectric constant.
The synthetic procedures for the preparation of triflate (1-OTf),
nitrate (1-NO3) and iodide (1-I) salts of 1,3-dimethyl-2-(4-
methylphenyl)-1H-benzimidazolium (1) salts are trivial and occur
with excellent yields under mild conditions (Scheme 1). o-Phenyle-
nediamine and p-tolualdehyde in ethanol at room temperature
undergo efficient (490%) oxidative condensation (i) to give the
benzimidazole core through hypervalent iodine generated in situ
2
6,27
24
Likewise a bigger downfield shift of signals has also been
reported for bromine-substituted N-alkylcarbazole as a result of
the strongest interaction between the halogen and nitrogen
2
3
atoms in derivatives with shortest linkers. In 1-salts, an
increase in the electronegative character of counterions results
in a higher net positive charge of nitrogen atoms shifting the
protons to lower fields. This agrees with the more deshielded
4 2 2
from Bu NI/H O . The following double alkylation of nitrogen atoms
(ii) with iodomethane and tBuOK in THF at 60 1C results in
benzimidazolium iodide, 1-I in very good yield (470%). 1-I can be
quantitatively reacted with appropriate silver salts through the
3 3
CH –N and Hc protons of 1-I compared to those of 1-NO and
1
-OTf. To exclude any possible effect due to the low solubility of
metathesis reaction thus giving 1-OTf and 1-NO . The high purity
3
1
the salts in a non-polar solvent, their H-NMR spectra have also
been recorded in CDCl at the same concentration (10 M) as
grade of the final products, required for photophysical measure-
ments, is obtained by flash chromatography through the automated
À5
3
s
used for photophysical studies (Fig. S2, ESI†). Under these
Biotage system and multiple recrystallizations with proper
1
conditions, H-NMR spectra show the same deshielding trend
solvents.
À3
as that displayed by more concentrated solutions (10 M). Very
similar behavior was previously reported by some of us regarding
the chemical shift of protons, in the a position to the positively
charged nitrogen atom, of various methyl-pyridinium salts
Solutions
Absorption spectra of 1-salts in a diluted solution of polar
solvents (MeOH/EtOH 1 : 4) show three maxima at 210, 240
and 280 nm (Fig. S4a, ESI†). Similarly, the photoluminescence
spectra (PL) at 298 K (lexc = 300 nm, see Table 1 and Fig. 1 left)
show intense fluorescence at 345 nm (lifetime, t, 1.54–1.57 ns)
with a slight variation of quantum yields among the three salts
2
8
3
recorded in CDCl .
1
The H-NMR spectra of the three samples have also been
obtained from CD OD solutions (Fig. S3, ESI†). The Hc and
3
CH –N signals of 1-salts are affected by the specific ionic pair
3
3 3
observed in CDCl solutions (1-I 4 1-NO Z 1-OTf) although to a
lesser extent. This could reasonably suggest that the counterion
still plays a role in influencing the photophysical properties in
alcoholic solution by explaining the slight variation in the
recorded quantum yields of the investigated salts in MeOH/EtOH
diluted solutions. Accordingly, the differences in the optical
properties become more prominent when the three compounds
À5
are dissolved in CHCl (10 M). The same absorption bands
3
(240 and 282 nm) are observed for the three salts in polar and
non-polar solvents (Fig. S4a and S4b respectively, ESI†), while the
emissive properties are more significantly affected. A severe
quenching of fluorescence intensities at 345 nm (F = 43.6,
3
3.4 and 1.7% for 1-OTf, 1-NO and 1-I respectively) is flanked by
long-lived (tav = 4.34–5.58 ms) components at 450 nm (Table 1 and
Fig. S6, ESI†) which become visible in the steady-state PL spectra
under direct triplet-excitation at 350 nm (Fig. S4c, ESI†). Direct
population of triplet states has been recently reported as a
feasible way to bypass S
cence quantum efficiency or as a means to visualize the
1 1
–T ISC and improve the phosphores-
Scheme 1 Schematic synthetic procedure for the preparation of 1-I,
29
1
3
-NO and 1-OTf.
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Table 1 Photophysical properties of 1-salts at room temperature and 77 K
Solution
Solid state
Crystal
Entry
-OTf
MeOH/EtOH
CHCl
3
Entry
Powder
1
F
300
F
300
1-OTf
F
300
F
350
F
300
F
350
23.2%
77.5%
43.6%
50.7%
34.7%
57.9%
FL
345 nm
0.94 ns
345 nm
1.17 ns
4.59 ms
FL
345 nm
0.89 nsa
a
1
.54 ns
0.94 ns
b
PH
M
450 nm
450 nm
PH
M
S
410 nm
530 nm
3.08 ms a
46.80 ms
327.33 ms
a
2446.00 ms
PH
a
a
a
a
1
096.10 ms
1
-NO
3
F
300
F
3.4%
345 nm
300
1-NO
3
ÁEtOH
F
3.7%
345 nm
300
F
73%
350
F
2.2%
300
F
350
71%
52.6%
FL
345 nm
1.21 ns
0.97 ns
5.85 ms
FL
0.65 nsa
0.66 ns
5.43 ms a
29.26 ms
8.23 ms
a
1
.57 ns
b
PH
M
450 nm
450 nm
PH
M
S
415 nm
530 nm
a
2230.30 ms
PH
1104.40 ms
1-NO
3
ÁH
2
O
U
300
F
25%
350
F
4.0%
300
F
350
34.4%
1.9%
FL
345 nm
415 nm
530 nm
0.42 ns
a
0.76 ns
PH
M
S
11.43 ms
a
3
5.31 ms
12.18 ms
178.50 ms
PH
1
1
-I
F
300
F
300
1-IÁCH
2
Cl
2
F
300
F
23.7%
350
F
15.6%
300
F
350
26.1%
53.6%
1.7%
345 nm
11.4%
345 nm
FL
345 nm
1.20 ns
1.05 ns
4.36 ms
FL
0.45 nsa
1.05 ns
6.15 ms a
59.08 ms
15.03 ms
a
1
.55 ns
b
PH
M
450 nm
450 nm
PH
M
S
460 nm
a
2232.60 ms
PH
530 nm
1145.50 ms
a
b
Measurements conducted at 77 K. PH
M
not detectable (only t in ns timescale attributed to the intense FL component).
quantum efficiencies of 1-salts in CHCl suggests that the fluores-
3
cence quenching is strongly correlated with the closeness of the
anion and its electronegative character (I 4 NO Z OTf). In
3
agreement, the highest fluorescence quantum efficiency has been
observed in 1-OTf, which is endowed with the weakest ion-pairing
in solutions. The theoretical study well elucidates the PL char-
acteristics observed for 1-salts in solution. The simulated TDDFT
absorption spectrum of the isolated cation 1 displays three bands
at 193, 220 and 245 nm (see Fig. S14 and Table S3 (ESI†) for the
first singlet and triplet excitation energies). Its overall shape
reproduces the experimental one collected in MeOH/EtOH
Fig. 1 Emission spectra (300 nm excitation wavelength) of diluted solutions
À5
(
10 M in MeOH/EtOH) of 1-OTf (green), 1-NO
3
(blue) and 1-I (orange).
(
l
v/v 1/4) for the three salts (all displaying three absorptions at
abs = 210, 240 and 280 nm), though the computed maxima are
shifted towards slightly higher energies. The S - S excitation
30
phosphorescent emission when overwhelmed by fluorescence.
0
1
Being visible also in polar solvents at 77 K, (Fig. 1, right side) such (oscillator strength f = 0.60) is mainly (75%) a HOMO - LUMO
a high energy long-lived emission is ascribed to molecular origin transition where both HOMO and LUMO are p orbitals,
(
PH
infra). Time-delayed spectra further show very weak phos- the first singlet and triplet states of 1 reveals the presence of a
phorescence emissions emerging over 500 nm (Fig. S4c, ESI†). triplet level (T ) of (p,p*) character just below S (DE = 0.162 eV,
M
) as also corroborated by computational investigation (vide delocalized over the whole molecule (Fig. S15, ESI†). Analysis of
7
1
As discussed in the section related to the solid-state, such a low- 8 nm). Such a small S–T energy gap could allow an otherwise
energy emission is attributed to supramolecular behavior (PHS), forbidden ISC from the singlet (S ) to the close triplet state (T )
1
7
associated with the possible formation of nanoaggregates in non- which decays to T1 by internal conversion. This explains the
polar solution. The switching on of PHM in CHCl solutions at observed molecular phosphorescence (from T ) accompanying
3
1
room temperature suggests an easier singlet–triplet (S–T) inter- the fluorescence (from S ) of luminogen 1 in MeOH/EtOH
1
system crossing (ISC) by ion-pairing assistance. The drop of solution at 77 K.
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To explore the possible effect of the counterion on the
electronic levels of 1, calculations on the 1-OTf, 1-NO and 1-I
3
ionic pairs have also been carried out, starting from the
respective X-ray structures of the three salts (see below) and
freezing angles and torsions to preserve the relative arrange-
ment of the ions as observed in the crystals (for 1-NO
3
the
structure of 1-NO O has been considered as the starting
3
ÁH
2
point). The first significantly populated (i.e. with f 4 0.1)
singlet excited state, despite its charge transfer (CT) character
from the anion towards 1 (Fig. S16, ESI†), is only slightly shifted
towards lower energies (248, 249 and 253 nm for 1-OTf, 1-NO3
and 1-I, respectively, see Table S4, ESI†) than that computed for
1
(245 nm), in accordance with the nearly superimposable
absorption spectra recorded in polar and non-polar solutions.
Looking at the closest triplet levels, it is found that the S–T
energy gap further reduces from 0.162 (1) to 0.068 (both 1-OTf
and 1-NO
3
pairs) and 0.010 eV (1-I), triggering an even easier
Fig. 2 (a) Steady state (solid line) and time-delayed (dotted line) photo-
S–T ISC with respect to the isolated cation in accordance with
luminescence emission at 300 nm excitation wavelength; (b) steady-state
FL quenching and PHM enhancement observed in non-polar photoluminescence emission at 350 nm excitation wavelength; and
(
c) photographs of 1-salt powders under natural and UV light (366 nm).
solvent. Notably, the lowest S–T energy gap calculated for 1-I
agrees well with the strongest electronegative character of
1
iodide as evidenced by H-NMR analysis.
To disclose the effect of counterions on solid state photo-
physical properties, the crystal structures of the four salts have
been determined by single crystal X-ray diffraction analysis.
Solid state
1
-OTf, 1-NO and 1-I crystallize in the P1% (1-OTf and 1-NO ÁH O)
3
3
2
In the solid state, 1-salts exhibit concomitant fluorescence and
multiple phosphorescence emissions whose relative position,
intensity and lifetime at room temperature are strongly depen-
dent on the counterion and crystal features.
and C2/c (1-NO Á0.5EtOH and 1-IÁ0.5CH Cl ) space groups (see
3
2
2
Fig. 3 and Fig. S12, ESI†). The asymmetric units of 1-NO
and 1-IÁ0.5CH Cl include one water and one-half dichloro-
methane cocrystallized molecule, respectively, while that of
-NO
Á0.5EtOH comprises three independent ionic pairs
labelled as A, B and C) and one and a half ethanol cocrystal-
3
ÁH O
2
2
2
Four different crystalline phases have been isolated from
acetonitrile/H O (1-NO ÁH O), ethanol (1-OTf and 1-NO Á
1
(
3
2
3
2
3
0
.5EtOH) and dichloromethane (1-IÁ0.5CH
2
Cl
2
). By exciting all
lized molecule. XRD analysis shows that 1 is far from being
planar, as indicated by the dihedral angle between the least-
squares (l.s.) planes through the benzimidazole moiety and the
phenyl ring, varying in the range 48.23–52.351 in the four
structures.
crystals at 300 nm at room temperature, fluorescence (t in the
ns regime, see Table 1 and Fig. S7–S11, ESI†) emission at about
350 nm is observed together with phosphorescent components
with properties depending on the specific anion and its relative
distance from the cation (see Table 1, Fig. 2 and Fig. S7a, ESI†).
In particular, while the steady-state spectrum of 1-OTf (Fig. 2a,
solid-line) displays only an intense fluorescent emission (FL),
the time-delayed spectrum (Fig. 2a, dotted-line) reveals the
presence of two phosphorescences at 410 nm (tav = 23.08 ms,
The benzimidazole moiety forms in all cases stacks of head-
to-tail p–p dimeric aggregates. Two independent stacks are
found in the crystal structure of 1-NO
3
Á0.5EtOH, one built up
of A molecules and the other from B and C molecules. Along the
stacks, the distances between benzimidazole centroids are
alternately equal to 3.610 and 4.316 Å (1-OTf), 3.555 and
PH
crystals of 1-NO
at 415 nm, tav = 11.43 and 5.43 ms, respectively) is already
observed in the steady-state spectrum together with FL at
50 nm (see Fig. 2a and Fig. S7a, ESI†). An additional long-
M
) and 530 nm (tav = 327.33 ms, PH
S
). Interestingly, for
3
ÁH O and 1-NO Á0.5EtOH the PH
2
3
M
component
(
3
lived component PH can be detected at 530 nm (t = 12.18 and
S
av
8
.23 ms for 1-NO
3
ÁH
2
O and 1-NO
3
Á0.5EtOH, respectively) in the
time-delayed spectra. The PL spectrum of 1-IÁ0.5CH
2 2
Cl dis-
plays an almost undetectable FL and an intense, broad phos-
phorescence band PH at 460 nm (tav = 6.15 ms). Again, PH is
M
S
detected only in the delayed spectrum at 530 nm (tav = 15.03 ms).
It is worth noting that the red shift of the PHM band observed in
nitrate (5 nm) and iodide (50 nm) salts with respect to that of
triflate, follows the trend of the electronegative character of the
anions.
Fig. 3 Fragment of crystal packing of (a) 1-OTf with shortest distances
between benzimidazole centroids (green circles) and (b) 1-NO
3
Á0.5EtOH
with shortest distances between the cation and anion (blue and red circles,
respectively) centroids for three independent ionic pairs.
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4
.526 Å (1-NO
3
ÁH
2
O), 3.416/3.487 and 5.227/5.180 Å (1-NO
Cl ). The short and weak PH
3
Á0.5EtOH, intensities (Fig. S7a, ESI†). 1-NO
3
ÁH
2
O displays intense FL
Á0.5EtOH displays the opposite,
stacks A/B,C) and 3.566 and 4.767 Å (1-IÁ0.5CH
2
2
M
while 1-NO
3
distances between centroids suggest the presence of H-dimers confirming that the ion-pairing effect increases with reducing
owing to very little slippages (0.49, 0.46, 0.53 and 0.62 Å for 1-OTf, distances between the ions thus making the ISC easier. Accordingly,
1
-NO ÁH O, 1-NO Á0.5EtOH and 1-IÁ0.5CH Cl , respectively, con- by decreasing the ionic distance, fluorescence is quenched and the
3
2
3
2
2
sidering, for 1-NO
3
Á0.5EtOH, the H-dimer of A molecules) and the intensity of phosphorescence increase. Although the co-crystallized
values of the angle y between the centroid–centroid vector and its solvent effect cannot be totally excluded, the impact of ionic distance
projection on the molecular plane (82, 83, 81 and 801 in the same on photophysical properties of the same ionic couple has never been
order as above). Such H-dimers are laterally shifted along the stacks investigated to date.
with much larger slippages (2.3, 2.8, 4.0 and 3.1 Å and y = 58, 52,
As expected by a reinforced ion-pairing, the decrease in
4
0 and 491 for 1-OTf, 1-NO ÁH O, 1-NO Á0.5EtOH and 1-IÁ0.5CH Cl , the fluorescence quantum efficiency in crystalline 1-OTf,
3
2
3
2
2
respectively). In all structures, the cations are laterally connected via 1-NO Á0.5EtOH, 1-NO ÁH O and 1-IÁ0.5CH Cl follows a trend
3
3
2
2
2
À
À
À
CF SO or NO3 or I anions through weak C–HÁÁÁX (X = F, O in similar to that observed in CHCl solutions (F equal to 51, 4, 2
3
3
3
triflate; X = N, O in nitrates and X = I in iodide) hydrogen bonds and 11%, respectively). The weak cation–anion interaction in 1-
HB). The strongest interactions correspond to HÁÁÁX distances/ OTf produces the highest quantum efficiency observed at
C–HÁÁÁX angles of 2.45 Å/1691 (X = O, 1-OTf), 2.49 Å/1771 (X = O, 300 nm excitation and guarantees ultrabright FL emission both
-NO O), 2.36 Å/1621 (X = O, 1-NO Á0.5EtOH) and 2.91 Å/1701 in the solution and in solid state.
ÁH
X = I, 1-IÁ0.5CH Cl ). In addition, in both nitrate structures the Concerning the low energy phosphorescence, PH
(
1
(
3
2
3
2
2
S
, of the four
cocrystallized solvent molecules behave as HB donors of medium salts, the single crystal XRD study allows explaining that its origin
À
strength with oxygen atoms of NO3 anions. The CH Cl cocrystal- is due to the presence of H-dimers in their crystal structure, as
lized molecule in 1-I, on the other hand, is weakly hydrogen bonded thoroughly demonstrated for other luminophores.
to both 1 and I , in agreement with a slow loss of crystallinity at
room temperature.
2
2
11,31
À
The presence of cocrystallized solvent, as well, may play a
role in the observed solid-state behavior introducing non
The separations between ionic pairs in the four structures radiative deactivation paths. In fact, shorter lifetimes are mea-
see Fig. 3b for 1-NO ÁH O (12.18 ms), 1-NO Á0.5EtOH (8.23 ms) and
Á0.5EtOH and Fig. S13, ESI†) reveal that sured for 1-NO
the shorter distances between cation–anion centroids increase 1-IÁ0.5CH Cl (15.03 ms) crystals compared with that of the
from 3.582 to 4.385, 4.596 and 5.296 Å moving from solvent free 1-OTf salt (327.33 ms) which exhibits ultralong RTP
(
3
3
2
3
2
2
1
-NO Á0.5EtOH to 1-IÁ0.5CH Cl , 1-NO ÁH O and 1-OTf, respec- with an afterglow of up to 2s. The different lifetimes of the two
3
2
2
3
2
tively. This trend clearly does not reproduce that expected from phosphorescence emissions at room temperature allow observing
NMR investigation, according to which the cation–anion dis- the blue to green color variation with the naked eye (Fig. 4).
tances should increase in the order I o NO
on the electronegativity effect. In the crystal state other factors
come into play and result in the shortest value for 1-NO
.5EtOH. This is obviously imputable to the reduced dimen- allows visualizing the PH
sions of the counter-ion (compared to that of 1-OTf) and of FL. The corresponding PL emission spectrum of 1-OTf crys-
the cocrystallized solvent molecule (compared to that of tals (Fig. 2b), in fact, shows only PHM at 410 nm (F350 = 34.7%).
3
r OTf based only
In agreement with the excitation spectra (Fig. S7, ESI†), the
state of the four crystals can be directly populated by exciting
T
1
3
Á
at 350 nm. As stated above, the direct excitation of triplet-states
emission overwhelmed by intense
0
M
3
0
1
-IÁ0.5CH Cl ) and to the 2 : 1 ratio between the ionic pair and Similarly, the PL spectra of nitrate and iodide crystals exhibit
2
2
cocrystallized solvent molecule (compared to the 1 : 1 ratio in their corresponding PH
-NO O). Thus, the strength of the ionic pair concomitantly enhancement of quantum efficiencies. Remarkably, 1-NO
ÁH
depends on the anion electronegativity and cation–anion dis- 0.5EtOH crystals show impressively high phosphorescence
M
bands (Fig. 2b) with a sizable
1
3
2
3
Á
1
5,32
tances and is expected to be reduced from the most electro- quantum efficiency (F350
=
73%).
1-NO
3
ÁH
2
O
and
negative iodide to the less electronegative and more distant 1-IÁ0.5CH
2
Cl crystals exhibit a minor, still consistent, increase
2
triflate. However, the close proximity of nitrate to the cation in their relative quantum yields (F350 = 25% and 23.7%
could reasonably provide a reinforced ion-pairing in 1-NO Á respectively). Such an efficient direct population of the triplet
3
0.5EtOH.
state, reflecting in quantum yields above 20% in all crystals, is
These analyses, together with theoretical calculations
reported above, allow justifying the photophysical behavior of
crystalline 1-salts. The presence of heavy atoms along with the
narrowest S–T gap computed for 1-IÁ0.5CH
2 2
Cl well agree with
an easier ISC, thus producing PH enhancement and conco-
M
mitant FL quenching as a result of the strong EHE. On the other
hand, the 1-OTf PL-spectrum shows only the FL band while in
1
-NO both FL and PHM are displayed. Noticeably, the emissive
3
patterns of 1-NO
the same counterion but with different anion–cation center of
mass separation, show the inverted ratio of FL and PH
3
Á0.5EtOH and 1-NO
3
ÁH
2
O salts, equipped with
Fig. 4 Afterglow of 1-OTf crystals at 298 K and 77 K before and after
ceasing UV excitation (366 nm).
M
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Journal of Materials Chemistry C
3
3–35
also evidenced by their bright luminescence under the excita- surface defects and the reduction of crystal sizes,
all of
tion of a 366 nm lamp (Fig. 2c). Except for 1-OTf, which shows a them affecting the photophysical properties of ground 1-salts.
lowering in the quantum efficiency when moving from 300 to
3
50 nm excitation wavelengths, most likely attributed to the
Conclusions
loss of the intense FL emission, a boosted quantum yield is
observed for 1-NO ÁH O and 1-IÁ0.5CH Cl
Á0.5EtOH, 1-NO
crystals. In agreement, these latter ones show, at 350 nm
excitation, higher ratios between radiative (K ) and non-
radiative rate constant (Knr) than those obtained by exciting
at 300 nm (Table S1, ESI†).
3
3
2
2
2
In this work, we develop a facile strategy to manipulate the
emissive properties of a new class of ionic luminogens, pre-
pared with different counterions, i.e. triflate, iodide and nitrate,
based on benzimidazolium ion 1. The photophysical study
along with theoretical calculations, single crystal X-ray diffrac-
r
Despite the presence of solvent molecules in the crystal
1
tion analysis and H-NMR investigation, performed in diluted
packing, the direct triplet-excitation of 1-NO Á0.5EtOH crystals
3
solutions of both polar and low polar solvents, have allowed
disclosing the effect of the counterion on molecular and
supramolecular emissive properties of 1-salts. Four crystals
have been obtained, i.e., 1-OTf, 1-IÁ0.5CH Cl , 1-NO ÁH O and
produces the highest phosphorescence quantum efficiency in
the series. Considering the same counterion in 1-NO
and 1-NO O, the former is characterized by the shorter
distance between cation–anion centers of mass (3.582 vs.
3
Á0.5EtOH
3
ÁH
2
2
2
3
2
1
-NO Á0.5EtOH. They display at room temperature multiple
À1
3
4.596 Å), and smaller non-radiative (Knr = 50.00 vs. 65.56 s
)
emissions, comprising fluorescence (FL), molecular phos-
phorescence (PH and supramolecular phosphorescence
PH ), the latter associated with the presence of strong p–p
À1
and greater radiative (K
r
= 135.19 vs. 21.85 s ) rate constants.
M
)
Once again this supports the role played by the counterion and
its vicinity to the luminophore in activating the phosphores-
cence emission. Furthermore, the differences in quantum
efficiencies observed between the two nitrate salts under
(
S
stacking interactions in the crystal structures. The position and
relative intensities of these emissions are found to depend on
the nature of the counterion as a combination of ion-pairing
effects and supramolecular interactions. In particular, crystals
of 1-OTf, displaying a weaker anion–cation interaction, provide
ultrabright fluorescence and supramolecular long-lasting
350 nm excitation suggest that the stronger the ion-pairing,
the more efficient the direct triplet-excitation.
RTP; 1-IÁ0.5CH
2 2
Cl crystals mainly show molecular phos-
Crystallinity dependent optical responses
phorescence, promoted by the EHE of iodide counter-ion granting
an efficient singlet–triplet ISC; nitrate salts, characterized by
different cation–anion distances, exhibit simultaneous fluores-
cence and molecular phosphorescence. More specifically, crystals
of 1-NO ÁH O display intense FL and weak PH while those of
The multi-emissive patterns of 1-salts vary with mechanical
stimuli produced by the grinding of crystalline samples in an
agate mortar. By excitation at 300 nm, ground 1-I and 1-NO3
(
(
both forms) exhibit FL enhancement and PHM quenching
Fig. 5) but almost unchanged overall quantum efficiency.
3
2
M
1
-NO Á0.5EtOH show an opposite trend. For the latter salt, having
3
Ground 1-OTf displays a slight increase only in its fluorescent
emission (QY300 = 57.9%).
Upon excitation at 350 nm, the effect of grinding on the
isolated phosphorescence emission can be analyzed. Ground
the shortest anion–cation distance within the series of examined
compounds, an impressive phosphorescence quantum efficiency
is obtained by the direct population of the triplet states.
These findings clearly demonstrate that strong ion-pairing is
associated with (i) severe quenching of fluorescence; (ii) enhance-
ment of molecular phosphorescence by facilitating singlet–triplet
ISC; (iii) efficient direct population of triplet states. The supra-
molecular long-lasting RTP observed at lower energies appears to
be less affected by the specific anion owing to the close similarity
in the stacking pattern of the examined structures. The presence
of co-crystallized solvent molecules, however, clearly affects the
1
-OTf shows a reduced QY350 (23.2%) with respect to that
obtained in highly ordered crystals (34.7%). On the other hand,
in 1-NO Á0.5EtOH, the variation of QY is almost negligible
3
350
after mechanical micronization. Finally, an increased QY350 is
observed for both ground 1-NO O (F350 = 34.4%) and
ÁH
Cl (F350 = 26.1%). Such variations could be ascribed
3
2
1-IÁ0.5CH
2
2
to a number of factors including, for example, the partial loss of
the loosely packed cocrystallized solvent, the introduction of
S
PH lifetimes, which are significantly longer for 1-OTf, free from
co-crystallized solvent molecules, than for the other salts.
The results obtained for the ion-pairing effects are expected
to contribute to the understanding of the key-principles governing
the emissive properties of ionic salts, providing a tool for the
design of new luminescent ionic molecules with fine-tailored
optical properties.
Author contributions
G. D. C. and P. M. synthesized and characterized 1-salts. D. M.
and orange lines respectively) and after grinding (grey lines) (lexc = 300 nm). performed the photophysical study. C. B. performed time-delayed
Fig. 5 PL spectra of 1-NO
3
Á0.5EtOH and 1-IÁ0.5CH
2 2
Cl solids before (blu
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Journal of Materials Chemistry C
Paper
spectroscopic investigation. A. F. performed crystallography and 16 Y. Li, Y. Lei, L. Dong, L. Zhang, J. Zhi, J. Shi, B. Tong and
computational investigation. F. T. recorded the absorption spectra
Y. Dong, Chem. – Eur. J., 2019, 25, 573–581.
and performed data curation and M. P. was involved in editing 17 W. Xi, J. Yu, M. Wei, Q. Qiu, P. Xu, Z. Qian and H. Feng,
processes. The project was conceived by E. C., A. F. and G. D. C.
Chem. – Eur. J., 2020, 26, 3733–3737.
The manuscript was written by E. C., A. F. and G. D. C. All authors 18 Z. Cheng, H. Shi, H. Ma, L. Bian, Q. Wu, L. Gu, S. Cai,
have given approval to the final version of the manuscript.
X. Wang, W. Xiong, Z. An and W. Huang, Angew. Chem., Int.
Ed., 2018, 57, 678–682.
1
2
9 J. Wang, X. Gu, H. Ma, Q. Peng, X. Huang, X. Zheng,
S. H. P. Sung, G. Shan, J. W. Y. Lam, Z. Shuai and
B. Z. Tang, Nat. Commun., 2018, 9, 2963.
0 G. Chen, H. Feng, F. Feng, P. Xu, J. Xu, S. Pan and Z. Qian,
J. Phys. Chem. Lett., 2018, 9, 6305–6311.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
21 G. Zhang, X. Zhang, L. Kong, S. Wang, Y. Tian, X. Tao and
J. Yang, Sci. Rep., 2016, 6, 37609.
The use of instrumentation purchased through the Regione
Lombardia-Fondazione Cariplo joint SmartMatLab Project is
gratefully acknowledged. G. D. C. greatly thanks the University
of Milan (Piano Sostegno alla Ricerca 2018 LINEA 2 Azione
A – Giovani Ricercatori) for financial support.
2
2 J. Wang, X. Gu, P. Zhang, X. Huang, X. Zheng, M. Chen,
H. Feng, R. T. K. Kwok, J. W. Y. Lam and B. Z. Tang, J. Am.
Chem. Soc., 2017, 139, 16974–16979.
23 X. Sun, B. Zhang, X. Li, C. O. Trindle and G. Zhang, J. Phys.
Chem. A, 2016, 120, 5791–5797.
2
4 S. Riera-Galindo, A. Orbelli Biroli, A. Forni, Y. Puttisong,
F. Tessore, M. Pizzotti, E. Pavlopoulou, E. Solano, S. Wang,
G. Wang, T.-P. Ruoko, W. M. Chen, M. Kemerink,
M. Berggren, G. di Carlo and S. Fabiano, ACS Appl. Mater.
Interfaces, 2019, 11, 37981–37990.
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