Near-Infrared Boron Difluoride Formazanate Dyes

Article abstract of DOI:10.1002/chem.202004793

Near-infrared (NIR) dyes are sought after for their utility in light harvesting, bioimaging, and light-mediated therapies. Since long-wavelength photoluminescence typically involves extensive π-conjugated systems of double bonds and aromatic rings, it is often assumed that NIR dyes have to be large molecules that require complex syntheses. We challenge this assumption by demonstrating that facile incorporation of tertiary amine groups into readily available 3-cyanoformazans affords efficient production of relatively simple NIR-active BF₂ formazanate dyes (λ_abs=691–760 nm, λ_PL=834–904 nm in toluene). Cyclic voltammetry experiments on these compounds reveal multiple reversible redox waves linked to the interplay between the tertiary amine and BF₂ formazanate moieties. Density-functional calculations indicate that the NIR electronic transitions in BF₂ formazanates are of π→π*-type, but do not always involve strong charge transfer.

Full text of DOI:10.1002/chem.202004793

Accepted Article
Accepted Article
Title: Near-Infrared Boron Difluoride Formazanate Dyes
Authors: Francis L Buguis, Ryan R Maar, Viktor N Staroverov, and Joe
B. Gilroy
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.202004793
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0
1/2020

Chemistry - A European Journal
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RESEARCH ARTICLE
Near-Infrared Boron Difluoride Formazanate Dyes
[a]
[a]
[a]
[a]
Francis L. Buguis, Ryan R. Maar, Viktor N. Staroverov, and Joe B. Gilroy*
[
a]
F. L. Buguis, Dr. R. R. Maar, Prof. Dr. V. N. Staroverov, Prof. Dr. J. B. Gilroy
Department of Chemistry and The Centre for Advanced Materials and Biomaterials Research (CAMBR)
The University of Western Ontario
1151 Richmond Street North, London, Ontario N6A 5B7 (Canada)
E-mail: joe.gilroy@uwo.ca
Abstract: Near-infrared (NIR) dyes are sought after for their utility in
light harvesting, bioimaging, and light-mediated therapies. Since
long-wavelength photoluminescence typically involves extensive -
conjugated systems of double bonds and aromatic rings, it is often
assumed that NIR dyes have to be large molecules that require
complex syntheses. We challenge this assumption by demonstrating
that facile incorporation of tertiary amine groups into readily available
rigidity of large -electron systems, this strategy often results in
dyes with small Stokes shifts and poor solubility, as in the case
of cyanines (e.g., I), the most common candidates for
[
15]
bioimaging applications. The second strategy aims to promote
twisted intramolecular charge transfer to enhance NIR
[
13a, 14e, 16]
photoluminescence.
Dyes of this type, exemplified by
[
6b, 13b]
compound II, have found utility in biomedical applications.
3
-cyanoformazans affords efficient production of relatively simple
NIR-active BF formazanate dyes (λabs = 691−760 nm, λPL
34−904 nm in toluene). Cyclic voltammetry experiments on these
compounds reveal multiple reversible redox waves linked to the
interplay between the tertiary amine and BF formazanate moieties.
Density-functional calculations indicate that the NIR electronic
transitions in BF formazanates are of *-type, but do not always
Molecular integration involving noncovalent interactions (as in
2
=
III) is another effective strategy for the realization of NIR-I and
[
13b, 16-17]
8
NIR-II photoluminescence.
The purpose of this work is to call attention to a novel
strategy of creating NIR dyes. This strategy involves formazans
1, redox-active nitrogen-rich analogs of -diketimines, which
have already gained prominence in the area of functional
2
2
[
18]
involve strong charge transfer.
molecular materials. BF
1
2
complexes 2 derived from formazans
[
18-19]
are readily accessible, easily tunable dyes
promise as cell-imaging agents,
light-emitters, and electron acceptors in photovoltaic cells.
that show
electrochemiluminescent
[20]
[21]
[22]
Introduction
Synthetic dyes that absorb and photoluminesce in the near-
infrared (NIR) spectral region have attracted tremendous interest
[
1]
[2]
due to their emerging utility in light harvesting, bioimaging,
[
3]
and biomedical applications. When used for light harvesting
e.g., in photovoltaic cells), NIR dyes harness those regions of
the solar energy spectrum that are untapped by conventional
dyes, thereby increasing the efficiency of energy conversion. In
the bioimaging arena, dyes that photoluminesce in the NIR-I
(
[
4]
(
700‒950 nm) and NIR-II (1000‒1700 nm) regions enable
deeper tissue penetration, improve signal-to-noise ratios, and
[5]
enhance imaging capabilities.
State-of-the-art imaging
techniques that rely on NIR dyes include fluorescence
[
6]
[7]
microscopy, two-photon excitation microscopy, photoacoustic
[
8]
imaging, and tomography. The deep-tissue penetration offered
by NIR dyes is also well-suited for light-mediated therapies,
where controlled and localized doses of thermal energy are used
[
5a, 9]
[
3d, 10]
for tumor treatment.
The utility of NIR dyes in many active research areas
provides motivation for designing readily accessible, high-
performance molecular architectures. Several strategies for the
production of NIR dyes have been proposed so far: i) extension
[
11],[12]
of -electron conjugation;
acceptor interactions to induce charge transfer;
and iii) modulation of properties via controlled aggregation.
ii) tuning of electron donor-
[
6b,
13]
[
14]
The first approach, while seemingly fail-safe, involves complex
[
11, 12e]
molecules with extended syntheses.
Due to the inherent
1
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Chemistry - A European Journal
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RESEARCH ARTICLE
1
the H NMR spectra between 12.01 and 12.70 ppm (depending
on the formazan), as well as the appearance of a quartet in the
1
9
11
F NMR spectra (: ‒132.3 to ‒137.4) and triplet in the B NMR
spectra (: ‒0.4 to ‒0.8). A Stille reaction was used to produce
BF formazanate 2d in a 65% yield (Scheme 1, Figures S13 and
S14). This particular compound was prepared in an effort to
2
1
Recently, we discovered that a BF
2
formazanate 2 with Ar =
CN exhibits both NIR
888 nm) and
electronically isolate the NPh
2
units from the BF
2
formazanate
5
3
Ar
=
p-C
6
H
4
NMe
2
and
(λPL
R
=
=
framework and promote charge transfer.
photoluminescence
[21c]
Analysis of the bond lengths in the X-ray structures of 2a
and 2c (Figure 1 and Table 1) suggests a significant quinoidal
character at the N-aryl-substituents: the N6−C6 [1.3689(17) Å]
and N7−C12 [1.3656(17) Å] bonds in 2a and N6−C6
electrochemiluminescence (λECL = 910 nm) despite its relatively
simple structure (MW = 383.21 g mol ) and small -electron
Given that these properties are extraordinary for
such a small molecule, we and others have wondered about
their chemical underpinnings. To address this question, we have
−
1
[21c]
system.
[
1.3739(12) Å] and N7−C12 [1.4027(11) Å] bonds in 2c are
3
[24]
shorter than a typical N(sp )−C(Ar) bond [~1.425 Å] and the N−C
bond [1.418(4) Å] in triphenylamine.
devised a novel synthetic route leading to amine-substituted BF
2
[25]
This observation is
formazanates 2, which enabled us to perform a detailed
experimental and theoretical investigation of the optoelectronic
properties of these compounds. We will show here that the
observed NIR photoluminescence of these complexes arises
from the interplay between the formazanate core and tertiary
amine substituents.
consistent with the planar nature of the amine nitrogen atoms
and the fact that the sums of the bond angles around nitrogen
are 359.7(1) for 2a and 360.03(8) for 2c. The boron atoms are
four-coordinate in each structure, adopting slightly distorted
tetrahedral geometries, and are displaced from the planes
defined by the N-aryl substituent and the four nitrogen atoms in
the formazanate backbone (N1, N2, N3, and N4), with average
dihedral angles of 18.63° and 15.61° for 2a and 2c, respectively.
The NNCNN backbone of the formazanate complex is
delocalized with the N−C [1.3315(16)−1.3356(17) Å] and N−N
Results and Discussion
A new synthetic pathway was developed for the production
of amine-substituted formazans 1 in appreciable yields. Addition
of the appropriate aryl diazonium tetrafluoroborate salt(s)
[1.3134(14)−1.3143(14)
[1.3371(11)−1.3384(11) Å] and N−N [1.3061(11)−1.3108(11) Å]
for 2c bond lengths intermediate of single and double bonds for
the respective atoms involved (Table 1),
efficient electronic delocalization.
Å]
for
2a
and
N−C
[
24]
(
Scheme S1) to THF solutions containing CH
3
CN and nBuLi at
suggesting an
−78 C produced 3-cyanoformazans 1a−1c in yields ranging
from 23 to 66% (Scheme 1, Figures S1−S4). The relatively low
yield of 1b (23%) is due to the use of a 50:50 mixture of two aryl-
diazonium salts (implying a 50% maximum theoretical yield). For
comparison, the previously reported synthesis of compound 1a
under conditions typical for the production of 3-cyanoformazans
When probed by cyclic voltammetry, amine-substituted BF2
formazanates 2a−2d display multiple reversible redox events.
Each of compounds 2a−2d exhibits two one-electron reduction
[
19b]
waves (Figure 2 and Table 2) typical of BF formazanates
2
at
half-wave potentials Ered1 = −0.67 to −1.03 V and Ered2 = −1.65 V
to −1.99 V relative to the ferrocene/ferrocenium redox couple in
CH Cl . The observed trend in the reduction potentials of 2a−2d
[
21c]
had a yield of just 11%,
while 1b and 1c could not be
produced at all under similar conditions. Formazan 3 was
2
2
1
5
prepared by adapting an existing procedure (Figure S5 and
correlates well with the electron-donating ability of the R and R
substituents (NMe2 > NPh2 > C H NPh ). In most cases the
[
23]
S6).
BF
yields ranging from 52 to 86% by stirring the corresponding 3-
cyanoformazans in toluene solutions containing excess NEt and
BF •OEt (Scheme 1, Figures S7−S12). This transformation was
accompanied by a distinctive loss of the diagnostic NH signal in
6
4
2
2
formazanate complexes 2a−2c and 4 were obtained in
values are significantly more negative than the reduction
potentials found for the analogous BF formazanate with simple
2
1
5
3
phenyl substituents (R = R = H; E
−1.68 V).
red1
= −0.53 V and Ered2 =
[
19b]
3
2
2
Scheme 1. Synthesis of amine-substituted BF formazanates 2a−2d.
2
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RESEARCH ARTICLE
2
a
b
N6
C6
N7
2
C12
α2
α1
C3
C9
B1
N4
N3
N1
N2
2c
C1
2d
25 µA
2
Figure 1. Solid-state structure of BF formazanate 2c. Thermal displacement
ellipsoids are shown at 50% probability level. Hydrogen atoms are omitted for
clarity.
-2.3
-1.8
-1.3
-0.8
-0.3
+
0.2
0.7
1.2
E vs. Fc/Fc (V)
Table 1. Selected bond lengths, bond angles, and additional structural metrics
[
21c]
Figure 2. Cyclic voltammograms recorded for dry, degassed 0.05 mM
6
solutions of BF formazanates 2a‒2d in CH Cl containing 0.1 M [nBu N][PF ]
as supporting electrolyte at a scan rate of 100 mV s . The arrows denote the
initial scan direction.
for BF
2
formazanates 2a
and 2c. Additional structural refinement data for
2
2
2
4
2c are reported in Table S1.
−1
[
21c]
Metric
2a
2c
Bond lengths (Å)
N1−N2
N3−N4
N2−C1
N4−C1
N1−C3
N4−C9
N6−C6
N7−C12
N1−B1
1.3134(14)
1.3143(14)
1.3315(16)
1.3356(17)
1.4143(16)
1.4140(16)
1.3689(17)
1.3656(17)
1.5670(17)
1.5703(17)
124.35(10)
123.83(10)
130.32(11)
106.71(10)
0.136
1.3061(11)
1.3108(11)
1.3384(11)
1.3371(11)
1.4107(11)
1.4214(11)
1.3739(12)
1.4027(11)
1.5754(12)
1.5628(12)
124.48(7)
123.87(7)
130.03(8)
106.83(7)
0.116
Table 2. Cyclic voltammetry data recorded for BF
2
formazanates 2a−2d in
CH
2
Cl
2
reported relative to the ferrocene/ferrocenium redox couple.
Ered2 (V)
Ered1 (V)
Eox1 (V)
Eox2 (V)
[
21c]
1
5
[a]
2
2
2
2
a
(R = R = NMe
2
)
‒1.99
‒1.83
‒1.72
‒1.65
‒1.03
‒0.92
‒0.83
‒0.67
0.25
0.38
0.47
0.54
0.66
0.73
0.79
1
5
b (R = NPh
c (R = R = NPh
2
, R = NMe
2
)
1
5
2
)
1
5
6 4 2
d (R = R = C H NPh )
N4−B1
[a]
Irreversible peak. Potential at maximum anodic current is reported.
Bond angles ()
N2−N1−B1
N3−N4−B1
N2−C1−N3
N1−B1−N4
electromagnetic
photoluminescence quantum yields.
phenyl derivative (R = R = H) has λ = 502 nm, λPL = 586 nm,
and ΦPL = 15% in toluene.
spectrum
and
exhibit
modest
[19b, 28]
For example, the
[
a]
Boron displacement (Å)
1
5
[
[
b]
b]
18.51
18.74
15.57
15.65
abs
Twist angle 
Twist angle 
1
()
()
[19b]
2
By contrast, dyes 2a−2d have λabs
[
a]
Distance between the
B atom and the plane defined by the NNCNN
values ranging from 691 to 760 nm, λPL values from 834 to 904
nm, and quantum yields of 2−7% in toluene (Figure 3, Table 3).
It is evident from these numbers that the introduction of tertiary
amine groups at the N-aryl substituents dramatically alters the
[
b]
backbone. Angles between the planes defined by the N-aryl substituents and
the NNCNN backbone.
2
BF formazanates 2a−2d also exhibit reversible oxidation
events (Figure 2). Similar oxidation events are not typically
observed within the stability window of common electrochemistry
solvents for other BF formazanates. In compounds 2a−2c,
2
2
electronic structure of BF formazanates. First, the maximum
absorption wavelengths for 2a−2c increase with the size of the
-electron system as expected. The largest complex 2d,
however, has the shortest λabs of the four. This suggests that the
oxidation occurs in two one-electron steps (Eox1 = 0.25 to 0.47 V
and Eox2 = 0.66 V to 0.79 V) and follows the same trend as the
reduction waves (Figure 2). We attribute these features to the
stepwise oxidation of the amine substituents to radical cations,
2 2
NPh groups of 2d are electronically isolated from the BF
formazanate scaffold in the ground state. Second, dyes 2a−2c
exhibit normal photoluminescence solvatochromism: their λPL
values increase with solvent polarity. The opposite trend is
[
26]
by drawing an analogy with other electron-rich amines.
stepwise oxidation is indicative of the electronic communication
between the terminal NR groups through a molecule-wide -
The
observed for 2d, which has λPL = 904 nm in toluene and λPL
881 nm in THF (no detectable photoluminescence in CH CN).
The quantum yields for NIR dyes 2a−2d are lower than
those of BF formazanate dyes that photoluminesce at shorter
wavelengths because non-radiative decay is enhanced by the
=
2
3
conjugated system. In 2d, however, only one oxidation wave
corresponding to two electrons was observed at Eox1 = 0.54 V,
indicating that the NPh groups are electronically isolated from
2
2
[
29]
each other and provide a platform for significant charge transfer.
This oxidation process likely results in the production of a
diradical dication (Scheme 2). Our attempts to chemically
oxidize dyes 2a−2d have not yielded clean reaction products,
perhaps due to competing decomposition pathways associated
narrowed optical bandgap according to the energy-gap law. It
is noteworthy that a related BF formazanate dye bearing
2
1
5
methoxy substituents (R = R = OMe; λabs = 572 nm, λPL = 656
nm, ΦPL = 77%) does not exhibit NIR bands and shows limited
sensitivity to solvent polarity (ΔλPL = 6 nm across a similar range
[
27]
[19b]
with amine-derived radical cations.
Nonetheless, amine
of solvents).
To understand these unusual photophysical
substituents at the N-aryl rings create an electronic environment
properties of amine-substituted BF
2
formazanates, we turned to
that is qualitatively different from that of BF
related compounds.
2
formazanates and
electronic structure calculations.
Time-dependent
density-functional theory (TDDFT)
Given the relatively simple molecular structures of BF
formazanates 2a−2d, their low-energy absorption and
2
calculations implicate the highest occupied molecular orbital
(HOMO,  type) and lowest unoccupied molecular orbital (LUMO,
* type) as the orbital pair that makes the dominant contribution
photoluminescence bands are unexpected. In general, BF
formazanates absorb and emit in the red region of the
2
3
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RESEARCH ARTICLE
Scheme 2. Stepwise reduction and oxidation of BF
2
formazanates 2c and 2d.
[
32]
[33]
to the observed absorption and photoluminescence maxima for
each of dyes 2a–2d. Figures 4 and S15 show that the HOMO
and LUMO of 2a–2c, both at the ground- and excited-state
geometries, span at least a dozen common atoms, whereas the
HOMO and LUMO of 2d are localized on different parts of the
molecule. This strongly suggests that the HOMOLUMO
transition in 2d involves significant charge transfer. Another
hybrid density functionals,
e.g., LC-ωhPBE.
When we
optimized the solvated structures of 2a–2d and calculated their
excitation energies using the LC-ωhPBE/DGDZVP2 method with
a tuned range-separation parameter ω=0.14, the maximum
absorption wavelengths of 2a−2c changed little, whereas the
λ
abs of 2d improved dramatically and all results were now within
6−52 nm of the experimental values (Tables 3 and S3).
Photoluminescence wavelengths of 2a–2d computed using the
LC-ωhPBE functional (Table 3) are in equally good agreement
with experiment (errors of 2–77 nm). The experimentally
observed trends in the wavelengths of absorption maxima (2c >
2b > 2a > 2d) and photoluminescence maxima
[
30]
telltale sign of strong charge transfer
in 2d is that standard
grossly underestimate the
lowest absorption energy for this particular dye but not for 2a−2c
Table S2). It is known that the charge-transfer error of standard
functionals can be remedied by using tunable range-separated
[
31]
density functionals such as PBE0
(
1.0
0.8
0.6
0.4
0.2
0.0
1.0
2a
2b
0.8
0.6
0.4
0.2
0.0
300
500
700
900
1100
300
500
700
900
1100
Wavelength (nm)
Wavelength (nm)
1
.0
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
2c
2d
300
500
700
900
1100
300
500
700
900
1100
Wavelength (nm)
Wavelength (nm)
Figure 3. Normalized UV-vis absorption (black lines) and photoluminescence (red lines) spectra of dyes 2a−2d in 3‒6 µM dry, degassed toluene solutions.
4
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Chemistry - A European Journal
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RESEARCH ARTICLE
Table 3. Experimental and calculated solution-state characterization data for BF
LC-hPBE( = 0.14)/DGDZVP2 SCRF=PCM level with equilibrium solvation.
2
formazanates 2a−2d. The theoretical values were obtained using TDDFT at the
Experiment
Theory
Solvent
−1
−1
[a]
−1
λ
abs (nm)
ε (M cm
)
λ
PL (nm)
ΦPL (%)
νST (nm)
νST (cm
)
λ
abs (nm)
λ
PL (nm)
[
21c]
1
5
2
a
2
(R = R = NMe )
Toluene
CH Cl
THF
CH CN
b (R = NPh
Toluene
CH Cl
THF
CH CN
c (R = R = NPh
Toluene
CH Cl
THF
CH CN
d (R = R = C
Toluene
CH Cl
THF
CH CN
728
733
731
728
41300
41300
38600
43600
834
866
855
888
7
4
106
133
124
160
1746
2095
1984
2475
680
732
726
755
771
868
858
925
2
2
2
3
<1
1
5
2
2 2
, R = NMe )
734
741
735
729
46000
47500
46500
47400
860
898
880
915
4
126
157
145
186
1996
2359
2242
2788
701
749
748
763
815
913
903
955
2
2
<1
<1
<1
3
1
5
2
2
)
760
760
745
735
47000
45800
45500
45600
902
924
917
935
2
<1
<1
2
142
164
172
200
2071
2335
2518
2910
731
766
774
783
874
910
2
2
961
3
1012
1
5
2
6 4 2
H NPh )
691
684
665
643
25900
24300
30100
26100
904
900
881
-
2
<1
<1
-
213
216
216
-
3410
3509
3687
-
667
690
688
695
844
946
936
988
2
2
[
b]
3
[
a]
[b]
3
Determined using integrating sphere method. Photoluminescence was not detected in CH CN.
(
2c ≈ 2d > 2b > 2a) are also adequately reproduced by the LC-
ωhPBE(ω=0.14)/DGDZVP2 calculations. Excitation of BF
formazanate dyes results in their structural reorganization
sensitive to solvent variation than the ground states (Table 3),
which is a common manifestation of solvatochromism. The
charge-transfer character of the * electronic transitions is
also responsible for the larger Stokes shifts of 2d compared to
those of 2a−2c.
2
[
21b, 28b, 34]
(
planarization).
In the case of compounds 2a−2c, our
calculations show that the excited states are significantly more
LUMOs
HOMOs
2
a
2b
2c
2d
Figure 4. Frontier molecular orbitals of compounds 2a–2d computed at the ground-state molecular geometries using the LC-hPBE( = 0.14)/DGDZVP2
SCRF=(PCM, Solvent=Toluene) method.
5
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Caram, O. T. Bruns, D. Franke, R. A. Day, E. P. Farr, M. G.
Bawendi, E. M. Sletten, Angew. Chem. Int. Ed. 2017, 56,
Conclusion
1
3126–13129; e) K. S. de Valk, H. J. Handgraaf, M. M.
We have developed a new synthetic route to tertiary amine-
substituted 3-cyanoformazans and used it to carry out a
systematic investigation of the unusually low absorption and
Deken, B. G. Sibinga Mulder, A. R. Valentijn, A. G. Terwisscha
van Scheltinga, J. Kuil, M. J. van Esdonk, J. Vuijk, R. F.
Bevers, K. C. Peeters, F. A. Holman, J. V. Frangioni, J.
Burggraaf, A. L. Vahrmeijer, Nat. Commun. 2019, 10, 3118.
a) R. R. Nani, A. P. Gorka, T. Nagaya, H. Kobayashi, M. J.
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2
photoluminescence energies of BF formazanate dyes 2a−2d.
[
3]
Cyclic voltammetry experiments and electronic structure
calculations suggest that the NIR transitions in all of these dyes
involve the HOMO-LUMO pair, but occur by different
mechanisms. Detailed TDDFT and photophysical studies reveal
that the electronic transitions of dyes 2a−2c (abs = 728−760 nm
and PL = 834−902 nm in toluene) are best described as *
with little to no charge transfer. The energies of these transitions
exhibit greater sensitivity to solvent variation in the near-planar
excited states (photoluminescence solvatochromism) than in the
nonplanar ground-state structures. The electronic transitions in
dye 2d (abs = 691 nm and PL = 904 nm in toluene) are also of
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In summary, this work demonstrates the exciting feasibility of
NIR dyes consisting of relatively small -electron systems and
suggests that significant charge transfer is not a prerequisite for
their NIR activity. These findings open new strategies for
enhancing the functionality of luminescent compounds used in
materials chemistry and chemical biology arenas.
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Acknowledgements
1715‒1724; c) B. Li, L. Lu, M. Zhao, Z. Lei, F. Zhang, Angew.
This work was supported by the Natural Sciences and
Engineering Research Council (NSERC) of Canada (F.L.B.:
PGS-D Scholarship; R.R.M.: CGS-D Scholarship; V.N.S.: DG,
RGPIN-2020-06420; J.B.G.: DG, RGPIN-2018-04240), the
Ontario Ministry of Research and Innovation (J.B.G.: ERA, ER-
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Bao, J. Wang, Y. Zhang, W. Wei, G. Ma, L. Zhao, Z. Tian,
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14-10-147), and the Canadian Foundation for Innovation (J.B.G.:
JELF, 33977).
Keywords: Boron formazanates • near-infrared dyes • charge
transfer
•
cyclic voltammetry
•
range-separated hybrid
functionals
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Entry for the Table of Contents
Abs.
PL
1.0
0.8
0.6
0.4
0.2
0.0
25 µA
4
00
600
800
1000
-2.3 -1.8 -1.3 -0.8 -0.3 0.2 0.7 1.2
+
Wavelength (nm)
E vs. Fc/Fc (V)
2
Simple and readily synthesizable BF complexes of tert-amine-substituted 3-cyanoformazans are shown to absorb and emit light at
wavelengths that are unusually long for such small molecules (λabs = 691−760 nm, λPL = 834−904 nm in toluene). Detailed
investigations of several such complexes reveal that strong charge transfer is not a prerequisite for their near-infrared activity.
Institute and/or researcher Twitter usernames: @westernuchem, @gilroygroup
8
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