Tuning J-aggregate Formation and Emission Efficiency in Cationic Diazapentacenium Dyes

Article abstract of DOI:10.1002/chem.202100123

Enhancement of the luminescence efficiency of two new diazapentacenium salts (D1 and D2) of more than 55 for D1 and 22 times for D2) in poor solvents, acetonitrile and/or dichloromethane, was observed and rationalized as formation of emissive J-aggregates. Both compounds displaying 4-n-decylphenyl substituents at the 7,14-carbons and phenyl (D1) or 2,6-difluorophenyl (D2) substituents at the quaternary nitrogen atoms in 5,12-positions have been synthetized in a two-step procedure involving a two-fold Buchwald-Hartwig-type CN cross-coupling and an electrophilic Friedel-Crafts-type cyclization. The optical properties of the dicationic diazapentacenium salts in various solvents and in thin films have been investigated by steady-state and time-resolved absorption and photoluminescence spectroscopies. In thin films and in good solvents, isolated molecules coexist with aggregates. Nonetheless, D1 is seven times more emissive than D2, reflecting a higher J-aggregate contribution in the former.

Full text of DOI:10.1002/chem.202100123

Communication
doi.org/10.1002/chem.202100123
Chemistry—A European Journal
Tuning J-aggregate Formation and Emission Efficiency in
Cationic Diazapentacenium Dyes
Ana Clara B. Rodrigues,[a] Dario Wetterling,[b] Ullrich Scherf,*[b] and J. Sérgio Seixas de Melo*[a]
heteroarene molecules showing different properties.[3] Azapen-
Abstract: Enhancement of the luminescence efficiency of
two new diazapentacenium salts (D1 and D2) of more than
55 for D1 and 22 times for D2) in poor solvents, acetonitrile
and/or dichloromethane, was observed and rationalized as
tacenes are, thereby, a particularly interesting subgroup of
these heteroarenes,[3a] which formally result from inserting
nitrogen atoms into the pentacene skeleton; such interest
mainly arose from the promise of tuning the electronic
formation of emissive J-aggregates. Both compounds
structure, molecular packing, stability, and processability of
displaying 4-n-decylphenyl substituents at the 7,14-carbons
and phenyl (D1) or 2,6-difluorophenyl (D2) substituents at
the quaternary nitrogen atoms in 5,12-positions have been
pentacene by replacing C atoms with N atoms of varied
number, position and valence state.[4] Synthetic routes to
prepare azaacenes often use condensation/alkynylation meth-
synthetized in a two-step procedure involving a two-fold
ods that have been developed for the preparation of all-carbon-
Buchwald-Hartwig-type CN cross-coupling and an electro-
acenes.[5] Known N-quaternized azaacenium salts were synthe-
philic Friedel-Crafts-type cyclization. The optical properties
sized by N-alkylation of azapentacenes or by introducing aryl
of the dicationic diazapentacenium salts in various solvents
and in thin films have been investigated by steady-state
groups into the precursors of azaacenes.[6] N,N’-dihydro com-
pounds, which may be obtained by reduction of larger
and time-resolved absorption and photoluminescence
azaacenes, are known for much longer time, and found more
spectroscopies. In thin films and in good solvents, isolated
stable than longer azaacenes themselves.[7]
molecules coexist with aggregates. Nonetheless, D1 is
seven times more emissive than D2, reflecting a higher J-
aggregate contribution in the former.
Aggregation induced emission (AIE) and AIE-luminogen
systems are, at the moment, topics of high interest and impact
due to the increment of the emission properties in aggregate
systems, which are usually poorly emissive. The occurrence of
AIE with a new fluorescence band originating from intermolec-
ular interactions was first reported by Scheibe et al.[8] and
Since John and Clar’s synthesis of pentacene in the early
1930’s,[1] the synthetic strategies and properties of higher
member acenes have challenged chemists. Early on, pentacene
and structurally related molecules were of curiosity-driven
research; nowadays, pentacene is considered a reference hole
semiconductor in organic field-effect transistors.[2] Propelled by
such curiosity, boron and nitrogen atoms have been introduced
into π-backbones of poly-aromatic hydrocarbons, structurally
similar to their hydrocarbon analogues, resulting in interesting
Jelley[9] who independently observed in 1936 an unusual
behavior for pseudoisocyanine chloride (also known as 1,1’-
diethyl-2,2’-cyanine chloride, PIC chloride) whereas in aqueous
solutions the absorption maximum was shifted to lower
energies, when compared to the absorption spectrum of the
same dye in ethanol, and upon concentration increase (in
water), this band became more intense and sharp.[10] Dye
aggregates with a narrow absorption band bathochromically
shifted, with
a very small Stokes’ shift and increased
fluorescence intensity relative to the monomer are generally
termed Scheibe aggregates or J-aggregates (J denotes Jelley).[11]
In contrast, aggregates with absorption bands shifted to shorter
wavelength (hypsochromically shifted) with respect to the
monomer band are called H-aggregates (H denotes hypsochro-
mic), in most cases coupled to the occurrence of low intensity,
or absence of fluorescence.[12]
Currently, AIE or aggregation induced enhanced emission
(AIEE) caused by the formation of J-aggregates have been
described in several systems. Oelkrug et al. has demonstrated
the role of J-aggregation for enhanced solid-state emission of a
series of oligophenylene vinylenes, with the increase of
fluorescence quantum yield from almost zero in solution to
60% in nanoparticles or in films.[13] They explained this
phenomenon as arising from the rigid environments provided
by viscous solvents or solid phases that suppress torsion-
induced radiationless deactivation.[14] In this work, we report, to
[a] Dr. A. C. B. Rodrigues, Prof. J. S. Seixas de Melo
Department of Chemistry, CQC
University of Coimbra
Rua Larga, 3004-535, Coimbra (Portugal)
E-mail: sseixas@ci.uc.pt
[b] D. Wetterling, Prof. U. Scherf
Macromolecular Chemistry Group (buwmakro) and Institute for Polymer
Technology
Bergische Universitat Wuppertal
Gauss-Str. 20, 42097, Wuppertal (Germany)
E-mail: scherf@uni-wuppertal.de
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202100123
© 2021 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-Com-
mercial NoDerivs License, which permits use and distribution in any medium,
provided the original work is properly cited, the use is non-commercial and
no modifications or adaptations are made.
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our best knowledge, for the first-time the formation of J-
aggregates in quaternized 5,12-diazapentacenium salts,
coupled to an increase in fluorescence intensity (AIE properties).
For synthesis of the diazapentacenium salts, N,N’-diaryl1-
N,N’-diaryl2-substituted 2,5-bis(4-n-decylbenzoyl)-1,4-phenylene
diamine precursors (aryl1: 4-n-decylphenyl, aryl2: phenyl (D1) or
2,6-difluorophenyl (D2)) were generated in Buchwald-Hartwig-
type aryl-amine couplings (Scheme 1). The Friedel-Crafts-acyla-
tion-type cyclization chemistry was already reported for syn-
thesis of monocationic N-arylacridinium salts, specifically for
reaction of triphenylamines with benzoic acid derivatives or
cyclization of 2-benzoyltriphenylamine into 9,10-diphenylacridi-
nium salts, both at (nearly) quantitative conversion of the
starting compounds into the respective products.[15]
The photophysical properties of the diazapentacenium salts
D1 and D2 (Scheme 1) were studied in five solvents of different
polarity (here given as solvent’s relative permittivity, ɛ): two low
polarity aprotic solvents, tetrahydrofuran (THF, ɛ=7.58), and
dichloromethane (DCM, ɛ=8.93) and acetonitrile (ACN, ɛ=
37.5) as polar aprotic solvents, and methanol (MeOH, ɛ=32.7)
as polar protic solvent. Thin films of the diazapentacenium salts
prepared by spin-coating supported in zeonex matrices were
also studied. Absorption, fluorescence emission and excitation
spectra for the two compounds D1 and D2 are depicted in
Figure 1. A first visual observation shows that the solution
colour strongly depends from the solvent used (Figure S1 and
Table S1). However, in DCM and ACN very similar solution
colours for both compounds are observed. In other solvents,
also significant differences between D1 and D2 are noticed (see
Figure S1).
MeOH for D1 and THF for D2 represent the best solvents for
each compound. The difference between D1 and D2 is
expected to be caused by the introduction of the fluorine
atoms into the N-phenyl substituents. Here, absorption and
emission, associated to a π-π* transition, lack vibronic reso-
lution and the compounds are practically non-emissive, with
ϕF �1% (Table 2). In contrast, ACN and DCM, behave as a poor
solvent for the two salts. Now, mirror-symmetry between
absorption and fluorescence excitation spectra is observed.
Especially D1 is much more emissive in ACN and DCM if
compared to MeOH as good solvent (see the ϕF values in
Table 2). The long-wavelength absorption bands are vibronically
resolved and red shifted, when compared to the spectra in the
good solvents MeOH and THF at the same concentrations. For
similar cationic compounds, e.g., quaternary N-phenyl azaace-
nium salts in dichloromethane, also small Stokes’ shifts and
vibronically well-resolved long wavelength absorption bands
have been reported.[7] The formation of the vibronically
structured, red shifted absorption bands, together with a small
Stokes’ shift (ΔSS ~600 cmÀ 1, Table 1) and an increase of the
fluorescence quantum yield (ϕF), indicates the formation of J-
aggregates in the poor solvents ACN and DCM (Figure 1). In
thin films, although the longest absorption and emission bands
are vibronically resolved for D1, the ϕF value is lower than in
poor solvents but higher than in good solvents (Table 2). For
thin films of D2, a broad absorption and emission bands are
°
Scheme 1. Synthetic route to the diazapentacenium salts D1 and D2. i) NaO-t-Bu, Pd(OAC)2, tri-t-Bu-phosphine, toluene, 120 C, 16 h; ii) trifluoromethane-
°
sulfonic acid (TMSA) under argon, toluene, 50 C, 3 h.
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Figure 1. Absorption, fluorescence excitation, and emission spectra of compounds D1 (left) and D2 (right) in THF, DCM, MeOH, ACN and in films. All spectra
were recorded with concentration=1.0×10À 5 molLÀ 1. Abs= absorption spectra; Ex= excitation spectra; Em= emission spectra.
Table 1. Spectroscopic data (Absorption, λabs, emission maxima, λem, and Stokes shift, ΔSS) of diazapentacenium salts D1 and D2 in different solvents.[a]
Dye
Solvent
λ
abs [nm]
λ
em [nm][b]
ΔSS [cmÀ 1
]
D1
THF
304/435/575/620/678
305/431/581/627/685
323/411/589
302/428/569/618/674
309/440/580/630/685
319/386/583
302/429/581/628/688
317/384/555
300/427/576/620/680
318/379/430/586
708/773(s)
709/773(s)
683
701/765(s)
713/770
716
710/778(s)
572/618(s)
709/773(s)
642
625
494
2337
571
573
3186
450
536
602
1489
DCM
MeOH
ACN
Film
D2
THF
DCM
MeOH
ACN
Film
[a] All data were collected with a concentration=1.0×10À 5 molLÀ 1. [b] Emission maxima were collected with with excitation into the S0!S1 band (λex
530 nm). s=shoulder.
=
observed, with a large Stokes shift and low ϕF value. The data
indicate that the C10H21 chain (present in D1 and D2) and the
fluorine atoms (only in D2) contribute differently to the degree
of organization of the salts in the solid state and in solutions of
poor solvents. To summarize at this stage, a comparison of all
spectra for D1 and D2 shows significant differences for the
solutions in different solvents, but also some differences
between D1 and D2: THF is a good solvent for D2, but not for
D1, while MeOH is a good solvent for D1 (Figure 2).
The observed spectral characteristics for D1 or D2 in ACN
and DCM (especially for the absorption and fluorescence
excitation spectra) together with the red shifted fluorescence
and increased quantum yields, ϕF, indicates J-aggregate for-
mation in these poor solvents. However, the most attractive
feature comes from the distinct increase, by 55 times (for D1)
and 22 times (for D2) in ϕF when going from the good solvents
to ACN as poor solvent (D1: MeOH!ACN; D2: THF!DCM).
Next, we varied the concentrations of D1 and D2; the results
(concentration range from 2.0 to 50×10À 6 molLÀ 1) show that
D1 (Figure S2) and D2 (Figure S3), both in THF or ACN, display
similar spectral profiles during the variation of the concen-
tration, with the intensities of absorption, fluorescence excita-
tion and emission linearly increasing with increasing concen-
tration (Figures S2 and S3).
However, for D2 in MeOH (Fig. S3), the spectral profile
changes, and the intensity of the observed bands does not
linearly respond to an increasing concentration, as indication
for the presence of mixtures of different absorbing/emitting
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Table 2. Room temperature fluorescence quantum yields (φF) and emission lifetimesa (τi) for compounds D1 and D2 in pure solvents. Also presented are the
associated pre-exponential factors (ai), the fractional contributions for each decay time (%Ci), and the chi-square values (χ2) for the judgment of the quality
of the fits. Radiative (kF) and radiationless rate (kNR) constants associated with the third decay component (τ3) are also listed.
c)
Dye Solvent Concentration [μM] φF
τ1 [ns] a1 [C1%]
τ2 [ns] a2 [C2%]
τ3 [ns] a3 [C3%]
χ2
b)kF ×10À 6 [sÀ 1
]
k
×10À 8 [sÀ 1
]
NR
D1
THF
50
50
50
50
–
50
50
5
10
25
50
50
–
0.013
0.102
0.002 0.16
4.07
5.41
1 (100)
1 (100)
0.015 (21)
1 (100)
0.032 (50)
0.010 (11)
0.854 (94.5) 0.68 6.2
0.974 (93)
0.972 (92)
0.081 (33)
0.184 (54)
1 (100)
0.69 3.19
0.81 18.9
1.04 0.46
0.70 25.2
1.09 1.64
0.92 0.18
2.43
1.66
2.27
2.04
2.36
1.77
2.75
2.05
2.18
1.98
1.99
4.20
2.40
DCM
MeOH
ACN
Film
THF
0.889 (44) 1.18
0.096 (35) 4.40
0.11
4.36
4.27
0.007 0.14
0.001 0.34
0.022 1.21
0.067
0.968 (50)
0.902 (62) 1.51
0.146 (5.5)
13.31
D2
0.089 (27) 5.65
3.55
0.026 (7.0) 4.56
0.028 (8.0) 4.5
0.025 (37) 5.03
0.019 (24) 5.02
2.36
DCM
MeOH
1.16 14.7
1.04 4.0
1.01 0.60
1.11 0.20
0.018
13.30
18.65
0.003 0.42
0.001 0.48
0.008
0.894(30)
0.797 (22) 21.21
ACN
Film
0.6
3.4
0.001 0.19
0.996 (92)
4.17
0.004 (8)
0.82 0.24
�
1À �F
[a] Experimental conditions: λex =460 nm [b] kF
¼
. [c] kNR
¼
.
F
F
�
�
F
To further rationalize these observations, time-resolved
studies were performed (Figure S4 and S5). The complete
photophysical data (quantum yields and decay times) of the
two compounds in different organic solvents and in films,
together with the concentration dependence for D2 also in
methanol, are presented in Table 2.
In DCM and ACN, the emission of both D1 and D2 decays
mono-exponentially with lifetimes τ=5.41 ns (DCM) and
4.36 ns (ACN) for D1, and τ=3.55 ns (DCM) and 2.36 ns (ACN)
for D2. This emission behaviour is assigned to J-aggregates.
Also in THF, as poor solvent for D1, a single-exponential decay
(τ=4.07 ns) is observed, indicative of J-aggregation. In MeOH
(for D1) and THF (for D2), as good solvents, it seems that, a few
amount of aggregated species coexist with the monomeric
component (showing the shortest lifetime of τ1 <1 ns). As
expected, D1 and D2 show the lowest ϕF in these solvents. In
MeOH, D2 displays a much higher lifetime decay component
(τ2 >13 ns). The contribution associated to this lifetime in-
creases with increasing concentration, accompanied by the
occurrence of an additional emission band at longer wave-
lengths, at >700 nm, and by an ongoing fluorescence quench-
ing (Figure S3). In thin films, the fluorescence decays for D1 and
D2 are bi-exponential, showing that both monomers and J-
aggregated coexist, yet with different contributions. Indeed, for
D1 J-aggregates and monomers are found with equal contribu-
tion to fluorescence (as seen by the %C1 and %C3, in Table 2),
whereas for D2, J-aggregates contribution is now of 8%. These
findings corroborate the steady-state data with D1 presenting
higher ϕF values (higher J-aggregate contribution), while D2 is
poorly emissive in the solid state (lower J-aggregates contribu-
tion). The radiative (kF) and non-radiative rate (kNR) constants
were determined by considering the lifetime τ3 as indicator of J-
aggregate formation. The kNR are at least 1 order of magnitude
higher than kF in solvents where the compounds show a higher
ϕF. The photophysical data (Table 2) also display an increase in
the radiative rate constant (when going from good to poor
solvents) of more than one order of magnitude with a nearly
constant rate constant of radiation-less deactivation. For D1, as
example, kF goes from 0.5×106 sÀ 1 to 25×106 sÀ 1 (MeOH!ACN),
Figure 2. Absorption and emission spectra of compounds D1 and D2 in
good and poor solvents pairs. [D1, D2]: 1.0×10À 5 molLÀ 1. Highlighted bands
were assigned to J aggregates (J-agg) of D1 and D2. Line breaks were added
to magnify visualization of the new absorption bands (at lower wave-
numbers) present in poor solvents.
species (isolated molecules, aggregates). The ϕF also decreases
with the concentration (Table 2). At the lowest concentration
(2 μM), the emission spectrum is poorly resolved (presence of
isolated, non-aggregated molecules), whereas at 5 μM the
spectrum displays the characteristic (vibronically structured)
emission of either D1 or D2 J-aggregates observed in poor
solvents such as DCM and ACN. Further increase in concen-
tration leads, best seen in the fluorescence excitation spectra,
to a progressive loss of the vibronic resolution and the
appearance of an additional broad emission band at emission
wavelengths >700 nm, that increases in intensity with increas-
ing concentration. We attribute this behaviour to an initial
coexistence of monomers and, possibly, a few J-aggregates at
low concentrations, with the relative concentration of J-
aggregates increasing with increasing concentration and reach-
ing its maximum for a concentration of ca. 5 μM. Further
increase of the concentration leads to the progressive formation
of disordered and/or ill-defined, non-emissive aggregates,
reflected in a gradual decrease of ϕF with the rise in
concentration for D2 in MeOH.
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whereas kNR only varies between 2.3×108 sÀ 1 and 2.0×108 sÀ 1 in
these solvents. This also reflects the enhancement of the
photoluminescence emission intensity caused by J-aggregate
formation.
gramme). Open access funding enabled and organized by
Projekt DEAL.
We hereby describe the synthesis and optical character- Conflict of Interest
ization of two cationic diazapentacenium dyes, with particular
emphasis to the photoluminescence properties in different
solvents. Our data indicate formation of emissive J-aggregates
in poor solvents, while in good solvents and in thin films,
isolated molecules coexist with aggregates. This is evidenced
by a strong increase of the fluorescence quantum yields along
with the appearance of a narrow absorption band bathochromi-
cally shifted in poor solvents. Therefore, the nature of the
fluorescence decay changes from single exponential (when J-
aggregates dominate, in poor solvents) to triexponential
decays, reflecting the coexistence of isolated molecules, J-
aggregates, and, probably, other disordered aggregate species;
the last presented especially at higher concentrations. The
simple structural change of the N-aryl substituents from phenyl
(D1) to 2,6-difluorophenyl (D2) also influences the optical
properties. As an example, THF is found to be the best solvent
for fluorinated derivative D2, while it is a poor solvent for D1.
This illustrates that simple structural modifications in these
compounds can dramatically change their optical properties,
thus paving the way for a rational control of the supramolecular
ordering, and consequently, of the luminescence efficiency.
The authors declare no conflict of interest.
Keywords: AIE · diazapentacene · fluorescence · J-aggregates ·
photochemistry
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Acknowledgements
This work was supported by Project “Hylight” (no. 031625) 02/
SAICT/2017, PTDC/QUI-QFI/31625/2017, which is funded by the
Portuguese Science Foundation (Fundação para Ciência e
Tecnologia, FCT) and Compete Centro 2020. We acknowledge
funding by Fundo Europeu de Desenvolvimento Regional
(FEDER) through COMPETE and project ROTEIRO/0152/2013.
CQC acknowledges FCT for financial support (Projects UIDB/
00313/2020, UIDP/00313/2020). We also acknowledge funding
from Laserlab-Europe (no. 284464, EC’s 7thFramework Pro-
Manuscript received: January 12, 2021
Accepted manuscript online: April 9, 2021
Version of record online: ■■■, ■■■■
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COMMUNICATION
Two new diazapentacenium salts
have been obtained from a new
synthetic methodology and shown to
display a drastic increase of
Dr. A. C. B. Rodrigues, D. Wetterling,
Prof. U. Scherf*, Prof. J. S. Seixas de Me-
lo*
1 – 6
fluorescence quantum yields induced
by aggregation (AIE phenomena) in
poor solvents and associated to a par-
ticular type of aggregates organiza-
tion: J-aggregates. A simple structural
modification (4 H in D1 vs 4 F in D2)
changes the contribution of J-aggre-
gates which is quantitatively followed
by time resolved fluorescence.
Tuning J-aggregate Formation and
Emission Efficiency in Cationic Diaza-
pentacenium Dyes