Oligotriarylamines with a Pyrene Core: A Multicenter Strategy for Enhancing Radical Cation and Dication Stability and Tuning Spin Distribution

Article abstract of DOI:10.1002/chem.201403847

Monoamine 1, diamines 2-4, triamine 5, and tetra-amine 6 have been synthesized by substituting dianisylami-no groups at the 1-, 3-, 6-, and/or 8-positions of pyrene. Diamines 2-4 differ in the positions of the amine substituents. No pyrene-pyrene interact

Full text of DOI:10.1002/chem.201403847

DOI: 10.1002/chem.201403847
Full Paper
&
Oligotriarylamines
Oligotriarylamines with a Pyrene Core: A Multicenter Strategy for
Enhancing Radical Cation and Dication Stability and Tuning Spin
Distribution
Hai-Jing Nie, Chang-Jiang Yao, Jiang-Yang Shao, Jiannian Yao, and Yu-Wu Zhong*[a]
Abstract: Monoamine 1, diamines 2–4, triamine 5, and tetra-
amine 6 have been synthesized by substituting dianisylami-
no groups at the 1-, 3-, 6-, and/or 8-positions of pyrene. Dia-
mines 2–4 differ in the positions of the amine substituents.
No pyrene–pyrene interactions are evident in the single-crys-
tal packing of 3, 4, and 6. With increasing numbers of amine
substituents, the first oxidation potential decreases progres-
sively from the mono- to the tetraamine. These compounds
show intense charge-transfer (CT) emission in CH2Cl2 at
around 530 nm with quantum yields of 48–68%. Upon step-
wise oxidation by electrolysis or chemical oxidation, these
compounds were transformed into radical cations 1·+–6·+
and dications 22+–62+, which feature strong visible and
near-infrared absorptions. Time-dependent density function-
al theory studies suggested the presence of localized transi-
tions from the pyrene radical cation and aminium radical
cation, intervalence CT, and CT between the pyrene and
amine moieties. Spectroscopic studies indicated that these
radical cations and dications have good stability. Triamine 5
and tetraamine 6 formed efficient CT complexes with tetra-
cyanoquinodimethane in solution. The results of EPR spec-
troscopy and density functional theory calculations suggest-
ed that the dications 22+–42+ have a triplet ground state,
whereas 52+ and 62+ have a singlet ground state. The dicat-
ion of 1,3-disubstituted diamine 4 exhibits a strong EPR
signal.
Introduction
Aminium radical cations, obtained by the oxidation of p-
conjugated arylamine derivatives, have been the subject of in-
tense research activity.[8] One of the earliest and most notable
examples is Wurter’s Blue, the radical-cation form from tetra-
methyl-p-phenylenediamine.[9] By using weakly coordinating
anions, Wang and co-workers recently succeeded in isolating
benzidine and aniline radical cations,[10] among other organic
radical cations.[11] Kamada, Yamamoto, and co-workers demon-
strated that bis(acridine) dimers with diradical character show
much larger two-photon absorption cross-sections than
closed-shell compounds with the same structural backbone
and composition.[12] Monoradical cations of bis(triarylamine)
compounds have been investigated as mixed-valence (MV) sys-
tems to probe basic electron-transfer processes.[13] Bis(triaryla-
mine) compounds with strong charge delocalization possess
intense near-infrared (NIR) absorptions in the MV state, and
polymeric films based on these materials show interesting NIR
electrochromism.[14] In addition, compounds containing multi-
ple aminium radical cations are promising high-spin organic
materials.[15] In all of these studies, the stability of the aminium
radical cations and/or dications is crucial for their physical
properties and optoelectronic performance.
Radical cations and dications are not only important intermedi-
ates in organic transformations, but also appealing molecular
materials for a wide range of applications because of their in-
triguing electronic, optical, and magnetic properties.[1] These
species normally possess high energies and are quite reactive.
However, considerable advances have been made recently in
the characterization of radical cations and dications with long
lifetimes and they have even been isolated as stable com-
pounds.[2] For example, Komatsu and co-workers obtained radi-
cal cations and dications of oligothiophenes stabilized by an-
nulation with bicyclo[2.2.2]octene units,[3] long-lived radical cat-
ions of monocyclic arenes were obtained by Pampaloni and
co-workers by oxidation with NbF5,[4] the radical cations of
poly(p-phenylene) oligomers were prepared and characterized
by Rathore and co-workers,[5] and Yamago and Jasti and their
co-workers were able to isolate and characterize the cyclopara-
phenylene radical cation and dication.[6] Recently, polycyclic ar-
omatic compounds with open-shell diradical ground states
have received considerable attention.[7]
Pyrene and pyrene derivatives are some of the most used
fluorescent probes and organic semiconductors.[16] Electrophilic
substitution at the 1-, 3-, 6-, and/or 8-positions of pyrene and
subsequent transformation have led to enormous organic mol-
ecules that have been used in electric and optoelectronic devi-
[a] H.-J. Nie, C.-J. Yao, J.-Y. Shao, Prof. Dr. J. Yao, Prof. Dr. Y.-W. Zhong
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Photochemistry
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: zhongyuwu@iccas.ac.cn
ces.[16] The pyrene radical cation (Pyr +) is normally highly reac-
C
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403847.
tive and its characterization is difficult. Dietrich and Heinze
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were able to observe reversible two-step oxidative waves for
pyrene by cyclic voltammetry (CV) in liquid SO2 at low temper-
ature,[17] however, the oxidation of pyrene is usually irreversible
at room temperature in conventional solvent. Pyrene deriva-
tives bearing peripheral dimethylaminoethynyl units show en-
hanced radical stability and electrogenerated chemilumines-
cence,[18] however, the electrochemical oxidation is still irrever-
sible in the timescale of CV. Interestingly, Rathore and co-work-
ers successfully isolated the radical cation of tetraisopropylpyr-
ene.[19] This compound showed a reversible oxidation wave at
gen atom during the palladium-catalyzed transformation.
These compounds have been fully characterized. In addition,
single-crystal X-ray structures for 3, 4, and 6 were determined
(Figure 2 and Table S1 in the Supporting Information). In these
+0.98 V versus SCE and an intense Pyr +-associated absorption
C
band at 494 nm. Similar absorption bands (ca. 460 nm) have
been observed by Oyama and Matsui for a number of 1-substi-
tuted pyrene radical cations by an electron-transfer stopped-
flow method.[20]
We present herein a combined experimental and theoretical
study of six triarylamine derivatives with a pyrene core (1–6,
Figure 1). From compound 1 to 2–4, 5, and 6, the number of
di-p-anisylamino groups progressively increases from 1 to 4.
Figure 2. Single-crystal X-ray structures of (a) 3, (b) 4, (c) top view of 6, and
(d) side view of 6. Ellipsoids are drawn at the 30% probability level. Hydro-
gen atoms have been omitted for clarity.
compounds, the nitrogen atoms are bonded to three aryl
groups to form three-wheel propeller structures. No pyrene–
pyrene interactions are evident in the crystal packing of these
compounds (see Figures S1, S3, and S4) owing to the steric
hindrance of the anisyl groups.
Figure 1. Compounds 1–6 studied in this paper.
Compounds 2–4 are diamine derivatives with two di-p-anisyla-
mino groups at different positions of the pyrene core. We ex-
pected that, by attaching multiple redox-active amino groups
to the pyrene core, the stability of the corresponding radical
cations and dications would be significantly enhanced. Com-
pounds 1–6 have been studied by single-crystal X-ray crystallo-
graphic, electrochemical, and spectroscopic analysis. In addi-
tion, the radical cations and dications of these compounds
have been studied by electronic absorption and electron para-
magnetic resonance (EPR) spectroscopy. Finally, density func-
tional theory (DFT) and time-dependent DFT (TDDFT) compu-
tations were carried out to elucidate the spin distributions and
absorption assignments.
Compounds 4 and 6 show interesting lamellar crystal pack-
ing (Figures 3 and 4). The pyrene core of 4 stands upright from
the side view of the lamellar structures. In each layer, the tert-
butyl groups of neighboring molecules are alternately directed
up and down. In the lamellar packing of 6, the pyrene core lies
in the plane of each layer and the eight anisyl groups of each
molecule keep each layer well separated from each other.
Electrochemical studies
Figure 5 shows the CVs and differential pulse voltammograms
(DPVs) of 1–6 in CH2Cl2. Monoamine 1 shows a chemically re-
versible redox wave at +0.72 V versus Ag/AgCl. Diamines 2–4
display two well-separated redox couples in a similar potential
window. The first redox wave of 2–4 is at +0.61 to +0.63 V
(Table 1). Triamine 5 shows three waves at +0.51, +0.60, and
+1.18 V, respectively. When the conventional electrolyte,
nBu4NClO4, was used during the measurement of tetraamine 6,
two two-electron processes at +0.44 and +1.21 V were ob-
served. However, the first wave split into two when
nBu4NB(C6F5)4 with weakly coordinating anions was used.[24]
These waves are associated with the oxidation of both amine
Results and Discussion
Synthesis and single-crystal X-ray structures
Mono- and diamine compounds 1–4 were synthesized from
the palladium-catalyzed CꢀN coupling reactions between di-p-
anisylamine and the corresponding pyrene bromides, 1-bromo-
pyrene, 1,8-dibromopyrene,[21] 1,6-dibromopyrene, and 1,3-di-
bromo-7-tert-butylpyrene,[22] respectively (see details in the Ex-
perimental Section). Triamine 5 and tetraamine 6 were isolated
from the reaction of di-p-anisylamine with 1,3,6,8-tetrabromo-
pyrene.[23] The isolation of 5 was unexpected but reasonable,
as a result of the replacement of one bromine atom by a hydro-
(N0/ +) and pyrene moieties (Pyr0/ +), which will be further dis-
cussed later. At more positive potentials, some chemically irre-
versible events can be observed (see Figure S2 in the Support-
C
C
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Figure 4. (a) Lamellar crystal packing of 6 viewed along the a axis. (b) Top
view of each layer.
core. This suggests that the stability of the resulting radical
cations can be efficiently enhanced by increasing the number
of amine substituents. Note that pristine pyrene exhibits an ir-
reversible oxidation at potentials more positive than +1.3 V
versus Ag/AgCl (see Figure S3 in the Supporting Information).
On the basis of the electrochemical data, the highest-occupied
molecular orbitals (HOMOs) were estimated to be ꢀ5.4 eV for
1, ꢀ5.3 eV for 2–4, ꢀ5.2 eV for 5, and ꢀ5.1 eV for 6 in
a vacuum. This means that the HOMOs of these compounds
are progressively destabilized with increasing amine substitu-
ents. This trend correlates with the results of the DFT calcula-
tions; the HOMOs of 1–6 were calculated to be ꢀ5.99, ꢀ5.85,
ꢀ5.85, ꢀ5.90, ꢀ5.77, and ꢀ5.70 eV, respectively (Figure 6). It is
interesting to note that the
Figure 3. (a) Lamellar crystal packing of 4 viewed along the b axis. (b) Top
view of the middle layer in (a).
ing Information). These are associated with the irreversible oxi-
dation of the in situ generated aminium cation radicals[25] or
the pyrene core. The radical cations of 2–4 and dications of 5
and 6 have large comproportionation constants, Kc (Table 1),
which suggests that they have good thermodynamic stability.
The above electrochemical results show that the first oxida-
tion potentials of 1–6 progressively shift to the less positive
region as more amine substituents are attached to the pyrene
lowest-unoccupied molecular or-
Table 1. Electrochemical, absorption, and emission data in CH2Cl2.[a]
bitals (LUMOs) of 1–6 are all do-
minated by the pyrene moiety;
[c]
[d]
Comp.
E1/2
[V]
DE(1,2)/DE(2,3)
KC(1,2)/KC(2,3)
Eanodic
[V]
labs,max
lemi,max
F[e]
t
however, the orbital composi-
tions of the HOMOs differ signifi-
cantly. The HOMO of 1 is domi-
nated by the triarylamine com-
ponent whereas the HOMOs of
the diamines 2–4 are localized
across the whole molecule. In
contrast, the pyrene frameworks
of 5 and 6 make a more impor-
tant contribution to their
HOMOs.
[mV][b]
[nm]
[nm]
[%]
[ns]
1
2
3
4
5
6
6[f]
+0.72
+0.61, +0.79
+0.62, +0.80
+0.63, +0.96
+0.51, +0.60, +1.18
+0.44 (2e), +1.21 (2e)
+0.31, +0.39, +1.30 (2e)
–/–
–/–
1120/–
1120/–
+1.35
+1.46
+1.47
+1.36
+1.55
+1.54
+1.92
416
442
445
434
465
487
–
529
530
531
523
529
525
–
48
52
58
66
48
68
–
12
11
10
10
9
180/–
180/–
330/–
90/580
–/770
80/910
3.9ꢁ105/–
33/6.8ꢁ109
–/1.1ꢁ1013
23/2.6ꢁ1015
8
–
[a] Potentials are reported versus Ag/AgCl. nBu4NClO4 was used as the electrolyte unless otherwise noted.
[b] DE(1,2) and DE(2,3) represent the potential difference between the mono- and dioxidized forms and the di-
and trioxidized forms, respectively. [c] The comproportionation constant Kc was determined from Kc =10(DE/59)
by using DE(1,2) and DE(2,3). [d] Anodic peak potential for an irreversible oxidation. [e] The emission quantum
yield was determined by using quinine sulfate as reference with an error of ꢁ5%. [f] nBu4NB(C6F5)4 was used
as the electrolyte.
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Figure 5. (a) CVs and (b) DPVs of 1–6 in CH2Cl2 at 100 mVsꢀ1. The bottom
CV and DPV curves of 6 were recorded with nBu4NB(C6F5)4 as electrolyte, the
others were recorded with nBu4NClO4 as electrolyte.
Figure 7. (a) Absorption and (b) emission spectra of 1–6 in CH2Cl2.
Figure 6. DFT-calculated energy diagrams (left) and isodensity plots of the LUMOs and HOMOs of 1–6.
Spectroscopic studies
substituents. The ICT absorptions of 1–6 show bathochromic
shifts of 3–6 nm in toluene and hypsochromic shifts of 2–6 nm
in CH3CN relative to those in CH2Cl2 (see Figure S5 and
Table S2).
The electronic absorption and emission spectra of compounds
1–6 in dilute solutions are shown in Figure 7 and the corre-
sponding data are summarized in Table 1. The absorption
bands between 350 and 550 nm have been attributed to intra-
molecular charge-transfer (ICT) transitions from the amine moi-
eties to the pyrene core. This is supported by the results of
a TDDFT analysis of 1 (see Figure S4 in the Supporting Informa-
tion). The predicted S1 (S=singlet) excitation of HOMO!
(LUMO) character is responsible for the observed absorptions
in the range 350–550 nm. The ICT absorption maxima for 1–6
are 416, 442, 445, 434, 465, and 487 nm, respectively, in CH2Cl2.
Those of the tri- and tetraamines are distinctly bathochromical-
ly shifted relative to the others. The data are consistent with
the results of the DFT analysis, and indicate that the HOMO–
LUMO energy gaps for 1–6 are 5.47, 5.26, 5.27, 5.34, 5.13, and
5.00 eV, respectively. Thus, the frontier MO energy gap be-
comes increasingly narrow with increasing numbers of amine
All the compounds are highly emissive in CH2Cl2 solutions
(lmax,emi ꢂ530 nm) with quantum yields of 48–68% and excited-
state lifetimes of around 10 ns. The emissions have been attrib-
uted to polarized ICT, as supported by the significant hypso-
chromic shifts observed in nonpolar solvents (ca. 30 nm shift,
see Figure S6 and Table S2 in the Supporting Information).[26]
No excimer emission was observed for any of the compounds
at a high concentration of 1ꢁ10ꢀ4 m. This again indicates that
possible pyrene–pyrene interactions are inhibited in these
compounds, making them appealing for use in light-emitting
devices.[27]
To characterize the spectroscopic properties of their radical
cations and dications, 1–6 were subjected to stepwise oxida-
tions with SbCl5 in CH2Cl2. The absorption spectral changes are
shown in Figures 8–10 and the absorption maxima and molar
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absorptivities of 1–6 bearing different charges are summarized
in Table 2.
Table 2. Absorption data of 1–6 with different charges obtained by oxi-
dation with SbCl5 in CH2Cl2.
When 1 was subjected to one-electron oxidation, the ICT
bands decreased significantly and strong visible/NIR absorp-
tions appeared (Figure 8). Absorption bands in the region 500–
labs,max [nm] (e [105 mꢀ1 cmꢀ1])
1
1
2
2
388 (0.12), 416 (0.13)
502 (0.066), 541 (0.12), 590 (0.19), 760 (0.21), 1122 (0.15)
442 (0.17)
628 (0.18), 678 (0.23), 864 (0.032), 1960 (0.12)
628 (0.13), 1048 (0.35)
+
+
C
600 nm with well-defined substructures are very likely from
+ [19,20]
C
Pyr
.
The intense absorption at 760 nm has been attribut-
C
+
[13–15]
C
22+
ed to the aminium radical cation (N
)
and the absorption
at 1122 nm has been assigned to the ICT transition from
3
403 (0.16), 445 (0.25)
624 (0.25), 678 (0.33), 1940 (0.18)
632 (0.14), 1033 (0.54)
+
pyrene to N +, which will be further discussed later. The occur-
C
3
C
32+
+
+
C
C
rence of N -localized and pyrene!N ICT transitions sug-
4
383 (0.14), 434 (0.19)
+
gests that the radical spin is dominated by the triarylamine
C
4
481 (0.14), 596 (0.29), 739 (0.056), 942 (0.091), 2130 (0.055)
579 (0.18), 790 (0.34), 1134 (0.099), 1750 (0.039)
392 (0.15), 465 (0.27)
452 (0.17), 640 (0.21), 703 (0.25), 1530 (0.12)
452 (0.18), 963 (0.27)
518 (0.16), 856 (0.38), 1059 (0.39)
398 (0.19), 487 (0.39)
462 (0.24), 667 (0.16), 732 (0.27), 1275 (0.11)
425 (0.25), 850 (0.30)
+
42+
5
C
unit. However, the appearance of Pyr absorptions suggests
that the pyrene framework was also partially oxidized.
5·+
52+
53+
Figure 9 shows the absorption spectral changes of the three
diamine compounds upon the first (mono-oxidation) and
second (dioxidation) one-electron oxidations with SbCl5. The
6
+
+
+
C
6
C
C
radical cations 2 and 3 display similar NIR absorptions at
62+
64+
around 2000 nm. These bands possibly have intervalence CT
544 (0.23), 814 (0.75), 1012 (0.27)
+
(IVCT) and amine!Pyr CT character. The dications 22+ and
C
32+ show intense absorption peaks at around 1040 nm, which
have been assigned to
+-localized and
C
a mixture of N
+
C
pyrene!N ICT transitions (see discussion on TDDFT later).
+
C
The absorption spectra of the 1,3-disubstituted diamine 4
and 42+ are different to those of the radical cations and dicat-
+
C
ions of diamines 2 and 3. The radical cation 4 shows rather
weak and broad NIR absorptions. In contrast, the dication 42+
exhibits three distinct bands at 790, 1134, and 1750 nm. The
former two peaks have been assigned to N +-localized and
C
+
C
pyrene!N ICT transitions, respectively, and appear in the
same region as the absorptions of 1 +. The peak at 1750 nm
C
Figure 8. Absorption spectral changes of 1 upon one-electron oxidation
with 1 equiv SbCl5 in CH2Cl2.
may have similar ICT character.
Figure 9. Absorption spectral changes of (a,b) 2, (c,d) 3, and (e,f) 4 upon (a,c,e) mono- and (b,d,f) dioxidation with SbCl5 in CH2Cl2. *: Artefacts.
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Figure 10. Absorption spectral changes of (a,b,c) 5 and (d,e,f) 6 upon (a,d) mono-, (b,e) di-, (c) tri-, and (d) tetra-oxidation with SbCl5 in CH2Cl2. *: Artefacts.
In comparison with the diamine radical cations, the NIR ab-
di- and triamine radical cations are slightly more stable than
the monoamine radical cation. The relatively low stability of
the tetraamine radical cation is due to the small value of Kc for
6·+. As for the dications, 52+ and 62+ are slightly more stable
than 22+–42+. The spectral changes show that around half of
the 22+–42+ dications are transformed into radical cations in
15 min, whereas around 30 min is needed for this conversion
+
+
C
C
sorptions of triamine 5 and tetraamine 6 are progressively
shifted to higher-energy regions (centered at 1530 and
1275 nm, respectively; Figure 10). As shown in Figure S7 in the
Supporting Information, these peaks can be deconvoluted into
multiple Gaussian functions and the origin of these peaks
should be very complex. The results of the TDDFT analyses
suggest that these peaks have a large contribution from
for 52+ and 62+
.
+
C
amine!Pyr ICT transitions. Similarly, the NIR absorptions of
Compounds 1–6 exhibit low oxidation potentials and desta-
bilized HOMOs, and therefore they can be considered as good
electron donors. The possibility of forming charge-transfer (CT)
complexes[28] of these compounds with the typical electron ac-
ceptor tetracyanoquinodimethane (TCNQ) was examined.
Figure 11 shows the absorption spectra of 1:1 mixtures of 1–6
with TCNQ in CH2Cl2 at a concentration of 1ꢁ10ꢀ2 m. It is clear
that considerable quantities of CT complexes are formed in the
solutions 5 or 6 with TCNQ. The three absorption peaks at 695,
dications 52+ and 62+, at 963 and 850 nm, respectively, consist
of several sub-bands. N +-localized transitions become more
C
important in higher redox states (856 nm for 53+ and 814 nm
for 64+). The assignments of these absorptions will be further
discussed later.
In addition to chemical oxidation, compounds 1–6 were sub-
jected to stepwise oxidation by electrolysis at an indium tin
oxide (ITO) glass electrode in CH2Cl2 (see Figures S8 and S9 in
the Supporting Information). In general, similar absorption
spectral changes are observed by chemical oxidation and elec-
trolysis, and the spectral changes can be reversed by decreas-
ing the applied potential during the spectroelectrochemical
Cꢀ
750, and 850 nm are characteristic of TCNQ . For example,
similar absorptions at 686, 748, and 850 nm were observed by
measurements. The high redox states of 5 and 6 (53+ and 64+
)
are only accessible by chemical oxidation; the attempt to
access 53+ and 64+ by electrolysis failed, possibly because the
oxidation potentials needed are too high.
The stabilities of the radical cations and dications were
tested by monitoring the absorption spectral changes during
spectroelectrochemical measurements. For example, when
monoamine 1 (1ꢁ10ꢀ4 m in CH2Cl2 in the presence of electro-
+
C
lyte) was oxidized to 1 by electrolysis, the applied potential
was switched off and the solutions were left to stand under
ambient conditions. The spectral changes show that around
+
+
C
C
Figure 11. Absorption spectra of 1:1 mixtures of 1–6 with TCNQ at 1ꢁ10ꢀ2
m
half of the radical cations 1 –6 turn into neutral 1–6 in 30,
60, 60, 60, 180, and 30 min, respectively. This means that the
in CH2Cl2.
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Cꢀ
Weiss and co-workers for TCNQ in CH2Cl2 following electroly-
details in the Experimental Section). The Mulliken spin (a–b)
sis of neutral TCNQ.[29] The appearance of the absorption band
density population plots are shown in Figure 13. In mono-
amine 1 +, the dianisylamine group and the pyrene core have
C
at 1300 nm in the spectrum of the mixture of 6 and TCNQ indi-
cates the formation of 6 +. However, the mixture of 5 and
TCNQ shows an absorption band at around 1000 nm, which
points to the formation of 52+ rather than 5 in this mixture.
C
spin densities of 0.664 and 0.205, respectively. This further indi-
cates that the oxidation of 1 is largely based on the pyrene
framework. In the di-, tri-, and tetraamine radical cations, the
+
C
The reason for this difference is unclear at this stage. The
amounts of CT complexes formed in other mixtures are much
smaller. In dilute solutions (e.g., c=1ꢁ10ꢀ4 m), none of the
compounds showed absorption of TCNQ·ꢀ.
contribution from the pyrene core increases significantly (0.4–
+
0.7). In 2 and 3 +, the spins are delocalized across the whole
C
C
molecule, with comparable spin contributions from both the
+
+
triarylamine and pyrene units. For 4 , 5 , and 6 +, the spin is
more biased towards the pyrene framework and the spin con-
tribution of each dianisylamino group is less than 0.2. Note
that care should be taken in the interpretation of these DFT re-
sults as it is well known that DFT calculations overestimate
charge delocalization.
C
C
C
EPR studies
The radical cations of 1–6 all display a single-line EPR signal at
around g=2.0 (Figure 12). In the dication form, 22+ and 32+
show a triple-line signal, however, the signal intensity is much
lower than that of their radical cations. In comparison, the EPR
signal of the dication 42+ is rather strong. The dications of tria-
mine 52+ and tetraamine 62+ show very weak EPR signals (es-
The geometries of the dications 22+–62+ were optimized for
both the closed-shell singlet state and the open-shell triplet
state. The singlet states of 22+–42+ were calculated to be 3.62,
3.64, and 10.48 kcalmolꢀ1 higher in energy than their triplet
sentially no signal for 62+). These results suggest that 22+, 32+
,
and 42+ are mainly in the triplet ground state, and that 52+
and 62+ are largely in the singlet state. This will be discussed
later with the aid of theoretical calculations.
DFT and TDDFT calculations on the radical cations and dicat-
ions
The radical cations 1·+–6·+ were subjected to DFT calculations
at the UCAM-B3LYP/6-31G*/CPCM/CH2Cl2 level of theory (see
+
+
C
C
Figure 13. DFT-calculated Mulliken spin distributions of (a–f) 1 –6 (in the
doublet state) and (g–i) 22+–42+ (in the triplet state) at the UCAM-B3LYP/6-
31G*/CPCM/CH2Cl2 level of theory and a schematic representation of the
spin contributions on the pyrene core and each dianisylamino group (NAr2).
+
+
Figure 12. EPR spectra of 1 –6 and 22+–62+ at room temperature ob-
C
C
tained by oxidation with SbCl5 in CH2Cl2.
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states. In contrast, the singlet states of 52+ and 62+ are 4.22
and 9.31 kcalmolꢀ1 lower in energy than their triplet states.
This is in agreement with the EPR results, which indicated that
the ground states of 22+–42+ are triplet states, and those of
52+ and 62+ are singlet states. The lower panel of Figure 13
shows the spin populations of triplets 22+–42+; in each mole-
cule the spins are dominated by two dianisylamino groups.
Next, TDDFT calculations were performed on the DFT-opti-
mized structures to rationalize the absorptions of these com-
pounds in different redox states. Figure 14 shows the TDDFT-
+
C
predicted doublet (D) excitations and absorption spectra of 1
. The predicted absorptions agree well with the experimental
data, although they are a little higher in energy than in the re-
corded spectrum. The D1 excitation at 900 nm is responsible
+
C
for the observed absorption of 1 at 1122 nm. This excitation
is associated with b-spin excitation from the highest-occupied
spin orbital (HOSO, localized on the pyrene framework) to the
lowest-unoccupied spin orbital (LUSO, localized on the dianisy-
lamino unit), which suggests an ICT transition from pyrene to
C
N
+. The D2 excitation at 683 nm of b-HOSO-1!b-LUSO charac-
ter is related to the observed N +-localized transition at
760 nm. The D3 excitation at 566 nm is related to the observed
C
Pyr +-localized transitions at 541 and 590 nm.
C
+
C
The TDDFT-predicted doublet excitations for 2 and triplet
(T) excitations for 22+ are shown in Figure 15. The D1 excitation
of 2 at 1516 nm has a large contribution from a b-HOSO (de-
+
C
localized across the whole molecule)!b-LUSO (more biased
towards the pyrene framework) transition (f=0.327), which is
related to the absorption at 1960 nm observed experimentally.
+
C
This transition is better interpreted as an amine!Pyr CT
transition. However, the occurrence of an IVCT transition be-
Figure 15. (a) TDDFT-predicted low-energy doublet (D) excitations and the
+
C
spin orbitals of 2 involved and (b) the predicted low-energy triplet (T) exci-
+
tations and the spin orbitals of 22+ involved. The absorption spectra of 2
and 22+ are included for comparison.
C
tween two amine components is also possible. The observed
NIR absorption at 1048 nm for the dication 22+ is mainly asso-
ciated with T1 and T3 excitations. The T1 excitation at 798 nm
has b-HOSO (localized on pyrene)!b-LUSO (localized on
amine) character (f=0.327) and has been attributed to the ICT
transition from pyrene to N +. The T3 excitation at 737 nm is of
C
b-HOSOꢀ2 (localized on amine)!b-LUSO character (f=0.693),
indicative of N +-localized transitions. The radical cations and
C
dications of 3 have very similar experimental and predicted ab-
sorption spectra to those of 2 (see Figures S10 and S11 in the
Supporting Information).
+
C
The results of TDDFT calculations for 4 are shown in Fig-
ure S12. The predicted D1 excitation at 1445 nm is of b-
HOSO!b-LUSO character (f=0.141). This can be regarded as
Figure 14. TDDFT-predicted low-energy doublet (D) excitations and the spin
+
+
C
an amine!Pyr CT transition, which appears as a broad NIR
C
orbitals of 1 involved. The experimentally observed absorption spectrum
of 1+ is included for comparison.
band at 2130 nm in the recorded spectrum (Figure 9e). Fig-
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ure S13 shows the TDDFT-predicted triplet excitations of 42+
,
S2 (HOMO-1!LUMO) transitions for 52+ exhibit very similar or-
which are unable to explain the observed low-energy absorp-
tions at 1134 and 1750 nm. The T2 and T3 excitations are
bital changes with respect to the D2 and D1 transitions for 5·+
+
C
and are also mainly of amine!Pyr CT character.
mainly related to the N +-localized transitions observed at
The TDDFT results for 6 and 62+ (see Figures S14 and S15
+
C
C
790 nm.
in the Supporting Information) show some similarities to those
+
+
Figure 16 shows the TDDFT results for 5 and 52+. The pre-
for 5 and 52+. The predicted D1, D2, and D3 transitions are re-
C
C
lated to the observed NIR absorptions of 6 +, and the predict-
C
dicted D1 and D2 transitions are related to the observed NIR
absorptions of 5 +. The D1 transition is of b-HOSOꢀ1 (dominat-
ed S1, S2, and S3 transitions are related to the observed NIR ab-
C
ed by the dianisylamino units)!b-LUSO (dominated by
pyrene) character (l=1143 nm, f=0.357), and the D2 transition
is of b-HOSO (dominated by the dianisylamino units)!b-LUSO
sorptions of 62+. All of these transitions have mainly been at-
+
C
tributed to amine!Pyr CT transitions. These results suggest
that the mono- and dioxidations of 5 and 6 are dominated by
pyrene oxidation with the amino units acting as electron-do-
nating substituents to stabilize the resulting radical cations
and dications.
character (l=1035 nm, f=0.141). These transitions have been
+
C
attributed to amine!Pyr CT excitations. The above EPR and
DFT results suggest that dication 52+ has a singlet ground
state. Therefore TDDFT calculations were also performed on
the closed-shell singlet state of 52+, which showed that the
predicted S1 and S2 transitions are related to the observed NIR
absorptions of 52+. Interestingly, the S1 (HOMO!LUMO) and
Conclusion
We have demonstrated in this work that by increasing the
number of dianisylamino substituents on the pyrene core, the
oxidation potentials of the materials decrease progressively
and the stabilities of the corresponding radical cations and di-
cations are enhanced. Such a multicenter strategy provides
new guidelines for the design and synthesis of organic optoe-
lectronic materials with multielectron redox capability. The oli-
gotriarylamine derivatives reported in this work are good elec-
tron donors, have interesting optoelectronic properties, and
have potential uses in many areas. In addition, triamine 5 and
tetraamine 6 are able to form efficient CT complexes with
TCNQ in solution. These oligotriarylamines have potential use
as hole-transporting materials in optoelectronic devices.[30] All
pyrene-amine adducts (1–6) reported in this work are highly
emissive. No pyrene–pyrene interactions are evident in the
single-crystal X-ray analyses of 3, 4, and 6, which may make
them appealing for use in light-emitting devices.[27] The 1,3-dia-
nisylaminopyrene compound shows an intense EPR signal in
the dication form and is useful in the preparation of organic
spin materials.[15]
The spin distribution and NIR transitions in different redox
states can be tuned by varying the number of dianisylamino
substituents. All the materials are in the doublet state in the
radical-cation form. The monoamine radical cation is better de-
scribed as an aminium radical cation, the free spin in the dia-
mine radical cations is delocalized across the whole molecule,
and the tri- and tetraamine radical cations are better described
as pyrene radical cations. In the dication form, the diamine
compounds are in the triplet ground state, whereas the tri-
and tetraamine dications are in the singlet ground state, as
supported by EPR and DFT analyses.
Experimental Section
Spectroscopic measurements: Absorption spectra were recorded
by using a Perkin-Elmer Lambda 750 UV/VIS/NIR spectrophotome-
ter at room temperature in the denoted solvents with a conven-
tional 1.0 cm quartz cell. Spectroelectrochemistry was performed in
a thin-layer cell (optical length=0.2 cm) in which an ITO glass elec-
trode (<10 W/square) was set in the indicated solvent containing
Figure 16. (a) TDDFT-predicted low-energy doublet (D) excitations and the
spin orbitals of 5+ involved and (b) the predicted low-energy singlet (S) exci-
tations and the molecular orbitals of 52+ involved. The absorption spectra of
5+ and 52+ are included for comparison.
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0.1m Bu4NClO4 and the compound to be measured (cꢂ1ꢁ10ꢀ4 m).
A platinum wire and Ag/AgCl in saturated aqueous NaCl solution-
was used as a counter electrode and reference electrode, respec-
tively. The cell was placed in the spectrophotometer to monitor
spectral changes during electrolysis.
J=9.2 Hz, 4H), 7.77 (d, J=8.0 Hz, 1H), 7.92 (d, J=9.2 Hz, 1H), 7.97
(t, J=7.6 Hz, 1H), 8.03 (s, 2H), 8.12 (m, 3H), 8.19 ppm (d, J=9.2 Hz,
1H); 13C NMR (100 MHz, CDCl3): d=55.6, 114.7, 123.7, 123.8, 125.0,
125.1, 125.1, 126.0, 126.3, 126.5, 126.8, 126.9, 127.4, 127.5, 127.5,
129.0, 131.2, 131.5, 142.2, 143.2, 154.8 ppm; HRMS (EI): calcd for
C30H23NO2 429.1729; found: 429.1733.
Electrochemical measurements: All CV and DPV measurements
were made by using a CHI 620D potentiostat with a one-compart-
ment electrochemical cell under an atmosphere of nitrogen. All
measurements were carried out at a scan rate of 100 mVsꢀ1 in the
indicated solvent containing 0.1m of Bu4NClO4 or Bu4NB(C6F5)4 as
the supporting electrolyte. The working electrode was a home-
made disk platinum electrode with a diameter of 2.0 mm. The elec-
trode was polished prior to use with 0.05 mm alumina and rinsed
thoroughly with water and acetone. A large-area platinum wire
coil was used as the counter electrode. All the potentials are refer-
enced to an Ag/AgCl electrode in saturated aqueous NaCl without
regard for the liquid junction potential. Potentials versus ferro-
cene0/+ can be estimated by subtracting 0.45 V.
Synthesis of 1,8-bis(di-p-anisylamino)pyrene (2): A suspension of
1,8-dibromopyrene (30 mg, 0.080 mmol), di-p-anisylamine (57 mg,
0.25 mmol), [Pd2(dba)3] (7.6 mg, 0.0080 mmol), dppf (4.6 mg,
0.0080 mmol), and NaOtBu (24 mg, 0.25 mmol) in 10 mL toluene
was heated at 1408C for 48 h under a N2 atmosphere in a sealed
pressure tube. The system was then cooled to room temperature.
The solvent was removed under vacuum and the crude product
was purified by silica gel chromatography (eluting with petroleum
ether/ethyl acetate, 60:1) to yield 19 mg of 1,8-bis(di-p-anisylami-
no)pyrene (2) as a yellow solid in a yield of 35%. 1H NMR
(400 MHz, CDCl3): d=3.75 (s, 6H), 3.76 (s, 6H), 6.75 (m, 8H), 6.93
(d, J=8.8 Hz, 4H), 6.97 (d, J=9.2 Hz, 4H), 7.71 (d, J=8.4 Hz, 2H),
7.85 (d, J=9.2 Hz, 1H), 7.96 (s, 1H), 8.01 (s, 1H), 8.02 (d, J=7.2 Hz,
1H), 8.07 (d, J=8.0 Hz, 1H), 8.11 ppm (d, J=9.2 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=55.5, 114.5, 120.5, 122.9, 123.2, 123.5, 123.6,
125.8, 125.9, 126.5, 126.6, 127.1, 127.3, 127.5, 127.7, 128.8, 129.1,
141.9, 142.0, 143.0, 154.6 ppm; MS (MALDI-TOF): m/z=656.3 [M]+;
elemental analysis calcd (%) for C44H36N2O4·2H2O: C 76.28, H 5.82, N
4.04; found: C 75.92, H 6.25, N 3.89.
X-ray crystallography: The X-ray diffraction data were collected by
using a Rigaku Saturn 724 diffractometer on a rotating anode
(MoKa radiation, 0.71073 ꢂ) at 173 K. The structures were solved by
direct methods using SHELXS-97[31] and refined with Olex2.[32] The
structure graphics were generated by using Olex2.
CCDC-989554 (3), 989553 (4), and 989555 (6) contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of 1,6-bis(di-p-anisylamino)pyrene (3): Compound 3
was prepared from 1,6-dibromopyrene (100 mg, 0.28 mmol), di-p-
anisylamine (191 mg, 0.83 mmol), [Pd2(dba)3] (25 mg, 0.028 mmol),
dppf (15 mg, 0.028 mmol), and NaOtBu (80 mg, 0.83 mmol) by
using the same procedure as described for the synthesis of 2. In
this reaction, 133 mg of 1,6-bis(di-p-anisylamino)pyrene (3) was iso-
Computational methods: DFT calculations are carried out by using
the long-range corrected hybrid functional CAM-B3LYP[33] imple-
mented in the Gaussian 09 package.[34] The electronic structures of
the complexes were determined by using a general basis set with
6-31G*.[35] Solvation effects in CH2Cl2 were included in all calcula-
tions, and the conductor-like polarizable continuum model (CPCM)
was employed.[36] No symmetry constraints were used in the opti-
mization (nosymm keyword was used). Frequency calculations
were performed with the same level of theory to ensure the opti-
mized geometries were local minima. All the orbitals were comput-
1
lated as a yellow solid in a yield of 73%. H NMR (400 MHz, CDCl3):
d=3.76 (s, 12H), 6.76 (d, J=8.8 Hz, 8H), 6.97 (d, J=8.8 Hz, 8H),
7.71 (d, J=8.0 Hz, 2H), 7.85 (d, J=9.2 Hz, 2H), 8.02 (d, J=8.4 Hz,
2H), 8.10 ppm (d, J=9.2 Hz, 2H); 13C NMR (100 MHz, CDCl3): d=
55.5, 114.6, 122.9, 123.5, 125.8, 126.6, 127.1, 127.3, 127.7, 128.8,
142.0, 143.0, 154.6 ppm; MS (MALDI-TOF): m/z=656.4 [M]+; ele-
mental analysis calcd (%) for C44H36N2O4·H2O: C 78.32, H 5.68, N
4.15; found: C 78.23, H 5.58, N 4.24.
ed at an isovalue of 0.02 ebohrꢀ3
.
EPR spectroscopy: EPR spectra were recorded on a Bruker ELEX-
SYS E500-10/12 spectrometer at room temperature in CH2Cl2 solu-
tions. The spectrometer frequency was 9.7ꢁ109 Hz.
Synthesis of 1,3-bis(di-p-anisylamino)-7-tert-butylpyrene (4):
Compound 4 was prepared from 1,3-dibromo-7-tert-butylpyre-
ne(100 mg, 0.24 mmol), di-p-anisylamine (165 mg, 0.72 mmol),
[Pd2(dba)3] (22 mg, 0.024 mmol), dppf (13 mg, 0.024 mmol), and
NaOtBu (69 mg, 0.72 mmol) by using the same procedure as de-
scribed for the synthesis of 2. In this reaction, 72 mg of 1,3-bis(di-
p-anisylamino)-7-tert-butylpyrene (4) was isolated as a yellow solid
Synthesis: NMR spectra were recorded in the indicated solvent on
a Bruker Avance 400 MHz spectrometer. Chemical shifts are report-
ed in ppm values by using the residual protons of the deuterated
solvent as reference. Mass spectra were recorded with a Bruker
Daltonics Inc. Apex II FT-ICR or Autoflex III MALDI-TOF mass spec-
trometer. The matrix for MALDI-TOF measurement was a-cyano-4-
hydroxycinnamic acid. Microanalysis was carried out by using
a Flash EA 1112 or Carlo Erba 1106 analyzer at the Institute of
Chemistry, Chinese Academy of Sciences.
1
in a yield of 42%. H NMR (400 MHz, CDCl3): d=1.53 (s, 9H), 3.76
(s, 12H), 6.73 (d, J=9.2 Hz, 8H), 6.94 (d, J=8.8 Hz, 8H), 7.50 (s,
1H), 7.82 (d, J=9.2 Hz, 2H), 8.06 (d, J=9.2 Hz, 2H), 8.07 ppm (s,
2H); 13C NMR (100 MHz, CDCl3): d=13.7, 19.2, 29.7, 30.6, 31.8, 35.1,
53.4, 55.5, 65.5, 114.5, 114.6, 120.4, 122.1, 123.2, 123.2, 123.3, 123.4,
125.3, 126.8, 127.5, 128.8, 130.9, 131.2, 132.3, 142.5, 142.7, 149.5,
154.5 ppm; MS (MALDI-TOF): m/z=712.3 [M]+; elemental analysis
calcd (%) for C48H44N2O4·H2O: C 78.88, H 6.34, N 3.83; found: C
78.40, H 6.45, N 3.85.
Synthesis of 1-(di-p-anisylamino)pyrene (1): A suspension of 1-
bromopyrene (100 mg, 0.36 mmol), di-p-anisylamine (122 mg,
0.53 mmol), tris(dibenzylideneacetone)dipalladium ([Pd2(dba)3],
16.0 mg, 0.018 mmol)], 1,1’- bis(diphenylphosphino)ferrocene
(dppf, 9.9 mg, 0.018 mmol), and NaOtBu (41 mg, 0.43 mmol) in tol-
uene (10 mL) was heated at 1408C for 48 h under a N2 atmosphere
in a sealed pressure tube. The system was then cooled to room
temperature. The solvent was removed under vacuum and the
crude product purified by silica gel chromatography (eluting with
petroleum ether/ethyl acetate, 120:1) to yield 72.0 mg of 1-(di-p-
Synthesis of 1,3,6-tris(di-p-anisylamino)pyrene (5) and 1,3,6,8-
tetrakis(di-p-anisylamino)pyrene (6): A suspension of 1,3,6,8-tetra-
bromopyrene(100 mg, 0.19 mmol), di-p-anisylamine (266 mg,
1.16 mmol), [Pd2(dba)3] (27 mg, 0.029 mmol), dppf (16 mg,
0.029 mmol), and NaOtBu (111 mg, 1.16 mmol) in toluene (10 mL)
was heated at 1408C for 96 h under a N2 atmosphere in a sealed
pressure tube. The system was then cooled to room temperature
and the solvent was removed under vacuum. The crude product
1
anisylamino)pyrene (1) as a yellow solid in a yield of 47%. H NMR
(400 MHz, CDCl3): d=3.77 (s, 6H), 6.78 (d, J=8.8 Hz, 4H), 7.00 (d,
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was purified by silica gel chromatography (eluting with petroleum
ether/ethyl acetate, from 7:1 to 3:2) to yield 27 mg of 1,3,6-tris(di-
p-anisylamino)pyrene (5) in a yield of 16% yield and 87 mg of
1,3,6,8-tetrakis(di-p-anisylamino)pyrene (6) in a yield of 40% as
yellow solids.
Data for 5: 1H NMR (400 MHz, CDCl3): d=3.74 (s, 18H), 6.71 (m,
12H), 6.91 (m, 12H), 7,49 (s, 1H), 7.66 (d, J=8.2 Hz, 1H), 7.78(d, J=
9.1 Hz, 1H), 7.95 (s, 2H), 7.97 (d, J=8.3 Hz, 1H), 8.05 ppm (d, J=
9.1 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=55.5, 114.5, 122.8, 122.9,
123.3, 123.5, 123.6, 125.7, 125.8, 125.9, 126.6, 126.9, 127.5, 127.9,
128.3, 128.5, 128.9, 129.2, 131.0, 141.8, 142.7, 143.0, 154.6 ppm; MS
(MALDI-TOF): m/z=883.2 [MꢀH]+; elemental analysis calcd (%) for
C58H49N3O6·2H2O: C 75.71, H 5.81, N 4.57; found: C 75.97, H 5.85, N
4.30.
1
Data for 6: H NMR (400 MHz, CDCl3): d=3.75 (s, 24H), 6.71 (d, J=
8.6 Hz, 16H), 6.89 (d, J=8.8 Hz, 16H), 7.47 (s, 2H), 7.92 ppm (s,
4H); 13C NMR (100 MHz, CDCl3): d=55.5, 114.5, 122.9, 123.3, 126.2,
128.5, 129.0, 142.5, 142.6, 154.6 ppm; MS (MALDI-TOF): m/z=
1110.2 [M]+; elemental analysis calcd (%) for C72H62N4O8: C 77.82, H
5.62, N 5.04; found: C 77.36, H 5.89, N 4.85.
Acknowledgements
We thank the National Natural Science Foundation of China
(grants 21271176, 91227104, 21472196, and 21221002), the Na-
tional Basic Research 973 Program of China (grant
2011CB932301), and the Strategic Priority Research Program of
the Chinese Academy of Sciences (grant XDB 12010400) for
funding.
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Keywords: density functional calculations
systems · mixed-valent compounds · radicals · amines
·
fused ring
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Received: June 6, 2014
Published online on October 28, 2014
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