Systematic Control of the Overlapping Energy Region for an Efficient Intramolecular Energy Transfer: Functionalized Salen-Al/Triphenylamine Guest-Host Assemblies

Article abstract of DOI:10.1021/acs.inorgchem.8b03005

A series of triphenylamine (TPA)-containing salen-Al assembly dyads, [salen(3-tBu-5-R)₂Al(OC₆H₄-p-N(C₆H₅)₂)] [salen = N,N′-bis(salicylidene)ethylenediamine; R = H (D1), tBu (D2), Ph (D3), OMe (D4), and NMe₂ (D5)], were prepared in good yield (50-80%) and fully characterized by NMR spectroscopy and elemental analysis. Both the UV/vis absorption and photoluminescence (PL) spectra of D1-D4, except for D5, in a tetrahydrofuran solution exhibited dual patterns, which are assignable to the salen-Al-centered α-α? transition (low-energy region) and the TPA-centered α-α? transition (high-energy region). In particular, the emission spectra of the dyads displayed interesting dual-emissive patterns via a significant intramolecular energy transfer (IET) process between the salen-Al moiety and TPA group. Notably, this IET process was systematically tuned by varying the substituents and dominantly observed in the rigid state. More interestingly, compared to the salen-Al complexes (A1-A4) without the TPA group, D1-D4 exhibited enhanced quantum efficiencies. Time-dependent density functional theory calculations on the S₁-optimized structures of D1-D5 further supported these experimental results by indicating the existence of independent transition states between the salen-Al moiety and TPA group in the assembly dyads. The present study reports the first example of salen-Al complexes bearing electron-rich TPA moieties.

Full text of DOI:10.1021/acs.inorgchem.8b03005

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Systematic Control of the Overlapping Energy Region for an
Efficient Intramolecular Energy Transfer: Functionalized Salen−Al/
Triphenylamine Guest−Host Assemblies
Sang Woo Kwak,†,§ Hyomin Jin,‡,§ Ji Hye Lee,‡ Hyonseok Hwang,‡ Min Kim,† Youngjo Kim,†
Yongseog Chung,*,† Kang Mun Lee,*,‡ and Myung Hwan Park*,⊥
†Department of Chemistry and ⊥Department of Chemistry Education, Chungbuk National University, Cheongju, Chungbuk 28644,
Republic of Korea
‡Department of Chemistry and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chuncheon,
Gangwon 24341, Republic of Korea
S
* Supporting Information
ABSTRACT: A series of triphenylamine (TPA)-containing salen−Al
assembly dyads, [salen(3-tBu-5-R)2Al(OC6H4-p-N(C6H5)2)] [salen =
N,N′-bis(salicylidene)ethylenediamine; R = H (D1), tBu (D2), Ph (D3),
OMe (D4), and NMe2 (D5)], were prepared in good yield (50−80%)
and fully characterized by NMR spectroscopy and elemental analysis.
Both the UV/vis absorption and photoluminescence (PL) spectra of
D1−D4, except for D5, in a tetrahydrofuran solution exhibited dual
patterns, which are assignable to the salen−Al-centered π−π* transition
(low-energy region) and the TPA-centered π−π* transition (high-energy
region). In particular, the emission spectra of the dyads displayed
interesting dual-emissive patterns via a significant intramolecular energy
transfer (IET) process between the salen−Al moiety and TPA group.
Notably, this IET process was systematically tuned by varying the
substituents and dominantly observed in the rigid state. More
interestingly, compared to the salen−Al complexes (A1−A4) without the TPA group, D1−D4 exhibited enhanced quantum
efficiencies. Time-dependent density functional theory calculations on the S1-optimized structures of D1−D5 further supported
these experimental results by indicating the existence of independent transition states between the salen−Al moiety and TPA
group in the assembly dyads. The present study reports the first example of salen−Al complexes bearing electron-rich TPA
moieties.
resulting from the intramolecular charge transfer (ICT)
interactions between each ligand.36,37 Recently, we have
developed carbazole-containing salen−aluminum (Al) com-
plexes (I in Figure 1), the quantum efficiency of which is
enhanced by an intramolecular energy transfer (IET) process,
not by ICT.42 Additionally, we have reported novel salen−Al-
INTRODUCTION
■
Since the pioneering works by Shirota on triphenylamine
(TPA) derivatives,1−3 a number of TPA-based molecules have
been extensively studied in various optoelectronic fields, such
as organic light-emitting diodes,4−10 organic field-effect
transistors,11−13 and organic photovoltaics,14−21 because of
their outstanding thermal, electron-donating, and hole-trans-
porting properties.1,22,23 As a well-known propeller-like
molecular building block with highly efficient optoelectronic
properties, the TPA molecule can be easily improved by the
proper and systematic combination with various functional
groups, and the resulting derivatives exhibit intriguing
photophysical and electrochemical properties. To date,
numerous π-extended conjugated donor (D)−acceptor (A)
systems based on ruthenium,24,25 osmium,26 iridium,27−30
platinum,31−33 gold,34−38 zinc,39 and europium40,41 complexes
bearing TPA derivatives have been developed for diverse
optoelectronic applications. Among these complexes, the TPA-
containing alkynylgold(III) complexes recently reported by
Yam and co-workers exhibit excellent optical properties,
Figure 1. Series of salen−Al-based assembly dyads showing an IET
process.
Received: October 23, 2018
© XXXX American Chemical Society
A
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Article
a
Scheme 1. Synthetic Routes for Assembly Dyads (D1−D5) and Their Respective Compounds (A1−A5)
a
Reaction conditions and reagents; (i) AlMe3, toluene, 110 °C, 1 h; (ii) N2, toluene, 110 °C, 4 h; (iii) phenol, toluene, 110 °C, 4 h; (iv)
iodobenzene, CuI, tBuOK, toluene, 110 °C, 12 h; (v) BBr3, DCM, rt, 10 h.
Figure 2. PL spectra in THF (3.0 × 10−5 M) for (a) D1, A1, and N2 (λex = 306 nm), (b) D2, A2, and N2 (λex = 310 nm), (c) D3, A3, and N2 (λex
= 312 nm), (d) D4, A4, and N2 (λex = 316 nm), and (e) D5, A5, and N2 (λex = 324 nm) at ambient temperature.
based dyad systems (II and III) that feature interesting
luminescent properties via IET phenomena.43,44 On the basis
of these previous results, we have found that the proper
selection of both functional groups and a practical synthetic
strategy can lead to attractive optical properties in salen−Al
complexes. In the continuous extension of our systematic
research on IET, we have taken an interest in the fact that the
TPA groups can sufficiently serve as intramolecular energy
B
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Table 1. Photophysical Data of the Assembly Dyads and Mixtures of the Corresponding Compounds
a
λem/nm
τ /ns
a
a
b
b
,c
d
compound
λabs /nm (ε × 10−3 M−1 cm−1
)
λex/nm
THF
film
ΦPL
380 nm
>450 nm
kIET /s−1
D1
D2
D3
D4
D5
N2
285 (40.3), 345 (13.0)
286 (40.7), 360 (15.2)
274 (81.1), 364 (8.7)
286 (39.9), 382 (10.5)
248 (46.6), 408 (6.6)
296 (25.2)
306
310
312
316
324
305
382, 458
379, 473
379, 484
375, 506
376
449
467
471
495
570
0.046
0.197
0.153
0.059
<0.01
0.190
4.14
2.02
2.01
1.67
4.63
1.23
7.27
9.91
9.00
3.29
e
2.42 × 108
4.95 × 108
4.98 × 108
5.99 × 108
2.16 × 108
375
a
b
c
d
Measured at 3.0 × 10−5 M in a THF solution. Measured in the film state (5 wt % doped in PMMA). Absolute PLQY. kIET = 1/(τ detected at
e
380 nm). Not observed because of weak emission.
hosts in a novel class of salen−Al dyad systems based on their
excellent stabilities and emission in the near-UV energy region.
Furthermore, to investigate the efficiency of the IET process in
guest−host dyad assemblies and its dependence on the
overlapping energy region between the absorption of the
guest moiety and the emission, a series of differently R5-
functionalized salen-ligand-based Al/TPA assembly dyads were
designed.
To the best of our knowledge, there has not yet been a
reported example of Al complexes bearing TPA groups. As a
new type of guest−host assembly dyads, salen−Al complexes
bearing the electron-rich TPA moiety could provide significant
insights into the future development of an effective approach in
optoelectronics. Herein, we report the detailed synthetic
procedures, characterization, and optical properties of the
first TPA-containing Al complexes in conjunction with
theoretical calculations.
compounds (A1−A5; λabs = 348−410 nm; Figure S9). On the
other hand, the intense high-energy absorptions in the region
around approximately 290 nm are associated with the TPA-
centered π−π* transition, which is also present in the
absorption spectra of N2 and other TPA-containing deriva-
tives.1−3,7−10,45,46 Thus, the absorption spectra of D1−D5 are
very similar to the convolution of the absorption bands for the
corresponding compounds and strongly suggest that the
electronic transitions of these assemblies occur independently
in the salen−Al moiety and TPA group. The PL spectra of
D1−D5 exhibited specific dual-emission patterns when excited
between 306 and 324 nm (Figure 2 and Table 1). These
excitation values were determined by the minimum point of
the excitation profile for each dyad and can be used to
selectively irradiate only the TPA group (Figure S10).
A comparison of the emission intensities of the assembly
dyads (D1−D5) with the corresponding salen−Al (A1−A5)
and TPA compound (N2) under the same conditions [in
tetrahydrofuran (THF)] indicates that the intensity of the
TPA group at approximately 380 nm (λem = 375 nm for N2)
was considerably reduced and that simultaneously the intensity
of the salen−Al moieties (λem = 458 nm for D1, λem = 473 nm
for D2, λem = 484 nm for D3, and λem = 506 nm for D4)
remarkably increased for D1−D4 compared to A1−A4, except
for D5 and A5. The fact that only the emission intensity of
each part changes without significant change in the emission
wavelength values reveals that the partial IET proceeds from
the TPA group to the salen−Al moiety.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthetic routes for
a series of novel salen−Al/TPA assembly dyads (D1−D5) and
the corresponding Al complexes (A1−A5) are illustrated in
Scheme 1. Two tert-butyl (tBu) groups were introduced to the
C3 position of the salicylidene rings to enhance the solubility
of all complexes in various organic solvents. In particular, all of
the dyads were obtained in good yield (50−80%) by the
reaction of the corresponding salen−Al precursors [M1, (3-
tBu-salen)Al−Me; M2, (3,5-tBu-salen)Al−Me; M3, (3-tBu-5-
Ph-salen)Al−Me; M4, (3-tBu-5-OMe-salen)Al−Me; M5, (3-
tBu-5-NMe2-salen)Al−Me)] with N2, which were prepared via
a well-known copper-mediated amination and subsequent
demethylation by BBr3. The formation of the dyads (D1−D5)
and the respective Al complexes (A1−A5) was fully confirmed
by 1H and 13C NMR spectroscopy (Figures S1−S8) and
Notably, compared to the salen−Al parts of A2−A4, the
intensities of the salen−Al parts of D2, D3, and D4 increased
by nearly 3, 4, and 2 times, respectively (Figure 2b−d). The
emission behavior of the assembly dyads strongly suggests that
the greater the match in the energy region between the
intramolecular energy host (TPA group) and guest moiety
(salen−Al moiety) , the more efficient the IET. Indeed, the
emission profile of N2 overlaps well with absorption profiles in
the order A3 > A2 > A4 > A1 > A5 in THF (Figure S11).
Thus, the IET process between the salen−Al moiety (as a
guest) and TPA group (as a host) in the assembly dyads can be
1
elemental analysis. The H NMR and 13C NMR spectra of
D1−D5 clearly exhibited the expected resonances, correspond-
1
ing to the salen−Al and TPA moieties. Among the H NMR
signals, in particular, the two multiplets observed at
approximately 4.2 and 3.7 ppm definitely revealed the
complete complexation between the salen moiety and Al.
Photophysical Properties of Assembly Dyads. The
photophysical properties of salen−Al/TPA assembly dyads
(D1−D5) were investigated by UV/vis absorption and
photoluminescence (PL) measurements (Figures S9 and 2
and Table 1) and compared with those of an equimolar
mixture between the corresponding salen−Al complexes (A1−
A5) and N2. All of the dyads showed major low-energy
absorption bands from approximately 345 to 410 nm, which
are assignable to the major salen−Al-centered π−π* transition;
the transition is also observed for the respective salen−Al
̈
depicted as the Forster mechanism caused by through-space
interaction. Further investigation of the emission spectra of
solutions of equimolar mixtures (A1, A2, A3, A4, and A5 with
N2) of each compound clearly demonstrates this type of IET
behavior in the assembly dyads. These results show that the
emission intensity of the salen−Al compounds in the mixed
solutions (Figure 2, red dashed line) is similar to (A1 + N2, A2
+ N2, A4 + N2, and A5 + N2) or lower than (A3 + N2) that
of A1, A2, A3, A4, and A5 (Figure 2, sky-blue solid line).
Accordingly, the IET characteristics of the mixed systems are
not much more efficient than those of the assembly dyads. To
C
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further check for any undesired physical interaction in
physically mixed systems, additional PL experiments were
performed under 10 times diluted conditions (3.0 × 10−6 M;
Figure S12). The results are significantly similar to those in
previous conditions (3.0 × 10−5 M), indicating that there is no
physical interaction in the physically mixed systems under the
presented condition. Furthermore, the PL spectra of N1 were
investigated and compared with D1−D5, A1−A5, and
equimolar mixtures (Figure S13). The results are similar to
the presented PL results; however, the emission pattern of N1
appears to be in not good agreement with the emission bands
for the TPA parts of D1−D5.
The fluorescence decay lifetime measurements (Figures
S14−S18) of the assembly dyads reveal that the emission at
380 nm assignable to TPA-centered fluorescence (1.7−4.6 ns)
is slightly shorter than that at >450 nm (3.3−10 ns; Table 1).
These results can also be attributed to the salen−Al-centered
fluorescence, implying the occurrence of IET from the TPA
group to the salen−Al moiety. Furthermore, the efficiency of
the IET process can be verified by the rate constant calculated
from the decay lifetime.47 The kIET values of D2−D4 [(5.0−
6.0) × 108 s−1; Table 1] are over 2-fold larger than those of D1
and D5 [(2.2−2.4) × 108 s−1]. This result shows that the IET
process of D2−D4 is much more effective than that of D1 and
D5. More importantly, the kIET values of D2−D4 are almost
twice as high as those of the physically mixed systems [A2 +
N2, A3 + N2, and A4 + N2; (1.9−3.8) × 108 s−1, respectively].
Meanwhile, the kIET values of D1 and D5 are similar and
considerably lower than those of A1 + N2 and A5 + N2,
respectively (Table 1 and Figures S20−S24). These findings
definitely demonstrate that the IET phenomenon can occur
efficiently when the energy region between the absorption of
the guest and the emission of the host overlaps well and when
the two species are intramolecularly linked to each other.
These results also indicate that the IET phenomena of the
assembly dyads can be quantitatively controlled by simple
changes in the substituents.
systems, consistent with the emissive results in solution (Figure
S25).
To quantify the IET behavior of the assembly dyads (D1−
D5), the absolute quantum efficiency (ΦPL) was investigated in
the film state (Table 1). The fact that the emission efficiency
can be improved by an IET process has already been verified in
our previously reported work.42 The ΦPL values of the dyads
were measured as 4.6% for D1, 19.7% for D2, 15.3% for D3,
and 5.9% for D4; with the exception of D5, these values are
significantly higher than those for the corresponding
compounds (2.6% for A1,42 11.7% for A2,42 11.7% for A3,
and 5.3% for A4). Among these complexes, D2 and D3
showed remarkable increases of up to 68% and 31% in
efficiency compared to those of A2 and A3, respectively.
Furthermore, the ΦPL values of the physically mixed systems
were estimated as 2.7% for A1 + N2, 16.2% for A2 + N2,
14.7% for A3 + N2, and 5.5% for A4 + N2, which are a little
lower than those for the dyads. As a result, these findings
strongly demonstrate that the greater the energy overlap
between the emission area of the host and the absorption area
of the guest in the assembly dyad systems, the better the ΦPL
value of the guest moiety because of the efficient IET process.
Theoretical Calculation and Orbital Analysis. To gain
insight into the nature of the electronic transitions of the
assembly dyads, time-dependent density functional theory
(TD-DFT) calculations on the ground (S0) and first excited
(S1) state optimized structures for D1−D5 were carried out
using the B3LYP functional and 6-31G(d) basis sets (Figure 4,
Table 2, and Figures S26−S36). The calculated electronic
transitions for the dyads in the S0 and S1 states matched well
with the experimentally observed photophysical data. The
results for the S0 state indicate that the major contributions
(fcalc > 0.03) regarding the low-energy absorptions below 375
nm are made by the HOMO−1 → LUMO transitions (λabs
=
376 nm for D1, 385 nm for D2, 402 nm for D3, 423 nm for
D4, and 448 nm for D5; Figure S26 and Table S1). Both
HOMO−1 and LUMO (HOMO = highest occupied
molecular orbital and LUMO = lowest unoccupied molecular
orbital) for D1−D5 are predominantly localized on the salen−
Al moiety (>97%; Tables S3, S7, S11, S15, and S19), revealing
that the absorptions originate from the salen−Al-centered
π−π* transition. On the other hand, the electronic transition
that most contributes (fcalc > 0.7) to the absorptions in the
high-energy regions above 360 nm is the HOMO−2 →
LUMO+1 transition. The orbital contribution of HOMO−2 is
mainly focused on the bridged phenoxy group (>60%), while
LUMO+1 mostly localizes on the diphenylamine (NPh2)
moiety (>76%; Tables S3, S7, S11, S15, and S19). Thus, the
absorption process corresponds to the ICT from the phenoxy
group to the NPh2 moiety. Therefore, these findings for the
calculated absorption transitions clearly indicate that the main
absorption processes in the assembly dyads occur independ-
ently in the salen−Al-centered and TPA groups, respectively.
Similar features are also present in the S1-optimized structures
of the assembly dyads (Figure 4 and Table 2), which were used
to confirm the intrinsic nature of the emissive properties. All of
the main contributions corresponding to the low-energy
emissions around the 430−600 nm region are made by the
HOMO−1 → LUMO transition, which could be assigned to
the salen−Al-centered emission. However, the high-energy
emissions above 390 nm are closely related to the HOMO →
LUMO+2 transitions observed in the experimental TPA-
centered emission. While HOMO−1 and LUMO are entirely
Interestingly, this IET process of D1−D5 was observed
more clearly in the rigid state than in solution. As illustrated in
Figure 3, in the film state [5 wt % doped in poly(methyl
methacrylate) (PMMA)], the low-energy emissions (450−570
nm) corresponding to the salen−Al moieties were more
intense than the high-energy emissions (above 400 nm)
assigned to the TPA group. The film PL spectra of each
mixture of the corresponding compounds also exhibited the
less intensive IET features than those of the assembly dyad
Figure 3. PL spectra of the film (5 wt % doped on PMMA) for
assembly dyads (D1−D5).
D
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Figure 4. Frontier molecular orbitals for D1−D5 in their first excited singlet state (S1) with their relative energies from DFT calculations (isovalue
0.04). The transition energy (in nm) was calculated using the TD-B3LYP method with 6-31G(d) basis sets.
the angles between each transition dipole and the line
connecting them. JDA is the spectral overlap between donor
emission and acceptor absorption. We calculated the transition
dipole moments for N2 and A1−A5 (Figure S37) and the
emission spectrum for N2 and absorption spectra for A1−A5
(Figure S38), so that we can indirectly estimate the IET
efficiency. On the basis of eqs 1 and 2, kDA increases as the
transition dipole moments of the donor and acceptor moieties
become parallel or antiparallel to each other along the line
connecting them. Figure S37 delineates the orientation of the
transition dipole moments of the donor (N2) and acceptor
(A1−A5) molecules, where the molecules are aligned such that
the transition dipole orientation of the corresponding parts in
the dyad complexes (D1−D5) can be estimated. For the
transition dipole of N2, the one for the S0 → S1 transition was
evaluated at the optimized geometry of S1, while the transition
dipole moments of S0 → S1 for A1, A2, and A3, S0 → S2 for
A4, and S0 → S3 for A5 were calculated at the optimized
geometry of S0 of each acceptor molecule. The excited states
for A1−A5 were selected as those with the largest transition
dipole moments among the states with larger absorption
wavelengths than the emission wavelength (385.64 nm) of N2.
Figure S35 clearly demonstrates that the transition dipoles of
the donor and acceptor molecules are oriented in similar or
opposite directions, so that they can interact effectively,
indicating the considerable possibility of IET between the
donor and acceptor parts in the dyad complexes.
Table 2. Major Low-Energy Electronic Transition for the
Assembly Dyads (D1−D5) in the S1 State Calculated Using
a
the TD-B3LYP Method with 6-31G(d) Basis Sets
λcalc/nm
fcalc
assignment
D1
D2
D3
D4
D5
389.73
433.32
390.61
445.88
407.35
498.60
389.61
552.64
392.40
591.88
0.0438
0.0511
0.0455
0.0541
0.0336
0.0575
0.0695
0.0558
0.0846
0.0590
HOMO → LUMO+2 (98.4%)
HOMO−1 → LUMO (99.5%)
HOMO → LUMO+2 (93.8%)
HOMO−1 → LUMO (99.3%)
HOMO → LUMO+2 (98.7%)
HOMO−1 → LUMO (98.5%)
HOMO → LUMO+2 (98.8%)
HOMO−1 → LUMO (99.5%)
HOMO → LUMO+2 (98.9%)
HOMO−1 → LUMO (84.8%)
a
Singlet energies for the vertical transition calculated at the S1-
optimized geometries.
concentrated in the salen−Al moieties (>99%), HOMO and
LUMO+2 are mainly localized on the TPA groups (HOMO,
bridged phenoxy groups ≈61% and NPh2 moieties ≈37%,
LUMO+2, NPh2 moieties ≈92%; Tables S5, S9, S13, S17, and
S21), which implies that ICT occurs on only the TPA part.
These calculated results for the assembly dyads strongly
confirm the dual-emission characteristics corresponding to
independent transitions in the salen−Al center and in the TPA
moiety in solution. Consequently, DFT calculations further
demonstrate that the assembly dyads exhibit only partial IET
from the TPA group to the salen−Al part.
CONCLUSION
■
Furthermore, the IET rate, kDA, can be estimated from the
transition dipole interaction and the spectrum overlap owing to
the following equation:48
A series of assembly dyads based on salen−Al/TPA complexes
(D1−D5) were designed, prepared, and thoroughly charac-
terized. In addition, their intriguing photophysical properties
were investigated. In particular, D1−D5 exhibited dual-
emission features in a THF solution, which originate from
the salen−Al-centered π−π* transition and the TPA-centered
π−π* transition. Simultaneously, the emission spectra of the
assembly dyads showed a significant IET process between the
salen−Al moiety and TPA group. Notably, this IET process
could be systematically controlled by alteration of the
substituents. More importantly, compared to the closely
relevant salen−Al complexes (A1−A4) without the TPA
group, D1−D4 exhibited enhanced luminescence efficiencies.
The theoretical results for the S1-optimized structures of the
2π
kDA
=
|VDA|2 J
DA
ℏ2
(1)
Here the transition dipole interaction, VDA, under a dipole−
dipole approximation is defined as
μ μ
̅
̅
A
r3
VDA
=
B (cos α − 3 cos βD cos β )
A
(2)
where μA and μB are the transition dipole magnitude of the
̅
̅
donor and acceptor parts, α is the angle between the transition
dipoles of the donor and acceptor moieties, and βD and βA are
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solution of S1 at −78 °C. Then, the mixture was slowly heated to 110
°C and refluxed for 1 h. After cooling to room temperature, N2
(0.047 g, 0.18 mmol) in toluene (10 mL) was added to the mixture,
and the mixture was refluxed for 4 h at 110 °C. After cooling to room
temperature, all of the volatiles were removed under reduced pressure.
The residue was washed with n-hexane (50 mL), and the desired
assembly dyads further supported the experimental results by
indicating the existence of independent transition states
between the two parts of the complexes (the salen−Al moiety
and TPA group). Furthermore, the calculated transition dipole
density and spectral overlaps for the donor (N2) and acceptors
(A1−A5) strongly verify the IET mechanism in the dyad
systems. As the first example of salen−Al complexes with
electron-rich TPA moieties and as a novel type of guest−host
assembly dyad, this work is anticipated to open up a new
synthetic strategy for the development of versatile optoelec-
tronic materials.
1
product D1 was obtained as a yellow solid (0.078 g, 59%). H NMR
(CDCl3): δ 8.35 (s, 2H), 7.45 (d, J = 7.6 Hz, 2H), 7.10 (m, 6H), 6.88
(d, J = 8.0 Hz, 4H), 6.82 (dd, J = 7.2 Hz, 2H), 6.72 (dd, J = 7.6 Hz,
2H), 6.63 (d, J = 8.8 Hz, 2H), 6.30 (d, J = 8.8 Hz, 2H), 4.10 (m, 2H),
3.74 (m, 2H), 1.48 (s, 18H). 13C NMR (CDCl3): δ 169.88, 165.24,
157.56, 148.39, 141.71, 137.16, 132.89, 131.63, 128.66, 127.73,
121.98, 120.76, 120.67, 119.27, 116.58, 55.00, 35.39, 29.67. Anal.
Calcd for C42H44AlN3O3: C, 75.77; H, 6.66; N, 6.31. Found: C,
75.54; H, 6.38; N, 5.97.
Synthesis of D2. This compound was produced in a manner
analogous to the synthesis of D1 using S2 (0.089 g, 0.18 mmol). The
desired product D2 was obtained as a yellow solid (0.077 g, 57%). 1H
NMR (CDCl3): δ 8.38 (s, 2H), 7.51 (d, J = 2.4 Hz, 2H), 7.10 (dd, J =
7.8 Hz, 4H), 7.02 (d, J = 2.4 Hz, 2H), 6.90 (d, J = 7.6 Hz, 4H), 6.81
(dd, J = 7.2 Hz, 2H), 6.63 (d, J = 8.8 Hz, 2H), 6.31 (d, J = 8.8 Hz,
2H), 4.10 (m, 2H), 3.73 (m, 2H), 1.47 (s, 18H), 1.28 (s, 18H). 13C
NMR (CDCl3): δ 169.88, 165.24, 157.56, 148.39, 141.71, 137.16,
132.89, 131.63, 128.66, 127.73, 121.98, 120.76, 120.67, 119.27,
116.58, 55.00, 35.39, 29.67. Anal. Calcd for C50H60AlN3O3: C, 77.19;
H, 7.77; N, 5.40. Found: C, 77.10; H, 7.39; N, 4.97.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out
under an inert N2 atmosphere using standard Schlenk and glovebox
techniques. All anhydrous grade solvents (dichloromethane and
toluene) purchased from Aldrich were dried by passing them through
an activated alumina column and stored over activated molecular
sieves (5 Å). Spectrophotometric-grade THF (Merck) was used as
received. Commercially available reagents were used without any
further purification after purchasing them from Aldrich [4-
methoxyaniline, iodobenzene, copper iodide (CuI), potassium tert-
butoxide (tBuOK), phenol, boron tribromide (BBr3), and trimethy-
laluminum (AlMe3, 2.0 M in toluene), 3,5-di-tert-butyl-2-hydrox-
ybenzaldehyde, 3-tert-butyl-2-hydroxybenzaldehyde]. Salen ligands
(S1−S5),49 Al precursors (M1−M5), and Al complexes (A1 and
A2)50−52 were prepared according to modified literature procedures.
A deuterated solvent (chloroform-d1) from Cambridge Isotope
Laboratories was used after drying over activated molecular sieves
(5 Å). NMR spectra were recorded on a Bruker Avance 400
spectrometer (400.13 MHz for 1H and 100.62 MHz for 13C) at
ambient temperature. Chemical shifts are given in ppm and
referenced against external Me4Si (1H and 13C). Elemental analyses
were performed on an EA3000 (Eurovector) in the Central
Laboratory of Kangwon National University. UV/vis absorption and
PL spectra were recorded on a Jasco V-530 and a Fluoromax-4P
(Horiba) spectrophotometer, respectively. Fluorescence decay life-
times were measured at 298 K using a time-correlated single-photon-
counting spectrometer (FLS920; Edinburgh Instruments) equipped
with an EPL-375 picosecond-pulsed semiconductor diode laser as an
excitation source and a microchannel plate photomultiplier tube
(MCP-PMT, 200−850 nm) as a detector.
Synthesis of D3. This compound was produced in a manner
analogous to the synthesis of D1 using S3 (0.16 g, 0.3 mmol). The
1
desired product D3 was obtained as a yellow solid (0.15 g, 61%). H
NMR (CDCl3): δ 8.48 (s, 2H), 7.72 (d, J = 2.4 Hz, 2H), 7.54 (dd, J =
8.4 Hz, 4H), 7.41 (dd, J = 7.2 Hz, 4H), 7.30 (m, 4H), 7.06 (dd, J =
8.4 Hz, 4H), 6.88 (d, J = 7.6 Hz, 4H), 6.79 (dd, J = 7.2 Hz, 2H), 6.66
(d, J = 8.8 Hz, 2H), 6.34 (d, J = 8.8 Hz, 2H), 4.18 (m, 2H), 3.83 (m,
2H), 1.52 (s, 18H). 13C NMR (CDCl3): δ 170.04, 164.80, 157.38,
148.35, 142.09, 140.84, 137.26, 132.26, 129.56, 129.43, 129.03,
128.75, 128.66, 127.73, 126.50, 122.01, 120.77, 120.61, 119.27, 55.02,
35.57, 29.69. Anal. Calcd for C54H52AlN3O3: C, 79.29; H, 6.41; N,
5.14. Found: C, 78.83; H, 6.25; N, 4.90.
Synthesis of D4. This compound was produced in a manner
analogous to the synthesis of D1 using S4 (0.22 g, 0.5 mmol). The
1
desired product D4 was obtained as a yellow solid (0.18 g, 50%). H
NMR (CDCl3): δ 8.23 (s, 2H), 7.13 (m, 6H), 6.86 (dd, J = 8.4 Hz,
4H), 6.71 (d, J = 8.0 Hz, 2H), 6.53 (dd, J = 7.2 Hz, 2H), 6.46 (d, J =
3.2 Hz, 2H), 6.37 (d, J = 7.8 Hz, 2H), 3.99 (m, 2H), 3.74 (s, 6H),
3.65 (m, 2H), 1.46 (s, 18H). 13C NMR (CDCl3): δ 169.36, 160.74,
157.55, 149.84, 148.37, 143.45, 137.12, 128.66, 127.70, 123.42,
121.97, 120.76, 120.68, 117.91, 110.76, 55.68, 55.01, 35.52, 29.52.
Anal. Calcd for C44H48AlN3O5: C, 72.81; H, 6.67; N, 5.79. Found: C,
72.51; H, 6.56; N, 5.42.
Synthesis of 4-Methoxy-N,N-diphenylaniline (N1).53 4-Methox-
yaniline (0.62 g, 5 mmol), iodobenzene (2.04 g, 10 mmol), CuI (0.05
g, 0.025 mmol), and tBuOK (1.68 g, 15 mmol) were dissolved in
toluene (30 mL) at room temperature. After being refluxed for 12 h at
110 °C, the reaction mixture was cooled to room temperature and
filtered. The solvent was removed under reduced pressure, and the
crude product was purified using column chromatography [hexane/
ethyl acetate = 10/1 (v/v)] to afford the desired product N1 (0.71 g,
52%). 1H NMR (CDCl3): δ 7.20 (m, 4H, Ar−H), 7.04 (m, 6H, Ar−
H), 6.93 (m, 2H, Ar−H), 6.83 (m, 2H, Ar−H), 3.79 (s, 3H, −OCH3).
1H NMR spectral data obtained for the compound are in good
agreement with the previously reported data.
Synthesis of D5. This compound was produced in a manner
analogous to the synthesis of D1 using S5 (0.23 g, 0.5 mmol). The
1
desired product D5 was obtained as a yellow solid (0.30 g, 80%). H
NMR (CDCl3): δ 8.34 (s, 2H), 7.18 (d, J = 3.2 Hz, 2H), 7.09 (dd, J =
8.4 Hz, 4H), 6.90 (d, J = 7.6 Hz, 4H), 6.81 (dd, J = 7.2 Hz, 2H), 6.64
(d, J = 8.8 Hz, 2H), 6.45 (d, J = 3.2 Hz, 2H), 6.30 (d, J = 8.8 Hz, 2H),
4.08 (m, 2H), 3.73 (m, 2H), 2.81 (s, 12H), 1.46 (s, 18H). 13C NMR
(CDCl3): δ 169.74, 159.68, 157.84, 148.45, 142.30, 142.08, 136.89,
128.64, 127.73, 123.98, 121.97, 120.75, 120.69, 118.42, 114.50, 55.05,
42.62, 35.66, 29.65. Anal. Calcd for C46H54AlN5O3: C, 73.48; H, 7.24;
N, 9.31. Found: C, 73.36; H, 6.95; N, 9.40.
Synthesis of A3. This compound was produced in a manner
analogous to the synthesis of D1 using S3 (0.16 g, 0.3 mmol) and
phenol (0.028 g, 0.3 mmol). The desired product A3 was obtained as
a yellow solid (0.12 g, 62%). 1H NMR (CDCl3): δ 8.39 (s, 2H), 7.74
(d, J = 2.4 Hz, 2H), 7.55 (d, J = 7.6 Hz, 4H), 7.41 (dd, J = 7.6 Hz,
4H), 7.30 (m, 4H), 6.87 (dd, J = 7.6 Hz, 2H), 6.53 (dd, J = 7.2 Hz,
1H), 6.39 (d, J = 8.0 Hz, 2H), 4.05 (m, 2H), 3.73 (m, 2H), 1.55 (s,
18H). 13C NMR (CDCl3): δ 170.02, 164.81, 160.17, 142.17, 140.89,
132.16, 129.56, 129.36, 128.77, 126.46, 119.74, 119.33, 117.24, 54.98,
Synthesis of 4-(Diphenylamino)phenol (N2).54 To a solution of
N1 (0.60 g, 2.2 mmol) in dichloromethane (20 mL) was slowly added
BBr3 (0.21 mL, 2.2 mmol) at 0 °C. After being stirred at 0 °C for 30
min, the reaction mixture was allowed to warm to room temperature.
Stirring was continued for 10 h, and then a saturated NH4Cl aqueous
solution (30 mL) was added to the reaction mixture to quench the
reaction. The resulting mixture was extracted with dichloromethane
(30 mL × 3), and the organic phases were dried over anhydrous
MgSO4. The solvent was removed under reduced pressure, and the
crude product was purified using column chromatography [hexane/
1
DCM = 1/2 (v/v)] to afford N2 as a white solid (0.48 g, 84%). H
NMR (CDCl3): δ 7.51−7.05 (m, 14H), 5.26 (s, 1H, OH).
Synthesis of D1. S1 (0.076 g, 0.2 mmol) was dissolved in toluene
(10 mL). AlMe3 (0.11 mL, 0.22 mmol) was added to the toluene
F
DOI: 10.1021/acs.inorgchem.8b03005
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
35.59, 29.68. Anal. Calcd for C42H43AlN2O3: C, 77.51; H, 6.66; N,
4.30. Found: C, 77.47; H, 6.48; N, 4.17.
Myung Hwan Park: 0000-0002-8858-5204
Author Contributions
Synthesis of A4. This compound was produced in a manner
analogous to the synthesis of D1 using S4 (0.13 g, 0.3 mmol) and
phenol (0.028 g, 0.3 mmol). The desired product A4 was obtained as
a yellow solid (0.10 g, 60%). 1H NMR (CDCl3): δ 8.30 (s, 2H), 7.13
(d, J = 2.8 Hz, 2H), 6.85 (dd, J = 7.6 Hz, 2H), 6.50 (m, 3H), 6.34 (d,
J = 8.4 Hz, 2H), 4.02 (m, 2H), 3.76 (s, 6H), 3.70 (m, 2H), 1.45 (s,
18H). 13C NMR (CDCl3): δ 169.33, 160.70, 160.27, 149.80, 143.50,
128.67, 123.23, 119.76, 117.92, 117.04, 110.78, 55.70, 54.99, 35.52,
29.45. Anal. Calcd for C32H39AlN2O5: C, 68.80; H, 7.04; N, 5.01.
Found: C, 68.37; H, 6.84; N, 5.01.
Synthesis of A5. This compound was produced in a manner
analogous to the synthesis of D1 using S5 (0.23 g, 0.5 mmol) and
phenol (0.047 g, 0.5 mmol). The desired product A5 was obtained as
a yellow solid (0.22 g, 80%). 1H NMR (CDCl3): δ 8.31 (s, 2H), 7.20
(d, J = 3.2 Hz, 2H), 6.85 (dd, J = 7.6 Hz, 2H), 6.49 (dd, J = 7.2 Hz,
1H), 6.45 (d, J = 2.8 Hz, 2H), 6.35 (d, J = 7.6 Hz, 2H), 4.01 (m, 2H),
3.68 (m, 2H), 2.82 (s, 12H), 1.47 (s, 18H). 13C NMR (CDCl3): δ
169.72, 160.43, 159.67, 142.30, 141.98, 128.62, 123.98, 119.83,
118.41, 116.85, 114.67, 54.99, 42.71, 35.65, 29.57. Anal. Calcd for
C34H45AlN4O3: C, 69.84; H, 7.76; N, 9.58. Found: C, 69.95; H, 7.51;
N, 9.22.
Theoretical Calculations. The optimized structures of the
assembly dyads (D1−D5) in the ground (S0) and first excited (S1)
states were obtained using the DFT method with the B3LYP
functional55,56 and 6-31G(d)57 basis sets. TD-DFT58 measurements
using the hybrid B3LYP functional (TD-B3LYP) were utilized to
obtain the electronic transition energies, which also included an
account of electron correlation. All of the calculations of D1−D5 were
performed in a THF solution. Solvent effects were evaluated with the
conductor-like polarizable continuum model.59,60 All calculations
were carried out using the Gaussian 09 program.61 The GaussSum 3.0
was used to calculate the percent contribution of a group in a
molecule to each molecular orbital.62
UV/Vis Absorption and PL Experiments. UV/vis absorption
and PL measurements in a THF solution (3.0 × 10−5 M) were
performed with a 1 cm quartz cuvette. The thin-film samples in
PMMA were obtained by spin-coating a THF solution (1 mL) of
PMMA (50 mg) containing compounds (5 wt % vs PMMA) on a 15
× 15 mm quartz plate (thickness = 1 mm). The absolute PL quantum
yields (PLQYs) of the dyad complexes (D1−D5) and the mixtures of
the corresponding compounds (A1−A5 + N2, respectively) in the
film state were obtained using a 3.2-in. integrating sphere (FM-sphere,
Horiba) equipped on a Fluoromax-4P spectrophotometer (Horiba) at
298 K.
§S.W.K. and H.J. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Basic Science Research
Program (Grant 2016R1C1B1008452 for M.H.P., Grant
2018R1D1A1B07040387 for K.M.L., and Grant
2017R1D1A1B03031911 for Y.C.) and Basic Research
Laboratory (Grant 2017R1A4A1015405 for K.M.L.) through
the National Research Foundation of Korea funded by the
Ministry of Science and ICT.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03005.
Experimental, computational, and spectroscopic data
1
and copies of the H and 13C data (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: ychung@chungbuk.ac.kr.
*E-mail: kangmunlee@kangwon.ac.kr.
*E-mail: mhpark98@chungbuk.ac.kr.
■
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