A salen-Al/carbazole dyad-based guest-host assembly: Enhancement of luminescence efficiency: Via intramolecular energy transfer

Article abstract of DOI:10.1039/c8cc01528g

A novel class of salen-Al/carbazole dyads (D1 and D2) was synthesized and fully identified. The emission spectra of the dyads presented intriguing dual-emission patterns via an intramolecular energy transfer (IET) state in solution. Furthermore, the IET feature of the dyads was clearly observed in the rigid state. Interestingly, the emission efficiency of the dyads was enhanced by the significant IET process from the carbazole group to the salen-Al moiety. Particularly, D1 exhibited a nearly three-fold enhanced luminescence efficiency compared to the corresponding mononuclear aluminum complexes (A1). Such an emission process of these guest-host systems was further supported by theoretical calculation.

Full text of DOI:10.1039/c8cc01528g

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: M. H. Park, S. W.
Kwak, J. Hyomin, H. Shin, J. H. Lee, H. Hwang, J. Lee, M. Kim, Y. Chung, Y. Kim and K. M. Lee, Chem.
Commun., 2018, DOI: 10.1039/C8CC01528G.
This is an Accepted Manuscript, which has been through the
Volume 52 Number
1 4 January 2016 Pages 1–216
Royal Society of Chemistry peer review process and has been
accepted for publication.
ChemComm
Accepted Manuscripts are published online shortly after
Chemical Communications
www.rsc.org/chemcomm
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1359-7345
COMMUNICATION
Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
=
3
rsc.li/chemcomm

Please do not adjust margins
ChemComm
Page 1 of 5
View Article Online
DOI: 10.1039/C8CC01528G
Chemical Communications
COMMUNICATION
Salen–Al/Carbazole Dyad-Based Guest–Host Assembly:
Enhancement of Luminescence Efficiency via Intramolecular
Energy Transfer
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel class of salen–Al/carbazole dyads (D1 and D2) was
synthesized and fully identified. The emission spectra of the dyads
presented intriguing dual-emission patterns via the intramolecular
energy transfer (IET) state in solution. Furthermore, the IET
feature of the dyads was clearly observed in the rigid state.
Interestingly, the emission efficiency of the dyads was enhanced
by the significant IET process from the carbazole group to the
salen–Al moiety. Particularly, D1 exhibited a nearly three-fold
enhanced luminescence efficiency compared to the corresponding
mononuclear aluminum complexes (A1). Such an emission process
of these guest–host systems was further supported by theoretical
calculation.
Scheme 1 Synthetic routes for salen–Al/carbazole dyads.
Carbazole-based compounds have been widely utilized in
prominent optoelectronic applications such as organic light-
emitting diodes (OLEDs) and photovoltaic cells, because of
their excellent thermal and electronic properties.1–5 These
intriguing properties are mainly attributed to the rigidity and
electron-abundant nature of the carbazole unit,6,7 and can be
further improved by systematic combination with various
functional groups, thus allowing carbazole derivatives to
broaden their applicability in optoelectronics.8–12
Fig. 1 X-ray crystal structures of D1 (40% thermal ellipsoids). The H atoms are omitted
and t-butyl groups are simply drawn for clarity.
Recently, a novel class of Pt-,13–16 Au-,17,18 Cr-,19 and Ru-
based20 organometallic complexes containing carbazole ligand
has received much attention because it can provide positive
effects altering optical properties such as emission-colour
tuning and enhancement of quantum efficiency. Among them,
Yam and co-workers developed novel platinum complexes via
systematic design between the platinum centre and electron-
donating carbazolyl ligands, exhibiting highly efficient
multicolour emission derived from the intramolecular charge
transfer (ICT) process arising from the donor–acceptor
conjugate systems.13 Until now, to the best of our knowledge,
there have been no reported studies on aluminum-carbazole
complexes for optical applications, though aluminum-based
complexes have been well known as prominent optoelectronic
materials. Therefore, we selected salen and carbazole ligands
as proper anchoring ligands into aluminum centre for the
induction of the desired luminescent properties and the
enhancement of emission efficiency via the IET process. Herein,
we describe the detailed synthesis and optical properties of
novel guest–host dyads (D1 and D2) based on salen–Al
complexes with
a carbazole unit in conjunction with
computational calculations.
The synthetic routes for salen–Al/carbazole dyads (D1 and
D2) are shown in Scheme 1. These dyads were prepared by the
reaction of the salen–Al precursors (M1: (3-tBu-salen)Al-Me
and M2
:
(3,5-tBu-salen)Al(CH3)) with 4-(2,7-di-tBu-9H-
carbazol-9-yl)phenol (C2) in moderate yield (68% for D1 and
64% for D2). D1 and D2 were fully characterized by 1H and 13C
NMR spectroscopy (Figs. S1–S4 in the ESI†) and elemental
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 5
View Article Online
DOI: 10.1039/C8CC01528G
COMMUNICATION
Journal Name
Table 1 Photophysical data of mononuclear (A1, A2, and C1) and dyad compounds (D1
and D2)
analysis. The solid-state molecular structure of D1 was
analysed by the single-crystal X-ray diffraction method (Fig. 1).
Single crystals of D1 suitable for X-ray diffraction studies were
obtained from the slow diffusion of MeOH into a THF solution.
The structural parameters including selected bond lengths (Å)
and angles (°) for D1 are presented in Tables S1 and S2 of the
ESI†. The phenoxy group connects the Al atom with a
carbazole group as the key group, indicating that D1 is an
assembly dyad with a five-coordinate aluminum centre. The
trigonality parameter 21,22 (τ) value for D1 is 0.47, indicating
that the geometry around the Al centre of D1 is an
intermediate between trigonal bipyramidal (tbp) and square
pyramidal (sp) structures. Furthermore, the carbazole-centred
plane is significantly distorted with respect to the phenoxy
group linked to the Al centre, owing to the dihedral angle of
54.6°. This structural feature confirms that the delocalization
of electron density between the salen–Al moiety and the
carbazole group in the assembly dyad cannot occur.
τa /ns
a
λabsa/nm (ε ×
10−3 M−1 cm−1
λem
b
Comp.
ΦPL
380
nm
460
nm
)
nm
348 (13.8)
280 (31.4)
A1
A2
C2
D1
D2
459c
478d
0.026c
0.117d
‒
‒
‒
3.923
6.923
‒
359 (10.3)
281 (26.4)
326 (4.60)
299 (23.9)
360c,d
0.38
5.0
6.4
346 (12.8)
288 (37.4)
364c
458c
360d
475d
0.072c
0.244d
7.7
8.2
358 (9.10)
288 (34.7)
a
b
Measured using a 5.0 × 10−5 M THF solution. Absolute PL quantum yield
measured in film state (5 wt% doped on PMMA). c λex = 310 nm. d λex = 316 nm.
To investigate the photophysical properties of the assembly
dyads (D1 and D2), UV−vis absorpꢀon (Fig. 2 and Table 1) and
photoluminescence (PL) measurements (Fig. 3 and Table 1)
were performed in THF at room temperature. D1 and D2
exhibited a dominant low-energy absorption band at ca. 346
nm and 358 nm, respectively, which is associated with the
major salen–Al-centred π‒π* transition that is also observed
for closely related aluminum compounds (λabs = 348 nm for A1
and 359 nm for A2, Table 1) without the carbazole unit. The
high-energy absorptions in the region of ca. 288 nm are
assignable to the carbazole-centred π‒π* transition that is also
present in the absorption spectra of C2 and other carbazole-
based compounds.13,14,16,18–20
Hence, the absorption spectra of D1 and D2 are quite similar
to the arithmetic sum of the absorption bands for the
corresponding mononuclear compounds, strongly suggesting
that the electronic transitions of these assembly dyads
independently occur on the salen–Al moiety and the carbazole
group, respectively. The PL spectra of D1 and D2 present
specific dual-emission patterns when excited at 316 nm and
310 nm, respectively (Fig. 3 and Table 1). These excited values
are the minimum points in each excitation graph for dyads,
and can only partially excite the carbazole moieties (Fig. S5 in
the ESI†). A comparison of the emission intensities of D1 and
D2 with their relevant mononuclear aluminum (A1 or A2) and
carbazole compound (C2) at the same condition reveals that
the intensity of the carbazole moiety (λem = 360 nm for C2) was
drastically reduced and simultaneously, the intensity of the
salen–Al part (λem = 459 nm for A1 and 478 nm for A2
)
increased nearly two-fold compared to that of mononuclear Al
compound (A1 or A2). Particularly, the fact that the PL
intensity of each part changed with no significant alteration in
the emission wavelengths shows the relevance of the partial
IET from the carbazole group to the salen–Al moiety. These IET
Fig. 2 (a) UV−vis absorpꢀon spectra in THF (5.0 × 10−5 M) for (a) D1, A1, and C2 and (b)
D2, A2, and C2.
Fig. 3 PL spectra in THF (5.0 × 10−5 M) for (a) D1, A1, and C2 (λex = 310 nm) and (b) D2,
A2, and C2 (λex = 316 nm) at room temperature.
Fig. 4 PL spectra of film (5 wt% doped on PMMA) for D1 and D2. The insets are
photographs of film under a handheld UV lamp (λex = 365 nm).
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 3 of 5
View Article Online
DOI: 10.1039/C8CC01528G
Journal Name
COMMUNICATION
Fig. 5 Frontier molecular orbitals for D1 and D2 at their ground (S0) and first singlet (S1) state optimized geometries in THF and the major electronic transitions from TD-DFT
calculations (isovalue = 0.04).
380 nm and 460 nm) for the equimolar mixtures (Figs. S12 and
Table 2 Major low-energy electronic transitions determined for D1 and D2 for the
S13). From the results (6.5 ns for A1+C2, 8.3 ns for A2+C2), the
ground state (S0) and first excited singlet state (S1) using TD-DFT calculationsa
kIET values for A1+C2 and A2+C2 at 380 nm, which is the
emission region of the energy donor (C2), were calculated as
1.5 respectively, resulting that the
108 s−1 and 1.2 108 s−1
values are lower than those of dyad complexes (2.0
108 s−1
for D1 and 1.6
108 s−1 for D2). These results clearly indicate
λcalc
fcalc
assignment
/nm
×
×
,
D1
D2
S0
S1
S0
S1
375.71
335.32
0.0313
0.0823
HOMO−1 → LUMO (70.6%)
HOMO−2 → LUMO+1 (92.0%)
×
×
that the IET process in the dyad systems is faster and more
effective than that of the physically mixed systems,24 further
implying the IET from the carbazole group to the salen–Al
moiety. Interestingly, such an IET phenomenon of D1 and D2 is
observed in the rigid state. The only intense emission assigned
to the salen–Al moiety appeared in the film state (5 wt%
doped in poly(methyl methacrylate) (PMMA)), indicating that
the IET feature completely occurs.
458.81
356.89
0.0501
0.0928
HOMO−1 → LUMO (99.9%)
HOMO−2 → LUMO+1 (93.0%)
384.98
342.97
0.0351
0.0796
HOMO−1 → LUMO (97.7%)
HOMO−2 → LUMO+1 (93.2%)
477.66
354.08
0.0490
0.0956
HOMO−1 → LUMO (98.9%)
HOMO−2 → LUMO+1 (94.8%)
a
Singlet energies for the vertical transition calculated at the optimized S0 and S1
In the PMMA film, importantly, the obtained quantum
efficiency (ΦPL) of these guest–host dyads (D1 and D2) clearly
indicates that the emission efficiency can be tuned by the
nature of the IET feature (Table 1). D1 (ΦPL = 0.072) and D2
(ΦPL = 0.244) containing a key carbazole group had higher
geometries.
features are also evident in the emission spectra of the
corresponding mononuclear compounds (A1
+C2 (λex = 310 nm)
and A2+C2 (λex = 316 nm)) in a solution with an equimolar
mixture (Fig. 3). However, the IET characteristics of the
mixture systems are not as efficient as those of the guest–host
dyads (D1 and D2). Additional considerations of significant
overlaps between the emission band of the carbazole
compound (C2) and the absorption bands of the salen–Al
compounds (A1 or A2) also confirm this phenomenon (Fig. S8
in the ESI†). Thus, the mechanism of the IET between the
carbazole moiety (as the donor) and the salen–Al moieties (as
the acceptor) in the dyads (D1 and D2) is the Förster
mechanism via through-space interaction. The emission decay
quantum efficiencies than A1 (ΦPL = 0.026) and A2 (ΦPL
=
0.117), respectively. Notably, the ΦPL of D1 was enhanced
approximately three-fold compared to that of A1. Accordingly,
both guest–host dyads showed significantly higher
luminescence efficiency than their corresponding aluminium
complexes without key carbazole groups. These findings
highlight the significance of the IET effect of guest–host dyads
on ΦPL.
To better elucidate the origin of the electronic transitions of
the dyad complexes, time-dependent density functional theory
(TD-DFT) calculations on the ground (S0) and first excited state
(S1) optimized structures for the two guest–host dyads were
performed using the B3LYP functional and 6-31G(d) basis sets
(Fig. 5, Table 2, and Figs. S14–S17 in the ESI†). The computed
electronic transitions for the dyads in their S0 and S1 states are
in good agreement with the experimentally observed
absorption and emission bands. The calculation results for the
lifetime measurements (Figs. S9‒S12 in the ESI†) of these
dyads indicate that the emissions at 380 nm assignable to
carbazole-centred fluorescence (5.0 ns for D1 and 6.4 ns for
D2) are slightly shorter than those at 460 nm (ca. 8.0 ns). To
check the rate constant (kIET) for the IET process in the dyads
(D1 and D2) and the equimolar systems (A1+C2 or A2+C2), we
additionally carried out the decay lifetime measurements (at
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 5
View Article Online
DOI: 10.1039/C8CC01528G
COMMUNICATION
Journal Name
organometallic complexes based on their different desirable
electronic features. Relevant studies on expanding novel Al-based
guest–host systems are in progress.
optimized structures in the S0 state demonstrate that the
major contributions (fcalc > 0.03) to the low-energy absorptions
below 370 nm are mainly associated with HOMO−1 → LUMO
transitions (λabs = 376 nm for D1 and 385 nm for D2; Fig. 5 and
Table 2). HOMO−1 is dominantly localized on the salen–Al
moiety (94.6% for D1 and 97.7% for D2), and LUMO also
predominantly occupies the salen–Al parts (99.4% for D1 and
D2, Tables S4 and S8 in the ESI†), indicating that the
absorptions originate from the salen–Al-centred π‒π*
transition. On the other hand, the largest electronic transitions
(fcalc ≈ 0.8) for the absorption regions above 370 nm are mainly
attributed to the HOMO−2 → LUMO+1 transiꢀon. The orbital
contribution of HOMO−2 and LUMO+1 is completely localized
on the carbazole moiety (> 99%, Tables S4 and S8 in the ESI†),
due to which the absorption process corresponds to a
carbazole–centred π‒π* transition. Hence, these findings for
the calculated absorption transitions clearly indicate that the
main absorption processes in D1 and D2 occur independently
of the salen–Al-centred and carbazole-centred π‒π*
transitions, respectively. The same features are also present in
the calculation results for the S1-optimized structures of dyads
(Fig. 5 and Table 2), which were used to clarify the intrinsic
character in emission bands. The main contribution to low-
energy emissions below 450 nm is the HOMO−1 → LUMO
transition, which could be assigned to the salen–Al-centred
emission, whereas the high-energy emissions above 350 nm
Notes and references
1
2
3
Z. Yang, Z. Mao, Z. Xie, Y. Zhang, S. Liu, J. Zhao, J. Xu, Z. Chi
and M. P. Aldred, Chem. Soc. Rev., 2017, 46, 915.
X. Yang, G. Zhou and W.-Y. Wong, Chem. Soc. Rev., 2015, 44
8484.
A. W. Schmidt, K. R. Reddy and H.-J. Knölker, Chem. Rev.,
2012, 112, 3193.
,
4
5
Y. Lin, Y. Li and X. Zhan, Chem. Soc. Rev., 2012, 41, 4245.
N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair, R.
Neagu-Plesu, M. Belletête, G. Durocher, Y. Tao and M.
Leclerc, J. Am. Chem. Soc., 2008, 130, 732.
D. F. O’Brien, P. E. Burrows, S. R. Forrest, B. E. Koene, D. E.
Loy and M. E. Thompson, Adv. Mater., 1998, 10, 1108.
B. E. Koene, D. E. Loy and M. E. Thompson, Chem. Mater.
1998, 10, 2235.
6
7
8
9
Y. H. Lee, S. H. Park, J. Oh, J. W. Shin, J. Jung, S. Yoo and M. H.
Lee, ACS Appl. Mater. Interfaces, 2017, , 24035.
A. Venkateswararao, K. R. J. Thomas, C.-P. Lee, C.-T. Li and
K.-C. Ho, ACS Appl. Mater. Interfaces, 2014, , 2528.
9
6
10 N. Prachumrak, S. Pojanasopa, S. Namuangruk, T. Kaewin, S.
Jungsuttiwong, T. Sudyoadsuk and V. Promarak, ACS Appl.
Mater. Interfaces, 2013, 5, 8694.
11 A. Michaleviciute, E. Gurskyte, D. Y. Volyniuk, V. V. Cherpak,
G. Sini, P. Y. Stakhira and J. V. Grazulevicius, J. Phys. Chem. C,
2012, 116, 20769.
12 S.-i. Kato, H. Noguchi, A. Kobayashi, T. Yoshihara, S. Tobita
and Y. Nakamura, J. Org. Chem., 2012, 77, 9120.
13 A. K.-W. Chan, M. Ng, Y.-C. Wong, M.-Y. Chan, W.-T. Wong
and V. W.-W. Yam, J. Am. Chem. Soc., 2017, 139, 10750.
14 F. K.-W. Kong, M.-C. Tang, Y.-C. Wong, M. Ng, M.-Y. Chan and
V. W.-W. Yam, J. Am. Chem. Soc., 2017, 139, 6351.
15 G. Li, A. Wolfe, J. Brooks, Z.-Q. Zhu and J. Li, Inorg. Chem.,
2017, 56, 8244.
16 F. K.-W. Kong, M.-C. Tang, Y.-C. Wong, M.-Y. Chan and V. W.-
W. Yam, J. Am. Chem. Soc., 2016, 138, 6281.
17 C.-H Lee, M.-C. Tang, Y.-C. Wong, M.-Y. Chan and V. W.-W.
Yam, J. Am. Chem. Soc., 2017, 139, 10539.
18 M.-C. Tang, D. P.-K. Tsang, M. M.-Y. Chan, K. M.-C. Wong and
V. W.-W. Yam, Angew. Chem. Int. Ed., 2013, 52, 446.
19 K. Shen, X. Tian, J. Zhong, J. Lin, Y. Shen and P. Wu,
Organometallics, 2005, 24, 127.
20 N. D. McClenaghan, R. Passalacqua, F. Loiseau, S. Campagna,
B. Verheyde, A. Hameurlaine and W. Dehaen, J. Am. Chem.
Soc., 2003, 125, 5356.
are closely related to the HOMO−2 → LUMO+1 transition
attributed to the carbazole-centred emission. While the
molecular orbital distributions of HOMO−1 and LUMO are
concentrated in the salen–Al parts (>99%, Tables S6 and S10 in
the ESI†), those of HOMO−2 and LUMO+1 are localized on the
carbazole parts (>99%, Tables S6 and S10 in the ESI†).
Consequently, the insights obtained from the computed
results for the S1-optimized structures of D1 and D2 strongly
suggest that the experimentally observed dual-emissions in
the THF solution resulted from the independently bounded
transition in the salen–Al centre and carbazole moieties,
respectively. Along with the experimental results, DFT
calculations further confirm that the dyads only exhibit partial
IET from the carbazole group to the salen–Al moiety.
In summary, novel guest–host systems (D1 and D2) based on
salen–Al/carbazole dyads were prepared and fully characterized.
These guest–host systems exhibited nearly two-fold enhanced
luminescence efficiencies compared to the corresponding
aluminum complexes through the significant IET feature from the
carbazole group to the salen–Al moiety in PMMA films. The
computed calculation results also showed the independently
bounded transition on the salen–Al centre and carbazole moieties,
respectively, further verifying the experimental results. Thus, it is
anticipated that these guest–host systems based on salen–
Al/carbazole dyads can constitute a novel class of optoelectronic
materials in OLEDs. In particular, the dyads can be used as a
promising candidate of guest–host systems in blue-emitting
fluorescent OLEDs (FLOLEDs). Furthermore, the present study may
provide important insights for the future development of
prominent optoelectronic materials using small-molecule
21 D. A. Atwood and M. J. Harvey, Chem. Rev., 2001, 101, 37.
22 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and G. C.
Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349.
23 S. W. Kwak, B. H. Choi, J. H. Lee, H. Hwang, J. Lee, H. Kwon, Y.
Chung, K. M. Lee and M. H. Park, Inorg. Chem., 2017, 56
6039.
24 R. Ziessel and A. Harriman, Chem. Comm., 2011, 47, 611.
,
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 5 of 5
View Article Online
DOI: 10.1039/C8CC01528G
Chemical Communications
COMMUNICATION
Graphical and Textual Abstract for TOC
A novel class of salen–Al/carbazole dyads was synthesized and
fully identified. The dyads exhibited an over two-fold
enhanced luminescence efficiency compared to the
corresponding aluminum complexes through the significant
intramolecular energy transfer process from the carbazole
group to the salen–Al moiety.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins