AEE-active cyclic tetraphenylsilole derivatives with?100% solid-state fluorescence quantum efficiency

Article abstract of DOI:10.1039/c5dt01846c

Two new strongly AEE active (I/I0 ≈ 94) tetraphenylsilole-containing cyclosiloxanes with cyan emissions (λ_em = 500 nm) and ?100% solid state fluorescence quantum yields are reported. The intraand intermolecular C-H ··· π interactions in the cry

Full text of DOI:10.1039/c5dt01846c

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10.1039/C5DT01846C.
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AEE-active Cyclic Tetraphenylsilole Derivatives with ~100% Solid-
State Fluorescence Quantum Efficiency
Received 00th January 20xx,
Accepted 00th January 20xx
Yuanjing Cai,*ab Kerim Samedov,b Brian S. Dolinar,b Haley Albright,b Zhegang Song,c Chaocan
c
b
*
Zhang,a
Ben
Zhong
Tang
and
Robert
West
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two new strongly AEE active (I/I0
≈ 94) tetraphenylsilole- emitting layer (host) has been accepted as the most effective
containing cyclosiloxanes with cyan emissions (λem = 500 nm) and approach.9 Although doping offers high ΦPL and other advantages
~100% solid state fluorescence quantum yields are reported. The such as an improvement of temperature dependence of external
intra- and intermolecular C-H•••π interactions in the crystal play a electroluminescence (EL) quantum efficiency (ΦPL),10 it complicates
major role in the observed high solid state fluorescence quantum the fabrication process and may increase production cost of OLEDs.
yields.
Thus, it would be ideal if one could achieve very high ΦPL in
undoped solid thin films.10 Apart from
few exceptions
a
Introduction
tetraphenylsilole and silafluorene derivatives are known to exhibit
low ΦPL in solution.11 On the other hand, fluorescent materials
comprised of silole-containing compounds show strong AEE
(aggregation enhanced emission) effects that usually enhance their
quantum yields (ΦPL) by up to two order of magnitude upon
aggregation,12 making them promising materials for solid state (thin
film) applications. However, to date, only a few silole based-
compounds with very high quantum yield (ΦPL > 90%) suitable for
application in OLEDs are known.13a Notably, in 2002 Kafafi et al
reported highly efficient MOLED (molecular organic light-emitting
diodes) with superior external electroluminescence quantum
efficiency (ηEL) of 4.8 % and luminous power efficiency of 9 lm/W
Organic conjugated molecules have been the subject of active
research in advanced materials due to their unique optical and
electronic properties. They have been found to have a wide variety
of applications in fundamental electron/energy transfer studies,1
biological fluorescent probes,2 photovoltaic devices,3 field-effect
transistors,4 and organic light-emitting diodes (OLEDs).5 Among
these compounds, five-membered aromatic heterocycles with a
silicon atom, siloles, have been shown to be excellent electron
transporting materials for OLEDs. Silole possesses a low-energy
LUMO due to the interaction between the σ* orbital of its two
exocyclic Si-C bonds and the π* orbitals of the butadiene moiety.6
Moreover, silole derivatives exhibit high thermal and air stability,
relatively good solubility, easily tunable photophysical properties
and good efficiency in electroluminescence (EL) devices.7
fabricated
using
1,2-bis(1-methyl-2,3,4,5-
silole derivative
tetraphenylsilacyclopentadienyl)ethane,
a
exhibiting blue-green fluorescence with an absolute quantum yield
of 97% in the solid state.13a In 2009, Tang et al reported the
Since high-performance optoelectronic materials are commonly
used as thin films in their real world applications, 8 materials with
high electron mobility and very high photoluminescence quantum
yield (ΦPL) in the solid state are desirable. For realizing high ΦPL in
solid state, doping a highly fluorescent material (guest) into the
synthesis
of
1,1-dimethyl-2,5-bis(trialkylsilylethynyl)-3,4-
solid-state quantum yield of 99.9%.13b
diphenylsiloles with
a
Although our research group has recently reported the synthesis
and photophysical properties of two further silole-based derivatives
displaying solid-state quantum yields (ΦPL) over 80%,13c, d a careful
analysis of their spectroscopic data indicates that the quinaldinate
ligand in these molecules plays a certain role in the observed
enhancement of the fluorescence quantum yield in the solid state.
As optical and photophysical properties of silole-based
compounds in the solid state are primarily influenced by their
molecular packing, modification of optical properties in the solid
state as compared to solution is mainly due to reduction of
rotationally induced non-radiative deactivation processes,
dissipation of excitation energy into various vibrational modes, but
most importantly, the nature of the intermolecular interactions. For
example, the quenching of excited states via non-radiative
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processes in the solid-state often originates from π-π interactions
Similarly, compound 2 can be isolated as pale yellow crystals in
encountered in the unit cell of planar conjugated molecules.14 π-π Et2O/hexane mixture by solvent evaporation. The yields of the
Interactions lead to the formation of aggregates in the ground-state above pale yellow crystals of 1a and 2 suitable for further
and excimers in the excited state that usually diminish photoluminescence studies are 4% and 10%, respectively.
photoluminescence efficiency.15 One strategy to prevent such
The isolated compounds 1a and 2 were characterized by 1H-NMR,
undesirable type of intermolecular interactions between adjacent 13C-NMR, 29Si-NMR, IR, UV-Vis spectroscopy and electrospray
molecules in the solid state is to introduce bulky substituents in the ionization-time of flight mass spectroscopy. Their structures were
molecules impeding their aggregation.16 An alternative strategy determined by single-crystal X-ray diffracꢀon (see ESI, Table S1† for
would be incorporating fluorophores into a bulky and/or space- details). The crystal structure of 1a, 2 and 1b are shown in Fig. 1,
filling rigid framework that would effectively prevent dense packing Fig. 2 and ESI-Fig.S12†, respecꢀvely. All the bond lengths and bond
of the molecules in the cell thus resulting in high solid state angles of the silole unit in these compounds are within the typical
quantum yields.13
range of those tetraphenylsilole derivatives found in the Cambridge
Combining these two approaches, we have used
a
simple Structural Database (ESI, Table S2†). In general, the cyclosiloxane in
condensation reaction of the suitable precursor 1,1-dihydroxy- these compounds has nearly planar ring structures.
2,3,4,5-tetraphenylsilole with appropriate dichlorosilane derivatives
Pure crystals of 1a and 2 were used in the photoluminescence
to incorporate a well-known silole derivative – tetraphenylsilole – property studies. Fig.3 displays the photophysical properties of 1a
into a siloxane framework to generate ring-shaped compounds in THF solution. The maximum of UV-Vis absorption is at 367 nm,
displaying high solid-state fluorescence quantum yields. Here, we which is attributed to the π-π* transition (vide infra) from the
report the synthesis, characterization, and photophysical properties tetraphenylsilole fluorophores. Compound 1a in THF solution emits
of two cyan-emitting ring-shaped silole derivatives 1-2 with solid cyan light with emission maximum at 497 nm. There is no red or
state fluorescence quantum yields ~100%.
blue shift of the emission maximum for compound 1a in solid state
(ESI, Fig.S17†) as compared to in soluꢀon. This indicates that no π-π
stacking exists in 1a in the crystalline state (vide infra).17 Similar
photophysical properties are found for compound 2, these results
are summarized in Table 1.
Results and discussion
The synthetic routes to compounds 1-2 are shown in Scheme 1. The
hydrolysis of 1,1-dichloro-2,3,4,5-tetraphenylsilole at -20°C
followed by condensation reaction with dichlorophenylmethylsilane
or dichlorotetramethyldisiloxane at -20°C gives compounds 1 and 2,
in raw yields of 50% and 40%, respectively. Lower reaction
temperatures (0°C ~ -20°C) and using triethylamine in these
reactions favor the formation of the cyclic structures of
tetraphenylsilole derivatives. Compound
1 has trans and cis
isomers. Although cis isomer 1b cannot be isolated pure neither by
column chromatography nor chiral gas chromatography nor
supercritical fluid chromatography (because of its high boiling point
(over 300°C) and bad solubility in EtOH), it readily crystalizes in form
of pale yellow crystals together with the trans isomer 1a from Et2O.
The trans isomer 1a can be isolated by column chromatography and
subsequent crystallization in hexane/THF mixture upon solvent
evaporation.
Fig. 1 Thermal ellipsoids drawing of compound 1a (all atoms are
shown as 50% thermal probability ellipsoids, all H atoms are
omitted for clarity).
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Cl
Ph
Ph
O
Ph
O
Si
Si
O
Ph
Ph
Cl
O
Ph
Me
Ph
Si
Ph
Ph
Si
O
Si
O
Si
Si
+
Me
Me
O
Me
O
Si
Si
Ph
Ph
Ph
Ph
NaH
CO
3, H2
O
,
Et2
O, -
20°C
Ph
Ph
Ph
Ph
C
°
0
2
l 2
C
i
-
•
S
,
1b
cis isomer
1a
e
N
M
H
t
3
l
h
E
,
C
P
H
F
trans isomer
N
T
t
3
Ph
Ph
OH
Ph
E
-
Ph
HO
C
Si
l
(
M
e
T
)
H
2
F
S
i
-
O
,
E
t
-
Ph
Ph
S
3
N
i
(
,
M
-
-
2
E
e
)
2 C
0
t
°
C
3
N
l
•
H
C
Ph
O
Ph
l
Si
O
Me
Si
i
Me
Me
S
O
Me
Fig. 2 Thermal ellipsoids drawing of compound 2 (all atoms are
shown as 50% thermal probability ellipsoids, all H atoms are
omitted for clarity).
2
Scheme 1 Condensation reactions between 1,1-dihydroxy-2,3,4,5-
tetraphenylsilole and dichlorosilane or dichlorosiloxane.
2 | J. Name., 2012, 00, 1-3
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[a] Fluorescence quantum yield. [b] ε, Extinction coefficient. [c] THF solution of compounds
1a and 2; [d] Excitation wavelength 370 nm; [e] Excitation wavelength 325 nm; [f] log10ε at
325 nm; [g] Excitation wavelength 350 nm.
Fig. 4 (A) Fluorescence Emission Spectra of 10-3 mg/mL compound
1a in THF/H2O mixtures with different fractions of H2O. (B) Plot of
I/I0 at 502 nm versus the water fraction, where I0 is the fluorescence
emission intensity in pure THF solution, λex=370 nm. (C) Photograph
of 1a in THF/H2O mixtures containing different volume fractions of
water taken under illumination of a UV lamp at 365 nm; from left to
right: water fractions 0%-95%.
Fig. 3 (a) Normalized UV absorption of 1a in THF solution; (b)
Normalized fluorescence emission of 1a in THF solution, λex= 370
nm; (c) Normalized fluorescence excitation spectrum of 1a in THF
solution, λactivation=497 nm.
The extinction coefficients of compounds 1a and 2 in THF solution
do not differ greatly (Table 1). The solution quantum yields of these
compounds are close to zero (ESI, Figures S15, S16†), which can be
predicted because there are known tetraphenylsilole compounds
with zero solution quantum yields.13 In this case, as long as a
compound contains a tetraphenylsilole fluorophore, its solution
quantum yield is close to zero. The rotation of phenyl moieties in
this fluorophore in solution can serve as the main relaxation
channel of fluorescence quenching.12 However, in line with our
expectations, the solid state quantum yields of 1a and 2 as crystals
are significantly higher (97±2% for 1a and 99±2% for 2), which
means that these compounds emit almost the exact number of
photons as they absorb. These exceptional photophysical properties
(500 nm cyan emission and ~100% solid state quantum yields) can
be hardly found in other compounds containing fluorophores such
Packing style and interactions, to a large extent, are related to
the solid state quantum yields.12d,14b Typically, a C-H···π interaction
(~4.2 KJ/mol) has the interaction distance less than 2.9 Å, a π-π
interaction (~2 KJ/mol) has the interaction distance less than 3.8
Å.20 While the former interaction can reduce the exciton-vibronic
coupling, the latter one can lead to a red shift in the emission
spectra and fluorescence decay by increasing the exciton-phonon
coupling.14a,16d,21
Whereas the X-ray crystal structures of compounds 1a, 1b and 2
do not contain any notable π-π interactions (Figures 5-7), each
structure does contain C-H···π interactions. Compound 1a (Fig. 5)
contains two strong intramolecular C-H···π interactions (distances:
2.663(4) Å, 2.800(4) Å; angles: 142.7(4)°, 161.1(4)°) and one
intermolecular C-H···π interaction (distance: 2.570(3) Å; angle:
159.6(10)°) per molecule. Compound 1b (Fig. 6) contains two
intramolecular C-H···π interactions (distances: 2.750(3) Å, 2.872(3)
Å; angles: 159.1(3)°, 149.9(3)°) and one weak intermolecular C-H···π
interaction (distance: 2.849(4) Å; angle: 147.3(4)°). Compound 2
(Fig. 7) does not contain intramolecular C-H···π interactions; it only
contains three intermolecular C-H···π interactions (distances:
2.7559(16) Å, 2.8130(14) Å, 2.8925(14) Å; angles: 147.68(13)°
156.44(13)°, 142.97°) per molecule in the asymmetric unit.
as
silafluorene,
phenyl
substituted
silafluorene,
tetraphenylethylene and aluminodiquinaldinate (-AlQ2), etc.13, 18, 19
To better understand the different emission intensities of these
compounds between the solution and crystalline state, we also
studied the aggregation enhanced emission (AEE) behavior of 1a
and 2 by comparing their fluorescence in pure THF and THF/H2O
mixtures.12,13 These results are shown in Fig.4 and ESI-Fig.S14†.
Addition of water to THF solutions of 1a and 2 causes little change
in the spectral profiles. Both compounds show strong AEE with ~94-
fold increases in I/I0 ratio when water fraction is over 70%. The
mechanism of AEE (fluorophores have weak emission in solution
but strong emission in aggregated state) or AIE (aggregation
induced emission: fluorophores have no emission in solution but
strong emission in aggregated state) can be explained by the
restriction of intramolecular motions (RIM) in compounds in the
aggregated state, which facilitates the emission efficiency of the
fluorophores.19
Table 1 Photoluminescent properties of 1a and
2
(λ given in [nm]).
[a]
UV λmax
soln [c]
367
fluorescence λmax
log10ε [b]
Фfl
soln
crystal
500[e]
501[e]
soln
soln
crystal
0.99[e]
0.97[e]
1a
2
497[d]
500[d]
3.98[f]
3.59[f]
0.00[g]
0.00[g]
373
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Fig. 5 Intra- and intermolecular interactions (distances in Å) in mechanisms among them, MO and excited state UV-Vis calculations
compound 1a.
were performed employing the Gaussian 09 package (see ESI† for
detail).23 The calculated geometrical parameters of the optimized
molecules of 1a, 1b and 2 in the ground states agree very well with
the X-ray crystal structure (for optimized geometries, see ESI-Tables
S4-S6†). The first five energeꢀcally lowest-lying excited states (S1-S5)
and corresponding oscillation strengths for 1a, 1b and 2 were
calculated from the geometry optimized single molecules. The
results are summarized in ESI-Table S3†.
The UV-Vis calculations indicate that compound 2 has three
optically allowed excited states (S1, S2, S5), two of which (S1 and S2)
correspond roughly to (π-π*) HOMO → LUMO (S1) and HOMO-1 →
LUMO (S2) transitions. The excited state S5 can be adequately
described as a HOMO → LUMO+1 transiꢀon. While both HOMO and
HOMO-1 in compound 2 are widely spread over the π-systems of
the silole moiety and appurtenant phenyl rings, the LUMO is mainly
located on the π-system of the silole ring with minor contribution of
its phenyl rings’ π-systems (ESI, Figure S22†).
Fig. 6 Intra- and intermolecular interactions (distances in Å) in
compound 1b.
The electronic situations in compounds 1a and 1b do not show
any significant differences. The results of the UV-Vis excited state
calculations for both 1a and 1b predict three optically allowed
excited states (S2, S3 and S5). They further indicate that the
experimentally observed absorption spectra correspond to
a
transition into the second excited state rather than first, since the
first excited state is optically forbidden in both 1a and 1b (ESI, table
S3†). Predicted excitation energies are remarkably close to the
experimentally observed ones. The second excited state wave
function (S2) in 1a and 1b has significant contributions from
configurations arising from HOMO → LUMO and HOMO →
LUMO+1 transitions. All aforementioned MOs in 1a and 1b are
almost exclusively localized on the π-systems of the silole ring and
(with a smaller contribution) appurtenant phenyl substituents on
the silole moieties (Fig.8, ESI-Figures S20-S22†). Notably, phenyl
groups of the PhMe-Si-moieties in both 1a and 1b show absolutely
no contribution to the frontier orbitals and thus to the observed
absorption spectra.
Fig. 7 Intermolecular interactions (distances in Å) in compound 2.
As intramolecular C-H···π interactions can more readily stabilize a
structure’s conformation than intermolecular C-H···π interactions
can, the molecules having inter- and intramolecular C-H···π
interactions lead to a much higher fluorescence quantum yield
compared to the compounds having only intermolecular C-H···π
interactions. In part, we attribute the higher solid state
fluorescence behavior of 1a to this effect.
Actually, for tetraphenylsilole fluorophores, it is not easy to form
π-π stacking. Because the phenyls on 2,3,4,5 positions of silole unit
prefer to form certain degree of angles to the silole plane (e.g.
dichlorotetraphenylsilole: 52°, 54°, 58°, 31°), which inhibits the π-π
stacking of siloles and enlarges the distance between siloles.
Compared to compounds 1a and 2, the starting material
dichlorotetraphenylsilole as crystals has an emission peak at 522
nm with the solid state quantum yield only 5% (ESI, Figure S19†).
The packing analysis shows that there are no effective C-H···π and
π-π interactions in the dichlorotetraphenylsilole crystal structure.22
In an attempt to gain more insight into electronic structure of
Fig. 8 (a) HOMO; (b) LUMO; (c) LUMO+1 in compound 1a.
Conclusions
In summary, we synthesized tetraphenylsilole cyclosiloxane
compounds with ~100% solid state fluorescence quantum
compounds 1a, 1b and
2 and explore possible fluorescence
4 | J. Name., 2012, 00, 1-3
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yields. These compounds can emit cyan light with emission
maxima at around 500 nm both in THF solution and as crystals.
The AEE studies reveal that all these cyclic tetraphenylsilole
derivatives are strongly AEE active with I/I0 ≈ 94. Packing
analysis suggests that the intramolecular C-H···π interactions
combined with the intermolecular C-H···π interactions in the
crystal structure can stabilize the lattice which reduces the
exciton-vibronic and exciton-phonon coupling, resulting in less
fluorescence decay and high solid state quantum yield. Excited
state UV-Vis calculations predict that the observed absorption
Mochida, Y. Asai, A. Yamatani, R. Kaki, T. Hiyama, N. Nagai,
H. Yamagishi and H. Furutani, J. Mater. Chem., 2012, 22
,
4337; (c) Z. Zhao, S. Chen, J. W. Y. Lam, C. K. W. Jim, C. Y. K.
Chan, Z. Wang, P. Lu, C. Deng, H. S. Kwok, Y. Ma and B. Z.
Tang, J. Phys. Chem. C., 2010, 114, 7963; (d) Y. Xu, T. Fujino,
H. Naito, T. Dohmaru, K. Oka, H. Sohn and R. West, Jpn. J.
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(a) C. W. Tang, S. A. VanSlyke and C. H. Chen, J. Appl. Phys.,
1989, 65, 3610; (b) H. Murata, C. D. Merritt and Z. H. Kafafi,
maxima most likely correspond to HOMO→LUMO and HOMO
→
LUMO+1 transitions. MO calculations demonstrate that all
8
9
MOs relevant for fluorescence processes of 1-2 are located on
tetraphenylsilole units. These findings can be very useful in
photophysical applications, and important guides for designing
new highly efficient photoluminescent materials.
IEEE J. Sel. Top. Quant. Electron, 1998, 4, 119.
10 (a) H. Murata, C. D. Merritt, H. Inada, Y. Shirota and Z. H.
Kafafi, Appl. Phys. Lett., 1999, 75, 3252; (b) L. Chen, Y. Jiang,
H. Nie, P. Lu, H. H. Y. Sung, I. D. Williams, H. S. Kwok, F.
Huang, A. Qin, Z. Zhao and B. Z. Tang, Adv. Funct. Mater.,
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Acknowledgements
Y. Cai thanks China Scholarship Council. We acknowledge Prof.
Trisha Andrew (University of Wisconsin-Madison) for providing
instruments for fluorescence measurements. We thank Ilia A.
Guzei (Molecular Structure Laboratory, University of Wisconsin-
Madison) for his contribution to single crystal related matters.
NMR spectra were financed by NSF CHE-9208463 and NSF CHE-
9629688. Mass spectrometry was partially funded by NSF Award
#CHE-9974839 to the Department of Chemistry. Quantum
chemical calculations were supported in part by National Science
Foundation Grant CHE-0840494.
11 (a) K. Hatano, H. Saeki, H. Yokota, H. Aizawa, T. Koyama, K.
Matsuoka and D. Terunuma, Tetrahedron Lett., 2009, 50
,
5816; (b) A. J. Boydston and B. L. Pagenkopf, Angew. Chem.,
2004, 116, 6496; (c) Z. Li, Y. Dong, B. Mi, Y. Tang, M.
Häussler, H. Tong, Y. Dong, J. W. Y. Lam, Y. Ren, H. H. Y. Sung,
K. S. Wong, P. Gao, I. D. Williams, H. S. Kwok and B. Z. Tang,
J. Phys. Chem. B., 2005, 109, 10061.
12 (a) S. Yin, Q. Peng, Z. Shuai, W. Fang, Y. H. Wang and Y. Luo,
Phys. Rev. B., 2006, 73, 205409; (b) Y. Dong, J. W. Y. Lam, A.
Qin, J. Sun, J. Liu, Z. Li, J. Sun, H. H. Y. Sung, I. D. Williams, H.
S. Kwok and B. Z. Tang, Chem. Commun., 2007, 3255; (c) J.
Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S.
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Bright tetraphenylsilole-containing cyclosiloxanes with 500 nm emission and ~100% solid-state fluorescence
quantum yields.
52x39mm (300 x 300 DPI)