Metal-free triplet phosphors with high emission efficiency and high tunability

Article abstract of DOI:10.1002/anie.201402199

Design of highly efficient phosphorescent emitters based on metal- and heavy atom-free boron compounds has been demonstrated by taking advantage of the singlet fission process. The combination of a suitable molecular scaffold and appropriate electronic nature of the substituents has been utilized to tailor the phosphorescence emission properties in solution, neat solid, and in doped PMMA thin films.

Full text of DOI:10.1002/anie.201402199

Angewandte
Chemie
DOI: 10.1002/anie.201402199
Organic Phosphorescence
Hot Paper
Metal-Free Triplet Phosphors with High Emission Efficiency and High
Tunability**
Michael Koch, Karthikeyan Perumal, Olivier Blacque, Jai Anand Garg, Ramanathan Saiganesh,
Senthamaraikannan Kabilan, Kallupattu Kuppusamy Balasubramanian, and
Koushik Venkatesan*
Abstract: Design of highly efficient phosphorescent emitters
based on metal- and heavy atom-free boron compounds has
been demonstrated by taking advantage of the singlet fission
process. The combination of a suitable molecular scaffold and
appropriate electronic nature of the substituents has been
utilized to tailor the phosphorescence emission properties in
solution, neat solid, and in doped PMMA thin films.
yields (QY) up to 200% to be achieved.[5a] Recently, an
external quantum efficiency of 160% for an organic solar cell
device employing pentacene molecules was demonstrated
and also a first observation of efficient singlet fission in
solution based on a bis(triisopropylsilylethynyl)-pentacene
excimer was reported.[8] Boron b-diketonate compounds
incorporated into a polylactide polymer were also reported
to show phosphorescence, albeit only in the solid-state and
the origin for the emission was ascribed to singlet fission.[9]
Therefore, until now most investigations on well-defined
small molecules have been limited to the acene family and the
design of novel chromophores that show singlet fission
induced phosphorescence based on purely organic com-
pounds and a systematic tuning of their photophysical and
chemical properties remains a major challenge.[5b] A key
factor to the successful realization of singlet fission based
materials relies on the design of molecules with appropriate
electronic structure requirements that enables judicious
disposition of the ground state as well as the excited state.[7]
The rich photoluminescent properties of organoboron com-
pounds prompted us to pursue them as molecules of choice
for our investigations.[10] Moreover, the chemical and the
electronic versatility of the molecular scaffold amenable to
suitable modifications allows for achieving superior photo-
luminescent behavior both in the solid state as well as in
solution.[11] This work purposedly combines the electronic and
structural requirements favoring singlet fission by satisfying
different criteria such as presence of strong dipole moments,
p–p interactions (see Scheme 1) and appropriate functional
groups such as an aromatic nitro group as a part of the
conjugate unit that can to some extent exhibit radical
character in the excited state.[7,12]
Herein, we report the synthesis, structural and detailed
photophysical investigations of a novel family of b-hydroxy-
vinylimine boron compounds that show singlet fission
induced room temperature phosphorescence emission with
high efficiency both in solution and when doped in PMMA
(poly(methyl methacrylate)) matrix. Two different classes of
molecules were chosen and synthesized through a common
approach as potential ligand candidates. The first (1–3)
consists of b-hydroxyvinylimines derived from a 3,4-dihydro-
naphthalene unit with scope for additional modulation of
electronic (and dipole moment) changes made possible by
varying the R1 and R2 substituents. The second set (4–7)
involves b-hydroxyvinylimines derived from a 2H-chromene
unit in which the donor moiety is already incorporated into
the structure (oxa group).
M
olecules that display phosphorescence play a significant
role in light- and energy-harvesting systems.[1] This is because
the spin statistics theoretically allow for achieving high
efficiencies in these systems by harnessing the triplet exci-
tons.[2] In this context, search for new triplet emitters with
high quantum yields and tunable emission properties is of
major interest for numerous applications such as solar cells,
organic light-emitting diodes (OLEDs), photocatalysis, sen-
sors etc.[3] Most of the triplet emitters contain transition
metals or heavy atoms that have large spin–orbit coupling to
enable efficient intersystem crossing to the triplet states.[4] In
contrast to the extensive number of examples of transition
metal based triplet phosphors, purely organic materials that
show room temperature phosphorescence are quite rare,
particularly in solution.[2b,5] Although it has long been
established that phosphorescence can be achieved through
singlet fission process in purely organic molecules,[6] the
design and experimental investigations on new molecular
scaffolds are limited.[7] Singlet fission is a process that involves
sharing of energy between a singlet-excited state and
a ground-state molecule to produce a correlated pair of
triplet-excited molecules. In theory, it allows for quantum
[*] M. Koch,[+] Dr. O. Blacque, Dr. J. A. Garg, Dr. K. Venkatesan
Department of Chemistry, University of Zurich
Winterthurerstrasse 190, CH-8057 Zurich (Switzerland)
E-mail: venkatesan.koushik@chem.uzh.ch
K. Perumal,[+] Dr. R. Saiganesh, Prof. Dr. K. K. Balasubramanian
Shasun Research Centre
27 Vandaloor-Kelambakkam Road, Keelakottaiyur, Chennai-600048
(India)
K. Perumal,[+] Dr. S. Kabilan
Department of Chemistry, Annamalai University
Annamalai Nagar, Chidambaram-600002 (India)
[+] These authors contributed equally to this work.
[**] This work was supported by the Swiss National Science Foundation
NRP 62 Smart Materials Program (Grant No. 406240-126142).
Support from the University of Zurich, Prof. em. Dr. Heinz Berke
and Prof. Dr. Roger Alberto are also gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402199.
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Initial examination of
the emission properties of
1–7 revealed strong lumi-
nescence
in
solution
marked by the presence
of two emission bands of
different intensities. While
the band with the lowest
intensity that showed
comparably small Stokes
shifts
(30–81 nm,
see
Table 1) was attributed to
fluorescence, the remark-
ably high intense band
that was significantly
Scheme 1. Synthesis of boron compounds 1–7 from hydroxyvinylimines 8–14.
The synthesis of the hitherto unknown b-hydroxyvinyli-
mines 8–14 was accomplished by treating b-chlorovinylalde-
hydes with 4-nitroaniline in ethanol/water/THF, in the
presence of p-toluenesulfonic acid (pTsOH). The correspond-
ing b-chlorovinylimines were not observed under these
reaction conditions. Since molecular scaffolds incorporating
a nitro group were targeted, the reaction methodology was
standardized primarily only for nitro aniline as a substrate in
this work. It is important to mention that the above utilized
synthetic strategy is previously unknown for accessing b-
hydroxyvinylimines and serves as an elegant approach to
prepare this family of molecules. In a subsequent step, the b-
hydroxyvinylimines precursors were further reacted with
BF3·Et2O in the presence of NEt3 to give the corresponding
boron compounds 1–7 in good to high yields.
Detailed investigations were carried out to evaluate the
photophysical properties of 1–7 in solution (Figure 1), in neat
solid (powder) and spin-coated PMMA films. The UV/Vis
profiles shown in the Supporting Information (Figure S1)
reveal a strong influence of the electronic properties of the
substituents on the absorption maxima (Figure 2). While
compound 7 absorbs at 476 nm, 2 exhibits a strong hypso-
chromic shift with maxima at 418 nm. All compounds show
high molar extinction coefficients in solution with the highest
found for compound 3 at 46200mÀ1 cmÀ1.
Figure 2. Normalized emission spectra of 1–7 in CH2Cl2 (0.1 OD, RT).
Table 1: Photophysical data of 1–7 in CH2Cl2 (0.1 OD, RT, under N2
atmosphere).
F
P
Compound
labs
[nm]
lexc
[nm]
lem,fl
[nm]
t0
lem,ph
[nm]
t0
fem
[%]
[ns]
[ns]
1
2
3
4
5
6
7
421
418
440
430
426
439
456
423
415
432
430
427
434
446
501
500
470
478
493
537
483
2.07
1.69
2.94
3.77
2.07
4.79
4.73
568
558
599
588
621
653
606
1233
5413
1382
3054
1311
2343
2543
29
86
104
100
70
98
49
shifted bathochromically with substantially large Stokes
shifts (140–197 nm at 0.1 OD, see also Tables S3 and S5)
was tentatively ascribed to phosphorescence. It is worthwhile
to note that such significant Stokes shifts are unprecedented
for boron compounds.[9a,13] The excited state lifetimes
obtained for the compounds are in the range 1.69–4.79 ns
and 1.2–5.4 ms for the low intense band and the high intense
band, respectively. This reiterated the nature of the attributed
emission to fluorescence and phosphorescence, respectively.
In order to further validate the excited-state nature of the
long-lived intense band, concentration dependent studies
were carried out. A significant shift of the emission wave-
Figure 1. Concentration dependence on the emission wavelengths of 3
in CH2Cl2 (RT, under N2 atmosphere).
2
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length maxima of the long-lived intense band was observed
upon varying the concentration of the complexes 1–7 in
CH2Cl2 (Figure 1). For instance, the emission of 3 in CH2Cl2
was found to be bathochromically shifted over 150 nm upon
increasing the concentration (509 nm at 4.33 ꢀ 10À9 m and
659 nm at 4.33 ꢀ 10À5 m). This systematic significant shift is
quite unique and a rare behavior, which is tentatively ascribed
to originate from polarization effects. A local polarization
field caused due to the molecular dipole moment interaction
with the dipole moment of a neighboring dye molecule. As
a consequence, a red shift of the emission band is expected for
an excited state with higher polarity than the ground state.
This effect decreases with 1/r3 and hence is less pronounced
for diluted solutions. This behavior was up to now only
observed for different concentrations of dye molecules doped
into a thin film exhibiting Dlem up to 75 nm.[14] While the
fluorescence emission maximum was hypsochromically
shifted for higher concentrations (Dlem = À33 nm for 3), the
phosphorescence emission showed an even stronger depend-
ency on concentration (Dlem = 124 nm for 3, see Figure 1 and
S11–S18). However, neither the absorption wavelength nor
the wavelength maximum in the excitation spectrum showed
any discernible changes upon varying the concentration,
ruling out aggregation induced effects and strongly indicative
of formation of an excimer being the origin of the long lived
intense emission band (see Figure S3 and S8). This hypothesis
is consistent with the single-crystal X-ray diffraction analysis
of 2 and 4, where adjacent molecules are in close proximity
allowing p–p stacking interactions between the benzene rings
(Figure 3). In the crystal structure of 2,[15] the crystallograph-
ically independent molecules are almost parallel to each
other, with an anti-parallel alignment probably favored by the
interaction of the dipole moments of the molecules, and are
linked through two p–p interactions along the c direction
(centroid–centroid distances of 3.7568(8) and 3.7834(8) ꢁ).
These pairs of molecules are further connected by additional
p–p interactions along the c axis with centroid–centroid
distances of 3.6970(7) and 3.8667(8) ꢁ. In the crystal of 4,[16]
pairs of CH···O interactions generate dimers along the a axis
(H···O distances of 2.45 and 2.53 ꢁ with the corresponding
C···O distances of 3.3027(13) and 3.4556(14) ꢁ). These dimers
are further connected by p–p interactions between benzene
rings (bearing the NO2 group) with a centroid–centroid
distance of 3.7875(6) ꢁ. These packing arrangements in the
solid state can be presumed to be present also in solution
leading to the formation of excimers.
Such an excimer formation favors the sharing of the
energy between the molecules in the singlet excited state and
the ground state, after which they combine to produce
a correlated pair of triplet state excited molecules.[17] These
molecules in the triplet excited state then return to the ground
state through a radiative decay process resulting in phosphor-
escence. As a consequence of this series of photophysical
events, two photons are being produced through a singlet
fission process, which ultimately leads to QY in excess of
100%. This is indeed consistent with the absolute QY found
for 3 in solution, which had a maximum value of 114% at
2.16 ꢀ 10À6 m in CH2Cl2 with lower values observed for more
diluted solutions. It has to be mentioned that the QY
remained comparably high (35%, see Table S1) even at
very low concentrations (4.33 ꢀ 10À9 m). On the other hand,
significantly higher concentrations also displayed consider-
ably lower QY (see also Table S1) which is most probably due
to the non-radiative decay behavior induced by enhanced
self-quenching. The phosphorescence was found to contribute
between 83.5% (7) and 90% (3) to the total emission at 0.1
OD with subsequent decrease of the phosphorescence
fraction as a consequence of dilution.[18] This finding renders
further support to the hypothesis that the observed phosphor-
escence process depend on the formation of excimers in
solution, which becomes less efficient upon dilution. The
presence of oxygen showed no significant influence on the
photophysical properties, for example, no quenching of the
long-lived intense band was observed. This can be attributed
to the energetically significantly lower lying T1 states of the
boron compounds compared to the HOMO of O2.[9a]
Investigations of 1–7 in the neat solid (powder) also
showed luminescence, albeit with quantum yields that were
found to be much lower than in solution or in PMMA ranging
between 5.8% and 11.9% (see Table S7). Although 3 exhibits
the highest QY of 52.8% in comparison to the other
compounds, it was still considerably lower in comparison to
what was obtained in solution and PMMA. The creation of
low-energy quenching sites due to increased aggregation in
conjunction with fast exciton migration in the powder is
deemed as a plausible reason for the low quantum yields
observed in the neat solid for the compounds. Since the
emitter molecules are doped into a matrix for most of the
relevant applications, the photophysical behavior of the
compounds doped into an inert polymer film such as
Figure 3. Top: Packing diagram of 2. Bottom: Measured distance
between two molecules of 2 in the crystal structure.
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PMMA was probed. The studies were carried out by spin-
coating a CH2Cl2 solution containing 2 wt% of the corre-
sponding boron compound in PMMA. The emission profiles
of the boron compounds spin-coated in PMMA resembled
the highly diluted samples in solution (Figure S26). However,
the quantum yields were found to even exceed the highest
solution quantum yields at 0.1 OD in most cases (Table 2)
rationalized on the basis of the expected highest molecular
dipole moment affirmed by the presence of methoxy (donor)
and nitro group (acceptor) which are aligned in the direction
of the molecular dipole vector of the molecule.
In summary, a new family of b-hydroxyvinylimine boron
compounds showing efficient and highly tunable phosphor-
escent emission at room temperature both in solution and
when doped in PMMA have been synthesized. Detailed
photophysical investigations confirmed the significantly
enhanced phosphorescence efficiency achieved through the
systematic variation of the electronic nature of the basic
chromophoric scaffold as well as the fine-tuning of the
electronic nature of the substituents. Among the investigated
complexes, an absolute quantum yield exceeding 100% was
obtained for 3 and 4 in solution and for 3, 4 and 7 in PMMA.
The superior efficiencies found for these selected compounds
can be explained on the basis of singlet fission favored by
amalgamation of factors such as permanent molecular dipole
moment, strong p–p stacking interactions and the presence of
a functional group that aids to promote radical character in
the excited state. The culmination in interesting photoem-
ission properties derived through a rational design approach
is expected to pave way for obtaining novel, heavy atom-free
compounds exhibiting singlet fission for potential optoelec-
tronic applications. Further investigations are currently
ongoing in our group pursuing the applications of the
reported compounds.
Table 2: Photophysical data of 2 wt% 1–7 in PMMA.
Compound
lexc [nm]
lem [nm]
fem [%]
1
2
3
4
5
6
7
425
424
437
430
426
436
450
512
512
498
528
512
528
556
73
67
118
117
63
63
106
with the exception of the fluorine-containing analogues 2 and
5 for which lower quantum yields were obtained under these
conditions than in solution. While a maximum quantum yield
of 118% was found for 3, compounds 4 and 7 exhibited 117%
and 106%, respectively, in PMMA. The high quantum yields
further lend support to the assumption that in addition to the
compounds being well dispersed in the polymer matrix, the
vibrational non-radiative decay in the solid state is minimized
due to increased rigidity in comparison to solution and
suppression of other non-radiative decay pathways that were
present in powder.
Received: February 7, 2014
Published online: && &&, &&&&
A closer look at the two different series of boron
compounds (1–3 and 4–7) highlights the importance of both
the electronic properties of the substituents as well as
a suitable chromophore scaffold in order to induce efficient
singlet fission based phosphorescence. It is evident that the
first series with the 3,4-dihydronaphthalene backbone shows
a stronger response upon variation of the electronics via R1
and R2. Compared to 1, the substitution with electron-
donating groups (3) leads to significant increase of the
quantum yield, while the substitution with electron-with-
drawing groups (2) facilitates a moderate augmentation. This
is probably due to presence of increased p–p interactions and
less vibrational quenching in the former versus the latter. In
this series, the parent compound 1 displayed much lower QY
in solution. In the second series consisting of the 2H-
chromene backbone, the parent compound 4 exhibited
higher QY in solution and in PMMA than 5–7. Since the
donor unit is already incorporated in the chromophore
scaffold in these latter compounds, further electronic tuning
of R1 and R2 leads to less beneficial photophysical properties
compared to the parent compound in solution and in PMMA.
Interestingly, strong p-donor functional groups like the
methoxy groups (6) showed similar quantum yields in
comparison to 4 in solution but much lower in PMMA. The
weaker donor methyl group (7) gave only comparable results
to the parent compound in PMMA and in powder, which is
indicative of the higher non-radiative decay rates in solution.
The highest fem in solution, in PMMA and in solid state
observed for 3 in comparison to the other compounds can be
Keywords: dual emission · excimer · organic emitter ·
phosphorescence
.
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www.angewandte.org
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Angewandte
Communications
Communications
Organic Phosphorescence
By taking advantage of the singlet fission
process highly efficient phosphorescent
emitters based on metal- and heavy
atom-free boron compounds were syn-
thesized. The combination of a suitable
molecular scaffold and appropriate elec-
tronic properties of the substituents was
utilized to tailor the phosphorescence
emission properties in solution, neat
solid, and in doped PMMA films.
M. Koch, K. Perumal, O. Blacque,
J. A. Garg, R. Saiganesh, S. Kabilan,
K. K. Balasubramanian,
K. Venkatesan*
&&&& — &&&&
Metal-Free Triplet Phosphors with High
Emission Efficiency and High Tunability
6
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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