Luminescent Mono-, Di-, and Triradicals: Bridging Polychlorinated Triarylmethyl Radicals by Triarylamines and Triarylboranes

Article abstract of DOI:10.1002/chem.201903007

Up to three polychlorinated pyridyldiphenylmethyl radicals bridged by a triphenylamine carrying electron withdrawing (CN), neutral (Me), or donating (OMe) groups were synthesized and analogous radicals bridged by tris(2,6-dimethylphenyl)borane were prepared for comparison. All compounds were as stable as common closed-shell organic compounds and showed significant fluorescence upon excitation. Electronic, magnetic, absorption, and emission properties were examined in detail, and experimental results were interpreted using DFT calculations. Oxidation potentials, absorption and emission energies could be tuned depending on the electron density of the bridges. The triphenylamine bridges mediated intramolecular weak antiferromagnetic interactions between the radical spins, and the energy difference between the high spin and low spin states was determined by temperature dependent ESR spectroscopy and DFT calculations. The fluorescent properties of all radicals were examined in detail and revealed no difference for high and low spin states which facilitates application of these dyes in two-photon absorption spectroscopy and OLED devices.

Full text of DOI:10.1002/chem.201903007

A Journal of
Accepted Article
Title: Luminescent mono-, di-, and tri-radicals: bridging polychlorinated
triarylmethyl radicals by triarylamines and triarylboranes
Authors: Yohei Hattori, Evripidis Michail, Alexander Schmiedel,
Michael Moos, Marco Holzapfel, Ivo Krummenacher, Holger
Braunschweig, Ulrich Müller, Jens Pflaum, and Christoph
Lambert
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To be cited as: Chem. Eur. J. 10.1002/chem.201903007
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10.1002/chem.201903007
Chemistry - A European Journal
Luminescent mono-, di-, and tri-radicals: bridging
polychlorinated triarylmethyl radicals by triarylamines and
triarylboranes
Yohei Hattori,[a] Evripidis Michail,[a] Alexander Schmiedel,[a] Michael Moos,[a] Marco
Holzapfel,[a] Ivo Krummenacher,[b] Holger Braunschweig,[b] Ulrich Müller,[c] Jens
Pflaum,[c] and Christoph Lambert*[a]
[a] Julius-Maximilians-University Würzburg, Institute of Organic Chemistry, Am Hubland, 97074 Würzburg
[b] Julius-Maximilians-University Würzburg, Institute of Inorganic Chemistry, Am Hubland, 97074 Würzburg
[c] Julius-Maximilians-University Würzburg, Institute of Physics, Am Hubland, 97074 Würzburg
Abstract:
Polychlorinated pyridyldiphenylmethyl mono-, di-, and tri-radicals bridged with triphenylamine
substituted by electron withdrawing (CN), neutral (Me), or donating (OMe) groups were
synthesized and di- and tri-radicals bridged with tris(2,6-dimethylphenyl)borane were prepared
for comparison. All compounds were as stable as common closed-shell organic compounds
and showed significant fluorescence upon excitation. Electronic, magnetic, absorption, and
emission properties were examined in detail, and experimental results were interpreted
referring DFT calculation results. Oxidation potentials, absorption and emission energies could
be tuned depending on the electron abundance of the bridges. Triphenylamines mediated
intramolecular weak antiferromagnetic interactions between radical spin, and high spin and
low spin calculation results for di- and tri-radicals were validated. The fluorescent properties of
all radicals were examined in detail and revealed no difference for high and low spin states
which facilitates application of these dyes in two-photon absorption and OLED devices.
Introduction
Organic -radicals or radical ions are typically nonemitting species. Among several molecules
that “violate” this rule1-8 are donor-acceptor compounds comprising chlorinated triphenylmethyl
radicals as electron acceptors in combination with arylamine donors.9-14 Emission in the red to
near infrared spectral region with sizable quantum yields have been reported. Furthermore, for
compounds with carbazole as the donor efficient OLEDs could be fabricated.15, 16
The topology of the triphenylmethyl radical and the triphenlyamine donor also allowed the
synthesis of linear oligomers17, polymers18 and branched chromophores19-21 that show charge
transfer upon optical excitation. However, while radicals are useful building blocks for
functional materials,22-28 almost nothing is known about the luminescent properties of such di-
1
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and polyradicals.29-31 Thus, the focus of this work is to elucidate the basic emission properties
of bi- and triradicals based on polychlorinated pyridyldiphenylmethyl radical as the spin bearing
unit, to compare them with suitable monoradical parent compounds, and to demonstrate some
potential applications in OLEDs and as two-photon absorption chromophores. A particular
focus will lie on the modification of the bridge moiety connecting the two (or three) radical
centers and its impact on the optical performance.
Figure 1. Structures of mono-, di-, tri-radicals.
2
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Results and Discussion
In this study, the polychlorinated pyridyldiphenylmethyl radical (PyBTM′′) is combined with a
triarylamine bridge. In our design of the compounds, three, two, or one PyBTM′′ moieties are
bonded to the 4, 4′, and 4′′ positions of triphenylamine (TPA) and the remaining free positions
are filled with cyano, methyl or methoxy groups in order to tune the donor strength of the
triarylamine (see Fig. 1). Compared to the previously used perchlorinated triphenylmethyl and
tris(trichlorophenyl)methyl radicals, where the chlorine atoms serve to enhance the acceptor
character and, concomitantly, shield the radical center to make it persistent, the
polychlorinated pyridyldiphenylmethyl radical is distinctly less light sensitive,5, 32 an issue which
is quite important for any (electro)optical application. Thus, TPA(PyBTM′′)3 triradical,
TPA(CN)(PyBTM′′)2,
TPA(Me)(PyBTM′′)2,
TPA(OMe)(PyBTM′′)2
diradicals,
and
TPA(CN)2(PyBTM′′), TPA(Me)2(PyBTM′′), TPA(OMe)2(PyBTM′′) monoradicals were
synthesized (Fig. 1, see also the SI). In order to compare the donor bridge with an acceptor
34
bridge,33,
we also prepared tris(2,6-dimethylphenyl)borane (TPB) compounds,
TPB(PyBTM′′)3 triradical and TPB(Me)(PyBTM′′)2 diradical.35
For estimating the donor strength of the bridging triarylamine group we measured cyclic
voltammograms of the radicals (Fig S6, Table S3). These show the reduction waves of the
PyBTM′′ groups at around −0.9 V (vs. Fc+/Fc). While for the TPB compounds the first oxidation
waves at +0.74 V (vs. Fc+/Fc) refer to the PyBTM′′ groups, the TPA compounds show the first
oxidation wave in a less oxidative region (+0.20 - 0.57 V vs. Fc+/Fc) which corresponds to the
oxidation of the triarylamine group. First oxidation of TPA rather than PyBTM′′ can be judged
from the ratios of the oxidation to reduction waves (1:1 for monoradicals, 1:2 for diradicals, 1:3
for triradicals) in addition to the expectation of the oxidation potentials. The phenomenon that
another group is oxidized before the radical group is called “SOMO-HOMO energy-level
37
conversion”.36,
The order of oxidation susceptibility, TPA(OMe)2(PyBTM′′)
>
TPA(Me)2(PyBTM′′) > TPA(OMe)(PyBTM′′)2 > TPA(Me)(PyBTM′′)2 > TPA(PyBTM′′)3 >
TPA(CN)(PyBTM′′)2 > TPA(CN)2(PyBTM′′) reflects the electron donating ability of the TPA
along the substituent R = OMe > Me > PyBTM′′ > CN.
The ground and excited spin doublet states are unique characteristics of monoradicals. For
the spin multiplicities of the di- or tri-radicals, orientation of two or three spins have to be
considered. Diradicals can possess triplet (high spin, HS, ferromagnetic coupling of spin
centers) or singlet states (low spin, LS, antiferromagnetic coupling), and triradicals can adopt
quartet (HS) or doublet (LS) states.
In order to estimate the magnetic interaction between the radical centers in the ground state,
temperature dependent EPR spectra were measured for the di- and tri-radicals in frozen
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toluene. In addition to g = 2 signals, half-field transitions typical of triplet states were observed
for the diradicals as well as for the quartet states of triradicals (see Fig. 2a and Fig. S2).
The temperature dependence of the double integral of the main signal of the TPA bridged
diradicals (see Fig 2 and Table S1, Fig. S3) were fitted with the Bleaney-Bowers equation (1),38
which describes the magnetic susceptibility of a two spin system. Energy differences between
triplet and singlet levels were fitted as E = 2J = −8.78 cm−1 for TPA(CN)(PyBTM′′)2, 2J =
−11.9 cm−1 for TPA(Me)(PyBTM′′)2, and 2J = −11.3 cm−1 for TPA(OMe)(PyBTM′′)2,
respectively. Here, J is the exchange interaction taken positive when the HS state is lower in
energy than the LS state. For the TPA(PyBTM′′)3 triradical, the magnetic susceptibility
equation for triangular system (2)39 was used and the energy difference between quartet and
doublet levels was optimized as 3J = −14.3 cm−1. These small negative values can be
interpreted as antiferromagnetic interactions between two spins or three spins inside a
molecule, that is, in all cases the low spin state is slightly more stable. The order of absolute
value of exchange interaction
J
[(TPA(CN)(PyBTM′′)2
<
TPA(PyBTM′′)3
<
TPA(OMe)(PyBTM′′)2 < TPA(Me)(PyBTM′′)2] roughly follows the expected electron density on
the N atom of TPA which are thought to mediate the interaction. Following a three-orbital
superexchange model40, 41 with four electrons (two unpaired electrons and one electron pair at
the bridge connecting the spin bearing orbitals, see SI), rising the energy of the bridge orbital
(e.g. by donor substituents as in the TPA moiety) leads to an increase of the singly occupied
MO constructed from the interaction of the three localized orbitals. This increase favours the
antiferromagnetic contribution and leads to an increase of │-J│. Since the spin-spin
interactions are not strong (3J / k < 30 K), both HS and LS states exist in almost equal amount
for diradicals and triradicals at room temperature.
2
2
Ng 
1
χA =
χA =
3 + exp (-2J/kT) + Nα
(1)
(2)
kT
2
2
Ng  5 + exp (-3J/kT)
12kT 1 + exp (-3J/kT)
+ Nα
Temperature dependence of the EPR signal strength of TPB(PyBTM′′)3 in toluene showed an
unexpected behavior at low temperature and the data points could not be fitted by equations
(1) or (2). However, a powder sample of TPB(PyBTM′′)3 diluted in KBr gave a Bleaney-Bowers
fit (eq. 1) with a large antiferromagnetic value (Table S2, Fig. S4, 2J = −349 cm−1), and no
meaningful fit was obtained with the equation for triangular system (2). The results indicate
that in the solid state some intermolecular magnetic interactions are much stronger than
intramolecular ones. Temperature dependence of the double integral of the signal of
TPB(Me)(PyBTM′′)2 in toluene was fitted by the equation (1) with 2J = −16.2 cm−1. This value
4
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is remarkably high compared to those of the other diradicals. However, using the same
superexchange model as described above but with only two unpaired electrons (because the
boron p-orbital is empty), we arrive at the same conclusion that a low lying vacant bridge orbital
increases the orbital energy gap and favors the antiferromagnetic state thereby increasing
│-J│. This is one of the rare cases in organic chemistry where an empty orbital mediates the
spin-spin interaction.
In order to verify the electronic structure of the compounds, the molecular structures were
optimized by DFT calculation at the UB3LYP/6-31G* level for all possible spin multiplicities
(see SI). However, because the wave function of the low spin state in all calculations was
calculated by the “broken symmetry” approach and in fact represents a mixture of LS and HS
state, the exact energy difference between the LS and the HS state was evaluated by eq (3)42-
44
taking the spin expectation values into account. The thereby estimated J values depend
strongly on the functional (see Fig. S12) but using CAMB3LYP yielded reasonable agreement
with the experiment for all TPA radicals, that is, the LS state is always more stable than the
HS state, see Fig. 2c. Therein we also give the J values for some TPA derivatives with
substituent R = NH2, Cl, and NO2 which have not been studied experimentally but which
illustrate the influence of the TPA substituent clearly. The trend expected from the three-orbital
superexchange model was indeed verified, that is, J varies smoothly along the electron
donating (withdrawing) strength of the substituent at the TPA unit (R = NH2, OMe, Me, Cl, CN,
NO2). The spin density at the TPA nitrogen also varies accordingly.
5
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a)
b)
12
10
8
10 K
20 K
40 K
60 K
80 K
100 K
120 K
6

4
2
0
331 332 333 334 335 336 337
0
20 40 60 80 100 120 140 160
B0 / mT
T / K
c)
0.016
0.014
0.012
0.010
0.008
0.006
0.004
0
-2

-4
-6
-8
-10
NH2
OMe
Me
Cl
CN
NO2
TPA diradical substituent R
Figure 2. a) EPR spectra of TPA(CN)(PyBTM′′)2 at different temperatures. b) Double integral
of EPR spectra vs. temperature (circles) and fit by Bleaney-Bowers equation (1) (red line). c)
Calculated (UCAMB3LYP/6-31G*) exchange coupling J (black), spin density at UB3LYP (blue)
of the HS state and J experimental (red) coupling from temperature dependent EPR
measurements for a series of TPA diradicals with varying substituent R.
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Figure 3. Frontier orbitals of singlet and triplet TPA(Me)(PyBTM’’)2 calculated at the UB3LYP
/ 6-31G* level. The orbital energy increases from right to left. See also Fig. S14 for orbitals of
the other radicals.
E
- E
LS
HS
2
J =
(3)
2
̂
〈
S
̂
〉
〈
〉
- S
HS
LS
Calculated spin density distributions of the molecules are shown in Fig. S13. In general, -spin
molecular orbital and -spin molecular orbital are not degenerate for open-shell compounds
such as radicals. While orbitals are delocalized by -conjugation, the largest coefficients are
on the radical carbon for the LUMOs, and on amine nitrogen for the HOMOs (see Fig. 3 for an
example, the orbitals of the other radicals can be found in Fig. S14). These are the centers
where reduction and oxidation occurs, respectively. There is one more -spin electron than -
spin electron in the (doublet) monoradicals. In these cases, the lowest unoccupied molecular
orbitals are -spin orbitals (-LUMOs), and the highest occupied molecular orbitals are -spin
orbitals (-HOMOs).
For the triplet diradicals with two excess -electrons the LUMOs are -LUMOs, and the
HOMOs are -HOMOs. In the singlet diradicals the number of -spin electrons and -spin
electrons are the same, and - and -LUMOs, and - and -HOMOs are almost degenerate.
The quartet triradicals behave similar to the triplet diradicals and the doublet triradicals similar
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to the singlet diradicals concerning the orbital energies. In all compounds irrespective of the
spin, the lowest unoccupied molecular orbital is mainly located on the PyBTM′′ groups. The
highest occupied molecular orbitals of the TPA compounds are centered on the TPA moiety
but show considerably delocalization onto the PyBTM′′ groups in some compounds. In
contrast, the highest occupied molecular orbitals of the TPB compounds are positioned on the
PyBTM′′ groups. These assignments correspond to the results obtained by electrochemistry
analysis where first oxidation occurs at the amine in the TPA radicals but at the PyBTM′′ in
TPB radicals. Reduction and oxidation potentials and the calculated energies of the LUMOs
and HOMOs correlated qualitatively (see Table S3).
Plotting the computed spin density (HS state) of the central nitrogen along with the exchange
interaction J indicates that the central atom mediates the exchange interaction for the TPA
compounds (see Fig. 2c). The spin density is tuned by the electron donating or withdrawing
substituent at the TPA.
l / nm
l / nm
1000
600
400
1000
600
400
TPA(OMe)2(PyBTM")
TPA(Me)2(PyBTM")
TPA(CN)2(PyBTM")
TPA(Me)(PyBTM")2
TPA(OMe)(PyBTM")2
TPA(CN)(PyBTM")2
a)
b)
60000
40000
20000
e
0
80000
60000
40000
20000
0
TPA(CN)2(PyBTM") × 10
TPB(Me)(PyBTM")2
TPB(PyBTM")3
TPA(CN)2(PyBTM")
c)
d)
× 10
TPA(CN)(PyBTM")2
TPA(PyBTM")3
× 10
× 10
10000 15000 20000 25000 30000 35000 40000 15000 20000 25000 30000 35000 40000
n / cm-1 n / cm-1
Figure 4. Absorption spectra of radicals in cyclohexane.
Electronic spectra of all compounds show strong absorption around 25000-30000 cm-1 which
is characteristic for chlorinated triarylmethyl radical compounds.45, 46 Furthermore, the TPA
radicals display somewhat weaker absorptions between 17000-25000 cm-1, and even weaker
8
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lowest energy absorptions between 12000 – 19000 cm-1 (Fig. 4). As the DFT calculation
revealed (see below), this band consists of several transitions, one for the monoradicals, two
for the diradicals and three for the triradicals. Therefore, it is difficult to give the energy of
maximum absorption of the lowest energy band because of varying intensities of these
overlapping bands. Thus, we evaluated the 00-energy by the intersection of a tangent at the
low energy flank with the baseline (see Table 1). These lowest energy absorptions have to
some extent CT character47 and their maxima are shifted towards lower energy the stronger
the donor is (see Fig. 4a and 4b: TPA(OMe)(PyBTM′′)2 > TPA(Me)(PyBTM′′)2
>
TPA(CN)(PyBTM′′)2 ) and the more radical centers are involved (see Fig. 4c
TPA(CN)2(PyBTM′′) > TPA(CN)(PyBTM′′)2 > TPA(PyBTM′′)3). In general the absorption
coefficients become larger as the number of PyBTM′′ groups increases.
For the TPB compounds the lowest energy band is rather weak and shifted towards higher
energy (see Fig. 4d). Furthermore, there is a very strong absorption peaking at ca. 22000
cm-1 which is neither seen in unsubstituted triarylboranes48, 49 nor in the TPA radical and thus
has to be assigned to interactions of the triarylborane with the radical center.
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Table 1. Experimental (E00) and TD-UDFT[a] computed lowest energy absorption data in the
gas phase.
[b]
E00
LScal
/cm-1[c]
f[d]
HScal
/cm-1 [e]
f[d]
/cm-1
TPB(PyBTM′′)3
14800
15800
15800
16000
16100
0.003 17900
0.005 18000
0.008 18200
0.002
0.072
0.070
0.001
TPB(Me)(PyBTM′′)2
14800
15700
16000
0.004 17900
0.005 18000
0.099
0.022
TPA(CN)2(PyBTM′′)
14400
14100
15700
0.12
TPA(CN)(PyBTM′′)2
13900[f]
13900[f]
0.14
0.13
14500
15200
0.25
0.061
TPA(PyBTM′′)3
13300
13000
12600[f]
12900[f]
13600[f]
0.16
0.22
0.10
13600
13600
14700
0.24
0.25
0.00
TPA(Me)(PyBTM′′)2
12300[f]
12500[f]
0.26
0.11
12700
13500
0.29
0.053
TPA(OMe)(PyBTM′′)2 12700
12100[f]
12200[f]
0.33
0.06
12400
13300
0.31
0.056
TPA(Me)2(PyBTM′′)
12600
12100
11500
0.18
0.20
TPA(OMe)2(PyBTM′′) 12000
[a]UB3LYP/6-31G* [b] 00-energy obtained by the intersection of a tangent at the
low energy flank of the absorption lowest energy band with the baseline [c]Low
[d]
spin state absorption energy Oscillator strength [e] High spin state absorption
energy. [f]Highly spin contaminated.
While the assignment of bands is quite clear for the TPA radicals, the electronic nature of the
lowest energy band and the very intense band of the boranes are unclear. Thus, the absorption
spectra were calculated by TD-UDFT (UB3LYP/6-31G*) for both LS and HS state of the
monoradicals, diradicals, and triradicals. The lowest excited state of the monoradical consists
mainly of -HOMO → -LUMO transition (see TD-UDFT calculations in the SI). For the
diradicals there are two almost degenerate excited singlet states corresponding to -HOMO
→ -LUMO and -HOMO → -LUMO transitions and two excitations for the triplet state with
-HOMO → -LUMO und -HOMO → -LUMO+1 transitions. For the TPA doublet triradical
there are three excitations corresponding to -HOMO → -LUMO, -HOMO → -LUMO and
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-HOMO → -LUMO+1 and also three excitations for the quartet states with -HOMO → -
LUMO, -HOMO → -LUMO+1 and -HOMO → -LUMO+2. For the boron triradical there is
a strong mixing of a large number of configurations for both doublet and quartet state.
For TPA compounds, both - and -HOMOs have the largest coefficients at the site of the
amine nitrogen atom, and the lowest absorption transitions can be described as charge transfer
absorptions from TPA to PyBTM′′. The calculated excitation energies and oscillator strengths
of the respective HS state are somewhat higher than that of the LS states (the difference is
more pronounced for the boron compounds) but in general are in excellent agreement with the
experimental E00 data (see Table 1) taking into account that the UDFT computations refer to
vertical excitations.
Moreover, for both spin states of the TPA diradicals and the triradical, the transitions are highly
allowed showing that the actual spin multiplicity of the ground state does not matter for the
optical transitions because, unlike singlet closed shell ground state molecules, there are two
ground states in the diradicals and triradicals.
For the boron radical, the situation is somewhat different. From the fact that the boron p-orbital
is vacant one clearly attributes acceptor character to the triarylborane which is at odd with the
assignment of the lowest energy transition to a CT. However, the boron has significant -donor
character which increases electron density in the aryl rings while the chlorines on the PyBTM′′
group have -acceptor character which decreases electron density in these aryl rings.
Although the -HOMO is on PyBTM′′ radical, the -HOMO is still located at the triarylborane
aryl groups (see SI). Thus, despite the vacant boron-p-orbital there is enough electron density
in the triarylborane to donate electron density to the very strong radical acceptor upon optical
excitation which therefore can be assigned to a CT transition. Vice versa, according to the TD-
UDFT computations, the high intensity transition at ca. 22000 cm-1 can be assigned to a
reverse CT from the PyBTM′′ radical center to the triarylborane acceptor (see SI).
All radicals display strong fluorescence in cyclohexane (Fig. 5). Luminescence of TPA radicals
was quenched in more polar solvents such as dichloromethane similarly to the previous
reports. The 00-energy of the radicals’ fluorescence shows a distinct Stokes shift of ca. 800-
1100 cm-1 (see Table 2) and thus displays the same trend as the absorption maxima (see
Table 1). Although the TPB core is a -acceptor, the fluorescence bands fit in a continuous
trend of the other donor-acceptor radicals supporting the argument that the TPA serves as a
(weak) donor in combination with a strong radical acceptor.
One may ask as to what extent the small energy difference between the HS and LS excited
state influences the fluorescence spectra. The almost identical shape of the emission spectra
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of TPA(OMe)(PyBTM′′)2 and TPA(Me)2(PyBTM′′) clearly shows that the fact that the former
compound may adopt two different spin states while the latter possesses only one has no
influence on the shape of the emission band. Similar is true for TPA(CN)(PyBTM′′)2 and
TPA(CN)2(PyBTM′′). We also measured the fluorescence lifetimes (τ) of all radicals by time-
correlated single photon counting (TCSPC) at 15240 cm-1 excitation (the boron radicals at
19420 cm-1) and, in one case (TPA(CN)(PyBTM′′)2), by broadband fluorescence upconversion
at 16900 cm-1 excitation. In all cases, we found a monoexponential decay as given in Table 2.
Even when measuring at emission energies lower than the fluorescence maximum the
lifetimes are monoexponential. Only for the shortest times, the fluorescence upconversion
spectra display weak band narrowing due to vibrational relaxation (see Fig. S7). Furthermore,
excitation spectra are in excellent agreement for different fluorescence detection energies.
Transient absorption spectra of TPA(CN)(PyBTM′′)2 at 16100 cm-1 excitation with fs-time
resolution corroborates the formation of an CT state by showing the typical signal for a
triarylamine radical cation at ca. 13300 cm-1 (see Fig. S8) also did not give any hint for the
population of two energetically different excited CT states. All these experiments show that
fluorescence from the HS and the LS state are virtually identical and cannot be distinguished.
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Table 2. Emission properties of radical compounds.
[a]
E00
kf
knr
휐̃


fl
/cm-1
/cm-1
/107 s–1 /107 s–1
/% /ns
TPB(PyBTM′′)3
15000 15800
15000 15800
14500 15300
14200 14900
13700 14400
13200 14000
0.3 1.3
0.4 1.5
2.4 3.2
0.2
0.3
0.8
77
66
31
22
16
13
14
8.9
51
TPB(Me)(PyBTM′′)2
TPA(CN)2(PyBTM′′)
TPA(CN)(PyBTM′′)2
TPA(PyBTM′′)3
3.7 4.4[b] 0.8
6.1 5.8
7.9 7.2
6.0 6.5
24 8.6
2.8 1.9
1.1
1.1
0.9
2.7
1.5
TPA(Me)(PyBTM′′)2
TPA(OMe)(PyBTM′′)2 12900 13800
TPA(Me)2(PyBTM′′) 12700 13500
TPA(OMe)2(PyBTM′′) 11800 12800
[a]
00-energy obtained by the intersection of a tangent at the low energy flank of the
fluorescence band with the baseline [b]Broadband fluorescence upconversion yields the
following time constants: 7.5 ps, 99 ps and 3.5 ns.
l / nm
1200
1000
800
600
1.0
0.8
0.6
0.4
0.2
0.0
8000
10000
12000
n / cm-1
14000
16000
Figure 5. Fluorescence spectra of all radicals in cyclohexane at r.t. Color code:
TPB(PyBTM′′)3,
TPA(PyBTM′′)3,
TPB(Me)(PyBTM′′)2,
TPA(Me)(PyBTM′′)2,
TPA(CN)2(PyBTM′′),
TPA(OMe)(PyBTM′′)2,
TPA(CN)(PyBTM′′)2,
TPA(Me)2(PyBTM′′),
TPA(OMe)2(PyBTM′′).
Absolute fluorescence quantum yields (ϕ) of the radicals were determined by means of an
integration sphere and are given in Table 2. The TPA compounds showed higher quantum
yields and longer lifetimes than the TPB compounds with TPA(Me)2(PyBTM′′) possessing the
13
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10.1002/chem.201903007
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highest quantum yield of 24 %. For luminescent radicals, in which intersystem crossing does
not play a role, the quantum yields are the result of a competition between rates of fluorescence
(kf) and non-radiative decay (knr) to the ground state (= internal conversion, IC). These
quantities were calculated from quantum yields and lifetime via kf = ϕ/τ and knr = (1-ϕ/τ),
respectively. There are two reasons for enhancement of fluorescence by addition of TPA:
increase of kf and decrease of knr. Increase of kf is explained by increase of oscillator strength
of the transition between the ground state and the lowest excited state as can be seen by the
intensity of the lowest energy absorption which is also reflected by the TD-UDFT calculations
(Table 1); here the TPA radicals have larger values of oscillator strengths and absorption
coefficients than TPB radicals. The value of knr was smallest for TPA(Me)2(PyBTM′′) and
second smallest for TPA(Me)(PyBTM′′)2. Largest knr values for TPB radicals and larger knr
values for cyano radicals indicate that complete charge transfer from TPA to PyBTM′′ promotes
the nonradiative decay pathway. TPA(OMe)2(PyBTM′′) also showed large knr value, probably
because of the lowest energy of the excited CT state (small D0-D1 gap).
Two-photon absorption (2PA)50-52 induced fluorescence in the near infrared is a highly sought
after property for biomarkers, bioimaging53-56 and micro structuring57-60. Linear and branched
acceptor-donor-acceptor type chromophores appear to be suitable design concepts.
Therefore, we measured the 2PA cross section of TPA(PyBTM′′)3, TPA(OMe)(PyBTM′′)2, and
TPA(OMe)2(PyBTM′′), see Fig. 6 (and Fig. S11), in comparison to the 1PA cross section
derived from the absorption spectra. While for the monoradical the 2PA cross section follows
roughly the lowest energy absorption between 12000-17000 cm-1 and the peak at ca. 19000
cm-1, the 2PA cross section of the diradical and even more that of the triradical are significantly
enhanced, reaching quite impressive 1000-3000 GM, particularly at around 19000 cm-1 and
15500 cm-1, which contrasts the behavior of recently investigated tris-
(tetrachlorophenyl)methyl radicals.20 The latter signal shows that the lowest energy absorption
band between 12000-17000 cm-1 is indeed composed of at least two or three electronic
transitions. For example, assuming ideal D3 symmetry in TPA(PyBTM′′)3, irrespective of the
spin multiplicity, the two lowest energy excited states are (almost) degenerate and 1PA is
allowed, but the third state at slightly higher energy is 1PA forbidden (see Table 1). In 2PA
spectroscopy this is reversed. In reality, the selection rules are not that strict because of
molecular distortion and vibrational coupling to asymmetric modes.61, 62 The enhancement of
2PA cross section of the triradical and the diradical vs. the monoradical cannot be explained
by the simple additivity of individual chromophore moieties in the three radicals but must be
caused by interactions between the three (two) donor-acceptor branches.
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2PA n / cm-1
6000 7000 8000 9000 10000 11000 12000 13000
4.0x10-16
3000
TPA(PyBTM'')3
3.5x10-16
3.0x10-16
2.5x10-16
2.0x10-16
1.5x10-16
1.0x10-16
5.0x10-17
0.0
TPA(OMe)(PyBTM'')2
TPA(OMe)2(PyBTM'')
2500
2000
1500
1000
500
0
12000 14000 16000 18000 20000 22000 24000 26000
1PA n / cm-1
Figure 6. 1PA (solid lines) and 2PA (dashed lines) cross sections of selected radicals in
cyclohexane solution.
Because of the good fluorescence properties we selected TPA(OMe)(PyBTM′′)2 diradical as
dopant for an OLED test device. This diradical is luminescent as guest in spin-coated PMMA
or p-terphenyl thin film matrices despite the slight polarity of the hosts (see Fig. S9). When
photo-exciting an OLED (ITO/PEDOT:PSS/PDY-132+TPA(OMe)(PyBTM’’)2/Ca/Al) at 532 nm
the photoluminescence of PDY-132 (Super-Yellow from Merck) is strongly quenched by the
TPA(OMe)(PyBTM′′)2 dopant indicating an highly efficient Förster resonance energy transfer
(Fig. S10). Upon electrical excitation of the OLEDs, luminescence of TPA(OMe)(PyBTM′′)2
was clearly observed for voltages above 8 V (Fig. 7), demonstrating that, as a proof-of-concept,
NIR OLEDs can be built from diradicals. As Ai and co-workers pointed out,16 this might also be
advantageous concerning the spin statistics of exciton formation in OLEDs but in-depth
investigation in this direction requires knowledge on the efficiency of the contributing
populations and emission channels which is beyond the scope of this work.
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10.1002/chem.201903007
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Figure 7.
Current density and electroluminescence vs. applied voltage for an
ITO/PEDOT:PSS/PDY-132+TPA(OMe)(PyBTM’’)2/Ca/Al
OLED
device.
Inset:
electroluminescence spectrum recorded at 14V.
Conclusion
In conclusion, mono-, di-, tri-radicals with significant fluorescence ranging from the red visible
to NIR region were synthesized. The lowest excited states are charge transfer states for both
the TPA and the TPB radicals. While the HS and LS spin states are almost degenerate for the
di- and triradicals, their actual spin multiplicity has almost no influence on the photophysics
thereby rendering weakly coupled diradicals useful emitting species for two-photon
applications and particularly for NIR OLED devices where spin statistics need to be considered.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft (GRK 2112) and by a
JSPS Overseas Research Fellowships to Y. H.
Keywords: radical • two-photon absorption • fluorescence • DFT calculation • NIR OLED
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In combination with steady state and time resolved fluorescence spectroscopy we
were able to characterise the fluorescence of chlorinated triphenylmethyl diradicals
both from the high spin state and from the low spin state. This is rather unexpected
and opens the way to use diradicals as emitters in molecular devices.
20
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