Controlling through-space and through-bond intramolecular charge transfer in bridged D-D′-A TADF emitters

Article abstract of DOI:10.1039/d1tc02316k

Donor-donor′-acceptor molecules where the donor′ bridges the donor and acceptor have different possible interaction pathways for charge transfer. Here we study a series of donor-donor′-acceptor molecules, having the same acceptor and donor′ but different donors, and donor′-phenyl spacer-acceptor to change the spatial separation and overlap between potential through-space donor-acceptor charge transfer (CT) in competition with donor′-acceptor through-bond CT. We determine that the charge transfer driving force plays a large role in dictating which charge transfer channel is favoured. Strong donors and acceptors with large driving force favour through-space CT. We also find that solid state host packing plays an important role, with small molecule hosts that pack tightly, distorting the guest molecules, reducing D-A separation to stabilise the through-space CT state over the through-bond state. Only the through-space CT states give fast reverse intersystem crossing and efficient TADF. These results give the first insight into the photophysics of through-space CT compared to through-bond states on the same molecule.

Full text of DOI:10.1039/d1tc02316k

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Materials Chemistry C
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Controlling through-space and through-bond
intramolecular charge transfer in bridged D–D0–A
TADF emitters†
Cite this: J. Mater. Chem. C, 2021,
9, 8819
Received 19th May 2021,
Accepted 24th June 2021
a
c
Hector Miranda-Salinas,
Yi-Tzu Hung,b Yi-Sheng Chen,b Dian Luo,
Hao-Che Kao,c Chih-Hao Chang, *c Ken-Tsung Wong
*
and
bd
a
DOI: 10.1039/d1tc02316k
Andrew Monkman
*
rsc.li/materials-c
Donor–donor0–acceptor molecules where the donor0 bridges the between the triplet (3CT) and the singlet (1CT) charge-transfer
donor and acceptor have different possible interaction pathways states, mediated by vibrionic coupling between 3CT and a local
for charge transfer. Here we study a series of donor–donor0– excited triplet state (3LE) that enables spin orbit coupling
acceptor molecules, having the same acceptor and donor0 but (SOC), when the energy gap between all three states becomes
different donors, and donor0–phenyl spacer–acceptor to change small, o100 meV. Mediation by the third state is required
3
1
the spatial separation and overlap between potential through- when CT and CT are so close in energy because direct spin
space donor–acceptor charge transfer (CT) in competition with orbit coupling (SOC) between them is forbidden because they
donor0–acceptor through-bond CT. We determine that the charge have effectively the same orbital and no change in orbital
transfer driving force plays a large role in dictating which charge angular momentum occurs during the direct transition
1
transfer channel is favoured. Strong donors and acceptors with 3CT - CT.2,3 One of the simplest ways to achieve extremely
large driving force favour through-space CT. We also find that solid small electron correlation energy, i.e. singlet–triplet splitting
state host packing plays an important role, with small molecule energy (DEST), is the use of conformational twisting between
hosts that pack tightly, distorting the guest molecules, reducing directly bridged donor (D) and acceptor (A) units in order to
D–A separation to stabilise the through-space CT state over the minimize the overlap between the highest occupied molecular
through-bond state. Only the through-space CT states give fast orbital (HOMO) and the lowest unoccupied molecular orbital
reverse intersystem crossing and efficient TADF. These results give (LUMO).4,5 Beyond the subtle manipulations of the degree of
the first insight into the photophysics of through-space CT com- intramolecular through-bond charge transfer to control TADF
pared to through-bond states on the same molecule.
properties, recent interest in through-space charge transfer
between D and A, mediated by strong D A intramolecular p–p
dipolar interactions has been motivated by the potential of
increasing photoluminescence quantum yield.6,7 Such
intramolecular through-space charge transfer is much like an
exciplex state except that it offers far more control over the
spatial seperation and orientation of D and A compared to the
random orientation of D and A in the intermolecular exciplex
state. In a conjugated system, the D and A can be arranged co-
facially, in close spatial proximity, using a non-coplanar mole-
cular scaffold (bridge).8 This approach has also been used in
the design of non-conjugated polymers with spatially separated
pendant D and A.9,10 Despite the growing interest of intra-
molecular through-space charge transfer states as a means to
TADF, very little photophysical study of the excited state
dynamics and molecular conformations of such systems has
been made.11 Here we investigate the excited state dynamics of
new molecules configured with asymmetrical donor–donor0–
acceptor (D–D0–A) architectures, where the co-facial overlap
between D and A is controlled by the introduction of a common
Introduction
Organic molecules showing thermally-activated delayed
fluorescence (TADF) are considered as the third generation of
materials for organic light-emitting diodes (OLEDs).1 Unlike
phosphorescent emitters, TADF molecules harvest triplet exci-
tons by a reverse intersystem crossing (rISC) mechanism
a Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
E-mail: a.p.monkman@durham.ac.uk
b Department of Chemistry, National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei, 10617, Taiwan. E-mail: kenwong@ntu.edu.tw
c Department of Electrical Engineering, Yuan Ze University, Taoyuan 32003,
Taiwan. E-mail: chc@saturn.yzu.edu.tw
d Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan
† Electronic supplementary information (ESI) available. CCDC 1499013 and
2043941–2043943. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d1tc02316k
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Fig. 1 The chemical structures (a) and X-ray structures (b) of Ph3TRZCzTPA, Ph2TRZCzTPA, Ph2TRZCzPhCz and Ph3TRZCzPhCz, thermal ellipsoids
were drawn at a 50% probability level.
highly twisted from the carbazole bridge (D0) due to congested
Table
1
The dihedral angles and distances of Ph2TRZCzTPA,
steric interactions. All dihedral angles between the D, A and
bridging D0, selected closest atom-to-atom distances between
the donor and the acceptor branches (3.09–3.37 Å), and dis-
tances between the donor N-atom and the centre of triazine
acceptor ring (4.93–5.30 Å) are summarized in Table 1.
Ph3TRZCzTPA, Ph2TRZCzPhCz, and Ph3TRZCzPhCz
j1 (1) j2 (1) j3 (1) j4 (1) j5 (1) d1 (Å) d2 (Å) d3 (Å) d4 (Å)
Ph2TRZCzTPA 49.3 55.2
Ph2TRZCzPhCz 36.5 47.1
Ph3TRZCzTPA
3.093 4.985
3.373 4.926
75.1 70.9 0.6
65.4 57.8 2.5
3.150 5.295
3.201 4.924
In general, Ph3TRZCzTPA and Ph2TRZCzTPA (triphenyl-
amine donor) show larger donor-bridge dihedral angles as
compared to those of the counterpart Ph2TRZCzPhCz and
Ph3TRZCzPhCz (PhCBZ donor). In addition, Ph3TRZCzTPA
and Ph2TRZCzTPA show shorter closest atom-to-atom
distances, but slightly longer distances between the donor
N-atom and the centre of triazine ring. These distorted con-
formations and short distances imply that a through-space
interaction between the donor and acceptor branches due to
the stronger electron-donating ability of TPA may stabilize the
twisted molecular structures. All of the molecules show excel-
lent thermal stability required for stable film and device
formation, and calculated HOMO/LUMO energy levels obtained
from cyclic voltammetry (Fig. S1, ESI†) are given in Table 2.
Ph3TRZCzTPA and Ph2TRZCzTPA show higher HOMO energy
levels (ꢀ5.28/ꢀ5.27 V) as compared to Ph2TRZCzPhCz and
Ph3TRZCzPhCz (ꢀ5.85/ꢀ5.57 V) in accord with donor strength.
Since the acceptor of all compounds is the same, the LUMO
energy levels are similar (ca. ꢀ2.75 V) in all cases.
Ph3TRZCzPhCz
weak (rigid) carbazole donor bridge (D0) and by the introduc-
tion of phenyl spacer units between the acceptor and bridge,
shown in Fig. 1. A common diphenyltriazine (dTRZ) acceptor is
used and either a weak phenyl carbazole (PhCBZ) or strong
triphenylamine (TPA) donor.
Results
DFT calculations
Synthetic methods and characterization of these new D–D0–A
molecules and model compounds used in this study are The ground state structures were first optimized at a B3LYP/
reported in ESI† (Scheme S1–S3). Fig. 1b shows their single 6-311G(d) level. The calculated HOMO and LUMO distributions
crystal structures and crystallographic data is summarized in as well as energy levels are shown in Fig. S2 (ESI†). The HOMOs
Table S1 (ESI†). As shown, the pendant (spacer) aryl rings are of Ph3TRZCzTPA and Ph2TRZCzTPA are mainly located at the
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Table 2 The electrochemical properties and thermal properties of Ph2TRZCzTPA, Ph3TRZCzTPA, Ph2TRZCzPhCz, and Ph3TRZCzPhCz
oxi
onset
a
red b
1/2
d
e
f
f
E
(V)
E
(V)
HOMOc (eV)
LUMOc (eV)
Eg (eV)
Td (1C)
Tg (1C)
Tm (1C)
Ph2TRZCzTPA
Ph3TRZCzTPA
Ph2TRZCzPhCz
Ph3TRZCzPhCz
0.98b
0.97b
1.21
ꢀ1.45
ꢀ1.53
ꢀ1.47
ꢀ1.50
ꢀ5.28
ꢀ5.27
ꢀ5.85
ꢀ5.57
ꢀ2.73
ꢀ2.74
ꢀ2.79
ꢀ2.77
2.55
2.53
3.06
2.80
324
389
340
419
111
111
127
129
212
279
n.d.g
338
1.23
a
b
Calculated from the onset potential. E = (Ep,a + Ep,c)/2, where Ep,a and Ep,c stand for the peak potential at which anodic- and cathodic-direction
c
d
scan, respectively. HOMO and LUMO were determined from the electrochemical results in CH2Cl2 fand DMF solution, respectively. Eg = LUMO–
e
HOMO using electrochemical results. Td was analyzed using TGA (5% weight loss). Tg and Tm were determined by DSC.
g
Not detected.
TPA moiety and slightly extend to the carbazole bridge. The 3LE state is the lowest triplet state. Notably, a distinguishable
HOMOs of Ph2TRZCzPhCz and Ph3TRZCzPhCz are mostly change between the S1 and T1 orbitals could be observed for
positioned at the pendant PhCBZ donor with limited contribu- Ph3TRZCzTPA and Ph2TRZCzTPA, which is a prerequisite for an
tion from the carbazole bridge. Not surprisingly, the LUMOs of allowed SOC spin–flip transition. Very interestingly, the differ-
Ph2TRZCzTPA and Ph2TRZCzPhCz are located at the dTRZ, ent orbital distributions of S1 and T1 states could also be
which are well separated from their HOMOs. Whereas, the observed in the case of Ph2TRZCzPhCz. The HONTO of the S1
LUMOs of Ph3TRZCzTPA and Ph3TRZCzPhCz are located at state distributes on the PhCBZ donor and the carbazole bridge,
the TRZ together with a small contribution on the carbazole while the LUNTO locates on the dTRZ. On the other hand, the
bridge, giving weak but non zero HOMO–LUMO overlap. HONTO of the T1 state of Ph2TRZCzPhCz delocalises mainly on
Obviously, the stronger donor, TPA, leads Ph3TRZCzTPA and the dTRZ group and slightly on the phenylene of the pendant
Ph2TRZCzTPA to have higher HOMO levels as compared to PhCBZ donor, whereas the LUNTO only locates on the dTRZ
those of Ph2TRZCzPhCz and Ph3TRZCzPhCz. Whereas group. Thus, Ph2TRZCzPhCz is anticipated to possess both
Ph2TRZCzTPA and Ph2TRZCzPhCz without the aryl spacer through-bond and through-space CT character due to the slight
between acceptor and bridge show slightly lower LUMOs. The overlap of HONTO and LUNTO. For Ph3TRZCzPhCz, the
calculated results are consistent with the observed CV data.
HONTO of the S1 state distributes on the pendant PhCBZ,
TD-DFT calculations were then made using a hybrid whereas the LUNTO distributes on the TRZ group, leading to
meta-generalized gradient-approximation functional, m06-2x/ a through-space CT transition character. The HONTO and
6-311(d), to optimize the S1 and T1 structure of these four LUNTO distributions of the T1 state are similarly delocalised
molecules and calculate the natural transition orbitals (NTOs). on the PhCRZ and slightly coupled with the carbazole bridge,
The HONTO and LUNTO distributions are shown in Fig. S3 resulting in an apparent LE transition character. Although
(ESI†). For the S1 state of Ph3TRZCzTPA and Ph2TRZCzTPA, the Ph3TRZCzPhCz shows considerable orbital deformation
HONTOs distribute on the TPA moiety, while the LUNTOs between S1 and T1 states, the larger DEST undermines the
delocalises at the dTRZ and TRZ, respectively. Since there is probability of reverse intersystem crossing process.
nearly no overlap between the HONTO and LUNTO, a through-
Steady state photophysics
space charge transfer character for the S1 state is suggested. For
the T1 state configuration, the HONTO of the Ph3TRZCzTPA is
delocalised on the TPA and TRZ, exhibiting both CT and LE
transition character. On the other hand, the HONTO of the
Ph2TRZCzTPA only distributes on the dTRZ, indicating the
Steady state absorption and emission of each compound was
measured in methylcyclohexane (MCH) are shown in Fig. 2a
and b, and in a range of different polarity solvents and solid
state hosts in Fig. 3, for direct comparison.
Fig. 2 Extinction coefficients (a) and emission spectra (b) of the molecules in MCH solution (20 mM Lꢀ1).
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Fig. 3 Emission spectra of the compounds (a) Ph3TRZCzTPA, (b) Ph2TRZCzTPA, (c) Ph2TRZCzPhCz and (d) Ph3TRZCzPhCz in film and different
solvents, methylcyclohexane (MCH), chlorobenzene (CB) and acetonitryl (MeCN).
The lowest energy absorption band, peaking at ca. 375 nm in Ph2TRZCzTPA, and Ph3TRZCzPhCz all show strong solvatochro-
Ph3TRZCzPhCz and Ph3TRZCzTPA is absent in Ph2TRZCzTPA mism, giving a positive Stokes shift indicative of CT emission.
and Ph2TRZCzPhCz. In Ph3TRZCzPhCz this band correlates Emission of Ph3TRZCzTPA and Ph2TRZCzTPA in MeCN is highly
very well to the lowest energy absorption band in the red shifted and extremely weak indicating very low emission yields.
model compound, 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-
In general, the molecules with a Cz donor emit at higher
9N-carbazole, CzPhTrz, as observed by Sharma et al.,12 having energy than those having a TPA donor, in line with donor
strong pp* character, and indicates a more delocalised acceptor strength. This also dictates the driving force for charge transfer
unit, i.e. the phenyl spacer between A and bridge forms part of in each pair. The CT driving force may be estimated from the
the acceptor unit itself in both cases, as suggested by our following equation;
TD-DFT calculations. The magnitude of the extinction coeffi-
DG0 = EAA ꢀ EAD ꢀ DEb
cients of these lowest energy bands is also 42 ꢁ 104 cmꢀ1 Mꢀ1
indicating that they are not direct CT absorption bands as Where EAx are the electron affinity of A and D fragment
observed in typical D–A–D through bond TADF molecules.4
respectively, and DEb is the difference in binding energy
Emission spectra measured for each compound in MCH, Fig. 2b, between the initially created exciton and the relaxed charge
show energy on-sets at; 2.89 eV, 2.79 eV, 3.59 eV and 3.32 eV for separated state, see Scheme S3 (ESI†). In the case of these rigid
Ph3TRZCzTPA, Ph2TRZCzTPA, Ph2TRZCzPhCz and Ph3TRZCzPhCz, orthogonal intramolecular D A molecules we assume that this
respectively. Ph2TRZCzPhCz gives dual emission in MCH from a difference in binding energy to be very small in non-polar
local donor at 3.59 eV and a charge transfer (CT) state at lower media. Taking the measured (electrochemical) ionization
energy, in excellent agreement with the TD-DFT calculations. potentials for the two families as that of the D units, the EA
Ph2TRZCzTPA emission shows some mixed local/CT character, of the D units where estimated by adding the D optical excita-
indicated by the weak structure on the emission band. The stronger tion energy, yielding EACz = ꢀ2.21 eV and EATPA = ꢀ1.76 eV. The
donor strength of Ph3TRZCzTPA and Ph2TRZCzTPA is clearly man- measured EA of the A unit in all cases being ꢀ2.75 eV from the
ifest by the large Stokes shift observed in their CT emission spec- CVs. For Ph3TRZCzTPA and Ph2TRZCzTPA we then estimate a
tra. With increased solvent polarity, Fig. 3. Ph3TRZCzTPA, CT driving force of ca. 1 eV, and for Ph2TRZCzPhCz and
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Ph3TRZCzPhCz, ca. 0.5 eV. We thus find a near double CT CT state between the bridge Cz (D0) and dTRZ unit. The higher
driving force for Ph3TRZCzTPA and Ph2TRZCzTPA compared to intensity, narrow linewidth blue emission band with vibronic
Ph2TRZCzPhCz and Ph3TRZCzPhCz. It is interesting to note structure, also observed in Ph2TRZCzPhCz, onset ca. 3.54 eV,
that for these rigid D–A intramolecular systems, in non-polar showing little or no solvatochromic shift, must be emission
environment (minimising the Coulomb attraction energy) we from a local excited state. Considering the individual D and A
also find that the difference between the (optical) D exciton units in the molecule, PhTRZ is only very weakly emissive
energy and CT energy also gives very similar values, 0.8 eV for through excitation at 280 nm, emission onset at 3.44 eV,
Ph3TRZCzTPA and Ph2TRZCzTPA and 0.5 eV for Ph2TRZCzPhCz with slight vibronic structure.15 Emission in our model CzTrz
and Ph3TRZCzPhCz, yielding a potential very simple way to peaking at ca. 290 nm seems highly plausible to be from the
estimate the driving energy for photoexcited charge transfer in dTRZ unit. This is not what we observe in Ph2TRZCzPhCz,
highly decoupled intramolecular D–A molecules.
however, carbazole shows strong emission, onset at 3.54 eV17
The solution measurements for Ph3TRZCzTPA and and so we conclude that this local emission band in
Ph2TRZCzTPA are similar as can be seen in Fig. 3a and b, the Ph2TRZCzPhCz comes from the pendent Cz D unit, consistent
bands shift by the same amount in different polarity solvents. with the DFT calculations. This Cz must be strongly decoupled
Comparing the film spectra, we see that the zeonex films from the rest of the molecule as the emission is always present
behave just like the MCH solutions for these materials with even in high polarity solvent, ruling out a through-space CT
Ph3TRZCzTPA even showing a small blue shift compare to state. In highly polar MeCN, there is a small population of CT
1
1
MCH. Ph2TRZCzPhCz also shows the same LE and CT dual states in giving rise to the highly red shifted band which we
emission bands, but the CT state has increased relative inten- believe to be through-space CT states, Fig. 3c.
1
sity in zeonex. However, in DPEPO films, no LE component is
With this in mind, we turn to Ph2TRZCzPhCz in zeonex
observed from Ph2TRZCzPhCz, only the 1CT emission is (a branched polyolefin) and DPEPO (bis[2-(diphenylphosphino)
observed, at about the same peak wavelength as the corres- phenyl]ether oxide) host films. In both cases the CT band is
ponding Ph2TRZCzPhCz CT emission in toluene solution.
much stronger than in solution. In zeonex dual emission is still
Ph3TRZCzPhCz, Fig. 3d, is seen to have a very similar observed but the CT emission has equal intensity to the local
spectrum with weakly structured emission in MCH compared emission, whereas in tightly packed DPEPO host, only a strong,
to the model systems CzPhTrz12 and meta-CzTRZ reported by red shifted and very broad CT emission is observed. As there
Liu et al.13 Thus, we ascribe its excited state as a through-bond can be no ‘solid state solvatochromic effect’ in a rigid solid
CT state between the bridge Cz and TRZ D0–A pair. The CT state host,18,19 this must be due to a packing effect of the host. If the
shows both weak solvatochromism and in solid state the host forces the D and A units closer together, improving
emission has very similar band shape and width as in solution, co-facial overlap, a through-space CT state can be stabilized
mostly unaffected by host packing effects. Only in highly polar and the increased charge transfer rate would effectively quench
MeCN does the band become highly red shifted and very broad, donor emission from the pendent Cz unit. Thus, in DPEPO we
which may point to a different excited state origin such as a conclude that a through-space CT state is stabilized on
through-space CT state.
Ph2TRZCzPhCz, in-line with TD-DFT calculations that strongly
Ph2TRZCzPhCz on the other hand is obviously different, suggest both through-space and through-bond CT states are
being the only compound showing dual CT emission and very possible. We also note that this CT emission band in DPEPO is
strong local emission, consistent with the X-ray data that shows much broader and more red shifted than the CT emission
a shallow 36.51 dihedral angle between D0–A. Cz donor units observed from Ph3TRZCzPhCz in DPEPO. This we believe
tend to have shallow ground state D-A dihedral angles giving indicates
a very inhomogeneous process stabalising the
strong LE character.14 This is further emphasised by the through-space CT state, clearly in-line with random host pack-
localisation of the HOMO on the pendant Cz unit found in ing effects. The stable through-bond CT state in Ph3TRZCzPhCz
our DFT calculations. We therefore synthesised the model remains narrow in DPEPO, which potentially indicates that the
compound, 2-(carbazol-9-yl)-4,6-diphenyl-1,3,5-triazine (CzTrz), extra flexibility introduced by the spacer phenyl ring enables
Fig. S4 (ESI†). The absorption spectrum of CzTrz indicates closer co-facial separation of D and A stabalising the through-
electronically decoupled D and A units, absorbing above space CT state so that host packing has little extra effect on this
350 nm. Excitation into the A (TRZ) band yields both TRZ15 conformation. To confirm this hypothesis we measured
and weak CT emission, whereas excitation into the D (Cz) band Ph2TRZCzPhCz in both mCP (1,3-bis(N-carbazolyl)benzene)
yields strong CT emission, onset at 3.26 eV. The CT emission and CBP (4,40-bis(N-carbazolyl)-1,10-biphenyl) hosts, both
on-set shifts to 3.1 eV in toluene and dominates the emission having very low dielectric and polarizability values compared
spectra. This shift is 35 nm more to the red than for the model in DPEPO. In both cases clear CT emission is observed from
CzPhTrz in toluene, which we ascribe to a smaller electron hole Ph2TRZCzPhCz, Fig. S5 (ESI†), confirming that host packing
separation in CzTrz therefore larger Coulomb attraction energy, plays a major role in stabilising through-space CT in these
which is relaxed more strongly by the increasing solvent materials.
polarity red shifting the energy of the CT state to a greater
To understand Ph3TRZCzTPA and Ph2TRZCzTPA we recall
extent.16 This clearly corresponds with the CT state seen in an important observation from our previous work. A CT
Ph2TRZCzPhCz which can then be identified as a through-bond complex can form between TRZ and TPA in solution.11 The
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Fig. 4 Time-resolved spectrally integrated intensity decay (left) and representative time resolved spectra (right) of Ph3TRZCzTPA (a) 300 K and (b) 80 K;
Ph2TRZCzTPA (c) 300 K and, (d) 80 K in zeonex matrix. Spectral times given in ns, and decay fitting data included in the decay plots.
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resulting TRZ:TPA excited state emits at 492 nm (peak) in intensity shows clear thermal activation of the DF mechanism.
toluene (with solvatochromism being observed, Fig. S6, ESI†). Phosphorescence at 80 K, Fig. 4b, also has a much lower onset
This is at the same energy with the same solvatochromic of 2.76 eV. This large difference in CT state energy between RT
behaviour as the band observed in both Ph3TRZCzTPA and and 80 K implies that thermal motion on average leads to a
Ph2TRZCzTPA. We therefore identify the CT emission in both higher energy CT state in Ph3TRZCzTPA. As this behaviour is
as coming from a through-space TRZꢂ ꢂ ꢂTPA (strong D – strong not observed in Ph2TRZCzTPA we conclude that it must be an
A) CT excited state. No contribution from a through-bond, effect due to the presence of the spacer phenyl ring between
strong A – weak D0 CT pair is observed. Concomitantly, emis- A and bridge. One explanation for this is that the thermal
sion is seen well to the red of either Ph2TRZCzPhCz and energy drives the spacer phenyl ring more planar (conjugated).
Ph3TRZCzPhCz CT emission in line with a strong-D strong-A So at 80 K the phenyl ring has a more orthogonal configuration
pair CT state. We note from X-ray measurements that this with respect to the A and bridge, changing the coupling of the
behaviour does not correlated simply to D–A co-facial separa- phenyl ring to the dTRZ and localising the LUNTO on the dTRZ
tion, indicating the role of CT driving force. We also previously unit analogous with Ph2TRZCzTPA as shown in our TDDFT
found that the intermolecular TRZꢂ ꢂ ꢂTPA state has a ground calculations. This will also greatly affect the localisation of the
state interaction, i.e. a heterodimer complex,11 and we postu- lowest energy triplet state as well.
1
late that such a ground state interaction between D and A units
Ph2TRZCzTPA, Fig. 4c and d, shows constant CT emission
may also help to drive and stabilise the through-space CT state behaviour, onset at 2.82 eV both at RT and 80 K with a
over a through-bond interaction. Such ground state absorption monotonic blue shift of 50 meV over the first 40 ms, Fig. S7
is also seen in other intramolecular through-space CT and S8 (ESI†). Phosphorescence, onset at 2.80 eV is seen at
systems.20
millisecond times. At RT, we see faster prompt decay, earlier
time DF and faster DF decay compared to Ph3TRZCzTPA. As the
DEST are less than 30 meV (at RT) in both cases, this could
Time-resolved spectroscopy
Starting with the compounds in zeonex host matrix. Ph3TRZCzTPA indicate stronger magnetic coupling between D and A in
at room temperature, Fig. 4a, shows initial CT like emission Ph2TRZCzTPA, so more efficient SOC, given by the different
(Gaussian band shape) in the first few ns, onset at 3.01 eV which spatial overlap and orientation of D and A units in the two
relaxes in energy over 100 ns to give strong prompt (PF) CT compounds, again an effect that can be related to the spacer
emission at 2.88 eV and delayed fluorescence (DF) at 2.87 eV phenyl unit. As seen in the DFT calculations the D–A separation in
(on-sets). The DF then undergoes a monotonic blue shift of some Ph2TRZCzTPA (4.985 Å) is smaller compared to Ph3TRZCzTPA
50 meV over the first 40 ms, Fig. S7 (ESI†). Phosphorescence, onset (5.295 Å) fully supporting this argument. Previously, Wada et al.20
at 2.90 eV is seen at millisecond times. Fitting the emission decay have proposed that the D A co-facial angle is critically in determin-
indicates a fast 11 ns and a slow 77 ns prompt CT decays along with ing the SOC and rISC rate in a similar through-space CT system.
a 1 ms and 13 ms DF components. All parameters are given in
Ph2TRZCzPhCz at RT in zeonex, Fig. 5a, has a PF energy of
Table 3. Here we note that the DF decays are highly non exponen- 3.54 eV that decays within a few nanoseconds as expected for D
tial at longer times, turning into a power law decay. This is very emission. A simultaneous CT emission band (on our 1 ns
typical of TADF materials in the solid state. Thus, here we fit the timescale), onset at 3.10 eV, is also observed that decays very
early time DF with one or two exponential terms to model the rapidly, within 100 ns, shown in detail in Fig. S18 (ESI†). At
majority of the DF emission and calculate rISC rates from a kinetic 100 ns we also see a weak transient CT emission band, onset at
model as detailed previously.21 The possible interpretation of the 2.95 eV, which also decays rapidly. This is similar to what we
power law DF region will be discussed later.
observe in DPEPO (see later) and may indicate a small popula-
At 80 K, Fig. 4b and Fig. S8 (ESI†), we see similar behaviour tion of weakly stabilised through-space CT states in
but the ns region prompt emission energy relaxation is slower. Ph2TRZCzPhCz, as predicted by our calculations, and that are
However, the emission band has a constant additional 60 meV stabilised in high polarity MeCN solution, Fig. 3c. This band is
red shift and exactly matches the band shape and energy better resolved at 80 K, Fig. 5b, with all prompt emission
position (in time) of the emission observed in Ph2TRZCzTPA decayed by 200 ns. From 200 ns to 10 ms we observe no emission
(both at RT and 80 K, Fig. 4c and d). The large decrease in DF (above the noise floor of the iCCD), Fig. S7 (ESI†). At
Table 3 Time resolved data of zeonex film measurement at RT
1LEa (eV)
tLE (ns)
1CTa (eV)
tPF1 (ns)
tPF2 (ns)
tDF1 (ms)
tDF2 (ms)
1CT0 (eV)
3LEc (eV)
DEST (meV)
b
b
b
b
a
d
Molecule
Ph3TRZCzTPA
Ph2TRZCzTPA
Ph2TRZCzPhCz
Ph3TRZCzPhCz
3.01
—
3.54
—
B1
—
3
2.88
2.82
3.10
3.35
11
15
3
77
54
9
7.6
5
342
—
54
74
4900
—
—
—
2.92
—
2.90
2.80
2.76
2.88
20
20
340
470
—
9
—
a
b
Local (LE) and charge transfer (CT) emission energies calculated from the on-set of the emission band. Prompt (tLE and tPF) and delayed
c
fluorescence (tDF) component lifetimes calculated from fits to the measured intensity decay curves. Local triplet energy taken from the on-set of
the phosphorescence measured at 80 K. DEST is the energy difference between the 1CT and 3LE energies.
d
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Fig. 5 Time-resolved spectrally integrated intensity decay (left) and representative time resolved spectra (right) of (a) Ph2TRZCzPhCz, (a) 300 K and
(b) 80 K; Ph3TRZCzPhCz, (c) 300 K and, (d) 80 K in zeonex matrix. Spectral times given in ns, and decay fitting data included in the decay plots.
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Fig. 6 Time-resolved spectrally integrated intensity decay (left) and representative time resolved spectra (right) of Ph3TRZCzTPA (a) 300 K and (b) 80 K;
Ph2TRZCzTPA (c) 300 K and (d) 80 K in DPEPO host. Spectral times given in ns, and decay fitting data included in the decay plots.
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millisecond times, we observe an emission with a new band observed, Fig. S11 (ESI†). At 40 ms we again see phosphores-
shape and onset energy of 2.92 eV, which we assume to be cence at about 2.73 eV but the signal is very weak here. In both
phosphorescence. At 80 K the prompt spectra show minor Ph3TRZCzTPA and Ph2TRZCzTPA the 3LE energy from low
shifts in energy of ca. 10 meV only. However, a much larger temperature zeonex measurements at 2.76 and 2.80 eV respec-
shift is seen in the phosphorescence which red shifts to an tively, is very consistent with the phosphorescence seen in
onset of 2.76 eV at 80 K. This is a large change in triplet energy, DPEPO. This would then set DEST at around 150–200 meV in
and looking critically at the band shape it is different. Careful DPEPO, consistent with the observed thermally activated
inspection of the 40 ms emission at room temperature (com- delayed emission.24 The calculated rISC rates of Ph3TRZCzTPA
pared to that at 4 ms) shows a shoulder has grown in on the and Ph2TRZCzTPA in DPEPO films where calculated using a
red side of the band which we believe is a contribution from the kinetic model. At RT, we estimated rates of 6.3 ꢁ 104 sꢀ1 and
low energy triplet species observed at 80 K. We have observed 1 ꢁ 105 sꢀ1 respectively, and at 80 K, 1.4 ꢁ 104 sꢀ1 and
such dual phosphorescence before in D–A–D systems and 1 ꢁ 104 sꢀ1 respectively, Fig. S12–S15 (ESI†).
this implies two coupled local triplet states in thermal equili-
For Ph2TRZCzPhCz in DPEPO at RT, Fig. 7a, emission is
brium co-existing on the molecule.22 As the lowest energy observed at 3.1 eV (onset) having a bi-exponential decay of
triplet state is at 2.76 eV, this yields a true DEST of 340 meV 5.8 ns and 32 ns. From 500 ns to 35 ms very weak emission is
and so it is not surprising that we observe little or no delayed seen with an onset of 2.88 eV, Fig. S10 (ESI†), and at 4 ms there
emission.
may be phosphorescence with onset at 2.92 eV but the signal is
Ph3TRZCzPhCz at RT and 80 K, Fig. 5c and d, behaves like a very weak. At 80 K, Fig. 7b, again we observe prompt emission
simple fluorescence emitter, having only a single emission at 3.1 eV and then very weak emission from 500 ns to 35 ms at
band, onset 3.35 eV with mono exponential lifetime of 9 ns. 2.88 eV, Fig. S11 (ESI†). At 40 ms emission is still seen at 2.83 eV
Over its lifetime, the blue edge of this band decays faster than which we ascribe to phosphorescence. The 3.1 eV CT band is
the red edge, a typical red edge effect. No DF is observed from red shifted by over 300 meV compared with that in zeonex and
this band at either temperature. At 80 K phosphorescence is so we believe this shows that in DPEPO the through-space CT
observed in the 1–10 ms range, having an extremely well state is stabilised by the packing effect of the DPEPO forcing the
resolved spectrum, onset at 2.88 eV which is a clear match to D and A units closer together. In order to confirm this hypoth-
a carbazole phosphorescence spectrum.23 Further, by 40 ms the esis, we measured Ph2TRZCzPhCz decays in both mCP and CBP
band shape has evolved, losing much of its structure, onset at hosts as well. In both cases, clear 3.1 eV CT emission is
2.91 eV. The spectral band shape can be deconvolved into a observed from Ph2TRZCzPhCz with long lifetime confirming
linear combination of the carbazole phosphorescence and the DF, Fig. S5 (ESI†). These observations confirm that host pack-
phosphorescence band we observe in Ph2TRZCzPhCz, Fig. S9 ing plays a major role in the underlying physics of the CT states
(ESI†), indicating dual phosphorescence in Ph3TRZCzPhCz as in Ph2TRZCzPhCz. All delayed emission in Ph2TRZCzPhCz is
well. This yields a DEST of ca. 470 meV.
rather weak, and if we assume the same triplet energy as found
Measurements were also made at 10% by weight loading in in zeonex then DEST will be of order 250 meV implying
DPEPO host, which has higher dielectric coefficient and polar- inefficient rISC in line with the very long lived weak DF. The
izability than zeonex, also the DPEPO matrix greatly restricts CT band is also broaden which we assume implies a high
the movement of the molecules because of tighter packing than degree of disorder within the host DPEPO, again consistent
the zeonex polymer matrix. Concomitantly, all CT emission is with an inhomogeneous, packing induced through-space
seen to be red shifted. In the case of Ph3TRZCzTPA the prompt CT state. Ph3TRZCzPhCz also shows similar behaviour to
emission at RT has onset at 3.0 eV, Fig. 6a. By 80 ns the Ph2TRZCzPhCz, Fig. 7c and d. At RT we observe bi-
emission has relaxed to 2.85 eV and from 500 ns until 4 ms exponential decay with lifetimes of 6 ns and 20 ns, emission
we see DF at 2.89 eV, Fig. S10 (ESI†). After 12 ms until 4 ms weak onset at 3.1 eV. From 100 ns to 4 ms this emission shifts to 2.91
emission is observed with onset at 2.85 eV. At 80 K, Fig. 6b, eV, Fig. S10 (ESI†). Again the CT band is red shifted compared
bi-exponential prompt emission, with lifetimes of 7 ns and to zeonex pointing to DPEPO stabilising the through-space CT
169 ns is observed, onset at 3.0 eV with a very weak blue knee excited state and giving weak DF. At 80 K, prompt emission
ascribed to rapid D emission along with the longer lived starts at 3.26 eV and shifts to 3.1 eV by 80 ns. The decay is bi-
prompt CT emission. The prompt and delayed CT emission exponential, 6.3 ns and 31 ns, in line with a D emission as well
behave similarly to room temperature but the DF intensity is as prompt through-space CT emission. There is little DF at
lower, again showing thermal activation, Fig. S11 (ESI†). From 80 K, Fig. S11 (ESI†), in line with inefficient TADF, but at 40 ms
120 ms to 40 ms emission is seen with slight structure and onset clear phosphorescence emission with some structure is seen,
at 2.75 eV consistent with phosphorescence from the lowest onset at 2.95 eV in line with that observed in zeonex. All
triplet state. Ph2TRZCzTPA at RT shows dual PF, onset at 3.26 lifetimes are summarised in Table 4.
eV with lifetimes of 8.3 ns and 48 ns, Fig. 6c. From 80 ns until
4 ms DF is observed at 2.64 eV, Fig. S10 (ESI†). From 12 ms until
Device results
4 ms very weak emission onset ca. 2.79 eV is seen, ascribed Ph3TRZCzTPA and Ph2TRZCzTPA were selected as the
to phosphorescence. At 80 K, Fig. 6d, the PF is again bi- emitters to investigate electroluminescent (EL) characteristics.
exponential and the same delayed emission behaviour is Four commonly used high-triplet energy hosts including;
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Fig. 7 Time-resolved spectrally integrated intensity decay (left) and representative time resolved spectra (right) of Ph2TRZCzPhCz, (a) 300 K and
(b) 80 K; Ph3TRZCzPhCz, (c) 300 K and (d) 80 K in DPEPO host. Spectral times given in ns, and decay fitting data included in the decay plots.
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Table 4 Time resolved data of DPEPO film measurement at RT
1LEa (eV)
tLE (ns)
1CTa (eV)
tPF1 (ns)
tPF2 (ns)
tDF1 (ms)
tDF2 (ms)
3LEc (eV)
DEST (meV)
b
b
b
b
d
Molecule
Ph3TRZCzTPA
Ph2TRZCzTPA
Ph2TRZCzPhCz
Ph3TRZCzPhCz
3.00
2.95
3.10
3.10
B1
B1
B1
B1
2.85
2.67
2.88
2.91
31
8.3
5.8
6
161
48
32
4
2.7
2
28
21
59
51
2.76
2.80
2.76
2.95
90
ꢀ130
B250
ꢀ40
20
4
a
b
Local (LE) and charge transfer (CT) emission energies calculated from the on-set of the emission band. Prompt (tPF) and delayed fluorescence
c
(tDF) component lifetimes calculated from fits to the measured intensity decay curves. Local triplet energy taken from the on-set of the
phosphorescence measured at 80 K. DEST is the energy difference between the 1CT and 3LE energies.
d
Fig. 8 (a) Structural drawings of the materials used in OLEDs; (b) schematic structures of the fabricated OLEDs with different emitters.
4,40-bis(N-carbazolyl)-1,10-biphenyl (CBP) (ET = 2.56 eV), 2,6-bis peak EQE of 16.3%, 49.7 cd Aꢀ1, and 59.3 lm Wꢀ1 found at
(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) (ET = 2.71 eV), 12 wt% Ph2TRZCzTPA.
1,3-bis(carbazol-9-yl)benzene (mCP) (ET = 2.9 eV), and 9-(4-tert-
26DCzPPy was found to be the best host for Ph3TRZCzTPA.
butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) (ET = 3.0 eV) In addition, MoO3-doped TAPC was adopted as a hole injection
were tried. To match the HOMO and the LUMO levels of the layer to decrease the energy barrier between the ITO anode and
emitting layer, di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) the organic layer, Fig. 8. Fig. S17 (ESI†) depicts the EL char-
and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) were selected as acteristics while the corresponding parameters are summarized
the hole and electron transport layers respectively, see Fig. 8. Devices in Table S5 (ESI†). A weak emission from 26DCzPPy was
using mCP as the host gave the best performance in line with the observed in the EL spectra at doping concentrations of
photophysics, including relatively low operation voltages, higher Ph3TRZCzTPA below 2 wt%, indicating the incomplete host–
maximum luminance, and improved efficiency. For doping concen- guest energy transfer. As the doping concentration increased
tration (2–20 wt%) of Ph2TRZCzTPA in the mCP further fine-tuning from 2 wt% to 24 wt%, the devices showed the EL spectra red-
gave optimal performance, Fig. S16 (ESI†) depicts the EL char- shifted from 518 nm to 530 nm. The current density of the
acteristics and parameters are summarized in Table S4 (ESI†). From devices increases with the doping concentration, indicating the
the EL spectra, no emission from mCP and/or carrier transport favourable carrier transport capability of Ph3TRZCzTPA. Given
materials were observed when the doping concentration was greater the inferior carrier mobility of 26DCzPPy; 2 wt% Ph3TRZCzTPA
than or equal to 2 wt%, implying effective charge transfer and the turn-on voltage was 5.0 V but continuously decreased to
exciton confinement on Ph2TRZCzTPA. The EL spectra of the 4.4 V with a higher doping concentration, to 24 wt%. The
devices were red-shifted from 505 nm to 529 nm as the doping optimized concentration was found at 20 wt% Ph3TRZCzTPA,
concentration increased from 2 wt% to 20 wt%. The current giving maximum EQE of 13.3%, 40.3 cd Aꢀ1, and 28.6 lm Wꢀ1
.
density–voltage ( J–V) curves shown in Fig. S16(b) (ESI†) indicate The corresponding EL characteristics of the optimized devices
that current density increases with doping concentration whilst with Ph3TRZCzTPA and Ph2TRZCzTPA (i.e. devices A and B) are
the turn-on voltages (at 1 cd mꢀ2) decrease from 3.3 V to 2.6 V, shown in Fig. 9 and the data are summarized in Table 5.
in line with Ph2TRZCzTPA being a bipolar charge transporting Overall, the results demonstrate that both Ph3TRZCzTPA and
material. Fig. S16(d) (ESI†) depicts the external quantum effi- Ph2TRZCzTPA give efficient TADF devices from through-space
ciency (EQE) as a function of Ph2TRZCzTPA concentration, with CT excited states.
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Fig. 9 (a) Normalized EL spectra at a luminance of 103 cd mꢀ2; (b) current density–luminance–voltage (J–V–L) characteristics; (c) external quantum
efficiency versus luminance, (d) luminance/power efficiency versus luminance for devices A and B.
Table 5 EL characteristics of the devices with Ph3TRZCzTPA and Ph2TRZCzTPA emitters
Von [V] Lmax [cd mꢀ2
]
EQE [%]
CE [cd Aꢀ1
]
PE [lm Wꢀ1
]
lPeak [nm] CIEd [x, y]
a
Device Host
Emitter
A
B
CBP
Ph2TRZCzTPA (12 wt%) 2.9
27 158 (12.4 V) 16.3,b 14.7,c 10.9d 49.7,b 44.9,c 33.4d 59.3,b 28.2,c 15.0d 522
26 434 (11.0 V) 13.3,b 12.4,c 9.7d 40.3,b 37.5,c 29.4d 28.6,b 23.3,c 15.8d 529
0.32, 0.55
0.34, 0.54
26DCzPPy Ph3TRZCzTPA (20 wt%) 4.4
a
b
c
d
Turn-on voltage measured at 1 cd mꢀ2
.
Maximum efficiency. Measured at 102 cd mꢀ2
.
Measured at 103 cd mꢀ2
.
Discussion
Ph3TRZCzPhCz show no D emission but instead a highly red
shifted, stable CT band. We propose that the packing forces of
the DPEPO host distorts the guest molecules, forcing the D and
A closer together, stabilizing a through-space CT excited state.
This was confirmed measuring Ph2TRZCzPhCz in both CBP
and mCP hosts which also gave the same strong CT stabalisa-
tion as DPEPO does with long lived CT delayed emission. This
DF, although weak, is seen to be temperature dependent in
both compounds, i.e. TADF. In the case of the through-space CT
state, with D and A still have a larger co-facial separation than
an A–D0 through-bond CT state (see X-ray data) and so the
Coulomb attraction energy between separated electron and
hole is reduced and thus the CT energy is red shifted. The
1CT through-space state being red shifted compared to the
through-bond state, reduces the 1CT–3LE energy gap to ca.
120 meV and we thus observe DF in both cases.
We find Ph2TRZCzPhCz and Ph3TRZCzPhCz to have different
photophysics compared to Ph3TRZCzTPA and Ph2TRZCzTPA,
primarily because of their relative weak donor strength and
smaller driving force for CT. A through-bond CT state is
observed to be favoured (by comparison to model systems)
and in Ph2TRZCzPhCz stabilization of any CT state is hard to
achieve so that D emission is always observed. In zeonex, the
lowest energy triplet state is always found with onset at 2.76 eV,
ascribed to the local triplet state of the pendent Cz donor,
1
giving large CT–3LE energy gaps, 40.35 eV, thus we observe
little or no TADF from the through-bond CT states. In
Ph3TRZCzPhCz highly resolved Cz donor phosphorescence is
observed indicative of a large local triplet population, remain-
ing uncoupled from the rest of the molecule. In DPEPO films
we observe a physical effect on the photophysics arising from
the small molecule host packing, whereas in zeonex, which is
an open polymer network having high free volume, the guest
molecules behave much like in MCH solution, i.e. they are not
constrained by the host. In DPEPO, Ph2TRZCzPhCz and
In all cases where we observe a measurable DF signal, we
observe a strongly non-exponential decay at long times. We
believe this comes from the inhomogeneous environment of
the doped films, with the hosts imposing various degrees
of restriction and packing effects on the TADF molecules.
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The through-space CT state systems seem to be particularly negligible mediation of rISC for the through-bond CT state. All
prevalent to this which is not surprising, showing a potentially of our observations clearly align very well with ideas recently
high sensitivity to exact spatial separation and orientation of put forward by Wada et al. from their through-space charge
the D and A in a host environment. Recently, Suresh et al.25 transfer TADF molecule.20 We also see that the effect of host
have discussed such a long time non-exponential DF tail in packing has profound effects on the through-space CT states
terms of combined TADF and TTA emission, however here we and these effects cannot be ignored, especially in devices.
have only used 1% loading of the TADF molecules in zeonex
and 10% in DPEPO which probably rules out TTA. However in
devices were much higher loadings are used a TTA contribution
along with TADF cannot be ruled out.
Conclusions
Ph3TRZCzTPA and Ph2TRZCzTPA, by comparison to the From this new series of D–D0–A materials we have gained a
model TRZꢂ ꢂ ꢂTPA excited state CT complex (exciplex), are seen great deal of new understanding about through-space CT
to have stable through-space CT excited states. The stabalising states, competition with through-bond CT formation and the
of the through space CT state over a A–D0 through bond CT role a host plays in controlling this equilibrium. In the two
state is ascribed to the larger CT driving force arising from their materials having strong D and strong A, Ph3TRZCzTPA and
strong D, strong A, structure. From the time resolved decays Ph2TRZCzTPA, we unambiguously identify stable through-
and spectra, we do though see that in Ph3TRZCzTPA, an initial space CT states via comparison to model compounds and
higher energy transient CT state which we tentatively ascribe to solution state D A complexes. This stabilisation we ascribe to
a short lived through-bond CT state (by comparison to model the large driving force for charge transfer (ca. 1 eV) and
systems), potentially stabalised by the effect of the extra phenyl subsequent larger charge separation distance. For the weak D
ring between acceptor and bridge. Relaxation of this phenyl strong A pair, Ph2TRZCzPhCz and Ph3TRZCzPhCz, a through-
ring configuration could then stabilise the through space CT bond CT state between the A and the bridge D0 is stabilised as
state. Alternatively the through bond state may occur on a small the CT driving force is much less, ca. 0.5 eV. Little DF is
population of Ph3TRZCzTPA molecules trapped in a non observed unless an external perturbation further stabilises
relaxed geometry in the film. This behaviour is observed in the CT state, i.e. solvent polarity or host packing forces. Indeed,
both zeonex and DPEPO which we believe favours the in DPEPO, CBP and mCP with large packing forces we observe
former explanation. Certainly we see no such behaviour in highly red shifted CT formation and subsequent TADF, which
Ph2TRZCzTPA which lacks the spacing phenyl unit. we ascribe to stabilization of a through-space CT state in
Ph3TRZCzTPA and Ph2TRZCzTPA show intense temperature Ph2TRZCzPhCz and Ph3TRZCzPhCz. This shows that such
dependent delayed emission, clearly TADF. The measurable external forces can distort the molecule, forcing the D and A
differences between rISC rates of the two molecules we believe branches closer together to fully stabilise the through-space CT
arises from differences in relative co-facial separation and state over the through-bond state. Very satisfyingly, our TDDFT
orientation of D and A due to the phenyl spacer between A calculations identified the possibility of Ph3TRZCzPhCz having
and D0 (bridge) in Ph3TRZCzTPA, as seen in the X-ray data. This both through-space and through-bond CT excited states. In
controls their magnetic coupling, i.e. spin orbit coupling matrix Ph3TRZCzTPA having an additional ‘spacer’ phenyl group
elements, that dictate rISC rate, given they have very similar, between the bridge D0 and A we observe a large temperature
small singlet triplet energy gaps. By comparing the photophy- dependent red shift in the energies of both the 1CT and
sics of these two systems, we see that strong acceptor, strong 3LE excited states. We ascribe this to thermal motion of this
donor pairs favour through-space charge transfer whereas phenyl ring which reduces planarity between the A and the
strong acceptor weak donor favours through-bond charge bridge, decoupling the phenyl ring from the dTRZ A moiety,
transfer, in line with both the driving force for charge transfer making the Ph3TRZCzTPA act in a very similar fashion to
and the spatial separation and orientation of the non- Ph2TRZCzTPA. This decrease in conjugation also localises the
conjugated D A pair. This clearly sets some basic design criteria 3LE state on the dTRZ unit, increasing the phosphorescence
for TADF intramolecular through-space charge transfer sys- energy as we observe. In view of these observations, having such
tems. The differences between the two molecules are also a spacing phenyl unit that can rotate between the D and A units,
reflected in the device performance. We find that Ph2TRZCzTPA might not be the optimum design strategy for TADF materials, as
gives better device performance than Ph3TRZCzTPA, very much we find that Ph2TRZCzTPA gives better performing devices than
in line with the difference in rISC rates and the lack of possible Ph3TRZCzTPA. We also find that Ph2TRZCzTPA has faster rISC
residual through-bond CT states in Ph3TRZCzTPA. From these rates than Ph3TRZCzTPA which we believe indicates more optimal
device results we again see sensitivity to D A orientation, flexibility co-facial orientation (not separation) of D and A forming the
of linkage to the bridge and also host packing effects. Much like through-space CT state, which is critical in controlling SOC
exciplex emitters, these effects need to be carefully managed and properties and thus rISC, as proposed by Wada et al.27 These
controlled.26
differences in photophysics are also reflected in device perfor-
In Ph2TRZCzPhCz and Ph3TRZCzPhCz, the lowest local mance with Ph2TRZCzTPA giving better performance in optimised
triplet state of the molecular resides on the pendant Cz unit devices. This first in-depth study of the physical properties and
which quenches higher energy D0 (bridge) triplets leading to correlated device performance of through-space compared to
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´
5 X.-K. Chen, D. Kim and J.-L. Bredas, Acc. Chem. Res., 2018,
51, 2215–2224.
through-bond CT states and TADF, gives many new pointers and
ideas to design more efficient and stable TADF emitters.
6 M.-Y. Zhang, Z.-Y. Li, B. Lu, Y. Wang, Y.-D. Ma and
C.-H. Zhao, Org. Lett., 2018, 20, 6868–6871.
7 K. Kawasumi, T. Wu, T. Zhu, H. S. Chae, T. Van Voorhis,
M. A. Baldo and T. M. Swager, J. Am. Chem. Soc., 2015, 137,
11908–11911.
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M. A. Baldo and T. M. Swager, J. Am. Chem. Soc., 2017,
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Methods
Full details of the synthesis and physical characterisation of the
materials reported here, along with photophysical measurements
and data analysis are given in the ESI† file published alongside
this paper.
9 Z. Ren, R. S. Nobuyasu, F. B. Dias, A. P. Monkman, S. Yan
and M. R. Bryce, Macromolecules, 2016, 49, 5452–5460.
10 S. Y. Shao, J. Hu, X. D. Wang, L. X. Wang, X. B. Jing and
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Author contributions
Hector Miranda-Salinas performed the majority of the optical
and photoluminescence measurements and contributed to the
analysis and helped in the preparation of the manuscript.
Yi-Tzu Hung synthesized and characterize the physical property
of titled compounds. Yi-Sheng Chen made the theoretical
analysis of the titled compounds. Dian Luo and Hao-Che Kao
fabricate the OLED device and collect the device data. Chih-Hao
Chang (analyse the device data and helped to prepare the
manuscript. Ken-Tsung Wong designed the project and mole-
cules, and helped in the preparation of the manuscript. APM
conceived and the model for the analysis of the photophysical
results, lead the analysis of the photophysics and helped write
the manuscript.
12 N. Sharma, E. Spuling, C. M. Mattern, W. Li, O. Fuhr,
¨
Y. Tsuchiya, C. Adachi, S. Brase, I. D. W. Samuel and
E. Zysman-Colman, Chem. Sci., 2019, 10, 6689–6696.
13 D. Liu, D. Li, H. Meng, Y. Wang and L. Wu, J. Mater. Chem.
C, 2019, 7, 12470–12481.
14 M. Aydemir, S. Xu, C. Chen, M. R. Bryce, Z. Chi and
A. P. Monkman, J. Phys. Chem. C, 2017, 121, 17764–17772.
15 H. Tanaka, K. Shizu, H. Nakanotani and C. Adachi, J. Phys.
Chem. C, 2014, 118, 15985–15994.
16 M. Colella, A. Danos and A. P. Monkman, J. Phys. Chem.
Lett., 2019, 10, 793–798.
Data
17 E. Stanislovaityte, J. Simokaitiene, S. Raisys, H. Al-Attar,
J. V. Grazulevicius, A. P. Monkman and V. Jankus,
J. Mater. Chem. C, 2013, 1, 8209.
18 P. L. dos Santos, M. K. Etherington and A. P. Monkman,
J. Mater. Chem. C, 2018, 6, 4842–4853.
All data resulting from this work is given in the paper and ESI.†
Conflicts of interest
There are no conflicts to declare.
19 T. Northey, J. Stacey and T. J. Penfold, J. Mater. Chem. C,
2017, 5, 11001.
Acknowledgements
20 Y. Wada, H. Nakagawa, S. Matsumoto, Y. Wakisaka and
H. Kaji, Nat. Photonics, 2020, 14, 643–649.
21 N. Haase, A. Danos, C. Pflumm, A. Morherr, P. Stachelek,
A. Mekic, W. Bru¨tting and A. P. Monkman, J. Phys. Chem. C,
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22 R. S. Nobuyasu, Z. Ren, G. C. Griffiths, A. S. Batsanov,
P. Data, S. Yan, A. P. Monkman, M. R. Bryce and
F. B. Dias, Adv. Opt. Mater., 2016, 4, 597–607.
HMS acknowledges the Mexican National Council for Science
and Technology, CONACYT for his studentship (2019-000021-
01EXTF-00308). APM acknowledges the EPSRC for funding
under grant number EP/P012167/1. KTW acknowledges fund-
ing from the Ministry of Science and Technology, Taiwan
(MOST 107-2113-M-002-019-MY3).
23 J. Pina, J. S. de Melo, H. D. Burrows, A. P. Monkman and
S. Navaratnam, Chem. Phys. Lett., 2004, 400, 441–445.
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