Introduction
The advent of thermally activated delayed fluorescence (TADF) has led to a revolution
in the field of organic light-emitting diodes (OLEDs).[1 ]
[2 ]
[3 ]
[4 ]
[5 ] By designing compounds with small singlet–triplet gaps, the triplet excitons formed
during OLED operation can be harvested through thermally reversible intersystem crossing.
External quantum efficiencies (EQEs) of 30% have been reported for OLEDs that include
intramolecular TADF emitters.[6 ]
[7 ]
[8 ]
[9 ] These small singlet–triplet gaps can be achieved through spatial separation of donor
and acceptor moieties, either through a twisted donor–acceptor approach[10 ] or homoconjugation ([Figure 1 ]).[11 ]
[12 ]
[13 ] Either approach spatially separates the HOMO and LUMO orbitals, which results in
a decreased exchange energy and small singlet–triplet gaps. The search for new TADF
materials has furthermore revived an interest in exciplexes as emissive materials
([Figure 1 ]).[14 ]
[15 ]
[16 ]
[17 ]
[18 ]
[19 ]
[20 ]
[21 ]
[22 ]
[23 ]
[24 ]
[25 ]
[26 ]
[27 ]
[28 ]
[29 ]
[30 ]
[31 ]
[32 ]
[33 ]
[34 ]
[35 ]
[36 ]
[37 ] Since the HOMO and LUMO orbitals of an exciplex are located on entirely different
molecules, exciplexes intrinsically have a small singlet–triplet gap. Exciplex-based
OLEDs have achieved remarkable EQEs,[38 ] either as pure exciplex emitters or when formed with an intrinsic TADF emitter,
with some devices exceeding 20%.[39 ]
[40 ]
Figure 1 Overview of classes of TADF emitters. Molecules featuring a large torsional angle
between the donor and acceptor (left), homoconjugated molecules (middle), and exciplexes
(right).
In addition to interfacial challenges,[41 ] low fluorescence quantum yields limit exciplex-based device efficiency. The excited
complex between two molecules is weakly bonded and its flat, shallow bonding potential
gives rise to non-radiative decay channels. These limitations impose the need for
an extensive structure and device optimization to achieve high efficiencies.
In order to address this limitation of exciplexes, we draw upon a central concept
of supramolecular chemistry: more than 100 years ago Emil Fischer introduced the lock-and-key
hypothesis to describe the action of enzymes on their substrates.[42 ]
[43 ] Enzymes can only work on substrates for which a geometrical fit is possible. Exciplexes
are supramolecular entities and, consequently, geometrical complementarity will also
play a role in determining their physical properties. The influence of geometrical
considerations has occasionally been mentioned in the literature on TADF exciplexes,[26 ]
[32 ] but no systematic study on these parameters has been performed. In the present work,
we propose a lock-and-key approach to exciplex design wherein an acceptor and a donor
are tailor-made to fit each other. The molecular recognition in the ground state leads
to a preorganization of the exciplex geometry, giving rise to a more rigid and consequently
more emissive exciplex upon excitation.
For our molecular design we envisioned a concave binding pocket that binds the other
component through a close fit. Such a binding pocket is realized in compound 1 ([Figure 2 ]) where the electron-accepting hexaazatriphenylene core is flanked by three triptycene
moieties. Hexaazatriphenylene derivatives have previously been used as OLED materials.[44 ]
[45 ]
[46 ]
[47 ]
[48 ]
[49 ]
[50 ]
[51 ]
[52 ]
[53 ] Triptycenes, on the other hand, have been shown to be very effective in introducing
free volume that can be used for molecular recognition processes.[54 ] Because of the C3
-symmetric nature of the acceptor 1 , we explored donor frameworks that reflect this symmetry. Three distinct donor families
were chosen for geometrical complementarity: triarylamines, triarylbenzenes, and triarylbenzotrithiophenes
([Figure 2 ]), and their exciplex properties with acceptor 1 were studied.
Figure 2 Tailor-made acceptor and shape-selected donors in this work.
Results and Discussion
Acceptor Synthesis and Properties
The acceptor (1 ) was prepared on a gram scale in 55% yield through condensation of hexaaminobenzene
trihydrochloride[55 ] (2 ) with diketone 3
[56 ] ([Scheme 1 ]). The colorless solid has a generally low solubility, with chloroform being the
best among the commonly used solvents. Crystals were grown by slow diffusion of pentane
into a dichloromethane solution of 1 . As a result of the large proportion of disordered solvent molecules in the crystal,
X-ray crystallographic data of quality sufficient for publication could not be obtained.
However, the general connectivity and crystal packing information regarding acceptor
1 could be determined (see the Supporting Information). We observed that the molecules
stacked in dimers that were mediated by π/π-interactions between two triptycene wings.
The large void spaces created by the remaining triptycene wings are filled with heavily
disordered dichloromethane and pentane molecules.
Scheme 1 Synthesis of the acceptor.
Acceptor 1 has absorption maxima at 336 and 356 nm ([Figure 3 ]). While the absorption spectra are essentially the same in solution and thin films,
marked differences were observed for the photoluminescence (PL) spectra. In solution
the compound emits at 383 nm with a shoulder at 363 nm. In contrast, the thin-film
emission exhibits three maxima at 392, 420, and 442 nm. We hypothesized that the peak
at 392 nm corresponds to unperturbed 1 similar to that observed in solution with a slight red-shift attributed to solid-state
effects. To gain better understanding of the broad features at 420 and 442 nm, we
measured the PL of acceptor 1 in a poly(methyl methacrylate) (PMMA) matrix. The resulting PL spectrum had a single
maximum at 383 nm similar to 1 's solution PL. This finding indicated that the thin-film peaks at 420 and 442 nm
arise from aggregation-induced effects. This is further supported through the observation
that the intensity ratios of the three maxima in the thin film vary between different
spin-coated samples.
Figure 3 Absorption and photoluminescence (PL) spectra of acceptor 1 in thin film, solution, and PMMA matrix (abs = absorption, em = emission, sol = solution).
The electronic structure of acceptor 1 was further elucidated by cyclic voltammetry ([Table 1 ]). The compound exhibited a reversible redox behavior in dichloromethane solution,
and a single redox peak was observed at −1.90 V (vs. Ag/Ag+ ). Compared to parent hexaazatriphenylene[57 ]
[58 ] (E
1/2 = −1.44 eV), the reduction potential of acceptor 1 is shifted to more negative potentials. This indicates a more electron-rich character
of 1 that can be explained by electron donation via homoconjugation from the triptycene
wings.[59 ] The LUMO level (−3.39 eV) was subsequently calculated from the onset reduction potential
of the compound relative to the internal ferrocene/ferrocenium standard potential.
The optical band gap estimated from the onset absorption wavelength was then subtracted
from the LUMO level to calculate a HOMO level of −6.69 eV.
Table 1
HOMO and LUMO energies of acceptor 1 derived from cyclic voltammetry and UV–vis absorption
E
redox
[V][a ]
E
onset
[V][b ]
LUMO
[eV][c ]
λonset
[nm][d ]
E
gap
[eV][e ]
HOMO
[eV][f ]
−1.90
−1.80
−3.39
376
3.30
−6.69
a 1st half-wave potential, 1/2 (E
pa + E
pc ); E
pa , anodic peak potential; E
pc , cathodic peak potential.
b Onset reduction potential.
c LUMO = − e (E
onset + 5.39 − E
Fc+/Fc ) [ref[60 ]]; E
Fc+/Fc = 0.2 V.
d Onset absorption wavelength in chloroform solution.
e Optical band gap = hc /λonset ; h , Planck's constant; c , speed of light.
f HOMO = LUMO − E
gap .
Exciplex Studies
Exciplex formation was studied with 1:1 mixtures of donors and acceptors. Measurements
were performed on spin-coated thin films. No exciplexes were observed in solution
as a result of the limited solubility of acceptor 1 that prevented the concentrations necessary for exciplex formation.
The thin-film PL spectra of all donors and their 1:1 mixture with acceptor 1 are shown in the Supporting Information. We discuss the general features of these
spectra with triphenylamine (X01 ) as an exemplary donor ([Figure 4 ]). The PL of triphenylamine has a maximum at 396 nm; the PL of acceptor 1 with its peaks at 392, 420, and 442 nm has been described above. The PL of a 1:1
mixture of 1 and X01 exhibited a single maximum at 500 nm. The pronounced red-shift compared to the individual
components is typical for an exciplex. Only the exciplex emission is visible in the
mixture; the contributions of the individual components are no longer visible. To
exclude the action of a solvatochromic effect, we performed ratio experiments in which
the acceptor was gradually added to the donor, and vice versa (see the Supporting
Information). We unambiguously observed that the exciplex peak develops independently
of the acceptor and donor peaks.
Figure 4 Fluorescence spectra of acceptor 1 , donor X01 , and their 1:1 mixture.
Triarylamine Donors
Molecular modeling indicated that the C3
-symmetrical core of triarylamines possesses a good geometrical fit to the cavity
of the acceptor ([Figure 5b ]). We first explored exciplex formation with a series of commercially available triarylamines
([Figure 5a ]). In addition to the triarylamines X01 –X03 , we also included X04 (TPD) and X05 (TCTA) that are well-known hole-transporting materials in OLED devices.[61 ] For the benzidine TPD (X04 ), we hypothesized that only one of the triarylamine units would bind to the acceptor.
The exciplexes of compounds X01 –X05 with acceptor 1 emitted in the range of 500–537 nm. Because of the general interest in blue OLED
materials, we focused on synthesizing electron-poorer triarylamines to achieve a blue-shift
of the emission (for an explanation of this rationale, see the section “Color Tuning
of Exciplex Emission”). Substituted triarylamines were prepared by a C − N coupling
methodology[62 ] and initial targets included cyano, trifluoromethyl, and fluorine substitution (compounds
B01 –B06 in [Figure 5c ]). The exciplex emission was, as expected, blue-shifted to 442–496 nm. Qualitative
comparison of the exciplexes indicated that cyano-substituted donors led to brighter
exciplexes. Therefore, the cyano motif was further explored by synthesizing B09 –B11 ([Figure 5c ]). Compound B09 constitutes a heavier analogue of B01 and improves practicability in the thermal evaporation step for OLED fabrication.
The benzidines B10 and B11 were inspired by the TPD motif (X04 in [Figure 5a ]). The respective exciplexes with acceptor 1 emitted in the range of 473–507 nm.
Figure 5 Triarylamine donors and their exciplex properties with acceptor 1 (exciplex emission wavelength and calculated binding energy at B3LYP-D/aug-cc-pVDZ//B3LYP-D/6-31 + G*).
Dispersion-corrected density functional theory was employed to calculate the binding
energies between the donor and acceptor in the ground state. For the triarylamine
family, binding energies of 25–46 kcal/mol were obtained. The parent triarylamine
X01 is at the low end of the scale, whereas triarylamine B09 exhibits the largest binding energy. These values compare nicely with the calculated
binding energies in molecular tweezers that are known to be effective hosts for aromatic
substrates.[63 ] Closer inspection of the calculated geometries revealed that only C3
-symmetric donors and B02 are perfectly poised at the center of the acceptor cavity. Non-C3
-symmetrical donors except B02 are either displaced towards a triptycene wing (X03 , X04 , B01 , B03 , B10 , B11 ) or towards the void between two triptycene wings (B04 , B09 ).
The large library of triarylamine donors with minute electronic differences allowed
us to exhaustively cover a broad range of emission wavelengths. For example, in [Figure 6 ] we plot the PL of seven selected exciplexes that cover the emission range from 461
to 537 nm. In fact, the growing data set in the course of this investigation allowed
us to derive empirical relationships that guided the synthesis of donors with exciplex
emission wavelengths that were not yet covered by our molecules. These relationships
are discussed in detail in the section “Color Tuning of Exciplex Emission.”
Figure 6 Tunability of exciplex emission through variation of triarylamine donor.
Triarylbenzene Donors
We next turned our attention to C3
-symmetrical triarylbenzenes because computational modeling demonstrated their fit
into the cavity of acceptor 1 ([Figure 7 ]). Compounds in this family were prepared by a standard Suzuki cross-coupling methodology.
The PL maximum of a 1:1 mixture of donor A01 with acceptor 1 was observed at 431 nm; this is a minimal redshift compared to the pure acceptor
emission, and therefore the formation of an exciplex had to be questioned. In order
to red-shift the emission into the visible region and away from the acceptor emission,
we synthesized the more electron-rich derivatives A02 and A03 ([Figure 7 ]). The design idea behind A03 was that the dioxymethylene bridges are more rigid than the methoxy groups in A02 and that increased rigidity will favor radiative emission over other decay channels.
The resulting exciplexes indeed exhibited a red-shift with emissions at 458 and 448 nm,
respectively (see the Supporting Information). These emission bands are clearly separated
from the potential acceptor emission, so that efficient exciplex formation can be
assumed. Again, density functional theory was employed to assess binding energies.
The calculated binding energies for the ground state complexes of A01 –A03 with acceptor 1 range from 34 to 39 kcal/mol, which is comparable to the values for the triarylamine
donor family above. Donor A02 has the highest binding energy of the triarylbenzene donors.
Figure 7 Triarylbenzene donors and their exciplex properties with acceptor 1 (exciplex emission wavelength and calculated binding energy at B3LYP-D/aug-cc-pVDZ//B3LYP-D/6-31 + G*).
Benzotrithiophene Donors
Another promising C3
-symmetrical motif for donors is the benzotrithiophene framework ([Figure 8 ]). We prepared the triphenyl derivative C01
[64 ] and studied its exciplex properties with acceptor 1 . A weak emission at 453 nm was observed that is in the range of both acceptor and
donor emission. As a result, no clear evidence for an exciplex could be observed in
this case despite the excellent geometric fit. The calculated binding energy of the
ground state complex amounts to 38.4 kcal/mol.
Figure 8 Triphenylbenzotrithiophene and its exciplex properties with acceptor 1 (exciplex emission wavelength and calculated binding energy at B3LYP-D/aug-cc-pVDZ//B3LYP-D/6-31 + G*).
Color Tuning of Exciplex Emission
We have shown that the emission wavelength of the exciplexes can be adjusted by synthesizing
donors with electron-donating or -withdrawing groups. It is well known that exciplexes
display a linear relationship between the exciplex emission energy and the difference
between the electron affinity (EA) of the acceptor and the ionization potential (IP)
of the donor: hλ
exciplex = (EAacceptor – IPdonor ) + const.[65 ]
[66 ] To study this relationship we calculated the EA of acceptor 1 (0.97 eV) and the IP of the various triarylamine donors X01–B11 (see the Supporting Information). In [Figure 9 ] we plot the experimental exciplex emission wavelengths against the calculated EA − IP
differences for the exciplexes of 1 with triarylamines X01 –B11 .[67 ]
[68 ] The resulting empirical relationship can be used to design donors with a specific
exciplex emission wavelength. For example, for blue exciplexes (450–480 nm) one should
synthesize a triarylamine that yields a theoretical EA − IP difference of 5.8–6.3 eV.
Since the exciplex emission wavelength also depends on the geometrical distortions
in the excited complex,[66 ] such relationships have to be established separately for every donor family.
Figure 9 Empirical relationship between experimental exciplex emission wavelength of acceptor
1 and triarylamine donors X01 –B11 and the calculated difference of electron affinity (EA) and ionization potential
(IP).
Delayed Fluorescence Measurements
Owing to its bright emission and preferable emission wavelength, we focus on the exciplex
with donor B01 . [Figure 10 ] shows the transient PL for the 1:1 mixture of donor B01 and acceptor 1 in a PMMA matrix. While exhibiting biexponential delayed fluorescence with lifetimes
of τ = 4.68 and 45.1 μs, we further confirm the existence of TADF by performing temperature-dependent
transient PL measurement. Upon cooling the sample down to 150 K, the relative contribution
from delayed fluorescence drops by approximately a factor of 2 (Supporting Information),
indicating that the delayed fluorescence at room temperature is thermally activated.
Lifetimes of further exciplexes can be found in the Supporting Information but were
generally similar.
Figure 10 Transient of delayed fluorescence from thin film of acceptor 1 and B01 (1:1 molar ratio) in PMMA.
OLED Device Studies
In order to check the feasibility for acceptor 1 + donor B01 exciplex in OLED device applications, we fabricated the test device with the structure
of: indium tin oxide (ITO, 132 nm)/di-[4[(N ,N -ditolyl-amino)-phenyl]-cyclohexane (TAPC, 150nm)/acceptor 1 :donor B01 approx. 1:1 molar ratio (15 nm)/1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB, 60 nm)/LiF
(1 nm)/Al (100 nm). ITO and Al are the anode and cathode, respectively. TAPC serves
as the hole-transport layer (HTL) and TmPyPB is the electron-transport layer (ETL).
In addition, we inserted LiF between the ETL and the cathode to boost electron injection
([Figure 11a ]).
Figure 11 (a) Device structure and the energy band diagrams for the OLED device. The energy
levels are labeled in electron volts and the thicknesses are in nanometers. (b) Electroluminescence
spectra of the device. (c) The device IV curve. (d) External quantum efficiencies
(EQEs) with different current densities.
The electroluminescence (EL) spectrum of a device is shown in [Figure 11b ]. Intriguingly, instead of showing blue emission as expected from the PL (461 nm,
[Figure 6 ]), the EL is red-shifted by 75 nm, having its emission peak at 536 nm. The red-shift
is too large to attribute to the optical cavity in the device. Transient PL measurements,
however, showed that the delayed fluorescence is red-shifted relative to the prompt
luminescence (Supporting Information). The shift is consistent with solid-state solvation[69 ]
[70 ]
[71 ] as the molecules surrounding an emissive state polarize in response to the excited-state
dipole. The effect is enhanced in the OLED relative to PL because EL is dominated
by carrier condensation to give long-lived triplet excited states rather than photoexcited
singlet states.
[Figure 11c,d ] shows the device characteristics. The large turn on voltage may originate from the
lack of a host material to facilitate charge transport in the emitting layer. From
[Figure 11d ], a maximum EQE of 5.4% is achieved, which is comparable to those of other exciplex
OLEDs.[38 ] Optimization of the device structure, such as introducing a suitable host material
for better charge transport and balance, or tuning thickness for each layer to enhance
out-coupling efficiency, can be done to further improve the device overall performance.
Conclusions
In summary, we have designed a supramolecular system featuring a lock-and-key concept,
guided by density functional theory calculations. The C3
-symmetric nature of the acceptor (lock) allows strong binding with three complementarily
shaped donor families (keys). Lock-and-keys are demonstrated to be exciplexes, with
emission colors covering a large portion of the visible spectrum. Empirical relationships
between experimental exciplex wavelengths are established and can be used for color
tuning. The exciplex with acceptor 1 and B01 displays TADF, which is supported by transients of the delayed fluorescence as well
as temperature-dependent studies. EQEs of up to 5.4% are achieved in an OLED.
Experimental Section
Commercial chemicals were used without further purification. NMR spectra were recorded
on a Bruker Avance 400 MHz spectrometer. The residual solvent peaks were used as internal
standards.[72 ] Mass spectra were measured with a Bruker Daltonics APEX IV 4.7 Tesla Fourier Transform
Ion Cyclotron Resonance Mass Spectrometer in electrospray ionization (ESI) or direct
analysis in real time (DART) mode. Infrared (IR) spectra were measured on a Thermo
Scientific Nicolet 6700 Fourier Transform Infrared (FT-IR) spectrometer in attenuated
total reflection (ATR) mode on a germanium crystal. Atmospheric suppression correction
was applied. Thin-films of single components were spin coated (4000 rpm, 30 s) from
10 mM solutions in chloroform; mixtures from 5 mM solutions respectively. Samples
with PMMA matrix contained 5 wt% of the single component. The films were spin coated
on micro cover glasses by VWR.
Solution UV/VIS absorption spectra were obtained with an Agilent Cary 4000 UV/VIS
spectrophotometer. Thin-film UV/VIS absorption spectra were obtained with an Agilent
8453 UV/VIS spectroscopy system. Photoluminescence spectra were measured with a Horiba
Jobin Yvon Fluorolog-3 spectrofluorometer (model FL3-21) employing a 450 W xenon short-arc
lamp. Excitation and emission bandpass slits were set at 3 nm and 5 nm, respectively.
Donor molecules were excited at their absorption maxima; acceptor 1 at 336 nm respectively. For 1:1 mixtures of donors with acceptor 1 an excitation wavelength of 336 or 356 nm was used.
The thin-film for the delayed fluorescence measurements was fabricated through drop-casting,
with 37 wt% of acceptor 1 + B01 in PMMA. Acceptor to B01 ratio is 1:1 in molar ratio. The transient PL was measured with a Hamamatsu streak
camera at 375 nm excitation, with a repetition rate of 220Hz to ensure full decay
of the delayed fluorescence. Temperature-dependent transient PL was performed by cooling
down the sample to 150 K with a Helium-cooled cryostat, using the same setup and measurement
parameters as room temperature measurement. Transient photoluminescence lifetime measurements
of all other exciplexes were performed with a 337 nm N2 laser (NL 100, Stanford Research Systems) as the excitation source. The photoluminescence
was detected with a Thorlabs PDA10A Si detector. The resulting time-dependent photoluminescence
data were recorded using a Tektronix TDS3054C oscilloscope.
Electrochemical measurements were carried out in a glove box under nitrogen, using
an Autolab PGSTAT 10 or PGSTAT 20 potentiostat (Eco Chemie) in a three-electrode cell
configuration. A Pt button (1.6 mm in diameter) electrode, a Pt wire, and a quasi-internal
Ag wire submerged in 0.01 M AgNO3 / 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6 ) in acetonitrile were used as a working electrode, a counter electrode, and a reference
electrode, respectively, in 0.1 M TBAPF6 dichloromethane solution. The ferrocene/ferrocenium (Fc/Fc+ ) redox couple was used as an internal standard, with the half-wave potentials observed
between 0.195–0.205 V vs Ag/Ag+ in dichloromethane solution.
Device Fabrication and Characterization
The testing OLED devices were made on a pre-cleaned and pre-patterened indium tin
oxide (ITO) on glass substrate. The ITO is 132 nm thick. All the other OLED layers
in the device were deposited subsequently by thermal evaporation in a vacuum chamber
with base pressure of <10−6 torr. These layers include: Di-[4[(N,N-ditolyl-amino)-phenyl]-cyclohexane (TAPC,
150 nm) / emitting materials layer (acceptor 1 :donor B01 = 1:1 in molar ratio, 15 nm) / 1,3,5-Tris(3-pyridyl-3-phenyl)benzene (TmPyPB, 60 nm)
/ LiF (1 nm) / Al (100 nm). The EML was fabricated by co-evaporation from two separate
evaporation sources at the same rate to achieve 1:1 volume ratio for acceptor 1 and donor B01 . The devices are then packaged under nitrogen glovebox with UV-cured epoxy to maintain
inert environment during measurements. The device currents and photocurrents are then
measured with a 4156C Precision Semiconductor Parameter Analyzer. The electroluminescence
spectra of the devices are measured with a Princeton Instruments SP2300 and PIXIS-100
spectrometer.
Density functional theory calculations were performed with the QChem 4.3 software
package[73 ] using the B3LYP functional[74 ] with Grimme's dispersion corrections.[75 ] A Lebedev (75, 302) exchange-correlation grid[76 ] was used. Geometry optimizations were performed using a 6-31 + G* basis set. Binding
energies, HOMOs, and IPs were computed in an aug-cc-pVDZ basis set.[77 ] Ionization potentials were computed by vertical detachment of an electron. Electron
affinities were computed by vertical attachment of an electron.
