Liquid-crystalline TADF materials based on substituted carbazoles and terephthalonitrile

Article abstract of DOI:10.1039/d1tc00443c

By functionalising 2,5-di(N,N′-carbazolyl)terephthalonitrile or 2,3,5,6-tetra(N,N′-carbazolyl)terephthalonitrile with alkoxy chains located on the carbazole moiety, a family of materials is realised, all of which show a TADF response and two of which are also liquid crystalline. This journal is

Full text of DOI:10.1039/d1tc00443c

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Liquid-crystalline TADF materials based on
substituted carbazoles and terephthalonitrile†
Cite this: DOI: 10.1039/d1tc00443c
a
bc
a
Alfiya F. Suleymanova,
Marsel Z. Shafikov,
Adrian C. Whitwood,
Rafał Czerwieniec *b and Duncan W. Bruce
*
a
By functionalising 2,5-di(N,N0-carbazolyl)terephthalonitrile or 2,3,5,6-tetra(N,N0-carbazolyl)terephthalonitrile
with alkoxy chains located on the carbazole moiety, a family of materials is realised, all of which show a
TADF response and two of which are also liquid crystalline.
Received 29th January 2021,
Accepted 30th April 2021
DOI: 10.1039/d1tc00443c
rsc.li/materials-c
S1 and T1 states can be achieved when the HOMO and LUMO
orbitals are separated significantly in space. In such a case, the
Introduction
There is great interest in luminescent liquid crystals as multi- exchange energy is small and the states resulting from the
functional materials, as they combine emissive properties with HOMO - LUMO excitation are close in energy. The ability to
the ability to self-organise over extended length scales. This realise high electroluminescence efficiency then no longer
combination of properties suggests potential applications in relies on the incorporation of a heavy metal to facilitate spin–
the general field of electronic devices, such as polarised orbit coupling.
emitters,1,2 organic field-effect transistors (OFETs),3,4 organic
photovoltaic devices5,6 and organic light emitting diodes, synthesis of liquid-crystalline materials that are either
OLEDs.7
phosphorescent or emit through conventional fluorescence
However, while there has been interest in the design and
Because of their ability to harvest 100% of the excitons mechanisms, to the best of our knowledge there are currently
produced in the OLED device, phosphorescent materials are no examples of liquid-crystalline TADF materials.
much more effective emitters comparing to fluorescence
Pioneered by Adachi and co-workers,11 the use of carbazole
equivalents and those based on iridium(III) in particular are donors with terephthalonitrile (1,4-dicyanobenzene) acceptors
used successfully in the current generation of commercial represents an archetypal class of purely organic TADF materials.
OLED displays.8,9 However, more recently there has been In these compounds, the HOMO is essentially centred at the
increasing interest in a new family of emissive materials based carbazole moieties whereas the LUMO is localised on the
on fluorescence which, in common with phosphorescent dicyanobenzene moiety. The separation of the frontier orbitals
materials, can utilise 100% of device excitons.10 This is possible arises as the donor and acceptor rings are essentially orthogonal
when the lowest excited singlet and triplet states, S1 and T1, are to each other.
close in energy. When the S1–T1 separation is sufficiently small,
As demonstrated by Giroud–Godquin12 and by Swager,13,14
the singlet state can be populated thermally from the lower- and then later used by ourselves with octahedral iridium(III)
lying triplet state generated in course of electroluminescent complexes,15,16 judicious functionalisation of the periphery of a
excitation in the OLED. Thus, thermally activated delayed more three-dimensional molecule by flexible alkyl chains can
fluorescence, TADF, is observed. The small separation of the generate an anisotropic motif capable of being stabilised in a
liquid-crystal phase. Indeed, fullerenes can also be made liquid
crystalline.17 Therefore, using this general approach, we generated
a Department of Chemistry, University of York, Heslington, YORK YO10 5DD, UK.
a family of four related compounds (3a, 3b, 4a, 4b) whose
preparation and formulae are shown in Scheme 1. The mono-
(1a) or di-(1b) alkoxyphenyl dioxaborolanes were obtained directly
from the corresponding alkoxybromobenzene and were then
E-mail: duncan.bruce@york.ac.uk; Tel: +44 1904 324085
b
¨
¨
¨
Institut fur Physikalische Chemie Universitat Regensburg, Universitatsstrasse 31,
Regensburg 93053, Germany. E-mail: rafal.czerwieniec@chemie.uni-regensburg.de;
Tel: +49 941 943 4463
c Ural Federal University, Mira 19, Ekaterinburg, 620002, Russia
† Electronic supplementary information (ESI) available. CCDC 2056911 and
2056912. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d1tc00443c
reacted with 3,6-dibromocarbazole using
a Suzuki–Miyaura
protocol to give the substituted carbazoles 2. Target compounds
3 and 4 were then obtained in a nucleophilic substitution reaction
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Scheme 1 Synthetic route to the target products 3 and 4. Conditions: (i) and (ii) K2CO3 (10 mol equiv.), [Pd3(OAc)6] (0.2 mol equiv.), S-Phos
(0.4 mol equiv.), toluene/ethanol/water, 80 1C, 24–48 h; (III) and (iv) NaH (6 mol equiv.), DMF or THF, 75 1C, 4 h.
with 2,5-difluoroterephthalonitrile or 2,3,5,6-tetrafluorotere- function of concentration in the emissive layer, in the solid
phthalonitrile, respectively. Full details are found in the ESI.†
state and devices on the basis of the existence of persistent
It was possible to obtain single crystals of the methoxy dimer states.
analogues of 3b and 4b (labelled 3b-Me and 4b-Me, respectively)
and their structures were determined using X-ray methods; the
Liquid-crystalline properties
data are recorded in Table S1 (ESI†), while the structures are
shown as Chart 1. Analysis of the structure of 3b-Me shows that
the plane described by the central phenyl ring of the terephtha-
lonitrile subtends an angle of 54.841 to a plane described by the
five atoms in the pyrrolic ring of the carbazole. This is somewhat
smaller than that found in related materials functionalised with
more carbazole groups, reflecting the lower steric crowding.
In terms of intermolecular interactions, there is a distance of
3.985 Å between the planes of phenyl groups bearing the
methoxy groups (Fig. S1, ESI†), but no short contacts between
carbazole units.18
In the structure of 4b-Me, the plane described by the central
phenyl ring of the terephthalonitrile subtends a somewhat
larger angle of 63.48 ꢀ 0.051 to the planes described by the
five atoms in the pyrrolic ring of the carbazole. Steric effects
open this angle up compared to that found in 4a-Me and
indeed, the larger angle is similar values to those found in
related structures described by Etherington et al.18 Also in
common with their observations, a close intermolecular ring–
ring contact of about 3.9 Å is found between carbazole phenyl
rings (Chart 1b) and an illustration of the overlap is shown in
Chart 1c. Such interactions were proposed as the origin of
changes in emission characteristics, sometimes also as a
The thermal properties of compounds 3 and 4 were first
determined by polarised optical microscopy, which showed
that neither 3a nor 4a was a liquid crystal, each simply melting
to give an isotropic fluid at 242 and 249 1C, respectively. Being
(short-chain) methoxy derivatives, neither 3b-Me nor 4b-Me
shows liquid crystal properties. However, for 3b and 4b the
situation was different, and both were found to show a columnar
hexagonal (Colh) mesophase; the observations are the same for
both compounds. Placing the as-obtained material on the micro-
scope slide, it was found to be rather viscous with the viscosity
decreasing as the temperature increased and while there was an
overall birefringence to the sample, there was no discernible
texture to help identify the phase. However, at 181 1C (3b) or
191 1C (4b) the birefringent fluid gave way to an isotropic fluid
which, on cooling developed well-defined optical textures as
shown in Fig. 1a and b, both being characteristic of a Colh
mesophase. The transitions were detected also by DSC (Fig. 1c
and d)
To confirm the symmetry of the observed mesophases,
small-angle X-ray scattering (SAXS) measurements were
performed for both compounds and the diffraction patterns
are shown in Fig. 2. Compound 3b showed a strong, low-angle
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Chart 1 (a) Molecular structure of compound 3b-Me; (b); molecular structure of compound 4b-Me (c) figure showing the close approach of two
molecules of 4b-Me in the solid state giving rise to the intermolecular ring–ring separation of 3.910 Å (different molecules color-coded for ease of
observation); (d) figure to show the detail of the overlap of the carbazole ring systems.
(10) reflection at 2y = 2.521 (Fig. 2a) corresponding to a also consistent with the results from TD-DFT calculations
d-spacing of 35 Å and a lattice (a) parameter of 40.4 Å. Much (below).
weaker reflections at 2y = 4.331 and 4.931 were assigned as the
All of the compounds are luminescent at ambient temperature
corresponding (11) and (20) reflections, confirming hexagonal both as dilute solutions in organic solvents and as solid samples
symmetry. For compound 4b, the strong reflection was found at and their emission spectra recorded in toluene are shown in
a very similar angle (2y = 2.791, Fig. 2b), although higher-order Fig. 4. The band maxima, decay times, and quantum yields are
reflections were not readily evident. The assignment of 4b as listed in Table 1.
showing a Colh phase was confirmed as it was found to be
The emission bands are broad and structureless, reflecting
continuously miscible with 3b (Fig. S2, ESI†). From the single the strongly charge-transfer character of the respective electronic
crystal structure of 4b-Me and from literature data for all-trans transitions. Owing to the measured decay times on the order of
C12 chains, it is possible to estimate the length of 3b and 4b to several nanoseconds and partial spectral overlap with the lowest-
be about 53 Å, which is significantly greater than the lattice energy absorption features, the emissions are identified as being
parameter evaluated for 3b (E 40 Å) or 4b (E 36 Å), suggesting dominated by prompt fluorescence (S1 - S0). However, in the
some tilting of the molecular units within the columns.19
decay profiles, longer-lived components with a decay time of
several hundred ns (4a, 4b) up to 3.5 and 5.5 ms (3b and 3a,
respectively) are present. The longer-lived emission component,
which is on a ms timescale and originates from the T1 state, is the
most evident in compound 3a (Fig. 5) and accounts for around
10% of the net intensity. Intrinsic phosphorescence (radiative
transition T1 - S0) lifetimes for organic compounds normally
amount to hundreds of milliseconds up to seconds. The T1
lifetime observed here, however, which is in the range o1 ms
to 5.5 ms is much shorter and points to another relaxation
mechanism, which, at ambient temperature, is most probably
TADF. This assignment results straightforwardly from the ana-
logy to related carbazole-terephthalonitrile derivatives, for which
Photophysical properties
UV-Vis absorption spectra of compounds 3a, 3b, 4a, and 4b
reveal distinct, low-energy absorption bands due to charge-
transfer transitions. Respective band maxima are, for example,
440 nm for 3a and 3b and 510 nm for 4a and 4b, respectively.
(Fig. 3 and Fig. S3 in ESI†). The charge-transfer character of the
excited states, resulting from an electron density shift from
carbazole moieties to terephthalonitrile, is in accordance with
literature data for analogous carbazole-cyanobenzenes11 and is
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Fig. 1 Polarising optical microscopy images: (a) compound 3b at 177 1C on cooling; (b) compound 4b at 186 1C on cooling. DSC traces (second heat
and second cool, scan rate of 5 1C minꢁ1): (c) compound 3b; (d) compound 4b.
significant TADF was reported,11 and results of quantum interactions between adjacent carbazoles (in ortho position one to
chemical computations (below), predicting small S1–T1 energy another) and decreases the LUMO energy, probably as an effect of
separations of less than 0.1 eV – a prerequisite for efficient TADF. the larger angles between the planes of terephthalonitrile acceptor
The energy of the emitting singlet state depends on both the and the carbazole donor moieties and the concomitantly reduced
number of alkoxy groups attached to each carbazole moiety LUMO delocalisation in 4a and 4b. (cf. Fig. S8, ESI†) The other
(two in compounds 3a and 4a and four in 3b and 4b, respectively) point of interest here is that while the effect of the addition of a
and the number of carbazole moieties in the molecule (two in methoxy group in the 4-position of the phenyl group is significant
3a and 3b, and four in 4a and 4b, respectively). Thus, the (e.g. lem = 615 nm for 4a compared to 577 nm for the equivalent
emission maximum of compound 3a with just two carbazoles compound with no methoxy groups–3,6-diphenylcarbazole
bearing two alkoxy groups is 545 nm while the emission donor),11 addition of the second methoxy group further red-
maximum of compound 4b with four carbazoles bearing four shifts the emission by only 5 nm to 620 nm because the p-
alkoxy groups is 620 nm. The maxima of compounds 3b and 4a donor in the 3-position of the phenyl ring is not in conjugation
have intermediate values of 560 and 615 nm, respectively and with the carbazole.
each alkoxy group increases the donor strength of the carbazole
The same trend found in solution is also observed for solid
moiety, thus shifting the S1 state to lower energy. Greater samples (Fig. S4 and Table S2 in ESI†) and frozen (77 K)
numbers of donor moieties in compounds 4a and 4b as solutions (Fig. S5 and Table S2 in ESI†). However, the
compared to 3a and 3b increase the HOMO energy due to p–p magnitudes of each particular energy shift are not exactly the
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Fig. 2 SAXS patterns for: (a) compound 3b at 39.4 1C; (b) compound 4b at 45.5 1C.
Table 1 Luminescence properties determined in toluene solution at
ambient temperature
l
max/ t/ns prompt t/ms delayed
nm flu. em.
f
PL/%
kr/sꢁ1
knr/sꢁ1
3a 545 12.2 (9.9)b 5.5 (10% int)a 18 (12)air 1.3 ꢂ 107 6.9 ꢂ 107
(1.2 ꢂ 107)b (8.9 ꢂ 107)b
3b 560 6.9 (6.4)b
3.5 (5% int)a
5
0.6 ꢂ 107 14 ꢂ 107
(0.8 ꢂ 107)b (15 ꢂ 107)b
1.3 ꢂ 107 24 ꢂ 107
1.3 ꢂ 107 82 ꢂ 107
4a 615 3.9
4b 620 1.2
r1
r1
5
1.5
a
‘int’ refers to the % emission intensity arising from the longer-lived
b
emission. Recorded in air.
Fig. 3 UV-Vis absorption spectra of compounds 3a, 3b, 4a and 4b in
toluene.
Fig. 5 Emission decay curves for compound 3a recorded in toluene at
ambient temperature. Degassed and air saturated samples are compared.
Fig. 4 Luminescence spectra of compounds 3a, 3b, 4a, and 4b recorded
in degassed diluted toluene solutions (ca 5 ꢂ 10ꢁ5 mol dmꢁ3) at ambient
temperature.
The emissions recorded in liquid samples, with quantum
yields from 1.5 to 18%, are moderate, whereas those deter-
mined for solid samples are significantly higher with the
maximum value of 30% found for compound 3a. (Table S1,
ESI†) This increase in quantum yield is a consequence of
diminished non-radiative losses in a rigid microenvironments
same, indicating that also other factors, such as local micro-
environment, modulate excited state energies significantly.
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expressed by lower values of the non-radiative relaxation rate
knr determined for solid samples (Table S1, ESI†) when
compared to solution (Table 1). The rates found for different
compounds increase with decreasing energy of the emitting
excited state, resembling the trend expected according to the
energy gap law.20 For example, for liquid samples of 3a and 4b,
knr values of 6.9 ꢂ 107 sꢁ1 and 82 ꢂ 107 sꢁ1 are found,
representing a difference by a factor of more than 10. On the
other hand, the radiative rate of prompt fluorescence (kr in
Table 1 and Tables S1 and S2, ESI†) is essentially invariant with
compound composition and rigidity of the matrix.
Theoretical studies
To gain more insight into the luminescent properties of the
materials, the electronic structure of the methoxy analogues
3b-Me, 4a-Me and 4b-Me was analysed theoretically. The com-
putations utilised the Gaussian 09 code21 with the M11L22,23
density functional and def2-SVP basis set.24 Structures of
3b-Me, 4a-Me and 4b-Me were optimised in the ground state
(S0) and the lowest excited triplet state electronic configurations.
The corresponding molecular geometries can be found in the
format of Cartesian coordinates in the ESI.† The relaxed ground-
state geometries calculated for 3b-Me and 4b-Me are comparable
to those determined by X-ray analysis and, for example, the
calculated interplanar angles between the mean planes of car-
bazoles and mean plane of the phenyl ring of terephthalonitrile
Fig. 6 The DFT (M11L/def2SVP) calculated iso-surface contour plots
(iso-value = 0.03) and energies of the frontier orbitals of 3a-Me (a), 4a-Me
(b) and 4b-Me (c) in the T1 state optimised geometries. Hydrogens are
omitted for clarity.
Intersystem crossing S1 - T1 (ISC) and reverse intersystem
(f) are rather similar: (Expt./Calc.): 3b-Me (54.841/57.601); 4b-Me crossing T1 - S1 (RISC) are non-radiative transitions occurring
(63.481/65.971). The notably larger values of parameter f in between isoenergetic vibrational levels of states S1 and T1. The
4a-Me and 4b-Me compared to 3b-Me, is due to the greater rates of these transitions determine the efficiency of thermal
degree of substitution, where planarisation of the molecule by equilibration of S1 and T1, which is essential for TADF to occur.
rotation of the carbazole moieties is hampered by increased The rates of transition where a change in multiplicity is
intramolecular steric hinderance.
involved (e.g. singlet 2 triplet) is determined by the efficiency
According to the calculations, the lowest excited singlet (S1) of the spin–flip of an unpaired electron during the transition
and triplet (T1) states of 3b-Me, 4a-Me, 4b-Me, are of HOMO - and is efficient only with conservation of the total momentum
LUMO orbital origin. The HOMOs of these compounds of the electron (spin plus orbital–El-Sayed’s rule26). In organic
represent p-orbitals localised predominantly on the carbazole molecules, the main mechanism facilitating this conservation
units (donors) whereas the LUMO is a p* orbital localised is spin–orbit coupling (SOC).27 For the S1 and T1 states of 3b-
predominantly on the terephthalonitrile (Fig. 6 and Tables Me, 4a-Me and 4b-Me, 1pp* 2 3 pp* ISC and 3 pp* - 1 pp* RISC
S3–S6 in the ESI†). Thus, transitions between the S1 and T1 processes are spin-forbidden in the zeroth-order approximation.
states of 3b-Me, 4a-Me, 4b-Me can be described as p - p* in The forbidden nature in this approximation can, however, be
character (1,3pp*) with strong charge transfer (1,3CT) from relaxed with perturbations caused by molecular vibrations. These
the donor carbazole to the acceptor terephthalonitrile. can induce vibronic and spin–orbit coupling of the S1 and T1 states
The unpaired electrons in these excited states are well separated with pp* states resulting from local excitations at the carbazole unit
spatially and have a weak exchange interaction resulting in a and at the terephthalonotrile (3LE triplets) as well as with sp* and
small triplet–singlet splitting DE(S1–T1). The DE(S1–T1) values, ps* states.28 For instance, the vibrations displacing the atoms out
estimated from the energies of vertical (Frank–Condon) of the plane of p-conjugation electronically perturb the S1 state
1
1
transitions S0 - T1 and S0 - S1 in the optimised T1 state (charge transfer pp*) admixing it with singlet states of sp* and
geometries, are 0.05 eV (400 cmꢁ1) for 3b-Me, 0.06 eV (480 cmꢁ1
)
1ps* character (vibronic coupling), giving a non-vanishing SOC
for 4a-Me and 0.04 eV for 4b-Me (320 cmꢁ1). These values are matrix element with a triplet state having (at least partial) 3pp*
3
comparable to those of other donor–acceptor (D–A) organic character. Also, SOC of the S1 state with triplet states of sp* and
materials designed to exhibit TADF25 and at ambient temperature 3ps* character or with 3pp* character LE triplets, which may
should allow efficient thermal activation of the T1 state to become close energetically to S1 during the molecular vibrations,
vibrational levels isoenergetic with the lower vibrational levels of relaxes the spin-forbidden nature of the S1 2 T1 transition.
the S1 state.
Similarly, the T1 state is perturbed by vibronic coupling to other
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triplet states and by SOC with electronically suitable singlet states. Acknowledgements
Therefore, the perturbed S1 and T1 states can have a non-vanishing
The authors would like to thank the University of York (AFS),
SOC matrix element relaxing the otherwise forbidden zeroth-order
ISC and RISC processes between them. It is noted that this
relaxation is weak due to the comparatively large energy gap to
the perturbing (admixing) states (sp*, ps* and LE pp* states) and
small spin–orbit coupling constants of the light elements (H, C, N)29
constituting 3b-Me, 4a-Me and 4b-Me.
¨
the Universitat Regensburg (MZS) and the German Research
Foundation (DFG) (Project No. 389797483) for financial
support.
References
Since the molecular vibrations facilitate the S1 2 T1 transi-
tion rates, it can be expected that ISC and RISC rates in 4a-Me
and 4b-Me are larger compared to 3b-Me. This is due to the
structural design of 4a-Me and 4b-Me with four carbazole
derivatives affording much larger number of vibrational modes
of the molecule (3N ꢁ 6) compared to 3b-Me with only two
carbazole units. Therefore, in 4a-Me and 4b-Me, the thermal
equilibration of the S1 and T1 states is more efficient than in
3a-Me. It is noted, however, that S1 - T1 ISC and T1 - S1 RISC
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exponential increase of non-radiative relaxation rates to the
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an anisotropic molecule from a rather three-dimensional core
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red-shift emission and consequently to diminish the emission
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There are no conflicts of interest to declare.
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