The influence of molecular geometry on the efficiency of thermally activated delayed fluorescence

Article abstract of DOI:10.1039/c9tc00720b

In this work we successfully developed a strategy for positively influencing the conformation of thermally activated delayed fluorescence (TADF) molecules containing phenothiazine as the electron donor (D) unit, and dibenzothiophene-S,S-dioxide as the acceptor (A), linked in D-A and D-A-D structures. In this strategy the effect of restricted molecular geometry is explored to maximize TADF emission. The presence of bulky substituents in different positions on the donor unit forces the molecules to adopt an axial conformer where the singlet charge transfer state is shifted to higher energy, resulting in the oscillator strength and luminescence efficiency decreasing. With bulky substituents on the acceptor unit, the molecules adopt an equatorial geometry, where the donor and acceptor units are locked in relative near-orthogonal geometry. In this case the individual signatures of the donor and acceptor units are evident in the absorption spectra, demonstrating that the substituent in the acceptor uncouples the electronic linkage between the donor and acceptor more effectively than with donor substitution. In contrast with the axial conformers that show very weak TADF, even with a small singlet triplet gap, molecules with equatorial geometry show stronger oscillator strength and luminescence efficiency and are excellent TADF emitters. Acceptor-substituted molecules 6 and 7 in particular show extremely high TADF efficiency in solution and solid film, even with a singlet-triplet energy gap around 0.2 eV. This extensive study provides important criteria for the design of novel TADF and room temperature phosphorescence (RTP) emitters with optimized geometry.

Full text of DOI:10.1039/c9tc00720b

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Materials Chemistry C
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The influence of molecular geometry on the
efficiency of thermally activated delayed
Cite this:DOI: 10.1039/c9tc00720b
fluorescence†
a
b
c
c
Roberto S. Nobuyasu,‡ Jonathan S. Ward, ‡ Jamie Gibson, Beth A. Laidlaw,
bd
ae
b
Zhongjie Ren,
Przemyslaw Data,
Andrei S. Batsanov,
c
b
a
Thomas J. Penfold, * Martin R. Bryce * and Fernando B. Dias
*
In this work we successfully developed a strategy for positively influencing the conformation of
thermally activated delayed fluorescence (TADF) molecules containing phenothiazine as the electron
donor (D) unit, and dibenzothiophene-S,S-dioxide as the acceptor (A), linked in D–A and D–A–D structures.
In this strategy the effect of restricted molecular geometry is explored to maximize TADF emission. The
presence of bulky substituents in different positions on the donor unit forces the molecules to adopt an
axial conformer where the singlet charge transfer state is shifted to higher energy, resulting in the oscillator
strength and luminescence efficiency decreasing. With bulky substituents on the acceptor unit, the molecules
adopt an equatorial geometry, where the donor and acceptor units are locked in relative near-orthogonal
geometry. In this case the individual signatures of the donor and acceptor units are evident in the absorption
spectra, demonstrating that the substituent in the acceptor uncouples the electronic linkage between the
donor and acceptor more effectively than with donor substitution. In contrast with the axial conformers that
show very weak TADF, even with a small singlet triplet gap, molecules with equatorial geometry show stronger
oscillator strength and luminescence efficiency and are excellent TADF emitters. Acceptor-substituted
molecules 6 and 7 in particular show extremely high TADF efficiency in solution and solid film, even with
a singlet–triplet energy gap around 0.2 eV. This extensive study provides important criteria for the design
of novel TADF and room temperature phosphorescence (RTP) emitters with optimized geometry.
Received 6th February 2019,
Accepted 3rd May 2019
DOI: 10.1039/c9tc00720b
rsc.li/materials-c
singlet states (S ) created upon charge recombination may also
1
Introduction
be converted into triplet states, due to intersystem crossing (ISC)
2
Three quarters of the excitons created by the electrical current caused by local hyperfine interactions, or due to spin–orbit
3
used when driving an organic light emitting diode (OLED) are coupling.
non-emissive triplet states. The localized nature of the excitons
Singlet and triplet states have significantly different photo-
in organic materials gives rise to large exchange interactions, physical characteristics. The singlet state lives for a few nano-
and leads to the formation of excited states with spin zero seconds and is often highly emissive. In contrast the decay of
1
(
singlet) and spin one (triplet). In addition, a fraction of the the triplet state occurs in the microsecond and even millisecond
time ranges. The triplet state is also often non-emissive in organic
3
a
b
c
molecules. Since singlet and triplet states are formed in a ratio
Durham University, Physics Department, South Road, Durham, DH1 3LE, UK.
E-mail: f.m.b.dias@durham.ac.uk
of 1 : 3 in OLEDs, the formation of triplet states is a major loss
mechanism in the efficiency of these devices. This problem has
driven research in OLEDs since they were first demonstrated in
the late 20th century, and has led to the application of organic
heavy-metal complexes, which have been the benchmark materials
for phosphorescent OLEDs. Remarkably, OLEDs with internal
quantum efficiency of nearly 100% have been fabricated with metal
complexes, owing to the fast ISC and room temperature phos-
phorescent properties of these materials, which are promoted
by the spin orbit coupling (SOC) of metals, such as iridium(III)
Durham University, Chemistry Department, South Road, Durham, DH1 3LE, UK.
E-mail: m.r.bryce@durham.ac.uk
Chemistry-School of Natural and Environmental Sciences, Newcastle University,
Newcastle upon Tyne, NE1 7RU, UK. E-mail: tom.penfold@ncl.ac.uk
Beijing University of Chemical Technology, State Key Laboratory of Chemical
Resource Engineering, Beijing 100029, China
4,5
d
e
6,7
Faculty of Chemistry, Silesian University of Technology, M. Strzody 9,
4
4-100 Gliwice, Poland
†
Electronic supplementary information (ESI) available. CCDC 1884007–1884011.
For ESI and crystallographic data in CIF or other electronic format see DOI:
1
0.1039/c9tc00720b
8,9
‡
These authors contributed equally to this work.
and platinum(II). However heavy-metal complexes show significant
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problems when applied in OLEDs. The heavy metals are scarce and a small DEST. The sole presence of CT states is not a sufficient
expensive materials, they may create environmental challenges, and condition for a small DEST; near-orthogonal geometry between
10,11
22–24
readily degrade in the blue spectral region.
These issues limit D and A units is also required.
The dihedral angle between the
the utilization of metal complexes in areas that require high volume D and A units is therefore important for the TADF contribution in
manufacturing, such as in lighting and display technologies. these molecules, which can significantly vary among regioisomers
25
Therefore, alternative materials free of heavy metals are needed. of the same emitter. Non-exponential luminescence decays often
Molecules showing thermally activated delayed fluorescence occur in the solid-state, with the emitter dispersed in amorphous
(
TADF) have been introduced in recent years and have emerged matrices, due to the presence of conformational heterogeneity
1
2,13
26,27
as a promising alternative to heavy-metal complexes.
To induced by molecular packing.
Additionally, a small DEST is
date the most successful TADF emitters have been designed with not the only important parameter for RISC. Recent literature
2
8,29
electron donor (D) and electron acceptor (A) units covalently highlights the importance of spin-vibronic coupling,
and
linked with conjugational separation between the units. This mixing of charge transfer and localized electronic states to
1
5,26,30,31
separation can lead to a very small excited state singlet–triplet promote RISC.
gap (DEST). In many cases DEST can be on the order of a few meV.
14
The motivation for the present study comes from literature
If DEST is small, a significant population of the triplet state that reports of a series of phenothiazine (D) and dibenzothiophene-
occupies upper vibrational levels is able to undergo reverse inter- S,S-dioxide (A) TADF molecules where the donor is substituted
1
5
system crossing (RISC), giving rise to delayed fluorescence.
with side bulky groups, aiming to force near D–A orthogonal
3
2
OLEDs fabricated with TADF emitters have shown impressive geometry. Remarkably, while in the unsubstituted D–A and
performances, sometimes with external efficiencies (EQE) above D–A–D molecules strong TADF is observed, even in solid thin films,
However, the performance of TADF-based OLEDs is for the substituted emitters the TADF emission is suppressed, and is
often weaker in the red and blue regions, and is in general almost entirely replaced by room temperature phosphorescence
1
6–18
30%.
1
9
30,31
affected by poor stability showing significant device efficiency (RTP) in some molecules.
Here, we expand the study of D–A
roll-off at high current densities. The causes of poor stability are and D–A–D molecules, with additional analogues utilizing bulky
not always clear, but triplet–triplet annihilation and triplet– substituents in different positions of both the D and A units. The
polaron processes are often responsible for the luminescence effect of the substituents on the energy of the singlet and triplet
quenching. Both processes can compete with TADF due to the excited states, and on molecular geometry is studied in detail
2
0
long lifetime of the triplet excited state. Faster decaying TADF with a variety of spectroscopic techniques and supporting
molecules are thus highly desirable for resolving the device density functional theory (DFT) and time-dependent density
efficiency roll-off issues. Crucially, deeper understanding of the functional theory (TDDFT) calculations.
TADF mechanism is necessary to overcome these drawbacks
2
1
and optimize device performance.
The formation of excited states with strong charge transfer
CT) character in D–A and D–A–D structures leads to a vestigial
overlap between the highest occupied and lowest unoccupied
Results and discussion
Molecular structures
(
frontier molecular orbitals, (HOMOs and LUMOs, respectively). The molecular structures studied in this work are shown in
3
2
33
This effect strongly minimizes the exchange energy and leads to Fig. 1. Compounds 1–5, 10, 11 and 9 have been reported
Fig. 1 Molecular structures of the emitters investigated in this work.
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previously; compounds 6–8 and 12 are new. Molecules 1 and 9 either case and the nitrogen atom’s lone pair is interacting with
are the unsubstituted D–A–D and D–A reference compounds, the arene rings of the donor rather than the acceptor (compare
which allows the influence of the bulky side-groups on the C(A)–N and C(D)–N bond distances in Table S3.2, ESI†). Molecule 8
excited state dynamics of the substituted molecules to be in the non-solvated crystal has crystallographic C symmetry, the
2
established. Molecules 2–5 are D–A–D, and are all substituted methylphenothiazine moiety is disordered (in a 0.86 : 0.14 ratio)
on the phenothiazine donor. Molecules 6 and 7 are also D–A–D, between two equatorial orientations with the methyl group lying
but substituted on the dibenzothiophene-S,S-dioxide acceptor; on either side of the dibenzothiophene plane. Thus, the prevalent
molecule 8 is substituted on both D and A units. Finally, isomer must have anti-disposition of methyls and genuine local C
molecules 10–12 are the asymmetric D–A versions substituted symmetry, although the syn-isomer (with approximate C symme-
, the molecule has crystallo-
2
s
on the donor unit, with molecule 12 substituted at a remote try) is not impossible. In 8ꢀCDCl
3
site, thus being a useful control molecule in assessing steric graphic C symmetry, with the mirror plane passing through the
s
hindrance at the D–A bond. The structures and high purity of the S(1) and both O atoms. Here, the methylphenothiazine moiety
molecules were confirmed by NMR spectroscopy, mass spectro- is similarly disordered in a 0.9 : 0.1 ratio, but in this case the
metry and elemental analysis, with X-ray crystal structures for prevalent conformation is syn. In contrast with all the other
compounds 6, 7, and 8.
structures, in 8ꢀCDCl
3
the dibenzothiophene system is not
With this comprehensive mapping of structural substitutions, planar but bent by 15.71. The distortion cannot be explained
the influence of the substituents on the TADF efficiency in by steric repulsion between syn methyl groups, which are too
phenothiazine-based emitters can be assessed in unprecedented distant for a van der Waals contact (Cꢀ ꢀ ꢀC 6.62 Å).
detail. The focus is on studying the intricate role of the steric
effects that influence molecular geometry, and the energy of Computational studies and electrochemistry
electronic excited states on the photophysics of TADF.
Molecules 1, 9 and their substituted D–A–D and D–A analogues
can exhibit more than one stable conformer, with both the
X-ray crystal data
H-intra and H-extra folded conformers of the phenothiazine
2
5
Fig. 2 shows the molecular structures obtained from X-ray possible. This allows formation of parallel quasi-axial (ax) and
experiments for molecules 6, 7 and 8. In each molecule, both perpendicular quasi-equatorial (eq) conformers, as shown in
thiazine moieties have equatorial orientations and are folded Fig. 3 for compounds 1 and 9. For the D–A–D molecules there is
inward along the Nꢀ ꢀ ꢀS vectors (Table S3.2, ESI†, dihedral also the possibility for mixed conformers, where one donor is
angles y
, t
6–881 and 66–741, showing the extent of conformational flexibility, labelled in the literature, and is in reference to the Nꢀ ꢀ ꢀS axis
although the donor–acceptor p-conjugation is precluded in and plane of the phenyl rings.
1 2
and y ). In molecules 6 and 7, the twist angles quasi-axial and the other quasi-equatorial. The nomenclature
(t
1
2
) around the C(A)–N bonds are somewhat unsymmetrical: of axial and equatorial conformers is defined as previously
3
4
8
Fig. 2 Molecular structures of 6 (a), 7 at 120 K (b), 8 (c) and 8ꢀCDCl
3
(d) showing ellipsoids at 50% probability. H atoms are omitted. Primed atoms are
generated by a twofold axis (c) and a mirror plane (d).
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Fig. 3 DFT(M062X) optimised structures and Kohn–Sham HOMO and LUMO orbitals for the (a) quasi-equatorial and (b) quasi-axial conformers of
molecules 1 and 9.
Molecules substituted in the 1-position of the phenothiazine this figure are also reproduced by a plot of the HOMO and
donor favour the axial conformation. This is the case for LUMO energies for each of the molecules, calculated using
molecules 2, 3, 4, 10 and 11. In contrast, molecules substituted DFT(M062X)/def2-SVP and shown in Fig. S2.2 (ESI†). The axial
on the acceptor favour the equatorial conformation. This is the conformers exhibit the largest energy gap between the HOMO
case for molecules 6 and 7 which exhibit no axial conformations. and LUMO due to the stabilisation of the HOMO in this form.
1 2
The twist angles (t , t ) around the C(A)–N bonds in 6 and 7 range While care should be exercised in interpreting excited state
between 86–881 and 66–741 (Table S3.2, ESI†), showing room for energies from DFT HOMO–LUMO energy gaps, this data set is
some conformational flexibility, but where significant donor– consistent with the higher CT energy observed for the axial
2
5
acceptor p-conjugation is prevented. Importantly, no inter- conformers. The stabilisation of the HOMO is due to its ability
conversion between ax. and eq. forms is possible in both sets, as to delocalise over both the donor and acceptor groups, as shown
only one form is observed in our measurements and calculations. in Fig. S2.2 (ESI†) for molecules 1 and 9.
Unsubstituted molecule 1 and methyl-donor substituted 5 favour
For compounds 1–4 the IP increases with increasing the size
the axial conformer, but show equatorial and mixed forms at of the alkyl groups, varying from ꢁ5.4 eV in 1, to ꢁ6.1 eV, in 4.
higher energy (Table S2.1, ESI†). Molecule 1 can interconvert The effect on the EA energy is the opposite and is less
between the ax., eq., and mixed forms in solution, as the barrier pronounced. With increasing the bulkiness of the donor side
that separates these conformers is relatively low (o0.15 eV). groups the EA energy varies from ꢁ3.09 eV in 1, to ꢁ2.68 eV in
This calculated barrier is consistent with previous calculations 4. This is associated with the increasing energy of the LUMO
3
5
on 1, and highlights the suitability of the DFT parameters related to orbital density across the strained N–C (donor–acceptor)
used in this more expanded study. In molecule 5 however, such bond. Interestingly, there is almost no significant difference in the
interconversion is more difficult as the energy barrier is higher IP and EA energy levels of the t-butyl mono-substituted 4, and the
(0.33 eV between ax. and mixed forms, and 0.64 between ax. and methyl bi-substituted analogue 5. This suggests that the steric
eq. forms). Molecule 8 shows eq., ax. and also the mixed form, effects caused by the presence of the two methyl groups on the
all separated by a considerable energy barrier, around 0.2 eV. phenothiazine donor are similar to that of a single t-Bu group.
Therefore, interconversion between conformers is also difficult
Similar behaviour is observed for the D–A derivatives, compounds
in compound 8. Finally, molecules 9 and 12 are both asym- 9–11. With increasing the bulkiness of the alkyl substituents the IP
metrical and do not show a significant energy barrier between energy is stabilised, while the EA is destabilised. The bulkier side
the ax. and eq. conformers. Therefore, interconversion between groups force the angle between donor and acceptor to increase,
both forms should occur in solution. Control molecule 12 is making oxidation more difficult. However, when the phenothia-
clearly useful to demonstrate that the effects observed here are zine is substituted with a methyl group at a remote position,
predominantly steric.
compound 12, no significant effect is observed, i.e. compounds 9
In summary, as shown in Table S2.1 (ESI†), all the molecules with and 12 have similar IP and EA energies. This indicates that there
the exception of 6 and 7 favour the axial or the mixed form in the is no influence on the dihedral angle between the donor and
electronic ground state. Interconversion to the eq. form is possible in acceptor groups when the substitution is not adjacent to the D–A
molecules 1, 9 and 12, with molecules 6 and 7 permanently in the bond, as expected.
eq. form. This is important, as it has been recently proposed that the
When the alkyl substitution occurs on the acceptor unit, the
different conformers of phenothiazine have profound influence on IP remains almost unaffected, i.e. compounds 1, 6 and 7 show
25,35
the electronic structure, and on TADF efficiency.
similar IP energies. There is however, a very small change in the
All the compounds show oxidation and reduction processes EA energy, but this is not correlated with the size of the alkyl
in cyclic voltammetry (CV) studies (Fig. S1.1, ESI†). The redox substituent. As shown in Fig. 2b, the alkyl substituent on the
potentials, electron affinity (EA) and ionization potential (IP) acceptor induces an equatorial conformation of the phenothiazine
values are shown in Fig. S1.2 (ESI†). The trends observed in units to minimise energy. In the equatorial conformation the steric
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penalty of the alkyl substituent is effectively negated. Therefore, 250 nm becomes broadened. The molar absorption coefficient
switching from methyl to isopropyl does not change the EA of at 250 nm decreases, suggesting a decrease of the oscillator
the acceptor significantly because the steric hindrance of both strength of the electronic transitions associated with the D unit.
substituents is quite similar in the equatorial conformation. Additionally, the CT absorption tail that extends to 450 nm in 1
Interestingly, when both donor and acceptor are substituted in and 9 is also suppressed with a bulkier substituent. These data
compound 8, the influence of the methyl group on the phenothia- indicate that the direct absorption of the CT state is weak in the
zine donor is less pronounced (see the relative changes observed D-substituted compounds. In contrast, the absorption spectra of
between compounds 1, 2, and 7, 8 in Fig. S1.2, ESI†).
A-substituted 6 and 7 show an enhanced molar absorption
coefficient around 250 nm (Fig. 4c). This is a clear signature
of the phenothiazine donor unit, and shows no broadening
UV-vis absorption in DCM solution
ꢁ
4
Fig. 4a shows the absorption spectra of the 10 M solutions of effect in the D region, as these two molecules are substituted
unsubstituted D–A–D, 1, and D–A, 9, in dichloromethane only on the acceptor unit. The absorption band of the acceptor
(
DCM), compared with the absorption of the D and A uncoupled unit is more clearly observed in compounds 6 and 7 than in 1–5.
fragments. The absorption of the phenothiazine unit is clearly The increased acceptor absorption suggests that the substitution
identified by the two well-defined bands, one peaking at 250 nm of the acceptor uncouples the electronic linkage between the D
and the other at E325 nm. The absorption of the dibenzothio- and A units more effectively than donor substitution. In molecule 8,
phene-S,S-dioxide acceptor is also clearly identified, showing the vibrational structure in the acceptor unit is lost and the peak
bands that peak at E280 nm, and below 250 nm.
around 250 nm is broad with weaker oscillator strength, confirming
In general, the absorption spectra of compounds 1 and 9 are the effects are due to D substitution.
well reproduced by the superposition of the absorption of the
For asymmetrical donor-substituted 10 and 11, the absorption
individual D and A fragments, with the exception of a tail with spectra are strongly suppressed at 250 nm as occurs in 1–4 (Fig. 4d).
low oscillator strength that is observed up to 450 nm, which is The behaviour is clearly different for molecule 12. The absorption
attributed to the direct absorption of the charge transfer (CT) spectrum of 12 is almost identical to the spectrum of 9 due to the
26,32,33
state.
This assignment is further confirmed by computational alkyl substitution in the remote position on the donor. The UV-vis
studies, as these peaks are very weak in the calculated absorption absorption data is entirely consistent with the CV data, showing the
spectrum (Section S2.4, ESI†). They are only expected to gain influence of substitution position on the electronic properties
3
6
intensity from thermal fluctuations. When the D unit is of the donor or acceptor regions due to the steric hindrance
substituted with bulky substituents in 2–5, marked differences introduced by the side groups.
are observed in the absorption spectra, when compared with the
The absorption spectra were calculated using TDDFT(M062X)
unsubstituted counterpart D–A–D (see Fig. 4b). With increasing at the DFT(M062X) optimised ground state geometry of the major
bulkiness of the substituent, the absorption band appearing at conformers for 1–12 (Section S2.4, ESI†). The spectra broadly
Fig. 4 Absorption spectra in dichloromethane. (a) Donor, acceptor and the unsubstituted D–A–D and D–A molecules, 1 and 9; (b) D–A–D molecules
with the substituents on the donor, 1–5; (c) D–A–D molecules with the substituents on the acceptor 6 and 7, and on both D and A units, molecule 8;
(
d) D–A molecules with substituents on the donor, 10–12.
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reproduce the trends observed experimentally. The calculated consistent with the CV data. The effect is particularly pronounced
1
absorption spectra for 1–5 indicate the axial conformer is in 4, where the CT emission is almost completely quenched. The
1
predominant in these molecules. In contrast, for 6 and 7 the
LE emission dominates the entire spectrum, showing that the
equatorial conformer is predominant, while 8 favours the axial. steric-effect imposed by the substituent also affects the formation
Finally, molecules 9–12 exhibit relatively broad featureless and stabilisation of the CT state.
absorption spectra due to the increased mixing of the donor
and acceptor electronic structures, consistent with a preference acceptor are shown in Fig. 5b. Compared with 1, the CT emissions
The emissions of the molecules 6–8 substituted on the
1
of the axial form.
in 6 and 7 that are substituted with methyl and i-propyl groups,
respectively, are significantly blue-shifted, relative to the emission
of 1, but no other significant effects are observed in these measure-
ments. The emission of D and A substituted molecule 8, shows a
slight blue shift, compared with 6 and 7. This is consistent with
the calculations in Table S2.3 (ESI†), where no axial conformer
is detected for 6 and 7, but in 8 the mixed axial–equatorial form
is favoured relative to the axial conformer.
Steady-state emission in toluene solution
Photoluminescence experiments were performed in toluene
solution, in the absence of oxygen. Fig. 5a shows the emission
spectra of molecules 1–5. Molecule 1 shows strongly red-shifted
broad emission, relative to the emission of the uncoupled
units, and consistent with the CT character of the emissive
singlet excited state, peaking around 600 nm. According to
calculations (Tables S2.1 and S2.3, ESI†), this red-shifted emission
can only be derived from the equatorial conformer of one of the
D–A groups, suggesting conformational reorganisation in the
excited state. This ability to reorganise derives from an elongation
of the D–A bond distance in the excited state, which lowers the
barrier between the two conformers. For 2 and 5 the scenario is
similar. However, the emissive CT state shifts to higher energies
due to the methyl substituent groups. In the D methyl substituted
molecules, the axial conformer becomes the most stable and
the equatorial conformer is raised in energy significantly. This
elevated energy barrier therefore prevents the occurrence of
axial to equatorial conformational reorganization in 2 and 5.
Molecules 3 and 4 show no equatorial or mixed conformers in
the DFT calculations (Table S2.1, ESI†). This is consistent with
the emission spectrum of 4, which is dominated by a well-
Fig. 5c shows the emission spectra of D–A molecules 9–12.
The spectral shifts induced by the substitution at the D unit are
significantly smaller, when compared with the shifts observed
for the D–A–D counterparts 1–5. The emission of 10 is slightly
red shifted relative to the emission of 9. A similar effect is
observed for 12. However, the emission of 11 appears broader than
all the others, peaking at a similar wavelength to 9. Therefore, in
general, the effect of the substituent on the emission of the
asymmetrical D–A molecules is weaker than in their D–A–D
counterparts. The emission of 9 appears significantly blue-
2
9
1
shifted when compared to 1, showing that the CT state is more
stabilised in the D–A–D structure than it is in the asymmetrical
D–A counterpart. The second donor in the D–A–D structures
weakens the strength of the acceptor.
Thermally activated delayed fluorescence
resolved band, peaking around 400 nm. 3 still shows emission The intensity of the emission in TADF molecules is often strongly
with a degree of CT character, but this appears at higher dependent on the oxygen concentration. This occurs due to triplet
energies than in 1, 2 and 5. The band peaking around 400 nm quenching mechanisms competing with the RISC rate, giving
is also more intense in 3 than in 1, 2 and 5. A clear trend is weaker TADF. In general terms the TADF mechanism involves
observed for molecules 1–5, where the steric hindrance caused the recycling of singlet (S ) and triplet (T ) states, mediated by
1
1
by the substituent on the donor narrows and shifts the CT the intersystem crossing kISC and reverse intersystem crossing
emission to higher energies. An emission band around 400 nm kRISC rates. Both singlet and triplet states are also affected by
S
T
is therefore observed, and grows in intensity with increasing internal conversion, represented by kIC and kIC, and by their
steric hindrance. The growing emission band is assigned to the radiative decay rates, k , and kPh, accounting for the fluorescence
radiative decay of the axial conformer, and is most likely mixed and phosphorescence emissions (Fig. 6a).
f
1
with emission of the singlet local excited state ( LE), as shown in
f
The total fluorescence emission from a TADF emitter (F )
the calculations in Table S2.1 (ESI†). These observations are comprises prompt (FPF) and delayed fluorescence (FDF) components
Fig. 5 Steady state emission in degassed toluene solution at room temperature for molecules: (a) 1–5, (b) 1, 6–8, and (c) 9–12.
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Fig. 6 (a) Schematic representation of the triplet harvesting mechanism responsible for TADF, (b) DF/PF ratio for the 12 molecules studied in this work,
measured in toluene by comparing steady-state fluorescence integrals in deoxygenated and aerated solutions.
due to the triplet harvesting, as described in Fig. 6a. Strong TADF shorter emission lifetimes. Fig. 7 shows the time-dependent
is easy to observe and analyse in molecules with a relatively fluorescence decays for all the molecules in this study. The
strong triplet yield, F , and where the yield of singlet states decays are in general complex, even in the PF region, requiring
T
formed by reverse intersystem crossing, FRISC, is close to 100%. bi-exponential and even tri-exponential fittings. The interpretation
Such conditions are met in compounds where the energy gap of complex fluorescence decays is not always obvious. This is
between the singlet and triplet states is small, (typically o0.1 eV), especially true in TADF compounds in general, and particularly in
and the pathways for vibrational decay affecting the triplet the compounds reported here which have a tendency to show
T
15
excited state are suppressed, i.e. kRISC c kPh + kIC, see eqn (1).
fluorescence from local excited states, pronounced spectral shifts
over time due to solvatochromism of the CT emission, emission
appearing from different conformers and thus different CT states,
and even potentially delayed fluorescence contributions to the
kRISC
FRISC ¼
ꢂ 1
(1)
T
kRISC þ k þ kPh
IC
Eqn (2), gives the product between the yield of triplet formation, tail of the PF decay due to fast RISC processes. Therefore, it is
F , and the yield of singlet states formed by reverse intersystem not possible to give a unique and well-defined reasoning for
T
crossing, FRISC defined in eqn (1). Simple examination of eqn (2) the observation of the multi-exponential decays. Instead, they
shows that triplet states are harvested with efficiency close to reflect the complexity of the CT and TADF mechanisms in these
1
00%, when the ratio between the delayed and prompt fluores- molecules. However, despite this complexity it is clear that some
cence components, FDF/FPF, is above three.
trends can be defined. The unsubstituted molecule 1 shows
mono-exponential TADF decay with 0.69 ms lifetime. This molecule
also has a large DF/PF ratio (Fig. 6b), showing that the fast DF
lifetime results from a fast RISC rate. In contrast, the substituted
FDF=FPF
FTFRISC ¼
(2)
1
þ FDF=FPF
Fig. 6b shows the FDF/FPF ratio determined in toluene solution molecules 2–5 exhibit increasingly longer and more complex TADF
for all the molecules in this study, obtained by comparing decays and increasingly smaller DF/PF ratios. Molecules 2–4,
deoxygenated and aerated solutions in steady-state conditions. As substituted on the donor unit with increasingly bulky groups,
oxygen quenches the triplet states very efficiently, the fluorescence show fast bi-exponential TADF decays with lifetimes in the 1 ms
intensity in aerated conditions is only the PF component. to 15 ms range. TADF emission is followed by a longer decay
Deoxygenating the solution allows the triplet states to contribute component that varies from 32 ms to 53 ms. Tetramethyl-
to the overall emission through delayed fluorescence via the substituted molecule 5 shows the slowest emission with 88 ms
RISC process. The emission in deoxygenated solutions contains lifetime. Molecule 4 shows complex three-component decay in
contributions from both prompt and delayed fluorescence.
the ns range and negligible delayed fluorescence. The trend of
Strong TADF is observed in the unsubstituted molecules 1 smaller DF/PF ratios and increasingly complex DF decays is
and 9, in acceptor-substituted molecules 6 and 7, and in control clearly associated with the high-energy barrier to the equatorial form,
molecule 12. For the other molecules, the TADF contribution is due to the steric hindrance imposed by the substituent groups.
weaker, and is absent in 4. Strong TADF emission is thus
observed only in molecules where the equatorial conformer is show clear mono-exponential TADF decays, with lifetimes of
favoured in the excited state geometry (Table S2.3, ESI†). 4.8 ms, and 14.6 ms, and larger DF/PF ratios (Fig. 6b) when
Molecules 6 and 7 adopt the equatorial conformation, and
Time dependent fluorescence decays were also collected to compared with molecules 1–5. The excellent TADF performance
evaluate the influence of the substituent groups on the delayed of molecules 6 and 7 indicates that the presence of the axial
fluorescence lifetime. TADF occurs while triplet excited states conformer acts as a quenching centre for TADF. Molecule 8
are available, so TADF emission decays with the triplet lifetime. shows mono-exponential TADF decay with a long lifetime of
1
5
This data gives insight into the triplet deactivation pathways.
77.9 ms, and small DF/PF ratio. This is consistent with the equatorial
Molecules with efficient triplet harvesting and TADF will have form only being accessible in 8 via the mixed conformer.
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Fig. 7 Time-dependent fluorescence decays for molecules (a) 1–5, (b) 6–8 and (c) 9–12, all collected in degassed toluene solution at room temperature.
D–A molecule 9 is a very strong TADF emitter (DF/PF ratio DF/PF ratio are those that are able to promote more triplets
4.5), and TADF decays mono-exponentially with 2.5 ms lifetime. back to the singlet manifold. Molecules 3 and 4 deviate from
Molecules 10 and 11 have slow TADF lifetimes of 34.8 ms and the correlation because they have a small TADF contribution,
25.9 ms, respectively, and small DF/PF ratios. Control molecule making determination of RISC/ISC rates problematic.
1
2 shows strong TADF (DF/PF ratio 3.4), and TADF decays
Steady-state luminescence in solid film
mono-exponentially with 1.5 ms lifetime. Molecules 9 and 12
show the equatorial conformation, whereas 10 and 11 show In contrast with the emission in solution, the steady-state
axial, again confirming that the equatorial conformation is a emission spectra of compounds 2–5 in solid zeonex film show
key parameter for TADF emission in these systems.
Fig. 8 shows a plot of the DF/PF ratio vs. the ratio between at short wavelengths (E400 nm) from a mix of axial LE and
the RISC and ISC rates, determined by fitting the decays in
dual luminescence. The dual emission is composed of fluorescence
1
1
CT fluorescence and red-shifted phosphorescence, peaking at
Fig. 7 using the kinetic equations developed in ref. 29. There is E500 nm (Fig. 9). The phosphorescence emission matches the
a clear correlation showing that molecules with high TADF emission of the separate donor and acceptor units, and is
3
efficiency measured by the DF/PF ratio in Fig. 6b, have a larger assigned to a local triplet excited state ( LE).
ratio between the RISC and ISC rates, confirming TADF con-
Room temperature phosphorescence (RTP) is also observed
tributions measured from steady-state and time-resolved data for molecules 10 and 11 in Fig. 9c, and in 8, where it is less
are consistent with each other. The compounds with larger pronounced (Fig. 9b). In summary, all the molecules that show
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3
D unit ( D), while in the other compounds some of the triplets are
trapped in the high energy triplet localised in the acceptor unit.
The contribution of the acceptor phosphorescence is particularly
strong in compounds 2–5 and 10–11. Molecules in this series with
strong acceptor phosphorescence show less efficient TADF, while
acceptor phosphorescence is absent in the strongest TADF emit-
ters 6 and 7. The question here is: what stops some of the triplets,
3
given their long lifetime, relaxing to the D triplet of lower energy?
Previous research shows that the triplet energy of the
3
6
phenothiazine unit does not change with alkyl substitution.
This is also confirmed here, by comparing molecules 1–5. The
phosphorescence from the phenothiazine changes only in inten-
Fig. 8 The DF/PF ratio vs. the ratio between the RISC and ISC rates in toluene sity and not in energy. Comparing molecules 1 and 8 it is
obtained from a fit of the time-decays using the equations developed in ref. 29
plotted in logarithmic scale (the dash-line in the figure is a guide for the eye).
concluded that the triplet energy of the acceptor fragment does
not change as a response to simple substitution; in both 1 and 8
the phosphorescence of the acceptor is observed at the same
energy. Therefore, the phosphorescence spectra of the unsubsti-
a large DF/PF ratio in Fig. 6b, e.g. 1, 6, 7, 9 and 12, show broad tuted phenothiazine and dibenzothiophene-S,S-dioxide were used
CT fluorescence in zeonex, and no RTP is observed. Molecules to fit the phosphorescence of all the molecules in Fig. 10.
2–5, 8, 10 and 11 show dual luminescence composed of fluores- Interestingly, the acceptor phosphorescence disappears entirely
cence at shorter wavelengths, and RTP. These data indicate that in 6 and 7. This leads to the conclusion that the change in the
3
3
quenching of delayed fluorescence is due to a slow RISC rate in the contributions of the D and A to overall phosphorescence is not
more geometrically restricted molecules, and is consistent with the purely due to the substitution, and that conformation is a key
correlation shown in Fig. 8. Triplets are formed in relatively high factor in explaining why the high energy triplets localised in the
yield, and are not significantly quenched by internal conversion as acceptor unit do not relax to the low energy triplet localised on
RTP is observed at ms delay times. However, the RISC rate is not the donor unit. In molecules 6 and 7 only the equatorial form is
fast enough to make triplet-to-singlet up-conversion a viable step. present and only the phosphorescence of the low energy triplet
Note also that the molecules showing weak DF are those where localised in the donor unit is observed. In the remaining
the interconversion from axial to equatorial conformation is molecules both equatorial and axial (or only axial) forms may
more difficult to occur, as described above.
exist independently, therefore, two different phosphorescence
Fig. 10 shows the phosphorescence of molecules 1–12 in emissions are observed. The observed phosphorescence originates
zeonex at 80 K. Molecules 6 and 7 stand out as their phosphores- from the donor and acceptor units at low and high energies,
cence is clearly dominated by the donor phenothiazine. No respectively. This is consistent with the calculations and the high
phosphorescence from the dibenzothiophene-S,S-dioxide acceptor and low energy triplets are assigned to the axial and equatorial
is observed. In all the other molecules both donor and acceptor forms, respectively. In some cases the calculations predict the
phosphorescence is observed. This indicates that the triplet states formation of the equatorial form at inaccessible energies. This
3
in 6 and 7 are able to relax to the low energy triplet localised in the is in apparent disagreement with our interpretation as no D
Fig. 9 Steady-state emission in solid zeonex film, in vacuum, at room temperature for molecules: (a) 1–5, (b) 1, 6–8, and (c) 9–12.
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Fig. 10 Phosphorescence in solid zeonex film, in vacuum, at 80 K for molecules 1–12. The phosphorescence of the single D phenothiazine and A
dibenzothiophene-S,S-dioxide fragments have been used to fit the phosphorescence of each compound.
1
phosphorescence should be observed for these molecules, this study. The CT energy was determined from the onset of
3
3
assuming that no triplet interconversion from A to D triplet the emission spectra using time dependent emission data. This
exists in the axial form. However, calculations are more suitable is to determine the energy of the most relaxed charge transfer
to describe the behaviour in gas and solution phases, while our state without being affected by the emission of the short lived
1
1
experimental data is obtained in solid zeonex films, which may
LE state. In general, the CT energy is more stabilised in
explain these differences. Also a potential triplet interconversion toluene than in zeonex, as expected. Four different groups can
3
3
rate between A and D may be sufficiently slow to allow direct be identified in Fig. 11. In the first group (blue line-squares)
3
1
phosphorescence from the A triplet to occur simultaneously with molecules 1–4, show a clear progression of the CT energy to
D phosphorescence.
3
higher energies with increasing bulkiness of the substituent.
This is observed in both toluene and zeonex. The unsubstituted
molecule 1 shows the most stabilised CT energy in toluene
1
The influence of substituent groups on the CT energy
1
1
Fig. 11 shows the variation of the CT energy determined in (2.43 eV) and zeonex (2.63 eV). Molecules 2 and 5 can be directly
toluene (a) and in zeonex (b) at RT, for all molecules used in compared, as they are both substituted with methyl groups at
1
Fig. 11 Energy of the singlet charge transfer state ( CT) collected in (a) toluene and (b) zeonex at room temperature. The blue and red dashed lines
3
3
represent the energy of the triplet of the acceptor and donor fragments, A and D, respectively. The short dark green mark represents the energy of the
triplet state of each molecule, measured from the onset of the phosphorescence. The DF/PF ratio measured in zeonex by comparing steady-state
fluorescence integrals in degassed and aerated solutions is represented in (c).
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the same positions of the donor. Each donor in 2 is substituted
with one methyl group and in 5 with two methyl groups. The
effect in 5 is expected to be more pronounced due to stronger
1
steric hindrance in 5. As expected the CT energy is shifted
further to high energies in 5, when compared to 2. Molecules
2–4 are weak TADF emitters, both in toluene and zeonex, and
the data appear to be independent of DEST. In zeonex the DEST is
larger than 0.1 eV, weak TADF is thus expected. However, in
1
toluene, molecules 2 and 4 in particular, have the CT state
3
3
almost isoenergetic with the triplets, D and A, respectively.
Despite this fact no strong TADF is observed in these molecules.
The second group (pink line, circles) are molecules 6–8.
1
The CT emission shifts to higher energies with increasing
Fig. 12 Time-dependent fluorescence decays for molecule 7 in toluene
degassed solution and zeonex film in vacuum, at room temperature.
bulkiness of the substituent. However, the variation is much
less pronounced compared with donor substituted molecules,
1
showing that the effect of the acceptor substitution on the CT
energy is smaller than with donor substitution. This observa-
tion is consistent with the CV data, and strongly suggests that
the substituents on the A unit could be sterically interacting
Table 1 Photoluminescence quantum yield in zeonex at RT for molecules
1
–12 measured under saturated N
2
atmosphere
with the donor imposing a strong restriction to the rotation Compound
PLQY
Compound
PLQY
around the D–A axis. The restricted movement prevents the
formation of the axial conformer due a high energy barrier.
Molecule 8 shows the CT energy shifting to slightly higher
1
2
3
4
5
6
0.32
0.07
0.08
0.09
0.07
0.32
7
8
9
10
11
12
0.38
0.20
0.42
0.13
0.11
0.21
1
energies, when compared with 7. This is due to some contribution
of axial conformers to the emission because of the methyl group,
as shown in the calculations (Table S2.1, ESI†). Molecules 6 and 7
both have large DF/PF ratios showing strong TADF contributions
in toluene and zeonex.
In the third group, (red line, triangles), molecules 9–11, a
similar trend to the molecules in group one is observed. The
variation in emission is less pronounced than in group one due
to D–A substitution. Molecule 10, which directly compares with
form 6–7, or that can adopt mixed conformations 1, 8, 9 and 12,
show consistently higher PLQY values.
Conclusions
2
, shows no TADF in zeonex even though the DEST gap is less In summary, the reason that different substitution leads to
than 0.01 eV, confirming that additional parameters affect the different conformers is due to the steric repulsion of the alkyl
TADF efficiency. substituent with the neighbouring arene rings. With alkyl sub-
1
Control molecule 12 shows CT energy directly comparable stitution on the donor, if the system was to remain in the
1
with the unsubstituted molecule 9. In this case, the CT energy equatorial conformation with perpendicular donor and acceptor,
is slightly more stabilised than in the unsubstituted molecule. there would be a large steric interaction between the donor alkyl
This effect is however dominated by electronic factors as demon- group and the acceptor aryl rings. The molecules with 1-alkyl
strated in the CV data. The HOMO–LUMO energy difference in 9 substitution exclusively on the donor (compounds 2–4, 10 and 11)
and 12 is 2.47 eV and 2.44 eV, respectively.
can reduce this steric repulsion by placing the phenothiazine in
Interestingly, in 7 the DEST increases from 0.01 eV in toluene the axial conformation. The axial phenothiazine donor tilts from
to 0.19 eV in zeonex. The RISC rate is thus slower in zeonex the plane of the acceptor, and this therefore tilts the alkyl
than in toluene, given its exponential dependence with DE . A substituent away from the acceptor, because this substituent is
ST
slower RISC rate is consistent with the longer TADF lifetime on the donor (as revealed in the X-ray data in ref. 32). This is the
observed in zeonex, as seen in Fig. 12. However, the large DF/PF reason why the axial conformation is preferred for the donor-
ratio shows that the RISC yield, FRISC, is close to unity in both substituted compounds, as the steric interactions are minimised
solution and zeonex film (according with eqn (2)). This indicates in this conformation.
that the RISC rate is significantly faster than any other process
In contrast, when only the acceptor is alkyl substituted
leading to luminescence quenching in 7.
(compounds 6 and 7), similar ‘tilting’ of the phenothiazine
Finally, PLQY measurements were performed in zeonex for unit as previously described does not move the alkyl substituent
the twelve molecules studied here. The results are shown in with respect to it, as the alkyl group is on the acceptor, not on
Table 1, giving clear indication for pronounced decreases in the the phenothiazine donor. The phenothiazine in this case is
luminescence efficiency of molecules that are exclusively in the forced equatorial and perpendicular with respect to the acceptor
axial conformation. This is especially evident for molecules 2–5. because the phenothiazine cannot rotate to an axial co-planar
In contrast, molecules that are exclusively in the equatorial orientation (see Fig. 2b). In this case the acceptor substituent is
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