Enhancing thermally activated delayed fluorescence by fine-tuning the dendron donor strength

Thermally activated delayed fluorescence (TADF) relies on a small energy gap between the emissive singlet and the nonemissive triplet state, obtained by reducing the wave function overlap between donor and acceptor moieties. Efficient emission, however, requires maintaining a good oscillator strength, which is itself based on sufficient overlap of the wave functions between donor and acceptor moieties. We demonstrate an approach to subtly fine-tune the required wave function overlap by employing donor dendrons of changing functionality. We use a carbazolyl-phthalonitrile based donor-acceptor core (2CzPN) as a reference emitter and progressively localize the hole density through substitution at the 3,6-positions of the carbazole donors (Cz) with further carbazole, (4-tert-butylphenyl)amine (tBuDPA), and phenoxazine (PXZ). Using detailed photoluminescence studies, complemented with density functional theory (DFT) calculations, we show that this approach permits a gradual decrease of the singlet-triplet gap, ΔEST, from 300 to around 10 meV in toluene, yet we also demonstrate why a small ΔEST alone is not enough. While sufficient oscillator strength is maintained with the Cz- and tBuDPA-based donor dendrons, this is not the case for the PXZ-based donor dendron, where the wave function overlap is reduced too strongly. Overall, we find the donor dendron extension approach allows successful fine-tuning of the emitter photoluminescence properties.


Introduction
Emitters showing thermally activated delayed fluorescence (TADF) can achieve comparable internal quantum efficiencies to those of commercialised phosphorescent emitters, but without the need to employ scarce heavy metals within an organic light emitting diode (OLED). 1-3 TADF allows non-coherent endothermic triplet exciton upconversion to the singlet state, mediated by the small singlet-triplet splitting. This is particularly useful when employing these materials for OLED applications, since in this way, all electrically generated excitons in the OLED may be used towards light emission. The energy gap between the lowest singlet and triplet states, ΔEST, that governs in part the efficiency of the TADF process depends on the magnitude of the wavefunction overlap in the exchange integral. [4][5] The singlettriplet gap can therefore be reduced by localizing hole and electron wavefunctions onto different parts of the molecule, and this is commonly achieved by employing, respectively, donor and acceptor moieties that adopt a highly twisted conformation in the emitter structure. Nevertheless, there needs to be sufficient wavefunction overlap to still allow for a reasonable transition dipole moment, and thus a good radiative decay rate. [4][5] The balance of these two factors is required to obtain a highperformance TADF emitter.
One way to adjust wavefunction overlap is by modulating the torsion between the donor and the acceptor moieties, which controls the extent of the conjugation between these two fragments. [6][7][8][9] An alternative way is to increase the delocalization of the hole over the donor unit, which effectively localizes the hole density on this fragment. 10 Implementation of this strategy is illustrated in dendrimers, which frequently contain dendritic donors surrounding an electron-acceptor. 5,[11][12][13][14] It has been shown that the more moderate D-A torsions in the dendrimers would ensure sufficient overlap between HOMO and LUMO wavefunctions to achieve the paradox of a small ΔEST and a large oscillator strength. 7 A further advantage of employing a dendritic structure for the emitter is to potentially obviate the need for a host matrix in the emitter layer of an OLED. 2,15 Commonly, TADF emitters are dispersed in a host matrix to reduce concentration quenching. The dendritic structure itself is often 4 sufficiently sterically crowded to mitigate concentration quenching of the vulnerable triplet states.
Concentration quenching can be synergistically further suppressed by the use of stericly bulky groups decorating the donor dendrons. This facilitates the device fabrication and avoids issues associated with host-guest interaction such as exciplex formation and undesirable energy shifts. 16 There are, however, few variants of donor dendrons that have been explored. An appreciation of the effect of structure modulation on the optoelectronic properties, particularly on the efficiency of the TADF process, is thus currently missing. A toolbox-like approach is desirable, where different donor units can be connected with ease to gently and gradually modify the properties. 17 Here we demonstrate such a modular fine-tuning approach by changing the nature of the peripheral donor groups in donor dendrons, which affects hole localization, while maintaining the central carbazole donor unit and its connection to the common phthalonitrile acceptor. This strategy avoids re-optimizing the entire molecular design. We are not aware of detailed and systematic studies on the impact of the nature of the donor dendrons on the singlet-triplet gap and TADF properties. An overview of literature-reported emitters containing donor dendrons, including dendrimers, is given in the ESI (Table S1, Figure S1). We address whether such a strategy can significantly improve the efficiency of the TADF process in an emitter that already shows TADF in thin films.
We implement our approach by changing the peripheral donor groups on the TADF-active sky-blue emitter 2CzPN (Figure 1). [18][19][20] 2CzPN consists of an electron-accepting phthalonitrile core connected to two carbazole donor moieties at the 4-and 5-positions. These donors can be substituted comparatively easily at the 3,6-positions of the carbazole, allowing for the extension of the conjugated π-system. The compound is thus well suited to study the impact of hole wavefunction delocalization on the singlettriplet gap and the TADF performance. 2CzPN shows a desirable set of photophysical properties in terms of its high reported photoluminescence quantum yield (PLQY) of 89% and the reported presence of TADF in an mCP matrix, 19-20 although we find a surprisingly high ΔEST gap of 230 meV in our 5 measurements in a film with mCP as host matrix, increasing to 300 meV in toluene solution. To gradually reduce electron-hole overlap we progressively altered the electronic structure of the donor dendron, by decorating the central carbazole with either peripheral carbazole (Cz), di(4-tertbutylphenyl)amine ( t BuDPA) or phenoxazine (PXZ) groups. We find that our approach allows for an enhancement of the intramolecular charge transfer (CT) character of the emitter and a progressively smaller ΔEST, from 300 meV for the parent compound 2CzPN to 160 meV for Cz-2CzPN and 10 meV for the other two compounds in toluene. Consistent with this, the optical spectroscopy as well as density functional theory (DFT) calculations suggest that the overlap of electron and hole density reduces along the series, until for PXZ-2CzPN, the hole density is localized only on the PXZ units, thus limiting the overlap with the electron density localized on the phthalonitrile unit. We also find that non-radiative losses are present in the case of Cz-2CzPN and 6 Theoretical calculations DFT calculations were undertaken to elucidate the effect of changing the donor in the 2CzPN derivatives on the distribution of electron and hole densities, the magnitude of the oscillator strengths for the vertical transition between the ground state and the singlet excited states, and the orbital and state energies. Figure 2 presents the hole and electron densities for the lowest-lying excited singlet state calculated within the attachment/detachment formalism and the orbital and state energies from the ground state geometry. 21 The lowest-lying triplet state electron and hole densities can be found in the ESI (Figure S2). 22 This approach provides information about the reorganization of the electron density upon excitation of the molecule. Left axis (black): HOMO and LUMO energies calculated for the ground state geometry at the PBE0/6-31G(d,p) level. Right axis (red): vertical transition energies for S1, T1 and T2 computed at the 7 TDA-DFT using the PBE0 functional together with the 6-31G(d,p) basis set. Also shown are the electron (top) and hole densities (bottom) for the for S1 state, where f is the oscillator strength.
For all compounds, the electron density is localized on the phthalonitrile acceptor. In contrast, the localization of the hole density varies across the different emitters. For 2CzPN, the hole density is mainly localized on the carbazole donors with some delocalization onto the phthalonitrile phenyl ring.
When extending the donor as in Cz-2CzPN and t BuDPA-2CzPN, however, the hole density is distributed across the now larger donor dendron moieties, with a reduced contribution on the phthalonitrile unit. respectively, for the S1 state suggesting an increase in CT character and supporting the decrease of the oscillator strength, f (see Figure 2). The electron and hole densities for the first triplet excited state closely resemble those of the S1 state and are given in the ESI (Figure S2). For each compound, we also calculated the energy of the T2 state and characterized its nature. The presence of intermediate triplet states energetically close to the T1 and S1 states is essential for efficient TADF to take place. Indeed, El Sayed's rules essentially forbid the direct T1 and S1 conversion in cases where the states have the same natures, as would be the case when both are pure CT states, due to the predicted vanishing spin-orbit coupling. We consider that for our compounds, the upconversion process from T1 to S1 involves reverse 8 internal conversion from T1 to T2 driven by vibronic coupling, followed by efficient Reverse Intersystem Crossing (RISC) to S1 in the case where T2 is of a different nature than S1. Such a mechanism supports the high RISC rates observed even in the case of ΔEST values exceeding 200 meV. [23][24] For 2CzPN, the T2 state lies between the T1 and S1, while in each of the other three compounds it is either the same energy or slightly higher in energy than that of the S1 state. Owing to the similar energy levels of S1 and T2 it is likely that these states exhibit non-vanishing intersystem crossing. Further, T1 and T2 are energetically close and could show efficient vibronic coupling.
The calculated HOMO energy progressively destabilizes, indicative of both the increasing strength of the donors as well as the delocalization of the HOMO orbital, across the series ( Table 1). An unexpected trend is observed with the calculated LUMO energy. Both 2CzPN and t BuDPA-2CzPN have similar energies at -2.41 eV and -2.42 eV, respectively. By contrast, a significant stabilisation is observed for Cz-2CzPN and PXZ-2CzPN, with LUMO energies of -2.85 eV and -2.92 eV, respectively. The change in behaviour is likely related to the calculated torsional angles between the carbazole donor and phthalonitrile acceptor, with the torsions being greatest for PXZ-2CzPN and Cz-2CzPN, with average angles between the respective planes of 60° and 67-70°, respectively, compared to 53° and 56° for both 2CzPN and t BuDPA-2CzPN. The smaller torsion in the latter ensures a larger delocalization of the LUMO orbital onto the electron-donating Cz groups. These trends were also observed by X-ray crystallography (vide infra). The HOMO-LUMO gap is strongly affected by the changes in HOMO and LUMO energies, with values ranging from 2.08 eV for PXZ-2CzPN to 3.65 eV for 2CzPN. The trend in the HOMO-LUMO gap ( Table 1) aligns also with the trend observed for the S1 energies ( Table 2) because this excited state is mainly described (> 90%) by a HOMO to LUMO transition. The increasing CT character along the series is associated with a smaller calculated ΔEST, with the values decreasing from 0.34 eV in 2CzPN to 0.01 eV in PXZ-2CzPN (Table S2). This is consistent with the values measured by photoluminescence spectroscopy ( Table 3). When M062X is employed as the functional in lieu of PBE0 in the DFT calculations, identical trends are observed; however, the magnitude of the CT 9 character changes slightly, owing to the changing HF content (56% 25 for M062X and 25% 26 for PBE0).
This increased HF contribution helps to alleviate overstabilisation of the CT states; 27 however, as the S1 and T1 states are CT in nature in these compounds, the differences between the two methods are minimal. Data from the M062X calculations, including oscillator strength, S, ΔEST, calculated HOMO LUMO energies, can be found in the ESI (Table S3). Overall, the DFT data suggest that the design concept of reducing the singlet-triplet gap by increasing the strength of the peripheral donors of the donor dendrons is reasonable. BuDPA-2CzPN was also synthesised and we hypothesise that the addition of t Bu groups should have a minimal impact on the photophysics while addressing the issues of solubility. This is corroborated by DFT calculations that predict nearly identical S1 (2.19 eV and 2.18 eV for DPA-2CzPN and t BuDPA-2CzPN, respectively) and ΔEST (0.10 for both) values for the two compounds ( Figure S3, Tables S2 and S3). No further photophysical analysis was undertaken for DPA-2CzPN owing to its insolubility but we discuss its crystal structure as crystals of t BuDPA-2CzPN were not obtained and the conformations are calculated to be similar (Table S5) Table S5. The differences between the two structures of PXZ-2CzPN include differences in torsions between rings, such as those mentioned above, as well as differences in the extent of puckering of the PXZ groups ( Figure S4). The PXZ groups adopt varying degrees of puckered conformation between the two structures, similar to that observed previously in structurally similar phenothiazine-containing compounds. 28  To experimentally determine the HOMO and LUMO levels, cyclic voltammetry (CV) and differential pulsed voltammetry (DPV) were performed on each of the four emitters as well as on the isolated donor groups (and partial donor groups) in dichloromethane (DCM) (Figure 4 and Figures S5-S9). A summary of the electrochemical data is found in Table 1 for the emitters, and in the ESI (Table S6)   We first investigated the electrochemical behaviour of the isolated donor groups in DCM. As expected, the oxidation of carbazole is not reversible and the compound degrades upon successive scans ( Figure   S6), via a putative electropolymerisation. 29 The E ox from the DPV of 1.15 V versus SCE matches that previously reported ( E ox = 1.16 V in MeCN). 30 Both t BuDPA and PXZ display quasi-reversible oxidation waves centred at 0.84 V and 0.66 V (Figure S5), respectively, indicating that PXZ is the strongest donor.

Synthesis, chemical and electrochemical characterisation
The E ox of Cz-Cz is cathodically shifted with respect to Cz at 1.08 V, in agreement with a similar derivative (wherein the N-H of Cz-Cz was replaced by N-C6H13) with E ox also at 1.08 V in DCM. 29,31 Only a single oxidation wave is observed, suggesting that the radical cation is delocalized over the Cz-Cz molecule. Cz-Cz is electrochemically unstable, as evidenced by the evolution of the CVs over multiple scans where a second oxidation wave appears (Figure S6), attributed to polymerisation of the carbazole species. For t BuDPA-Cz there are two observed reversible oxidation waves (Figure S7) assigned to the oxidation of carbazole and then t BuDPA (Figure 4a). These are both cathodically shifted compared those of Cz (1.16 V) and t BuDPA (0.84 V), the E ox of the first wave is 0.49 V. For PXZ-Cz, there is a single reversible oxidation wave at 0.71 V assigned to oxidation of PXZ (E ox of PXZ = 0.66 V, Figure S7). Although PXZ is a stronger donor than t BuDPA based on its cathodically shifted oxidation 13 potential, the ability for t BuDPA to effectively conjugate to carbazole ensures that t BuDPA-Cz (E ox of 0.49 V) is a stronger donor than PXZ-Cz (E ox of 0.71 V).
We next investigated to electrochemistry of the emitters. The CV of 2CzPN shows an irreversible oxidation wave (Figure S5) with E ox at 1.54 V. The calculated HOMO at -5.87 eV reproduces that previously reported (EHOMO = -5.88 eV). 20 The irreversible oxidation wave (Figure S5) for Cz-2CzPN is cathodically shifted at E ox of 1.28 V due to the stronger donor. There are two observed oxidation processes in the DPV for t BuDPA-2CzPN (Figure 4b). The first oxidation peak potential is significantly cathodically shifted at 0.72 V compared to that of Cz-2CzPN, reflecting the much stronger donor character of the t BuDPA-Cz donor dendron. The E ox for t BuDPA-2CzPN is anodically shifted compared to t BuDPA-Cz due to the conjugation with the electron-accepting phthalonitrile. The reversible oxidation ( Figure S7) of PXZ-2CzPN occurs almost exclusively on the PXZ moieties, with an E ox of 0.80 V, which is slightly anodically shifted compared to the oxidation potential of PXZ-Cz (E ox of 0.71 V). This assignment is supported by DFT calculations that reveal that the HOMO density resides on the PXZ units (Figure 2). The increasing donor strength is largely reflected in the oxidization potentials, with E ox of 1.54 V, 1.28 V, 0.72 V and 0. 80 V for 2CzPN, Cz-2CzPN, t BuDPA-2CzPN and PXZ-2CzPN, respectively.
The relative order of the oxidation potentials reflects the interplay between the degree of conjugation between the peripheral donors and the central carbazole and the strength of the peripheral donors.
Reversible reduction waves are observed for all four emitters, assigned to the reduction of the phthalonitrile acceptor ( Figure S9). The reduction potentials are more catholically shifted for 2CzPN and t BuDPA-2CzPN with E red of -1.46 V and -1.43 V, compared to those of Cz-2CzPN and PXZ-2CzPN (E red of -1.36 V and -1.30 V, respectively, Figure S9). This trend is corroborated by DFT calculations (Figure 2). We attribute these trends to an increased electronic interaction between the donors and  eV, 33 where E ox and E red are the anodic and cathodic peak potentials, respectively measured from DPV relative to Fc/Fc + .

Photophysics in solution and film
The photophysical data is collated in Table 2. We first consider the compounds in dilute toluene solution (0.05 mg/mL, corresponding to 10 -4 -10 -5 M, depending on compound) where intermolecular interactions can be expected to be supressed. This should provide a clear photophysical understanding related to monomolecular processes. All compounds show a broad, unstructured, low intensity absorption peak at the low energy side of the absorption band, with the peak positions indicated in Table 2. Along the series, the absorption maxima shift to lower energy, consistent with the trends observed for the HOMO-LUMO gap found from electrochemistry, and they reduce in intensity ( Table 2). We assign these bands to a transition with a predominant CT character between the electron-rich donors and the electron-poor  Table 2.   Figure S10). The absorption band associated with the phthalonitrile acceptor is located at even higher energies with peaks from 4.6 eV (270 nm) onwards. 36 Regarding the emission in toluene, all steady-state spectra are broad and structureless, characteristic of emission from a state with a dominant CT character. The photoluminescence maxima shift to lower energies across the family of derivatives, analogous to the shifts in the absorption maxima of the lowenergy CT bands and in alignment with the TDA-DFT predicted S1 energies. ΔEST was determined from the measurements of fluorescence and phosphorescence spectra taken at the same temperature, namely 77 K, to exclude different temperature-dependent energy shifts of the two emissions. This is shown in Figure 6a, with the room temperature steady-state emission added for comparison. The steady-state emission at 77 K contains contributions from the prompt emission and delayed emission, which is mostly phosphorescence and, except for t BuDPA-2CzPN, a negligible contribution of delayed fluorescence. For t BuDPA-2CzPN, the delayed fluorescence was found to be still competing in intensity with phosphorescence at 77 K, so that ΔEST was determined from the 5 K measurements. The decrease in temperature leads to a blue-shift of the steady-state emission compared to its room-temperature spectrum. We attribute this to the freezing of the solvation shell at low temperatures that prevents reorganization of the solvent molecules after photoexcitation of the emitter molecules. 37 To obtain the phosphorescence spectrum without contribution from fluorescence, we recorded the emission with detection in the millisecond range (see ESI for details). Table 3 below summarizes the energies obtained for the onsets of the singlet and triplet states along with the singlet-triplet gaps that are determined from them. Table 3. Singlet and triplet energies in toluene solution and mCP film (10 wt%).

BuDPA-2CzPN
Energy (eV) To understand how the photophysical properties in solution transfer into film, we next investigated the compounds in 10 wt% mCP film [mCP = 1,3-bis(N-carbazolyl)benzene], where mCP is a host matrix with sufficiently high triplet energy (2.90 eV) to ensure confinement of the triplet excitons on the emitter. 39 Figure 6b shows the thin film PL emission for the series at 300 K and 77 K (5 K for t BuDPA-2CzPN). We find the same trend in the singlet-triplet gap as in solution with ΔEST decreasing along the series (within the experimental error margin of 10 meV, Table 3). In contrast to solution, the roomtemperature steady-state spectra almost coincide with the low-temperature steady-state spectra. We attribute this to the absence of solvent reorganization after excitation in the solid matrix. 37 It implies that the spectra and energy gaps obtained at low-temperature represent the room temperature situation well.
For pronounced TADF, a small ΔEST value is a necessary but not a sufficient condition. To evaluate the potential of a molecule as a TADF emitter, we also need to consider the relevant rates of radiative and non-radiative decay, as reflected in the PLQY, delayed emission lifetimes, and the contribution of the TADF to the total emission. With a view to their use as emitters in OLEDs, we focus the discussion on the TADF-related parameters in film, comparing with the solution data wherever relevant. Figure 6c shows the transient PL decays for the compounds in 10 wt% mCP film. The analogous data for solution are given in the ESI, along with a complete list of parameters used in the mono-exponential fittings for transient decay profiles in solution and film ( Figure S11, Table S7). The photophysical parameters obtained are summarized in Table 4 for solution and Table 5 for films.
For both solution and film, we can identify two regimes in the PL decay, that is, a prompt decay in the nanosecond range and a delayed emission that takes place in the microsecond range. We can safely attribute the delayed emission to TADF, based on the small ΔEST gap and the increase of the emission in the microsecond range with increasing temperature. From the temperature dependence of the 21 delayed fluorescence (DF) in the film we derive the activation energy for TADF (Table 5, Figure S12).
As with the singlet-triplet gap, the activation energy also decreases along the series. The lower or comparable activation energy than the ΔEST measured from the steady-state emission spectra together with the prediction from TDA-DFT calculations of closely-lying T1, T2 and S1 point to the involvement of higher-lying triplet states (Table S2). We note that the difference between ΔEST and the measured activation energy is strongest for 2CzPN, which is the compound with the highest LE contribution in T2 which is expected to lead to the highest spin-orbit coupling with the S1 CT state. (a) ΔEST = S1 -T1. Estimated from the onsets at 77 K (5 K for t BuDPA-2CzPN) as in Table 3.
(b) Total PLQY at 300 K measured in the integrating sphere under N2. exc = 355 nm.
(c) Ratio between the integrated area under the delayed and prompt emission at 300 K.
(d) Prompt emission lifetime at 300 K extracted from a mono-exponential fitting ( Figure S11).
(e) Delayed emission lifetime at 300 K extracted from a mono-exponential fitting ( Figure S11).
(f) Constant of radiative decay rate extracted using r S = PF/ DF.
(b) Total PLQY at 300 K measured in the integrating sphere under N2. exc = 355 nm.
(c) Ratio between the integrated area under the delayed and prompt emission.
(d) Prompt emission lifetime at 300 K extracted from a mono-exponential fitting ( Figure S11).
(e) Delayed emission lifetime at 300 K extracted from a mono-exponential fitting ( Figure S11).
(f) Constant of radiative decay rate extracted using r S = PF/ DF.
We note that for Cz-2CzPN and t BuDPA-2CzPN the radiative decay rate from the singlet state r S is only slightly reduced relative to the parent compound, while it is significantly reduced for PXZ-2CzPN, indicating a too strong decoupling of the hole and electron wavefunctions for this compound. This is consistent with both the low calculated oscillator strength and the low PLQY in PXZ-2CzPN. For t BuDPA-2CzPN, the ΔEST is small and a high radiative rate from the CT singlet is maintained, yet a high non-radiative CT singlet decay rate precludes a high photoluminescence quantum yield. This finding is consistent with the work of Yang et al., [40][41] where they reported a diphenylsulfone core linked to acridine-based donor dendrons. When they attached diphenylamine or carbazole as peripheral donor groups they also found a smaller gap and lower PLQY in toluene for the former (40 meV, 12%) than for the latter (90 meV, 68%). [40][41] Despite the small ΔEST, we observe very little TADF in solution except for Cz-2CzPN (c.f. column 3 in Table 4, In the film, the rigid matrix removes this collisional quenching mechanism, so that TADF has a stronger contribution to the overall PLQY. Nevertheless, the ratio between the integrated emission intensities of delayed and prompt fluorescence is less than 4 for all compounds, including the parent 2CzPN, so that the commonly used approximation forwarded by Dias et al. to calculate the RISC rate does not apply. 43 An assessment of the efficiency of RISC is still possible by considering the DF/PF ratio (PF = prompt fluorescence) and the DF lifetime together. As summarized in Table 5, the three 2CzPN derivatives show a shorter DF lifetime than the parent compound. While this by itself could indicate a faster RISC rate, the lower DF/PF ratios in the extended compounds indicate that there must be an additional dominant process present. We conclude that the shorter lifetime of the DF is thus rather associated with a quenching channel for the triplet states from which the DF is fed. We confirmed this by measuring the phosphorescence lifetimes. The triplet lifetime of t BuDPA-2CzPN and PXZ-2CzPN at 5 K is about 50 and 1000 times shorter than that for the parent 2CzPN. For Cz-2CzPN, it is comparable with 2CzPN at 5K, yet decreases more strongly upon heating, for example, to 80 K, thus also indicating 24 thermally activated non-radiative decay channels ( Table 5, and Figure S13). The nearly identical triplet energy of 2CzPN and Cz-2CzPN excludes the energy-gap law as the origin. Most likely, the shorter triplet lifetime results from the higher conformational degrees of freedom in the three 2CzPN derivatives, so that deactivation by torsional modes is facilitated. Overall, we find that employing donor dendrons indeed reduces the singlet-triplet gap, and concomitantly the activation energy for TADF. However, in the film this advantage is compromised by an increased non-radiative decay from the triplet state, which occurs in different degrees for the different extensions.

Concluding discussion
While employing donor dendrons has been explored before as a means to increase the TADF yield, often with the same donor units used throughout the dendron, there are few investigations into the relationship between the nature of the donor dendron and the reduction in the singlet-triplet gap. 44 In our work, we synthesized a series of molecules based on the TADF-active 2CzPN where we increased the hole-localization through suitable decoration of the carbazole donors. Our results show that the singlet-triplet gap can be reduced while maintaining a sufficiently high oscillator strength for efficient emission. Indeed, Cz-2CzPN demonstrates the most suitable balance in charge distribution where in solution this compound significantly exceeds the PLQY and TADF properties of the parent 2CzPN.
However, we also find that, in the film, a high non-radiative rate of the triplet state prohibits a stronger role of TADF in the extended donor molecules despite a reduced gap between the singlet and triplet state. Our results imply that the approach of fine-tuning the donor dendron strength indeed can improve the TADF emission, yet for implementation, this still requires more control over the nonradiative decay of the triplet state in the film by, for instance, ensuring that the donor dendrons are sufficiently rigid.

Methods
We prepared solutions of 0.05 mg/mL in toluene (corresponding to 10 -4 -10 -5 M depending on molecular weight) as well as spin-coated 10 wt% films in mCP and measured them at different temperatures in steady-state mode using a Jasco FP-8600 spectrofluorimeter. In time-resolved mode they were measured using an iCCD camera (Andor iStar A-DH334T-18F-03) by exponentially increasing delay and gating times where the gating time is kept lower by 10 times compared to the delay time. For the steady-state mode, variable excitation wavelengths were used (see figure captions) while for the iCCD measurements samples were excited at 355 nm. The quantum chemical calculations were performed using the Gaussian 09 (revision D.0184) suite for the DFT. 45 Molecular structures were optimised in the ground state geometry using PBE0 46 functionals each employing the 6-31G(d,p) basis set with dispersion effects included. 47 Excited state calculations were performed within the Tamn-Dancoff approximation (TDA) at the same level of theory. 48 Full experimental and computational details as well as the information on the synthesis and chemical characterization are provided in the ESI.