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Optically Triggered Planarization of Boryl Substituted Phenoxazine: Another Horizon of TADF Molecules and High Performance OLEDs Deng-Gao Chen, Tzu-Chieh Lin, Chi-Lin Chen, Yi-Ting Chen, Yi-An Chen, Gene-Hsiang Lee, Pi-Tai Chou, Chia-Wei Liao, Po-Chen Chiu, Chih-Hao Chang, Yi-Jyun Lien, and Yun Chi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00053 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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ACS Applied Materials & Interfaces

Optically Triggered Planarization of Boryl Substituted Phenoxazine: Another Horizon of TADF Molecules and High Performance OLEDs

Deng-Gao Chen,a Tzu-Chieh Lin,a Chi-Lin Chen,a Yi-Ting Chen,a Yi-An Chen,a Gene-Hsiang Lee,a Pi-Tai Chou*,a Chia-Wei Liao,b Po-Chen Chiu,b Chih-Hao Chang,*,b Yi-Jyun Lien,c and Yun Chi,*,c,d a

Department of Chemistry, National Taiwan University, Taipei, 10617 Taiwan

b

Department of Photonics Engineering, Yuan Ze University, Chungli 32003, Taiwan

c

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan

d

Department of Materials Science and Engineering and Department of Chemistry,

City University of Hong Kong, Hong Kong SAR

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Keywords: thermally activated delay fluorescence, N-donor, borane, phenoxazine, charge transfer, solvatochromism, structural relaxation

Abstract: We report the unprecedented dual properties of excited-state structural planarization and thermally activated delay fluorescence (TADF) of 10-dimesitylboryl phenoxazine, i.e., PXZBM. Bearing a non-planar phenoxazine moiety, PXZBM shows the lowest lying absorption onset at ∼390 nm in nonpolar solvents such as cyclohexane but reveals an anomalously large Stokes shifted (∼ 14500 cm-1) emission maximized at 595 nm. In sharp contrast, when a phenylene spacer is added between phenoxazine and dimesitylboryl moieties of PXZBM, the 10-(4-dimesitylborylphenyl) phenoxazine PXZPBM in cyclohexane reveals a much blue-shifted emission at 470 nm despite its red-shifted absorption maximized at 420 nm (cf. PXZBM). The emission of PXZBM further reveals solvent polarity dependence, being redshifted from 595 nm in cyclohexane to 645 nm in CH2Cl2. For rationalization, the steric hindrance between phenoxazine and the dimesitylboryl unit in PXZBM caused a puckered arrangement −1− ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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of phenoxazine at the ground state. Upon electronic excitation, as supported by the femtosecond early relaxation dynamics, spectral-temporal evolution and energetics calculated along the reaction potential energy surfaces, the diminution of N→B electron transfer reduces π-conjugation and elongates the N-B bond length, inducing the fast phenoxazine planarization with a time constant of 890 ± 100 fs. The associated charge-transfer reaction from phenoxazine (donor) to dimesitylboryl unit (acceptor) results in a further red-shifted emission in polar solvents. In stark contrast, PXZPBM shows a planar phenoxazine and undergoes excited-state charge transfer only. Despite the distinct difference in excited-state relaxation dynamics, both PXZBM and PXZPBM exhibit efficient TADF capable of producing highly efficient orange and green OLEDs with peak efficiencies of 10.9 % (30.3 cd∙A-1 and 18.7 lm∙W-1) and 22.6% (67.7 cd∙A-1 and 50.0 lm∙W-1). -------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction Recently, the strategy of using boron as either a bridging atom or terminal substituent to spatially separate electron donating (D) and accepting (A) moieties has been pervasive in the preparation of thermally activated delayed fluorescence (TADF) emitters for high performance organic light emitting diodes (OLEDs).1-11 On the one hand, this is mainly due to the existence of lower energy, the vacant 2p orbital on a typical trivalent boron atom, which allows this fragment to act as an efficient acceptor unit. In addition, the mesityl substituents on the boryl group also take part in stabilizing the D-A architecture through larger steric encumbrance, affording the kinetic stability needed for the OLED emitter.12-13 As a result, upon photo-induced charge transfer (CT), HOMO and LUMO can be effectively separated to reduce the electron exchange energy.14-16 The resultant small singlet-triplet energy gap leads to thermal reversibility between singlet and triplet states such that efficient TADF can be achieved. On the other hand, incorporating a pyridyl or heteroaromatic substituent into this trivalent boryl fragment gives tetravalent boron-containing −2− ACS Paragon Plus Environment

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molecules, in which the π*-orbital of the added pyridyl or heteroaromatic unit replaces the vacant p orbital of the trivalent boryl group to serve as the acceptor unit.8-9,

17-18

Hence, both the trivalent boryl compounds and tetravalent,

heteroaromatic coordinated boron-containing compounds are ideal platforms for constructing TADF emitters, showing superiority as well as versatility in the chemical derivation and functionalization of boryl compounds. In yet another approach, the structural relaxation of molecules in the excited state has been attracting much attention due to its fundamental importance in probing the transient species and hence possible multiple emissions along the potential energy surface. For example, a recent advance has shown that saddle-shaped N,N-disubstituted-dihydrodibenzo[a,c]phenazines undergo structural planarization in the excited state, which, accompanied by various degrees of charge transfer amid structural relaxation, gives rise to multiple emission bands suitable for white light generation in a single molecule identity.19-20 Since the prerequisite of TADF requires excited-state D-A charge transfer as well as spatial separation in terms of distance and orientation, structural relaxation may add another dimension to broaden the horizon for the chemical derivatization and spectral tunability of TADF molecules. Even though many phenothiazine-based TADF molecules have been reported to exhibit a non-planar skeleton at the ground state,21-26 correlation for photophysical behavior versus structural relaxation has yet to be reported. This leads us to consider the possibility of geometry relaxation that alters the relative spatial D-A separation, which may couple with D-A charge transfer and consequently channel into the TADF spectroscopy and dynamics.

Scheme 1. Structural drawing of PXZBM and PXZPBM. −3− ACS Paragon Plus Environment


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We herein report on the properties of 10-dimesitylboryl phenoxazine, PXZBM (Scheme 1), bearing a non-planar phenoxazine framework at the ground state, which shows the unique feature of excited-state planarization. This feature, together with D-A charge transfer, leads to a drastic solvatochromism with prominent TADF behavior. Conversely, if a phenylene spacer to PXZBM is added between the dimesitylboryl and phenoxazine moieties, the resulting 10-(4-dimesitylborylphenyl) phenoxazine PXZPBM (Scheme 1) reveals a planar phenoxazine, which only undergoes photoinduced charge transfer. For PXZBM, it turns out that the steric constraint imposed on the phenoxazine plays a key role that accounts for this photoinduced planarization effect. Elaborated below are details of spectroscopy, relaxation dynamics and a computational approach, together with their associated high-performance TADF OLEDs, to shed light on these unique photophysical properties.

2. Results and Discussion Structure analyses The required boryl compounds PXZBM and PXZPBM were prepared in a single-step

procedure

that

involved

treatment

of

phenoxazine

and

10-(4-bromophenyl)-10H-phenoxazine with n-BuLi, followed by the addition of dimesitylfluoroborane to yield PXZBM and PXZPBM, respectively. The procedure is akin to that reported for the phenothiazine and acridine derivatives.10, 27 Synthetic details and the structural and spectroscopic data are provided in the supporting information. As can be seen in Figure 1, the boron centers in PXZBM and PXZPBM display a trigonal planar arrangement; i.e., the sum of bond angles around B(1) atom is close to 360°, as shown by X-ray structural analyses. As for PXZBM, although the B-C distances of the dimesitylboryl unit are similar in length to the BPh2 unit in the tetravalent boron compounds reported in the literature, the recorded B-N distance of 1.438(2) Å is akin to those observed in N-borylated pyrroles (1.442 Å ∼ 1.472 Å),28 −4− ACS Paragon Plus Environment

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but shorter than the B-N bond (∼1.64 Å) of the tetravalent boryl compounds.9, 29-30 This can be rationalized by the increased B-N bond strength exerted by the greater s-character of the sp2-hybridized, trivalent boron atom vs. that of sp3-hybridized boron compounds. Moreover, as depicted in Figure 1, the O(1)⋅⋅⋅N(1)-B(1) nonbonding angle (ф) and bending angle (Θa) between the benzo groups of phenoxazine were recorded to be 142.70°and 146.39°, respectively, indicating a nonplanar and saddle-like skeletal arrangement for phenoxazine. At the same time, the torsional angle Θb, i.e. the angle between the planes C(12)-N(1)-C(1) and C(13)-B(1)-C(22), was recorded to be 25.20°. Such a nonplanar distortion plausibly originates from the accumulated steric interaction between phenoxazine and the dimesitylboryl unit. Similar situation was also

noted

for

overcrowded

bistricyclic

aromatics31

and

folded

fluorenylidene-acridanes.32 Concurrently, adding a phenylene spacer between phenoxazine and dimesitylboryl moieties to form PXZPBM lifts the steric constraint. As a result, PXZPBM shows a nearly planar phenoxazine with a di-benzo bending angle (Θa) of 171.23° (see Figure S1 and S2). Figure 2 reveals the absorption and emission spectra of PXZBM and PXZPBM in various solvents. The absorption onset of PXZBM appeared at ∼390 nm in all solutions, and the absorption extinction coefficient gradually increased upon shifting to the shorter wavelength. In sharp contrast, the onset of PXZPBM occurred in a much lower energy region, near 450 nm, and a distinct absorption band appeared with a peak maximum at ∼420 nm. As supported by the later computational approach (vide infra), the lowest lying transition for both PXZBM and PXZPBM is assigned to a charge-transfer (CT) transition from the electron donor (phenoxazine) to dimesitylboryl or 4-dimesitylborylphenyl acceptor (-B(mes)2 or -C6H4B(mes)2). Despite the nearly solvent-independent absorption profile, the corresponding emission is subject to great changes. As shown in Figure 2a, in cyclohexane, PXZBM exhibits an emission band maximized at 595 nm, which is red-shifted by ∼14500 cm-1 related to the absorption peak maximum at ∼320 nm. Additional red shift of PXZBM −5− ACS Paragon Plus Environment


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was observed upon increasing the solvent polarity. For example, while the absorption onset remained unchanged, the emission maximum of PXZBM occurred at 618 nm in toluene, 645 nm in CH2Cl2 (see Figure 2a), and down to ∼656 nm in acetonitrile. Although the decrease in the emission energy gap upon increasing the solvent polarity can be ascribed to photoinduced charge transfer, followed by solvent relaxation, which is commonly dubbed as emission solvatochromism, the origin of the ∼14500 cm-1 Stokes shift for the emission in cyclohexane cannot be attributed to the

solvent

dipolar

relaxation.

This

viewpoint

is

evidenced

by

the

Lippert-Mataga-Plot, in which the difference between absorption and emission peak energies (Δν) in cm-1, for a CT compound can be a function of solvent dielectric constant ɛ, refractive index n, molecular size (a3, a is the radius of the molecule, assuming a spherical shape) and the difference between ground and excited state static dipole moments, denoted as μg and μe, respectively, expressed below (eq. (1)): 2  ε −1 n2 − 1  ( µ g − µe ) ∆ν = − + constant   hc  2ε + 1 2n 2 + 1  a3 2

(1)

whereas the expression “(ɛ‒1)/(2ɛ+1) − (n2‒1)/(2n2+1)” is defined as the solvent polarity parameter Δf. In theory, the other term “2(μg − μe)2/hca3” is independent of solvent polarity. Accordingly, the plot of Δν as a function of Δf should result in a straight line if Δν is only affected by solvent polarity. Conversely, the data shown in Figure 3a clearly indicate a different situation, in which the result in cyclohexane reveals a large deviation from the plot for the rest of the data recorded in polar solvents. If we simply assume Δν in cyclohexane to be an offset and subtract it from Δν obtained in other solvents, the Lippert-Mataga-Plot is linear (Figure 3b). According to eq. (1), Figure 3b gives a change of dipole moment of 11.9 Debye between ground and excited states, manifesting the lowest lying CT state for PXZBM, and the solvatochromic effect, in part, plays a role in the observed emission shift. Clearly, in addition to the emission solvatochromism originating from the CT property, another excited-state relaxation pathway must take place in PXZBM to

−6− ACS Paragon Plus Environment

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account for the anomalously large Stokes shifted emission even in nonpolar solvent. In light of this, the nonplanar phenoxazine in PXZBM identified by X-ray structural analyses leads us to propose the possibility of phenoxazine planarization in the excited state. Support of this viewpoint is given by the 495 nm emission for PXZBM in the crystalline state, which is blue shifted as much as ∼3395 cm-1 compared with 595 nm emission in cyclohexane. Firm fundamental support is elaborated in the section of excited-state relaxation dynamics and computational approach (vide infra). In yet another approach, PXZPBM, for which phenoxazine and dimesitylboryl moieties are separated by a phenylene spacer, exhibits a 470 nm emission in cyclohexane that is mirror imaged with respect to the lowest-lying 420 nm absorption band. The emission peak wavelength is also subject to the solvent polarity, being red-shifted from 470 nm in cyclohexane to 650 nm in acetonitrile (Figure 2b). As shown in Figure 3c, the Lippert-Mataga-Plot is a straight line, indicating that the shift of the CT emission is governed solely by the solvent relaxation. From a molecular structure point of view, the addition of phenylene in PXZPBM may reduce both the N→B dative bonding and steric hindrance between phenoxazine and dimesitylboryl moieties, resulting in relief of the strain energy imposed on phenoxazine. Evidence is given by the X-ray structural analysis (see Figure 1), in which the phenoxazine moiety in PXZPBM is nearly planar, as opposed to the non-planar conformation in PXZBM. Upon excitation, PXZPBM undergoes CT reaction only, followed by solvent relaxation to exhibit emission solvatochromism. For PXZPBM, the best linear fit of the Lippert-Mataga-Plot, shown in Figure 3c, renders a change of dipole moment vector (∆ߤԦ) of ∼23 Debye between the ground and excited states. In comparison to PXZBM, it is predictable that PXZPBM undergoes a larger change of dipole moment in the excited state. Under the same electron D-A strength and charge separation ∆q, in a qualitative manner, the distance of charge separation v r is expected to be longer in PXZPBM due to the added phenylene, resulting in a v larger change of dipole moment ∆q × r .

−7− ACS Paragon Plus Environment


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Thermally activated delay fluorescence Except for PXZBM and PXZPBM in acetonitrile, as shown in Figure 4 and Figures S3-S4, both PXZBM and PXZPBM exhibit prominent TADF, consisting of a prompt (several tens of nanoseconds) and a long (several microseconds) decay component. Representative cases are shown in Figures 4a and 4b for PXZBM and PXZPBM in CH2Cl2 solution, and their decay dynamics are fitted to be 3.86 ns (prompt), 3.05 μs (delay) and 20.0 ns (prompt), 0.89 μs (delay), respectively. Figures S3 and S4 show the results for other solvents, and Table S1 lists all pertinent photophysical data. Qualitatively, under the assumption of equilibrium between S1 and T1 states, the difference in energy ΔET-S (defined as T1-S1) can be estimated according to the relationship between ΔET-S and equilibrium constant Keq, expressed as ΔET-S = -RTln(Keq/3), where the equilibrium constant Keq can be deduced from the pre-exponential factors for prompt fluorescence versus delayed components. The results (see Table S1) deduce a ΔET-S (T1-S1) value in various solvents ranging from -4.8 to -0.7 kcal/mol, showing effective thermal reversibility between S1 and T1. Note that the results show an increase of ΔET-S upon increasing the solvent polarity. This trend predicts ΔET-S to be more negative in a polar solvent such as acetonitrile, which may prohibit the thermally activated reverse intersystem crossing T1 → S1, evidenced by the lack of TADF in acetonitrile for both PXZBM and PXZPBM. Further support of a small ΔET-S is provided by the time-resolved PL spectra of these two samples in 77 K CH2Cl2 matrix. As shown in Figure S5, the prompt emission component acquired by an intensified charge coupled detector (ICCD) at a zero delay time and a gate width of 10 ns is assigned to fluorescence, while the component acquired at a delay time of 1 ms and gate width 100 μs is ascribed to the phosphorescence. The proximity of spectral onset between fluorescence and phosphorescence was apparent, and ΔET-S is calculated to be -0.2 and -0.7 kcal/mol for PXZBM and PXZPBM, respectively, which is on the same order of magnitude as that in solution at room temperature. Last but not the least, also shown are the similar TADF characteristics of PXZBM and PXZPBM obtained in neat film (Figures 4c and 4d) as well as in doped thin solid −8− ACS Paragon Plus Environment

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film (vide infra). These, together with high emission quantum yield of > 50 % in thin solid film, leads to excellent OLED performance for both PXZBM and PXZPBM (vide infra).

Excited-state structural relaxation for PXZBM In this section, we focus on the relaxation dynamics of PXZBM probed by the femtosecond

fluorescence-upconversion

technique.

Because

the

delayed

fluorescence has revealed slow relaxation kinetics up to several microseconds, the experiment using high repetition-rate femtosecond laser led to serious repeated signal accumulation. Instead, a low repetition rate 1 kHz femtosecond laser system was used as the pump-probe source for the fluorescence up-conversion measurements (see supporting information for detail). Figure 5a and Table S2 show the relaxation dynamics of PXZBM at various emission wavelengths in cyclohexane (λex = 380 nm). Upon monitoring at a short wavelength region, e.g. 460 nm, where negligible steady-state emission intensity was observed, the emission dynamics consisted of a resolvable decay component fitted to be 890 ± 100 fs, followed by a relatively slower, small-amplitude decay component of several picoseconds (∼13 ps) and a very long population decay of ∼14 ns (see Figure S6). Upon increasing of the emission wavelength, the pre-exponential value (at t = 0) of the 890 ± 100 fs decay component gradually decreased, accompanied by the growth of a long decay component. Monitoring at the tail of the emission such as ∼700 nm revealed an apparent rise component prior to the long decay, which was fitted to be ∼890 ± 100 fs. Within the experimental error, the 890 fs decay of the 460 nm emission correlates well with the rise of 700 nm emission, indicating a precursor (reactant)-successor (product) type of reaction pattern. The small but non-negligible decay of ∼13 ps in the short wavelength region should be the result of vibrational relaxation. The rate of vibronic relaxation in nonpolar solvents, depending on the energy gap, is normally on a time scale of a few to tens of picoseconds.33-36 Unfortunately, upon monitoring at the long wavelength, we were not able to resolve this ∼13 ps rise dynamics −9− ACS Paragon Plus Environment


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associated with vibrational relaxation. This is plausibly due to the cancellation between the vibrational relaxed rise and decay intensity at long emission wavelengths, which has been a common phenomenon.33-36 We also performed a fluorescence upconversion experiment for PXZBM in toluene (the solubility of PXZBM is sparse in high polar solvents). The results in toluene, shown in Figure S7 and Table S2, give a time constant of ∼950 ± 100 fs, presumably for the structural relaxation plus polar solvent relaxation, which is on the same magnitude as 890 fs in the cyclohexane. The solvent relaxation dynamics has been well established during the past three decades. For a low viscous solvent like that of toluene, the solvent relaxation time constant is around subpico- to pico-seconds, which is nearly indistinguishable from that of structural relaxation for PXZBM obtained in cyclohexane. Therefore, it is believed that PXZBM undergoes structural coupled solvent relaxation in polar solvents, giving rise to further red-shifted emission in addition to the structurally relaxed Stokes shifted emission. We then integrated the emission intensity at the acquired wavelengths for each specific time interval and then plotted the temporal emission spectra as a function of delay time on the time-scale of 0 - 100 ps. The results shown in Figure 5b clearly reveal a gradual red shift of the emission for PXZBM in cyclohexane, which corresponds to the structural relaxation toward the energy minimum. Note that in this study, we intentionally excited PXZBM at the onset of the lowest lying absorption; therefore, the spectral temporal evolution contributed to the vibrational relaxation has been minimized.

Computational approach Computational efforts were then made to gain insight into the transition properties and to support the experimental results. Based on the density functional theory (DFT) and time-dependent (TD)-DFT calculation (see the experimental section for details), the lowest singlet optical transition S0 → S1 is assigned to HOMO → LUMO for both PXZBM and PXZPBM (Figure 6). The electron density distributions in − 10 − ACS Paragon Plus Environment

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HOMO and LUMO are mainly localized at phenoxazine (donor) and dimesitylboryl (acceptor) fragments, respectively, manifesting the charge transfer property in the lowest lying excited state. In sharp contrast to the previously reported tetravalent boron complexes,8-9 for which the boron atom has a rather small contribution in LUMO (< 1 %), the boron atom of these two studied dimesitylboryl complexes has a much greater contribution in the corresponding LUMO (> 38%). In other words, being quite different from the cases of tetravalent boron complexes, in which the boron atom acts virtually as a bridge in supporting donor and acceptor, the boron atom in the current complexes PXZBM and PXZPBM serves virtually as part of an acceptor fragment. Despite the sameness of the charge transfer properties of PXZBM and PXZPBM, the distinct difference lies in the structure of the phenoxazine moiety. In the full geometry optimized PXZBM in the ground state, shown in Figure 6a, the phenoxazine is in a nonplanar configuration, in which the bending angle Θa between two half-phenoxazine benzo groups is calculated to be ∼144°, which is similar to that obtained from the X-ray structural analysis. Upon full geometry optimization in the excited state, the phenoxazine moiety reaches a planar configuration with a bending angle Θa of 179° in vacuum and in cyclohexane and other solvents used in this study. The computational result indicates the occurrence of excited-state structural relaxation for PXZBM, accompanied by energy minimization along the phenoxazine planarization coordinate. The latter is evidenced by the calculated S0 → S1 Franck Condon absorption of 274 nm versus the anomalously long wavelength 510 nm for S1 → S0 Franck-Condon emission, in which the calculated emission gap is consistent with the onset (∼500 nm) of the experimentally observed 580 nm (peak wavelength) emission for PXZBM in cyclohexane. In stark contrast, as shown in Figure 6b, the phenoxazine of PXZPBM holds the planar configuration (179°) in both ground and excited states. As a result, the calculated S1 → S0 Franck-Condon transition of 420 nm in cyclohexane is much higher in energy than that (510 nm) calculated in PXZBM, despite the much lower energy of the S0 → S1 transition (350 nm) in PXZPBM versus − 11 − ACS Paragon Plus Environment


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that (274 nm) in PXZBM. Again, the calculated 420 nm of the S1 → S0 Franck-Condon transition for PXZPBM matches well the onset of ∼430 nm of the observed PXZPBM emission in cyclohexane, supporting the validity of the current computational methodology. Realizing that the structural relaxation of organoboryl compounds may involve multidimensional pathways,19,28,37-38 we then conducted an investigation by scanning the potential energy surface (PES) along the bending angle Θa, B-N distance, torsional angle Θb between the planes C(12)-N(1)-C(1) and C(13)-B(1)-C(22) (see Figure 1), followed by the full geometry optimization to gain more insight into the structural relaxation. Figure 7 shows a 2-dimensional (2D) variation of PES along Θa and B-N bond distance, which clearly indicates that upon Franck-Condon excitation of PXZBM the energy minimum relaxation pathway is along the increase of both Θa and B-N distance (see black solid line). Particularly, Θa changes significantly from 145° to the 179°, accompanied by the increase of B-N distance from 1.44 Å in the ground state to 1.56 Å in the Franck Condon excited state (Figure 8a), and a change of torsional angle Θb from 23° to 62° in the excited state after optimization (see Figure 6a and Figure S8). Therefore, the excited-state structure relaxation of PXZBM involves the B-N bond elongation together with both planarization and twisting of the dimesitylboryl fragment and phenoxazine moiety. The result fits well to the chemical interpretation, in which the electron is donated from the nitrogen lone pair to the empty p-orbital of boron atom at the ground state. It then induces the excessive congestion between phenoxazine and dimesitylboryl unit and hence causes the puckered arrangement of phenoxazine in PXZBM. Upon electronic excitation, the B-N bond elongates and releases the strain energy, such that the phenoxazine relaxes to the planar structure, concomitantly accompanied by change of torsional angle Θb between the phenoxazine and dimesitylboryl unit. For comprehension, the calculations were performed in S0 (black), S1 (blue), and S0 states (red) that were calculated from the direct S1 → S0 transition. The results − 12 − ACS Paragon Plus Environment

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shown in Figure 8b clearly indicate that PES in the S1 state decreases with increases in Θa and achieves the minimum energy at ∼180°. Oppositely, the PES of the geometry optimized S0 increases as Θa increases from 144° to 180°. Moreover, the PES of the S0 state (red) calculated from the direct S1 → S0 transition holds nearly flat, which is nevertheless higher than that of the geometry optimized S0 by 5-10 kcal/mol (see Figure 8b). The decrease in S1 energy with a simultaneous increase in S0 energy leads to the anomalously large Stokes shifted PXZBM emission observed in nonpolar solvents. Because the emission spectra of PXZBM in solid and thin film are significantly blue shifted compared with the emission in all solvents (see Figure 2a) the excited-state structural relaxation that requires large amplitude motion does not seem to take place. The less blue shift of the emission in spin-casted thin film (cf. in the solid state) may be due to the amorphous phase and hence different environment perturbation (lattice energy and proximal dipole-dipole interaction, etc.) from that in the solid state.

OLED fabrication and performance Both PXZBM and PXZPBM were used as dopants to investigate their electroluminescence (EL) performance. Three potential wide-bandgap materials combined with the emitters were examined to achieve adequate host-guest exothermic

energy

transfer

4,4'-N,N'-dicarbazolebiphenyl (26DCzppy),40-41

and

as

well

(CBP),39

as

carrier

balance.

They

were

2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine

3-bis(9-carbazolyl)benzene

(mCP).42

Furthermore,

1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC)43-44 was adopted as the hole transport layer (HTL) because of its high carrier transport capability and high triplet energy gap of 2.87 eV. In addition, MoO3-doped TAPC was adopted as a hole injection layer (HIL) to decrease the energy barrier between the ITO anode and the organic layer.45 On

the

other

hand,

3,5,3’,5’-tetra(m-pyrid-3-yl)-phenyl[1,1’]biphenyl

(BP4mPy)46 and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB)47 were used as the − 13 − ACS Paragon Plus Environment


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electron transport layer (ETL) to optimize the carrier balance. In addition, the thicknesses of the HIL, HTL, ETL and ITO layers were carefully regulated to enhance radiation output. Our preliminary tests revealed that the 26DCzPPy host could achieve the optimal carrier balance and be suitable for both emitters. Furthermore, BP4mPy was the appropriate ETL for PXZBM, while TmPyPB was proper for PXZPBM. Consequently, the optimal architecture consisted of ITO (90 nm)/ TAPC doped with 20 wt.% of MoO3 (20 nm)/TAPC (30 nm)/26DCzPPy and x wt.% PXZBM (30 nm)/ BP4mPy (40 nm)/ LiF (0.8 nm)/Al (150 nm) for device A; meanwhile, the optimal structure of device B was set to ITO (70 nm)/ TAPC doped with 20 wt.% of MoO3 (20 nm)/TAPC (70 nm)/26DCzPPy and x wt.% of PXZPBM (20 nm)/ TmPyPB (50 nm)/ LiF (0.8 nm)/Al (150 nm), for which LiF and aluminum were respectively used as the electron injection layer and reflective cathode. Figure S9 shows the external quantum efficiency of various devices A and B as a function of dopant concentration, for which the best performances were achieved using x = 14 and 4 wt.% of PXZBM and PXZPBM, respectively. Figure 9 presents the chemical structures of the employed materials along with the schematic architecture of the as-fabricated OLEDs. The transient PL decay of the doped thin film of PXZBM and PXZPBM in 26DCzppy was first recorded to confirm their TADF characteristics. As shown in Figure S10, the nanosecond-scale prompt and microsecond-scale delayed components were clearly observed at RT, for which the prompt lifetimes (τp) of 33 ns and 23 ns are assigned to conventional fluorescence decay, whereas the delayed lifetimes (τd) of 2.21 μs and 1.76 μs are caused by the slower intersystem crossing to the triplet excited state, followed by a reverse intersystem crossing process. Figure 10 and Table 1 show the electroluminescence (EL) and the associated numerical data for the OLED devices. Figure 10a shows the EL spectra of devices measured at 100 cd∙m-2. As indicated, the EL of device A was similar to the PL of its spin-casted thin film, while the EL of device B was akin to the PL recorded in cyclohexane. These are attributed to the differences in the doping concentrations − 14 − ACS Paragon Plus Environment

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(i.e., 14 wt.% vs. 4 wt.%) of the emitter in EML. From the current density-voltage-luminance (J-V-L) curves shown in Figure 10b, the current density of B was much higher than that of device A despite the thicker HTL used in device B. For instance, devices A and B presented a difference in current density of nearly one order of magnitude, operating at 6 V (cf. 0.295 vs. 2.02 mA∙cm-2). The lower HOMO of PXZBM and the high doping concentration might also enhance carrier trapping and thus reduce the current density. As shown in Figure S9, the current density of device A decreased with increasing PXZBM concentrations, which showed an opposite trend to that of device B, demonstrating that PXZPBM possesses definite carrier transport capability. Furthermore, the turn-on voltages defined at a luminance of 1 cd∙m-2 for devices A and B were respectively recorded at 4.9 V and 4.0 V. The insertion of a phenylene spacer in PXZPBM decreased the onset of the lowest lying absorption such that the turn-on voltage of the PXZPBM-based OLEDs was much lower than that obtained for PXZBM. On the other hand, the maximum luminance of devices A and B were recorded to be 4414 and 37,497 cd∙m-2. The higher luminance obtained in device B was attributed to the higher human eye sensitivity to the color green and higher EL efficiency. In terms of the eﬃciency characteristics shown in Figures 10c and d, device A exhibited a maximum EL eﬃciency of 10.9% (30.3 cd∙A-1, and 18.7 lm∙W-1), and a superior peak efficiency of 22.6% (67.7 cd∙A-1, and 50.0 lm∙W-1) was achieved in device B. The PLQY of PXZPBM-doped film was estimated to be 0.84, revealing good agreement with the recorded peak efficiency of device B. In addition, these results showed that nearly 100% internal quantum efficiency was achieved in this architecture. Furthermore, device A gave efficiencies of 9.8% (27.2 cd∙A-1, and 14.1 lm∙W-1) and 4.0% (11.1 cd∙A-1, and 4.3 lm∙W-1), respectively recorded at luminance levels of 102 and 103 cd∙m-2. Significant efficiency roll-off was observed in device A with PXZBM, which might have resulted from the longer relaxation-time induced exciton quenching or the polar quenching induced by carrier imbalance;48 This could be attributed to the long time constant of reverse intersystem crossing caused by − 15 − ACS Paragon Plus Environment


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larger ΔET-S. It is worth noting that device B with PXZPBM maintained high efficiency levels of 22.4% (67.1 cd∙A-1, and 43.7 lm∙W-1) and 20.8% (62.4 cd∙A-1, and 33.5 lm∙W-1) respectively at practical luminance levels of 102 and 103 cd∙m-2. An efficiency drop of only 8% was estimated from the peak to the value recorded at the luminance levels of 103 cd∙m-2. The mitigated efficiency roll-off illustrated that the device architecture design was adequate and that PXZPBM has high potential for EL applications.

3. Conclusions In

summary,

we

10-(4-dimesitylborylphenyl)

report

a

new

phenoxazine

class

of

compounds

10-dimesitylboryl PXZBM

and

and

PXZPBM

possessing TADF properties, in which PXZBM exhibits the remarkable excited-state structural planarization. Bearing a non-planar phenoxazine moiety, PXZBM shows an anomalously large Stokes shifted (∼14500 cm-1) emission in nonpolar solvent. The steric hindrance between phenoxazine and dimesitylboryl fragments in PXZBM, causes the puckered arrangement of phenoxazine at the ground state. Upon excitation, supported by the relaxation dynamics and calculated potential energy surfaces, the diminution of N→B electron transfer reduces π-conjugation and elongates the N-B bond distance, inducing the fast phenoxazine planarization with a time constant of 890 fs in cyclohexane. When a phenylene spacer is added between phenoxazine and the dimesitylboryl moieties of PXZBM, the resultant PXZPBM possesses planar phenoxazine and a regular CT character. Despite the distinction in excited-state relaxation dynamics, both PXZBM and PXZPBM exhibit efficient TADF, in which the use of PXZBM and PXZPBM as an emitter affords high performance OLEDs, giving orange and green emissions with EQE of 10.9 % and 22.6 %, luminance efficiency (ηl) of 30.3 cd∙A-1 and 67.7 cd∙A-1, and power efficiency (ηp) of 18.7 lm∙W-1 and 50.0 lm∙W-1, respectively. It is believed that the non-planar phenoxazine is subject to a twisting-induced radiationless deactivation channel, resulting in an inferior EL property. This excited-state planarization is also expected for TADF molecules bearing analogous donors such as − 16 − ACS Paragon Plus Environment

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acridine and phenothiazine,10, 27 giving inferior PL and EL performances vs. analogues with 10-(4-dimesitylborylphenyl) acceptor. Evidently, these results open another horizon for both the N-borylated TADF molecules and common TADF emitters bearing a structurally flexible donor entity in harnessing the photophysics and hence the emission properties via structural relaxation.

Supporting information Detailed synthetic procedures, X-ray structural data, transient and time resolved PL spectra, fluorescence up-conversion decay and relaxation dynamics, and EL characteristics of the studied boron compounds.

Corresponding authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]

ORCID Chih-Hao Chang: ORCID id: orcid.org/0000-0002-5586-9526 Pi-Tai Chou: ORCID id: orcid.org/0000-0002-8925-7747 Yun Chi: ORCID id: orcid.org/0000-0002-8441-3974

Acknowledgements C.-H. Chang, Y. Chi and P.-T. Chou thank the Ministry of Science and Technology of Taiwan, for the financial support with grant numbers MOST 106-2221-E-155-035 and MOST 106-2628-M-007-004.

Notes The authors declare no competing financial interest.

− 17 − ACS Paragon Plus Environment


Figure 1. Single crystal x-ray structure of PXZBM (left) and PXZPBM (right) with thermal ellipsoids shown at the 50 % probability.

Abs. (A. U.)

(a) 1.0

PXZBM Emission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5

(b) 0.0 1.0

PXZPBM

0.5

0.0

300

400

500

600

700

800

Wavelength (nm) Figure 2. Absorption and emission spectra of (a) PXZBM and (b) PXZPBM recorded in cyclohexane (black), toluene (blue), CH2Cl2 (green) and acetonitrile (red), emission spectra of crystalline solid (pink solid circle) and spin-casted thin film (brown open cycle) at 298 K.


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Figure 3. The Lippert-Mataga-Plot (Δν versus Δf) for (a) PXZBM, (b) PXZBM after subtracting each Δν from that in cyclohexane, and (c) PXZPBM. Δν: the difference in cm-1 between absorption and emission peak maximum. Δf: the solvent polarity parameter (Δf = (ɛ‒1)/(2ɛ+1) ‒ (n2‒1)/(2n2+1)) where ɛ and n are dielectric constant and refractive index, respectively, for the chosen solvent). Δf is 0.00, 0.01, 0.15, 0.17, 0.22 and 0.31 for cyclohexane, toluene, chloroform, ethyl ether, CH2Cl2 and acetonitrile.



100000

(a)

PXZBM in DCM IRF fitting

Log (Count.)

1000 100 10 1

0

5000

10000 Time (ns)

15000

(b)

PXZPBM in DCM IRF fitting

10000 Log (Count.)

10000

1000 100 10 1

20000

0

2000

4000 6000 Time (ns)

8000

10000

100000 PXZBM_neat film IRF fitting

1000 100 10 1

0

5000

10000 Time (ns)

15000

10000 Log (Count.)

(c)

10000 Log (Count.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(d)

PXZPBM_neat film IRF fitting

1000 100 10

20000

1

0

2000

4000 Time (ns)

6000

Figure 4. The transient decay (black line) characteristics of (a,c) PXZBM, and (b,d) PXZPBM in CH2Cl2 solution and as neat film, respectively. IRF: instrument response function (red line) and fitting curve (blue line).


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(a)

460nm IR F

1.0 0.5 0.0 1.0

5 00 n m

Normalized Intensity (A. U.)

0.5 0.0 1.0

5 50 n m

0.5 0.0 1.0

6 00 n m

0.5 0.0 1.0

650nm

0.5 0.0 1.0

7 00 n m

0.5 0.0 -2

(b ) 1.0 A. U.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5

-1

0

1 2 T im e (ps)

3

4

5

0.25 ps 0.5 p s 1 ps 5 ps 100 p s

0.0 450

500

550 600 650 W avelen gth (nm )

700

Figure 5. (a) The early relaxation dynamics of PXZBM (∼10-4 M) in cyclohexane monitored at different emission wavelengths. λex ∼ 380 nm, repetition rate 1 kHz. (b) The temporal evolution of the normalized emission of PXZBM in cyclohexane within 0 - 100 ps.



Page 22 of 30

Figure 6. The optimized molecular structures and the associated HOMO and LUMO in the geometry optimized ground and excited states for (a) PXZBM and (b) PXZPBM. Angles Θa and Θb indicated the bending angle of phenoxazine and the dihedral angle between the planes C(12)-N(1)-C(1) and C(13)-B(1)-C(22) of phenoxazine and dimesitylboryl fragment, respectively.


Page 23 of 30

Relative energy

B-N Bond distance (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.557

85.00

1.556

75.67

1.555

74.73

1.554

73.78

1.553

72.83

1.552

71.88

1.551

70.94

1.550

69.99

1.549 145

69.04

150

155

160

165

170

175

Bending angle, Θa (deg.)

Figure 7. Calculated PES for PXZBM in the first excited state scanned along the change of bending angle Θa from 145° to 180° in increments of 5° and the bond distance from 1.548 Å (optimized at Θa = 145°) to 1.557 Å.



B-N distance (Å)

1.57 1.56

(a)

1.55 1.54 1.46 1.45 1.44 110

(b)

100

70

510 nm

75

274 nm

Relative energy (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

65 10 0 145

150

155 160 165 170 175 Bending angle,Θa (deg.)

180

Figure 8. (a) The B-N bonding distance as a function of bending angle Θa for PXZBM in the first excited state S1 (blue triangle), and ground state S0 (black square), (b) calculated PES scanned along the change of bending angles Θa from 145° to 180° in increments of 5°, in which S0 (red cycle) denotes the vertical transition from S1 in a specific Θa. S0 is a ground-state optimized state at the same Θa. Open data symbols (i.e. both square and triangle) mark the global (or local) minimum of the PES for S0 and S1, among which the black hollow triangle shows the Franck-Condon absorption point.


Page 25 of 30

Figure 9. (a) Structural drawings of the employed materials; (b) schematic OLED structures with PXZBM and PXZPBM as emitters. (a)

(b) A : PXZBM B : PXZPBM

0.8

2

EL spectra @ 100 cd/m 1.0

Current Density (mA/cm )

2

2

10

5

10

0

10

0.6

3

10 -2

10

0.4

0.0 350

1

10

-4

10

0.2

A : PXZBM B : PXZPBM

-1

-6

450

550

650

750

850

10

0

2

4

6

8

10

12

14

10

Voltage (V)

Wavelength (nm)

(d)

(c) 20 16 12 8 4 0 -1 10

A : PXZBM B : PXZPBM 0

10

1

10

2

10

3

10

2

4

10

70

70

60

60

50

50

40

40

30

30

20 10 0 -1 10

Luminance (cd/m )

20 A : PXZBM B : PXZPBM 0

10

1

10

10 2

10

3

10

2

4

Power Efficiency (lm/W)

Luminance Efficiency (cd/A)

24

Quantum Efficiency η ext (%)

2

Normalized Intensity (a.u.)

1.2

Luminance (cd/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

10

Luminance (cd/m )

Figure 10. EL characteristics of OLEDs with emitters PXZBM and PXZPBM: (a) normalized EL spectra; (b) current density-voltage-luminance (J-V-L) curves; (c) external quantum efficiency vs. luminance; (d) luminance efficiency and power efficiency vs. luminance of the studied OLEDs.


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 26 of 30

Table 1. EL Characteristics of the fabricated TADF OLEDs. EQE (%)

LE (cd∙A-1)

PE (lm∙W-1)

Von (V)

Max. Lumin.

λmax. (nm)

CIE1931 coordinates

[a]/[b]/[c]

[a]/[b]/[c]

[a]/[b]/[c]

[d]

(cd∙m-2) [V]

[b]

[b]/[c]

PXZBM

10.9/9.8/4.0

30.3/27.2/11.1 18.7/14.1/4.3

4.9

4144 [14.2]

567

(0.454, 0.507)/(0.443, 0.502)

PXZPBM

22.6/22.4/20.8

67.7/67.1/62.4 50.0/43.7/33.5

4.0

37497 [14.6]

505

(0.247, 0.543)/(0.245, 0.540)

Emitters

[a] Maximum efficiency; [b] recorded at 102 cd∙m-2; [c] measured at 103 cd∙m-2; [d] turn-on voltage measured at 1 cd∙m-2.

− 26 −

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