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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Rigidity and Polarity Effects on the Electronic Properties of Two Deep Blue Delayed Fluorescence Emitters Christian M. Legaspi, Regan E. Stubbs, Md. Wahadoszamen, David J. Yaron, Linda A. Peteanu, Abraham Kemboi, Eric Fossum, Yongli Lu, Qi Zheng, and Lewis J. Rothberg J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12025 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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The Journal of Physical Chemistry

Rigidity and Polarity Effects on the Electronic Properties of Two Deep Blue Delayed Fluorescence Emitters 1 1 Christian M. Legaspi , Regan E. Stubbs , Md. Wahadoszaman2, David J. Yaron1, and Linda A. Peteanu1,* 1 Department of Chemistry, Carnegie Mellon University 2 Department of Physics, University of Dhaka Abraham Kemboi and Eric Fossum Department of Chemistry, Wright State University Yongli Lu, Qi Zheng, and Lewis J. Rothberg Department of Chemistry, University of Rochester

Abstract Molecules that undergo reverse intersystem-crossing (RISC) which enables thermally-activated delayed fluorescence (TADF) represent an important advance in the development of organic-based light emitting diodes (OLEDs). The current study focuses on two blue-emitting RISC molecules employing carbazole as the donor and benzothiazole or benzoxazole derivative as the acceptor (BTZ/BOX-CBZ). While the emission maxima of these compounds is deep blue (~410nm) in hydrocarbon solvents, their spectra broaden, red shift, and decrease in intensity with even a modest increase in solvent polarity due to their strong charge-transfer (CT) character. These effects are qualitatively predicted from TD-DFT calculations using the state-specific polarizable continuum (SS-PCM) model though the emission spectral shifts are significantly over-estimated. The desired blue emission peak of both compounds (~425nm) is recovered by rigidifying the environment, either in low temperature glasses or in room-temperature polymer films, independent of local polarity. The polarity-induced emission red shift is therefore due to the solvent orientational polarizability. The effects of an applied electric field on the spectra (Stark effect) are used to quantify the CT character of the absorbing and emitting states. Significantly less field-induced emission quenching is observed in BOX-CBZ versus BTZ-CBZ. Minimizing this effect is important to performance in the large (1-10MV) fields present within OLED devices. *[email protected]

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Introduction Organic light-emitting diodes (OLEDs) have been a very popular research area since the fabrication of the first small molecule and polymer devices.1-2 OLEDs are particularly desirable for use in electronics, achieving high color contrast, having the potential for use with flexible substrates, and being easily manufactured on a large scale. Despite many advances, there remains a dearth of organic emitters with high efficiencies and long-term operating stabilities. In the last several years, several new classes of organic small-molecule emitters have shown promise for the creation of efficient devices by capturing both singlet and triplet excitations through efficient RISC. One employs thermal repopulation of the singlet state from the relaxed triplet state (thermallyactivated delayed fluorescence or TADF)3-5 while another exhibits RISC from ‘hot’ triplet exciton states that is competitive with internal conversion within the triplet manifold.6-7 Recovery of the excited triplet states, which may otherwise be non-emissive, enables a maximum theoretical internal quantum efficiencies of 100% and a projected external conversion efficiency of 20-30% in OLED devices.8-9 The efficiencies of both these RISC mechanisms are known to be enhanced in donor-acceptor substituted (D-A) organic molecules having significant excited state charge-transfer character. In the case of TADF molecules, high efficiency depends on controlling the singlet-triplet energy gap, Δ ,10 to be on the order of kT (~26 meV at room temperature),11 though TADF has been demonstrated in systems with much larger gaps.12 Spatial separation of the HOMO and LUMO orbitals favors small Δ values because the reduced exchange energy raises the energy of the triplet state over its typical position in non-D-A molecules, pushing it closer to that of the singlet. 8, 12 Molecules designed to undergo RISC from ‘hot’ triplet excitons exhibit extensive mixing of nearly iso-energetic charge-transfer (CT) and locally excited (LE) states to form a hybrid excited state.6-7 The emission of D-A compounds is highly sensitive to the polarity of their surroundings. They demonstrate a red shift (positive solvatochromism) with increasing solvent polarity that is typically accompanied by a broadening of their emission spectra. The latter is detrimental to purity of the emission color in an OLED device if it persists in the solid state. For TADF-capable compounds, the resulting decrease in excited state energy can negatively impact RISC efficiency by increasing Δ .13-14 In molecules that undergo hot RISC, increasing the energy separation of the LE and CT states will decrease their mixing leading to lower efficiency.6, 15 A mechanistic understanding of these effects and their accurate modeling is therefore important to optimizing RISC properties.16 Deep blue emitters having Commission internationale de l’éclairage (CIE) y-coordinates less than 0.2 are important for creating RGB full-color displays and varying the color warmth of OLED-based white lights. 17-19 Though organic TADF and RISC emitters have been synthesized in a wide variety of colors, creating blue emitters with bright emission that are long-lived in devices has been particularly challenging.11, 19-26 In general, injecting electrons into blue emitters is inherently difficult due to the high LUMO energy which does not match well with typical materials available for electron transport.27 The high LUMO energy also makes blue emitters more susceptible to photochemical reactions than their green and red counterparts. In D-A systems, obtaining pure deep blue emission is difficult because small variations in 2 ACS Paragon Plus Environment

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the electron-donating or electron-withdrawing characteristics of the molecule can increase the effective π conjugation length and therefore shift the emission away from the desired blue color.28-29 In recent years, the use of a carbazole donor (Scheme 1a) has proven to produce efficient blue emitters as carbazole is a weaker π-electron donor than other amines that have been used as donors.30 Focusing on the effects of the local environment polarity and rigidity on the emission properties of TADF/RISC compounds, we present a spectroscopic analysis of two novel blue-emitting compounds in this class, BOX-CBZ and BTZ-CBZ (Scheme 1) that utilize a carbazole donor moiety. Steady-state solutionphase measurements indicate strong positive solvatochromism in their emission, as is typical of D-A compounds. Interestingly, when these compounds are frozen in solvent glasses or cast in polymer matrices, this behavior is suppressed, independent of the polarity of the solvent(s) comprising the glass. This paper will discuss this phenomenon in terms of solvation free energy and the polarity changes between the ground and excited states. F

F

X

N

R N

R (a) (b) Scheme 1. Template structures of the (a) donor moiety and (b) acceptor moiety. BOX-CBZ contains a benzoxazole derivative (X=O) as its acceptor. BTZ-CBZ contains a benzothiazole derivative (X=S) as its acceptor. Both contain the carbazole donor. In the donor-acceptor compounds, the moieties are connected at the “R” positions. In the structures of the isolated subunits (BOX-H, BTZ-H, and H-CBZ), R = H.

Experimental Sample Preparation For spectroscopic analysis, all solvents were of HPLC grade or better and used as received. Concentrations of 1-10 µM were used for solution phase measurements and 1-100 µM for lowtemperature solvent glass emission measurements. For Stark spectroscopy measurements, both compounds were prepared at 0.1-1 mM. Films were spin-cast onto quartz slides (Chemglass) from solutions containing 100 µM D-A compound and 10-25 mg/mL polymer in chloroform. Prior to spincasting, the quartz slides were washed with isopropanol, dried with nitrogen, and passed quickly over a butane torch. Samples were spin-casted by depositing 60 µL of solution on the quartz substrate spinning at 3000 rpm for 5 seconds, 300 rpm for 30 seconds to dry, and 3000 rpm for 5 seconds to remove any remaining solvent.



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Solution-Phase Steady-State Absorption and Emission Measurements Solution-phase steady-state absorption and emission measurements were performed in 10x10 mm path length quartz cuvettes (Starna Cells) using a Varian Cary 50 Bio UV/Vis and a Jobin-Yvon FluoroMax 2 spectrophotometer, respectively. Orientation polarizability (Δ) is a measure of solvent polarity which captures both the fast and slow solvent dielectric responses, and is used in the Lippert-Mataga solvatochromism analysis.31-34 It is calculated by =

−

(1)

where is the static dielectric constant and n is the refractive index of the solvent. Values for these quantities in solvents and in polymer films were obtained from Ref. 35 unless otherwise noted. The quantum yields (QY) for both compounds in methylcyclohexane (MCH) were determined using quinine sulfate in 0.1 M sulfuric acid as the standard. Each D-A compound was excited at the maximum of its excitation spectrum. Stark Spectroscopy Stark spectroscopy consists of measuring the effects of an external electric field applied to a sample on its transmission (electroabsorption or EA) or its fluorescence (electrofluorescence or EF) spectrum. These measurements were performed on a custom-built instrument detailed in previous work.36-37 Analyses of the EA and EF spectra were performed using the standard Liptay method.38 Briefly, for a rigid, isotropic, bulk sample, the EA/EF spectra are fit to a linear combination of the zeroth, first, and second derivatives of the zero-field absorption/emission spectra. In the case of EA, the spectrum is fit to √ ()

= () +

#) ! %(" !"# $ "# & +

'

#) ! %(" $ "# &( # !"

(2)

and for EF to 2√2 () = () +

#) ! ,(" + !"# $ "#- & +

' +

#) ! ,(" $ # - &( # " !"

(3)

where is wavenumber, Feff is the effective field strength in volts per centimeter, () (()) is the zero-field absorption (emission) spectrum as a function of wavenumber, Δ() (Δ()) is the EA (EF) spectrum as a function of wavenumber, and , , and ' are the linear fitting coefficients to the

zeroth, first, and second derivatives, respectively. The factor of 2√2 is to account for the change to DC voltage from root-mean-square voltage as read by the lock-in amplifier, and the factor of ln(10) in 4 ACS Paragon Plus Environment

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equation (2) is from treating the change in transmitted light intensity by the field as a perturbation. is related to the change in transition moment due to the applied field and can be used to calculate fieldinduced quenching efficiency.37 is related to the average change in the polarizability (Δ.). As the contributions of these coefficients to the Stark spectra are minor and the related properties are not pertinent to this work, they will not be discussed further in this text. The values of these coefficients and Δ. are reported in the Supporting Information. When the field is applied at the magic angle (~54.7°) relative to the electric field of the incident light beam, the angular dependence in the linear coefficients vanishes allowing facile estimation of the associated molecular parameters. At the magic angle condition, the magnitude of the difference dipole moment between the ground and excited state is determined by |0| = ℎ236'

(4)

To determine the linear coefficients in eq. 2 and 3, the absorption or emission spectra were first fit to a sum of n symmetric Gaussians to reproduce their shape and provide smooth derivative curves () = ∑7@ 67 89: ;−

(" # =

?

(5)

where ai, bi, and ci are the Gaussian fitting parameters representing maximum intensity, energy at maximum intensity, and line width, respectively. The number of Gaussians (n) was determined by minimizing the root-mean-square error (RMSE) of the fit using the least number of Gaussians necessary. Using a minimal value for n is necessary to avoid overfitting and representing the spectrum with more transitions than are necessary. The linear coefficients were then determined using a least-squares fit to the Gaussian-fit curves and their weighted first and second derivatives. In some cases, a poor fit results from modeling the spectra with one set of molecular parameters. This is often due to the presence of multiple accessible excited states that each respond differently to the electric field and must therefore be fit with different molecular parameters.39-41 Assuming that the spectrum corresponding to each transition can be approximated by one or more Gaussians, these were sorted according to their maximal energy and divided into multiple sets, j, to emulate different continuous energy regions of the spectrum, A (), A () = ∑7∈A 67 89: ;−

(" # =

?.

(6)

The linear coefficients for each region were then simultaneously determined by a least-squares fit according to √ () ln

∑A ,A A () + =

#) ! % (" !"# $ G"# & + ,A

',A

#) ! %G (" $ "# &( # !"

(7)



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for EA and ∑A ,A A () + 2√2 () =

#) ! , (" + !"# $ G# - & + ,A "

',A +

#) ! ,G (" $ # - &( # " !"

(8)

for EF. The grouping of the Gaussians was optimized by iterating over all possible groupings of consecutive Gaussians, fitting each according to the appropriate equation above, and selecting the grouping which minimizes the RMSE of the fit, minimizes the number of sets (HA ), and has all positive values of ',A . When fits having similar RMSE values were generated, those with smaller HA values were selected to avoid overfitting and an unphysical representation of the spectra. The errors in the reported dipole moments are ±15%. Solvent Glass Measurements Low-temperature absorption and emission spectra were recorded using the same instrumental setup as for Stark spectroscopy, but without the applied field. Sample cells were constructed from uncoated quartz slides (Chemglass). An optical chopper wheel (Palo Alto Research) was coupled to the lock-in amplifier to isolate the transmitted/emitted light signal. Samples were prepared at 100 µM, except for BTZ-CBZ in 1:4 (v/v) isopentane/methylcyclohexane, which was prepared at 1 µM due to aggregation at low temperature. Fluorescence Lifetime Measurements The fluorescence lifetime was measured using the time-correlated single photon counting (TCSPC) method. A 375 nm diode laser (PicoQuant) pulsed at 10 MHz was attenuated using a neutral density filter and focused onto the sample, contained in a 10x10 mm quartz cuvette. The emission was collected at 90 degrees through a 10 nm band-pass filter centered at the fluorescence maximum and detected by an MPD detector (PicoQuant). Each photon was time-tagged using the PicoHarp 300 system (PicoQuant). The emission decays were fit using MATLAB. Quantum Chemical Calculations Ground- and excited-state geometries, energies, and dipole moments were calculated using density functional theory (DFT) or time-dependent DFT (TD-DFT) implemented in the Gaussian09 software package.42 The M06-2X functional43 and the cc-pVDZ basis set44 were used. The solvent was represented using the integral equation formalism polarizable continuum model (IEF-PCM)45 applied with the statespecific (SS) approach.46-47 Orbital iso-density surfaces were generated using the AMPAC software package using an iso-density value of 0.03.48 Orbital contributions to vertical excitations were calculated by % contribution =

>= × .Q

100%

(9)


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where ci is the configuration interaction (CI) coefficient for the orbital contribution in question and 0.5 is the normalization value of the square of the CI coefficients for closed-shell systems in Gaussian09. CI coefficients were determined as a part of the TD-DFT excited-state energy calculations. Electron-hole spatial distribution overlaps were calculated from output of Gaussian09 using the Multiwfn program.49 Briefly, the overlap (T) is calculated as T = U VWHXYZ[ (\]), Y ^ (\])_ `+ \]

(10)

where YZ[ (\]) and Y ^ (\]) are the spatial distributions of the hole and electron wavefunctions, respectively, and the integral over all space of each distribution is 1. Because of this normalization condition, T is therefore the fractional overlap of the wavefunctions, where T = 1 represents complete overlap. The values of Δ were calculated using two protocols to establish a range of values for this parameter because computational values can vary considerably based on the chosen DFT method.50 Moreover, there is considerable uncertainty in the spectroscopic results to which the computational values are compared.51 All calculations of Δ were performed using the B3LYP functional52 and the 6-31G* basis set.53 The first protocol used was that of Van Voorhis et al..54 First, the ground-state geometry is optimized using restricted, closed-shell DFT. Using this geometry as a starting point, the geometry and energy of the lowest excited singlet state were obtained using the restricted, open-shell Kohn-Sham (ROKS) method.55-56 The geometry and energy of the lowest excited triplet state were obtained using restricted, open-shell TD-DFT. The calculated value of Δ was taken to be the energy difference between these singlet and triplet states at their respective minimized geometries. The calculated Δ was scaled based on a linear least-squares fit of the calculated values of Δ obtained by Van Voorhis et al. and their respective experimental values obtained from the literature (see SI Sections S2 and S3). These calculations were performed using the QChem 4.4.2 software package57 because the ROKS method is not available in Gaussian09. The second protocol used to obtain Δ was that of Aspuru-Guzik et al..51 The ground state geometry was first optimized using restricted, closed-shell DFT. Then the vertical excitation energies to the lowest singlet and triplet states were determined using restricted closed shell TD-DFT and subtracted to obtain the calculated value of Δ . This value was also scaled in a manner equivalent to that used above from experimental and calculated values provided by Aspuru-Guzik et al.51 (see SI Sections S2 and S3). These calculations were performed using Gaussian09.42 Results and Discussion BOX-CBZ and BTZ-CBZ contain a carbazole donor subunit (Scheme 1a) and a benzoxazole or benzothiazole acceptor subunit (Scheme 1b, X = O or S), respectively, connected at the “R” positions. Both were established to undergo TADF via measurements of their emission decays which show a component in the microsecond regime (SI Figure S1) that is absent in compounds undergoing RISC from 7 ACS Paragon Plus Environment


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'hot' triplet states.6 The relatively small values of Δ calculated using the Aspuru-Guzik method.51 (171 meV for BOX-CBZ and 177 meV for BTZ-CBZ) are more consistent with TADF than are those we obtained using the van Voorhis method54 (801 meV and 856 meV, respectively). However, as no phosphorescence was observed for either compound, even at 77K and under de-oxygenated conditions, the calculated Δ values could not be verified experimentally. To assign the transitions in BOX/BTZ-CBZ compounds, the absorption spectra of the D-A compounds and the hydrogen-capped donor and acceptor (BOX-H and BTZ-H, Scheme 1b, R = H, X = O and S, respectively) subunits were obtained (Figure 1). At energies above 350nm, the absorption features of the D-A compounds and those of the corresponding subunits are similar, indicating that these higherenergy states are localized on one of the subunits. The most significant difference between the absorption spectra of subunits and those of the D-A compounds is the broad feature near 350 nm. It is therefore attributed to the presence of a charge-transfer (CT) state arising from the interaction of the subunits in the D-A scaffold.

4

(a)

3

4

(b)

BOX-CBZ BOX-H H-CBZ

BTZ-CBZ BTZ-H H-CBZ

3

2

2

1

1

0

0 250

300 350 Wavelength (nm)

400

250


400

Figure 1. Steady-state solution phase absorption spectra for (a) BOX-CBZ and its hydrogen-capped donor (H-CBZ) and acceptor (BOX-H) subunits, and (b) BTZ-CBZ and its donor (H-CBZ) and acceptor (BTZ-H) in tetrahydrofuran (THF).

To quantify the CT character of these absorption transitions, EA was performed on both compounds in frozen 2-methyltetrahydrofuran (MT) solvent glass (Figure 2). The low-temperature absorption spectra of both compounds (Figure 2a,d) show a broad transition at low energy (~2.8 x 104 cm-1 or 350nm) and additional transitions at higher energy. Analysis of the corresponding EA spectra (see Experimental) demonstrates that they cannot be modeled using a single set of molecular parameters. There are therefore multiple excited states underlying the absorption bands in this energy region, only some of which have significant CT character. When the EA spectrum for BOX-CBZ (Figure 2b) is fit to three parameter sets (see SI Section S4 for compiled parameters), the low-energy portion of the spectrum (G1) is found to have a large change in dipole moment upon absorption (|Δ0% | = 17 D). For comparison, a full electron transferred between the 8 ACS Paragon Plus Environment

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two subunits would give a |Δ0% | of ~36D. The two higher-energy features (G2/G3 and G4/G5) are associated with a smaller dipole moment change (|Δ0% | = 2 and 6 D, respectively), as expected for a localized transition. When BTZ-CBZ (Figure 2e), is modeled with two parameter sets (G1/G2 and G3/G4, SI Table S3), the results are similar to BOX-CBZ, with a large dipole moment change for the low-energy band (|Δ0% | = 23 D) and a smaller one for the high-energy band (|Δ0| = 8 D). When the fit is decomposed into the respective contributions of the derivatives of the spectra (Figure 2c,f), a large second-derivative component is observed in the low-energy region, indicating a large change in the dipole moment. These values are generally consistent with the results of a Lippert-Mataga solvatochromism analysis31-34 (see SI Section S5). Overall, the low-energy bands are associated with large values of |Δ0|, indicating CT-type transitions, and the high-energy bands are associated with small values of |Δ0|, indicating more localized transitions. The contribution of the CT bands to the overall oscillator strength is larger in BTZ-CBZ than in BOX-CBZ but it is nonetheless rather small compared to the localized transitions. One striking difference between the field responses of the two compounds is that the magnitude of the zeroth-derivative component is significantly larger in BOX-CBZ than in BTZ-CBZ. This is commonly interpreted to mean that the applied field significantly affects the oscillator strength of the transition. This would be expected in D-A compounds if, for example, the field affects the mixing between nearby localized and CT states. At present it is not clear why this effect would be significantly greater in BOXCBZ versus BTZ-CBZ, particularly as the oscillator strengths of the two are comparably affected by increasing solvent polarity (see below).


10 15

10 15 A (cm 2 V -2)

Absorbance

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A (cm 2 V -2)

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Absorbance


Figure 2. Absorption (a,d) and electroabsorption (EA) (b,e) for BOX-CBZ (a-c) and BTZ-CBZ (d-f) in 2-methyltetrahydrofuran (MT) solvent glass with the Liptay model fit and the constituent derivative components (c,f). The individuals Gaussians used to fit the absorption spectra are shown and labeled in order of increasing peak energy (G1-Gn).

Overall, the EA results demonstrate that the spectra of TADF compounds are significantly more complex than a simple HOMO-LUMO transition with CT character. TD-DFT calculations find two main transitions that occur in the energy window considered (see SI Tables S1 and S2). The low-energy state, predicted to lie at 350 nm (358 nm) for BOX-CBZ (BTZ-CBZ) in THF, is dominated by a transition from the HOMO to the LUMO (BOX-CBZ: 76% contribution, BTZ-CBZ: 77% contribution). The HOMO (Figure 3c,g) and LUMO (Figure 3d,h) reside primarily on the donor and acceptor subunits, respectively, indicating significant CT character. The main contribution to the higher-energy state, predicted to lie at 261 nm (268 nm) for BOX-CBZ (BTZ-CBZ), is a transition between the orbital the HOMO-2 level to the LUMO (BOX-CBZ: 85%, BTZ-CBZ: 88%). Both the HOMO-2 (Figure 3b,f) and the LUMO reside on the acceptor, indicating this is a transition localized on the acceptor subunit. This higher-energy transition has a small contribution (BOXCBZ: 8%, BTZ-CBZ: 4%) from the HOMO to LUMO transition, indicating some mixing of the HOMO to LUMO excitation into this state.


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(a)

(b)

(c)

(d)

(e) (f) (g) (h) Figure 3. Orbital iso-density surface diagrams for BOX-CBZ (a-d) and BTZ-CBZ (e-h) at ground state geometry computed in THF. For reference, the bare structures are shown in (a,e). The orbitals shown are HOMO-2 (b,f), HOMO (c,g), and LUMO (d,h).

Effects of Solvent Polarity on Solution-Phase Emission The CT character of these compounds means their emission properties will be affected by the polarity of the local environment. With increasing solvent polarity (as measured by orientation polarizability, see Methods), the emission spectra of both compounds broaden and red shift with a significant drop in quantum yield (Figure 4) whereas their absorption spectra are essentially unchanged (SI Figure S3). Particularly striking is the structured appearance of both spectra in MCH which resembles that of carbazole itself but red shifted by ~4000 cm-1. One possible explanation of the evolution of the emission spectra with increased solvent polarity would be a change in the emissive state from LE to CT character with the LE emission arising predominantly from the carbazole subunit. However, as discussed later, this model is not clearly supported by the computational results or by measurements of the CT character via Stark spectroscopy. Moreover, a Lippert-Mataga analysis of absorption and emission data from 11 solvents ranging in polarity from cyclohexane to acetonitrile gives a good linear fit (SI Figure S2), indicating that the electronic structure does not change significantly in solvents of varying polarities. For comparison, molecules in which the emission evolves from LE- to CT-like with solvent polarity show two distinct linear regimes corresponding to two different values of the excited state dipole moment.6, 58



1

(a) Relative Intensity

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(b) MCH TOL DXN CFM THF ACN

0.8 0.6 0.4 0.2 0 350

400


550

600

Figure 4. Solution-phase steady-state emission spectra for (a) BOX-CBZ and (b) BTZ-CBZ in an array of solvents with varying polarities. BOX-CBZ (BTZ-CBZ) samples were excited at ~292 nm (~350 nm). Solvent abbreviations and orientation polarizability (Δ, see Methods): methylcyclohexane (MCH, ≈ 0), toluene (TOL, 0.014), 1,4-dioxane (DXN, 0.021), chloroform (CFM, 0.148), tetrahydrofuran (THF, 0.210), and acetonitrile (ACN, 0.305).

To further explore the solvent effects, TD-DFT calculations were performed using the state-specific polarizable continuum model (SS-PCM) for the solvent (see Methods). The SS-PCM model assumes that the fast component of the solvent dielectric response, due to electronic polarization of solvent, follows the charge distribution of the molecule without lag. However, the orientational (i.e. slow) component is equilibrated to the ground state charge distribution of the solute in absorption and to the excited state charge distribution for emission. Computationally, the dipole moments of both compounds are predicted to increase substantially on excitation and to rotate ~ 90 degrees so as to point along the D-A axis. This can be used to rationalize the trends in the computed solvent shifts. For absorption, the excitation energy increases minimally with solvent polarity, leading to a predicted blue shift of 0.40 eV (0.37 eV) between MCH and ACN for BOXCBZ (BTZ-CBZ). This is because the slow dielectric response of the solvent remains polarized to the ground-state dipole moment and therefore interacts unfavorably with the large excited state dipole. For emission, the energy decreases significantly with solvent polarity, leading to a predicted red shift of 1.48 eV between MCH and ACN (Fig. 5a, x’s) for both BOX-CBZ and BTZ-CBZ. This is because, before emission, the slow dielectric response equilibrates to and substantially stabilizes the large excited-state dipole. This stabilization enhances the magnitude of the excited-state dipole (Fig. 5c, x’s), which comes from increased spatial separation between the electron and hole, computed as discussed in Methods (Fig. 5c). A consequence of the increased electron-hole separation is a decrease in computed oscillator strength (Fig. 5b, x’s). This is expected for transitions having high CT character because the transition moment requires some spatial overlap between the HOMO orbitals comprising the hole and the LUMO orbitals


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comprising the electron. The computed results can, therefore, be rationalized in terms of a solute dipole that changes direction and increases substantially in magnitude upon excitation. Comparison with experiment, however, indicates that the computations significantly overestimate the solvent effects on emission. The observed red shift between MCH and ACN is 0.65 eV (0.57 eV) for BOXCBZ (BTZ-CBZ), which is 2.3 (2.6) times smaller than the computed value of 1.48 eV for both. The absorption energy shows little discernable solvent shift for both compounds. In this limit, this behavior is consistent with the computed blue shift of 0.40 eV (0.37 eV) for BOX-CBZ (BTZ-CBZ) provided this computed value is also overestimated by a factor of 2.3 (2.6). This overestimation of solvent effects may arise from a deficiency in the description of CT excited states by the method,50 despite the use of a functional that includes corrections for long-range interactions.43 Experimental estimates of the oscillator strengths can be derived from the fluorescence quantum yields, Φ, and lifetimes, τf (Table 1). Assuming that neither the ground nor the excited state is degenerate, the oscillator strength can be calculated from experimental values by bc > - Z e &f d g hem

=$

(11)

where m is the electron mass, is vacuum permittivity, c is the speed of light, h is Planck’s constant, e is the electron charge, Φ is the quantum yield, l is the fluorescence lifetime, and em is the emission energy, with all quantities in SI units (see SI Section S7 for derivation).59 Experiment and theory both find that the oscillator strength decreases with solvent polarity though the magnitude of the decrease is larger in experiment (Fig. 5b). Experiment and computation also disagree on the relative oscillator strengths of BTZ-CBZ and BOX-CBZ in the non-polar solvent MCH. Computations predict the oscillator strength of BOX-CBZ to be twice that of BTZ-CBZ, whereas experiments find BTZ-CBZ to be 4.2 times that of BOX-CBZ. It is not clear whether the disagreement between computation and experiment originates from a disagreement regarding the magnitude of the transition moment, or from the indirect measurement of oscillator strength via Eq. 11. Table 1: Solution-phase experimental properties of BOX-CBZ and BTZ-CBZ BOX-CBZ BTZ-CBZ a b c d b c d lf b Φ lf b Solvent Φ MCH 0.30 3.4 3.10 0.75 2.1 3.03 THF 0.16 8.8 2.66 0.52 4.6 2.69 ACN 0.08 7.9 2.45 0.18 5.2 2.46 a

Solvent abbreviations: methylcyclohexane (MCH), tetrahydrofuran (THF), acetonitrile (ACN). bQuantum yield. cFluorescence lifetime (in ns). d Energy at emission maximum (in eV).



(a)

(b)

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(c)

Figure 5. (a) Experimental and TD-DFT emission energies, (b) experimental and TD-DFT emission oscillator strengths relative to methylcyclohexane, and (c) TD-DFT excited state dipole moment (0n ) and electron-hole spatial overlap percentage (S) for BOXCBZ and BTZ-CBZ in solution phase.

Because the solvent shifts in the emission energy are large, though smaller than computed, it is useful to consider whether solvent polarity alters the excited state geometry and/or the electronic properties of these molecules. For example, decreases in quantum yield with increasing solvent polarity are often associated with twisted intra-molecular charge transfer (TICT) molecules.58, 60-61 Upon excitation, the geometry of a TICT molecule undergoes changes in the dihedral angle between the donor and acceptor that enhance the CT character which results in lower electron-hole overlap and quantum yield. In fact, one method to enhance RISC yields is to tune the mixing of LE and CT character in the excited state by tuning the degree of twisting between the donor and acceptor groups.6, 58 However, no evidence for significant changes in excited-state geometry were found computationally for the D-A systems studied here. Comparing the Franck-Condon and relaxed excited state geometries for both compounds, the dihedral angle between the donor and acceptor planes decreases by 11-12 degrees in non-polar solvent and by only an additional ~2 degrees in polar solvent. The total energy of the Franck-Condon geometries in different solvents also varies by no more than 160 meV and the relaxed excited state geometries by 116 meV. Both are an order of magnitude lower than the calculated solvent shifts. The solvent-polarity-induced changes in the solute geometry are therefore small and do not significantly affect the computed solvent shifts. The possibility that solvent polarity alters the electronic character of the excited state was also considered. However, this was found not to be the case as the computed contribution of the HOMOLUMO transition to the excited state increases only by 4% with increasing solvent polarity for both compounds. The computations do indicate however that the excited state is more polar in ACN than in MCH due to the cavity field effect which causes the excited state dipole moment of both compounds to be ~ 6-7D larger in the more polar solvent.


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Both the large polarity-induced shift in the excited state energy and the associated drop in quantum yield is concerning for organic TADF compounds with strong D-A character. The implications of the reduction in the quantum yield for OLED device performance is fairly unambiguous. The effect of broadening is to reduce the purity of the emission color which is problematic for display applications. Additionally, the change in energy may affect the efficiency of TADF by changing the singlet state energy and therefore the singlet-triplet energy gap (Δ ).13 As Δ should be on the order kT for efficient triplet conversion, this energy shift can either enhance or decrease TADF efficiency. Effect of Rigidifying the Local Environment on Emission As mentioned earlier, there are two components to the dielectric response of the solvent: polarization of the solvent molecules (fast) and reorientation of the solvent molecules (slow). If the solvatochromic response is primarily due to the slow dielectric response, then it should be suppressed in the solid state. If it is primarily due to the fast dielectric response then the energy shift will persist. To determine the relative contribution of the fast and slow dielectric response to the solvatochromic response in these D-A compounds, their absorption and emission spectra were measured in frozen solvent glasses. The solvent glasses used, in order of increasing polarity, were: 1:4 (v/v) isopentane/methylcyclohexane (IPM), 2-methyltetrahydrofuran (MT), and 4:5 (v/v) propionitrile/nbutyronitrile (PBN). The emission spectra in all glasses were found to be nearly identical (Figure 6, solid lines), exhibiting deep blue emission with emission maxima at ~420 nm for BOX-CBZ and ~425 nm for BTZ-CBZ. In contrast, the compounds exhibited strong positive solvatochromism in these solvents in fluid phase (Figure 6, dashed lines). To verify that the suppressed solvatochromism is not a temperature effect, the D-A compounds were cast in poly(methyl methacrylate) (PMMA) and polystyrene (PS) films and the emission spectra were collected at room temperature (Figure 7). Both molecules exhibited suppressed solvatochromism and deep blue emission in the polymer films, with emission maxima at ~411 nm and ~415 nm and CIE coordinates (0.16, 0.07) and (0.16, 0.06) for BOX-CBZ and BTZ-CBZ, respectively.



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(a)

0.8 0.6 0.4 0.2 0 400


550

600

(b)

Figure 6. Emission from (a) BOX-CBZ and (b) BTZ-CBZ in room-temperature solution (RT, dashed lines) and low-temperature solvent glass (LT, solid lines). The BTZ-CBZ IPM solvent glass spectrum was smoothed with a moving average filter. Samples were excited at 340 nm.


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Figure 7. Emission for BOX-CBZ and BTZ-CBZ cast in PMMA and PS films. Samples were all excited at 340 nm.

This suppressed solvatochromism can be understood by using a non-polar solvent as a model of the solid state. In MCH, the D-A compounds exhibit high-energy emission because the CT excited state induces polarization of the solvent cavity primarily via the fast dielectric response. A similar situation applies when these molecules are in a solvent glass or polymer film. There the effect of the slow dielectric response is eliminated and only the fast response remains. The large solvatochromic shifts in solution are therefore primarily induced by the slow solvent dielectric response. This is particularly clear when comparing toluene to PS. Toluene solution is often used to infer the expected properties of emissive compounds in OLED host matrices because their dielectric properties are similar.51 Though PS is nominally slightly more polar and polarizable than toluene (dielectric constant: 2.379 (TOL), 2.6 (PS) and refractive index: 1.4941 (TOL), 1.6033 (PS)62) the emission spectra of BOX/BTZ-CBZ are more blue shifted in the polymer matrix (compare Figures 4 and 7). It is striking that none of the solid-state matrices studied are able to fully reproduce the desirable blue emission maxima and relatively narrow spectral features for these compounds in MCH fluid solution, particularly given the strong similarity between the optical parameters of MCH and that of all of the other polymers/solvents (MCH dielectric constant: 2.03 and refractive index: 1.423). The remarkable sensitivity of D-A compounds to the polarity and polarizability of their environment highlights the importance of carefully controlling these parameters to achieving optimal emission color in TADF emitters. Comparing the change in dipole moment in absorption (|Δ0% |) with that on emission (|Δ0, |) is a useful measure of the evolution of CT character between the Franck-Condon region and the relaxed excited state. These quantities would be different if the molecule absorbs to an LE state but emits from a state with CT character. Such a model has been invoked to explain the behavior of some CT compounds that undergo a geometry change in the excited state that favors charge separation63 or that form exciplexes.64-66 The change in dipole moment on emission of compounds in the solid state is also a measure whether the CT character of the emission transition is reduced significantly when the slow dielectric response is frozen. 17 ACS Paragon Plus Environment


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In Figure 8, the results of EF measurements on both D-A compounds in MT solvent glass are presented for comparison to the parameters derived from EA. Analysis of the EF for BOX-CBZ (Figure 8a-c and Table S3) yielded a good fit to one set of molecular parameters, as expected for emission from a single state. This is in notable contrast with the EA results that required consideration of multiple electronic transitions to achieve good agreement with the data. The derived value of |Δ0, | from EF (~12 D) is smaller than |Δ0% | for the CT transition in EA. The EF spectra of BTZ-CBZ (Figure 8d-f and Table S3) was best fit to two parameter sets, giving |Δ0, | of ~24 D for the low-energy group (G1/G2/G3) and ~11 D for the high-energy group (G4). The poor one-parameter-set fit for BTZ-CBZ may be due to aggregation. In IPM glass, the emission peak of BTZ-CBZ shifts ~1100 cm-1 lower in energy when the concentration is increased from 1 µM to 100 µM. While the solubility of BTZ-CBZ in MT is much better than in IPM, emissive aggregates could still be present due to the high sample concentrations required for Stark spectroscopy. This may contribute to the red tail in emission (Figure 8d) and result in a second species with an electric field response for BTZCBZ. No aggregation was apparent for BOX-CBZ. If the low-energy band identified in the fitting corresponds to an aggregated species then the high-energy band is the CT state and |Δ0, | is again lower than |Δ0% | for BTZ-CBZ. As noted above, the maximum value of |Δ0| is ~36-37 D assuming the electron and hole wavefunctions are uniformly spread over the acceptor and donor, respectively. The measured |Δ0| values for the CT state represent an over 50% charge transfer in absorption, but only ~30% in EF. Two possible explanations of this decrease in |Δ0| are a loss of CT character in the electronic excited state due to external perturbations or the internal effect of planarization of the excited state geometry. As the emission spectra in the solid state are similar to those in non-polar solvent, a significant alteration of the electronic character of the excited state is unlikely. Likewise, the calculations in solution show only a slight planarization of the molecules between the Franck-Condon and emission geometries and this motion is likely to be further restricted at low temperature. Overall, the fact that |Δ0% | is significantly larger than |Δ0, | in a non-polar matrix argues against a picture in which the LE state is accessed in absorption whereas emission occurs from a state with more CT character. However, it is difficult to assess to what degree the difference between |Δ0% | and |Δ0, | reflects the greater complexity of the absorption versus the emission spectra and the need to de-convolve the contributions of multiple electronic states of similar energies in the former. Ultrafast transient spectroscopy measurements are therefore underway to address the question of whether these systems evolve from LE to CT character with increasing solvent polarity.


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Figure 8. Emission (a,d) and electrofluorescence (EF) (b,e) for BOX-CBZ (a-c) and BTZ-CBZ (d-f) in 2-methyltetrahydrofuran (MT) solvent glass with the Liptay model fit and the constituent derivative components (c,f). The individuals Gaussians used to fit the emission spectra are shown and labeled in order of increasing peak energy (G1-Gn). Both samples were excited at 347 nm, a zero-crossing in the EA spectra. The dotted line at zero EF signal is for illustrative purposes.

Some quenching of the emission intensity due to the applied field is observed in BTZ-CBZ but not BOXCBZ (Figure 8, compare zeroth derivative component in panels c and f). One possible cause is a fieldinduced change in the degree of charge separation, similar to what has been observed for exciplexes64-66 and some conjugated materials.37, 67-68 Though it is not obvious why BTZ-CBZ would be more susceptible to this than BOX-CBZ, minimizing field-induced emission quenching is favorable for the operation of the material in OLED devices that operate under fields one or two orders of magnitude larger than those utilized here.68-69 Overall, the emission behavior of BOX/BTZ-CBZ in the solid state is promising for mitigating unwanted emission energy shifts due to the polarity of the local environment, leading to higher purity of the emission color in devices. Likewise, the CT character is retained in the solid state which supports small values of Δ and therefore efficient TADF. Follow-up studies are underway to determine the effect of varying the local polarity and rigidity on the TADF efficiencies of these compounds. 19 ACS Paragon Plus Environment


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Conclusions Two novel blue-emitting donor-acceptor TADF compounds, BOX-CBZ and BTZ-CBZ, are shown to have a rich electronic state structure including a CT state accessible by absorption that is the emissive state for both compounds. This state broadens, red shifts, and becomes less emissive with increasing solvent polarity. TD-DFT calculations indicate that these effects are due to the slow dielectric response of the solvent stabilizing the CT state. When these compounds are confined to the solid state, the solvatochromic response is suppressed and the emission maximum is independent of the bulk dielectric properties of the environment. Stark fluorescence measurements show that the emission transition nonetheless retains its high CT character in the solid state as is required for efficient TADF. Acknowledgement L. A. P. acknowledges NSF-CHE 1363050, E. F. acknowledges NSF CHE- 1307117, and L. R acknowledges NSF-DMR 1609451 for support. Y. L. is grateful for a fellowship from The Professor Richard F. Eisenberg and Harriet Rippey Eisenberg Fund. Supporting Information for Publication Spectra as a function of time and prompt and delayed emission kinetics for BOX/BTZ-CBZ in toluene under air and argon, linear fitting and scaling information for calculated Δ , detailed TD-DFT results, Stark EA and EF coefficients and values for Δ., Lippert-Mataga solvatochromism analysis, absorption spectra in solvents of differing polarity, and a derivation of eq. 11. The Supporting Information is available free of charge on the ACS Publications website.


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