Tuning the color of thermally activated delayed fluorescent properties for spiro-acridine derivatives by structural modification of the acceptor fragment: a DFT study

Abstract

A theoretical investigation on the relationship between the electronic structures and emission properties of thermally activated delayed fluorescent materials based on donor–acceptor type spiro-acridine derivatives has been performed. Efficient color tuning over the whole visible range has been achieved via structural modification of the acceptor fragment.

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Introduction

Recently, thermally activated delayed fluorescent (TADF) materials as a promising alternative solution for the conversion of triplet into singlet excitons for highly efficient OLEDs were reported,¹ corresponding to a high external quantum efficiency of close to 20% for TADF.² Pure organic TADF materials will be useful for the realization of the rare-metal free, low-cost fabrication of flat-panel displays and general lighting.³ TADF is significant only when the quantum yields of the triplet formation (φ_T) and singlet formation (φ_S) are both high. This in turn implies that a small energy gap (ΔE_ST) between the first singlet excited state (S₁) and the triplet excited state (T₁), a long T₁ lifetime, and a temperature that is not too low, are necessary for TADF.⁴ The ΔE_ST is proportional to the exchange integral (J_LU) between the spatial wave functions of a highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).⁵ Thus, TADF materials must have an effective HOMO and LUMO separation in order to obtain a near zero ΔE_ST, and the effective intermolecular charge transfer from the π to π* orbital, while maintaining a reasonable high radiative decay rate of fluorescence.⁶ To achieve the separation of the HOMO and LUMO, an effective method for steric separation is to introduce a spiro-junction between the acceptor and donor fragments.⁷ Adachi and co-workers have reported a series of efficient TADF materials based on spiro structures, such as a spirobifluorene derivative^6a and a spiro-acridine derivative (ACRFLCN),⁸ whose emitting colors resided in the yellow region. Therefore, it is eagerly anticipated that the emission color of spiro-based TADF materials can be tuned from blue to red by modifying the electron donor or acceptor fragment.

In this communication, using ACRFLCN as the parent molecule, a series of derivatives were designed by utilizing “CH”/N substitution and changing the π-conjugation degree on the acceptor fragment in order to tune the emitting color.⁹ As shown in Fig. 1, an A series containing 1a–c and 2a–c, where 1 and 2 refer to mono- and di-substitutions, respectively, and a B series corresponding to the reduced conjugation derivatives 2d and 2e. The calculated emitting wavelengths cover the blue to red region, giving a high quantum emission yield. An in depth theoretical understanding of the delayed fluorescent properties of the designed spiro-acridine derivatives could be revealed.

Fig. 1 Molecular structures of the investigated spiro-acridine derivatives.

Results and discussion

A crucial step in theoretical investigations employing the density functional method (DFT) and the time dependent density functional method (TD-DFT) is the choice of an appropriate exchange correlation functional. The ground state and excited state geometries of the studied compounds were optimized using the DFT and TD-DFT methods with the restricted hybrid-type Perdew–Burke–Ernzerhof exchange correlation functional (PBE0), a functional which uses a 25% exchange and 75% correlation weighting.¹⁰ On the basis of the optimized S₁ structure, we have chosen different functionals for the computations of delay wavelengths (λ_TADF) using the 6-31G (d) and 6-311+G (d, p) basis sets, respectively. The calculated λ_TADF values for ACRFLCN together with the experimental data are shown in Fig. 2. The λ_TADF value calculated at the PBE0/6-31G (d) theoretical level (540 nm) is quite close to the experimental value (530 nm), with the deviation being 10 nm. The calculated λ_TADF values using the traditional B3LYP¹¹ and newly developed TPSSh¹² functionals are larger than the experimental value. The hybrid functional (M062X)¹³ and long range corrected functionals (CAM-B3LYP¹⁴ and wB97XD¹⁵) are also unacceptable for the current investigation, and overestimated the transition energy between the LUMO and HOMO.¹⁶ Meanwhile, the basis set effect was also considered in this study. The λ_TADF value calculated using the 6-31G (d) basis set is similar to that of the 6-311+G (d, p) basis set, which was augmented with a diffuse function. Therefore, the 6-31G (d) functional was selected for the following calculations in order to save computational resources. All of the calculations are performed with the Gaussian 09 package.¹⁷

Fig. 2 The calculated λ_TADF values for ACRFLCN with different functionals and basis sets on the basis of the resulting optimized S₁ state at the TD-PBE0/6-31G* level, together with the experimental result.

For the asymmetric mono-substitution of “CH”/N derivatives, the dihedral angles between the acceptor and donor fragments are 89.9°, 95.4°, and 99.6° for 1a–1c in the S₁ state, respectively. While the symmetric “CH”/N di-substituted derivatives 2a–2e in S₁ state have an orthogonal orientation between the donor and acceptor fragments with the distortion angles being 90.0° (Table S1†). This might be more favourable for the steric separation of the frontier molecular orbital (FMO). For all of the designed molecules, the largest difference between the bond lengths in the S₀ and S₁ states was found to be between the N and its neighbouring C atom with a length of about 0.03 Å (Fig. S1†). The bond lengths and dihedral angles of these spiro-acridine derivatives change slightly between the T₁ and S₁ states to within 0.02 Å and 1°, respectively, indicating that it is easy to get an efficient up-conversion from triplet to singlet excitons. Furthermore, the FMOs for all of these derivatives in the S₀ state show π character (Fig. S2†). The electron density distribution of the HOMO is located on the acridine donor fragment, while that of LUMO is located on the acceptor fragment. Hence, all of the complexes show a great separation of the HOMO and LUMO because of the spiro-conjugation. We also give the density of state (DOS) analysis to support the effect of the separation of the HOMO and LUMO (Fig. S3†). Meanwhile, the distribution of the electron density plots for the T₁ states (Fig. S4†) are similar to those for the S₁ states. It is also of benefit for the reverse intersystem crossing (RISC) from the T₁ to S₁ states.

Moreover, we also analyzed the ΔE_ST values for all of the derivatives in Table 1. The smaller the ΔE_ST value at a given temperature, the easier it is for RISC from the T₁ to S₁ states. Firstly, we discussed the substituent position effect on the ΔE_ST values. The calculated ΔE_ST value for ACRFLCN is 0.0102 eV at the PBE0/6-31G (d) level, which is close to the Adachi et al. calculated value of 0.0083 eV at the B3LYP/6-31G (d) level of theory.⁸ This small difference arises from the use of different density functionals. For the asymmetric mono-substitution of the “CH”/N derivatives 1a–c, “CH”/N substitution at the Z-position does not significantly affect the ΔE_ST value, whereas the ΔE_ST values of the “CH”/N substituted derivatives at the Y- and X-positions are increased to 0.0164 eV and 0.0157 eV. For the symmetric di-substitution of “CH”/N derivatives, the order of the ΔE_ST values is 2b (0.0168 eV) > 2a (0.0109 eV) > 2c (0.0064 eV). 2c, i.e. with a di-substitution at the X, X′-positions, has the smallest ΔE_ST value, which can be traced back to the more effective separation between its HOMO and LUMO, and donor/acceptor compositions (%) of the LUMO in the S₀ state that are 0.7 : 99.3 (Table S2 and Fig. S5†).

Table 1 The vertical excitation energy (E_S1 and E_T1) and vertical singlet-triplet energy gap (ΔE_ST) of these studied spiro-acridine derivatives at the TD-PBE0/6-31G (d) level (in eV)

	ES1	ET1	ΔEST
ACRFLCN	2.7277	2.7175	0.0102
1a	2.6010	2.5907	0.0103
2a	2.4737	2.4628	0.0109
1b	2.6807	2.6643	0.0164
2b	2.6261	2.6093	0.0168
1c	2.2792	2.2635	0.0157
2c	2.0834	2.0770	0.0064
2d	3.2893	3.2786	0.0107
2e	2.7629	2.7530	0.0099

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The slight discrepancies in the ΔE_ST values between the symmetric di-substituted and the asymmetric mono-substituted derivatives for a and b are 0.0006 eV and 0.0004 eV, respectively, i.e. the asymmetric mono-substitution and symmetric di-substitution of “CH”/N substituted derivatives at the Z-position and Y-position do not significantly affect the ΔE_ST value due to a slight change in the optimized structural parameters for the S₀ states. However, for the X-position substituted derivatives, the di- and mono-substitution exhibited a significant effect on the ΔE_ST value. The twisted angle between the donor and acceptor fragments of 1c in the ground state is 79.7°, which is smaller than that for 2c (90.0°). The electronic separation between the donor and acceptor moieties of 2c is more effective than that for 1c due to the larger twisted angles of 2c, indicating that the ΔE_ST value for 2c is smaller compared to the value for 1c. Thus, we only considered the “CH”/N di-substitution at the X_B, X^′_B-positions for the B series. The calculated ΔE_ST values for 2d (0.0107 eV) and 2e (0.0099 eV) are similar to that for the parent molecule ACRFLCN. The ΔE_ST value for 2e, which is the derivative with “CH”/N di-substitution at the X_B, X^′_B-positions, is smaller than that for the unsubstituted 2d. Thus, smaller ΔE_ST values were obtained via “CH”/N di-substitution at the X, X′-positions for spiro-junction TADF materials. It is instructive to estimate the RISC rate constant (k_RISC) using the equation k_RISC = A exp(−ΔE_ST/RT). It is known that high values of k_RISC are favored by a small ΔE_ST at a given temperature in the absence of oxygen. Consequently, the k_RISC values of 2c and 2e with the smaller ΔE_ST values should be higher than for those of other investigated derivatives.

It is known that the photophysical properties of the complexes depend strongly on the character of the FMOs. The electron density plots of the HOMOs and LUMOs and the energy gaps (ΔE_H–L) of the studied spiro-acridine derivatives in the S₁ state are depicted in Fig. 3. It is noted that the HOMOs are located on the donor fragments and the LUMOs reside in the acceptor fragments in the S₁ states. The ΔE_H–L values for the A series are reduced compared to the value for ACRFLCN due to the “CH”/N substitution effect. Compared with the asymmetric mono-substituted derivatives at the same position, the ΔE_H–L values for these symmetric di-substituted derivatives decreased, and the emission wavelengths are predicted to red-shift. The HOMO energy levels in 2a and 2b are decreased, whereas that in 2c is increased. On the other hand, the LUMO energy levels in 2a–2c are lower than those in ACRFLCN. A larger change in the LUMO energy levels than those in the HOMOs is observed, because the LUMO is mostly located on the “CH”/N substituted derivative acceptor fragment. In contrast, the LUMO energy levels in the B series are higher than that in ACRFLCN, and consequently the ΔE_H–L values are enlarged due to a decrease in the degree of π-conjugation. Therefore, we anticipate that blue- and red-shifted emission wavelengths could be observed for 2d and 1(2)a–c, respectively, compared with that for ACRFLCN.

Fig. 3 The electron density plots and energy levels of the HOMOs and LUMOs for these investigated spiro-derivations of the S₁ states.

The time dependent density functional theory (TD-DFT) calculated delayed fluorescent properties of these spiro-acridine derivatives are listed in Table 2. The FMO distribution shows that the delayed fluorescent band for all of the derivatives can be assigned to a π* → π transition. Similar to the order of the ΔE_H–L values, the singlet vertical emission energies of the asymmetric and symmetric derivatives follow the same tendency: 1c < 1a < 1b < ACRFLCN, 2c < 2a < 2b < ACRFLCN ≈ 2e < 2d, respectively. For the symmetric di-substituted derivatives 2a–2c, the λ_TADF values (611 nm, 567 nm, and 766 nm) are bathochromically-shifted by 71 nm, 27 nm, and 226 nm, respectively, which is caused by the “CH”/N substitution at different positions. The derivative 2c with the largest Stokes shift among the symmetric di-substituted derivatives exhibits a red emission. However, it is interesting to note that 2d exhibits a blue emission centred at 432 nm with the smaller Stokes shift, which might result from a decrease in the degree of π-conjugation. We also calculated the phosphorescence emission wavelength (λ_ph) using the TD-PBE0 method on the basis of the optimized T₁ states. The λ_ph data are similar to the λ_TADF data, and the small variation between the λ_TADF and λ_ph values is of benefit for achieving the TADF phenomenon.

Table 2 The delayed fluorescence emission wavelength (λ_TADF) and phosphorescence emission wavelength (λ_ph), corresponding to the absorption wavelength (λ_abs) and Stokes shift of the explored complexes

	λabs (nm)	λTADF (nm)	λph (nm)	Stokes shift
ACRFLCN	455	540	540	85
1a	477	571	575	94
2a	501	611	615	110
1b	463	560	565	97
2b	472	567	571	95
1c	544	723	731	179
2c	595	766	770	171
2d	377	432	430	55
2e	449	536	535	87

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Conclusion

In summary, we have successfully demonstrated and elucidated the broad range color tuning of spiro-acridine TADF materials via “CH”/N substitution and conjugation degree control on the acceptor fragment. All of the designed TADF materials possess small ΔE_ST values, which may provide an efficient up-conversion from the T₁ to S₁ levels. The calculated delayed emission wavelengths cover the whole visible region. The λ_TADF values for the “CH”/N substituted derivatives are bathochromically-shifted compared with that for the parent compound. The derivative 2c, which is “CH”/N substituted at the X, X′-positions, exhibits a red emission, and shows the highest k_RISC value, a factor that contributes to an increase in the emission quantum yield, observed among all of the investigated derivatives. The derivative 2d exhibited a blue emission centred at 432 nm with the smallest Stokes shift due to a decrease in the degree of π-conjugation. Our theoretical studies provide hints for the design of efficient spiro-conjugated TADF-OLED emitting materials with higher k_RISC values as blue and red emission materials in the future.

Acknowledgements

The financial support from NSFC (21173037 and 21203021) and Specialized Research Fund for the Doctoral Program of Higher Education for New Teachers (20120043120008) are gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Methodology in detail, molecular geometries in the ground and excited states, frontier molecular orbital analysis, density of states (DOS), transition density matrix, and constrained density functional theory (C-DFT) calculations. See DOI: 10.1039/c4ra15155k

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