Article Cite This: J. Phys. Chem. C 2018, 122, 23091−23101
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Fluorescence, Phosphorescence, or Delayed Fluorescence?A Theoretical Exploration on the Reason Why a Series of Similar Organic Molecules Exhibit Different Luminescence Types Ying-Chen Duan,† Li-Li Wen,† Ying Gao,† Yong Wu,‡ Liang Zhao,† Yun Geng,*,† Guo-Gang Shan,*,† Min Zhang,† and Zhong-Min Su*,† †
Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China School of Pharmaceutical Sciences, Changchun University of Chinese Medicine, 1035 Boshuo Road, Changchun 130117, P. R. China
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‡
S Supporting Information *
ABSTRACT: In contrast to the traditional view that the small organic molecules emit fluorescence, more and more experiments manifest their special luminescence types, such as the thermally activated delayed fluorescence (TADF) and roomtemperature phosphorescence. Why the similar organic molecules exhibit different luminescence types is focused on in this work on the basis of density functional theory/timedependent density functional theory calculations on a series of small organic molecules with phenoxazine or carbazole as a donor and diphenyl-triazine as an acceptor. The deep analysis of the geometrical and electronic structures shows how the structure, especially for the donor−acceptor dihedral angle, determines the singlet−triplet energy gap and the property of excited states. The explorations on the electron−hole pairs of natural transition orbitals and the contribution of the key heteroatom (N) to different molecular orbitals reveal the distinct electron transition processes of excitation to singlet and triplet states, explain the reason for different energy-level distributions of excited states, and identify which pairs have more favorable intersystem crossing for these molecules, while the calculations of spin−orbit coupling and reorganization energy display the efficiency of the different luminescence types. Meanwhile, considering the potential application of TADF materials in organic light-emitting diodes, we also separately modified the phosphorescent molecule and the prompt fluorescent molecule through the introduction of methyls to increase the steric hindrance and realize the perpendicular orientation of donor and acceptor unit, and finally to screen the excellent TADF molecules theoretically.
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INTRODUCTION In recent years, the luminescence type of small organic molecules has been developed from traditional fluorescence to delayed fluorescence and phosphorescence. Thermally activated delayed fluorescence (TADF)1,2 materials, which perform upconversion through reverse intersystem crossing (RISC) from triplet excitons, achieve as high internal quantum efficiency of 100% as phosphorescent transition-metal complexes. The non-noble-metal Cu complexes or small organic molecules as common emitting materials in this mechanism have the advantages of both low cost and high quantum efficiency.3−7 Phosphorescence materials with long lifetime could eliminate the interference of background fluorescence,8 and the discovery of phosphorescent pure organic molecules enriches this sort of materials.9−11 Thanks to the advantage in mass production, these two types of materials have favorable prospect in practical application. Compared with the traditional fluorescence molecules, TADF and pure organic phosphorescence molecules have © 2018 American Chemical Society
higher internal quantum efficiency because of the utilization of triplet excitons. They generally have stronger spin−orbit coupling (SOC) compared with that of traditional molecules and have heteroatoms such as N or O assisting the intersystem crossing (ISC) and RISC between excited singlet and triplet states.5−7,9−15 However, their luminescence mechanisms are different. The former radiates after the excited electron undergoes ISC from singlet to triplet state and RISC back to singlet state, while the latter radiates directly after the excited electron performs ISC to the triplet state at room temperature. What is the difference between them on geometrical and electronic structures? What is the reason for the different properties of their triplet states? Also, which factors influence the final luminescence type? These questions arouse our interest to explore their common and different properties from Received: July 9, 2018 Revised: September 16, 2018 Published: September 18, 2018 23091
DOI: 10.1021/acs.jpcc.8b06533 J. Phys. Chem. C 2018, 122, 23091−23101
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The Journal of Physical Chemistry C
ering that the calculations are for pure organic molecules, we adopted the 6-31G* basis set, which is enough to describe the common organic small molecules.25,26 As we know that the long-range corrected hybrid functional CAM-B3LYP is suitable for describing the charge transfer (CT) excited states,27 we used it to carry out the geometrical calculation of excited states, taking their distinct charge transfer characterization into account. We should mention that to reduce the errors during the structure comparison between S0 and excited states and the calculation of reorganization energy, we employed the CAMB3LYP functional in S0 state calculation as well. In addition, the S0 geometries optimized by functional CAM-B3LYP basically match the X-ray diffraction crystal structure obtained from the experimental reports22,28 and the results are close to those calculated by functional B3LYP, which is commonly used for organic small molecules (see Table S1), certifying the reliability of the chosen functional. Because the energy differences of the excited states are crucial to estimate the RISC ability, we should use an appropriate functional to obtain relatively accurate excitation energy. However, the conventional density functional often seriously underestimates the charge transfer transition energy,29,30 which goes against our calculation of the molecules with obvious donor and acceptor fragments. Range-separated exchange density functionals can lead to a balance between the electron correlation that describes the delocalization features and the exact-exchange effect that depicts the localization features by searching for the optimal range-separation parameter ω; thus, the electronic structures could be described accurately.31,32 According to the research of Jean-Luc Brédas’s group,33 long-range corrected functional LC-ωPBE can make accurate prediction of ionization potentials (IPs) and electron affinities (EAs). Furthermore, the Tamm−Dancoff approximation (TDA) method34 can mingle the part of configuration interaction with single excitations (CIS), which overestimates the excitation energy to offset the underestimation of charge transfer transition energy in triplet excitation energy calculation. In this way, more accurate singlet−triplet energy gap ΔEST could be obtained. Hence, we adopted TDA-LCωPBE/6-31+G* to conduct all electronic structure calculations of the excited states. The exchange term is divided into a short-range domain characterized by density functional theory (DFT) [the former on the right of formula (1)] and a long-range domain characterized by Hartree−Fock (HF) [the latter on the right of formula (1)]
a theoretical perspective although there have been works focusing on their respective luminescence mechanisms either in experiment or in theory.16−21 On the basis of the questions above, we chose a series of small molecules with similar geometrical structures but different luminescence types (see Figure 1) to conduct a
Figure 1. Molecular structures of PXZ-TRZ, DPhCzT, Cz-TRZ, and DPhPXZT.
theoretical research. The phenothiazine-2,4,6-triphenyl-1,3,5triazine (PXZ-TRZ) designed by Chihaya Adachi’s group is an efficient TADF molecule.22 The 4,6-diphenyl-2-carbazole1,3,5-triazine (DPhCzT) synthesized by Wei Huang’s group was identified to emit ultralong phosphorescence, whose Htype aggregation was speculated to extend the lifetime of the excited states. The 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazole (Cz-TRZ) reported by Kim et al. is a traditional fluorescent molecule.23 Additionally, we also add 4,6-diphenyl-2-phenothiazine-1,3,5-triazine (DPhPXZT) to have a theoretical comparison. Because both TADF and room-temperature phosphorescence involve the crossing process between singlet and triplet states, we investigated ISC/RISC and their determining factors in detail, expecting to ascertain why these molecules have different luminescence types. Furthermore, organic light-emitting diodes (OLEDs) are a significant application of organic semiconductor molecules, whose widespread use is still restricted by the efficiency and stability of the luminescent materials. Because of the high internal quantum efficiency of TADF materials, we predict that they have tremendous potential application prospect in OLEDs. According to the theoretical analysis in this article, we attempt to modify the low-efficiency normal fluorescent molecule and the phosphorescent molecule and turn them into TADF molecules. Therefore, we designed a series of molecules through the introduction of methyls to enlarge steric hindrance and change the structure, and finally adjust the energy difference between the lowest singlet and triplet excited states (S1 and T1). At the same time, we assessed their TADF performances from a theoretical perspective and synthesized a representative compound to prove the strategy.
1 − [α + β ·erf(ωr12)] α + β ·erf(ωr12) 1 = + r12 r12 r12
(1)
Here, r12 represents the interelectronic distance, α and β are constants (0 ≤ α ≤ 1, 0 ≤ β ≤ 1), erf(ωr12) denotes the error function, and ω is the reciprocal of the demarcation point where the exchange term separates into long-range and shortrange domains. To get the optimal ω value, ω must make the exact functional obey the exact Kohn−Sham (KS) or generalized KS (GKS) theory,35 which means for an N-electron system, the opposite number of the highest occupied molecular orbital (HOMO) energy value must equal the vertical IP and J = |εH(N) + IP(N)| should be as small as possible.36 For a donor−acceptor system, in addition to the IP that closely relates to the donor fragment, the vertical EA in connection
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METHODOLOGY AND COMPUTATIONAL DETAILS Geometrical and electronic structures of all S0 state molecules were calculated by Gaussian 09 program package.24 Consid23092
DOI: 10.1021/acs.jpcc.8b06533 J. Phys. Chem. C 2018, 122, 23091−23101
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The Journal of Physical Chemistry C
Table 1. Bond Lengths (Å) and Dihedral Angles (°) between the Donor and the Acceptor at Optimized S0, S1, and T1 States, together with Their Differences (Δ) at the CAM-B3LYP/6-31G* Level PXZ-TRZ DPhCzT Cz-TRZ DPhPXZT PXZ-TRZ DPhCzT Cz-TRZ DPhPXZT
bond length/dihedral angle
S0
S1
Δ(S1 − S0)
T1
Δ(T1 − S0)
N2−C3 N2−C3 N2−C3 N2−C3 C1−N2−C3−C4 C1−N2−C3−N4 C1−N2−C3−C4 C1−N2−C3−N4
1.426 1.382 1.413 1.379 85.68 −17.65 52.21 −9.18
1.434 1.466 1.412 1.477 89.71 −37.90 52.34 −89.22
0.008 0.084 0.000 0.098 4.03 −20.25 0.13 −80.04
1.404 1.389 1.413 1.486 124.13 −20.20 52.67 −64.32
−0.022 0.007 0.000 0.106 38.45 −2.55 0.47 −55.14
Figure 2. Calculated HOMO and LUMO diagram of the investigated molecules.
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RESULTS AND DISCUSSION Comparative Analysis of the Molecular Luminescence Properties. Geometrical Structure and Frontier Molecular Orbital. As our previous work certifies, geometrical and electronic structures have a great influence on the energy gap between singlet and triplet states and the relevant ISC and RISC processes;43 thus, they are fully analyzed first in this work. For the four investigated molecules, the bond lengths and dihedral angles between donor and acceptor units at the S0 and excited states as well as the variations are listed in Table 1. It shows both the N2−C3 bond lengths and C1−N2−C3−N4 dihedral angles in PXZ-TRZ and Cz-TRZ are larger than those in DPhCzT and DPhPXZT. The larger dihedral angles in PXZ-TRZ and Cz-TRZ are ascribed to the steric hindrance between the H atoms in central phenyl and those in phenothiazine or carbazole. For DPhCzT and DPhPXZT, the formation of π bond between C3 and N2 atoms leads to their shorter bond length, which is related to the electron in p orbital of C3 atom vertical to the plane of triazine as well as the lone pair electrons of N2 atom perpendicular to the plane consisting of N2 atom and the three adjacent C atoms. This is clearly illustrated by the HOMO and LUMO maps in Figure 2, wherein the distinct bonding π orbital between C3 and N2 in DPhCzT and DPhPXZT and n orbital in PXZ-TRZ and CzTRZ are observed. Compared with those of the S0-state structures, the geometries at excited states (S1 and T1) undergo diverse changes, which may result in their different luminescence types to some extent. First, for the TADF molecule PXZ-TRZ and the traditional fluorescence molecule Cz-TRZ, the structures of S1 state change little compared with those of S0 state, referring to the small Δ(S1 − S0) in both bond length and dihedral angle (see Table 1), which may favor fluorescence emission from S1 to S0. However, for PXZ-TRZ, the structure of T1 state changes a lot, especially the C1−N2−C3−N4
with the acceptor should be considered as well. Also, the latter of an N-electron system could be taken as the IP of an (N + 1)electron system.37 Therefore, we should adjust ω to make the value of the following equation as small as possible 1
J2 =
∑ [εH(N + i) + IP(N + i)]2 i=0
(2)
Figure S1 shows the relation between ω and the sum of HOMO energy and ionization energy deviations J2 at the electronic state of N and (N + 1) electrons for PXZ-TRZ, DPhCzT, Cz-TRZ, and DPhPXZT. It is seen that the deviations are minimized when the ω values are near 0.18. On the basis of the diagram, we can have further calculations and obtain more precise ω values. The J2 values corresponding to the ω values are presented in Table S2, and the optimal ω values are screened out. We also use this way to find the optimal ω values of the modified molecules. The SOC matrix elements were calculated by ADF2013.01 program package,38 and the zeroth-order regular approximation method39 was used to consider the relativistic effects for all atoms under all electron TZP basis set. All overlap integrals of the norms of HOMO and lowest unoccupied molecular orbital (LUMO), overlap integrals of the norms of natural transition orbitals (NTOs), and the centroid distance between NTOs were obtained by Multiwfn program.40 The reorganization energies from S1 to S0 and from T1 to S0 were computed by DUSHIN program package.41,42 Furthermore, we should note in passing that the purpose of this article is to explore the basic mechanism for different luminescence types at the level of a single molecule; the intermolecular interaction or the packing effect is not involved herein. 23093
DOI: 10.1021/acs.jpcc.8b06533 J. Phys. Chem. C 2018, 122, 23091−23101
Article
The Journal of Physical Chemistry C dihedral angle, which may hinder its phosphorescence emission from T1 to S0 since the geometry relaxation between S0 and excited states would increase nonradiation energy dissipation. Second, for the organic phosphorescence molecule DPhCzT, S1 structure changes more than T1 structure according to the much smaller variations in N2−C3 bond length and C1−N2−C3−N4 dihedral angle at T1 state than those at S1 state compared with S0 state, which indicate its more favorable phosphorescence emission from T1 to S0. Finally, does the little change in geometry for both S1 and T1 compared with that for S0 state impart the favorable fluorescence and phosphorescence for Cz-TRZ? This will be further explained in the next section. It is well known that the electronic structure has a dominant influence on the photoluminescence property; thereby, it is important to have a clear insight into the frontier molecular orbitals of these compounds. It is illustrated in Figure 2 that the HOMOs mainly locate at phenothiazine or carbazole and the LUMOs mainly delocalize at triazine and benzene rings. For the TADF molecule PXZ-TRZ, the small HOMO− LUMO overlap results from the near orthogonal orientation of phenothiazine relative to triazine and benzene ring. For the HOMOs of DPhCzT and DPhPXZT, there are still some distributions at triazine in addition to carbazole or phenothiazine induced by the smaller C1−N2−C3−C4 dihedral angle, which increase their respective HOMO− LUMO overlaps. For Cz-TRZ, the LUMO distribution resembles that of PXZ-TRZ, but the relative smaller C1− N2−C3−C4 dihedral angle than that of PXZ-TRZ gives rise to its overlapped distribution with HOMO at the benzene ring. One of the frontier molecular orbital distinctions of these four compounds is their respective HOMO−LUMO overlaps, which may give rise to different photoluminescence properties. Decreasing as much as possible the HOMO−LUMO overlap is favorable to obtain smaller ΔEST, which is one important characteristic of TADF materials.44 Therefore, we calculated the orbital overlap integral ⟨ΨH|ΨL⟩ for all compounds and list them in Table S3 to quantitatively scale the HOMO−LUMO overlap. We observe that PXZ-TRZ has the smallest overlap integral value among these four compounds, which drops its ΔEST and promotes the RISC. This gives us an inspiration that we could modify the molecules to increase the steric hindrance in other ways to turn the donor−acceptor relative position from planar to nearly perpendicular and obtain TADF features. On the other hand, Cz-TRZ showing the highest overlap integral is suspected to be related with its traditional fluorescence. DPhCzT and DPhPXZT, in which the benzene ring is absent, also show much larger overlap integral than that of PXZ-TRZ. Excited-State Properties. The excited-state properties such as excitation energy, transition configuration, and hole− electron distribution at excited states could directly reflect the luminescence of a compound. First, the ΔEST value influences the ISC and RISC ability, which imparts whether it is easy to reach the triplet excited state and go back. Thus, the excitation energies of S1 and T1 as well as the corresponding vertical ΔEST values for each molecule were calculated, and the results are collected in Table 2. Comparing the results, we find that the energy gap of PXZTRZ is an order of magnitude smaller than those of the other three molecules, and the ΔEST values of the latter molecules do not meet the qualification of typical TADF molecules for which the ΔEST is less than 0.1 eV,45 although the values are
Table 2. Calculated Excitation Energies of S1 (ES/eV) and T1 (ET/eV) as Well as the Vertical Singlet−Triplet Energy Gap ΔEST (eV) for Each Molecule in S0 Geometry with Their Optimal ω Values at the LC-ωPBE/6-31+G* Level ω ES ET ΔEST
PXZ-TRZ
DPhCzT
Cz-TRZ
DPhPXZT
0.1845 2.904 2.883 0.015
0.1785 3.839 3.466 0.373
0.1798 3.678 3.146 0.532
0.1760 3.680 3.381 0.300
also pretty small (
