Theoretical study of the optical and charge transport properties of π-conjugated three-coordinate organoboron compounds as organic light-emitting diodes materials

Abstract

A series of π-conjugated three-coordinate organoboron compounds has been designed as luminescent and charge transport materials for organic light-emitting diodes (OLEDs). Their optical, electronic, and charge transport properties have been explored with theoretical method. The frontier molecular orbital (FMOs) analysis has turned out that the vertical electronic transitions of absorption and emission are characterized as intramolecular charge transfer (ICT). The optical, electronic, and charge transport properties of the designed compounds are affected by the introduction of different conjugate π-bridges. Our results suggest that the designed compounds can serve as luminescent materials for OLEDs. In addition, the predicted mobility indicates that some designed compounds are expected to be promising candidates for hole or electron transport materials for OLEDs.

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Introduction

Organic π-conjugated luminophores have recently received considerable interest due to their wide range of applications in optoelectronic devices such as emitters and electron-transport materials for organic light-emitting diodes (OLEDs), organic solid-state lasers (OSLs), and fluorescent sensors.^1–6 Especially, π-conjugated organoboron compounds have gained great attention for use in OLEDs because of their excellent optical properties such as, strong fluorescence with high quantum yields, high molar extinction coefficients, sharp absorption and fluorescence emission spectra, and high photo and chemical stability.^7–9 Unfortunately, the lower efficiency of OLEDs limits their commercialization application. In order to obtain high efficiency of OLEDs, the intense luminescence and high carrier mobility of materials are the two most important parameters. It is therefore necessary to develop new multifunctional materials, which exhibit high carrier mobility and excellent luminous efficiencies for use in OLEDs.^10,11 The π-conjugated three-coordinate organoborons display strong electron accepting ability¹² and spin carrier properties¹³ owing to the p_π–π* conjugation between vacant π-orbital of boron with π* of π-conjugated framework. Particularly, the introduction of an electron-donating group for the triarylboron-systems can improve the intense luminescence and carrier mobility. Thus, this type of materials can serve as bifunctional materials as both light emitters and electron transporters for OLEDs.^14,15 On the other hand, theoretical study is capable of providing useful insights into the understanding of the nature of molecules for design and synthesis of materials.^16–18 Recently, some π-conjugated three-coordinate organoboron compounds have been reported. It was found that these molecules exhibited long emission wavelength, large Stokes shift, and high solid-state fluorescence efficiency properties.¹⁹

In this contribution, in order to investigate the influence of topology for π-conjugated three-coordinate organoboron compounds, several novel organoboron derivatives, as shown in Scheme 1, have been designed to investigate their optical and charge transport properties with theoretical methods. The frontier molecular orbitals (FMOs), including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies, the HOMO–LUMO gaps (E_g), the reorganization energy as well as the absorption and fluorescence spectra were predicted with theoretical method. The carrier mobility property of the compounds under investigation was also investigated. It provides a demonstration for the rational design of novel luminescent and charge transporting organoboron materials for OLEDs.

Scheme 1 Molecular structures of the investigated molecules.

Computational methods

All the calculations were performed with aid of the Gaussian 09 software.²⁰ The PBE0 (ref. 21) method was employed to optimize the geometries of the compounds under investigation in ground states (S₀). The corresponding the geometries in the first excited singlet states (S₁) were optimized using TD-PBE0. All geometry optimizations were performed using the 6-31G(d,p) basis set.²² The harmonic vibrational frequency calculations using the same methods as for the geometry optimizations were used to ascertain the presence of a local minimum. The absorption and fluorescent spectra of the compounds under investigation were predicted by TD-PBE0/6-31G(d,p) based on the optimized S₀ and S₁ geometries, respectively.

The charge transport in the organic solid can be viewed as hopping process according Marcus electron transfer theory.^23,24 The charge transfer reaction is the self-exchange reaction. Namely, the free energy difference (ΔG°) is approximately taken as zero in the transfer process between the initial and final states. Thus, the transfer rate constant is represented as:

K = (V²/ℏ)(π/λk_BT)^½ exp(−λ/4k_BT) (1)

where, T, k_B, V, and λ are the absolute temperature, the Boltzmann constant, the charge transfer integral, and the reorganization energy, respectively. It can be seen from eqn (1) that the large V and small λ values are the two most important parameters for effective transport.

The charge transfer integral V for hole (electron) transfer in this scheme can be written as^25,26

V_ij = 〈ϕ⁰₁|F̂⁰|ϕ⁰₂〉 (2)

here, ϕ⁰₁ and ϕ⁰₂ represent the unperturbed frontier orbital of the two adjacent molecules 1 and 2, respectively. F̂⁰ is the Kohn–Sham–Fock matrix of the dimer obtained with non-interacting molecular orbitals, which can be separately studied by using the standard self-consistent field procedure. In the present study, the transfer integrals have been calculated at the DFT/pw91pw91/6-31G(d) level,²⁷ which method gave reasonable description for intermolecular coupling term reported in literature.²⁸

For the reorganization energy λ, they can be divided into external reorganization energy (λ_ext) and internal reorganization energy (λ_int) two parts. The λ_ext corresponds to the effect of polarized medium on charge transfer, while the λ_int represents a measure of structural change between ionic and neutral states.²⁹ The calculated λ_ext values in pure organic condensed phases are much smaller than their λ_int and are not expectable to be the main factor of reorganization energy because of the low dielectric constant of medium.^30–32 Therefore, in this paper, the λ_int is considered exclusively, ignoring any environmental relaxation and changes. The electron (λ_e) and hole (λ_h) reorganization energy can be calculated by definition:³³

λ_e = (E₀⁻ − E₋⁻) + (E⁰₋ − E⁰₀) (3)

λ_h = (E₀⁺ − E₊⁺) + (E⁰₊ − E⁰₀) (4)

where, E₀⁻ (E₀⁺) represents the energy of the anion (cation) calculated with the optimized structure of the neutral molecule, E₋⁻ (E₊⁺) is the energy of the anion (cation) calculated with the optimized anion (cation) structure, E⁰₋ (E⁰₊) corresponds to the energy of the neutral molecule calculated at the anionic (cationic) state, and E⁰₀ is the energy of the neutral molecule in ground state. For comparing with the interested results reported in literature,^34,35 the λ_e and λ_h of the molecules under investigation were calculated from the single point energy at the B3LYP³⁶/6-31G(d,p) level on the basis of the PBE0/6-31G(d,p) optimized neutral, anionic, and cationic geometries.

The drift mobility of hopping μ can be obtained from the Einstein relation

where, D represents the diffusion coefficient, which can be evaluated as follows:³⁷here, d is the center mass distance to adjacent molecules, n = 3 means the spatial dimension of the crystal, k is the hopping rate due to charge transfer to neighbour. Then the hopping time between two neighbour molecules corresponds to the inverse of the rate constant 1/k. P represents the probability of the specific hopping route for charge transfer to a particular neighbour, which can be evaluated from .

In this work, because the crystal structures of the designed compounds are not available, the molecular crystal structures were predicted by the module Polymorph of software package Materials Studio.³⁸ This method have been used to predict the crystal structures reported in the literature.³⁹ The Dreiding force field was used for the prediction. Coulomb and van der Waals interactions were evaluated by using the Ewald summation method. The cutoff and accuracy tolerance of Ewald were set to 6 Å and 0.0001 kcal mol⁻¹, respectively. The geometry of the cluster models used in present study were taken from PBE0/6-31G(d,p) level.

The stability is a useful criterion to evaluate the nature of devices for charge transport and luminescent materials. To predict the stability of compounds under investigation, the absolute hardness, η, of compounds under investigation were calculated using operational definitions given by:⁴⁰

where, μ and N represents the chemical potential and the total electron number, respectively. In this work, the values for IP (adiabatic ionization potential) and EA (adiabatic electron affinity) were determined according to the equation IP = E_cr − E_p and EA = E_p − E_ar, where p, cr, and ar indicate the parent molecule and the corresponding cation and anion radical generated after electron transfer.

Results and discussion

Frontier molecular orbital

The Cartesian coordinates of 1–8 in the S₀ and S₁ are given in Tables S1 and S2 in ESI,† respectively. Since the FMOs and E_g are heavily related to the optical, electronic, and charge transport properties, it is useful to analyze the HOMOs and LUMOs of the designed compounds. The distributions of the FMOs for the compounds under investigation were investigated, and their FMOs plots are present in Fig. 1. The total and partial densities of states (TDOS and PDOS) on each fragment of the investigated molecules around the HOMO–LUMO gaps were calculated using the same methods as for the geometry optimizations. The contributions of the dimesityl(phenyl)borane (DMPB), conjugate bridge (CB), and N,N-dimethylbenzenamine (MBA) fragments (in%) to the FMOs of the designed compounds were calculated on the basis the Mulliken population analysis at the current level of theory, as shown in Table 1. As visualized in Fig. 1, the FMOs all show π characteristics. The S₀ → S₁ excitation process can be mainly assigned to the HOMOs → LUMOs transitions, which corresponds to a π–π* excited singlet state. One can see that the HOMOs and LUMOs are spread over the whole conjugated molecule. It indicates that the spatial overlap between the HOMOs and LUMOs are strong, which may result in stronger optical absorption corresponding to the transition from HOMOs to LUMOs. Furthermore, the distribution patterns of HOMOs and LUMOs also provide a remarkable signature for the intramolecular charge-transfer (ICT) character of the vertical S₀ → S₁ transition. The results displayed in Table 1 reveal that the contributions of DMPB fragments for 1–8 to LUMOs are increased, while the corresponding contributions of MBA fragments are decreased compared with those of to HOMOs, respectively. For CB fragments, the contributions of 1–6 are decreased, while the corresponding contributions of 7 and 8 increased compared with those of to HOMOs, respectively. It implies that the excitations of the electron from the HOMOs to LUMOs lead the electronic density to flow mainly from the MBA and CB fragments to DMPB fragments for 1–6, while the corresponding electronic density flow mainly from MBA fragments to CB and DMPB fragments for 7 and 8. The percentages of charge transfer are the differences between the contributions of fragments for LUMOs and HOMOs. The percentages of charge transfer from MBA and CB fragments to DMPB fragments are 54.3–80.4% for 1–6. The percentages of charge transfer values is in the sequence 1 (54.3%) < 6 (57.1%) < 3 (63.0%) < 4 (65.0%) < 5 (70.5%) < 2 (80.4%). The corresponding percentages of charge transfer from MBA fragments to CB and DMPB are 66.7 and 70.5% for 7 and 8, respectively. It suggests that MBA and CB fragments serve as donors and DMPB fragments serve as acceptors for 1–6. The MBA fragments serve as donors and CB and DMPB fragments serve as acceptors for 7 and 8. Furthermore, the photophysical properties of intramolecular charge transfer are well known and highly dependent on the electron donor/acceptor strength.^41,42 The introduction of different conjugate π-bridges strengthens the electron-donating (-withdrawing) abilities of donors (acceptors), which can induce strong intramolecular CT transition. As a result, the π-conjugated three-coordinate organoboron compounds are capable of fulfilling both high fluorescence efficiency and large Stokes shifts, which make them promising materials for applications in OLEDs.^43–45

Fig. 1 The electronic density contours of the frontier orbital for the studied compounds at the PBE0/6-31G(d,p) level.

Table 1 The FMOs energies E_HOMO and E_LUMO, HOMO–LUMO gaps E_g (eV), and HOMOs and LUMOs contributions (%) to the FMOs of 1–8 at PBE0/6-31G(d,p) level

Species	HOMO				LUMO				Eg
	EHOMO	DMPB^a	CB^b	MBA^c	ELUMO	DMPB^a	CB^b	MBA^c
a DMPB: dimesityl(phenyl)borane moieties.b CB: conjugate bridge moieties.c MBA: N,N-dimethylbenzenamine moieties.
1	−5.062	17.4	19.0	63.6	−1.616	71.7	16.0	12.3	3.446
2	−5.194	4.5	16.6	78.9	−1.545	84.9	12.3	2.8	3.649
3	−5.066	11.4	29.3	59.3	−1.642	74.4	19.8	5.8	3.424
4	−4.931	14.2	34.2	51.6	−1.572	79.2	14.9	5.9	3.341
5	−4.840	14.9	40.0	45.1	−1.484	85.4	10.8	3.8	3.355
6	−5.046	6.2	47.0	46.8	−1.642	63.3	32.9	3.5	3.404
7	−5.213	6.1	21.4	72.5	−2.177	14.0	80.2	5.8	3.036
8	−5.313	5.9	20.3	73.8	−1.551	74.1	22.6	3.3	3.762

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With the aim to gain insight into the influence of the optical, electronic, and charge transport properties, another way is to examine the E_HOMO, E_LUMO, and E_g values. The E_HOMO, E_LUMO, and E_g of the designed compounds are given in Table 1. From Table 1, one can find that the E_HOMO values of 2, 3, 7, and 8 decrease, while the corresponding values of 4–6 increase compared with that of parent compound 1, respectively. The E_LUMO values are in the order of 5 > 4 > 6 > 1 > 3 > 2 > 7 > 8. However, the E_HOMO values of 2, 4, 5, and 8 increase, while the corresponding values of 3, 6, and 7 decrease compared with that of parent compound 1, respectively. The sequence of E_LUMO is 5 > 2 > 8 > 4 > 1 > 3 ≈ 6 > 7. Thus, the E_g values of 1–8 are in the order of 8 > 2 > 5 > 4 > 1 > 3 > 6 > 7. It suggests that the E_g values of the molecules with benzene (2), furan (4), 1H-pyrrole (5), and benzo[c][1,2,5]thiadiazole (8) conjugate π-bridges are smaller than that of the parent compound molecule with ethene conjugate π-bridge. The molecules with thiophene (3), benzo[d]thiazole (6), and benzo[c]thiophene (7) conjugate π-bridges are larger than that of the parent compound with ethene conjugate π-bridge. It suggests that the E_g values are affected by the introduction of different conjugate π-bridges to these molecules.

Absorption and fluorescence spectra

The most intense absorption (λ_abs) and fluorescence (λ_fl) wavelengths, the corresponding oscillator strength f, and main assignments of the designed compounds are listed in Tables 2 and 3, respectively. The λ_abs and λ_fl values of parent compound 1 are all in agreement with experimental results,¹⁹ the deviations are 18 and 6 nm, respectively. The Stokes shift of 1 is 33 nm (1808 cm⁻¹), which is comparable to the experimental 57 nm (3223 cm⁻¹). Therefore, the selected computational approach is reasonable for the investigated system.

Table 2 The most intense absorption wavelengths λ_abs (in nm), the oscillator strength f, and main assignments (coefficient) of 1–8 at the TD-PBE0/6-31G(d,p)//PBE0/6-31G(d,p) level, along with available experimental data

Species	λabs	f	Assignment
a Experimental result of 1 was taken from ref. 19.
1	411	1.30	H → L (0.70)
2	386	0.68	H → L (0.69)
3	419	1.04	H → L (0.70)
4	429	0.99	H → L (0.70)
5	426	0.96	H → L (0.70)
6	431	0.68	H → L (0.70)
7	499	0.49	H → L (0.70)
8	379	0.76	H → L (0.69)
Exp^a	393

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Table 3 The most intense fluorescence wavelengths λ_fl (in nm), the oscillator strength f, and main assignments (coefficient) of 1–8 at the TD-PBE0/6-31+G(d,p)//TD-PBE0/6-31(d,p) level, along with available experimental data

Species	λfl	f	Assignment
a Experimental result of 1 was taken from ref. 19.
1	444	1.34	L → H (0.70)
2	431	0.95	L → H (0.70)
3	469	1.28	L → H (0.70)
4	468	0.99	L → H (0.70)
5	350	0.18	L → H−3 (0.68)
			L → H−1 (0.12)
6	498	0.94	L → H (0.70)
7	594	0.47	L → H (0.70)
8	433	1.11	L → H (0.70)
Exp^a	450

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For the absorption spectra, the results presented in Table 2 show that the absorptions of the designed compounds are essentially originates from the S₀ → S₁ electronic transitions and HOMOs → LUMOs excitations play a dominant role. One can find that the λ_abs of 3–7 have bathochromic shifts 8, 18, 15, 20, and 88 nm (465, 1021, 857, 1129, and 4291 cm⁻¹) compared with that of the parent compound 1, respectively. The λ_abs of 2 and 8 show hypsochromic shifts 25 and 32 nm (1576 and 2054 cm⁻¹) compared with that of the parent compound 1, respectively. Moreover, the oscillator strengths f values of the compounds under investigation are slightly less than of the parent compound 1. The oscillator strength for an electronic transition is proportional to the transition moment.⁴⁶ In general, larger oscillator strength corresponds to larger experimental absorption coefficient or stronger fluorescence intensity. This indicates that the designed compounds show large absorption intensity.

For the fluorescence spectra, the LUMOs → HOMOs excitations play a dominant role for 1–4 and 6–8. The fluorescence's of the designed compounds are assigned to the S₁ → S₀ electronic transitions except for 5. As shown in Table 3, the λ_fl values of 3, 4, 6, and 7 show bathochromic shifts 25, 24, 54, and 150 nm (1201, 1155, 2443, and 5688 cm⁻¹) compared with that of the parent compound 1, respectively. On the other hand, the λ_fl values of 2, 5, and 8 have hypsochromic shifts 13, 94, and 11 nm (679, 6048, and 572 cm⁻¹) compared with that of the parent compound 1, respectively. The Stokes shifts of 1–4 and 6–8 are 3, 45, 50, 39, 67, 95, and 54 nm (1808, 2705, 2544, 1942, 3122, 3205, and 3290 cm⁻¹), respectively. Furthermore, the f values of the designed compounds are slightly less than of the parent compound 1. This implies that the designed compounds have large fluorescent intensity and large Stokes shifts. Therefore, they are promising luminescent materials for OLEDs.

Interestingly, the electronic transition of fluorescence for 5 is different from other designed compounds. The LUMO → HOMO−3 and LUMO → HOMO−1 excitations play a dominant role for 5. The main electronic transitions, λ_fl, the oscillator strength f, and main assignments (coefficient) of 5 are listed in Table S3 in ESI.† From Table S3,† one can find that the f values of both S₁ → S₀ and S₂ → S₀ electronic transitions are almost equal to zero, indicating that 5 shows weak fluorescence intensity in these two electronic transitions. However, the f values S₃ → S₀ electronic transition is 0.18. Therefore, the fluorescence of 5 is mainly attributed to the S₃ → S₀ electronic transition. Furthermore, as shown in Tables 2 and 3, the λ_abs value of 5 is larger than that of λ_fl value (the deviation is 76 nm). This can be explained by the fact that the 5 has worse conjugation due to large twist angle between DMPB with CB (−88.7°) in S₁. It suggests that the conjugative effect between DMPB, CB, and MBA fragments in 5 is weaker than that in S₀ (the corresponding twist angle between DMPB with CB is 17.2°). This worse conjugative effect weakens the ICT character of 5, resulting in the λ_fl value of 5 is smaller than that of λ_abs value (see detail discussion and Table S3 in ESI†).

Reorganization energy and stability

It is well-known that the lower the reorganization energy values, the higher the charge transfer rate.^23,24 Table 4 presents the calculated λ_e, λ_h, IP, EA, and η of the designed compounds. The results displayed in Table 4 show that the λ_e values of the designed compounds (0.285–0.404 eV) are larger than that of tris(8-hydroxyquinolinato)aluminum(III) (Alq3) (λ_e = 0.276 eV), a typical electron transport material.³⁴ It indicates that the electron transfer rates of 1–8 might be lower than that of Alq3. On the other hand, the λ_h values of 1, 2, 4, 7, and 8 (0.254–0.286 eV) are smaller than that of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD), which is a typical hole transport material (λ_h = 0.290 eV).³⁵ However, the λ_h values of 3, 5, and 6 are slightly larger than that of TPD. It suggests that the hole transfer rates of 3, 5, and 6 might be slightly lower, while the corresponding hole transfer rates of 1, 2, 4, 7, and 8 may be higher than that of TPD. Therefore, these molecules are potential hole transport materials only under the proper operating conditions for OLEDs from the stand point of the smaller reorganization energy. The λ_h values are predicted in the order 8 < 7 < 2 < 1 < 4 < 6 < 3 < 5. This shows that the introduction of benzo[c][1,2,5]thiadiazole (8) benzo[c]thiophene (7) benzene (2) conjugate π-bridges lead to the increase of hole transfer rates, while introduction of thiophene (3), furan (4), 1H-pyrrole (5), and benzo[d]thiazole (6) conjugate π-bridges decrease the hole transfer rates compared with that of the parent compound with ethene conjugate π-bridge. It suggests that the hole transfer rates values are affected by the introduction of different conjugate π-bridges to these molecules.

Table 4 Calculated molecular λ_e, λ_h, IP, EA, and η (all in eV) of 1–8 at the B3LYP/6-31G(d,p) level

Species	λh	λe	IP	EA	η
1	0.275	0.334	5.980	0.753	2.613
2	0.265	0.344	6.111	0.696	2.707
3	0.337	0.350	5.894	0.825	2.535
4	0.286	0.296	5.811	0.708	2.552
5	0.347	0.285	5.682	0.605	2.538
6	0.323	0.353	5.867	0.848	2.509
7	0.261	0.404	6.088	1.237	2.425
8	0.254	0.385	6.168	0.766	2.701

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The absolute hardness η is the resistance of the chemical potential to change in the number of electrons.⁴⁰ Inspection of Table 4 reveals clearly that η values of 2–8 are almost equal to that of the parent compound 1. It implies that the introduction of different conjugate π-bridges to these molecules does not significantly affect the stability of the substituted derivatives.

Mobility

With the aim to study the charge transport property, the mobilities of the designed compounds were calculated. The total energies of 1–8 in different space groups are listed in the Table S4 in the ESI.† The lattice constants of 1–8 with the lowest total energies in different space groups are listed in Table S5 in the ESI.† 7 and 8 are selected as representation to study and discuss their electron and hole integral values. The predicted crystal structures of 7 and 8 are shown in Fig. 2. The calculated crystal structures of 7 and 8 with the lowest total energies both belong to C2 space group. According to the optimized crystal structures, the transmission paths were selected. One molecule is arbitrarily chosen as the initial position for the charge to diffuse. All its neighboring molecules are taken as paired elements. Each pair is defined as a transmission path. The transfer integrals of 7 and 8 for hole and electron can be calculated according to the transmission path. Fig. 3 shows the most important eight pathways for 7 and 8. The calculated transfer integrals of 7 and 8 for holes and electrons in space groups C2 are listed in Table 5. The holes and electrons transfer integrals of 1–6 with the lowest total energies in different space groups are listed in the Table S6 in the ESI.† The mobility can be estimated from the Einstein relation. The calculated mobility of 1–8 with the lowest total energies in different space groups are listed in the Table 6. The results presented in Table 5 show that the electronic coupling is determined by the relative distance and orientations of the interacting molecules. One can see that 7 has the largest both electron and hole coupling values in pathways 5 and 6. On the other hand, 8 possesses the electron coupling value in pathways 1 and 2, and the absolute largest hole coupling value in pathway 3. Since the cofacial stacking structure is expected to provide more efficient orbital overlap,⁴⁷ orientation of the interacting molecules is more important parameter of hole or electron coupling for 7 and 8. Furthermore, from Table 6, one can find that the values of hole mobility for 7 and 8 in C2 space group are larger than those of electron mobility, respectively. The values of hole mobility of 7 and 8 (2.81 × 10⁻² and 9.06 × 10⁻³ cm² V⁻¹ s⁻¹) in C2 space group are much larger, while the corresponding values of 1–6 in different space groups are smaller than that of TPD (1.0 × 10⁻³ cm² V⁻¹ s⁻¹),⁴⁸ respectively. For the electron mobility, 4 and 5 have the largest electron mobility values. It indicates that the stacking structure is the most important factor for molecular mobility property. The theoretical prediction shows that 7 and 8 are expected to be promising candidates for hole transfer materials, while 4 and 5 can serve as electron transfer materials using for OLEDs.

Fig. 2 Herringbone structures of 7 and 8 in C2 space group.

Fig. 3 Crystal structures and hopping routes of 7 and 8 in C2 space group.

Table 5 Center–center distance (Å) and the corresponding hole and electron coupling (eV) between the dimer in all of the nearest neighbor pathways for 7 and 8 in C2 space group

Pathway	Distance	Electron coupling	Hole coupling
7
1	11.999	3.42 × 10^−5	1.07 × 10^−5
2	11.999	3.42 × 10^−5	1.07 × 10^−5
3	11.062	−2.13 × 10^−6	−1.02 × 10^−6
4	33.165	−3.31 × 10^−7	−5.64 × 10^−6
5	19.590	−5.30 × 10^−4	−6.60 × 10^−3
6	10.102	−7.60 × 10^−4	−5.41 × 10^−5
8
1	10.276	1.40 × 10^−3	3.86 × 10^−4
2	10.276	1.40 × 10^−3	3.86 × 10^−4
3	18.727	−4.34 × 10^−4	3.80 × 10^−3
4	11.996	−2.98 × 10^−4	−1.62 × 10^−5
5	14.630	1.06 × 10^−4	−4.48 × 10^−5
6	11.188	9.80 × 10^−5	3.56 × 10^−7

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Table 6 Calculated electron and hole mobility of 1–8 with the lowest total energies in different space groups [T = 298 K, in cm² (V⁻¹ s⁻¹)]

Species	Space groups	Electron mobility	Hole mobility
1	C2	2.80 × 10^−5	2.31 × 10^−5
2	P21	7.91 × 10^−6	4.27 × 10^−7
3	P21	5.16 × 10^−5	2.35 × 10^−6
4	P21	2.53 × 10^−2	8.78 × 10^−5
5	P21	7.44 × 10^−3	6.05 × 10^−5
6	P21	9.67 × 10^−4	1.30 × 10^−5
7	C2	2.51 × 10^−5	2.81 × 10^−2
8	C2	8.10 × 10^−5	9.06 × 10^−3

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Conclusions

In the present work, a series of π-conjugated three-coordinate organoboron compounds have been designed to explore optical and charge transporting properties with theoretical method. The FMOs analysis has turned out that the vertical electronic transitions of absorption and emission are characterized as intramolecular charge transfer (ICT). The calculated results show that their optical, electronic, and charge transport properties are affected by the introduction of different conjugate π-bridges to these molecules. Our results suggest that π-conjugated three-coordinate organoboron compounds with ethene (1), benzene (2), thiophene (3), furan (4), 1H-pyrrole (5), benzo[d]thiazole (6), benzo[c]thiophene (7), and benzo[c][1,2,5]thiadiazole (8) conjugate π-bridges can serve as luminescent materials for OLEDs. In addition, the predicted mobility indicates that 7 and 8 are expected to be promising hole transfer materials, while 4 and 5 can serve as electron transfer materials for OLEDs. On the basis of investigated results, it is possible to predict reasonable optical and electronic properties of the π-conjugated organoboron compounds for OLEDs using theoretical methodologies.

Acknowledgements

Financial supports from the NSFC (No. 21563002), the Natural Science Foundation of Inner Mongolia Autonomous Region (No. 2015MS0201), and the Research Program of Sciences at Universities of Inner Mongolia Autonomous Region (No. NJZZ235) are gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The total energies of the compounds under investigation in different space groups. The calculated crystal cell parameters of the compounds under investigation with the lowest total energies. The center–center distance and the corresponding hole and electron coupling between the dimer in all of the nearest neighbor pathways for 1–6 with the lowest total energies in different space groups. See DOI: 10.1039/c6ra24301k

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