Computational evaluation of optoelectronic and photophysical properties of unsymmetrical distyrylbiphenyls

Abstract

Unsymmetrical distyrylbiphenyls (UDSBs) have been evaluated for their suitability for optoelectronic applications. Totally 14 UDSBs including four already reported have been investigated using DFT/TD-DFT calculations. The computed results reveal that the UDSB 1–12 can be used as good hole transport materials and UDSB 13 and 14 can be used as good electron transport materials. The newly designed UDSBs show promising optoelectronic properties and they can be used as a ‘trifunctional materials’ (emitter, hole and electron transport) in OLEDs. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials, electron affinities, reorganization energies and exciton binding energies for these complexes are affected by different donor and acceptor groups. The photophysical characterization of the UDSBs show that, except UDSB 11–14 the absorption and emission spectra of all other molecules have π → π* character as revealed by natural transition orbital (NTO) analysis. The results obtained confirm that the optical properties of UDSBs can be significantly tuned by suitable substitution and these compounds can be used to make efficient OLEDs.

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Introduction

The development of first generation organic light-emitting diodes (OLED) has progressed tremendously and they have received considerable attention as potential candidates for applications¹ in the next-generation full-color flat-panel displays.² In general, efficient and stable multifunctional luminescent materials can be used for enhancing the device performance and stability.³ In the last two decades, significant efforts have been made to study the electronic and structural properties of π-conjugated organic molecules such as bistriphenylenyl,⁴ spirobifluorene,⁵ Fluorene,⁶ thiazole,⁷ imidazole,⁸ diarylanthracene⁹ and distyrylbiphenyl derivatives¹⁰ for their potential applications in light harvesting, data processing, modern communication and photonic technologies.¹¹ Among them, distyrylbiphenyl derivatives have drawn significant interest in this field as they are efficient candidates for optoelectronic applications.¹² The distyrylarylene derivatives with symmetric structures crystallize better than the distyrylarylene with an unsymmetrical structure.¹³ The unsymmetrical distyrylbiphenyl derivatives have been reported to have longer lifetimes by prohibiting intermolecular chromophoric π–π stacking and having an unsymmetrical structure and this would make them better candidates for OLED applications. Unsymmetrical distyrylbiphenyls (UDSBs) synthesized by Bon Hyeong Koo et al.¹⁴ have been shown to be efficient light-emitting materials with satisfactory multifunctional properties and high thermal stability.¹⁴ These properties have stimulated a great deal of interest in such molecules in recent years and motivated the present study. In the present work we systematically investigate the photophysical and optoelectronic properties of UDSBs and design newer UDSB candidates with enhanced properties viz improved charge-transfer/injection performance. Particularly the effect of various substituents on the UDSB skeleton in modulating the optoelectronic properties and photophysical properties of them have been investigated using density functional theory (DFT), single excitation configuration interaction,¹⁵ and time-dependent density functional theory (TD-DFT) calculations by examining their highest occupied molecular orbitals (HOMOs), lowest unoccupied molecular orbitals (LUMOs), band gaps, ionization potentials (IPs), electron affinities (EAs), reorganization energies (λ_{hole/electron}) and exciton binding energies (E_b). By computing the electron transport and hole transport properties of the UDSBs, their suitability for better OLED performance can be analyzed.

Computational methodology

The ground-state geometry of the molecules were fully optimized at the DFT level using B3LYP¹⁶ with 6-311G(d,p) basis set. The excited-state geometries were optimized by the ab initio configuration interaction singles method.^15,17 These fully optimized stationary points were further characterized by harmonic analysis to ensure that all the structures are minima on the potential energy surface. Over the years, polarized continuum model (PCM)¹⁸ has been reported to be the most successful and cost effective model to examine the solvent effect. Therefore, the influence of solvent on the UDSBs is investigated within the same framework. Further the time dependent density functional theory (TD-DFT)¹⁹ has been used to compute the electronic absorption and emission spectra, singlet vertical excitation energies and oscillator strengths on the ground state optimized geometries. All computations were carried out with Gaussian 09W program package.²⁰

It becomes essential to find out a functional that offers better accuracy in predicting absorption spectra of UDSBs. Therefore, in TD-DFT calculations different functionals such as B3LYP, CAM-B3LYP,²¹ PBEPBE,²² PBE0 ²³ and the dispersion corrected M06 ²⁴ have been attempted here and the computed absorption maxima are compared with the experimental values. Fig. 1 represents the performance of various functionals along with the experimental values. From this figure, it is clear that CAM-B3LYP agrees well with experimentally observed absorption maxima. Hence CAM-B3LYP is used for all the TD-DFT calculations employing 6-311G(d,p) basis set.

Fig. 1 Computed absorption maxima at various levels with experimentally reported values.

Finally, to understand the nature of excited states involved in absorption and emission processes, the natural transition orbital (NTO) analysis was performed for all the molecules considered here. This approach provides the most compact representation of the electronic transitions in terms of an expansion into single particle orbitals by diagonalizing the transition density matrix associated with each excitation.²⁵ In addition, the positive and negative ions with regard to the “electron–hole” creation are relevant to their use as OLED materials. Thus, ionization potentials (IPs), electron affinities (EAs), and reorganization energy (λ_{hole/electron}) were obtained by comparing the energy levels of neutral molecules with positive ions and negative ions, respectively. To gain further insights into the various stabilizing interactions in the ground state Weinhold's Natural Bond Orbital (NBO)²⁶ analysis has been carried out.

Results and discussion

The structures of 14 UDSBs are shown in Fig. 2 and of them UDSB 1–4 are experimentally reported. In the remaining 10 candidates, UDSB 5–8 bear electron releasing –OH and –NH₂ groups and UDSB 9–12 have electron withdrawing –CN and –NO₂ groups in the skeleton. The UDSB 13 have –NH₂ and –CN groups with respect to R and R′ position, whereas UDSB 14 have –NH₂ and –NO₂ groups with respect to R and R′ position.

Fig. 2 Structures of UDSB 1–14.

The optimized ground-state geometries of 14 UDSBs are given in the ESI Fig. 1 (SIF1†). Fig. SIF2† reveals that the C–C bond lengths fall in between their respective single and double bond limits and this shows that the bond is no longer a pure single bond. This is an evidence for the enhanced delocalization of π-electrons due to coplanarity of the benzene rings in the molecule. The geometry of the cationic and anionic UDSBs have been computed in order to arrive at the IP and EA values and it is found that the geometry of the molecules in the charged states are comparable to that of their corresponding neutral molecule. ESI Table 1 (SIT1†) lists the bond parameters of the UDSB 2 and UDSB 7. Except in the C5–C4 of the UDSB 2 which is slightly longer in the neutral molecule than the anionic and the cationic molecules, all the other variations in the bond parameters of the ionic species from that the corresponding neutral molecules are well within 0.1 Å.

Frontier molecular orbital (FMO) analysis

For optical and electronic characterization, the HOMOs, LUMOs and energy gaps (ΔE_H–L) are examined. The relative ordering of the molecular orbitals qualitatively indicate the excitation properties and the ability of the molecule for electron or hole transportation.²⁷ Theoretically, the energy gap is the orbital energy difference between HOMO and LUMO, termed the HOMO–LUMO gap (ΔE_H–L).²⁸ Experimentally, the band gap is obtained from the absorption spectra, as the lowest transition energy from the ground-state to the first dipole allowed excited state and is termed the optical band gap.²⁹ In fact, the optical band gap is not the orbital energy difference between HOMO and LUMO, but the energy difference between the S₀ state and the S₁ states. Only when the excitation to the S₁ state corresponds exclusively to the promotion of the electron from the HOMO to the LUMO, the optical band gap will be approximately equal to the ΔE_H–L in quantity.^28a,b In this work, the optical band gaps of UDSBs 1–14 are calculated from the absorption spectra at the TD-DFT level and abbreviated as E_g.

The frontier molecular orbital energy level graph starting from HOMO − 3 to LUMO + 3 is given in Fig. 3. As seen from the figure, the HOMO–LUMO gap lie over a range of 2.53 eV to 3.67 eV and band gap is found to be maximum for UDSB 1 and minimum for UDSB 14. The HOMOs are spread over −5.12 eV to −5.82 eV and the LUMOs lie in the range of −1.66 eV to −2.80 eV. When comparing the band gaps of UDSB 1–4, it can be noted that the substitution of electron releasing groups such as –OH and –NH₂ at R and R′ reduces the band gap; particularly, NH₂ substituted UDSBs are found to have lower band gaps than the corresponding –OH substituted UDSBs. Generally, the hole transporting material (HTM) with the smaller negative value of HOMO should lose their electron more easily, while the electron transport material (ETM) with large negative value of LUMO should accept electrons more easily.²⁹ Therefore the FMO energies are compared. In UDSB 1–6, the HOMO energies are slightly higher than those of UDSB 9–12, where as the HOMOs of UDSB 13 and UDSB 14 are higher than those of UDSB 7 and UDSB 8. The high HOMO energy (−5.48 eV to −5.12 eV) and low LUMO (−1.66 eV to −1.82 eV) energy largely reduces the energy barrier for the hole creation and electron acceptance process. The HOMO levels are stabilized in UDSB 9–12 compared to UDSB 1–8 and this is due to the presence of electron withdrawing groups in UDSB 9–12 as they gradually reduce the band gap. Among the investigated UDSBs, the donor–acceptor type UDSB 13 and UDSB 14 have the lowest band gap and are considered to be suitable candidates for OLEDs and further require structural modification to enhance their luminescence ability. Therefore it is clear that the ability of the charge injection becomes enhanced from UDSB 1 to UDSB 8. These findings suggest that electron releasing groups have little effect on the band gap while the electron withdrawing groups reduce them to a higher degree.

Fig. 3 FMO energy levels of UDSB 1–14.

Mes₂B[p-4,4′-biphenyl-NPh(1-napthyl)] (BNPB) reported earlier in literature is compared with UDSBs due to their wider usability as HTM in OLEDs. HOMO energy level of BNPB was reported by experiments³⁰ to be −5.30 eV (−5.01 eV at b3lyp/6-31G(d)³¹ and −5.27 eV at b3lyp/6-311G(d,p) by DFT calculations). UDSBs generally show lower HOMO levels than BNPB. In UDSB 7 and UDSB 8 the HOMO energies have been increased considerably compared to the other members. Therefore efforts to find suitable substituents which can still increase the energy levels may be necessary in future to make UDSBs better hole transporters as BNPBs. The LUMO levels of UDSBs are lower than that of the BNPB (experiments to be −2.44 and −1.63 eV (at b3lyp/6-31G(d)), 1.88 eV (at b3lyp/6-311G(d,p)) by DFT calculations). In addition, HOMO level of UDSBs match well with the work function of the indium tin oxides (ITO; from −4.8 to −5.1 eV),³² and the LUMO levels are close to that of tris(8-hydroxyquinoine)aluminium (Alq₃, −1.81 eV),³³ which is one of the widely used electron transport material. Therefore the UDSBs may act as ‘trifunctional materials’ (emitter, hole and electron transporters) in OLEDs.

The frontier molecular orbitals such as HOMO and LUMO are given in Fig. 4. It can be seen that there are some common characteristics observed in HOMOs and LUMOs of these UDSBs. Particularly the HOMOs, LUMOs and LUMO + 1 s are evenly localized on the entire molecule. This indicates that the HOMO → LUMO transitions bear a significant π → π* transition. The electronic density distribution in molecules UDSB 1–6 majorly spread over the entire biphenyl core. While in the case of UDSB 9 and UDSB 11 HOMOs are mainly localized on the R′ part, whereas the LUMOs are centered on the R part. On the other hand, in UDSB 12–14, HOMOs are mainly localized on the R part, but LUMOs are localized on the R′ part. This indicates that the HOMO → LUMO transitions bear a significant intramolecular charge transfer character.

Fig. 4 Highest occupied molecular orbital and lowest unoccupied molecular orbitals of UDSB 1–14.

The contributions of various fragments of the molecules to the HOMOs and LUMOs have been computed using QMForge program.³⁴ The whole molecule has been segmented into three fragments, namely Unsymmetrical distyrylbiphenyl part (UDSB), R and R′ groups, and their corresponding percentage contributions are shown in Fig. 5. From the Figures, it is clear that in all the molecules, the UDSB unit contributes largely towards LUMO and HOMO. Invariably in UDSB 5–8, the electron donating groups significantly contributes (20%) towards HOMO, while in UDSB 9–12 the electron withdrawing group contributes significantly towards LUMO (20%). In UDSB 11–14 the NO₂ groups were found to have a greater share of the LUMOs. This gives an idea as to the share of various fragments to the FMOs that are responsible directly or indirectly to the absorption, emission and electron transport processes.

Fig. 5 Molecular orbital composition (%) in the ground state for UDSB 1–14 (a) for HOMO (b) for LUMO.

Internal reorganization energy

The charge transport/injection and their balance are crucial for optoelectronic compounds.³⁵ Therefore, it is important to investigate their ionization potentials (IPs), electron affinities (EAs),³⁶ and reorganization energies (λ_{hole/electron}) to evaluate the energy barrier for injection and transport rates of the holes and electrons. We calculated IPs and EAs both vertical (v, at the geometry of the neutral molecule) and adiabatic (a, optimized structure for both the neutral and charged molecule), together with the hole extraction potential (HEP), which is the energy difference from M⁺ (cationic) to M₀ (neutral molecule) using the M⁺ geometric structure, and the electron extraction potential (EEP), which is the energy difference from M⁻ (anionic) to M₀ using the M⁻ geometric structure.³⁷

To understand the charge transfer rate and balance, the reorganization energy was calculated for the investigated molecules. Generally, Organic π-conjugated materials are assumed to transport charge at room temperature via a thermally activated hoping-type mechanism.^15,38 The hole and electron transfer process through which the charge hops between two molecules, can be summarized as follows:

M⁺ + M₀ → M₀ + M⁺

M⁻ + M₀ → M₀ + M⁻

where M is the neutral molecule interacting with neighboring oxidized (M⁺) or reduced (M⁻) ion. The rates of charge transfer (K_h/e) can be approximately described by Marcus theory³⁹ using the following equation,

K_h/e = (π/λ_h/e k T)^1/2 × V_h/e²/ħ × exp(−λ_h/e/4kT)

where T is the temperature, λ_h/e is the hole/electron reorganization energy due to geometric relaxation accompanying charge transfer, and V_h/e is the electronic coupling matrix element between the two species M and M^+/−. The electron-transfer rate (K_h/e) depends on two important parameters, electron coupling V_h/e and the reorganization energy λ_h/e. However, experimentally determined V_h/e shows a rather narrow range of values,⁴⁰ and an even more limited range of V_h/e is expected due to the intermolecular charge-transfer processes considered in OLEDs. Hence, the reorganization energy is just the internal reorganization energy of the isolated active organic π-conjugated systems and any environmental relaxation and changes can be ignored. Hence, the hole/electron reorganization energy can be redefined as follows.^38c

λ_hole = λ₁ + λ₂ = [E⁺(M₀) − E⁺(M⁺)] + [E(M⁺) − E(M₀)] = IP(V) − HEP

λ_electron = λ₃ + λ₄ = [E⁻(M₀) − E⁻(M⁻)] + [E(M⁻) − E(M₀)] = EEP − EA(v)

As illustrated in Fig. 6, E⁺ and E represent the energies of the cation and neutral species based on their lowest energy geometries respectively. While (M⁺) and (M₀) denote their optimized structures at charged and neutral states, E⁺(M₀) is the energy of a cation calculated with the optimized structure of the neutral molecule M₀. λ₁ is the relaxation energy of a neutral molecule M₀ that captured a hole going toward the M⁺ optimum geometry on the potential energy surface of M⁺, and λ₂ is the relaxation energy from M⁺ extracting a hole going toward the M₀ optimum geometry on the potential energy surface of M. The sum of λ₁ and λ₂ is the hole reorganization energy λ_hole. Similarly, in the electron-transport process, λ_electron = λ₃ + λ₄.

Fig. 6 Schematic diagram for internal reorganization energy for hole transfer.

The main challenge for the application of organic molecules in OLEDs is the achievement of high EA and low IP to improve the electron and hole transport electronic devices. For OLED material, the lower the IP easier the entry of holes from indium tin oxide (ITO)⁴¹ to hole transporting layer (HTL) and the higher the EA easier the entry of electron from cathode to electron transport layer (ETL).⁴² It has been experimentally proved that BNPB is a good trifunctional molecule.^30,43 In Table 1, it is shown that the IPs of UDSBs are close to that of BNPB (6.09 eV), therefore they can be used as HTL materials. By analysis of the values of EAs, UDSB 1–12 are expected to accept the electron easily than BNPB (0.74 eV).³¹ Therefore UDSB 1–12 exhibit more excellent properties as ETL materials than BNPB due to the presence of various donor and acceptor groups. The trends in the IPs and EAs of UDSBs are similar to those of the negative HOMO and LUMO energies. The small value of the IP for UDSB 7 and 8 and the large value of the EA for UDSB 12 (1.51 eV) demonstrate that the incorporation of substituents can improve the hole creating ability.

Table 1 Ionization potentials (IPs), electron affinities (EAs), extraction potentials and reorganization energy (eV) of UDSB 1–14

Molecule	IP (V)	IP (a)	HEP	EA (v)	EA (a)	EEP	λhole	λelectron
UDSB1	6.54	6.41	6.30	0.76	0.97	1.16	0.24	0.40
UDSB2	6.55	6.42	6.29	0.77	0.98	1.17	0.27	0.39
UDSB3	6.47	6.34	6.22	0.75	0.96	1.15	0.25	0.40
UDSB4	6.47	6.33	6.2	0.76	0.98	1.16	0.27	0.40
UDSB5	6.48	6.33	6.19	0.73	0.94	1.12	0.29	0.39
UDSB6	6.47	6.34	6.21	0.71	0.93	1.13	0.26	0.42
UDSB7	6.29	6.09	5.91	0.67	0.88	1.06	0.37	0.39
UDSB8	6.27	6.09	5.94	0.63	0.86	1.06	0.33	0.44
UDSB9	6.83	6.71	6.59	1.18	1.41	1.61	0.23	0.42
UDSB10	6.84	6.71	6.58	1.26	1.43	1.59	0.26	0.33
UDSB11	6.87	6.75	6.63	1.42	1.63	1.82	0.24	0.40
UDSB12	6.89	6.75	6.62	1.51	1.68	1.85	0.28	0.34
UDSB13	6.46	6.25	6.06	1.18	1.34	1.5	0.40	0.33
UDSB14	6.50	6.28	6.08	1.44	1.62	1.78	0.41	0.34

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As emitting layer materials, it needs to achieve the balance between hole injection and electron acceptance. Thus, lower the λ values have bigger the charge-transport rate.^39a,44 Both injection and transport of the hole and electron can be modified significantly via different donors and acceptors. Table 1 shows that, the λ_hole values of the UDSB 1–12 are smaller than that of λ_electron values, which suggests that the hole-transfer rate is better than the electron transfer rate. Interestingly the UDSB 13 and 14 has higher λ_hole than λ_electron suggesting that it can be used as an electron transport material. Therefore UDSB 1–12 are better hole transport material with high quantum efficiency. The λ_hole values decrease from UDSB7 to UDSB8 due the replacement of NH₂ group from R to R′ position. From UDSB 9 to UDSB 10, the λ_hole value increases and λ_electron value decreases, as the result the UDSB 10 has better hole- and electron transporting balance. Furthermore, the difference between λ_hole and λ_electron for UDSB 12 (0.06 eV) is less compared to UDSB 11 (0.16 eV) and this is due to the position of NO₂ group. In addition, the differences between the λ_hole and λ_electron of UDSB 7 (0.02 eV), UDSB 10 (0.07 eV), UDSB 12 (0.06 eV), UDSB 13 (0.07 eV), and UDSB 14 (0.07 eV) are small enough, suggesting that these molecules have a delicate electron–hole transporting balance and therefore can act as ambipolar materials.⁴⁵

Dipole moment

The fundamental importance in structural chemistry is the electric dipole moment, which is known to be sensitive to small changes in the structure and the electronic charge distribution in the molecule.⁴⁶ The calculated dipole moments of UDSBs both in gas and chloroform medium are summarized in Table 2. A similar trend in the computed dipole moments has been observed in the gas and the solvent phases and as expected an increase in the dipole moment has been observed in the presence of the polar solvent. It has been observed that the substitution increases the dipole moment value and the effect is significant. This is also revealed in the increased dipole moment values of the UDSBs with electron withdrawing substituent (UDSB 9–12 than UDSB 1–8).UDSB 13 and UDSB 14 show higher polarity due to the push–pull nature which is pronounced in the solvent phase.

Table 2 Ground-state dipole moment (Debye) of UDSB derivatives in gas phase and in CHCl₃ solution calculated at B3LYP/6-311G(d,p) level

Molecule	Gas phase	CHCl3
UDSB1	1.07	1.05
UDSB2	0.61	0.66
UDSB3	1.13	1.16
UDSB4	1.10	1.11
UDSB5	1.32	1.82
UDSB6	1.78	2.22
UDSB7	2.56	3.22
UDSB8	3.50	4.16
UDSB9	5.67	6.88
UDSB10	5.98	7.11
UDSB11	5.97	7.17
UDSB12	6.49	7.62
UDSB13	7.07	8.29
UDSB14	7.56	8.76

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Absorption and emission spectra

The absorption and emission spectra of the investigated UDSBs in both gas and solvent phases were calculated using TD-DFT method to rationalize the nature of electronic transitions, contributing configurations to the transitions and charge transfer probability. The calculated wavelengths from absorption (Tables 3 and 4) and emission spectra (Table 5), excitation energies, main transition configurations and oscillator strengths for the most relevant singlet excited states are summarized in this section.

Table 3 Computed absorption maxima (λ_max nm), electronic excitation energies (E, eV), and oscillator strength (f) of UDSB 1–4 in both gas and solvent phase (CHCl₃) using TD-DFT method at the CAM-B3LYP/6-311G(d,p) level

Molecule	States	Electron transition	Cal. λmax (nm)	*Exp. (nm)	Oscillator strength f	E (eV)	Main contributing configurations
*Experimental values from ref. 14.
UDSB1	Gas-phase	S0 → S1	324		1.75	3.82	HOMO → LUMO (84%), HOMO − 1 → LUMO + 1 (8%)
	CHCl3	S0 → S1	333	341	1.89	3.73	HOMO → LUMO (83%), HOMO − 1 → LUMO + 1 (8%)
UDSB2	Gas-phase	S0 → S1	323		1.58	3.84	HOMO → LUMO (76%), HOMO − 1 → LUMO + 1 (10%)
	CHCl3	S0 → S1	331	320	1.74	3.75	HOMO → LUMO (76%), HOMO − 1 → LUMO + 1 (10%), HOMO − 1 → LUMO (9%)
UDSB3	Gas-phase	S0 → S1	326		1.67	3.81	HOMO → LUMO (81%), HOMO − 1 → LUMO + 1 (10%)
	CHCl3	S0 → S1	334	332	1.83	3.72	HOMO → LUMO (81%), HOMO − 1 → LUMO + 1 (10%)
UDSB4	Gas-phase	S0 → S1	326		1.77	3.80	HOMO → LUMO (81%), HOMO − 1 → LUMO + 1 (10%)
	CHCl3	S0 → S1	334	331	1.91	3.72	HOMO → LUMO (81%), HOMO − 1 → LUMO + 1 (9%)

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Table 4 Computed absorption maxima (λ_max, nm), electronic excitation energies (E, eV), and Oscillator strength (f) of UDSB 5–14 in both gas and solvent phase (CHCl₃) using TD-DFT method at the CAM-B3LYP/6-311G(d,p) level

Molecule	States	Electron transition	Cal. λmax (nm)	Oscillator strength f	E (eV)	Major contribution
UDSB5	Gas-phase	S0 → S1	324	1.54	3.83	HOMO → LUMO (63%), HOMO − 1 → LUMO (19%)
	CHCl3	S0 → S1	332	1.71	3.74	HOMO → LUMO (62%), HOMO − 1 → LUMO (22%)
UDSB6	Gas-phase	S0 → S1	327	1.71	3.80	HOMO → LUMO (83%)
	CHCl3	S0 → S1	336	1.86	3.69	HOMO → LUMO (81%)
UDSB7	Gas-phase	S0 → S1	329	1.30	3.77	HOMO → LUMO (44%), HOMO − 1 → LUMO (28%), HOMO → LUMO + 1 (20%)
	CHCl3	S0 → S1	337	1.42	3.68	HOMO → LUMO (41%), HOMO − 1 → LUMO (30%), HOMO → LUMO + 1 (19%)
UDSB8	Gas-phase	S0 → S1	334	1.80	3.71	HOMO → LUMO (75%)HOMO → LUMO + 1 (13%)
	CHCl3	S0 → S1	346	1.93	3.58	HOMO → LUMO (73%), HOMO → LUMO + 1 (16%)
UDSB9	Gas-phase	S0 → S1	328	1.31	3.78	HOMO → LUMO (51%), HOMO → LUMO + 1 (21%), HOMO − 1 → LUMO (16%)
	CHCl3	S0 → S1	336	1.43	3.69	HOMO → LUMO (49%), HOMO → LUMO + 1 (23%), HOMO − 1 → LUMO (15%)
UDSB10	Gas-phase	S0 → S1	335	1.88	3.70	HOMO → LUMO (58%), HOMO − 1 → LUMO (31%)
	CHCl3	S0 → S1	346	2.02	3.59	HOMO → LUMO (62%), HOMO − 1 → LUMO (28%)
UDSB11	Gas-phase	S0 → S1	337	0.92	3.67	HOMO − 1 → LUMO (37%), HOMO → LUMO (36%), HOMO → LUMO + 1 (19%)
	CHCl3	S0 → S1	351	0.90	3.54	HOMO − 1 → LUMO (43%), HOMO → LUMO (34%), HOMO → LUMO + 1 (13%)
UDSB12	Gas-phase	S0 → S1	346	1.76	3.59	HOMO − 1 → LUMO (40%), HOMO → LUMO (45%)
	CHCl3	S0 → S1	361	1.83	3.43	HOMO → LUMO (47%), HOMO − 1 → LUMO (36%)
UDSB13	Gas-phase	S0 → S1	342	1.61	3.63	HOMO − 1 → LUMO (48%), HOMO → LUMO (31%), HOMO → LUMO + 1 (15%)
	CHCl3	S0 → S1	350	1.82	3.54	HOMO − 1 → LUMO (56%), HOMO → LUMO (26%), HOMO → LUMO + 1 (12%)
UDSB14	Gas-phase	S0 → S1	351	1.59	3.53	HOMO − 1 → LUMO (52%), HOMO → LUMO (26%), HOMO → LUMO + 1 (11%)
	CHCl3	S0 → S1	365	1.76	3.40	HOMO − 1 → LUMO (60%), HOMO → LUMO (19%)

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Table 5 Computed emission spectra (λ_max, nm), electronic excitation energies (E, eV), exciton binding energy (E_b, eV), oscillator strength (f) and radiative lifetimes (τ, ns) of UDSB 1–14 in both gas and solvent phase (CHCl₃) using TD-DFT method at the CAM-B3LYP/6-311G(d,p) level^a

Molecule	States	Electron transition	Cal. λmax (nm)	Oscillator strength f	E (eV)	Eb (eV)	Major contribution	τ (ns)
a values in the parenthesis are from ref. 14.
UDSB1	Gas-phase	S1 → S0	387	1.95	3.21	0.46	HOMO → LUMO (94%)	1.15
	CHCl3	S1 → S0	402 (422)	2.07	3.08	0.59	HOMO → LUMO (93%)	1.17
UDSB2	Gas-phase	S1 → S0	385	1.85	3.22	0.44	HOMO → LUMO (92%)	1.20
	CHCl3	S1 → S0	400.1 (423)	2.00	3.10	0.56	HOMO → LUMO (92%)	1.20
UDSB3	Gas-phase	S1 → S0	388	1.93	3.20	0.45	HOMO → LUMO (93%)	1.16
	CHCl3	S1 → S0	402.8 (421)	2.07	3.08	0.57	HOMO → LUMO (93%)	1.18
UDSB4	Gas-phase	S1 → S0	389	2.04	3.19	0.47	HOMO → LUMO (93%)	1.11
	CHCl3	S1 → S0	403 (425)	2.16	3.08	0.58	HOMO → LUMO (93%)	1.12
UDSB5	Gas-phase	S1 → S0	386	1.84	3.21	0.38	HOMO → LUMO (91%)	1.22
	CHCl3	S1 → S0	401	1.99	3.09	0.50	HOMO → LUMO (90%)	1.21
UDSB6	Gas-phase	S1 → S0	389	1.91	3.19	0.42	HOMO → LUMO (93%)	1.18
	CHCl3	S1 → S0	405	2.05	3.06	0.55	HOMO → LUMO (93%)	1.20
UDSB7	Gas-phase	S1 → S0	502	0.49	2.47	0.87	HOMO → LUMO (94%)	7.76
	CHCl3	S1 → S0	526	0.57	2.36	1.0	HOMO → LUMO (94%)	7.29
UDSB8	Gas-phase	S1 → S0	396	1.97	3.13	0.33	HOMO → LUMO (93%)	1.19
	CHCl3	S1 → S0	416	2.09	2.98	0.48	HOMO → LUMO (92%)	1.24
UDSB9	Gas-phase	S1 → S0	491	0.54	2.52	0.95	HOMO → LUMO (95%)	6.68
	CHCl3	S1 → S0	512	0.63	2.42	1.05	HOMO → LUMO (95%)	6.25
UDSB10	Gas-phase	S1 → S0	395	2.01	3.14	0.22	HOMO → LUMO (91%)	1.16
	CHCl3	S1 → S0	413	2.14	3.00	0.36	HOMO → LUMO (91%)	1.19
UDSB11	Gas-phase	S1 → S0	494	0.56	2.51	0.55	HOMO → LUMO (93%)	6.55
	CHCl3	S1 → S0	521	0.65	2.38	0.68	HOMO → LUMO (92%)	6.22
UDSB12	Gas-phase	S1 → S0	404	1.94	3.07	0.05	HOMO → LUMO (87%)	1.26
	CHCl3	S1 → S0	428	2.05	2.90	0.12	HOMO → LUMO (86%)	1.34
UDSB13	Gas-phase	S1 → S0	400	1.96	3.10	0.21	HOMO → LUMO (68%), HOMO − 1 → LUMO (25%)	1.22
	CHCl3	S1 → S0	417	2.11	2.97	0.08	HOMO → LUMO (65%), HOMO − 1 → LUMO (28%)	1.26
UDSB14	Gas-phase	S1 → S0	409	1.90	3.03	0.49	HOMO → LUMO (58%), HOMO − 1 → LUMO (33%)	1.32
	CHCl3	S1 → S0	432	2.03	2.87	0.65	HOMO → LUMO (54%), HOMO − 1 → LUMO (37%)	1.37

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The calculated absorption spectra of UDSB 1–4 are in good agreement with the experimental results with the largest deviation of 11 nm. Except in UDSB 11–14, all other electronic transitions are of the π → π* character, and excitation to S₁ state corresponds exclusively to the promotion of an electron from HOMO → LUMO. From Table 4 it is observed that, in UDSB 1, the experimental band found at 341 nm corresponds to the transition calculated at 332 nm, which originates from a HOMO → LUMO transition (83%) with π → π* character. In UDSB 2, the experimental absorption band of 320 nm corresponds to the transition calculated at 331 nm, which originates from HOMO → LUMO (76%) with π → π* character. Similarly, in UDSB 3 and UDSB 4 the experimental band found at 332 and 331 nm, corresponds to the transitions calculated at 333 nm with larger oscillator strengths. This transition has significant π → π* character due to the exclusive promotion of an electron from HOMO → LUMO (81%).

It is important to note that UDSB 12 and UDSB 14 have a higher wavelength of absorption (361 and 365 nm) with the oscillator strength of 1.83 and 1.76 respectively. They mainly come from HOMO → LUMO and HOMO − 1 → LUMO with significant intramolecular charge transfer character. The calculated λ_max for UDSB 13 is 350 nm and this correspond to the promotion of electron from HOMO − 1 → LUMO (56%) and HOMO → LUMO (26%). Over all these excitations are mainly due to π → π* (UDSB 1–10) and ICT (UDSB 11–14) character. The suitable substitution on UDSB unit greatly increases the absorption wavelength. As expected, molecules with a small ΔE_H–L possess maximum absorption and emission wavelength.

The absorption and emission spectra of the UDSBs chosen here are calculated in solvents (dioxane, chloroform, ethanol, THF and DMSO) and the results are depicted in Fig. SIF3 and SIF4.† By comparing the simulated absorption spectra in the gas phase with those in chloroform solution, we find that there is a consistent red shift in solution and this is due to the solute–solvent interaction.⁴⁷ As expected, the absorption of all the molecules in various solvents corresponds to S₀ → S₁ transitions and is responsible for the most intense bands with higher oscillator strength and is mainly contributed by HOMO → LUMO transition.

Theoretical emission spectra for UDSBs based on optimized excited-state geometries are presented in Table 5. The emission peaks in chloroform, with the largest oscillator strength for the UDSBs are assigned to π → π* character, arising from the HOMO → LUMO transition (∼92%). The calculated values of fluorescence wavelength in CHCl₃ for UDSB 1–4 are located at 402, 400, 402 and 403 nm respectively. The highest oscillator strengths of the S₁ → S₀ transition for UDSBs imply that they have a large fluorescent intensity, and that they are useful as fluorescent OLED materials. In comparison with UDSB 12, the λ_max of UDSB 9 is red shifted by about 89 nm. The influence of various solvents on the emission spectra was simulated by using the PCM method. The results reveal that the emission wavelengths, coefficients and configurations are nearly identical for all the UDSBs, but a 14–24 nm red-shift has been observed and this is due to the solute–solvent interaction (Fig. SIF5 and SIF6†).

The radiative lifetimes (τ) have been computed for spontaneous emission by using the Einstein transition probabilities according to the formula (in au).⁴⁸

where c is the velocity of light, E_Flu is the excitation energy, and f is the oscillator strength. OLED molecules with short radiative lifetime have been known to have high light-emitting efficiency, while those with long radiative lifetime facilitate the electron and energy transfer and the attack of active species.²⁹ The calculated life time for the studied 14 molecules is listed in Table 5. The substitution of different functional groups leads to changes in excitation energy, which results in variation in radiative lifetime. The short radiative lifetime of UDSB 1–4 indicates that they are good light-emitting materials. These results further support the fact that the optical and electronic properties of the UDSBs can be modified or tuned by suitable substitutions. In UDSB 7, 9 & 11 the lifetimes are 6.5 to 7 ns while that in the UDSB 13 and UDSB 14 are 1.32 and 1.37 ns respectively which is closer to the UDSB 1 to UDSB 5. Therefore, it is clear that the UDSBs with only electron withdrawing substituent are expected to have better electron transport properties than light emitting properties. This is also revealed in the reorganization energies. For instance, the UDSB 7, which has the highest radiative lifetime has better electron transport property (λ_electron = 0.32 eV). UDSB 4 with t-butyl group at both the ends of the molecule seems to have better light emitting property (1.11 ns). This may be due to the increase in the length of the molecule that enhances absorption and also the raditiative emission. It is a key point toward the development of this type of materials for OLEDs.

It is well-known that fluorescence emission is accompanied by energy ejection. When the energy of the fluorescence excitation is E_Flu and the energy difference between the HOMO and LUMO are ΔE_H–L, the exciton binding energy (E_b) can be defined as E_b = ΔE_H–L − E_Flu. Therefore the exciton binding energy is the energy required to destroy a hole–electron exciton. As shown in Table 5, the values of E_b for UDSB 1–14 indicate that the energy required to destroy a hole–electron exciton follows the order UDSB 13 < UDSB 12 < UDSB 10 < UDSB 8 < UDSB 5 < UDSB 6 < UDSB 2 < UDSB 3 < UDSB 4 < UDSB 1 < UDSB 14 < UDSB 11 < UDSB 7 < UDSB 9. This is driven by the nature of the substituent group. When the R group is electron withdrawing it is higher and when it is electron releasing it is lower.

Natural transition orbital analysis

In order to analyze the nature of absorption, we performed NTO analysis based on the calculated transition density matrices.²⁵ This method offers the most compact representation of the transition density between the ground and excited states in terms of an expansion into single-particle transitions (hole and electron states for each given excitation). Here we refer to the unoccupied and occupied NTOs as “electron” and “hole” transition orbitals respectively. Note that NTOs are not the same as virtual and occupied MO pairs from the ground state calculations. The natural transition orbitals (NTOs) for UDSB 1–14 are given in the Fig. SIF7–SIF10.† Based on our TD-DFT NTO analysis the absorption and emission bands of all the molecules (except UDSB 11–14) can be characterized as a π → π* transition. As shown in Fig. SIF7–SIF10,† optical excitations occur from the occupied (hole) transition orbitals to the unoccupied (electron) transition orbitals.

Natural bond orbital analysis

NBO analysis offers useful insights on the intramolecular delocalization and donor–acceptor interactions based on the second order interactions between filled and vacant orbitals. The ground state stabilization interactions have been examined and the results are summarized in ESI Table 2 (SIT2†). This table lists the major second order perturbation interactions along with the corresponding donor and acceptor NBOs. It is interesting to note that the most predominant stabilization arises through π → π* interactions (π_C–C → π*_C–C). Yet the major interactions in UDSBs (5–8 and 11–14) towards stabilization are due to the n → π* interactions arising from lone pair on the oxygen/nitrogen to the π* of adjacent C–C bond which is dominant than the π → π* interactions by a small margin. In UDSB 7, UDSB 8 and UDSB 13 the lone pair on N atom participates in the stabilization through n → π* interactions.

The most predominant interactions such as π_C–C → π*_C–C and n_N → π*_N–C are depicted in the Fig. 7, where the interacting orbitals are in proper position and orientation, facilitating the ground state stabilization of the UDSB 1 and UDSB 11. Overall the results highlight the importance of the incorporation of heteroatom towards the ground state stabilization of UDSBs.

Fig. 7 Ground state stabilizing interactions of UDSB1 and UDSB11, showing π → π* and n → π* interactions.

Conclusions

Totally 14 UDSBs, 4 of them reported by Bon Hyeong Koo et al. and ten more UDSBs newly designed here, have been studied using DFT/TD-DFT methods in search for candidates with better optoelectronic properties. Our results show that the experimentally synthesized UDSBs were found to be efficient with lowest band gap and high wave length of absorption. The designed UDSBs show promising optoelectronic properties and it can be used as a trifunctional materials (emitter, hole and electron transport) in OLEDs. From UDSB 1 to UDSB 8, the HOMO energy slightly increases and LUMO energy gradually decreases. This indicates that hole creating and electron accepting abilities have been improved with suitable substituents. The longest wavelength absorption corresponds to the excitation to the lowest singlet excited state, where the electronic transitions are of the π → π* type, and excitation to S₁ state corresponds exclusively to the promotion of an electron from HOMO → LUMO in UDSB 1–10. Accordingly, the energy (E_g) of the S₀ → S₁ electronic transition follows the same trend with the ΔE_H–L of the molecule. The λ_hole values of the UDSB 1–12 are smaller than that of λ_electron values, which suggest that the hole-transfer rate is better than the electron transfer rate. The λ_hole for UDSB 13 and UDSB 14 have higher than that of λ_electron suggesting that it can act as a good electron transport material. The differences between the λ_hole and λ_electron of UDSB 7, 10, 12, 13 and 14 are small enough to act as ambipolar materials. The π → π* (UDSB 1–10) and ICT (UDSB 11–14) transition plays an important role both in absorption and emission spectra which was further characterized by NTO analysis. Further NBO analysis show that π → π* and n → π* interactions stabilize the ground state of the molecules. Overall, the present study clearly demonstrates the optical properties of UDSBs can be significantly tuned by suitable substitution and these compounds can be used to make efficient OLEDs.

Acknowledgements

G. V thanks University Grants Commission (UGC), India for the financial support in the form of BSR Meritorious Student Fellowship. SAV thanks the University Grants Commission (UGC), India and Bishop Heber College for FDP program (Ref. no. F.ETFTNBD030/FIP-XI PLAN). P. V thanks CSIR, India for the Award of an Emeritus Scientistship (Award Letter No. 21(0936)/12/EMR-II) and Department of Science & Technology (DST), India for Major Research Project (Ref. no. SB/S1/PC-52/2012). The authors also thank the reviewers for their very helpful suggestions and critical comments.

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Footnote

† Electronic supplementary information (ESI) available: The optimized geometries of the chosen molecules, results of natural transition orbital analysis and the results of second order perturbation theory analysis, effect of solvent on computed absorption and emission spectra of UDSBs. See DOI: 10.1039/c4ra07809h

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