Phenothiazine-based bipolar green-emitters containing benzimidazole units: synthesis, photophysical and electroluminescence properties

Abstract

A series of bipolar green emitters bearing hole-transporting phenothiazine and electron-transporting N-phenylbenzimidazole are synthesized and characterized by their photophysical, electrochemical and thermal properties. All the dyes exhibited solvent dependent emission characteristics indicative of a more polarized excited state probably arising from the enhanced intramolecular charge transfer (ICT) character in the photo-excited state. Dyes possessing two N-phenylbenzimidazole units and pyridine showed a larger degree of ICT in the excited state than the remaining dyes as evidenced from the huge solvatochromic shifts in emission spectra. The dyes exhibited facile oxidation attributable to the removal of an electron predominantly from the phenothiazine donor. However, the oxidation is positively shifted due to the electronic perturbations arising from the electron-withdrawing N-phenylbenzimidazole and pyridine units. The compounds exhibited marked thermal stability with high decomposition temperatures in the range, 412–454 °C. Solution processed organic light-emitting diodes fabricated using the dyes as neat hole-transporting emitting layer or dopant emitter in CBP host showed promising device parameters. A dye possessing three phenothiazine units exhibited bright green electroluminescence with maximum luminance of 1365 cd m⁻², CIE coordinates (0.314, 0.593) resembling the prescribed NTSC 1987 standards with a current efficiency of 1.9 cd A⁻¹ at 100 cd m⁻².

---

Introduction

During the last few decades electroluminescent devices such as fluorescent and phosphorescent organic light emitting diodes (FOLEDs & PHOLEDs) have received immense academic and commercial interest owing to their potential lighting applications such as full-colour flexible flat-panel displays, solid-state lighting¹ as well as alternatives to liquid-crystal displays.² Pioneering work on multi-layered OLEDs by Tang et al.,³ polymeric OLEDs by Burroughes et al.,⁴ PHOLEDs by Forrest et al. and OLEDs exploiting thermally activated delayed fluorescence (TADF) by Adachi have led the technology to commercial realization.⁵ The majority of the work on OLEDs has been devoted to improve the device efficiency by structural modifications of the active materials as well as modifications in device configurations.⁶ The colour of electroluminescence is controlled by employing different types of bipolar hybrids such as triphenylamine–oxadiazole,⁷ triphenylamine–benzimidazole,⁸ carbazole–oxadiazole,⁹ carbazole–benzimidazole¹⁰ and triphenylamine/carbazole–benzothiadiazole,¹¹ etc. in either single layer or multiple layer OLED device configurations or manipulating the exciton confinement. Though a huge number of solution-processable or vacuum deposited blue, green or red FOLEDs were reported with small molecules and polymers with encouraging device performances they require practical viability in external quantum efficiency and brightness. The low OLED parameters of FOLEDs are presumably due to improper alignment of energy levels between emitters and adjacent hole/electron injection or hole/electron transport materials leading to large hole/electron injection drifts, poor carrier mobility and unbalanced charges and consequently less charge recombination confinement in the emitter region, increased driving voltages, and lowered device efficiency.^1b,6 One strategy to improve the efficiency is the use of phosphorescent or TADF chromophores as emitting component.^6f,12 In this type of OLEDs dispersing of different types of triplet harvesters in various large band-gap hosts has been proven as an efficient strategy to achieve RGB (blue, green, red) colours with high luminance and quantum efficiency. Inherently, PHOLEDs cater high external quantum efficiency (EQE) due to the harvesting of both singlet and triplet excitons¹² but involves the use of expensive inorganic metal complexes and often shows poor device stability, high power consumption and sharper efficiency roll-off at high brightness is not enough for their commercialization.¹³ Therefore, improving the performance of FOLEDs rather than PHOLEDs is essential to realize commercially cheaper products. It can be easily achieved by the following criteria: (i) design and synthesize of emitting materials possessing of suitable HOMO and LUMO energy levels with respect to adjacent hole/electron injection or hole/electron transport materials by clamping of suitable electron donors (D) and electron acceptors (A) in a bipolar D–A configuration. Eventually that gives a balanced electron and hole transport and their recombination confinement in the emitting materials by lowering the hole/electron injection drifts,¹⁴ (ii) high quantum yield (Φ_F) of an emitting material is also desirable to get high EQE in OLED devices,¹⁵ (iii) the emitting material should have tendency to restrict the formation of aggregates in solid state to avoid the self-quenching pathways. It can be achieved by either the introduction of bulky substituent and attachment of branched chains in the molecular design or mixing of emitting material with polymers which possess high glass transition temperature.¹⁶ But, in contrary to this recently some groups have reported the use of aggregation induced emission (AIE) luminogens as an effective way to construct promising OLED devices,¹⁷ (iv) emitting materials should possess amorphous nature with high thermal decomposition temperature (T_d) and glass transition temperature (T_g) to give prolonged stability for the device.¹⁸ Hence, the design and synthesize of organic emitting materials featuring above characteristics is required to attain highly stable and efficient OLEDs.

Phenothiazines have strong tendency to donate electrons attributed to the presence of electron-releasing sulphur and nitrogen hetero atoms in comparison to well documented triarylamines and carbazole derived chromophores.¹⁹ In addition to this the nonplanar butterfly or bowl-shaped conformation of phenothiazine ring prevents the molecular aggregation, formation of intermolecular excimers and consequently reduces the intermolecular interactions between molecules in concentrated solution and solid state.¹⁹ By, encasing these points phenothiazine-based small molecules²⁰ and dye sensitizers¹⁹ have been developed for application in bulk heterojunction solar cells (BHSCs) and dye-sensitized solar cells (DSSCs), respectively. Youngil Park et al.²¹ reported a new hole injecting material for OLEDs based on phenothiazine with high T_g and device life time in comparison to triphenylamine based commercial hole-injection layer (HIL) material 4,4′,4′′-tris-(N-(naphthalen-2-yl)-N-phenyl-amino) triphenylamine (2-TNATA). Phenothiazine-based polymers,²² dendrimers,²³ TADF chromphores,²⁴ aggregation induced enhanced emission (AIEE) compounds,²⁵ mechanochromic materials,²⁶ and sensors²⁷ were also well documented in the literature. On the other hand due to high electron affinity²⁸ of N-phenylbenzimidazole, it serves as good electron acceptor or electron transport/injection component.⁸ Due to this reason in a huge number of bipolar compounds it was grafted as electron acceptor along with donors.²⁹ In few reports, N-phenylbenzimidazole was also used as ligand in the iridium³⁰ and copper³¹ complexes used as triplet emitters in PHOLEDs. Since systematic structure–property investigations on phenothiazine–benzimidazole conjugates are lacking in the literature we set a goal to make a series of compounds, 4a–d (Fig. 1) possessing chromophores such as vinyl, phenothiazine and benzimidazole. Additionally, the role of end-capping chromophores such as phenyl, pyridine, phenothiazine and N-phenylbenzimidazole on the optical and electrochemical properties was studied by systematically changing the molecular structures. All of the dyes exhibited green emission which is highly dependent on the solvent polarity indicating solvent specific interaction in the excited state. The new dyes are also tested as green emitters in the organic light-emitting diodes fabricated by solution-processed method. A bright green electroluminescence observed for the devices showed trends that can be correlated to the differences in the electronic properties of the dyes originating from the variation in the structure.

Fig. 1 Structures of the phenothiazine–benzimidazole based bipolar dyes.

Experimental section

The precursor 3-bromo-10-butyl-7-(1-phenyl-1H-benzo[d]imidazol-2-yl)-10H-phenothiazine (1) was synthesized according to our earlier report^19a and vinylphenothiazines such as 10-butyl-3-vinyl-10H-phenothiazine (2) and 10-butyl-3,7-divinyl-10H-phenothiazine (3) were synthesized according to literature procedures.^25b All other chemicals were procured from commercial sources and used as received. Solvents used for spectroscopic measurements were distilled over suitable drying agents according to standard protocols. All synthesized compounds were purified by column chromatography using 100–200 mesh silica gel. ¹H and ¹³C NMR spectra were recorded using a Bruker NMR spectrometer operating at 500.13 and 125.77 MHz, respectively. The chemical shifts were calibrated from the residual peaks observed for the deuterated solvent chloroform (CDCl₃) at δ = 7.26 and δ = 77.0 ppm for ¹H and ¹³C, respectively. High resolution mass spectra were obtained using a Bruker TOF-Q ESI mass spectrometer. The optical absorption and emission spectra of the dyes were measured for the freshly prepared air equilibrated solutions at room temperature by using UV-Vis spectrophotometer and spectrofluorimeter, respectively. Electrochemical measurements were performed with an electrochemical analyser using a conventional three-electrode assembly comprising glassy carbon working electrode, a platinum wire auxiliary electrode, and a non-aqueous (acetonitrile) Ag/AgNO₃ reference electrode. The E_1/2 values were determined as (E^a_p + E^c_p)/2, where E^a_p and E^c_p are the anodic and cathodic peak potentials, respectively. Ferrocene was used as an internal potential marker.

Synthesis of the dyes

(E)-10-Butyl-3-(1-phenyl-1H-benzo[d]imidazol-2-yl)-7-styryl-10H phenothiazine (4a). A mixture of 3-bromo-10-butyl-7-(1-phenyl-1H-benzo[d]imidazol-2-yl)-10H-phenothiazine (0.53 g, 1.0 mmol), styrene (0.12 g, 1.1 mmol), CH₃COONa (0.83 g, 10.0 mmol), Pd(OAc)₂ (10 mg, 2 mol%), (Bu)₄NBr (64 mg, 0.2 mmol) and DMF (10 mL) was charged in a pressure tube, maintained in nitrogen atmosphere and heated at 95 °C for 24 h. After the reaction completed, the crude reaction mixture was extracted with dichloromethane, dried over Na₂SO₄ and solvent removed under reduced pressure. The residue thus obtained was purified by column chromatography using hexane/dichloromethane (1 : 6 v/v) as eluant. Yellow solid; yield 0.25 g (60%); mp 170 °C; IR (KBr, cm⁻¹) 1639 (ν_C=C); ¹H NMR (CDCl₃, 500.13 MHz) δ 0.94 (t, J = 7.5 Hz, 3H), 1.42–1.48 (m, 2H), 1.73–1.79 (m, 2H), 3.81 (t, J = 7.0 Hz, 2H), 6.70 (d, J = 9.0 Hz, 1H), 6.80–6.87 (m, 2H), 7.13–7.24 (m, 6H), 7.27–7.35 (m, 6H), 7.45 (d, J = 2.0 Hz, 1H), 7.50–7.56 (m, 3H), 7.86 (d, J = 8.0 Hz, 1H), 8.56 (d, J = 6.0 Hz, 2H); ¹³C NMR (CDCl₃, 125.77 MHz) δ 13.76, 20.06, 28.77, 47.34, 110.26, 114.52, 115.38, 119.55, 122.91, 123.08, 123.85, 124.17, 124.39, 124.90, 125.86, 126.32, 127.18, 127.32, 127.41, 127.48, 128.08, 128.39, 128.56, 128.65, 129.94, 132.25, 137.04, 137.33, 142.96, 143.59, 145.81, 151.56; HRMS calcd for C₃₇H₃₁N₃S (M + Na) m/z 572.2136, found 572.2135.

(E)-10-Butyl-3-(1-phenyl-1H-benzo[d]imidazol-2-yl)-7-(2-(pyridin-4-yl)vinyl)-10H-phenothiazine (4b). It was synthesized by following the procedure described above for 4a, except using of 4-vinyl pyridine instead of styrene. Yellow solid; yield 64%; mp 190 °C; IR (KBr, cm⁻¹) 1630 (ν_C=C); ¹H NMR (CDCl₃, 500.13 MHz) δ 0.94 (t, J = 7.5 Hz, 3H), 1.41–1.50 (m, 2H), 1.71–1.79 (m, 2H), 3.81 (t, J = 7.0 Hz, 2H), 6.69 (d, J = 8.5 Hz, 1H), 6.79–6.80 (m, 1H), 6.96 (s, 1H), 7.19–7.7.31 (m, 7H), 7.32–7.36 (m, 5H), 7.43 (d, J = 2.0 Hz, 1H), 7.47–7.54 (m, 4H), 7.85 (d, J = 8.5 Hz, 1H); ¹³C NMR (CDCl₃, 125.77 MHz) δ ¹³C NMR (CDCl₃, 125.77 MHz) δ 13.80, 20.07, 28.74, 47.43, 110.33, 114.68, 115.41, 119.58, 120.71, 123.01, 123.20, 123.98, 124.12, 124.35, 124.54, 125.36, 126.66, 127.49, 128.14, 128.47, 128.73, 129.99, 130.94, 131.72, 137.00, 137.32, 142.92, 144.64, 144.72, 145.49, 149.89, 150.10, 151.44; HRMS calcd for C₃₆H₃₀N₄S (M + Na) m/z 573.2089, found 573.2114.

(E)-10-Butyl-3-(2-(10-butyl-10H-phenothiazin-3-yl)vinyl)-7-(1-phenyl-1H benzo[d]imidazol-2-yl)-10H-phenothiazine (4c). It was synthesized by following the procedure described above for 4a, but using 10-butyl-3-vinyl-10H-phenothiazine instead of styrene. Yellow solid; yield 50%; mp 192 °C; IR (KBr, cm⁻¹) 1653 (ν_C=C); ¹H NMR (CDCl₃, 500.13 MHz) δ 0.92–0.96 (m, 6H), 1.43–1.49 (m, 4H), 1.73–1.81 (m, 4H), 3.79–3.87 (m, 4H), 6.69 (d, J = 9.0 Hz, 1H), 6.77–6.82 (m, 3H), 6.85 (d, J = 8.0 Hz, 1H), 6.89–6.92 (m, 1H), 7.12–7.16 (m, 2H), 7.19–7.7.25 (m, 8H), 7.30–7.35 (m, 3H), 7.42 (d, J = 2.0 Hz, 1H), 7.49–7.55 (m, 3H), 7.85 (d, J = 8.0 Hz, 1H); ¹³C NMR (CDCl₃, 125.77 MHz) δ 13.72, 13.77, 20.00, 20.09, 28.68, 28.88, 47.10, 47.25, 110.22, 114.42, 144.77, 115.24, 115.31, 116.53, 119.49, 122.30, 122.85, 123.02, 123.72, 124.07, 124.21, 124.59, 124.67, 124.92, 125.23, 125.44, 125.60, 126.01, 127.16, 127.34, 127.40, 128.00, 128.16, 128.31, 128.60, 128.97, 129.88, 131.78, 132.33, 136.97, 137.26, 142.93, 143.19, 144.36, 144.86; HRMS calcd C₄₇H₄₂N₄S₂ (M + Na) m/z 749.2749, found 749.2778.

7,7′-(1E,1′E)-2,2′-(10-Butyl-10H-phenothiazine-3,7-diyl)bis(ethene-2,1-diyl)bis(10-butyl-3-(1-phenyl-1H-benzo[d]imidazol-2-yl)-10H-phenothiazine) (4d). It was obtained by following the procedure described above for 4a but using 3-bromo-10-butyl-7-(1-phenyl-1H-benzo[d]imidazol-2-yl)-10H-phenothiazine (0.53 g, 1.0 mmol), 10-butyl-3,7-divinyl-10H-phenothiazine (0.54 g, 1.7 mmol), CH₃COONa (5.6 g, 68.4 mmol), (Bu)₄NBr (0.39 g, 0.70 mmol), Pd(OAc)₂ (35 mg, 2 mol%) and DMF (10 mL). The desired compound was obtained on elution with hexane/dichloromethane (1 : 1 v/v). Yellow solid; yield 0.51 g (25%); mp 200 °C; IR (KBr, cm⁻¹) 1638 (ν_C=C); ¹H NMR (CDCl₃, 500.13 MHz) δ 0.91–0.97 (m, 9H), 1.41–1.47 (m, 6H), 1.74–1.79 (m, 6H), 3.78–3.86 (m, 6H), 6.69 (d, J = 9.0 Hz, 2H), 6.76–6.82 (m, 8H), 7.20–7.24 (m, 14H), 7.30–7.35 (m, 6H), 7.42 (d, J = 2.0 Hz, 2H), 7.49–7.55 (m, 6H), 7.86 (d, J = 8.0 Hz, 2H); ¹³C NMR (CDCl₃, 125.77 MHz) δ 13.80, 20.12, 28.90, 29.67, 31.63, 47.33, 110.27, 114.49, 115.21, 115.38, 119.52, 123.74, 124.15, 124.33, 124.38, 124.67, 124.72, 125.58, 126.02, 128.06, 128.40, 128.67, 129.94, 131.92, 132.39, 137.02, 137.29, 142.90, 143.30, 143.98, 145.85, 151.57; HRMS calcd for C₇₈H₆₇N₇S₃ (M + Na) m/z 1220.4518, found 1220.

OLED fabrication and performance evaluation

The OLED devices for the dyes 4a–d were fabricated on a precleaned glass substrate containing a 125 nm layer indium tin oxide as anode, 35 nm poly(3,4-ethylene-dioxythiophene) poly(styrenesulphonate) (PEDOT:PSS) as hole-injection layer (HIL), emissive layer (EML), 32 nm 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) as electron transporting layer (ETL), a 0.7 nm LiF electron injection layer (EIL), and a 150 nm Al layer as cathode. The aqueous solution of PEDOT:PSS was spin coated at 4000 rpm for 20 s to form a 40 nm HIL layer. The dyes 4a–d doped in 4,4′-bis(9H-carbazol-9-yl)biphenyl (CBP), and non-doped were deposited by spin-coating at 2500 rpm for 20 s and served as emissive layer. Subsequently, lithium fluoride and aluminium cathode were thermally evaporated at 1.0 × 10⁻⁵ Torr.

Results and discussion

Synthesis and characterization

The new dyes (4a–d) containing phenothiazine and N-phenylbenzimidazole chromophores are synthesized conveniently as illustrated in Scheme 1 by following a single step Heck cross-coupling³² involving 3-bromo-10-butyl-7-(1-phenyl-1H-benzo[d]imidazol-2-yl)-10H-phenothiazine (1) and suitable vinyl precursors such as styrene, 4-vinylpyridine, 10-butyl-3-vinyl-10H-phenothiazine and 10-butyl-3,7-divinyl-10H-phenothiazine. All the dyes are greenish yellow in colour and soluble in most common organic solvents such as dichloromethane, toluene, tetrahydrofuran, etc. All these new compounds were thoroughly analysed by NMR (¹H & ¹³C) spectroscopy and high resolution mass spectrometry (HRMS) and the data found consistent with the proposed structures.

Scheme 1 Synthetic pathway for the dyes 4a–d.

Photophysical properties

The absorption spectra of the dyes recorded in dichloromethane are displayed in Fig. 2 and the corresponding absorption data summarized in Table 1. All the dyes exhibited two absorption bands at ca. 300–305 nm and ca. 385–415 nm. The intense band appearing at higher energy wavelength (∼300 nm) is assigned to the localized π–π* transitions originating from the phenothiazine and N-phenylbenzimidazole units,^19a,33 while the longer wavelength absorption (>385 nm) is attributed to the delocalized π–π* electronic transition of the entire molecule. The above assignment is supported by the fact that the shorter wavelength remains unaltered in all the compounds while the longer wavelength absorption shifts progressively to the red region reflective of the variation in the conjugation length in the molecules. Thus the dye 4d exhibited the most red-shifted absorption with high molar extinction coefficient owing to the presence of elongated π-conjugation.^34,35 A slightly red-shifted absorption observed for 4b when compared to 4a is reflective of the donor–acceptor interaction between the phenothiazine and pyridine units. All the dyes show red-shifted absorption (Δλ = ∼30 nm) when compared to the phenothiazine and benzimidazole hybrids known in the literature³³ which suggests the effect of additional chromophores such as vinyl, phenyl, pyridine and phenothiazine in extending the electronic conjugation in the molecules. The absorption peak positions were red-shifted by 32, 11, 16 and 9 nm, respectively for dyes 4a–d in the film (Fig. S5†), when compared to those recorded in the solution, indicating a weak aggregation interaction in the solid state.³⁶

Fig. 2 Absorption and emission spectra of the dyes recorded in dichloromethane.

Table 1 Optical data for the dyes 4a–d

Dye	λabs, nm (εmax × 10^3 M^−1 cm^−1)^a	λem, nm (ΦF)^a^,^b	Stokes shift, cm^−1
a Measured for dichloromethane solutions.b Absolute quantum yield (ΦF) was obtained with the use of integrating sphere method.
4a	377 (28.4), 302 (85.4)	501 (0.41)	6565
4b	392 (28.9), 303 (82.3)	537 (0.89)	6888
4c	389 (29.9), 300 (75.5)	502 (0.42)	5787
4d	414 (57.7), 303 (126.0)	518 (0.81)	4850

---

To probe the nature of ground state in these dyes and its sensitivity to solvent polarity, we had measured the absorption and emission spectra in solvents having different polarity parameters such as cyclohexane (CH), toluene (TOL), dichloromethane (DCM), tetrahydrofuran (THF), methanol (MeOH), acetonitrile (ACN) and N,N-dimethylformamide (DMF). The absorption spectra of the dyes are not affected by the polarity of the solvents indicating a non-polar ground state.

The emission spectra of the dyes measured in dichloromethane are also shown in Fig. 2. All the dyes displayed a broad structureless emission profile in the green region. The emission maximum of the dyes follows the trend, 4a (501 nm) < 4c (502 nm) < 4d (518 nm) < 4b (537 nm) which is contrary to the absorption maximum trend. This suggests a probable structural reorganization in the excited state for the dyes.³⁷ Considering the Stokes shift as the measure of structural reorganization, the degree of structural deformation in the excited state for the dyes may be in the order: 4d (4850 cm⁻¹) < 4c (5787 cm⁻¹) < 4a (6565 cm⁻¹) < 4b (6888 cm⁻¹). Significant structural perturbations in 4b may be due to the acceptor nature of pyridine which may induce intramolecular charge transfer in the excited state.³⁸

In contrast to the absorption spectra, the fluorescence spectra of the dyes showed significant solvent dependence. The emission spectrum gradually shifted to longer wavelength region on increasing the polarity from CH to DMF. The largest shift was observed for the pyridine dye 4b (Fig. 3). The solvatochromism of the dyes may originate either due to the formation of more polarized excited state by the rotation of aromatic chromophores with respect to ethylene bridge^25c or a large induced dipole moment due to the alternation in the donor or acceptor strength of the chromophores in the excited state.²⁶ The polar excited state are prone to be stabilized by polar solvents than the non-polar solvents. This is further confirmed by the large Stokes shifts observed for the dyes in the polar solvents such as methanol.³⁹ The compounds showing photo-induced ICT are found to be beneficial for the functioning in OLED.⁴⁰ The dyes exhibited slightly red-shifted emission in film when compared to those recoded in dichloromethane. This may be due to different dielectric surroundings in the solid state⁴¹ or solid state effect.⁴² The absolute quantum yield (Φ_F) of the dyes was recorded in TOL and DCM with using an integrating sphere and in general all the dyes exhibited moderate to good quantum yield (Tables 1 and 2). Interestingly, the dyes showed higher quantum efficiency in polar solvent, DCM when compared to that of toluene. This points that the dipolar quenching is not an effective energy decay channel in these dyes.^15c,34,44 Due to high Φ_F the electroluminescent devices fabricated with the dyes 4b and 4d are expected to deliver high L_max, and EQE%.

Fig. 3 Emission spectra of 4b recorded in different solvents.

Table 2 Emission data for the dyes recorded in different solvents and as thin film

λem (ΦF)^a, nm					Stokes shift, cm^−1
Solvent	4a	4b	4c	4d	4a	4b	4c	4d
a Obtained using an integrating sphere.
CH	481	496	482	494	5877	5745	5227	4388
TOL	494 (0.33)	512 (0.61)	496 (0.18)	509 (0.47)	6282	6176	6017	4625
THF	495	526	498	507	6465	6762	5693	4548
MeOH	504	544	508	510	6897	7128	6088	4962
ACN	503	552	505	516	6786	7724	5708	4951
DMF	506	551	504	515	6692	7232	5348	4563
Film	513	528	510	523

---

The compound, 4b contains a pyridine unit which is capable of interacting with metal salts and hydrogen ions.⁴⁵ Such an interaction may alter the electronic properties of the molecule. For instance, protonation of pyridine unit will form a pyridinium ion which is a much stronger acceptor group when compared to pyridine. Incrimination of acceptor strength by protonation may result in a strong donor–acceptor interaction with phenothiazine which might affect the position of the corresponding charge transfer transition in the ground state as well as excited state. Changes in the absorption and emission spectra of 4b in the presence of trifluoroacetic acid (TFA) was examined in dichloromethane and shown in Fig. 4. On addition of TFA, 4b showed a dramatic red-shift in absorption and emission profiles suggestive of protonation at the pyridine unit as hypothesized above. On addition of TFA, the absorption shifted to 478 nm from 392 nm (Δλ = 86 nm) and the emission peak moved to 612 nm from 539 nm (Δλ = 73 nm) for 4b. All these observations suggested that 4b is converted to a more polar molecule on protonation and highly stabilized in polar solvents. No such changes were witnessed for the other dyes which indicate the involvement of pyridine in the reaction.

Fig. 4 Absorption and emission spectra of 4b recorded in dichloromethane before and after addition of TFA.

The effect of solvent polarity on emission properties of the dyes can be further understood by correlation of Stokes shift with the orientation polarizability (Δf) (Lippert–Mataga plot) as well as E_T(30) parameter. Especially, Lippert–Mataga⁴⁶ plots are useful to predict change in dipole moment between ground and excited states and the plot of Stokes shift vs. E_T(30) is useful for the prediction of nature of solvatochromism.⁴⁷ Lippert–Mataga equation, expresses the Stokes shift as a function of the Δf.

where h is Planck's constant (6.6256 × 10⁻²⁷ erg), c is the velocity of light (2.9979 × 10¹⁰ cm s⁻¹), a is radius of cavity in which molecule resides, [small nu, Greek, macron]_A and [small nu, Greek, macron]_F are the wave numbers (cm⁻¹) of the absorption and emission respectively. Dielectric constant is represented by ε while n denotes refractive index. The orientation polarizability is calculated by the following equation:

Fig. 5 shows the Lippert–Mataga and Stokes shift vs. E_T(30) parameter plots for the dyes. The positive slopes in the plots indicate a general positive solvatochromism. The general solvation effect and dipolar interactions dictate the dye–solvent associations. Among the dyes, 4a and 4b exhibited relatively good linearity in the correlation plots and 4b showed largest slope in the series in accordance with the more polarized excited state deduces for this molecule from its emission spectra.⁴⁸

Fig. 5 (a) Lippert–Mataga plot (b) Stokes shift vs. E_T(30) plot for the dyes 4a–d.

Electrochemical characteristics

Since the injection of holes and electrons from anode and cathode, respectively into the active layer and their recombination are mainly dependent on the energy level alignment of the interfaces, we had probed the redox behaviour of the dyes to gain insights into the HOMO and LUMO energy levels of the molecules. The cyclic voltammograms recorded for the dyes are displayed in Fig. 6 and the electrochemical parameters are listed in Table 3. All the dyes exhibited a quasi-reversible oxidation couple attributable to the removal of electron from the phenothiazine segment.^19a The dyes 4c and 4d exhibited an additional oxidation originating from the second phenothiazine unit. The first oxidation potential showed a trend (4d < 4c < 4a < 4b) which is reflective of the electron-richness of the molecule. Consequently, the dye, 4b containing pyridine unit showed the highest oxidation potential in the series owing to the donor–acceptor interaction which reduces the electron density of phenothiazine. Out of the two oxidations observed for the dyes 4c and 4d, the low oxidation potential is attributed to the phenothiazine farthest away from the benzimidazole unit. The electron-withdrawing benzimidazole segment is expected to deplete the electron density from the closely lying phenothiazine moiety and reduce its donor strength. Generally, the phenothiazine oxidation occurs close to ferrocene oxidation potential.^19,49 In the present compounds, the anodic shift of the oxidation potentials is attributed to the electron-withdrawing effect of the benzimidazole unit. The HOMO energy values estimated from the first oxidation potential of the dyes fall in the range, 4.93–5.11 eV which lies slightly above the HOMO of hole injection layer PEDOT:PSS (4.9 eV). Comparatively small hole injection barrier (Φ_h) between PEDOT:PSS and emitting layer comprising these dyes will be beneficial for the operation of the OLED devices. Particularly, the Φ_h between PEDOT:PSS and the dyes 4c and 4d are 0.09 and 0.03 eV, respectively and suggest a relatively better performance for these dyes. The LUMO values of the dyes were obtained by subtracting of optical band gap (E_0–0) from HOMO values, while the E_0–0 deduced from the intersection of absorption and emission spectra. The calculated LUMO values are in the range, 2.22–2.42 eV. This leads to a small barrier for the injection of electrons from the electron transport layer (TPBi) into the emitting layer. However, we see an improvement in the LUMO values attributable to the electron-accepting nature of benzimidazole.⁵⁰

Fig. 6 Cyclic voltammograms recorded for the compounds 4a–d in dichloromethane.

Table 3 Electrochemical and thermal data of the compounds 4a–d

Dye	Eox, V (ΔEp, mV)^a	HOMO^b, eV	LUMO^c, eV	E0–0^d, eV	Td, °C
a The oxidation potential of the dyes were recorded in DCM (2 × 10^−4 M) with 0.1 M Bu4NClO4 as electrolyte (working electrode: glassy carbon; counter electrode: Pt wire; reference electrode: non-aqueous Ag/Ag^+ ferrocene was used as internal standard for potential calibration).b Obtained from the oxidation potential using the formula HOMO = 4.8 + Eox.c Calculated using the formula LUMO = HOMO − E0–0.d Estimated from the intersection between the normalized absorption and emission spectra.
4a	0.285 (66)	5.09	2.33	2.76	467
4b	0.311 (64)	5.11	2.42	2.69	476
4c	0.185 (67), 0.297 (69)	4.99	2.22	2.77	456
4d	0.127 (69), 0.229 (70)	4.93	2.27	2.66	479

---

Theoretical calculations

To gain more insight into the electronic structure of the investigated dyes, we have performed density functional theoretical (DFT)⁵¹ calculations using Gaussian 09 program package. The ground state geometry of the approximated dyes (butyl groups replaced with methyl unit) was optimized by using B3LYP⁵² functional and 6-31 G (d, p) basis set. The computed frontier molecular orbitals HOMO, HOMO−1, LUMO and LUMO+1 of the dyes are shown in Fig. 7. Vertical excitation energies, their oscillator strengths and orbital contribution are listed in Table 4. In all of the dyes, the HOMO is predominantly contributed by the phenothiazine chromophore and ethylene linker. The LUMO is also spread on the same units, except for 4b in which pyridine also participates in the LUMO construction. Thus the HOMO to LUMO electronic transitions in the dyes can be mainly assigned to the π–π* transition with a little charge-transfer character for the dye 4b. The calculation absorption wavelength corresponding to this transition assumes the order 4d > 4b > 4c > 4a which is consistent with the trend observed in dichloromethane solution (vide supra). The N-phenylbenzimidazole unit is involved in the HOMO−1 or LUMO+1 orbitals. This explains the difficulty in the removal of second electron from the molecules.

Fig. 7 Electronic distribution in the frontier molecular orbitals of the model dyes.

Table 4 Computed vertical transitions and their oscillator strengths, configurations,^a ionization potentials and electron affinities and related parameters

Dye	B3LYP			Adiabatic IP, eV	Vertical IP, eV	Adiabatic Ea, eV	Vertical Ea, eV	Reorganization Energies^b
	λmax, nm	f	Configuration					λ^+, eV	λ^−, eV
a Contributions less than 10% are omitted.b computed by following procedure reported earlier.^55
4a	406.0	0.5791	HOMO → LUMO (92%)	−5.877	−6.063	0.424	0.284	0.34	0.27
	317.7	0.6409	HOMO−2 → LUMO (52%), HOMO → LUMO+2 (23%)
	307.7	0.5397	HOMO−1 → LUMO (64%), HOMO → LUMO+4 (22%)
	303.1	0.3781	HOMO → LUMO+4 (40%), HOMO−2 → LUMO (19%), HOMO−1 → LUMO (17%)
	287.0	0.2461	HOMO−1 → LUMO+1 (85%)
4b	423.8	0.4966	HOMO → LUMO (94%)	−6.067	−6.255	0.684	0.542	0.34	0.28
	331.8	0.1085	HOMO−1 → LUMO (59%), HOMO−3 → LUMO (22%)
	313.9	0.7731	HOMO−3 → LUMO (35%), HOMO−1 → LUMO (27%), HOMO−2 → LUMO (13%)
	308.2	0.1599	HOMO → LUMO+3 (68%)
	303.4	0.4084	HOMO → LUMO+4 (59%), HOMO → LUMO+3 (14%)
4c	419.6	1.0155	HOMO → LUMO (92%)	−5.595	−5.753	0.477	0.345	0.30	0.26
	318.0	0.2271	HOMO → LUMO+4 (29%), HOMO−3 → LUMO (20%)
	309.4	0.7382	HOMO F022 LUMO+6 (34%), HOMO−2 F022 LUMO (20%), HOMO−1 F022 LUMO+4 (10)
	305.7	0.5982	HOMO → LUMO+5 (24%), HOMO−2 → LUMO (19%), HOMO−3 → LUMO (17%), HOMO → LUMO+4 (10%)
4d	451.3	1.8140	HOMO → LUMO (93%)	−5.351	−5.463	0.767	0.692	0.22	0.15
	393.9	0.2178	HOMO−2 → LUMO (46%), HOMO−1 → LUMO+1 (40%)

---

According to Marcus–Hush electron-transfer theory, the charge transport rate depends mainly on reorganization energy and electronic coupling.⁵³ The hole and electron reorganization energies computed for the dyes are also listed in Table 4. In general, the dyes possess relatively small electron reorganization energies than the hole reorganization energies. This suggests that the molecules possess favourable electron-transport characteristics. Among the dyes, 4d possesses the lowest reorganization energies for the hole and electron suggesting a more favourable condition for charge transport. Since the hole and electron reorganization energies are similar for 4d, one can expect a balanced charge transport in its molecular layer.⁵⁴

Thermal properties

In OLEDs, thermal stability of the active materials plays a key role. In order to ascertain the thermal behaviour of the dyes, we had performed thermogravimetric analysis (TGA) for the dyes under nitrogen atmosphere at a heating rate of 10 °C min⁻¹. Thermogravimetric traces of the compounds are shown in Fig. 8 and the thermal decomposition temperatures (T_d) of the dyes fall in the range, 412–454 °C (Table 3). Furthermore the present dyes displayed superior thermal stability than the reported phenothiazine–benzimidazole containing emitter 3,7-bis(1-(biphenyl-4-yl)benzimidazol-2-yl)10-methylphenothiazine (PhBBMP),⁵⁶ suggesting the beneficial role of additional vinyl linked chromophores.

Fig. 8 Thermogravimetric traces of the dyes.

Electroluminescent properties

To evaluate the function of the synthesized dyes as green emitters, we have fabricated two types of electroluminescent devices by solution-processed method. The configurations of Type 1 and Type 2 devices are ITO/PEDOT:PSS/4a–d/TPBi/LiF/Al and ITO/PEDOT:PSS/CBP:10 wt% of 4a–d/TPBi/LiF/Al respectively. In both the devices, ITO and Al are anode and cathode, respectively. PEDOT:PSS and LiF acts as hole injection and electron injection layers, respectively while TPBi serve as electron transport layer (ETL). The differences in the devices come from the function of the newly synthesized dyes. In Type 1 devices 4a–d act as hole transporting emitting layer (HTL & ETL), while in Type 2 devices they were used as emitting dopants in CBP host. Though N-phenylbenzimidazole moiety is expected to import electron-transporting capability to the dyes, we inserted a thin layer of TPBi to reduce the electron injection barrier.^43a

The energy levels alignment of the materials used in the fabrication of the devices, current density–voltage–luminescence (I–V–L) curves and electroluminescence emission spectra are shown in Fig. 9–11, respectively. The device performance characteristics are summarized in Table 5.

Fig. 9 Energy levels alignment of the materials used in the fabrication of the devices.

Fig. 10 Current density–voltage–luminance (I–V–L) characteristics of the devices, (a) Type 1 devices and (b) Type 2 devices.

Fig. 11 Electroluminescence spectra of (a) Type 1 devices and (b) Type 2 devices.

Table 5 Electroluminescent (EL) characteristics of the devices based on the dyes 4a–d

Dye	Von^a, V	λem, nm	At 100/1000 cd m^−2			Lmax^b, cd m^−2	CIE (x, y)
			ηc, cd A^−1	ηp, lm W^−1	EQE (%)
a Driving voltage (V) at 10 cd m^−2.b Maximum luminance.
4a	5.4	588	0.3/—	0.1/—	—/—	164	—/—
4b	8.3	548	0.4/—	0.1/—	—/—	245	—/—
4c	5.6	572	0.6/—	0.2/—	0.2/—	510	(0.438, 0.519)/—
4d	3.7	540	1.2/0.5	0.8/0.3	0.4/—	1071	(0.424, 0.554)/—
CBP:10 wt% 4a	5.4	508	0.9/0.6	0.5/0.1	0.4/—	1036	(0.319, 0.494)/—
CBP:10 wt% 4b	7.1	524	1.0/1.2	0.4/0.3	0.3/—	1407	(0.339, 0.564)/—
CBP:10 wt% 4c	5.2	508	1.4/0.6	0.6/0.2	0.5/—	1016	(0.289, 0.529)/—
CBP:10 wt% 4d	4.7	520	1.9/1.1	0.9/0.3	0.6/—	1365	(0.314, 0.593)/—

---

When the dyes are applied as non-doped emitting layer (EML) in Type 1 devices, they delivered poor electroluminescent characteristics. Among the devices, the one fabricated with 4d as emitting layer rendered green electroluminescence with high luminance (L_max = 1071 cd m⁻²), CIE coordinates (0.424, 0.554), current density (η_c) 1.2 cd A⁻¹, power efficiency (η_p) 0.8 lm W⁻¹ and EQE 0.4%. The better electroluminescent characteristics of 4d device are attributed to the following aspects: (1) small operating voltage probably arising due the presence of two electron-withdrawing benzimidazole units which might comparatively enhance the electron-transport in the molecular layer.^14d,57,58 (2) The presence of more number of non-planar phenothiazine segments in the 4d molecular structure may reduce the intermolecular aggregations and consequently close-packing of the molecules in the solid state lead to improved device efficiency.⁵⁹ (3) High T_d value of 4d also help to form stable films during the device operation. (4) Relatively high Φ_F of 4d when compared to the other dyes. However, the order of L_max, η_c and η_p of the devices fabricated with these dyes is 4d > 4c > 4b > 4a.

In order to improve the electroluminescent characteristics of the OLED devices, we have further fabricated another type of devices employing the dyes as emitting dopants in CBP host. We found 10% dopant concentration is required to give satisfactory results. Generally, the Type 2 devices exhibited better performance when compared to the Type 1 devices. These devices showed blue-shifted electroluminescence when compared to the Type 1 devices (Fig. 11). This suggests the possibility of dye aggregation in the neat films.⁶⁰ The EL maxima observed for the Type 2 devices are consistent with the solid film PL maxima of the dyes (vide supra).

In Type 2 devices also, the device made with 10 wt% doped 4d in CBP exhibited the high EQE of 0.6% with a η_c and a η_p of 1.9 cd A⁻¹ and 0.9 lm W⁻¹ at 100 cd m⁻², respectively. Apparently for the same device considerable high L_max of 1365 cd m⁻² with CIE coordinates (0.314, 0.593) resembling the prescribed NTSC 1987 standards observed. Interestingly device fabricated with 10 wt% 4b doped in CBP produced high L_max of 1407 cd m⁻² compared to other devices but at the same time it gave inferior other electroluminescent characteristics such as low EQE (0.3%), η_c (1.0 cd A⁻¹), η_p (0.4 lm W⁻¹) at 100 cd m⁻². Meanwhile the EQE% of the devices mainly related to the current density of the dyes in the I–V curves (Fig. 10). The order of EQE of the dyes at 100 cd m⁻² is 4d (0.6%) > 4c (0.5%) > 4a (0.4%) > 4b (0.3%) and it is consistent with the order of current density of the dyes 4d > 4c > 4a > 4b. The better electroluminescent characteristics of the device made with 10 wt% 4d doped in CBP can be explained by the facile energy transfer from the CBP host to 4d through Forster-type energy transfer mechanism.^35,59,61 resulting in facile formation and decay of excitons at the interface of CBP doped emitting layer of 4d.

Conclusions

We have synthesized bipolar green emitting compounds based on phenothiazine and N-phenyl benzimidazole by Heck cross-coupling reaction of 3-bromo-10-butyl-7-(1-phenyl-1H-benzo[d]imidazol-2-yl)-10H-phenothiazine scaffold with suitable vinyl derivatives to understand the effect of molecular design on photophysical, electrochemical, thermal and electroluminescent properties. The absorption, emission, redox and thermal properties of these compounds were investigated by UV-Vis, fluorescence, cyclic voltammetry techniques and thermogravimetric analysis, respectively. Furthermore, the organization of frontier molecular orbitals such as HOMO, HOMO−1, LUMO and LUMO+1 in the present dyes were studied using DFT studies. It was predicted that the absorption and emission properties of these dyes greatly depend on the electron delocalization in the ground state and ICT character in the excited state, respectively. Dye 4d having more elongated structure exhibited most red-shifted absorption maximum while dye 4b having strong ICT character in the excited state rendered the most red-shifted emission maximum. These compounds have an appreciable Φ_F in the range of 0.41–0.89% in dichloromethane solution. Electrochemical studies revealed that the propensity of oxidation of phenothiazine donor mainly depended on the electron richness of the dyes. Dyes 4c and 4d exhibited two reversible oxidation waves attributable to the presence of two different types of phenothiazine donors, while the presence of single phenothiazine chromophore dyes 4a and 4b displayed only one reversible oxidation wave. Finally, the electroluminescent properties of these dyes were characterized by fabricating the OLED devices. Among the devices, the device fabricated with the 4d exhibited promising EQE and L_max presumably due to balanced electron and hole transport and their recombination confinement in the emitting material.

Acknowledgements

KRJT is thankful to DST, New Delhi (ref. No. DST/TSG/PT/2013/09) for financial support. GBB acknowledges a senior research fellowship (SRF) from CSIR, New Delhi. FIST grant from DST for the purchase of ESI mass spectrometer in the Department of Chemistry, IIT Roorkee is also acknowledged.

Notes and references

1. (a) Y. Im and J. Y. Lee, Chem. Mater., 2014, 26, 1413; (b) M. Zhu and C. Yang, Chem. Soc. Rev., 2013, 42, 4963; (c) D. Li, H. Zhang and Y. Wang, Chem. Soc. Rev., 2013, 42, 8416.
2. (a) M. A. Wolak, J. Delacamp, C. A. Landis, P. A. Lane, J. Anthony and Z. Kafafi, Adv. Funct. Mater., 2006, 16, 1943; (b) J. Li, D. Liu, Y. Li, C. S. Lee, H. L. Kwong and S. T. Lee, Chem. Mater., 2005, 17, 1208.
3. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
4. J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Mackay, K. Marks, R. H. Friend, P. L. Burns and A. B. Holmes, Nature, 1990, 347, 539.
5. (a) M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151; (b) S. Y. Lee, T. Yasuda, Y. S. Yang, Q. Zhang and C. Adachi, Angew. Chem., Int. Ed., 2014, 53, 6402; (c) H. Tanaka, K. Shizu, H. Miyazaki and C. Adachi, Chem. Commun., 2012, 48, 11392; (d) H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 2012, 492, 234.
6. (a) H. Sasabe and J. Kido, J. Mater. Chem. C, 2013, 1, 1699; (b) J.-H. Jou, S. Kumar, A. Agrawal, T.-H. Li and S. Sahoo, J. Mater. Chem. C, 2015, 3, 2974; (c) B. Minaev, G. Baryshnikov and H. Agren, Phys. Chem. Chem. Phys., 2014, 16, 1719; (d) K. S. Yook and J. Y. Lee, Adv. Mater., 2014, 26, 4218; (e) Y. Tao, C. Yang and J. Qin, Chem. Soc. Rev., 2011, 40, 2943; (f) A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz and A. B. Holmes, Chem. Rev., 2009, 109, 897; (g) H.-C. Yeh, L.-H. Chan, W.-C. Wu and C.-T. Chen, J. Mater. Chem., 2004, 14, 1293.
7. F. Dumur and F. Goubard, New J. Chem., 2014, 38, 2204.
8. (a) S. Gong, C. Zhong, Q. Fu, D. Ma, J. Qin and C. Yang, J. Phys. Chem. C, 2013, 117, 549; (b) M. Y. Lai, C. H. Chen, W. S. Huang, J. T. Lin, T. H. Ke, L. Y. Chen, M. H. Tsai and C. C. Wu, Angew. Chem., Int. Ed., 2008, 47, 581; (c) Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto, T. Kajita and M. Kakimoto, Chem. Mater., 2008, 20, 2532.
9. (a) A. L. Fisher, K. E. Linton, K. T. Kamtekar, C. Pearson, M. R. Bryce and M. C. Petty, Chem. Mater., 2011, 23, 1640; (b) k. Brunner, A. Dijken, H. Bӧrner, J. J. A. M. Bastiaansen, N. M. M. Kiggen and B. M. W. Langeveld, J. Am. Chem. Soc., 2004, 126, 6035; (c) Y. Zheng, A. S. Batsanov, V. Jankus, F. B. Dias, M. R. Bryce and A. P. Monkman, J. Org. Chem., 2011, 76, 8300; (d) Y. T. Tao, Q. Wang, C. L. Yang, Q. Wang, Z. Q. Zhang, T. T. Zou, J. G. Qin and D. G. Ma, Angew. Chem., Int. Ed., 2008, 47, 8104; (e) K. R. J. Thomas, J. T. Lin, Y.-T. Tao and C.-H. Chuen, Chem. Mater., 2004, 16, 5437; (f) Q. Li, L.-S. Cui, C. Zhong, Z.-Q. Jiang and L.-S. Liao, Org. Lett., 2014, 16, 1622; (g) K. E. Linton, A. L. Fisher, C. Pearson, M. A. Fox, L.-O. Pålsson, M. R. Bryce and M. C. Petty, J. Mater. Chem., 2012, 22, 11816.
10. (a) W.-Y. Hung, L.-C. Chi, W.-J. Chen, Y.-M. Chen, S.-H. Chou and K.-T. Wong, J. Mater. Chem., 2010, 20, 10113; (b) S. Gong, Y. Zhao, C. Yang, C. Zhong, J. Qin and D. Ma, J. Phys. Chem. C, 2010, 114, 5193.
11. (a) N. Prachumrak, S. Pojanasopa, S. Namuangruk, T. Kaewin, S. Jungsuttiwong, T. Sudyoadsuk and V. Promarak, ACS Appl. Mater. Interfaces, 2013, 5, 8694; (b) T. Khanasa, N. Prachumrak, R. Rattanawan, S. Jungsuttiwong, T. Kaewin, T. Sudyoadsuk, T. Tuntulani and V. Promarak, Chem. Commun., 2013, 49, 3401; (c) Y. Li, B.-X. Li, W.-Y. Tan, Y. Liu, X.-H. Zhu, F.-Y. Xie, J. Chen, D. Ma, J. Peng, Y. Cao and J. Roncali, Org. Electron., 2012, 13, 1092; (d) X. Yang, S. Zheng, R. Bottger, H. S. Chae, T. Tanaka, S. Li, A. Mochizuki and G. E. Jabbour, J. Phys. Chem. C, 2011, 115, 14347; (e) W. Qin, J. W. Y. Lam, Z. Yang, S. Chen, G. Liang, W. Zhao, H. S. Kwok and B. Z. Tang, Chem. Commun., 2015, 51, 7321.
12. (a) A. Baldo and D. F. O'Brien, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 60, 14422.
13. S. Gong, Y.-L. Chang, K. Wu, R. White, Z.-H. Lu, D. Song and C. Yang, Chem. Mater., 2014, 26, 1463.
14. (a) C. H. Chang, M. C. Kuo, W. C. Lin, Y. T. Chen, K. T. Wong, S. H. Chou, E. Mondal, R. C. Kwong, S. Xia, T. Nakagawa and C. Adachi, J. Mater. Chem., 2012, 22, 3832; (b) A. Benor, S. Y. Takizawa, C. Pérez-Bolivar and P. Anzenbacher, Appl. Phys. Lett., 2010, 96, 243310; (c) S. Lee, K. H. Kim, D. Limbach, Y. S. Park and J. J. Kim, Adv. Funct. Mater., 2013, 23, 4105; (d) Y. Yuan, J.-X. Chen, F. Lu, Q.-X. Tong, Q.-D. Yang, H.-W. Mo, T.-W. Ng, F.-L. Wong, Z.-Q. Guo, J. Ye, Z. Chen, X.-H. Zhang and C.-S. Lee, Chem. Mater., 2013, 25, 4957.
15. (a) U. Mitschke and P. Bauerle, J. Mater. Chem., 2000, 10, 1471; (b) A. J. Heeger, Solid State Commun., 1998, 107, 673; (c) A. P. Kulkarni, P.-T. Wu, T. W. Kwon and S. A. Jenekhe, J. Phys. Chem. B, 2005, 109, 19584.
16. (a) C. Liu, Y. H. Li, Y. F. Li, C. L. Yang, H. B. Wu, J. G. Qin and Y. Cao, Chem. Mater., 2013, 25, 3320; (b) J. Wang, Y. Zhao, C. Dou, H. Sun, P. Xu, K. Ye, J. Zhang, S. Jiang, F. Li and Y. Wang, J. Phys. Chem. B, 2007, 111, 5082; (c) S. H. Lee, B.-B. Jang and Z. H. Kafafi, J. Am. Chem. Soc., 2005, 127, 9071.
17. (a) W. Qin, Z. Yang, Y. Jiang, J. W. Y. Lam, G. Liang, H. S. K. Kwok and B. Z. Tang, Chem. Mater., 2015, 27, 3892; (b) J. Chen, B. Xu, X. Ouyang, B. Z. Tang and Y. Cao, J. Phys. Chem. A, 2004, 108, 7522.
18. (a) B. E. Konne, D. E. Loy and M. E. Thompson, Chem. Mater., 1998, 10, 2235; (b) N. Prachumrak, S. Pansay, S. Namuangruk, T. Kaewin, S. Jungsuttiwong, T. Sudyoadsuk and V. Promarak, Eur. J. Org. Chem., 2013, 6619.
19. (a) G. B. Bodedla, K. R. J. Thomas, C.-T. Li and K.-C. Ho, RSC Adv., 2014, 4, 53588; (b) W.-I. Hung, Y.-Y. Liao, T.-H. Lee, Y.-C. Ting, J.-S. Ni, W.-S. Kao, J. T. Lin, T.-C. Wei and Y.-S. Yen, Chem. Commun., 2015, 51, 2152; (c) Y. Hua, S. Chang, J. He, C. Zhang, J. Zhao, T. Chen, W.-Y. Wong, W.-K. Wong and X. Zhu, Chem.–Eur. J., 2014, 20, 6300; (d) Z.-S. Huang, C. Cai, X.-F. Zang, Z. Iqbal, H. Zeng, D.-B. Kuang, L. Wang, H. Meier and D. Cao, J. Mater. Chem. A, 2015, 3, 1333; (e) Y. Hua, S. Chang, D. Huang, X. Zhou, X. Zhu, J. Zhao, T. Chen, W.-Y. Wong and W.-K. Wong, Chem. Mater., 2013, 25, 2146; (f) H. Tian, X. Yang, R. Chen, Y. Pan, L. Li, A. Hagfeldt and L. Sun, Chem. Commun., 2007, 3741; (g) R. K. Konidena, K. R. J. Thomas, S. Kumar, Y.-C. Wang, C.-J. Li and J.-H. Jou, J. Org. Chem., 2015, 80, 5812.
20. (a) J.-H. Huang and K.-C. Lee, ACS Appl. Mater. Interfaces, 2014, 6, 7680; (b) Q. Tan, X. Yang, M. Cheng, H. Wang, X. Wang and L. Sun, J. Phys. Chem. C, 2014, 118, 16851.
21. Y. Park, B. Kim, C. Lee, A. Hyun, S. Jang, J.-H. Lee, Y.-S. Gal, T. H. Kim, K.-S. Kim and J. Park, J. Phys. Chem. C, 2011, 115, 4843.
22. (a) W. Jang, F. Lyu, H. Park, Q. B. Meng, S.-H. Lee and Y.-S. Lee, Chem. Phys. Lett., 2013, 584, 119; (b) M.-J. Park, J. Lee, I. H. Jung, J.-H. Park, D.-H. Hwang and H.-K. Shim, Macromolecules, 2008, 41, 9643; (c) D.-H. Hwang, S.-K. Kim, M.-J. Park, J.-H. Lee, B.-W. Koo, I.-N. Kang, S.-H. Kim and T. Zyung, Chem. Mater., 2004, 16, 1298; (d) G. Kim, H. R. Yeom, S. Cho, J. H. Seo, J. Y. Kim and C. Yang, Macromolecules, 2012, 45, 1847.
23. G. W. Kim, M. J. Cho, Y.-J. Yu, Z. H. Kim, J.-I. Jin, D. Y. Kim and D. H. Choi, Chem. Mater., 2007, 19, 42.
24. (a) H. Tanaka, K. Shizu, H. Nakanotani and C. Adachi, J. Phys. Chem. C, 2014, 118, 15985; (b) Z. Xie, C. Chen, S. Xu, J. Li, Y. Zhang, S. Liu, J. Xu and Z. Chi, Angew. Chem., Int. Ed., 2015, 54, 7181.
25. (a) X. Zhang, Z. Chi, J. Zhang, H. Li, B. Xu, X. Li, S. Liu, Y. Zhang and J. Xu, J. Phys. Chem. B, 2011, 115, 7606; (b) J. Zhang, B. Xu, J. Chen, L. Wang and W. Tian, J. Phys. Chem. C, 2013, 117, 23117; (c) G. Zhang, J. Sun, P. Xue, Z. Zhang, P. Gong, J. Peng and R. Lu, J. Mater. Chem. C, 2015, 3, 2925.
26. (a) P. Xue, B. Yao, J. Sun, Q. Xu, P. Chen, Z. Zhang and R. Lu, J. Mater. Chem. C, 2015, 3, 1018; (b) S. A. Jenekhe, L. Lu and M. M. Alam, Macromolecules, 2001, 34, 7315; (c) J. Herbich, A. Kapturkiewicz, J. Nowacki, J. Golinski and Z. Dabrowski, Phys. Chem. Chem. Phys., 2001, 3, 2438.
27. (a) B. Garg and Y.-C. Ling, Chem. Commun., 2015, 51, 8809; (b) Y. Weng, Y. Shouhai, G. Qirun, Z. Tuan, W. Keyi and H. Yonghong, Sens. Actuators, B, 2015, 213, 404.
28. (a) Y. Shirota and H. Kageyama, Chem. Rev., 2007, 107, 953; (b) C. H. Chen, W. S. Huang, M. Y. Lai, W. C. Tsao, J. T. Lin, Y. H. Wu, T. H. Ke, L. Y. Chen and C. C. Wu, Adv. Funct. Mater., 2009, 19, 2661.
29. Z. Zhang, W. Jiang, X. Ban, M. Yang, S. Ye, B. Huang and Y. Sun, RSC Adv., 2015, 5, 29708; W. Jiang, J. Tang, X. Ban, Y. Sun, L. Duan and Y. Qiu, Org. Lett., 2014, 16, 5346; B. Pan, B. Wang, Y. Wang, P. Xu, L. Wang, J. Chen and D. Ma, J. Mater. Chem. C, 2014, 2, 2466; X. Ouyang, X.-L. Li, L. Ai, D. Mi, Z. Ge and S.-J. Su, ACS Appl. Mater. Interfaces, 2015, 7, 7869.
30. (a) W.-S. Huang, J. T. Lin, C.-H. Chien, Y.-T. Tao, S.-S. Sun and Y.-S. Wen, Chem. Mater., 2004, 16, 2480; (b) J. Ding, B. Wang, Z. Yue, B. Yao, Z. Xie, Y. Cheng, L. Wang, X. Jing and F. Wang, Angew. Chem., Int. Ed., 2009, 48, 6664.
31. (a) J. Min, Q. Zhang, W. Sun, Y. Cheng and L. Wang, Dalton Trans., 2011, 40, 686; (b) M. J. Leitl, V. Krylov, P. I. Djurovich, M. E. Thompson and H. Yersin, J. Am. Chem. Soc., 2014, 136, 16032.
32. C. He, Q. He, Y. Yi, G. Wu, F. Bai, Z. Shuaia and Y. Li, J. Mater. Chem., 2008, 18, 4085.
33. X.-H. Zhang, S. H. Kim, I. S. Lee, C. J. Gao, S. I. Yang and K.-H. Ahn, Bull. Korean Chem. Soc., 2007, 28, 1389.
34. K. R. J. Thomas, M. Velusay, J. T. Lin, C.-H. Chuen and Y.-T. Tao, Chem. Mater., 2005, 17, 1860.
35. S. J. Lee, J. S. Park, K. J. Yoon, Y. I. Kim, S. H. Jin, S. K. Kang, Y. S. Gal, S. W. Kang, J. Y. Lee, J. W. Kang, S. H. Lee, H. D. Park and J. J. Kim, Adv. Funct. Mater., 2008, 18, 3922.
36. X. Qiu, R. Lu, H. Zhou, X. Zhang, T. Xu, X. Liu and Y. Zhao, Tetrahedron Lett., 2007, 48, 7582.
37. H. Detert and V. Schmitt, J. Phys. Org. Chem., 2006, 19, 603.
38. X. Ban, W. Jiang, K. Sun, X. Xie, L. Peng, H. Dong, Y. Sun, B. Huang, L. Duan and Y. Qiu, ACS Appl. Mater. Interfaces, 2015, 7, 7303.
39. J. Pina, J. S. Seixas de Melo, R. M. F. Batista, S. P. G. Costa and M. M. M. Raposo, J. Org. Chem., 2013, 78, 11389.
40. W. Li, D. Liu, F. Shen, D. Ma, Z. Wang, T. Feng, Y. Xu, B. Yang and Y. Ma, Adv. Funct. Mater., 2012, 22, 2797.
41. H.-C. Ting, Y.-M. Chen, H.-W. You, W.-Y. Hung, S.-H. Lin, A. Chaskar, S.-H. Chou, Y. Chi, R.-H. Liu and K.-T. Wong, J. Mater. Chem., 2012, 22, 8399.
42. J. Salbeck, F. Weiss€ortel, N. Yu, J. Baner and H. Bestgen, Synth. Met., 1997, 91, 209.
43. (a) J. Huang, J.-H. Su, X. Li, M.-K. Lam, K.-M. Fung, H.-H. Fan, K.-W. Cheah, C. H. Chen and H. Tian, J. Mater. Chem., 2011, 21, 2957; (b) Z. Cai, M. Zhou, B. Li, Y. Chen, F. Jin and J. Huang, New J. Chem., 2014, 38, 3042.
44. (a) J. M. Hancock, A. P. Gifford, Y. Zhu, Y. Lou and S. A. Jenekhe, Chem. Mater., 2006, 18, 4924; (b) N. Kapoor and K. R. J. Thomas, New J. Chem., 2010, 34, 2739; (c) Z.-H. Guo, Z.-X. Jin, J.-Y. Wang and J. Pei, Chem. Commun., 2014, 50, 6088.
45. (a) A. J. Zucchero, J. N. Wilson and U. H. F. Bunz, J. Am. Chem. Soc., 2006, 128, 11872; (b) K. Tateno, R. Ogawa, R. Sakamoto, M. Tsuchiya, T. Otani and T. Saito, Org. Lett., 2014, 16, 3212; (c) M. A. Saeed, H. T. M. Le and O. Š. Miljanić, Acc. Chem. Res., 2014, 47, 2074.
46. (a) E. Z. Lippert and A. Naturforsch, J. Phys. Chem., 1955, 10, 541; (b) N. Mataga, Y. Kaifu and M. Koizumi, Bull. Chem. Soc. Jpn., 1956, 29, 465.
47. C. Reichardt, Chem. Rev., 1994, 94, 2319.
48. X. Y. Shen, Y. J. Wang, E. Zhao, W. Z. Yuan, Y. Liu, P. Lu, A. Qin, Y. Ma, J. Z. Sun and B. Z. Tang, J. Phys. Chem. C, 2013, 117, 7334.
49. T. Meyer, D. Ogermann, A. Pankrath, K. Kleinermanns and T. J. J. Müller, J. Org. Chem., 2012, 77, 3704.
50. X. Ban, W. Jiang, K. Sun, X. Xie, L. Peng, H. Dong, Y. Sun, B. Huang, L. Duan and Y. Qiu, ACS Appl. Mater. Interfaces, 2015, 7, 7303.
51. R. G. Parr and W. T. Yang, Annu. Rev. Phys. Chem., 1995, 46, 701.
52. (a) A. D. Becke, J. Chem. Phys., 1993, 98, 1372; (b) C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37, 785.
53. (a) R. A. Marcus, Rev. Mod. Phys., 1993, 65, 599–610; (b) V. Coropceanu, J. Cornil, A. Demetrio, Y. Olivier, R. Silbey and J. L. Bredas, Chem. Rev., 2007, 107, 926.
54. (a) B. C. Lin, C. P. Cheng and Z. P. M. Lao, J. Phys. Chem. A, 2003, 107, 5241; (b) S. E. Koh, C. Risko, D. A. S. Filho, O. Kwon, A. Facchetti, J.-L. Brédas, T. J. Marks and M. A. Ratner, Adv. Funct. Mater., 2008, 18, 332.
55. H. Li, L. Duan, D. Zhang and Y. Qiu, J. Phys. Chem. C, 2014, 118, 14848.
56. S.-H. Lee, M. S. Kim, Y.-B. Cha, K.-H. Ahn and Y. C. Kim, Mol. Cryst. Liq. Cryst., 2010, 520, 36[312]–43[319].
57. (a) J. Littman and P. Martic, J. Appl. Phys., 1992, 72, 3610; (b) S.-H. Cheng, W.-Y. Hung, M.-H. Cheng, H.-F. Chen, A. Chaskar, G.-H. Lee, S.-H. Chou and K.-T. Wong, J. Mater. Chem. C, 2014, 2, 8554.
58. (a) S. H. Kim and J. Y. Lee, Appl. Phys. Lett., 2007, 90, 223505; (b) S.-K. Kim, Y.-I. Park, I.-N. Kang and J.-W. Park, J. Mater. Chem., 2007, 17, 4670.
59. K. H. Lee, J. K. Park, J. H. Seo, S. W. Park, Y. S. Kim, Y. K. Kim and S. S. Yoon, J. Mater. Chem., 2011, 21, 13640.
60. X. Yang, S. Zheng, R. Bottger, H. S. Chae, T. Tanaka, S. Li, A. Mochizuki and G. E. Jabbour, J. Phys. Chem. C, 2011, 115, 14347.
61. M.-Y. Chang, Y.-K. Han, C.-C. Wang, S.-C. Lin, Y.-J. Tsai and W.-Y. Huang, J. Electrochem. Soc., 2008, 155, J365.

---

Footnote

† Electronic supplementary information (ESI) available: Absorption and emission spectra of the dyes recorded in different solvents, DPV recorded in DCM, ¹H, and ¹³C NMR spectra of the dyes. See DOI: 10.1039/c5ra18372c

---