Synthesis and studies of electrochemical properties of lophine derivatives

Abstract

A versatile series of lophine (2,4,5-triphenyl-1H-imidazole) derivatives (1–13) were synthesized and characterized using melting point investigation, Fourier transform infrared spectroscopy (FTIR), Liquid Chromatography-Mass Spectrometry (LC-MS), and ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. Their photo physical and electrochemical parameters were evaluated using Ultraviolet-visible (UV-Vis) spectroscopy, fluorescence spectrophotometry and cyclic voltammetric experiments. The optical band gap and quantum yield for the derivatives were in the range of 3.05–3.55 eV and 0.06 to 0.36, respectively. The HUMO–LUMO and associated energy gaps (E_g) calculated through the cyclic voltammetric experiments were found to be in the range of 0.068–0.194 eV. Among them, the derivatives with electron donating groups such as N(Me)₂, phenyl and methoxy more efficiently facilitated the redox process as compared to lophine. N(Me)₂ was found to be more effective because of the enhancement of electron density on nitrogen because of the electron donating ability of the two methyl groups.

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Introduction

The hetero aromatic push–pull system with various functional groups exhibits interesting chemical and physical properties. The introduction of various chromophores through extended π-conjugated molecules, with readily polarizable hetero atoms and groups, via facile synthetic routes plays an important role in the area of optoelectronic, optical data storage and light emitting materials.¹ These push–pull systems that arise from optimal donor-π-acceptor functional groups can effectively enhance the charge transfer ability, and thus find various new applications as functional materials.^2–5 The chemical nature, including the electronic effect of the substituent and molecular back bone, plays a crucial role on the linear and non-linear optical properties.⁶ Recently, hetero aromatic π-conjugated molecules have been investigated to explore the charge transport properties for their potential in the field of electronic devices.⁷ Moreover, these organic molecules are susceptible for structural modifications, which in turn can alter the optical band gap, redox potentials and charge transport properties.⁸ Among the heterocyclic systems, imidazole derivatives have attracted considerable attraction because of their unique optical, thermal and electronic properties.⁹ The coupling of strong electron donating groups and strong electron withdrawing moieties are commonly used to generate dipolar push–pull systems that feature low-energy and intense charge transfer absorption. Moreover, the substitution of various electron donating–accepting groups to the imidazole moiety increases the polarisability, stability, and thermal and chemical robustness.¹⁰ Imidazole derivatives with various donor and acceptor groups are attractive blue-emitting materials.¹¹ Lophine (2,4,5-triphenyl-1H-imidazole) is an attractive fluorescent and chemiluminescent molecule with the property to emit yellow light on reaction with oxygen in the presence of a strong base.¹² The derivatives of lophine are also known for efficient hole transportation,¹³ enhancement of chemiluminescence¹⁴ and fluorescent chemosensing.¹⁵ Moreover, lophine derivatives can be tailored with various donor and acceptor groups on the phenyl ring present in the C-2, C-4 and C-5 positions of the imidazole ring. Tailoring the electronic nature of these derivatives by the substitution of different electron donating and/or accepting functional groups, or the elongation of π-conjugation through these phenyl rings makes them promising candidates for light emitting applications.¹⁶ These modifications could also be used to modulate the band gap of the imidazole derivatives.¹⁷ The energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), often called the band gap of the material, must be fairly small to achieve multi-functional material applications.¹⁸

Absorption/emission spectral studies reveal a basic understanding about the photo physical properties, including Stokes shift, quantum yield and optical band gap.¹⁹ Furthermore, these imidazole derivatives exhibit halochromism and have been shown to exhibit discrete color changes on variation of pH;²⁰ the absorption maxima (λ_(max)) increases with the increase of pH.²¹ The protonation–deprotonation equilibria for donor–acceptor under varying pH conditions for imidazole derivatives have already been established.²²

Therefore, understanding the effect of substituents on imidazole moieties using its absorption/emission spectra and the effect of pH on the spectral characteristics would provide a more clear insight about its successive applications in various fields, including fluorescent labelling reagents²³ and fluorescent probes.²⁴ This fact has encouraged the development of imidazole based functional material with improved transport properties by the balanced injection of charges. Therefore, to develop a material with the aforementioned properties, the photo physical and electrochemical properties of lophine derivatives were studied by modifying lophine with electron donating and attracting groups at the para-position of the C-2 phenyl ring.

Results and discussion

Characterization

The structure of all the synthesized lophine and their derivatives (1–13) was ascertained using the melting point (mp), Fourier transform-infrared spectroscopy (FTIR), Liquid Chromatography-Mass Spectrometry (LC-MS), ¹H and ¹³C nuclear magnetic resonance (NMR) spectral techniques. The results were in good agreement with the previously reported literature. The FTIR spectra of all the derivatives showed four important characteristic stretching vibration bands. The band near 3440 cm⁻¹ can be ascribed to NH stretching; the band near 3100 cm⁻¹ to aromatic C–H stretching; the band near 1600 cm⁻¹ to aromatic C=C stretching; and the band near 1500 cm⁻¹ to C=N stretching vibrations.²⁵ To affirmatively determine the structure of all the synthesized products, ¹H and ¹³C NMR spectral analyses were performed. The NMR chemical shift of the N–H proton of the imidazole ring was observed at around δ 12.80 ppm.²⁶ In addition, the peaks in the range of δ 6.8–8.0 ppm were attributed to the C-2, C-4 and C-5 phenyl ring protons of the imidazole moiety.²⁷ Because of the complex nature of the NMR, the splitting pattern was not clear for these derivatives. In ¹³C NMR spectra, the peaks at around 120–140 ppm for the aromatic carbons of the derivatives and the peak above 140 ppm for the C-2 carbon of the imidazole ring confirmed all the synthesized derivatives.²⁸ The M + 1 peak of the LC-MS spectra also added evidence to the product formation.

2,4,5-Triphenyl-1H-imidazole (1). Mp: 267–268 °C; IR (KBr tablet) ν_max/cm⁻¹: 3444, 3082, 1641, 1462; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 7.24–7.62 (m, 15H), 12.57 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 125.20, 126.52, 127.09, 127.78, 128.19, 128.26, 128.47, 128.66, 128.69, 130.36, 131.09, 135.18, 137.13, 145.52; LCMS: calculated 296.1, found 297.2 (M + 1).

4-(4,5-Diphenyl-1H-imidazol-2-yl)phenol (2). Mp: 232–233 °C; IR (KBr tablet) ν_max/cm⁻¹: 3448, 3178, 1641, 1240; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 6.86–8.31 (m, 14H), 9.75 (s, OH), 12.42 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 115.45, 121.67, 126.37, 126.89, 127.08, 127.40, 127.53, 128.16, 128.33, 128.62, 131.34, 135.44, 136.63, 146.12, 157.82; LCMS: calculated 312.1, found 313.2 (M + 1).

2-(4-Methoxyphenyl)-4,5-diphenyl-1H-imidazole (3). Mp: 230–232 °C; IR (KBr tablet) ν_max/cm⁻¹: 3439, 1612, 1492, 1247; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 3.85 (s, 3H), 7.07–8.06 (m, 14H), 12.55 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 54.73, 113.62, 122.63, 125.95, 126.23, 126.58, 127.17, 127.70, 127.88, 128.17, 130.74, 134.82, 136.28, 145.16, 158.94; LCMS: calculated 326.1, found 327.2 (M + 1).

2-(2,5-Dichlorophenyl)-4,5-diphenyl-1H-imidazole (4). Mp: 223–225 °C; IR (KBr tablet) ν_max/cm⁻¹: 3415, 3174, 1448, 1398; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 7.36–8.32 (m, 13H), 12.77 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 127.37, 128.44, 129.72, 130.10, 130.57, 131.29, 131.73, 132.05, 142.02; LCMS: calculated 364.0, found 365.1 (M + 1).

4,5-Diphenyl-2-p-tolyl-1H-imidazole (5). Mp: 233–235 °C; IR (KBr tablet) ν_max/cm⁻¹: 3446, 3176, 1629, 1492, 1400; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 2.38 (s, 3H), 7.26–7.6 (m, 12H), 8.04 (m, 2H), 12.67 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 20.90, 125.21, 126.46, 127.11, 127.67, 127.74, 127.97, 128.18, 128.42, 128.62, 129.26, 131.20, 135.30, 136.98, 137.68, 145.74; LCMS: calculated 310.0, found 311.1 (M + 1).

2-(3,4-Dimethoxyphenyl)-4,5-diphenyl-1H-imidazole (6). Mp: 215–216 °C; IR (KBr tablet) ν_max/cm⁻¹: 3417, 3132, 1496, 1253; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 3.85–3.89 (s, 6H), 7.09–7.71 (m, 13H), 12.60 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 55.54, 55.59, 108.82, 111.82, 117.94, 123.20, 127.09, 127.76, 145.70, 148.80, 149.07; LCMS: calculated 356.1, found 357.2 (M + 1).

2-(3-Nitrophenyl)-4,5-diphenyl-1H-imidazole (7). Mp: 264–265 °C; IR (KBr tablet) ν_max/cm⁻¹: 3435, 1541, 1521, 1348; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 7.37–8.52 (m, 13H), 8.96 (s, 1H), 13.11 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 119.4, 122.61, 127.21, 128.49, 130.43, 131.18, 131.83, 143.39, 148.36; LCMS: calculated 341.1, found 342.2 (M + 1).

2-(4-Fluorophenyl)-4,5-diphenyl-1H-imidazole (8). Mp: 190–192 °C; IR (KBr tablet) ν_max/cm⁻¹: 3441, 3132, 1492, 1400; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 7.26–8.15 (m, 14H) 12.73 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 115.54, 115.76, 126.54, 126.98, 127.01, 127.04, 127.27, 127.35, 127.79, 128.18, 128.23, 128.39, 128.66, 131.01, 135.08, 137.05, 144.68, 160.89, 163.33; LCMS: calculated 314.1, found 315.2 (M + 1).

2-(4-Chlorophenyl)-4,5-diphenyl-1H-imidazole (9). Mp: 263–265 °C; IR (KBr tablet) ν_max/cm⁻¹: 3417, 3130, 1485, 1402; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 7.25–8.12 (m, 14H) 12.80 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 126.81, 127.05, 127.86, 128.23, 128.39, 128.64, 128.76, 129.16, 130.87, 132.72, 134.96, 137.24, 144.40; LCMS: calculated 330.0, found 331.1 (M + 1).

2-(4-Bromophenyl)-4,5-diphenyl-1H-imidazole (10). Mp: 247–248 °C: IR (KBr tablet) ν_max/cm⁻¹: 3417, 3136, 1483, 1402; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 7.35–8.05 (m, 14H), 12.80 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 121.38, 127.09, 128.42, 129.52, 131.66, 144.46; LCMS: calculated 374.0, found 375.0 (M + 1).

4-(4,5-Diphenyl-1H-imidazol-2-yl)-N,N-dimethylaniline (11). Mp: 256–258 °C; IR (KBr tablet) ν_max/cm⁻¹: 3446, 3132, 1508, 1406, 1361; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 3.00 (s, 6H), 6.82–7.95 (m, 14H), 12.03–12.35 (b, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 111.92, 118.24, 126.32, 126.87, 127.70, 128.37, 146.47, 150.28; LCMS: calculated 339.17, found 340.3 (M + 1).

2-(Biphenyl-4-yl)-4,5-diphenyl-1H-imidazole (12). Mp: 229–230 °C; IR (KBr tablet) ν_max/cm⁻¹: 3468, 3132, 1485, 1400; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 7.23–8.32 (m, 19H), 12.76 (b, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 125.72, 126.54, 126.88, 127.09, 127.60, 127.78, 128.19, 128.39, 128.44, 128.65, 128.97, 129.37, 131.04, 135.16, 137.29, 139.48, 139.66, 145.21; LCMS: calculated 372.2, found 373.3 (M + 1).

1,4-Bis(4,5-diphenyl-1H-imidazol-2-yl)benzene (13). Mp: 232–235 °C; IR (KBr tablet) ν_max/cm⁻¹: 3500, 1637, 1402; ¹H NMR (400 MHz, DMSO-d₆): δ_ppm 7.23–8.18 (m, 24H) 12.75 (s, NH); ¹³C NMR (100 MHz, DMSO-d₆): δ_ppm 125.37, 126.58, 127.09, 127.82, 128.19, 128.40, 128.66, 130.98, 135.10, 145.14; LCMS: calculated 514.1, found 515.2 (M + 1).

Absorption and emission spectra

Fig. 1 shows the absorption spectra of lophine and its derivatives in N,N-dimethylformamide (DMF) solution. The absorption maxima (λ_(max)), molar absorptivity coefficient (ε) and optical band gap (E_(opt)) for all the derivatives (1–13) are presented in Table 1. Highly aromatic substituted molecules, such as lophine, act to be an efficient π-conjugated system. This type of extended π-conjugated molecules exhibit charge transfer processes that play a vital role to achieve new electronic applications. The charge transfer ability of these molecules could be modified by substituting suitable donor and/or acceptor substituents.¹⁶ The blue or red shift will be based on the nature of the substituents in the π-conjugated system.²⁹ Furthermore, the π-electron delocalization because of either effective donor–acceptor group or efficient π-conjugation resulted in a red shifted peak as compared with the absorption pattern of parent molecule.¹⁰

Fig. 1 Absorption spectra of lophine derivatives.

Table 1 Physical properties of lophine derivatives 1–13

Compound	λ(max) (nm)	ε (M^−1 cm^−1)	Eopt^a (eV)	λ(em)max^b (nm)	Stokes shift (cm^−1)	Φf^c
a Eopt = optical band gap derived from the onset wavelength of the UV-vis absorption spectra.b molecules are excited at their respective absorption maximum.c fluorescence quantum yield were measured using lophine as the standard.
Lophine	311	32 890	3.53	390	6513	0.27
4-OH	303	32 500	3.49	406	8373	0.28
4-OMe	304	38 080	3.51	401	7957	0.23
2,5-Dichloro	307	33 160	3.45	338	2988	0.06
4-F	308	31 590	3.55	393	7022	0.17
4-Me	311	25 970	3.49	395	6838	0.40
3-NO2	313	34 500	3.46	340	2537	0.04
3,4-(OMe)2	314	32 500	3.47	403	7034	0.27
4-Cl	317	34 000	3.47	389	5839	0.28
4-Br	319	39 570	3.45	343	2193	0.05
4-N(Me)2	323	37 190	3.34	452	8836	0.15
4-Ph	338	37 200	3.29	416	5547	0.36
Bis	361	49 420	3.08	425	4171	0.29

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The absorption maximum of all these derivatives lies in the range of 300–361 nm.³⁰ The planarity and dependence of the polarizing ability of the substituents were clearly shown by the absorption patterns of these synthesized lophine derivatives.¹⁰ Moreover, the derivatives with substituents such as CH₃, Cl, Br, N(Me)₂, Ph, NO₂, and 3,4-dimethoxy groups showed a red shift, whereas those with F, OH, OMe, and 2,5-dichloro showed a slight blue shift in the absorption spectrum. This behaviour could be because of the charge transfer property from the benzene ring to the imidazole controlled by the substituent.⁹ Derivatives substituted with Cl and Br showed a red shift, but fluorine substituted derivative showed a blue shift. This might be due to the mesomeric effect (electron donating) of the fluorine atom substituted at the para-position.³¹ In addition, the ortho substituted chlorine atom disturbed the planarity between the benzene ring and imidazole in the 2,5-dichloro substituted derivative.¹⁰ The derivatives containing substituents such as N(Me)₂, phenyl and NO₂ showed a large red shift with enhanced intensity, and the extended π-conjugation in bis-imidazole derivative (13) showed a red shift with a higher molar extinction coefficient.³²

Effect of pH

The effect of pH on the absorption spectra of all the derivatives in DMF was studied to understand the influence of pH on the charge transport behaviour of these derivatives. All the derivatives showed halochromic behaviour at different pH values in DMF. Moreover, all the derivatives showed a significant shift when the pH was lowered from 7 to 1, but no significant shift was observed when the pH was increased from 7 to 14. The results are summarized in Table 2. The λ_(max) value of lophine shifted from 311 nm to 295 nm with reduced absorption intensity when the pH was decreased from 7 to 1. The isobestic point was observed at 285 nm. These observations clearly indicated the formation of protonated lophine with the decrease in pH. Protonation occurred at the N-3 position of the imidazole ring when the pH was lowered, as shown in Fig. 2.²²

Table 2 Halochromic behaviour of lophine derivatives 1–13

Compound	λ(max) (absorbance (arb. units))
	pH 7	pH 6	pH 4	pH 2	pH 1
Lophine	311 (0.288)	310 (0.252)	310 (0.251)	310 (0.251)	310 (0.251)
4-OH	303 (0.353)	305 (0.345)	305 (0.339)	305 (0.335)	305 (0.326)
4-OMe	304 (0.384)	305 (0.383)	305 (0.382)	305 (0.382)	305 (0.382)
2,5-Dichloro	307 (0.310)	297 (0.286)	297 (0.286)	297 (0.286)	297 (0.286)
4-F	308 (0.315)	305 (0.309)	305 (0.309)	305 (0.309)	305 (0.309)
4-Me	311 (0.259)	304 (0.257)	304 (0.256)	304 (0.254)	304 (0.254)
3-NO2	313 (0.396)	311 (0.382)	311 (0.368)	311 (0.368)	311 (0.368)
3,4-(OMe)2	314 (0.366)	310 (0.346)	310 (0.342)	310 (0.342)	310 (0.342)
4-Cl	317 (0.285)	305 (0.257)	305 (0.256)	305 (0.254)	305 (0.254)
4-Br	319 (0.394)	317 (0.336)	317 (0.333)	317 (0.333)	317 (0.333)
4-N(Me)2	323 (0.473)	345 (0.451)	345 (0.462)	345 (0.467)	345 (0.475)
4-Ph	338 (0.355)	320 (0.343)	320 (0.342)	320 (0.341)	320 (0.331)
Bis	361 (0.526)	364 (0.418)	364 (0.411)	364 (0.410)	364 (0.398)

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Fig. 2 Protonation of lophine with a decrease in pH.

The protonation at the imidazole ring may be attributed to the shift in the λ_(max) value of the absorption spectrum. The electron donation ability of N-3 of the imidazole ring can be greatly influenced by the substituents present at the C-2 phenyl ring. As a consequence, the various electron donating and accepting groups could cause a significant shift in their absorption spectra as compared to lophine. The derivatives with substituents such as 2,5-dichloro, CH₃, 3,4-dimethoxy, NO₂, Cl, F and Ph showed a blue shift with the decrease in pH from 7 to 1. The derivatives with substituents such as OH, OMe and highly conjugated bis-imidazole showed a very slight red shift with decreased absorption intensity with the reduction of pH from 7 to 1. This might be due to the less pronounced conjugation between these substituent bearing phenyl ring and protonated imidazole ring. The N(Me)₂ group imparted good electron donating ability to the N-3 of imidazole and showed a larger red shift. Moreover, the λ_(max) value of N(Me)₂ substituted lophine (11) showed a red shift from 323 nm to 345 nm with a decrease in pH from 7 to 1. The isobestic point was observed at 333 nm. Moreover, the absorption intensity of the derivative increased with a decrease in pH from 7 to 1, as shown in Fig. 3. The bromine substituted lophine also showed a significant red shift compared with chlorine and fluorine because of its lower electronegativity. These observations clearly indicate the influence of substituents present at the C-2 phenyl ring of the imidazole moiety and the pH on charge transfer.

Fig. 3 Absorption spectra of (11) at different pH values.

The optical band gap corresponds to the formation of Frenkel exciton with the electron and hole in the same molecule or a charge transfer exciton with the electron and hole with adjacent molecules.³³ Extended π-conjugation and the nature of substituents could affect the optical band gap.³⁴ The difference in the band gap value compared with lophine clearly showed the donor–acceptor effect of the substituent, which destabilized the HOMO and LUMO levels. The optical band gap determined by the absorption edge of the solution spectra was found to be in the range of 3.08–3.54 eV, as given in Table 1. Among the investigated substituents, the strongly electron donating N(Me₂) group showed considerably lower optical band gap in 11. The blue shifted optical band gap values for 12 and 13 could be due to the extended π-conjugation.¹⁰ Moreover, the optical band gap value clearly indicated that lophine materials can be used as potential candidates in optoelectronic applications.³⁵

The emission band maxima, λ(em)_max and the Stokes shift along with fluorescence quantum yield of lophine and its derivatives are summarized in Table 1. The normalized emission spectra of derivatives 1–13 were measured in dimethyl sulfoxide (DMSO) solution (Fig. 4), and the characteristic band in the blue region (386–452 nm) was observed.³⁰ Further, the value of the emission wavelength (Table 1) clearly showed that the substituents significantly influenced the fluorescence property of lophine derivatives. The reason for this behavior might be the charge transfer from the benzene ring to imidazole ring; these materials emitted blue light. The lophine derivatives exhibited red shift in their emission spectra relative to the parent lophine, except 4, 7 and 10. Among all the derivatives, 11–13 showed a higher red shift value. The exhibited fluorescence in these derivatives might be because of the resonance effect between the phenyl ring and imidazole ring. This red shift could be due to the substituent on the phenyl ring, which carried the unshared pair of electron and delocalized π-electrons in the ground state.

Fig. 4 Emission spectra of lophine derivatives.

The effect of extended conjugation was also observed for the fluorescence quantum yield (Φ_f). Derivative 11 showed higher Stokes shift compared to others. Derivative 11, with N(Me)₂ group, exhibited higher red shift compared to the highly π-conjugated 12 and 13 as well. This might be due to their electron donating ability.²² Derivatives 5 and 12 with electron donating substituents CH₃ and Ph also exhibited higher Φ_f value as compared to highly π-conjugated 13. Fluorescence quantum yield (Φ_f) for all the synthesized derivatives was measured and compared with lophine as a reference (Φ_f of 0.27 in methanol).³⁰ The quantum yield Φ_f was calculated by the following formula:

Φ_f = Φ_std(I/I_std)(A/A_std)(η/η_std)²

where Φ_f and Φ_std are the fluorescence quantum yield of the sample and standard and I and I_std are the integrated emission intensities of the sample and standard, respectively. A and A_std are the absorbance of the sample and standard and η and η_std are the refractive indices of the solution.³⁶ The derivatives with substituents such as N(Me)₂ and OH showed a larger Stokes shift. The larger Stokes shift indicated the structural reorganization of the nucleus after the conformational changes that resulted due to this emission.³⁷ The low Stokes shift in derivatives 4, 7, 12 and 13 might be because of the distortion in the planarity. In the halogen substituted derivatives, the Stokes shift exhibited the trend F > Cl > Br. This phenomenon may be due to the increase in the atomic weight of the substituents.³⁸ From the results, it was clearly evident that the lophine molecules could be effectively used as versatile fluorescent materials with extended π-conjugation without affecting their planarity.

Cyclic voltammetry

The electronic structures (HOMO and LUMO) of the lophine derivatives were characterized by cyclic voltammetry (CV) at a scan rate of 20 mV s⁻¹. Moreover, from the oxidation (E_ox) and reduction (E_re) potential, the HOMO and LUMO energy levels were calculated using the relationship E_HOMO = −(E_ox + 4.4) and E_LUMO = −(E_red + 4.4), respectively.³⁹ All the synthesized lophine derivatives exhibited stable and consistent voltammograms, indicating the stability of the compounds, as depicted in Fig. 5.

Fig. 5 Cyclic voltagramms of 2-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazole (3).

All the derivatives of lophine showed one electron reversible redox process. It can be seen from Table 3 that all the lophine derivatives (1–13) were oxidized in the potential range of 0.505–0.702 V and were reduced in the potential range of 0.572–0.821 V. The impact of various substituents on the electrochemical properties of the lophine derivative was evaluated (Table 3) by variations in E_ox and E_re values.

Table 3 Electrochemical properties of lophine derivatives

Compound	Eox^a (V)	Ere^a (V)	HOMO^b (eV)	LUMO^b (eV)	Eg^c (eV)
a Redox potentials with reference to ferrocene, which is used as an internal standard.b HOMO = Eox + 4.4 and LUMO = Ere + 4.4.c Eg = HOMO–LUMO.
Lophine	0.544	0.738	5.344	5.538	0.194
4-OH	0.516	0.643	5.316	5.443	0.127
4-OMe	0.553	0.674	5.353	5.474	0.121
2,5-Dichloro	0.680	0.762	5.480	5.562	0.082
4-F	0.587	0.727	5.387	5.527	0.140
4-Me	0.576	0.700	5.376	5.500	0.124
3-NO2	0.654	0.788	5.454	5.588	0.134
3,4-(OMe)2	0.538	0.686	5.338	5.486	0.148
4-Cl	0.604	0.712	5.404	5.512	0.108
4-Br	0.600	0.717	5.400	5.517	0.117
4-N(Me)2	0.505	0.572	5.305	5.372	0.068
4-Ph	0.537	0.624	5.337	5.424	0.087
Bis	0.702	0.821	5.502	5.621	0.119

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Lophine derivatives possessing OH (2), OMe (3), N(Me₂) (11) and Ph (12) exhibit a low oxidation potential value as compared to the parent lophine. This trend evidently showed that the oxidation process was efficiently facilitated by the electron-donating group N(Me)₂ (11) as compared to other groups such as CH₃ (5) and Ph (12). Moreover, these 2, 3 and 11 derivatives showed a red shifted peak in their absorption spectra when the pH was lowered from 7 to 1. The fluoro substituted derivatives showed a different trend compared to chloro and bromo substituted derivatives. This could be due to the anomalous behaviour of fluorine. Fluorine atom as a substituent exhibited both an inductive effect (electron-withdrawing) and mesomeric effect (electron-donating). Inductive effect will have the same influence at any position, whereas the mesomeric effect will be dominant over the inductive effect when the fluorine atom is present at the para-position.³²

In the redox processes, N(Me)₂ (11) and Ph (12) substituted derivatives efficiently facilitated the reduction. The extensively π-conjugated bis-imidazole (13) had a minimal impact on the redox process. This behavior was observed on the HOMO–LUMO level as well, where the lophine and its various derivatives considerably differed with regard to their E_gap value.

The E_gap value was found to mainly depend on the charge transport ability or the promotion of the electron or hole in organic molecules. Here, the lophine derivatives and charge transport ability lies on the imidazole ring; therefore, the substitution in the aromatic ring could make a predominant impact on it. From the E_gap value, it was evident that the substituent present on the phenyl ring could play a vital role on the charge transport ability of the lophine. Among the derivatives, the lophine substituted with N(Me)₂ (11) and Ph (12) showed a significant impact on the charge transport phenomena.

Experimental

Materials

Benzil, substituted aldehydes, ammonium acetate and solvents were purchased from Aldrich and S.D. fine chemicals and were used as received. Thin-layer chromatography (TLC) was conducted on aluminium sheets coated with silica gel 60 F obtained from Merck, with visualization by a UV lamp (254 or 360 nm). The melting points were measured on a Büchi B-540 melting-point apparatus in open capillaries and are uncorrected. The ¹H and ¹³C NMR spectra were recorded with a Bruker AVANCE 400 MHz instrument at 25 °C. Chemical shifts are reported in ppm relative to the signal of trimethylsilane. Apparent resonance multiplicities are described as s (singlet), br (broad), m (multiplet) and NH (secondary amine). LC-MS were recorded in Agilent 6420 Triple quadrupole LC-MS instrument. The FTIR spectra were recorded using a Perkin-Elmer Spectrum-1 instrument with freshly dried KBr pellets in the range between 4000 and 400 cm⁻¹. Ultraviolet-visible spectroscopy (UV-Vis) was analyzed using a JASCO V-670 UV-VIS Spectrophotometer. The emission spectra were measured using a Perkin-Elmer LS 50-B spectro-fluorimeter. For the absorption and emission measurements, the sample concentration was maintained at 10⁻³ M. Spectroscopic grade solvents were used for the spectral measurements, without further purification. CV was conducted in a DMSO medium in the presence of 0.1 M Bu₄NPF₆ (tetrabutylammonium hexafluorophosphate) with the use of a three-electrode cell, in which a glassy carbon electrode as a working electrode, platinum wire as an auxiliary electrode and Ag/AgCl as the reference electrode were used.

General procedure for the synthesis of lophine derivatives

In a round-bottomed flask, Benzil (1 mmol), suitable benzaldehyde (1 mmol), and ammonium acetate (7 mmol) were added to boiling glacial acetic acid (16 ml) and refluxed for 5–6 h. The completion of the reaction was monitored by TLC. After the completion of the reaction, the crude mixture was poured into ice-cooled water and neutralized with sodium bicarbonate to obtain a precipitate. The precipitate was washed with water and recrystallized using suitable solvents such as methanol and ethanol. The expected products were obtained in 80–90% yield (Scheme 1).⁴⁰

Scheme 1 Synthesis of lophine derivatives (1–13).

Conclusion

Lophine derivatives with different electron donating–accepting groups in the para-position of the C-2 phenyl ring were synthesized, and their photo physical properties were studied. These electron donor–acceptor substituted lophine derivatives and their characteristic absorption and emission spectra showed that their electronic nature was significantly influenced by their π-extension and substituent. Moreover, from the electrochemical properties, it was clear that the HOMO–LUMO energy levels could be engineered by the extension of π-conjugation. Moreover, N(CH₃)₂ and Ph substituted lophine derivatives showed a higher absorption value and calculated quantum yield. These results confirm that the synthesized imidazole derivatives could also be used as luminescent materials.

Acknowledgements

The authors sincerely thank the management of VIT-University for the financial support and infrastructure provided for the research work. In addition, the instrumentation facility of SAS, VIT University for UV-VIS-NIR, FTIR NMR and fluorescence studies, along with the cyclic voltammetric facility provided by Dr A. Anand Prabu, are greatly acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08601e

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