Exclusive excited state intramolecular proton transfer from a 2-(2′-hydroxyphenyl)benzimidazole derivative

Abstract

The excited state intramolecular proton transfer (ESIPT) of a newly designed 2-(2′-hydroxyphenyl)benzimidazole derivative (bis-HPBI), has been investigated in different nonpolar, polar aprotic, and polar protic solvents. Unlike 2-(2′-hydroxyphenyl)benzimidazole, it exhibits exclusive ESIPT even in protic solvents. The existence of trans enol was made unviable by crafting a steric hindrance that stops the normal emission of bis-HPBI. Though bis-HPBI has two HPBI units, the tautomer emission of bis-HPBI is due to only single proton transfer. The experimental studies and theoretical calculations corroborate the finding. Protonation and deprotonation studies on bis-HPBI are also performed. The enhancement in the tautomer band intensity upon deprotonation of one of the OH groups also supports the single proton transfer in bis-HPBI. On the other hand, the initial addition of acid quenches the tautomer emission by hydrogen bonding interactions. After protonation of imidazole nitrogen, bis-HPBI acts as a photoacid. The dissociation of ‘OH’ protons and reorganization of the molecule leads to partial recovery of tautomer emission. In a strongly acidic medium where deprotonation of the ‘OH’ group is not possible, the emission is observed from the cation of bis-HPBI.

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1. Introduction

Organic molecules possessing luminescence properties are useful for various applications in different fields. The utility of fluorophores is enhanced tremendously when they undergo excited state intramolecular proton transfer (ESIPT).^1–3 ESIPT is basically a phototautomerisation process in which a proton transfers from a protic acidic group to a nearby basic group in the electronic excited state of the molecule.¹ This phototautomer generally produces a large Stokes shifted emission, which has the desired features to avoid self-absorption and the inner filter effect. ESIPT exhibiting molecules have potential application in the field of lasers, UV photostabilizers, sensors, molecular logic gates, and light-emitting materials due to their excited state proton transfer ability.^4–10 However, in ESIPT exhibiting fluorophores the normal emission also competes with the ESIPT process which sometimes limits the application of these molecules.

Among variety of ESIPT dyes, ESIPT of 2-(2′-hydroxyphenyl)benzazoles such as 2-(2′-hydroxyphenyl)benzimidazole (HPBI), 2-(2′-hydroxyphenyl)benzoxazole (HPBO), 2-(2′-hydroxyphenyl)benzothiazole (HPBT) and their derivatives have been extensively studied.^11–32 In recent time, these dyes are used for the study as a tailoring molecule with different organic backbone. The idea behind these studies are to tune the absorption or emission to a longer wavelength or to obtain a largely Stoke shifted emission.^23–27 Zhao et al. reported the absorption spectrum of the HPBT was red shifted upon conjugated with naphthalimide chromophore.²³ Douhal et al. found a red shift in the absorption and fluorescence spectra in 5′-substituted HPBI derivatives.²⁴ Park et al. observed that the tautomer fluorescence of HPBO is red shifted upon substitution with electron donor on the phenyl ring and blue shifted upon substitution of electron acceptor on benzoxazole moiety.²⁵ Theoretical study by Stefani et al. suggest that the red shift in the absorption spectra is due to effect of substituents in the HPBO frame.²⁶ It was found that when methoxy is substituted at 3′ position of HPBO, the fluorescence yield decreases and the spectral characteristics are nearly similar when the methoxy group is substituted at 4′-position.²⁷ We found that the presence of ground state keto and reduction in fluorescence when pyridyl nitrogen is substituted in HPBO.²⁸ Recently Douhal et al. and Liu et al. proposed that the modulation of ESIPT of HPBO moiety due to the effect of position and presence of amino group in HPBO moiety.^29,30 Araki et al. witness that the tautomer emission of 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridine is red shifted with introduction of electron withdrawing group and blue shifted upon addition of electron donating group to phenyl ring.³¹ The acidity and the basicity of the groups are affected by the substitutions thereby the rate of proton transfer is also altered. Zhao et al. also reported that the absorption and emission of 2-(2′-hydroxyphenyl)-benzothiazole-rhodamine dyad is switched by the addition of acids.³² Still it looks more room is left to investigate the effect of substituents on ESIPT of these dyes. One of the characteristics of these 2-(2′-hydroxyphenyl)benzazole derivatives is that in the ground state they exist as both cis-enol and trans-enol which are in equilibrium. Only, the cis-enol is capable to undergo ESIPT but not the trans-enol. In addition since trans-enol exhibits the less Stokes shifted normal emission it overlaps with the absorption spectrum.

HPBI also exist as cis-enol and trans-enol.^16–18 The relative population of trans-enol rises with increase in polarity and hydrogen bonding ability of the solvent.¹⁷ If the trans-enol is excluded from the equilibrium one can achieve exclusive ESIPT. Since, it will make the fluorophore to exhibit only tautomer emission it will help to avoid the fluorescence quenching due to self-absorption (inner filter effect). Having a hope to peruse this, a new HPBI derivative, 4-(3-(1H-benzo[d]imidazol-2-yl)-5-tert-butyl-4-hydroxybenzyl)-2-(1H-benzo[d]imidazol-2-yl)-6-tert-butylphenol (bis-HPBI, Chart 1), is designed and investigated. The special features of bis-HPBI are that it contains a bulky tert-butyl group ortho to ‘OH’ groups and also have two HPBI moieties which are not in conjugation.

Chart 1 Structures of HPBI and bis-HPBI.

Two ESIPT moieties containing benzoxazole derivatives bis-2,5-(2-benzoxazolyl)hydroquinone and bis-3,6-(2-benzoxazolyl)-pyrocatochol were investigated.³⁴ In the excited state, bis-2,5-(2-benzoxazolyl)hydroquinone was hypothesized to undergo single proton transfer and bis-3,6-(2-benzoxazolyl)-pyrocatochol was reported to undertake double proton transfer.³⁴ Recently based on theoretical calculations it was predicted that both single and double proton transfer are possible in bis-2,5-(2-benzoxaroiyl)hydroquinone.³⁵ Unlike bis-HPBI, in these benzoxazole derivatives the ESIPT units are in conjugation. Therefore, it is also interesting to find that whether bis-HPBT bears a single or double proton transfer in the excited state.

2. Materials and methods

A mixture of 2-phenylendiamine, 5,5′-methylene-di-3-tert-butyl-salicylaldehyde and NaHSO₃ was stirred in dimethylformamide at 80 °C for 7 h. The reaction was quenched by the addition of water. Precipitate was obtained immediately which was filtered and dried in air. The compound was purified by column chromatography.

¹H NMR (400 MHz, DMSO-d₆): 13.91 (s, 2H), 13.22 (s, 2H), 7.84 (d, J = 28 Hz, 2H), 7.71 (d, J = 8 Hz, 2H), 7.59 (d, J = 4 Hz, 2H), 7.27–7.22 (m, 6H), 3.95 (s, 2H), 1.38 (s, 18H).

HRMS (ESI) m/z: [M + H]⁺ Calcd for C₃₅H₃₆N₄O₂ 545.2872; found 545.2908.

Solvents used for the spectroscopic studies, were of HPLC grade from Rankem or Spectrochem, India and were used as received. Analytic grade H₂SO₄, NaOH and H₃PO₄ from Merck, India were used as received. All the spectral measurements in organic solvents were performed with 1 μM of dye in order to avoid aggregation and self-quenching. 3 μM of the dye in aqueous solution having 1% methanol is used for the prototropic study. Millipore water was used for the preparation of aqueous solution. Aqueous solutions of the sample in the pH range 3–11 were prepared by adding appropriate amounts of dilute solutions of NaOH and H₃PO₄. Hammett's acidity scales (H₀) and Yagil's basicity scale (H_) were used for preparing solutions of pH <1 and >13, respectively.^36,37

The pH of the solutions were measured using Jenway (model No 3510) pH meter. The absorption spectra were recorded on a Perkin Elmer Lambda 25 UV-Visible spectrophotometer. The steady state fluorescence spectra were collected on a HORIBA Jobin Yvon Fluoromax-4 spectrofluorimeter. Fluorescence lifetimes were measured using an Edinburgh instrument Life-Spec II instrument using 308 nm LED and 375 nm laser as light source. The data were analysed by the reconvolution method by using the software provided by Edinburgh instrument. The quality of the fit was assessed by the χ² values and the distribution of the residuals. Quinine sulphate in 1 N H₂SO₄ (Φ = 0.51) used as a standard for the quantum yield determination.³⁸

Theoretical calculations were performed by using Gaussian 09 program.³⁹ Acetonitrile is considered as the solvent for the calculation by using integral equation formalism-polarizable continuum (IEF-PCM) model.^40,41 The ground state geometries are optimized at density functional theory (DFT) level and the excited state geometries are optimized by the time dependent DFT (TDDFT) method without any symmetry constraints. Becke's three-parameter hybrid functional B3LYP and the 6-31G(d,p) basis set were employed for the calculations.^42,43

3. Results and discussion

3.1. Effect of solvents on the ESIPT of bis-HPBI

3.1.1. The absorption and steady state fluorescence. The absorption spectra of bis-HPBI in few selected solvents are provided in Fig. 1 and the complete data are compiled in Table 1. Like HPBI,³³ the absorption spectra of bis-HPBI are structured in all solvents and blue shifted with increase in polarity and hydrogen bond capacity of the solvent. However, bis-HPBI absorbs at a longer wavelength than HPBI and the molar extinction co-efficient obtained for bis-HPBI is also higher than that of HPBI in any given solvent. This suggests that the alkyl substitution at 3′ and 5′ positions have affected the electronic transition.⁴⁴ A tailing towards longer wavelength is found in the absorption spectra of bis-HPBI in protic solvents and it may be due to small amount of anions (phenoxide ion) present in the ground state.^44,45

Fig. 1 Absorption spectra of bis-HPBI in selected solvents.

Table 1 Absorption band maxima (λ^ab_max, nm), log ε_max (in the parenthesis), fluorescence band maxima (λ^fl_max, nm) and Stoke shift (cm⁻¹) of bis-HPBI in different solvents

Solvents	λ^abmax (log εmax)	λ^flmax		Stokes shift
		Normal emission	Tautomer emission
Cyclohexane	332 (4.28), 343 (4.20)		482	8408
1,4-Dioxane	329 (4.91), 341 (5.03)		480	8361
Diethyl ether	328 (4.89), 341 (5.06)		479	8449
Ethylacetate	327 (4.87), 340 (5.04)		477	8449
Tetrahydrofuran	329 (4.95), 341 (5.06)		477	8361
Acetonitrile	326 (4.89), 339 (4.90)	385	478	8490
Dimethylformamide	327 (4.95), 340 (5.04)		481	8623
Dimethylsulphoxide	328 (4.95), 341 (5.06)		478	8405
Butanol	328 (4.98), 341 (5.04)		471	8139
2-Propanol	328 (4.94), 340 (5.07)		471	8135
1-Propanol	328 (4.95), 340 (5.07)		470	8180
Ethanol	328 (4.95) 340 (5.07)		472	8180
Methanol	327 (4.93), 338 (5.08)		470	8309
Glycol	327 (4.95), 340 (5.08)	393	477	8447

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In most of the solvents, bis-HPBI exhibits almost exclusively single emission (Table 1 and Fig. 2). In acetonitrile and glycol a very weak shorter wavelength emission is observed along with a strong longer wavelength emission. Except in few nonpolar solvents, HPBI emits two emissions in most of the solvents.³³ The shorter wavelength emission is the normal emission from the excited trans-enol and the longer wavelength emission is from the tautomer which is formed by ESIPT.^16–18 The tautomer emission has high Stokes shift. The Stokes shift calculated for bis-HPBI is higher than that of normal emission of HPBI and comparable to that of tautomer emission. Therefore, it can be inferred that the single emission at longer wavelength of bis-HPBI is the tautomer emission. Following points substantiate this assignment, (i) the other very weak emission observed at shorter wavelength in acetonitrile and glycol indicates that it is the normal emission and the strong longer wavelength emission is the tautomer emission, (ii) the longer wavelength emission band is blue shifted with increase in polarity and hydrogen bonding capacity of the solvent (Table 1). This is a characteristic of tautomer emission. But the normal emission undergoes a bathochromic shift with rise in polarity and hydrogen bonding capacity of the solvents. The blue shift in the tautomer emission is due to smaller dipole moment of the tautomer in the excited state compared to that in the ground state. Therefore, upon increasing the polarity and hydrogen bond capacity of the solvent, the excited state is less stabilized than the ground state. This leads to increase in the energy gap between these states. The negative solvatochromism is established from the plot of emission maximum against E_T(30) parameter⁴⁶ (Fig. 3). But the extent of blue shift in bis-HPBI is less than that in HPBI. However, the tautomer emission of bis-HPBI is red shifted compared to that of HPBI in the same solvent. For example, in methanol it is 34 nm red shifted to that of HPBI. These changes indicate the effect of substitution.

Fig. 2 Fluorescence spectra of bis-HPBI in selected solvents, (λ_exc = 340 nm and ‘*’ denotes normal band in acetonitrile).

Fig. 3 Plot of tautomer band maximum of solvents versus solvent polarity parameter E_T(30).

3.1.2. Tautomer lifetime and quantum yield. Since bis-HPBI has two HPBI units, it is possible to form monoketo by ESIPT of one unit of HPBI and diketo by ESIPT of both the units of HPBI. bis-HPBI exhibits single longer wavelength emission, which indicates that the presence of either monoketo or diketo. Sometimes if one emission is very weak and buried underneath the other then it is very difficult to distinguish the single emission and the dual emission. In those cases, the time resolved emission is handy to detect the existence of weak emission that is hidden beneath a strong emission.⁴⁷ Therefore, the fluorescence decays of tautomer emission are recorded in different solvents. A single exponential decay is observed in all the solvents and the lifetimes thus obtained are compiled in Table 2. It clearly tells that only one species is responsible for the tautomer emission. The fluorescence lifetimes of bis-HPBI in methanol and acetonitrile are less and this indicates that non-radiative channel become active in these solvents.

Table 2 Lifetime (τ_T, ns), quantum yield (Φ_T), radiative rate constant (k_r, 10⁷ s⁻¹) and non-radiative rate constant (k_nr, 10⁷ s⁻¹) of tautomer emission of bis-HPBI in different solvents

Solvents	τT	ΦT	kr	knr
Cyclohexane	4.0	0.82	20.50	4.50
1,4-Dioxane	3.8	0.41	10.78	15.52
Diethyl ether	4.2	0.51	12.14	11.67
Ethylacetate	3.7	0.39	10.54	16.48
Tetrahydrofuran	3.8	0.40	10.52	15.79
Acetonitrile	2.6	0.26	9.62	27.40
Dimethyformamide	4.0	0.48	12.00	13.00
Dimethylsulphoxide	4.3	0.51	11.86	11.40
Butanol	3.8	0.41	10.78	15.52
2-Propanol	3.6	0.39	10.83	16.95
1-Propanol	3.5	0.39	11.14	17.43
Ethanol	3.2	0.29	9.06	22.18
Methanol	2.2	0.17	7.72	37.73
Glycol	3.9	0.51	13.07	12.56

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Since, almost exclusive tautomer emission is observed the quantum yield of tautomer emission as well as the radiative and the nonradiative rates are calculated in different solvents (Table 2). The quantum yield of tautomer emission is less in methanol and acetonitrile relative to other solvents. Therefore, the radiative rate constants are low and the nonradiative rate constants are high in methanol and acetonitrile. In HPBI and related molecules the torsional rotation around phenolic/azole rings of keto in the excited state leads to a nonemissive ICT state which acts as a major nonradiative channel.^16,48,49 Such a process is expected to be high in polar nonviscous solvents like methanol and acetonitrile. Grellinann et al. observed a relatively intense tautomer emission when the cis–trans conversion of keto tautomer is practically disabled by bridging the benzimidazole and phenolic ring of HPBI in a bridged HPBI.⁵⁰ In bis-HPBI, the rotation of the phenolic ring is difficult due to bulky substitution, but, the rotation of benzimidazole ring is not hampered that much. As a result torsional rotation of the keto tautomer is feasible. However, the torsional rotation of excited keto of HPBI is also predicted to be retarded by viscosity of the medium.¹⁷ The quantum yields of bis-HPBI are higher in relatively more viscous solvents like dimethylformamide, dimethylsulphoxide and glycol despite their higher polarity. Similarly, the fluorescence lifetimes are also longer in these solvents.

3.1.3. Ground state and excited state species. HPBI exists as cis- and trans-enol in the ground state.³³ Due to the presence of two HPBI units, three enol conformers are possible for bis-HPBI. The optimized structures of different forms are presented in Chart 2 and the data are compiled in Table 3. The cis,cis-enol is more stable than cis,trans-enol by 0.34 eV which more stable than trans,trans-enol by 0.26 eV. cis,trans-Enol or trans,trans-enol is required to produce normal emission after photoexcitation. As cis,cis-enol has very high stability than other two conformers and bis-HPBI is present almost exclusively as cis,cis-enol in the ground state. This explains the negligible presence of normal emission in acetonitrile and its almost complete nonexistence in other solvents. Unlike in bis-HPBI, in HPBI the normal emission is significant in polar and protic solvents.²⁸ In bis-HPBI, the substitution of bulky tert-butyl group ortho to ‘OH’ group helped in suppressing the population of the trans conformer. Unlike HPBI, the trans-enol conformer of bis-HPBI has steric hindrance between the hydrogen atom of the ‘OH’ group and hydrogen atom of the tert-butyl group (Fig. 4). Consequently, the population of cis,trans-enol is negligible and almost no existence of trans,trans-enol in the ground state. Excitation of the cis-enol will result in keto tautomer in the excited state. Both, cis,cis-enol and cis,trans-enol has cis-enol conformer. As mentioned earlier, bis-HPBI can form monoketo or diketo. The excitation of cis conformer of cis,cis-enol and cis,trans-enol can lead to monoketo. Whereas, excitation on both cis conformer of cis,cis-enol may lead to diketo. As suggested by the calculation bis-HPBI exist primarily as cis,cis-enol. Therefore, both one proton transfer and two proton transfers are probable. However, in a nonconjucated system like bis-HPBI at least one of the cis-enol of cis,cis-enol has to be excited to form monoketo and both the cis-enols have to be excited to form diketo. Such an excitation of two chromophores has to be a two photon absorption processes. To find the dependence of fluorescence intensity on excitation intensity, using a set of neutral density filters the excitation intensity is attenuated. The relative spectral areas of bis-HPBI fluorescence spectrum with no filter (F₀) to with different filters (F) are obtained. Similarly, the relative spectral areas of standard quinine sulfate solution in 1 N sulphuric acid with no filter (I₀) to with different filters (I) are measured. A linear fit with a slope of 0.96 ± 0.05 was obtained for the plot of relative fluorescence of both the solutions (Fig. 5). This shows that the formation of keto tautomer is a single photon processes. Therefore, it can be inferred that the emitting species of bis-HPBI is the monoketo tautomer. This is further substantiated by the theoretical calculation which suggests that the formation of monoketo is energetically favored over diketo (Table 3). The calculated excitation and emission energies are in good agreement with the experimental values (Table 3). The tautomer emission is observed along with the anion emission even after deprotonation of one of the ‘OH’ group of bis-HPBI (see latter). This further supports the assignment of tautomer band to emission form monoketo formed by single proton transfer.

Chart 2 DFT optimized geometries of the different conformers of the bis-HPBI.

Table 3 Optimized parameters of different conformers of bis-HPBI in acetonitrile and experimental transition energy is provided in the parenthesis^a

		cis–cis-Enol	cis–trans-Enol	trans–trans-Enol	Monoketo	Diketo
a E (relative energy with respect to most stable cis–cis-enol energy −46 940.30 eV).
S0 state
	E (eV)	0.00	0.34	0.60
	μ (D)	6.8	9.3	8.3
	Transition energy (S0→ S1)/nm	331 (339)	329 (326)	314
S1 state
	E (eV)		3.76		3.11	3.36
	μ (D)		9.2		7.7
	Transition energy (S1→ S0)/nm		378 (385)		493 (477)

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Fig. 4 Absence and presence of steric effect in trans-enol's of HPBI and bis-HPBI.

Fig. 5 Logarithmic plot of relative fluorescence intensity (F₀/F) of bis-HPBI in acetonitrile versus relative excitation intensity log(I₀/I), λ_exc = 330 nm.

3.2. Effects of acids and bases on ESIPT

Besides the ESIPT process, the acid–base chemistry of bis-HPBI will be an interesting one as it has two HPBI moieties. The variation in hydrogen ion concentrations is expected to modulate the spectral characteristics of bis-HPBI due to the presence of acidic and basic groups in bis-HPBI.

3.2.1. Effect of base. At pH 7.2 bis-HPBI exists in neutral form. Upon increasing the pH, a new band starts to appear in the absorption spectra on the red side of the neutral band of the bis-HPBI (Fig. S1†). The new band at ∼360 nm co-exists with neutral band upto pH ∼12.5. Nonetheless, with further rise in pH, the molecular band totally disappeared and a single band at ∼360 nm is observed around pH ∼12.9 to H_ ∼14.2 (Fig. S1†). The red shift illustrates the increase in conjugation due to deprotonation of hydroxyl group.³³

The emission spectrum of bis-HPBI in water at pH 7.2 is also having predominantly a single emission around 475 nm. Therefore the emitting species in aqueous medium is also the tautomer (Fig. 6a) and the lifetime is 1.64 ns. Upon addition of the base, the fluorescence intensity increases and a new band appear at shorter wavelength (∼435 nm). But the intensity of the tautomer also increases with base concentration till pH 12.5 along with the band at 435 nm (Fig. 6a). In contrast, it was reported that the tautomer emission of HPBI decreases and that of its anion increases upon addition of base to aqueous solution.³³ This difference in behaviour is due to the presence of two HPBI units in bis-HPBI. The step wise deprotonation is taking place in bis-HPBI for initial addition of base only one units of ‘OH’ group is deprotonated to form anion. Since, the other HPBI unit possess the ‘OH’ group and it can undergo ESIPT. As a result, the tautomer emission is also observed along with the emission from the anion. At higher concentration of base both the ‘OH’ groups are deprotonated and bis-HPBI exhibits only single emission (Fig. 6b). The single exponential decay is also obtained for bis-HPBI at H_ 14.2 (Table S1†). Such an enhancement in lifetime compared to that of tautomer is also obtained for HPBI upon deprotonation of ‘OH’ group (the fluorescence lifetimes of tautomer and anion in aqueous solution are 1.86 ns and 2.95 ns, respectively).⁵¹

Fig. 6 Fluorescence spectra of bis-HPBI in basic solution, λ_exc = 360 nm.

3.2.2. Effect of acid. Under acidic condition, the vibrational structure in the absorption spectrum of bis-HPBI is lost and the absorbance of the longer wavelength band decreases and that of shorter wavelength band increases (Fig. S2†). In other words, the addition of acid brings minimal changes in the absorption spectrum of bis-HPBI in aqueous medium. This shows the interaction of protons with bis-HPBI in the ground state, but no clear picture emerges from the absorption studies. However, the emission spectra gave a clear picture about the interactions. The neutral bis-HPBI emits the tautomer emission at 475 nm. For initial addition of acids, the intensity of the tautomer emission decreases (Fig. 7a). When acidity of the solution increased a new band appeared at a shorter wavelength at 388 nm and the tautomer emission also recovered its fluorescence partially (Fig. 7b). Interestingly, both the bands co-exist up to H₀ −7.8. However, with further increase in acid concentration, the two band system slowly moves toward a single band system. The emission at 388 nm gains the intensity and the tautomer emission intensity diminishes. In a strongly acidified solution (∼H₀ −10.0) the tautomer band totally vanishes and bis-HPBI exclusively emits 388 nm emission (Fig. 7c). The fluorescence lifetimes of bis-HPBI in aqueous medium at different conditions are summarized in Table S1.†

Fig. 7 Fluorescence spectra of bis-HPBI in acidic solution, λ_exc = 300 nm.

The initial decrease in tautomer emission suggests weaker hydrogen bonding interaction of protons with bis-HPBI. Such hydrogen bond induced quenching of tautomer emission was observed in other dyes also.^19,52 The recovery of tautomer emission and appearance of new band at 388 nm are due to protonation of imidazole nitrogen of HPBI unit. The emission band at 388 nm can be assigned to emission of cation formed by the protonation of the imidazole nitrogen. The appearance of neutral tautomer emission along with cationic emission is due to dissociation of proton from the ‘OH’ group of cation which reorganizes to form tautomer in the excited state. Rodríguez-Prieto et al. also reported that in aqueous acidic solution (pH ∼ 3), the cation of HPBI upon excitation emits the neutral keto emission.⁵¹ Similar events are also encountered by HPIP-b in water.^53,54 Rodríguez et al. observed two emissions of HPBI in acidified ethanol, due to excited state cation and the neutral tautomer produced from cation.⁵¹ The results of bis-HPBI in acidified aqueous solution is little similar to results of HPBI in acidified ethanol rather than acidified aqueous solution. Recently, Rodríguez-Prieto et al. demonstrated that when electron donating amino group was substituted in HPBI, the excited cation was unable to transfer its ‘OH’ proton to solvent, due to electron donating nature of –NH₂ group.⁵⁵ In bis-HPBI, the electron donating alkyl groups are substituted, but as the electron donating ability is less than that of amino group. Bis-HPBI exist in both cationic form and neutral tautomeric form in the electronic excited state. As, the deprotonation is prevented in strongly acidic solution emission is observed only from the cationic form of bis-HPBI.

4. Conclusion

The absorption and emission spectra of bis-HPBI are bathochromically shifted than those of HPBI. Unlike HPBI, bis-HPBI undergoes almost exclusively ESIPT and emits intense tautomer emission even in protic solvents. The presence of bulky tert-butyl group ortho to ‘OH’ group hindered the formation of trans-enol and as a result of which normal emission of bis-HPBI is not observed in most of the solvents. Though bis-HPBI has two HPBI units, the tautomer emission of bis-HPBI is due to only single proton transfer. The theoretical predicted emission energy of monoketo also matches with experimental result. Enhancement of tautomer band intensity upon deprotonation of one of the ‘OH’ also supports the single proton transfer hypothesis in bis-HPBI. The tautomer emission decreases in acidic solution with initial addition of acid due to quenching by hydrogen bonding. When the imdazole nitrogen is protonated the tautomer emission is partially recovered due to dissociation of proton of ‘OH’ group and reorganization of cation. In strongly acidic solution the dissociation of ‘OH’ group from cation is prevented and emission is observed only from cation of bis-HPBI.

Acknowledgements

The authors thank Department of Science and Technology (DST), India, for the financial support and the Central Instruments Facility (CIF), Indian Institute of Technology Guwahati for the LifeSpec II instrument. S. K. B thanks CISR, India for the senior research fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Absorption spectra of bis-HPBI in aqueous medium at different conditions and fluorescence wavelength maximum and lifetimes of bis-HPBI in aqueous medium at different conditions. See DOI: 10.1039/c6ra11780e

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