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Spectral characteristics of the monocations of 2-(2′-aminophenyl)benzimidazole in different solvents
Journal of Molecular Structure 478 (1999) 169–183
Spectral characteristics of the monocations of 2-(2 -aminophenyl)benzimidazole in different solvents 0
Swadeshmukul Santra, Sneh K. Dogra* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Received 3 August 1998; received in revised form 13 October 1998; accepted 13 October 1998
Abstract The absorption and fluorescence spectra of 2-(2 0 -aminophenyl)benzimidazole (2-APBI) were studied in three solvents and at different concentrations of acids. Trifluoroacetic acid was used as an acid in cyclohexane and sulphuric acid was used to control the acid concentrations for acetonitrile and water. The broadness of the long wavelength absorption band of the monocation in three solvents indicates that the monocation is either existing in different isomeric forms or it is formed by protonating different basic centres. Fluorescence and fluorescence excitation spectra of the monocation recorded at different emission wavelengths establish the presence of two forms of the monocations in cyclohexane and three forms of the monocations in acetonitrile and water. Semi-empirical quantum mechanical calculations were carried out to confirm the presence of different forms of the monocations. 䉷 1999 Elsevier Science B.V. All rights reserved. Keywords: Monocations of 2-APBI; Spectral characteristics; Semi-empirical calculations
1. Introduction Acid–base properties of aromatic compounds, especially heterocyclic molecules containing more than one heteroatom are always of interest in chemistry and biochemistry [1–13]. This property in general is associated with the so called non-bonding pair of electrons at the heteroatoms. The heteroatom can be a part of the aromatic ring (–Ny) or it can be present in the substituent (–OH, –NH2 etc.) of the aromatic ring. The presence of the lone pair of electrons is reflected by the basic character of many of these compounds and their ability to participate in hydrogen bonding. Various attempts have been made to establish a correlation between the electronic structure of aza aromatic molecules and the acidity or * Corresponding author. Tel.: ⫹ 91-512-597-163; Fax: ⫹ 91512-590-007; e-mail: [email protected]
basicity of the basic centre. Of these, the successful ones are the concept of: (i) the average local ionization energy on the molecular surface [14,15], (ii) the effective valence electron potential as a reactivity index for the protonation reaction [16–18]. In this method the reactivity index reflects the local ionization, serving as a measure of nucleophilicity of the basic centre and (iii) the electrostatic potential energy mapping [19]. This method also considers the charge density at a particular basic centre of interest plus the effect of charges of rest of the atoms in the molecule. The greater the depth of potential well the greater is the chances of protonation on the basic centre. In our recent sudies on 2-(4 0 -aminophenyl)benzoxazole (4-APBO) [20], 2-(4 0 -aminophenyl)benzothiazole (4-APBT) [21] and others [22], we observed that the monocations are formed by either protonating the –Ny atom or the amino group. The red and blue shifts observed in the spectral characteristics of these
0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(98)00754-6
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Fig. 1. Charge densities at different basic centres and dipole moments of different species of 2-APBI in S0 and S1 states, ground state energies and dihedral angle f .
compounds in the acidic medium are because of formation of the monocation by the protonation of the –Ny atom and the –NH2 group respectively [22]. In our recent study on 2-(2 0 -aminophenyl)benzimidazole (2-APBI) [23,24], it was confirmed that
there are two different kinds of rotamers (I and II), which are nearly equally stable. The activation barrier for the conversion of II to I is only 2.71 kJ mol ⫺1 in the lowest singlet state. The tautomer III is highly unstable (Fig. 1). Looking at the two rotamers and
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Table 1 Absorption maxima (nm) and log 1 max ( in parenthesis ) of 2-APBI (5 × 10 ⫺5 M) in its neutral, monocationic and dicationic forms, at 298 K Solvent
Species N abs l max (log 1 max)
Cyclohexane 360 (sh) 348 (4.05) 299 (3.98) 291 (3.98) 288 (sh) 278 (3.88) 252 (4.12)
Acetonitrile 345 (4.15) 297 (4.14) 289 (4.10) 285 (sh) 278 (sh) 251 (4.23) Water 322 (3.60) 295 (sh) 288 (3.70) 275 (sh) 242 (3.70)
MC abs l max (log 1 max)
DC abs l max (log 1 max)
[TFA] 1 × 10 ⫺4 M 360 (sh) 348 (3.58) 330 (sh) 298 (3.66) 291 (3.68) 286 (sh) 278 (sh) 252 (3.70)
[TFA] 0.1 M 284 (4.22)
[H2SO4] 1 × 10 ⫺4 M 348 (3.91) 288 (4.17)
[H2SO4] 1.0 M 282 (4.23)
pH 2.0 340 (3.82) 285 (4.17)
[H2SO4] 2.5 M 284 (4.53)
the tautomer, three possible structures of the monocations can be written (Fig. 1). The spectral characteristics of 2-APBI were studied in three different solvents (cyclohexane, acetonitrile and water) containing different amounts of acids. The absorption, fluorescence and flourescence excitation spectra and the lifetimes measured at different emission wavelengths were used to establish these three monocations. Semi-empirical quantum mechanical calculations were also carried out to supplement the above assignment.
2. Materials and methods 2-APBI was procured from Aldrich Chemical Company, UK and was purified by recrystallization from methanol. The purity was checked by a single spot on TLC and by the production of similar fluorescence bands on excitation with different wavelengths. Cyclohexane (SD Fine Chemicals) and
acetonitrile (E. Merck) were of AnalR grade and were further purified as described in the literature [25]. Triple distilled water was used to prepare aqueous solutions. The pH of the aqueous solutions in the range of 3–10 was adjusted by adding required amounts of o-phosphoric acid and sodium hydroxide, as such an amount of buffers does not quench the fluorescence intensities of the investigated species [26]. Trifluoroacetic acid (TFA) was used as an acid in cyclohexane, and H2SO4 was used to control the acid concentration in acetonitrile and water. The absorption spectra were recorded on a Shimadzu UV-190 spectrophotometer and fluorescence excitation spectra were recorded on PerkinElmer luminescence spectrofluorometer, model LS 50B, equipped with a WIPRO Genius PC 386X (Slit width used for both excitation and emission was 2.5 nm). Fluorescence spectra were recorded on a recording spectrofluorometer, fabricated in our laboratory. Details are available elsewhere [27]. Fluorescence spectra were corrected according to
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Fig. 2. Absorption spectra of 2-APBI at different acid concentrations in (A) cyclohexane ⫹ TFA, (B) acetonitrile ⫹ H2SO4; (a): 0.0 M, (b): 1 × 10 ⫺5M, (c): 5 × 10 ⫺5 M, (d): 1 × 10 ⫺4 M, (e): 1 × 10 ⫺3 M, (f): 5 × 10 ⫺3 M, (g): 1 × 10 ⫺2 M, (h): 1 × 10 ⫺1 M and (C) water at various pH; (a) 2.0, (b) 3.0, (c) 4.0, (d) 4.2, (e) 4.6, (f) 5.0, (g) 6.0, (h) 7.0; [2-APBI] 2 × 10 ⫺5M.
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Fig. 2. (continued)
the procedure suggested by Parker [28] and fluorescence quantum yields mentioned in the text are relative to each other (details given in the text). The excited state lifetimes were measured on a time-correlated single photon counting nanosecond spectrofluorimeter supplied by Applied Photophysics Limited. Details are available in our recent paper [29]. Semi-empirical quantum mechanical calculations were carried on different species and details are given later on. The data are compiled in Fig. 1. The energy of the solvated states may be estimated by taking into account the dipole–dipole interactions between the dipole moment (m ) and the solvent according to [30,31]: h i E solv E ⫺ m2 =4p10 a3 f 1
1
where a is the Onsager’s cavity radius and f 1
1 ⫺ 1 21 ⫹ 1
2
where 1 is the static dielectric constant of the solvent and E is the energy of the state under isolated conditions. The effective electrostatic potential at different basic centres has been calculated using the VSSPC computer program [19].
3. Results 3.1. Absorption spectra The absorption spectra of 2-APBI were recorded at different acid concentrations in cyclohexane, acetonitrile and water. The results are compiled in Table 1. Figs. 2a–c depict the effect of acid concentrations on the absorption spectrum of 2-APBI in cyclohexane, acetonitrile and water respectively. The isosbestic points observed in the monocation-neutral equilibrium in these solvents are 378, 370 and 342 nm respectively in the same order. As the concentration of TFA is increased in cyclohexane containing 2APBI, the absorbance of both the band maxima (i.e. at ⬃ 348 nm and at the structured band system at ⬃290 nm) decrease (Fig. 2A). The structure in the 290 nm band is lost and ⬃ 348 nm band becomes very broad at TFA concentration equal to 1 × 10 ⫺4 M and 1 × 10 ⫺3 M. At TFA concentration equal to 0.1 M, only one band at 284 nm is observed. The absorption spectra of 2-APBI in acetonitrile at H2SO4 concentration of 1 × 10 ⫺4 M and 1.0 M (Fig. 2B) and in water at pH equal to 2.0 and at H2SO4 concentration of 2.5 M (Fig. 2C), are all very similar. At lower acid concentration, the long wavelength band is red shifted and the structure is lost in the short wavelength band. At high acid concentration
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Table 2 flu of 2-APBI (5 × 10 ⫺5 M) at different excitation wavelengths in the following solvents at 298 K Fluorescence band maxima l max Solvents
l exc (nm)
Species l max (N)
flu l max (MC)
flu l max (DC)
Cyclohexane
290 310 350 375 390 410
383 384 386, 527 ^ 2 — — —
409, 472 408, 471 405, 462 417, 471 —, 472 —, 475
369 ^ 2, 455 370 ^ 2, 448 466 ^ 2 — — —
Acetonitrile
290 310 350 370 390 400 410 420
399, 525 ^ 10 399, 522 ^ 10 400, 518 ^ 05 401, 523 ^ 05 —, — —, — —, — —, —
449, 370 449, 370 445, 370 447, — 460, — 467, — 472, — 473, —
386 389 — — — — — —
Water
287 305 325 342 370 430
— 412 — 413 — —
375, 452 375, 455 446, 475 (sh) 464 474 474
391 — — — — —
only one large blue shifted band is observed at 283 ^ 1 nm. The broadness of the long wavelength band of the monocation in cyclohexane as depicted by the band width at half the maximum height (BWHMH) is 5770 cm ⫺1 and decreases to 4920 cm ⫺1 in acetonitrile. The BWHMH observed for the dication is the same (4880 cm ⫺1) in all the three solvents. The pKa value for the monocation–neutral equilibrium in water is found to be 4.2 agreeing with earlier results [23]. 3.2. Fluorescence spectra Fluorescence spectra of 2-APBI were also studied in three different solvents at different acid concentrations and by exciting at different wavelengths (l exc). The relevant data are compiled in Table 2. From the data of Table 2 and Fig. 3A, it is obvious that with cyclohexane as solvent and a TFA concentration 1 × 10 ⫺4 M, dual fluorescence is observed from the monocation. The fluorescence intensities of the two bands observed depend upon l exc. With l exc in the range of 290–350 nm, fluorescence bands are observed with maxima at 407 ^ 2 and 467 ^ 5 nm respectively,
whereas at l exc greater than 350 nm only the long wavelength band is present. In contrast, at TFA concentration 1 × 10 ⫺3 M, only one fluorescence band (472 ^ 1 nm, Fig. 3B) is observed when l exc is in the range of 290–410 nm, but BWHMH are different for different l exc i.e. 3410, 3480 and 3030 cm ⫺1 at l exc 290, 350 and 410 nm. From our earlier study [23,24] it is known that dual fluorescence is observed from neutral 2-APBI in cyclohexane. 384 nm emission band is assigned to the normal fluorescence and 530 nm band is assigned to the tautomer fluorescence. Thus, in comparison to the normal fluorescence of neutral 2-APBI both the fluorescence bands of the monocation of 2-APBI are red shifted, whereas when excited at 410 nm, the fluorescence band of the monocation is red shifted with respect to the short wavelength emission (384 nm) and blue shifted in comparison to the long wavelength emission (530 nm) of the neutral 2-APBI. The fluorescence quantum yield of the short wavelength band decreases and that of the long wavelength band increases with increasing l exc, while BWHMH of both the bands remain unchanged.
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Fig. 3. Fluorescence spectra of 2-APBI at different excitation wavelengths in cyclohexane; (A): [TFA] 1 × 10 ⫺4 M and (B): [TFA 1 × 10 ⫺3 M; [2-APBI] 2 × 10 ⫺5 M.
In acetonitrile as solvent and H2SO4 of concentration 1 × 10 ⫺4 M, we observed two fluorescence bands (370 ^ 2 nm, 447 ^ 2 nm) for l exc in the range 290– 350 nm, one fluorescence band 447 ^ 2 nm when l exc is between 350–370 nm and another band at 470 ^ 3 nm for l exc greater than 390 nm (Fig. 4A). The 370 nm band is blue shifted and the other two bands red shifted in comparison to the short wavelength emission (400 nm) of the neutral 2-APBI. All these bands are blue shifted in comparison to the long wavelength fluorescence band (520 nm) of neutral 2-APBI. The fluorescence quantum yields of the bands at 370 and 447 nm decrease with increase of l exc and that of the 470 nm band increases with increase of l exc. A similar behaviour is also observed when 2-APBI is
excited in water at pH 2 (Fig. 4B). The only difference is that If 370=If 447 0:18 in acetonitrile compared to 0.083 in water. At high acid concentrations (0.1 M TFA), two fluorescence bands are observed in cyclohexane when excited at 290 and 310 nm, whereas only one band is present when excited at l exc 350 nm. In contrast, only one fluorescence band (389 ^ 3 nm) is observed in acetonitrile containing 1.0 M H2SO4 and water at 2.5 M H2SO4. Based on these results observed in cyclohexane, the long wavelength band is assigned to the monocation and the short wavelength band to a dication formed by the protonation of the –NH2 group and the –Ny atom of 2-APBI. The long wavelength band (445 nm) assigned to the
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Fig. 4. Fluorescence spectra of 2-APBI at different excitation wavelengths in (A): acetonitrile, [H2SO4] 1 × 10 ⫺4 M and (B): water at pH 2; [2-APBI] 2 × 10 ⫺5 M.
monocation of 2-APBI in cyclohexane at TFA concentration 0.1 M does not agree with the one observed at TFA concentration 1 × 10 ⫺4 M (407 nm) but it agrees nicely with the one observed at H2SO4 concentration 1 × 10 ⫺4 M in acetonitrile. This observation could be because of the fact that the polarity of cyclohexane at TFA concentration 0.1 M may resemble with that of acetonitrile containing 1 × 10 ⫺4 M H2SO4. The assignment of the long wavelength band to the monocation is further substantiated with the fact that its fluorescence band maximum and its fluorescence intensity depend upon l exc.
3.3. Fluorescence excitation spectra The fluorescence excitation spectra of 2-APBI were recorded at different emission wavelengths in cyclohexane containing 1 × 10 ⫺4 M and 1 × 10 ⫺3 M TFA. The fluorescence excitation spectra recorded at two acid concentrations are exactly similar to each other. Fig. 5A represents the fluorescence excitation spectra recorded at 400 and 500 nm emission and the absorption spectra of the monocation at 1 × 10 ⫺3 M TFA. The fluorescence excitation spectra recorded at 400 and 425 nm are similar to each other, having broad
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Fig. 5. Fluorescence excitation spectra of 2-APBI in (A): cyclohexane at [TFA] 1 × 10 ⫺3 M, (B): acetonitrile at [H2SO4] 1 × 10 ⫺4 M and (C): water at pH 2; [2-APBI] 2 × 10 ⫺5M.
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Fluorescence excitation spectra recorded in water (Fig. 5C) at pH 2 and at the above mentioned fluorescence wavelengths are exactly similar to those observed in acetonitrile. The absorption spectra of the monocation in all the solvents do not show any band or shoulder at 390 nm but the tail is extended towards red. This could be because of the monocation V (see later). As the fluorescence excitation spectrum is more sensitive and represents different species present in the system, the 390 nm band could be buried under the strong band at 350 nm of the absorption spectrum of the monocation. The fluorescence excitation spectra in cyclohexane (0.1 M TFA) were recorded in the range of ⬃ 370– 390 and 440–470 nm. The former are similar to each other and resemble with the absorption spectrum of the dication and the latter resemble with the excitation spectra of the monocation. Whereas in acetonitrile (1 M H2SO4) and water 2.5 M H2SO4),
band around 340 nm and structured band around 278– 300 nm. Similarly, the fluorescence excitation spectra recorded at 480, 500, 520 and 540 nm emission wavelengths resemble each other depicting large broad bands at ⬃ 290 and at ⬃ 355 nm with a shoulder at ⬃ 390 nm, whereas fluorescence excitation spectra recorded at 455 nm is a mixture of two. In case of acetonitrile containing 1 × 10 ⫺4 M H2SO4, the fluorescence excitation spectra of the monocation of 2-APBI were recorded at the following emission wavelengths: 380, 420, 440, 470, 485, 500, 520 and 540 nm. Fig. 5B shows only the fluorescence excitation spectra recorded at 380, 440 and 540 nm, along with absorption spectra. The fluorescence excitation spectrum recorded at 380 nm gives two main bands at 290 and 240 nm. At all other emission wavelengths less than 470 nm an additional band is observed at ⬃ 350 nm. Above 470 nm, a shoulder appears at ⬃ 390 nm in addition to 350 nm band.
Table 3 Excited singlet state lifetime (ns) of the monocations of 2-APBI measured in different solvents and at different emission wavelengths
l em (nm)
Single exponential decay t x2
t1
Double exponential decay A1 t2
A2
x2
Cyclohexane ⫹ [TFA] 1 × 10 ⫺4 M, l exc 354 nm 380 420 460 500
3.0 4.3 5.2 6.7
1.13 1.30 1.63 1.80
3.0 2.95 3.2 3.1
96.4 56.3 38.2 20.4
2.0 2.0 1.95 1.96
3.6 43.7 61.8 79.2
1.13 1.18 1.08 1.10
Acetonitrile ⫹ [H2SO4] 1 × 10 ⫺4 M, l exc 354 nm 400 420 460 500
2.88 2.84 2.66 2.80
1.96 1.27 1.32 1.28
3.33 3.66 3.60 3.56
68.5 55.5 36.0 28.9
1.81 1.83 2.03 1.95
31.5 44.5 64.0 71.1
0.94 1.17 1.20 0.96
Water at pH 2.0, l exc 354 nm 430 460 500
0.77 1.16 6.50
1.30 4.00 4.60
0.57 0.50 0.58
90.1 72.0 52.1
10.90 10.60 11.90
9.9 28.0 47.9
0.90 1.09 1.10
Water at pH 2.0, l exc 310 nm 430 460 500
2.10 3.10 5.80
2.11 5.10 4.20
0.80 0.68 0.80
75.7 55.2 46.1
10.50 10.20 11.00
24.3 44.8 53.9
1.10 1.21 1.10
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Table 4 Spectral transitions (l , nm) and their oscillator strengths (f) of the monocation of 2-APBI, predicted by CNDO/S-CI
not be measured because of its low intensity as well as the low intensity of the flash lamp at 290 nm.
Monocations IV l f
3.5. Quantum mechanical calculations V l
347 307 275 260 241 225
353 307 289 260 241 236
0.223 0.070 0.031 0.012 0.010 0.067
f
VI l
f
0.353 0.028 0.060 0.018 0.021 0.015
313 292 277 261 245 226
0.186 0.015 0.071 0.057 0.023 0.028
the fluorescence excitation spectra were recorded only at emission wavelengths in the range of ⬃ 370– 390 nm and resemble with the absorption spectra of the dication in these solvents.
3.4. Excited state lifetimes Excited state lifetimes of the monocations of 2APBI were measured at TFA concentration 1 × 10 ⫺4 M in cyclohexane, H2SO4 concentration 1 × 10 ⫺4 M in acetonitrile and at pH 2 in water. In acetonitrile and cyclohexane the species were excited at 354 nm, whereas in water, the l exc used were 310 and 354 nm. The decay curves were observed at different emission wavelengths as mentioned in Table 3. The fluorescence decay followed a single exponential when measured at l em 380 nm in cyclohexane. This is substantiated by the fact that the x 2 observed for a single exponential and a double exponential analysis were similar and that the major (90%) amplitude in the double exponential analysis is the same as found with only one exponential. The fluorescence decay followed a double exponential when measured at l em 460 and 500 nm. The amplitude of the short lived species (2 ns) increases from 3.6% to 79.2%, whereas the amplitude of the long lived species (3 ns) decreases with the increase of l em. A similar behaviour is also observed in acetonitrile, except that the amplitude of the species with a lifetime 2 ^ 0.2 ns increases from 31% to 70% with increase of l em. In case of water, the x 2 values correspond to a double exponential decay at all three l em and at both l exc, except that the amplitudes are different (Table 3). The lifetime of the species emitting at 370 nm could
2-APBI in its neutral form can be depicted by two rotamers (I and II) and one tautomer (III) as shown in Fig. 1. It has been shown earlier [24] and it is also depicted in Fig. 1 that I and II are nearly of similar stability in the ground state and that the barrier for interconversion between rotamer II and I is 2.71 kJ mol ⫺1, whereas the tautomer III is 99 kJ mol ⫺1 less stable. This indicated that rotamers I and II are both present at room temperature. Three possible monocations are shown in Fig. 1. The ground state geometries of the three monocations were first optimized using the MM2 force field [32]. A more precise geometry was then obtained using AM1 [33] (MOPAC, QCPE program number 455, version 4.0). The values of energies (Ei), dipole moment (m g) and charge densities at the basic centres obtained from these calculations are compiled in Fig. 1. Transition energies DEi!j for the three different monocations were calculated using CNDO/S-CI method [34]. 64 singly excited configurations were generated by exciting one electron from the eight highest occupied molecular orbitals to the eight lowest unoccupied molecular orbitals. The calculated values of the dipole moment and the charge densities at the basic centres in S1 state are included in Fig. 1. The transition wavelengths are given in Table 4. As free molecules, the species V and VI are predicted to be 63.8 and 90.97 kJ mol ⫺1 less stable than species IV. The relative stability of species V and VI changes to 36.5 and 57 kJ mol ⫺ respectively when solvated by methanol. The solvation energies were calculated using Eq. (1). For V and VI the solvent stabilization is considerably large because of the greater dipole moment of these species. The low stability of VI is because of the absence of a hydrogen bonding interaction which reflects in a large dihedral angle (f ) as shown in Fig. 1 (67⬚). In V, the f is small (23.7⬚) but the intramolecular hydrogen bond distance ˚ ) than in IV. Thus as a free (N3 –H17) is larger (2.48 A molecule (non-polar solvents) major contribution for all the monocationic species will be from IV, but the contribution from V and VI will increase with the increase in the polarity of the medium.
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Fig. 6. Electrostatic potential map for 2-APBI in the ground state (values shown in kJ mol ⫺1).
We have constructed the electrostatic potential energy map for both the rotamers I and II by using VSSPC computer program [19] as shown in Fig. 6. It is found that in S0 state and in the molecular plane (Z 0.0), both the rotamers I and II give rise to identical results. The value of absolute minima (depth of the well) is 118.04 kJ mol ⫺1 and it is located near the – Ny atom (Fig. 6). The contours are spread over N16 and N3 indicating that the chances of protonating N3 are larger than that of N16. In the S1 state, rotamers I and II behave differently with respect to the depth of the potential well, although the location of the absolute minima still remains near the N3 atom in the molecular plane. In rotamer I, the well is located almost in between N16 and N3 atoms, indicating the equal chances of protonation, although structure protonated at N3 will be more stable than that at N16. This indicates that even if the protonation occurs at N16, proton will be transferred to N3 via ESIPT to get more stable structure. The depths of the potential well for I and II are ⫺183.3 and ⫺149.4 kJ mol ⫺1 respectively. Although we have taken the fully optimized geometry of the
ground state, the present calculations for the S1 state might be little less reliable because of the change of geometry in the S1 state.
4. Discussion Based on the results, obtained using semi-empirical quantum mechanical calculations (Fig. 1) and those obtained from the electrostatic potential energy mapping (Fig. 6), it is clear that the monocation IV will be the most stable and will be the predominant species in the solution phase as a free molecule (in cyclohexane). The red shift observed in the long wavelength absorption spectra, in comparison to the neutral 2-APBI, is consistent with the fact that protonation of the –Ny atom leads to the red shift in the absorption and fluorescence spectra if p ! p * is the lowest energy transition [1]. But the spectral transitions obtained using the CNDO/S-CI calculations do not agree with the absorption spectra (specially the long wavelength band) of the monocation of 2-APBI observed in any of the three solvents. This could be a
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result of the presence of different monocations present in the solutions formed by protonating different basic centres. Thus the absorption spectra recorded at moderate acid concentrations are the composite spectra of the three monocations, whereas the CNDO/S-CI calculations were done on a particular monocation. This is substantiated by: (i) the large BWHMH in the absorption spectra, (ii) the contours are spread over N3 to N16 (Fig. 6) and (iii) the observation of different fluorescence excitation spectra recorded at different emission wavelengths. The fluorescence and fluorescence excitation spectral data clearly show the presence of two kinds of monocations in cyclohexane and three kinds of monocations in the polar/aprotic and polar/protic solvents. The lifetime data, wherever available also supported this. In acetonitrile and water, 370 nm fluorescence band can be assigned to the monocation VI, formed by the protonation of the amino group. This is based on the fact that: (i) the blue shift observed in the spectral characteristics lead to the protonation of the –NH2 group [1], (ii) the absorption and fluorescence spectra of VI should resemble with those of 2-phenylbenzimidazole [36] (l abs 299 nm, l fl 370 nm). The fluorescence excitation spectrum recorded at 380 nm shows the presence of only bands at 240 and 290 nm and thus supports this assignment, (iii) the fluorescence intensity ratio (I370/I447) decreases in going from acetonitrile to water. This agrees with the fact that the –NH3⫹ ion becomes stronger acid in S1 [35] and its dissociation will be accelerated in polar/protic solvents. A similar behaviour is also observed when the fluorescence spectrum of 5-aminoindazolium chloride [37,38] was recorded in solvents of different polarity. The absence of this band in cyclohexane could be because of the strong ionic and polar nature of VI (m g 9.85 D) and thus is very unstable in non-polar medium. Because of the low fluorescence intensity of VI, in acetonitrile and water its lifetime could not be measured to substantiate its further presence. The 405 nm fluorescence band in cyclohexane, 445 nm in acetonitrile and 450 nm in water can be assigned to the monocation IV. This is based on the facts that: (i) the long wavelength absorption band maximum at ⬃ 348 nm is very close to the
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fluorescence excitation band maximum observed at ⬃ 350 nm. It is also in agreement with the transitions calculated using CNDO/S-CI calculations (Table 4), (ii) it is very sensitive to the solvent polarity, for example, 405 nm fluorescence band observed in cyclohexane containing 1 × 10 ⫺4 M TFA shifts to 445 nm containing 0.1 M TFA. This is consistent with the fact that the change in the dipole moment of IV on excitation to the S1 state is maximum (Dm 6.18 D), (iii) the fluorescence intensity decreases with the increase of l exc. The changes observed in the fluorescence intensity is more pronounced in case of cyclohexane than that in polar solvents because the 405 nm and ⬃ 470 nm bands are well separated. Lastly ⬃ 475 nm fluorescence band and its corresponding fluorescence excitation spectra (350 nm with a shoulder at ⬃ 390 nm) can be assigned to the monocation V. This is consistent with the fact that: (i) although the agreement between the predicted (Table 4) and the observed band maximum is not very good still it is at the maximum. The absence of the distinct band maximum at ⬃ 390 nm wavelength could be because of the presence of strong band at 350 nm, (ii) the most significant feature is its insensitivity to the environments which agrees nicely with the dipole moment values in the S0 (8.4 D) and S1 (8.2 D) states and (iii) the lifetime analysis shows that the amplitude of the long lived species in cyclohexane and acetonitrile decrease with the increase of the emission wavelength and thus can be assigned to the monocation IV. This is in agreement with the fact that as the l em increases, the relative contribution of the emission spectra because of the monocation IV decreases. Since the long lived lifetime obtained from the double exponential analysis in these solvents agrees with the lifetime obtained from the single exponential analysis, it supports our earlier assignments, whereas the short lived lifetime can be assigned to the monocation V. In contrast in water, based on the aforementioned explanation the short lived species is the monocation IV and the long lived species is the monocation V. It is further concluded that the equilibrium between the two species (IV and V) is not established in the S1 state, because of the observation of the two different lifetimes at all l em. The decrease in the lifetime of IV in going from acetonitrile to water could be either
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because of the solvent induced fluorescence quenching [39,40] or as a result of the increase in the rate of intersystem crossing to the triplet state [41]. The stabilization of the emitting state in polar solvents decreases the gap between the singlet and triplet states. 5. Spectral characteristics of dication Unlike monocation, the dication can have only one kind of species, because there are only two kinds of basic centres. The large blue shift observed in the absorption and the fluorescence spectra of the monocation with increase of acid concentration, similar BWHMH in the absorption and fluorescence spectra of the dication in all three solvents suggest that the dication of 2-APBI is formed by the protonation of the amino group and the –Ny atom. Thus unlike the monocation, the dication can have only one kind of species. Although the formation of dication is complete at the acid concentration used in the S0 state, but –NH3⫹ decomposes on excitation [35] to yield monocation obtained by protonating N3 because it is well known that –NH3⫹ becomes stronger acid on excitation to S1 state. The presence of latter species is confirmed by its fluorescence band and the excitation spectra recorded at the red part of the fluorescence band. This agrees with our earlier results observed in water [23], i.e. the formation of dication is complete at 2.5 M H2SO4 in the S0 state, but requires 9.5 M H2SO4 in the S1 state.
Acknowledgements The authors are thankful to the Department of Science and Technology, New Delhi for the financial support to the project No. SP/S1/H-19/91.
References [1] J.F. Ireland, A.A.H. Waytt, Adv. Phys. Org. Chem. 12 (1976) 160. [2] S.G. Schulman, in: E.L. Wehry (Ed.), Modern Fluorescence Spectroscopy, Plenum Press, New York, 1976. [3] L.G. Arnaut, S.J.J. Formoshino, J. Photochem. Photobiol. A: Chem. 75 (1993) 1.
[4] S.J.J. Formoshino, L.G. Arnaut, J. Photochem. Photobiol. A: Chem. 75 (1993) 21. [5] J.K. Dey, S.K. Dogra, J. Photochem. Photobiol. A: Chem. 66 (1992) 15. [6] M. Krishnamurthy, S.K. Dogra, J. Photochem. Photobiol. A: Chem. 32 (1986) 235. [7] E.L. Roberts, J.K. Dey, I.M. Werner, J. Phys. Chem. 100 (1996) 19681. [8] J.K. Dey, J.L. Haynes III, I. M. Werner, J. Phys. Chem. A 101 (1997) 2271. [9] J.K. Dey, J.L. Haynes III, I. M. Werner, J. Phys. Chem. A 101 (1997) 4872. [10] J.K. Dey, I.M. Werner, J. Photochem. Photobiol. 101 (1996) 21. [11] J.K. Dey, I.M. Werner, J. Lumin. 71 (1997) 105. [12] M. Mosquera, J.C. Penedo, M.C.R. Rodriguez, F. RodriguezPrieto, J. Phys. Chem. 100 (1996) 5398. [13] M. Mosquera, M.C.R. Rodriguez, F. Rodriguez-Prieto, J. Phys. Chem. A 101 (1997) 2766. [14] T. Brinck, J.S. Murray, P. Politzer, J. Org. Chem. 56 (1991) 2934. [15] T. Brinck, J.S. Murray, P. Politzer, Int. J. Quantum Chem., 48 (1993) 73 and references listed therein. [16] J. Spanget-Larsen, J. Chem. Soc. Perkin Trans. 2 (1985) 417. [17] J. Waluk, W. Rettig, J. Spanget-Larsen, J. Phys. Chem. 92 (1988) 6930. [18] J. Spanget-Larsen, J. Phys. Org. Chem., 8 (1995) 496 and references listed therein. [19] P. C. Mishra, B. P. Asthana, Quantum Chemistry, QCPE Bull. Programme No. QCMP 039, 7 (1987) 176. [20] J.K. Dey, S.K. Dogra, Chem. Phys. 149 (1990) 97. [21] J.K. Dey, S.K. Dogra, Bull. Chem. Soc. Jpn. 64 (1991) 3142. [22] J.K. Dey, S.K. Dogra, J. Phys. Chem. 98 (1994) 3638. [23] A.K. Mishra, S.K. Dogra, J. Photochem. 31 (1985) 333. [24] S. Santra, S.K. Dogra, Chem. Phys. 226 (1998) 285. [25] J.A. Riddick, W.B. Bunger, Techniques in Organic Chemistry – Organic Solvents, Wiley Interscience, New York, 1970 p. 592, 798. [26] A.K. Mishra, M. Swaminathan, S.K. Dogra, J. Photochem. 26 (1984) 49. [27] M. Swaminathan, S.K. Dogra, Indian J. Chem. 22A (1983) 853. [28] C.A. Parker, Photoluminescence of Solutions with Applications to Photochemistry and Analytical Chemistry, Elsevier, Amsterdam, 1968. [29] R.S. Sarpal, S.K. Dogra, J. Chem. Soc. Trans Faraday I 88 (1992) 2725. [30] M. Mataga, T. Kubota, Molecular Interactions and Electronic Spectra, Marcel Dekker, New York, 1970. [31] J.F. Letard, R. Lapouyado, W. Rettig, Chem. Phys. Lett. 222 (1994) 209. [32] J.C. Tai, N.L. Allinger, J. Am. Chem. Soc. 110 (1988) 2050. [33] J. Delbene, H.H. Jaffe, J. Chem. Phys. 48 (1968) 1801. [34] M.J.S. Dewar, E.G. Zoebish, E.F. Healy, J.J. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [35] H. Shizuka, Acc. Chem. Res. 18 (1985) 141.
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[39] M. Sow, G. Durocher, J. Photochem. Photobiol. A: Chem. 59 (1990) 349. [40] S.K. Saha, S.K. Dogra, J. Photochem. Photobiol. A: Chem. 110 (1997) 257. [41] K. Bhattacharyay, M. Chowdhury, Chem. Rev. 93 (1993) 507.
Spectral characteristics of the monocations of 2-(2 -aminophenyl)benzimidazole in different solvents 0
Swadeshmukul Santra, Sneh K. Dogra* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Received 3 August 1998; received in revised form 13 October 1998; accepted 13 October 1998
Abstract The absorption and fluorescence spectra of 2-(2 0 -aminophenyl)benzimidazole (2-APBI) were studied in three solvents and at different concentrations of acids. Trifluoroacetic acid was used as an acid in cyclohexane and sulphuric acid was used to control the acid concentrations for acetonitrile and water. The broadness of the long wavelength absorption band of the monocation in three solvents indicates that the monocation is either existing in different isomeric forms or it is formed by protonating different basic centres. Fluorescence and fluorescence excitation spectra of the monocation recorded at different emission wavelengths establish the presence of two forms of the monocations in cyclohexane and three forms of the monocations in acetonitrile and water. Semi-empirical quantum mechanical calculations were carried out to confirm the presence of different forms of the monocations. 䉷 1999 Elsevier Science B.V. All rights reserved. Keywords: Monocations of 2-APBI; Spectral characteristics; Semi-empirical calculations
1. Introduction Acid–base properties of aromatic compounds, especially heterocyclic molecules containing more than one heteroatom are always of interest in chemistry and biochemistry [1–13]. This property in general is associated with the so called non-bonding pair of electrons at the heteroatoms. The heteroatom can be a part of the aromatic ring (–Ny) or it can be present in the substituent (–OH, –NH2 etc.) of the aromatic ring. The presence of the lone pair of electrons is reflected by the basic character of many of these compounds and their ability to participate in hydrogen bonding. Various attempts have been made to establish a correlation between the electronic structure of aza aromatic molecules and the acidity or * Corresponding author. Tel.: ⫹ 91-512-597-163; Fax: ⫹ 91512-590-007; e-mail: [email protected]
basicity of the basic centre. Of these, the successful ones are the concept of: (i) the average local ionization energy on the molecular surface [14,15], (ii) the effective valence electron potential as a reactivity index for the protonation reaction [16–18]. In this method the reactivity index reflects the local ionization, serving as a measure of nucleophilicity of the basic centre and (iii) the electrostatic potential energy mapping [19]. This method also considers the charge density at a particular basic centre of interest plus the effect of charges of rest of the atoms in the molecule. The greater the depth of potential well the greater is the chances of protonation on the basic centre. In our recent sudies on 2-(4 0 -aminophenyl)benzoxazole (4-APBO) [20], 2-(4 0 -aminophenyl)benzothiazole (4-APBT) [21] and others [22], we observed that the monocations are formed by either protonating the –Ny atom or the amino group. The red and blue shifts observed in the spectral characteristics of these
0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(98)00754-6
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Fig. 1. Charge densities at different basic centres and dipole moments of different species of 2-APBI in S0 and S1 states, ground state energies and dihedral angle f .
compounds in the acidic medium are because of formation of the monocation by the protonation of the –Ny atom and the –NH2 group respectively [22]. In our recent study on 2-(2 0 -aminophenyl)benzimidazole (2-APBI) [23,24], it was confirmed that
there are two different kinds of rotamers (I and II), which are nearly equally stable. The activation barrier for the conversion of II to I is only 2.71 kJ mol ⫺1 in the lowest singlet state. The tautomer III is highly unstable (Fig. 1). Looking at the two rotamers and
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171
Table 1 Absorption maxima (nm) and log 1 max ( in parenthesis ) of 2-APBI (5 × 10 ⫺5 M) in its neutral, monocationic and dicationic forms, at 298 K Solvent
Species N abs l max (log 1 max)
Cyclohexane 360 (sh) 348 (4.05) 299 (3.98) 291 (3.98) 288 (sh) 278 (3.88) 252 (4.12)
Acetonitrile 345 (4.15) 297 (4.14) 289 (4.10) 285 (sh) 278 (sh) 251 (4.23) Water 322 (3.60) 295 (sh) 288 (3.70) 275 (sh) 242 (3.70)
MC abs l max (log 1 max)
DC abs l max (log 1 max)
[TFA] 1 × 10 ⫺4 M 360 (sh) 348 (3.58) 330 (sh) 298 (3.66) 291 (3.68) 286 (sh) 278 (sh) 252 (3.70)
[TFA] 0.1 M 284 (4.22)
[H2SO4] 1 × 10 ⫺4 M 348 (3.91) 288 (4.17)
[H2SO4] 1.0 M 282 (4.23)
pH 2.0 340 (3.82) 285 (4.17)
[H2SO4] 2.5 M 284 (4.53)
the tautomer, three possible structures of the monocations can be written (Fig. 1). The spectral characteristics of 2-APBI were studied in three different solvents (cyclohexane, acetonitrile and water) containing different amounts of acids. The absorption, fluorescence and flourescence excitation spectra and the lifetimes measured at different emission wavelengths were used to establish these three monocations. Semi-empirical quantum mechanical calculations were also carried out to supplement the above assignment.
2. Materials and methods 2-APBI was procured from Aldrich Chemical Company, UK and was purified by recrystallization from methanol. The purity was checked by a single spot on TLC and by the production of similar fluorescence bands on excitation with different wavelengths. Cyclohexane (SD Fine Chemicals) and
acetonitrile (E. Merck) were of AnalR grade and were further purified as described in the literature [25]. Triple distilled water was used to prepare aqueous solutions. The pH of the aqueous solutions in the range of 3–10 was adjusted by adding required amounts of o-phosphoric acid and sodium hydroxide, as such an amount of buffers does not quench the fluorescence intensities of the investigated species [26]. Trifluoroacetic acid (TFA) was used as an acid in cyclohexane, and H2SO4 was used to control the acid concentration in acetonitrile and water. The absorption spectra were recorded on a Shimadzu UV-190 spectrophotometer and fluorescence excitation spectra were recorded on PerkinElmer luminescence spectrofluorometer, model LS 50B, equipped with a WIPRO Genius PC 386X (Slit width used for both excitation and emission was 2.5 nm). Fluorescence spectra were recorded on a recording spectrofluorometer, fabricated in our laboratory. Details are available elsewhere [27]. Fluorescence spectra were corrected according to
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Fig. 2. Absorption spectra of 2-APBI at different acid concentrations in (A) cyclohexane ⫹ TFA, (B) acetonitrile ⫹ H2SO4; (a): 0.0 M, (b): 1 × 10 ⫺5M, (c): 5 × 10 ⫺5 M, (d): 1 × 10 ⫺4 M, (e): 1 × 10 ⫺3 M, (f): 5 × 10 ⫺3 M, (g): 1 × 10 ⫺2 M, (h): 1 × 10 ⫺1 M and (C) water at various pH; (a) 2.0, (b) 3.0, (c) 4.0, (d) 4.2, (e) 4.6, (f) 5.0, (g) 6.0, (h) 7.0; [2-APBI] 2 × 10 ⫺5M.
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173
Fig. 2. (continued)
the procedure suggested by Parker [28] and fluorescence quantum yields mentioned in the text are relative to each other (details given in the text). The excited state lifetimes were measured on a time-correlated single photon counting nanosecond spectrofluorimeter supplied by Applied Photophysics Limited. Details are available in our recent paper [29]. Semi-empirical quantum mechanical calculations were carried on different species and details are given later on. The data are compiled in Fig. 1. The energy of the solvated states may be estimated by taking into account the dipole–dipole interactions between the dipole moment (m ) and the solvent according to [30,31]: h i E solv E ⫺ m2 =4p10 a3 f 1
1
where a is the Onsager’s cavity radius and f 1
1 ⫺ 1 21 ⫹ 1
2
where 1 is the static dielectric constant of the solvent and E is the energy of the state under isolated conditions. The effective electrostatic potential at different basic centres has been calculated using the VSSPC computer program [19].
3. Results 3.1. Absorption spectra The absorption spectra of 2-APBI were recorded at different acid concentrations in cyclohexane, acetonitrile and water. The results are compiled in Table 1. Figs. 2a–c depict the effect of acid concentrations on the absorption spectrum of 2-APBI in cyclohexane, acetonitrile and water respectively. The isosbestic points observed in the monocation-neutral equilibrium in these solvents are 378, 370 and 342 nm respectively in the same order. As the concentration of TFA is increased in cyclohexane containing 2APBI, the absorbance of both the band maxima (i.e. at ⬃ 348 nm and at the structured band system at ⬃290 nm) decrease (Fig. 2A). The structure in the 290 nm band is lost and ⬃ 348 nm band becomes very broad at TFA concentration equal to 1 × 10 ⫺4 M and 1 × 10 ⫺3 M. At TFA concentration equal to 0.1 M, only one band at 284 nm is observed. The absorption spectra of 2-APBI in acetonitrile at H2SO4 concentration of 1 × 10 ⫺4 M and 1.0 M (Fig. 2B) and in water at pH equal to 2.0 and at H2SO4 concentration of 2.5 M (Fig. 2C), are all very similar. At lower acid concentration, the long wavelength band is red shifted and the structure is lost in the short wavelength band. At high acid concentration
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Table 2 flu of 2-APBI (5 × 10 ⫺5 M) at different excitation wavelengths in the following solvents at 298 K Fluorescence band maxima l max Solvents
l exc (nm)
Species l max (N)
flu l max (MC)
flu l max (DC)
Cyclohexane
290 310 350 375 390 410
383 384 386, 527 ^ 2 — — —
409, 472 408, 471 405, 462 417, 471 —, 472 —, 475
369 ^ 2, 455 370 ^ 2, 448 466 ^ 2 — — —
Acetonitrile
290 310 350 370 390 400 410 420
399, 525 ^ 10 399, 522 ^ 10 400, 518 ^ 05 401, 523 ^ 05 —, — —, — —, — —, —
449, 370 449, 370 445, 370 447, — 460, — 467, — 472, — 473, —
386 389 — — — — — —
Water
287 305 325 342 370 430
— 412 — 413 — —
375, 452 375, 455 446, 475 (sh) 464 474 474
391 — — — — —
only one large blue shifted band is observed at 283 ^ 1 nm. The broadness of the long wavelength band of the monocation in cyclohexane as depicted by the band width at half the maximum height (BWHMH) is 5770 cm ⫺1 and decreases to 4920 cm ⫺1 in acetonitrile. The BWHMH observed for the dication is the same (4880 cm ⫺1) in all the three solvents. The pKa value for the monocation–neutral equilibrium in water is found to be 4.2 agreeing with earlier results [23]. 3.2. Fluorescence spectra Fluorescence spectra of 2-APBI were also studied in three different solvents at different acid concentrations and by exciting at different wavelengths (l exc). The relevant data are compiled in Table 2. From the data of Table 2 and Fig. 3A, it is obvious that with cyclohexane as solvent and a TFA concentration 1 × 10 ⫺4 M, dual fluorescence is observed from the monocation. The fluorescence intensities of the two bands observed depend upon l exc. With l exc in the range of 290–350 nm, fluorescence bands are observed with maxima at 407 ^ 2 and 467 ^ 5 nm respectively,
whereas at l exc greater than 350 nm only the long wavelength band is present. In contrast, at TFA concentration 1 × 10 ⫺3 M, only one fluorescence band (472 ^ 1 nm, Fig. 3B) is observed when l exc is in the range of 290–410 nm, but BWHMH are different for different l exc i.e. 3410, 3480 and 3030 cm ⫺1 at l exc 290, 350 and 410 nm. From our earlier study [23,24] it is known that dual fluorescence is observed from neutral 2-APBI in cyclohexane. 384 nm emission band is assigned to the normal fluorescence and 530 nm band is assigned to the tautomer fluorescence. Thus, in comparison to the normal fluorescence of neutral 2-APBI both the fluorescence bands of the monocation of 2-APBI are red shifted, whereas when excited at 410 nm, the fluorescence band of the monocation is red shifted with respect to the short wavelength emission (384 nm) and blue shifted in comparison to the long wavelength emission (530 nm) of the neutral 2-APBI. The fluorescence quantum yield of the short wavelength band decreases and that of the long wavelength band increases with increasing l exc, while BWHMH of both the bands remain unchanged.
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Fig. 3. Fluorescence spectra of 2-APBI at different excitation wavelengths in cyclohexane; (A): [TFA] 1 × 10 ⫺4 M and (B): [TFA 1 × 10 ⫺3 M; [2-APBI] 2 × 10 ⫺5 M.
In acetonitrile as solvent and H2SO4 of concentration 1 × 10 ⫺4 M, we observed two fluorescence bands (370 ^ 2 nm, 447 ^ 2 nm) for l exc in the range 290– 350 nm, one fluorescence band 447 ^ 2 nm when l exc is between 350–370 nm and another band at 470 ^ 3 nm for l exc greater than 390 nm (Fig. 4A). The 370 nm band is blue shifted and the other two bands red shifted in comparison to the short wavelength emission (400 nm) of the neutral 2-APBI. All these bands are blue shifted in comparison to the long wavelength fluorescence band (520 nm) of neutral 2-APBI. The fluorescence quantum yields of the bands at 370 and 447 nm decrease with increase of l exc and that of the 470 nm band increases with increase of l exc. A similar behaviour is also observed when 2-APBI is
excited in water at pH 2 (Fig. 4B). The only difference is that If 370=If 447 0:18 in acetonitrile compared to 0.083 in water. At high acid concentrations (0.1 M TFA), two fluorescence bands are observed in cyclohexane when excited at 290 and 310 nm, whereas only one band is present when excited at l exc 350 nm. In contrast, only one fluorescence band (389 ^ 3 nm) is observed in acetonitrile containing 1.0 M H2SO4 and water at 2.5 M H2SO4. Based on these results observed in cyclohexane, the long wavelength band is assigned to the monocation and the short wavelength band to a dication formed by the protonation of the –NH2 group and the –Ny atom of 2-APBI. The long wavelength band (445 nm) assigned to the
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Fig. 4. Fluorescence spectra of 2-APBI at different excitation wavelengths in (A): acetonitrile, [H2SO4] 1 × 10 ⫺4 M and (B): water at pH 2; [2-APBI] 2 × 10 ⫺5 M.
monocation of 2-APBI in cyclohexane at TFA concentration 0.1 M does not agree with the one observed at TFA concentration 1 × 10 ⫺4 M (407 nm) but it agrees nicely with the one observed at H2SO4 concentration 1 × 10 ⫺4 M in acetonitrile. This observation could be because of the fact that the polarity of cyclohexane at TFA concentration 0.1 M may resemble with that of acetonitrile containing 1 × 10 ⫺4 M H2SO4. The assignment of the long wavelength band to the monocation is further substantiated with the fact that its fluorescence band maximum and its fluorescence intensity depend upon l exc.
3.3. Fluorescence excitation spectra The fluorescence excitation spectra of 2-APBI were recorded at different emission wavelengths in cyclohexane containing 1 × 10 ⫺4 M and 1 × 10 ⫺3 M TFA. The fluorescence excitation spectra recorded at two acid concentrations are exactly similar to each other. Fig. 5A represents the fluorescence excitation spectra recorded at 400 and 500 nm emission and the absorption spectra of the monocation at 1 × 10 ⫺3 M TFA. The fluorescence excitation spectra recorded at 400 and 425 nm are similar to each other, having broad
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177
Fig. 5. Fluorescence excitation spectra of 2-APBI in (A): cyclohexane at [TFA] 1 × 10 ⫺3 M, (B): acetonitrile at [H2SO4] 1 × 10 ⫺4 M and (C): water at pH 2; [2-APBI] 2 × 10 ⫺5M.
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Fluorescence excitation spectra recorded in water (Fig. 5C) at pH 2 and at the above mentioned fluorescence wavelengths are exactly similar to those observed in acetonitrile. The absorption spectra of the monocation in all the solvents do not show any band or shoulder at 390 nm but the tail is extended towards red. This could be because of the monocation V (see later). As the fluorescence excitation spectrum is more sensitive and represents different species present in the system, the 390 nm band could be buried under the strong band at 350 nm of the absorption spectrum of the monocation. The fluorescence excitation spectra in cyclohexane (0.1 M TFA) were recorded in the range of ⬃ 370– 390 and 440–470 nm. The former are similar to each other and resemble with the absorption spectrum of the dication and the latter resemble with the excitation spectra of the monocation. Whereas in acetonitrile (1 M H2SO4) and water 2.5 M H2SO4),
band around 340 nm and structured band around 278– 300 nm. Similarly, the fluorescence excitation spectra recorded at 480, 500, 520 and 540 nm emission wavelengths resemble each other depicting large broad bands at ⬃ 290 and at ⬃ 355 nm with a shoulder at ⬃ 390 nm, whereas fluorescence excitation spectra recorded at 455 nm is a mixture of two. In case of acetonitrile containing 1 × 10 ⫺4 M H2SO4, the fluorescence excitation spectra of the monocation of 2-APBI were recorded at the following emission wavelengths: 380, 420, 440, 470, 485, 500, 520 and 540 nm. Fig. 5B shows only the fluorescence excitation spectra recorded at 380, 440 and 540 nm, along with absorption spectra. The fluorescence excitation spectrum recorded at 380 nm gives two main bands at 290 and 240 nm. At all other emission wavelengths less than 470 nm an additional band is observed at ⬃ 350 nm. Above 470 nm, a shoulder appears at ⬃ 390 nm in addition to 350 nm band.
Table 3 Excited singlet state lifetime (ns) of the monocations of 2-APBI measured in different solvents and at different emission wavelengths
l em (nm)
Single exponential decay t x2
t1
Double exponential decay A1 t2
A2
x2
Cyclohexane ⫹ [TFA] 1 × 10 ⫺4 M, l exc 354 nm 380 420 460 500
3.0 4.3 5.2 6.7
1.13 1.30 1.63 1.80
3.0 2.95 3.2 3.1
96.4 56.3 38.2 20.4
2.0 2.0 1.95 1.96
3.6 43.7 61.8 79.2
1.13 1.18 1.08 1.10
Acetonitrile ⫹ [H2SO4] 1 × 10 ⫺4 M, l exc 354 nm 400 420 460 500
2.88 2.84 2.66 2.80
1.96 1.27 1.32 1.28
3.33 3.66 3.60 3.56
68.5 55.5 36.0 28.9
1.81 1.83 2.03 1.95
31.5 44.5 64.0 71.1
0.94 1.17 1.20 0.96
Water at pH 2.0, l exc 354 nm 430 460 500
0.77 1.16 6.50
1.30 4.00 4.60
0.57 0.50 0.58
90.1 72.0 52.1
10.90 10.60 11.90
9.9 28.0 47.9
0.90 1.09 1.10
Water at pH 2.0, l exc 310 nm 430 460 500
2.10 3.10 5.80
2.11 5.10 4.20
0.80 0.68 0.80
75.7 55.2 46.1
10.50 10.20 11.00
24.3 44.8 53.9
1.10 1.21 1.10
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Table 4 Spectral transitions (l , nm) and their oscillator strengths (f) of the monocation of 2-APBI, predicted by CNDO/S-CI
not be measured because of its low intensity as well as the low intensity of the flash lamp at 290 nm.
Monocations IV l f
3.5. Quantum mechanical calculations V l
347 307 275 260 241 225
353 307 289 260 241 236
0.223 0.070 0.031 0.012 0.010 0.067
f
VI l
f
0.353 0.028 0.060 0.018 0.021 0.015
313 292 277 261 245 226
0.186 0.015 0.071 0.057 0.023 0.028
the fluorescence excitation spectra were recorded only at emission wavelengths in the range of ⬃ 370– 390 nm and resemble with the absorption spectra of the dication in these solvents.
3.4. Excited state lifetimes Excited state lifetimes of the monocations of 2APBI were measured at TFA concentration 1 × 10 ⫺4 M in cyclohexane, H2SO4 concentration 1 × 10 ⫺4 M in acetonitrile and at pH 2 in water. In acetonitrile and cyclohexane the species were excited at 354 nm, whereas in water, the l exc used were 310 and 354 nm. The decay curves were observed at different emission wavelengths as mentioned in Table 3. The fluorescence decay followed a single exponential when measured at l em 380 nm in cyclohexane. This is substantiated by the fact that the x 2 observed for a single exponential and a double exponential analysis were similar and that the major (90%) amplitude in the double exponential analysis is the same as found with only one exponential. The fluorescence decay followed a double exponential when measured at l em 460 and 500 nm. The amplitude of the short lived species (2 ns) increases from 3.6% to 79.2%, whereas the amplitude of the long lived species (3 ns) decreases with the increase of l em. A similar behaviour is also observed in acetonitrile, except that the amplitude of the species with a lifetime 2 ^ 0.2 ns increases from 31% to 70% with increase of l em. In case of water, the x 2 values correspond to a double exponential decay at all three l em and at both l exc, except that the amplitudes are different (Table 3). The lifetime of the species emitting at 370 nm could
2-APBI in its neutral form can be depicted by two rotamers (I and II) and one tautomer (III) as shown in Fig. 1. It has been shown earlier [24] and it is also depicted in Fig. 1 that I and II are nearly of similar stability in the ground state and that the barrier for interconversion between rotamer II and I is 2.71 kJ mol ⫺1, whereas the tautomer III is 99 kJ mol ⫺1 less stable. This indicated that rotamers I and II are both present at room temperature. Three possible monocations are shown in Fig. 1. The ground state geometries of the three monocations were first optimized using the MM2 force field [32]. A more precise geometry was then obtained using AM1 [33] (MOPAC, QCPE program number 455, version 4.0). The values of energies (Ei), dipole moment (m g) and charge densities at the basic centres obtained from these calculations are compiled in Fig. 1. Transition energies DEi!j for the three different monocations were calculated using CNDO/S-CI method [34]. 64 singly excited configurations were generated by exciting one electron from the eight highest occupied molecular orbitals to the eight lowest unoccupied molecular orbitals. The calculated values of the dipole moment and the charge densities at the basic centres in S1 state are included in Fig. 1. The transition wavelengths are given in Table 4. As free molecules, the species V and VI are predicted to be 63.8 and 90.97 kJ mol ⫺1 less stable than species IV. The relative stability of species V and VI changes to 36.5 and 57 kJ mol ⫺ respectively when solvated by methanol. The solvation energies were calculated using Eq. (1). For V and VI the solvent stabilization is considerably large because of the greater dipole moment of these species. The low stability of VI is because of the absence of a hydrogen bonding interaction which reflects in a large dihedral angle (f ) as shown in Fig. 1 (67⬚). In V, the f is small (23.7⬚) but the intramolecular hydrogen bond distance ˚ ) than in IV. Thus as a free (N3 –H17) is larger (2.48 A molecule (non-polar solvents) major contribution for all the monocationic species will be from IV, but the contribution from V and VI will increase with the increase in the polarity of the medium.
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Fig. 6. Electrostatic potential map for 2-APBI in the ground state (values shown in kJ mol ⫺1).
We have constructed the electrostatic potential energy map for both the rotamers I and II by using VSSPC computer program [19] as shown in Fig. 6. It is found that in S0 state and in the molecular plane (Z 0.0), both the rotamers I and II give rise to identical results. The value of absolute minima (depth of the well) is 118.04 kJ mol ⫺1 and it is located near the – Ny atom (Fig. 6). The contours are spread over N16 and N3 indicating that the chances of protonating N3 are larger than that of N16. In the S1 state, rotamers I and II behave differently with respect to the depth of the potential well, although the location of the absolute minima still remains near the N3 atom in the molecular plane. In rotamer I, the well is located almost in between N16 and N3 atoms, indicating the equal chances of protonation, although structure protonated at N3 will be more stable than that at N16. This indicates that even if the protonation occurs at N16, proton will be transferred to N3 via ESIPT to get more stable structure. The depths of the potential well for I and II are ⫺183.3 and ⫺149.4 kJ mol ⫺1 respectively. Although we have taken the fully optimized geometry of the
ground state, the present calculations for the S1 state might be little less reliable because of the change of geometry in the S1 state.
4. Discussion Based on the results, obtained using semi-empirical quantum mechanical calculations (Fig. 1) and those obtained from the electrostatic potential energy mapping (Fig. 6), it is clear that the monocation IV will be the most stable and will be the predominant species in the solution phase as a free molecule (in cyclohexane). The red shift observed in the long wavelength absorption spectra, in comparison to the neutral 2-APBI, is consistent with the fact that protonation of the –Ny atom leads to the red shift in the absorption and fluorescence spectra if p ! p * is the lowest energy transition [1]. But the spectral transitions obtained using the CNDO/S-CI calculations do not agree with the absorption spectra (specially the long wavelength band) of the monocation of 2-APBI observed in any of the three solvents. This could be a
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result of the presence of different monocations present in the solutions formed by protonating different basic centres. Thus the absorption spectra recorded at moderate acid concentrations are the composite spectra of the three monocations, whereas the CNDO/S-CI calculations were done on a particular monocation. This is substantiated by: (i) the large BWHMH in the absorption spectra, (ii) the contours are spread over N3 to N16 (Fig. 6) and (iii) the observation of different fluorescence excitation spectra recorded at different emission wavelengths. The fluorescence and fluorescence excitation spectral data clearly show the presence of two kinds of monocations in cyclohexane and three kinds of monocations in the polar/aprotic and polar/protic solvents. The lifetime data, wherever available also supported this. In acetonitrile and water, 370 nm fluorescence band can be assigned to the monocation VI, formed by the protonation of the amino group. This is based on the fact that: (i) the blue shift observed in the spectral characteristics lead to the protonation of the –NH2 group [1], (ii) the absorption and fluorescence spectra of VI should resemble with those of 2-phenylbenzimidazole [36] (l abs 299 nm, l fl 370 nm). The fluorescence excitation spectrum recorded at 380 nm shows the presence of only bands at 240 and 290 nm and thus supports this assignment, (iii) the fluorescence intensity ratio (I370/I447) decreases in going from acetonitrile to water. This agrees with the fact that the –NH3⫹ ion becomes stronger acid in S1 [35] and its dissociation will be accelerated in polar/protic solvents. A similar behaviour is also observed when the fluorescence spectrum of 5-aminoindazolium chloride [37,38] was recorded in solvents of different polarity. The absence of this band in cyclohexane could be because of the strong ionic and polar nature of VI (m g 9.85 D) and thus is very unstable in non-polar medium. Because of the low fluorescence intensity of VI, in acetonitrile and water its lifetime could not be measured to substantiate its further presence. The 405 nm fluorescence band in cyclohexane, 445 nm in acetonitrile and 450 nm in water can be assigned to the monocation IV. This is based on the facts that: (i) the long wavelength absorption band maximum at ⬃ 348 nm is very close to the
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fluorescence excitation band maximum observed at ⬃ 350 nm. It is also in agreement with the transitions calculated using CNDO/S-CI calculations (Table 4), (ii) it is very sensitive to the solvent polarity, for example, 405 nm fluorescence band observed in cyclohexane containing 1 × 10 ⫺4 M TFA shifts to 445 nm containing 0.1 M TFA. This is consistent with the fact that the change in the dipole moment of IV on excitation to the S1 state is maximum (Dm 6.18 D), (iii) the fluorescence intensity decreases with the increase of l exc. The changes observed in the fluorescence intensity is more pronounced in case of cyclohexane than that in polar solvents because the 405 nm and ⬃ 470 nm bands are well separated. Lastly ⬃ 475 nm fluorescence band and its corresponding fluorescence excitation spectra (350 nm with a shoulder at ⬃ 390 nm) can be assigned to the monocation V. This is consistent with the fact that: (i) although the agreement between the predicted (Table 4) and the observed band maximum is not very good still it is at the maximum. The absence of the distinct band maximum at ⬃ 390 nm wavelength could be because of the presence of strong band at 350 nm, (ii) the most significant feature is its insensitivity to the environments which agrees nicely with the dipole moment values in the S0 (8.4 D) and S1 (8.2 D) states and (iii) the lifetime analysis shows that the amplitude of the long lived species in cyclohexane and acetonitrile decrease with the increase of the emission wavelength and thus can be assigned to the monocation IV. This is in agreement with the fact that as the l em increases, the relative contribution of the emission spectra because of the monocation IV decreases. Since the long lived lifetime obtained from the double exponential analysis in these solvents agrees with the lifetime obtained from the single exponential analysis, it supports our earlier assignments, whereas the short lived lifetime can be assigned to the monocation V. In contrast in water, based on the aforementioned explanation the short lived species is the monocation IV and the long lived species is the monocation V. It is further concluded that the equilibrium between the two species (IV and V) is not established in the S1 state, because of the observation of the two different lifetimes at all l em. The decrease in the lifetime of IV in going from acetonitrile to water could be either
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because of the solvent induced fluorescence quenching [39,40] or as a result of the increase in the rate of intersystem crossing to the triplet state [41]. The stabilization of the emitting state in polar solvents decreases the gap between the singlet and triplet states. 5. Spectral characteristics of dication Unlike monocation, the dication can have only one kind of species, because there are only two kinds of basic centres. The large blue shift observed in the absorption and the fluorescence spectra of the monocation with increase of acid concentration, similar BWHMH in the absorption and fluorescence spectra of the dication in all three solvents suggest that the dication of 2-APBI is formed by the protonation of the amino group and the –Ny atom. Thus unlike the monocation, the dication can have only one kind of species. Although the formation of dication is complete at the acid concentration used in the S0 state, but –NH3⫹ decomposes on excitation [35] to yield monocation obtained by protonating N3 because it is well known that –NH3⫹ becomes stronger acid on excitation to S1 state. The presence of latter species is confirmed by its fluorescence band and the excitation spectra recorded at the red part of the fluorescence band. This agrees with our earlier results observed in water [23], i.e. the formation of dication is complete at 2.5 M H2SO4 in the S0 state, but requires 9.5 M H2SO4 in the S1 state.
Acknowledgements The authors are thankful to the Department of Science and Technology, New Delhi for the financial support to the project No. SP/S1/H-19/91.
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