Spectral characteristics of the methylated derivatives of 2-(2′-aminophenyl)benzimidazole: effects of solvents

Journal of Molecular Structure 559 (2001) 25–39 www.elsevier.nl/locate/molstruc

Spectral characteristics of the methylated derivatives of 2-(2 0 -aminophenyl)benzimidazole: effects of solvents S. Santra, G. Krishnamoorthy, S.K. Dogra* Department of Chemistry, Indian Institute of Technology, Kanpur-208016, India Received 25 April 2000; revised 30 May 2000; accepted 30 May 2000

Abstract Absorption, fluorescence and fluorescence excitation spectra of 2-(2 0 -N,N-dimethylaminophenyl)-N-methylbenzimidazole (TMAPBI), 2-(2 0 -N,N-dimethylamino phenyl)benzimidazole (2-DMAPBI) and 2-(2 0 -N-methylaminophenyl)-N-methyl benzimidazole (2-N(Me)(sNMe)DMAPBI), have been studied in different solvents. Absorption spectra and the semi-empirical quantum mechanical calculations establish the presence of two rotamers in each molecule. Besides the short wavelength (SW) fluorescence band, a long wavelength (LW) emission is observed as a tail to the SW emission band in each molecule. Band maxima of both the emissions are red shifted and the fluorescence quantum yield increased with the increase in the polarity and hydrogen bonding nature of the solvents. Single exponential decay observed in the SW emission in non-polar solvents suggests that the equilibrium is established between the two rotamers of each molecule. Biexponential decay in the SW emission in polar solvents indicates that the equilibrium is not established between the two rotamers in the S1 state. These results are substantiated by the semi-empirical quantum mechanical calculations. The SW emission is assigned to rotamer I and the tail part of the emission is assigned to rotamer IV. 䉷 2001 Elsevier Science B.V. All rights reserved. Keywords: Spectral characteristics; Methyl derivatives of 2-APBI; Semi-empirical calculations; Dual emission

1. Introduction In the recent past, lot of interest has been developed in studying the photophysics of the excited state intramolecular proton transfer (ESIPT) reactions [1–6]. This is not only because of its importance in understanding the ultrafast reactions at the molecular level [7–9], but the molecules undergoing ESIPT process find lot of applications ranging from polymer UV stabilizers [10,11] and laser dyes [12–16] to photochromic materials [17]. The fundamental requirement for the ESIPT process is the presence of intramole* Corresponding author. Tel.: ⫹91-512-597163; fax: ⫹91-512590007. E-mail address: [email protected] (S.K. Dogra).

cular hydrogen bonding between the acidic proton and the basic moiety and the suitable geometry of the molecular system. The acidic groups mostly used are –OH, –NH2, etc. and the basic centres are yN–, carbonyl oxygen. Large number of potential molecules [18–32] undergoing ESIPT are available in the literature and have been studied using absorption, fluorescence, fluorescence excitation and time correlated fluorescence spectroscopy under different environments, e.g. solvents, temperature variation, etc. Theoretical calculations have also been carried out to substantiate the experimental observations. In non-polar solvents, two rotamers and one tautomer have been established to explain the dual fluorescence observed in molecules undergoing ESIPT [2–6] reactions, rotamer I leads to the normal

0022-2860/01/$ - see front matter 䉷 2001 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(00)00684-0

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Scheme 1. Fig. 1. Different rotamers and tautomer of 2-HPBI and molecule 2-APBI.

fluorescence (SW) and tautomer III, formed by the ESIPT in rotamer II leads to the tautomer emission (LW). Some of these molecules are depicted in Fig. 1. It is further established that: (i) the energy of tautomer III is lower than that of rotamer II [19,20] in the S1 state; (ii) the activation barrier for the excited state proton transfer is very small [7,8]; and (iii) the tautomer is having more or less a planar structure. In polar and protic solvents rotamer IV is also formed at the expense of both the rotamers, especially for II. Our laboratory is actively involved for a long time in synthesizing the molecules showing ESIPT behaviour and studying their photophysics [18,27,33–38]. Recently, our study [38] on 2-(2 0 -aminophenyl)benzimidazole (2-APBI) has shown that the energy of the first excited singlet state of the tautomer of type III is higher than that of rotamer II by ⬃96 kJ mol ⫺1 and thus ESIPT process is endothermic. On the other hand, semi-empirical quantum mechanical calculations using the configuration interactions have shown that the energy of the S1 twisted form (rotamer V) is lower than that of rotamer

II. Thus LW fluorescence band is observed from the twisted rotamer V, formed by following the sequence II ! III ! V. The involvement of this sequence is necessary because in polar/protic solvents, the band maximum of the LW emission gets blue shifted and its fluorescence quantum yield decreases with the increase in the hydrogen bonding nature of solvents, completely absent in water. This is only possible because the intramolecularly hydrogen bonding structure II is replaced by the intermolecular hydrogen bonding structure IV. This was substantiated by the agreement between the predicted energy of the transition S1 ! S0 and the location of the LW peak in the fluorescence spectrum in cyclohexane. The present study has been carried out keeping in mind the following two points: (i) is it necessary to have intramolecular hydrogen bonding in the ground state to observe the LW tautomer fluorescence band?; (ii) if it is not so and the LW fluorescence band is observed, what is its origin and what are its spectral characteristics? We have synthesized three methylated derivatives of 2-APBI, as shown in Scheme 1 and have studied the absorption, fluorescence, fluorescence excitation and time correlated fluorescence spectroscopic studies in different environments. The

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Table 1 Calculated characteristics of the ground state and first excited singlet state of different molecules using AM1 method (MOPAC 6.0) Parameters

2-DMAPBI S0 state E (eV) E (eV) (solvated) r [C9 –C10 (nm)] f [C11 –C10 –C9 –N8, (⬚)] m g (D) Charge density N7 N8 N16 S1 state E (eV) E (eV) (solvated) r [C9 –C10 (nm)] f [C11 –C10 –C9 –N8 (⬚)] m e (D) Charge density N7 N8 N16 2-DMAPBI S0 state E (eV) E (eV) (solvated) r [C9 –C10 (nm)] f [C11 –C10 –C9 –N8 (⬚)] m g (D) 2-N(Me)(sNMe)DMAPBI S0 state E (eV) E (eV) (solvated) r [C9 –C10 (nm)] r (Hydrogen bond) (nm) f [C11 –C10 –C9 –N8 (⬚)] m g (D)

Rotamers IV 0

IV

I

I0

⫺2745.4517 ⫺2745.6328 0.1471 15.4 2.7

⫺2745.5582 ⫺2745.7700 0.1467 53.1 3.3

⫺2745.7288 ⫺2746.0000 0.1468 ⫺46.0 3.6

⫺2745.6512 ⫺2746.0744 0.1471 ⫺18.0 3.9

5.2472 5.1110 5.2456

5.2477 5.1063 5.2497

5.2150 5.1425 5.2502

5.2087 5.1499 5.2532

⫺2742.2144 ⫺2742.6000 0.1417 15.4 4.0

⫺2742.0572 ⫺2742.4252 0.1422 53.1 4.3

⫺2742.0101 ⫺2742.3890 0.1424 ⫺46.0 4.1

⫺2742.1982 ⫺2742.9660 0.1418 ⫺18.0 5.5

5.2429 5.1643 5.1303 IV

5.2349 5.1452 5.1332 I

5.2061 5.1837 5.2289

5.2143 5.1658 5.1858

⫺2900.8501 ⫺2901.0828 0.1468 55.5 3.5 II

⫺2900.9494 ⫺2901.2380 0.1470 ⫺59.3 3.7 I

III

⫺2745.8162 ⫺2745.9176 0.1467 N8 –H27:0.2477 57.7 2.3

⫺2745.8057 ⫺2746.0181 0.1468 – ⫺64.0 3.3

⫺2744.8588 ⫺2745.1693 0.1400 N16 –H27:0.2153 16.8 3.0

nomenclature used to depict the various rotamers of the methylated derivatives of 2-APBI is the same as that used for molecule 2-APBI, even though in some rotamers intramolecular hydrogen bonding will be absent. For example, rotamer IV will not involve the solvent molecules in the non-polar and polar/aprotic solvents.

2. Materials and methods 2-APBI was obtained from Aldrich Chemical Company UK. TMAPBI (melting point (m.p.) 30⬚C), 2-DMAPBI (m.p. 28⬚C) and 2-(NMe)(sNMe)DMAPBI (m.t. 25⬚C) were prepared by methylating 2-APBI as suggested in the literature [39,40].

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Fig. 2. Absorption spectra of the molecules TMAPBI, 2-DMAPBI and 2-(NMe)(sNMe)DMAPBI in cyclohexane at concentration 2 × 10⫺5 M: A–A–A TMAPBI; K–K–K, 2-DMAPBI and W–W–W, and 2-(NMe)(sNMe)DMAPBI.

AnalR grade solvents cyclohexane (S.D. Fine), dioxane, acetonitrile and methanol (E. Merck) were further purified by the methods described in the literature [41]. Triply distilled water was used for the preparation of aqueous solutions. Instruments used to record absorption, fluorescence, fluorescence excitation spectra and measurements of excited state lifetimes are the same as described in our recent papers [38,42,43]. Lifetimes in different solvents were measured on a nanosecond single-photon counting spectrofluorimeter (SP 70/80), supplied by Applied Photophysics England. The electronic processing equipment and the multichannel analyser were from Ortec and Norland, respectively. Nitrogen gas was used in the flash lamp. The lamp profile, defined by full width at half the maximum height (FWHM), was 2 ns at the lamp frequency of 30 kHz. The fluorescence decay was analysed by the reconvolution technique (software supplied by IBH Consultants, UK). The lifetime so obtained possess x 2 in the range of 1 ^ 0:2: The error involved in the measurements of emission lifetime, taking into account the experimental facts is 10% and 0.2 ns is the shortest value of the emission lifetime which can be measured

under the best condition of the experiment. Fluorescence spectra were corrected according to the Parker’s method [44]. The fluorescence quantum yields (f fl) were determined with solutions having absorbance less than 0.1 and quinine sulphate in 0.1 N H2SO4 [45] …ffl ˆ 0:55† in different solvents except water. The concentration of each compound used was 2 × 10⫺5 M: Semi-empirical quantum mechanical calculations were carried out on different rotamers and tautomer of all the molecules. The details are given later and the data are compiled in Table 1. The energies of the solvated species in S0 and S1 states were estimated by taking into account the dipole–dipole interactions between the dipoles and the solvent according to [46,47] Ei …solv:† ˆ Ei ⫺ …m2 =4pe0 a30 †f …e†

…1†

where a is the Onsager’s cavity radius and f …e† ˆ …e ⫺ 1†=…2e ⫹ 1†

e is the static dielectric constant of the solvents.

…2†

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Table 2 ab (nm)) and log e max of TMAPBI, 2-DMAPBI and 2-(NMe)(sNMe)DMAPBI in different solvents and at 298 K Absorption band maxima (l max (concentration of each compound ˆ 2 × 10⫺5 M Molecules Solvent/pH Cyclohexane Dioxane Acetonitrile Methanol Water (pH ˆ 8.0) Cyclohexane Water

TMAPBI 260 (4.00) 260 (4.02) 261 (4.01) 260 (3.98) 265 (3.88) 2-APBI 251 (4.03) 244 (3.70)

288 (3.85) 288 (3.86) 285 (3.86) 284 (3.85) 282 (sh)

310 (sh) 312 (sh) 310 (3.52) 310 (3.50) 320 (3.28)

278 (3.79) 275 –

287 289 (3.70)

2-DMAPBI 257 (4.11) 258 (4.11) 256 (4.05) 251 (4.02) 245 (4.01) 291 (3.90) 295 –

3. 2-DMAPBI 3.1. Results 3.1.1. Absorption spectrum The absorption spectra of 2-DMAPBI have been studied in five different solvents and Fig. 2 depicts the absorption spectrum only in cyclohexane. The absorption band maxima …lab max † and the molecular extinction coefficients (log e max) have been compiled in Table 2. The spectral characteristics of 2-APBI are also compiled in Table 2. The LW absorption band maximum of 2-DMAPBI in cyclohexane is at the same wavelength (350 nm) as observed for rotamer I of 2-APBI in the same solvent except that the shoulder

289 (3.89) 290 (3.90) 288 (3.86) 283 (3.81) 281 (3.87)

350 (3.81) 350 (3.77) 342 (3.68) 323 (3.59) 310 (3.56)

298 (3.91)

2-(NMe)(sNMe)DMAPBI 253 288 (4.02) (3.91) 252 288 (4.06) (3.92) 250 285 (3.99) (3.90) 250 283 (sh) (3.88) – 282 (3.90) 346 (3.99)

330 (3.85) 330 (3.81) 322 (3.75) 311 (3.68) 303

363

325 (3.60)

appearing at 363 nm in 2-APBI is absent in 2DMAPBI, but the tail part of the absorption spectrum of 2-DMAPBI is extended towards red. The absorption band maxima are blue shifted with increase in the polarity and hydrogen bond forming capacity of the solvents. The effect of solvents on the absorption spectra of 2-DMAPBI is similar to that observed for 2-APBI and 2-(2 0 -hydroxyphenyl)benzimidazole (2HPBI) [18]. Further the structure observed in the middle band (⬃298 nm) of 2-APBI is absent in 2DMAPBI in all the solvents. 3.1.2. Fluorescence spectrum The fluorescence spectrum of 2-DMAPBI was also recorded in five different solvents. The fluorescence

Table 3 fl (nm)) and fluorescence quantum yield (f fl) of TMAPBI, 2-DMAPBI and 2-(NMe)(sNMe)DMAPBI in Fluorescence band maxima (l max different solvents and at 298 K (concentration of each compound ˆ 2 × 10 ⫺5 M; lexc ˆ 330 nm† Solvent/pH

Cyclohexane Dioxane Acetonitrile Methanol Water (pH ˆ 8.0) a

Molecules TMAPBI

2-DMAPBI a

2-(NMe)(sNMe)DMAPBI

2-APBI

398, 408, 414, 418, 435,

410, – (0.012) 409, – (0.05) 418, 480 (0.06), (0.02) 420, 488 (0.25), (0.04) 435, 496 (0.28), (0.06)

395, – (0.04) 397, 480 (0.12), – 400, 485 (0.14), (0.03) 410, 487 (0.33), (0.03) (0.35), (0.04)

384, 528 (0.013), (0.005) 399, 530 (0.07), (0.007) (0,061), (0.008) 410 – (0.125) (0.286)

– – – – –

(0.43) (0.68) (0.72) (0.80) (0.87)

FWHM observed in SW emission are 3560, 3740, 3800 and 4090 cm ⫺1, respectively.

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Fig. 3. Fluorescence spectra of the 2-DMAPBI in different solvents. [2-DMAPBI] ˆ 2 × 10⫺5 M: lexc ˆ 330 nm; A–A–A cyclohexane; K–K–K, dioxane; ⫹– ⫹ – ⫹ , acetonitrile; W–W–W, methanol; B–B–B, water.

band maxima …lflmax † and fluorescence quantum yield (f fl) of 2-DMAPBI, along with those of 2-APBI, are compiled in Table 3 and the emission spectra are shown in Fig. 3. Fig. 4 depicts the fluorescence spectra of 2-DMAPBI in acetonitrile when excited at different excitation wavelengths. Besides the SW fluorescence band …420 ^ 10†; the emission also consists of a long tail towards the red, except in acetonitrile where the fluorescence band is noticed. This is because of the very weak LW emis-

sion, submerged under the strong SW emission. Inset at the corner of Fig. 4 confirms this. Thus the error in locating the band maxima in LW emission (^5 nm) is larger than the SW emission (^2 nm). The f fl (LW) was measured by drawing a bell shaped curve in the tail part of the fluorescence spectra and the values reported are not as accurate as for the fN fl : Both the fluorescence band maxima are red shifted and their fluorescence quantum yields increase with the increase in the polarity and the protic nature of the

Fig. 4. Effect of excitation wavelength on the fluorescence spectra of molecule 2-DMAPBI in acetonitrile. [2-DMAPBI] ˆ 2 × 10⫺5 M: lexc : (a) 290, (b) 330, (c) 350, (d) 370, (e) 380 and (f) 390 nm.

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Fig. 5. Fluorescence excitation spectra of molecule 2-DMAPBI recorded at different emission wavelengths in acetonitrile. lem : (a) 380, (b) 460 and (c) 500 nm.

for 2-DMAPBI, recorded at 380, 460 and 500 nm in acetonitrile as solvent. In non-polar solvents (cyclohexane and dioxane) fluorescence excitation spectra recorded at any emission wavelength in the range of 380–450 nm were similar to each other, whereas the fluorescence excitation spectra recorded at lem ⬎ 450 nm were broad band and difficult to assign a maximum. On the other hand, in polar solvents, another weak maximum in the fluorescence excitation spectra was observed at ⬃380 nm, when recorded at the emission wavelengths in the range of 460– 540 nm. Data of Table 2 and spectra in Fig. 2 clearly indicate that the absorption spectrum and the fluorescence excitation spectra do not match with each other.

solvents. Further, the SW emission band maxima and its fN fl are insensitive to the excitation wavelength in each solvent, whereas the fluorescence intensity of the tail, when compared with that of SW emission (I490/ I390) increases with the increase of the excitation wavelength (Fig. 4) and the solvent polarity. This behaviour is different from that observed in 2-APBI [38] in the sense that the LW fluorescence band of 2APBI was blue shifted and its fluorescence quantum yield decreased with the increase in the solvent polarity. The fluorescence excitation spectra for 2-DMAPBI were recorded at different emission wavelengths in the range of 380–540 nm. Fig. 5 depicts the same

Table 4 Excited singlet state lifetimes (ns) and the values of non-radiative decay rate (knr (10 8 s ⫺1)) of TMAPBI, 2-DMAPBI and 2-(NMe)(sNMe)DMAPBI measured at lem ˆ 390 nm in different solvents; lem ˆ 332 nm; concentration of each compound ˆ 5 × 10⫺4 M Solvent

Molecules a TMAPBI

Cyclohexane Dioxane Acetonitrile Methanol Water (pH ˆ 8.0) a

2-DMABI

2-(NMe)(⬎NMe)DMAPBI

t1

knr

t2

t1

knr

t2

knr

t1

knr

t2

knr

1.2 2.3 (65.9) 2.2 (53.1) 2.16 (32.3) 1.6 (23.2)

4.8 1.4 1.3 0.9 0.8

– 3.9 (34.1) 5.2 (46.9) 5.7 (67.7) 9.4 (76.8)

– – 1.9 (45.0) 1.6 (35.4) 1.6 (29.9)

– – 4.9 4.7 4.3

– 4.1 5.13 (55.0) 5.5 (64.6) 7.3 (70.1)

– 2.4 1.9 1.7 1.3

– 2.0 1.84 (69.1) 1.4 (51.3) 1.38

– 4.4 4.7 4.8 4.7

– – 5.6 (30.9) 6.7 (48.7) 7.8 (54.6)

– – 1.7 1.5 1.2

Figures in parenthesis give the relative amplitude.

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amplitude of the short-lived species decreased and that of long-lived species increased with increase in l em. The lifetimes of the short- and long-lived species do not depend on the l em. The lifetime and the amplitude of the short-lived species decrease and those of the long-lived species increase in going from dioxane to water. The data are compiled in Table 4.

Fig. 6. Energies of the rotamers I, I 0 , IV and IV 0 of 2-DMAPBI as a free and solvated (acetonitrile) molecules in the S0 and S1 states.

The SW emission band corresponds to the species with absorption band at ⬃320 nm, whereas the LW emission corresponds to the species with maximum at 380 nm, establishing clearly the presence of two species. The fluorescence decay was observed by monitoring at the emission wavelengths (l em) as 380, 400, 430 and 480 nm using 332 nm as the excitation wavelength. In non-polar solvents, emission at each wavelength followed a single exponential decay, whereas in the case of polar and protic solvents, the decay followed a double exponential at each l em. The

3.1.3. Semi-empirical quantum mechanical calculations PC MODEL program [48] was used to draw the structures of the rotamers of 2-DMAPBI (IV and I, Fig. 1). Their geometries were roughly optimized using the MM2 force field in the S0 state. A more precise geometry optimization was then obtained using AM1 method [49] (MOPAC, QCPE program: QCMP 137, version 6.0). Similarly, the geometries of rotamers of IV and I of 2-DMAPBI were also optimized in the S1 state using the above program and by keeping the dihedral angle same as that in the ground state. The values of the energies thus obtained represent the energies of the Franck–Condon states and thus can be used to calculate the transition energies as vertical transitions. Excited singlet state calculations were also performed taking into account the configuration interaction (CI ˆ 5 in MOPAC, total 100 configurations). Rotamers of 2-DMAPBI (labelled as IV 0 and I 0 ) were also fully optimized in the S1 state using the above program with configuration interactions and without any constraint. The molecular parameters for rotamers IV 0 and I 0 of 2DMAPBI were also calculated exactly in the similar manner in the S0 state as done for IV and I except by keeping the angle f as that obtained in the S1 state. The energies of the solvated (in acetonitrile) rotamers

Table 5 Assignment of emission and excitation spectra of 2-DMAPBI in terms of the calculated energies (eV) with and without solvation energy Excitation

Emission

Assignment

Cal.

Exptl.

Assignment

Cal.

Exptl.

Without solvation a, I (S1) ← I (S0) a 0 , IV (S1) ← IV (S0)

3.72 3.50

3.54

c, I 0 (S1) ! I 0 (S0) c 0 , IV 0 (S1) ! IV 0 (S0)

3.45 3.24

3.14

With solvation b, I (S1) ← I (S0) b 0 , IV (S1) ← IV (S0)

3.61 3.35

3.63 3.27

d, I 0 (S1) ! I 0 (S0) d 0 , IV 0 (S1) ! IV 0 (S0)

3.11 3.03

2.99 2.60

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Fig. 7. Potential energy curves for different molecules as free molecules in the S0 state. The potential energy for the most stable rotamer is taken as 0.0 eV. A–A–A TMAPBI; K–K–K, 2-DMAPBI; W–W–W, 2-(NMe)(sNMe)DMAPBI.

IV and IV 0 , I and I 0 , both in the S0 and S1 states were calculated using Eq. (1) and the dipole moments of each rotamers in the S0 and S1 states. The Onsager’s cavity radius was obtained using Prabhumirashi et al. [50] procedure. The relevant data are also given in Table 1. The energies of the free and solvated rotamers are plotted in Fig. 6. The respective absorption and emission transitions are also depicted in Fig. 6. The Franck–Condon absorption and emission energies obtained for each rotamer are given in Table 5. The potential energy curve (Fig. 7) for the interconversion of rotamer I to IV of 2-DMAPBI in the S0 state was also constructed by varying the angle f by 10⬚. The interconversion barrier height in the S0 state from rotamer I to rotamer IV is found to be 19.2 kJ mol ⫺1, which is very large in comparison to that obtained for 2-APBI (2.71 kJ mol ⫺1) [38]. The angles f obtained for the minimum energy configurations from Fig. 7 are the same as obtained earlier. 3.1.4. Discussion Our earlier studies on 2-APBI [37,38] have clearly established that the SW fluorescence band is due to rotamer I in non-polar solvents and the IV and I in polar and protic solvents. Whereas, the LW fluorescence band is due to rotamer V, formed by the ESIPT in rotamer II leading to the formation of the tautomer

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III and then to the twisted species V. In other words, the formation of the tautomer III requires the presence of intramolecular hydrogen bonding in the ground state. This is because the absorption and the LW fluorescence spectra of 2-APBI are blue shifted in polar protic and aprotic solvents. Further the f fl of the LW fluorescence band decreases with the increase in the protic nature of the solvents and thus disrupting the intramolecular hydrogen bonding in rotamer-II. Although it will also disrupt the intramolecular hydrogen bonding in rotamer-I, but this rotamer cannot give rise to the LW emission. The fluorescence excitation spectra have established that 350 and 363 nm bands in the LW absorption spectrum of 2APBI are due to rotamers I and II, respectively. The absence of LW emission in water could be either due to the solvent induced fluorescence quenching or due to the replacement of rotamer II by IV because of the inter- and intra-molecular hydrogen bonding competition. In 2-DMAPBI, rotamer II cannot be formed as there is no hydrogen atoms on the amino group and thus tautomer III and rotamer V formed from tautomer III will be absent. This is substantiated by: (i) the absence of 363 nm shoulder in the LW absorption band and the absence of 520 nm LW fluorescence band in nonpolar solvents; (ii) the presence of 350 nm absorption band and the presence of SW fluorescence band. Thus one needs a different mechanism to explain the dual emission in 2-DMAPBI. Two possible rotamers were considered for 2DMAPBI in the S0 state, closed ring rotamer I and open ring rotamer IV. The latter rotamer will involve the solvating molecules in polar/proptic solvents. The data of Table 1 show that rotamer I is more stable than rotamer IV by 16.5 and 22.2 kJ mol ⫺1 under the free and solvated (acetonitrile) molecular conditions, respectively. This is because rotamer I involves the intramolecular hydrogen bonding between H25 –N16, whereas rotamer IV involves the interelectronic repulsion between the lone pairs on N8 and N16. Although the dihedral angle f in rotamer IV (53.1⬚) is not very different from that in rotamer I (⫺46.0), it still substantiates the interelectronic repulsion in rotamer IV of 2-DMAPBI. Further, the interconversion barrier height for rotamer I to IV is 19.2 kJ mol ⫺1 under isolated condition (Fig. 6), which is much larger than that observed in 2-APBI (2.7 kJ mol ⫺1) and

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will further increase when solvation energy is included. This is because of the higher m g for rotamer I (3.6 D) than rotamer IV (3.3 D) of 2-DMAPBI. Although the interconversion between rotamers I to IV of 2-DMAPBI at room temperature will not be facile as in 2-APBI, but the population ratio between rotamers IV and I of 2-DMAPBI, based on Maxwell– Bolzmann’s distribution, will be 1:3 × 10⫺3 at room temperature. It thus suggests that the two rotamers of 2-DMAPBI will be present in the ground state, but the contribution to the absorption spectrum by rotamer IV of 2-DMAPBI will be negligible due to very low concentration. This is also manifested by the two emission bands and different fluorescence excitation spectra recorded at two emission wavelengths. The agreement between the Frank–Condon absorption transitions obtained from the theoretical studies and determined experimentally is quite good under isolated conditions and reasonably nice under solvated conditions, considering the assumptions involved in these calculations. On the other hand, the agreement between the absorption spectra and the excitation spectra is not very good, specially the tail part towards the red as the absorption spectrum of 2-DMAPBI is a composite spectra of the absorption spectra of the two rotamers I (major contribution) and IV (minor contribution). This is supported by the fact that the FWHM of the absorption spectrum (4300 cm ⫺1) is slightly larger than that of the fluorescence excitation spectrum (4080 cm ⫺1) recorded at lem ˆ 380 nm: From the absorption and excitation spectra, it may be concluded that ⬃380 nm band corresponds to rotamer IV and ⬃340 nm to rotamer I. The blue shift observed in the absorption spectrum of 2-DMAPBI with increase of polarity and hydrogen bonding capacity of the solvents is due to the competition between the inter- (solvent) and intra-molecular (H25 –N16) hydrogen bonding. Since the intramolecular hydrogen bonding is not strong in this molecule [37], intermolecular hydrogen bonding may lead to the removal of co-planarity among the two rings and thus leads to poor conjugation. Similar behaviour has also been observed in 2-APBI [37,38], 2-HPBI [18] and other similar molecules [33–36]. Semi-empirical calculations (Table 1) have been carried out on two rotamers (I 0 and IV 0 ) of 2-DMAPBI in the S1 state after using the configuration interactions. The results have indicated that these two

rotamers attain planarity in the S1 state, i.e. (i) dihedral angle f decreases from ⫺46 to ⫺18⬚ in rotamer I 0 and 53.1 to 15.4⬚ in rotamer IV 0 , and (ii) the bond length C9 –C10 decreases from 0.1471 nm to 0.1417 in both the rotamers. Further the S1 state energies of both the rotamers in the free molecular state (in non-polar solvents) are equal. The S1 state energy of rotamer I 0 decreases more than that of rotamer IV 0 with the increase in the polarity of solvents. This is because the m e of rotamer I 0 is larger than that of m e of rotamer IV 0 . This may lead to the equilibrium and non-equilibrium between the rotamers I 0 and IV 0 of 2-DMAPBI in non-polar and polar solvents, respectively, in their S1 states. Time dependence studies carried out at both the emission bands substantiate this observation in the sense that: (i) fluorescence decays followed a single exponential decay in the non-polar solvents and double exponential decay in the polar solvents; and (ii) the rate of interconversion of rotamer I 0 to IV 0 in the polar solvents in the excited singlet state will be slower than the radiative decay, as the stability of the rotamer I 0 is larger in the S1 state (35.3 kJ mol ⫺1) than that in the S0 state (16.5 kJ mol ⫺1). In other words two fluorescence bands, two fluorescence excitation bands and two different lifetimes in the polar/protic solvents do suggest the presence of two species in the S1 state which are not in equilibrium. Results of Table 3 do suggest that the SW emission band is slightly more sensitive to the solvent polarity and hydrogen bonding nature than the LW emission. The results of Table 1 also indicate that the increase in the dipole moment of rotamer I 0 on excitation to the S1 state …mg ˆ 3:9 D; me ˆ 5:5 D† is slightly greater than that of rotamer IV 0 …mg ˆ 2:7 D; me ˆ 4:0 D†: Thus comparing the results of Tables 1, 3 and 5, it may be concluded that the SW emission, SW excitation band and shorter lifetime can be assigned to rotamer I 0 and the LW emission, LW excitation band and longer lifetime to rotamer IV 0 . The assignment of the lifetimes to different rotamers are based on the facts that: (i) the amplitude of the short lived species decreases with the increase of l em; and (ii) knowing that m e for rotamer I 0 (5.5 D) is larger than that of rotamer IV 0 (4.0) the former will have larger solvent interactions and may possess the solvent induced fluorescence quenching, thereby decreasing the lifetime of rotamer I 0 . Similar behaviour is also observed in case of 7-aminoindazole [51] and

S. Santra et al. / Journal of Molecular Structure 559 (2001) 25–39

Fig. 8. Fluorescence spectra of the TMAPBI in different solvents. [TMAPBI] ˆ 2 × 10⫺5 M: lexc ˆ 330 nm; A–A–A (multiplied by a factor of 3), cyclohexane; K–K–K, dioxane; ⫹– ⫹ – ⫹ , acetonitrile; W–W–W, methanol; B–B–B, water.

6-hydroxy-1-ethyl-5,7,8-trimethyl-1,2-,3,4-tetraquinoline [52]. It may also be mentioned here that the LW emission does not belong to the TICT state because the change in the dipole moment on excitation to the S1 state is very small and thus inconsistent with the changes that should have been observed in the emission spectra if belonged to the TICT emission. The fluorescence bands of 2-DMAPBI are more sensitive to the nature of the solvents than the absorption spectra. This is in agreement with the fact that greater charge transfer takes place from –NMe2 group to the aromatic ring in the excited state in comparison to that in the ground state. A continuous red shift observed in the fluorescence band maximum with increase in the polarity of the solvents indicates the increase in the delocalization of the lone pair of electrons of –NMe2 group throughout the aromatic ring in the S1 state. This is clearly depicted by the changes in the charge densities at different nitrogen atoms of 2DMAPBI (e.g. charge densities increase from 5.1063 to 5.1643 at N8, decrease from 5.2497 to 5.1303 at N16 and nearly remain unchanged at N7 in rotamer IV of 2DMAPBI when it is excited to S1. Similar behaviour is also noticed in rotamer I). This also suggests that both the rotamers attain better co-planarity in the S1 state than in the S0 state. This is supported by the facts that: (i) decrease in the dihedral angles and C9 –C10 bond length as mentioned earlier; (ii) the SW emission band

35

is sharper (full width at the half the maximum height, FWHM is 3600 cm ⫺1) than the absorption band (FWHM 4300 cm ⫺1) in cyclohexane and similar behaviour is also observed in other solvents. Further similar FWHM for the LW emission band in acetonitrile (⬃3700 cm ⫺1) is also observed when excited at 380 and 390 nm. The FWHM for the LW emission band in other solvents could not be determined because of its poor intensity; and (iii) it is well known [51,53] that the loss of flexibility of the fluorophore in the S1 state reduces the non-radiative decay rate (knr). This is consistent with the values of knr (Table 4) and FWHM of the emission (3560 cm ⫺1) and absorption (4300 cm ⫺1) spectra. The increase in the FWHM with increase in the solvent polarity and protic nature suggests the presence of solvated species. Observation of the similar fluorescence band maxima and similar f fl at the l exc in the range of 290–350 nm and in different solvents indicate that both the emissions are taking place from the lowest and the most relaxed electronically excited states. This also suggests that the solvent relaxation times around 2-DMAPBI in these solvents in the S1 state are much shorter than the radiative lifetime. This is in agreement with the literature results [54,55]. Lastly, the tail observed towards the red of the SW fluorescence band of 2-DMAPBI is different from that noticed in 2-APBI in protic solvents. This emission cannot be assigned to the tautomer fluorescence from tautomer III, because: (i) the intramolecular hydrogen bonding in rotamer II, a prime condition for ESIPT is absent in the S0 state; (ii) instead of blue shift and decrease in the fluorescence intensity a small red shift and increase in the fluorescence intensity is observed with the increase in the polarity of the solvents; and (iii) fluorescence intensity at any given wavelength of the tail part increases with the increase in the l exc specially after 360 nm. In other words, the tail part of the SW emission originates from rotamer IV and is consistent with the results as explained earlier.

4. TMAPBI The spectral characteristics of TMAPBI, similar to 2-DMAPBI, are compiled in Tables 2–4. Absorption spectrum in cyclohexane only and the fluorescence

36

S. Santra et al. / Journal of Molecular Structure 559 (2001) 25–39

spectra in different solvents are shown in Figs. 2 and 8, respectively. The absorption spectral characteristics of TMAPBI are different from those of 2-DMAPBI in any given solvent. For example, the lab max of TMAPBI (310 nm) is largely blue shifted as compared to 2DMAPBI (350 nm), but similar to that of 2-phenylbenzimidazole (2-PBI) [56]. Unlike 2-DMAPBI, but similar to the absorption band maximum of 2-PBI the LW lab max of TMAPBI is insensitive to the solvent polarity except that it is red shifted by 10 nm in water. Emission characteristics of TMAPBI, i.e. effect of solvents and l exc on the fluorescence band maxima, fluorescence excitation spectra recorded at different l em, and the SW emission decay, are similar to those observed for 2-DMAPBI with the following differences: (i) the LW emission band in all the solvents appears as a tail towards the red. This could be because of very weak LW emission, submerged of TMAPBI ⬎ under the strong SW emission …fSW fl fSW of 2-DMAPBI). Since the error in locating the fl band maxima of LW emission is quite large, the lflmax of the LW emission are not reported. Same is the of TMAPBI; and reason for not reporting the fLW fl (ii) the Stokes shift (7150 cm ⫺1) of TMAPBI in cyclohexane is greater than that observed for 2-DMAPBI (4180 cm ⫺1) in the same solvent. This shows and is consistent with the fact that greater change in the geometry is observed in TMAPBI than that in 2DMAPBI on excitation. Decrease in the values of knr with increase in polarity/protic nature of solvents substantiates that similar to 2-DMAPBI, TMAPBI is planar in the S1 state. The semi-empirical calculations for TMAPBI have been carried out only for rotamers I and IV in the ground state. Since there are no hydrogen atoms either on the amino group or on the sN–H moiety of TMAPBI, this molecule will not be having any intramolecularly hydrogen bonded rotamer I or II as in 2-APBI (Fig. 1). But for the convenience sake, if – NMe2 group is cis to sN–Me moiety, it is labelled as rotamer I and if the two groups are trans to each other, it is labelled as rotamer IV. The relevant data are compiled in Table 1 and the potential energy curve for the interconversion of rotamer I to IV of TMAPBI is given in Fig. 7. Similar to 2-DMAPBI in the S0 state, rotamer I is 9.6 and 15.0 kJ mol ⫺1 more stable than rotamer IV of TMAPBI under the free and solvated conditions, respectively. The activation

barrier for the interconversion of rotamer I to IV of TMAPBI is 11.6 kJ mol ⫺1 under the free molecular environments and will increase with the increase in the solvent polarity when dipolar interaction energy is included. As mentioned above, the intramolcular hydrogen bonding is completely absent in TMAPBI. Thus the two rings may not be as much co-planar in the S0 sate as those in other molecules. This is substantiated by: (i) the larger angle f (59⬚ in rotamer in I and ⫺55.5⬚ in rotamer IV) of TMAPBI as compared to those in the other molecules; (ii) the stability of rotamer I (16.5 kJ mol ⫺1 in cyclohexane and 22.2 kJ mol ⫺1 in acetonitrile) over rotamer IV of 2-DMAPBI is larger than that of rotamer I (9.6 kJ mol ⫺1 in cyclohexane and 15.0 kJ mol ⫺1 in acetonitrile) over rotamer IV of TMAPBI; (iii) absence of intramolecular hydrogen bonding in rotamer I. A similar behaviour is also observed in 2-(3 0 -hydroxy-2 0 -pyridyl)benzimidazole [31]. Although we have not carried out the semi-emipirical calculations on the S1 state of TMAPBI, as carried out for 2-DMAPBI, the similarity of the emission spectra and the effect of l exc on l em, the similarity of the fluorescence excitation spectra recorded at different l em, the similarity of the emission decay profile recorded at different l em and the presence of tail towards the red of lab max ; suggests the presence of two species (rotamers I and IV) for TMAPBI also. The results can be explained on the same lines as has been done for 2-DMAPBI. The minor differences observed in the spectral characteristics of TMAPBI, as compared to those of 2-DMAPBI can be explained as follows: (i) the large blue shift observed in the lab max of TMAPBI, as said earlier, is due to the absence of the intramolecular hydrogen bonding between the two rings. Because of the above reasons, there will be a poor conjugation between the two rings in TMAPBI and thus large blue shift in the absorption spectrum of TMAPBI is observed when compared to the other three molecules; (ii) the large Stokes shifts (7150 cm ⫺1) observed in TMAPBI are due to large changes observed in the geometries of TMAPBI on excitation, as compared to that (4180 cm ⫺1) in 2DMAPBI. This is substantiated by the smaller values of the knr for TAMPBI. The latter one is consistent with the fact that the loss in the flexibility of the fluorophore decreases the value of knr [51,53]. Further,

S. Santra et al. / Journal of Molecular Structure 559 (2001) 25–39

Fig. 9. Fluorescence spectra of the 2-(NMe)(sNMe)DMAPBI in different solvents.[2-(NMe)(sNMe)DMAPBI] ˆ 2 × 10⫺5 M: lexc ˆ 330 nm; A–A–A (divided by a factor of 5), cyclohexane; K–K–K, dioxane; ⫹– ⫹ – ⫹ , acetonitrile; W–W–W, methanol; B–B–B, water.

the large value of f fl in TMAPBI than that in 2DMAPBI is due to the smaller value of knr and the absence of intramolecular hydrogen bonding, as well as, better co-planarity of TMAPBI in the S1 state as compared to that in 2-DMAPBI.

5. –(NMe)(sNMe)DMAPBI The spectral characteristics of 2-(NMe)(sNMe)DMAPBI, similar to other molecules are compiled in Tables 2–4. The absorption spectrum in cyclohexane only and the emission spectra of 2-(NMe)(sNMe)DMAPBI in different solvents are shown in Figs. 2 and 9, respectively. The LW absorption band maximum of 2-(NMe)(sNMe)DMAPBI is blue shifted in comparison to 2-DMAPBI and 2-APBI, whereas red shifted when compared with that of TMAPBI. Unlike TMAPBI, but similar to other two molecules, the LW absorption band maximum is blue shifted with the increase in the polarity of the solvents. The emission characteristics of 2(NMe)(sNMe)DMAPBI are similar to those of TMAPBI and 2-DMAPBI. 2-(NMe)(sNMe)DMAPBI can have two rotamers (I and II) and one tautomer (III). The semi-empirical calculations for this molecule have been carried out only for the ground state and the relevant data have

37

been compiled in Table 1. As expected, under the free molecular conditions, rotamer II is stable by 1.0 and 92.4 kJ mol ⫺1 in comparison to rotamer I and tautomer III, respectively, whereas under solvation conditions (acetonitrile) rotamer I is stabilized by 9.7 and 81.9 kJ mol ⫺1 in comparison to II and III. In comparison to other two molecules, the activation barrier for the interconversion of I to II is very small (2.0 kJ mol ⫺1, Fig. 7) under the free molecular environments. Based on the small value of interconversion energy barrier, both rotamers I and II can be present in 2(NMe)(sNMe)DMAPBI in the S0 state. Slightly higher stability (1.0 kJ mol ⫺1, under free molecule) of II than I, unlike other molecules, is due to the presence of intramolecular hydrogen bonding between N8 and H27. On the other hand, greater stability of rotamer I (9.7 kJ mol ⫺1) in comparison to rotamer II in polar and protic solvents is due to the loss of intramolecular hydrogen bonding and higher m g for rotamer I (3.28 D) than that of rotamer II (2.62 D). The blue shift in the LW absorption band of rotamer II (330 nm) of 2-(NMe)(sNMe)DMAPBI in comparison to that of rotamer II of 2-APBI (365 nm) is due to the poor intramolecular hydrogen and H27 [r(N8 – bonding between N8 H27) ˆ 0.2477 nm] and larger angle f (57.7⬚) in rotamer II of 2-(NMe)(sNMe)DMAPBI than those present in rotamer II of 2-APBI [r(N8 – H27) ˆ 0.2435 nm, f ˆ 50:2⬚Š: The structure of 2-(NMe)(sNMe)DMAPBI is such that the tautomer emission can be observed if the ESIPT reaction is complete within the lifetime of this molecule. But the behaviour of the emission towards the red of SW emission is such that it resembles with those observed in TMAPBI and 2-DMAPBI where ESIPT is not possible. The results so obtained thus can be explained on the same line as done for the other two molecules. This clearly suggests that the rate of ESIPT in this molecule is very slow as compared to that of the radiative decay and the tautomer III formation is inhibited. Weak intramolecular hydrogen bonding, large angle f and smaller acidity of the amino hydrogen atom may be the reason for this slow rate of ESIPT. In our other study [57–59], we have found that the substitution of an amino hydrogen atom by an electron withdrawing benzoyl or palmitoyl or acetyl group in 2-APBI has

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increased the fluorescence quantum yield of the tautomer emission and thus also increases the rate of the ESIPT process. 6. Conclusions The following conclusions can be drawn from the above study: (i) all the methylated derivatives of 2APBI are less co-planar in comparison to 2-APBI in the S0 state and becomes co-planar in the S1 state. This is substantiated by the large sensitivity of the SW emission towards the polarity of the solvents and FWHM of the absorption spectrum is larger than that of the SW emission; (ii) the spectral characteristics of these molecules have shown the presence of two rotamers in each case; (iii) the SW absorption, fluorescence excitation and fluorescence spectra and short-lived species have been assigned to rotamer I, whereas LW absorption, fluorescence excitation and fluorescence and the long-lived species have been assigned to rotamer IV; (iv) the energies of both the rotamers (I and IV) in the S1 state in non-polar solvents are similar, whereas in polar and hydrogen bonding solvents, rotamer I is more stable than rotamer IV. In the former case equilibrium is established, whereas in the latter case equilibrium between rotamers I and IV is not established in the S1 state, confirmed by a single exponential decay in the nonpolar solvents and biexponential decay in the polar solvents. Acknowledgements The authors are thankful to the Department of Science and Technology, New Delhi for the financial support to the project no. SP/SI/H-39/96. References [1] M. Kasha, J. Chem. Soc. Faraday Trans. II 82 (1986) 2379. [2] L.G. Arnaut, S.J.J. Formosinho, J. Photochem. Photobiol. A: Chem. 75 (1993) 1. [3] S.J.J. Formosinho, L.G. Arnaut, J. Photochem. Photobiol. A: Chem. 75 (1993) 21. [4] A. Douhal, F. Lahmani, A.H. Zewail, Chem. Phys. 207 (1996) 477. [5] J.L. Herek, S. Pederson, L. Banares, A.H. Zewail, J. Phys. Chem. 97 (1992) 9046.

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