Photoluminescence of 2-aminofluorene: a relook

Journal of Molecular Structure 470 (1998) 301±311

Photoluminescence of 2-amino¯uorene: a relook Subit K. Saha, Sneh K. Dogra* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Received 10 October 1997; revised 16 March 1998; accepted 16 March 1998

Abstract The absorption and ¯uorescence spectra of 2-amino¯uorene (2-AF) have been studied as a function of solvent polarity and acid concentration. Using the multiparametric approach of Taft et al., it is clear that 2-AF is a better proton acceptor in the S0 state and a proton donor in the S1 state. Excitation of 2-AF to three electronically excited states has shown that ¯uorescence is always observed from the lowest excited singlet state, but ¯uorescence quantum yield increases with the increase of lexc. The decrease in ¯uorescence quantum yield with increase in solvent polarity and hydrogen bonding is due to solvent-induced quenching. A correspondence is observed between the decrease and increase in ¯uorescence intensities of the neutral and monocation, respectively, in the pH range from 6 to 3. Proton-induced ¯uorescence quenching of neutral 2-AF is noticed in the pH range 3 to 1. pKa and pKap values were determined for different prototropic equilibria and are discussed. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Absorption spectrum; Fluorescence spectrum; Excited-state dipole moment; Prototropic reactions; Proton-induced ¯uorescence quenching

1. Introduction The spectral characteristics and prototropic reactions of aromatic amines have been studied extensively. This is because aromatic amines are very good ¯uorophores [1] (that is, either the ¯uorescence band maximum or ¯uorescence intensity is very sensitive to the environment) and have been used widely to probe the characteristics of biomimetic systems and biomembranes. Important ®ndings of all of these reports are [2±4]: (1) aromatic amines become stronger acids in the S1 state. Monocation±neutral and neutral±monoanion equilibria are established in the S1 state, with a few exceptions in the former prototropic reactions [5±7]; (2) proton-induced ¯uorescence * Corresponding author. Tel.: 1 91-512-597163; fax: 1 91512-590260; e-mail: [email protected]

quenching of neutral aromatic amines is observed prior to formation of the monocation; and (3) the absorption spectra are hardly affected but the ¯uorescence spectra are very sensitive to the environment. Recently we have started to use these aromatic amines as ¯uorescence probes to study the characteristics of micelles and the prototropic reactions of these molecules in the micellar medium [8±12]. During these studies, we found that the prototropic reactions of 2-amino¯uorene (2-AF) are quite different from those observed by Ritchol and Fitch [13]. This led us to re-investigate the spectral characteristics in different solvents and prototropic reactions of 2-AF at various acid/base concentrations. The previous study [13] on this molecule had been carried out in 33% dioxane/water (v/v) mixture. The ground-state pKa value for the protonation reaction of the amino group has been reported to be 4.3 and the excited-state

0022-2860/98/$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII S0 022-2860(98)003 75-5

302

S.K. Saha, S.K. Dogra / Journal of Molecular Structure 470 (1998) 301±311

Table 1 fl Absorption band maxima (lab max , nm), log emax, ¯uorescence band maxima (lmax , nm), ¯uorescence quantum yields of 2-AF, and refractive index and dielectric constants of different solvents, lexc ˆ 325 nm Solvent 1 2 3 4 5 6 7 8 9 10 11 12

Cyclohexane n-Hexane n-Heptane Dioxane Ether CH2Cl2 Ethylacetate Acetonitrile Methanol Ethanol 2-Propanol Water (pH ˆ 8)

13

Water (H0 ˆ 2 3)

n

D

lab max

log emax

lflmax

f¯

1.426 1.372 1.385 1.420 1.352 1.424 1.370 1.342 1.326 1.359 1.375 1.332

2.015 1.890 1.970 2.209 4.235 9.08 6.02 38.88 32.63 24.30 18.30 78.54

318 318 317 322 318 319 325 325 317 317 317 315 283 299 289

Ð Ð Ð 3.74 3.75 3.82 3.92 3.73 4.16 3.87 3.93 3.87 4.29 3.93 3.85

347 344 344 363 349 353 360 363 367 368 369 377

0.74

pKa, determined with the help of ¯uorometric titrations, is 0.4. No proton-induced ¯uorescence quenching of neutral amine has been reported. The present work was carried out to remove some of these anomalies and to report new ®ndings.

0.73

0.52 0.84 0.20

314

was (2.0 ^ 0.1) £ 10 25 M, containing 1% (v/v) dioxane/water solution.

3. Results and discussion 3.1. Absorption spectra

2. Materials and methods 2-AF was procured from Aldrich Chemical Company and was puri®ed repeatedly by crystallization from a 95% (v/v) ethanol/water mixture. The purity of the compound was con®rmed by a sharp melting point, thin-layer chromatography, ¯uorescence spectra when excited at different wavelengths and ¯uorescence excitation spectra. The solvents used were of analytical grade and were further puri®ed by the usual techniques reported in the literature [14]. The procedures used to prepare solutions, adjust H0/pH/H2 of the solutions, record absorption and ¯uorescence spectra, calculate the ¯uorescence quantum yields and measure excited-state lifetimes are the same as used in our recent papers [15±18]. Isosbestic wavelengths were used for calculating pKa in the S1 state and the values are 268 nm and 271 nm in 1% dioxane/ water and 33% dioxane/water (v/v) solutions, respectively. A stock solution of 2-AF was prepared in dioxane, whereas the concentration of the ®nal solution

Absorption band maxima (lab max , nm) and log emax of 2-AF in different solvents are compiled in Table 1. Unlike those of ¯uorene [19], the absorption bands of 2-AF are broad and structureless and the long-wavelength band at 318 nm appears as a shoulder to the main 288 nm band. Furthermore, the ®rst two bands of 2-AF are largely red-shifted, whereas the 217 nm band of 2-AF is slightly red-shifted when compared with those of ¯uorene in any given solvent. The results of Bree and Zwarich [20] established that the ®rst two long-wavelength bands of ¯uorene are long-axispolarized, whereas the third one is short-axis-polarized. Since the amino group in 2-AF is present along the long axis, this will perturb the long-axis transitions of ¯uorene more than the short-axis one. The results of Table 1 are consistent with the above explanation and the values of the transition moment integral (Table 2) con®rm these assignments. The above assignment is also consistent with the effect of an amino group present either at the 1 or 2 position on the spectrum of naphthalene [21].

S.K. Saha, S.K. Dogra / Journal of Molecular Structure 470 (1998) 301±311

303

Table 2 Absorption band maxima (lab max , nm), transition moments and oscillator strength for the ®rst ®ve transitions of 2-AF, calculated by CNDO/S-CI

lab max

Polarization

Oscillator strength

Exp.

Theor.

~x M

~y M

~z M

318 288

301 292 283 226 215

0.0303 0.3849 0.3712 20.9943 0.8895

20.9995 20.9229 20.9285 0.1040 20.4569

0.0025 0.0035 0.0042 0.023 0.002

225

Similar to the other amines, the long-wavelength absorption band maximum gets red-shifted with increase in polarity of the solvent, but blue-shifted with increase in the hydrogen-bonding capacity of solvents, indicating that the amino group is acting as a proton acceptor in the S0 state. Moreover, this is in agreement with values of the coef®cients obtained when the long-wavelength-band maximum is analysed using the multiparametric approach developed by Taft et al. [22, 23]. The equation is given as:

n ab …cm21 † ˆ 3:145 £ 104 2 6:86 £ 102 p 1 9:69 £ 102 a 2 2:58 £ 102 b2

(1)

…r ˆ 0:93; n ˆ 12† where p, a and b are parameters indicating dipolar interaction, hydrogen-bond-donor ability and hydrogen-bond-acceptor ability of the solvent, respectively (r and n are the regression coef®cient and number of solvents used, respectively). The positive value of the coef®cient a con®rms the proton-accepting nature of the amines. Further, the magnitudes of coef®cients of a and b reveal that the proton-accepting nature of 2AF is larger than the proton-donating nature in the S0 state. The geometries of 2-AF in the S0 and S1 states were optimized by means of the AM1 method [24], whereas CNDO/S-CI [25] calculations were carried out on 2AF to prove the above assignments of the transitions. lab max , the transition moment integral along the x, y and z directions, and the oscillator strengths of the ®rst ®ve transitions are compiled in Table 2. As expected, all of the transitions were p ! p* in character. The agreement with the lab max recorded in cyclohexane is quite

0.052 0.157 0.207 0.014 0.103

satisfactory except for the longest wavelength band, where the discrepancy is , 1800 cm 21. This could be due to the inaccuracy in measuring the band maximum because it appears as a shoulder. These results also indicate that the lone pair of electrons present on the amino group is parallel to the p cloud of the ¯uorene. The charge is migrating from the amino group to the ring and charge migration increases further in the S1 state. This is consistent with the fact that the bond order between the nitrogen atom and the carbon atom is slightly greater than unity (i.e., 1.09) and increases to 1.15 on excitation to the S1 state. Similarly, the HNH angle also increases from 1138 to 1198 on going from the S0 and S1 state, consistent with earlier results that hybridization on the amino nitrogen atom changes from sp 3 to sp 2 [26, 27]. The major contribution of various molecular orbitals (MOs) involved in the ®rst four transitions and their localization at various centres are listed in Table 3. The molecular orbitals involved in the ®rst three transitions are the ®rst three highest occupied molecular orbitals (HOMOs) and the ®rst three lowest unoccupied molecular orbitals (LUMOs). All of the HOMOs and the ®rst LUMO are nearly delocalized over the complete molecule, whereas the second and third LUMOs are localized over the ®rst and third ring, respectively. In the longest wavelength transition, a contribution of 52% is from the one-electron transition involving migration of the charge towards the ®rst ring, whereas in the second and third transitions, the percentages of LUMO-37 (localized on the third ring) involved are 25 and 31% respectively. This indicates that in the ®rst transition charge density migrates towards the ®rst ring, and in the second and third transitions charge migration takes place towards the third ring. This is con®rmed by the

304

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Table 3 Contribution of various molecular orbitals in the ®rst four transitions of 2-AF and their localization Transition (nm)

Con®guration

Composition (%)

MO localization

301

34 ! 34 ! 33 ! 32 !

35 36 35 35

12 52 13 10

32p 1,3,4,6,8,9,11,12 33p 4,8,9,11,12 34p 2,5,7,8,10 35p* 2,3,5,7,10,12 36p* 1,3,6 37p* 8,9,11,12 38p* 2,5,6,7,9,10

292

34 ! 35 34 ! 37 33 ! 35

37 25 16

283

34 ! 35 34 ! 37 32 ! 35

47 31 6

226

34 ! 38 33 ! 37

79 8

electron densities present at each atom in the molecule (Fig. 1). 3.2. Fluorescence spectra The ¯uorescence band maxima in different solvents are compiled in Table 1. This ¯uorescence band, unlike that of ¯uorene [28], is broad and more sensitive to the nature of the solvent. This is in agreement with the fact that greater charge transfer takes place from the amino group to the aromatic ring in the excited singlet state than in the ground state. A continuous red shift observed in the ¯uorescence band maxima with increasing solvent polarity indicates the increase in delocalization of the lone pair of electrons of the amino group throughout the aromatic ring in the S1 state. By using the multiparametric approach [22, 23], the ¯uorescence band maximum can be described by:

n fl …cm21 † ˆ 2:90 £ 104 2 12:84 £ 102 p 21:37 £ 102 a 2 14:39 £ 102 b

(2)

…r ˆ 0:97; n ˆ 12† The magnitudes and signs of the coef®cients indicate that dipolar interactions and proton-donor capabilities

of 2-AF increase on excitation, consistent with the earlier results [3]. The ¯uorescence excitation spectra recorded at three emission wavelengths (i.e., 360, 380 and 400 nm) in cyclohexane are similar to each other and also resemble the absorption spectrum of 2-AF. This suggests that there is only one conformer for 2AF in the S0 state. Modi®ed absorption and ¯uorescence spectra (using the method suggested by Birks and Dyson [29]) in cyclohexane are shown in Fig. 2. A discrepancy between the modi®ed absorption and ¯uorescence spectra is observed, but not as large as observed for different aminoindazoles [30]. This indicates a small change in the geometry of 2-AF when excited to the S1 state. This is supported by the fact that the Stokes shift observed in cyclohexane is small (2200 cm 21) for 2-AF and teFM is always less than ttFM in all the solvents (Table 4). The excited-state lifetimes of 2-AF were measured by exciting at 325 nm and monitoring at the ¯uorescence band maxima in four different solvents. The data are recorded in Table 4. The ¯uorescence decay followed a single exponential with a good x 2 value and autocorrelation functions. The effect of lexc (285, 305 and 325 nm) on the photophysical properties of 2-AF was also studied in ®ve different solvents and data are compiled in Table

S.K. Saha, S.K. Dogra / Journal of Molecular Structure 470 (1998) 301±311

305

Fig. 1. Charge densities at the various atoms of 2-AF in the ground and ®rst three excited singlet states.

5. The similar ¯uorescence band maximum observed in all cases indicates that the ¯uorescence is taking place from the lowest and the most relaxed electronically excited state, in agreement with Kasha's rule [31], even though 2-AF is excited to three different

electronically excited states. On the other hand, a considerable increase in the ¯uorescence quantum yield (34% in cyclohexane and 53% in water) is observed when excited with long-wavelength radiation. This could be due to the competition between the rate of

Fig. 2. Modi®ed absorption spectra [e…n †=n , ± z ±], modi®ed ¯uorescence spectrum [F…n †=n 3 , ± + ±] and its re¯ection [F…2n 0 2 n f †=…2n 0 2 n f †3 , ± A ±] for 2-AF in cyclohexane.

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S.K. Saha, S.K. Dogra / Journal of Molecular Structure 470 (1998) 301±311

Table 4 Excited-state lifetimes (tf, ns), ¯uorescence quantum yields, radiative (kr, 10 8 s 21) and non-radiative (knr, 10 8 M 21 s 21) rate constants, ttFM (ns) and teFM (ns) of 2-AF in different solvents, lexc ˆ 325 nm Solvent

tf

f¯

kr

knr

ttFM

teFM

Cyclohexane Acetonitrile Methanol Water

5.3 3.2 4.5 2.1

0.74 0.52 0.84 0.20

1.40 1.60 1.87 0.94

0.50 1.50 0.36 3.77

15.0 22.7 7.5 18.5

7.2 6.1 5.4 10.6

ttFM ˆ radiative lifetime, calculated with Strickler and Berg's equation. teFM ˆ tf/f¯.

intersystem crossing from the higher singlet states to the triplet states, and the rate of emission. The values of the radiative (kr) and non-radiative (knr) rate constants were calculated from the following relationships: kr ˆ ffl =tf ;

knr ˆ 1=tf 2 kr

and are compiled in Table 4, when 2-AF was excited at 325 nm. Although no de®nite order is observed either in the ¯uorescence quantum yield or in the lifetime values when measured in different solvents, the values of knr increase with increasing polarity and hydrogen-bond-formation capacity of the solvent. The values of the theoretical or natural radiative lifetime (ttFM , characterizing the ground-state geometry and the transition moment in the absorption process) in four solvents were calculated by using Strickler and Berg's equation [32]; the values of radiative lifetime (teFM ˆ tf/f¯, indicative of the relaxed excited singletstate geometry and the transition moment involving the spontaneous emission process) are compiled in Table 4. The values of teFM are always less than ttFM observed in each solvent.

It is well known that ¯exibility in molecules increases the non-radiative decay constants in their excited states [33]. Our earlier results have shown that the amino group of 2-AF becomes more planar (changes the hybridization on the nitrogen atom from sp 3 to sp 2 as observed in other aromatic amines) and the hydrogen bonding in the excited state is much stronger than that in the ground state. All of these factors will reduce the ¯exibility of the amino group and therefore should reduce the rate of non-radiative decay, whereas the trend observed in the case of 2-AF is different. It may thus be concluded that the increase in non-radiative decay rate observed in solvents with increasing polarity and hydrogen-bond-formation capacity could be due to solvent-induced ¯uorescence quenching. Similar results have also been observed in the case of 6-hydroxy-1-ethyl-5,7,8-trimethyl1,2,3,4-tetrahydroquinoline [34] and 7-aminoindazole [30]. 3.3. Dipole moments The ground-state dipole moment (mg ˆ 1.79 D) for 2-AF has been calculated using the AM1 program after optimizing the geometry. Many equations are available to determine the excited-state dipole moment (me) from absorption and ¯uorescence data. We shall use the BK (3) and BK 0 (4) equations [35] to calculate the excited-state dipole moments. BK and BK 0 polarity parameters for a ˆ 0 and a ˆ 1 have been taken from the literature [35].

n ab 2 n fl ˆ m1 f …D; n† 1 const

(3)

n ab 1 n fl ˆ 2m2 ‰f …D; n† 1 2g…n†Š 1 const

(4)

Table 5 Fluorescence band maxima (lflmax , nm) and ¯uorescence quantum yield (f¯) of 2-AF when excited at different wavelengths Solvent

Cyclohexane Dioxane Acetonitrile Methanol Water

lexc ˆ 285 nm

lexc ˆ 305 nm

lexc ˆ 325 nm

lflmax

f¯

lflmax

f¯

lflmax

f¯

349 360 363 366 375

0.55 0.58 0.37 0.44 0.13

348 360 363 365 375

0.69 0.68 0.48 0.66 0.16

347 363 363 367 377

0.74 0.73 0.52 0.84 0.20

S.K. Saha, S.K. Dogra / Journal of Molecular Structure 470 (1998) 301±311

307

Fig. 3. Plot of Stokes shift versus BK parameters: a ˆ 0 and a ˆ 1 and ET (30) parameter. [2-AF] ˆ 2 £ 10 25 M.

where m1 ˆ

m2 ˆ



me 2 mg ba 3

m2e 2 m2g ba 3

2 (5)

(6)

where b ˆ 2pe0hc ˆ 1.105 £ 10 235 C 2. In the case of isotropic polarizability of molecules, the condition 2a/4pe0a 3 ˆ 1 is frequently satis®ed and eqn (3) will represent the BK equation. When the polarizability of the ¯uorophore is neglected, eqn (3) reduces to eqn (7), derived by Lippert [36] and Mataga et al. [37], ! D21 n2 2 1 2 n ab 2 n fl ˆ m1 1 const (7) 2D 1 1 2n2 1 1 Fig. 3 presents plots of Stokes shifts versus BK parameters when a ˆ 0 (7) and a ˆ 1 (3). A plot of Stokes shifts versus defaultET (30) parameters is also included in Fig. 3. It is clear from Fig. 3 that the Stokes shifts observed in protic solvents are much larger than expected on the basis of linear relations. As explained earlier for other aromatic amines, the large deviation from linearity shown by protic solvents is due to the fact that the hydrogen bond between the protic solvent and the lone pair of the

amino group in the defaultS0 state is broken on excitation and a hydrogen bond is formed between the amino proton and the lone pair of the solvent molecule [38]. Because of this a large red shift is observed in the ¯uorescence spectra of aromatic amines. This is also re¯ected by the positive value of the coef®cient a (1) in absorption spectra and the negative value in ¯uorescence spectra (2). This is supported by the fact that the plot of Stokes shifts versus defaultET (30) parameters (Fig. 3), which also includes the speci®c interactions, is linear in all solvents. The peculiar properties of dioxane as a solvent are well known and the large deviation from linearity observed can be explained on these lines [39]. Nice correlations are not observed between the plot of (nab 1 n fl ) versus BK 0 parameters (®gure not shown) and thus the results are not used to calculate the me values. If the hydroxylic solvents and dioxane are omitted, the regression coef®cients observed for BK (a ˆ 0) and (a ˆ 1) parameters (Fig. 3) are found to be 0.99 and 0.98, respectively. Based on these values of regression coef®cients, it is not possible to draw any conclusion about whether polarizability of the ¯uorophore plays a role or not in ®nding out the excited-state dipole moment. me was determined from the slope of the linear part of the plots of Fig. 3 using mg and Onsager's cavity radius obtained theoretically. The Onsager's cavity radius was calculated by using the method of

308

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Table 6 Excited singlet-state lifetimes of 2-AF and its monocation at different pH values pH

tf (ns)

Neutral 2-AF (lexc ˆ 311 nm; lem ˆ 375 nm) 7.02 2.12 6.04 1.93 5.04 2.10 4.11 1.94

pH

3.15 2.08 1.56 1.09

tf (ns)

2.05 1.83 1.60 1.55

Prabhumirashi et al. [40]; i.e., measure the maximum distances along the X, Y and Z directions obtained from the optimized geometry, calculate the volume of ellipsoid so formed and then calculate the value of radius of a sphere of volume equal to 0.64 times Ê. the volume of ellipsoid. This comes out to be 2.1 A The values of me are found to be 3.40 D (when a ˆ 0) and 2.8 D (when a ˆ 1), indicating that me is a function of polarizability of the ¯uorophore. In order to solve this problem, we have tried to calculate me by doing semi-empirical quantum-mechanical calculations (AM1 method) by taking into account con®guration interaction calculations on 100, 400 and 1225 con®gurations (CI ˆ 5, 6 and 7 in mopac). The values so obtained are 2.18, 3.78 and 3.84 D, indicating that constant values of me are obtained for nearly 400 con®gurations. Similar to experimental results, theory also predicts that me . mg. The theoretical results show that for 2-AF better agreement is achieved with experiment if BK parameters are used without taking into account the polarizability factor. 3.4. Effects of acid/base concentration The absorption spectrum of 2-AF was recorded in the H0/pH/H2 range of 210.6 to 16 in 1% dioxane/ water (v/v) mixture. 2-AF is neutral between pH 7 and H_16, monocation in the pH range ,3 and no further change was observed below pH 1. The absorption spectra of 2-AF at pH 2.9 resemble that of ¯uorene, indicating the formation of a monocation by protonating the amino group. The isosbestic point observed at 268 nm at various acid concentrations re¯ects the equilibrium H2 O 1 2-HAF1 O 2-AF 1 H3 O1 where 2-HAF 1 is the corresponding anilinium ion.

H0

tf (ns)

Monocation (lexc ˆ 294 nm; lem ˆ 315 nm) 22.5 1.1 23.0 0.94

The ground-state pKa value for the above equilibrium is found to be 4.5. This is slightly greater than that reported [13] in the literature (4.3), determined in 33% (v/v) dioxane/water mixture. The small difference could be due to the effect of the medium, supported by the fact that the pKa value of the above equilibrium in 33% (v/v) dioxane/water mixture is found to be 4.3. The ¯uorescence spectrum of 2-AF was studied over the range H_16 to H0 2 10.6 in 1% dioxane/ water (v/v) mixture. Only the neutral and monocation species were observed to ¯uoresce and the data are recorded in Table 1. The ¯uorescence intensity of neutral 2-AF was quenched after pH 11, without the appearance of any new ¯uorescence band, indicating as observed in many cases (with few exceptions [41]) that the monoanion is non-¯uorescent and that the non-¯uorescent nature of the monoanion is due to solvent (water) induced ¯uorescence quenching [8, 11]. The monocation band was observed at a wavelength close to that of ¯uorene, i.e., blue-shifted. The ¯uorescence intensity of the monocation started decreasing at H0 , 24, without the appearance of any new ¯uorescence band. This could be due to protonation of the hydrocarbon, which in general is non¯uorescent [42]. Excited singlet-state lifetimes of neutral 2-AF were measured at various acid concentrations (pH range 1 to 7, lexc ˆ 311 nm, lem ˆ 375 nm). Below pH 1, the ¯uorescence intensity of 2-AF was too low to measure tf. Similarly, the excited-state lifetimes of the monocation of 2-AF were measured at H0 2 2.5 and 23 (lexc ˆ 294 nm, lem ˆ 315 nm); at other acid concentrations the ¯uorescence intensity was too weak to measure tf. In both cases, the ¯uorescence decay followed a single exponential with good x 2 value (1.1 ^ 0.1) and the data are compiled in Table 6. It is observed that tf (2-AF) is nearly constant in the pH

S.K. Saha, S.K. Dogra / Journal of Molecular Structure 470 (1998) 301±311

Fig. 4. Fluorometric titration curves for different prototropic equilibria of 2-AF in 1% dioxane/water (v/v) mixture.

range from 7 to 3 and then decreases, whereas that of the monocation is same at both acid concentrations. Unlike for other aromatic amines [43], the lifetime of the monocation of 2-AF is smaller than that of 2-AF. The ¯uorometric titration curves for the complete prototropic equilibria are drawn in Fig. 4. The behaviour of the ¯uorometric titration curves is different from that of absorptiometric curves, as well as from those of other aromatic amines [43, 44]. Unlike for other aromatic amines, the increase and decrease of the ¯uorescence intensities of monocation and neutral 2-AF, respectively, correspond to each other in the pH range 6 to 3; i.e., f¯(N) 1 f¯(MC) < 1.0. Below pH 3 the decrease in ¯uorescence intensity of 2-AF is very fast, whereas the increase in ¯uorescence intensity of the monocation is slow up to pH 0 and then increases sharply, reaching a maximum at H0 2 3. Below H0 2 3, the ¯uorescence intensity of the monocation is quenched, without the appearance of any new ¯uorescence band. The behaviour of the ¯uorometric curve at pH $ 11 is consistent with earlier results [44]; i.e., the amino group becomes a stronger acid in the S1 state, with pKap for neutral±monoanion equilibrium being 13.0. In the pH range 6 to 3, only deprotonation of the monocation and protonation of neutral species are occurring. Had these reactions been the only path even below pH 3, the ¯uorometric titration curves would have given ground-state pKa values which is not so. It is thus concluded that, in this pH range, the rate of protonation (k21[H 1]) or the proton-induced ¯uorescence quenching rate (kq[H 1]) of 2-AF is less

309

than the radiative decay constant (kr) (f¯ ˆ 0.20, tf ˆ 2.10 ns, kr ˆ 9.5 £ 10 7 s 21). This is consistent with the value of kq[H 1] at pH 4 (2 £ 10 9 £ 10 24 ˆ 2 £ 10 5 s 21), as discussed later. Similarly, the rate of protonation of 2-AF in this pH range (k21[H 1]) varies between 10 4 and 10 7 s 21 and is also slower than the radiative rate, assuming k21 to be equal to diffusioncontrolled rate constant (kdiff < 10 10 M 21 s 21). Assuming that the lifetime of the monocation in this pH range is the same as it is at H0 2 2.5 and 2 3 (i.e., 1.0 ^ 0.1 ns), it is clear that the rate of deprotonation will be less than 1 £ 10 9 s 21. Furthermore, the two species between pH 6 and 3 and in their S1 states are behaving independently as their lifetimes are different, thus re¯ecting the ground-state concentrations of the species. Because of this ¯uorometric titration curves will give the ground-state pKa, which is in agreement with the observation. The very sharp decrease in the ¯uorescence intensity of neutral species and nearly no change in the ¯uorescence intensity of the monocation indicate the presence of proton-induced ¯uorescence quenching of neutral 2-AF in the pH range from 3 to 1. Thus the pKap for the monocation±neutral equilibrium has been determined from the formation curve of the monocation, i.e., 21.6, indicating that anilinium ions are a stronger acid in the S1 state. These results are different from those reported by Ritchol and Fitch [13] (pKap ˆ 0.4). We have also carried out the ¯uorometric titration in 33% dioxane/water mixture. The behaviour is found to be similar to that observed in 1% dioxane/water mixture, except that the increase in ¯uorescence intensity of the monocation is slower than that in water in the pH range 4 to 0, and the pKap comes out to be 22.3. This is consistent with the fact that one requires a higher concentration of acid to protonate neutral species in a less polar medium. Further, pKap obtained from the decrease in ¯uorescence intensity of 2-AF comes out to be 0.4. It could be that the value of pKap reported by Ritchol and Fitch [13] has been obtained from the changes in the ¯uorescence intensity of 2-AF. 3.5. Proton-induced ¯uorescence quenching As discussed earlier, no correspondence was observed between the decrease in ¯uorescence intensity of neutral 2-AF and the increase in ¯uorescence

310

S.K. Saha, S.K. Dogra / Journal of Molecular Structure 470 (1998) 301±311

Fig. 5. Stern±Volmer plot of 2-AF: I0/I (± z ±) and t0/t (± A ±) versus [H 1].

intensity of its monocation in the pH range from 3 to 1. This has been attributed to proton-induced ¯uorescence quenching, as observed in other cases also [43, 44]. Since the ¯uorescence intensity of the monocation either remains constant or increases slowly in this pH range, it is assumed that the ¯uorescence intensity of the neutral 2-AF at pH 3 can be taken as I0. The Stern±Volmer plot is shown in Fig. 5. It is linear up to [H 1] ˆ 0.08 M and departure from linearity at high acid concentration could be due either to other prototropic reactions or the effect of ionic strength. The Stern±Volmer plot using t0/t has also been plotted in Fig. 5. Both plots give the same slope and the small departure in the case of the t0/t plot from the I0/I plot could be due to a small error in lifetime measurement as the lifetimes are very small. These results suggest that ¯uorescence quenching is dynamic in nature and the value of kq (proton-induced ¯uorescence quenching constant) is found to be 2.0 £ 10 9 M 21 s 21, which is similar to those observed for other amines [2]. 4. Conclusions The following conclusions can be drawn from the above study. 1. Multiparametric analysis has shown that 2-AF is a proton acceptor in the S0 state and a proton donor in the S1 state. 2. The increase in the non-radiative decay constant is due to solvent-induced ¯uorescence quenching.

3. A change in the geometry of the molecule is observed upon excitation to the S1 state. 4. Better agreement between the me values determined experimentally and theoretically is achieved if BK parameters are used with a ˆ 0. 5. The correspondence between the decrease in ¯uorescence intensity of the neutral sepecies and increase in ¯uorescence intensity of the monocation in the pH range 6 to 3 indicates that the rates of protonation of 2-AF, deprotonation of 2-HAF 1 and proton-induced quenching are much less than the radiative decay rate of 2-AF. 6. Proton-induced ¯uorescence quenching of 2-AF observed in the pH range from 3 to 1 is dynamic in nature and the value of kq is 2 £ 10 9 M 21 s 21. 7. pKap for the monocation±neutral and neutral± monoanion equilibra in 1% (v/v) dioxane/water solution are found to be 21.6 and 13.0, respectively.

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