Journal of Photochemistry

and Photobiology,

A:

Chemistry,

41 (1988)

337

- 346

337

DIANION OF NAPHTHYLAMINES OR A CASE OF DEPROTONATION FROM AROMATIC RING CARBON? t NITIN

CHATTOPADHYAY

and MIHIR

Department of Physical Chemistry, Jadavpur, Calcutta 700032 (India) (Received

CHOWDHURY

Indian Association

for the Cultivation

of Science,

June 23, 1987)

Summary

Proton transfer reactions have been studied in aqueous solutions for lnaphthylamine (lN), 2-naphthylamine (2N), N,N-dimethyl-1-naphthylamine (MlN) and N,N-dimethyl-2-naphthylamine (M2N) by both steady-state and time-resolved methods. In the presence of a large excess of alkali both MlN and M2N produce new absorption and emission bands. Similar behaviour has been observed for 1N and 2N under the same conditions. On the basis of the time-resolved studies, it is suggested that ring deprotonation occurs in the ground state at very high alkali concentrations while hydroxyl-ion-catalysed tautomeric interconversion between two monoanionic forms occurs in the excited state.

1. Introduction An understanding of the acid-base property in the excited states is needed for a proper interpretation of photochemical reactions in acidic or basic media. Photoexcitation of organic molecules, causing redistribution of electrons over the atomic constituents, has a marked effect on their acidbase properties [ 1 - lo]. The Farster cycle technique, Weller’s steady state fluorescence quenching technique and time-resolved kinetic measurements have been exploited for the determination of pK* in the lowest excited state [ 1 - 3, 8 - 211. All the studies have shown that, as a general rule, the basic&y of aromatic amines is much decreased in the first excited state [ 3 - 7,191, so much so that, in the presence of a large excess of alkali, dianions of some excited aromatic primary amines may also be formed in aqueous media [203. In our present study, we have investigated the decay profiles and steady state absorption and fluorescence spectra of l-naphthylamine (lN), 2naphthylamine (2N), N,N-dimethyl-1-naphthylamine (MlN) and N,NTPresented in part at the 4th Quantum 29,1986, to January 1,1987. lOlO-6030/88/$3.60

Electronics

Symp.,

Cochin,

0 Elsevier Sequoia/Printed

India, December

in The Netherlands

338

dimethyl-2naphthylamine (M2N) in the presence of added caustic soda to establish the reaction scheme and the nature of the anions produced in highly alkaline solutions.

2. Experimental details 1N and 2N (E. Merck, West Germany) were purified by vacuum sublimation and recrystallization from 90% ethanol. MIN (Aldrich) was vacuum distilled; M2N was prepared from 2N following the method of Hodgson and Crook [22] and purified by vacuum sublimation. The purity of all the compounds was checked by matching their absorption and fluorescence spectra with those reported in the literature. Sodium hydroxide (E. Merck, AnalaR) and triply distilled water were used for preparation of the solutions. The solutions were not degassed after it was verified that dissolved gases have no perceptible effect on the lifetime of the excited species. Above pH = 14, the H_ values were calculated using the Yagil scale [23]. Owing to the extremely low solubility, only saturated aqueous solutions of the compounds were used in the experiments. The instrumental details have been described elsewhere [ 91. 3. Results and discussion 3.1. Absorption study The absorption spectra of aqueous solutions of 1N and MlN in neutral medium (pH = 7) and in the presence of a large excess of caustic soda (H- = 16) are shown in Fig. 1. Figure 2 shows similar spectra for 2N and M2N. The absorption spectra of the solutions remain unchanged over the pH range 7 - 14. However, at higher H_ values they shift towards higher wavelengths and at H_ = 16 attain a steady spectral position. The shift from the maxima of the neutral species is least for 2N (6 nm only) and for each of the other three cases it is around 40 nm. All the spectral changes are reversible. The shift in the absorption spectra at high H- values points to the formation of new species in the ground state from each of the abovementioned compounds. The spectral similarity indicates that the species formed are similar in nature in all cases. Let us designate the new anionic ground state species as S. 3.2. Fluorescence spectral study Figures 1 and 2 summarize the salient features of steady state emissions. As the pH of the solution increases from 7 to 14, a red-shifted band (with a maximum at 520 nm) due to the monoanion is observed [17, 19, 201 in the fluorescence spectrum of 2N. As the excitation spectrum remains unchanged over this range of pH, no new ground state species is postulated

339

220

260

300

3403;0380

WaIvelo

40”o’0

n g th

440 (nm)

4.60

520

-

Fig. 1. Absorption and emission spectra of aqueous solutions of 1N and MlN: at pH 7; .m-.---,MlN at pH 7; - - -, 1N at EL 16; -x---, MlN at IL 16.

-,

1N

and the new emission band is ascribed to the formation of the monoanion (N-*) from the deprotonation of an amine hydrogen [I91 in the excited state. The similar monoanion of 1N is non-luminescing [20]. Because of the absence of amine hydrogen, no such emission band was observed for MlN and M2N over this pH range, nor is there any quenching of emission from the neutraJspecies. At still higher H_ values all four compounds show a characteristic blueshifted emission band (even blue shifted from the fluorescence of the neutral species), e.g. at around 395 nm for 2N. A change occurs in the corresponding excitation spectra, indicating that these emissions originated from the excited state of the newly formed ground state species S. Previously this new fluorescence was ascribed to a dianionic species [20] formed from further deprotonation of RNH-. In view of our observation that a new species S- is formed in the ground state itself at these H_ ranges, even for MlN and M2N where no dissociable hydrogen is attached to the amine nitrogen, we maintain that the luminescent species is tbe excited form of S. When the solution is frozen to reduce the diffusion-controlled rate of monoanion (N*) formation it is found that the 520.nm band of 2N (ascribed to monoanion N*) disappears, as expected. However, the 395 nm fluorescence remains,

220

260

Fig. 2.

300

340 ' 380 co4o2O 440 360 Wavelength

(nm)

480

520

560

600

-

Absorption and emission spectra of aqueous solutions of 2N and M2N: -, 2N -, 2N at pH 14; - - -, 2N at H_ 16; -X--, M2N at at pH 7; .-...-., M2N at pH 7; -.

H-16.

clearly indicating that it originates from excitation of the ground state species S. However, it may still be possible that, in addition to the excited anionic g-round state species S*, which luminesces, there is an additional dianionic species. This dianion may luminesce at the same wavelength or may not luminesce at all. In order to see whether there is any overlapping luminescence at 395 nm from two species, we compared the 77 K frozen medium spectrum with the room temperature spectrum (Fig. 3); apart from a slightly greater resolution at low temperature there is hardly any difference from the room temperature spectrum. The lifetime measurement over the emission profile did not give any indication of overlapping emissions. We also tried to perturb the system by changing the ionic strength of the medium (to create a difference in energies of the two species, if they happen to be degenerate) but the emission spectrum showed no sign of splitting. A timeresolved study with a variable time window and temperature variation failed to show any superimposition of dual emission around 395 nm. We therefore conclude that this 395 nm emission originates only from the excited form of the ground state deprotonated species S. The dianion, if formed, is either non-fluorescent or fluoresces at much longer wavelength which is beyond the capability of our instrumentation .

341

.,. . ; :. .. . : : . : : .: : - .: : : ,- i 300

420

460

Wavelength

Fig. 3. Room

temperature

500 (nm)

(-

540

580

-

) and 77 K (...a--.) emission spectra of 2N at H_ 14.4.

3.3. Time-resolved decay study Table 1 shows the decay parameters for various species of 2N and also the temperature variation results. Some of the characteristic decay profiles of aqueous solutions of 2N and M2N at various alkali concentrations are shown in Fig. 4. As reported earlier [5, 191, at low concentrations of alkali (pH 12.7) the decay profile for the 520 nm emission band of 2N (2N*) shows a growth followed by a decay of single exponential nature. When H_ 2 14, the decay becomes biexponential. With an increase in hydroxyl ion concentration, 71, the shorter of the two 7 values, decreases but the longer 72 remains constant. This behaviour is consistent with a monoanion e dianion process in the excited state, the delayed part indicating the back formation of monoanion from the newly formed dianionic species. However, the 395 nm emission band does not show a growth corresponding to monoanion decay. This is understandable if this luminescence is ascribed to a new excited ground state species S*. The profile for the 395 nm band of 2N in the region H_ 2 14 clearly shows a double decay, The two lifetimes do not change further with increase in alkali concentration. 72, the longer of the two decay constants, is identical with the 72 value for the 520 nm band. On freezing the solution, the decay becomes single exponential with a lifetime 7 equal to that of 71 at room

11.2 10.0 12.1

5.5

5.4 4.5 5.7 6.0

3.85

3.77 3.71 Al = 0.40 3.28

5

20 55 -196 20

16.0

16.0 16.0 16.0 16.0

Hz0

Monoanion W)

395

D2O

11.3

4.2

1.25

20

14.4

H2O

Monoanion (N-1

520

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11.0

-

04

18.2

72

Al = 0.42

W

20

71

7.0

AI/AZ

H2O

Temperature P3

Neutral

pH or H_

415

Solvent

excited species

Emission wavelength @ml

TYPO of

Decay parameters for various species of 2-naphthylamine

TABLE 1

“I .

-_

343

.

50

150

250 CHANNEL

350 NUMBER

1 450

c

-

Fig. 4. Characteristic decay profiles (excited at 295 nm; one channel corresponds to 0.16 ns): ~ pump profile; - - - -, 415 nm emission of neutral 2N at pH 7; -.--.e., 520 nm monoanioh emission of 2N at pH 12.7; -X-, 520 nm biexponential emission of 2N at H14.4; - * -, 395 nm biexponential emission of 2N at H- 16; - - * - -, 395 nm single exponential emission of 2N at H_ 16 at 77 K; - - . -, 415 nm single exponential emission of M2N at IL 16.

temperature. This indicates that the excited form of the anionic ground state species has an intrinsic lifetime r I. The biexponential nature of the decay can be explained from a consideration of the formation of the excited form of the ground state monoanionic species (S*) via two routes: (i) direct excitation from the generated ground state species S- followed by its normal radiative decay and (ii) the slow formation of S* at room temperature by the tautomeric reformation from NY* through the dianion produced in the excited state. r2 therefore is related to the lifetime of the intermediate dianion whose luminescence we have not been able to record so far. Similar tautomeric transformations through the dianion have been proposed by Schulman and coworkers [24, 251 and by Swaminathan and Dogra [ 261 for methyl salicylate and 5aminoindazole respectively. For lN, the biexponentiality of the decay of the 430 nm band at H_ 16 is not very clear. It is probably due to the closeness of the values of the two lifetimes; both are too short to be resolved by our measuring apparatus. For the dimethyl derivatives, channel (ii) is absent and, as a consequence, the decay is a neat single exponential for both MlN and M2N. An explicit discussion is necessary, at this stage, to decide the nature of the anionic ground state species S. Two models offer themselves as candidates: (i) an adduct formation between the amine and Na+ OH- (something similar to Meisenheimer complex formation [ 271) and (ii) dissociation of an aromatic ring proton. Both possible models are somewhat unusual. While Meisenheimer complexes for nitro-substituted aromatic compounds are known [28], they are not expected for amines where the substituent is an electron donor, and carbon acids are known only for aliphatic and alicyclic compounds 129, 301. The proton dissociation ability should, of course, depend on the state of hybridization and the charge density on the carbon. The acidic property decreases in the order sp > sp2 > sp3 and, from

344

that point of view, the dissociation of an aromatic proton is not unexpected in the presence of a large amount of strong alkali. In fact, in a few cases aromatic carbon acidity has been speculated for intermediates, such as benzyne, in chemical reactions [31]. It might be argued that the charge densities on the carbon atom should increase when donor substituents, such as the NH2 group, are introduced, and hence its acidity should be less than that of the corresponding parent hydrocarbon. Apart from the fact that insolubility in water does not allow a similar experiment to be carried out for the hydrocarbon at H- > 14, a detailed charge density calculation shows that the charge densities on certain carbon atoms in naphthylamines, particularly lN, may be substantially less than that of the parent hydrocarbon [32, 331; this will make the proton dissociation facile. We therefore tentatively propose that S- represents a form where one of the hydrogen atoms of the aromatic ring is dissociated as a proton. Since in 2N, C(4) has been shown to have the minimum charge density, we consider

@pH2 -

as the most likely form of S- for 2N. On the basis of our observations, we tentatively propose the scheme shown in Fig. 5. The proposed model shows that the 395 nm luminescent S*, which we presume to be is f&rmed directly from the ground state as well as from the excited lQQJNH-* The hydroxyl ion catalyses the tautomerization process via the dianion. When the dianion recombines with H+ to form the monoanion, it has two possibilities: the proton attaches itself either to NH to form S’ or to CT to form N*. This explains why 72 is the same for both N-* and S-* emission. We prefer the carbon acid model for S- to the Meisenheimer adduct model

-

Fig. 5. Scheme

for ground and excited

state proton transfer reactions of 2N.

345

for, in the latter case, it is difficult to understand the role of the intermediate and the closeness of r2 for the N* and S-* emissions. In principle, nuclear magnetic resonance (NMR) of the photoproduct formed in D20 should be able to provide evidence of the carbon acid model but, unfortunately, the concentrations are too low to permit NMR spectral measurements.

Acknowledgments The work has been supported by Department of Science and Technology Project SERC 23(IP-2)/81-STP II. Thanks are due to D. Sengupta of the Organic Chemistry Department, Indian Association for the Cultivation of Science, for assisting in the preparation of M2N, to Professor S. K. Dogra of the Department of Chemistry, Indian Institute of Technology, Kanpur, for discussion, and to T. Kundu for experimental assistance. References 1 T. Fiirster, 2. Elektrochem., 54 (1950) 42, 531. 2 A. Weller, 2. Elektrochem., 56 (1952) 662; 61 (1957) 956. 3 A. Weller, Progr. React. Kinet., 1 (1961) 189. 4 J. B. Birks, Photophysics of Aromatic Molecules, Wiley, New York, 1970. 5 J. F. Ireland and P. A. H. Wyatt, Adu. Phys. Org. Chem., I2 (1976) 131. 6 E. Vanderdonct, Progr. Reuct Kinet., 5 (1970) 273. 7 H. Shizuka, Ace. Chem. Res., 18 (1985) 141. 8 A. Samanta, N. Chattopadhyay, D. Nath, T. Kundu and M. Chowdhury, Chem. Phys. Lett., 121 (1985) 507. 9 N. Chattopadhyay and M. Chowdhury, J. Photochem.. 38 (1987) 301. 10 H. Shizuka, K. Kameta and T. Shinozaka, J. Am. Chem. Sot., 107 (1985) 3956. 11 T. C. Werner and D. M. Hercules, J. Phys. Chem., 73 (1969) 2005. 12 R. Itoh, J. Phys. Sot. Jpn., 13 (1958) 389. 13 M. Ofran and J. Feitelson, Chem. Phys. Lett., 19 (1973) 427. 14 A. Gafni and L. Brand, Chem. Phys. Lett., 58 (1978) 346. 15 A. Gafni, R. L. Modlin and L. Brand, J. Phys. Chem., 80 (1976) 898. 16 C. J. Marzzacco, G. Deckey and A. M. Halpern, J. Phys. Chem., 86 (1982) 4937. 17 A. C. Capomacchia and S. G. Schulman, Anal. Chim. Acta, 59 (1972) 471. 18 D. D. Rosebrook and W. W. Brandt, J. Phys. Chem., 70 (1966) 3857. 19 J. Shah, H. C. Pant and D. D. Pant, Chem. Phys. Lett., 215 (1985) 192. 20 A. K. Mishra, M. Swaminathan and S. K. Dogra, J. Photochem., 28 (1985) 87. 21 H. H. Jaffe and H. L. Jones, J. Org. Chem., 30 (1965) 964. 22 H. H. Hodgsonand J. Crook, J. Chem. Sot., (1936) 1500. 23 G. Yagil, J. Phys. Chem., 71 (1967) 1034. 24 P. J. Kovi, C. L. Miller and S. G. Schulman, Anal. Chim. Acta, 61 (1972) 7. 25 P. J. Kovi and S. G. Schulman, Anal. Chim. Acta, 67 (1973) 259. 26 M. Swaminathan and S. K. Dogra, J. Am. Chem. Sot., IO5 (1983) 6223. 27 J. Meisenheimer, Justus Liebigs Ann. Chem., 323 (1902) 205. 28 J. March, Advanced Organic Chemistry, McGraw-Hill, New York, 1977, 2nd edn., pp. 29

585, 603,606. H. Rapoport and G. Smolinsky,

J. Am.

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Sot..

80 (1958)

2910.

346 30 W. K. McEwen,J. Am. Chem. Sot., 58 (1936) 1124. 31 R. W. Hoffmann, Dehydrobenzene and Cycloalkynes,

Academic Press, New York, 1967. Jr., Dictionary of R electron Calculations, 32 C. A. Coukon and A. Streitwieser, Pergamon, Oxford, 1965, pp. 307, 311. 33 B. Pullman and G. Tarrago, J. Chim. Phys., 55 (1958) 502.