- Email: [email protected]
Reversible photochemistry of 1-methyl-4-ethoxycarbonylpyridinium iodide
Journal of Photochemistry
and PhotobioZogy,
A: Chemistry,
REVERSIBLE PHOTOCHEMISTRY OF l-METHYL-4-ETHOXYCARBONYLPYRIDINIUM P. MARKOV Faculty
357
- 365
357
IODIDE
and M. NOVKIRISHKA
of Chemistry,
(Received
46 (1989)
University
May 20, I988
of Sofia, Sofia 1126
; in revised form August
(Bulgaria)
2, X 988)
Summary The effect of UV light on a methanol-water solution of l-methyl-4ethoxycarbonylpyridinium iodide was studied by spectroscopic and potentiometric methods. The irradiation of an air-saturated solution by UV light with a wavelength shorter than 240 nm initiates the formation of species which absorb at 358 nm. The photoprocess involved is not related to any change in the concentration of iodide ion. Irradiation at 280 nm or an increase in temperature of the solution containing the newly formed particles causes a reverse process and the gradual diminution of the concentration of these particles. The molar absorptivity E, of the complex, the equilibrium constant K,, of formation of the complex and the enthalpy N and entropy AS changes were determined. In an atmosphere of argon, the irradiation of the solution causes a different effect which is manifested by the appearance of absorption at 345 nm. A possible mechanism of the effect is discussed.
1. Introduction A limited amount of widely scattered and isolated photochemical data on quaternary pyridinium salts are now available. Kaplan et al. [l] have shown that the excited state properties of l-methylpyridinium chloride substantially differ from those of pyridine and its substituted derivatives. It is believed [2] that unlike pyridine, the photochemistry of pyridinium salts ensues from the ?r -+ 7~* excitation of the pyridine ring system. The photochemical behaviour of alkylpyridinium iodides is especially interesting because an intramolecular electron transfer from the iodide anion is possible_ Since 1952 the spectral aspects of this important problem have been extensively studied (see, for example, refs. 3 - 6). The influence of UV light on l-methylpyridinium iodide has been described in an earlier investigation [7]. During the course of this study it was found that the introduction of an ethoxycarbonyl group at position lOlO-6030/89/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
4 in the pyridine ring produces a significant change in the photochemical properties of the pyridinium salt. This work extends our preliminary studies in this field [ 81.
2. Experimental
details
2.1. I-Methyl-4-ethoxycarbonyipyridinium iodide (MEPI) 4-Ethoxycarbonylpyridine (11 g; 0.07 mol) was dissolved in anhydrous ethanol (30 cm’) and colourless methyl iodide (20 g; 0.14 mol) was gradually added. The mixture was refluxed for 5 h. The excess of ethanol and methyl iodide was distilled off and the resulting crystals were extracted in a Soxhlet apparatus with ethyl ether. On cooling the acetone solution prepared at room temperature the residue crystallized to give bright orangeyellow crystals. Found: C, 36.69%; H, 4.96%; N, 4.84%; I, 43.76%. Calculated for CgH1202NI: C, 36.86%; H, 4.10%; N, 4.78%; I, 43.34%. 2.2. Solvent Spectral grade methanol (Merck) was used to prepare 80% water solution which was used as medium.
methanol-
2.3. Spectra Absorption spectra were recorded with a Specord UV VIS spectrophotometer using quartz cells with widths of 0.2, 0.5 and 2.0 cm. The data obtained were recalculated for a cell width of 1.0 cm. The peak maxima were measured by scanning over the maximum absorption three to five times and then averaging. The good reproducibility of the band position and absorption intensities indicated that the spectroscopic data were valid. All solutions were prepared in a box under an argon atmosphere and all cells used for the measurement of deaerated solutions were filled with the same protection. Argon used for removing the traces of oxygen from the irradiated solutions was purified by passing the gas flow through a quartz tube containing titanium sponge heated at 800 “C. 2.4. Potentiometric measurements The potentiometric assay of concentration of the iodide anion in alcohol-water solutions of 1-methyl-4-ethoxycarbonylpyridinium iodide was carried out using a precise pH-meter (type OP-208/l Radelkis) with an iodide-selective electrode (type Orion LWl) and a reference calomel electrode (OP-083OP). The iodide-selective electrode was calibrated with wateralcohol solutions (approximately (1 X 10e2) - (1 X 1O-5) mol) of NaI. 2.5. Irradiation The samples (2 cm3) were placed in quartz glass vessels and were kept in a thermostat at 20 “C. They were exposed to UV light under standard conditions. UV irradiation was obtained from a Hanau medium pressure
359
mercury arc lamp, with chlorine filter or filters for 280 and 350 nm wavelengths. The light quanta falling on the quartz cell were counted using a uranyl oxalate actinometer [ 91.
3. Results and discussion The UV Fig. 1.
200
spectra of water-alcohol
250
300
Fig. 1, UV absorption spectra of in 80% methanol (approximately 2.0 cm.
350
solutions of MEPI are shown in
c
nw
l-methyl-4ethoxycarbonylpyridinium 1 x 10m3 mol 1-l): curve a, Z = 0.1
iodide (MEPI) cm; curve b, 1 =
The irradiation of an air-saturated solution of MEPI by light with a wavelength shorter than 240 nm (chlorine filter) results in the appearance of a new band at 358 nm whose intensity increases gradually during the time of irradiation (Fig. 2). In an atmosphere of pure argon the irradiation causes a different effect which is manifested by the appearance of an absorption band at 345 nm (Fig. 3). The presence of traces of oxygen in the irradiated solutions is almost sufficient for the onset of a conversion related to the formation of species which absorb at 358 nm. The irradiated air-saturated solutions were placed in the dark at 20 “C. The 358 nm absorbance A358 vs. time t after termination of the irradiation is given in Fig. 4. The UV light initiates the formation of species which absorb at 358 nm. The intensity vs. temperature for the new band is shown in Fig. 5. The absorption intensity at 358 nm is also found to diminish when the solution is irradiated with 280 nm UV light (Fig. 6). These changes are reversible, i.e. the original spectra of the solutions are restored completely. The reverse process after the 280 nm irradiation is not too fast and conventional spectrophotometric recording may be used.
300
400
350
500
nm
Fig. 2. Sequence of absorption spectra of MEPI (approximately 1 X lop3 mol 1-l airsaturated methanol solution) during UV irradiation (chlorine filter): 1, 0 min; 2, 1 min; 3, 2 min; 4,3 min; 5,4 min; 6,5 min; 7, 6 min; 8, 7 min; 9,8 min; 10, 9 min; 11,lO min. Intensity of the irradiation, 0.58 x lOi quanta cmY3 s-l; 1 = 1 cm.
al
300
350
400
nm
*
Fig. 3. UV absorption spectra of MEPI (approximately 1 x 10e3 mol 1-l deaerated methanol solution) during UV irradiation (chlorine filter): 1, 0 min; 2, 60 min; 3, 120 min. Intensity of the irradiation, 0.58 X 1016 quanta cmm3 s-l; 1 = 1 cm.
The most serious difficulty in measuring the UV spectra of the pyridinium iodide salts in the 280 - 360 nm range is that there is always the possibility that the spectra may include some absorption by triiodide ion,
361
Fig. 4. Plot of the absorbance A 358at 358 nm us. the time t (hours), after the end of UV irradiation. UV irradiation of air-saturated (1 x 10m3 mol 1-1) solutions of MEPI in 80% methanol for: 1,6 min; 2,lO min; 3,20 min;4,60 min. 350
900
500
nm
0 Fig. 5. Effect 2, 30 “C; 3,40
of temperature on the intensity “C; 4,50 “C; 5,60 “C.
of the absorption
at 358 nm:
1, 20 “C;
which in chloroform solution displays two intense maxima at 362.5 and 295 nm. Since the long wavelength band of Is- is nearly the same as the new band in the UV spectrum of the irradiated MEPI solutions, it is essential to control the concentration of the iodide ion, The potentiometric assay, provided using an iodide-selective eIectrode, showed that there was no change in the actual concentration of the iodide ion during the irradiation of the solutions. Consequently, it can be concluded that under the chosen experimental conditions (sohent, duration of the irradiation, irradiation intensity) no triiodide ion is formed.
362
4.2
Fig. 6. Sequence of absorption spectra of MEPI (equilibrated 1 X lop3 mol 1-l airsaturated solution in 80% methanol) during 280 nm irradiation: 1, 0 min; 2, 15 min; 3, 35 min; 4, 55 min; 5, 100 min; 6, 145 min; 7, 261) min; 8, 320 min. Intensity of the irradiation, 0.77 x 1016 quanta cmY3 s-l; I = 1 cm.
The data obtained undoubtedly show (see refs. 10 - 13) that the photochemical behaviour of the compound under discussion (as observed from the changes in its UV spectrum) is closely related to the photoexcitation of the iodide anion. According to the views of Frank and Platzman [ 141 light absorption by iodide ion results in ionization, the electron being transferred to the potential well formed by the dipoles of the solvent molecules. In the case of pyridinium salts the charge transfer from iodide ion is believed to be directed to the pyridinium ring [ 123. The basic presumption of these considerations is the photoinduced formation of iodine atoms as a result of an electron transfer to the solvent molecule or to the pyridinium ring system. Some arguments in favour of electron transfer within the framework of the 1-methylpyridinium iodide molecule leading to the formation of iodine atoms have been reported previously [ 71. In the case considered here, however, the potentiometric assay of the solutions studied showed no decrease in the concentration of iodide ion during the irradiation. On the other hand special attention must be paid to the reversibility of the photoinduced process (Figs. 5 and 6). The results obtained reveal that the iodide ion photoexcitation produces two different types of particles absorbing at 345 and 358 nm.
363
Species absorbing &
PIyt + I- &
at 345 nm yDE+
I
280
Specie;5a;;zing
at
nm UV light or
rise in temperature
It is instructive to note that the position of the absorption band after the irradiation of the deaerated solution of MEPI is nearly the same as that reported by Kosower [ 121 for a transition involving charge transfer from the iodide ion to the alkylpyridinium ion in a solution of 1-ethyl-4-methoxycarbonylpyridinium iodide. A quantitative description of the species which absorbs at 358 nm is now given. It can be assumed that the 358 nm absorption is due to an oxygen-sensitized photoprocess which yields a relatively stable complex formed between the existing ionic particles hv,traces py+ + I- 7
K ea
of O2
Complex absorbing at 358 nm
(1)
The equilibrium constant K,, and the molar absorptivity E, of the particles absorbing at 358 nm can be obtained from the study of the variation in the UV absorption at 358 nm with the concentration of MEPI. The Keefer-Andrews equation [ 151 was used
co*
1 1 -= + A, G(2Co - C,) &, E, where Co is the initial concentration of the compound under consideration, C, is the concentration of the complex, E, is the molar absorptivity of the complex and A, is the absorbance due to the complex. The first-order approximation was obtained from eqn. (2) with C, = 0, and the resulting K,, value was used to calculate a C, value. The iteration process was repeated until the resulting Keg value was approximately constant. The values found for K,, and ec are 1.33 X lo3 1 mol-l and 11.1 X lo3 1 mol-’ cm-’ respectively. As readily seen from Fig. 5, the equilibrium (1) is dependent on the temperature of the solution. The data accumulated enabled a calculation to be made of the enthalpy m and entropy AS changes related to complex formation. The numerical values of these quantities, calculated on the basis of the dependence In K,, us. l/T, are m = -1.86 kcal mole1 and AS = 7.32 kcal mole1 K-l. From the negative value of N it can be concluded that the complex is weak. However, AS is slightly positive, which means that the formation of the complex increases the disorder in the system to
364
some degree. It is interesting to note that the value of AH found is very close to the values observed by Verhoven et al. [6] for the complexes between N-methyl-4-cyanopyridinium ion and neutral aromatic donors. A series of irradiated and equilibrated solutions of MEPI was prepared with different ratios of ~-methyl-~-ethoxyc~bonylpyrid~ium ion (Pyi) and iodide ion (I-); the total concentration of ions was kept constant at mol 1-l. A maximum value of A358 was found at a ratio of c=2.0x1Q-3 1:l for the two ionic species. This implies that the UV irradiation leads to the formation of a 1:I complex. The kinetics of the photoinduced conversion were studied using 1 X 10e3 mol 1-i solutions in 80% methanol at 20 “C. In the region of high optical densities of the iodide anion, the process follows zero-order kinetics. As seen from Fig. 4 the UV irradiation initiates further complex formation in the dark. A hyperbolic relationship of the type A358 _-
=
(at
+
(3)
by
t
between the absorbance A 358 at 358 nm and the time t after termination of UV irradiation is strictly obeyed. The rate of formation in the dark of the species which absorbs at 358 nm is given by
dcc
J.7=___~--
dt
b
1
(4)
le, at2 + bt
where a and b are coefficients. It is worth mentioning that the rate of the dark process V increases with increasing time of irradiation 7, e.g. for 7-= 5 min, V = 0.69 X 10w3 and for T = 120 min, V = 6.66 X 1W3 {t = 100 h). The reverse process of complex decomposition, which results from the photoexcitation of alkylpyridinium ion (Py’) (Fig. 6), satisfies the function In C,O- In CFrf = hTrr which is a solution of the rate law for a first-order reaction. C!: denotes the initial concentration of the complex and CC(7r)is the concentration of the complex 7, min after the onset of the 280 nm irradiation of the solution. A possible route of the oxygen-sensitized photoinduced formation of the complex which absorbs at 358 nm is presented below
py+ + I240 nm, (FIS or SSIP) * 280
nm hv or
hv traces of 02
l?y+1- -
temperature increase (c Ip )
f
py+ + I-* _If
py+. * . * {EC) ‘I +
(PyY)* (E)
-
py+ I-” (CC)
{FIS, free ions; SSIP, solvent separated ion pairs; CIP, contact ion pairs; EC, encounter complex; CC, collision complex; E, exciplex).
365
In fluid medium, the photoexcited iodide ion I-* and alkylpyridinium ion Py+ may form encounter and collision complexes. The latter is transformed into an exciplex (see, for example, refs. 16 - 18). Quenching via exciplexes is a particularly important pathway when the reactants are planar molecules capable of forming “sandwich” complexes [ 191. It is remarkable that as early as 1958 Kosower [ 121 noted the presence of this type of intermolecular complex in solutions of alkylpyridinium iodides. In our opinion the exciplex decay produces a contact ion pair which is easily destroyed when the pyridinium component is excited (280 nm irradiation) or as a result of the rise in temperature of the irradiated solutions. The most striking feature of the photochemical behaviour of l-methyl4-ethoxycarbonylpyridinium iodide is the existence of a dark process which results in an increase in the amount of particles which absorb at 358 nm. The available experimental data do not allow us to explain its intrinsic mechanism.
References 1 L. Kaplan, J. W. Pavlik and K. E. Wilzhach, J. Am. Chem. SW., 94 (1972) 3283. 2 J. A. Barltrop and J. D. Coyle, Excited States in Organic Chemistry, Wiley, London, 1978, p. 355 (Russian translation). 3 E. M. Kosower, J. Am, Chem. Sot., 78 (1956) 3497. 4 E, M. Kosower and P. E. Kiinedinst, Jr., J, Am. Chem. Sot., 78 (1956) 3493. 5 E. M. Kosower and J. A. Skorcz, J. Am. Chem. Sot., 82 (1960) 2195. 6 J. W. Verhoven, I. P. Dirkx and Th. J. de Boer, Tetrahedron, 25 (1969) 3395. 7 P. Markov and M. Novkirishka, Annu. Univ. Sofia, 79 (1985). 8 P. Markov and M. Novkirishka, Monatsh. Chem., 119 (1988) 1185. 9 C. R. Masson, V. Boekelheide and W. A. Noyes, Jr., Technique ofOrganic Chemistry, Vol. 2, Wiley-Interscience, New York, 1956, p. 281. 10 E. Lederle, 2. Phys. Chem. Abt. B, IO (1930) 121. 11 M. Smith and M. C. R. Symons,J. Chem. Phys., 25 (1956) 1074. 12 E. M. Kosower, J, Am. Chem. Sot., 80 (1958) 3267. 13 E. Rahinowitch, Rev. Mod. Phys., I4 (1942) 118. 14 J. Frank and R. Platzman, L. Farkas Memorial Volume, Jerusalem, 1952, p. 21. 15 R. M. Keefer and L. J. Andrews,J. Am. Chem. Sot., 74 (1952) 1891. I.6 J. Moore and R. G. Pearson, Kinetics and Mechanism, Wiley, New York, 1981, p. 237. 17 N. Sutin,Acc. Chem. Rea, 15 (1982) 275. 18 R, D. Cannon, Electron Transfer Reactions, Butterworths, London, 1980. p. 97. 19 G. J. Kavarnos and N. J. Turro, Chem. Rev., 86 (1986) 401.
and PhotobioZogy,
A: Chemistry,
REVERSIBLE PHOTOCHEMISTRY OF l-METHYL-4-ETHOXYCARBONYLPYRIDINIUM P. MARKOV Faculty
357
- 365
357
IODIDE
and M. NOVKIRISHKA
of Chemistry,
(Received
46 (1989)
University
May 20, I988
of Sofia, Sofia 1126
; in revised form August
(Bulgaria)
2, X 988)
Summary The effect of UV light on a methanol-water solution of l-methyl-4ethoxycarbonylpyridinium iodide was studied by spectroscopic and potentiometric methods. The irradiation of an air-saturated solution by UV light with a wavelength shorter than 240 nm initiates the formation of species which absorb at 358 nm. The photoprocess involved is not related to any change in the concentration of iodide ion. Irradiation at 280 nm or an increase in temperature of the solution containing the newly formed particles causes a reverse process and the gradual diminution of the concentration of these particles. The molar absorptivity E, of the complex, the equilibrium constant K,, of formation of the complex and the enthalpy N and entropy AS changes were determined. In an atmosphere of argon, the irradiation of the solution causes a different effect which is manifested by the appearance of absorption at 345 nm. A possible mechanism of the effect is discussed.
1. Introduction A limited amount of widely scattered and isolated photochemical data on quaternary pyridinium salts are now available. Kaplan et al. [l] have shown that the excited state properties of l-methylpyridinium chloride substantially differ from those of pyridine and its substituted derivatives. It is believed [2] that unlike pyridine, the photochemistry of pyridinium salts ensues from the ?r -+ 7~* excitation of the pyridine ring system. The photochemical behaviour of alkylpyridinium iodides is especially interesting because an intramolecular electron transfer from the iodide anion is possible_ Since 1952 the spectral aspects of this important problem have been extensively studied (see, for example, refs. 3 - 6). The influence of UV light on l-methylpyridinium iodide has been described in an earlier investigation [7]. During the course of this study it was found that the introduction of an ethoxycarbonyl group at position lOlO-6030/89/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
4 in the pyridine ring produces a significant change in the photochemical properties of the pyridinium salt. This work extends our preliminary studies in this field [ 81.
2. Experimental
details
2.1. I-Methyl-4-ethoxycarbonyipyridinium iodide (MEPI) 4-Ethoxycarbonylpyridine (11 g; 0.07 mol) was dissolved in anhydrous ethanol (30 cm’) and colourless methyl iodide (20 g; 0.14 mol) was gradually added. The mixture was refluxed for 5 h. The excess of ethanol and methyl iodide was distilled off and the resulting crystals were extracted in a Soxhlet apparatus with ethyl ether. On cooling the acetone solution prepared at room temperature the residue crystallized to give bright orangeyellow crystals. Found: C, 36.69%; H, 4.96%; N, 4.84%; I, 43.76%. Calculated for CgH1202NI: C, 36.86%; H, 4.10%; N, 4.78%; I, 43.34%. 2.2. Solvent Spectral grade methanol (Merck) was used to prepare 80% water solution which was used as medium.
methanol-
2.3. Spectra Absorption spectra were recorded with a Specord UV VIS spectrophotometer using quartz cells with widths of 0.2, 0.5 and 2.0 cm. The data obtained were recalculated for a cell width of 1.0 cm. The peak maxima were measured by scanning over the maximum absorption three to five times and then averaging. The good reproducibility of the band position and absorption intensities indicated that the spectroscopic data were valid. All solutions were prepared in a box under an argon atmosphere and all cells used for the measurement of deaerated solutions were filled with the same protection. Argon used for removing the traces of oxygen from the irradiated solutions was purified by passing the gas flow through a quartz tube containing titanium sponge heated at 800 “C. 2.4. Potentiometric measurements The potentiometric assay of concentration of the iodide anion in alcohol-water solutions of 1-methyl-4-ethoxycarbonylpyridinium iodide was carried out using a precise pH-meter (type OP-208/l Radelkis) with an iodide-selective electrode (type Orion LWl) and a reference calomel electrode (OP-083OP). The iodide-selective electrode was calibrated with wateralcohol solutions (approximately (1 X 10e2) - (1 X 1O-5) mol) of NaI. 2.5. Irradiation The samples (2 cm3) were placed in quartz glass vessels and were kept in a thermostat at 20 “C. They were exposed to UV light under standard conditions. UV irradiation was obtained from a Hanau medium pressure
359
mercury arc lamp, with chlorine filter or filters for 280 and 350 nm wavelengths. The light quanta falling on the quartz cell were counted using a uranyl oxalate actinometer [ 91.
3. Results and discussion The UV Fig. 1.
200
spectra of water-alcohol
250
300
Fig. 1, UV absorption spectra of in 80% methanol (approximately 2.0 cm.
350
solutions of MEPI are shown in
c
nw
l-methyl-4ethoxycarbonylpyridinium 1 x 10m3 mol 1-l): curve a, Z = 0.1
iodide (MEPI) cm; curve b, 1 =
The irradiation of an air-saturated solution of MEPI by light with a wavelength shorter than 240 nm (chlorine filter) results in the appearance of a new band at 358 nm whose intensity increases gradually during the time of irradiation (Fig. 2). In an atmosphere of pure argon the irradiation causes a different effect which is manifested by the appearance of an absorption band at 345 nm (Fig. 3). The presence of traces of oxygen in the irradiated solutions is almost sufficient for the onset of a conversion related to the formation of species which absorb at 358 nm. The irradiated air-saturated solutions were placed in the dark at 20 “C. The 358 nm absorbance A358 vs. time t after termination of the irradiation is given in Fig. 4. The UV light initiates the formation of species which absorb at 358 nm. The intensity vs. temperature for the new band is shown in Fig. 5. The absorption intensity at 358 nm is also found to diminish when the solution is irradiated with 280 nm UV light (Fig. 6). These changes are reversible, i.e. the original spectra of the solutions are restored completely. The reverse process after the 280 nm irradiation is not too fast and conventional spectrophotometric recording may be used.
300
400
350
500
nm
Fig. 2. Sequence of absorption spectra of MEPI (approximately 1 X lop3 mol 1-l airsaturated methanol solution) during UV irradiation (chlorine filter): 1, 0 min; 2, 1 min; 3, 2 min; 4,3 min; 5,4 min; 6,5 min; 7, 6 min; 8, 7 min; 9,8 min; 10, 9 min; 11,lO min. Intensity of the irradiation, 0.58 x lOi quanta cmY3 s-l; 1 = 1 cm.
al
300
350
400
nm
*
Fig. 3. UV absorption spectra of MEPI (approximately 1 x 10e3 mol 1-l deaerated methanol solution) during UV irradiation (chlorine filter): 1, 0 min; 2, 60 min; 3, 120 min. Intensity of the irradiation, 0.58 X 1016 quanta cmm3 s-l; 1 = 1 cm.
The most serious difficulty in measuring the UV spectra of the pyridinium iodide salts in the 280 - 360 nm range is that there is always the possibility that the spectra may include some absorption by triiodide ion,
361
Fig. 4. Plot of the absorbance A 358at 358 nm us. the time t (hours), after the end of UV irradiation. UV irradiation of air-saturated (1 x 10m3 mol 1-1) solutions of MEPI in 80% methanol for: 1,6 min; 2,lO min; 3,20 min;4,60 min. 350
900
500
nm
0 Fig. 5. Effect 2, 30 “C; 3,40
of temperature on the intensity “C; 4,50 “C; 5,60 “C.
of the absorption
at 358 nm:
1, 20 “C;
which in chloroform solution displays two intense maxima at 362.5 and 295 nm. Since the long wavelength band of Is- is nearly the same as the new band in the UV spectrum of the irradiated MEPI solutions, it is essential to control the concentration of the iodide ion, The potentiometric assay, provided using an iodide-selective eIectrode, showed that there was no change in the actual concentration of the iodide ion during the irradiation of the solutions. Consequently, it can be concluded that under the chosen experimental conditions (sohent, duration of the irradiation, irradiation intensity) no triiodide ion is formed.
362
4.2
Fig. 6. Sequence of absorption spectra of MEPI (equilibrated 1 X lop3 mol 1-l airsaturated solution in 80% methanol) during 280 nm irradiation: 1, 0 min; 2, 15 min; 3, 35 min; 4, 55 min; 5, 100 min; 6, 145 min; 7, 261) min; 8, 320 min. Intensity of the irradiation, 0.77 x 1016 quanta cmY3 s-l; I = 1 cm.
The data obtained undoubtedly show (see refs. 10 - 13) that the photochemical behaviour of the compound under discussion (as observed from the changes in its UV spectrum) is closely related to the photoexcitation of the iodide anion. According to the views of Frank and Platzman [ 141 light absorption by iodide ion results in ionization, the electron being transferred to the potential well formed by the dipoles of the solvent molecules. In the case of pyridinium salts the charge transfer from iodide ion is believed to be directed to the pyridinium ring [ 123. The basic presumption of these considerations is the photoinduced formation of iodine atoms as a result of an electron transfer to the solvent molecule or to the pyridinium ring system. Some arguments in favour of electron transfer within the framework of the 1-methylpyridinium iodide molecule leading to the formation of iodine atoms have been reported previously [ 71. In the case considered here, however, the potentiometric assay of the solutions studied showed no decrease in the concentration of iodide ion during the irradiation. On the other hand special attention must be paid to the reversibility of the photoinduced process (Figs. 5 and 6). The results obtained reveal that the iodide ion photoexcitation produces two different types of particles absorbing at 345 and 358 nm.
363
Species absorbing &
PIyt + I- &
at 345 nm yDE+
I
280
Specie;5a;;zing
at
nm UV light or
rise in temperature
It is instructive to note that the position of the absorption band after the irradiation of the deaerated solution of MEPI is nearly the same as that reported by Kosower [ 121 for a transition involving charge transfer from the iodide ion to the alkylpyridinium ion in a solution of 1-ethyl-4-methoxycarbonylpyridinium iodide. A quantitative description of the species which absorbs at 358 nm is now given. It can be assumed that the 358 nm absorption is due to an oxygen-sensitized photoprocess which yields a relatively stable complex formed between the existing ionic particles hv,traces py+ + I- 7
K ea
of O2
Complex absorbing at 358 nm
(1)
The equilibrium constant K,, and the molar absorptivity E, of the particles absorbing at 358 nm can be obtained from the study of the variation in the UV absorption at 358 nm with the concentration of MEPI. The Keefer-Andrews equation [ 151 was used
co*
1 1 -= + A, G(2Co - C,) &, E, where Co is the initial concentration of the compound under consideration, C, is the concentration of the complex, E, is the molar absorptivity of the complex and A, is the absorbance due to the complex. The first-order approximation was obtained from eqn. (2) with C, = 0, and the resulting K,, value was used to calculate a C, value. The iteration process was repeated until the resulting Keg value was approximately constant. The values found for K,, and ec are 1.33 X lo3 1 mol-l and 11.1 X lo3 1 mol-’ cm-’ respectively. As readily seen from Fig. 5, the equilibrium (1) is dependent on the temperature of the solution. The data accumulated enabled a calculation to be made of the enthalpy m and entropy AS changes related to complex formation. The numerical values of these quantities, calculated on the basis of the dependence In K,, us. l/T, are m = -1.86 kcal mole1 and AS = 7.32 kcal mole1 K-l. From the negative value of N it can be concluded that the complex is weak. However, AS is slightly positive, which means that the formation of the complex increases the disorder in the system to
364
some degree. It is interesting to note that the value of AH found is very close to the values observed by Verhoven et al. [6] for the complexes between N-methyl-4-cyanopyridinium ion and neutral aromatic donors. A series of irradiated and equilibrated solutions of MEPI was prepared with different ratios of ~-methyl-~-ethoxyc~bonylpyrid~ium ion (Pyi) and iodide ion (I-); the total concentration of ions was kept constant at mol 1-l. A maximum value of A358 was found at a ratio of c=2.0x1Q-3 1:l for the two ionic species. This implies that the UV irradiation leads to the formation of a 1:I complex. The kinetics of the photoinduced conversion were studied using 1 X 10e3 mol 1-i solutions in 80% methanol at 20 “C. In the region of high optical densities of the iodide anion, the process follows zero-order kinetics. As seen from Fig. 4 the UV irradiation initiates further complex formation in the dark. A hyperbolic relationship of the type A358 _-
=
(at
+
(3)
by
t
between the absorbance A 358 at 358 nm and the time t after termination of UV irradiation is strictly obeyed. The rate of formation in the dark of the species which absorbs at 358 nm is given by
dcc
J.7=___~--
dt
b
1
(4)
le, at2 + bt
where a and b are coefficients. It is worth mentioning that the rate of the dark process V increases with increasing time of irradiation 7, e.g. for 7-= 5 min, V = 0.69 X 10w3 and for T = 120 min, V = 6.66 X 1W3 {t = 100 h). The reverse process of complex decomposition, which results from the photoexcitation of alkylpyridinium ion (Py’) (Fig. 6), satisfies the function In C,O- In CFrf = hTrr which is a solution of the rate law for a first-order reaction. C!: denotes the initial concentration of the complex and CC(7r)is the concentration of the complex 7, min after the onset of the 280 nm irradiation of the solution. A possible route of the oxygen-sensitized photoinduced formation of the complex which absorbs at 358 nm is presented below
py+ + I240 nm, (FIS or SSIP) * 280
nm hv or
hv traces of 02
l?y+1- -
temperature increase (c Ip )
f
py+ + I-* _If
py+. * . * {EC) ‘I +
(PyY)* (E)
-
py+ I-” (CC)
{FIS, free ions; SSIP, solvent separated ion pairs; CIP, contact ion pairs; EC, encounter complex; CC, collision complex; E, exciplex).
365
In fluid medium, the photoexcited iodide ion I-* and alkylpyridinium ion Py+ may form encounter and collision complexes. The latter is transformed into an exciplex (see, for example, refs. 16 - 18). Quenching via exciplexes is a particularly important pathway when the reactants are planar molecules capable of forming “sandwich” complexes [ 191. It is remarkable that as early as 1958 Kosower [ 121 noted the presence of this type of intermolecular complex in solutions of alkylpyridinium iodides. In our opinion the exciplex decay produces a contact ion pair which is easily destroyed when the pyridinium component is excited (280 nm irradiation) or as a result of the rise in temperature of the irradiated solutions. The most striking feature of the photochemical behaviour of l-methyl4-ethoxycarbonylpyridinium iodide is the existence of a dark process which results in an increase in the amount of particles which absorb at 358 nm. The available experimental data do not allow us to explain its intrinsic mechanism.
References 1 L. Kaplan, J. W. Pavlik and K. E. Wilzhach, J. Am. Chem. SW., 94 (1972) 3283. 2 J. A. Barltrop and J. D. Coyle, Excited States in Organic Chemistry, Wiley, London, 1978, p. 355 (Russian translation). 3 E. M. Kosower, J. Am, Chem. Sot., 78 (1956) 3497. 4 E, M. Kosower and P. E. Kiinedinst, Jr., J, Am. Chem. Sot., 78 (1956) 3493. 5 E. M. Kosower and J. A. Skorcz, J. Am. Chem. Sot., 82 (1960) 2195. 6 J. W. Verhoven, I. P. Dirkx and Th. J. de Boer, Tetrahedron, 25 (1969) 3395. 7 P. Markov and M. Novkirishka, Annu. Univ. Sofia, 79 (1985). 8 P. Markov and M. Novkirishka, Monatsh. Chem., 119 (1988) 1185. 9 C. R. Masson, V. Boekelheide and W. A. Noyes, Jr., Technique ofOrganic Chemistry, Vol. 2, Wiley-Interscience, New York, 1956, p. 281. 10 E. Lederle, 2. Phys. Chem. Abt. B, IO (1930) 121. 11 M. Smith and M. C. R. Symons,J. Chem. Phys., 25 (1956) 1074. 12 E. M. Kosower, J, Am. Chem. Sot., 80 (1958) 3267. 13 E. Rahinowitch, Rev. Mod. Phys., I4 (1942) 118. 14 J. Frank and R. Platzman, L. Farkas Memorial Volume, Jerusalem, 1952, p. 21. 15 R. M. Keefer and L. J. Andrews,J. Am. Chem. Sot., 74 (1952) 1891. I.6 J. Moore and R. G. Pearson, Kinetics and Mechanism, Wiley, New York, 1981, p. 237. 17 N. Sutin,Acc. Chem. Rea, 15 (1982) 275. 18 R, D. Cannon, Electron Transfer Reactions, Butterworths, London, 1980. p. 97. 19 G. J. Kavarnos and N. J. Turro, Chem. Rev., 86 (1986) 401.