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Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino-substituted fluorenones
ARTICLE IN PRESS
Radiation Physics and Chemistry 72 (2005) 711–722
Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino-substituted fluorenones Vaishali Samant, Ajay K. Singh, Tulsi Mukherjee, Dipak K. Palit* Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Received 28 January 2004; accepted 13 April 2004
Abstract Electron pulse radiolysis of four differently substituted amino derivatives of fluorenone, namely, 1-amino-, 2-amino3-amino-, and 4-aminofluorenone, has been carried out to study the effect of structure on the spectroscopic and kinetic characteristics of the triplet excited states as well as the transient free radical intermediates formed under reducing and oxidizing conditions. The triplet states of these compounds have been generated in benzene by pulse radiolysis and in other solvents by flash photolysis technique and their spectral and kinetic properties have been investigated. Hydrated electron ðe aq Þ has been found to react with these fluorenone derivatives to form the anion radical species with a diffusion-controlled rate constant. The spectral and kinetic properties of the transient ketyl and anion radicals have been studied by generating them in aqueous solutions of suitable pH. The pKa values of ketyl"anion radical equilibria are in the range of 6.8–7.7 for these derivatives. The oxidized species have been generated by reaction with the azide radical. Hydrogen atom adducts as well as the cation radicals of these derivatives have also been generated by pulse radiolysis and characterized. r 2004 Elsevier Ltd. All rights reserved. Keywords: Amino-substituted fluorenones; Flash photolysis and electron pulse radiolysis; Triplet states; Reaction with hydrated electron; Ketyl and anion radicals; Hydrogen atom adduct; Oxidized and cation radicals
1. Introduction Photo-reduction of aromatic ketones and their derivatives in the presence of H atom donors is one of the most well-studied fundamental processes in photochemistry. It is now well-established that in photochemical reduction of ketones in various solvents, an intermediate step involves the reaction of the excited triplet state of the ketone with the solvent or any other suitable hydrogen atom donor present in solution to form the ketyl radical of the ketone and the free-radical of the donor molecule (Wagner et al., 1986; Wong and King, 1978; Arimitsu et al., 1975; Roth and Manion, *Corresponding author. Tel.: +91-2225-595-091; fax: +9122-550-5151. E-mail address: [email protected] (D.K. Palit).
1975). The excited triplet states of the ketones are capable of abstracting hydrogen atoms from a variety of substrates including hydrocarbons, alcohols, phenols and amines (Gilbert and Baggot, 1991; Wagner et al., 1986). These reactions are known to occur either via a direct H atom transfer process or via an electron transfer process followed by transfer of a proton. However, it is evident that the efficiency of the photo-reduction process is primarily dependent on the quantum yield of the triplet state production as well as other factors, such as, the electronic structure of the lowest excited triplet (T1) state (i.e., whether it is pp or np type), as well as the nature of the solvent or hydrogen atom donor (Beckett and Porter, 1963; Porter and Suppan, 1966; Wagner, 1967). The reactivity of the aromatic ketones towards hydrogen atom abstraction reaction has long been associated with the fundamental difference between
0969-806X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2004.04.132
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V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
the two kinds of T1 states, namely np and pp, and this in turn is governed by the nature and position of the substituent on the aromatic ring and also by the solvent property (Wagner et al., 1971; Wagner and Siebert, 1981). If the T1 state has the npcharacter, the ketyl radical yield is normally high in hydrogen atom donating solvents, but pp kind of T1 state is nearly unreactive towards hydrogen atom abstraction reaction (Das et al., 1981; Leigh et al., 1996; Berger et al., 1978; Wolf et al., 1977; Bhasikuttan et al., 1998). The electron pulse radiolysis technique, which is complimentary to the flash photolysis technique, has been used effectively for studying the properties of the triplet state as well as the ketyl and anion radicals of the ketones. An advantage of this technique is that the ketyl radical is formed by protonation of the radical anion generated due to interaction between hydrated electron and the ketones, thus avoiding the intermediacy of the triplet state. The ketyl radicals are formed in good yield because the parent ketones have high reactivity towards solvated or hydrated electrons (vide infra). Other kind of radicals, such as cation and anion, also could be produced selectively and characterized spectroscopically by this technique (Bensasson et al., 1983). There has been considerable interest in determining the specific role of quinones, ketones and related carbonyl compounds in biological reaction. Their importance in oxidative phosphorylation mechanisms has been established for some of them (Morton, 1965). Along with a few other aromatic carbonyl compounds, Hayon et al. have characterized the spectral and kinetic properties of the ketyl and anion radicals as well as hydrogen atom adduct of fluorenone (Hayon et al., 1972). Inoue and coworkers have reported the properties of the singlet states of amino derivatives of fluorenone by fluorescence spectroscopy as well as those of triplet states in toluene by nanosecond laser flash photolysis study (Biczok et al., 2001). However, to the best of our knowledge, no report is available in the literature on the radiation chemical properties of the amino-substituted fluorenones. In this paper, the spectroscopic and kinetic properties of the triplet state and the cation, anion and ketyl radical species of four different amino-substituted fluorenone derivatives, have been studied both by pulse radiolysis and flash photolysis techniques.
grade (Spectrochem, India) were used without further purification. High-purity nitrogen gas (Indian Oxygen Co., purity>99.9%) was used to deaerate the samples. All experiments were carried out at room temperature (29871 K) unless specified otherwise. Buffer solutions were prepared by mixing solutions of 1 102 mol dm3 of Na2HPO4 and 1 102 mol dm3 of NaH2PO4 in suitable proportions. Aqueous solutions were prepared in nano-pure water obtained from Barnstead System (resistivity 18.3 MO cm). pH of the aqueous solutions was measured by using an ELICO India model LI-120 pH meter. Details of the experimental set-up for the electron pulse radiolysis and the kinetic-absorption spectrophotometric technique used here have been described elsewhere (Guha et al., 1989; Mukherjee, 1997). Sample solutions were taken in a suprasil cuvette of 1 cm pathlength to irradiate by electron pulses of 50 ns duration (fwhm) from a 7 MeV linear electron accelerator at a radiation dose of 15–16 Gy per pulse as measured by an air-saturated 5 102 mol dm3 KSCN dosimeter, tak3 1 ing Ge for (SCN)d cm1 at 500 nm 2 as 21 522 dm mol (G being defined as the number of molecules formed per 100 eV energy absorbed). The temporal absorption profiles at different wavelengths were monitored by kinetic-absorption spectrophotometric arrangement consisting of an OSRAM 450 W pulsed xenon lamp in conjunction with a Krato model GMA 301D-2 monochromator and Hamamatsu R955 photomultiplier tube connected to a Larsen & Toubro model 4072 (100 MHz, 400 Ms/s) digital storage oscilloscope. The data were transferred to an IBM PC, where data analysis was carried out with indigenously developed programme. The nanosecond laser flash photolysis equipment has been described in detailed elsewhere (Palit et al., 1992). Briefly, the third harmonic (355 nm, 3 mJ) output pulses (35 ps) from an active-passive mode locked Nd:YAG laser (Continuum, model 501-C-10) were used for excitation and a cw tungsten lamp in combination with a monochromator, PMT and 500 MHz digital storage oscilloscope (Tektronix, TDS-540A) connected to a PC for data acquisition and analysis.
3. Results and discussion 3.1. Ground-state absorption spectra
2. Experimental Scintillation-grade fluorenone, obtained from Thomas-Becker sensitizers’ kit, has been further purified by recrystallization from methanol. The amino derivatives of fluorenone, namely 1-aminofluorenone (1AF), 2aminofluorenone (2AF), 3-aminofluorenone (3AF) and 4-aminofluorenone (4AF) were obtained from Aldrich, and used as they were received. Solvents of spectroscopic
Ground-state absorption characteristics of fluorenone and its amino-derivatives in different kinds of nonaqueous solvents have been well-studied and the different bands have been well-assigned to np or pp kind of transition (Yatsuhashi et al., 1998; Moog et al., 1991). However, no study has yet been reported in aqueous solutions because of low solubility of these compounds. For the present study, the fluorenone
ARTICLE IN PRESS V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
derivatives have been dissolved in aqueous-alcoholic mixtures (in aqueous solution of 1 101 mol dm3 tertbutanol) and the ground-state absorption spectra have been recorded at three different pHs, namely 1, 5 and 13. The absorption spectra recorded in 270–600 nm region are shown in Fig. 1. In the case of each of the aminofluorenones, the spectra recorded at pH 5 and 13 are similar and could be assigned to the neutral form of them (structure I). At pH 1, the absorption spectrum of 1AF in 350–500 nm region is very similar to those recorded at pH 5 and 13, but the former has higher molar absorption coefficient or oscillator strength. In the case of other three derivatives, the spectra recorded at pH 1, are very different from those at pH 5 and 13, and they could be assigned to the protonated form of these amino-derivatives (structure II). Hence, it is evident that in the case of 2AF, 3AF and 4AF, the pKa values representing the conversion of the protonated form (II) to the neutral form (I), lie between pH 1 and 4, but in the case of 1AF it possibly lies below pH 1. In the cases of 2AF, 3AF and 4AF, the absorbance of the solutions were measured at 480, 440 and 480 nm, respectively, as a function of pH in the range 1–5 and the pKa values for the protonation–deprotonation equili-
8
Molar Absorption Coefficient ( 103 dm3mol-1cm-1)
1AF 4
0 1
2AF
0
3AF 4
0
4AF
2
0 400
500
600
Wavelength (nm) Fig. 1. Ground state absorption spectra of the aminofluorenones in aqueous solution of 1 101 mol dm3 of tertbutanol at pH 1 (—) and pH 5 or 13 (—J—). Absorption spectra are similar at pH 5 and 13.
713
brium in the ground state were determined by sigmoid fitting of the curves. The pKa values are 3.2, 1.7 and 2.3 for 2AF, 3AF and 4AF, respectively. The difference in behaviour in the case of 1AF can possibly be explained by the presence of intramolecular hydrogen bonding between the carbonyl and amino groups, preventing protonation at the nitrogen atom (Moog et al., 1991). +H
+
- H+
H2N
+ H3N
O
O
I
II
ð1Þ
3.2. Triplet state 3.2.1. Generation of the triplet state using pulse radiolysis Pulse radiolysis is an excellent technique for selective creation of excited triplet states of organic molecules using triplet–triplet energy transfer method in aromatic solvents, such as benzene (Bensasson et al., 1983). The radiolysis of benzene produces the benzene triplet (3BZ) in high yield. The energy level (ET ¼ 353 kJ mol1) of 3 BZ is higher than that of most of the organic molecules, but its lifetime is only 1 ns. 3BZ can transfer its energy to another solute having T1 state energy lower than 353 kJ mol1, thus generating the triplet state of the latter. Hence, in pulse radiolysis of benzene containing a relatively higher concentration (>103 mol dm3) of the scavenger species, 3BZ can serve as a sensitizer for initiating the triplet–triplet energy transfer process. In this case, the low intersystem crossing efficiency for the conversion of the singlet to the triplet state of the solute molecule does not pose any limitation to the production of the triplet state. Also, the triplet production process being selective, conclusive evidence for assignment of the photolytically generated transient species to the triplet state can be obtained from the pulse radiolysis studies of the solute in de-aerated solutions of benzene. The absorption spectra of the triplet state produced due to electron pulse radiolysis of four amino-substituted fluorenones in deaerated benzene solutions are presented in Fig. 2A. Each of the triplet absorption spectra consists of two bands, one in 300–400 nm region and the another in 400–700 nm region, and these features are very similar to those of their parent, fluorenone (Biczok et al., 2001), and also to those of aminofluorenones reported using laser flash photolysis technique (Biczok et al., 1999). The molar absorption coefficients for triplet–triplet (T–T) absorption at suitable wavelengths, at which absorption due to the ground state is negligible, have been determined by the method of energy transfer from biphenyl triplet to that of aminofluorenone (Bensasson et al., 1983). The energy of the lowest
ARTICLE IN PRESS V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
excited triplet state (ET ) of biphenyl (273 kJ mol1) lies at much higher level than those of fluorenone (211 kJ mol1) and aminofluorenones (220 and 192 kJ mol1 for 1AF and 2AF, respectively) (Biczok et al., 2001). Using these molar absorption coefficient values, the differential triplet absorption spectra of the aminofluorenones have been corrected for the effect of ground-state bleaching using a standard method (Amouyal et al., 1974) and these corrected spectra are presented in Fig. 2B. The position of the maxima on the corrected T–T absorption spectra as well as the values of molar absorption coefficients at these wavelengths along with the lifetimes of the triplet states of the aminofluorenones in benzene are given in Table 1.
0.012
1AF
0.008
8000
0.004
4000
0.000
0
0.005
0.000 0.006
3AF
0.004 0.002
Extinction Coefficient
2AF
0.010
∆Absorbance
12000
0.000
3.2.2. Generation of the triplet state using flash photolysis The triplet states of four amino-substituted fluorenones in acetonitrile (ACN), methanol and isopropanol (IP) have also been generated by flash photolysis method. In this method, the singlet state formed due to absorption of light undergoes intersystem crossing to form the triplet state. The differential triplet absorption spectra obtained in acetonitrile are shown in Fig. 3. Due to large molar absorption coefficient of absorption of the ground state at shorter wavelength region below 450 nm, the T–T absorption spectra could not be recorded for a few of them. The assignment of these spectra to the triplet states of fluorenone derivatives has been made from significant quenching of the transient
0.000
10000
0
3AF
20000
4AF
0.000 3AF
0.005
10000
0.000 4AF
15000
4AF
0.006
0.003
10000 0.004 5000
0.002 0.000
(A)
2AF
0.003
2AF
20000
0
0.008
1AF
0.004
1AF
∆Absorbance
714
0 300
400
500
600
300
(B)
400
500
0.000
600
400
Wavelength, nm Fig. 2. (A) Differential transient spectra due to T–T absorption obtained following electron pulse radiolysis of the aminofluorenones in benzene. (B) Corrected or true T–T absorption spectra of the aminofluorenones in Benzene.
500
600
Wavelength (nm) Fig. 3. Differential T–T absorption spectra of the aminofluorenones in acetonitrile obtained due to laser flash photolysis at 355 nm.
Table 1 Triplet characteristics of aminofluorenone derivatives Compound
Benzene 3
3
1
lmax (nm) (e; 10 dm mol 1AF 2AF 3AF 4AF a b
450 340 320 320
(14.6), 540 (2.4) (24.970.1), 490 (8.5) (14.0), 450 (5.5) (16.4), 440 (6.7)
Error in determining the value is 15%. Error in determining the value is 10%.
1 a
cm )
t (ms)
b
3473 3173 1872 4274
ACN
Methanol
IP
t (ms)
t (ms)
t (ms)
2572 1371 2873 2372
3973 771 — —
3572 6.772 — —
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species in presence of oxygen in solution. In alcoholic solvents, while the triplet yield was very low for 1AF and 2AF, no triplet absorption could be detected for 3AF and 4AF. Low triplet yield in alcohols could be assigned to the efficient energy degradation process from the singlet state via intermolecular hydrogen bond stretching vibrations (Yatsuhashi et al., 1998). The lifetimes of the triplet states in these solvents are given in Table 1. In alcoholic solvents the lifetimes of the triplet states of aminofluorenones, except 1AF, are considerably reduced as compared to those in benzene and acetonitrile. However, the lifetime of the triplet state of 1AF is not significantly affected, possibly due to presence of weak intramolecular hydrogen bond between the carbonyl and the amino groups, which prevents intermolecular hydrogen bonding with the solvents. The molar absorption coefficient of T–T absorption and hence the triplet yield could not be determined in these solvents with reasonable accuracy using flash photolysis technique due to unavailability of proper standard triplet energy donor or acceptor for the aminofluorenones. Since the ground state absorption and T–T absorption spectra of them are extended throughout the near uv and visible region up to 650 nm, suitable excitation and monitoring wavelengths for the selected donors and acceptors could not be found out. We observe complete decay of the triplet state at each of the wavelengths on the triplet absorption spectra of 1AF and 2AF in alcohols. These observations lead us to conclude that the yield of ketyl radical due to abstraction of hydrogen atom from the alcoholic solvents by the triplet states of the aminofluorenones is insignificant. Lack of reactivity of the triplet states of the aminofluorenones towards H atom abstraction reaction with alcoholic solvents can be assigned to its pp character. Hence it becomes obvious that the pulse radiolysis in aqueous solution is the only suitable method for generation and characterization of the ketyl and the anion radicals of the aminofluorenones. 3.3. Reaction of e aq Electron pulse radiolysis of water generates three d d major primary reactive species, e aq ; OH and H with Gvalues 2.7, 2.7 and 0.5, respectively (Spinks and Wood, 1990). The G-value for e aq is constant (2.7) in the pH range 4–10. A few experiments have been performed in solutions with pH > 10, at which Gðe aq Þ is greater than 2.7 and in such cases actual values of Gðe aq Þ reported at respective pH’s have been used (Spinks and Wood, 1990). The experiments carried out at pH=1, at which the hydrated electron is converted into H atom via the reaction with H+ (Eq. (2)) and hence the G-value of H atom becomes 3.45 (Anbar and Neta, 1967). Among the three primary species, dOH is the oxidizing species while the other two are reducing in nature. By adding suitable scavengers in the solution, the reaction condition can be
715
adjusted in such a way that only the reaction of e aq with the solutes could be extensively studied. dOH radicals are scavenged by 1 101 mol dm3 tert-butanol (Spinks and Wood, 1990). þ d e aq þ H -H ;
keaq ¼ 2:3 1010 dm3 mol1 s1 ;
ð2Þ
d d
OH þ ðCH3 Þ3 COH-H2 O þ H2 C CðCH3 Þ2 OH;
k ¼ 5:5 108 dm3 mol1 s1 :
ð3Þ e aq
with the aminofluorKinetics of the reactions of enones have been studied in de-aerated aqueous solutions containing 1 101 mol dm3 tert-butyl alcohol and different concentrations of aminofluorenones (varying in the range 1 106–2 105 mol dm3). tert-Butyl alcohol not only scavenges the oxidizing dOH radical efficiently but also increases the solubility of the aminofluorenones in aqueous solutions. The decay kinetics of e aq has been monitored at 720 nm and the second order rate constants ðkeaq Þ for the reaction between e aq and the aminofluorenones have been evaluated from the slopes of the linear plot of the pseudo first-order rate constants vs. the concentration of the aminofluorenones. The values of keaq are found to vary between 2.5 and 4 1010 dm3 mol1 s1 (listed in Table 2). They are found to be comparable with that of parent fluorenone, i.e. the reactions are diffusion controlled (Table 2). We do not observe any noticeable difference in the rate constant values for differently substituted aminofluorenones and hence we infer that all of them are equally reactive towards e aq and the rate constants are independent of the position of the amino group. Even intramolecular hydrogen bonding in 1AF does not make any noticeable difference in the reactivity with e aq : We compare the values of keaq associated with the amino-substituted benzophenones (keaq values are 4.4 109 dm3 mol1 s1 for 2-aminobenzophenone, 1 1010 dm3 mol1 s1 for 4-aminobenzophenone and 9.8 109 dm3 mol1 s1 for 4-(N,N-dimethylamino)-benzophenone) with those given in Table 2 (Singh et al., 2002). The higher reactivities of the amino-substituted fluorenones towards e aq as compared to those of benzophenone analogues may possibly be associated with the cyclopentadienyl nature of the central ring of the fluorenone derivatives (Hayon et al., 1972). Fluorenone differs from benzophenone by the linkage at the ortho positions of the phenyl groups. In its anion radical, which is formed following capture of an electron by fluorenone or its derivatives, has an electron distribution analogous to that of cyclopentadienyl anion (structure IV), which is unusually stable because of completion of the aromatic hextet. 3.3.1. Transient absorption at different pH The time-resolved spectra of the transient radical species, which are produced due to the reaction of the
ARTICLE IN PRESS
∆Absorbance
6.0 104
Molar Absorption Coefficient
6.870.2 1.0 106
3.5 105
6.970.1 1.0 105
3.2 1010 4AF
Error in determining value is 10%. Error in determining the value is 15%. c Hayon et al. (1972).
2.3 1010 3AF
b
3.6 1010 2AF
a
2.2 1010
6
0.007
λ=540 nm
0.006
0.005
4 4
6
8
10
12
pH 2
0 500
600
700
Fig. 4. (A) Differential transient absorption spectra obtained due to electron pulse radiolysis of 1AF (1 103 mol dm3) in nitrogen-saturated aqueous solution in the presence of 1 101 mol dm3 tert-butanol (pH=1 (n), pH=5 (—J—) and pH=13 (—K—)). (B) Corrected or true absorption spectra of hydrogen atom adduct (—n—), ketyl radical (—J—) and anion radical (—K—) of 1AF. Inset of (A): plot of the first-order rate constants for the reaction of e aq with 1AF at pH 7 vs. concentrations of 1AF. The slope corresponds to the bimolecular rate constant, keaq : Inset of (B): plot of the differential transient absorption monitored at 540 nm obtained due to pulse radiolysis of 1AF (1 103 mol dm3) in the pH range 4–13. Sigmoid curve has been fitted according to Eq. (6) to obtain the pKa value of 1AF.
1.6 104 1.3 104 1.1 104 5.1 103 1.8 103 1.9 103 1.3 104 3.4 103 6.0 103 2.0 103 2.3 103 1.3 103 1AF
80
Wavelength (nm)
347 362 520 390 430 520 350 460 370 450 390 510 3.3 1010
(B)
400
cm ) Flc
40
[1AF] (10-6 mol dm-3)
∆Absorbance
8.5 105
4.2 103 4.6 103 1.8 103 5.8 103 4.2 103 4.7 103 3.1 103 4.8 103 1.3 103 380 430 510 370 460 370 440 400 470 7.770.2 1.2 106
1.1 104 6.2 104 6.370.2 360c 450
lmax (nm) e (dm mol
10 0
(103 dm3mol-1cm-1)
1.6 106 1.5 105
3.8 105
4.5 105
270 348 500 350 420 550 340 460 420 510 350 530
3.8 105
2k=e (s1)a lmax (nm) cm )
1 b
2k=e (s )
20
0.01
8
1 b
lmax (nm) e (dm mol
0.02
30
-0.01
1
3
Anion radical at pH=13 pKa
1 a
(A)
0.00
1
3
Compound ke-aq (dm3 mol1 s1 at pH=7)a Ketyl radical at pH=5
Table 2 Transient characteristics of fluorenone and aminofluorenones on pulse radiolytic reduction in aqueous solutions at pHs 1, 5 and 13
2k=e (s )
1 a
Hydrogen Adduct at pH=1
0.03
ke-aq x 10-5
V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
716
aminofluorenones (conc. 1 103 mol dm3) with primary radical species in aqueous solutions (i.e. e aq at pH’s 5 and 13 and H atom at pH 1) have been recorded soon after the electron decay (i.e. at 1 ms) and at 20 ms after the electron pulse. However, the time-resolved transient absorption spectra in each case did not reveal any kind of evolution of the spectral shapes within 20 ms time-domain, thus indicating the formation of only one kind of transient species. The transient absorption spectra recorded at 20 ms after the electron pulse are shown in the Figs. 4A-7A. These figures show that for each of the aminofluorenones, the transient spectra of the radical species recorded in aqueous solution at three different pHs are quite different. Considering the fact + that at pH=1, e to aq is completely quenched by H produce H atom (Eq. (2)), the transient absorption
ARTICLE IN PRESS V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
ðlÞ eðlÞ R ¼ eAF þ
½DA ½GðSCNÞ:-2 ½eðSCNÞ: 2 ; ½AðSCNÞ:-2 GP
ð4Þ
where eðlÞ AF is the molar absorption coefficient in dm3 mol1 cm1 for the aminofluorenones at wavelength l; DA is the observed difference in absorbance of the transient radical species at l: AðSCNÞ: is the absorbance of 2 (SCN)d 2 which is produced in the dosimeter solution under iso-dose condition and measured at 500 nm. GP is the d G-value of the primary radical species (e.g. e aq or H ) responsible for the generation of the particular radical species. The corrected transient absorption spectra of the ketyl and anion radicals as well as that of H atom adduct are presented in the Figs. 4B-7B. The decay rate constants for the H atom adduct as well as the ketyl and the anion radicals are given in Table 2
+
H2N
O I
eaq
.
0.03
0.02
0
To obtain pKa values for the acid base equilibrium for the ketyl radical—anion radical conversion process (i.e. II"III), the absorbance values of the transient radical species surviving at 20 ms after the electron pulse were measured as a function of pH at a particular wavelength selected in such a way that at this wavelength the difference in the differential transient absorption between those of the ketyl and the anion radicals is
40
80
[2AF] (10-6mol dm-3)
0.01
0.00 12
(B) 8
0.012
λ=450nm
0.009 0.006 6
4
8
pH
10
0 400
500
600
700
Wavelength (nm)
Fig. 5. (A) Differential transient absorption spectra obtained due to electron pulse radiolysis of 2AF (1 103 mol dm3) in nitrogen-saturated aqueous solution in the presence of 1 101 mol dm3 tert-butanol (pH=1 (n), pH= 5 (—J—) and pH=13 (—K—)). (B) Corrected or true absorption spectra of hydrogen atom adduct (—n—), ketyl radical (—J—) and anion radical (—K—) of 2AF. Inset of (A): plot of the first-order rate constants for the reaction of e aq with 2AF at pH 7 vs. concentrations of 2AF. The slope corresponds to the bimolecular rate constant, keaq : Inset of (B): plot of the differential transient absorption monitored at 450 nm obtained due to pulse radiolysis of 2AF (1 103 mol dm3) in the pH range 4–13. Sigmoid curve has been fitted according to Eq. (6) to obtain the pKa value of 2AF.
maximum, as well as the ground state molar absorption coefficient is also negligible (see Figs. 4A-7A). The
.
H2N OII
20
0
∆Absorbance
∆Absorbance
(A)
+H -H
H2N
k x 10-5
40
Molar Absorption Coefficient (103 dm3mol-1cm-1)
spectra recorded at pH 1, is assigned to hydrogen atom adduct. Like other aromatic ketones, addition of H atom is not specific in the case of the aminofluorenones (Hayon et al., 1972). However, attachment of electrons to the aminofluorenones, like other aromatic ketones, occurs almost exclusively with the carbonyl group. Hence, the transient absorption spectra recorded at pH 5 and 13 can be assigned to the ketyl and anion radicals formed due to attachment of the solvated electron with the fluorenone derivatives (Eq. 5). Considering the fact that G-values of the solvated electron at pH 5 and 13 are 2.7 and 2.9, respectively, and that of H atom is 3.45. The molar absorption coefficient values for absorption of the ketyl and anion radicals as well as H atom adduct at wavelengths, at which ground state absorption for the compound is not significant, have been determined. Using the same, the true or corrected absorption spectra of the radical species have been obtained by correcting the corresponding difference spectra due to bleaching of the ground state. If at a particular pH, a single form, say the ketyl or the anion radical or the hydrogen atom adduct, is present, then at any wavelength l, the molar 3 1 absorption coefficient (eðlÞ cm1) of the R ; dm mol transient radical species is given by
717
ð5Þ
OH III
wavelengths thus selected are 540, 450 and 400 nm for 1AF, 2AF and 4AF, respectively (see Figs. 4A-7A). With increase in pH of the solution, the equilibrium concentration of the anion radical increases and that of the ketyl radical decreases. Hence, we observe an increase in absorption at 450 and 400 nm for 2AF and 4AF, respectively, and a decrease in absorption at 540 nm in the case 1AF. pKa of 3AF could not be
ARTICLE IN PRESS
(A)
aq
∆Absorbance
0.02
ke- x 10-5
V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
718
20 10 0
0.01
none (pKa=9.25) and its amino-substituted derivatives (pKa values are 8.8 and 9.8 for 2-aminobenzophenone and 4-aminobenzophenone, respcetively) could also be explained by the stability of cyclopentadienyl anion (structure IV) (Singh et al., 2002).
30
40
80
[3AF] (10-6 mol dm-3)
0.00
6
(B)
H2N
O.
3
IV
Protonation of oxygen removes an electron from the central ring with concomitant loss of stability. In case of
0
0.02
determined because of unavailability of any such suitable wavelength. It is important to note that throughout the pH range 4–13, each of the aminofluorenones exists in solution as a species in the neutral form, as shown by structure I. Hence, it is straight forward to determine the pKa value of the ketyl—anion radical conversion process by fitting the sigmoid curves obtained, as shown in the insets of the Figs. 4B-7B, to the following Eq. (6) (Bensasson et al., 1983), AII AIII þ ; 1 þ 10ðpH-pK1 Þ 1 þ 10ðpK1 -pHÞ
ð6Þ
where, AII and AIII represent the absorbance of the species II and III, respectively, measured at a particular wavelength and at pH values, where these are the only absorbing species in solution. The pKa values thus obtained for the aminofluorenes are listed in Table 2. pKa values of 2AF and 4AF are comparable to that of the parent fluorenone. The higher pKa (7.7) value for the deprotonation of the OH group at the carbonyl site of the ketyl radical species of 1AF can be explained by considering the presence of weak intramolecular hydrogen bonding in 1AF. Higher basicity of the anion radicals of fluorenone (pKa=6.3) and its derivatives as compared to those of the anion radicals of benzophe-
30
aq
(A)
ke-
Fig. 6. (A) Differential transient absorption spectra obtained due to electron pulse radiolysis of 3AF (1 103 mol dm3) in nitrogen saturated aqueous solution in the presence of 1 101 mol dm3 tert-butanol (pH=1 (—n—), pH=5 (—J—), pH=13 (—K—)). (B) Corrected or true absorption spectra of hydrogen atom adduct (—n—), ketyl radical 5 (—J—) and anion radical (—K—) of 3AF. Inset of (A): plot of the first-order rate constants for the reaction of e aq with 3AF at pH 7 vs. concentrations of 3AF. The slope corresponds to the bimolecular rate constant keaq :
Aobs ¼
x 10-5
700
15
0
40
80
[4AF]
0.01
(10-6 mol dm-3)
0.00 5
(B)
4 3
∆Absorbance
500 600 Wavelength (nm)
∆Absorbance
400
Molar Absorption Coefficient (103 dm3mol-1cm-1)
Molar Absorption Coefficient (103 dm3mol-1cm-1)
-
0.02
λ = 400 nm
0.01
6
8
10
12
pH
2 1 0 400
500 600 Wavelength (nm)
700
Fig. 7. (A) Differential transient absorption spectra obtained due to electron pulse radiolysis of 4AF (1 103 mol dm3) in nitrogen-saturated aqueous solution in the presence of 1 101 mol dm3 tert-butanol (pH=1 (n), pH=5 (—J—) and pH = 13 (—K—)). (B) Corrected or true absorption spectra of hydrogen atom adduct (—n—), ketyl radical (—J—) and anion radical (—K—) of 4AF. Inset of (A): plot of the first-order rate constants for the reaction of e aq with 4AF at pH 7 vs. concentrations of 4AF. The slope corresponds to the bimolecular rate constant, keaq : Inset of (B): plot of the differential transient absorption monitored at 400 nm obtained due to pulse radiolysis of 4AF (1 103 mol dm3) in the pH range 4–13. Sigmoid curve has been fitted according to Eq. (6) to obtain the pKa value of 4AF.
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3.4. Oxidation using Nd3 radical
0.10 Molar Abs. Coeff. (2x103dm3 mol-1 cm-1)
3
0.08 ∆Absorbance (arb. unit)
benzophenone with amino groups substituted at different positions of the phenyl ring, the basicity of the anion radical is reduced and this can be explained by considering the positive inductive effect of the amino or the dimethylamino group (Singh et al., 2002). However, in the case of the amino-substituted fluorenones, this effect seems to be insignificant as compared to the effect of filling of the aromatic sextet in the cyclopentadienyl ring. Hence, the pKa values of the aminofluorenones are little affected by amino substitution as compared to that of the parent fluorenone.
The most convenient way for obtaining a condition, which is suitable exclusively for oxidation of the solute, is to saturate the solution with N2O, which converts e aq to dOH (Eq. (7))
0.06
d
d OH þ N 3 -N3 þ OH ;
k ¼ 1:2 1010 dm3 mol1 s1 ;
ð8Þ
Nd3 radical can be generated by irradiation of N2Osaturated aqueous solutions containing NaN3 (1 101 mol dm3) at pHB5. In this condition, almost all primary dOH species generated by electron pulse were converted into Nd3 with GB6:9 molecules per 100 eV (Spinks and Wood, 1990). One electron reduction potential of Nd3 /N 3 couple is +1.33 V vs. NHE. For generating oxidized radicals, solutions containing 2.5 104 mol dm3 of the solutes, 3 mol dm3 of acetonitrile and 0.1 mol dm3 of NaN3 at pH 5 were radiolyzed by electron pulse. The differential absorption spectra of the transient species produced due to reaction of the Nd3 radical with the amino-substituted fluorenones are presented in Fig. 8. However, no transient radical species, which can be characterized as an oxidized radical species, has been detected for Fl and 1AF. The true absorption spectra of the oxidized radical species have been obtained by applying Eq. (4) to the corre-
500
0.00
-0.02
2
OH is known to be a strong oxidizing radical species (one-electron reduction potential of dOH /OH couple is +2.8 V vs. NHE) but non-selective in its reactions (Spinks and Wood, 1990). Hence, dOH species reacts with the solute to give oxidized species along with other products, such as OH-adduct and deprotonated product. It is possible to generate a milder but more selective oxidizing condition by converting dOH into another inorganic radical. In this work we have used azidyl radical Nd3 , which is known as specific oneelectron oxidant, for generating the oxidized radical species of fluorenone and aminofluorenones.
0
0.02
400
d
1
400
d
ð7Þ
2
0.04
O H þ OH ; e aq þ N2 O-N2 þ O ! H O
k ¼ 9:1 109 dm3 mol1 s1 ;
719
500 600 Wavelength (nm)
Fig. 8. Differential transient absorption spectra obtained due to electron pulse radiolysis of fluorenone and the aminofluorenones (2.5 104mol dm3) in N2O saturated aqueous solution in the presence of 3 mol dm3 acetronitrile and 1 101 mol dm3 NaN3. Inset: Ground state absorption spectra of fluorenone and aminofluorenones (2.5 104 mol dm3) in aqueous solution in presence of 3 mol dm3 acetronitrile and 1 101mol dm3 NaN3; 2AF (—J—), 3AF (—,—) and 4AF (—&—).
sponding difference spectra using the value of GP (6.5 molecules per 100 eV). The true absorption spectra thus obtained for fluorenone, 2AF, 3AF and 4AF are shown in Fig. 9. The absorption maxima, molar absorption coefficient, growth life time and 2k=e values for oxidized species are listed in Table 3. The oxidized radical species formed in the aminofluorenones have an absorption band above 400 nm. It has already been reported that the oxidation of amino group in aniline gives anilino radical that has an absorption band in this wavelength region (Solar et al., 1986). So the transient absorption in case of the aminofluorenones is due to formation of oxidized species by oxidation of the amino group. Absence of formation of oxidized radical species in case of Fl and 1AF, which is intramolecularly hydrogen bonded, also suggest the fact that the amino group in other aminofluorenones undergoes oxidation by Nd3 radical, not in the aromatic ring (Eq. 9).
.
N3
H2N
.
HN O
O
ð9Þ
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720
3.5. Cation radical in dichloroethane Detection of the cationic species in any photochemical or radiation chemical process reveals the involvement of charge or electron-transfer reactions. Hence, it is essential to have information regarding the spectroscopic and kinetic properties of the cation radical by
2AF
Molar Absorption Coeff. (103 dm3 mol-1 cm-1)
8
4 0 3AF 4 2 0 4AF
4
2
0 400
500 Wavelength (nm)
600
Fig. 9. True transient absorption spectra of the oxidized radicals of 2AF, 3AF and 4AF generated by pulse radiolysis of aqueous solution in the presence of azidyl radical.
generating it in isolated condition. The electron pulse radiolysis in chlorinated solvents has been used as a standard method to generate and characterize the radical cations of different kinds of solute molecules (Alfassi et al., 1989). However, in this method, several other oxidants are generated in solution, in addition to the radical cation of the solvent molecule, which is the main oxidant, and different kinds of transient species, such as ion pairs, radicals and chlorine atom adducts, may be formed. Hence, this method becomes nonselective and one should be careful in characterizing the transient species formed. We have studied the cationic species generated by pulse radiolysis of the aminofluorenones in 1,2-dichloroethane (DCE) solvent. Aminofluorenones have good solubility in this solvent because of its higher dielectric constant (eB10) as compared to other chloro-hydrocarbon solvents, such as, carbon tetrachloride and dichloromethane, which are common for generation of cation radicals pulse radiolytically (Wang et al., 1979). According to the reports available in the literature (Berman et al., 1979), for radiolysis in low-temperature glasses and liquid DCE, it is well known that aromatic solutes such as biphenyl, anthracene and many others form respective radical cations, very likely by charge transfer to the radical cation of the solvent. In the gasphase studies, the parent radical cation, ClCH2CH2Cld+ and the radical cation ClCH2CH+ 2 (Berman et al., 1979) were observed. Differential transient absorption spectra of the radical species produced due to pulse radiolysis of Fl and aminofluorenones (concentration 2.5 104 mol dm3) in DCE thus obtained are presented in Fig. 10. The absorption maxima, and 2k=e values for cation radicals determined in the present experimental condition are
Table 3 Transient characteristics of Fluorenone and Aminofluorenone derivatives on pulse radiolytic oxidation at pH=5 and cation radical in 1,2-dichloromethane Compound
Fl
1AF 2AF 3AF
4AF a
Oxidized radical
Cation radical 3
1
lmax (nm)
eðlmax Þ (dm mol
410
3.6 102
1 a
cm )
2k=e (s )
0.570.05
500
2.5 103
0.670.05
3.5 105
360 440
5.5 103 3.9 103
c
c
350 500
3
3.4 10 1.3 103
Error in determining the value is 15%. Error in determining value is 10%. c Due to very weak signal values cannot be determined accurately. b
tg (ms)
1 b
0.970.04
5
6.0 10
lmax (nm) 330 460 670 390 370 490 340 410 520 330 450
2k=e (s1)b 4.5 106 8.0 106 1.0 106
3.0 106 1.6 106
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unreactive towards the H-atom abstraction reaction with the alcoholic solvents. Amino-substituted fluorenones reacts with the hydrated electron at the diffusioncontrolled rate (B2 1010 dm3 mol1 s1). Higher reactivity of the aminofluorenones as well as the higher pKa values of the ketyl-anion radical equilibrium as compared to those of the benzophenone analogues has been explained by the extra stability of the aminofluorenone radical anion due to completion of the aromatic sextet in the cyclopentadienyl anion formed at the central ring of the fluorenone moiety. Oxidized radicals have been generated in aqueous solution using Nd3 as the oxidizing species and the cation radicals by electron transfer from the cation radical of 1,2 dichloroethane. The difference in spectral characteristics of the radicals generated by these two methods could be attributed to the formation of only aminyl radicals in aqueous solution while the radicals formed in DCE include other species as well.
Fl
0.01
0.00 0.01 0.00
1AF
∆Absorbance
-0.01
2AF
0.02
0.00 0.010
3AF
0.005 0.000 0.006
721
4AF
References
0.003 0.000 400
500 600 Wavelength (nm)
700
Fig. 10. Differential transient absorption spectra of 2.5 104 mol dm3 solution of fluorenone and its amino derivatives in dicholoroethane.
listed in Table 3. Comparing the spectral shapes in the Figs. 8 and 10, we observe that the spectra of the radical species of the aminofluorenones generated in DCE are found to have different shapes from those of the oxidized radicals generated in aqueous solution using Nd3 radical. Fl could not be oxidized by Nd3 radical in aqueous solution. Since Fl as well as all four aminosubstituted fluorenones form the radical species in DCE, this possibly suggests that the initial cation radical formed in DCE is centreed in the central aromatic ring of the fluorenone derivatives. However, other radicals may be present as well, and only from the spectral shapes it is not possible to comment on the structure of the radical species formed in this experiment.
4. Conclusion Spectroscopic and kinetic properties of the triplet excited states as well as the transient free radical intermediates of the differently substituted aminofluorenones have been investigated by using flash photolysis and electron pulse radiolysis techniques. Due to pp nature of the triplet states of aminofluorenones, they are
Alfassi, Z.B., Mosseri, S., Neta, P., 1989. Reactivitives of chlorine atoms and peroxyl radicals formed in radiolysis of dichloromethane. J. Phys. Chem. 93, 1380–1385. Amouyal, E., Bensasson, R.V., Land, E.J., 1974. Triplet states of ubiquinone analogs studied by ultra violate and electron nanosecond irradiation. Photochem. Photobiol. 20, 415–422. Anbar, M., Neta, P., 1967. Reactions of halogenoaliphatic acids with free radicals in aqueous solution. Part I. Reactions with hydrogen atoms. J. Chem. Soc. (A), 834–837. Arimitsu, S., Masuhara, H., Mataga, N., Tsubomura, H., 1975. Laser photolysis studies on quenching processes of triplet benzophenones by amines in fluid solutions. J. Phys. Chem. 79, 1255–1259. Beckett, A., Porter, G., 1963. Primary photochemical processes in aromatic molecules. Trans. Faraday Soc. 59, 2038–2050. Bensasson, R.V., Land, E.J., Truscott, T.G., 1983. Flash Photolysis and Pulse Radiolysis. Pergamon Press, Oxford, UK. Berger, M., McAlpine, E., Steel, C., 1978. Substituted acetophenones. Importance in activation energies in mixed state models of photoreactivity. J. Am. Chem. Soc. 100, 5147–5151. Berman, D.W., Anicich, V., Beauchamp, J.L., 1979. Stabilities of isomeric ions C2H4X+(X=Cl, Br) by photoionization mass spectrometry and ion cyclotron resonance spectroscopy. General considerations of the relative stabitities of cyclic and acyclic isomeric onium ions. J. Am. Chem. Soc. 101, 1239–1248. Bhasikuttan, A.C., Singh, A.K., Palit, D.K., Sapre, A.V., Mittal, J.P., 1998. Laser flash photolysis studies on the monohydroxy derivatives of benzophenone. J. Phys. Chem. A 102, 3470–3480. Biczok, L., Berces, T., Inoue, H., 1999. Effects of molecular structure and hydrogen bonding on the radiationless
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deactivation of singlet excited fluorenone derivatives. J. Phys. Chem. 103, 3837–3842. Biczok, L., Berces, T., Yatsuhashi, T., Tachibana, H., Inoue, H., 2001. The role of intersystem crossing in the deactivation of the singlet excited aminofluorenones. Phys. Chem. Chem. Phys. 3, 980–985. Das, P.K., Encinas, M.V., Scaiano, J.C., 1981. Laser flash photolysis study of the reactions of carbonyl triplets with phenols and photochemistry of p-hydroxy propiophenone. J. Am. Chem. Soc. 103, 4154–4162. Gilbert, A., Baggot, J., 1991. Essentials of Molecular Photochemistry. CRC Press, Boston, pp. 302–328. Guha, S.N., Moorthy, P.N., Kishore, K., Naik, D.B., Rao, K.N., 1989. One-electron reduction of thionine studied by pulse radiolysis. Proc. Indian Acd. Sci. (Chem. Sci.) 99, 261–271. Hayon, E., Ibata, T., Lichtin, N.N., Simic, M., 1972. Electron and hydrogen atom attachment to aromatic carbonyl compounds in aqueous solution. Absorption spectra and dissociation constant. J. Phys. Chem. 76, 2072–2078. Leigh, W.J., Lathioor, E.C., Pierre, St.M.J., 1996. Photoinduced hydrogen abstraction from phenols by aromatic ketones. A new mechanism for hydrogen abstraction by carbonyl np and pp triplets. J. Am. Chem. Soc. 118, 12339–12348. Moog, R.S., Burozski, N.A., Desai, M.M., Good, W.R., Silvers, C.D., Thompson, P.A., Simon, J.D., 1991. Solution photo physics of 1- and 3-aminofluorenone: The role of inter- and intramolecular hydrogen bonding in radiationless deactivation. J. Phys. Chem. 95, 8466–8473. Morton, R.A., 1965. Biochemistry of Quinones. Academic Press, New York. Mukherjee, T., 1997. In: Ahmed, S.A. (Ed.), Atomic, Molecular and Cluster Physics. Narosa, New Delhi, pp. 299. Palit, D.K., Sapre, A.V., Mittal, J.P., Rao, C.N.R., 1992. Photophysical properties of fullerenes, C60 and C70. Chem. Phys. Lett. 195, 1–6. Porter, G., Suppan, P., 1966. Primary photochemical processes in aromatic molecules. Trans. Faraday Soc. 62, 3375–3383. Roth, H.D., Manion, M.L., 1975. Photoreactions of ketones with amines CIDNP criteria for the intermediacy of
aminoalkyl radicals and aminium radical ions. J. Am. Chem. Soc. 97, 6886–6888. Singh, A.K., Palit, D.K., Mukherjee, T., 2002. Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino and dimethylamino derivatives of benzophenone. J. Phys. Chem. A 106, 6084–6093. Spinks, J.W.T., Wood, R.J., 1990. An Introduction to Radiation Chemistry. Wiley-Interscience, New York, (a) p. 262, (b) p. 327. Solar, S., Solar, W., Getoff, N., 1986. Resolved multisite OH attack on aqueous aniline studied by pulse radiolysis. Rad. Phys. Chem. 28, 229–234. Wagner, P.J., 1967. Solvent Effects on Type-II Photoelimation of Phenyl Ketones. J. Am. Chem. Soc. 10389, 5898–5901. Wagner, P.J., Kemppainen, A.E., Schott, H.N., 1971. Effect of ring substituents on the type-II photoreactions of phenyl ketones. How nearby excited triplets affect chemical reactivity. J. Am. Chem. Soc. 95, 5604–5614. Wagner, P.J., Siebert, E.J., 1981. Deactivation of triplet phenyl alkyl ketones by conjugatively electron withdrawing substituents. J. Am. Chem. Soc. 103, 7329–7335. Wagner, P.J., Truman, R.J., Puchalski, A.E., Wake, R., 1986. Photoreactivity of a-fluorinated phenyl alkyl ketones. J. Am. Chem. Soc. 108, 7727–7738, 7739–7745. Wang, Y., Tria, J.J., Dorfman, L.M., 1979. Identity and yield of positive charge centers in irradiated chloro hydrocarbon liquids and the rates of their interaction with solute molecules. J. Phys. Chem. 83, 1946–1951. Wolf, M.W., Brown, R.E., Singer, L.A., 1977. Deactivation of benzophenone triplets via exiplex formation. Evidance for dual reaction pathways. J. Am. Chem. Soc. 99, 526–531. Wong, S., King, 1978. Electron transfer vs. hydrogen abstraction in photoreduction of quinones. An application of chemically induced dynamic electron polarization. J. Am. Chem. Soc. 100, 5488–5490. Yatsuhashi, T., Nakajima, Y., Shimada, T., Inoue, H., 1998. Photophysical properties of intramolecular charge-transfer excited singlet state of aminofluorenone derivatives. J. Phys. Chem. A 102, 3018–3024.
Radiation Physics and Chemistry 72 (2005) 711–722
Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino-substituted fluorenones Vaishali Samant, Ajay K. Singh, Tulsi Mukherjee, Dipak K. Palit* Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Received 28 January 2004; accepted 13 April 2004
Abstract Electron pulse radiolysis of four differently substituted amino derivatives of fluorenone, namely, 1-amino-, 2-amino3-amino-, and 4-aminofluorenone, has been carried out to study the effect of structure on the spectroscopic and kinetic characteristics of the triplet excited states as well as the transient free radical intermediates formed under reducing and oxidizing conditions. The triplet states of these compounds have been generated in benzene by pulse radiolysis and in other solvents by flash photolysis technique and their spectral and kinetic properties have been investigated. Hydrated electron ðe aq Þ has been found to react with these fluorenone derivatives to form the anion radical species with a diffusion-controlled rate constant. The spectral and kinetic properties of the transient ketyl and anion radicals have been studied by generating them in aqueous solutions of suitable pH. The pKa values of ketyl"anion radical equilibria are in the range of 6.8–7.7 for these derivatives. The oxidized species have been generated by reaction with the azide radical. Hydrogen atom adducts as well as the cation radicals of these derivatives have also been generated by pulse radiolysis and characterized. r 2004 Elsevier Ltd. All rights reserved. Keywords: Amino-substituted fluorenones; Flash photolysis and electron pulse radiolysis; Triplet states; Reaction with hydrated electron; Ketyl and anion radicals; Hydrogen atom adduct; Oxidized and cation radicals
1. Introduction Photo-reduction of aromatic ketones and their derivatives in the presence of H atom donors is one of the most well-studied fundamental processes in photochemistry. It is now well-established that in photochemical reduction of ketones in various solvents, an intermediate step involves the reaction of the excited triplet state of the ketone with the solvent or any other suitable hydrogen atom donor present in solution to form the ketyl radical of the ketone and the free-radical of the donor molecule (Wagner et al., 1986; Wong and King, 1978; Arimitsu et al., 1975; Roth and Manion, *Corresponding author. Tel.: +91-2225-595-091; fax: +9122-550-5151. E-mail address: [email protected] (D.K. Palit).
1975). The excited triplet states of the ketones are capable of abstracting hydrogen atoms from a variety of substrates including hydrocarbons, alcohols, phenols and amines (Gilbert and Baggot, 1991; Wagner et al., 1986). These reactions are known to occur either via a direct H atom transfer process or via an electron transfer process followed by transfer of a proton. However, it is evident that the efficiency of the photo-reduction process is primarily dependent on the quantum yield of the triplet state production as well as other factors, such as, the electronic structure of the lowest excited triplet (T1) state (i.e., whether it is pp or np type), as well as the nature of the solvent or hydrogen atom donor (Beckett and Porter, 1963; Porter and Suppan, 1966; Wagner, 1967). The reactivity of the aromatic ketones towards hydrogen atom abstraction reaction has long been associated with the fundamental difference between
0969-806X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2004.04.132
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the two kinds of T1 states, namely np and pp, and this in turn is governed by the nature and position of the substituent on the aromatic ring and also by the solvent property (Wagner et al., 1971; Wagner and Siebert, 1981). If the T1 state has the npcharacter, the ketyl radical yield is normally high in hydrogen atom donating solvents, but pp kind of T1 state is nearly unreactive towards hydrogen atom abstraction reaction (Das et al., 1981; Leigh et al., 1996; Berger et al., 1978; Wolf et al., 1977; Bhasikuttan et al., 1998). The electron pulse radiolysis technique, which is complimentary to the flash photolysis technique, has been used effectively for studying the properties of the triplet state as well as the ketyl and anion radicals of the ketones. An advantage of this technique is that the ketyl radical is formed by protonation of the radical anion generated due to interaction between hydrated electron and the ketones, thus avoiding the intermediacy of the triplet state. The ketyl radicals are formed in good yield because the parent ketones have high reactivity towards solvated or hydrated electrons (vide infra). Other kind of radicals, such as cation and anion, also could be produced selectively and characterized spectroscopically by this technique (Bensasson et al., 1983). There has been considerable interest in determining the specific role of quinones, ketones and related carbonyl compounds in biological reaction. Their importance in oxidative phosphorylation mechanisms has been established for some of them (Morton, 1965). Along with a few other aromatic carbonyl compounds, Hayon et al. have characterized the spectral and kinetic properties of the ketyl and anion radicals as well as hydrogen atom adduct of fluorenone (Hayon et al., 1972). Inoue and coworkers have reported the properties of the singlet states of amino derivatives of fluorenone by fluorescence spectroscopy as well as those of triplet states in toluene by nanosecond laser flash photolysis study (Biczok et al., 2001). However, to the best of our knowledge, no report is available in the literature on the radiation chemical properties of the amino-substituted fluorenones. In this paper, the spectroscopic and kinetic properties of the triplet state and the cation, anion and ketyl radical species of four different amino-substituted fluorenone derivatives, have been studied both by pulse radiolysis and flash photolysis techniques.
grade (Spectrochem, India) were used without further purification. High-purity nitrogen gas (Indian Oxygen Co., purity>99.9%) was used to deaerate the samples. All experiments were carried out at room temperature (29871 K) unless specified otherwise. Buffer solutions were prepared by mixing solutions of 1 102 mol dm3 of Na2HPO4 and 1 102 mol dm3 of NaH2PO4 in suitable proportions. Aqueous solutions were prepared in nano-pure water obtained from Barnstead System (resistivity 18.3 MO cm). pH of the aqueous solutions was measured by using an ELICO India model LI-120 pH meter. Details of the experimental set-up for the electron pulse radiolysis and the kinetic-absorption spectrophotometric technique used here have been described elsewhere (Guha et al., 1989; Mukherjee, 1997). Sample solutions were taken in a suprasil cuvette of 1 cm pathlength to irradiate by electron pulses of 50 ns duration (fwhm) from a 7 MeV linear electron accelerator at a radiation dose of 15–16 Gy per pulse as measured by an air-saturated 5 102 mol dm3 KSCN dosimeter, tak3 1 ing Ge for (SCN)d cm1 at 500 nm 2 as 21 522 dm mol (G being defined as the number of molecules formed per 100 eV energy absorbed). The temporal absorption profiles at different wavelengths were monitored by kinetic-absorption spectrophotometric arrangement consisting of an OSRAM 450 W pulsed xenon lamp in conjunction with a Krato model GMA 301D-2 monochromator and Hamamatsu R955 photomultiplier tube connected to a Larsen & Toubro model 4072 (100 MHz, 400 Ms/s) digital storage oscilloscope. The data were transferred to an IBM PC, where data analysis was carried out with indigenously developed programme. The nanosecond laser flash photolysis equipment has been described in detailed elsewhere (Palit et al., 1992). Briefly, the third harmonic (355 nm, 3 mJ) output pulses (35 ps) from an active-passive mode locked Nd:YAG laser (Continuum, model 501-C-10) were used for excitation and a cw tungsten lamp in combination with a monochromator, PMT and 500 MHz digital storage oscilloscope (Tektronix, TDS-540A) connected to a PC for data acquisition and analysis.
3. Results and discussion 3.1. Ground-state absorption spectra
2. Experimental Scintillation-grade fluorenone, obtained from Thomas-Becker sensitizers’ kit, has been further purified by recrystallization from methanol. The amino derivatives of fluorenone, namely 1-aminofluorenone (1AF), 2aminofluorenone (2AF), 3-aminofluorenone (3AF) and 4-aminofluorenone (4AF) were obtained from Aldrich, and used as they were received. Solvents of spectroscopic
Ground-state absorption characteristics of fluorenone and its amino-derivatives in different kinds of nonaqueous solvents have been well-studied and the different bands have been well-assigned to np or pp kind of transition (Yatsuhashi et al., 1998; Moog et al., 1991). However, no study has yet been reported in aqueous solutions because of low solubility of these compounds. For the present study, the fluorenone
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derivatives have been dissolved in aqueous-alcoholic mixtures (in aqueous solution of 1 101 mol dm3 tertbutanol) and the ground-state absorption spectra have been recorded at three different pHs, namely 1, 5 and 13. The absorption spectra recorded in 270–600 nm region are shown in Fig. 1. In the case of each of the aminofluorenones, the spectra recorded at pH 5 and 13 are similar and could be assigned to the neutral form of them (structure I). At pH 1, the absorption spectrum of 1AF in 350–500 nm region is very similar to those recorded at pH 5 and 13, but the former has higher molar absorption coefficient or oscillator strength. In the case of other three derivatives, the spectra recorded at pH 1, are very different from those at pH 5 and 13, and they could be assigned to the protonated form of these amino-derivatives (structure II). Hence, it is evident that in the case of 2AF, 3AF and 4AF, the pKa values representing the conversion of the protonated form (II) to the neutral form (I), lie between pH 1 and 4, but in the case of 1AF it possibly lies below pH 1. In the cases of 2AF, 3AF and 4AF, the absorbance of the solutions were measured at 480, 440 and 480 nm, respectively, as a function of pH in the range 1–5 and the pKa values for the protonation–deprotonation equili-
8
Molar Absorption Coefficient ( 103 dm3mol-1cm-1)
1AF 4
0 1
2AF
0
3AF 4
0
4AF
2
0 400
500
600
Wavelength (nm) Fig. 1. Ground state absorption spectra of the aminofluorenones in aqueous solution of 1 101 mol dm3 of tertbutanol at pH 1 (—) and pH 5 or 13 (—J—). Absorption spectra are similar at pH 5 and 13.
713
brium in the ground state were determined by sigmoid fitting of the curves. The pKa values are 3.2, 1.7 and 2.3 for 2AF, 3AF and 4AF, respectively. The difference in behaviour in the case of 1AF can possibly be explained by the presence of intramolecular hydrogen bonding between the carbonyl and amino groups, preventing protonation at the nitrogen atom (Moog et al., 1991). +H
+
- H+
H2N
+ H3N
O
O
I
II
ð1Þ
3.2. Triplet state 3.2.1. Generation of the triplet state using pulse radiolysis Pulse radiolysis is an excellent technique for selective creation of excited triplet states of organic molecules using triplet–triplet energy transfer method in aromatic solvents, such as benzene (Bensasson et al., 1983). The radiolysis of benzene produces the benzene triplet (3BZ) in high yield. The energy level (ET ¼ 353 kJ mol1) of 3 BZ is higher than that of most of the organic molecules, but its lifetime is only 1 ns. 3BZ can transfer its energy to another solute having T1 state energy lower than 353 kJ mol1, thus generating the triplet state of the latter. Hence, in pulse radiolysis of benzene containing a relatively higher concentration (>103 mol dm3) of the scavenger species, 3BZ can serve as a sensitizer for initiating the triplet–triplet energy transfer process. In this case, the low intersystem crossing efficiency for the conversion of the singlet to the triplet state of the solute molecule does not pose any limitation to the production of the triplet state. Also, the triplet production process being selective, conclusive evidence for assignment of the photolytically generated transient species to the triplet state can be obtained from the pulse radiolysis studies of the solute in de-aerated solutions of benzene. The absorption spectra of the triplet state produced due to electron pulse radiolysis of four amino-substituted fluorenones in deaerated benzene solutions are presented in Fig. 2A. Each of the triplet absorption spectra consists of two bands, one in 300–400 nm region and the another in 400–700 nm region, and these features are very similar to those of their parent, fluorenone (Biczok et al., 2001), and also to those of aminofluorenones reported using laser flash photolysis technique (Biczok et al., 1999). The molar absorption coefficients for triplet–triplet (T–T) absorption at suitable wavelengths, at which absorption due to the ground state is negligible, have been determined by the method of energy transfer from biphenyl triplet to that of aminofluorenone (Bensasson et al., 1983). The energy of the lowest
ARTICLE IN PRESS V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
excited triplet state (ET ) of biphenyl (273 kJ mol1) lies at much higher level than those of fluorenone (211 kJ mol1) and aminofluorenones (220 and 192 kJ mol1 for 1AF and 2AF, respectively) (Biczok et al., 2001). Using these molar absorption coefficient values, the differential triplet absorption spectra of the aminofluorenones have been corrected for the effect of ground-state bleaching using a standard method (Amouyal et al., 1974) and these corrected spectra are presented in Fig. 2B. The position of the maxima on the corrected T–T absorption spectra as well as the values of molar absorption coefficients at these wavelengths along with the lifetimes of the triplet states of the aminofluorenones in benzene are given in Table 1.
0.012
1AF
0.008
8000
0.004
4000
0.000
0
0.005
0.000 0.006
3AF
0.004 0.002
Extinction Coefficient
2AF
0.010
∆Absorbance
12000
0.000
3.2.2. Generation of the triplet state using flash photolysis The triplet states of four amino-substituted fluorenones in acetonitrile (ACN), methanol and isopropanol (IP) have also been generated by flash photolysis method. In this method, the singlet state formed due to absorption of light undergoes intersystem crossing to form the triplet state. The differential triplet absorption spectra obtained in acetonitrile are shown in Fig. 3. Due to large molar absorption coefficient of absorption of the ground state at shorter wavelength region below 450 nm, the T–T absorption spectra could not be recorded for a few of them. The assignment of these spectra to the triplet states of fluorenone derivatives has been made from significant quenching of the transient
0.000
10000
0
3AF
20000
4AF
0.000 3AF
0.005
10000
0.000 4AF
15000
4AF
0.006
0.003
10000 0.004 5000
0.002 0.000
(A)
2AF
0.003
2AF
20000
0
0.008
1AF
0.004
1AF
∆Absorbance
714
0 300
400
500
600
300
(B)
400
500
0.000
600
400
Wavelength, nm Fig. 2. (A) Differential transient spectra due to T–T absorption obtained following electron pulse radiolysis of the aminofluorenones in benzene. (B) Corrected or true T–T absorption spectra of the aminofluorenones in Benzene.
500
600
Wavelength (nm) Fig. 3. Differential T–T absorption spectra of the aminofluorenones in acetonitrile obtained due to laser flash photolysis at 355 nm.
Table 1 Triplet characteristics of aminofluorenone derivatives Compound
Benzene 3
3
1
lmax (nm) (e; 10 dm mol 1AF 2AF 3AF 4AF a b
450 340 320 320
(14.6), 540 (2.4) (24.970.1), 490 (8.5) (14.0), 450 (5.5) (16.4), 440 (6.7)
Error in determining the value is 15%. Error in determining the value is 10%.
1 a
cm )
t (ms)
b
3473 3173 1872 4274
ACN
Methanol
IP
t (ms)
t (ms)
t (ms)
2572 1371 2873 2372
3973 771 — —
3572 6.772 — —
ARTICLE IN PRESS V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
species in presence of oxygen in solution. In alcoholic solvents, while the triplet yield was very low for 1AF and 2AF, no triplet absorption could be detected for 3AF and 4AF. Low triplet yield in alcohols could be assigned to the efficient energy degradation process from the singlet state via intermolecular hydrogen bond stretching vibrations (Yatsuhashi et al., 1998). The lifetimes of the triplet states in these solvents are given in Table 1. In alcoholic solvents the lifetimes of the triplet states of aminofluorenones, except 1AF, are considerably reduced as compared to those in benzene and acetonitrile. However, the lifetime of the triplet state of 1AF is not significantly affected, possibly due to presence of weak intramolecular hydrogen bond between the carbonyl and the amino groups, which prevents intermolecular hydrogen bonding with the solvents. The molar absorption coefficient of T–T absorption and hence the triplet yield could not be determined in these solvents with reasonable accuracy using flash photolysis technique due to unavailability of proper standard triplet energy donor or acceptor for the aminofluorenones. Since the ground state absorption and T–T absorption spectra of them are extended throughout the near uv and visible region up to 650 nm, suitable excitation and monitoring wavelengths for the selected donors and acceptors could not be found out. We observe complete decay of the triplet state at each of the wavelengths on the triplet absorption spectra of 1AF and 2AF in alcohols. These observations lead us to conclude that the yield of ketyl radical due to abstraction of hydrogen atom from the alcoholic solvents by the triplet states of the aminofluorenones is insignificant. Lack of reactivity of the triplet states of the aminofluorenones towards H atom abstraction reaction with alcoholic solvents can be assigned to its pp character. Hence it becomes obvious that the pulse radiolysis in aqueous solution is the only suitable method for generation and characterization of the ketyl and the anion radicals of the aminofluorenones. 3.3. Reaction of e aq Electron pulse radiolysis of water generates three d d major primary reactive species, e aq ; OH and H with Gvalues 2.7, 2.7 and 0.5, respectively (Spinks and Wood, 1990). The G-value for e aq is constant (2.7) in the pH range 4–10. A few experiments have been performed in solutions with pH > 10, at which Gðe aq Þ is greater than 2.7 and in such cases actual values of Gðe aq Þ reported at respective pH’s have been used (Spinks and Wood, 1990). The experiments carried out at pH=1, at which the hydrated electron is converted into H atom via the reaction with H+ (Eq. (2)) and hence the G-value of H atom becomes 3.45 (Anbar and Neta, 1967). Among the three primary species, dOH is the oxidizing species while the other two are reducing in nature. By adding suitable scavengers in the solution, the reaction condition can be
715
adjusted in such a way that only the reaction of e aq with the solutes could be extensively studied. dOH radicals are scavenged by 1 101 mol dm3 tert-butanol (Spinks and Wood, 1990). þ d e aq þ H -H ;
keaq ¼ 2:3 1010 dm3 mol1 s1 ;
ð2Þ
d d
OH þ ðCH3 Þ3 COH-H2 O þ H2 C CðCH3 Þ2 OH;
k ¼ 5:5 108 dm3 mol1 s1 :
ð3Þ e aq
with the aminofluorKinetics of the reactions of enones have been studied in de-aerated aqueous solutions containing 1 101 mol dm3 tert-butyl alcohol and different concentrations of aminofluorenones (varying in the range 1 106–2 105 mol dm3). tert-Butyl alcohol not only scavenges the oxidizing dOH radical efficiently but also increases the solubility of the aminofluorenones in aqueous solutions. The decay kinetics of e aq has been monitored at 720 nm and the second order rate constants ðkeaq Þ for the reaction between e aq and the aminofluorenones have been evaluated from the slopes of the linear plot of the pseudo first-order rate constants vs. the concentration of the aminofluorenones. The values of keaq are found to vary between 2.5 and 4 1010 dm3 mol1 s1 (listed in Table 2). They are found to be comparable with that of parent fluorenone, i.e. the reactions are diffusion controlled (Table 2). We do not observe any noticeable difference in the rate constant values for differently substituted aminofluorenones and hence we infer that all of them are equally reactive towards e aq and the rate constants are independent of the position of the amino group. Even intramolecular hydrogen bonding in 1AF does not make any noticeable difference in the reactivity with e aq : We compare the values of keaq associated with the amino-substituted benzophenones (keaq values are 4.4 109 dm3 mol1 s1 for 2-aminobenzophenone, 1 1010 dm3 mol1 s1 for 4-aminobenzophenone and 9.8 109 dm3 mol1 s1 for 4-(N,N-dimethylamino)-benzophenone) with those given in Table 2 (Singh et al., 2002). The higher reactivities of the amino-substituted fluorenones towards e aq as compared to those of benzophenone analogues may possibly be associated with the cyclopentadienyl nature of the central ring of the fluorenone derivatives (Hayon et al., 1972). Fluorenone differs from benzophenone by the linkage at the ortho positions of the phenyl groups. In its anion radical, which is formed following capture of an electron by fluorenone or its derivatives, has an electron distribution analogous to that of cyclopentadienyl anion (structure IV), which is unusually stable because of completion of the aromatic hextet. 3.3.1. Transient absorption at different pH The time-resolved spectra of the transient radical species, which are produced due to the reaction of the
ARTICLE IN PRESS
∆Absorbance
6.0 104
Molar Absorption Coefficient
6.870.2 1.0 106
3.5 105
6.970.1 1.0 105
3.2 1010 4AF
Error in determining value is 10%. Error in determining the value is 15%. c Hayon et al. (1972).
2.3 1010 3AF
b
3.6 1010 2AF
a
2.2 1010
6
0.007
λ=540 nm
0.006
0.005
4 4
6
8
10
12
pH 2
0 500
600
700
Fig. 4. (A) Differential transient absorption spectra obtained due to electron pulse radiolysis of 1AF (1 103 mol dm3) in nitrogen-saturated aqueous solution in the presence of 1 101 mol dm3 tert-butanol (pH=1 (n), pH=5 (—J—) and pH=13 (—K—)). (B) Corrected or true absorption spectra of hydrogen atom adduct (—n—), ketyl radical (—J—) and anion radical (—K—) of 1AF. Inset of (A): plot of the first-order rate constants for the reaction of e aq with 1AF at pH 7 vs. concentrations of 1AF. The slope corresponds to the bimolecular rate constant, keaq : Inset of (B): plot of the differential transient absorption monitored at 540 nm obtained due to pulse radiolysis of 1AF (1 103 mol dm3) in the pH range 4–13. Sigmoid curve has been fitted according to Eq. (6) to obtain the pKa value of 1AF.
1.6 104 1.3 104 1.1 104 5.1 103 1.8 103 1.9 103 1.3 104 3.4 103 6.0 103 2.0 103 2.3 103 1.3 103 1AF
80
Wavelength (nm)
347 362 520 390 430 520 350 460 370 450 390 510 3.3 1010
(B)
400
cm ) Flc
40
[1AF] (10-6 mol dm-3)
∆Absorbance
8.5 105
4.2 103 4.6 103 1.8 103 5.8 103 4.2 103 4.7 103 3.1 103 4.8 103 1.3 103 380 430 510 370 460 370 440 400 470 7.770.2 1.2 106
1.1 104 6.2 104 6.370.2 360c 450
lmax (nm) e (dm mol
10 0
(103 dm3mol-1cm-1)
1.6 106 1.5 105
3.8 105
4.5 105
270 348 500 350 420 550 340 460 420 510 350 530
3.8 105
2k=e (s1)a lmax (nm) cm )
1 b
2k=e (s )
20
0.01
8
1 b
lmax (nm) e (dm mol
0.02
30
-0.01
1
3
Anion radical at pH=13 pKa
1 a
(A)
0.00
1
3
Compound ke-aq (dm3 mol1 s1 at pH=7)a Ketyl radical at pH=5
Table 2 Transient characteristics of fluorenone and aminofluorenones on pulse radiolytic reduction in aqueous solutions at pHs 1, 5 and 13
2k=e (s )
1 a
Hydrogen Adduct at pH=1
0.03
ke-aq x 10-5
V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
716
aminofluorenones (conc. 1 103 mol dm3) with primary radical species in aqueous solutions (i.e. e aq at pH’s 5 and 13 and H atom at pH 1) have been recorded soon after the electron decay (i.e. at 1 ms) and at 20 ms after the electron pulse. However, the time-resolved transient absorption spectra in each case did not reveal any kind of evolution of the spectral shapes within 20 ms time-domain, thus indicating the formation of only one kind of transient species. The transient absorption spectra recorded at 20 ms after the electron pulse are shown in the Figs. 4A-7A. These figures show that for each of the aminofluorenones, the transient spectra of the radical species recorded in aqueous solution at three different pHs are quite different. Considering the fact + that at pH=1, e to aq is completely quenched by H produce H atom (Eq. (2)), the transient absorption
ARTICLE IN PRESS V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
ðlÞ eðlÞ R ¼ eAF þ
½DA ½GðSCNÞ:-2 ½eðSCNÞ: 2 ; ½AðSCNÞ:-2 GP
ð4Þ
where eðlÞ AF is the molar absorption coefficient in dm3 mol1 cm1 for the aminofluorenones at wavelength l; DA is the observed difference in absorbance of the transient radical species at l: AðSCNÞ: is the absorbance of 2 (SCN)d 2 which is produced in the dosimeter solution under iso-dose condition and measured at 500 nm. GP is the d G-value of the primary radical species (e.g. e aq or H ) responsible for the generation of the particular radical species. The corrected transient absorption spectra of the ketyl and anion radicals as well as that of H atom adduct are presented in the Figs. 4B-7B. The decay rate constants for the H atom adduct as well as the ketyl and the anion radicals are given in Table 2
+
H2N
O I
eaq
.
0.03
0.02
0
To obtain pKa values for the acid base equilibrium for the ketyl radical—anion radical conversion process (i.e. II"III), the absorbance values of the transient radical species surviving at 20 ms after the electron pulse were measured as a function of pH at a particular wavelength selected in such a way that at this wavelength the difference in the differential transient absorption between those of the ketyl and the anion radicals is
40
80
[2AF] (10-6mol dm-3)
0.01
0.00 12
(B) 8
0.012
λ=450nm
0.009 0.006 6
4
8
pH
10
0 400
500
600
700
Wavelength (nm)
Fig. 5. (A) Differential transient absorption spectra obtained due to electron pulse radiolysis of 2AF (1 103 mol dm3) in nitrogen-saturated aqueous solution in the presence of 1 101 mol dm3 tert-butanol (pH=1 (n), pH= 5 (—J—) and pH=13 (—K—)). (B) Corrected or true absorption spectra of hydrogen atom adduct (—n—), ketyl radical (—J—) and anion radical (—K—) of 2AF. Inset of (A): plot of the first-order rate constants for the reaction of e aq with 2AF at pH 7 vs. concentrations of 2AF. The slope corresponds to the bimolecular rate constant, keaq : Inset of (B): plot of the differential transient absorption monitored at 450 nm obtained due to pulse radiolysis of 2AF (1 103 mol dm3) in the pH range 4–13. Sigmoid curve has been fitted according to Eq. (6) to obtain the pKa value of 2AF.
maximum, as well as the ground state molar absorption coefficient is also negligible (see Figs. 4A-7A). The
.
H2N OII
20
0
∆Absorbance
∆Absorbance
(A)
+H -H
H2N
k x 10-5
40
Molar Absorption Coefficient (103 dm3mol-1cm-1)
spectra recorded at pH 1, is assigned to hydrogen atom adduct. Like other aromatic ketones, addition of H atom is not specific in the case of the aminofluorenones (Hayon et al., 1972). However, attachment of electrons to the aminofluorenones, like other aromatic ketones, occurs almost exclusively with the carbonyl group. Hence, the transient absorption spectra recorded at pH 5 and 13 can be assigned to the ketyl and anion radicals formed due to attachment of the solvated electron with the fluorenone derivatives (Eq. 5). Considering the fact that G-values of the solvated electron at pH 5 and 13 are 2.7 and 2.9, respectively, and that of H atom is 3.45. The molar absorption coefficient values for absorption of the ketyl and anion radicals as well as H atom adduct at wavelengths, at which ground state absorption for the compound is not significant, have been determined. Using the same, the true or corrected absorption spectra of the radical species have been obtained by correcting the corresponding difference spectra due to bleaching of the ground state. If at a particular pH, a single form, say the ketyl or the anion radical or the hydrogen atom adduct, is present, then at any wavelength l, the molar 3 1 absorption coefficient (eðlÞ cm1) of the R ; dm mol transient radical species is given by
717
ð5Þ
OH III
wavelengths thus selected are 540, 450 and 400 nm for 1AF, 2AF and 4AF, respectively (see Figs. 4A-7A). With increase in pH of the solution, the equilibrium concentration of the anion radical increases and that of the ketyl radical decreases. Hence, we observe an increase in absorption at 450 and 400 nm for 2AF and 4AF, respectively, and a decrease in absorption at 540 nm in the case 1AF. pKa of 3AF could not be
ARTICLE IN PRESS
(A)
aq
∆Absorbance
0.02
ke- x 10-5
V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
718
20 10 0
0.01
none (pKa=9.25) and its amino-substituted derivatives (pKa values are 8.8 and 9.8 for 2-aminobenzophenone and 4-aminobenzophenone, respcetively) could also be explained by the stability of cyclopentadienyl anion (structure IV) (Singh et al., 2002).
30
40
80
[3AF] (10-6 mol dm-3)
0.00
6
(B)
H2N
O.
3
IV
Protonation of oxygen removes an electron from the central ring with concomitant loss of stability. In case of
0
0.02
determined because of unavailability of any such suitable wavelength. It is important to note that throughout the pH range 4–13, each of the aminofluorenones exists in solution as a species in the neutral form, as shown by structure I. Hence, it is straight forward to determine the pKa value of the ketyl—anion radical conversion process by fitting the sigmoid curves obtained, as shown in the insets of the Figs. 4B-7B, to the following Eq. (6) (Bensasson et al., 1983), AII AIII þ ; 1 þ 10ðpH-pK1 Þ 1 þ 10ðpK1 -pHÞ
ð6Þ
where, AII and AIII represent the absorbance of the species II and III, respectively, measured at a particular wavelength and at pH values, where these are the only absorbing species in solution. The pKa values thus obtained for the aminofluorenes are listed in Table 2. pKa values of 2AF and 4AF are comparable to that of the parent fluorenone. The higher pKa (7.7) value for the deprotonation of the OH group at the carbonyl site of the ketyl radical species of 1AF can be explained by considering the presence of weak intramolecular hydrogen bonding in 1AF. Higher basicity of the anion radicals of fluorenone (pKa=6.3) and its derivatives as compared to those of the anion radicals of benzophe-
30
aq
(A)
ke-
Fig. 6. (A) Differential transient absorption spectra obtained due to electron pulse radiolysis of 3AF (1 103 mol dm3) in nitrogen saturated aqueous solution in the presence of 1 101 mol dm3 tert-butanol (pH=1 (—n—), pH=5 (—J—), pH=13 (—K—)). (B) Corrected or true absorption spectra of hydrogen atom adduct (—n—), ketyl radical 5 (—J—) and anion radical (—K—) of 3AF. Inset of (A): plot of the first-order rate constants for the reaction of e aq with 3AF at pH 7 vs. concentrations of 3AF. The slope corresponds to the bimolecular rate constant keaq :
Aobs ¼
x 10-5
700
15
0
40
80
[4AF]
0.01
(10-6 mol dm-3)
0.00 5
(B)
4 3
∆Absorbance
500 600 Wavelength (nm)
∆Absorbance
400
Molar Absorption Coefficient (103 dm3mol-1cm-1)
Molar Absorption Coefficient (103 dm3mol-1cm-1)
-
0.02
λ = 400 nm
0.01
6
8
10
12
pH
2 1 0 400
500 600 Wavelength (nm)
700
Fig. 7. (A) Differential transient absorption spectra obtained due to electron pulse radiolysis of 4AF (1 103 mol dm3) in nitrogen-saturated aqueous solution in the presence of 1 101 mol dm3 tert-butanol (pH=1 (n), pH=5 (—J—) and pH = 13 (—K—)). (B) Corrected or true absorption spectra of hydrogen atom adduct (—n—), ketyl radical (—J—) and anion radical (—K—) of 4AF. Inset of (A): plot of the first-order rate constants for the reaction of e aq with 4AF at pH 7 vs. concentrations of 4AF. The slope corresponds to the bimolecular rate constant, keaq : Inset of (B): plot of the differential transient absorption monitored at 400 nm obtained due to pulse radiolysis of 4AF (1 103 mol dm3) in the pH range 4–13. Sigmoid curve has been fitted according to Eq. (6) to obtain the pKa value of 4AF.
ARTICLE IN PRESS V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
3.4. Oxidation using Nd3 radical
0.10 Molar Abs. Coeff. (2x103dm3 mol-1 cm-1)
3
0.08 ∆Absorbance (arb. unit)
benzophenone with amino groups substituted at different positions of the phenyl ring, the basicity of the anion radical is reduced and this can be explained by considering the positive inductive effect of the amino or the dimethylamino group (Singh et al., 2002). However, in the case of the amino-substituted fluorenones, this effect seems to be insignificant as compared to the effect of filling of the aromatic sextet in the cyclopentadienyl ring. Hence, the pKa values of the aminofluorenones are little affected by amino substitution as compared to that of the parent fluorenone.
The most convenient way for obtaining a condition, which is suitable exclusively for oxidation of the solute, is to saturate the solution with N2O, which converts e aq to dOH (Eq. (7))
0.06
d
d OH þ N 3 -N3 þ OH ;
k ¼ 1:2 1010 dm3 mol1 s1 ;
ð8Þ
Nd3 radical can be generated by irradiation of N2Osaturated aqueous solutions containing NaN3 (1 101 mol dm3) at pHB5. In this condition, almost all primary dOH species generated by electron pulse were converted into Nd3 with GB6:9 molecules per 100 eV (Spinks and Wood, 1990). One electron reduction potential of Nd3 /N 3 couple is +1.33 V vs. NHE. For generating oxidized radicals, solutions containing 2.5 104 mol dm3 of the solutes, 3 mol dm3 of acetonitrile and 0.1 mol dm3 of NaN3 at pH 5 were radiolyzed by electron pulse. The differential absorption spectra of the transient species produced due to reaction of the Nd3 radical with the amino-substituted fluorenones are presented in Fig. 8. However, no transient radical species, which can be characterized as an oxidized radical species, has been detected for Fl and 1AF. The true absorption spectra of the oxidized radical species have been obtained by applying Eq. (4) to the corre-
500
0.00
-0.02
2
OH is known to be a strong oxidizing radical species (one-electron reduction potential of dOH /OH couple is +2.8 V vs. NHE) but non-selective in its reactions (Spinks and Wood, 1990). Hence, dOH species reacts with the solute to give oxidized species along with other products, such as OH-adduct and deprotonated product. It is possible to generate a milder but more selective oxidizing condition by converting dOH into another inorganic radical. In this work we have used azidyl radical Nd3 , which is known as specific oneelectron oxidant, for generating the oxidized radical species of fluorenone and aminofluorenones.
0
0.02
400
d
1
400
d
ð7Þ
2
0.04
O H þ OH ; e aq þ N2 O-N2 þ O ! H O
k ¼ 9:1 109 dm3 mol1 s1 ;
719
500 600 Wavelength (nm)
Fig. 8. Differential transient absorption spectra obtained due to electron pulse radiolysis of fluorenone and the aminofluorenones (2.5 104mol dm3) in N2O saturated aqueous solution in the presence of 3 mol dm3 acetronitrile and 1 101 mol dm3 NaN3. Inset: Ground state absorption spectra of fluorenone and aminofluorenones (2.5 104 mol dm3) in aqueous solution in presence of 3 mol dm3 acetronitrile and 1 101mol dm3 NaN3; 2AF (—J—), 3AF (—,—) and 4AF (—&—).
sponding difference spectra using the value of GP (6.5 molecules per 100 eV). The true absorption spectra thus obtained for fluorenone, 2AF, 3AF and 4AF are shown in Fig. 9. The absorption maxima, molar absorption coefficient, growth life time and 2k=e values for oxidized species are listed in Table 3. The oxidized radical species formed in the aminofluorenones have an absorption band above 400 nm. It has already been reported that the oxidation of amino group in aniline gives anilino radical that has an absorption band in this wavelength region (Solar et al., 1986). So the transient absorption in case of the aminofluorenones is due to formation of oxidized species by oxidation of the amino group. Absence of formation of oxidized radical species in case of Fl and 1AF, which is intramolecularly hydrogen bonded, also suggest the fact that the amino group in other aminofluorenones undergoes oxidation by Nd3 radical, not in the aromatic ring (Eq. 9).
.
N3
H2N
.
HN O
O
ð9Þ
ARTICLE IN PRESS V. Samant et al. / Radiation Physics and Chemistry 72 (2005) 711–722
720
3.5. Cation radical in dichloroethane Detection of the cationic species in any photochemical or radiation chemical process reveals the involvement of charge or electron-transfer reactions. Hence, it is essential to have information regarding the spectroscopic and kinetic properties of the cation radical by
2AF
Molar Absorption Coeff. (103 dm3 mol-1 cm-1)
8
4 0 3AF 4 2 0 4AF
4
2
0 400
500 Wavelength (nm)
600
Fig. 9. True transient absorption spectra of the oxidized radicals of 2AF, 3AF and 4AF generated by pulse radiolysis of aqueous solution in the presence of azidyl radical.
generating it in isolated condition. The electron pulse radiolysis in chlorinated solvents has been used as a standard method to generate and characterize the radical cations of different kinds of solute molecules (Alfassi et al., 1989). However, in this method, several other oxidants are generated in solution, in addition to the radical cation of the solvent molecule, which is the main oxidant, and different kinds of transient species, such as ion pairs, radicals and chlorine atom adducts, may be formed. Hence, this method becomes nonselective and one should be careful in characterizing the transient species formed. We have studied the cationic species generated by pulse radiolysis of the aminofluorenones in 1,2-dichloroethane (DCE) solvent. Aminofluorenones have good solubility in this solvent because of its higher dielectric constant (eB10) as compared to other chloro-hydrocarbon solvents, such as, carbon tetrachloride and dichloromethane, which are common for generation of cation radicals pulse radiolytically (Wang et al., 1979). According to the reports available in the literature (Berman et al., 1979), for radiolysis in low-temperature glasses and liquid DCE, it is well known that aromatic solutes such as biphenyl, anthracene and many others form respective radical cations, very likely by charge transfer to the radical cation of the solvent. In the gasphase studies, the parent radical cation, ClCH2CH2Cld+ and the radical cation ClCH2CH+ 2 (Berman et al., 1979) were observed. Differential transient absorption spectra of the radical species produced due to pulse radiolysis of Fl and aminofluorenones (concentration 2.5 104 mol dm3) in DCE thus obtained are presented in Fig. 10. The absorption maxima, and 2k=e values for cation radicals determined in the present experimental condition are
Table 3 Transient characteristics of Fluorenone and Aminofluorenone derivatives on pulse radiolytic oxidation at pH=5 and cation radical in 1,2-dichloromethane Compound
Fl
1AF 2AF 3AF
4AF a
Oxidized radical
Cation radical 3
1
lmax (nm)
eðlmax Þ (dm mol
410
3.6 102
1 a
cm )
2k=e (s )
0.570.05
500
2.5 103
0.670.05
3.5 105
360 440
5.5 103 3.9 103
c
c
350 500
3
3.4 10 1.3 103
Error in determining the value is 15%. Error in determining value is 10%. c Due to very weak signal values cannot be determined accurately. b
tg (ms)
1 b
0.970.04
5
6.0 10
lmax (nm) 330 460 670 390 370 490 340 410 520 330 450
2k=e (s1)b 4.5 106 8.0 106 1.0 106
3.0 106 1.6 106
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unreactive towards the H-atom abstraction reaction with the alcoholic solvents. Amino-substituted fluorenones reacts with the hydrated electron at the diffusioncontrolled rate (B2 1010 dm3 mol1 s1). Higher reactivity of the aminofluorenones as well as the higher pKa values of the ketyl-anion radical equilibrium as compared to those of the benzophenone analogues has been explained by the extra stability of the aminofluorenone radical anion due to completion of the aromatic sextet in the cyclopentadienyl anion formed at the central ring of the fluorenone moiety. Oxidized radicals have been generated in aqueous solution using Nd3 as the oxidizing species and the cation radicals by electron transfer from the cation radical of 1,2 dichloroethane. The difference in spectral characteristics of the radicals generated by these two methods could be attributed to the formation of only aminyl radicals in aqueous solution while the radicals formed in DCE include other species as well.
Fl
0.01
0.00 0.01 0.00
1AF
∆Absorbance
-0.01
2AF
0.02
0.00 0.010
3AF
0.005 0.000 0.006
721
4AF
References
0.003 0.000 400
500 600 Wavelength (nm)
700
Fig. 10. Differential transient absorption spectra of 2.5 104 mol dm3 solution of fluorenone and its amino derivatives in dicholoroethane.
listed in Table 3. Comparing the spectral shapes in the Figs. 8 and 10, we observe that the spectra of the radical species of the aminofluorenones generated in DCE are found to have different shapes from those of the oxidized radicals generated in aqueous solution using Nd3 radical. Fl could not be oxidized by Nd3 radical in aqueous solution. Since Fl as well as all four aminosubstituted fluorenones form the radical species in DCE, this possibly suggests that the initial cation radical formed in DCE is centreed in the central aromatic ring of the fluorenone derivatives. However, other radicals may be present as well, and only from the spectral shapes it is not possible to comment on the structure of the radical species formed in this experiment.
4. Conclusion Spectroscopic and kinetic properties of the triplet excited states as well as the transient free radical intermediates of the differently substituted aminofluorenones have been investigated by using flash photolysis and electron pulse radiolysis techniques. Due to pp nature of the triplet states of aminofluorenones, they are
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