Photophysics and Photochemistry of 2-Aminobenzoic Acid Anion in Aqueous Solution

Photophysics and Photochemistry of 2-Aminobenzoic Acid Anion in Aqueous Solution

(Parte 1 de 2)

Photophysics and Photochemistry of 2-Aminobenzoic Acid Anion in Aqueous Solution

Ivan P. Pozdnyakov,* Victor F. Plyusnin, and Vjacheslav P. Grivin

Institute of Chemical Kinetics and Combustion, NoVosibirsk, Institutskaya 3, 630090 Russia NoVosibirsk State UniVersity, NoVosibirsk, PirogoVa 2, 630090 Russia

ReceiVed: July 3, 2009; ReVised Manuscript ReceiVed: October 9, 2009

Nanosecond laser flash photolysis and absorption and fluorescence spectroscopy were used to study photochemicalprocesses of 2-aminobenzoicacid anion (ABA-) in aqueous solutions.Excitationof this species gives rise to the ABA- triplet state to the ABA• radical and to the hydrated electron (eaq-). The last two species result from two-photon processes. In a neutral medium, the main decay channels of ABA- triplet state, the ABA• radical, and eaq- are T-T annihilation, recombination, and capture by the ABA- anion, respectively.

1. Introduction

Organic acids (R-CO2H) are a class of compounds abundant in natural water.1 These acids can form complexes with many transient metals (including Fe(I)) whose photochemistry can contribute substantially to the balance of organic compounds in water.2-8 It is worth noting that aromatic acids have their own strong absorption bands in the UV range and can be subjected to photochemical transformationunder solar radiation in a free noncoordinated state.2

Salicylic acid (2-hydroxybenzoic acid, H2SA) and its derivatives (SAD) are considered to be representatives of the com- plexing functional groups in humic substances9 and can serve as model compounds for investigating the photochemical properties of natural acids. Photophysics of SAD is widely investigated because these compounds are the simplest systems for studyingexcited-stateintramolecularproton transfer(ESIPT) in a gas phase and liquids.10-14 However, the photochemical properties of SAD are poorly characterized. It was not until recently that the spectrum of TTA (triplet-triplet absorption) of SAD in organic solvents recorded by laser flash photolysis was published.1 In our previous works,15-17 the photochemistry of H2SA and 5-sulfosalicylicacid (H3SSA) in aqueous solutions was studied. In a wide pH range, monoanion HSA- and dianion

HSSA2- were found to be the main photoactive forms of H2SA and H3SSA, accordingly. Excitation of these species (308 nm, XeCl laser) gives rise to the tripletstate, to the hydratedelectron, and to the corresponding phenoxyl radical. The last two species result from two-photon processes.15-17

In the present work, the photochemical and photophysical processes have been studied for 2-aminobenzoic acid anion (ABA-) in aqueous solutions in the framework of the program for studying photochemical transformations of organic compounds in natural water. This species is the main photoactive form of the acid at pH values that are typical for natural waters. The main attention was concentrated on the determination of the spectral and kinetic characteristics of primary intermediate states and species formed upon the excitation of ABA- and comparison of the photochemical properties of anions of 2-aminobenzoic and salicylic acid.

2. Experimental Details

Laser flash photolysis experiments were performed using a setup with a YAG laser excitation (355 nm, pulse duration 6 ns, irradiation spot 0.03 cm2, mean energy up to 10 mJ/pulse) analogous to that described elsewhere.16 The laser pulse power was measured using a SOLO-2 “Gentec” power meter.

Nanosecond fluorescence dynamics was measured using the time-correlatedsinglephotoncounting(TCSPC)setup described elsewhere.18 Excitation was performed using a diode laser PLC- 340 (LDH series, PicoQuant GmbH) at 350 nm and 40 MHz. The data were analyzed using the convolution of the instrument response function with the exponential function. The overall time resolution was ca. 300 ps.

The steady-state fluorescence spectra were measured in a 1 cm cell with a Varian CARY Eclipse spectrofluorimeter. We determined the fluorescence quantum yields by integrating the corrected emission spectra19 and using a solution of quinine bisulfate in1MH 2SO4 (φ ) 0.546) as standard.20 The absorption spectra were recorded using an Agilent 8453 spectrophotometer. 2-Aminobenzoic acid (HABA, 98%, Aldrich) was recrystallized from aqueous solution before use. The solutions were prepared using bidistilled or deionized water. Unless otherwise specified, all experiments were carried out with oxygen-free samples ina1c m optical cell at 298 K. We removed oxygen by bubbling solutions with gaseous argon. Samples for flash photolysis were used until the 10% decrease in UV absorption.

3. Results and Discussion

3.1. Absorption and Fluorescence Spectra of ABA. Dependingon the pH, the 2-aminobenzoicacid can exist in cationic and anionic (dissociation of carboxylic group, pKa(COOH) ) 4.9521) forms. At circumneutral pH, which is typical for natural waters,the main form of the acid is anionABA-. The absorption and emission spectra of this species in aqueous solutions are depicted in Figure 1a. ABA- exhibits a long-wave absorption band with a maximum at 310 nm (ε ≈ 2.8 × 103 M-1 cm-1) corresponding to π-π* transition,2 which is typical behavior of SAD.14 Large Stokes shift of ABA- fluorescence (∼6900 cm-1) is observed indicating that the geometry of the emissive excited state of ABA- differs significantly from that of the ground state due to ESIPT process. This observation is in* Corresponding author. E-mail:

10.1021/jp906269a 2009 American Chemical Society Published on Web 1/30/2009

agreement with previous photophysical investigations on the neutral and anionic forms of SAD in nonpolar11-13 and polar13,14 solvents.

The fluorescencequantumyieldof ABA- is fl ) 0.64,which makes it possible to determine the rate constant of the radiative on the measured fluorescence lifetime of the anion (τfl ) 8.75 ns, Figure 1b).

3.2. Laser Flash Photolysis of ABA- Aqueous Solution.

Excitation of the oxygen-free aqueous solutions of ABA- by a laser pulse gives rise to intermediate absorption consisting of two bands with maxima at 450 and 720 nm (Figure 2a). The bands at 450 and 720 nm decay at substantially different rates (Figure 2b), which indicates the formation of several intermediate species after the laser pulse. The band lifetime at 450 nm decreases sharply in the presence of oxygen, which is evidence of the band belonging to ABA- absorption from the triplet (T1) state. It worth to note that TTA band of the anionic forms of salicylic and 5-sulfosalicylic acids in aqueous solution has a maximum at 455 and 470 nm, accordingly.15,17 On the basis of data from pulse radiolysis, it is known that a wide band with a maximum at 720 nm belongs to the absorption of hydrated

ABA- and eaq- on laser pulse intensity. The TTA (450 nm) yield increases linearly, allowing one to determine the product nm (Figure 2a). The eaq- absorption yield (720 nm) depends on the laser pulse intensity according to the square law (Figure

3). This indicates a two-photon process of hydrated electron formation.

In the oxygen-saturatedaqueous solutions, eaq- and the triplet state decay rapidly with the characteristic times <50 and ∼160 ns, respectively. These conditions make it possible to detect one more long-lived intermediate, whose absorption spectrum consists of absorption band with maximum at 405 nm (Figure 4a). The decay kinetics of this intermediate are presented in Figure 4b. The absorption amplitude at 405 nm (∆A405) depends on the laser pulse intensity according to the square law and depends linearly on the absorption of the hydrated electron at 720 nm (∆A720) (Figure 5). These data indicate that the longlived intermediate is generated in the two-photon process together with the electron.

In our previous works,15,16 when studying the photochemistry of the sulfosalicylic acid dianion, we have shown that the twophoton ionization of HSSA2- occurs because of the absorption of the second photon by the excited singlet (S1) state of the dianion to form the hydrated electron-HSSA•- radical anion pair. It can be assumed that in the case of the 2-aminobenzoic acid anion, photoionization occurs from the S1 state of ABA- to form the hydrated electron and 2-carboxyanilinyl radical

(ABA•). It is worth noting that anilinyl-type radicals have an absorptionmaximumat 400-420 nm.24,25The lineardependence of ∆A405 on ∆A720 (Figure 5) allows one from the known absorption coefficient of the hydrated electron to determine a similar parameter for the ABA• radical (ε405 ) 2.6 × 103 M-1 cm-1).

The observed quantum yield of eaq- at a laser pulse intensity of 100 mJ/cm2 (∼0.26) is five times higher than the corre-

Figure 1. (a) Normalized (1) absorption and (2) fluorescence spectrum of ABA- in aqueous solution at pH 8. (b) Kinetic curve of (1) ABA- fluorescence at 400 nm and (2) the instrument response function. Smooth line is the best exponential fit after reconvolution with the instrument response function.

Figure 2. (a) Transient absorption spectra at (1) 0, (2) 0.4, (3) 1.2, and (4) 10 µs after excitation of ABA- (4 × 10-4 M, pH 8) in oxygenfree aqueous solution. (b) Kinetic curves at (1) 330, (2) 450, and (3) 720 nm.

Figure 3. Dependence of the optical density of ABA- TTA (450 nm)

14110 J. Phys. Chem. A, Vol. 113, No. 51, 2009 Pozdnyakov et al.

spondingvaluefor the HSASA- anion(∼0.06).17 This fact could be explained by the following reasons: (1) Ionization potentials of aromatic amines are less than corresponding values for phenols.26 (2) The lifetime of the S1 state of ABA- is two times longer than the one of HSA- (4.3 ns13), which leads to a higher probability of absorbing second light quantum. 3.3. Absorption Coefficient of the Triplet State of ABA.

In our previous works,15,17 the triplet-triplet energy transfer method was used to determine the absorption coefficient from

(ET ) 26 100 cm-12 7). In both cases, 2,2′-dipyridyl(DP), which has the TTA band with a maximum at 350 nm and absorption

was used because ET of 2-aminobenzoic acid (25200 cm-13 0) is close to the corresponding values for HSA- and HSSA2- .

The excitation energy was adjusted to <10 mJ/cm2 to eliminate the two-photon formation of the hydrated electron.

It was observed that in the presence of DP, the TTA band of ABA- (450 nm) rapidly transforms into two new absorption bands with maxima at 365 and ∼620 nm (Figure 6a) belonging to the DPH• radical (ε365 ) 3 × 104 M-1 cm-1)31 and not to the triplet state of DP. Figure 6b represents the kinetic curves of the absorption change at 450 and 365 nm. Therefore, in contrast with the triplet states of HSSA2- and HSA-, the reaction of

DP with T1 state of ABA- leads to the electron transfer and not to the energy transfer

The thermodynamic favorability for a given electron transfer reactionmaybe estimatedby theuseof thegeneralRehm-Weller relationship32 where E°(D+/D) is the standard redox potential for oxidation of the donor, E°(A/A-) is the standard redox potential for reduction of the acceptor, ∆E00 is the energy of the excited state for the photoactive component, and wp is the Coulombic work term. The last three terms are the same in the case of ABA- and HSA-. The only difference is the E°(D+/D) value for these species. Unfortunately, exact values of E°(ABA•/ABA-) and E°(HSA•/HSA-) are unknown (there is only a rough estimate:

E°(HSA•/HSA-) < 1.3 V3), and direct determinationof ∆GET° for both ABA- and HSA- is impossible; however, one can estimatethe differenceof ∆GET° in the case of ABA- and HSA- if we assume that E°(ABA•/ABA-) - E°(HSA•/HSA-) ≈

E°(PhNH•/PhNH2) - E°(PhO•/PhOH) )- 0.3 V.34 Therefore, one can assume that electron transfer should be more favorable in the case of ABA- anion.

Kinetic curves of DPH• formation and decay at 365 nm are well fitted by the two-exponential model where A2, k1obs365, and k2obs365 with the DPH• absorption yield at 365 nm and the observed constants of formation and decay of this radical, accordingly. k1obs365 increases linearly with an increase in the DP concentration (Figure 7a). This allows one to determine the rate constant of electron transfer k1 ) 5.5 × 109 M-1 s-1 that is close to diffusionone. Decay of DPH• radical

Figure 4. (a) Transient absorption spectra at (1) 0.8; (2) 3.2, and (3) 48 µs after excitation of ABA- (1.1 × 10-3 M, pH 8) in oxygensaturated aqueous solution. (b) Kinetic curves at (1) 405 and (2) 350 nm.

Figure 6. (a) Transient absorption spectra at (1) 0, (2) 2, and (3) 7 µs after excitation of ABA- (4 × 10-4 M, pH 8) and DP (8 × 10-5 M) in oxygen-free aqueous solution. Curve (4) represents the absorption spectrum of DPH• radical.25 (b) Kinetic curves at (1) 450 and (2) 365 nm.

2-Aminobenzoic Acid Anion in Aqueous Solution J. Phys. Chem. A, Vol. 113, No. 51, 2009 14111

The ratio A2 to the initial TTA of ABA- at 450 nm (∆A450) increases at low concentration of DP and reaches the plateau at concentrationof DP > 10-4 M correspondingto 100%quenching of triplet state of ABA- by the acceptor (Figure 7b). In this case, the ratio A2 to ∆A450 is equal to the ratio of the absorption coefficient of DPH• at 365 nm (ε365)t ot he T-T absorption coefficient of ABA- at 450 nm (εT450). The calculated value of εT450 (8.6 × 103 M-1 cm-1) was found to be close to corresponding values for anionic forms of salicylic and 5-sul-

calculate the quantum yield of the triplet state of ABA- ( (T1) ) 0.4). The known lifetime of the S1 state (8.75 ns) and the

(T1) makes it possible to calculate the intersystem crossing rate constant (kisc ) (T1)/τfl )4.7 × 107 s-1). It is worth noting that the sum of quantum yields of the triplet state and fluorescence is close to unity.

3.4. Decay Reactions of eaq-,T 1 State of ABA-, and ABA• Radical. Hydrated Electron. The observed decay rate constant of the hydrated electron (kobs720) obtained upon the processing of the kinetic curves of electron absorption decay (720 nm) in oxygen-free solutions depends on the initial absorption of eaq -

(∆A720) and the ABA- concentration (Figure 8a). The intercept on the ordinate by straight lines in Figure 8a depends linearly on the ABA- concentration and corresponds to electron capture by the anion (Figure 8b)

(Parte 1 de 2)