Experimental and Theoretical Studies on the Mechanism of

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J. Phys. Chem. B 2010, 114, 11920–11926

Experimental and Theoretical Studies on the Mechanism of Photochemical Hydrogen Transfer from 2-Aminobenzimidazole to nπ* and ππ*Aromatic Ketones Dolors Jornet,† Pavel Bartovsky´,† Luis R. Domingo,‡ Rosa Tormos,*,† and Miguel A. Miranda*,† Departamento de Quı´mica/Instituto de Tecnologı´a Quı´mica UPV-CSIC, UniVersidad Polite´cnica de Valencia, AVenida de los Naranjos s/n, E-46022 Valencia, Spain, and Departamento de Quı´mica Orga´nica, UniVersidad de Valencia, Dr. Moliner 50, E 46100 Burjassot, Valencia, Spain ReceiVed: June 10, 2010; ReVised Manuscript ReceiVed: August 3, 2010

This work has examined the photoreactivity of benzophenone (3), 2-benzoylthiophene (4), 4-methoxybenzophenone (5), 4,4′-dimethoxybenzophenone (6), and 4-carboxybenzophenone (7) with 2-aminobenzimidazole (1). Laser flash photolysis (LFP) revealed quenching of the aromatic ketone triplets by 1, leading to formation of ketyl radicals plus aminyl radical 1-H•. The quenching rate constants obtained for 3 (nπ* triplet) and 4 (ππ* triplet) were 6.2 × 109 and 3.9 × 109 M-1 s-1, respectively. The similarity between the two values suggests that the process is not a pure hydrogen abstraction but rather a charge transfer followed by proton transfer. This is in agreement with thermodynamic calculations, using the Rehm-Weller equation. In the case of 5 and 6, the transient absorption spectra showed distinct bands corresponding to both types of triplets (nπ* and ππ*); their ratio was found to depend on solvent polarity. In the presence of 1, spectral changes were also consistent with formation of the aminyl/ketyl radical pairs. The rate constants for quenching of both types of triplets were very high, in the range 109-1010 M-1 s-1. When an electron acceptor substituent was attached to the aromatic ring, as in 7 (nπ* triplet), the quenching rate constant was higher (8 × 109 M-1 s-1), close to diffusion control. The reaction mechanism for hydrogen abstraction from 1 by triplet excited 3 or 4 was theoretically studied using density functional theory (DFT) methods. The results suggest formation of ground state molecular complexes, where one electron is transferred from the 2-aminobenzimidazole to the benzophenone or benzoylthiophene moieties upon excitation, giving radical ion pairs; subsequent proton transfer from the amino group to the carbonyl oxygen atoms leads to the neutral biradicals. A comparison between the relative energies and geometries of the species involved in the photochemical reactions indicates that all ketones follow a similar mechanism. Introduction Compounds containing the benzimidazole (BZ) moiety have found application as anthelmintics and are effective as antinematode and antiprotozoal agents.1 Some of them show antifungal activity and are used on fruits and vegetables.2 In addition, the antitumoral behavior exhibited by several members of the family has attracted considerable attention in the last years.3 By contrast, it is surprising that little attention has been paid to the effects of light on the BZ chromophore. Thus, photochemical studies are limited to the parent molecule and some of its alkyl derivatives;4 however, due to their toxicity,5 more recently the interest has been centered on photoelimination of antifungal xenobiotics from the environment. Specifically, 2-aminobenzimidazole (ABZ, 1) is the core of a number of bioactive BZ derivatives (Chart 1). This moiety is also a substructure of chemosensor receptors used for selective recognition of anions with an important role in a variety of biological activities, such as phosphate,6 acetate,7 iodide,8 dicarboxylate,9 etc. In spite of these applications, little is known on the photobehavior of 1. The studies have focused on the effect of solvents and pH on the absorption and emission spectra; in addition, semiempirical molecular orbital calculations have been used as an approach to characterize the lowest singlet excited state.10 The triplet * Corresponding author. Fax: +34 963877809. E-mail: mmiranda@ qim.upv.es; [email protected]. † Universidad Polite´cnica de Valencia. ‡ Universidad de Valencia.

CHART 1: Chemical Structures of Benzimidazole and Benzophenone Derivatives

excited state and the neutral radical (1-H•) have been recently characterized by our group.11 On the other hand, hydrogen abstraction is probably the best known chemical reaction of aromatic carbonyl triplets.12 Thus, photoreduction of benzophenone (BP, 3) by hydrocarbons,13 phenols,14 thiols,15 ethers,16 and amines17 has been thoroughly studied over five decades. Basically, the reactivity seems to be governed by two factors: the nature of the hydrogen donor and the electronic configuration of the lowest triplet excited state. Depending on the donor, the process can occur by a variety of mechanisms, ranging from pure hydrogen atom abstraction (hydrocarbons) to electron transfer (amines), with the excited carbonyl compound as an acceptor. In regard to the electronic nature of the triplet state, it is known that nπ* aromatic ketones

10.1021/jp1053327  2010 American Chemical Society Published on Web 08/24/2010

Mechanism of Photochemical Hydrogen Transfer efficiently abstract aliphatic or benzylic hydrogens, leading to ketyl radicals;18 by contrast, direct hydrogen abstraction by ππ* ketone triplets is rare.19 In this context, the nature of the lowest lying triplet state is conditioned by the type and position of the substituents at the aromatic rings and by solvent polarity. Thus, electron-donating groups and polar solvents stabilize ππ* triplets, resulting in a decreased reactivity.20 For example, benzophenone (3) has a characteristic nπ* triplet both in polar and nonpolar solvents. By contrast, 2-benzoylthiophene (4) presents a ππ* triplet in both types of solvents.21 Interestingly, in the case of 4-methoxybenzophenone (MBP, 5) and 4,4′-dimethoxybenzophenone (DMBP, 6), nπ* to ππ* inversion is observed with increasing polarity of the medium.14b,22 However, 4-carboxybenzophenone (CBP, 7) behaves as the parent unsubstituted compound.23 In spite of a number of studies on the photoreduction of benzophenone, the use of benzimidazole or its derivatives as potential hydrogen donors has not yet been investigated. With this background, the aim of the present work is to perform a detailed study on the photoreactivity of benzophenone (BP, 3) and some derivatives with 2-aminobenzimidazole (ABZ, 1). The parent benzimidazole (BZ, 2) has also been studied for comparison. Using the laser flash photolysis (LFP) technique, formal hydrogen abstraction has been assessed by detection of the transient absorption spectra of BP ketyl radicals 3H•, together with ABZ-derived radicals 1-H •. Furthermore, theoretical calculations on the interaction between BP or BT and ABZ at the triplet excited state show that the process is initiated by electron transfer from 1 to the aromatic ketones. Experimental Section Materials and Solvents. 2-Aminobenzimidazole (1), benzimidazole (2), benzophenone (3), 2-benzoylthiophene (4), 4-methoxybenzophenone (5), 4,4′-dimethoxybenzophenone (6), and 4-carboxybenzophenone (7) were purchased from Aldrich. Their purity was checked by 1H nuclear magnetic resonance (NMR) spectroscopy and high performance liquid chromatography (HPLC) analysis. Reagent grade solvent acetonitrile was purchased from Scharlau and used without further purification. Absorption Spectra. Optical spectra in different solvents were measured on a Perkin-Elmer Lambda 35 UV/vis spectrophotometer. Laser Flash Photolysis Experiments. The LFP experiments were carried out by using a Q-switched Nd:YAG laser (Quantel Brilliant, 355 nm, 14 mJ per pulse, 5 ns fwhm) coupled to mLFP-111 Luzchem miniaturized equipment. This transient absorption spectrometer includes a ceramic xenon light source, 125 mm monochromator, Tektronix 9-bit digitizer TDS-3000 series with 300 MHz bandwidth, compact photomultiplier and power supply, cell holder and fiber optic connectors, fiber optic sensor for laser-sensing pretrigger signal, computer interfaces, and a software package developed in the LabVIEW environment from National Instruments. The LFP equipment supplies 5 V trigger pulses with programmable frequency and delay. The rise time of the detector/digitizer is approximately 3 ns up to 300 MHz (2.5 GHz sampling). The laser pulse is probed by a fiber that synchronizes the LFP system with the digitizer operating in the pretrigger mode. All transient spectra were recorded using 10 × 10 mm2 quartz cells with 4 mL capacity, and all were bubbled for 20 min with N2. The absorbance of the samples was kept between 0.2 and 0.3 at the laser wavelength. All the experiments were carried out at room temperature. Computational Methods. Density functional theory (DFT) calculations were carried out using the B3LYP24 exchange-

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Figure 1. (A) Transient absorption spectra obtained by 355 nm irradiation of 4 × 10-3 M acetonitrile solutions of benzophenone (9) and in the presence of 2-aminobenzimidazole 1 × 10-4 M (b) registered at 2.56 µs after the pulse. (B) Transient absorption spectra obtained by 355 nm irradiation of 1 × 10-3 M acetonitrile solutions of 2-benzoylthiophene (9) and in the presence of 2-aminobenzimidazole 7 × 10-4 M (b) registered at 0.48 µs after the pulse.

correlation functional, together with the standard 6-31G(d) and 6-311+G(d,p) basis sets.25 For triplet and radical species, the unrestricted formalism (UB3LYP) was employed. B3LYP/631G* optimizations were carried out using the Berny analytical gradient optimization method.26 Energies were obtained by single point energy calculation at the (U)B3LYP/6-311+G(d,p) level. Vertical energies of the singlet-excited states were calculated using the time-dependent (TD-DFT) method.27 Electronic structures of stationary points were analyzed by the natural bond orbital (NBO) method.28 All calculations were carried out with the Gaussian 03 suite of programs.29 Results and Discussion Dynamic studies on the reaction of 2-aminobenzimidazole (1) with the aromatic ketone triplets (3-7) (see Chart 1) were performed in acetonitrile or mixtures of acetonitrile and buffered aqueous solution, using 355 nm laser excitation (Nd:YAG). Laser flash photolysis of 3 in deareated acetonitrile led to a transient absorbing in the 300-700 nm range. The spectrum matched that previously reported for the BP triplet state.14 Quenching of the triplet by 1 led to formation of two new absorption bands peaking at 450 and 530 nm. The spectral changes are shown in Figure 1A. They are consistent with generation of BP ketyl radical (3H•) plus the ABZ derived radical (1-H•).11 Similar results were obtained for 4 (Figure 1B), although the maxima of the triplet and ketyl radical (4H•) transients appeared expectedly at different wavelengths (600 and 580 nm, respectively).21,30 In principle, formation of two 1-H• radicals could be possible, depending on the H atom

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SCHEME 1: Photoreaction between ABZ and Ketones 3 or 4 and Formation of Possible Radical Pairs

abstracted: H transfer from the amino group would lead to an aminyl radical A, whereas abstraction of the H bound to the ring nitrogen would give rise to radical B (Scheme 1). The rate constants for triplet quenching by ABZ were determined by measuring triplet lifetimes of 3 and 4 in the absence and presence of increasing concentrations of 1, using the kinetic traces at 620 and 640 nm, respectively. At low quencher concentrations (below 0.1 mM), the corresponding kq values (6.2 × 109 and 3.9 × 109 M-1 s-1) were obtained from the linear regions of the Stern-Volmer plots (Figure 2A) according to eq 1

1/τ ) 1/τ0 + kq[1]

(1)

where τ0 is the triplet lifetime in acetonitrile solution and τ is the measured lifetime in the presence of quencher. The similarity between the two rate constants suggests that reduction of 3 and 4 is not a pure hydrogen abstraction process but rather a charge transfer followed by proton transfer. Interestingly, at higher quencher concentrations, clear deviations from linearity were observed, indicating formation of encounter complexes (Figure 2B);31 the contribution of this phenomenon can explain the fact that the determined quenching rate constants (see above) are somewhat lower than the diffusion limit in the employed solvent.

In a control experiment, LFP of BP was performed in the presence of the parent BZ (2), which could only lead to a radical of type B. On the other hand, formation of the BZ triplet by energy transfer is not possible, due to its high triplet energy (75 kcal/mol)32a as compared with BP (69.2 kcal/mol).33 Moreover, electron transfer from BZ to excited BP would be thermodynamically disfavored. Actually, using the oxidation potential of BZ (Eox ) 1.56 V),32b the reduction potential of BP (Ered ) -1.78 V),14b and the above-mentioned triplet energy of BP, the Rehm-Weller equation34 afforded ∆G ) 8 kcal mol-1, indicating that the process is endergonic. As a matter of fact, LFP of 3 in the presence of 2 gave rise to a transient spectrum that matched with that of the ketone triplet, so no new signal was observed. In addition, no change in the BP triplet lifetime was detected, ruling out a possible dynamic quenching by BZ. This shows the key role of the electron donating amino group of 1 assisting the BP reduction processes and giving rise to a radical of type A. Accordingly, thermodynamic calculations using the Rehm-Weller equation and the known ABZ oxidation potential (Eox ) 0.60 V)35 led to ∆G ) -12.8 kcal mol-1, confirming the spontaneity of the redox processes. The same trend was observed in the case of ketone 4, for which a ∆G value of -6.56 kcal mol-1 was obtained through application of the Rehm-Weller equation, using Ered ) -1.8 (V)34 and ET* ) 63.2 kcal mol-1.21a

Figure 2. Stern-Volmer plots obtained for triplet quenching by 2-aminobenzimidazole of benzophenone (b) and 2-benzoylthiophene (9) upon LFP at 355 nm: (A) quencher concentration in the range 0-0.1 mM; (B) quencher concentration in the range 0-0.5 mM.

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J. Phys. Chem. B, Vol. 114, No. 36, 2010 11923 TABLE 1: Quenching of the nπ* and ππ* Triplets of Benzophenone Derivatives by 2-Aminobenzimidazole compound 3 4 5 5 5 6 6 6 7

Figure 3. Transient absorption spectra obtained by LFP (355 nm) of (A) 4-methoxybenzophenone in acetonitrile at 0.56 µs after the laser pulse (2), in PBS:acetonitrile (3:1, v/v) at 0.88 µs after the laser pulse (b), and in cyclohexane at 0.12 µs after the pulse (9). (B) (Normalized) 4,4′-dimethoxybenzophenone in acetonitrile at 0.56 µs after the pulse (b), in PBS:acetonitrile (3:1, v/v) at 0.88 µs after the pulse (2), and in cyclohexane at 0.12 µs after the pulse (9).

In principle, attachment of electron donor substituents to the aromatic rings of benzophenone should have a strong influence on the process. For this reason, abstraction of the 2-aminobenzimidazole hydrogen by excited 4-MBP (5) and 4,4′-MPB (6) was also studied. In nonpolar solvents, the triplet of these aromatic ketones presents the nπ* configuration, showing the typical BP-like behavior. However, in aqueous solution, the transient absorption spectra display maxima at ca. 450 and 680 nm, attributed to the ππ* triplet state22 (Figure 3A). Actually, LFP of 5 in acetonitrile gave a transient spectrum similar to that described in cyclohexane22 but with enhanced contribution of the 450 and 680 nm bands (Figure 3A). In the presence of 1, spectral changes were consistent with formation of the aminyl/ketyl radical pair, with characteristic bands of 1-H• at 450 nm11 and 5H• at 550 nm.36 The rate constant for quenching of triplet 5 by 1 in acetonitrile was determined using the kinetic traces at 520 (nπ*) and 680 (ππ*) nm. The values obtained were 4.0 × 109 and 3.8 × 109 M-1 s-1, respectively (see Table 1). Here, it is remarkable that the quenching kinetics of two triplets of different electronic configuration, corresponding to the same molecule, has been investigated simultaneously under identical experimental conditions. When the 355 nm LFP of 5 was carried out in a mixture of aqueous buffer and acetonitrile (3:1, v/v), the absorption spectrum was very different from that obtained in net acetonitrile. Thus, three bands centered at 350 (medium intensity), 450 (strong), and 680 (broad) nm were observed, matching the previously described spectrum of the ππ* triplet of 5 (Figure

λ (nm) 620 640 680 520 680 680 680 550 680 680 680

(n π*) (π π*) (π π*) (n π*) (π π*) (π π*) (π π*) (n π*) (π π*) (π π*) (n π*)

solvent ACN ACN ACN ACN ACN:PBS ACN:PBS ACN ACN ACN:PBS ACN:PBS ACN

(1:3) (D2O) (1:3) (1:3) (D2O) (1:3)

k (M-1 s-1) 6.2 × 109 3.9 × 109 3.9 × 109 4.0 × 109 5.4 × 109 4.2 × 109 2.8 × 109 3.2 × 109 3.9 × 109 3.3 × 109 8.0 × 109

3A).22 By comparison of the traces obtained 0.24 µs after the laser pulse in the absence and presence of 1, the residual signals at 450 and 550 nm corresponding to the aminyl and ketyl radicals were distinguishable. The quenching rate constant was measured at 680 nm and resulted to be 5.4 × 109 M-1 s-1. The studies were extended to 4,4′-DMBP in organic and aqueous media. Thus, LPF of 6 in acetonitrile gave rise to a spectrum with four maxima at 360 (strong), 425 and 550 (medium), and 675 nm (broad). In a mixture of aqueous buffer and acetonitrile (3:1 v/v), the spectrum obtained showed two medium intensity bands at 370 and 670 nm and a strong band centered at 450 nm. By comparison with the spectra of 5 in the same media, it is possible to assign the 425-450 nm band to the ππ* triplet and the 550 nm band to the nπ* triplet. In the presence of 1, in acetonitrile, a diminished intensity of the 425 and 550 nm bands was observed, and the residual absorption after long delay times was consistent with formation of the aminyl/ketyl36 radical pair; however, in this case, the spectra were masked by overlap with the triplet bands. The quenching rate constants were obtained from the Stern-Volmer plots; they were found to be 2.7 and 3.2 × 109 M-1 s-1 for the ππ* and nπ* triplet, respectively (see Table 1). When the experiment was conducted in aqueous medium, where only ππ* triplet is formed, the quenching rate constant was measured at 680 nm and resulted to be 3.9 × 109 M-1 s-1. To gain further insight into the mechanistic aspects of the reaction, the behavior of triplet 5 and 6 was examined in acetonitrile-PBS (D2O) (1:3). Under these conditions, the obtained rate constants were 4.2 × 109 and 3.3 × 109 M-1 s-1, respectively (Table 1). Hence, the observed deuterium isotope effects were kH/kD ) 1.23 for 5 and 1.18 for 6. These values are within the expected range for hydrogen abstraction by ππ* ketone triplets14c,37 and are compatible with an electron transfer/ proton transfer pathway. Finally, LPF of 4-carboxybenzophenone (7), where the substituent attached to the aromatic ring is an electron acceptor in nature, was also studied. In acetonitrile solution, the obtained spectrum showed maxima at ca. 350 and 550 nm, safely assigned to the previously described nπ* triplet of 7.23 In the presence of 1, disappearance of this band was accompanied by formation of new transients peaking at 450 and 570 nm that were ascribed to the aminyl/ketyl radical pair. The quenching rate constant was 8 × 109 M-1 s-1, somewhat higher than the values obtained for the other benzophenone derivatives in the same solvent. Theoretical DFT Calculations. The reaction mechanism for hydrogen abstraction from 2-aminobenzimidazole (ABZ, 1) by triplet excited benzophenone (3) or 2-benzoylthiophene (4) was theoretically studied using density functional theory (DFT) methods. In this context, it has to be taken into account that this type of calculation may serve as a model study, but it is

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TABLE 2: Total (E, in au) and Relative (∆E, in kcal/mol) Energies of the Stationary Points Involved in the Photoreaction of ABZ 1 with BP 3 and BT 4 6-31G(d) E 1 (S0) 3 (S0) 3 (T1) MC3 (S0) RIP3 RP3 RP3′ 4 (S0) 4 (T1) MC4 (S0) RIP4 RP4 RP4′

-435.226323 -576.632268 -576.530767 -1011.874473 -1011.786074 -1011.803358 -1011.804253 -897.389704 -897.292160 -1332.631911 -1332.549395 -1332.561298 -1332.564711

6-311+G(d,p) ∆E

63.7 -10.0 45.5 34.7 34.1 61.2 -10.0 41.8 34.3 32.2

E -435.3508632 -576.7818064 -576.6791541 -1012.145192 -1012.058090 -1012.074515 -1012.075141 -897.5470666 -897.4488589 -1332.910681 -1332.828951 -1332.840753 -1332.843527

∆E

64.4 -7.6 47.1 36.8 36.4 61.6 -8.0 43.3 35.9 34.1

not intended to provide a complete description of the real situation. In the ground state, molecular complexes can be formed between 1 and 3 or 4 (MC3 and MC4), in which the carbonyl oxygen atom of the ketones is hydrogen-bonded to the two acidic hydrogens of 1 (H1 and H10). Actually, at the B3LYP/6-311+G(d,p) level, MC3 and MC4 are located 7.6 and 8.0 kcal/mol below the separated reagents 1+3 or 1+4, respectively; therefore, hydrogen transfer was studied at the excited molecular complexes. The energies of 1, 3, 4, the corresponding excited states, the molecular complexes MC3 and MC4, as well as the resulting radical ion pairs (RIP3, RIP4) and neutral radical pairs (RP3, RP3′, RP4, RP4′) are given in Table 2. At MC3 and MC4, electronic excitation is associated with promotion of one electron from the HOMO to the LUMO. Analysis of the MO shape indicates that, while HOMOs are mainly located at the ABZ framework, LUMOs mainly reside at the BP and BT framework (see Figure 4); the HOMO/LUMO

Figure 4. B3LYP/6.31G* HOMO and LUMO of the MC3 and MC4.

Figure 5. Geometries of the most relevant intermediate species involved in formal hydrogen donation: RIP3, RIP4, RP3, RP4, RP3′, and RP4′.

energy gaps are as low as 58.0 kcal/mol (MC3) and 53.3 kcal/ mol (MC4). Consequently, excitation of the molecular complexes can be considered as electron donation from the HOMO of 1 to the LUMO of 3 or 4, to yield radical ion pairs RIP3 or RIP4. These triplet species are located 47.1 or 43.3 kcal/mol above the ground states and 17.3 or 18.3 kcal/mol below the separated excited states 1 (S0) plus 3 (T1) or 4 (T1), pointing to a strong hydrogenbond interaction. Proton transfer within the radical ion pairs would lead to the triplet radical pairs RP3, RP4, RP3′, or RP4′, which are located 36.8, 36.4, 35.9, and 34.1 kcal/mol above the corresponding ground states; hence, this step would be thermodynamically favorable by ca. 10 kcal/mol. All attempts to locate the transition states for this process were unsuccessful, suggesting that it proceeds basically in a barrierless fashion. Figure 5 shows the geometries of the most relevant intermediate species involved in formal hydrogen donation: RIP3, RIP4, RP3, RP4, RP3′, and RP4′. The most significant data are the N1-H1 and N10-H10 bond lengths, as well as the distances between these acidic hydrogens of 1 and the carbonyl oxygen atom of 3 or 4 (see Table 3). At the ground state molecular complexes, the lengths of the acidic N-H bonds are 1.013 Å (N1-H1) and 1.015 Å (N10-H10), whereas the H1-O and H10-O distances are 2.072 and 2.222 Å (MC3) or 2.092 and 2.230 Å at (MC4). In the radical ion pairs, the lengths of the acidic NH bonds are 1.093 Å (N1-H1) and 1.063 Å (N10-H10) at RIP3 or 1.047 Å (N1-H1) and 1.080 Å (N10-H10) at RIP4. The H1-O and H10-O distances are 1.507 and 1.683 Å at RIP3 or 1.732 and 1.568 Å at RIP4. Thus, while the N-H bonds are longer, the H-O distances are shorter at the RIPs; this is consistent with

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TABLE 3: (U)B3LUP/6-31G(d) Selected Geometrical Parameters, Bond Lengths in Å, of the Triplet States of the Process Involved in the Photoreactions of MC3 and MC4 1 (S0) MC3 RIP3 RP3 RP3′ MC4 RIP4 RP4 RP4′

N1-H1

N10-H10

H1-O

H10-O

1.009 1.013 1.093 1.859 1.018 1.013 1.047 1.827 1.017

1.015 1.015 1.063 1.018 1.859 1.015 1.080 1.018 1.856

2.072 1.507 0.994 2.041 2.092 1.732 0.998 2.063

2.222 1.683 2.035 0.992 2.230 1.568 2.042 0.994

TABLE 4: (U)B3LUP/6-311+G(d,p) Total Mulliken Spin and Total Natural Charges at the ABZ Moiety of the MCs and at the Resulting RIPs and RPs MC3 RIP3 RP3 RP3′

spin

charge

0.00 1.01 1.01 1.01

-0.01 0.81 0.03 0.03

MC4 RIP4 RP4 RP4′

spin

charge

0.00 0.99 1.01 1.01

-0.01 0.81 0.04 0.04

a strong hydrogen-bond interaction. In the case of radical pairs RP3 and RP4, the N1-H1 and N10-H10 distances are 1.859 and 1.018 Å or 1.827 and 1.018 Å, respectively. Accordingly, the corresponding H1-O and H10-O distances are 0.994 and 2.035 Å or 0.998 and 2.042 Å, respectively. Finally, at RP3′ and RP4′, the N1-H1 and N10-H10 distances are 1.018 and 1.859 Å or 1.017 and 1.856 Å, while the H1-O and H10-O distances are 2.041 and 0.992 Å or 2.063 and 0.994 Å, respectively. These values indicate that the transferred hydrogen atom is H1 at RP3 and RP4 or H10 at RP3′ and RP4′. In order to assess the ionic versus neutral nature of RIPs and RPs, the degree of charge transfer was analyzed. In these species, the values obtained for Mulliken atomic spin densities indicate that both subunits have a radical character (see Table 4). Interestingly, the B3LYP/6-311+G(d,p) natural atomic charges were found to be shared between the donor and the acceptor fragments. Thus, while at the radical ion pairs RIP3 and RIP4 charge separation is 0.81 e, at the neutral radical pairs RP3, RIP3′, RP4, and RP4′ the values are lower than 0.04 e. These results suggest that a large amount of electron density (ca. 1 e) has been transferred from ABZ to BP or BT frameworks along the course of the reaction and are consistent with a sequential electron transfer/proton transfer mechanism. Conclusion Both experimental and DFT calculations suggest that at the triplet excited states of the molecular complexes one electron is transferred from 2-aminobenzimidazole to benzophenone or benzoylthiophene, giving radical ion pairs RIP3 or RIP4. Subsequent proton transfer from the amino group to the carbonyl oxygen atoms leads to the neutral biradicals RP3′ or RP4′. A comparison between the relative energies and geometries of the species involved in the photochemical reactions indicates that all ketones follow a similar mechanism. Acknowledgment. Financial support from the MICINN (grants CTQ2007-67010 and CTQ2009-11027/BQU and predoctoral fellowship to P.B.) and the Generalitat Valenciana (Prometeo Program) is gratefully acknowledged. References and Notes (1) (a) Behnke, J. M.; Buttle, D. J.; Stepek, G.; Lowe, A.; Duce, I. R. Parasites Vectors 2008, 1. (b) Prichard, R. K. Parasitology 2007, 134, 1087.

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