Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 78– 84
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Journal of Photochemistry and Photobiology A:Chemistry
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ffect of co-ligands on photoredox pathways in Cr(III) oxalate complexes
oanna Wisniewskaa, Hasan Maraia, Andrzej Karockib, Grzegorz Stopab, Ewa Kitaa, Zofia Stasickab,∗
Department of Chemistry, N. Copernicus University, 87-100 Torun, PolandFaculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland
r t i c l e i n f o
rticle history:eceived 18 July 2012eceived in revised form1 September 2012ccepted 29 September 2012vailable online 8 October 2012
eywords:
a b s t r a c t
The photochemical behaviour of some mixed ligand chromium(III) complexes with amino acids,[Cr(C2O4)2(Aa)]n− (where Aa = alanine, valine, serine, cysteine, asparagine, aspartic acid) was studied.The attention was focused on the photoredox mode, which proceeded via inner- or intramolecular path-way yielding Cr(II) species and hydrated electrons, respectively. The secondary thermal processes weredependent on the O2 presence and solution pH: (i) in oxygen-free media the regeneration of substrateand photoaquation induced by the Cr(III) → Cr(II) reduction were observed, (ii) in the presence of O2 bothCr(II) and ligands were oxidized and the former was transformed not only into Cr(III) but also to Cr(VI)
hromium(III) complexeshromate(VI)xalatemino acidhotoreductionhotoinduced electron transfer
(provided that pH > 7). Prolonged irradiation resulted in photoreduction of Cr(VI) accompanied by pho-todegradation of oxalate and/or amino-acid ligands. The photoreaction modes were independent of theco-ligand nature, but the secondary reaction rates and efficiencies were sensitive both to the co-ligandnature and its side substituent. Environmental consequences of the chromium photoreduction are dis-cussed in the paper: the parameters affecting production and consumption of Cr(VI) are analysed, andthe tools of controlling the photoredox behaviour of the Cr(III) and Cr(VI) compounds are suggested.
. Introduction
Photochemistry of chromium compounds is one of the mostntensively studied fields and an essential progress in recognitionf the Cr(III) and Cr(VI) photoredox behaviors results from theirelevance to the biochemical and environmental processes [1–7].
The phenomenon of Cr(III) photoreduction to Cr(II) is character-stic of the complexes in which at least one of the ligands (L) is ableo reduce the Cr(III) centre in the ligand-to-metal charge transferLMCT) excited state [8–10]:
CrIIIL6]hv(LMCT)−→
∗[CrII(L
•)L5] (1)
Unstable radical complexes *[CrII(L•)L5] decay mostly by releasef the L• radical followed by completion or reorganization of theoordination sphere, as in the case of the Cr(III) citrate or tartrateomplex [11,12]:
CrIII(cit)]hv(LMCT)−→
∗[CrII(cit
•)] → Craq
II + cit•2− (2)
In the case of polydentate ligand, like EDTA, the photoinducedlectron transfer can result in cleavage of only one M L bond andormation of a dangling radical grouping, sensitive to O2 like freeadical [13–16].
Recently, the study of mixed ligand [CrIII(C2O4)2(pda)]3− com-plexes, where pda = pyridinedicarboxylate, has shown still anothermechanism of the LMCT deactivation in which the innersphereredox process:
[CrIII(C2O4)2(pda)]3−h�(LMCT)−→ [CrII(C2O4
•−)(C2O4)(pda)]3−
(3)
is accompanied by the outersphere electron transfer:
[CrIII(C2O4)2(pda)]3−h�(LMCT)−→ [Cr(C2O4
•−)(C2O4)(pda)]2− + e−
aq
(4)
and followed by the radical ligand substitution and generation ofdiaqua Cr(III) complexes [17].
Both Cr(II) species and radical ligands readily react with molec-ular oxygen and thus the most important consequence of theCr(III)→Cr(II) photoreduction in aerobic atmosphere is oxidation ofligands (and/or external electron donors involved in the photore-dox process) accompanied by oxidation of Cr(II) to Cr(III) and/or toCr(VI). These processes have two important environmental aspects:positive as stimuli of the photocatalytic degradation of organic pol-lutants and negative as producing the harmful chromate(VI).
To take advantage of the former aspect, the Cr(VI) concentra-
tion should be efficiently suppressed. This effect can be achievedin two ways: (i) by stimulation of the efficient Cr(VI)→Cr(III) pho-toreduction and (ii) by suppression of the Cr(II)→Cr(VI) oxidationvia selection of optimal ligands and reaction medium.
he former process (Eq. (5)) involves two electrons and occurshen Cr(VI) is covalently bonded with an electron donor, whereas
he latter (Eq. (6)) involves one electron transfer between a Cr(VI)xcited species and an outersphere donor. The outersphere PETechanisms, similar to that shown in Eq. (6), were found also in
ther systems containing chromate(VI) and electron donors, suchs oxalate or phenol and its halogen derivatives [2,6,18–23].
(ii) The most simple way to avoid the Cr(II)→Cr(VI) oxidations keeping the acidic medium because the reaction is favoured bylkaline medium and a relatively large O2 excess over the Cr(II)oncentration:
r(II)aq + O2OH−−→CrO4
2− (7)
echanism of this reaction has been interpreted in terms of gener-tion of transient Cr(III) superperoxide, CrO2aq2+, which in reactionith the OH− ions is transformed into CrO4
2− [24,25]. Recently, theH• radicals, generated either during photoreduction of a Cr(III)ydroxo-complex, e.g.:
CrIII(cit)(OH)]−hv(LMCT)−→ CrII
aq + cit3− + OH•
(8)
r from the O2•−/H2O2 intermediates were suggested as respon-
ible for Cr(VI) formation [11,12]. The photoredox behaviour ofhe Fe(III)–EDDS complex was interpreted similarly [26], but theiverse redox reactions proceeding in alkaline solution of Cr(VI)nd H2O2 seem to challenge this thesis [26–29].
Recent investigation of some mixed-ligand oxalato Cr(III) com-lexes has shown that pyridinedicarboxylate co-ligands can controlhe photoreduction of these compounds, especially they affect theroduct output [17]. To get more information on the co-ligandffects we studied photochemical behavior of Cr(III) oxalate com-lexes with �-amino acids of the [Cr(C2O4)2(Aa)]n− type (wherea = RCH(NH2)COO), which exemplify the complexes with almost
dentical coordination sphere differing only the side group R = CH3alanine), CH(CH3)2 (valine), CH2OH (serine), CH2SH (cysteine),H2CO(NH2) (asparagine), CH(NH2)COO (aspartic acid).
. Experimental
.1. Reagents
Chromium(III) complexes of the general formulaCr(C2O4)2(Aa)]n−, where Aa = ala, val, ser, cys, asn or asp wereynthesized from cis-[Cr(C2O4)2(H2O)2]− and relevant �-aminocid and analysed as described previously [30].
For the continuous photolysis the 2 mM solutions of ther-complexes were used, whereas for laser experiments loweroncentrations (0.1–0.4 mM) were applied. In all cases pH was sta-ilized at the 10.5 value with Britton-Robinson buffer containing.04 M H3PO4, 0.04 M H3BO3, 0.04 M CH3COOH and NaOH. Oxygen-ree solutions were prepared by 30-min bubbling of the samplesith argon.
.2. Procedures, instrumentation and data analysis
Continuous irradiation was performed using a high-pressureercury HBO-200 lamp (OSRAM) as a light source, equippedith 10 cm IR water filter. Irradiations were carried out with full
Photobiology A: Chemistry 250 (2012) 78– 84 79
light from the mercury lamp or with an additional cut-off glassfilter (� ≥ 360 nm). Solutions were held in 1 cm quartz cells ther-mostated at 298 K. Reaction progress was monitored spectrallywithin 200–800 nm. Spectral detection was made using a ShimadzuUVPC 2100 spectrophotometer. The solution pH was measuredusing a CX-741 pH-meter (Elmetron, Poland) with a glass electrode,calibrated with standard buffers (POCH, Poland).
Quantum yields were measured on a computer controlledhome-made equipment according to the method previouslydescribed [31]. Increase in the Cr(VI) concentration wascalculated from the absorbance changes at 374 nm (takingε374 = 4880 M−1cm−1 as molar absorption coefficient for theCrO4
2− anion at pH 10.5) [22].Laser flash photolysis within nano- to microseconds was carried
out at 283 K using a LKS.60 Spectrometer (Applied Photophysics,UK) equipped with Nd:YAG laser pump source Surlite I-10 (Con-tinuum), operating in fourth harmonic (266 nm, max 75 mJ pulses,6 ns FWHM). Spectral analysis of the flash photolysis results wasmade using a Perkin Elmer Lambda 950 UV/vis/NIR spectropho-tometer. Further experimental details were the same as describedpreviously [17].
3. Results and discussion
The UV–vis spectra of the studied [Cr(C2O4)2(Aa)]n− ions aretypical of the Cr(III) complexes: they consist of two spin allowedMC bands in the visible region (ca. 405 and 550 nm), one spin for-bidden band (ca. 750 nm) and an intense LMCT band (within �max
260–266 nm) (Table 1).The [Cr(C2O4)2(Aa)]n− complexes in aqueous solution undergo
a very slow dark aquation yielding cis-[Cr(C2O4)2(H2O)2]− and rel-evant amino acid. Preliminary kinetic measurements performed inbuffered solutions showed that at room temperature the pseudofirst-order rate constants of the aquation (kobs) were of the 10−3 s−1
order and their values were the lower the higher solution pH (theresults of the studies on kinetics and mechanism of the acid- andbase-catalyzed thermal aquations of the [Cr(C2O4)2(Aa)]n− com-plexes are published elsewhere [30]).
The only exception is [Cr(C2O4)2(cys)]2−, for which the initialUV/vis spectrum observed at pH 10.5 is remarkably different fromthose of other [Cr(C2O4)2(Aa)]n− complexes but some fast reac-tions (within minutes) cause bathochromic shifts of main bandsaccompanied by increase in absorption at � > 240 nm. The spec-tral shapes are only negligibly sensitive to the oxygen presence.The final spectrum approaches that recorded in neutral solutionalthough the molar absorption coefficient of the LMCT band is sub-stantially higher (cf. Table 1).
The spectral changes can be interpreted in terms of unique prop-erties of cysteine, which has three dissociable protons (pKa = 1.91,8.16 and 10.29) and is ambidentate [32]. Thus, at pH = 10.5 depro-tonation of the SH group can lead to the fleeting disorder in thecoordination sphere, which can lead to formation either triden-tate (S,N,O) or bidentate stable form with different combination ofdonor atoms (N,O), (S,N) or (S,O). As spectrum of this form is similarto other amino-acid complexes, which can be only bidentate (N,O)we assumed that in all studied complexes the amino-acid ligandsare bidentate, although the contribution of the S-donor atoms can-not be excluded due to substantially higher absorption within theCT band (cf. Table 1).
The slow thermal aquation at pH 10.5 of the [Cr(C2O4)2(Aa)]n−
complexes did not disturb to follow the photochemical behaviors
of their 2 mM freshly prepared solutions at 298 K, and 0.4 mM solu-tions at 283 K.
All studied [Cr(C2O4)2(Aa)]n− complexes undergo photochem-ical reactions, which mode depends on both energy of absorbed
80 J. Wisniewska et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 78– 84
Table 1Spectral characteristics of 0.002 M [Cr(C2O4)2(Aa)]n− in Britton–Robinson buffer at pH = 10.5 as compared with those of homo- and heteroleptic oxalato and aqua complexes.
adiation and presence of molecular oxygen, whereas it is indepen-ent of the Aa co-ligand nature. Spectral changes recorded duringontinuous irradiation of [CrIII(C2O4)2(ala)]2− (Fig. 1) illustrate typ-cal behavior of all [Cr(C2O4)2(Aa)]n− complexes.
Excitation within the MC bands (Fig. 1a) leads to decrease inhe absorption intensity accompanied with a bathochromic shift,hat is consistent with photoaquation yielding mostly the rela-
ively stable cis-[Cr(C2O4)2(H2O)2]− isomer and the relevant aminocid [30]:
CrIII(C2O4)2(Aa)]n− + 2H2O
hv(MC)−→ [CrIII(C2O4)2(H2O)2]− + Aa(1−n)
(9)
pparently the similar effect is observed in result of CT excitation ofhe complexes under oxygen-free conditions (Fig. 1b). The percep-ible differences consist only in the higher yield of photoaquationnd significant absorption increase at � < 260 nm. However, in theresence of atmospheric oxygen (Fig. 1c), the CT excitation leadso significant increase in absorption with maximum at 374 nm,haracteristic of the CrO4
2− ions [22].The results let us to conclude that full-light irradiation besides
he photosubstitution (Eq. (9)) induces also a photoredox mode (Eq.10)). The hypothesis was verified by the experiment, in which thentermediate generated by UV exposure under oxygen-free atmo-phere, was transformed into chromate(VI) during subsequentaturation by molecular oxygen (Fig. 2). The electronic spectrumf the intermediate and its conversion into chromate(VI) owing toxygenation resemble the results reported earlier for the [CrIIedta•]pecies (cf. Fig. 5 in Ref. [13]). It suggests, that LMCT excitation leadso photoreduction:
CrIII(C2O4)2(Aa)]n−h�(LMCT)−→ [CrII(C2O4
•−)(C2O4)(Aa)]n−
(10)
The labile Cr(II) species under oxygen-free atmospherendergoes back electron transfer either regenerating initialCrIII(C2O4)2(Aa)]n− complex:
hus, the aquation induced by the Cr(II) radical complex enhanceshe photoaquation yield (cf. Fig. 1a and b) similarly to other pho-oreduced chromium(III) complexes [22,33].
In the presence of O2, however, even more processes areobserved: the photoreduction (Eq. (10)) is followed not only byback electron transfer (Eqs. (11) and (12)), but also by the Cr(II)oxidation to Cr(III) or/and Cr(VI) at the expense of O2 (cf. Eq. (7)).The latter is characterized by the absorption increase in the nearUV region with two clearly isolated maxima which position andintensity are highly sensitive to the solution pH (i.e. they dependon the relationship between the CrO4
2− and HCrO4− forms); at
pH = 10.5 chromate(VI) is characterized by �max = 374 and 275 nm,ε = 4880 and 3600 M−1 cm−1, respectively [22,34]. Unfortunately, inthese experiments the band at 275 nm could not be observed dueto its overlap with the intense LMCT bands of the studied Cr(III)complexes (cf. Table 1).
Although the photoredox behavior was qualitatively almostidentical for all studied [CrIII(C2O4)2(Aa)]n− complexes and alsosimilar to other Cr(III) oxalate compounds, it is interesting to notethat the side group in the amino acid co-ligand significantly influ-ences the quantum yield of the chromate(VI) production (Table 2).
Comparing these results with those previously reported [17],the co-ligands LL′ in the [CrIII(C2O4)2LL′] complexes (where LL′ isa bidentate co-ligand with the same or different donor atoms) canbe arranged in the series of decreasing quantum yields of the Cr(VI)production (in parenthesis ˚VI × 101):
The quantum yield values allow to conclude that presence ofthe N,O-bonded LL′ co-ligand in the coordination sphere of Cr(III)-oxalato complexes is of crucial importance for the Cr(VI) output.In detail, the ˚VI value depends on: (i) the co-ligand nature (pdaor Aa); (ii) the nature of side-substituent in the Aa ligand [espe-cially large difference is observed between R = CH2OH (serine) andR = CH(NH2)COO− (aspartic acid)]; and (iii) the isomeric form of thepda ligand (cf. 2,3-pda and 2,5-pda).
The investigation substantiates the earlier observation [17] thatthe oxalate ligand is responsible for the photoredox reaction of theCr(III) complex and thereby for chromate(VI) generation. It maybe also concluded, that different ligands coordinated to the Cr(III)centre play diverse roles: they can be either electron donors in theexcited Cr(III) complex (such as oxalate) or mediators not engageddirectly in the photoredox reaction but affecting the secondary pro-cesses (such as amino acids or pda ligands).
To receive more mechanistic information on the[CrIII(C2O4)2(Aa)]n− photoreduction the time-resolved spec-tra upon the 266-nm laser pulse excitations were recorded.The results shown in Fig. 3 are representative for all studied
J. Wisniewska et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 78– 84 81
Table 2Effect of amino-acid co-ligands on rate constant (kobs) and quantum yield of chromate(VI) production (˚VI × 101) from aerated, alkaline (pH = 10.5) 2 mM oxalatochromate(III)solutions exposed to 254-nm continuous irradiation at 298 K.
mino-acid co-ligands, although they differ in some quantitativeetails. The experiments revealed that decrease in the substrateoncentration (recorded at ∼270 nm) is accompanied by anbsorption increase at � > 300 nm. The most pronounced transient
ig. 1. Spectral changes recorded in every minute of continuous irradiation of 2 mMCrIII(C2O4)2(ala)]2− in buffer solution at pH = 10.5: (a) irradiation within MC range� > 360 nm) under atmospheric conditions; (b) exposure within both MC and LMCTands (full light of the mercury lamp) under oxygen free atmosphere; and (c) fullercury lamp irradiation in the presence of atmospheric oxygen.
Fig. 2. Effect of post-irradiation oxygenation: curve 1: absorbance of 0.4 mM[Cr(C2O4)2(val)]2− solution at pH = 10.5; curve 2: absorbance differences,
�Airr = 10 (Airr − A0), upon 1 min continuous irradiation with full light of the 250 Wxenon high-pressure lamp in oxygen-free atmosphere; and curve 3: �Aox = 100(Airr,ox − Airr) upon subsequent O2 saturation; temperature 298 K.
absorption, recorded within 450–750 nm with a maximum at680–700 nm, is characteristic of hydrated electrons [35,36]. Thee−
aq generation is accompanied by production of relatively long-lived intermediate weakly absorbing at ∼330–340 nm, which isattributed to the Cr(II) species [13].
The spectral changes presented in Fig. 3 are interpreted in termsof two competitive photoreduction pathways, namely innersphere
(Eq. (10)) and outersphere (Eq. (13)):
[CrIII(C2O4)2(Aa)]n− + H2O
hv(CT)−→ [Cr(C2O4)2(Aa)](n−1)− + e−aq (13)
Fig. 3. Time-resolved spectra of deoxygenated 0.4 mM [Cr(C2O4)2(asp)]3− solutionat pH = 10.5 recorded within microseconds upon flashing by 266 nm laser pulse;�A = Airr,t − A0; T = 283 K.
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2 J. Wisniewska et al. / Journal of Photochemist
The latter reaction is followed by fast decay of hydrated elec-rons (within �s), which in deaerated medium is accompanied byess intensive spectral changes at shorter wavelengths (at about60 and 320 nm), that can be interpreted in terms of possible post-
rradiation changes in the substrate and product concentrations. Itmplies that in the studied systems e−
aq can be scavenged by: (i) TheCr(C2O4)2(Aa)](n−1)− intermediate in reaction reverse to Eq. (13)egenerating the substrate [easily observed for alanine (R = CH3)nd valine (R = CH(CH3)2)]:
Cr(C2O4)2(Aa)](n−1)− + e−aq → [CrIII(C2O4)2(Aa)]
n−(14)
ii) The substrate, what leads to post-irradiation generation of ar(II) species (pronounced in the case of R = CH2OH, which justifieshe high ˚VI of the serine complex):
CrIII(C2O4)2(ser)]2− + e−
aq → [CrII(C2O4)2(ser)]3−
(15)
nd (iii) The ligand radicals C2O4•− and/or Aa• - then the e−
aq decay isccompanied by no changes in the complex concentrations (notice-ble for R = CH(NH2)COO, cf. Fig. 3). The oxalate radicals, known asast e−
aq scavengers [22]:
−aq+C2O4
•− → C2O42− (16)
re released from the transient [CrII(C2O4•−)(C2O4)2]3− or
CrII(C2O4•−)(C2O4)(Aa)]n−, e.g.:
CrII(C2O4•−)(C2O4)(Aa)]n− + 2H2O
→ [CrII(C2O4)(Aa)(H2O)2](n−1)− + C2O4•− (17)
Amino-acid ligands can be as well involved in the e−aq release and
cavenging. Similar to the pda• radical in the [CrIII(C2O4)2(pda)]3−
hotochemistry [17], the Aa• radical is rapidly substituted by two2O molecules, e.g.:
Rates of the e−aq decay recorded for the UV irradiated
CrIII(C2O4)2(Aa)]n− complexes (Table 3) show that the naturef the co-ligand controls the scavenging efficiency. Theseesults completed by those reported previously for theCrIII(C2O4)2(pda)]3− complexes [17] allow to arrange the co-igands in the series of decreasing electron scavenging properties ofhe [CrIII(C2O4)2(LL′)]n− complexes (in parenthesis rate constants,obs [106 s−1]):
ox (4.43) � asn (2.37) > val (2.17) > asp (2.08) > ser (1.68) > ala1.21) > cys (1.16) (T = 283 K)
The homoleptic oxalate complex turned out to be the most effec-ive scavenger, less efficient were complexes with amino-acids,hereas complexes with the pyridinedicarboxylate co-ligandsere found to be the weakest scavengers. It is worthwhile to note,
hat the nature of the side substituent of the Aa co-ligand is also anmportant factor.
In aerated media the reaction with molecular oxygen
−aq + O2 → O2
− (20)
s considered as main pathway of the e−aq scavenging. This reaction is
nown as initiator of successive processes generating OH• radicalshat were suggested as responsible for the Cr(II) → Cr(VI) oxidation
11,12,26,37]. The hypothesis is supported by the lowest ˚VI valueor the [Cr(C2O4)3]3− complex (Table 2), as the C2O4
•− radicals com-ete with O2 in the e−
aq scavenging more effectively then the Aa• orda• radicals. However, the relation between e−
aq scavenging and
Photobiology A: Chemistry 250 (2012) 78– 84
Cr(VI) efficiency for the [CrIII(C2O4)2(Aa)]n− complexes is not sounequivocal (cf. Tables 2 and 3), because of strong involvement ofthe OH• radicals in the amino-acid oxidation [38,39].
At prolonged UV irradiation of aerated [CrIII(C2O4)2(Aa)]n− solu-tions the Cr(VI) production becomes slower or even the Cr(VI)depletion is recorded (Fig. 4a and b). The subsequent changes inCr(VI) concentration depends on the R substituent nature: stabi-lization of the Cr(VI) level (for R = CH3 or CH(NH2COO) can resultfrom the enhanced consumption of the OH• radicals by the Aareleased in the redox reactions (Eqs. (7), (9) and (18)); whereasthe Cr(VI) depletion (recorded for R = CH2CONH2, CH(CH3)2, CH2OHand CH2SH), must be a consequence of its photoinduced reduction:
CrO42− + D
h�(LMCT)−→ CrVO43− + D+ (21)
Reaction (21) is possible if (i) accumulation of the CrO42− reaches
concentration high enough to compete with other UV absorberswithin its LMCT band, and (ii) the external electron donors able toreduce the excited Cr(VI) to Cr(V) in alkaline medium are gener-ated. The Cr(VI) photoreduction (Eq. (21)) is followed by a series ofsecondary thermal com- and disproportionation reactions yieldingCr(III) and Cr(VI), summarized in Eq. (22):
3Cr(V) → 2Cr(VI) + Cr(III) (22)
The effect of the decreased Cr(VI) concentration at prolongedirradiation was never observed either for [CrIII(C2O4)2(pda)]3−
or [Cr(C2O4)3]3− complexes [17]. In the latter case, some of the[Cr(C2O4)3]3− photoproducts (CrO4
2− and C2O42−) can react with
each other (Eq. (23)), provided that chromate(VI) is in the excitedstate and solution pH is not higher than 7; the photoreduction wasfound to be mediated by an ion-pair formation [22]:
∗CrO2−4 + C2O2−
4M+−→CrVO3−
4 + C2O4•− (23)
However, some electron donors (D) are, known, which areable to reduce the excited CrO4
2− ions also in alkaline medium,e.g. D = phenol and its halogen derivatives, butane-2,3-diol or tri-ethanoloamine [6,19,23].
Thus, the depletion of the Cr(VI) concentration recorded duringprolonged irradiation of the [CrIII(C2O4)2(Aa)]n− complexes can beinterpreted in terms of chromate(VI) photoreduction in result ofphotoinduced electron transfer from an exterior electron donortowards the excited chromate(VI) ion. The role of electron donoractive in alkaline medium must be played either by reduced formsof the [CrIII(C2O4)2(Aa)]n− complex or by the amino acid, e.g.:
2CrO42− + 2cys−hv(LMCT)−→ 2CrVO4
3− + (cys)2 (24)
In any case, cysteine is known as effective reducer of chromate(VI)even in its ground state and the reaction is then accompanied byformation of the stable Cr(III)–cys complex containing the directCr S bonding [40].
The results of this paper allow to order the studied amino acidsin the series of decreasing efficiency in the *CrO4
2− reduction:cys � ser > asn ≈ val > asp > alawhich to some extend resembles
the series of their rate constants in reaction with the OH• radicals(in parenthesis rate constants, k [107 M−1 s−1]) [39]:
cys (340) � val (76) ∼ asp (75) > ser (32) > ala (7.7) > asn (4.9)Among the investigated amino acids, alanine is known as one of
the most stable against the electrochemical oxidation. This stabi-lization was motivated presence of the methyl group in its chemicalstructure [41].
The Cr(VI) photoreduction (Eq. (21)) is accompanied by oxi-
dation of the external organic donors, which are often treatedas pollutants. Thus, the photoredox generation and consequentconsumption of chromate(VI) is more effective for the environmen-tal cleaning purposes than the Cr(III) photoreduction proceeding
J. Wisniewska et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 78– 84 83
Table 3Decay rates and lifetimes of hydrated electrons generated upon 266-nm laser pulse in deoxygenated solutions of the oxalatochromates(III) at 283 K.
ig. 4. Effect of prolonged irradiation by full light of the mercury lamp of 2 mM
or Aa = serine (successive curves were recorded in every 1 min of exposure); aCrIII(C2O4)2(Aa)]n− complexes.
ithout any Cr(VI) production. The amino-acid co-ligands enhancehe ˚VI values but prolonged exposure of the [CrIII(C2O4)2(Aa)]n−
omplexes to UV irradiation induces the Cr(VI) photoreductionccompanied by oxidation not only ligands but also exterior donorsEqs. (5), (6), (21), and (23)) enhancing and enriching thereby theange of degraded organic pollutants. The amino acid co-ligandslay in these processes a special role (cf. Fig. 4). The highest yield ofoth the Cr(VI) production and Cr(VI) photoreduction was observedor serine and cysteine (R = CH2OH or CH2SH). The theoretical cal-ulation reported earlier showed that for serine and cysteine cationadical the loss of the side chain, CH2OH+ and CH2SH+, respectively,ith formation of glycyl radical is clearly preferred, although for
ther amino acids the loss of COOH+ is the most favorable process42].
In any case the Cr(VI) production can be easily suppressed bycidifying the medium; thus considering the usefulness of Cr com-ounds for the purposes of the environmental cleaning one has twoptions to choose: avoiding or favouring chromate(VI) formationnd its photoredox activity; the former path is more safe, whereashe latter is more efficient.
. Conclusions
The results of this paper demonstrate that oxalatochromates(III)oth homoleptic and heteroleptic undergo intra- and intermolec-lar photoreduction induced by the LMCT excitation (Eqs. (10) and13)). The secondary processes proceeding in oxygen-free mediumonsist in e−
aq scavenging that regenerates Cr(III) complexes in theirnitial or hydrated form (Eqs. (14–19)).
In presence of molecular oxygen the redox processes becomeore complex, as O2 is not only efficient e−
aq scavenger (Eq.20)), but also oxidizes Cr(II) to Cr(III) and Cr(VI) (Eq. (7)). The
fficiency of the latter process depends considerably on theo-ligand nature: it is higher in the case of the N,O-bondedo-ligands than for homoleptic oxalate and decreases in therder:
d [Cr(C2O4)2(Aa)]n− solutions at pH = 10.5: (a) an example of absorption changes) Cr(VI) concentration changes (C in 0.1 mM) vs. irradiation time for different
The ˚VI is sensitive to the Aa side chain substituent (Table 2)as well as to the isomeric form of the pda co-ligand [17]. The pho-toreduction of the Cr(III) complexes is accompanied by oxidation ofligands, not only oxalate (Eq. (17)), but also amino acid (Eq. (18)).
Considering the environmental consequences of the Cr(III) pho-toreduction we conclude that it contributes to the self-cleaningprocesses, provided that (i) sunlight energy is sufficient to inducethe LMCT excitation (Eqs. (1) and (10)) and (ii) the yield of Cr(VI)(Eq. (7)) is suppressed or Cr(VI) is efficiently photoreduced. The firstprerequisite is fulfilled in the case of homoleptic oxalate for whichthe ˚VI value is the least among the studied Cr(III) complexes; itscleaning efficiency at sunlight is, however, relatively low. On theother hand, the [CrIII(C2O4)2(Aa)]n− complexes, where Aa = cysteineor serine, are characterized by the highest yield of both the Cr(VI)production and Cr(VI) photoreduction. Hence, the photochemicalcleaning in the case of these co-ligands is much more effective.
Acknowledgements
Authors thank Mr. Zygmunt Wołek for his assistance in prepa-ration of the manuscript. Part of the research was carried out usingthe Perkin Elmer Lambda 950 UV/vis/NIR spectrophotometer pur-chased thanks to the financial support of the European RegionalDevelopment Fund in the framework of the Polish Innovation Econ-omy Operational Program (contract no. POIG.02.01.00-12-023/08).
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