Photosubstitution quantum yields

The failure to observe photosubstitution in the presence of a sensitizer in which the latter is the principal absorber, the invariance of product quantum yield with wavelengths shorter than 350 nm (onset of n -> -n absorption), and the observation that chloride and bromide ions (known to catalyze S-+T intersystem crossing) strongly diminish the quantum yields of these reactions, strongly points to the lowest excited ir- n singlet state as the reactive species in these transformations. Excitation into the n->ir absorption band results in little product formation. A triplet state may, however, be involved in the photoamination of nitrobenzene.a41)... 

Figure 1. Spectrum of Ru3(CO) 2 in octane. Quantum yields for photosubstitution and photofragmentation in 25°C argon flushed octane in the presence in 0.012 M P(OCH3>3 represented as a function of irradiation wavelength (from reference 5).

On irradiation, the cluster molecule Os3(CO)12 is resistant to Os-Os bond cleavage and instead undergoes photosubstitution with a low quantum yield ... 

Generally, clusters of nuclearity >3 undergo photosubstitution with low quantum yield ... 

When o-, m- and p-nitroanisole with 14C-labelled at the methoxy group were irradiated under identical conditions in methanol in the presence of sodium methoxide, only m-nitroanisole underwent methoxy exchange, with the limiting quantum yield (

labelled isotope experiments support a a complex intermediate and indicate an Sjv23Ar mechanism (direct substitution in the triplet state) for this reaction (equation 12) and for 4-nitroveratroles (equation 13). Further evidence from quenching and lifetime experiments also support a direct displacement SAr2Ar mechanism for the photosubstitution reaction of nitroaryl ethers with hydroxide ions13. 

Nitropyrrole does not seem to undergo any photosubstitution with either methoxide, cyanate, cyanide or water 123> 2-Nitrothiophene 1 a) and 5-bromo-2-nitrothiophene (1b) undergo photocyanation smoothly and efficiently. The disappearance quantum yield for 7 a,b equals the product quantum yield. 

Already, with one of the first nucleophilic aromatic photosubstitutions encountered, curious behaviour was found when studying the rate of reaction as a function of pH. m-Nitrophenyl sulphate shows no increase in the quantum yield of photohydrolysis with increase of hydroxide ion concentration up to values as high as 0-1 M. This behaviour, also found with one other compound (5-chloro-3-nitroanisole), is in clear contradistinction to what is... 

With the reactions discussed so far, variation of the wavelength of irradiation has only a minor influence on the quantum yield of nucleophilic aromatic photosubstitution (Section 1). Nitroazulenes are found to be interesting exceptions (Lok et al., 1973). 

Chlorine and bromine react under favourable conditions (activation by other substituent, appropriate nucleophile). Iodine may also be photosubstituted by nucleophiles but is easily induced to enter into homolytic reaction pathways. One has to bear in mind that the heavier substituents (iodine, bromine and even chlorine) increase the rate of intersystem crossing which, depending on the conditions, increases or decreases the quantum yield. 

Table 3 Photosubstitution Quantum Yields for some d6 Complexes...

Photocalorimetry offers a convenient alternative to other methods of AH determination and, in some instances, may be the only practical method. The ligand substitution reactions of robust Werner-type complexes are a case in point. Conventional thermochemical measurements are complicated by the slowness of the substitution process and/or by competing reactions. Some of these same complexes, however, undergo clean photosubstitutions with high quantum yields and thus are excellent candidates for photocalorimetry. Examples include [Cr(NH3)6]3+, [Cr(CN)6]3-and [Co(CN)6]3-.192 Photocalorimetric measurements of AH have also been obtained for isomerization and redox reactions of coordination compounds.193194... 

The complexation of coordination compounds may make it possible to control their photochemical behaviour via the structure of the supramolecular species formed. For instance, the binding of cobalt(m) hexacyanide by macrocyclic polyammonium receptors markedly affects their photoaquation quantum yield in a structure-dependent manner [8.73-8.77]. It thus appears possible to orient the photosubstitution reactions of transition-metal complexes by using appropriate receptor molecules. Such effects may be general, applying to complex cations as well as to complex anions [2.114]. 

There is no general consensus on why the difference in the quantum yield of photosubstitutions is so large for 02-adducts (4> 10 3) and CO-adducts and on which excited states are responsible for this difference. An explanation based on a different efficiency of the recoordination of released 02 or CO molecules (geminate recombination) can be ruled out, as in the systems with the same biocomplex (e.g. Hb02 and HbCO) both molecules (02 and CO) have nearly identical escaping probability from the protein cage due to their similar size, mass and polarity. The reason could, therefore, lie in the different photoreactive excited states involved. 

However, Reveco et al.181 observed a pronounced temperature dependence for the emission quantum yield in [Ru(bpy)2(NPP)]+ and concluded there to be an energy separation between emitting and deactivating states. The complete absence of photosubstitution in this complex led them to propose that the deactivating state was MLCT with significant singlet character, and not ligand field. Allen et al.182 had previously considered the possibility of this interpretation. 

The second photochemical reaction which was studied was the reaction of CotCO NO with Lewis base ligands L (J 6 ). The observed solution phase photochemical reaction is carbonyl photosubstitution. This result initially did not appear to be related to the proposed excited state bending. Further reflection led to the idea that the bent molecule in the excited state is formally a 16 electron coordinatively unsaturated species which could readily undergo Lewis base ligand association. Thus, an associative mechanism would support the hypothesis. Detailed mechanistic studies were carried out. The quantum yield of the reaction is dependent on both the concentration of L and the type of L which was used, supporting an associative mechanism. Quantitative studies showed that plots of 1/ vs. 1/[L] Were linear supporting the mechanism where associative attack of L is followed by loss of either L or CO to produce the product. These studies support the hypothesis that the MNO bending causes a formal increase in the metal oxidation state. 

Figure 3- Electronic absorption spectrum of Fe(C0)T(l, 4-4102%) and quantum yields for photosubstitution of CO by PPI13. Reproduced from Ref. 21. Copyright 1981, American Chemical Society.

The work reports wavelength dependent quantum yields for the photoreaction and photosubstitution yields for the Os(III) complex are much lower than those of the iron or ruthenium analogs. The authors also report that the complexes exhibit weak luminescence in the green the Os(III) complex has a reported emission yield of 0.008 [35]. 

Of the Group VIII systems which have been studied Co(III) complexes are the best understood photochemically and electronically. Generally, Co(III) - cyanide and Co(III) — ammine complexes have been most well-studied and the former are the most photosensitive 46 but the latter have recently been examined.47 With respect to photosubstitution though the quantum yields for reaction are very small (< 10-2) upon population of the LF excited states. The Oh Co(CN)6- complex undergoes reaction (10) with a quantum yield of 0.3 at either 254 or 313 nm, i.e., direct population of either the 1 Tig or the state yields substitution with the same quantum ef-... 

The Rh(III)- and lr(lll)-ammines undergo efficient photosubstitution in marked contrast to the Co(III) analogues as mentioned above. However, the efficient photosubstitution found for the second and third row metal systems does appear to obey the essential rationalizations outlined in the models for LF substitutional reactivity. Two sets of complexes seem to behave quite nicely in this regard C4v, Rh(NH3)s X2+ 58) and DAh trans-M(en)2Xl (M = Ir, Rh X = Cl, Br, I).59,6°) In both sets of complexes the lowest excited state is likely one which features population of the d22(a ) orbital. Consistent with this fact, the photosubstitution chemistry can be viewed as resulting from loss of a ligand on the z-axis. For the Rh(III)-amines a dissociative mechanism is suggested for Cl photoaquation by the quantum yield data in Table 8.60) The... 

In both Re(CO)5X and M(CO)s(amine) higher energy photolysis leads to population of LF states which feature population of the dxzy2 bi )° y orbital which labilizes the equatorial CO s and leads to larger CO substitution quantum yields. In all of these C4v complexes the ligand photosubstitution most likely occurs by strictly a dissociative mechanism to yield coordinatively unsaturated intermediates. For the Re(CO)sX, photolysis in the absence of added nucleophiles yields the dimeric species [Re(CO)4X]2 reaction (12), which likely form via coupling of two coordinatively unsaturated Re(CO)4X intermediates.68 ... 

Styrylpyridine photochemistry has been important in one other system. The photoprocesses in the complexes W(CO)sX (X = pyridine, 2-styrylpyridine, and 4-styrylpyridine) have been investigated.129 Unlike the XRe(CO)3(taz s-styryl-pyridine)2 and Ru(II)-styrylpyridine complexes having lowest IL and CT states, respectively, the W(CO)5X complexes have lowest LF excited states with only a small contribution from W - pyridyl CT. The one-electron diagram for low-spin d6, Cnv complexes shown in Scheme 7 is appropriate here. Both photosubstitution and photoisomerization reactions are found for the W(CO)5X complexes and some quantum yield data are found in Table 23.129 The data show that substitution efficiencies for... 

Quantum yields have been measured for the photoaquation of a large range of substituted pyridine complexes of the type [Ru(NH3)5(pyX)]2+.53 The marked dependence of quantum yields on the nature of X indicates that metal-to-ligand-charge-transfer (MLCT) excited states are not involved in the photosubstitution. Presumably, a ligand-field excited state is responsible. Evidence has been reported for a simple outer-sphere reduction of cytochrome c by [Ru(NH3)6]2+.54 Such a... 

A particularly promising feature of the Ru(terpy)(phen)(L)2+ series, in relation to future molecular machine and motors, is related to the pronounced effect of steric factors on the photochemical reactivity of the complexes [84]. When the bulkiness of the spectator phenanthroline moiety was increased, the steric congestion of the coordination sphere of the ruthenium complex also increased. This increased congestion was qualitatively correlated to the enhanced photoreactivities of these complexes (Fig. 14). More specifically, changing phen for dmp increased by one to two orders of magnitude the quantum yield of the photosubstitution reaction of L by pyridine with L = dimethylsulfide or 2,6-dimethoxybenzonitrile. 

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