Miquel Mir

Efficiency of singlet oxygen generation from the triplet states of nitrophenyl ethers

Abstract The bimolecular rate constants for oxygen quenching, k O 2 T and the quantum yields of singlet oxygen production, Δ , for 4-nitroveratrole, 4-nitroanisole and 3-nitroanisole triplet states in air-equilibrated deuterated water and acetonitrile solutions were determined where possible from nanosecond laser flash photolysis and time-resolved O 2 ( 1 Δ g ) phosphorescence measurements. The results show that the efficiency of singlet oxygen production, ƒ Δ T, for 4-nitroveratrole and 3-nitroanisole is moderately high and is related with the nature of the low-lying triplet state and that 4-nitroanisole is not able to sensitize singlet oxygen under these conditions.

Solid state photochemistry of ternary cyclodextrin complexes: Total selectivity in the photoreduction of nitrophenyl ethers by 1-phenylethylamine

Abstract The irradiation of the ternary complexes of β-cyclodextrin with several nitrophenyl ethers, and 1-phenylethylamine in the solid state, produces selective photoreduction to the corresponding nitroso derivatives.

Primary photophysical properties of 4H-1-benzopyran-4-thione in cyclodextrin complexes

The influence of cyclodextrins (CDs) on the photophysics of 4H-1-benzopyran-4-thione (BPT) has been studied in solution and in the solid state by steady-state absorption, emission and circular dichroism as well as by time-resolved diffuse reflectance laser flash photolysis. The spectral changes and induced circular dichroism indicate the formation of the cyclodextrin complexes of BPT. The very low fluorescence quantum yield from the S 2 state for this complex, similar to that measured for BPT alone in water, suggests that there is an interaction between water and guest molecules in the CD complexes leading to shortening of the S 2 state lifetime. The Albery model was used to analyse the phosphorescence of the BPT·β-CD solid complex. The values obtained for the average rate constants, , are similar to values of the triplet state decay constant of the thioketone in inert solvents at infinite dilution, k 0 . Quenching by oxygen does not play an important role when thioketones are included within cyclodextrins in the solid state.

Selective photoreduction of nitrophenyl ethers by amines in ternary β-cyclodextrin complexes. A spectroscopic and mechanistic study

The photoreduction of nitrophenyl ethers by amines has been studied in aqueous solution in the presence of β-cyclodextrin (β-CD), and in ternary β-cyclodextrin complexes in the solid state. NMR (NOESY) and induced circular dichroism (ICD) studies in solution suggest the operation of a cooperative effect by the amine in the complexation of the nitrophenyl ether by the β-CD, evidencing the existence of interactions between complementary groups that govern the structure of the ternary complexes. Irradiation of the solutions shows an inhibitory effect of the β-CD, together with a significant increase in the chimioselectivity of the photoreaction. This tendency is confirmed by irradiation of the solid ternary complexes that show a complete selectivity, only producing the nitroso derivative. A time-resolved diffuse reflectance laser flash photolysis (DRLFP) mechanistic study on the solid complexes has demonstrated that in the ternary β-CD complexes, the evolution of the first transient (assigned to the triplet excited state of the nitrophenyl ether) is completely different than in solution or in mechanical cellulose mixtures, this approach being a very valuable tool to investigate bimolecular photoinduced reactions difficult to study in solution due to the existence of other competitive processes.

Study of bimolecular processes in ternary cyclodextrin complexes in the solid state by diffuse reflectance laser flash photolysis

Abstract The photochemistry of binary inclusion complexes of nitroveratrole (NV) and p -dinitrobenzene (DNB) with β-cyclodextrin (β-CD) and of ternary inclusion complexes of NV or DNB and L-phenylethylamine (PhEA) with β-CD has been examined in the solid state by diffuse reflectance laser flash photolysis. The binary NV/β-CD complex shows a transient absorption signal assigned to the triplet excited state of NV. The binary DNB/β-CD complex shows a transient signal assigned to the protonated anion of DNB formed via hydrogen abstraction from the β-CD cavity. Laser excitation of the ternary complexes generates the corresponding radical ion pairs via photoinduced electron transfer from the amine to the nitroaromatic triplet state. Comparison with previously described conventional laser flash photolysis studies in solution shows that the investigation of solid ternary CD inclusion complexes by diffuse reflectance laser flash photolysis is valuable in the case of bimolecular photoreactions that are difficult to study in solution due to the existence of other competitive processes.

Study of the effect of β-cyclodextrin on the photophysicsof alloxazines in the solid state

The influence of β-cyclodextrin (β-CD) on the photophysics of alloxazines† (lumichrome, Lch; 7-methylalloxazine, 7MA11; and 8-methylalloxazine, 8MA11) has been studied in the solid state by ground-state absorption and steady-state fluorescence methods. We support the existence of inclusion complexes between Lch and β-CD in water. The presence of similar complexes for 7MA11 and 8MA11 has also been found. Complexes of alloxazines and β-CD are also present in the solid state. The formation of hydrogen bonds in solid inclusion complexes between alloxazines and β-CD have been postulated in order to explain the luminescence behaviour of these solid complexes. New spectroscopic evidence has been found in the solid state of doubly hydrogen-bonded lumichrome dimers capable of executing double proton transfer in the excited state.

Photosubstitution-photoreduction mechanistic duality in the SET photoreactions of nitrophenyl ethers with amines. The role of the steps that follow the ET

Abstract Nitrophenyl ethers are photoreduced by primary amines in water through a mechanism initiated by single electron transfer that is in direct competition with the single electron transfer photosubstitution mechanism (S N Ar∗-SET). Our results indicate that the preferred pathway does not depend on the electron donor or proton donor ability of the amine. The key factor that determines the progress of the photoreaction is the structure of the carbon skeleton of the amine, particularly the type of hydrogens on the carbon α to the amino group. A mechanistic rationale that includes hydrogen atom transfer as a key step is discussed.

Photophysics of xanthione and 4H-1-benzopyran-4-thione inβ-cyclodextrin complexes

The phosphorescence decays of β-cyclodextrin (β-CD) complexes of xanthione (XT) and 4H-1-benzopyran-4-thione (BPT) show that self-quenching and oxygen quenching are not important in such complexes, and that the existence of bimodal inclusion in solid complexes can be identified using the width of the distribution parameter of the Gaussian distribution of the logarithm of the observed rate constants for decay.

Reductively activated “Polar” nucleophilic aromatic substitution. III : The reactions of polyfluoronitrobenzenes with methanol☆

The nucleophilic aromatic substitution reactions of pentafluoronitrobenzene (PFNB) and 2,3,4,5-tetrafluoronitrobenzene with methanol are both reductively activated. However, the first one is sensitive to the presence of radical scavengers but the second is not. In addition, the kinetic behavior of both reactions in the absence of stimulation is very similar suggesting they are examples of second stage rate determining SNAr mechanism. Kinetic studies also show that no real photochemical stimulation exist in those reactions. The reported facts are not compatible with the previously proposed (for the reaction of PFNB with methanol) SRN2 mechanism. A new mechanistic rationale is discused.

Reductively activated “Polar” nucleophilic aromatic substitution. II. The reaction of p-dinitrobenzene and p-nitrobenzonitrile with charged and neutral nucleophiles

Electrochemical studies indicate that although the reactions of p-dinitrobenzene and p-nitrobenzonitrile with phenolate or phenol in DMF show radical features, they can not be attributed to the direct reaction of the nucleophile on the substrate radical anion. Reductive activation is feasible in certain cases (reaction of p-dinitrobenzene with phenol). However, substrate radical anion formation is not responsible for it.