Photophysics and photochemistry of the ß-lapachone derived diphenyldihydrodioxin: generation and characterization of its cation radical.

The photophysics and photochemistry of the ß-lapachone derived diphenyldihydrodioxin 3 were investigated using steady-state and time resolved techniques. Laser excitation of 3 leads to the formation of its cation radical 4 (absorption maxima at 410 and 450 nm and a lifetime of 10 µs), which was confirmed by its thermal generation employing tris(2,4-dibromophenyl)-aminium hexachloroantimonate (BAHA) as the electron acceptor. The cation radical 4 was also formed via the triplet excited state of 3 through a triplet sensitized process using benzophenone (ET = 69 kcal mol(-1)) as the sensitizer.

3,3-Dimethyl-cis-9a,13a-diphenyl-2,3,9a,11,12,13a-hexa-hydro-1H-benzo[h][1,4]dioxino[2',3':5,6][1,4]dioxino[2,3-f]chromene.

In the title di-hydro-dioxin, C31H28O5, the dioxane ring has a chair conformation, whereas each of the pyran and dioxine rings has an envelope conformation with methyl-ene and quaternary C atoms, respectively, being the flap atoms. The phenyl rings are cis and form a dihedral angle of 82.11 (10)°. The molecular structure is stabilized by C-H⋯O contacts. In the crystal packing, supra-molecular layers parallel to (101) are sustained by C-H⋯π inter-actions.

Photosensitizing properties of triplet beta-lapachones in acetonitrile solution.

The photochemical reactivity of beta-lapachone (1), nor-beta-lapachone (2) and 1,2-naphthoquinone (3) towards amino acids and nucleobases or nucleosides has been examined employing the nanosecond laser flash photolysis technique. Excitation (lambda = 355 nm) of degassed solutions of 1-3, in acetonitrile, resulted in the formation of their corresponding triplet excited states. These transients were efficiently quenched by l-tryptophan, l-tryptophan methyl ester, l-tyrosine, l-tyrosine methyl ester and l-cysteine (k(q) approximately 10(9) L mol(-1) s(-1)). For l-tryptophan, l-tyrosine and their methyl esters new transients were formed in the quenching process, which were assigned to the corresponding radical pair resulting from an initial electron transfer from the amino acids or their esters to the excited quinone, followed by a fast proton transfer. No measurable quenching rate constants could be observed in the presence of thymine and thymidine. On the other hand, efficient rate constants were obtained when 1-3 were quenched by 2'-deoxyguanosine (k(q) approximately 10(9) L mol(-1) s(-1)). The quantum efficiency of singlet oxygen ((1)O(2)) formation from 1 to 3 was determined employing time-resolved near-IR emission studies upon laser excitation and showed considerably high values in all cases (Phi(Delta) = 0.6), which are fully in accord with the pipi character of these triplets in acetonitrile.

Characterization, reactivity and photosensitizing properties of the triplet excited state of alpha-lapachone.

alpha-Lapachone is a natural 1,4-naphthoquinone with promising biological activity. The fused dihydropyran ring present in its structure, acting as formal 2-alkoxy and 3-alkyl substituents to the quinone moiety, endows this compound with milder redox properties and lower toxicity, when compared with other bioactive 1,4-quinones. Its photochemistry, here reported, seems to originate from the triplet state, which shows pipi* character. Triplet quenching in acetonitrile solution with added hydrogen-atom donors such as 1,4-cyclohexadiene or 2-propanol is inefficient, independent of solvent polarity, and leads to formation of the semiquinone radical. With phenol and indole, quenching rate constants are two orders of magnitude higher, but smaller than the value for triethylamine. In the first two cases the semiquinone radical can be detected by laser flash photolysis and in the last case, the anion radical derived from alpha-lapachone is readily detected. The semiquinone radical can also be observed in the quenching of triplet alpha-lapachone by 2'-deoxyguanosine and by the methyl esters of L-tryptophan and L-tyrosine, whereas for L-cysteine methyl ester the quenching rate constant is very slow. Triplet alpha-lapachone is not quenched by thymine, thymidine, 2'-deoxycytosine or 2'-deoxyadenosine; this is probably due to its pipi* character and low energy, which prevents oxetane formation and triplet-triplet energy transfer, respectively. Steady-state photolysis of aerated solutions of these compounds in the presence of alpha-lapachone does not show evidence of decomposition, whereas similar experiments with 2'-deoxyguanosine result in efficient consumption of the nucleoside. Singlet oxygen is formed from triplet alpha-lapachone, and a quantum yield of 0.68 is measured.

Laser flash photolysis study of the triplet reactivity of beta-lapachones.

The photochemical reactivity of beta-lapachone (1), nor-beta-lapachone (2) and beta-lapachone 3-sulfonic acid (3) has been examined by laser flash photolysis. Excitation (lambda = 266 nm) of degassed solutions of , in acetonitrile or dichloromethane, resulted in the formation of detectable transients with absorption maxima at 300, 380 and 650 nm. These transients, with lifetimes of 5.0 micros, were quenched by beta-carotene at a diffusion-controlled rate constant and assigned to the triplet excited states of 1-3. Addition of hydrogen donors, such as 2-propanol, 1,4-cyclohexadiene, 4-methoxyphenol or indole led to the formation of new transients, which were assigned to the corresponding ketyl radicals obtained from the hydrogen abstraction reaction by the triplets 1-3 . In the presence of triethylamine it was observed the formation of the long-lived anion radical derived from , which shows absorption maxima at 300 and 380 nm. The low values observed for the hydrogen abstraction rate constants for the beta-lapachones 1-3 using 2-propanol and 1,4-cyclohexadiene as quenchers led us to conclude that their triplet excited states show pi pi* character.