António Dias

Dipole–dipole interactions between the terminal groups of 1,n-diarenecarboxy alkanes, n= 1, 2, …, 6

The mutual interaction between the end groups of bichromophoric molecules is studied using absorption and fluorescence techniques (steady-state and time-resolved). With three series of di-1,n-arenecarboxyalkanes (arene = benzene, naphthalene and anthracene; n= 1, 2, 3, 5 and 6) and a series of 1-pyrenecarboxy, n-(N-methyl, N-phenylamino) alkanes (n= 2, 3, 4, 5, 6 and 9), it is found that model compounds such as the methyl, ethyl and hexyl esters of benzene, naphthalene, anthracene and pyrene differ from the bichromophoric compounds in their absorption and emission spectra, molar absorption coefficients and radiative rate constants. These differences decrease with the mean value of the sixth power of the end-to-end distance, being attributed to dipole–dipole interactions between the end groups. The terminal group induces mixing of the lowest excited singlet state with higher energy states and increases the local polarizability of the medium. Two methods for evaluating the unquenched fluorescence lifetimes of bichromophoric compounds are discussed.

Photochemistry of flavothione and hydroxyflavothiones: mechanisms and kinetics.

In this work we present a detailed study of the mechanism of photochemistry and thermal reactions, as well as of the kinetics of flavothione (FLT) in ethanol. Furthermore, we analyzed how the hydroxysubstitution pattern of FLT influenced both the kinetics and the mechanism relative to the parent FLT. We show that the primary photochemical reaction of FLT in the absence of oxygen is hydrogen (H)‐atom abstraction from the solvent by way of the excited triplet state of FLT. Several products result from thermal reactions of the resulting semireduced FLTH· radical, including more than one dimer. A full mechanism is proposed, and the relevant rate constants are evaluated. On the other hand, in the presence of oxygen and a low concentration of FLT, we found that the principal photoproduct is the parent flavone (FL). The reaction leading to photoxidation is not via1O2 attacking a thione, but instead, it is via a reaction of the FLTH· radical with ground state oxygen. The kinetic data also demonstrate that the relative values of concentrations of reactants and the rate constants of the reactions can control the dominance of one mechanism over others. We also have examined the photochemical mechanisms and kinetics for several hydroxyflavothiones (n‐OHFLT) and compared them with FLT itself. We have found that the photochemical mechanism radically changes depending on the positions of substitution. These differences are directly related to the ordering of the excited states of the n‐OHFLT. Specifically, FLT with lowest 3n,π* states (FLT, 6‐hydroxyflavothione, 7‐hydroxyflavothione and 7,8‐dihydroxyflavothione) efficiently abstract H atoms to give the semireduced radical of the thione. The radical can ( 1 ) dimerize to form two different dimers; ( 2 ) react with oxygen to produce the parent FL; and ( 3 ) recombine with the solvent radical to yield the original FLT. In contrast, FLT with lowest 3π,π* states (3‐hydroxyflavothione, 3,6‐dihydroxyflavothione and 3,7‐dihydroxyflavothione) behave as photosensitizers of oxygen to form singlet oxygen, which then reacts with the ground state of the substituted FLT. Finally, when T2(π,π*) is above S1(n,π*), as for 5‐hydroxyflavothione and 5,7‐dihydroxyflavothione, both the S1(n,π*) → T1(n,π*) intersystem crossing and photodegradation are inefficient.