The triplet state of N-(n-butyl)-5-nitro-2-furamide by laser flash photolysis. Spectrum, lifetime, energy and electron-transfer reactions

Abstract

Upon nanosecond laser flash photolysis (347.1 nm) of N-(n-butyl)-5-nitro-2-furamide (BNFA) in solution (at 298 K), a short-lived transient absorption has been observed with λ_max= 500 ± 5 nm, which shows a small bathochromic shift on going to hydroxylic solvents (irrespective of polarity) and which is attributed to its lowest excited triplet state (³BNFA). A long-lived species, formed in high yield in alkanols, is assigned to the radical anion (BNFA^˙–) and has λ_max⩽ 360 nm with a weaker band peaking ca. 510 ± 5 nm in propan-2-ol. The lifetime of ³BNFA depends on both the solvent and the ground-state concentration of BNFA, having its smallest value in water (ca. 19 ns). Sensitization and quenching experiments enabled the triplet energy, E_T, to be estimated as 238 ± 4 kJ mol^–1; this value, together with the ground-state reduction potential, E¹₇, yields E¹₇(³BNFA/BNFA^˙–)= 2.23 V vs NHE for the one-electron reduction potential of ³BNFA in neutral aqueous solution. ³BNFA is rather oxidizing, thus it abstracts a hydrogen atom from diphenylmethanol with k₂=(3.01 ± 0.12)× 10⁷ dm³ mol^–1 s^–1 in acetone solution, and the second-order rate constants for the one-electron oxidation of a number of substrates by ³BNFA have been measured in 1 : 4 (v/v) water + acetonitrile mixtures and range from ca. 2 × 10⁸ for CH₃CO^–₂ to ca. 6 × 10⁹ dm³ mol^–1 s^–1 for I^–. Evidence for complete electron transfer arises, in several cases, from the observation of the semi-oxidized substrate. The correlation of the logarithm of k₂ with the standard free-energy change, obtained from the estimated thermodynamic potentials of the reactants, are discussed in terms of the Rehm–Weller and Marcus free-energy relationships. Absolute rate constants for the one-electron reduction of the lowest triplet states of duroquinone, 2-nitrothiophen and 5-nitro-2-furoic acid are also included in this correlation. Pulse photolysis of a solution of 2-nitropyrrole showed no absorbing transient in the spectral region 390–700 nm. The implications of some of these findings for nucleophilic photosubstitution reactions of 5-membered nitroheterocyclic compounds are discussed.