Triplet state of 5-nitro-2-furoic acid by laser flash photolysis. Spectrum, lifetime and reactivity

Abstract

Nanosecond laser flash photolysis (347.1 nm) of 5-nitro-2-furoic acid (NFA) in both polar and non-polar solvents at 295 K shows a transient absorption centred at 490 ± 5 nm with εca. 2 × 104 dm3 mol–1 cm–1 which is assigned to the lowest triplet excited state of NFA, denoted 3NFA. While λmax of 3NFA is independent of the solvent polarity, its lifetime is strongly solvent-dependent, being 22.7 ns in water and 274 ns in MeCN. Triplet–triplet energy transfer experiments showed the triplet energy ET to be 240 ± 4 kJ mol–1. 3NFA is extremely reactive, thus it abstracts a hydrogen atom from diphenyl-amine with k2=(8.64 ± 0.44)× 109 dm3 mol–1 s–1 and from diphenylmethanol with k2=(6.94 ± 0.55)× 107 dm3 mol–1 s–1, both in acetone solution. Representative of electron-transfer rates to 3NFA are those of CO2–3 and NNN′ N′-tetramethyl-p-phenylenediamine with k2=(1.56 ± 0.10)× 108 dm3 mol–1 s–1 and k2=(5.98 ± 0.20)′× 109 dm3 mol–1 s–1, respectively. The logarithm of the rate constant of oxidation of a series of electron donors by 3NFA correlates well with the corresponding electrode potentials up to the diffusion limit, although the figure for F– notably fails to fit the plot. The reduction potential of 3NFA is estimated to be 2.17 V. Both the lack of spectral shift with increasing solvent polarity and the reactivity towards hydrogen-atom donors indicate that 3NFA is an nπ* state which retains its nπ* character even in polar solvents.